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Institute: search.filters.UBSInstitut.Institut für Technische ChemieAuthor: search.filters.author.Frey, Wolfgang

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    Structural characterization of surface immobilized platinum hydrides by sensitivity-enhanced 195Pt solid state NMR spectroscopy and DFT calculations
    (2024) Atterberry, Benjamin A.; Wimmer, Erik J.; Klostermann, Sina; Frey, Wolfgang; Kästner, Johannes; Estes, Deven P.; Rossini, Aaron J.
    Supported single-site platinum hydride compounds are promising heterogeneous catalysts for organic transformations. Few methods exist to describe the structures of single-site Pt catalysts with atomic resolution because of their disordered structures and low Pt loadings. Here, we study the compounds formed when bis(tri-tert-butylphosphino)platinum, Pt(PtBu3)2, is supported on dehydroxylated SiO2 or SiO2-Al2O3. First, we obtain magic angle spinning (MAS) 1H, 31P and 195Pt ssNMR spectra of four model Pt phosphine compounds with oxidation states of 0 or +2 and coordination numbers between 2 and 4. These compounds are analogs of potential structures present in the supported compounds. MAS 195Pt ssNMR spectra were obtained using 31P{195Pt} sideband selective J-resolved and J-HMQC experiments. The measured 1H and 31P chemical shifts, 31P-195Pt J-couplings and 195Pt chemical shift (CS) tensors are shown to be diagnostic of oxidation state and coordination number. Room temperature 1H ssNMR spectra of Pt(PtBu3)2 supported on SiO2 or SiO2-Al2O3 show diagnostic hydride NMR signals, suggesting that Pt(PtBu3)2 undergoes oxidative addition, resulting in surface hydrides and Pt–oxygen bonds to the support surface. MAS dynamic nuclear polarization (DNP) enables 31P{195Pt} correlation NMR experiments on the supported compounds. These experiments enable the measurement of the 31P-195Pt J-coupling constants and 195Pt CS tensors. Combined NMR and DFT analyses suggest that the primary surface platinum species are [HPt(PtBu3)2OSi] on SiO2 and [HPt(PtBu3)2]+[Si-O--Al] on SiO2-Al2O3. The Pt-oxygen bond length is dependent on the support and estimated as 2.1-2.3 Å and 2.7-3.0 Å for SiO2 and SiO2-Al2O3, respectively.
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    Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids
    (2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, Sabine
    Covalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.
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    Efficient and spatially controlled functionalization of SBA‐15 and initial results in asymmetric Rh‐catalyzed 1,2‐additions under confinement
    (2021) Beurer, Ann‐Katrin; Kirchhof, Manuel; Bruckner, Johanna R.; Frey, Wolfgang; Baro, Angelika; Dyballa, Michael; Giesselmann, Frank; Laschat, Sabine; Traa, Yvonne
    Selectively functionalized mesoporous silica may considerably advance heterogeneous catalysis through the controlled immobilization of highly selective complex catalysts inside the mesopores. However, spatially controlled functionalization and the precise analytical verification are still a challenge. In this publication, we report a method, which ensures a selective functionalization of the mesopore surface with a clickable linker and thus makes it possible to study confinement effects during catalyzed reactions. First, we passivate the silanol groups on the particle surface and in the pore entrances of the mesoporous silica material SBA‐15 with 1,1,1‐trimethyl‐N‐(trimethylsilyl)silanamine. Then we remove the template by solvent extraction and functionalize the pore walls with 3‐azidopropyltriethoxysilane before we click the catalyst. In initial experiments of asymmetric Rh‐catalyzed 1,2‐addition, we investigate the performance of a catalyst clicked selectively in the mesopores and compare it to the dissolved catalyst as well as to the catalyst immobilized exclusively on the external surface of SBA‐15.