Two-state Reactivity Scenario Research Articles

Mechanistic insight into the carboxylic derivatives formation from CO2 and ethylene over iron(0)-based catalyst

Since the 1980s, nickel/palladium-based catalysts have been extensively studied for the coupling reactions of CO2 and ethylene. In this study, the CO2 and ethylene coupling mediated by the iron(0)-based catalyst with a bis(dicyclohexylphosphino)ethane (dcpe) ligand was systematically investigated by means of DFT calculations. The DFT functionals were first subjected to a benchmark test in order to describe an iron-catalyzed reaction with a two-state reactivity (TSR). For the Fe/dcpe catalytic system, the formation of the five-membered iron-lactone 3 follows a stepwise route, which is easy to occur with a low energy barrier of 13.8 kcal/mol. The competing reactions by CO2 insertion and β-H elimination determine the fate of the five-membered iron-lactone 3, which may lead to the formation of three different products, including the formation of i) succinic acid, ii) iso-methylmalonic acid, and iii) acrylic acid. The calculated TOFs showed that CO2 insertion into the five-membered iron-lactone 3 is more likely to occur (1.67 h−1 for the formation of succinic acid) compared with the β-H elimination leading to iso-methylmalonic acid and acrylic acid. Interestingly, β-H elimination can be effectively facilitated by addition of the electrophiles, and a comparable TOF of 0.042 h−1 was obtained for the formation of methyl acrylate in the presence of CH3I. Unlike nickel-/palladium-catalyzed CO2 and ethylene coupling reactions, all four iron(0)-catalyzed reaction routes investigated herein showed a two-state reactivity scenario involving spin crossover between triplet and quintet PESs. This study provides a mechanistic understanding of the intriguing iron(0)-catalyzed CO2 and ethylene coupling reactions, which may shed some light on the development of green catalysts for CO2 utilization.

Infrared photodissociation spectroscopy of mass-selected [TaO3(CO2)n]+ (n = 2-5) complexes in the gas phase.

We report a joint experimental and theoretical study on the structures of gas-phase [TaO3(CO2)n]+ (n = 2-5) ion-molecule complexes. Infrared photodissociation spectra of mass-selected [TaO3(CO2)n]+ complexes were recorded in the frequency region from 2200 to 2450 cm-1 and assigned through comparing with the simulated infrared spectra of energetically low-lying structures derived from quantum chemical calculations. With the increasing number of attached CO2 molecules, the larger clusters show significantly enhanced fragmentation efficiency and a strong band appears at around 2350 cm-1 near the free CO2 antisymmetric stretching vibration band, indicating only a small perturbation of CO2 molecules on the secondary solvation sphere while higher frequency bands corresponding to the core structure remain largely unaffected. A core structure [TaO3(CO2)3]+ is identified to which subsequent CO2 ligands are weakly attached and the most favorable cluster growth path is verified to proceed on the triplet potential energy surface higher in energy than that of ground states. Theoretical exploration reveals a two-state reactivity (TSR) scenario in which the energetically favored triplet transition state crosses over the singlet ground state to form a TaO3+ core ion, providing new information on the cluster formation correlated with the reactivity of tantalum metal oxides towards CO2.

Front Cover: Ab Initio Methods in First‐Row Transition Metal Chemistry (Eur. J. Inorg. Chem. 15/2022)

The Front Cover shows the reaction profiles, calculated at different levels of theory, of H-atom abstraction and epoxidation processes of a non-heme iron(IV)-oxo biomimetic catalyst featuring a two-state reactivity scenario. The accurate calculation of the spin-state energetics can be very challenging for these methodologies, as symbolized by the question marks in the thoughts of the operator in front of the screen. More information can be found in the Review by M. Feldt and Q. M. Phung.

Hydrogen- and oxygen-atom transfers in the thermal activation of benzene mediated by Cu2O2+ cations.

The mass-selected copper oxide cluster cations Cu2O2+ are successfully prepared by laser ablation and reacted with benzene in a linear ion trap reactor. The cluster reaction is characterized by reflectron mass spectrometry in conjunction with density functional theory calculations. Four types of reaction channels are observed: (1) Cu2OH+ + C6H5O˙, (2) Cu2C6H6+ + O2, (3) Cu(C6H6)2+ + Cu and (4) Cu2O2C6H6+, in which the first one is the major product. Observation of the products Cu2OH+ indicates that oxygen atom transfer (OAT) accompanying hydrogen atom transfer (HAT) occurs, and the oxygen-centered radical (O-˙) in Cu2O2+ is crucial to the cleavages of C-H and Cu-O bonds. It is interesting that HAT and OAT occur in a single reaction channel to form C6H5O˙, which has not been reported in the reactions of gas-phase ions with benzene. The calculated results are in agreement with the experimental observations, and no two-state reactivity scenario is present in the potential energy surface of channel 1. This work can provide useful insights into the nature of active sites over copper oxides and reaction mechanisms in the corresponding heterogeneous reactions.

