Four N,O-Bidentate-Chelated Ligand-Tunable Copper(II) Complexes: Synthesis, Structural Characterization and Exceptional Catalytic Properties for Chan-Lam Coupling Reactions

Four N,O-bidentate chelating ligands [L-1=N-(4-thiazolylmethyl)morpholine-N-oxide, L-2=N-(2-pyridylmethyl)pyrrolidine-N-oxide, L-3=N-(4-thiazolylmethyl)pyrrolidine-N-oxide, and L-4=N-(2-pyridylmethyl)morpholine-N-oxide] were designed and synthesized. The treatment of N,O-bidentate ligands with CuCl2 in ethanol at room temperature afforded four new copper complexes-[Cu(L-1)(2)]Cl-2 (I), [Cu(L-2)(2)][CuCl4] (II), [Cu(L-3)(2)Cl](2)[CuCl2(H2O)(2)]Cl-2 center dot 10H(2)O (III) and [Cu(L-4)(2)][Cu(L-4)(2)(CH3OH)(2)][CuCl4](2) (IV)-which were fully characterized by single-crystal X-ray diffraction, IR spectroscopy and elemental analyses. Most interestingly, the single-crystal X-ray diffraction analyses revealed that the N,O-bidentate ligands L-1-L-4 can tune the corresponding geometry configuration of copper atoms in complexes I-IV. The copper atoms in I and II showed four-coordinated distorted square planar geometry, whereas the central copper atom in III adopted a distorted square pyramidal geometry. It should be noted that complexIV was constructed by a distorted square planar CuN2O2 unit, one octahedral CuN2O4 unit and two [CuCl4](2-) anions. Importantly, these copper(II) complexes, especially complexIV, showed high catalytic activity in Chan-Lam coupling reactions of primary or secondary amines with arylboronic acid under mild conditions. Substrates with many functional groups were tolerated and the C-N coupling products were afforded in good to excellent yields (up to 96% yield).