Quantum chemical studies by the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metal–oxo species are reviewed. The activation of the O–O bond of dioxygen at the mono- and dimetal sites of metalloenzymes is an essential process in the initial catalytic stages of naturally occurring oxygenation reactions. A way of orbital thinking about dioxygen activation at diiron and dicopper enzymes is developed at the extended Hückel level of theory. Two different reaction pathways that lead from dimetal peroxo complexes with µ-η1:η1-O2 and µ-η2:η2-O2 modes to the corresponding dioxo complexes are discussed in detail. In considering the mechanism of methane hydroxylation, special attention to FeO+-mediated methane hydroxylation that occurs under ion cyclotron resonance (ICR) conditions is given. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ions ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+ are systematically analyzed on the basis of density functional theory (DFT) calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process. The spin inversion from the high-spin state to the low-spin state effectively decreases the barrier height of C–H activation. Another feature in the reaction is that no radical species is involved in the course of the hydroxylation because the methyl species formed as a result of the C–H bond cleavage can directly coordinate to the metal active site. The coupling of the OH and CH3 ligands occurs to produce methanol at the metal active center. The reaction profiles obtained from DFT calculations are similar to those of the Gif chemistry proposed by D. H. R. Barton, in particular the involvement of the HO–M–CH3 species as an intermediate in the hydroxylation of methane. The nonradical mechanism is extended to methane hydroxylation by the diiron and dicopper species of methane monooxygenase (MMO). This mechanism is possible when the metal active center is coordinatively unsaturated to have a space for the coordination of the OH and CH3 groups as ligands. Although compelling discussion to support the involvement of oxygen- and carbon-centered radicals is provided, the nonradical mechanism still seems to be applicable for methane hydroxylation from the viewpoint of reaction selectivity. Kinetic isotope effects (KIEs) in the C–H activation process by the bare FeO+ complex and diiron and dicopper enzyme models are compared with respect to the radical and nonradical mechanisms by using transition state theory.
In this article quantum chemical studies of the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metal–oxo species are reviewed.
Gas storage and separation are becoming a high priority area of research due to economic, industrial, and environmental reasons. The last two decades have therefore witnessed dramatic growth in the search for more efficient and adaptable nanoporous materials. In particular, much attention has been focused on porous coordination polymers (PCPs) or metal organic frameworks (MOFs) as new nanoporous materials. Based on the unlimited combination of metal ions and organic ligands, PCPs can provide infinite variety of nanospace in their pores. As molecular adsorption is dependent on the size, shape, and surface nature of nanospace, many unique molecular adsorption or trapping phenomena have been reported in this class of compounds. In this account, I focus on how thoughtful design can lead to the synthesis of porous coordination polymers that demonstrate unprecedented adsorption behavior not found in other porous materials. Examples include selective adsorption of acetylene over carbon dioxide in the CPL series of PCPs, using charge-transfer to induce selective adsorption of nitric oxide and oxygen in TCNQ (7,7,8,8-tetracyano-p-quinodimethane) based PCP and light-induced on-demand adsorption and structural transformations in CID-based PCPs. The guidelines underpinning such unique, highly selective guest adsorption are discussed.
Porous coordination polymers (PCPs), a new class of porous materials, can be designed by functionalized building blocks. In this accounts we focus on the design and synthesis of PCPs which show unusual sorption behavior in particular highly selective adsorption.
1,2-Bis(ferrocenyl)dipnictenes bearing a Pn=Pn π-spacer (Pn: P (1), Sb (2), and Bi (3)) between two ferrocenyl units have been synthesized as stable compounds. Not only their molecular structures and fundamental properties but also their redox behavior have been systematically disclosed. Interestingly, in the reduction region, the dipnictenes showed two pseudo-reversible one-electron redox couples at low temperature, suggesting possible generation of the corresponding radical anion and dianion species. On the other hand, they showed three-step one-electron oxidation processes in the oxidation region. The first two oxidation steps would correspond to those of the two ferrocenyl moieties, while the third step would be that of the Pn=Pn π-spacer moiety, respectively. Thus, these 1,2-bis(ferrocenyl)dipnictenes with a Pn=Pn π-spacer should be stable multiredox systems reflecting unique properties of a double bond between heavier 15 group elements. As a result, all Pn=Pn units (Pn: P, Sb, and Bi) were found to work as a more effective π-spacer than those of 2nd row elements such as C=C and N=N.
