Reversible cyclometalation at RhI as a motif for metal-ligand bifunctional bond activation and base-free formic acid dehydrogenation

Reversible cyclometalation is demonstrated as a strategy for the activation of small protic molecules, with a proof-of-principle catalytic application in the dehydrogenation of formic acid in the absence of an exogenous base. The well-defined Rh-I complex Rh(CO)(L) 1, bearing the reactive cyclometalated PN(C) ligand L (L-H = PNCH = 2-di.tert-butylphosphinomethyl)-6-phenylpyridine), undergoes protonolysis of the Rh-C-Ph bond with weak protic reagents, such as thiols and trifluoromethanesulfonamide. This system also displays bifunctional metal-ligand protonolysis reactivity with formic acid and subsequent decarboxylation of the formate complex. Density functional theory (DFT) calculations show that H-2 evolution from putative Rh(CO)(H)(L-H) complex A is very facile, proposedly encompassing formal C-H oxidative addition at Rh to give C via agostic intermediate B and subsequent reductive elimination of H-2. Complex 1 is a catalytically competent species for base-free formic acid dehydrogenation, with the intermediacy of formate complex 4. DFT calculations reveal accessible barriers for involvement of a flanking phenyl group for both initial activation of formic acid and release of H-2, supporting a cooperative pathway. Reversible C-H activation is thus a viable mechanism for metal-ligand bifunctional catalysis.