Synthesis and Structure of the Small Superelectrophile [C2(OH)2Me2]2.

The acid-activation of 1,2-dicarbonyl compounds plays a key role in a variety of electrophilic reactions, some of which are only accessible in superacidic media when a superelectrophilic dication is formed. To obtain structural and electronic information about these elusive species, the vicinal dication [C2(OH)2Me2]2+ is synthesized and characterized by Raman spectroscopy and X-ray diffraction. Since this superelectrophile could not be stabilized in convenient superacids, the usage of liquid SO2 turned out to be crucial. The experimental data are discussed together with quantum-chemical calculations on the B3LYP/aug-cc-pVTZ level of theory. Natural Bond Orbital (NBO) analyses quantify the superelectrophilic interactions found in the solid state.

Synthesis and Structure of Protonated Propiolic Acid.

Propiolic acid was investigated in the superacidic system XF/SbF5 (X = H, D). The salts of the monoprotonated species of propiolic acid were characterized by vibrational and NMR spectroscopy as well as single-crystal analyses. The rotational conformers of the protonated species can be distinguished by NMR spectroscopy via the temperature-dependent rotational barrier. In the solid state, they can be detected by H/D exchange and packing effects due to different anions. The experimental results are discussed together with an IRC calculation of the rotational barrier. After acetic acid and formic acid, this is the third protonated carboxylic acid for which the energy differences between the conformers have been determined.

Synthesis and Stabilization of Highly Reactive Haloacetylium Ions.

The reactions of selected haloacetyl fluorides with the strong Lewis acids AsF5 and SbF5 were investigated in the aprotic solvent SO2ClF. Depending on the stoichiometric ratio of AsF5 or SbF5 and the haloacetyl fluorides, either O-coordinated adducts or the respective haloacetylium ions were isolated as solids. The compounds were characterized by low-temperature vibrational spectroscopy and single-crystal X-ray diffraction. [CClH2CO][Sb2F11] and [CH2FCO][Sb2F11] crystallize in the monoclinic space group P21 with two formula units per unit cell. The reactivity of the haloacetyl fluorides under Lewis acidic conditions is discussed based on quantum chemical calculations of fluoride ion affinities on the BP/def2SVPP level of theory. The haloacetylium ions are stabilized in the solid-state phase by intermolecular C···F contacts and electron donation of the fluorine ligands of the anions as well as intramolecular hyperconjugation.

Synthesis and Structure of Protonated Sulfur Dioxide.

Salts of protonated sulfur dioxide were synthesized by recrystallization of [FS(OX)2][SbF6] (X=H, D) in aprotic solvents at low temperatures. Hemiprotonated sulfur dioxide [(SO2)2H][Sb2F11] was obtained from the solvent SO2 and the monoprotonated sulfur dioxide [OSOD][Sb2F11], using 1,1,1,2-tetrafluoroethane as solvent. For both compounds, single crystals were obtained and an X-ray structure analysis was conducted. Furthermore, the salts were characterized by Raman spectroscopy and the results were discussed together with quantum chemical calculations on the M06-2X/aug-cc-pVTZ level of theory.

Accumulation, Characterization and Reactivity of Chiral Ammonium-Carboxonium Dications in Superacid.

The accumulation of chiral ammonium-oxocarbenium dications in superacid is evidenced by low-temperature NMR spectroscopy, X-ray diffraction analysis and confirmed by DFT calculations. Its potential for the diastereoselective remote hydrofunctionalization of non-activated alkene is also explored.

Strengthening of the C-F Bond in Fumaryl Fluoride with Superacids.

The reaction of fumaryl fluoride with the superacidic solutions XF/MF5 (X=H, D; M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C4 H3 F2 O2 ]+ [MF6 ]- (M=As, Sb) and [C4 H2 X2 F2 O2 ]2+ ([MF6 ]- )2 (X=H, D; M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low-temperature vibrational spectroscopy. Low-temperature NMR spectroscopy and single-crystal X-ray structure analyses were carried out for [C4 H3 F2 O2 ]+ [SbF6 ]- as well as for [C4 H4 F2 O2 ]2+ ([MF6 ]- )2 (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C4 H4 F2 O2 ⋅ 2 HF]2+ and [C4 H3 F2 O2 ⋅ HF]+ at the B3LYP/aug-cc-pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge-related properties of the diprotonated species. The C-F bond lengths in the protonated dication are considerably reduced on account of the +R effect.

