bluuebunny/crossref_metadata_2025_split huggingface dataset:
Vibrational and Quantum‐Chemical Study of Push–Pull Chromophores for Second‐Order Nonlinear Optics from Rigidified Thiophene‐Based π‐Conjugating Spacers
Authors: Ruiz Delgado Mari Carmen , Hernández Víctor , Casado Juan , López Navarrete Juan T. , Raimundo Jean‐Manuel , Blanchard Philippe , Roncali Jean 
Published: July 2003

AbstractTwo types of push–pull chromophores built around thiophene‐based π‐conjugating spacers rigidified by either covalent bonds or noncovalent intramolecular interactions have been analysed by means of IR and Raman spectroscopical measurements in the solid state as well as in a variety of solvents. Comparison of the Raman features of NLO‐phores based on a covalently rigidified dithienylene (DTE) spacer with those of their open chain DTE analogues shows that the bridging of the central double bond of DTE with the nearest β‐positions of the thienyl units through two ethylene bridges significantly improves the intramolecular charge transfer. This also occurs for NLO‐phores based on a 2,2′‐bi(3,4‐ethylenedioxythiophene) (BEDOT) spacer as compared with their corresponding parent compounds based on an unsubstituted bithiophene (BT) spacer. For NLO‐phores based on a BEDOT spacer, noncovalent intramolecular interactions between sulfur and oxygen atoms are responsible for the rigidification of the spacer. The Raman spectra of these NLO‐phores obtained in the form of solutes in dilute solutions reveal two different behaviours: i) chromophores based on covalently bridged or open chain DTE spacers display Raman spectral profiles in solution quite similar to those of the corresponding solids, with a very little dependence on the polarity of the solvent, while ii) larger spectral changes are noticed for NLO‐phores built around BEDOT or BT spacers on going from solids to solutions. In the second case, spectral changes must be ascribed not solely to conformational distortions of the donor and acceptor end groups with respect to the π‐conjugated backbone mean‐square‐plane (as for the DTE‐based NLO‐phores) but also to distortions of the thienyl units of the π‐conjugating spacer from coplanarity. The insertion of vinylenic bridges between the thienyl units of the π‐conjugating spacer and between the spacer and the donor and acceptor end groups is a suitable strategy to reach a fairly large intramolecular charge transfer both in polar and nonpolar solvents. Density functional theory (DFT) calculations have been carried out to assign the relevant electronic and vibrational features and to derive useful information about the molecular structure of these NLO‐phores.
Stabilization of CoI by ZnII in Pure Acetonitrile and its Reaction with Aryl Halides
Authors: Seka Sylvaine , Buriez Olivier , Périchon Jacques 
Published: July 2003

AbstractThe study of the electrochemical behavior of cobalt(II) bromide (CoBr2) in pure acetonitrile allowed us to demonstrate that Co2+ is the catalyst precursor involved in the electrochemical and chemical conversions of arylhalides, ArX, to arylzinc compounds in that solvent. The reduction of Co2+ leads to the Co+ species, which disproportionates too rapidly to react further with aryl halides. However, the presence of zinc(II) bromide allows us to stabilize the electrogenerated cobalt(I) and to observe it on the timescale of slow cyclic voltammetry. Under such conditions, the CoI species has time to react with aryl halides and produce [CoIIIArX]+ complexes that are reduced into [CoIIArX] by a single electron uptake at the same potential at which Co2+ is reduced. Rate constants for the oxidative addition of ArX to CoI have been determined for various aryl halides and compared to the values obtained in an acetonitrile (ACN)/pyridine (9:1, v/v) mixture. It is shown that CoI is stabilized more by ZnBr2 than by pyridine. A transmetallation reaction between [CoIIArX] and ZnBr2 has also been observed. We finally propose a mechanism for the cobalt‐catalyzed electrochemical conversion of aryl bromides into organozinc species in pure acetonitrile.
GdIII Complexes with Fast Water Exchange and High Thermodynamic Stability: Potential Building Blocks for High‐Relaxivity MRI Contrast Agents
Authors: Laus Sabrina , Ruloff Robert , Tóth Éva , Merbach André E. 
Published: July 2003

