Lewis Superacids Research Articles

Front Cover: Association of Pyramidal Boron Lewis Superacids with Pyridines: Bending 2,4,6‐Collidine with the 10‐Sulfonium‐9‐boratriptycene (Z. Anorg. Allg. Chem. 9‐10/2023)

This cover picture shows a Lewis adduct between a pyramidal boron Lewis superacid and a bulky pyridine. Due to the high Lewis acidity and pyramidalization at the boron atom, the boron-nitrogen bond is shorter than that in other Lewis adducts. This induces large steric repulsions resulting in the deformation of the collidine Lewis base. This fundamental observation illustrating the importance of the structural reorganization of Lewis bases in a process of association with a bulky Lewis acid is relevant to the field of frustrated Lewis pairs chemistry (DOI: 10.1002/zaac.202300009).

Taming the Lewis Superacid Al(ORF)3 (RF=C(CF3 )3 ): DFT Guided Identification of the "Stable yet Reactive" Adduct SiPr2 →Al(ORF)3 ; Its Use as ORF- Abstractor from a "Ni-ORF" complex.

A DFT study of several L→Al(ORF )3 (L=Lewis bases) adducts allowed the identification of (i Pr2 S)→Al(ORF )3 1-Si Pr2 as a "stable yet reactive" adduct. 1-Si Pr2 was shown to act as a masked Lewis superacid able to release Al(ORF )3 under mild conditions. It could be used to abstract a ORF- ligand from (bipyMe2 )Ni(ORF )2 (bipyMe2 : 6,6'-dimethyl-2,2'-dipyridyl) and generate the nickel alkoxide complex [(bipyMe2 )Ni(ORF )(i Pr2 S)]+ [(RF O)3 Al-F-Al(ORF )3 ]- 5. Ligand exchange of i Pr2 S by Ph3 P yielded [(bipyMe2 )Ni(ORF )(PPh3 )]+ [(RF O)3 Al-F-Al(ORF )3 ]- 6.

An Amorphous Teflate Doped Aluminium Chlorofluoride: A Solid Lewis‐Superacid for the Dehydrofluorination of Fluoroalkanes

AbstractAn anion‐doped aluminium chlorofluoride AlCl0.1F2.8(OTeF5)0.1 (ACF‐teflate) was synthesized. The material contains pentafluoroorthotellurate (teflate) groups, which mimic fluoride ions electronically, but are sterically more demanding. They are embedded into the amorphous structure. The latter was studied by PDF analysis, EXAFS data and MAS NMR spectroscopy. The mesoporous powder is a Lewis superacid, and ATR‐IR spectra of adsorbed CD3CN reveal a blue‐shift of the adsorption band by 73 cm−1, which is larger than the shift for SbF5. Remarkably, ACF‐teflate catalyzes dehydrofluorination reactions of monofluoroalkanes to yield olefins in C6D6. In these cases, no Friedel‐Crafts products were formed.

Association of Pyramidal Boron Lewis Superacids with Pyridines: Bending 2,4,6‐Collidine with the 10‐Sulfonium‐9‐boratriptycene

AbstractThe association of pyridine and of 2,4,6‐trimethylpyridine (collidine) with a pyramidal triarylborane belonging to the 9‐boratriptycene family is investigated by NMR spectroscopy and by X‐ray diffraction analysis. Despite of the large size of the ortho‐disubstituted collidine Lewis base and of the large steric hindrance of the boron Lewis superacid used (9‐boratriptycene‐10‐sulfonium), the superacidity at its boron atom is enabling the formation of a B‐N bond and is resulting in a very stabe Lewis adduct. Single‐ crystal X‐ray diffraction analysis revealed an exceptional deformation of the collidine moiety with a deviation from planarity of nearly 41° between the pyridine C−N bonds and the ortho‐methyl groups C−Me bonds, which is even resulting a small twist of the collidine six‐membered heteroaromatic ring into a boat‐like conformation. Computational investigations indicate that the steric repulsions are reducing the Gibbs free energy of Lewis adduct formation by approximately 50 kJ mol−1.

Aluminabenzene-based Lewis superacids and weakly coordinating anions.

Quantum-chemical calculations were used to describe both the acidity of aluminabenzene-based Lewis acids and stability of aluminabenzene-based anions. Aluminabenzene itself was found to exhibit greater acidity than antimony pentaflouride, and thus can be qualified as a Lewis superacid. Substitution of the heterocyclic ring with electron withdrawing groups results in formation of extremely strong Lewis superacids. Two of them, namely AlC5 Cl5 and AlC5 (CN)5 are the strongest Lewis acids described in the literature so far. Whereas, anions formed after the addition of fluoride anion to substituted aluminabenzene-based Lewis acids, while characterized by somewhat lower electronic stability than the least coordinating anions hitherto known, are considerably more stable in terms of thermodynamic stability (measured by the propensity to electrophile attack). On this account they are expected to act as counterions for the most reactive cations. The proposed Lewis acids may be prone to the isomerization and dimerization, whereas studied anions are expected to be stable with regard to such processes.

