A Solid-State NMR and ab Initio Study of Sodium Metallocenes

Abstract

Sodium metallocenes (sodocenes) are a fascinating class of molecules that form polymeric chains of repeating [Cp‘Na]n units. A variety of sodium metallocenes have been studied by 23Na (I = 3/2) and 13C solid-state NMR, where Cp‘ = Cp (C5H5-), CpMe (C5H4Me-), CpiPr (C5H4iPr-), and Cp* (C5Me5-). Simulations of 23Na MAS NMR spectra yield the 23Na quadrupolar coupling constants (CQ) and asymmetry parameters (ηQ) for all compounds. Values of CQ range from 2.97 to 3.89 MHz for the linear Cp‘Na compounds. A new bent base-substituted species, CpNa·THF (THF = tetrahydrofuran), is identified which has CQ = 1.82(2) MHz. The sodium nuclei in sodocenes are extremely shielded with respect to the standard sodium chemical shift range, with chemical shifts (δiso) ranging from −45.5 to −61.9 ppm. All of the linear base-free compounds exhibit values of ηQ near zero, which reflect the axial symmetry of the sodium environments, while the nonlinear CpNa·THF has ηQ = 0.39(2), indicative of the electronic asymmetry about the sodium nucleus. Static 23Na NMR experiments reveal subtle instances of 23Na chemical shielding anisotropy, with spans (Ω) ranging from 9.5 to 12.5 ppm. Variable-temperature 23Na NMR experiments show that the magnitude of CQ(23Na) grows with increasing temperature in the linear metallocenes. Ab initio calculations were performed on a variety of structural models to gain insight into the nature of the electric field gradient (EFG) and chemical shielding (CS) tensors. Rotation of the Cp‘ rings and changes in the inter-ring distances were examined in order to see how these motions affect the 23Na EFG tensors. A combination of experimental and theoretical data indicate that inter-ring distances decrease in the linear sodocenes as the samples are heated. Solid-state 13C CPMAS NMR experiments are used to probe the purity and crystallinity of the metallocene samples to confirm the existence of the bent ligated metallocene CpNa·THF and to examine the effect of dynamic ring motion on observed carbon chemical shielding tensors of the Cp‘ ring carbons.