From mixed group 13 cations [M(AlCp*)3]+ (M = Ga/In/Tl) to an Al4+ cluster

Abstract

AlCp*-complexes with transition metals have shown to be highly reactive and enable C–H or Si–H bond activation. Yet, complexes of AlCp* with low-valent main-group metals are scarce. Here, we report the syntheses of [M(AlCp*)₃][Al(OR^F)₄] (R^F = C(CF₃)₃) with M = Ga, In, Tl, which include the first covalent Al–In and Al–Tl bonds. For M = Ga, AlCp*-coordination induced the formation of the dication [Ga₂(AlCp*)₆]²⁺ in the solid state, which exhibits a solvent and temperature dependent monomer–dimer equilibrium in solution. By contrast, the In and Tl complexes are monomeric and prone to reduction to the metal by the electron-rich AlCp*-moieties. The QTAIM analysis suggests that the metal centres are already highly reduced in the complexes, while the positive charge is distributed onto the AlCp* units. Addition of Me₃TACN (1,4,7-trimethyl-1,4,7-triazacyclononane) to the Ga- and Tl-complex salts resulted in an isomerization to the novel low-valent Al₄⁺ cation [(Me₃TACN)Al(AlCp*)₃][Al(OR^F)₄]. Intermittently formed tetrahedral GaAl₃⁺ clusters could be structurally characterized. From a detailed mechanistic study of this isomerization, the very high yield and clean preparation of [(Me₃TACN)Al(AlCp*)₃][Al(OR^F)₄] was devised from [M(Me₃TACN)][Al(OR^F)₄] (M = Ga, Tl) and [(AlCp*)₄].