Actinide cyclooctatetraene

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. 

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. 

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... 

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. 

The bis-cyclooctatetraene actinides, M(t -C8H8)2, are structurally the closest analogues, for the 5f elements, of the d-block transition metal sandwich compounds. Their orbital structure (see Fig. 6) differs, however, in that the e2u orbital is occupied by... 

The pseudocontact term is simply the axially symmetric form of eq. 3. The contact term is eq. 2, where 3 and g have been evaluated using a crystal field model for bis-cyclooctatetraene-actinide sandwich compounds proposed by Karraker (31). 

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. 

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. 

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

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