Synthesis and Structure of Solvated Protons Incorporating Weakly Coordinating Anions. Precursors of Superacids

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The Strongest Brønsted Acid: Protonation of Alkanes by H(CHB11F11) at Room Temperature

What is the strongest acid? Can a simple Brønsted acid be prepared that can protonate an alkane at room temperature? Can that acid be free of the complicating effects of added Lewis acids that are typical of common, difficult‐to‐handle superacid mixtures? The carborane superacid H(CHB₁₁F₁₁) is that acid. It is an extremely moisture‐sensitive solid, prepared by treatment of anhydrous HCl with [Et₃Si? H? SiEt₃][CHB₁₁F₁₁]. It adds H₂O to form [H₃O][CHB₁₁F₁₁] and benzene to form the benzenium ion salt [C₆H₇][CHB₁₁F₁₁]. It reacts with butane to form a crystalline tBu⁺ salt and with n‐hexane to form an isolable hexyl carbocation salt. Carbocations, which are thus no longer transient intermediates, react with NaH either by hydride addition to re‐form an alkane or by deprotonation to form an alkene and H₂. By protonating alkanes at room temperature, the reactivity of H(CHB₁₁F₁₁) opens up new opportunities for the easier study of acid‐catalyzed hydrocarbon reforming.     

A whole zoo of hydrogen bonds in one crystal structure: tris(isonicotinium) hydrogensulfate sulfate monohydrate

Depending on the reaction partner, the organic ditopic molecule isonicotinic acid (Hina) can act either as a Brønsted acid or base. With sulfuric acid, the pyridine ring is protonated to become a pyridinium cation. Crystallization from ethanol affords the title compound tris(4‐carboxypyridinium) hydrogensulfate sulfate monohydrate, 3C₆H₆NO₂⁺·HSO₄⁻·SO₄²⁻·H₂O or [(H₂ina)₃(HSO₄)(SO₄)(H₂O)]. This solid contains 11 classical hydrogen bonds of very different flavour and nonclassical C—H…O contacts. All N—H and O—H donors find at least one acceptor within a suitable distance range, with one of the three pyridinium H atoms engaged in bifurcated N—H…O hydrogen bonds. The shortest hydrogen‐bonding O…O distance is subtended by hydrogensulfate and sulfate anions, viz. 2.4752 (19) Å, and represents one of the shortest hydrogen bonds ever reported between these residues.     

A Spiro Phosphamide Catalyzed Enantioselective Proton Transfer of Ylides in a Free Carbene Insertion into N−H Bonds

Free carbene readily causes multiple side reactions due to its high energy, thus its asymmetric transformation is very difficult. We present here our findings of high-pK_a Brønsted acid catalysts that enable free carbene insertion into N−H bonds of amines to prepare chiral α-amino acid derivatives with high enantioselectivity. Under irradiation with visible light, diazo compounds produce high-energy free carbenes that are captured by amines to form free ylide intermediates, and then the newly designed high-pK_a Brønsted acids, chiral spiro phosphamides, promote the proton transfer of ylides to afford the products. Computational and kinetic studies uncover the principle for the rational design of proton-transfer catalysts and explain how the catalysts accelerate this transformation and provide stereocontrol.     

Synthesis and Structure of (N,)C,N‐chelated Organoantimony(III) and Bismuth(III) Cations and Isolation of Their Adducts with Ag[CB11H12]

