Triplet Resting States Govern E / Z Selectivity and Chemoselectivity in Carbene-Pincer Cobalt-Catalyzed Alkyne Semihydrogenation

Abstract

Rigid, electronically tunable carbene-based pincer ligands constitute a highly promising platform for controlling selectivity in cobalt-catalyzed alkyne semihydrogenation, but the influence of catalyst spin-state characteristics on Z/E selectivity and over-reduction remains poorly understood. Herein, we compare the mechanistic pathways governing Z-selective, E-selective, and overhydrogenation of alkynes catalyzed by the mono(carbene)-based pincer cobalt complex [(PCNHCP)Co(N2)]+. The observed chemoselectivity between Z-selective semihydrogenation and overhydrogenation is found to be primarily from the energy difference between the two triplet resting states─B2triplet and A2triplet, which has not ever been reported. The β-hydride elimination transition state, TSB5-4_Esinglet, lies at a high energy of 17.5 kcal/mol, rendering the E-selective pathway kinetically inaccessible. Steric and electronic analyses indicate that the coordinated E-alkene in the β-hydride elimination step experiences greater steric hindrance and less πC═C → π*carbene electron-transfer stabilization than the Z-alkene, thereby suppressing β-hydride elimination. Comparative analysis reveals that the carbene ligand preferentially stabilizes the singlet state relative to the triplet state in these Co systems, and the stabilizing effect of the carbene ligands can be accumulated across multiple carbene units.