Catalytic Oxidation of Water with High-Spin Iron(IV) - American

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Catalytic oxidation of water with high-spin iron(IV)oxo species: The role of the water solvent Leonardo Bernasconi, Andranik Kazaryan, Paola Belanzoni, and Evert Jan Baerends ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Catalytic oxidation of water with high-spin iron(IV)-oxo species: The role of the water solvent Leonardo Bernasconi,∗,† Andranik Kazaryan,‡ Paola Belanzoni,¶ and Evert Jan Baerends∗,‡ †1 STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX, United Kingdom ‡Theoretical Chemistry Section, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands ¶Department of Chemistry, Biology and Biotechnology, University of Perugia and Institute of Molecular Science and Technologies (ISTM-CNR), Via Elce di Sotto 8, I-06123 Perugia, Italy E-mail: [email protected]; [email protected]

Abstract We use density-functional theory (DFT) and ab initio molecular dynamics to study the conversion of H2 O into H2 O2 in water solution by the Fe(IV)O2+ group at room temperature and pressure conditions. We compute the free energy of formation of an O(water)-O(oxo) bond using thermodynamic integration with explicit solvent and we examine the subsequent generation of H2 O2 by proton transfer. We show that the O-O bond formation follows the standard reactivity pattern observed in hydroxylation reactions catalysed by high-spin (S = 2) iron(IV)-oxo species, which is initiated by

1

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the transfer of one electron from the highest occupied molecular orbital of the moiety attacking the Fe(IV)O2+ group, either a -C-H bonding orbital (hydroxylation) or a lone pair of a water molecule (water oxidation). The highly electrophilic character exhibited by the Fe(IV)O2+ ion, which is related to the presence of an acceptor 3σ ∗ orbital at low energy with a large contribution on the O end of the Fe(IV)O2+ ion, is the crucial factor promoting the electron transfer. The electron transfer occurs at an O(water)O(oxo) distance of ca. 1.6 ˚ A and the free energy required to orient favourably a solvent H2 O molecule for the O(oxo) attack and to bring it to the transition state amounts to only 35 kJ mol−1 . The ensuing exoergonic O-O bond formation is accompanied by the progressive weakening of one of the O-H bonds of the attacking H2 O assisted by a second solvent molecule, and leads to the formation of an incipient Fe2+ -[O-OH]− [H3 O+ ] group. Simultaneously, three additional solvent molecules correlate their motion and form a hydrogen bonded string which closes to form a loop within 5 ps. The migration of the H+ ion in this loop via a Grotthuss mechanism leads to the eventual protonation of the [O-O-H]− moiety, its progressive removal from the Fe2+ coordination sphere and the formation of free H2 O2 in solution.

1

Introduction

The extraordinary reactivity of non-heme iron(IV)-oxo (ferryl) complexes in the oxidation of poorly reactive hydrocarbons is widely known and well documented in both biological and inorganic processes. 1–17 Extensive experimental and theoretical work has been devoted to understanding the origin of this reactivity, and attempts have been made to recreate abiotically optimal conditions for promoting the generation of ferryl species and enhance their reactivity and/or specificity in organic oxidations. 18–30 One of the most technologically important and sought after goals in this effort is the development of efficient and environmentally friendly processes for the chemical oxidation of methane (the main component of natural gas) to produce methanol. 31–35 This is one of the core processes underpinning future

2


Page 2 of 28

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

scenarios for energy production and utilisation at a planetary scale. 36 Much work has also been devoted to study the prospective role of iron(IV)- and iron(V)oxo species in the catalytic oxidation of water to H2 and O2 , 37–43 one of the most important processes in natural and artificial photosynthesis. 44 One of the crucial challenges for the success of artificial photosynthesis is the development of viable catalytic processes for the O-O bond formation reaction, and the most efficient catalysts for water splitting prepared to date are based on Ru, Co, and Ir complexes (see Ref. 45–50 and references therein). The function of these species in artificial fuel generation from solar energy is analogous to the natural Mnbased O2 evolving complex (OEC) of photosystem II in natural photosynthesis. Iron-oxo species can potentially offer an attractive alternative to these systems, and notably to the Ru and Ir based synthetic analogues, because of the large abundance and low toxicity of iron and of the recent advances made in the isolation and characterisation of (non-heme) iron-oxo complexes. 14,51,52 Furthermore, strong parallels exist between the oxidation chemistry of iron and manganese oxides, 53 and mechanistic details unveiled in the study of Fe-based O-O bond formation processes can therefore be carried over to elucidate the chemistry of the OEC. Interestingly, extensive computational work has indicated that inorganic Fe(IV)O2+ species in water solution can be generated by direct reduction of atmospheric dioxygen (rather than hydrogen peroxide or ozone, as in standard Fenton chemistry) in the presence of suitable ligands stabilising an incipient ferryl group. 27,29,32,34 If this prediction is verified experimentally, it may pave the way for the creation of a new generation of inexpensive and environmentally friendly Fe(IV)O-based catalysts for organic reactions at mild working conditions, including hydrocarbon and, potentially, water, oxidation. The reactivity of the ferryl ion in C-H bond hydroxylation is driven by the Fe-3dz2 -O-2pz antibonding acceptor orbital 3σ ∗ which is at particularly low energies in high-spin (quintet, S=2) iron-oxo systems, 24,54 and imparts strong electrophilic character to the Fe(IV)oxo group. 22,24,27,53–58 The electrophilicity can be further enhanced by a judicious choice of ligands. In particular, a weak ligand field, from, e.g. oxygen based rather than nitrogen based

3


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

coordination, is essential to achieve reactivities comparable to those observed in biological systems. 24,30 The same electronic feature can make the iron(IV)oxo group potentially suitable for water oxidation reactions, which are initiated by a nucleophilic attack on the ferryl group by a water molecule and lead to the formation of an O-O bond between Ooxo and Ow . The electrophilicity of the ferryl ion may even be strong enough to carry out a H-atom abstraction from a water O-H bond, even though this O-H bond is electronically less suitable than the aliphatic C-H bond, because the corresponding orbital is at lower energies compared to -C-H. 59 In this study we use ab initio molecular dynamics (AIMD) simulations to examine the O-O bond formation reaction involving an Fe(IV)oxo species and a water molecule in water solution. The existence of the (H2 O)5 FeO2+ complex in water solution has been spectroscopically demonstrated by Bakac et al.. 60 The lifetime of this species is short (of the order of minutes), and the direct oxidation of water is a possible decay route. In this work we will address the plausibility of this reaction pathway and study the mechanistic details of the early stages of an O-O bond formation and of how this process leads to the evolution of hydrogen peroxide in aqueous solution, h

H2 O + Fe(IV)O2+ −→ H2 O2 + Fe(II)2+

i

. aq

(1)

This reaction involves the initial transfer of one electron from a water molecule to Fe(IV) via the Ooxo atom. In the case of hydrocarbon activation in the gas phase and in aqueous solution, it is now recognised that this process is driven by a low-lying anti-bonding orbital (3σ ∗ ) in high-spin (quintet) iron-oxo systems, which imparts strong electrophilic character to the Fe(IV)oxo group and regulates its reactivity in hydroxylation reactions. 27,53–58 In this work, we will show that the electron-acceptor character of the Fe(IV)O2+ group acts again as the crucial factor in driving the early stages of the oxidation of water molecules in solution, specifically during the formation of an O-O bond involving the oxygen atom of the

4


Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fe(IV)O2+ group. In the hydroxylation of hydrocarbons, solvent effects have been shown to be particularly important in influencing the overall reactivity of the Fe(IV)O2+ moiety through a subtle orbital control mechanism. 29 In the case of the water oxidation reaction, we will show that solvent molecules play an even more fundamental and specific role in promoting O-O bond formation. In this case they are actual participants in the reaction and they drive the eventual generation of free H2 O2 in solution. Our computational approach uses DFT-based AIMD simulations and free-energy calculations in the presence of explicit solvent water molecules. These approaches have previously been used to examine the generation and reactivity of Fe(IV)oxo2+ species in solution in a variety of conditions. 29,34,61–64 At variance with classical molecular dynamics, AIMD provides real-time information about the evolution of the electron density distribution and therefore makes it possible to examine bond breaking/forming events in some detail. Furthermore, the explicit inclusion of solvent molecules in the simulation provides an unbiased picture (within system size and propagation time limitations) of how solvent molecules may contribute, individually or collectively, to a chemical reaction. This information is crucial, as we will show, to understand how Fe(IV)O2+ moieties activate water molecules in their vicinity to promote the creation of stable O-O bonds. The paper is organised as follows. In Section 2 we describe the calculation set up and the approach used to estimate free energies of reaction in solution. The main results concerning the O-O bond formation and H2 O2 evolution are presented in Section 3. The work is summarised in Section 4.

2

Details of calculations

The liquid solution structure was modelled using periodic boundary conditions applied on a cubic supercell of side a = 15.9 ˚ A, containing one FeO2+ ion and 64 water molecules, in the presence of a homogeneous charge background to enforce neutrality and formally replace

