Komplexe flexibler ditopischer Catecholphosphane : Synthese, metallunterstützte Selbstorganisation und Anwendung in der Katalyse

The present work deals with the syntheses of flexible, ditopic catechol phosphines and their self-assembly to produce di- or multinuclear complexes. A set of bidentate ligands with a phosphine functionality at one end and a catechol unit at the other end was synthesised via hydrophosphanation followed by LiAlH4 reduction. The rationale behind the design was to have ligands which can differentiate between hard and soft metals, and react with a combination of both selectively to produce hetero di- or multi-nuclear assemblies in a single step. Reaction of the catechol phosphine with boronic acid produced a phosphine functionalised benzo-1,3,2-dioxaborol. The ability of this species to act as a Lewis-base via the P-centered donor and as Lewis-acid via the B centered acceptor was demonstrated in reactions with (COD)PdCl2 and some nitrogen donors. Reaction of a palladium complex of the phosphine functionalised benzo-1,3,2-dioxaborol with DABCO gave a triple Lewis acid/base complex which contains both P-Pd and B-N dative bonds. Taking advantage of the selective reactivity of the ligand functionalities the condensation of two catechol phosphines with boric acid furnished a borate templated anionic bis-phosphine ligand. This species was further reacted with silver salts to produce neutral bis-phosphine silver complexes. The main focus of the present work was on performing the assembly of catechol phosphine fragments on a hard acid center and the coordination to a catalytic metal in one step. This working concept was successfully demonstrated by using various hard acid centers as templates and catalytically relevant metals. In detail, bi- or multi-metallic complexes of type L2MEXn with palladium and platinum as metal (M), and gallium, tin, bismuth, zirconium and yttrium as template (E) were synthesised. The experimental findings allow to state that the coordination geometry of the template and its size have considerable impact on the assembly process. The complexes studied here enabled to vary the P-Pd-P bite angles between 97º to 102º, depending on the size and nature of the substituents on the template. Interestingly, all the synthesized complexes displayed μ2-bridging oxygen donors. The investigations were further focused on building chiral complexes by synthesizing a chiral phosphine and employing it in complexation. Another approach was to induce chirality at E by placing two different substituents on E. Intensive NMR studies indicated that the supramolecular ligands in L2MEXn show hemilabile behaviour in donor solvents like DMF. A closer look revealed that the constitution of the above complexes resembles those used in C-C coupling reactions. Catalytic activity was indeed demonstrated by employing selected complexes in the Sonogashira coupling of phenyl acetylene with p-iodo-nitrobenzene.Die vorliegende Arbeit behandelt die Synthese flexibler, ditopischer Katecholphosphane und deren Selbstanordnung zur Darstellung zwei- und mehrkerniger Komplexe. Eine Reihe zweizähniger Liganden mit einer Phosphan-Funktion auf der einen und einem Katechol auf der anderen Seite wurde durch Hydrophosphanierung und nachfolgende Reduktion mit LiAlH4 dargestellt. Ziel ist dabei Liganden zur Verfügung zu stellen, die sowohl zwischen harten und weichen Metallen unterscheiden können als auch selektiv mit einer Kombination von beiden zu zwei- oder auch mehrkernigen, heteronuclearen Aggregaten reagieren können. Die Reaktion des Katecholphosphans mit Boronsäurederivaten lieferte ein phosphansubstituiertes Benzo-1,3,2-dioxaborol. Die Fähigkeit dieser Spezies, als Lewis-Base über das Phosphoratom und gleichzeitig als Lewis-Säure über das Boratom zu reagieren wurde anhand von Reaktionen mit (COD)PdCl2 sowie mit einigen Stickstoff-Liganden aufgezeigt. Die Umsetzung eines Palladiumkomplexes des phosphansubstituierten Benzo-1,3,2-dioxaborols mit DABCO führte zu einem dreifachen Lewis-Säure/Base-Komplex, der sowohl dative P-Pd- als auch B-N-Bindungen aufweist. Unter Nutzung der selektiven Reaktivität der Ligandenfunktionen gelang die Kondensation zweier Katecholphosphane mit Borsäure unter Bildung eines anionischen Bis-phosphan-Liganden, mit einem Borat-Templat. Diese Spezies wurde weiter mit Silbersalzen zu neutralen Bis-phosphan-Silberkomplexen umgesetzt. Das Hauptaugenmerk der vorliegenden Arbeit liegt auf der Verknüpfung von Katecholphosphan-Fragmenten mit einem harten Lewis-sauren Zentrum und der Koordination mit einem katalytisch aktiven Metallzentrum in einem Schritt. Dieses Arbeitskonzept wurde erfolgreich demonstriert, indem verschiedene harte Säurezentren als Template und verschiedene katalytisch wirksame Metalle eingesetzt wurden. Insbesondere wurden zwei- oder mehrkernige Komplexe des Typs L2MEXn mit Palladium und Platin als Metall (M) und Gallium, Zinn, Bismut, Zirkonium und Yttrium als Templat (E) dargestellt. Die experimentellen Befunde stützen die Aussage, dass die Koordinationsgeometrie des Templats sowie dessen Größe beträchtlichen Einfluss auf den Aggregationsprozess ausüben. Die hier untersuchten Komplexe erlaubten eine Variation des P-Pd-P-Bisswinkels zwischen 97 und 102°, abhängig von der Größe und Art der Substituenten am Templat. Interessanterweise wiesen alle dargestellten Komplexe µ2-verbrückende Sauerstoff-Donoratome auf. Die Untersuchungen wurden weiterhin auf den Aufbau chiraler Komplexe ausgeweitet, indem zunächst ein chirales Phosphan synthetisiert und dieses anschließend bei der Komplexbildung eingesetzt wurde. Ein anderer Zugang war die Chiralität durch Verwendung zweier unterschiedlicher Substituenten an E einzuführen. Sorgfältige NMR-Untersuchungen sprachen dafür, dass die supramolekularen Liganden in L2MEXn in Lösungsmitteln wie DMF hemilabiles Verhalten aufweisen. Eine genauere Betrachtung zeigte, dass die Konstitution der dabei entstandenen Komplexe denen ähnelt, die bei C-C-Kupplungsreaktionen beobachtet werden. Die katalytische Aktivität konnte auch tatsächlich durch Einsatz ausgewählter Komplexe in der Sonogashira-Kupplung von Phenylacetylen mit p-Iod-nitrobenzol gezeigt werden

Enhancing the hydrogen storage capacity of Pd-functionalized multi-walled carbon nanotubes

We demonstrate that the hydrogen storage capacity of multi-walled carbon nanotubes can be enhanced by polyvinylpyrrolidone functionalization. The polyvinylpyrrolidone acts as a stabilizing agent for Pd-nanoparticles, reduces their size and facilitates their uniform and enhanced loading onto multi-walled carbon nanotubes. According to sorption studies, the polyvinylpyrrolidone capping and consequent nanostructural modification enables 2.3 times more hydrogen adsorption than mere Pd-functionalization of multi-walled carbon nanotubes. Corresponding morphological changes before and after polyvinylpyrrolidone capping, characterized using Raman Spectroscopy, X-ray diffraction, TEM and thermal analysis techniques, are also presented. The results contribute towards increasing the efficiency of hydrogen based sustainable energy sources

Enhancing the hydrogen storage capacity of Pd-functionalized multi-walled carbon nanotubes

We demonstrate that the hydrogen storage capacity of multi-walled carbon nanotubes can be enhanced by polyvinylpyrrolidone functionalization. The polyvinylpyrrolidone acts as a stabilizing agent for Pd-nanoparticles, reduces their size and facilitates their uniform and enhanced loading onto multi-walled carbon nanotubes. According to sorption studies, the polyvinylpyrrolidone capping and consequent nanostructural modification enables 2.3 times more hydrogen adsorption than mere Pd-functionalization of multi-walled carbon nanotubes. Corresponding morphological changes before and after polyvinylpyrrolidone capping, characterized using Raman Spectroscopy, X-ray diffraction, TEM and thermal analysis techniques, are also presented. The results contribute towards increasing the efficiency of hydrogen based sustainable energy sources

Synthesis of renewable copolyacetals with tunable degradation

Acetal metathesis copolymerization (AMCP) of renewable isohexide diacetals and aliphatic long-chain diacetals is reported and access to a small family of copolyacetals has been established. Crucial 1–2D NMR and MALDI-ToF-MS findings unambiguously confirm the existence of a copolymeric structure. In a stark contrast to the earlier reported isohexide-polyacetals, the current copolyacetals reveal very slow degradation. Hydrolytic degradation of copolyacetal pellets is extremely slow at pH 7, whereas only 30% degradation over a period of 15 d is observed in 9 m hydrochloric acid solution. GPC investigations reveal that with increasing chain-length the rate of degradation reduces, whereas copolyacetals with short-chain aliphatic segments display a faster degradation profile. The reduced rate of degradation can be attributed to the hydrophobic nature of long-chain acetal segments. In situ NMR spectroscopy reveals the existence of formates, hemiacetals, and diols as degradation products. Thus, the rate of degradation can be tuned by the judicious choice of isohexide-diacetal and linear-diacetals in a copolyacetal.publishe

