Catalytic nitrogen fixation using visible light energy.

The synthesis of ammonia from atmospheric dinitrogen, nitrogen fixation, is one of the essential reactions for human beings. Because the current industrial nitrogen fixation depends on dihydrogen produced from fossil fuels as raw material, the development of a nitrogen fixation reaction that relies on the energy provided by renewable energy, such as visible light, is an important research goal from the viewpoint of sustainable chemistry. Herein, we establish an iridium- and molybdenum-catalysed process for synthesizing ammonia from dinitrogen under ambient reaction conditions and visible light irradiation. In this reaction system, iridium complexes and molybdenum triiodide complexes bearing N-heterocyclic carbene-based pincer ligands act as cooperative catalysts to activate 9,10-dihydroacridine and dinitrogen, respectively. The reaction of dinitrogen with 9,10-dihydroacridine is not thermodynamically favoured, and it only takes place under visible light irradiation. Therefore, the described reaction system is one that affords visible light energy-driven ammonia formation from dinitrogen catalytically.

Utilization of Low-Concentration CO2 with Molecular Catalysts Assisted by CO2-Capturing Ability of Catalysts, Additives, or Reaction Media.

Increasing concentration of atmospheric CO2 is a worldwide concern and continues to trigger various environmental problems. Photo- or electrocatalytic CO2 reduction (CO2-Red) using solar energy, i.e., artificial photosynthesis, is a prospective technique owing to its sustainability and the usefulness of the reaction products. Concentrations of CO2 in exhaust gases from industries are several % to 20%, and that in the atmosphere is about 400 ppm. Although condensation processes of CO2 require high energy consumption and cost, pure CO2 has been used in most of the reported studies for photo- and electrocatalytic CO2-Red because the reaction between CO2 and the catalyst could be one of the rate-limiting steps. To address these issues and provide a repository of potential techniques for other researchers, this perspective summarizes the catalytic systems reported for the reduction of low-concentration CO2, which utilize a combination of catalytic CO2-Red and CO2-capturing reactions (or CO2 adsorption). First, we describe CO2 insertions into M-X bonds of the catalysts, which increase the rate constants and/or equilibrium constants for CO2 binding on the catalysts, and modifications of the second coordination sphere to stabilize the CO2-bound catalysts. Furthermore, we discuss the reaction media used for catalytic CO2-Red that have the unique effect of increasing CO2 concentrations around the catalysts. These reaction media include typical CO2-capturing additives, ionic liquids, and metal-organic frameworks.

Synthesis and Light-Harvesting Functions of Ring-Shaped Re(I) Trinuclear Complexes Connected with an Emissive Ru(II) Complex.

Ring-shaped Re(I) multinuclear complexes (Re(I) rings) in which Re(I)-diimine-biscarbonyl complexes are connected to each other through bisphosphine bridging ligands exhibit very suitable photophysical and electrochemical properties as redox photosensitizers. We developed two approaches for synthesizing Re(I) rings connected with a Ru(II) complex: cyclization of a linear Re(I) trinuclear complex connected with a Ru(II) complex and Mizoroki-Heck coupling of a ring-shaped Re(I) trinuclear complex and a Ru(II) complex. Photophysical measurements of these heteromultinuclear complexes and comparisons with their model complexes indicated that they exhibit efficient light-harvesting abilities, where energy transfer from the excited ring-shaped Re(I) trinuclear complex unit to the Ru(II) complex unit proceeds efficiently.

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)-.

Dual mechanoluminescence system comprising a solid-state di-copper(I) complex containing N-heterocyclic carbene ligands.

A simple N-heterocyclic carbene (NHC) ligand linked to a flexible propylene linker allows the formation of "Cu-Cu"- and "2 Cu"-type geometries inside a molecular framework. The incorporation of two Cu(i) ions in close proximity was observed in the Cu-Cu-type geometry but not in the 2 Cu-type geometry. In this study, the ground-state geometries of solid-state di-copper(i) complexes containing NHC ligands with ethyl substituents were modulated by external stimuli. A crystal with the 2 Cu-type geometry was obtained by the mechanical grinding and heating of a crystal with the Cu-Cu-type geometry, as confirmed by the disappearance of the absorption peak attributed to cuprophilic interaction in the diffuse reflection spectrum. The mechanical grinding of both crystals afforded composite states comprising small crystallites of the corresponding crystalline phases and an amorphous domain. This structural transition was accompanied by tribochromism and chronochromism. The results suggest that these di-copper(i) complexes show promise for the development of stimuli-responsive photoluminescent Cu(i) complexes.

