Bridging Moiety Research Articles

Application of a chiral copper-1,1-bis{2-[(4 S)- tert-butyloxazolinyl]}cyclopropane catalyst for asymmetric cyclopropanation of styrene

The structural effects of the bridge moiety and 5-position on bisoxazoline ligands were studied for the copper-catalyzed asymmetric cyclopropanation of styrene with ethyl diazoacetate. The 1,1-bis{2-[(4 S)- tert-butyloxazolinyl]}cyclopropane ligand showed a remarkable enhancement in the stereoselectivities ( trans/ cis = 84/16, >99.9% ee for the trans product) compared with the previously reported best ligand, 2,2-bis{2-[(4 S)- tert-butyloxazolinyl]}propane ( trans/ cis = 75/25, 99.0% ee for the trans product).

Using Meta Conjugation To Enhance Charge Separation versus Charge Recombination in Phenylacetylene Donor−Bridge−Acceptor Complexes

A pair of donor-bridge-acceptor electron-transfer complexes, with a carbazole donor and a naphthalimide acceptor connected by either a para- or meta-conjugated phenylacetylene bridge, are synthesized and studied using time-resolved and steady-state spectroscopy. These experiments show that the charge separation times, which depend on the coupling of the donor and acceptor through the excited bridge moiety, are similar for the two molecules (Meta and Para). The charge recombination time, however, is a factor of 10 slower for Meta than for Para. These results are related to changes in the electronic coupling of the bridge depending on its electronic state, and show that meta-conjugated bridges provide a possible motif for the design of asymmetric molecular wires.

5-Endo-Dig Radical Cyclizations: “The Poor Cousins” of the Radical Cyclizations Family

Kinetics and thermodynamics of 5-endo-dig radical cyclizations were studied using a combination of DFT computations and Marcus theory. When the reactant is stabilized by conjugation of the radical center with the bridge pi-system, the cyclization starts with reorientation of the radical orbital needed to reach the in-plane acetylene pi-orbital in the bond-forming step. This reorientation leads to loss of the above conjugative stabilization, increases the activation energy, and renders such cyclizations less exothermic. As a result, even when the radical needed for the 5-endo cyclization is formed efficiently, it undergoes either H-abstraction or equilibration with an isomeric radical. Only when the bridging moiety is saturated or when intramolecular constraints prevent the overlap of the bridge pi-orbital and the radical center, 5-endo cyclizations may be able to proceed with moderate efficiency under conditions when H-abstraction is slow. The main remaining caveat in designing such geometrically constrained 5-endo-dig cyclizations is their sensitivity to strain effects, especially when polycyclic systems are formed. The strain effects can be counterbalanced by increasing the stabilization of the product (e.g., by introducing heteroatoms into the bridging moiety). Electronic effects of such substitutions can be manifested in various ways, ranging from aromatic stabilization to a hyperconjugative beta-Si effect. The 4-exo-dig cyclization is kinetically competitive with the 5-endo-dig process but less favorable thermodynamically. As a result, by proper design of reaction conditions, 5-endo-dig radical cyclizations should be experimentally feasible.

Unexpected Ferromagnetic Interaction in a New Tetranuclear Copper(II) Complex: Synthesis, Crystal Structure, Magnetic Properties, and Theoretical Studies

The new tetranuclear carbonate complex [Cu2L)2(CO3)] x 8H2O (1 x 8H2O) (H3L = (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) has been obtained by two different synthetic routes and fully characterized. Recrystallization of 1 x 8H2O in methanol yields single crystals of {[(Cu2L)2(CO3)]}2 x 12H2O (1 x 6H2O), suitable for X-ray diffraction studies. The crystal structure of 1 x 6H2O shows two crystallographically different tetranuclear molecules in the asymmetric unit, 1a and 1b. Both molecules can be understood as self-assembled from two dinuclear [Cu2L]+ cations, joined by a mu4-eta(2):eta(1):eta(1) carbonate ligand. The copper atoms of each crystallographically different [(Cu2L)2(CO3)] molecule present miscellaneous coordination polyhedra: in both 1a and 1b, two metal centers are in square pyramidal environments, one displays a square planar chromophore and the other one has a geometry that can be considered as an intermediate between square pyramid and trigonal bipyramid. Magnetic studies reveal net intramolecular ferromagnetic coupling between the metal atoms. Density functional calculations allow the assignment of the different magnetic coupling constants and explain the unexpected ferromagnetic behavior, because of the presence of an unusual NCN bridging moiety and countercomplementarity of the phenoxo (or carbonate) and NCN bridges.

