The stability of coordination compounds of some crown ethers with lanthanide(III) ions

Abstract

Moncyclic polyethers, the ‘crown ethers’, are noted for their affinity and high selectivity towards alkali and alkaline earth elements, and a large amount of work has been published concerning the coordination chemistry of crown ethers with these elements. However. information about the coordination behavior of crown ethers towards the lanthanides is relatively scarce and is badly needed because of its practical as well as theoretical significance. t001 Stability Constants, β, Coordination Compounds of Crown Ethers with Na + and Ln 3+ (Solvent: MeOH; Ionic Strength: 0.1 M Me 4NCl). Crown 18C6 DB18C6 26 ± 0.5 °C DCH18C6 26 ± 0.5 °C Logβ 15 ± 0.5 °C 26 ± 0.5 °C 35 ± 0.5 °C Izatt [2] Ion Na + 4.35 ± 0.05 4.17 ± 0.02 4.01 ± 0.05 4.33 4.18 ± 0.01 3.32 ± 0.01 La 3+ 3.21 +- 0.05 3.25 ± 0.01 3.31 ± 0.03 3.29 ± 0.03 no 2.49 ± 0.03 Ce 3+ 2.76 ± 0.02 2.81 ± 0.01 2.93 ± 0.04 3.57 ± 0.20 no 2.05 ± 0.05 Pr 3+ 2.36 ± 0.02 2.47 ± 0.01 2.57 ± 0.04 2.63 ± 0.28 no 1.62 +- 0.07 Nd 3+ 2.05 ± 0.11 2.16 ± 0.01 2.30 ± 0.09 2.44 ± 0.16 no <1.3 Sm 3 - <1.6 - 2.03 ± 0.07 no no Eu 3+ - <1.5 - 1.84 ± 0.14 no no Gd 3+ - <1.5 - 1.34 ± 0.12 no no Tb 3+ - <1.2 - no <1.5 no Dy 3+ - <1.2 - no <1.7 no Ho 3+ - <1.5 - no <1.7 no Er 3+ - <1.4 - no <1.5 no Tm 3+ - <1.3 - no <1.6 no Yb 3+ - no - no <1.4 no Lu 3+ - no - no <1.4 no t002 Thermodynamic Functions for the Coordination of 18-C-6 with Na + and Ln 3+ in Methanol at 25 °C Ion ΔG° (kcal/mol) ΔH° (kcal/mol) TΔS° (kcal/mol) Na + −5.72 −8.01 −2.3 La 3+ −4.47 2.30 6.7 Ce 3+ −3.89 3.84 7.7 Pr 3+ −3.38 4.45 7.8 Nd 3+ −2.96 6.14 9.1 A pNa method is developed for the determination of the stability constants of the coordination compounds of crown ethers and lanthanide ions, Ln 3+, based on the competition between Na + and Ln 3+ ions towards coordination with a given crown. It consists of setting up a cell using Na + as the competing ion, Na +-selective electrode as the indicating electrode and AgAgCl electrode as the reference electrode. The electromotive forces of such a cell (C = drown ether) AgAgCl¦0.1 M Me4NCl, x M Na +, y M C, z M Ln 3+¦ Na +-electrode are measured successively under those conditions: (1) only Na +, but neither crown nor Ln 3+ is present; (2) crown ether is present but not Ln 3+ ion is present; (3) both crown ether and Ln 3+ are present. From the electromotive forces measured, concentrations of free ions and then stability constants can be calculated. Details of this method were published in a short note [1]. With this method, stability constants of coordination compounds of 18-crown-6 (18-C-6), dibenzo 18-crown-6 (DB 18-C-6), dicyclohexyl 18-crown-6 (DCH 18-C-6), 15-crown-5 (15-C-5) and benzo 15-crown-5 (B 15-C-5) with all Ln 3+ ions except Pm 3+ in methanol are determined. The results are given in Table I and agree quite well with those reported by Izatt et al. [2] except for Ce 3+. It can be seen that for the lighter lanthanides, the stability constants of the coordination compounds with 18-C-6 or DCH 18-C-6 decrease with increasing atomic number of the lanthanides, a fact which can be explained by the better matching of the crown ether cavity size with cation diameter. The 18-C-6 also shows weak coordination with the heavier lanthanides whereas the DB 18-C-6 shows weak coordination only with the heavier lanthanides and no coordination is indicated for 15-C-5 or 15-C5 with any lanthanides. The stability constants of Na +, La 3+, Ce 3+ Pr 3+ and Nd 3+ are determined at three different temperatures for the determination of ΔH° calculated by plotting log β against 1/T, Δ° is calculated from the relation ΔG° = RTlnβ and then ΔS° calculated from the relation ΔG° = ΔH° - TΔS°. The results obtained are given in Table II. From the ΔH° and ΔS° values it can be seen that the observed stability is entropic in origin.