Electrochemical and chemical reduction of disulfur dinitride: formation of [S4N4], EPR spectroscopic characterization of the [S2N2H]• radical and X-ray structure of [Na(15 crown-5)][S3N3]

Abstract

Voltammetric studies of S2N2 employing both cyclic voltammetry (CV) and rotating disc electrode (RDE) methods on GC electrodes at RT revealed two irreversible reduction processes at ca. −1.4 V and −2.2 V in CH3CN, CH2Cl2 and THF (vs. ferrocene) and no observable oxidation processes up to the solvent limit when the scan is initially anodic. However, after cycling the potential through −1.4 V, two new couples appear near −0.3 V and −1.0 V due to [S3N3]−/0 and [S4N4]−/0 respectively. The diffusion coefficient D for S2N2 was determined to be 9.13 × 10−6 cm2 s−1 in CH2Cl2 and 7.65 × 10−6 cm2 s−1 in CH3CN. Digital modeling of CVs fits well to a mechanism in which [S2N2]−• couples rapidly with S2N2 to form [S4N4]−•, which then decomposes to [S3N3]−. In situ EPR spectroelectrochemical studies of S2N2 in both CH2Cl2 and CH3CN resulted in the detection of strong EPR signals from [S4N4]−• when electrolysis is conducted at −1.4 V; at more negative voltages, spectra from transient adsorbed radicals are observed. In moist solvent or with added HBF4, a longer-lived spectrum is obtained due to the neutral radical [S2N2H]•, identified by simulation of the EPR spectrum and DFT calculations. The chemical reduction of S2N2 with Na[C10H8] or Na[Ph2CO] produces [Na(15-crown5)][S3N3], while reduction with cobaltocene gives [Cp2Co][S3N3]. The X-ray structure of the former reveals a strong interaction (Na···N = 2.388(5) Å) between the crown ether-encapsulated Na+ cation and one of the nitrogen atoms of the essentially planar six-membered cyclic anion [S3N3]−.