Synthesis and Reactions of fac-[Re(dmbpy)(CO)3X] (dmbpy = 4,4‘-Dimethyl-2,2‘-bipyridine; X = COOH, CHO) and Their Derivatives

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).

Solar CO2 reduction using a molecular Re(I) catalyst grafted on SiO2via amide and alkyl amine linkages

Heterogenized molecular catalysts have shown interesting activities in different chemical transformations. In our previous studies, a molecular catalyst, Re(bpy)(CO)3Cl where bpy is 2,2'-bipyridine, was covalently attached to silica surfaces via an amide linkage for use in photocatalytic CO2 reduction. Derivatizing the bpy ligand with electron-withdrawing amide groups led to detrimental effects on the catalytic activity of Re(bpy)(CO)3Cl. In this study, an alkyl amine linkage is utilized to attach Re(bpy)(CO)3Cl onto SiO2 in order to eliminate the detrimental effects of the amide linkage by breaking the conjugation between the bpy ligand and the amide group. However, the heterogenized Re(I) catalyst containing the alkyl amine linkage demonstrates even lower activity than the one containing the amide linkage in photocatalytic CO2 reduction. Infrared studies suggest that the presence of the basic amine group led to the formation of a photocatalytically inactive Re(I)-OH species on SiO2. Furthermore, the amine group likely contributes to the stabilization of a surface Re(I)-carboxylato species formed upon light irradiation, resulting in the low activity of the heterogenized Re(I) catalyst containing the alkyl amine linkage.

Photochemistry of Rhenium(i) Diimine Tricarbonyl Complexes in Biological Applications

Luminescent rhenium complexes continue to be the focus of growing scientific interest for catalytic, diagnostic and therapeutic applications, with emphasis on the development of their photophysical and photochemical properties. In this short review, we explore such properties with a focus on the biological applications of the molecules. We discuss the importance of the ligand choice to the contribution and their involvement towards the most significant electronic transitions of the metal species and what strategies are used to exploit the potential of the molecules in medicinal applications. We begin by detailing the photophysics of the molecules; we then describe the three most common photoreactions of rhenium complexes as photosensitizers in H₂ production, photocatalysts in CO₂ reduction and photochemical ligand substitution. In the last part, we describe their applications as luminescent cellular probes and how photochemical ligand substitution is utilized in the development of photoactive carbon monoxide-releasing molecules as anticancer and antimicrobial agents.

Positive effect of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) on homogeneous photocatalytic reduction of CO2

Addition of DBU enables reducing the amount of sacrificial electron donor and increases catalytic performance in photocatalytic CO₂ reduction.

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-R)(CO)3 Br] Complexes

The electrochemical behavior of fac-[Mn(pdbpy)(CO)₃ Br] (pdbpy=4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine) (1) in acetonitrile under Ar, and its catalytic performances for CO₂ reduction with added water, 2,2,2-trifluoroethanol (TFE), and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V versus the standard calomel electrode (SCE) in CO₂ -saturated acetonitrile, reveal that the process selectivity is extremely sensitive to the acid strength, producing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV/Vis) study under Ar and CO₂ atmospheres shows that 1 undergoes fast solvolysis; however, dimer formation in acetonitrile is suppressed, resulting in an atypical reduction mechanism in comparison with other reported Mn^I catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO₂ reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)₃ Br] (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst (2), bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO₂ .

