Redox‐active ligands as a challenge for electronic structure methods

To improve the catalytic activity of 3d transition metal catalysts, redox‐active ligands are a promising tool. These ligands influence the oxidation state of the metal center as well as the ground spin‐state and make the experimental determination of both properties challenging. Therefore, first‐principles calculations, in particular employing density functional theory with a proper choice of exchange‐correlation (xc) functional, are crucial. Common xc functionals were tested on a simple class of metal complexes: homoleptic, octahedral tris(diimine) iron(II) complexes. The spin‐state energy splittings for most of these complexes showed the expected linear dependence on the amount of exact exchange included in the xc functionals. Even though varying redox‐activity affects the electronic structure of the complexes considerably, the sensitivity of the spin‐state energetics to the exact exchange admixture is surprisingly small. For iron(II) complexes with highly redox‐active ligands and for a broad range of ligands in the reduced tris(diimine) iron(I) complexes, self‐consistent field convergence to local minima was observed, which differ from the global minimum in the redox state of the ligand. This may also result in convergence to a molecular structure that corresponds to an energetically higher‐lying local minimum. One criterion to detect such behavior is a change in the sign of the slope for the dependence of the spin‐state energy splittings on the amount of exact exchange. We discuss possible protocols for dealing with such artifacts in cases in which a large number of calculations makes checking by hand unfeasible. The 3d transition metal complexes with redox‐active ligands are highly attractive for catalysis, but a challenge for first‐principles electronic structure methods such as density functional theory (DFT). The ligands’ redox‐activity can lead to the self‐consistent field algorithm converging to high‐energy solutions. We analyze such cases and suggest ways to deal with them.