5


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a solvated counter-ion. A similar set up has been tested in an extensive series of previous calculations, 29,34 and has been shown to provide a satisfactory representation of the solvent structure in the vicinity of FeO2+ ion, which is stabilised, in solutions, by coordination to a first solvation shell of 5 water molecules (FeO2+ .(H2 O)5 ). AIMD simulations were performed using the hybrid Gaussian/plane-wave 65 package CP2K Quickstep 66 version 2.1.347 with standard double-ζ VB basis sets and Goedecker-TeterHutter pseudopotentials 67 for all atomic species and a cut-off of 280 Ry in the plane-wave expansion. Exchange-correlation effects were described at the OPBE 68 level of theory, as in our earlier work. 24,29,34,64 This functional has been extensively validated, e.g. by Swart et al. concerning its performance for (spin states) of iron complexes, 69,70 and by Kazaryan and Baerends specifically for the water oxidation reaction with a metaloxo group. 71 The system was constrained to its quintet ground state by fixing the occupations of the Kohn-Sham spinorbitals to yield a total of 4 unpaired electrons. AIMD propagation of the atomic positions was carried out using the Born-Oppenheimer scheme 72 with a time-step of 0.5 fs. The simulation temperature (298 K) was controlled through a Nosé-Hoover thermostat. Trajectory analysis and visualisation were carried out using VMD 73 version 1.8.6 and XCrySDen 74 version 1.5.17. Free energies for O-O bond formation were computed using standard thermodynamic integration techniques (see e.g. Ref. 75 ). The reaction was modelled by constraining the distance between the oxygen atom of a water molecule in the vicinity of the Fe(IV)O2+ group (Ow ) and the Fe(IV)O2+ oxygen atom (Ooxo ) to a series of fixed values xi , with 1.05 ˚ A ≤ xi ≤ 2.35 ˚ A, following a preliminary equilibration of 2.5 ps at 298 K, during which the system was propagated in the absence of constraints. At each value of xi , an AIMD simulation of 2.5 ps was then performed by solving equations of motion with the constraint imposed in the form of a Lagrangian multiplier λ(xi ). The mean force of the constraint f (xi )

6


Page 6 of 28

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

was estimated from the unbiased time averaged value of λ(xi ),

f (xi ) − f0 = hλ(xi )i −

2kB T , xi

(2)

where f0 is the value of f (xi ) for the largest value of the Ow -Ooxo distance considered (x0 = 2.35 ˚ A). f0 represents the residual mean force which is required to maintain the two O atoms at a fixed distance corresponding to the equilibrium distance during free (unconstrained) dynamics. f0 differs from zero because of the small difference between the value of the imposed constraint and the actual unconstrained O-O mean distance in the limit of infinite simulation time. In the second term in the right hand side of Eqn. 2, kB = 1.38×10−23 J K−1 and T = 298 K. This term is used to circumvent one of the difficulties encountered in the general definition of the mean force in a constrained simulation, namely the fact that a new complete set of coordinates is required containing the reaction coordinate as an independent variable, to represent the dependence of the mean force on the directional derivatives of the force and the Jacobian of the coordinate transformation. 76 In the approach of Ciccotti and Sprik 77 , this correction term is used to avoid any reference to directional derivatives and Jacobians for scalar reaction coordinates. xi was varied in steps of 0.1 ˚ A, in sequence from x0 to its shortest value xi = 1.05 ˚ A. The potential of mean force (free energy) ∆G(x) was then estimated by numerical integration,

∆G(x) = −

Z

x

[f (x′ ) − f0 ]dx,

(3)

x0

after interpolating the mean force function f (xi ) − f0 from Eqn. (2) with a 100 point Akima spline function.

7


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

Results and discussion

According to our AIMD simulations, Reaction (1) occurs in two stages (Figure 1): (1) Ow -Ooxo bond formation and release of a proton to a second solvent water molecule to + form a coordinated peroxo bridge, (H2 O)5 Fe2+ · · · (Ooxo − Ow H)− + H3 O+ (Steps 1-3);

(2) protonation and cleavage of the FeOoxo bond to yield free H2 O2 (Step 4). Overall, the oxidation of a water molecule is therefore coupled to the reduction of the metal centre from aqua-Fe(IV)O2+ to aqua-Fe(II)2+ , which ensures the overall charge neutrality of the process.

3.1

Step 1: O-O bond formation

In Figure 2 we show representative snapshots of selected atomic positions before and during the constrained shortening of the Ow -Ooxo distance x. Initially, the Fe(IV)O2+ ion is coordinated by 5 water molecules, with an additional water molecule bonded to the Ooxo atom (Figure 2 (a)). We note that this is not a case of traditional hydrogen bonding, for which the interaction of the O-H σ ∗ empty acceptor orbital with a lone pair donor orbital on the substrate (the H acceptor) is an important component. Here we have a case of complexation caused by electron donation from the O-H σ bonding orbital to the low-lying 3σ ∗ acceptor orbital of the FeO2+ fragment. According to the present, as well as previous, AIMD simulations, this configuration, with complexation of a single water molecule to Fe(IV)O2+ , is stable, at 298 K, for at least 10 ps. We note that, in the gas phase, the complex with the water molecule oriented with oxygen lone pairs toward the σ ∗ acceptor orbital of FeO2+ has the lowest energy. 54 It is therefore the hydrogen bonding with the surrounding solvent water molecules that makes the orientation with H to the Ooxo more favorable. In this work, we assume that this water molecule is the one eventually oxidised by the Fe(IV)O2+ group, and we therefore study the O-O bond formation by constraining the distance between the O atom of this molecule (Ow ) and Ooxo , irrespective of the fact that the initial orientation of the molecule is unsuitable for a direct nucleophilic attack to the Fe(IV)O2+ group with

