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Fe(ii) complexes with pentadentate ligands, including N-heterocyclic carbene moieties, were prepared and electrostatically attached onto the perfluorinated surface of a mesoporous aluminosilicate.

Recently, porphyrin derivatives having ring-fused structures at the periphery have attracted attention by virtue of their unique optoelectronic characteristics. This manuscript surveys the syntheses and properties of porphyrin derivatives having fused five-membered rings at the periphery. Porphyrin derivatives with fused five-membered rings have been revealed to exhibit unique properties, which are different from those having fused six-membered rings at the periphery. For instance, porphyrins with fused five-membered rings exhibit expanded aromatic and anti-aromatic circuits passing through the outer rim of the fused rings. The expanded aromatic and anti-aromatic circuits severely affect the electronic structures of the porphyrin derivatives.

Efficient and selective functionalization of methane is one of the most important tasks in chemistry in light of its utilization as naturally abundant feedstock toward the development of a sustainable...

“Push–pull” derivatives (2 – 4) of zinc(II) complexes having quadruply fused porphyrinato (QFP) ligands have been synthesized to elucidate the impact of the “push–pull” effects on the electronic structure of QFP. Each ZnQFP derivative of 2 – 4 has an electron-withdrawing group (EWG) such as nitro group and a methoxy group as an electron-donating group (EDG) at the diagonal positions of the molecule. Owing to the extended [Formula: see text]-conjugation of 2– 4 reaching to the substituted positions of the EWG and EDG, charge-transfer (CT) bands, which are sensitive to the solvent polarity, have been observed in the UV-vis spectra. In contrast, the corresponding “push–pull” derivatives of tetraphenylporphyrin, whose [Formula: see text]-conjugation is disconnected at the bonds between the meso-carbon and meso-aryl group, do not exhibit any CT transitions derived from the substituents.

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

We report herein the solution and solid-state studies of conformationally flexible multidecker naphthalenediimides (NDIs) in which the chromophoric NDI units intramolecularly assemble into a series of discrete π-stacks. The X-ray crystallography reveals the existence of exclusively all-syn NDIs orientations in lower congeners while all-anti in a higher congener, suggesting short- to long-range π···π interactions throughout the slipped πNDI chromophoric array. The UV/vis and fluorescence spectra evaluate the discrete π-stacks by remarkable optical changes upon cooling in solution. Furthermore, we carried out a systematic electrochemical investigation to gain an insight into redox properties of the long-range π-stacked structures. The higher congener (5NDI) shows a ten-electron reversible reduction process in a small working potential window (∼0.8 V). To our knowledge, this is an unusual observation in an organic molecular system to undergo up to ten-electron reduction. These results pave the way to design multidecker π-stacks in which structural control with specific electronic properties would be engineered.

Heme binds to a parallel-stranded G-quadruplex DNA to form a peroxidase-mimicking heme-DNAzyme. An interpolyelectrolyte complex between the heme-DNAzyme and a cationic copolymer possessing protonated amino groups was characterized and the peroxidase activity of the complex was evaluated to elucidate the effect of the polymer on the catalytic activity of the heme-DNAzyme. We found that the catalytic activity of the heme-DNAzyme is enhanced through the formation of the interpolyelectrolyte complex due to the general acid catalysis of protonated amino groups of the polymer, enhancing the formation of the iron(IV)oxo porphyrin π-cation radical intermediate known as Compound I. This finding indicates that the polymer with protonated amino groups can act as a cocatalyst for the heme-DNAzyme in the oxidation catalysis. We also found that the enhancement of the activity of the heme-DNAzyme by the polymer depends on the local heme environment such as the negative charge density in the proximity of the heme and substrate accessibility to the heme. These findings provide novel insights as to molecular design of the heme-DNAzyme for enhancing its catalytic activity.

In this award account are described our achievements in proton-coupled electron transfer reactions performed by transition metal complexes. Topics include formation and reactivity of RuIV-oxo and R...

Four kinds of possible atropisomers of a porphyrin derivative (1), having mesityl groups at one of the o-positions of each meso-aryl group, can be selectively converged to targeted atropisomers amo...

We have thoroughly investigated oxidation of benzyl alcohol (BA) derivatives by a RuIV(O) complex (RuIV(O)) in the absence or presence of Bronsted acids in order to elucidate the proton-coupled ele...

