Two Unexpected Roles of Water: Assisting and ... - ACS Publications

10684

J. Phys. Chem. B 2008, 112, 10684–10688

Two Unexpected Roles of Water: Assisting and Preventing Functions in the Oxidation of Methane and Methanol Catalyzed by Porphyrin-Fe and Porphyrin-SH-Fe Xingbang Hu, Congmin Wang, Yong Sun, Hang Sun, and Haoran Li* Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, P. R. China ReceiVed: NoVember 24, 2007; ReVised Manuscript ReceiVed: May 21, 2008

The C-H activation of methane and the O-H activation of methanol catalyzed by porphyrin-Fe and porphyrin-SH-Fe with water molecules in the microenvironment were investigated. It was found that there were two absolutely opposite regions in the vicinity of porphyrin-Fe. In one region (W1), water molecule can prevent the reaction, whereas in the other region (W2), water molecule can assist the reaction. The roles of W1 and W2 are unexpected compared with those reported in previous papers. Furthermore, the previous experimental phenomena can be explained by these results to some extent. These results are useful for understanding the influences of water on the oxidations with homogeneous catalysts and controlling the reactions by changing the microenvironment of the catalysts. 1. Introduction To understand the roles of water in chemical reactions is of great importance,1 especially for the reactions where water molecules inevitably exist, such as most of the oxidation reactions. On the one hand, many experiments had revealed that water can assist the oxidations.1c,d,2,3 Taking the CO oxidation catalyzed by gold as an example, it has been found that this reaction was greatly influenced by moisture in the reactant gas, and the presence of water was required to obtain appreciable reaction rates.1c,d,3 On the other hand, opposing experiments which suggest that water prevents the oxidations have been reported too.4 Interestingly, there are also experimental evidence which suggested that the roles of water differed in the same reaction depending on the amount of water5 or the form of the catalysts.6 The oxidations catalyzed by metal porphyrin are typical examples. Previous experimental researches have revealed that trace water can assist the oxidation catalyzed by metal porphyrin.5a,b However, when the amount of water increases, the reaction will be suppressed to some extent.5a All of these experimental phenomena indicated that the roles of water in the oxidation were quite complicated and should be diversiform. In the fields of theoretic research on the role of water in the reaction, it has been widely accepted that the water molecule can act as a bridge to assist the proton transfer processes and reduce the barrier of the reaction by about 25 kcal/mol.1a,b,7 As we know, the proton transfer process is the rate determining step in many oxidation reactions.8 If no further theoretic researches were performed, at a venture, we may think that water can also assist the substrate oxidation by acting as a bridge. Is it true? Furthermore, though water suppressing the oxidation has been observed by the experiments,4,5a there is no theoretic evidence for these phenomena. Do all water molecules in the microenvironment of the catalyst act the same role theoretically? Herein, aiming at above questions, we present a direct theoretic study on the oxidation of methane and methanol catalyzed by porphyrin-Fe (Porph-Fe) and porphyrin-SH-Fe (Porph-SH-Fe) with water molecule in the microenvironment. In the theoretic fields related to the metal porphyrin catalyst, it * Corresponding author. Fax: +86-571-8795-1895. E-mail: [email protected].