AbstractOn the basis of structural considerations in the inner sphere of nine‐coordinate, monohydrated GdIII poly(aminocarboxylate) complexes, we succeeded in accelerating the water exchange by inducing steric compression around the water binding site. We modified the common DTPA5− ligand (DTPA=(diethylenetriamine‐N,N,N′,N″,N″‐pentaacetic acid) by replacing one (EPTPA5−) or two (DPTPA5−) ethylene bridges of the backbone by propylene bridges, or one coordinating acetate by a propionate arm (DTTA‐prop5−). The ligand EPTPA5− was additionally functionalized with a nitrobenzyl linker group (EPTPA‐bz‐NO25−) to allow for coupling of the chelate to macromolecules. The water exchange rate, determined from a combined variable‐temperature 17O NMR and EPR study, is two orders of magnitude higher on [Gd(eptpa‐bz‐NO2)(H2O)]2− and [Gd(eptpa)(H2O)]2− than on [Gd(dtpa)(H2O)]2− (k${_{{\rm ex}}^{\,298}}$=150×106, 330×106, and 3.3×106 s−1, respectively). This is optimal for attaining maximum proton relaxivities for GdIII‐based, macrocyclic MRI contrast agents. The activation volume of the water exchange, measured by variable‐pressure 17O NMR spectroscopy, evidences a dissociative interchange mechanism for [Gd(eptpa)(H2O)]2− (ΔV ≠=(+6.6±1.0) cm3 mol−1). In contrast to [Gd(eptpa)(H2O)]2−, an interchange mechanism is proved for the macrocyclic [Gd(trita)(H2O)]− (ΔV ≠=(−1.5±1.0) cm3 mol−1), which has one more CH2 group in the macrocycle than the commercial MRI contrast agent [Gd(dota)(H2O)]−, and for which the elongation of the amine backbone also resulted in a remarkably fast water exchange. When one acetate of DTPA5− is substituted by a propionate, the water exchange rate on the GdIII complex increases by a factor of 10 (k${_{{\rm ex}}^{\,298}}$=31×106 s−1). The [Gd(dptpa)]2− chelate has no inner‐sphere water molecule. The protonation constants of the EPTPA‐bz‐NO25− and DPTPA5− ligands and the stability constants of their complexes with GdIII, ZnII, CuII and CaII were determined by pH potentiometry. Although the thermodynamic stability of [Gd(eptpa‐bz‐NO2)(H2O)]2− is reduced to a slight extent in comparison with [Gd(dtpa)(H2O)]2−, it is stable enough to be used in medical diagnostics as an MRI contrast agent. Therefore both this chelate and [Gd(trita)(H2O)]− are potential building blocks for the development of high‐relaxivity macromolecular agents.
From Supramolecular Block Copolymers to Advanced Nano‐Objects
Authors: Gohy Jean‐François , Lohmeijer Bas G. G. , Schubert Ulrich S. 
Published: July 2003

AbstractThe formation of asymmetric bis‐complexes, based on terpyridine ligands and ruthenium ions, is described as a powerful tool for the self‐assembly of polymer blocks end‐functionalized with terpyridine units. This is illustrated in this contribution for the synthesis of amphiphilic metallo‐supramolecular block copolymers, which are further used to produce aqueous micelles. Finally, the reversibility of the supramolecular bond opens new avenues for the preparation and manipulation of these nano‐objects.
Synthesis and Properties of the First Stable Silylene–Isocyanide Complexes
Authors: Takeda Nobuhiro , Kajiwara Takashi , Suzuki Hiroyuki , Okazaki Renji , Tokitoh Norihiro 
Published: July 2003

