C–H-Bond Activation by Rare-Earth-Metallocene Compounds

Abstract

The synthesis of complexes [Cp*2LnMe]x and the examination on the exceptional reactivity toward various substrates resulted in the development of σ-bond metathesis as the most important mechanism for C–H-bond activation by rare-earth-metal alkyl complexes. Ever since, complexes [Cp*2LnMe]x emerged as archetypical compounds for σ-bond-metathesis chemistry. However, owing to the high reactivity of these compounds, their synthesis is challenging and the reaction pathways hard to control. As such, it is of interest to develop synthesis strategies based on stabilized derivatives of [Cp*2LnMe]x such as [Cp*2LnMe(do)] and [Cp*2Ln(MMe4)] (M = Al, Ga) to investigate into σ-bond metathesis reactions. In a first approach, thermal treatment of [Cp*2YMe(THF)] resulted in the activation of THF to afford [Cp*2Y(OC2H3)(THF)] and the substituted tetrahydrofuranyl compound [Cp*2Y(2 C2H2 OC4H7)]. The attempted synthesis of elusive [Cp*2LaMe(THF)] by donor induced aluminate cleavage of [Cp*2La(AlMe4)] resulted in formation of polymeric [Cp*2La(AlMe4)(THF)]n. This compound was found to be thermally labile cleanly converting into AlMe3-stabilized tetrahydrofuranyl compound [Cp*2La(2 AlMe3-OC4H7)]. Thermal treatment of [Cp*2Y(MMe4)] (M = Al, Ga) in benzene solutions led to C–H-bond activation of benzene to afford complexes [Cp*2Y(Me2MPh2)] and [Cp*2Y(MPh4)]. The proposed mechanism involves thermally induced dissociation of MMe3 to liberate highly reactive [Cp*2YMe]. In a similar vein, the sterically demanding boryl moiety {B(NDippCH)2}– was implemented to promote dissociative aluminate cleavage. As a result, [Cp*2YMe3Al{B(NDippCH)2}] activates benzene already at ambient temperatures affording [Cp*2YMe2AlPh{B(NDippCH)2}] and [Cp*2YPh3Al{B(NDippCH)2}]. This concept was successfully extended to the corresponding lutetium and lanthanum compounds [Cp*2Ln(MMe4)] (Ln = Lu, La). Accordingly, the sandwich complexes [Cp*2Ln(MMe4)] gave access to lanthanide-based C–H-bond activation chemistry covering the full range of ionic radii in the lanthanide series. In a final approach, the reactivity of [Cp*2Ln(GaMe4)] (Ln = Lu, Y) toward an excess of GaMe3 at elevated temperatures was explored. Interestingly, complexes [Cp*2Ln(GaMe4)] triggered the formation of molecular homoleptic gallium methylene [Ga8(CH2)12] from GaMe3 via a cascade C–H-bond activation involving dodecametallic clusters [Cp*6Ln3Ga9(CH2)15] as crucial intermediates.