Precise Manipulation of Electron Transfers in Clustered Five Redox Sites.

Electron transfers in multinuclear metal complexes are the origin of their unique functionalities both in natural and artificial systems. However, electron transfers in multinuclear metal complexes are generally complicated, and predicting and controlling these electron transfers is extremely difficult. Herein, we report the precise manipulation of the electron transfers in multinuclear metal complexes. The development of a rational synthetic strategy afforded a series of pentanuclear metal complexes which composed of metal ions and 3,5-bis(2-pyridyl)pyrazole (Hbpp) as a platform to probe the phenomena. Electrochemical and spectroscopic investigations clarified overall picture of the electron transfers in the pentanuclear complexes. In addition, unique electron transfer behaviors, in which the reduction of a metal center occurs during the oxidation of the overall complex, were identified. We also elucidated the two dominant factors that determine the manner of the electron transfers. Our results provide comprehensive guidelines for interpreting the complicated electron transfers in multinuclear metal complexes.

Iron-Complex-Based Supramolecular Framework Catalyst for Visible-Light-Driven CO2 Reduction.

Molecule-based heterogeneous photocatalysts without noble metals are one of the most attractive systems for visible-light-driven CO2 reduction. However, reports on this class of photocatalysts are still limited, and their activities are quite low compared to those containing noble metals. Herein, we report an iron-complex-based heterogeneous photocatalyst for CO2 reduction with high activity. The key to our success is the use of a supramolecular framework composed of iron porphyrin complexes bearing pyrene moieties at meso positions. The catalyst exhibited high activity for CO2 reduction under visible-light irradiation (29100 µmol g-1 h-1 for CO production, selectivity 99.9%), which is the highest among relevant systems. The performance of this catalyst is also excellent in terms of apparent quantum yield for CO production (0.298% at 400 nm) and stability (up to 96 h). This study provides a facile strategy to create a highly active, selective, and stable photocatalyst for CO2 reduction without utilizing noble metals.

Brønsted Acid/Base Site Isolated in a Pentanuclear Scaffold.

Invited for the cover of this issue is the group of Shigeyuki Masaoka at Osaka University. The image depicts a Brønsted acid/base pair based on a pentanuclear scaffold, and the formation thermodynamically metastable state of H+ by the steric isolation of a Brønsted acid/base site. Read the full text of the article at 10.1002/chem.202203253.

Brønsted Acid/Base Site Isolated in a Pentanuclear Scaffold.

The concept of Brønsted-Lowry acids and bases is long and widely recognized as the most reasonable theory to explain the behavior of H+ ions. Here, we report a Brønsted acid/base pair that does not follow this theory. Two heteronuclear metal complexes, in which Brønsted acid/base sites are sterically isolated, were synthesized and characterized. These sterically isolated sites exhibited anomalous behavior, wherein the H+ species encapsulated in the Brønsted acid site did not undergo a deprotonation reaction, and the corresponding protonation reaction at the Brønsted base site failed to proceed. As a result, two states that are in a relationship of a Brønsted acid/base pair stably exist over a wide pH range without any interconversion, generating a thermodynamically metastable state. Additionally, these two states exhibited distinct electron transfer abilities and reactivities. The system presented in this study is in sharp contrast with the traditional concept of Brønsted-Lowry acids and bases.

A Catalytic Alkylation of Ketones via sp3 C-H Bond Activation.

We identified a ternary hybrid catalyst system composed of an acridinium photoredox catalyst, a thiophosphoric imide (TPI) catalyst, and a titanium complex catalyst that promoted an intermolecular addition reaction of organic molecules with various ketones through sp3 C-H bond activation. The thiyl radical generated via single-electron oxidation of TPI by the excited photoredox catalyst abstracted a hydrogen atom from organic molecules such as toluene, benzyl alcohol, alkenes, aldehydes, and THF. The thus-generated carbon-centered radical species underwent addition to ketones and aldehydes. This intrinsically unfavorable step was promoted by single-electron reduction of the intermediate alkoxy radical by catalytically generated titanium(III) species. This reaction provided an efficient and straightforward route to a broad range of tertiary alcohols and was successfully applied to late-stage functionalization of drugs or their derivatives. The proposed mechanism was supported by both experimental and theoretical studies.

