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

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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.

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