Article pubs.acs.org/JPCA
Fluxionality in the Chemical Reactions of Transition Metal Oxide Clusters: The Role of Metal, Spin State, and the Reactant Molecule Raghunath O. Ramabhadran, Edwin L. Becher, III, Arefin Chowdhury, and Krishnan Raghavachari* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: The phenomenon of fluxionality in the reactions of transition metal oxide clusters provides many opportunities in various catalytic and industrial processes. To gain a better understanding into the various factors influencing this phenomenon, we have carried out an electronic structure investigation of the fluxionality pathways when W3O6− and Mo3O6− clusters react with hydrogen sulfide and ammonia. The study illustrates how the metal (W vs Mo), different spin states of the anionic metal oxide cluster (doublet vs quartet), and the nature of the nonmetal in the small molecule (O vs S vs N), all affect the fluxionality pathway. The thermodynamically facile fluxionality pathway with H2S detaches both the hydrogens from hydrogen sulfide and can thus be very useful in the petrochemical and desulfurization industries. The fluxionality pathway with NH3 results in interesting metal-bound imines and bridged amines. However, the overall fluxionality process with NH3 is found to be thermodynamically unfavorable.
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manner, we believe that such fluxionality pathways can be useful in other catalytic and industrial processes such as autoexhaust catalysis and gas-sensing. Herein, we perform a theoretical study of the fluxionality profiles of W3O6− and Mo3O6− clusters in their reactions with two other small molecules, H2S and NH3. Our study enables us to explain how various important factors, such as the role played by the metal (W vs Mo), different spin states of the anionic metal oxide cluster (doublet vs quartet), and the nature of the nonmetal in the small molecule (O vs S vs N), affect the fluxionality pathway. In particular, H2S reactivity with TMOs has been of particular interest in the field of desulfurization catalysis44 and the petrochemical industry.45 The fluxionality pathway (Figure 1) provides the opportunity to strip the S of both the hydrogens (vide infra). With NH3, the presence of the additional hydrogen highlights interesting structural aspects in the course of the fluxionality pathway that have not been studied previously. Also, S and N are both immediate neighbors of O in the periodic table. Therefore, a comparative study in the reactivity of the hydrides of these important elements (N, O, and S) with TMO clusters helps in the systematic understanding of how the periodic trends affect the fluxionality pathway.
INTRODUCTION Catalysis is central to life today1 and transition metal oxides (TMOs) play a key role in catalysis.2 Many surface defects in the TMOs, principally responsible for their catalytic action in heterogeneous catalysis, can be adequately represented by cluster models.3 Previous findings have clearly suggested that gas phase studies of neutral and ionic TMO clusters assist in identifying the reactive sites in chemical reactions4−6 relevant to alternate ways of energy production (e.g., hydrogen evolution from water). The chemical reactivity of TMO clusters with small molecules has therefore been an exciting area of contemporary research.7−20 Furthermore, with the view of advancing the catalytic applications of the TMOs, various novel physical and chemical features of TMO clusters have recently been investigated theoretically as well as experimentally.21−39 Fluxionality is recognized as an important factor in cluster science and has been widely used in catalysis, mostly with purely metallic clusters.41−43 In a previous publication, we demonstrated the interesting chemical phenomenon of protonassisted fluxionality in TMO clusters.40 In our study, wherein the chemical reactions of W3O6− and Mo3O6− clusters with water were investigated, an apparent hydroxyl migration was shown to be actually effected by a proton-hop. Such a protonhop alters the reactive site of the TMO clusters and hence can affect their catalytic activity. Also, the proton-assisted fluxionality pathway was found to lead to a bridging oxygen in the cluster being replaced by a bridging oxygen originally belonging to water. This interchange of the oxygen positions is similar to the mechanism proposed for the industrially significant ammoxidation process of propylene to yield acrolein and acrylonitrile using bismuth molybdates.2 In a similar © 2012 American Chemical Society
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COMPUTATIONAL DETAILS The Gaussian suite of programs46 has been used for all our calculations. All the geometries were optimized with the Received: April 13, 2012 Revised: June 5, 2012 Published: June 5, 2012 7189
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Figure 1. Illustration of proton-assisted fluxionality using the example of Mo3O6− (doublet) with hydrogen sulfide. A proton-hop results in the conversion of a bridged oxygen to a bridged sulfur.