Selective two-electron reduction of dioxygen (O2 ) to hydrogen peroxide (H2 O2 ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me2 P) and an N21,N'23-dimethylated porphyrin (syn-Me2 P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me2 P with higher turnover number (TON=250 for 30 min) than that by syn-Me2 P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me2 Iph or syn-Me2 Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O2 with isophlorins based on kinetic analysis. The ORR rate by anti-Me2 Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me2 Iph by external protons was observed. The different mechanisms in the O2 reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O2 with inner NH protons of the isophlorins.

Novel heterodinuclear IrIII–MII complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and emplo...

The design and synthesis of an enantiomeric pair of 1,8-diethynylanthracene-bridged naphthalenediimide (NDI)-based cyclophanes (Cyclo-NDIs) are reported. Each enantiomer of Cyclo-NDI exhibits a circularly polarized luminescence signal with a relatively large luminescence dissymmetry factor (glum =±8×10-3 ). We have further investigated the modulation of through-space electronic communication between co-facially oriented NDIs in a discrete Cyclo-NDI with changes in the temperature. Tuning of the electronic communication results from the conformational transformation of monomer- versus dimer-like features of Cyclo-NDI, as confirmed by UV/Vis, fluorescence, circular dichroic, and NMR spectroscopic analysis. The temperature-dependent optical response in the Cyclo-NDI through the conformational transformation could be utilized as a highly sensitive and reversible optical thermometer in a wide temperature range (100 to -80 °C).

Development of a near-infrared (NIR) light-induced hydrogen (H2) evolution system is indispensable for constructing a sustainable society wherein the utilization of solar energy is maximized. Here, we report NIR light-driven H2 evolution catalyzed by a combination of a diprotonated saddle-distorted porphyrin as a photosensitizer and platinum nanoparticles as a H2-evolving catalyst. The quantum yield at 710 nm was determined to be 17%, which is the highest value among photocatalytic H2-evolution systems ever reported.

A RuII complex, [RuII(tpphz)(bpy)2]2+ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [RuII(tpphz-HH)(bpy)2]2+ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of 1 is firstly formed upon photoirradiation and the 3MLCT* state is reductively quenched with an electron donor to afford [RuII(tpphz˙-)(bpy)2]+, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3MLCT* state to react with a proton and aldehydes. In photocatalytic H2 evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

An iron(II) complex of a quadruply ring-fused porphyrin (QFP), Fe-1, in which four mesityl groups were introduced at the periphery to improve the solubility in organic solvents, has been newly synthesized and characterized. In the pyridine solution, two pyridine molecules bind to the low-spin Fe[Formula: see text] center of Fe-1 as axial ligands to make the complex stable even under air. Characterization of Fe-1 was performed using 1H NMR spectroscopy, MALDI-TOF-MS spectrometry and single-crystal X-ray diffraction analysis. The 1H NMR signals of Fe-1 were observed in the diamagnetic region, reflecting the low-spin state of the FeII center. In the differential pulse voltammogram of Fe-1, three oxidation waves and four reduction waves were observed in pyridine; the first oxidation wave at -0.08 V vs. Fc/Fc[Formula: see text] can be ascribed to the oxidation process of the FeII center, FeII/FeIII, and other six waves can be assigned to the redox processes of the QFP ligand. Furthermore, the ESR measurement of 1e[Formula: see text]-reduced Fe-1 upon controlled-potential bulk electrolysis in pyridine exhibited a signal at [Formula: see text] = 2.003 with a well-resolved 45-line hyperfine splitting at room temperature, due to the coupling with four nitrogen nuclei and twelve hydrogen ones of the QFP ligand. This indicates that the ligand radical anion of Fe-1 is stabilized by delocalization of the spin density owing to the peripheral ring fusion and resultant expansion of the [Formula: see text]-conjugation.

We have synthesized a new Ni(II) complex having an S2N2-tetradentate ligand with two noncoordinating pyridine pendants as binding sites of Lewis-acidic metal ions in the vicinity of the Ni center, ...

Heme in the ferric state (heme(Fe3+)) binds to G-quadruplex DNAs to form stable complexes that exhibit enhanced peroxidase activities. The complexes are considered DNAzymes possessing heme as a pro...

