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.