Kinetic and Mechanistic Effects of Bipyridine (bpy) Substituent, Labile Ligand, and Brønsted Acid on Electrocatalytic CO2 Reduction by Re(bpy) Complexes

Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Testing functional anchor groups for the efficient immobilization of molecular catalysts on silver surfaces

Modifications of complexes by attachment of anchor groups are widely used to control molecule-surface interactions. This is of importance for the fabrication of (catalytically active) hybrid systems, viz. of surface immobilized molecular catalysts. In this study, the complex fac-Re(S-Sbpy)(CO)3Cl (S-Sbpy = 3,3'-disulfide-2,2'-bipyridine), a sulfurated derivative of the prominent Re(bpy)(CO)3Cl class of CO2 reduction catalysts, was deposited onto the clean Ag(001) surface at room temperature. The complex is thermostable upon sublimation as supported by infrared absorption and nuclear magnetic resonance spectroscopy. Its anchoring process has been analyzed using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The growth behavior was directly contrasted to the one of the parent complex fac-Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine). The sulfurated complex nucleates as single molecule at different surface sites and at molecule clusters. In contrast, for the parent complex nucleation only occurs in clusters of several molecules at specifically oriented surface steps. While this shows that surface immobilization of the sulfurated complex is more efficient as compared to the parent, symmetry analysis of the STM topographic data supported by DFT calculations indicates that more than 90% of the complexes adsorb in a geometric configuration very similar to the one of the parent complex.

Redox Behavior and Kinetics of Hydroxo Ligand Exchange on Iron Tetraphenylporphyrin: Comparison with Chloro Exchange and Consequences for Its Role in Self-Modulation of Molecular Catalysis of Electrochemical Reactions

Thermodynamics and kinetics of hydroxide ion binding to iron tetraphenylporphyrin (TPPFe) at different redox states is investigated by electrochemistry and UV-vis spectroscopy. The reduction of initial TPPFe(III) drastically decreases the binding affinity of hydroxide ions. An activation-driving force correlation is revealed showing that the strongest the binding affinity, the largest the association rate constant and vice versa. Comparison with chloride ions shows that hydroxide ions are stronger ligands for iron tetraphenylporphyrin. However, kinetic data indicate that coordination and decoordination of chloride ions is intrinsically faster than coordination and decoordination of hydroxide ions. Finally, the consequence of hydroxide ion binding dynamics when TPPFe is used as a molecular catalyst for electrochemical reactions liberating hydroxides is discussed in the framework of self-modulation of catalytic processes.

Synthesis and Photocatalytic CO2 Reduction of a Cyclic Zinc(II) Porphyrin Trimer with an Encapsulated Rhenium(I) Bipyridine Tricarbonyl Complex

We previously reported a cyclic Zn(II) porphyrin trimer in which three Zn porphyrins are alternately bridged by three 2,2'-bipyridine (bpy) moieties, enabling the encapsulation of metal complexes within the nanopore formed by the Zn porphyrins. In this study, we introduced a [Re(CO)3 Br] fragment into one of the bpy moieties of the cyclic trimer to form the catalytic Re(4,4'-R2 -bpy)(CO)3 Br center (R=methyl ester). The ester groups (R) play an important role in the synthesis of the cyclic structure. However, it was observed that these ester groups significantly deactivated the photocatalytic CO2 reduction reaction. Therefore, we converted the ester groups with a suitable reducing reagent into hydroxymethyl groups, followed by acetylation to form acetoxymethyl groups. This modification remarkably enhanced the photocatalytic activity of the cyclic trimer=Re complex system for CO2 reduction. Moreover, in the modified system, the presence of the Re complex induced room-temperature phosphorescence of the Zn porphyrin. The phosphorescence was significantly quenched by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole, indicating that efficient electron transfer mediated by the excited triplet state of the Zn porphyrin occurs during the photocatalytic CO2 reduction.

