Takahiko Kojima

Photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors

Recent development of artificial photosynthetic systems using photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors such as fullerenes, carbon nanotubes and a new type of nanocarbon is reviewed by focusing on their photoinduced electron-transfer properties. Electron donor–acceptor ensembles composed of porphyrins and fullerenes are described, covering small electron donor–acceptor dyads to larger multi-component systems including porphyrin-based photofunctional nanometerials. Such photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors (in particular C60) have been utilized to construct efficient light energy conversion systems such as photovoltaic devices.

Ruthenium‐Catalyzed Selective and Efficient Oxygenation of Hydrocarbons with Water as an Oxygen Source

The development of methods for the highly selective and efficient conversion of abundant organic resources into valuable products is crucial for a sustainable society. To achieve this goal, extensive studies on the methodology of efficient material conversion with metal complexes as catalysts have been made for a long time. High-valent metal–oxo species are key intermediates in biological oxidations by metalloenzymes (mainly heme and non-heme iron enzymes), which catalyze the oxygenation of hydrocarbons in metabolic and catabolic processes. These oxygenases involve high-valent metal–oxo species as reactive species that arise by reductive activation of molecular oxygen coupled with proton transfer. Peroxides such as hydrogen peroxide can lead to a so-called “peroxide shunt” to perform the catalytic oxygenation; this mechanism is found for cytochrome P450 and methane monooxygenase. Thus, a number of model systems for these enzymatic oxidations have been developed to elucidate the reaction mechanisms and to perform effective catalytic oxygenation of external substrates with metal complexes involving the formation of high-valent metal–oxo species. These systems usually require organic solvents and excess amount of organic or inorganic peroxides as both oxidants and oxygen sources. Moreover, in such cases, the reaction pathways become complicated and give multiple products. Consequently it is difficult to control the product distribution that arises mainly from the inevitably produced radical species. Another strategy to generate a high-valent metal–oxo species has been recognized in the oxygen-evolving complex (OEC) in Photosystem II (PSII) for the photosynthesis to oxidize water to produce dioxygen. At the OEC, a manganese(V)–oxo species has been proposed to be formed by proton-coupled electron transfer (PCET), and the deprotonation of coordinated water and the oxidation of the metal center are thought to occur concertedly. This strategy has been applied to form and isolate high-valent metal–oxo species to perform stoichiometric oxidation reactions; however, it has not been applied to catalytic oxidations with transition-metal complexes as catalysts in water. Inspired by the reactions at the OEC in photosynthesis, we have tried to establish a novel catalytic oxygenation system using water as both the solvent and the oxygen source by virtue of PCET. We report herein the formation of a novel ruthenium(IV)–oxo complex and its reactivity toward highly efficient and selective catalytic oxygenation and oxidation reactions of various hydrocarbons in water, which can be used as an oxygen source. We synthesized a novel bis-aqua Ru complex, [Ru(tpa)(H2O)2](PF6)2 (1; tpa= tris(2-pyridylmethyl)amine) (Figure 1a,b), by the treatment of [RuCl(tpa)]2(PF6)2 [20] with AgPF6 in water. Complex 1 exhibits a reversible twostep deprotonation–protonation equilibrium, and the two pKa values were determined by UV/Vis spectroscopic titration (see Figure S1 in the Supporting Information) in the range of

A Discrete Supramolecular Conglomerate Composed of Two Saddle-Distorted Zinc(II)-Phthalocyanine Complexes and a Doubly Protonated Porphyrin with Saddle Distortion Undergoing Efficient Photoinduced Electron Transfer†