Photocatalysis of a Dinuclear Ru(II)-Re(I) Complex for CO2 Reduction on a Solid Surface.

The development of CO2-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO2-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru(II)-complex photosensitizer and a Re(I)-complex catalyst fixed on the surface of insulating Al2O3 particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO2 atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru(II) and Re(I) complexes attached to an Al2O3 surface, the photocatalytic activity was much lower. The activity of the Al2O3-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO2 reduction. Coadsorption of a Ru(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the Al2O3 particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO2 reduction, which are composed of metal-complex photocatalysts and solid materials.

A Series of Mixed-Ligand Cu(I) Complexes Comprising Diphosphine-Disulfide Ligands: Effects of Diphosphine Ligands on Luminescent Properties.

Mixed-ligand Cu(I) complexes have attracted attention as alternatives to the noble- and/or rare-metal complexes, because of their remarkable photofunctions. To develop mixed-ligand Cu(I) complexes with rich photofunctions, an investigation of a suitable combination of ligands has captured more and more research interests. Herein, we report the first examples of emissive heteroleptic diphosphine-disulfide Cu(I) complexes combined with diphosphine ligands. The systematic study using a series of diphosphine ligands revealed that large π-conjugated bridging moieties between the two P atoms in the diphosphine ligands result in higher light-emission performance. When the diphosphine ligand was (R)-BINAP ((R)-BINAP = (R)-(+)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), the Cu(I) complex had an emission quantum yield (Φem) of 0.13 and a long emission lifetime (τem = 118 µs).

Photocatalytic CO2 reduction using metal complexes in various ionic liquids.

Aiming to diversify photocatalytic systems for CO2 reduction using metal complexes, this study investigated the use of various ionic liquids as reaction solvents. The photophysical properties of an Ir(iii) complex, functioning as a photosensitiser, and the photocatalytic ability of mixed systems consisting of the Ir(iii) photosensitiser and a Re(i) catalyst in twelve kinds of ionic liquids were systematically investigated by comparison with those in N,N-dimethylacetamide (DMA), which is a standard solvent for photocatalytic CO2 reduction. Even though the photophysical properties of the Ir(iii) complex in ionic-liquid solutions were quite similar to those in DMA, both the photosensitising ability of the Ir complex and the photocatalytic abilities of the systems strongly depended on the structures of the ionic liquids. Several ionic liquids were successfully used as new solvents for the photocatalytic systems showing durability similar to or higher than DMA solutions. The results demonstrated that even a small modification of the molecular structures of ionic liquids can control the efficiencies of both the photosensitising cycles and the catalytic cycles for CO2 reduction.

Synthesis of an Emissive Spectacle-Shaped Hexanuclear Rhenium(I) Complex.

Ring-shaped multinuclear Re(I) complexes (Re(I) rings) have the potential to function as useful units in multifarious photofunctional compounds because of their flexibility in molecular design and various photofunctions. The first synthetic example of the coupling reaction using a Re(I) ring as a building block to synthesize a novel spectacle-shaped hexanuclear complex is reported herein. This complex shows an efficient intramolecular energy transfer to accumulate the excitation energy into the central Re(I) units, and has suitable photophysical properties as a photosensitizer.

Kinetics and Mechanism of Intramolecular Electron Transfer in Ru(II)-Re(I) Supramolecular CO2-Reduction Photocatalysts: Effects of Bridging Ligands.

The supramolecular photocatalysts in which a Ru(II) complex as a molecular redox photosensitizer unit and a Re(I) complex as a molecular catalyst unit are connected with a various alkyl or ether chain have attracted attention because they can efficiently photocatalyze CO2 reduction with high durability and high selectivity of CO formation, especially on various solid materials such as semiconductor electrodes and mesoporous organosilica. The intramolecular electron transfer from the one-electron reduced photosensitizer unit to the catalyst unit, which follows excitation of the photosensitizer unit and subsequent reductive quenching of the excited photosensitizer unit by a reductant, is one of the most important processes in the photocatalytic reduction of CO2. We succeeded in determining the rate constants of this intramolecular electron transfer process by using subnanosecond time-resolved IR spectroscopy. The logarithm of rate constants shows a linear relationship with the lengths of the bridging chain in the supramolecular photocatalysts with one bridging alkyl or ether chain. In conformity with the exponential decay of the wave function and the coupling element in the long-distance electron transfer, the apparent decay coefficient factor (ß) in the supramolecular photocatalysts with one bridging chain was determined to be 0.74 Å-1. In the supramolecular photocatalyst with two ethylene chains connecting between the photosensitizer and catalyst units, on the other hand, the intramolecular electron transfer rate is much faster than that with only one ethylene chain. These results strongly indicate that the intramolecular electron transfer from the one-electron reduced species of the Ru photosensitizer unit to the Re catalyst unit proceeds by the through-bond mechanism.

