Complex solution chemistry behind the simple “one-pot” synthesis of vanadium-substituted polyoxometalates unraveled by electrospray ionization mass spectrometry

Rationale: The “one-pot” method is a deceptively simple and straightforward method for synthesis of polyoxometalates (POMs). However, the complex solution chemistry behind the simple method is neither clear nor elucidated thoroughly by a suitable method. In general, POM chemists are more focused on what final products are obtained by deliberately choosing the ingredients used in the “one-pot” process with a limited knowledge of what is actually happening in the reaction solution. Methods: Time-resolved electrospray ionization mass spectrometry (ESI-MS) was developed to monitor the dynamic solution speciation changes occurred in the reaction mixtures ({SiW_12-x} (x = 1–3) + NaVO₃) against reaction time. The reactions were conducted as normal, with 2 μL aliquots removed at specific time intervals during the reaction. The aliquots were immediately diluted with 1 mL of HPLC grade water and then analyzed by ESI-MS. As a control, solutions of each {SiW_12-x} were analyzed alongside the vanadium reactions to corroborate which fragments were due to the lacunary precursors and which were due to the reactions with sodium metavanadate. Results: It was discovered that the reaction solution speciation was sensitively changed with the reaction conditions such as concentration, temperature, and reaction time. Spontaneous transformations from mono- into di- and eventually tri-substituted products ({SiW₁₁V} → {SiW₁₀V₂} → {SiW₉V₃}) were observed for the reactions of {SiW₁₁} + VO₃ ⁻ and {SiW₁₀} + VO₃ ⁻, respectively. Higher concentration and temperature definitely accelerate the transformation reaction rates whereas different molar ratios of reactants have minimal effects on changing the solution speciation in the cases studied. Conclusions: Time-resolved ESI-MS is effectively and successfully used in the monitoring of the dynamic speciation changes in the reaction mixtures consisting of lacunary POMs with transition salts. This study will provide guidance for the oriented design and controlled synthesis of POMs with novel structures.