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Modulation of the magnetic anisotropy of octahedral cobalt(II) single-ion magnets by a fine-tuning of axial coordination microenvironment
Abstract
Two mononuclear cobalt(II) complexes, with formulas: [Co(2,6-dfba)2(bpp)2(H2O)2]n (1) and [Co(2,6-dfba)2(bpe)2(H2O)2]n (2)
(2,6-Hdfba = 2,6-difluorobenzoic acid, bpp = 1,3-bis(4-pyridyl)propane, bpe = 1,2-bis(4-pyridyl)ethylene), have been
synthesized by combing Co(II) ion with benzoate derivatives and two homogeneous N-donor ligands, respectively.
Constrained by the analogous CoN2O4 coordination spheres, the discretely hexa-coordinated Co(II) cores in both complexes
display stretched octahedral geometries. The equatorial environments in both complexes are identical, whereas the axial
sites are finely modulated by the different chemical nature of the terminal N-donor ligands. The combined analyses of the
magnetic data, the high frequency electron paramagnetic resonance (HF-EPR) and the ab initio calculations unveil large easyplane magnetic anisotropies for both complexes (D = +53.19 and +65.67 cm-1 for 1 and 2, respectively), which uniformly
perform field-induced single-ion magnet (SIM) behaviors with effective barriers (Ueff) of 45.34 (1) and 57.97 K (2). This work
demonstrates how a fine-tuning the coordination microenvironment of the metal ions results in a non-negligible
manipulation of their magnetic anisotropy.
... To put the observed changes in magnetic relaxation in context with the change of geometry of coordination environment, the difference of symmetry measure parameter S(Oh) from 0.198 (I) to 0.306 (II) is associated with decrease of relaxation time limit τ0 from 3.2·10 −4 s to 1.0·10 −5 s at the field of 0.1 T. For comparison, a recent work by Liu et al. [73] reported the effect of subtle geometry changes in isolated octahedral complexes with the same coordination environment CoN2O4 but an easy-plane magnetic anisotropy. In their work, the difference of symmetry measure parameter from S(Oh) = 0.025 (Co1) and S(Oh) = 0.067 (Co2) to S(Oh) = 0.117 induced a small increase of relaxation time limit τ0 from 1.67·10 −8 s to 1.88·10 −8 s at the field of 0.2 T. ...
... To put the observed changes in magnetic relaxation in context with the change of geometry of coordination environment, the difference of symmetry measure parameter S(O h ) from 0.198 (I) to 0.306 (II) is associated with decrease of relaxation time limit τ 0 from 3.2 × 10 −4 s to 1.0 × 10 −5 s at the field of 0.1 T. For comparison, a recent work by Liu et al. [73] reported the effect of subtle geometry changes in isolated octahedral complexes with the same coordination environment CoN 2 O 4 but an easy-plane magnetic anisotropy. In their work, the difference of symmetry measure parameter from S(O h ) = 0.025 (Co1) and S(O h ) = 0.067 (Co2) to S(O h ) = 0.117 induced a small increase of relaxation time limit τ 0 from 1.67 × 10 −8 s to 1.88 × 10 −8 s at the field of 0.2 T. ...
Two previously synthesized cobalt(II) coordination polymers; {[Co(μ2-suc)(nia)2(H2O)2]·2H2O}n (suc = succinate(2−), nia = nicotinamide) and [Co(μ2-fum)(nia)2(H2O)2]n (fum = fumarate(2−)) were prepared and thoroughly characterized. Both complexes form 1D coordination chains by bonding of Co(nia)2(H2O)2 units through succinate or fumarate ligands while these chains are further linked through hydrogen bonds to 3D supramolecular networks. The intermolecular interactions of both complexes are quantified using Hirshfeld surface analysis and their infrared spectra, electronic spectra and static magnetic properties are confronted with DFT and state-of-the-art ab-initio calculations. Dynamic magnetic measurements show that both complexes exhibit single-ion magnet behaviour induced by a magnetic field. Since they possess very similar chemical structure, differing only in the rigidity of the bridge between the magnetic centres, this chemical feature is put into context with changes in their magnetic relaxation.
... Previous experimental and theoretical studies have demonstrated that MA is affected by various factors, including: (i) SOC of the metal ion [36][37][38][39], (ii) shape of metal electron density in the ground state [40,41], (iii) surrounding ligand environment and symmetry [36,40,[42][43][44][45][46], (iv) orbital ordering and degeneracy [46][47][48], (v) intermolecular and intramolecular exchange interactions, etc. [49,50]. The structures of TM molecules are highly sensitive to all these parameters, posing significant challenges for theoretical and experimental analyses [29,[37][38][39]51]. ...
Single-molecule magnets (SMMs) possess a crucial property called magnetic anisotropy (MA), which has an exceedingly delicate correlation with their structures. In recent years, the study on magneto-structural correlations has emerged as a challenging area in singlemolecule science. Understanding the fundamental physical mechanisms underlying the magneto-structural correlations is essential for building excellent high-temperature SMMs. In this work, we screened various four-coordinated nickel(II) SMMs and studied several key structural factors, such as the lengths and angles of the coordination bonds that may be closely associated with MA. Following that, we developed simple molecular models to deduce the evolution trends of MA with coordination bond angles and lengths. The findings on the magneto-structural correlations stimulated our interest to further explore the crystal structure database. We revealed that the magneto-structural correlation can be well described by a logarithmic function. Guided by such a relationship, we discovered a nickel(II) complex with the strongest MA to date among the tetrahedral-coordinated ones. Our work may be helpful for the empirical synthesis of exceptional high-temperature SMMs.
... The positive sign of the zfs parameter arises from the interaction between the ground and excited electronic states coupled through the SOC and illustrates the strong easy-plane anisotropy of the high-spin cobalt(II) in 1. The value of D for 1 lies within the range of those reported for other examples of SIMs with six-coordinate cobalt(II) ions[17,[56][57][58][59][60][61][62][63][64][65][66][67][68][69][70]. ...
A new mixed-valence one-dimensional coordination polymer of formula {[CoII(MeOH)2][(μ-NC)2CoIII(dmphen)(CN)2]2}n·2nH2O (1) was obtained by reacting the Ph4P[CoII(dmphen)(CN)3] metalloligand (dmphen = 2,9-dimethyl-1,10-phenanthroline and Ph4P+ = tetraphenylphosphonium ion) with cobalt(II) acetate tetrahydrate. The structural analysis shows the formation of a neutral 4,2-ribbon-like chain of vertex-sharing cyanido-bridged {CoIII2CoII2} squares in which the metalloligand underwent an oxidation process and a reorganization to form {CoIII(dmphen)(CN)4}− linkers that coordinate to the [CoII(MeOH)2]2+ units through single cyanido ligands. Both cobalt(II) and Co(III) cations are six-coordinated in distorted octahedral environments. The shortest intrachain distance between the paramagnetic cobalt(II) ions is 7.36 Å, a value which is shorter than the shortest interchain one (10.36 Å). Variable-temperature (1.9–300 K) static (dc) magnetic measurements for 1 indicate the occurrence of magnetically isolated high-spin cobalt(II) ions with a D value of +67.0 cm−1. Dynamic alternating current (ac) magnetic measurements between 2.0–13 K reveal that 1 exhibits slow magnetic relaxation under non-zero applied dc fields, being thus a new example of field-induced SIM with easy-plane magnetic anisotropy. Theoretical calculations by CASSCF/NEVPT2 on 1 support the results from magnetometry. The relaxation of the magnetization occurs in the ground state under external dc fields through a two-phonon Raman process and one intra-Kramers mechanism.
... Let's emphasize that the compound's magnetic behavior is tightly related to the type, number and coordination of ligands with respect to the composing magnetic centers [25][26][27]. These has been the subject of extensive interest, here we mention only some prominent and most recent examples, such as the mononuclear β-diketonate Dy 3+ single molecular magnet [28], tetranuclear lanthanide metallocene complexes [29], the tetravanadate [V 4 O 12 ] 4− anion bridged Cu 2+ complexes [30] and the single-ion Co 2+ one [31]. For an overview on the properties of 3d 2 type systems the reader may consult Refs. ...
We introduce a multi-configurational approach to study the magneto-structural correlations in $3d^2$ systems. The theoretical framework represents a restricted active space self-consistent field method, with active space optimized to the number of all non-bonding orbitals. To demonstrate the validity and effectiveness of the method, we explore the physical properties of the trigonal bipyramidal spin-one single-ion magnet (C$_6$F$_5$)$_3$trenVCN$^t$Bu. The obtained theoretical results show a good agreement with the experimental data available in the literature. This includes measurements for the magnetization, low-field susceptibility, cw-EPR and photoluminescence spectroscopy. The proposed method may be reliably applied to a variety of $3d^2$ magnetic systems. To this end, and for the sake completeness, we provide detailed analytical and numerical representations for the generic Hamiltonian's effective matrix elements related to the crystal field, exchange, spin-orbit and Zeeman interactions.
... This indicates that the linker effectively separates the magnetic subunits and no SCM behavior would be expected, only SMM. [73][74][75][76] The observed behavior and anisotropic values are similar to other published mononuclear Co(II) complexes in pseudo-octahedral geometry and also in other Co(II) CPs. [77][78][79][80][81][82] Dynamic susceptibility measurements were performed to study the possibility of slow relaxation of the magnetization. Without an applied static dc eld, no c 0 signal was observed; however, when an external dc eld was used, a dependence of c 0 appeared. ...
The synthesis of 1D cobalt and zinc monometallic and heterometallic coordination polymers (CPs) was carried out applying one-pot synthetic methods by using either supercritical carbon dioxide or ethanol as the solvent. A collection of four 1D CPs were thus obtained by the combination of a metal (or a mixture of metals) with the linker 1,4-bis(4-pyridylmethyl)benzene. The used metallic complexes were zinc and cobalt hexafluoroacetylacetonate, which can easily incorporate pyridine ligands in the coordination sphere of the metal centre. Independently of the used solvent, the precipitated phases involving Zn(II), i.e., homometallic CP of Zn(II) and bimetallic CP of Zn(II)/Co(II), were isostructural. Contrarily, homometallic CPs of Co(II) were precipitated as an isostructural phase of Zn(II) or with a different structure, depending on the used solvent. All the structures were resolved by XRD using synchrotron radiation. In addition, the magnetic properties of the new CPs involving Co(II) were studied. Remarkably, at low temperatures with the application of an external field, they acted as field-induced single molecule magnets.
... Most of them are complexes of trivalent rare-earth cations (for instance Dy III , Nd III , etc.) [45][46][47][48][49][50][51], while examples concerning 3d metal ions such as Co II , Fe II , Mn III or Ni II are comparatively fewer [43,[52][53][54][55][56][57][58][59]. Recent studies evidenced the influence of the coordination environment around the metal ion on the magnitude of the magnetic anisotropy [60][61][62][63][64][65]. Coordinative and supramolecular architectures enclosing magnetically isolated SIMs and SMMs moieties are very appealing considering their potential use in molecular electronics [66,67]. ...
The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.
A new metallosupramolecular compound of formula Na6Ni3(dabtzox)3·10H2O (1) was prepared by the metal-mediated self-assembly of the potentially tris(chelating) N,N’-2,2’-(4,4’-bithiazole)bis(oxamate) (dabtzox) ligand and its structure was determined by single-crystal X-ray diffraction....
In this paper, a mononuclear Co(II) compound [Co(H 2 O) 6 ]Cl 2 ·2C 6 H 12 N 4 ·4H 2 O ( 1 ) was synthesized and characterized. X‐ray single crystal diffraction analyses shows that the Co(II) ion adopts almost perfect octahedral coordination geometry with O6 donors provided by six water molecules. Magnetic properties studies show it has strong in‐plane anisotropy with D = +66.7 cm ⁻¹ . AC susceptibility measurements indicate it is a field‐induced SMM with U eff = 56.11 K via Raman relaxation process and/or direct process. It is rare for Co(II) ion in cobalt‐based compound showing almost perfect octahedral coordination geometry.