Understanding the Interplay of the Brønsted Acidity of Catalyst Ancillary Groups and the Solution Components in Iron-porphyrin-Mediated Carbon Dioxide Reduction

The rapid and efficient conversion of carbon dioxide (CO2) to carbon monoxide (CO) is an ongoing challenge. Catalysts based on iron-porphyrin cores have emerged as excellent electrochemical mediators of the two proton + two electron reduction of CO2 to CO, and many of the design features that promote function are known. Of those design features, the incorporation of Brønsted acids in the second coordination sphere of the iron ion has a significant impact on catalyst turnover kinetics. The Brønsted acids are often in the form of hydroxyphenyl groups. Herein, we explore how the acidity of an ancillary 2-hydroxyphenyl group affects the performance of CO2 reduction electrocatalysts. A series of meso-5,10,15,20-tetraaryl porphyrins were prepared where only the functional group at the 5-meso position has an ionizable proton. A series of cyclic voltammetry (CV) experiments reveal that the complex with -OMe positioned para to the ionizable -OH shows the largest CO2 reduction rate constants in acetonitrile solvent. This is the least acidic -OH of the compounds surveyed. The turnover frequency of the -OMe derivative can be further improved with the addition of 4-trifluoromethylphenol to the solution. In contrast, the iron-porphyrin complex with -CF3 positioned opposite the ionizable -OH shows the smallest CO2 reduction rate constants, and its turnover frequency is less enhanced with the addition of phenols to the reaction solutions. The origin of this effect is rationalized based on kinetic isotope effect experiments and density functional calculations. We conclude that catalysts with weaker internal acids coupled with stronger external acid additives provide superior CO2 reduction kinetics.

Overall reaction mechanism of photocatalytic CO2 reduction on a Re(i)-complex catalyst unit of a Ru(ii)-Re(i) supramolecular photocatalyst

Rhenium(i) complexes fac-[ReI(diimine)(CO)3(L)]n+ are mostly used and evaluated as photocatalysts and catalysts in both photochemical and electrochemical systems for CO2 reduction. However, the selective reduction mechanism of CO2 to CO is unclear, although numerous mechanistic studies have been reported. A Ru(ii)-Re(i) supramolecular photocatalyst with fac-[ReI(diimine)(CO)3{OC(O)OCH2CH2NR2}] (R = C2H4OH) as a catalyst unit (RuC2Re) exhibits very high efficiency, selectivity, and durability of CO formation in photocatalytic CO2 reduction reactions. In this work, the reaction mechanism of photocatalytic CO2 reduction using RuC2Re is fully clarified. Time-resolved IR (TR-IR) measurements using rapid-scan FT-IR spectroscopy with laser flash photolysis verify the formation of RuC2Re(COOH) with a carboxylic acid unit, i.e., fac-[ReI(diimine)(CO)3(COOH)], in the photocatalytic reaction solution. Additionally, this important intermediate is detected in an actual photocatalytic reaction using steady state irradiation. Kinetics analysis of the TR-IR spectra and DFT calculations demonstrated the reaction mechanism of the conversion of the one-electron reduced species of RuC2Re with a fac-[ReI(diimine˙-)(CO)3{OC(O)OCH2CH2NR2}]- unit, which was produced via the photochemical reduction of RuC2Re by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), to RuC2Re(COOH). The kinetics of the recovery processes of the starting complex RuC2Re from RuC2Re(COOH) accompanying the release of CO and OH- was also clarified. As a side reaction of RuC2Re(COOH), a long-lived carboxylate-ester complex with a fac-[ReI(diimine)(CO)3(COOC2H4NR2)] unit, which was produced by the nucleophilic attack of TEOA to one of the carbonyl ligands of RuC2Re(CO) with a fac-[ReI(diimine)(CO)4]+ unit, was formed during the photocatalytic reaction. This complex works not only as a precursor in another minor CO formation process but also as an external photosensitiser that photochemically reduces the other complexes i.e., RuC2Re, RuC2Re(COOH), and the intermediate that is reductively converted to RuC2Re(COOH).

Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO2 reduction catalyst

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2'-bipyridine), by these radicals [kTEA• = (4.4 ± 0.3) × 109 M-1 s-1 and kTEOA• = (9.3 ± 0.6) × 107 M-1 s-1]. The ∼50× smaller rate constant for TEOA• indicates, that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ∼0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals.

