Imidazole substituent effects on oxidative reactivity of tripodal(imid)2(thioether)CuI complexes

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, b- 6a, 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 7a- 9a (L=CO), specifically nu(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(CH 3CN)]PF6 complexes reacts with dioxygen to form [(BIT)Cu(II) 2(mu-OH) 2](PF6)2 derivatives, 10- 12, but they vary considerably in their relative reactivity, following the same trend as the ease of their electrochemical oxidation, that is, [(2-BIT (NMe))Cu(CH 3CN)](+) ( 9b')>[(4-BIT (Ph,NMe))Cu(CH3CN)](+) ( 8b')>[(2-BIT (Ph2,NMe))Cu(CH3CN)](+) (1a')>[(4-BIT (Ph,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.