Electronic Complexity of Active Site Models for FeNC Catalysts: A Systematic Study of Truncation Effects in Molecular and Periodic Models

While single-atom catalysts have been widely studied
experimentally
and computationally due to their high potential for small molecule
activation reactions, the structures and electronic details of their
active sites remain elusive. Much progress has been made with nuclei-specific
spectroscopy methods, such as Mössbauer spectroscopy, to probe
FeNC catalysts for the oxygen reduction reaction. These studies are
often complemented by computational studies of active site models.
We here report on the optimal model size for computational studies
of FeNC catalysts with molecular and periodic approaches using two
prominent FeNC active site models, FeN4C10 (pyridinic
nitrogen coordination) and FeN4C12 (pyrrolic
nitrogen coordination). We furthermore unveil the electronic complexity
of these models to include not only the expected low-spin, intermediate-spin,
and high-spin configurations but in addition intrasystem redox events
and unpaired electrons in the graphene-like environment that ferromagnetically
or antiferromagnetically couple with the unpaired electrons located
on iron. A key conclusion is that square-planar structures fail to
explain the experimentally observed high-spin species. Instead, axial
displacement of iron or binding of axial ligands is needed to stabilize
the high-spin configuration, which has implications for the interpretation
of experimental data and thus the mechanism of the oxygen reduction
reaction.