ANIONIC GROUP-6B METAL-CARBONYLS AS HOMOGENEOUS CATALYSTS FOR CARBON-DIOXIDE HYDROGEN ACTIVATION - THE PRODUCTION OF ALKYL FORMATES
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The production of alkyl formates from the hydrocondensation of carbon dioxide in alcohols utilizing anionic group 6B carbonyl hydrides as catalysts is herein reported. HM(CO)5- (M = Cr, W; derived from -H[M2(CO)10]-) and their products of carbon dioxide insertion, HCO2M(CO)5-, have been found to be effective catalysts for the hydrogenation of CO2 in alcohols under rather mild conditions (loading pressures of CO2 and H2, 250 psi each, and 125 C) to provide alkyl formates. The only metal carbonyl species detected in solution via infrared spectroscopy, both at the end of a catalytic period and during catalysis, were M(CO)6 and HCO2M(C0)5-. The metal hexacarbonyls were independently shown to be catalytically inactive. A catalytic cycle is proposed which initially involves release of formic acid from the metal center, either by reductive elimination of the hydrido formato ligands or ligand-assisted heterolytic splitting of dihydrogen with loss of formic acid. In a rapid subsequent process HCOOH reacts with alcohols to yield HCOOR. The addition of carbon monoxide retards alkyl formate production, strongly implying CO2 to be the primary source of the carboxylic carbon atom in HCOOR. This was verified by carrying out reactions in the presence of HCO2W(13CO)5- which provided only H12COOR after short reaction periods. However, in the absence of hydrogen and carbon dioxide -H[M2(CO)10]- species were observed to be effective catalyst precursors for converting CO and methanol into methyl formate. 1984, American Chemical Society. All rights reserved.