Quantum-chemical stability study of coordination ion-metal complexes with EDTA

Etilendiamintetraocetna kislina (EDTA) je spojina, ki z večino kovin tvori zelo stabilne vodotopne komplekse. Zaradi te lastnosti se v raznih industrijah in zdravstvu uporablja za kompleksiranje kovinskih ionov. Kljub široki uporabi ima tudi negativne učinke, saj ni biorazgradljiva in bi posredno lahko povzročala škodo okolju. Zaradi te pomanjkljivosti je potrebno najti okolju prijazne kelatorje. Za iskanje ustreznih okolju prijaznejših kelatorjev je zelo pomembno razumevanje stabilnosti takšnih koordinacijskih kompleksov. V delu sem s pomočjo štirih kvantno-kemijskih metod študiral geometrijo in stabilnost koordinacijskih kompleksov Cu2+, Ni2+, Co2+ in Cd2+ z EDTA4-. Uporabil sem semiempirični metodi PM3 in PM6, ab initio Hartree-Fockovo metodo z baznim setom 6-31G* in metodo teorije gostotnega funkcionala B3LYP z baznim setom 6-31G*. Komplekse [M(EDTA)]2- sem glede na centralni kovinski kation (M) razvrstil po ocenjeni stabilnosti. Kot mero stabilnosti sem uporabili dolžino vezi M-N, tvorbeno energijo, vrzel HOMO-LUMO ter dipolni moment. Našel sem linearno korelacijo med energijo tvorbe in eksperimentalno določeno konstanto formacije, z izjemo kadmijevega kompleksa. S pomočjo vrzeli HOMO-LUMO sem pravilno napovedal zaporedje stabilnosti kompleksov glede na centralni ion. Z izjemo bakrovega kompleksa je energija kompleksa linearno korelirana z dipolnim momentom. Ugotovil sem, da je metoda teorije gostotnega funkcionala (B3LYP6-31G*) najboljša za napoved eksperimentalnih trendov stabilnosti kompleksov glede na centralni atom, vrzel HOMO-LUMO pa količina, ki eksperimentalne trende najbolje odraža.Ethylenediaminetetraacetic acid (EDTA) is a molecule that forms very stable water-soluble complexes with most metals. Because of this property, it is widely used for metal-ion complexation in different industries and healthcare. Despite its wide use, it could be harmful to the environment due to its inability to biodegrade. Therefore it must be replaced with environment friendly chelators. Understanding the stability of such complexes plays an essential role in resolving this problem. Four quantum-chemical methods were used to study geometry and stability of coordination complexes of Cu2+, Ni2+, Co2+ and Cd2+ with EDTA4-. The methods used were semi-empirical PM3 and PM6, ab initio Hartree-Fock method with 6-31G* basis set and density functional theory B3LYP with 6-31G* basis set. The complexes [M(EDTA)]2- were arranged in sequence of central metal ion (M) stability. As a measure of stability I used M-N bond length, energy of formation, HOMO-LUMO gap and the dipole moment. Linear coorelation between complexation energy and experimental formation constant, apart from cadmium complex, was found. With the help of HOMO-LUMO gap, the predicted sequence of central metal ion complex stability was correct. With the exception of the copper complex, linear correlation between complexation energy and dipole moment was found. I determined that density functional theory (B3LYP6-31G*) gives best predictions for the experimental stability trends with respect to the central atom. The HOMO-LUMO gap was the calculated property which gave best qualitative trends with experiment