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Environmental friendly Fe substitutive of Ru in water oxidation catalysis
CATCOM-03586; No of Pages 4 Catalysis Communications xxx (2013) xxx–xxx
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Environmental friendly Fe substitutive of Ru in water oxidation catalysis Albert Poater ⁎ Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Catalonia, Spain
a r t i c l e
i n f o
Article history: Received 29 April 2013 Received in revised form 3 July 2013 Accepted 12 July 2013 Available online xxxx Keywords: Water oxidation Catalysis Nonprecious metal Ruthenium Iron Oxygen reactivity
a b s t r a c t The present study pretends to unravel by means of DFT calculations how the energy profile change replacing the precious Ru by the nonprecious Fe for the Ru-Hbpp (in,in-{[RuII(trpy)(H2O)]2(μ-bpp)}3+, trpy = 2,2′:6′,2″terpyridine, bpp = bis(2-pyridyl)-3,5-pyrazolate) water oxidation catalyst. This change might remove or at least decrease the potential carcinogenic toxicity due to Ru, however computationally supposes to increase the difficulty of the right assignment of the ground state multiplicity of dinuclear iron complexes when interacting with oxygen. The reaction pathway for Fe-Hbpp arises promising results and now it is a challenge its synthesis. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The prediction of energy demand in 2050 is predicted to range from 30 to 50 TW which means 100% of increase at least with respect to current values [1]. In addition, the generation of CO2 might generate catastrophic consequences environmentally. Thus society has this issue as one of the most challenging to face, searching for renewable energy sources that do not involve the atom of carbon. One appealing alternative source that is clean is the molecular hydrogen. Although its storage and cleavage of the H\H bond have been successfully improved, the source of H2 is still uncertain, knowing that a sustainable source is mandatory. Nowadays the main hydrogen source comes from the natural gas, with the inconvenient of the generation of CO2 during the process. However, the generation of hydrogen is also feasible by water electrolysis. Although this reaction is endothermic, photovoltaic cells can balance favorably this issue [2]. 2H2 O→O2 þ 2H2 þ
2H2 O→O2 þ 4H þ 4e þ
ð1Þ −
−
2H þ 2e →H2 :
ð2Þ ð3Þ
Bearing in mind that water is the most abundant molecule on earth, its splitting in H2 and O2 in the endothermic Eq. (1), thanks to the energy of the sunny light could turn out to be a sustainable source of H2 [3]. Although water is not able to absorb the electromagnetic radiation of ⁎ Tel.: +34 669594001. E-mail address: [email protected].
the sun, molecular assembling reactions for the generation of protons in Eq. (2) together with the necessary energy from the sunny light in Eq. (3) that then would finally generate H2 are feasible by metal catalysts. Similarly the photosynthesis converts water together with CO2 in hydrated carbons and molecular oxygen. Nowadays, biomimetic efforts are carried out for the oxidation of water to molecular oxygen [4] because this is currently the main issue for the photoproduction of H2 from water [5]. Overall, it is necessary to get a catalyst able to face the four-electron reduction of molecular oxygen to water. During the last five years a race to get more efficient catalysts for water oxidation started [6], among them Ru-based catalysts [7], monomeric and dimeric species, where the dominant pathway leading to O\O bond formation is thought to involve nucleophilic attack of a water molecule to the Ru\O moiety of the catalyst. These complexes biomimethize the blue dimmer and the smart chemistry developed by Meyer et al. 30 years ago, dealing with systems such as [Ru2(bpy)4(H2O)2(μ2-O)]4 + [8]. Llobet located in 2004 an active catalyst without the oxo bridge, [Ru2(trpy)2(H2O)2(μ2-bpp)]2+ (trpy = 2,2′:6′,2″-terpyridine, bpp = bis(2-pyridyl)-3,5-pyrazolate) [9], with the suitable architecture thanks to the bridge bpp ligand to fit the water oxidation, increasing its efficiency [10]. Later Bernhard, Crabtree and Nocera extended these studies to other metal transition metals such as iridium [11,12], cobalt [13], or rhodium [14], and Bonchio and Hill showed that polyoxometalates are suitable candidates [15,16], actually getting more robust catalysts [17]. And again Meyer in 2008 demonstrated the catalytic activity of the water oxidation for monomeric complexes, with a feasible characterization of the mechanism [18]. Mechanistic insights for the catalytic four electron reduction of molecular oxygen describe a peroxo intermediate to play the key role, followed by a further reduction to water [19,20]. However, the
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.026
Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
2
A. Poater / Catalysis Communications xxx (2013) xxx–xxx
Fig. 1. {[RuII(trpy)(H2O)][RuII(trpy)(OH)](μ-bpp)]}2+ optimized catalyst.
