Cyclopentadienyl Removal Assisted Nuclearity Expansion in Thermolabile Titanium and Zirconium Organophosphates Sourced from Metallocene Dichlorides

Abstract

Reaction of titanocene dichloride [Cp2TiCl2] (Cp = C5H5) with (tBuO)2PO2H (di-tert-butylphosphate, dtbpH) in THF at room temperature results in the formation of mononuclear titanophosphate [Cp2Ti(dtbp)2].H2O (1), whereas the reaction of potassium salt of dtbpH (dtbpK) with [Cp2TiCl2] in THF yields tetranuclear cluster [Cp4Ti4(µ-O)3(µ-dtbp)2(dtbp)2(µ-mtbpH)2].2C6H5CH3 (2) (mtbpH = (tBuO)PO3H). Change of the reaction medium to acetonitrile for the reaction dtbpK with [Cp2TiCl2] leads to the isolation of novel hexanuclear titanophosphate cluster [Ti6(µ-O)6(µ-OH)3(µ-dtbp)9] (3), which is free of cyclopentadienyl ligands. Compound 1, when dissolved in CH3CN and allowed to stand under aerobic conditions, converts to either 2 or 3, depending on the dilution (water content in the medium) and the duration of crystallization. While lesser water content and shorter crystallization periods produce 2 as orange crystals, longer exposure of the solution results in the complete hydrolysis of Cp-Ti linkages and produce colorless, Cp-free hexanuclear cluster 3. In contrary, the reaction of either dtbpH or its potassium salt with zirconocene dichloride exclusively leads to the formation of [CpZr(µ-OH)(µ-dtbp)(dtbp)]2 (4) as the only product. Compounds 1-4 have been characterized by analytical and spectroscopic methods. Their molecular structures have further been established by single-crystal X-ray diffraction studies. Thermal decomposition of 1 reveals its thermolabile nature with the initial loss of alkyl substituents of the organophosphate ligands through β-hydride elimination followed by the loss of C5H5 groups to form an organic-free crystalline titanium phosphate phases which sinter finally to titanium pyrophosphate TiP2O7 at 800 ˚C.