Four octamolybdate-based complexes based on flexible bis-imidazole-bis-amide ligands with different lengths: Structure, electrochemical and photocatalytic properties

Abstract

By adjusting the spacer length of bis-imidazole-bis-amide ligands and solvents, four octamolybdate-based metal-organic complexes (MOCs), [(Co2(L1)2(H2O)3)2(γ-Mo8O28)(MoO3)2]·6H2O (1), [(Ni2(L1)2(H2O)3)2(γ-Mo8O28)(MoO3)2]·6H2O (2), [Co2(L2)2(H2O)4](β-Mo8O26)·4DMA·6H2O (3) and [Ni(L3)0.5(H2O)(β-Mo8O26)0.5]·3H2O (4), (L1 = 1,2-bis(1H-imidazole-4-carboxamido)ethyl, L2 = 1,4-bis(1H-imidazole-4-carboxamido)butyl, L3 = 1,6-bis(1H-imidazole-4-carboxamido)hexyl, DMA = N,N′-dimethylacetamide), have been synthesized. Complexes 1 and 2 were obtained in H2O:EtOH and H2O:MeOH mixed solvents, respectively, which are isostructural 2D supramolecular structures, both containing binuclear metal organic cation rings and [(γ-Mo8O28)(MoO3)2]8− anion. Interestingly, such a [(γ-Mo8O28)(MoO3)2]8− anion is formed in 1 and is very rare in the polymolybdate-based MOCs. By increasing the length of ligands in H2O:DMA mixed solvent system at the presence of Co2+/Ni2+ ion and [Mo7O24]6− polyoxoanion, a 3D supramolecular complex 3 based on [Co2(L2)2(H2O)4]4+ binuclear units and [β-Mo8O26]4− anions was constructed, while complex 4 shows a 2D supramolecular network based on a 1D [Ni2(L3)2(H2O)2(β-Mo8O26)] wave-like chain. The results reveal that the solvents have great effect on the formation of the target MOCs, and the length of ligand plays important role in tuning the diverse architectures of polymolybdate and polymolybdate-based complexes. The electrochemical and photocatalytic properties of the title complexes were studied.