Ring Opening Reactions of ß-Propiolactam in Superacidic Media.

The reaction of ß-propiolactam in the superacidic systems HF/MF5 (M=Sb, As) led to the formation of monoprotonated 3-aminopropanoyl fluoride in the form of [C(O)F(CH2 )2 NH3 ][SbF6 ] and [C(O)F(CH2 )2 NH3 ][AsF6 ]. In the presence of traces of water, the diprotonated species ß-alanine [C(OH)2 (CH2 )2 NH3 ][AsF6 ]2 was synthesized for the first time. All salts were characterized by low-temperature infrared and Raman spectroscopy. Additionally, single-crystal X-ray analyses were conducted in the case of [C(O)F(CH2 )2 NH3 ][SbF6 ] and [C(OH)2 (CH2 )2 NH3 ][AsF6 ]2 . By using SO2 instead of HF as the solvent, the salt [C(OH)2 (CH2 )2 NHSO][SbF6 ]2 was obtained, and single-crystal X-ray analysis of this salt containing a thionylimide moiety was conducted. For the formation of these open-chain compounds, an acyl cationic species as intermediate is assumed, which is formed from N-protonated ß-propiolactam. Quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory were carried out to gain a better understanding of the formation and the structural properties of protonated ß-propiolactam.

Fluoromethyl-2,4,6-trinitrophenylsulfonate: A New Electrophilic Monofluoromethylating Reagent.

Fluoromethyl-2,4,6-trinitrophenylsulfonate has been prepared for the first time and qualified as a simple to use monofluoromethylating reagent. Its molecular structure in the solid state has been determined by single-crystal X-ray diffraction studies. This reagent proves to be effective for the electrophilic introduction of a CH2F group into selected chalcogen and nitrogen nucleophiles. Monofluoromethyl derivatives of various bifunctional N,O-nucleophiles have been synthesized using fluoromethyl-2,4,6-trinitrophenylsulfonate. Due to the good crystallizing properties of the anion, the fluoromethylated products as well as side products that are difficult to identify by nuclear magnetic resonance spectroscopy can readily be characterized by X-ray crystallographic techniques.

Evidence of Phosphonium-Carbenium Dication Formation in a Superacid: Precursor to Fluorinated Phosphine Oxides.

Unambiguously confirmed by low-temperature in situ NMR experiments, X-ray diffraction and vibrational spectroscopy, phosphonium-carbenium superelectrophiles are shown to be generated in strong acidic conditions. Representing crucial intermediates, their exploitation allows for the synthesis of unprecedented fluorinated (cyclic) phosphine oxides.

Diprotonation of Guanidine in Superacidic Solutions.

Guanidinium chloride reacts with the superacidic solutions HF/MF5 (M=As, Sb) at a molar ratio of 1:2 under formation of the diprotonated guanidinium salts [C(NH2 )2 (NH3 )][AsF6 ]2 and [C(NH2 )2 (NH3 )][SbF6 ]2 . The compounds were characterized by using infrared and Raman spectroscopy. Furthermore, single-crystal X-ray structure analysis of the guanidinium(2+) salts [C(NH2 )2 (NH3 )][SbF6 ]2 ⋅HF, [C(NH2 )2 (NH3 )]2 [Ge3 F16 ]⋅HF, and [C(NH2 )2 (NH3 )]2 [Ge3 F16 ]⋅2 HF and the guanidinium(1+) salt [C(NH2 )3 ][SbF6 ] is reported. The discussion of the experimental data is supported by quantum-chemical calculations of the [C(NH2 )2 (NH3 )]2+ and [C(NH2 )3 ]+ ions to investigate the modification of the resonance stabilization during the protonation process at the PBE1PBE/6-311G++(3df,3pd) level of theory. The planar CN3 skeleton of the guanidinium(2+) ion has two carbon-nitrogen bonds in the range 1.286(4)-1.293(4) Šand one carbon-nitrogen bond of 1.453(4) Å, which can be explained with a decreased resonance stabilization relative to the guanidinium(1+) ion.