Thermal degradation kinetics of polyesters containing mesogenic aromatic diols

AbstractA series of polyesters were synthesized by reacting structurally differing aromatic diols with either saturated (flexible) or unsaturated (rigid) dicarboxylic acid halide by a stirred interfacial polycondensation technique. Thermal degradation kinetics of these polyesters were investigated by applying Coats–Redfern and Horowitz–Metzger nonisothermal procedures. The dynamic thermogravimetry experiments were conducted in nitrogen to obtain differential thermogravimetric plots. Thermal stability of these polyesters was discussed on the basis of semiquantitative methods such as differential procedural decomposition temperature, integral procedural decomposition temperature, and fraction decomposition temperature (e.g., 10% DT). Degradation proceeded in multiple stages. The thermal degradation patterns and activation energies in these stages were discussed in relation to central bridging moieties of aromatic diol. The activation energies of these polyesters were found to be in the range of 100 to 200 kJ/mol. The effect of spacer type on activation energy was also reported. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 784–792, 2005

Ni(cyclam)(μ1,3-dca)2Cu(μ1,5-dca)2]: A Genuine 3D Bimetallic Coordination Polymer Containing Both μ1,3- and μ1,5-Bidentate Dicyanamide Bridges and a Ferromagnetic Interaction between Copper(II) and Nickel(II) Ions

The structure of [Ni(cyclam)(mu(1,3)-dca)2Cu(mu(1,5)-dca)2], a genuine 3D dicyanamide-bridged bimetallic coordination polymer, is made up of 2D [Cu(mu(1,5)-dca)2]n layers connected by [Ni(cyclam)(mu(1,3)-dca)2] bridging moieties; it exhibits a ferromagnetic exchange interaction between copper(II) and nickel(II) ions through the mu(1,3)-bidentate dicyanamide bridges.

Two New Diphenoxo‐Bridged Discrete Dinuclear CuIIGdIII Compounds with Cyclic Diimino Moieties: Syntheses, Structures, and Magnetic Properties

AbstractTwo Schiff base compartmental ligands, H2L1 and H2L2, derived from the condensation of 3‐ethoxysalicylaldehyde with 1,2‐diaminocyclohexane and orthophenylenediamine, respectively, have been utilized to isolate two diphenoxo‐bridged discrete dinuclear CuIIGdIII complexes, [CuII(H2O)L1GdIII(NO3)3] (3) and [CuIIL2GdIII(NO3)3] (4). This is the first report on the 3d‐4f compounds derived from compartmental Schiff base ligands with cyclic diamines. The compounds 3 and 4 crystallize in the triclinic P and orthorhombic Pna2(1) space groups, respectively, with the following unit cell parameters – 3: a = 8.8713(2) Å, b = 12.8399(3) Å, c = 14.0067(3) Å, α = 80.6649(5)°, β = 77.4059(5)°, γ = 76.8879(5)°, and Z = 2; 4: a = 9.2210(1) Å, b = 16.5407(2) Å, c = 19.9248(4) Å, and Z = 4. Structural analysis reveals that both are discrete dinuclear complexes. In 3, one water molecule is coordinated to the copper(II) ion to result in a square‐pyramidal coordination geometry, while the geometry of the coppe(II) center in 4 is square planar. In both complexes, the gadolinium(III) center has an O10 coordination environment. In contrast to expectation, although the N2O2 cavity affords a better planar environment for the copper(II) center in 4, the bridging moiety in complex 3 is more planar than that in 4 or in most of the previously reported examples (the dihedral angle between two CuO2Gd planes: 2.1° for 3 and 7.1° for 4). Variable‐temperature and variable‐field magnetic measurements reveal that the metal centers in both the complexes are ferromagnetically coupled (J values: 6.3 cm–1 for 3 and 5.4 cm–1 for 4; H = –2JSCu·SGd). Interestingly, complex 3 exhibits strongest ferromagnetic interaction among the related compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Tetramanganese(II) Cluster with Centered Trigonal Topology: Structure and Magnetic Properties

