Palladium(II) complexes of N,N-co-ordinating arylazouracil ligands: infrared spectroscopy, thermal properties, and X-ray crystal structure of trans-bis(6-amino-1,3-dimethyl-5-phenylazouracilato)palladium(II)

Abstract

Palladium(II) forms the complexes [PdL₂](1)–(4) on reaction of tetrachloropalladate(II) with 6-amino-5-arylazouracil derivatives (uracil = pyrimidine-2,4-dione) in ethanolic media, with the exception of 6-amino-5-(2′-chlorophenylazo)-3-methyluracil (HL⁵) which leads to the complex [Pd(HL⁵)₂Cl₂](5). The crystal structure of the complex bis(6-amino-1,3-dimethyl-5-phenylazouracilato)palladium(II), [PdL¹₂](1), was determined by X-ray crystallography. The complex crystallizes in the monoclinic system, space group P2₁/c, with a= 7.404(2), b= 9.537(1), c= 17.420(4)Å, β= 99.38(2)°, Z= 2, and R= 0.043. The structure consists of discrete [PdL¹₂] molecules (L¹= 6-amino-1,3-dimethyl-5-phenylazouracilate), in which L¹ acts as a bidentate ligand through the nitrogen atom of the deprotonated amino group and the nitrogen atom of the azo group bonded to the phenyl ring, to give a square-planar PdN₄ geometry with palladium(II) at the inversion centre. The entire molecule is planar with the exception of the phenyl rings which are twisted by 84.4(2)° about the C–N bond away from the co-ordination plane in order to overcome interligand steric crowding. On the basis of i.r. and thermal data a trans-square-planar structure is proposed for (5), the ligand being co-ordinated through the nitrogen atom of the 6-amino group. In the temperature range 230–320 °C this complex eliminates two molecules of HCl to give [PdL⁵₂](6). The solid-state parameters and mechanism for this reaction were determined.