Preparation and properties of alkyl- and aryl-substituted closo-closo- and closo-nido-hydrazocarbaboranes

Abstract

closo-closo-1,1′-Azo-o-carbaboranes RCB₁₀H₁₀CNNCB₁₀H₁₀CR react with alkyl- and aryl-lithiums to afford the derivatives RCB₁₀H₁₀CNHNR′CB₁₀H₁₀CR (R = H, Me, or Ph; R′= alkyl or aryl group)(3) along with the hydrazo-carbaboranes RCB₁₀H₁₀CNHNHCB₁₀H₁₀CR (2). Further substitution of these compounds with alkyl halides resulted in the formation of the NN′-disubstituted compound RCB₁₀H₁₀CNR″NR′CB₁₀H₁₀CR (4). The closo-closo compounds (2) and (4) can be converted into the corresponding closo-nido ones in the presence of piperidine base. By means of variable-temperature ¹H n.m.r. spectroscopy, the free-energy barriers to conformational interconversion have been estimated [15.5 kcal mol^–1 for (3e; R = Me, R′= Et), >22 kcal mol^–1 for (4b; R = H, R′= R″= Et), and (4e; R = Me, R′= R″= Et)]. In addition, the existence of interconvertible diastereoisomers has been evidenced for RCB₁₀H₁₀CNMeNMeCB₉H₁₀CR^–(6a; R = H) and (6b; R = Me) for which the free energies of activation are also estimated.