Non-aqueous protonation, protonolysis and related reactions of polyoxometalates

Abstract

A sound knowledge of the complex solution processes underlying molecular metal oxide aggregation and structural inter-conversion is crucial to developing efficient synthetic routes to accessing unprecedented structures and understanding their properties. The nonaqueous aggregation of (TBA)2[W6O19] from WO(OMe)4 and (TBA)2[WO4] was studied systematically using 17O NMR spectroscopy. The study resulted in the development of a novel and efficient synthetic route to a series of heterometallic Lindqvist POMs, {M′M5} from [M6O19]2– anion. Using a combination of NMR techniques (1H, 17O and 2D 1H EXSY) and DFT calculations, the hydrolysis and condensation of (TBA)3[(MeO)TiW5O18] and (TBA)3[(MeO)SnW5O18] were investigated to understand factors influencing subtle differences in the POMs. The study led to the isolation and characterization of (TBA)3[(HO)TiW5O18] - a new member of the [M′M5] family. The study further revealed that the tin hydroxido POM, (TBA)3[(HO)SnW5O18] was more readily accessible than the titanium anologue, (TBA)3[(HO)TiW5O18] under similar conditions whereas the tin oxo-bridge dimer, (TBA)6[(μ-O)(SnW5O18)2] was less stable and hence more difficult to isolate compared to the titanium oxo-bridged POM, (TBA)6[(μ-O)(TiW5O18)2]. These behaviours were ascribed to differences in relative free energies. Furthermore, the surface oxygen basicity of the POMs, (TBA)3[(MeO)TiW5O18] and (TBA)6[(μ-O)(TiW5O18)2] and the behaviour of (TBA)6[(μ-O)(TiW5O18)2] towards a range of electrophiles were explored using 17O NMR spectroscopy. Generally, it was demonstrated that the TiOW oxygens are the most basic sites in (TBA)6[(μ-O)(TiW5O18)2]. The reactions resulted in the isolation and characterization of the dmso adduct (TBA)4[(μ-O)(TiW5O18H)2(dmso)] and (TBA)4[(μ-O)(TiW5O18)2(SnMe2)] wherein Sn(IV) is bonded to the Ti-O-W oxygens suggesting the possibility of “POM-pincer” complexes. Additionally, [O=TiW5O18]4–, which is possibly the first member of the oxo-titanium Lindqvist family was isolated and characterized in an excellent yield by treating [(CH3O)TiW5O18]3– with an organic base. The new POM was subsequently reacted with alkyl and aromatic isocyanate providing insights into the reactivity of the titanyl bond. Generally, products were further characterised by FT-IR, Multinuclear NMR (1H, 13C, 31P, 119Sn, and 183W) and/or single crystal XRD. While DFT calculations provided support for experimental observations.