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_nXM₁₂O₄₀] (X = P, Si; M = Mo, W) were studied by electrospray ionization tandem mass spectrometry. The bare polyoxoanions XM₁₂O₄₀]^n- as well as the non-covalent complexes {TBAXM₁₂O₄₀]}^(n-1)- and {TBA_mXM₁₂O₄₀]₂}^3- displayed characteristic dissociation pathways. Fragmentation of XM₁₂O₄₀]^n- 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 XM₁₂O₄₀]^n- and TBA⁺ was addressed in detail via the example of {TBAXM₁₂O₄₀]}^(n-1)-. The non-covalent interaction 1] primarily dominated by the Coulombic attraction of the opposite charges completely changed the dissociation chemistry of XM₁₂O₄₀]^n-. The non-covalent complexes {TBAXM₁₂O₄₀]}^(n-1)- and {TBA_mXM₁₂O₄₀]₂}^3-, formed by the charge reduction during the electrospray process, underwent distinct dissociation routes: {TBAXM₁₂O₄₀]}^(n-1)- fragmented to give rise to its product ion {(C₄H₉)XM₁₂O₄₀]}^(n-1)- by cleaving the N−C covalent bond inside the TBA⁺ cation whereas {TBA_mXM₁₂O₄₀]₂}^3- dissociated into a pair of product ions, {TBA_iXM₁₂O₄₀]}^2- and {TBA_m-iXM₁₂O₄₀]}^-, by breaking the non-covalent bond between XM₁₂O₄₀]^n- 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.