A series of half-sandwich complexes of iridium, rhodium, and ruthenium are shown to be active catalysts for the conversion of aldehydes and water to carboxylic acids. Depending on the catalyst, H2 is either released (the “aldehyde–water shift”) or transferred to a second equivalent of aldehyde (aldehyde disproportionation). Mechanistic studies suggest hydride transfer to be the selectivity-determining step along the reaction pathway. Using [(p-cymene)­Ru­(bpy)­OH2]­[OTf]2 as precatalyst, we demonstrate a novel example of a highly selective aldehyde–water shift in the absence of a hydrogen acceptor or base.

A family of (para-cymene)Ru II complexes are shown to be competent precatalysts for the oxidation of aldehydes to carboxylic acids using water as the oxidant. This reaction, known as the "aldehyde−water shift" (AWS), has been previously demonstrated to be in competition with aldehyde disproportionation. For the few reported mononuclear catalysts for this reaction, either high selectivity for AWS and low conversion or low AWS selectivity and high conversion is observed. A homogeneous precatalyst which is both highly selective for the desired AWS and is highly efficient for conversion of the aldehyde to products is reported herein. In addition, catalyst activity is found to be general to a variety of sterically unencumbered aliphatic aldehydes producing the corresponding carboxylic acid and hydrogen gas.