A pentanuclear iron catalyst designed for water oxidation.

Although the oxidation of water is efficiently catalysed by the oxygen-evolving complex in photosystem II (refs 1 and 2), it remains one of the main bottlenecks when aiming for synthetic chemical fuel production powered by sunlight or electricity. Consequently, the development of active and stable water oxidation catalysts is crucial, with heterogeneous systems considered more suitable for practical use and their homogeneous counterparts more suitable for targeted, molecular-level design guided by mechanistic understanding. Research into the mechanism of water oxidation has resulted in a range of synthetic molecular catalysts, yet there remains much interest in systems that use abundant, inexpensive and environmentally benign metals such as iron (the most abundant transition metal in the Earth's crust and found in natural and synthetic oxidation catalysts). Water oxidation catalysts based on mononuclear iron complexes have been explored, but they often deactivate rapidly and exhibit relatively low activities. Here we report a pentanuclear iron complex that efficiently and robustly catalyses water oxidation with a turnover frequency of 1,900 per second, which is about three orders of magnitude larger than that of other iron-based catalysts. Electrochemical analysis confirms the redox flexibility of the system, characterized by six different oxidation states between Fe(II)5 and Fe(III)5; the Fe(III)5 state is active for oxidizing water. Quantum chemistry calculations indicate that the presence of adjacent active sites facilitates O-O bond formation with a reaction barrier of less than ten kilocalories per mole. Although the need for a high overpotential and the inability to operate in water-rich solutions limit the practicality of the present system, our findings clearly indicate that efficient water oxidation catalysts based on iron complexes can be created by ensuring that the system has redox flexibility and contains adjacent water-activation sites.

Design of molecular water oxidation catalysts with earth-abundant metal ions.

The four-electron oxidation of water (2H2O → O2 + 4H+ + 4e-) is considered the main bottleneck in artificial photosynthesis. In nature, this reaction is catalysed by a Mn4CaO5 cluster embedded in the oxygen-evolving complex of photosystem II. Ruthenium-based complexes have been successful artificial molecular catalysts for mimicking this reaction. However, for practical and large-scale applications in the future, molecular catalysts that contain earth-abundant first-row transition metal ions are preferred owing to their high natural abundance, low risk of depletion, and low costs. In this review, the frontier of water oxidation reactions mediated by first-row transition metal complexes is described. Special attention is paid towards the design of molecular structures of the catalysts and their reaction mechanisms, and these factors are expected to serve as guiding principles for creating efficient and robust molecular catalysts for water oxidation using ubiquitous elements.

Modulation of Self-Assembly Enhances the Catalytic Activity of Iron Porphyrin for CO2 Reduction.

Electrochemical reduction of CO2 in aqueous media is an important reaction to produce value-added carbon products in an environmentally and economically friendly manner. Various molecule-based catalytic systems for the reaction have been reported thus far. The key features of state-of-the-art catalytic systems in this field can be summarized as follows: 1) an iron-porphyrin-based scaffold as a catalytic center, 2) a dinuclear active center for the efficient activation of a CO2 molecule, and 3) a hydrophobic channel for the accumulation of CO2 . This article reports a novel approach to construct a catalytic system for CO2 reduction with the aforementioned three key substructures. The self-assembly of a newly designed iron-porphyrin complex bearing bulky substituents with noncovalent interaction ability forms a highly ordered crystalline solid with adjacent catalytically active sites and hydrophobic pores. The obtained crystalline solid serves as an electrocatalyst for CO2 reduction in aqueous media. Note that a relevant iron-porphyrin complex without bulky substituents cannot form a porous structure with adjacent active sites, and the catalytic performance of the crystals of this relevant iron-porphyrin complex is substantially lower than that of the newly developed catalytic system. The present study provides a novel strategy for constructing porous crystalline solids for small-molecule conversions.

Pentanuclear Scaffold: A Molecular Platform for Small-Molecule Conversions.

