Mechanisms for CO production from CO2 using reduced rhenium tricarbonyl catalysts

Metal-Dependent Mechanism of the Electrocatalytic Reduction of CO2 by Bipyridine Complexes Bearing Pendant Amines: A DFT Study

In this study, the electrocatalytic reduction of carbon dioxide by MnI, ReI, and RuII bipyridine complexes bearing pendant amines is evaluated by DFT methods. Prior experimental studies showed that introducing pendant amines in the secondary coordination sphere of the catalyst shifts product selectivity from CO to HCOO- (in the presence of a proton source) in the case of Mn. In contrast, CO is the major product with Re and Ru. This work includes a comprehensive study of the pathways leading to CO, HCOO-, and H2 to elucidate the energetic preferences that underlie product selectivity. Our results show that switching the metal center leads to changes in the preferred mechanism. While with Mn, the reaction is preferred in an endo configuration, allowing the participation of amines in the hydride formation, reactivity on the exo configuration is preferred with Re. In addition, the distinct redox properties of Re allow for the formation of Re OCOCO2-bridged adducts that lead to CO without a proton source. Further, the ability of Ru to exchange the two Cl- anions changes the preferred coordination number of Ru compared to Mn and Re and, consequently, its reaction mechanism. Overall, this study provides the structure and reactivity insight needed for further catalyst design.

Push-pull effect - how to effectively control photoinduced intramolecular charge transfer processes in rhenium(I) chromophores with ligands of D-A or D-π-A structure

Over the last five decades, diimine rhenium(I) tricarbonyl complexes have been extensively investigated due to their remarkable and widely tuned photophysical properties. These systems are regarded as attractive targets for design functional luminescent materials and performing fundamental studies of photoinduced processes in transition metal complexes. This review summarizes the latest developments concerning Re(I) tricarbonyl complexes bearing donor-acceptor (D-A) and donor-π-acceptor (D-π-A) ligands. Such compounds can be treated as bichromophoric systems with two close-lying excited states, metal-to-ligand charge transfer (MLCT) and intraligand-charge-transfer (ILCT). A role of ILCT transitions in controlling photobehaviour was discussed for Re(I) tricarbonyls with six different diimine cores decorated by various electron-rich amine, sulphur-based and π-conjugated aryl groups. It was evidenced that this approach is an effective tool for enhancement of the visible absorptivity, bathochromic emission shift and significant prolongation of the excited-state, opening up new possibilities in the development of more efficient materials and expand the range of their applications.

Charge Transport Approaches in Photocatalytic Supramolecular Systems Composing of Semiconductor and Molecular Metal Complex for CO2 Reduction

The design of photocatalytic supramolecular systems composing of semiconductors and molecular metal complexes for CO2 reduction has attracted increasing attention. The supramolecular system combines the structural merits of semiconductors and metal complexes, where the semiconductor harvests light and undertakes the oxidative site, while the metal complex provides activity for CO2 reduction. The intermolecular charge transfer plays crucial role in ensuring photocatalytic performance. Here, we review the progress of photocatalytic supramolecular systems in reduction of CO2 and highlight the interfacial charge transfer pathways, as well as their state-of-the-art characterization methods. The remaining challenges and prospects for further design of supramolecular photocatalysts are also presented.

Photocatalytic conversion of CO2 to CO by Ru(II) and Os(II) octahedral complexes: a DFT/TDDFT study

The reaction mechanisms of the photocatalytic reduction of CO2 to CO catalyzed by [(en)M(CO)3Cl] complexes (M = Ru, Os, en = ethylenediamine) in the presence of triethanolamine (TEOA), R3N (R = -CH2CH2OH), in DCM and DMF solvents, were studied by means of DFT/TDDFT electronic structure calculations. The geometric and free energy reaction profiles for two possible reaction pathways were calculated. Both reaction pathways studied, start with the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ generated from the first triplet excited state, T1 upon reductive quenching by TEOA which acts as a sacrificial electron donor. In the first possible pathway, TEOA- anion binds to the metal center of the catalytically active intermediate, [(en)M(CO)3]˙+ followed by CO2 insertion into the M-OCH2CH2NR2 bond. The latter upon successive protonations releases a metal 'free' [R2NCH2CH2OC(O)(OH)] intermediate which starts a new and final catalytic cycle, leading to the formation of CO and H2O while regenarating TEOA. In the second possible pathway, the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ captures CO2 molecule, forming an η1-CO2 complex. Upon 2H+/2e- successive protonations and reductions, CO product is obtained along with regenarating the catalytically active intermediate [(en)M(CO)3]˙+. The nature of the proton donor affects the reaction profiles of both mechanisms. The nature of the solvent does not affect significantly the reaction mechanisms under study. Finally, since photoexcitation and T1 reductive quenching are common to both pathways, we have srutinized the photophysical properties of the [(en)M(CO)3Cl] complexes along with their T1 excited states reduction potentials, . The [(en)M(CO)3Cl] complexes absorb mainly in the UV region while the absolute are in the range 6.4-0.9 eV.

