Shift in uranocenes

Uranocenes. Edelstein and co-workers (5) proposed that the XH isotropic shift in uranocene can be approximated by... 

Subsequently, the H NMR of 1,1, 3,3, 5,5, 7,7 -octamethyl-uranocene was analyzed in a similar manner (6). The contact shifts for the ring and ot-protons were found to be similar in magnitude, but opposite in sign, implying spin density in a 7T-MO, and transfer of spin density via a spin polarization type mechanism (entries 3 and 4 in Table II). In this paper, a new, significantly smaller, value for the pseudocontact shift in uranocene... 

B. The Temperature Dependent H NMR of Uranocene and Octamethyluranocene. Our initial interest was in verifying the temperature dependence of the isotropic shift in uranocene and the reported non-zero intercept at T =0. Recent laser Raman studies by Spiro and co-workers (41) have established that the first excited state in uranocene is 466 cm- above the ground state. Thus, the isotropic shift may not vary linearly with the inverse of the temperature from -100°C to 100°C. Indeed, below 100°K some controversy exists concerning the temperature dependence of the magnetic moment in uranocene (42,43). 

The temperature dependence of the isotropic shift in uranocene was measured on two independent samples from -80°C to 100°C. At the same nominal temperature slight differences in the shift between the two samples are undoubtedly due to slight differences in the true temperature of the samples and provide an estimate of the error in temperature measurement or measurement of the resonance frequency in this study. 

The discussions above have shown that the pseudocontact component of the isotropic shift in 1,1 -dialkyluranocenes is accurately given by the axially symmetric form of eq. 3 and thus, these systems can be used in evaluating both the assumptions employed in deriving, and the value of the anisotropy term (xn Xj[ ) used, by previous workers in factoring isotropic shifts in 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. ... 

Another important result of this study is the confirmation of Fischer s demonstration that Xj is not equal to zero in uran-ocene. Early attempts to factor isotropic shifts in uranocene have generally assumed that Xj[=0, and leads to overestimation of the anisotropy term. A precise value of Xj is difficult to determine rigorously from analysis of available NMR data. We have found that y 2 - yj 2 = 12.5 BM2 leads to the best internal consistency factored isotropic shifts for a wide variety of 1,1 -dialkyluranocenes. Assuming yav2 = 5.76 BM2 and yjj - yj 2 = 12.5 BM2, at 30°C/ the corresponding values of y. 2 and yj2 are 14.09 and 1.59 BM2, respectively. This implies that Xj j/X L 8 uran ocene, a value substantilly larger than Fischer s ratio of X /Xj = 2.8 (15). 

As a result of Xjj 0, early work on factoring the isotropic shift of the ring protons in uranocene underestimated the magnitude of the contact shift. Using our value of Uj2 yj = 12.5 BM2, the pseudocontact and contact shifts for uranocene ring protons are -8.30 ppm and -34.2 ppm, (G = -2.34 x 1021 cm-3), respectively. Thus, this study confirms that both contact and pseudocontact interactions contribute to the observed isotropic shifts in uranocenes. The contact component is dominant for ring protons, but rapidly attenuates with increasing number of Q-bonds between the observed nucleus and the uranium such that the contact shift is effectively zero for g-protons. 

Figure 14. Contact and dipolar shifts in uranocene and l,l -dimethyl-uranocene. Thorocenes are used as the diamagnetic references.

Figure 15. Spin polarization model for contact shifts in uranocenes.

Fischer has proposed useful and important methods for factoring the isotropic shifts of uranocenes into contact and pseudocontact components (15) values were reported for uranocene, 1,-1, 3,3, 5,5, 7,7 -octamethyluranocene, and 1 1 -bis(trimethyl-si lyl) uranocene using a non-zero value of Xj Fischer arrived at values of yjj2 and y 2 at several temperatures from the ratio of the geometry factor and the isotropic shift for methyl protons in bis(trimethylsilyl)-uranocene, and bulk magnetic susceptibility data, assuming no contact contributions to the isotropic shift of the methyl protons. From the published data of Fischer, the value of y( - y2 at 30°C is 8.78 BM2. 

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). ... 

There have been several studies of shift anisotropy in paramagnetic solids, obtained from the orientation analysis of powder lineshapes. Bloembergen [76], Rundle [77], and Poulis and Hardeman [78] analyzed the orientation dependent shifts in single crystals of hydrated Cu salts at low temperatures. McGarvey and Nagy [79, 80] have investigated the temperature dependence of static powder spectra of uranocene U(CgHg)2, a system where all the protons are chemically equivalent. 

McGarvey BR, Nagy S (1987) Evidence for an anisotropic contact shift, proton NMR study of line shapes in uranocenes and (C5H5)3 UCl powders. Inoig Chem 26 4198-4203... 

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 ). ... 

Since the calculated pseudocontact shifts are smaller in magnitude than the observed isotropic shift, Edelstein, et.al., concluded that an upfield contact component contributes to the total isotropic shift, indicative of covalency in the ligand metal bonds of uranocene. 

Octamethyluranocene, 35, has effective 4-fold symmetry and Xx and Xy are constrained to be equal on the nmr time scale. The temperature dependence of the ring protons of this compound is compared with uranocene in Fig. 6 and Table V. The non-zero intercept is probably due to referencing the isotropic shift to the tetramethylCOT dianion note in Table IV that the ring protons of dimethylthorocene differ from methylCOT dianion by almost 1 ppm. 

The temperature dependence of the unsubstituted ring proton resonances are linear functions of T 1 and the slopes of shift vs. T-- - are identical within experimental error to that of uranocene (fig. 7, Table V). The slight difference in intercepts at T-1=0 undoubtedly result from using the proton resonance of cycloocta-tetraene dianion as a diamagnetic reference for all the compounds. 

Figure 6. Isotropic shift vs. T 1 for uranocene and the ring protons in 5,5, 7,7 -octamethyluranocene, 3 5...

The high degree of linearity in the temperature dependence of the ring proton shifts is evident from the correlation coefficients of the least squares regression lines (Table IX). The slopes of the lines are all negative and similar in magnitude to that of uranocene. However, the standard deviations of the extrapolated intercepts at T-1=0 indicate that a number of the intercepts are non-zero. Ideally, eq. 3 predicts that all of the intercepts should be zero at T 1=0. 

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-... 

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