A mixed-addenda Nb/W polyoxometalate containing dimeric Dawson subunit: Synthesis, structure, and characterization

A mixed-addenda Nb/W polyoxometalates, K4Na4[H6P2W12Nb4O59(NbO2)2]2 · 48H2O (1), has been isolated under acidic condition. The single crystal structural analysis showed that the six vacant sites of the lacunary {P2W12} precursor are occupied by six Nb atoms, two at the polar sites and four at the equatorial sites to form a {P2W12Nb6} mixed-addenda subunit. Two {P2W12Nb6} subunits are joined together in an inversion mode by two Nb–O–Nb bridges from the Nb atoms at equatorial sites to form a dimeric unit, which can be considered as a Nb12 core wrapped by two lacunary {P2W12} polyoxoanions. It is notable that the terminal peroxo groups bonded niobium atoms in the Nb12 core are only partially eliminated during the formation of the dimeric unit.