bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electronic Structure and Linear and Nonlinear Optical Properties of Symmetrical and Unsymmetrical Squaraine Dyes
Authors: Meyers Fabienne , Chen Chin‐Ti , Marder Seth R. , Brédas Jean‐Luc , Meyers F. 
Published: July 2007

AbstractIn this theoretical study, we investigate the molecular and electronic structures of symmetrical and unsymmetrical squaraine dyes. Such dyes can be represented by the formula D‐A‐D, where D is a donor group and A an acceptor moiety. We analyze the evolution in geometric structure that results from changing both donor substituents simultaneously or from varying only one donor group to produce an asymmetrical system. The changes in geometric and electronic structures are compared and found to be consistent. The trends in linear and nonlinear optical properties, and in particular in second‐order polarizabilities, are investigated in several ways. The two‐state model appears to be inadequate in describing the second‐order polarizability, β. Consequently, we are unable to deduce simple structure/property relationships that might help in the design of quadrupolar compounds for nonlinear optics. Finally, a series of unsymmetrical squaraines with OH substitution and enol‐ketone isomerism are investigated; the calculated nonlinear properties follow a similar trend to the experimental results: the OH substitution and isomerization contribute to increasing the ground‐state polarization.
Thermal Ring‐Opening Polymerization of Hydrocarbon‐Bridged [2]Ferrocenophanes: Synthesis and Properties of Poly(ferrocenylethylene)s and Their Charge‐Transfer Polymer Salts with Tetracyanoethylene
Authors: Nelson James M. , Nguyen Paul , Petersen Ruth , Rengel Heiko , Macdonald Peter M. , Lough Alan J. , Manners Ian , Raju Nandyala P. , Greedan John E. , Barlow Stephen , O'Hare Dermot 
Published: July 2007

AbstractThe poly(ferrocenylethylene)s [Fe(η‐C5H3RCH2)2]n 5a and 5b (a: R = H, b: R = Me) have been prepared by thermal ring‐opening polymerization of the corresponding strained hydrocarbon‐bridged [2]ferrocenophanes [Fe(η‐C5‐H3RCH2)2] (4a and 4b). An X‐ray diffraction study of 4a indicated significant strain. Polymer 5a was crystalline and insoluble in common organic solvents and was characterized by solid‐state 13C NMR. Polymer 5b, which was soluble in organic solvents, was characterized by 1H and 13C NMR, UV/visible spectroscopy and elemental analysis. Its molecular weight distribution was bimodal (gel permeation chromatography: Mw = 9.6 × 104, Mn = 8.6 × 104 for the high molecular weight fraction, Mw = 4.8 × 103, Mn = 3.5 × 103 for the oligomeric fraction), suggesting two polymerization mechanisms. The UV/visible spectrum implied a localized structure for the polymer backbone. Cyclic voltammetry revealed that 5b undergoes two reversible oxidations in CH2Cl2 solution at–0.25 and–0.16V. The redox coupling is indicative of only a small degree of interaction between the iron centres. Thermogravimetric analysis indicated that 5a and 5b are thermally stable to ca. 300–350°C under N2. At higher temperatures they yield ferromagnetic iron carbide ceramics 6a and 6b (ca. 50% and 32%, respectively, at 600°C) together with molecular depolymerization products. The reaction of 5b with tetracyanoethylene (TCNE) yielded insoluble and soluble oxidized products 11 and 12, which differed in the degree of oligomerization of the TCNEy‐x counterions. These products were characterized by IR, elemental analysis, ESR spectroscopy, and magnetic susceptibility measurements. The last revealed the presence of significant antiferromagnetic interactions in 12.
Dendritic Bipyridine Ligands and Their Tris(Bipyridine)Ruthenium(II) Chelates—Syntheses, Absorption Spectra, and Photophysical Properties
Authors: Issberner Jörg , Vögtle Fritz , Cola Luisa De , Balzani Vincenzo 
Published: July 2007

