Showing papers in "Chemistry: A European Journal in 1996"

N‐Heterocyclic Carbenes: Generation under Mild Conditions and Formation of Group 8–10 Transition Metal Complexes Relevant to Catalysis

Abstract: Stable mono- and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloro- and acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene). A general novel method for the deprotonation of N-substituted azolium salts in liquid ammonia–the liquid ammonia route–is described. This method yields not only the known free monimeric 1,3-dimethylimidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3-dimethyl-imidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of RuII, RhI PdII, OsII and IrI are presented. Reaction of the potentially chelating ligand 1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diylidene with [(COD)-RhCl]2 yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metalcarbene complexes (Rh, Os) are reported. N-Heterocyclic carbene complexes of Group 8–10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal–carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis-basic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.

Heterocyclic Carbenes: A High‐Yielding Synthesis of Novel, Functionalized N‐Heterocyclic Carbenes in Liquid Ammonia

Abstract: To date the only free carbenes of the imidazoline-2-ylidene type to have been described in the literature are those bearing simple hydrocarbon or haloalkyl and -aryl substituents. We report here a novel, versatile and high-yielding method for the synthesis of N-heterocyclic carbenes in a mixture of liquid ammonia and aprotic organic solvents. Deprotonation of the imidazolium precursor salts proceeds under mild conditions within a few minutes at temperatures below 30 \"C, and side reactions are thus avoided. The imidazolium salts are much more soluble in organic solvents if liquid ammonia is added. Furthermore, the acidity of the C2 protons appears to be enhanced by hydrogen bonding. Not only are the known free ylidenes conveniently and quantitatively accessible by this procedure, but also novel functionalized derivatives that are not accessible by known procedures. Imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted (0, N, P) and chiral hydrocarbon residues are accessible through the novel route. Stable carbene-metal \"adducts\" are conveniently obtained by treating the free carbenes with chloroor acetato-bridged dinuclear metal complexes, or by displacement of coordinated ligands such as carbon monoxide, THF or acetonitrile by the free carbenes. The syntheses of novel imidazolium salts, N-heterocyclic carbenes and carbene adducts of Ru\", Rh', Wo and sulfur are reported, and the structures of five products analysed by single-crystal X-ray diffraction. N-Heterocyclic carbenes bearing functionalized side chains are important because a number of these complexes show excellent activity in catalytic reactions. They do not show the typical reactivity of metal-carbon \"double bonds\" and are remarkably stable both thermally and chemically. For a number of reasons, they are best viewed as donor adducts of the highly Lewis basic imidazoline-2-ylidene ligands and the Lewis acidic organometallic fragments. The new synthetic procedure reported here makes Nheterocylic carbenes a generally accessible class of useful ligands in coordination chemistry and catalysis.

Self-Assembled Monolayers on Gold Nanoparticles

Abstract: Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecular terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-defined. The characterization of the thermal behaviour of SAMs is important not only for the design of stable, well-ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20–30 A in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT-IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long-chain thiols (≥ C16), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25°C. Furthermore, calorimetry, variable temperature transmission FT-IR spectroscopy, and solid-state 13C NMR studies have established that a cooperative chain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the extended chain conformation and the geometry of the spherical nanoparticles. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this suggests that chain ordering arises from an interdigitation of chains on neighboring particles. The thermotropic behavior is sensitive to the alkyl chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much-studied planar SAMs.

Fluorescent Sensors for Transition Metals Based on Electron‐Transfer and Energy‐Transfer Mechanisms

Abstract: Fluorescent sensors for 3d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal-receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one-electron oxidation of the metal, quenching takes place through a photoinduced metal-to-fluorophore electron-transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy-transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., CuII from NiII) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam-containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.

