冠醚配合物热力学性质的研究——Ⅷ 无水硝酸镧与若干冠醚在乙腈中的量热滴定

稀土离子与冠醚反应的热函至今报道不多。仅1977年Izatt等用量热滴定对稀土氯化物在甲醇中与18-冠-6的反应热函进行过研究。他们发现轻稀土从La~(3+)至Gd~(3+)与冠醚反应是吸热反应,而重稀土从Tb~(3+)至Lu~(3+)与冠醚反应却没有热量产生或△H近于零。此外他们还得出了若干反常结果,我们认为,反应热量的大小与稀土盐和有机溶剂的含水量有密切关系,因三价稀土离子电荷高,牛径又不大,水合能力强,在不严格去水的条件下,所得的△H和稳定常数不甚可靠。为此,我们研究了无水硝酸镧与18-冠-6(18C6),2,3,11,12-二环已基-18-冠-6(DC-18C6),及2,3-苯并-11-甲基-18-冠-6(BC1-18C6)在无水乙腈中的配位反应,得出与Izatt等不同的结果。实验表明,所有冠醚与La~(3+)的反应均为放热反应,它们有较大的热函和较高的稳定性。

STUDIES ON THERMODYNAMICS OF CROWN COORDINATION COMPOUNDS (Ⅷ) CALORIMETRIC TITRATION FOR COORDINATION REACTION OF DEHYDRATED LANTHANUM NITRATE WITH SOME CROWNS IN ACTONITRILE

The coordination reaction of lanthanum nitrate with 18-crown-6, dicyclohexyl-18-crown-6 and 2,3-benzo-11-methyl-18-crown-6 in acetonitrile was studied. The reaction enthalpies and stability constants were obtained by home-made titration calorimeter and computer program. We demonstrated that the reaction is exothermic on condition that the lanthanum nitrate was dehydrated rigorously. Supplementary experiments showed that the reaction heat is dependent obviously on water content in lanthanum nitrate. Because lanthanum (III) ion has strong hydrophilic ability and large hydration heat, large energy consumption is necessary when crown replaces the coordinated water. Therefore, if the lathanum nitrate has not been dehydrated rigorously, the reaction heat measured may be small negative or positive value.