has been found that the water molecule can assist the selfoxidation of the heme.9a Recent theoretical researches found that the water molecule could assist the formation of ferric-hydroperoxide species of horseradish peroxidase (HRP) by forming a water bridge.9b There are also researches that revealed that the water interacting with the oxo ligand of cytochrome P450 can lower the barrier of H-abstraction by about 4 kcal/ mol.10 To the best of our knowledge, no substrate oxidation influenced by water in different regions of the metal prophyrin catalyst has been reported, though the oxidation of methane and methanol catalyzed by the following catalysts have been widely illuminated, such as metal porphyrin catalyst,11 methane monooxygenase hydroxylase component,12 hydrated iron(IV) oxo species,13 MO+ (M ) Fe, Mn, Cu, Co),14 V4O10+,15 Mo3O9,16 chromyl chloride,17 bipyrimidine-PtCl2 or PtCl2 (NH3)2, 18 N-heterocyclic carbene Pd(II) complexes,19 gold complexes,20 and all-metal aromatic complexes.21 To obtain the physical origin of the water effect on the oxidation catalyzed by metal prophyrin is not only essential to give a comprehensive understanding of the reaction but also fundamental in homogeneous catalysis in general. 2. Computational Methods B3LYP has been used with satisfying results for the methane oxidation reaction catalyzed by various catalysts.11–19 Therefore, all of the structures were optimized with the B3LYP theory in the presented work. Generally, the 6-31G basis set is used for all atoms except the transition metal in the reaction catalyzed by porphyrin metal complex, and it is necessary to take the relativistic effect of transition metal into consideration.8,11 For the ground-state of the porphyrin systems, the valence electron / orbital for the reactant and there is an should filled in the πFe / during the C-H electron transfer from the σCH to the πFe 8b,11 As we know, the porphyrin activation process (Scheme 1). systems are notorious for its multiple electron configurations even within the same multiplicity. In some calculations, parts of the electrons may transfer into the σ*xy orbital. To ensure the correctness of the obtained results, the initial structure of the reactant was obtained by the intrinsic reaction coordinate analysis of the transition state. Furthermore, three different

10.1021/jp8028903 CCC: $40.75  2008 American Chemical Society Published on Web 08/01/2008

Two Unexpected Roles of Water

J. Phys. Chem. B, Vol. 112, No. 34, 2008 10685

SCHEME 1: Orbital Diagrams Showing the Low Spin C-H Activation Processes of Porph-Fe and Porph-SH-Fe Systems

calculation methods were used in this manuscript to evaluate the effect of different electron configurations. In this manuscript, the LANL2DZ basis set was used for Fe and three different basis sets (6-31G, 6-31+G* and 6-311+G**) were used for other atoms here, which was abbreviated as LANL2DZ(6-31G), LANL2DZ(6-31+G*) and LANL2DZ(6311+G**), respectively. For the Porph-SH-Fe system, optimizations were further performed with the following basis set: the CEP-121G basis set for Fe and 6-31+G* for other atoms (CEP-121G(6-31+G*)). Energy calculations as well as zeropoint energy correction have been done by using the same theory. To estimate the environment effect, a IPCM calculation22 was performed by using the structure optimized with the LANL2DZ(6-31+G*) basis set. The solvent used here was water (
) 78.4). All calculations have been performed with the Gaussian 03 suite of programs.23

Figure 1. Methane oxidation catalyzed by Porph-Fe with and without water molecule. The shown values represent the change of the bond length (in Å) from the reactant to the transition state. Values out of (in) the parentheses belong to the low (high) spin state. Optimizing with LANL2DZ(6-31G) basis set. See the Supporting Information for the LANL2DZ(6-31+G*) results.

3. Results and Discussion 3.1. Porph-Fe System. For the methane oxidation catalyzed by Porph-Fe, only the rate determining steps8,11 (C-H activation steps) were investigated. Both the low spin and high spin state reaction processes were considered.24 Besides this, the O-H activation of methanol catalyzed by Porph-Fe was also studied to strengthen and enrich the obtained results. 3.1.1. C-H ActiWation of Methane. When there is no explicit water, the proton of methane transfers to the catalyst directly (Figure 1). The corresponding transition state exhibits a negative spin density on the migrating hydrogen, which is a typical feature of the hydrogen abstraction process by a radical.11 The barriers of these processes are 25.1 and 18.6 kcal/mol for low and high spin states, respectively (Table 1). 3.1.1.1. Water PreVents the C-H ActiVation of Methane. Previous researches have suggested that water molecule can bind the FedO part of Porph-Fe.8b,9b,10 Herein, the calculated binding energies between Porph-Fe and water are 7.5 and 6.2 kcal/mol larger than that between Porph-Fe and methane for LANL2DZ(6-31G) and LANL2DZ(6-31+G*) basis set, respectively (Table 2). If Porph-Fe has binded with methane, the binding energy between Porph-Fe-CH4 and water is -7.2 kcal/ mol at least. These values indicate that when there is water in the reaction system, it can bind with the catalyst easily. How does the water influence the oxidation reaction? Inspired by the idea of the assisting ability of the water bridge (W1 in Figure 1) in the proton transfer processes,1a,b,7,9b we investigated the process that one hydrogen of methane moves to W1 and W1 gives its proton to the catalyst (Porph-CH4-W1 f Porph-CH4-W1-TS). Interestingly, it was found that the water bridge could not assist the proton transfer of the C-H activation process. Contrarily, the barrier with water as bridge is 7.1 kcal/ mol higher than that without water for the low spin state with LANL2DZ(6-31G) basis set. This value is 9.8 kcal/mol calculating with LANL2DZ(6-311+G**) basis set. When the environment effect was taken into consideration, this value is 11.4

