[Mo^VIO₂(S₂C₂(CN)₂)₂]₂₋ (┘1) and [Mo^IVO(S₂C₂(CN)₂)₂]²⁻ (2) mimick oxidoreductase enzymatic activities of sulphite oxidase with biological electron donor, SO₃²⁻, andin vitro electron acceptor, [Fe(CN)₆]³⁻, demonstrating proton coupled electron transfer reaction in water and inhibition of the oxidation of (2) in the presence of KCN. The sulphite exidizing system is characterized by substrate saturation kinetics indicating the biological significance of the reactions

(NEt₄)₂[W^IVO(S₂C₂(CN)₂)₂] (1), isolated by reaction of Na₂ WO₄, Na₂S₂C₂(CN)₂ (Na₂mnt) in acidified (pH5.5) aqueous medium in the presence of excess of sodium dithionite and NEt₄Br, reduces CO₂/HCO₃⁻ (pH 7.5) to yield HCOO⁻ and (NEt₄)₂[W^VIO₂(S₂C₂(CN)₂)₂] (2) mimicking tungsten-formate dehydrogenase (W-FDH) activity. (1) reacts with Na₂MoO₄ in acidic medium to produce [Mo^IvO(S₂C₂(CN)₂)₂]²⁻ implicating the displacement of tungsten by molybdenum from the cofactor complex in W-FDH.

Facile reduction of the triangular {Mo₃^IV}⁶-core containing [Mo₃S₄ (CN)₉]⁵⁻ derivative is achieved in aqueous medium, containing the KOH-KCN-K₂ [Zn(CN)₄] ternary system as supporting electrolyte, in a two-step one-electron reduction reaction followed by an irreversible one. The diamagnetic species containing the {Mo₃}⁸ core, obtained by chemical reduction of [Mo₃S₄(CN)₉]⁵⁻ with zinc in aqueous medium containing KOH-KCN, is oxidized by air in two steps. The partially oxidized species with {Mo₃}⁷ core in the first step exhibits characteristic ESR signals which on further oxidation quantitatively reverts to the original compound with {Mo₃}⁶ core displaying distinctly separate isosbestic points in the stepwise conversion of {Mo₃}⁸ to {Mo₃}⁷ and {Mo₃}⁷ to {Mo₃}⁶ cores. Under drastic reduction conditions with aluminium in 40% aqueous KOH solution the {Mo₃}⁶ core is reduced to unstable trinuclear {Mo₃^III}⁹ core which expands to tetrahedral {Mo₄^III}¹²-core containing [Mo₄S₄(CN)₁₂]⁸⁻ derivative according to electron-precise cluster rule.

The compound (NH₄)₇[ V^vO₄W₁₀^VIV₂^VIO₃₆]·ca.22H₂O (1) has been synthesized from an aqueous ammonium acetate buffer (pH 4) containing sodium vanadate, sodium rungstate_and sodium dithionite. Compound (1) crystallizes in a cubic space groupFm — 3, witha = 22.2001(6) Å and Z = 8. The anion [V^vO₄W₁₀^VIV₂^IVO₃₆]^7- is a typical Keggin type structure with V^VO₄ as the central tetrahedron. (1) has further been characterized by elemental analyses, redox titration, IR, EPR, and electronic spectroscopy and room temperature magnetic moment measurement.

Synthesis and structural characterization of a polyoxometalate compound Na₃(H₂O)₆ [Al(OH)₆Mo₆O₁₈]-2H₂O (1) have been described. Compound1 exhibits three-dimensional network structure in the solid state, which is assembled by Anderson-type heteropolyanions, [Al(OH)₆Mo₆O₁₈]_n^3n-, as building blocks sharing sodium cations.1 possesses “sinuous” channels occupied by supramolecular water dimers as guests. Anderson anions, sodium-coordinated water and crystal water are additionally involved in an intricate hydrogen-bonding network in the crystal of1.

The open air reaction of the chiral Schiff base ligand H₂L, prepared by the condensation of L-phenylalaninol and 5-bromosalicylaldehyde, with Mn^II(CH₃COO)₂.4H₂O yielded dark brown complex [Mn^IV L₂]·0·5 DMF (1). Compound 1 was characterized by elemental analysis, IR, UV-visible, CD and EPR spectroscopy, cyclic voltammetry and room temperature magnetic moment determination. Single-crystal X-ray analysis revealed that compound 1 crystallises in the monoclinicP2₁ space group with six mononuclear [Mn^IV L₂] units in the asymmetric unit along with three solvent DMF molecules. In the crystal structure, each Mn(IV) complex, acting as the building unit, undergoes supramolecular linking through C-H...0 bonds leading to an intricate hydrogen bonding network.

