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

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. 3d-Transition metal ions such as Fe(III) were sandwiched by our original Co2Mo3 inorganic cluster as a new kind of binucleating ligands. Their Fe(III) centers with a ca. 5.4 Å separation interact each other via bifurcated three-centered hydrogen bonds which assist stepwise reductions at two Fe(III) centers. [Display omitted] •A Co2Mo3 inorganic cluster acts as a new dinucleating ligand for 3d-transition metal ions.•Dinuclear 3d-transition metal units are sandwiched between inorganic cubane ligands.•The inorganic cubane ligands interact each other by intramolecular hydrogen bonds.•The intramolecular hydrogen bonds by OH groups have a bifurcated three-centered form.•The dinuclear Fe(III) cluster shows stepwise redox waves due to Fe(III/II).