Back Cover: Boron‐Centered Lewis Superacid through Redox‐Active Ligands: Application in C−F and S−F Bond Activation (Angew. Chem. Int. Ed. 13/2023)

One-electron oxidation converts the ferrocene-substituted borane into a Lewis superacid, as reported by Frank Breher, Jan Paradies, and co-workers in their Communication (e202216959). The Lewis superacid activates almost inert carbon–fluorine and sulfur–fluorine bonds for reductions, alkylations, and arylations to form new chemical bonds.

Rücktitelbild: Bor‐Zentrierte Lewis‐Supersäuren durch Redoxaktive Liganden: Anwendung in der C‐F‐ und S‐F‐Bindungsaktivierung (Angew. Chem. 13/2023)

One-electron oxidation converts the ferrocene-substituted borane into a Lewis superacid, as reported by Frank Breher, Jan Paradies, and co-workers in their Communication (e202216959). The Lewis superacid activates almost inert carbon–fluorine and sulfur–fluorine bonds for reductions, alkylations, and arylations to form new chemical bonds.

Gold Teflates Revisited: From the Lewis Superacid [Au(OTeF5 )3 ] to the Anion [Au(OTeF5 )4

A new synthetic access to the Lewis acid [Au(OTeF5 )3 ] and the preparation of the related, unprecedented anion [Au(OTeF5 )4 ]- with inorganic or organic cations starting from commercially available and easy-to-handle gold chlorides are presented. In this first extensive study of the Lewis acidity of a transition-metal teflate complex by using different experimental and quantum chemical methods, [Au(OTeF5 )3 ] was classified as a Lewis superacid. The solid-state structure of the triphenylphosphine oxide adduct [Au(OPPh3 )(OTeF5 )3 ] was determined, representing the first structural characterization of an adduct of this highly reactive [Au(OTeF5 )3 ]. Therein, the coordination environment around the gold center slightly deviates from the typical square planar geometry. The [Au(OTeF5 )4 ]- anion shows a similar coordination motif.

Bor‐Zentrierte Lewis‐Supersäuren durch Redoxaktive Liganden: Anwendung in der C‐F‐ und S‐F‐Bindungsaktivierung

AbstractEine Reihe redox‐responsiver, Ferrocenyl‐substituierter Borane und Boronsäureester wurde dargestellt. Oxidation der Ferrocenyl‐Einheit zum entsprechenden Ferrocenium führte zu einer drastischen Erhöhung der Lewis‐Acidität, wobei sogar die des SbF5übertroffen wird. Dies wurde sowohl experimentell als auch durch quantenchemische Rechnungen untersucht. Die resultierenden, stark Lewis‐aciden Borverbindungen wurden für katalytische C‐F‐ und S‐F‐Aktivierung verwendet.

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.

Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3 and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3 (1) with one or two equivalents of AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imide) or AgSbF6 gives stibaalkene mono- and dications of the form [(CDC)SbBr3-n][A]n (2-4; n = 1,2; A = NTf2 or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3 (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3 (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2 (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

Synthesis, Characterization, and Properties of Three-Dimensional Analogues of 9-Borafluorenes.

Three-dimensional (3D) analogues of 9-borafluorenes, (C2B10H10)2BR (R = Cl (1), Br (2), H (3, merely as ether or silane adduct), phenyl (4), mesityl (5)), were synthesized and fully characterized. Gutmann-Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies confirmed the Lewis superacidity of 1-4, with the Lewis acidity of 1-3 being higher than that of the corresponding 3D analogues of 9,10-diboraanthracenes (DBA). The phenyl group and the C4B borole unit are nearly coplanar in the solid state, while the rotation of the mesityl group of 5 is hindered, forcing 5 to adopt a large C4B/Mes dihedral angle, which leads to distinct properties: air-stable and bluish-violet fluorescence arising from the intramolecular charge-transfer (ICT) transition. The 29Si NMR spectra indicated a high silylium cation character of 3·Et3SiH. The reaction of 4 with Ph3CBr yielded (Ph3C)+[(C2B10H10)2B(Ph)Br]- (6). The 4/PPh3 Lewis pair was capable of splitting dihydrogen and Si-H bond of triethylsilane.

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 ].

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

AbstractThe 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 ofortho‐carborane and lack of pi‐delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb3is a superior catalyst for promoting C−F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C6F5)3].

