A computational study of electrocatalytic CO2 reduction by Mn(I) complexes: Role of bipyridine substituents

Mechanistic insights of electrocatalytic CO2 reduction by Mn complexes: synergistic effects of the ligands

The electrocatalytic mechanisms of CO2 reduction catalyzed by pyridine-oxazoline (pyrox)-based Mn catalysts were investigated by DFT calculations. In-depth comparative analyses of pyrox-based and bipyridine-based Mn complexes were carried out. C-OH cleavage is the rate-determining step for both the protonation-first path and the reduction-first path. The free energy of CO2 activation (ΔG1) and the electrons donated by CO ligands in this step are effective descriptors in regulating the C-OH cleavage barrier. The reduction of carboxylate complex 6 (E6) is the potential-determining step for the reduction-first path. Meanwhile, for the protonation-first path, the initial generation (E2) or the regeneration (E8) of active catalyst might be potential-determining. Hirshfeld charge and orbital contribution analysis indicate that E6 is definitely based on the heterocyclic ligand and E2 is related to both the heterocyclic ligand and three CO ligands. Therefore, replacement of the CO ligand by a stronger electron donating ligand can effectively boost the catalytic activity of CO2 reduction without increasing the overpotential in the reduction-first path. This hypothesis is supported by the mechanism calculations of the Mn complex in which the axial CO ligand is replaced by a pyridine or PMe3.

Analyzing Structure-Activity Variations for Mn-Carbonyl Complexes in the Reduction of CO2 to CO

Contemporary electrocatalysts for the reduction of CO₂ often suffer from low stability, activity, and selectivity, or a combination thereof. Mn-carbonyl complexes represent a promising class of molecular electrocatalysts for the reduction of CO₂ to CO as they are able to promote this reaction at relatively mild overpotentials, whereby rare-earth metals are not required. The electronic and geometric structure of the reaction center of these molecular electrocatalysts is precisely known and can be tuned via ligand modifications. However, ligand characteristics that are required to achieve high catalytic turnover at minimal overpotential remain unclear. We consider 55 Mn-carbonyl complexes, which have previously been synthesized and characterized experimentally. Four intermediates were identified that are common across all catalytic mechanisms proposed for Mn-carbonyl complexes, and their structures were used to calculate descriptors for each of the 55 Mn-carbonyl complexes. These electronic-structure-based descriptors encompass the binding energies, the highest occupied and lowest unoccupied molecular orbitals, and partial charges. Trends in turnover frequency and overpotential with these descriptors were analyzed to afford meaningful physical insights into what ligand characteristics lead to good catalytic performance, and how this is affected by the reaction conditions. These insights can be expected to significantly contribute to the rational design of more active Mn-carbonyl electrocatalysts.

Mechanistic exploration of CO2 conversion to dimethoxymethane (DMM) using transition metal (Co, Ru) catalysts: an energy span model

The conversion of CO₂ to DMM is an important transformation for various reasons. Co and Ru-based triphos catalysts have been investigated using density functional theory (DFT) calculations to understand the mechanistic pathways of the CO₂ to DMM conversion and the role of noble/non-noble metal-based catalysts. The reaction has been investigated sequentially through methylformate (MF) and methoxymethane (MM) intermediates as they are found to be important intermediates. For the hydrogenation of CO₂ and MF, the hydrogen sources such as H₂ and methanol have been investigated. The calculated reaction free energy barriers for all the possible pathways suggest that both hydrogen sources are important for the Co-triphos catalyst. However, in the case of the Ru-triphos catalyst, molecular H₂ is calculated to be the only hydrogen source. Various esterification and acetalization possibilities have also been explored to find the most favorable pathway for the conversion of CO₂ to DMM. We find that the hydride transfer to the CO₂ is the rate determining step (RDS) for the overall reaction. Our mechanistic investigation reveals that the metal center is the active part for the catalysis rather than the Brønsted acid and the redox triphos ligand plays an important role through the push-pull mechanism. The implemented microkinetic study shows that the reaction is also quite dependent on the concentration of the gaseous reactants and the rate constant increases exponentially above 363 K.

