SYNTHESIS AND CHARACTERIZATION OF SQUARE-PLANAR PLATINUM(II) AND PALLADIUM(II) COMPLEXES WITH PYRIDINE-2,6-DICARBOXYLIC ACID (H2DIPIC) - X-RAY CRYSTAL-STRUCTURE OF TRANS-NA2[PT(DIPIC)2]-6H2O AND K[PT(DIPIC)I]-1/2H2O

The tetra-coordinate complexes trans-[Pt(Hdipic)2].2H2O (1) and trans-[Pd(HdiPic)2].2H2O (2) as well as their potassium and sodium derivatives trans-K2[Pt(dipic)2] (3), trans-Na2[Pt(dipic)2].6H2O (4) and trans-Na2[Pd(dipic)2].2H2O (5) have been obtained in high yield by reacting aqueous solutions of K2[ptCl4] and K2[PdCl4] with pyridine-2,6-dicarboxylic acid (H2dipic) or its potassium or sodium salts, respectively, in 1:2 molar ratio. 1 and 2 are also formed on reacting equimolar amounts of reagents in water (pH 1.4) leaving in solution unreacted K2[PtCl4] and K2[PdCl4]. The resulting mixtures brought to pH 5 with KOH undergo a redistribution reaction with formation of the monoanionic species K[Pt(dipic)Cl].H2O (6) and K[Pd(dipic)Cl].H2O (7), respectively. The iodo derivative K[Pt(dipic)l]. 1/2H2O (8) was obtained from the chloro species by replacement of the coordinated chloride with iodide in aqueous solution. All the complexes have been characterized by a number of physico-chemical measurements including the X-ray analysis of trans-Na2[Pt(dipic)2].6H2O (4) and K[Pt(dipic)I].1/2H2O (8). Crystals of 4 are monoclinic, space group P2(1)/c, Z = 2, with unit-cell dimensions a = 11.912(1), b = 6.544(1), c = 12.699(1) angstrom, beta = 91.56(1)-degrees. Crystals of 8 are triclinic, space group P1BAR, Z = 4, with unit-cell dimensions a = 11.018(1), b = 11.585(2), c = 9.946(1) angstrom, alpha = 97.99(1), beta = 115.07(1), gamma = 73.14(1)-degrees. The structures were solved from three dimensional counter data by Patterson and Fourier methods and refined by full-matrix least-squares to R = 0.019 and 0.030 for 1556 and 4118 observed reflections, respectively. The crystal of 4 is built up by centrosymmetric Pt(dipic)2(2-) anions and Na+ cations hexa-coordinated by four water molecules and two carboxylate oxygens belonging to the ligands. Each dipicolinate ion behaves as a bidentate ligand, and the resulting coordination around the platinum is square planar. The asymmetric unit of compound 8 contains two independent square planar Pt(dipic)l-anions, in each of them the dipicolinate ion acts as a tridentate ligand and the fourth position is occupied by an iodine atom. The planar complex anions Pt(dipic)l- are stacked but not parallel and Pt atoms form infinite zigzag chains.