A new octahedral cobalt(III) complex of (1,8)- bis (2-hydroxybenzamido)-3,6-diazaoctane incorporating phenolate-amide-amine coordination: synthesis, X-ray crystal structure, ligand substitution and redox activity with sulfur(IV) and L -ascorbic acid

Abstract

The octahedral complex, [Co^III(HL)]·9H₂O (H₄L = (1,8)-bis(2-hydroxybenzamido)-3,6-diazaoctane) incorporating bis carboxamido-N-, bis sec-NH, phenolate, and phenol coordination has been synthesized and characterized by analytical, NMR (¹H, ¹³C), e.s.i.-Mass, UV–vis, i.r., and Raman spectroscopy. The formation of the complex has also been confirmed by its single crystal X-ray structure. The cyclic voltammetry of the sample in DMF ([TEAP] = 0.1 mol dm^-3, TEAP = tetraethylammonium perchlorate) displayed irreversible redox processes, [Co^III(HL)] → [Co^IV(HL)]⁺ and [Co^III(HL)] → [Co^II(HL)]^- at 0.41 and -1.09 V (versus SCE), respectively. A slow and H⁺ mediated isomerisation was observed for the protonated complex, [Co^III(H₂L)]⁺ (p^K = 3.5, 25 °C, / = 0.5 mol dm^-3). H₂Asc was an efficient reductant for the complex and the reaction involved outer sphere mechanism; the propensity of different species for intra molecular reduction followed the sequence: [{[Co^III(HL)],(H₂Asc)}–H]^-<<< {[Co^III(H₂L)],(H₂Asc)}⁺ < {[Co^III(HL)],(H₂Asc)}. A low value (ca. 3.7 × 10^-10 dm³ mol^-1 s^-1, 25 °C, / = 0.5 mol dm^-3) for the self exchange rate constant of the couple [Co^III(HL)]/[Co^II(HL)]^- indicated that the ligand HL^3- with amido (N-) donor offers substantial stability to the Co^III state. HSO₃^- and [Co^III(HL)] formed an outer sphere complex {[Co^III(HL)],(HSO₃^-)}, which was slowly transformed to an inner sphere S-bonded sulfito complex, [Co^III(H₂L)(HSO₃)] and the latter was inert to reduction by external sulfite but underwent intramolecular S^IV → Co^III electron transfer very slowly.