Search Results
Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy
By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase.
Cooperative catalytic methoxycarbonylation of alkenes: Uncovering the role of palladium complexes with hemilabile ligands
Mechanistic studies of the catalyst [Pd2(dba)3/1,1′-bis(tert-butyl(pyridin-2-yl)phosphanyl)ferrocene, L2] for olefin alkoxycarbonylation reactions are described. X-ray crystallography reveals the coordination of the pyridyl nitrogen atom in L2 to the palladium center of the catalytic intermediates. DFT calculations on the elementary steps of the industrially relevant carbonylation of ethylene (the Lucite α-process) indicate that the protonated pyridyl moiety is formed immediately, which facilitates the formation of the active palladium hydride complex. The insertion of ethylene and CO into this intermediate leads to the corresponding palladium acyl species, which is kinetically reversible. Notably, this key species is stabilized by the hemilabile coordination of the pyridyl nitrogen atom in L2. The rate-determining alcoholysis of the acyl palladium complex is substantially facilitated by metal-ligand cooperation. Specifically, the deprotonation of the alcohol by the built-in base of the ligand allows a facile intramolecular nucleophilic attack on the acyl palladium species concertedly. Kinetic measurements support this mechanistic proposal and show that the rate of the carbonylation step is zero-order dependent on ethylene and CO. Comparing CH3OD and CH3OH as nucleophiles suggests the involvement of (de)protonation in the rate-determining step.
Determining the location of Co2+ in zeolites by UV-Vis diffuse reflection spectroscopy : A critical view
UV-Vis spectroscopy as well as in situ FTIR spectroscopy of pyridine and CO adsorption were applied to determine the nature of Co species in microporous, mesoporous, and mixed oxide materials like Co-ZSM-5, Co/Na-ZSM-5, Co/Al-SBA-15, and Co/Al2O3-SiO2. Because all sample types show comparable UV-Vis spectra with a characteristic band triplet, the former described UV-Vis band deconvolution method for determination and quantification of individual cationic sites in the zeolite appears doubtful. This is also confirmed by results of pyridine and CO adsorption revealing that all Co-zeolite samples contain two types of Co2+ species located at exchange positions as well as in oxide-like clusters independent of the Co content, while in Co/Al-SBA-15 and Co/Al2O3-SiO2 only Co2+ species in oxide-like clusters occur. Consequently, the measured UV-Vis spectra represent not exclusively isolated Co2+ species, and the characteristic triplet band is not only related to γ-, β-, and α-type Co2+ sites in the zeolite but also to those dispersed on the surface of different oxide supports. The study demonstrates that for proper characterization of the formed Co species, the use of complementary methods is required. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
General and selective synthesis of primary amines using Ni-based homogeneous catalysts
The development of base metal catalysts for industrially relevant amination and hydrogenation reactions by applying abundant and atom economical reagents continues to be important for the cost-effective and sustainable synthesis of amines which represent highly essential chemicals. In particular, the synthesis of primary amines is of central importance because these compounds serve as key precursors and central intermediates to produce value-added fine and bulk chemicals as well as pharmaceuticals, agrochemicals and materials. Here we report a Ni-triphos complex as the first Ni-based homogeneous catalyst for both reductive amination of carbonyl compounds with ammonia and hydrogenation of nitroarenes to prepare all kinds of primary amines. Remarkably, this Ni-complex enabled the synthesis of functionalized and structurally diverse benzylic, heterocyclic and aliphatic linear and branched primary amines as well as aromatic primary amines starting from inexpensive and easily accessible carbonyl compounds (aldehydes and ketones) and nitroarenes using ammonia and molecular hydrogen. This Ni-catalyzed reductive amination methodology has been applied for the amination of more complex pharmaceuticals and steroid derivatives. Detailed DFT computations have been performed for the Ni-triphos based reductive amination reaction, and they revealed that the overall reaction has an inner-sphere mechanism with H2metathesis as the rate-determining step. © The Royal Society of Chemistry 2020.
Addressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction
Reproducibility of photocatalytic reactions, especially when conducted on small scale for improved turnover numbers with in situ formed catalysts can prove challenging. Herein, we showcase the problematic reproducibility on the example of attractive photocatalytic CO2 reduction utilizing [FeFe] hydrogenase mimics. These Fe complexes, well-known for their application in proton reduction reactions, were combined with a heteroleptic Cu photosensitizer and produced CO/H2/HCO2H mixtures of variable constitution. However, the reactions indicated a poor reproducibility, even when conducted with well-defined complexes. Based on our experience, we make suggestions for scientists working in the field of photocatalysis on how to address and report the reproducibility of novel photocatalytic reaction protocols. In addition, we would like to highlight the importance of studying reproducibility of novel reaction protocols, especially in the fields of photocatalytic water splitting and CO2 reduction, where TONs are widely used as the comparable measure for catalytic activity. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
Enzyme Activity by Design: An Artificial Rhodium Hydroformylase for Linear Aldehydes
Artificial metalloenzymes (ArMs) are hybrid catalysts that offer a unique opportunity to combine the superior performance of natural protein structures with the unnatural reactivity of transition-metal catalytic centers. Therefore, they provide the prospect of highly selective and active catalytic chemical conversions for which natural enzymes are unavailable. Herein, we show how by rationally combining robust site-specific phosphine bioconjugation methods and a lipid-binding protein (SCP-2L), an artificial rhodium hydroformylase was developed that displays remarkable activities and selectivities for the biphasic production of long-chain linear aldehydes under benign aqueous conditions. Overall, this study demonstrates that judiciously chosen protein-binding scaffolds can be adapted to obtain metalloenzymes that provide the reactivity of the introduced metal center combined with specifically intended product selectivity.
Synthetic strategies to bicyclic tetraphosphanes using P1, P2 and P4 building blocks
Different reactions of Mes* substituted phosphanes (Mes* = 2,4,6-tri-tert-butylphenyl) led to the formation of the bicyclic tetraphosphane Mes*P4Mes* (5) and its unknown Lewis acid adduct 5·GaCl3. In this context, the endo–exo isomer of 5 was fully characterized for the first time. The synthesis was achieved by reactions involving “self-assembly” of the P4 scaffold from P1 building blocks (i.e. primary phosphanes) or by reactions starting from P2 or P4 scaffolds (i.e. a diphosphene or cyclic tetraphosphane). Furthermore, interconversion between the exo–exo and endo–exo isomer were studied by 31P NMR spectroscopy. All compounds were fully characterized by experimental as well as computational methods.
Synthesis of Single Atom Based Heterogeneous Platinum Catalysts: High Selectivity and Activity for Hydrosilylation Reactions
Catalytic hydrosilylation represents a straightforward and atom-efficient methodology for the creation of C-Si bonds. In general, the application of homogeneous platinum complexes prevails in industry and academia. Herein, we describe the first heterogeneous single atom catalysts (SACs), which are conveniently prepared by decorating alumina nanorods with platinum atoms. The resulting stable material efficiently catalyzes hydrosilylation of industrially relevant olefins with high TON (≈105). A variety of substrates is selectively hydrosilylated including compounds with sensitive reducible and other functional groups (N, B, F, Cl). The single atom based catalyst shows significantly higher activity compared to related Pt nanoparticles.
3,3′-Dimethyl-1,1′-methylenediimidazolium tetrabromidocobaltate(II)
The title compound, (C9H14N4)[CoBr4], was obtained as single crystals directly in very low yield as a side product in the reaction of 1,1′-bis(1-methylimidazolium)acetate bromide and CoBr2. The title compound consists of an imidazolium-based dication and a tetrabromidocobaltate(II) complex anion, which are connected via C—H...Br interactions in the crystal. The dihedral angle between the imidazolium rings in the cation is 72.89 (16)°. The CoII ion in the anion is coordinated tetrahedrally by four bromide ligands [Co—Br = 2.4025 (5)–2.4091 (5) Å and Br—Co—Br = 106.224 (17)–113.893 (17)°]. The compound exhibits a high melting point (>300°C) and is a light-blue solid under ambient conditions.
Specific Signal Enhancement on an RNA-Protein Interface by Dynamic Nuclear Polarization
Sensitivity and specificity are both crucial for the efficient solid-state NMR structure determination of large biomolecules. We present an approach that features both advantages by site-specific enhancement of NMR spectroscopic signals from the protein-RNA binding site within a ribonucleoprotein (RNP) by dynamic nuclear polarization (DNP). This approach uses modern biochemical techniques for sparse isotope labeling and exploits the molecular dynamics of 13C-labeled methyl groups exclusively present in the protein. These dynamics drive heteronuclear cross relaxation and thus allow specific hyperpolarization transfer across the biomolecular complex's interface. For the example of the L7Ae protein in complex with a 26mer guide RNA minimal construct from the box C/D complex in archaea, we demonstrate that a single methyl-nucleotide contact is responsible for most of the polarization transfer to the RNA, and that this specific transfer can be used to boost both NMR spectral sensitivity and specificity by DNP.