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Coordination Chemistry of Gold Catalysts in Solution: A Detailed NMR Study

Authors

  • Dipl.-Chem. Alexander Zhdanko,

    1. Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen (Germany), Fax: (+49) 7071-295137
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  • Dr. Markus Ströbele,

    1. Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen (Germany), Fax: (+49) 7071-295702
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  • Prof. Dr. Martin E. Maier

    Corresponding author
    1. Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen (Germany), Fax: (+49) 7071-295137
    • Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen (Germany), Fax: (+49) 7071-295137
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Abstract

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh3, JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S-tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOH≪hex-3-yne <MeCN≈OTf≪Me2S<2,6-lutidine<4-picoline<CF3CO2≈DMAP<TMTU<PPh3<OH≈Cl. Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)2 and S-tolBINAP(AuOTf)2 showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh3 causes abstraction of one gold atom to give mononuclear complexes LLAuPPh3+ and (Ph3P)nAu+, but other N and S ligands give ordinary dicationic species LL(AuNu)22+. In reactions with different bases, LAu+ provided new oxonium ions whose chemistry was also studied: (DTBPAu)3O+, (L2Au)2OH+, (L2Au)3O+, (L3Au)2OH+, and (IMesAu)2OH+. Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand- or base-assisted interconversions between (L2Au)2OH+, (L2Au)3O+, and L2AuOH are described. Reactions of dppf(AuOTf)2 and S-tolBINAP(AuOTf)2 with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)2]3O22+, L8(Au)2OH+, and [L8(Au)2]3O22+, but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe+ with S2− was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined.

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