Theoretical Analysis Of Three Structural Effects On Electronic Circular Dichroism Spectra Of Cobalt(â…¢)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-N.N’-diacetate), DMEDDA(N,N’-dimethylethylenediamine-N,N’diacetate), DEEDDA(N, N’-diethyl ethylenediamine-N,N’-diacetate), and a bidentate ancillary ligand L (L=ethylene diamine, oxalate, carbonate,S-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, oscillator and rotational strengths have been calculated using TDDFT method with the same functional and basis set. The calculated CD curves are in excellent agreement with the observed ones except for some small red or blue shift in peak wavelengths. Particularly, the coB97XD is used to calculate the [Co(EDDA)(NO2)2]-and its derivativesDetailed analysis of the transitions reveals that the CD bands in the wavelength range400nm～650nm are originated from the metal-centered d-d transitions11A→21A,11A→11B and11A→21B. The major CD band results from the partial cancelation of the first two transitions.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 s-cis-isomers, the influence is negligible due to the perturbation is symmetric. For the uns-cis-isomers, the perturbation is un-symmetric. 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 S-type chiral nitrogens could be considered as a good measurement 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 chiroptical properties of these chelates, because it not only dominates the relative stabilities of the s-cis-A(SS)-diastereoisomers with the result of λ>δ, but also affect the major CD band by changing the order of the first two transitions. Moreover, the twist angle of E-ring is reversely related to the vicinal effect of chiral nitrogens.For [Co(EDDA)(NO2)2]-and its derivatives, the restricted rotation of two planar-type monodentate ligands play a dominate role in rotational strength, particularly affecting the negative or positive of CDs. And CD spectra can be used as an effective method to confirm the accurate orientation of each monodentate ligand.These findings provide a deep insight into the vicinal effects and chiroptical properties of the cobalt EDDA-type complexes.