First structural characterisation of a 2,1,12-MC2B9 metallacarborane, [2,2,2-(NMe2)3-closo-2,1,12-TaC2B9H11]. Trends in boron NMR shifts on replacing a {BH} vertex with a metal {MLn} vertex in icosahedral carboranes

Abstract

Reactions of M(NMe₂)₅ (M = Ta or Nb) with nido-2,9-C₂B₉H₁₃ and the salt [Me₃NH][nido-7,9-C₂B₉H₁₂] gave the isomeric dicarbollide complexes [2,2,2-(NMe₂)₃-closo-2,1,12-MC₂B₉H₁₁] (M = Ta 1, Nb 4) and [2,2,2-(NMe₂)₃-closo-2,1,7-MC₂B₉H₁₁] (M = Ta 2, Nb 5) respectively. The structures of 1 and 2 were determined by single crystal X-ray diffraction and 1 represents the first structurally characterised example of a 2,1,12-MC₂B₉ metallacarborane. Comparison of ¹¹B NMR data of the tantalum complexes, along with the isomeric [3,3,3-(NMe₂)₃-closo-3,1,2-TaC₂B₉H₁₁] 3, with that of 1,2-, 1,7- and 1,12-C₂B₁₀H₁₂, reveals that the metal vertex {Ta(NMe₂)₃}, on replacing a {BH} vertex, influences significantly the boron NMR shifts of the neighbouring and antipodal cage atoms. Based on this observation the assignments of the reported peaks in the boron NMR data for the seven isomers of (η⁵-C₅H₅)CoC₂B₉H₁₁ are tentatively predicted.