The treatment of N,C,N‐chelated antimony(III) and bismuth(III) chlorides [C₆H₃‐2,6‐(CH=NR)₂]MCl₂ [R = tBu and M = Sb ( 1 ) or Bi ( 2 ); R = Dmp and M = Sb ( 3 ) or Bi ( 4 )] (Dmp = 2,6‐Me₂C₆H₃) with one molar equivalent of Ag[CB₁₁H₁₂] led to a smooth formation of corresponding ionic pairs {[C₆H₃‐2,6‐(CH=NR)₂]MCl}⁺[CB₁₁H₁₂]^– [R = tBu and M = Sb ( 7 ) or Bi ( 8 ), R = Dmp and M = Sb ( 9 ) or Bi ( 10 )]. Similarly, the reaction of C,N‐chelated analogues [C₆H₂‐2‐(CH=NDip)‐4,6‐(tBu)₂]MCl₂ [M = Sb ( 5 ) or Bi ( 6 ), Dip = 2′,6′‐iPr₂C₆H₃] gave compounds {[C₆H₂‐2‐(CH=NDip)‐4,6‐(tBu)₂]MCl}⁺[CB₁₁H₁₂]^– [M = Sb ( 11 ) or Bi ( 12 )]. All compounds 7 – 12 were characterized with ¹H, ¹¹B and ¹³C{¹H} NMR spectroscopy, ESI‐mass spectrometry, IR spectroscopy, and molecular structures of 7 – 9 and 12 were determined by the help of single‐crystal X‐ray diffraction analysis. In contrast, all attempts to cleave also the second M–Cl bond in 7 – 12 using another molar equivalent Ag[CB₁₁H₁₂] remained unsuccessful. Nevertheless, the reaction between 7 (or 8 ) and Ag[CB₁₁H₁₂] produced unprecedented adducts of both reagents namely {[C₆H₃‐2,6‐(CH=NtBu)₂]SbCl}₂²⁺[Ag₂(CB₁₁H₁₂)₄]^2– ( 13 ) and {[C₆H₃‐2,6‐(CH=NtBu)₂]BiCl}⁺[Ag(CB₁₁H₁₂)₂]^– ( 14 ) in a reproducible manner. The molecular structures of these sparingly soluble compounds were determined by single‐crystal X‐ray diffraction analysis.     

The Strongest Acid: Protonation of Carbon Dioxide

The strongest carborane acid, H(CHB₁₁F₁₁), protonates CO₂ while traditional mixed Lewis/Brønsted superacids do not. The product is deduced from IR spectroscopy and calculation to be the proton disolvate, H(CO₂)₂⁺. The carborane acid H(CHB₁₁F₁₁) is therefore the strongest known acid. The failure of traditional mixed superacids to protonate weak bases such as CO₂ can be traced to a competition between the proton and the Lewis acid for the added base. The high protic acidity promised by large absolute values of the Hammett acidity function (H₀) is not realized in practice because the basicity of an added base is suppressed by Lewis acid/base adduct formation.     

Origin and Location of Electrons and Protons during the Formation of Intermetalloid Clusters [Sm@Ga3−xH3−2xBi10+x]3− (x=0, 1)

Reaction of [GaBi₃]^2? with [Sm(C₅Me₄H)₃] yielded the first protonated ternary intermetalloid clusters [Sm@Ga_3?xH_3?2xBi_10+x]^3? ( 1 ; x=0,1). The presence of the Ga? H bonds and the transfer of electrons and protons during the formation of 1 were elucidated by a combination of experimental and quantum chemical methods, thereby rationalizing the role of the solvent ethane‐1,2‐diamine as a Brønsted acid. As an organic by‐product, we observed the previously unknown octamethylfulvene ( 2 ) upon C? C coupling of (C₅Me₄H)^?.     

Eliminations from aryl bis(<Emphasis Type="Italic">p</Emphasis>-chlorophenyl)acetates promoted by R<Subscript>2</Subscript>NH in MeCN. Effects of base-solvent and <Emphasis Type="Italic">β</Emphasis>-aryl group

Elimination reactions of (4′-ClC₆H₄)₂CHCO₂C₆H₃-2-X-4-NO₂ promoted by R₂NH in MeCN have been studied kinetically. The observed second-order kinetics, Brønsted β = 0.44–0.86 and |β_blog| = 0.46–0.69 are consistent with the E2 mechanism. The Brønsted β decreased as the leaving group was made more nucleofugic and the |β_log| increased with a weaker base. Comparison with existing data reveals that the structure of the transition state is relatively insensitive to the base-solvent and β- aryl substituent on the E2 transition state.     

