Water Splitting Promoted by a Ruthenium(II) PNN Complex: An

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Water Splitting Promoted by a Ruthenium(II) PNN Complex: An Alternate Pathway through a Dihydrogen Complex for Hydrogen Production K. S. Sandhya and Cherumuttathu H. Suresh* Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology, Trivandrum 695 019, India

bS Supporting Information ABSTRACT: A new mechanism involving the formation of a dihydrogen intermediate is described using the TPSS level of the DFT method for the generation of hydrogen from water catalyzed by the PNN Ru(II) pincer complex 4 (Science 2009, 324, 74). The Mulliken charge of the hydride ligand in 4 is highly negative (
1.574) and facilitates a dihydrogen bonding with one of the solvated water molecules to yield 4 3 3 3 H2O. In the next step, water coordinates to the metal center by forcing a conformational change in the orientations of the hydride and CO ligands to form a transient intermediate, 40 (H2O). The activation free energy, Gact for this step is 14.92 kcal/mol. Subsequently, the intermediate liberates the dihydrogen with a Gact of 10.47 kcal/mol. With respect to (4 + H2O) as reference, the effective Gact of the entire mechanism is calculated to be 32.56 kcal/mol. Unlike the previously reported mechanism, the new mechanism operates without the cooperation of the aromatization
dearomatization processes of the pincer ligand and bypasses a highly reversible pathway involving the water-mediated 4 to 1 conversion. Further, a direct pathway for the formation of the cis-dihydroxo intermediate is possible with the new mechanism.

H

ydrogen production from water under mild reaction conditions is a technologically challenging problem, and advancement in this area gives hope for clean and renewable energy for the future.1 Although many ways exist to produce hydrogen by reducing protons, truly functional and efficient systems for water oxidation are yet to be achieved due to a high energy requirement (1.23 eV) of this four-electron oxidation process.2 One of the most promising methods for a low-energy water splitting pathway can be envisaged if the reaction occurs in the presence of a single metal system acting as homogeneous catalyst for both hydrogen and oxygen production under the influence of heat and light.1d,3 A breakthrough discovery is reported in this area in a recent paper by Milstein and co-workers, where they showed homogeneous catalytic activity for a trans-hydrido-hydroxo ruthenium(II) complex containing a PNN pincer ligand (1) toward water oxidation (Figure 1).4 Complex 1 in refluxing water for three days yielded H2 with concomitant formation of the cis-dihydroxo complex 2. Further, 2 under photochemical activation led to an intramolecular reaction to liberate O2. The O2 formation was explained on the basis of reductive elimination of H2O2 from the dihydroxo system and subsequent catalytic disproportion of the peroxide to water and oxygen under the influence of light. Recently light-driven O2 formation was theoretically well explored, supporting experimental findings.5 Yoshizawa et al.6 were the first to theoretically propose a mechanism by the solvent-effect-incorporated B3LYP method of r 2011 American Chemical Society

the thermal production of hydrogen using a model system for 1 (P and N substituents were Me). In the rate-determining transition state (TS1), a proton from the P side arm migrated to the hydride ligand and led to the formation of H2 and monohydroxide complex 3. The reaction of 3 with water produced the trans isomer of the dihydroxo complex 2. One drawback of this mechanism is that it could not satisfactorily explain the formation of the cis-dihydroxo complex, an essential system to describe the subsequent photochemical step of the reaction. The activation free energy (Gact) based on TS1 in the solvent phase was 37.6 kcal/mol, while that in gas phase was 33.8 kcal/mol. In a more detailed mechanistic study, Yang and Hall described the thermal hydrogen evolution reaction of 1 at the TPSS (Tao
Perdew
Staroverov
Scuseria) level of the DFT method (solvent effect incorporated) as well as reported time-dependentDFT results for a photolytic process.5b The Gact for the ratedetermining step of the hydrogen formation step was 32.8 kcal/mol. They also described the reaction of the nonaromatic pincer Ru(II) complex 4 with water, yielding the aromatic transhydrido-hydroxo complex 1. Milstein et al. previously reported the use of 4 as a powerful catalyst for the coupling of alcohols to Received: January 19, 2011 Published: June 27, 2011 3888

dx.doi.org/10.1021/om200046u | Organometallics 2011, 30, 3888–3891

Organometallics

NOTE

Figure 1. Selected PNN Ru(II) complexes and transition states studied in the Yoshizawa and Yang
Hall mechanism.

