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

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(ORF)(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 [Ga-2(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 Me(3)TACN (1,4,7-trimethyl-1,4,7-triazacyclononane) to the Ga- and Tl-complex salts resulted in an isomerization to the novel low-valent Al-4(+) cation [(Me(3)TACN)Al(AlCp*)(3)][Al(ORF)(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 [(Me(3)TACN)Al(AlCp*)(3)][Al(ORF)(4)] was devised from [M(Me(3)TACN)][Al(ORF)(4)] (M = Ga, Tl) and [(AlCp*)(4)].

Automated summary

This study reports the synthesis of AlCp*-complexes with low-valent main-group metals, including the first covalent Al-In and Al-Tl bonds. The Ga complex exhibits a solvent and temperature dependent equilibrium between monomeric and dimeric forms, while the In and Tl complexes are monomeric and can be reduced by AlCp*. Addition of Me(3)TACN leads to the isomerization to a novel low-valent Al-4(+) cation. This research provides valuable insights into the reactivity of metal-main-group element complexes.