Proton isotropic shifts, ring

The temperature dependence behavior of the ring proton isotropic shifts also reflects the effects of lower symmetry. While the ring proton shift in Cp U shows a linear dependence on T from -106°C to 133°C, the ring proton shifts of Cp3U-X compounds 2-5, 8-10, 12, 17-18, and 23-25 all show marked deviations from linearity. The alkyl-substituted systems show linear behavior from ca. -150°C to room temperature but deviate from linearity above room temperature. The alkoxy compounds show apparent linearity from ca. 200°C to 400°C but deviations from linearity below 200°C. All of the halides except for the fluoride display a slight curvature from 200°C to 400°C. The variable temperature behavior of the fluoride is solvent dependent and reflects the formation of dimers. 

Table V displays the XH-NMR data of [Cp2Yb(ketim)]2 at various temperatures. The rather different isotropic shifts(opposite signs ) of the two methyl groups suggest their location in fairly different "magnetic environments". It is, however, not possible to decide if there are N- or O-bridging links. As only one Cp ring proton resonance appears, some fluxional behaviour of the bridging ketim ligand is not unlikely.

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. 

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

Figure 8. Isotropic shift vs. T1 for the ring protons in l,r-dimethyluranocene, 27...

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. 

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