A novel CrII-dimeric complex, [CrIIN(SiiPr3)2(μ-Cl)(THF)]2 (1), has been successfully constructed using a bulky silyl-amide ligand. Single-crystal structure analysis reveals that complex 1 exhibits a binuclear motif, with a Cr2Cl2 rhombus core, where two equivalent tetra-coordinate CrII centers in the centrosymmetric unit display quasi-square planar geometry. The crystal structure has been well simulated and explored by density functional theory calculations. The axial zero-field splitting parameter (D < 0) with a small rhombic (E) value is unambiguously determined by systematic investigations of magnetic measurements, high-frequency electron paramagnetic resonance spectroscopy, and ab initio calculations. Remarkably, ac magnetic susceptibility data unveil that 1 features slow dynamic magnetic relaxation typical of single-molecule magnet behavior with Ueff = 22 K in the absence of a dc field. This increases up to 35 K under a corresponding static field. Moreover, magnetic studies and theoretical calculations point out that a non-negligible ferromagnetic coupling (FMC) exists in the dimeric Cr-Cr units of 1. The coexistence of magnetic anisotropy and FMC contributes to the first case of CrII-based single-molecule magnets (SMMs) under zero dc field.
Four air-stable mononuclear Co(II) complexes, with the formulas [Co(dapbh)(H2O)(CH3OH)] (1), [Co(Hdapbh)(N3)(CH3OH)]·(CH3OH) (2), [Co(H2aapbh)(CH3OH)2]·(NO3)2 (3) and [Co(H2bapbh)(H2O)(NO3)]·(NO3) (4), have been synthesized and structurally characterized by single crystal X-ray diffraction. In all of the complexes, the Co(II) centers constrained by the rigid pentadentate ligand H2LR with two protonable hydrogens adopt a heptacoordinated pentagonal-bipyramidal geometry. The combined analyses of magnetic data and ab initio calculations unveil large easy-plane magnetic anisotropies for these complexes (D = +37.338, +37.273, +41.138 and +41.139 cm-1 for 1-4, respectively), which indicate that the chemical alterations of the equatorial ligand and the ligand field strength in the axial positions synergistically fine-tune the magnitude of the D values. Magnetic investigations demonstrate the field-induced single-ion magnetic behavior in complexes 1, 2 and 4 with diverse energy barriers (Ueff) of 12.25 for 1, 44.15 for 2 and 48.72 K for 4, corresponding to the geometrical distortion of the heptacoordinated Co(II) ion. That is, the greatest deviation from the ideal D5h symmetry in 4 is responsible for the highest barrier.
Five cobalt(II) complexes of formula [CoCl2(Ln)2] [1 with L1 = 1-benzyl-2-phenyl-1H-benzimidazole, 2 with L2 = 2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzimidazole, 3 with L3 = 1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-benzimidazole, 4 with L4 = 1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1H-benzimidazole and 5 with L5 = 2-(thiophen-2-yl)-1-(thiophen-2-ylmethyl)-1H-benzimidazole] have been synthesised, spectroscopically characterised and cryomagnetically investigated. The crystal structures of 1, 3, 4 and 5 have been determined by X-ray diffraction on single crystals. Each cobalt(II) ion is four-coordinate in a distorted tetrahedral environment built by two chloride anions and two benzimidazole ligands. The neutral molecules are well separated from each other, shortest intermolecular cobalt⋯cobalt distances being greater than 9.0 Å. Static (dc) magnetic susceptibility measurements in the temperature range 2.0-300 K of 1-5 reveal the occurrence of a Curie law behaviour of magnetically non-interacting spin quadruplets in the high-temperature domain with a downturn at low temperatures due to magnetic anisotropy. The values of the D and E/D parameters for these compounds vary in the ranges -8.75 to +8.96 cm-1 and 0.00140 to 0.23, respectively. Dynamic (ac) magnetic susceptibility measurements of 1-5 show slow magnetic relaxation in the lack (1) or under the presence (1-5) of applied dc magnetic fields, a feature which is typical of single-molecule magnet behaviour (SMM). The analysis of the ac data shows that a thermally activated Orbach relaxation mechanism dominates this behaviour. Complexes 1-5 also act as efficient and highly selective eco-friendly catalysts in the coupling reaction between CO2 and epoxides to produce cyclic carbonates under solvent-free conditions. Under optimized reaction conditions, different epoxides were converted to the respective cyclic carbonate, with excellent conversions, using catalyst 4.
A one‐dimensional (1D) coordination polymer [Co(btca)1/2(mbpy)]n (1, mbpy=4,4′‐Dimethyl‐2,2′‐bipyridyl, H4btca=1,2,4,5‐Benzenetetracarboxylic acid) constructed by a mixed tetracarboxylic‐dipyridyl ligand strategy with a ribbon topology was synthesized and magnetically characterized. Magnetic studies combined with theoretical calculations reveal large easy‐axis magnetic anisotropy of the central Co(II) ions in a highly distorted five‐coordination geometry. Interestingly, field‐induced single‐ion magnet (SIM) behavior was evidenced in the chain compound, making the cobalt(II) coordination ribbon to be the first five‐coordinate SIM chain.
Four cobalt(II) complexes, [Co(L1)2(NCX)2(MeOH)2] (X=S (1), Se (2)) and {[Co(L2)2(NCX)2]}n (X=S (3), Se (4)) (L1=2,5‐dipyridyl‐3,4,–ethylenedioxylthiophene and L2=2,5‐diethynylpyridinyl‐3,4‐ethylenedioxythiophene), were synthesized by incorporating ethylenedioxythiophene based redox‐active luminescence ligands. All these complexes have been well characterized using single‐crystal X‐ray diffraction analyses, spectroscopic and magnetic investigations. Magneto‐structural studies showed that 1 and 2 adopt a mononuclear structure with CoN4O2 octahedral coordination geometry while 3 and 4 have a 2D [4×4] rhombic grid coordination networks (CNs) where each cobalt(II) center is in a CoN6 octahedral coordination environment. Static magnetic measurements reveal that all four complexes displayed a high spin (HS) (S=3/2) state between 2 and 280 K which was further confirmed by X‐band and Q‐band EPR studies. Remarkably, along with the molecular dimensionality (0D and 2D) the modification in the axial coligands lead to a significant difference in the dynamic magnetic properties of the monomers and CNs at low temperatures. All complexes display slow magnetic relaxation behavior under an external dc magnetic field. For the complexes with NCS⁻ as coligand observed higher energy barrier for spin reversal in comparison to the complexes with NCSe⁻ as coligand, while mononuclear complex 1 exhibited a higher energy barrier than that of CN 3. Theoretical calculations at the DFT and CASSCF level of theory have been performed to get more insight into the electronic structure and magnetic properties of all four complexes.
Based on a β-diketiminate ligand, an iron(III) tetrahedral high-spin complex, [LFeIII(Cl)2] (1), and an iron(II) high-spin triangular planar complex, [LFeIICl] (2), have been synthesized and structurally characterized. Also, complex 1 can be considered as a precursor to produce 2via a reduction process, with a concomitant change in its magnetic anisotropy. Combined experiments and ab initio calculations support the observation of field-induced single-ion magnet (SIM) behaviour for 2 with an effective spin-reversal energy barrier of Ueff = 40.02 K, as a direct consequence of the large intrinsic magnetic anisotropy (easy axis) arising from out-of-state spin-orbit coupling in the trigonal planar FeII complex. Noteworthy butterfly hysteresis loops were observed from 2 up to 8 K, which represents the first case in Fe(II) complexes.
A new mononuclear Dy(iii) complex, with the formula [Dy(Hcpt)3]·2H2O (1), has been successfully prepared via self-assembly between Dy(iii) ions and 2-cyano-N'-(1-(pyridin-2-yl)amido)acetyl (Hcpt) ligand. X-ray diffraction study shows that the Dy(iii) ion is nine-coordinated by three Hcpt ligands with a tridentate chelating mode, leading to an approximately monocapped square-antiprismatic (C 4v) geometry. Magnetic data analysis demonstrates that 1 performs field-induced slow magnetic relaxation with a relaxation barrier of 97.90 K, due to the quantum tunneling effect suppressed upon a static dc field of 2000 Oe. To deeply understand the magnetic behaviors, the relaxation mechanisms and magneto-structure relationship are rationally discussed using ab initio calculations as well.
Metal-organic frameworks or coordination polymers showing interesting magnetic properties have attracted much attention. By using 4,4′-bipyridine (4,4′-bpy) as auxiliary ligand, three new cobalt phosphonates based on 1-naphthalene phosphonic acid (1-napH2) were assembled under hydrothermal conditions at different pH, namely, Co(1-napH)2(4,4′-bpy)2(H2O)2 (1), Co2(1-nap)2(4,4′-bpy) (2), and Co2(1-nap)2(4,4′-bpy)(H2O)·2H2O (3). Compound 1 has a 0D mononuclear structure, where the Co(II) ion has a distorted octahedral geometry and 4,4′-bpy serves as an unusual terminal ligand. A 2D layer structure is found in compound 2, where the ladder-like chains made up of corner-sharing CoO3N and PO3C tetrahedra are connected by 4,4′-bpy. Interestingly, compound 3 with similar chemical composition to 2 except additional water molecules shows a 3D open-framework structure, where undulating chains of corner-sharing CoO4N trigonal bipyramids and PO3C tetrahedra are cross-linked by 4,4′-bpy. Magnetic studies reveal that compound 1 exhibits field-induced slow magnetization relaxation, whereas compounds 2 and 3 show field-induced spin-flop behavior and paramagnetism, respectively.
We introduce a multi‐configurational approach to study the magneto‐structural correlations in 3d2 systems. The theoretical framework represents a restricted active space self‐consistent field method, with active space optimized to the number of all non‐bonding orbitals. To demonstrate the validity and effectiveness of the method, we explore the physical properties of the trigonal bipyramidal spin‐one single‐ion magnet (C6 F5)3 tren VCN t Bu. The obtained theoretical results show a good agreement with the experimental data available in the literature. This includes measurements for the magnetization, low‐field susceptibility, cw‐EPR and photoluminescence spectroscopy. The proposed method may be reliably applied to a variety of 3d2 magnetic systems. To this end, and for the sake completeness, we provide detailed analytical and numerical representations for the generic Hamiltonian’s effective matrix elements related to the crystal field, exchange, spin‐orbit and Zeeman interactions. This article is protected by copyright. All rights reserved.
Two novel Cu(II) coordination polymers, namely, [Cu2(HL1)2]n•2nCH3CN•nH2O (1), [Cu(HL2)(H2O)]n (2), were solvothermally synthesized by isomeric N-heterocyclic multicarboxylate ligands (H3L1= 5-(3,4-dicarboxylphenoxy)nicotic acid, H3L2 = 5-(2,3-dicarboxylphenoxy)nicotic acid). Complex 1 possess the [Cu2(COO)4] SBUs and displays a 2D kgd topological network with the point symbol of (4³)2(4⁶·6⁶·8³). Complex 2 shows a three-dimensional supramolecular structure with a single-node SP 2-periodic net (6,3)Id topology with a topological symbol of (4²•6⁴). Meanwhile, magnetic studies show that complex 2 exhibit antiferromagnetic behavior.
By using the iridium(IV) complex (NBu4)2[IrBr6] (1) as a metalloligand towards a Cu(II) metal ion, three novel Ir(IV) one-dimensional (1D) compounds of formula {IrBr5(μ-Br)Cu(Meim)4}n (2), {IrBr5(μ-Br)Cu(Viim)4}n (3) and {IrBr5(μ-Br)Cu(Buim)4}n (4), [Meim = 1-methylimidazole; Viim = 1-vinylimidazole; Buim = 1-butylimidazole] have been prepared and structurally and magnetically characterised. Compounds 2, 3 and 4 crystallise in the triclinic, monoclinic and orthorhombic crystal systems with space groups P1̄, C2/c and Pccn, respectively. Each Ir(IV) ion in 1-4 is six-coordinate and bonded to six bromide ions in a quasi regular octahedral geometry. In compounds 2-4, the CuII ion shows an axially elongated octahedron with four N atoms, from four monodentate imidazole derivative ligands, that form the equatorial plane and two bromide ions that occupy the axial positions. Cu(II) and Ir(IV) ions are linked through bridging bromide anions generating Ir(IV)-Cu(II) chains [with intrachain Cu(II)⋯Ir(IV) distances covering the range of ca. 5.10-5.42 Å]. In the crystal lattice of 2 and 3 are observed significant intermolecular Ir-Br⋯Br-Ir contacts and π⋯Br interactions, which organize arrangements that contribute to stabilizing the crystal structure of these Ir(IV)-based compounds. DC magnetic susceptibility measurements reveal that 1 displays magnetic behaviour typical of noninteracting mononuclear centres with S = 1/2. Besides, antiferromagnetic behaviour (2 and 3) and ferromagnetic (4) exchange coupling occur between the Cu(II) and Ir(IV) metal ions in the one-dimensional bromo-bridged compounds 2-4. Moreover, the study of the AC magnetic susceptibility shows a field-induced slow relaxation of the magnetisation for 1, indicating the presence of the single-ion magnet (SIM) phenomenon for the magnetically isolated hexabromoiridate(IV) complex.
