Gas-Phase Fragmentation Pathways of Mixed Addenda Keggin Anions: PMo12-nW nO 40 3- (n = 0-12).

We report a collision-induced dissociation (CID) investigation of the mixed addenda polyoxometalate (POM) anions, PMo(12-n)W(n)O(40)(3-) (n = 0-12). The anions were generated in solution using a straightforward single-step synthesis approach and introduced into the gas phase by electrospray ionization (ESI). Distinct differences in fragmentation patterns were observed for the range of mixed addenda POMs examined in this study. CID of molybdenum-rich anions, PMo(12-n)W(n)O(40)(3-) (n = 0-2), generates an abundant doubly charged fragment containing seven metal atoms (M) and 22 oxygen atoms (M(7)O(22)(2-)) and its complementary singly charged PM(5)O(18)(-) ion. In comparison, the doubly charged Lindqvist anion, (M(6)O(19)(2-)) and its complementary singly charged PM(6)O(21)(-) ion are the dominant fragments of Keggin POMs containing more than two tungsten atoms, PMo(12-n)W(n)O(40)(3-) (n = 3-12). The observed transition in the dissociation pathways with an increase in the number of W atoms in the POM may be attributed to the higher barrier of tungsten-rich anions towards isomerization. We present evidence that the observed distribution of Mo and W atoms in the major M(6)O(19)(2-) and M(7)O(22)(2-) fragment ions is different from that predicted by a random distribution, indicating substantial segregation of the addenda metal atoms in the POMs. Charge reduction of the triply charged precursor anion resulting in formation of doubly charged anions is also observed. This is a dominant pathway for mixed POMs having a majority (8-11) of W atoms and a minor channel for other precursors indicating a close competition between fragmentation and charge loss pathways in CID of POM anions.