CoII Complexes with Mixed Amino-N and Thiolato-S Donor Sets − Structural Characterization and Electronic Properties of a Stable Bis(μ-thiolato)-Bridged Binuclear CoII Complex

Two different ligands [S2N3py] and [S2N2], prepared from 2,6-bis[1-(2-mercaptoanilino)ethyl]pyridine and 2,3-bis(2-mercaptoanilino)butane, respectively, have been used to investigate cobalt coordination with mixed amine nitrogen/sulfur donor sets. The pentadentate [S2N3py] ligand gave rise to a mononuclear [CoII(S2N3py)] complex, which was found to be stable only at low temperatures under argon, and was characterized as having a high-spin CoII state on the basis of 1H NMR and EPR measurements. In contrast, the tetradentate [S2N2] ligand led to a binuclear bis(μ-thiolato) [CoII(S2N2)]2 complex, the structure of which was solved by X-ray crystallography. Each CoII centre was found to reside in an N2S3 square-pyramidal environment in the crystal, the two Co atoms being bridged by one of the two thiolates of an [S2N2] ligand. On the basis of the temperature dependence of the magnetic susceptibility, the two CoII centres were found to be in a low-spin state and slightly antiferromagnetically coupled with an absolute J value of less than 3 cm−1. In aerated CH2Cl2 in the presence of CH3SO3H, the binuclear complex proved to be stable in its mixed valence state, [CoII/CoIII], which could be converted back to [CoII/CoII] upon addition of NBu4OH in MeOH. Three different stable oxidation states could be characterized by electro- and spectroelectrochemistry (E [CoII/CoII]/[CoII/CoIII] = −0.19 V, E [CoII/CoIII]/[CoIII/ CoIII] = 0.2 V vs. SCE). The electronic spectrum of the [CoII/ CoII] state features a broad absorption at 630 nm and a sharp band at 565 nm, while that of the [CoII/CoIII] state shows two bands at 657 and 800 nm. The Raman spectrum of the [CoII/CoII] state was found to be dominated by a Co2S2 core vibration at 223 cm−1, which proved to be strongly resonanceenhanced within the 565 nm absorption band, thus giving a sound basis for its assignment as an SCoII charge-transfer band. For the [CoII/CoIII] state, a significant resonance Raman enhancement was observed within the 657 nm absorption band for a larger number of vibrations involving the Co2S2 core as well as the phenyl ring.