Unexpected photochemistry and charge-transfer complexes of [CB₁₁H₁₂]⁻carborane

Abstract

Although the [CB 11 H 12 ] 2 carborane does not exhibit an absorption band in UV, its triplet excited state can be generated upon 308 nm laser excitation; also unexpectedly carborane acts as electron donor forming a charge transfer complex with methylviologen that upon illumination gives rise to viologen radical cation.Since the initial report of Reed and co-workers describing the superacidity of carboranes in their protonic form, there is a renewed interest in this molecule. 1-4 Carborane CB 11 H 12 2 results from the apical isomorphous substitution of a boron in dodecaborane with a carbon atom. 5 As result of this substitution and the extra carbon electron, a negative charge is introduced in the system and the carborane becomes an anionic species. Carboranes are among the less nucleophilic species due to electron delocalization over the structure. [1][2][3][4] The chemistry and properties of carboranes have recently been reviewed. [5][6][7][8][9] Carborane does not exhibit any absorption band in the UV/Vis region and, therefore, no photochemistry is expected upon irradiation in this wavelength region. 5 Also, carborane cannot be oxidized in the potential range commonly available in conventional organic solvents, 5 although oxidation could occur at higher potentials. In sharp contrast with these two well-known properties of CB 11 H 12 2 carborane, herein we report the unexpected observation of photochemical activity of Cs [CB 11 H 12 ] and the formation of photoactive charge transfer complexes with a strong electron acceptor. Optical UV/Vis spectrum of Cs [CB 11 H 12 ] in acetonitrile solution shows no peaks in the 200-800 nm range and only an absorption tail growing to shorter wavelengths is observed (Fig. 1) (Fig. 2). The transient spectrum consists of a broad band expanding from 350 to 650 nm and peaking at 420 nm. The temporal profile of the signal was coincident in the complete spectral range, this being consistent with the photochemical generation of a single transient species (inset of Fig. 2). The signal decay fits well to a first-order kinetics with a half life of 16.2 ms. The signal from Cs [CB 11 H 12 ] generated upon the laser flash is quenched by oxygen (inset of Fig. 2). Taken together, these observations are consistent with the unexpected characterization of carborane triplet excited state. Since carboranes do not possess absorption bands in the UV, we performed an energy-dependence study to determine whether or COMMUNICATION www.rsc.org/chemcomm | ChemCommThis journal is ß

Cu-MOFs as active, selective and reusable catalysts for oxidative C–O bond coupling reactions by direct C–H activation of formamides, aldehydes and ethers

“…20% yield, whereupon 1 mmol of 2-hydroxyacetophenone and 1 mL of DMF were added at the same reaction temperature. [Cu-2pymo) 2 ] was selected as catalyst since we know from previous works that this material is highly selective to CmHP at low cumene conversions 11,12 . Under these conditions, the carbamate product 3 was quantitatively formed in 97% yield after 1.5 h of reaction.…”

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