Syntheses, structures and properties of three Keggin-based metal–organic complexes derived from flexible bis-pyridyl-bis-amide and rigid phenanthroline mixed ligands

Three Keggin-type polyoxometalate (POM)-based metal–organic complexes (MOCs) constructed from mixed ligands of phenanthroline (phen) and flexible bis-pyridyl-bis-amide ligands with different spacer length: [Cu2(SiW12O40)L1(phen)2(H2O)]·3H2O (1), [Cu2(SiW12O40)L2(phen)2(H2O)4] (2) and [Cu2(SiW12O40)L3(phen)2(H2O)4]·6H2O (3) (L1 = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, L2 = N,N′-bis(3-pyridinecarboxamide)-1,4-butane, L3 = N,N′-bis(3-pyridinecarboxamide)-1,6-hexane), have been synthesized under hydrothermal conditions and structurally characterized by single crystal X-ray diffraction analyses, elemental analyses, thermogravimetric (TG) analyses, IR spectra and powder X-ray diffraction (PXRD) analyses. Complex 1 exhibits a 2D network, in which [SiW12O40]4− (SiW12) anions act as tetradentate linkages. Complex 2 is a 0D structure containing a dinuclear complex cation [Cu2L2(phen)2(H2O)4]4+ and a discrete SiW12 anion. Complex 3 shows a 1D chain structure, in which the SiW12 anions as bidentate linkages bridge the adjacent complex cations [Cu2L3(phen)2(H2O)4]4+. The spacer lengths of the bis-pyridyl-bis-amide ligands play important roles in tuning the structures of the title complexes. In addition, the introduction of the secondary ligand phen shows significant effect on the dimensionalities of POM-based bis-pyridyl-bis-amide complexes. The electrochemical behavior and electrocatalytic properties of complex 3, as well as the photocatalytic activities of the title complexes have been studied.