Characteristics of five-coordinate nickel-cysteine centers

Monomeric five-coordinate nickel-cysteine complexes were prepared using ani onic tris(3,5-disubstituted pyrazolyl)borates (Tp* (-) and Tp(PhMe-)) and l -cysteine (ethyl ester and amino acid forms). Tp*NiCysEt crystallizes with a single methanol of solvation in the monoclinic space group P2(1): a = 7.8 145(18), b = 24.201(6), c = 7.9925(14) Angstrom; beta = 117.991(16)degrees. [Tp*NiCys(-)][K+] and Tp(PhMe)NiCysEt show magnetic and electronic charact eristics similar to Tp*NiCysEt, so that the trigonal bipyramidal coordinati on geometry confirmed for Tp*NiCysEt in the solid state likely applies to a ll three. All three complexes have high spin magnetic ground states at room temperature (mu(eff) = 2.9-3.2 mu(B), S =1). Their electronic spectra are dominated by sulfur to nickel charge-transfer bands (388-430 nm in chlorofo rm) with energies that correlate to respective thiolate basicities and Tp(X -) donor strengths. The Tp* derivatives undergo a rapid reaction with molec ular oxygen. Stoichiometric, infrared, and electronic spectroscopy measurem ents are consistent with formation of a sulfinate as a result of reaction w ith dioxygen. Kinetics measurements for the reaction of Tp*NiCysEt and O-2 fit the following composite rate law: rate k(1)[Tp*NiCysEt] + k(2)[O-2][Tp* NiCysEt] with k(1) = 0.013(1) min(-1) and k(2) = 4.8(1) M-1.min(-1) at 22 d egrees C. Increased nucleophilicity of the nickel-sulfur center enhanced by electron donation from Tp*- (vs Tp(PhMe-)) and encouraged by a trigonal bi pyramidal geometry (vs square planar Ni(CysEt)(2)) is hypothesized as the r eason for the susceptibility of Tp*NiCys complexes to oxygen.