Cooperative H₂ Activation across a Metal–Metal Multiple Bond and Hydrogenation Reactions Catalyzed by a Zr/Co Heterobimetallic Complex

Abstract

In the quest for active and selective catalysts featuring nonprecious metals, bimetallic cooperativity poses a unique opportunity to promote catalytic reactions and influence selectivity. While examples of stoichiometric H2 activation across metal–metal bonds have been reported, there have been limited advances toward the incorporation of a well-defined cooperative bimetallic H2 activation process into a catalytic cycle for the hydrogenation of unsaturated hydrocarbons. Herein, we demonstrate that facile activation of H2 by two nonprecious metals is facilitated by metal–metal cooperativity in the coordinatively unsaturated Zr/Co bis(phosphinoamide) complexes (THF)(I)Zr(XylNPiPr2)2CoPR3 (3-PMe3 and 3-PMePh2, Xyl =3,5-dimethylphenyl), which feature highly polar Zr–Co triple bonds. Owing to the stabilizing nature of the metal–metal bond, the H2 activation products (THF)(I)Zr(μ-H)(XylNPiPr2)2Co(H)(PR3) (4-PMe3 and 4-PMePh2), which feature one terminally bound Co hydride and one hydride bridging the two metals, have been isolated and crystallographically characterized. The Zr/Co bimetallic complex 3-PMePh2 is an active catalyst for the hydrogenation of alkenes and semihydrogenation of alkynes, and relevant intermediates including 4-PMePh2 and alkene (5) and alkyne (6) adducts have been identified spectroscopically in situ and isolated and characterized through independent synthesis. The alkyne semihydrogenation reaction catalyzed by 3-PMePh2 exhibits high selectivity for alkene over alkane products and generates an unselective distribution of (E)- and (Z)-alkenes via direct formation of both stereoisomers. These findings lend insight into the roles that metal–metal bonding and cooperativity play in the activation of small molecules and the promotion and selectivity of subsequent catalytic transformations.