03 Fakultät Chemie
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Item Open Access Stereoretentive regio‐ und enantioselektive Allylierung von Isoxazolinonen per planar chiralem Palladacyclus‐Katalysator(2022) Yu, Xin; Hu, Lingfei; Frey, Wolfgang; Lu, Gang; Peters, RenéDie katalytische allylische Substitution ist eines der wichtigsten Werkzeuge in der asymmetrischen Synthese zur enantioselektiven Bildung von C-C-Bindungen. Während in vorigen Arbeiten eine hohe Effizienz in Bezug auf Enantio- und Regiokontrolle unter Verwendung verschiedener Katalysatortypen erreicht wurde, besteht eine starke allgemeine Einschränkung in einer sehr ausgeprägten Präferenz für die Bildung von allylischen Substitutionsprodukten mit (E)-konfigurierten C=C-Doppelbindungen. Hier berichten wir, dass mit einem planar-chiralen Palladacyclus-Katalysator unter Verwendung von Isoxazolinonen und Allylimidaten als Substrate ein diastereospezifisches Reaktionsergebnis erzielt wird, wodurch die C=C-Doppelbindungsgeometrie der Allylsubstrate in den hoch enantiomerenangereicherten Produkten beibehalten wird. DFT-Rechnungen zeigen, dass die Reaktionen über einen SN2-Mechanismus und nicht über π-Allyl-Pd-Komplexe ablaufen. Entscheidend für die hohe Kontrolle ist die Stabilisierung des allylischen Fragments im SN2-Übergangszustand durch π-Wechselwirkungen mit den Phenylsubstituenten des Pentaphenylferrocen-Katalysatorkerns.Item Open Access Tetra-μ3-iodido-tetrakis[(tri-n-butylphosphane-κP)copper(I)](2014) Klenk, Simon; Frey, Wolfgang; Bubrin, Martina; Laschat, SabineTetrameric phosphane complexes of copper(I) halides are extensively used as reagents for copper-mediated conjugate additions. Furthermore, theoretical interest stems from the fact that all group 11 elements in the oxidation state +1 are prone to form clusters with potential metal-metal distances. Thus, theoretical work on such complexes (XCuPR3)4 has been carried out to study structures and stabilities in detail. However, the plethora of structural information on these compounds came from X-ray crystal structure analyses. Although the known tri-n-butyl phosphane complex [n-Bu3PCuI]4 had already been characterized by using X-ray crystallography, no atomic coordinates, bond lengths or bond angles were reported. Thus, we reinvestigated the crystal structure of [n-Bu3PCuI]4. We were able to confirm the previously postulated tetrameric complex with a distorted heterocubane structure similar to the AsEt3 derivative.Item Open Access Chromium(VI) bisimido dichloro, bisimido alkylidene, and chromium(V) bisimido iodo N‐heterocyclic carbene complexes(2020) Panyam, Pradeep K. R.; Stöhr, Laura; Wang, Dongren; Frey, Wolfgang; Buchmeiser, Michael R.Reaction of CrCl2(N-tBu)2 with 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl2), 1,3-di(2-propyl)imidazol-2-ylidene (IPr), 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-bis(2,6-(2-Pr)2C6H3)imidazol-2-ylidene (IDipp) yields the corresponding N-heterocyclic carbene (NHC) adducts CrCl2(IMe)(N-tBu)2 (1), CrCl2(IMeCl2)(N-tBu)2 (2), CrCl2(IPr)(N-tBu)2 (3), CrCl2(IMes)(N-tBu)2 (4) and CrCl2(IDipp)(N-tBu)2 (5). Likewise, reaction of CrCl2(N-2,6-(2-Pr)2C6H3)2 and CrCl2(N-adamantyl)2 with IMes yields CrCl2(N-2,6-(2-Pr)2C6H3)2(IMes) (6) and CrCl2(N-adamantyl)2(IMes) (7), respectively. Reaction of CrCl2(N-tBu)2 with the bidentate NHCs 1-R-3-(1-(2-LiO-C6H4))imidazol-2-ylidene yields the corresponding pentacoordinated Cr(VI) complexes CrCl2(1-R-3-(1-(2-O-C6H4))imidazol-2-ylidene)2C6H3)2(IMes) (R = 2,4,6-(CH3)3C6H2, 8), (R = tBu, 9), (R = 2-phenyl-C6H4, 10). Reaction of the chromium(VI) complex Cr(N-2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 with 1,3-dimethylimidazol-2-ylidene·AgI yields the bimetallic silver adduct of the chromium alkylidene complex (11) along with the tetrahedral chromium(V) complex CrI(N-2,6-(2-Pr)2-C6H3)2(1,3-dimethylimidazol-2-ylidene) (12). Compounds 1-4, 7, 9-12 were characterized by single-crystal X-ray analysis. Finally, the chromium(VI) bisimido-amido complexes 13-14 bearing the N-6-(2-(diethylboryl)phenyl)pyridyl-2-yl-motif are reported.Item Open Access Absolute configuration of small molecules by co‐crystallization(2020) Krupp, Felix; Frey, Wolfgang; Richert, ClemensThe most reliable method to determine the absolute configuration of chiral molecules is X‐ray crystallography, but small molecules can be difficult to crystallize. We report rapid co‐crystallization of tetraaryladamantanes with small molecules as different as n‐decane to nicotine to produce crystals for X‐ray analysis and the assignment of absolute configuration when the molecules are chiral. A screen of 52 diverse compounds gave inclusion in co‐crystals for 88 % of all cases and a high‐resolution structure in 77 % of cases. Furthermore, starting from three milligrams of analyte, a combination of NMR spectroscopy and X‐ray crystallography produced a full structure in less than three days using an adamantane crystallization chaperone that encapsulates the analyte at room temperature.Item Open Access 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.Item Open