NMR evaluation of the configurational stability of Cu(I) complexes

Association of chiral [CuL2]+ complexes (L = 2-R-phen, 6-R-bpy, and 2-iminopyridine) with TRISPHAT (tris(tetrachlorobenzenediolato)phosphate(V)) anion leads to NMR enantiodifferentiation, which can be used to determine the kinetics of racemization of the complexes.     

Biphasic enantioselective olefin epoxidation using Tropos dibenzoazepinium catalysts

Several novel chiral iminium TRISPHAT [tris(tetrachlorobenzenediolato)phosphate(V)] salts combining a diphenylazepinium core, chiral exocyclic appendages, and lipophilic counterions have been prepared and tested in biphasic enantioselective olefin epoxidation conditions. Interestingly, the iminium salts derived from commercially available (S)- or (R)-1,2,2-trimethylpropylamine can display efficiency similar to those made from L-acetonamine. Variable-temperature NMR spectroscopy (VT-NMR) and circular dichroism (CD) experiments were performed in search of a correlation between good enantioselectivity in the products and high diastereomeric control of the biphenyl axial chirality of the catalysts.     

Chiral anion-based NMR enantiodiscrimination of a dinuclear, cationic Ir(I) NHC complex with a figure-of-eight loop structure

A dinuclear Ir(I) complex with a bis-NHC ligand was formed in situ from 1,1'-((4,6-dimethyl-1,3-phenylene)bis(methylene))bis(3-methyl-1H-imidazol-3-ium) in the presence of Cs(2)CO(3) and [Ir(μ-Cl)(cod)](2). Its solid-state structure, determined by X-ray diffraction, shows a figure-of-eight loop with both the P and M enantiomers being present. Solution NMR studies were performed to examine the enantiodiscrimination between them by applying the enantiopure anions tris(tetrachlorobenzenediolato)phosphate(v) (Δ-TRISPHAT) and bis(tetrachlorobenzenediolato)mono([1,10]binaphthalenyl-2,20-diolato)phosphate(v) (Λ-BINPHAT).     

Towards the stereoselective synthesis of inherently chiral pseudorotaxanes

Herein is reported an investigation towards the stereoselective synthesis of inherently chiral pseudorotaxanes. Chiral ammonium threads were readily prepared in five steps from racemic or enantiopure (M or P) salts of di-n-propyl-1,13-dimethoxyquinacridinium cation. Their self-assembly with DB24C8 or disymmetrically oriented DB24C8F6 rings formed pseudorotaxanes as shown by 1H and 19F NMR spectroscopy as well as MS measurements. A determination of the association constants (Ka) was afforded. The crucial role played by the ammonium counter-ion in the threading process was further demonstrated as salts of TRISPHAT (tris(tetrachlorobenzenediolato)phosphate(V)) anion were quite more effective than their PF6- analogues (x 7.3). A general lack of diastereoselectivity (de 相似文献   

Kinetics of ion-exchange of alkaline earth metals on antimony(V) phosphate cation exchanger

Forward and reverse ion exchange kinetics of Mg(II), Ca(II), Sr(II) and Ba(II) exchange with H(I) has been studied on antimony(V) phosphate cation exchanger applying the Nernst-Planck equations. As a result, some kinetic parameters like diffusion coefficients, activation energies, and entropies of activation have been evaluated under the conditions favoring a particle diffusion-controlled mechanism. Such kinetic parameters have also been compared with the kinetic parameters of different single and double salts.     

Enantioselective self-assembly of bimetallic [Mn(II) (delta)-Cr(III)(C2O4)3]- and [Mn(II) (lambda)-Cr(III)(C2O4)3]- layered anionic networks templated by the optically active (Rp)- and (Sp)-[1-CH2N(n-C3H7)3-2-CH3-C5H3Fe-C5H5]+ ions

