Main Group Lewis Acids Research Articles

Oxygen Atom Stabilization by a Main-Group Lewis Acid: Observation and Characterization of an OBeF2 Complex with a Triplet Ground State.

Terminal oxygen radicals involving p- and d-block atoms are quite common, but s-block compounds with an oxygen radical character remain rare. Here, we report that alkaline-earth metal beryllium atoms react with OF2 to form the oxygen beryllium fluorides OBeF and OBeF2. These species are characterized by matrix-isolation infrared spectroscopy with isotopic substitution and quantum-chemical calculations. The linear molecule OBeF has a 2Π ground state with an oxyl radical character. The 3A2 (C2v) ground state of OBeF2 represents the unusual case of a triplet oxygen atom stabilized by a relatively weak interaction by the Lewis acidic BeF2. The interaction involves both a donor component from oxygen to empty Be orbitals and a back-bonding contribution from fluorine substituents toward oxygen.

A Sterically Accessible Monomeric Stibine Oxide Activates Organotetrel(IV) Halides, Including C-F and Si-F Bonds.

Phosphine oxides and arsine oxides are common laboratory reagents with diverse applications that stem from the chemistry exhibited by these monomeric species. Stibine oxides are, in contrast, generally dimeric or oligomeric species because of the reactivity-quenching self-association of the highly polarized stiboryl (Sb=O/Sb+-O-) group. We recently isolated Dipp3SbO (Dipp = 2,6-diisopropylphenyl), the first example of a kinetically stabilized monomeric stibine oxide, which exists as a bench-stable solid and bears an unperturbed stiboryl group. Herein, we report the isolation of Mes3SbO (Mes = mesityl), in which the less bulky substituents maintain the monomeric nature of the compound but unlock access to a wider range of reactivity at the unperturbed stiboryl group relative to Dipp3SbO. Mes3SbO was found to be a potent Lewis base in the formation of adducts with the main-group Lewis acids PbMe3Cl and SnMe3Cl. The accessible Lewis acidity at the Sb atom results in a change in the reactivity with GeMe3Cl, SiMe3Cl, and CPh3Cl. With these species, Mes3SbO formally adds the E-Cl (E = Ge, Si, C) bond across the unsaturated stiboryl group to form a 5-coordinate stiborane. The biphilicity of Mes3SbO is sufficiently potent to activate even the C-F and Si-F bonds of C(p-MeOPh)3F and SiEt3F, respectively. These results mark a significant contribution to an increasingly rich literature on the reactivity of polar, unsaturated main-group motifs. Furthermore, these results highlight the utility of a kinetic stabilization approach to access unusual bonding motifs with unquenched reactivity that can be leveraged for small-molecule activation.

Pyridinium‐abgeleitete mesoionische N‐heterocyclische Olefine (py‐mNHOs)

AbstractMesoionische Polarisation ermöglicht den Zugang zu elektronenreichen Olefinen, die als Organokatalysatoren, Liganden oder Nucleophile Anwendung finden. Wir berichten hier über die Synthese und Charakterisierung einer Reihe von 3‐Methylpyridinium‐abgeleiteten mesoionischen Olefinen (py‐mNHOs). Wir verwendeten ein mit DFT‐Rechnungen unterstütztes Designkonzept, das zeigte, dass Arylgruppen an den 1‐, 2‐, 4‐ und 6‐Positionen des heterocyclischen Kerns die kinetische Stabilisierung der neuartigen mesoionischen Verbindungen ermöglicht. Die elektronischen Tolman‐Parameter zeigen, dass py‐mNHOs bemerkenswert starke σ‐Donor‐Liganden für Übergangsmetalle und Hauptgruppen‐Lewis‐Säuren sind. Außerdem gehören sie zu den stärksten Nucleophilen auf der Mayr‐Reaktivitätsskala. Bei Reaktionen von py‐mNHOs mit elektronenarmen π‐Systemen wurde experimentell ein sukzessiver Übergang von der Bildung zwitterionischer Addukte über schrittweise zu konzertierten 1,3‐dipolaren Cycloadditionen beobachtet und durch quantenchemische Rechnungen analysiert.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs).

