Electrochemical and Electronic Structure Investigations of the [S3N3]• Radical and Kinetic Modeling of the [S4N4]n/[S3N3]n (n = 0, −1) Interconversion

Abstract

Voltammetric studies of S4N4 employing both cyclic (CV) and rotating disk (RDE) methods in CH2Cl2 at a glassy carbon electrode reveal a one-electron reduction at −1.00 V (versus ferrocene/ferrocenium), which produces a second redox couple at −0.33 V, confirmed to be the electrochemically generated [S3N3]− by CV studies on its salts. Diffusion coefficients (CH2Cl2/0.4 M [nBu4N][PF6]) estimated by RDE methods: S4N4, 1.17 × 10−5 cm2 s−1; [S3N3]−, 4.00 × 10−6 cm2 s−1. Digital simulations of the CVs detected slow rates of electron transfer for both couples and allowed for a determination of rate constants for homogeneous chemical reaction steps subsequent to electron transfer. The common parameters (kf1 = 2.0 ± 0.5 s−1, ks1 = 0.034 ± 0.004 cm s−1 for [S4N4]−/0; kf2 = 0.4 ± 0.2 s−1, ks2 = 0.022 ± 0.005 cm s−1 for [S3N3]−/0 at T = 21 ± 2 °C) fit well to a “square-scheme” mechanism over the entire range of data with first order decay of both redox products. An alternate model could also be fit wherein [NS]• liberated in the first step reacts with formed [S3N3]• to reproduce S4N4 with an apparent second order rate constant kf2′ = 1.1 ± 0.3 × 103 M−1 s−1. The crystal structure of [PPN][S3N3] was determined by X-ray crystallography indicating the solvation of the anion by 1 equiv of methanol. The generated [S4N4]−• radical anion was detected by the Simultaneous Electrochemical Electron Paramagnetic Resonance (SEEPR) method to give: (a) [32S414N4]−•, 9 lines, a(14N) = 0.118 mT; (b) [32S415N4]−•, 5 lines, a(15N) = 0.164 mT; (c) [33S414N4]−•, estimated a(14N) = 0.118, a(33S = 0.2 mT); g = 2.0008(1). Equivalence of 33S hyperfine splittings is consistent with dynamic averaging of the C2v geometry in solution. High-level electronic structure calculations provide evidence for an open-shell doublet triradicaloid character to the ground state wave function of [S3N3]•.