Photocatalytic CO2 Reduction Using Various Heteroleptic Diimine-Diphosphine Cu(I) Complexes as Photosensitizers.

The development of efficient redox-photosensitizers based on the earth-abundant metal ions as an alternative toward noble- and/or rare-metal based photosensitizers is very desirable. In recent years, heteroleptic diimine-diphosphine Cu(I) complexes have been well investigated as one of the most remarkable candidates because of their great potentials as efficient photosensitizers. Here, we investigated the effects of the structure of the diphosphine ligands on the photosensitizing abilities using a series of Cu(I) complexes bearing 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (dmpp) and various diphosphine ligands in order to explore the suitable structure for the photosensitizing reactions. The number of methylene chains between the two phosphorous atoms in the diphosphine ligands was systematically changed from two to four, and the relationship between the length of the carbon chains and the photosensitizing abilities were investigated by conducting photocatalytic CO2 reduction with the Cu(I) complexes as photosensitizers. Turnover frequencies of the CO2 reduction drastically increased with increasing the length of the carbon chains. The systematic study herein reported suggests that the large P-Cu-P angles should be one of the most important factors for enhancing the photosensitizing abilities.

Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption.

Ruthenium(II) picolinate complex, [Ru(dmb)2(pic)]+ (Ru(pic); dmb = 4,4'-dimethyl-2,2'-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N∧N)3]2+ (N∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λabs < 670 nm) and stronger reduction ability in a one-electron reduced state ( E 1 / 2 red = -1.86 V vs. Ag/AgNO3), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO2 to CO under red-light irradiation (λex > 600 nm). TONCO reached 235 and ΦCO was 8.0%. Under these conditions, [Ru(dmb)3]2+ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOFCO = 6.7 min-1) and larger TONCO (2347) than Ru(dmb) (TOFCO = 3.6 min-1; TONCO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.

Electrocatalytic reduction of low concentration CO2.

Utilization of low concentration CO2 contained in the exhaust gases from various industries and thermal power stations without the need for energy-consuming concentration processes should be an important technology for solving global warming and the shortage of fossil resources. Here we report the direct electrocatalytic reduction of low concentration CO2 by a Re(i)-complex catalyst that possesses CO2-capturing ability in the presence of triethanolamine. The reaction rate and faradaic efficiency of CO2 reduction were almost the same when using Ar gas containing 10% CO2 or when using pure CO2 gas, and the selectivity of CO formation was very high (98% at 10% CO2). At a concentration of 1% CO2, the Re(i) complex still behaved as a good electrocatalyst; 94% selectivity of CO formation and 85% faradaic efficiency were achieved, and the rate of CO formation was 67% compared to that when using pure CO2 gas. The electrocatalysis was due to the efficient insertion of CO2 into the Re(i)-O bond in fac-[Re(dmb)(CO)3{OC2H4N(C2H4OH)2}] (dmb = 4,4'-dimethyl-2,2'-bipyridine).

Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode.

A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO2 at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.

Synthesis of Re(I) Rings Comprising Different Re(I) Units and Their Light-Harvesting Abilities.

Trimethylamine N-oxide (Me3NO) could selectively remove only one CO ligand from fac-[Re(N^N)(CO)3(PR2R')]+ (N^N = diimine ligand), whereby only the CO ligand in the trans position to the phosphorus ligand was selectively removed to give cis,trans-[ReI(N^N)(CO)2(PR2R')(L)] n+ in good yields. This decarbonylation reaction using Me3NO was found to be especially useful for synthesizing biscarbonyl Re(I) complexes with electron-withdrawing groups in the diimine ligand, which could not be synthesized or were obtained only in low yields by the photochemical method. Me3NO also selectively removed the carbonyl ligands in the trans position to the phosphorus ligands from the edge Re(I) complex units, which have the fac-[Re(N^N)(CO)3(PR2R')]+ structure, in linear-shaped Re(I) multinuclear complexes. This reaction was successfully applied to synthesize a novel precursor with ring-shaped multinuclear Re complexes (Re-rings) comprising different kinds of Re(I) units. The newly synthesized Re-rings, which consist of one Re unit with a 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (CF3bpy) ligand and one or two Re unit(s) with a 2,2'-bipyridine (bpy) ligand, showed almost quantitative excitation-energy harvesting ability from the Re unit(s) with bpy to that with CF3bpy.