electron-transfer reduction for dinuclear metal-peroxo complexes has not been described yet by X-ray crystallography, and thus there are open questions about the role of the dinuclear complex [21], despite the smart computational efforts of Baik [22,23] and Cramer [24]. The still scarce number of computational papers is due to the complexity in the evaluation of the right electronic correlation and spin coupling between both metallic centers, in addition to the problems associated with the oxidation states of the metals during the catalytic cycle. Bearing in mind the high loadings of catalyst that are required to carry out the water oxidation catalysis, new nonprecious metallic species appear to be the right solution and it is a challenge to get efficient catalysts [25,26] not only chemically but also economically [27,28]. Llobet et al. recently substituted Ru by Co for the Ru-Hbpp (in,in-{[RuII(trpy)(H2O)]2(μ-bpp)}3 +, trpy = 2,2′:6′,2″-terpyridine, bpp = bis(2-pyridyl)-3,5-pyrazolate) [29,30]. And thus there is an open door to evaluate the former successful Ru-based complexes with nonprecious metals. Although the reliability of computational studies in the field of water oxidation is still unclear, and even there are contradictory results for the same species [23,24], here computationally we report the substitution of Ru by Fe for the Ru-Hbpp catalyst in Fig. 1. DFT calculations have been performed for all the species involved in the water oxidation catalytic cycle for M-Hbpp (M = Ru, Fe). In Fig. 2 we include the overall reaction pathway for Fe, and later in Fig. 4 both Fe and Ru energy profiles for terms of comparison. But first, to test the reproducibility and accuracy of the geometries with the computational method M06L/6-31G(d), DFT calculations on the
Fig. 2. Stationary points of water oxidation catalytic cycle for {[FeII(trpy)(H2O)]2(μ-bpp)}3+(energies in kcal/mol, selected distances in Å). For the sake of clarity non-relevant H atoms have been omitted. The imaginary frequencies characterizing the transition states structures are given in brackets.
Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
A. Poater / Catalysis Communications xxx (2013) xxx–xxx
Fig. 3. Optimized intermediate G (bond distances in Å). For the sake of clarity non-relevant H atoms have been omitted.
crystal of {[RuII(trpy)(H2O)][RuII(trpy)(OH)](μ-bpp)]}2+ in Fig. 1 have been performed [24], and overall the optimized geometry is in an absolutely excellent agreement with the X-ray structure (rmsd 0.047 Å on distances and 0.9° on angles using Eq. (4)) [31]. Second, all bond distances and angles of stationary points of the catalytic cycle range within the expected values for this type of complexes [32]. Third, a comparison with the previous work of Cramer et al. describes a high coincidence of the relative energy values for Ru structures [24], and thus validating the choice of the computational DFT approach [33,34] including the M06L correlation exchange functional.
