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.     

Water-soluble nitric oxide-releasing gold nanoparticles

The synthesis and characterization of water-soluble nitric oxide (NO)-releasing monolayer-protected gold clusters (MPCs) are reported. Tiopronin-protected MPCs ( approximately 3 nm) were functionalized with amine ligands and subsequently exposed to 5 atm of NO to form diazeniumdiolate NO donors covalently bound to the gold MPC. Diazeniumdiolate formation conditions, NO-release, and nanoparticle stability were examined as a function of the structure of the protecting ligand, pH, and storage time. Despite their aqueous solubility, proton-initiated decomposition of the diazeniumdiolate-modified Tio-MPCs resulted in only modest NO-release (<0.023 micromol/mg) for short durations (<1.5 h). To increase the NO storage capacity of gold nanoparticles, polyamine-stabilized MPCs ( approximately 5 nm) were synthesized with significantly enhanced NO-release properties (0.386 micromol/mg) and durations (up to 16 h). Transmission electron microscopy, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, elemental analysis, and UV-vis spectroscopy were used to characterize both nanoparticle systems before and after NO exposure. The MPCs represent the smallest water-soluble NO-release nanoparticles to date (3-5 nm).     

Reactive oxygen species produced upon photoexcitation of sunscreens containing titanium dioxide (an EPR study)

Commercial sunscreen products containing titanium dioxide were irradiated with lambda>300 nm and the formation of oxygen- (.OH, O2.-/.OOH) and carbon-centered radicals was monitored by EPR spectroscopy and spin trapping technique using 5,5-dimethyl-1-pyrroline N-oxide, alpha-phenyl-N-tert-butylnitrone (PBN), alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone as spin traps, and free nitroxide radical 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl. The photoinduced production of singlet oxygen was shown by 4-hydroxy-2,2,6,6-piperidine. The generation of reactive oxygen radical species upon irradiation of sunscreens significantly depends on their composition, as the additives present (antioxidants, radical-scavengers, solvents) can transform the reactive radicals formed to less harmful products. The continuous in situ irradiation of titanium dioxide powder, recommended for cosmetic application, investigated in different solvents (water, dimethyl sulfoxide, isopropyl myristate) resulted in the generation of oxygen-centered reactive radical species (superoxide anion radical, hydroxyl and alkoxyl radicals).     

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.     

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 Br?nsted 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.