Synthesis and Structure of Cyclic Aluminum Disiloxides

The synthesis and structure of cyclic five- and six-membered aluminum disiloxides containing the
chelating diolate ligands SiMe2[(Me3Si)2SiO]22- (1a)-2H, [(Me3Si)2SiO]22- (1b)-2H, and [Me(Me3Si)3SiSiO]22-
(rac-1c)-2H are reported. The dinuclear aluminum disiloxides [SiMe2{(Me3Si)2SiO}2AlMe]2 (2a) and
[(E)-{Me(Me3Si)3SiSiO}2AlMe]2 (2c) were prepared in n-pentane by treatment of AlMe3 with 1 equiv
of the free ligands 1a and 1c, respectively. The analogous reaction of 1c with AlMe3 in THF as solvent
yielded the monomeric THF adduct (E)-[Me(Me3Si)3SiSiO]2AlMe·THF (4c). Irrespective of the molar
ratio, treatment of 1b with AlMe3 in pentane gave the trinuclear complex [{(Me3Si)2SiO}2]2Al3Me5 (3b)
as the major product, whereas the thermally stable AlMe3 adduct (E)-[Me(Me3Si)3SiSiO]2AlMe·AlMe3
(5c) was synthesized by employing a 2-fold excess of AlMe3 in the reaction with 1c. Treatment of 4c
with B(C6F5)3 led to full exchange of the methyl group by C6F5 with quantitative formation of (E)-[Me(Me3Si)3SiSiO]2Al(C6F5)·THF (6c) and a mixture of B(C6F5)3-nMen, where n = 1, 2, 3. The reaction of
4c with phenol did not yield the expected complex (E)-[Me(Me3Si)3SiSiO]2AlOPh·THF; rather, quantitative
ligand transfer to give the spirocyclic complex {[(E)-{Me(Me3Si)3SiSiO}2]2Al}-H+ (7c) occurred. The
structures of the complexes 2a, 2c, 3b, 4c, and 7c were determined by X-ray crystallography.