Uranium clusters

A nonstandard synthesis of the first uranium cluster containing an isopolyoxo-metalate core was reported recently [310b], Reduction of [Cp UCl2] (Cp7 — 1,2,4-/-Bu3C5H2) with two equivalents of KC8 in THF, followed by an addition of two equivalents of pyridine N-oxide, was conducted in an attempt to produce the organometallic dioxo species [Cp7U02]. However, the cluster compound [Cp (bipy)2][U6013] was isolated instead as the main product (54% yield) from this reaction. 

Very recently we have communicated on the role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocyclic and heterocyclic uranium clusters [185]. Using electronic structure calculation methods (DFT) we demonstrated that the model planar isocyclic cjc/o-U X (n = 3, 4 X = 0,NH) and heterocyclic cjc/o-U (/u.2-X) (n = 3, 4 X = C, CH, NH) clusters are thermodynamically stable molecules with respect to their dissociation either to free U and X moieties or to their monomeric UX species. The equilibrium geometries of the cyclo- J X and cyclo-U iJ,2-X) clusters are shown in Fig.42. 

Tsipis AC, Kefalidis CE, Tsipis CA (2008) The role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocycUc and heterocyclic uranium clusters. J Am Chem Soc 130 9144-9155... 

Preliminary Calculations Performance of DFT Approaches IN Uranium Clusters... 

Duval, P. B., Bums, C. J., Clark, D. L., et al. 2001. Synthesis and structural characterization of the first uranium cluster containing an isopolyoxometalate core. Angew. Chem. Int. Ed. Engl. 40 3357-3361. 

An interesting development from a magnetic point of view is the recent preparation of large uranium-oxo clusters [61], reminiscent of the M30Mo72 (M = Fe3+, V4+, Cr3+) polyoxometallates, that were the subject of many investigations on magnetism [62]. 

This affirmed necessity of positioning around the cluster, U (U ) cations is a very relevant fact, especially if coupled with the result of the channeling experiment reported above, which shows a displacement of uranium ions from their equilibrium positions. The Willis clusters can then be described also as molecularities in which the electronic charge... 

In the cellular multiple scattering model , finite clusters of atoms are subjected to condensed matter boundary conditions in such a manner that a continuous spectrum is allowed. They are therefore not molecular calculations. An X type of exchange was used to create a local potential and different potentials for up and down spin-states could be constructed. For uranium pnictides and chalcogenides compounds the clusters were of 8 atoms (4 metal, 4 non-metal). The local density of states was calculated directly from the imaginary part of the Green function. The major features of the results are ... 

FIGURE 5.27 (a) The fluorite structure of UO2 with a unit cell marked in bold, (b) Interstitial defect cluster in U02+jf. Uranium positions... 

Figure 5.16 Willis clusters in UOj+y Open circles, oxygens small filled circles, uranium atoms in the UO2 structure. A 2 2 2 defect cluster consists of two interstitial oxygen atoms (large filled circles) and two normal oxygen atoms displaced from their ideal positions (open squares) to new interstitial positions (hatched circles). A 2 1 2 defect cluster would consist of one interstitial oxygen and two normal oxygens displaced from their ideal positions. (After Anderson, 19726.)...

Since the first electron-microscopical observation of a heavy atom on a surface different studies have looked at effects related to individual atomic adsorbates. These include diffusion along the surface (atoms can be tracked in real time), giving results in agreement with equivalent FIM observations, and pair spacing distributions, showing for example a peak in the distribution near 4-5 A for uranium atoms on a carbon surface Clustering can be studied in some cases as well. 

An observation of motion of single atoms and single atomic clusters with STEM was reported by Isaacson et al,192 They observed atomic jumps of single uranium atoms on a very thin carbon film of —15 A thickness or less. Coupled motion of two to three atoms could also be seen. As the temperature of the thin film could not be controlled, no Arrhenius plot could be obtained. Instead, the Debye frequency , kTIh, was used to calculate the activation energy of surface diffusion, as is also sometimes done in field ion microscopy. That the atomic jumps were not induced by electron bombardment was checked by observing the atomic hopping frequencies as a function of the electron beam intensity. 

The periodic table is rich and complex, and very heavy elements pose rather distinct challenges to MO theory. First, there is the purely technical hurdle that such elements have large numbers of electrons, and there is thus a concomitant requirement to use a large number of basis functions to describe them. Of course, these extra electrons are mostly core electrons, and finis a minimal representation will probably be adequate. Nevertheless, if one wants to model a small cluster of uranium atoms, for instance, the basis set size quickly becomes intractable. Not surprisingly, more electrons means more energy associated with electron correlation, too. 

Aminothiazoles, with trinuclear Os clusters, 6, 730 Amino(triamido)-pentamethylcyclopentadieny complexes, with thorium and uranium, 4, 208-209 Aminotroponiminato ligands, in Ti(IV) dialkyl complexes,... 

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