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

Cobalt-(II) and -(III) complexes of pentadentate N4S ligands based on methy l 2-aminocyclopent-1-ene-1-carbodithioate with appended pyrazolyl groups (3 ,5-Me2C3HN2CH2)(2)NCH(R)CH2NHC5H6C(=S)SCH3 (R=H, Hmmecd; CH3, Hmmpcd) have been prepared and characterised by IR, H-1 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, show s a five-coordinate, trigonal bipyramidal geometry while its cobalt(III) co unterpart, [Co(mmpcd)Cl]ClO4, 2, reveals a six-coordinated distorted octahe dral structure by the inclusion of a chloride ligand in its equatorial plan e. In dmf or acetonitrile solution, 1 can bind dioxygen reversibly as indic ated by EPR spectra recorded at cryogenic temperatures. Metal-dioxygen bind ing in 1 appears to be weak, possibly due to its trigonal bipyramidal struc ture 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 re gion, tentatively assigned to S --> Co-III charge transfer.