Uranocene mono-

The investigation of actinide cyclooctatetraenyl complexes continued to be an area of active research in recent years. Especially notable among the more recent developments is the synthesis and derivative chemistry of various cationic mono(cyclooctatetraenyl) uranium(iv) and (v) complexes.71,97 The chemistry of mono(cyclooctatetraenyl)uranium complexes has been reviewed by Ephritikhine et al.lso A highly interesting essay on the discovery and chemistry of uranocene has been published by Seyferth under the title Uranocene. The First Member of a New Class of Organometallic Derivatives of the/Elements. 151... 

C. Monosubstituted UrariOCeheg. Some monosubstituted uranocenes are known, compounds with one COT and one substituted COT ligands. The mono-t-butoxycarbonyluranocene, 42, was prepared by reaction of one mole of the corresponding COT dianion with one mole of COT dianion itself and UC14(4 ). It could be separated from the disubstituted compound, 41, also formed, by its greater stability towards hydrolysis. Mono-t-butyluranocene, 32, was obtained and measured as a 1.8 1 mixture with the disubstituted compound, 33. A separate preparation of pure 33 allowed complete analysis of the mixture. Mono-(di-t-butylphosphino)uranocene has also been reported by Fischer, et al(45). 

At 30°C, the protons of the unsubstituted ring in mono-t-butyluranocene resonate at 0.51 ppm lower field and those in the mono-ester resonate at 0.43 ppm higher field than the ring protons in uranocene. These differences are small but real and were established independently by observing the spectrum of mixtures of these compounds. 

Figure 7. Isotropic shift vs. T 1 for uranocene and the ring protons in the unsubstituted ring of mono-t-butyl, 32, and mono-t-butoxycarbonyluranocene, 42...

In all of the mono- and 1,1-disubstituted uranocenes prepared to date, the NMR resonances of the non-equivalent protons in... 

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. 

The coordination chemistry of other n ligands has also been explored (Chart 1 Bombieri and Paolucci, 1998 Edelmaim et al., 2002). The cyclooc-tatetraene dianion, (ri -CgHg) , commonly referred to as COT, also famous for the isolation of uranocene (Avdeef et al., 1972 Streitwieser and Mueller-Westerhoff, 1968), is known to stabilize lanthanides in all three oxidation states (+2, +3, and +4). In fact, the first COT lanthanide complexes to be prepared were solvated Ln(COT) species with Ln = Eu, Yb (Hayes and Thomas, 1969). Most rare earth COT complexes, which feature both mono-and bis-COT, are, however, of the trivalent lanthanides (Bombieri and Paolucci, 1998). Cerocene, Ce(COT)2, merits a special mention since its electronic structure is still being actively investigated (Ferraro et al., 2012 Kerridge, 2013 Kumari et al., 2013 Walter et al., 2009). 

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