Theory Revealing Unusual Non-Rebound Mechanisms Responsible

Research Article pubs.acs.org/acscatalysis

Theory Revealing Unusual Non-Rebound Mechanisms Responsible for the Distinct Reactivities of OMnIVO and [HO−MnIV−OH]2+ in C−H Bond Activation Dibyendu Mallick and Sason Shaik* Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel S Supporting Information *

ABSTRACT: This article uses theory to address the origins of the reactivity differences between MnIVO and MnIV−OH complexes, having identical ligand spheres and metal oxidation states, toward 9,10-dihydroanthracene (DHA) under different pH conditions. Theory discovers different non-rebound mechanisms leading to unique products for the two complexes. One of these is a novel mechanism that operates under basic conditions and that rationalizes the formation of anthraquinone through an anthracene radical anion intermediate. In addition, the calculations reveal a rich mechanistic scheme having blended hydrogen atom transfer and proton-coupled electron transfer (HAT/PCET) with both proton transfer/electron transfer (PT/ET) and electron transfer/proton transfer (ET/PT) characters. The distinct nature of the transition states, such as PT/ET and ET/PT, for the second H-abstraction reactions from the substrate radical by the MnIVO and MnIV−OH complexes accounts for the observed product distributions for these two species. The formation of an anthracene radical anion, and its participation in a unique non-rebound mechanism, is a testable prediction. KEYWORDS: Mn−oxo, Mn−hydroxo, PT/ET, ET/PT, non-rebound, anthracene radical anion

1. INTRODUCTION The C−H bond activation of hydrocarbons by high-valent metal ions has been a subject of great interest in bioinorganic and oxidation chemistry for the last few decades.1 In many cases, the high-valent metal−oxo moieties, such as iron−oxo, manganese−oxo, copper−oxo, etc., have been identified among the key active species for many chemical and biological oxidation processes.2 However, various studies revealed the existence of other species, such as metal−hydroxo and metal− hydroperoxo, which also activate C−H bonds.3 In iron and manganese lipoxygenase, the FeIII−OH and MnIII−OH moieties were suggested as the key active species for performing H-abstraction reactions from unsaturated fatty acids.4 Theoretical calculations showed that the high-valent heme−Mn−OH species is also a potent H abstractor.5 Because of their significance in biological and chemical processes, it is important to understand the differences in reactivity between the metal−oxo and metal−hydroxo moieties. In some model systems,6 metal−oxo and metal−hydroxo complexes, which differ only in their protonation states, were found to be hydrogen abstractors (H-abstractors) of equal potency. In contrast, in manganese complexes, the hydroxo © XXXX American Chemical Society

species were found to be poorer H-abstractors in comparison to the manganese−oxo species, even though the MnO(H)−H bond dissociation energies, which determine the thermodynamic driving forces, were virtually identical at 84.3 and 83.0 kcal/mol, respectively.7 Thus, Yin et al.7,8 carried out a detailed H-abstraction study using two Mn(IV) complexes, MnIV(Me2EBC)(O)2 (I) and [MnIV(Me2EBC)(OH)2]2+ (II) shown in Figure 1, which possess MnIVO and MnIV−OH groups in identical ligand spheres. They found that these two complexes (I and II) interconvert with changes in the pH of the medium. Under basic conditions (pH 13.4) manganese− dioxo compound I was the major species, whereas under acidic conditions (pH 4.0) the major species was the manganese− dihydroxo compound II. Yin et al.7b also found that I reacts 40 times faster than II in H-abstraction reactions from dihydroanthracene (DHA). Another interesting observation from these experiments is the different product distributions found at different pH conditions, as shown in Table 1.9 Under Received: January 10, 2016 Revised: March 21, 2016

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DOI: 10.1021/acscatal.6b00085 ACS Catal. 2016, 6, 2877−2888

