Molecular ions of 3,3′-bi(2-R-5,5-dimethyl-4-oxopyrrolinylidene) 1,1′-dioxides

Abstract

The electrochemical reduction of 3,3´ bi(2 R 5,5 dimethyl 4 oxopyrrolinylidene) 1,1´ dioxides (R = CF 3 , Me, Ph, Bu t ), which are cyclic dinitrons with conjugated С=С bond, in acetonitrile is an EE process producing stable radical anions and dianions, whereas the electrochemical oxidation is an EEC (R = Me, Ph) or EE process (R = Bu t ) with formation of radical cations (except for the case of R = CF 3 ) and dications (R = Bu t ) stable under standard conditions. Radical cations of the dioxides with R = Me, Ph, and Bu t and radical anions of the whole series of the compounds studied, including R = CF 3 , were characterized by ESR spectroscopy combined with electrochemical measurements and quantum chemical calcula tions. The electrochemical behavior of the Bu t substituted dinitron is unique: the EE processes in the region of negative and positive potentials with formation of the dianion, radical anion, radical cation, and dication stable at Т = 298 K were observed for the first time within one cycle of potential sweep in the CV curve measured in MeCN.It has previously 1 been shown that 3,3´ bi(2 R 5,5 di methyl 4 oxopyrrolinylidene) 1,1´ dioxides 1-4, being cyclic dinitrons with conjugated C=C bond, have a con siderable electron withdrawing ability comparable with that of tetracyanoethylene, 2 and their electrochemical re duction (ECR) in DMF is an EE process (includes two one electron electrochemical steps) with formation of radical anions (RA) and dianions (DA) stable at Т = 298 K.The one electron reduction of dinitrons 1-4 is ac companied by the formation of the corresponding RA detected in the ESR spectra. According to these data, in the radical anion states, the substituents in positions 2 of the pyrrolinone rings disturb weakly the π system O←N=C-C=C-C=N→O, both 14 N nuclei are spec trally equivalent, the corresponding isotropic hyperfine interaction (IHFI) constants a N are close in magnitude and vary in a range of ±0.004 mT with R variation, and hyperfine coupling with the substituent nuclei appears only in the cases of R = CF 3 and Me.Dinitrons 1-4 are formed due to the mild chemical oxidation of the starting pyrrolinones 5. 3 The extension of the π system in compounds 1-4 compared to the starting pyrrolinones should decrease an energy gap between the levels of the frontier MOs (∆ε = ε LUMO -ε HOMO , LUMO and HOMO are the lowest unoc cupied and highest occupied MOs, respectively) and result in an easier one electron oxidation and, prob ably, stabilization of the positively charged molecular ions (cf. Ref. 4).To study a possibility of formation and estimate stability limits of radi cal cations (RC) and dications (DC), we studied the electro chemical oxidation (ECO) and, for comparison, ECR of compounds 1-4 in MeCN by cyclic voltammetry (CV) and ESR spectroscopy. ExperimentalCyclic voltammograms of dinitrons 1-4 were measured on an SVA 1BM electrochemical system (Bulgaria) equipped with a LAB MASTER polyfunctional interface (Institute of Nuclear Physics, Novosibirsk, Russia), which enables one complete d...

2-Butyl-2-tert-butyl-5,5-diethylpyrrolidine-1-oxyls: Synthesis and Properties

“…The samples of similar dimers are known to have quintet EPR spectra, initially attributed to a contribution of the biradical form [28]. It has been demonstrated later that the signal belongs to paramagnetic impurities [29], presumably to radical ions of the dinitrones [30]. Exchange with these radical ions may account for the strong broadening of some signals in 13 C NMR spectra of 12 (see Supplementary Materials).…”

Radical anions of 3,3′-bi(2-R-5,5-dimethyl-4-oxopyrrolinylidene) 1,1′-dioxides in MeCN-H2O solvent mixtures

“…For RA 2´ and 3´, only HFS on the 14 N nuclei are observed due to the remoteness of the sub stituents R protons, which is in agreement with the pub lished data. 2 The dependencies of the IHFI constants for RA 1´-4f rom χ are shown in Fig. 2.…”

Electrochemical Reduction, Oxidation and Molecular Ions of 3,3´-bi(2-R-5,5-dimethy-1-4- oxopyrrolinylidene) 1,1´-dioxides

“…1, example for DN 1). The replacement of the DMF [10] solvent by MeCN [11] does not qualitatively change the reduction processes. In MeCN, the potentials of reduction peaks are by approximately 0.1 V shifted to the region of more negative values compared to the potentials measured in DMF (Table1).…”

“…The transition from the Et 4 NClO 4 supporting electrolyte (see Ref. [11]) to LiClO 4 shifts the potentials of the first cathodic peaks (E () 1c ) measured in MeCN by 0.1 V, on the average, toward less negative potentials. The diffusion nature and reversible one electron character of the first reduction peak are retained upon this transition.…”

“…ESR spectra of the corresponding RA are detected under ECR at potentials of the first cathodic peaks (E (-) 1c ) [11] (Fig. 6).…”