Linear Free Energy Relationships and Transition State Analysis of CO2 Reduction Catalysts Bearing Second Coordination Spheres with Tunable Acidity

The development of molecular catalysts for electrochemical CO2 reduction is a promising approach to the valorization of this stable small molecule. Drawing inspiration from enzymes, protic functional groups in the secondary coordination sphere (SCS) work in conjunction with an exogenous acid to relay proton equivalents to the active site of CO2 reduction. However, it is not well understood how the acidity of the SCS and exogenous acid together determine the kinetics of catalytic turnover. To gain insight into the relative contributions of proton transfer driving forces, we synthesized a series of iron tetraphenylporphyrin electrocatalysts bearing SCS amide groups of tunable pKa (17.6–20.0 in DMSO) and employed phenols of variable acidity (15.3–19.1) as exogenous acids. The modularity of this system allowed us to (1) evaluate contributions from proton transfer driving forces associated with either the SCS or exogenous acid, and (2) obtain mechanistic insights into CO2 reduction as a function of pKa. Plots of catalytic rate constants as a function of the various acidities reveal a series of linear free energy relationships: kinetics become increasingly sensitive to variations in SCS pKa when more acidic exogenous acids are used (0.82 ≥ Brønsted α ≥ 0.13), as well as to variations in exogenous acid pKa when acidity of the SCS is increased (0.62 ≥ Brønsted α ≥ 0.32). An Eyring analysis reveals that the rate-determining transition state is highly ordered and trends with SCS acidity (-88 ± 4 ≥ ΔSǂ ≥ -139 ± 3 J K-1 mol-1). These results are consistent with the proposal that SCS acidity modulates the degree of charge accumulation and solvation at the rate-limiting transition state. Together, this system provides key insights that enable optimization of catalytic activation barriers as a function of the acidity of all participants in a proton relay. The implications of this work can be used to guide rational design of electrocatalysts in which SCS acidity is considered in conjunction with that of the exogenous proton source.