Substituted uranocenes

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. 

Numerous substituted uranocenes are now known and could, in principle, provide useful tests. Other factors now, however, become involved and need to be evaluated. The lower symmetry of these compounds means that X and Y are no longer constrained to be equal and the eq. 3 needs to be considered in its entirety. Moreover, the substituent could have an effect on magnetic anisotropy. Finally, some substituents have more than one possible conformation which would need to be considered. 

In this section we summarize the e gperimental results for a number of substituted uranocenes. The compounds studied are listed in Table III and Fig. 3. 

We next inquire whether this result is consistent with other physical properties of uranocenes. Bulk magnetic susceptibility measurements at low temperature on several substituted uranocenes appear to suggest that within experimental error the magnetic properties of all uranocenes are essentially identical and equal to 2.4 0.2 B.M. (Table VII). This result is consistent with the idea confirmed by Xa Scattered Wave (47) and Extended Huckel MO (48) calculations that the magnetic properties of uranocenes are determined principally by the two unpaired electrons that are primarily metal electrons. 

E. H NMR of Substituted Uranocenes. Table VIII summarizes the chemical shifts relative to TMS for a number of uranocenes at a common temperature (30°C). The results are summarized for ring and substituent protons for convenience. 

For purposes of convenient identification, the ring proton resonances in the NMR of substituted uranocenes will be labeled alphabetically starting with the lowest field resonance. This does not imply that the "A" resonancees in two different uranocenes correspond to the same ring position. We shall discuss below the assignment of the individual ring proton resonances. 

Note in these results that the total difference between the highest and lowest field resonance of the non-equivalent ring protons in all of the uranocenes increases as the temperature decreases. Moreover, the relative pattern of the ring proton resonances in each uranocene remains constant as a function of temperature except for the two phenyl-substituted uranocenes and 1,1 -... 

In fact, studies of a number of substituent protons in substituted uranocenes provide linear correlations with (54). 

Figure 12. Conformations of the substituent in substituted uranocenes shown in Newman projection form with the uranium atom below the plane of the ring in each figure...

Ill. Identification of Ring Proton Resonances in Substituted Uranocenes. 

Previous attempts at factoring the isotropic NMR shifts in uranocene and substituted uranocenes have assumed that these systems can be viewed as having effective axial symmetry. The temperature dependent 1h NMR spectra of uranocene and a variety of substituted uranocenes clearly verify this assumption and show that eq. 9 can be used to evaluate the pseudocontact contribution to the total isotropic shift in uranocenes. In this equation xx = Xy f°r substituted uranocenes and are replaced by Xj. ... 

Some ring-substituted uranocenes " have been scrutinized and a similar bonding model assessed by PE spectra. Einally PE spectra of di-7r-(8)annulenecerium(rv), cerocene, are similar to those of thorocene, and spectroscopic/... 

Few reactions have been noted for the 8]annulene compounds of actinides and lanthanides except for uranocene itself. The reactions of uranocene with nitro-compounds o and of some substituted uranocenes with or-... 

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). 

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