Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity.

The possibility to alter properties of metal complexes without significant steric changes is a useful tool to tailor the reactivity of the complexes. Herein we present the synthesis of iron complexes with the tripodal phosphane ligands Triphos and Triphos(Si) and report on their different coordination properties. Whereas reaction of Triphos(Si) and FeX2 (X = Cl, Br) exclusively afforded (Triphos(Si))FeX2 with a κ(2)-coordinated ligand, the homologous C-derived Fe complexes show rapid conversion in solution to afford [(Triphos)Fe(CH3CN)3][Fe2Cl6] or [(Triphos)Fe(CH3CN)3][FeBr4], respectively. The structural conversion was found to be temperature- and solvent-dependent and was accompanied by a linear change of the overall magnetization. The different ligand influence was shown to have a significant effect on the ability of (Triphos(Si))FeCl2 and (Triphos)FeCl2 to perform the Sonogashira cross-coupling reaction of 4-iodotoluene and phenyl acetylene as well as the hydrosilylation of acetophenone. The results presented herein show the different coordination properties of two structurally homologous tripodal ligands and demonstrate the importance of geometrically controlled ligand field splitting on the stability and reactivity of metal complexes. The C/Si exchange therefore provides a simple and straightforward tool to manipulate properties and reactivity of metal complexes.

Intermolecular Interactions of a Chiral Amine Borane Adduct Revealed by VCD Spectroscopy.

Amine boranes feature strong hydrogen bonding acceptor and donor moieties in close proximity, leading, for instance, to dihydrogen bonding driven self-aggregation. In this work, the infrared (IR) and vibrational circular dichroism (VCD) spectra of the bulky bis(α-phenylethyl)amine borane 1 in chloroform and acetonitrile solution are reported. By comparison with calculated spectra, the VCD spectral features observed in chloroform solution can clearly be associated with the presence of monomeric species. A shift of the conformational preferences occurs when changing the solvent to acetonitrile, which can only be deduced from the VCD spectral signatures but not from the IR spectrum. Using variable-temperature IR and VCD spectroscopy, the dihydrogen bonded dimeric species is characterized experimentally at -50 °C and theoretically by means of density functional theory calculations.

Behavior of Supported Palladium Oxide Nanoparticles under Reaction Conditions, Studied with near Ambient Pressure XPS.

Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is a promising method to close the "pressure gap", and thus, study the surface composition during heterogeneous reactions in situ. The specialized spectrometers necessary for this analytical technique have recently been adapted to operate with a conventional X-ray source, making it available for routine quantitative analysis in the laboratory. This is shown in the present in situ study of the partial oxidation of 2-propanol catalyzed with PdO nanoparticles supported on TiO2, which was investigated under reaction conditions as a function of gas composition (alcohol-to-oxygen ratio) and temperature. Exposure of the nanoparticles to 2-propanol at 30 °C leads to immediate partial reduction of the PdO, followed by a continuous reduction of the remaining PdO during heating. However, gaseous oxygen inhibits the reduction of PdO below 90 °C, and the oxidation of 2-propanol to carboxylates only occurs in the presence of oxygen above 90 °C. These results support the theory that metallic palladium is the active catalyst material, and they show that environmental conditions affect the nanoparticles and the reaction process significantly. The study also revealed challenges and limitations of this analytical method. Specifically, the intensity and fixed photon energy of a conventional X-ray source limit the spectral resolution and surface sensitivity of lab-based NAP-XPS, which affect precision and accuracy of the quantitative analysis.

Estimation of Lattice Enthalpies of Ionic Liquids Supported by Hirshfeld Analysis.

New measurements of vaporization enthalpies for 15 1:1 ionic liquids are performed by using a quartz-crystal microbalance. Collection and analysis of 33 available crystal structures of organic salts, which comprise 13 different cations and 12 anions, is performed. Their dissociation lattice enthalpies are calculated by a combination of experimental and quantum chemical quantities and are divided into the relaxation and Coulomb components to give an insight into elusive short-range interaction enthalpies. An empirical equation is developed, based on interaction-specific Hirshfeld surfaces and solvation enthalpies, which enables the estimation of the lattice enthalpy by using only the crystal-structure data.

Influence of deuteration and fluorination on the supramolecular architecture of pyridine N-oxide crystals.

