An infrared nuNH scale for weakly basic anions. Implications for single-molecule acidity and superacidity.

The N-H stretching frequencies of tri-n-octylammonium salts are reported for a series of weakly basic anions (A(-)), many of which are the conjugate bases of known strong acids and superacids. Data have been collected primarily in carbon tetrachloride, where Oct(3)N(+)-H...A(-) contact ion pairs are formed. In the more polar solvent 1,2-dichloroethane, some salts form both contact and solvent-separated ion pairs. Salts have also been studied in crystalline form or as oils. In general, the nuNH frequencies decrease in the order fluoroanions > carboranes > oxyanions, reflecting the relative basicities of the anions. By inference, the data reflect differences in the acidity of the corresponding conjugate acids (HA). This qualitative indicator of acid strength is useful because it reflects acidity on an individual molecule basis rather than in bulk. In this respect, it provides a condensed-phase analogy to gas-phase ("intrinsic") acidity and gives insight into the aggregation phenomena that determine bulk acidity. The data also reveal the importance of the chemical stability of conjugate base anions in attaining high acidity and suggest where acids stronger than those presently known may be discovered.

The nature of the H3O+ hydronium ion in benzene and chlorinated hydrocarbon solvents. Conditions of existence and reinterpretation of infrared data.

Salts of the C(3v) symmetric hydronium ion, H(3)O(+), have been obtained in the weakly basic solvents benzene, dichloromethane, and 1,2-dichloroethane. This is made possible by using carborane counterions of the type CHB(11)R(5)X(6)(-) (R = H, Me, Cl; X = Cl, Br, I) because they combine the three required properties of a suitable counterion: very low basicity, low polarizability, and high chemical stability. The existence of the H(3)O(+) ion requires the formation of three more-or-less equivalent, medium-to-strong H-bonds with solvent or anion bases. With the least basic anions such as CHB(11)Cl(11)(-), IR spectroscopy indicates that C(3v) symmetric trisolvates of formulation [H(3)O(+) .3Solv] are formed with chlorocarbon solvents and benzene, the latter with the formation of pi bonds. When the solvents and anions have comparable basicity, contact ion pairs of the type [H(3)O(+).nSolv.Carborane] are formed and close to C(3v) symmetry is retained. The conditions for the existence of the H(3)O(+) ion are much more exacting than previously appreciated. Outside of the range of solvent basicity bounded at the lower end by dichloromethane and the upper end by tributyl phosphate, and with anions that do not meet the stringent requirements of weak basicity, low polarizability of high chemical stability, lower symmetry species are formed. One H-bond from H(3)O(+) to the surrounding bases becomes stronger than the other two. The distortion from C(3v) symmetry is minor for bases weaker than dichloromethane. For bases stronger than tributyl phosphate, H(2)O-H(+)-B type species are formed that are more closely related to the H(5)O(2)(+) ion than to H(3)O(+). IR data allow criteria to be defined for the existence of the symmetric H(3)O(+) ion. This includes a linear dependence between the frequencies of nu(max)(OH) and delta(OH(3)) within the ranges 3010-2536 cm(-1) for nu(max)(OH) and 1597-1710 cm(-1) for delta(OH(3)). This provides a simple way to assess the correctness of the formulation of the proton state in monohydrated acids. In particular, the 30-year-old citation classic of the IR spectrum believed to arise from H(3)O(+) SbCl(6)(-) is re-interpreted in terms of (H(2)O)(x)().HSbCl(6) hydrates. The correctness of the hydronium ion formulation in crystalline H(3)O(+)A(-) salts (A(-) = Cl(-), NO(3)(-)) is confirmed, although, when A(-) is a fluoroanion, distortions from C(3)(v)() symmetry are suggested.

The structure of the H3O+ hydronium ion in benzene.

Infrared, X-ray structural, 1H NMR, and computational evidence for pi-solvation of H3O+ by benzene molecules is presented. A salt with a discrete [H3O.3benzene]+ cation can be isolated using a very weakly interacting carborane counterion, CHB11Cl11-. pi-Arene solvation of H3O+ explains the solubility of this salt in benzene solution. Similar results are indicated for the "Zundel-type" H5O2+ ion. These findings suggest structures for the active protonating species when strong acids are used as catalysts in arene solvents containing trace water. They are also relevant to structures that may be present in biological proton transport.