Investigations on Squaric Acid in Superacidic Media.

The syntheses of [OC(COX)3 ][MF6 ] and [(COX)4 ][MF6 ]2 ⋅2 HF were carried out in superacidic media XF/MF5 (M=As, Sb; X=H, D). The degree of protonation is highly dependent on the stoichiometric ratio of the Lewis acid with regard to squaric acid. The salts of diprotonated squaric acid were characterized by Raman spectroscopy and, in the case of [(COH)4 ][MF6 ]2 ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH)4 ][AsF6 ] crystallizes in the monoclinic space group P21 /n with two formula units per unit cell. Analysis of the vibrational spectra was achieved with the support of quantum chemical calculations of the cation [(COH)4 ]2+ ⋅4 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH)3 ][AsF6 ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P21 /n with four formula units per unit cell. The protonation of squaric acid leads to a change of the carbon skeleton, which is discussed for the entire series, starting with the dianion of squaric acid and ending with the tetrahydroxy dication.

Tuning the Anomeric Effect in Sulfamide with Superacids.

This study shows that the anomeric effect (negative hyperconjugation) that arises in sulfamide, as a result of the relatively short S-N bonds, can be tuned by the utilization of superacidic media. Sulfamide was reacted in binary superacidic systems XF/MF5 (M=As, Sb; X=H, D) and HF/BF3 . The colorless salts formed, [X2 NSO2 NX3 ]+ [MF6 ]- and [H2 NSO2 NH3 ]+ [BF4 ]- were characterized by low-temperature vibrational spectroscopy. In the case of [H2 NSO2 NH3 ]+ [BF4 ]- , a single crystal X-ray diffraction study was performed. The salt crystallizes in the monoclinic space group P21 /c with four formula units per unit cell. An exclusive N,N'-diprotonation was observed in the superacidic system HF/SbF5 when using several equivalents of the Lewis acid. Low-temperature vibrational spectra as well as a single-crystal X-ray structure of [H3 NSO2 NH3 ]2+ 2 [SbF6 ]- ⋅2 HF are reported. The salt crystallizes in the orthorhombic space group Pna21 with four formula units per unit cell. Upon mono- or diprotonation of sulfamide, remarkable structural changes of the sulfur-nitrogen bond lengths were observed. Herein, these changes are discussed together with quantum chemical calculations.

The Existence of Tricyanomethane.

Calcium tricyanomethanide reacts with hydrogen fluoride under formation of tricyanomethane and Ca(HF2)2. Tricyanomethane is stable below -40 °C and was characterized by IR, Raman, and NMR spectroscopy. The vibrational spectra were compared to the quantum-chemical frequencies at the PBE1PBE/6-311G(3df,3dp) level of theory and confirm the predicted C(3v) symmetry of the molecule with regular C-H (109.8 pm), C-C (146.7 pm), and C≡N (114.7 pm) bonds.

The existence of hexafluoroarsenic(V) acid.

The homogeneous mixture of anhydrous hydrogen fluoride aHF and antimony pentafluoride AsF5 is known as a superacidic system. The high acidity is derived from the formation of [H2F](+) [AsF6](-). No experimental evidence exists for the existence of the free acid molecule HAsF6. The reaction of trimethylsilyl N,N-dimethylcarbamate in the binary system aHF/AsF5 led to decomposition of trimethylsilyl N,N-dimethylcarbamate at -50 °C to dimethylammonium hexafluoridoarsenate and cocrystallization of HAsF6. The single-crystal X-ray structure displays an HAsF6 molecule involved in an asymmetric hydrogen bridge to the hexafluoridoarsenate anion. As a result of the incalculable situation in the crystal lattice, the molecular structure of HAsF6 is calculated by quantum chemical structure optimization of the extreme cases of [FHF-AsF5](-) (strong hydrogen bond) and HAsF6 (no hydrogen bond) at the PBE1PBE/6-311G(3df,3pd) level of theory.

