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

Abstract

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 CO 2 for a 24-h period. The most probable mechanism involves the generation of the [(CO) 5WOCH 3] − 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)W 2(CO) 10] −. Carbon dioxide insertion into the complex [(CO) 5WH] −, followed by reaction of NaOMe regenerates the alkoxide compound [(CO) 5WOCH 3] − 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)W 2(CO) 10] − and formaldehyde. For higher base/metal ratio (>8) the formation of metal cluster is observed.