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Since recently Nam and co-workers10 have shown that the rebound mechanism seldom operates in reactions of nonheme metal−oxo species due to favorable dissociation of the initially formed radical, we felt that the mechanism in Scheme 1 merits theoretical investigation. As such, we set out to do a detailed mechanistic study using density functional theory (DFT) with an aim of comprehending the different reactivity patterns of manganese−oxo and −hydroxo species, and their different products of DHA oxidation, under two different pH conditions. In so doing, we shall also explore the possibility of a nonrebound mechanism for these systems.10 Finally, since there are several spin states possible for the intermediate species formed during the course of the reaction, the roles of two-state reactivity2c and the exchange-enhanced reactivity11 (EER) will be probed. As shall be demonstrated, theory reveals a rich mechanistic scheme having blended hydrogen atom transfer and proton-coupled electron transfer (HAT/PCET) with both proton transfer/electron transfer (PT/ET) and electron transfer/proton transfer (ET/PT) characters. Furthermore, theory discovers a novel non-rebound mechanism which takes place under basic conditions, wherein the monohydroanthracenyl radical intermediate transfers a proton to HO−MnIIIO, leading to the formation of anthracene radical anion and [HO−MnIII−OH]+, which dissociate despite their opposite charges, eventually leading to the production of anthraquinone.

Figure 1. Dominant forms of MnIV complexes present in solution at different pH conditions: (a) at basic pH (13.4); (b) at acidic pH (4.0).

basic conditions, anthraquinone was the major product with anthracene as a minor product, whereas anthracene was the sole product under acidic conditions. Table 1. Experimentally Observed9a Product Distributions for the Reactions of I with DHA and II with DHA product distribution (%) pH

MnIV moiety

anthracene

anthraquinone

anthrone

4.0 13.4

MnIV−OH MnIVO

17.6 6.3

0.8 21.8

none trace

These experimental observations raised an interesting question regarding the mechanisms whereby these two species, MnIVO and MnIV−OH, react under the employed different pH conditions. Shi et al.9a proposed the mechanism in Scheme 1 to rationalize the observed product distributions under different pH conditions. According to their proposal, after the H-abstraction step, the substrate radical can undergo two competing pathways, desaturation and rebound. Under acidic conditions, desaturation to anthracene was proposed to be highly favored over rebound, whereas highly basic conditions favored the rebound mechanism to form a hydroxylated product, which was subsequently oxidized to anthraquinone.

2. COMPUTATIONAL DETAILS All of the structures were optimized at the UB3LYP/LACVP* level of theory12 (B1) using the Gaussian 09 package.13 Since the manganese−dihydroxo complex II possesses a +2 charge, the system is prone to be afflicted by self-interaction errors (SIEs), which can generally be minimized by performing solvent-phase optimization. Our recent study shows that the SIE can also be removed by performing the calculation in the presence of counterions in the gas phase.14 We also tested this protocol, and we observed an H-abstraction barrier almost similar to that obtained using solvent-phase optimization

Scheme 1. Proposed Mechanisms (in Ref 9a) for the Reactions of (a) Manganese−Dioxo Species I with DHA and (b) Manganese−Dihydroxo Species II with DHA

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ACS Catalysis techniques (see Figure S1 in the Supporting Information). Hence, we followed only the solvent-phase optimization protocol for all other calculations after the hydrogen atom abstraction step. The PCM solvent model15 was used in order to take account of the solvent effects. Acetone is used in the theoretical study, as it was used in the experiment. The nature of stationary points was characterized by frequency calculations at the same level of theory (using B1). Single-point energy evaluations were carried out at the UB3LYP/Def2-TZVP level16 (B2) including the solvent. Free energies were evaluated at 25 °C, and the corresponding free energies of activation for H abstraction were calculated relative to the separated reactants. The reaction barriers for the desaturation, rebound, and radical recombination processes were evaluated from the respective intermediate clusters (ICs). In addition to B3LYP, we have also tested the M06-L functional16c (0% HF exchange with embedded dispersion) for the relative spin-state ordering of complexes I and II as well as for a few key reaction steps, such as rebound, desaturation, and dissociation processes. M06-L was found to give results similar to those of B3LYP for most of these cases, as given in Tables S1−S3 in the Supporting Information. The impact of dispersion corrections was tested using Grimme’s DFT-D3 program16d implemented in the Gaussian 09 program. The dispersion-corrected values have the same trends as those of the uncorrected B3LYP, albeit the absolute numbers changed. The results are given in Tables S1−S3. As such, the B3LYP results were doubly validated. Spin natural orbitals (SNOs) have been used in characterizing the HAT/PCET features of a given transition state (TS). Spin densities (ρ) and natural charges (q) of stationary points were analyzed for the identification of the type of mechanism: namely either the PT/ET or ET/PT type.