A one-pot synthesis, that includes CuCl₂.2H₂O, Na₂mnt, H₂salph and Mn(CH₃COO)₃.H₂O, leads to the isolation of a trinuclear heterometallic compound [Mn^III(salph)(H₂O)₂Cu^II(mnt)₂].4DMF (1) formed by Mn…S-Cu-S…Mn supramolecular interactions. Compound1 crystallizes in the monoclinic space groupP21/c witha = 13.433(4),b = 16.283(5),c = 15.072(4) å, Β= 107.785(4)‡, Z = 2. In the crystal structure, the complex anion [Cu^II(mnt)₂]^2- bridges two [Mn^III(salph)(H₂O)]¹⁺ cations through Mn…S contacts. The non-covalent hydrogen bonding and π-π interactions among the trinuclear [Mn^III (salph)(H₂O)₂Cu^II(mnt)₂)] complexes lead to an extended chain-like arrangement of [Mn^III(salph) (H₂O)]¹⁺ cations with [Cu^II(mnt)₂]^2- anions embedded in between these chains.

An inorganic−organic hybrid material, [HMTAH]₂[{Zn(H₂O)₅}{Zn(H₂O)₄}{Mo₇O₂₄}].2H₂O (1) (where HMTAH = protonated hxamethylenetetramine) has been synthesized and structurally characterized. The compound 1 crystallizes in a monoclinic space group $C2/c$. The crystal data of 1: 𝑎 = 43.12(3), 𝑏 = 12.399(10), 𝑐 = 16.285(13), 𝛽 = 111.131(11), 𝑍 = 8. Its crystal structure shows that two different Zn(II)-aqua complexes, [Zn(H₂O)₅]²⁺ and [Zn(H₂O)₄]²⁺ are covalently coordinated to a heptamolybdateanion [Mo₇O₂₄]^6- resulting in an anionic species of polyoxometalate supported zinc-aqua complexes, [{Zn(H₂O)}{Zn(H₂O)₄}{Mo₇O₂₄}]^2-, that is stabilized with two protonated hexamethylenetetramine cations in the title compound 1. In the crystal structure, both lattice water molecules are found to interact with the heptamolybdate cluster anion and the protonated hexamethylenetetramine cation resulting in an intricate three-dimensional hydrogen bonding network. Interestingly, compound 1 exhibits catalytic activity towards oxidation of some primary alcohols.

An inorganic-organic hybrid material, [2-AmpH]₄[Mo₈O₂₆] (1) has been isolated by the reaction of Na₂MoO$_4·2$H₂O with 2-aminopyrimidine (2-Amp) from an acidic aqueous medium. In this low pH synthesis, the organic molecule (2-Amp) gets mono-protonated (2-AmpH⁺) and acts as the cation in stabilizing the octamolybdate anion in 1. Compound 1 crystallizes in the triclinic space group 𝑃-1 with 𝑎 = 9.925(3), 𝑏 = 10.020(3), 𝑐 = 10.414(3) Å, 𝛼 = 88.811(4), 𝛽 = 64.907(4), 𝛾 = 89.506(4)°, 𝑍 = 1. An interesting three-dimensional supramolecular structure, having well-defined channels, is formed through N-H$\cdots$O and C-H$\cdots$O hydrogen bonds, in which the protonated organic cation plays a significant role. The crystal structure also reveals an unusual cluster-cluster (non-covalent O$\cdots$O) interaction using cis-(MoO₂} moieties of the isopolyanion. N-H$\cdots$O hydrogen bonds, originated from pyridimidinium cation and isopolyanion, are found to influence to attain such non-covalent O$\cdots$O interactions among polyoxometalate anions. An interesting helical arrangement, formed from isopolyanion and organic cation, is observed.

The violet-coloured compound [Ni^II(opda)₂(NCS)₂] (1) undergoes colour change to strawyellow colour retaining its molecular composition on standing over long period of time at room temperature in the solid state. Compound 1 (violet form) and its yellow-form [Ni^II(opda)₂(NCS)₂] (2) (opda = orthophenylenediammine) have been characterized by routine spectroscopic methods and single crystal X-ray diffraction analysis. Compound 1 crystallizes in monoclinic space group $P2_1/c$ and its yellow form (compound 2) retains same space group. Their crystal structures show an intricate supramolecular network based on N-H$\cdots$S hydrogen bonds, that involve amine and thiocyanate groups coordinated to nickel(II).

This article describes an account of some of our polyoxometalate (POM)-based research, we have been doing in our laboratory last several years. There are several well-defined POM cluster anions, that are structurally characterized. We have chosen Anderson-type of heteropolyanion [Al(OH)₆Mo₆O₁₈]$^{3−}$ and explored its linking propensity in different dimensions using `s', `d' and `f' block elements as linkers.We have demonstrated how a lanthanide linker provides a new pathway in forming a two-dimensional linked {As₈V₁₄} system [{Ln(H₂O)₆}₂As₈V₁₄O₄₂(SO₃)]$_n$.8nH₂O, that is derived from discrete {As₈V₁₄} cluster containing compound (NH₄)₆[As₈V₁₄O₄₂(SO₃)]. A polyoxometalate compound has been described in which a reduced tungstovanadate-heteropolyanion clusters get linked via capped V = O groups into one-dimensional chains. All these systems have already been reported elsewhere. The last portion of this article will be described by a new system [3-ampH]₆[V₁₀O₂₈]$\cdot$2H₂O having discrete molecular structure and extended supramolecular structure.

