Effects of Isolobal Heteroatoms in Divanadium-Substituted γ-Keggintype Polyoxometalates on (OV)2(μ-OH)2 Diamond and (OV)2(μ-0) Core Structures and the Transformation.

Effects of isolobal heteroatoms in divanadium-substituted γ- Keggin-type polyoxometalates, (TBA)4[γ-XV2W10O38(μ-OH)2] IX and (TBA)4[γ-XV2W10O38(μ-O)] 2X (where X = Ge or Si), on (OV)2(μ-OH)2 and (OV)2(μ-O} core structures and transformations from 2X to 1x have been investigated. X-ray crystallography of 1x and 2X reveals that larger Ge (covalent radius 1.22 Å; covalent radius of Si 1.11 Á) induces (a) expansion of (OV)2(μ-OH)2 and (OV)2(μ-O) cores, (b) expansion of lacunary sites, and (c) deep location of divanadium cores inside their lacunary sites. Density functional theory (DFT) calculations for anionic moieties of 1x and 2X reveal that energy levels of the highest occupied molecular orbital (HOMO)--1 in 1Ge and HOMO in 2Ge are lower than those in 1Si and 2Si, respectively, because of smaller contribution of pz orbitals of oxygen atoms in 1Ge and 2Ge, which would result from shorter V┅O(--Ge) distances. Compound 2Ge reacts with water vapor to form (TBA)4[γ-GeV2W10O38(μ-OH)2] 1'Ge via a crystal-to-crystal transformation, and the water dissociation proceeds heterolytically. DFT calculations reveal that the reaction proceeds through (1) coordination of water on a coordinatively unsaturated site of vanadium in the lowest unoccupied molecular orbital (LUMO), followed by (2) proton transfer to the bridging oxo moiety. The order is different from that in 2Si, which would result from the lower energy level of HOMO of 2Ge (i.e., lower nudeophilicity toward a proton of water) than that of 2Si. [ABSTRACT FROM AUTHOR]