Probing the electronic structure of early transition metal oxide clusters: Molecular models towards mechanistic insights into oxide surfaces and catalysis

Selected recent works from the authors’ laboratory on the intrinsic electronic and structural properties of early transition metal oxide clusters are reviewed. These clusters provide well-defined molecular models pertinent to mechanistic understandings of complex oxide surface chemistry and catalysis. The energy gap evolution with cluster size was probed for the stoichiometric (TiO2)n−, (V2O5)n−, and (CrO3)n− clusters, and each system was shown to approach the band gap of bulk oxides in a unique way. A variety of other model clusters have been characterized, such as the oxygen radical or diradical on a single W6+ site in WO4−/WO4, the superoxide (WO3)n(O2−) complexes for dioxygen activation, and terminal versus bridging oxygen in M3O2− (M = Nb, Ta) clusters. Novel chemical bonding has been observed in a number of oxide clusters. The W3O9− and W3O92− clusters were found to possess d-orbital aromaticity, whereas δ-aromaticity was discovered in the Ta3O3− cluster.