To understand how deuterium and fluorine substituents influence the supramolecular architecture of pyridine N-oxide crystals, the crystal structure of 3-fluoropyridine N-oxide (PNO-3F) was determined and the crystal packing motives of non-deuterated pyridine-N-oxide (PNO), partial-deuterated pyridine-N-oxide (PNO-D) and PNO-3F were analyzed based on ab initio quantum-chemical calculations of the intermolecular interaction energy, using the MP2/6-311G(d,p) method. The appearance of the weak-directing substituents deuterium and fluorine leads to significant changes in the crystal organization of the isotropic packing of PNO molecules.

Polymorphism and Multi-Component Crystal Formation of GABA and Gabapentin.

This study exploits the polymorphism and multi-component crystal formation of γ-amino butanoic acid (GABA) and its pharmaceutically active derivative, gabapentin. Two polymorphs of GABA and both polymorphs of gabapentin are structurally revisited, together with gabapentin monohydrate. Hereby, GABA form II is only accessible under special conditions using additives, whereas gabapentin converts to the monohydrate even in the presence of trace amounts of water. Different accessibilities and phase stabilities of these phases are still not fully clarified. Thus, indicators of phase stability are discussed involving intermolecular interactions, molecular conformations, and crystallization environment. Calculated lattice energy differences for polymorphs reveal their similar stability. Quantification of the hydrogen bond strengths with the atoms-in-molecules (AIM) model in conjunction with non-covalent interaction (NCI) plots also shows similar hydrogen bond binding energy values for all polymorphs. We demonstrate that differences in the interacting modes, in an interplay with the intermolecular repulsion, allow the formation of the desired phase under different crystallization environments. Salts and co-crystals of GABA and gabapentin with fumaric as well as succinic acid further serve as models to highlight how strongly HBs act as the motif-directing force in the solid-phase GABA-analogs. Six novel multi-component entities were synthesized, and structural and computational analysis was performed: GABA fumarate (2:1); two gabapentin fumarates (2:1) and (1:1); two GABA succinates (2:1) and (1:1); and a gabapentin:succinic acid co-crystal. Energetically highly attractive carboxyl/carboxylate interaction overcomes other factors and dominates the multi-component phase formation. Decisive commonalities in the crystallization behavior of zwitterionic GABA-derivatives are discussed, which show how they can and should be understood as a whole for possible related future products.

Influence of the metal center and linker on the intracellular distribution and biological activity of organometal-peptide conjugates.

Organometallic complexes conjugated to cell-penetrating peptides (CPPs) are promising systems for diagnostic imaging and therapeutic applications in human medicine. Recently, we reported on the synthesis of cymantrene(CpMn(CO)(3))-CPP conjugates with biological activity on different cancer cell lines. However, the precise mechanism of cytotoxicity remained elusive in these studies. To investigate the role of the metal center and the linker between the CpM(CO)(3) moiety and the peptide, a number of derivatives with manganese replaced by rhenium and the keto linker originally used substituted by a methylene group were prepared and fully characterized by (1)H NMR spectroscopy, infrared spectroscopy, electrospray ionization mass spectrometry, and elemental analysis as well as X-ray structure determination. The organometal-peptide conjugates as well as carboxyfluorescein-labeled derivatives thereof were prepared by solid-phase peptide synthesis, purified by high-performance liquid chromatography, and analyzed by mass spectrometry. Fluorescence microscopy studies of MCF-7 human breast cancer cells revealed an efficient cellular uptake and pronounced nuclear localization of the bioconjugates with the methylene linker compared with systems with the keto group. In addition, the latter also showed a higher cytotoxicity. In contrast, the variation of the metal center from manganese to rhenium had a negligible effect. The structure-activity relationships determined in the present work will aid in the further tuning of the biological activity of organometal-peptide conjugates.

Temperature-driven mixing-demixing behavior of binary mixtures of the ionic liquid choline bis(trifluoromethylsulfonyl)imide and water.

The ionic liquid (2-hydroxyethylammonium)trimethylammonium) bis(trifluoromethylsulfonyl)imide (choline bistriflimide) was obtained as a supercooled liquid at room temperature (melting point=30 degrees C). Crystals of choline bistriflimide suitable for structure determination were grown from the melt in situ on the X-ray diffractometer. The choline cation adopts a folded conformation, whereas the bistriflimide anion exhibits a transoid conformation. The choline cation and the bistriflimide anion are held together by hydrogen bonds between the hydroxyl proton and a sulfonyl oxygen atom. This hydrogen bonding is of importance for the temperature-dependent solubility properties of the ionic liquid. Choline bistriflimide is not miscible with water at room temperature, but forms one phase with water at temperatures above 72 degrees C (equals upper critical solution temperature). 1H NMR studies show that the hydrogen bonds between the choline cation and the bistriflimide anion are substantially weakened above this temperature. The thermophysical properties of water-choline bistriflimide binary mixtures were furthermore studied by a photopyroelectric technique and by adiabatic scanning calorimetry (ASC). By photothermal analysis, besides highly accurate values for the thermal conductivity and effusivity of choline bistriflimide at 30 degrees C, the detailed temperature dependence of both the thermal conductivity and effusivity of the upper and lower part of a critical water-choline bistriflimide mixture in the neighborhood of the mixing-demixing phase transition could be determined with high resolution and accuracy. Together with high resolution ASC data for the heat capacity, experimental values were obtained for the critical exponents alpha and beta, and for the critical amplitude ratio G+/G-. These three values were found to be consistent with theoretical expectations for a three dimensional Ising-type of critical behavior of binary liquid mixtures.