Small-molecule conversions involving multielectron transfer processes enable the conversion of earth-abundant materials into valuable chemicals and are regarded as a solution for environmental and energy shortage problems. In this context, the development of artificial catalysts that promote these reactions is an important research target. In nature, metalloenzymes that contain multinuclear metal complexes as active sites are known to efficiently catalyze reactions under mild conditions. Therefore, using multinuclear metal complexes as artificial catalysts can be an attractive strategy for small-molecule conversions involving multielectron transfer processes. However, multinuclear-metal-complex-based catalysts for these reactions have not been well established. In this Account, we describe our recent advances in the development of multinuclear metal complexes as catalysts for small-molecule conversion, mainly focusing on water oxidation. As small-molecule conversions involving multielectron transfer processes consists of two essential processes, (1) the transfer of multiple electrons and (2) the formation/cleavage of covalent bond(s), catalysts for these reactions should facilitate both steps. Therefore, we assumed that the assembly of redox-active metal ions and the cooperative effect of neighboring coordinatively unsaturated metal ions can promote these processes. On the basis of this assumption, we employed a pentanuclear metal complex as a molecular scaffold for the catalyst. The scaffold has a pentanuclear structure with quasi-D3 symmetry and consists of a [M3(µ3-X)] core (X = O2- or OH-) wrapped by two [M(µ-bpp)3] units (Hbpp = 3,5-bis(2-pyridyl)pyrazole). The metal ions in the triangular core are coordinatively unsaturated, whereas the metal ions at the apical positions are coordinatively saturated. In other words, the pentanuclear scaffold possesses multiple redox-active centers and coordinatively unsaturated sites. It should also be noted that the electron transfer ability of the complex changes dramatically depending on the identity of the constituent metal ions. The iron derivative of the pentanuclear scaffold was found to serve as an electrocatalyst for water oxidation (2H2O → O2 + 4e- + 4H+) with a high reaction rate and excellent robustness. The substitution of metal ions in the pentanuclear scaffold to cobalt ions resulted in the development of a catalyst for CO2 reduction. Furthermore, we investigated the effect of substituents on the ligands of the pentanuclear iron complex and succeeded in precisely manipulating the electron transfer possess. These results clearly demonstrate that the pentanuclear scaffold is an attractive platform for catalysts for small-molecule conversions. Additionally, the intrinsic features of the multinuclear catalytic system, which are totally different from those of conventional mononuclear-metal-complex-based catalysts, are disclosed. In reactions mediated by multinuclear complexes, the multinuclear core can initially accumulate the charge required for catalysis to reach the catalytically active state. Subsequently, the catalyst in the active state reacts with the substrate, initiating electron transfer to the substrate and rearrangement of covalent bonds in the substrate to afford the product. In such a mechanism, the desired number of electrons can be transferred to the substrates in an on-demand fashion, and the formation of undesired chemical species in the targeted catalysis may be prevented. This feature of multinuclear-metal-complex-based catalysts will achieve demanding small-molecule conversions with a high reaction rate, selectivity, and durability.

Dirhodium-Based Supramolecular Framework Catalyst for Visible-Light-Driven Hydrogen Evolution.

The direct conversion of solar energy to clean fuels as alternatives to fossil fuels is an important approach for addressing the global energy shortage and environmental problems. Here, we introduce a new dirhodium-complex-based framework assembly as a heterogeneous molecule-based photocatalyst for hydrogen evolution using visible light. Two dirhodium complexes bearing visible-light-harvesting BODIPY (boron dipyrromethene, BDP) moieties were newly designed and synthesized. The obtained complexes were self-assembled to framework structures (supramolecular framework catalysts), which are stabilized intermolecular noncovalent interactions. These frameworks retained excellent visible-light-harvesting properties of BDP moieties. Investigation of the catalytic performance of the supramolecular framework catalysts revealed that the supramolecular framework catalyst with heavy atoms at BDP moieties exhibited excellent performance in the formation of hydrogen with a reaction rate of 275.8 µmol g-1 h-1 under irradiation of visible light, whereas the supramolecular framework catalyst without heavy atoms at BDP moieties was inactive. Moreover, the system has the additional benefits of high durability (up to 96 h), reusability, and facile removal from the reaction mixture. We also disclosed the effect of heavy atoms at BDP moieties on the catalytic activity and proposed a reaction mechanism.

