Halogen Bonding and Electron Transfer Between Diiodine and Aromatic (Di)amines

ABSTRACT UV–Vis and X‐ray crystallographic measurements, together with the analysis of the potential energy surfaces, demonstrated a critical role of halogen‐bonded (HaB) complexes in the electron transfer (ET) reactions between diiodine and aromatic amines. Specifically, UV–Vis spectral studies showed that the addition of I 2 to solutions of tetramethyl‐ p ‐phenylenediamine (TMPD) or various N,N ‐dimethylanilines resulted in the immediate formation of HaB complexes. In systems with TMPD, this was followed by rapid ET—even at −85°C—resulting in the formation of persistent TMPD +• cation radicals (which were crystallized as salts with iodide counterions). The formation of HaB complexes upon mixing I 2 with dimethylanilines was followed by the generation of I 3 − , indicating the occurrence of redox processes. Subsequent chemical reactions led to the crystallization of triiodide salts of protonated m ‐methoxy‐ N,N ‐dimethylaniline, p ‐iodo‐ N,N ‐dimethylaniline (from the reaction with N,N ‐dimethylamine), or, surprisingly, 4,4′‐bis(dimethylamino)benzhydrylium cations (from the reaction of I 2 with p ‐bromo‐ N,N ‐dimethylaniline). Most notably, the rates of the redox processes in all these systems were many orders of magnitude faster than those predicted by the classical Marcus (outer‐sphere) ET theory. These high rates were accounted for by considering the donor–acceptor electronic coupling in the HaB precursor complexes, which drastically lowers the activation barriers for inner‐sphere ET. These results confirm that HaB complexes are vital intermediates in the ET reactions of halogenated electrophiles, and their role should be taken into account when analyzing potential ET steps in chemical transformations involving such molecules.