Coordination chemistry of gold catalysts in solution: a detailed NMR study.

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh(3), 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(-) ≪Me(2)S<2,6-lutidine<4-picoline<CF(3)CO(2)(-) ≈DMAP<TMTU<PPh(3) <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, PPh(3) causes abstraction of one gold atom to give mononuclear complexes LLAuPPh(3)(+) and (Ph(3)P)(n)Au(+), but other N and S ligands give ordinary dicationic species LL(AuNu)(2)(2+). In reactions with different bases, LAu(+) provided new oxonium ions whose chemistry was also studied: (DTBPAu)(3)O(+), (L2Au)(2)OH(+), (L2Au)(3)O(+), (L3Au)(2)OH(+), and (IMesAu)(2)OH(+). Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand- or base-assisted interconversions between (L2Au)(2)OH(+), (L2Au)(3)O(+), 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)](3)O(2)(2+), L8(Au)(2)OH(+), and [L8(Au)(2)](3)O(2)(2+), 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 S(2-) was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined.