Photocatalytic dimerization of olefins by decatungstate(VI), [W10O32]4–, in acetonitrile and magnetic resonance studies of photoreduced species

U.v. photolysis of homogeneous solutions containing tetrakis(tetrabutylammonium)decatungstate(VI), [NBu4]4[W10O32], and olefins in acetonitrile leads to dimerization of olefins via abstraction of an allyl hydrogen, with accompanying formation of reduced decatungstates, up to two electrons. Coexistence of heterogeneous catalysts such as RuO2 and Pt results in a hydrogen-forming reoxidation of the protonated reduced decatungstate to [W10O32]4–, establishing a catalytic dimerization of olefins with evolution of hydrogen. The photoassisted catalytic dimerization is sustained over 50 h. Flash photolysis experiments indicate that the O → W charge-transfer excited state of [W10O32]4– undergoes an electron transfer from olefins to yield [W10O32]5–. Production of [W10O32]5– is followed by protonation to give [HW10O32]4–(pKa for [HW10O32]4–= 5.4 ± 0.5), which competes with a back reaction between [W10O32]5– and a semi-oxidized olefin with a rate constant of (7.2 ± 0.2)× 1010 dm3 mol–1 s–1. Disproportionation of [HW10O32]4– to [H2W10O32]4– and [W10O32]4– occurs with rate constants of (2.6–10.3)× 104 dm3 mol–1 s–1 which increase with increasing ratio of olefin concentration in the solvents. The peak to peak linewidth (ΔHpp= 20 G) of the e.s.r. signal due to [W10O32]5– or [HW10O32]4– is almost constant over the range 4.4–100 K. In connection with the single-crystal e.s.r. spectrum of [HW10O32]4– at 77 K, this is attributed to the semi-occupied molecular orbital consisting of orbital mixing among four sets of OW–O–WO multiple bonds at eight equatorial WO6 sites. At temperatures higher than 100 K, line broadening with increasing temperature is observed and gives an activation energy of 0.06 eV for an electron hopping between equatorial and capped WO6 sites. 183W N.m.r. chemical shifts for [H2W10O32]4– are discussed in terms of two effects on the electron delocalization, the orbital-mixing delocalization among equatorial WO6 sites and the thermally activated hopping process between equatorial and capped WO6 sites (at T > 100 K): the former results in an upfield shift relative to the resonance for the eight equatorial W atoms of [W10O32]4–, while the latter causes a downfield shift relative to the resonance for the two capped W atoms.