Synthesis, Structural and Spectroscopic Characterization, Catalytic Properties, and Thermal Transformations of New Cyclic Di- and Trisiloxanediolato Tantalum Complexes

The reaction between Ta(OEt)5 and 1,1,3,3-tetramethyl-1,3-disiloxanediol, (HOSiMe2OSiMe2OH), leads to new siloxy
complexes in which the dimeric nature of Ta(OEt)5 is maintained with both bridging ethoxide and disiloxanediolato
bridges. With equal amounts of the reagents, two terminal OEt groups are replaced to form [Ta(OEt)2]2(μ-OEt)2(μ-OSiMe2OSiMe2O)2, 1, whereas with an excess of diol, the remaining terminal OEt groups are also replaced but
with a trisiloxanediolato unit to form [Ta(OSiMe2OSiMe2OSiMe2O)]2(μ-OEt)2(μ-OSiMe2OSiMe2O)2, 2. Complexes 1
and 2 catalyze the transformation of HOSiMe2OSiMe2OH to polysiloxanes. Thermal treatment of 1 results in the
formation of a 1:2 mixture of Ta2O5/SiO2; no new phases are observed. The molecular structures of 1 and 2 are
confirmed by X-ray crystallography.