A new tetranuclear manganese(II) compound, [Mn4L6](ClO4)2 (L is the Schiff base derived from 2-pyridylaldehyde and 2-aminophenolate), has been synthesized and characterized structurally and magnetically. The compound is the first example of tetramanganese clusters with centered planar trigonal topology in which a central Mn(II) ion is connected with three peripheral Mn(II) ions by double phenoxo bridges, generating a trigonal Mn[(mu-phenoxo)2Mn]3 core. Magnetic studies have demonstrated a very weak ferromagnetic interaction between the central and peripheral Mn(II) ions, which leads to a high-spin ground state (S = 10). The ferromagnetic interaction has been tentatively related to distortion of the metal coordination environments after comparing the magnetic and structural data of the known Mn(II) complexes with similar bridging moieties.

Low-Valent Titanocene Products from Attempted Syntheses of Titanocene Bearing Dimethyl(3,3,3-trifluoropropyl)silyl Groups

Reduction of silyl-substituted titanocene dichloride [TiCl2{η5-C5Me4(SiMe2CH2CH2CF3)}2] (1) with one molar equivalent of magnesium afforded a mixture of products, thus precluding the isolation of the possibly formed titanocene [Ti{(η5-C5Me4(SiMe2CH2CH2CF3)}2]. The presence of isolable monochloride [TiCl{η5-C5Me4(SiMe2CH2CH2CF3)}2] (2) in the mixture indicates that the mangnesium is consumed in concurrent reactions, that produce various titanocene compounds of which some were obtained by the reduction of 1 with excess magnesium. Those include the trinuclear TiIII-MgII-TiIII hydride-bridged complex [Ti{η5-C5Me4- (SiMe2CH2CH2CF3)}2(μ-H)2]2Mg (3) and a dimeric dinuclear Ti-Mg complex 4 containing the [TiIII(μ-H)2Mg(μ-X)]2 core where, however, the nature of the bridging moiety X remains unknown. The reduction of 1 with excess magnesium in the presence of bis(trimethylsilyl)ethyne afforded the product of C-H activation [Ti{η5-C5Me4(SiMe2CH2CH2CF3)}- {η5:η1-C5Me3(CH2)(SiMe2CH2CH2CF3)}] (5) in 47% yield. This compound reacted rapidly with tert-butylethyne to give the TiIII-acetylide complex [Ti(η1-C≡CCMe3){η5-C5Me4- (SiMe2CH2CH2CF3)}2] (6). All the reductions of 1 at molar ratios Mg:Ti ≥ 1 gave mixtures, where a good deal of the reduction products remained in the mother liquors unidentified. The structures of 1, 2, 3, 5, and 6 were determined by X-ray diffraction analysis and, for 2, 3, 4, 5, and 6, further corroborated by ESR spectra.

