Preparation, structural characterization and physical properties of palladium(II) and platinum(II) complexes of the biheteroaromatic ligand 2-(2′-pyridyl)quinoxaline (L)

Abstract

The 1:1 and 1:2 reactions of [MX 4] 2− (M=Pd, Pt; X=Cl, Br) with the biheteroaromatic ligand 2-(2′-pyridyl)quinoxaline (L) were studied. The new complexes [PdCl 2L] ( 1), [PdL 2]Cl 2 ( 2), [PtCl 2L] ( 3), [PdBr 2L] ( 4) and [PtBr 2L] ( 5) were isolated and studied by a combination of conductivity measurements, X-ray powder patterns, thermal techniques (TG/DTG, DTA) and spectroscopy (IR, far-IR, Raman). Compounds 1 and 2 have also been characterized by using 1H and 13C NMR spectroscopies. The complex [PdCl 2L] crystallizes in space group P2 1/ c with a=9.842(1), b=10.223(1), c=15.130(1) A, β=122.70(1)°, V=1281.0(6) A 3 and Z=4. Refinement of the structure gave a final R factor of 0.017 ( R w=0.024) for 2050 unique reflections having I>2.0 σ( I). The X-ray crystal structure of 1 shows a slightly distorted square planar geometry around the Pd II atom. The ligand L behaves as a bidentate chelate with the ligated atoms being the pyridine nitrogen and the nearest quinoxaline nitrogen. Similar monomeric, square planar structures are also assigned for 3, 4 and 5. A trans square planar structure is proposed for the [PdL 2] 2+ cation in 2. All data are discussed in terms of the nature of bonding and known and assigned structures. Compounds with the empirical formulae PtCl 2L 0.5 ( 3a) and PtBr 2L 0.25 ( 5c) were isolated during the thermal decomposition of 3 and 5; the vibrational studies of these thermally stable intermediates show that L acts as a tridentate bridging ligand and support complicated polymeric structures. The 1H NMR study of the 1:1 [PdI(dien)]I/L (dien=diethylenetriamine) reaction solution in DMSO-d 6/MeOD reveals a monodentate quinoxaline coordination of L.