8


Page 8 of 28

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

the lone pairs of the water oxygen as donor orbitals. The mean force (see below) exhibits a shallow minimum at x = 2.35 ˚ A, which corresponds to a reactant complex (RC) in which the attacking water molecule retains its initial orientation relative to Ooxo ((Figure 2 (b)). The transition state (TS) for the O-O bond formation appears at x = 1.75 ˚ A ((Figure 2 (c)). Before reaching the TS, the attacking water molecule changes its orientation, after which Ow can interact directly with Ooxo . Formally, the O-O bond formation is a consequence of the partial donation of first one electron to the anti-bonding 3σ ∗ α orbital of the Fe(IV)O2+ group (Figure 1, Step 1), which weakens the Fe-Ooxo bond, followed by the partial transfer of a further electron of Ow to the 3σ ∗ β orbital of Fe(IV)O2+ . This process eventually leads to the formation of an electron pair bond between the oxygens. With two electrons being transferred from Ooxo to Fe, Fe(IV) is reduced to Fe(II) and a HOO− ligand is generated. In addition to the O-O bond electron pair, the HOO− group has two π and one σ (sp hybrid) lone pair on the former Ooxo , and it is therefore formally an O− ion. The involvement of the 3σ ∗ gives step 1 some similarity to the standard mechanism of hydrocarbon activation by quintet Fe(IV)O2+ systems, 22,24,54,55 with the important difference that the process is now triggered by an electron transfer to Fe(IV)O2+ , rather than by a proton-coupled electron-transfer reaction, as in the hydroxylation of, e.g., methane. 29,78 We also note that, in exploratory gas phase calculations of the water oxidation reaction, 79 the analogous proton-coupled electron transfer reaction (or H-atom transfer, HAT) of the H of the O-H bond is also feasible, but it exhibits a slightly higher reaction barrier than the direct O-O coupling reaction considered here. Further shortening the Ow -Ooxo bond leads to a product complex (PC) with an Ow -Ooxo distance of 1.45 ˚ A (Figure 2 (d)), which is suggestive of a typical peroxo bond (for comparison, the O-O distance in molecular hydrogen peroxide is 1.47 ˚ A). The evolution of the TS toward the PC also involves the transfer of a proton to a nearby solvent molecule (not shown in Figure 2). In the PC, this water molecule remains bound to the Fe-coordinated O-O-H− group, with an essentially planar arrangement of the two oxygen atoms, the H atom bound to Ow and the proton transferred to the second solvent water molecule (see Figure 2

9


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(d)). In Figure 3 we show the evolution of selected Mulliken atomic charges and spin moments obtained from gross populations of spin-orbitals during the O-O bond formation. The charges and spins are obtained by averaging over 50 atomic configurations extracted from AIMD trajectories at fixed values of x. In the ferryl complex, the atomic charge of Ooxo is ca. −0.5 , which is consistent with the substantial degree of covalency of the Fe(IV)-O bond with the consequent large deviation from the ideal ionic charges of +4 of −2 for Fe and O respectively. During the O-O bond formation, its value decreases to ca. −1.0, consistent with a HOO− hydroperoxide ligand. The intial charge of Ow (ca. −0.5) (compensated in water molecules by charges of ∼ 0.25 on the H atoms) increases to ca −0.2 during the reaction, consistent with the evolution to a formally neutral O atom in HOO− . The spin moment of Ow remains essentially zero, whereas that of Ooxo decreases, as the reaction proceeds. When the bond of Ooxo with Fe weakens, the spin density of Fe delocalizes less onto the Ooxo , i.e. spin density shifts from Ooxo to Fe. The number of unpaired electrons on Fe formally remains constant, as both Fe(IV) in the ferryl group and Fe(II) at the end of the reaction carry 4 unpaired electrons. The strong covalent bonding of Fe to Ooxo in Fe(IV)O2+ leads to larger spin delocalization than with coordinative bonds to Fe, as for OOH− . The spin moment of Ow changes by a modest amount during the reaction and it exhibits a weak negative maximum at the TS (x = 1.75 ˚ A). This may be related to Ow acquiring some (negative) net spin density as the attacking water approaches Ooxo and starts donating a spin α electron to σ ∗ α. Its absolute value decreases again after the TS, once the spin β electron donated by Ow pairs with the α electron in the incipient O-O bond. The mean force profile and the free energy obtained from Eqn. 3 are shown in Figure 4. The mean force at large O-O distance is initially positive, relative to f0 , as work has to provided to force the two O-O atoms involved in the O-O bond formation to move to closer distances. Once the TS is reached, the incipient chemical bonding reverses the sign of the mean force, and this explains the local minimum in the free energy profile. At O-O

10


Page 10 of 28

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

distances shorter than the equilibrium bond length, the mean force changes sign again, in consequence to strong Pauli repulsion between the atoms. The free-energy maximum at x = 1.65 ˚ A corresponds to the free-energy barrier for O-O bond formation, and amounts to 35 kJ mol−1 . This is a remarkably low barrier compared to other systems, like the ironoxo moieties derived from iron centred tetraamido macrocyclic (Fe-TAML) complexes. 37 Theoretical investigations on these systems have lead to postulate reaction barriers of the order of 126 kJ mol−1 . 39 Comparable reaction barriers have also been reported for Ir-oxo complexes on the basis of CCSD(T) and DFT calculations. 71 The unusual reactivity of Fe(IV)O2+ is a consequence of its peculiar electronic structure, which is dominated by the low energy virtual 3σ ∗ orbital, stabilised by the weak ligand field of the water ligands and the concomitant high-spin state, see Ref. e.g. 30 The role of the solvent in this reaction is essential. We have already observed that a second solvation shell molecule, in addition to the attacking water, is necessary to stabilise the incipient Fe-coordinated O-O-H− group and to act as an acceptor of a proton at the end of the reaction. In Figure 5 we show a total energy (enthalpy) profile obtained by extracting from the constrained AIMD simulations subsets of atoms corresponding to those shown in Figure 2, plus the additional water molecule acting as a proton acceptor, and removing all other water molecules. 50 atomic configurations were considered for each value of x and total energies calculated using the same computational parameters as for the solutions. This approach models a hypothetical reaction occurring in the gas phase with modalities analogous to those observed in solution. Within statistical uncertainty, the mean enthalpy does not appear to show a well defined reaction profile (cf. Figure 4). Other attempts to model the reaction directly in the gas phase (with or without the inclusion of a small number of water molecules) were similarly unsuccessful, leading to unphysical reaction paths or to extremely large reaction barriers (details will be given elsewhere). This demonstrates the importance of including a sufficiently large number of solvent molecules in modelling this class of reactions, and also of removing spurious surface effects that may affect cluster calculations through the

11


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

use of periodic boundary conditions. The AIMD approach adopted in this work also makes the determination of optimised geometries for the solute+solvent system unnecessary and obviates the essentially unsurmontable difficulties encountered when large water clusters are treated using open boundary conditions. 80,81 In the next subsection, we will also show that an explicit description of the solvent is necessary to account properly for the generation of H2 O2 after O-O bond formation.

3.2

Step 2: Generation of free H2 O2

Once the Ow -Ooxo bond is formed at x = 1.45 ˚ A, the H-O-O− group remains coordinated to the partially reduced Fe centre and the second hydrogen atom of the attacking water molecule is shared with a second solvent water molecule. Analysis of the trajectory at x = 1.45 ˚ A indicates that this O-H bond is only partially broken. During the dynamics, the H atom remains most of the time closer to the second water molecule, although it occasionally returns in the vicinity of the parent oxygen. A complete breaking of the O-H bond occurs when the Ow -Ooxo distance is further constrained to shorten to 1.35 ˚ A (Figure 6). According to the calculated free energy profile (Figure 4), the energetic cost of this further bond contraction amounts to only ca. 5 kJ A and at the local mol−1 (it can be estimated from the free energy difference at x = 1.35 ˚ minimum x ≃ 1.4 ˚ A). This modest energetic cost is sufficient to trigger the three processes that lead to the eventual evolution of free H2 O2 : (1) the complete cleavage of the O-H bond involved in hydrogen bonding with a second water molecule, (2) the breaking of the HOO-Fe bond with the detachment of the HOO− ion from the metal ion centre and (3) the transfer of a proton from the solvent to generate H2 O2 and aqua-Fe2+ with a vacant coordination site. According to our simulation, once initiated, this series of events complete within 5 ps at 298 K. We do however remark that, owing to the application of a constrain in the AIMD, this time estimate should be considered purely indicative. The mechanism appears to be concerted, with the three events contributing essentially 12


Page 12 of 28

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

simultaneously to the overall process. Initially, the Fe centre is coordinated by the five water molecules of the first solvation shell and by the HOw O− oxo ion, with a further water molecule sharing the second proton of the attacking water (Figure 6 (a)). Within 2 ps, the coordinated motion of three additional water molecules (whose distance from the metal centre can be as large as ca. 7 ˚ A) leads to the formation of a short open chain of water molecules {[Fe]-[OOH][H3 O]-[H2 O]3 }2+ (Figure 6 (b)). Within 2 ps, the chain closes with the final water molecule orienting one of its O-H bonds toward Ooxo in the peroxo ion (Figure 6 (c)). This closed arrangement creates a path for H+ migration from the first water molecule in the chain to the final H+ acceptor Ooxo (Figure 6 (d)). Mulliken charges (Figure 7) confirm that it is a proton, and not a hydrogen atom, that is being transferred along the ring of water molecules, as in the standard Grotthuss mechanism. The atomic charge on the metal ion centre is consistent with an oxidation state II at the end of the reaction. The proton migration occurs within 1 ps. Again, we note the crucial role of solvent molecules not initially coordinated to the Fe(IV)O2+ ion, and potentially even outside its second solvation shell, in making the H+ transfer to the peroxo group thermodynamically and kinetically favourable. In agreement with previous studies based on AIMD simulations, 82,83 our work highlights the importance of an explicit description of solvent molecules in modelling proton transfer processes in water solution.

