Isolation, X-ray single crystal and theoretical study of quinquevalent metal oxoisopropoxides, Nb6O8(iPrO)14(iPrOH)2 and Re4O6(OiPr)10

Oxoisopropoxide complexes of niobium, Nb6O8(iPrO)14(iPrOH)2 (I), and rhenium, Re4O6(OiPr)10 (II), were isolated as byproducts of anodic oxidation of these metals in iPrOH in the presence of LiCl as conductive additive. The common feature of both structures consists in the occurrence in their molecules of an M4O16 planar core formed in the formal absence of M–M bonding for I and in the presence of electron-deficient but surprisingly short (Re–Re 2.52–2.54 Å) metal–metal bonds in II. The stability of this core for the oxoalkoxide derivatives of rhenium(V,VI) and niobium(V) and poor stability for those of tantalum(V) are discussed based on the results of quantum-chemical calculations.