Structural Studies of Rare Earth/Transition Metal Complex Ion Systems as a Basis for Understanding Their Thermal Decomposition to Mixed Oxides

Thermal decomposition in air of the series of complexes [Co(NH3)6][Ln(dipic)3]·x H2O, [Cr(en)3][Ln(dipic)3]·y H2O and [Cr(urea)6][Ln(dipic)3]·z H2O (Ln = La – Lu, excluding Pm but plus Y; dipic = 2,6-pyridinedicarboxylate) to give oxide phases has been examined. The products were characterised by powder X-ray diffraction, magnetic and surface area measurements. From the Co-containing species, either Co3O4 + Ln2O3 or perovskite (CoLnO3) products result, while the Cr-containing species provide CrLnO4 phases in addition, the monazite form of CrNdO4 being detected for the first time in this way. Underpinning consideration of the solid state decomposition mechanisms, single-crystal X-ray structure de-terminations have been performed on [Co(NH3)6][Ln(dipic)3]·n H2O across the Ln series, defining differing phases (alltriclinic P) for n = 5 (encompassing Ln = La–Pr), n = 8(.5) (Ln = Tm – Eu) and n = 10 (Ln = Pr – Er, including Y), and also on one of the low-temperature decomposition products, [Co(NH3)6]Cl·dipic·2 H2O (1). The structure of 1 provides evidence that conventional π-stacking of the dipicolinate anions may be of some importance in determining the solid state array, whereas consideration of all structures involving [Ln(dipic)3]3– anions indicates that “edge-to-face” interactions between dipicolinate ligands of adjacent complex anions may also be determinants of the formation of sheet-like arrays of the trianions commonly observed in these compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)