Charge-transfer structures of aromatic EDA complexes with N-heteroatom-substituted pyridinium cations

Abstract

X-Pyridinium cations with N-heteroatomic substituents such as X = halogen, nitro, alkoxy and acyloxy (i.e., X-NC₅H₅⁺), in common with the ubiquitous N-alkylpyridinium cations, interact spontaneously with different benzene, naphthalene and anthracene donors (ArH) to afford a series of 1 : 1 charge-transfer complexes showing characteristic absorption spectra in the visible region. Quantitative evaluation of the electron-acceptor properties of X-Py⁺(via an electrochemical reduction potential E_p^c) indicates that the N-nitropyridinium (E_p^c=+0.10 V), N-fluoropyridinium (E_p^c=–0.66 V), N-acetoxypyridinium (E_p^c=–0.81 V), and N-methoxypyridinium (E_p^c=–1.02 V) cations are strong electrophiles in their charge-transfer interaction with aromatic donors to produce cofacial acceptor–donor pairs [X–Py⁺, ArH] that are structurally related to the more common N-methylpyridinium (E_p^c=–1.32 V) analogues.