Dimolybdenum(III) Complexes of −OSi(OtBu)3, −O2P(OtBu)2, and −OB[OSi(OtBu)3]2as Single-Source Molecular Precursors to Molybdenum-Containing, Multi-Component Oxide Materials

Abstract

The following dimolybdenum complexes containing −OSi(OtBu)3, −O2P(OtBu)2, and −OB[OSi(OtBu)3]2 ligands have been synthesized and structurally characterized: Mo2(NMe2)4[OSi(OtBu)3]2 (1), Mo2(OtBu)4[OSi(OtBu)3]2 (2), Mo2(NMe)4{OB[OSi(OtBu)3]2}2 (3), Mo2(NMe2)2[μ-O2P(OtBu)2]2[O2P(OtBu)2]2 (4), Mo2(NMe2)2[OSi(OtBu)3]2[μ-O2P(OtBu)2]2 (5), and Mo2(NMe2)2[μ-O2P(OtBu)2]2{OB[OSi(OtBu)3]2}2 (6). The isolation and structural characterization of trans- and cis-isomers of complexes 4 and 5 (4a and 4b, 5a and 5b, respectively) are also reported. Studies of the thermal decompositions of the complexes (by thermogravimetric analysis and solution 1H NMR spectroscopy) were performed. Xerogels with approximate compositions of 2MoO1.5·2P2O5 and 2MoO1.5·2P2O5·2SiO2 were derived from 4a and 5a or 5b, respectively, via solution thermolyses (toluene). The as-synthesized (and dried) xerogels contain one equiv of HNMe2 per molybdenum center (by combustion analysis, IR spectroscopy, and thermogravimetric analysis), and these materials have high surface areas (up to 270 m2 g-1). Upon calcination at 300 °C, the coordinated amines are lost and the surface areas are significantly reduced (to 40 m2 g-1 and <5 m2 g-1 for the materials derived from 4 and 5a or 5b, respectively). Solid-state 31P 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 −O2P(OtBu)2 ligands. Upon calcination at 300 °C, the resonances for bridging −O2P(OtBu)2 groups are replaced by those for PO43-. The material derived from 4 exhibits low activity and poor selectivity for the oxidative dehydrogenation (ODH) of propane to propylene. Cothermolyses of 4 and Bi[OSi(OtBu)3]3 resulted in formation of Bi/Mo/P/Si/O materials with improved performance for the ODH of propane.