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Heteronuclear Cobalt(III)/Sodium complexes with salen type compartmental Schiff base ligands: Methylene spacer regulated variation in nuclearity and exploration of σ/π-hole carbon bonding interactions
Abstract
Three heteronuclear cobalt(III)/sodium Schiff base complexes have been synthesized and characterized by elemental and spectral analysis. The structures of all three complexes have been confirmed by single crystal X-ray analyses. Each of these three complexes crystallizes in monoclinic space group P21/c. In each complex, cobalt(III) is placed in the inner N2O2 compartment and sodium is placed in the outer O2O'2 compartment (O and O' denote the phenolic and ethoxy/methoxy oxygen atoms, respectively) of the Schiff bases. With decrease in steric hindrance around the O2O'2 compartment by the replacement of ethyl groups with methyl groups, tetranuclear complexes resulted instead of dinuclear ones. Interesting carbon bonding interactions in the solid state of the complexes have been studied by means of DFT calculations using several computational tools such as “atoms-in-molecules” (AIM) and natural bond orbital (NBO) analyses.
... Recently, transition metal ions [1,2] have attracted considerable attention because of their interesting molecular structures when prepared in conjunction with co-ligands [3,4]. Moreover, multinuclear metal complexes also are of continual interest of the chemists because their functionalities, connectivity, porosities and structural variations make them promising materials for applications in the field of magnetism, materials, biology sensors, and catalysis [5][6][7][8][9][10]. ...
... Recently, transition metal ions [1,2] have attracted considerable attention because of their interesting molecular structures when prepared in conjunction with co-ligands [3,4]. Moreover, multinuclear metal complexes also are of continual interest of the chemists because their functionalities, connectivity, porosities and structural variations make them promising materials for applications in the field of magnetism, materials, biology sensors, and catalysis [5][6][7][8][9][10]. ...
The reaction of Co(OAc)2·4H2O with multisite coordinated salamo-based ligand H2L containing six coordinating sites in presence of co-ligand NCS- anions afforded successfully a trinuclear Co(II) complex [Co3(L)2(NCS)2]. The trinuclear Co(II) complex has been characterized by elemental analyses, UV-visible, Fourier transform infrared spectroscopic methods and DFT calculation. In addition, the structure of the Co(II) complex has been confirmed by single crystal X-ray crystallography. X-ray crystal structure analysis of the Co(II) complex revealed that the Co(II) complex consists of three Co(II) atoms coordinated by two fully deprotonated ligand (L)2- units and co-ligand NCS- anions. The close surveillance of the crystal structure of the Co(II) complex discloses some notable non-covalent interactions like H-bonding, C-H···π and π···π. The luminescent property of the Co(II) complex has been studied in methanol solution. Apart from, as a complementary revelation, intermolecular interactions with respect to percentages of hydrogen bondings in the X-ray crystal structure of the trinuclear Co(II) complex was quantified by analyses of Hirshfeld surfaces and fingerprint plots.
... After calculation, W = 0.59 [ 55 ], the Na(I) atom (Na1) has a fivecoordinate twisted triangular double-cone geometry. This is consistent with the results for the previously reported complexes [ 56 ]. It is well known that Cd(II) is located in the fifth period of the periodic table of elements and has a large ionic radius, and the Cd(II) atom generally has a higher coordination number. ...
A novel heterometallic Cd(II)–Na(I) coordination polymer (Cd(II)–Na(I) CP), [{Cd(H2L)(OAc)(μ2-OAc)Na(OAc)(MeOH)(EtOH)}2]n is self-assembled via a reaction between Cd(OAc)2·H2O, NaN(CN)2, and a salamo-type ligand H2L containing terminal pyridine. It is structurally characterized. There are two kinds of metal atoms with different coordination modes. The N2O2 cavity of H2L is not involved in coordination, while the terminal pyridine nitrogen atoms are linked to Cd(II) atoms. Two kinds of acetate groups (OAc– and μ2-OAc–) are coordinated with Cd(II) atoms in different coordination modes; two identical Cd(II) atoms are bridged by oxygen atoms of μ2-OAc– groups. The Na(I) atoms are linked to Cd(II) atoms via one oxygen atom of μ2-OAc–; one acetate group (OAc–), one undeprotonated methanol and ethanol molecules are also coordinated with Na(I) atoms. With Cd(II) atoms as the node and the undeprotonated H2L ligand molecule as the linker, a coordination polymer with a larger pore size (25.04·17.66(2) Ų) is formed. Cd(II)–Na(I) CP can emit brighter green fluorescence, hence, it can be applied to the development of fluorescent materials. Various short-range interactions on the Cd(II)–Na(I) CP surface are studied by the Hirshfeld surface analysis.
A trinuclear linear mixed-valence centrosymmetric cobalt(III)-cobalt(II)-cobalt(III) complex, [Co II {(m-L)(m-Hglu)Co III (OH 2)} 2 ](ClO 4) 2 $6H 2 O has been synthesized during tetradentate N 2 O 2 donor 'Schiff base' ligand, H 2 L {N,N 0-bis(salicylidene)-1,3-diaminopropane} and glutaric acid (H 2 glu) as anionic co-ligand. The complex has been characterized by spectroscopic measurements and its solid state structure has been determined by single crystal X-ray diffraction analysis. The supra-molecular assembly formed by the hydrogen bonding interactions in the solid state of the complex has been analysed using DFT calculations.
Two mixed-valence trinuclear cobalt(III)-cobalt(II)-cobalt(III) complexes, [CoII{(μL1
)(μ-OOCC6H5NO2)CoIII(N3)}2] (1) and [CoII{(μ-L2)(μ-OOCC6H4NO2CH3)CoIII(N3}2] (2) have been synthesized using two tetradentate N2O2 donor 'reduced Schiff base' ligands, H2L1 {2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol}, and H2L2 {2-hydroxy-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol}, and 3-nitrobenzoic
benzoic acid or 4-methyl 3-nitrobenzoic benzoic acid as anionic co-ligands. The complexes have been characterized by spectroscopic measurements and their solid state structures have been determined by single crystal X-ray diffraction analysis. The trinuclear cobalt(III)-cobalt(II)-cobalt(III) skeleton of both the complexes are linear and centrosymmetric. Both the central cobalt(II) and terminal cobalt(III) of both the complexes are hexa-coordinated. DFT calculations have been used to analysed some supramolecular assemblies of the complex that have been observed in the solid state, specifically the Cl···Cl interactions that result in the creation of a stable 2D supramolecular assembly in the solid state. QTAIM and NCI Plot computational tools have been used to characterize the interactions. Thermo-gravimetric (TG) analysis was used to examine the thermal behaviour of both complexes. The redox behaviors of both the complexes were studied by using Cyclic Voltammetry (CV).
Designed synthesis and DFT study of R22(8) motifs in the solid state structures of trinuclear cobalt complexes of N2O2 donor Schiff bases using dicar-boxylic acids as bridging co-ligands Sovana Maity, Abstract Three trinuclear, linear, centrosymmetric, Co complexes, [Co{(μ-L 1)(μ-Hsuc)Co(DMF)}2](ClO4)2 (1), [Co{(μ-L2)(μ-Hsuc)Co(DMF)}2](ClO4)2 (2) and [Co{(μ-L3)(μ-Hmal)Co(DMF)}2](ClO4)2 (3) have been synthesized using tetradentate N2O2 donor 'Schiff base' ligands, H2L1 {N,N'-bis(salicylidene)-1,3-diaminopropane}, H2L2 {N,N'-bis(salicylidene)-2,2-dimethyl-1,3-diaminopropane}, H2L3 {N,N'-bis(5-chlorosalicylidene)-2,2-dimethyl-1,3-diaminopropane} and succinic acid (H2suc) or malonic acid (H2mal) as anionic co-ligands. The complexes have been characterized by spectroscopic measurements and their solid state structures have been determined by single crystal X-ray diffraction analysis. Some supra-molecular assemblies observed in the solid state of the complexes have been analysed using DFT calculations, in particular hydrogen bonding interactions. QTAIM computational tool has been used to characterize the interactions. Taking into consideration that each trinuclear complex is dicationic, the dimerization energy cannot be estimated using the supramolecular approach. Consequently, we have used the Lagrangian kinetic energy density (Gr) measured at the bond CPs that characterize the H-bonds to estimate the strength of the R22(8) motifs free from the pure Coulombic repulsion between the cations.
Crystallization of Co( ii ) and Ni( ii ) complexes with a 3-imidazoline nitroxide derivative from a mixture of MeOH and EtOH (1 : 10) leads to the formation of complexes with an ordered alternation of supramolecular MeOH- and EtOH-containing layers.
Four manganese(III) complexes, [MnL1(H2O)2]ClO4$H2O (1), [MnL2(H2O)2]ClO4 (2), [MnL3(DMSO)(H2O)]ClO4 (3) and [MnL4(DMSO)(H2O)]ClO4 (4), where H2L1 = N,N′-bis(5-bromosalicylidene)-1,3-diaminopropane, H2L2 = 2,2-dimethyl-N,N-bis(3-methyloxysalicylidene)-1,3-diaminopropane, H2L3 = N,N′-bis(5-chlorosalicylidene)-2,2-dimethyl-1,3-diaminopropane and H2L4 = 2-hydroxy-N,N′-bis(3-ethyloxysalicylidene)-1,3-diaminopropane are tetradentate N2O2-donor ligands and DMSO = dimethyl sulfoxide, have been synthesized and characterised by elemental analysis, IR and UV-vis spectroscopy and single-crystal X-ray diffraction studies. All are monomeric complexes. Complex 1 crystallises in orthorhombic space group P212121, complex 3 crystallises in triclinic space group P-1, whereas complexes 2 and 4 crystallize in monoclinic space groups, C2/c and C2/m respectively. In all the complexes, manganese(III) has a six-coordinated pseudo-octahedral geometry in which imine nitrogen atoms and phenolate oxygen atoms of the deprotonated di-Schiff base constitute the equatorial plane. In complexes 1 and 2, water molecules are present in the fifth and sixth coordination sites in the axial positions while in complexes 3 and 4 they are occupied by one water and one DMSO. The coordinated water molecules initiate hydrogen-bonded networks in all complexes. DFT calculations have been carried out to analyze two aspects of these complexes viz. the formation of halogen (HaB) and chalcogen bonding (ChB) interactions in complexes 1 and 3 where the electron donor is the perchlorate anion and the acceptor either bromine or chlorine atoms for the HaBs and the sulfur atom of the coordinated DMSO for the ChB. In addition, other intermolecular effects are discussed in the solid state for complexes 1, 2 and 4, where the hydrogen atoms of the coordinated water molecules interact with the electron rich cavities formed by the phenolate and alkyloxy oxygen atoms of the Schiff-base ligand.
A flexible polydentate Salamo-Salen-Salamo hybrid ligand H4L was designed and synthesized, which has rich pockets (salamo and salen pockets) so that it may have fascinating coordination patterns with transition metal(II) ions. Four multinuclear transition metal(II) complexes, novel butterfly-shaped homotetranuclear [Ni4(L)(μ1-OAc)2(μ1,3-OAc)2(H2O)0.5(CH3CH2OH)3.5]·4CH3CH2OH (1), helical homotrinuclear [Zn3(L)(μ1-OAc)2]·2CH3CH2OH (2), double-helical homotrinuclear [Cu2(H2L)2]·2CH3CN (3), and mononuclear [Ni(H2L)]·1.5CH3COCH3 (4), have been synthesized and characterized by single-crystal X-ray diffraction. The effects of different anions [OAc- and (O2C5H7)2-] on the complexation behavior of H4L with transition metal(II) ions were studied by UV-vis spectrophotometry. The fluorescent properties of the four complexes were studied with zebrafish, which are expected to be a potential light-emitting material. Ultimately, interaction region indicator (IRI) valuations, Hirshfeld surface analyses, density functional theory (DFT & TD-DFT), electrostatic potential analyses (ESP), and simulations were carried out to further demonstrate the weak interactions and electronic properties of the free ligand and its four complexes.
In this manuscript we report the preparation of four new zinc complexes, [{(μ-
CH3COO)ZnL1}2Zn]·2DMSO (1), [{(DMF)ZnL1(μ-CH3COO)ZnL1Zn(N3)}]·0.64DMSO·0.36EtOH (2) and [(μ-CH3COO)2Zn2L1]n (3) and
[Zn2L2Cl2] (4) that were characterized by elemental and spectral analyses, where H2L1 is 1,3-propanediylbis(iminomethylene)bis(4-chlorophenol) and H2L2 is 1,3-
propanediylbis(iminomethylene)bis(6-methoxyphenol). The structures have been confirmed by single crystal X-ray diffraction analysis. The synthetic strategy for complexes 1 and 2 involvesthe use of drops of DMSO, which is essential to stabilize crystals of 1 and 2, respectively, indicating the importance of DMSO in the stabilization of the complexes. A thorough DFT calculation, using the quantum theory of atoms-in-molecules and the non-covalent interaction plot (NCI Plot) indicates the formation of strong H-bonds, CH···π and ancillary SpBs in complexes 1 and 2. DMSO has not been added for the synthesis of complexes 3 and 4, where no spodium bonds have been observed. Strong self-assembled (π-π stacked or H-bonded) dimers are found in its solid state structures of 3 and 4.
The coordination behaviors of asymmetric quinoline-decorated half-salamo-type ligand HL¹ with Cu(NO3)2·3H2O and Cu(OAc)2·H2O were studied. Two Cu(II) complexes, [Cu(L¹)(NO3)]·CH3CH2OH (1) and [Cu2(L²)2] (2), were isolated and characterized by X-ray crystallography, FTIR, UV–vis, and fluorescence spectroscopies. Structural analyses showed that 1 is a mononuclear species, whereas 2 is a binuclear entity. Spontaneous cleavage along the oxime N-O bond close to quinoline ring of the ligand HL¹ occurred upon reaction with Cu(OAc)2·H2O, yielding an unexpected complex 2. Moreover, DFT theoretical calculations and Hirshfeld surfaces analyses of the ligand HL¹ and its corresponding Cu(II) complexes were investigated in detail.
Two new dinuclear cobalt(II) complexes, [(H2O)CoIIL1 (μ-O 2 CR 1)Co II (NCS)] (1) and [(DMSO)CoIIL2(μ-O2CR2)CoII(NCS)] (2) {H2L1 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol); H2L2 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol); R1CO2H= 3-methyl-4-nitrobenzoic acid; R2CO2H= 4-methyl-3-nitrobenzoic acid} have been synthesized using multidentate N, O-donor ligands having two pockets. Each complex has been characterized by single crystal X-ray diffraction and spectral analysis. The noncovalent interactions have been analyzed using density functional theory (DFT) calculations. Moreover, the optical properties of the complexes have been studied focusing on the band gaps. Both complexes have been utilized for the fabrication of semiconducting Schottky devices.
Salen-type ligands, prepared by the 1:2 condensation of a diamine and a salicylaldehyde derivative, represent a well known class of symmetrical tetra-dentate ligands, which have been used in the synthesis of a variety of complexes with several transition metals since long. Syntheses of ‘salen type’ asymmetric Schiff base ligands containing two different salicylaldehyde moieties, on the other hand, are tricky. This review focuses light on the synthesis of different ‘salen type’ asymmetric Schiff base ligands, and their mono-nuclear transition metal complexes. The x-ray structures and application of these complexes are also discussed.
Using two different metalloligands [NiLA] and [NiLB], developed by using two different N2O2 donor Schiff bases, H2LA 6,6'-((1E, 1'E)-((2,2 -dimethylpropane-1,3-diyl)bis(azaneylylidene))bis(2-methoxyphenol) and H2LB 6,6'-((1E, 1'E)-((2,2 (2-ethoxyphenol), two new tetra and tri...
The cyanide groups of [Cr(CN)6]³⁻, [Mo(CN)8]³⁻ and [W(CN)8]³⁻ may be used to bridge several mono-nuclear transition metal-salen complexes to form different di-, tri-, tetra- hexa- and poly-nuclear complexes of different molecular architectures. The synthetic strategy of the complexes generally involves the formation of H2salen type ligands by the 1:2 condensation of the appropriate diamine and salicylaldehyde derivatives in appropriate solvent at the beginning, followed by the formation of mononuclear transition metal-salen complexes, which are finally connected by [Cr(CN)6]³⁻, [Mo(CN)8]³⁻ and [W(CN)8]³⁻ bridges to form poly-nuclear complexes. In some cases, however, transition metal-salen units were not isolated, but they have been used in situ to synthesize cyanide bridged poly-nuclear complexes. The magnetic properties of these complexes are interesting. This review represents an overview on the synthetic strategies, structures, magnetic properties of these complexes. It is noteworthy that only those complexes are considered in this review whose X-ray structures are available.
A cyclic tetranuclear cobalt(III)/ (2) have been prepared and their molecular structures have been determined by X-ray crystallography [H 2 L 1 = N,N'-bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-diaminoproane, H 2 L 2 = N,N'-bis(2-hydroxynaphthylmethylidene)propane-1,3-diamine]. Complex 1 contains two cobalt(III) and two sodium centers, while on the other hand, complex 2 contains a high-spin Co(II) and two terminal low-spin Co(III) centers in a Co III (S=0)-Co II (S=3/2)-Co III (S=0) trimer, as confirmed by DFT calculations Theoretical calculations have been used to analyze the importance of CH···π interactions involving the π-systems of the pseudohalide ligands. Indeed apart from the formation of conventional π···π stacking and CH···π(arene) interactions both complexes exhibit CH···π(pseudohalide) interactions that have been evaluated energetically by using mutated dimers and characterized by means of QTAIM and NCI plot computational methods which can be used to clarify interactions in real space.
The swastika-shaped anion, [Cu(N 3) 4 ] 2-in the pentanuclear hetero-metallic copper(II)-Sodium complex, [(CuLNa) 2 (µ-N 3) 2 Cu(N 3) 2 ] (1), where H 2 L = N,N'-bis(3-ethoxysalicylidene)-2,2-dimethylpropane-1,3-diamine (N 2 O 4 donor compartmental Schiff base) is nearly planar and provides a π-basic surface adequate for establishing non-covalent interactions in the solid state as electron donor. This unit participates in strong CH···[Cu(N 3) 4 ] 2-interactions at both sides of the molecular plane. They have been analyzed using DFT calculations and MEP analysis and characterized using NCI Plot index analysis along with π-stacking interactions involving the aromatic and chelate rings of the Na, Cu-coordinated Schiff base ligand. Finally, cation-π interactions between the pentacoordinated Na-atom and the aromatic ring of the Schiff base ligand are described and compared with two X-ray structures retrieved from the CSD. These unconventional interactions are important for the X-ray packing of 1, particularly the CH···[Cu(N 3) 4 ] 2-that has not been studied before, as far as our knowledge extends.
We present the synthesis, structure and magnetic characterization of four dinuclear cobalt(III,II) complexes: [(NCS)CoIII(Lⁿ)(µ–OAc)CoII(NCS)]·G, with n/G = 1/CH3OH (1), 2/CH3CN (2), 3/- (3) and 4/CH3OH (4), being H2Lⁿ, four closely related reduced Schiff base ligands: H2L¹ = (1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol), H2L² = (1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol), H2L³= (2,2-dimethyl-1,3-propanediyl)-bis(iminomethylene)bis(6-methoxyphenol) and H2L⁴= (2,2-dimethyl-1,3-propanediyl)-bis(iminomethylene)bis(6-ethoxyphenol). The single crystal X-ray structures of complexes 1-4 show that both cobalt centres in the four complexes adopt a distorted octahedral geometry with Co(III) and Co(II) centres residing at inner N2O2 and outer O4 pockets of the reduced Schiff base, respectively. Magnetic measurements show that all complexes behave as Co(II) monomers, confirming the presence of diamagnetic low spin Co(III) ions, which have also been supported by the spin-density calculation using Density Functional Theory. Additionally, complexes 1-4 show a field-induced slow relaxation of the magnetization at low temperatures that follow direct and Raman mechanisms.
Spontaneous self-assembly is one of the available synthetic routes to achieve structurally versatile and unique crystal complexes with selected metal-ligand combinations in the spirit of pseudohalides. In this endeavour, we designed a novel 1D coordination polymer (CP), [(Cd)(Pb)(L)(h 1-NCS)(h 1-SCN)] n (1), using a compartmental Salen ligand (H 3 L) in the presence of NaSCN. The characterization of the CP was accomplished using several spectroscopic techniques: MALDI-TOF, PXRD, SEM, EDX mapping, and single-crystal X-ray crystallography. The CP crystallizes in the monoclinic space group P2 1 /c with Z ¼ 4. SCXRD reveals Cd(II) and Pb(II) metal ions fulfilled distorted square pyramidal and hemi-directed coordination spheres. Cd(II) is placed in the inner N 2 O 2 and heavier Pb(II) in the outer O 4 compartments of the de-protonated form of the ligand [L] 2À. Supramolecular interactions in the intricate crystal structure produced attractive molecular architectures of the compound. The flexible aliphatic-OH pendent group coordinates with the Pb(II) ions. This unique binding further elevates the supramolecular crystal topographies. The supramolecular interactions were authenticated by Hirshfeld surface analysis (HSA). The observation of the recurring unconventional tetrel bonds was rationalized by DFT calculations and surface plots of molecular electrostatic potential (MEP). In the 1D polymeric chain in the complex, the O-atom of the-OH groups shows a tetrel bonding interaction with the Pb atom. We have found that the combination of QTAIM/NCI and QTAIM/ELF plots helps reveal the nature of these contacts. Moreover, the QTAIM/ELF plot determines the donor-acceptor interaction between the O-atom and the Pb atom, establishing the s-hole. Agreeably, the s-hole interaction also helps Pb(II) serve as a Lewis acid in the complex. Finally, spodium and tetrel bonds are formed, possible thanks to a hemi-directional coordination sphere of the Pb atoms in the polymer described.
