Investigating the Formation of “Molybdenum Blues” with Gel Electrophoresis and Mass Spectrometry

The reduction of solutions of acidified
molybdate leads to the
formation of a family of nanostructured molybdenum blue (MB) wheels
which are linked together in a series of complex reaction networks.
These networks are complex because the species which define the nodes
are extremely labile, unstable, and common to many different networks.
Herein, we combine gel electrophoresis and electrospray ionization
mass spectrometry (ESI-MS) to investigate the effect of the pH and
the ratio of reactants and reducing agents, R (R = [S2O42–]/[MoO42–]), on the complex underlying set of equilibria
that make up MBs. By mapping the reaction parameter space given by
experimental variables such as pH, R, solvent medium,
and type of counterion, we show that the species present range from
nanostructured MB wheels (comprising ca. 154 Mo atoms) to smaller
molecular capsules, [(SO3)2MoV2MoVI16O54]6– ({S2Mo18}), and templated hexameric [(μ6-SO3)­MoV6O15(μ2-SO3)3]8–({S4Mo6}) anions. The parallel effects of templation and reduction
on the self-assembly process are discussed, taking into consideration
the Lewis basicity of the template, the oxidation state of the Mo
centers, and the polarity of the reaction medium. Finally, we report
a new type of molecular cage (TBA)5[Na­(SO3)2(PhPO3)4MoV4MoVI14O49]·nMeCN
(1), templated by SO32– anions
and decorated by organic ligands. This discovery results from the
exploration of the cooperative effect of two anions possessing comparable
Lewis basicity, and we believe this constitutes a new synthetic approach
for the design of new nanostructured molecular metal oxides and will
lead to a greater understanding of the complex reaction networks underpinning
the assembly of this family of nanoclusters.