Photoconductive Coordination Polymer with a Lead-Sulfur Two-Dimensional Coordination Sheet Structure.

Coordination polymers with metal-sulfur (M-S) bonds in their nodes have interesting optical properties and can be used as photocatalysts for water splitting. A wide range of inorganic-organic hybrid materials with M-S bonds have been prepared in recent years. However, there is a dearth of structural information because of their low crystallinity, which has hampered the understanding of their underlying chemistry and physics. Thus, we conducted a structural study of a novel, highly crystalline coordination polymer with M-S bonds. Theoretical calculations were performed to elucidate its photoconductivity mechanism. The photoconductive, three-dimensional coordination polymer [Pb(tadt)]n (denoted as KGF-9; tadt = 1,3,4-thiadiazole-2,5-dithiolate) was synthesized and confirmed to have a three-dimensional structure containing a two-dimensional Pb-S framework by single-crystal X-ray diffraction. We also performed diffuse-reflectance ultraviolet-visible-near-infrared spectroscopy, time-resolved microwave conductivity, and photoelectron yield spectroscopy measurements on the bulk powder samples, as well as first-principles calculations. Additionally, direct-current photoconductivity measurements were conducted on a single-crystal sample.

Quick and Easy Method to Dramatically Improve the Electrochemical CO2 Reduction Activity of an Iron Porphyrin Complex.

The development of artificial molecular catalysts for CO2 reduction is the key to solving energy and environmental problems. Although chemical modifications can generally improve the catalytic activity of this class of compounds, they often require complicated synthetic procedures. Here, we report a simple procedure that dramatically enhances electrochemical CO2 reduction activity. A one-step counteranion-exchange reaction increased the solubility of a commercially available catalyst, iron(III) tetraphenylporphyrin chloride, in a variety of solvents, allowing the investigation of its catalytic performance under various conditions. Surprisingly, the turnover frequency for CO evolution in acetonitrile (MeCN) reached 7 300 000 s-1 , which is the highest among those of current best-in-class molecular catalysts. This excellent catalytic activity originates from the unique reaction between the generated FeI species and CO2 in MeCN during catalysis. The present study offers a "quick and easy" method for obtaining an efficient catalytic system for electrochemical CO2 reduction.

Electrochemical Polymerization Provides a Function-Integrated System for Water Oxidation.

Water oxidation is a key reaction in natural and artificial photosynthesis. In nature, the reaction is efficiently catalyzed by a metal-complex-based catalyst surrounded by hole-transporting amino acid residues. However, in artificial systems, there is no example of a water oxidation system that has a catalytic center surrounded by hole transporters. Herein, we present a facile strategy to integrate catalytic centers and hole transporters in one system. Electrochemical polymerization of a metal-complex-based precursor afforded a polymer-based material (Poly-1). Poly-1 exhibited excellent hole-transporting ability and catalyzed water oxidation with high performance. It was also revealed that the catalytic activity was almost completely suppressed in the absence of the hole-transporting moieties. The present study provides a novel strategy for constructing efficient molecule-based systems for water oxidation.

Semiconductive Nature of Lead-Based Metal-Organic Frameworks with Three-Dimensionally Extended Sulfur Secondary Building Units.

Recently, metal-organic frameworks (MOFs) composed of sulfur secondary building units (sulfur-SBUs) have attracted significant attention as unique electronic materials with high conductivities and photo- and electrocatalytic properties. Herein we report the crystal structure of KGF-1, an example of a Pb-MOF composed of three-dimensionally extended sulfur-SBUs that displays molecular sieving behavior, visible-light absorption, and a semiconductor band structure and is a hydrogen-evolution photocatalyst.

Unveiling Latent Photoreactivity of Imines.

