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

Abstract

Nickel superoxide dismutase (NiSOD) is a metalloenzyme that converts O2*- into H2O2 and O2 by cycling between Ni(II) and Ni(III) oxidation states. Reduced NiSOD contains Ni(II) in a square-planar N2S2 coordination environment formed by two cysteinate S atoms, an amide N, and an amine N to Ni(II). [Me4N](Ni(II)(BEAAM)) represents the first NiN2S2 complex containing Ni in a mixed amine/amide environment. [Me4](Ni(II)(BEAAM)) contains Ni-S bonds at 2.177(2) and 2.137(2) A and Ni-N bonds at 1.989(7) and 1.858(6) A, which compare well with the metalloenzyme. Orange solutions of [Me(4)N](Ni(II)(BEAAM)) in MeCN are diamagnetic and stable toward O2 for weeks. A quasireversible Ni(II/III) redox couple is observed for [Ni(II)(BEAAM)](NMe4) at 0.12(1) V vs Ag/AgCl. These data suggest that NiSOD utilizes the mixed amine/amide ligands to modulate the Ni(II/III) redox couple to best match the O2*- reduction/oxidation couples while maintaining O2 stability.