Ibuprofen as linker for calcium(II) in a 1D-coordination polymer: A solid state investigation complemented with solution studies

The ibuprofen alkaline-earth compounds Ibuprofen-Mg and Ibuprofen-Ca were investigated by using combined solid state and solution studies. The molecular and crystal structures of the calcium (Ca_Ibu_Hy, a 1D-coordination polymer based on ibuprofen) and magnesium (Mg_Ibu_Hy_RT and Mg_Ibu_Hy_LT) ibuprofen dihydrate salts have been analyzed in depth by using X-ray diffraction (both single crystal and powder) and in-silico tools. Differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), variable temperature X-ray diffraction experiments were carried out to study the stability of the solid forms under different temperature conditions. The Mg_Ibu_Hy_LT -> Mg_Ibu_Hy_RT transition, which occurs between 250 and 300 K, is fully reversible and consists of a mild rearrangement of the crystal structure. Upon raising the temperature, dehydration of the Mg_Ibu_Hy_RT phase occurs, which leads to a completely amorphous phase at T > 340 K. As a consequence, the RT phase of the MgII compound is stable in a quite narrow temperature range. The calcium salt, in turn, gives a partially crystalline anhydrous phase with rising temperature. The coordination behavior of ibuprofen towards MgII, CaII and SrII in a water-ethanol mixture has been studied by means of potentiometric titrations. Based on the almost identical formation constants for CaII- and MgII-Ibuprofen complexes and on the well-known great affinity of MgII-for water molecules, we speculated the formation of solvent-shared ion pairs in the latter case, where the MgII-carboxylate interaction is of the outer-sphere type.