form esters with the liberation of H2.7 The results of Yang and Hall on the water-mediated conversion of 4 to 1 suggested a very facile reaction, as the Gact was 12.9 kcal/mol. Further, the Yang
Hall mechanism could explain the formation of the cisdihydroxo complex 2 by invoking the condition that 3 will undergo a conformational change to a more stable isomer 5 and then undergo reaction with water via TS2 to yield 2. In this paper we describe an alternate low-energy pathway for the hydrogen evolution step of the reaction of Milstein et al.4 To describe such a mechanism, the attack of water on the hydride ligand of 4 is considered. This mode of reaction will lead to the formation of a weak complex 4 3 3 3 H2O, which then undergoes dihydrogen bond formation. For this study, all calculations were carried out with the TPSS level of DFT methodology, the same method used by Yang and Hall.5b The Stuttgart fully relativistic effective core potential (ECP) with augmented correlation consistent polarized valence double-zeta basis set for Ru and the 6-31++G(d,p) basis set for all other atoms were used.8 One imaginary frequency was confirmed in every transition state geometry by vibrational frequency calculations. Solvent effects were considered through singlepoint calculations on optimized geometries using the polarizable continuum method (PCM) implemented in Gaussian03.9 In Figure 2, the 4 3 3 3 H2O complex is shown. An interesting observation is the short H 3 3 3 H distance of 1.649 Å between the hydride ligand and hydrogen of the water molecule. An attractive electrostatic H 3 3 3 H interaction exists in 4 3 3 3 H2O, as the H of the water is positively charged (Mulliken charge = 0.397) while the hydride ligand is negatively charged (Mulliken charge =
1.574). The high negative Mulliken charge on the hydride ligand is rather a surprising feature. The interaction distance of 1.649 Å is in the range of a reasonably strong dihydrogen interaction. For instance, in the case of (HMgH)2 3 3 3 (HNC)2, Alkorta et al.10 reported a dihydrogen interaction distance of 1.658 Å and interaction energy of 12.7 kcal/mol. In some cases, dihydrogen bonds formed by transition metal hydrides show comparable strength to classical hydrogen bonds,11 and in such systems, the H 3 3 3 H interaction distances were typically in the range 1.7
2.2 Å. Thus the charge and distance features of the H 3 3 3 H interaction in 4 3 3 3 H2O accounts for a typical X-Hδ+ 3 3 3 δ
H-M type dihydrogen bonding.10,12 Bader's atoms in molecule (AIM) analysis is used to confirm the dihydrogen bonding in metal hydrides.13 In 4 3 3 3 H2O, a characteristic (3,
1) bond critical point is identified for the

Figure 2. (a) Bridged fashion attack of water to 4. (b) Molecular graph of 4 3 3 3 H2O.

dihydrogen bond. At the critical point, the electron density, F, Laplacian of F (r2F), and total electron energy density (H) were 0.0234, 0.0448, and
0.0016, respectively, in atomic units. According to Popelier,14 a dihydrogen bond will have a F value within the range 0.002
0.035, whereas its r2F will fall in the range 0.024
0.139. Thus the F and r2F values of 4 3 3 3 H2O are much larger than the proposed lower limits of Popelier and suggest a reasonably strong dihydrogen bond.14 Further, the negative value of H suggests a slightly covalent character of the dihydrogen interaction. The strong H 3 3 3 H interaction in 4 3 3 3 H2O facilitates the coordination of the water to the metal center by forcing a change in the orientation of the hydride ligand in the axial position to the equatorial position as well as the CO ligand in the equatorial position to the axial position. The intermediate complex 40 (H2O) thus formed via TS4 required an activation free energy, Gact, of 14.92 kcal/mol. The relative free energy of the intermediate is 22.09 kcal/mol higher than the reactants (4 + H2O). In the subsequent step, 40 (H2O) passes through the transition state TS3 (Gact = 10.48 kcal/mol) to yield the dihydrogen complex 6 3 3 3 H2 (Figure 3). Apparently with respect to the infinitely separated 4 and H2O, the effective Gact for the dihydrogen formation can be considered as 32.56 kcal/mol, the relative free energy of TS3. However, since the formation of the solvated 4 3 3 3 H2O structure is a distinct possibility in excess water, the effective Gact could be the difference in the relative free energy of 4 3 3 3 H2O and TS3 (20.17 kcal/mol). The highly charged hydride ligand of 4 is vulnerable to attack by water, and such a process is justifiable because the water as solvent is always in contact with the first solvation shell of the complex. 3889

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Figure 3. Solvent-corrected relative free energy profile for the formation of hydrogen through the dihydrogen complex. The Et and tBu substituents are shown with notations. All values are in kcal/mol.

It may be noted that Milstein et al.4 reported a reversible process for the water-mediated 4 to 1 conversion. In the Yang
Hall mechanism, 1 is formed from 4 when H2O coordinates trans to the hydride ligand followed by a subsequent proton migration from water to the unsaturated P side arm (Gact = 12.9 kcal/mol) of the pincer ligand. They also used 1 in subsequent steps of the mechanism of dihydrogen formation. Interestingly, in the Yang
Hall mechanism, relative free energies of (4 + H2O) and 1 were nearly the same, and therefore the reverse process (conversion of 1 to 4) was as likely as the forward reaction. This result is in good support of Milstein’s result that 1 and 4 are in equilibrium. In the immediate next step from 1, Yang and Hall described the migration of the proton from the saturated P side arm (in the previous step it was unsaturated) to the hydride ligand to generate dihydrogen. Obviously, this step of the reaction (Gact = 32.8 kcal/mol) has to compete with the much easier 1 to 4 conversion. The new mechanism described herein for the formation of 6 3 3 3 H2 can be considered as a viable alternative to that of the Yang
Hall mechanism only if we invoke the condition that 4 3 3 3 H2O is present in the reaction mixture. Otherwise, considering the associative nature of the complexation process for 4 3 3 3 H2O formation, the loss of entropy must be included in the calculation of Gact. Even with (4 + H2O) as the reference point, the Gact for the formation of 6 3 3 3 H2 (32.56 kcal/mol) is 0.24 kcal/mol lower than the Yang
Hall mechanism. Complex 6, obtained by eliminating H2 from 6 3 3 3 H2, showed a structure somewhat similar to 5. In 5, the OC
Ru
OH angle

