Selective interaction of lower rim calix[4]arene derivatives and bivalent cations in solution. Crystallographic evidence of the versatile behavior of acetonitrile in lead(II) and cadmium(II) complexes

The interaction of lower rim calix(4)arene derivatives containing ester (1) and ketone (2) functional groups and bivalent (alkaline-earth, transition- and heavy-metal) cations has been investigated in various solvents (methanol, N,N-dimethylformamide, acetonitrile, and benzonitrile). Thus, 1H NMR studies in CD3OD, C3D7NO, and CD3CN show that the interaction of these ligands with bivalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+, Cd2+) is only observed in CD3CN. These findings are corroborated by conductance measurements in these solvents including benzonitrile, where changes upon the addition of the appropriate ligand (1 or 2) to the metal-ion salt only occur in acetonitrile. Thus, in this solvent, plots of molar conductance against the ligand/metal cation ratio reveal the formation of 1:1 complexes between these ligands and bivalent cations. Four metal-ion complex salts resulting from the interaction of 1 and 2 with cadmium and lead, respectively, were isolated and characterized by X-ray crystallography. All four structures show an acetonitrile molecule sitting in the hydrophobic cavity of the ligand. The mode of interaction of the neutral guest in the cadmium(II) complexes differs from each other and from that found in the lead(II) complexes and provides evidence of the versatile behavior of acetonitrile in binding processes involving calix(4)arene derivatives. The thermodynamics of complexation of these ligands and bivalent cations in acetonitrile is reported. Thus, the selective behavior of 1 and 2 for bivalent cations is for the first time demonstrated. The role of acetonitrile in the complexation process in solution is discussed on the basis of 1H NMR and X-ray crystallographic studies. It is suggested that the complexation of 1 and 2 with bivalent cations is likely to involve the ligand-solvent adducts rather than the free ligand. Plots of complexation Gibbs energies against the corresponding data for cation hydration show a selectivity peak which is explained in terms of the predominant role played by cation desolvation and ligand binding energy in complex formation involving metal cations and macrocycles in solution. A similar peak is found in terms of enthalpy suggesting that for most cations (except Mg2+) the selectivity is enthalpically controlled. The ligand effect on the complexation process is quantitatively assessed. Final conclusions are given highlighting the role of the solvent in complexation processes involving calix(4)arene derivatives and metal cations.     

The Structures and Hydrogen Bonding Networks in Crystals of Alkylenediammonium Salts of Galactaric Acid

The crystal structures of five alkylenediammonium galactarates (1– 5) were determined because the information from these structures may provide some insight into the solid state structures of the poly(alkylene galactaramides) derived from these salts. In each case the meso-galactarate anion is in the extended conformation. In four out of the five cases associations between galactarate units led to alternating layers of anions and cations rather than the expected alternation of anion and cation found in ionic solids. All five salts display extensive hydrogen bonding involving ammonium and carboxylate groups and in some cases hydroxyl groups of the anion.     

Chiral heterocyclic ligands. XVI: Synthesis and crystal structures of four metal complexes of a tridentate,biheterocyclic ligand derived from l-cysteine

The readily available homochiral ligand (1) was used to prepare four mononuclear nickel and cobalt complexes with a 2:1 ligand:metal ratio. X-ray crystal structures of these revealed that in all cases the N,N,O-tridentate ligand adopts facial coordination to the metal with the pyridine rings trans-disposed and the carboxylate and amine groups cis. The individual crystal structures of these compounds differ significantly in the way the complexes pack, which is mediated by hydrogen bonding involving the cations, anions and water molecules as well as, in one case, π-stacking.     

Nonstoichiometric spinel ferrites obtained from alpha-NaFeO2 via molten media reactions

Different solid/liquid "exchange" reactions involving divalent cations, protons, or ammonium ions have been performed at low/moderate temperatures (between 80 and 500 degrees C) on alpha-NaFeO2 dipped in molten salts (or acid) media. Several ferrites have been obtained which are nonstoichiometric with partially inverse spinel structures. When sodium is replaced by divalent cations (Mg2+, Co2+, Ni2+, and Zn2+), the obtained ferrites are hyperstoichiometric (cation/oxygen ratio higher than 3/4) whereas proton or ammonium reactions result in hypostoichiometric materials (cation/oxygen lower than 3/4). All these ferrites present a platelet-like morphology and show ferrimagnetic, soft magnet behavior.     

