First Evidence for the Existence of Hexafluoroantimonic(V) Acid

Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages

Theoretical investigation of the H(CHB₁₁ X₁₁ ) (X=H, F, Cl, CN), H(CHB₁₁ X_n Y_11-n ) (X,Y=F, Cl; n=1,5), and dimeric (H(CHB₁₁ X₁₁ ))₂ (X=F, Cl) carborane superacids performed at the B3LYP/6-311++G(d,p) theory level revealed the similarity of their equilibrium structures and the possibility of nearly barrierless hydrogen atom migration among the substituents attached to one side of the icosahedral CB₁₁ cage. The vertical electron detachment energies predicted at the OVGF/6-311++G(3df,2pd) theory level for the conjugate bases (CHB₁₁ X₁₁ )^- were found to span the 5.82-9.00 ev range. The acid strengths (manifested by the Gibbs free deprotonation energies spanning the 213-266 kcal/mol range) predicted for the icosahedral H(CHB₁₁ X₁₁ ) carborane systems confirm their superacidic properties which might be increased even further by the attachment of the second carborane H(CHB₁₁ X₁₁ ) unit that leads to a dimeric structure mimicking a part of an experimentally observed H-bridged polymeric chain. The Gibbs free deprotonation energy of the dimeric (H(CHB₁₁ Cl₁₁ ))₂ acid was predicted to be smaller by 17 kcal/mol than that of the corresponding monomeric H(CHB₁₁ Cl₁₁ ) acid.

Designing aromatic heterocyclic superacids in terms of Brønsted and Lewis perspectives

The unexplored area of organic superacids was investigated in terms of both Brønsted and Lewis concepts of acids and bases. The primary requirement of a superacid-high affinity for electron/fluoride ions was fulfilled using two strategies: (i) using the superhalogen-type heterocyclic framework and (ii) selecting systems that have an electron count one short of attaining (4n + 2) Hückel aromaticity. With these in mind, eleven systems were considered throughout the study, expected to cross the target of 100% H₂SO₄ acidity and/or the fluoride affinity of SbF₅. To enhance the pK_a and F^- affinity values of the considered systems, electron-withdrawing ligands F and CN were employed. The superhalogen and aromaticity properties were verified by vertical detachment energy (VDE) and nucleus independent chemical shift (NICS) calculations, respectively. Finally, the collective effect of the potential super Lewis acids was looked into using a BL₃ skeleton with them acting as ligands.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

The Acid Strength of the Lewis-Brønsted Superacids - A QSPR Study

The acidity of Lewis-Brønsted superacids can be derived from the theoretical calculations as the Gibbs free energy of the deprotonation reaction (ΔG_acid ), which describes the tendency of a studied compound to donate a proton. This paper presents the first Quantitative Structure - Property Relationship (QSPR) model that correlates the ΔG_acid of superacid (HF/MeX₃ formula (X=F, Cl, Br)) with their structure. Developed model is well fitted, roubustness, has good predictive abilities, fulfills all OECD recommendation for good model. Obtained results provide the insight into the relation of structural features of superacids, which are responsible for their acid strength - the structures characterized by strong F-Me dative bond (with relatively large vibrational frequency), small positive partial atomic charge on Me central atom, possibly large polarity exhibit large acid strength. Such assumption can be used in the future as valuable information in the process of the designing new, stronger, more effective superacids.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Revealing the Activity of π-Acid Catalysts using a 7-Alkynyl Cycloheptatriene

A compound that isomerizes into distinct products depending on the particular Lewis acid or Brønsted acid catalyst used is disclosed. One product can only be obtained with the softest π-acids, such as Au, Pt, Ga, or In complexes. Another is formed only with harder π-acids incorporating Ag or Cu salts. The formation of the third category requires even harder π-acids or protons. This simple benchmark reaction allows for prediction of the category of transformations that can be catalyzed by a new complex. It also informs whether protons have been unintentionally generated in the reaction mixture.

Superhalogens as building blocks of a new series of superacids

A new series of superacids by the protonation of B_nH_3n+1⁻ superhalogen anions has been proposed. The resulting B_nH_3n+2 species behave as superacids for n ≥ 2 due to their smaller free energy of deprotonation than that of H₂SO₄. These B_nH_3n+2 superacids do not require Brønsted/Lewis acid components.

The saturation of the gas phase acidity of nHF/AlF3 and nHF/GeF4 (n = 1–6) superacids caused by increasing the number of surrounding HF molecules

The acidic strength of selected Brønsted/Lewis superacids is evaluated on the basis of theoretical calculations carried out at the QCISD/6-311++G(d,p) level.

Catalytic applications of [IPr·GaX2][SbF6] and related species

The broad applicability of cationic NHC complexes of gallium in molecular catalysis is revealed.