AbstractThe first stable silylene–isocyanide complexes, [Tbt(Mes)SiCNAr] (5 c: Ar=Tip, 5 d: Ar=Tbt, 5 e: Ar=Mes*; Tbt=2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl, Mes=mesityl, Tip=2,4,6‐triisopropylphenyl, Mes*=2,4,6‐tri‐tert‐butylphenyl) were successfully synthesized by the reaction of a kinetically stabilized disilene, [Tbt(Mes)SiSi(Mes)Tbt] (1), with bulky isocyanides, ArNC (3c–e). The spectroscopic data of 5 c–e and theoretical calculations for a model molecule indicated that 5 c–e are not classical cumulative compounds but the first stable silylene–Lewis base complexes. The reactions of 5 c–e with triethylsilane and 1,3‐dienes gave the corresponding silylene adducts, and they underwent isocyanide‐exchange reactions in the presence of another isocyanide at room temperature. These results indicate dissociation of complexes 5 c–e to the corresponding silylene 2 and isocyanides 3 c–e under very mild conditions. The reaction of 5 c with methanol gave the MeOH adduct 16, [Tbt(Mes)SiHC(OMe)NTip], which has a hydrogen atom on the silicon atom. This regioselectivity can be explained in terms of the contribution of zwitterionic resonance structures D and E, which have an anion on the silicon atom. This result indicates that 5 c is not a classical cumulene having SiC double bonds that should react with methanol to give adducts bearing a methoxyl group on the silicon atom. Although the reactions of 5 c–e with electrophilic reagents such as methanol, hydrogen chloride, and methyl iodide gave the formal silylene adducts, the studies on the reaction mechanism by trapping experiments and the observation of the intermediate suggested that the reaction mainly or partially proceeds by initial nucleophilic attack of the silicon atom, as is the case in the formation of 16 in the reaction of 5 c with methanol. It was revealed that 5 c–e show the nucleophilicity of the silicon atom, most likely resulting from the contribution of the zwitterionic resonance structures D and E.
Synthesis and Reactivity of Calix[4]arene‐Supported Group 4 Imido Complexes
Authors: Dubberley Stuart R. , Friedrich Andreas , Willman David A. , Mountford Philip , Radius Udo 
Published: July 2003

AbstractNew mononuclear titanium and zirconium imido complexes [M(NR)(R′2calix)] [M=Ti, R′=Me, R=tBu (1), R=2,6‐C6H3Me2 (2), R=2,6‐C6H3iPr2 (3), R=2,4,6‐C6H2Me3 (4); M=Ti, R′=Bz, R=tBu (5), R=2,6‐C6H3Me2 (6), R=2,6‐C6H3iPr2 (7); M=Zr, R′=Me, R=2,6‐C6H3iPr2 (8)] supported by 1,3‐diorganyl ether p‐tert‐butylcalix[4]arenes (R′2calix) were prepared in good yield from the readily available complexes [MCl2(Me2calix)], [Ti(NR)Cl2(py)3], and [Ti(NR)Cl2(NHMe2)2]. The crystallographically characterised complex [Ti(NtBu)(Me2calix)] (1) reacts readily with CO2, CS2, and p‐tolyl‐isocyanate to give the isolated complexes [Ti{N(tBu)C(O)O}(Me2calix)] (10), [{Ti(μ‐O)(Me2calix)}2] (11), [{Ti(μ‐S)(Me2calix)}2] (12), and [Ti{N(tBu)C(O)N(‐4‐C6H4Me)}(Me2calix)] (13). In the case of CO2 and CS2, the addition of the heterocumulene to the TiN multiple bond is followed by a cycloreversion reaction to give the dinuclear complexes 11 and 12. The X‐ray structure of 13⋅4(C7H8) clearly establishes the N,N′‐coordination mode of the ureate ligand in this compound. Complex 1 undergoes tert‐butyl/arylamine exchange reactions to form 2, 3, [Ti(N‐4‐C6H4Me)(Me2calix)] (14), [Ti(N‐4‐C6H4Fc)(Me2calix)] (15) [Fc=Fe(η5‐C5H5)(η5‐C5H4)], and [{Ti(Me2calix)}2{μ‐(N‐4‐C6H4)2CH2}] (16). Reaction of 1 with H2O, H2S and HCl afforded the compounds [{Ti(μ‐O)(Me2calix)}2] (11), [{Ti(μ‐S)(Me2calix)}2] (12), and [TiCl2(Me2calix)] in excellent yields. Furthermore, treatment of 1 with two equivalents of phenols results in the formation of [Ti(O‐4‐C6H4R)2(Me2calix)] (R=Me 17 or tBu 18), [Ti(O‐2,6‐C6H3Me2)2(Me2calix)] (19) and [Ti(mbmp)(Me2calix)] (20; H2mbmp=2,2′‐methylene‐bis(4‐methyl‐6‐tert‐butylphenol) or CH2({CH3}{C4H9}C6H2‐OH)2). The bis(phenolate) compounds 17 and 18 with para‐substituted phenolate ligands undergo elimination and/or rearrangement reactions in the nonpolar solvents pentane or hexane. The metal‐containing products of the elimination reactions are dinuclear complexes [{Ti(O‐4‐C6H4R)(Mecalix)}2] [R=Me (23) or tBu (24)] where Mecalix=monomethyl ether of p‐tert‐butylcalix[4]arene. The products of the rearrangement reaction are [Ti(O‐4‐C6H4Me)2 (paco‐Me2calix)] (25) and [Ti(O‐4‐C6H4tBu)2(paco‐Me2calix)] (26), in which the metallated calix[4]arene ligand is coordinated in a form reminiscent of the partial cone (paco) conformation of calix[4]arene. In these compounds, one of the methoxy groups is located inside the cavity of the calix[4]arene ligand. The complexes 24, 25 and 26 have been crystallographically characterised. Complexes with sterically more demanding phenolate ligands, namely 19 and 20 and the analogous zirconium complexes [Zr(O‐4‐C6H4Me)2(Me2calix)] (21) and [Zr(O‐2,6‐C6H3Me2)2(Me2calix)] (22) do not rearrange. Density functional calculations for the model complexes [M(OC6H5)2(Me2calix)] with the calixarene possessing either cone or partial cone conformations are briefly presented.
Electron Delocalization in Cross‐Conjugated p‐Phenylenevinylidene Oligomers
Authors: Klokkenburg Mark , Lutz Martin , Spek Anthony L. , van der Maas John H. , van Walree Cornelis A. 
Published: July 2003

