Charge transfer complexation of N,N-dimethyl-p-toluidine with nitrogen heteroaromatic cations as a model for association complexes in hydride transfer reactions

Abstract

Charge-transfer complexation of N,N-dimethyl-p-toluidine (DMT) with each of the following N-methyl cations has been investigated in acetonitrile solution at 25 °C: acridinium; phenanthridinium; 3-X-quinolinium and 4-X-isoquinolinium (X = H, Br, CONH2, CN); 5-nitroisoquinolinium; 3-X-pyridinium (X = CONH2, CN) (also the N-benzyl pyridinium cations); N,N′-dimethyl-4,4′-dipyridylium dication. Charge-transfer absorption maxima are reported, and extinction coefficients (εmax) at these maxima and association constants (K) for 1:1 complex formation have been evaluated from the dependence of absorbance upon [DMT]. In general, the longest wavelength absorption maximum increases with increasing association constant. There is a strictly linear relationship between εmax and 1/K. These observations are considered in the context of theories of charge-transfer spectra. Such charge-transfer species are considered as models for the association complexes that are believed to exist upon the reaction pathway for hydride transfer between heteroaromatic cations. Although some significant qualitative relationships are apparent, variations in susceptibility of heteroaromatic cations to charge-transfer complexation are much smaller than variations in the reactivity of these cations towards hydride donor species.