unrestricted version of the B3LYP47,48 hybrid density functional. The 60 core electrons of tungsten and 28 core electrons of molybdenum were replaced with the Stuttgart−Dresden relativistic pseudopotentials. The remaining 14 valence electrons on both these metal centers have been treated with an augmented version of the associated double-ζ basis sets including diffuse and polarization functions. The D95 basis set including diffuse and polarization functions was used to treat H, O, N, and S in these optimizations. Vibrational analyses have been carried out to confirm the nature of all the minimum energy structures and transition states. The associated zero point energies and thermal corrections have been included in all the free energies obtained. Intrinsic reaction coordinate (IRC) calculations were carried out to verify that key transition states obtained connect to the minima on either side. The effects of larger basis sets on the computed reaction energies were then evaluated by single point calculations using larger augmented triple-ζ basis sets.49 The aug-cc-pVTZ basis set was used to treat H, O, N, and S in these single point calculations. The exponents of the additional diffuse and polarization functions used in both the double-ζ and the triple-ζ basis sets are provided in the Supporting Information
Figure 2. Comparison of the fluxionality pathways obtained for the reaction of hydrogen sulfide with Mo oxide (black line) and W oxide (green line) in their doublet (solid line) and quartet (dotted line) states. M refers to Mo or W, as indicated in the legend. The doublet states have been chosen as the origin along the reaction energy coordinate (kcal/mol).
Inspection of the optimized minimum energy structures obtained in step 3 (Figure 3a,b) readily accounts for the larger
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RESULTS AND DISCUSSIONS a. Fluxionality Pathway with H2S: Doublet Electronic States of W3O6− and Mo3O6−. The lowest energy isomers of both W3O6− and Mo3O6− correspond to doublet electronic states and have three bridging oxygens and three terminal oxygens. As in the case of the reactions with water, the fluxionality pathway with hydrogen sulfide (Figure 2) involves five mechanistic steps. The first step is the electrostatic complexation of H2S with the metal oxides (step 1). This is followed by the formation of the intermediate adduct (step 2), the preparation step (step 3) that creates the right conformation for the proton-hop to occur, the subsequent proton-hop (step 4), and finally the formation of the rearranged adduct (step 5). For the doublet electronic states (Figure 2, bold lines), until step 3, only the first proton of H2S is involved. At the preparation step 3, where a bridging oxygen opens up by forming a metal−oxygen π bond, Mo3O6− has a significantly larger reaction energy barrier than W3O6−. This is consistent with the metal−oxygen σ and π bond-energies obtained by Dixon and Peterson.50 In our previous study with water,40 it was seen that the proton-hop step (step 4) involved a substantially lower energy barrier than the preparation step (step 3). However, in the case of H2S, the preparation step is energetically not too different (vide infra) from the proton-hop step; i.e., relatively larger barriers for the proton-hop are observed.
Figure 3. Optimized geometries of the conformations obtained in the preparation step (Figure 2, step 3). With the doublet states (3a and 3b), the hydrogen-bonded conformation facilitating proton-hop is not obtained. With the quartet states (3c and 3d); however, the right conformation facilitating subsequent proton-hop is obtained.
