Showing papers by "Shinji Inagaki published in 2019"

Cooperative Catalysis of an Alcohol Dehydrogenase and Rhodium‐Modified Periodic Mesoporous Organosilica

Abstract: The combined use of a metal-complex catalyst and an enzyme is attractive, but typically results in mutual inactivation. A rhodium (Rh) complex immobilized in a bipyridine-based periodic mesoporous organosilica (BPy-PMO) shows high catalytic activity during transfer hydrogenation, even in the presence of bovine serum albumin (BSA), while a homogeneous Rh complex exhibits reduced activity due to direct interaction with BSA. The use of a smaller protein or an amino acid revealed a clear size-sieving effect of the BPy-PMO that protected the Rh catalyst from direct interactions. A combination of Rh-immobilized BPy-PMO and an enzyme (horse liver alcohol dehydrogenase; HLADH) promoted sequential reactions involving the transfer hydrogenation of NAD+ to give NADH followed by the asymmetric hydrogenation of 4-phenyl-2-butanone with high enantioselectivity. The use of BPy-PMO as a support for metal complexes could be applied to other systems consisting of a metal-complex catalyst and an enzyme.

Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica.

Abstract: A hybrid vapoluminescent system exhibiting fast and repeatable response was constructed using periodic mesoporous organosilica with bipyridine moieties (BPy-PMO) and a Pt(II) complex bearing a potentially luminescent 2-phenylpyridinato (ppy) ligand. An intense red luminescence appeared when the Pt(II)-complex immobilised BPy-PMO was exposed to methanol vapour and disappeared on exposure to pyridine vapour. The ON-OFF vapochromic behaviour occurred repeatedly in a methanol/pyridine/heating cycle. Interestingly, a rapid response was achieved in the second cycle and cycles thereafter. Scanning and transmission electron microscopies (SEM/TEM), absorption and emission, and nuclear magnetic resonance spectroscopies, mass spectrometry, and powder X-ray diffraction indicated that methanol vapour induced Si-C cleavage and thus liberated [Pt(ppy)(bpy)]Cl (bpy = 2,2′-bipyridine) from the BPy-PMO framework. Furthermore, the self-assembling properties of the Pt(II) complex resulted in the formation of highly luminescent micro/nanocrystals that were homogeneously dispersed on the porous support. The unique vapoluminescence triggered by the unprecedented protodesilylation on exposure to protic solvent vapour at room temperature is attributable to BPy-PMO being a giant ligand and an effective vapour condenser. Consequently, this hybrid system presents a new strategy for developing sensors using bulk powdery materials.

Heterogeneous hydrosilylation reaction catalysed by platinum complexes immobilized on bipyridine-periodic mesoporous organosilicas

Abstract: The utility of a bipyridine periodic mesoporous organosilica, BPy-PMO, as a support material of a hydrosilylation catalyst was investigated in the hydrosilylation of phenylacetylene with trimethoxysilane. [PtMe2(BPy-PMO)] (1) exhibited a moderate catalytic activity, whereas the reaction was successfully catalysed by [PtMe2(BPy-PMO-TMS)] (2) bearing end-capped TMS groups on the surface. Spectroscopic analyses of 2 revealed that the porous structure of BPy-PMO-TMS remained almost unchanged through the reaction. The hot filtration test supported the nonleaching property of 2, thereby exhibiting good reusability without the loss of the product yields.

Catalytic Disproportionation of Formic Acid to Methanol by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

Abstract: We report the first example of using heterogeneous Iridium (Ir) catalysts under atmospheric pressure conditions to achieve the catalytic disproportionation of formic acid to methanol. IrCp* complex (Cp*=pentamethylcyclopentadienyl (ɳ5‐C5Me5)) was immobilized on the pore surface of the periodic mesoporous organosilica synthesized from a precursor mixture of ethane (Et) and 2,2′‐bipyridine (BPy)‐bridged organosilanes (Ir‐Et‐BPy‐PMO). Ir‐Et‐BPy‐PMO showed unique catalysis with higher selectivity of methanol compared to homogeneous Ir‐BPy complexes. Furthermore, Ir‐Et‐BPy‐PMO exhibited high reusability without any noticeable decrease in activity for at least five times with no leaching of Ir species during the reaction. The increase in methanol selectivity of Ir‐Et‐BPy‐PMO would be attributed to the unique confinement effect in the mesochannels with their high aspect ratio for the retention of generated H2/CO2, which suppresses the competing formic acid dehydrogenation and promotes the formic acid hydrogenation.

Well-controlled radical-based epoxidation catalyzed by copper complex immobilized on bipyridine-periodic mesoporous organosilica

Abstract: The development of synthetic methods and reaction systems for safe radical reactions is of extremely industrial importance. Here we proposed new concept for a safe radical reaction system based on combined use of a mesoporous catalyst and an insoluble solid scavenger. We selected Mukaiyama epoxidation of olefin as a model radical reaction and investigated the catalysis of Cu-bipyridine complexes immobilized on trimethylsilylated bipyridine-periodic mesoporous organosilica as a solid support. The immobilized Cu complex exhibited high catalytic activity and reusability for Mukaiyama epoxidation at low substrate concentration (1 mmol) but free-radical auto-oxidation also occurred at high substrate concentration (7 mmol). Although both epoxidation reactions outside and inside the mesochannels were almost completely quenched by addition of molecular scavenger, addition of solid scavenger allowed quenching the reaction outside the mesopores but not inside the mesopores because the solid scavenger could not access the interior of the mesochannels. Thus, the combined use of a mesoporous catalyst and a solid radical scavenger would offer new reaction system for safe radical reactions.

