12-Vertex Zwitterionic Bis-phosphonium-nido-carborates through Ring-Opening Reactions of 1,2-Diphosphetanes

Carborane-substituted 1,2-diphosphetanes (Ia,b) react with elemental lithium in THF with cleavage of the P-P bond to give a deep red solution from which, in the case of Ia, red crystals of a lithiated intermediate, [{1-Li(THF)PtBu-6-PtBu-4,1,6-closo-Li(THF)C₂ B₁₀ H₁₀ }{Li(THF)₃ }]₂ ⋅2 THF (2 a), are obtained. The compound is dimeric, C₂ -symmetric and contains six lithium and four phosphorus atoms. Two lithium atoms cap the six-membered C₂ B₄ faces, resulting in two 13-vertex closo-clusters (according to Wade's rules) with docosahedral geometry. The addition of methyl iodide resulted in the formation of zwitterionic bis-phosphonium-nido-carborates 7,10-bis(tert-butyldimethylphosphonium)dodecahydro-7,10-dicarba-nido-dodecaborate(2-) (1 a) and 7,10-bis(N,N-diisopropylaminodimethylphosphonium)dodecahydro-7,10-dicarba-nido-dodecaborate(2-) (1 b) in moderate to good yields. Compounds 1 a and 1 b exhibit short C_cluster -P bonds and large C_cluster ⋅⋅⋅C_cluster distances in the solid state. Further insight into the ring opening and reduction potential of the alkyl halide was obtained from methylation reactions of different 1,2-bis-phosphinocarboranes. The reaction of rac-/meso-1,2-bis(tert-butylmethylphosphino)-1,2-dicarba-closo-dodecaborane(12) (3 a) with two equivalents of methyl iodide also resulted in the formation of 1 a (as shown by NMR spectroscopy), whereas the reaction of 1,2-bis(diphenylphosphino)-1,2-dicarba-closo-dodecaborane(12) with methyl triflate afforded the phosphonium salt 1-methyl-diphenylphosphonium-2-diphenylphosphino-1,2-dicarba-closo-dodecaborane(12) triflate (4) without reduction of the cluster.