barriers. Whereas with water, the proton-hop was facilitated by conformations favoring O−H···O hydrogen bonding, with hydrogen sulfide (containing the less electronegative sulfur), the preparation step (step 3, Figure 2, bold lines) does not lead to the appropriate conformation containing S−H···O hydrogen bonds (vide infra).51 Hence, additional rotation of the hydrogens is needed, and the proton-hop from sulfur to oxygen proceeds with a larger barrier. 7190
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In the final step, the metal−sulfur π bond is converted to a bridging metal−sulfur σ bond to complete the fluxionality pathway. It is very easily noticed that, for the doublet states (Figure 2, solid lines) step 5 has a lower barrier than the reverse of step 3. This clearly indicates that a metal−sulfur−metal bridging bond is kinetically more favorable than a metal− oxygen−metal bond. The final products obtained in step 5 (Figure 4a,b), after both of H2S’s hydrogens have been utilized, have two metal−
than the metal−oxygen−metal bond angles in the intermediate adducts (Table 1), illustrating the increased p character of S over O. Table 1. Showing the Bond Angle Differences between the Metal−Sulfur−Metal or Metal−Nitrogen−Metal Bonds Obtained in the Final Products in Step 5 and the Metal− Oxygen−Metal Bond Formed in Step 3 in the Intermediate Adducts structure 4a (Mo oxide, product) 4b (W oxide, product) 4c (Mo oxide, product) 4d (W oxide, product) 5a (Mo oxide, adduct) 5b (W oxide, adduct) 5c (Mo oxide, adduct) 5d (W oxide, adduct) 9a (Mo oxide, product) 9b (W oxide, product) 9c (Mo oxide, product) 9d (W oxide, product) 10a (Mo oxide, adduct) 10b (W oxide, adduct) 10c (Mo oxide, adduct) 10d (W oxide, adduct)
Figure 4. Final products obtained in the fluxionality pathway with H2S. 4a and 4b correspond to doublet electronic states, and 4c and 4d correspond to the quartet states.
oxygen bridging σ bonds, one metal−sulfur bridging σ bond and two OH bonds. In contrast, the intermediate adducts obtained in step 2 (Figure 5a,b) have three metal oxygen bridging bonds, an OH and an SH bond. The metal−sulfurmetal bond angles in the final products are consistently smaller
spin state
bridging bond angle indexa
bond angle (degrees)
doublet
Mo−S−Mo
74.0
doublet
W−S−W
73.1
quartet
Mo−S−Mo
79.1
quartet
W−S−W
70.1
doublet
Mo−O−Mo
85.6
doublet
W−O−W
85.1
quartet
Mo−O−Mo
99.3
quartet
W−O−W
94.4
doublet
Mo1−N−Mo2
89.2
doublet
W −N−W
88.8
quartet
Mo1−N−Mo2
99.1
quartet
W1−N−W2
85.6
doublet
Mo1−O−Mo2
85.4
doublet
W1−O−W2
85.3
quartet
Mo1−O−Mo2
98.6
quartet
W1−O−W2
95.5
1
2
a
Bridging bond-angle indices are the same labels used in the corresponding structures 4a−4d, 5a−5d, 9a−9d, and 10a−10d.
Considering the overall thermodynamic behavior of the Mo oxide, the final product is only slightly (2 kcal/mol) more stable than the intermediate adduct. With the W oxide, however, the corresponding final product is significantly more stable (4 kcal/ mol) than the intermediate adduct, suggesting that W3O6− clusters are more suitable than Mo3O6− clusters to strip both the hydrogens from H2S. b. Fluxionality Pathway with H2S: Quartet Electronic States of W3O6− and Mo3O6−. Two major differences are noted in the fluxionality pathway for the excited quartet electronic states (Figure 2, dotted lines) of W3O6− and Mo3O6− in their chemical reaction with H2S. (1) The barrier for the proton-hop step (step 4, Figure 2, dotted lines) is lower than that of the corresponding doublet states. Parts c and d of Figure 3 help in explaining the lower barrier noticed. For the quartet states, the preparation step (step 3, Figure 2, dotted lines) results in the right conformation facilitating S−H···O hydrogen bonds (Figure 3c,d).51 However, because this conformation is not achieved with the doublet
Figure 5. Intermediate adducts obtained in the second step of the fluxionality pathway with H2S. 5a and 5b correspond to doublet electronic states, and 5c and 5d correspond to the quartet states. 7191
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states (Figure 3a,b), a higher barrier for the proton-hop is observed with the doublet states. It is likely that the metal−metal distance plays a role in facilitating the right conformation in the quartet electronic states (Table 2). The MS−MO distances (where M is either W Table 2. Key Metal−Metal Bond Lengths That Help in Elucidating the Role of the Metal as Well as the Role of the Spin State in the Fluxionality Pathway structure
spin state
bond-length labela
bond length (Å)
3a (Mo oxide) 3b (W oxide) 3c (Mo oxide) 3d (W oxide) 7a (Mo oxide) 7b (W oxide) 7c (Mo oxide) 7d (W oxide)
doublet doublet quartet quartet doublet doublet quartet quartet
MoS−MoO WS−WO MoS−MoO WS−WO MoN−MoO WN−WO MoN−MoO WN−WO
2.58 2.60 2.85 2.85 2.71 2.61 2.88 2.89
Figure 6. Comparison of the fluxionality pathways obtained for the reaction of ammonia with Mo oxide (black line) and W oxide (green line) in their doublet (solid line) and quartet (dotted line) states. M refers to Mo or W, as indicated in the legend. The doublet states have been chosen as the origin along the reaction energy coordinate (kcal/ mol).
