LANTHANIDE IONS water

Electric conductance measurements have been widely used in the study of lanthanide complexes to determine the nature of the anions in the complexes and hence to indicate the possible coordination number of the lanthanide ion. Water is a strong donor toward the lanthanides and is seldom used for the purpose of measuring electric conductance, since the complex is completely dissociated on dissolution in water. The complete dissociation of lanthanide complexes in water has been shown by molecular weight determinations in water as in the case of the complexes of DMSO (246,249, 250), PyO (147,157,158), and DMF (41, 43). Most useful data are obtained in non-aqueous solvents like nitromethane, acetonitrile, nitrobenzene, and acetone (317). 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

The higher coordinating ability and Lewis acidity of Zn(H) ion in addition to the low pK of the metal-bound water molecule and the appearance of this metal ion in native phosphatases inspired a number of research groups to develop Zn(II)-containing dinuclear artificial phosphatases. In contrast, very few model compounds have been published to mimic the activity of Fe(III) ion in dinuclear centers of phosphatase enzymes. Cu(II) or lanthanide ions are not relevant to natural systems but their chemical properties in certain cases allow extraordinarily high acceleration of phosphate-ester hydrolysis [as much as 108 for copper(II) or 1013 for lanthanide(III) ions]. 

It is quite difficult to measure an accurate enthalpy of solution A//( olutioni with a calorimeter, but we can measure it indirectly. Consider the example of sodium chloride, NaCl. The ions in solid NaCl are held together in a tight array by strong ionic bonds. While dissolving in water, the ionic bonds holding the constituent ions of Na+ and Cl- in place break, and new bonds form between the ions and molecules of water to yield hydrated species. Most simple ions are surrounded with six water molecules, like the [Na(H20)6]+ ion (VI). Exceptions include the proton with four water molecules (see p. 235) and lanthanide ions with eight. 

Basinska and Domka512 also reported enhanced water-gas shift rates when the Fe oxide was modified by the addition of lanthanide ions. Both a and 8 Fe oxides were prepared and calcined at 600 °C prior to addition of the lanthanide (La, Ce, or Sm) via aqueous solutions of the nitrate yielding Ru to lanthanide ratios of 1 0.5, 1 0.75, 1 1, 1 5, and 1 10. The Ru content was 0.5%. Activity data at 300 °C are reported in Table 119. 

As noted in table 11.1, the ability of THFTCA to separate LJO from trivalent lanthanide ions is mainly of enthalpic origin. Reaction 11.33 has a considerably more unfavorable enthalpic contribution than reaction 11.32. The complexation is, however, predominantly entropy driven because the T ArS° term dominates the ArH° contribution for all systems. The large positive entropy changes observed for reactions 11.32 and 11.33 result from the release of water molecules coordinated to the metal on complexation with the tridentate THFTCA2- ligand. Note that a negative entropy contribution would be expected if these reactions were truly 2 particle = 1 particle reactions [226]. 

Complexes of PyzO with lanthanide perchlorates (2 79) and hexafluorophosphates 180) are eight coordinate. However, La(III) perchlorate gives the complex La(Pyz0)7(C104)3 2 H20 in which both the water molecules are coordinated to La(III). In the case of complexes of PyzO with lanthanide chlorides 180), the number of coordinated ligands increases as the ionic radius of the lanthanide ion decreases. These complexes probably contain bridging ligands. 

In the above structures, the uncoordinated chloride ion is surrounded by a polyhedron of six water molecules forming a distorted octahedron with average Cl—H2O distances of 3.18 and 3.21 A for Eu(III) and Gd(III) complexes respectively. The coordinated chloride ions are, however, heptacoordinated being bonded to six water molecules and to a lanthanide ion. The average Cl—H2O distances for this chlorine is somewhat larger than the uncoordinated case and are 3.23 and 3.16 A for Eu(III) and Gd(III) complexes respectively. 

The classical example of nonacoordinated lanthanide ion is, of course, the enneaaquo ions, [M(OH2)9] +, present in bromate, sulphate and ethylsulphate salts. The nine water molecules form a tricapped trigonal prism around the central lanthanide ion. The nine M—0 distances for La and Nd in La2(S04)s-9 H2O and in Nd(Br03)3 9 H2O are virtually the same, being 2.72 and 2.49 A (average) respectively (Table 17) 182—186). The three equatorial M—0 distances for M = Pr, Er and Y, in ethylsulphates are somewhat larger than other six M—0 distances to the oxygens situated on the prism corners (Table 17). 

The parent ligand tris(pyrazolyl)borate, [L4]-, forms 1 1 and 1 2 metakligand complexes [Ln(L4)(N03)2] and [Ln(L4)2]+ the molecular structure of the latter is shown in Fig. 6. In solution, dissociation of the nitrate anions in the former complex leave the lanthanide ion open to coordination by solvent molecules (47). Photophysical studies in water and methanol (48) confirm this use of the Horrocks equation... 

The bipyridyl chromophore has been extensively used in lanthanide coordination chemistry. In addition to those based on the Lehn cryptand (see Section IV.B.4), a number of acyclic ligands have also employed this group. One such ligand is L17, which binds to lanthanide ions such that one face of the ligand is left open (Scheme 3) (60). As expected, luminescence is extremely weak in water and methanol, but stronger in acetonitrile ( = 0.30, 0.14 for europium and terbium, respectively). In addition, the nature of the counter ion can... 

Horrocks, W. de W. Jr. Sudnick, D. R. Lanthanide ion probes of structure in biology. Laser-induced luminescence decay constants provide a direct measure of the number of metal-coordinated water molecules. J. Am. Chem. Soc. 1979,101(2), 334-340. 

It is believed that the presence of small concentrations of water in the sulfuric acid solutions corresponds to a solution structure in which the H20 molecules are not in the first coordination sphere around the lanthanide ions. A further increase in the water, however, changes the structure of the solution. It is thought that the H2S04 in the first coordination sphere... 

Association between lanthanide ions and azide or thiocyanate ions has been studied in solution by electronic, Raman and NMR spectroscopy.186,187 The complexation constant in water between Nd3+ and N3 is approximately 2.5. Longitudinal relaxation time studies for Gd-Dy indicate that the M—N—NN angle is bent (135° approximately). 

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