Uranyl cations

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) 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 iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene 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)J j)/H20 (x = 2-5). 

Detection is primarily based on the principle of fluorescence quenching by substances absorbing UV light. It is also possible to detect certain substances whose absorption wavelengths interfere with the uranyl cation [1]. 

It is interesting to note that some metal ions may exist in solution partly as cationic and partly as anionic species. The uranyl ion is a good example. From sulfate solution it is possible to extract species such as U02(S04)2 with amines. The uranyl cation, on the other hand, can be extracted with acidic extractants such as D2EHPA. 

Rossini, and G. Wipff, Comput. Phys. Commun., 2, 591 (2001). Do Perchlorate and Triflate Anions Bind to the Uranyl Cation in an Acidic Aqueous Medium A Combined EXAFS and Quantum Mechanical Investigation. 

J. Phys. Chem. B, 107, 3051 (2003). Modeling of Uranyl Cation-Water Clusters. 

Importance of Charge Transfer and Polarization Effects for the Modeling of Uranyl-Cation Complexes. 

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

These are photoinduced electron transfer reactions between two ions. The closed-shell ions then form free radicals which can be charged or neutral, these primary photochemical products being very reactive. One example of this process is the electron transfer between a uranyl cation and a nitrate anion... 

The uranyl cation (closed shell, U022+) absorbs light in the visible (VIS)... 

Uranyl nitrate has been extracted into solvents such as diethyl ether as the entity [U02(N03)2(H20)4] solvated by two to six molecules of the organic solvent.254-256 The solvation is usually considered to be of the secondary type, in which the solvent molecules are attached by hydrogen bonding to the water molecules in the primary hydration shell. However, IR data have been presented257 258 to support Muller s earlier hypothesis259 that only two of the water molecules are directly bound to the uranyl cation, the remaining coordination sites being occupied by solvent molecules. 

Ce(IV) ions efficiently catalyse the hydrolysis of phospho monoesters in nucleotides under physiological conditions. The proposed mechanism for the hydrolysis is illustrated in (217).189 Uranyl cations (U021) catalyse the hydrolysis of aggregated and non-aggregated p-nitrophenyl phosphodiesters such as (218)/(219) and (220), respectively.190 Bis(/>-nitrophenyl) phosphate (218) hydrolysis is accelerated ca 2.8 x 109-fold by Th(IV) cations in aqueous Brij micelles.191 The reactivity of Th(IV) towards (219) and (221 R = Et, C16H33) also exceeds that of uranyl ion190 and is comparable to that of Ce(IV) and exceeds that of other metal cations. 

Alkyl-substituted monoamides are known as extractants for the uranyl cation and they could potentially be considered as alternatives to organophosphorus compounds in nuclear fuel reprocessing. In toluene, the uranyl cation forms complexes with two monoamide molecules. These relatively simple molecules were selected for computer-aided design,14 taking into account a lot of synthetic and experimental work that must be done to prove the modeling predictions. 

Parallel experiments carried out with cyclo[6]pyrroles showed that only the hexa-iV-methyl derivative could be efficiently obtained, most likely because of steric limitations [169], Interestingly, however, cyclo[6]pyrrole 54a was found to coordinate the uranyl cation [U02]2+ with a concomitant two-electron oxidation of the macrocycle to a [20]annulenoid structure 55a [170], As expected, this system is strongly paratropic, as evidenced by the shielding of peripheral ethyl substituents in the II NMR spectra (-1.42 ppm for CH2 and -1.12 ppm for CH3). Additionally, electronic absorptions of 55a are much less intense that those of the free base 54a. 

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]. 

Uranium can be dissolved in dilute acids, when uranium(IV) ions, U4+, and hydrogen gas are formed. Uranium(IV) ions can easily be oxidized to the hexavalent state, which is the most stable oxidation state of uranium. At this oxidation state, depending on the pH of the solution, two ions can be formed the uranyl cation, UO +, is stable in acid solutions, while the diuranate anion, u2or, in alkaline media. The two ions are in equilibrium with each other ... 

Initial reports served to indicate that the pentaphyrin macrocycle is capable of complexing Zn " ", Co, and Hg [158]. The exact nature of this coordination, however, was not determined. Nonetheless, it was speculated to be via ligation to only two of the five pyrrolic nitrogen centers (c.f. 238-240 in Scheme 41). Also, a complex 237 was found to form between what is formally the doubly deprotonated pentaphyrinato dianion of 232 and uranyl cation (Scheme 41). The uranyl center may be readily displared by treatment with add [158, 187]. 

Another expanded porphyrin previously known to form a complex with the uranyl cation was the pentaphyrin 232 [158, 187]. An improved synthesis of a new pentaphyrin derivative and its corresponding, structurally characterized, uranyl complex was recently reported [240]. This new uranyl pentaphyrin, has a very distorted solid state structure reminiscent of the closely related uranyl superphthalocyanine complex 160 [112] (Figures 22 and 23). 

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). 

The tetraketone 1,7-diphenyl 1,3,5,7 heptanetetraone (dbbaHs, 134), containing three enolizable protons, has been prepared and several heterotrinuclear metal complexes 135, containing the uranyl cation U02 + and other bivalent metal cations, have been isolated and characterized ... 

Consistent with the original supposition that sapphyrins should be good ligands for large cations, Woodward predicted that sapphyrins might form particularly stable chelate complexes with uranyl cation (U02 ), a species that is known to favor pentagonal planar coordination environments. " Unfortunately,... 

Several years after the initial report of Woodward, et al., Sessler and coworkers found that sapphyrin 5.21 would in fact form a stable complex with uranyl cation. It was determined, however, that 1102 " complexation was accompanied by reduction of the sapphyrin macrocycle. Specifically, it was found that metal insertion occurred concurrent with addition of methanol to one of the meso-Xike carbon centers. The net result was an overall neutral complex of a modified, nonaromatic sapphyrin-like system (Scheme 5.5.2). 

Through a series of experiments, Sessler and coworkers were able to determine a likely mechanism for the formation of complex 5.96. This mechanism, shown in Scheme 5.5.3, involves the initial coordination of uranyl cation to sapphyrin giving a complex such as 5.97 or 5.98. Addition of methanol (or methoxide anion) to one of... 

Sapphyrins bearing peripheral substituents other than those present in 5.21 also react with uranyl cation in methanol to form complexes akin to 5.96. Analogous reactions, wherein the methoxide nucleophile is replaced by cyanide were also described by Sessler and coworkers. Unfortunately, when cyanide anion is used as the nucleophile, the reaction yield was very poor. It thus proved impossible to isolate and characterize fully the resulting uranyl complex. 

Subsequent to submitting this manuscript, a single crystal x-ray diffraction structure of the neutral uranyl complex of monoxasapphyrin 5.74 was obtained. The uranyl cation is pentaligated (with a long furan oxygen-uranium bond) and resides within the macrocyclic plane. " ... 

Sessler and coworkers prepared two similar uranyl complexes 6.48 and 6.49. A single crystal X-ray diffraction study of the former revealed a saddle-shaped penta-phyrin macrocycle with the uranyl cation centrally coordinated in a nearly pentagonal bipyramidal fashion (Figure 6.4.3). Distortion from planarity arises, presumably. 

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