Carbene-for-Phosphine Substitution Inverts the Mechanism for Hydrogen-Borrowing Amination

We introduce 1, a (pyridyl)-N-heterocyclic carbene-ligated ruthenium(II) homolog of catalyst 2, and show that the NHC ligand prevents catalyst cluster formation and catalyst death while changing the rate-determining step to imine hydrogenation. We previously introduced (pyridyl)phosphine-ligated ruthenium(II) precursor (2) as a hydrogen-borrowing amination catalyst, but it undergoes carbonylative deactivation at high catalyst loadings (>2.5 mol %), converting from an active metal cluster to inactive dimers. Stunningly, we show here that carbene for phosphine ligand modification results in a longer-lived catalyst, an inverted Hammett plot, orthogonal substrate compatibility, and a change of rate-limiting step.