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Imidazole Substituent Effects on Oxidative Reactivity of Tripodal(imid)2(thioether)CuI Complexes

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posted on 19.05.2008, 00:00 by Lei Zhou, Kenneth M. Nicholas
In the search for new bis(imidazole)thioether (BIT) copper complexes that accurately mimic the electronic and reactivity features of the CuM site of copper hydroxylase enzymes, a set of tripodal BIT ligands 4a,b6a,b has been synthesized that vary according to the imidazole C-(Ph or H) and N-(H or Me) substituents, as well as the position (2- or 4-) of the tripodal attachment. Corresponding [(BIT)Cu(L)](PF6) complexes 7a,b′, 8a,b′, and 9a′,b′ [L = CO (a), CH3CN (b)] have been prepared and characterized spectroscopically. The IR spectra of 7a9a (L = CO), specifically ν(CO), show little variation (2090–2100 cm−1), suggesting a similar electronic character of the Cu centers. In contrast, cyclic voltammetric analysis of these compounds (L = CH3CN) reveals quasi-reversible oxidation waves with significant variation of Epa in the range of + 0.45–0.57 V vs Fc/Fc+, depending on the imidazole substituents. Each of the [(BIT)Cu(CH3CN)]PF6 complexes reacts with dioxygen to form [(BIT)CuII2(μ-OH)2](PF6)2 derivatives, 1012, but they vary considerably in their relative reactivity, following the same trend as the ease of their electrochemical oxidation, that is, [(2-BITNMe)Cu(CH3CN)]+ (9b′) > [(4-BITPh,NMe)Cu(CH3CN)]+ (8b′) > [(2-BITPh2,NMe)Cu(CH3CN)]+ (1a′) > [(4-BITPh,NH)Cu(CH3CN)]+ (7b′). Thus, N-Me substitution and 4-tethering on the imidazole unit increase oxidation and oxygenation reactivity, while Ph-substitution and 2-tethering decrease reactivity. PM3 and DFT calculations are employed to analyze the relative stability, the electronic features, the Cu−CO vibrtional frequency, and the electrochemical and oxidative reactivity of the complexes.