Catalytic and mechanistic studies of some anionic transition metal carbonyl hydrides

Abstract

As PPN⁺ (bistriphenylphosphineimminium) salts the anionic metal hydrides HFeM(CO)₉⁻ (M = Cr, Mo, W) were shown to be olefin isomerization catalysts under mild conditions (25°C, fluorescent lighting) toward the conversion of allylbenzene to the cis- and trans-propenylbenzenes and 1-hexene to internal olefins. The use of DFeM(CO)₉⁻ as an olefin isomerization catalyst was found to lead to the incorporation of d-label into the olefinic products. The activity of HFe(CO)₄⁻ was examined in the presence of alkali metal ions (Li⁺ or Na⁺) as well as in the presence of hydride abstracting agents (Ph₃C⁺ or BF₃). Evidence presented suggests the role of M(CO)₅⁰ to be similar to the alkali cations in promoting CO labilization and thus promoting catalysis on the Fe-H⁻ center. The reaction of H₂ with species of the type BM(CO)₅⁻ (B = Bronsted base; M = Cr, Mo, W) was shown to lead to the formal products of heterolytic cleavage, HB and HM(CO)₅⁻. For B = OAc⁻ or HM(CO)₅⁻, the catalytic hydrogenation of aldehydes, ketones, and α,β-unsaturated ketones was carried out. Mechanistic models consistent with observations made using in situ high pressure FTIR and isotopic labelling studies include (1) ligand assisted heterolytic activation of H₂, and (2) an alkoxide stabilized oxidative addition (dihydride) product or a base stabilized η²-H₂-metal carbonyl intermediate. Subsequent steps involve hydride attack on the carbonyl carbon followed by protonation to produce the alcohol product. For M(CO)₅R⁻, with B formally equal to R⁻ (R = Me, Ph, Bzl), products of simple elimination (or R⁻/H⁻) exchange), RH and HM(CO)₅⁻, were observed for M = W with activities Me > Ph >> Bzl. For M = Cr, carbonyl insertion products, i.e., aldehydes and alcohols were observed. Comparison studies were performed on the anionic iron alkyls, RFe(CO)₄⁻ (R = Me, Bzl). Like the W alkyls, products were those of simple elimination, RH and HFe(CO)₄⁻. A mechanistic model consistent with observed results involves the intermediacy of a molecular H₂ complex for the group 6 complexes. For the iron alkyls, the oxidative addition product RFe(H)₂(CO)₃⁻ is assumed to be an intermediate. For the heterobimetallic system RFeW(CO)₉⁻, with B = RFe(CO)₄⁻ (R = H, Me), products formed were HFe(CO)₄⁻, W(CO)₆, μ-HW₂(CO)₁₀⁻, and HFe₃(CO)₁₁⁻. Mechanisms proposed include (1) CO labilization of the heterobimetallic system followed by oxidative addition of H₂ on the Fe center, and (2) heterolytic activation of dihydrogen between the metal centers.