A Two‐State Reactivity Rationale for Counterintuitive Axial Ligand Effects on the CH Activation Reactivity of Nonheme Fe^IVO Oxidants

Abstract

This paper addresses the observation of counterintuitive reactivity patterns of iron-oxo reagents, TMC(L)FeO(2+,1+); L=CH(3)CN, CF(3)CO(2) (-), N(3) (-), and SR(-), in O-transfer to phosphines versus H-abstraction from, for example, 1,4-cyclohexadiene. Experiments show that O-transfer reactivity correlates with the electrophilicity of the oxidant, but H-abstraction reactivity follows an opposite trend. DFT/B3 LYP calculations reveal that two-state reactivity (TSR) serves as a compelling rationale for these trends, whereby all reactions involve two adjacent spin-states of the iron(IV)-oxo species, triplet and quintet. The ground state triplet surface has high barriers, whereas the excited state quintet surface features lower ones. The barriers, on any single surface, are found to increase as the electrophilicity of TMC(L)FeO(2+,1+) decreases. Thus, the counterintuitive behavior of the H-abstraction reactions cannot be explained by considering the reactivity of only a single spin state but can be rationalized by a TSR model in which the reactions proceed on the two surfaces. Two TSR models are outlined: one is traditional involving a variable transmission coefficient for crossover from triplet to quintet, followed by quintet-state reactions; the other considers the net barrier as a blend of the triplet and quintet barriers. The blending coefficient (x), which estimates the triplet participation, increases as the quintet-triplet energy gap of the TMC(L)FeO(2+,1+) reagent increases, in the following order of L: CH(3)CN > CF(3)CO(2) (-) > N(3) (-) > SR(-). The calculated barriers predict the dichotomic experimental trends and the counterintuitive behavior of the H-abstraction series. The TSR approaches make a variety of testable predictions.

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

“…This conclusion is in agreement with previous density functional theory studies (10,12,14). Our detailed analysis of the electronic structure changes along the reaction coordinate suggests that the real C-H bond cleaving agent is an oxyl-ferric species that is generated by lengthening of the Fe-oxo bond in the ferryl reactant en route to TS.…”

“…The reaction of the C-H bond cleavage by this complex turned out to be approximately 100 times faster than the corresponding mononuclear IS oxo-iron(IV) complex (8). This experimental finding is in accord with the theoretical prediction that quintet ferryl species are more aggressive oxidants than the corresponding triplet counterparts (9,10,11). However, there is a long-term debate on how to rationalize the differential reactivity between quintet and triplet oxo-iron(IV) species.…”

“…This way of looking at the reaction is simply an approach to rationalize the electronic structure changes that occur en route to the H-atom abstraction TS. As a consequence of this interpretation, the electron acceptor for the quintet σ-pathway is best viewed to be the O-p z based σ*-orbital rather than the Fe-d z2 orbital as proposed before (10,12,14). 5 IN and 7 IN are essentially energetically degenerate, because they have the same orbital occupation pattern except for the different spin coupling between the alkyl-radical and the HS ferric center.…”

Experiment and Theory Reveal the Fundamental Difference between Two‐State and Single‐State Reactivity Patterns in Nonheme FeIVO versus RuIVO Oxidants

“…[5] Theoretical studies proposed that the puzzling reactivity trends arose from the fact that these nonheme iron(IV) oxo reagents have two closely lying spin states, a ground state with a triplet spin state (T) and a low-lying quintet spin state (Q), as shown schematically in Figure 1 b. [6,7] Thus, the triplet state has a high energy barrier, while the quintet state has a much lower barrier that crosses through the larger triplet barrier. Therefore, the H-abstraction reactions proceed on the two energy surfaces, and the resulting blended reactivity is behind the unusual reactivity patterns.…”

“…Therefore, the H-abstraction reactions proceed on the two energy surfaces, and the resulting blended reactivity is behind the unusual reactivity patterns. [6] Without the two-state blend, the reactivity on any one of the spin-state surfaces was shown to follow the electrophilicity of 1-X. [6] How can one test this two-state reactivity (TSR) concept for the dichotomic reactivity pattern in oxo-transfer and H-abstraction processes by [Fe IV …”