Figure 2. UB3LYP/B1 optimized key geometrical parameters (distances in Å, angles in deg) and quartet−doublet energy gaps, ΔE QD (in kcal/mol), for oxidants I and II. Mn−O/N A /N B corresponds to the average distances between Mn and O/NA/NB atoms, where NA represents nitrogen atoms on the same axis as oxygen atoms and NB represents other nitrogen atoms. The ΔEQD values are presented as follows: ΔE(B2//B1)/ΔG(B2//B1). ΔE includes a zero point energy correction at the B1 level. Hydrogen atoms, except for those in the O−H groups, are omitted for clarity.

for II. Note, in Scheme 2a, that unlike the case for other nonheme metal complexes, in I the added electron during HAT goes to σ*x2−y2 rather than to σ*z2, whereas during the same process in II the added electron moves to σ*z2 (Scheme 2b). 3.3. Transition State Features and Mechanism for HAbstraction by I and II. Figure 4a shows key geometric features and charge and spin distributions on the TSs, TSI-H and TSII-H, for the first H-abstraction steps by I and II, respectively. Inspection of the TS geometries in Figure 4a shows that, for C−H bond activation by I, the TS is quite late (extensive C−H bond cleavage and lesser H−O bond making distances), whereas for II the TS is early with small C−H bond cleavage and more extensive H−O bond making distances. These are unusual trends, considering that the barrier for II is larger. In TSI-H and TSII-H, the charges on the H-abstracted species (qDHA‑H) are −0.54 and 0.10, respectively, and the corresponding spin densities (ρDHA‑H) are −0.15 and −0.54 (Figure 4a). High negative charge and low spin density values on the H-abstracted moiety of TSI-H indicate that it possesses considerable proton transfer (PT) character. The representative H-abstraction SNO (Figure 4b) for the corresponding TS has characteristics of both HAT and PCET type mechanisms, according to the definition proposed by Borden and Mayer.17 Thus, on the basis of the charge, spin density, and the nature of SNO we can designate that this TS has HAT character with a considerable PT feature. In contrast, TSII-H has a small negative charge but a high negative spin density on the Habstracted moiety. This is also attended by a large degree of charge transfer (qCT = 0.48) from the substrate to the oxidant in the TS such that the positive charge on the oxidant decreases from 2.00 to 1.52. These findings indicate that the corresponding TS possesses high electron transfer (ET) character and can be labeled as TSPCET. Examination of the representative H-abstraction SNO (Figure 4b) for TSII-H also supports the PCET type mechanism with some blended HAT character. 3.4. Hydroxylation (Rebound) vs Desaturation vs Dissociation for the Reactions I + DHA and II + DHA.

3. RESULTS AND DISCUSSION 3.1. Electronic Structure. Both I and II possess d3 configurations, with quartet (Q; S = 3/2) ground states. The optimized geometries and the relative energies for both spin states for I and II are given in Figure 2. The doublet states (D; S = 1/2) for I and II are 18.4 and 25.5 kcal/mol higher in energy at B2 relative to the corresponding ground states. The Kohn−Sham molecular orbitals (KSMOs) and their occupations for complex I in S = 3/2 and 1/2 states are given in Figure 3. The doublet state (S = 1/2) of complex I has a broken spin symmetry solution having two unpaired electrons antiferromagnetically coupled to a third unpaired electron, as shown in Figure 3b. The electronic structures of complex II in S = 3/2 and 1/2 states are mostly similar to those of complex I in S = 3/2 and 1/2 states with little different ordering of the singly occupied KSMOs, as shown in Figure S2 in the Supporting Information. Since the doublet states are rather higher lying, we have not considered these states for the reactivity. 3.2. Hydrogen Atom Abstraction (HAT) Reactions by Oxidants I and II. Table 2 collects the zero point energy corrected reaction barriers (ΔE⧧) and the corresponding free energy barriers (ΔG⧧) for the H-abstraction reactions from DHA by I and II. It is seen that I has a lower free energy barrier (21.9 kcal/mol) for H-abstraction than II (27.5 kcal/mol), which is qualitatively consistent with experimental observations.7b The experimentally observed free energy barriers for the H-abstraction, 21.1 and 23.5 kcal/mol for complexes I and II, respectively, display the same trend as the computations, but apparently the computations overestimate slightly the barrier 2879


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Figure 3. Electronic structures of I with (a) S = 3/2 and (b) S = 1/2, with the corresponding Kohn−Sham molecular orbital (KSMO) occupancies.