Uniform-sign cross-peak double-quantum-filtered correlation spectroscopy.

We detail the uniform-sign cross-peak double-quantum-filtered correlation spectroscopy (UC2QF COSY) experiment, a new through-bond correlation method for disordered solids. This experiment is a refocused version of the popular double-quantum-filtered correlation spectroscopy experiment in liquids. Its key feature is that it provides in-phase and doubly absorptive line shapes, which renders it robust for chemical shift correlation in solids. Both theory and experiment point to distinct advantages of this protocol, which are illustrated by several experiments under challenging conditions, including fast magic-angle spinning (30kHz), anisotropic molecular motion, and (13)C correlation spectroscopy at the natural abundance isotope level.

Synthesis of the C59N+ carbocation. A monomeric azafullerene isoelectronic to C60.

A heterofullerene isoelectronic to C60 is reported. The azafullerenium cation C59N+ can be isolated in good yield as a carborane salt via the two-electron oxidation of the C-C bond of (C59N)2 dimer. [C59N][Ag(CB11H6Cl6)2] has been characterized by electronic, IR, Raman, and 13C NMR spectroscopies, MALDI spectrometry, DFT calculations, and X-ray crystallography.

Isolating benzenium ion salts.

When partnered with carborane anions, arenium ions are remarkably stable. Previously investigated only at subambient temperatures in highly superacidic media, protonated benzene is readily isolated as a crystalline salt, thermally stable to >150 degrees C. Salts of the type [H(arene)][carborane] have been prepared by protonating benzene, toluene, m-xylene, mesitylene, and hexamethylbenzene with the carborane superacid H(CB(11)HR(5)X(6)) (R = H, Me; X = Cl, Br). They have been characterized by elemental analysis, X-ray crystallography, NMR and IR methods. Solid-state (13)C NMR spectra are similar to those observed earlier in solution, indicating that lattice interactions are comparable to solution solvation effects. The acidic proton(s) of the arenium cations interact weakly with the halide substituents of the anion via ion pairing. This is reflected in the dependence of the C-H stretching frequency on the basicity of the carborane anion. Bond lengths in the arenium ions are consistent with predominant cyclohexadienyl cation character, but charge distribution within the cation is less well represented by this resonance form. Structural and vibrational comparison to theory is made for the benzenium ion (C(6)H(7)(+)) with density functional theory at B3LYP/6-31G and B3P86/6-311+G(d,p) levels. The stability of these salts elevates arenium ions from the status of transients (Wheland intermediates) to reagents. They have been used to bracket the solution-phase basicity of C(60) between that of mesitylene and xylene.

Molecular structure of the solvated proton in isolated salts. Short, strong, low barrier (SSLB) H-bonds.

Large, inert, weakly basic carborane anions of the icosahedral type CHB(11)R(5)X(6)(-) (R = H, Me; X = Cl, Br) allow ready isolation and structural characterization of discrete salts of the solvated proton, [H(solvent)(x)][CHB(11)R(5)X(6)], (solvent = common O-atom donor). These oxonium ion Brønsted acids are convenient reagents for the tuned delivery of protons to organic solvents with a specified number of donor solvent molecules and with acidities leveled to those of the chosen donor solvent. They have greater thermal stability than the popular [H(OEt(2))(2)][BAr(F)] acids based on fluorinated tetraphenylborate counterions because carborane anions can sustain much higher levels of acidity. When organic O-atom donors such as diethyl ether, tetrahydrofuran, benzophenone, and nitrobenzene are involved, the coordination number of the proton (x) in [H(solvent)(x)()](+) is two. A mixed species involving the [H(H(2)O)(diethyl ether)](+) ion has also been isolated. These solid-state structures provide expectations for the predominant molecular structures of solvated protons in solution and take into account that water is an inevitable impurity in organic solvents. The O.O distances are all short, lying within the range from 2.35 to 2.48 A. They are consistent with strong, linear O.H.O hydrogen bonding. Density functional theory calculations indicate that all H(solvent)(2)(+) cations have low barriers to movement of the proton within an interval along the O.H.O trajectory, i.e., they are examples of so-called SSLB H-bonds (short, strong, low-barrier). Unusually broadened IR bands, diagnostic of SSLB H-bonds, are observed in these H(solvent)(2)(+) cations.

