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 μ -1,2-[ o -C 6 H 4 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 ( 1 ), μ -1,2-[1,8-C 10 H 6 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 ( 2 ), and μ -1,2-[1,1‘-(C 6 H 4 ) 2 -2,2‘-(CH 2 ) 2 ]-1,2-C 2 B 10 H 10 ( 3 ) were prepared from the reactions of Li 2 C 2 B 10 H 10 with o -C 6 H 4 (CH 2 Br) 2, 1,8-C 10 H 6 (CH 2 Cl) 2, and 1,1‘-(C 6 H 4 ) 2 -2,2‘-(CH 2 Br) 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 -C 6 H 4 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 } 2 Na 4 (THF) 6 ] n ( 4 ), [{ μ -1,2-[ o -C 6 H 4 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 } 2 K 3 (18-crown-6) 2 ][K(18-crown-6)(CH 3 CN) 2 ] ( 5 ), and [{ μ -1,2-[1,8-C 10 H 6 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 } 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 [{ μ -1,4-[1,1‘-(C 6 H 4 ) 2 -2,2‘-(CH 2 ) 2 ]-1,4-C 2 B 10 H 10 }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 [{ μ -1,2-[ o -C 6 H 4 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 }Li 4 (THF) 6 ] 2 ( 13 ) and [{ μ -1,2-[1,8-C 10 H 6 (CH 2 ) 2 ]-1,2-C 2 B 10 H 10 }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.