Aryl−aryl bond formation through dehydrogenative coupling is an attractive transformation due to the atom and step economy of working with unfunctionalized C−H bonds. Pd catalysts are most common for this process, but Ni complexes have also been targeted as less expensive and more abundant alternatives. Here we report the ability of Ni 0 to activate a sterically encumbered phosphino-pyridine ligand with resulting dehydrogenative aryl−aryl homocoupling. The net H 2 equivalent is transferred to the coligand, resulting in hydrogenation of an olefin unit. We have investigated the mechanism of this process by stirring Ni(1,5-COD) 2 (COD = cyclooctadiene) and the PNPh ligand (PNPh = 2-((di-tert-butylphosphino)methyl)-6phenylpyridine) at mild temperatures to afford a Ni II complex. Isolation and characterization shows a PNPh ligand coordinated to Ni II through an activated pyridine carbon and the directing phosphine. The coligand is an activated allylic cyclooctenyl fragment resulting from partial hydrogenation of 1,5-COD. We propose that this intermediate reacts intermolecularly with 1 equiv of itself, enabling the isolation of a bi-PNPh compound (bi-PNPh = 2,2′-bis((di-tert-butylphosphino)methyl)-6,6′diphenyl-3,3′-bipyridine), coupled through the activated pyridyl position. This dehydrogenative coupling, although stoichiometric, demonstrates the potential of Ni 0 -mediated C(sp 2 )−H activation and homocoupling for synthetic applications.
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