Reaction mechanism of acid dissociation of bis (dipyrrolylmethanes) d-metals and quantum chemistry simulation of their molecular structure
Helicates on the basis of 3.3-bis(dipyrrolylmethanes) [M2L2] have pronounced chromophoric properties and, in some cases, intensive fluorescence, which determines the prospects of their application as active components of polymer silicate, organo-inorganic matrices in optical devices.
Differences in the electronic and geometric structure of the helicates [M2L2] as well as their impact on the stability of optical properties, kinetic and thermal stability of the compounds were analyzed.
Obtained regularities in width change of HOMO - LUMO energy gaps associated with the changes of helicates [M2L2] properties were observed and analyzed. The comparison of the width changes of the forbidden band in the range of [Cu2L2], [Co2L2], [Ni2L2], [Zn2L2], [Cd2L2] and [Hg2L2] showed good agreement with the experimental data on the fluorescent helicates properties.
In order to develop ideas on the stability of the complexes in acidic environment a kinetic model of the of bis(dipyrrolylmethanes) protolytic dissociation process was introduced with the method PM6 useing quantum - mechanical programs Gaussian 09W.
It was shown that the main contribution to the overall speed of the dissociation reaction resulted from the limiting stage of a nitrogen atom attack of the second molecule of acetic acid from one of the focal helicates nodes causing the activated complex formation. Then another fast stage involved two more molecules of the acids into the second coordination centre and the dissociation of the complex to the molecular ligand and zinc acetate occurred. Subsequently protonation ligand also very quickly developed to dihydroacetate H2L•2АсОН. On the basis of the results of computer calculations a kinetic model of the helicates [M2L2] protolytic dissociation process was experimentally confirmed, the results of spectral studies of the kinetics of the reactions of the helicates [M2L2] dissociation in the mixed solvent СН3СООН - С6Н6 (298.15-318.15K) being obtained.
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