DFT Study on the pH Dependence of the Reactivity of Ferrate(VI)

Chapter 18

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DFT Study on the pH Dependence of the Reactivity of Ferrate(VI) Takashi Kamachi, Mayuko Miyanishi, and Kazunari Yoshizawa* Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan *E-mail: [email protected]

Density-functional-theory (DFT) study on methanol oxidation by ferrate (FeO42-) in water is reviewed in this chapter. The oxidizing power of three species, non-protonated, monoprotonated, and diprotonated ferrates was evaluated by using the B3LYP-D method. The oxidizing power increases in the order non-protonated ferrate < monoprotonated ferrate < diprotonated ferrate. The reaction pathway is initiated by C–H bond activation, which is the rate-determining in the overall reaction. Kinetic aspects of the reaction are analyzed from calculated energy profiles and experimentally known pKa values. The pH dependence of this reaction in water is explained well in terms of a multi-oxidant scheme.

1. Introduction High-valent transition metal oxides such as manganese dioxide (MnO2), potassium permanganate (KMnO4), chromium trioxide (CrO3), potassium chromate (K2CrO4), and potassium dichromate (K2Cr2O7) are frequently used for oxidation of organic compounds in laboratory and industry (1). However, in addition to their lack in selectivity and difficulty in controlling the experimental conditions, these reagents are corrosive and violently toxic to human beings and to the environment. In view of ever compelling environmental constraints, it is unacceptable for industrial wastes to contain such highly toxic transition metal complexes (2). Ferrate (FeO42-), derived from mineral salts such as the potassium (K2FeO4) and barium (BaFeO4) forms, can mediate oxidation of a wide variety of organic compounds such as alcohols (3, 4), amines (4), hydrazines (5), © 2016 American Chemical Society Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


thiosulfates (6), peroxides (7), and hydrocarbons (8) with excellent selectivity. Primary and secondary alcohols are successfully converted into aldehydes and ketones, respectively, but tertiary alcohols are not oxidized (3). Due to its friendliness to the environment, special interests are directed to the catalytic functions of ferrate (9–12). From an x-ray analysis, the crystal structure of FeO42is slightly distorted from Td symmetry (13). An isotope labeling experiment of oxygen (7) and IR spectroscopy (14) demonstrated that its mononuclear structure is kept in aqueous solution. Ferrate salt once dried is stable in air, while in water the stability is limited depending on the temperature and pH of solution (15). For instance, in acidic and neutral media ferrate is reduced by water to evolve O2, while it is considerably stable in strongly alkaline solution around pH = 10 (16). Since the reaction rate of alcohol oxidation by ferrate is dependent on the pH of solvent, proton or hydroxide is considered to play an important role in the alcohol oxidation (17–20). In an acidic media, ferrate is protonated and its oxidation ability is greatly increased. In conjunction with the chemical and biochemical importance for the activation of O–H and C–H bonds by high-valent transition metal-oxides, the widespread use of ferrate in the functionalization of O–H and C–H bonds has spurred considerable interests in the underlying activation mechanism. Under such circumstances, theoretical calculations are useful to obtain detailed information about the oxidation processes. Early pioneering theoretical studies of alcohol oxidation by oxo-metal compounds were performed by Goddard et al. (21–24), who applied the generalized valence bond (GVB) method to the oxidation of alcohol, alkane, and alkene by chromyl chloride (CrO2Cl2) and molybdenyl chloride (MoO2Cl2). Ziegler et al. (25–27) have extensively studied the activation of the C–H and O–H bonds of methanol by a series of d0 transition metal oxides MO2X2, where M = V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, and Os; X = Cl, from a thermodynamic point of view by using a density-functional-theory (DFT) method. They reported detailed reaction pathways of methanol oxidation by MO2X2, where M = Cr, Mo, X = Cl; M = Ru, X = O, from density-functional-theory (DFT) calculations and intrinsic reaction coordinate (IRC) analyses (27). In this chapter, we examine the oxidizing power of ferrate and protonated ferrates in water solvent from DFT calculations in the framework of the polarizable continuum model (PCM). Computational results clearly indicate that inclusion of solvent effects is essential for a proper description of the alcohol oxidation process by ferrate from the viewpoint of structure and energetics. We also investigate the reaction kinetics using information about obtained potential energy diagrams to refine the reaction mechanism. These analyses will increase our understanding of the mechanistic aspects of oxidation reactions by ferrate.

