Structural, spectroscopic, and electrochemical properties of a series of high-spin thiolatonickel(II) complexes

A series of high-spin thiolatonickel(II) complexes, [PhTttBu]Ni(SR) (PhTttBu = phenyltris((tert-butylthio)methyl)borate; 2, R = triphenylmethyl; 3, R = pentafluorophenyl; 4, R = phenyl), were synthesized via the reaction of [PhTttBu]Ni(NO3) (1) with thiols (RSH) in the presence of triethylamine. The [PhTttBu]Ni(SR) products were isolated and characterized by various physicochemical measurements including X-ray diffraction analyses. These thiolatonickel(II) complexes have a distorted trigonal pyramidal geometry with somewhat different tau values: 0.80 and 0.90 for two crystalline phases of 2, 0.74 for 3, and 0.69 for 4, where tau is a normalized measure of pyramidalization (tau = 0 for tetrahedron, tau = 1 for trigonal pyramid). The electronic absorption spectra display characteristic sulfur-to-nickel(II) charge transfer (CT) bands at 532 nm (7500 M(-1) cm(-1)) for 2, 510 nm (4800 M(-1) cm(-1)) for 3, and 569 nm (4100 M(-1) cm(-1)) for 4. The cyclic voltammograms show a quasi-reversible redox couple at E1/2 = -1.11 V for 2, and reversible redox couples at E1/2 = -1.03 V for 3 and E1/2 = -1.17 V for 4 (vs Fc+/Fc). Correlation between the tau value and the CT intensity was observed: the strong CT intensity results from the high tau value, which provides for strong orbital overlap (2 > 3 > 4). Additionally, the CT transition energy correlates with the reduction potential: both the CT transition energy and potential decrease in the order 3 > 2 > 4, consistent with the influence of decreasing electron withdrawing abilities, R = pentafluorophenyl > triphenylmethyl > phenyl. The three thiolatonickel complexes exhibit dramatically different thermal stabilities. Complex 4 is the least stable, undergoing decomposition to [kappa2-PhBttBuSPh]Ni(eta2-CH2SBut) (5) via net exchange of Ni-SPh and B-CH2SBut groups.