Theoretical Analysis of the Circular Dichroism Spectra of Chiral Metal Complexes with Different Ligands
|Keywords||Ru(Ⅱ)/Os(Ⅱ) chelates with different ligands [Rh（thiox）3]3- complexes Circular Dichroism TDDFT calculation Chiroptical Properties|
The theoretical analysis of circular dichroism spectra is not only the demand of discriminating the absolute configuration of chiral molecules, but also is a powerful means to test the related theoretical methods. Many chiral transition metal complexes, especially the six-coordinated chelates with bidentate ligands such as bipyridine and phenanthroline, have been receiving much attention due to their important applications in the domain of photochemistry, structure recognition of DNA molecules, and chiral catalysis etc. However, only a few works involves the theoretical calculations of electronic structure of excited states for such chelates, especially for the chiroptical properties of the chelates with different ligands, but most of them were performed with the semi-empirical methods. In this paper, the electronic structures and chiroptical properties of the chelates [Ru(bpy)2(phen)]2+, [Ru(phen)2(bpy)]2+, [Os(bpy)2(phen)]2+, [Os(phen)2(bpy)]2+and [Rh(thiox)3]3-have been theoretically analysed using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). The main works and conclusions are as follows.The ground-state geometries of the C2-symmetric chelates [M(phen)2 (bpy)]2+(M=Ru,Os) and [M(bpy)2(phen)]2+were determined at the DFT/ B3LYP level including solvent effects. The main optimized geometric parameters are agreemnt with those of [M(bpy)3]2+and [M(phen)3]2+with D3 symmetry. The calculated CD spectra are good agreement with the observed ones except for some red or blue shift in the calculated band wavelength. The results reveals that CD bands in the long wavelength region (320-700nm) are dominated by the charge transfer transitions d-π, while in the short wavelength region (200-320nm), the stronger CD bands are mainly from the exciton coupling of the long-axis-polarizedπ-πtransitions on the ligands. The chelates [M(bpy)2(phen)]2+shows only two exciton absorption bands, in which the negative one can be assigned to theπ-πtransition of the phenanthroline, and the positive one to those of bipyridine ligands. However, the chelates [M(phen)2(bpy)]2+has three exciton bands, in which the left two with opposite signs can be assigned to theπ-πtransitions of phenanthroline ligand, and the right positive one to that of the bipyridine. These findings provide a deep insight into both the absolute configurations and chiroptical properties of the chelates with different ligands.The ground-state geometry of the chelate A-[Rh(thiox)3]3-has a D3 symmetry, and its experimental CD spectrum consists of five broad bands in the UV-visible region. TDDFT calculations show that each of the CD bands involves several transitions, which accounts for the broad band-shape. Detailed analyses on the involved transitions show that, the first CD band in the long wavelength region is originated from the d-d transition, and displays a positive Cotton effect; the second CD band can be assigned to the ns-πtransition with a negative cotton effect. The stronger positive band at 338nm is dominated by the charge transfer transition ns*Rh-s, mixed with little contribution of ns-πtransition. The negative CD band at 292nm and the postive CD band at 254nm are mainly from the ns/o-πand ns/oσRh-s transitions. The two strong CD bands at short wavelength region can be used to determine the absolute configuration of such kind of chelates containing chromogenic agent ligands.