A Study of Intramolecular Electron Exchange in Copper-Radical Complexes Involved in Catalysis Using ESR Spectroscopy

Ahn WS; Zhong Y; Lim PK

Korean Journal of Chemical Engineering, Vol.14, No.5, 394-398, September, 1997

ESR simulation and experimental results have been presented to show that transition metal-radical complexes can engage in intra molecular electron exchange and that the exchange gives rise to ESR asymmetric line broadening effect. Depending on the relative concentration of the redox-exchange pair, metal or radical-like spectrum can be obtained. Simulation results show that a metal-radical complex can be masked by its redox counterpart upto a relative concentration of 1:2 at a modest exchange rate of 3×10⁸/s. Asymmetric line broadening was predicted to occur upon such metal-radical complexation. Experimentally, a pronounced ESR asymmetric line broadening was observed for Cu(Ⅱ) complexes of various redox-active ligands. Cu(Ⅱ) complex with redox-inert ammonia, however, showed no such evidence. Ligand displacement experiments established the reversibility of metal-radical complexation and the associated ESR line broadening.

Keywords:Intramolecular Electron Exchange;Cu(Ⅱ) Complex;Redox-ligand;ESR Spectroscopy;Asymmetric Line Broadening

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[3] Connelly NG, Geiger WE, Adv. Org. Chem., 23, 1 (1984)

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[5] Hanaya M, Iwaizumi M, J. Org. Chem., 417, 407 (1991)

[6] Hanaya M, Iwaizumi M, J. Org. Chem., 435, 337 (1992)

[7] Halpem J, Pure Appl. Chem., 58, 575 (1986)

[8] Halpem J, "The Role of Free Radicals in Organometallic Catalysis," in Paramagnetic Organometallic Species in Activation/Selectivity Catalysis, ed. by Chanon, M., Julliard, M. and Poite, J.C., Kluwer Academic Press, Boston, 423 (1989)

[9] More JK, More KM, Eaton GR, Eaton SS, J. Am. Chem. Soc., 106, 5397 (1984)

[10] Nielson RM, Wherland S, Inorg. Chem., 25, 2437 (1986)

[11] Razuvaev GA, Cherkasov VK, Abakumov GA, J. Org. Chem., 160, 361 (1978)

[12] Tkac A, Vesely K, Omelka L, J. Phys. Chem., 75, 2575 (1971)