Active peroxo titanium complexes: syntheses, characterization and their potential in the photooxidation of 2-propanol.

Two novel peroxo titanium complexes, Li(2)(NH(4))(4)[Ti(2)(O(2))(2)(cit)(Hcit)](2).5H(2)O and Zn(NH(4))(4)[Ti(4)(O(2))(4)(Hcit)(2)(cit)(2)].12H(2)O (cit = citrate), show encouraging results in the photochemical oxidation of 2-propanol.

Photoinduced CO release, cellular uptake and cytotoxicity of a tris(pyrazolyl)methane (tpm) manganese tricarbonyl complex.

Cell viability studies of HT29 colon cancer cells treated with the CO-releasing compound [Mn(CO)(3)(tpm)]PF(6) revealed a significant photoinduced cytotoxicity comparable to that of established agent 5-fluorouracil (5-FU), while controls kept in the dark were unaffected at up to 100 microM.

Lithium-promoted hydrogenation of carbon dioxide to formates by heterobimetallic hydridozinc alkoxide clusters.

The remarkably distinct reactivity of hydridozinc heterobimetallic cubanes [(HZnOtBu)4-n(thf LiOtBu)n] 1a-1d towards CO2 is reported--the hydride transfer from Zn-H to CO2 is drastically accelerated in the presence of Li ions in 1b-1d which led to the respective metal formate hydrates; the systems are inspiring models for the selective conversion of water gas into formates on lithium-promoted ZnO supports.

Controlled usage of H/D exchange to circumvent concomitant polymorphs of ROY.

The hypothesis that H/D exchange affects the structural formation of organic compounds in the solid state is supported by a deeper understanding of the altering polymorphism of ROY (a substance striking for its high number of polymorphic forms) through deuteration. Therefore, ROY was deuterated at its amine function, which leads to a seemingly small yet effective modification of the hydrogen-bond strength. In contrast to the crystallization of the non-deuterated ROY in methanol or ethanol, which leads to the simultaneous formation of two forms (OP and Y polymorphs), so-called concomitant polymorphs, the crystallization of d1-ROY leads to the selective formation of the Y polymorph exclusively. The preferred aggregation behavior of the Y form of d1-ROY is assigned to the weakening of an intramolecular hydrogen bond and a consequently strengthened intermolecular hydrogen bond after deuteration.

Isoindoles and dihydroisoquinolines by gold-catalyzed intramolecular hydroamination of alkynes.

The title compounds are enantioselectively synthesized in just two preparative steps, making use of the Ugi-four-component reaction with an amino acid as chiral component, followed by a gold-catalyzed hydroamination.

Self-addition of a sterically hindered alkynylselenolate.

[structure: see text] The unprecedented formation of a complex macrocyclic selenium-carbon ring system, which is initiated by the action of coordinating reagents upon a sterically hindered alkynylselenolate, has been observed. The crystal structure of the product could be obtained and shows a rigid tricyclic arrangement consisting only of selenium and sp(2)-hybridized carbon atoms. This reactivity stands in contrast to the corresponding unsubstituted alkynylselenolates and is an example where a bulky substituent destabilizes an adjacent unsaturated pi-system.

Synthesis of the first fluoro(phosphanyl)- and diphosphanyl-stannanes and surprising formation of [P(SnMe3)4]+ SiF5-.

Monophosphanylation of the difluorostannane Is2SnF2 (Is = 2,4,6-triisopropylphenyl) with Si(PH2)4 furnishes the first isolable fluoro(phosphanyl)stannane Is2Sn(F)PH2 1, which readily undergoes unique SiO2-assisted decompostion to [Is2Sn(PH)]2 2 and HF, while further phosphanylation of 1 leads to the remarkably stable diphosphanylstannane Is2Sn(PH2)2 3; the latter reacts with Me3SnF at the glass-wall to give the first perstannylphosphonium pentafluorosilicate [P(SnMe3)4]+ SiF5- 4.

