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A Pulsed EPR and DFT Investigation of the Stabilization of Coordinated Phenoxyl Radicals in a Series of Cobalt Schiff-Base Complexes

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Abstract

We recently demonstrated how the aerobic addition of acetic acid to N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino CoII, [Co(1)], leads to the formation of an unusual coordinated CoIII-phenoxyl radical. In this work, some of the structural aspects associated with the Schiff-base-derived ligand (1) that are crucial for the acid-mediated formation of the phenoxyl radical are investigated. For comparison with [Co(1)], we therefore studied the influence of acetic acid on two complexes: (1) the N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-ethane-diamino CoII complex, [Co(2)], that lacks the cyclohexyl group of [Co(1)], and (2) the N′-disalicylidene-ethylenediamine CoII salen complex, [Co(3)], that lacks both the tertiary butyl groups and the cyclohexyl groups. It is shown that the cyclohexyl group of [Co(1)] is not involved in the formation or stabilization of the phenoxyl radical, whereas the tertiary butyl groups of [Co(1)] play a crucial role. In addition, the characteristics of the phenoxyl radical, formed after aerobic addition of acetic acid to [Co(2)], are analyzed in detail by pulsed electron paramagnetic resonance, in combination with isotopic labeling. The experimental data are compared to density functional theory computations and to previous data on the acid-mediated phenoxyl radical of [Co(1)].

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Acknowledgments

The research was supported by the Fund of Scientific Research-Flanders (FWO) (Project G.0312.05N to S.V.D.). E.V. is a research assistant of the FWO. D.M.M. and I.A.F. acknowledge the support of Engineering and Physical Sciences Research Council (EP/E030122). It should be noted that we first made the link between the interpretation of the 8-line EPR spectrum observed in acetic acid activated [Co(1)] and the formation of phenoxyl radicals through the early studies of M. Baumgarten and W. Lubitz on reactions of CoII salen with phenols. It is therefore a pleasure to present here some of our results on this topic in honor of Wolfgang Lubitz’ 60th birthday.

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Authors and Affiliations

  1. Department of Physics, University of Antwerp, 2610, Antwerp, Belgium

    Evi Vinck & Sabine Van Doorslaer

  2. School of Chemistry, Cardiff University, Cardiff, CF10 3TB, Wales, UK

    Damien M. Murphy & Ian A. Fallis

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  1. Evi Vinck
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  2. Damien M. Murphy
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  3. Ian A. Fallis
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  4. Sabine Van Doorslaer
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Correspondence to Sabine Van Doorslaer.

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Vinck, E., Murphy, D.M., Fallis, I.A. et al. A Pulsed EPR and DFT Investigation of the Stabilization of Coordinated Phenoxyl Radicals in a Series of Cobalt Schiff-Base Complexes. Appl Magn Reson 37, 289–303 (2010). https://doi.org/10.1007/s00723-009-0059-6

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