Pushing the Lewis Acidity Boundaries of Boron Compounds With Non‐Planar Triarylboranes Derived from Triptycenes

Boron-Centered Lewis Superacid through Redox-Active Ligands: Application in C-F and S-F Bond Activation

A series of redox-responsive ferrocenyl-substituted boranes and boronic esters were synthesized. Oxidation of the ferrocenyl ligand to the ferrocenium resulted in a drastic increase in the Lewis acidity beyond the strength of SbF₅ , which was investigated experimentally and computationally. The resulting highly Lewis acidic boron compounds were used for catalytic C-F and S-F bond activation.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid

The synthesis of tris(ortho-carboranyl)borane (BoCb₃ ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb₃ is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ].

Dual Reactivity of a Geometrically Constrained Phosphenium Cation

A geometrically constrained phosphenium cation in bis(pyrrolyl)pyridine based NNN pincer type ligand (1⁺) was synthesized, isolated and its preliminary reactivity was studied with small molecules. 1⁺ reacts with MeOH and Et₂NH, activating the O−H and N−H bonds via a P‐center/ligand assisted path. The reaction of 1⁺ with one equiv. of H₃NBH₃ leads to its dehydrogenation producing 5. Interestingly, reaction of 1⁺ with an excess H₃NBH₃ leads to phosphinidene (P^I) species coordinating to two BH₃ molecules (6). In contrast, [1⁺][OTf] reacts with Et₃SiH by hydride abstraction yielding 1‐H and Et₃SiOTf, while [1⁺][B(C₆F₅)₄] reacts with Et₃SiH via an oxidative addition type reaction of Si−H bond to P‐center, affording a new P^V compound (8). However, 8 is not stable over time and degrades to a complex mixture of compounds in matter of minutes. Despite this, the ability of [1⁺][B(C₆F₅)₄] to activate Si−H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1⁺ closely mimics transition metal behaviour.

What Distinguishes the Strength and the Effect of a Lewis Acid: Analysis of the Gutmann-Beckett Method

IUPAC defines Lewis acidity as the thermodynamic tendency for Lewis pair formation. This strength property was recently specified as global Lewis acidity (gLA), and is gauged for example by the fluoride ion affinity. Experimentally, Lewis acidity is usually evaluated by the effect on a bound molecule, such as the induced ³¹ P NMR shift of triethylphosphine oxide in the Gutmann-Beckett (GB) method. This type of scaling was called effective Lewis acidity (eLA). Unfortunately, gLA and eLA often correlate poorly, but a reason for this is unknown. Hence, the strength and the effect of a Lewis acid are two distinct properties, but they are often granted interchangeably. The present work analyzes thermodynamic, NMR specific, and London dispersion effects on GB numbers for 130 Lewis acids by theory and experiment. The deformation energy of a Lewis acid is identified as the prime cause for the critical deviation between gLA and eLA but its correction allows a unification for the first time.

Taming the Lewis Superacidity of Non-Planar Boranes: C-H Bond Activation and Non-Classical Binding Modes at Boron

The rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It allowed the formation of non-classical electron deficient B-H-B type of bonding which was supported by spectroscopic and X-ray diffraction parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations such as Csp 2 -H and Csp 3 -H activation under frustrated Lewis pair regime and the cleavage of Csp 3 -Si bonds. The demonstration of such type of rich chemical behavior and flexibility on a single molecular compound make it a unique mediator of chemical transformations generally restricted to transition metals.

Rational Development of a Metal-Free Bifunctional System for the C-H Activation of Methane: A Density Functional Theory Investigation

The activation or heterolytic splitting of methane, a challenging substrate usually restricted to transition metals, has so far proven elusive in experimental frustrated Lewis pair (FLP) chemistry. In this article, we demonstrate, using density functional theory (DFT), that 1-aza-9-boratriptycene is a conceptually simple intramolecular FLP for the activation of methane. Systematic comparison with other FLP systems allows to gain insight into their reactivity with methane. The thermodynamics and kinetics of methane activation are interpreted by referring to the analysis of the natural charges and by employing the distortion-interaction/activation strain (DIAS) model. These showed that the nature of the Lewis base influences the selectivity over the reaction pathway, with N Lewis bases favoring the deprotonation mechanism and P bases the hydride abstraction one. The lower barrier of activation for 1-aza-9-boratriptycene and the higher products stability are due to a better interaction energy than its counterparts, itself due to electrostatic interactions with the methane moiety, favorable orbital overlaps allowed by the side-attack, and space proximity between the B and N atoms.

