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

The reaction of [CpTiCl] (Cp = CMe) with monoalkyl phosphates (RO)POH (R = Me, Et, and Pr) in tetrahydrofuran (THF) at 25 °C leads to the formation of binuclear complexes [CpTi(μ-OP(OH)OR)(μ-OP(O)OR)] [R = Me (1), Et (2), and Pr (3)]. On the other hand, the reaction of ( BuO)POK with [CpTiCl] in acetonitrile or THF results in isolation of either the dinuclear [CpTi(μ-OP(OH)O Bu)(μ-OP(O)O Bu)] (4) or the trinuclear titanophosphate [CpTi(μ-OPO Bu)(μ-O)(μ-OP(O Bu))] (5), respectively. The formation of compounds 4 and 5 is facilitated by partial hydrolysis of the tert-butoxy groups of ( BuO)POK. 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 CMe groups to form an organic-free amorphous titanophosphate in the temperature range 300-500 °C. This material transforms to highly crystalline titanium pyrophosphate TiPO at 800 °C. Compounds 1-5 and the TiPO materials obtained at 300, 500, and 800 °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 °C.