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A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 26 to 80 K
Abstract
The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with FeII affords the solid (Me2NH2)2[Fe2L3]·6DMF·2H2O. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of FeIII centers with mixed-valence ligands of the form (L3)8- that result from a spontaneous electron-transfer from FeII to L2-. Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m2/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally-characterized extended solid that contains semiquinone radical ligands.
... Therefore, utilizing larger ligands would weaken the exchange interaction, thus resulting in a low magnetic ordering temperature of magnets that limits their practical application in spin devices. [41] Currently, several strategies have been proposed to strengthen the exchange coupling between spin centers, including employing π-conjugated organic ligands, [31,[42][43][44][45][46][47] introducing radical-based ligands, [48,49] adopting antiaromatic ligands [60] and so on. ...
... In 2015, a semiquinone radical-based 2D MOF, (Me 2 NH 2 ) 2 [Fe 2 L 3 ]·2 H 2 O·6 DMF (DMF = N,N-dimethylformamide), was synthesized through the reaction of 2,5dichloro-3,6-dihydroxy-1,4-benzoquinone (LH 2 ) with Fe-(BF 4 ) 2 ·6 H 2 O (Figure 3a,b). [48] The spontaneous electron transfer from Fe II to L 2À lead to the formation of high-spin Fe III centers and mixed-valence ligands of (L 3 ) 8À , resulting in a magnetic ordering transition temperature (T c ) of 80 K. Upon activation, DMF and water solvent molecules were removed from the frameworks to yield a desolvated phase (Me 2 NH 2 ) 2 [Fe 2 L 3 ] with a faint structural distortion, thereby resulting in a decrease in magnetic ordering temperature (T c ) to 26 K. The electrical conductivities of these compounds before and after activation were 1.4×10 À 2 and 1.0×10 À 3 S cm À 1 , respectively. ...
... Orbital analysis revealed that the high T c observed in these systems is ascribed to the enhanced orbital hybridization between the d orbitals of Cr atoms and the π* orbitals of organic linkers. Reproduced with permission from Ref. [48]. Copyright 2015, American Chemical Society. ...
The intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two‐dimensional metal–organic frameworks (2D MOFs) highly prospective candidates for next‐generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF‐based spintronics materials. Then, we highlight the spin‐transport properties of 2D MOF‐based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.
... 30 As for Raman spectroscopy, the typically observed peak for benzoquinones is at 1600 cm À1 , while for the corresponding semiquinones, it is downshifted to 1400 cm À1 . 103,104 The spectroscopic data along with single crystal structural data elucidate the presence of radical species in the 2D frameworks and, hence, provide relevant structure/properties relationships. ...
... An example of a permanently porous 2D framework incorporating the linker in its semiquinone form, was reported by Harris et al. in 2015. 103,104 The porous semiquinoid magnet is formulated as (Me 2 NH 2 ) 2 [Fe 2 (Cl 2 An) 3 ]Á2H 2 OÁ6DMF (2), with a BET surface area of 885(105) m 2 g À1 . It was synthesized through a solvothermal method in DMF, by combining Fe II ions with H 2 Cl 2 An which lead to a spontaneous electron transfer from Fe II to Cl 2 An 2À , leading to the simultaneous formation of both Fe II /Fe III and benzoquinone/ semiquinone couples. ...
... It was synthesized through a solvothermal method in DMF, by combining Fe II ions with H 2 Cl 2 An which lead to a spontaneous electron transfer from Fe II to Cl 2 An 2À , leading to the simultaneous formation of both Fe II /Fe III and benzoquinone/ semiquinone couples. 103 2 is a porous antiferromagnet, showing a spontaneous magnetization below 80 K, while the magnetic ordering temperature (T c = temperature above a material behaves as paramagnet) decrease to 26 K when the solvent molecules are removed from the channels. Due to its delocalized nature, 2 shows also conductivity values of s = 1.4(7) ...
Exploiting redox activity in supramolecular frameworks such as coordination polymers, metal-organic frameworks and related nanostructures is of paramount importance both at the molecular level and for their technological applications, since the modulation of their redox states is an emerging strategy to enhance their physical properties. In the plethora of organic linkers, quinone derivatives are excellent redox-active ligands, widely used for various applications such as electrode materials, flow batteries, pseudocapacitors, etc. Benzoquinones undergo a one-electron reversible reduction to form a semiquinone radical species that can be further reduced to form hydroquinone. Remarkably, the quinoid ring can be functionalized with various functional groups making these systems excellent linkers to construct supramolecular frameworks as well as challenging platforms to tune the redox potential and therefore the stability of radical anions and electrochemical performances of the obtained materials. The recent advances on benzoquinone-based 2D CPs/MOFs and related nanostructures are reported, highlighting the extreme versatility of this class of redox-active linkers in tailoring the physical properties of the obtained materials. The current/future potential of these materials in electrochemical and technologically relevant applications will also be envisioned. This journal is
... 45 These materials can be synthesized with a large number of different metal centers or organic linkers, giving rise to a diverse range of electronic and magnetic phenomena, 46−52 and post-synthetic reduction or oxidation of these materials enables further modification of their properties. 32,53,54 Of p a r t i c u l a r i n t e r e s t i s t h e f r a m e w o r k m a t e r i a l (H 2 NMe 2 ) 2 [Fe 2 L 3 ]·6DMF·2H 2 O (1) (H 2 L = 3,6-dichloro-2,5-dihydroxybenzoquinone), which is a ferrimagnetic semiconductor with a characteristic temperature of T C = 80 K. 48 The conductivity of this material arises from organic linker-centered mixed valency, while the magnetic order arises from strong metal−radical coupling. Both the magnetic ordering temperature and conductivity of this material can be readily modified through post-synthetic reduction. ...
... 32,55 Importantly, while the spin carriers of 1 are isotropic S = 5 / 2 Fe III metal centers and S = 1 / 2 L 3−• linkers, magnetic measurements on nonoriented powders reveal a large coercive field of H c = 2630 Oe at 1.8 K, suggesting the presence of magnetic anisotropy. 48 Although single crystals of 1 are too small to perform oriented magnetic measurements, the chemical versatility of these materials enables the exploration of related compounds for which large single crystals can be grown. Herein, we report the synthesis and magnetic characterization of the material (NMe 4 ) 2 [Fe 2 L 3 ]·7DMF (2), which contains a 2D honeycomb framework isostructural to that in 1 with a similar electronic and magnetic structure. ...
... Mossbauer and Raman spectroscopies were used to confirm that the metal−organic layers of 2 possess a similar electronic structure to those in 1. Mossbauer spectroscopy confirms the presence of a high-spin Fe III center with a similar isomer shift (δ = 0.574(1) mm/s) and quadrupole splitting (ΔE Q = 1.125(5) mm/s) as in 1 ( Figure S4), and Raman spectroscopy indicates partial ligand reduction and electron delocalization ( Figure S5). 48 Together, these results suggest an in situ metal-to-ligand electron transfer during framework formation and a formal approximate composition of [Fe 2 III (L 2− )(L 3−• ) 2 ] 2− . This formulation is further supported by the observation of an intervalence charge-transfer band in the near-infrared diffuse reflectance spectrum and a room-temperature electronic conductivity comparable to that of 1 ( Figures S6 and S7). ...
... For example, metal-dioxolene units have been shown to display stimulidependent metal-to-ligand electron transfer ("valence tautomerism"), leading to switchable physical properties. [3][4][5][6] Strong metal-radical coupling and magnetic ordering can be engendered by their capacity to stabilize ligand radicals, [6][7][8] and high electrical conductivity can result from the close energy alignment between metal and ligand orbitals. [9][10][11][12] Thanks to these desirable properties, metal-dioxolene-based coordination solids and metal-organic frameworks have been investigated for diverse applications, including spintronics, 5,6 electrocatalysis, 13 and electrochemical energy storage. ...
... For example, while H 2 dhbq and its substituted derivatives (H 2 R 2 dhbq, where R = F, Cl, Br, NO 2 , etc.) have been shown to bind metals to form 1-, 2-, and 3D networks (Fig. 1), 16 how to select for each structure remains poorly understood. In the case of iron, 3D cubic frameworks, 11 2D honeycomb sheets, 8,[18][19][20][21][22][23][24][25] 1D linear chains, 49,50 and 1D zig-zag chains 6,26 have all been reported. ...
Redox-active tetraoxolene ligands such as 1,4-dihydroxybenzoquinone provide access to a diversity of metal-organic architectures, many of which display interesting magnetic behavior and high electrical conductivity. Here, we take a closer look at how structure dictates physical properties in a series of 1D iron-tetraoxolene chains. Using a diphenyl-derivatized tetraoxolene ligand (H2Ph2dhbq), we show that the steric profile of the coordinating solvent controls whether linear or helical chains are exclusively formed. Despite similar ligand environments, only the helical chain displays temperature-dependent valence tautomerism, switching from (FeII)(Ph2dhbq2-) to (FeIII)(Ph2dhbq3˙-) at temperatures below 203 K. The stabilization of ligand radicals leads to exceptionally strong magnetic exchange coupling (J = -230 ± 4 cm-1). Meanwhile, the linear chains are more amenable to oxidative doping, leading to Robin-Day class II/III mixed-valency and an increase in electrical conductivity by nearly three orders of magnitude. While previous studies have focused on the effects of changing metal and ligand identity, this work highlights how altering the metal-ligand connectivity can be a similarly powerful tool for tuning materials properties.
... Many coordination polymers [15] have elaborate lattices with large geometric frustration, which often results in flat bands similar to the kagome lattice [16][17][18][19][20][21][22][23][24][25][26][27]. Coordination polymers are also often strongly correlated, displaying phenomena such as Mott insulators, Kondo physics [26,27], and unconventional superconductivity [28][29][30]. ...
... With the recent prediction of uncon-ventional superconductivity on the decorated honeycomb lattice near half-filling [34], there is an open question about the connection between the strongly correlated insulator found in our study and unconventional superconductivity. A useful direction to explore this connection may be found in coordination complexes and polymers where the decorated honeycomb lattice is often found [16][17][18][19][20][21][22][23][24][25][26][27]. ...
We study the single-orbital Hubbard model on the half-filled decorated honeycomb lattice. In the non-interacting theory at half-filling the Fermi energy lies within a flat bland where strong correlations are enhanced and the lattice exhibits frustration. We find a correlation driven first-order metal-insulator transition to two different insulating ground states - a dimer valence bond solid Mott insulator when inter-triangle correlations dominate, and a broken $\mathcal{C}_3$ symmetry antiferromagnet that arises from frustration when intra-triangle correlations dominate. The metal-insulator transitions into these two phases have very different characters. The metal-broken $\mathcal{C}_3$ antiferromagnetic transition is driven by spontaneous $\mathcal{C}_3$ symmetry breaking that lifts the topologically required degeneracy at the Fermi energy and opens an energy gap in the quasiparticle spectrum. The metal-dimer valence bond solid transition breaks no symmetries of the Hamiltonian. It is caused by strong correlations renormalizing the electronic structure into a phase that is adiabatically connected to both the trivial band insulator and the ground state of the spin-1/2 Heisenberg model in the relevant parameter regime. Therefore, neither of these metal-insulator transitions can be understood in either the Brinkmann-Rice or Slater paradigms.