AbstractSeveral synthetic strategies have been explored to prepare dendrimers having the [Ru(bpy)3]2+ complex as their core (bpy = 2,2′‐bipyridine). Dendritic ligands have been synthesized by attaching branches in the 4,4′‐positions of bpy. The largest dendritic bipyridine ligand contains 54 peripherical methylester units. Four RuII dendritic complexes have been prepared. Their absorption and emission spectra are very similar to those of the unsubstituted parent RuII–bipyridine complexes. The large dendritic complexes, however, exhibit a more intense emission and a longer excited‐state lifetime than [Ru(bpy)3]2+ in aerated solutions. This is due to the shielding effect of the dendrimer branches on the Ru–bipyridine core, which limits the quenching effect of molecular oxygen. For the largest dendritic complex, which contains 54 peripherical methylester units, the excited‐state lifetime in aerated acetonitrile solution is longer than 1 μs, and the rate constant for dioxygen quenching is twelve times smaller than for [Ru(bpy)3]2+.
A Theoretical Study of the Indigoid Dyes and Their Chromophore
Authors: Serrano‐Andrés Luis , Roos Björn O. 
Published: July 2007

AbstractA theoretical study of the electronic spectra of indigo, bispyrroleindigo, and the H‐chromophore, thought to be the basic structure leading to the intense absorption in the visible spectrum of the indigoid dyes, has been performed by means of the complete active space (CAS) SCF method and the multiconfigurational second‐order perturbation theory (CASPT2). The calculations include excitation energies, oscillator strengths, and transition moment directions for the valence singlet states of the molecules. The assumption that the H‐chromophore is responsible for the absorption in the visible spectrum is quantitatively revisited with the help of the modern ab initio methodologies and the main features of these molecules are theoretically analyzed, some of them for the first time.
Complexes [(P2)Rh(hfacac)] as Model Compounds for the Fragment [(P2)Rh] and as Highly Active Catalysts for CO2 Hydrogenation: The Accessible Molecular Surface (AMS) Model as an Approach to Quantifying the Intrinsic Steric Properties of Chelating Ligands in Homogeneous Catalysis
Authors: Angermund Klaus , Baumann Wolfgang , Dinjus Eckhard , Fornika Roland , Görls Helmar , Kessler Magnus , Krüger Carl , Leitner Walter , Lutz Frank 
Published: July 2007

AbstractThe complexes [(P2)Rh(hfacac)] 1 [P2 = R2P‐(X)‐PR2] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(P2)Rh] fragment. The ligand exchange processes that occur during the syntheses of 1 from [(cod)Rh(hfacac)] and the appropriate chelating diphosphanes 3 were studied by variable‐temperature multinuclear NMR spectroscopy. The molecular structures of eight examples of 1 with systematic structural variations in 3 were determined by X‐ray crystallography. The steric repulsion of the PR2 groups within the chelating fragment was found to significantly influence the coordination geometry of [(P2)Rh], depending on the nature and length of the backbone (X). A linear correlation between the P‐Rh‐P angles in the solid state and the 103Rh chemical shifts reveals a similar geometric situation in solution. A unique molecular modeling approach was developed to define the accessible molecular surface (AMS) of the rhodium center within the flexible [(P2)Rh] fragment. The potential of this model for application in homogeneous catalysis was exemplified by the use of 1 as catalysts in a test reaction, the hydrogenation of CO2 to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.
Self‐Assembly of Novel [2]Catenanes and [2]Pseudorotaxanes Incorporating Thiacrown Ethers or Their Acyclic Analogues
Authors: Asakawa Masumi , Ashton Peter R. , Dehaen Wim , L'abbé Gerrit , Menzer Stephan , Nouwen Jan , Raymo Françisco M. , Stoddart J. Fraser , Tolley Malcolm S. , Toppet Suzanne , White Andrew J. P. , Williams David J. 
Published: July 2007

