Non-aqueous synthetic methodology for TiW5 polyoxometalates: protonolysis of [(MeO)TiW5O18]3− with alcohols, water and phenols
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, C6H4tBu-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.