The effects of tripodal ligands on charge distribution in cobalt(III)-catecholate and cobalt(II)-semiquinonate compounds

Three monomeric cobalt compounds, [Co(PzPy(2))(3,5-DBcat)]center dot PF6 (1), [Co(Pz(2)Py)(3,5-DBsq)]center dot BPh4 (2) and [Co(Pz(3))(3,5-DBsq)]center dot PF6 (3), have been synthesized, where PzPy(2) = (3,5-dimethyl-1H-pyrazol-1-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine, Pz(2)Py = (3,5-dimethyl-1H-pyrazol-1-yl)-N-((3,5-dimethyl-1H-pyrazol-1-yl) methyl)-N-(pyridine-2-ylmethyl) methanamine and Pz(3) = tris((3,5-dimethyl-1H-pyrazol-1yl)methyl)amine, respectively. Compound 1 consist of low-spin Co(III) ion and diamagnetic cat(2) (Cols-III -DBcat(2)) over the temperature regions observed, while compounds 2 and 3 as high-spin cobalt(II) ion and paramagnetic semiquinonate radical (Cohs-II-DBsq(center dot)) state are stable in the whole temperature range. No temperature-dependent valence tautomeric transitions were observed in these compounds, and the reason caused the distinct charge distribution on cobalt ion and dioxolene ligands assigned to the natural flexibility and steric hindrance of the tripodal ligands. Magnetic studies indicate that very weak antiferromagnetic interactions are presented between the high-spin Co(II) ion and semiquinonate ligands in 2 and 3. Pressure effects on the magnetic properties were performed, which showed that no pressure-induced valence tautomeric transitions were occurred on 2 and 3. Meanwhile, antiferromagnetic coupling in 2 and 3 are clearly enlarged under external pressure. (C) 2012 Elsevier B. V. All rights reserved.