Cobalt-catalysed unactivated C(sp3)–H amination: two-state reactivity and multi-reference electronic character

A remarkable two-state reactivity scenario and an unusual multi-reference character have been computationally found in Co-catalysed C(sp3)–H amination. In addition, the investigation on the additive, aminating reagent, metal center, and auxiliary ligand provides implications for development of new catalytic C–H functionalization systems.

Spin-Selective, Competitive Hydrogen-Atom Transfer versus CH2 O-Generation from the CH4 /[ReO4 ]+ Couple at Ambient Conditions.

The thermal gas-phase reactions of [ReO4 ]+ with methane have been explored by using Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. Upon reacting with methane, this cluster oxide, having an even-number of valence electrons, brings about both hydrogen-atom abstraction (HAT) to generate [ReO4 H].+ and the formation of formaldehyde. Mechanistically, HAT occurs on the ground-state triplet surface, while for the generation of formaldehyde a two-state reactivity scenario prevails. The branching ratio of these competing processes is affected by the rather inefficient spin-orbit coupling to bring about the required triplet-singlet intersystem crossing.

Substrate-Dependent Two-State Reactivity in Iron-Catalyzed Alkene [2+2] Cycloaddition Reactions.

Iron-catalyzed alkene [2+2] cycloaddition reactions represent a promising stepwise pathway to effect the kinetically hindered concerted [2+2] cycloaddition. However, the fundamental reactivity paradigm of these reactions remains unclear. Based on high level combined CASPT2/DFT modelings, herein we reveal an unprecedented substrate-dependent two-state reactivity scenario for the key C-C coupling in this iron catalysis, in which the representative substrates of mono-olefins only and mono-olefin plus 1,3-diene exhibit different reactivity paradigms. The role of the redox-active ligand is found to generate a ferric oxidation state for the metallacyclic intermediate of C-C coupling, thereby rendering a thermodynamically more accessible FeIII/FeI reductive elimination process compared with the otherwise FeII/Fe0 one. The enhancement of the spin state transition efficiency between the singlet and triplet states is predicted as an alternative way to increase the C-C coupling reactivity in the cross [2+2] cycloaddition reactions between mono-olefins and dienes. This work highlights the ab initio multi-reference method in describing very complicated open-shell iron catalysis.

Spin-Selective Thermal Activation of Methane by Closed-Shell [TaO3 ](.).

Thermal reactions of the closed-shell metal-oxide cluster [TaO3 ](+) with methane were investigated by using FTICR mass spectrometry complemented by high-level quantum chemical calculations. While the generation of methanol and formaldehyde is somewhat expected, [TaO3 ](+) remarkably also has the ability to abstract two hydrogen atoms from methane with the elimination of CH2 . Mechanistically, the generation of CH2 O and CH3 OH occurs on the singlet-ground-state surface, while for the liberation of (3) CH2 , a two-state reactivity scenario prevails.

Two-State Reactivity in Low-Valent Iron-Mediated C-H Activation and the Implications for Other First-Row Transition Metals.

C-H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C-H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C-H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C-C coupling and finalizing the C-H functionalization. Regeneration of the Fe(II) catalyst for the next round of C-H activation involves SET oxidation of the Fe(I) species generated after the C-C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C-H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C-H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C-H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C-H activation reactions.

Penetrating the Elusive Mechanism of Copper-Mediated Fluoromethylation in the Presence of Oxygen through the Gas-Phase Reactivity of Well-Defined [LCuO](+) Complexes with Fluoromethanes (CH(4-n)Fn, n = 1-3).

