Proton-phosphorous connectivities revealed by high-resolution proton-detected solid-state NMR

Abstract

Proton-detected solid-state NMR enables atomic-level insight in solid-state reactions, for instance in heterogeneous catalysis, which is fundamental for deciphering chemical reaction mechanisms. We herein introduce a phosphorus-31 radiofrequency channel in proton-detected solid-state NMR at fast magic-angle spinning. We demonstrate our approach using solid-state ¹H/³¹P and ¹H/¹³C correlation experiments at high magnetic fields (850 and 1200 MHz) and high spinning frequencies (100 kHz) to characterize four selected PH-containing compounds from the chemistry of phosphane-borane frustrated Lewis pairs. Frustrated Lewis pairs have gained high interest in the past years, particularly due to their capabilities of activating and binding small molecules, such as di-hydrogen, however, their analytical characterization especially in the solid state is still limited. Our approach reveals proton-phosphorus connectivities providing important information on spatial proximity and chemical bonding within such compounds. We also identify protons that show strongly different chemical-shift values compared to the solution state, which we attribute to intermolecular ring-current effects. The most challenging example presented herein is a cyclotrimeric frustrate Lewis pair-associate comprising three crystallographically distinct phosphonium entities that are unambiguously distinguished by our approach. Such ³¹P spin-filtered proton-detected NMR can be easily extended to other material classes and can strongly impact the structural characterization of reaction products of hydrogen-activated phosphane/borane FLPs, heterogeneous catalysts and solid-state reactions in general.