molecules Review Solid-State NMR Techniques for the Structural Characterization of Cyclic Aggregates Based on Borane–Phosphane Frustrated Lewis Pairs Robert Knitsch 1, Melanie Brinkkötter 1, Thomas Wiegand 2, Gerald Kehr 3 , Gerhard Erker 3, Michael Ryan Hansen 1 and Hellmut Eckert 1,4,* 1 Institut für Physikalische Chemie, WWU Münster, 48149 Münster, Germany;
[email protected] (R.K.);
[email protected] (M.B.);
[email protected] (M.R.H.) 2 Laboratorium für Physikalische Chemie, ETH Zürich, 8093 Zürich, Switzerland;
[email protected] 3 Organisch-Chemisches Institut, WWU Münster, 48149 Münster, Germany;
[email protected] (G.K.);
[email protected] (G.E.) 4 Instituto de Física de Sao Carlos, Universidad de Sao Paulo, Sao Carlos SP 13566-590, Brazil * Correspondence:
[email protected] Academic Editor: Mattias Edén Received: 20 February 2020; Accepted: 17 March 2020; Published: 19 March 2020 Abstract: 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.