Lanthanide coordination

Absorption spectra and structure of lanthanide coordination compounds in solution. K. B. Yatsi-mirskii and N. K. Davidenko, Coord. Chem. Rev., 1979, 27, 223-273 (255). 

Aminopyridinato ligands form a special class of anionic ligands in which an aromatic ring is part of an amidinate system. These ligands have frequently been employed in early transition metal and lanthanide coordination chemistry. Their diverse and interesting chemistry has been described in detail by Kempe et al. ° and will thus be covered here only briefly. Typical reaction pathways leading to titanium aminopyridinato complexes are outlined in Scheme 169. Metathetical as well as salt-free routes have been developed. 

In the REC model, the ligand is modelled through an effective point charge situated in the axis described by the lanthanide-coordinated atom axis, at a distance R, which is smaller than the real metal-ligand distance (Figure 2.6). To account for the effect of covalent electron sharing, a radial displacement vector (Dr) is defined, in which the polar coordinate R is varied. At the same time, the charge value (q) is scanned in order to achieve the minimum deviation between calculated and experimental data, whereas 9 and cp remain constant. 

Parker D, Dickins RS, Puschmann H, Crossland C, Howard JAK (2002) Being excited by lanthanide coordination complexes aqua species, chirality, excited-state chemistry, and exchange dynamics. Chem Rev 102 1977-2010... 

In conclusion, the search for improved Gd(III)-based agents has been highly beneficial for the growth of lanthanide coordination chemistry and has created a very fertile interdisciplinary area with contributions from Chemistry Biology Medicine, and Imaging technology. The requisite for... 

Lanthanide coordination chemistry has seen a considerable increase in interest in recent years due to potential applications in a variety of fields, including catalysis (9), biochemical analysis (3-8,10-14), and non-invasive diagnostics (8,15,16). 

An attempt to synthesize binuclear complexes led to the development of the back-to-back ligand [L7]2-. The lanthanide coordination environment of the resultant [ ( )2 2( 7)] complexes is the same as that in the parent mononuclear [L4]- complexes, as shown in Pig. 7. Despite the possibility of displacement of the nitrate anions by solvent molecules, a reasonable lifetime (0.9 ms) and quantum yield (0.13) is recorded for aqueous solutions of the terbium complex (49). 

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... 

Due to the presence of hard anionic oxygen atoms, phenolate and carboxylate groups are often employed as donors in lanthanide coordination chemistry. Ligand [L18]4- is reported as an excellent triplet sensitizer for lanthanide luminescence (61). Indeed aqueous lifetimes of 0.57 and 1.61 ms are reported for europium and terbium, respectively quantum yields of 0.20 and 0.95 respectively refer to the efficiency of the energy transfer process alone. 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

D networks based on the lanthanides with their high coordination numbers (typically 7-11) are less predictable than those based on transition metals and the high oxophilicity of the lanthanide (III) ions means that they do not bind strongly to ligands such as pyridine derivatives. As a result lanthanide coordination networks are less common in the literature however, they represent tremendous potential in terms of ever more complex topologies. 

One of the most studied class of macrocyclic ligands in lanthanide coordination chemistry is without any doubt the molecules derived from cyclen, a 12-membered ring bearing four amino functions. In particular, its tetracarboxylic derivative H4DOTA (see Fig. 4.40) was synthesized in 1976 by a German chemist, H. Stetter from the University of Aachen,... 

The hydration munber, or the nimiber of bound water molecules in the lanthanide coordination sphere, can be calculated using a method introduced by Horrocks and Sudnick for terbimn and europium complexes (50). The relationship between Tb or Eu excited state lifetimes (t), which are experimentally determined in H2O and D2O, and the hydration number (q) is given in Eq. (1)... 

We conclude that the simple explanation based only on steric interactions in the lanthanide coordination sphere is not sufficient, and that there are more complex forces at work. We therefore recommend that ligand screens be performed to determine the optimal ancillary ligand for a given target analyte when designing lanthanide-based sensors. 

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