4

Summary and conclusions

We have studied the oxidation of a water molecule to hydrogen peroxide by the ferryl ion, Fe(IV)O2+ , in water solution at room temperature using DFT-based AIMD and thermodynamic integration. The reaction occurs in two stages: initial formation of a Fe-coordinated H-O-O− group, assisted by a second solvation shell solvent molecule acting as a proton acceptor, followed by the back-donation of the proton from the solvent via a Grotthuss mechanism of proton migration involving four water molecules in a closed configuration, which leads to

13


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the generation of a free H2 O2 molecule. Potentially, the involvement of non-stoichiometric water in the H2 O oxidation reaction can be verified experimentally using kinetic measurements. Interestingly, the involvement of non-stoichiometric water has also been observed in AIMD simulations of the aqueous Fenton reaction. 61–63,84 The O-O bond formation is made possible by the strong electrophilic character of the Fe(IV)O2+ group, similar to what is observed in the hydroxylation of hydrocarbons in biological and abiotic conditions, which is explained by the existence of a virtual 3σ ∗ acceptor orbital at unusually low energy with a large contribution on the Ooxo atom (cf. e.g. Refs. 24,27,29,54,55 for details). A free energy barrier for O-O bond formation can be estimated of 35 kJ mol−1 , with a potentially further free energy cost of only 5 kJ mol−1 required to generate H2 O2 and remove it from the coordination shell of the reduced Fe(II) ion. These low barriers for water oxidation are consistent with the short lifetime observed experimentally for (H2 O)5 FeO2+ in water solution. 60 Our work highlights the fundamental role of the solvent in driving this oxidation reaction, both by providing a local environment that promotes the formation of the hydroperoxide ligand by acting as a proton acceptor and by subsequently channeling the proton through the solvent by a Grotthus type mechanism leading to protonation of the originally Ooxo end of the coordinated hydroperoxide ion.

Acknowledgement This work was supported by EPSRC through a Service Level Agreement with STFC Scientific Computing Department and by the UK Materials Chemistry Consortium (Grant EP/L000202). Computer resources were provided by the Netherlands’ Scientific Research Council (NWO) through a grant from Stichting Nationale Computerfaciliteiten (NCF), and by STFC Rutherford Appleton Laboratory.

14


Page 14 of 28

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

References (1) Shu, L.; Nesheim, J. C.; Kauffmann, K.; Muenck, E.; Lipscomb, J. D.; Que Jr., L. Science 1997, 275, 515–518. (2) Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Mller, J.; Lippard, S. J. Angew. Chem. Int. Ed. 2001, 40, 2782–2807. (3) Kopp, D. A.; Lippard, S. J. Curr. Opin. Chem. Biol. 2002, 6, 568–576. (4) Guallar, V.; Gherman, B. J.; Lippard, S. J.; Friesner, R. A. Curr. Opin. Chem. Biol. 2002, 6, 236–242. (5) Gherman, B. F.; Baik, M.-H.; Lippard, S. J.; Friesner, R. A. J. Am. Chem. Soc. 2004, 126, 2978–2990. (6) Groves, J. T. J. Inorg. Biochem. 2006, 100, 434–447. (7) Shaik, S.; Hirao, H.; Kumar, D. Acc. Chem. Res. 2007, 40, 532. (8) Kovaleva, E. G.; Neibergall, M. B.; Chakrabarty, S.; Lipscomb, J. D. Acc. Chem. Res. 2007, 40, 475–483. (9) Tinberg, C. E.; Lippard, S. J. Biochemistry 2010, 49, 79027912. (10) Xue, G. Q.; Hont, R. D.; Munck, E.; Que, Jr., L. Nature Chem. 2010, 2, 400–405. (11) Friedle, S.; Reisner, E.; Lippard, S. J. Chem. Soc. Rev. 2010, 39, 2768–2779. (12) Solomon, E. I.; Light, K. M.; Liu, L. V.; Srnec, M.; Wong, S. D. Acc. Chem. Res. 2013, 46, 2725 – 2739. (13) Nam, W. W.; Lee, Y. M.; Fukuzumi, S. Acc. Chem. Res. 2014, 47, 1146 – 1154. (14) Puri, M.; Que Jr, L. Acc. Chem. Res. 2015, 48, 2443 – 2452.

