Cyclooctatetraene actinide complexes

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. 

Table 1. Summary of crystal and molecular data for actinide and lanthanide cyclooctatetraene complexes...

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. 

Figure 11.2. Molecular orbital scheme for cyclooctatetraene complexes of the actinides.

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. 

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. 

Bis(cyclooctatetraene) complexes, actinide Bls([8]annulene) complexes, actinide Plutonocene... 

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... 

I. Fragala, G. Condorelh, P. Zanella, E. Tondello, Photoelectron spectroscopy of actinide organometaUic compounds I. Bis(cyclooctatetraene)actinide(fV) complexes, J. Organomet. Chem., 122, 357-363 (1976). 

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... 

Controversy exists in naming sandwich organometallic actinide and lanthanide compounds of cyclooctatetraene dianion [8]annulene dianion and [8]annulene-metal(X) complex are chosen here because these names properly describe the delocalization of charge in the ligand and also emphasize the formal oxidation state of the metal in the complex. 

Cyclooctatetraene forms complexes with both lanthanide and actinide elements. They are prepared by reacting the anhydrous metal halides with K2(CgHg) in tetrahydrofuran. The anion CgHJ" is a planar ten-electron aromatic system and this planar structure is retained by the CgHg ligand in the complexes. 

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 ). 

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