Modular preparation of cationic bipyridines and azaarenes via C-H activation

Bipyridines are ubiquitous in organic and inorganic chemistry because of their redox and photochemical properties and their utility as ligands to transition metals. Cationic substituents on bipyridines and azaarenes are valuable as powerful electron-withdrawing functionalities that also enhance solubility in polar solvents, but there are no general methods for direct functionalization. A versatile method for the preparation of trimethylammonium- and triarylphosphonium-substituted bipyridines and azaheterocycles is disclosed. This methodology showcases a C-H activation of pyridine N-oxides that enables a highly modular and scalable synthesis of a diverse array of cationically charged azaarenes. The addition of trimethylammonium functionalities on bipyridine derivatives resulted in more anodic reduction potentials (up to 700 mV) and increased electrochemical reversibility compared to the neutral unfunctionalized bipyridine. Additonally, metallation of 4-triphenylphosphinated biquinoline to make the corresponding Re(CO)3Cl complex resulted in reduction potentials 400 mV more anodic than the neutral derivative.

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.

Mechanistic insights into CO2 conversion to CO using cyano manganese complexes

Without the use of a photosensitizer, [Mn(bpy)(CO)₃(CN)] (MnCN) can photochemically form [Mn(bpy)(CO)₃]^-, the active species for CO₂ reduction. While cases of the axial X-ligand dissociating upon irradiation of fac-[M(N-N)(CO)₃X] complexes (M = Mn or Re; N-N = bipyridine (bpy) ligand; X = halogen or pseudohalogen) are well documented, the axial cyanide ligand is retained when either [Mn(bpy)(CO)₃(CN)] or [Mn(mesbpy)(CO)₃(CN)], MnCN(mesbpy), are irradiated anaerobically. Infrared and UV-vis spectroscopies indicate the formation of [Mn(bpy)(CO)₂(MeCN)(CN)] (s-MnCN) as the primary product during the irradiation of MnCN. An in-depth analysis of the photochemical mechanism for the formation of [Mn(bpy)(CO)₃]^- from MnCN is presented. MnCN(mesbpy) is too sterically hindered to undergo the same photochemical mechanism as MnCN. However, MnCN(mesbpy) is found to be electrocatalytically active for CO₂ reduction to CO. Thus providing an interesting distinction between photochemical and electrochemical charge transfer.

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.

In situ x-ray absorption investigations of a heterogenized molecular catalyst and its interaction with a carbon nanotube support

A highly active heterogenized molecular CO₂ reduction catalyst on a conductive carbon support is investigated to identify if its improved catalytic activity can be attributed to strong electronic interactions between catalyst and support. The molecular structure and electronic character of a [Re⁺¹(tBu-bpy)(CO)₃Cl] (tBu-bpy = 4,4'-tert-butyl-2,2'-bipyridine) catalyst deposited on multiwalled carbon nanotubes are characterized using Re L₃-edge x-ray absorption spectroscopy under electrochemical conditions and compared to the homogeneous catalyst. The Re oxidation state is characterized from the near-edge absorption region, while structural changes of the catalyst are assessed from the extended x-ray absorption fine structure under reducing conditions. Chloride ligand dissociation and a Re-centered reduction are both observed under applied reducing potential. The results confirm weak coupling of [Re(tBu-bpy)(CO)₃Cl] with the support, since the supported catalyst exhibits the same oxidation changes as the homogeneous case. However, these results do not preclude strong interactions between a reduced catalyst intermediate and the support, preliminarily investigated here using quantum mechanical calculations. Thus, our results suggest that complicated linkage schemes and strong electronic interactions with the initial catalyst species are not required to improve the activity of heterogenized molecular catalysts.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)₅Cl to form complexes of the type Re(bpy)(CO)₃Cl, which are related to known catalysts for CO₂ reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO₂ on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO₂ reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Two-Electron-Induced Reorganization of Cobalt Coordination and Metal-Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO2 Reduction

