Dihydropteridine/Pteridine as a 2H+/2e- Redox Mediator for the Reduction of CO2 to Methanol: A Computational Study

Mechanism of photocatalytic CO2 reduction to HCO2H by a robust multifunctional iridium complex

The tetradentate PNNP-type IrIII complex Mes-IrPCY2 ([Cl-IrIII-H]+) is reported to be an efficient catalyst for the reduction of CO2 to formate with excellent selectivity under visible light irradiation. Density functional calculations have been carried out to elucidate the mechanism and the origin of selectivity in the present work. Calculations suggest that the double-reduced complex 1-H (1[IrI-H]0) demonstrates higher activity than the single-reduced complex 2-H (2[IrIII(L˙-)-H]+), possibly owing to the higher hydride donor ability of the former compared to the latter; thus 1-H functions as the active species in the overall CO2 reduction reaction. In the HCOO- formation pathway, the hydride of 1-H performs a nucleophilic attack on CO2via an outer-sphere fashion to generate species 1-OCHO (1[IrI-OCHO]0), which then releases HCOO- to produce an IrI intermediate. A subsequent protonation and chloride coordination of the Ir center leads to the regeneration of catalyst 1[Cl-IrIII-H]+. For the CO production, a nucleophilic attack on CO2 takes place by the Ir atom of 1-Hvia an inner-sphere manner to afford complex O2C-3-H (1[O2C-IrIII-H]0), followed by a two-proton-one-electron reduction to furnish the OC-2-H complex (2[OC-IrIII(L˙-)-H]+) after liberating a H2O. Ultimately, CO is released to form 2-H. The stronger nucleophilicity as well as smaller steric hindrance of the hydride than the Ir atom of the active species 1-H (1[IrI-H]0) is found to account for the favoring of formate formation over CO formation. Meanwhile, the CO2 reduction reaction is calculated to be preferred over the hydrogen evolution reaction, and this is consistent with the experimental product distributions.

Beyond Continuum Solvent Models in Computational Homogeneous Catalysis

In homogeneous catalysis solvent is an inherent part of the catalytic system. As such, it must be considered in the computational modeling. The most common approach to include solvent effects in quantum mechanical calculations is by means of continuum solvent models. When they are properly used, average solvent effects are efficiently captured, mainly those related with solvent polarity. However, neglecting atomistic description of solvent molecules has its limitations, and continuum solvent models all alone cannot be applied to whatever situation. In many cases, inclusion of explicit solvent molecules in the quantum mechanical description of the system is mandatory. The purpose of this article is to highlight through selected examples what are the reasons that urge to go beyond the continuum models to the employment of micro-solvated (cluster-continuum) of fully explicit solvent models, in this way setting the limits of continuum solvent models in computational homogeneous catalysis. These examples showcase that inclusion of solvent molecules in the calculation not only can improve the description of already known mechanisms but can yield new mechanistic views of a reaction. With the aim of systematizing the use of explicit solvent models, after discussing the success and limitations of continuum solvent models, issues related with solvent coordination and solvent dynamics, solvent effects in reactions involving small, charged species, as well as reactions in protic solvents and the role of solvent as reagent itself are successively considered.

Diazaphospholenes as reducing agents: a thermodynamic and electrochemical DFT study

Diazaphospholenes have emerged as a promising class of metal-free hydride donors and have been implemented as molecular catalysts in several reduction reactions. Recent studies have also verified their radical reactivity as hydrogen atom donors. Experimental quantification of the hydricities and electrochemical properties of this unique class of hydrides has been limited by their sensitivity towards oxidation in open air and moist environments. Here, we implement quantum chemical density functional theory calculations to analyze the electrochemical catalytic cycle of diazaphospholenes in acetonitrile. We report computed hydricities, reduction potentials, pK_a values, and bond dissociation free energies (BDFEs) for 64 P-based hydridic catalysts generated by functionalizing 8 main structures with 8 different electron donating/withdrawing groups. Our results demonstrate that a wide range of hydricities (29-66 kcal mol^-1) and BDFEs (58-81 kcal mol^-1) are attainable by functionalizing diazaphospholenes. Compared to the more common carbon-based hydrides, diazaphospholenes are predicted to require less negative reduction potentials to electrochemically regenerate hydrides with an equivalent hydridic strength, indicating their higher energy efficiency in the tradeoff between thermodynamic ability and reduction potential. We show that the tradeoff between the reducing ability and the energetic cost of regeneration can be optimized by varying the BDFE and the reorganization energy associated with hydride transfer (λ_HT), where lower BDFE and λ_HT correspond to more efficient catalysts. Aromatic phosphorus hydrides with predicted BDFEs of ∼62 kcal mol^-1 and λ_HT's of ∼20 kcal mol^-1 are found to require less negative reduction potentials than dihydropyridines and benzimidazoles with predicted BDFEs of ∼68 and ∼84 kcal mol^-1 and λ_HT's of ∼40 and ∼50 kcal mol^-1, respectively.

Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate

We report a novel metal-free chemical reduction of CO₂ by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble-metal-based hydrides such as [Ru(tpy)(bpy)H]⁺ and [Pt(depe)₂H]⁺. Chemical CO₂ reduction to the formate anion (HCOO^-) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. ¹³CO₂ experiments confirmed the formation of H¹³COO^- by CO₂ reduction with the formate product characterized by ¹H NMR and ¹³C NMR spectroscopy and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium toward the ionic products. After CO₂ reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO₂ reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO₂ by sustainable, recyclable, and metal-free organo-hydrides.

Renewable Hydride Donors for the Catalytic Reduction of CO2: A Thermodynamic and Kinetic Study

Increasing atmospheric CO₂ concentration and dwindling fossil fuel supply necessitate the search for efficient methods for CO₂ conversion to fuels. Assorted studies have shown pyridine and its derivatives capable of (photo)electrochemically reducing CO₂ to methanol, and some mechanistic interpretations have been proposed. Here, we analyze the thermodynamic and kinetic aspects of the efficacy of pyridines as hydride-donating catalytic reagents that transfer hydrides via their dihydropyridinic form. We investigate both the effects of functionalizing pyridinic derivatives with electron-donating and electron-withdrawing groups on hydride-transfer catalyst strength, assessed via their hydricity (thermodynamic ability) and nucleophilicity (kinetic ability), and catalyst recyclability, assessed via their reduction potential. We find that pyridines substituted with electron-donating groups have stronger hydride-donating ability (having lower hydricity and larger nucleophilicity values), but are less efficiently recycled (having more negative reduction potentials). In contrast, pyridines substituted with electron-withdrawing groups are more efficiently recycled, but are weaker hydride donors. Functional group modification favorably tunes hydride strength or efficiency, but not both. We attribute this problematic coupling between the strength and recyclability of pyridinic hydrides to their aromatic nature and suggest several avenues for overcoming this difficulty.

Thermodynamic and kinetic hydricities of metal-free hydrides

Thermodynamic and kinetic hydricities provide useful guidelines for the design of hydride donors with desirable properties for catalytic chemical reductions.