Mechanistic Study of Tungsten Bipyridyl Tetracarbonyl Electrocatalysts for CO2 Fixation: Exploring the Roles of Explicit Proton Sources and Substituent Effects

Tungsten bipyridyl tetracarbonyl complexes were shown to reduce CO2 to CO in acetonitrile [Chem. Sci., 2014, 5, 1894-1900]. Here, we employ density functional theory (DFT) calculations to investigate the electronic structure and reactivity of a series of tungsten electrocatalysts, [W(bpy-R)(CO)4] (where R = H, CH3, tBu, OCH3, CF3, and CN), for the CO2 reduction reaction (CO2RR). Our proposed mechanism suggests that initial reduction of the starting material by two electrons is required to access the active catalyst upon CO dissociation, which is slightly endergonic, consistent with the slow product release observed experimentally. The doubly reduced species, which has a closed-shell singlet ground state, can bind CO2 via an η2-CO2 binding mode to yield the metallocarboxylate intermediate. Based on the energy span model, CO2 addition is the TOF-determining transition state (TDTS) in the presence of water as the proton source. Different substituents at the 4,4'-positions of the bipyridine ligand in [W(bpy-R)(CO)4] (R = H, CH3, tBu, OCH3, CF3, and CN) were considered to comprehend the substituent effects for CO2RR. DFT results show that electron-withdrawing substituents, such as CN and CF3, do not yield efficient CO2 reduction catalysts due to the necessity of forming high energy intermediates for the protonation steps, resulting in low TOFs and high overpotentials. Among electron-donating groups, the parent compound and tert-butyl substituted complex are the most active catalysts for CO2RR due to higher TOFs at low overpotentials. Overall, based on the energy span model and theoretical Tafel plots, our computational approach provides quantitative information for designing CO2 reduction electrocatalysts.

Computational study on the reactivity of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2 and Me4) for CO2-to-CO conversion over H2 formation

We have recently reported a series of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts, [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2, and Me4), for CO2-to-CO conversion in the presence of H2O as the proton source [J. Am. Chem. Soc., 2019, 141, 6569]. These catalysts feature slightly acidic imidazolium moieties in the secondary coordination sphere and reduce CO2 at mild electrochemical potentials. Here, we employ density functional theory (DFT) calculations to understand the electronic structure and reactivity for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) using [Mn[bpyMe(ImMe)](CO)3Br]+ (1+). Our work indicates that, in the absence of water, the imidazolium ligand stabilizes the Mn-CO2 adduct through hydrogen bonding-like interactions, similar to the activated CO2 molecule in the C-cluster of the Ni,Fe-carbon monoxide dehydrogenase II, and assists the protonation steps during CO2RR and HER. More significantly, based on the energy span model, we demonstrate that the selectivity for CO2 fixation over proton reduction results from a higher activation energy for yielding the manganese dihydrogen intermediate before H2 release, which is the TOF determining transition state (TDTS) under an applied potential of Φ = -1.82 V versus Fc0/+. The calculated TOF also reflects the selectivity for CO2RR, which is four orders of magnitude larger than for HER, consistent with the CPE experiments that show no hydrogen was obtained. In the case of CO2 reduction, the TOF determining intermediate (TDI) corresponds to the doubly reduced active catalyst, 1C2(red2), which features a manganese(0) center that couples ferromagnetically with one unpaired electron in the π* orbital of bipyridine. On the other hand, for HER, the metal-hydride intermediate, 1C2(I11-R), is the TDI. Finally, second-order perturbation analyses imply that the strongest hydrogen bonding-like interaction at the C2 position in 1+ contributes to the higher catalytic activity with respect to [Mn[bpyMe(ImMe2)](CO)3Br]+ (2+) and [Mn[bpyMe(ImMe4)](CO)3Br]+ (3+) for CO2 fixation, consistent with the experimental data.

Sterically hindered Re- and Mn-CO2 reduction catalysts for solar energy conversion

