Solvent effect on the electrode reduction mechanism of phenyl substituted ethylenes

Abstract

This communication presents some preliminary results on the electrochemical reduction of tri- and tetraphenylethylenes in dimethylformamide (DMF) and hexamethylphosphoramide (HMPA), as part of a more extensive study on the correlation between kinetic and thermodynamic properties of radical anions and electrode reaction mechanism of the parent molecules. Triphenylethylene (ø 3E) Cyclic voltammetry of ø 3E in DMF with 0.1 M TBAP as supporting electrolyte shows two reversible reduction peaks, corresponding to the formation of the anion radical (R + e ⇌ R ⨪) and of the dianion (R ⨪ + e ⇌ R =), provided the solvent is carefully anhydrified, the last traces of water being eliminated by addition of alumina or molecular sieves directly into the cell. Traces of H 2O, in fact, cause the irreversibility of the second reduction peak (disappearance of the corresponding anodic peak) even at a scan rate of 400 V sec −1. Further addition of H 2O affects also the reversibility of the first reduction peak, which increases in height giving eventually a single irreversible two-electron peak. Macroscale electrolysis in the presence of H 2O yields quantitatively triphenylethane. The anion radical R ⨪, stable in absence of proton donors, undergoes a relatively slow decay in the presence of H 2O at low temperatures (−30 °C) according to the following mechanism: ▪ HS being the proton donor. Holding for R = the stationary state hypothesis the decay rate of R ⨪ results −dC R⨪/dt = 2k′ pk dC 2 R⨪/(k′ p + k −dC R) with k′ p = k pC HS. This results in a second order reaction when C R ⪢ C R⨪, so that C R, is practically constant. Values of k d, k −d and k′ p can thus be obtained, the equilibrium dismutation constant having been derived from the peak potential separation. On the other hand, for k −dC R ⪡ k′ p one has −dC R⨪/dt = 2k dC 2 R⨪. These conditions, which can be realised at higher temperatures in voltammetric conditions, correspond to an irreversible dismutation and allow an easy determination of k d; the latter results 9 × 10 3 M −1 sec −1 at 40 °C with an activation energy of 10 kcal mol −1, which compares very well with the value of 9 × 10 1 M −1 sec −1 obtained at −30 °C. The voltammetric behaviour of ø 3E in HMPA is analogous to that in DMF, except for a higher peak separation, which is consistent with a lower tendency to ion pair formation in HMPA. Tetraphenylethylene (ø 4E) In HMPA, ø 4E presents two one-electron reversible reduction peaks, the second one being about 220 mV less negative than the corresponding one of ø 3E, in the same conditions. This is to be attributed to a higher delocalisation of the negative charge in the dianion of ø 4E. In DMF only a single reversible two-electron wave can be observed, resulting from the overlapping of two one-electron processes, separated by about 60 mV. This lower peak separation is in agreement with the effect already observed for ø 3E, passing from HMPA to DMF, the latter solvent enhancing the formation of ion pairs. Kinetic runs carried out in DMF at 0 °C have shown that the decay mechanism of the radical anion of ø 4E is the same already observed for ø 3E. Further investigations are in progress with the aim of evaluating the role of the solvent and of the cations of the supporting electrolyte on the kinetic and electrochemical behaviour of the radical anions of substituted ethylenes.