Lewis acid effects on donor–acceptor associations and redox reactions: ternary complexes of heteroaromatic N-oxides with boron trifluoride and organic donors

Structural and spectral features of the novel supramolecular [D·NXO·BF₃] complexes formed via simultaneous n-coordination of a Lewis acid (BF₃) and π-complexation of organic donors (D) to polyfunctional nitrosubstituted N-oxide molecules (NXO = 4-nitropyridine-N-oxide or 4-nitroquinoline-N-oxide) are reported. X-Ray studies of [pyrene·NPO·BF₃]·CH₂Cl₂ and [(pyrene)₂·NQO·BF₃] salts revealed that the Lewis acid is coordinated to the oxygen atom of the N-oxide group with O–B distance of 1.52 Å similar to that in the separate [NXO·BF₃] adducts. Aromatic rings of N-oxide molecules in the ternary systems are π-stacked with pyrene moieties, and their interplanar separations of 3.35 Å are common for conventional charge-transfer complexes. In dichloromethane, associations between [NXO·BF₃] adduct and organic donors are characterized by higher formation constants (and charge-transfer bands of [D·NXO·BF₃] complexes are red-shifted) as compared to the complexes between the same donors and NXO acceptors. Spectral data and electrochemical measurements point out enhanced acceptor abilities of [NXO·BF₃] adducts relative to the separate N-oxides which correspond to 0.5 V positive shift of reduction potentials of acceptors. Synergetic oxidation of strong organic donors by [NXO·BF₃] dyads (which results in the formation of the corresponding cation radicals and products of transformation of both BF₃ and NXO components) is discussed.