A systematic study of the interactions between chemical partners (metal, ligands, counterions, and support) involved in the design of Al2O3-supported nickel catalysts from diamine-Ni(II) chelates

1.5 Ni wt %/Al2O3 catalysts have been prepared by incipient wetness impregnation using [Ni(diamine)x(H2O)(6-2x)]Y2 precursors (diamine = 1,2-ethanediamine (en) and trans-1,2-cyclohexanediamine (tc); x = 0, 1, and 2; Y = NO3- and Cl-), to avoid the formation, during calcination, of difficult-to-reduce nickel aluminate. N2 was chosen for thermal treatment to help reveal and take advantage of the reactions occurring between Ni2+, ligands, counterions, and support. In the case of [Ni(en)2(H2O)2]Y2 salts used as precursors, in situ UV-vis and DRIFT spectroscopies show that after treatment at 230 degrees C Ni(II) ions are grafted to alumina via two OAl bonds and that the diamine ligands still remain coordinated to grafted nickel ions but in a monodentate way, bridging the cation with the alumina surface. With Y = Cl-, the chloride counterions desorb as hydrogen chloride, and hydrogen released upon decomposition of the en ligands is able to reduce a fraction of nickel ions into metal as evidenced by XPS. In contrast, with Y = NO3-, compounds such as CO or NO are formed during thermal treatment, indicating that nitrate ions burn the en ligands. After thermal treatment at 500 degrees C, a surface phase containing Ni(II) ions forms, characterized by XPS and UV-vis spectroscopy. Temperature-programmed reduction shows that these ions can be quantitatively reduced to the metallic state at 500 degrees C, in contrast with the aluminate obtained when the preparation is carried out from [Ni(H2O)6]2+, which is reduced only partly at 950 degrees C. On the other hand, a total self-reduction of nickel complexes leading to 2-5-nm metal particles is obtained upon thermal treatment via the hydrogen released by a hydrogen-rich ligand such as tc, whatever the Y counterion. An appropriate choice of the ligand and the counterion allows then to obtain selectively Ni(II) ions or a dispersed reduced nickel phase after treatment in N2, as a result of the reactions occurring between the chemical partners present on alumina.     

Thermo‐ and pH‐Responsive Micelles of Poly(acrylic acid)‐block‐Poly(N,N‐diethylacrylamide)

Summary: The bis‐hydrophilic block copolymer, poly(acrylic acid)₄₅‐block‐poly(N,N‐diethylacrylamide)₃₆₀, was obtained after hydrolysis of poly(tert‐butyl acrylate)₄₅‐block‐poly(N,N‐diethylacrylamide)₃₆₀, synthesized by sequential anionic polymerization of tert‐butyl acrylate (tBA) and N,N‐diethylacrylamide (DEAAm) in the presence of Et₃Al. The polymer is stimuli‐sensitive with respect to both pH and temperature in aqueous solution, reversibly forming spherical crew‐cut micelles with PDEAAm‐core (〈R_h〉_z = 21.5 nm) under alkaline conditions for T > 35 °C as well as inverse star‐like micelles with an expanded PAA‐core (〈R_h〉_z = 43.8 nm) under acidic conditions for T < 35 °C, as indicated by dynamic light scattering.

Modes of micelle formation for poly(acrylic acid)₄₅‐block‐poly(N,N‐diethylacrylamide)₃₆₀ in aqueous solution depending on the pH and temperature.     

A Novel Method for Analysis of Xanthate Group Distribution in Viscoses

Analytical monitoring of xanthation in the viscose process along with xanthate group analysis in the viscose material is a long-debated problem in cellulose chemistry. The task is rendered extremely intricate by the lability of the starting material and the harshness of the reaction medium, which adds to a lack of suitable analytical approaches. In a four-years' endeavor in our lab, a method is being developed which allows to analyze the distribution of xanthate groups in viscoses relative to the anhydroglucose units and along the cellulose chain. In a first step the xanthate groups are stabilized by alkylation, which was optimized towards quantitative conversion. In a second step, the remaining free hydroxyl groups are protected by carbanilation, followed by selective removal of the stabilized xanthate groups. Steps two and three thus generate an inverse image of the initial xanthate pattern. In the forth and fifth step, the liberated hydroxyl groups are methylated, and the carbanilates are removed, so that in the overall process the xanthates were replaced by methyl groups. All reaction steps have been comprehensively tested with regard to completeness of conversion and orthogonality of the protecting groups.     

