Pentamethylcyclopentadienyl-rhodium and -iridium complexes. Part 33. The metal-catalysed disproportionation of acetaldehyde into acetic acid and ethanol

Abstract

Acetaldehyde is converted into ethanol and acetic acid (1 : 1) by dilute neutral aqueous solutions of pentamethyl-cyclopentadienyl-rhodium or -iridium complexes, e.g.[{Rh(C₅Me₅)}₂(OH)₃]⁺. Areneruthenium complexes catalyse a similar reaction but more acetic acid than ethanol is formed; the ratios change with added ligand. The reaction is first-order in aldehyde and half-order in the binuclear catalysts [{Rh(C₅Me₅)}₂(OH)₃]Cl or [{Ru(C₆Me₆)}₂(OH)₃]Cl. When the reactions using [{Rh(C₅Me₅)}₂(OH)₃]OH were quenched with PF₆^– or when aqueous solutions of [{Rh(C₅Me₅)}₂(OH)₃][PF₆] were reacted with acetaldehyde, µ-hydrido-complexes [{Rh(C₅Me₅)}₂H(O₂CMe)₂][PF₆] and [{Rh(C₅Me₅)}₂H₂(O₂CMe)][PF₆] were isolated. Addition of acetaldehyde to [{Ru(C₆Me₆)}₂(OH)₃][PF₆] gave [{Ru(C₆Me₆)}₂(O₂CMe)₃][PF₆]. In base (pH 12.8) the disproportionation of acetaldehyde was very much (>10⁴ fold) accelerated in the presence of catalyst and gave alcohol and acetate in a 1 : 1 ratio for all catalysts. The rhodium-catalysed reaction competed successfully with the aldol condensation even at high base strength (1.5 mol dm^–3 NaOH) but the ruthenium- and iridium-catalysed reactions were slower. Other aldehydes (EtCHO, PhCHO, MeCHCHCHO) reacted similarly. Hydride complexes [{Rh(C₅Me₅)}₂H(O₂CR)₂][PF₆] could again be isolated from the neutral regime reactions and addition of alkali caused significant accelerations in rate. The mechanism is suggested to be related to that of the Cannizzaro reaction, except that the attack by the aldehyde hydrate anion is at the metal to which hydride transfer initially occurs. Formation of the carboxylate (or carboxylic acid) and of the alcohol (or alkoxide) thus occur in the co-ordination sphere of the complex. A variety of non-half-sandwich complexes of Rh and Ru as well as the metals themselves were essentially inactive.