2. Method of Calculation We used the hybrid B3LYP-D (28–30) methods to investigate methanol oxidation implemented with the Gaussian 09 program (31). The B3LYP method has been reported to provide excellent descriptions of various reaction profiles, 474 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


particularly in geometries, heats of reaction, barrier heights, and molecular vibrations (32, 33). Dispersion correction was included in the calculations by the DFT-D3 approach of Grimme (34). For the Fe atom the (14s9p5d) primitive set of Wachters’ all electron basis set (35) added by one polarization f-function (α = 1.05) (36) resulting in a (611111111|51111|311|1) [9s5p3d1f] contraction was used, and for the other atoms the 6-311++G** basis set was used (37, 38). The dielectric effect of water solvent was incorporated using the polarized continuum model (PCM) (39–42). All geometries for reaction intermediates and transition states were fully optimized in the aqueous phase. Vibrational frequencies were systematically computed for all stationary points in order to confirm that each optimized geometry corresponds to a local minimum that has no imaginary frequency or to a saddle point that has only one imaginary frequency. Zero-point-energy corrections were taken into account for calculating the energetics of the reaction pathways. The spin state of all species in this study was set to be a triplet according to magnetic susceptibility measurements of the FeO42– salts (43, 44).

3. Results and Discussion 3.1. Structures of Ferrate and Protonated Ferrates As mentioned above, protonated ferrates are considered to exhibit stronger oxidation ability (17–19). We thus built ferrate FeO42- and protonated ferrates HFeO4- and H2FeO4 for our theoretical analyses, as shown in Figure 1. The formal charge of the iron atom is +6. Calculated Fe–O distances of 1.65 Å and O–Fe–O bond angles of 109.5o for FeO42- in water are in good agreement with an X-ray structure (13) of K2FeO4 (Fe–O, 1.65 ± 0.01 Å; O–Fe–O, 109.5o). The Fe–Ooxo bonds of ferrate are significantly increased in length upon protonation, while nonprotonated Fe–Ooxo bonds are decreased in length.

Figure 1. Optimized geometries of ferrate and protonated ferrates in aqueous (gas) phase. Bond distances in Å and angles (italic) in deg. Figure 2 shows isosurface spin-density plot for ferrate and protonated ferrates in gas phase. Obviously, spin densities of oxo ligands increase upon the protonation (11), this result being consistent with the experimental findings (19) that protonated ferrate has stronger oxidizing ability. On the other hand, the OH ligands in mono- and diprotonated ferrates have almost no spin density, 475 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and therefore the protonation of the oxo ligands of ferrate increases the spin density of the remaining oxo ligands after the reconstruction of the four Fe–O bonds. In general, solvent effects reduce the LUMO energies of anions. Pearson (45) determined electron affinities in gas and aqueous phases for a large number of anions from gas-phase proton affinities and aqueous pKa values. All anions exhibit a significant increase of their electron affinity in aqueous phase, and the solvent effects reduce their LUMO energies compared with those in gas phase. Safi et al. (46) also observed a decrease in the LUMO energy levels of anions in their ab initio study using the effective fragment potential (EFP) model. As shown in Figure 3, the LUMO shows a strong energy lowering from 5.5 eV to –1.8 eV in FeO42- and from 1.3 eV to –3.3 eV in HFeO4–, whereas the LUMO of H2FeO4 remains almost unchanged in energy. Thus, the LUMO energy levels of all these species have a minus sign in aqueous phase. This result clearly shows that the oxidizing abilities of FeO42- and HFeO4– would be enhanced in aqueous phase because of the increased electron accepting ability of the two species. In a previous study, we could not locate the transition states and intermediates for a direct hydrogen-atom abstraction from methanol (47) and adamantine (48) by FeO42- and HFeO4– in contrast to H2FeO4. The underlying reason of this result may be the high-lying LUMOs of FeO42- and HFeO4– in gas phase. FeO42- and HFeO4– are able to mediate a hydrogen-atom abstraction from the C–H and O–H bonds in aqueous phase, as discussed in the following sections.

Figure 2. Isosurface spin-density plot for ferrate and protonated ferrates.

3.2. Reaction Pathways for the Conversion of Methanol to Formaldehyde by Ferrate We considered two reaction pathways for the methanol oxidation process by non-protonated and protonated ferrates taking solvent effects into account: (1) an addition–elimination mechanism that begins with coordination of methanol to the iron atom; (2) a direct abstraction mechanism that begins with a hydrogen-atom abstraction from the O–H or C–H bonds of methanol (Figure 4). The addition–elimination mechanism is initiated by the formation of a methanol-coordinating complex that has an Fe–Omethanol bond. We tried in vain to find the methanol-coordinating complex for FeO42- and HFeO4–, while 476 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the oxygen atom of a methanol molecule can coordinate to the iron center of diprotonated ferrate with a binding energy of 6.3 kcal/mol. Optimized geometries of the methanol-coordinating complex for diprotonated ferrate and relevant transitions state for O–H bond activation are shown in Figure 5. Activation energies for the cleavage of the O–H bond of methanol by the oxo and hydroxo ligands of diprotonated ferrate were computed to be 25.0 and 17.5 kcal/mol, respectively. These barriers are rather high compared with a corresponding barrier of 4.3 kcal/mol in the direct abstraction mechanism. In addition, the Fe–O bond is cleaved in the geometry optimization of the transition state for C–H bond activation despite our best effort. Thus, we can reasonably rule out the addition–elimination mechanism and focus on the direct abstraction mechanism.

Figure 3. The LUMO energy level of ferrate and protonated ferrates in gas and aqueous phases. Solvent effects greatly reduce the LUMO energy of FeO42– and HFeO4–.

Figure 4. Direct abstraction (A) and addition-elimination (B) mechanisms of alcohol oxidation by ferrate. 477 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 5. Optimized geometries of a reactant complex and transition states in the addition–elimination mechanism. Bond distance in Å.

3.3. Non-Protonated Ferrate Figure 6 shows a computed energy diagram and optimized geometries of the reaction intermediates and transition states for the conversion of methanol to formaldehyde by non-protonated ferrate. We named non-protonated ferrate 1n for the discussion below. The conversion of methanol is initiated by a direct hydrogen-atom abstraction from a C–H bond of methanol by 1n. The C–H bond is cleaved by an oxo ligand via TS(1n→2n) to yield a radical intermediate (2n) in which hydroxymethyl radical (•CH2OH) is bound to an oxo ligand of ferrate. The activation energy for the hydrogen-atom abstraction was computed to be 14.0 kcal/mol relative to the dissociation limit (1n + methanol). DFT studies (27, 49) on analogous oxidants such as CrO2Cl2, MoO2Cl2, RuO4, and MnO4demonstrated that these oxidants should require a higher activation energy for a hydrogen-atom abstraction from a C–H bond. However, diprotonated ferrate is found to mediate the activation process of C–H bonds of adamantane with a much lower barrier of 9.0 kcal/mol from tertiary carbon atoms and of 7.5 kcal/mol from secondary carbon atoms in the triplet spin state (48). Thus, non-protonated ferrate is suggested to be a weak oxidant for the C–H bond cleavage as expected from the spin density of the oxo ligand and the energy level of the LUMO. Indeed, non-protonated ferrate does not have sufficient ability to abstract a hydrogen atom from the more rigid O–H bond of methanol; bond dissociation energies of the C–H and O–H bonds were computed to be 91.8 and 97.6 kcal/mol, respectively, at the B3LYP/6-311++G** level of theory. The transition state involves a nearly linear arrangement with respect to the (Fe)O•••H•••C moiety. The imaginary frequency mode of the transition state (1908i cm-1) includes stretching motion of the C–O and O–H bonds. The transition state has an O–H bond of 1.234 Å and a C–H bond of 1.303 Å; these bond distances are typical of the C–H bond activation 478 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


by various FeO species. In the next step, the O–H bond of hydroxymethyl radical in 2n is cleaved in a virtually barrierless fashion to give rise to an intermediate (3n), where the product aldehyde molecule is weakly bound to complex 4n.

Figure 6. Energy profile (in kcal/mol) for the methanol-formaldehyde conversion by ferrate in water. Optimized parameters are shown in Å.

3.4. Monoprotonated Ferrate Figure 7 shows the computed energy diagram and optimized geometries of the reaction intermediates and transition states for the conversion of methanol to formaldehyde by monoprotonated ferrate. In contrast to nonprotonated ferrate, monoprotonated ferrate has an oxidizing ability to activate not only the C–H bond of methanol but also the more rigid O–H bond. In the initial stages of the methanol–formaldehyde conversion, the C–H and O–H activations are comparable in energy (50). Thus, there are two possible reaction pathways for methanol oxidation by monoprotonated ferrate. 479 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 7. Energy profile (in kcal/mol) for the methanol-formaldehyde conversion by monoprotonated ferrate in water. Optimized parameters are shown in Å.

In path 1, the hydrogen atom of the OH group of methanol is first abstracted by an oxo group of monoprotonated ferrate (1m) via transition state TS(1m→2m) to form 2m, in which a methoxy radical is weakly bonded to ferrate. On the other hand, a hydrogen atom abstraction form a C–H bond of methanol occurs in path 2 via TS(1m→3m) to lead to an organometallic intermediate 3m with an Fe–C bond. The activation barrier of 11.7 kcal/mol for these transition states is 2.3 kcal/ mol lower than that by non-protonated ferrate, which supports that protonation of ferrate increases the oxidation power for the C–H bond activation. In the next step of path 1, a C–H bond of the methoxy radical in 2m is activated either by the oxo or hydroxo ligand to form formaldehyde and complexes 5m and 6m. The C–H bond activation via TS(2m→5m) and TS(2m→6m) is energetically competitive. In path 2, the O–H bond of the hydroxymethyl moiety in 3m is cleaved by an oxo ligand via TS(3m→4m) to form a formaldehyde–ferrate complex 4m. In view of the calculated energy diagrams the reaction pathway that proceeds through 3m is the most favorable reaction pathway. This calculational result agrees with the proposal by Lee’s group that a key organometallic intermediate with an Fe–C bond is involved in the course of alcohol oxidation by ferrate (51). 480 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 8. Energy profile (in kcal/mol) for the methanol-formaldehyde conversion by diprotonated ferrate in water. Optimized parameters are shown in Å. 3.5. Diprotonated Ferrate Figure 8 shows the computed energy diagram and optimized geometries of the reaction intermediates and transition states for the conversion of methanol to formaldehyde by diprotonated ferrate. The activation energies for a hydrogen atom abstraction from the O–H bond (TS(1d→2d)) and a C–H bond (TS(1d→4d)) of methanol are 9.7 kcal/mol and 4.3 kcal/mol, respectively. This result suggests that the C–H bond is preferentially activated by an oxo ligand of diprotonated ferrate. We can reasonably conclude from the calculated activation barriers that diprotonated ferrate has the strongest oxidizing power for methanol oxidation among the three species. A stable organometallic intermediate 4d with an Fe–C bond was found to be produced in this pathway. We cannot find the corresponding intermediate for the non-protonated ferrate and the binding energy between the carbon radical center of hydroxymethyl radical and the iron atom is small in 3m. On the other hand, 4d has a relatively strong Fe–C bond as indicated by a 481 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

significant energy of 25 kcal/mol that is released in the course of the Fe–C bond creation. The intermediate is rather stable in energy compared with intermediate 3d in which the OH group of hydroxymethyl radical species coordinates to the iron atom. Since the overall reaction is 55 kcal/mol exothermic and the transition states involved in this pathway are low-lying, the reaction mediated by diprotonated ferrate should easily take place in water.


3.6. Kinetics of Methanol Oxidation by Ferrate in Water Our calculational results demonstrate that the order of oxidizing power in water is diprotonated ferrate > monoprotonated ferrate > non-protonated ferrate. However, the oxidizing power of the three species is not necessarily a good index to determine which is a main oxidant in methanol oxidation mediated by ferrate. Diprotonated ferrate exists in aqueous solution in extremely small quantities under experimental conditions. Such a trace amount of diprotonated ferrrate is unlikely to be involved as a main oxidant in the reaction. In this section, we consider kinetic aspects of the reaction on the basis of simple kinetics calculations to clarify the relationship between the concentration of these species and actual reaction rate. The calculated potential energy diagrams show that this oxidation reaction is downhill and highly exothermic and that there is no high barrier after the first step. Thus, we assumed that the rate law for each pathway is first order with respective to the concentration of ferrate and substrate. The net rate of methanol oxidation is written as follows:

where [M] is the concentration of methanol and kn, km, and kd are reaction rate constants for FeO42–, HFeO4–, and H2FeO4, respectively. Mono- and diprotonation of FeO42– reduce the activation barriers for the hydrogen-atom abstraction from the C–H bond of methanol by 2.3 kcal/mol and 9.7 kcal/mol. The barrier reductions lead to the acceleration of reaction rate: km/kn = 4.844 × 10 and kd/kn = 1.188 × 107. We estimated the pH dependence of concentration of these three oxidants from the following equilibria (52):

Figure 9 shows calculated relative reaction rates of FeO42–, HFeO4–, and H2FeO4 for methanol oxidation as a function of pH. This illustration clearly demonstrates that the identity of main oxidant for the reaction is dependent on pH. Interestingly, the main oxidant is FeO42– in strongly basic media, and not H2FeO4 that has the strongest oxidizing power among the three oxidants. This result leads us to propose that the concentration of diprotonated ferrate is so low that this powerful oxidant cannot participate in the oxidation reaction. 482 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 9. Estimated reaction rate fraction of FeO42–, HFeO4–, and H2FeO4 for methanol oxidation as a function of pH. Alcohol oxidation by ferrate is experimentally performed in a pH range of 9-13 to diminish interference from the decomposition of ferrate in aqueous solution as much as possible. Thus, it is difficult to uniquely determine the identity of active oxidant because the hydrogen atom abstraction processes by the three oxidants compete, as shown in Figure 9. In fact, numerous kinetic experiments (6, 19, 53) have suggested the involvement of multiple active oxidants in ferrate reactions. Norcross et al. (19) revealed that the observed pH dependence for the oxidation of 1,1,1,3,3,3-hexafluoro-2-propanol by ferrate was explained well by a kinetic model for the multi-oxidant process of FeO42– and HFeO4–. Our result on the basis of theoretical calculations and experimentally determined pKa’s of ferrate is in good agreement with the complex pH dependence of methanol oxidation by ferrate in water.

4. Conclusions In this chapter, we have discussed from DFT calculations the reactivity of three active species, FeO42–, HFeO4–, and H2FeO4, in water using the polarizable continuum model (PCM). The lowering of the LUMO energy levels of FeO42and HFeO4– in water greatly increases the oxidation ability of the two species. The rate-determining step is the C–H bond activation process by the oxo ligand of ferrate in methanol oxidation. The order of oxidizing power in water is diprotonated ferrate > monoprotonated ferrate > non-protonated ferrate. To gain 483 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

a better understanding of the oxidation mechanism in water, we have analyzed the complex pH dependence of this reaction using a simple kinetic model. The calculated relative reaction rate in water indicates that the three oxidants compete in these reactions under experimental conditions, which is in good agreement with the experimentally observed pH dependence of this reaction.

Acknowledgments Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 16, 2017 | http://pubs.acs.org Publication Date (Web): December 19, 2016 | doi: 10.1021/bk-2016-1238.ch018

We are thankful to Professor V. K. Sharma for his kind advice and discussion.

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487 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

DFT Study on the pH Dependence of the Reactivity of Ferrate(VI)

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