Aluminum-Triggered Condensation of Vicinal Silicate Groups into a Bicyclic Alumosilicate

The molecular alumosilicates AlL{OSi(OBu)₂O}[OSi{(μ₃-O)(MR₂)₂(μ-OBu)}(OBu)] (L = HC[CMeNAr]₂^-, where M = Al, R = Me (2), Et (3), and Bu (4) and M = Ga, R = Me (5)) were obtained from the reaction of AlL{OSi(OBu)₂(OH)}₂ (1) with 1 or 2 equiv of the respective organometallic precursor. These compounds have a central bicyclic inorganic core formed by a six-membered AlSi₂O₃ alumosilicate ring with a Si-O-Si unit connected via a Si-O bond to a four-membered Al₂O₂ alumoxane ring. These compounds are formed even though 1 is specifically designed to yield 4R alumosilicate rings that would obey the Löweinstein's and Dempsey's rules about concatenation between silicon and aluminum tetrahedra in alumosilicates. We propose a mechanism for this rearrangement, based on the experimental evidence and density functional theory calculations, that involves a κ³μ₂ coordination of a silicate unit to two AlMe₂ groups, which weakens one Si-O bond and explains how aluminum atoms can cleave Si-O bonds. Furthermore, formation of the products experimentally confirms the theory that Al-O-Al groups can exist in alumosilicates if the oxygen atom belongs to an OH moiety.