Schiff bases, usually synthesized from the condensation of an amino group with carbonyl compounds and their transition complexes have interesting chemistry and are widely used for industrial purposes exhibiting a broad range of biological activities, including antifungal, antibacterial, antimalarial, antiproliferative, anti-inflammatory, antiviral, and antipyretic properties. This review is focused on the use and properties of substituted Schiff bases with ONS donating sites and their metal complexes, discussing in brief about biological activities.
Keywords: Schiff bases; ONS donor sites; metal complexes; biological activity.
Two counteranion-solvent-dependent homopolynuclear Ni(II) complexes, [Ni2(L)(OAc)(MeOH)]·CH3CN·MeOH (1) and [Ni4(L)2(acac)2(EtOH)2]·2CH3CN (2) have been constructed by exploiting the flexibility of a symmetrical multi-halogen-substituted bis(salamo)-based ligand H3L. The structures of complexes 1 and 2 were confirmed by single crystal X-ray diffraction studies. Complex 1 is a di-nuclear Ni(II) complex, where two octahedral Ni(II) atoms are located in the N2O2 cavities of the salamo units, and bridged via a acetate ion. Complex 2 possesses a centrosymmetric tetra-nuclear structure with two di-nuclear [Ni2(L)(acac)(EtOH)] units. It is worth noting that one acetylacetonate anion is coordinated to Ni2 atom in bidentate fashion along with one methanol molecule that completes its hexa-coordinated geometry. It is possible that the existence of the acetylacetone anion makes the ligand (L)³⁻ unit exhibit its flexibility, which promotes the formation of tetra-nuclear complex 2. The close surveillance of the crystal structure of complexes 1 and 2 discloses some notable non-covalent interactions like H-bonding, C-H···π and π···π. Moreover, both complexes 1 and 2 form three-dimensional supramolecular structures with the help of intermolecular hydrogen bonds. It was found that the synthesis of complexes 1 and 2 weakened the fluorescence intensity of H3L until quenching. Hirshfeld surfaces analyses confirmed that hydrogen bond interactions play the most important role in the stability of crystal structures. The high kinetic stability and low chemical reactivity of complexes 1 and 2 were investigated based on the band gap energy.
A variety of polynuclear cobalt complexes with H2salen type Schiff bases and their reduced analogues have been synthesized and their structures and properties have been reported in literature. This review represents an overview on the synthetic strategies, structures, magnetic properties and biological activities of these complexes. It is worthy to mention here that only those complexes are considered in this review whose X-ray structures are available. The complexes may grossly be divided into three major sub classes; Cobalt(II) complexes, cobalt(III) and mixed valence cobalt(III)/cobalt(II) complexes. Different synthetic strategies have been applied for their preparation. In most of the cases, mixed valence complexes have been synthesized by the in situ partial (aerial) oxidation of cobalt(II) to cobalt(III). However, reduction of cobalt(III) precursors has also been identified as another synthetic route. Trinuclear cobalt(II)-cobalt(II)-cobalt(II) complexes were usually synthesised from cobalt(II) precursors in anaerobic condition. Different methods of synthesising di and polynuclear complexes are also discussed. The less explored areas of potential applications of these complexes are also discussed.
A new symmetrical multi-halogen-substituted bis(salamo)-based tetraoxime ligand H3L and its corresponding penta-coordinated homo-binuclear Co(II) and Zn(II) complexes have been successfully synthesized. The three complexes, [{Co2(L)(OCH3)}2]·CHCl3 (1), [{Zn2(L)(OCH3)}2]·CH3OH (2) and [{Co2(L)(OCH2CH3)}3]·2CH3CH2OH·CHCl3 (3), have been characterized by elemental analyses, FT-IR and UV-Vis absorption spectroscopy as well as X-ray single-crystal structure analyses. In complexes 1 and 2, two crystallographically independent but chemically identical binuclear complex molecules A and B were detected by X-ray analysis. However, in complex 3, three crystallographically independent but chemically identical binuclear complex molecules A, B and C were detected. All the penta-coordinated Co(II) and Zn(II) atoms in complexes 1 and 2 are located in the N2O2 cavities of the completely deprotonated (L)³⁻ units and coordinated to one oxygen atom from one μ2-bridged methoxyl group. The coordination environment of the Co(II) atoms in complexes 1 and 3 are similar, except that the μ2-bridged methoxyl group is replaced with the μ2-bridged ethoxyl group. Additionally, both of complexes 1 and 3 form 3D supramolecular structures through abundant intermolecular interactions, while complex 2 forms 1D supramolecular structure through intermolecular hydrogen bonds. The C-H···π interactions have been established in complexes 1 and 2, where the π-system of H3L participates as π-donor. Through the means of Hirshfeld surfaces and 2D fingerprint plot analyses, existing different non-covalent supramolecular interactions in the Co(II) and Zn(II) complexes have been explained. Furthermore, to obtain a better understanding of the fluorescence properties of H3L and its complexes 1-3, fluorescence titration experiments have been performed.
Three heteronuclear Cd−Ln clusters based on open-chain ether Schiff (bis(5-bromine-3-methoxysalicylidene)-3-oxapentane-1,5-diamine (H2L) and niacin (NA), with composition [Cd6Ln4L5 (OAc)5(NA)5(CH3OH)5](NO3)4·x(CH3OH)·y(CH2Cl2) (1 Ln = Sm, x=1, y=1; 2 Ln =Eu, x=0, y=1; 3 Ln =Tb, x=2, y=1), have been synthesized and structural characterizated. The structure analysis reveals that 1–3 contain the similar sandwich-type polycationic unit [Cd6Ln4L5(OAc)5(NA)5(CH3OH)5]⁴⁺. Solid-state luminescence studies show that the emission peaks of the H2L ligand located in the range of 340∼373 nm in 1−3. The characteristic emission peaks of Sm³⁺ ion appear in cluster 1, but there is no Eu³⁺ or Tb³⁺ ion emission peak in clusters 2 and 3. The fluorescence spectra of 1−3 in N,N-dimethylformamide (DMF) solution only appear emission peaks of π*→π transition of [CdL] coordination units, but characteristic emission peaks of Ln³⁺ ions didn’t emerge. It is possible that the DMF molecules impeded the energy transfer from [CdL] coordination units to the lanthanum(III) ions. Moreover, antioxidation properties indicate that H2L and 1−3 have outstanding biological activity in scavenging oxygen free radicals.
The synthesis and X-ray characterization of a new dinuclear Zn complex using a tetradentate N2O2 donor Schiff base and azide as anionic co-ligand of formula [(DMSO)2 ZnL(µ1,1-N3)Zn(N3)2 ] (1) is reported herein. This complex forms self-assembled dimers in the solid state governed by σ-hole tetrel bonding interactions (C···N) involving the Zn-coordinated methoxy group. The σ-hole tetrel bonding interaction has been differentiated from a trifurcated CH3 ···N H-bonding interaction using the NBO analysis and the inspection of the donor-acceptor orbital interactions. Other interactions (like π-stacking) are also important governing the solid state architecture of the dinuclear Zn-complex. The energy of π-stacking interaction has also been estimated using DFT calculations and several computational tools (MEP surfaces, QTAIM and NCI plot analyses).
Two new zinc(II) been synthesized and characterized. The structures of both complexes have been confirmed by X-ray crystallography. A subtle difference in the aromatic ring of the Schiff base ligand (6-ethoxy in H 2 L 2 and 6-methoxy in H 1 L 1) has a strong influence on the final solid state structure. In addition, the O-atom of the methoxy groups forms stronger intramolecular spodium bonds with Zn(II) than the O-atom of the ethoxy group in 2. This has been analysed using density functional theory (DFT) calculations and a combination of the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) plot index computational tools.
The synthesis and X-ray characterization of two new dinuclear Zn complexes using a tridentate N2O donor reduced Schiff base and acetate or azide as anionic coligands of formula [Zn2L2 (OAc)2 ] (1) and [Zn2L2(N3)2] (2). The presence of acidic NH groups in the complexes due to the utilization of a reduced Schiff-based ligand and also strong H-bond acceptors (electron rich atoms from the anionic coligands) facilitates the formation of infinite 1D assemblies governed by H-bonding interactions in the solid state that have been analyzed energetically using DFT calculations and several computational tools (SAPT, MEP surfaces, QTAIM and NCI plot analyses).
Two new dinuclear nickel(II) complexes, [2-{(2-(ethylamino)ethylimino)methyl}-6-ethoxyphenol] and HL2 [2-{(2-(dimethylamino)ethylimino)-methyl}-6-ethoxy-phenol], have been synthesized and characterized. Variable temperature (2-300 K) magnetic susceptibility measurements indicate the presence of moderate ferromagnetic exchange coupling between nickel(II) centers. In each complex, antiferromagnetic exchange takes place through the phenoxido bridge and ferromagnetic through the μ 1,1-azido bridge. The competitive interactions therefore reduce the overall magnetic coupling. In a theoretical complex, where the bridging azido ligand has been eliminated and the rest of the geometry kept frozen, the magnetic coupling becomes antiferromagnetic which suggests that the ferromagnetic exchange occurs via the μ1,1-azido bridge. Mulliken population analysis and spin density plots clearly show that the spin distributed spherically in the Ni centers is due to the presence of one unpaired electron in both the dx2-y2 and dz2 orbitals. The shape of the spin density at the bridging O-atom and azide evidences the participation of their p orbitals in the magnetic coupling. The SOMO is basically constituted by the dz2 orbital of one nickel(II) center with the participation of the azide π-system. The SOMO-1 is constituted by the dx2-y2 orbital of the other nickel(II), oxygen atom and the azide π-system.
A heteronuclear cobalt(III)/potassium complex, [(L1)2Co2K(L2)2 ]I3, {where, H2L1 = N,N-bis(3-ethoxysalicylidene)2,2-dimethyl-1,3-propanediamine and HL2 = 3-ethoxysalicylaldehyde}, has been synthesized and characterized by several analytical techniques including single crystal X-ray diffraction analysis. The I 3-anions form discrete dimers in the solid state with a I···I distance of 3.55 Å. The molecular electrostatic potential (MEP) surface shows a depletion of the charge density at the extension of I-I covalent bond (σ-hole). Although this value is negative due to the overall negative charge of the system, the value is significantly smaller (in absolute value) than the value at the negative belt around the I-atoms, indicating that the binding mode observed in the X-ray structure minimizes the electrostatic repulsion. The I3-···I3-dimer in the gas phase is not stable, however it is energetically favourable in water and acetonitrile, thus suggesting the possibility that anti-electrostatic halogen···halogen interaction may exist in solution. This gives a plausible explanation for the formation of these counterintuitive dimers in the solid state where the effect of the surrounding molecules is expected to be higher than water solvation. Agreeably, the NCI plot computational tool also suggests that the interaction is attractive in nature.
