Lanthanide chemical substitution

Although the number of actinides is the same as that of the lanthanides, their availability and chemical characteristics have so far largely restricted the study of their ligand substitution mechanisms to dioxouranium(VI), which is the ionic form of uranium most amenable to such studies in solution. In the solid state, the oxo ligands occupy axial sites above and below the U(VI) center, and four (328), five (329, 330), and six (331, 332) oxygen donor atoms have been reported to occupy equatorial positions. From a mechanistic point of view, this variability of the occupancy of the equatorial plane suggests the possibility of both d- and a-activated ligand substitution pro-... 

The relaxation equations discussed here and in Section 3.4 and 3.5 take a different form in the case of lanthanides and actinides. For these systems, in fact, the J quantum number substitutes the S quantum number and gj substitutes ge. In the absence of chemical exchange phenomena (r 1 r ) the equations for the... 

Intermolecular electron-transfer rates have been studied for uranocene and substituted derivatives of uranium, neptunium, and plutonium by examining the variable-temperature NMR spectra of mixtures of (CgH8)2An and [(C8Hg)2An]. In all cases, electron-transfer rates are rapid. Specific rates could not be derived for uranium and plutonium derivatives owing to the small chemical shift differences between analogous An(fV) and An(III) compounds, but in the case of (f-BuC8H7)2Np, the rate has been estimated to be of the same order of magnitude as comparable lanthanide cyclooctatetraene compounds ( 10 s ). ... 

Albrecht, M., Osetska, O., Klankermayer, J., etal. (2007) Enhancement of near-lR emission by bromine substitution in lanthanide complexes with 2-carboxamide-8-hydroxyquinoline. Chemical Communications, 1834. 

Borbas, K.E. and Bruce, J.I. (2006) Synthesis of asymmetrically substituted 1,4,7,10-tetraazacyclododecanes for the triggered near infrared emission from lanthanide complexes. Chemical Communications, 4596. 

Conformational analysis of substituted 1,3,2-dioxaphosphorinanes has been achieved using a topological approach (429) rather than the more elaborate random search method. This simplified approach only considers chemically and physically accessible positions for the lanthanide. Using this approach the LIS calculations are found to predict the conformational equilibrium for complexed cis conformers which is in good agreement with that based on the response of the coupling constants J(H-H) and J(P-H) to lanthanide addition. For the trans conformers the two approaches are not in agreement. 

These comparisons should be taken into account when substituting lanthanide ions for first-row transition-metal ions in order to induce chemical shifts or change the relaxation behavior of a n.m.r. spectrum to elicit ion sequestering and structural information (see later). 

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