Effects of pore surfaces on the electronic states of metal complexes formed on bipyridine periodic mesoporous organosilica

Abstract: Bipyridine periodic mesoporous organosilica (BPy-PMO) is a porous material in which the pore walls consist of silica frameworks bridged by 2,2′-bipyridine (bpy) functional groups, and is expected to be a novel platform for catalysts and photocatalysts. Metal complexes formed on the pore surfaces of BPy-PMO are under unusual environmental conditions that are entirely different from those in the bulk solids or in solution. To understand the effects of the unique structural features of the pore surfaces on the metal complexes, this study focused on the combination of BPy-PMO with Re(bpy)(CO)3Cl (Re-BPy-PMO) and analyzed the metal-to-ligand charge transfer (MLCT) absorption band of this material. UV/vis and infrared analyses demonstrated that the Re-complex in Re-BPy-PMO interacts with water molecules adsorbed on the pore surfaces. Quantum chemical calculations suggest that the Re-complex also interacts with SiOH groups on the silica framework. Combined interactions are responsible for the unexpectedly short MLCT absorption wavelength of Re-BPy-PMO. This hypothetical mechanism is consistent with a red shift of the MLCT absorption following trimethylsilylation, which transforms SiOH groups to SiOSiMe3 (TMS) groups and removes adsorbed water. Calculations also suggest that the TMS group is more electron-donating than the SiOH group. Therefore, the trimethylsilylation should result in a blue shift of the MLCT absorption with regard to the substituent effect, which is inconsistent with the experimental observations. Evidently, the effect of the removal of water and SiOH groups is sufficiently powerful to counteract the blue shift. Thus, the pore-surface features have a significant influence on the metal complex.

Synthesis and Applications of Periodic Mesoporous Organosilicas

Abstract: Periodic mesoporous organosilicas (PMOs) represent a new class of hybrid organic–inorganic porous materials, in which organic groups are densely and covalently embedded within the pore walls in the framework of the base material. PMOs have the distinct advantage of allowing various functional groups to be integrated into the pore walls via the appropriate selection of organic precursors, and also allow tuning of the spatial arrangement of such moieties within the framework. This chapter describes the synthesis of PMOs with various structures and organic functionalities, as well as the use of these materials in diverse applications, including light-harvesting antennae, heterogeneous molecular photocatalysts, photovoltaic devices, and as substrates for laser desorption/ionization mass spectrometry and solid supports for heterogeneous metal complex catalysis.

Excited-State Dynamics of 2,2′-Bipyridine Moieties Embedded in the Framework of Periodic Mesoporous Organosilica

Abstract: We investigate the excited-state dynamics of 2,2′-bipyridine (BPy) moieties in a framework of BPy-bridged periodic mesoporous organosilica (BPy-PMO), which is a unique solid chelating ligand that has densely and regularly arranged BPy units that are exposed on the pore surfaces. The steady-state absorption spectrum of BPy-PMO shows broad absorption bands and a prolonged absorption tail compared to its precursor molecule, which suggests that a part of the BPy moieties form dimers and/or aggregates in the ground state. Transient absorption spectra of BPy-PMO after excitation with 295 nm light show mainly intersystem crossing from the lowest singlet excited state (S1) to the lowest triplet excited state (T1) of BPy moieties with a lifetime of 52 ps. Energy transfer (7.2 ps) from the S1 of a BPy monomer to BPy dimers and/or aggregates is also suggested. The lifetime of the excited BPy dimers and/or aggregates is estimated by selective excitation (355 nm) to be 3.4 ns. The lifetime for the T1 of the BPy monome...

Excited-State Dynamics of 2,2′-Bipyridine Moieties Embedded in the Framework of Periodic Mesoporous Organosilica

Abstract: We investigate the excited-state dynamics of 2,2′-bipyridine (BPy) moieties in a framework of BPy-bridged periodic mesoporous organosilica (BPy-PMO), which is a unique solid chelating ligand that has densely and regularly arranged BPy units that are exposed on the pore surfaces. The steady-state absorption spectrum of BPy-PMO shows broad absorption bands and a prolonged absorption tail compared to its precursor molecule, which suggests that a part of the BPy moieties form dimers and/or aggregates in the ground state. Transient absorption spectra of BPy-PMO after excitation with 295 nm light show mainly intersystem crossing from the lowest singlet excited state (S₁) to the lowest triplet excited state (T₁) of BPy moieties with a lifetime of 52 ps. Energy transfer (7.2 ps) from the S₁ of a BPy monomer to BPy dimers and/or aggregates is also suggested. The lifetime of the excited BPy dimers and/or aggregates is estimated by selective excitation (355 nm) to be 3.4 ns. The lifetime for the T₁ of the BPy monomer is estimated to be 700 ns from transient absorption spectra of BPy-PMO after nanosecond 266 nm pulse irradiation. The characteristics of the excitation-relaxation processes of BPy-PMO are discussed.

Observation of electron decay dynamics in Pt nano-structures by femtosecond infrared luminescence

Abstract: Femtosecond infrared luminescence has been observed in bulk and nano-structured platinum. Very broad spectra ranging from 0.3 to 1.05 eV are ascribed to hot luminescence within metal like continuous states both in bulk and nano-structured Pt. In addition, an excitation power dependent lifetime was found in Pt nano-structures.