The Alkylaluminate/Gallate Trap: Metalation of Benzene by Heterobimetallic Yttrocene Complexes [Cp*2Y(MMe3R)] (M = Al, Ga).

Abstract

Yttrocene derivatives [Cp*2Y(MMe4)] (Cp* = C5Me5; M = Al, Ga) and Cp*2Y[Me3Al{B(NDippCH)2}] (Dipp = C6H3iPr2-2,6) deprotonate benzene at elevated temperatures via the release of methane. The formation of [Cp*2Y(Me2MPh2)] (M = Al, Ga), Cp*2Y(MPh4) (M = Al, Ga), Cp*2Y[Me2AlPh{B(NDippCH)2}], and Cp*2Y[AlPh3{B(NDippCH)2}] can be controlled via the temperature applied. The activation temperature and formation of the coordinatively unsaturated "reactive" [Cp*2YMe] strongly depend on the coordination strength of the displaceable Lewis acids [AlMe3]2, GaMe3, and [Me2Al{B(NDippCH)2}]2. Hence, [Cp*2Y(AlMe4)] requires temperatures above 100 °C to metalate benzene, while Cp*2Y[AlMe3{B(NDippCH)2}] undergoes C-H-bond activation even at ambient temperatures. A kinetic deuterium isotope effect was observed for the reactions in C6D6 solutions. Distinct differences in the stabilities of the bulky Group 13 anions ([Me2MPh2]-, [MPh4]-, [Me3Al{B(NDippCH)2}]-, [Me2AlPh{B(NDippCH)2}]-, and [AlPh3{B(NDippCH)2}]-) are assessed by detailed studies of the coordination chemistry with tetrahydrofuran (THF) and by variable-temperature 1H NMR spectroscopy. Thus, increased steric bulk or a reduced Lewis acidity of the Group 13 metal center promote temperature-sensitive dissociation of trivalent Group 13 alkyl entities. Consequently, compound Cp*2Y[AlPh3{B(NDippCH)2}] was found to engage in a dissociation equilibrium with [Cp*2YPh] and AlPh2{B(NDippCH)2} in a C6D6 solution at ambient temperature. The reaction of Cp*2Y[AlPh3{B(NDippCH)2}] with THF results in the concomitant formation of monometallic Cp*2YPh(THF) and the solvent-separated ion pair [Cp*2Y(THF)2][AlPh3{B(NDippCH)2}].