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments.

New developments in the field of Lewis acidity are highlighted, with the focus of novel Lewis acids and Lewis superacids of group 2, 13, 14, and 15 elements. Several important basics, illustrated by modern examples (classification of Donor-Acceptor (DA) complexes, amphoteric nature of any compound in terms of DA interactions, reorganization energies of main group Lewis acids and the role of the energies of frontier orbitals) are presented and discussed. It is emphasized that the Lewis acidity phenomena are general and play vital role in different areas of chemistry: from weak "atomophilic" interactions to the complexes of Lewis superacids.

Binding, Sensing, And Transporting Anions with Pnictogen Bonds: The Case of Organoantimony Lewis Acids.

Motivated by the discovery of main group Lewis acids that could compete or possibly outperform the ubiquitous organoboranes, several groups, including ours, have engaged in the chemistry of Lewis acidic organoantimony compounds as new platforms for anion capture, sensing, and transport. Principal to this approach are the intrinsically elevated Lewis acidic properties of antimony, which greatly favor the addition of halide anions to this group 15 element. The introduction of organic substituents to the antimony center and its oxidation from the + III to the + V state provide for tunable Lewis acidity and a breadth of applications in supramolecular chemistry and catalysis. The performances of these antimony-based Lewis acids in the domain of anion sensing in aqueous media illustrate the favorable attributes of antimony as a central element. At the same time, recent advances in anion binding catalysis and anion transport across phospholipid membranes speak to the numerous opportunities that lie ahead in the chemistry of these unique main group compounds.

XeF2 Coordination Complexes of the [BrO2]+ Cation, [O2Br(FXeF)n][AsF6] (n = 1, 2) and [O2Br(FXeF)2][SbF6]; Their Syntheses and Structural Characterizations.

The syntheses and structural characterizations of the first XeF2 coordination complexes of the [BrO2]+ cation are described. The reactions of [BrO2][PnF6] (Pn = As, Sb) with XeF2 in anhydrous HF solvent yield the salts [O2Br(FXeF)n][AsF6] (n = 1, 2) and [O2Br(FXeF)2][SbF6], which were characterized by low-temperature (LT) Raman spectroscopy and single-crystal X-ray diffraction (SCXRD). The XeF2 ligands and [PnF6]- coordinate to the Lewis acidic [BrO2]+ cation through primarily electrostatic BrV---FXe σ-hole bonds that result from coordination of the F atoms into regions of high positive electrostatic potential on the Br(V) atom and have bond trajectories that avoid the stereoactive valence electron lone-pair of Br(V). The complexes and their structural characterizations by LT Raman spectroscopy and SCXRD significantly extend the coordination chemistry of Br(V) and provide rare examples of a noble-gas difluoride coordinated to a strong oxidant main-group Lewis acid center.

Exploring the Lewis Acidity and Reactivity of Neutral Pentacoordinate Dithienophospholes.

A series of luminescent, neutral pentacoordinate dithieno[3,2-b:2',3'-d]phosphole compounds was synthesized by [4+1] cycloaddition of o-quinones with the corresponding trivalent phospholes. The electronic and geometrical modification of the π-conjugated scaffold implemented here impacts the aggregation behavior of the species in solution. It proved successful in generating species with improved Lewis acidity of the phosphorus center that was then leveraged for small-molecule activation. Hydride abstraction from an external substrate involving the hypervalent species is followed by an intriguing P-mediated umpolung from the hydride to a proton and supports the catalytic potential of this class of main-group Lewis acids for organic chemistry. This study is a comprehensive investigation into various methods, including electronic, chemical, geometric modifications (and sometimes combinations of these approaches) to systematically improve the Lewis acidity of neutral and stable main-group Lewis acids with practical value for a range of chemical transformations.

Cationic Triarylchlorostibonium Lewis Acids.

