Hexaaquamagnesium(II) hydrogenphosphite: Synthesis, crystal structure, vibrational study, Hirshfeld surface, DFT insights, and anti-corrosion efficacy

Abstract

This work focused on the synthesis of Hexaaquamagnesium(II) hydrogenphosphite, Mg(H₂O)₆.HPO₃, 1. The structural analysis reveals that the crystalline framework, composed of Mg(H₂O)₆²⁺ cations and HPO₃^2- anions, features a magnesium ion in an octahedral arrangement, flanked by six oxygen atoms originating from water molecules and phosphite ions. The three-dimensional framework is stabilized by an extended set of intermolecular hydrogen bonds between hydrogen atoms on coordinated water molecules and oxygen atoms in the phosphite units. The functional groups were confirmed using Fourier-transform infrared spectroscopy (FTIR). TGA–DSC reveals a single endothermic dehydration step (∼48% mass loss) due to loss of six coordinated water molecules, yielding a stable anhydrous MgHPO₃ with no further thermal decomposition up to 350 °C. The reactivity and electronic properties were investigated using DFT calculations performed with the DMol3 code, including solvent effects modeled through the COSMO approach. In addition, the anticorrosion properties of Hexaaquamagnesium(II) hydrogenphosphite hexahydrate have been evaluated in an acid medium (HCl 1 M) on C38 steel, employing electrochemical techniques (Tafel polarization, EIS impedance). The inhibition efficiency reached 85.82% and 89.73% based on potentiodynamic polarization and EIS measurements, respectively, attributed to the formation of a protective film that suppresses both cathodic and anodic reactions. This sorption follows the Langmuir-isotherm model, which suggests an electrostatic interaction between phosphite anions and the metal substrate. Complementary theoretical studies have established a correlation between electronic reactivity and inhibitory activity, opening up prospects for the rational design of phosphite inhibitors.