Abstract
(Poly)phosphonic acids (of the general formula R-P(O)(OH)2) constitute a class of phosphorus compounds that is intensively investigated, due to the attractive properties of the phosphonic functional group. Mono- or di-anionic species are generated with its gradual deprotonation (with an increase in pH), and these can act as strong, multifunctional ligands for a multitude of metal ions, producing metal phosphonate compounds. These exhibit a wide variety of crystal structures that can be either coordination polymers (1D, 2D, or 3D), or molecular complexes (0D). The nature of the R group determines some of their physicochemical properties, and, consequently, the specific field of application. Aminophosphonic acids are a distinct sub-class of phosphonic acids, with the presence of (at least) one protonated amino group, which confers a zwitterionic character to the molecule. In recent decades the creation of new phosphonic acids with increased complexity, has attracted the interest of synth ...
(Poly)phosphonic acids (of the general formula R-P(O)(OH)2) constitute a class of phosphorus compounds that is intensively investigated, due to the attractive properties of the phosphonic functional group. Mono- or di-anionic species are generated with its gradual deprotonation (with an increase in pH), and these can act as strong, multifunctional ligands for a multitude of metal ions, producing metal phosphonate compounds. These exhibit a wide variety of crystal structures that can be either coordination polymers (1D, 2D, or 3D), or molecular complexes (0D). The nature of the R group determines some of their physicochemical properties, and, consequently, the specific field of application. Aminophosphonic acids are a distinct sub-class of phosphonic acids, with the presence of (at least) one protonated amino group, which confers a zwitterionic character to the molecule. In recent decades the creation of new phosphonic acids with increased complexity, has attracted the interest of synthetic chemists who have made important advances in their targeted synthesis based on the specific application for which they were intended. One of the most important applications of aminophosphonic acids is their use as corrosion inhibitors in industrial water systems, as they have the ability to create passive coatings on the metal surface, thus preventing corrosion and achieving its protection.This PhD thesis is based on three principal axes: (a) the synthesis and characterization of new amino-disphosphonic acids with the general formula R-N(CH2PO3H2)2, R = aliphatic chain, R = aliphatic chain, C1, C2, C3, C4, C6, C8, and C12], (b) their use as ligands to divalent metal ions in the synthesis of new metal phosphonate materials, and (c) the study of both amino-diphosphonic acids (in their “free” form) and their metal phosphonate derivatives as corrosion inhibitors under various experimental conditions, by changing variables such as pH, inhibitor concentration, metal:phosphonate molar ratio, etc.The synthesis of amino-disphosphonic acids was carried out based on the Mannich type reaction methodology (also known as Moedritzer-Irani) with high yields (70–80 %). For the synthetic access to these compounds, primary aliphatic amines of variable carbon chain length (in one case the R side group was benzyl-) were selected. Their characterization was implemented by nuclear magnetic resonance spectroscopy (1H, 13C and 31P) and vibrational spectroscopy (ATR-IR).Subsequently, a mapping of their coordination chemistry with divalent alkaline earth metals (Mg2+, Ca+2, Sr2+, Ba2+) but also with selected transition metals (Co2+, Cu2+) was investigated. The bridging co-ligand 4,4'-dipyridine was used in certain syntheses. The isolated compounds were characterized by various physicochemical methods, while the determination of their crystal structures was performed by single crystal X-ray diffraction. The metal centers presented a variety of coordination geometries, while the amino-disphosphonate ligands were found to possess various deprotonation degrees and displayed chelating and/or bridging modes. These compounds are coordination polymers of various dimensionalities, 1D, 2D or 3D. The crystal structures of thirteen new metal phosphonate materials were determined.Last but not least, the effectiveness of the amino-disphosphonates in inhibiting the corrosion on metal (carbon steel) surfaces, in the presence or absence of metal cations (Mg2+, Ca2+, Sr2+, Ba2+, Zn2+), was studied in detail. Exposure of steel surfaces (C1010) to solutions containing various inhibitor concentrations (0.10 mM, 0.10 mM, 1.00 mM) and of a wide pH range (2 to 7) was selected. At the lower pH (2 to 4) the inhibitory efficiency is low, and in most cases it is less than 30 %. This can be attributed to the mono-deprotonated state of the phosphonic group, which limits its ability to form bonds with the metal surface. Also, it was observed that the lowest concentration (0.01 mM) is ineffective in almost the entire studied pH range. In contrast, the higher concentrations of 0.10 mM and 1.00 mM demonstrated enhanced inhibition, both in the presence and absence of metal ions. The maximum inhibitory efficiency (%) was observed at pH 6 and 7 at inhibitor concentration of 0.10 mM. There was no clear trend in the increase inhibitory efficiency with increasing size of the R chain. Interestingly, enhanced inhibition was noted in the case of the benzyl analog, in more acidic conditions. Each system exhibits its maximum inhibition performance at different pH and concentration conditions.
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