AbstractA series of π electron rich macrocyclic polythioethers and their acyclic analogues have been synthesized in good yields. The association constants for the complexation of the π electron deficient bis(hexafluorophosphate) bipyridinium‐based salt, paraquat, by these macrocycles, as well as those for the complexation of corresponding acyclic compounds by the bipyridinium‐based tetracationic cyclophane, cyclobis(paraquat‐p‐phenylene), are significantly lower than those observed in the case of the “all‐oxygen” analogues. Nonetheless, yields as high as 86% were recorded in the template‐directed syntheses of [2]catenanes composed of cyclobis(paraquat‐p‐phenylene) and the macrocyclic polythioethers. Single‐crystal X‐ray crystallographic analyses of the [2]catenanes incorporating constitutionally unsymmetrical π electron rich macrocyclic polythioethers revealed that, in all cases, the dioxyaromatic units are located inside the cavity of the tetracationic cyclophane component in preference to the dithiaaromatic units. A similar selectivity was observed in solution by variable‐temperature 1H NMR spectroscopy. However, inversion of the ratio between the two translational isomers of the two [2]catenanes bearing 1,5‐dithi‐anaphthalene, as one of their π electron rich ring systems, and either 1,4‐dioxy‐benzene or 1,5‐dioxynaphthalene, as the other, occurs upon increasing the temperature from –30 to +30 πC. These [2]catenanes can be viewed as temperature‐responsive molecular switches.
A Novel Synthetic Route to Tantalum–Zinc Neophylidyne Complexes Stabilized by ortho‐Chelating Arylamine Ligands; the X‐Ray structure of [TaCl2(μ‐C6H4CH2NMe2−2)(μ‐CCMe2Ph)ZnCl(THF)]
Authors: Rietveld Marco H. P. , Lohner Pierre , Nijkamp Marije G. , Grove David M. , Veldman Nora , Spek Anthony L. , Pfeffer Michel , van Koten Gerard 
Published: July 2007

AbstractA dimeric tantalum‐zinc neophylidyne complex proposed to be [{{TaCl2(μ‐CCMe2Ph)(μ‐Cl)(THF)2}{Zn(μ‐Cl)}}2](5a), which can be prepared easily on a scale of 50 mmol, is an ideal starting material for the synthesis of mono‐ and bis(ortho)‐chelated arylamine alkylidyne species such as the red Ta‐Zn neophylidyne complexes [TaCl2(μ‐C6H4CH2NMe2−2)(μ‐CCMe2Ph)ZnCl(THF)] (6) and [TaCl2‐{μ‐C6H3(CH2NMe2)2−2,6}(μ‐CCMe2Ph)ZnCl] (7), which have been isolated in high yields. Reaction of 7 with tmeda affords the Tav alkylidene complex [TaCl(=CHCMe2Ph){C6H3(CH2N(Me)CH2)−2‐CH2NMe2)‐6}] (8), which shows catalytic activity in the ring‐opening metathesis polymerization of norbornene.
Molecular Structures and Energetics of Corrole Isomers: A Comprehensive Local Density Functional Theoretical Study
Authors: Ghosh Abhik , Jynge Knut 
Published: July 2007

AbstractBy using geometry optimizations with local density functional theory and double‐π plus polarization basis sets, an extensive study has been carried out on the molecular structures and stabilities of free‐base and metal‐complexed corrole isomers. The optimized structures of normal metallocorroles have been found to agree well with crystallographic results. For both free‐base and metal‐complexed derivatives, the [1.1.1] ring system is found to be the most stable. The [2.0.1]‐ and [2.1.0]corrole isomers are unequivocally predicted to exist as stable materials. Of these, the [2.0.1] ring system, known as isocorrole, has been recently synthesized. Various derivatives of these two ring systems lie only about 10–20 kcal mol−1 above analogous derivatives of normal corrole. In general, the cis‐ and trans−[3.0.0]corrole derivatives are predicted to be significantly less stable than the other corrole isomers. However, the ScIII complexes of these two stereoisomeric ring systems are surprisingly stable. Direct C−α‐Cα linkages between pyrrole rings are identified as a principal source of strain in the molecular structures of corrole isomers. The N4 cores of [1.1.1]‐ and [2.0.1]corrole isomers are significantly smaller than the porphyrin core. Thus, these corrole isomers are predicted to have a strong preference for smaller metal ions such as GaIII. The [2.1.0] core is somewhat larger, as evidenced by longer metal‐nitrogen distances in [2.1.0]‐metallocorroles. These differences in core geometry account for an interesting reversal of the relative stabilities of [2.0.1]‐and [2.1.0]metallocorroles with increasing ionic radius of the complexed metal ion. Analogous to porphyrin isomer chemistry, the trans stereoisomer of [3.0.0]‐corrole is found to be more stable than the cis stereoisomer for the free‐base and for the ScIII and InIII derivatives. For the free bases of any particular isomer, the tautomers are quite similar in energy, differing by only 2–7 kcal mol−1. This, together with the presence of short, strong N‐HN hydrogen bonds, suggests that N‐H tautomerization in at least some free‐base corrole isomers should be considerably faster than that in porphyrins. Overall, it has been possible in most cases to establish a good correlation between the energetics trends and structural differences among molecules.
Formation of Hydrido–η3‐Allyl Complexes of IrIII by Sequential Olefinic CH Bond Activation and CC Coupling of Alkenyl and Olefin Ligands
Authors: Alvarado Ysaias , Boutry Olivier , Gutiérrez Enrique , Monge Angeles , Nicasio M. Carmen , Poveda Manuel L. , Pérez Pedro J. , Ruíz Caridad , Bianchini Claudio , Carmona Ernesto 
Published: July 2007

