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.