Unsymmetrical 1λ3-1,2,4,6-Thiatriazinyls with Aryl and Trifluoromethyl Substituents: Synthesis, Crystal Structures, EPR Spectroscopy, and Voltammetry

A general synthetic route to 3-trifluoromethyl-5-aryl-1λ3-1,2,4,6-thiatriazinyl radicals was developed. X-ray structures were obtained for all five neutral radicals and show that they exist in the solid state as cofacial dimers linked by S···S contacts. X-ray structures were also obtained for two of the precursor chlorothiatriazines along with several aryl N-imidoylamidines, p-methoxybenzamidine, and N-chlorosulfonyl-N,N′-benzamidine. Cyclic voltammetric studies were performed on the [R2C2N3S]• radicals in CH3CN and CH2Cl2 with [nBu4N][PF6] as the supporting electrolyte under vacuum conditions in an all-glass electrochemical cell. The results provide quasi-reversible formal potentials for the [R2C2N3S]−/0 process in the range of −0.61 to −0.47 V, irreversible peak potentials for the [R2C2N3S]0/+ process from 0.59 to 0.91 V at lower concentrations, and the appearance of a second, reversible oxidation process from 0.69 to 0.94 V at higher concentrations (versus the Fc0/+ couple; Fc = ferrocene). This behavior was indicative of monomer–dimer equilibrium in solution, as ascertained from digital models of the voltammograms. There is a small but measurable trend in both the oxidation and reduction potentials with varying remote aryl substituents. EPR spectra were obtained for all five neutral radicals in CH2Cl2 solutions, which confirm the concentration of the unpaired electron density on the heterocyclic core. Trends were also seen in the hyperfine splitting constants aN with varying remote aryl substituents. Calculations were performed for all three oxidation states of the [R2C2N3S]−/•/+ monomeric rings; the resulting theoretical redox energies correlate well with solution phase voltammetric data.

本研究开发了一条合成3-三氟甲基-5-芳基-1λ³-1,2,4,6-硫三嗪基自由基的通用路线。获取了全部五种中性自由基的X射线晶体结构，结果表明它们在固态下以通过S···S相互作用连接的共面二聚体形式存在。同时还解析了两种前体氯代硫三嗪，以及多种芳基N-酰亚胺脒、对甲氧基苯甲脒和N-氯磺酰基-N,N′-苯甲脒的X射线晶体结构。以[nBu4N][PF6]为支持电解质，在全玻璃电化学池的真空条件下，于乙腈（CH3CN）和二氯甲烷（CH2Cl2）溶剂中对[R2C2N3S]•自由基开展了循环伏安研究。研究结果显示，[R2C2N3S]−/0氧化还原过程的准可逆式量电位处于−0.61至−0.47 V区间；在低浓度下，[R2C2N3S]0/+过程呈现不可逆峰电位，范围为0.59至0.91 V；而在高浓度下则出现第二个可逆氧化过程，电位区间为0.69至0.94 V（均相对于Fc0/+电对，Fc=二茂铁）。通过伏安图的数字模拟分析证实，该溶液行为表明体系存在单体-二聚体平衡。随着远端芳基取代基的变化，氧化与还原电位均呈现出微弱但可观测的变化趋势。在二氯甲烷溶液中对全部五种中性自由基采集了电子顺磁共振（EPR）谱，结果证实未成对电子密度主要集中于杂环核心。随着远端芳基取代基的变化，氮原子超精细分裂常数（aN）也呈现出相应的变化趋势。针对[R2C2N3S]−/•/+三种氧化态的单体环开展了理论计算，所得理论氧化还原能与溶液相伏安数据具有良好的相关性。