... 6 Honeycomb molecular magnets are no exception to this, particularly those based on chelating ligands such as (ox 2 -=C 2 O 2 -4 ), 7-9 tetraoxolene (C 6 O 4 R n -2 ) 10 amongst others. 11,12 These structures often have multiple different paramagnetic species producing ferrimagnetic order, whether this is alternating metal sites A[M II M III ](ox) 3 , 7 A[M II M III ](C 6 O 4 R 2 ) 3 13,14 or the presence of radical ligands in A 2 [M 2 (C 6 O 4 R 2 ) 3 ], 10 where A is charge balancing cation (typically alkylammonium). Equally, distorted honeycomb structures, with multiple different exchange pathways are common. ...
We report the magnetic structure and properties of a thiocyanate-based honeycomb magnet [Na(OH 2 ) 3 ]Mn(NCS) 3 which crystallises in the unusual low-symmetry trigonal space group P3. Magnetic measurements on powder samples show this...
... Exchange-correlation effects were treated using the Perdew-Burke-Ernzerhof (PBE) GGA density functional [61], where a 7 × 7 × 11 Γ-centered k-point mesh was used to sample the Brillouin zone of the primitive cell. We used the pristine MOF structure as determined by Jeon et al. [62] with the inert solvents removed. A total energy tolerance of 10 −6 eV was used as the convergence criteria on the self-consistent charge density. ...
We predict the magnetic and electronic properties of a novel metal-organic framework. By combining density functional theory and density matrix renormalization group approaches, we find the diatomic Kagome crystal structure of the metal-semiquinoid framework (H$_2$NMe$_2$)$_2$M$_2$(Cl$_2$dhbq)$_3$ (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) to host a rich variety of antiferromagnetic (AFM) and ferromagnetic (FM) Dirac semimetallic, spin-polarized Dirac fermions, and flat band magnetic insulators and metallic phases. Concomitantly, the spin excitation spectrum of the various magnetic systems display multiple Dirac-like and nodal-ring crossings. This suggests that the metal-semiquinoid system is an ideal platform for examining the intertwining of Dirac fermions and magnons.
... These complexes often exhibit spin-state changes as well as dielectric/ferroelectric switching owing to the presence of metal centers. [6][7][8][9] For example, spin crossover (SCO) complexes represent spin state switching between lowand high-spin states controlled by external stimuli, including heat, pressure, magnetic field, and light irradiation. [10,11] Another example is electron-transferred-coupled spin transition (ETCST) compounds, wherein metal-to-metal charge transfer between two metal centers influenced by external stimuli results in spinstate change and valence tautomerism. ...
Significant research has been conducted on molecular ferroelectric materials, including pure organic and inorganic compounds; however, studies on ferroelectric materials based on coordination metal complexes are scarce. Ferroelectric materials based on coordination metal complexes have tunable structures and designs, with coexistence or synergy between the ferroelectric behavior and magnetic properties. Compared to inorganic compounds, few coordination metal complexes exhibit coupling between the magnetic and dielectric properties. Coordination metal complexes with strong coupling between the magnetic and dielectric properties exhibit dielectric permittivity variations under external magnetic fields. Therefore, they have attracted substantial interest for their potential use in magnetoelectric devices. In this review, we discuss recent advances in coordination metal complexes, that exhibit coupled magnetic functionalities and ferroelectricity or dielectric properties, including single‐molecule magnets, electron delocalization systems, and external stimuli responsive compounds.
... Among the large list of redox-active ligands used to promote conductivity in porous frameworks, dihydroxybenzoquinone (dhbq) and its derivatives stand promising to engender mixed-valence MOFs due to their ability to exist in three different redox states through aromatic-quionid-aromatic transitions ( Figure 1). [27] The 3 À charge state is a radical and thus carries a spin compared to the diamagnetic 2 À and 4 À states. The π*-orbitals of the ligand are energetically matched with transition metal 3d-orbitals, enabling effective metal-ligand coordination and long-range conjugated pathways along the framework struts for efficient charge transport. ...
The crystal structures of metal‐organic frameworks (MOFs) are typically determined by the strong chemical bonds formed between the organic and inorganic building units. However, the latest generation of redox‐active frameworks often rely on counterions in the pores to access specific charge states of the components. Here, we model the crystal structures of three layered MOFs based on the redox‐active ligand 2,5‐dihydroxybenzoquinone (dhbq): Ti2(Cl2dhbq)3, V2(Cl2dhbq)3 and Fe2(Cl2dhbq)3 with implicit and explicit counterions. Our full‐potential first‐principles calculations indicate that while the reported hexagonal structure is readily obtained for Ti and V, the Fe framework is stabilised only by the presence of explicit counterions. For high counterion concentrations, we observe the formation of an electride‐like pocket in the pore center. An outlook is provided on the implications of solvent and counterion control for engineering the structures and properties of porous solids.
... Elemental analysis calc. for C 16 2 ] (abbreviated as cis-1D-M). cis-1D-Mn and cis-1D-Zn were isomorphic. ...
The coordination polymers (CPs) with binary ligands, including 2,5-dihydroxy-1,4-benzoquinone (H2DHBQ) and 4,4'-bipyridyl (bpy), were synthesized using in situ hydrolysis of 2,5-dimethoxy-1,4-benzoquinone (DMBQ). Three kinds of CPs were obtained depending on the metal ions. For M = Mn and Zn, a 1D zigzag chain structure with cis conformation ( cis-1D-M) was obtained, whereas Co, Ni, and Cu compounds afforded a 2D net structure with trans conformation (trans -2D-M) with a 1D pore. A linear chain structure was also obtained for M = Cu. Magnetic susceptibility (χM T) at 300 K in cis -1D-Mn and trans -2D-Co was evaluated to be 4.421 and 2.950 cm3 K mol-1, respectively, indicating that both compounds are in the high-spin state. According to the N2 adsorption isotherms at 77 K, trans -2D-Ni showed microporosity with the BET surface area of 177 m2 g-1, whereas the isomorphic trans -2D-Co rarely adsorbed N2 at 77 K. This phenomenon was explained by the difference of diffusion kinetics of the adsorbent molecules, which was supported by the CO2 adsorption isotherms at 195 K. The optical band gaps of cis -1D-Mn, cis -1D-Zn, trans -2D-Co, and trans -2D-Ni were estimated to be 1.6, 1.8, 1.0, and 1.1 eV, respectively, by using UV-vis-NIR spectroscopy.
... Zinc cations have proven to be a reasonably convenient platform for testing methods for obtaining coordination polymers based on anilate ligands [3,[45][46][47][48][49][50][51][52][53]. The first examples of such derivatives, 1-3, with a tert-butyl-substituted ligand were obtained in this work. ...
We report the synthesis, structural investigation, and thermal behavior for three zinc-based 1D-coordination polymers with 3,6-di-tert-butyl-2,5-dihydroxy-p-benzoquinone, which were synthesized in the presence of different glycols. The interaction of zinc nitrate with glycols, followed by using the resulting solution in solvothermal synthesis with the anilate ligand in DMF, makes it possible to obtain linear polymer structures with 1,2-ethylene or 1,2-propylene glycols coordinated to the metal. The reaction involving 1,3-propylene glycol under similar conditions gives a crystal structure that does not contain a diol. The crystal and molecular structures of the synthesized compounds were determined using single crystal by X-ray structural analysis. The influence of glycol molecules coordinated to the metal on the thermal destruction of synthesized compounds is shown.
... Progress in the science of molecular magnets has eventually enabled a variety of metal-organic framework (MOF)-based magnetic materials, [1][2][3][4] which are endowed with properties of highly configurable porosity and tunable magnetic coupling in the framework moiety. Therefore, MOF-based magnetic materials, such as spontaneous magnets (MOFmagnets) [5][6][7][8][9][10][11][12][13][14][15][16][17][18] and spin-crossover materials (MOF-SCO), [19][20][21] exhibit tremendous potential as chemo-responsive magnetic materials in which the guest play a crucial role in inducing a change in magnetism or spin state. ...
Host–guest electron transfer (HGET) in molecular framework systems is a critical trigger for drastic functional changes in both host framework and guest. A reversible magnetic phase transition was achieved via HGET in a layered framework, [{Ru2(2,6‐F2PhCO2)4}2(BTDA‐TCNQ)] (1), where 2,6‐F2PhCO2⁻ and BTDA‐TCNQ represent 2,6‐difluorobenzoate and bis[1,2,5]dithiazolotetracyanoquinodimethane, respectively. The guest‐free 1 with an antiferromagnetic ground state transformed into a paramagnet, [{Ru2(2,6‐F2PhCO2)4}2(BTDA‐TCNQ)]I3 (1‐I3), by adsorbing iodine (I2). The local charge distribution of [{Ru2II,III}⁺‐(BTDA‐TCNQ).−‐{Ru2II,II}] in 1 was reversibly modified to [{Ru2II,III}⁺‐(BTDA‐TCNQ)⁰‐{Ru2II,II}](I3⁻) in 1‐I3 through HGET. Theoretical calculations of 1‐I3 indicated a partial charge delocalization as [{Ru2}(1−δ)+‐(BTDA‐TCNQ)⁰‐{Ru2}δ+](I3⁻) with δ≈0.2, aided by weak ferromagnetic coupling. 1‐I3 exhibited a hundred‐fold enhancement in electrical conductivity compared to that of 1.
... 14,15 On the other hand, CPs based on 3,5-disubstituted-2,6-dihydroxy-1,4-benzoquinone, commonly called anilates, have been widely studied over the past decade. [16][17][18][19] Anilates are excellent building blocks for constructing layered mono/multifunctional materials showing magnetic, [20][21][22][23][24][25] conducting, [26][27][28][29] and luminescent properties. [30][31][32] Among them, the first example of heterosubstituted anilate, the chlorocyananilate ligand (3-chloro-6-cyano-2,5-dihydroxybenzoquinone-ClCNAn 2-), is an interesting antenna linker for constructing Ln-CPs, due to the absence of IR quenchers in its molecular structure. ...
Controlled modulation of electronic and magnetic properties in stimuli‐responsive materials provides valuable insights for the design of magnetoelectric or multiferroic devices. This paper demonstrates the modulation of electrical and magnetic properties of a semiconductive, paramagnetic metal−organic framework (MOF) Cu 3 (C 6 O 6 ) 2 with small gaseous molecules, NH 3 , H 2 S, and NO. This study merges chemiresistive and magnetic tests to reveal that the MOF undergoes simultaneous changes in electrical conductance and magnetization that are uniquely modulated by each gas. The features of response, including direction, magnitude, and kinetics, are modulated by the physicochemical properties of the gaseous molecules. This study advances the design of multifunctional materials capable of undergoing simultaneous changes in electrical and magnetic properties in response to chemical stimuli.
Conductive metal-organic frameworks (c-MOFs) are promising thermoelectric (TE) materials due to their low thermal conductivity and tunable electronic properties. Theoretical results show that two dimensional semiconuctors have natural advantages in...
Six new coordination polymers (CPs) of cadmium based on an extended anilate-type ligand - 4,4'-(1,4-phenylenebis(azanylylidene))bis(3,6-di-tert-butyl-2-hydroxycyclohexa-2,5-dienone) (H2L) have been synthesized. The series of coordination polymers includes [Cd(L)(DMF)] (1), 1D-CP [Cd(L)(dipy-1)]·2DMF (2A)...
Controlled modulation of electronic and magnetic properties in stimuli‐responsive materials provides valuable insights for the design of magnetoelectric or multiferroic devices. This paper demonstrates the modulation of electrical and magnetic properties of a semiconductive, paramagnetic metal−organic framework (MOF) Cu3(C6O6)2 with small gaseous molecules, NH3, H2S, and NO. This study merges chemiresistive and magnetic tests to reveal that the MOF undergoes simultaneous changes in electrical conductance and magnetization that are uniquely modulated by each gas. The features of response, including direction, magnitude, and kinetics, are modulated by the physicochemical properties of the gaseous molecules. This study advances the design of multifunctional materials capable of undergoing simultaneous changes in electrical and magnetic properties in response to chemical stimuli.
