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Synthesis, Structural Characterization, and Reactivity of ‘Carbons-Adjacent' nido- and arachno-Carborane Anions of the C2B10 Systems and Their Metal Complexes

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posted on 30.10.2002, 00:00 authored by Guofu Zi, Hung-Wing Li, Zuowei Xie
Three cage carbons-linked o-carboranes μ-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10 (1), μ-1,2-[1,8-C10H6(CH2)2]-1,2-C2B10H10 (2), and μ-1,2-[1,1‘-(C6H4)2-2,2‘-(CH2)2]-1,2-C2B10H10 (3) were prepared from the reactions of Li2C2B10H10 with o-C6H4(CH2Br)2, 1,8-C10H6(CH2Cl)2, and 1,1‘-(C6H4)2-2,2‘-(CH2Br)2, respectively. The cage carbon atom adjacency of 1 and 2 was maintained during the reactions with excess Na or K metal in THF, leading to high yields of ‘carbons-adjacent' nido-carborane group 1 complexes [{μ-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10}2Na4(THF)6]n (4), [{μ-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10}2K3(18-crown-6)2][K(18-crown-6)(CH3CN)2] (5), and [{μ-1,2-[1,8-C10H6(CH2)2]-1,2-C2B10H10}2Na4(THF)6]n (8), respectively. In sharp contrast, reaction of 3 with excess Li or Na metal resulted in the complete cleavage of the cage carbon−carbon bond, giving [{μ-1,4-[1,1‘-(C6H4)2-2,2‘-(CH2)2]-1,4-C2B10H10}M2(THF)3]n (M = Li (10), Na (11)), in which the two cage carbon atoms are in para positions. Surprisingly, Li metal was able to reduce directly 1 and 2 to the ‘carbons-adjacent' arachno-carborane species, forming [{μ-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10}Li4(THF)6]2 (13) and [{μ-1,2-[1,8-C10H6(CH2)2]-1,2-C2B10H10}Li4(THF)6]2 (17), respectively. These results showed that the bridge length controls the formation of ‘carbons-adjacent' or ‘carbons-apart' carborane anions, and the ‘carbons-adjacent' carboranes are more reactive than the ‘carbons-apart' isomers. All these carborane anions are very air- and moisture-sensitive. Oxygen oxidizes them back to the neutral o-carboranes. Hydrolysis converts both nido-carborane dianions and arachno-carborane tetraanions into the corresponding carborane monoanions by one- or three-proton uptake from water molecules. All complexes were fully characterized by various spectroscopic data and elemental analyses. Some were subjected to single-crystal X-ray analyses.