Isolation and structure elucidation of transient (colored) complexes of arenediazonium with aromatic hydrocarbons as intermediates in arylations and azo couplings

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 Delta G(EDA) < 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 (N-2(+)) center. The mechanistic implication of such pi-pi interactions of ArH/ArN2+ pairs to the transition state for electron-transfer arylation is discussed in its relationship to electrophilic azo coupling.