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

The 1:1 and 1:2 reactions of [MX4](2-)(M=Pd, Pt; X=Cl, Br) with the biheteroaromatic ligand 2-(2'-pyridyl)quinoxaline (L) were studied. The new complexes [PdCl2L] (1), [PdL2]Cl-2 (2), [PtCl2L] (3), [PdBr2L] (4) and [PtBr2L] (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 H-1 and C-13 NMR spectroscopies. The complex [PdCl2L] crystallizes in space group P2(1)/c with a=9.842(1), b=10.223(1), c=15.130(1) A, beta=122.70(1)degrees, 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 sigma(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 [PdL2](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 PtCl2L0.5 (3a) and PtBr2L0.25 (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 H-1 NMR study of the I:1 [PdI(dien)]I/L (dien=diethylenetriamine) reaction solution in DMSOd(6)/MeOD reveals a monodentate quinoxaline coordination of L.