New Metformin–Citric Acid Pharmaceutical Molecular Salt: Improving Metformin Physicochemical Properties

Crystal engineering and, more specifically, the development of multicomponent materials has become an effective technique to rationally modify important physicochemical properties of solids, such as solubility and thermal stability. In this work, in order to overcome some of the problems that metformin has as a pharmaceutical, a new metformin base salt with citric acid (MTF–CIT) has been developed, which improves the thermal stability and solubility (two-fold) compared to metformin base (MTF). A complete characterization of the new crystalline form through PXRD, DSC, SCXRD, and FT–IR was conducted to ensure the purity of the new phase and provide a comprehensive view of its physicochemical behavior, thus correlating the improvement in stability and solubility with the crystal structure. The MTF–CIT salt crystallizes in the monoclinic P21/c1 spacegroup with z′ = 1. Intermolecular interactions found in MTF–CIT structure and simulated crystal morphology suggest a steric protection effect on the metformin ion that leads to the enhancement of stability in several orders of magnitude compared with MTF, as well as an improvement in solubility due to the exposition of polar groups in the biggest facets, making this new multicomponent salt a promising pharmaceutical solid.