1,2-Bis(ferrocenyl)dipnictenes bearing a Pn=Pn π-spacer (Pn: P (1), Sb (2), and Bi (3)) between two ferrocenyl units have been synthesized as stable compounds. These 1,2-bis(ferrocenyl)dipnictenes with a Pn=Pn π-spacer should be stable multiredox systems reflecting unique properties of double-bond between heavier 15 group elements. All Pn=Pn units (Pn: P, Sb, and Bi) were found to work as a more effective π-spacer than those of 2nd row elements such as C=C and N=N.
We investigated anodization of gold in aquaous solutions of carboxylic acids as a continuation of anodization of gold in oxalic acid. In contrast to the anodization in aquaous oxalic acid, in which nanoporous gold film is formed, nanoporous gold oxide (Au3+) films are formed in other aquaous carboxylic acids. The orange oxide films were unstable at room temperature: the films blackened within a week and then lightened in color about a month after the anodization. Although the nanoporous framework looked unchanged by SEM observation during the changes in the color, reduction of gold oxide and growth of nanoparticulate gold proceeded in the film. The isothermal adsorption measurement of the nanoporous gold, which was formed in citric acid and kept more than a month at room temperature, revealed that the mean pore size and specific surface area were 13 nm and 60 m2 g−1, respectively. These results demonstrated that bulk gold is easily anodized in aqueous carboxylic acids to form nanoporous gold oxide film. Then, it spontaneously reduces to nanoporous gold at room temperature owing to the noble redox properties of the gold.
Vertical alignment of mesoscaled and phase-separated structures in organic–inorganic hybrid materials has significant potential for various applications. However, particular procedures and treatments are required to generate such pattern geometries. We demonstrate herein a simple and spontaneous process for vertical alignment of mesoscopic phase-separated structures in a hybrid material consisting of an amphiphilic block copolymer that possesses a thermotropic liquid crystal framework and silicate sol. By combining the thermotropic and lyotropic liquid crystal properties, the sol–gel condensation in this system spontaneously leads to the vertical alignment of mesoscopic lamellae. Most likely, the vertical molecular orientation formed in the thermotropic smectic liquid crystal domains promotes the vertical alignment of the mesoscaled lyotropic liquid crystalline structure of the hybrid film. The organic block components in the film could be selectively etched by ultraviolet light/ozone treatment and, consequently, provide a topologically undulated silica film that retains the vertical lamella structure of the precursor hybrid film. This approach via the hybridization with a metal oxide sol solution is advantageous in tuning the lateral pitch of the structure simply by adjusting the volume fraction of precursor sol content.
Quantum mechanical theory of electrochemical kinetics based on Fermi’s golden rule was formulated by introducing the concept of electron-transfer distance. The expressions for the exchange current density and standard rate constant in electrochemistry were derived in analytical form, as well as exponential current overpotential dependence. The theory corresponds well to the electrode kinetics based on the transition state theory that is familiar at present with many electrochemists. It was applied to various kinds of electrode reactions to analyze the standard rate constants and the exchange current densities reported in past literature. The evaluated magnitudes of the electron exchange energy were very small, being in the order of 10−3–10−5 eV. This theory supports the ordinary electron-transfer mechanism due to the overlap of wave functions between the electrode and redox species, denying tunneling mechanism.