Bioinspired Binickel Catalyst for Carbon Dioxide Reduction: The Importance of Metal-ligand Cooperation

Catalyst design for the efficient CO2 reduction reaction (CO2RR) remains a crucial challenge for the conversion of CO2 to fuels. Natural Ni-Fe carbon monoxide dehydrogenase (NiFe-CODH) achieves reversible conversion of CO2 and CO at nearly thermodynamic equilibrium potential, which provides a template for developing CO2RR catalysts. However, compared with the natural enzyme, most biomimetic synthetic Ni-Fe complexes exhibit negligible CO2RR catalytic activities, which emphasizes the significance of effective bimetallic cooperation for CO2 activation. Enlightened by bimetallic synergy, we herein report a dinickel complex, NiIINiII(bphpp)(AcO)2 (where NiNi(bphpp) is derived from H2bphpp = 2,9-bis(5-tert-butyl-2-hydroxy-3-pyridylphenyl)-1,10-phenanthroline) for electrocatalytic reduction of CO2 to CO, which exhibits a remarkable reactivity approximately 5 times higher than that of the mononuclear Ni catalyst. Electrochemical and computational studies have revealed that the redox-active phenanthroline moiety effectively modulates the electron injection and transfer akin to the [Fe3S4] cluster in NiFe-CODH, and the secondary Ni site facilitates the C-O bond activation and cleavage through electron mediation and Lewis acid characteristics. Our work underscores the significant role of bimetallic cooperation in CO2 reduction catalysis and provides valuable guidance for the rational design of CO2RR catalysts.

Neutral and Cationic Re(I) Complexes with Pnictogen-Based Coligands and Tunable Functionality: From Phosphorescence to Photoinduced CO Release

In this work, we have explored Re(I) complexes featuring triphenylpnictogen (PnPh3, Pn = P, As, or Sb)-based coligands and bidentate (neutral or monoanionic) luminophores derived from 1,10-phenantroline (phen), as well as from 2-(3-(tert-butyl)-1H-1,2,4-triazol-5-yl)pyridine (H(N-tBu)). The effect of the increasingly heavy elements on the structural parameters, photoexcited-state properties, and electrochemical behavior as well as the hybridization defects and polarization of the Pn atoms was related to the charges of the main luminophores (i.e., phen vs N-tBu) and explored in terms of photoluminescence spectroscopy, X-ray diffractometry, and quantum-chemical methods. Therefore, an in-depth analysis of the bonding, crystal packing, excited-state energies, and lifetimes was assessed in liquid solutions, frozen glassy matrices, and crystalline phases along with a semiquantitative photoactivation study. Notably, by changing the main ligand from phen to N-tBu, an increase in radiative and radiationless deactivation rates (kr and knr, respectively) at 77 K together with a faster photoinduced CO release and fragmentation at room temperature was detected. In addition, a progressively red-shifted phosphorescence was observed with the growing atomic number of the pnictogen atom, along with a boost in kr and knr at 77 K. Down the Vth main group and upon coordination of the Pn atom to the Re(I) center, an increasingly prominent jump of s-orbital participation on the binding sxp3.00-orbitals of the Pn atoms is evidenced. Based on these findings, the ability of these complexes to act as tunable photoluminescent labels able to perform as light-driven CO-releasing molecules is envisioned.

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).

Photocatalytic Reduction of CO2 into CO with Cyclometalated Pt(II) Complexes of N^C^N Pincer Dipyridylbenzene Ligands: A DFT Study

In this work, density functional theory (DFT) calculations were employed to study the photocatalytic reduction of CO2 into CO using a series of Pt(II) square planar complexes with the general formula [Pt(5-R-dpb)Cl] (dpb = 1,3-di(2-pyridyl)benzene anion, R = H, N,N-dimethylaniline,T thiophene, diazaborinine). The CO2-into-CO conversion process is thought to proceed via two main steps, namely the photocatalytic/reduction step and the main catalytic step. The simulated absorption spectra exhibit strong bands in the range 280-460 nm of the UV-Vis region. Reductive quenching of the T1 state of the complexes under study is expected to be favorable since the calculated excited state redox potentials for the reaction with sacrificial electron donors are highly positive. The redox potentials reveal that the reductive quenching of the T1 state, important to the overall process, could be modulated by suitable changes in the N^C^N pincer ligands. The CO2 fixation and activation by the three coordinated Pt(II) catalytically active species are predicted to be favorable, with the Pt-CO2 bond dissociation energies D0 in the range of -36.9--10.3 kcal/mol. The nature of the Pt-CO2 bond of the Pt(II) square planar intermediates is complex, with covalent, hyperconjugative and H-bonding interactions prevailing over the repulsive electrostatic interactions. The main catalytic cycle is estimated to be a favorable exergonic process.

Photocatalytic Reduction of CO2 to HCOOH and CO by a Phosphine-Bipyridine-Phosphine Ir(III) Catalyst: Photophysics, Nonadiabatic Effects, Mechanism, and Selectivity

Photocatalytic CO2 reduction is one of the best solutions to solve the global energy crisis and to realize carbon neutralization. The tetradentate phosphine-bipyridine (bpy)-phosphine (PNNP)-type Ir(III) photocatalyst, Mes-IrPCY2, was reported with a high HCOOH selectivity but the photocatalytic mechanism remains elusive. Herein, we employ electronic structure methods in combination with radiative, nonradiative, and electron transfer rate calculations, to explore the entire photocatalytic cycle to either HCOOH or CO, based on which a new mechanistic scenario is proposed. The catalytic reduction reaction starts from the generation of the precursor metal-to-ligand charge transfer (3 MLCT) state. Subsequently, the divergence happens from the 3 MLCT state, the single electron transfer (SET) and deprotonation process lead to the formation of one-electron-reduced species and Ir(I) species, which initiate the reduction reaction to HCOOH and CO, respectively. Interestingly, the efficient occurrence of proton or electron transfer reduces barriers of critical steps. In addition, nonadiabatic transitions play a nonnegligible role in the cycle. We suggest a lower free-energy barrier in the reaction-limiting step and the very efficient SET in 3 MLCT are cooperatively responsible for a high HCOOH selectivity. The gained mechanistic insights could help chemists to understand, regulate, and design photocatalytic CO2 reduction reaction of similar function-integrated molecular photocatalyst.

