Boyko Y. Synthesis and properties of calixarene-containing sorbents for binding Eu(III)

Boyko Yu.S. Synthesis and properties of calixarene-containing sorbents for binding Eu(III). - Qualification scientific work on manuscript rights. Dissertation for the Doctor of Philosophy degree in specialty 102 – Chemistry. National University "Kyiv-Mohyla Academy", Ministry of Education and Science of Ukraine, Kyiv, 2023. The dissertation is devoted to the synthesis and investigation of the properties of calixarene-containing sorbents based on the silica gel matrix. In this work we synthesized calixarenes modified on the upper rim of the macrocycle with phosphonate and phosphine oxide groups capable to effective coordination of metals. At the same time, two methods of immobilization of calixarenes on the surface of silica gel were developed. Phosphorus-containing calixarenes were synthesized by the Arbuzov reaction, which consists in the interaction of chloromethylcalixarene with a phosphorylating agent. The reaction proceeds in a chloroform solution at room temperature for 8-12 hours and leads to the corresponding tetraphosphorylated calixarenes in high yields. Similar to the original chloromethylcalixarene, its phosphorylated derivatives exist in a cone conformation. The results of the 1H and 13C NMR spectra evidence the cone-like conformation. The molecular structure of tetrahydroxycalixarene phosphine oxide and bis-methoxycarbonylmethoxycalixarene phosphine oxide were confirmed by X-ray structural analysis.The first method of calixarenes immobilization on the silica gel surface consists in the interaction of methoxycarbonylmethoxy groups of calixarenes with amino groups on the silica gel surface with the formation of amide bonds. The reaction was carried out by boiling in toluene for 30 hours. The phosphorus-containing calixarenes with methoxycarbonylmethoxy groups on the lower rim were synthesized with yields of 81-83%. Regioselective dialkylation of the hydroxyl groups of phosphorylated tetrahydrocalixarenes with an excess of bromoacetic acid methyl ester in the presence of potassium carbonate is occurred according to Williamson reaction. The synthesis of aminopropylated silica gel was carried out by reaction of silica gel with 3-aminopropylethoxysilane in dry toluene at 110°C. The second method of immobilization consists in the direct grafting of phosphorus-containing tetrahydroxycalixarenes onto the surface of silica gel (without an alkylamide spacer). Immobilization of tetraphosphorylated tetrahydroxy(thia)calix[4]arenes on silica gel was carried out in two stages. Silica gel was treated with tetrachlorosilane in the presence of triethylamine, at the first stage, to obtain reactive trichlorosilyl groups on its surface. In the second step, the modified silica gel was heated with an excess of phosphorylated tetrahydroxy(thia)calixarene and triethylamine in toluene for 24 hours. The resulting silica gels with iso-propoxyphosphonate and iso-propoxyphosphinate groups were treated with an excess of trimethylbromosilane. Subsequent treatment of obtained trimethylsilyl esters with methanol excess leads to silica gels modified with dihydroxyphosphonic or hydroxyphenylphosphinic acids. For a better understanding of the spatial structure of phosphorylated calixarenes, the molecular structure of tetrahydroxycalixarene phosphinoxide and bis-methoxycarbonylmethoxycalixarene phosphineoxide was investigated by X-ray analysis. The macrocyclic skeleton of tetrahydroxycalixarene of phosphine oxide has the conformation of a flattened cone.To test the possibilities of practical use of the obtained sorbents, the process of europium(III) sorption from nitric acid solutions in the pH range of 1-7 was investigated. Sorption isotherms with Langmuir and Freundlich models were constructed using nonlinear regression. The Langmuir model characterizes monolayer adsorption without lateral interaction between adsorbates, while the Freundlich model characterizes adsorption at binding sites with different affinities for the adsorbate. The behavior of the sorption isotherms as well as the close values of R2 for the two models indicate the energetic heterogeneity of the sorbent surfaces and the application of several binding mechanisms. To understand the nature of metal complexation by synthesized phosphorylated calixarenes, quantum chemical modeling was carried out. Calculations in the DFT approximation (RI-BP86/def-TZVP) were performed for the ground states of complexes of calixarene phosphonates and Eu(III) with six unpaired electrons. Possible variants of Eu3+ cation coordination by ligand groups of different nature were analyzed: phosphonate groups on the upper rim, carbonyl oxygen atoms on the lower rim and aromatic π-systems of benzene rings. Keywords: calixarenes, complexing agents, sorbents, silica gel, europium, quantum chemical calculation, X-ray structural analysis.