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 = [S₂O₄^2–]/[MoO₄^2–]), 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)₂Mo^V₂Mo^VI₁₆O₅₄]^6– ({S₂Mo₁₈}), and templated hexameric [(μ₆-SO₃)­Mo^V₆O₁₅(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)₄Mo^V₄Mo^VI₁₄O₄₉]·nMeCN (1), templated by SO₃^2– 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.