Lanthanide ions stability

The reason why lanthanides of high atomic number emerge first is that the stability of a lanthanide ion-citrate ion complex increases with the atomic number. Since these complexes are formed by ions, this must mean that the ion-ligand attraction also increases with atomic number, i.e. that the ionic radius decreases (inverse square law). It is a characteristic of the lanthanides that the ionic radius... 

Sarcoplasmic reticulum vesicles prepared from rabbit skeletal muscle were crystallized in a medium of 0.1 M KCl, lOmM imidazole (pH 8), and 5mM MgCl2 by the addition of either CaCl2 (100/rM) or lanthanide ions (1-8 M) that stabilize the E conformation of the Ca -ATPase [119]. After incubation at 2°C for 5-48 hours, crystalline arrays were observed on the surface of about 10 20% of the vesicles in sarcoplasmic reticulum preparations obtained from fast-twitch rabbit skeletal muscles. 

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

In a potentiometric study in propylene carbonate, using Pb11 or Tl1 as auxiliary ions, stability constants have been determined for a variety of crown ethers. Some results464 are shown in Table 8. They show that the wrap-around ligand dibenzo-30-crown-10 is relatively quite effective, while the 2 1 complexes, presumably of the sandwich type, are favoured for larger lanthanides and smaller crowns. 

Solutions of dipositive lanthanide cations have been obtained in liquid ammonia, ethanol, THF, acetonitrile and hexamethylphosphoramide. Ions stabilized, or for which there is evidence for stabilization, include Nd2+, Dy2+ and Tm2+ as well as Eu2+, Yb2+ and Sm2+. The non-hydroxylic solvents are best at stabilizing M2+ ions. Thus NdCl2(THF)2 has been reported from the reduction of NdCl3 in THF by Na(naphthalene),657 and corresponding reductions of MC13 (M = Eu, Yb or Sm) have also been achieved.658 Solutions of solvated MI2 (M = Sm or Yb) in THF may easily be made by the quantitative reaction of the metal with 1,2-diiodoethane, producing ethane. The solid THF adducts may be isolated.659... 

By redesigning the above acyclic podand-type ligand 3 into a cyclic cryptate, the issue of stability can be resolved resulting in kinetically stable complexes (Scheme 4) [102]. The Tb(III) and Eu(III) complexes of cryptate 5 show an increase in lanthanide emission lifetimes of 0.72 ms and 0.41 ms, respectively, upon excitation at 310 nm. Similar results are found with the phenanthroline analogue 6 with Eu(III). A large number of modifications of these cryptates have been reported, all showing enhancements in the lanthanide ion emission [103-106]. 

Apart from Eu3+ and Tb3+, few studies have been reported on optical properties of lanthanide ions doped in ZnS nanociystals. Bol et al. (2002) attempted to incorporate Er3"1" in ZnS nanociystal by ion implantation. They annealed the sample at a temperature up to 800 °C to restore the crystal structure around Er3"1", but no Er3"1" luminescence was observed. Schmidt et al. (1998) employed a new synthesis strategy to incorporate up to 20 at% Er3"1" into ZnS (1.5-2 nm) cluster solutions which were stabilized by (aminopropyl)triethoxysilane (AMEO). Ethanolic AMEO-stabilized Er ZnS clusters in solutions fluoresce 200 times stronger at 1540 nm than that of ethanolic AMEO-Er complexes. This is explained by the very low phonon energies in ZnS QDs, and indicates that Er3+ ions are trapped inside chalcogenide clusters. However the exact position of Er3+ in ZnS clusters remains unknown. Further spectroscopic and structural analyses are required in order to obtain more detailed information. 

Thus the separation of the two lanthanide ions from each other depends upon their respective complex formation constants. The lanthanide whose stability constant is higher will desorb and elute from the column in preference to the lanthanide whose stability constant is lower. 

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