Non-aqueous synthetic methodology for TiW5 polyoxometalates: protonolysis of [(MeO)TiW5O18]3− with alcohols, water and phenols

Abstract

The tetra-n-butylammonium (TBA) salt of [(MeO)TiW₅O₁₈]³⁻1 was reacted with alcohols ROH to give primary, secondary and tertiary alkoxide derivatives [(RO)TiW₅O₁₈]³⁻ (R = Et 2, ⁱPr 3 and ^tBu 4), whilst hydrolysis afforded [(µ-O)(TiW₅O₁₈)₂]⁶⁻5 rather than the hydroxo derivative (R = H). In reactions with ⁱPrOH and ^tBuOH, impurity peaks observed at 1015 and 1020 ppm in the ¹⁷O NMR spectra indicate alkoxide degradation and TiO bond formation via reactions analogous to those occurring at the surfaces of solid heteropolyacids. Aryloxides [(ArO)TiW₅O₁₈]³⁻ were prepared by reacting 1 with phenols ArOH (Ar = C₆H₅6, C₆H₄Me-4 7, C₆H₄^tBu-4 8, C₆H₄OH-4 9, C₆H₄OH-3 10, C₆H₃(OH)₂-3,5 11 and C₆H₄CHO-2 13). TiW₅O₁₈ units were linked by reacting 1 with 9 to give [(µ-1,4-OC₆H₄O)(TiW₅O₁₈)₂]⁶⁻12. ¹⁷O and ¹⁸³W NMR spectra are reported and X-ray crystal structures were obtained for TBA salts of anions 3–10 and 13, which showed that the titanium is six-coordinate in all cases. Reactions were monitored by ¹H NMR, including a 2D-EXSY study of methoxo exchange, and the slow rates observed are probably associated with the reluctance of titanium in these anions to achieve seven-coordination.