Dimolybdenum(III) Complexes of −OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>, −O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub>, and −OB[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub> as Single-Source Molecular Precursors to Molybdenum-Containing, Multi-Component Oxide Materials

2004-03-23T00:00:00Z (GMT) by Kyle L. Fujdala T. Don Tilley
The following dimolybdenum complexes containing −OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>, −O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub>, and −OB[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub> ligands have been synthesized and structurally characterized:  Mo<sub>2</sub>(NMe<sub>2</sub>)<sub>4</sub>[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub> (<b>1</b>), Mo<sub>2</sub>(O<i><sup>t</sup></i><sup></sup>Bu)<sub>4</sub>[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub> (<b>2</b>), Mo<sub>2</sub>(NMe)<sub>4</sub>{OB[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub>}<sub>2</sub> (<b>3</b>), Mo<sub>2</sub>(NMe<sub>2</sub>)<sub>2</sub>[μ-O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub>]<sub>2</sub>[O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub>]<sub>2</sub> (<b>4</b>), Mo<sub>2</sub>(NMe<sub>2</sub>)<sub>2</sub>[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub>[μ-O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub>]<sub>2</sub> (<b>5</b>), and Mo<sub>2</sub>(NMe<sub>2</sub>)<sub>2</sub>[μ-O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub>]<sub>2</sub>{OB[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub>}<sub>2</sub> (<b>6</b>). The isolation and structural characterization of trans- and cis-isomers of complexes <b>4</b> and <b>5</b> (<b>4a</b> and <b>4b</b>, <b>5a</b> and <b>5b</b>, respectively) are also reported. Studies of the thermal decompositions of the complexes (by thermogravimetric analysis and solution <sup>1</sup>H NMR spectroscopy) were performed. Xerogels with approximate compositions of 2MoO<sub>1.5</sub>·2P<sub>2</sub>O<sub>5</sub> and 2MoO<sub>1.5</sub>·2P<sub>2</sub>O<sub>5</sub>·2SiO<sub>2</sub> were derived from <b>4a</b> and <b>5a</b> or <b>5b</b>, respectively, via solution thermolyses (toluene). The as-synthesized (and dried) xerogels contain one equiv of HNMe<sub>2</sub> per molybdenum center (by combustion analysis, IR spectroscopy, and thermogravimetric analysis), and these materials have high surface areas (up to 270 m<sup>2</sup> g<sup>-1</sup>). Upon calcination at 300 °C, the coordinated amines are lost and the surface areas are significantly reduced (to 40 m<sup>2</sup> g<sup>-1</sup> and <5 m<sup>2</sup> g<sup>-1</sup> for the materials derived from <b>4</b> and <b>5a</b> or <b>5b</b>, respectively). Solid-state <sup>31</sup>P MAS NMR spectroscopy suggests that the as-synthesized xerogels retain structural features of the starting molecular precursors, as indicated by the presence of resonances that correspond to both bridging and terminal −O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub> ligands. Upon calcination at 300 °C, the resonances for bridging −O<sub>2</sub>P(O<i><sup>t</sup></i><sup></sup>Bu)<sub>2</sub> groups are replaced by those for PO<sub>4</sub><sup>3-</sup>. The material derived from <b>4</b> exhibits low activity and poor selectivity for the oxidative dehydrogenation (ODH) of propane to propylene. Cothermolyses of <b>4</b> and Bi[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>3</sub> resulted in formation of Bi/Mo/P/Si/O materials with improved performance for the ODH of propane.