Scandium Reduced Arene Complex: Protonation and Reaction with Azobenzene

The reactivity of the reduced anthracene complex of scandium [Li(thf)₃][Sc{N(tBu)Xy}₂(anth)] ( 2-anth-Li ) (Xy=3,5-Me₂C₆H₃; anth=C₁₄H₁₀²⁻, thf=tetrahydrofuran) toward Brønsted acid [NEt₃H][BPh₄] and azobenzene is reported. While a stepwise protonation of 2-anth-Li with two equivalents of [NEt₃H][BPh₄] afforded the scandium cation [Sc{N(tBu)Xy}₂(thf)₂][BPh₄] ( 3 ), reduction of azobenzene gave a thermolabile, anionic scandium reduced azobenzene complex [Li(thf)][Sc{N(tBu)Xy}₂(η²-PhNNPh)] ( 4 ), which slowly lost one of the anilide ligands to form the neutral scandium azobenzene complex dimer [Sc{N(tBu)Xy}(μ-η²:η²-Ph₂N₂)]₂ ( 5 ). Exposure of 3 to CO₂ produced the scandium carbamate complex [Sc{κ²-O₂CN(tBu)(Xy)}₂][BPh₄] ( 6 ) as a result of CO₂ insertion into the Sc−N bonds. In an attempt to prepare scandium hydrides, the reaction of 3 with the hydride sources LiAlH₄ and Na[BEt₃H] led to the terminal aluminum hydride [AlH{N(tBu)Xy}₂(thf)] ( 7 ) and the scandium n-butoxide [Sc{N(tBu)(Xy)}₂(μ-OnBu)] ( 8 ) after Sc/Al transmetalation and nucleophilic ring-opening of THF, respectively. All reported compounds isolated in moderate to good yields were fully characterized.     

Mercury derivatives of polyhedral boranes,carboranes, and metallacarboranes

The review concerns the synthesis, structure, and chemical transformations of mercury derivatives of various polyhedral boron hydrides (carboranes, metallacarboranes, and boranes) and covers a period of time since the pioneering studies of Prof. V. I. Bregadze on chemistry of C- and B-substituted derivatives of icosahedral carboranes C₂B₁₀H₁₂ up to date. The emphasis is placed on recent results obtained in this field, in particular, Hg-promoted substitution in nido-carboranes as well as the synthesis and properties of mercury derivatives of monocarbon carborane anion [CB₁₁H₁₂]^–.

     

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by ¹⁷O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.     

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C₁–n‐C₄ alkanes on Zn‐modified high‐silica zeolites ZSM‐5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C? H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n‐butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C₁–n‐C₄ alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.     

Homolytic C−H Bond Activation by Phosphine−Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine−quinone systems and explore their potential for the activation of C−H bonds. PMes₃ (Mes=2,4,6-Me₃C₆H₂) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P−O bonded zwitterionic adduct Mes₃P−DDQ ( 1 ), while the reaction with the sterically more crowded PTip₃ (Tip=2,4,6-iPr₃C₆H₂) afforded C−H bond activation product Tip₂P(H)(2-[CMe₂(DDQ)]-4,6-iPr₂-C₆H₂) ( 2 ). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip₃]⋅⁺[DDQ]⋅⁻, and subsequent homolytic C−H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip₂P(H)(2-[CMe₂{TCQ−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 4 , TCQ=tetrachloro-1,4-benzoquinone) and Tip₂P(H)(2-[CMe₂{oQ^tBu−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 8 , oQ^tBu=3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip₃ with Lewis-acid activated quinones, TCQ−B(C₆F₅)₃ and oQ^tBu−B(C₆F₅)₃, respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C−H bond activation by the synergistic action of radical ion pairs.     

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by ¹⁷O DNP surface‐enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub‐picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.     

Taming Gold(I)–Counterion Interplay in the De‐aromatization of Indoles with Allenamides

A careful interplay between the π electrophilicity of a cationic Au^I center and the basicity of the corresponding counterion allowed for the chemo‐ and regioselective inter‐ as well as intramolecular de‐aromatization of 2,3‐disubstituted indoles with allenamides. The silver‐free bifunctional Lewis acid/Brønsted base complex [{2,4‐(tBu)₂C₆H₃O}₃PAuTFA] assisted the formation of a range of densely functionalized indolenines under mild conditions.     

Bismuth Undecahydro‐closo‐dodecaborane: A Retainable Intermediate of B−H Bond Activation by Bismuth(III) Cations

The [B₁₂H₁₂]^2? anion shows an extensive substitutional chemistry based on its three‐dimensional aromaticity. The replacement of functional groups can be attained by electrophilically induced substitution caused by Brønsted or Lewis acidic electrophiles (e.g. Pt²⁺). Until now, it was impossible to structurally characterize a metal‐substituted [B₁₂H₁₂]^2? cage. When an aqueous solution containing both Bi³⁺ cations and [B₁₂H₁₂]^2? anions was heated, the charge‐neutral bismuth undecahydro‐closo‐dodecaborane BiB₁₂H₁₁ was obtained, representing a new class of metalated [B₁₂H₁₂]^2? clusters. The title compound was characterized by single‐crystal X‐ray diffraction and NMR spectroscopic methods. Compared to the typical B?H bond, the short B?Bi single bond (230 pm) exhibits inverted polarity.     