Trinuclear Re(I)-Coordinated Organic Cage as the Supramolecular Photocatalyst for Visible-Light-Driven CO2 Reduction

Photocatalytic reduction of excess CO₂ in the atmosphere to value-added chemicals by visible light can be an effective solution to fuel shortage and global warming. Considering these issues, we designed and successfully synthesized a trinuclear Re(I)-coordinated organic cage (Re-C₄^R) as the supramolecular photocatalyst. Photophysical, electrochemical properties, and photocatalytic performance comparison of Re-C₄^R and its mononuclear analogue Re-bpy are discussed in detail. Notably, the covalent linkage of three Re(I) subunits in Re-C₄^R leads to TON_CO = 691 (per Re(I) site in 4 h) more than three times as much as TON_CO = 208 of Re-bpy. Compared to Re-bpy, higher current enhancement in the control CV experiments under CO₂ was observed for Re-C₄^R. CO₂ adsorption process can be promoted because of the cryptand structure and multiple amine groups of Re-C₄^R. Moreover, decay lifetimes of Re-C₄^R are shorter than those of Re-bpy in the ultrafast transient absorption (TA) and photoluminescence (PL) decay spectra, indicating that the trinuclear cryptate structure of Re-C₄^R could facilitate electron transfer efficiency during CO₂ reduction.

Manipulating the Reaction Pathway of CO2 Photoreduction via the Microenvironment of a Re Molecular Catalyst

Re molecular complexes incorporated into two metal-organic frameworks were investigated to disclose the host-guest interaction by infrared and ¹H nuclear magnetic resonance and to explore the microenvironment around the Re complex by absorption and photoluminescence spectra. ZIF-8 provides a confined space to isolated Re via an electrostatic interaction, while UiO-66 exerts a relaxed space to accessible Re via a coordination interaction. For CO₂ two-electron photoreduction to CO, the turnover number of 28.6 in Re@ZIF-8 is 10-fold that of 2.7 in Re@UiO-66. The electron transfer is promoted in Re@ZIF-8 by a local electrostatic field with a cross-space pathway, whereas it is retarded in Re@UiO-66 as the solvation shell surrounding Re. In the following CO₂ activation, the charged intermediate species could be stabilized in Re@ZIF-8 by spatial confinement, while Re-triethanolamine adducts prevailed in Re@UiO-66 with the accessibility of the Re complex. This work demonstrates a feasibility of diverting the CO₂ activation pathway by the microenvironment of a molecular catalyst in the field of artificial photosynthesis.

Tandem Photocatalysis of CO2 to C2H4 via a Synergistic Rhenium-(I) Bipyridine/Copper-Porphyrinic Triazine Framework

The photocatalytic conversion of CO₂ into C₂₊ products such as ethylene is a promising path toward the carbon neutral goal but remains a big challenge due to the high activation barrier for CO₂ and similar reduction potentials of many possible multi-electron-transfer products. Herein, an effective tandem photocatalysis strategy has been developed to support conversion of CO₂ to ethylene by construction of the synergistic dual sites in rhenium-(I) bipyridine fac-[Re^I(bpy)(CO)₃Cl] (Re-bpy) and copper-porphyrinic triazine framework [PTF(Cu)]. With these two catalysts, a large amount of ethylene can be produced at a rate of 73.2 μmol g^-1 h^-1 under visible light irradiation. However, ethylene cannot be obtained from CO₂ by use of either component of the Re-bpy or PTF(Cu) catalysts alone; with a single catalyst, only monocarbon product CO is produced under similar conditions. In the tandem photocatalytic system, the CO generated at the Re-bpy sites is adsorbed by the nearby Cu single sites in PTF(Cu), and this is followed by a synergistic C-C coupling process which ultimately produces ethylene. Density functional theory calculations demonstrate that the coupling process between PTF(Cu)-*CO and Re-bpy-*CO to form the key intermediate Re-bpy-*CO-*CO-PTF(Cu) is vital to the C₂H₄ production. This work provides a new pathway for the design of efficient photocatalysts for photoconversion of CO₂ to C₂ products via a tandem process driven by visible light under mild conditions.

Rhenium(I)-tricarbonyl complexes with methimazole and its selenium analogue: Syntheses, characterization and cell toxicity

This study explores the effect of a thione/selone ligand on the cell toxicity (in vitro) and light activity of diimine Re(CO)₃⁺ complexes. Six rhenium(I) complexes with general formula fac-[Re(CO)₃(N,N')X]⁺ were prepared, where X = 2-mercapto-1-methylimidazole (methimazole; MMI), and 1-methylimidazole-2-selone (MSeI); N,N' = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (dmphen). Their triflate salts were characterized using single-crystal X-ray diffraction, ¹H, ¹³C and 2D NMR, UV-vis and vibrational spectroscopy. Their cytotoxic properties were tested, showing significant cytotoxicity (IC₅₀ = 8.0-55 μM) towards the human breast cancer cell line MDA-MB-231. The half-inhibitory concentration (IC₅₀) for fac-[Re(CO)₃(dmphen)(MMI)]⁺, the most toxic complex in this series (8.0 ± 0.2 μM), was comparable to that of the corresponding aqua complex fac-[Re(CO)₃(dmphen)(H₂O)]⁺ with IC₅₀ = 6.0 ± 0.1 μM. The fac-[Re(CO)₃(bpy)(MMI/MSeI)]⁺ complexes were somewhat less toxic towards the human embryonic kidney cell line HEK-293 T after 48 h of exposure. The stability of the complexes upon irradiation was monitored using UV-vis spectroscopy, with no CO released when exposed to UV-A light (λ = 365 nm).