Table 2. Activation Energy Barriers (kcal/mol) and the Reaction Energies for H Abstraction from DHA by I and II complex

spin state S

ΔE⧧

ΔG⧧

ΔG⧧exp

ΔGr

I II

3/2 3/2

9.2 15.7

21.9 27.5

21.1 23.5

2.1 10.2

Scheme 2. Orbital Shift Diagrams for H-Abstraction Reactions from DHA by Complexes (a) I and (b) II

Figure 4. (a) Optimized structures of the TSs with key geometrical parameters (distances in Å, angles in deg) for the H-abstraction reactions by I and II from DHA along with natural charges (q) and spin densities (ρ) on the H-abstracted moieties (DHA-H) and on the oxidants. The amount of charge transferred (qCT) from the substrate to the oxidant or vice versa is also given. (b) Representative Habstraction SNOs in TSs for I + DHA and II + DHA. Only relevant hydrogen atoms are shown for clarity.

of radical traps) by Nam and his co-workers for many nonheme cases.10 It is noted that for the reaction I + DHA the second Habstraction can transpire at the second oxygen center and hence the substrate does not need any reorganization, as needed for mono-oxygenated metal complexes for performing desaturation. These mechanistic varieties for complex I are depicted in Figure 5a, showing all of the possibilities mentioned above. Let us now explore all these possibilities one by one for I in the following. From Figure 5a it is apparent that the rebound step for I has a prohibitively high barrier (46.6 kcal/mol) in comparison to the barrier for the desaturation step (2.1 kcal/mol). We have also found that the rebound pathway is highly endothermic

The subsequent step following H-abstraction poses an interesting mechanistic trifurcation. The substrate radical, monohydroanthracenyl radical (MHA•), may either undergo rebound to form the hydroxylated product or desaturate to anthracene in a second H-abstraction step. The radical may also dissociate in the solution, as amply demonstrated (e.g., by usage 2880


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Figure 5. Different mechanistic possibilities (hydroxylation or desaturation or dissociation) that the substrate radical can undergo after first Habstraction for (a) complex I and (b) complex II. The energies are given in kcal/mol, calculated at the UB3LYP/B2//UB3LYP/B1 level of theory.

clearly rule out the rebound pathway and expect a competition between the desaturation and dissociative pathways. Figure 5b shows the various mechanistic possibilities after the first H-abstraction step by complex II. In this case, the radical center on the substrate is far from the OH group of the reduced oxidant (IIa), [HO−MnIII−OH2]2+ (see Figure S3b in the Supporting Information), and hence in situ rebound of the OH group to form the hydroxylated product is not possible. However, due to the close proximity of the OH group with the other hydrogen atom of the substrate, the desaturation pathway

(44.0 kcal/mol) whereas the desaturation is exothermic, which is understandable, as the desaturation leads to the formation of the aromatic anthracene as a product, while some aromatization is lost by the rebound to alcohol. The huge rebound barrier is due primarily to the high endothermicty of the process. The hydroxylation pathway via the rebound mechanism in the sextet state was also found to have a very high barrier (37.5 kcal/mol). The dissociative pathway for the radical is found to be the most favorable process among all three possibilities. Thus, we can 2881


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Scheme 3. Reaction Network of the Possible Pathways Following the First H-Abstraction, for the Reaction of I with DHAa

a

The favorable pathways (having low barriers) are marked in green, whereas unfavorable pathways (high barriers) are marked in red. Shown also are the follow-up reactions after dissociation, such as the hydroxylation via the non-rebound (recombination) pathway and desaturation via the second H-abstraction pathway, for the reactions I + MHA• and Ia + MHA•.