Two homometallic class-I dinuclear mixed valence cobalt complexes, [(N3)CoIIIL1(µ-(m-NO2)C6H4COO)CoII(N3)] (1) and [(N3)CoIIIL2(µ-C6H4(NO2)CO2)CoII(N3)] (2) have been synthesized using multisite N2O4 coordination ligands, H2L1 {where H2L1 = 2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol) and H2L2 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)}. Each complex has been structurally characterized by single crystal X-ray diffraction and spectral analysis. Both the cobalt centers in these dinuclear complexes adopt distorted-octahedral geometry, where cobalt(III) center resides at the inner N2O2 cavity and cobalt(II) center resides at the outer O4 cavity of the reduced Schiff base. Both of them show good electrical conductivity, which has been rationalized by band gap measurement. The band gap in the solid state has been determined by experimental as well as by DFT calculations and it confirms that each of the two complexes behaves as a semiconductor. Space-charge-limited current (SCLC) theory is employed to evaluate the charge transport parameters such as effective carrier mobility and transit time for both complexes. The difference in conductivity values of the complexes may be correlated with the strengths of extended supramolecular interactions in the complexes. Bader’s Quantum Theory of Atoms-in-Molecules (QTAIM) is applied extensively to get quantitative and qualitative insight into the physical nature of weak non-covalent interactions. Additionally, the Non-covalent Interactions Reduced Density Gradient (NCI-RDG) methods support nicely the presence of such non-covalent intermolecular interactions.
A N2O4 donor compartmental reduced Schiff base ligand, H2L [(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol)], obtained on 1:2 condensation of 2,2-dimethyl-1,3-propanediamine with ortho-vanillin followed by reduction with NaBH4 in methanol solution, has been used to prepare two cobalt complexes, [(N3)CoIIIL(µ-OAc)CoII(N3)] (1) and [(μ-N3)2{(AcO)CoIIILNa(CH3OH)}2]·2CH3OH (2). Complex 1 is a dinuclear mixed valence cobalt(III)/cobalt(II) complex with CoIIIO2CoII core. Complex 2, on the other hand, is a tetranuclear cobalt(III)/sodium complex with CoO2Na(N3)2NaO2Co core. Formation of complex 1 or 2 is mainly governed by the amount of cobalt(II) precursors present in the reaction mixture. Each complex has been characterized by elemental and spectral analysis. X-ray diffraction analysis has confirmed their structures. Complex 1 crystallized in a chiral space group Pna21 where both the cobalt(III) and cobalt(II) centers adopt six-coordinate distorted octahedral geometry with cobalt(III) and cobalt(II) centers residing respectivelyat inner N2O2 and outer O4 cavities of the reduced Schiff base. Complex 2 crystallized in triclinic system with P1-space group, where both cobalt(III) and sodium centers adopt distorted octahedral geometry. Oxidation states of cobalt centers have been confirmed by bond length consideration, BVS calculations as well as from room temperature magnetic moment measurement. Both complexes 1 and 2 show phenoxazinone synthase mimicking activity with kcat values 250.21 and 493.73 h⁻¹ respectively.
Two centrosymmetric dinuclear iron(III) complexes, [(mu-O)(FeL)2] (1) and [Fe2L2(N3)2 ] (2) {H2L= 1,2-Bis[[1-(2-hydroxyphenyl)ethylidene]amino]ethane} have been synthesized and characterized by X-ray crystallographic studies. The structural correlation of these two complexes has been reported. The energies of interesting C-H···π interactions have been estimated by DFT calculations. Three different types of C-H···π interactions have been observed in the solid state of complexes 1 and 2, which involve a classical π-system (arene), and two non-classical ones (chelate ring and azide co-ligand). Moreover, the NCI plot index has been used to characterize these interactions. The C-H···π(arene) interactions in complex 2 have been found to be weaker than the C-H···π(Chelate Ring) interaction in complex 1, thus confirming the higher ability of the chelate ring to participate in C-H···π interactions.
A dinuclear class-I mixed valence cobalt(III)/cobalt(II) complex, [(N3)CoIIIL(µ-C6H5COO)CoII(N3)]·CH3OH [where H2L = (1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol)] has been prepared and characterized by elemental and spectral analysis. The structure of the complex has been confirmed by single crystal X-ray diffraction analysis. The cobalt(III) center is placed in the inner N2O2 compartment, whereas cobalt(II) center is present in the outer O4 compartment of the ligand. The complex may act as active functional model for the oxidation of 2-aminophenol to 2-aminophenoxazine-3-one with kcat = 153.90 h⁻¹.
The nature and characteristics of the C-H···π interactions that play an important role in crystal packing of two iron(III) complexes have been discussed. [Fe(HL1)2(NCSe)2]Cl (1) and [Fe(HL2)2]Cl (2),where HL1= 2-((3-(methylamino)propylimino)methyl)-6-ethoxyphenol and H2L2=2-((3-(methylamino)propylimino)methyl)-6-ethoxyphenol, have been synthesized and characterized by single crystal X-ray diffraction, IR, UV-visible spectroscopy and elemental analyses. Their structures have been confirmed by single crystal X-ray diffraction study. The crystal packing of 1 shows infinite 1D supramolecular network of C-H···π interactions involving either the π-cloud the aromatic ring or the π-system of the SeCN ligand with several C-H bonds (both aliphatic and aromatic). Formation of 2D supramolecular sheet in the solid state structure of 2 is facilitated by C-H···π and π···π interactions The DFT calculations has been done to determine the interaction energies in these complexes. The Molecular electrostatic potential (MEP) surface analyses implies that the values are large and negative at the SeCN ligand (in 1) and over the aromatic ring (both in 1 and 2). Thus both π-systems are well suited for interacting with positive MEP regions at the aliphatic and aromatic H-atoms. Moreover, the interactions have also been characterized using the non-covalent Interaction (NCI) Plot index computational tool.
A trinuclear zinc complex, [Zn{Zn(N3)L}2] {H2L= 2,2'-[(1-ethyl-1,3-propanediyl)bis(iminomethylene)]bis[6-ethoxyphenol]}, has been synthesized and characterized. Its structure has been confirmed by X-ray crystallographic analysis. The complex acts as a sensor for the detection of nitroaromatics in CH3CN solution via turn-off fluorescence response. The solution phase sensing phenomenon has been studied and a strong quenching efficiency has been observed with a quenching constant (KSV) 13.7952 x 10² M⁻¹ for 2-chloro-4-nitrobenzoic acid. H2L displayed high specificity, sensitivity as well as selectivity for zinc ion in its turn-on fluorescence activity. The mechanism of fluorescence enhancement has been proposed, while the metal-ligand binding stoichiometry and binding constant have also been determined. The detection limit of zinc ion is 7.74 X 10⁻⁷ M., with the binding constant K = 2.857 × 10⁵ M⁻¹.
Two dinuclear zinc(II) complexes, [(SCN)Zn(L)(μ1,3-OAc)Zn(DMSO)] (1) and [(dca)Zn(L)(μ1,3-OAc)Zn(DMSO)] (2), {H2L=2,2'-[(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)]bis[6-ethoxy-phenol]and dca= dicyanamide} have been synthesized and characterized. X-ray crystallographic analyses have confirmed their structures. Both the complexes have shown excellent phosphatase mimicking activity {kcat ∼ 266 h⁻¹ for complex 1 and 204 h⁻¹ for complex 2 at 298 K}, evaluated spectrophotometrically by monitoring the increase in absorbance at ∼425 nm, indicating the gradual release of 4-nitrophenolate by the hydrolysis of 4-nitrophenylphosphate ester with time, when 4-NPP (disodium 4-nitrophenylphosphatehexahydrate) has been used as substrate. Both complexes have also been found to be efficient photocatalyst for the degradation of methylene blue (MB). The degradation efficiency reaches to ∼90% (for both complexes) using 15 mg of catalyst in 100 mL 20 mg/L MB solution within 35 minutes.
An ionic cobalt(III/II) complex, [CoIII(L)2][CoII(NCS)3(H2O)], where H2L = 2-((3-aminopropylimino)methyl)-6-methoxyphenol, has been synthesized and characterized by several analytical techniques, including single crystal X-ray diffraction analysis. The energetic features of the solid state non-covalent interactions involved in the titled ionic coordination complex have been studied by means of DFT computations, which indicate that a combination of strong CH3···π and H-bonding interactions is the main reason behind the stabilization of this complex.
The synthesis of a mononuclear octahedral [Prolinate2-Na(MeOH)4]⁻H⁺ complex is achieved from a [3 + 2] cycloadduct with excess of NaOMe in methanol. The slow crystallization provides the hexacoordinated complex stabilized by four molecules of solvent. The antibacterial, antifungal and anti-tuberculosis activity of this compound is also evaluated.
Four new mononuclear Schiff base manganese(III) and cobalt(III) complexes viz. [Mn(L¹)(NCS)] (1), [Mn(L²)(NCS)] (2), [Co(L³)(NCS)] (3), and [Co(L⁴)(NCS)]·0.5CH3OH·0.5H2O (4), containing thiocyanate as a common pseudohalide ion are reported. The pentadentate Schiff base ligands H2L¹, H2L², H2L³, and H2L⁴ were obtained by the condensation of substituted salicylaldehydes with N-(3-aminopropyl)-N-methylpropane-1,3-diamine. The syntheses of the complexes have been achieved by the reaction of manganese(II) perchlorate or cobalt(II) perchlorate with the respective Schiff bases in the presence of thiocyanate in methanol medium. Complexes 1–4 have been characterized by microanalytical, spectroscopic, single-crystal X-ray diffraction and other physicochemical studies. Structural studies reveal that 1–4 adopt nearly similar structures containing the MN4O2 (M = Mn, Co) chromophore in which each central M(III) ion adopts a distorted octahedral geometry. Weak intermolecular H-bonding interactions are operative in these complexes to bind the molecular units. The antibacterial activity of 1–4 and their constituent Schiff bases has been tested against some common bacteria.
A dinuclear copper(II) complex [LCu(μ1,1-N3)2CuL] {where HL = (2-(3-aminopropylimino)methyl-6-methoxyphenol)} has been synthesized and characterized by several analytical techniques including single crystal X-ray diffraction analysis. Interesting supramolecular interactions are observed in the solid state of the complex. An unusual N–H⋯π interaction is observed where the chelate ring, Cu(1)–N(1)–Cu(1)b–N(1)b, [where b = 1 − x, −y, 1 − z] behaves as a π system. Supramolecular interactions have been related with Hirshfeld surface calculations. The complex has been evaluated as functional model for phenoxazinone synthase enzyme by using o-aminophenol (OAPH) as a model substrate in acetonitrile medium. Kinetics of the reaction was followed spectrophotometrically, which confirm that catalytic reactions follow Michaelis–Menten enzymatic reaction kinetics.
