The Fundamental Role of Exchange‐Enhanced Reactivity in CH Activation by S=2 Oxo Iron(IV) Complexes

Abstract

It is shown that H-abstraction reactivity by oxoiron(IV) complexes with a quintet ground state is highly enhanced due to exchange-stabilization endowed by the increased number of the exchange Correspondence to: Lawrence Que, Jr, larryque@umn.edu; Sason Shaik, sason@yfaat.ch.huji.ac.il. One of us, [6] has recently prepared two such S=2 reagents and compared their H-abstraction activities to those of the synthetic complexes that possess the more common S=1 ground state. These results generated however, a bag full of surprises, which are addressed herein by means of DFT calculations. Shown in Figure 1 are DFT calculated iron(IV)-oxo complexes along with their key geometric features, and spin state information. The isolated complex with an S=2 ground state is TMG 3 trenFe(IV)O 2+ (1),[6a] which possesses a trigonal bipyramidal iron coordination, typified by two-below-two-below-one d-orbital block, [3b] and hence a quintet ground state, well below the S=1 state. Surprisingly, however, 1 exhibited a rather sluggish H-abstraction reactivity even towards the weak C-H bonds of 1,4-cyclohexadiene (CHD). Thus, 1 was slightly less reactive than N4PyFe(IV)O 2+ 2 and five times more reactive than TMC(AN)Fe(IV)O 2+ , 3;[6b] both of which are thought to react via TSR. NIH Public Access[4] To add to the puzzle, the putative Tp(OBz)Fe(IV)O, 4, which was proposed to form upon oxygenation of Tp(benzoylformate)Fe(II) as a model for TauD, was found to be highly reactive and capable of activating even the strong C-H bond of cyclopentane (BDE = 96.3 kcal mol −1 ).[6b] Note that in the S=2 state, 5 4, Fe loses one of the benzoate arms and becomes a pentacoordinated square pyramid with a basal Fe(IV)-oxo moiety (Fig. 1). Thus, it is this weaker ligand field that stabilizes S=2 relative to the hexacoordinated S=1. Indeed, as can be seen from Figure 1, 4 is computed to involve degenerate S=1 and S=2 states.[7] So, in 4 a competition is expected between the two spin states to effect C-H activation; which state dominates? In summation, the experimental relative reactivities of the four Fe=O reagents order in a puzzling sequence:What is the origin of this reactivity pattern, and what are the electronic and steric factors that shape this trend? Answering this question is important for establishing rules of design of effective catalysts for C-H activation.To answer these questions we studied the reactivities of 1-4 towards H-abstraction from CHD. The geometries of all the critical species along the H-abstraction paths of 1-3, which are di-positively charged, were optimized at the B3LYP/B1(CH 3 CN) (B1 is LACVP) level at the reaction solvent, to minimize self-interaction errors which cause artificial electron transfer in some of these systems.[8] For 4, which is neutral and hence less subject to these particular errors, [8] we used B3LYP/B1. All energies were subsequently estimated using a NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript larger basis set, B2 (B2 is LACV3P+*), and solvent corrections, using th...

Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C–H bond activation transition state

“…Thus, unexpectedly the quintet channel is only marginally energetically disfavored. This may be ascribed to the much stronger spin-polarization stabilization created by five unpaired electrons in the HS ferric center in 5 TS1 compared to only one unpaired electron in the LS iron(III) in the corresponding 3 TS1 (12,13). Hence, the formation of oxyl-ferric species is not the key factor for the differential reactivity.…”

“…However, there is a long-term debate on how to rationalize the differential reactivity between quintet and triplet oxo-iron(IV) species. De Visser, Shaik, and coworkers argued that the enhanced exchange interaction upon approaching the transition state (TS) on the quintet surface flattens the potential energy surface (PES) and hence lowers the barrier (12,13). Baerends, Solomon, and coworkers proposed that the greater degree of exchange stabilization in the HS ferryl reactant relative to the IS analogues significantly stabilizes the Fe-d z2 based σ-antibonding molecular orbital (MO) (14)(15)(16)(17)(18).…”

The biology and chemistry of high-valent iron–oxo and iron–nitrido complexes

“…So far, all theoretical studies have led to the common conclusion that the ferryl species are better oxidants on the quintet-state than the corresponding triplet-state. There is a long-term debate, however, on how to rationalize the higher reactivity of the quintate state [66][67][68][69] . Moreover, direct experimental evidence for the higher reactivity of the S = 2 state is lacking in the literature.…”

“…Moreover, direct experimental evidence for the higher reactivity of the S = 2 state is lacking in the literature. Presumably, because the oxoiron(IV) core is protected by the sterically bulky chelator in the recently reported S = 2 [(TMG 3 tren)Fe IV (O)] 2 + complex 59,60,67 , its reactivity towards C-H bond cleavage is only comparable with triplet ferryl analogues. Indirect evidence of the higher reactivity of the quintate state is, however, provided by Seo et al 64 65 .…”

Nuclear Resonance Vibrational Spectroscopy on the FeIVO S=2 Non‐Heme Site in TMG3tren: Experimentally Calibrated Insights into Reactivity

“…Our DFT calculations for their reaction energetics ( Figure 5) are in agreement with the reaction barriers determined from experimental kinetics [10] as well as a recent DFT study. [21] From Figure 5, both 1 along an S = 2 (quintet) surface and 2 along an S = 1 (triplet) surface have similar electronic and Gibbs free energies associated with this reaction and similar O-H bond strengths of their initial Fe III -OH products. We estimated the steric contribution to these total reaction energies using two methods: (i) with the Fe IV =O core frozen at the transition state structure, an undistorted CHD (from the reactants) at the transition state is allowed to geometry-optimize away from the Fe IV =O core; (ii) with the same frozen core, the undistorted CHD is moved stepwise away (linearly) from the Fe IV =O core (Scheme S1, Supporting Information).…”