Gas-Phase Fragmentation of Polyoxotungstate Anions

Abstract

A series of phospho-polyoxotungstate anions was transferred to the gas phase via electrospray ionization (ESI), and the anions' fragmentation was examined by collision-induced dissociation (CID). The anions included [PW12O40]3-, [P2W18O62]6-, and {Co4(H2O)2][PW9O34]2}10- as well as lacunary and metal-substituted derivatives such as [PW11O39]7- and [MPW11O39]5- (M = Co(II), Ni(II), Cu(II)). Common species observed in the mass spectra arose from protonation and alkali metal cationization of the precursor ions. Additional species arising from the formal loss of oxide from the precursor species were also observed, presumably formed via protonation and the loss of an oxo ligand as water. These processes of protonation/cationization and the loss of water both led to species with reduced gas-phase anionic charges, and their formation appears to be driven by the enhanced effects of Coulombic repulsion in the desolvated species generated during transfer to the gas phase via ESI. Fragmentation of selected species was examined by multistage mass spectrometry experiments employing CID. Fragmentation occurred via multiple reaction channels, leading to pairs of complementary product anions whose total stoichiometry and charge matched those of the precursor anion. For example, [PW12O40]3- fragmented to give pairs of product ions of general formulas [W(x)O(3x+1)]2- and [PW(12-x)O(39-3x)]- (x = 6-9), with the most intense pair being [W6O19]2- and [PW6O21]-. Similar ions were also observed for fragmentation of [P2W18O61]4- (derived from the loss of water from [P2W18O62]6-). The lacunary and M(II)-substituted lacunary systems fragmented via related pathways, with the latter generating additional fragment ions due to the presence of M(II). These results highlight the usefulness of ESI-MS in the characterization of complex polyoxometalate anion clusters.

Intriguing Role of a Quaternary Ammonium Cation in the Dissociation Chemistry of Keggin Polyoxometalate Anions

“…Although a few systematic studies of the gas-phase fragmentation of POM clusters have been reported so far [12,[20][21][22], little attention has been paid to the interaction between the cations and the POM anions in the gas phase, which reflects intrinsic nature of interaction between these two entities. The interaction is simple in the case of alkali metal cations (i.e., Na + , K + , etc).…”

Gas-Phase Fragmentation Pathways of Mixed Addenda Keggin Anions: PMo12-nWnO403– (n = 0–12)

“…The larger energy gap between α and γ isomers for tungsten-rich POM may inhibit isomerization into and fragmentation from the γ-Keggin structure. This likely explains the preferential formation of the M 6 O 19 2-Lindqvist anion, which is known to be the most abundant fragmentation product of the α isomer of W 12 POM [31].…”

“…In the present study, we use a straightforward method of preparing mixed addenda Keggin POMs, PMo 12-n W n O 40 3-(n = 0-12), for analysis by electrospray ionization mass spectrometry (ESI-MS) and examine their dissociation pathways and relative stability towards fragmentation by means of collision-induced dissociation (CID) experiments. A few groups have previously studied the Keggin Mo 12 POM, W 12 POM, and selected mixed addenda POMs employing CID to analyze fragmentation patterns of the respective species [27][28][29][30][31][32]. However, to the best of our knowledge, there are no prior reports of systematic atom-by-atom CID studies of the full range of mixed addenda Keggin POMs.…”

Generation and detection of gaseous W12O41−· and other tungstate anions by laser desorption ionization mass spectrometry

“…By electrospray ionization mass spectrometry of aqueous tungstate solutions, Walanda et al [12] showed some peaks as multiply charged W 12 species; however, those peaks were of such low abundance and such poor resolution that their composition remained ambiguous. Recently, using ESI-MS, Ma et al [15] unambiguously assigned gas-phase Keggin-type structures to phosphotungstate species with 12 or 18 tungsten atoms. However, these researchers did not work with tungsten compounds without heteroatoms.…”

“…As a consequence to this low signal-to-noise ratio, an unambiguous assignment of a chemical formula to an ion, based on isotope cluster determinations, appeared less reliable. In a more recent ESI-MS report by Ma et al [15], possible structures for gas-phase phosphotungstate fragments, and charge assignments to tungsten species have been discussed on the basis of isotopic peak separations. They also carried out MS-MS analysis on phosphotungstate species, and some important neutral losses that take place were identified.…”