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

19 April 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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.


carbon dioxide
secondary coordination sphere
CO2 reduction
catalyst design
iron porphyrin
small molecule activation

Supplementary materials

Supplementary Information
Supplemental experimental, electrochemical, synthetic, and crystallographic details.


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.