Steric titration of arylthiolate coordination modes at pseudotetrahedral nickel(II) centers

Several derivatives of the pseudotetrahedral phenylthiolate complex Tp ^Me,MsNi-SPh (1), Tp^Me,Me- = hydrotris(3,5dimethyl-1- pyrazolyl)borate, were prepared incorporating substituted arylthlolates, including a series of orthosubstituted ligands Tp^Me,MeNi-SR (R = 2,6-Me₂C₆H₃,2; 2,4,6-Me₃C ₆H₂,3; 2,4,6- Pr₃C₆H₂,4; and 2,6-Ph₂C₆H₃,5) and a series of para-substituted complexes (R = C₆H₄-4-OMe, 6; C ₆H₄-4-Me, 7; and C₆H₄-4-CI, 8). The products were characterized by ¹H NMR and UV-vis spectroscopy. Spectra of 6-8 were consistent with retention of a common structure across the para-substituted series with modest perturbation of the spectral features of 1 assisting their assignment. In contrast, spectra of 2-5 were indicative of a significant change in configuration across the orthodisubstituted series. The structure of complex 5 was determined by X-ray crystallography and a distinctive arylthiolate ligation mode was found, in which the N₃S ligand field was significantly distorted toward a sawhorse, compared to a more common trigonal pyramidal shape (e.g., 1). Moreover, the arylthiolate substituent rotated from a vertical orientation co-directional with the pyrazole rings and disposed between two of them in 1, to a horizontal orientation perpendicular to and over a single pyrazole ring in 5. This reorientation is necessary to accommodate the large ortho substituants of the latter complex. The divergent Ni-S coordination modes result in distinct ¹H NMR and electronic spectra that were rationalized by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. These results demonstrate rich coordination chemistry for arylthlolates that can be elicited by steric manipulation at the periphery of pseudotetrahedral ligand fields.