The ferrocenic ammonium (Rp)- and (Sp)-[1-CH2NR(3-)-2-CH3-C5H3Fe-C5H5] iodide salts with R=CH3, C2H5, n-C3H7, n-C4H9, were synthesized starting from the (Rp)- and (Sp)-[1-CH2N(CH3)2-2-CH3-C5H3Fe-C5H5] amines obtained in their optically active forms through asymmetric cyclopalladation of [C5H5Fe-C5H4CH2N(CH3)2]. 1H NMR studies of these planar chiral 1,2-disubstituted ferrocenic ammonium iodide salts in the presence of the (Delta)-(tris(tetrachlorobenzenediolato)phosphate(V) anion), [(Delta)-Trisphat] support the formation of specific diastereomeric ion pairs. Such intermolecular interactions can be related to the self-assembly of the two-dimensional optically active compounds [[(Sp)-1-CH2N(n-C3H7)3-2-CH3-C5H3Fe-C5H5][Mn (Delta)-Cr(C2O4)3]] and [[(Rp)-1-CH2N(n-C3H7)3-2-CH3-C5H3Fe-C5H5][Mn (Lambda)-Cr(C2O4)3]] starting from the resolved (Rp)- and (Sp)-[1-CH2N(n-C3H7)3-2-CH3-C5H3Fe-C5H5]+ ion associated to the racemic anionic building block rac-[Cr(C2O4)3]3- and Mn2+. Both enantiomeric forms of the networks behave as ferromagnets with a Curie temperature of 5.7 K.     

C3-symmetric trinuclear molybdenum cluster sulfides: configurational stability, supramolecular stereocontrol, and absolute configuration assignment

The chiral C3-symmetric [Mo3S4Cl3(dppe)3]+ cluster [dppe = 1,2-bis(diphenylphosphinoethane), P or M enantiomers] with incomplete cuboidal structure is shown to be configurationally stable at room temperature and configurationally labile at elevated temperature using enantiopure Delta- or Lambda-TRISPHAT [(tris(tetrachlorobenzenediolato)phosphate(V)] anions both as chiral NMR solvating and asymmetry-inducing reagents. It is evidenced that the enantiomers of this trinuclear cluster cation can equilibrate at higher temperature (typically 72 degrees C), and in the presence of the hexacoordinated phosphate anion, a moderate level of stereocontrol (1.2:1) can be achieved. It results in a diastereomeric enrichment of the solution in favor of the heterochiral ion pairs, e.g., M+ Delta- or P+ Lambda-. At higher temperature, a partial racemization of the TRISPHAT anion is also observed, and precipitation at room temperature of [rac-Mo3S4Cl3(dppe)3][rac-TRISPHAT] salts from the diastereomeric enriched solution improves the diastereomeric purity of the mother liquor to a 4:1 ratio. A low-energy pathway for the inverconversion between the [P-Mo3S4Cl3(dppe)3]+ and [M-Mo3S4Cl3(dppe)3]+ enantiomers has been found using combined quantum mechanics and molecular mechanics methodologies. This pathway involves two intermediates with three transition state regions, which result from the partial decoordination of the diphosphane coordinated at each metal center. Such decoordination creates a vacant position on the metal, producing a Lewis acidic site that presumably catalyzes the TRISPHAT epimerization.     

Bimetallic iridium(III) complexes consisting of Ir(ppy)(2) units (ppy = 2-phenylpyridine) and two laterally connected N/\N chelates as bridge: synthesis, separation, and photophysical properties

Reaction of the precursor complex Ir2(ppy)4Cl2 (ppy = 2-phenylpyridine) with the bridging ligand 3,8-dipyridyl-4,7-phenanthroline (L) affords, in 94% yield, the cyclometalated iridium dinuclear complex [(ppy)2Ir(mu-L)Ir(ppy)2]2+ (12+) as a mixture of three stereoisomers. This mixture consists of a meso form Delta,Lambda and a racemic form (enantiomeric pair Delta,Delta and Lambda,Lambda) in the ratio 1:1.5. Single-crystal X-ray characterization of the perchlorate salt of the meso form reveals (i) the distortion of the bridging ligand from the planarity and (ii) the location of the two iridium subunits above and below the medium plane of the bridging ligand. Ion-pair chromatography with Delta-TRISPHAT anion (TRISPHAT = tris(tetrachlorobenzenediolato)phosphate(V)) as resolving anion permits the separation of the three stereoisomers. The 1H NMR spectroscopic analysis of each fraction indicates high diastereomeric purity. Electronic circular dichroism properties and comparison with literature data establish their absolute configuration. The absorption and emission properties of the three stereoisomers show only very small variations. The anisotropic properties can be interpreted as distinct interactions of the isomers with the chiral resolving Delta-TRISPHAT anion.     