Electrochemical reorganization of complex structures is directly related to catalytic reactivity; thus, the geometric changes of catalysts induced by electron transfer should be considered to scrutinize the reaction mechanism. Herein, we studied electron-induced reorganization patterns of six-coordinate Co complexes with neutral N-donor ligands. Upon two-electron transfer into a Co center enclosed within a bulky π-acceptor ligand, the catalytic site exhibited different reorganization patterns depending on the ligand characteristics. While a bipyridyl ligand released Co-bound solvent (CH₃CN) to open a reaction site, a phenanthroline ligand caused Co-N_arm (side "arm" of NNN-ligand) bond dissociation. The first electron transfer occurred in the Co(II/I) reduction step and the second electron entered the bulky π-acceptor, of which redox steps were assigned from cyclic voltammograms, magnetic moment measurements, and DFT calculations. In comparison, the Co complex of [NNN^NCH₃-Co(CH₃CN)₃](PF₆)₂ ([1-(CH₃CN)₃](PF₆)₂) showed a high H₂ evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNN^NCH₃-Co(L)(CH₃CN)](PF₆)₂ (L = phen ([2-CH₃CN](PF₆)₂), bpy ([3-CH₃CN](PF₆)₂), [NNN^NCH₃-Co(tpy)](PF₆)₂ ([4](PF₆)₂), and [NNN^CH₂-Co(phen)(CH₃CN)](PF₆)₂ ([5-CH₃CN](PF₆)₂)) suppressed the HER but rather enhanced the CO₂ reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO₂ reduction. Additionally, the amine pendant attached to the NNN^NCH₃-ligand could stabilize the CO₂ reduction intermediate through the hydrogen-bonding interaction with the Co-CO₂H adduct.

Decoupled Solar Energy Storage and Dark Photocatalysis in a 3D Metal-Organic Framework

Materials enabling solar energy conversion and long-term storage for readily available electrical and chemical energy are key for off-grid energy distribution. Herein, the specific confinement of a rhenium coordination complex in a metal-organic framework (MOF) unlocks a unique electron accumulating property under visible-light irradiation. About 15 C g_MOF ^-1 of electric charges can be concentrated and stored for over four weeks without loss. Decoupled, on-demand discharge for electrochemical reactions and H₂ evolution catalysis is shown and light-driven recharging can be conducted for >10 cycles with ≈90% of the initial charging capacity retained. Experimental investigations and theoretical calculations link electron trapping to MOF-induced geometry constraints as well as the coordination environment of the Re-center, highlighting the key role of MOF confinement on molecular guests. This study serves as the seminal report on 3D porous colloids achieving photoaccumulation of long-lived electrons, unlocking dark photocatalysis, and a path toward solar capacitor and solar battery systems.

Electrocatalytic Conversion of CO2 to Formate at Low Overpotential by Electrolyte Engineering in Model Molecular Catalysis

An electrolyte engineering strategy was developed for CO₂ reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H₂ O or acetic acid), and the electrode/solution interface with imidazolium- and pyrrolidinium-based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π⁺ -π interactions between the imidazolium-based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO₂ reduction reaction (CO₂ RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FE_HCOO -) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO₂ conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO₂ RR.

Potential- and Buffer-Dependent Selectivity for the Conversion of CO2 to CO by a Cobalt Porphyrin-Peptide Electrocatalyst in Water

A semisynthetic electrocatalyst for carbon dioxide reduction to carbon monoxide in water is reported. Cobalt microperoxidase-11 (CoMP11-Ac) is shown to reduce CO₂ to CO with a turnover number of up to 32,000 and a selectivity of up to 88:5 CO:H₂. Higher selectivity for CO production is favored by a less cathodic applied potential and use of a higher pK _a buffer. A mechanistic hypothesis is presented in which avoiding the formation and protonation of a formal Co(I) species favors CO production. These results demonstrate how tuning reaction conditions impact reactivity toward CO₂ reduction for a biocatalyst previously developed for H₂ production.

Ligands modification strategies for mononuclear water splitting catalysts

Artificial photosynthesis (AP) has been proved to be a promising way of alleviating global climate change and energy crisis. Among various materials for AP, molecular complexes play an important role due to their favorable efficiency, stability, and activity. As a result of its importance, the topic has been extensively reviewed, however, most of them paid attention to the designs and preparations of complexes and their water splitting mechanisms. In fact, ligands design and preparation also play an important role in metal complexes’ properties and catalysis performance. In this review, we focus on the ligands that are suitable for designing mononuclear catalysts for water splitting, providing a coherent discussion at the strategic level because of the availability of various activity studies for the selected complexes. Two main designing strategies for ligands in molecular catalysts, substituents modification and backbone construction, are discussed in detail in terms of their potentials for water splitting catalysts.

Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re(^Rbpy)(CO)₃H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, ^tBu, Me, H, Br, COOMe, CF₃) to CO₂ and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re(^Rbpy)(CO)₃H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

Making and breaking of chemical bonds in single nanoconfined molecules

Nanoconfinement of catalytically active molecules is a powerful strategy to control their chemical activity; however, the atomic-scale mechanisms are challenging to identify. In the present study, the site-specific reactivity of a model rhenium catalyst is studied on the subnanometer scale for complexes confined within quasi-one-dimensional molecular chains on the Ag(001) surface by scanning tunneling microscopy. Injection of tunneling electrons causes ligand dissociation in single molecules. Unexpectedly, while half of the complexes show only the dissociation, the confined molecules show also the reverse reaction. On the basis of density functional theory calculations, this drastic difference can be attributed to the limited space in confinement. That is, the split-off ligand adsorbs closer to the molecule and the dissociation causes less structural disruption. Both of these facilitate the reverse reaction. We demonstrate formation and disruption of single chemical bonds of nanoconfined molecules with potential application in molecular data storage.

A Mesoionic Carbene-Pyridine Bidentate Ligand That Improves Stability in Electrocatalytic CO2 Reduction by a Molecular Manganese Catalyst

Tricarbonyl Group 7 complexes have a longstanding history as efficacious CO₂ electroreduction catalysts. Typically, these complexes feature an auxiliary 2,2'-bipyridine ligand that assists in redox steps by delocalizing the electron density into the ligand orbitals. While this feature lends to an accessible redox potential for CO₂ electroreduction, it also presents challenges for electrocatalysis with Mn because the electron density is removed from metal-ligand bonding orbitals. The results presented here thus introduce a mesoionic carbene (MIC) as a potent ligand platform to promote Mn-based electrocatalysis. The strong σ donation of the N,C-bidentate MIC is shown to help centralize the electron density on the Mn center while also maintaining relevant redox potentials for CO₂ electroreduction. Mechanistic investigation supports catalytic turnover at two operative potentials separated by 400 mV. In the low operating potential regime at -1.54 V, Mn(0) species catalyze CO₂ to CO and CO₃^2-, which has a maximum rate of 7 ± 5 s^-1 and is stable for up to 30.7 h. At higher operating potential at -1.94 V, "Mn(-1)" catalyzes CO₂ to CO and H₂O with faster turnovers of 200 ± 100 s^-1, with the trade-off being less stability at 6.7 h. The relative stabilities of Mn complexes bearing MIC and 4,4'-di-tert-butyl-2,2'-bipyridine were compared by evaluation under the same electrolysis conditions and therefore elucidated that the MIC promotes longevity for CO evolution throughout a 5 h period.

Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel-Bipyridine Photoredox Catalysts

Ni 2,2'-bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited-state Ni(II)-C bond homolysis. Here, we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(^Rbpy)(^R'Ph)Cl (R = MeO, t-Bu, H, MeOOC; R' = CH₃, H, OMe, F, CF₃), to illuminate the mechanism of excited-state bond homolysis. At given excitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer (MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited-state barriers (ΔH^‡ ∼ 4 kcal mol^-1) and a minimum energy excitation threshold (∼55 kcal mol^-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me₂ daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂ daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands. Electronic absorption spectroscopy and single-crystal X-ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂ daf), reveal features consistent with charge delocalization driven by π-backbonding into the LUMO of Me₂ daf, reminiscent of behavior displayed by the workhorse 2,2'-bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9'-dialkylated daf-type ligands are promising building blocks for applications in reductive chemistry and catalysis.

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.

