MOLECULAR ROUTES ES METAL-OXIDES SAND METAL SILICATES, SYNTHESIS AND THERMAL-DECOMPOSITION STUDIES OF ECLIPSED MO-2[O2SI(O(T)BU)(2)](3) AND(2))(2)[O2SI(O(T)BU)(2)](2)[OSI(OH)(O(T)BU)(2)](2)

Reaction of Mo-2(NMe(2))(6) with 3 equiv of ((t)BuO)(2)Si(OH)(2) affor ded a new siloxide complex, Mo-2[O2Si(O(t)Bu)(2)](3) (1), as the only metal-containing product. An X-ray crystal structure determination rev eals that I adopts an eclipsed structure with three di-tert-butoxysila nediolato ligands bridging themolybdenum centers in a symmetrical fash ion. Lewis acidic complex 1 forms an isolable adduct with pyridine. Re action of W-2(NMe(2))(6) with 3 equiv of ((t)BuO)(2)Si(OH)(2) afforded (2))(2)[O2Si(O(t)Bu)(2)](2)[OSi(OH)(O(t)Bu)(2)](2) (3), which was iso lated as air-sensitive orange crystals. In this molecule, two O2Si(O(t )Bu)(2) ligands bridge the W=W bond in an eta(1),eta(1) fashion, and O Si(OH)(O(t)Bu)(2) ligands are coordinated to each W atom. This molecul e adopts a highly eclipsed conformation and has a 2-fold axis of symme try which perpendicularly bisects the tungsten-tungsten bond. Complex 1 undergoes a two-step weight loss (by TGA), which corresponds to the initial loss of isobutene and water (at ca. 200 degrees C), followed b y the loss of molybdenum by evaporation of MoO3 at 700-1050 degrees C. The pyrolysis of 1 at 550 degrees C for 2 h under argon gave a cerami c material with the approximate formula Mo2Si3O10. When the sample was heated at 1200 degrees C for 1 h, evaporation of MoO3 occurred and th e final ceramic material had a composition of Mo0.5Si3O6.5. Very low c arbon contents for these materials are consistent with a clean elimina tion process during the pyrolysis. The material heated to 550 degrees C is amorphous, while the material taken to 1200 degrees C contains Mo O2 as the only crystalline phase. Pyrolysis of 1 in air at 1200 degree s C resulted in the extensive loss of molybdenum (2.14% Mo; no crystal line phases by XRD). Interestingly, pyrolysis of small crystals of 1 o ccurs with retention of the crystals' shape and size, despite a great loss in weight (75%). Pyrolysis of 3 at 550 and 1200 degrees C under a rgon produced ceramic materials consistent with the formulas W2.2Si4O1 7.4 and W2.2Si4O13, respectively. While the 550 degrees C ceramic is a morphous, the 1200 degrees C material contains crystalline W and WO2 ( by XRD analysis).