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Mechanism for the Light-Induced O2 Evolution from H2O Promoted by Ru(II) PNN Complex: A DFT Study Yue Chen and Wei-Hai Fang* College of Chemistry, Beijing Normal UniVersity, Beijing 100875, China ReceiVed: July 14, 2010; ReVised Manuscript ReceiVed: August 18, 2010
The density functional theory (DFT) method was used to explore the light-induced O2 formation from H2O promoted by Ru(II) PNN complex in the present work. The elimination of H2O2 was found to be highly endothermic, which is not in competition with the H2O elimination and hydrogen transfer. The calculated results reported here do not support the mechanism proposed in a recent experiment, where H2O2 was suggested as an important intermediate for formation of O2. We proposed a new mechanism for formation of the triplet O2 molecule, which contains the two steps of the concerted hydrogen transfer and dehydration. The lightinduced O2 evolution from water promoted by the Ru(II) complex was found to be a nonadiabatic process. The O-O bond is formed along the T1 pathway as a result of the efficient S1 f T1 intersystem crossing. All experimental findings on the light-induced O2 evolution can be explained by the mechanism proposed in the present work. Introduction Light-driven production of molecular hydrogen and molecular oxygen from water is a promising strategy to solve the growing problems of global energy consumption, pollution, and climate change,1 since water and solar energy are abundant, cheap, sustainable, and environmentally friendly resources. The direct conversion of sunlight into chemical energy constitutes breakthrough technology and is one of the most important challenges that scientists need to address within the next few decades. Actually, dioxygen formation from water is one of the most fundamental reactions for the evolution of life on the earth and understanding of these fundamental processes has been a highpriority scientific goal for many decades.1b The active site of the oxygen-evolving complex of photosystem II converts water into dioxygen by using sunlight, which has been the source of inspiration for the development of transition metal-based complexes to mimic water oxidation by the natural enzyme.1,2 Multinuclear ruthenium complexes have received a lot of attention since the first functional Ru-based water oxidation catalyst, the “blue dimer”, was reported by Meyer and coworkers.3 Milstein and co-workers recently reported a consecutive thermal H2 and light-induced O2 evolution from water promoted by a Ru(II) hydride-hydroxo complex with PNN pincer ligand without the use of sacrificial reductants or oxidants.4 The complex A (Scheme 1) has been found to be powerful for the coupling of alcohols to form esters with the liberation of H2 and for the dehydrogenative coupling of alcohols with amines to produce amides.5 Discovery of the complex A for the sunlight-driven splitting of water into O2 and H2 has attracted extensive attention6-10 for a number of reasons.6 First, the O2 and H2 molecules were found to form in one catalytic cycle, which consists of the heat- and light-driven steps. Second, the water oxidation reaction does not involve high-valent oxo species (RudO), which are generally very reactive and can lead to catalyst degradation. Third, the reductive elimination of * To whom correspondence should be addressed. E-mail: fangwh@ bnu.edu.cn.
SCHEME 1: Proposed Pathways for a Consecutive Thermal H2 and Light-Induced O2 Evolution from Water Promoted by a Ru(II) Hydride-Hydroxo Complex
hydrogen peroxide is a remarkable new fundamental step in water oxidation catalysis. As highlighted by Reek and coworkers, these findings could well mark the onset of a new paradigm in the area of water splitting.6 The heat-driven reaction (A f B) and the subsequent process (D f A) have been studied by Yoshizawa et al. using density functional theory (DFT).9 They found that the metal center and the PNN ligand function jointly in H2 production and subsequent H2O decomposition.9 Photolysis of isotopically labeled complex B and the subsequent IR, NMR, and GC-MS experiments provided strong evidence that the O2 formation is an intramolecular process and involves a single metal center.4 The O2 formation mechanism involving the OH radical was discarded on the basis of several experiments with radical traps and the use of the enzyme catalysis. Hydrogen peroxide was suggested as the key intermediate for the O2 formation, but the H2O2 molecule was not detected experimentally.4 Hall and coworkers10 attempted to study the light-driven O2 evolution step by the DFT and TD-DFT calculations, but it is still unclear how the H2O2 molecule is formed. In the present work, we performed DFT calculations to explore mechanistic details of the light-
10.1021/jp1065105 2010 American Chemical Society Published on Web 08/30/2010