Supramolecular Self-assembly of Dimeric Dendrons with Aromatic Bridge Units

The dimeric dendrons with amide branch units, alkyl periphery, and the aromatic bridging units such as naphthyl, biphenyl, or isopropylidenebiphenyl were synthesized via a convergent method. The self-assembly characteristics of these dimeric dendrons were investigated not only in organic media but also at the solid−liquid interface. The gel formation behavior of the dimeric dendrons in organic media and their nanostructures were highly dependent on the bridging moiety. The dendrons with naphthyl and biphenyl bridges form immobile gel in organic media such as n-octanol, while those with isopropylidenebiphenyl unit did not form gel. The dry gel obtained via self-assembly formed either lamellae or columnar hexagonal arrays depending on the structure of the bridge moiety. The dendron dimer with the 1,5-naphthoxy bridging unit which formed a lamella structure in a dry gel state also self-assembled at the solid−liquid interface in 0.1% n-octanol solution to exhibit well-defined nanoribbons extending to several ...

Synthesis and anomalous magnetic properties of heterometal dinuclear Fe(II)–Cr(III) complex bridged by 3,5-bis(pyridin-2-yl)-pyrazolate

A Cr(III)–Fe(II) dinuclear complex [(nta)Cr(μ-bpypz)Fe(dpea)]BF 4, where bpypz=3,5-bis(pyridin-2-yl)-pyazolate, nta=nitrilotriacetate and dpea=(2-aminoethyl)-bis(2-pyridylmethyl)amine, has been synthesized and structurally characterized by ESI-MS and 2H NMR spectra. The proposed structure is mostly more planar in the bridging moiety as compared to that of the previously reported [(nta)Cr(μ-bpypz)Fe(picen)]BF 4( N, N ′-bis-(2-pyridylmethyl)ethylenediamine(picen)). The dpea complex is found to be a high spin state, but to exhibit no spin-crossover by magnetic susceptibility and Mössbauer spectra measurements; showing a large antiferromagnetic interaction, which could make the spin transition temperature lower and/or destroy spin-crossover, suggesting complementarity between spin-crossover and magnetic interaction.

4-Ethynylpyridine as bridging moiety in mixed Ru/Re complexes

Two ruthenium(II) 4-ethynylpyridine-hydride complexes bearing one Lewis-basic nitrogen atom as coordination site, namely trans-Ru(dppe)2H(CCpy-4) (1) and trans-Ru(dppm)2H(CCpy-4) (2) (dppe=1,2-bis(diphenylphosphino) ethane, dppm=1,2-bis(diphenylphosphino) methane) and Ru(dmpe)2(CCpy-4)2 (3) (dmpe=1,2-bis(dimethylphosphino) ethane) with two nitrogen donor atoms were applied to synthesize the heterobimetallic mixed Ru/Re complexes 4–7. The X-ray crystal structure of the binuclear complex [Re(CO)3(t-bu2bipy)Ru(dppe)2(CCpy-4)H] [OS(O)2CF3] (t-bu2bipy=4, 4′-di (t-butyl)-2,2′-bipyridine, (7)) has been determined. Besides the usual characterization (IR, NMR, UV/Vis, EA) of the compounds 1–7, thermogravimmetry (TG) and cyclovoltammetry (CV) of selected complexes were measured in order to study the interaction between the organometallic building blocks. The existence of long-range Ru⋯Re interactions has been observed.

The synthesis and structure of heteroleptic tris(diimine)ruthenium(II) complexes.

The reactions of bidentate diimine ligands (L2) with cationic bis(diimine)[Ru(L)(L1)(CO)Cl]+ complexes (L, L1, L2 are dissimilar diimine ligands), in the presence of trimethylamine-N-oxide (Me3NO) as a decarbonylation reagent, lead to the formation of heteroleptic tris(diimine) ruthenium(II) complexes, [Ru(L)(L1)(L2)]2+. Typically isolated as hexafluorophosphate or perchlorate salts, these complexes were characterised by UV-visible, infrared and mass spectroscopy, cyclic voltammetry, microanalyses and NMR spectroscopy. Single crystal X-ray studies have elucidated the structures of K[Ru(bpy)(phen)(4,4'-Me(2)bpy)](PF(6))(3).1/2H(2)O, [Ru(bpy)(5,6-Me(2)phen)(Hdpa)](ClO(4))(2), [Ru(bpy)(phen)(5,6-Me(2)phen)](ClO(4))(2), [Ru(bpy)(5,6'-Me(2)phen)(4,4'-Me(2)bpy)](PF(6))(2).EtOH, [Ru(4,4'-Me(2)bpy)(phen)(Hdpa)](PF(6))(2).MeOH and [Ru(bpy)(4,4'-Me(2)bpy)(Hdpa)](ClO(4))(2).1/2Hdpa (where Hdpa is di(2-pyridyl)amine). A novel feature of the first complex is the presence of a dinuclear anionic adduct, [K(2)(PF(6))(6)](4-), in which the two potassium centres are bridged by two fluorides from different hexafluorophosphate ions forming a K(2)F(2) bridging unit and by two KFPFK bridging moieties.