E(SiMe3 )4+ Ions (E=P, As): Persilylated Phosphonium and Arsonium Ions.

Only strong Lewis acidic, arene-solvated Me3 Si+ ions react with E(SiMe3 )3 compounds (E=P, As) to give the crystallographically characterized E(SiMe3 )4+ onium ions 1 (left hand picture), which contain highly negative polarized P and As atoms, respectively. The masked Me3 Si+ ions in 1 can be easily transferred to Et2 O, to give the first structurally characterized planar silyloxonium ion [Et2 (Me3 Si)O]+ 2 (right hand picture).

[(OC)5 Cr=BSi(SiMe3 )3 ]: A Terminal Borylene Complex with an Electronically Unsaturated Boron Atom.

An extremely low-field shifted 11 B NMR signal at δ=204.3 and a very short Cr-B distance of 187.8 pm characterize the title compound [(OC)5 Cr=B-Si(SiMe3 )3 ], which is the first borylene complex in which the boron atom is both coordinatively and electronically unsaturated.

Isothermal and isochoric crystallization of highly hygroscopic pyridine N-oxide of aqueous solution.

Highly hygroscopic pyridine N-oxide, C5H5NO, dissolves in water absorbed from atmospheric air, but it crystallizes in the neat form of the aqueous solution under high pressure. The crystals grown at high-pressure isochoric conditions are of the same phase as that obtained from anhydrous crystallization at ambient pressure. This feature can be employed for retrieving compounds highly soluble in water from their aqueous solutions. The crystal structure is strongly stabilized by CH...O contacts. The crystal compression and thermal expansion as well as three shortest H...O distances comply with the inverse-relationship rule of pressure and temperature changes.

Inorganic chemistry in a nanoreactor: Au/TiO2 nanocomposites by photolysis of a single-source precursor in miniemulsion.

An original synthetic route, based on the combination of a single-source precursor, UV-photodegradation and inverse w/o miniemulsion, is used to prepare Au nanoparticles (NPs) dispersed on titania. The source of the nanocomposite materials is the photolabile single-source precursor AuCl4(NH4)7[Ti2(O2)2(cit)(Hcit)]2·12H2O, which is suspended in a w/o miniemulsion consisting of different surfactant/hydrocarbon/water formulations (surfactant: sodium dodecylsulfate (SDS) or Triton X-100) and subsequently irradiated with a UV lamp to promote its decomposition in the confined space of the droplets. Gold NPs that form at room temperature are found to be crystalline, while titanium dioxide occurs as an amorphous phase. Moreover, the average crystallite size of gold NPs ranges between 20 and 24 nm when using SDS and between 26 and 40 nm in the case of Triton X-100, after 4 and 8 hours of irradiation time, respectively. Scanning and transmission electron microscopies (SEM and TEM) are used to get information about the nanocomposite morphology and nanostructure, revealing that gold NPs are uniformly distributed on the titanium oxide surface. Furthermore, X-ray photoelectron spectroscopy (XPS) outcomes, besides confirming the formation of both metallic gold and titania, provide information about the high dispersion of Au NPs on the TiO2 surface. In fact, the Au : Ti atomic ratio is found to be 0.45-1.5 (1 : 2-1.5 : 1), which is higher than the value determined by starting from the precursor stoichiometry (0.25). Catalytic testing in the oxidation of 2-propanol shows that decomposition of the precursor in a miniemulsion provides a nanocomposite with enhanced activity compared to the decomposition in the aqueous phase.

Stereochemistry rules: a single stereocenter changes the conformation of a cyclic tetrapeptide.

Two novel cyclo(Boc-Cys-Pro-Leu-Cys-OMe) peptides 1 and 2 containing the enantiomeric amino acids d-Leu and l-Leu, respectively, were synthesized to investigate the effect of chiral centers on peptide conformations. By combining a variety of experimental techniques (X-ray crystallography, 2D NMR spectroscopy, temperature-dependent (1)H NMR and IR spectroscopy, and UV-CD spectroscopy) with replica exchange molecular dynamics (REMD) techniques and quantum mechanics/molecular dynamics (QM/MM) calculations, we establish that the stereochemistry of just one residue can noticeably influence the properties of the whole peptide and rationalize the origins of this effect, with potential implications for the rational design of peptides of chemical and biological relevance.