Catalytic olefin hydrogenation reactions are ubiquitous in organic synthesis. Most proposed catalytic cycles for the homogeneous hydrogenation of olefins using molecular H2 start with the oxidative addition of H2 by metal complexes to form two reactive M–H bonds, often via a non-classical metal dihydrogen (M–H2) intermediate. Previous reports had provided indirect evidence for an alternative mechanism involving direct hydrogen transfer from a metal-bound H2 molecule to a metal-bound olefin without the oxidative addition step. However, the key metal(olefin)(H2) and the corresponding ligand-to-ligand hydrogen transfer (LLHT) step had not been directly observed. Herein, we show that incorporating a precoordinated olefin in a P(C=C)P pincer ligand framework allows for the observation of both a non-classical Ni-(H2) complex and the Ni(alkyl)(hydrido) product of LLHT in rapid equilibrium with dissolved H2. The utility of this cooperative H2-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene under mild conditions. Mechanistic investigations supported by DFT calculations back the central role of LLHT for both cooperative H2 activation and catalytic semihydrogenation. These results provide an experimental basis for the role of LLHT steps in olefin hydrogenation mechanisms and demonstrate the utility of olefin-based pincer ligands for cooperative catalysis with non-noble metals.
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