In solution the PCy3/B(C6F5)3 pair is rapidly deactivated by nucleophilic aromatic substitution. In the solid state (or in a fluorous liquid), however, it serves as an active frustrated Lewis pair to effectively split dihydrogen.

The reactive five-membered frustrated P/B Lewis pair 6a was generated from Tipp-P(vinyl)2 (Tipp: 2,4,6-triisopropylphenyl) by a series of anti-Markovnikov and Markovnikov hydroboration reactions. The in situ generated compound dimerized to give 7a under kinetic control and the dimer pair 8a/9a under thermodynamic control. The Mes*-P(vinyl)2 analogue (Mes*: 2,4,6-tri-tert-butylphenyl) reacts in a similar way, but in this case the thermodynamic dimers are observed at r.t. whereas the monomer 6b is the dominant species at 80 °C in solution. It is an active dihydrogen splitting reagent. The in situ generated monomeric five-membered P/B FLPs undergo addition reactions to various organic π-reagents. With benzaldehyde or phenylacetylene this gave the respective zwitterionic heteronorbornene or -norbornadiene type products, the latter with liberation of ethylene.

Modern solid-state NMR techniques offer a wide range of opportunities for the structural characterization of frustrated Lewis pairs (FLPs), their aggregates, and the products of cooperative addition reactions at their two Lewis centers. This information is extremely valuable for materials that elude structural characterization by X-ray diffraction because of their nanocrystalline or amorphous character, (pseudo-)polymorphism, or other types of disordering phenomena inherent in the solid state. Aside from simple chemical shift measurements using single-pulse or cross-polarization/magic-angle spinning NMR detection techniques, the availability of advanced multidimensional and double-resonance NMR methods greatly deepened the informational content of these experiments. In particular, methods quantifying the magnetic dipole–dipole interaction strengths and indirect spin–spin interactions prove useful for the measurement of intermolecular association, connectivity, assessment of FLP–ligand distributions, and the stereochemistry of adducts. The present review illustrates several important solid-state NMR methods with some insightful applications to open questions in FLP chemistry, with a particular focus on supramolecular associates.