Theoretical Analysis Of The Circular Dichroism Spectra Of Chiral Metal Complexes

As a kind of special electronic spectrum, the CD spectrum is very important in determining the absolute configurations, stable conformations and transition properties of molecules. It has recently been used widely in chirality domain, such as the coordination chemistry, medicinal chemistry and non-symmetric catalysts. The origin of CD absorption bands is, theoretically, quite different for different complexes:some come from the d-d transitions of central metal ions, some from the charge transfer transition between the central metal ions and the ligands, or from transitions within ligands. Quantitative analyse of the transition properties are not only necessary for the determination the absolute configurations of chiral compounds, but also important for the exploration of new chiral materials.In this paper, the CD spectra of two kinds of transition metal complexes have been investigated theoretically:2,2'-bipy compounds (1), A-cis-[CoBr(NH3)(en)2]2+Br2(2). The CD spectra of 1 have been measured experimentally in early years. Lots of theoretical work has also been done on calculating and analyzing the properties of such chelates by molecular orbital and semi-experiential theories. However, little work has analyzed the transition properties on the first principle in detail and seldom investigation has been done to calculate the exciton coupling bands of such chelates consisting of extended conjugated ligands by density functional theory (DFT). Moreover, the assignment of the first CD transition band for [Fe(bipy)3]2+is still undetermined. The CD spectra of 2, are alternated partly by the coordination environment and the switch of different conformations usually. Besides, the dominate conformation cannot be determined just from the experimental CD spectra. To solve these questions, the ground state geometries were optimized in solution first. Then the excitation energies, rotational and oscillator strengths have been calculated with the same funcitional and basis set. Based on the rotational strengths calculated, the circular dichroism spectra were drawn. For cis-[CoBr(NH3)(en)2]2+, a statistically averaged CD spectrum has been calculated as a sum of the possible isomers. By comparing the calculated and the observed spectra, analyzing the corresponding transition properties, we can draw conclusions as follows:For [M(bipy)3]2+(M=Fe,Ru,Os), the calculated CD spectra are good agreement with the observed ones except for some red or blue shift in the calculated band wavelengths. The CD bands in the long wavelength region are dominated by the charge transfer transitions d-n*, except for the first band of [Fe(bipy)3]2+, which is dominated by the degenerated d-d transitions. The different band types in the long wavelength region of the three complexes are mainly due to the different properties of corresponding transitions. In the short wavelength region, however, the two stronger CD bands with opposite signs are mainly originated from the exciton splitting ofπ-π* transitions. The observed ratio of the intensity of positive exciton band to that of negative one can be well reproduced at the TDDFT level for the [Fe(bipy)3]2+ chelate, while the calculated ratios are much smaller than the observed ones for the [Ru(bipy)3]2+and [Os(bipy)3]2+chelates, which may caused by underestimating the contribution of charge tranfer in exciton transition using the pseudopotential basis set. This paper presents some suggestions to solve this problem for further studies.In addition, the CD spectra of the three isomers,Λ(δδ)-, A(δλ)- andΛ(λλ)-cis-[CoBr(NH3)(en)2]2+, have been calculated incuding their Boltzmann-weighted averaged CD spectrum. The averaged CD spectrum is generally good agreement with the observed one, especially for the short wavelength region. Interestingly, it was found that the shape of the observed CD in the long region is dorminated by the two high-energry isomers other than the most stable one.