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

Abstract

The potential of the redox couple FeIV=O / FeIII–O is of interest for the reactivity of the high‐valent nonheme iron oxidants in enzymes and bioinspired small molecule systems but, unfortunately, experimentally it so far is very poorly described. Discussed are three computational methods that are used in combination with available experimental data derived from titrations of FeIV=O species with ferrocene derivatives in dry acetonitrile, and from spectroelectrochemical titrations of FeIII–OH complexes in wet acetonitrile, i.e. describing the FeIV=O / FeIII–OH couple – both data sets are known to have some ambiguities. First, a DFT‐based method is used to compute the E° values of 14 FeIV=O / FeIII–O couples with an error margin of around 110 mV. A subset of four species of the original data set is used to evaluate a DLPNO‐CCSD(T) based approach, and another subset of complexes, where the spectroelectrochemically determined FeIV=O / FeIII–OH potentials are also known, are used for a Bordwell‐Polanyi analysis, which also yield pKa values. It is shown that the three approaches lead to a consistent picture but due to possible ambiguities with the experimental data, it currently is not possible to fully evaluate the accuracy of the used approaches.

Non‐Heme‐Iron‐Mediated Selective Halogenation of Unactivated Carbon−Hydrogen Bonds

“…The current DFT analysis does not allow to unambiguously relate the barrier heights to specific electronic properties (see the Supporting Information for details), and in view of the limits of the DFT model this is not surprising. An important feature of the energetics shown in Figure 3 is the driving force, that is, it is by far largest (by >30 kJ mol −1 ) for [(L 1 )(Cl)Fe IV =O] + , and this is due to the large and rigid bispidine cavity and agrees with experimental observations and computational studies: bispidine Fe IV =O complexes are among the strongest ferryl oxidants known to date [32–35] . Another feature of importance is the energy difference between the triplet and the quintet states, and this is very small for [(L 1 )(Cl)Fe IV =O] + and tunable with the coligand: the computed triplet‐quintet gap for MeCN, Cl − and Br − as coligands is in good agreement with observed d electronic transitions and with the observed reactivities (see above) [23] .…”

“…[ 25 , 26 , 27 , 28 ] Therefore, the L 1 based ferryl complex discussed here has “the wrong spin ground state” but is the most reactive low molecular weight non‐heme‐iron model system, as fast as some enzymes and much faster than the L 4 based complex with a quintet ground state (see the Supporting Information for a comparison of relevant kinetic parameters). [23] A plausible reason for the unprecedented reactivity of this S =1 Fe IV =O species is the rigid and for Fe IV =O slightly too large bispidine cavity that provides four nitrogen donors and enforces a short Fe IV −N3 bond ( z ‐axis) and a longer and more flexible Fe IV −N7 bond ( xy ‐plane):[ 29 , 30 , 31 ] the large cavity leads to one of the thermodynamically strongest ferryl oxidants,[ 32 , 33 , 34 , 35 ] and the relatively long Fe IV −N7 distance together with the possibility to select a weak monodentate ligand trans to N7 yields a small in‐plane ligand field and therefore a small triplet‐quintet gap. This is supported by the observation that the reactivities strongly depend on the monodentate coligand in the predicted order (MeCN<Cl − <Br − ).…”

Ab Initio Methods in First‐Row Transition Metal Chemistry

“…For example, Comba and co-workers calculated the spin gaps of different nonheme iron-oxo complexes using DLPNO-CCSD(T). [239,240] They obtained DLPNO-CCSD(T) data in agreement with CASPT2/CC results from the work of Phung et al [241] However, a closer inspection of Comba et al calculations revealed that the calculated quintet state is in fact not the lowest quintet.…”

“…Finally, we note that in an extreme case, one can accidentally converge to a wrong HF reference, thus the following CC correlation energy is erroneous and a wrong conclusion might be drawn. For example, Comba and co‐workers calculated the spin gaps of different nonheme iron‐oxo complexes using DLPNO‐CCSD(T) [239,240] . They obtained DLPNO‐CCSD(T) data in agreement with CASPT2/CC results from the work of Phung et al [241] .…”

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

“…Also, [(L 1 )Fe IV =O(Cl)] + is believed to have (one of) the highest Fe IV/III =O redox potentials although one needs to be cautious with reported Fe IV/III potentials. [66] An interesting question is, how much the observed reactivity relates to the Fe IV/III potential, how much to the electrophilicity of the oxido‐group and how much it depends on the quintet‐triplet gap of the ferryl complex. We believe to have shown with the data presented here that some of these interesting questions may be discussed thoroughly with the bispidine‐based complexes studied here.…”