a
Bond-length labels as used in the corresponding structures 3a−3d and 7a−7d.
or Mo, as labeled in Figure 3a−d) are clearly larger for the quartet states. Consequently, the weakened O−H···S hydrogen bond (S and O as labeled in Figure 3a−d) present in the doublet states (3a and 3b) is nonexistent in the quartet states (3c and 3d). The sulfur in the quartet states instead engages in an S−H···O hydrogen bond with the other oxygen to result in a conformation favoring the proton-hop step. (2) The other major difference is in step 5. Unlike with the doublet electronic states, we do not notice a kinetic preference for the metal−sulfur-metal bridging bond over the metal− oxygen−metal bridging bonds in the quartet states; i.e., the barriers for step 5 (Figure 2, dotted lines) are comparable to the barriers for step 3. Overall, for the quartet states as well, the final products obtained in step 5 (Figure 4c,d) for both Mo and W oxides are thermodynamically more stable than the intermediate adducts obtained in step 2 (Figure 5c,d) by about 2−3 kcal/mol. c. Fluxionality Pathway with NH3: Doublet Electronic States of W3O6− and Mo3O6−. In reactions with ammonia, the electrostatic complexations, intermediate adduct formations (leading to a metal-bound amine group), and the preparation steps (steps 1−3, Figure 6, bold lines) are very similar for the doublet electronic states of both Mo and W oxides. But the proton-hop step shows a strongly metal-dependent behavior, with the barrier for Mo3O6− being much lower than that for W3O6−. This is again due to the preparation step leading to the appropriate conformation only in the case of Mo3O6− (Figure 7a,b).51 The MoN−MoO distance (Figure 7a, Table 2) is only 0.1 Å longer than the WN−WO distance (Figure 7b, Table 2). The metal−metal distance (MN−MO distances, M = W or Mo, as labeled in Figure 7a,b) thus directly does not seem to play a major role here. Instead, the metal dependence noticed here is due to a competition between two different N−H···O hydrogen bonds. With W3O6− the N−H···O hydrogen bond (N and O as labeled in Figure 7a,b) occurs preferentially with the oxygen not involved in the proton-hop step (labeled as O). With Mo3O6−, however, the N−H···O hydrogen bond occurs with the oxygen involved in the proton-hop step, the result being a facile proton-hop step for the Mo oxide.
Figure 7. Optimized geometries of the conformations obtained in the preparation step (Figure 6, step 3). With the doublet state of the Mo oxide (7a), the hydrogen-bonded conformation facilitating proton-hop is obtained. But with the W oxide, the right conformation is not obtained (7b). Thus, a metal dependence is noticed. With all the quartet states (7c and 7d), however, the conformation facilitating subsequent proton-hop is obtained.
Following proton-hop, interesting metal-bound imines are obtained (Figure 8a,b) in step 4 (Figure 6, bold lines). These are molecules with metal−nitrogen π bonds. As expected, such M−N distances (M = Mo or W, and N as labeled in Figure 8a,b) are consistently shorter (Table 3) than the metal− nitrogen distances for the metal-bound amines (obtained in step 2) containing metal−nitrogen σ bonds. The HOMOs of both the tungsten-bound imine and the molybdenum-bound imine appear to be very similar (Figure 8a,b). In the HOMOs of both 8a and 8b, we observe back-bonding between the filled d orbital of the metal center labeled M3 (M = W or Mo, Figure 8a,b) and a p orbital on a terminal oxygen. 7192
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Figure 9. Final products containing bridged metal amines obtained in the fluxionality pathway with ammonia. The HOMOs for the doublet states are shown in 9a and 9b. Figure 8. Metal-bound imines. The HOMO for the doublets is shown in 8a and 8b.