NMR, PGSE diffusion, and X-ray diffraction studies of lithium and potassium salts derived from diphenylphosphino(o-cyanophenyl)aniline and their crown ether complexes

1H, 31P, and 7Li pulsed-gradient spin-echo (PGSE) diffusion and variable-temperature NMR results for THF solutions of the lithium and potassium salts derived from diphenylphosphino(o-cyanophenyl)aniline are reported and compared to the solid-state results obtained via X-ray diffraction studies. The solution results favor mononuclear salts, sometimes strongly ion paired, whereas the solid-state data reveal dinuclear species. The structures of the products from reactions of these salts with crown ethers are determined via PGSE and 1H Overhauser NMR methods.     

Counterion-induced translational isomerism in a bistable [2]rotaxane

Translational isomerization can be induced by changing the anions associated with a bistable rotaxane in which the tetracationic cyclophane (blue box), cyclobis(paraquat-p-phenylene), encircles a dumbbell component containing bispyrrolotetrathiafulvalene (green) and a dioxynaphthalene (red) recognition sites. The rotaxane was isolated as both its hexafluorophosphate and tris(tetrachlorobenzenediolato)phosphate(v) (TRISPHAT(-)) salts. Photophysical measurements and NMR spectroscopy carried out in acetone (CD(3)COCD(3)) and acetonitrile (CD(3)CN) solutions reveal that the much larger TRISPHAT(-) anion favors predominantly the encirclement of the green site by the blue box.     

Synthesis, characterization, and PGSE (1H and 19F) NMR diffusion studies on cationic (eta6- arene)Mn(CO)3+ complexes: boron counterion, ion pairing, and solvent dependences

The synthesis, characterization, and PGSE ((1)H and (19)F) NMR diffusion studies on the cationic [(eta(6)-arene)Mn(CO)(3)][X] (arene = anisole, 4-chloroanisole, and 1,3,5-trimethoxybenzene; X = BPh(4) and BArF) are reported. The tetraphenyl borate complexes of anisole and 4-chloroanisole show surprisingly strong ion pairing in dichloromethane solution, whereas the BArF salts do not. (1)H,(1)H-NOESY data support this anion selectivity. In chloroform solution one finds the usual strong ion pairing for both anions. The solid-state structure of [(eta(6)-1,3,5-trimethoxybenzene)Mn(CO)(3)][BPh(4)] has been determined. (13)C NMR and IR data for the new complexes are reported. The observed IR frequencies are higher for the BArF complexes than for the BPh(4) complexes.     

PGSE NMR diffusion overhauser studies on [Ru(Cp*)(eta6-arene)][PF6], plus a variety of transition-metal, inorganic, and organic salts: an overview of ion pairing in dichloromethane

PGSE diffusion, 19F, 1H HOESY and 13C NMR studies for a series of [Ru(Cp*)(eta6-arene)][PF6] (1) salts are presented. The solid-state structure of [Ru(Cp*)(eta6-fluorobenzene)][PF6] (1 c) is reported. The extent of the ion pairing and the relative positions of the ions are shown to depend on the arene. For the solvent dichloromethane, new and literature PGSE data for PF6(-) salts of transition-metal, inorganic, and organic salts are compared. Taken together, these new results show that the charge distribution and the ability of the anion to approach the positively charged positions (steric effects due to molecular shape) are the determining factors in deciding the amount of ion pairing. DFT calculations of the charges in four salts of type 1, as well as in a variety of other salts, using a natural population analysis (NPA), support this view. This represents the first attempt, using experimental data, to understand, correlate, and partially explain the various degrees of ion pairing in a widely different collection of salts.     