Porphyrin based Metal–Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m² g⁻¹). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li⁺/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.
Providing the reader with an up-to-date digest of the most important current research carried out in the field, this volume is compiled and written by leading experts from across the globe. It reviews the trends in electrochemical sensing and its applications and touches on research areas from a diverse range, including microbial fuel cells, 3D printing electrodes for energy conversion and electrochemical and electrochromic colour switching in metal complexes and polymers.
Coverage is extensive and will appeal to a broad readership from chemists and biochemists to engineers and materials scientists. The reviews of established and current interests in the field make this book a key reference for researchers in this exciting and developing area.
The intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two‐dimensional metal–organic frameworks (2D MOFs) highly prospective candidates for next‐generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF‐based spintronics materials. Then, we highlight the spin‐transport properties of 2D MOF‐based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.
We report the synthesis, characterization, and electronic properties of the quinoid-based three-dimensional metal-organic framework [Fe2(dhbq)3]. The MOF was synthesized without using cations as a template, unlike other reported X2dhbq3-based coordination polymers, and the crystal structure was determined by using single-crystal X-ray diffraction. The crystal structure was entirely different from the other reported [Fe2(X2dhbq3)]2-; three independent 3D polymers were interpenetrated to give the overall structure. The absence of cations led to a microporous structure, investigated by N2 adsorption isotherms. Temperature dependence of electrical conductivity data revealed that it exhibited a relatively high electrical conductivity of 1.2 × 10-2 S cm-1 (Ea = 212 meV) due to extended d-π conjugation in a three-dimensional network. Thermoelectromotive force measurement revealed that it is an n-type semiconductor with electrons as the majority of charge carriers. Structural characterization and spectroscopic analyses, including SXRD, Mössbauer, UV-vis-NIR, IR, and XANES measurements, evidenced the occurrence of no mixed valency based on the metal and the ligand. [Fe2(dhbq)3] upon incorporating as a cathode material for lithium-ion batteries engendered an initial discharge capacity of 322 mAh/g.
Since the characterization of the extended coordination network Prussian blue as a mixed‐valence material last century, the phenomenon has continued to be a source of inspiration for understanding fundamental aspects of electron transfer in three‐dimensional coordination space. Over the past 20 years, burgeoning interest in the field of coordination polymers and metal–organic frameworks has delivered a new suite of exquisite materials that have shed further light on structure–function relationships underpinning mixed valency in extended solids. This chapter provides an overview of the historical context for mixed‐valence materials and discusses the very latest developments in the field, along with new avenues for the application of the phenomenon.
Mechanochemistry, a resurging synthetic approach, has been developed into an effective and controllable method to access a family of crystalline porous catechol-derived metal-organic frameworks (MOFs) for the first time. We have identified that the obtained crystalline phase is readily tunable by precursors and the addition of solvents or drying agents. The described mechanochemistry allows us to synthesize these materials in a highly sustainable manner. Thus, mechanochemistry is expected to pave a promising avenue to access a broader class of MOF materials, in addition to those based on the motifs of carboxylic acid or imidazole.
To investigate the effect of the remote (> 10 Å) substituents of the ligand on the bridging metal center on the metal‐to‐metal charge transfer (MMCT) properties in cyanidometa‐bridged complexes, a series of new cyanidometal‐bridged complexes and their one‐electron and two‐electron oxidation products trans‐[Cp(dppe)Fe‐NC‐(L)Ru(PPh3)‐CN‐Fe(dppe)Cp][PF6]n (n = 2, 3, 4) (L = dmptpy, 1[PF6]n; L = meoptpy, 2[PF6]n; L = t‐Buptpy, 3[PF6]n) (Cp = 1,3‐cyclopentadiene, dppe = 1,2‐bis(diphenylphosphino)ethane, PPh3 = triphenylphosph‐ine, dmptpy = 4’‐(4‐dimethylaminophenyl)‐2,2’,6’,2”‐terpyridine, meoptpy = 4’‐(4‐methoxyphenyl)‐2,2’,6’,2”‐terpyridine, t‐Buptpy = 4’‐(4‐tertbutylphenyl)‐2,2’,6’,2”‐terpyridine) have been synthesized and well characterized. The investigations suggest that the cyanido‐stretching (νCN) vibration energy for the complexes is unsensitive to the electron‐donating ability change of the remote (> 10 Å) substituents of the cyanidometal bridging auxiliary ligand from tertbutyl, methoxy to dimethylamino group. However, the MMCT energies of the one‐ and two‐electron oxidation complexes are still sensitive to the remote (> 10 Å) substituents of the ligand on the bridging metal center and decreases with the increase of the electron‐donating ability of the remote (> 10 Å) substituents from tertbutyl, methoxy to dimethylamino group. All one‐ and two‐electron oxidation products belong to Class II mixed valence compounds according to the classification of Robin and Day.
This work reports on the design, synthesis and characterization of novel anilate-based functional molecular materials showing luminescent, magnetic and/or conducting properties. The family of anilate ligands comprises several derivatives obtained by introducing various substituents (H, F, Cl, Br, I, CN, etc.) at the 3 and 6 positions of the common 2,5-dihydroxy-1,4-benzoquinone framework. Among the anilate ligands, the only known heterosubstituted anilate with Cl/CN substituents at the 3,6 positions, ClCNAn2-, have been selected for preparing a novel family of 2D layered coordination polymers (2D CP) with both 3d metal ions and 4f lanthanide ions, through a general and straightforward synthetic strategy. i) Mixed-valence FeIIFeIII 2D CP, formulated as [TAG][FeIIFeIII(ClCNAn)3], containing, the tris(amino)-guanidinium (TAG) cation for the first time in such 2D networks has been synthesized and thoroughly characterized. ii) 2D CPs based on NIR-emitting lanthanides (YbIII, NdIII, ErIII) and the ClCNAn2- ligand, have been prepared and characterized. These layered compounds were exfoliated to nanosheets, by sonication-assisted solution synthesis. Time-resolved photoluminescence studies performed on both the bulk and nanosheets are also highlighted. iii) Novel family of heteroleptic 2D CPs based on NIR-emitting lanthanides and mixed ligands (ClCNAn2- and carboxylate ligands (DOBDC and F4-BDC)), were prepared and characterized. vi) Novel family of 2D CPs based on DyIII and ClCNAn2- were prepared in order to investigate their magnetic properties. v) Furthermore, the ability of anilate ligands to work as components of BEDT-TTF- based molecular conductors have been demonstrated through the synthesis, via electrocrystallization technique. vi) П-d hybrid multifunctional paramagnetic molecular conductors BEDT-TTF and [Fe(ClCNAn)3]3-) were also studied.
Magnetism of layered magnets depends on the inter-layer through-space magnetic interactions (J NNNI). Using guest sorption to address inter-layer pores in bulk-layered magnets is an efficient approach to magnetism control because the guest-delicate inter-layer distance (l trans) is a variable parameter for modulating J NNNI. Herein, we demonstrated magnetic changes induced by the adsorption of CO2, N2, and O2 gases in various isostructural layered magnets with a π-stacked pillared-layer framework, , (M = Co, 1, Fe, 2, Cr, 3; Cp* = η5-C5Me5; 2,3,5,6-F4PhCO2 - = 2,3,5,6-tetrafluorobenzoate; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane). Each compound had almost identical adsorption capability for the three types of gases; only CO2 adsorption was found to have a gated profile. A breathing-like structural modulation involving the extension of l trans occurred after the insertion of gases into the isolated pores between the [Ru2]2-TCNQ ferrimagnetic layers, which is more significant for CO2 than for O2 and N2, due to the CO2-gated transition. While adsorbent 1 with M = Co (S = 0) was an antiferromagnet with T N = 75 K, 1⊃CO2 was a ferrimagnet with T C = 76 K, whereas 1⊃N2 and 1⊃O2 were antiferromagnets with T N = 68 K. The guest-insertion effect was similarly confirmed in 2 and 3, and was characteristically dependent on the type of sandwiched spin in as M = Fe (S = 1/2) and Cr (S = 3/2), respectively. This study reveals that common gases such as CO2, O2, and N2 can serve as crucial triggers for the change in magnetism as a function of variable parameter l trans.
Conjugated coordination polymers (CCPs) with extended π–d conjugation, which can effectively promote long‐range delocalization of electrons and enhance conductivity, are superior to traditional metal‐organic frameworks (MOFs) and attracted great attention for potential applications in chemical sensors, electronics, energy conversion/storage devices, etc. However, the precise construction of CCPs is still challenging due to the complex and uncontrollable reactions of CCPs. Herein, two different framework dimensions of CCPs are controllably realized by employing the same ligand (2,3,5,6‐tetraaminobenzoquinone (TABQ)) and the same metal (copper) as center ions. The manipulation of reaction leads to different valences of ligands and metal ions, different coordination geometries, and thereby 1D‐CuTABQ and 2D‐CuTABQ frameworks, respectively. High performance of charge storage is hence achieved involving the storage of both cations and anions, and therein, 2D‐CuTABQ shows a high reversible capacity of ≈305 mAh g⁻¹, good rate capability and high capacity retention (≈170 mAh g⁻¹ after 2000 cycles at 5 A g⁻¹ with 0.01% decay per cycle), which outperforms 1D‐CuTABQ and almost all of the reported MOFs as cathodes for batteries. These results highlight the delicate structural control of CCPs for high‐performance batteries and other various applications.
Mononuclear complexes within a particular coordination geometry have been well recognized for high-performance single-molecule magnets (SMMs), while the incorporation of such well-defined geometric ions into multinuclear complexes remains less explored. Using the rigid 2-(di(1H-pyrazol-1-yl)methyl)-6-(1H-pyrazol-1-yl)pyridine (PyPz3) ligand, here, we prepared a series of benzoquinone-bridged dicobalt(II) SMMs [{(PyPz3)Co}2(L)][PF6]2, (1, L = 2,5-dioxo-1,4-benzoquinone (dhbq2-); 2, L = chloranilate (CA2-); and 3, L = bromanilate (BA2-)), in which each Co(II) center adopts a distorted trigonal prismatic (TPR) geometry and the distortion increases with the sizes of 3,6-substituent groups (H (1) < Cl (2) < Br (3)). Accordingly, the magnetic study revealed that the axial anisotropy parameter (D) of the Co ions decreased from -78.5 to -56.5 cm-1 in 1-3, while the rhombic one (E) increased significantly. As a result, 1 exhibited slow relaxation of magnetization under a zero dc field, while both 2 and 3 showed only the field-induced SMM behaviors, likely due to the increased rhombic anisotropy that leads to the serious quantum tunneling of the magnetization. Our study demonstrated that the relaxation dynamics and performances of a multinuclear complex are strongly dependent on the coordination geometry of the local metal ions, which may be engineered by modifying the substituent groups.
Layered metal-organic frameworks and metal-organic nanosheets have been regarded as an important series of low-dimensional materials. Their flexible designability and intrinsic porosity allow layered metal-organic frameworks and metal-organic nanosheets to be differentiated from inorganic counterparts. The present review article is devoted to collecting pieces of research aiming at exploiting layered metal-organic frameworks and metal-organic nanosheets as innovative nanomaterials. Spearheaded by an introductory part as Section 1, the present review article includes Section 2 (Physical properties), Section 3 (Electrocatalysis), Section 4 (Separation and filtration), Section 5 (Energy devices), Section 6 (Sensors), Section 7 (Theories for unveiled properties), and Section 8 (Conclusions and outlook).
