Uranocene preparation

In all of the mono- and 1,1-disubstituted uranocenes prepared to date, the NMR resonances of the non-equivalent protons in... 

The discovery in 1968 by Streitwieser and Muller-Westerhoff [52,53] that the dianion of cyclo-octatetraene, CgHl, can act as a ligand to the actinide elements signaled an important new development in organoactinide chemistry. The first compound of this class was uranocene, prepared by reaction of UCI4 with the potassium salt of cyclo-octatetraene (COT) ... 

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 ... 

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. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres, Dyusengaliev, Ustynyuck 1982 Todres, Dyusengaliev, Sevast yanov 1985 Todres Tsvetkova 1987 Charoenkwan and others, in preparation). The reduction develops as in process (a) in Scheme 2-7 if the mercury cathode as well as the cyclooctatetraene dianion are electron sources, and as in process (b) if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For (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). 

Shortly after the synthesis of uranocene (see Uranocene), the first lanthanide complexes containing the cyclooctate-traenyl dianion, K[Ln(COT)2], have been prepared by the reaction of K2[COT] with lanthanide trichlorides (equation 21). 

It has recently been reported83 that di-n-cyclooctatetraene complexes of uranium, thorium, and plutonium can be prepared by the direct reaction of the finely divided metals with cyclooctatetraene. The finely divided metals were prepared by a procedure described by Seaborg and Katz.84 Several cycles of formation and descomposition of the hydride gave a finely divided reactive metal. However, treatment of the pyrophoric uranium with cyclopentadiene under the same conditions used to form uranocene failed to produce any detectable amounts of the known compounds U(CsH5)3 or U(C5H5)4. 

C. Monosubstituted UrariOCeheg. Some monosubstituted uranocenes are known, compounds with one COT and one substituted COT ligands. The mono-t-butoxycarbonyluranocene, 42, was prepared by reaction of one mole of the corresponding COT dianion with one mole of COT dianion itself and UC14(4 ). It could be separated from the disubstituted compound, 41, also formed, by its greater stability towards hydrolysis. Mono-t-butyluranocene, 32, was obtained and measured as a 1.8 1 mixture with the disubstituted compound, 33. A separate preparation of pure 33 allowed complete analysis of the mixture. Mono-(di-t-butylphosphino)uranocene has also been reported by Fischer, et al(45). 

Lithium naphthalide and uranocene in thf give the monoanion [U(r/-cot)2], isolated as a solvated lithium salt (405) neutral (406) and anionic (407) neptunium and plutonium analogs have also been prepared. 

Preparation of ferrocene was reported at about the same time by two research groups, and a sandwich structure was proposed, based on ferrocene s physical properties (Kauffman, pp. 185-186). The sandwich structnre was confirmed by x-ray diffraction studies. Since then, other metallocenes composed of other metals and other carbon ring molecules, such as dibenzenechromium (see Figure 3) and uranocene (see Figure 4), have been prepared. 

The first actinide organometallic complex, red-brown (C5H5)3UC1, was prepared and characterized by Reynolds and Wilkinson in 1956 or shortly after the discovery of ferrocene see Ferrocene, (C5H5)2pe. However, it was not until the preparation and structural characterization of bis(cyclooctatetraenyl)uranium, (C8H8)2U ( uranocene , see Uranocene) in 1968 and the isolation and characterization of uranium alkyls see Alkyl Complexes) in 1973 that the field developed momentum. The purpose of this chapter is to provide an overview of organoactinide chemistry that reflects historical as well as recent developments and achievements in the field it is not meant to be an exhaustive review of the primary literature. More detailed discussion, background material, and extensive references to the literature can be found in several excellent reviews. 

A single compound has been reported in this section, namely the uranocene derivative (3a)2UCl2, which was prepared by C. Qian et al. in 31% yield... 

Cyclooctatetraenyl derivatives of the rare earths were synthesized shortly after the characterization of uranocene first for europium and ytterbium in the oxidation state R by Hayes and Thomas (1969a). But one year later, Mares et al. (1970) prepared complexes of the type K[R(Cj,Hg)2], where R = Ce, Pr, Nd, Sm, and Tb. The corresponding yttrium, lanthanum and gadolinium derivatives are described by Hodgson et al. (1973) and the scandium compound was made by Westerhof and De Liefde Meijer in 1976 ... 

Bis([8]annulene)uranium(IV)1, uranocene, 1, was prepared as an expected f-orbital analogue to d-transition metal metallocenes. 

Thorocene, uranocene and plutonocene can also be synthesized by the reaction of cyclooctatetraene with the finely divided metals obtained by heating the metal hydrides.i7 in this reaction, a sealed Pyrex tube containing the actinide-cyclooctatetraene mixture was heated in a tube furnace at 150-160 C. The reaction product was isolated by sublimation. This preparative method provides demonstration of the thermodynamic stability of the bis([8]annulene)actinide complexes. 

Streitwieser and Miiller-Westerhoff (1968) prepared the uranocene, U(C8Hj)2, sandwich complex. They speculated that the U(5f) orbitals might play an important role in the metal-ligand bonds in uranocene. The reactivity properties of uranocene have been studied quite extensively by Streitwieser et al. (1973) and Streitwieser and Kinsley (1985). Other actinocenes and substituted uranocenes have been experimentally synthesized as well (Karraker et al. (1970), Karraker 1972). 

Uranocene, like other actinocenes, is readily prepared by the following reactions ... 

Streitwieser noticed that the HOMOs of C Hg have the correct symmetry to interact with the 5/orbitals of an actinide (or 4/of a lanthanide). This led to the preparation of the sandwich complex bis(cyclooctatetraene)uranium (uran-ocene). Uranocene is pyrophoric in air, but it thermally very robust. It forms deep green crystals which can be sublimed under vacuum. X-ray diffraction shows that the rings are parallel and eclipsed (Dg ). 

The coordination chemistry of other n ligands has also been explored (Chart 1 Bombieri and Paolucci, 1998 Edelmaim et al., 2002). The cyclooc-tatetraene dianion, (ri -CgHg) , commonly referred to as COT, also famous for the isolation of uranocene (Avdeef et al., 1972 Streitwieser and Mueller-Westerhoff, 1968), is known to stabilize lanthanides in all three oxidation states (+2, +3, and +4). In fact, the first COT lanthanide complexes to be prepared were solvated Ln(COT) species with Ln = Eu, Yb (Hayes and Thomas, 1969). Most rare earth COT complexes, which feature both mono-and bis-COT, are, however, of the trivalent lanthanides (Bombieri and Paolucci, 1998). Cerocene, Ce(COT)2, merits a special mention since its electronic structure is still being actively investigated (Ferraro et al., 2012 Kerridge, 2013 Kumari et al., 2013 Walter et al., 2009). 

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