Closed Shell Iron(IV) Oxo Complex with an Fe–O Triple Bond: Computational Design, Synthesis, and Reactivity

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.T ochallenge this hypothesis,w ed esigned 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 quinisoxl igand 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 À1 ,c onsistent with an Fe-O triple bond, which was also confirmed by multireference calculations.T he unprecedented bond strength is accompanied by high gas-phase reactivity of 1 + in oxygen atom transfer (OAT)a nd in proton-coupled electron transfer reactions.T his challenges the current view of the spin-state driven reactivity of the Fe-O complexes.

Role of “S” Substitution on C–H Activation Reactivity of Iron(IV)–Oxo Cyclam Complexes: a Computational Investigation

“…The advancement of transition-metal oxo complexes as intermediates in a variety of catalytic reactions has been pivotal in the field of bioinorganic C–H activation chemistry latterly, with groups 3–10 being extensively explored. − Fe­(IV)O complexes have also been investigated vastly . They act as bio-inspired catalysts for several important reactions such as dioxygen activation and C–H activation resulting in hydroxylation/halogenation/epoxidation and other products. − In 2003, the first high-resolution structure of a mononuclear non-heme Fe­(IV)O complex was reported in the reaction of Fe­(II)­(TMC)­(CF 3 SO 3 ) 2 (TMC = 1,4,8,11-tetramethyl 1,4,8,11-tetraazacyclotetradecane), and the intermediate was characterized as Fe­(IV)O with an Fe–O double-bond character and a triplet ( S = 1) Fe­(IV) ground oxidation state . Since then, a number of mononuclear non-heme iron­(IV)–oxo complexes bearing tetradentate N 4 ligand entities have been studied such as TMC and amino-pyridyl and even porphyrin units. − TMC is recognized as one of the most popular macrocyclic fragments for the Fe­(IV)O complexes, which has been extensively investigated by many experimental and computational groups. − A number of ligand frameworks which mimic enzyme reactivity have also been reported, demonstrating how altering a ligand architecture (axial and equatorial position) leads to increased selectivity and reactivity. − Multiple investigations on [(TMC)­Fe­(IV)­O­(NCCH 3 )] 2+ have been performed largely because the sixth ligand acetonitrile in the axial position is conveniently supplanted by other ligands, for example, N 3 – , SR – , and so forth, wherein the rates of hydrogen atom transfer (HAT) from DHA (dihydroanthracene) hiked upon the introduction of more electron-donating axial ligands …”

Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity

“…Judicious ligand design in coordination complexes can also help to clarify the effect of crystal field (energy difference between spin states), sterics and electronics in defining the oxidation abilities of oxoiron(IV) complexes. Several studies have addressed the first two aspects in a quite systematic manner [18–27] . On the contrary, studies describing systematic tuning of the ligand electronic properties of the oxoiron(IV), without modifying sterics, are rather scarce and limited to pentadentate ligands [28] …”