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

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.

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF3 -Et2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C-11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C6 F5 )2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp2 -carbon-sp2 -boron σ-bond with 1 J(13 C-11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.

Synthesis and Characterizations of Polythiophene Networks with Nonplanar BN Lewis Pair Building Blocks

Doping the boron (B) element endowed organic π-conjugated polymers (OCPs) with intriguing optoelectronic properties. Herein, we introduce a new series of thienylborane-pyridine (BN) Lewis pairs via the facile reactions between thienylborane and various pyridine derivatives. Particularly, we developed a "one-pot" synthetic protocol to access BN2 with an unstable 4-bromopyridine moiety. Polycondensations between the BN Lewis pairs and distannylated thiophene afforded a new series of BN-cross-linked polythiophenes (BN-PTs). Experiments revealed that BN-PTs exhibited highly uniform chemical structures, particularly the uniform chemical environment of B-centers. BN-PTs showed good stability in the solid state. PBN2 even maintained the uniform B-center under high temperature or moisture conditions. The studies further suggested that the presence of topological BN structures endowed the polymers with strong intramolecular charge separation character. As a proof of concept, a representative BN-PT was tested as the catalyst for photocatalytic hydrogen evolution.

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.

Solid State NMR Spectroscopy a Valuable Technique for Structural Insights of Advanced Thin Film Materials: A Review

Solid-state NMR has proven to be a versatile technique for studying the chemical structure, 3D structure and dynamics of all sorts of chemical compounds. In nanotechnology and particularly in thin films, the study of chemical modification, molecular packing, end chain motion, distance determination and solvent-matrix interactions is essential for controlling the final product properties and applications. Despite its atomic-level research capabilities and recent technical advancements, solid-state NMR is still lacking behind other spectroscopic techniques in the field of thin films due to the underestimation of NMR capabilities, availability, great variety of nuclei and pulse sequences, lack of sensitivity for quadrupole nuclei and time-consuming experiments. This article will comprehensively and critically review the work done by solid-state NMR on different types of thin films and the most advanced NMR strategies, which are beyond conventional, and the hardware design used to overcome the technical issues in thin-film research.