Organic–inorganic hybrid photocatalyst for carbon dioxide reduction

Efficient hybrid photocatalysts for carbon dioxide reduction were developed from dye-sensitized TiO₂ nanoparticles and their catalytic performance was optimized by ternary organic/inorganic components. Thus, the hybrid system consists of (E)-2-cyano-3-(5′-(5′′-(p-(diphenylamino)phenyl)thiophen-2′′-yl)thiophen-2′-yl)-acrylic acid as a sensitizer and fac-[Re(4,4′-bis(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO)₃Cl] as a reduction catalyst (ReP), both of which have been fixed onto TiO₂ semiconductors (s-TiO₂, h-TiO₂, d-TiO₂). Mott–Schottky analysis on flat-band potential (E_fb) of TiO₂ mesoporous films has verified that E_fb can be finely modulated by volume variation of water (0 to 20 vol%). The increase of added water resulted in substantial positive shifts of E_fb from −1.93 V at 0 vol% H₂O, to −1.74 V (3 vol% H₂O), to −1.56 V (10 vol% H₂O), and to −1.47 V (20 vol% H₂O). As a result, with addition of 3–10 vol% water in the photocatalytic reaction, conversion efficiency of CO₂ to CO increased significantly reaching a TON value of ∼350 for 30 h. Catalytic activity enhancement is mainly attributed to (1) the optimum alignment of E_fb by 3–10 vol% water with respect to the of the dye and E_red of ReP for smooth electron transfer from photo-excited dye to RePvia the TiO₂ semiconductor and (2) the water-induced acceleration of chemical processes on the fixed ReP. In addition, the energy level was further tuned by variation of the dye and ReP amounts. We also found that the intrinsic properties of TiO₂ sources (morphology, size, agglomeration) exert a great influence on the overall photocatalytic activity of this hybrid system. Implications of the present observations and reaction mechanisms are discussed in detail.

Raman Spectroscopy as a Method to Investigate Catalytic Intermediates: CO2 Reducing [Re(Cl)(bpy-R)(CO)3] Catalyst

Complexes of the type [Re(Cl)(bpy-R)(CO)3] (1, bpy = bipyridine, R = (t)Bu, H, CF3) show high catalytic activity for electrochemical CO2 reduction. Application of Raman spectroscopy to these complexes as well as to the doubly reduced species [Re(bpy-R)(CO)3](-) (3), which are the postulated active species, and the monoreduced complex [Re(Cl)(bpy-CF3)(CO)3](-) (2) and comparison with state-of-the-art quantum chemical calculations allows accurate investigation of electronic structures as well as geometries. For doubly reduced complexes, calculations point out a formal closed-shell singlet state only compatible with a formal {Re(I)(bpy-R)(2-)} moiety. In contrast, based on molecular orbital analysis and the change of the actual charge distribution during the overall two-electron reduction, the system is better described as {Re(0)(bpy-R(•))(-)}. Interestingly, the Raman spectra of the monoreduced and doubly reduced complexes with the CF3-substituted bpy ligand are virtually identical, which points to the same overall electronic structure of the bpy species in both complexes. Additional Raman experiments and calculations of [Re(COOH)(bpy)(CO)3] (4) and [Re(bpy)(CO)4]OTf (5), which are proposed to be intermediates of the catalytic cycle for CO2 reduction, confirm the presence of neutral bpy showing that the reducing equivalent stored at the bidentate ligand is involved in the activation of CO2. As such, Raman spectroscopy combined with quantum chemical calculations is an ideal tool to investigate catalysts with redox active ligands, since the spectra give local information about the electronic and geometric structure of the molecule.

Understanding the hydrolysis mechanism of ethyl acetate catalyzed by an aqueous molybdocene: a computational chemistry investigation

