Structural, Spectroscopic, and Electrochemical Properties of a Series of High-Spin Thiolatonickel(II) Complexes

A series of high-spin thiolatonickel(II) complexes, [PhTt^tBu]Ni(SR) (PhTt^tBu = phenyltris((tert-butylthio)methyl)borate; 2, R = triphenylmethyl; 3, R = pentafluorophenyl; 4, R = phenyl), were synthesized via the reaction of [PhTt^tBu]Ni(NO₃) (1) with thiols (RSH) in the presence of triethylamine. The [PhTt^tBu]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 τ values:  0.80 and 0.90 for two crystalline phases of 2, 0.74 for 3, and 0.69 for 4, where τ is a normalized measure of pyramidalization (τ = 0 for tetrahedron, τ = 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 E_1/2 = −1.11 V for 2, and reversible redox couples at E_1/2 = −1.03 V for 3 and E_1/2 = −1.17 V for 4 (vs Fc⁺/Fc). Correlation between the τ value and the CT intensity was observed:  the strong CT intensity results from the high τ 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 [κ²-PhBt^tBuSPh]Ni(η²-CH₂SBu^t) (5) via net exchange of Ni−SPh and B−CH₂SBu^t groups.