Towards redox-switchable organocatalysts based on bidentate halogen bond donors.

Redox-active bidentate halogen bond donors based on halopyridinium groups as halogen-bond donating units were synthesized and their structures were elucidated by X-ray diffraction analyses and DFT calculations. Via reversible twofold reduction, these dicationic species can be transformed to neutral compounds which should be much weaker Lewis acids. The corresponding electrochemical data were obtained, and CV as well as UV-vis and NMR techniques were also used to determine binding constants of these halogen bond donors to halides. While all titrations agree on the relative order of binding strengths (with chloride being bound strongest), there are marked deviations in the overall affinity constants which are discussed. In contrast to earlier azo-bridge analogues, the ethylene-linked variants presented herein do not oxidize halides, and thus the novel halogen bond donors could also be used as Lewis acidic organocatalysts in a halide abstraction benchmark reaction, yielding a performance similar to bis(haloimidazolium)-derived catalysts.