Amine Hole Scavengers Facilitate Both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst

A well-known catalyst, fac-Re(4,4'-R₂-bpy)(CO)₃Cl (bpy = bipyridine; R = COOH) (ReC0A), has been widely studied for CO₂ reduction; however, its photocatalytic performance is limited due to its narrow absorption range. Quantum dots (QDs) are efficient light harvesters that offer several advantages, including size tunability and broad absorption in the solar spectrum. Therefore, photoinduced CO₂ reduction over a broad range of the solar spectrum could be enabled by ReC0A catalysts heterogenized on QDs. Here, we investigate interfacial electron transfer from Cd₃P₂ QDs to ReC0A complexes covalently bound on the QD surface, induced by photoexcitation of the QD. We explore the effect of triethylamine, a sacrificial hole scavenger incorporated to replenish the QD with electrons. Through combined transient absorption spectroscopic and computational studies, we demonstrate that electron transfer from Cd₃P₂ to ReC0A can be enhanced by a factor of ∼4 upon addition of triethylamine. We hypothesize that the rate enhancement is a result of triethylamine possibly altering the energetics of the Cd₃P₂-ReC0A system by interacting with the quantum dot surface, deprotonation of the quantum dot, and preferential solvation, resulting in a shift of the conduction band edge to more negative potentials. We also observe the rate enhancement in other QD-electron acceptor systems. Our findings provide mechanistic insights into hole scavenger-quantum dot interactions and how they may influence photoinduced interfacial electron transfer processes.

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.

Highly Robust Rhenium(I) Bipyridyl Complexes Containing Dipyrromethene-BF2 Chromophores for Visible Light-Driven CO2 Reduction

New rhenium bipyridyl complexes with dipyrromethene-BF₂ chromophores (A-ReBDP-CZ, A-ReBDP₂ , ReBDP-CZ, and ReBDP₂ ) were developed for highly efficient photocatalytic carbon dioxide (CO₂ ) reduction to carbon monoxide (CO). These catalysts consisted of two moderate electron-deficient groups (dipyrromethene-BF₂ , BDP) as the visible-light-harvesting antenna as well as both electron donor (N-phenylcarbazole, CZ) and acceptor (BDP) on Re bipyridyl framework. Among ReBDP-CZ and ReBDP₂ complexes, the ReBDP₂ incorporating two electron-deficient BDP chromophores had a longer-lived photoexcited state (182.4 μs) and a twofold enhanced molar absorption coefficient (ϵ=157000 m^-1  cm^-1 ) compared with ReBDP-CZ. Thus, ReBDP₂ achieved the superior photocatalytic reactivity and stability with a CO turnover number (TON_CO ) value as high as 1323 and quantum yield (Φ_CO ) up to 55 %, which was the most excellent photocatalysis efficiency among the single-active-site Re catalysts without additional photosensitizer. Furthermore, the acetylene-bridged linker was detrimental to the photoactivity and durability of the catalyst. In brief, two BDP-based Re bipyridyl systems with outstanding catalytic performance and significant visible-light-harvesting capabilities in the solar spectrum offer a promising strategy for solar-to-fuel conversion schemes.

Cooperative Activation of CO2 and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms

Activation of inert molecules like CO2 is often mediated by cooperative chemistry between two reactive sites within a catalytic assembly, the most common form of which is Lewis acid/base bifunctionality observed in both natural metalloenzymes and synthetic systems. Here, we disclose a heterobinuclear complex with an Al-Fe bond that instead activates CO2 and other substrates through cooperative behavior of two radical intermediates. The complex Ldipp(Me)AlFp (2, Ldipp = HC{(CMe)(2,6-iPr2C6H3N)}2, Fp = FeCp(CO)2, Cp = η5-C5H5) was found to insert CO2 and cyclohexene oxide, producing LdippAl(Me)(μ:κ2-O2C)Fp (3) and LdippAl(Me)(μ-OC6H10)Fp (4), respectively. Detailed mechanistic studies indicate unusual pathways in which (i) the Al-Fe bond dissociates homolytically to generate formally AlII and FeI metalloradicals, then (ii) the metalloradicals add to substrate in a pairwise fashion initiated by O-coordination to Al. The accessibility of this unusual mechanism is aided, in part, by the redox noninnocent nature of Ldipp that stabilizes the formally AlII intermediates, instead giving them predominantly AlIII-like physical character. The redox noninnocent nature of the radical intermediates was elucidated through direct observation of LdippAl(Me)(OCPh2) (22), a metalloradical species generated by addition of benzophenone to 2. Complex 22 was characterized by X-band EPR, Q-band EPR, and ENDOR spectroscopies as well as computational modeling. The "radical pair" pathway represents an unprecedented mechanism for CO2 activation.

