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Two new dinuclear cobalt(ii) complexes were synthesized using compartmental N,O-donor ligands. The optical properties of the complexes were studied with focus on the band gaps.

The study is devoted to the analysis of the Zn⋯O contacts established between the square-pyramidal Zn metal centers and the co-crystallized DMF and DMSO molecules.

A hooked cross-shaped anion, [Cu(N3)4]2−, in a synthesized hetero-metallic pentanuclear copper(ii)–sodium complex provides a π-basic surface adequate for establishing a strong CH⋯[Cu(N3)4]2− interaction.

The optoelectronic properties of two cadmium based complexes have been examined both experimentally and theoretically. The energy associated with non-covalent interactions observed in the solid state of both complexes has been calculated.

The coordinated amino groups in a trinuclear zinc complex participate in strong H-bonding interactions that have been analysed. The dimerization energy is very large for both the homodimer and the heterodimer in the 1D supramolecular chain.

The synthesis and X-ray characterization of a new dinuclear Zn complex using a tetradentate N2O2 donor Schiff base and azide as an anionic co-ligand of formula [(DMSO)2ZnL(μ1,1-N3)Zn(N3)2] (1) are 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).

The crystal structure and magnetic properties of two mononuclear iron(III) Schiff base complexes, [FeL1(NCS)2] (1), HL1 = 2-[1-[[2-[(2-aminoethyl)amino]ethyl]imino]ethyl]phenol and [FeL2(N3)Cl] (2), HL2 = 2-(-1-(2-(2-aminoethylamino)ethylimino)ethyl)-4-methylphenol are reported. Each complex contains a Fe(III) ion surrounded by a N3O Schiff base ligand and two NCS− ligands (in 1) or one N3− and one Cl− ligands (in 2). The magnetic properties can be well reproduced with zero field splittings in the high spin S = 5/2 Fe(III) ions and weak intermolecular Fe–Fe interactions mediated by hydrogen bonds. This intermolecular antiferromagnetic interaction has been validated by using DFT calculations in complex 2. Moreover, the interaction energies of the H-bonded dimers in both complexes have been estimated using DFT calculations and characterized using a combination of QTAIM and NCI plot computational tools. Complexes 1 and 2 constitute two rare examples of Fe(III) complexes with magnetic interactions through H-bonds.

Two new dinuclear nickel(II) complexes, [(H2O)Ni(N3)(L1)(μ1,1-N3)Ni(L1)] and [(H2O)Ni(N3)(L2)(μ1,1-N3)Ni(L2)]·MeOH, derived from two isomeric Schiff base ligands, HL1 [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 μ1,1-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 is 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 and 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 orbital of one nickel(II) center with the participation of the azide π-system. The SOMO−1 is constituted by the orbital of the other nickel(II), an oxygen atom and the azide π-system.

A dimeric copper(II) complex, [Cu2(μ1,3-L)4(DMSO)2] (HL = 2-chloro-4-nitrobenzoic acid), was synthesized and characterized by standard spectroscopic methods and X-ray analysis. In the solid state, conventional C–H⋯O and π–π stacking interactions are important forces governing the X-ray packing, as evidenced by Hirshfeld surface analysis. The formation of two chalcogen bonding (ChB) interactions involving coordinated DMSO molecules as σ-hole donors and the O atoms of carboxylate groups as acceptors has been also described and studied by means of DFT calculations. The effect of the Cu coordination of DMSO on the intensity of the sigma holes has been studied using MEP surface analysis. The S⋯O and S⋯π contacts have been characterized by using a combination of QTAIM and NCI plot computational tools.

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 I3− anions form discrete dimers in the solid state with an I⋯I distance of 3.55 A. 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 an 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.

To understand the packing in a particular crystal, we need to investigate supramolecular interactions. Here, we report a hydronium ion encapsulated within a dimeric assembly of two different nickel(II) Schiff base complexes (NiL). The structure has been confirmed by single crystal X-ray diffraction analysis. In the structure, the two NiL complexes are placed in such a way that the outer cores form a cavity in which the hydronium ion resides. The crystal packing is mainly directed by strong hydrogen bonding interactions observed between the hydronium ion and NiL complexes but, in addition, weak interactions such as π⋯π and C–H⋯π are also observed. The overall crystal packing and the strong hydrogen bonds due to the encapsulated H3O+ ion are evaluated using DFT calculations. The reason for the strength of these hydrogen bond interactions can be accounted for by charge transfer from the lone pair of oxygen atoms in the two nickel(II) Schiff base complexes to the antibonding orbital of the O–H (of H3O+) bond.