Light-Induced O2 Evolution from H2O driven O2 evolution (B f C), which plays a crucial role in the mechanism proposed experimentally.4 It was found that the elimination of H2O2 occurs with little possibility from the B complex. A new mechanism was determined for the O2 release, where the two oxygen atoms come from a single metal center. All experimental findings on the light-induced O2 evolution from the B complex can be explained by the new mechanism reported in the present work. More importantly, the light-induced O2 evolution from water promoted by the Ru(II) complex was found to be a nonadiabatic process and the O2 formation occurs along a triplet pathway as a result of the efficient intersystem crossing (ISC). Computational Details To examine the effect of different density functionals on the calculated energy barriers in the Ru complexes, nine of the most popular density functionals were selected for evaluation.10 The PBE functional was found to have a good performance for the relative energies. Thus, all the calculations were carried out here by using the PBE hybrid functional implemented in the Gaussian 03 program package.11 The SDD basis set was used for the Ru atom and cc-pVDZ basis set for all other atoms,12,13 which are referred to as Gen hereafter. For computational tractability, the ethyl and tert-butyl of PNN ligand were replaced by a methyl group. The same model was used in the DFT calculations by Yoshizawa and co-workers.9 The geometric structures of all species were optimized at the PBE/Gen level and were confirmed to be minimum or the first-order saddle point by the calculated vibrational frequencies. On the basis of the PBE/ Gen optimized structures for all stationary points, the effect of solvent was taken into account by using the polarizable continuum model (PCM) for water (ε ) 78.4) on the same level of theory (PBE/Gen-PCM). Results and Discussion Irradiation at λ >320 nm causes primarily the complex B in an excited singlet state (S1). The direct conversion of B into C involves a cleavage of two Ru-O bonds and formation of the O-O bond simultaneously. In addition, the reactant B in the S1 state correlates adiabatically with the product C in an excited state and the reaction in the S1 state is of high endothermic character. It is evident that the direct conversion of B into C in the S1 state is not in competition with internal conversion (IC) to the ground state (S0) or intersystem crossing to the lowest triplet state (T1). Therefore, we attempted to determine the concerted pathway for the elimination of H2O2 from the B complex in the S0 and T1 states (referred to as BS0 and BT1 hereafter). The BS0 and BT1 complexes were optimized at the level of PBE/Gen and the resulting BS0 and BT1 structures are schematically shown in Figure 1, along with the selected bond parameters. The initial excitation was found to be of Ru(II)to-pyridine charge transfer character. But the two singly occupied electrons are mainly distributed in the Ru(II) atom after relaxation to the BT1 equilibrium structure. The calculated natural orbital populations and atomic spin densities for the BT1 complex clearly show that the electronic configuration at the BT1 equilibrium structure can be approximately represented as d2xy d2yz d1xz dz12, where the x and z Cartesian axes are respectively defined as the two N-Ru bond directions with the y axis perpendicular to the N-Ru-N plane. As a result of repulsion interaction of the d1xz and d1z2 electrons with the lone-pair electrons of ligands, the Ru-N bonds are significantly weakened in the
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10335 BT1 structure. This is the reason why the Ru-N distances are longer in BT1 than those in BS0. On the basis of the optimized structures and calculated energies for B, C, and H2O2 in the S0 state, the concerted elimination of H2O2 is estimated to be endothermic by 100.4 kcal · mol-1 with vibrational zero-point energy correction. The potential energy surfaces were scanned as a function of the O-O distance with other geometric parameters optimized fully, which is plotted in Figure 2. The relative energies were increased monotonically with a decrease of the O-O distance in the S0 and T1 states and the H2O2 molecule was not observed to be formed. On the basis of the scanned PES, a number of initial structures were selected for full optimization of a transition state for the concerted H2O2 elimination, but all attempts failed finally. No transition state was found for the elimination of H2O2 in previous theoretical studies,9,10 and the free energy barrier was estimated to be larger than 100.0 kcal · mol-1 for the H2O2 elimination.10 As pointed out by Reek and co-worker,6 the reductive elimination of H2O2 is a highly endothermic reaction and the subsequent disproportionation of H2O2 is obviously exothermic. As a result, the excess solar energy is required for the synthesis of H2O2 in the proposed mechanism by Milstein and co-workers.4 The excess energy is subsequently lost as heat upon disproportionation of H2O2, therefore rendering a lower overall efficiency. Figure 2 also shows the scanned potential energy surface as a function of the Ru-O1 distance (O1 is the oxygen atom