Elucidating the Geometric and Electronic Structure of a Fully Sulfided Analog of an Anderson Polyoxomolybdate Cluster

The biological activity of transition metal sulfide (TMS) clusters in enzymatic reactions, small molecule reduction, and charge transfer has sparked interest in designing novel TMS clusters for reductive catalysis and other chemical and materials applications. Polyoxometalates (POMs), known for their diverse structures, can be formed into TMS clusters, but these have seldom been isolated in the fully sulfided state, likely due to the tendency of uncapped TMS clusters to agglomerate. Here, we report the geometric and electronic structure of a capping ligand-free fully sulfided analog of heptamolybdate Anderson POM [MoVI7O24]6–, synthesized through the sulfidation of a nanoconfined POM secured within a porous Zr-metal organic framework (NU-1000). A combined computational and experimental analysis indicates that the sulfided counterpart of the Anderson POM is geometrically and electronically more sophisticated than the parent POM. Comparison of experimental pair distribution function (PDF) data with computationally simulated structures confirms that, unlike only oxygen anions in [MoVI7O24]6– cluster, the [MoIV7(3-S)6(2-SH)6(S2)6]2– polythiometalate (PTM) contains various sulfur anions (S2–, HS–, S22–). DFT calculations show that H2S acts as a reducing agent and along with the presence of terminal disulfides (S22–) in the PTM’s structure converts all seven Mo(VI) of the parent POM into seven Mo (IV) in the PTM. X-ray photoelectron spectroscopy (XPS) confirms the presence of Mo (IV) in the PTM cluster.