Fast dimerisation of the triparaquat radical dication

Abstract

The one-electron reduction of 2,4,6-tris(N-methylpyridinium-4-yl)pyridine trication, triparaquat, TPQ³⁺, synthesized in the form of the perchlorate salt, was studied by cyclic voltammetry on a platinum disc electrode. It was found that the reduction peak is centered at − 880 mV (s. SCE) at 500 mV s⁻¹ scanning rate. The subsequent oxidation takes place at + 75 mV, indicating an irreversible process, but no changes in the solution content were found after the oxidation. The blue colored reduction product reacts anomalistically slowly with oxygen with a second-order rate constant of (0.65 ± 0.01) mol⁻¹ dm³ s⁻¹. EPR experiments showed that the reduction product in ethanol solution does not possess an unpaired electron. A low temperature gamma radiolysis experiment revealed that the spectral peak maximum of the reduction product of TPQ³⁺ solubilized in PVA film is shifted ∽200 nm with respect to that obtained in aqueous solution at room temperature. Pulse radiolysis experiments proved that this seemingly inconsistent set of data was a consequence of the very fast dimerisation process 2TPQ^2+•(TPQ)₂⁴⁺ (k_dim = 1.4 × 10⁹ mol⁻¹ dm³ s⁻¹). Cyclic voltammetry experiments on a mercury electrode revealed that the second reduction step corresponds to the dimer reduction. The third and fourth reduction steps were irreversible and initiated further chemical reactions of triparaquat, which was consistent with the data obtained by the prolonged steady state gamma radiolysis.