A Solid-State NMR and Computational Study of Sodium and Potassium Tetraphenylborates:  ²³Na and ³⁹K NMR Signatures for Systems Containing Cation−π Interactions

Abstract

Sodium and potassium tetraphenylborates were examined by solid-state 23 Na and 39 K NMR spectroscopy. Analyses of solid-state NMR spectra obtained at 4.70, 11.75, and 19.60 T yielded the following 23 Na and 39 K NMR parameters: Na[BPh 4 ], | C Q | = 1.24 ± 0.05 MHz, η Q = 0.0 ± 0.1, δ iso = −45.6 ± 0.5 ppm, and Ω = 14 ± 2 ppm; K[BPh 4 ], | C Q | = 1.32 ± 0.05 MHz, η Q = 0.0 ± 0.1, and δ iso = −92 ± 1 ppm. In both Na[BPh 4 ] and K[BPh 4 ], the electric field gradient and chemical shift tensors at the metal site are axially symmetric, in agreement with the crystallographic symmetry. Extensive quantum mechanical calculations were performed for Na[BPh 4 ] and K[BPh 4 ] as well as for a large number of model cation−π systems containing Na + and K + ions and common aromatic compounds. Experimental and theoretical studies confirm that a highly shielded environment at the metal cation site is a characteristic feature for cation−π interactions, making it useful as a NMR signature for identifying cation−π interactions in proteins and nucleic acids. In this study, powder X-ray diffraction spectra for Na[BPh 4 ] and K[BPh 4 ] were also reported.