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Title: 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
Authors: NAYAK, S
Keywords: Manganese(Iv) Complex
Iron(Iii) Complexes
Catalyzed Aquation
Issue Date: 2008
Publisher: SPRINGER
Citation: TRANSITION METAL CHEMISTRY, 33(1), 39-53
Abstract: The octahedral complex, [Co-III(HL)]center dot 9H(2)O (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 (H-1, C-13), 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)](+) (pK = 3.5, 25 degrees C, I = 0.5 mol dm(-3)). H(2)Asc 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)],(H(2)Asc)}- H](-) <<< {[Co-III(H2L)],(H(2)Asc)}(+) \{[Co-III(HL)],(H(2)Asc)}. A low value (ca. 3.7 x 10(-10) dm(3) mol(-1) s(-1), 25 degrees 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 HL3- 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.
ISSN: 0340-4285
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