Mechanism of Alcohol–Water Dehydrogenative Coupling into Carboxylic Acid Using Milstein’s Catalyst: A Detailed Investigation of the Outer-Sphere PES in the Reaction of Aldehydes with an Octahedral Ruthenium Hydroxide

In aqueous basic media, the square-pyramidal complex [Ru­(PNN)­(CO)­(H)] (<b>1</b>-Ru, where PNN is a dearomatized bipyridyl-CH-P^tBu₂ pincer ligand) catalyzes the transformation of alcohols and water into carboxylates and H₂. A previous theoretical investigation reported the following mechanism for the reaction: (i) metal-catalyzed dehydrogenation of the alcohol into an aldehyde, (ii) metal–ligand cooperation (MLC) addition of water to <b>1</b>-Ru to give an octahedral ruthenium hydroxide (<b>2</b>-Ru–OH), (iii) concerted MLC hydration of the aldehyde by <b>2</b>-Ru–OH to give separated <b>1</b>-Ru and a gem-diol, and (iv) concerted MLC dehydrogenation of the gem-diol by <b>1</b>-Ru into an octahedral ruthenium dihydride (<b>2</b>-Ru–H) and a carboxylic acid. We calculate the outer-sphere PES in the reaction between the aldehyde and <b>2</b>-Ru–OH to start with a localized coupling step yielding an ion-pair minimum (<b>7</b>-ip-OH) in which the hydroxyl group of an α-hydroxyl-alkoxide (gem-diolate) is coordinated to the metal of a cationic square-pyramidal complex. From <b>7</b>-ip-OH, we identify a route to carboxylic acid that circumvents ligand deprotonation involving (i) 1,1-rearrangement of the gem-diolate within the contact ion pair through an α-OH/O^– slippage TS into the octahedral <b>2</b>-Ru–OCH­(OH)­R and (ii) a second 1,1-rearrangement through an α-O^–/H slippage TS that gives a new ion-pair minimum in which the α-hydrogen of the anion is coordinated to the metal, followed by a localized hydride-transfer TS that gives a carboxylic acid and the octahedral hydride complex (<b>2</b>-Ru–H). The net transformation from <b>2</b>-Ru–OH and the aldehyde to the carboxylic acid and <b>2</b>-Ru–H can be viewed as a H/OH metathesis in which a hydride and a hydroxide are exchanged between the acyl group of the aldehyde and the metal center of <b>2</b>-Ru–OH. The MLC mechanism gives the same metathesis products through the intermediacy of a gem-diol. When the SMD solvent continuum model is applied during geometry optimization with water as the solvent, the Gibbs free energy profile of the slippage pathway is predicted to be much lower than that predicted for MLC. The possibility of dissociation of the ion pair <b>7</b>-ip-OH into free ions and reassociation is also briefly addressed. Some calculations are also performed to address why no esters are observed in the given system