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

Abstract

The nonheme iron(IV)-oxido complex trans-N3-[(L 1 ) Fe IV = O(Cl)] + , where L 1 is a derivative of the tetradentate bispidine 2,4-di(pyridine-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1one, is known to have an S = 1 electronic ground state and to be an extremely reactive oxidant for oxygen atom transfer (OAT) and hydrogen atom abstraction (HAA) processes. Here we show that, in spite of this ferryl oxidant having the "wrong" spin ground state, it is the most reactive nonheme iron model system known so far and of a similar order of reactivity as nonheme iron enzymes (CÀ H abstraction of cyclohexane, À 90°C (propionitrile), t 1/2 = 3.5 sec). Discussed are spectroscopic and kinetic data, supported by a DFT-based theoretical analysis, which indicate that substrate oxidation is significantly faster than self-decay processes due to an intramolecular demethylation pathway and formation of an oxido-bridged diiron(III) intermediate. It is also shown that the iron(III)-chlorido-hydroxido/cyclohexyl radical intermediate, resulting from CÀ H abstraction, selectively produces chlorocyclohexane in a rebound process. However, the lifetime of the intermediate is so long that other reaction channels (known as cage escape) become important, and much of the CÀ H abstraction therefore is unproductive. In bulk reactions at ambient temperature and at longer time scales, there is formation of significant amounts of oxidation product -selectively of chlorocyclohexane -and it is shown that this originates from oxidation of the oxido-bridged diiron (III) resting state.

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

“… [23] This can be monitored by optical spectroscopy, where the corresponding d–d transition is low in energy (for the L 1 ‐based ferryl complex shifting from 768 to 850 and 900 nm for the complexes with MeCN, Cl − and Br − trans to N7, respectively) but still indicates an intermediate‐spin ( S =1) ground state albeit with decreasing triplet‐quintet gap. [ 23 , 24 ] C−H abstraction by high‐valent non‐heme‐iron oxidants generally proceeds on the quintet surface, with a preference for the σ channel with a linear [Fe−O⋅⋅⋅H⋅⋅⋅C−R] transition state and transfer of the electron into the d orbital. All non‐heme‐iron enzymes have in contrast to many intermediate‐spin ( S =1) model systems a high‐spin ( S =2) ground state.…”

“…[ 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 − ).…”

“…This is supported by the observation that the reactivities strongly depend on the monodentate coligand in the predicted order (MeCN<Cl − <Br − ). [23] The in‐plane ligand field may be further tuned by the substituent at N7 (L 1 vs. L 2 )[ 23 , 36 ] and substituents at the pyridine groups. [ 34 , 38 ]…”

“…The proposed mechanism for the halogenation of cyclohexane by [(L n )(Cl)Fe IV =O] + ( n =1, 2) is shown in Scheme 1 (data from the published kinetic analysis are given in the Supporting Information). The initial step is C−H abstraction by the ferryl oxidant, and for the L 1 ‐based system, this is inferred from second‐order kinetics with the substrate [23] and experimentally determined kinetic isotope effects and product distributions with adamantane as substrate. [ 14 , 39 , 40 ] The rebound step at the iron(III)/radical intermediate is supported by the observation that addition of external Cl − decreases the yield of halogenated product (see below) because free Cl − is oxidized to ClO − .…”

“…From the kinetic analysis it follows that the halogenation (pathway with k 4 ) competes with inner‐sphere C−H activation – demethylation at the tertiary amine N7 (a first‐order process with k 3 that, depending on the conditions, is 1–2 orders of magnitude slower than the reaction with substrate and can be excluded with L 2 ) – and with the second‐order comproportionation of the iron(IV) product with unreacted iron(II) precursor, producing the very stable diiron(III) “resting state” (a second‐order process with k 2 that depends on the formation rate of the ferryl complex and, depending on the conditions can be of similar efficiency as the halogenation pathway). [23] ii) Part of the cyclohexyl radical intermediate (pathway with k 4 ) might decay unproductively, that is, by cage escape. Pathways other than rebound, that is, combination of the organic radical either with the Fe III −OH or the Fe III −Cl site at the intermediate (Scheme 1 ), generally called cage escape, become important with increasing life‐time of the intermediate and this was probed experimentally for the bulk reaction (see the Supporting Information for details).…”