Rh complexes of 1-(2,4,6-triisopropylphenyl-)3-methyl-1H-phosphole: preparation and use as catalysts in the hydroformylation of styrene

In the series of 1-(2,4,6-trialkylphenyl-)3-methyl-1H-phospholes (1a–c) that are to a certain extent of aromaticity, only the isopropyl substituted one (1a) entered into reaction with dimeric (pentamethylcyclopentadienyl)rhodium dichloride to afford Rh(III) complex (2a) in a reversible manner. After a careful workup, (2a) could be prepared and characterized whose stereostructure was elucidated by B3LYP/3–21G*, B3LYP/6–31G* and LANL2DZ calculations. Complex (2a) as a preformed catalyst, as well as the Rh(acac)(CO)₂ + 2(1a) in situ catalytic system were useful in the hydroformylation of styrene and gave the branched aldehyde in regioselectivities of 65–96%.     

Supramolecular complex of [60]fullerene-grafted polyelectrolyte and surfactant: mechanism and nanostructures

Water-soluble, pH-responsive mono- and di-[60]fullerene end-capped poly(acrylic acid)s (PAA-C60 and C60-PAA-C60) were synthesized using the atom transfer radical polymerization technique. Isothermal titration calorimetry, dynamic light scattering, UV-vis spectroscopy, and transmission electron microscopy were employed to study the supramolecular complexation between fullerene end-capped PAAs and nonionic surfactant, polyethylene glycol (9-10) tert-octylphenyl ether, also known as Triton X100 (TX100) at different pH values. At pH < 4, TX100 bound specifically to C60 domains driven by hydrophobic and pi-pi interactions between TX100 and fullerene molecules. The binding was exothermic, and the magnitude of the interaction decreased gradually with increasing pH. The amount of polymer-bound TX100 was proportional to the fullerene content, which was approximately 1.3 and approximately 2.5 mM for 5 mM (concentration of carboxylic groups) PAA-C60 and C60-PAA-C60, respectively. Morphological transformations resulting in the formation of polymer/surfactant complex (PSC) precipitates in the course of binding were observed for both polymers. The PSC of PAA-C60 possessed a dense spherical structure, whereas the PSC of C60-PAA-C60 possessed a lamellar stacking structure. The PSC precipitates resolubilized in excess amounts of TX100 to form stable aggregates.     

Ring strain energies: substituted rings, norbornanes, norbornenes and norbornadienes

Ring strain energies (RSEs) are predicted using homodesmotic reactions at the B3LYP/6-31G^* level of theory. Substituents are conserved in the acyclic reference and any difference in energy between the ring and the acyclic reference corresponds exclusively to RSE. Small rings are stabilized by alkyl substituents and this stabilization decreases as the size of the ring increases. There is a destabilization of medium sized rings. Greater stabilization is found upon alkyl substitution at a double bond in an unsaturated ring and this stabilization decreases as ring size increases. The effects of cis-1,2-disubstitution on RSEs have been evaluated and indicate stabilization for both small and medium sized rings. RSEs of saturated and unsaturated polycyclic systems agree well with the RSEs derived from experimental thermochemical data. RSEs are reported for substituted norbornanes, norbornenes, and norbornadienes to complement experimental studies.     

N,N'-ethylenebis(pyridoxylideneiminato) and N,N'-ethylenebis(pyridoxylaminato): synthesis, characterization, potentiometric, spectroscopic, and DFT studies of their vanadium(IV) and vanadium(V) complexes

The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds.     

Structure and catalytic activity of some soluble polyethylene glycol-peptide conjugates

Soluble catalysts for the Juliá-Colonna asymmetric epoxidation reaction have been constructed in three different ways, using NH₂-PEG-OMe as the support system: suitable solvents have been identified and it is shown that the degree of helicity of the conjugates correlates with the extent of conversion and (to a lesser extent) the enantioselectivity of epoxidation.     

Synthesis and application of novel carbohydrate-based ammonium and triazolium salts

The synthesis of nine new carbohydrate-based quaternary ammonium salts and two new triazolium salts starting from d-glucose has been accomplished. Our synthesis utilized the regio- and stereoselective ring opening reaction of 2,3-anhydro sugars by nucleophilic reagents to afford the key intermediates. In these new types of phase transfer catalysts, the ammonium and triazolium functions are directly attached to the carbohydrate scaffold in different positions (2-, 3-, 6-positions of the sugar). The efficiency of the altrose- and glucose-based quaternary salts were tested in the alkylation of N-(diphenyl) methylene glycine tert-butyl ester with benzyl bromide. To our knowledge this is the first example when sugar-based quaternary ammonium or triazolium salts were used successfully as phase transfer catalysts. The enantiomeric recognition ability of the synthesized salts towards racemic Mosher’s acid silver salt was also investigated by ¹⁹F NMR spectroscopy.     

