Scorpionate Complexes [(κ ^<i>n</i> -Tp ^Ph,Me )NiS ₂ CNR ₂ ] ( <i>n</i> = 2, 3) as a Structural and Spectroscopic Model for Reduced Nickel-Dependent Superoxide Dismutase

Scorpionato-dithiocarbamate Ni(II) complexes [(Tp^Ph,Me)NiS₂CNR₂], where Tp^Ph,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate and NR₂ = NMe₂, NEt₂, NPh₂, carbazolyl, are advanced as structural and spectroscopic models for the active site of nickel-dependent superoxide dismutase in its reduced state. The Tp^Ph,Me ligand can adopt variable κ²- and κ³-coordination modes in the synthetic complexes that give rise to several ligand field conformations: square-planar, elongated square-pyramidal, square-pyramidal, and trigonal-bipyramidal. The first two are red and diamagnetic (<i>S</i> = 0), and the latter two are green and paramagnetic (<i>S</i> = 1). The different spin states are facilitated by a range of resonance structures within the dithiocarbamate coligand. These different geometric and spin isomers have been captured in the solid state and structurally characterized by X-ray crystallography. Electronic structures of a simplified [(Tp)NiS₂CNMe₂] model in relevant conformations, spin and oxidation states were calculated using Density Functional Theory and compared to the enzyme. Reversible one-electron redox couples were observed at biologically relevant potentials. We propose that the diamagnetic, square-planar and elongated square-pyramidal conformations at Ni(II) are relevant to the base-off and base-on conformations, respectively, reported for NiSOD.