15


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(15) Que, Jr., L.; Puri, M. Bull. Jpn. Soc. Coord. Chem. 2016, 67, 10–18. (16) Cho, K. B.; Hirao, H.; Shaik, S.; Nam, W. W. Acc. Chem. Res. 2016, 45, 1197 – 1210. (17) Impeng, S.; Siwaipram, S.; Bureekaew, S.; Probst, M. Phys. Chem. Chem. Phys. 2017, 19, 3782–3791. (18) Buda, F.; Ensing, B.; Gribnau, M. C. M.; Baerends, E. J. Chem. Eur. J. 2001, 7, 2775–2783. (19) Buda, F.; Ensing, B.; Gribnau, M. C. M.; Baerends, E. J. Chem. Eur. J. 2003, 9, 3436–3444. (20) Kumar, D.; Hirao, H.; Que, Jr., L.; Shaik, S. J. Am. Chem. Soc. 2005, 127, 8026. (21) Hirao, H.; Kumar, D.; Que, Jr., L.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 8590. (22) Decker, A.; Clay, M. D.; Solomon, I. J. Inorg. Biochem. 2006, 100, 697–706. (23) Shan, X.; Que, Jr., L. J. Inorg. Biochem. 2006, 100, 421–433. (24) Bernasconi, L.; Louwerse, M. J.; Baerends, E. J. Eur. J. Inorg. Chem. 2007, 2007, 3023–3033. (25) Cho, K.-B.; Shaik, S.; Nam, W. ChemComm 2010, 46, 4511–4513. (26) England, J.; Guo, Y.; Farquhar, E. R.; Young Jr., V. G.; Munck, E.; Que Jr., L. J. Am. Chem. Soc. 2010, 132, 8635–8644. (27) Bernasconi, L.; Baerends, E. J. Eur. J. Inorg. Chem. 2008, 2008, 1672–1681. (28) Shaik, S.; Chen, H.; Janardanan, D. 2011, 3, 19–27. (29) Bernasconi, L.; Baerends, E. J. J. Am. Chem. Soc. 2013, 135, 8857–8867. (30) Kazaryan, A.; Baerends, E. J. ACS Catal. 2015, 5, 14751488. 16


Page 16 of 28

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(31) Xue, G.; Wang, D.; Hont, R. D.; Fiedler, A. T.; Shan, X.; Munck, E.; Que, Jr., L. PNAS 2007, 52, 20713–20718. (32) Belanzoni, P.; Bernasconi, L.; Baerends, E. J. J. Phys. Chem. A 2009, 113, 1192611937. (33) Wang, D.; Farquhar, E. R.; Stubna, A.; M¨ unk, E.; Que, Jr., L. Nature Chemistry 2009, 1, 145–150. (34) Bernasconi, L.; Belanzoni, P.; Baerends, E. J. Phys. Chem. Chem. Phys. 2011, 13, 1527215282. (35) Dietl, N.; Schlangen, M.; Schwarz, H. Angew. Chemie Int. Ed. 2012, 51, 5544–5555. (36) Crabtree, R. H. Chem. Rev. 1995, 95, 987–1007. (37) Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J. J. Am. Chem. Soc. 2010, 132, 10990–10991. (38) Lloret Fillol, J.; Codolà, Z.; Garcia-Bosch, I.; Gómez, L.; Pla, J. J.; Costas, M. Nature Chem. 2011, 3, 807–813. (39) Ertem, M. Z.; Gagliardi, L.; Cramer, C. J. Chem. Sci. 2012, 3, 1293. (40) Hetterscheid, D. G.; Reek, J. N. Angew Chem Int Ed Engl. 2012, 51, 9740–9747. (41) Wasylenko, D. J.; Palmera, R. D.; Berlinguette, C. P. Chem. Comm. 2013, 49, 218–227. (42) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; ˚ Akermark, B. Chem. Rev. 2014, 114, 1186312001. (43) Acuna-Parés, F.; Codolà, Z.; Costas, M.; Luis, J. M.; Lloret Fillol, J. Chem. Eur. J. 2014, 20, 5696 – 5707. (44) Lubitz, W.; Reijerse, E. J.; Messinger, J. Energy Environ. Sci. 2008, 1, 15–31. (45) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072 – 1075. 17


