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

The octahedral complex, [CoIII(HL)]·9H2O (H4L = (1,8)-bis(2-hydroxybenzamido)-3,6-diazaoctane) incorporating bis carboxamido-N-, bissec-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, [CoIII(HL)] → [CoIV(HL)]+ and [CoIII(HL)] → [CoII(HL)]− at 0.41 and −1.09 V (versus SCE), respectively. A slow and H+ mediated isomerisation was observed for the protonated complex, [CoIII(H2L)]+ (pK = 3.5, 25 °C, I = 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: [{[CoIII(HL)],(H2Asc)}–H]− <<< {[CoIII(H2L)],(H2Asc)}+ < {[CoIII(HL)],(H2Asc)}. A low value (ca. 3.7 × 10−10 dm3 mol−1 s−1, 25 °C, I = 0.5 mol dm−3) for the self exchange rate constant of the couple [CoIII(HL)]/[CoII(HL)]− indicated that the ligand HL3− with amido (N-) donor offers substantial stability to the CoIII state. HSO3− and [CoIII(HL)] formed an outer sphere complex {[CoIII(HL)],(HSO3−)}, which was slowly transformed to an inner sphere S-bonded sulfito complex, [CoIII(H2L)(HSO3)] and the latter was inert to reduction by external sulfite but underwent intramolecular SIV → CoIII electron transfer very slowly.