Uranocen

However, uranocene can be made more air-stable by use of sufficiently bulky substituents, and 1,3,5,7-tetraphenylcyclo-octatetraene yields the completely air-stable [U( -CgH4Ph4)2], in which the parallel ligands are virtually eclipsed but the phenyl substituents staggered and rotated on average 42° out of the Cg ring plane (Fig. 31.10). 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

The first and thus far only silsesquioxane complex of an actinide element is [Cy7Si70i2]2U (100). This colorless, nicely crystalline uranium(VI) compound is formed upon reaction of 3 with any uranium precursor, e.g., UCI4 in the presence of NEt3. In all cases oxidation of uranium to the hexavalent oxidation state is observed. The best synthetic route leading to 100 in ca. 80% yield is the reaction of 3 with uranocene as outlined in Scheme 33. 

Uranocene itself was prepared by allowing cyclooctatetraene (COT) to react with potassium in dry, oxygen-free tetrahydrofuran (THF) at —30° followed by the addition of a THF solution of anhydrous uranium tetrachloride ... 

Many substituted uranocenes have been made and there is a substantial body of organometallic chemistry of uranocene derivatives now known 16, 17). Some of this chemistry will be mentioned in passing but wiU not be covered in a systematic way since other reviews of the organic chemistry are available 18). The only other actinide cyclooctatetraene complex structurally characterized to date is bis[(l,3,5,7-tetramethylcyclooctatetraenyl]uranium(IV) 19), which was of interest because the presence of methyl groups allowed the planarity and relative orientation of the dianion rings to be determined. Crystal and molecular parameters for these three actinide compounds are summarized in Table 1. 

It is interesting to compare these actinide(IV) cyclooctatetraene complexes with similar compounds of the group IVB transition elements Ti, Zr and Hf. Bis (cyclooctatetraene) complexes of aU three are known although structural data is only available for the first two. All would appear to involve both planar and non-planar COT rings and to exhibit a sHpped sandwich structure rather than the true sandwich structure of uranocene. 

The nature and extent of /-orbital participation in the bonding of uranocene and other bis(cyclooctatetraenyl) actinides has never been satisfactorily established, although a good deal of effort has been expended on it. The X-ray structures do not resolve the issue because an ionically bonded model would also lead to a sandwich-type structure (for example, MgCp2 has essentially the same structure as ferrocene). Other physical techniques have been used, but the complexity of the electronic structures often leads to ambiguous interpretations. 

The synthesis of bis(rj8-cyclooctatetraene)uranium(IV) (uranocene)J from uranium tetrachloride and (cyclooctatetraene)dipotassium was first published in 1968.1 The method reported here is a modification of that procedure and is suitable for a large variety of cyclooctatefraene complexes.2-4 BisO 8-cyclo-octatetraene)uranium(IV) has also been prepared by the reaction of uranium tetrafluoride with (cyclooctatetraene)magnesium in the absence of solvent.5 Direct reaction of finely divided uranium metal with cyclooctatetraene vapors at 150° also gives some uranocene.5 However, both methods give low yields. 

The suffix ocene is formally reserved for bis(t]-cyclopentadienyl) metal complexes. The name uranocene does not fit this convention, but has been used as a trivial name in the literature. 

Uranocen wurde ebenfalls auf seine reduktiven Eigenschaften in bezug auf Nitrobenzole uberpruft5. Hierbei zeigte sich, daB die ort/zo-substituierten Nitrobenzole niedrigere Aus-beuten an Azo-Verbindungen liefern als die entsprechenden para-substituierten ... 

No reports have been made on photochemical transformations in other metallocenes such as uranocene and ruthenocene. These are doubtless forthcoming for the metallocenes and their derivatives offer fertile grounds for photochemical study. 

Condensation of uranium vapors with CgHg affords uranocene, U(CgHg)2 (33). However, this may not be a metal atom reaction simply by vaporizing and condensing uranium vapor by itself would afford... 

Continuing with examples of proper axes in typical molecules, we may cite the planar PtCl4 ion, which has a C4 axis perpendicular to the plane of the ion and four C2 axes in the plane of the ion. The cyclopentadienyl anion, C5H5, possesses a C5 axis perpendicular to the molecular plane and five C2 axes in the molecular plane. Benzene possesses a C6 axis and two sets of three Ci axes. Probably the only known example of a molecule with a C7 axis is the planar [C7H7]+, the tropylium ion. An example of a molecule with a C8 axis is (CaHg U (uranocene). 

Fig. 15.44 Structure of uranocene showing the two eclipsed cydooctatetmenyl rings.

Many metallocene derivatives are known of other conjugated cyclic polyenes. Examples are bis(cyclooctatetraene)uranium (uranocene, 7) and bis-(pentalenylnickel), 8 (see Section 22-12B) ... 

Also crystallizing in space group P2 ]c, uranocene has two molecules per unit cell, so that the U(CsH8)2 molecule occupies a special position of site symmetry 1. In other words, the molecule has an eclipsed conformation, and it may be assigned to special position 2(a). Similarly, the two halves of the [Re2Clg]2- dianion in K2 [Rc2C lmolecular dimensions (indicating that the symmetry of the dianion is Z>4h within experimental error) and crystal structure are shown in Fig. 9.6.5. 

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