Uranocene substituted

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. 

Intermolecular electron-transfer rates have been studied for uranocene and substituted derivatives of uranium, neptunium, and plutonium by examining the variable-temperature NMR spectra of mixtures of (CgH8)2An and [(C8Hg)2An]. In all cases, electron-transfer rates are rapid. Specific rates could not be derived for uranium and plutonium derivatives owing to the small chemical shift differences between analogous An(fV) and An(III) compounds, but in the case of (f-BuC8H7)2Np, the rate has been estimated to be of the same order of magnitude as comparable lanthanide cyclooctatetraene compounds ( 10 s ). ... 

Sf1 complex, Pa(Cot)2, but uranocene has now been extensively studied, and a fairly consistent picture of its magnetic behaviour between 1.25 and 300 K is currently emerging. The magnetic data for each An(Cot)2 system and its substituted analogues will therefore be considered, whereafter the more limited results for the [An(Cot)2]-system will be surveyed. 

More definitive information has however been forthcoming from a study of the charge-transfer spectra of the actinocenes and their alkyl substituted derivatives. Thus uranocene shows an extremely characteristic absorption in the visible region of the spectrum between about 14.5 and 17 kK, consisting of a sharp peak at 16.27 kK (e = 1850), with weaker subsidiary peaks towards lower energies at 15.6 (890),... 

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. 

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). 

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. ... 

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