Density functional approximations for consistent spin and oxidation states of oxoiron complexes

Abstract

We report here a computational study on a series of FeII, FeIII, and FeIV hydroxo/oxoiron complexes with a broad palette of ligands. We are interested in assessing the robustness of widely used density functionals for their prediction and description of structures and spin states for the examined oxoiron complexes. We have used a variety of density functional approximations (S12g, LDA, BP86‐D3, OPBE, SSB‐D, B3LYP‐D3, S12h, and MVS), in all cases including solvation and relativistic effects explicitly. One of the main observations of this detailed study is the excellent performance of S12g for both accurate structures and spin state splittings. Moreover, our results show that in general all density functionals can be used as a reliable computational tool for reproducing and predicting geometries, determining the oxidation state of iron, and most are able as well to providing good descriptions of spin state energetics.

Pathways of the Extremely Reactive Iron(IV)‐oxido complexes with Tetradentate Bispidine Ligands

“…The computed free energy profile of the demethylation process of [(L 1 )Fe IV =O(Cl)] + , involving the three spin surfaces starting with the singlet, triplet and quintet ferryl species and including both pathways a and b is shown in Figure 7 – the corresponding plot for [(L 1 )Fe IV =O(MeCN)] 2+ is given as Supporting Information (Figure S31; plots of the optimized structures of all relevant intermediates and transition states, together with selected structural data, as well as computed spin densities are given in Figures S33–S39). Note that DFT generally has problems in predicting the correct spin state energies,[ 55 , 56 , 57 , 58 , 59 , 60 ] and a recent DLPNO‐CCSD(T) study, also involving [(L 1 )Fe IV =O(X)] n+ (X=Cl − , MeCN) shows in agreement with experimental data that the DFT computed spin ground states of the ferryl complexes are, not unexpectedly, wrong. [30] As usual, we assume, that the computed activation barriers for the various pathways are nevertheless of acceptable accuracy.…”

Computational Approaches for Redox Potentials of Iron(IV)‐oxido Complexes

“…[29] The principal challenge is that DFT has difficulties to correctly describe metal ligand bonding, [30] and with Fe IV =O complexes in particular, it is difficult to correctly predict the spin ground state. [9,[31][32][33][34] We therefore have tested the published DFT-based approach [29] on a relatively large series of ferryl complexes, where the redox potentials all have been determined with the same method, i.e., the titration of ferryl complexes with ferrocene derivatives. With a small subset of these complexes, where ab initio based spin state energies are known, [35] we have also used these for a correlation with the experimental ARTICLE values of the Fe IV/III potentials.…”

“…[ 29 ] The principal challenge is that DFT has difficulties to correctly describe metal ligand bonding, [ 30 ] and with Fe IV =O complexes in particular, it is difficult to correctly predict the spin ground state. [ 9,31–34 ]…”

Calibration of computational Mössbauer spectroscopy to unravel active sites in FeNC catalysts for the oxygen reduction reaction

“…When using DFT, the selection of an appropriate density functional and basis set in combination with a good knowledge of ligand field theory and molecular orbital (MO) theory appears to be sufficient to identify shortcomings in many cases, for example, when the electronic structure obtained by the self-consistent field (SCF) procedure is not the lowest lying electronic state. [62][63][64] In other cases, of course, the F I G U R E 2 Typical values for the isomer shift (mm s −1 ) and the absolute value of the quadrupole splitting (mm s −1 ) for iron in oxidation states, A, I to VI and B, I to IV. The reference compounds for the isomer shift are both molecular complexes and solid state materials, whereas for the quadrupole splitting only complexes with an FeN 4 environment were chosen.…”

“…When using DFT, the selection of an appropriate density functional and basis set in combination with a good knowledge of ligand field theory and molecular orbital (MO) theory appears to be sufficient to identify shortcomings in many cases, for example, when the electronic structure obtained by the self‐consistent field (SCF) procedure is not the lowest lying electronic state. [ 62–64 ] In other cases, of course, the electronic structure will have non‐negligible multireference character or substantial mixing of low‐lying excited states, and in such scenarios DFT is prone to failure. Wavefunction approaches such as the complete active space SCF or density matrix renormalization group methods for large active spaces in combination with extensive basis sets and a subsequent perturbation theory treatment can lead to accurate predictions of relative spin state energies.…”