reacting either with a second molecule of the oxidant I (O MnIVO) which is present in the reaction medium or with the reduced oxidant Ia (HO−MnIIIO). As shown in Scheme 3, only desaturation is allowed, while hydroxylation remains a forbidden process. Similar follow-up reactions are also possible for the reaction of substrate radical with II, [HO−MnIV− OH]2+, or with the reduced oxidant IIa, [HO−MnIII−OH2]2+ (see Scheme S1 in the Supporting Information). Thus, Scheme 3 and Scheme S1 represent the allowed reaction networks for both I and II after the respective H-abstraction steps. Scheme 4 displays the electron-shift diagrams for the followup processes, discussed above in Scheme 3. Thus, since the substrate radical is highly delocalized and is quite stable, its spin may undergo reorientation and thereby create a two-state reactivity scenario. As such, for the radical reacting with a second molecule of I, we have two spin state possibilities. The

to generate anthracene could be feasible. However, the second H-abstraction by the second OH group has a high barrier (22.5 kcal/mol). As with I, here too the dissociative pathway is possible after the first H-abstraction step and it is found that the dissociation of the radical is highly exothermic (−9.5 kcal/mol) and is the most favorable among all the pathways discussed. To ease the discussion of these many mechanistic steps, we present in Scheme 3 an overview of the potential mechanistic pathways using color coding. All of the pathways marked with red arrows are the roads not taken, while those marked with green arrows are the allowed pathways through which the mechanism proceeds. Thus, it is seen that, past the Habstraction step, the intermediate undergoes either desaturation and/or dissociation, while the high barrier forbids the respective rebound step. Once the substrate radical diffuses away, it can undergo follow-up hydroxylation or desaturation by 2882


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Scheme 4. Electron Shift Diagrams for All Possible Spin States of I ((a) S = 2, (b) S = 1) and Ia ((c) S = 5/2, (d) S = 3/2), Showing the Second H-Abstraction (2H) and Radical Recombination (Rec) Steps

second molecule of the oxidant I in the Soxi = 3/2 state reacts with either the Ssub = 1/2 or −1/2 state of the substrate (total spin S = 2 or 1), as shown in Scheme 4a,b, respectively. Similarly for the reaction of reduced oxidant Ia in the Soxi = 2 state with the radical, we have two spin state possibilities, S = 5/ 2 or 3/2, shown in Scheme 4c,d. We will explore all these spin state possibilities for both hydroxylation and desaturation reactions in the next section. 3.5. Hydroxylation vs Desaturation for the Reactions of I + MHA• and Ia + MHA•. The free energy profiles for the substrate radical reacting with a second molecule of I to form the hydroxylated or desaturated products for both spin states are given in Figure 6. In accord with the color coding in the

reaction network in Scheme 3, Figure 6 shows that the desaturation pathway is favored over the radical attack step (Rec), which would lead to substrate’s hydroxylation products, for both S = 2 and S = 1 spin states. Similar free energy profiles for the reaction of the reduced oxidant (Ia) with the substrate radical (shown in Figure 7) also reveal the preference of the

Figure 7. Free energy profiles of the desaturation and hydroxylation reactions by Ia (HO−MnIIIO) in S = 5/2 and S = 3/2 states as calculated at the UB3LYP/B2//UB3LYP/B1 level of theory. The energies are given in kcal/mol.

desaturation pathway over the hydroxylation pathway for both possible spin states (S = 5/2 and 3/2). Hence, our theoretical calculations suggest that the formation of anthracene via the desaturation pathway will always be favored over the hydroxylation pathway, irrespective of the oxidants and their spin states that are reacting. This result is in fact consistent with the previous results for various nonheme systems where DHA always

Figure 6. Free energy profiles of the desaturation and hydroxylation reactions by I (OMnIVO) in S = 2 and S = 1 states as calculated at the UB3LYP/B2//UB3LYP/B1 level of theory. The energies are given in kcal/mol. 2883