Traveling wave ion mobility spectrometry (TWIMS) isomer separation was exploited to react the particularly well-defined ionic species [LCuO](+) (L = 1,10-phenanthroline) with the neutral fluoromethane substrates CH(4-n)Fn (n = 1-3) in the gas phase. Experimentally, the monofluoromethane substrate (n = 1) undergoes both hydrogen-atom transfer, forming the copper hydroxide complex [LCuOH](•+) and concomitantly a CH2F(•) radical, and oxygen-atom transfer, yielding the observable ionic product [LCu](+) plus the neutral oxidized substrate [C,H3,O,F]. DFT calculations reveal that the mechanism for both product channels relies on the initial C-H bond activation of the substrate. Compared to nonfluorinated methane, the addition of fluorine to the substrate assists the reactivity through a lowering of the C-H bond energy and reaction preorganization (through noncovalent interaction in the encounter complex). A two-state reactivity scenario is mandatory for the oxidation, which competitively results in the unusual fluoromethanol product, CH2FOH, or the decomposed products, CH2O and HF, with the latter channel being kinetically disfavored. Difluoromethane (n = 2) is predicted to undergo the analogous reactions at room temperature, although the reactions are less favored than those of monofluoromethane. The reaction of trifluoromethane (n = 3, fluoroform) through C-H activation is kinetically hindered under ambient conditions but might be expected to occur in the condensed phase upon heating or with further lowering of reaction barriers through templation with counterions, such as potassium. Overall, formation of CH(3-n)Fn(•) and CH(3-n)FnOH occurs under relatively gentle energetic conditions, which sheds light on their potential as reactive intermediates in fluoromethylation reactions mediated by copper in the presence of oxygen.

On the Mechanisms of Degenerate Ligand Exchange in [M(CH3)]+/CH4 Couples (M=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as Explored by Mass Spectrometric and Computational Studies: Oxidative Addition/Reductive Elimination versus σ‐Complex‐Assisted Metathesis

The degenerate ligand exchange in [M(CH(3))](+)/CH(4) couples occurs in the gas phase at room temperature for M=Ni, Ru, Rh, Pd, and Pt, whereas the complexes containing Fe and Co are unreactive. Details of hydrogen-atom scrambling versus direct ligand switch have been uncovered by labeling experiments with CD(4) and (13)CH(4), respectively. The reactivity scale ranges from unreactive (M=Fe, Co) or inefficient (M=Ni, Pd) to moderately (M=Ru) and rather reactive (M=Rh, Pt). Quite extensive, but not complete, H/D exchange between the hydrogen atoms of the incoming and outgoing methyl groups is observed for M=Pt, whereas for M=Ni and Pd a predominantly direct ligand switch prevails. DFT calculations performed at the B3LYP level of theory account well for the thermal nonreactivity of the Fe and Co couples. For [Ni[CH(3))](+)/CH(4), a sigma-complex-assisted metathesis (sigma-CAM) is operative such that, in a two-state reactivity (TSR) scenario, two spin flips between the (3)A ground and (1)A excited states take place at the entrance and exit channels of the encounter complexes. For M=Ru and Rh, only oxidative addition/reductive elimination (OA/RE) is favored energetically, and the reaction is confined to the electronic ground states (3)A and (2)A. In contrast, for the [Pd(CH(3))](+)/CH(4) system, on the (1)A ground-state potential-energy surface both the OA/RE and sigma-CAM variants are energetically comparable, and the small reaction efficiency for the ligand switch is reflected in transition states located energetically close to the reactants. For the [M(CH(3))](+)/CH(4) complexes of the 5d elements, the sigma-CAM mechanism does not play a role. For M=Pt, the energetically most favored path proceeds in a spin-conserving manner on the (1)A potential-energy surface, which accounts for the extensive single and double hydrogen-atom exchange preceding ligand exchange. Although for M=Os and Ir the [M(CH(3))](+) complexes could not be generated experimentally, computational studies predict that both systems may undergo thermal reaction with CH(4), and an OA/RE mechanism will commence on the respective high-spin ground states; however, the bond-activation and ligand-exchange steps will occur on the excited low-spin surfaces in a TSR scenario.

The “Rebound Controversy”: An Overview and Theoretical Modeling of the Rebound Step in C—H Hydroxylation by Cytochrome P450

AbstractFor Abstract see ChemInform Abstract in Full Text.

