Pendant Acid–Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays? Mechanisms of the Conversion of CO₂ to CO by Electrogenerated Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities

Abstract

Two derivatives of iron tetraphenylporphyrin bearing prepositioned phenolic functionalities on two of the opposed phenyl groups prove to be remarkable catalysts for the reduction of CO2 to CO when generated electrochemically at the Fe(0) oxidation state. In one case, the same substituents are present on the two other phenyls, whereas in the other the two other phenyls are perfluorinated. They are taken as examples of the possible role of pendant acid-base groups in molecular catalysis. The prepositioned phenol groups incorporated into the catalyst molecule induce strong stabilization of the initial Fe(0)CO2 adduct through H-bonding, confirmed by DFT calculations. This positive factor is partly counterbalanced by the necessity, resulting from the same stabilization, to inject an additional electron to trigger catalysis. Thanks to the preprotonation of the initial Fe(0)CO2 adduct, the potential required for this second electron transfer is not very distant from the potential at which the adduct is generated by addition of CO2 to the Fe(0) complex. The protonation step involves an internal phenolic group and the reprotonation of the phenoxide ion thus generated by added phenol. The prepositioned phenol groups thus play both the role of H-bonding stabilizers and high-concentration proton donors. They play the same role in the second electron transfer step which closes the catalytic loop concertedly with the breaking of one of the two C-O bonds of CO2 and with proton transfer. It is also remarkable that reprotonation by added phenol is concerted with the three other events.

Ultraefficient homogeneous catalyst for the CO 2 -to-CO electrochemical conversion

“…Although the favorable effect of pendant acid groups has been noted in several cases (see ref. 27 and references therein), this was, to our knowledge, the first time their exact role was deciphered. The difference in the role played by the phenol moieties takes place within the framework of two different mechanisms (see Scheme 2 for CAT and FCAT and Scheme 3 for FeTPP) (27).…”

“…27 and references therein), this was, to our knowledge, the first time their exact role was deciphered. The difference in the role played by the phenol moieties takes place within the framework of two different mechanisms (see Scheme 2 for CAT and FCAT and Scheme 3 for FeTPP) (27). With FeTPP, the first step is, as with CAT and FCAT, the addition of CO 2 on the electrogenerated Fe 0 complex (et 1 in Schemes 2 and 3).…”

“…At first blush, the comparison with the role of phenol in the case of FeTPP would entail attributing this considerable enhancement of catalysis to a local concentration of phenol much larger than can be achieved in solution. In fact, as analyzed in detail elsewhere (27), the role played by the internal phenol moieties is twofold. They indeed provide a very large local phenol concentration, favoring proton transfers, but they also considerably stabilize the initial Fe 0 -CO 2 adduct through H bonding.…”

“…However, a deeper analysis of the meaning of these figures in terms of effective catalysis is required. The mechanism of the reaction (Scheme 2) has been shown to be the same with FCAT as with CAT, and the various kinetic parameters indicated in Scheme 2, whose values are recalled in Table 1, have been determined (26,27). Comparison of the two catalysts may then be achieved more rationally based on the determination, in each case, of the catalytic Tafel plots, which relates the turnover frequency (TOF) to the overpotential (η).…”

“…1, derives from secondary phenomena related to the observed high catalytic efficiencies, such as substrate consumption, inhibition by products, and deactivation of the catalyst. These factors and the ways to go around their occurrence to finally obtain a full characterization of the mechanism and kinetics of the catalysis process are discussed in detail elsewhere (27). Under pure kinetic conditions, the active catalyst molecules are then confined within a thin reaction-diffusion layer adjacent to the electrode surface.…”