Two-dimensional (2D) metal-organic framework (MOF) materials with large perpendicular magnetic anisotropy energy (MAE) are important candidates for high-density magnetic storage. The MAE-targeted high-throughput screening of 2D MOFs is currently limited by the time-consuming electronic structure calculations. In this study, a machine learning model, namely, transition-metal interlink neural network (TMINN) based on a database with 1440 2D MOF materials is developed to quickly and accurately predict MAE. The well-trained TMINN model for MAE successfully captures the general correlation between the geometrical configurations and the MAEs. We explore the MAEs of 2583 other 2D MOFs using our trained TMINN model. From these two databases, we obtain 11 unreported 2D ferromagnetic MOFs with MAEs over 35 meV/atom, which are further demonstrated by the high-level density functional theory calculations. Such results show good performance of the extrapolation predictions of TMINN. We also propose some simple design rules to acquire 2D MOFs with large MAEs by building a Pearson correlation coefficient map between various geometrical descriptors and MAE. Our developed TMINN model provides a powerful tool for high-throughput screening and intentional design of 2D magnetic MOFs with large MAE.
The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.
Current research fields of metal-organic frameworks (MOFs), which are being developed in the last 5-10 years by Russian scientific institutions and universities, are generalized. The review encompasses the design, synthesis, topological description, and prediction of MOF properties, the development of methods for their chemical engineering and modification, their investigation by modern physicochemical techniques, and the creation of functional materials based on porous frameworks (heterogeneous catalysts, highly efficient and highly selective sorbents of the new generation, conducting materials, systems for the target drug delivery).
Group 4 metallocene complexes Cp2M[OC] (Cp = η⁵-C5H5; M = Ti (1) or Zr (2); [OC] = κ²-O,C-OC6H2-2-CPh2-4,6-tBu2) supported with a redox-active bidentate O,C-ligand were successfully synthesized through salt metathesis. X-ray crystallographic results showed that both complexes maintained similar bent metallocene coordination geometry. A close inspection of the structural parameters revealed a remarkably long Ti–C bond in 1, indicating concomitant intramolecular charge transfer upon metathesis and coordination. The electronic structure of 1 was investigated by electron paramagnetic resonance (EPR) measurements, revealing the presence of a Ti(III) center and one [OC]•– radical anion. In contrast, the diamagnetic complex 2 was found to comprise a Zr(IV) center and an alkyl/aryloxo dianion. Divergent catalytic reactivity was observed for the two group 4 derivatives in the reaction between alkenes and HBpin. Specifically, dehydrogenative boration products (i.e., vinyl boronate esters) were obtained exclusively in the presence of the Zr complex (18 examples, up to 90% yield), while the Ti compound selectively promoted the formation of alkyl boronate esters via hydroboration (19 examples, up to 99% yield).
The syntheses and structures of a pair of neutral one-dimensional (1D) Fe-anilate based coordination polymers, Fe(Fan)(4,4'-bipy)2 (Fann- = deprotonated 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone; 4,4'-bipy = 4,4'-bipyridine) and Fe(Clan)(OPPh3)2 (Clann- = deprotonated 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone; OPPh3 = triphenylphosphine oxide), are reported. In the case of Fe(Fan)(4,4'-bipy)2, the Fe centre is in the +2 oxidation state and the Fan ligand is present in its quinoidal, dianionic form. In contrast, the structurally similar Fe(Clan)(OPPh3)2 chain contains Fe centres and chloranilate ligands in oxidation states close to +3 and -3 respectively at low temperature. It is suggested that intrachain π-π interactions aid electron transfer from the Fe centres to the bridging ligands.
We study the single-orbital Hubbard model on the half-filled decorated honeycomb lattice. In the noninteracting theory at half filling the Fermi energy lies within a flat band where strong correlations are enhanced. The lattice is highly frustrated. We find a correlation driven first-order metal-insulator transition to two different insulating ground states—a dimer valence bond solid Mott insulator when intertriangle correlations dominate, and a broken C3-symmetry antiferromagnet that arises from frustration when intratriangle correlations dominate. The metal-insulator transitions into these two phases have very different characters. The metal-broken C3 antiferromagnetic transition is driven by spontaneous C3 symmetry breaking that lifts the topologically required degeneracy at the Fermi energy and opens an energy gap in the quasiparticle spectrum. The metal-dimer valence bond solid transition breaks no symmetries of the Hamiltonian. It is caused by strong correlations renormalizing the electronic structure into a phase that is adiabatically connected to both the trivial band insulator and the ground state of the spin-1/2 Heisenberg model in the relevant parameter regime. Therefore, neither of these metal-insulator transitions can be understood in either the Brinkmann-Rice or Slater paradigms.
To investigate the effects of cis/trans‐configuration of the cyanidometal bridge and the electron donating ability of the auxiliary ligand on the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal‐bridged mixed valence compounds, two groups of trinuclear cyanidometal‐bridged compounds cis/trans‐[Cp(dppe)Fe(μ‐NC)Ru(4,4’‐dmbpy)2(μ‐CN)Fe(dppe)Cp][PF6]n (n=2 (cis/trans‐1[PF6]2), 3 (cis/trans‐1[PF6]3), 4 (cis/trans‐1[PF6]4)) and cis/trans‐[Cp(dppe)Fe(μ‐NC)Ru(bpy)2(μ‐CN)Fe(dppe)Cp][PF6]3 (cis/trans‐2[PF6]3) were synthesized and fully characterized. The experimental results indicate that for these one‐ and two‐electron oxidation mixed valence compounds, the trans‐configuration compounds are more beneficial for MMCT than the cis‐configuration compounds, and increasing the electron donating ability of the auxiliary ligand on the cyanidometal bridge is also conductive to MMCT. Moreover, compounds cis/trans‐1[PF6]n (n=3, 4) and cis/trans‐2[PF6]3 belong to localized compounds by analyzing the experimental characterization results, supported by the TDDFT calculations.
The incorporation of organic radicals into coordination polymers was considered as a promising strategy to promote metal‐ligand exchange interactions, but there are only a very limited number of stable organic radical‐based ligands that can serve well such a purpose. Herein, we report two new tris(2,4,6‐trichlorophenyl)methyl (TTM) radical‐based ligands L1 and L2 with two and three imidazole substituents, respectively. The imidazole unit serves as a coordination site and it can also stabilize the TTM radical by intramolecular donor–acceptor interaction. Coordination of L1 and L2 with cobalt(II) ions gave the corresponding one‐ (CoCP‐1) and two‐dimensional (CoCP‐2) coordination polymers, the structures of which were confirmed by X‐ray crystallographic analysis. Magnetic measurements and theoretical calculations suggest antiferromagnetic coupling between the paramagnetic cobalt(II) ions and the radical ligands. Our study provides a rational design for stable organic radical‐based ligands and further demonstrated the feasibility of a metal–radical approach toward magnetic materials.
Host–guest electron transfer (HGET) in molecular framework systems is a critical trigger for drastic functional changes in both host framework and guest. A reversible magnetic phase transition was achieved via HGET in a layered framework, [{Ru 2 (2,6‐F 2 PhCO 2 ) 4 } 2 (BTDA‐TCNQ)] ( 1 ), where 2,6‐F 2 PhCO 2 − and BTDA‐TCNQ represent 2,6‐difluorobenzoate and bis[1,2,5]dithiazolotetracyanoquinodimethane, respectively. The guest‐free 1 with an antiferromagnetic ground state transformed into a paramagnet, [{Ru 2 (2,6‐F 2 PhCO 2 ) 4 } 2 (BTDA‐TCNQ)]I 3 ( 1‐I 3 ), by adsorbing iodine (I 2 ). The local charge distribution of [{Ru 2 II,III } + –(BTDA‐TCNQ) • − –{Ru 2 II,II }] in 1 was reversibly modified to [{Ru 2 II,III } + –(BTDA‐TCNQ) 0 –{Ru 2 II,II }](I 3 − ) in 1‐I 3 through HGET. Theoretical calculations of 1‐I 3 indicated a partial charge delocalization as [{Ru 2 } (1– d )+ –(BTDA‐TCNQ) 0 –{Ru 2 } d + ](I 3 − ) with d ≈ 0.2, aided by weak ferromagnetic coupling. 1‐I 3 exhibited a hundred‐fold enhancement in electrical conductivity compared to that of 1 .
In the process of solvothermal synthesis of one-dimensional (1D) Zn–Dy Schiff base coordination polymers, 25% of tetraoxolene anions (X2tetraox2) underwent a one-electron reduction in situ reaction to
form X2tetraox3 radical anions, which was confirmed by using the nCO vibration Raman spectra. Both
the X2tetraox2 anion and the X2tetraox3 radical anion act as bridging ligands to connect two Zn–Dy
Schiff base structural units, [DyZn(LSchiff)(NO3)(MeOH)]2+ (H2LSchiff = N,N0
-bis(3-methoxysalicylidene)-1,3-
diaminopropane) and [DyZn(LSchiff)(MeOH)2]
3+, and finally assemble into two ionic zigzag chain-like
coordination polymers, [Dy2Zn2(LSchiff)2(X2tetraox2)1.5(X2tetraox3)0.5(NO3)(MeOH)3](ClO4)0.5�2MeOH
[1: X = Cl and 2: X = Br]. The two 1D Zn–Dy heterometallic coordination polymers exhibit the properties
of single-molecule magnets (SMMs) and exhibit two-step magnetic relaxation behaviors under a dc field
of 1500 Oe. Their effective energy barriers are adjusted by halogenated groups on the bridging ligands
X2tetraox2 and X2tetraox3.
Multi-dimensional coordination frameworks whose charge states are controllable by the sophisticated chemical modification of the components or by the application of stimuli are fascinating targets for the design of electronic/magnetic functional materials. A simple way to design such frameworks is to assemble electron donor (D) and electron acceptor (A) units in a DmAn ratio with electronically conjugated linkages; we call this type of framework a D/A metalorganic framework (D/A-MOF). In this account article, our previous studies on D/A-MOFs composed of carboxylate-bridged paddlewheel-type diruthenium units ([Ru2]) and polycyano organic molecules such as N,N¤-dicyanoquinodiimine (DCNQI) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) as the D and A subunits, respectively, are summarized. In this family of D/A-MOFs, the charge distribution between the internal D and A subunits can be precisely tuned by varying their electronic structure, i.e., depending on what kind of D and A we choose. Crucially, the diverse charge states, as well as anisotropic framework and often porous nature, of D/A-MOFs are well correlated with their bulk electronic and magnetic properties.
The isotypic compounds Na2(H2O)24[M2(C6H2O4)3] (M = Mn2+, Cd2+) form infinite net-like layers. The transition metal ion is coordinated octahedrally. The dianion of 2,5-dihydroxy-l,4-benzoquinone, consisting of two allyl systems interlinked by two long C—C distances (152—153 pm), acts as a chelating ligand. Adjacent layers are connected by hydrogen bonds. The stacking sequence of these layers, ••.AAA., leads to channels, which are filled by hydrated Na+ ions. The Na+ ions are coordinated in a distorted octahedral fashion, with neighbouring octahedra sharing common faces. © 1986, Verlag der Zeitschrift für Naturforschung. All rights reserved.
