Structure and Dynamics of Homoleptic Beryllocenes: A Solid‐State ⁹Be and ¹³C NMR Study

Abstract

The correlation between anisotropic 9Be NMR (quadrupolar and chemical shielding) interactions and the structure and dynamics in [Cp2Be], [Cp2*Be], and [(C5Me4H)2Be] is examined by solid-state 9Be NMR spectroscopy, as well as by ab initio and hybrid density functional theory calculations. The 9Be quadrupole coupling constants in the three compounds correspond well to the relative degrees of spherical ground-state electronic symmetry of the environment about beryllium. Theoretical computations of NMR interaction tensors are in excellent agreement with experimental values and aid in understanding the origins of NMR interaction tensors and their correlation to molecular symmetry. Variable-temperature (VT) 9Be and 13C NMR experiments reveal a highly fluxional structure in the condensed phase of [Cp2Be]. In particular, the pathway by which the Cp rings of [Cp2Be] 'invert' coordination modes is examined in detail using hybrid density functional theory in order to inspect variations of the 9Be NMR interaction tensors. The activation energy for the 'inversion' process is found to be 36.9 kJ mol(-1) from chemical exchange analysis of 13C VT CP/MAS NMR spectra. The low-temperature (ca. -100 degrees C) X-ray crystal structures of all three compounds have been collected and refined, and are in agreement with previously reported structures. In addition, the structure of the same Cp2Be crystal was determined at 20 degrees C and displays features consistent with increased intramolecular motion, supporting observations by 9Be VT NMR spectroscopy.

Alkaline Earth Organometallics

“…The isotropic 9 Be chemical shift has been found to vary with the coordination of the Cp 0 rings, from À19.8 ppm in 11 (Z 5 /Z 1 ) to À24.4 ppm in 10 (Z 5 /Z 5 ). 50 Calculations at various levels of theory have been used to analyze the bonding in 10 and 12 and how it differs from that in 9. 50 Calculations at various levels of theory have been used to analyze the bonding in 10 and 12 and how it differs from that in 9.…”

“…38 The results of electron-diffraction data, 41,42 Raman spectroscopy, 43 and X-ray diffraction results [44][45][46] were not always consistent with one another, but the consensus in recent years, bolstered by molecular dynamics 47,48 and DFT calculations, 49 is that 9 adopts an Z 5 /Z 1 -slip-sandwich structure in solution and the solid state. 50 Beryllocene is thought to adopt its easily distorted structure because of a balance between Be-C bond strength, favored by s-bonding, and Cp delocalization energy, which is favored by p-bonding. 49 With some density functionals (BP86, B3P86), this conclusion is basis-set dependent, and the use of a relatively large basis set (6-311G(d,p)) is required to establish that the C s structure is in fact the lowest-energy conformer.…”

“…49 With some density functionals (BP86, B3P86), this conclusion is basis-set dependent, and the use of a relatively large basis set (6-311G(d,p)) is required to establish that the C s structure is in fact the lowest-energy conformer. 50 The nature of the fluxional process has been studied with a combination of DFT, the Car-Parrinello projector augmented wave method, and Car-Parrinello molecular dynamics calculations. 50 Beryllocene is thought to adopt its easily distorted structure because of a balance between Be-C bond strength, favored by s-bonding, and Cp delocalization energy, which is favored by p-bonding.…”

Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives

“…11 During the course of our work, the dynamics of the known beryllocenes have been investigated by solid-state 13 C and 9 Be NMR spectroscopy. 12 Unfortunately, the dynamic range of this method is limited.…”

Solid state NMR spectroscopy

“…805 9 Be and 13 C variable-temperature MAS NMR studies of Cp 2 Be, Cp* 2 Be and (C 5 Me 4 H) 2 Be show highly fluxional structures in the solid phases. 806 MAS NMR spectra ( 1 H, 23 Na, 27 Al and 29 Si) were used to characterise lanthanideexchanged NaFAU-Y zeolite. The 23 Na and 29 Si data were related to the migration tendencies of rare-earth cations from the supercages to the small cages in the zeolite structure.…”