Two-electron-transfer redox systems. Part 8. Proving the electron stoichiometry for the electrochemical two-electron oxidation of N,N'-bis(ferrocenoyl)-1,2-diaminoethane

摘要

N,N'-Bis(ferrocenoyl)-1,2-diaminoethane is electrochemically oxidized at Pt electrodes in non-aqueous solvents with NBu_4PF_6 or NBu_4[Al(pftb)_4] (pftb=OC(CF_3)_3) as supporting electrolytes. Cyclic voltammetry (CV) at macro and ultramicro disk electrodes in propylene carbonate (PC) in the presence of the former electrolyte shows a single signal from which the number of transferred electrons n (equal to 2) and the diffusion coefficient D are estimated and confirmed by simulation. In a series of other solvents (DMF, DMSO, CH_3CN, THF, CH_2Cl_2) with the same electrolyte, similar behavior is observed. The latter supporting electrolyte, however, induces signal broadening as compared to NBu_4PF_6 in PC for cyclic and differential pulse voltammetry as well as peak splitting in the case of THF and CH_2Cl_2. This is attributed to interaction between the two ferrocene moieties in the molecule, which is not relayed through a conjugated bridge. Diffusion coefficients in DMF and DMSO were determined by pulse gradient spin echo NMR experiments and compared to electrochemical values. The solvent dependence of D follows the Stokes-Einstein relation. Comproportionation equilibrium constants as calculated from the difference of the formal potentials E~0 range from below 10 (PC, NBu_4PF_6) to approximately 150 (THF and CH_2Cl_2, NBu_4[Al(pftb)_4]).