Cobalt Pincer Complexes in Catalytic C-H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans-((PNP)-P-iPr)-CoH2(BPin) as the resting state in the borylation of five-membered heteroarenes and 4-BPin-((PNP)-P-iPr)Co(N-2)BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-((PNP)-P-iPr)-CoBPin (R = H, BPin), were generated by reductive elimination of H-2 in the former, through Berry pseudorotation to the cis isomer and N-2 loss in the latter. The catalytic mechanism of the resulting Co(I) complex was computed to involve three main steps: C H oxidative addition of the aromatic substrate (C6H6), reductive elimination of PhBPin, and regeneration of the active complex. The oxidative addition product formed through the most favorable pathway, where the breaking C-H bond of C6H6 is parallel to a line between the two phosphine atoms, leaves the complex with a distorted PNP ligand, which rearranges to a more stable complex via dissociation and reassociation of HBPin. Alternative pathways, sigma-bond metathesis, and the oxidative addition in which the breaking C-H bond is parallel to the Co-B bond are predicted to be unlikely for this Co(I) complex. The thermodynamically favorable formation of the product PhBPin via reductive elimination drives the reaction forward. The active species regenerates through the oxidative addition of B(2)Pin(2) and reductive elimination of HBPin. In the overall reaction, the flipping (refolding) of the five-membered phosphine rings, which connects the species with two phosphine rings folded in the same direction and that with them folded in different directions, is found to play an important role in the catalytic process, as it relieves steric crowding within the PNP ligand and opens Co coordination space. Metal ligand cooperation based on the ligand's aromatization/dearomatization, a common mechanism for heavy-metal pincer complexes, and the dissociation of one phosphine ligand do not apply in this system. This study provides guidance for understanding important features of pincer ligands with first-transition row metals that differ from those in heavier metal complexes.