Isolation and Structure Elucidation of Transient (Colored) Complexes of Arenediazonium with Aromatic Hydrocarbons as Intermediates in Arylations and Azo Couplings

Abstract

Brightly colored (1:1) complexes are observed immediately upon the mixing of electron-poor arenediazonium (ArN2+) salts and aromatic hydrocarbons (ArH). Binding energies of these intermolecular electron donor−acceptor or EDA complexes are typically ΔGEDA < 1 kcal mol-1 in acetonitrile. As such, the weak donor−acceptor interaction in [ArN2+ ArH] is reminiscent of that in other transient complexes which exhibit charge-transfer (CT) absorption bands of the type originally described by Mulliken. Successful isolation of crystalline (colored) complexes between various ArH/ArN2+ pairs allows X-ray crystallography to identify the pertinent donor−acceptor interaction responsible for the distinctive (CT) colors. Indeed, the cofacial stacking of the aromatic donor on top of the aromatic moiety of ArN2+ is uniquely different from the usual bonding modes of electron-rich anions directly to the diazono (N2+) center. The mechanistic implication of such π−π interactions of ArH/ArN2+ pairs to the transition state for elec...