Benzene and hexamethylbenzene partake in the rapid formation of unique electron donor-acceptor (EDA) complexes with the nitrosonium cation, as reflected in the association constants that vary markedly from K = 0.5 to 31000 M−1, respectively. The 1:1 EDA complexes of electron-rich arene donors exhibit particularly high degrees of charge transfer that are established by X-ray crystallography coupled to the infrared spectroscopic analysis of the N-O stretching frequencies. Thus the N-O bond distance undergoes a dramatic lengthening from 0.95 ° in the simple salt NO+SbCl6− to 1.12 ° in the hexamethylbenzene (HMB) complex. Such a bond elongation of the nitrosonium acceptor is equivalent to that extant in nitric oxide (1.15 °), and it is accompanied by a corresponding change of the aromatic donor in the ground state of the EDA complex to that of the cation radical (HMB+), as deduced from the changes in the l3C NMR and the IR spectra. The same structural and spectroscopic analysis of the other NO+ complexes reveals a graded series of the EDA complexes in which the degree of charge transfer (Z) varies linearly with the donor strength as measured by the ionization potential of the arene (ArH). The increase of Z from benzene (0.52) to hexamethylbenzene (0.97) in the charge-transfer complex tracks their increasing proclivity to undergo electron transfer and afford arene cation radicals (ArH+) as reactive intermediates. A general mechanistic formulation based on electron transfer from the charge-transfer complex is developed, in which the facility of the followup reactions (such as fragmentation, cycloreversion, rearrangement, ion-pair annihilation, etc.) of the labile ArH+ is critical. When the electron transfer is reversible, this 1-electron mechanism is shown not to be readily distinguished from the more conventional electrophilic (2-electron) pathways.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry