Mechanism of Water Oxidation Catalyzed by a Mononuclear Iron

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Mechanism of Water Oxidation Catalyzed by a Mononuclear Iron Complex with a Square Polypyridine Ligand: A DFT Study Ying-Ying Li,† Lian-Peng Tong,‡ and Rong-Zhen Liao*,† †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China S Supporting Information *

ABSTRACT: The mononuclear [Cl−FeIII(dpa)−Cl]+ (1Cl) complex containing a square planar tetradentate polypyridine ligand has been reported to catalyze water oxidation in pH = 1 aqueous medium with ceric ammonium nitrate (CAN) as a chemical oxidant. The reaction mechanism of the oxygen evolution driven by this catalyst was investigated by means of density functional calculations. The results showed that one chloride ligand of 1Cl has to exchange with a water molecule to generate 1, [Cl−FeIII(dpa)−OH2]2+, as the starting species of the catalytic cycle. The initial one-electron oxidation of 1 is coupled with the release of two protons, generating [Cl−FeIV(dpa) O]+ (2). Another one-electron transfer from 2 leads to the formation of an FeVO complex [Cl−FeV(dpa)O]2+ (3), which triggers the critical O−O bond formation. The electronic structure of 3 was found to be very similar to that of the high-valent heme-iron center of P450 enzymes, termed Compound I, in which a π-cation radical ligand is believed to support a formal iron(IV)-oxo core. More importantly, 3 and Compound I share the same tendency toward electrophilic reactions. Two competing pathways were suggested for the O−O bond formation based on the present calculations. One is the nitrate nucleophilic attack on the iron(V)-oxo moiety with a total barrier of 12.3 kcal mol−1. In this case, nitrate functions as a co-catalyst for the dioxygen formation. The other is the water nucleophilic attack on iron(V)-oxo with a greater barrier of 16.5 kcal mol−1. In addition, ligand degradation via methyl hydrogen abstraction was found to have a barrier similar to that of the O−O bond formation, while the aromatic carbon hydroxylation has a higher barrier.

1. INTRODUCTION

due to their high abundance on earth as well as due to their direct relation to the metal centers of natural enzymes. Recently, Thummel and co-workers reported the synthesis of a mononuclear iron complex [Cl−FeIII(dpa)−Cl]+ (1Cl, Figure 1, dpa = N,N-di(1,10-phenanthrolin-2-yl)-N-isopentylamine) with a square planar tetradentate polypyridyl ligand. It is capable of catalyzing water oxidation using ceric ammonium nitrate (CAN) as the chemical oxidant at pH = 1.23 The turnover frequency (TOF) of O2 evolution was estimated to be 842 h−1 under such catalytic conditions. The cyclic voltammogram (CV) of 1Cl at pH = 1.0 showed a catalytic current with an onset potential of +1.47 V vs SHE (reported as +1.25 V vs Ag/AgCl). Kinetic studies indicated linear dependence of the reaction rate upon the catalyst concentration, ruling out the intermolecular O−O bond formation of two iron species as the rate limiting step. In addition, the participation of nanoparticles was safely excluded, which confirms that the active catalyst is a molecular complex.23 In the same paper by Thummel and coworkers, they also reported a dinuclear ferric WOC with a 2-

Water splitting is a promising technology to produce hydrogen as a clean and sustainable fuel and thus to convert solar energy to chemical energy. Since the report of the “blue dimer” by Meyer and co-workers,1 the study of artificial water oxidation catalysts (WOCs) has drawn great attention. Water oxidation is a complex process that involves the removal of four protons and four electrons from two water molecules to produce one dioxygen molecule and, at the same time, form a dioxygen bond.2 It is both thermodynamically and kinetically demanding and occurs at a standard redox potential of 1.23 V vs SHE (standard hydrogen electrode). In the past few decades, enormous efforts have been dedicated to the development of both heterogeneous and homogeneous WOCs.3−7 While the heterogeneous catalysts are mainly aimed at industrial applications, homogeneous catalysts offer us a unique opportunity to probe the ligand effect and reaction mechanism regarding the water oxidation reaction. First-row transitionmetal-based WOCs have emerged recently, including complexes with vanadium,8 manganese,9−16 iron,17−27 cobalt,28−40 nickel,41−47 and copper48−56 centers. They are of great interest © XXXX American Chemical Society

Received: February 5, 2018

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DOI: 10.1021/acs.inorgchem.8b00333 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The calculation protocol for the redox potentials and pKas is the same as that in our previous studies,61,75−78 and details of the protocol have been discussed in our recent review.57 Therefore, only some critical features are described here. The experimental solvation free energies of water (−6.3 kcal mol−1),79 chloride (−72.7 kcal mol−1),80,81 and nitrate (−65.0 kcal mol−1)82 in aqueous solution were used herein, whereas the calculated values using the present methodology are −7.5, −65.2, and −61.1 kcal mol−1, respectively. Notably, the use of either experimental or calculated values does not affect any of the conclusions of this study. In addition, when the chloride and nitrate become a part of the molecular complex, a large part of the first solvation shell is replaced by strong interactions with the rest of the complex. Therefore, the use of a continuum solvation model should lead to much smaller errors. For the calculation of pKa values for all species, the gas phase Gibbs energy of a proton (−6.3 kcal mol−1, corresponding to 1 atm in the gas phase) and the experimental solvation free energy of a proton (−264.0 kcal mol−1, corresponding to 1 M in the solution phase) were used.79 The absolute potential (1.72 + 4.281 V)83 of the Ce4+/Ce3+ redox couple was used as a reference, which corresponds to an electron affinity of 138.4 kcal mol−1 and was used to set up the thermodynamic diagrams of all redox steps.

Figure 1. Schematic representation of the [Cl−FeIII(dpa)−Cl]+ complex.

(pyrid-2′-yl)-8-(1″,10-phenanthrolin-2″-yl)-quinoline (ppq) ligand. Its TOF was measured as ca. 4000 h−1 per iron core, much greater than that of 1Cl. The catalytic mechanism of the diferric catalyst proposed is more complicated because of the potential synergistic effect of the two iron centers. Quantum chemical methods have been successfully applied to illustrate the catalytic pathway of a large number of homogeneous water oxidation catalysts,57 including a number of molecular iron complexes58−65 and many other transitionmetal complexes.57 The dpa and ppq ligands have very similar structural features. Both of them, for instance, are tetradentate and are composed of conjugated pyridyl units. The study of mononuclear iron WOCs with dpa, therefore, will provide inspiring information for the investigation of intricate multinuclear iron WOCs with ppq or other analogue ligands. Here, we conducted density functional theory calculations to elucidate the catalytic mechanism of water oxidation by the mononuclear iron catalyst 1Cl. Importantly, nitrate was proposed as a co-catalyst to facilitate the oxygen evolution, which is more favorable than the water nucleophilic attack. Ligand degradation pathways were also explored so that better strategies of ligand design can be adapted in the future to improve the stability of the catalyst.

3. RESULTS AND DISCUSSION 3.1. Ligand Exchange. Our investigation starts from the identification of iron-containing species in the catalytic pathway. In order to increase computational efficiency and avoid the multiminima dilemma of various rotational conformers, the isopentyl group of the dpa ligand is represented by a methyl group in all calculations of this work. This modification has no major effect on the results. We first examined 1Cl (Figures 2 and 3), in which a ferric center

2. EXPERIMENTAL SECTION 2.1. Computational Details. All density functional calculations presented here were performed with the Gaussian 09 program.66 The geometries were optimized using the B3LYP-D3 functional (with Grimme’s D3 dispersion)67,68 with the SDD69 pseudopotential for Fe and the 6-31G(d,p) basis set for the C, N, O, H, and Cl elements. Analytical frequency calculations were carried out at the same level of theory as the geometry optimizations to verify the nature of various stationary points and to obtain the Gibbs free energy corrections. The final and solvation energies in the aqueous solvent were calculated by employing the SMD70 continuum solvation model with larger basis sets, in which all elements, except Fe (SDD), were described by 6311+G (2df,2p) at the B3LYP*-D3 (15% Hartree−Fock exchange, including D3 dispersion and Gibbs free energy corrections from the B3LYP-D3 level)71 level. Single-point calculations at the B3LYP-D3,68 M06L-D3,72 M06-D3,73 and M11L74 levels were also performed to evaluate the sensitivities of the redox potentials and barriers to the choice of the functionals (vide infra). The concentration correction of 1.9 kcal mol−1 at room temperature (derived from the free energy change of 1 mol of an ideal gas at 1 atm (24.5 L mol−1) to 1 M (1 mol L−1 in water solution)) was added for all species except water, for which the corresponding value is 4.3 kcal mol−1, as the standard state of water is 55.6 mol L−1.

Figure 2. Processes of chloride ligand exchange by a water molecule at the FeIII and FeIV states considered in the present work.

coordinates to one equatorial dpa ligand84 and two axial chlorides. The ground state of 1Cl is a sextet, while its quartet and doublet states are 12.1 and 2.8 kcal mol−1 higher in energy, respectively. Before the initiation of water oxidation catalysis, water molecules have to displace at least one chloride ligand of 1Cl, leading to the formation of [Cl−FeIII(dpa)−OH2]2+ (1, Figures 2 and 3). This ligand exchange may undergo either a concerted associative pathway or a stepwise dissociative pathway.85,86 Both scenarios were considered in our study. The transition state for the concerted associative pathway (labeled as TS0, Figure S1) has been located and associated with a barrier of 6.4 kcal mol−1 in the sextet relative to the initial sextet of 1Cl plus a water molecule. At TS0, the critical Fe−O and Fe−Cl distances are 2.17 and 3.26 Å, respectively. B


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Inorganic Chemistry

Figure 3. Optimized structures of 1Cl, 1, 2, and 3. Distances are given in Å. Spin densities on selected atoms are indicated in italics. Superscripts indicate the multiplicity of the structures. For all structures, only the ground state is shown.

Figure 4. Energy diagram for water oxidation catalyzed by 1.

The doublet and quartet barriers are 14.6 and 0.6 kcal mol−1 higher, respectively. On the other side, the dissociative pathway proceeds via a penta-coordinated ferric intermediate Int0 (Figure S1). The dissociation of chloride from 1Cl is endergonic by 1.1 kcal mol−1. The subsequent water association was calculated to be exergonic by 0.5 kcal mol−1. Via combination of these two steps, the dissociative water/chloride exchange process is endergonic by 0.6 kcal mol−1. It should be pointed out that the water concentration is much higher than the chloride concentration (μM),23 which should favor the exchange process. The comparison of the energy demand of these two ligand exchange pathways thus suggests a more favorable dissociative pathway. The pKa of 1 was calculated to be 5.9, suggesting protonation of 1 at pH = 1. Like its chloride precursor, 1 prefers a high-spin sextet to the quartet and doublet, which are 4.0 and 5.1 kcal mol−1 higher in energy, respectively. Further water/chloride exchange of 1 (Figure 2) has also been considered and calculated to be endergonic by 8.9 kcal mol−1, because the pKa of the resulting diaqua iron(III) species was calculated to be −1.3. At pH = 1, it is prone to being

deprotonated, affording a hydroxyl species denoted as 1W2 (Figure S2). The conversion of 1 to 1W2 is thus endergonic by 5.8 kcal mol−1, ruling out the prevalence of 1W2 under such conditions. The nitrate anion, from either CAN or nitric acid, was also considered as a coordinating ligand of the ferric center. The processes of ligand exchange involving nitrate are shown in Figure S3. All of the processes are thermodynamically unfavorable. As concluded from the evidence above and the redox potentials (vide infra), complex 1 is determined as the dominant iron(III) species and the authentic water catalyst when [Cl−FeIII(dpa)−Cl]+ is dissolved in the pH = 1.0 aqueous solution. 3.2. Redox Properties. The pKa values of [Cl−FeIV(dpa)− OH2]3+ (2dpt, Figure S4) and [Cl−FeIV(dpa)−OH]2+ (2pt, Figure S4) were calculated to be −5.7 and −0.7, respectively, which suggests that both protons are released into the water solution at pH = 1. The one-electron oxidation of 1, therefore, is coupled with the release of two protons from the axial aqua ligand, leading to the formation of a triplet FeIVO complex 2 (Figure 3). Complex 2 is best described as a low-spin FeIII (SFe C


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Inorganic Chemistry

Figure 5. Optimized structures of TS1, Int1, TS1′, and Int1′. Distances are given in Å. Spin densities for selected atoms are indicated in italics. Superscripts indicate the multiplicity of the structures. The imaginary frequencies for transition states are also shown. For all structures, only the ground state of every structure is shown.

quartet are close to degenerate due to the relatively weak coupling between the ligand radical and the ferric center. On the contrary, the sextet has an intermediate spin ferric center (SFe = 3/2) ferromagnetically coupled with both the oxyl radical and the ligand radical. The electronic structure of 3 is thus reminiscent of that of Compound I in P450 enzymes.89−92 The formal oxidation state of Compound I can be described as FeVO, whereas the calculations suggested the interaction of a porphyrin ligand radical with the ferric-oxyl radical moiety in either a ferromagnetic (quartet) or an antiferromagnetic (doublet) fashion. Further oxidation of 3 to a formally FeVI complex 4 (Figure S4) is very unlikely because the oxidation potential was calculated to be as enormously positive as 2.42 V. The oxidations of other ferric species present in the solution have also been investigated. The one-electron oxidation of 1Cl to form the iron(IV) complex 2Cl (Figure S4) occurs at a redox potential of 1.71 V. One chloride displacement of 2Cl by a water molecule was calculated to be endergonic by 11.6 kcal mol−1. Compared with the water/chloride ligand exchange at the iron(III) state, the ligand exchange at the iron(IV) state is significantly less favorable. The oxidation of 1W2 occurs at 1.46 V, which is less positive than the redox potential of 2/1 and certainly less positive than that of Ce4+/Ce3+. The oxidation of 1 W2 , however, is unlikely to happen because of the thermodynamically unfavorable formation of 1W2 and also because of the somewhat higher potential (1.93 V). 3.3. O−O Bond Formation and O2 Release. Concerning our previous computational study of the iron tetraamido macrocyclic WOC,61 two mechanistic scenarios were considered for the O−O bond formation triggered by 3, namely water attack and nitrate attack on the FeVO moiety of 3. Even though the water attack pathway was often discussed in literature studies,57 it should be noted that the involvement of nitrate in the O−O bond formation has been proposed according to the experimental evidence of Ru-based WOCs.93,94

= 1/2) center ferromagnetically coupled to an oxyl radical (SO = 1/2). The quintet and the closed-shell singlet are 4.0 and 28.3 kcal mol−1 higher in energy, respectively. The redox potential of this 2/1 oxidation was calculated to be 1.69 V, suggesting that this step is slightly exergonic by 0.7 kcal mol−1 (Figure 4) using Ce4+ as the chemical oxidant (reference potential of 1.72 V).83 The calculated potential of the 2/1 couple is somewhat greater than the experimental onset potential (1.47 V), which is reasonable since an onset potential is generally expected to be slightly lower than the “exact” limiting oxidation potential, and a large rising current was observed during the anodic voltammetry scan up to about 1.8 V.23 The subsequent one-electron oxidation of 2 (potential of 1.73 V) leads to the generation of a quartet FeVO complex [Cl−FeV(dpa)O]2+ (3, Figure 3) that is capable of initiating the O−O bond formation (vide infra). The calculated value of 1.73 V is barely above that for the Ce4+/Ce3+ redox couple. The accessibility of 3 is thus a very important issue which depends on whether a subsequent irreversible chemical reaction takes place. If there were large potential differences and no irreversible reactions occurring in 3, it would be difficult to accumulate due to the thermodynamic hindrance. However, it becomes a different scenario if a subsequent irreversible reaction occurs, such as O−O bond formation as shown below. The reaction can, in principle, proceed from 3 and go through the subsequent transition state if the total barrier is reasonable. Similar analysis has been performed in our previous studies on Mn- and Ir-catalyzed water oxidation.87,88 The quartet of 3 can be interpreted as a low-spin ferric center (SFe = 1/2) interacting with an oxyl radical (SO = 1/2) and a dpa ligand radical (S = 1/2) in a ferromagnetic fashion. The spin densities on Fe and O1 are 1.12 and 0.96, respectively, and it is 0.77 on the dpa ligand. The sextet and the doublet lie at +2.0 and +0.3 kcal mol−1 relative to the quartet, respectively. In the case of the doublet state, the ligand radical (S = 1/2) interacts with the ferric-oxyl center (SFe = 1/2, SO = 1/2) in an antiferromagnetic fashion. Consequently, the doublet and the D


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Inorganic Chemistry

Figure 6. Orbital diagrams of electron transfer during the O−O bond formation.

(TS1′′ in Figure S5) becomes 13.0 kcal mol−1 relative to that of 3 plus nitrate, which is only 2.2 kcal mol−1 higher than what was obtained from gas phase geometry optimizations. If one water molecule is explicitly included in the model so as to account for the local hydrogen bonding effect, in which a nitrate water complex was used as the substrate, the barrier (TS1−H2O in Figure S5) increases by 6.1 kcal mol−1. Of course, more water molecules may be added to account for the environment in a better way, but molecular dynamics simulations would be needed to sample all possible configurations, which is beyond the scope of the present study. We also considered O−O bond formation via nitrate attack from the iron(IV) state. The transition state 2-TS1 and the resulting intermediate 2-Int1 are displayed in Figure S6. 2-TS1 has a total barrier of 39.2 kcal mol−1 in the quintet state (56.1 kcal mol−1 for the triplet and 46.5 kcal mol−1 for the singlet), which is 27.0 kcal mol−1 higher than the barrier of O−O bond formation through TS1 as described above. The results confirm O−O bond formation that is triggered by the formally FeV species, as proposed for other iron-based WOCs.60,64,65 In the study of the alternative water attack mechanism, four water molecules were included in the model as a proton is released into the water solution during the O−O bond formation.58,95−97 The optimized transition state TS1′ is shown in Figure 5. At TS1′, the critical O1−O2 distance is 1.82 Å. TS1′ is a doublet, and the barrier was calculated to be 16.3 kcal mol−1 relative to that of 3 (Figure 4). A spin crossing from quartet to doublet is thus also required for the water attack pathway. The sextet and quartet barriers for the water attack are +12.1 and +13.6 kcal mol−1 higher, respectively. The dominant doublet spin state of TS1′ is similar to that of TS1 of the nitrate attack pathway. For the quartet, the average equatorial Fe−N distance does not change from 43 to 4TS1′, whereas the axial Fe−O1 bond increases from 1.64 Å in 43 to 1.87 Å in 4TS1′ (Table S2). These geometric disparities strongly suggest the transfer of a substrate α-electron to the unoccupied dz2* orbital of iron. This is different from the manner of electron transfer during O−O bond formation proceeding through the nitrate attack pathway, where the dx2−y2* orbital accepts the electron in the quartet. For the sextet, the electron transfer manner for the water attack pathway is the same as that for the nitrate attack pathway. The iron center dz2* orbital accepts the electron in both cases. The total barrier for O−O bond formation via the water attack mechanism (16.5 kcal mol−1) is thus 5.5 kcal mol−1 higher than that for the nitrate attack mechanism (11.0 kcal mol−1). The afforded hydroperoxide intermediate (Int1′) undergoes two more facile oxidation steps and subsequently releases a dioxygen molecule. The whole catalytic cycle and the energy diagram of the water attack pathway are shown in Scheme S1 and Figure S7, respectively. TS1′ is the rate limiting step for the whole catalytic cycle.

The transition state (TS1) for the nitrate attack pathway is displayed in Figure 5. Excessive nitrate anions come from either nitric acid as the pH adjustor or CAN as the oxidant. TS1 prefers a doublet state, and a spin crossing from the quartet to doublet state is needed during the O−O bond formation. The oxo group and the nitrate have opposite spin densities (0.78 for O1, −0.49 for O2, −0.30 for O3, and −0.15 for O4), which renders the most efficient O−O bond formation between O1 and O2. TS1 was confirmed to be a true transition state with only one imaginary frequency of 117.5i cm−1, which mainly corresponds to O−O bond formation. At TS1, the critical O1− O2 distance is 2.73 Å. The barrier of this step was calculated to be 10.8 kcal mol−1 relative to that of 3. The quartet and sextet barriers were calculated to be 6.5 and 19.0 kcal mol−1 higher, respectively. Figure 6 shows the orbital diagrams of the electron transfer from nitrate to 3 in different spin states during the O− O bond formation. The doublet state, first, involves the transfer of an α-electron from the oxygen 2p orbital of nitrate to the radical ligand, generating a closed-shell dpa ligand. Meanwhile, the corresponding β-electron from the same oxygen 2p orbital is combined with the α-electron of the oxyl radical, affording an O−O bond. For the quartet state, second, the α-electron of the nitrate substrate has to transfer into the antibonding dx2−y2* or dz2* orbital of the ferric ion, in company with the shift of a βelectron from the dyz* orbital to the dpa radical cation to form a closed-shell dpa ligand. It is very likely that the dx2−y2* orbital of iron accepts the nitrate electron, indicated by the substantial elongation of the average equatorial Fe−N distance from 2.00 Å in 43 to 2.14 Å in 4TS1. In contrast, the average equatorial Fe−N distances are 2.00 Å in both 23 and 2TS1 (Table S2), ruling out the involvement of the dx2−y2* orbital in the doublet state during the O−O bond formation. The sextet, third, includes the substrate α-electron transfer to the dz2* orbital, coupled with the transfer of a β-electron from the dxy orbital to the ligand. The involvement of the dz2* orbital is again evident because of the elongation of the axial Fe−O1 bond from 1.63 Å in 63 to 1.89 Å in 6TS1 (Table S2). For all three spin states discussed above, the nitrate group and the oxyl moiety have opposite spin densities at the transition state. This is how the interaction of the substrate β-electron and the oxyl α-electron is deduced, as indicated by the blue curve arrows in Figure 6. The resulting peroxynitrate-coordinated ferric intermediate (Int1, Figure 5) lies at +3.4 kcal mol−1 relative to 3. At Int1, the O1− O2 bond length is 1.40 Å and the Fe−O bond is 1.88 Å, which is significantly longer than that of 3 (1.64 Å). In Int1, the doublet and sextet are close with an energy difference of only 0.5 kcal mol−1, favoring the doublet, while the quartet is 9.5 kcal mol−1 higher than that of the doublet state. The nitrate attack involves intermolecular electron transfer, which might be quite sensitive to the water environment. Due to this technical reason, we performed geometry optimizations of 3, nitrate, and TS1 in a water environment. The barrier E


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Figure 7. Optimized structures of TS2, Int2, Int3, TS3, and Int4. Distances are given in Å. Spin densities for selected atoms are indicated in italics. Superscripts indicate the multiplicity of the structures. The imaginary frequencies for TS2 and TS3 are also shown. Only the ground state of every structure is shown.

One-electron oxidation of Int2 (at a potential of 1.24 V) leads to the generation of a septet intermediate Int3 (Figure 7). Its electronic structure can be regarded as a nitrogen dioxide radical (S = 1/2) ferromagnetically coupled with a high-spin ferric center (SFe = 5/2). The spin densities on the Fe center and the NO2 moiety are 4.12 and 0.97, respectively. The singlet, triplet, and quintet are 15.9, 5.1, and 3.8 kcal mol−1 higher, respectively. Int3 also has two isomers: either one oxygen (Figure 7) or the nitrogen (Figure S9) of the nitrogen dioxide coordinated to the iron center. The former coordination mode is more stable than the latter by 6.0 kcal mol−1. The long O3−Fe bond distance of 2.96 Å at Int3 suggests a partial dissociation of the nitrogen dioxide. An alternative pathway for the formation of Int3 is the direct oneelectron oxidation of Int1, which results in the barrierless N−O bond cleavage and dioxygen release. This step is associated with a potential of 0.54 V. This pathway should compete with that of the N−O bond cleavage via TS2 followed by oxidation. Subsequent replacement of the nitrogen dioxide radical by a water molecule regenerates 1, which was calculated to be exergonic by 0.1 kcal mol−1. It might be followed by the oxidation of 1 to 2. Further reaction of 2 with the free nitrogen dioxide radical can produce a nitrate intermediate Int4 (Figure 7). The quartet transition state (TS3) of the 2 to Int4 conversion is shown in Figure 7, and the barrier was calculated to be 9.6 kcal mol−1 relative to that of 2 plus a nitrogen dioxide

The screening of important intermediates and transition states (Figure S8) bearing the isopentyl substituent group, instead of the methyl group, does not demonstrate any alteration of the catalytic mechanism discussed above. As the catalytic cycle proceeds after nitrate nucleophilic attack, the N−O bond cleavage takes place via TS2 (Figure 7) coupled with the release of a triplet O2. TS2 prefers to be a sextet, and the associated barrier is 8.7 kcal mol−1 relative to that of Int1 (Figure 4). The doublet and quartet barriers are 6.7 and 4.9 kcal mol−1 higher, respectively. The N−O bond cleavage thus requires a spin crossing from doublet to sextet. The dioxygen evolution mediated by this catalyst involves spin crossing through all three possible spin states. The optimization of the minimum energy crossing points may be necessary to obtain a more accurate barrier, but this is beyond the scope of the present study. When the energy penalty for the formation of Int1 is taken into account, the total barrier becomes 12.3 kcal mol−1. This is slightly higher than that for TS1. At TS2, the scissile N−O bond is 1.77 Å, and the spin densities on O1 and O2 are 0.55 and 0.35, respectively. The ferric core of the afforded intermediate, denoted as Int2 (Figure 7), is bound to one oxygen atom of a nitrite group. Coordination via the nitrogen atom (Figure S9), instead of the oxygen, raises the energy of the intermediate to an extremely minor extent (0.01 kcal mol−1 higher). F


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3.4. Catalyst Degradation. DFT calculations have been used to understand the deactivation of some molecular WOCs.61,76,77 The degradation of 3 owing to the oxidation of the dpa ligand was investigated, aimed at development of reasonable strategies of enhancing the stability of the WOCs. Three possible oxidative pathways were considered: (i) the aromatic carbon oxidation, (ii) the C−H activation of the methyl group, and (iii) the pyridine nitrogen oxidation. The barriers for the oxidation of certain atoms of dpa via these three pathways using B3LYP*-D3 and M06L-D3 are listed in Table 2, and the structures of three representative transition states are shown in Figure 8.

