Abstract
Iron(IV)‐oxo intermediates in nature contain two unpaired electrons in the Fe–O antibonding orbitals, which are thought to contribute to their high reactivity. To challenge this hypothesis, we designed and synthesized closed‐shell singlet iron(IV) oxo complex [(quinisox)Fe(O)]+ (1+; quinisox‐H=(N‐(2‐(2‐isoxazoline‐3‐yl)phenyl)quinoline‐8‐carboxamide). We identified the quinisox ligand by DFT computational screening out of over 450 candidates. After the ligand synthesis, we detected 1+ in the gas phase and confirmed its spin state by visible and infrared photodissociation spectroscopy (IRPD). The Fe–O stretching frequency in 1+ is 960.5 cm−1, consistent with an Fe–O triple bond, which was also confirmed by multireference calculations. The unprecedented bond strength is accompanied by high gas‐phase reactivity of 1+ in oxygen atom transfer (OAT) and in proton‐coupled electron transfer reactions. This challenges the current view of the spin‐state driven reactivity of the Fe–O complexes.
Highlights
Iron(IV)-oxo units act as powerful oxidants in many enzymatic systems.[1,2,3,4,5] The scope of their reactivity includes reactions such as hydrogen atom transfer (HAT)[6] or electrophilic attacks to arene rings.[7,8] It has been well established that the reactivity correlates with the spin state[9] that may vary along the reaction coordinate.[10]
Our goal was to obtain a set of ligands that would form [(L)FeIV(O)]+ in the S = 0 ground state favored by more than 2 kcal molÀ1, predicted by both functionals and that would be synthetically accessible
After initial rounds of screening we found that ligands with pyridine rings, which form a plane perpendicular to the Fe-O unit seemed to have the most stable S = 0 states
Summary
Iron(IV)-oxo units act as powerful oxidants in many enzymatic systems.[1,2,3,4,5] The scope of their reactivity includes reactions such as hydrogen atom transfer (HAT)[6] or electrophilic attacks to arene rings.[7,8] It has been well established that the reactivity correlates with the spin state[9] that may vary along the reaction coordinate (multi-state reactivity).[10] In particular, HAT reactions in non-heme iron(IV)-oxo complexes have been generally predicted to proceed via high spin transition states,[11,12,13] irrespective of the spin state of the initial iron-oxo complexes, a phenomenon known as two-state reactivity.[14,15] The HAT reactions are frequently of a radical character,[16] but closed-shell oxidants can initiate them as well,[17,18] for example via proton-coupled electron transfer.[19] In this respect, reactivity of so far unknown closed shell ironoxo complexes can elucidate the role of unpaired electrons in Fe-O mediated oxidations.[20,21]
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