A mononuclear manganese(III) complex, [MnL(CH3OH)(H2O)]ClO4, has been synthesized and structurally characterized {H2L = N,N'-bis(5-bromo-3-methoxysalicylidene)2,2-dimethyl-1,3-propanediamine}. The energetic features of significant supramolecular interactions present in the complex, i.e. Br···Br, hydrogen-bonding and π···π stacking interactions, have been calculated using DFT calculations and further corroborated with NCI plot index computational tool. Catalase mimicking activity (catalytic decomposition of hydrogen peroxide into oxygen and water) of the complex has been investigated. The complex catalyzes the decomposition of hydrogen peroxide effectively in solution. The efficiency of the complex toward catalytic decomposition of hydrogen peroxide is related to its structure.
Octahedral cobalt(III) complexes of the type [Co(N2O2)X2] with N2O2 donor tetradentate salen type Schiff bases may form cis and trans isomers. DFT calculations are used to explore detailed analyses of a series of octahedral cobalt(III) complexes. The cis isomers are found to be more stable in most of the cases. One such complex, [CoIIILa(N3)(DMSO)] {H2La = N,N-bis(3,5-dichlorosalicylidene)2,2-dimethyl-1,3-propanediamine}, is synthesized and its structure is confirmed by single crystal X-ray diffraction analysis. As predicted from the theoretical calculations, this structure corresponds to a cis isomer. A Cl•••Cl close contact exists in the solid-state structure of the synthesized complex and the nature of the interaction are analyzed from theoretical computations. Bader’s theory of atoms in molecules (AIM) approach is applied to calculate the Cl•••Cl interaction energy in the crystal via the electron density features at the Cl•••Cl bond critical point. The values total energy density which is the sum of kinetic and potential energy densities at the bond critical point (BCP) of Cl•••Cl close contact is 3.99 kJ mol-1 bohr-3.
Two heteronuclear cobalt(III)/sodium complexes, [(H2O)2Co(H2L 1)Na(N3)2] (1) and (μ-N3)2[(N3)Co(H2L 2)Na]2 (2) have been synthesized by the reaction of two compartmental reduced Schiff bases, H2L 1 [(1,2-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)] and H2L 2 [(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)], with cobalt(II) nitrate hexahydrate in methanol. Structures of both the complexes have been confirmed by single crystal X-ray diffraction analysis. In each complex, cobalt(III) is located at the inner N2O2 compartment and sodium is placed in the outer O2O′2 compartment of the respective ligands. In complex 1, the saturated five-membered chelate ring assumes half-chair conformation, thereby facilitating the anti-orientations of two N-H bonds, which in turn favours the formation of very strong hydrogen bonds forming infinite one-dimensional assembly. Formation of this one-dimensional chain is also supported by C-H···π (arene) interactions. On the other hand, the best hydrogen bond donor NH groups are in syn disposition (as the saturated chelate ring is six-membered and assumes chair conformation) and do not participate in the crystal packing in complex 2. However, very strong C-H···π(N3) interactions has been established in complex 2, where the π-system of the bridging azide ligand participate as π-donor. A search in the Cambridge structural database (CSD) to has also been carried out to investigate the abundance and directionality of the interaction using different pseudohalides. Energies of all these supramolecular interactions were estimated by DFT calculations including Grimme's dispersion correction and characterized by the NCI plot index computational tool.
Two heteroleptic cobalt(III) complexes, [CoL¹(acac)(N3)] (1) and [CoL¹(acac)(NCS)] (2) {where HL¹ = 1-((2-(dimethylamino)ethylimino)methyl)naphthalen-2-ol and Hacac = acetylacetone} have been synthesized and structurally characterized by several analytical techniques including single crystal X-ray diffraction analysis. Extended supra-molecular assemblies were generated in them through weak noncovalent interactions. Synthesized complexes have also been found to mimic the role of quite a lot of metalloenzymes like catechol oxidase, phenoxazinone synthase and phosphatase efficiently by catalytic areal oxidation of respective substrates. Detailed kinetic studies of catalytic reactions are performed spectrophotometrically, which confirm that catalytic reactions follow Michaelis-Menten enzymatic reaction kinetics. Role of co-ligands in catalytic activity has also been assessed.
The structures of the alkali metal chloride SO 2 solvates (Li - Cs) in conjunction with 12-crown-4 or 1,2-disila-12-crown-4 show strong discrepancies, despite the structural similarity of the ligands. With LiCl both types of crown ethers form 1:1 complexes and give [Li(1,2-disila-12-crown-4)(SO 2 Cl)] (1) and [Li(12-crown-4)Cl]·4SO 2 (2). 1,2-disila-12-crown-4 turned out to be unable to coordinate cations too large for the cavity diameter, for example under formation of sandwich-type complexes. As a result, exclusively 12-crown-4 reacts with the heavier alkali metal chlorides NaCl, KCl and RbCl. [Na(12-crown-4) 2 ]Cl·4SO 2 (3) and [M(12-crown-4) 2 (SO 2 )]Cl·4SO 2 (4: M = K; 5: M = Rb) all show S coordination to the chloride ions by four SO 2 molecules, respectively. Compounds 4 and 5 additionally exhibit the first crystallographically evidenced non-bridging O,O' coordination mode of SO 2 . Unexpectedly, the disila-crown ether supports the dissolution of RbCl and CsCl in the solvent and yields the homoleptic SO 2 solvated alkali metal chlorides [MCl·3SO 2 ] (6: M = Rb; 7: M = Cs), which incorporate bridging μ-O,O' coordinating moieties as well as the unprecedented side on O,O' coordination mode. All compounds were characterised by single crystal X-ray diffraction. The crown ether complexes were additionally studied by means of NMR spectroscopy, the presence of SO 2 at ambient temperature was proven by IR spectroscopy on the neat compounds.
Identifying electron donating and accepting moieties is crucial to understanding molecular aggregation, which is of pivotal significance to biology. Anions such as NO3− are typical electron donors. However, computations predict that the charge distribution of NO3− is anisotropic and minimal on nitrogen. Here we show that when the nitrate’s charge is sufficiently dampened by resonating over a larger area, a Lewis acidic site emerges on nitrogen that can interact favourably with electron rich partners. Surveys of the Cambridge Structural Database and Protein Data Bank reveal geometric preferences of some oxygen and sulfur containing entities around a nitrate anion that are consistent with this ‘π-hole bonding’ geometry. Computations reveal donor–acceptor orbital interactions that confirm the counterintuitive Lewis π–acidity of nitrate.
In this manuscript, we combine high-level ab initio calculations on some model systems (XCH3 σ-hole/H-bond donors) and a Protein Data Bank (PDB) survey to distinguish between trifurcated H-bonds and noncovalent carbon bonds in XCH3· · ·O complexes (X = any atom or group). Recently, it has been demonstrated both experimentally and theoretically the importance of noncovalent carbon bonds in the solid state. When an electron-rich atom interacts with a methyl group, the role of the methyl group is commonly viewed as a weak H-bond donor. However, if the electron-rich atom is located equidistant from the three H atoms, the directionality of each individual H-bond in the trifurcated binding mode is poor. Therefore, the XCH3· · ·O interaction could be also defined as a tetrel bond (C· · ·O interaction). In this manuscript, we shed light into this matter and demonstrate the importance of XCH3· · ·O noncovalent carbon bonding interactions in two relevant protein-substrate complexes retrieved from the PDB.
New heteronuclear Zn2Ln2 clusters [Zn2L2Ln2(hfac)6)] (Ln = Eu (1), Tb (2) and Dy (3)) have been synthesized based on a flexible Schiff base (H2L = N,N'-bis(salicylidene)-3,6-dioxa-1,8-diaminooctane) and β-diketonate ligand (hfac = hexafluoroacetylacetonate). The structures of the isomorphous complexes 1–3 were determined by single-crystal X-ray crystallography. Photophysical studies indicate that the short intramolecular distance of Zn•••Ln allows energy transfer from Zn2Ln2-based sensitizers to the Eu(III) and Tb(III) centres, the lanthanide luminescence is indeed “lighted up” by excitation of the zinc complex. More interestingly, white-light emission was realized by codoping Eu(III) ion into the Dy(III) complex for the first time.
The 1,1,2,2-tetracyanocyclopropane (TCCP) unit presents a synthetically accessible and versatile synthon that can interact with lone-pair or π-electrons by 'non-covalent carbon bonding'. Complexes of TCCP with common small molecules, anions, aromatics like fullerenes, amino acids and nucleobases were computed at the DFT BP86-D3/def2-TZVP level of theory. Binding energies vary between about -10 kcal mol(-1) for neutral guests and -15 to -50 kcal mol(-1) for anionic species. This is comparable to strong and very strong hydrogen bonding respectively. Thus, in addition to synthons that contain polarized hydrogen or halogen atoms, TCCP presents a new supramolecular synthon that awaits experimental exploitation.
Two new uranyl(vi) Schiff base complexes [UO2(L1)(DMSO)] (1), where L1 = N,N′-di(5-bromosalicylidene)-1,2-cyclohexyldiaminate ligand and [UO2(L2)(MeOH)] (2), where L2 = N,N′-di(5-bromosalicylidene)-o-phenylenediaminate ligand, were synthesized and characterized by elemental analysis, 1H NMR, FTIR, UV-vis, fluorescence spectroscopy and molar conductivity measurements. The structure of the H2L1 free ligand and both complexes (1) and (2) were also determined by single crystal X-ray diffraction. According to the results obtained, the title complexes have a distorted pentagonal bipyramidal geometry where positions around the U(vi) centre are occupied by the ONNO donors of the deprotonated dibasic Schiff base ligands (L1 for 1 and L2 for (2)), two oxido groups and the oxygen of coordinated solvent. The antimicrobial activity of ligands and complexes was also screened against Gram positive bacteria Staphylococcus aureus PTCC 1112, Micrococcus luteus PTCC 1110, Bacillus cereus PTCC 1015, Enterococcus faecalis; Gram negative bacteria Pseudomonas aeruginosa PTCC 1214, Escherichia coli PTCC1330, Pseudomonas sp., Klebsiella pneumoniae; and fungus strain (Candida albicans). The molecular docking of GlcN-6-P synthase with the synthesized compounds was also performed. According to the results, complex 2 displayed minimum binding energy and can be considered as a good antimicrobial agent.
Two mononuclear cobalt(III) Schiff base complexes with azide [Co(L)(N3)(L′)] (1) and [Co(L)(N3)(L′′)] (2) {where HL = 1-((2-(diethylamino)ethylimino)methyl)naphthalene-2-ol, HL′ = 2-hydroxy-1-naphthaldehyde and HL′′ = acetylacetone} have been synthesized and characterized by elemental analysis, IR and UV–Vis spectroscopy and single crystal X-ray diffraction studies. Both complexes show mononuclear structures with azide as terminal coligand. Structural features have been examined in detail that reveal the formation of interesting supramolecular networks generated through non-covalent forces including hydrogen bonding, C–H•••H–C and C–H/π interactions. These interactions have been studied energetically by means of theoretical DFT calculations. We have also analyzed the unexpected O•••O interactions observed in one complex between the oxygen atoms of the coordinated aldehyde groups using several computational tools, including Bader’s “atoms-in-molecules” (AIM) and natural bond orbital (NBO) analyses.