A thoroughly mechanistic investigation on the [Cp2Mo(OH)(OH2)](+)-catalyzed hydrolysis of ethyl acetate has been performed using density functional theory methodology together with continuum and discrete-continuum solvation models. The use of explicit water molecules in the PCM-B3LYP/aug-cc-pVTZ (aug-cc-pVTZ-PP for Mo)//PCM-B3LYP/aug-cc-pVDZ (aug-cc-pVDZ-PP for Mo) computations is crucial to show that the intramolecular hydroxo ligand attack is the preferred mechanism in agreement with experimental suggestions. Besides, the most stable intermediate located along this mechanism is analogous to that experimentally reported for the norbornenyl acetate hydrolysis catalyzed by molybdocenes. The three most relevant steps are the formation and cleavage of the tetrahedral intermediate immediately formed after the hydroxo ligand attack and the acetic acid formation, with the second one being the rate-determining step with a Gibbs energy barrier of 36.7 kcal/mol. Among several functionals checked, B3LYP-D3 and M06 give the best agreement with experiment as the rate-determining Gibbs energy barrier obtained only differs 0.2 and 0.7 kcal/mol, respectively, from that derived from the experimental kinetic constant measured at 296.15 K. In both cases, the acetic acid elimination becomes now the rate-determining step of the overall process as it is 0.4 kcal/mol less stable than the tetrahedral intermediate cleavage. Apart from clarifying the identity of the cyclic intermediate and discarding the tetrahedral intermediate formation as the rate-determining step for the mechanism of the acetyl acetate hydrolysis catalyzed by molybdocenes, the small difference in the Gibbs energy barrier found between the acetic acid formation and the tetrahedral intermediate cleavage also uncovers that the rate-determining step could change when studying the reactivity of carboxylic esters other than ethyl acetate substrate specific toward molybdocenes or other transition metal complexes. Therefore, in general, the information reported here could be of interest in designing new catalysts and understanding the reaction mechanism of these and other metal-catalyzed hydrolysis reactions.

Photoreduction of CO2 to CO by a mononuclear Re(i) complex and DFT evaluation of the photocatalytic mechanism

A new method for the preparation of fac-[Re(phen-dione)(CO)₃Cl] and its application for the photochemical reduction of CO₂ to CO have been reported.

Direct detection of key reaction intermediates in photochemical CO2 reduction sensitized by a rhenium bipyridine complex

Photochemical CO2 reduction sensitized by rhenium-bipyridyl complexes has been studied through multiple approaches during the past several decades. However, a key reaction intermediate, the CO2-coordinated Re-bipyridyl complex, which should govern the activity of CO2 reduction in the photocatalytic cycle, has never been detected in a direct way. In this study on photoreduction of CO2 catalyzed by the 4,4'-dimethyl-2,2'-bipyridine (dmbpy) complex, [Re(dmbpy)(CO)3Cl] (1), we successfully detect the solvent-coordinated Re complex [Re(dmbpy)(CO)3DMF] (2) as the light-absorbing species to drive photoreduction of CO2. The key intermediate, the CO2-coordinated Re-bipyridyl complex, [Re(dmbpy)(CO)3(COOH)], is also successfully detected for the first time by means of cold-spray ionization spectrometry (CSI-MS). Mass spectra for a reaction mixture with isotopically labeled (13)CO2 provide clear evidence for the incorporation of CO2 into the Re-bipyridyl complex. It is revealed that the starting chloride complex 1 was rapidly transformed into the DMF-coordinated Re complex 2 through the initial cycle of photoreduction of CO2. The observed induction period in the time profile of the CSI-MS signals can well explain the subsequent formation of the CO2-coordinated intermediate from the solvent-coordinated Re-bipyridyl complex. An FTIR study of the reaction mixture in dimethyl sulfoxide clearly shows the appearance of a signal at 1682 cm(-1), which shifts to 1647 cm(-1) for the (13)CO2-labeled counterpart; this is assigned as the CO2-coordinated intermediate, Re(II)-COOH. Thus, a detailed understanding has now been obtained for the mechanism of the archetypical photochemical CO2 reduction sensitized by a Re-bipyridyl complex.

High-turnover visible-light photoreduction of CO2 by a Re(i) complex stabilized on dye-sensitized TiO2

Higher photocatalytic CO₂ conversion to CO with a turnover number of 435 was achieved by the ternary dye-sensitized systems comprising a dye/TiO₂/Re platform.

