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.