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 H(2)L = 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 displays a rhombic S = 1/2 signal consistent with a low spin configuration for the d(7) 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 (59)Co and (14)N 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 CH(3)I, generating the mono S-methylated complex, [CoL(Me)I] (1(Me)). The X-band EPR spectrum of 1(Me) displays a typical signal of a high spin (S = 3/2) Co(II) species. An optimized structure of 1(Me), 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(Me). 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(Me), 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.

A Bio‐Inspired Switch Based on Cobalt(II) Disulfide/Cobalt(III) Thiolate Interconversion

“…Each Co II ion displays a highly distorted square-pyramidal geometry, with two N atoms and two m-S atoms of the ligand forming the square planar base and one S atom of the disulfide bridge in axial position. As expected, the axial Co II ÀS S-S bond (2.5341 (8) ) is significantly longer than the basal Co II À (m-S thiolate ) bonds (2.3796(8) and 2.3272 (7) ), the latter being in the expected range (2.21-2.39 ). [10] The long distance between the two Co ions (3.1188(5) ) implies the absence of a direct metal-metal bond.…”

“…As expected, the Co À S and the CoÀN bonds in 1 are slightly longer than those in [Co II LS]. [8] From the magnetic properties of 1, it can be concluded that it is an S = 1 Co III species both in solid state and in solution. The c T product at 320 K of 1.3 cm 3 K mol À1 (Figure 2) is in agreement with the expected values for such S = 1 species.…”

“…When this reaction becomes reversible, it can switch "on" or "off" the reactivity of a protein. [7] The mononuclear complex 1 was obtained as black crystals by electrochemical oxidation of [Co II LS] [8] at À0.50 V vs Fc/Fc + (Fc = [(h-C 5 H 5 ) 2 Fe]) in presence of Et 4 NCl as electrolyte in CH 3 CN. [2] The idea that metal ions can play an active role in the disulfide/thiolate interconversion of essential biological processes has only emerged during the last decade.…”

“…[2] The idea that metal ions can play an active role in the disulfide/thiolate interconversion of essential biological processes has only emerged during the last decade. [8] From the magnetic properties of 1, it can be concluded that it is an S = 1 Co III species both in solid state and in solution. [4] In chemistry, the disulfide/thiolate interconversion has been largely explored as well as the effect of a metal ion on the kinetics.…”

“…[6] The removal of the Cl À ion from the Co III complex leads to the formation of a bis(m-thiolato) m-disulfido dicobalt(II) complex, [Co 2 II,II LSSL] 2+ (2 2+ ), containing the unusual {M 2 S 2 (SS)} core, only found in Mn, Re and Ir complexes until now. [7] The mononuclear complex 1 was obtained as black crystals by electrochemical oxidation of [Co II LS] [8] at À0.50 V vs Fc/Fc + (Fc = [(h-C 5 H 5 ) 2 Fe]) in presence of Et 4 NCl as electrolyte in CH 3 CN. The X-ray structure of 1 ( Figure 1) shows that the Co III ion is in the center of a distorted square pyramidal N 2 S 2 Cl coordination sphere, with the two N and two S atoms of the ligand located in the equatorial plane, and the Cl À ion occupying the axial position.…”