Determination of absolute configuration of chiral hemicage metal complexes using time-dependent density functional theory

Time-dependent density functional theory (TD-DFT) is applied to the UV-vis absorption and circular dichroism (CD) spectra of a series of transition metals (M=Ru, Zn, Fe) complexed with an enantiopure hemicage ligand, (-)-(5R,5'R,5' 'R,7R,7'R,7' 'R,8S,8'S,8' 'S)-8,8',8' '-[(2,4,6-trimethyl-1,3,5-benzenetriyl)tris(methylene)]tris[5,6,7,8-tetrahydro-6,6-dimethyl-3-(2-pyridinyl)-5,7-methanoisoquinoline (1). The electronic spectra of the Ru and Fe complexes contain two regions, one featuring low-energy 1MLCT transitions and the other higher energy 1LC transitions; the Zn analog possesses only the 1LC transitions due to its filled 3d shell. TD-DFT is able to identify correctly these transitions in the spectra, as well as to reproduce experimental spectra accurately, with regard to both the transition energies and the relative intensities of the different transitions. Additionally, it is possible to use TD-DFT to assign the absolute configuration at the metal center with high confidence by matching the experimental and calculated spectra.