NMR Linewidths of Metallocenes as a Function of Magic Angle Sample Spinning Frequency

In this paper we report high-resolution solid state 1 C NMR investigations of various metallocenes [(Ti5-C5H5)M(Ti5-C5H5); M = Fe, Ru, Ni], carried out as a function of magic angle sample spinning (MAS) frequency. Specifically, we focus on how the linewidth of the isotropic peak varies with MAS frequency at fixed temperature. Unexpectedly, it has been found that the linewidth increases as the MAS frequency is increased, and it is demonstrated that the underlying reason is that the *H decoupling becomes less efficient as the MAS frequency is increased. It is suggested that molecular motion within these solids is an important factor underlying this phenomenon. It is well known that, at room temperature, there is substantial molecular motion in crystalline metallocenes [1-3]. In ferrocene, for example, it has been shown [4-6] that there is rapid reorientation of the cyclopentadienyl (C5H5) rings via a five-fold jump mechanism, with correlation time xc = 5 X 10~ 2 s at 293 K. The correlation time for ring reorientation in nickelocene at room temperature is essentially the same as that for ferrocene, whereas the ring reorientation in ruthenocene is associated with a significantly longer correlation time (tc ~ 5 X 100 s at 293 K) [4,5]. It has been suggested [2] that there may be some additional slower molecular motions in crystalline ferrocene at ambient temperature. All experiments reported in this paper were carried out at constant temperature, and hence the correlation time for molecular motion for a given metallocene can be assumed to be constant for the series of experiments discussed here. Before presenting our results, we discuss briefly relevant aspects of magic angle sample spinning (MAS) and high power *H decoupling in relation to the measurement of high-resolution C NMR spectra for organic solids, with particular emphasis on the application of the technique to systems in which there is substantial molecular motion.