AbstractThe synthesis, structure, and electronic properties of a series of cross‐conjugated p‐phenylenevinylidene oligomers with one to four double bonds are reported. The X‐ray crystal structure of the compound with two double bonds reveals a nonplanar conformation with torsion angles about the C(phenylene)–C(vinylidene) and C(phenyl)–C(vinylidene) formal single bonds of 39.5(2)° and 30.5(2)°, respectively. Admixture of quinoid character in the ground state is observed. Infrared and Raman spectroscopy do not provide a clear picture of the degree of electron delocalization in the series, since the CC stretching mode does not adequately reflect the CC bond order and has a local nature. In contrast, electronic spectra and electrochemical data, as well as AM1 and PPP/SCF calculations, reveal that the cross‐conjugated compounds basically behave as linearly π‐conjugated systems in the sense that molecular orbitals are delocalized over the entire structure and systematically change in energy. The electronic interaction between the repeating units is, however, not very strong, which has the consequence that spatial extension of the molecular orbitals does not lead to a red shift of the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) electronic transition. This is related to the feature that the modest narrowing of the HOMO–LUMO gap with the chain length is accompanied by a relatively large reduction of electron repulsion. This finding implies that care should be taken in the use of electronic spectra for the evaluation of conjugation phenomena.
High‐Pressure Preparation, Crystal Structure, and Properties of α‐(RE)2B4O9 (RE=Eu, Gd, Tb, Dy): Oxoborates Displaying a New Type of Structure with Edge‐Sharing BO4 Tetrahedra
Authors: Emme Holger , Huppertz Hubert 
Published: July 2003

AbstractHigh‐pressure/high‐temperature conditions of 10 GPa and 1150 °C were used to synthesize the new rare‐earth oxoborates α‐(RE)2B4O9 (RE=Eu, Gd, Tb, Dy) in a Walker‐type multianvil apparatus. Single‐crystal X‐ray structure determination of α‐(RE)2B4O9 (RE=Eu, Gd, Tb) revealed: C2/c, Z=20, α‐Eu2B4O9: a=2547.9(5), b=444.3(1), c=2493.8(5) pm, β=99.82(3)°, R1=0.0277, wR2=0.0693 (all data); α‐Gd2B4O9: a=2539.0(1), b=443.3(1), c=2490.8(1) pm, β=99.88(1)°, R1=0.0457, wR2=0.0643 (all data); α‐Tb2B4O9: a=2529.4(1), b=441.6(1), c=2484.3(1) pm, β=99.88(1)°, R1=0.0474, wR2=0.0543 (all data). The isotypic compounds exhibit a new type of structure that is built up of BO4 tetrahedra to form a network that incorporates the rare‐earth cations. The most important feature is the existence of the new structural motif of edge‐sharing BO4 tetrahedra next to the known motif of corner‐sharing BO4 tetrahedra, which is realized in the presented compounds α‐(RE)2B4O9 (RE=Eu, Gd, Tb, Dy) for the second time. Furthermore, we report the temperature‐resolved in‐situ powder‐diffraction measurements, DTA, IR/Raman spectroscopic investigations, and magnetic properties of the new compounds.
Comparison of Side‐On and End‐On Coordination of E2 Ligands in Complexes [W(CO)5E2] (E=N, P, As, Sb, Bi, Si−, Ge−, Sn−, Pb−)
Authors: Esterhuysen Catharine , Frenking Gernot 
Published: July 2003

