MOLECULAR ROUTES FOR THE SYNTHESIS OF METAL CARBIDES, NITRIDES, AND OXIDES .1. STUDIES OF THE THERMAL-DECOMPOSITION OF M(2)(OR)(6) AND M(2)(CH(2)PH)(2)(OR)(4) COMPOUNDS WHERE M=MO AND W

Thermolysis of the homoleptic alkoxides M(2)(OR)(6) and the mixed benzyl alkoxides M(2)(CH(2)Ph)(2)(O-i-Pr)(4), where M = Mo and W, have been studied under a purified helium flow by a coupled thermogravimetric analyzer/mass spectrometer (TGA-MS) system installed in a VAC atmosphere Dri-Lab. Under rigorously purified He, Mo-2(OR)(6), where R = t-Bu and i-Pr, sublimed without decomposition at ca. 100-200 degrees C, 1 atm. In the presence of a trace of O-2/H2O, Mo-2(O-t-Bu)(6) gave MoO2 The related W-2(OR)(6) compounds were transformed to WO2 when R = t-Bu at ca. 200 degrees C and when R = i-Pr to a mixture of WO2 and W(m) after heating to 750 degrees C. In contrast, the cyclohexoxides M(2)(O-c-C6H11)(6) gave upon heating Mo2C and W(m), respectively, by way of a metastable amorphous phase of approximate composition M(2)C(4)O(4). The introduction of two benzyl ligands in the compound M(2)(CH(2)Ph)(2)(O-i-Pr)(4) significantly alters the molecular decomposition. For instance, for M = Mo the addition of the benzyl Ligands causes the compound to decompose prior to sublimation and gives Mo2C with traces of Mo(m). For M = W the decomposition pathway is altered considerably, with the initial decomposition temperature range lower than that of the homoleptic alkoxide W-2(O-i-Pr)(6) by ca. 100-250 degrees C. This decomposition gives the product W2C (>65%) with traces of W(m) and WC as the only detectable products by X-ray diffraction (XRD) powder patterns. The formation of the residues after thermal decomposition is based on XRD and the percentage weight retained in the TGA experiments. A variety of isotopically labeled compounds were prepared and the gases evolved during thermolysis were analyzed by MS, CC-MS, and H-1 and C-13 NMR spectroscopy. On the basis of these studies, some insights into the initial mechanisms of the thermolyses are proposed.