Biladienones from the photooxidation of a meso-gem-disubstituted phlorin: crystal and molecular structures of the 3N + O coordinated nickel(II) and copper(II) complexes

The photooxidation of a meso-gem-disubstituted phlorin gave two isomeric biladienones in an equilibrium involving a Z-E double bond photoisomerism. The structures of these bile pigments were elucidated using NMR techniques and show terminal benzoyl and pyrrolone moieties. Complexation with divalent cations (nickel(II), copper (II)) gave stable compounds whose crystal and molecular structure could be determined by X-ray diffraction. The metal is coordinated with three pyrrole nitrogen atoms and one oxygen atom of a terminal benzoyl group. In the crystal, the molecules are arranged in pairs through hydrogen bonds between the free terminal pyrrolones.     

Exploiting phenyl embraces and pi-stacking in the assembly of arrays of tetraphenylphosphonium p-sulfonatocalix[4]arene

Phenyl embraces involving tetraphenylphosphonium cations feature in complexes of p-sulfonatocalix[4]arene where a phenyl ring of a cation is included in the cavity of the calixarene. The overall structures are based on pseudo-polymorphic supramolecular arrays and their formation is templated or induced by lanthanide ions.     

Synthesis and isolation of polytrityl cations by utilizing hexaphenylbenzene and tetraphenylmethane scaffolds

The successful isolation of stable (and soluble) hexa- and tetratrityl cations based on hexaphenylbenzene and tetraphenylmethane scaffold has been accomplished by using readily available starting materials. These robust polytrityl cations can be isolated in crystalline form and stored indefinitely at 0 degrees C. Their structures have been established by (1)H/(13)C NMR spectroscopy and by UV-vis spectroscopy. The structures of these polytrityl cations were further confirmed by quantitative transformations to the reduced (poly)triphenylmethyl derivatives by hydride transfer from triethylsilane, cycloheptatriene, or a borane-dimethyl sulfide complex.     

Structures and relative energies of polylithiated benzenes

The geometries of dilithiobenzene, trilithiobenzene, tetralithiobenzene, pentalithiobenzene, and hexalithiobenzene were optimized at the B3LYP/6-311+G** level. The lowest energy structures can be understood in terms of maximizing the electrostatic interactions between carbanions and lithium cations. In particular, in-plane lithium cations bridging ortho dianions is a geometric arrangement that is repeatedly found, as epitomized in the star-shaped D6h hexalithiobenzene structure 11a. Disproportionation reactions involving phenyllithium leading to polylithiated benzenes are exothermic, suggesting that it may be posible to prepare highly lithiated aromatic species.     

Ab initio study of trimethyloxonium and disilylmethyloxonium cations as intermediates in reactions of CH3 cations with ethers and disiloxanes

The study of the gas-phase reactions ofCH₃⁺ cations generated from tert-butyl ether and hexamethyldisiloxane by the nuclear chemical method shows that these cations are produced via oxonium cations as intermediates. Trimethyloxonium and disilylmethyloxonium cations were taken as model structures for these intermediates, and their ab initio equilibrium geometries were refined at the HF/4-21 (O^*)G level of theory. The relative yields of the products of these reactions under competitive conditions found experimentally are in keeping with the theoretical relative stability of the (CH₃)₃O⁺ and (SiH₃)₂CH₃O⁺ cations. The geometry of the latter was also used for the discussion of the surface structures that arise when methanol chemisorbs onto silica. Some previously unassigned bands in the IR spectra of these surface species, the frequencies of which coincide with calculated CH stretching frequencies of the disilylmethyloxonium cation, were attributed to surface oxonium methoxy groups.     

Probing the surface chemical structure of some novel poly(ortho esters) prepared with N-methyl and N-phenyl ethanolamine by time-of-flight secondary ion mass spectrometry (ToF-SIMS)

The surface chemical structures of a series of novel poly(ortho ester) homopolymer and copolymer films prepared using N-methyl- and N-phenyl-ethanolamine (MDE and PDE) have been investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Systematic fragmentation patterns were observed within the ToF-SIMS spectra up to m/z = 1600, which were readily interpreted in terms of the polymer structures. Cations were detected which could be assigned to structures arising from the diols, the ortho ester species and diads, triads etc. of the monomer, from both the homopolymer and the random portion of the copolymer chains. The analysis of the proposed ion structures suggested two major mechanisms of fragmentation based on the cleavage of the ortho ester bonds involving either the exocyclic or the endocyclic alkoxy group. The Tof-SIMS peak area ratios for ions diagnostic of the diols and the ortho ester species reflected the bulk copolymer composition in a semi-quantitative fashion. The observation of such relationships for high mass cations (up to m/z = 500) is particularly noteworthy and has not been reported previously. These studies allow a dramatic insight into the surface chemical structure of these poly(ortho esters), information which has formed the foundation of current investigations on the mechanisms of the acid-catalysed surface degradation of these polymers.     