Switching Catalyst Selectivity via the Introduction of a Pendant Nitrophenyl Group

The conversion of abundant small molecules to value-added products serves as an attractive method to store renewable energy in chemical bonds. A family of macrocyclic cobalt aminopyridine complexes was previously reported to reduce CO₂ to CO with 98% faradaic efficiency through the formation of hydrogen-bonding networks and with the number of secondary amines affecting catalyst performance. One of these aminopyridine macrocycles, (NH)₁(NMe)₃-bridged calix[4]pyridine (L⁵), was modified with a nitrophenyl group to form L^NO2 and metalated with a cobalt(II) precursor to generate CoL^NO2, which would allow for probing the positioning and steric effects on catalysis. The addition of a nitrophenyl moiety to the ligand backbone results in a drastic shift in selectivity. Large current increases in the presence of added protons and CoL^NO2 are observed under both N₂ and CO₂. The current increases under N₂ are ∼30 times larger than the ones under CO₂, suggesting a change in the selectivity of CoL^NO2 to favor H₂ production versus CO₂ reduction. H₂ is determined to be the dominant reduction product by gas chromatography, reaching faradaic efficiencies up to 76% under N₂ with TFE and 71% under CO₂ with H₂O, in addition to small amounts of formate. X-ray photoelectron spectroscopy (XPS) reveals the presence of a cobalt-containing heterogeneous deposit on the working electrode surface, indicating the addition of the nitrophenyl group reduces the electrochemical stability of the catalyst. These observed catalytic behaviors are demonstrably different relative to the tetra-NH bridged macrocycle, which shows 98% faradaic efficiency for CO₂-to-CO conversion with TFE, highlighting the importance of pendant hydrogen bond donors and electrochemically robust functional groups for selective CO₂ conversion.

Ordering a rhenium catalyst on Ag(001) through molecule-surface step interaction

Atomic scale studies of the anchoring of catalytically active complexes to surfaces may provide valuable insights for the design of new catalytically active hybrid systems. In this work, the self-assembly of 1D, 2D and 3D structures of the complex fac-Re(bpy)(CO)₃Cl (bpy = 2,2'-bipyridine), a CO₂ reduction catalyst, on the Ag(001) surface are studied by a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. Infrared and sum frequency generation spectroscopy confirm that the complex remains chemically intact under sublimation. Deposition of the complexes onto the silver surface at 300 K leads to strong local variations in the resulting surface coverage on the nanometer scale, indicating that in the initial phase of deposition a large fraction of the molecules is desorbing from the surface. Low coverage regions show a decoration of step edges aligned along the crystal's symmetry axes <110>. These crystallographic directions are found to be of major importance to the binding of the complexes to the surface. Moreover, the interaction between the molecules and the substrate promotes the restructuring of surface steps along these directions. Well-aligned and decorated steps are found to act as nucleation point for monolayer growth (2D) before 3D growth starts.

Boosting CO2-to-CO evolution using a bimetallic diketopyrrolopyrrole tethered rhenium bipyridine catalyst

The use of homogeneous electro- and photo-catalysis involving molecular catalysts offers valuable insight into reaction mechanisms as it relates to the structure–function of these tunable systems.

Electrocatalyst decomposition pathways: torsional strain in a second sphere proton relay shuts off CO2RR in a Re(2,2′-bipyridyl)(CO)3X type electrocatalyst

Group 7 tris(carbonyl) bipyridine complexes have been well explored as important CO2 reduction reaction (CO2RR) electrocatalysts and now represent an excellent platform for catalyst design.

Strategies for Breaking Molecular Scaling Relationships for the Electrochemical CO2 Reduction Reaction

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion...

Electronic structure analysis of electrochemical CO2 reduction by iron-porphyrins reveals basic requirements to design catalysts bearing non-innocent ligands

Electrocatalytic CO2 reduction is a possible solution to the increasing CO2 concentration in the earth atmosphere, because it enables storage of energy while using the harmful CO2 feedstock as starting...

