Журнал "Макрогетероциклы"

Valentin K. Kochnev,^@ Elizaveta P. Simonenko, Vladimir G. Sevast’yanov, and Nikolay T. Kuznetsov

N.S. Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, 119991 Moscow, Russia

^@Corresponding author E-mail: valentine878@gmail.com

DOI: 10.6060/mhc150249k

Macroheterocycles 2015 8(2) 185-192

“Host-guest” complex cations consisted from crown ether macrocycle and a double-charged metal cation, were studied in order to evaluate the stability of [M(crown)A₂] / [M(crown)A]A type molecules to water (M – double charged metal cation, crown – a crown ether, and A – suitable ligands). The cations M²⁺ = Cu²⁺, Zn²⁺ were chosen in this study due to different stability of their aqua-complexes. While complex cations [M(crown)]²⁺ are very stable for many metals, the consistency between metal ion radius and crown ether cavity has some influence on hydrolytic properties. To study hydrolytic stability of these compounds their interaction with water molecule was considered in gas phase. Stability of [M(crown)A₂] / [M(crown)A]A type molecules is influenced by the activation barrier of the H2O entering in the inner sphere and bond energy between H2O and [M(crown)A₂] / [M(crown)A]A in a starting associate [M(crown)A₂]∙H₂O / M(crown)A]A∙H₂O. The activation barrier depends on matching between the M²⁺ ionic radius and the size of the macrocyclic cavity. The energetic profiles of the H2O entering into [M(crown)Cl₂]/[M(crown)Cl]Cl were determined for Cu²⁺ and Zn²⁺ cations and three crown ethers (12-crown-4, 15-crown-5 and 18-crown-6). In the case of discrepancy between the radius of metal ion and the cavity of crown ether the entering of water molecule into complex and its binding to central atom proceeds almost without energy barrier, while with consistent geometric parameters it is required to overcome the activation barrier ~15-20 kcal/mol. Hydrolytic stability appears dependent on the strength of molecular associates [M(crown)A₂]∙H₂O/[M(crown)A]A∙H₂O as compared with activation barrier of water entering into complexes [M(crown)A₂]/[M(crown)A]A, as well as on the energy effect of water coordination to the central atom (after overcoming the barrier). The latter depends on electronic nature of central atom and it appears this may be characterized in terms of values of electronic chemical potential μ and Pearson’s hardness η of compounds M(crown)A₂. The calculations concretize the concept of crown ether complexes [M(crown)A₂]/[M(crown)A]A stability to water in gas phase. Hydrolytic activity of M(crown)A₂ compounds depends on the range of factors: a consistency/discrepancy of M²⁺ ion radius with a size of macrocyclic cavity, on the nature of entering molecule and on the electronic properties of central atom M. While stability of associates [M(crown)A₂]∙H₂O/[M(crown)A]A∙H₂O with H₂O depends on hydrogen bonds formed between H₂O and macrocycle, the geometric consistency provides not only flat structure of [M(crown)]A2, but also higher activation barrier of water entering into complexes [M(crown)A₂]/[M(crown)A]A. The barrier is then higher than the stability of the [M(crown)A₂]∙H₂O/[M(crown)A]A∙H₂O associates. When geometries are consistent the hydrolysis reactions are endothermic, while otherwise they are exothermic.

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