Novel molecular Re and Mn tricarbonyl complexes bearing a bipyridyl ligand functionalised with sterically hindering substituents in the 6,6'-position, [M(HPEAB)(CO)3(X)] (M/X = Re/Cl, Mn/Br; HPEAB = 6,6'-{N-(4-hexylphenyl)-N(ethyl)-amido}-2,2'-bipyridine) have been synthesised, fully characterised including by single crystal X-ray crystallography, and their propensity to act as catalysts for the electrochemical and photochemical reduction of CO2 has been established. Controlled potential electrolysis showed that the catalysts are effective for electrochemical CO2-reduction, yielding CO as the product (in MeCN for the Re-complex, in 95 : 5 (v/v) MeCN : H2O mixture for the Mn-complex). The recyclability of the catalysts was demonstrated through replenishment of CO2 within solution. The novel catalysts had similar reduction potentials to previously reported complexes of similar structure, and results of the foot-of-the-wave analysis showed comparable maximum turnover rates, too. The tentative mechanisms for activation of the pre-catalysts were proposed on the basis of IR-spectroelectrochemical data aided by DFT calculations. It is shown that the typical dimerisation of the Mn-catalyst was prevented by incorporation of sterically hindering groups, whilst the Re-catalyst undergoes the usual mechanism following chloride ion loss. No photochemical CO2 reduction was observed for the rhenium complex in the presence of a sacrificial donor (triethylamine), which was attributed to the short triplet excited state lifetime (3.6 ns), insufficient for diffusion-controlled electron transfer. Importantly, [Mn(HPEAB)(CO)3Br] can act as a CO2 reduction catalyst when photosensitised by a zinc porphyrin under red light irradiation (λ > 600 nm) in MeCN : H2O (95 : 5); there has been only one reported example of photoactivating Mn-catalysts with porphyrins in this manner. Thus, this work demonstrates the wide utility of sterically protected Re- and Mn-diimine carbonyl catalysts, where the rate and yield of CO-production can be adjusted based on the metal centre and catalytic conditions, with the advantage of suppressing unwanted side-reactions through steric protection of the vacant coordination site.

Electronic Effects of Substituents on fac-M(bpy-R)(CO)3 (M = Mn, Re) Complexes for Homogeneous CO2 Electroreduction

Synthesis and characterization of 14 new 2,2'-bipyridine metal complexes fac-M(bpy-R)(CO)₃X (where M = Mn, X = Br or M = Re, X = Cl and R = -CF₃, -CN, -Ph, -PhOH, -NMe₂) are reported. The complexes have been characterized by NMR, IR spectroscopy and elemental analysis. Single crystal X-Ray diffraction structures have been solved for Re(dpbpy)(CO)₃Cl (dpbpy = 4,6-diphenyl-2,2'-bipyridine) and Re(hpbpy)(CO)₃Cl (hpbpy = 4-(2-hydroxy-phenyl)-6-phenyl-2,2'-bipyridine). Electrochemical behaviors of the complexes in acetonitrile under Ar and their catalytic performances for CO₂ reduction with added water and MeOH have been investigated by cyclic voltammetry and controlled potential electrolysis. The role of the substituents on the electrochemical properties and the related over potentials required for CO₂ transformation have been analyzed. The complexes carrying only electron withdrawing groups like -CF₃, -CN totally lose their catalytic activities toward CO₂ reduction, whereas the symmetric -NMe₂ substituted and push-pull systems (containing both -NMe₂ and -CF₃) still display electrocatalytic current enhancement under CO₂ atmosphere. The complexes carrying a phenyl or a phenol group in position 4 show catalytic behaviors similar to those of simple M-bpy systems. The only detected reduction product by GC analysis is CO: for example, fac-Re (bpy-4,4'-NMe₂)(CO)₃Cl gives CO with high faradic efficiency and a TON of 18 and 31, in absence of external proton source and with 5% MeOH, respectively. DFT calculations were carried out to highlight the electronic properties of the complexes; results are in agreement with experimental electrochemical data.

Catalytic upgrading of ethanol to n-butanol using an aliphatic Mn–PNP complex: theoretical insights into reaction mechanisms and product selectivity

Understanding the product selectivity, conversion, and rate-determining steps in the catalytic upgrading of ethanol to butanol.

A rational design of manganese electrocatalysts for Lewis acid-assisted carbon dioxide reduction

Herein, the mechanisms of Brønsted acid- and Lewis acid-assisted CO2 electroreduction by Mn(mesbpy)(CO)3Br (1) were investigated by density functional theory calculations. Our results indicate that for the Lewis acid-assisted cycle, an energy sink (13) is present owing to the interaction between Mg(OTf)2 and activated CO2, which is disadvantageous to the apparent activation energy (ΔG≠). Moreover, a series of substituted 13 counterparts were investigated to reduce the energy sink and decrease ΔG≠. Based on our study on the substituent effect, an excellent linear relationship was found between 2e reduction potentials and LUMO energies of substituted 1, and a moderate linear relationship was observed between ΔG of substituted 13 and the 2e reduction potential of substituted 1 counterparts. Moreover, for the CO2 reduction assisted by a Lewis acid, the formyl-substituted complex R8 has been predicted to be a more effective catalyst with lower overpotential and higher catalytic activity than its parent complex 1.