kcal/mol (Table 3). Though adding polarization and diffuse functions in the basis set can enlarge the difference between the barriers with and without water, and the considering environment effect can further increase this difference, all these results suggest that the water (W1) prevent the proton transfer of the C-H activation process catalyzed by Porph-Fe. This theoretical result makes an exception to the well-known assisting role of the water bridge (Table 4).1a,b,7,9b The proton transfer distance is directly related to the barrier of the H-atom transfer reaction.7b The reason why W1 prevents the proton transfer can be explained by the proton transfer distance. 3.1.1.2. Water Assists the C-H ActiVation of Methane. If water locates only in the W1 region, the obtained theoretic result is inconsistent with the experimental fact that trace water can accelerate the oxidation reaction catalyzed by metal porphyrin.5 Another way of water binding (W2 in Figure 1) is also energy favorable (Table 2). In this case, water does not take part in the proton transfer process of the C-H activation. It just binds with the proton acceptor of the catalyst at the site opposite to the methane. This water molecule shows absolutely different influence compared with the water acting as “bridge”. Compared with the isolated reaction Porph-CH4 f Porph-CH4-TS, water located in W2 region lowers the barrier of the C-H activation by 2.6 kcal/mol for the low spin states. This value is 4.2 kcal/mol calculating with LANL2DZ(6-311+G**) basis set. When the environment effect was taken into consideration, this value is 2.0 kcal/mol (Table 3). All these results suggest that the water (W2) can assist the C-H activation process catalyzed by Porph-Fe. It is interesting that the water molecule located in the W2 region always shows the ability to prevent the proton transfer in the previous system without a metal organic catalyst (Table 4).7b,25 Why does W2 assist the reaction here? There are two main reasons. First, our previous works have revealed that a further proton transfer distance brought a higher reaction barrier.7b Here,

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Hu et al.

TABLE 1: Barriers (in kcal/mol) of the Transition State of C-H and O-H Activation with and without Watera system

water

barrierb

CH4 + Porph-Fe

none

25.1 [30.3] 18.6g 32.2 [33.2] 19.0g 22.5 [27.8] 15.7g 21.6 [26.8] 28.6 [33.6] 19.1 [23.9] 16.8 23.4 13.5

W1 W2 CH4 + Porph-SH -Fe CH3OH + Porph-Fe

none W1 W2 none W1 W2

changeb,c

barrierd

barriere

changee

25.9 [31.0]

[30.4]

v7.1

34.8 [39.9]

[40.2]

v[9.8]

V2.6

26.4 [29.4]

[26.2]

V[4.2]

v7.0 V2.5

22.1 [27.3] 31.9 [35.8] 20.8 [25.3]

[30.1] [35.6] [28.5]

v[5.5] V[1.6]

barrierf

23.1 [27.0] 32.7 [36.8] 21.6 [25.6]

v6.6 V3.3

a Values out of [in] the square brackets mean enthalpy change [absolute energy change] for the low spin state. b Optimizing structure and calculating energy with LANL2DZ(6-31G). c The values were obtained by comparing the barriers with and without water. d Optimizing structure and calculating energy with LANL2DZ(6-31+G*). e Optimizing structure whith LANL2DZ(6-31+G*) and calculating energy with LANL2DZ(6-311+G**). f Optimizing structure and calculating energy with CEP-121G(6-31+G*). g Values belong to the high spin state.