Na[GeF5]·2HF: the first quarternary phase in the H-Na-Ge-F system.

The structure of cis- or trans-bridged [GeF5]- anionic chains have been investigated [Mallouk et al. (1984). Inorg. Chem. 23, 3160-3166] showing the first crystal structures of µ-F-bridged pentafluorogermanates. Herein, we report the second crystal structure of trans-pentafluorogermanate anions present in the crystal structure of sodium trans-pentafluorogermanate(IV) bis(hydrogen fluoride), Na[GeF5]·2HF. The crystal structure [orthorhombic Pca21, a = 12.3786 (3), b = 7.2189 (2), c = 11.4969 (3) Šand Z = 8] is built up from infinite chains of trans-linked [GeF6]2- octahedra, extending along the b axis and spanning a network of pentagonal bipyramidal distorted Na-centred polyhedra. These [NaF7] polyhedra are linked in a trans-edge fashion via hydrogen fluoride molecules, in analogy to already known sodium hydrogen fluorides and potassium hydrogen fluorides.

The crystal structure of the ammonium salt of 2-aminomalonic acid.

The salt ammonium 2-aminomalonate (systematic name: ammonium 2-azaniumylpropanedioate), NH4+·C3H4NO4-, was synthesized in diethyl ether from the starting materials malonic acid, ammonia and bromine. The salt was recrystallized from water as colourless blocks. In the solid state, intramolecular medium-strong N-H...O, weak C-H...O and weak C-H...N hydrogen bonds build a three-dimensional network.

The electrophilic methylating agent [SO2Me]+ parent of two cationic species.

Methylfluoride and hydrogen fluoride react with powerful methylating agent [SO2Me]+ in a temperature-dependent addition reaction to form methylated fluorosulfuric acid [FS(OMe)2][Sb2F11] and methylated fluorosulfuric acid methyl ester [FS(OH)(OMe)][SbF6], respectively. The obtained methylated fluorosulfinic acid and methylated fluorosulfinic acid methyl ester were characterized by single X-ray structure analysis and vibrational spectroscopy.

Two Roomtemperature-stable Trications - Triprotonated Triamino- and Tricyanobenzene.

Protonation of 1,3,5-tricyano- and 1,3,5-triaminobenzene was achieved in various superacidic media, resulting in the formation of the respective trinitrilium and triammonium species. Furthermore, the respective N-methyl nitrilium species was synthesized by methylation. Characterization was performed by NMR and vibrational spectroscopy, followed by single-crystal X-ray diffraction analyses of selected species. Fourfold protonation of the amine, which would have led to the triammonium arenium species, could not be achieved. Quantum chemical calculations are employed to enable full vibrational assignment as well to quantify charge localization.

Synthesis and characterization of tetramethylammonium trifluorosulfate.

[Me(4) N](+) [SO(2) F(3) ](-) , the first example of a [SO(2) F(3) ](-) salt, has been prepared from Me(4) NF and SO(2) F(2) . The colorless, microcrystalline solid was characterized by its infrared and Raman spectra. The trigonal bipyramidal structure of C(2v) symmetry of the [SO(2) F(3) ](-) anion is predicted by ab initio calculations. Two oxygen atoms with d(SO)=143.2 pm and one fluorine atom with d(SF)=157.9 pm occupy the equatorial plane. The two fluorine atoms in the axial position with d(SF)=168.5 pm are repulsed by the two oxygen atoms forming a bent axis with ∡(F(ax) SF(ax) )=165.2°.

2-Sulfoethylammonium hexafluoridoantimonate.

The title salt, (C(2)H(8)NO(3)S)[SbF(6)], which contains the protonated form of taurine (2-aminoethanesulfonic acid), was synthesized in anhydrous hydrofluoric acid and recrystallized as colourless block-shaped crystals from liquid SO(2). In the solid state, a three-dimensional network is observed. This is formed by intra- and intermolecular N-H···O, N-H···F and O-H···F hydrogen bonds.