Probing the Self-Assembly of Inorganic Cluster Architectures in Solution with Cryospray Mass Spectrometry: Growth of Polyoxomolybdate Clusters and Polymers Mediated by Silver(Ⅰ) Ions

摘要

Cryospray mass spectrometry (CSI-MS) has been used to probe the mechanism of self-assembly of polyoxometalate clusters in solution. By using CSI-MS and electronic absorbance spectroscopy it was possible to monitor in real-time the self-assembly of polymeric chains based on [Ag_2Mo_8O_(26)]~(2-)_n building blocks. The role of the Ag ion in the solution state rearrangement of molybdenum Lindqvist ({Moe_6}) into the silver-linked β-octamolybdate ({Mo_8}) structure ((n-C_4H_9)_4N)_(2n)[Ag_2Mo_8O_(26)]n (1) is revealed in unprecedented detail. A monoanionic series, in particular [AgMo_mO_(3m+1)]~- where m = 2 to 4, and series involving mixed oxidation state polyoxomolybdate species, which illustrate the in-solution formation of the (Ag{Mo_8}Ag) building blocks, have been observed. CSI-MS detection of species with increasing metal nuclearity concomitant with increasing organic cation contribution supports the hypothesis that the organic cations used in the synthesis play an important structure-directing role in polyoxometalate (POM) growth in solution. A real-time decrease in [{Mo_6}] and associated increase in [{Mo_8}] have been observed using CSI-MS and electronic absorbance spectroscopy, and the rate of {Mo_6} interconversion to {Mo_8} was found to decrease on increasing the size of the countercation. This result can be attributed to the steric bulk of larger organic groups hindering {Mo_6} to {Mo_8} rearrangement and hindering the contact between silver cations and molybdenum anions.