In the crystal lattice of two novel Re IV compounds, the paramagnetic [ReCl 6 ] ²⁻ and [ReBr 6 ] ²⁻ anions are well separated from each other through two protonated forms of the antibiotic ciprofloxacin. These compounds behave as single-ion magnets (SIMs).
A tetranuclear cubane-type complex [Co4(ntfa)4(CH3O)4(CH3OH)4] (1) with a {Co4O4} core, and a mononuclear complex [Co(ntfa)2(CH3OH)2] (2) have been rationally obtained by adjusting the ratio of the β-diketonate and Co(II) ions,...
We describe herein the first examples of six-coordinate CoII single-ion magnets (SIMs) based on the β-diimine Mebik ligand [Mebik = bis(1-methylimidazol-2-yl)ketone]: two mononuclear [CoII(Rbik)2L2] complexes and one mixed-valence {CoIII2CoII}n chain of formulas [CoII(Mebik)(H2O)(dmso)(μ-NC)2CoIII2(μ-2,5-dpp)(CN)6]n·1.4nH2O (3) [L = NCS (1), NCSe (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine (3)]. Two bidentate Mebik molecules plus two monodentate N-coordinated pseudohalide groups in cis positions build somewhat distorted octahedral surroundings around the high-spin cobalt(II) ions in 1 and 2. The diamagnetic [CoIII2(μ-2,5-dpp)(CN)8]2- metalloligand coordinates the paramagnetic [CoII(Mebik)(H2O)(dmso)]2+ complex cations in a bis-monodentate fashion to afford neutral zigzag heterobimetallic chains in 3. Ab initio calculations, and cryomagnetic dc (2.0-300 K) and ac (2.0-12 K) measurements as well as EPR spectroscopy for 1-3 show the existence of magnetically isolated high-spin cobalt(II) ions with D values of 59.84-89.90 (1), 66.32-93.90 (2) and 70.40-127.20 cm-1 (3) and field-induced slow relaxation of the magnetization, being thus new examples of SIMs with transversal magnetic anisotropy. The analysis of their relaxation dynamics reveals that the relaxation of the magnetization occurs by the Raman (with values of the n parameter covering the range 6.0-6.8) and direct spin-phonon processes.
The very recent literature on single molecule magnets is reviewed. The focus is on single molecule magnets (SMMs) that show high energy barriers to relaxation of magnetisation. The first section covers monometallic dysprosium based SMMs and shows correlations between the energy barrier and structural parameters. The second section includes a discussion of single molecule toroids and SMMs where lanthanide centres are bridged by radicals. The literature of polymetalic lanthanide SMMs reported in 2019 is précised. In the final section monometallic 3d-metal SMMs are reviewed.
A large single crystal of a compound from the family of coordination polymer [Co(NCS)2(L)2]n chains was synthesized and its magnetic properties are reported. [Co(NCS)2(4-(3-phenylpropyl)pyridine)2]n is ferromagnetic with Tc = 3.39 K. Single-ion ab initio calculations predict an almost Ising-type magnetic anisotropy and the direction of the magnetic easy-axis nearly along the Co-Npy bond of the apical pyridine-based co-ligand. Both predictions are confirmed by single-crystal magnetic measurements. The magnetic relaxation of the single crystal sample significantly differs from the powder sample data, and clearly shows the presence of two separate relaxation processes. The process dominant below 3.2 K demonstrates a single chain magnet (SCM) behaviour, with a crossover between single-wall and two-wall processes, in spite of the fact that the system is ferromagnetically ordered. The faster process that dominates just below Tc is attributed to spin waves. Micromagnetic Monte Carlo simulations of the investigated compound show that the dipolar field cancels for some chains located at the border between 3-dimensional domains. Such chains are responsible for the measured ac signal, and demonstrate the SCM behaviour. The quantitative analysis of the SCM relaxation time is supported by preparing and examining a corresponding diamagnetically diluted compound, [CoxCd1-x(NCS)2(4-(3-phenylpropyl)pyridine)2]n (x = 0.013), which behaves as a field-induced single-ion magnet. The relaxation pathways for single Co(ii) spins are determined to be Raman, direct, and quantum tunneling processes, which were included in an improved approach to describe the magnetic relaxation in the Co(ii)-based SCM compound.
The 1 : 2 and 1 : 1 Co(ii) complexes of the L ligand (L = 6-(3,5-diamino-2,4,6-triazinyl)2,2'-bipyridine) with formulas [CoII(L)2](ClO4)2·0.5MeCN·Et2O (1) and [CoII(L)(CH3CN)2(H2O)](ClO4)2·MeCN (2) have been prepared. The structural and magnetic characterization of the two compounds shows that they contain octahedral high-spin Co(ii) and present a field-induced slow relaxation of the magnetization. 1 has been inserted into a bimetallic oxalate-based network leading to a novel achiral 3D compound of formula [CoII(L)2][MnIICrIII(ox)3]2·(solvate) (3) exhibiting ferromagnetic ordering below 4.6 K. EPR measurements suggest a weak magnetic coupling between the two sublattices.
Two CoII based complexes namely [Co(IAP)(SCN)2] (1) and {[Co(IPEH)2(SCN)2]·H2O}n (2) (Where IPEH = (((1E,2E)-1,2-bis(1-(4-(1H-imidazol-1-yl)phenyl)ethylidene)hydrazine) and IAP = (4’-(Imidazol-1-yl)acetophenone)) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic measurements and ab initio calculations. Structural analysis revealed that complex 1 has the zero-dimensional mononuclear structure and complex 2 has a two-dimensional framework where the CoII centers are bridged by bis(monodentate) ligand IPEH. In both the complexes CoII center has the distorted octahedral geometry with CoN6 coordination environment, formed by four equatorial N atoms from the neutral ligand and two NCS- at the axial positions. Detailed magnetic measurements reveal the presence of easy-plane magnetic anisotropy for both the complexes whereas the field-induced slow relaxation of magnetization is observed at relatively higher temperature in 1 (above 2 K, Ueff = 30 K) as compared to 2 (below 2 K, Ueff = 6.5 K). The ab initio calculations show negligible effect of the first coordination sphere as similar anisotropic parameters are obtained for both the complexes. Additionally, it was found that the anisotropic axes are nicely aligned in a particular direction for complex 1 whereas it is randomly oriented in the framework of 2. The parallel orientation of the anisotropic axis enhances the axial anisotropy and reduces the transverse component and result in a higher energy barrier (Ueff = 30 K) in complex 1. The detailed analysis of field and temperature dependence of relaxation time indicates that Raman, direct and QTM processes mainly play an important role in relaxation dynamics of complex 1.
Three mononuclear Co(II) complexes with the formulas of [Co(L1)2] (1), [Co(L2)2(CH3OH)2] (2), and [Co(L3)2(CH3OH)2] (3) (HL1 = 4-nitro-2-((E)- (propylimino)methyl)phenol, HL2 = 2,4-dinitro-6-((E)-(propylimino) methyl)phenol, HL3 = 2-(methoxymethyl)-4-nitro-6-((E)-(propylimino)methyl)phenol) have been synthesized and structurally characterized. The -CH2OCH3 group in the ligand of complex 3 was in situ formed during the reaction. The Co(II) ion of complex 1 is in a distorted tetrahedral environment, while the Co(II) centers in complexes 2 and 3 adopt a deformed octahedral geometry. The static magnetic data can be well fitted by the spin (1) or Griffith-Figgis (2 and 3) Hamiltonian and negative D and B20 values were obtained. Quantum chemical calculations confirm the presence of significant easy-axial magnetic anisotropy with non-negligible transversal contributions in all the three complexes. All the three complexes show field-induced slow magnetic relaxation with one (2) or two (1 and 3) relaxation processes. Interestingly, their coordination geometry and magnetic relaxation behaviors can be tuned by ligand substitutions.
Four novel coordination polymers, namely, [Zn3 (L1)2(H2O)2]n·n (CH3CN)·n (H2O) (1), [Cd3 (L1)2(H2O)4]n (2), [Cd3 (L2)2Cl(H2O)2]n·n (H3O) (3), [Pb(HL2)]n (4) were solvothermally synthesized based on isomeric N-heterocyclic multicarboxylate ligands (H3L1 = 5-(2,3-dicarboxylphenoxy)nicotinic acid, H3L2 = 5-(3,4-dicarboxylphenoxy)nicotinic acid). Compound 1 is a 2D layer framework with a (4,8)-connected new topology with the point symbol of (4¹⁸·6¹⁰) (4⁵·6)2 based on the [Zn3(COO)4] SBUs (Secondly Building Units). Compound 2 displays a 3D topological network with the point symbol of (3·4²·5³) (3⁵·4⁶·5⁹·6⁸) and possess the uncommon trinuclear [Cd3 (μ2-H2O) (COO)2] SBUs. Compound 3 shows an interesting (4,4,8)-connected 3D network based on the [Cd3(COO)3] SBUs with the point symbol of (4¹⁴·6¹³·8) (4⁴·6²) (4⁵·6). Compound 4 has the [Pb2(COO)2] SBUs and further spreaded by (HL2)²⁻ to give rise to 2D structure with the uncommon (4,8)-connected sqc169 topology with the point symbol of (4¹²·6¹²·8⁴). H3L1 in 1, 2 and H3L2 in 3, 4 show different coordination modes, respectively, and are found easy to form various metal clusters (secondary building units, SBUs) in final structures. Meanwhile, the solid state luminescence properties of compounds 1–4 were studied at room temperature, and compound 1 showed the high selectivity and sensitivity for Cu²⁺ ion by a luminescence quenching effect, while 3 can detect nitrobenzene with high selectivity and sensitivity. Moreover, the mechanisms for the luminescence quenching of 1 and 3 have been further studied.
This paper describes a tetrahedral mononuclear Co(II) complex [CoL2](ClO4)2 (1) in which L = 2,9-diphenyl-1,10-phenanthroline. The structure of 1, which was determined by single crystal X-ray diffraction, indicates that it exists in the triclinic space group P. Magnetic property studies were conducted by reduced magnetization measurements, ab initio calculations and X-band EPR experiments, the results of which revealed a large zero-field splitting, with D ∼ −45.9 cm⁻¹. The Arrhenius equation indicates that the kinetic energy barrier of 1 is Ueff = 46.9 cm⁻¹. This study describes a very rare case of a Co(II) single ion magnet (SIM) that is purely tetrahedrally coordinated by pyridine like ligands.
A new chain compound consisting of Mn 2 (salen) 2 building blocks bridged by aromatic selenite was synthesized and characterized.
