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

CeO2-Based Frustrated Lewis Pairs via Defective Engineering: Formation Theory, Site Characterization, and Small Molecule Activation

Activation of small molecules is considered to be a central concern in the theoretical investigation of environment- and energy-related catalytic conversions. Sub-nanostructured frustrated Lewis pairs (FLPs) have been an emerging research hotspot in recent years due to their advantages in small molecule activation. Although the progress of catalytic applications of FLPs is increasingly reported, the fundamental theories related to the structural formation, site regulation, and catalytic mechanism of FLPs have not yet been fully developed. Given this, it is attempted to demonstrate the underlying theory of FLPs formation, corresponding regulation methods, and its activation mechanism on small molecules using CeO2 as the representative metal oxide. Specifically, this paper presents three fundamental principles for constructing FLPs on CeO2 surfaces, and feasible engineering methods for the regulation of FLPs sites are presented. Furthermore, cases where typical small molecules (e.g., hydrogen, carbon dioxide, methane oxygen, etc.) are activated over FLPs are analyzed. Meanwhile, corresponding future challenges for the development of FLPs-centered theory are presented. The insights presented in this paper may contribute to the theories of FLPs, which can potentially provide inspiration for the development of broader environment- and energy-related catalysis involving small molecule activation.

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

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...

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts

The first comprehensive solid-state nuclear magnetic resonance (NMR) characterization of geminal alane-phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect 27 Al-31 P spin-spin coupling constants, and 27 Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the 31 P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences. The 27 Al NMR central transition signals are spread out over a broad frequency range (>200 kHz), owing to the presence of strong nuclear electric quadrupolar interactions that can be well-reproduced by the static 27 Al wideband uniform rate smooth truncation (WURST) Carr-Purcell-Meiboom-Gill (WCPMG) NMR experiment. 27 Al chemical shifts and quadrupole tensor components offer a facile and clear distinction between three- and four-coordinate aluminum environments. For measuring internuclear Al⋅⋅⋅P distances a new resonance-echo saturation-pulse double-resonance (RESPDOR) experiment was developed by using efficient saturation via frequency-swept WURST pulses. The successful implementation of this widely applicable technique indicates that internuclear Al⋅⋅⋅P distances in these compounds can be measured within a precision of ±0.1 Å.

Silica Nanopowder Supported Frustrated Lewis Pairs for CO2 Capture and Conversion to Formic Acid

Treatment of hydroxylated silica nanopowders S1 and allyl-functionalized silica nanopowders S2 with 3-(diphenylborano)- or 3-bis(pentafluorophenylborano)propyltrimethoxysilane or 2-(diphenylphosphino)- or 2-(dicyclohexylphosphino)ethyltriethoxysilane generates silica nanopowder supported Lewis acids S3 and silica nanopowder supported Lewis bases S4. These surfaces were characterized by ¹³C, ¹¹B, and ³¹P cross-polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR), X-ray photoelectron spectroscopy (XPS), and attenuated total reflection Fourier transform infrared (ATR FTIR). When S3 is combined with solution-phase Lewis bases PR₃ (R = C₆F₅, C₆H₅, mesityl), six associated silica nanopowder supported frustrated Lewis pairs (FLPs) are formed. In another set of six reactions, the interactions between the supported Lewis bases S4 and solution-phase Lewis acids BR₃ with R = C₆F₅, C₆H₅, mesityl produced six more associated supported FLPs. The capture of CO₂ by these FLPs producing FLP-CO₂ Lewis pair adducts S5 and S6 were highlighted by ATR FTIR, and it was found that FLP S5e with R = C₆H₅ on both the supported Lewis acid and solution-phase Lewis base trapped the largest quantities of CO₂ on the silica nanopowder supports. Conversion of CO₂ to HCOOH was achieved by first activating H₂ to generate activated FLP-H₂ surfaces S7 and S9. Addition of CO₂ then generated HCOOH via the silica nanopowder supported FLP-HCOOH adducts S8 and S10. Qualitative identification of HCOOH generation was achieved by ATR FTIR measurements, and surface 10b with R = C₆H₅ proved to be the most successful silica nanopowder surface bound FLP in HCOOH generation. In some cases, diborano formates (-BO(CH)OB-) S11 and S12 were also identified as side products during HCOOH formation. Spectroscopic characterization of purposefully synthesized S11 and S12 included ¹¹B and ³¹P CP MAS NMR.

Solid-State NMR Techniques for the Structural Characterization of Cyclic Aggregates Based on Borane-Phosphane Frustrated Lewis Pairs

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.

