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

Abstract

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)Å 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)Å, 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 Co^II state relative to Co^III arises from the presence of pyridyl and imine π acceptors.