pH and amine-induced various octamolybdate-based metal–organic complexes: assembly, structures and properties

Abstract

Three octamolybdate-based metal–organic complexes (MOCs), namely, [H₂en]₂[Cu(pzca)₂(Mo₈O₂₆)]·4H₂O (1), [Cu₂(pzca)₂(Mo₈O₂₆)_0.5(H₂O)₄]·H₂O (2) and [Cu^I₃Cu^II₄(pzca)₇(Mo₈O₂₆)(H₂O)₂]·4H₂O (3) (en = ethylenediamine, pzca = 2-pyrazinecarboxylic acid), have been successfully synthesized under hydrothermal conditions via adding different kinds of amine and adjusting the pH values. The single crystal X-ray diffraction, elemental analyses, IR spectra, and thermogravimetric analyses have been used to characterize the title complexes. In compound 1, the [Cu(pzca)₂] units bridge the [β-Mo₈O₂₆]⁴⁻ clusters to generate a 1D chain, which is extended by hydrogen-bonding interactions to a 3D supramolecular structure. Compound 2 exhibits a 3D coordination framework based on the 1D infinite [Cu₂(pzca)₂]_n²ⁿ⁺ zigzag chains and β-Mo₈O₂₆⁴⁻ anions, and shows a trinodal 3,3,4-connected {8³}₄{8⁶} topology containing a dodecagonal channel with the dimension of 13.496 × 19.642 Ų. Compound 3 is a 3D metal–organic network derived from the “λ”-like [Cu₅(pzca)₄] subunits and [β-Mo₈O₂₆]⁴⁻ anions, which represents a novel {4·6·8³·10}₂{4·6·8}₂{4²·10⁴}{4²·8³·10}{6·8²}₂{6²·10³·12}{6²·8²·10²}{6²·8³·10}{6³}₄ topology in octamolybdate-based MOCs. The structural diversities show that the different amines and pH value of the reaction system play key roles in the construction of high dimensional architectures. Moreover, the electrochemical behaviors and photocatalytic activity for compound 1 have been investigated in detail.