bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spectroscopic Properties of Orthometalated Platinum(II) Bipyridine Complexes Containing Various Ethynylaryl Groups
Authors: Clark Meaghan L. , Diring Stéphane , Retailleau Pascal , McMillin David R. , Ziessel Raymond 
Published: July 2008

AbstractNeutral orthometalated platinum(II) complexes of the deprotonated 6‐phenyl‐2,2′‐bipyridine ligand (bearing a trialkoxygallate, tolyl, ethynyltrialkoxygallate, or ethynyltolyl substituent) and a σ‐bonded Cl, ethynyltolyl, or ethynyltrialkoxygallate coligand have been prepared by a stepwise procedure based on copper‐promoted cross‐coupling reactions. The X‐ray structure of the [2‐(p‐tolyl)ethynyl][4‐{2‐(p‐tolyl)ethynyl}‐6‐phenyl‐2,2′‐bipyridyl)]platinum(II) complex revealed a coplanar arrangement of all residues bound to platinum, although the tolylethynyl groups exhibit position‐dependent bending in the solid state. The complexes exhibit charge‐transfer absorption in the visible region. All except two of the complexes also exhibit charge‐transfer emission, typically from an excited state that has a submicrosecond lifetime at room temperature in deoxygenated dichloromethane solution. In accordance with the presence of a carbometalated polypyridine ligand, the emitting state is assumed to have a mixture of metal‐to‐ligand charge‐transfer (MLCT) and intra‐ligand charge‐transfer (ILCT) character. However, spectral comparisons and electrochemical data suggest that the emissive state also exhibits interligand charge‐transfer (LLCT) character when an electron‐rich ethynylaryl group is bound to platinum. In keeping with altered orbital parentage in the latter systems, the emission occurs at longer wavelength. The excited‐state lifetime is also shorter, evidently due to vibronic interactions. The decay is so efficient when an ethynyltrialkoxygallate group binds to platinum that there is no detectable emission in fluid solution, although the complexes do emit in a frozen glass. The excited states are subject to associative (exciplex) quenching by Lewis bases, but the admixture of ILCT and/or LLCT character diminishes efficiency, except for relatively strong bases like dimethyl sulfoxide and dimethylformamide.
Cyclophanes of Perylene Tetracarboxylic Diimide with Different Substituents at Bay Positions
Authors: Feng Junqian , Zhang Yuexing , Zhao Chuntao , Li Renjie , Xu Wei , Li Xiyou , Jiang Jianzhuang 
Published: July 2008

AbstractCyclophanes of perylene tetracarboxylic diimides (PDIs) with different substituents at the bay positions, namely four phenoxy groups at the 1,7‐positions (1), four piperidinyl groups at the 1,7‐positions (2), and eight phenoxy groups at the 1,6,7,12‐positions (3) of the two PDI rings, have been synthesized by the condensation of perylene dianhydride with amine in a dilute solution. These novel cyclophanes were characterized by 1H NMR spectroscopy, MALDI‐TOF mass spectrometry, electronic absorption spectroscopy, and elemental analysis. The conformational isomers of cyclophanes substituted with four piperidinyl groups at the 1,7‐positions (2 a and 2 b) were successfully separated by preparative TLC. The main absorption band of the cyclophanes shifts significantly to the higher energy side in comparison with their monomeric counterparts, which indicates significant π–π interaction between the PDI units in the cyclophanes. Nevertheless, both the electronic absorption and fluorescence spectra of the cyclophanes were found to change along with the number and nature of the side groups at the bay positions of the PDI ring. Time‐dependent DFT calculations on the conformational isomers 2 a and 2 b reproduce well their experimental electronic absorption spectra. Electrochemical studies reveal that the first oxidation and reduction potentials of the PDI ring in the cyclophanes increase significantly compared with those of the corresponding monomeric counterparts, in line with the change in the energy of the HOMO and LUMO according to the theoretical calculations.
Ultrafast Excited State Dynamics Controlling Photochemical Isomerization of N‐Methyl‐4‐[trans‐2‐(4‐pyridyl)ethenyl]pyridinium Coordinated to a {ReI(CO)3(2,2′‐bipyridine)} Chromophore
Authors: Busby Michael , Hartl František , Matousek Pavel , Towrie Mike , Vlček Antonín 
Published: July 2008