Electrochemical CO2 Reduction Catalyzed by Binuclear LRe2(CO)6Cl2 and LMn2(CO)6Br2 Complexes with an Internal Proton Source

The synthesis of certain commodity chemicals, e.g., methanol and acetic acid, relies on CO, which is currently mainly produced by the combustion of carbon or natural gas. Photo- or electrochemical conversion of atmospheric CO₂ to CO represents an attractive alternative strategy as this approach is carbon-neutral. Such photo- or electrochemically formed CO can also be used in the Fischer-Tropsch process forming liquid hydrocarbons for energy storage applications. The multiple electroreduction of CO₂ is preferably coupled with proton transfer steps as this requires less energy than the single outer-sphere 1e^- reduction of CO₂.In 1984 and 2011, it was shown that [(L^bpy)Re(CO)₃Cl] (1) and [(L^bpy)Mn(CO)₃Br] (2), respectively, mediate the electrochemical 2e^-/2H⁺ reduction of CO₂ forming CO and water (L^bpy = 2,2'-bipyridine). Since proton management is crucial for catalysis, recently the impact of internal proton sources close to the axial position in such complexes has been investigated. However, binuclear complexes have been used rarely as mediators although it has been shown very early for 1 that electron management is also important: the 2e^-/2H⁺ reduction pathway with 1 exhibits a higher reaction rate than going via the singly reduced species, though the pathway requires a higher overpotential. In this Account, we focus on recent developments of binuclear LMn₂(CO)₆ and LRe₂(CO)₆ mediators with an internal phenol group in the electroreduction of CO₂. In contrast to mononuclear derivatives, for which the impact of the internal proton source on catalysis is very diverse, we always observed a higher reaction rate and for the Mn complexes also a lower overpotential with the binuclear complexes compared to the mononuclear variants. Spectroscopic, electrochemical, and computational studies on the mono- and binuclear complexes shed light on their reactivity under reductive conditions, elucidated the structure of reduced species, unraveled the kinetics for catalytically productive and unproductive (side) reactions, and allowed us to derive some hypothesis on the CO₂ reduction mechanism. Finally, I emphasize that the electrohydrogenation of the polar double bonds by the binuclear complex LMn₂(CO)₆ with a central phenol unit is not restricted to CO₂ but is also applicable to organic compounds with C═O bonds.

Ultrafast charge transfer dynamics in 2D covalent organic frameworks/Re-complex hybrid photocatalyst

Rhenium(I)-carbonyl-diimine complexes have emerged as promising photocatalysts for carbon dioxide reduction with covalent organic frameworks recognized as perfect sensitizers and scaffold support. Such Re complexes/covalent organic frameworks hybrid catalysts have demonstrated high carbon dioxide reduction activities but with strong excitation energy-dependence. In this paper, we rationalize this behavior by the excitation energy-dependent pathways of internal photo-induced charge transfer studied via transient optical spectroscopies and time-dependent density-functional theory calculation. Under band-edge excitation, the excited electrons are quickly injected from covalent organic frameworks moiety into catalytic Rhenium^I center within picosecond but followed by fast backward geminate recombination. While under excitation with high-energy photon, the injected electrons are located at high-energy levels in Rhenium^I centers with longer lifetime. Besides those injected electrons to Rhenium^I center, there still remain some long-lived electrons in covalent organic frameworks moiety which is transferred back from Rhenium^I. This facilitates the two-electron reaction of carbon dioxide conversion to carbon monoxide.

Re complexes within covalent organic frameworks have emerged as promising photocatalysts for CO₂ reduction. Here, authors identify a high-energy electron transfer pathway during CO₂ reduction that results in longer-lived excited states than a low-energy electron transfer pathway.

Inorganometallic Photocatalyst for CO2 Reduction

ConspectusDuring the last few decades, the design of catalytic systems for CO₂ reduction has been extensively researched and generally involves (1) traditional approaches using molecular organic/organometallic materials and heterogeneous inorganic semiconductors and (2) combinatory approaches wherein these materials are combined as needed. Recently, we have devised a number of new TiO₂-mediated multicomponent hybrid systems that synergistically integrate the intrinsic merits of various materials, namely, molecular photosensitizers/catalysts and n-type TiO₂ semiconductors, and lower the energetic and kinetic barriers between components. We have termed such multicomponent hybrid systems assembled from the hybridization of various organic/inorganic/organometallic units in a single platform inorganometallic photocatalysts. The multicomponent inorganometallic (MIOM) hybrid system onto which the photosensitizer and catalyst are coadsorbed efficiently eliminates the need for bulk-phase diffusion of the components and avoids the accumulation of radical intermediates that invokes a degradation pathway, in contrast to the homogeneous system, in which the free reactive species are concentrated in a confined reaction space. In particular, in energetic terms, we discovered that in nonaqueous media, the conduction band (CB) levels of reduced TiO₂ (TiO₂(e^-)) are positioned at a higher level (in the range -1.5 to -1.9 V vs SCE). This energetic benefit of reduced TiO₂ allows smooth electron transfer (ET) from injected electrons (TiO₂(e^-)) to the coadsorbed CO₂ reduction catalyst, which requires relatively high reducing power (at least more than -1.1 V vs SCE). On the other hand, the existence of various shallow surface trapping sites and surface bands, which are 0.3-1.0 eV below the CB of TiO₂, efficiently facilitates electron injection from any photosensitizer (including dyes having low excited energy levels) to TiO₂ without energetic limitation. This is contrasted with most photocatalytic systems, wherein successive absorption of single high-energy photons is required to produce excited states with enough energy to fulfill photocatalytic reaction, which may allow unwanted side reactions during photocatalysis. In this Account, we present our recent research efforts toward advancing these MIOM hybrid systems for photochemical CO₂ reduction and discuss their working mechanisms in detail. Basic ET processes within the MIOM system, including intervalence ET in organic/organometallic redox systems, metal-to-ligand charge transfer of organometallic complexes, and interfacial/outer-sphere charge transfer between components, were investigated by conducting serial photophysical and electrochemical analyses. Because such ET events occur primarily at the interface between the components, the efficiency of interfacial ET between the molecular components (organic/organometallic photosensitizers and molecular reduction catalysts) and the bulk inorganic solid (mainly n-type TiO₂ semiconductors) has a significant influence on the overall photochemical reaction kinetics and mechanism. In some TiO₂-mediated MIOM hybrids, the chemical attachment of organic or organometallic photosensitizing units onto TiO₂ semiconductors efficiently eliminates the step of diffusion/collision-controlled ET between components and prevents the accumulation of reactive species (oxidatively quenched cations or reductively quenched anions) in the reaction solution, ensuring steady photosensitization over an extended reaction period. The site isolation of a single-site organometallic catalyst employing TiO₂ immobilization promotes the monomeric catalytic pathway during the CO₂ reduction process, resulting in enhanced product selectivity and catalytic performance, including lifetime extension. In addition, as an alternative inorganic solid scaffold, the introduction of a host porphyrin matrix (interlinked in a metal-organic framework (MOF) material) led to efficient and durable photocatalytic CO₂ conversion by the new MOF-Re(I) hybrid as a result of efficient light harvesting/exciton migration in the porphyrinic MOF and rapid quenching of the photogenerated electrons by the doped Re(I) catalytic sites. Overall, the case studies presented herein provide valuable insights for the rational design of advanced multicomponent hybrid systems for artificial photosynthesis involving CO₂ reduction.

