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A new type of copper-oxygen intermediate that reacts with C-H bonds

Recent research from the research groups of Professor William TolmanProfessor Christopher Cramer

A variety of copper-oxygen species are implicated as intermediates in oxidation catalysis by metalloenzymes, synthetic complexes, and materials. In efforts to characterize such species, a variety of core motifs have been identified (A-E, Figure 1) or postulated (F). In a communication published in the Journal of the American Chemical Society (2011, 133, 17602-17605), postdoctoral associate Patrick J. Donoghue, graduate student Jacqui Tehranchi, Stanford University collaborators Ritimukti Sarangi and Edward I. Solmon, and Professors William B. Tolman and Christopher J. Cramer report the preparation, spectroscopic and theoretical characterization, and initial reactivity studies of a new copper-oxygen intermediate (G). The title complex is prepared by 1-electron oxidation of a copper(II)-hydroxide precursor. Importantly, computational and X-ray absoption spectroscopy results, in combination with electrochemistry data, showed that the deep purple complex (Figure 2) contains Cu(III), and thus can be viewed as a protonated version of core F. It is extraordinarily reactive with dihydroanthracene, a commonly used substrate in studies of metal-oxygen complexes. Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M-1 s-1 at -80 °C, ΔH = 5.4(2) kcal mol-1, ΔS = -30(2) eu) and with very large kinetic isotope effects (cf. kH/kD = 44 at -70 °C). The findings suggest that a Cu(III)OH moiety is a viable reactant in oxidation catalysis.

Figure 1. Copper oxygen intermediate cores. Figure 2. The structure of the new Cu(III)-hydroxide complex alongside a cuvette showing its deep purple color.