Two new microporous metal–organic frameworks, [Zn3(NH2BDC)3(H2O)2]·5DMF (1) and (Me2NH2)2[Cd3(NH2BDC)4]·2DMF·2.5 H2O (2) (NH2BDC = 2-amino-1,4-benzenedicarboxylate), based on linear trinuclear secondary building units of Zn3(COO)6 or Cd3(COO)8 that have different connected numbers, have been synthesized and characterized, which exhibit usual pcu and rare hex-like topologies, respectively. Both 1 and 2 are three-dimensional porous frameworks, having solvent accessible volumes of ca. 53 and 39%, respectively. There are unremovable guest Me2NH2+ cations clogged in the channels of 2. In contrast, being of good thermo-stability, 1 has a Langmuir surface area about 570 m2 g−1 and type I isotherm behaviour for nitrogen gas sorption at 77 K, being typical of microporous material.
Crystalline compounds (H3O)2(phz)3M2(C6O4Cl2)3·(CH3COCH3)n
·(H2O)n
(n = 0−2, M = Mn (1), Fe (3)) were obtained in an acetone-water-tetrahydrofuran medium by the reaction of metal sulfates with chloranilic acid and phenazine (phz). The molecular and crystal structure of 1 was studied by X-ray diffraction at 300, 200, and 150 K. The crystal structure is composed of polymeric cationic [(H3O)2(phz)3]n
2n+ and anionic [Mn2(C6O4Cl2)3]n
2n− layers having honeycomb structure and stacked in such a way that open-ended through-going channels accommodating H2O and CH3COCH3 solvent molecules are formed; the H3O+ cations in the crystal structure are disordered. Magnetic studies indicate antiferromagnetic coupling of Mn2+ ions through chloranilate ligands; transition to a magnetically ordered state occurs at T = 5 K. According to powder X-ray diffraction data, complex 3 is isostructural with compound 1 but differs crucially from 1 in the electronic structure. According to Fe57 Mössbauer spectroscopy, complex 3 exists in delocalized mixed-valence Fe2+/Fe3+ state and, as a consequence, shows ferromagnetic character of magnetic correlations and semiconductor type of electrical conductivity. These features were ascribed to the valence tautomerism Fe2+ + (C6O4Cl2)2− → Fe3+ + (C6O4Cl2)·3−, which was observed for the first time for iron in a honeycomb structure.
The use of organic molecules as an alternative strategy for achieving new nanoporous metal - organic materials has become an attractive prospect. So far, the use of chemical coordination or crystal engineering techniques allows the systematic design of open-framework structures with a considerable range of modulatable pore sizes and functionalities using different organic ligands such as phosphane, cyano groups, N,N'-type ligands and polycarboxylic acids. Furthermore, the use of transition metal ions opens the possibility to obtain nanoporous materials with additional electrical, optical or magnetic properties. Among them, the search for magnetic open-framework structures has become a major challenge due to their potential applications in the development of low-density magnetic materials, magnetic sensors and intelligent or multifunctional materials.
The literature on open-framework materials has shown numerous examples of porous solids with additional structural, chemical, or physical properties. These materials show promise for applications ranging from sensing, catalysis and separation to multifunctional materials. This critical review provides an up-to-date survey to this new generation of multifunctional open-framework solids. For this, a detailed revision of the different examples so far reported will be presented, classified into five different sections: magnetic, chiral, conducting, optical, and labile open-frameworks for sensing applications. (413 references.)
Breathtaking are the highly magnetic and porous vanadocarboxylates (TN=95 K) MIL-47as (left) and evacuated MIL-47 (right) that exhibit three-dimensional large-pore structures built up from infinite chains of corner-sharing VIIIO4(OH)2 (MIL-47as) and VIVO6 (MIL-47) octahedra linked by rigid terephthalate anions. The reversible adsorption–desorption corresponds to a breathing of the structure. The high TN value results from the combination of magnetic chains with the presence of the π electrons of the terephthalate.
Reaction of FeCl2 and H4DSBDC (2,5-disulfhydrylbenzene-1,4-dicarboxylic acid) leads to the formation of Fe2(DSBDC), an analog of M2(DOBDC) (MOF-74, DOBDC(4-) = 2,5-dihydroxybenzene-1,4-dicarboxylate). The bulk electrical conductivity of both Fe2(DSBDC) and Fe2(DOBDC) is approximately six orders of magnitude higher than that of the Mn(2+) analogs, Mn2(DEBDC) (E = O, S). While the metals are of the same formal oxidation state, the increase in conductivity is attributed to the loosely bound Fe(2+) β-spin electron. These results provide important insight for the rational design of conductive metal-organic frameworks, highlighting in particular the advantages of iron for synthesizing such materials.
A very rare structurally characterized CO2-coordinated metal-organic framework was synthesized by solvothermal reaction. The CO2 ligand links two open Zn metal centers in a linear and symmetrical u(O,O’) coordination mode with C=O distances of 1.107(4) Å. The new complex reported here is stable under ambient conditions and may provide a new strategy for CO2 fixation.
In this short critical review, selected examples of current (within the past two years) synthetic efforts toward the construction of high-spin molecules are explored, including the use of metal complexes containing stable free radical ligands, lanthanide or actinide complexes, and other coordination clusters, or a completely different approach, taking advantage of non-Heisenberg exchange in fully delocalized mixed-valence complexes (spin-dependent delocalization, SDD, or double exchange). A description of reported work in this regard is followed by a brief general discussion that highlights what the future may hold for high-spin molecule design.
The syntheses, structures, and magnetic properties of a family of bimetallic anilate-based compounds with inserted spin-crossover cationic complexes are reported. The structures of 1-4 present a two-dimensional anionic network formed by Mn(II) and Cr(III) ions linked through anilate ligands with inserted [Fe(III)(sal2-trien)](+) (1), [Fe(III)(4-OH-sal2-trien)](+) (2), [Fe(III)(sal2-epe)](+) (3), or [Fe(III)(5-Cl-sal2-trien)](+) (4) complexes. The structure of 5 is formed by anionic [Mn(II)Cl2Cr(III)(Cl2An)3](3-) chains surrounded by [Fe(II)(tren(imid)3)](2+), Cl(-), and solvent molecules. The magnetic properties indicate that 1-4 undergo a long-range ferrimagnetic ordering at ca. 10 K. On the other hand, the inserted Fe(III) cations remain in the low-spin (in 4) or high-spin state (in 1, 2, and 3). In the case of 5, half of the inserted Fe(II) cations undergo a complete and gradual spin crossover from 280 to 90 K that coexists with a magnetic ordering below 2.5 K.
Crystalline compounds of general composition (NBu4)2[MII2(dhbq)3] (where M = Mn, Fe, Co, Ni, Zn, and Cd and dhbq2– is the dianion of 2,5-dihydroxybenzoquinone) are obtained by reaction of the divalent metal acetate (or the sulfate in the case of Fe) with 2,5-diaminobenzoquinone and an excess of NBu4Br in aqueous solution at 115 °C in sealed tubes. The dhbq2– ligand is generated in situ by hydrolysis of the 2,5-diaminobenzoquinone. We have been unable to obtain these compounds in a crystalline form suitable for single crystal X-ray diffraction studies directly from H2dhbq itself. A structural feature common to this series is the presence of two interpenetrating [MII2(dhbq)3]2– coordination networks, each with the chiral (10,3)-a topology, with the two independent nets being of opposite hand—unprecedented circumstances for dhbq-based coordination polymers. Crystals of the same zinc compound as that obtained above from 2,5-diaminobenzoquinone can alternatively be obtained by in situ aerial oxidation of 1,2,4,5-tetrahydroxybenzene in the presence of Zn(OAc)2 and NBu4Br in aqueous methanol at room temperature. Analogous in situ aerial oxidation of 1,2,4,5-tetrahydroxy-3,6-dichlorobenzene in the presence of Mn(OAc)2 and NBu4Br affords crystalline samples of the chloranilate (NBu4)2[MnII2(can)3], which contains two interpenetrating (10,3)-a [MnII2(can)3]2– networks of opposite hand.
We present the syntheses, structural characterization, gas sorption, I2 uptake and magnetic properties of a double-walled porous metal-organic framework, [CoII3(lac)2(pybz)2]•3DMF (1•3DMF, purple, where pybz = 4-pyridyl benzoate, lac = D and L-lactate), and of its post-synthetic modified (PSM) congeners, [CoII3(lac)2(pybz)2]•xGuest (xGuest = 6MeOH, purple; 4.5EtOH, purple; 3PrOH, purple; 2C6H6, purple; 2.7I2, black), [CoII3(lac)2(pybz)2] (1, purple), [CoII3(pybz)2(lac)2(H2O)2]•7H2O (1a•7H2O, green) and [CoIIICoII2(pybz)2(lac)2(H2O)2]I•2H2O•1.5DMSO (1b•I-•2H2O•1.5DMSO, yellow, DMSO = dimethyl sulfoxide). Crystallography shows that the framework is not altered by the replacement of DMF by different solvents or by the removal of the solvent molecules during the single-crystal to single-crystal (SC-SC) transformations while upon exchange with H2O or partial oxidation by molecular iodine, the crystallinity is affected. 1 absorbs N2, H2, CH4, CH3OH, C2H5OH, PrOH, C6H6 and I2 but once it is in contact with H2O the absorption efficiency is drastically reduced. Upon PSM, the magnetism is transformed from a canted antiferromagnet (1•3DMF and 1•xGuest) to single-chain magnet (1), to a ferrimagnet (1a•7H2O) and to a ferromagnet (1b•I-•2H2O•1.5DMSO). Raman spectroscopy suggests the color change (purple to green 1a•7H2O or yellow 1b•I-•2H2O•1.5DMSO) is associated with a change of geometry from a strained octahedron due to the very acute chelating angle (~ 60°) of the lactate of a cobalt center to a regular octahedron with a monodentate carboxylate and one H2O. The magnetic transformation is explained by the different interchain exchanges (J'), antiferromagnetic for 1•3DMF and 1•xSolvent (J' < 0), SCM for 1 (J' verge to 0) and ferromagnetic for 1a•7H2O (J' > 0), between homometal topological ferrimagnetic chains (two octahedral and one tetrahedral CoII ions) connected by the double walls of pybz at 13.3 Å (shortest Co•••Co). For 1b•I-•2H2O•1.5DMSO the moment of the tetrahedral site is turned off, thus stabilizing a ferromagnetic state (J' > 0). The present stabilization of four magnetic ground states is unique in the field of metal-organic frameworks as well as the electrical conductivity of 1•2.7I2.
One-electron reduction of the complex [(TPyA)2Fe(II)2((NPh)L(2-))](2+) (TPyA = tris(2-pyridylmethyl)amine, (NPh)LH2 = azophenine = N,N',N'',N'''-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)2Fe(II)2((NPh)L(3-•))](+). X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on (NPh)L(2-) to give an S = ½ radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between S = 2 Fe(II) centers and (NPh)L(3-•) in the reduced complex, giving an S = 7/2 ground state with an estimated coupling constant magnitude of |J| ≥ 900 cm(-1). Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of Ueff = 50(1) cm(-1). To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet.
This review presents a survey of the literature dedicated to the design of metal complexes of stable free radical ligands that have a ground spin
state of high multiplicity but excluding extended species. Most stable free radicals have a sophisticated chemistry allowing the design of multi-si
te
coordination ligands whose metal complexes are oligonuclear with a fairly high ground spin state. The versatile magnetic behavior of these species
associated with the direct bonding of metal and organic spin carriers is described. The advantages of using organic free radical ligands for building
up single-molecule magnets (SMM) is discussed.