The “Rebound Controversy”: An Overview and Theoretical Modeling of the Rebound Step in C−H Hydroxylation by Cytochrome P450

AbstractC−H hydroxylation by the enzyme cytochrome P450 is one of Nature’s important and most ubiquitous processes. There is strong evidence that the mechanism proceeds by initial hydrogen abstraction, from the alkane, by the high‐valent iron−oxo species of the enzyme, followed by a rebound of the alkyl radical to form the ferric−alcohol product complex (the Groves “rebound” mechanism). Nevertheless, the “rebound” mechanism is still controversial due to the ultrashort radical lifetimes deduced from radical‐clock experiments. This review describes the main elements of the controversy and its updated resolution by theory, with an emphasis on the controversial rebound step. The theoretically derived model for alkane hydroxylation is found to involve two‐state reactivity (TSR). In TSR, radicals are produced on two different spin‐state surfaces, and thereafter they react differently; on the low‐spin surface, the rebound proceeds with no product rearrangement and the lifetime of the radicals is ultrashort (or zero), while on the high‐spin surface the barrier for rebound is substantial and the lifetime of the radical is sufficiently long that rearrangement may compete with product formation by rebound. A new valence‐bond model is developed to model the rebound barrier on the high‐spin surface and conceptualize its dependence on the nature of the alkyl radical. The possible intermediacy of carbocationic intermediates alongside radicals is discussed. It is shown that the TSR scenario provides a satisfactory rationale for the controversial findings in the field, and makes verifiable predictions. One of the predictions of TSR is that the ratio of unrearranged to rearranged products, [U/R], will be subject to an intrinsic isotope effect that is substrate‐dependent. This prediction and its possible verification by experiment are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Can a single oxidant with two spin states masquerade as two different oxidants? A study of the sulfoxidation mechanism by cytochrome p450.

Density functional calculations were performed on the sulfoxidation reaction by a model compound I (Cpd I) of cytochrome P450. By contrast to previous alkane hydroxylation studies, which exhibit a dominant low-spin (LS) pathway, the sulfoxidation follows a dominant high-spin (HS) reaction. Thus, competing hydroxylation and sulfoxidation processes as observed for instance by Jones et al. (Volz, T. J.; Rock, D. A.; Jones, J. P. J. Am. Chem. Soc. 2002, 124, 9724) are the result of a two-state reactivity scenario, whereby the hydroxylation originates from the LS pathway and the sulfoxidation from the HS pathway. In this manner, two spin states of a single oxidant (Cpd I) can be disguised as two different oxidants. The calculations rule out the possibility that a second oxidant (the ferric peroxide, Cpd 0 species) interferes in the observed results of Jones et al.

A Model “Rebound” Mechanism of Hydroxylation by Cytochrome P450: Stepwise and Effectively Concerted Pathways, and Their Reactivity Patterns

A two-state rebound mechanism of alkane hydroxylation by a model active species of the enzyme cytochrome P450 is studied using density functional theoretic calculations. Theory corroborates Groves's rebound mechanism (Groves, J. T. J. Chem. Educ. 1985, 62, 928), with a key difference, namely that in the two-state rebound the reactivity and product distribution result from the interplay of two reactive states of the active ferryl-oxene (Por+•FeO) species of the enzyme: one state is low-spin (doublet) and the other high-spin (quartet). Transition-state structures, intermediates, and product complexes are identified for the two states. The bond activation in either one of the two states involves a hydrogen abstraction-like transition structure. However, while in the high-spin state there forms a radical that has a significant barrier for rebound, in the low-spin state the rebound is virtually barrierless. Even though one cannot ignore incursion of a small amount of radicals in the low-spin state, it is clear that the radical has a significant lifetime mainly on the high-spin surface. The results are used to gain insight into puzzling experimental data which emerge from studies of ultrafast radical clocks (e.g., Toy, P. H.; Newcomb, M.; Hollenberg, P. F., J. Am. Chem. Soc. 1998, 120, 7719), vis à vis the nature the transition state, deduced from kinetic isotope effect measurements (Manchester, J. I.; Dinnocenzo, J. P.; Higgins, L. A.; Jones, J. P. J. Am. Chem. Soc. 1997, 119, 5069) and stereochemical scrambling patterns (Groves, J. T.; McClusky, G. A.; White, R. E.; Coon, M. J. Biochem. Biophys. Res. Commun. 1978, 81, 154). Understanding the electronic structure of the various species leads to a predictive structure−reactivity picture, based on the two-state reactivity scenario (Shaik, S.; Filatov, M.; Schröder, D.; Schwarz, H. Chem. Eur. J. 1998, 4, 193). The model makes it possible to predict the dependence of the relative rates of the two states, and of the corresponding steps as a function of the nature of the alkane, the resulting alkyl radical, and the binding capability of the thiolate proximal ligand of the active species.