Regioselective Synthesis and Structures of (+)‐Catechin‐Aldehyde Polycondensates

AbstractThe polymerization of (+)‐catechin with various aldehydes has been performed in the presence of an acid catalyst to produce catechin‐aldehyde polycondensates in high yields. NMR analysis of the product showed that the polymerization regioselectively proceeded to form the polymer linked by an ethyl bridge through C‐6 and C‐8 positions of catechin A ring. Specific rotation and CD measurements of the polymer showed the change of chemical bonding and conformation at near chiral center. The direction of specific rotation was changed from dextrorotatory (+) to levorotatory (−) and a new chiral center was formed on the bridge moiety between the catechin units. Computer simulation suggests the coiled interactions in the polymer by hydrogen bonds between hydroxyl groups and pyranic oxygens. magnified image

Syntheses and Properties of Dithia‐tetrahomodiaza‐calix[4]arenes Bridged by Cystine Unit.

Dithia-tetrahomodiaza-calix[4]arenes were synthesized by the cyclization reactions of bis(3-(chloro-methyl)-2-hydroxyphenyl)sulfide with cystine peptides in moderate yields. Conformational analysis of the macrocycles by using nmr spectroscopy reveled that the cyclophanes adopt a cone-like form as a preferable conformation and the cystine bridge moiety is incorporated in the cavity. The calixarene analogs can extract transion metals such as Zn2+ and Cu2+ ions from an aqueous phase into chloroform.

Preparation of siloxane–silsesquioxane hybrid thin films for large‐scale‐integration interlayer dielectrics with excellent mechanical properties and low dielectric constants

AbstractSeveral kinds of homogeneous organic–inorganic hybrid polymer thin films were designed with improved mechanical properties and low dielectric constants (<3.0). Novel soluble siloxane–silsesquioxane hybrid polymers were synthesized with cyclic and/or cage silane monomers, which had triorganosiloxy (R3Si1/2), diorganosiloxane (R2SiO2/2), and organosilsesquioxane (RSiO3/2) moieties with ethylene bridges at the molecular level, by the hydrolysis and condensation of 2,4,6,8‐tetramethyl‐2,4,6,8‐tetra(trimethoxysilylethyl)cyclotetrasiloxane (a cyclic monomer). The electrical properties of these films, including the dielectric constant (∼2.51), leakage current (6.4 × 10−11 A/cm2 at 0.5 MV/cm), and breakdown voltage (∼5.4 MV/cm) were fairly good. Moreover, the mechanical properties of the hybrid films, including the hardness (∼7 GPa), modulus (∼1.2 GPa), and crack‐free thickness (<2 μm), were excellent in comparison with those of previous spin‐on‐glass materials with low dielectric constants. The excellent mechanical properties were proposed to be due to the high contents of SiOH groups (>30%) and the existence of ethylene bridge and siloxane moieties in the hybrid polymer precursors. In addition, the mechanical properties of the hybrid films were affected by the contents of the cagelike structures. The more cagelike structures a hybrid film contained, the worse its mechanical properties were. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 626–634, 2003

Density functional characterization of the chemoselective oxidation of catechol by using molecular oxygen: thermodynamics of the reaction between [(triphos)Ir(dtbc)]+ and O2.

