FeN4 Environments upon Reduction: A Computational Analysis of Spin States, Spectroscopic Properties, and Active Species

Abstract

FeN₄ motifs, found, for instance, in bioinorganic chemistry as heme-type cofactors, play a crucial role in man-made FeNC catalysts for the oxygen reduction reaction. Such single-atom catalysts are a potential alternative to platinum-based catalysts in fuel cells. Since FeNC catalysts are prepared via pyrolysis, the resulting materials are amorphous and contain side phases and impurities. Therefore, the geometric and electronic nature of the catalytically active FeN₄ site remains to be clarified. To further understand the behavior of FeN₄ centers in electrochemistry and their expected spectroscopic behavior upon reduction, we investigate two FeN₄ environments (pyrrolic and pyridinic). These are represented by the model complexes [Fe(TPP)Cl] and [Fe(phen₂N₂)Cl], where TPP = tetraphenylporphyrin and phen = 1,10-phenanthroline. We predict their Mössbauer, UV-vis, and NRV spectral data using density functional theory as windows into their electronic structure differences. By varying the axial ligand, we further show how well small chemical changes in both complexes can be discerned. We find that the differences in ligand field strength in pyrrolic and pyridinic coordination result in different spin ground states, which in turn leads to distinct Mössbauer spectroscopic properties. As a result, pyrrolic nitrogen donors with a weaker ligand field are predicted to show more pronounced spectroscopic differences under in situ and operando conditions, while pyridinic nitrogen donors are expected to show less pronounced spectroscopic changes upon reduction and/or ligand loss. We therefore suggest that a weaker ligand field leads to better detectability of catalytic intermediates in in situ and operando experiments.

Figure 1

Sketch of some of the open questions around the active site(s) in single-atom catalysts for the oxygen reduction reaction, left, and the FeN₄ model complexes [Fe(TPP)Cl] (top) and [Fe(phen₂N₂)Cl] (bottom) with ligand spheres P₁ and P₂, respectively, studied here in terms of their electronic and spectroscopic properties.

Figure 2

Structural formulas of the complexes studied here: (a) pyrrolic [Fe^IIP₁] and its congeners (P_1a to P_1e) and (b) pyridinic [Fe^IIP₂] and variants (P_2a, P_2b). Optimized geometries of (c) [Fe^IIIP₁Cl] and (d) [Fe^IIIP₂Cl]. Iron atoms are depicted in orange, nitrogen in blue, chlorine in green, carbon in gray, and hydrogen in white.

Figure 3

Schematic representations of the MOs dominated by Fe 3d atomic orbitals for (a) P₁ and (b) P₂ ligand environment before and after reduction. The energy spacings are not drawn to scale; detailed MO schemes are shown in Supporting Information. In a square-planar ligand field, orbitals with z-components would be stabilized, so that the d(z²) orbital may become the most stabilized orbital. For multideterminant cases, one representative electronic configuration is shown.

Figure 4

(a) Square scheme for the reduction and ligand abstraction steps. Gibbs energy diagrams calculated at the TPSS/def2-TZVP:def2-SVP level of theory for (b) [FeP₁] and (c) [FeP₂]. The red and orange lines in part b refer to the axial ligand being Cl^– and OH^–, respectively. The dark blue and light blue lines in (c) refer to the axial ligand being Cl^– and OH^–, respectively.

Figure 5

UV–vis spectra predicted with TDDFT at the B3LYP/def2-TZVP(-f) level of theory for all species in the square scheme in their lowest energy spin states; (a) [FeP₁] and (b) [FeP₂]. The spectrum of the S = 1 [Fe^IIP₁L]⁻ case is shown in Figure S14. The y-axis reflects the calculated oscillator strength of the vertical transitions scaled by applying an artificial broadening of 0.25 eV to obtain the line spectrum and is hence labeled as intensity with arbitrary units (a.u.).

Figure 6

Mössbauer data predicted for all species in the square scheme; (a) [FeP₁] and (b) [FeP₂]. The error bars for isomer shift and quadrupole splitting from a calibration study are shown in the bottom right corner (trust regions for δ_iso: ±0.065 mm s^–1, ΔE_Q: ±0.18 mm s^–1).

Figure 7

NRV spectra predicted at the TPSS/def2-TZVP:def2-SVP level of theory for all species in the square scheme in their lowest energy spin states; (a) [FeP₁] and (b) [FeP₂]. A line broadening of 15 cm^–1 was applied.