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)-Re(i) supramolecular photocatalysts for CO2 reduction using time-resolved IR measurements.

Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO2 reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the 3MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 104 s-1 as a major component and (3.5-4.3) × 106 s-1 as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 104 s-1. Thus, although it has a large driving force (-ΔG0CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (kq[BNAH (0.2 M)] = (3.5-3.8) × 106 s-1) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (kET > 2 × 107 s-1) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of kET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), kET was measured as kET = (1.4 ± 0.1) × 109 s-1. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO2, which is one of the main reasons why they work so efficiently.

Synthesis of Os(ii)-Re(i)-Ru(ii) hetero-trinuclear complexes and their photophysical properties and photocatalytic abilities.

Photofunctional trinuclear complexes containing three different central metals, i.e. Os(ii), Re(i) and Ru(ii), were synthesised for the first time using stepwise Mizoroki-Heck reactions. The vinylene groups in the bridging ligands of the Os(ii)-Re(i)-Ru(ii) trinuclear complexes were selectively reduced by photochemical hydrogenation in moderate yield, affording novel supramolecular photocatalysts which can absorb a wide range of visible light up to 730 nm and induce CO2 reduction with high selectivity and durability. The turnover numbers of CO formation were over 4300. Details of the photophysical properties of these new trinuclear complexes, especially their intramolecular excitation-energy transfer phenomena, are also reported.

Ruthenium Tris-bipyridine Single-Molecule Junctions with Multiple Joint Configurations.

Single-molecule junctions are of particular interest in molecular electronics. To realize molecular electronic devices, it is crucial that functional single-molecule junctions are connected to each other by using joint units on the atomic scale. However, good joint units have not been reported because controlling the charge transport directions through the junctions is not trivial. Here, we report a joint unit that controls and changes the charge transport directions through the junctions, by using a ruthenium-tris-bipyridine (RuBpy) complex. The RuBpy single-molecule junction was fabricated with scanning tunnelling microscopy-based break junction techniques. The RuBpy single-molecule junction showed two distinct high and low conductance states. The two states were characterized by the conductance measurement, the correlation analysis, and the comparative experiment of bipyridine (Bpy), which is the ligand unit of RuBpy. We demonstrate that the Ru complex has multiple charge transport paths, where the charge is carried vertically and horizontally through the complex depending on the path.

Selectivity control between Mizoroki-Heck and homo-coupling reactions for synthesising multinuclear metal complexes: unique addition effects of tertiary phosphines and O2.

The addition of a tertiary phosphine and O2 to reaction solutions strongly affected the reactivity and selectivity of coupling reactions between transition metal complexes. The Mizoroki-Heck reaction between metal complexes with bromo and those with vinyl groups in the diimine ligand did not proceed using Pd(OAc)2 in the presence of 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (Sphos) under Ar but proceeded selectively after injection of air into the reaction vessel. In the absence of the phosphine ligand, on the other hand, not only the Mizoroki-Heck reaction but also a homo-coupling reaction between the metal complexes with the bromo groups proceeded at the same time. Mechanistic investigation showed that nanoparticles of Pd species were produced in the absence of the phosphine ligand and worked as catalysts for both the Mizoroki-Heck and homo-coupling reactions. On the other hand, larger Pd particles, which were produced in the presence of Sphos but after addition of air for oxidising Sphos, selectively catalysed the Mizoroki-Heck reaction. 'Molecular' Pd species that were stabilised in the presence of non-oxidised Sphos could not catalyse both coupling reactions under the reaction conditions. Based on these results, reaction conditions were established for the selective progress of the Mizoroki-Heck and the homo-coupling reactions.

Photochemical Hydrogenation of π-Conjugated Bridging Ligands in Photofunctional Multinuclear Complexes.

Vinylene or ethynylene linkers in the bridging ligands of photofunctional multinuclear complexes synthesized by various coupling reactions, such as the Mizoroki-Heck reaction, olefin metathesis, and Sonogashira coupling, were successfully converted to their corresponding saturated carbon chains using photochemical hydrogenation, which proceeded in an MeCN-pyridine-CF3COOH (3:1:0.1 v/v/v) mixed solution containing the starting metal complexes and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor under visible light irradiation in high yields. Hydrogenation of linkers in a Ru2-Re trinuclear complex improved the photocatalytic ability for CO2 reduction.