A RuII—NH3 complex, 2, was oxidized through a proton-coupled electron transfer (PCET) mechanism with a CeIV complex in water at pH 2.5 to generate a RuV═NH complex, 5. Complex 5 was characterized with various spectroscopies, and the spin state was determined by the Evans method to be S = 1/2. The reactivity of 5 in substrate C–H oxidation was scrutinized in acidic water, using water-soluble organic substrates such as sodium ethylbenzene-sulfonate (EBS), which gave the corresponding 1-phenylethanol derivative as the product. In the substrate oxidation, complex 5 was converted to the corresponding RuIII—NH3 complex, 3. The formation of 1-phenylethanol derivative from EBS and that of 3 indicate that complex 5 as the oxidant does not perform nitrogen-atom transfer, in sharp contrast to other high-valent metal–imido complexes reported so far. Oxidation of cyclobutanol by 5 afforded only cyclobutanone as the product, indicating that the substrate oxidation by 5 proceeds through a hydride-transfer mechanism. In ...

The development of highly efficient catalysts for electrochemical oxygen reduction reactions (ORRs) is crucial for energy applications such as metal–air batteries and fuel cells. Here, we show an e...

We have designed and synthesized a hetero-dinuclear RuII–CoII complex with a dinucleating ligand inspired by hetero-dinuclear active sites of metalloenzymes. A synergistic effect between the adjace...

We have successfully constructed hydrogen-bonded supramolecular assemblies based on a diprotonated saddle-distorted porphyrin (H4DPP2+) and redox-active molecules bearing a carboxylate group, such ...

Interconversion between dioxygen (O2) and hydrogen peroxide (H2O2) has attracted much interest because of the growing importance of H2O2 as an energy source. There are many reports on O2 conversions to H2O2; however, no example has been reported on O2/H2O2 interconversion. Herein, we describe successful achievement of a reversible O2/H2O2 conversion based on an N21, N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O2 to afford the corresponding porphyrin and H2O2; conversely, the porphyrin also reacted with excess H2O2 to reproduce the corresponding isophlorin and O2. The isophlorin-O2/porphyrin-H2O2 interconversion was repeatedly proceeded by alternate bubbling of Ar or O2, although no reversible conversion was observed in the case of an N21, N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner N H protons in hydrogen-bond formation of the isophlorin core with O2 as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H2O2. The intriguing isophlorin-O2/porphyrin-H2O2 interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O2/porphyrin-H2O2 states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O2/porphyrin-H2O2 interconversion using 1H NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21, N23-dimethylated porphyrin and H2O2 as an intermediate.

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O2-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O2-evolving rate constants, indicating that the O2-releasing step is the rate-determining step in the O2 evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Bulk heterojunction (BHJ) strategy requires morphology of wide area donor–acceptor interfaces with high charge carrier mobilities through the bicontinuous charge transporting layers. Here, we repor...

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h−1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis.

Isolation and characterisation of RuIV(O) complexes were accomplished to investigate their fundamental electron transfer (ET) and proton-coupled ET (PCET) properties. Reorganisation energies (λ) in electron transfer (ET) and proton-coupled ET (PCET) from electron donors to the isolated RuIV(O) complexes have been determined for the first time to be in the range of 1.70–1.88 eV (ET) and 1.20–1.26 eV (PCET). It was suggested that the reduction of the λ values of PCET in comparison with those of ET should be due to the smaller structural change in PCET than that in ET on the basis of DFT calculations on 1 and 1e−-reduced 1 in the absence and presence of TFA, respectively. In addition, the smaller λ values for the RuIV(O) complexes than those reported for FeIV(O) and MnIV(O) complexes should be due to the lack of participation of dσ orbitals in the ET and PCET reactions. This is the first example to evaluate fundamental ET and PCET properties of RuIV(O) complexes leading to further understanding of their reactivity in oxidation reactions.