Dioxomolybdenum(VI) complexes containing 1-alkyl-2-ethyl-3-hydroxy-4-pyridin-4(1H)-ones

Dioxomolybdenum(VI) complexes derived from ethyl maltol (2-ethyl-3-hydroxy-4-pyrone) and 1-alkyl-2-ethyl-3-hydroxy-4-pyridin-4(1H)-ones have been prepared and characterized using physical methods including ¹H- and ¹³C-n.m.r. spectroscopy, i.r., elemental analysis, and X-ray diffraction for the pyrone and 3-methylpyridine pyridinone derivatives. The octahedral complexes have the general formula cis-MoO₂L₂ where L = the deprotonated pyrone or pyridinone ligand.     

Mimicking catalase and catecholase enzymes by copper(II)-containing complexes

An imidazolate-bridged copper(II)-zinc(II) complex (Cu(II)-diethylenetriamino-μ-imidazolato-Zn(II)-tris(2-aminoethyl)amine perchlorate (denoted as “Cu,Zn complex”) and a simple copper(II) complex (Cu(II)-tris(2-aminoethyl) amine chloride (“Cu-tren”) were prepared and immobilised on silica gel (by hydrogen or covalent bonds) and montmorillonite (by ion exchange). The immobilised substances were characterised by FT-IR spectroscopy and their thermal characteristics were also studied. The obtained materials were tested in two probe reactions: catalytic oxidation of 3,5-di-tert-butyl catechol (DTBC) (catecholase activity) and the decomposition of hydrogen peroxide (catalase activity). It was found that the catecholase activity of the Cu,Zn complex increased considerably upon immobilization on silica gel via hydrogen bonds and intercalation by ion exchange among the layers of montmorillonite. The imidazolate-bridged copper(II)-zinc(II) complex and its immobilised versions were inactive in hydrogen peroxide decomposition. The Cu(II)-tris(2-aminoethyl)amine chloride complex displayed good catalase activity; however, immobilisation could not improve it.     

FTIR study of CO adsorption on molybdena-alumina catalysts for surface characterization

CO adsorption at low temperature has been used to probe Lewis acid sites created upon dehydroxylation of γ-Al₂O₃ and reduction of Mo/Al₂O₃ catalysts, using Fourier Transform Infrared spectroscopy (FTIR). Carbon-monoxide adsorption on γ-Al₂O₃ and Mo/Al₂O₃ catalysts dehydroxylated and reduced at different temperatures was studied at 78 K by IR spectroscopy. However, our results indicate that there is an approximately linear correlation between the increase either of dehydroxylation or the extent of reduction of the catalysts and the increasing absorbance of CO due to CO adsorption on Lewis acid sites created upon dehydroxylation of γ-Al₂O₃ and reduction of Mo/Al₂O₃.     

Synthesis and aqueous solution properties of sterically stabilized pH-responsive polyampholyte microgels

Emulsion copolymerization of poly(methacrylic acid) and poly(2-(diethylamino)ethyl methacrylate) (PMAA/PDEA) yielded pH-responsive polyampholyte microgels of 200-300 nm in diameter. These microgels showed enhanced hydrophilic behavior in aqueous medium at low and high pH, but formed large aggregates of approximately 2500 nm at intermediate pH. To achieve colloidal stability at intermediate pH, a second batch of microgels of identical monomer composition were synthesized, where monomethoxy-capped poly(ethylene glycol)methacrylate (PEGMA) was grafted onto the surface of these particles. Dynamic light-scattering measurements showed that the hydrodynamic radius, Rh, of sterically stabilized microgels was approximately 100 nm at intermediate pH and increased to 120 and 200 nm at pH 2 and 10, respectively. Between pH 4 and 6, these microgels possessed mobility close to zero and a negative second virial coefficient, A2, due to overall charge neutralization near the isoelectric pH. From the Rh, mobility, and A2, cross-linked MAA-DEA microgels with and without PEGMA retained their polyampholytic properties in solution. By varying the composition of MAA and DEA in the microgel, it is possible to vary the isoelectric point of the colloidal particles. These new microgels are being explored for use in the delivery of DNA and proteins.