The role of macrocyclic ligands in the peroxo/superoxo nature of Ni–O 2 biomimetic complexes

The impact of the macrocyclic ligand on the electronic structure of two LNiO 2 biomimetic adducts, [Ni(12‐TMC)O 2 ] + (12‐TMC = 1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane) and [Ni(14‐TMC)O 2 ] + (14‐TMC = 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane), has been inspected by means of difference‐dedicated configuration interaction calculations and a valence bond reading of the wavefunction. The system containing the 12‐membered macrocyclic ligand has been experimentally described as a side‐on nickel(III)‐peroxo complex, whereas the 14‐membered one has been characterized as an end‐on nickel(II)‐superoxide. Our results put in evidence the relationship between the steric effect of the macrocyclic ligand, the O 2 coordination mode and the charge transfer extent between the Ni center and the O 2 molecule. The 12‐membered macrocyclic ligand favors a side‐on coordination, a most efficient overlap between Ni 3 d and O 2 π* orbitals and, consequently, a larger charge transfer from LNi fragment to O 2 molecule. The analysis of the ground‐state electronic structure shows an enhancement of the peroxide nature of the NiO 2 interaction for [Ni(12‐TMC)O 2 ] + , although a dominant superoxide character is found for both systems. © 2012 Wiley Periodicals, Inc.