Synthesis and Speciation-Dependent Properties of a Multimetallic Model Complex of NiSOD that Exhibits Unique H-Bonding

Superoxide (O2•‒) is a cytotoxic byproduct of aerobic metabolism. As a result, all aerobic organisms possess superoxide dismutases (SODs) to catalyze the disproportionation of O2•‒ into hydrogen peroxide (H2O2) and oxygen (O2) via alternate oxidation and reduction of their respective catalytic metal centers. In 1996, a new class of SOD was isolated from Streptomyces soil bacteria containing Ni (NiSOD) in an unusual, mixed N/S coordination sphere. The principal question remaining is how this unique coordination sphere promotes both Ni-based redox activity and S-oxidative-stability. One potential mechanism to prevent unwanted S-oxygenation/oxidation chemistry includes the extensive H-bonding networks in the hexameric enzyme. To address the impact of this mechanism on the oxidative-stability of Ni-coordinated thiolates, we have constructed multimetallic Ni-N2S2 complexes as synthetic analogues of NiSOD. We designed and synthesized a trimeric platform utilizing a trithiolate base to connect three monomeric Ni(N2S) complexes to generate new [{Ni(N2S)}3S'] complexes. Our work has shown that these trimetallic complexes interact via unique inter/intra-molecular H-bonding networks, which result in physical and reactive properties distinct from their monometallic counterparts. The presence of H-bonds allows for
speciation that is temperature-, concentration-, and solvent-dependent and has been characterized as a monomeric complex (1M) with intramolecular NH···S bonds and a dimeric complex (1A) that aggregates through intermolecular NH···O=C bonds. Notably, 1A and 1M exhibit remarkable stability in protic solvents such as MeOH and H2O and the reactivity of 1 with excess O2 and O2•− is species-dependent. These results offer new insight into H-bonding in NiSOD and assess the role this mechanism plays in oxidative-stability and catalysis.