The catalytic mechanism of intra and extra catechol cleaving dioxygenases is still a matter of debate, because the intermediate transient oxygen adduct has never been isolated in these enzymes. The complex [(triphos)Ir(dtbc)](+) (triphos=MeC(CH(2)PPh(2))(3), DBTC(2-)=3,5-di-tert-butylcatecholate), reacts with molecular oxygen and exhibits a ring-cleaving catalytic activity, and a stable oxygen intermediate appears; this stimulates an opportunity for obtaining information that can be transferable to the above-mentioned enzymatic catalytic centers, therefore, giving a breakthrough into the strategy of oxygen activation in these natural systems. Here, we present the results of our computational approach, based on density functional theory, focusing on the following aspects: to explain the interaction with molecular oxygen and how to avoid the existing spin problem, to understand the formation of the bridging moiety, to clarify, if any, the possible influence on the structure of the formed adduct of the bidentate versus monodentate binding of the catechol ligand, and to disclose, eventually, the nature of final decomposition products with the aim to rationalize the intra versus extra cleavage of the catechol molecule itself.

Syntheses and properties of dithia‐tetrahomodiaza‐calix[4]arenes bridged by cystine unit

AbstractDithia‐tetrahomodiaza‐calix[4]arenes were synthesized by the cyclization reactions of bis(3‐(chloro‐methyl)‐2‐hydroxyphenyl)sulfide with cystine peptides in moderate yields. Conformational analysis of the macrocycles by using nmr spectroscopy reveled that the cyclophanes adopt a cone‐like form as a preferable conformation and the cystine bridge moiety is incorporated in the cavity. The calixarene analogs can extract transion metals such as Zn2+ and Cu2+ ions from an aqueous phase into chloroform.

4‐Ethynylpyridine as a Bridging Moiety in Mixed Rh/Re Complexes

AbstractA paramagnetic, dimeric RhIIIRhII complex of the formula [Rh2(form)4(C≡CC5H4N)] (1) [form = N,N′‐di‐p‐tolylformamidinate; C5H4N = pyridyl (py)] has been synthesized and used as starting material to prepare heterotrimetallic complexes of composition [Rh2(form)4(C≡Cpy){Re(CO)3(bipy)}][OS(=O)2CF3] (2) and [Rh2(form)4(C≡Cpy){Re(CO)3(tbu2bipy)}][OS(=O)2CF3] (3). These complexes have been examined by vibrational spectroscopy, EPR spectroscopy, thermogravimetry and cyclic voltammetry. They are heat‐stable and can be handled in air without problems. The observed interactions between the Rh2 and the Re subunits, however, are weak, and a significant charge transfer does not seem to take place. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Characterisation of crystalline C-S-H phases by X-ray photoelectron spectroscopy

We have prepared a number of crystalline calcium-silicate-hydrate (C-S-H) phases hydrothermally, with calcium–silicon ratios varying from approximately 0.5 (K-phase) to 2.0 (hillebrandite and α-dicalcium silicate hydrate). The phases were then analysed using X-ray photoelectron spectroscopy (XPS). Increasing calcium–silicon ratios resulted in decreased silicon binding energies. Additionally, changes in the O 1s spectra could be explained in terms of bridging (BO) and nonbridging oxygen (NBO) moieties. Finally, the modified Auger parameter has proved particularly useful in determining the extent of silicate anion polymerisation. Of note also are the apparently unusual spectra for 11 Å tobermorite. The silicon and oxygen photoelectron spectra indicate a phase with a lower degree of silicate polymerisation than predicted from its composition. The main contributing factor is the intrinsic disorder within the tobermorite structure. This study has shown how XPS may be used to obtain valuable structural information from C-S-H phases, and our analysis of the crystalline phases is the first step towards the analysis of real C-S-H-based cement systems.