Synthesis, structure and properties of cobalt(III) complexes of pentadentate ligands with pyridyl pendant arms

Cobalt(III) complexes of the pentadentate ligands 3-[4-(2-pyridyl)-3-azabut-3-enyl]-3-azapentane-1,5-diamine (L1), 3-[4-(2-pyridyl)-3-azabutyl]-3-azapentane-1,5-diamine (L2) and 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane (L4) have been synthesized. Various characterization studies have confirmed the formation of the complexes [CoL4(OH2)]3+, [Co(L4)Cl]2+, [Co(L1)Cl]2+, [Co(L2)Cl]2+ and [Co(HL3)Cl]2+(HL3= 3-{2-[hydroxy(2-pyridyl)methyleneamino]ethyl}-3-azapentane-1,5-diamine) the product of oxidation of [Co(L1)Cl]2+ in acidic solution. The structure of [CoL4(OH2)][ClO4]3·H2O has been determined by single crystal X-ray diffraction. The complex crystallizes in the orthorhombic space group P212121, with a= 15.090(3), b= 16.659(4), c= 10.192(4)A and Z= 4. Refinement gave final R and R′ values of 0.045 and 0.043, respectively, for 1708 observed reflections. The pyridyl pendant arms were found to introduce significant distortion from ideal octahedral geometry. The Co–N(tacn) distances are shorter than in related complexes of ligands derived from 1,4,7-triazacyclononane (tacn). The complex [Co(HL3)Cl][ClO4]2·H2O crystallizes in the monoclinic space group P21/c with a= 13.496(5), b= 10.253(3), c= 15.371(6)A, β= 101.65(3)° and Z= 4. Refinement gave final R and R′ values of 0.054 and 0.054, respectively, for 1686 observed reflections. The bond lengths within the amide portion of the pentadentate ligand are intermediate between keto and enol resonance 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 stabilization of the CoII state relative to CoIII arises from the presence of pyridyl and imine π acceptors.

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