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

The molecular alumosilicates Al{OSi(OBu)O}[OSi{(μ-O)(MR)(μ-OBu)}(OBu)] ( = HC[CMeNAr], where M = Al, R = Me (), Et (), and Bu () and M = Ga, R = Me ()) were obtained from the reaction of Al{OSi(OBu)(OH)} () with 1 or 2 equiv of the respective organometallic precursor. These compounds have a central bicyclic inorganic core formed by a six-membered AlSiO alumosilicate ring with a Si-O-Si unit connected via a Si-O bond to a four-membered AlO alumoxane ring. These compounds are formed even though 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 κμ coordination of a silicate unit to two AlMe 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.