Water Encapsulated [(Fe₄Se₄)Se₄]^6– Clusters in [Na₆(H₂O)₁₈][Fe₄Se₈]

[Na₆(H₂O)₁₈][Fe₄Se₈] was synthesized using hydrothermal methods and characterized by single-crystal X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, magnetization, and muon spin resonance (μSR) measurements. The cubic crystal structure (space group <i>I</i>23, <i>a</i> = 11.785 Å, <i>Z</i> = 2) contains heterocubane-type <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mo>[</mml:mo><mml:msub><mml:mrow><mml:mi>F</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>S</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:mo>]</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math> clusters with <i>T</i>_d symmetry. Four additional selenium atoms complete the tetrahedral coordination around the iron atoms, forming <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mrow><mml:mi>F</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>S</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:msub><mml:mrow><mml:mi>S</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:mo>]</mml:mo></mml:mrow><mml:mrow><mml:mn>6</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msup></mml:math> units. Notably, the selenide ligands do not interact with the sodium cations, as the Fe₄Se₈^6- cluster in [Na₆(H₂O)₁₈][Fe₄Se₈] is exclusively surrounded by water molecules via weak Se···H-O hydrogen bonds. This is particularly remarkable given that [Fe₄Q₄] clusters are typically unstable in water. Quantum chemical calculations (DFT) confirm the crystal structure and improve the positions of the hydrogen atoms. ⁵⁷Fe Mössbauer spectroscopy reveals that the spin ordering of the mixed-valence [Fe₄^2.5+Se₄]²⁺ cluster core mirrors the antiparallel spin arrangement found in analogous iron-sulfur clusters, such as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mo>[</mml:mo><mml:msub><mml:mrow><mml:mi>F</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mi>S</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:mo>]</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>, in protein-bound systems. An increase of the quadrupole splitting at low temperatures indicates changes of the orbital occupations. μSR Experiments show a reduction of the spin fluctuation frequency until the system becomes static at low temperatures, but no evidence of a <i>S</i> = 0 state above 2 K.