Homogeneous catalytic synthesis of formaldehyde using the tungsten carbonyl complex [(CO)5WCl]− in the presence of sodium methoxide

Formaldehyde can be synthesized (Turnover number=10) from sodium methoxide and carbon dioxide using the anionic tungsten carbonyl complex [(CO)₅WCl]⁻ as catalyst precursor and a molar ratio NaOMe/W lower than 8 at 125°C, 400 psi of CO₂ for a 24-h period. The most probable mechanism involves the generation of the [(CO)₅WOCH₃]⁻ species by the reaction of [(CO)₅WCl]⁻ with NaOMe. The methoxide complex can undergo β-hydrogen abstraction to yield formaldehyde and the terminal hydride [(CO)₅WH]⁻, which in turn, decomposes under reaction conditions to provide the bridging hydride [(μ-H)W₂(CO)₁₀]⁻. Carbon dioxide insertion into the complex [(CO)₅WH]⁻, followed by reaction of NaOMe regenerates the alkoxide compound [(CO)₅WOCH₃]⁻ and sodium formate. A kinetic study of the reaction of [(CO)₅WCl]⁻ with NaOMe suggests that the mechanism involves nucleophilic attack of the base on the metal center, and proceeds by two different pathways depending on the molar ratio base/metal. For a ratio NaOMe/W < 8 the β-hydrogen reaction takes place with the formation of [(μ-H)W₂(CO)₁₀]⁻ and formaldehyde. For higher base/metal ratio (>8) the formation of metal cluster is observed.