Two new zinc(II) complexes, [{N(CN)2}Zn(L1)Zn{μ-1,5-N(CN)2}]2 (1) and [{μ-1,5-N(CN)2}{N(CN)2}Zn(L2)Zn{μ-1,5-N(CN)2}]n (2) {H2L1 = 2,2′-[(2,2-dimethyl-1,3-propanediyl)bis(nitrilomethylidyne)]bis[6-methoxyphenol] and H2L2 = 2,2′-[(2,2-dimethyl-1,3-propanediyl)bis(nitrilomethylidyne)]bis[6-ethoxyphenol]} have 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 H2L2 and 6-methoxy in H1L1) has a strong influence on the final solid state structure. In addition, the O-atom of the methoxy groups in 1 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 the anionic coligands with the formula [Zn2L2(OAc)2] (1) and [Zn2L2(N3)2] (2) is described. The presence of acidic NH groups in the complexes, owing 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. These assemblies have been analysed energetically using density functional theory (DFT) calculations and several computational tools (symmetry adapted perturbation theory (SAPT), molecular electrostatic potential (MEP) surfaces, quantum theory of atoms in molecules (QTAIM) and non-covalent interaction index (NCI) plot analyses).

A reduced Schiff base has been synthesized and characterized and used as a fluorescence chemo-sensor for the selective detection of zinc(II). Fluorescence titrations have also been conducted for the ligand and the binding constant for the ligand (K = 1.056 × 106 M−1) has been evaluated using the Benesi–Hildebrand equation. Two mononuclear zinc(II) complexes have also been synthesized with the ligand and S0 and S1 of their electronic structures were calculated. The HOMO–LUMO energy difference in each complex is ca. 4.69 eV in S0 states and the energy gap is reduced to ca. 4.3 eV in S1 facilitating easier electronic transition. Their strong fluorescence behaviors may be correlated with the presence of a rigid hydrogen-bonded network in their solid state structure. Two types of intermolecular hydrogen bonding are noticed in the dimeric form of the complexes. The hydrogen bonding environment is well supported qualitatively and quantitatively with the help of NCI-RDG (noncovalent interaction reduced density gradient) and QTAIM. The physical nature of other weak non-covalent interactions in both complexes was also examined. Based on the optimized ground state geometry (S0), the TDDFT/B3LYP method combined with the SMD solvation model in methanol media was used to calculate the absorption properties of the investigated complexes. Additionally, analysis on the electronic structure of the excited states employing NTO (natural transition orbital) representation showed that the S1 state can be mainly characterized by an inter-ligand charge-transfer (ILCT) transition, populating the highest-occupied (HO) NTO and lowest-unoccupied (LU) NTO, which describe the hole and the excited electron state, respectively. The calculation indicates that the fluorescence originates from the charge transfer from N3/NCS to the reduced Schiff base.

A new copper(II) complex [CuL(NCS)] with a naphthalene based Schiff base, HL = (1-(2-(diethylamino)ethylimino)ethyl)naphthalene-2-ol, has been synthesized and characterized by elemental analysis as well as single-crystal X-ray diffraction studies. The complex crystallizes in the monoclinic space group P21/a. The complex shows C–H⋯π interactions to form a supramolecular chain structure. The electrical conductivity of the complex has been recorded under dark and irradiated conditions and the measured I–V characteristics of the complex under dark and light conditions indicate highly non-linear rectifying behavior, thus signifying a Schottky diode nature. The material is also photosensitive as the values of the electrical parameters are enhanced after exposure to illumination of incident radiation. The optical band gap of the complex in the solid state (2.92 eV) has been determined by experimental measurements and compared with the theoretical value obtained from DFT calculations (2.99 eV).

