Characterization of polyoxometalates by electrospray ionization mass spectrometry

Insights into the self-assembly of giant polyoxomolybdates from building blocks to supramolecular structures

Giant polyoxomolybdates are a special class of polyoxometalate clusters which can bridge the gap between small molecule clusters and large polymeric entities. Besides, giant polyoxomolybdates also show interesting applications in catalysis, biochemistry, photovoltaic and electronic devices, and other fields. Revealing the evolution route of the reducing species into the final cluster structure and also their further hierarchical self-assembly behaviour is undoubtedly fascinating, aiming to guide the design and synthesis. Herein, we reviewed the self-assembly mechanism study of giant polyoxomolybdate clusters, and the exploration of a new structure and new synthesis methodology is also summarized. Finally, we emphasize the importance of in-operando characterization in revealing the self-assembly mechanism of giant polyoxomolybdates, and especially for the further reconstruction of intermediates into the designable synthesis of new structures.

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 ({SiW12-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 {SiW12-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 ({SiW11 V} → {SiW10 V2 } → {SiW9 V3 }) 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. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of vanadium valence state on the solution chemistry and the stability of vanadium substituted polyoxometalates

ESI-MS combined with DFT calculation revealed that the vanadium valence state and the vanadium substitution degrees can substantially affect the stabilities of vanadium-substituted POMs.