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ACS Catalysis produces anthracene as a major product.18 Moreover, the preferred desaturation is a common computational result,18,19 which follows simple physical logic (see above). At the same time, however, this conclusion appears to mismatch the experimental finding9a of anthraquinone as the major product under basic conditions (pH 13.4). The disagreement of the theoretical and experimental product distributions suggests that there might exist an entirely dif ferent mechanism through which anthraquinone is formed. A base-catalyzed autoxidation of DHA to anthraquinone as found by Hawthorne and co-workers could have been one of the mechanisms responsible for anthraquinone formation.20 However, as the present reaction is conducted under an inert atmosphere, the possibility for autoxidation is quite low. An experiment using 18O-labeled oxidant can prove useful to verify whether the autoxidation is really operative or not. As we shall see, in the following sections, an unusual mechanism involving anthracene radical anion, produced by a proton transfer pathway, will lead to the formation of anthraquinone. Hence, in the next section, we will focus on the desaturation mechanisms by the oxidants I and IIa in detail. 3.6. Comparison of Desaturation Pathways for All Spin States by I and Ia. The free energy profiles for the desaturation pathways by I and Ia in all possible spin states (S = 2 and 1 for I and S = 5/2 and 3/2 for Ia) are shown in Figures 6 and 7. Comparison of the second H-abstraction reactions by oxidant I (OMnIVO) and the reduced oxidant Ia (HO− MnIIIO) reveals that Ia has a lower H-abstraction barrier than I. The second H-abstraction barrier for Ia in the S = 5/2 (sextet) state has the lowest barrier (1.8 kcal/mol). The same processes in the S = 2 and 1 states of I are higher: 5.2 and 7.4 kcal/mol, respectively. Surprisingly, the second H-abstraction barrier in the S = 3/2 (quartet) state of Ia is quite similar (2.1 kcal/mol) to that of the sextet barrier, which indicates no exchange-enhanced reactivity for the sextet TS (6TS-2H). In order to comprehend this observation, we have analyzed the Mulliken spin densities and the natural charges of the two TSs, 6 TS-2H and 4TS-2H, shown in Figure 8. Inspection of the spin

density of 6TS-2H shows that a very slight electron shift occurs to the oxidant (ρIa = 4.03) at the TS but that a significant positive charge developed on the oxidant (qIa = 0.24). This indicates that the corresponding TS might be qualified as a proton transfer species. The carbanionic nature of the substrate residue (qMHA‑H = −0.63) also supports a proton transfer mechanism. It should be noted that the TS for the first Habstraction by I from DHA already has proton transfer characteristics, but for the second H-abstraction reaction by Ia, the PT character is more pronounced. Since Ia is a reduced Mn(III) species, in comparison to the Mn(IV) species I, the former has a lower tendency to accept an electron during the second H-abstraction process, which is in turn reflected in the higher PT character in 6TS-2H. Similarly, a PT process is observed in 4TS-2H, for which the charges on the oxidant and the hydrogen-abstracted moiety are 0.27 and −0.64, respectively. Since there is no single-electron shift in the TS, we do not expect any EER for the sextet state. As such, both spin states have similar H-abstraction barriers (Figure 7). 3.7. Formation of Anthraquinone via Anthracene Radical Anion. Following the intrinsic reaction coordinate (IRC) for 6TS-2H (shown in Figure 9) leads to the formation

Figure 9. IRC of the second H-abstraction pathway for the reaction Ia + MHA• in the S = 5/2 state.

of anthracene and HO−MnII−OH species (Ib), which indicates that the electron transfer transpires once the reaction passes the TS. Since there is no proton transfer intermediate, the entire process is concerted but asynchronous, having discrete PT and ET events along the path.21 On the other hand, by following the IRC for 4TS-2H, the reaction leads to the formation of an anthracene radical anion (see Figure 10) along with a protonated Mn(III) species, [HO−MnIII−OH]+ (Id). The formation of anthracene radical anion opens up new reaction possibilities that can account for the formation of anthraquinone under basic conditions. We note that the back-electron-transfer process from the anthracene radical anion to [HO−MnIII−OH]+, to form anthracene, is thermodynamically disfavored by 9.1 kcal/mol (4IC-PT to 4P-2H step in Figure 7), and hence we rule out back electron transfer. The rebound possibility from [HO−MnIII− OH]+ onto the anthracene radical anion was also tested and revealed a very high barrier. We could not locate any rigorous TS species for this process, even after several different attempts.