sn−1 ¼
hX i¼1 →N
i1=2 2 ðCV−EVÞ =ðN−1Þ :
ð4Þ
3
Taking the labeling scheme of the previous study with the Ru-Hbpp catalyst [24], we envisaged calculations to get the key structures due to the interaction of Fe-Hbpp with water, in Fig. 2. With an overall +3 charge, bearing in mind that species A contains two FeIV\O functionalities the multiplicity ground state. The relative energy of species A modifying its multiplicity from singlet to nonucaplet ranges only 1.5 kcal/mol, being the open-shell singlet just 0.1 and 0.8 kcal/mol more stable than the triplet and nonucaplet multiplicity states, respectively. Thus, in comparison with Ru that displays two metaloxo moieties in triplet state, coupled, and thus quintuplet overall, the Fe-based system does not display a clear electronic conformation. Anyway, since B all species in Fig. 2 display an open-shell singlet as a multiplicity ground state, except for the intermediate F that displays a septuplet multiplicity ground state. However, for F the window of energies from the open-shell singlet to the undecaplet spans a range of less than 6 kcal/mol. The same argument is valid for the relative stability of higher multiplicities with respect to the singlet for the other complexes. Take for instance, for B the triplet, quintuplet and septuplet are less than 2 kcal/mol higher in energy. Moving to the mechanistic insights of the reaction pathway, bearing in mind that water is a potential nucleophilic agent, thanks to its facility to ionize and behave as a combination of hydroxyls plus protons, the attack of a water molecule to species A can take place in two alternative ways to get species F. First, through a concerted process a water molecule can split and the hydroxyl moiety interact with one oxygen to create the \OOH functionality in one iron center, apart from the protonation of the other Fe\O functionality. This pathway is described by the expensive transition state B that is 39.3 kcal/mol above in energy with respect to A. Second, the entering water molecule can bond directly to an iron center, but first it is necessary that both oxygen atoms of each Fe\O moiety collapse, describing a simple O\O bond (1.39 Å) in intermediate D, after overcoming a cheap transition state C, which is only 16.6 kcal/mol higher in energy than A. From D, the water insertion in one metal center costs 24.1 kcal/mol due to the Fe\O bond
Fig. 4. Water oxidation catalytic cycle for M-Hbpp (M = Ru, Fe), energies in kcal/mol.
Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
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A. Poater / Catalysis Communications xxx (2013) xxx–xxx
cleavage where the water ligand links. Overall, this alternative mechanism A–C–D–E–F requires to overcome an upper barrier placed 32.6 kcal/mol above in energy than A. Thus, the mechanism described by species B, i.e. following the path A–B–F, has an upper barrier placed 6.7 kcal/mol above with respect to the energy described by the transition state E, thus the existence of B is ruled out. From species F previous studies have shown that the generation of O2 including another water molecule is a cheaper and thus less important step [24], confirming that the rate-determining step that defines the free energy of activation is placed before intermediate F. From species F, an entering water molecule allows the bis-aquo complex G, displayed in Fig. 3, releasing a free O2 molecule. Bearing in mind that the experimental value of free energy of activation in aqueous solution for Ru is 23.1 kcal/mol [24], DFT calculations for Ru homologous complexes, displayed in Fig. 4, are in good agreement (26.0 kcal/mol), pointing out that previous computational studies suggested that the addition of water molecules might decrease the computational value [24,35]. The direct comparison of the reaction pathway for Ru and Fe-based complexes in Fig. 4, reveals quite similar behavior, but more disfavored barriers described for iron complexes. Moving to the structural characterization, the Fe⋯Fe distances are lower, which do not facilitate the water insertion. For the transition state E, the Ru⋯Ru distance is 4.26 Å to compare with 4.17 Å for the iron system. The same is valid and a bit more evident for the previous intermediate D that shows metal⋯metal distances of 4.02 and 3.82 Å for Ru and Fe, respectively. Indeed, at the former intermediate A again this distance was 0.15 Å longer for Ru. Differently to Ru, for Fe the transition state E does not stabilize when including the interaction of a second water molecule, by means of a hydrogen bond to the Ru–O–O moiety; thus Fe suffering a destabilization of 1.4 kcal/mol, whereas for Ru the favorable interaction decreases the barrier by 0.4 kcal/mol. The inclusion of more water molecules might refine these data, however the existing good agreement between experimental and in silico energy activation precludes the necessity of further studies. To sum up, environmental friendly chemistry is one of the main concerns and here the substitution of the precious Ru by the nonprecious Fe for the Ru-Hbpp water oxidation catalyst by DFT calculations gives promising results. Despite the complexity of working with Fe with respect to Ru, and the upper barrier of the reaction pathway for Fe is higher in energy, the low cost and non toxicity of the Fe-Hbpp makes the challenging synthesis and application of this prototype catalyst be more feasible.
Acknowledgments AP is grateful to the European Commission (CIG09-GA-2011-293900), Spanish MICINN (Ramón y Cajal contract RYC-2009-05226), and Generalitat de Catalunya (2012BE100824), and to Prof. A. Llobet and Dr. M. Swart for helpful comments.
Appendix A. Supplementary data Computational details; Cartesian coordinates and absolute SCF energies of all computed species. Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.catcom.2013.07.026.
