Conformations of [(R,R)-1,5-diaza-cis-decalin] copper (II) complex and its hydrogen bonding interaction with the crystal water: A combined experimental VA, UV–Vis and ECD spectroscopic and DFT study

Chiral 1,5-diaza-cis-decalin ligand and its copper-complexes have found considerable applications in catalyzing enantioselective organic reactions. In the present article, (R,R)-1,5-diaza-cis-decalin ligand and its copper (II) hydroxide iodine hydrate complex have been investigated by using vibrational absorption (VA), ultraviolet–visible (UV–Vis), and electronic circular dichroism (ECD) experimental spectroscopic measurements, as well as density functional theory modeling. An extensive theoretical conformational analysis of the ligand has revealed that the ligand can adopt a good number of orientations in terms of its frame conformations, i.e. Chair–Chair, Chair–Boat, and Boat–Boat, the proximal (also denoted as in) and distal (out) positions for its two N atoms, and the equatorial (e) and axial (a) positions of its two amine H atoms. The most dominant conformation at room temperature was predicted to take on the Chair–Chair N-in H-ea configuration, based on the relative Gibbs free energy ordering at the B3LYP/6-31++G(d,p) level. The VA, vibrational CD, UV–Vis, and ECD spectra of the ligand conformers have been simulated. Conformational searches have also been carried out for the titled copper complex at the B3LYP/LanL2DZ level and the dominant conformer was predicted to have the Chair–Chair N-in H-ee configuration for its ligands, with a close to 100% Boltzmann population factor at room temperature. Furthermore, geometry optimizations and spectral simulations have been performed for the hydrogen bonded cluster of the complex with the crystal water. The effects of hydrogen bonding interaction with the crystal water on the VA, VCD, UV–Vis, and ECD spectra have been discussed. The interaction with the water molecule has been found to have significant impacts on the appearance of VA, VCD and ECD spectra and its inclusion is essential to obtain satisfactory interpretations of the experimental VA and ECD spectra.