Porphyrins (Por) and phthalocyanines (Pc) exhibit lightharvesting efficiency for producing charge-separated states as models of the reaction center in photosynthetic bacteria and photovoltaic cells for energy conversion. The use of supramolecular assemblies to model the functionality of the reaction center is an attractive and fruitful strategy to develop photofunctional materials and devices. Porphyrins exhibit strong Soret bands around 400 to 450 nm, whereas phthalocyanines show strong Q bands around 700 to 800 nm. Thus, the combination of those two p systems can cover nearly the whole range of the visible region and can be a useful strategy for development of photofunctional materials for efficient light-energy conversion. Attempts have so far been made to synthesize covalently linked Por–Pc heterodyad molecules and construct Por–Pc heterosupramolecules. Recently, ZnPor and ZnPc have been reported to form two-dimensional arrays on gold surfaces, and the formation of a cofacial ZnPor–ZnPc coordination tetrad has also been reported. However, a crystal structure determination of a discrete supramolecular assembly composed of both Por and Pc has yet to be reported. In addition, since the Q-band absorption of Pc usually overlaps the wavelength of fluorescence of Por, energy transfer is favored over electron transfer in most heterodyads. We have developed supramolecular assemblies based on a saddle-distorted nonplanar porphyrin, dodecaphenylporphyrin (H2DPP), and its metal complexes. [11–13] The saddle distortion facilitates protonation of pyrrole nitrogen atoms to allow access to a stable diprotonated porphyrin, which can act as an electron acceptor. In addition, the saddle distortion affords higher Lewis acidity at the metal center to maintain axial coordination of ligands, as a result of poor overlap of the pyrrole nitrogen lone pair orbitals with d orbitals of the metal center. In contrast, the Zn complex of the saddle-distorted phthalocyanine 1,4,8,11,15,18,22,25-octaphenylphthalocyanine (H2OPPc) exhibits a lower oxidation potential relative to the corresponding porphyrin complex. To construct supramolecular conglomerates composed of both porphyrin and phthalocyanine in a well-defined manner, we have taken advantage of saddle distortion of both components. Herein, we report formation of a discrete supramolecular assembly composed of H4DPP 2+ and [Zn(OPPc)] connected by 4-pyridinecarboxylate (4-PyCOO ) with coordination and hydrogen bonding (Figure 1). The supramolecular conglomerate [(H4DPP){Zn(OPPc)(k-N-4-PyCOO)}2] (1) was synthesized by reaction of [H4DPP](4-PyCOO)2 (2) and Zn(OPPc) (3) in toluene. We crystallized and isolated 1 in pure form by vapor diffusion of hexanes into solution of the mixture in toluene. X-ray crystallography of 1 unambiguously established its structure (Figure 2a).

Photochemical and Thermal Isomerization of a Ruthenium(II)−Alloxazine Complex Involving an Unusual Coordination Mode

A Ru(II) complex having a flavin analogue as a ligand in a unusual coordination mode exhibits a photochemical and thermal isomerization; the bistability of the complex is attained by chelate effect and intramolecular CH···O interaction.

Control of redox reactivity of flavin and pterin coenzymes by metal ion coordination and hydrogen bonding

The electron-transfer activities of flavin and pterin coenzymes can be fine-tuned by coordination of metal ions, protonation and hydrogen bonding. Formation of hydrogen bonds with a hydrogen-bond receptor in metal–flavin complexes is made possible depending on the type of coordination bond that can leave the hydrogen-bonding sites. The electron-transfer catalytic functions of flavin and pterin coenzymes are described by showing a number of examples of both thermal and photochemical redox reactions, which proceed by controlling the electron-transfer reactivity of coenzymes with metal ion binding, protonation and hydrogen bonding.

Synthesis and characterization of novel ferrocene-containing pyridylamine ligands and their ruthenium(II) complexes: electronic communication through hydrogen-bonded amide linkage.

Tris(2-pyridylmethyl)amine (TPA) derivatives with one or two ferrocenoylamide moieties at the 6-position of one or two pyridine rings of TPA were synthesized. The compounds, N-(6-ferrocenoylamide-2-pyridylmethyl)-N,N-bis(2-pyridylmethyl)amine (Fc-TPA; L1) and N,N-bis(6-ferrocenoylamide-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (Fc2-TPA; L2), were characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. Their Ru(II) complexes were also prepared and characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. [RuCl(L1)(DMSO)]PF(6) (1) that contains S-bound dimethyl sulfoxide (DMSO) as a ligand and an uncoordinated ferrocenoylamide moiety exhibited two redox waves at 0.23 and 0.77 V (vs ferrocene/ferrocenium ion as 0 V), due to Fc/Fc(+) and Ru(II)/Ru(III) redox couples, respectively. [RuCl(L2)]PF(6) (2) that contains both coordinated and uncoordinated amide moieties showed two redox waves that were observed at 0.27 V (two electrons) and 0.46 V (one electron), assignable to Ru(II)/Ru(III) redox couples overlapped with the uncoordinated Fc/Fc(+) redox couple and the coordinated Fc/Fc(+), respectively. In contrast to 2, an acetonitrile complex, [Ru(L2)(CH(3)CN)](PF(6))(2) (3), exhibited three redox couples at 0.26 and 0.37 V for two kinds of Fc/Fc(+) couples, and 0.83 V for the Ru(II)/Ru(III) couple (vs ferrocene/ferrocenium ion as 0 V). In this complex, the redox potentials of the coordinated and the uncoordinated Fc-amide moieties were discriminated in the range of 0.11 V. Chemical two-electron oxidation of 1 gave [RuIIICl(L1+)(DMSO)](3+) to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm-1 (H = -JS(1)S(2)) which was estimated by its variable-temperature electron spin resonance (ESR) spectra in CH(3)CN. The electron spins at the Ru(III) center and the Fe(III) center are ferromagnetically coupled via an amide linkage. In the case of 2, its two-electron oxidation gave the same ESR spectrum, which indicates formation of a similar triplet state. Such electronic communication may occur via the amide linkage forming the intramolecular hydrogen bonding.