AbstractTwo multifunctional photoactive complexes [Re(Cl)(CO)3(MeDpe+)2]2+ and [Re(MeDpe+)(CO)3(bpy)]2+ (MeDpe+=N‐methyl‐4‐[trans‐2‐(4‐pyridyl)ethenyl]pyridinium, bpy=2,2′‐bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet–visible–infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time‐resolved absorption spectroscopy in the visible and infrared regions; and time‐resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about −1.1 V versus Fc/Fc+ and is localized at MeDpe+. Reduction alone does not induce a trans→cis isomerization of MeDpe+. [Re(Cl)(CO)3(MeDpe+)2]2+ is photostable, while [Re(MeDpe+)(CO)3(bpy)]2+ and free MeDpe+ isomerize under near‐UV irradiation. The lowest excited state of [Re(Cl)(CO)3(MeDpe+)2]2+ has been identified as the Re(Cl)(CO)3→MeDpe+ 3MLCT (MLCT=metal‐to‐ligand charge transfer), decaying directly to the ground state with lifetimes of ≈42 (73 %) and ≈430 ps (27 %). Optical excitation of [Re(MeDpe+)(CO)3(bpy)]2+ leads to population of Re(CO)3→MeDpe+ and Re(CO)3→bpy 3MLCT states, from which a MeDpe+ localized intraligand 3ππ* excited state (3IL) is populated with lifetimes of ≈0.6 and ≈10 ps, respectively. The 3IL state undergoes a ≈21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited‐state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe+)(CO)3(bpy)]2+ reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe+)(CO)3(bpy)]2+ emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.
Evaluation of Electron Population Terms for 〈rSe−3〉4p, 〈rS−3〉3p, and 〈rO−3〉2p: How Do HOMO and LUMO Shrink or Expand Depending on Nuclear Charges?
Authors: Nakanishi Waro , Hayashi Satoko , Narahara Kenji , Yamaki Daisuke , Hada Masahiko 
Published: July 2008

AbstractElectron population terms 〈${r{{- 3\hfill \atop N\hfill}}}$〉 are evaluated for N=Se, S, and O. Calculations are performed on HOMO and LUMO constructed by pure atomic 4p(Se), 3p(S), and 2p(O) orbitals, employing the 6‐311+G(3d) and/or 6‐311++G(3df,3pd) basis sets at the HF, MP2, and DFT (B3 LYP) levels. Se4+, Se2+, Se0, and Se2− with the Oh symmetry are called G(A: Se) and HSe+, H2Se, and HSe− with the C∞h or C2v symmetry are named G(B: Se), here [G(A+B: Se) in all]. HOMO and LUMO in G(A+B: N) (N=Se, S, and O) satisfy the conditions of the calculations for 〈${r{{- 3\hfill \atop N\hfill}}}$〉. The 〈${r{{- 3\hfill \atop {\rm Se}\hfill}}}$〉4p, 〈${r{{- 3\hfill \atop {\rm S}\hfill}}}$〉3p, and 〈${r{{- 3\hfill \atop {\rm O}\hfill}}}$〉2p values correlate well with the corresponding MO energies (εN) for all calculation levels employed. Plots of 〈${r{{- 3\hfill \atop N\hfill}}}$〉HOMO and 〈${r{{- 3\hfill \atop N\hfill}}}$〉LUMO versus Q(N) (N=Se, S, and O) at the HF and MP2 levels are analyzed as two correlations. However, the plots at the DFT level can be analyzed as single correlation. A regression curve is assumed for the analysis. Behaviors of 〈${r{{- 3\hfill \atop N\hfill}}}$〉 clarify how valence orbitals shrink or expand depending on Q(N). The applicability of 〈${r{{- 3\hfill \atop N\hfill}}}$〉 is examined to establish a new method that enables us to analyze chemical shifts with the charge effect separately from others. A utility program derived from the Gaussian 03 (NMRANAL‐NH03G) is applied to evaluate 〈${r{{- 3\hfill \atop N\hfill}}}$〉 and examine the applicability to the NMR analysis.
A Rigid, Chiral, Dendronized Polymer with a Thermally Stable, Right‐Handed Helical Conformation
Authors: Zhang Afang , Rodríguez‐Ropero Francisco , Zanuy David , Alemán Carlos , Meijer E. W. , Schlüter A. Dieter 
Published: July 2008

