pH-Driven Rotational Configuration of Keggin-Fe 13 Clusters and Their Transformations.

Keggin-Fe ₁₃ clusters are considered foundational building blocks or prenucleation precursors of ferrihydrite. Understanding the factors that influence the rotational configuration of these clusters, and their transformations in water, is vital for comprehending the formation mechanism of ferrihydrite. Here, we report syntheses and crystal structures of four lanthanide-iron-oxo clusters, namely, [Dy ₆ Fe ₁₃ (Gly) ₁₂ (μ ₂ -OH) ₆ (μ ₃ -OH) ₁₈ (μ ₄ -O) ₄ (H ₂ O) ₁₇ ]·13ClO ₄ ·19H ₂ O ( 1 ), [Dy ₆ Fe ₁₃ (Gly) ₁₂ (μ ₃ -OH) ₂₄ (μ ₄ -O) ₄ (H ₂ O) ₁₈ ]·13ClO ₄ ·14H ₂ O ( 2 ), [Pr ₈ Fe ₃₄ (Gly) ₂₄ (μ ₃ -OH) ₂₈ (μ ₃ -O) ₃₀ (μ ₄ -O) ₄ (H ₂ O) ₃₀ ]·6ClO ₄ ·20H ₂ O ( 3 ), and [Pr ₆ Fe ₁₃ (Gly) ₁₂ (μ ₃ -OH) ₂₄ (μ ₄ -O) ₄ (H ₂ O) ₁₈ ]·13ClO ₄ ·22H ₂ O ( 4 , Gly = glycine). Single-crystal analyses reveal that 1 has a β-Keggin-Fe ₁₃ cluster, marking the first documented instance of such a cluster to date. Conversely, both 2 and 4 contain an α-Keggin-Fe ₁₃ cluster, while 3 is characterized by four hexavacant ε-Keggin-Fe ₁₃ clusters. Magnetic property investigations of 1 and 2 show that 2 exhibits ferromagnetic interactions, while 1 exhibits antiferromagnetic interactions. An exploration of the synthetic conditions for 1 and 2 indicates that a higher pH promotes the formation of α-Keggin-Fe ₁₃ clusters, while a lower pH favors β-Keggin-Fe ₁₃ clusters. A detailed analysis of the transition from 3 to 4 emphasizes that lacunary Keggin-Fe ₁₃ clusters can morph into Keggin-Fe ₁₃ clusters with a decrease in pH, accompanied by a significant change in their rotational configuration.