Conformational analysis and vibrational circular dichroism of Tris(ethylenediamine)ruthenium(II) complex: a theoretical study

Abstract

The conformational preferences and vibrational circular dichroism of tris(ethylenediamine)ruthenium complex in two main configurations (Λ) and (Δ), have been performed using density functional theory. We find that for the free [Ru(en)₃]²⁺ ion in the Δ-configuration, the conformational stability order is Δ)(δδδ) > Δ)(λδδ) > Δ)(λλδ) > Λ(λλλ) and that for the Λ-configuration it is Λ(δδδ) < Λ(δδδ) < Λ(λλδ) < Λ(λλλ). The energy differences between the four conformers for both the configurations Δ and Λ are relatively small, but the activation barriers for ring inversion from one conformation to another are significant, as compared to other such systems. We trace the origin of these results to the lower oxidation state of Ru and relatively larger Ru-N bond length. We have also studied the effect of counterions on the conformational stability for Ru(en)₃Cl₂. Our results indicate a reverse stability order for the associated complex, Ru(en)₃ Cl₂ and higher activation barriers for ring inversion as compared to the free complex ion Ru(en)₃²⁺. It is because of larger hydrogen bonding interactions between the three N-H bonds and the chloride ion in these two conformers as compared to other conformations, which is also evident from the VCD spectra of N-H stretching modes. We also investigate IR spectra for all conformations in Δ- and Λ-configurations and together with energetics and VCD spectra elucidate the spectroscopic characteristics of Ru(en)₃²⁺ complexes with and without the associated counterions.