A discrete conglomerate of a distorted Mo(V)-porphyrin with a directly coordinated keggin-type polyoxometalate

The reaction of a saddle-distorted Mo(v)-dodecaphenylporphyrin complex and a Keggin-type polyoxometalate gives a discrete and nanosized molecule, [{Mo(DPP)(O)}(2)(H(2)SiW(12)O(40))], which involves direct coordination between the Mo(v) centers and terminal oxo groups of the polyoxometalate and exhibits excellent stability in solution to show reversible multi-redox processes.

Proton Shift upon One‐Electron Reduction in Ruthenium(II)‐Coordinated Pterins

Pterins are ubiquitous heteroaromatic coenzymes that are involved in many biological redox reactions in the vicinity of various metal ions. The redox processes of pterins proceed through proton-coupled electron transfer (PCET) involving the pyrazine moiety, in which up to four protons and electrons are manipulated in a concerted manner. Such processes can convert fully oxidized biopterin into fully reduced 5,6,7,8tetrahydrobiopterin. In the course of the redox processes of pterins, as shown in Scheme 1, a 5,8-dihydropterin is formed as a two-electronreduced species of the pterin. In contrast, the two-electron oxidation of a 5,6,7,8-tetrahydrobiopterin gives a quinonoid 6,7-dihydropterin, which contains a C=N bond involving the carbon atom at the 2-position. Both dihydropterins undergo thermal rearrangement to form a 7,8-dihydropterin as a thermodynamic sink. Fully oxidized pterins are known to release a proton from the nitrogen atom at the 3-position or the oxygen atom at the 4-position to coordinate to metal ions in a deprotonated imidate form (Scheme 2). Subsequent protonation gives a neutral pterin ligand, which undergoes reduction. Recently, we reported the one-electron reduction of monoprotonated pterins coordinated to a ruthenium(II)–tris(2-pyridylmethyl)amine (tpa) unit. The reduction gives ruthenium-bound monohydropterin radicals in which an unpaired electron is delocalized over the PCET region of the pyrazine moiety. Herein, we report the unprecedented observation of a proton shift from the nitrogen atom at the 1-position of the pyrimidinone moiety to the nitrogen atom at the 8-position of the pyrazine moiety upon the one-electron reduction of novel monoprotonated pterins in ruthenium(II) complexes (Scheme 2). [Ru(Hdmp)(tpa)](ClO4)2 (1; Hdmp = 6,7-dimethylpterin) and [Ru(Hdmdmp)(tpa)](ClO4)2 (2 ; Hdmdmp = N,Ndimethyl-6,7-dimethylpterin) were prepared through protonation of the corresponding precursor complexes [Ru(dmp)(tpa)](ClO4) (3) [9b] and [Ru(dmdmp)(tpa)](ClO4) (4), [9, 10] respectively, which have deprotonated, anionic pterin ligands, by adding 1 equivalent of HClO4 in CH3CN. Vapor diffusion of diethyl ether into the CH3CN solutions of 1 and 2 gave single crystals suitable for X-ray crystallography. ORTEP drawings of 1 and 2 are shown in Figure 1 (see also Figure S1 in the Supporting Information). As a common feature of 1 and 2, one of the perchlorate anions forms a hydrogen bond with an NH group at the 1position of the pterin ligand, as indicated by the close contact between one oxygen atom and N7 (Figure 1). In the crystal of 1, one of the perchlorate anions forms two hydrogen bonds to the neutral Hdmp ligand, one to the amino group at the 2position (O···N 2.93(1)–3.046(9) ) and one to the NH group at the 1-position (O···N 2.84(1)–2.88(1) ). This result clearly indicates that the proton is attached to the nitrogen atom at the 1-position in 1. In the crystal of 2, the NH group at the 1-position of the neutral Hdmdmp ligand forms a hydrogen bond with a perchlorate anion (O3···N7 2.94(1) ) or a water molecule of crystallization (O···N 2.78(1) ), confirming the protonation of the nitrogen atom at the 1position in 2. These results indicate that the first protonation occurs at the nitrogen atom at the 1-position of the coordinated pterins, rather than the nitrogen atom at the 8position. Prior to this work, it was thought that the first site of Scheme 1. PCET processes of pterins.

A porphyrin nanochannel: formation of cationic channels by a protonated saddle-distorted porphyrin and its inclusion behavior

A highly saddle-distorted dodecaphenylporphyrin dication (H4DPP2+) was revealed by X-ray crystallography to form positively charged porphyrin nanochannels which were 1 nm in diameter; chloride anion and redox-active hydroquinone could be incorporated in the channels.

Novel cofacial oxidative coupling reaction of phosphinine in the presence of Cu(I) and ClO4

2,4,6-triphenylphosphinine (TPP) underwent unprecedented cofacial oxidative coupling to form a novel C2-symmetric cage compound, having extremely long C-C bonds.