Probing solvation and ion-pairing effects on the redox behavior of Cyclopentadienyl Cobalt Dicarbonyl, CpCo(CO)2, in the presence of [B(C6F5)4]- anion

The redox behavior of CpCo(CO)2, 1, (Cp = η5 -C5H5) has been investigated in a number of organic solvents having different polarity and donor strength, in the presence of [B(C6F5)4] - as the supporting electrolyte anion. Voltammetric oxidation in weakly-donor (donor number, DNsolv, ~ 0) and low dielectric (ε) solvents such as benzotrifluoride (BTF, ε = 9.2), 1,2-dichloroethane (DCE, ε = 10.36), and 1,2-difluorobenzene (DFB, ε = 13.8) is identical to that observed in dichloromethane (DCM, ε = 8.9)/[NBu4][B(C6F5)4] media. Thus, in these gentle solvent/electrolyte media, the electrochemically generated 17e- cation, [CpCo(CO)2] + , 1 + , is postulated to undergo "radical-substrate" (R-S) dimerization via reaction with its neutral counterpart, 1, to eventually form, upon further one-electron oxidation [Cp2Co2(CO)4] 2+ , 2 2+ . However, this R-S coupling reaction is eliminated when the strongly donor tetrahydrofuran (THF, DNsolv = 20) or the highly polar nitromethane (NM, ε = 37.3) are used with [NBu4][B(C6F5)4], owing to rapid attack by solvent molecules on 1 + and formation of solvated species that dissociate into different oxidation products such as Cp2Co. These results attest that the redox behavior of CpCo(CO)2 is highly affected by the sequential changes in solvation energies and/or ion pairing, which can be manipulated via utilization of different solvent/electrolyte media.