Reactions of ZrOCl2·8H2O with carboxylic acids and thionyl chloride: crystal and molecular structures of K4[Zr(mal)4]·2H2O and K4[Zr(dipic)3]2·13.5H2O

Reactions of ZrOCl2·8H2O in aqueous solution with a carboxylic acid in the presence of K2CO3 have been studied as a route to ZrIV-carboxylates. With malonic acid (HO2CCH2CO2H) (H2mal) the product has been identified as K4[Zr(mal)4]·2H2O (1) by X-ray crystallography. The individual eight-coordinate zirconium anions contain four bidentate (OO′) malonate anions with the metal geometry approximating to a square antiprism with each chelating ligand spanning the two square faces, Zr—O 2.091(3)–2.288(3) Å. The four potassium cations feature irregular coordination spheres of oxygen atoms [from both H2O and (mal) ligand molecules] with a 7–9 coordination range. With 2,6-dicarboxypicolinic␣acid (HO2CC5NH3CO2H) (H2dipic) the product has been characterised as K4[Zr(dipic)3]2·13.5H2O (2) following X-ray diffraction studies. The structure consists of two [Zr(dipic)3]2- anions, four potassium cations and lattice solvate (H2O) molecules. Individual anions feature nine-coordinate zirconium in which each dipic ligand is terdentate, being bonded via one N (pyridine) and two O (carboxylate) atoms. The metal geometry approximates to tricapped trigonal prismatic with each nitrogen atom capping a regular face of four oxygen atoms, Zr—O, 2.216(6)–2.261(6) Å; Zr—N, 2.343(8)–2.361(7) Å. The potassium cations show similar environments to those observed in structure (1). Dehydration of ZrOCl2·8H2O using SOCl2 in the presence of an excess of THF effects removal of coordinated H2O molecules and hydroxy bridging groups to provide the anhydrous bis-adduct ZrCl4(thf)2 in good yield (72%).