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

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journal contribution
posted on 06.06.2011, 00:00 by René T. Boeré, Tracey L. Roemmele, Xin Yu
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.