Theoretical Analysis on the Importance of Achiral Unidentate Ligands to Electronic Circular Dichroism of cis-Bis(ethylenediamine) Cobalt(III) Complexes.

Compared with the importance of didentate ligands to chiral chelates, little is known about the influence of unidentate ligands on the chiroptical properties of related chelates. To assess the importance of achiral unidentate ligands to electronic circular dichroism (ECD) spectra, calculations of the excitation energies and oscillator and rotational strengths for all the Λ-enantiomers of bis(ethylenediamine) cobalt(III) complexes cis-Co(en)2(X)2 (X = Cl(-), CN(-), NH3, N3(-), NO2(-), H2O; n = 1, 3) were performed at the TDDFT/B3LYP/6-311++G(2d,p)//DFT/B3LYP/6-311++G(2d,p) level of theory, including solvent effects. The individual contributions of the chiral arrays, Δ/Λ octahedral core, δ/λ twists of the en ligands, and δ/λ relative orientations of the unidentate ligands to the rotational strengths of related transitions were quantitatively determined and graphically presented. It was found that, for the chelates with nonaxially symmetric unidentate ligands (N3(-), NO2(-), H2O), the chiral orientation (δ/λ) of the unidentate ligands not only dominates the ECD spectra of the Λ-diastereoisomers but also dominates the relative energies in solution with the δ-orientation preferred. For those complexes with axially symmetric unidentate ligands (Cl(-), CN(-), NH3), the inherent dissymmetry within the metal ion-donor atom cluster was found to be unidentate ligand-dependent. The Boltzmann-weighted averaged ECD spectra of the complexes with Λ-octahedral core are in excellent agreement with the observed ones, except that for the diazido complex, they are opposite in sign. This demonstrates that the absolute configuration of the complex cis-(-)-Co(en)2(N3)2 is Δ, not the Λ-form assigned by McCaffery et al. in 1965. These findings not only reveal the importance of unidentate ligands to the ECD spectra of related chelates but also provide an insight for the influence of unidentate ligands on both the inherent dissymmetry and the distributional chirality.