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 ₁₂O ₄₀] (X = P, Si; M = Mo, W) were studied by electrospray ionization tandem mass spectrometry. The bare polyoxoanions [XM ₁₂O ₄₀] ^n- as well as the non-covalent complexes {TBA[XM ₁₂O ₄₀]} ^(n-1)- and {TBA _m[XM ₁₂O ₄₀] ₂} ^3- displayed characteristic dissociation pathways. Fragmentation of [XM ₁₂O ₄₀] ^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 [XM ₁₂O ₄₀] ^n- and TBA ⁺ was addressed in detail via the example of {TBA[XM ₁₂O 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 ₁₂O ₄₀] ^n-. The non-covalent complexes {TBA[XM ₁₂O ₄₀]} ^(n-1)- and {TBA _m[XM ₁₂O ₄₀] ₂} ^3-, formed by the charge reduction during the electrospray process, underwent distinct dissociation routes: {TBA[XM ₁₂O ₄₀]} ^(n-1)- fragmented to give rise to its product ion {(C ₄H ₉)[XM ₁₂O ₄₀]} ^(n-1)- by cleaving the N-C covalent bond inside the TBA ⁺ cation whereas {TBA _m[XM ₁₂O ₄₀] ₂} ^3- dissociated into a pair of product ions, {TBA _i[XM ₁₂O ₄₀]} ^2- and {TBA _m-i[XM ₁₂O ₄₀]} ^-, by breaking the non-covalent bond between [XM ₁₂O ₄₀] ^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.