Figure 8. Optimized structures of the TSs (6TS-2H and 4TS-2H) with key geometrical parameters (distances in Å, angles in deg) for the second H-abstraction reactions by Ia from MHA• for S = 5/2 and 3/2, along with natural charges (q) and spin densities (ρ) on the Habstracted moieties (MHA-H) and on the oxidants. The amount of charge transferred (qCT) from the substrate to the oxidant or vice versa is also given. For clarity, only relevant hydrogen atoms are shown. 2884


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which eventually oxidizes to anthraquinone through an anthrone intermediate. 3.8. Mn(III) or Mn(II) as Final Product? Formation of a Mn(III) species as one of the final products during the reaction indicates that the substrate radical will react with a second molecule of oxidant I, which produces the Mn(III) species, rather than with the reduced oxidant Ia, which forms a Mn(II) species as the final product.7b,9b Since the concentration of I is higher in comparison to that of Ia which is produced during the first HAT step, the substrate radical has more chance of reacting with I, thus accounting for the detection of a Mn(III) species as the final product. There is another way in which the Mn(III) species can be formed under basic conditions. The excess oxidant, the Mn(IV) species, can produce a stable Mn(III) species by a comproportionation reaction with the Mn(II) species in basic medium. More detailed experimental work will be needed in order to explain the actual pathway. 3.9. Fate of the Substrate Radical under Acidic Conditions. After successfully explaining the mechanisms for different product formations under basic conditions, we now explore the fate of the MHA• radical under acidic conditions. After dissociation, MHA• can either react with a second molecule of II, [HO−MnIV−OH]2+, or with the reduced oxidant IIa, [HO−MnIII−OH2]2+. Figures 12 and 13 show the free energy profiles for the desaturation and hydroxylation pathways for II and IIa, respectively. Thus, since II has a +2 charge, it is capable of accepting an electron from the monohydroanthracenyl radical (MHA•), which is also a good electron donor. Indeed, we have found a spontaneous electron transfer (ET) event f rom the substrate radical (MHA•) to the oxidants (II or IIa) before the second H-abstraction or hydroxylation event. For the reaction II + MHA• in the quintet

Figure 10. Formation of anthracene radical anion via a proton transfer process during the reaction of Ia with MHA• in the quartet state. The energies are given in kcal/mol, calculated at the UB3LYP/B2// UB3LYP/B1 level of theory.

In contrast, this anthracene radical anion can diffuse away with a thermodynamically favorable free energy of dissociation (ΔGDiss = −8.2 kcal/mol). Once dissociated, the anthracene radical anion (a resonance form of the anthracene radical anion is shown in Figure 11a) will react with another molecule of OMnIVO (I) as shown in Figure 11b. The free energy barrier (ΔG⧧) for this reaction is 12.9 kcal/mol, and the reaction is exothermic by 5.0 kcal/mol (Figure 11b). Then the resultant intermediate will undergo aromatization followed by hydrolysis to produce 9-hydroxyanthracene (Figure 11b). Under basic conditions, 9-hydroxyanthracene will readily convert to anthrone, which is subsequently oxidized to anthraquinone (Figure 11c). The trace amount of anthrone observed in the reaction mixture supports the mechanism of anthraquinone formation under basic conditions via an anthrone intermediate.9a Thus, for the quartet state, the second H-abstraction leads to the formation of anthracene radical anion,

Figure 11. (a) Resonance form of anthracene radical anion. (b) Anthrone formation via the reaction of an anthracene radical anion with the oxidant I. (c) Oxidation of anthrone to anthraquinone. The free energies are given in kcal/mol, calculated at the UB3LYP/B2//UB3LYP/B1 level of theory. 2885


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and hydroxylation pathways, respectively (Figure 13). However, as we already found above, here too for both of the oxidants, II and IIa, we found the proton transfer pathway leading to anthracene to be favored over the hydroxylation pathway. Hence, the experimental results of Yin et al.7−9 and our present theoretical predictions for II are f ully consistent; both predict anthracene as the sole product under acidic conditions. The pathway for II in the S = 1 state is not discussed, as the electron transfer intermediate for the triplet state (3IC-ET) is very high in energy: 17.8 kcal/mol above the corresponding quintet state. No electron transfer intermediate has been observed for IIa in the quartet (S = 3/2) state, and it has a very high barrier (24.4 kcal/mol) for the second H-abstraction step (Figure S4 in the Supporting Information). 3.10. Comparison of the Second H-Abstraction Mechanisms by Ia and IIa. Comparison of the second Habstraction mechanisms for oxidants Ia and IIa reveals a uniquely distinct transition state features for this step, which gives rise to different product distributions under distinct experimental conditions. Scheme 5 provides an overview of these processes. Thus, although the first H-abstraction reactions by I and II follow net H atom transfers, the second Habstraction by oxidant Ia follows a concerted PT/ET mechanism (for S = 5/2) or PT mechanism (for S = 3/2) to generate anthracene or anthracene radial anion, whereas IIa involves a separate ET/PT pathway to produce anthracene as the sole product. Such uniquely distinct mechanisms for the desaturation processes by Ia and IIa explain the different product distributions shown in Table 1.

Figure 12. Free energy profiles of the desaturation and hydroxylation reactions by II, [HO−MnIV−OH]2+, in the S = 2 state as calculated at the UB3LYP/B2//UB3LYP/B1 level of theory. The energies are given in kcal/mol.

4. CONCLUSIONS Density functional studies of the reactions of DHA with two Mn(IV) oxidants, having Mn(O)2 (I) and Mn−(OH)2 (II) moieties, are reported. Theory shows higher H-abstraction reactivity of the MnO group in comparison to Mn−OH, in accord with the experimental findings.7−9 Mechanistic studies revealed that the H-abstraction TS for I + DHA has HAT character with a considerable proton transfer feature, whereas for II + DHA, the TS has PCET character with a considerable electron transfer feature. After the first H-abstraction step, both resulting complexes (Ia and IIa) preferred the desaturation pathway, while the very high barriers of the rebound step prohibited hydroxylation. The second H-abstraction step by Ia was found to have similar activation barriers irrespective of the spin state and to feature a novel nonrebound mechanism. Thus, while the sextet state follows a concerted but asynchronous PT/ET pathway to yield anthracene, the quartet state follows a PT pathway to generate anthracene radical anion. The formation of anthraquinone, as experimentally observed under basic conditions, is produced in a follow-up reaction of the anthracene radical anion intermediate. For complex IIa, the desaturation process produces anthracene, in accord with experiment, via discrete ET/PT pathways. The present results provide insights into the differences in reactivity and the mechanisms through which the two Mn(IV) moieties, such as MnO and Mn−OH, react with the substrate. All in all, therefore, the calculations are in good agreement with the experimental results7−9 and provide a clear rationale for the different products of the two oxidants I and II. In addition, theory finds that the reaction of Ia generates anthracene radical anion, which follows a unique nonrebound mechanism. This is a novel aspect that merits experimental testing.

Figure 13. Free energy profiles of desaturation and hydroxylation reactions by IIa, [HO−MnIII−OH2]2+, in the S = 5/2 state as calculated at the UB3LYP/B2//UB3LYP/B1 level of theory. The energies are given in kcal/mol.

state, the electron transfer process leads to the formation of electron transfer intermediates, such as 5IC-ET and 5IC-ET′, involving a cationic MHA+ species and the oxidant reduced to [HO−MnIII−OH]+. Such electron transfer mechanisms involving II were also observed in experiments9a,22 by Yin et al., who showed that II spontaneously accepts an electron from the electron transfer reagents, such as ABTS (2,2′-azinobis(3ethylbenzthiazoline-6-sulfonic acid), to form ABTS+•, as well as from PPh3 during oxygen atom transfer reactions.23 After formation, these electron transfer intermediates, 5ICET and 5IC-ET′, can subsequently undergo proton transfer and hydroxylation pathways, respectively, as shown in Figure 12. Similarly, the reaction IIa + MHA• in the sextet state also leads to the formation of electron transfer intermediates, such as 6ICET and 6IC-ET′, which subsequently follow proton transfer 2886


Research Article

ACS Catalysis

Scheme 5. Schematic Representation of the Second H-Abstraction Processes, from MHA• by Oxidants Ia and IIa, Showing the Origins of the Different Products Observed by theory (and Matching Experiment) under Different pH Conditions

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00085. Energy profiles, geometries, orbital pictures, Mulliken spin and natural charges for a few TSs and intermediates, other functional test values, Mulliken spin densities for all stationary points, and optimized Cartesian coordinates for all of the stationary points (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The paper is dedicated to Prof. E. D. Jemmis on the occasion of his 65th birthday. The research was supported in part by the Israel Science Found grant to S.S. (ISF grant 1183/13). D.M. thanks the Israeli PBC for a postdoctoral fellowship.

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REFERENCES

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Theory Revealing Unusual Non-Rebound Mechanisms Responsible

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