Theoretical Studies of Diiron(II) Complexes that Model Features of the Dioxygen-Activating Centers in Non-Heme Diiron Enzymes

Abstract

We have applied high-level Density Functional Theory to investigate the properties of recently characterized carboxylate-bridged diiron(II) complexes supported by 2,6-di(p-tolyl)benzoate (ArTolCO2-) ligands. These compounds, prepared as synthetic models for the reduced non-heme diiron centers in the enzymes MMO, RNR-R2, and Δ9D, reproduce the composition of the first coordination sphere ligands as well as the core geometry. The experimentally observed flexibility of the diiron cores in the model compounds, a main design target, was confirmed computationally. Details of a possible interconversion mechanism that transforms quadruply and doubly carboxylate-bridged isomers of [Fe2(ArTolCO2)4L2], L = pyridine or related ligand, were examined. The orientation of the pyridine ligands plays a major role and promotes an initial carboxylate shift of the bridging carboxylate ligand that is orthogonal to the pyridine ring plane. Alternative mechanisms were explored and evaluated. Structural features of the strongly coupled diiron centers could only be reproduced reliably by using the experimentally determined antiferromagnetic spin-coupling properties of the high-spin d6 iron(II) centers. Use of the ferromagnetic-coupling scheme gave rise to a poor correlation of the computed structure with the experiment. The broken-symmetry orbitals required to describe the antiferromagnetic coupling are compared to the MOs as classical symmetry-adapted linear combinations of atomic orbitals that form the basis for the magnetic coupling scheme. The molecular orbitals responsible for the dependence of the structural results on spin coupling were identified and used to evolve an intuitive explanation for the structural differences observed.