AbstractComplexes of W(CO)5 with neutral diatomic pnictogen ligands N2, P2, As2, Sb2, and Bi2 and anionic Group 14 ligands Si22−, Ge22−, Sn22−, and Pb22− coordinated in both side‐on and end‐on fashion have been optimized by using density functional theory at the BP86 level with valence sets of TZP quality. The calculated bond energies have been used to compare the preferential binding modes of each respective ligand. The results were interpreted by analyzing the nature of the interaction between the ligands and the metal fragment using an energy partitioning method. This yields quantitative information regarding the strength of covalent and electrostatic interactions between the metal and ligand, as well as the contributions by orbitals of different symmetry to the covalent bonding. Results show that all the ligands studied bind preferentially in a side‐on coordination mode, with the exception of N2, which prefers to coordinate in an end‐on mode. The preference of the heavier homologues P2–Bi2 for binding in a side‐on mode over the end‐on mode in the neutral complexes [(CO)5WE2] comes mainly from the much stronger electrostatic attraction in the former species. The energy difference between the side‐on and end‐on isomers of the negatively charged complexes with the ligands Si22−, Ge22−, Sn22−, and Pb22− is much less and it cannot be ascribed to a particular bonding component.
N‐Heterocyclic Carbene Functionalized Iridium Phosphinidene Complex [Cp*(NHC)IrPMes*]: Comparison of Phosphinidene, Imido, and Carbene Complexes
Authors: Termaten Arjan T. , Schakel Marius , Ehlers Andreas W. , Lutz Martin , Spek Anthony L. , Lammertsma Koop 
Published: July 2003

AbstractThe novel phosphinidene complex [Cp*(NHC)IrPMes*] (3; NHC=1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene) was prepared in high yield from [Cp*(NHC)IrCl2] (2) and [LiPHMes*]⋅3 THF. It represents the first example of an NHC ligated transition metal phosphinidene complex. The X‐ray crystal structure for 3 is also reported. DFT calculations on the N‐heterocyclic carbene containing parent complexes [Cp(NHC)IrE] (E=PH, NH, CH2) show that the NHC ligand acts as good σ‐donor/weak π‐acceptor ligand and forms strong IrC(NHC) single bonds. The IrE double bonds result from strong triplet–triplet interactions between [Cp(NHC)Ir] and E.
Covalent Chemistry and Conformational Dynamics of Topologically Chiral Amide‐Based Molecular Knots
Authors: Lukin Oleg , Müller Walter M. , Müller Ute , Kaufmann Astrid , Schmidt Claus , Leszczynski Jerzy , Vögtle Fritz 
Published: July 2003