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Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Environmental friendly Fe substitutive of Ru in water oxidation catalysis Albert Poater ⁎ Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Catalonia, Spain
a r t i c l e
i n f o
Article history: Received 29 April 2013 Received in revised form 3 July 2013 Accepted 12 July 2013 Available online xxxx Keywords: Water oxidation Catalysis Nonprecious metal Ruthenium Iron Oxygen reactivity
a b s t r a c t The present study pretends to unravel by means of DFT calculations how the energy profile change replacing the precious Ru by the nonprecious Fe for the Ru-Hbpp (in,in-{[RuII(trpy)(H2O)]2(μ-bpp)}3+, trpy = 2,2′:6′,2″terpyridine, bpp = bis(2-pyridyl)-3,5-pyrazolate) water oxidation catalyst. This change might remove or at least decrease the potential carcinogenic toxicity due to Ru, however computationally supposes to increase the difficulty of the right assignment of the ground state multiplicity of dinuclear iron complexes when interacting with oxygen. The reaction pathway for Fe-Hbpp arises promising results and now it is a challenge its synthesis. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The prediction of energy demand in 2050 is predicted to range from 30 to 50 TW which means 100% of increase at least with respect to current values [1]. In addition, the generation of CO2 might generate catastrophic consequences environmentally. Thus society has this issue as one of the most challenging to face, searching for renewable energy sources that do not involve the atom of carbon. One appealing alternative source that is clean is the molecular hydrogen. Although its storage and cleavage of the H\H bond have been successfully improved, the source of H2 is still uncertain, knowing that a sustainable source is mandatory. Nowadays the main hydrogen source comes from the natural gas, with the inconvenient of the generation of CO2 during the process. However, the generation of hydrogen is also feasible by water electrolysis. Although this reaction is endothermic, photovoltaic cells can balance favorably this issue [2]. 2H2 O→O2 þ 2H2 þ
2H2 O→O2 þ 4H þ 4e þ
ð1Þ −
−
2H þ 2e →H2 :
ð2Þ ð3Þ
Bearing in mind that water is the most abundant molecule on earth, its splitting in H2 and O2 in the endothermic Eq. (1), thanks to the energy of the sunny light could turn out to be a sustainable source of H2 [3]. Although water is not able to absorb the electromagnetic radiation of ⁎ Tel.: +34 669594001. E-mail address: [email protected].
the sun, molecular assembling reactions for the generation of protons in Eq. (2) together with the necessary energy from the sunny light in Eq. (3) that then would finally generate H2 are feasible by metal catalysts. Similarly the photosynthesis converts water together with CO2 in hydrated carbons and molecular oxygen. Nowadays, biomimetic efforts are carried out for the oxidation of water to molecular oxygen [4] because this is currently the main issue for the photoproduction of H2 from water [5]. Overall, it is necessary to get a catalyst able to face the four-electron reduction of molecular oxygen to water. During the last five years a race to get more efficient catalysts for water oxidation started [6], among them Ru-based catalysts [7], monomeric and dimeric species, where the dominant pathway leading to O\O bond formation is thought to involve nucleophilic attack of a water molecule to the Ru\O moiety of the catalyst. These complexes biomimethize the blue dimmer and the smart chemistry developed by Meyer et al. 30 years ago, dealing with systems such as [Ru2(bpy)4(H2O)2(μ2-O)]4 + [8]. Llobet located in 2004 an active catalyst without the oxo bridge, [Ru2(trpy)2(H2O)2(μ2-bpp)]2+ (trpy = 2,2′:6′,2″-terpyridine, bpp = bis(2-pyridyl)-3,5-pyrazolate) [9], with the suitable architecture thanks to the bridge bpp ligand to fit the water oxidation, increasing its efficiency [10]. Later Bernhard, Crabtree and Nocera extended these studies to other metal transition metals such as iridium [11,12], cobalt [13], or rhodium [14], and Bonchio and Hill showed that polyoxometalates are suitable candidates [15,16], actually getting more robust catalysts [17]. And again Meyer in 2008 demonstrated the catalytic activity of the water oxidation for monomeric complexes, with a feasible characterization of the mechanism [18]. Mechanistic insights for the catalytic four electron reduction of molecular oxygen describe a peroxo intermediate to play the key role, followed by a further reduction to water [19,20]. However, the
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.026
Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
2
A. Poater / Catalysis Communications xxx (2013) xxx–xxx
Fig. 1. {[RuII(trpy)(H2O)][RuII(trpy)(OH)](μ-bpp)]}2+ optimized catalyst.
