Tatsuki Morimoto

Photocatalytic CO2 reduction to formic acid using a Ru(II)-Re(I) supramolecular complex in an aqueous solution.

In an aqueous solution, photophysical, photochemical, and photocatalytic abilities of a Ru(II)-Re(I) binuclear complex (RuReCl), of which Ru(II) photosensitizer and Re(I) catalyst units were connected with a bridging ligand, have been investigated in details. RuReCl could photocatalyze CO2 reduction using ascorbate as an electron donor, even in an aqueous solution. The main product of the photocatalytic reaction was formic acid in the aqueous solution; this is very different in product distribution from that in a dimethylformamide (DMF) and triethanolamine (TEOA) mixed solution in which the main product was CO. A (13)CO2 labeling experiment clearly showed that formic acid was produced from CO2. The turnover number and selectivity of the formic acid production were 25 and 83%, respectively. The quantum yield of the formic acid formation was 0.2%, which was much lower, compared to that in the DMF-TEOA mixed solution. Detail studies of the photochemical electron-transfer process showed back-electron transfer from the one-electron-reduced species (OERS) of the photosensitizer unit to an oxidized ascorbate efficiently proceeded, and this should be one of the main reasons why the photocatalytic efficiency was lower in the aqueous solution. In the aqueous solution, ligand substitution of the Ru(II) photosensitizer unit proceeded during the photocatalytic reaction, which was a main deactivation process of the photocatalytic reaction. The product of the ligand substitution was a Ru(II) bisdiimine complex or complexes with ascorbate as a ligand or ligands.

Photocatalytic Reduction of Low Concentration of CO2

A novel molecular photocatalytic system with not only high reduction ability of CO2 but also high capture ability of CO2 has been developed using a Ru(II)-Re(I) dinuclear complex as a photocatalyst. By using this photocatalytic system, CO2 of 10% concentration could be selectively converted to CO with almost same photocatalysis to that under a pure CO2 atmosphere (TONCO > 1000, ΦCO > 0.4). Even 0.5% concentration of CO2 was reduced with 60% initial efficiency of CO formation by using the same system compared to that using pure CO2 (TONCO > 200). The Re(I) catalyst unit in the photocatalyst can efficiently capture CO2, which proceeds CO2 insertion to the Re-O bond, and then reduce the captured CO2 by using an electron supplied from the photochemically reduced Ru photosensitizer unit.

Ring-Shaped Rhenium(I) Multinuclear Complexes: Improved Synthesis and Photoinduced Multielectron Accumulation

We successfully developed selective synthesis of strongly emissive ring-shaped Re(I) multinuclear complexes (RnP(x)(n+) in Chart 1) with much higher yields compared with the previously reported method. This improved method could also be employed to prepare a novel ring-shaped multinuclear complex composed of structurally different Re(I) units. Each Re unit in RnP(x)(n+) could electrochemically accept one electron, and the multielectron reduced states of RnP(x)(n+) were stable. In the presence of triethanolamine, the ring-shaped tetranuclear and hexanuclear complexes can be photochemically reduced and accumulate 2.9-3.6 and 4.4 electrons in one molecule, respectively.

Halide Anion Mediated Dimerization of a meso‐Unsubstituted N‐Confused Porphyrin

The new N-confused porphyrin (NCP) derivatives, meso-unsubstituted beta-alkyl-3-oxo N-confused porphyrin (3-oxo-NCP) and related macrocycles, were synthesized from appropriate pyrrolic precursors by a [3+1]-type condensation reaction. 3-Oxo-NCP forms a self-assembled dimer in dichloromethane that is stabilized by complementary hydrogen-bonding interactions arising from the peripheral amide-like moieties. The protonated form of 3-oxo-NCP was observed to bind halide anions (F(-), Cl(-)) through the outer NH and the inner pyrrolic NH groups, thus affording a dimer in dichloromethane. The structure of the chloride-bridged dimer in the solid state was determined by X-ray diffraction analysis.

Selective H2 and CO production with rhenium(I) biscarbonyl complexes as photocatalyst

Rhenium(I) biscarbonyl complexes with two phosphine ligands photocatalyzed not only CO2 reduction under CO2 atmosphere but also H2 evolution under Ar. The reductant 1-benzyl-1,4-dihydronicotinamide (BNAH) worked only as a one-electron donor, and it was quantitatively converted to its corresponding oxidized dimer (BNA2). The photocatalytic reactions required addition of a base such as triethanolamine, because deprotonation from the oxidized BNAH (BNAH•+) is essential for the suppression of the back electron transfer from the reduced rhenium(I) complex to BNAH•+. 1H, 13C, and 31P NMR studies under vacuum or 13CO2 atmosphere indicated that the rhenium(I) complex is relatively stable under the CO2 reduction conditions, but it is converted to some other complexes under the H2 evolution conditions.

Supramolecular Interaction of Keto-substituted Pyrroles

Several types of pyrrole derivatives bearing carbonyl groups were synthesized and their structures in the solid states were elucidated by single crystal X-ray analysis. In the case of α-ketopyrroles, the molecules exist as hydrogen bonding dimers between the pyrrole NH and carbonyl groups, while in the case of β-ketopyrrole derivatives, hydrogen bonding networks are formed. Both the α,β′-diketo-substituted pyrroles and their derivatives are suggested to be suitable for the construction of 1-D chain networks. Overall, the effects of substituents on the supramolecular interactions are discussed in detail.

Doubly N-Methylated Porphyrinoids

Chirality-induced aromatic π-electronic macrocycles, porphyrin and corroles, were synthesized through doubly inner N-methylation through multistep and one-pot reactions, respectively. The exact structures of doubly N-methylated porphyrin and corroles were revealed by single-crystal synchrotron X-ray analysis, exhibiting two N-methyl groups located on neighboring pyrrole rings in up/down conformations. These doubly inner N-substitutions of the π-electronic macrocycles induced distorted geometries, resulting in chiroptical properties after optical resolutions.

Programmed asymmetrical trimer formation of β-alkyl N-confused porphyrin zinc(II) complex

Self-coordination of β-alkyl N-confused porphyrin (NCP) zinc(II) complexes has been studied to reveal that asymmetrical trimers formed exclusively. A key factor to control the configuration of the trimers (symmetric or asymmetric) has been studied theoretically, and revealed that the steric environment around the peripheral nitrogen atoms as well as rotational angles of the confused pyrrole rings play a crucial role in trimer formation of β-alkyl NCP zinc(II) complexes.

Supramolecular dimeric structures of pyrazole-containing meso-oxo carbaphlorin analogues

Abstract Synthesis and properties of a novel meso-oxo carbaphlorin analogue embedded with an N-free pyrazole moiety are described. The N-benzyl precursor was prepared by a [3 + 1]-MacDonald condensation of N-benzyl pyrazole dialdehyde and β-alkyl-substituted tripyrrane dicarboxylic acid and subsequent oxidation by ferric chloride. Upon deprotection of the benzyl group, the resulting N-free oxophlorin analogue formed a unique supramolecular dimer through mutual hydrogen bonding interactions between the pyrazole NH and meso-carbonyl group. The assembled behaviour was characterised by various spectroscopies, X-ray crystallographic analysis and vapour pressure osmometry. Under the similar reaction conditions, the condensation of meso-phenyl-substituted tripyrrane derivative afforded an unprecedented tetrapyrrolic macrocycle fused with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone unit at their α-pyrrolic positions. The X-ray crystallographic analysis of the macrocycle revealed a twisted core structure, and nonaromatic nature was elucidated.