Development of strategy in energy conversion and storage is indispensable for a sustainable society. So far, redox-active transition metal complexes have been used as efficient catalysts and photosensitizers in energy-converting reactions such as hydrogen evolution through thermal and photoinduced electron transfer (ET). We have been working on development of redox and photochemical functionality of diprotonated saddle-distorted porphyrins based on ET reactions. Herein we would like to highlight recent advance in our study of catalytic dioxygen reduction and hydrogen evolution using diprotonated forms of dodecaphenylporphyrin (H2DPP) derivatives as organocatalysts and photosensitizers. A diprotonated form of a water-soluble H2DPP derivative can act as a photosensitizer (PS) in photocatalytic oxidation of organic substrates in water using Na2S2O8 as an oxidant under visible-light irradiation.1 On the contrary, diprotonated H2DPP has been used as a PS in photocatalytic H2 evolution using platinum nanoparticles as catalysts under near-infrared (710 nm) irradiation in the quantum yield of 17%.2 Diprotonated H2DPP (H4DPP2+) can serve as an organo-photocatalyst in highly selective and efficient O2 reduction to afford H2O2 under photoirradiation at 480 nm; the quantum yield of H2O2 generation was 12% and turnover frequency was 500 h–1.3 Interconversion of O2 and H2O2 has been achieved using an N21,N23-dimethylated H2DPP derivative.4 The porphyrin is reduced by H2O2 to form the corresponding isophlorin derivative as a two-electron-reduced product, which is in turn oxidized by O2 to afford the porphyrin and H2O2. We also developed efficient O2 reduction using N21,N22- and N21,N23-dimethylated H2DPP derivatives as catalysts; in the catalysis, catalytic mechanisms and performance depend on the structures of catalysts used in terms of interaction between O2 and the corresponding isophlorins.5 References T. Ishizuka et al., Green Chem. 2018, 20, 1975. H. Kotani et al., ACS Appl. Energy Mater. 2020, 3, 3193. E. Aoki et al., Chem. Commun. 2019, 55, 4925. W. Suzuki et al., J. Am. Chem. Soc. 2019, 141, 5987. W. Suzuki et al., Chem. Eur. J. 2020, 26, 10480.

An Fe

Novel heterodinuclear Ir

The design and synthesis of a tweezer-shaped naphthalenediimide (NDI)-anthracene conjugate (2NDI) are reported. In the structure of the closed form (πNDI ⋅⋅⋅πNDI stack) of 2NDI, which was elucidated by single-crystal XRD, the existence of C-H⋅⋅⋅O hydrogen bonding involving the nearest carbonyl oxygen atom of an NDI unit was suggested. The tunability of πNDI ⋅⋅⋅πNDI interactions was studied by means of UV/Vis absorption, fluorescence and NMR spectroscopy and molecular modelling. This revealed that the πNDI ⋅⋅⋅πNDI interactions in 2NDI affect the absorption and emission properties depending on the temperature. Furthermore, in polar solvents, 2NDI prefers the stronger πNDI ⋅⋅⋅πNDI stack, whereas the πNDI ⋅⋅⋅πNDI interaction is diminished in nonpolar solvents. Importantly, the conformational variations of 2NDI can be reversibly switched by variation in temperature, and this suggests potential application for fluorogenic molecular switches upon temperature changes.

Proton-coupled electron transfer (PCET) is one of the ubiquitous and fundamental processes in various redox reactions performed by transition-metal complexes. In this article, we describe our remarkable achievements related to PCET in metal complexes, including proton manipulation of ligands to afford molecular bistability involving reversible intramolecular PCET and emergence of novel electronic structures, formation and reactivity of RuIV–oxo and RuIII–oxyl complexes, and mechanistic insights into PCET reactions from O–H and C–H bonds to RuIII–pterin complexes.

Efficient degradation of harmful benzene rings in water is indispensable for achieving a clean water environment. We report herein unprecedented catalytic oxidative benzene cracking (OBC) in water ...

A new chromium(V)–oxo complex, [CrV(O)(6-COO–-py-tacn)]2+ (1; 6-COO–-py-tacn = 1-(6-carboxylato-2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane), was synthesized and characterized to evaluate the reactivity of CrV(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported CrV(O) complex, [CrV(O)(6-COO–-tpa)]2+ (2; 6-COO–-tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine). Definitive differences of these two CrV(O) complexes were observed in resonance Raman scatterings of the Cr–O bond (ν = 911 cm–1 for 1 and 951 cm–1 for 2) and the reduction potential (0.73 V vs SCE for 1 and 1.23 V for 2); this difference should be derived from that of the ligand bound at the trans position to the oxo ligand, a tertiary amino group in 1, and a pyridine nitrogen in 2. When we employed 9,10-dihydroanthracene as a substrate, the second-order rate constant (k) of 1 was 4000 times smaller than that of 2. Plots of normalized k values for both complexes r...

We report homogeneous electrocatalytic and photocatalytic H2 evolution using two Ni(II) complexes with S2N2-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SC2S–Ni chelate ring (1) exhibited higher activity than that with a six-membered SC3S–Ni chelate ring (2) in both electrocatalytic and photocatalytic H2 evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e–/1H+ proton-coupled electron transfer reaction to afford a putative Ni(II)-hydrido (NiII–H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H2 evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic ...

