Synthesis, structure, reactivity, and thermal isomerization of boron-substituted 13-vertex cobaltacarboranes (eta5-Cp)Co(eta6-R2C2B10Me8H2) (R = H, Et)

Reduction of boron-substituted carboranes o-R2C2B10Me8H2 (R = H, Et), thermal isomerization, and nucleophilic reaction of the resultant 13-vertex cobaltacarboranes were studied. Reaction of o-C2B10Me8H4 (1) with excess potassium metal in tetrahydrofuran (THF) gave, after recrystallization from a THF solution of 18-crown-6 ether, [[K(18-crown-6)(THF)2][K(18-crown-6)]][[4-(18-crown-6)-2,3,5,8,9,11,12,13-Me8-4,1,6-KC2B10H4]2] (2) in 78% yield. Interaction of 1 with excess sodium or potassium metal in THF, followed by treatment with CoCl2/CpNa and then aerobatic oxidation, afforded two boron-substituted 13-vertex cobaltacarboranes, 4-Cp-2,3,5,8,9,11,12,13-Me8-4,1,6-CoC2B10Me8H4 (3) and 4-Cp-2,3,5,9,10,11,12,13-Me8-4,1,6-CoC2B10Me8H4 (4), in 15% and 8% yield, respectively. Subsequently, thermal isomerization of 3 and 4 yielded another two new isomers, 4-Cp-2,3,5,6,8,11,12,13-Me8-4,1,9-CoC2B10Me8H4 (5) and 4-Cp-2,3,5,6,7,11,12,13-Me8-4,1,9-CoC2B10Me8H4 (6). Treatment of 3 or 4 with strong bases such as nBuLi and MeLi generated unexpected nucleophilic substitution products 4-nBuCp-2,3,5,8,9,11,12,13-Me8-4,1,6-CoC2B10Me8H4 (7), 4-nBuCp-2,3,5,9,10,11,12,13-Me8-4,1,6-CoC2B10Me8H4 (8a), and 4-MeCp-2,3,5,9,10,11,12,13-Me8-4,1,6-CoC2B10Me8H4 (8b) in good yields. Under the same reaction conditions, however, only one 13-vertex cobaltacarborane, 4-Cp-1,9-Et2-2,5,6,7,8,11,12,13-Me8-4,1,9-CoC2B10Me8H4 (10), was isolated when o-Et2C2B10Me8H2 (9) was used as the starting material. Complex 10 is a thermodynamically stable product and has a substitution pattern different from that of 3-6. These results show that the substituents on either the cage carbon or boron atoms have an important effect on the formation and thermal stability of the 13-vertex metallacarboranes. The formation of these complexes can be rationalized by the diamond-square-diamond mechanism.