Solvothermal Synthesis of [Cr₇ S₈ (en)₈ Cl₂ ]Cl₃  ⋅ 2H₂ O with Magnetically Frustrated [Cr₇ S₈ ]⁵⁺ Double-Cubes

A novel transition metal chalcohalide [Cr₇ S₈ (en)₈ Cl₂ ]Cl₃  ⋅ 2H₂ O, with [Cr₇ S₈ ]⁵⁺ dicubane cationic clusters, has been synthesized by a low temperature solvothermal method, using dimethyl sulfoxide (DMSO) and ethylenediamine (en) solvents. Ethylenediamine ligand exhibits bi- and monodentate coordination modes; in the latter case ethylenediamine coordinates to Cr atoms of adjacent clusters, giving rise to a 2D polymeric structure. Although magnetic susceptibility shows no magnetic ordering down to 1.8 K, a highly negative Weiss constant, θ=-224(2) K, obtained from Curie-Weiss fit of inverse susceptibility, suggests strong antiferromagnetic (AFM) interactions between S=3/2 Cr(III) centers. Due to the complexity of the system with (2S+1)⁷ =16384 microstates from seven Cr³⁺ centers, a simplified model with only two exchange constants was used for simulations. Density-functional theory (DFT) calculations yielded the two exchange constants to be J₁ =-21.4 cm^-1 and J₂ =-30.2 cm^-1 , confirming competing AFM coupling between the shared Cr³⁺ center and the peripheral Cr³⁺ ions of the dicubane cluster. The best simulation of the experimental data was obtained with J₁ =-20.0 cm^-1 and J₂ =-21.0 cm^-1 , in agreement with the slightly stronger AFM exchange within the triangles of the peripheral Cr³⁺ ions as compared to the AFM exchange between the central and peripheral Cr³⁺ ions. This compound is proposed as a synthon towards magnetically frustrated systems assembled by linking dicubane transition metal-chalcogenide clusters into polymeric networks.