Establishing through-bond connectivity in solids with NMR: structure and dynamics in HC(60)(+).

We present a novel nuclear magnetic resonance experiment for establishing through-bond connectivity in solids using scalar coupling-driven correlation. This method, a variant of the popular double-quantum-filtered correlation spectroscopy experiment in liquids, is robust under fast magic-angle-spinning conditions and in the presence of dynamics. In HC(60)(+), where anisotropic molecular motion renders through-space dipolar-driven correlation ineffective, this through-bond correlation method answers a significant structural question by accurately identifying the direct bond between the protonated sp(3) hybridized carbon site and the sp(2) hybridized cationic site.

Crystallographic evidence for a free silylium ion.

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes)3Si][H-CB11Me5Br6].C6H6 (where Mes is 2,4,6-trimethylphenyl). Free (Mes)3Si+ cations are well separated from the carborane anions and benzene solvate molecules. Ortho-methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The downfield 29Si nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in "gas phase" calculations, indicating that three-coordination can be preserved in all phases.

Et(2)Al(+) alumenium ion-like chemistry. synthesis and reactivity toward alkenes and alkene oxides.

Inert weakly coordinating carborane anions, CB(11)H(6)X(6)(-) (X = Cl, Br), allow access to the long sought, highly electrophilic diethylaluminum moiety in Et(2)Al(CB(11)H(6)X(6)). X-ray crystallography reveals ion-like structural features reminiscent of the corresponding trialkylsilylium species. Et(2)Al(CB(11)H(6)X(6)) is a potent catalyst for the electrophilic ethenation of benzene, the polymerization of cyclohexene oxide, and the oligomerization of ethene to a low molecular weight, highly branched product.

Artifacts in the electron paramagnetic resonance spectra of C60 fullerene ions: inevitable C120O impurity.

Aspects of the electron paramagnetic resonance (EPR) spectra of C60n- fulleride ions (n = 2, 3) and the EPR signal observed in solid C60 are reinterpreted. Insufficient levels of reduction and the unrecognized presence of C120O, a ubiquitous and unavoidable impurity in air-exposed C60, have compromised most previously reported spectra of fullerides. Central narrow line width signals ("spikes") are ascribed to C120On- (n = odd). Signals arising from axial triplets (g approximately 2.0015, D = 26-29 G) in the spectrum of C602- are ascribed to C120On- (n = 2 or 4). Their D values are more realistic for C120O than C60. Less distinct signals from "powder" triplets (D approximately 11 G) are ascribed to aggregates of C120On- (n = odd) arising from freezing nonglassing solvents. In highly purified samples of C60, we find no evidence for a broad approximately 30 G signal previously assigned to a thermally accessible triplet of C60(2-). The C60(2-) ion is EPR-silent. Signals previously ascribed to a quartet state of the C60(3-) ion are ascribed to C120O4-. Uncomplicated, authentic spectra of C60- and C60(3-) become available when fully reduced samples are prepared under strictly anaerobic conditions from freshly HPLC-purified C60. Solid off-the-shelf C60 has an EPR signal (g approximately 2.0025, DeltaH(pp) approximately 1.5 G) that is commonly ascribed to the radical cation C60*+. This signal can be reproduced by exposing highly purified, EPR-silent C60 to oxygen in the dark. Doping C60 with an authentic C60*+ salt gives a signal with much greater line width (DeltaH(pp) = 6-8 G). It is suggested that the EPR signal in air-exposed samples of C60 arises from a peroxide-bridged diradical, *C60-O-O-C60* or its decomposition products rather than from C60*+. Solid-state C60 is more sensitive to oxygen than previously appreciated such that contamination with C120O is almost impossible to avoid.