The assembly of a Co(II) salt with two [Au(CN)2]⁻ anions and ancillary ligands L afforded 2D complexes of the general formula {Co(L)2[Au(CN)2]2}n (where L = DMSO (1), DMF (2), Py (3) and PyPhCO (4); PyPhCO is benzoylpyridine). The structure of these complexes consists of parallel sheets, which are built from edge-sharing slightly distorted square-planar {NC–Au–CN–Co}4 units with the Co(II) ions located at the corners and the [Au(CN)2]⁻ bridging anions at the edges. Co(II) atoms exhibit a slightly tetragonally distorted CoN4X2 coordination sphere (X = O, N), where the L molecules occupy the axial positions. These molecules are oriented in such a way that they penetrate the holes of neighbouring layers, giving rise in the case of 1, 2 and 4 to AB bilayers held together by Au⋯Au aurophilic interactions, whereas in 3, there are no aurophilic interactions between neighbouring layers, so they are not arranged in pairs but equally separated. Dc magnetic properties, HFEPR (high-frequency and -field EPR) and FIRMS (far-infrared magnetic spectroscopy) measurements and ab initio calculations demonstrate that Co(II) ions in compounds 1–4 possess large and positive D values (≳+70 cm⁻¹). The experimental D values follow the same order as that established from ab initio calculations including gold(I) atoms: D (2) > D (4) > D (3) > D (1), which highlights the important role of Au(I) in determining the anisotropy of the Co(II) ions. All the complexes show field-induced slow relaxation of magnetization through a predominant Raman mechanism above 3 K. Neither the anisotropy order nor the Co(II)⋯Co(II) distances are clearly correlated with the phenomenological Ueff parameter (or the Raman parameters). This fact suggests that other factors, such as the flexibility of the axial ligands, could significantly contribute to the fast relaxation observed for these complexes.
The reactions of LnIII ions with a versatile pyridyl-decorated dicarboxylic acid ligand lead to the formation of a series of novel three-dimensional (3D) Ln-MOFs, [Ln3(pta)4(Hpta)(H2O)]·xH2O (Ln = Dy (1), Eu (2), Gd (3), Tb (4), H2pta = 2-(4-pyridyl)-terephthalic acid, x = 6 for 1, 2.5 for 2, 1.5 for 3 and 2 for 4). The Ln3+ ions act as nine-coordinated muffin spheres, linking to each other to generate trinuclear {Ln3(OOC)6N2} SBUs, which are further extended to be interesting 3D topological architectures. To the best of our knowledge, the Dy-MOF exhibits zero-field single-molecule magnet (SMM) behaviour with the largest effective energy barrier among the previously reported 3D MOF-based Dy-SMMs. The combined analyses of a diluted sample (1@Y) and ab initio calculations demonstrate that the thermally assisted slow relaxation is mainly attributed to the single-ion magnetism. Furthermore, fluorescence measurements reveal that H2pta can sensitize EuIII and TbIII characteristic luminescence.
Effective recognition and detection of heavy metal Cu²⁺ and Fe³⁺ ions from water are highly desirable because of their serious harms to human health and environmental safety. However, the existing detection techniques are still high cost, time-consuming and highly demanding for operators. As an alternative approach to address this issue, an indium-based MOF labeled as In-MOF of 1 with high water stability and opposite charges to Cu²⁺ and Fe³⁺ ions was successfully prepared and deployed as optical sensor to sense target analytes, according to our previous work. Prominently, this anionic framework exhibits high photoluminescence property and excellent fluorescence quenching response to target analytes with high selectivity, sensitivity and detection limits of 3.88 ppb for Cu²⁺, 4.2 ppb for Fe³⁺ ions, respectively, both of which have been confirmed to be ultro-low values among all reported MOFs-based photochemical sensors. Furthermore, the quick and accurate probing capability of 1 also has been proved even under the interference of dozens of anions or cations. The possible photoluminescence and fluorescence quenching mechanisms were studied in detail, revealing that rapid coordination reactions between free imidazole nitrogen and carboxyl oxygen atoms of the framework (Lewis basic sites) and the metal ions of Cu²⁺/Fe³⁺ (Lewis acid sites) should be proposed to be the key reason for high efficiency fluorescence quenching.
A stable 3D In(III)-based metal-organic framework, [In(L)(μ2-OH)]·0.5H2O (1) (where H2L = 5-(1H-imidazol-1-yl)isophthalic acid), was successfully synthesized and fully characterized. As established by single-crystal X-ray diffraction, compound 1 is a 4, 6-connected 3D architecture crystalized in the monoclinic C2/c space group. On the basis of thermodiffractometry, 1 remains crystallinity and main framework below 450 °C. The structural integrity of 1 can be sustainable in water, harsh acidic and basic conditions (pH 2-13), and common organic solvents, presenting excellent water stability and chemical stability. The photoluminescence property of 1 is investigated as well. Remarkably, luminescence studies reveal 1 can recognize Fe3+ ions with high selectivity from mixed metal ions in aqueous solution through a luminescence quenching phenomenon. The results suggest that the MOF may serve as potentially useful sensory materials for the detection of metal cations, and this work also demonstrates that highly hydrolytically stable In(III)-MOFs could be constructed and used for some meaningful applications in water.
Two penta-coordinate complexes of the general formula [Co(Lⁿ)(NCS)]ClO4, where L¹ = {bis[(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]-[(3,4-dimethoxypyridin-2-yl)methyl]}amine and L² = {bis[(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]-[(4-methoxy-3,5-dimethylpyridin-2-yl)methyl]}amine, have been synthesized and thoroughly characterized. Each of the cobalt(II) atoms is penta-coordinated in the {CoN5} donor set with a distorted square-pyramidal geometry in [Co(L¹)(NCS)]ClO4·MeOH (1), while the vicinity of the central atom can be described as a distorted trigonal–bipyramidal geometry in [Co(L²)(NCS)]ClO4 (2) as revealed using the SHAPE analysis. Differences in interatomic parameters among the cobalt(II) and donor atoms in 1 and 2 have definite impact on the magnetic features of both compounds. The complexes show an easy-axis magnetic anisotropy (D = −38.5 cm⁻¹ for 1 and D = −8.5 for 2), and both complexes reveal a large rhombicity with E/D = 0.21 for 1 and E/D = 0.29 for 2. The ZFS parameters (g, D and E) were also calculated using CASSCF/NEVPT2 methods and they are in good agreement with those determined from experimental data. A frequency dependent out-of-phase susceptibility has been observed in external magnetic field (Bdc = 0.1 T) revealing the following parameters of slow relaxation of magnetization for 1: energy of the spin reversal barrier, Ueff = 16.0 cm⁻¹ (Ueff/kB = 23.0 K) and the relaxation time, τ0 = 1.28 × 10⁻⁶ s. In the case of complex 2, no maxima of frequency dependent out-of-phase susceptibility have been observed and thus, the value of Ueff = 17 cm⁻¹ has been estimated using the expression Ueff = |D| × (S² − 1/4). It has been demonstrated that the degree of substitution and the type of substituents on the pyridyl moieties of the tripodal ligands (L¹ and L²) used in these penta-coordinate cobalt(II) complexes have significant impact on structural and magnetic features.
A new luminescent EuIII complex, namely [Eu2(BTFA)4(OMe)2(dpq)2] (1), in which BTFA = 3-benzoyl-1,1,1-trifluoroacetone and dpq = dipyrido [3,2-d:2′,3′-f] quinoxaline, has been designed and synthesized by employing two different ligands as sensitizers. Crystal structure analysis reveals that complex 1 is composed of dinuclear EuIII units crystallized in the monoclinic P space group. Notably, 1 exhibits high thermal stability up to 270 °C and excellent water stability. The photoluminescence property of the complex is investigated. Further studies show 1 can recognize Fe³⁺ ions with high selectivity from mixed metal ions in aqueous solution through the luminescence quenching phenomenon. Furthermore, the recyclability and stability of 1 after sensing experiments are observed to be adequate. By virtue of the superior stability, detection efficiency, applicability and reusability, the as-prepared EuIII complex can be a promising fluorescent material for practical sensing.
The geometry of cobalt(II) ions in the axially distorted octahedral cation in [Co(MeCN)6](BF4)2 (1) was compared to the trigonal prismatic cation in [CoTppy]PF6 (2) which revealed significant differences in magnetic anisotropy. Combined experimental and ab initio CASSCF/NEVPT2 calculations support the observed zero field SMM behaviour for 2, with easy axis anisotropy, attributed to the rigidity of the trigonal prismatic ligand. Strong transverse anisotropy for 1 leads to significant quantum tunnelling processes.
Two novel mixed valence CoII-CoIII complexes, namely [CoIICoIII(L1)(ab)(mb)2(H2O)]∙dmf (1) and [CoIII2CoII(L2)4(H2O)4]∙2H2O (2) [H2L1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol, ab = 2-amino-butan-1-ol anion, mb = p-methyl benzoate, H2L2= 3-((2-hydroxy-3-methoxy-benzylidene)-amino)-propionic acid, dmf = N,N-dimethyl-formamide],were synthesized and characterized by single crystal X-ray diffraction and magnetic studies at low temperature. The structure determination reveals that both complexes belong to the monoclinic system with P21/c (1) and I2/a (2) space groups. Complex 1 is a dinuclear CoIIICoII species with distorted octahedral cobalt centers showing different coordination environments. In 2, a bent trinuclear CoIII2CoII complex, the coordination environments around the two terminal CoIII sites are alike, whereas it is different in the central CoII ion. Alternating current/direct current (ac/dc) magnetic studies revealed that both complexes show field-induced slow magnetic relaxation. The dc magnetic susceptibility and magnetization data were analyzed with the following Hamiltonian H @#x0302;_(zfs+Zeeman)=D[S @#x0302;_z^2-1/3 S(S+1)]+E(S @#x0302;_x^2-S @#x0302;_y^2 )+βH[g_∥ S @#x0302;_z+g_⊥ (S @#x0302;_x 〖+S @#x0302;〗_y )], where D and E are the axial and rhombic zero-field splitting (zfs) parameters, respectively, and a good agreement between experimental and simulated results was found using the parameters g = 2.585, g|| = 2.437, D = +98.1 cm−1, E/D = 0.008 and F = 8.2 10−5 for 1 and g = 2.580, g|| = 2.580, D = +55.4 cm−1, and E/D = 0.000 for 2.
Construction of efficient multifunctional materials is one of the largest challenges of our time. We herein report the magnetic and catalytic characterization of dinuclear [CoIIICoII(HL¹)2(EtOH)(H2O)]Cl•2H2O (1) and trinuclear [CoIIICoII2(HL²)2(L²⁾Cl2]•3H2O (2) mixed valence complexes. Relevant structural features of the complexes have been mentioned to correlate with the magnetic and catalytic outcomes. Unique structural features, especially in terms of significant distortion around CoII centre(s) prompted us to test both spin-orbit coupling (SOC) and zero field splitting (ZFS) methodologies on the systems. The positive sign of D value has been established from X-band EPR spectra recorded in the 5 – 40 K temperature range and reaffirmation has been obtained from CAS/NEVPT2 calculations. ZFS tensors are also extracted for the compounds along with CoIIGaIII and CoIIZnIICoIII model species. Interestingly, 1 shows slow relaxation of magnetization below 6.5 K in presence of 1000 Oe external dc field with two relaxation processes (Ueff = 37.0 K with τ0 = 1.57×10⁻⁸ s for the SR process and Ueff = 7 K and τ0 = 1.66×10⁻⁶ s for the FR process). As mixed valent cobalt complexes of varying nucleaity are central to the quest for water oxidation catalysts, we were tempted to explore the feature and to our surprise, water oxidation ability has been realized for both 1 and 2 with significant nuclearity control.
Three dinuclear cobalt(II) compounds with different fluoro-substituted benzoate ligands, named as [Co2(3,4-dfba)4(bpp)2]n (1), [Co2(3-fba)4(bpp)2]n (2) and {[Co2(2,4-dfba)4(bpp)2(H2O)]·(bpp)(H2O)}n (3) (3,4-Hdfba = 3,4-difluorobenzoic acid, 3-Hfba = 3-fluorobenzoic acid, 2,4-Hdfba = 2,4-difluorobenzoic acid, bpp = 1,3-bis(4-pyridyl)propane), have been successfully synthesized. In all compounds, Co(II) centers feature hexa-coordinated environments with distorted octahedrons. Among them, compounds 1 and 2 are isomorphic which perform three-dimensional (3D) network, whereas 3 displays a 2D layer structure. This modulation leads to different antiferromagnetic interactions between the metal ions (J1 = -6.43 cm⁻¹, J2 = -2.28cm⁻¹ for 1, J1 = -19.44 cm⁻¹, J2 = -6.08 cm⁻¹ for 2, J = -12.65 cm⁻¹ for 3).