The role of substituted pyridine Schiff bases as ancillary ligands in the optical properties of a new series of fac-rhenium(i) tricarbonyl complexes: a theoretical view

Over the last few years, luminescent Re(i) tricarbonyl complexes have been increasingly proposed as fluorophores suitable for fluorescence microscopy to visualize biological structures and cells. In this sense, incorporating an asymmetrical pyridine Schiff base (PSB) as the ancillary ligand strongly modifies the staining and luminescent properties of Re(i) tricarbonyl complexes. In this work, we analyzed two series of Re(i) tricarbonyl complexes with their respective PSB ligands: (1) fac-[Re(CO)3(2,2'-bpy)(PSB)]1+ and (2) fac-[Re(CO)3(4,4'-bis(ethoxycarbonyl)-2,2'-bpy)(PSB)]1+, where the PSB exhibits substitutions at positions 4 or 6 in the phenolic ring with methyl or halogen substituents. Thus, we performed computational relativistic DFT and TDDFT studies to determine their optical properties. The ten complexes analyzed showed absorption in the visible light range. Furthermore, our analyses, including zero-field splitting (ZFS), allowed us to determine that the low-lying excited state locates below the 3LLCT states. Interestingly, seven of the ten analyzed complexes, whose corresponding PSB harbors an intramolecular hydrogen bond (IHB), exhibited luminescent emission that could be suitable for biological purposes: large Stokes shift, emission in the range 600-700 nm and τ in the order of 10-2 to 10-3 s. Conversely, the three complexes lacking the IHB due to two halogen substituents in the corresponding PSB showed a predicted emission with the lowest triplet excited state energy entering the NIR region. The main differences in the complexes' photophysical behavior have been explained by the energy gap law and time-resolved luminescence. These results emphasize the importance of choosing suitable substituents at the 4 and 6 positions in the phenolic ring of the PSB, which determine the presence of the IHB since they modulate the luminescence properties of the Re(i) core. Therefore, this study could predict Re(i) tricarbonyl complexes' properties, considering the desired emission features for biological and other applications.

DFT insights into the photocatalytic reduction of CO2 to CO by Re(I) complexes: the crucial role of the triethanolamine "magic" sacrificial electron donor

The reaction mechanism for the photocatalytic reduction of CO₂ to CO catalyzed by the [Re(en)(CO)₃Cl] complex in the presence of triethanolamine, R₃N (R = CH₂CH₂OH) abbreviated as TEOA, in DMF solution was studied in-depth with the aid of DFT computational protocols by calculating the geometric and free energy reaction profiles for several possible reaction pathways. The reaction pathways studied start with the "real" catalytic species [Re(en)(CO)₃], [Re(en)(CO)₃]^- and/or [Re(en)(CO)₂Cl]^- generated from the excited triplet T1 state upon single and double reductive quenching by a TEOA sacrificial electron donor or photodissociation of a CO ligand. The first step in all the catalytic cycles investigated involves the capture of either CO₂ or the oxidized R₂NCH₂CH₂O˙ radical. In the latter case, the CO₂ molecule is captured (inserted) by the Re-OCH₂CH₂NR₂ bond forming stable intermediates. Next, successive protonations (TEOA also acts as a proton donor) lead to the release of CO either from the energy consuming 2e^- reduction of [Re(en)(CO)₄]⁺ or [Re(en)(CO)₂Cl]⁺ complexes in the CO₂ capture pathways or from the released unstable diprotonated [R₂NCH₂CH₂OC(OH)(OH)]⁺ species regenerating TEOA and the catalyst. The CO₂ insertion reaction pathway is the favorable pathway for the photocatalytic reduction of CO₂ → CO catalyzed by the [Re(en)(CO)₃Cl] complex in the presence of TEOA manifesting its crucial role as an electron and proton donor, capturing CO₂ and releasing CO.

Accessing the triplet manifold of naphthalene benzimidazole-phenanthroline in rhenium(I) bichromophores

The steady-state and ultrafast to supra-nanosecond excited state dynamics of fac-[Re(NBI-phen)(CO)₃(L)](PF₆) (NBI-phen = 16H-benzo[4',5']isoquinolino[2',1':1,2]imidazo[4,5-f][1,10]phenanthrolin-16-one) as well as their respective models of the general molecular formula [Re(phen)(CO)₃(L)](PF₆) (L = PPh₃ and CH₃CN) has been investigated using transient absorption and time-gated photoluminescence spectroscopy. The NBI-phen containing molecules exhibited enhanced visible light absorption with respect to their models and a rapid formation (<6 ns) of the triplet ligand-centred (LC) excited state of the organic ligand, NBI-phen. These triplet states exhibit an extended excited state lifetime that enable the energized molecules to readily engage in triplet-triplet annihilation photochemistry.