© 2007 Elsevier B.V. All rights reserved.
A simple change of the substituents in the bridging ligand allows tuning of the ordering temperatures, Tc, in the new family of layered chiral magnets A[M(II)M(III)(X2An)3]·G (A = [(H3O)(phz)3](+) (phz = phenazine) or NBu4(+); X2An(2-) = C6O4X2(2-) = 2,5-dihydroxy-1,4-benzoquinone derivative dianion, with M(III) = Cr, Fe; M(II) = Mn, Fe, Co, etc.; X = Cl, Br, I, H; G = water or acetone). Depending on the nature of X, an increase in Tc from ca. 5.5 to 6.3, 8.2, and 11.0 K (for X = Cl, Br, I, and H, respectively) is observed in the MnCr derivative. Furthermore, the presence of the chiral cation [(H3O)(phz)3](+), formed by the association of a hydronium ion with three phenazine molecules, leads to a chiral structure where the Δ-[(H3O)(phz)3](+) cations are always located below the Δ-[Cr(Cl2An)3](3-) centers, leading to a very unusual localization of both kinds of metals (Cr and Mn) and to an eclipsed disposition of the layers. This eclipsed disposition generates hexagonal channels with a void volume of ca. 20% where guest molecules (acetone and water) can be reversibly absorbed. Here we present the structural and magnetic characterization of this new family of anilato-based molecular magnets.
The homologous series M(TCNQ)(2) (M = Mn(II), Fe(II), Co(II), and Ni(II); TCNQ = 7,7,8,8-tetracyanoquinodimethane) prepared from reactions of [M(CH(3)CN)(6)][BF(4)](2) and [n-Bu(4)N][BF(4)] in CH(3)CN has been carefully analyzed from the perspective of synthetic issues and physical characterization, including complete magnetic analyses by the tools of de and ac magnetometry. The preparative method was optimized to definitively establish the reproducibility of the chemistry as judged by infrared spectroscopy, thermal gravimetric analysis, powder X-ray crystallography, and elemental analysis. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies results are also in accord with the conclusion that these materials are pure, isostructural phases. The dc magnetic measurements reveal a spontaneous magnetization for the four materials at low temperatures with a weak field coercivity of 20, 750, 190, and 270 G at 2 K for Mn(TCNQ)(2), Fe(TCNQ)(2), CO(TCNQ)(2), and Ni(TCNQ)(2), respectively. At low temperatures, ac susceptibility measurements confirm the presence of a magnetic phase at 44, 28, 7, and 24 K for Mn(TCNQ)(2), Fe(TCNQ)(2), Co(TCNQ)(2), and Ni(TCNQ)(2), respectively, but do not support the description of this system as a typical magnet. In the absence of the ac magnetic data, the behavior is indicative of ferri- or ferromagnetic ordering (depending on the metal), but in fact a complete investigation of their physical properties revealed their true nature to be a glassy magnet. The glassiness, which is a high magnetic viscosity known to originate from randomness and frustration, is revealed by a frequency dependence of the ac susceptibility data and is further supported by a lack of a lambda peak in the heat capacity data. These results clearly demonstrate that molecule-based materials with a presumed magnetic ordering may not always be exhibiting truly cooperative behavior.
The crystal structures of lanthanide complexes involving the dianions of dihydroxybenzoquinone (H2dhbq) or chloranilic acid (H2can) are presented. The complexes, Ln2dhbq3·24H2O (Ln = Y, La, Ce, Gd, Yb and Lu) are 2-D coordination polymers with metal centres linked by dhbq2− ligands. Each of these species contain a Ln2(H2O)18 pentagonal dodecahedral cage. Despite the variation in the radii of lanthanide ions this series is isostructural. In contrast, the hydrated structures of Ln2can3 (Ln = Sc, Y, La, Pr, Nd, Gd, Tb, Yb and Lu) show considerable structural variation. The crystal structures of the diamond-like networks Ycan2− and Thcan2 are also presented.
The reaction of tris-tetrahalogeno-o-benzosemiquinonate chromium(III) complexes, [CrIII(X4SQ)3] (X = Cl 1a or Br 1b), with bis(cyclopentadienyl) cobalt [CoIICp2], tetramethyltetraselenafulvalene (TMTSF) and tetrakis(methylsulfanyl)tetrathiafulvalene (TMT-TTF) afforded four charge-transfer compounds, [CoIIICp2][CrIII(X4SQ)2(X4Cat)] (X = Cl 2a or Br 2b), [TMT-TTF][CrIII(Br4SQ)2(Br4Cat)] 3b and [TMTSF][CrIII(Br4SQ)2(Br4Cat)] 4b, where Cat is catecholate. The paramagnetic [CrIII(X4SQ)2(X4Cat)]– complexes are commonly formed by cocrystallization with a diamagnetic [CoIIICp2]+ cation for 2a·C6H6 and 2b, and paramagnetic TMT-TTF˙+ and TMTSF˙+ cations for 3b·C6H5CH3 and 4b·2CH2Cl2, respectively. The one-electron reduced complexes, [CrIII(X4SQ)2(X4Cat)]–, with two semiquinonate and one catecholate ligands were isolated and crystallographically characterized. The crystal structures of 2a·C6H6 and 3b·C6H5CH3 consist of alternating stacks of cations and anionic complexes, which form one-dimensional column structures. On the other hand, the anionic complexes in 4b·2CH2Cl2 form a hexagonal honeycomb network, whose cavities are occupied by the dimerized cation molecules. The temperature dependence of the magnetic susceptibilities reveals that all the [CrIII(X4SQ)2(X4Cat)]– complexes are in a ground state of S = 1/2, which results from the intramolecular antiferromagnetic interaction between CrIII(d3) and two semiquinonates. In addition, 3b and 4b have a contribution of the paramagnetic TMT-TTF˙+ (3b) and TMTSF˙+ (4b) cations. In all compounds weak intermolecular magnetic interactions were recognized from the decrease of the χmT values at low temperature.
The two compounds CoCu(obbz)(H2O)4·2H2O (I) and CoCu(obze)(H2O)4·2H2O (II) have been synthesized, and their crystal structures have been determined at room temperature. Both compounds crystallize in the monoclinic system, space group P21/n. The lattice parameters are a = 7.8190(6) Å, b = 12.534(2) Å, c = 20.2770(10) Å, β = 97.306(6)°, and Z = 4 for CoCu(obbz)(H2O)4·2H2O and a = 17.966(5) Å, b = 6.907(1) Å, c = 13.678(3) Å, β = 97.69(1)°, and Z = 4 for CoCu(obze)(H2O)4·2H2O. For both compounds, the structure consists of isolated Co2+Cu2+ pairs (in which the metal ions are bridged by an oxamide group) and noncoordinated water molecules. A thermogravimetric analysis has been carried out. Three water molecules are removed at a temperature T2 (96 °C for I and 120 °C for II), and five water molecules are removed at a temperature T3 (205 °C for I and 190 °C for II). When cooling down, the two compounds reabsorb water slowly and then return to the initial state. The dehydration−rehydration process is totally reversible, provided that the temperature does not exceed 275 °C. The magnetic properties of the starting materials, as well as the materials containing three water molecules and then one, have been investigated, both in the dc and ac modes. The two starting materials, CoCu(obbz)(H2O)4·2H2O and CoCu(obze)(H2O)4·2H2O, possess a nonmagnetic ground state. The compounds CoCu(obbz)(H2O)3 and CoCu(obze)(H2O)3 behave as one-dimensional ferrimagnets without long-range magnetic ordering down to 2 K. The compounds CoCu(obbz)(H2O) and CoCu(obze)(H2O), finally, also behave as ferrimagnets, but with a long-range magnetic ordering occurring at 25 and 20 K, respectively. Upon rehydration, the compounds return to the initial state with a nonmagnetic ground state. Infrared and Raman vibrational spectroscopies have been used to obtain further insights into the stuctural modifications accompanying the removal and the uptake of water molecules.
The ferrimagnetic one-dimensional chain 1a was obtained by photolysis of the 1:1 complex 1 between [MnIIL2] and diazodi(4-pyridyl)methane. The correlation length extended over 40 units at 3 K. L = hexafluoroacetylacetonate.
The 9,10-dioxophenanthrenesemiquinonato adduct of a nickel(II)–CTH acceptor (CTH =
dl-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) was synthesized and structurally characterized. Temperature dependent magnetic susceptibility measurements show that this compound has a quartet electronic ground state arising from the strong ferromagnetic coupling between the S
= 1 metal ion and the radical ligand. A computational DFT study carried out using the broken symmetry approach supports the observed magnetic properties as well those of the other nickel(II)–semiquinonato analogues. These results elucidate the electronic properties of the related cobalt–dioxolene complexes when undergoing valence tautomerism.
Ce2(dhbq)3·24H2O (dhbq2–= the dianion derived from 2,5-dihydroxybenzoquinone) contains gas hydrate-like pentagonal dodecahedral Ce2(H2O)18 cages which link together hexagonal grid Ce2(dhbq)3 sheets to form a three-dimensional network related to diamond and which pass through the Ce6(dhbq)6 rings of an independent, identical, interpenetrating diamond-related network.
The synthesis and crystal structure of a diiron(ii) complex containing a bridging semiquinonate radical are presented. The unique electronic structure of this S = 7/2 complex is examined with spectroscopic (absorption, EPR, resonance Raman) and computational methods.
Many features of the structural, magnetochemical, spectral, electrochemical, and chemical behavior of transition metal complexes with redox-active dioxolene ligands indicate that most of them are valence-localized species with well defined and experimentally distinguishable metal- and dioxolene-oxidation states. The valence localization results mainly from large structural differences between individual oxidation states of the dioxolene ligands. It is shown that dioxolene complexes may be treated like weakly coupled mixed-valence compounds, and a qualitative “localized-valence” model is developed in order to interpret physical and chemical properties of dioxolene complexes, including their bistability, in a unifying way.
Magnetic susceptibility and electron paramagnetic resonance studies are reported for two Cu(II) semiquinonato compounds, for which the molecular structures have been previously described. These compounds are [Cu(NH(py)2)(DTBSQ)](ClO4) (1) and [Cu(DTBSQ)2]2 (2). with NH(py)2 = di-2-pyridylamine and DTBSQ = 3,5-di-tert-butyl-o-semiquinonato. In compound 1, the magnetic behavior reveals an exceptionally strong ferromagnetic interaction with a triplet ground state stabilized by ca. 200 cm-1 with regard to the singlet excited state. This behavior results from the strict orthogonality of the σ magnetic orbital of the copper(II) chromophore and the π* magnetic orbital of the semiquinone radical. In compound 2, the Cu(II)-semiquinone interaction is found weakly antiferromagnetic, so that the semilocal ground state within the Cu(DTBSQ)2 mononuclear unit is a doublet. This reversal of the sign of the coupling is attributed to the tetrahedral distortion around copper(II), which mixes the σ and π* symmetries. In addition, the two Cu(DTBSQ)2 units are coupled antiferromagnetically and the ground state of the compound as a whole is a singlet, with a triplet excited state at 7.9 cm-1 above the singlet state.
