Computational and experimental characterization of a cagelike Fe ₁₅ polycation

Polynuclear open-shell transition-metal clusters are synthetically challenging, yet fascinating from the perspective of their diverse properties. Iron clusters in particular can exhibit interesting magnetic and electrochemical behaviour, and they can potentially be exploited as models for geochemical processes at iron oxide surfaces. From solvothermal synthesis, we have obtained a cagelike polycationic Fe^III cluster, [Fe₁₅Cl ₆O₆(OH)₂(C₃H₆O ₂)₁₂]⁺, which features the common polyoxometalate (POM) Keggin ion trimer fragments oriented in an inverse fashion relative to the centre of the cluster. In addition to solid-state structural characterization, we have examined the magnetic properties, which revealed the presence of antiferromagnetic exchange within the polynuclear system. Electrospray-ionization mass spectrometry (ESI-MS) revealed that the Fe ₉ core of the cluster remains intact upon dissolution, whereas the Fe-Cl²⁺ caps are more labile. Computational studies agree well with the observed antiferromagnetic character, as well as the HOMO-LUMO bandgap, and also describe these frontier orbitals.