Thermolabile Organotitanium Monoalkyl Phosphates: Synthesis, Structures, and Utility as Epoxidation Catalysts and Single-Source Precursors for TiP₂O₇

The reaction of [Cp*TiCl₃] (Cp* = C₅Me₅) with monoalkyl phosphates (RO)­PO₃H₂ (R = Me, Et, and ^<i>i</i>Pr) in tetrahydrofuran (THF) at 25 °C leads to the formation of binuclear complexes [Cp*₂Ti₂(μ-O₂P­(OH)­OR)₂(μ-O₂P­(O)­OR)₂] [R = Me (<b>1</b>), Et (<b>2</b>), and ^<i>i</i>Pr (<b>3</b>)]. On the other hand, the reaction of (^<i>t</i>BuO)₂PO₂K with [Cp*TiCl₃] in acetonitrile or THF results in isolation of either the dinuclear [Cp*₂Ti₂(μ-O₂P­(OH)­O^<i>t</i>Bu)₂(μ-O₂P­(O)­O^<i>t</i>Bu)₂] (<b>4</b>) or the trinuclear titanophosphate [Cp*₃Ti₃(μ-O₃PO^<i>t</i>Bu)₂(μ-O)₂(μ-O₂P­(O^<i>t</i>Bu)₂)] (<b>5</b>), respectively. The formation of compounds <b>4</b> and <b>5</b> is facilitated by partial hydrolysis of the <i>tert</i>-butoxy groups of (^<i>t</i>BuO)₂PO₂K. New titanophosphates <b>1</b>–<b>5</b> have been characterized by spectroscopic and analytical methods, and the molecular structures have further been confirmed by single-crystal X-ray diffraction studies. Thermal decomposition studies of <b>1</b>–<b>5</b> reveal the initial loss of thermally labile alkyl substituents of the organophosphate ligands, followed by the loss of C₅Me₅ groups to form an organic-free amorphous titanophosphate in the temperature range 300–500 °C. This material transforms to highly crystalline titanium pyrophosphate TiP₂O₇ at 800 °C. Compounds <b>1</b>–<b>5</b> and the TiP₂O₇ materials obtained at 300, 500, and 800 °C through the thermal decomposition of <b>3</b> have been employed as efficient homogeneous catalysts for the alkene epoxidation reaction. Using hydrogen peroxide as the oxidant in an acetonitrile medium, these catalysts exhibit >90% alkene conversion with >90% epoxide selectivity in 4 h at temperatures below 100 °C