Cyclooctatetraene uranium complex

Reduction of nitrobenzene (Grant and Streitwieser 1978, Todres et al. 1985) and 4-methoxy-nitrobenzene (Todres et al. 1985) by uranium, thorium, and lanthanum-di(cyclooctatetraene) complexes leads to azo compounds. Scheme 1.8 illustrates these reductive reactions using the di(cyclooctatetraene)-uranium complex as an example. 

Uranium complexes analogous to these compounds were also described, but with the cot (cyclooctatetraene dianion) ligand rather than the Cp or Cp ones. Both the dianionic U(IV) [(cot)U(dddt)2]2 [47] and monoanionic U(V) [(cot)U(dddt)2] [48] complexes were isolated and structurally characterized (Fig. 5). Spectacular distortions of the US2C2 metallacycles were rationalized on the basis of DFT calculations, which reproduced the spectacular folding of the endo US2C2 metalla-cycle when the dianionic species undergoes an oxidation. 

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. 

In analogy to the cyclooctatetraene dianion, 71 also forms a very stable sandwich complex 75 upon reaction with UC14 in THF (Scheme 18) [69]. The H NMR spectrum of this complex 75 revealed a very characteristic singlet at 6 = -21 ppm ( ) which is in good agreement with the chemical shift of urano-cene,the bis(cyclooctatetraenediyl)uranium complex [70]. 

Another interesting molecule is the cyclooctatetraene sandwich complex, U(CgHg)2. The Sf contribution is important to the stability of uranium-containing molecules. Semi-empirical calculations have been carried out on this molecule as well as other actinocene compounds.Relativistic calculations on UF, NpFg and PuFg have been reported. "... 

The reaction of bis(cyclooctatetraene)uranium with air occurs explosively however, the derivatives of U(COT)2 are considerably more stable, for example, [U(l,3,5,7-C8H4Ph4)2] is stable in air for several weeks. This happens because of steric hindrance which prevent the attack of oxygen on the uranium atom (Table 11.1). Other cyclooctatetraene complexes of the actinides and lanthanides may be oxidized in a similar way. 

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 actinide SMMs discussed so far were based on uranium. In 2011, the first mononuclear transuranic SMM in the form of the Np4+ complex [Np(COT)2] (10, COT = cyclooctatetraene, C8H82-), was published [9]. Although first published in 1970, the dynamics of its magnetic moment had never been investigated. The relaxation is slow enough to be measured when applying an external dc magnetic... 

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. 

The dication adopts a three-legged piano-stool configuration in which the O-U-O angles vary from 84.0° to 88.2° with a mean value of 87(3)°, and average COT-U-O angles of 127(1)°. The uranium atom is 1.92(2) A away from the planar cyclooctatetraene ring, and the mean U-G bond distance is 2.65(3) A. These values compare well with those determined in the monocationic cyclooctatetraene complexes [(COT)U(NEt2)(THF)3]+.161... 

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. 

This article has reviewed the synthesis and reactivity towards small molecules of a range of U(lll) cyclooctatetraene and pentalene complexes. It is evident that in many cases the uranium centre is capable of tt back-bonding through the 5/ orbitals additionally, a C - C agostic interaction between a bound substrate and a U(IV) centre has been observed. Clearly uranium is capable of bonding with a degree of covalency , and this is perhaps why the reduction chemistry of U(III) is so rich and diverse, and not simply an iteration of low-valent lanthanide chemistry. 

Mono(pentamethylcyclopentadienyl) complexes, actinide Mono(pentamethylcyclopentadienyl) amides, actinide Triscyclopentadienyl complexes, uranium(III) Trlscyclopentadienyl complexes, thorium(III) Bls(peralkylcyclopentadlenyl) complexes, uranlum(III) Cyclooctatetraene complexes, actinide [8]Annulene complexes, actinide Pyrazolylborate complexes, actinide Alkoxlde complexes, actinide... 

An alkyne may also be used in place of the diene component. With the cyclic sulfone complex 11.174a, this leads to an interesting synthesis of cyclooctatetraenes as the initial cycloadduct 111.76 is photochemically unstable and readily loses SO2 (Scheme 11.59). The adduct may be isolated if uranium glass filters are employed to exclude the high-energy wavelengths. Subsequent photolysis with the more transparent vycor filters causes chelotropic elimination of sulfur dioxide to give the tetraene 11.177. 

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