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 O 2  and H 2 O 2 , without the generation of any C-heteroatom products. Overall, these results suggest that the ( R N4)Ni II Me 2  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.