Challenging The Metal-Ligand Bifunctional Mechanism Pavel A. Dub1 Brian L. Scott2 and John C. Gordon1 1Chemistry Division, and 2Materials and Physics Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States jgordon@lanl.gov Abstract Such a mechanism implies that carbonyl compounds are reduced in the outer-sphere via a concerted multi-bond transfer of both the hydride from the metal and the proton from an N–H group within the ligand, respectively. Within this pericyclic transition state, the nitrogen containing chelating ligand thus directly participates in the act of proton transfer (in concert with hydride transfer from the metal) via its N–H group and is said to be “non-innocent”. This mechanism is usually supported by gas-phase computations, wherein the corresponding multi-bond type transition state is indeed located. However, recent state-of-the-art computional analysis which includes solvent effects suggests that such a pericyclic transition state does not physically exist in solution.1 In turn, the reduction of a carbonyl substrate proceeds via one-bond processes as follows; (a) Outer-sphere hydride transfer from the metal center followed by; (b) protonation of the anion within the resultant ion-pair. The proton source may be either the NH group of the ligand (i.e. the ligand is non-innocent), a h2-H2 ligand or a protic solvent molecule (i.e the ligand is innocent in the latter two cases). The metal-ligand bifunctional mechanism has also been supported via alkylation of the NH functionality within these ligands, which results in a dramatic decrease in catalytic activity. Herein we present the first experimental evidence that such a substitution does not inhibit catalytic activity, and in fact in some cases, can lead to more active catalysts for the hydrogenation of ketonic substrates. References (1) Dub, P. A.; Gordon, J. C. Invited Review, ACS Catalysis, submitted.

Challenging the metal-ligand bifunctional mechanism

251st National Meeting (2016)

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