Synthesis, Characterization, and Evaluation of the Cobalt-Carbon Bond Strength of (PNNP)Co^III-R Complexes

Square pyramidal cobalt-(III)-alkyl complexes ((PNCH₂CH₂NP)-Co^III-R (1-R) and (PNCHCHNP)-Co^III-R (2-R), R = CH₃, ⁿ Bu, or Bn) were synthesized via oxidative addition of alkyl halides to [(PNCH₂CH₂NP)-Co^I]-[Na-(THF)_n] (1-Na) or [(PNCHCHNP)-Co^I]-[Na-(THF)_n] (2-Na). The redox-active ligand of (PNCHCHNP)-Co^II (2) provides access to the one-electron oxidized species [(PNCHCHNP)-Co-(THF)]-[PF₆] (2-PF ₆ ), allowing synthesis of 2-R compounds through alkylation of 2-PF ₆ with carbanions. Radical trapping experiments with TEMPO and analysis of the thermal decomposition products suggest cobalt-carbon bond homolysis as the primary decomposition pathway for these molecules, although the homolysis products are dependent on the equatorial ligand framework and the identity of the alkyl substituent. The stability of the cobalt-carbon bond was evaluated through kinetic and computational methods. We suggest a more distorted equatorial ligand destabilizes the cobalt-carbon bond, while the electron-rich phosphine substituents stabilize the cobalt-carbon bond compared to previously reported cobalamin model compounds. Catalytic investigations into radical Heck-type cross-coupling found (PNCH₂CH₂NP)-Co^II (1) to be a more active catalyst than 2, although unwanted side products resulted in reduced yields for the desired cross-coupled product.