PREPARATION, STRUCTURAL CHARACTERIZATION AND PHYSICAL-PROPERTIES OF PALLADIUM(II) AND PLATINUM(II) COMPLEXES OF THE BIHETEROAROMATIC LIGAND2-(2'-PYRIDYL)QUINOXALINE (L)

The 1:1 and 1:2 reactions of [MX4](2-)(M=Pd, Pt; X=Cl, Br) with the bi heteroaromatic 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 conduc tivity measurements, X-ray powder patterns, thermal techniques (TG/DTG , DTA) and spectroscopy (IR, far-IR, Raman). Compounds 1 and 2 have al so been characterized by using H-1 and C-13 NMR spectroscopies. The co mplex [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 cry stal structure of 1 shows a slightly distorted square planar geometry around the Pd-II atom. The ligand L behaves as a bidentate chelate wit h the ligated atoms being the pyridine nitrogen and the nearest quinox aline nitrogen. Similar monomeric, square planar structures are also a ssigned for 3, 4 and 5. A trans square planar structure is proposed fo r the [PdL2](2+) cation in 2. All data are discussed in terms of the n ature of bonding and known and assigned structures. Compounds with the empirical formulae PtCl2L0.5 (3a) and PtBr2L0.25 (5c) were isolated d uring 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) reactio n solution in DMSOd(6)/MeOD reveals a monodentate quinoxaline coordina tion of L.