Supramolecular Motifs: Concerted Multiple Phenyl Embraces between Ph4P+ Cations Are Attractive and Ubiquitous

Abstract: Examination of the Cambridge Structural Database reveals that Ph4P+ cations in crystals associate through phenyl-phenyl nonbonded interactions which are attractive, concerted, and widespread. Intermolecular phenyl-phenyl conformations, which are offset-face-to-face (off), edge-to-face (ef) or vertex-to face (vf), combine in five classes of supramolecular motifs for {Ph4P+}2 pairs, namely the sextuple phenyl embrace (SPE) with (ef)6 and offset sextuple phenyl embrace (OSPE) containing (off)1 (ef)2(ef/vf)2, the translational quadruple phenyl embrace (TQPE) with (ef)4, the parallel quadruple phenyl embrace (PQPE) with (off)1(vf)2, and the double phenyl embrace (DPE) with (off)1. Typical intermolecular attractive energies (kJ per mol of {Ph4P+}2) for these motifs are SPE 85, OSPE 57, TQPE 70, PQPE 41, DPE 34. There is strong interpenetration of the cations in these motifs: 489/770 structures in the CSD have P P≤7 A (spherical Ph4P+ has a van der Waals diameter of 13.6 A). Of the 812 instances of P P ≤7 A, 86% are SPE, 10% are OSPE, 2% are TQPE, and only 2% are unclassified, Average P P separations in the PQPE and DPE are 8.3 A. Centrosymmetry is prevalent in all except the TQPE, which has implications for the engineering of noncentric crystals.

Polysaccharides of Intracrystalline Glycoproteins Modulate Calcite Crystal Growth In Vitro

Abstract: Assemblies of glycoproteins from within the mineralized tissues of sea urchins and mollusks both interact in vit- ro in a similar manner with growing cal- cite crystals. A protein-rich fraction, a polysaccharide-rich fraction, and a frac- tion composed of densely glycosylated peptide cores were obtained by chemical and enzymatic treatment of the glyco- proteins from sea-urchin spines. Each fraction was partially purified and charac- terized (amino acid composition, FTIR and NMR spectroscopy). A comparison of the interactions of these fractions with growing calcite crystals in vitro shows that the polysaccharide moieties of these

Self‐Assembly of Heterobimetallic Complexes and Reactive Nucleophiles: A General Strategy for the Activation of Asymmetric Reactions Promoted by Heterobimetallic Catalysts

Abstract: Heterobimetallic asymmetric catalysts, such as the lanthanum–lithium–binaphthol complex (La Li–BINOL), the aluminum–lithium–binaphthol complex (AlLi–BINOL), and a newly prepared gallium–sodium–binaphthol complex (Ga Na–BINOL), have been self-assembled with reactive nucleophiles, such as lithium nitronates and sodium malonates, to generate more efficient catalysts than the parent heterobimetallic catalysts. For example, by the combined use of La Li–BINOL (1 mol%; contains one H2O molecule) and BuLi (0.9 mol%) as the catalyst system, asymmetric nitroaldol reactions are greatly accelerated in all cases without a decrease in the optical purity of the nitroaldol products. Kinetic analyses have also been carried out on the Ga Na–BINOL-catalyzed Michael reaction of dibenzyl malonate with cyclohexenone, with or without NaOtBu. The calculated rate constants show that the combined use of Ga Na–BINOL and NaOtBu as the catalyst gives reaction rates that are about 50 times faster than with Ga Na–BINOL alone. This activation method should be useful for other asymmetric reactions catalyzed by heterobimetallic complexes.

A General Scheme Based on Empirical Increments for the Prediction of Hydrogen-Bond Associations of Nucleobases and of Synthetic Host–Guest complexes

Abstract: Association energies ΔGt in chloroform, in part also in carbon tetrachloride, were determined by NMR titrations of suitably substituted nucleosides and several synthetic analogues. Based on these and on many literature data, two simple free energy increments were derived describing the ΔGt values of 58 complexes within 1.8 kJ mol-1. With chloroform as solvent the increment for the primary interaction between donor and acceptor is 7.9 kJ mol-1, for the secondary one 2.9 kJ mol-1, irrespective of whether the latter is attractive or repulsive. Addition of only 1% methanol to CCl4 led to a decrease in association constants by a factor of 25. Calorimetric titrations of G-C nucleoside derivatives in CCl4 showed substantial contributions from G dimers, in line with NMR titrations, and surprisingly small decreases in entropy. Preliminary NOE measurements allowed us to single out some of the possible association modes; they are also in line with expected self- and triple-association modes of the nucleobases. These modes are generally in accord with nucleobase associations predicted by MM calculations in the literature, which in turn agree with predictions based solely on the increments derived in the present work.