Promoting photocatalytic CO2 reduction through facile electronic modification of N-annulated perylene diimide rhenium bipyridine dyads

The development of CO₂ conversion catalysts has become paramount in the effort to close the carbon loop. Herein, we report the synthesis, characterization, and photocatalytic CO₂ reduction performance for a series of N-annulated perylene diimide (NPDI) tethered Re(bpy) supramolecular dyads [Re(bpy-C2-NPDI-R)], where R = –H, –Br, –CN, –NO₂, –OPh, –NH₂, or pyrrolidine (–NR₂). The optoelectronic properties of these Re(bpy-C2-NPDI-R) dyads were heavily influenced by the nature of the R-group, resulting in significant differences in photocatalytic CO₂ reduction performance. Although some R-groups (i.e. –Br and –OPh) did not influence the performance of CO₂ photocatalysis (relative to –H; TON_co ∼60), the use of an electron-withdrawing –CN was found to completely deactivate the catalyst (TON_co < 1) while the use of an electron-donating –NH₂ improved CO₂ photocatalysis four-fold (TON_co = 234). Despite being the strongest EWG, the –NO₂ derivative exhibited good photocatalytic CO₂ reduction abilities (TON_co = 137). Using a combination of CV and UV-vis-nIR SEC, it was elucidated that the –NO₂ derivative undergoes an in situ transformation to –NH₂ under reducing conditions, thereby generating a more active catalyst that would account for the unexpected activity. A photocatalytic CO₂ mechanism was proposed for these Re(bpy-C2-NPDI-R) dyads (based on molecular orbital descriptions), where it is rationalized that the photoexcitation pathway, as well as the electronic driving-force for NPDI²⁻ to Re(bpy) electron-transfer both significantly influence photocatalytic CO₂ reduction. These results help provide rational design principles for the future development of related supramolecular dyads.

Seven N-annulated perylene diimide tethered rhenium (2,2′-bipyridine) supramolecular dyads are evaluated as photocatalysts for the reduction for carbon dioxide, highlighting the importance of photoexcitation pathway and electronic driving-force.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives

Urgent solutions are needed to efficiently convert the greenhouse gas CO₂ into higher-value products. In this work, fac-Mn(bpy)(CO)₃Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO₂ conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et₃N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)₃Br and [Mn(bpy)(CO)₃]^- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et₃N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et₃N is replaced with Et₂NH. The origin of this difference lies in the ability of Et₃N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO₂, while with Et₂NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO₂ in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO₂-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO₂ to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

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%).

Molecular Transition Metal Oxide Electrocatalysts for the Reversible Carbon Dioxide-Carbon Monoxide Transformation

Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidation-CO₂ reduction reaction and are of interest in the context of CO₂ abatement and carbon-neutral solar fuels. Bioinspired by the active-site composition of the CODHs, polyoxometalates triply substituted with first-row transition metals were modularly synthesized. The polyanions, in short, {SiM₃ W₉ } and {SiM'₂ M''W₉ }, M, M', M''=Cu^II , Ni^II , Fe^III are shown to be electrocatalysts for reversible CO oxidation-CO₂ reduction. A catalytic Tafel plot showed that {SiCu₃ W₉ } was the most reactive for CO₂ reduction, and electrolysis reactions yielded significant amounts of CO with 98 % faradaic efficiency. In contrast, Fe-Ni compounds such as {SiFeNi₂ W₉ } preferably catalyzed the oxidation of CO to CO₂ similar to what is observed for the [NiFe]-CODH enzyme. Compositional control of the heterometal complexes, now and in the future, leads to control of reactivity and selectivity for CO₂ electrocatalytic reduction.

Lowering Electrocatalytic CO2 Reduction Overpotential Using N-Annulated Perylene Diimide Rhenium Bipyridine Dyads with Variable Tether Length

