Theoretical Analysis of the Circular Dichroism Spectra of Transition Metal Schiff-Base Complexes
|Keywords||Transition metal Schiff-base complexes Metal complexes with the derivation of 2，2’-bipyridine ligand Circular dichroism spectrum TDDFT calculation Chiroptical properties|
Circular dichroism (CD) spectra can provide the information of absolute configurations and stable conformations of chiral molecules as well as their reaction mechanism. In this aspect, it has greater advantage than other spectroscopy techniques. To fully use the information provided by CD spectra, theoretical analyses on the experimental CD are indispensable. However, since CD is an electronic absorption spectrum, it is difficult to quantitatively analyze it. In recent years, as the continuous improvement both in theoretical method and in computational techniques, it is possible to make such an analysis for moderate-size organic molecules as well as for transition metal complexes.In this paper, the chiroptical properties of following three kinds of transition metal complexes have been theoretically analyzed using the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the first time:(1) the salenCobalt(Ⅲ) complexes with different axial imidazole ligandsλ(RR)[Co(MeO-salen)L2]+[L=imidazole (Im), 1-methylimidazole (Melm),2-methylimidazole (2-MeIm)]; (2) the tetra-coordinated salenNi(Ⅱ) complexδ(SS)[Ni(Me-salen)]; and (3) the Ru(Ⅱ) and Os(II) complexes with three 4,5,4’,5’- bis(pinene)-2,2’-bipyridine ligands. The main conclusions are as follows:1. For the [Co(MeO-salen)L2]+ complexes, the first CD band in the long wavelength region is dominated by the ligand-to-metal charge transfer transitionπ→d, which was wrongly assigned to a d→d transition in some literatures. The introduction of imidazole ligands to the axial position has no major influence on the sign of the first two CD bands but it does have a significant influence on the shape and intensity of the CD spectra. For complexes with theλ(RR) chiral configuration the first CD absorption band is positive and the second is negative.2. For the [Ni(Me-salen)] complexes, the first CD band in the long wavelength region is dominated by both theπ→d transition and the transition, which is different from that of [Co(MeO-salen)L2]+. For complex with theδ(SS) chiral configuration the first CD absorption band is negative and the second is positive. In addition, the salen-type ligand tends to form a planar chelating structure with the central metal ion when axial ligands are introduced.3. ForΔ(S12)-[M(bpbpy)3]2+(M=Ru,Os) complexes, the CD absorption bands in the long-wavelength region are dominated by the transition d→π*, mixed with a little contribution of d→d transition. There are two strong CD absorption bands at the short-wavelength region, which are originated from the exciton coupling ofπ→π* transitions on the ligands, and displays a negative chiral exciton splitting for complexes with theΔoctahedral core. The influence of chiral carbon atoms (R/S) to the exciton splitting not only makes the splitting pattern from inequality in [M(bipy)3]2+ complexes become equality in [M(bpbpy)3]2+, but also leads the satellite peak in the CD spectra of [M(bipy)3]2+ disappear in [M(bpbpy)3]2+.These results not only provide a deep insight to the chiroptical properties of the complexes and their asymmetric catalytic mechanism at the atomic and electronic levels, but also have important meaning to explore new chiral complexes.