Theoretical rationalization for the equilibrium between (μ-η²:η²-peroxido)Cu^IICu^II and bis(μ-oxido)Cu^IIICu^III complexes: Perturbational effects from ligand frameworks

DFT calculations are carried out to investigate the geometric effects of the supporting ligands in the relative energies of the (μ-η²:η²-peroxido)Cu^IICu^II complex 1 and the bis(μ-oxido)Cu^IIICu^III complex 2. The N₃-tridentate ligand bearing acyclic propane diamine framework La preferentially provided 1, whereas the N₃-tridentate ligand with cyclic diamine framework such as 1,4-diazacycloheptane Lb gave 2 after the oxygenation of the corresponding Cu^I complexes as reported previously [S. Itoh, et al., Inorg. Chem., 2014, 53, 8786-8794]. Calculations at the B3LYP∗-D3 level of theory can reasonably explain the experimental results in relative energies, structures and harmonic frequencies of 1 and 2. Perturbational effects of the diamine chelates of La and Lb especially on the equilibrium of 1 and 2 are investigated in detail. In the range from 2.30 Å to 3.40 Å of the N-N distance in the diamine moiety, 1 is more stable than 2 by 8.4 kcal mol^-1 at the distance of 3.40 Å. Calculated potential energies indicate that the decrease in the N-N distance is associated with a decrease in energy of 2, leading that 2 can be most stabilized at the N-N distance of 2.60 Å. Furthermore, molecular orbitals analyses are performed to explain that the energy gaps between the σ∗ orbital of the O-O bond and the d_x²-y² orbitals of the Cu^II ions of 1 get small as the diamine moiety is shrunk, leading to facilitate the O-O bond cleavage from 1 to 2.