An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM4–), (NEt4)2[FeIII(DTTM)Cl] (1), was synthesized and characterized by crystallographic, spectroscopic, and electrochemical methods. Complex 1 has a square-pyramidal structure in the S = 3/2 spin state. The complex exhibited two reversible redox waves at +0.36 and +0.68 V (vs SCE) in the cyclic voltammogram measured in CH2Cl2 at room temperature. The stepwise oxidation of 1 using chemical oxidants allowed us to observe clear and distinct spectral changes with distinct isosbestic points for each step, in which oxidation occurred at the phenylenediamido moiety rather than the iron center. One-electron oxidation of 1 by 1 equiv of [RuIII(bpy)3](ClO4)3 (bpy = 2,2′-bipyridine) in CH2Cl2 afforded square-pyramidal (NEt4)[Fe(DTTM)Cl] (2), which was in the S = 1 spin state involving a ligand radical and showed a slightly distorted square-pyramidal structure. Complex 2 showed an intervalence charge-transfer band at 900 nm, which was as...

We have synthesized 2, a derivative of zinc(II) quadruply fused porphyrinato (ZnIIQFP) that is tetrabrominated at the β-positions of the two nonfused pyrroles, by treatment of ZnIIQFP with N-bromosuccinimide. X-ray diffraction analysis of a single crystal obtained from a THF solution of 2 by vapor diffusion of ethanol (EtOH) revealed that 2 formed an unprecedented dimeric structure, (2)2-L (L = EtOH), in which one of the brominated QFP ligands acts as a bridging ligand in an unprecedented μ-η3:η1 coordination mode. In the dimeric structure, the two QFP ligands showed a unique η3 coordination mode for both ZnII centers. In (2)2-EtOH, one of the pyrrolic nitrogen atoms of the two nonfused pyrroles dissociates from the ZnII center, and the dissociated pyrrolic nitrogen atom coordinates to the ZnII center of the other molecule in the dimer. The ZnII center having the μ-η3:η1-QFP ligand is coordinated by an EtOH molecule, and the other ZnII center is coordinated by the η3-QFP ligand and one nitrogen atom of th...

A diprotonated form of a cationic water-soluble saddle-distorted porphyrin (H416+) forms stable supramolecular assemblies with multianionic polyoxometalates (POMs) by electrostatic interactions. An assembly of H416+ with a Ru-substituted POM can perform photocatalytic oxidation of organic substrates in water under visible-light irradiation through adduct formation of the H416+ moiety with an oxidant.

A Ni(II) complex bearing an S2N2-type tetradentate ligand inspired by the active site of carbon monoxide dehydrogenase was found to selectively catalyze CO2 reduction to produce CO in a photocatalytic system using [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. The Ni(II) complex shows a high turnover number over 700 with high CO selectivity of >99% and quantum yield of 1.42% in the photocatalytic system.

High-valent metal-oxo complexes are produced by successive electron-transfer oxidation of metal complexes with one-electron oxidants in the presence of water, which is an oxygen source in the generation of the metal-oxo complexes. Then, metal-oxo complexes oxidize substrates to yield oxygenated substrates, accompanied by the regeneration of reduced metal complexes. Thus, the oxidation of substrates using one-electron oxidants can be catalyzed by metal complexes via formation of high-valent metal-oxo complexes by the electron-transfer oxidation of metal complexes in the presence of water as an oxygen source. When water is used as a substrate, water is oxidized by one-electron oxidants to evolve dioxygen via an O O bond formation process. The one-electron oxidants used for the formation of high-valent metal-oxo complexes can be replaced by much weaker oxidants, when a photosensitizing metal complex, such as [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), is employed as a photocatalyst, an oxidized form of the photocatalyst, which is generated via photoinduced electron transfer from the excited state to a weaker oxidant, can oxidize metal complexes in the presence of water to afford the high-valent metal-oxo complexes. Thus, the oxidation of substrates, including water oxidation, by weak oxidants can be catalyzed by metal complexes under photoirradiation of the photocatalyst using water as an oxygen source.

The thermodynamic stability of diprotonated saddle-distorted dodecaphenylporphyrin (H4DPP2+(X−)2) was controlled by the hydrogen-bonding strength of conjugate bases (X−) of strong acids (HX) or acids (R+-COOH) having positively charged moieties. The thermodynamic control of H4DPP2+(X−)2 made it possible to achieve selective formation of supramolecular hetero-triads, H4DPP2+(X−)(Cl−).

We have synthesized a new Ni(II) complex having an S

Metal-oxyl (Mn+-O•) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal-oxo (M(n+1)+═O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of Mn+-O• species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of Mn+-O• complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the Mn+-O• species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h-1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis.