Dancing multiplicity states supported by a carboxylated group in dicopper structures bonded to O2

The present study pretends to assign the correct multiplicity state to dinuclear copper complexes when interacting with free molecular oxygen. Recently, the formation of a bridge butterfly μ–η2:η2-peroxo dicopper core structure stabilized by the direct interaction of the counterion, a carboxylate group that allows the double bridge linking both metal-centre atoms, was characterized by crystallography. This system was assigned as a diradical singlet with Ms = 0. However, after new calculations it has turned out to be triplet (Ms = 1) despite the stabilization for this latter multiplicity state is not high. Here, the factors that contribute to make this structure display a multiplicity different with respect to the previously expected diradical singlet are described. In the present theoretical study, the roles of the αSp ligand constraints and the counterion are unravelled. On the other hand, the relative stability between the butterfly μ–η2:η2-peroxo structure and the isomeric bis(μ-oxo) species is also on discussion. Despite the relative stabilities of all these either structural or electronic isomeric species are supposed to depend on the computational method, which is a difficulty to reach a definite conclusion about the nature of the active species, all DFT methods using either pure or not pure DFT functionals here reach the same conclusion, favoring the triplet as the ground state for the butterfly μ–η2:η2-peroxo dicopper core structure, and the bis(μ-oxo) species when removing the benzoate counterion.