Linear Free Energy Relationships and Transition State Analysis of CO2 Reduction Catalysts Bearing Second Coordination Spheres with Tunable Acidity
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".