Novel ionic liquid N,N-diethyl-N-sulfoethanaminium hydrogen sulfate: Design,characterization, and application as a highly efficient catalyst for the production of triazolo[1,2-a]indazole-triones and 2H-indazolo[2,1-b]phthalazine-triones

A novel Brønsted acidic ionic liquid namely N,N-diethyl-N-sulfoethanamminium hydrogen sulfate ([Et₃N-SO₃H]HSO₄) was synthesized, and characterized using FT-IR, ¹H NMR, ¹³C NMR, and mass data. Then, its catalytic activity was examined for the preparation of triazolo[1,2-a]-indazole-triones and 2H-indazolo[2,1-b]phthalazine-triones by the one-pot multi-component condensation of arylaldehydes with dimedone and 4-phenylurazole/2,3-dihydrophthalazine-1,4-dione under solvent-free conditions. [Et₃N-SO₃H]HSO₄ efficiently promoted the reaction to afford the products in high yields and in short reaction times.     

A 3D Analogue of Phenyllithium: Solution‐Phase,Solid‐State,and Computational Study of the Lithiacarborane [Li−CB11H11]−

The lithiacarborane [Li?CB₁₁H₁₁]^? plays a central role in carborane chemistry, as it is a key intermediate to achieve the selective functionalization of the monocarba‐closo‐dodecaborate [CB₁₁H₁₂]^? for applications in various fields. Also, it is an organometallic species of fundamental interest because it represents a 3D analogue of phenyllithium featuring an exo C?Li bond in addition to the delocalized negative endo charge of the spherical cluster. For the first time, the elusive and highly reactive endo/exo formal dianion [CB₁₁H₁₁]^2? has been isolated as its lithiate as well as zincate in pure form and fully characterized. DFT calculations corroborate the experimental findings and underscore the remarkably high reactivity of the lithiacarborane. Subsequent derivatizations demonstrate the relevance of its initial clean formation.     

Iron-Catalyzed Intermolecular C−H Amination Assisted by an Isolated Iron-Imido Radical Intermediate

Here we report the use of a base metal complex [(^tBupyrpyrr₂)Fe(OEt₂)] ( 1 -OEt₂) (^tBupyrpyrr₂²⁻=3,5-tBu₂-bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of C_sp3−H bonds of 9,10-dihydroanthracene ( 2 a ) using 2,4,6-trimethyl phenyl azide ( 3 a ) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1 -OEt₂ with control in selectivity for single C−H amination versus double C−H amination. Catalytic C−H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(^tBupyrpyrr₂)Fe{=N(2,6-Me₂-4-tBu-C₆H₂)] ( 1 -imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 Fe^IV complex. Complex 1 -imido reacts with 2 a to produce the ferrous aniline adduct [(^tBupyrpyrr₂)Fe{NH(2,6-Me₂-4-tBu-C₆H₂)(C₁₄H₁₁)}] ( 1 -aniline) in 45 % yield. Lastly, it was found that complexes 1 -imido and 1 -aniline are both competent intermediates in catalytic intermolecular C−H amination.     

A 3D Analogue of Phenyllithium: Solution‐Phase,Solid‐State,and Computational Study of the Lithiacarborane [Li−CB11H11]−

The lithiacarborane [Li?CB₁₁H₁₁]^? plays a central role in carborane chemistry, as it is a key intermediate to achieve the selective functionalization of the monocarba‐closo‐dodecaborate [CB₁₁H₁₂]^? for applications in various fields. Also, it is an organometallic species of fundamental interest because it represents a 3D analogue of phenyllithium featuring an exo C?Li bond in addition to the delocalized negative endo charge of the spherical cluster. For the first time, the elusive and highly reactive endo/exo formal dianion [CB₁₁H₁₁]^2? has been isolated as its lithiate as well as zincate in pure form and fully characterized. DFT calculations corroborate the experimental findings and underscore the remarkably high reactivity of the lithiacarborane. Subsequent derivatizations demonstrate the relevance of its initial clean formation.