Mixed ligand Cu(II) complexes as new precursors for the synthesis of CuO nanomaterials: development of homogeneous and heterogeneous catalysts for styrene epoxidation and phenol hydroxylation under mild conditions.

Abstract

A new synthetic strategy for obtaining CuO nanocatalysts has been devised using a pair of newly synthesized mixed ligand Cu(II) complexes as the precursors. The new Cu(II) complexes are of the types [Cu(def)(bpy)(NO3)] 1 and [Cu(def)(phen)(H2O)]NO3·(CH3OH) 2, where def (deferiprone) is the primary ligand and bpy (2,2'-bipyridine) and phen (1,10-phenanthroline) are the co-ligands. They have been employed as the synthetic precursors for the straightforward synthesis of CuO nanomaterials. The single crystal X-ray structures of 1 and 2 reveal that both the complexes possess square pyramidal coordination geometries. The mixed ligand-templated CuO nanomaterials 3 and 4 have been obtained via thermal degradation of the complexes 1 and 2, respectively. They have been comprehensively characterized using FT-IR, UV-DRS, XRD, SEM, TEM, and BET analyses. The CuO nanocomposites have been found to be efficient heterogeneous catalysts for the selective styrene epoxidation with H2O2, yielding a striking 99.7% conversion and 99.3% selectivity under organic solvent-free conditions. Also, the same nanocatalysts displayed remarkable efficiency as water tolerant heterogeneous catalysts to facilitate phenol hydroxylation (65.9/67.8% conversion) with H2O2 in aqueous medium. They could be easily recovered and recycled for at least three consecutive cycles without significant lowering of their selectivity or activity profile. The precursor mixed ligand complexes 1 and 2, on the other hand, served as homogeneous catalysts in the epoxidation of styrene under similar solvent-free reaction conditions with 98.5/97.3% conversion and around 99% selectivity, as well as in the phenol hydroxylation in aqueous medium with 63.2/69.3% conversion.