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Abstract:
A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I-+III) in a series of structurally characterized complexes (1-3). Redox interconversion of [LCu](+/2+/3+) is electrochemically (quasi)reversible and occurs at moderate potentials (E-1/2=-0.45 V and +0.82 V (vs. Fc/Fc(+))). A linear C-NHC-Cu-C-NHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of Cu-I versus Cu-II/Cu-III. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu K beta high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1-3 are textbook examples for Cu-I, Cu-II, and Cu-III species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced Cu-I form suggest that [LCu](+/2+/3+) may operate in catalysis via coupled 2 e(-)/2 H+ transfer.
