Synthesis, structural characterization, and reactivity of 'Carbons-adjacent' nido- and arachno-carborane anions of the C2B10 systems and their metal complexes

Three cage carbons-linked o-carboranes mu-1,2-[o-C6H4(CH2)(2)]-1,2-C2B10H10 (1), mu-1,2-[1,8-C-10 H-6(CH2)(2)]-1,2-C2B10H10 (2), and mu-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 [{mu-1,2-[o-C6H4(CH2)(2)]-1,2-C2B10H10}(2)-Na-4(THF)(6)](n) (4), [{mu-1,2-[o-C6H4(CH2)(2)]-1,2-C2B10H10](2)K-3(18-crown-6)(2)][K(18-crown-6)(CH3-CN)(2)] (5), and [{mu-1,2-[1,8-C10H6(CH2)(2)]-1,2-C2B10H10}(2)Na-4(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 [{mu-1,4-[1,1'-(C6H4)(2)-2,2'-(CH2)(2)]-1,4-C2B10H10}M-2(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 [{mu-1,2-[o-C6H4(CH2)(2)]-1,2-C2B10H10}Li-4(THF)(6)](2) (13) and [{mu-1,2-[1,8-C10H6-(CH2)(2)]-1,2-C2B10H10}Li-4(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.