Access The coordination behaviour of CuI photosensitizers bearing multidentate ligands investigated by X‐ray absorption spectroscopy(2020) Rentschler, Martin; Iglesias, Sirma; Schmid, Marie‐Ann; Liu, Cunming; Tschierlei, Stefanie; Frey, Wolfgang; Zhang, Xiaoyi; Karnahl, Michael; Moonshiram, DooshayeA systematic series of four novel homo‐ and heteroleptic CuI photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.Item Open Access Transitions in solvate crystals of a tetraaryladamantane(2023) Frey, Wolfgang; Schwenger, Alexander; Berking, Tim; Richert, ClemensObtaining high-resolution structures of liquid compounds can be difficult. Encapsulating them in the lattice of a larger organic molecule acting as crystallization chaperone is one option to overcome this difficulty. Tetraaryladamantane ethers can play the role of chaperones, accommodating a range of different guest molecules in their crystals. How well-ordered crystalline arrangements for molecules of different shape are achieved is not clear. Cases in which more than one structure is found may shed light on this phenomenon. Here, we report low-order cubic crystal structures of 1,3,5,7-tetrakis(2,4-dimethoxyphenyl)adamantane (TDA) encapsulating ortho-xylene or cyclohexane, together with better ordered structures obtained after warming the crystals to 60 °C. Evidence for cubic crystal systems was also found for limonene, hexachlorobutadiene and eucalyptol, with a transition to a triclinic system for the former two, but no transition up to 70 °C for the latter. These findings indicate that some solvate structures of TDA can readily undergo structural transitions to less solvated, better ordered systems. Crystals obtained by rapid thermal crystallization may be in kinetically trapped states, and the transition to a solvate-free crystal system appears to have a kinetic barrier that depends strongly on the structure of the liquid guest molecules encapsulated in the lattice.Item Open Access How the way a naphthalimide unit is implemented affects the photophysical and -catalytic properties of Cu(I) photosensitizers(2022) Yang, Yingya; Doettinger, Florian; Kleeberg, Christian; Frey, Wolfgang; Karnahl, Michael; Tschierlei, StefanieDriven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two heteroleptic Cu(I) complexes CuNIphen (NIphen = 5-(1,8-naphthalimide)-1,10-phenanthroline) and Cubiipo (biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthroline-16-one) were prepared and compared with the novel unsubstituted reference compound Cuphen (phen = 1,10-phenanthroline). Beside a comprehensive structural characterization, including two-dimensional nuclear magnetic resonance spectroscopy and X-ray analysis, a combination of electrochemistry, steady-state and time-resolved spectroscopy was used to determine the electrochemical and photophysical properties in detail. The nature of the excited states was further examined by (time-dependent) density functional theory (TD-DFT) calculations. It was found that CuNIphen exhibits a greatly enhanced absorption in the visible and a strong dependency of the excited state lifetimes on the chosen solvent. For example, the lifetime of CuNIphen extends from 0.37 µs in CH2Cl2 to 19.24 µs in MeCN, while it decreases from 128.39 to 2.6 µs in Cubiipo. Furthermore, CuNIphen has an exceptional photostability, allowing for an efficient and repetitive production of singlet oxygen with quantum yields of about 32%.Item Open Access 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, YvonneSelectively 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.Item Open Access Predicting catalytic activity from 13CCH alkylidene chemical shift in cationic tungsten oxo alkylidene N‐heterocyclic carbene complexes(2021) Musso, Janis V.; Schowner, Roman; Falivene, Laura; Frey, Wolfgang; Cavallo, Luigi; Buchmeiser, Michael R.A series of cationic tungsten oxo alkylidene N‐heterocyclic carbene (NHC) complexes was synthesized and structurally characterized by single crystal X‐ray diffraction. The 13C NMR chemical shifts of the alkylidene C atoms of these complexes were correlated with the diamagnetic, paramagnetic and spin‐orbit chemical shifts calculated by DFT. A good correlation (R2=0.90) between the DFT isotropic chemical shifts and the experimental chemical shift as well as a strong correlation between the DFT isotropic chemical shifts and the TOF1min for the RCM of 1,7‐octadiene was found. Further, a comparison of the catalyst geometries allowed for assigning tetracoordinate pseudotetrahedral catalysts to the most deshielded alkylidenes and to the highest TOF1min, pentacoordinate square‐planar catalysts to the intermediate deshielded alkylidenes and intermediate TOF1min, and hexacoordinate and octahedral catalyst to the most shielded alkylidene and lowest TOF1min. Analysis of the magnetic shielding tensors allowed for ascribing variations in the chemical shifts to electronic transitions between occupied molecular orbitals corresponding to the alkylidene‐C and alkylidene‐H σ‐bonds and the empty molecular orbital corresponding to the W‐alkylidene σ*‐bond.