Organopnictogen cations show promise as powerful, tunable main-group Lewis acid catalysts. The synthesis, solid-state structures, and reactivity of a series of weakly coordinated triarylchlorostibonium salts [Ar3SbCl][B(C6F5)4] (Ar = Ph, 3-FC6H4, 4-FC6H4, 3,5-F2C6H3, 2,4,6-F3C6H2) are reported. The cation in each adopts a tetrahedral coordination environment of antimony, with near complete separation from the anion. Structural, computational, and reactivity studies reveal that the Lewis acidity of [Ar3SbCl]+ generally increases with increased fluorination of the Ar substituents, with a secondary quenching effect from para fluorination. [Ar3SbCl]+ is reduced to Ar3Sb in the presence of Et3SiH, and the mechanism of this reaction has been modeled computationally. Preliminary studies demonstrate that they are useful catalysts for the dimerization of 1,1-diphenylethylene and the Friedel-Crafts alkylation of benzene.

Photochemical and Electrochemical Strategies for Hydrodefluorination of Fluorinated Organic Compounds.

Hydrodefluorination (HDF) is a very important fundamental transformation for conversion of the C-F bond into the C-H bond in organic synthesis. In the past decade, much progress has been achieved with HDF through the utility of low-valent metals, transition-metal complexes and main-group Lewis acids. Recently, novel methods have been introduced for this purpose through photo- and electrochemical pathways, which are of great significance, due to their considerable environmental and economical advantages. This Review highlights the HDF of fluorinated organic compounds (FOCs) through photo- and electrochemical strategies, along with mechanistic insights.

Boron-Based Lewis Acid Catalysis: Challenges and Perspectives

In the last two decades, boron-based catalysis has been gaining increasing traction in the field of organic synthesis. The use of halogenated triarylboranes as main group Lewis acid catalysts is an attractive strategy. It has been applied in a growing number of transformations over the years, where they may perform comparably or even better than the gold standard catalysts. This review discusses methods of borane synthesis and cutting-edge boron-based Lewis acid catalysis, focusing especially on tris(pentafluorophenyl)-borane [B(C6F5)3], and other halogenated triarylboranes, highlighting how boron Lewis acids employed as catalysts can unlock a plethora of unprecedented chemical transformations or improve the efficiency of existing reactions.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen.

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H2 can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

Syntheses and Structural Characterizations of the Cl(V) Coordination Complex, [O2Cl(FXeF)2][AsF6], and β-[ClO2][AsF6

The synthesis and structural characterization of the first Cl(V) coordination complex of XeF2, [O2Cl(FXeF)2][AsF6], is described. The reaction of α-[ClO2][AsF6] with XeF2 at −78 °C in anhydrous HF (aHF) solvent yields [O2Cl(FXeF)2][AsF6], a rare example of noble-gas difluoride coordination to a main-group Lewis acid center. The low-temperature (LT) phase of β-[ClO2][AsF6] was obtained by recrystallization of α-[ClO2][AsF6] from aHF solvent at –10 °C. The compounds were characterized by LT single-crystal X-ray diffraction and LT Raman spectroscopy. Quantum-chemical calculations were carried out for the model gas-phase ([O2Cl(FXeF)2][AsF6]2)− anion to aid in the assignments of fundamental vibrational frequencies and 35/37Cl isotope shifts for [O2Cl(FXeF)2][AsF6] and to assess its chemical bonding by use of an NBO analysis. The [O2Cl(FXeF)2]+ cations and [AsF6]− anions of [O2Cl(FXeF)2][AsF6] are intimate ion-pairs with secondary Cl---FAs and Cl---FXe bonds that are significantly shorter than the sums of the Cl and F van der Waals radii, and are consistent with noncovalent σ-hole bonding. MEPS and NBO analyses were also carried out for the gas-phase [XO2]+ (X = Cl, Br, I) cations.

Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes.

