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

The synthesis and characterization of (^tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (^tBu PBP)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 (^tBu PBOP)₂ Ni₄ (μ-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 (^tBu P₂ BO)Ni-acyl intermediate (A) that leads to a (^tBu P₂ BO)-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 (^tBu P₂ BO)Ni^II (η² -TEMPO) (7). Additionally, ¹³ C and ¹ H NMR spectroscopy analysis using ¹³ C-enriched CO₂ provides information about the species involved in the CO₂ activation process.