was 110° (Yang
Hall mechanism), while in 6, it was 149°. A conformational change from 6 to 5 is expected to be nearly barrierless due to a small amount of structural reorganization involving OC
Ru
OH angle bending and small twists around alkyl substituents on P and N. However, all attempts to locate a transition state for this conformational change failed due to the shallow nature of the stationary point. Starting from 5, the formation of the cis-dihydroxo complex 2 can be easily explained by following the Yang
Hall pathway. Otherwise, a direct coordination of water on 6 can yield the complex 7 3 3 3 H2O (Figure 3). Formation of 2, the cis-dihydroxo complex from 7 3 3 3 H2O, is easily obtained (Gact = 2.35 kcal/mol) and also illustrated in Figure 3. The ZPE- and solvation-effect-corrected total energy profile is provided in the Supporting Information, which showed that the rate-determining step involves TS3 and has the highest activation energy of 21.96 kcal/mol (with respect to 4 + H2O). Overall the reaction is exothermic by 5.40 kcal/mol. In conclusion, we have described an alternate mechanism for the first stage of hydrogen formation from a ruthenium catalyst developed by Milstein and co-workers. This mechanism operates through the formation of a dihydrogen complex, 4 3 3 3 H2O. The new mechanism may be more attractive than the Yang
Hall mechanism due to the fact that it can bypass the highly reversible 1 to 4 conversion pathway. Moreover, the binding energy of water for coordinating with Ru in 4 to form complex 4(H2O) was only 10.58 kcal/mol, and under reflux experimental conditions such a complex may not be formed, as the water can easily dissociate from the complex. In the present mechanism, water is 3890

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Organometallics playing the role of a solvent and not of a coordinating ligand to 4. Therefore, the chances are quite high for the development of dihydrogen contact interactions, which will activate the OH bond of water at the outside of the coordination sphere of the metal center. Therefore, we suggest that the pathway via a dihydrogen complex is a viable alternate route for liberating H2 from water, or at least it may coexist with the Yang
Hall pathway.

’ ASSOCIATED CONTENT

bS

Supporting Information. Cartesian coordinates of all the systems are available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +91-471-2491712.

NOTE

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’ ACKNOWLEDGMENT This research is supported by the EMPOWER scheme (OLP132839) of CSIR, Govt. of India. A fellowship from CSIR, received by S.K.S., is also gratefully acknowledged. ’ REFERENCES (1) (a) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729–15735. (b) Lubitz, W.; Tumas, W. Chem. Rev. 2007, 107, 3900–3903. (c) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem. 2008, 1, 26–58. (d) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141–145. (2) Ruttinger, W.; Dismukes, G. C. Chem. Rev. 1997, 97, 1–24. (3) (a) Lazarides, T.; McCormick, T.; Du, P.; Luo, G.; Lindley, B.; Eisenberg, R. J. Am. Chem. Soc. 2009, 131, 9192–9194. (b) Sala, X.; Romero, I.; Rodriguez, M.; Escriche, L.; Llobet, A. Angew. Chem., Int. Ed. 2009, 48, 2842–2852. (c) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159–244. (d) McCormick, T. M.; Calitree, B. D.; Orchard, A.; Kraut, N. D.; Bright, F. V.; Detty, M. R.; Eisenberg, R. J. Am. Chem. Soc. 2010, 132, 15480–15483. (e) Wang, M.; Na, Y.; Gorlov, M.; Sun, L. Dalton Trans. 2009, 33, 6458–6467. (4) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; David, Y. B.; Iron, M. A.; Milstein, D. Science 2009, 324, 74–77. (5) (a) Chen, Y.; Fang, W. H. J. Phys. Chem. A 2010, 114, 10334–10338. (b) Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2010, 132, 120–130. (6) Li, J.; Shiota, Y.; Yoshizawa, K. J. Am. Chem. Soc. 2009, 131, 13584–13585. (7) (a) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790–792. (b) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840–10841. (8) (a) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phy. Rev. Lett. 2003, 91, 146401–146404. (b) Peterson, K. A.; Figgen, D.; Dolg, M.; Stoll, H. J. Chem. Phys. 2007, 126, 124101–124112. (c) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724–729. (9) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; 3891

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