Tuning the magnetic anisotropy of metal ions remains highly interesting in the design of improved single‐molecule magnets (SMMs). We herein report synthetic, structural, magnetic, and computational studies of four mononuclear CoII complexes, namely [Co(hfac)2(MeCN)2] (1), [Co(hfac)2(Spy)2] (2), [Co(hfac)2(MBIm)2] (3), and [Co(hfac)2(DMF)2] (4) (MeCN=acetonitrile, hfac=hexafluoroacetylacetone, Spy=4‐styrylpyridine, MbIm=5,6‐dimethylbenzimidazole, DMF=N,N‐dimethylformamide), with distorted octahedral geometry constructed from hexafluoroacetylacetone (hfac) and various axial ligands. By a building block approach, complexes 2–4 were synthesized by recrystallization of the starting material of 1 from various ligands containing solution. Magnetic and theoretical studies reveal that 1–4 possess large positive D values and relative small E parameters, indicating easy‐plane magnetic anisotropy with significant rhombic anisotropy in 1–4. Dynamic alternative current (ac) magnetic susceptibility measurements indicate that these complexes exhibit slow magnetic relaxation under external fields, suggesting field‐induced single‐ion magnets (SIMs) of 1–4. These results provide a promising platform to achieve fine tuning of magnetic anisotropy through varying the axial ligands based on Co(II) bis(hexafluoroacetylacetonate) complexes.
Two mononuclear azido-cobalt(II) complexes, with formulas [Co(3,3-Hbpt)2(N3)2(H2O)2] (1) and [Co(abpt)2(N3)2]·H2O (2) (3,3-Hbpt = 1H-3,5-bis(3-pyridyl)-1,2,4-triazole, abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole), have been prepared by alternating the pyridyl-triazole coligands. In both complexes, Co(II) centers feature hexa-coordinated environments with distorted octahedrons in which the axial sites are identical, whereas the equatorial environments are finely modulated by the varying chemical natures of the different coligands. It is worth noting that the distinct intermetallic distances in two complexes (10.302 Å for 1 and 6.576 Å for 2) unambiguously cause the disparity of intermolecular interactions, implying the dissimilar magnetic behaviours. As a result, alternating current dynamic susceptibility measurements show that only 2 exhibits field-induced slow relaxation of the magnetization with an effective energy barrier of 11.29 K, though large easy-axis magnetic anisotropies for both complexes are unveiled by the combined analyses of the magnetic data and the ab initio calculations.
Two families of lanthanide(III) metal-organic frameworks (Ln-MOFs) constructed from 2-(4-pyridyl)-terephthalic acid (H2pta) ligand, namely [Ln4(pta)5(Hpta)2(H2O)4]·xH2O (1-Eu, 2-Gd), [Ln(Hpta)(C2O4)]·3H2O (3-Tb, 4-Dy, 5-Er and 6-Ho) (C2O4 = oxalic acid), have been discovered by solvothermal method. On the basis of thermodiffractometry, all MOFs remain their crystallinity and main structure below 370 °C. Utilizing their excellent water stability and chemical stability, the luminescence studies have been carried out. Notably, Eu-MOF and Tb-MOF are indicative of pronounced luminescence, as well as high stability in water and other organic solvents. The luminescence explorations reveal that Eu-MOF possesses good selectivity for testing Fe3+ in aqueous phase via fluorescence quenching, while Tb-MOF behaves as a multifunctional chemical sensor for the detections of Fe3+ cation and Cr2O72- anion in aqueous media and the recognition of nitrofuran (NIF) in DMF solvent. The recyclability and stability of Eu-MOF and Tb-MOF after sensing experiments are observed to be adequate. By virtue of the superior stability, detection efficiency, applicability and reusability, Eu-MOF and Tb-MOF can be the promising fluorescent materials for practical sensing.
Complexation of dysprosium(iii) ions with a multidentate hydrazone ligand, N-[(E)-pyridin-2-ylmethylideneamino]pyridine-2-carboxamide (L), in the presence of different β-diketonate coligands, leads to the formation of two novel DyIII dimers, with formulas Dy2(BTFA)4(L)2 (1) and Dy2(TTA)4(L)2 (2) (BTFA = 3-benzoyl-1,1,1-trifluoroacetone and TTA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate). They exhibit slightly different coordination geometries around DyIII centers and discrepant binuclear motifs - as a result of altering the β-diketonate coligands - which has an impact on the magnetic interactions between metal centers, the local tensor of anisotropy on each DyIII site and their relative orientations, therefore contributing to distinct magnetization dynamics. Compared to 2, complex 1 exhibits a more significant slow magnetic relaxation of SMM behavior in the absence of a dc field. The QTM effect is effectively repressed under a static field, resulting in the energy barriers of 57 K for 1 and 38 K for 2. Ab initio calculations clarify that, strong single-ion magnetic anisotropies exist in both complexes, whereas intermetallic ferromagnetic interaction and antiferromagnetic interaction are observed in 1 and 2, respectively, therefore resulting in dissimilar magnetization dynamics.
The iridium(IV) complex (NBu4)2[IrCl6] (1) has been synthetised, characterised and used as a precursor to prepare the new chloro-bridged heterobimetallic IrIV-CuII chain of formula {IrCl5(µ-Cl)Cu(viim)4}n (2) [viim = 1-vinylimidazole]. The crystal structure and magnetic properties of 1 and 2 have been investigated. Both compounds crystallise in the monoclinic system with space group C2/c. Each IrIV ion in both 1 and 2 is six-coordinate and bonded to six chloride ions in a regular octahedral geometry. In compound 2, the CuII ion exhibits an axially elongated octahedron with four N atoms, from four monodentate viim ligands, that form the equatorial plane, and two chloride ions that occupy the axial positions. The way in which the anionic [IrCl6]2- units are arranged in the crystal packing of 1, well separated from each other by means of the bulky NBu4+ cations, avoids significant intermolecular Ir−Cl•••Cl−Ir interactions. The crystal lattice of 2 shows adjacent IrIV-CuII chains that are connected through π•••π stacking interactions, and are organized adopting perpendicular arrangements. The study of the magnetic properties of 1 and 2 through dc magnetic susceptibility measurements reveals that 1 shows magnetic behaviour typical of noninteracting mononuclear centres with S = 1/2, whereas 2 exhibits ferromagnetic exchange coupling between the CuII and IrIV metal ions linked through chloride ligand. In addition, ac magnetic susceptibility measurements show a field-induced slow relaxation of the magnetisation for 1, indicating single-ion magnet (SIM) behaviour for this mononuclear IrIV system.
The utilization of two isomorphic ligands with different substituents leads to two centrosymmetric DyIII dimers, namely, [Dy2(bfbpen)2(H2O)2]·2I- (1) and [Dy2(bcbpen)2(H2O)2]·2I-·0.5H2O (2) (H2bfbpen = N,N′-bis-(2-hydroxy-5-fluoro-benzyl)-N,N′-bis-(pyridin-2-ylmethyl)ethylenediamine and H2bcbpen = N,N′-bis-(2-hydroxy-5-chloro-benzyl)-N,N′-bis-(pyridin-2-ylmethyl) ethylenediamine). Although Dy ions in both compounds uniformly exhibit a square-antiprism geometry, the critical difference found in terminal substituents of two ligands fine-tunes the local crystal field around Dy centers and the dinuclear molecular structures of 1 and 2. Magnetic investigations unveil that both 1 and 2 display dynamic magnetic relaxation of single-molecule magnets (SMMs) behaviour with different energy barriers of 20.9 K for 1 and 72.7 K for 2 under zero direct-current (dc) field, as well as 26.9 K for 1 under 1200 Oe dc field. Compared to 1, the stronger uniaxial anisotropy and magnetic exchange in 2 render it as a better SMM as evidenced by the higher energy barrier. Ab initio calculations are also performed on both Dy2 compounds to rationalize the observed discrepancy in their magnetic behaviours. The contribution illustrates that the SMM behaviour could be effectively enhanced by means of deliberate local structural manipulation.
A new Co(II) complex, [Co(pta)(H 2 O) 2 ] n ( 1 ), with the 2-(4-pyridyl)-terephthalate ligand (pta ²⁻ ) has been synthesized and structurally and magnetically characterized. Single crystal X-ray analysis indicates that the unique Co(II) ion in the asymmetric unit of 1 displays stretched octahedral geometry. Compound 1 presents a bimetallic layer structure which is further expanded to a 3D supramolecular network through hydrogen bonding interactions. Magnetic measurements have revealed the temperature-dependent existence of antiferromagnetic and ferromagnetic interactions in compound 1 .
In this study, we report the syntheses and structures of two new mononuclear coordination complexes ([Co(H2pimdc)2(phen)]n (1) and [Fe(H2pimdc)2(phen)]n (2), H2pimdc = 2-propyl-imidazole-4, 5-dicarboxylate, phen = 1, 10-Phenanthroline monohydrate) exhibiting single-ion magnet (SIM) behavior. Single-crystal X-ray diffraction analyses reveal that 1 and 2 are isomorphic, crystallized in the monoclinic crystal system with P21/n space group. In both structures, the Co(II)/Fe(II) centers adopt distorted octahedral {Co/FeN4O2} geometries. In addition, the mononuclear structures of 1 and 2 are linked by strong intermolecular N−H···O hydrogen bonds, leading to form a two-dimensional (2D) sheet in which Co(II)/Fe(II) ions are spatially separated from each other. The study of magnetism reveals that compound 1 presents an easy-plane magnetic anisotropy (D = +3.47 cm-1). Alternating current dynamic susceptibility measurements show that 1 exhibits field-induced slow relaxation of the magnetization with effective energy barrier value of 48.49 K.
We herein report on synthetic, structural, magnetic, and compuational studies of six air-stable mononuclear Co(II) complexes with distorted octahedral geometry from PyBox type ligands. Magnetic and theoretical studies reveal that these complexes all exhibit field-induced type single-molecule magnet behaviour and a large energy splitting between ground and first excited Karmers doublets. Dynamic magnetic property analysis shows that Raman relaxation process is the predominant process in all six compounds. For complexes 1, 2, 3, 5, and 6, the contribution of direct process also existed. Importantly, the axial zero-field splitting parameter D in this series varies from positive to negative with the increased distortion of the octahedral geometry for Co(II) center, indicating that the fine-tuning of molecular symmetry is an effective approach to manipulate the magnetic anisotropy in SIMs.
Based on 1H-3-(3-pyridyl)-5-(3′-pyridyl)-1,2,4-triazole (3,3′-Hbpt) ligand, {[Co(3,3′-Hbpt) 2 (SCN) 2 ]·2H 2 O} n (1) and {[Ni(3,3′-Hbpt) 2 (SCN) 2 ]·2H 2 O} n (2) have been prepared and structurally determined by single-crystal X-ray crystallography. The Co(II)/Ni(II) ions are bridged by the curved 3,3′-Hbpt ligands to generate helix chains, further forming a two-dimensional (2D) sheet in which Co(II)/Ni(II) ions are spatially separated from each other by the long spacer 3,3′-Hbpt ligand. Alternating-current magnetic susceptibility measurements show that the individual octahedral Co(II) ions in 1 exhibit field-induced slow magnetic relaxation, dominated by Raman-like process. Compound 1 exhibits a very large positive axial anisotropy parameter (D = +70.1 cm −1) and a small transverse anisotropy parameter (|E| = 0.7 cm −1) by analysis of direct current magnetic data, which is further confirmed by high-field electron paramagnetic resonance (HF-EPR) spectroscopy and ab initio calculations. Furthermore, semiconductor behaviors of 1 and 2 were also studied, which could be as wide-gap semiconductors.
Structural, spectroscopic and magnetic methods have been used to characterize the tris(carbene)borate compound PhB(MesIm)3MnN as a four-coordinate manganese(iv) complex with a low spin (S = 1/2) configuration. The slow relaxation of the magnetization in this complex, i.e. its single-molecule magnet (SMM) properties, is revealed under an applied dc field. Multireference quantum mechanical calculations indicate that this SMM behavior originates from an anisotropic ground doublet stabilized by spin-orbit coupling. Consistent theoretical and experiment data show that the resulting magnetization dynamics in this system is dominated by ground state quantum tunneling, while its temperature dependence is influenced by Raman relaxation.
