Synthesis and structural studies on 1:1 metal complexes with 1,3–dimethyllumazine and 1,3,6,7–tetramethyllumazine. Crystal structure of two dichloro copper (II) and mercury (II) compounds and a polymeric chloro–nitrato bridged–mercury (II) complex

Abstract

In absolute methanol medium, eight new 1:1 metal complexes of general formula MIIX2L·nH2O have been isolated (L=1,3–dimethyllumazine (DLM) or 1,3,6,7–tetramethyllumazine (DLMD) ; M=Cu, Cd, Hg, Pd and Pt; X=Cl− or NO−3) . The experimental techniques used for their structural characterization were thermal (TG and DSC) and spectral methods (IR, 13C and 1H–NMR and EPR) and magnetic measurements (77–290 K) . Single-crystal X-ray diffraction studies have been performed on the complexes [CuCl2 (DLMD) (H2O) ] ·H2O (1) , [HgCl2 (DLMD) ] (2) and [HgCl (NO3) (DLMD) ] (3) . In all the complexes, the lumazine-derived ligands seem to be coordinated in bidentate form through the N5 and O4 atoms. The complex 1 shows a monomeric five-coordinated structure in which the coordination polyhedron can be best described as a trigonal bipyramid. The equatorial plane is occupied by two chlorine atoms (Cu–Cl 2.268 and 2.261 Å) and an oxygen O (4) from the organic ligand (Cu–O (4) 2.294 Å) , whereas the apical positions were designed by a coordinated water molecule and the N (5) atom of the bidentate pteridine ligand (Cu–O (1W) 1.950 Å and Cu–N (5) 2.038 Å) . The crystal structure is conformed by an extense hydrogen bonds scheme in which the non-coordinated water molecule plays an important role. In the compound 2, the pteridine ligand interacts only weakly with the Hg (II) ion (Hg–O (4) 2.772 Å and Hg–N (5) 2.675 Å) ; the structure must be described as an adduct between the HgCl2 and the organic ligand in which the molecules of metallic salt retain their identity (Hg–Cl 2.284 and 2.288 Å) . These units are weakly paired through long interactions of each metal centre with chloride ions of a neighbour molecule (Hg···Cl 3.664 Å) . The mixed complex 3 displays a polymeric structure in which the nitrate groups act as bridging bifurcate ligands between two Hg (II) ions through only one of their three oxygen atoms. The coordination polyhedron around the Hg (II) ion must be described as a trigonal bipyramid in which the metal is surrounded by the O (4) , O (1N) and O (1N′) atoms in the equatorial plane at distances of 2.551, 2.563 and 2.618 Å, respectively. In the axial positions, a terminal chloride ligand (Hg–Cl (1) 2.307 Å) and the N (5) atom of pteridine ligand (Hg–N (5) 2.208 Å) can be found.