Uranocene substituent

Least Squares Regression Data for Alkyl Uranocene Substituent Proton Data vs T ... 

However, uranocene can be made more air-stable by use of sufficiently bulky substituents, and 1,3,5,7-tetraphenylcyclo-octatetraene yields the completely air-stable [U( -CgH4Ph4)2], in which the parallel ligands are virtually eclipsed but the phenyl substituents staggered and rotated on average 42° out of the Cg ring plane (Fig. 31.10). 

However, uranocene can be made more air-stable by use of sufficiently bulky substituents, and... 

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. 

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. 

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...

Figure 14. Several conformations of 1,1 -disubstituted uranocenes about the central axis. The 1 -substituent is shown with the dotted line.

Using Fischer s value of y j2 - yj 2=8.78 BM2 the calculated pseudocontact shifts for the t-butyl groups in 1,1 -di-t-butyl-and 1,1 -dineopentyl uranocene are -12.1 ppm and 7.28 ppm, respectively, for coplanar substituents, and -14.6 ppm and 3.22 ppm, respectively, for tipped substituents. Agreement between the calculated pseudocontact shifts and the observed isotropic shifts is rather good. Calculation of the pseudocontact shifts for the eye-... 

We can now return to our conclusion in the last section where we deduced from the pattern of ring proton resonances and from steric considerations that t-butyl substituents in uranocenes must be tilted away from uranium. This argument does not apply to the neopentyl group which is a normal primary alkyl substituent for which the ring-CH2 bond can be tilted towards uranium without difficulty. 

Early attempts to factor the isotropic shifts in alkyl-uranocenes using eq. 9 were not completely successful because of failure to correctly assess the conformation of the substituent in solution and overestimation of the value of the anisotropy term jj-Xj (5,6,14). ... 

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