Synthesis and X-Ray Crystal Structures of Three New Nickel(II) Complexes of Benzoylhydrazones: Catalytic Applications in the Synthesis of 2-Arylbenzoxazoles

Abstract

Three new complexes of nickel(II), having general formula [Ni(L1)(PPh<inf>3</inf>)] (1), [Ni(L1)(4-picoline)] (2), and [Ni(L2)(4-picoline)] (3) were synthesized by the reaction of Ni(OAc)<inf>2</inf>·4H<inf>2</inf>O with corresponding benzolyhydrazine-derived Schiff base ligands i.e. [4-(diethylamino)-2-hydroxybenzylidene]-benzohydrazonic acid (H<inf>2</inf>L1) or [2-(hydroxynaphthalen-1-yl)methylene]-benzohydrazonic acid (H<inf>2</inf>L2) and PPh<inf>3</inf>/4-picoline as co-ligand in 1:1:1 ratio in methanol. All the three complexes were air-stable, isolated as reddish brown solids and characterized by Elemental analysis, FT-IR, 1H, 13C{1H}, 31P{1H} NMR spectroscopy and mass spectrometry. The structures of all three complexes were determined by single crystal X-ray diffraction studies which revealed the distorted square planar geometry of the complexes. In these complexes, three coordination sites were occupied by ONO pincer type Schiff base ligand and the fourth site was blocked by phosphorus (P) or nitrogen (N) atom of the co-ligand. The catalytic potential of all three complexes was explored in the synthesis of a series of 2-arylbenzoxazoles from aldehydes and 2-aminophenol, using low catalyst loading (0.5 mol%). Complex 1 was found to be the best catalyst among three complexes, for the synthesis of a series of 2-aryl benzoxazoles. The ease of synthesis, air-stability and robustness of the catalyst, and good TONs are some of the key characteristics of the described catalytic system. © 2019 Elsevier B.V.