Three tetranuclear zinc dicyanamide complexes with three potential hexadentate reduced Schiff base ligands have been synthesized and characterized. In the solid state structure of complex 1, the aromatic ring of one complex is located in the U-cavity formed by two aromatic rings and the (Zn2)L2 (dicyanamide)2 macrocycle of the adjacent complex establishing π⋯π and C–H⋯π interactions. In sharp contrast, the presence of two methyl groups in addition to the longer methylene chain in 2 and 3 provokes a totally different packing, where the methyl groups are located in the U-cavity instead of the aromatic ring, establishing several C–H⋯π contacts. Thus, the presence of a methyl spacer and an additional methyl substituent in the alkyl chain of 2 and 3 is crucial since it has an enormous effect on the crystal packing. The DFT study is devoted to the analysis of the competition between C–H⋯π and π⋯π interactions observed in the solid state of complexes 1 and 2 and also to the characterization of the interactions using the NCI (non-covalent interaction) plot index computational tool. The molecular electrostatic potential (MEP) surface of complex 2 as a model of complexes 1–3 has also been calculated.

A heterodinuclear copper(II)/lead(II) complex with a compartmental ‘reduced Schiff base’ ligand (having inner N2O2 and outer compartments) has been synthesized and characterized. Single-crystal X-ray diffraction analysis confirmed its structure. The X-ray data indicate that the inner N2O2 compartment of the compartmental reduced Schiff base is occupied by copper(II) while the outer compartment is occupied by lead(II). A thiocyanate anion is coordinated to the copper(II) center via its N-end, whereas, another thiocyanate ion is linked to the lead(II) center via its S-end and is semi-coordinated to the copper(II) center through its N-end. A hemidirectionally coordinated lead(II) center is well suited for establishing tetrel bonding interactions. We estimated the BSSE (basis set superposition error) corrected energies of non-covalent S⋯S, Pb⋯π, and π⋯π interactions and N–H hydrogen bonding along with tetrel bonding by DFT calculations. To obtain an insight into the physical nature of these bonds, we extensively used Bader's quantum theory of atoms-in-molecules (QTAIM). Additionally, the non-covalent interaction reduced density gradient (NCI-RDG) method established nicely the presence of such non-covalent intermolecular interactions. Here, we also used natural bond orbital (NBO) analysis to find out the origin of S⋯S and Pb⋯S bonding.

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-[[[2-[(2-hydroxyethyl)amino]ethyl]imino]methyl]-6-methoxyphenol, have been synthesized and characterized by single crystal X-ray diffraction, IR, UV-visible spectroscopy and elemental analyses. Their structures have been confirmed by a single crystal X-ray diffraction study. The crystal packing of 1 shows an infinite 1D supramolecular network of C–H⋯π interactions involving either the π-cloud of the aromatic ring or the π-system of the SeCN ligand with several C–H bonds (both aliphatic and aromatic). Formation of a 2D supramolecular sheet in the solid state structure of 2 is facilitated by the C–H⋯π and π⋯π interactions. The DFT calculations have been conducted to determine the interaction energies in these complexes. The molecular electrostatic potential (MEP) surface analysis 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.

Two compartmental N2O4 donor Schiff bases, H2L1 [N,N-bis(3-methoxysalicylidene)-2,2-dimethylpropane-1,3-diamine] and H2L2 [N,N-bis(3-ethoxysalicylidene)-2,2-dimethylpropane-1,3-diamine], each having inner N2O2 and outer O4 compartments, have been used to prepare four heterotrinuclear nickel(II) complexes [Ni(L1)Na(L1)Ni(NCS)]·H2O (1), [Ni(L1)Na(L1)Ni(N3)] (2), [Ni(L2)Na(L2)Ni]NCS·CH3OH (3), and [Ni(L2)Na(L2)Ni]ClO4 (4). All the complexes have been characterized by elemental and spectral analyses. Single crystal X-ray diffraction analyses have confirmed their structures. In each complex, nickel(II) is placed in the inner N2O2 compartment and sodium is placed in the outer O4 compartment of the respective Schiff base ligand. In complexes 1 and 2, one nickel(II) centre is tetra-coordinated (square planar) and the other is penta coordinated (square pyramidal), whereas in complexes 3 and 4, both nickel(II) centres are tetra-coordinated. The geometries of complexes 1 and 3 have been optimized without a counterion (denoted as 1+ and 3+). The electrophilic nickel(II) centre is found to be accessible in complex 1+ and, conversely, it is unreachable in complex 3+, in agreement with the experimental result. Starting from the hypothetical 1+ complex where both nickel(II) centres are tetra-coordinated, the energy change for replacing four methyl groups with four ethyl groups has been computed using ethane and methane to complete the homodesmotic equation. The result is that the intra-molecular interaction of the four ethyl groups favours complex 3+ by ΔE = −26.1 kcal mol−1.

