SYNTHESIS, STRUCTURE AND PROPERTIES OF COBALT(III) COMPLEXES OF PENTADENTATE LIGANDS WITH PYRIDYL PENDANT ARMS

Cobalt(III) complexes of the pentadentate ligands pyridyl)-3-azabut-3- enyl]-3-azapentane-1,5-diamine (L(1)), 4-(2-pyridyl)-3-azabutyi]-3-aza pentane-l,5-diamine (L(2)) and 1,4-bis(2-pyridylmethyl)-1, 4,7-triazac yclononane (LC) have been synthesized. Various characterization studie s have confirmed the formation of the complexes [CoL(4)(OH2)](3+), [Co (L(4))Cl](2+), [Co(L(1))Cl](2+), [Co-(L(2))Cl](2+) and [Co(HL(3))Cl](2 +) (HL(3) = 3-{2-[hydroxy (2-pyridyl)methyleneamino] ethyl}-3-azapenta ne-1,5-diamine) the product of oxidation of [Co(L(1))Cl](2+) in acidic solution. The structure of [CoL(4)(OH2)][ClO4](3) H2O has been determ ined by single crystal X-ray diffraction. The complex crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 15.090(3), b = 16 .659(4), c = 10.192(4) Angstrom and Z = 4. Refinement gave final R and R' values of 0.045 and 0.043, respectively, for 1708 observed reflect ions. The pyridyl pendant arms were found to introduce significant dis tortion from ideal octahedral geometry. The Co-N(tacn) distances are s horter than in related complexes of ligands derived from 1,4,7-triazac yclononane (tacn). The complex [Co(HL(3))Cl][ClO4](2) H2O crystallizes in the monoclinic space group P2(1)/c with a = 13.496(5). b = 10.253( 3), c = 15.371(6) Angstrom, beta = 101.65(3)degrees and Z = 4. Refinem ent gave final R and R' values of 0.054 and 0.054, respectively, for 1 686 observed reflections. The bond lengths within the amide portion of the pentadentate ligand are intermediate between keto and enol resona nce forms. Protonation of the amide oxygen is implied by the number of counter ions present. No evidence of imine oxidation was observed in electrochemical studies. These studies indicated that some stabilizati on of the Co-II state relative to Co-III arises from the presence of p yridyl and imine pi accepters.