A comparative vibrational CD study of homo- and heteroleptic complexes of the type...

Vibrational circular dichroism (VCD) spectroscopy is a powerful tool to characterize absolute configurations and conformations of chiral organometallic systems. Such characterizations rely on the corresponding density functional theory (DFT) spectral simulations which may become very time consuming and sometimes even impossible for systems with increasing complexity. Systematic studies on small model compounds can lead to empirical structure-spectra relationships. The present work continues the systematic investigations of transition metal complexes of chiral trans-1,2-diamino-cyclohexane (chxn). VA and VCD spectra of the mixed-ligand complexes [Cu(chxn)2L](2+) with L being either ethylene diamine (en) or N,N'-dimethylethylene diamine (dmen) are measured. The comparison of the experimental spectra of the mixed-ligand complexes with those of the distorted octahedral complex [Cu(chxn)3](2+) reveals that the VA and VCD patterns below 1500 cm(-1) of the three complexes are not significantly affected by the nature of the third ligand, while the VCD pattern of the NH2-bending modes in the 1500 to 1800 cm(-1) region features some characteristic changes. Comparison with the corresponding DFT spectral calculations shows that these spectral differences are related to the relative abundance of the Δ- and Λ-configurations at the metal ion. In addition, the results of this study highlight that the VCD pattern of the NH2-bending modes is characteristic of the coordination number and the configuration of the metal center.