radical. At TS3, the nascent N−O bond distance is 2.10 Å. Finally, the exchange of the nitrate ion by a water molecule regenerates 1 and closes the catalytic cycle. From Figure 4, it can be seen that the N−O bond cleavage has the highest barrier, 12.3 kcal mol−1, assuming an oxidation of the complex takes place after the N−O bond cleavage. However, it is difficult to assign whether TS1 or TS2 is the rate limiting step because their energy difference is only 1.3 kcal mol−1. The apparent barrier would become slightly higher than 12.3 kcal mol−1 if an energetic span model as proposed by Kozuch and Shaik were applied.98 It is also likely that the oxidation of Int1 is faster than the N−O bond cleavage from Int1, so then the total barrier becomes 11.0 kcal mol−1. The experimental TOF of 0.23 s−123 corresponds to a barrier of 18.3 kcal mol−1 according to the classical transition state theory. The calculated barrier is somewhat lower than the experimental kinetic data, and the reason may come from the density functional used. It should be pointed out that a potential of 1.72 V was used as the reference to set up the energy diagram. If a value of 1.47 V was used, which corresponds to the onset potential observed experimentally, the total barrier increases to 23.2 kcal mol−1. Single-point calculations using B3LYP-D3, M06L-D3, M06-D3, and M11L functionals have been performed in order to illustrate how sensitive the calculated potentials and barriers are regarding the choice of density functionals used. As shown in Table 1, the calculated potentials

Table 2. Calculated Barriers of Ligand Degradation by Using B3LYP*-D3 and M06L-D3 O−O formation N2 N3 C1H C2 C3 C4 C5 C6 C7 C8 C9 C10 C11

Table 1. Comparison of Redox Potentials and Barriers Calculated Using Different Density Functionals redox potential (V) B3LYP*-D3 B3LYP-D3 M06L-D3 M06-D3 M11L a

Fe(IV)/Fe(III)

Fe(V)/Fe(IV)

total barrier (kcal mol−1)a

1.69 2.12 1.60 2.25 1.66

1.73 1.76 1.54 1.88 1.92

12.3 (16.5) 21.8 (30.1) 7.6 (23.1) 31.6 (47.6) 21.3 (32.4)

B3LYP*-D3

M06L-D3

12.3 (16.5)a 28.5 25.3 11.1 22.6 23.5 27.3 18.7 21.1 17.6 13.9 16.4 20.8 17.1

7.6 (23.1)a 38.2 32.4 25.0 36.7 38.5 42.3 32.9 34.9 29.7 28.4 30.4 35.1 32.2

a The total energy barriers of O−O formation refer to the nitrate attack mechanism. The barriers of the WNA mechanism are shown in parentheses.

The C−H oxidative functionalization of the methyl group (TSC1H), assisted by the hydrogen bonding network of three water molecules, has a total barrier energy of 11.1 kcal mol−1 in the quartet state (TSC1H, Table 2). This is 1.2 kcal mol−1 lower than that for the O−O bond formation at the B3LYP*-D3 level. When M06L-D3 was used, however, the barrier of C−H activation (25.0 kcal mol−1) is much greater than that of the O−O bond formation (7.6 kcal mol−1). When the isopentyl ligand was used, the barrier of C−H oxidation is also similar to the barrier of O−O bond formation at the B3LYP*-D3 level (Figure S8). Oxidation of the pyridine nitrogen needs to overcome much higher barriers (Table 2), over 25.0 kcal mol−1 at the B3LYP*D3 level, than the water oxidation. Similar tendencies have been found in our recent study regarding a mononuclear manganese WOC, in which the oxidation of a pyridine nitrogen or an amine nitrogen was found to require considerably more energy than that of water oxidation.76 Oxidation of various aromatic carbons (C2 to C11 as shown in Figure 1) of the dpa ligand was also studied. The oxidation of C8 assisted by two water molecules proceeding via a quartet transition state (TSC8) is associated with a total barrier of 13.9 kcal mol−1 (Table 2). This barrier is substantially higher than the overall barrier of the nitrate attack pathway at the B3LYP*-D3 level. Addition of another water molecule as a proton relay in the model does not further lower the barriers of the ligand degradation pathways (Table S1).

