Copper compounds play a valuable role in both synthetic and biological chemistry. Copper catalyzes a vast array of reactions, primarily oxidation-reduction reactions which make use of the Cu(I)/Cu(II) redox cycle. Copper is found in the active site of many enzymes and proteins, including the oxygen carrying proteins called hemocyanins.
Common oxidation states of copper include the less stable copper(I) state, Cu+; and the more stable copper(II) state, Cu2+. Copper (I) has a d10 electronic configuration with no unpaired electrons, making it undetectable by EPR. The d9 configuration of Cu2+ means that its compounds are paramagnetic making EPR of Cu(II) containing species a useful tool for both structural and mechanistic studies. Two literature examples of how EPR can provide insight into the mechanisms of reactivity of Cu(II) are discussed herein.
Copper (II) centers typically have tetrahedral, or axially elongated octahedral geometry. Their spectra are anisotropic and generally give signals of the axial or orthorhombic type. From EPR spectra of copper (II) compounds, the coordination geometry can be determined. An example of a typical powder Cu(II) spectrum is shown in Figure 1.
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The spectrum above shows four absorption-like peaks corresponding to g‖ indicating coordination to four identical atoms, most likely nitrogen. There is also an asymmetric derivative peak corresponding to g⊥ at higher field indicating elongation along the z axis.
Determination of an intermediate
The reactivity and mechanism of Cu(II)-peroxy systems was investigated by studying the decomposition of the Cu(II) complex 1 with EPR as well as UV-Vis and Raman spectroscopy. The structure (Figure 2) and EPR spectrum Figure 3 of 1 are given. It was postulated that decomposition of 1 may go through intermediates LCu(II)OOH, LCu(II)OO•, or LCu(II)O• where L = ligand.
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To determine the intermediate, a common radical trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was added. A 1:1 complex of intermediate and DMPO was isolated, and given the possible structure 2 (Figure 4), which is shown along with its EPR spectrum (Figure 5).
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The EPR data show similar though different spectra for Cu(II) in each compound, indicating a similar coordination environment – elongated axial, and most likely a LCu(II)O• intermediate.
Determination of a catalytic cycle
The mechanism of oxidizing alcohols to aldehydes using a Cu(II) catalyst, TEMPO, and O2 was investigated using EPR. A proposed mechanism is given in Figure 6.
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EPR studies were conducted during the reaction by taking aliquots at various time points and immediately freezing the samples for EPR analysis. The resulting spectra are shown in Figure 7.
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The EPR spectrum (a) in Figure 6, after 1.2 hours shows a signal for TEMPO at g = 2.006 as well as a signal for Cu(II) with g‖= 2.26, g⊥ = 2.06, A‖ = 520 MHz, and A⊥ < 50 MHz. After 4 hours, the signal for Cu(II) is no longer in the reaction mixture, and the TEMPO signal has decreased significantly. Suggesting that all the Cu(II) has been reduced to Cu(I) and the majority of TEMPO has been oxidized. After 8 hours, the signals for both Cu(II) and TEMPO have returned indicating regeneration of both species. In this way, EPR evidence supports the proposed mechanism.
Bibliography
- M. Contel, P. R. Villuendas, J. Fernández-Gallardo, P. Alonso, J. M. Vincent, and R. Fish, Inorg. Chem., 2005, 44, 9771.
- A. Kunishita, H. Ishimaru, S. Nakashima, T. Ogura, S. Itoh, J. Am. Chem. Soc., 2008, 130, 4244.
- G. Palmer, In Physical Methods in Bioinorganic Chemistry Spectroscopy and Magnetism. Ed. L. Que, University Science Books: Sausalito, CA (2000).
- J. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarenden Press, Oxford (1990).
- J. Weil, J. Bolton, and J. Wertz, Electron Paramagnetic Resonance: Elemental Theory and Practical Applications John Wiley and Sons: New York (1994).
