Electroreduction and acid-base properties of dipyrrolylquinoxalines

Abstract

The electroreduction and acid-base properties of dipyrrolylquinoxalines of the form H2DPQ, H2DPQ(NO2), and H 2DPQ(NO2)2 were investigated in benzonitrile (PhCN) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). This study focuses on elucidating the complete electrochemistry, spectroelectrochemistry, and acid-base properties of H2DPQ(NO2)n (n = 0, 1, or 2) in PhCN before and after the addition of trifluoroacetic acid (TFA), tetra-n-butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), or tetra-n-butylammonium acetate (TBAOAc) to solution. Electrochemical and spectroelectrochemical data provide support for the formation of a monodeprotonated anion after disproportionation of a dipyrrolylquinoxaline radical anion produced initially. The generated monoanion is then further reduced in two reversible one-electron-transfer steps at more negative potentials in the case of H2DPQ(NO2) and H 2DPQ(NO2)2. Electrochemically monitored titrations of H2DPQ(NO2)n with OH-, F-, or OAc- (in the form of TBA+X- salts) give rise to the same monodeprotonated H2DPQ(NO 2)n produced during electroreduction in PhCN. This latter anion can then be reduced in two additional one-electron-transfer steps in the case of H2DPQ(NO2) and H2DPQ(NO 2)2. Spectroscopically monitored titrations of H 2DPQ(NO2)n with X- show a 1:2 stoichiometry and provide evidence for the production of both [H 2DPQ(NO2)n]- and XHX-. The spectroscopically measured equilibrium constants range from log β2 = 5.3 for the reaction of H2DPQ with TBAOAc to log β2 = 8.8 for the reaction of H2DPQ(NO 2)2 with TBAOH. These results are consistent with a combined deprotonation and anion binding process. Equilibrium constants for the addition of one H+ to each quinoxaline nitrogen of H2DPQ, H2DPQ(NO2), and H2DPQ(NO2) 2 in PhCN containing 0.1 M TBAP were also determined via electrochemical and spectroscopic means; this gave rise to log β2 values ranging from 0.7 to 4.6, depending upon the number of nitro substituents present on the H2DPQ core. The redox behavior of the H2DPQ(NO2)n compounds of the present study were further analyzed through comparisons with simple quinoxalines that lack the two linked pyrrole groups, i.e., Q(NO2)n where n = 0, 1, or 2. It is concluded that the pyrrolic substituents play a critical role in regulating the electrochemical and spectroscopic features of DPQs. © 2012 American Chemical Society.