The bonding properties of the 3,5-di-tert-butyl-1,2-dioxobenzene ligand in the title complexes were experimentally investigated as a function of the dioxolene ligand oxidation state. The electronic absorption spectra are reported for all studied complexes. For pentacoordinated catecholate species and hexacoordinated quinone complexes, resonance Raman spectra were obtained as well. Using these data, the characters of the frontier orbitals of the complex molecules were determined. It follows that pi-delocalization exists in the five-coordinated catecholate species where the catecholate ligand behaves as a strong pi-donor. The lowest electronic transition is characterized as pi --> pi* with some LMCT character, the pi and pi* orbitals being partly delocalized over the Mn(DBCatecholate)- chelate ring. This stabilizing pi-delocalization is lost in six-coordinated weak adducts with pyridine or P(OEt)3, which possess only the pi*(DBCat) --> Mn LMCT transition in the violet region. In the DBSQ complexes, the radical-anionic ligand exerts no special pi-bonding properties to the Mn atom. Its spectrum is dominated by a MLCT transition. On the other hand, the quinone ligand is a very strong pi-acceptor. The absorption spectrum of [Mn(CO)2{P(OEt)3}2(DBQuinone)]+ complex is dominated by an intense absorption band at 582 nm (epsilon = 7400 M-1 cm-1). A Raman band at 571 cm-1 corresponding to the nu(s)(Mn-O) vibration is very strongly enhanced in intensity by resonance with this electronic transition. It points to a very strong pi-delocalization within the Mn(DBQ)+ chelate ring, which is probably also the main factor responsible for the unusual chemical stability of this complex. Despite the metal-ligand pi-orbital mixing in the catecholate and, especially, quinone complexes, the localized oxidation state picture appears to be appropriate, with the Mn atom possessing a formal oxidation state of I in all studied complexes except [Mn0(CO)3(DBCat)]2-. The above interpretation of the electronic absorption and resonance Raman spectra is fully supported by IR and EPR data and by the substitution effects on the spectral properties. Strong influence of the change of the number of CO ligands on the electrode potential of the DBSQ/DBQ ligand-localized redox couple is fully in line with the strong delocalization in the quinone complexes.
Metal complexes of formula M2(CTH)2(DHBQ)Y2 (M = Mn, Fe, Ni; CTH = dl-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; Y = ClO4, PF6) have been obtained by reaction of the appropriate M(CTH)2+ cation and 2,5-dihydroxy-1,4-benzoquinone (H2DHBQ) in alkaline medium. On the basis of their spectral and magnetic properties, it is suggested that these complexes are dinuclear with the DHBQ2- ligand bridging the two dipositive metal ions. The coupling between the metal ions is weakly antiferromagnetic. When Cr(CTH)2+ is used as reacting species, complexes of formula Cr2(CTH)2(DHBQ)Y3 are obtained. A compound of the same stoichiometry is obtained by oxidizing the above diiron(II) derivative. On the basis of IR, electronic, EPR, and Mossbauer spectra, evidence is found that these compounds contain the radical species DHBQ3- bridging the tripositive metal ions. Electrochemical data support this formulation. Magnetic susceptibility measurements in the range 4-298 K are consistent with a S = 5/2 and a S = 9/2 ground state for the chromium(III) and iron(III) derivatives, respectively, as a result of a strong antiferromagnetic coupling between the metal ions and the bridging radical. The stabilization of the radical ligand in the latter derivatives is discussed on the basis of the electronic properties of the tetraoxolene moiety. The unusual EPR spectra are discussed and interpreted in terms of a general approach valid for high-spin values.
Reactions of [M(MeCN)n][BF4]2 salts with Bu4N(TCNQ) in MeOH yield M(TCNQ)2(MeOH)x (M = Mn, 1; Fe, 2; Co, 3; Ni, 4, x = 2−4), and reactions of MnCl2, FeSO4·7H2O, CoSO4·7H2O and NiCl2·6H2O in H2O produce M(TCNQ)2(H2O)2 (M = Mn, 5; Fe, 6; Co, 7; Ni, 8). Infrared spectroscopy, powder X-ray diffraction and thermogravimetric analyses of 1−8 indicate that the products prepared in the same solvent constitute isostructural families of compounds. [Mn(TCNQ−TCNQ)(MeOH)4]∞ (9) crystallizes in the triclinic space group P1̄, a = 7.2966(8) Å, b = 7.4289(8) Å, c = 14.060(2) Å, α = 76.112(2)° β = 87.242(2)°, γ = 71.891(2)°, V = 702.91(13) Å3, Z = 1; [Mn(TCNQ)(TCNQ−TCNQ)0.5(MeOH)2]∞ (10) crystallizes in the monoclinic space group C2/c, a = 14.4378(5) Å, b = 27.3067(11) Å, c = 13.1238(5) Å, β = 90.057(1)°, V = 5174.0(3) Å3, Z = 8. Compound 9 contains TCNQ- ligands that have undergone an unusual σ-dimerization to [TCNQ−TCNQ]2- that acts as a tetradentate ligand to Mn(II) ions to give a 2-D staircase polymeric motif. The layers are connected by hydrogen-bonds between axially coordinated MeOH from adjacent layers and MeOH located between the layers. Compound 10 exhibits a zigzag polymeric motif with equatorially bound TCNQ-derived ligands of two types; the edges of the layers consist of cis-μ-TCNQ- molecules involved in π-stacking with TCNQ- units from another layer and σ-[TCNQ−TCNQ]2- acting as a tetradentate bridging ligand. Axial MeOH ligands are hydrogen-bonded to dangling nitrile groups of cis-μ-TCNQ- ligands in the next layer. Mn(TCNQ)2(H2O)2 (11) crystallizes in the monoclinic space group I2/a, a = 12.5843(7) Å, b = 13.7147(7) Å, c = 13.3525(70 Å, β = 92.887(1)°, V = 2301.58 Å3, Z = 4. This material adopts a double-layer motif consisting of Mn(II) ions bonded to syn-μ2-TCNQ- equatorial ligands and axial H2O molecules. Compounds 1−11 exhibit Curie−Weiss behavior with weak antiferromagnetic coupling being observed at low temperatures. The Zn(II) analogue of 10, [Zn(TCNQ)(TCNQ−TCNQ)0.5(MeOH)2]∞ (12) was also prepared: space group C2/c, a = 14.2252(1) Å, b = 27.3290(4) Å, c = 13.1177(2) Å, β = 90.074(1)°, V = 5099.64(11) Å3, Z = 8. Powder X-ray diffraction was used to probe structures 1−11, and it was found that 9 converts to a new phase with heating or exposure to X-rays that is related to disruption of the σ-dimer (TCNQ−TCNQ)2- ligands and loss of MeOH. The new phase, whose powder pattern is identical with that of a phase prepared from MeCN, exhibits ferromagnetic behavior.
Molecular materials are characterized by being made up by discrete molecules. This structural property gives in principle many possibilities, to modulate the bulk electrical, magnetic, and optical properties of the material by choosing appropriately the constituent molecules. At the same time, however, it is a challenge to develop synthetic strategies that allow the control of the spatial distribution of the molecules in the lattice. In fact, the bulk properties are always determined by cooperative interactions between the constituent molecules, which consequently must be assembled in the lattice in such a way as to maximize the bulk response. In this Account we provide a concise resume of magnetic phenomena, report briefly on the different strategies that have been developed up to the moment for designing molecular magnetic materials, and then summarize our own approach and the main results that have been obtained in this area.
Introduction – Bistability, Hysteresis and Electronically Labile Materials Bistability and HysteresisElectronically Labile MaterialsValence Tautomerism in Dioxolene Complexes of Cobalt Valence Tautomerism – A General Chemical DescriptionValence Tautomerism – A Simplified MO DescriptionVT ThermodynamicsExperimental Determination of Thermodynamic ParametersDependence of KVT Equilibrium on Ancillary LigandsPressure-induced VTLight-induced VT and Rates of VTVT Complexes of Other Quinone Ligands and Redox Chemistry of VT ComplexesPolymeric VT MaterialsFuture Directions Bistability and HysteresisElectronically Labile Materials Valence Tautomerism – A General Chemical DescriptionValence Tautomerism – A Simplified MO DescriptionVT ThermodynamicsExperimental Determination of Thermodynamic ParametersDependence of KVT Equilibrium on Ancillary LigandsPressure-induced VTLight-induced VT and Rates of VTVT Complexes of Other Quinone Ligands and Redox Chemistry of VT ComplexesPolymeric VT Materials
The synthesis and characterization of a three-dimensional inorganic-organic hybrid network [{Cu2(CA)3}{(H3O)2(phz)3}·G]n (1; H2CA = chloranilic acid, phz = phenazine; G = 2CH3COCH3·2H2O) is described. The crystal structure of 1 shows that the 3D network assembly is based on a combination of two types of grid-building sub-units of (6,3) topology: an anionic metal-organic coordination honeycomb grid, and a cationic hydronium-ion-mediated organic honeycomb grid. The inorganic and organic honeycomb nets are perfectly π−π-stacked, creating an infinite number of open-end channels (channel size about 8 Å in diameter). The solid framework of 1 retains its rigidity upon removal of the guest molecules under high vacuum to form compound [{Cu2(CA)3}{(H3O)2(phz)3}]n (2) as revealed by a single-crystal X-ray diffraction analysis. Compounds [{M2(CA)3}{(H3O)2(phz)3}·G]n (M = Cd, 3; M = Zn, 4; M = Co, 5; G = 2CH3COCH3·2H2O) were also prepared and characterized as isostructures of 1 [crystal data for 1: Cu2(CA)3(H3O)2(phz)3·G, trigonal, space group P 1m, formula mass = 1478.86, a = 13.7593(2) Å, c = 9.1869(2) Å, V = 1506.22(5) Å3, Z = 1; crystal data for 2: Cu2(CA)3(H3O)2(phz)3, trigonal, space group P 1m, formula mass = 1326.62, a = 13.7565(4) Å, c = 9.1544(5) Å, V = 1500.3(1) Å3, Z = 1]. The magnetic-exchange coupling between the copper centers for compound 1 was analyzed on the basis of the Curie−Weiss expression and a dinuclear magnetic model. The negative values of the Weiss constant and the magnetic-exchange coupling constant indicate an antiferromagnetic interaction between the copper centers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
Slow magnetic relaxation and hysteresis effects of dynamic origin have been observed above liquid helium temperature in a chain compound (see picture), comprising ConII centers and organic radicals, without any evidence of phase transition to three-dimensional magnetic order. These results are the first evidence of the slow dynamics predicted for one-dimensional magnetic systems with Ising anisotropy, and they open the possibility of storing information in a single magnetic nanowire.
Manganese(II) complexes of imidazolyl-substituted chelating nitronylnitroxides are characterized by a stereoselective arrangement of the metal centers and the ligands to give a honeycomblike layer structure (shown on the right). The magnetic properties at high temperature indicate the presence of a ferrimagnetic structure within the layers. At low temperatures, ferromagnetic ordering is observed.
Remarkable thermal stability is exhibited by [FeII(tcne)2]⋅x CH2Cl2 (57Fe Mössbauer spectra reveal that the oxidation state of iron is II), which was prepared by treating the MeCN solvate of FeI2 with tetracyanoethylene (TCNE). This method, which can also be applied to synthesize the analogous Mn, Co, and Ni compounds, provides a simple route to novel molecule-based magnets with interesting properties.
