The Fundamental Noninnocent Role of Water for the Hydrogenation of

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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The Fundamental Noninnocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-based Catalysts Jesús A. Luque-Urrutia‡ and Albert Poater*,‡ ‡

Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Campus Montilivi, 17003 Girona, Catalonia, Spain S Supporting Information *

Scheme 1. Hydrogenation of N2O by the PNP-pincer Ru based catalyst 1 (P = P(iPr)2)

ABSTRACT: The hydrogenation of nitrous oxide by PNP pincer ruthenium complexes supposes a promising way to functionalize a hazardous gas and reduce the greenhouse effect, generating dinitrogen and water. Here, by DFT calculations we describe not only the whole mechanism for such a green transformation but we unravel the fundamental role of water, without which the reaction could not go forward. Water assists mandatorily in the H transfer to generate the hydroxyl group together with the release of dinitrogen.

T

he greenhouse effect (GE) has become a worldwide problem that keeps on affecting the world climate and the beings that live in it. The main perpetrators for this phenomenon are the greenhouse gases: carbon dioxide (CO2), methane (CH4), ozone-depleting substances (ODS), hydrofluorocarbons (HFCs), sulfur hexafluoride (SF6), perfluorocarbons (PFCs), and nitrous oxide (N2O).1,2 The gas with the most impact right now is CO2, being over 60% of the GE cause; however, the other ones should not be overlooked. N2O has been making ground over the years due to the increase in nitrogenized substances for our daily life,3 such as fertilizers and pesticides or burning fossil fuels.4 These processes add N2O to the atmosphere, and due to its steadystate lifetime (120 years), it has been increasing for decades.5,6 The reductive process that consists of the hydrogenation of N2O by means of H2 to generate innocuous N2 and H2O has turned out to be feasible by heterogeneous7 and homogeneous8,9 catalytic systems. For the latter systems, the reaction was indeed not catalytic, but simply stoichiometric,8 and further, computationally, this reaction was scarcely studied. Thus, key mechanistic insights are missing. In order to address these drawbacks, and following the recent Milstein et al. catalytic work,9 we have focused our efforts on unveiling the full mechanism for catalyst 1 in Scheme 1. The latter catalyst displayed the highest TON for the reaction that converts N2O and H2 into innocuous N2 and H2O. This catalyst bears a PNP pincer ligand,10,11,12 which consists of an equatorial tridentate ligand that binds to the ruthenium center by two phosphorus atoms and one nitrogen atom.13−15 Recently, the computational studies by Gonçalves and Huang turned out to be fundamental for the rationalization of the reactivity of pincer based complexes from an aromatic point of view.16 These pincer type ligands have shown many experimental applications such as the formation of NH3 starting from N217 and what © XXXX American Chemical Society

could be its counterpart, the cleavage of the N−H bond in NH318 as well as many others. In terms of transition metal centers, Fe,19,20 Ru,21,22 Ir,23,24 Mo,25,26 Ni,27,28 W,29,30 and Pd31,32 have been studied, showing a growing interest in this kind of pincer ligands and a high applicability to many areas.15 From the initial catalyst 1, i.e., a nearly symmetrical species, a N2O molecule is added by the attack of its oxygen to the Ru−H moiety, the bond of which breaks at the same time, and consequently the former hydride protonates the terminal nitrogen atom (see Figure 1).33 Overall, the transition state that defines this concerted attack of the entering N2O turns out to be the highest energy barrier, with 26.7 kcal/mol. The next step implies the formation of the hydroxo ligand on ruthenium with the concerted release of a N2 molecule. However, this step is again not trivial. It requires an energy amount of 33.5 kcal/ mol (see Figure 2a), whereas when assisted by a water molecule, the energy barrier decreases to 25.5 kcal/mol (see Figure 2b). This means that in order to go forward, traces of water in the solvent used, THF, are mandatory.34,35 THF was found to not assist this step, even not facilitating it. Anyway, the barrier corresponding to the I → I−II + H2O step represents the rate-determining step (rds) since it describes the upper energy point of the whole reaction pathway, even though the previous energy barrier corresponding to the step 1 → 1−I is slightly higher in energy by 1.2 kcal/mol.36 Once the hydroxo species II is formed, any of both methylidene moieties of the pincer ligand provides a proton, generating the aqua complex III, overcoming a facile transition state. Then, the H2O liberation means the generation of catalyst 2. From 2, the coordination of H2 to the vacant position of the ruthenium center, followed by a H-transfer to the close Received: October 13, 2017

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DOI: 10.1021/acs.inorgchem.7b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. Full mechanism for the hydrogenation of N2O. Isopropyl ligand represented. tert-Butyl and phenyl ligands’ energy shown as well (green and pink energies respectively). (Gibbs free energies in kcal/mol, and referred to catalyst 1.)

