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Issue 40, 2011
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Switching on oxygen activation by cobalt complexes of pentadentate ligands

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Abstract

The monoanionic N4O ligand N-methyl-N,N′-bis(2-pyridylmethyl)ethylenediamine-N′-acetate (mebpena) undergoes oxidative C–N bond cleavage in the presence of Co(II) and O2. The two resultant fragments are coordinated to the metal ion in the product [CoIII(2-pyridylformate)(mepena)]ClO4 (mepena = N-methyl-N′-(2-pyridylmethyl)ethylenediamine-N′-acetato). Bond cleavage does not occur in the presence of chloride ions and [CoIII(mebpena)Cl]+, containing intact mebpena, can be isolated. The oxidative instability of the mebpena in the presence of Co(II) and air stands in contrast to the oxidative stability of the family of very closely related penta- and hexa-dentate ligands in their cobalt complexes. Cyclic voltammetry on the matched pair [CoIIICl(mebpena)]+ and [CoIICl(bztpen)]+, bztpen = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethylenediamine, shows that substitution of a pyridine donor for a carboxylato donor results in a relatively small cathodic shift of 150 mV in the E°(Co(II)/Co(III)) oxidation potential, presumably this is enough to determine the contrasting metal oxidation state in the complexes isolated under ambient conditions. DFT calculations support a proposal that [CoII(mebpena)]+ reacts with O2 to form a Co(III)-superoxide complex which can abstract an H atom from a ligand methylene C atom as the initial step towards the observed oxidative C–N bond cleavage.

Graphical abstract: Switching on oxygen activation by cobalt complexes of pentadentate ligands

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Publication details

The article was received on 07 Apr 2011, accepted on 26 Jul 2011 and first published on 24 Aug 2011


Article type: Paper
DOI: 10.1039/C1DT10594A
Citation: Dalton Trans., 2011,40, 10698-10707
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    Switching on oxygen activation by cobalt complexes of pentadentate ligands

    M. S. Vad, A. Nielsen, A. Lennartson, A. D. Bond, J. E. McGrady and C. J. McKenzie, Dalton Trans., 2011, 40, 10698
    DOI: 10.1039/C1DT10594A

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