Transition metal complexes containing first row metal ions chelated by catecholate (Cat) and semiquinonate (SQ) ligands have been found to have localized electronic structures with quinone ligands bonded in both SQ and Cat electronic forms. In exceptional cases the balance of metal and quinone orbital energies is sufficiently close as to permit magnetic and spectroscopic observations on isomers differing in charge distribution together under equilibrium conditions. Equilibria occurring between Mn(SQ−) and Mn+1(Cat2−) redox isomers have been observed in solution and in the solid state for complexes of Co, Mn, and Cu. The temperature range over which equilibria may be observed, is defined by enthalpy and entropy changes associated, mainly, with changes in charge and spin-state of the metal ion. This effect has been considered as an example of valence tautomerism (VT). In this review we present an overview of recent research on complexes of quinone ligands that exhibit reversible shifts in charge distribution under equilibrium conditions.
Metal complexes of 1,4-dihydroxy-benzoquinone and its homologues (H2C6X2O4) are reviewed, focusing on assembled structures based on their X-ray crystallographic structures reported so far. A wide variety of crystal structures based on mononuclear, binuclear, polynuclear complexes and extended structures of coordination polymers or hydrogen bond linked networks are listed in relation to the coordination modes such as the monodentate, bidentate, and bis-bidentate form of the ligands. Their physico-chemical properties such as redox, UV–vis, EPR and magnetic properties are described. The ligand, H2C6X2O4, discussed here is one of the most useful multifunctional ligands, affording not only various self-assembled frameworks via coordination, hydrogen, and coulomb linkages, but also unique electronic structures, which are accompanied by charged and spin states. Their characteristics are mentioned in detail.
Combining porosity and magnetic ordering in a single material presents a significant challenge since magnetic exchange generally requires short bridges between the spin carriers, whereas porosity usually relies on the use of long diamagnetic connecting ligands. Despite this apparent incompatibility, notable successes have been achieved of late in generating truly microporous solids with high magnetic ordering temperatures. In this critical review, we give an overview of this emerging class of multifunctional materials, with particular emphasis on synthetic strategies and possible routes to new materials with improved properties (149 references).
The charge-transfer compound [{Ru(2)(O(2)CPh-o-Cl)(4)}(2)TCNQ(MeO)(2)] x CH(2)Cl(2) (1; o-ClPhCO(2)(-) = o-chlorobenzoate; TCNQ(MeO)(2) = 2,5-dimethoxy-7,7,8,8-tetracyanoquinodimethane) was synthesized from the reaction of the neutral precursors [Ru(2)(II,II)(O(2)CPh-o-Cl)(4)] (abbreviated as [Ru(2)(II,II)] or [Ru(2)(4+)]) and TCNQ(MeO)(2) in a CH(2)Cl(2)/nitrobenzene solution. The structure consists of two-dimensional layers consisting of an infinite array in which [Ru(2)(II,II)] units are involved in charge transfer to TCNQ(MeO)(2) to give a formal charge of [{Ru(2)(4.5+)}-TCNQ(MeO)(2)(*-)-{Ru(2)(4.5+)}]. Interstitial CH(2)Cl(2) molecules are located in the void spaces between the layers. Strong intralayer magnetic coupling between the units [Ru(2)(II,II)] with S = 1 or [Ru(2)(II,III)] with S = 3/2 and TCNQ(MeO)(2)(*-) with S = 1/2, as well as long-range ordering due to antiferromagnetic interlayer interactions, was observed. An antiferromagnetic ground state exists below T(N) = 75 K, which undergoes a metamagnetic transition under applied fields less than 2 T to a field-induced canted antiferromagnetic state with large coercivities up to H(c) = 1.6 T at 1.8 K. Compound 1 gradually loses the interstitial CH(2)Cl(2) molecule at room temperature to form a dried sample (1-dry) without loss of crystallinity and converts nearly reversibly back to 1 after being exposed to CH(2)Cl(2) vapor for 72 h (distinguished as 1'). Interestingly, during this process there is no significant change in lattice dimensions and bond distances or angles with a volume change of only 1.2 vol %. The only discernible difference is ordering/disordering of a pendant ligand orientation, but the magnetism is dramatically altered to a ferromagnetic state with T(c) approximately 56 K for 1-dry. The magnetic property changes are gradual and depend on the degree of interstitial CH(2)Cl(2) molecule loss with reversibility in the process of going between 1 and 1-dry. In addition, in the case of partially desolvated crystals that have mixed domains of ferromagnetically and antiferromagnetically ordered domains for desolvated and solvated segments, respectively, the complete change to ferromagnet can also be triggered by magnetic fields even if the desolvated segments are comparatively minor compared to the solvated segments in a crystal. Surprisingly, the information of the existence of ferromagnetically ordered domains is dynamically recorded in the entire crystal after applying significant magnetic fields as if the majority of the antiferromagnetically ordered domains for solvated segments were never present.
("Figure Presented") Something for nothing? A chiral induction reagent catalyzes the growth of chiral crystals and controls their bulk chirality. Time-dependent experiments show that an initial achiral phase is slowly converted into enantioenriched crystals in the pres-ence of the chiral induction agent.
Pink crystals composed of antiferromagnetic chains (1) can be transformed into blue crystals composed of a ferromagnetic diamondoid framework (2) with structural and magnetic changes.
This critical review focuses on a strange behaviour of crystallized solid matter: its reversible swelling with large magnitude. This will be of interest for experts in porous solids but also for solid state chemists and physicists. Some examples, classified according to the dimensionality of the inorganic subnetwork, present the general requirements and the structural rules which govern the existence of this phenomenon. Its consequences concern specific applications related to sensors, energy savings, sustainable development and health (100 references).
The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal-organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).
The mu-eta(2):eta(2)-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the bis-diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED) with O(2) is a functional and spectroscopic model of the coupled binuclear copper protein tyrosinase. This complex reacts with 2,4-di-tert-butylphenolate at low temperature to produce a mixture of the catechol and quinone products, which proceeds through three intermediates (A-C) that have been characterized. A, stabilized at 153 K, is characterized as a phenolate-bonded bis-mu-oxo dicopper(III) species, which proceeds at 193 K to B, presumably a catecholate-bridged coupled bis-copper(II) species via an electrophilic aromatic substitution mechanism wherein aromatic ring distortion is the rate-limiting step. Isotopic labeling shows that the oxygen inserted into the aromatic substrate during hydroxylation derives from dioxygen, and a late-stage ortho-H(+) transfer to an exogenous base is associated with C-O bond formation. Addition of a proton to B produces C, determined from resonance Raman spectra to be a Cu(II)-semiquinone complex. The formation of C (the oxidation of catecholate and reduction to Cu(I)) is governed by the protonation state of the distal bridging oxygen ligand of B. Parallels and contrasts are drawn between the spectroscopically and computationally supported mechanism of the DBED system, presented here, and the experimentally derived mechanism of the coupled binuclear copper protein tyrosinase.
Dinuclear [(TPyA)M(II)(DBQ(2-))M(II)(TPyA)](BF(4))(2) [TPyA = tris(2-pyridylmethyl)amine; DBQ(2-) = 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate; M = Co (1(2+)), Fe (2(2+)), Ni (3(2+))] complexes have been prepared by the reaction of M(2+), TPyA, H(2)DBQ, and triethylamine in MeOH solution. Their monooxidized form [(TPyA)M(III)(DBQ(*3-))M(III)(TPyA)](3+) [Co = (1(3+)), Fe (2(3+))] has been synthesized by using ferrocenium tetrafluoroborate, and the dioxidized form of 1(2+), [(TPyA)Co(III)(DBQ(2-))Co(III)(TPyA)](4+) (1(4+)), has been obtained by using thianthrinium tetrafluoroborate. These dinuclear compounds were characterized by X-ray crystallography, electrochemistry, magnetism, and EPR spectroscopy. Valence ambiguous 1(3+) forms via redox-induced electron transfer, whereby the one-electron oxidation of the [Co(II)(DBQ(2-))Co(II)](2+) core forms [Co(III)(DBQ(*3-))Co(III)](3+), and it also exhibits spin crossover behavior to the core [Co(III)(DBQ(2-))Co(II)](3+) above room temperature. The M ions in 1 and 2 have a distorted octahedral geometry by coordination with four nitrogens of a TPyA, two oxygens of a DBQ(2-/*3-). Due to the interdimer offset face-to-face pi-pi and/or herringbone interactions, 1(2+), 1(3+), and 2(2+) show extended 1-D and/or 2-D supramolecular structures. The existence of DBQ(*3-) in 1(3+) is confirmed from both solid-state magnetic and solution EPR data. Co- and Ni-based 1(2+) and 3(2+) show weak antiferromagnetic interactions [1(2+): g = 2.44, J/k(B) = -3.20 K (-2.22 cm(-1)); 3(2+): g = 2.13, J/k(B) = -3.22 K (-2.24 cm(-1)), H = -2JS(1)*S(2) for 1(2+) and 3(2+)], while Fe-based 2(2+) exhibits strong spin crossover behavior above room temperature. 1(2+) has three reversible one-electron transfer waves at E(1/2) (vs SCE in MeCN) = -1.121, 0.007, and 0.329 V, and a fourth wave at -1.741 V that exhibits a slight chemical irreversibility. The first three correspond to [Co(II)DBQ(2-)Co(II)](2+) reduction to [Co(II)DBQ(*3-)Co(II)](+), and oxidation to [Co(III)DBQ(*3-)Co(III)](3+) and [Co(III)DBQ(2-)Co(III)](4+), respectively. The mechanism of the multielectron transfer oxidation from [Co(II)DBQ(2-)Co(II)](2+) to [Co(III)DBQ(*3-)Co(III)](3+) is unknown; the energy of stabilization for oxidizing the Co(II) centers in the presence of DBQ(*3-), relative to oxidizing the Co(II) centers in the presence of DBQ(2-) is computed to be 1.45 eV. 2(2+) also has three reversible one-electron transfer waves at 0.802, 0.281, and -1.007 V that correspond to two successive one-electron oxidations (2(2+)/2(3+) and 2(3+)/2(4+)), and a one-electron reduction (2(2+)/2(+)). 2(2+) has the [Fe(hs)(II)(DBQ(2-))Fe(hs)(II)](2+) electronic structure that becomes [Fe(hs)(III)(DBQ(*3-))Fe(hs)(III)](3+) upon oxidation. The latter undergoes spin crossover above room temperature to populate the [Fe(hs)(III)(DBQ(2-))Fe(hs)(II)](3+) excited state.
Dehydration of the Prussian blue analogues CsNi[Cr(CN)(6)] x 2 H(2)O (1) and Cr(3)[Cr(CN)(6)](2) x 10 H(2)O (2) affords two new microporous magnets: CsNi[Cr(CN)(6)] (1d) and Cr(3)[Cr(CN)(6)](2) x 6 H(2)O (2d). Compounds 1d and 2d maintain the Prussian blue structure, and N(2) adsorption measurements at 77 K show them to be microporous with BET surface areas of 360 and 400 m(2)/g, respectively. Both solids largely retain the magnetic properties of their parent hydrates, with 1d ordering at 75 K and 2d ordering at 219 K, by far the highest ordering temperature yet observed for a microporous magnet. The compounds further show unexpected changes in their magnetic properties upon adsorption of O(2). In 2d, adsorption of O(2) results in a reversible decrease in the magnetic moment of the system, as well as a reduction of the coercivity from 110 to 10 G and of the remnant magnetization from 1200 to 400 emu.G/mol, indicating a net antiferromagnetic interaction between O(2) and the framework. In 1d, adsorption of O(2) instead results in a reversible increase in the magnetic moment of the system, indicating a net ferromagnetic interaction between O(2) and the framework. Together, the results suggest that ferromagnetic exchange coupling between O(2) and the [Cr(CN)(6)](3-) units provides the predominate magnetic interaction of the adsorbate with the framework.