Protonolysis of [((i)PrO)TiMo5O18]3-: access to a family of TiMo5 Lindqvist type polyoxometalates

The tetra-n-butylammonium (TBA) salts of ((i)PrO)TiMo(5)O(18)](3-) 1 and ((i)BuO)TiMo(5)O(18)](3-) 2 were prepared by hydrolysis of mixtures of (TBA)(2)Mo(2)O(7)], (TBA)(4)α-Mo(8)O(26)] and Ti(OR)(4) (R = (i)Pr or (i)Bu) in acetonitrile. Treatment of (TBA)(3)1 with alcohols ROH afforded primary and tertiary alkoxide derivatives (RO)TiMo(5)O(18)](3-) (R = Me 3, (t)Bu 4), whilst aryloxides (ArO)TiMo(5)O(18)](3-) were prepared by reacting 1 with phenols ArOH (Ar = C(6)H(4)Me-4 5, and C(6)H(4)CHO-2 6). Oxo-bridged (μ-O)(TiMo(5)O(18))(2)](6-)7 rather than the hydroxo derivative (HO)TiMo(5)O(18)](3-) was obtained upon hydrolysis of 1. X-Ray crystal structures of TBA salts of anions 3-7 show that titanium is six-coordinate in all cases, although titanium sites are disordered over two trans positions in 3. Mo-O bond length alternation is observed in the Mo(4)O(4) planes of 4 and 7 and in one of the two independent anions in the structure of 3. In solution, (17)O NMR spectra are consistent with the higher anionic charge compared to Mo(6)O(19)](2-) and reveal an order of basicity for the anions LM'Mo(5)O(18)](3-) associated with the ability of {LM'}(3+) to donate/withdraw electron density from {Mo(5)O(18)}(6-). Protonolysis reactions of 1 and 3 were slower than for tungstate analogues and the possibility of initial protonation at TiOM (M = Mo) rather than TiOR (M = W) in a proton-assisted S(N)1 mechanism for ligand exchange in (RO)TiM(5)O(18)](3-) is discussed.