AbstractThe bis(ethene) complex [Tp*Ir‐(C2H4)2] (1*) (Tp* = tris(3,5‐dimethyl‐1‐pyrazol‐1‐yl)hydroborato) undergoes thermal rearrangement to the hydrido–allyl complex [Tp*IrH(π3‐C3H4Me)] (6*), through the intermediacy of the hydrido–vinyl complex [Tp*IrH(C2H3)(C2H4)] (2*). The overall conversion of 1* into 6* corresponds formally to the dimerisation of ethene by an unprecedented pathway that involves sequential C–H bond activation of a coordinated olefin molecule and C–C bond formation by coupling of the resulting vinyl and ethene moieties. Similar transformations have been observed for monosubstituted olefins like propene and 1‐butene, while the internal alkene cis−2‐butene experiences allylic activation of an sp3 C–H bond, which provides an alternative route to 6*. The extension of these investigations to the analogous complexes of the unsubstituted tris(pyrazolyl)hydroborato ligand Tp is also reported. Mechanistic studies on the formation of the C–C bond by coupling of the vinyl and the olefin ligands suggest the participation of a vinylidene complex (formed by α‐H abstraction from the vinyl group), which then rearranges to an allene species. Evidence for the involvement of these and other key reaction intermediates is provided.
Synthesis and Biological Evaluation of a Backbone‐Modified Phytoalexin Elicitor
Authors: Timmers C. M. , Turner J. J. , Ward C. M. , van der Marel G. A. , Kouwijzer M. L. C. E. , Grootenhuis P. D. J. , van Boom J. H. 
Published: July 2007

AbstractTwo suitably protected building blocks (11 and 33) for the preparation of amide‐linked heptaglucoside mimetic 2, an analogue of the naturally occurring phytoalexin elicitor 1a, were readily accessible by glycal chemistry. Sequential elongation of terminal glucuronide 21 with laminaribiosyl hemiaminal 33 and anomeric amine 11 by EDC/HOBt‐catalyzed condensation and two‐step conversion of the C6‐OTr moiety into the corresponding carboxylate function afforded homogeneous carbopeptoid 2 in high overall yield. It was found that replacement of the acetal linkages by the more rigid amide bonds destroys the phytoalexin‐elicitor activity.
Molecular Receptors for Adenine and Guanine Employing Metal Coordination, Hydrogen‐Bonding and π‐Stacking Interactions
Authors: Kickham James E. , Loeb Stephen J. , Murphy Shannon L. 
Published: July 2007

