Cinchona Alkaloids as Versatile Ambivalent Ligands – Coordination of Transition Metals to the Four Potential Donor Sites of Quinine

A series of metal complexes in which quinine (L1) and cinchonine (L2) are coordinated via the four possible donor atoms was prepared and characterized. Coordination of the tertiary N atom of quinine is observed in [L1MPPh3]+NO3− [M = Au (1), Ag (2)], [L1Pd(COD)(Cl)]+ NO3− (3) and L1Pd(Cl)(allyl) (4). The mono anion of quinine functions as N,O chelate ligand in (L1–H+)M(Cl)(PR3) (M = Pd, Pt, 5–10), [(L1–H+)Pd(en)]+NO3− (11) and (L1–H+)2TiCl2 (12). Protection of the tertiary N atom of quinine by methylation or protonation allows the synthesis of the quinoline complexes [(L1CH3+)M(Cl)2(PR3)]+ BF4− (M = Pd, Pt, 14–18), [(L1CH3+)Ir(Cl)2(Cp*)]+ BF4− (19), [(L1+H+)M(Cl)2(PR3)]+ CF3SO3− (M = Pd, Pt, 20–22) and (L2+H+)(ZnCl3−) (23). Quinine and cinchonine act as N,O–N bridge in the complexes (R3P)(Cl)M(μ-L1–H+)M(Cl)2(PR3) (M = Pd, Pt, 24, 25, 27, 28), (Cp*)(Cl)Ir(μ-L1–H1)Ir(Cl)2(Cp*) (29), (EtP3)(Cl)Pd(μ-L2–H+)Pd(Cl)2(PEt3) (26), and as N–N bridge in the complexes [(Ph3P)Au(μ-L1)Au(PPh3)]+NO3− (30) and (allyl)(Cl)Pd(μ-L1)Pd(Cl)(allyl) (31). Coordination of quinine via the C=C double bond occurs in [(L1+2 H+)PtCl3]+Cl− (32). The structures of 11, 13, 19, 23, 26 and 32 were determined by X-ray diffraction. In most cases the coordination mode and the conformation of quinine can also be derived from the 1H-NMR spectra.