Highly Enantioselective Pd-Catalyzed Synthesis of P‑Stereogenic Supramolecular Phosphines, Self-Assembly, and Implication

Metal-catalyzed asymmetric addition of a secondary phosphine to an aryl halide is one of the most efficient and reliable approaches for the construction of enantiopure carbon–phosphorus bonds. An isolated Pd­(II) complex (<b>5</b>) catalyzes the carbon–phosphorus coupling reaction between tolylphenylphosphine (<b>1a</b>) and 3-iodophenylurea (<b>2b</b>), which proceeds with an unprecedented enantiomeric excess (ee) of 97%. The generality of the strategy has been demonstrated by preparing a small library of a new class of P-stereogenic phosphines with an in-built hydrogen bonding motif for the first time. The P-stereogenic phosphines self-assemble on a metal template via deliberately installed hydrogen-bonding motifs and mimic the bidentate ligand coordination. Interestingly, when it was employed in asymmetric hydrogenation, the supramolecular phosphine {1-(3-(phenyl­(<i>o</i>-tolyl)­phosphanyl)­phenyl)­urea} (<b>6b</b>) produced the corresponding hydrogenated product with the highest enantiomeric excess of 99% along with excellent conversion, demonstrating the potential of these enantioenriched P-chirogenic supramolecular phosphines in asymmetric catalysis

Highly Enantioselective Pd-Catalyzed Synthesis of P‑Stereogenic Supramolecular Phosphines, Self-Assembly, and Implication

Metal-catalyzed asymmetric addition of a secondary phosphine to an aryl halide is one of the most efficient and reliable approaches for the construction of enantiopure carbon–phosphorus bonds. An isolated Pd­(II) complex (<b>5</b>) catalyzes the carbon–phosphorus coupling reaction between tolylphenylphosphine (<b>1a</b>) and 3-iodophenylurea (<b>2b</b>), which proceeds with an unprecedented enantiomeric excess (ee) of 97%. The generality of the strategy has been demonstrated by preparing a small library of a new class of P-stereogenic phosphines with an in-built hydrogen bonding motif for the first time. The P-stereogenic phosphines self-assemble on a metal template via deliberately installed hydrogen-bonding motifs and mimic the bidentate ligand coordination. Interestingly, when it was employed in asymmetric hydrogenation, the supramolecular phosphine {1-(3-(phenyl­(<i>o</i>-tolyl)­phosphanyl)­phenyl)­urea} (<b>6b</b>) produced the corresponding hydrogenated product with the highest enantiomeric excess of 99% along with excellent conversion, demonstrating the potential of these enantioenriched P-chirogenic supramolecular phosphines in asymmetric catalysis

Highly Enantioselective Pd-Catalyzed Synthesis of P‑Stereogenic Supramolecular Phosphines, Self-Assembly, and Implication

Metal-catalyzed asymmetric addition of a secondary phosphine to an aryl halide is one of the most efficient and reliable approaches for the construction of enantiopure carbon–phosphorus bonds. An isolated Pd­(II) complex (<b>5</b>) catalyzes the carbon–phosphorus coupling reaction between tolylphenylphosphine (<b>1a</b>) and 3-iodophenylurea (<b>2b</b>), which proceeds with an unprecedented enantiomeric excess (ee) of 97%. The generality of the strategy has been demonstrated by preparing a small library of a new class of P-stereogenic phosphines with an in-built hydrogen bonding motif for the first time. The P-stereogenic phosphines self-assemble on a metal template via deliberately installed hydrogen-bonding motifs and mimic the bidentate ligand coordination. Interestingly, when it was employed in asymmetric hydrogenation, the supramolecular phosphine {1-(3-(phenyl­(<i>o</i>-tolyl)­phosphanyl)­phenyl)­urea} (<b>6b</b>) produced the corresponding hydrogenated product with the highest enantiomeric excess of 99% along with excellent conversion, demonstrating the potential of these enantioenriched P-chirogenic supramolecular phosphines in asymmetric catalysis