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 I23, a = 11.785 Å, Z = 2) contains heterocubane-type [ F e 4 S e 4 ] 2 + clusters with T d symmetry. Four additional selenium atoms complete the tetrahedral coordination around the iron atoms, forming [ ( F e 4 S e 4 ) S e 4 ] 6 − 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 [ F e 4 S 4 ] 2 + , 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 S = 0 state above 2 K.