AbstractFirst‐ and second‐generation dendronized polymethacrylates PG1 and PG2 carrying chiral 4‐aminoproline‐based dendrons were obtained on the half‐gram scale in high molar masses (PG1: Mn=5×106 g mol−1, PG2: Mn=1×106 g mol−1) by spontaneous (radical) polymerization of the corresponding vinyl macromonomers. NMR spectroscopic studies on PG2 together with its unprecedented high glass transition temperature (Tg>200 °C, decomp) and structural parameters provided by atomistic MD simulations show this polymer to be rather rigid. Optical rotation and CD measurements revealed that PG2 adopts a helical conformation that remains unchanged over wide ranges of temperature and solvent polarity. It is also retained when the polymer is deprotected (and thus positively charged, de‐PG2) at its terminal amino groups, by which the mass and steric demand of the dendrons is reduced by roughly 50 %. Molecular dynamics simulations on models of PG2 reveal its helical conformation to be right‐handed, irrespective of backbone tacticity, and initial results also indicate that de‐PG2 retains the right‐handedness.
Laves Phases, γ‐Brass, and 2×2×2 Superstructures: A New Class of Quasicrystal Approximants and the Suggestion of a New Quasicrystal
Authors: Berger Robert F. , Lee Stephen , Johnson Jeffreys , Nebgen Ben , So Adrian Chi‐Yau 
Published: July 2008

AbstractOf the most common cubic intermetallic structure types, several (MgCu2, Cu5Zn8, Ti2Ni, and α‐Mn) have superstructures with unusual symmetry properties. These superstructures (Be5Au, Li21Si5, Sm11Cd45, and Mg44Ir7) have the unusual property of pairs of perpendicular pseudo fivefold axes, most apparent in their X‐ray diffraction patterns. The current work shows that an 8D to 3D projection method cleanly describes most (and in one case, all) of the atomic positions in the four superstructures mentioned above. This type of projection, which maps the E8 lattice (a mathematically simple 8D crystal) into 3D space, combines the desired higher dimensional point group's perpendicular fivefold rotations with 3D translational symmetry—exactly what we see in the experimental crystal structures. The projection method successfully accounts for all heavy atom positions in the four superstructures, and at least 60–70 % of the light atom positions. The results suggest that all of these structures, previously known to be connected only by qualitative similarities in their atomic “clusters”, are approximants of a single, as‐yet unknown, class of quasicrystal.
Oriented Immobilization of a Fully Active Monolayer of Histidine‐Tagged Recombinant Laccase on Modified Gold Electrodes
Authors: Balland Véronique , Hureau Christelle , Cusano Angela Maria , Liu Yingli , Tron Thierry , Limoges Benoît 
Published: July 2008

AbstractThe formation of a dense monolayer of histidine‐tagged recombinant laccase on gold electrodes by using a short thiol‐NTA linker is described, as well as a kinetic analysis of the process by cyclic voltammetry. From a detailed analysis of the catalytic reduction of dioxygen by laccase in the presence of a one‐electron redox mediator it can be concluded that the immobilized enzyme remains as active as in homogeneous solution.
Ligand‐Free Sonogashira Coupling Reactions with Heterogeneous Pd/C as the Catalyst
Authors: Mori Shigeki , Yanase Takayoshi , Aoyagi Satoka , Monguchi Yasunari , Maegawa Tomohiro , Sajiki Hironao 
Published: July 2008

AbstractA variety of aryl iodides were coupled with aromatic and aliphatic terminal alkynes to give the corresponding 1,2‐disubstituted aromatic alkynes in good yields by using only 0.4 mol % of the heterogeneous 10 % Pd/C as the catalyst without a ligand, copper salt, or amine in an aqueous medium.
Non‐natural Amino Acids as Modulating Agents of the Conformational Space of Model Glycopeptides
Authors: Fernández‐Tejada Alberto , Corzana Francisco , Busto Jesús H. , Jiménez‐Osés Gonzalo , Peregrina Jesús M. , Avenoza Alberto 
Published: July 2008

AbstractThe synthesis and conformational analysis in aqueous solution of different α‐methyl‐α‐amino acid diamides, derived from serine, threonine, β‐hydroxycyclobutane‐α‐amino acids, and their corresponding model β‐O‐glucopeptides, are reported. The study reveals that the presence of an α‐methyl group forces the model peptides to adopt helix‐like conformations. These folded conformations are especially significant for cyclobutane derivatives. Interestingly, this feature was also observed in the corresponding model glucopeptides, thus indicating that the α‐methyl group and not the β‐O‐glucosylation process largely determines the conformational preference of the backbone in these structures. On the other hand, atypical conformations of the glycosidic linkage were experimentally determined. Therefore, when a methyl group was located at the Cβ atom with an R configuration, the glycosidic linkage was rather rigid. Nevertheless, when the S configuration was displayed, a significant degree of flexibility was observed for the glycosidic linkage, thus showing both alternate and eclipsed conformations of the ψs dihedral angle. In addition, some derivatives exhibited an unusual value for the ϕs angle, which was far from a value of −60° expected for a conventional β‐O‐glycosidic linkage. In this sense, the different conformations exhibited by these molecules could be a useful tool in obtaining systems with conformational preferences “à la carte”.
Half‐Sandwich Bis(tetramethylaluminate) Complexes of the Rare‐Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization
Authors: Zimmermann Melanie , Törnroos Karl W. , Sitzmann Helmut , Anwander Reiner 
Published: July 2008

