Lanthanide-crown ether

Table 8 Log Association Constant Values for Lanthanide-Crown Ether Complexes in Propylene Carbonate1164...

Various crown ethers (p. 96) with differing cavity diameters provide a range of coordination numbers and stoichiometries, although crystallographic data are sparse. An interesting series, illustrating the dependence of coordination number on cationic radius and ligand cavity diameter, is provided by the complexes formed by the lanthanide nitrates and the 18-crown-6 ether (i.e. 1,4,7,10,13,16-... 

NMR investigation of molecular structure of paramagnetic lanthanide complexes with crown ethers in solutions 98ZSK714. 

In a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

Table 7 Lanthanide-Ether Distances in Crown Ether Complexes...

Where the lanthanide ionic radius and the macrocyclic cavity are incompatible, though in hydrous conditions the crown ether is likely to be displaced by water ligands, the crown may still be present in the structure of the crystal as a hydrogen-bonded adduct. This behaviour is seen in [Gd(N03)3(H20)3]-(18-crown-6).445 This type of compound is quite well known in the case of s block metals also, e.g. [Mg(H20)6]Cl2 (12-crown-4)454 and, a more subtle case, [Ca(nitrobenzoate)2(benzo-15-crown-5)]-3H20(benzo-15-crown-5)455 in which an apparent 2 1 complex has only half its crown ligand coordinated to Ca2+. 

Although they are not macrocyclic ligands, polyethyleneglycols behave somewhat similarly to the crown ethers regarding lanthanide complexation, as established by a number of crystal structure determinations. Thus a series of neodymium complexes shows consistent 10-coordination, namely [Nd(N03)3(tri-eg)],458 where tri-eg is triethyleneglycol, [Nd(N03)2(penta-eg)]N03,458 and [Nd(N03)2(N03)(tetra-eg)],459 where N03 is monodentate. The larger La3+ ion shows 11-coordination in [La(N03)3(tetra-eg)].460... 

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. 

There has been recent interest in lanthanide complexes of ligands of the type (21). These are in some sense nitrogen analogues of the crown ethers. As Schiff bases, their lanthanide complexes may be synthesized by a template condensation, for example from 2,6-diacetylpyridine, 1,2-diaminoethane and lanthanum nitrate. These components react in alcohols to give [La(N03)3L], where the aza-crown L = (21 R = Me). This compound has a structure (Figure 6), quite analogous to that of [La(N03)3(18-crown-6)], in which La—N... 

These complexes, unlike the crown ether complexes but similar to the aza-crown and phthalocyanine complexes, are fairly stable in water. Their dissociation kinetics have been studied and not surprisingly they showed marked acid catalysis.504 Association constant values for lanthanide cryptates have been determined.505,506 A study in dimethyl sulfoxide solution by visible spectroscopy using murexide as a lanthanide indicator showed that there was little lanthanide specificity (but surprisingly the K values for Yb are higher than those of the other lanthanides). The values are set out in Table 9.507... 

In related work,20 these same authors evaluated a fluorinated crown ether carboxylic acid,. rym-difluorobenzo-16-cniwn-5 oxyacetic acid, for use in the SCF extraction of uranyl ion and lanthanides, again from a cellulose matrix. Although this compound... 

Liu, Y., Zhang, H. Y., Bai, X. P., Wada, T., and Inoue, Y. (2000) Molecular design of crown ethers. 21. Synthesis of novel double-armed benzo-15-crown-5 lariats and their complexation thermodynamics with light lanthanoid nitrates in acetonitrile, J. Org. Chem. 65, 7105-7109 see also Danil de Namor, A. F. D., Chahine, S., Jafou, O., and Baron, K. (2003) Solution thermodynamics of lanthanide-cryptand 222 complexation processes, J. Coord. Chem 56, 1245-1255 Israeli, Y., Bonal, C., Detellier, C., Morel, J. P., and Morel-Desrosiers, N. (2002) Complexation of the La(III) cation by p-sulfonatocalix[4]arene - A La-139 NMR study, Canad. J. Chem. 80, 163-168. 

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