New Insights into Frustrated Lewis Pairs: Structural Investigations of Intramolecular Phosphane–Borane Adducts by Using Modern Solid-State NMR Techniques and DFT Calculations

Covalent bonding interactions between the Lewis acid
and Lewis
base functionalities have been probed in a series of “frustrated
Lewis pairs” (FLPs) (mainly substituted vinylene linked intramolecular
phosphane–borane adducts), using solid-state nuclear magnetic
resonance techniques and accompanying DFT calculations. Both the 11B NMR isotropic chemical shifts and nuclear electric quadrupolar
coupling parameters turn out to be extremely sensitive experimental
probes for such interactions, revealing linear correlations with boron–phosphorus
internuclear distances. The principal component Vzz of the 11B electric field
gradient tensor is tilted slightly away (∼20°) from the
boron–phosphorus internuclear vector, leading to an improved
understanding of the remarkable reactivity of the FLPs. Complementary 31P­{1H}-CPMAS experiments reveal significant 31P–11B scalar spin–spin interactions
(1J ≈ 50 Hz), evidencing covalent
bonding interactions between the reaction centers. Finally, 11B­{31P} rotational echo double resonance (REDOR) experiments
show systematic deviations from calculated curves based on the internuclear
distances from X-ray crystallography. These deviations suggest non-zero
contributions from anisotropic indirect spin–spin (J anisotropy) interactions, thereby offering additional
evidence for covalent bonding.