Generation of Cationic [Zr‐{ tert ‐Butyl Enolate}] Reactive Species: Methyl Abstraction versus Hydride Abstraction

Treatment of the neutral methyl–Zr–enolate [Cp 2 Zr(Me){O( t BuO)CCMe 2 }] ( 1 ) with one equivalent of B(C 6 F 5 ) 3 or [HNMe 2 Ph][B(C 6 F 5 ) 4 ] as a methyl abstractor in THF at 0 °C leads to the selective formation of the free ion pair complex [Cp 2 Zr(THF){O( t BuO)CCMe 2 }] +  [anion] − ( 2 ) (anion=MeB(C 6 F 5 ) 3 − , B(C 6 F 5 ) 4 − ), which is relevant to the controlled polymerization of methacrylates. Cation 2 rapidly decomposes at 20 °C in THF with release of one equivalent of isobutene to form the cationic Zr–carboxylate species [Cp 2 Zr(THF)(O 2 C i Pr)] + ( 3 ), through a proposed intramolecular proton transfer process from the tert ‐butoxy group to the enolate. The reaction of 1 with one equivalent of B(C 6 F 5 ) 3 or [HNMe 2 Ph][B(C 6 F 5 ) 4 ] in CH 2 Cl 2 leads to the direct, rapid formation of the dimeric μ‐isobutyrato–Zr dicationic species [{Cp 2 Zr[μ‐(O 2 C i Pr)]} 2 ] 2+ ( 4 ), which gives 3 upon dissolution in THF. Contrastingly, when [Ph 3 C][B(C 6 F 5 ) 4 ] is used to generate the cationic Zr–enolate species from 1 in CD 2 Cl 2 , a 15:85 mixture of dicationic complexes 4 and [{Cp 2 Zr[ μ ‐(O 2 CC(Me)CH 2 )]} 2 ] 2+ [B(C 6 F 5 ) 4 ] ${{{- \hfill \atop 2\hfill}}}$ ( 5 ‐[B(C 6 F 5 ) 4 ] 2 ) is obtained quantitatively. The formation of 5 is proposed to arise from initial hydride abstraction from a methyl enolate group by Ph 3 C + , as supported by the parallel production of Ph 3 CH, and subsequent elimination of methane and isobutene. In addition to standard spectroscopic and analytical characterizations for the isolated complexes 2 – 5 , complexes 4 and 5 have also been structurally characterized by X‐ray diffraction studies.