Intermolecular hydrogen bonding of steroid compounds: PFG NMR diffusion study, cold-spray ionization (CSI)-MS and X-ray analysis

An extensive analysis of hydrogen bonding of steroid compounds in diluted solution is preformed by pulsed field gradient (PFG) NMR and cold-spray ionization (CSI)-MS, in the solid state by X-ray crystallographic analysis. The formation of hydrogen bond interaction are quantified and discussed. Although X-ray analysis in the crystalline state and CSI-MS measurement in solution suggested that the observed diffusion coefficient D(obs) of the steroid compounds may vary in accordance with the number of hydrogen bonds, the actual observed D(obs) value determined from the diffusion studies diminished constantly without correlation on the decreasing numbers of hydrogen bonds. Comparison of two different calibration profiles of calculated molecular volume (V(cal)) vs. D(obs), which are obtained from compounds possessing no hydrogen bonding and the steroid compounds, formation of a chain structure (cluster) based on intermolecular hydrogen bonding of the steroid compounds is unambiguously confirmed.     

7Li, 31P, and 1H pulsed gradient spin-echo (PGSE) diffusion NMR spectroscopy and ion pairing: on the temperature dependence of the ion pairing in Li(CPh3), fluorenyllithium, and Li[N(SiMe3)2] amongst other salts

7Li, 31P, and 1H variable-temperature pulsed gradient spin-echo (PGSE) diffusion methods have been used to study ion pairing and aggregation states for a range of lithium salts such as lithium halides, lithium carbanions, and a lithium amide in THF solutions. For trityllithium (2) and fluorenyllithium (9), it is shown that ion pairing is favored at 299 K but the ions are well separated at 155 K. For 2-lithio-1,3-dithiane (13) and lithium hexamethyldisilazane (LiHMDS 16), low-temperature data show that the ions remain together. For the dithio anion 13, a mononuclear species has been established, whereas for the lithium amide 16, the PGSE results allow two different aggregation states to be readily recognized. For the lithium halides LiX (X = Br, Cl, I) in THF, the 7Li PGSE data show that all three salts can be described as well-separated ions at ambient temperature. The solid state structure of trityllithium (2) is described and reveals a solvent-separated ion pair formed by a [Li(thf)4]+ ion and a bare triphenylmethide anion.     

Formation and diffusion of oxygen-vacancy pairs on TiO2(110)-(1x1)

We present the measurements for the diffusion of bridging oxygen vacancy (OV) crossover Ti rows via OV pairs (OVPs). Using a high-resolution scanning tunneling microscope (STM), we show that the OVs can be moved along the bridging oxygen rows driven by the STM tip at voltages higher than 3.0 V on TiO(2)(110)-(1x1) surface. It is found that the combination of OVPs leads to the formation of OVPs, which can diffuse crossover Ti rows under the mediation of OVs in adjacent bridging oxygen rows. The deduced diffusion activation energy for the diffusive OVPs from experiments is in agreement with first-principles calculations. The reaction activation energy of the OVPs with O(2) is lower than that of the OVs by 82 meV.     

Intrinsic diffusion of hydrogen on rutile TiO2(110)

The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO 2(110) is presented. Sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting the presence of a long-lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. Density functional theory calculations show that there are two comparable low energy diffusion pathways for hydrogen motion along the BBO from one BBO to its neighbor, one by a direct hop and the other by an intermediate minimum at a terrace O. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via a two-step mechanism: the initial diffusion of localized charge, followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.     

Adsorption and diffusion of Mg, O, and O2 on the MgO(001) flat surface

A thorough investigation of the adsorption and diffusion of Mg, O, and O(2) on MgO(001) terraces is performed by first-principles calculations. The single Mg adatom weakly binds to surface oxygens, diffuses, and evaporates easily at room temperatures. Atomic O strongly binds to surface oxygens, forming peroxide groups. The diffusion of the O adatom is strongly influenced by the spin polarization, since energy barriers are significantly different for the singlet and triplet states. The crossing of the two Born-Oppenheimer surfaces corresponding to the distinct spin states is also analyzed. Although the O(2) molecule does not stick to the perfect surface, it chemisorbs on surface nonstoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. We show that the oxidation rate limiting factor in an O(2) atmosphere is the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers are used to estimate the Mg and O surface lifetimes and diffusion lengths, and some implications for the homoepitaxial growth of MgO are discussed.     

