Mechanism of the Mechanochemical Synthesis of Mixed-Ligand Terbium(III) Quinolinates

Abstract

Mechanochemical synthesis of terbium quinolinates with nitrogen-containing neutral ligands involves several stages; no self-propagation reaction is observed. The resulting terbium(II) quinolinate exhibits paramagnetic properties.Previously we have studied feasibility of solidphase mechanochemical techniques for synthesis of mixed-ligand compounds of europium(III) and terbium(III) with monocarboxylic acids [1,2]. In [3] we have described the mechanochemical synthesis of terbium quinolinate hydrate Tb 2 [C 5 H 3 N(COO) 2 ] 3 . 3H 2 O and mixed-ligand terbium(III) quinolinates [C 5 H 3 N(COO) 2 2! ] with nitrogen-containing neutral ligands Tb 2 [C 5 H 3 N(COO) 2 ] 3 D . 2H 2 O, where D is 1,10-phenanthroline (Phen) or 2,2`-dipyridyl (Dipy).In the present work we studied the mechanism of the mechanochemical synthesis of previoulsly prepared terbium(III) compounds with the dicarboxylic quinolinic acid by means of X-ray phase analysis, thermal gravimetric analysis, and electron spin resonance (ESR).Mechanochemical treatment in a vibrating mill (1000 rpm) of a mixture of a terbium(III) salt, quinolinic acid, and a nitrogen-containing ligand results, within 132 min, to homogenization of the mixture and formation of a colorless powder.The X-ray patterns of the reaction mixture after 1-min mechanical activation and of the starting reagents before mechanical treatment are identical to each other. The X-ray patterns show the same interplanar spacings and reflection intensities. This finding suggests that 1-min mechanical treatment gives rise to no new fractions and results in grinding and stirring of the starting compounds. On further mechanical treatment (90 s), as seen from the X-ray patterns, reflections of the start-ing crystal phases disappear, the intensities of reflections and interplanar spacings change, and a number of broadened reflections appear. The X-ray patterns of the samples prepared under these conditions are not identical to those of complexes prepared by a traditional procedure in aqueousorganic media. The X-ray diffraction data suggest activation of the reaction mixture of the starting compounds. It is only after 2-min mechanical treatment, that the X-ray pattern contain reflections whose intensities and interplanar spacings correspond to mixedligand terbium(III) compounds and suggest completion of their synthesis (comparison with the X-ray patterns of compounds synthesized by a traditional procedure in aqueous ethanol).After mechanical treatment for 30, 60, and 90 s, the reaction mixture [Tb(NO 3 ) 3 . 6H 2 O + C 5 H 3 N . (CO 2 H) 2 ] was subjected to thermal gravimetric analysis. It was found that mechanical treatment produces a slight exothermal effect at 903100oC, assignable to loss of traces of water, as well as an endothermic effect at 2803310oC, assignable to melting of terbium(III) quinolinate. The lack of new exothermal effects in the mechanochemical process suggests that product formation in the activated reaction mixture occurs not by the expense of the reaction heat. The reaction is not self-...