AbstractThe protonolysis reaction of [Ln(AlMe4)3] with various substituted cyclopentadienyl derivatives HCpR gives access to a series of half‐sandwich complexes [Ln(AlMe4)2(CpR)]. Whereas bis(tetramethylaluminate) complexes with [1,3‐(Me3Si)2C5H3] and [C5Me4SiMe3] ancillary ligands form easily at ambient temperature for the entire LnIII cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4‐(Me3C)3C5H3] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X‐ray structure analyses of seven representative complexes of the type [Ln(AlMe4)2(CpR)] reveal a similar distinct [AlMe4] coordination (one η2, one bent η2). Treatment with Me2AlCl leads to [AlMe4] → [Cl] exchange and, depending on the Al/Ln ratio and the CpR ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe4)(μ‐Cl)(CpR)}2] and [{Ln(μ‐Cl)2(CpR)}n], respectively, have been identified. Complexes [{Y(AlMe4)(μ‐Cl)(C5Me4SiMe3)}2] and [{Nd(AlMe4)(μ‐Cl){1,2,4‐(Me3C)3C5H2}}2] have been characterized by X‐ray structure analysis. All of the chlorinated half‐sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe4)2(CpR)] with boron‐containing cocatalysts, such as [Ph3C][B(C6F5)4], [PhNMe2H][B(C6F5)4], or B(C6F5)3, produces initiators for the fabrication of trans‐1,4‐polyisoprene. The choice of rare‐earth metal cation size, CpR ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe4)2(C5Me4SiMe3)]/B(C6F5)3 (trans‐1,4 content: 95.6 %, Mw/Mn=1.26) and [La(AlMe4)2(C5Me5)]/B(C6F5)3 (trans‐1,4 content: 99.5 %, Mw/Mn=1.18).
Enhanced Cooperativity through Design: Pendant CoIIISalen Polymer Brush Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin (Salen=N,N′‐Bis(salicylidene)ethylenediamine Dianion)
Authors: Gill Christopher S. , Venkatasubbaiah Krishnan , Phan Nam T. S. , Weck Marcus , Jones Christopher W. 
Published: July 2008

AbstractThe CoIIIsalen‐catalyzed (salen=N,N′‐bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C3 building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant CoIIIsalen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site–site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer‐based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Cosalen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.
Analysis of Linear Free‐Energy Relationships Combined with Activation Parameters Assigns a Concerted Mechanism to Alkaline Hydrolysis of X‐Substituted Phenyl Diphenylphosphinates
Authors: Um Ik‐Hwan , Han Jeong‐Yoon , Hwang So‐Jeong 
Published: July 2008

AbstractA kinetic study is reported for alkaline hydrolysis of X‐substituted phenyl diphenylphosphinates (1 a–i). The Brønsted‐type plot for the reactions of 1 a–i is linear over 4.5 pKa units with βlg=−0.49, a typical βlg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σo and σ− constants are linear but exhibit many scattered points, while the corresponding Yukawa–Tsuno plot results in excellent linear correlation with ρ=1.42 and r=0.35. The r value of 0.35 implies that leaving‐group departure is partially advanced at the rate‐determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO− ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (DN + AN) mechanism is also ruled out on the basis of the small βlg value. As the substituent X in the leaving group changes from H to 4‐NO2 and 3,4‐(NO2)2, ΔH ≠ decreases from 11.3 kcal mol−1 to 9.7 and 8.7 kcal mol−1, respectively, while ΔS ≠ varies from −22.6 cal mol−1 K−1 to −21.4 and −20.2 cal mol−1 K−1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1 a–i.
A Multilateral Mechanistic Study into Asymmetric Transfer Hydrogenation in Water
Authors: Wu Xiaofeng , Liu Jianke , Di Tommaso Devis , Iggo Jonathan A. , Catlow C. Richard A. , Bacsa John , Xiao Jianliang 
Published: July 2008

