Synthesis, structure, and alkyne insertion of a mixed-sandwich zirconacarborane alkyl

Abstract

A neutral mixed-sandwich zirconacarborane alkyl [η¹:σ:η⁵-{MeN(CH₂)CH₂CH₂}C₂B₉H₁₀]Zr(η⁵-Cp′′) (Cp′′ = 1,3-(Me₃Si)₂C₅H₃) was prepared via methane elimination reaction of 7-Me₂N(H)CH₂CH₂-7,8-C₂B₉H₁₁ with (η⁵-Cp′′)ZrMe₃, followed by an intramolecular C–H activation. It reacted with internal alkynes and trimethylsilylacetylene to give the Zr–C σ bond mono-insertion products; however, the terminal alkyne 3,3-dimethyl-1-butyne underwent an acid–base reaction to produce a zirconium alkynyl complex. Both electronic and steric factors affected the regioselectivity of the insertion process. The results showed that the coordination of the sidearm nitrogen atom could enhance the thermal stability of the resulting zirconacarborane alkyl, and on the other hand, lowered the Lewis acidity of the Zr atom, leading to relatively poor reactivity. All complexes were fully characterized by NMR spectroscopy and elemental analyses. Most of them were further confirmed by single-crystal X-ray analyses.