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. 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. Keywords: Frustrated Lewis pairs; aggregation; solid-state NMR; dipolar spectroscopy; internuclear distance measurement 1. Introduction During the past decade, borane–phosphane frustrated Lewis pairs (FLPs) attracted great interest as metal-free systems for a large variety of catalytic and stoichiometric chemical transformations [1–17]. Their Lewis centers can be incorporated in the same molecule (intramolecular FLPs) or in two different entities (intermolecular FLPs). In these molecules, both Lewis centers are shielded by sterically demanding groups, which prevent quenching, i.e., the formation of a covalent bond between them. This so-called frustration phenomenon results in a remarkable reactivity rarely encountered in metal-free organic molecules. The most prominent example of such a reaction is the ability of borane–phosphane (B/P) FLPs to split dihydrogen and to transfer the resulting proton/hydride pair to other organic substrates such as unsaturated carbon bonds [1–7]. In addition to their ability of splitting dihydrogen, B/P FLPs can give rise to various types of adducts with a large number of small molecules, leading to interesting heterocycles and complex organic structures, which might be valuable building Molecules 2020, 25, 1400; doi:10.3390/molecules25061400 www.mdpi.com/journal/molecules Molecules 2019, 24, x FOR PEER REVIEW 2 of 40 organic substrates such as unsaturated carbon bonds [1–7]. In addition to their ability of splitting Moleculesdihydrogen,2020, 25 B/P, 1400 FLPs can give rise to various types of adducts with a large number of small2 of 39 molecules, leading to interesting heterocycles and complex organic structures, which might be valuable building blocks for novel organoborane materials. These small molecules include CO2, CO, blocksSO2, NO, for alkenes, novel organoborane alkynes, carbonyls, materials. isonitriles, Thesesmall and many molecules more, include as summarize CO2, CO,d in SO Figure2, NO, 1 [1,7–17]. alkenes, alkynes,Beginning carbonyls, in 2010, isonitriles,we developed and a many comprehensive more, as summarized research program in Figure exploring1[ 1,7–17 the]. Beginning relationship in 2010,between we developedstructure aand comprehensive chemical activity research of program these systems, exploring using the relationship advanced betweensolid-state structure NMR andtechniques chemical [18–20]. activity These of these techniques systems, were using not advanced only found solid-state to assist in NMR the structural techniques characterization [18–20]. These techniquesof disordered were solids not only that found elude to single-crystal assist in the structural diffraction, characterization but to provide of disordered suitable anisotropic solids that eludeobservables single-crystal (magnetic diff raction,shielding, but direct to provide and suitableindirect anisotropic spin–spin observablescoupling, and (magnetic nuclear shielding, electric directquadrupolar and indirect coupling spin–spin tensors) coupling, that are and able nuclear to characterize electric quadrupolar the centers coupling individually, tensors) including that are able the toextent characterize of intramolecular the centers electronic individually, interactions including between the extent them of intramolecular[20–22]. As such, electronic NMR spectroscopy interactions betweenwas helpful them to [20predict–22]. Asthe such,degree NMR of frustration spectroscopy of such was systems, helpful which to predict rela thetes degreeto their of reactivity. frustration ofIn suchaddition, systems, NMR which methods relates proved to their reactivity.to be instrumental In addition, in NMR characterizing methods proved reaction to bepathways instrumental and inintermediates characterizing associated reaction with pathways FLP chemistry, and intermediates as well as associatedfor the stereochemical with FLP chemistry, analysis of as the well final as forproducts the stereochemical obtained [12,21–25]. analysis ofAs the discussed final products in previous obtained reviews [12,21– 25[18,19],]. As discussed NMR spectroscopic in previous reviewsparameters [18, 19allow], NMR a distinction spectroscopic between parameters classical allow and a distinctioncooperative between adducts, classical the identification and cooperative and adducts,quantification the identification of different andpossible quantification isomers and of diepimers,fferent possibleand the isomersdescription and of epimers, intermolecular and the descriptionaggregation of and intermolecular oligomerization aggregation effects. and oligomerization effects. Figure 1. 1. SchematicSchematic overview overview for various for various chemical chemical reactions reactions performed performed with intramolecular with intramolecular borane– borane–phosphanephosphane (B/P) frustrated (B/P) frustrated Lewis pairs Lewis (FLPs). pairs (FLPs). The formationformation of of supramolecular supramolecular macrocyclic macrocyclic structures structures by FLPs by andFLPs their and small-molecule their small-molecule adducts wasadducts developed was developed further duringfurther theduring past the five past years, five and years, it forms and it the forms focus the of focus the present of the present review. Suchreview. supramolecular Such supramolecular assemblies assemblies are not onlyare not interesting only interesting in their ownin their right own from right a sheer from esthetics a sheer viewpoint,esthetics viewpoint, but also because but also they because represent they smallrepres spin-clusterent small spin-cluster systems, allowing systems, advanced allowing solid-stateadvanced NMRsolid-state methodologies NMR methodologies to be tested, to validated, be tested, and validat optimizeded, and with optimized the help with of theoretical the help of calculations theoretical andcalculations simulations. and Insimulations. the present In review, the present we illustrate review, how we advanced illustrate solid-state how advanced NMR methodologysolid-state NMR can bemethodology used to obtain can essential be used information to obtain onessential the structural information organization on the ofstructural such complex organization materials, of which such provide important insights into their reactivity and self-assembling features. The use of solid-state NMR spectroscopy is crucial for these purposes, as the supramolecular arrangements may fall apart Molecules 2020, 25, 1400 3 of 39 into their monomeric constituents upon dissolution. Furthermore, the anisotropic interactions, like dipolar and quadrupolar couplings, which are most intimately linked to structural information, are not accessible in the liquid state. Here, we focus on measurements of crystallographically well-characterized materials, thereby laying the foundations for future applications to FLP-based systems in poly-, nano-, or disordered crystalline and amorphous states of matter. 2. Solid-State NMR—Basics and Methodology 2.1. Fundamental Principles The basic principles of nuclear magnetic resonance (NMR) were summarized in numerous excellent books and review articles devoted to applications to various fields in solid-state chemistry [26–30]. The method is based on nuclear spin motion, represented by an angular momentum operator J. J = I} (1) where } is the reduced Planck’s constant, and I is a dimensionless operator, satisfying angular momentum commutation