Structural, spectroscopic and redox properties of a mononuclear CoII thiolate complex – the reactivity toward S-alkylation: an experimental and theoretical study

Abstract

The structural, spectroscopic, redox properties and also the reactivity toward S-alkylation of a new mononuclear N2S2 dithiolate Co^II complex [CoL] (1), with H₂L = 2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1,1-diphenylethanethiol), have been investigated. The X-ray structure of 1 has revealed an unusual distorted square planar geometry for a Co^II ion within a thiolate environment. The X-band EPR spectrum of 1 displays a rhombic S = 1/2 signal consistent with a low spin configuration for the d⁷ Co^II ion with a large g-anisotropy (g_x = 2.94, g_y = 2.32 and g_z = 2.01). By pulsed EPR experiments (HYSCORE), two weak hyperfine couplings (hfc) of 3.2 and 2.2 MHz have been measured and attributed respectively to protons and nitrogen nuclei of the bipyridine unit. In addition, another hyperfine coupling (hfc) of 7.5 MHz has been attributed to the cobalt ion. DFT calculations have successfully reproduced the ⁵⁹Co and ¹⁴N hfc parameters. However, multiconfigurational ab initio calculations were required to predict the g-tensor of 1. The cyclic voltammogram (CV) displays two one-electron metal based processes: a quasi-reversible Co^III/Co^II oxidation wave at E_1/2 = −0.5 V vs. Fc⁺/Fc and a quasi-reversible Co^II/Co^I reduction wave at E_1/2 = −1.7 V. 1 reacts with CH₃I, generating the mono S-methylated complex, [CoL^MeI] (1^MeMe). The X-band EPR spectrum of 1^MeMe displays a typical signal of a high spin (S = 3/2) Co^II species. An optimized structure of 1^MeMe, calculated by DFT, is consistent with its EPR and UV-visible spectra. Time dependent density functional theory (TD-DFT) calculations attribute the most prominent features observed in the electronic absorption spectra of 1 and 1^MeMe. The singly occupied MO (SOMO) of 1 shows a notable delocalization of the unpaired electron over the metal (85%) and the ligand, especially over the sulphur atoms (10.5%), indicating a certain degree of covalency for the Co–S bonds. In 1^MeMe, for two of the three SOMOs, the unpaired electron is notably delocalized over the metal (78.5 and 77.6%, respectively) and the ligand (12.5 and 7.8%, respectively over the sulphur of the thiolate function). For the third SOMO, the unpaired electron is mainly localized on the metal (92.2%). There is no electronic density spread on the sulphur atom of the thioether function in any of these SOMOs. The reactivity and the electronic properties of 1 are also compared with those of the analogous [ZnL] and [NiL] complexes.