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Cocrystallisation of ortho-metaand para-carboranes with hmpa (hexamethylphosphoramide) results in the isolation of 1 : 1 carborane:hmpa adducts which provide the first definitive X-ray structures of the unsubstituted carboranes as part of three very different C-H-0 hydrogen-bonded supramolecular structures.

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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 = NMe 2 , NH 2 , OH, OMe, Me, CF 3 or NO 2 ), principally at the para positions, has been prepared. The carbon-13 NMR shifts of the monoaryl compounds (4-XC 6 H 4 )CB 10 H 10 CH showed a linear increase in shielding of the unsubstituted (antipodal) cage carbon, C12, and a linear decrease at the substituted position, C1, with increasing electrondonating 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 13 C 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.* Michl and co-workers 17 have effected coupling between p-carboranylcopper() and aryl iodides using chloride bis(triphenylphosphine)palladium() as catalyst. We have observed efficient phenylation of this copper intermediate with idobenzene using triphenylphosphine instead of pyridine, and suggest that catalysis by the palladium complex may be due to exchange with this ligand rather than to the palladium. † 1,7-Bis(3-nitrophenyl)-m-carborane is an exception which gives several products under these conditions; all are destroyed by hydrogenation at higher temperatures.

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