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Z. Naturforsch. 69a, 371 – 384 (2014)
doi:10.5560/ZNA.2014-0032
Theoretical Analysis of the Unusual Vicinal Effects on Electronic Circular Dichroism Spectra of Cobalt(III) Complexes with ED3A-Type and Related Ligands
Yuekui Wang and Chunxia Zhang
Key Laboratory of Chemical Biology and Molecular Engineering of the Education Ministry, Institute of Molecular Science, Shanxi University, Taiyuan, Shanxi 030006, P. R. China
Received March 10, 2014 / April 4, 2014 / published online July 15, 2014
Reprint requests to: Y. W.; E-mail: ykwang@sxu.edu.cn
To investigate the origin of unusual N-vicinal effects, the geometries of the two series of cobalt(III) complexes, [Co(ED3A-type)(X)] (X = CN, NO2) and [Co(EDDS-type)], with the pentadentate ethylenediamine-N, N, N′-triacetate (ED3A), hexadentate (S,S)-ethylenediamine-N, N′-dissuccinate (EDDS), and their N-alkyl-substituted ligands in aqueous solution have been optimized at the DFT/B3P86/6-311++G(2d,p) level of theory. Based on the optimized geometries, the excitation energies and rotational strengths have been calculated using the time dependent density functional theory (TDDFT) method with the same functional and basis set. The optimized geometries and calculated electronic circular dichroism (ECD) curves are in good agreement with the observed ones. Based on this agreement, the characteristics of usual and unusual N-vicinal effects as well as the related chiral stereochemistry phenomena have been discussed. To reveal the origin of the unusual N-vicinal effects, a novel calculation scheme has been proposed, which permits efficiently assessing the contribution of the octahedral core to the optical activities of the chelates. The results show that the substituent effects and conformational relaxation effects make opposite contributions to the overall N-vicinal effects with the former being dominant. The unusual N-vicinal effects originate from the negligible chirality of the octahedral core in the unsubstituted [Co(ED3A)(X)] chelates. For this reason, their optical activity is dominated by the asymmetric nitrogens and behaves different from the normal cases. The unusual vicinal effects observed in the N-alkyl-substituted ED3A-type chelates reflect an increase in the contribution of the octahedral core to their optical activity, which recovers the ECD spectra from the special cases to the normal ones.These findings provide some insight into the unusual N-vicinal effects as well as the chiroptical properties of the chelates.
Key words: Co(III)-ED3A Chelates; Co(III)-EDDS Chelates; ECD Spectra; TDDFT Calculations.
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