Density functional theory calculations of vibrational circular dichroism in transition metal complexes: Identification of solution conformations and mode of chloride ion association for (+)-tris(ethylenediaminato)cobalt(III)

Density functional theory calculations of the vibrational circular dichroism (VCD) spectra of a transition metal complex, (+)-tris(ethylenediaminato)cobalt(III) as a free ion and with associated chloride ions, are reported. By comparison with experimental data obtained in our laboratory in 1986 (Young et al. J. Am. Chem. Soc. 1986, 108, 7255) and new measurements, this study identifies, for the first time, the geometry and absolute configuration of the association complex as A-deltadeltadelta-Co(en)(3)Cl-2(4), in which two chloride ions each associate with three NH bonds along the C-3-axis of a D-3-symmetry complex. The DFT calculations, carried out with both the LanL2DZ and 6-31G(d)[C, H, N]/Stuttgart ECP [Cc] basis sets, predict the energy order of ethylenediamine ring conformations as lambdalambdalambda < lambdalambdadelta < lambdadeltadelta < deltadeltadelta for the free (gas phase) A-Co-(en)(3)(3+) ion, an order opposite to that deduced from solution studies. The VCD studies identify the delta-conformation as the most prevalent in solution for halide and sulfate counterions, suggesting an interaction with solvent or weakly associated counterions that stabilizes this conformation.