Synthesis, characterization and pulse radiolysis of cobalt(ii) complexes of 2-picolinate and polypyridyl ligands

Abstract

Complexes of the type [Co(pic)₂(LL)], where pic = picolinate; LL = (H₂O)₂ (1), 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz) (4) have been synthesized and characterized by elemental analysis, IR, UV-visible and ¹H-NMR spectroscopy. Complex 2 crystallized from a mixture of chloroform and methanol in orthorhombic crystal system, space group Iba2. Complex 3 crystallizes from chloroform in monoclinic crystal system, space group P12₁/a1 and in a mixture of water and methanol in triclinic crystal system, space group P. Co(II)/Co(III) oxidation potentials have been determined by cyclic voltammetry and the concomitant spectral changes measured by spectroelectrochemistry. The reactions of one electron reducing (e⁻_aq, (CH₃)₂˙COH and CO₂˙⁻) and one electron oxidizing (˙OH) radicals with the above metal complexes have been studied by pulse radiolysis. The rate constants for the reaction of e⁻_aq and ˙OH radicals with the complexes are of the order of 10¹⁰ and 10⁹ dm³ mol⁻¹ s⁻¹, respectively. The reaction of hydrated electron (e⁻_aq) with the complexes results in the formation of a ligand radical anion species which decays by either protonation of ligand or reduction of metal by intramolecular electron transfer resulting in cobalt(I) species. The reaction of ˙OH radical forms the OH-adducts of the ligand.