Unusual Weakly Coordinating Anion Reactivity in Metallocene Chemistry. Formation of Tantalocene Cation−Dinuclear Anion Pairs

Abstract

Unlike the clean reaction of group 4 metallocene alkyls with B(C6F5)3, the reaction of Cp2TaMe3 with M(C6F5)3 (M = B, Al) in benzene or toluene produces an oily product mixture containing the expected Cp2TaMe2+CH3M(C6F5)3- as well as the unexpected Cp2TaMe2+[(C6F5)3MCH3M(C6F5)3]- in equilibrium, along with unreacted Cp2TaMe3. The surprising reactivity of weakly coordinating anions CH3M(C6F5)3- in the abstractive metallocene chemistry renders formation of novel μ-Me-bridged dinuclear anions. Subsequently, the reaction of Cp2TaMe3 with 2 equiv of Al(C6F5)3 cleanly generates the tantalocene cation−dinulcear anion ion pair Cp2TaMe2+[(C6F5)3AlCH3Al(C6F5)3]- as colorless crystals in a quantitative yield. A crystallographic study confirms the structure, which reveals unassociated cation−anion pairs with nearly symmetrical μ-CH3 bridging in the anion portion.