The total barriers for the WNA mechanism are shown in parentheses.

of the Fe(IV)/Fe(III) redox couple by B3LYP*-D3, M06L-D3, and M11L are in the range of 1.6−1.7 V. Yet, the two potentials, 2.12 and 2.25 V, by B3LYP-D3 and M06-D3, respectively, are also positive. The calculated potentials of the Fe(IV)/Fe(V) redox couple, by B3LYP*-D3, B3LYP-D3, and M06L-D3, are quite close to each other, being around 1.7 V. Nevertheless, the M11L and M06-D3 functionals gave somewhat greater values of 1.92 and 1.88 V, respectively. Notably, all functionals favor the nitrate attack pathway rather than the water attack pathway. The barrier difference varies from 4.2 kcal mol−1 using B3LYP*-D3 to 16.0 kcal mol−1 using M06-D3. The large variation of iron oxidation potentials using different density functionals is not unexpected, and different functionals may favor different ground spin states of 1, 2, and 3 (Table S4). Similar observations have been reported in the study of electronic structures of 14 Fe(II), Fe(III), and Fe(IV) complexes with 20 exchange-correlation functionals.99 Benchmark calculations using high-level ab initio methods are needed to validate the accuracy of these density functionals. Further experimental studies using isotope-labeled nitrate oxygens should be able to validate the presented mechanism, as it is proposed here that one oxygen atom of the evolved dioxygen molecule comes from nitrate. G


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Inorganic Chemistry

Figure 8. Optimized structures of TSC1H, TSC8, and TSN3. Distances are given in Å. Spin densities for selected atoms are indicated in italics. Superscripts indicate the multiplicity of the structures. The imaginary frequencies for transition states are also shown. For TSC1H, unimportant hydrogen atoms are not shown for clarity.

Scheme 1. Suggested Catalytic Water Oxidation Cycle Based on Calculationsa

a

The dpa ligand is omitted for clarity.

4. CONCLUSIONS

constructed, and the whole catalytic cycle (Scheme 1) has been proposed on the basis of our calculations. The complex 1Cl undergoes ligand exchange of a chloride by a water molecule, leading to the aqua species 1 as the authentic catalyst. The one-electron oxidation of 1 is coupled with the release of two protons, generating the formally ferryl-oxo species 2. The subsequent one-electron oxidation of 2

In this work, we have investigated the mechanism of water oxidation catalyzed by the [Cl−FeIII(dpa)−Cl]+ complex using density functional calculations. Two O−O bond formation pathways, namely nitrate attack and water attack, have been thoroughly examined. The full energy diagram has been H


Inorganic Chemistry

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generates a formally FeVO species 3, which is the catalytically competent species and enables the O−O bond formation via a nitrate attack mechanism. The radical ligand of 3 may interact with the low-spin ferric-oxyl radical in either a ferromagnetic fashion or an antiferromagnetic manner. The electronic structure of 3 is thus reminiscent of Compound I in P450 enzymes. The alternative water attack pathway is kinetically unfavored compared to the nitrate attack pathway, no matter which density functional is applied. The preference of a doublet transition state during the O−O bond formation was interpreted by orbital diagrams of electron transfer. The substrate α-electron is transferred to the ligand π orbital in the doublet state but to the dx2−y2* or dz2* orbitals, which have an energy level higher than that of the π orbital, in the quartet or sextet states. The nitrate attack on 3 results in the formation of a ferric peroxonitrate intermediate. Simultaneous N−O bond cleavage and triplet dioxygen liberation via TS2 produce a ferric nitrite intermediate Int2. This step was calculated to have a slightly higher barrier compared to that of TS1, being 12.3 kcal mol−1. Further oxidation results in a nitrogen dioxide radical being loosely bound to the ferric center Int3, which may also be generated by the other competing pathway, namely, oneelectron oxidation of Int1. Ready replacement of the nitrogen dioxide radical by a water molecule regenerates 1. Consequent oxidation of 1 by the nitrogen dioxide radical generates a nitrate intermediate. Final nitrate dissociation regenerates 1 and completes the catalytic cycle. Nitrate thus functions as a cocatalyst by delivering one oxygen atom to the FeVO moiety to evolve a dioxygen molecule and is regenerated by oxidation of nitrite. Last but not least, catalyst degradation via diverse pathways has been investigated, among which the C−H oxidation of the methyl group was found to have the lowest barrier. The similar barriers of C−H oxidation and water oxidation explain the relatively low TON of this iron complex. Introduction of substituents on the isopentyl group and on the C8 position can substantially promote the resistance of dpa toward oxidation degradation.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00333. Structures, energy profiles, and coordinates (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rong-Zhen Liao: 0000-0002-8989-6928 Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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Article

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21503083) and the Fundamental Research Funds for the Central Universities (2017KFKJXX014). I


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Mechanism of Water Oxidation Catalyzed by a Mononuclear Iron

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