electron-transfer reduction for dinuclear metal-peroxo complexes has not been described yet by X-ray crystallography, and thus there are open questions about the role of the dinuclear complex [21], despite the smart computational efforts of Baik [22,23] and Cramer [24]. The still scarce number of computational papers is due to the complexity in the evaluation of the right electronic correlation and spin coupling between both metallic centers, in addition to the problems associated with the oxidation states of the metals during the catalytic cycle. Bearing in mind the high loadings of catalyst that are required to carry out the water oxidation catalysis, new nonprecious metallic species appear to be the right solution and it is a challenge to get efficient catalysts [25,26] not only chemically but also economically [27,28]. Llobet et al. recently substituted Ru by Co for the Ru-Hbpp (in,in-{[RuII(trpy)(H2O)]2(μ-bpp)}3 +, trpy = 2,2′:6′,2″-terpyridine, bpp = bis(2-pyridyl)-3,5-pyrazolate) [29,30]. And thus there is an open door to evaluate the former successful Ru-based complexes with nonprecious metals. Although the reliability of computational studies in the field of water oxidation is still unclear, and even there are contradictory results for the same species [23,24], here computationally we report the substitution of Ru by Fe for the Ru-Hbpp catalyst in Fig. 1. DFT calculations have been performed for all the species involved in the water oxidation catalytic cycle for M-Hbpp (M = Ru, Fe). In Fig. 2 we include the overall reaction pathway for Fe, and later in Fig. 4 both Fe and Ru energy profiles for terms of comparison. But first, to test the reproducibility and accuracy of the geometries with the computational method M06L/6-31G(d), DFT calculations on the
Fig. 2. Stationary points of water oxidation catalytic cycle for {[FeII(trpy)(H2O)]2(μ-bpp)}3+(energies in kcal/mol, selected distances in Å). For the sake of clarity non-relevant H atoms have been omitted. The imaginary frequencies characterizing the transition states structures are given in brackets.
Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
A. Poater / Catalysis Communications xxx (2013) xxx–xxx
Fig. 3. Optimized intermediate G (bond distances in Å). For the sake of clarity non-relevant H atoms have been omitted.
crystal of {[RuII(trpy)(H2O)][RuII(trpy)(OH)](μ-bpp)]}2+ in Fig. 1 have been performed [24], and overall the optimized geometry is in an absolutely excellent agreement with the X-ray structure (rmsd 0.047 Å on distances and 0.9° on angles using Eq. (4)) [31]. Second, all bond distances and angles of stationary points of the catalytic cycle range within the expected values for this type of complexes [32]. Third, a comparison with the previous work of Cramer et al. describes a high coincidence of the relative energy values for Ru structures [24], and thus validating the choice of the computational DFT approach [33,34] including the M06L correlation exchange functional.
sn−1 ¼
hX i¼1 →N
i1=2 2 ðCV−EVÞ =ðN−1Þ :
ð4Þ
3
Taking the labeling scheme of the previous study with the Ru-Hbpp catalyst [24], we envisaged calculations to get the key structures due to the interaction of Fe-Hbpp with water, in Fig. 2. With an overall +3 charge, bearing in mind that species A contains two FeIV\O functionalities the multiplicity ground state. The relative energy of species A modifying its multiplicity from singlet to nonucaplet ranges only 1.5 kcal/mol, being the open-shell singlet just 0.1 and 0.8 kcal/mol more stable than the triplet and nonucaplet multiplicity states, respectively. Thus, in comparison with Ru that displays two metaloxo moieties in triplet state, coupled, and thus quintuplet overall, the Fe-based system does not display a clear electronic conformation. Anyway, since B all species in Fig. 2 display an open-shell singlet as a multiplicity ground state, except for the intermediate F that displays a septuplet multiplicity ground state. However, for F the window of energies from the open-shell singlet to the undecaplet spans a range of less than 6 kcal/mol. The same argument is valid for the relative stability of higher multiplicities with respect to the singlet for the other complexes. Take for instance, for B the triplet, quintuplet and septuplet are less than 2 kcal/mol higher in energy. Moving to the mechanistic insights of the reaction pathway, bearing in mind that water is a potential nucleophilic agent, thanks to its facility to ionize and behave as a combination of hydroxyls plus protons, the attack of a water molecule to species A can take place in two alternative ways to get species F. First, through a concerted process a water molecule can split and the hydroxyl moiety interact with one oxygen to create the \OOH functionality in one iron center, apart from the protonation of the other Fe\O functionality. This pathway is described by the expensive transition state B that is 39.3 kcal/mol above in energy with respect to A. Second, the entering water molecule can bond directly to an iron center, but first it is necessary that both oxygen atoms of each Fe\O moiety collapse, describing a simple O\O bond (1.39 Å) in intermediate D, after overcoming a cheap transition state C, which is only 16.6 kcal/mol higher in energy than A. From D, the water insertion in one metal center costs 24.1 kcal/mol due to the Fe\O bond
Fig. 4. Water oxidation catalytic cycle for M-Hbpp (M = Ru, Fe), energies in kcal/mol.
Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026
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A. Poater / Catalysis Communications xxx (2013) xxx–xxx
cleavage where the water ligand links. Overall, this alternative mechanism A–C–D–E–F requires to overcome an upper barrier placed 32.6 kcal/mol above in energy than A. Thus, the mechanism described by species B, i.e. following the path A–B–F, has an upper barrier placed 6.7 kcal/mol above with respect to the energy described by the transition state E, thus the existence of B is ruled out. From species F previous studies have shown that the generation of O2 including another water molecule is a cheaper and thus less important step [24], confirming that the rate-determining step that defines the free energy of activation is placed before intermediate F. From species F, an entering water molecule allows the bis-aquo complex G, displayed in Fig. 3, releasing a free O2 molecule. Bearing in mind that the experimental value of free energy of activation in aqueous solution for Ru is 23.1 kcal/mol [24], DFT calculations for Ru homologous complexes, displayed in Fig. 4, are in good agreement (26.0 kcal/mol), pointing out that previous computational studies suggested that the addition of water molecules might decrease the computational value [24,35]. The direct comparison of the reaction pathway for Ru and Fe-based complexes in Fig. 4, reveals quite similar behavior, but more disfavored barriers described for iron complexes. Moving to the structural characterization, the Fe⋯Fe distances are lower, which do not facilitate the water insertion. For the transition state E, the Ru⋯Ru distance is 4.26 Å to compare with 4.17 Å for the iron system. The same is valid and a bit more evident for the previous intermediate D that shows metal⋯metal distances of 4.02 and 3.82 Å for Ru and Fe, respectively. Indeed, at the former intermediate A again this distance was 0.15 Å longer for Ru. Differently to Ru, for Fe the transition state E does not stabilize when including the interaction of a second water molecule, by means of a hydrogen bond to the Ru–O–O moiety; thus Fe suffering a destabilization of 1.4 kcal/mol, whereas for Ru the favorable interaction decreases the barrier by 0.4 kcal/mol. The inclusion of more water molecules might refine these data, however the existing good agreement between experimental and in silico energy activation precludes the necessity of further studies. To sum up, environmental friendly chemistry is one of the main concerns and here the substitution of the precious Ru by the nonprecious Fe for the Ru-Hbpp water oxidation catalyst by DFT calculations gives promising results. Despite the complexity of working with Fe with respect to Ru, and the upper barrier of the reaction pathway for Fe is higher in energy, the low cost and non toxicity of the Fe-Hbpp makes the challenging synthesis and application of this prototype catalyst be more feasible.
Acknowledgments AP is grateful to the European Commission (CIG09-GA-2011-293900), Spanish MICINN (Ramón y Cajal contract RYC-2009-05226), and Generalitat de Catalunya (2012BE100824), and to Prof. A. Llobet and Dr. M. Swart for helpful comments.
Appendix A. Supplementary data Computational details; Cartesian coordinates and absolute SCF energies of all computed species. Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.catcom.2013.07.026.
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Please cite this article as: A. Poater, Environmental friendly Fe substitutive of Ru in water oxidation catalysis, Catalysis Communications (2013), http://dx.doi.org/10.1016/j.catcom.2013.07.026