Hidekazu Arii

SOD activities of the copper complexes with tripodal polypyridylamine ligands having a hydrogen bonding site

Abstract As a structural mimic of the Arg 141 residue near copper site in native bovine Cu,Zn–superoxide dismutase (Cu,Zn–SOD), four mononuclear copper complexes with tris(2-pyridylmethyl)amine derivatives having a hydrogen bonding site (pivalamido, neopentylamino, or amino groups) at the pyridine 6-position, [Cu(tnpa)(OH)]ClO4 (1), [Cu(tapa)Cl]ClO4 (2), [Cu(tapa)(OH)]ClO4 (3), and [Cu(bppa)](ClO4)2 (4), and two analogous dinuclear copper complexes, [Cu2(tppen)(H2O)2](ClO4)4 (5) and [Cu2(tppen)Cl4] (6), were prepared, and the correlation between the coordination structures of the copper complexes and their superoxide dismutation activities were examined. Their structures in both solution and solid states were characterized by electronic absorption and ESR spectroscopic (for all the complexes) and by X-ray analytical methods (for 1, 2, 5 and 6), respectively. The coordination geometries around the copper ions were determined to be five-coordinate trigonal bipyramidal for 1, 2 and 3 and to be an intermediate of five-coordinate trigonal bipyramidal and square pyramidal for 5 and 6 in both crystal and solution phases, in contrast to four-coordinate square planar structure for 4 reported previously. The cyclic voltammetry measurement of the mononuclear complexes 1–4 showed quasi-reversible redox potentials (Cu(II)/Cu(I) couple) in the range between −330 mV (vs. NHE at pH 7; O2/O2 −) and +890 mV (vs. NHE at pH 7; O2 −/H2O2), which are in the range responsible for superoxide dismutation (SOD) reaction, although the dinuclear copper complexes, 5 and 6, gave only reduction potentials. The SOD activities of complexes 1, 2, and 3 were moderate and those of 5 and 6 were rather high, although 4 showed the lowest activity of all. Those of 1, 2 and 3 with a trigonal bipyramidal structure and hydrogen bonding interaction site are slightly high in comparison with that of the corresponding Cu(II) complex without hydrogen bonding site, [Cu(tpa)(H2O)](ClO4)2 (7). Relatively higher SOD activity observed in complexes 5 and 6 may be explained in terms of higher flexibility in the conformation and cooperativity by dinuclear copper ions.

m-Carborane-Based Chiral NBN Pincer-Metal Complexes: Synthesis, Structure, and Application in Asymmetric Catalysis

We have succeeded in synthesizing m-carborane-based chiral NBN-pincer ligands, 1,7-bis(oxazolinyl)-1,7-dicarba-closo-dodecaborane (Carbox) (7-9). The combination of bis(hydroxyamides) and 3 equiv of diethylaminosulfur trifluoride (DAST) is a key step for cyclization to form oxazoline rings in excellent yields. X-ray crystal structures of these ligands confirmed three donor sites, one central B and two flanking N atoms in fixed positions. The electrophilic halogenation of the Carbox pincer ligands with iodine and a catalytic amount of Lewis acid led to ring-opening of the oxazolines and afforded bis(haloamides) (13 and 14). The air- and moisture-stable Carbox pincer complexes of rhodium(III), nickel(II), and palladium(II) were synthesized by the oxidative addition of RhCl(3)·3H(2)O, Ni(COD)(2), and Pd(CH(3)CN)(4)[BF(4)](2) to the Carbox pincer ligands (7-9), respectively. The catalytic activity of the rhodium(III) complexes (18-20) was examined for the asymmetric conjugate reduction of α,β-unsaturated esters and reductive aldol reaction. Among these catalysts, [(S,S)-Carbox-iPr]Rh(OAc)(2)·H(2)O (18) showed the highest enantioselective catalytic ability for both asymmetric conjugate reduction and reductive aldol reaction.

Self-assembled monolayer electrode of a diiron complex with a phenoxo-based dinucleating ligand: observation of molecular oxygen adsorption/desorption in aqueous media

The phenoxo-based dinucleating ligand, 2,6-bis[bis(6-pivalamido-2-pyridylmethyl)amino-methyl]-4-aminophenol (1), and its Fe2(II) complex, [Fe2(II)(1)(PhCOO)2](CF3SO3) (2), were prepared and 2 deposited on the Au surface (2/Au) is much more stable than in solution and exhibits redox behavior in aqueous media as well as reversible adsorption/desorption of oxygen at room temperature.

