Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of ((PBP)-P-tBu)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of ((PBP)-P-tBu)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster ((PBOP)-P-tBu)(2)Ni-4(mu-CO)(2) (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate ((P2BO)-P-tBu)Ni-acyl intermediate (A) that leads to a ((P2BO)-P-tBu)-Ni-I complex (B) via a likely radical pathway. The Ni-I species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give ((P2BO)-P-tBu)Ni-II(eta(2)-TEMPO) (7). Additionally, C-13 and H-1 NMR spectroscopy analysis using C-13-enriched CO2 provides information about the species involved in the CO2 activation process.