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(46) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. T.; Bernard, S. J. Am. Chem. Soc. 2008, 130, 210 – 217. (47) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. J. Am. Chem. Soc. 2009, 131, 8730–8731. (48) Blakemore, J. D.; Schley, N. D.; Balcells, D.; Hull, J. F.; Olack, G. W.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. J. Am. Chem. Soc. 2010, 132, 16017– 16029. (49) Betley, T. A.; Wu, Q.; Van Voorhis, T.; Nocera, D. Inor. Chem. 2008, 47, 1849–1861. (50) Vallés-Pardo, J. L.; Guijt, M. C.; Iannuzzi, M.; Joya, K. S.; de Groot, H. J. M.; Buda, F. ChemPhysChem 2012, 13, 140–146. (51) Kärkäs, M. D.; ˚ Akermark, B. Dalton Trans. 2016, 45, 1442114461. (52) To, W.-P.; Chow, T. W.-S.; Tse, C.-W.; Guan, X.; Huang, J.-S.; Che, C.-M. Chem. Sci. 2015, 6, 58915903. (53) Michel, C.; Baerends, E. J. Inorg. Chem. 2009, 48, 3628–3638. (54) Louwerse, M. J.; Baerends, E. J. Phys. Chem. Chem. Phys. 2007, 9, 156–166. (55) Decker, A.; Rohde, J. U.; Klinker, E. J.; Wong, S. D.; Que Jr, L.; Solomon, E. I. J. Am. Chem. Soc. 2007, 129, 15983–15996. (56) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110, 10601081. (57) Janardanan, D.; Wang, Y.; Schyman, P.; Que, Jr., L.; Shaik, S. Angew. Chem. Int. Ed. 2010, 49, 3342 3345. (58) Geng, C.; Ye, S.; Neese, F. Angew. Chem. Int. Ed. 2010, 49, 1–6.

18


Page 18 of 28

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(59) Michel, C.; Belanzoni, P.; Gamez, P.; Reedijk, J.; Baerends, E. J. Inorg. Chem. 2009, 48, 11909–11920. (60) Pestovsky, O.; Stoian, S.; Bominaar, E. L.; Shan, X.; M¨ unck, E.; Que, Jr., L.; Bakac, A. Angew. Chem. Int. Ed. 2005, 44, 6871–6874. (61) Ensing, B.; Buda, F.; Blöchl, P.; Baerends, E. J. Angew. Chem. Int. Ed. 2001, 40, 2893–2895. (62) Ensing, B.; Buda, F.; Blöchl, P.; Baerends, E. J. Phys. Chem. Chem. Phys. 2002, 4, 3619–3627. (63) Ensing, B.; Buda, F.; Gribnau, M. C. M.; Baerends, E. J. J. Am. Chem. Soc. 2004, 126, 4355–4365. (64) Bernasconi, L.; Baerends, E. J. Inorg. Chem. 2009, 48, 527–540. (65) Lippert, G.; Hutter, J.; Parrinello, M. Mol. Phys. 1997, 92, 477–487. (66) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Comp. Phys. Comm. 2005, 167, 103–128. (67) Goedecker, S.; Teter, M.; Hutter, J. Phys. Rev. B 1996, 54, 1703–1710. (68) Swart, M.; Ehlers, A. W.; Lammertsma, K. Mol. Phys. 2004, 102, 2467–2474. (69) Swart, M. J. Chem. Theor. Comp. 2008, 4, 2057–2066. (70) Swart, M.; Gruden, M. Acc. Chem. Res. 2016, 49, 2690–2697. (71) Kazaryan, A.; Baerends, E. J. J. Comput. Chem. 2013, 34, 870–878. (72) Marx, D.; Hutter, J. Modern Methods and Algorithms of Quantum Chemistry; John von Neumann Institute for Computing, Julich, NIC Series: Julich, 2000; Vol. 1; pp 301–449. 19


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(73) Humphrey, W.; Dalke, A.; Schulten, K. Journal of Molecular Graphics 1996, 14, 33–38. (74) Kokalj, A. Comp. Mater. Sci. 2003, 28, 155. (75) Bernasconi, L.; Baerends, E. J.; Sprik, M. J. Phys. Chem. B 2006, 110, 11444–11453. (76) Carter, E. A.; Ciccotti, G.; Hynes, J. T.; Kapral, R. Chem. Phys. Lett. 1989, 156, 472. (77) Sprik, M.; Ciccotti, G. J. Chem. Phys. 1998, 109, 7737. (78) Bataineh, H.; Pestovsky, O.; Bakac, A. Inorg. Chem 2016, 55, 6719–6724. (79) Belanzoni, P.; Baerends, E. J. unpublished work. (80) Infante, I.; Visscher, L. J. Comput. Chem. 2004, 25, 386 – 392. (81) Infante, I.; Visscher, L. J. Comput. Chem. 2006, 27, 1156 – 1162. (82) Ma, C.; Piccinin, S.; Fabris, S. ACS Catalysis 2012, 2, 1500–1506. (83) Monti, A.; de Ruiter, J. M.; de Groot, H. J. M.; Buda, F. J. Phys. Chem. C 2016, 120, 2307423082. (84) Ensing, B.; Baerends, E. J. J. Phys. Chem. A 2002, 106, 7902–7910.

20


Page 20 of 28

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

!"#$%&'%()"#*%)"")+,%

!"#$%-'%./.%0123%41*5)"612% %

!

!"#$%7'%$*1"12%"*)284#*%"1%)%819:#2"%519#+;9#% %

!

!"#$%

Catalytic Oxidation of Water with High-Spin Iron(IV) - American

Recommend Documents