Good quality single crystals of 4-hydroxy tetramethylpiperazinium picrate (TMPP) were grown by slow evaporation solution growth method at room temperature. The average dimensions of the grown crystals were $0.6 \times 0.2 \times 0.2$ cm³. The solubility of the compound was estimated using methanol and acetone. The elemental analysis confirms the formation of the compound in the stoichiometric proportion. The UV-visible transmittance study indicates that the crystal possesses minimum transmittance at 370 nm and no absorption at 470-900 nm. The Bragg peaks obtained in the powder X-ray diffraction pattern confirm its crystallinity. The thermal behaviour of the crystal was investigated using thermogravimetric (TG) and differential thermal analysis (DTA) techniques. The structure of the compound was determined by using single crystal X-ray diffraction method. The compound was found to be crystallize in the monoclinic space group P2₍₁₎/c (a = 6.9513(8)Å, b = 11.8016(14)Å, c = 22.018(2)Å, 𝛼 = 90.00°, 𝛽 = 92.575(2)°, 𝛾 = 90.00°). The thermal anomalies observed in the differential scanning calorimetry (DSC) heating and cooling cycles indicate the occurrence of first order phase transition. Fourier transform infrared (FTIR) and polarized Raman spectral analyses were used to confirm the presence of various functional groups in the compound. The nonlinear optical property (NLO) of the crystal was analysed by Kurtz-Perry powder technique and found that the compound has SHG efficiency 1.5 times greater that of potassium dihydrogen phosphate (KDP).

One-dimensional isomorphous inorganic polymers containing Anderson type heteropoly anion as a basic building unit, namely [La(H₂O)₇Cr(OH)₆Mo₆O₁₈]$_n$·4nH₂O (1), [Gd(H₂O)₇Cr(OH)₆Mo₆O₁₈]$_n$·4nH₂O (2), [Gd(H₂O)₇Al(OH)₆Mo₆O₁₈]$_n$·4nH₂O (3), and [Eu(H₂O)₇Al(OH)₆Mo₆O₁₈]$_n$·4nH₂O (4) have been synthesized and studied by the powdered X-ray diffraction, TGA, IR, electronic and ESR spectroscopy, and unambiguously by single crystal X-ray crystallography. Isomorphous compounds 1-4 are crystallized in orthorhombic system with $Pca2_1$ space group. The crystal structure analysis reveals a one-dimensional extended chain in which the Anderson type heteropolyanion, acting as the building unit, is linked by rare earth metal ions in a zig-zag fashion. In the crystal structure, all types of oxygens of the heteropolyanion, lattice waters, lanthanum coordinated waters are extensively involved in O—H…O hydrogen bonding interactions. Compounds are additionally characterized by UV-visible and ESR spectroscopy.

Oxidation of styrene is carried out by using heptamolybdate coordinated transition metal (Co²⁺, Zn²⁺) complexes, [2-ampH]₄[{Co(H₂O)₅}Mo₇O₂₄]·9H₂O (1), [3-ampH]₄[{Co(H₂O)₅}Mo₇O₂₄]·9H₂O (2), [2-ampH]₄[{Zn(H₂O)₅}Mo₇O₂₄]·4H₂O (3) and [3-ampH]₄[{Zn(3-ampy)(H₂O)₄}Mo₇O₂₄]·4H₂O (4) as catalysts and H₂O₂ as an oxidant at 80°C. The leaching study has been carried out to check the quality of catalyst and it has been reused for three times with good percentage of conversion. For the first two catalysts (compounds 1 and 2), the major product obtained is benzaldehyde, and benzoic acid is the major product for next two catalysts (compounds 3 and 4). Stability of the catalysts has been analyzed by IR, UV-spectroscopy and powder X-ray crystallography.

Compounds [Fe₂{𝜇-pydt}(CO)₆] (pydt = pyrazine-2,3-dithiolate) (1), [Fe₂{𝜇-qdt}(CO)₆] (qdt = quinoxaline-2,3-dithiolate) (2), [Fe₂{𝜇-ppdt}CO)₆] (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) (3), [Fe₂{𝜇-pydt}(CO)₅PPh₃] (4), [Fe₂{𝜇-qdt}(CO)₅PPh₃] (5) and [Fe₂{𝜇-ppdt}(CO)₅PPh₃] (6) have been synthesized in order to model the active sites of `[FeFe]-hydrogenase’. Compounds 1–6 have been characterized by routine spectral studies and unambiguously by single crystal X-ray crystallography. Supramolecular chemistry of compounds 1–6 have been described in terms of intermolecular interactions, observed in their respective crystal structures. Electro-catalytic hydrogen evaluation studies (from acetic acid) have been performed using compounds 1–6 as electro-catalysts. The mechanistic aspects of relevant electro–catalytic proton reductions have been discussed in detail.