Low‐Coordinate Germanium(II) Centers Within Distorted Axially Chiral Seven‐Membered Chelates: Stereo‐ and Enantioselective Cycloadditions

The heavier group 14 analogues of carbenes (R2ED) have attractive structures and interesting reactivity toward unsaturated compounds. R2ED compounds have two covalent bonds with high p character, a single electron lone pair with high s character, and a single vacant p orbital in the singlet ground state. The intrinsic ambiphilic nature of R2ED compounds allows their participation in a variety of cycloaddition reactions with compounds containing unsaturated bonds. (2 + 2 + 1) Cycloaddition reactions of divalent group 14 element species provide one of the most interesting ways for synthesizing a heterocycle in one step but have been limited to a number of transient silylenes and [Si(Cp*)2] (Cp* = pentamethylcyclopentadienyl). 5] The reactions of these silylenes with a ketone or a aldehyde afforded the corresponding 1:2 cycloadducts, although different reaction intermediates were proposed for the used silylenes. In both cases, the high Lewis acidity of the group 14 element atom was believed to be an essential factor in promoting the formation of these intermediates. Since the appearance of a report on diaminogermylene by Harris and Lappert, most germylenes have been synthesized by taking advantage of the thermodynamic stabilization achieved through their binding with p-donor atoms (Scheme 1); in this stabilized form, their Lewis acidity is presumed to be relatively low. We envisioned that a sevenmembered N-heterocyclic germylene containing an N,N’-bis(trimethylsilyl)-2,2’-diamino-6,6’-dimethylbiphenyl (LH2) unit would engender a germanium center with relatively high Lewis acidity. We expected that the sevenmembered chelate would exhibit significantly lower p donation from the nitrogen atoms to the vacant p orbitals of the germanium center than planar fiveor six-membered chelates because of the twisted conformation of the seven-membered chelate. Indeed, a number of Nand O-heterocyclic germylenes of larger ring size form adducts with Lewis donors, however, their reactivity was not explored. On the other hand, seven-membered N-heterocyclic carbenes, such as 1,3-disubstituted 1,3-diazacycloheptane-2-ylidenes and dibenzo-1,3diazaazepine-2-ylidenes, have been reported. Palladium complexes containing these carbenes were reported to be very useful catalysts for Mizoroki–Heck coupling reactions. However, there has been no discussion on the Lewis acidity of the divalent carbon centers, although there has been discussion about the influence of ring size and steric hindrance of the carbenes on catalytic performance. Herein, we describe the synthesis and characterization of [GeL] (1) and a stereoselective cycloaddition reaction of an in situ generated carbonyl germaylide derived from the reaction of 1 with benzaldehyde. The reaction of dilithio derivative [LLi2] and GeCl2·dioxane in THF at 80 8C under Ar afforded the corresponding germylene 1 as yellow crystals in a 19% yield upon isolation. The crystal structure of 1 (Figure 1) revealed that a twofold rotation axis exists through the germanium center and the midpoint of the C(1) C(1)’ bond and that the distance between germanium atoms on neighboring molecules is approximately 6.6 , thus indicating that there are no intermolecular Ge···Ge interactions (Figure 1). The divalent germanium species has a bent geometry, thus indicating the presence of a vacant p orbital and a lone pair of electrons on the germanium center. The Ge N bonds (1.8651(13) ) of 1 are slightly longer than those of the previously reported planar five-membered N-heterocyclic germylenes; this difference can be attributed to the twisted geometry of the seven-membered chelate, wherein there is weak p donation from the nitrogen atoms to the germanium center. The large N-Ge-N bond angle (98.13(8)8) indicates that the the Ge N bond has higher s character and thus a lone pair of higher p character than that of typical non-distorted Scheme 1. Two-coordinate N-heterocyclic germylenes.

Regioselective and Stereospecific Dehydrogenative Annulation Utilizing Silylium Ion-Activated Alkenes.

