Synthesis and characterisation of labelled diphenylcarboranes

The synthesis, by either acid-catalysed electrophilic substitution or recapitation, and characterisation of four new B-labelled diphenyl carboranes is described. Two of these, 1,2-Ph₂-9,12-I₂-1,2-closo-C₂ B₁₀H₈ (1) and 1,2-Ph₂-3-Me-1,2-closo-C₂B₁₀H₉ (2) were subjected to crystallographic study, whilst the 3-Et (3) and 3-F (4) analogues of 2 were studied spectroscopically. Decapitation of the mono-iodo analogue of I affords [5-I-7,8-Ph₂-7,8-nido-C₂B₉ H₉]^-, isolated as [HNEt₃]⁺ (5a) [HNMe₃]⁺ (5b) and [C₆H₅CH₂NMe₃]⁺ (5c) salts, the last of which was subjected to crystallographic analysis. Decapitation of 3 and 4 selectively removes B6, yielding the 3-labelled nido-carboranes [3-Et-7,8-Ph₂-7,8-nido-C₂B₉ H₉]^- and [3-F-7,8-Ph₂-7,8-nido-C₂B₉ H₉]^-, respectively, both isolated as [HNMe₃]⁺ salts (6a and 7). The ethyl species was also prepared as the [HNEt₃]⁺ salt (6b), and was structurally characterised. Salts 5b, 6a and 7 are all afforded in good yield and the anions are ideal candidates for subsequent deprotonation and metallation, which should result in low-temperature isomerisation of the transient 3,1,2-MC₂B₉ species thereby produced, and thereby yield important, robust, mechanistic information on the rearrangement process. © 2003 Elsevier Science Ltd. All rights reserved.