cis to CO ligand) in the S0 state. With the increase of the Ru-O1 distance, a H2O molecule was gradually formed with a low barrier. The transition states for intramolecular dehydration in the S0 and T1 states were optimized and confirmed to be the first-order saddle points to connect B in the reactant side and E as the products. As shown in Figure 3, the energy barriers to the dehydration are 32.2 and 12.5 kcal · mol-1 in S0 and T1 states, respectively. They become 32.7 and 15.4 kcal · mol-1 with solvent effect considered. The scanned potential energy surface as a function of the Ru-O2 distance (O2 is the oxygen trans to CO ligand) indicates that the H2O elimination takes place between the hydroxyl (O2H) and the methylene group at the phosphine side arm of the PNN ligand. This reaction has been reported by Yoshizawa et al.9 and Hall et al.10 and the energy barrier was determined to be smaller than 40.0 kcal · mol-1 in the S0 state. The H2O elimination is endothermic by 28.7 kcal · mol-1 with a barrier of 32.4 kcal · mol-1 in the ground state, while the concerted H2O2 elimination is endothermic by 100.4 kcal · mol-1 in the S0 state and the free energy barrier to the elimination of H2O2 was estimated to be larger than 100 kcal · mol-1 in the previous study.10 The present and previous calculations clearly show that the concerted H2O2 elimination is not in competition with a direct intramolecular dehydration from the B complex. The calculated results do not support the mechanism proposed experimentally, where H2O2 was suggested as an intermediate in a consecutive thermal H2 and light-induced O2 evolution from water promoted by the Ru(II) hydride-hydroxo complex.4 To explain the experimental findings, here we designed a new pathway for formation of the triplet O2 molecule. It has been recognized that formation of a dimer might provide the mechanistic basis for water oxidation by the Ru complex.14 The dimerization process of the B complex was predicted to be exothermic by 12.3 kcal · mol-1 in the ground state by the PBE/ Gen calculation, which indicates that the B dimer, referred to as F hereafter, can be formed easily in the aqueous solution of the B complex. As pointed out before, the reactions on the S1
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Chen and Fang
Figure 1. The optimized stationary structures in the S0 and T1 states, along with the selected bond lengths (Å). The CH3 groups connected P and N atoms and the H atoms of the methylene are not shown for clarity.
Figure 2. Potential energy surfaces scanned as a function of the O-O distance (the S0 and T1 states) and the Ru-O distance (the S0 state).
state proceed with little possibility. The formation of the O-O bond from the F complex takes place either in the S0 state or in
Figure 3. Potential energy profiles of dehydration from the complex B in the S0 and T1 states. The relative energies are given in kcal · mol-1 with zero-point energy correction.
the T1 state. The ISC time constant from S1 to T1 was determined to be about 55 fs for the [Ru(bpy)3]2+ complex, due to strong spin-orbit interaction from the heavy Ru atom.15-17 This gives
Light-Induced O2 Evolution from H2O
Figure 4. The relative energies of the stationary structures on the T1 pathway were given with the S0 vibrational ground state of the B complex as zero-point of energy.
us reason to expect that the S1 f T1 transition is very efficient for the F complex. More importantly, the O2 molecule has the triplet ground state. If reactions start from the S0 state, the S0 and T1 states intersect a few times on the pathway to the triplet O2 molecule.10 Because of these spin-forbidden ISC processes, the rate and efficiency for formation of the O2 molecule are reduced substantially. However, the reaction starts from the T1 state only involve spin-conservation processes to the triplet O2 molecule. Therefore, we mainly focus on the spin-conservation triplet pathway for formation of the O2 molecule in the following. The proposed pathways for formation of the triplet O2 molecule are summarized in Figure 4, along with the relative energies of the related stationary structures (FT1, TSFGT1, GT1, TSGHT1, and HT1) plotted in Figure 1. More details on bond parameters and energies for these structures can be found in the Supporting Information. Upon photoexcitation of the F complex, the vertical transition is localized on one B monomer. The FT1 complex can be considered as a combination of BS0 and BT1. The F complex in the T1 state has its energy of 25.1 kcal · mol-1 with respect to the vibrational zero-level of the B complex in the ground state. The first step of hydrogen transfer is accompanied by dehydration, giving rise to the intermediate G in the T1 state (GT1). A transition state of TSFGT1 was found on the pathway from FT1 to GT1. The energy barrier on the first step was calculated to be 11.5 kcal · mol-1 at the level of PBE/ Gen. The PBE/Gen-PCM calculations predict the barrier to be 11.8 kcal · mol-1 on the basis of the PBE/Gen optimized FT1 and TSFGT1 structures.