AbstractThiacyclophane ligands 1 and 2, containing a meta‐xylyldithiaether unit, an aromatic spacing unit and a polyether chain, were prepared in good yield in a three‐step synthesis. The macrocyclic organopalladium complexes [Pd(L)‐(MeCN)][BF4] (3: L = 1; 4: L = 2) were prepared through palladation of the respective thiacyclophane ligand by reaction with [Pd(MeCN)4][BF4]2. These complexes act as metalloreceptors to aromatic amines such as p‐aminopyridine (pap), m‐aminopyridine (map) and the DNA nucleobases adenine and guanine. Binding occurs through simultaneous first‐ and second‐sphere coordination. This involves three separate interactions: first‐sphere σ donation from an aromatic N atom to the Pd centre, second‐sphere hydrogen bonds between the NH2 group and polyether O atoms, and π stacking between the electron‐poor aromatic rings of the substrate and the electron‐rich aromatic spacing units of the receptor. 1H NMR spectra exhibit chemical shift changes indicative of the H‐bonding and π‐stacking interactions in solution. X‐ray structures for thiacyclophane 1, metalloreceptor [Pd(1)(MeCN)][BF4] (3), metalloreceptor/model substrate complexes [Pd(1)(pap)][BF4 (5) and [Pd(2)‐(pap)][BF4] (7), and metalloreceptor/nucleobase complexes [Pd(1)(adenine)][BF4] (13), [Pd(2)(adenine)][BF4] (14) and [Pd(1)(guanine‐BF3)][BF4] (15b) show details of these interactions in the solid state.
The Origin of the Remarkable Regioselectivity of Fe+‐Mediated Dehydrogenation in Benzocycloalkenes
Authors: Hertwig Roland H. , Seemeyer Katrin , Schwarz Helmut , Koch Wolfram 
Published: July 2007

AbstractCombined mass spectrometric experiments and density functional theory (DFT) calculations have been performed to determine the regioselectivity and uncover the origin of the CH bond activation of benzocycloalkenes by “bare” FeI cations. The hydrocarbons investigated are benzocyclohexene, ‐heptene, and ‐octene. Extensive labeling experiments demonstrate that the site of CH bond activation in the non‐aromatic ring is dependent on the ring size. The barriers for the dehydrogenation process as predicted by DFT calculations are in agreement with the experimental findings. Further, the combined experimental and theoretical approach applied in the present study allows the principles underlying the remarkable ring size dependent regioselectivity of CH bond activation in these hydrocarbons to be unraveled.
Bis(Trimethylsilyl)Ammonium Salts Obtained by Reaction of Hexamethyldisilazane with TiCl4, ZrCl4, or SnCl4
Authors: Bettenhausen Regina , Milius Wolfgang , Schnick Wolfgang 
Published: July 2007

AbstractThe reaction of hexamethyldisilazane with TiCl4 in CH2Cl2 leads to the formation of [(Me3SiNTiCl2)8] (3). According to the crystal structure analysis, 3 is an octameric titanium trimethylsilylimido chloride. The mechanism of the formation of 3 was studied in detail by means of time‐dependent 14N NMR spectroscopy. [(Me3Si)2NTiCl3] was identified as an intermediate during the formation of 3. Two by‐products were identified: [(NH4)2TiCl6] and the hitherto unknown bis(trimethylsilyl)ammonium salt [(Me3Si)2NH2]+[Ti2Cl9]− (5). According to the crystal structure analysis, protonation of hexamethyldisilazane to give the cation of 5 causes a significant lengthening of the Si‐N bond from 173.5 to 186 pm. Other tetravalent metal chlorides, such as ZrCl4 and SnCl4, also reacted with hexamethyldisilazane to yield bis(trimethylsilyl)ammonium salts. However, the constitution of the corresponding anions differs significantly. In the case of ZrCl4, an anionic chlorotrimethylsilylimidozirconate (6) [(Me3Si)2NH2]+2‐[{Cl3ZrCl3Zr(Cl)(NSiMe3)}2]2− was obtained, while SnCl4 reacted to give a novel spiro anion [N(SnCl3)2(SnCl2)4‐(NSiMe3)2NH)2]− together with the bis(trimethylsilyl)ammonium ion in 7; this anion is the first species containing a tetrastannylated nitrogen.
Palladacycles: Efficient New Catalysts for the Heck Vinylation of Aryl Halides
Authors: Herrmann Wolfgang A. , Brossmer Christoph , Reisinger Claus‐Peter , Riermeier Thomas H. , Öfele Karl , Beller Matthias 
Published: July 2007

