Abstract The supramolecular complex [Mg(H 2 O) 6 Br 2 ] · [(C 7 H 14 N 4 ) 2 ] · 4 (H 2 O) ( 1 ) was synthesized and characterized by combination of techniques as NMR, FT-IR, elemental analysis and argentometric titration. The molecular structure was evidenced by X-ray crystallography. It underwent a spontaneous phase transition to a second monoclinic phase ( 2 ). Phase ( 1 ) crystallizes in the monoclinic space group P 2 1 / c with a = 13.79430(12), b = 14.80930(13), c = 14.29182(12) Å, β = 91.3976(5)°, and Z = 4. The crystal structure was partially solved by direct methods from intensities obtained from a microcrystal of the material measured by single-crystal method, and finalized from powder data. The second phase ( 2 ) with monoclinic space group P 2 1 / a , a = 9.19007(6), b = 21.38080(17), c = 8.12027(6) Å, β = 113.7766(5)°, and Z = 2, was solved from powder data, ab initio , by a charge flipping algorithm. The diffraction experiments showed an irreversible phase change occurs as a function of temperature with apparent gain of thermodynamic stability.