Two mononuclear iron(III) complexes, [Fe(L1)(N3)] and [Fe(L2)(N3)], {H2L1 = N,N′-bis(3-methoxysalicylidene)diethylenetriamine and H2L2 = N,N′-bis(3-ethoxysalicylidene)diethylenetriamine} have been synthesized and characterized by elemental, spectral and X-ray crystallographic studies. Structural features have been examined in detail that reveal the formation of interesting supramolecular networks generated through weak non-covalent interactions. The current–voltage characteristic curves for an Al/complex Schottky-barrier diode (SBD) exhibit non-ohmic behavior. Important parameters like ideality factor, barrier height and series resistance are measured with the help of thermionic emission (TE) theory. 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. Complex 1 is found to be more conductive. To obtain insight into the physical nature of weak non-covalent interactions, Bader's quantum theory of atoms-in-molecules (QTAIM) is used extensively. Additionally, the non-covalent interaction reduced density gradient (NCI-RDG) methods established nicely the presence of such non-covalent intermolecular interactions.

Metal-salen complexes have been widely used in inorganic chemistry to synthesize polynuclear complexes. This review highlights the synthetic strategy of multi-metallic complexes using metal-salen complexes as building blocks and structures of the resulting complexes. Three different processes have been used to prepare multi-metallic complexes using metal-salen complexes: (i) use of metal-salen complexes as metalloligands, (ii) self-assembly processes involving [M(salen)] complexes and various cyanaometallates and (iii) use of salen-type compartmental ligands. Complexes formed by these methods are included in the review. X-ray single crystal diffraction analysis confirmed the structures of these complexes. The properties of these complexes are also briefly discussed. This review will be of interest to researchers and synthetic chemists for synthetic strategy of homo- and heterometallic polynuclear complexes using salen-type Schiff bases.

Five heteronuclear nickel(II)/lead(II) complexes with two compartmental reduced Schiff base ligands were prepared and characterized. Their structures were confirmed by single crystal X-ray diffract...

Two centrosymmetric hetero-tetranuclear copper(II)/cadmium(II) complexes, (µ1,1-N3)2[(H2O)Cu(LR)Cd(N3)]2∙2CH3OH (1) and (µ1,1-NCS)2[CuLRCd(SCN)]2∙2CH3OH (2), have been prepared with a compartmental reduced Schiff base ligand, H2LR = 2,2′-[(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)]bis[6-methoxy-phenol]. Both complexes have been characterized elemental and spectral analyses. Structures of both complexes have been confirmed by single crystal X-ray diffraction technique. Both complexes contain CuO2Cd cores. Here the bridging mode of azide and thiocyanate is quite different. Complex 1 contains µ-1,1-azide bridges between two cadmium(II) centres whereas in complex 2, µ-1,1-thiocyanate bridges occur between copper(II) and cadmium(II) centres. The ability of both complexes to be used as photo-catalyst in degrading methylene blue (MB) has been explored. The difference in photocatalytic performance may be correlated with their structures. Complex 1 shows better catalytic activity compared to complex 2, and can degrade almost 62% MB in 18 min. Comparative IR studies (before and after the photocatalytic degradation process) confirmed the stability of both catalysts.

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.

Trivalent 3d metal complexes having the general formula [MIII(L)(X)(Y)] [where H2L represents the tetradentate N2O2 donor ‘salen type’ Schiff base, and X and Y are monodentate ligands], which were characterized by single crystal X-ray diffraction studies, have been analyzed in this review. In any octahedral complex of tetradentate N2O2 donor ‘salen type’ Schiff base ligand, two different arrangements of four donor atoms (two imine nitrogen atoms and two phenoxy oxygen atoms) around the metal centre are found. In one arrangement, the equatorial positions of M(III) are occupied by the respective deprotonated tetradentate ligands, whereas the axial positions are occupied by monodentate ligands. The second group of complexes follows a different arrangement where one oxygen atom is displaced from the equatorial plane, occupying an axial position of the coordination octahedron. The formation of the complexes of second type is favoured by the presence of a chelating bidentate ligand occupying two cis positions. On the other hand, the tetradentate ligands in the first type of complexes have also been found to be severely distorted from planarity in some complexes. This distortion may be quantified by measuring the angle between the plane of the phenyl ring(s) of the ligand and the equatorial plane of the complex.

