Metal ions trivalent lanthanides

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 15 trivalent lanthanide, or/ -block, ions La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+, which may be collectively denoted Ln3+, represent the most extended series of chemically similar metal ions. The progressive filling of the 4/orbitals from La3 + to Lu3 + is accompanied by a smooth decrease in rM with increase in atomic number as a consequence of the increasingly strong nuclear attraction for the electrons in the diffuse / orbitals (the lanthanide contraction). Thus, the nine-coordinate rM decrease from 121.6 to 103.2 pm from La3+ to Lu3+, and the eight-coordinate ionic radii decrease from 116.0 to 97.7 pm from La3+ to Lu3+ (2). Ligand field effects are small by comparison with those observed for the first-... 

As with (105), ligand (106, dota) also forms strong complexes with a range of both non-transition and transition metal ions (Stetter Frank, 1976 Delgado da Silva, 1982 Spirlet, Rebizant, Desreux Loncin, 1984) which are often more stable than the corresponding edta complexes. In particular, the calcium complex shows extremely high stability and very stable complexes are also formed with the trivalent lanthanides (Desreux, 1980 Spirlet, Rebizant, Desreux Loncin, 1984). 

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]. 

A novel approach for determining metal-metal distances in proteins in solution was applied to thermolysin by Horrocks et al. (16), As mentioned earlier, thermolysin has four Ca2+ binding sites. Three of the four can be replaced by trivalent lanthanide ions. Tb3 + fluorescence is enhanced when Tb3+ is bound to the so-called 1 Ca2+ site of thermolysin. When Co2+ is added to the Zn2 + active site, the fluorescence of bound Tb3+ is quenched by... 

Another technique that uses the fluorescence properties of trivalent lanthanides is that of the detection of fluorescence emission decay induced by pulsed dye laser excitation. Horrocks and Sudnick (17) have applied this technique to the study of water molecules bound to metal ions in small complexes and proteins. In one study they found that the exponential decay of Tb3+ fluorescence is altered when H20 is replaced by D20 and that this change can be used to determine the number of coordinated water molecules on the metal ion. With thermolysin, bound Tb3+ had 1-2 water molecules in the first coordination shell. This number is consistent with the x-ray structure. 

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium. Such a trend is determined by the... 

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