Solvent control on the selective, nonselective, and absent response of a partially substituted lower rim calix(4)arene derivative for soft metal cations (mercury(II) and silver(I)). Structural and thermodynamic studies

The solvent control on the ability of a partially substituted lower rim calix(4)arene derivative 5,11,17,23,tetra-tert-butyl[25,27-bis(hydroxy)-26,28-bis(ethylthioethoxy)]-calix(4)arene, 1 to host soft metal cations (Hg(II) and Ag(I)) is demonstrated through 1H NMR, electrochemical (conductance measurements), and thermodynamic characterization of the complexation process in a wide variety of solvents. Solvent-ligand interactions were assessed from 1H NMR measurements involving 1 and various solvents in CDCl3. Thus, the formation of a 1:1 1-CH3CN adduct is reported. As far as metal cations are concerned, depending on the medium their complexation with 1 was only observed for Hg(II) and Ag(I). Thus, in acetonitrile, 1 is more selective for Hg(II) relative to Ag(I) by a factor of 2.2 x 10(3). In methanol the selectivity is reversed to an extent that the affinity of 1 for Ag(I) is 1.4 x 10(3) higher than that for Hg(II). However, 1 is unable to recognize selectively these cations in N,N-dimethylformamide while in propylene carbonate the ability of 1 to interact with these cations is lost. An outstanding feature of thermodynamics emerges when an assessment is made of the ligand effect on the complexation of these cations and analogues calix(4)arene derivatives. Thus, in acetonitrile the thermodynamics of cation complexation by the hydrophilic cavity of a calix(4)arene containing mixed pendant groups is built up from thermodynamic data for the same process involving derivatives with common functionalities at the narrow rim. This is a unique example of the additive contribution of pendant arms in the field of thermodynamics of calixarene chemistry.     

Room-temperature synthesis of manganese oxide monosheets

Preparation of single-layer manganese oxide nanosheets (monosheets) comprised of edge-shared MnO(6) octahedra has relied on multistep processing involving a high-temperature solid-state synthesis of bulk templates, and ion-exchange and exfoliation reactions in solutions, requiring high cost and long processing time. Here we demonstrate the first single-step approach to directly access the MnO(2) monosheets, by the chemical oxidation of Mn(2+) ions in the presence of tetramethylammonium cations in an aqueous solution. Of importance is that this template-free reaction readily proceeds within a day at room temperature. The ability of the MnO(2) monosheets to self-assemble allows aggregation, to form layered structures with potassium cations and cationic tetrathiafulvalene analogues as intercalants. Furthermore, Langmuir-Blodgett (LB) films composed of the MnO(2) monosheets were successfully fabricated by the LB deposition method, in which about one layer of the monosheets was deposited for each process.     

Clays as architectural units at modified-electrodes

Since Ghosh and Bard [80] first established the field of clay-modified electrodes some 15 years ago, great strides have been made in understanding the nature of the clay structural units and their impact on transport of a variety of electroactive probes (anions, neutrals, small cations, large cations, and compounds with distributed charge). Great strides have also been made in understanding the nature of the layered material in creating access of interlayer sites (size, charge, iron content, pillaring, and organic tailoring). In the last five years several successful applications of clay-modified electrodes have been achieved. Given the explosive growth in tailoring and construction of novel clay structures it seems reasonable to predict further significant advances in applications involving clay-modified electrodes.     

Origin of facial diastereoselection in 2-adamantyl cations. Theoretical evidence against the Felkin-Anh and the Cieplak models

Quantitative analysis of the transition state (TS) structures for the reactions between 5-Cl- or 5-SiMe₃-2-adamantyl cations and methanol revealed that the magnitude of antiperiplanar hyperconjugative stabilization involving the incipient bond (the AP effect) decrease at TS, strongly suggesting that the AP effect contributes to `net destabilization' of TS (i.e., increase in activation energy), in sharp contrast to the proposals of the Felkin-Anh and the Cieplak models. The equilibrium population between two bridge(C1-C2-C3)-flipping E-Z cation conformers was found to be the origin of facial diastereoselection of these carbocations.     