Affinity interactions between phenylboronic acid-carrying self-assembled monolayers and flavin adenine dinucleotide or horseradish peroxidase

A method is provided for the recognition of glycated molecules based on their binding affinities to boronate-carrying monolayers. The affinity interaction of flavin adenine dinucleotide (FAD) and horseradish peroxidase (HRP) with phenylboronic acid monolayers on gold was investigated by using voltammetric and microgravimetric methods. Conjugates of 3-aminophenylboronic acid and 3,3'-dithiodipropionic acid di(N-hydroxysuccinimide ester) or 11-mercaptoundecanoic acid were prepared and self-assembled on gold surfaces to generate monolayers. FAD is bound to this modified surface and recognized by a pair of redox peaks with a formal potential of -0.433 V in a 0.1 M phosphate buffer solution, pH 6.5. Upon addition of a sugar to the buffer, the bound FAD could be replaced, indicating that the binding is reversible. Voltammetric, mass measurements, and photometric activity assays show that the HRP can also be bound to the interface. This binding is reversible, and HRP can be replaced by sorbitol or removed in acidic solution. The effects of pH, incubation time, and concentration of H(2)O(2) were studied by comparing the catalytic reduction of H(2)O(2) in the presence of the electron-donor thionine. The catalytic current of the HRP-loaded electrode was proportional to HRP concentrations in the incubation solution in the range between 5 microg mL(-1) and 0.1 mg mL(-1) with a linear slope of 3.34 microA mL mg(-1) and a correlation coefficient of 0.9945.     

Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study

We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol.     

Highly enantioselective syntheses of heterocycles via intramolecular Ir-catalyzed allylic amination and etherification

[reaction: see text] Enantioselective Ir-catalyzed intramolecular allylic aminations and etherifications are described. Up to 97% ee was achieved using catalysts prepared by in situ activation of mixtures of phosphorus amidites and [Ir(COD)Cl]2. Sequential aminations of bis-allylic carbonates, involving an inter- followed by an intramolecular reaction, gave trans-N-benzyl-2,5-divinylpyrrolidine and trans-N-benzyl-2,6-divinylpiperidine with > or = 99% ee. New phosphorus amidites as well as improved conditions for intermolecular aminations are reported.     

First total synthesis of cichorine and zinnimidine

The first total synthesis of the phytotoxins cichorine and zinnimidine is described. The synthetic tactics involve the sequential connection of the dense and diverse functionalities on the aromatic nucleus followed by a Parham cyclization process, giving rise to the lactam unit embedded in the title compound framework.     

Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: reactivity studies of a functional model for molybdenum oxotransferases

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (L-OH) with MoO2Cl2 in methanol in the presence of NaOMe and PF6- results in the formation of [MoO2(L-O)]PF6. Similarly, the reaction of N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L-SH) with MoO2(acac)2 leads to the formation of [MoO2(L-S)]+. The dioxo-molybdenum complex [MoO2(L-O)]+ reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L-O)(OCH3)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L-O)]PF6 with PPh3 in other solvents than methanol leads to the formation of the Mo(V) dimer [(L-O)OMo(micro-O)MoO(L-O)](PF6)2. The crystal structures of [MoO2(L-O)]PF6 and the micro-oxo bridged dimer are presented.     

Synthesis of Cyclic Peptides by Photochemical Decarboxylation of N‐Phthaloyl Peptides in Aqueous Solution

The synthesis of a variety of cyclic peptides from N‐phthaloyl‐protected di‐, tri‐, tetra‐, and pentapeptides with different aminocarboxylic acid tethers by photodecarboxylation – initiated by intramolecular electron transfer – has been explored in aqueous media. The progress and the chemoselectivity of the follow‐up processes after CO₂ extrusion were traced by the respective pH/time‐profiles, as well as by the overall change in pH after completion of the reaction. The competition between cyclization and simple oxidative decarboxylation depends on spacer length and geometry, H‐bonding interaction between the electron accepting phthalimide C?O groups and amide H‐atoms, as well as the geometric reorganization coupled with the radical combination step and the formation of the lactam rings. With progressing reaction, hydrolysis of the phthalimide chromophore becomes an increasingly important side reaction due to the constant increase in pH. The use of phosphate‐buffered aqueous media consequently improved the cyclization yields. The ground‐state interactions between amide groups and the terminal COO^? group with the imide C?O groups were studied for the model system [N‐(phthaloyl)glycyl]sarcosine ( 1 ) by NMR spectroscopy where the amide (E/Z)‐equilibrium depends on the presence of carboxylate vs. free carboxylic acid, demonstrating the role of H‐bonding and metal coordination.