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

Abstract

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–(pK_a 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 (ΔH_pp= 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.