We have prepared and characterized a series of unprecedented group 6–group 11, N2-bridged, heterobimetallic [ML4(η1-N2)(μ-η1:η1-N2)Au(NHC)]+ complexes (M = Mo, W, L2 = diphosphine) by treatment of trans-[ML4(N2)2] with a cationic gold(I) complex [Au(NHC)]+. The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M–N–N–Au motif. To verify how far the analogy goes, we computed the electronic structures of [W(depe)2(η1-N2)(μ-η1:η1-N2)AuNHC]+ (10W+) and [W(depe)2(η1-N2)(μ-η1:η1-N2)B(C6F5)3] (11W). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N2 with a d orbital of the group 6 metal overlaps in 10W+ with an empty sd hybrid orbital at gold, whereas in 11W with an sp3 hybrid orbital at boron. The bent N–N–LA arrangement maximizes these interactions, providing a similar level of N2 “push–pull” activation in the two compounds. In the gold case, the HOMO–2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the μ-N2–[Au(NHC)]+ bond.

Cyclometalated Iridium Bipyridine Complexes with Peripheral Antimony Substituents

As part of our ongoing efforts in the development of main group Lewis acids as anion receptors, we have investigated the synthesis of cyclometalated bipyridine iridium(III) complex decorated by antimony moieties. Reaction of 4‐(diphenylstibino)‐2,2'‐bipyridine (L) with [(ppy)2Ir(μ‐Cl)]2 afforded the corresponding tris‐chelate iridium complex [(ppy)2IrL]+ ([1]+), which was isolated as a hexafluorophosphate salt ([1][PF6]). Reaction of [1][PF6] with excess PhICl2 in DMSO induced the conversion of the diphenylantimony moiety of [1]+ into an anionic diphenyltrichloroantimonate leading to a zwitterionic complex referred to as 2‐Cl. Complexes [1][PF6] and 2‐Cl were characterized by NMR and the structure of 2‐Cl confirmed using X‐ray diffraction. DFT calculations and electrochemical measurments show that the electron‐rich diphenyltrichloroantimonate moiety in 2‐Cl cathodically shifts the Ir(III/IV) redox couple. Luminescence measurements also show that 2‐Cl is less emissive than [1]+.

Transition Metal Cooperative Lewis Pairs Using Platinum(0) Diphosphine Monocarbonyl Complexes as Lewis bases

The platinum(0)-diphosphine complex [Pt(CO)(L1)] (3, where L1 = 1,2-C6H4(CH2PtBu2)2) and its diphosphinite analogue [Pt(CO)(L2)] (11, where L2 = 1,2-C6H4(OPtBu2)2) act as Lewis bases in conjunction with the main group Lewis acid B(C6F5)3 to form frustrated or cooperative Lewis pairs. These systems activate dihydrogen, ethene/carbon monoxide, and phenylacetylene, leading to products that depend on the exact ligand used. These subtle changes to ligand structure influence reactivity, most notably in hydrogen activation where a variety of dinuclear species of the type [(diphos)Pt(μ-H)3Pt(diphos)]+ or [(diphos)Pt(μ-H)(μ-CO)Pt(diphos)]+ are observed. Activation of ethene with the Lewis pair leads to a previously reported coupling product and the mechanism is probed. The basicity of [Pt(CO)(L)] is demonstrated by deprotonation of phenylacetylene. Preliminary studies with an analogous palladium complex [Pd(CO)(L1)] 33 suggests related chemistry may be exploited for this metal. These results provide further examples of cooperative Lewis pair behavior in which one of the components is based on a transition metal complex.

Variational Forward-Backward Charge Transfer Analysis Based on Absolutely Localized Molecular Orbitals: Energetics and Molecular Properties.