One of the remaining challenges in single-crystal structure refinement is the proper description of disorder in crystal structures. This paper describes a computer program that performs semi-automatic modelling of disordered moieties in SHELXL [Sheldrick (2015) Acta Cryst. C71, 3–8.]. The new program contains a database that includes molecular fragments and their corresponding stereochemical restraints, and a placement procedure to place these fragments on the desired position in the unit cell. The program is also suitable for speeding up model building of well ordered crystal structures.
The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallographic Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as `a CIF') containing embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to extract the .hkl and .ins files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the determination of absolute structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors.
A statistical survey of the Cambridge Structural Database reveals that the interaction between the pi-hole of nitro-groups and electron rich atoms is somewhat directional. High level ab initio computations indicate energies up to –6.6 kcal/mol-1.
Three new hetero-bimetallic coordination complexes [Na(CuIIL1)2](ClO4)·0.5H2O (1), [Na(CuIIL2)2][CuI2(μ1,3-NCS)3]n (2), and {[Na(CuIIL3)2](μ1,5-dca)}n (3; dca = dicyanamide) have been synthesized by using different Schiff base ligands [e.g., L1H2 = N,N′-bis(3-methoxysalicylidenimino)-1,3-diaminopentane, L2H2 = N,N′-bis(3-ethoxysalicylidenimino)-1,3-diaminopropane, and L3H2 = N,N′-bis(5-bromo-3-methoxysalicylidenimino)-1,3-diaminopropane] in the presence of pseudohalide coligands N3–, SCN–, and N(CN)2– (dca), respectively. The ligands and the complexes have been characterized by microanalytical and spectroscopic techniques. The structures of the complexes, determined by single-crystal X-ray diffraction studies, show that in all cases a trinuclear Na(CuIIL)2 unit is formed, but of different configurations. 1 does not include N3– anions. In contrast, in 2, SCN– extrudes partial in situ reduction of CuII to lead to the formation of an infinite [CuI2(μ1,3-NCS)3]n anionic chain; and in 3, N(CN)2– bridges the metal–ligand assemblies to form a 1D polymeric chain. ESI-MS, UV/Vis spectroscopy, and cyclic voltammetry were performed to investigate the solution-state behavior of the complexes. Theoretical calculations of the optimized geometries of the complexes were carried out at the BLYP/DNP level to determine their relative stabilities from the HOMO–LUMO gap and chemical softness values.
Sodium-assisted self-assembly of two nickel(II) Schiff base complexes under similar reaction conditions yield hetero-metallic compounds [{Ni(salpn)}2Na(ClO4)] (1) and [{Ni(salpr)}3Na][Ni(salpr)]2ClO4·2H2O (2) (where salpn = N,N′-bis-(salicylidene)-1,3-diaminopropane and salpr = N,N′-bis-(salicylidene)-1,2-diaminopropane). Both have been characterized by physico-chemical techniques and single-crystal X-ray diffraction. Crystal structure reveals that in the tri-metallic system of 1 sodium is sandwiched between two [Ni(salpn)] units while the hexametallic system of 2 consists of tetrametallic cluster ion [{Ni(salpr)}3Na]+ with encapsulated sodium by three [Ni(salpr)] units. In both complexes, sodium adopts distorted trigonal prismatic geometry leaving nickel(II) in a distorted square-planar environment. Structural characterization also reveals that 2 : 1 (for 1) and 3 : 1 (for 2) self-assemblies of metallo-ligand and sodium were achieved with slight variation in ligand backbone.
Two zinc(II) complexes, [Zn2(L¹)2(N3)2] (1) (HL¹= [2-((2-(dimethylamino)ethylimino)methyl)phenol] and [Zn2(L²)2(NCO)2] (2) (HL²= [1-(1-(2-(dimethylamino)ethylimino)ethyl)naphthalen-2-ol] have been synthesized and characterized by X-ray crystallography. Density functional theory (DFT) calculations have been employed to estimate the contribution of various non-covalent interactions in the formation of self assembly using several theoretical models. Excellent phosphatase mimicking activities {kcat =135 h⁻¹ (for 1) and 190 h⁻¹ (for 2) at 298 K} on disodium 4-nitrophenylphosphate hexahydrate substrate have been exhibited by both complexes. The phosphatase mimicking activity has been evaluated by monitoring the increase in absorbance at 427 nm (spectrophotometrically) indicating gradual release of p-nitrophenolate by the hydrolysis of 4-nitrophenylphosphate ester with time.
On the basis of the bis(4‐methylbenzoxazol‐2‐yl)methanide ligand three new alkali or alkaline earth metal compounds were synthesized, and their solid‐state structures are discussed in detail. On the way to new alkaline earth metal species a promising potassium precursor complex [K{(4‐MeNCOC 6 H 3 ) 2 CH}{η ⁵ ‐K(4‐MeNCOC 6 H 3 ) 2 CH}(THF)] ∞ ( 2 ) and a homoleptic magnesium [Mg{(4‐MeNCOC 6 H 3 ) 2 CH} 2 ] ( 3 ) and calcium complex [Ca{(4‐MeNCOC 6 H 3 ) 2 CH} 2 (THF) 2 ] · THF ( 4 ) were obtained.
In this manuscript ab initio calculations have been combined with a search in the Protein Data Bank (PDB) to demonstrate the importance of σ-hole tetrel bonding interactions in biological systems. In particular, we focus our attention to the ability of the –CF3 group to participate in noncovalent interactions as Lewis acid and we show the importance of this interaction in the inhibition mechanism of a NADP+-dependent isocitrate dehydrogenase (IDH) enzyme that converts isocitrate to α-ketoglutarate. IDH mutations are found in multiple hematologic and solid tumors, inducing premalignant disorders. A potent triazine-based inhibitor of the mutant IDH (enasidenib) presents two –CF3 groups in the structure. One establishes a tetrel bonding interaction with an aspartate residue that contributes to the binding and selectivity of the inhibitor to the active site.
Two complexes, [Ni2(HL¹)2(NCS)2].CH2Cl2 (1) and [Ni3(L²)2(NCS)2(H2O)2].CH3CN (2) have been synthesized using two different tetradentate ONNO donor Mannich base ligands N,N′-bis(3,5-dimethyl-2-hydroxybenzyl)-N,N'-dimethylethylenediamine(H2L¹) and N,N′-bis(3-methoxy-5-methyl-2-hydroxybenzyl)-N,N'-dimethylethylenediamine(H2L²), respectively. Compound 1 is μ1,1 isothiocyanato bridged dinuclear compound whereas compound 2 is phenoxido bridged trinuclear compound with SCN⁻ as a terminal ligand. In compound 1 one of the two phenoxido oxygen atoms from each ligand remains protonated and hence makes the compound electrically neutral. X-ray-crystallography reveals that the two Ni(II) centres in compound 1 are in a distorted octahedral geometry and N atom of each of the two SCN⁻ groups bridge the two metal centres. On the other hand, compound 2 possesses a centrosymmetric trinuclear structure where two distorted square pyramidal terminal Ni(II) centres are connected to a central octahedral Ni(II) via phenoxido bridge. Here SCN⁻ ligand occupies the axial positions of the terminal Ni(II) centres by coordinating through N atom. Variable temperature magnetic susceptibility measurements of solid samples reveal that the Ni(II) paramagnetic centres in both 1 and 2 are antiferromagnetically coupled with J values of -30.66 cm⁻¹ and -5.92 cm⁻¹ for 1 and 2, respectively. These values are in accordance with the magneto structural correlations of previously reported isothiocyanato bridged dinuclear and phenoxido bridged trinuclear Ni(II) compounds.
A new complex ternary amide Rb2[Mn(NH2)4], which contains both transition and alkali metal catalytic sites, is synthesized by mechanochemical reaction. It exhibits two temperature-dependent polymorphs i.e. low-temperature orthorhombic and high-temperature monoclinic structures. Rb2[Mn(NH2)4] decomposes to N2, H2, NH3, Mn3N2, and RbNH2 under inert atmosphere; whereas releases NH3 at 80 °C under H2 atmosphere. Those unique behaviors enable Rb2[Mn(NH2)4] and K2[Mn(NH2)4] as excellent materials for NH3 decomposition and synthesis. Our experimental results show both NH3 decomposition onset temperatures and conversion rates over these amides are similar to those of Ru-based catalysts. More importantly, these ternary amides exhibit superior capabilities in catalyzing NH3 synthesis, which are more than 3 orders of magnitude higher than that of MnN and twice of that of Ru/MgO. The SR-PXD measurement discloses that Mn3N2 synergistic with Rb/KH or Rb/K(NH2)xH1-x are likely the active sites.
A new Schiff base derived from the antihypertensive hydralazine (LH) and its Co(III) complexes [CoL2]X∗4H2O, where X= NO3¯ (1) or Cl¯ (2), were synthesized and characterized using FTIR, UV-Vis, ¹H-NMR spectroscopy as well as thermal analysis. The crystal structures of the three compounds have been determined using single crystal X-ray diffraction technique. The two complexes structure is strictly divided in a highly ordered part of the [CoL2]⁺ units and a disordered section containing the anions and sixteen crystal water molecules per unit cell. In the complexes, the two ligand anions (L¯) chelate to the Co(III) through the nitrogen atoms in a tridentate meridional manner. The pyridine and phthalazine rings showed deviations from co-planarity in all compounds which attributed to the crystal packing effects as revealed by analysis of their molecular packing using Hirshfeld analysis and further supported by DFT calculations. The optimized structure of the [CoL2]NO3 as a model showed no co-planarity of the two rings while perfectly planar ligand units were predicted in the [CoL2]⁺ cation indicating that the intermolecular interactions with the nitrate counter anion have a major role in the deviations of the two ring moieties from planarity. The ligand anion (L¯) optimized structure using B97D function which include dispersion model showed similar results while the functions (B3LYP and mPW1PW91) which include no dispersion model predicted perfectly planar arrangement. Atoms in molecules (AIM) studies predicted the strength of the Co-N interactions is in the order, Co-N(azomethine) > Co-N(phthalazine) > Co-N(pyridine) which are generally stronger in 2 than 1. The developmental toxicity of LH and its complexes were tested in zebrafish embryos. The ligand itself was very toxic with LC50 of just 2µM as compared to its metal complexes, whereas, at sub-lethal concentration the ligand perturbed the neurogenesis in zebrafish embryos.
of tetradentate Schiff bases derivedfrom salicylaldehyde and a variety of diamines (1:2 ratio)have been carried out previously. They have been shown thatan increase in the methylene chain length allows adequateflexibility for the complexes to change their structure from aplanar towards a distorted or pseudo-tetrahedral motif. Inaddition, the longer chains cause the ligand field strength todecrease.
