Intriguing role of a quaternary ammonium cation in the dissociation chemistry of Keggin polyoxometalate anions.

The gas-phase fragmentations of a series of Keggin polyoxometalate anions with molecular formula of TBA(n)[XM(12)O(40)] (X = P, Si; M = Mo, W) were studied by electrospray ionization tandem mass spectrometry. The bare polyoxoanions XM(12)O(40) as well as the non-covalent complexes {TBA[XM(12)O(40)]}((n-1)-) and {TBA(m)XM(12)O(40)}(3-) displayed characteristic dissociation pathways. Fragmentation of XM(12)O(40) led to pairs of complementary product anions whose total stoichiometry and charge matched those of the precursor anion, consistent with the previous study by Ma et al. The nature of the non-covalent interaction between XM(12)O(40) and TBA(+) was addressed in detail via the example of {TBA[XM(12)O(40)]}((n-1)-). The non-covalent interaction [1] primarily dominated by the Coulombic attraction of the opposite charges completely changed the dissociation chemistry of XM(12)O(40). The non-covalent complexes {TBA[XM(12)O(40)]}((n-1)-) and {TBA(m)XM(12)O(40)}(3-), formed by the charge reduction during the electrospray process, underwent distinct dissociation routes: {TBA[XM(12)O(40)]}((n-1)-) fragmented to give rise to its product ion {(C(4)H(9))[XM(12)O(40)]}((n-1)-) by cleaving the N-C covalent bond inside the TBA(+) cation whereas {TBA(m)XM(12)O(40)}(3-) dissociated into a pair of product ions, {TBA(i)[XM(12)O(40)]}(2-) and {TBA(m-i)[XM(12)O(40)]}(-), by breaking the non-covalent bond between XM(12)O(40) and TBA(+). In addition, energy-variable CID was used to map the relative stabilities of the ion clusters in the gas phase, which was in excellent agreement with the relative orders of thermal stability in the condensed phase.