A dimeric phenoxo bridged manganese(III) complex, [Mn2L2(OH2)(CH3OH)](ClO4)2 [H2L = N,N′-bis((2-hydroxy-1-phenyl)ethylidene)ethane-1,2-diamine] has been synthesized and characterized. The structure has been confirmed by single crystal X-ray crystallography. The χMT value at room temperature is slightly higher than that expected for two non-interacting species with S = 2 (g = 2), that corresponds to two high spin manganese(III) centers. That χMT value increases gradually with lowering temperature to reach a maximum and then an abrupt decrease of χMT value is observed at lower temperature. A ferromagnetic interaction (J = 1.44 cm−1) between the manganese(III) centers within the dimer is counter balanced by the presence of zero-field splitting (ZFS, D = −2.03 cm−1) at very low temperature and possibly also by antiferromagnetic interaction between dimers (zJ = -0.06 cm−1). In order to investigate the single-molecule magnet (SMM) nature of the complex, alternating current (ac) magnetic measurements were carried out from 2 to 7 K under a 3.5 Oe ac field between 1 and 1000 Hz.

A centrosymmetric square planar bis-ligand copper(II) complex, [Cu(HL)2](ClO4)2, with a potential tetradentate Schiff base, HL = 2-(2-(ethylamino)ethyliminomethyl)-6-ethoxyphenol, containing pendant side arm, has been prepared and characterized by elemental and spectral analysis and single crystal X-ray diffraction studies. The potential tetradentate Schiff base is trapped in its zwitterionic form and shows a bidentate bonding mode. Several weak forces like hydrogen bonding and C H∙∙∙π interactions lead to the formation of supra-molecular architecture. The existence of an anion⋯anion interaction between the perchlorate anions have been analyzed using DFT calculations, the Bader’s theory of atoms-in-molecules (AIM) and the non-covalent interaction index (NCI plot). The O⋯O distance is 2.848(2) A.

A dinuclear cobalt(III) complex, [(N3)L1CoIIIL2CoIIIL1] (1), has been synthesized and characterized {H2L1 = 2-((1-hydroxybutan-2-ylimino)methyl)-4-bromophenol and HL2 = 2-amino-1-butanol}. Single crystal X-ray diffraction analysis confirmed its structure. Extended supra-molecular architectures were generated in the complex through weak noncovalent interactions. The energetic features of significant supramolecular interactions have been investigated using DFT calculation and further corroborated with NCI plot index computational tool. The complex has been found to mimic the role of phosphatase enzyme efficiently by transforming 4-nitrophenylphosphate to 4-nitrophenolate at room temperature. The reaction follows Michaelis–Menten enzymatic reaction kinetics with turnover numbers of ∼1.4 s−1 in aqueous DMF (98% DMF, v/v) medium. The complex also acts as an efficient photocatalyst for degradation of organic pollutants. Methylene blue (MB), a widely used dye in various industries, was selected as a model pollutant in aqueous media to evaluate the photocatalytic effectiveness of the complex.

A compartmental Schiff base ligand, N,N′-bis(3-ethoxysalicylidene)-2,2-dimethylpropane-1,3-diamine (H2L) has been used to prepare a hetero-trimetallic cadmium(II)/zinc(II) complex, Cd{LZn(NCS)}2. Single crystal X-ray diffraction analysis has confirmed the structure of the complex. The complex behaves as a sensor for the detection of various nitroaromatics via turn-off fluorescence response.

The review is focused to discuss the synthetic strategy and properties of mixed valence cobalt(III)/cobalt(II) complexes with N,O-donor Schiff base ligands. Use of compartmental ligand is a good approach to prepare such complexes. Polydentate hydroxy-rich Schiff base ligands have also been used by various groups to prepare mixed valence cobalt complexes. Salen types Schiff bases have also been used by many synthetic inorganic chemists to prepare this type of complexes. The oxidation states of cobalt centers in these complexes may be assigned from BVS (Bond valence sum) calculation for each centre. Different oxidation states (i.e. +2 and +3) of two cobalt centers in the complexes could also be inferred comparing the relative bond lengths involving the two metal centers, e.g. shorter cobalt(III)–oxygen distances compared to cobalt(II)–oxygen distances. Same has also been true for cobalt-nitrogen bond lengths. Cobalt(II) is paramagnetic in nature whereas cobalt(III) is usually diamagnetic. The magnetic properties of mixed valence cobalt complexes are therefore important. Only a few complexes are showing SMM (single molecule magnet) behavior. Catalytic activities of few such complexes have also been explored and included in this review.