Are intramolecularly stabilized compounds of aluminum suitable structural models of the S(N)2 transition state? Molecular structure of [(Bu-t)(2)Al(mu-OC6H4-2-OMe)](2)

The synthesis and crystallographic characterization of [(Bu-t)(2)Al(mu-OC6H 4-2-OMe)](2), when combined with the previously reported methyl, ethyl and iso-butyl analogs, allows for the structural comparison of the homologous s eries [R2Al(mu-OC6H4-2-OMe)](2) with the ideal structural changes that occu r during the S(N)2-like cleavage of an aluminum alkoxide dimer with an intr amolecular Lewis base. Ab initio calculations on the model system, [H2Al(mu -OCH2CH2OH)](2), confirm that in the absence of steric effects, intramolecu larly stabilized compounds of aluminum are suitable structural models of th e S(N)2 transition state. However, real compounds are not good models since the bond distances are controlled by the steric bulk of the alkyl substitu ents on adjacent aluminum centers rather than the extent of coordination of the fifth ligand. (C) 1999 Elsevier Science Ltd. All rights reserved.