Zinc complexes supported by pyridine- N -oxide ligands: synthesis, structures and catalytic Michael addition reactions.

New dipyridylpyrroleN-oxide ligands HL1 and HL2 are designed and synthesizedviaoxidation of 2-(5-(pyridin-2-yl)-1H-pyrrol-2-yl)pyridine (Hdpp) by using 3-chloroperbenzoic acid (m-CPBA) in CH2Cl2. The treatment of ZnEt(2)with two equiv. of HL1 and HL2 affords [Zn(L1)(2)] and [Zn(L2)(2)] in medium yield, respectively. These ligands and zinc complexes are fully characterized by NMR, IR, UV-vis and ESI-MS spectroscopy and X-ray diffraction analysis. The structure of HL1 and HL2 shows a planar geometry. The intramolecular hydrogen-bond interactions between the imino hydrogen andN-oxide oxygen atom are observed. In [Zn(L1)(2)] and [Zn(L2)(2)], two ligands chelate to the zinc metal with a cross perpendicular geometry. The zinc complexes were employed as a highly efficient catalyst for the thiol-Michael addition of thiols to alpha,beta-unsaturated ketones in EtOH at room temperature. The loading of the catalyst is lowered to 0.01 mol%. The catalytic mechanism was proposed based on NMR and ESI-MS experiments.