In this manuscript, we report the design, synthesis and X-ray characterization of nitrodiene derivatives that present crucial π-hole interactions involving the nitro group as a π-hole donor. The solid state structures of 1,4-dinitro-1,3-butadiene (1), its co-crystal 1·Diox, and homologous 1,4-dinitro-1,3-pentadiene (2) and 2,4-dinitro-2,4-hexadiene (3) feature competition of lone pair-π-hole interactions with common weak CH⋯O bonding and gradually increased role of the NO2⋯NO2 interactions. Regular evolution of the supramolecular patterns (1 to 3) results in generation of an unprecedented 3D non-covalent framework in 3 that is controlled exclusively by short π-hole contacts (O⋯N = 2.9615(18), 3.1304(18) Å). These findings complement the results of high level ab initio calculations (MP2/def2-TZVP) and unite theory and experiment, thus supporting the functional relevance of this novel π-hole interaction.
The effect of different solvents on synthesis and recrystallization of Ni²⁺ complex with the tetradentate Schiff base was studied. The polar protic (MeOH and EtOH) and aprotic (ACN, DMF and THF) solvents have been used. The synthesis and recrystallization from these solvents allow obtaining seven different solvates or solvent free compounds. The products were characterized using UV–vis and FITR spectroscopy, X-ray diffraction analysis and thermogravimetry. Only in the case of DMF, the synthesis and recrystallization lead to formation of the same compound. The remaining pairs of complexes differ in their composition or/and structure. In almost all compounds the coordination environment around Ni²⁺ centre consists of two nitrogen and two oxygen atoms of Schiff bases ligand which forms the distorted square planar geometry. The compound synthesised form THF shows significant paramagnetism which suggests that the coordination geometry around metal centre is a distorted octahedral.
Three new mononuclear cobalt(III) complexes, [Co(L¹)2]ClO4⋅2H2O (1) {HL¹ = 2((2(2-hydroxyethylamino)ethylimino)methyl)-6-methoxyphenol}, [Co(L²)(acna)(N3)] (2) { HL² = 2((2(2-hydroxyethylamino)ethylimino)methyl)-6-ethoxyphenol, Hacna = 2-acetyl-1-naphthol} and [Co(L³)(bzan)(N3)] (3) {HL³ = 1((2(diethylamino)ethylimino)methyl)naphthalen-2-ol, Hbzan = 1-benzoylacetone} were prepared and characterized by elemental and spectral analysis. The structures of the complexes were confirmed by single crystal X-ray analyses. Weak noncovalent interactions lead to the formation of extended supra-molecular assemblies in them. Both complexes 2 and 3 may be used as moderate catalyst for the oxidation of 2-aminophenol to 2-aminophenoxazine-3-one.
Two mononuclear square planar nickel(II) complexes, [Ni(HL)2](ClO4)2 (1) and [Ni(HL)2]Cl2 2H2O (2), [where HL = 3-(dimethylaminopropyliminomethyl)naphthalen-2-ol] have been synthesised and characterized by elemental and spectral analysis. The structures have been confirmed by single-crystal X-ray diffraction. In each complex, the Schiff base is present in its zwitter ionic form. Interesting supra-molecular networks were generated through various non-covalent forces. Density functional theory (DFT) calculations were employed to estimate the contribution of each interaction in the formation of the assembly using several theoretical models.
A bicompartmental N 2 O 4 donor symmetric Schiff-base ligand has been deployed to synthesize a trinuclear zinc complex [Zn 3 (L) 2 Cl 2 ], which upon treatment with sodium azide produces a new µ 1,1-azido bridged 1D polymer [Zn 2 (L) 2 (Na)N 3 ] n. Both complexes have been characterized using IR, NMR, UV-Vis and X-ray diffraction techniques. In order to have better understanding of electronic transitions of the complexes, a time dependent DFT study has been performed. Lifetime measurements have also been performed to learn about the stability of excited states of both complexes. The average fluorescence decay lifetime has been found to be 1.42 ns and 0.59 ns for 1 and 2, respectively. In Hirshfeld surface mapping X…H/H…X (X = O, Cl) contacts are found to be only 2.7% of the total surface, which indicates that no significant X…H/H…X contacts are present in either of the complexes. Unconventional interactions such as C-H···π and π···π stacking interactions are found in supramolecular architectures of both complexes.
The MEPs of a variety of nitro compounds (R-NO2) suggest the existence of a π-hole with a potential of up to +54 kcal/mol in 10 (R = CF3). Several of these nitro compounds were considered as partners for anions (F⁻, Cl⁻, NC⁻) and the electron rich molecules acetonitrile and dimethyl ether. In most cases a π-hole complex was obtained with calculated binding energies of up to 20 kcal/mol with anions and 5 kcal/mol with the neural molecules. A thorough analysis of the CSD revealed that nitrate esters are highly directional π-holes in the solid state, for at least sp² O atoms. This was further illustrated by highlighting several crystal structures where more than 0.2 Å van der Waals overlap was observed between the N atom of the nitrate ester and an electron rich atom like oxygen.
A new cobalt(II) coordination polymer 2 with µ1,5 dicyanamide (dca) and a bidentate ligand ‘pypz’ [3,5-dimethyl-1-(2΄-pyridyl)pyrazole] has been prepared in stepwise manner using the newly synthesized 1D linear Co(II) coordination polymer 1 as precursor. The structural and thermal characterization elucidates that the more stable complex 2 shows a 2D layer structural feature. Here, Co(II) atoms with µ1,5 dicyanamido bridgers are linked by ‘pypz’ ligand forming a macrocylic chain that runs along crystallographic ‘c’ axis having ‘sql’ (Shubnikov notation) net topology with 4-connected uninodal node having point symbol {44.62}. The remarkable noncovalent carbon bonding contacts observed in the solid state of compound 1 have been studied and characterized by DFT calculations and Bader’s theory of “atoms in molecules”.
In this present work we report the synthesis and structural characterisation of a trinuclear cadmium(II) (1) and a di(phenoxido)-bridged dinuclear cadmium(II)–nickel(II) (2) complex derived from a bicompartmental (N2O4) Schiff base ligand, H2L. It has been observed that, in bicompartmental ligands the relatively small inner core is suitable for 3d metal ions and outer core can be occupied by different metal centers like 3d, 1s, 2s, 4d and 4f. We have experimentally established the above fact. In homotrinuclear complex 1 both inner (N2O2) and outer (O4) core has been occupied by cadmium(II) ions. Complex 1 upon reaction with NiCl2·6H2O produces heterodinuclear complex 2. Structural studies reveal that, in complex 1 terminal Cd units acquire trigonal prismatic geometry whereas the central Cd unit is eight coordinated. In case of complex 2 both nickel(II) and cadmium(II) ions are hexa-coordinated in a distorted octahedral environment. Both the complexes are studied using different spectroscopic techniques. Complexes 1 and 2 exhibit important and relatively unexplored group of supramolecular interactions like π–hole, C–H⋯π and C–H⋯H–C along with other hydrogen bonding interactions. Theoretical DFT calculations are devoted to analyze these non covalent interactions. Several computational tools like MEP surface analysis and NCI analysis are utilized to explain and illustrate such interactions.
Two mononuclear cobalt(III) complexes, [Co(L1)(N3)2(deen)] (1) and [Co(L2)(N3)2(DMSO)] (2), where HL1 {2-(2-(diethylamino)ethyliminomethyl)-4-bromophenol} and HL2 {2-(2-(diethylamino)ethyliminomethyl)-6-methoxyphenol} are tridentate and tetradentate Schiff bases, respectively and deen is a bidentate chelating ligand, N,N-diethyl-1,2-diaminoethane, were prepared and characterized by elemental and spectral analysis. X-ray crystal structure determination confirmed the structures of the complexes. Both complexes are meridional isomers. The solid state structures show the participation of the organic ligands in concurrent conventional and unconventional C-H···π interactions along with hydrogen bonding. The energetic features of these interactions have also been studied by means of DFT calculations.
Two bis-μ-phenoxido-bis-μ-acetato heterobimetallic {Co(III)Dy(III)} complexes and , formulated as [Co(III)Dy(III)L(μ-OAc)2(NO3)2] derived from the comparable hexadentate Schiff bases N,N'-ethylenebis(3-ethoxysalicylaldimine) and N,N'-ethylenebis(3-methoxysalicylaldimine) were synthesized and X-ray structure analysis confirms their nearly identical structures. These are the first examples of bis(μ-phenoxido)-bis(μ-carboxylato) {Co(III)Dy(III)} systems. The AC susceptibility measurements show that both complexes exhibit a field-induced slow magnetic relaxation with two relaxation branches. While the high-frequency process spans the usual range of the relaxation time for analogous single molecule magnets (τ0 ∼ 10(-7) s), the low-frequency branch is as slow as τ ∼ 0.1 s at T = 1.9 K and B = 0.2 T.
The ability of several pnicogen sp3 derivatives ZF3 (Z = N, P, As, Sb) to interact with electron rich entities by means of the opposite face to the lone pair (lp) is investigated at the RI-MP2/aug-cc-pVQZ level of theory. The strength of the interaction ranges between -1 to -87 kJ·mol-1, proving its favorable nature, especially in case the lp is coordinated to a metal center, where the strength of the interaction is significantly enhanced. The NBO analysis showed that orbital effects are modest contributors to the global stabilization of the pnicogen σ-hole bonded complexes studied. Finally, some CSD examples are shown in order to analyze the impact of this counterintuitive binding mode in the solid state.
Three mononuclear facial cobalt(III) complexes, [Co(L)2]ClO4 (1), [Co(L)(bzan)(N3)] (2) and [Co(L)(bzan)(NCS)] (3) {HL = 2-(3-(dimethylamino)propyliminomethyl)-6-ethoxyphenol and Hbzan = 1-benzoylacetone} were prepared and characterized by elemental and spectral analysis. X-ray crystal structure determination confirmed the structures of the complexes. Extended supra-molecular assemblies were generated in all three complexes through weak noncovalent interactions. Density functional theory (DFT) calculations were employed to understand and estimate the contribution of each interaction in the formation of the assembly using several theoretical models. Bader’s theory of “atoms in molecules” (AIM) was also used to further describe the long range interactions and its role to stabilize different molecular assemblies. Complexes 2 and 3 are found to be efficient catalysts to perform the aerial oxidation of 2-aminophenol to 2-aminophenoxazine-3-one and therefore may be used as functional models for copper(II) containing enzyme, phenoxazinone synthase.
Tetrel (Tr) bonding is first placed into perspective as a σ-hole bonding interaction with atoms of the Tr family. An sp(3) R4 Tr unit has four σ-holes with which a Lewis base can form a complex. We then highlight some inspiring crystal structures where Tr bonding is obvious, followed by an account of our own work. We have shown that Tr bonding is ubiquitous in the solid state and we have highlighted that Tr bonding with carbon is possible when C is placed in the appropriate chemical context. We hope that this account serves as an initial guide and source of inspiration for others wishing to exploit this vastly underexplored interaction.
