Trace Metal Role on Crown Ethers Stability by Dft Methods

Journal of Environmentally Friendly Processes: Volume 3. Issue 1. June 2015

Petrotex Library Archive

Journal of Environmentally Friendly Processes

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TRACE METAL ROLE ON CROWN ETHERS STABILITY BY DFT METHODS

R. Razavi1* 1Department of Chemistry, Faculty of Science, University of Jiroft, Jiroft, Iran

Abstract:

Crown ethers strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis. The denticity of the polyether influences the affinity of the crown ether for various cations. For example, 18-crown-6 has high affinity for potassium cation, 15- crown-5 for sodium cation, and 12-crown-4 for lithium cation. The high affinity of 18-crown-6 for potassium ions contributes to its toxicity. Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. Ionophores such as valinomycin also display a marked preference for the potassium cation over other cations. In this study different role of crown ethers in stability of trace metal complexes were considered by DFT methods. Keyword: Crown ether, DFT, Stability, trace metal Introduction

In 1967, Charles Pedersen, who was a chemist working at DuPont, discovered a simple method of synthesizing a crown ether when he was trying to prepare a complexing agent for divalent cations.[1][2] His strategy entailed linking two catecholate groups through one hydroxyl on each molecule. This linking defines a polydentate ligand that could partially envelop the cation and, by ionization of the phenolic hydroxyls, neutralize the bound dication. He was surprised to isolate a by-product that strongly complexed potassium cations. Citing earlier work on the dissolution of potassium in 16-crown-4,[3][4] he realized that the cyclic polyethers represented a new class of complexing agents that were capable of binding alkali metal cations. He proceeded to report systematic studies of the synthesis and binding properties of crown ethers in a seminal series of papers. The fields of organic synthesis, phase transfer catalysts, and other emerging disciplines benefited from the discovery of crown ethers. Pedersen particularly popularized the dibenzo crown ethers.[5] Pedersen shared the 1987 Nobel Prize in Chemistry for the discovery of the synthetic routes to, and binding properties of, crown ethers. Apart from its high affinity for potassium cations, 18-crown-6 can also bind to protonated amines and form very stable complexes in both solution and the gas phase. Some amino acids, such as lysine, contain a primary amine on their side chains. Those protonated amino groups can bind to the cavity of 18-crown-6 and form stable complexes in the gas phase. Hydrogen-bonds are formed between the three hydrogen atoms of protonated amines and three oxygen atoms of 18-crown-6. These hydrogen-bonds make the complex a stable adduct. By incorporating luminescent substituents into their backbone, these compounds have proved to be sensitive ion probes, as changes in the absorption or fluorescence of the photoactive groups can be measured for very low concentrations of metal present.[6] Some attractive examples include macrocycles, incorporating oxygen and/or nitrogen donors, that are attached to polyaromatic species such as anthracenes (via the 9 and/or 10 positions)[7] or naphthalenes (via the 2 and 3 positions). 21- and 18-membered diazacrown ether derivatives exhibit excellent calcium and magnesium selectivities and are widely used in ion-selective electrodes. Some or all of the oxygen atoms in crown ethers can be replaced by nitrogens to form cryptands. A well- known tetrazacrown is cyclen in which there are no oxygens.

Industrial And Mining Research Centre – Cycle Science & Industry Company, Tehran, Iran Email: [email protected] – phone Number: +982188853416 Authors /Journal of Environmentally Friendly Processes 3 (2014) 45-47

Lariat crown ethers have sidearms that can augment complexation of cation. The lariat is typically attached to an amine centre in an azacrown.

Materials and Methods Optimization of chemical structure of crown ether was carried out by Guassian 09 in different basis set of DFT methods. Figure 1 shows the structure of crown ethers. The optimizing structure of trace metal(Pt and Pd) complexes was done the same as crown ethers.

Fig1 structure of common crown ethers: 12-crown-4, and 18-crown-6

Table 1. Optimized structure of crown ethers Compounds Optomized structure 12-crown-4

18-crown-6

Table 2. Optimized structure of crown ethers complex Pd Compounds Optomized structure 12-crown-4

Authors /Journal of Environmentally Friendly Processes 3 (2014) 45-47

18-crown-6

Table 3 Optimized structure of crown ethers complex Pt Compounds Optomized structure 12-crown-4

18-crown-6

Table 4. Energy of crown ether complexes Compounds Pd complex Pt complex 12-crown-4 -2154.72kJ -2134.72 18-crown-6 -2756.12kJ -2736.14

According to table 4 energy of stability of crown ether complexes were calculated by B3LYP/LAN2DZ basis set for Pd and Pt metals. Compared energy with Pd and Pt showed Pd complex of 18-crown-6 is more stable than Pd complex of 12-crown-4. Stability of Pt complex of 18-crown-6 is higher than 12-crown-4 Pt complex. Pd complexes of two crown ethers are more stable than Pt complexes of them.

Authors /Journal of Environmentally Friendly Processes 3 (2014) 45-47

1. References:

[1] Pedersen, C. J. "Cyclic polyethers and their complexes with metal salts". Journal of the American Chemical Society 89 (26): 7017–7036 (1967)..

[2] Pedersen, C. J. "Cyclic polyethers and their complexes with metal salts". Journal of the American Chemical Society 89 (10): 2495–2496 (1967)..

[3] D. G. Stewart. D. Y. Waddan and E. T. Borrows, GB 785229Oct. 23, 1957.

[4] J. L. Down, J. Lewis, B. Moore and G. W. Wilkinson, Proc. Chem. Soc.; J. Chem. Soc., 209, 3767, 1959.

[5] Charles J. Pedersen "Macrocyclic Polyethers: Dibenzo-18-Crown-6 Polyether and Dicyclohexyl-18- Crown-6 Polyether". Org. Synth.; Coll. Vol. 6, 395, (1988).

[6] Fabbrizzi, L.; Francese, G.; Licchelli, M.; Pallavicini, P.; Perotti, A.; Poggi, A.; Sacchi, D.; Taglietti, A. Chemosensors of Ion and Molecule Recognition; NATO ASI Ser., Ser. C, Vol. 492; Desvergne, J. P., Czarnik, A. W., Eds., Kluwer Academic Publishers, Dordrecht, 1997, 75.

[7] Bouas-Laurent, H.; Desvergne, J. P.; Fages, F.; Marsau, P. Fluorescent Chemosensors for Ion and Molecule Recognition ACS Symposium Series 538, Czarnik, A. W. (Editors) American Chemical Society, Washington D.C., 1993, 59.

[8] H Sharghi, S Ebrahimpourmoghaddam Helvetica Chimica Acta, 91 (7), 1363-1373, DOI: 10.1002/hlca.200890148, 2008