Scorpionato-dithiocarbamate Ni(II) complexes (TpPh,Me)NiS2CNR2, where TpPh,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 TpPh,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 (S = 0), and the latter two are green and paramagnetic (S = 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.
Ma et al. (Wed,) studied this question.