Pulsed-EPR evidence of a manganese(II) hydroxycarbonyl intermediate in the electrocatalytic reduction of carbon dioxide by a manganese bipyridyl derivative

A key intermediate in the electroconversion of carbon dioxide to carbon monoxide, catalyzed by a manganese tris(carbonyl) complex, is characterized. Different catalytic pathways and their potential reaction mechanisms are investigated using a large range of experimental and computational techniques. Sophisticated spectroscopic methods including UV/Vis absorption and pulsed-EPR techniques (2P-ESEEM and HYSCORE) were combined together with DFT calculations to successfully identify a key intermediate in the catalytic cycle of CO2 reduction. The results directly show the formation of a metal-carboxylic acid-CO2 adduct after oxidative addition of CO2 and H(+) to a Mn(0) carbonyl dimer, an unexpected intermediate.

Mechanisms for CO production from CO2 using reduced rhenium tricarbonyl catalysts

The chemical conversion of CO(2) has been studied by numerous experimental groups. Particularly the use of rhenium tricarbonyl-based molecular catalysts has attracted interest owing to their ability to absorb light, store redox equivalents, and convert CO(2) into higher-energy products. The mechanism by which these catalysts mediate reduction, particularly to CO and HCOO(-), is poorly understood, and studies aimed at elucidating the reaction pathway have likely been hindered by the large number of species present in solution. Herein the mechanism for carbon monoxide production using rhenium tricarbonyl catalysts has been investigated using density functional theory. The investigation presented proceeds from the experimental work of Meyer's group (J. Chem. Soc., Chem. Commun.1985, 1414-1416) in DMSO and Fujita's group (J. Am. Chem. Soc.2003, 125, 11976-11987) in dry DMF. The latter work with a simplified reaction mixture, one that removes the photo-induced reduction step with a sacrificial donor, is used for validation of the proposed mechanism, which involves formation of a rhenium carboxylate dimer, [Re(dmb)(CO)(3)](2)(OCO), where dmb = 4,4'-dimethyl-2,2'-bipyridine. CO(2) insertion into this species, and subsequent rearrangement, is proposed to yield CO and the carbonate-bridged [Re(dmb)(CO)(3)](2)(OCO(2)). Structures and energies for the proposed reaction path are presented and compared to previously published experimental observations.

Doping metal-organic frameworks for water oxidation, carbon dioxide reduction, and organic photocatalysis

Catalytically competent Ir, Re, and Ru complexes H(2)L(1)-H(2)L(6) with dicarboxylic acid functionalities were incorporated into a highly stable and porous Zr(6)O(4)(OH)(4)(bpdc)(6) (UiO-67, bpdc = para-biphenyldicarboxylic acid) framework using a mix-and-match synthetic strategy. The matching ligand lengths between bpdc and L(1)-L(6) ligands allowed the construction of highly crystalline UiO-67 frameworks (metal-organic frameworks (MOFs) 1-6) that were doped with L(1)-L(6) ligands. MOFs 1-6 were isostructural to the parent UiO-67 framework as shown by powder X-ray diffraction (PXRD) and exhibited high surface areas ranging from 1092 to 1497 m(2)/g. MOFs 1-6 were stable in air up to 400 °C and active catalysts in a range of reactions that are relevant to solar energy utilization. MOFs 1-3 containing [Cp*Ir(III)(dcppy)Cl] (H(2)L(1)), [Cp*Ir(III)(dcbpy)Cl]Cl (H(2)L(2)), and [Ir(III)(dcppy)(2)(H(2)O)(2)]OTf (H(2)L(3)) (where Cp* is pentamethylcyclopentadienyl, dcppy is 2-phenylpyridine-5,4'-dicarboxylic acid, and dcbpy is 2,2'-bipyridine-5,5'-dicarboxylic acid) were effective water oxidation catalysts (WOCs), with turnover frequencies (TOFs) of up to 4.8 h(-1). The [Re(I)(CO)(3)(dcbpy)Cl] (H(2)L(4)) derivatized MOF 4 served as an active catalyst for photocatalytic CO(2) reduction with a total turnover number (TON) of 10.9, three times higher than that of the homogeneous complex H(2)L(4). MOFs 5 and 6 contained phosphorescent [Ir(III)(ppy)(2)(dcbpy)]Cl (H(2)L(5)) and [Ru(II)(bpy)(2)(dcbpy)]Cl(2) (H(2)L(6)) (where ppy is 2-phenylpyridine and bpy is 2,2'-bipyridine) and were used in three photocatalytic organic transformations (aza-Henry reaction, aerobic amine coupling, and aerobic oxidation of thioanisole) with very high activities. The inactivity of the parent UiO-67 framework and the reaction supernatants in catalytic water oxidation, CO(2) reduction, and organic transformations indicate both the molecular origin and heterogeneous nature of these catalytic processes. The stability of the doped UiO-67 catalysts under catalytic conditions was also demonstrated by comparing PXRD patterns before and after catalysis. This work illustrates the potential of combining molecular catalysts and MOF structures in developing highly active heterogeneous catalysts for solar energy utilization.

