Connecting theory with experiment to understand the initial nucleation steps of heteropolyoxometalate clusters.

A complimentary combination of Density Functional Theory (DFT) methodology and Electrospray Ionization-Mass Spectrometry (ESI-MS) has been utilized to increase our limited understanding of the first nucleation steps in the formation of the [XM(12)O(40)](n-) Keggin polyoxometalates (POMs) (where addenda metal atom M = W or Mo, and the heteroatom X = P or As). We postulate that the first key steps of nucleation into discrete, high nuclearity heteropolyanions proceed via the formation of isodinuclear species (e.g. [M(2)O(7)](2-)), which undergo successive steps of protonation and water condensation to form a heterotrinuclear fragment, which acts as a template for the constituent parts required for subsequent aggregation and formation of the plenary Keggin heteropolyanion. The stability of calculated structures of the numerous postulated intermediates has been analysed and discussed in detail, and these results complemented using experimental mass spectrometry, using an assembly (reaction solution analysis) and disassembly (fragmentation of single crystals) approach. Overall, no significant differences between the Keggin POMs were found when changing the addenda metal atom (W or Mo) or the heteroatom (P or As); although small differences among the lowest-energy structures were detected.