Structure and Polymorphism in M(ethylenediamine)₃MoS₄ (M = Mn, Co, Ni)

Polymorphism in hybrid inorganic−organic materials has not been explored as extensively as that in organic compounds, yet differences in solid-state structure can significantly affect the physical properties central to application of these materials. A new polymorph of Ni(en)₃MoS₄ (en = ethylenediamine), a hydrodesulfurization catalyst precursor, has been synthesized solvothermally and structurally characterized by single-crystal X-ray diffraction. The new structure (2) assumes the orthorhombic Pcab space group with a = 14.020(5) Å, b = 14.821(7) Å, and c = 16.230(6) Å. The structure of a polymorph that had been found previously (1) was redetermined at 100 K, confirming the orthorhombic Pna2₁ structure with a = 15.916(13) Å, b = 7.610(3) Å, and c = 14.093(6) Å. Solvothermal reaction conditions including temperature, solvent water content, and nickel source were important in controlling polymorph formation. Differential scanning calorimetry and solvent-mediated conversion studies were used to compare the stabilities of the two nickel-containing polymorphs. The system was characterized as enantiotropic, with 2 favored at ambient temperature and 1 favored at 120 °C. However, kinetic factors are influential in the intermediate temperature range, and conversion is kinetically hindered under certain conditions. The new structures Co(en)₃MoS₄ (3) and Mn(en)₃MoS₄ (4) were determined through single-crystal methods to be isostructural to 2. Compounds 1−4 were also characterized by elemental analysis, infrared spectroscopy, variable-temperature magnetic susceptibility measurements, and thermogravimetric analysis.