observed in the intermediate adducts 10a and 10b (containing the metal−nitrogen σ bonds) and structures 8a and 8b (containing metal−nitrogen π bonds), suggesting the partial-π bond characteristic of the bridged metal amines. Interestingly, the bridging nitrogen is asymmetric in 9a as well as 9b; i.e., the M1−N distances are much shorter than the M2−N distances (Table 3). The HOMOs of both the metal amines (Figure 9a,b) show considerable metal−oxygen back-bonding with one of the terminal oxygens. In comparison with the HOMOs of 8a and 8b, the back-bonding in 9a and 9b occurs at M1 instead of M3. In a way, this is indicative of the shift in the electron density on the molecules as we proceed from the metal-bound imine to the bridging amine. Also, the metal−nitrogen−metal angles (Table 1) in the final products (9a and 9b) obtained in step 5 are only slightly larger than the metal−oxygen−metal angles in the adducts, 10a and 10b. Kinetically, the final products 9a and 9b are both more favorable than the intermediate adducts 10a and 10b, the barriers for the formation of a metal−nitrogen bridging bond in step 5 being much lower than the formation of a metal−oxygen bridging bond (reverse of step 3). Thermodynamically, however, with both Mo−oxide and W−oxide, the intermediate adducts are substantially more stable than the final products. d. Fluxionality Pathway with NH3: Quartet Electronic States of W3O6− and Mo3O6−. The fluxionality pathway for the quartet states of W3O6− and Mo3O6− (Figure 6, dotted lines) differs significantly from the pathway for the doublet electronic states only in the proton-hop step and in the final step. In the proton-hop step, as noted with H2S, the quartet states 7c and 7d both have the hydrogen-bonded conformation for proton-hop to occur readily51 and the metal−metal distances (Table 2, Figure 7c,d) in the quartet states are larger than those observed for the doublet states. The similarity with H2S extends to the final step as well (step 5, Figure 6, dotted
The final step in the fluxionality-pathway (step 5, Figure 6, bold lines) then involves the generation of a bridged metal amine. The M−N distances (M = Mo or W, and N as labeled in Figure 9a,b) for these bridged metal amines (Table 3), in the case of both Mo and W oxides, are between the distances Table 3. Metal−Nitrogen Distances in the Metal-Bound Imines, Metal-Bound Bridging Amines, and Metal-Bound Amines structure
spin state
bond length labelb
bond length (Å)
8a (Mo oxide) 8b (W oxide) 8c (Mo oxide) 8d (W oxide) 9a (Mo oxide) 9a (Mo oxide)a 9b (W oxide) 9b (W oxide)a 9c (Mo oxide) 9c (Mo oxide)a 9d (W oxide) 9d (W oxide)a 10a (Mo oxide) 10b (W oxide) 10c (Mo oxide) 10d (W oxide)
doublet doublet quartet quartet doublet doublet doublet doublet quartet quartet quartet quartet doublet doublet quartet quartet
Mo−N W−N Mo−N W−N Mo1−N Mo2−N W1−N W2−N Mo1−N Mo2−N W1−N W2−N Mo1−N W1−N Mo1−N W1−N
1.82 1.83 1.81 1.82 1.94 1.99 1.96 2.00 1.93 2.02 1.96 2.01 2.05 2.05 2.03 2.03
a
Because there are two metal−nitrogen bonds in the bridging metalbound amine, we have two metal−nitrogen distances. Indices 1 and 2 correspond to the labels shown in Figure 9a−d. bBond length labels as used in the corresponding structures 8a−8d, 9a−9d, and 10a−10d. 7193
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lines) wherein with the quartet states, the final products (Figure 9c,d) are not kinetically favorable over the intermediate adducts (Figure 10c,d) with the barriers for the formation of a metal−
formation of a metal−oxygen bridging bond in the doublet electronic states. Interestingly, the quartet electronic states (vide supra) appear to be relatively insensitive to the nature of the nonmetal in the reacting small molecule. The enlarged metal−metal distances in the quartet states seem to contribute to this facet.
