Preparation, separation and characterisation of two regioisomers of a N-hydroxyalkylpyridylpyrazole ligand: A structural investigation of their coordination to Pd(II), Pt(II) and Zn(II) centres

Abstract

The reaction of the β-diketone 1-phenyl-3-(pyridyn-2-yl)propane-1,3-dione, and the monosubstituted hydrazine 2-hydroxyethylhydrazine has been investigated. Two regioisomers were identified, 2-(3-phenyl-5-(pyridyn-2-yl)-1H-pyrazol-1-yl)ethanol (pzol.1) and 2-(5-phenyl-3-(pyridyn-2-yl)-1H-pyrazol-1-yl)ethanol (pzol.2) in 57:43 ratio. The separation of the regioisomers was done by silica column chromatography using ethyl acetate as eluent.Palladium(II) and platinum(II), [MCl2(pzol.1)2], [MCl2(pzol.2)], and zinc(II), [ZnCl2(pzol.1)], [ZnCl2(pzol.2)] complexes were synthesised and characterised. The crystals and molecular structures of [PdCl2(pzol.2)]·H2O and [ZnCl2(pzol.2)] were solved by X-ray diffraction, and consist of mononuclear complexes. In complex [PdCl2(pzol.2)]·H2O, the Pd(II) centre has a typical square planar geometry, with a slight tetrahedral distortion. The tetra-coordinated atom is bonded to one pyridinic nitrogen, one pyrazolic nitrogen and two chlorine atoms in cis disposition. The pzol.2 ligand acts as a bidentate chelate forming a five-membered metallocycle ring. In complex [ZnCl2(pzol.2)], the Zn(II) is five-coordinated with two Zn–N bonds (Zn–Npz and Zn–Npy), one Zn–OH bond and two Zn–Cl bonds. The coordination geometry is intermediate between a trigonal bipyramid and a square pyramid. In this complex, the ligand pzol.2 is tridentated and forms two metallocycle rings.