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

摘要

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,I3C),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=tetraethylammoniumper-chlorate)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)]~+(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:[{[Co~(III)(HL)],(H2Asc)}-H]~-<{[Co~(III)(H2L)],(H2Asc)}~+<{[Co~(III)(HL)],(H2Asc)}.Alow value(ca.3.7X10~(-10)dm3 mol~(-1)s~(-1),25°C,I=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.HSO_3~-and[Co~(III)(HL)]formed an outer sphere complex{[Co~(III)(HL)],(HSO_3~-).},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.