Reaction Pathway to the Only Open‐Shell Transition‐Metal Keggin Ion without Organic Ligation

Abstract

Aqueous metal‐oxo clusters without organic ligands (including polyoxometalates and group 13 polycations) are useful for energy, environmental and biomolecular applications. While ligands stabilize clusters of different elements, such as open‐shell transition metals, they inhibit reactivity and solubility. Therefore a grand challenge is to conceive and execute synthetic strategies to isolate metal‐oxo clusters of any composition, without ligation. Here we crystallize and structurally characterize a Cr3+ dodecamer Keggin ion without organic ligation. Detailed characterization of its formation leads to understanding its reaction pathway, which may translate to isolation of other open‐shell transition metal‐oxo clusters. We employed electrospray ionization mass spectrometry, Raman spectroscopy and X‐ray scattering (small‐angle and total) to identify important reaction intermediates and processes. The aqueous synthesis presented here is simple to implement and scalable, producing pure phase materials in gram quantities.