Unusual coexistence of magnetic and nonmagnetic Mo₆ octahedral clusters in a chalcohalide solid solution: Synthesis, X-ray diffraction, EPR, and DFT investigations of Cs₃Mo₆I₆ ⁱI_2-xⁱ_Se_xⁱI ₆^a

The C₃Mo₆I₆ⁱI _2-xⁱSe_xⁱI₆^a series has been obtained by a solidstate route. There is evidence for a solid solution between the compositions Cs₃Mo₆T₆ ⁱI_0.8ⁱSe_1.2ⁱI ₆^a and Cs₂Mo₆I₆ ⁱI_0.4ⁱSe_1.6ⁱI ₆^a (space group: R3̄c, Z = 6; a = 16.7065(4), c = 20.5523(4) Å, V = 4967.8(2) ų and a = 16.6354(3), c = 20.5444(4) Å. V = 4923.7(2) ų, respectively). The structure of this new series is based on magnetic [Mo₆I ₆ⁱSe₂ⁱI₆ ^a]^3- and diamagnetic [Mo₆I₇ ⁱSeⁱI₆^a]^3- units with 23 and 24 valence electrons per Mo₆ cluster, respectively. For a particular x, the structure of Cs₃Mo₆I₆ ⁱI_2-xⁱSe_xI₆^a is based on a mixture of (x-1)[Mo₆I₆ⁱSe ₂ⁱI₆^a]^3- with 2-x) [Mo₆I₇ⁱSeⁱI₆ ^a]³.This leads to an average [Mo₆I ₆ⁱI_2-xⁱSe_xI ₆^a]^3- ionic unit deduced from single-crystal X-ray diffraction investigations. The two inner positions of the average face-capped [Mo₆I_8-xⁱI₆ ^a]³ ionic units (located on the threefold axis of the unit) are randomly occupied by iodine and selenium, whereas the other ligand positions are fully occupied by iodine. Low-temperature electron paramagnetic resonance (EPR) studies reveal a signal split into two components with g ∥ > g ∏. The reciprocal double integration intensity of the EPR signal versus T graph reveals a typical Curie law behavior. A density functional theory (DFT) study indicates that occupation of the inner position on the threefold axis by selenium atoms is preferred energetically among the three possible distributions of selenium atoms. The comparison of experimental and theoretical g values confirms the crystallographic analysis and agrees with the axial elongation of the Mo₆ cluster within the crystal structure.