Département de chimie moléculaire

{S-rich Ni metalloenzymes are capable of stabilizing NiI and NiIII oxidn. states in catalytically relevant species. In an effort to better understand the structural and electronic features that allow the stabilization of such species, the authors have studied the electrochem. properties of two mononuclear N2S2 NiII complexes that differ in their S environment. Complex 1 features aliph. dithiolate coordination ([NiL], 1), and complex 2I was characterized by mixed thiolate/thioether coordination ([NiLMe]I, 2I). The latter results from the methylation of a single S of 1. The x-ray structure of 2I reveals a distorted square planar geometry around the NiII ion, similar to what was previously reported by the authors for 1. The electrochem. study of 1 and 2+ shows that the addn. of a Me group shifts the potentials of both redox NiII/NiI and NiIII/NiII redox couples by ∼0.7 and 0.6 V to more pos. values. Through bulk electrolyzes, only the mononuclear dithiolate [NiIL]- (1-) and the mixed thiolate/thioether [NiIIILMe]2+ (22+) complexes were generated, and their electronic properties were studied by UV-visible and EPR spectroscopy. For 1- (NiI, d9 configuration) the EPR data are consistent with a dx2-y2 based singly occupied MOs (SOMOs). However, DFT calcns. suggest that there is also pronounced radical character. This is consistent with the small g-anisotropy obsd. in the EPR expts. The spin population (Mulliken anal.) anal. of 1- reveals that the main contribution to the SOMO (64{%}) is due to the bipyridine unit. Time dependent d. functional theory (TD-DFT) calcns. attribute the most prominent features obsd. in the electronic absorption spectrum of 1- to metal to ligand charge transfer (MLCT) transitions. Concerning 22+, the EPR spectrum displays a rhombic signal with gx = 2.236