[Me₄N](Ni^II(BEAAM)): A Synthetic Model for Nickel Superoxide Dismutase That Contains Ni in a Mixed Amine/Amide Coordination Environment

Nickel superoxide dismutase (NiSOD) is a metalloenzyme that converts O2•- into H2O2 and O2 by cycling between NiII and NiIII oxidation states. Reduced NiSOD contains NiII in a square-planar N2S2 coordination environment formed by two cysteinate S atoms, an amide N, and an amine N to NiII. [Me4N](NiII(BEAAM)) represents the first NiN2S2 complex containing Ni in a mixed amine/amide environment. [Me4N](NiII(BEAAM)) contains Ni−S bonds at 2.177(2) and 2.137(2) Å and Ni−N bonds at 1.989(7) and 1.858(6) Å, which compare well with the metalloenzyme. Orange solutions of [Me4N](NiII(BEAAM)) in MeCN are diamagnetic and stable toward O2 for weeks. A quasireversible NiII/III redox couple is observed for [NiII(BEAAM)](NMe4) at 0.12(1) V vs Ag/AgCl. These data suggest that NiSOD utilizes the mixed amine/amide ligands to modulate the NiII/III redox couple to best match the O2•- reduction/oxidation couples while maintaining O2 stability.