Implications for FeNC Systems
From the data presented above, the isomer shift can be predicted with
reasonable accuracy for any coordination environment represented in the
test set. This explicitly includes the square-planar Fe(II) complexes
with difficult electronic structures, implying that for the correct
oxidation state, multiplicity and coordination geometry the prediction
can be good enough regardless of the exact details of the d-electron
configuration or precise nature of the spin state. The trust region for
isomer shift predictions is taken as double the mean absolute deviation
in the correlation plots of Figure 4, i.e. 0.10 mm s−1(TPSSh), 0.13 mm s−1 (B3LYP) and 0.12 mm
s−1 (PBE0) depending on the choice of density
functional. In line with results from previous studies, the quadrupole
splitting is predicted with lower accuracy. The trust region, i.e. twice
the mean absolute deviation among the penta- and tetracoordinate
complexes, is 0.51 mm s−1 (TPSSh), 0.45 mm
s−1 (B3LYP) and 0.51 mm s−1 (PBE0)
for the three density functionals considered. We note that these trust
regions should not be considered as absolute values since individual
data points have larger deviations from the correlation line. As
detailed above, predictions for the quadrupole splitting of
square-planar Fe(II) systems are much more sensitive to the valence
electronic structure and thus for any computational FeNC model system
care must be taken that all plausible electronic configurations are
considered.
In the literature, the Mössbauer data of different species are often
presented in graphs where the isomer shift and the quadrupole splitting
values of the same signal are plotted on the x- and y-axes,
respectively. Figure 7A illustrates this concept for the experimental
and computed Mössbauer parameters of the 20 reference complexes. It can
be clearly seen that deviations between experiment and calculation,
highlighted with connecting lines, are significantly more pronounced in
the direction of the quadrupole splitting.
Exemplary room temperature Mössbauer parameters observed in FeNC
catalysts are shown in Figure 7B in an analogous graph. The selection
includes FeNC catalysts prepared from various preparation approaches:
starting from porphyrins,49,134-142 metal organic
frameworks,38,41,143 and polyaniline in combination
with an iron source,38,144-147 formation of active
sites by an ammonia treatment38,137,148 and
others.149,150 Note that the labels
Si correspond to characteristic doublets known as
e.g. D1 or D2 in the original references; however, because some distinct
signals carried the same or similar labels in different references we
have chosen an independent nomenclature here. Signals labelled with
roman numerals are obtained with various treatments, e.g. poisoning by
sodium sulphite (IV),142 presence of excess sulphur
during the pyrolysis (II and III) or unknown origin38of their specific character.