Spectral assignments and molecular modeling of dinuclear cobalt(II) complexes with a bridged bidentate b-diketone and a cyclic octaamine ligand

Four new cobalt(II) complexes of the general formula [Co2(Rac)tpmc](ClO4)3, where tpmc and Rac refer to N,N0,N00,N000-tetrakis(2- pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane and b-diketonato ligands i.e. 2,4-pentanedionato (acac), 1,3-diphenyl-1,3-propanedionato (dibzac), 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato (hfac) or 2,2,6,6-tetramethyl-3,5-heptanedionato (tmhd) ions, respectively, have been prepared. Elemental analyses, conductometric measurements, UV/VIS, IR, EPR and mass spectroscopy were used for the complex characterization. Each cobalt(II) ion is coordinated with four nitrogen atoms in an exo arrangement of tpmc groups while the additional b-diketone bridges metal-ion centers through the both enolate oxygens. The presence of different R-groups on the diketone influence the nðC OÞ and nðC CÞ vibrations in the IR spectra. These frequencies decrease in the order of the complexes with coordinated hfac . dibzac . acac . tmhd ligands. The mechanism of the mass spectral fragmentation of the complexes entails a multi-step decomposition process. The geff-factors estimated from EPR spectra suggest a similar molecular character of the complexes. Molecular modeling calculations were used to characterize the complex species. q 2004 Elsevier B.V. All rights reserved