Treatment of dialkylbenzylsilanes (1) with trityl tetrakis(pentafluorophenyl)borate (TPFPB) afforded the corresponding silylium ions in equilibrium with their intra- or intermolecular π-complexes, which underwent dehydrogenative annulation with various alkenes to form 1,2,3,4-tetrahydro-2-silanaphthalenes (4) in up to 82% isolated yield. Sterically bulkier substituents on the silicon atom tended to increase the yield of cyclic products 4. The annulation products retained the stereochemistry in cases of the reactions using internal alkenes. The use of diisopropyl(1-naphthyl)silane (2) instead of 1 also resulted in annulation to obtain the 2,3-dihydro-1-sila-1H-phenalene derivatives 6. Electrophilic aromatic substitution at the 8-position was predominant, despite the two potentially reactive positions on the naphthyl group. The steric hindrance of the naphthyl group prevented addition of the cis-alkene to the silylium ion, which would considerably decrease yields of the desired products from 2 compared to those from 1.

Intramolecular Chain Hydrosilylation of Alkynylphenylsilanes Using a Silyl Cation as a Chain Carrier

Diorganyl[2-(trimethylsilylethynyl)phenyl]silanes 1a–c and methyl-substituted phenylsilanes 1d and 1e were treated with a small amount of trityl tetrakis(pentafluorophenyl)borate (TPFPB) as an initiator in benzene to afford the corresponding benzosiloles (2a–e) in moderate to good yields. However, no reaction was observed for the reaction using [2-(1-hexynyl)phenyl]diisopropylsilane lf. The methyl substituent was tolerated under the reaction conditions and increased the yield of the corresponding benzosilole depending on the substitution position. From the result using 1f, the current reaction was found to require the trimethylsilyl group, which can stabilize intermediary alkenyl carbocations by the β-silyl effect. The current reaction can be considered an intramolecular chain hydrosilylation of alkynylarylsilanes involving silyl cations as chain carriers. Therefore, the silyl cations generated by hydride abstraction from hydrosilanes 1 with the trityl cation causes intramolecular electrophilic addition to the C-C triple bond to form ethenyl cations, which abstract a hydride from 1 to afford benzosiloles 2 with the regeneration of the silyl cations.

Preparation and structural characterization of a novel dicopper(II) complex with a terminal hydroxide: a structural model of an active site in phosphohydrolases

As an intermediate model for active dinuclear metal sites in phosphohydrolases, a novel dicopper(II) complex with a terminal hydroxide has been prepared using a new dinucleating ligand with intramolecular hydrogen-bonding N–H sites and sterically hindered tert-butyl groups. It has been characterized by X-ray structure analysis, magnetic susceptibility, and ESR and electronic absorption spectroscopies.

A shallow origin of so-called ultrahigh-pressure chromitites, based on single-crystal X-ray structure analysis of the high-pressure Mg2Cr2O5 phase, with modified ludwigite-type structure

Abstract The crystal structure of the high-pressure Mg2Cr2O5 phase was studied by single-crystal X-ray diffraction (XRD) analysis for the recovered samples. The 61 parameters including anisotropic displacement parameters of each atom and site occupancies of Mg and Cr in cation sites were refined with R1 = 1.26%, wR2 = 4.33%, and Sfit = 1.265 for 470 unique reflections. The results show that the structure of the recovered Mg2Cr2O5 phase is the same as modified ludwigite (mLd)-type Mg2Al2O5 [space group: Pbam (no. 55)], and the lattice parameters are a = 9.6091(2), b = 12.4324(2), c = 2.8498(1) Å (Z = 4). The refined structure of the Mg2Cr2O5 phase has four (Mg,Cr)O6 octahedral sites and a MgO6 trigonal prism site, and is similar to but distinct from that of CaFe3O5-type Mg2Fe2O5 phase, which has two octahedral sites and a bicapped trigonal prism site with two longer cation-oxygen bonds. The isotropic atomic displacement parameter of the trigonal prism site cation in mLd-type Mg2Cr2O5 is relatively small compared with that of CaFe3O5-type Mg2Fe2O5, suggesting that the trigonal prism site is less flexible for cation substitution than that of CaFe3O5-type structure. To stabilize mLd-type A22+B23+O5

Theoretical Estimation of Catalytic Reactivity as Iron-Oxygen Nano-Materials

\begin{array}{} A_2^{2+}B_2^{3+}\text{O}_5 \end{array}

Synthesis and Characterization of Diiron(II) Complex with the Asymmetric Ligand Fixed into Internal Surface of Mesoporous Silica

phase, it would be an important factor for the B3+ cation to have high octahedral-site preference, resulting in only A2+ cation being accommodated in the tight trigonal prism site. Our study also suggests that mLd-type phase with (Mg,Fe2+)2Cr2O5 composition would crystallize as one of decomposed phases of chromitites, when the chromitites were possibly subducted into the mantle transition zone.