The Role of Water Exchange in Attaining Maximum Relaxivities for Dendrimeric MRI Contrast Agents

Abstract: Macrocyclic GdIII complexes attached to dendrimers represent a new class of potential MRI contrast agents. They have an extended lifetime in the blood pool, which is indispensable for their application in magnetic resonance angiography, and high relaxivities, which reduce the dose required to produce quality images. We performed a variable-temperature and -pressure 17O NMR study in aqueous solution and at 14.1, 9.4, and 1.4 T on the water exchange and rotational dynamics of three macrocyclic GdIII complexes based on polyamidoamine dendrimers, as well as on the GdIII complex of the monomer unit with the linker group. The water exchange rates k298ex for generation 5 [G5(N{CS}N-bz-Gd-{DO3A}{H2O})52], generation 4 [G4(N-{CS}N-bz-Gd{DO3A}{H2O})30], generation 3 [G3(N{CS}N-bz-Gd{DO3A}-{H2O})23], and the monomer [Gd(DO3A-bz-NO2)(H2O)] complexes are 1.5±0.1, 1.3±0.1, 1.0±0.1, and 1.6±0.1 × 106 s-1, respectively, and the activation volumes ΔV≢ of water exchange on the latter two compounds are + 3.1±0.2 and + 7.7±0.5 cm3 mol-1, indicating dissociatively activated exchange reactions ({CS}N-bz-{DO3A}=1-(4-isothiocyanatobenzyl)amido-4,7,10-tri(acetic acid)tetraazacyclododecane). The rotational correlation times for the dendrimers are 4 to 8 times longer than for monomeric or dimeric GdIII poly(amino carboxylates). As a consequence of the slow rotation, the proton relaxivities of these dendrimer complexes are considerably higher than those of smaller complexes. However, the low water exchange rates prevent the dendrimer proton relaxivities from attaining the values expected from the increase in the rotational correlation times. Modifications of the chelating ligand may result in a faster water exchange and thus allow the full benefit of slow rotation to be achieved.

Regiochemistry of Twofold Additions to [6,6] Bonds in C60: Influence of the Addend‐Independent Cage Distortion in 1,2‐Monoadducts

Abstract: Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2-(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2-monoadducts C61(COOEt)2 (1), C61-(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62-(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.

1,3-Alternate Calix[4]arenecrown-5 Conformers: New Synthetic Ionophores with Better K+/Na+ Selectivity than Valinomycin

Abstract: New 25,27-dialkoxycalix-[4]arenecrown-5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3-alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27-dialkoxycalix[4]arenes 7a-b with tetraethylene glycol di-p-toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27-dimethoxycalix[4]arenecrown-5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X-ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3-alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3-alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.

Ruthenium(II)‐Catalyzed Oppenauer‐Type Oxidation of Secondary Alcohols

Abstract: Highly efficient ruthenium-catalyzed Oppenauer-type oxidations of secondary alcohols to ketones have been developed. The catalytic system consists of [(PPh(3))(3)RuCl2] (1) and K2CO3 or [(C(4)Ph(4) ...

Mineralization of Gold Nanoparticles in a Block Copolymer Microemulsion

Abstract: Controlled mineralization of gold nanoparticles has been performed in a microemulsion of polystyrene-block-poly(2-vinylpyridine). The starting point was the formation of a thermodynamically stable dispersion of HAuCl4 in inverse micelles of the block copolymer in toluene, which became metastable when the gold was reduced. Kinetic control of the transformation allowed the following stages of the mineralization/coagulation process to be stabilized: 1) one gold particle per micelle, 2) aggregated micelles containing two or three gold particles, and 3) a state in which empty micelles coexist with larger polymer-stabilized gold particles. Distinctive variations in the spectra were observed depending on the particle size and whether two particles had formed a couple with orientation-dependent dipolar interactions.

