Co-II complexes with mixed amino-N and thiolato-S donor sets - Structural characterization and electronic properties of a stable bis(mu-thiolato)-bridged binuclear Co-II complex

Two different ligands [S2N3py] and [S2N2], prepared from 2,6-bis[1-(2-merca ptoanilino)ethyl]pyridine and 2,3-bis(2-mercaptoanilino)butane, respectivel y, have been used to investigate cobalt coordination with mixed amine nitro gen/sulfur donor sets. The pentadentate [S2N3py] ligand gave rise to a mono nuclear [Co-II(S2N3py)] complex, which was found to be stable only at low t emperatures under argon, and was characterized as having a high-spin Co-II state on the basis of H-1 NMR and EPR measurements. In contrast, the tetrad entate [S2N2] ligand led to a binuclear bis(mu -thiolato) [Co-II(S2N2)](2) complex, the structure of which was solved by X-ray crystallography. Each C o-II 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 magneti c susceptibility, the two Co-II centres were found to be in a low-spin stat e and slightly antiferromagnetically coupled with an absolute J value of le ss than 3 cm(-1). In aerated CH2Cl2 in the presence of CH3SO3H, the binucle ar complex proved to be stable in its mixed valence stale, [Co-II/Co-III], which could be converted back to [Co-II/Co-II] upon addition of NBu4OH in M eOH. Three different stable oxidation states could be characterized by elec tro- and spectroelectrochemistry (E [Co-II/Co-II]/[Co-II/Co-III] = -0.19 V, E [Co-II/Co-III]/[Co-III/ Co-III] = 0.2 V vs. SCE). The electronic spectru m of the [Co-II/ Co-II] state features a broad absorption at 630 nm and a s harp band at 565 nm, while that of the [Co-II/Co-III] state shows two bands at 657 and 800 nm. The Raman spectrum of the [Co-II/ Co-II] state was foun d to be dominated by a Co2S2 core vibration at 223 cm-l, which proved to be strongly resonance-enhanced within the 565 nm absorption band, thus giving a sound basis for its assignment as an S --> Co-II charge-transfer band. F or the [Co-II/Co-III] state, a significant resonance Raman enhancement was observed within the 657 nm absorption band for a larger number of vibration s involving the Co2S2 core as well as the phenyl ring.