Transmission of electronic effects by icosahedral carboranes; skeletal carbon-13 chemical shifts and ultraviolet–visible spectra of substituted aryl-p-carboranes (1,12-dicarba-closo-dodecaboranes)

Abstract

A series of para-carboranes (1,12-dicarba-closo-dodecaboranes) substituted at one or both carbon atoms by phenyl groups bearing a range of electron-releasing or -withdrawing substituents (X = NMe₂, NH₂, OH, OMe, Me, CF₃ or NO₂), principally at the para positions, has been prepared. The carbon-13 NMR shifts of the monoaryl compounds (4-XC₆H₄)CB₁₀H₁₀CH showed a linear increase in shielding of the unsubstituted (antipodal) cage carbon, C12, and a linear decrease at the substituted position, C1, with increasing electron-donating power of the substituent represented by its Hammett σ_P value. Purely inductive, σ_I, or resonance σ_R, parameters gave less satisfactory plots. The diaryl series of 4-nitrophenyl derivatives showed the same relationships. Calculations of electron densities showed that the ¹³C NMR shifts correspond with charge density in the tangential p orbitals. The UV absorptions of the monoaryl p-carboranes have substantial red-shifts compared with the corresponding substituted benzenes, while those of the 4-nitrophenyl compounds having 4-dimethylamino or 4-oxide anion substituents in the second phenyl group extend into the visible. The colour is attributed to changes in the nitro n → π* absorption rather than conjugation or intermolecular charge transfer. Two anhydrous polymorphs and a hydrate of the (18-crown-6)potassium salt of this nitrophenoxide have been identified (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane). The results establish that p-carborane transmits electronic effects, perhaps as a formally unsaturated system or possibly in a similar manner to the staffanes (bicyclo[1.1.1]pentanes) and other formally saturated hydrocarbon linkages.