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CONCLUSIONS In this mechanistic study, we have studied the fluxionality pathways of W3O6− and Mo3O6− clusters when they react with NH3 and H2S to gain valuable insight into the various factors that play a role in the fluxionality pathway of transition metal oxide clusters. With H2S, the fluxionality pathway provides a way to utilize both the hydrogens of H2S, thereby providing interesting opportunities in the area of desulfurization catalysis and the petrochemical industry. The doublet electronic states of both W3O6− and Mo3O6− show similar reactivity patterns. The preparation step with both metals leads to an intermediate which does not facilitate S−H···O hydrogen bonding necessary for a facile proton-hop. Thus, a substantial barrier for the key proton-hop step is subsequently noticed. With the quartet states, however, the preparation step results in the right conformation and, hence, the barrier for the proton-hop is not large. Overall, the fluxionality pathway with H2S results in final products lower in energy than the intermediate adducts formed before the proton-hop. In the reactions with NH3, whereas the quartet states exhibit similar behavior, the doublet states exhibit metal-dependent behavior in the preparation step and the proton-hop step. With the doublet electronic states of Mo3O6−, the right conformation for achieving the proton-hop is obtained and hence the barrier for the proton-hop is found to be small. With W3O6−, however, the barrier for the proton-hop step is high as the preparation step does not lead to the favored conformation. Although the fluxionality pathway with ammonia is not thermodynamically favorable overall, the bridging nitrogen at the end of the fluxionality pathway still remains attached to a hydrogen that opens up more channels for further reactivity.
Figure 10. Intermediate adducts obtained in the second step of the fluxionality pathway with NH3. 10a and 10b correspond to doublet electronic states, and 10c and 10d correspond to the quartet states.
nitrogen bridging bond being larger. However, the major difference between NH3 reactivity and H2S reactivity with the Mo and W oxides is the metal dependence shown by the doublet states in the proton-hop step (vide supra). Overall, with the quartet states as well, the final products 9c and 9d are less stable than the adducts 10c and 10d. e. Differences due to Oxygen, Sulfur, and Nitrogen in the Fluxionality Pathway. Both herein as well as in our initial theoretical study40 of proton-assisted fluxionality in TMO clusters, we observe that the proton-hop step and the preparation step leading to the proton hop are the key steps in the fluxionality process. This study with NH3 and H2S allows us further to point out the differences arising due to the nature of the nonmetal in the reacting small molecule. Most of these differences predominantly seem to arise in the doublet electronic states of the metal oxides. In the reactions with hydrogen sulfide, the different hydrogen bond strengths of O−H and S−H donors52 play a key role. In the preparation step with the doublet electronic states involving stronger O− H···O hydrogen bonds, the right conformation for the protonhop is obtained in the preparation step.40 However, between the S−H···O or O−H···S hydrogen bonds, the stronger O− H···S hydrogen bonds are preferred as indicated by structures 3a and 3b obtained in the corresponding preparation steps. Subsequently, higher energy barriers are seen for the protonhop step (vide supra). In the ammonia reactions, the metal dependence noticed stems from the competition between N− H···O hydrogen bonds with two different acceptor oxygen atoms. Overall, hydrogen bonding plays a crucial role in determining the fate of the fluxionality pathway for the doublet states. Other minor differences due to the nature of the nonmetal in the small molecule (i.e., O or N or S) are observed in the final step, wherein the formation of a metal−sulfur or a metal− nitrogen bridging bond is kinetically more favorable than the
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ASSOCIATED CONTENT
S Supporting Information *
Additional functions added to the double-ζ and the triple-ζ basis sets as well as the Cartesian coordinates of all the optimized structures. This information is available free of charge via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Shivnath Mazumdar for fruitful discussions. This work was supported by Department of Energy Grant No. DEFG02-07ER15889.
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REFERENCES
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dx.doi.org/10.1021/jp303593d | J. Phys. Chem. A 2012, 116, 7189−7195