Single-molecule magnets display magnetic bistability of molecular origin, which may one day be exploited in magnetic data storage devices. Recently it was realised that increasing the magnetic moment of polynuclear molecules does not automatically lead to a substantial increase in magnetic bistability. Attention has thus increasingly focussed on ions with large magnetic anisotropies, especially lanthanides. In spite of large effective energy barriers towards relaxation of the magnetic moment, this has so far not led to a big increase in magnetic bistability. Here we present a comprehensive study of a mononuclear, tetrahedrally coordinated cobalt(II) single-molecule magnet, which has a very high effective energy barrier and displays pronounced magnetic bistability. The combined experimental-theoretical approach enables an in-depth understanding of the origin of these favourable properties, which are shown to arise from a strong ligand field in combination with axial distortion. Our findings allow formulation of clear design principles for improved materials.
Geometry and magnetic relaxation modulations in a series of mononuclear dysprosium complexes, [DyLz2(o-vanilin)2]·X·solvent (Lz = 6-pyridin-2-yl-[1,3,5]triazine-2,4-diamine; X = Br− (1), NO3− (2), CF3SO3− (3)), were realized by changing the nature of the counter-anion. The DyIII ions in all complexes are eight-coordinate and in approximate D4d symmetry environments. The magnetic relaxation and anisotropy of these complexes were systematically investigated, both experimentally and from ab initio calculations. All complexes exhibit excellent single-molecule magnetic behavior. Remarkably, magneto-structural studies show that the rotation of the coordinating plane of the square-antiprismatic environment in complex 2 induces a magnetic relaxation path through higher excited states, yielding a high anisotropy barrier of 615 K (696 K for a diluted sample). Additionally, obvious opening of the hysteresis loop is observed up to 7 K, which is the highest blocking temperature ever reported for dysprosium single-molecule magnets.
This work introduces a novel family of CoII species having attached a curcuminoid (CCMoid) ligand, 9Accm, namely [Co(9Accm)2(py)2] (1) and [Co(9Accm)2(2,2'-bpy)] (2), achieved in high yields by the use of a microwave reactor, and exhibiting two different arrangements for the 9Accm ligands, described as “cis”(2) and “trans”(1). The study of the similarities/differences of the magnetic, luminescent and surfaces behaviors of the two new species, 1 and 2, is the main objective of the present work. The determined single-crystal structures of both compounds are the only CoII-CCMoid structures described in the literature so far. Both compounds exhibit large positive D values, that of 1 (D = +74 cm-1) being three times larger than that of 2 (D = +24 cm-1), and behave as mononuclear Single-Molecule Magnets (SMMs) in the presence of an external magnetic field. Their similar structures but different anisotropy and SMM characteristics provide, for the first time, deep insight on the spin-orbital effects thanks to the use of CASSCF/NEVPT2 calculations implementing such contributions. Further magnetic studies were performed in solution by means of paramagnetic 1H NMR, where both compounds (1 and 2) are stable in CDCl3 and display high symmetry. Paramagnetic NMR appears to be a useful diagnostic tool for the identification of such molecules in solution, where the resonance values found for the methine group (-CH-) of 9Accm vary significantly depending on the cis or trans disposition of the ligands. Fluorescence studies show that both systems display chelation enhancement of quenching (CHEQ) with regards to the free ligand, while 1 and 2 display similar quantum yields. Deposition of 1-2 on HOPG and Si(100) surfaces using spin-coating was studied by AFM; UV photoemission experiments under the same conditions display 2 as the most robust system. The measured occupied density of states of 2 with UV photoemission is in excellent agreement with theoretical DFT calculations.
Single-ion magnets (SIMs) are the smallest possible magnetic devices for potential applications in quantum computing and high-density information storage. Both, their addressing in surfaces and their organization in metal-organic frameworks (MOFs) are thus current challenges in molecular chemistry. Here we report a two-dimensional 2D MOF with a square grid topology built from cobalt(II) SIMs as nodes and long rod-like aromatic bipyridine ligands as linkers, and exhibiting large square channels capable to host a large number of different guest molecules. The organization of the cobalt(II) nodes in the square layers improves the magnetic properties by minimizing the intermolecular interactions between the cobalt(II) centres. Moreover, the SIM behaviour was found to be dependent on the nature of the aromatic guest molecules. The whole process could be followed by single-crystal X-ray diffraction reaching a comprehensive evidence on the putative role of the solvent guest molecules leaving a "fingerprint" on the 2D structures and thus, on the cobalt environment.
A novel two-dimensional (2D) coordination polymer, [Co(ppad)2]n (1), resulted from the assembly of Co(II) ions based on a versatile ligand termed N(3)-(3-pyridoyl)-3-pyridinecarboxamidrazone. Alternating/direct-current magnetic studies of compound 1 indicate that the spatially separated high-spin Co(II) ions act as single-ion magnets (SIMs). The present work represents the first case of a 2D Co(II)-based SIM composed of a monocomponent organic spacer.
The mononuclear complex [Ni(pydc)(pydm)].H2O, pyridine-2,6-dicarboxylato-2,6-bis(hydroxymethyl)pyridine nickel(II) monohydrate, features a highly axially compressed pseudo-octahedral chromophore with significant rhombic contribution that is responsible for the sizable magnetic anisotropy of the Ni(II) ion, D/hc ≈ -14 cm-1. This complex displays the superparamagnetic behavior in an applied external field that culminates at BDC = 0.2 T. Two relaxation processes are evidenced; the faster can be analyzed in terms of the Orbach, direct, and Raman processes yielding U/kB = 21.2 K and t0 = 3.83 × 10-7 s. This is the first example of field-induced single-molecule magnet in the class of hexacoordinate Ni(II) complexes.
The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallographic Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as `a CIF') containing embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to extract the .hkl and .ins files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the determination of absolute structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors.
The new computer program SHELXT employs a novel dual-space algorithm to solve the phase problem for single-crystal reflection data expanded to the space group P1. Missing data are taken into account and the resolution extended if necessary. All space groups in the specified Laue group are tested to find which are consistent with the P1 phases. After applying the resulting origin shifts and space-group symmetry, the solutions are subject to further dual-space recycling followed by a peak search and summation of the electron density around each peak. Elements are assigned to give the best fit to the integrated peak densities and if necessary additional elements are considered. An isotropic refinement is followed for non-centrosymmetric space groups by the calculation of a Flack parameter and, if appropriate, inversion of the structure. The structure is assembled to maximize its connectivity and centred optimally in the unit cell. SHELXT has already solved many thousand structures with a high success rate, and is optimized for multiprocessor computers. It is, however, unsuitable for severely disordered and twinned structures because it is based on the assumption that the structure consists of atoms.
An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand. Magnetic studies revealed that this complex exhibits slow magnetic relaxation in the absence of an applied dc field, which is one of the main characteristics of single-molecule magnets (SMMs). The relaxation barrier is as high as 109 K, which is quite large among transition-metal ion-based SMMs. The complex represents the first example of zero-field SMM behavior in a mononuclear six oxygen-coordinate Co(II) complex.
Nanomagnetism is a rapidly expanding area of research which appears to be able to provide novel applications. Magnetic molecules are at the very bottom of the possible size of nanomagnets and they provide a unique opportunity to observe the coexistence of classical and quantum properties. The discovery in the early 90's that a cluster comprising twelve manganese ions shows hysteresis of molecular origin, and later proved evidence of quantum effects, opened a new research area which is still flourishing through the collaboration of chemists and physicists. This book is the first attempt to cover in detail the new area of molecular nanomagnetism, for which no other book is available. In fact research and review articles, and book chapters are the only tools available for newcomers and the experts in the field. It is written by the chemists originators and by a theorist who has been one of the protagonists of the development of the field, and is explicitly addressed to an audience of chemists and physicists, aiming to use a language suitable for the two communities.
A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar0]2 (3), Fe[N(H) Ar*]2 (4), and Fe(OAr0)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe [N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)� (Dipp 1⁄4 C6H3-2,6-Pri2; Ar0 1⁄4 C6H3-2,6-(C6H3- 2,6-Pri2)2; Ar* 1⁄4 C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# 1⁄4 C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 [ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff 1⁄4 181, 146, 109, 104, and 43 cm�1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous DNh geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.
Two one-dimensional (1-D) cobalt(II) compounds, namely, {[Co(L1)(2,2'-bipy)]•0.5DMF}n (1), and {[Co(bimb)(H2O)4]∙(L2)∙2DMF}n (2) (H2L1= 2,2'-[benzene-1,4-diylbis(methanediylsulfanediyl)]dibenzoic acid, H2L2= 2,2'-(1,4-Phenylenebis(methylene))bis(sulfanediyl)dinicotinicacid, 2,2'-bipy= 2,2'-bipyridine, bimb =1,4-bis(benzoimidazo-1-ly)benzene), have been synthesized using cobalt(II) salt with long flexible carboxylic acids and N-containing ligands. Compounds 1 and 2 were systematically characterized by elemental analysis, infrared spectroscopy, and single crystal X-ray diffraction analysis. Both compounds display 1-D chain structures with two types of well-separated six-coordinated cobalt(II)ions possessing the same coordination sphere CoO4N2 but different coordination geometries: distorted trigonal prism and octahedron, respectively, which lead to different magnetic properties. Compound 1 exhibits more significant frequency-dependence signals and larger field-induced slow relaxation magnetization than compound 2. High frequency electron paramagnetic resonance (HF-EPR) studies further demonstrated the large anisotropy of both compounds with D values of -56.2 and +57.5 cm-1, respectively.
Reaction of a rigid tridentate ligand o-[(1H-imidazol-2-yl)methylideneamino]phenol (2-H2imap) with Co(ClO4)2 in the presence of NaN3, or Co(NO3)2 without a base yields [CoII(2-Himap)2] 1 and [CoIII(2-Himap)2]NO3·MeOH 2, respectively. Both complexes exhibit a mer-octahedral geometry with the cobalt centre being distorted along an octahedral-trigonal prismatic pathway. The packing in 1 and 2 is dominated by H-bonding forming 2D sheets and 1D chains, respectively. Detailed dc and ac magnetic studies indicate that 1 is a field-induced single-ion magnet (SIM) with D = 36.7 cm-1 and E = 2.0 cm-1. Extensive ab initio calculations support these conclusions and suggest that relaxation of the magnetization occurs principally through direct quantum tunnelling in the ground state, with the Raman process dominant in an applied field. This contrasts with the recently reported series of mer-[Co(L)2] (L = monoanionic NNO donor ligand; Inorg. Chem., 2017, 56, 6056–6066) complexes where D is negative, as these compounds have a more ambiguous geometry than 1, and highlights the importance of supramolecular interactions in subtly altering the coordination sphere thereby impacting the magnetic behaviour.
A series of mononuclear Dy(III) complexes with the general formula [DyLz2(salicylaldehyde)2]·X·solvent (Lz = 6-pyridin-2-yl[1,3,5]triazine-2,4-diamine; X = OH(-) (1·OH), Cl(-) (2·Cl), Br(-) (3·Br)) have been synthesized using mixed salicylaldehyde/pyridinyl-triazine ligands and discriminative counteranions. The Dy(III) ion in these three complexes resides in a similar D4d coordination geometry with counteranions perturbing the coordination environment and bond lengths and angles in the lattice. Magnetostructural studies reveal that the asymmetric distribution of salicylaldehyde/pyridinyl-triazine ligands and the presence of discriminative counteranions result in the coexistence of large anisotropy and quantum tunneling of magnetization. The magnetic anisotropy is dominated by the axial ligand field with short Dy-Osali distances and large ∠Osali-Dy-Osali angles, while the quantum tunneling relaxation is probably dictated by the π-π stacking of the Lz ligands, which induces an axial constriction of the coordinating plane. Ab initio calculations substantiate the diversity of the magnetic behaviors in these complexes and highlight the importance of axial ligand field with short Dy-Osali distances, large ∠Osali-Dy-Osali angles and less ligand stacking in these pseudo-D4d-symmetrical single-molecule magnets.
A mononuclear hexacoordinate complex [Co(pydm)2](dnbz)2 formed of 2,6-pyridinedimethanol in the coordination sphere of Co(II) and dinitrobenzoato anions exhibits magnetic anisotropy of an easy axis type and a field induced slow...
