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

摘要

The reaction of [Cp*TiCl3] (Cp* = C5Me5) with monoalkyl phosphates (RO)PO3H2 (R = Me, Et, and Pr-i) in tetrahydrofuran (THF) at 25 degrees C leads to the formation of binuclear complexes [Cp*Ti-2(2)(mu-O2P(OH)OR)(2)(mu-O2P(O)OR)(2)] [R = Me (1), Et (2), and Pr-i (3)]. On the other hand, the reaction of ((BuO)-Bu-t)(2)PO2K with [Cp*TiCl3] in acetonitrile or THF results in isolation of either the dinuclear [Cp*Ti-2(2)(mu-O2P(OH)(OBu)-Bu-t)(2)(mu-O2P(O)(OBu)-Bu-t)(2)] (4) or the trinuclear titanophosphate [Cp*Ti-3(3)(mu-(O3POBU)-B-t)(2)(mu-O)(2)(mu-O2P((OBu)-Bu-t)(2))] (5), respectively. The formation of compounds 4 and 5 is facilitated by partial hydrolysis of the tert-butoxy groups of ((t)3uO)(2)PO2K. New titanophosphates 1-5 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 1-5 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 degrees C. This material transforms to highly crystalline titanium pyrophosphate TiP2O7 at 800 degrees C. Compounds 1-5 and the TiP2O7 materials obtained at 300, 500, and 800 degrees C through the thermal decomposition of 3 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 degrees C.