Nonchelated ZrIV−Alkoxide−Alkyne Complexes
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Abstract
Addition of alkynes to [Cp‘2Zr(OtBu)][B(C6F5)4] (Cp‘ = C5H4Me, 2) at low temperature in CD2Cl2 solution yields equilibrium mixtures of Cp‘2Zr(OtBu)(RC⋮CR‘)+ (3a−g), 2, and free alkyne. The 13C NMR Cint resonance of terminal alkynes shifts far downfield upon coordination, while the Cterm resonance shifts upfield, consistent with polarization of the C⋮C bond with partial positive charge on Cint and partial negative charge on Cterm, probably due to unsymmetrical coordination. Alkynes bind more strongly to 2 than do similar alkenes, but are readily displaced by Lewis bases such as THF. Increasing the steric bulk of the alkyne decreases the coordination strength. Propargyltrimethylsilane coordinates particularly strongly, due to β-Si stabilization of the positive charge on Cint of the coordinated alkyne. Compounds 3a−g undergo two dynamic processes: reversible alkyne decomplexation and rotation around the metal−(alkyne centroid) axis.
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