TRANSMISSION OF ELECTRONIC EFFECTS BY ICOSAHEDRAL CARBORANES - SKELETAL C-13 CHEMICAL-SHIFTS AND ULTRAVIOLET-VISIBLE SPECTRA OF SUBSTITUTEDARYL-P-CARBORANES (1,12-DICARBA-CLOSO-DODECABORANES)

A series of para-carboranes (1,12-dicarba-closo-dodecaboranes) substit uted at one or both carbon atoms by phenyl groups bearing a range of e lectron-releasing or -withdrawing substituents (X = NMe2, NH2, OH, OMe , Me, CF3 or NO2), principally at the para positions, has been prepare d. The carbon-13 NMR shifts of the monoaryl compounds (4-XC6H4)CB10H10 CH showed a linear increase in shielding of the unsubstituted (antipod al) cage carbon, C12, and a linear decrease at the substituted positio n, C1, with increasing electron-donating power of the substituent repr esented by its Hammett sigma(P) value. Purely inductive, sigma(I), or resonance sigma(R), parameters gave less satisfactory plots. The diary l series of 4-nitrophenyl derivatives showed the same relationships. C alculations of electron densities showed that the C-13 NMR shifts corr espond with charge density in the tangential p orbitals. The UV absorp tions of the monoaryl p-carboranes have substantial red-shifts compare d with the corresponding substituted benzenes, while those of the 4-ni trophenyl compounds having Lt-dimethylamino or 4-oxide anion substitue nts in the second phenyl group extend into the visible. The colour is attributed to changes in the nitro n --> pi absorption rather than co njugation 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-hexaoxacyclooctadec ane). The results establish that p-carborane transmits electronic effe cts, perhaps as a formally unsaturated system or possibly in a similar manner to the staffanes (bicyclo[1.1.1]pentanes) and other formally s aturated hydrocarbon linkages.