Synthesis and Reactivity of [closo-1-CB9H9-1-N2]: Functional Group Interconversion at the Carbon Vertex of the {closo-1-CB9} Cluster

The dinitrogen derivative [closo-1-CB9H9-1-N-2] (1) was prepared from amine [closo-1-CB9H9-1-NH3](2) and reacted with three types of nucleophiles: activated arenes (phenolate and aniline), divalent sulfur compounds (Me2S and Me2NCHS), and pyridine, giving products of substitution at C-cage. The reaction of 1 with pyridine gave all four isomers 11a-11d, indicating the Gomberg-Bachmann mechanism, which involves radical anion [closo-1-CB9H9](-center dot) (26). The radical and also closed-shell electrophilic aromatic substitution mechanisms were probed with the aid of DFT and MP2 computational methods and compared to those of phenylation of pyridine. Overall, experimental results supported by computational analysis suggest two mechanisms for the substitution of the N-2 group in 1: (i) thermal heterolytic cleavage of the C-cage-N bond and the formation of electrophilic carbonium ylide [closo-1-CB9H9] (19) and (ii) electron-transfer-induced homolytic cleavage of the C-cage-N bond and the formation of 26. Decomposition of 1 in MeCN is believed to proceed by the nonradical mechanism involving formation of the ylide 19 as the rate-determining step with experimental activation parameters Delta H-double dagger = 38.4 +/- 0.8 kcal mol(-1) and Delta S-double dagger = 44.5 +/- 2.5 cal mol(-1) K-1. The electron-transfer-induced formation of 26 is consistent with the relatively high reduction potential of 1 (E-pc = -0.54 V), which is more cathodic than that of PhN2+ by 0.38 V. Transformations of the phenol 8a and the MONS adduct 10 were demonstrated by O-methylation of the former and hydrolysis of 10 followed by S-alkylative cyclization. Direct products and their derivatives were investigated by UV-vis spectroscopy and analyzed with the ZINDO computational method.