TABLE 2: Binding Energies (BE, in kcal/mol) in Forming AB Cluster from A and Ba A

B

AB cluster

BEb

BEc

Porph Porph Porph-CH4

H2O CH4 H2O

Porph-H2O

CH4

Porph-H2O Porph-CH4 Porph-CH4-W1 Porph-CH4-W2 Porph-CH4-W1 Porph-CH4-W2

-7.8 (-8.6) -0.3 (-0.3) -8.8 (-9.1) -7.2 (-8.2) -1.3 (-0.8) 0.2 (0.1)

-5.2 1.0 -6.3 -4.5 -0.1 1.8

a Values out of (in) the parentheses belong to the low (high) spin state. Calculation was performed with. b LANL2DZ(6-31G) basis set. c LANL2DZ(6-31+G*) basis set.

TABLE 3: Barriers (in kcal/mol) of C-H Activation in Water Solutiona system CH4 + Porph-Fe CH4 + Porph-SH-Fe

water

barrier

none W1 W2 None W1 W2

30.5 41.9 28.5 26.9 35.8 25.0

a Calculation was performed with ICPM solution model for the low spin state.

TABLE 4: Comparing the Roles of Water Molecule in the Proton Transfer watera

proton transfer in this work

without any catalyst

W1 W2

preventing assisting

assistingb preventingc

a W1 refers to the water acting as a “bridge”. W2 refers to the water binding with the proton acceptor at the site opposite to the region that proton transfers. b References 1a, b, 7, 9b. c References 7b and 23.

W2 can shorten the C-H proton transfer distance from the reactant to the transition state by 0.064 and 0.021 Å for low and high spin states, respectively. As a result, a lower barrier was observed. Second, the O-HW2 · · · O-Fe hydrogen bond in the transition state is stronger than that in the reactant for both low and high spin state. It indicates that the hydrogen bond stabilizes the transition state more than it does for the reactant, which can lower the barriers. Though the model used here is much simpler compared with the real reaction system, the idea of assisting and preventing functions of water molecules is helpful for understanding the

Figure 2. Methanol oxidation catalyzed by Porph-Fe with and without water molecule. The shown values represent the change of the bond length (in Å) from the reactant to the transition state. Optimizing with LANL2DZ(6-31G) basis set. See the Supporting Information for the LANL2DZ(6-31+G*) results.

previous experimental phenomena on metal prophyrin catalysts.5a,b The binding energy of the water molecule entering into the W1 region is close to that entering into the W2 region. However, the reaction barrier with water in the W2 region is much lower than that with water in the W1 region. It is reasonable that when there is trace water in the reaction system, the most possible reaction pathway is that with water in the W2 region. As a result, the oxidation reaction was facilitated. When the amount of water increases, the FedO part will be surrounded by water molecules and there may be water entering into the W1 region. As a result, the reaction will be suppressed to some extent. 3.1.2. O-H ActiWation of Methanol. The O-H activation of methanol was also investigated (Figure 2). This study was limited to the low spin state because the above researches had shown that the influence of water for the high spin state was the same to that of the low spin state. Similar to the C-H activation, there are two absolutely opposite regions in the vicinity of Porph-Fe. In one of the regions (W1), the water

Two Unexpected Roles of Water

J. Phys. Chem. B, Vol. 112, No. 34, 2008 10687 The result of W2 agrees with previous QM/MM researches on the role of W2 which had suggested that this water could assist the C-H activation by about 4 kcal/mol.10 It was found that W2 in Porph-SH-Fe system can shorten the C-H proton transfer distance from the reactant to the transition state by 0.039 Å and the O-HW2 · · · O-Fe hydrogen bond was shortened by 0.044 Å, both of which can lower the reaction barriers. This fact is similar to the Porph-Fe system. 4. Conclusions

Figure 3. Methane oxidation catalyzed by Porph-SH-Fe with and without water molecule. The shown values represent the change of the bond length (in Å) from the reactant to the transition state. Optimizing with LANL2DZ(6-31+G*) basis set. See the Supporting Information for the LANL2DZ(6-31G) and CEP-121G(6-31+G*) results.

