Axial Donor Effects on Oxidatively Induced Ethane Formation from Nickel-Dimethyl Complexes

Tetradentate pyridinophane ligands have been shown to stabilize uncommon high-valent palladium and nickel organometallic complexes. Described herein are the synthesis and detailed characterization of a series of Ni^II- and Ni^III-dimethyl complexes supported by modified tetradentate pyridinophane ligands in which one or both of the N-methyl substituents were replaced with electron-withdrawing p-toluenesulfonyl groups, thus reducing the amine N atom donicity and favoring the formation of Ni complexes with lower coordination numbers. The corresponding Ni^II-dimethyl complexes exhibit accessible oxidation potentials, and their oxidation generates Ni^III species that were characterized by EPR and X-ray crystallography. Moreover, the Ni^II-dimethyl complexes exhibit selective ethane formation upon oxidatively induced reductive elimination using various oxidants - including O₂ and H₂O₂, without the generation of any C-heteroatom products. Overall, these results suggest that the (^RN4)Ni^IIMe₂ complexes with more weakly donating axial ligands are more reactive toward ethane formation, likely due to destabilization of the corresponding high-valent Ni intermediates and formation of 5- and 4-coordinate conformations for these Ni species.