Browsing by Author "Birchall, Nicholas"

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    Annellated 1,3,4,2-triazaphospholenes-simple modular synthesis and a first exploration of ligand properties
    (2022) Richter, Ferdinand; Birchall, Nicholas; Feil, Christoph M.; Nieger, Martin; Gudat, Dietrich
    The successful use of 1,3,4,2-triazaphospholenes (TAPs) as organo-catalysts stresses the need for efficient synthetic routes to these molecules. In this study, we establish the [1 + 4]-cycloaddition of PBr3 to azo-pyridines as a new approach to preparing pyrido-annellated TAPs in a single step from easily available precursors. The modular assembly of the azo-component via condensation of primary amines and nitroso compounds along with the feasibility of post-functionalization at the P-Br bond under conservation of the heterocyclic structure allows, in principle, to address a wide range of target molecules, which is illustrated by prototypical examples. The successful synthesis of a transition metal complex confirms for the first time the ability of a TAP to act as a P-donor ligand. Crystallographic studies suggest that hyperconjugation effects and intermolecular interactions induce a qualitatively similar ionic polarization of the P-Br bonds in TAPs as in better known isoelectronic diazaphospholenes.
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    Chromium complexes with benzanellated N‐heterocyclic phosphenium ligands : synthesis, reactivity and application in catalytic CO2 reduction
    (2024) Birchall, Nicholas; Hennhöfer, Fridolin; Nieger, Martin; Gudat, Dietrich
    A chromium complex carrying two benzanellated N‐heterocyclic phosphenium (bzNHP) ligands was prepared by a salt metathesis approach. Spectroscopic studies suggest that the anellation enhances the π‐acceptor ability of the NHP‐units, which is confirmed by the facile electrochemical reduction of the complex to a spectroscopically characterized radical anion. Co‐photolysis with H2 allowed extensive conversion into a σ‐H2‐complex, which shows a diverse reactivity towards donors and isomerizes under H-H bond fission and shift of a hydride to a P‐ligand. The product carrying phosphenium, phosphine and hydride ligands was also synthesized independently and reacts reversibly with CO and MeCN to yield bis‐phosphine complexes under concomitant Cr‐to‐P‐shift of a hydride. In contrast, CO2 was not only bound but reduced to give an isolable formato complex, which reacted with ammonia borane under partial recovery of the metal hydride and production of formate. Further elaboration of the reactions of the chromium complexes with CO2 and NH3BH3 allowed to demonstrate the feasibility of a Cr‐catalyzed transfer hydrogenation of CO2 to methanol. The various complexes described were characterized spectroscopically and in several cases by XRD studies. Further insights in reactivity patterns were provided through (spectro)electrochemical studies and DFT calculations.
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    N-heterozyklische Phospheniumionen : eine noble Ergänzung zu einem unedlen Metall
    (2024) Birchall, Nicholas; Gudat, Dietrich (Prof. Dr. Dr h.c.)
    Die vorliegende Arbeit befasst sich mit Untersuchungen zur Aktivierung und Funktionalisierung kleiner Moleküle (H2, CO2, CO, Alkene, Alkine) an Komplexen mit einem unedlen Metallzentrum. Hierbei kommen neue Formen der Metall-Ligand-Kooperativität (engl. MLC) zum Einsatz, die eine Nutzbarkeit dieser (unedlen) Metallkomplexe in einer Vielzahl an katalytischen Transformationen ermöglichen.
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    Silyl‐ and germyl‐substituted diorganophosphonites
    (2023) Hettich, Thomas D.; Rudolf, Richard; Birchall, Nicholas; Nieger, Martin; Gudat, Dietrich
    Reactions of metalated diorganophosphonite boranes with triorganosilyl and ‐germyl halides provided borane adducts of diorgano(tetryl)phosphonites. Further treatment with excess Et3N or DABCO yielded the borane‐free species (RO)2P‐ER′>3 (E=Si, Ge; R, R′=alkyl, aryl). The products of all reactions were characterized by elemental analyses and NMR data, and in selected cases by MS and single‐crystal XRD studies. Reactions of selected ligands with Ni(CO)4 and selenium were shown to produce Ni(CO)‐complexes or diorgano(tetryl) phosphonoselenoates (RO)(R′3E)P=Se, respectively, which were identified spectroscopically but could not be isolated. Evaluation of the TEP and 1JPSe coupling constants were used for a first assessment of the electron donor properties of the new molecules.
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    Synthesis and ambiphilic reactivity of metalated diorgano‐phosphonite boranes
    (2021) Hettich, Thomas D.; Rudolf, Richard; Feil, Christoph M.; Birchall, Nicholas; Nieger, Martin; Gudat, Dietrich
    Unprecedented metalated phosphonite boranes were prepared from PH‐substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self‐condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.