We report the design, synthesis, and characterization of four N-annulated perylene diimide (NPDI) functionalized rhenium bipyridine [Re(bpy)] supramolecular dyads. The Re(bpy) scaffold was connected to the NPDI chromophore either directly [Re(py-C0-NPDI)] or via an ethyl [Re(bpy-C2-NPDI)], butyl [Re(bpy-C4-NPDI)], or hexyl [Re(bpy-C6-NPDI)] alkyl-chain spacer. Upon electrochemical reduction in the presence of CO₂ and a proton source, Re(bpy-C2/4/6-NPDI) all exhibited significant current enhancement effects, while Re(py-C0-NPDI) did not. During controlled potential electrolysis (CPE) experiments at E_appl = -1.8 V vs Fc^+/0, Re(bpy-C2/4/6-NPDI) all achieved comparable activity (TON_co ∼ 25) and Faradaic efficiency (FE_co ∼ 94%). Under identical CPE conditions, the standard catalyst Re(dmbpy) was inactive for electrocatalytic CO₂ reduction; only at E_appl = -2.1 V vs Fc^+/0 could Re(dmbpy) achieve the same catalytic performance, representing a 300 mV lowering in overpotential for Re(bpy-C2/4/6-NPDI). At higher overpotentials, Re(bpy-C4/6-NPDI) both outperformed Re(bpy-C2-NPDI), indicating the possibility of coinciding electrocatalytic CO₂ reduction mechanisms that are dictated by tether-length and overpotential. Using UV-vis-nearIR spectroelectrochemistry (SEC), FTIR SEC, and chemical reduction experiments, it was shown that the NPDI-moiety served as an electron-reservoir for Re(bpy), thereby allowing catalytic activity at lower overpotentials. Density functional theory studies probing the optimized geometries and frontier molecular orbitals of various catalytic intermediates revealed that the geometric configuration of NPDI relative to the Re(bpy)-moiety plays a critical role in accessing electrons from the electron-reservoir. The improved performance of Re(bpy-C2/4/6-NPDI)dyads at lower overpotentials, relative to Re(dmbpy), highlights the utility of chromophore electron-reservoirs as a method for lowering the overpotential for CO₂ conversion.

Secondary Coordination Effect on Monobipyridyl Ru(II) Catalysts in Photochemical CO2 Reduction: Effective Proton Shuttle of Pendant Brønsted Acid/Base Sites (OH and N(CH3)2) and Its Mechanistic Investigation

While the incorporation of pendant Brønsted acid/base sites in the secondary coordination sphere is a promising and effective strategy to increase the catalytic performance and product selectivity in organometallic catalysis for CO₂ reduction, the control of product selectivity still faces a great challenge. Herein, we report two new trans(Cl)-[Ru(6-X-bpy)(CO)₂Cl₂] complexes functionalized with a saturated ethylene-linked functional group (bpy = 2,2'-bipyridine; X = -(CH₂)₂-OH or -(CH₂)₂-N(CH₃)₂) at the ortho(6)-position of bpy ligand, which are named Ru-bpy^OH and Ru-bpy^diMeN, respectively. In the series of photolysis experiments, compared to nontethered case, the asymmetric attachment of tethering ligand to the bpy ligand led to less efficient but more selective formate production with inactivation of CO₂-to-CO conversion route during photoreaction. From a series of in situ FTIR analyses, it was found that the Ru-formate intermediates are stabilized by a highly probable hydrogen bonding between pendent proton donors (-diMeN⁺H or -OH) and the oxygen atom of metal-bound formate (Ru^I-OCHO···H-E-(CH₂)₂-, E = O or diMeN⁺). Under such conformation, the liberation of formate from the stabilized Ru^I-formate becomes less efficient compared to the nontethered case, consequently lowering the CO₂-to-formate conversion activities during photoreaction. At the same time, such stabilization of Ru-formate species prevents the dehydration reaction route (η¹-OCHO → η¹-COOH on Ru metal) which leads toward the generation of Ru-CO species (key intermediate for CO production), eventually leading to the reduction of CO₂-to-CO conversion activity.

Electrochemical Degradation of a Dicationic Rhenium Complex via Hoffman-Type Elimination

Electrocatalytic reduction of carbon dioxide (CO₂) by transition-metal catalysts is an attractive means for storing renewably sourced electricity in chemical bonds. Metal coordination compounds represent highly tunable platforms ideal for studying the fundamental stepwise transformations of CO₂ into its reduced products. However, metal complexes can decompose upon extended electrolysis and form chemically distinct molecular species or, in some cases, catalytically active electrode deposits. Deciphering the degradative pathways is important for understanding the nature of the active catalyst and designing robust metal complexes for small-molecule activation. Herein, we present a new dicationic rhenium bipyridyl complex capable of multielectron ligand-centered reductions electrochemically. Our in-depth experimental and computational study provides mechanistic insight into an unusual reductively induced Hoffman-type elimination. We identify benzylic tertiary ammonium groups as an electrolytically susceptible moiety and propose key intermediates in the degradative pathway. This investigation highlights the complex interplay between the ligand and metal ion and will guide the future design of metal-organic catalysts.