Silicon Catalyzed C−O Bond Ring Closing Metathesis of Polyethers

AbstractThe Lewis superacid bis(perchlorocatecholato)silane catalyzes C−O bond metathesis of alkyl ethers with an efficiency outperforming all earlier reported systems. Chemoselective ring contractions of macrocyclic crown ethers enable substrate‐specific transformations, and an unprecedented ring‐closing metathesis of polyethylene glycols allows polymer‐selective degradation. Quantum chemical computations scrutinize a high Lewis acidity paired with a simultaneous low propensity for polydentate substrate binding as critical for successful catalysis. Based on these mechanistic insights, a second‐generation class of silicon Lewis superacid with enhanced efficacy is identified and demonstrated.

Silicon Catalyzed C-O Bond Ring Closing Metathesis of Polyethers.

The Lewis superacid bis(perchlorocatecholato)silane catalyzes C-O bond metathesis of alkyl ethers with an efficiency outperforming all earlier reported systems. Chemoselective ring contractions of macrocyclic crown ethers enable substrate-specific transformations, and an unprecedented ring-closing metathesis of polyethylene glycols allows polymer-selective degradation. Quantum chemical computations scrutinize a high Lewis acidity paired with a simultaneous low propensity for polydentate substrate binding as critical for successful catalysis. Based on these mechanistic insights, a second-generation class of silicon Lewis superacid with enhanced efficacy is identified and demonstrated.

Lewis Superacidic Divalent Bis(m‐terphenyl)element Cations [(2,6‐Mes2C6H3)2E]+of Group 13 Revisited and Extended (E=B, Al, Ga, In, Tl)

AbstractIn a combined experimental and computational study, the molecular and electronic structures of the divalent bis(m‐terphenyl)element cations [(2,6‐Mes2C6H3)2E]+of group 13 (1, E=B;2, E=Al;3, E=Ga;4, E=In;5, E=Tl) were investigated. The preparation and characterization of2,3and5were previously reported by Wehmschulte's (Organometallics2004, 23, 1965–1967;J. Am. Chem. Soc.2003,125, 1470–1471) and our groups (Organometallics2009,28, 6893–6901). The indinium ion4was prepared and fully characterized for the first time. Attempts to prepare the borinium ion1by fluoride or hydride abstraction were unsuccessful. The electronic structures of1–5and the stabilization by the bulkym‐terphenyl substituents were analyzed using quantum chemical calculations and compared to the divalent bis(m‐terphenyl)pnictogenium ions [(2,6‐Mes2C6H3)2E]+of group 15 (6, E=P;7, E=As;8, E=Sb;9, E=Bi) previously investigated by our group (Angew. Chem. Int. Ed.2018,57, 10080–10084). The calculated fluoride ion affinities (FIA) of1–9are higher than that of SbF5, which classifies them as Lewis superacids.

Calix[4]pyrrolato Stibenium: Lewis Superacidity by Antimony(III)-Antimony(V) Electromerism.

Lewis superacids enable the activation of highly inert substrates. However, the permanent presence of a Lewis superacidic center comes along with a constantly increased intolerance toward functional groups or ambient conditions. Herein, we describe a strategy to unleash Lewis superacidity by electromerism. Experimental and computational results indicate that coordinating a Lewis base to Δ-calix[4]pyrrolato-antimony(III) triggers a ligand redox-noninnocent coupled transfer into antimony(V)-state that exhibits Lewis superacidic features. Lewis acidity by electromerism establishes a concept of potential generality for powerful yet robust reagents and on-site substrate activation approaches.

Calix[4]pyrrolato Stibenium: Lewis Superacidity by Antimony(III)‐Antimony(V) Electromerism

AbstractLewis superacids enable the activation of highly inert substrates. However, the permanent presence of a Lewis superacidic center comes along with a constantly increased intolerance toward functional groups or ambient conditions. Herein, we describe a strategy to unleash Lewis superacidity by electromerism. Experimental and computational results indicate that coordinating a Lewis base to Δ‐calix[4]pyrrolato‐antimony(III) triggers a ligand redox‐noninnocent coupled transfer into antimony(V)‐state that exhibits Lewis superacidic features. Lewis acidity by electromerism establishes a concept of potential generality for powerful yet robust reagents and on‐site substrate activation approaches.

Insights on the Lewis Superacid Al(OTeF5 )3 : Solvent Adducts, Characterization and Properties.

Preparation and characterization of the dimeric Lewis superacid [Al(OTeF5 )3 ]2 and various solvent adducts is presented. The latter range from thermally stable adducts to highly reactive, weakly bound species. DFT calculations on the ligand affinity of these Lewis acids were performed in order to rank their remaining Lewis acidity. An experimental proof of the Lewis acidity is provided by the reaction of solvent-adducts of Al(OTeF5 )3 with [PPh4 ][SbF6 ] and OPEt3 , respectively. Furthermore, their reactivity towards chloride and pentafluoroorthotellurate salts as well as (CH3 )3 SiCl and (CH3 )3 SiF is shown. This includes the formation of the dianion [Al(OTeF5 )5 ]2- .