Protonolysis of [(iPrO)TiMo5O18]3−: access to a family of TiMo5 Lindqvist type polyoxometalates

Abstract

The tetra-n-butylammonium (TBA) salts of [(ⁱPrO)TiMo₅O₁₈]³⁻1 and [(ⁱBuO)TiMo₅O₁₈]³⁻2 were prepared by hydrolysis of mixtures of (TBA)₂[Mo₂O₇], (TBA)₄α-[Mo₈O₂₆] and Ti(OR)₄ (R = ⁱPr or ⁱBu) in acetonitrile. Treatment of (TBA)₃1 with alcohols ROH afforded primary and tertiary alkoxide derivatives [(RO)TiMo₅O₁₈]³⁻ (R = Me 3, ^tBu 4), whilst aryloxides [(ArO)TiMo₅O₁₈]³⁻ were prepared by reacting 1 with phenols ArOH (Ar = C₆H₄Me-4 5, and C₆H₄CHO-2 6). Oxo-bridged [(μ-O)(TiMo₅O₁₈)₂]⁶⁻7 rather than the hydroxo derivative [(HO)TiMo₅O₁₈]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 Mo₄O₄ planes of 4 and 7 and in one of the two independent anions in the structure of 3. In solution, ¹⁷O NMR spectra are consistent with the higher anionic charge compared to [Mo₆O₁₉]²⁻ and reveal an order of basicity for the anions [LM′Mo₅O₁₈]³⁻ associated with the ability of {LM′}³⁺ to donate/withdraw electron density from {Mo₅O₁₈}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_N1 mechanism for ligand exchange in [(RO)TiM₅O₁₈]³⁻ is discussed.