Effects of Zr doping on stoichiometric and reduced ceria: a first-principles study

The Zr doping in CeO(2) may change the reduction properties and therefore the redox properties of CeO(2). Using first-principles density functional theory with the inclusion of on-site Coulomb interaction for a 96-atom supercell, these effects are studied by comparing the differences in atomic structures, electronic structures, and reduction energies of the doped CeO(2) and those of the nondoped CeO(2). It is found that (1) Zr doping of the ceria structure results in important modifications involving nonequivalent O atoms; (2) the oxygen anions (still four-coordinated) next to the doping center show considerably lower reduction energies (by 0.6 eV) and larger displacements ("higher mobilities"); (3) an O vacancy is most easily created close to the Zr centers, therefore the Zr-doping centers might serve as nucleation centers for vacancy clustering; and (4) the electrons left by the released oxygen localize on two Ce cations neighboring the vacancy, which results in the reduction of two Ce(4+) ions.     

Supramolecular systems based on the melamine salt of bis(hydroxymethyl)phosphinic acid (melafen) dihydrate and surfactants

Bis(hydroxymethyl)phosphinic acid and melamine were shown by FTIR and UV spectroscopy to form the salt (melafen), whose cations and anions involving water molecules can be joined into supramolecular structures due to electrostatic interactions and hydrogen bonds. The conductometry, dielcometric titration, and dynamic light scattering methods showed that melafen in water and chloroform in a concentration range of 10⁻¹⁰–10⁻⁴ mol L⁻¹ involving the solvent structures exist as supramolecular polymeric nanostructures, whose size and properties change nonlinearly depending on the melafen concentration. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1207–1214, June, 2008.     

高分散隔离活性位催化剂中过渡金属离子结构对丙烷选择氧化反应性能的影响

考察了碱金属K修饰的SiO2负载极低含量过渡金属的高分散隔离活性位系列催化剂(K-M/SiO2(n(K)∶n(M)∶n(Si)=10∶1∶1000),M=V,Cr,Mn,Fe,Co,Ni,Cu,Zn)的丙烷选择氧化催化性能,并运用UV-Raman和CO2-TPD等方法,对该系列催化剂进行了结构和物化性能表征,探讨了催化剂的表面离子结构与催化反应性能之间的关系。 发现钾修饰的SiO2负载过渡金属高分散隔离活性位催化剂上过渡金属离子的组态结构,对丙烷选择氧化反应性能有重要影响。相对稳定的全充满或无d电子的表面离子结构有利于选择氧化反应进行,而存在多种价态的相对不稳定的离子结构有利于深度氧化的进行。 这一离子结构与丙烷选择氧化催化性能的关系与前期乙烷选择氧化规律相似,进一步说明在高分散隔离活性位催化剂中,过渡金属离子的电子组态结构是影响低碳烷烃选择氧化反应性能的最重要因素。     

A new family of ionic liquids based on N,N-dialkyl-3-azabicyclo[3.2.2]nonanium cations: organic plastic crystal behaviour and highly reversible lithium metal electrodeposition

A new family of ionic liquids based on N,N-dialkyl-3-azabicyclo[3.2.2]nonanium cations exhibits wide electrochemical windows, excellent lithium deposition-stripping behaviour and plastic crystal properties and therefore these cation structures, which are extensions from pyrrolidinium- and piperidinium-based cations, complement a set of related structures for a systematic study on the physicochemical properties of ionic liquids.     

Functionalized fullerene cations {R-C60}+ from a theoretical point of view

The first comprehensive theoretical investigation of a series of functionalized fullerene cations, {R-C(60)}(+), where R = H, CH(3), CH(2)Cl, CHCl(2), and CCl(3), is accomplished. A detailed consideration of conformational energetic profiles of these systems resulted in the structures of most stable conformers that fit all available experimental data. The trends in stability of {R-C(60)}(+) cations as well as transition barriers for migration of alkyl groups over the C(60)-surface are provided. A thorough study of energetics of {R-C(60)}(+) is augmented by in-depth investigation of their electronic structures and aromaticity. As the final part of this study, the detailed comparison of structures, energetics and properties of {R-C(60)}(+) with those of corannulene cations, {R-hub-C(20)H(10)}(+), is performed at the same level of theory. The similarities and differences between the surface functionalized ball- and bowl-shaped carbocations are illuminated.