Understanding the photophysical properties of rhenium(I) compounds coordinated to 4,7-diamine-1,10-phenanthroline: synthetic, luminescence and biological studies

In the present study, the photophysical properties and preliminary time-dependent density functional theory (TD-DFT) data of new rhenium(i) polypyridyl compounds, fac-[Re(L)(Am2phen)(CO)3]0/+, where Am2phen = 4,7-diamine-1,10-phenanthroline and L = Cl and ethyl isonicotinate (et-isonic), provided new insights into excited-state deactivation through an unusual inversion between two metal-to-ligand charge-transfer excited states. In addition, their cellular uptake using breast cancer (MCF-7) and melanoma (SkMel-147 and SkMel-29) cell lines and bioactivity were investigated and their cell-killing mechanism and protein expression were also studied. Preliminary TD-DFT results showed that both compounds exhibited a strong and broad absorption band around 300-400 nm which corresponds to a combination of ILAm2phen and MLCTRe→Am2phen transitions, and a strong contribution of charge transfer transition MLCTRe→et-isonic for fac-[Re(et-isonic)(Am2phen)(CO)3]+ is also observed. In contrast to typical Re(i) polypyridyl complexes, the substitution of Cl with the et-isonic ligand showed a bathochromic shift of the emission maxima, relatively low emission quantum yield and fast lifetime. Photophysical investigation of the fac-[ReCl(et-isonic)2(CO)3] compound provided meaningful information on the excited state manifold of the fac-[Re(L)(Am2phen)(CO)3]0/+ complexes. As shown in the absorption profile, a remarkable inversion of the lowest-lying excited state takes place from the usually observed MLCTRe→Am2phen to the unusual MLCTRe→et-isonic. The lipophilicity of the positive-complex was higher than that of the non-charge compound and the same trend for the activity against cells was observed, in the absence of light. In addition, flow cytometry and Western Blot analyses showed an overexpression of pro-caspase-9, suggesting a caspase proteolytic cascade through an intrinsic-pathway apoptosis mechanism. The photophysical properties of these compounds reported herein provide new fundamental insights into the understanding of substituent groups on polypyridyl ligands which are relevant to practical development.

Reaction of a Molybdenum Bis(dinitrogen) Complex with Carbon Dioxide: A Combined Experimental and Computational Investigation

Refluxing Mo(CO)₆ in the presence of the phosphine-functionalized α-diimine ligand ^Ph2PPrDI allowed for substitution and formation of the dicarbonyl complex, (^Ph2PPrDI)Mo(CO)₂. Oxidation with I₂ followed by heating resulted in further CO dissociation and isolation of the corresponding diiodide complex, (^Ph2PPrDI)MoI₂. Reduction of this complex under a N₂ atmosphere afforded the corresponding bis(dinitrogen) complex, (^Ph2PPrDI)Mo(N₂)₂. The solid-state structures of all three compounds were found to feature a tetradentate chelate and cis-monodentate ligands. Notably, the addition of CO₂ to (^Ph2PPrDI)Mo(N₂)₂ is proposed to result in head-to-tail CO₂ coupling to generate the corresponding metallacycle and ultimately a mixture of (^Ph2PPrDI)Mo(CO)₂ and the bis(oxo) dimer, [(κ³-^Ph2PPrDI)Mo(O)(μ-O)]₂. Computational studies have been performed to gain insight into the reaction and evaluate the importance of cis-coordination sites for selective head-to-tail CO₂ reductive coupling, CO deinsertion, disproportionation, and stepwise CO₂ deinsertion.

Mechanistic study of photocatalytic CO2 reduction using a Ru(ii)-Re(i) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)₃]²⁺ photosensitiser and fac-[Re(diimine)(CO)₃{OC(O)OC₂H₄NR₂}] catalyst units can be used to reduce CO₂ to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO₂ to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO₂, e.g., 10% CO₂, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO₂ was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)−, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)− is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)− produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)− proceeds with an observed first-order rate constant of approximately 1.8 s⁻¹ at 298 K and is a unimolecular reaction, independent of the concentrations of both CO₂ and RuC2(Re)−.

Formation processes and reactivity of an important intermediate of photocatalytic CO₂ reduction, one-electron reduced species of a Ru(ii)–Re(i) supramolecular photocatalyst with a carbonate ester ligand, were investigated in detail.

[Re(CO)3(5-PAN)Cl], a rhenium(I) naphthalimide complex for the visible light photocatalytic reduction of CO2

A rhenium(i) naphthalimide complex [Re(CO)3(5-PAN)Cl] (Re(5-PAN); 5-PAN = 1-(1,10-phenanthroline)-4-nitro-naphthalimide) was synthesized, characterized, and evaluated as a photocatalyst for CO2 reduction. Characterization included use of MALDI-ToF mass spectrometry, FT-IR, RAMAN, 1H and 13C NMR, elemental analysis, electronic absorption and emission spectroscopy, single crystal X-ray diffraction, DFT and cyclic voltammetry. Photocatalytic (406 nm) reduction of 13CO2 to formate (H13COO) in the presence of this catalyst was tracked via13C NMR. Results support Re5-PAN (φ = 0.021) functioning as a catalyst for the reduction of CO2 (maximum turn-over 48-50 at 300 equiv. triethylamine as the sacrificial electron donor).

Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br₂]⁻ anion (2, where ^tBu₂bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), which can be isolated from the one electron reduction of the corresponding 17-electron complex (1). Compound 2 is stable in the solid state, but in solution it is oxidized by molecular oxygen back to 1. Replacement of a single bromide of 2 by σ-donor monodentate ligands (Ls) yields stable neutral 18-electron cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)₂(^tBu₂bpy)Br(L)] species can be further reacted with Ls to prepare stable cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes in good yield. Ligand substitution of Re(i) complexes proceeds via pentacoordinate intermediates capable of Berry pseudorotation. In addition to the cis–cis–trans-complexes, cis–cis–cis- (all cis) isomers are also formed. In particular, cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes establish an equilibrium with all cis isomers in solution. The solid state crystal structure of nearly all molecules presented could be elucidated. The molecules adopt a slightly distorted octahedral geometry. In comparison to similar fac-[Re(CO)₃]⁺complexes, Re(i) diacarbonyl species are characterized by a bend (ca. 7°) of the axial ligands towards the α-diimine unit. [Re(CO)₂(^tBu₂bpy)Br₂]⁻ and [Re(CO)₂(^tBu₂bpy)Br(L)] complexes may be considered as synthons for the preparation of a variety of new stable diamagnetic dicarbonyl rhenium cis-[Re(CO)₂]⁺ complexes, offering a convenient entry in the chemistry of the core.

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes offer a convenient entry in the chemistry of the cis-[Re(CO)₂]⁺ core.

Tuning of Ionic Second Coordination Sphere in Evolved Rhenium Catalyst for Efficient Visible-Light-Driven CO2 Reduction

Developing an efficient and easy-to-handle strategy in designing catalysts for CO₂ reduction into CO by harnessing sunlight is a promising project. Here, a facile strategy was developed to design a Re catalyst modified with an ionic secondary coordination sphere for photoreduction of CO₂ to CO by visible light. By adding ionic liquids or tuning a different ionic secondary coordination sphere, it was discovered that an outstanding optical property, other than CO₂ absorption ability or the ability to dissociation of chloride anion, is the prerequisite for catalyst design. Accordingly, a novel Re catalyst, {Re[BpyMe(tris(2-hydroxyethyl)amine)](CO)₃ Cl}Br (Re-THEA), was designed, screened, and resulted in a relative high quantum yield (up to 34 %) for visible-light-induced CO₂ reduction with a single-molecule system. DFT calculations, combined with experimental outcomes, suggested the pendant ionic tris(2-hydroxyethyl)amino (THEA) group on Re-THEA can enhance visible-light absorption, stabilize reaction intermediates, and suppress the Re-Re dimer formation.

Enhancing Molecular Electrocatalysis of CO2 Reduction with Pressure-Tunable CO2 -Expanded Electrolytes

Electrochemical studies of CO₂ conversion by molecular catalysts are typically carried out in a narrow range of near-ambient CO₂ pressures wherein low CO₂ solubilities in the liquid phase can limit the rate of CO₂ reduction. In this study, five-fold rate enhancements are enabled by pairing CO₂ -expanded electrolytes (CXEs), a class of media that accommodate multimolar concentrations of CO₂ in organic solvents at modest pressures, with a homogeneous molecular electrocatalyst, [Re(CO)₃ (bpy)Cl] (1, bpy=2,2'-bipyridyl). Analysis of cyclic voltammetry data reveals pressure-tunable rate behavior, with first-order kinetics at moderate CO₂ pressures giving way to zero-order kinetics at higher pressures. The significant enhancement in the space-time yield of CO demonstrates that CXEs offer a simple yet powerful strategy for unlocking the intrinsic potential of molecular catalysts by mitigating CO₂ solubility limitations commonly encountered in conventional liquid electrolytes. Moreover, our findings reveal that 1, a workhorse molecular catalyst, performs with intrinsic kinetic behavior, which is competitive with fast enzymes under optimal conditions in CXEs.

Covalent organic framework photocatalysts: structures and applications

In the light of increasing energy demand and environmental pollution, it is urgently required to find a clean and renewable energy source. In these years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or environmental pollutant degradation, has shown great potential to achieve this goal. Among the various photocatalysts, covalent organic frameworks (COFs) are very attractive due to their excellent structural regularity, robust framework, inherent porosity and good activity. Thus, many studies have been carried out to investigate the photocatalytic performance of COFs and COF-based photocatalysts. In this critical review, the recent progress and advances of COF photocatalysts are thoroughly presented. Furthermore, diverse linkers between COF building blocks such as boron-containing connections and nitrogen-containing connections are summarised and compared. The morphologies of COFs and several commonly used strategies pertaining to photocatalytic activity are also discussed. Following this, the applications of COF-based photocatalysts are detailed including photocatalytic hydrogen evolution, CO₂ conversion and degradation of environmental contaminants. Finally, a summary and perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts are given.

Aqueous electrocatalytic CO2 reduction using metal complexes dispersed in polymer ion gels

We use fac-[Re(bpy)(CO)₃Cl] ([Re-Cl]) dispersed in polymer ion gel (PIG) ([Re]-PIG) to carry out electrocatalytic CO₂ reduction in water. Electrolysis at -0.68 V vs. RHE in a CO₂-saturated KOH and K₂CO₃ solution produces CO with over 90% Faradaic efficiency. The PIG electrode is readily combined with water oxidation catalysts to generate a full electrochemical cell. Additionally, we provide evidence that the PIG electrode can be implemented with other molecular catalysts.

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO2 Reduction

Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three Re^I complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The Re^I compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO₂ to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive Re^I complexes and for electrocatalytic CO₂ reduction.