AbstractThe mechanism of aqueous‐phase asymmetric transfer hydrogenation (ATH) of acetophenone (acp) with HCOONa catalyzed by Ru‐TsDPEN has been investigated by stoichiometric reactions, NMR probing, kinetic and isotope effect measurements, DFT modeling, and X‐ray structure analysis. The chloride [RuCl(TsDPEN)(p‐cymene)] (1), hydride [RuH(TsDPEN)(p‐cymene)] (3), and the 16‐electorn species [Ru(TsDPEN‐H)(p‐cymene)] (4) were shown to be involved in the aqueous ATH, with 1 being the precatalyst, and 3 as the active catalyst detectable by NMR in both stoichiometric and catalytic reactions. The formato complex [Ru(OCOH)(TsDPEN)(p‐cymene)] (2) was not observed; its existence, however, was demonstrated by its reversible decarboxylation to form 3. Both 1 and 3 were protonated under acidic conditions, leading to ring opening of the TsDPEN ligand. 4 reacted with water, affording a hydroxyl species. In a homogeneous DMF/H2O solvent, the ATH was found to be first order in the concentration of catalyst and acp, and inhibited by CO2. In conjunction with the NMR results, this suggests that hydrogen transfer to ketone is the rate‐determining step. The addition of water stabilized the ruthenium catalyst and accelerated the ATH reaction; it does so by participating in the catalytic cycle. DFT calculations revealed that water hydrogen bonds to the ketone oxygen at the transition state of hydrogen transfer, lowering the energy barrier by about 4 kcal mol−1. The calculations also suggested that the hydrogen transfer is more step‐wise in nature rather than concerted. This is supported to some degree by the kinetic isotope effects, which were obscured by extensive H/D scrambling.
Belizeanic Acid: A Potent Protein Phosphatase 1 Inhibitor Belonging to the Okadaic Acid Class, with an Unusual Skeleton
Authors: Cruz Patricia G. , Fernández José Javier , Norte Manuel , Daranas Antonio Hernández 
Published: July 2008

AbstractBelizeanic acid (BA), a novel metabolite belonging to the okadaic acid class of protein phosphatase inhibitors, was isolated from artificial cultures of the dinoflagellate Prorocentrum belizeanum. The structure and conformational behaviour of BA was elucidated by a combination of NMR spectroscopy and conformational analysis. The isolation of this metabolite, which possesses a simplified version of the okadaic acid skeleton, supports the biogenetic pathway previously reported for this class of compounds. BA showed potent inhibitory activity against protein phosphatase 1 (PP1) within the nM range. A plausible model for the interaction of BA with the PP1 binding pocket was derived from computational docking studies.
Rigid–Flexible Block Molecules Based on a Laterally Extended Aromatic Segment: Hierarchical Assembly into Single Fibers, Flat Ribbons, and Twisted Ribbons
Authors: Lee Eunji , Huang Zhegang , Ryu Ja‐Hyoung , Lee Myongsoo 
Published: July 2008

AbstractSelf‐assembling rigid–flexible block molecules consisting of a laterally extended aromatic segment and different lengths of hydrophilic coils were synthesized and characterized. The block molecule based on a long poly(ethylene oxide) coil (1), in the melt state, shows an unidentified columnar structure, whereas the molecule with a shorter poly(ethylene oxide) coil (2) self‐organizes into an oblique columnar structure. Further decrease in the poly(ethylene oxide) coil length as in the case of 3, on heating, induces a rectangular columnar structure in addition to an oblique columnar mesophase. In diethyl ether, 1 and 2 were observed to self‐assemble into uniform nanofibers with bilayer packing. Remarkably, these elementary fibers were observed to further aggregate in a lateral way to form well‐defined flat ribbons (1) and twisted ribbons (2) with solvent exchange of diethyl ether into methanol. Furthermore, the ribbons formed in methanol dissociated into elementary fibers in response to the addition of aromatic guest molecules. This transformation between ribbons and single fibers in response to the addition of guest molecules is attributed to the intercalation of aromatic substrates within the rigid segments and subsequent loosening of the aromatic stacking interactions. These results demonstrate that the introduction of a laterally extended aromatic segment into an amphiphilic molecular architecture can lead to the hierarchical formation from elementary fibers of nanoribbons with a tunable twist through controlled lateral interactions between aromatic segments.