Theoretical Analysis of the Influence of Chelate-Ring Size and Vicinal Effects on Electronic Circular Dichroism Spectra of Cobalt(III) EDDA-Type Complexes

To assess the contributions of configurational and vicinal effects as well as chelate-ring size to rotational strengths, the geometries of a series of cobalt­(III) complexes [Co­(EDDA-type)­(L)]^± with the tetradentate EDDA-type ligands, EDDA (ethylenediamine-<i>N</i>,<i>N</i>′-diacetate), DMEDDA (<i>N</i>,<i>N</i>′-dimethyl­ethylene­diamine-<i>N</i>,<i>N</i>′-diacetate), DEEDDA (<i>N</i>,<i>N</i>′-diethyl­ethylene­diamine-<i>N</i>,<i>N</i>′-diacetate), and a bidentate ancillary ligand L (L = ethylenediamine, oxalate, carbonate, (<i>S</i>)-alanine, and malonate) in aqueous solution have been optimized at the DFT/B3LYP/6-311++G­(2d,p) level of theory. Based on the optimized geometries, the excitation energies and oscillator and rotational strengths have been calculated using the time-dependent density functional theory (TDDFT) method with the same functional and basis set. The calculated circular dichroism (CD) curves are in excellent agreement with the observed ones except for some small red or blue shifts in peak wavelengths. For the influence of chelate-ring size of the bidentate ligands on the CD intensities, a qualitative analysis together with the quantitative TDDFT calculation reveal that it depends on the symmetry of the cobalt-EDDA backbone. For the <i>s</i>-<i>cis</i>-isomers, the influence is negligible due to the perturbation is symmetric. For the <i>uns-cis</i>-isomers, the perturbation is unsymmetric. Since a small ring size means a large perturbation, this leads to the integral CD intensities decreasing with increasing the chelate ring size. The vicinal effects of asymmetric nitrogens incorporate both the substitutent effects and conformational relaxation effects, with the former being dominant. By analyzing the contributions of chiral arrays to rotational strengths, we found that the part of contributions dominated by the <i>S</i>-type chiral nitrogens could be considered as a good measure for the vicinal effects of chiral nitrogens. In addition, we found that the twist form (δ/λ) of the backbone ethylenediamine ring (E-ring) of the coordinated EDDA-type ligands is a key factor to understand the properties of these chelates, because it not only dominates the relative stabilities of the <i>s-cis</i>-Λ­(<i>SS</i>)-diastereoisomers with the result that λ > δ but also affects the major CD band by changing the order of the first two transitions. Moreover, the twist angle of E-ring is inversely related to the vicinal effect of chiral nitrogens. These findings may help us to understand the chelate ring size as well as vicinal effect related chiroptical phenomenon of the cobalt EDDA-type chelates