Artificial bioinorganic clusters of dinuclear 3d-transition metal ions coordinated by an inorganic coordination...

A new biologically relevant cubane-type artificial inorganic ligand was developed to support 3d-transition metal ions of Mn(II), Co(II), Ni(II), Zn(II), Fe(III), and also In(III). All of the reactions produce dinuclear cationic clusters of [({Co(tacn)}₂HMo₃O₁₃∙H₂O){M(H₂O)₂}]₂ⁿ⁺ (M = Mn²⁺ (4Mn), Co²⁺ (4Co), Ni²⁺ (4Ni), Zn²⁺ (4Zn), n = 2; M = Fe³⁺ (5), In³⁺ (6), n = 4), where tacn is the abbreviation of 1,4,7-triazacyclononane ligand. Clusters 4 do not dissolve in any solvents at all to prevent the recrystallization or the further characterization whereas the high solubility of clusters 5 and 6 allowed us to characterize the solution state properties. The dinuclear cores are separated at a fixed-distance in the range of 5.66-5.77 Å for M(II) complexes and 5.45-5.65 Å for M(III) complexes. Each 3d-transition metal ion is supported by two pairs of cis-oxido groups from a cubane unit formed by CoMo₃O₃(OH) group, and remaining two sites on the 3d-transition metal centers are occupied by two water molecules. The hydroxido group on the cubane unit provides an interaction route through bifurcated hydrogen bondings between dinuclear centers. Three consecutive stepwise reduction waves were observed at -0.73, -0.91, and -1.16 mV vs Fc⁺/Fc for 5 in cyclic voltammetry and the comparison with the redox inactive In³⁺ complex revealed that two iron centers are interacted each other through the inorganic ligand which gave a reduction wave observed at -0.91 mv vs Fc⁺/Fc.