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

The molecular alumosilicates AlL{OSi((OBu)-Bu-t)(2)O}-[OSi{(mu(3)-O)(MR2)(2)(mu-(OBu)-Bu-t)}((OBu)-Bu-t)] (L = HC [CMeNAr](2)(-), where M = Al, R = Me (2), Et (3), and Bu-t (4) and M = Ga, R = Me (5)) were obtained from the reaction of AIL{OSi-((OBu)-Bu-t)(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 Loweinstein'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 kappa(3)mu(2) 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.