Pressure induced superconductivity in a CeRhSi3single crystal-the high pressure study.

Pressure induced superconductivity in non-centrosymmetric CeRhSi3and CeIrSi3compounds has attracted significant attention of the scientific community since its discovery 15 years ago. Up-to-date, all reported experimental results were obtained employing the hybrid-cylinder piston pressure cells with a maximum reachable pressure of 3 GPa. Present study focuses on the superconducting state at higher, so far unreported, pressures using the Bridgman anvil cell and a CeRhSi3single crystal synthesized by the Sn-true-flux method. The initial increase of superconducting critical temperature from 0.4 K at 1.1 GPa to 1.1 K at 2.4 GPa is followed by a gradual suppression of superconducting state upon increasing the pressure above 3.0 GPa, forming a typical dome. The pressure induced superconductivity is expected to be completely suppressed in the pressure region between 4.5 and 5.0 GPa. Temperature dependence of electrical resistivity in constant magnetic fields and high pressures, as well as the magnetoresistance measurements, reveal a large critical field, exceeding 19 T at 0.6 K and 2.4 GPa, sharply decreasing receding the superconductivity dome. The previously reportedT-pandH-Tphase diagrams are completed by our high-pressure data and discussed in the frame of previous results.

Twisted, Z-shaped perylene bisimide.

The first derivative of a new class of perylene bisimide chromophores, N,N'-bis(octyl)-3,9-bis(phenyl)perylene-1,2,7,8-tetracarboxyl bisimide, 1, has been synthesized and its fundamental photophysical and electrochemical properties assessed. The extended, Z-shaped structure was achieved by use of the classic photoenolization of an o-methylbenzophenone analogue, 1,5-dibenzoyl-9,10-dihydroanthracene, and in situ Diels-Alder trapping of the resulting o-xylylenol intermediates with N-octylmaleimide. Subsequent dehydration and aromatization of the resulting bisadduct afforded 1. In dichloromethane, bisimide 1 has an absorption lambdamax at 491 nm (epsilon = 29,600 M-1 cm-1), a fluorescence lambdamax at 517 nm with a high quantum yield (Phi = 0.70), and a single-exponential fluorescence decay (tau = 5.01 ns). Pure crystals of 1 have red emission, suggesting exciplex formation in the solid state. X-ray crystallographic analysis of 1 revealed significant twisting of its perylene core.

An addition to the oxoacid family: H2B12(OH)12.

The acid H(2)B(12)(OH)(12) can be isolated as a crystalline solid by protonation of the hydroxylated borane anion, B(12)(OH)(12)(2)(-). This acidic compound has low solubility in water, conducts protons in the solid state, and has thermal stability to a temperature of 400 degrees C. The conductivity mechanism is a Grotthuss mechanism with a low activation enthalpy (9-13 kcal/mol). This new acid represents an addition to the class of oxoacids, of which sulfuric and phosphoric acid are the most prominent examples.

Mixed halo/hydroxy-carborane anions: thermally stable platforms for hydronium ion isolation.

Hydroxylation of the robust, weakly coordinating hexahalo-carborane anion system, CB(11)H(6)Br(6)(-1), produces a new class of anion with mixed halo/hydroxyl substituents, HCB(11)(OH)(5)Br(6)(-1) which can be used to isolate a number of hydronium cation salts including an 'ice tautomer' composed of hydronium cation, anion hydroxyl groups and coordinated water molecules.

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.

Optimizing the least nucleophilic anion. A new, strong methyl(+) reagent.

The icosahedral carborane anions H-CB11X6H5- (X = Cl, Br, I) are among the most inert, least coordinating, and least basic anions known. These properties are enhanced by 2,3,4,5,6-pentamethylation with methyl triflate. The resulting anions, H-CB11X6Me5-, are more inert than their unmethylated precursors, have improved NMR handles, and their salts have higher solubility in low dielectric media. They sustain superacidity in H(H-CB11X6Me5). Protonated benzene has been isolated and characterized by X-ray crystallography, moving Wheland intermediates from the status of spectroscopically observable transients to weighable reagents. The new anions sustain extreme Lewis acidity in silylium ion-like R3Si(H-CB11X6Me5) species. Treatment of Et3Si(H-CB11Br6Me5) with methyl triflate leads to a new methyl+ reagent CH3(H-CB11Br6Me5) that is more potent than methyl triflate. It methylates benzene without heating or acid catalysis to give the toluenium ion. The H-CB11X6Me5- anions come as close as any to the concept of a univeral weakly coordinating anion and, with cheaper starting materials now available, promise to become specialty chemicals of wide usage.