Uranyl halide complexes

Torsional barriers for trimethylphosphine derivatives (63) have been obtained from Raman spectra.56 Vibrational spectra for the uranyl nitrate complex of (63a) have been published.66 Complexes of triarylphosphine derivatives (64) with iodine,67 and of (64b) with metal halides,68 have been the subject of thermodynamic67 and spectroscopic67 68 study. 

Binding of hard anions occurs strongly at the hard Lewis acidic uranyl center, whereas cation- tt interactions are established between the aromatic side arms and the cation counterpart of the ion pair. Thus, complexation of alkali metal salts (MX) such as CsCl and RbCl with 15 resulted in the formation of isomorphous supramolecular assemblies in the solid state. In the dimeric [15-CsCl], each cation is coordinated to six oxygens, three from each receptor, thus creating a pseudo-crown-ether-Uke environment for the cation. Additionally, each metal ion in the dimeric unit is coordinated to both halide ions and, most importantly, to two aromatic side arms, one from each of the receptors giving decacoordination for the cation. The closest metal ion-aromatic carbon distances of 3.44(1) A for CsCl, 3.34-3.38(1) A for RbCl, and 3.58(1) A for CsF are observed in the respective alkali halide complexes [15 MX] indicating the conformational flexibility of the side arms and adaptability of the receptors 15 and 16 to form multiple cation- rt interactions with the hosted cations. 

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

The majority of U(VI) coordination chemistry has been explored with the trans-dioxo uranyl cation, U02+2. The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatility as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned in the introduction have been complexed with U(VI) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples include U02X2Lw (X = halide, OR, N03, RC02, L = NH3, primary, secondary, and tertiary amines, py, n = 2-4), U02(N03)2Lk (L = en, diaminobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J2 xj H20 (x = 2-5). 

Other uranyl salts are given by organic acids,41 sulfate, halides, and so on the water-soluble acetate in the presence of an excess of sodium acetate in dilute acetic acid gives a crystalline precipitate of NaU02(0C0CH3)3. There is considerable research on the use of cyclic hexadentate and related ligands to form strong complexes with the U02+ ion. 

The best-studied aqueous actinide halide systems are the fluorides and the chlorides. Fluoride and chloride ions are added to the actinyl centers in a stepwise fashion (Scheme 4). The end member in the actinyl fluoride system is the pentafluoride, An02F5 . For the uranyl analog, the U = O and U-F bond distances were found to be 1.79 and 2.18A respectively. The uranium tetrafluoride ion, U02F4 , has been isolated as a dimer with two bridging fluoride ligands. The U = O, U-Fterminai, uud U-Fbridging distauces for this complex were found to be 1.79, 2.15 to 2.20 A, and 2.30 A, respectively. 

Hexavalent. The majority of An(VI) coordination chemistry with N-donors has been explored with the uranyl cation, 50i. Stable adducts with the hgands discussed in the tri- and tetravalent complexes have been described, for example, U02X2L (X = halide, OR, NO3, RCO2). The coordination numbers observed for these complexes are typically 6, 7, or 8 with octahedral, pentagonal bipyramidal, or hexagonal bipyramidal geometries, respectively. Neutral and anionic thiocyanates have also been isolated, for example [U02(NCS)j2- yH20(x = 2 5). 

For example, halide ions may form soluble actinide complexes without addition of nitric acid vapor. Preliminary tests have shown, however, that uranyl chloride, UO2CI2, is not soluble in either a 6-mole% potassium chioride/94-mole% potassium nitrate melt, or a 5-mole% sodium chloride/95-mole% sodium nitrate melt. Uranyl fluoride, UO2F2, was slightly soluble in a 9-mole% potassium fluoride/91-mole% potassium nitrate system. There was no evidence of solubility of uranyl fluoride in 3.5 mole% sodium fluoride/96.5-mole% sodium nitrate. 

The structures of the metal, hydrides, and carbides are described in other chapters, as also are the halides MX3, MX4, and MX5. Here we devote sections to certain halide structures peculiar to U, complex fluorides of Th and U, oxides of U, uranyl compounds and uranates, nitrides and related compounds, and conclude with a note on the sulphides of U, Th, and Ce. 

Diethyl H-phosphonate forms a a-donor complex with uranyl nitrate U02(N03)2 similar to those already described for BXj and organotin halides [419]. 

Uranium compounds include acetates, carbonates, halides, nitrates, oxalates, oxides, phosphates, and sulfates. In broad terms uranyl (hexavalent, VI) compounds, which exist primarily as the U02 complex ion, tend to be the most soluble. The uranous (tetravalent, IV) compounds, such as UF4, are less soluble. Uranium oxides are the least soluble [3]. 

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