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10337 A transition state, referred to as TSGHT1 hereafter, was optimized and confirmed to be the first-order saddle point on the second step from GT1 to HT1. In addition to a hydrogen transfer and dehydration that occur in the second step, the O-O bond is nearly formed in this step, which can be seen from the optimized structures of GT1, TSGHT1, and HT1, where the O-O distance is gradually decreased from 2.843 to 1.648 and to 1.481 Å. It is evident that the second step involves a concerted hydrogen transfer, dehydration, and the O-O bond formation. This concerted process has a barrier height of 48.8 kcal · mol-1 in the gas phase and becomes 46.7 kcal · mol-1 in water solution. The present calculations clearly show that solvent effect has a little influence on barrier heights of the proposed pathways. It should be pointed out that the concerted second-step process is the rate-determining step for formation of the triplet O2 molecule. As discussed before, the O-O bond is nearly formed in the HT1 complex. It can be expected that the subsequent release of the triplet O2 molecule takes place very easily under the experimental conditions.4 Overall, the mechanism proposed here for formation of the triplet O2 molecule can be summarized in Scheme 2. In the designed pathway, the H2O molecule produced in the first step does not departure and instead acts as a part of the system, assisting subsequent reactions. It was found that the energy barrier was reduced by 2.5 kcal · mol-1 due to one H2O molecule H-bonded to the O atom in the second step. Test DFT calculations were performed with more H2O molecules in the F complex. The calculated results indicate that the relative energies of the related stationary structures (FT1, TSFGT1, GT1, TSGHT1, and HT1) are decreased by a few kcal · mol-1, but barrier heights are nearly unchanged. Overall, the reactivity is not significantly influenced with more water molecules included in the F complexes. It should be pointed out that the rate-determining step for formation of the O2 molecule has to overcome a high barrier on the triplet pathway, which indicates that the O2 release proceeds with considerable difficulty. Irradiation of the aqueous solution of complex B over two days resulted in a color change of the solution from green to greenish-yellow.4 The liberated gas was identified as dioxygen by GC-mass spectrometry and the yield was detected to be about 0.23. The high energy barrier for formation of the triplet O2 molecule through the newly proposed mechanism in the present study is consistent with the experimental observation.4 To verify that the O-O bond formation is an intramolecular process, the isotopically mixed-labeled dihydroxo complex B-18O16O was also prepared.4 Upon photolysis of B-18O16O, 34O2 was formed predominantly with only small amounts of 32O2 and
SCHEME 2: The Mechanism for Formation of the Triplet O2 Molecule
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O2 observed. In addition, photolysis of equimolar amounts of complexes B-18O18O and B-16O16O results in formation of 36O2 and 32O2 with only a small amount of 34O2. The above experimental results unambiguously show that the O-O bond formation is the intramolecular process.4 As shown in Figure 3, the two oxygen atoms of the O2 molecule come from a single metal center in the proposed mechanism, which is in good agreement with the isotopically labeled experimental findings. In addition, the 36O2 molecule was observed as the major dioxygen product4 upon irradiation of B-18O18O in H216O and no exchange was found to take place between B-18O18O and H216O, indicating that there is no substantial Ru-OH dissociation. The proposed mechanism here only involves hydrogen transfer and dehydration. Although all experimental findings4 on the light-induced O2 evolution from the aqueous B complex can be explained by the mechanism proposed in the present work, there might exist other mechanisms responsible for the experimental findings. We hope that the present calculation can stimulate further experimental and theoretical studies to explore the mechanism of the light-induced O2 evolution from water promoted by Ru(II) complexes. In summary, the elimination of H2O2 is highly