1-(4-Methoxyphenyl)pyrene (PyrPhOMe(1)), 1,3,6,8-tetrakis(4-methoxyphenyl)pyrene (PyrPhOMe(4)), 1-(4-methoxyphenylethynyl)pyrene (PyrC≡CPhOMe(1)), 1,3,6,8-tetrakis(4-methoxyphenylethynyl)pyrene (PyrC≡CPhOMe(4)), 1-(4-hydroxyphenylethynyl)pyrene (PyrC≡CPhOH(1)), and 1,3,6,8-tetrakis(4-hydroxyphenylethynyl)pyrene (PyrC≡CPhOH(4)) were synthesized via organometallic complex catalysis. Deprotection of the methoxy groups of PyrPhOMe(1) and PyrPhOMe(4) was conducted by treatment with BBr3. Deprotonation of the OH groups of PyrPhOH(1), PyrPhOH(4), PyrC≡CPhOH(1), and PyrC≡CPhOH(4) through treatment with NaH caused a bathochromic shift in the absorption and photoluminescence (PL) peaks. The bathochromic shift of the deprotonated species increased with the donor number (DN) of the solvents. These observations can be explained as the consequence of intramolecular charge transfer (ICT) from the ONa groups to the pyrene core.
Magnesium-promoted reductive coupling of chlorotrialkylsilanes and tropone or tropone acetal in DMF, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in benzene gives 2-trialkylsilyltropones as the main product, and the reductive coupling of tropone with acid anhydrides under similar reaction conditions yields benzene derivatives after ring contraction.
Practical syntheses of substituted ketene dithioacetal monoxides (KDMs) from the corresponding aldehydes or ethyl trifluoroacetate are described. Newly developed procedures are scalable and versatile to provide aryl-, alkyl-, and trifluoromethyl-substituted KDMs in gram scales.
In medicine, microarray chips are desired in order to improve personal medicine. In this paper, we propose a fabrication method for microarray chips using electroplated soft magnetic films on glass substrates using a simple process. Electroless plated Ni–P films were formed on soda lime glass as conductive substrates and were subsequently annealed at 150 °C for 30 min to improve adhesion strength between the films and glass substrates. Microspot array photoresists were patterned on the Ni–P films by photolithography. Co–Ni–Fe alloy films were then electroplated and finely the photoresist removed. As determined by electron probe microanalysis, the Co–Ni–Fe films were composed of 15–74 atom % Co, 12–72 atom % Fe, and 1.6–24 atom % Ni under several plating bath compositions. Vibrating sample magnetometry showed saturation magnetic flux density of 1400–2200 emu cm−3, residual magnetic flux density of 1100–1600 emu cm−3 and coercive force of 90–100 Oe on the Co–Ni–Fe films. A microspot array prepared by electroplated Co–Ni–Fe soft magnetic films/electroless plated Ni–P films on soda lime glass with simple patterning successfully trapped pseudo B-lymphocytes conjugated with magnetic beads.
A citric acid-assisted solid-state method is developed to prepare highly active Cu/ZnO catalysts in different atmospheres (air and argon). The entire catalyst preparation process is accomplished with inexpensive raw materials. It is environmentally friendly in that no water is used and no waste water is produced during the entire process. These as-prepared catalysts are investigated in low-temperature methanol synthesis with CO2-containing syngas. The properties of these catalysts are systematically studied by thermogravimetry differential thermal analysis, X-ray diffraction, Fourier transform-infrared spectroscopy, Raman spectroscopy, temperature-programmed reduction, Brunauer–Emmett–Teller, and N2O chemisorption techniques. The activity and methanol selectivity of Cu/ZnO catalysts are closely related to the metallic Cu0 surface areas and Cu crystalline sizes. Compared with traditional solid-state or sol–gel autocombustion methods, the catalysts prepared by this citric acid-assisted solid-state method exhibit much higher activity and methanol selectivity.
Stable nanometric colloidal sols consisting of fluorite-type metal oxides were prepared for use as chloride-free precursors for automotive catalysts. The aqueous solution containing metal carbonates and tetramethylammonium (TMA) ions was degassed and hydrothermally treated at 140–160 °C. Colloidal suspensions thus formed were purified by ultrafiltration to yield very stable and condensed sol solutions of CeO2 (CE), ZrO2 (ZR) and their solid solutions, CeO2–ZrO2 (CZ), which were stabilized by TMA. Each oxide sol contained 6–8 nm primary particles, but they formed aggregates larger than 40 nm. Local structural analysis by means of XAFS suggested that CZ sol exhibited two types of solid solution domains containing more or less Ce. The dried CZ sol exhibited higher thermal stability and oxygen storage capacity than CE and ZR sols.