Choice of Reference Systems
We have chosen 20 mononuclear complexes that are representative of the likely iron coordination environment in FeNC catalysts (see Figure 3) and for which Mössbauer data recorded at low temperatures (≤80 K) are available. This set of complexes contains tetra-, penta- and hexacoordinate iron in oxidation states II, III, IV, V and VI with high spin, intermediate spin and low spin electronic configurations where possible (see Table 1). The ligand spheres are predominantly composed of N-donor ligands, where in many cases the nitrogen atom is part of a π-system such as a porphyrin, bipyridine or phenanthroline ligand. Other ligands are small molecules expected to have σ- and π-interactions with the metal center, e.g. CO, O2– or N3–. Ligands that would lead to significant complications in the description of the electronic structure, e.g. nitrosyl ligands, have been excluded. Similarly, no oligonuclear iron complexes have been included to avoid the description of electronic structures that are dominated by antiferromagnetic coupling, e.g. in oxo-bridged dimers, with broken-symmetry DFT.
Although all of the complexes included here except complex 20have been used in previous calibration studies,21 we note that in the present study none of the ligand spheres were truncated. Hence, the full steric and electronic effects are recovered in the geometry optimizations and subsequent property calculations.