Unlike carbonyl compounds, it has long been common understanding that excited imines show virtually no photoreactivity, and hence their properties and potential utility in chemical science remain largely unexplored. Now, a strategy is presented for eliciting latent photoreactivity of imines based on the introduction of a donor-acceptor (D-A) structure to extend the lifetime of their photoexcited states. A series of spectroscopic analyses and density functional theory calculations reveal unique photophysical properties of the D-A-type imines. Furthermore, the reactivity of the D-A-type imines is demonstrated by using them as a photoredox catalyst for atom-transfer radical addition. These findings illuminate a previously neglected chemical space in the field of photochemistry, which will be exploited by taking advantage of the inherent structural modularity of imines.

Function-Integrated Ru Catalyst for Photochemical CO2 Reduction.

Visible-light-driven catalytic reduction of CO2 is at the heart of artificial photosynthesis. Here, we demonstrate the first example of a Ru complex that can function both as a photosensitizer and catalyst for CO2 reduction. The catalyst exhibited excellent activity for CO evolution with a high turnover number (TON, 353 for 24 h), reaction rate (TOF, 14.7 h-1), and product selectivity (97%) under visible-light irradiation. We also succeeded in selective product formation (CO or HCOOH) by changing the basicity of the reaction media. This finding will open new avenues for visible-light-driven photoredox catalysis using Ru-based function-integrated photocatalysts.

C(sp3 )-H Cyanation Promoted by Visible-Light Photoredox/Phosphate Hybrid Catalysis.

Inspired by the reaction mechanism of photo-induced DNA cleavage in nature, a C(sp3 )-H cyanation reaction promoted by visible-light photoredox/phosphate hybrid catalysis was developed. Phosphate radicals, generated by one-electron photooxidation of phosphate salt, functioned as a hydrogen-atom-transfer catalyst to produce nucleophilic carbon radicals from C(sp3 )-H bonds with a high bond-dissociation energy. The resulting carbon radicals were trapped by a cyano radical source (TsCN) to produce the C-H cyanation products. Due to the high functional-group tolerance and versatility of the cyano group, the reaction will be useful for realizing streamlined building block syntheses and late-stage functionalization of drug-like molecules.

Hybrid Catalysis Enabling Room-Temperature Hydrogen Gas Release from N-Heterocycles and Tetrahydronaphthalenes.

Hybrid catalyst systems to achieve acceptorless dehydrogenation of N-heterocycles and tetrahydronaphthalenes-model substrates for liquid organic hydrogen carriers-were developed. A binary hybrid catalysis comprising an acridinium photoredox catalyst and a Pd metal catalyst was effective for the dehydrogenation of N-heterocycles, whereas a ternary hybrid catalysis comprising an acridinium photoredox catalyst, a Pd metal catalyst, and a thiophosphoric imide organocatalyst achieved dehydrogenation of tetrahydronaphthalenes. These hybrid catalyst systems allowed for 2 molar equiv of H2 gas release from six-membered N-heterocycles and tetrahydronaphthalenes under mild conditions, i.e., visible light irradiation at rt. The combined use of two or three different catalyst types was essential for the catalytic activity.

Syntheses and CO2 reduction activities of π-expanded/extended iron porphyrin complexes.

The construction of molecular catalysts that are active toward CO2 reduction is of great significance for designing sustainable energy conversion systems. In this study, we aimed to develop catalysts for CO2 reduction by introducing aromatic substituents to the meso-positions of iron porphyrin complexes. Three novel iron porphyrin complexes with π-expanded substituents (5,10,15,20-tetrakis(pyren-1-yl)porphyrinato iron(III) chloride (Fe-Py)), π-extended substituents (5,10,15,20-tetrakis((1,1'-biphenyl)-4-yl)porphyrinato iron(III) chloride (Fe-PPh)) and π-expanded and extended substituents (5,10,15,20-tetrakis(4-(pyren-1-yl)phenyl)porphyrinato iron(III) chloride (Fe-PPy)) were successfully synthesized, and their physical properties were investigated by UV-vis absorption spectroscopy and electrochemical measurements under Ar in comparison with an iron complex with a basic framework, 5,10,15,20-tetrakis(phenyl)porphyrinato iron(III) chloride (Fe-Ph). Moreover, the catalytic activity of the complexes was studied by electrochemical measurements under CO2, and it is found that the complex with the π-expanded substituents exhibits the highest activity among these complexes.