Linear Free Energy Relationships and Transition State Analysis of CO₂ Reduction Catalysts Bearing Second Coordination...

In molecular catalysts, protic functional groups in the secondary coordination sphere (SCS) work in conjunction with an exogenous acid to relay protons to the active site of electrochemical 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 evaluate the relative contributions of proton transfer driving forces, we synthesized a series of modular iron tetraphenylporphyrin electrocatalysts bearing SCS amides of tunable pK_a (17.6 to 20.0 in dimethyl sulfoxide (DMSO)) and employed phenols of variable acidity (15.3 to 19.1) as exogenous acids. 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 pK_a. A series of linear free-energy relationships show that kinetics become increasingly sensitive to variations in SCS pK_a when more acidic exogenous acids are used (0.82 ≥ Brønsted α ≥ 0.13), as well as to variations in exogenous acid pK_a when SCS acidity is increased (0.62 ≥ Brønsted α ≥ 0.32). An Eyring analysis suggests that the rate-determining transition state becomes more ordered with decreasing SCS acidity, which is consistent with the proposal that SCS acidity modulates charge accumulation and solvation at the rate-limiting transition state. Together, these insights enable the optimization of activation barriers as a function of both SCS and exogenous acid pK_a and can further guide the rational design of electrocatalytic systems wherein contributions from all participants in a proton relay are considered.