Synthesis of isomeric B-methylated tantalum carboranes, (Me2N)3TaC2B9H10Me

Abstract

The cage-alkylated metallacarborane complex [4,4,4-(NMe₂)₃-3-Me-4,1,2-closo-TaC₂B₉H₁₀] 8, is obtained as the only product from the reaction of Ta(NMe₂)₅ with nido-11-Me-2,7-C₂B₉H₁₂4, which is prone to skeletal rearrangement under basic conditions. That no rearrangement occurs indicates that the hydroamminolysis reaction is a clean synthetic method. The isomeric metallacarboranes [3,3,3-(NMe₂)₃-4-Me-3,1,2-closo-TaC₂B₉H₁₀] 9 and [2,2,2-(NMe₂)₃-3-Me-2,1,7-closo-TaC₂B₉H₁₀] 10 are produced from the reaction of Ta(NMe₂)₅ with [Me₃NH][nido-9-Me-7,8-C₂B₉H₁₁] 5 and [Me₃NH][nido-8-Me-7,9-C₂B₉H₁₁] 6 respectively. Identities of the metallacarboranes 8, 9 and 10 are deduced by detailed multinuclear (¹¹B, ¹³C and ¹H) NMR spectroscopy. These geometries are supported by boron NMR shift predictions based on observed shifts of the known non-methylated analogues [3,3,3-(NMe₂)₃-closo-3,1,2-TaC₂B₉H₁₀] 1 and [3,3,3-(NMe₂)₃-closo-3,1,2-TaC₂B₉H₁₀] 2 and calculated shifts (GIAO-B3LYP/6-311G*//MP2/6-31G*) of the B-methyl carboranes, closo-3-Me-1,2-C₂B₁₀H₁₁11, 4-Me-1,2-C₂B₁₀H₁₁12 and 2-Me-1,7-C₂B₁₀H₁₁13. The molecular structure of 8 has been determined by X-ray diffraction.