To facilitate the understanding of charge-transfer (CT) effects in dative complexes, we propose a variational forward-backward (VFB) approach to decompose the overall CT stabilization energy into contributions from forward and backward donation in the framework of energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA). Such a decomposition is achieved by introducing two additional constrained intermediate states in which only one direction of CT is permitted. These two "one-way" CT states are variationally relaxed such that the associated nuclear forces can be readily obtained. This allows for a facile integration into the previously developed adiabatic EDA scheme, so that the molecular property changes arising from forward and back donation can be separately assigned. Using ALMO-EDA augmented by this VFB model, we investigate the energetic, geometric, and vibrational features of complexes composed of CO and main group Lewis acids (BH3, BeO/BeCO3) and complexes of the N2, CO, and BF isoelectronic series with [Ru(II)(NH3)5]2+. We identify that the shift in the stretching frequency of a diatomic π-acidic ligand (XY), such as CO, results from a superposition of the shifts induced by permanent electrostatics and backward CT: permanent electrostatics can cause an either red or blue shift depending on the alignment of the XY dipole in the dative complex, and this effect becomes more pronounced with a more polar XY ligand; the back-donation to the antibonding π orbital of XY always lowers the X-Y bond order and thus red-shifts its stretching frequency, and the strength of this interaction decays rapidly with the intermolecular distance. We also reveal that while σ forward donation contributes significantly to energetic stabilization, it affects the vibrational feature of XY mainly by shortening the intermolecular distance, which enhances both the electrostatic interaction and backward CT but in different rates. The synergistic effect of the forward and backward donations appears to be more significant in the transition-metal complexes, where the forward CT plays an essential role in overcoming the strong Pauli repulsion. These findings highlight that the shift in the XY stretching frequency is not a reliable metric for the strength of π back-donation. Overall, the VFB-augmented EDA scheme that we propose and apply in this work provides a useful tool to characterize the role played by each physical component that all together lead to the frequency shift observed.

Redox-controlled chalcogen-bonding at tellurium: impact on Lewis acidity and chloride anion transport properties.

Our interests in the chemistry of atypical main group Lewis acids have led us to devise strategies that augment the affinity of chalcogen-bond donors for anionic guests. In this study, we describe the oxidative methylation of diaryltellurides as one such strategy along with its application to the synthesis of [Mes(C6F5)TeMe]+ and [(C6F5)2TeMe]+ starting from Mes(C6F5)Te and (C6F5)2Te, respectively. These new telluronium cations have been evaluated for their ability to complex and transport chloride anions across phospholipid bilayers. These studies show that, when compared to their neutral Te(ii) precursors, these Te(iv) cations display both higher Lewis acidity and transport activity. The positive attributes of these telluronium cations, which originate from a lowering of the tellurium-centered σ* orbitals and a deepening of the associated σ-holes, demonstrate that the redox state of the main group element provides a convenient handle over its chalcogen-bonding properties.

Isolation, characterization and reactivity of three-coordinate phosphorus dications isoelectronic to alanes and silylium cations.

Three-coordinate main group Lewis acids are exceedingly important reagents in chemical synthesis. In contrast to the well-established chemistries of neutral group 13 and cationic group 14 species, isoelectronic group 15 element dications are unknown. In this work, we use stabilizing N-heterocyclic imine substituents to isolate and characterize phosphorandiylium dications ([R3P]2+) and show that the electrophilicity at the phosphorus atoms is controlled by the π-electron-donating ability of these subtituents. Structural, spectroscopic and theoretical results reveal that the phosphorus dications adopt a perfectly trigonal-planar geometry with the electron-deficient phosphorus centres being well separated from the borate anions. The reactivity of the dications reveal their exceptional Lewis acidity at phosphorus; the adjacent nitrogen atoms, however, are weakly basic, resulting in transformations such as chloride ion abstraction from Me3SiCl and the selective monodefluorination of trifluoromethyl groups.

C-F Bond Activation by a Saturated N-Heterocyclic Carbene: Mesoionic Compound Formation and Adduct Formation with B(C6 F5 )3.

The reaction of SIPr, [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] (1), with C6 F6 led to the formation of an unprecedented mesoionic compound (2). The formation of 2 is made accessible by deprotonation of the SIPr backbone with simultaneous elimination of HF. The C-F bond para to the imidazolium ring in 2 is only of 1.258(4) Å, which is the one of the shortest structurally authenticated C-F bonds known to date. The liberation of HF during the reaction is unequivocally proved by the addition of one more equivalent of SIPr, which leads to the imidazolium salt with the HF2 - anion. To functionalize 2, the latter reacted with B(C6 F5 )3 to give an unusual donor-acceptor compound, where the fluoride atom from the C6 F5 moiety coordinates to B(C6 F5 )3 and the carbanion moiety remains unaffected. Such coordination susceptibility of the fluoride atom of a nonmetallic system to a main-group Lewis acid (Fnon-metal →BR3 ) is quite unprecedented.