Dihydrogen Splitting by Intramolecular Borane-Phosphane Frustrated Lewis Pairs: A Comprehensive Characterization Strategy Using Solid State NMR and DFT Calculations

Four hydrogenated intramolecular phosphane-borane frustrated Lewis pair (B/P FLP) compounds bearing unsaturated cyclic or aromatic carbon backbones have been synthesized and structurally characterized using ¹¹ B, ³¹ P, ¹ H and ² H solid-state NMR spectroscopy. A comparison of the spectra with those of the corresponding free B/P FLPs shows that both ¹¹ B isotropic chemical shifts as well as nuclear electric quadrupolar coupling constants decrease significantly upon FLP hydrogenation, revealing the breakage of the partial B-P bond present in the starting materials. Likewise, the ³¹ P isotropic chemical shift, the chemical shift anisotropy, and the asymmetry parameter decrease significantly upon FLP hydrogenation, reflecting the formation of a more symmetric, C_3v -like local environment. ¹¹ B{³¹ P} rotational echo double resonance (REDOR) experiments can be used to measure the B-P internuclear distance (about 3.2 Å) of these compounds. Observation of the hydrogen atoms bound to the Lewis centers is best accomplished via ³¹ P{¹ H} and ¹¹ B{¹ H} cross-polarization-heteronuclear correlation experiments or by direct observation of the ² H MAS NMR signals on especially prepared FLP-D₂ adducts. For accurately measuring the phosphorus-deuterium distance via ³¹ P{² H} rotational echo adiabatic passage double resonance (REAPDOR), it is essential to take the secondary dipolar coupling of ³¹ P with the boron-bonded ² H nuclei explicitly into consideration, by simulating a ² H_P -³¹ P-² H_B three-spin system based on structural input. All of the experimental NMR interaction parameters are found in excellent agreement with values calculated by DFT methods, using the geometries obtained either by energy optimization or from single-crystal structures.

Characterization of H2 -Splitting Products of Frustrated Lewis Pairs: Benefit of Fast Magic-Angle Spinning

Proton spectroscopy in solid-state NMR on catalytic materials offers new opportunities in structural characterization, in particular of reaction products of catalytic reactions such as hydrogenation reactions. Unfortunately, the ¹ H NMR line widths in magic-angle spinning solid-state spectra are often broadened by an incomplete averaging of ¹ H-¹ H dipolar couplings. We herein discuss two model compounds, namely the H₂ -splitting products of two phosphane-borane Frustrated Lewis Pairs (FLPs), to study potentials and limitations of proton solid-state NMR experiments employing magic-angle spinning frequencies larger than 100 kHz at a static magnetic field strength of 20.0 T. The ¹ H lines are homogeneously broadened as illustrated by spin-echo decay experiments. We study two structurally similar materials which however show significant differences in ¹ H line widths which we explain by differences in their ¹ H-¹ H dipolar networks. We discuss the benefit of fast MAS experiments up to 110 kHz to detect the resonances of the H⁺ /H^- pair in the hydrogenation products of FLPs.

Aggregation Behavior of a Six-Membered Cyclic Frustrated Phosphane/Borane Lewis Pair: Formation of a Supramolecular Cyclooctameric Macrocyclic Ring System

A new six-membered cyclic frustrated phosphane/borane Lewis pair was liberated from its HB(C₆ F₅ )₂ adduct by treatment with vinylcyclohexane. The system is an active frustrated Lewis pair that undergoes cycloaddition reactions with suitable π reagents and it splits dihydrogen. At room temperature in solution the new compound is a monomer, however, in the crystal and in solution at low temperature it aggregates to a thermodynamically favoured supramolecular macrocyclic cyclooctamer.

Solid state frustrated Lewis pair chemistry

In solution the PCy3/B(C6F5)3 pair is rapidly deactivated by nucleophilic aromatic substitution. In the solid state deactivation is effectively suppressed and the active frustrated phosphane/borane Lewis pair splits dihydrogen or adds to sulfur dioxide. A variety of phosphane/B(C6F5)3 pairs have been used to carry out active FLP reactions in the solid state. The reactions were analyzed by DFT calculations and by solid state NMR spectroscopy. The solid state dihydrogen splitting reaction was also carried out under near to ambient conditions with suspensions of the non-quenched phosphane/borane mixtures in the fluorous liquid perfluoromethylcyclohexane.

Hydrogen cleavage by solid-phase frustrated Lewis pairs

We report the direct synthesis of a solid-phase frustrated Lewis pair (s-FLP) by combining a silica-supported Lewis acid ([triple bond, length as m-dash]SiOB(C6F5)2, s-BCF) with a Lewis base (tri-tert-butylphosphine, (t)Bu3P) to give [[triple bond, length as m-dash]SiOB(C6F5)2][(t)Bu3P]. Reaction of this s-FLP with H2 under mild conditions led to heterolytic H-H bond cleavage and the formation of [[triple bond, length as m-dash]SiOB(H)(C6F5)2][(t)Bu3PH].

Formation and reactions of active five-membered phosphane/borane frustrated Lewis pair ring systems

The five-membered P/B FLP ring systems featuring very bulky Tipp or Mes* aryl groups at phosphorus undergo addition reactions with organic π-reagents.

The Chemistry of a Non-Interacting Vicinal Frustrated Phosphane/Borane Lewis Pair

The dimesitylphosphinocyclopentene/HB(C₆ F₅ )₂ -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO₂ . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C₆ F₅ )₂ ] to generate a FLP-η² -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.