Electro- and Photochemical Reduction of CO2 by Molecular Manganese Catalysts: Exploring the Positional Effect of Second-Sphere Hydrogen-Bond Donors

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H

The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.

Unveiling the Activity and Mechanism Alterations by Pyrene Decoration on a Co(II) Macrocyclic Catalyst for CO2 Reduction

Mechanistic studies involving characterization of crucial intermediates are desirable for rational optimization of molecular catalysts toward CO2 reduction, while fundamental challenges are associated with such studies. Herein we present the systematic mechanistic investigations on a pyrene-appended CoII macrocyclic catalyst in comparison with its pyrene-free prototype. The comparative results also verify the reasons of the higher catalytic activity of the pyrene-tethered catalyst in noble-metal-free CO2 photoreduction with various photosensitizers, where a remarkable apparent quantum yield of 36±3 % at 425 nm can be obtained for selective CO production. Electrochemical and spectroelectrochemical studies in conjunction with DFT calculations between the two catalysts have characterized the key CO-bound intermediates and revealed their different CO-binding behavior, demonstrating that the pyrene group endows the corresponding CoII catalyst a lower catalytic potential, a higher stability, and a greater ease in CO release, all of which contribute to its better performance.

Electrochemical CO2 Reduction by Urea Hangman Mn Terpyridine species

Based on our previous study in chemical subtleties of the proton tunneling distance for metal hydride formation (PTD-MH) to regulate the selectivity of CO2 reduction reaction (CO2RR), we have developed a family of Mn terpyridine derivatives, in which urea groups functions as multipoint hydrogen-bonding hangman to accelerate the reaction rate. We found that such changes to the second coordination sphere significantly increased the turnover frequency (TOF) for CO2 reduction to ca. 360s - 1 ${{s}^{-1}}$ with this family of molecular catalysts while maintaining high selectivity (ca. 100 %±3) for CO even in the presence of a large amount of phenol as proton source. Notably, the compounds studied in this manuscript all exhibit large value fori c a t / i p ${{{\bf i}}_{{\bf c a t}}/{{\bf i}}_{{\bf p}}}$ as that achieved by Fe porphyrins derivates, while saving up to 0.55 V in overpotential with respect to the latter.

Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(I)-tpy complexes

A series of organometallic Re(I)(L)(CO)3Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO2 reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO2 electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)3Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75-0.95 V) with faradaic yields of 62-98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pKa of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.

Exploring the Parameters Controlling Product Selectivity in Electrochemical CO2 Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes

Selective reduction of CO₂ is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental-computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO₂ to HCOOH, CO, and H₂. In contrast to previous Mn(bpy-R)(CO)₃Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H₂. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)₃ moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H₂ and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO₂ to the metal hydride favoring the formation of H₂ over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO₂ adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

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.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)₂bpy vs mes₂bpy ligands) and Lewis basic ((Ph)₂bpy vs (MeOPh)₂bpy vs (MeSPh)₂bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO₂ reduction mechanisms with [Mn^I(R₂bpy)(CO)₃(CH₃CN)]⁺ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)₂bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO₂ binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes₂bpy-based precatalysts. Cyclic voltammetry conducted in CO₂-saturated electrolyte for the (Ph)₂bpy-based precatalyst [2-CH₃CN]⁺ confirms that CO₂ binding occurs at the two-electron-reduced activated catalyst [2]^- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO₂ binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]⁰ dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO₂ binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH₃CN]⁺ maintains a lower overpotential than [2-CH₃CN]⁺ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOF_max is observed at its reduction-first pathway with an almost identical overpotential as [2-CH₃CN]⁺. Notably, excessive steric bulk in the mes₂bpy-based precatalyst [3-CH₃CN]⁺ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH₃CN]⁺ and [2-CH₃CN]⁺. In fact, [3-CH₃CN]⁺ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO₂ with 2.8 M (5%) H₂O establishes [2-CH₃CN]⁺ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]⁰ dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH₃CN]⁺ possesses an increased steric presence relative to [2-CH₃CN]⁺, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst

This work demonstrates photocatalytic CO₂ reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4′-{Et₂O₃PCH₂}₂-2,2′-bipyridyl)(CO)₃] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO₂ to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl₄ (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO₂ reduction. The incorporation of −P(O)(OEt)₂ groups, decoupled from the core of the catalyst by a −CH₂– spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.

A Mn(I) bipyridyl tricarbonyl complex, where the diimine ligand is functionalized with water-solubilizing phosphonate ester groups, has been prepared and is shown to catalytically convert CO₂ to CO in aqueous solution following photosensitization from a water-soluble Zn(II) porphyrin under red-light irradiation.

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.

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Effects of Protonation State on Electrocatalytic CO2 Reduction by a Cobalt Aminopyridine Macrocyclic Complex

A critical component in the reduction of CO₂ to CO and H₂O is the delivery of 2 equiv of protons and electrons to the CO₂ molecule. The timing and sequencing of these proton and electron transfer steps are essential factors in directing the activity and selectivity for catalytic CO₂ reduction. In previous studies, we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO₂ to CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant (k_obs) and the number of pendant amine hydrogen bond donors minus one, suggesting the presence of a deprotonated active catalytic state. Herein, we investigate the feasibility of these proposed deprotonated complexes toward CO₂ reduction. Two deprotonated derivatives, Co(L^4-) and Co(L^2-), of the tetraamino macrocycle Co(L) were independently synthesized and structurally characterized revealing extensive delocalization of the negative charge upon deprotonation. ¹H nuclear magnetic resonance spectroscopy and ultraviolet-visible titration studies confirm that under catalytic conditions, the active form of the catalyst gradually becomes deprotonated, supporting thus the n_donor - 1 relationship with k_obs. Electrochemical studies of Co(L^4-) reveal that this deprotonated analogue is competent for electrocatalysis upon addition of an exogenous weak acid source, such as 2,2,2-trifluoroethanol, resulting in faradaic efficiencies for CO₂-to-CO conversion identical to those observed with the fully protonated derivative (>98%).

Click-Derived Triazoles and Triazolylidenes of Manganese for Electrocatalytic Reduction of CO2

A series of new fac-[Mn(L)(CO)₃Br] complexes where L is a bidentate chelating ligand containing mixed mesoionic triazolylidene-pyridine (MIC^py, 1), triazolylidene-triazole (MIC^trz, 2), and triazole-pyridine (trz^py, 3) ligands have been prepared and fully characterized, including the single crystal X-ray diffraction studies of 1 and 2. The abilities of 1–3 and complex fac-[Mn(MIC^MIC)(CO)₃Br] (4) to catalyze the electroreduction of CO₂ has been assessed for the first time. It was found that all complexes displayed a current increase under CO₂ atmosphere, being 3 and 4 the most active complexes. Complex 3, bearing a N^N-based ligand exhibited a good efficiency and an excellent selectivity for reducing CO₂ to CO in the presence of 1.0 M of water, at low overpotential. Interestingly, complex 4 containing the strongly electron donating di-imidazolylidene ligand exhibited comparable activity to 3, when the experiments were performed in neat acetonitrile at slightly higher overpotential (−1.86 vs. −2.14 V).