An inorganic–organic hybrid supramolecular framework based on the γ-[Mo8O26]4− cluster and cobalt complex of aspartic acid: X-ray structure and DFT study

Abstract

A new inorganic–organic hybrid based on an aspartate functionalized polyoxomolybdate, [pentaaquacobalt(II)]-μ-aspartate-[γ-octamolybdate]-μ-aspartate-[pentaaquacobalt(II)] tetrahydrate, [Co2(C4H6NO4)2(γ-Mo8O26)(H2O)10]·4H2O (1), has been synthesized under hydrothermal conditions from the reaction of an Evans–Showell-type polyoxometalate, (NH4)6[Co2Mo10H4O38], and L-aspartic acid. The complex exhibits a supramolecular three-dimensional framework structure in the crystal lattice. Compound 1 was structurally characterized by elemental analyses, IR and UV–Vis (diffuse reflectance) spectroscopy and single-crystal X-ray diffraction. In this compound, aspartic acid acts as a bridge between the two Co atoms and the Mo centres, with the –CH2COOH side chain directly linked to the Mo centre in γ-[Mo8O26]4− and the α-carboxylate side chain bound to the Co centre. Commonly, the binding of transition-metal complexes to POMs involves coordination of the metal to a terminal O atom of the POM so that 1, with a bridging ligand between Mo and Co atoms, belongs to a separate class of hybrid materials. While the starting materials are both chiral and one might expect them to form a chiral hybrid, the decomposition of the chiral Evans–Showell-type POM and its conversion to the centrosymmetric γ-octamolybdate POM, plus the presence of two aspartate ligands centrosymmetrically placed on either side of the POM, leads to the formation of an achiral hybrid. We have studied energetically by means of density functional theory (DFT) calculations and using the Bader's `atoms-in-molecules' analysis the electrostatically enhanced hydrogen bonds (EEHBs) observed in the solid state of 1, which are crucial for the formation of one-dimensional supramolecular assemblies.