Small Molecule Activation with Intramolecular "Inverse" Frustrated Lewis Pairs

The intramolecular "inverse" frustrated Lewis pairs (FLPs) of general formula 1-BR₂ -2-[(Me₂ N)₂ C=N]-C₆ H₄ (3-6) [BR₂ =BMes₂ (3), BC₁₂ H₈ , (4), BBN (5), BBNO (6)] were synthesized and structurally characterized by multinuclear NMR spectroscopy and X-ray analysis. These novel types of pre-organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via an ortho-phenylene linker, are capable of activating H-H, C-H, N-H, O-H, Si-H, B-H and C=O bonds. 4 and 5 deprotonated terminal alkynes and acetylene to form the zwitterionic borates 1-(RC≡C-BR₂ )-2-[(Me₂ N)₂ C=NH]-C₆ H₄ (R=Ph, H) and reacted with ammonia, BnNH₂ and pyrrolidine, to generate the FLP adducts 1-(R₂ HN→BR₂ )-2-[(Me₂ N)₂ C=NH]-C₆ H₄ , where the N-H functionality is activated by intramolecular H-bond interactions. In addition, 5 was found to rapidly add across the double bond of H₂ CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise, 5 is capable of cleaving H₂ , HBPin and PhSiH₃ to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules.

Calix[4]pyrrolato Aluminates: The Effect of Ligand Modification on the Reactivity of Square-Planar Aluminum Anions

Structural constraint represents an attractive tool to modify p-block element properties without the need for unusual oxidation or valence states. The recently reported methyl-calix[4]pyrrolato aluminate established the effect of forcing a tetrahedral aluminum anion into a square-planar coordination mode. However, the generality of this structural motif and any consequence of ligand modification remained open. Herein, a systematic ligand screening was launched, and the class of square-planar aluminum anions was extended by two derivatives that differ in the meso-substitution at the calix[4]pyrrolato ligand. Strikingly, this modification provoked opposing trends in the preference for a Lewis acidic binding mode with σ-donors versus the aluminum-ligand cooperative binding mode with carbonyls. Insights into the origin of these counterintuitive experimental observations were provided by computation and bond analysis. Importantly, this rationale might allow to exploit mode-selective binding for catalytic rate control.

Lewis Acidic Boranes, Lewis Bases, and Equilibrium Constants: A Reliable Scaffold for a Quantitative Lewis Acidity/Basicity Scale

A quantitative Lewis acidity/basicity scale toward boron-centered Lewis acids has been developed based on a set of 90 experimental equilibrium constants for the reactions of triarylboranes with various O-, N-, S-, and P-centered Lewis bases in dichloromethane at 20 °C. Analysis with the linear free energy relationship log KB =LAB +LBB allows equilibrium constants, KB , to be calculated for any type of borane/Lewis base combination through the sum of two descriptors, one for Lewis acidity (LAB ) and one for Lewis basicity (LBB ). The resulting Lewis acidity/basicity scale is independent of fixed reference acids/bases and valid for various types of trivalent boron-centered Lewis acids. It is demonstrated that the newly developed Lewis acidity/basicity scale is easily extendable through linear relationships with quantum-chemically calculated or common physical-organic descriptors and known thermodynamic data (ΔHBF 3 ). Furthermore, this experimental platform can be utilized for the rational development of borane-catalyzed reactions.

Reversible Silylium Transfer between P-H and Si-H Donors

The Mo=PR₂ π* orbital in a Mo phosphenium complex acts as acceptor in a new P^III -based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si-H abstraction from E₃ SiH to give a Mo-bound secondary phosphine ligand, Mo-PR₂ H. The resulting Et₃ Si⁺ ion remains associated with the Mo complex, stabilized by η¹ -P-H donation, yet undergoes rapid exchange with an η¹ -Si-H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo-bound phosphine-silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane-silylium adduct or "free" silylium.

Pincer-Type Ligand-Assisted Catalysis and Small-Molecule Activation by non-VSEPR Main-Group Compounds

In 2005, a facile dihydrogen activation was reported by the Power group using an alkyne analog of germanium [ArGe≡GeAr; Ar=2,6-Trip₂ -C₆ H₃ (Trip=2,4,6-ⁱ Pr₃ -C₆ H₂ )]. After that, a significant progress has been made in the activation of various small molecules by main-group compounds, and a variety of stoichiometric and catalytic processes have been formulated using the p-block elements. In this regard, compounds containing low-valent main-group elements with a frontier orbitals of relatively small energy gaps or compounds forming frustrated Lewis pair (FLP) became quite successful. In spite of these promising stoichiometric and catalytic transformations, redox-cycling catalysts based on main-group elements remain extremely rare. Recently, it has been observed that pincer type ligands supported geometry constrained main-group compounds are capable of acting as redox catalysts similar to those of the transition metals. In this review, we focus on the synthesis and the structural aspects of the geometry constrained main-group compounds using pincer ligands. Emphasis has been placed on their applications on catalytic activity and small molecules activation.