Reduction of CO₂and Other Substrates Using Photochemical Reactions of The W₂(CO)₁₀²“ Complex

The photochemistry of the W₂(CO)₁₀^2-complex was investigated with the goal of determining if irradiation of this dimer generates a powerful reducing agent, presumably a 19-electron species. In general, the photochemistry of the W₂(CO)₁₀^2-complex is comparable to that of other metal-metal-bonded carbonyl dimers. Irradiation into the low-energy tail of the dπₒσ7* electronic transition of the W₂(CO)₁₀²~ complex led to W-W bond homolysis. The resulting 17-electron W(CO)₅~ radicals could be trapped with suitable ligands such as 4-cyanopyridine to give “19-electron adducts”. (See ref 3 for an important definition of the phrase “19-electron adduct”.) Evidence is presented that the ligands PPh₃and PBu₃also react with photogenerated W(CO)₅“ to form adducts: W(CO)₅“ + PR_3-°W(CO)₅PR₃'. These adducts are powerful reducing agents, and they were used to reduce C0₂to formate and CO. The only organometallic product formed in the reaction was W(CO)₅PPh₃, the oxidized form of the 19-electron complex. In a similar manner, Mn₂(CO)₁₀was reduced to Mn(CO)₅“, Cp₂Co⁺to Cp₂Co, benzophenone to the radical anion, and methylviologen (MV²⁺) to MV⁺. Alternative reduction mechanisms involving the W(CO)₅~ radical, W(CO)₅²“, or HW₂(CO)₁₀” as reductants were shown not to be operating. Nineteen-electron complexes generated by irradiation of Cp₂Mo₂(CO)₆proved incapable of reducing CO₂.