双冠醚配位作用的热力学性质 Ⅴ.双(苯并-15-冠-5)与稀土(Ⅲ)硝酸盐在无水乙腈溶液中分子内夹心配位作用

Complexation Thermodynamics of Bis (crown ether)s V. Intramolecular Sandwich Complexation of Light Lanthanoid Nitrates with Bis(benzo-15-crown-5) in Anhydrous Acetonitrile

本文用分光光度滴定法测定了双(苯并-15-冠-5)(1)与轻稀土(Ⅲ)硝酸盐在无水乙腈溶液中,20.0～35.0℃时分子内夹心配位作用的稳定常数,进而计算了配位焓和配位熵,并与母体苯并15-冠-5(2)的实验结果作了比较.化学计量法表明,所有稀土硝酸盐均与双(苯并-15-冠-5)形成了1:1的配合物.从热力学的观点,讨论了双冠醚分子结构、尺寸效应和空间构型等对配位稳定性的影响.研究结果发现,双冠醚(1)对于Eu^(3+)具有较强的配位能力和配位选择性,Nd^(3+)次之.配合物的稳定性主要来自于熵的贡献.

The spectrophotometric titrations have been performed in anhydrous acetonitrile at 20.0-35.0℃ to give the stability constants (logKs) and the thermodynamic parameters (△H° andT△S°) for the intramolecular sandwich complexation of light lanthanoid (III) nitrates (La, Ce, Pr, Nd, Sm, Eu, Gd) with bis(benzo- 15 - crown- 5) (1), and the results were compared with the data reported for the parent benzo - 15 - crown - 5. The stoichiometry of the complex formed is 1:1 for all light lanthanoid nitrates complexation with bis(benzo - 15 - crown - 5) (1). The effects of the bis (crown ether)' s size - fit concept, spatial arrangement, intramolecular constitution and other major factors upon complex stability are discussed from a viewpoint of thermodynamics. The thermodynamic parameters obtained indicate that the coordination of Eu3+ with bis(benzo-crown ether) (1) shows the strongest complexation ability and selectivity, and Nd3+the second. The stabilities of light lanthanoid nitrates complexation with bis (crown ether) are contributed mainly through the coordination entropy.