Three new closely related CoIIYIII complexes of general formula [Co(-L)(-X)Y(NO3)2] (X- = NO3- 1, benzoate, 2 and 9-anthracenecarboxylato, 3) have been prepared with the compartmental ligand N,N',N"-trimethyl-N,N"-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2L). In these complexes CoII and YIII are triply bridged by two phenoxide groups belonging to the dideprotonated ligand (L2-) and one ancillary anion X-. The change of the ancillary bridging group connecting CoII and YIII ions induces small differences in the trigonally distorted CoN3O3 coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Dc-magnetic, HFEPR and FD-FT THz-EPR measurements and ab initio theoretical calculations demonstrate that CoII ions in compounds 1-3 have large and positive D values (50 cm-1) which decrease with increasing the distortion of the pseudo-octahedral CoII coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compound 1 exhibits field induced SMMs behaviour, whereas compound 2 and 3 only display this behaviour when are magnetically diluted with diamagnetic ZnII (Zn/Co=1/10). So, it is always advisable to use magnetic diluted complexes, in which intermolecular interactions and QTM would be at least partly suppressed, so that "hidden SIM" could emerge. Field and temperature dependence of the relaxation times indicate the prevalence of the Raman process in these complexes above approximately 3 K.
Abstract Molecular magnetism has travelled a long way from the pioneering studies on electron exchange and double exchange or spin crossover and valence tautomerism in small oligonuclear complexes, from mono- to di- and tetranuclear species, to the current investigations about magnetic anisotropy and spin dynamics or quantum coherence of simple mono- or large polynuclear complexes, behaving as switchable bistable molecular nanomagnets for potential applications in information data storage and processing. In this review, we focus on the origin and development of the research in the field of molecular magnetism from a coordination chemistry viewpoint, which dates back to the establishment of magnetochemistry as a novel discipline among the molecular sciences. This overview is conceived as an attempt to orientate coordination chemists regarding their role in the future direction that molecular magnetism will undergo in its further evolution toward molecular spintronics and quantum computation. A particular emphasis will be given to some selected recent advances in single-molecule spintronic circuitry and quantum computing devices based on the large class of multiresponsive and multifunctional magnetic metal complexes to stimulate the progress in the field of molecular magnetism.
A bimetallic Co(ii) compound [Co(dppm(O,O))3][CoBr4] consisting of cationic octahedral and anionic tetrahedral units in the crystal lattice shows a sizable magnetic anisotropy and field-supported slow magnetic relaxation with the relaxation time τ = 0.1-0.3 s at T = 1.9 K.
This paper describes the correlation between Ising-type magnetic anisotropy and structure in trigonal bipyramidal Co(II) complexes. Three sulfur-containing trigonal bipyramidal Co(II) complexes were synthesized and characterized. It was shown that we can engineer the magnitude of the Ising anisotropy using ligand field theory arguments in conjunction with structural parameters. To prepare this series of compounds, we used, on the one hand, a tetradentate ligand containing three sulfur atoms and one amine (NS3(tBu)) and on the other hand three different axial ligands, namely, Cl(-), Br(-), and NCS(-). The organic ligand imposes a trigonal bipyramidal arrangement with the three sulfur atoms lying in the trigonal plane with long Co-S bond distances. The magnetic properties of the compounds were measured, and ab initio calculations were used to analyze the anisotropy parameters and perform magneto-structural correlations. We demonstrate that a smaller axial zero-field splitting parameter leads to slower relaxation time when the symmetry is strictly axial, while the presence of very weak rhombicity decreases the energy barrier and speeds the relaxation of the magnetization.
The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co=N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm−1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ±7/2> ground Kramers doublet.
We herein reported the synthetic, structural, computational, and magnetic studies of four air-stable heptacoordinated mononuclear cobalt(II) complexes, namely, [Co(II)(tdmmb)(H2O)2][BF4]2 (1), [Co(II)(tdmmb)(CN)2]·2H2O (2), [Co(II)(tdmmb)(NCS)2] (3), and [Co(II)(tdmmb)(SPh)2] (4) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3](2,6)pyridino[3](2,9)-1,10-phenanthrolinophane-2,10-diene; SPh(-) = thiophenol anion). Constrained by the rigid pentadentate macrocyclic ligand tdmmb, the Co(II) centers in all of these complexes are in the heptacoordinated pentagonal-bipyramidal geometry. While the equatorial environments of these complexes remain very similar to each other, the axial ligands are systematically modified from C to N to O to S atoms. Analyses of the magnetic data and the ab initio calculations both reveal large easy-plane magnetic anisotropy (D > 0) for all four complexes. While the experimentally obtained D values do not show any clear tendency when the axial coordinated atoms change from C to N to O atoms (complexes 1-3), the largest value is for the heavier and softer S-atom-coordinated complex 4. Because of significant magnetic anisotropy, all four complexes are field-induced single-ion magnets. This work represents a delicate modification of the magnetic anisotropy by tuning the chemical environment of the metal centers.
A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L1)4](NO3)2 (1) and [Co(Lx)4](ClO4)2 where x = 2 (2), 3 (3), 4 (4) (where L1 = thiourea, L2 = 1,3-dibutylthiourea, L3 = 1,3-phenylethylthiourea, and L4 = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L1)4](NO3)2 (5) and [Zn(L2)4](ClO4)2 (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest Ueff value of 62 cm⁻¹ and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = ³/2/I(Co) = ⁷/2 hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.
In this article we report the synthesis and structure of the new Co(II) complex Et4N[Co(II)(hfac)3] (I) (hfac = hexafluoroacetylacetonate) exhibiting single-ion magnet (SIM) behavior. The performed analysis of the magnetic characteristics based on the complementary experimental techniques such as static and dynamic magnetic measurements, electron paramagnetic resonance spectroscopy in conjunction with the theoretical modeling (parametric Hamiltonian and ab initio calculations) demonstrates that the SIM properties of I arise from the nonuniaxial magnetic anisotropy with strong positive axial and significant rhombic contributions.
Magnetic anisotropy is the key element in the construction
of single-ion magnets, a kind of nanomagnets for high-density
information storage. Here we show, for the first time, an unusual
large easy-plane magnetic anisotropy (with a zero-field splitting
parameter D of +40.2 cm -1 ) mainly arising from the second order
spin-orbit coupling effect in a trigonal-planar Co(II) complex
[Li(THF) 4 ][Co(NPh 2 ) 3 ], revealed by combined studies of magnetism,
high frequency/field electron paramagnetic resonance spectroscopy
and ab initio calculations. Meanwhile, the field-induced slow
magnetic relaxation in this complex was mainly attributed to the
Raman process.
A mononuclear cobalt(II) complex [Co(3,5-dnb)2(py)2(H2O)2] {3,5-Hdnb = 3,5-dinitrobenzoic acid; py = pyridine} was isolated in two polymorphs, in space groups C2/c (1) and P21/c (2). Single-crystal X-ray diffraction analyses reveal that 1 and 2 are not isostructural in spite of having equal formulas and ligand connectivity. In both structures, the Co(II) centers adopt octahedral {CoN2O4} geometries filled by pairs of mutually trans terminal 3,5-dnb, py, and water ligands. However, the structures of 1 and 2 disclose distinct packing patterns driven by strong intermolecular O–H···O hydrogen bonds, leading to their 0D→2D (1) or 0D→1D (2) extension. The resulting two-dimensional layers and one-dimensional chains were topologically classified as the sql and 2C1 underlying nets, respectively. By means of DFT theoretical calculations, the energy variations between the polymorphs were estimated, and the binding energies associated with the noncovalent interactions observed in the crystal structures we
Single-molecule magnets (SMMs) and single-chain magnets (SCMs), also known as molecular nanomagnets, are molecular species of nanoscale proportions with the potential for high information storage density and spintronics applications. Metal-organic frameworks (MOFs) are three-dimensional ordered assemblies of inorganic nodes and organic linkers, featuring structural diversity and multiple chemical and physical properties. The concept of using these frameworks as scaffolds in the study of molecular nanomagnets provides an opportunity to constrain the local coordination geometries of lanthanide centers and organize the individual magnetic building blocks (MBBs, including both transition-metal and lanthanide MBBs) into topologically well-defined arrays that represent two key factors governing the magnetic properties of molecular nanomagnets. In this tutorial review, we summarize recent progress in this newly emerging field.
Direct current (dc) and alternating current (ac) magnetic measurements have been performed on the three Ni(I) complexes: [NiCl(PPh3)3], [NiCl(PPh3)2]·C4H8O, and [Ni(N(SiMe3)2)(PPh3)2]. Fits of the dc magnetic data suggest an almost similar behavior of the three compounds, which display only moderate deviations from the spin-only values. The ac magnetic investigations reveal that the two complexes with trigonal planar coordination-[NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)(PPh3)2]-display slow magnetic relaxation at low temperatures under applied dc fields, whereas tetrahedral [NiCl(PPh3)3] does not. Ground and excited states as well as magnetic data were calculated by ab initio wave function based multi-configurational methods, including dynamic correlation as well as spin-orbit coupling. The two trigonal planar complexes comprise well-isolated S = (1)/2 ground states, whereas two S = (1)/2 states with a splitting of less than 100 cm(-1) were found in the tetrahedral compound.
High-magnetic-field, high-frequency electronic spin resonance (ESR) facility has been first developed in Wuhan National High Magnetic Field Center, China. The facility can achieve a frequency range of 210-370 GHz, a temperature range of 2-300 K and magnetic fields up to 50 T. The ESR facility has been tested with a sample of ruby. Clear ESR spectra of Cr 3+ ion are obtained.
A new compound, {[Co(bmzbc)2]·2DMF}n (JXNU-1, JXNU denotes Jiangxi Normal University), based on the 4-(benzimidazole-1-yl)benzoate (bmzbc–) ligand has been synthesized and structurally characterized. The Co(II) ions are bridged by the rod-like bmzbc– ligands to give a two-dimensional (2D) sheet wherein the Co(II) ions are spatially separated from each other by the long bmzbc– rods. The 2D sheets are further stacked into a 3D framework with 1D channels occluding the guest DMF molecules. Detailed magnetic studies show that the individual octahedral Co(II) ions in JXNU-1 exhibit field-induced slow magnetic relaxation, which is characteristic behavior of single-ion magnets (SIMs). The rarely observed positive value of zero-field splitting (ZFS) parameter D for the Co(II) ion in JXNU-1 demonstrates that JXNU-1 is a unique example of Co(II)-based SIMs with easy-plane anisotropy, which is also confirmed by the calculations. The microporous nature of JXNU-1 was established by measuring CO2 sorption isotherms. The abrupt changes observed in the C3H8 and C2H6 adsorption isotherms indicate that a structural transformation occurred in the gas-loading process. The long connection between the magnetic metal centers in JXNU-1 meets the requirements for construction of porosity and SIM in a well-defined network, harmoniously providing a good candidate of functional molecular materials exhibiting SIM and porosity.
The possibility of controlling magnetic anisotropy by tuning contribution of second order perturbation to spin-orbit coupling through modulation of the coordination environment is investigated. Subtle variation of the coordination environment triggers a remarkable deviation in the axial zero field splitting parameter of seven coordinate Co(ii) complexes.
We report on a novel manganese(III)-porphyrin complex with the formula [Mn(III) (TPP)(3,5-Me2 pyNO)2 ]ClO4 ⋅CH3 CN (2; 3,5-Me2 pyNO=3,5-dimethylpyridine N-oxide, H2 TPP=5,10,15,20-tetraphenylporphyrin), in which the Mn(III) ion is six-coordinate with two monodentate 3,5-Me2 pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero-field splitting (D=-3.8 cm(-1) ), low rhombicity (E/|D|=0.04) of the high-spin Mn(III) ion, and, ultimately, for the observation of slow magnetic-relaxation effects (Ea =15.5 cm(-1) at H=1000 G) in this rare example of a manganese-based single-ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single-crystal diffraction studies, variable-temperature direct- and alternating-current measurements and high-frequency and -field EPR spectroscopic analysis followed by quantum-chemical calculations. Slow magnetic-relaxation effects were also observed in the already known analogous compound [Mn(III) (TPP)Cl] (1; Ea =10.5 cm(-1) at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.