AbstractCyclopalladated complexes of the general formula [Pd2(μ‐L)2(PC)2] (L = bridging ligand, e.g. OAc, Cl, Br, I; PC = cyclometallated P donor, e.g. o‐CH2C6H4P(o‐Tol)2 or o‐CH2C6H2(CH3)2‐P(Mes)2) are highly efficient catalysts for the Heck vinylation of aryl halides. The isolated complexes are easily accessible from palladium(II) acetate by spontaneous metallation of ortho‐methyl‐substituted arylphosphines. They display improved activity and stability compared to conventional catalyst mixtures (e.g. [Pd(OAc)2] +nPPh3), and also exhibit a higher stability towards air than conventional Pd0‐based systems (e.g. [Pd(PPh3)4]). Turnover numbers (TON) of up to 1000000 and turnover frequencies (TOF) in the range of 5000‐20000 are achieved in catalytic coupling reactions of aryl bromides. Even technically interesting aryl chlorides undergo the Heck reaction (TON = 600‐40000) if promoting salts are added to the catalyst ((NBu4)Br, LiBr). The new structural type for catalysts is compared to palladacycles formed in situ from mixtures of [Pd(OAc)2] + P(o‐tolyl)3 and the established [Pd(OAc)2] + nPPh3 system. The scope of the new CC coupling catalysts is outlined for the vinylation of aryl halides by the use of different mono‐ and disubstituted olefins. Mechanistic consequences for the Heck reaction in general are discussed.
Stereoselective Synthesis of (S)‐3,4‐Methylenedioxyamphetamines from (R)‐Cyanohydrins
Authors: Effenberger Franz , Jäger Jürgen 
Published: July 2007

AbstractA stereoselective synthesis of (S)‐3,4‐methylenedioxyamphetamines (S)‐7, which are highly interesting as psychoactive compounds, is described. Starting from readily available (R)‐cyanohydrins (R)‐2 the 2‐amino‐1‐aryl alcohols (1R,2S)‐4 were obtained with high diastereoselectivity by addition of Grignard reagents to the O‐protected cyanohydrins (R)‐3, transimination of the addition products A with primary amines, and hydrogenation of the imino intermediates B with NaBH4. For the hydrogenation of the benzylic hydroxyl group in the 1,2‐amino alcohols (1R,2S)‐4 a new, very efficient method was developed. The optically pure amphetamines (S)‐7 were obtained under very mild conditions by catalytic hydrogenation of the oxazolidinones (4S,5R)‐6, which were readily available by phosgenation of the amino alcohols (1R,2S)‐4.
From Dendrimers to Knedel‐like Structures
Authors: Wooley Karen L. 
Published: July 2007

AbstractShell‐crosslinked knedels are nanometer‐sized (5–200 nm diameters), amphiphilic core‐shell spheres that are prepared by the self‐assembly of amphiphilic block copolymers into polymer micelles, followed by crosslinking of side chain functionalities along the block composing the shell of the polymer micelles. Such structures resemble dendrimers in the basic structural components, and have the advantages of a rapid synthetic approach and the production of larger and broader size ranges.
Theory of the d10–d10 Closed‐Shell Attraction: 1. Dimers Near Equilibrium
Authors: Pyykkö Pekka , Runeberg Nino , Mendizabal Fernando 
Published: July 2007

AbstractWe study the dependence of the aurophilic attraction (AuI–AuI) in perpendicular model systems of the type [(ClAuPH3)2] on the ab initio method, basis set and different pseudopotentials used, and on relativity. The effects of varying the „halogen”︁ (X = F, H, Cl, Me, Br, ‐C≡CH, I), the „phosphine”︁ (L = PH3, PMe3, ‐N≡CH) and the metal (M = Cu, Ag, Au) on the M–M′ interaction of the [(XML)2] dimer are also studied. The depth of the interaction potential increases with the softness of the group X. It decreases by 27% for M = Au, X = Cl and L = PH3 if relativistic effects are omitted at fixed geometry.
Theory of the d10–d10 Closed‐Shell Attraction: 2. Long‐Distance Behaviour and Nonadditive Effects in Dimers and Trimers of Type [(x‐Au‐L)n] (n = 2, 3; X = Cl, I, H; L = PH3, PMe3, ‐N≡CH)
Authors: Pyykkö Pekka , Mendizabal Fernando 
Published: July 2007

AbstractWe study the nature of the aurophilic attraction (AuI–AuI) at its long‐range limit for the model systems [(X‐Au‐L)n] (n = 2, 3; X = Cl, I, L = PH3, PMe3; X = H, L = ‐N≡CH) at the ab initio MP2 and Hartree‐Fock levels. The nature of the interactions and nonadditive effects at various orientations are related to simple electrostatic induction and dispersion expressions involving the individual properties of each monomer.