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

Abstract

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) + NaVO 3 ) 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 ({SiW 11 V} → {SiW 10 V 2 } → {SiW 9 V 3 }) were observed for the reactions of {SiW 11 } + VO 3 À and {SiW 10 } + VO 3 À , 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.Polyoxometalates (POMs) represent a class of inorganic materials with an almost unmatched range of structure types, combined with a vast range of possible uses and physical properties from materials science to biology. [1,2] In contrast to a great range of cluster types and structural diversity of POM clusters, the synthetic method is deceptively simple and straightforward, which is called "one-pot" synthesis. The apparently simple "one-pot" reactions can hide very complex solution chemistry processes arising from the presence of multiple equlibria that depends on a multitude of factors such as pH, ionic strength, reaction time, temperature, counterions, and concentration of starting materials. [3][4][5][6][7][8][9][10][11][12][13] A barely detectable change in any given parameter can alter the solution speciation, thus affecting the outcome of the reaction. The sensitive dependence of the cluster architectures on the solution reaction as well...

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“…Among these, MS is an important analytical technique for accurately determining the structures and compositions of POMs. Both ESI and MALDI have been used as ionization techniques for POMs for MS analysis. Figure a–b shows negative ion SALDI mass spectra of dodecatungstosilicic acid (H 4 SiW 12 O 40 , SiW12) and phosphomolybdic acid (H 3 PMo 12 O 40 , PMo12), which are typical POMs, obtained by using TNC substrates.…”

Recent Approaches for Chemical Speciation and Analysis by Electrospray Ionization (ESI) Mass Spectrometry

“…Exploiting more complex MS experiments and adopting time-resolved ESI-MS, Cao et al (2016) monitored the "onepot" method for the synthesis of polyoxometalates (POMs), produced using silicotungstates and vanadium salts. These authors discovered that the reaction conditions, such as concentration, temperature, and reaction time, sensitively changed the speciation.…”

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Recovery of High-Purity Vanadium from Aqueous Solutions by Reusable Primary Amines N1923 Associated with Semiquantitative Understanding of Vanadium Species

“…ACCs were employed to produce intermediates relying on their momentary capacity of phase separation. 14,15 ESI-TOF-MS, a "soft" ionization technique to characterize polyoxometalate (POM) compounds in polar solvents, 16,17 became a powerful tool for the detecting and tracking of vanadium species. 18 Furthermore, this on-line monitoring system is also promising for other transition metal species, which can further expose transform pathways of various transition metal species in secondary resource recovery.…”

“…In this paper, observation of the vanadium species transformation pathway in the extraction process was realized on the basis of a novel on-line monitoring system combined with annular centrifugal contactors (ACCs) and electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). ACCs were employed to produce intermediates relying on their momentary capacity of phase separation. , ESI-TOF-MS, a “soft” ionization technique to characterize polyoxometalate (POM) compounds in polar solvents, , became a powerful tool for the detecting and tracking of vanadium species . Furthermore, this on-line monitoring system is also promising for other transition metal species, which can further expose transform pathways of various transition metal species in secondary resource recovery.…”