On the Crystal Structures of the Polymorphs of Manganese(II) Chloride Tetrahydrate: α-MnCl₂·4H₂O and β-MnCl₂·4H₂O

Abstract: The manganese(II) chloride tetrahydrate MnCl₂·4H₂O (α-form) crystallizes in the space group P2₁/n with a = 6.1918(4) Å, b = 9.5230(6) Å, c = 11.1918(6) Å, β = 99.757(3)°, and V = 650.37(7) ų (Z = 4). A second polymorph of MnCl₂·4H₂O (β-form) is mentioned in the literature to be isomorphous with FeCl₂·4H₂O but its crystal structure was never confirmed. In this study, we report for the first time on the crystal structure of this second polymorph of manganese(II) chloride tetrahydrate, β-MnCl₂·4H₂O, using room temperature single-crystal X-ray diffraction. The β-form crystallizes in space group P2₁/c with a = 5.9893(5) Å, b = 7.2877(6) Å, c = 8.5838(7) Å, β = 110.952(4)°, and V = 349.89(5) ų (Z = 2), confirming that β-MnCl₂·4H₂O is isotype to Fe(II)Cl₂·4H₂O. To the best of our knowledge this is the first compound reported to crystallize in both MnCl₂·4H₂O-type (α-form) and FeCl₂·4H₂O-type (β-form) structures. The crystal structure of both α-form and β-form of MnCl₂·4H₂O consists of discrete octahedra complexes [MnCl₂(H₂O)₄] linked by O–H⋅⋅⋅Cl⁻ hydrogen bonds. However, chlorine atoms are in trans-configuration in the β-form (trans-[MnCl₂(H₂O)₄] complexes) and in cis-configuration in the α-form (cis-[MnCl₂(H₂O)₄] complexes). Moreover, the coordination polyhedra of the manganese(II) atoms are less distorted in the β-form. This leads to two different hydrogen bond networks and to a slightly more compact [MnCl₂(H₂O)₄] complexes arrangement in the α-form. Graphic Abstract: The title compound is the first representative among divalent metal tetrahydrate halogenides to present both the MnCl₂·4H₂O-type (α-form) and FeCl₂·4H₂O-type (β-form) structures, leading to different configuration of complexes [MnCl₂(H₂O)₄], crystal packing, hydrogen bond networks, and possibly magnetic properties. [Figure not available: see fulltext.]