Two mononuclear seven-coordinate cobalt(ii) complexes [Co(L)3(NO3)2] (L = 4-tert-butylpyridine, ; L = isoquinoline, ) were prepared and structurally analyzed by single-crystal X-ray crystallography. The coordination spheres of and exhibit distorted pentagonal bipyramid geometry. Analysis of their direct-current magnetic data reveals the existence of easy plane anisotropy (D > 0) with a small transverse anisotropy (E), which was further confirmed by high-field electron paramagnetic resonance (HFEPR) spectroscopy. Field-induced slow magnetic relaxations were observed under the applied dc field in complexes and by alternating-current magnetic susceptibility measurements. Importantly, these complexes are new instances of mononuclear high-coordinate cobalt(ii)-based single-molecule magnets.
The synthesis, structures, and magnetic properties of a family of air-stable star-shaped CoIICoIII3 complexes were investigated. These complexes contain only one paramagnetic Co(II) ion with the approximate D3 coordination environment in the center and three diamagnetic Co(III) ions in the peripheral. Magnetic studies show their slow magnetic relaxation in the absence of an applied dc field, which is characteristic behavior of single-molecule magnets (SMMs), caused by the individual Co(II) ion with approximate D3 symmetry in the center. Most importantly, it was demonstrated that the anisotropy energy barrier can be finely tuned by the periphery substituent of the ligand and the countercation. The anisotropy energy barrier can be increased significantly from 38 K to 147 K.
Reversible switching of single-molecule magnet (SMM) behaviors was demonstrated by structural transformation of dinuclear dysprosium cores in polyoxometalates [{Dy(H2O)2(CH3COCH3)}2(γ-SiW10O36)2]10− (1) and [Dy2(μ2-OH)2(γ-SiW10O36)2]12− (2). Whereas 1 hardly showed a SMM behavior, 2 with bis(μ2-OH) bridging ligands between two dysprosium cations showed a SMM behavior with a thermal energy barrier ΔE/kB of 65.7 K. This is mainly because of large magnetic anisotropy in 2 obtained by the design of a weak interaction between ligands and the oblate-shaped electron density of dysprosium cations. The SMM behaviors could reversibly be manipulated by coordination and elimination of bis(μ2-OH) bridging ligands and transformation of the coordination geometry around dysprosium cations between distorted monocapped trigonal prism (1) and distorted trigonal prism (2).
The quest for the single molecular magnets (SMMs) based on mononuclear transition metal complexes is focused on the low-coordinate species. No metal complex with a coordination number of eight has been shown to exhibit SMM properties. Here the magnetic studies have been carried out for a mononuclear, eight-coordinate cobalt(II)-12-crown-4 (12C4) complex [CoII(12C4)2](I3)2(12C4) (1) with a large axial zero-field splitting. Magnetic measurements show field-induced, slow magnetic relaxation under an applied field of 500 Oe at low temperature. The magnetic relaxation time τ was fitted by the Arrhenius model to afford an energy barrier of Ueff = 17.0 cm-1 and a preexponential factor of τ0 = 1.5×10-6s. The work here presents the first example of the eight-coordinate, mononuclear, 3d metal complex exhibiting the slow magnetic relaxation.
Transition metal ions with long-lived spin states represent minimum size magnetic bits. Magnetic memory has often been associated with the combination of high spin and strong uniaxial magnetic anisotropy. Yet, slow magnetic relaxation has also been observed in some Kramers ions with dominant easy-plane magnetic anisotropy, albeit only under an external magnetic field. Here we study the spin dynamics of cobalt(II) ions in a model molecular complex. We show, by means of quantitative first-principles calculations, that the slow relaxation in this and other similar systems is a general consequence of time-reversal symmetry that hinders direct spin-phonon processes regardless of the sign of the magnetic anisotropy. Its magnetic field dependence is a subtle manifestation of electronuclear spin entanglement, which opens relaxation channels that would otherwise be forbidden but, at the same time, masks the relaxation phenomenon at zero field. These results provide a promising strategy to synthesize atom-size magnetic memories.
Pseudooctahedral mononuclear cobat(II) complex [Co(abpt)2(tcm)2] (1), where abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole and tcm = tricyanomethanide anion, shows field-induced slow relaxation of magnetization with U = 86.2 K and large axial and rhombic single-ion zero-field-splitting parameters, D = +48(2) cm(-1) and E/D = 0.27(2) (D = +53.7 cm(-1) and E/D = 0.29 from ab initio CASSCF/NEVPT2 calculations), thus presenting a new example of a field-induced single-ion magnet with transversal magnetic anisotropy.
Five dinuclear lanthanide complexes with formula [Ln2L2(OAc)4(MeOH)a(H2O)b]·cMeOH·dH2O (a = 2, b = 0, c = 2, d = 0, Ln = Sm (), Gd (), Dy (); a = 0, b = 2, c = 4, d = 2, Ln = Tm ()) and [Yb2L2(OAc)4(MeOH)2]·[Yb2L2(OAc)4(H2O)2]·2H2O () (HL = (E)-N'-(2-hydroxybenzylidene)-2-mercaptonicotinohydrazide), have been synthesized and their crystal structures and magnetic properties are reported. All five complexes are centrosymmetric, showing a similar dinuclear core with two lanthanide ions in each complex being bridged by acetate groups in the η(1):η(2):μ2 mode. The various coordination modes of acetate groups result in two kinds of coordination geometries for Ln ions with the ones in complexes and the Yb2 in being nine-coordinated with a mono-capped square antiprism geometry, while the Yb1 ions in the other part of complex are eight-coordinated with a triangular dodecahedron geometry. Magnetic susceptibility studies reveal that complex shows single molecule magnet behaviour with an energy barrier of 39.1 K. In addition, comparison of the structural parameters among the similar Dy2 SMMs with a η(1):η(2):μ2 coordination mode of carboxylate groups reveals the significant role played by coordination geometry in modulating the relaxation dynamics of SMMs.
A magnetic Co-production: The complex [Co(μ-L)(μ-OAc)Y(NO3 )2 ] (see structure O red, N blue, C gray), in which the Co(II) ion exhibits a D value of approximately +45 cm(-1) , as determined by magnetic and inelastic neutron scattering experiments, exhibits slow magnetic relaxation and single-ion magnet behavior.
Single-molecule magnets that contain one spin centre may represent the smallest possible unit for spin-based computational devices. Such applications, however, require the realization of molecules with a substantial energy barrier for spin inversion, achieved through a large axial magnetic anisotropy. Recently, significant progress has been made in this regard by using lanthanide centres such as terbium(III) and dysprosium(III), whose anisotropy can lead to extremely high relaxation barriers. We contend that similar effects should be achievable with transition metals by maintaining a low coordination number to restrict the magnitude of the d-orbital ligand-field splitting energy (which tends to hinder the development of large anisotropies). Herein we report the first two-coordinate complex of iron(I), [Fe(C(SiMe3)3)2](-), for which alternating current magnetic susceptibility measurements reveal slow magnetic relaxation below 29 K in a zero applied direct-current field. This S = complex exhibits an effective spin-reversal barrier of Ueff = 226(4) cm(-1), the largest yet observed for a single-molecule magnet based on a transition metal, and displays magnetic blocking below 4.5 K.
A family of segmented all-electron relativistically contracted (SARC) basis sets for the elements Hf-Hg is constructed for use in conjunction with the Douglas-Kroll-Hess (DKH) and zeroth-order regular approximation (ZORA) scalar relativistic Hamiltonians. The SARC basis sets are loosely contracted and thus offer computational advantages compared to generally contracted relativistic basis sets, while their sufficiently small size allows them to be used in place of effective core potentials (ECPs) for routine studies of molecules. Practical assessments of the SARC basis sets in DFT calculations of atomic (ionization energies) as well as molecular properties (geometries and bond dissociation energies for MHn complexes) confirm that the basis sets yield accurate and reliable results, providing a balanced description of core and valence electron densities. CCSD(T) calculations on a series of gold diatomic compounds also demonstrate the applicability of the basis sets to correlated methods. The SARC basis sets will be of most utility in calculating molecular properties for which the core electrons cannot be neglected, such as studies of electron paramagnetic resonance, Mossbauer and X-ray absorption spectra, and topological analysis of electron densities.
A general approach to spin-lattice relaxation is given for salts to which a crystalline field theory is appropriate. In particular, the theory of Elliott & Stevens for the interaction of a rare-earth ion with static ionic surroundings is generalized phenomenologically to represent the interaction of the rare-earth ion with the lattice vibrational modes. Evaluation of the spin-lattice interaction in terms of a few constants is possible. One- and two-phonon processes are investigated and the relaxation times for non-Kramers and Kramers salts computed. For the one-phonon (or direct) process the non-Kramers salts exhibit the typical behaviour T_1 propto H-2T-1, and the Kramers salts T_1 propto H-4T-1. It is shown that, for a given Zeeman splitting of the ground doublet, the latter may exhibit an enormous anisotropy with respect to the direction of the external field, approximately proportional to the anisotropy of the temperature-independent part of the susceptibility. Application of the general theory is made to two salts, holmium and dysprosium ethyl sulphate; the former a non-Kramers, the latter a Kramers salt. It is shown that the dysprosium salt would be expected to show a relaxation time in the direct process region which will vary as sin-2 theta cos-2 theta H-4T-1, where theta is the angle the external magnetic field makes with the crystallographic symmetry axis. For two-phonon processes, the additional distinction of whether the Debye energy (Ktheta_D) is less than or greater than the crystalline field splitting Delta between the ground state and the first excited state must be made. Non-Kramers salts to which the former condition apply (Ktheta_D < Delta) are shown to possess two-phonon relaxation processes of the usual Raman type. The relaxation time is proportional to T-7 and is independent of magnetic field. When Ktheta_D > Delta, there is present in addition a term arising from a resonance process, analogous to the resonance radiation effect in gases. Phonons of energy ~ Delta are absorbed and emitted by the spin system preferentially because of a phonon resonance with the crystalline field splitting of the spin states. As normally KT is much less than Delta, this leads to a relaxation time proportional to exp (Delta/KT). This process will dominate the Raman process except at very high and low temperatures. It is shown to be significant right down to the liquid-helium range by comparison with the relaxation rate due to direct processes. Kramers salts, when Ktheta_D < Delta, owing to a cancellation in the rate equation, exhibit a Raman relaxation time proportional to T-9 and independent of field. This `Van Vleck cancellation' is shown to be a consequence of time reversal symmetry. When Ktheta_D > Delta, the resonance process is also present, the relaxation time again being proportional to exp (Delta/KT). The resonance process is now shown to be dominant down to 1 or 2^circK for many rare-earth salts. Experimental verification is found for the resonance relaxation process in the rare-earth ethyl sulphates. In general, it is expected that this mechanism will be significant for any magnetic salt in which Ktheta_D > Delta.
A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d-block and f-block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well-known program MAGPACK, limited only by available hardware. © 2013 Wiley Periodicals, Inc.
The dispersion and absorption of a considerable number of liquid and dielectrics are represented by the empirical formula ε*−ε∞=(ε0−ε∞)/[1+(iωτ0)1−α]. In this equation, ε* is the complex dielectric constant, ε0 and ε∞ are the ``static'' and ``infinite frequency'' dielectric constants, ω=2π times the frequency, and τ0 is a generalized relaxation time. The parameter α can assume values between 0 and 1, the former value giving the result of Debye for polar dielectrics. The expression (1) requires that the locus of the dielectric constant in the complex plane be a circular arc with end points on the axis of reals and center below this axis.
If a distribution of relaxation times is assumed to account for Eq. (1), it is possible to calculate the necessary distribution function by the method of Fuoss and Kirkwood. It is, however, difficult to understand the physical significance of this formal result.
If a dielectric satisfying Eq. (1) is represented by a three-element electrical circuit, the mechanism responsible for the dispersion is equivalent to a complex impedance with a phase angle which is independent of the frequency. On this basis, the mechanism of interaction has the striking property that energy is conserved or ``stored'' in addition to being dissipated and that the ratio of the average energy stored to the energy dissipated per cycle is independent of the frequency.