Synthesis of 10-Methylsulfide and 10-Alkylmethylsulfonium nido-Carborane Derivatives: B-H•••π Interactions between the B-H-B Hydrogen Atom and Alkyne Group in 10-RC≡CCH2S(Me)-7,8-C2B9H11

The 10-dimethylsulfonium derivative of nido-carborane 10-Me2S-7,8-C2B9H11 was prepared by reaction of the protonated form of nido-carborane with dimethylsulfide in toluene. Its partial demethylation with sodium amide in toluene gave the corresponding 10-methylsulfide derivative [10-MeS-7,8-C2B9H11](-). This 10-methylsulfide derivative was alkylated with various alkylating agents to produce a series of 10-alkylmethylsulfonium derivatives 10-R(Me)S-7,8-C2B9H11 (R = Et, Pr, Bu, CH2CH=CH2, CH2Ph, CH2CCH, CH2CCPh, CH2CCSiMe3). The B-11 NMR spectra of the latter three compounds containing an acetylene group demonstrate unusually strong signal splitting that can be explained by strong intramolecular B-H center dot center dot center dot pi(C equivalent to C) interactions. The existence of such interactions in the solid state was supported by the X-ray crystal structure of 10-HC equivalent to CCH2(Me)S-7,8-C2B9H11 with a short B-H...C(C equivalent to C) distance of 2.658 angstrom. This is the first example of B-H center dot center dot center dot pi(C equivalent to C) hydrogen bonding.