The Role of Macrocyclic Ligands in the Peroxo/Superoxo Nature of Ni-O2 Biomimetic Complexes

The impact of the macrocyclic ligand on the electronic structure of two LNi-O-2 biomimetic adducts, [Ni(12-TMC)O-2](+) (12-TMC 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and [Ni(14-TMC)O2](+) (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 3d and O-2 pi* 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 Ni-O-2 interaction for [Ni(12-TMC)O-2](+), although a dominant superoxide character is found for both systems. (C) 2012 Wiley Periodicals, Inc.