The reaction of hydrated nickel(II) salts (chloride or nitrate) and various hydrated lanthanide nitrate salts with the Schiff base ligand 2-methoxy-6-[(E)-phenyliminomethyl] phenol (HL) in methanol resulted in the isolation of three isostructural linear heterometallic trinuclear complexes and a heterometallic tetranuclear complex. The molecular structures of these complexes were determined via single crystal X-ray diffraction, revealing molecular structures of formulae [Ni2Ln(L-)6](NO3) where Ln = La (1), Pr (2) and Tb (3) and [Ni2Dy2(L-)2(o-vanillin)2(CO3)2(NO3)2(MeOH)2] (4). Structural analysis for 1-3 reveals that the lanthanide ion is sandwiched between two Ni(II) ions and the Ni…Ln...Ni metallic core displays a linear arrangement, with an average angle Ni…Ln...Ni bond angle of 179.7 degree. Analysis of 4 reveals the metal ions are arranged such that two Ni-Dy subunits are bridged by two carbonate ligands via the Dy sites. Direct current magnetic susceptibility measurements for complexes 1-4 reveal that the Ni(II) ions are coupled ferromagnetically with the Tb(III) (3) and Dy(III) (4) ions, and antiferromagnetically with the Pr(III) ion (2). For complex 1 a long range intramolecular ferromagnetic interaction is witnessed between the Ni(II) ions (Ni....Ni = 6.873(9) Å) via a closed shell La(III) ion. The magnetic data of 1 were fitted using the HDVV Hamiltonian revealing the following parameters; J = +0.46 cm-1, g = 2.245, D = +4.91 cm-1. Alternating current magnetic susceptibility measurements performed on complexes 2-4 revealed that 3 and 4 displayed frequency dependent Chi” signals (Hac = 3.5 Oe and Hdc = 0 Oe) which is a characteristic signature of a single-molecule magnet behaviour.
Two square pyramidal copper(II) complexes, [Cu(bzan)(mdap)(OClO3)] (1) and [Cu(bzan)(dmdap)(OClO3)] (2) [Hbzan = 1-benzoylacetone, mdap = N-methyl-1,3-diaminopropane, dmdap = N,N-dimethyl-1,3-diaminopropane], have been prepared and characterized. Single crystal X-ray diffraction studies have confirmed their structures. Both complexes form supramolecular dimers via π/π interactions between the chelate rings. The energies associated with π-stacking interactions to form self-assembled dimers have been computed using DFT calculations. The energy associated with N, C–H/π contributions have also been evaluated. Bader’s theory of “atoms in molecules” (AIM) was also used to further describe the non-covalent π-stacking and N, C–H/π interactions.
Non-covalent interactions play a crucial role in (supramolecular) chemistry and much of biology. Supramolecular forces can indeed determine the structure and function of a host-guest system. Many sensors, for example, rely on reversible bonding with the analyte. Natural machineries also often have a significant non-covalent component (e.g. protein folding, recognition) and rational interference in such 'living' devices can have pharmacological implications. For the rational design/tweaking of supramolecular systems it is helpful to know what supramolecular synthons are available and to understand the forces that make these synthons stick to one another. In this review we focus on σ-hole and π-hole interactions. A σ- or π-hole can be seen as positive electrostatic potential on unpopulated σ* or π(() *()) orbitals, which are thus capable of interacting with some electron dense region. A σ-hole is typically located along the vector of a covalent bond such as XH or XHlg (X=any atom, Hlg=halogen), which are respectively known as hydrogen and halogen bond donors. Only recently it has become clear that σ-holes can also be found along a covalent bond with chalcogen (XCh), pnictogen (XPn) and tetrel (XTr) atoms. Interactions with these synthons are named chalcogen, pnigtogen and tetrel interactions. A π-hole is typically located perpendicular to the molecular framework of diatomic π-systems such as carbonyls, or conjugated π-systems such as hexafluorobenzene. Anion-π and lone-pair-π interactions are examples of named π-hole interactions between conjugated π-systems and anions or lone-pair electrons respectively. While the above nomenclature indicates the distinct chemical identity of the supramolecular synthon acting as Lewis acid, it is worth stressing that the underlying physics is very similar. This implies that interactions that are now not so well-established might turn out to be equally useful as conventional hydrogen and halogen bonds. In summary, we describe the physical nature of σ- and π-hole interactions, present a selection of inquiries that utilise σ- and π-holes, and give an overview of analyses of structural databases (CSD/PDB) that demonstrate how prevalent these interactions already are in solid-state structures.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
In an effort to better understand the nature of noncovalent carbon-bonding interactions, we undertook accurate high-resolution X-ray diffraction analysis of single crystals of 1,1,2,2-tetracyanocyclopropane. We selected this compound to study the fundamental characteristics of carbon-bonding interactions, because it provides accessible σ holes. The study required extremely accurate experimental diffraction data, because the interaction of interest is weak. The electron-density distribution around the carbon nuclei, as shown by the experimental maps of the electrophilic bowl defined by a (CN)2 CC(CN)2 unit, was assigned as the origin of the interaction. This fact was also evidenced by plotting the Δ(2) ρ(r) distribution. Taken together, the obtained results clearly indicate that noncovalent carbon bonding can be explained as an interaction between confronted oppositely polarized regions. The interaction is, thus electrophilic-nucleophilic (electrostatic) in nature and unambiguously considered as attractive.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
To examine the influence of the metal ions and their counter ions on crystalline networks, we have designed and synthesized six MX2/8-aminoquinoline (8-aq) (M = MnII, CuII, CdII and X = Cl-, Br-, I-, NO3-, SCN-) complexes, having the formula [Mn(8-aq)2 I2]•(1), [Mn(8-aq)2 (H2O)2](8-aq)3•Br2 (2), [Mn(8-aq)2(SCN)2] (3), [Cu(8-aq)2Cl(H2O)]•Cl•H2O (4), [Cu(8-aq)2(NO3)(H2O)].NO3 (5), and Cd(8-aq)2I2 (6). Single crystal X-ray diffraction analyses showed that all of complexes have a distorted octahedral geometry, in which each 8-aq molecule acts as a bidentate ligand and coordinates to the central metal ion with its common coordination mode, to form a N,N' chelating motive. Remarkably, the influence of the counter ion on the geometry of the complex is very significant since both I(-) and SCN(-) anions are coordinated to the metal ion in compounds1, 3 and 6 adopting a cis-configuration, whilst a single anion occupies an axial position in compounds 4 and 5 (Cl- and NO3-, respectively) and the other counterion is not coordinated. Finally, both Br‾ anions are not coordinated in the cationic complex 2 (Mn metal centre). In all cases, there are extended supramolecular networks due to cooperativity hydrogen-bonding and pi-pi stacking interactions that play an essential role in the formation and stability of the crystalline materials. The binding energies attributed to the different interactions have been evaluated using DFT calculations.
We report three new Cu-Na heterometallic complexes namely [Cu(L-1(2)) Na(NO3)(CH3OH)] (1), [Cu(L-22)Na(NO3) (CH3OH)] (2) and [Cu Na (L-33)] n (3) where the topology of the synthon is directly governed by the p-p interaction involving the Cu-Schiff base chelate rings. The crystal structure analysis also reveals how the systematic variation in the ligand framework influences the supramolecular assembly and the mutual cooperation of different pi-forces and the hydrogen bonding forces in the supramolecular assembly. The pi-forces are more important than the hydrogen bonding forces in such compounds is evident from the supramolecular assembly of compound 3. Most interesting revelation of the studies are the presence of relatively uncommon pi-forces such as chelate ring pi...pi chelate ring and metal-pi interactions in the supramolecular architecture of the complexes. The analysis of the supramolecular assembly of the complexes 1-3 reveals that metal-chelate rings play prominent role in the organization of the molecular complexes and should seriously consider along with other p-stacking forces.
Two enantiomorphic (P and M) helical coordination polymers of [Cu(μ-1,3-N3)(L)]n {HL = 2-(dimethylamino)ethyliminomethylnaphthalen-2-ol} have been synthesized by the self-assembly of the achiral precursors via spontaneous resolution. The structures have been confirmed by single crystal X-ray crystallography. Solid-state circular dichroism spectra have confirmed the chirality of both P and M helical chains.
Three new copper(II) complexes, [Cu(L)(Cl)]ClO4 (1), [Cu(L)(Br)]ClO4 (2) and [Cu(L)(I)]ClO4 (3), have been prepared from a tetradentate symmetrical Schiff base, N,N’-bis-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine (L) and characterized by elemental analysis, IR and UV–Vis spectroscopy and single-crystal X-ray diffraction studies. Density functional theory (DFT) calculations were employed to estimate the contribution of each interaction to the formation of the assembly using several theoretical models. The interplay between the anion–Π and Π-Π interactions are also analyzed and a mutual reinforcement of both interactions is demonstrated. The assignment of the contribution of each interaction and its mutual influence is certainly important to shed light into the delicate mechanism that governs the molecular recognition and crystal packing.
High-level calculations (RI-MP2/def2-TZVP) disclosed that the σ-hole in between two C atoms of cycloalkane X2CCX2 structures (X=F, CN) is increasingly exposed with decreasing ring size. The interacting energy of complexes of F−, HO−, N≡C−, and H2CO with cyclopropane and cyclobutane X2CCX2 derivatives was calculated. For X=F, these energies are small to positive, while for X=CN they are all negative, ranging from −6.8 to −42.3 kcal mol−1. These finding are corroborated by a thorough statistical survey of the Cambridge Structural Database (CSD). No clear evidence could be found in support of non-covalent carbon bonding between electron-rich atoms (El.R.) and F2CCF2 structures. In marked contrast, El.R.⋅⋅⋅(CN)2CC(CN)2 interactions are abundant and highly directional. Based on these findings, the hydrophobic electrophilic bowl formed by 1,1′,2,2′-tetracyano cyclopropane or cyclobutane derivatives is proposed as a new and synthetically accessible supramolecular synthon.
A new method for dividing a crystalline electron distribution into molecular fragments is proposed, based on Hirshfeld's partitioning scheme. Unlike other approaches, the method partitions the crystal into smooth molecular volumes as well as intermolecular voids of low electron density. To compare the new method with several other schemes which subdivide a crystal into molecules, numerical integration is performed on two model electron densities (one representing a superposition of isolated molecules, the other interacting molecules) for ice VIII, formamide and urea. The new scheme is simply to apply, aesthetically appealing, and offers some promise in routine partitioning of crystalline electron densities or in computer graphics to provide additional insight into molecular packing in crystals.
The prototypical directional weak interactions, hydrogen bonding and σ-hole bonding (including the special case of halogen bonding) are reviewed in a united picture that depends on the anisotropic nature of the molecular electrostatic potential around the donor atom. Qualitative descriptions of the effects that lead to these anisotropic distributions are given and examples of the importance of σ-hole bonding in crystal engineering and biological systems are discussed.