Porphyrin Synthesis by the “3+1” Approach: New Applications for an Old Methodology

Abstract: Acid-catalyzed condensation of tripyrranes with pyrrole-2,5-dicarboxaldehydes, followed by oxidation with an electron-deficient quinone, affords porphyrin products in excellent yields. This previously little used methodology has now been exploited in the synthesis of novel porphyrin structures, including tetrapyrrolic compounds with fused aromatic rings. By utilizing other aromatic or unsaturated dialdehydes, the “3+1” approach also allows the synthesis of new aromatic porphyrinoid systems, including benzene- and pyridine-containing macrocycles and carbaporphyrins.

X-Ray Structural Studies of Highly Enantioselective Mn(salen) Epoxidation Catalysts

Abstract: The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(sa1en) complexes (1 – 10) was examined. The X-ray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6′= -tBu; 4,4 = -tBu]+ClO4−), and 10 (6,6′= -tBu; 4,4′=-Br) were determined. Catalysts 1 – 9 were derived from (R,R)-1, 2 diaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethylenediamine. Catalysts 1–9 differ in the stereoelectronic substitution of the ortho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five- and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1 – 10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6′ = -H; 4,4′ = -H),2(6,6′ = -CH3; 4,4′ = -CH3),3(6,6′=-tBu;4,4′=-H),4(6,6′=-tBu; 4,4′ =-CH3), 5 (6,6′=-tBu; 4,4′=-tBu), and 6 (6,6′=-tBu; 4,4′= -trityl), and electronically modifying the para substituents in 7 (6,6 = -tBu; 4,4 =-OMe) and 8 (6.6′ = -tBu; 4,4′=-OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.

High-Spin → Low-Spin Relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and Thermal Spin-State Trapping—Dynamics of Spin Transition Versus Dynamics of Phase Transition

Abstract: The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≈285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mossbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal spin-state trapping was monitored at temperatures between 104 and 118 K by magnetic susceptibility measurements. The relaxation mechanisms of the HS states generated by LIESST and thermal spinstate trapping are completely different. We suggest that the HS → LS relaxation after thermal spin-state trapping is triggered by an additional structural phase transition of the system.

Synthesis, Crystal Structure, Magnetism, and Magnetic Anisotropy of Cyclic Clusters Comprising six Iron(III) Ions and Entrapping Alkaline Ions

Abstract: The reaction of ferric chloride and β-diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6-(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single-crystal X-ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in organic solution, as demonstrated by 23Na and 7Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high-spin ferric ions. The energy of the excited states was studied in detail by high-field DC and pulsed-field differential magnetization experiments at 0.7 and 1.5 K. Single-crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single-ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic-type behavior in transition-metal clusters.

Anion–Cation Redox Competition and the Formation of New Compounds in Highly Covalent Systems

Abstract: The increase in energy of the sp anionic band on going from oxides to the less electronegative sulfides, selenides, or tellurides (with a similar trend in neighboring columns) facilitates anion–cation redox interactions involving the d levels of transition metal cations and the sp levels of anionic species. The interactions can induce phase transitions or gradual change within a given structural model. When a cation is reduced by electron transfer to its d levels from the sp band, holes appear at the top of the latter. Interesting soft chemistry can be carried out based on redox processes that neutralize the holes with electrons. This approach also allows particular structural types to be stabilized. Three structural domains can be recognized amongst the transition elements: 1) on the left-hand side of the periodic table layered structures are observed involving M4+ and (chalcogen)2- ions; 2) formation of sets of metal-metal bonds is then observed, the geometry of which depends on both the initial electron population and the electron transfer to the metal; 3) on the right-hand side the metals in their highest oxidation state are no longer active, and the holes at the top of the sp band are taken up by a catenation of the anions, which can lead to full polymerization of the anionic sublattice.

