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]

Voltammetric studies of S2N2 employing both cyclic voltammetry (CV) and rotating disk electrode (RDE) methods on GC electrodes at room temperature (RT) revealed two irreversible reduction processes at about -1.4 V and -2.2 V in CH3CN, CH2Cl2, and tetrahydrofuran (vs ferrocene) and no observable oxidation processes up to the solvent limit when the scan is initially anodic. However, aftercycling 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 x 10(-6) cm(2) s(-1) in CH2Cl2 and 7.65 x 10(-6) cm(2) s(-1) in CH3CN. Digital modeling of CVs fits well to a mechanism in which [S2N2](-center dot) couples rapidly with S2N2 to form [S4N4](-center dot), which then decomposes to [S3N3](-). In situ electron paramagnetic resonance (EPR) spectroelectrochemical studies of S2N2 in both CH2Cl2 and CH3CN resulted in the detection of strong EPR signals from [S4N4](-center dot) 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](-)center dot, identified by simulation of the EPR spectrum and density functional theory (DFT) calculations. The chemical reduction Of S2N2 with Na[C10H8] or Na[Ph2CO] produces [Na(15-crown-5)][S3N3], while reduction with cobaftocene gives [Cp2Co][S3N3]. The X-ray structure of the former reveals a strong interaction (Na center dot center dot center dot N = 2,388(5) angstrom) between the crown ether-encapsulated Na+ caton and one of the nitrogen atoms of the essentially planar six-membered cyclic anion [S3N3](-).