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

Abstract

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.