Electrospray mass spectral study of isopolyoxomolybdates

Abstract

Electrospray mass spectrometry (ESMS) has been performed on aqueous solutions of dilute (10^–3 M) isopolyoxomolybdate systems. There is direct evidence that the evaporation process in the ESMS technique involves significant chemical effects, resulting in the detection of many new anions and cations. For ammonium polyoxomolybdate systems, negative-ion ESMS yields ions of the form [HMo_mO_3m + 1]^–, [Mo_mO_3m + 1]^2–, [Mo_mO_3m + 2]^4– as well as [Mo₇O₂₄]^6–, whereas for alkali metal polyoxomolybdate systems ions of the form [Mo_mO_3m + 1A]^– and [Mo_mO_4mA_2m – 2]^2– (where A = Li⁺, Na⁺ or K⁺) were observed. In positive-ion mode two series of polyoxomolybdate cations, namely [Mo_mO_4mA_2m + 1]⁺ and [Mo_mO_4mA_2m + 2]²⁺ could be assigned. Aggregates of both the [HMo_mO_3m + 1]^– and [Mo_mO_3m + 1]^2– series in the ammonium polyoxomolybdate system can be classified in terms of open-chained structures of tetrahedra that are corner shared, whereas the highly charged anions [Mo_mO_3m + 2]^4– and [Mo_mO_3m + 3]^6– are consistent with closed-packed structures. For the alkali metal polyoxomolybdate anion and cation systems the spectra are consistent with open-chained structures of octahedral units that are edge shared, with a terminating tetrahedral unit. Linear correlations suggest that the assembly of these aggregates occurs via an addition polymerization mechanism. This model, consistent with the ESMS data, may identify the additive moieties (MoO₃, MoO₂²⁺ and Mo₂O₈A₄) required for aggregation of polyoxomolybdate species in aqueous solution.