Structural Characterization of Frustrated Lewis Pairs and Their Reaction Products Using Modern Solid-State NMR Spectroscopy Techniques

Frustrated Lewis pair (FLP) chemistry has provided a new strategy for small-molecule binding and/or catalytic activation. The most prominent FLPs are based on intramolecular phosphane-borane adducts, the catalytic properties of which can be tailored over wide ranges of reactivity and selectivity. Advanced solid-state NMR spectroscopic techniques, together with DFT calculations, can provide new structural insights in these systems. This review illustrates the utility of P-31 and B-11 NMR chemical shifts, 11B electric field gradient tensors, and P-31-B-11 indirect and direct dipole-dipole interactions for characterizing intramolecular borane-phosphane FLPs. We demonstrate the potential of this method to 1) quantify the extent of boron-phosphorus bonding interactions (and hence, the "degree of frustration"); 2) reveal specific structural details (i.e., boron-phosphorus distances and other local geometric aspects) related to their catalytic activities; and (3) characterize products of FLP reactions with regard to molecular structure, stereochemistry, and aggregation properties in terms of internuclear distances, bonding connectivities, and orientational parameters.