Opening and Closure of the Fullerene Cage in cis‐Bisimino Adducts of C60: The Influence of the Addition Pattern and the Addend

Abstract: The synthesis, isolation, and spectroscopic characterization of the bisimino[60]fullerenes C60(NCOOR)2 (1a: R = Et, 1b. R = tBu) with a cis-1 addition pattern as well as of their regioisomeric analogues 2–7 with different addition patterns are described. Whereas compounds 2–7 are typical fulleroaziridines and obey the rule of the minimization of [5,6] double bonds, the cis-1 isomers 1a and 1b represent the first examples of fullerene derivatives with open transannular [6,6] bonds. Characteristic features within the fullerene framework of these valence isomers VI are the presence of a doubly bridged open 14-membered ring with a phenanthrene perimeter as well as of an 8-membered 1,4-diazocine heterocycle. Moreover, it is shown that, by transforming cis-1-C60(NCOOtBu)2 (1b) into cis-1-C60(NH)2 (1c), the fullerene cage can be closed in an intraring 2π→2s isomerization to valence isomer V. These are the first chemical modifications of the fullerene core that allow the synthesis of both open and closed valence isomers with the same addition pattern. Density-functional as well as AM1 calculations corroborate the experimental findings that it depends on the addition pattern as well as on the nature of the addends whether or not the transannular [6,6] bonds are closed. Only in the cis-1 adducts C60(NR)2 that prefer planar imino bridges (e.g., carbamates or amides) are the open forms VI more stable than the closed isomers V.

Catalytic Properties of Layered Gold–Palladium Colloids

Abstract: Layered bimetallic gold-palladium colloids in the size range of 20–56 nm have been synthesized by the seed=growth method: gold seeds were covered by palladium layers of various thickness and vice versa. The outer metal was coordinated by trisulfonated triphenylphosphine and sodium sulfanilate ligands to stabilize the bimetallic particles to such an extent that they could be isolated in the solid state. Owing to the hydrophilic ligand shell, redispersion in water was possible in any concentration. High-resolution transmission electron microscopy and energy-dispersive X-ray analysis was used to characterized the colloids. Stabilized and nonstabilized gold-palladium and palladium-gold systems on a TiO2 support were used as heterogeneous catalysts for the hydrogenation of hex-2-yne to cis-hex-2-ene. Both the palladium-plated gold seeds and the gold-plated palladium particles showed considerably increased activities compared with the pure metals. The ligand shell seems not to influence the catalytic behavior because protected and unprotected colloids behave very similarly.

Oxidative Degradation of Polychlorinated Phenols Catalyzed by Metallosulfophthalocyanines

Abstract: 2,4,6-trichlorophenol (TCP) is oxidized by potassium monopersulfate or hydrogen peroxide in the presence of iron or manganese tetrasulfonatophthalocyanines (FePcS or MnPcS) to yield not only the corresponding 2,6-dichloro-1,4-benzoquinone but also ring-cleavage products. Catalytic oxidation of the TCP ring by hydrogen peroxide is more efficient than by potassium monopersulfate, despite a slower substrate conversion, suggesting that different mechanisms are involved for these two catalytic systems: a metal-oxo mechanism for FePcS/KHSO5 and a metal-peroxo mechanism for FePcS/H2O2. Eight different final oxidation products and four quinone intermediates have been identified in the oxidation of TCP by the FePcS/H2O2 catalytic system. Chloromaleic acid is the main product of the oxidative ring cleavage. An iron-peroxo complex PcS-FeOOH is probably the active species responsible for the epoxidation of 2,6-dichloro-1,4-benzoquinone and the C–C bond cleavage of 3,5-dichloro-2-hydroxy-1,4-benzoquinone ring, both intermediates generated during the catalytic TCP degradation. The oxidation of pentachlorophenol (PCP) is also catalyzed by FePcS or MnPcS with KHSO5 or H2O2.

