Syntheses, structure and properties of cobalt-(II) and -(III) complexes of pentadentate N4S ligands with appended pyrazolyl groups: evidence for cobalt(II)–dioxygen reversible binding

Abstract

Cobalt-(II) and -(III) complexes of pentadentate N₄S ligands based on methyl 2-aminocyclopent-1-ene-1-carbodithioate with appended pyrazolyl groups (3,5-Me₂C₃HN₂CH₂)₂NCH(R)CH₂NHC₅H₆C(S)SCH₃ (R = H, Hmmecd; CH₃, Hmmpcd) have been prepared and characterised by IR, ¹H NMR and electronic spectroscopy. Two of these compounds have also structurally been characterised by X-ray single crystal diffraction analyses. Cobalt(II) in [Co(mmpcd)]ClO₄, 1, shows a five-coordinate, trigonal bipyramidal geometry while its cobalt(III) counterpart, [Co(mmpcd)Cl]ClO₄, 2, reveals a six-coordinated distorted octahedral structure by the inclusion of a chloride ligand in its equatorial plane. In dmf or acetonitrile solution, 1 can bind dioxygen reversibly as indicated by EPR spectra recorded at cryogenic temperatures. Metal–dioxygen binding in 1 appears to be weak, possibly due to its trigonal bipyramidal structure and the presence of a sulfur donor in the ligand framework. Electronic spectra of the cobalt(III) complexes show two LMCT bands in the near UV region, tentatively assigned to S→Co^III charge transfer.