Preparation, structure, and properties of tetranuclear vanadium(III) and (IV) complexes bridged by diphenyl phosphate or phosphate

Three novel tetranuclear vanadium(III) or (IV) complexes bridged by diphenyl phosphate or phosphate were prepared and their structures characterized by X-ray crystallography. The novel complexes are [{V(III)(2)(μ-hpnbpda)}(2){μ-(C(6)H(5)O)(2)PO(2)}(2)(μ-O)(2)]·6CH(3)OH (1), [{V(III)(2)(μ-tphpn)(μ-η(3)-HPO(4))}(2)(μ-η(4)-PO(4))](ClO(4))(3)·4.5H(2)O (2), and [{(V(IV)O)(2)(μ-tphpn)}(2)(μ-η(4)-PO(4))](ClO(4))(3)·H(2)O (3), where hpnbpda and tphpn are alkoxo-bridging dinucleating ligands. H(3)hpnbpda represents 2-hydroxypropane-1,3-diamino-N,N'-bis(2-pyridylmethyl)-N,N'-diacetic acid, and Htphpn represents N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine. A dinuclear vanadium(IV) complex without a phosphate bridge, [(VO)(2)(μ-tphpn)(H(2)O)(2)](ClO(4))(3)·2H(2)O (4), was also prepared and structurally characterized for comparison. The vanadium(III) center in 1 adopts a hexacoordinate structure while that in 2 adopts a heptacoordinate structure. In 1, the two dinuclear vanadium(III) units bridged by the alkoxo group of hpnbpda are further linked by two diphenylphosphato and two oxo groups, resulting in a dimer-of-dimers. In 2, the two vanadium(III) units bridged by tphpn are further bridged by three phosphate ions with two different coordination modes. Complex 2 is oxidized in aerobic solution to yield complex 3, in which two of the three phosphate groups in 2 are substituted by oxo groups.     

Acid recovery using diffusion dialysis with poly(4-vinylpyridine)-filled microporous membranes

Two series of membranes have been produced by photoinitiated polymerization of 4-vinylpyridine (4VP) and divinylbenzene (DVB) within the pores of polypropylene microfiltration membranes. The first series was comprised of membranes with varying mass gain and constant DVB content. The second series of membranes had similar mass gains but varying DVB content. The membranes were tested by diffusion dialysis of acid/salt solutions (HCl/NaCl/MgCl₂) in order to determine the effects of both mass gain and degree of crosslinking on dialysis coefficients and acid/salt separation. It was found for the first series of membranes that the dialysis coefficients of the acid and salts decreased and then leveled off with increasing mass gain while separation increased and then also leveled off. The second series of membranes showed a decrease in acid and salt dialysis coefficients but a dramatic increase in separation as the DVB content was increased. These results are interpreted in terms of the fixed charge concentration and the water content of the membranes. A comparison is made with a commercial diffusion dialysis membrane.     

Effect of ionic environments on the adsorption and diffusion characteristics of serine alkaline protease enzyme in polyethersulfone ultrafiltration membranes

Static adsorption of serine alkaline protease (SAP) enzyme on hydrophobic polyether sulfone (PES) ultrafiltration membranes in different ionic environments was investigated. The amount of SAP adsorbed on membranes was the lowest at its isoelectric point (IEP) where the maximum adsorption was obtained below the IEP of the enzyme. The extent of SAP adsorption in the phosphate buffer solutions including different salts followed the order: (NH4)2HPO4 > KH2PO4 > Na2HPO4-NaH2PO4 (buffer) > CaCl2 > ((NH4)2HPO4 + H2PO4 + CaCl2), which was consistent with the Hofmeister series. The zeta potentials of membranes contacted with the ionic species were calculated by streaming potential measurements and found that the increase in ionic strength decreased the electrical double layer thickness leading to a decrease in adsorption. A model based on mass balance was developed to calculate the diffusion coefficient of SAP in PES membranes. Employing experimental data evaluated in a diffusion cell along with the data of adsorption isotherms, diffusion coefficients of SAP in PES membranes in the presence of different ionic species were calculated. To detect the structural changes occurred, membrane surfaces were analysed by Fourier transform infrared-attenuated total reflectance (FTIR-ATR) measurements.