Substituted uranocenes resonances

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 -... 

Ill. Identification of Ring Proton Resonances in Substituted Uranocenes. 

The mechanism of the reaction to form the bis([8]annulene) complexes is not known. It is clear, however, that there are differences in the mechanism in the formation of uranocene compared to thorocene. When the reaction of two equivalents of n-butyl[8]annulene dianion with thorium tetrachloride was carried out in an NMR tube and monitored by iH-NMR, a resonance was observed which cannot be attributed to the bis([8]annulene) complex.22 Further experiments showed that this apparent intermediate in the reaction is the mono-ring complex, [8]annulenethorium(IV) dichloride (vide infra).22 The mechanism for the formation of bis([8]annulene)thoriuro evidently involves the stepwise substitution of [8]annulene dianion for two chloride ions. 

---