Dimerization of Aldehydes into Esters by an Octahedral d⁶‑Rhodium <i>cis</i>-Dihydride Catalyst: Inner- versus Outer-Sphere Mechanisms

The tripodal ligated octahedral complex d6-[Rh­(PhB­(CH2PPh2)3)­(H)2(NCMe)] (1-RhH) was discovered by Tejel and co-workers to catalyze the Tishchenko reaction in which two aldehydes are dimerized into an ester. Two fundamentally different mechanisms can be envisaged for this system: (i) an inner-sphere mechanism starting with substitution of the acetonitrile ligand of 1-RhH by an aldehyde and (ii) an outer-sphere mechanism starting with direct insertion of an aldehyde into a Rh–H bond of the intact 1-RhH to make an octahedral Rh-alkoxide intermediate. We use DFT methods to investigate the two mechanisms. The inner-sphere mechanism is computed to be energetically favorable. The outer-sphere one, in contrast, is prohibitively high in energy. This is opposite to catalysis of the same reaction using Gusev’s pincer-ligated octahedral catalyst trans-[(PHNN)­Os­(H)2(CO)] (2-OsH) where the outer-sphere mechanism was previously reported to have very low energy. The different behaviors of 1-RhH and 2-OsH can be attributed to a role from the different metals in the two catalysts as well as a role from their different ligands. Specifically, the higher oxidation state of the metal in 1-RhH, Rh­(III) versus Os­(II), greatly diminishes its thermodynamic hydricity leading to separated ions compared to 2-OsH, whereas the amino functionality of the ligand in 2-OsH greatly favors the kinetic hydricity in the reaction with an aldehyde by hydrogen bonding with the carbonyl group being reduced. Comparisons are also made with Milstein’s trans-[PNN-Ru­(H)2(CO)] alcohol dehydrogenative coupling catalyst (3-RuH) which also lacks the amino functionality.