AbstractThe readily available in gram quantities tris(allyloxy)knot of the amide‐type 5 (knotane) can be completely and partially deprotected with nBu3SnH in the presence of a palladium catalyst resulting in hydroxyknotanes 7–9. These, in turn, react with diethylchlorophosphate giving rise to knotanes equipped with between one and three phosphoryl groups. Sulfonylation of bis(allyloxy)monohydroxyknotane 8 with p‐toluenesulfonyl chloride and, following removal of one or two allyl groups from the intermediate monosulfonate 13, give rise to sulfonyloxy‐allyloxy‐hydroxy‐ and sulfonyloxy‐dihydroxy‐knotanes 15 and 14, respectively. This provides a convenient method for the preparation of knotanes with any substitution pattern. All new knotanes have been isolated in preparative amounts and as highly pure substances with an exception of allyloxy‐dihydroxyknotane 9. This compound could only be obtained as a mixture with the corresponding monohydroxy‐derivative 8. The structures of all synthesized compounds were established by means of FAB and MALDI TOF mass spectrometry, 1H and 31P NMR spectroscopy. The triphosphorylated knotane 10 exhibits high solubility in alcohols, allowing its complete enantiomeric resolution with a commercially available chiral HPLC column. 1H,1H DQF‐COSY correlation spectroscopy along with H/D exchange experiments and ab initio calculations provided the first detailed 1H NMR signal assignments of knotanes in [D6]DMSO solution. The combination of variable temperature 1H and 31P NMR spectroscopy and molecular modeling has been applied to study the conformational behavior of the new knotanes in different solvents. It has been shown that in DMSO solution at room temperature knotanes exist in a relatively rigid nonsymmetrical conformation similar to that found in the solid state while faster conformational exchange leading to the average D3 symmetrical structure was detected in a number of other solvents.
Lipopeptaibol Metabolites of Tolypocladium geodes: Total Synthesis, Preferred Conformation, and Membrane Activity
Authors: Rainaldi Mario , Moretto Alessandro , Peggion Cristina , Formaggio Fernando , Mammi Stefano , Peggion Evaristo , Galvez José Antonio , Díaz‐de‐Villegas Maria Dolores , Cativiela Carlos , Toniolo Claudio 
Published: July 2003

AbstractWe have synthesized by solution methods and characterized the lipopeptaibol metabolite LP237‐F8 extracted from the fungus Tolypocladium geodes and five selected analogues with the Etn→Aib or Etn→Nva replacement at position 8 and/or a triple Gln→Glu(OMe) replacement at positions 5, 6, and 9 (Etn=Cα‐ethylnorvaline, Aib=α‐aminoisobutyric acid, Nva=norvaline). Conformation analysis, performed by FT‐IR absorption, NMR, and CD techniques, strongly supports the view that the six terminally blocked decapeptides are highly helical in solution. Helix topology and amphiphilic character are responsible for their remarkable membrane activity. At position 8 the combination of high hydrophobicity and Cα tetrasubstitution, as in the Etn‐containing LP237‐F8 metabolite, has a positive effect on membrane interaction.
Cofactor Recycling Mechanism in Asymmetric Biocatalytic Reduction of Carbonyl Compounds Mediated by Yeast: Which is the Efficient Electron Donor?
Authors: Zhang Ben‐Li , Pionnier Sébastien 
Published: July 2003

AbstractIn asymmetric reduction of carbonyl compounds mediated by microorganisms, the cofactors that transfer hydride should be regenerated by using a recycling system. In most cases, this recycling system consists of carbohydrate molecules, especially glucose or sucrose. Other molecules such as ethanol and acetate have been used as electron donors too. The reduction can even be conducted without added electron donors. To improve biocatalytic synthesis, it is important to understand the cofactor recycling mechanism. In this work, the hydride‐transfer mechanism in cofactor regeneration, which takes place in bioreduction mediated by yeast, was studied by means of an isotope tracing technique. The results show that, when glucose was used, the NADH involved in the glycolysis was consumed directly in the formation of ethanol and was not used in the bioreduction. Hence, the regeneration of cofactors in the reduction is not coupled with glycolysis. Nevertheless, glucose is an efficient electron donor that transfers hydride through the hexose monophosphate (HMP) pathway in which the main hydrogen source is C‐1 and C‐3 hydrogen of glucose. Ethanol is not a good electron donor, since, when it was used, only a small quantity of hydrogen was transferred from this molecule, and the main hydrogen source was water. Therefore, the ethanol oxidation pathway may not be efficient. In the absence of added auxiliary substrates, the yeast cells may use electron donors stored in its cellules. However, in this case we observed that the main hydrogen source for cofactor recycling was water, while only very few hydrogen atoms were from unexchangeable sites. This is similar to the case in which ethanol is used, and is in contradiction with the HMP pathway if stored glucose was the electron donor. The question that remains to be investigated is “what is the efficient electron donor recycling mechanism in the yeast cellules?”