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

Abstract

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 ϵ-{Fe13 } Keggin cluster in their core is reported. The {Fe13 } metal-oxo cluster motif is the building block of two important iron oxyhydroxyide phases in nature and technology, ferrihydrite (as the δ-isomer) and magnetite (the ϵ-isomer). The reported ϵ-{Fe13 } 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 [Y12 Fe33 (TEOA)12 (Hyp)6 (μ3 -OH)20 (μ4 -O)28 (H2 O)12 ](ClO4 )23 ⋅50 H2 O (1, Y12 Fe33 ), [Gd12 Fe33 (TEOA)12 (Hyp)6 (μ3 -OH)20 (μ4 -O)32 (H2 O)12 ](ClO4 )15 ⋅50 H2 O (2, Gd12 Fe33 ) and [Ln16 Fe29 (TEOA)12 (Hyp)6 (μ3 -OH)24 (μ4 -O)28 (H2 O)16 ](ClO4 )16 (NO3 )3 ⋅n H2 O (Ln=Y for 3, Y16 Fe29 , n=37 and Ln=Gd for 4, Gd16 Fe29 n=25; Hyp=trans-4-Hydroxyl-l-proline and TEOA=triethanolamine). The next metal layer surrounding the ϵ-{Fe13 } 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.