J. Phys. Chem. C 2007, 111, 7711-7719
7711
The Electronic Structure of Reduced Phosphovanadomolybdates and the Implications on Their Use in Catalytic Oxidation Initiated by Electron Transfer Hajime Hirao,† Devesh Kumar,† Hui Chen,† Ronny Neumann,*,‡ and Sason Shaik*,† Department of Organic Chemistry and the Lise Meitner-MinerVa Center for Computational Quantum Chemistry, The Hebrew UniVersity, Jerusalem, Israel 91904, and Department of Organic Chemistry, Weizmann Institute of Science, RehoVot, Israel 76100 ReceiVed: December 18, 2006; In Final Form: March 23, 2007
The geometries and electronic structures of the five isomers of [PV2Mo10O40]5- and its mono- and direduced species were theoretically investigated by means of B3LYP calculations, with an attempt to understand their role as oxidative catalysts in electron-transfer-initiated processes. The calculations reveal the following features: (a) Either in the gas phase or in a solvent, the reduction of the [PV2Mo10O40]5- produces [PV2Mo10O40]6,7- ions in which the excess electrons are localized on the vanadium atoms in the corresponding δ orbitals. By contrast, the direduced [PV2Mo10O40]7- isomers where the one electron is localized on molybdenum are high in energy. Consequently, whereas the five isomers of [PV2Mo10O40]5- have roughly a statistical distribution, in the direduced species, the 1,4-[PV2Mo10O40]7- isomer becomes more stable than others. (b) The gas-phase reduction of the parent [PMo12O40]3- anion is exceedingly more favorable (by ca. 250 kcal/mol) than the reduction of [PV2Mo10O40]5-. By contrast, contribution of the solvent exerts a strong leveling effect and makes the reduction potentials of the two species very similar. (c) Since the reduction of [PV2Mo10O40]5- concentrates high negative charge near the OdVO4 moiety, this negative charge accumulation will act as an attractor that binds the organic radical cation (cation), which causes a subsequent proton transfer to an oxo ligand of the vanadium. As such, in terms of an electron-transfer catalytic effect, the presence of vanadium in [PV2Mo10O40]5- creates function for the catalyst. (d) The computations reveal that [PV2Mo10O40]7is a diradicaloid that possesses two virtually degenerate states, with singlet and triplet spins. Thus, we may expect two-state reactivity (TSR) in the catalytic reactions of [PV2Mo10O40]5-. Some predictions are made based on these features.
Introduction Polyoxometalates are versatile compounds that have been investigated as oxidation catalysts for the past twenty five years or so.1 Research within the field of homogeneous oxidation catalysis on aerobic oxidation catalyzed by phosphovanadomolybdates, [PVxMo12-xO40](3+x)- (x ) 0-6 but most often 2), has attracted considerable attention. The [PV2Mo10O40]5compound is a polyoxometalate of the R-Keggin-type structure, and due to the presence of two vanadium centers, it has five isomers that are depicted in Figure 1. To date, it has not been possible to separate these isomers and to study them individually. The literature also indicates that the isomer distribution may well be a function of the solvent.2 Also, depending on the ratio of Mo/V used in its synthesis, [PV2Mo10O40]5- may contain varying small amounts (usually