A Convergent Synthesis of Carbohydrate-Containing Dendrimers

Abstract: The synthesis of carbohydratecontaining dendrimers has been achieved by a convergent growth approach. The synthetic strategy involves: 1) the synthesis of the triglucosylated derivative of tris(hydroxymethyl)methylamine (TRIS), 2) the introduction of a glycine-derived spacer and 3,3′-iminodipropionic acid derived branching units on to the TRIS derivative by amide bond formation, 3) condensation of the above saccharidecontaining dendrons with a trifunctional 1,3,5-benzenetricarbonyl derivative, used as the core, by formation of amide bonds, and 4) deprotection of the saccharide units. A 9-mer and an 18-mer, carrying nine and eighteen saccharide units at the periphery, respectively, have been synthesized, in high yields at each step, by this synthetic strategy. By a variety of chromatographic and spectroscopic techniques, the dendrimers were shown to be structurally homogeneous, monodisperse, and error-free at all steps in their growth. These investigations were complemented by molecular modeling studies on the dendrimers. The presence of slightly distorted C3 symmetry was noted in both the 9-mer and the 18-mer.

Dimensional reduction in II-VI materials: A2Cd3Q4 (A = K, Q = S, Se, Te; A = Rb, Q = S, Se), novel ternary low-dimensional cadmium chalcogenides produced by incorporation of A2Q in CdQ

Abstract: The synthesis of the isomorphous, layered chalcogenides K2Cd3S4 (I), Rb2Cd3S4 (II), K2Cd3Se4 (III), Rb2Cd3Se4 (IV), and K2Cd3Te4 (V) in molten A2Qx fluxes in reported (A = K, Rb; Q = S, Se, Te; x = 2 to 3). The compounds form as (Cd3Q4) layers interspersed with A + cations; the layers are composed of Cd3Q2−4 units shaped as truncated cubes. The compounds have room-temperature band gaps of 2.75, 2.92, 2.36, 2.37, and 2.26 eV for I, II, III, IV, and V, respectively, and also display strong photoluminescence. The thermal analysis data for all compounds are reported. The properties of these compounds are compared with those of the three-dimensional compounds CdS, CdSe, and CdTe, as well as those of the nanometer-sized CdQ clusters. A conceptual context is presented to connect all these different types of compounds.

A Modified Cyclodextrin with a Fully Encapsulated Dansyl Group: Self-Inclusion in the Solid State and in Solution

Abstract: A monofunctionalized β-cyclodextrin containing a dansyl moiety, 6- deoxy- 6 - N - ( N′- (5- dimethylamino - 1 - naphthalenesulfonyl)diaminoethane) - β-cyclodextrin (CD-en-DNS, 2), was synthesized and its crystal structure determined. It was shown that the dansyl group is fully encapsulated within the cyclodextrin cavity, with the dimethylamino and sulfonyl groups emerging from opposite sides. The shape of the cavity is considerably flattened, since O(4)–O(4) distances parallel to the naphtalene ring were found to be longer than the others. The conformation of the diaminoethane linker was found to be determined by the inclusion of the dansyl group and by a hydrogen bond between the sulfonamide NH and one of the O(6)–H groups on the cyclodextrin rim. The self-inclusion features of the aromatic moiety were found to be consistent with the solution data: 1H NMR ROESY spectra suggested that the orientation of the dansyl moiety observed in the solid state was retained in aqueous solution; the circular dichroism spectrum was consistent with an axial complexation model. Fluorescence spectra showed that the inclusion of the dansyl group in the cyclodextrin cavity considerably increases the quantum yield: time-resolved fluorescence experiments showed the presence of a long-lifetime component (16.1 ns), which was attributed to the included fluorophore. The ability of 2 to act as a fluorescence sensor was evaluated by the addition of several guests of different shape: fluorescence intensity was lowered, especially upon addition of adamantanecarboxylic acid. All the data obtained were consistent with the model of the in-out movement of the dansyl group from the self-included conformation observed in the solid state to a position more exposed to the bulk solvent. Copper(II) was shown to enhance the difference in the fluorescence of 2 in the presence of guests by additional static quenching.