Anisotropic ¹¹B and ¹³C NMR Interaction Tensors in Decamethylcyclopentadienyl Boron Complexes

Abstract

Solid-state double-resonance and triple-resonance 13C, 11B, and 1H NMR experiments are used to investigate two main group metallocene complexes: the decamethylcyclopentadienylborinium cation, [Cp*2B]+, and bis(pentamethylcyclopentadienyl)methylborane, Cp*2BMe. The crystal structure for the latter complex is reported herein. A combination of magic-angle-spinning and static 11B{1H} NMR experiments are used to measure 11B nuclear quadrupole coupling constants (CQ) and rare instances of anisotropic boron chemical shielding tensors. Boron-11 nuclear quadrupole coupling constants reflect the higher spherical symmetry of [Cp*2B]+ compared to Cp*2BMe, with CQ(11B) = 1.14 MHz in the former and CQ(11B) = 4.52 MHz in the latter. Chemical shielding tensor spans are measured for [Cp*2B]+ and Cp*2BMe as Ω = 73.0 and 146.0 ppm, respectively. Hartree−Fock and hybrid density functional theory (B3LYP) calculations of electric field gradient and chemical shielding tensors are in quantitative agreement with experiment, and are applied to examine the relationships between the anisotropic NMR interaction tensors and the structure and symmetry of these chemically analogous but structurally dissimilar boron complexes. Variable-temperature 11B MAS NMR, 13C CPMAS NMR, and 13C/11B/1H CP TRAPDOR NMR experiments are applied to make a preliminary investigation of motion of the Cp* rings of [Cp*2B]+.