Polymorphism and phase transformations in cobaltacarborane molecular crystals

Abstract

The full study of polymorphism in the cobaltacarborane closo-[3-Co(η⁵-C₅H₅)-1,2-C₂B₉H₁₁] (1) compound by a combination of X-ray diffraction, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), IR and hot-stage microscopy (HSM) is reported for the first time. The previously reported Form I (orthorhombic Form, noncentrosymmetric space group P2₁2₁2) crystallizes as dark yellow to orange flat prisms from polar dichloromethane. A new centrosymmetric modification (Form II) that crystallizes from a mixture of polar and non-polar solvents has been found. Form II crystallizes as pleochroic orange blocks in the monoclinic crystal system and space group P2₁/c. Phase transformation between both polymorphs has been observed by DSC, IR and HSM, showing that the centrosymmetric Form II (kinetic) converts irreversibly to the noncentrosymmetric Form I (thermodynamic).