molecule can prevent the O-H activation of methanol, whereas in another region (W2), water molecule can assist the oxidation reaction. A shorter proton transfer distance and a stronger O-HW2 · · · O-Fe hydrogen bond were also observed when water was located in the W2 region. 3.2. Porphyrin-SH-Fe System. A more realistic catalytic system for the methane oxidation is investigated. Compared with the Porph-Fe discussed above, a sixth ligand (SH) was added (Figure 3). Porph-Fe plus SH ligand (Porph-SH-Fe) was generally thought as an effective model of Cytochrome P450 enzymes.8b,11 The barriers of C-H activation catalyzed by Porph-SH-Fe calculated with LANL2DZ(6-31G) and LANL2DZ(6-31+G*) listed in Table 1 are almost the same to that reported in previous paper.11a Compared with the reaction Porph-CH4f Porph-CH4-TS, the adding of the SH ligand can reduce the barrier by 3.5 kcal/mol for the low spin state. Previous research has demonstrated that the Fe-S bond is shortened from the reactant to the transition state, which resulted in reducing the reaction barrier.11a For Porph-SH-Fe system, when calculation was performed in the gas phase, the barrier of the C-H activation with water as a bridge (W1) is 7.0 kcal/mol higher than that without water, whereas the barrier with water located in the W2 region is 2.5 kcal/mol lower than that without water. These values are 5.5 and 1.6 kcal/mol calculating with the LANL2DZ(6-311+G**) basis set (Table 1). When the environment effect was taken into consideration, it was found that W1 can enlarge the barrier by 8.9 kcal/mol and W2 can lower the barrier by 1.9 kcal/mol (Table 3). When calculations were performed with the CEP121G(6-31+G*) basis set, similar results can also be obtained. These results with SH as axial ligand further confirm that W1 prevents the C-H activation whereas W2 assists this process.

The effects of water molecule in the C-H activation of methane and the O-H activation of methanol catalyzed by porphyrin-Fe and porphyrin-SH-Fe have been investigated. It was found that water molecules in different regions of Porph-Fe and porphyrin-SH-Fe had absolutely opposite roles for the methane and methanol oxidation reactions. Furthermore, the roles of W1 and W2 are unexpected (Table 4). The water bridge (W1) can assist the proton transfer in previously publications.1a,b,7,9b Interestingly, it prevents this process here. The water molecule W2 prevents the proton transfer without any metal catalysts in previous papers.7b,25 Surprisingly, it assists this process for the metal porphyrin system reported here and in previous papers.10 The biggest difference between the proton transfer in Porph-Fe and Porph-SH-Fe system and those in previous papers is that the proton acceptor is a part of the metal organic compound and the proton transfers to FedO part directly in this manuscript, whereas the proton acceptor is a part of the pure organic compound in previous researches. Whether the center metal of the catalyst is the key factor needs further evidence. Whatever, these results are useful for understanding the influences of water on the oxidations with homogeneous catalysts and controlling the reactions by changing the microenvironment of the catalysts. It is worth noticing that Porph-SH-Fe is a more realistic system for the C-H activation compared with Porph-Fe. Furthermore, for the Porph-Fe system, the vacant position for the sixth ligand will be occupied by a water molecule when they are full with water from the environment.26 Hence, we believe the results of Porph-SH-Fe are more realistic and meaningful. However, the investigation on the Porph-Fe system is helpful for understanding the origin of the roles of water molecules here, because it eliminates the possible cooperation effects between the SH and water. With this research, we can understand the pure influence induced by the water. Acknowledgment. We thank the reviewers for the valuable proposals and illuminating questions on the calculation and discussion. This work was supported by the National Natural Science Foundation of China (No. 20573093, No. 20704035, and No. 20773109) and China Postdoctoral Science Foundation funded project (No. 20070420227). Supporting Information Available: Tables giving the coordinates of the optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Antonczak, S.; Ruiz-Mpez, M. F.; Rivail, J. L. J. Am. Chem. Soc. 1994, 116, 3912. (b) Gorb, L.; Leszczynski, J. J. Am. Chem. Soc. 1998, 120, 5024. (c) Sanchez-Castillo, M. A.; Couto, C.; Kim, W. B.; Dumesic, J. A. Angew. Chem., Int. Ed. 2004, 43, 1140. (d) Date, M.; Okumura, M.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43, 2129. (e) Wang, L.; Yu, X.; Hu, P.; Broyde, S.; Zhang, Y. J. Am. Chem. Soc. 2007, 129, 4731.

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