Atomically Precise Lanthanide-Iron-Oxo Clusters Featuring the ε-Keggin Ion

Atomically precise molecular metal-oxo clusters provide ideal models to understand metal oxide surfaces, self-assembly, and form-function relationships. Devising strategies for synthesis and isolation of these molecular forms remains a challenge. Here, the synthesis of four Ln-Fe oxo clusters that feature the epsilon-{Fe-13} Keggin cluster in their core is reported. The {Fe-13} metal-oxo cluster motif is the building block of two important iron oxyhydroxyide phases in nature and technology, ferrihydrite (as the delta-isomer) and magnetite (the epsilon-isomer). The reported epsilon-{Fe-13} Keggin isomer as an isolated molecule provides the opportunity to study the formation of ferrihydrite and magnetite from this building unit. The four currently reported isostructural lanthanide-iron-oxo clusters are fully formulated [Y12Fe33(TEOA)(12)(Hyp)(6)(mu(3)-OH)(20)(mu(4)-O)(28)(H2O)(12)](ClO4)(23).50 H2O (1, Y12Fe33), [Gd12Fe33(TEOA)(12)(Hyp)(6)(mu(3)-OH)(20)(mu(4)-O)(32)(H2O)(12)](ClO4)(15).50 H2O (2, Gd12Fe33) and [Ln(16)Fe(29)(TEOA)(12)(Hyp)(6)(mu(3)-OH)(24)(mu(4)-O)(28)(H2O)(16)](ClO4)(16)(NO3)(3).n H2O (Ln=Y for 3, Y16Fe29, n=37 and Ln=Gd for 4, Gd16Fe29 n=25; Hyp=trans-4-Hydroxyl-l-proline and TEOA=triethanolamine). The next metal layer surrounding the epsilon-{Fe-13} core within these clusters exhibits a similar arrangement as the magnetite lattice, and Fe and Ln can occupy the same positions. This provides the opportunity to construct a family of compounds and optimize magnetic exchange in these molecules through composition tuning. Small-angle X-ray scattering (SAXS) and high-resolution electrospray ionization mass spectrometry (HRESI-MS) show that these clusters are stable upon dissolution in both water and organic solvents, as a first step to performing further chemistry towards building magnetic arrays or investigating ferrihydrite and magnetite assembly from pre-nucleation clusters.