endothermic and has to overcome a high energy barrier on the pathway. The H2O2 elimination is not in competition with the H2O elimination and hydrogen transfer. The PBE/Gen calculated results reported here do not support the mechanism proposed in a recent experiment, where H2O2 was suggested as an important intermediate for formation of O2. We proposed a new mechanism for formation of the triplet O2 molecule, which contains the two steps of the concerted hydrogen transfer and dehydration. The two oxygen atoms of the formed O2 molecule come from a single metal center in the proposed mechanism, which is in good agreement with the isotopically labeled experimental findings. More importantly, the light-induced O2 evolution from water promoted by the Ru(II) complex was found to be a nonadiabatic process and the O2 formation occurs along a triplet pathway as a result of the efficient intersystem crossing. All experimental findings on the light-induced O2 evolution from the B complex can be explained by the mechanism proposed in the present work. However, it is possible to propose other mechanisms responsible for the experimental findings. Thus, further experimental and theoretical studies are necessary to explore the light-induced O2 evolution from water promoted by the Ru(II) complex. In addition, the mechanism for the O2 formation can be optimized by using Ru complexes with different ligands (PNP and SNN) and could be extended to other
Chen and Fang metal complexes (Fe and Co). Theoretical calculations of these processes are still in progress and will be reported in due time. Acknowledgment. This work was supported by grants from the NSFC (Grant No. 20720102038) and from the Major State Basic Research Development Programs (Grant No. 2011CB8085003). Supporting Information Available: Cartesian coordinates and absolute energies of the stationary structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Dau, H.; Zaharieva, I. Acc. Chem. Res. 2009, 42, 1861–1870. (b) Siegbahn, P. E. M. Acc. Chem. Res. 2009, 42, 1871–1880. (c) Barber, J. Chem. Soc. ReV. 2009, 38, 185–196. (d) Tinker, L. L.; McDaniel, N. D.; Bernhard, S. J. Mater. Chem. 2009, 19, 3328–3337. (e) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26–58. (f) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729–15735. (2) (a) Yano, J.; Kern, J.; Sauer, K.; Latimer, M. J.; Pushkar, Y.; Biesiadka, J.; Loll, B.; Saenger, W.; Messinger, J.; Zouni, A.; Yachandra, V. K. Science 2006, 314, 821. (b) Meyer, T. J.; Huynh, M. H. V.; Thorp, H. H. Angew. Chem., Int. Ed. 2007, 46, 5284. (c) Dasgupta, J.; Ananyev, G. M.; Dismukes, G. C. Coord. Chem. ReV. 2008, 252, 347. (d) Mullins, C. S.; Pecoraro, V. L. Coord. Chem. ReV. 2008, 252, 416. (3) (a) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4029. (b) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (c) Sala, X.; Romero, I.; Rodrguez, M.; Escriche, L.; Llobet, A. Angew. Chem., Int. Ed. 2009, 48, 2842. (4) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74–77. (5) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840–10841. Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790–792. (6) Hetterscheid, D. G. H.; van der Vlugt, J. I.; de Bruin, B.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8178–8181. (7) Kubas, G. J. J. Organomet. Chem. 2009, 694, 2648–2653. (8) Eisenberg, R. Science 2009, 324, 44–45. (9) Li, J.; Shiota, Y.; Yoshizawa, K. J. Am. Chem. Soc. 2009, 131, 13584–13585. (10) Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2010, 132, 120–130. (11) Frisch, M. J., Gaussian 03, Revision B.02; Gaussian, Inc., Pittsburgh, PA, 2003. (12) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866–872. (13) Kendall, R. A.; Dunning, J. T. H.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796–6806. (14) Binstead, R. A.; Chronister, C. W.; Ni, J.; Hartshorn, C. M.; Meyer, T. J. J. Am. Chem. Soc. 2000, 122, 8464–8473. (15) Bhasikuttan, A. C.; Suzuki, M.; Nakashima, S.; Okada, T. J. Am. Chem. Soc. 2002, 124, 8398–8405. (16) D’Alessandro, D. M.; Dinolfo, P. H.; Davies, M. S.; Hupp, J. T.; Keene, F. R. Inorg. Chem. 2006, 45, 3261–3274. (17) Kaledin, A. L.; Huang, Z. Q.; Geletii, Y. V.; Lian, T.; Hill, C. L.; Musaev, D. G. J. Phys. Chem. A 2010, 114, 73–80.
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