Photocatalytic reduction of CO₂: from molecules to semiconductors

We are facing three serious problems related to fossil resources, i.e., shortage of energy, shortage of carbon resources, and the global worming problem. Development of practical systems for converting CO₂ to useful chemicals using solar light, i.e., photocatalytic CO₂ reduction systems, should be one of the best solutions for these problems. In this article, we review photocatalytic CO₂ reduction systems, which are classified in two categories: (1) homogeneous reaction systems mainly using transition metal complexes, and (2) heterogeneous systems mainly using inorganic semiconductor as a light absorber.

Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels

The scientific community now agrees that the rise in atmospheric CO(2), the most abundant green house gas, comes from anthropogenic sources such as the burning of fossil fuels. This atmospheric rise in CO(2) results in global climate change. Therefore methods for photochemically transforming CO(2) into a source of fuel could offer an attractive way to decrease atmospheric concentrations. One way to accomplish this conversion is through the light-driven reduction of carbon dioxide to methane (CH(4(g))) or methanol (CH(3)OH((l))) with electrons and protons derived from water. Existing infrastructure already supports the delivery of natural gas and liquid fuels, which makes these possible CO(2) reduction products particularly appealing. This Account focuses on molecular approaches to photochemical CO(2) reduction in homogeneous solution. The reduction of CO(2) by one electron to form CO(2)(*-) is highly unfavorable, having a formal reduction potential of -2.14 V vs SCE. Rapid reduction requires an overpotential of up to 0.6 V, due at least in part to the kinetic restrictions imposed by the structural difference between linear CO(2) and bent CO(2)(*-). An alternative and more favorable pathway is to reduce CO(2) though proton-assisted multiple-electron transfer. The development of catalysts, redox mediators, or both that efficiently drive these reactions remains an important and active area of research. We divide these reactions into two class types. In Type I photocatalysis, a molecular light absorber and a transition metal catalyst work in concert. We also consider a special case of Type 1 photocatalysis, where a saturated hydrocarbon links the catalyst and the light absorber in a supramolecular compound. In Type II photocatalysis, the light absorber and the catalyst are the same molecule. In these reactions, transition-metal coordination compounds often serve as catalysts because they can absorb a significant portion of the solar spectrum and can promote activation of small molecules. This Account discusses four classes of transition-metal catalysts: (A) metal tetraaza-macrocyclic compounds; (B) supramolecular complexes; (C) metalloporphyrins and related metallomacrocycles; (D) Re(CO)(3)(bpy)X-based compounds where bpy = 2,2'-bipyridine. Carbon monoxide and formate are the primary CO(2) reduction products, and we also propose bicarbonate/carbonate production. For comprehensiveness, we briefly discuss hydrogen formation, a common side reaction that occurs concurrently with CO(2) reduction, though the details of that process are beyond the scope of this Account. It is our hope that drawing attention both to current mechanistic hypotheses and to the areas that are poorly understood will stimulate research that could one day provide an efficient solution to this global problem.