Electrochemical and spectrophotometrical investigation of the electron-accepting strength of organic superelectrophiles: X-ray structure of their charge transfer complexes with tetrathiafulvalene

Nitro Benzoxadiazoles (benzofurazans), benzoxadiazoles-N-oxide (benzofuroxans) and benzothiadiazoles are ranked amongst the strongest electrophiles known to date. In the past twenty years, their propensity to act as electron organic acceptors has been less studied. In this paper, we report on the study of their electrochemical behavior and on the structural characterization of charge transfer complexes (CTC) deriving from their interaction with tetrathiafulvalene (TTF) derivatives, both in solution and in the solid state. The first half wave reduction potentials (E-1/2(I)) associated with a reversible monoelectronic transfer process of a large set of nitro substituted benzoxadiazoles (benzofurazans), benzoxadiazoles-N-oxide (benzofuroxans) and benzothiadiazoles have been determined through a detailed electrochemical approach in acetonitrile with a microelectrode network using the ferrocene as an internal reference potential in this electrochemical study. Determination of the electron affinity (EA(CT)) of this series of substituted electrodeficient heteroaromatics as well as their LUMO energy was performed using the Charge Transfer Spectroscopic (CTS) method in solution and by DFT calculations, respectively. The use of the correlation EA(CT) versus the reversible half wave potential (E-1/2(I)) appears to be a useful tool to estimate readily the E-1/2(I) or EA(CT) values when they cannot be experimentally determined. The diffusion coefficient of these electrophiles has, for the first time, been determined in acetonitrile. These air stable electrodeficient heteroaromatics have been explored as potential new organic acceptors in the formation of charge transfer (CT) complexes with TTF derivatives. Crystallographic data of two CT complexes with TTF (especially the C-C and C-S bond lengths of the TTF moieties) indicate that these complexes exhibit weak electron delocalization and that both molecules remain neutral. Their resulting levels of charge transfer were probed using UV-visible, IR spectroscopy and by DFT calculations.