Influence of the nature of solvent and substituents on the oxidation potential of 2,2,6,6-tetramethylpiperidine 1-oxyl derivatives

The influence of solvent (DMF, MeCN, and water) and R1, R2 substituent nature on the formal oxidation potential (E°´) of 4-R1,R2-2,2,6,6-tetramethylpiperidine 1-oxyls (1a—f) on a glass carbon electrode was studied by cyclic voltammetry. It was shown that for all the solvents the observed dependence had the form E°´ = ρ″σ″ + b, where σ″ is the substituent constant. The b values decreased with an increase of the solvent solvating ability, while the values ρ″ are similar for all the solvents, surpassing the corresponding values for nitroxyls of the imidazoline series with substituents at position 3, which can be interpreted as an abnormally strong influence of the substituent remote from the reaction center in 1a—f. The experimental values E°´ were linearly correlated with the reaction free energy values (ΔG) calculated by DFT B3LYP and MP2 for the gas phase contribution and by HF/PCM for the contribution of solvation effects. When applying the B3LYP and the HF/PCM approaches in combination, the dependence of ^E°´ on Δ^G for all the considered solvents was described by a linear correlation equation with a slope close to unity and a constant term which was close to the theoretical value of the absolute potential of the reference electrode used.