For the sake of a direct comparison, after replicating their calculations (M05-2X/6-311++G(2d,p) ∼ sdd //M05-2X/631G(d,p) ∼ sdd) with the computational scheme used here (M06/aug-cc-PVTZ ∼ sdd//M06/TZVP ∼ sdd), we obtained an energy difference of 4.7 kcal/mol. We attribute the increase of the preference for 1 with respect to 3 to the increased flexibility of the pincer ligand that bears 1, thanks to the second P atom, over a N atom of 3. This is explained due to the increased preference for the sp 3 hybridization of the phosphorus over the nitrogen atom, together with the smaller size of the latter atom (98 pm for P vs 65 pm for N in Bohr’s atomic radius model).38 Alternative pathways were also tested. First, instead of coordination to the ruthenium center of N2O via the O atom, it was done via the terminal N atom, which turns out to be 12.9 kcal/mol more expensive. Then, without the presence of traces of water, the formation of the isomer I′ is favored by 1.4 kcal/ mol with respect to the direct formation of II (see Figure 1). This step has to be considered as a possible catalyst death since it leads to a peculiar Ru−P bond cleavage, where the oxygen atom is inserted between both atoms (see Figure 3). Once we achieve this undesired species I′, the energy required to reorder the atoms in order to generate II is 58.2 kcal/mol, which is an energy barrier too high bearing the experimental conditions. Thus, this confirms that at an initial stage, traces of water must assist the mechanism development before water molecules formed as a byproduct of the hydrogenation of N2O are released. Furthermore, the assistance of either more water or THF molecules was tested. Among the computed data, for the hydrogenation step IV → 1, a water molecule increased the energy barrier by 25.9 kcal/mol, while a second water molecule also increased the energy barrier of steps I → II and II → III by 9.1 and 2.0 kcal/mol, respectively. Actually, going into detail,

Figure 2. Transition states I → II (a) not assisted or (b) assisted by a water molecule (most of hydrogens omitted for clarity, selected distances given in Å).

methylylidene of the pincer ligand, generates the dihydride species 1, thus closing the catalytic cycle, in a strongly exergonic manner (71.7 kcal/mol). Here, the first thing that should be remarked is that the thermodynamic equilibrium between catalysts 1 and 2 with intermediate IV in between is strongly displaced toward 1, by 14.3 kcal/mol. This was unexpected since the same equilibrium for complex 3 of Wang et al. (see Scheme 2),37 which resembles our catalyst, was found to be only slightly displaced to the hydrogenated species, by only 2.2 kcal/mol. Scheme 2. Catalyst Screening for Hydrogenation of N2O

B

DOI: 10.1021/acs.inorgchem.7b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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therefore, we can correlate this increase in bulkiness to its poorer catalytic performance. However, we noticed that system 2-Ph is indeed less hindered than 2, yet it is slightly worse. This poorer performance bearing phenyls may be due to the stronger C−P double bond that exists between the phosphorus moiety and the pyridine of the PNP pincer in 2-Ph that later must be hydrogenated to regenerate the catalytic hydride species 1. According to the Mayer bond order data for the bond between the P atom and the methylylidene fragment that gets hydrogenated, we observed a clear difference: for 2 being 1.079, while for 2-Ph it is 1.131, whereas upon hydrogenation of the CH moiety, MBOs for species 1 are 0.894 for the Ph and 0.881 for iPr. We have determined the whole reaction mechanism for the pincer based catalyst 1. The possibility of a water autocatalysis at the initial stages of the reaction was determined, while it would be hindered as time progresses and water is formed as a product of the hydrogenation of N2O. In complete absence of water, a peculiar bond cleavage of any of the Ru−P bonds in order to form a Ru−O−P moiety has been computationally highly stable, and we consider it a plausible catalyst poisoning due to the high energy required in order to continue the mechanism and close the catalytic cycle. Between the three studied ligands on P atoms, iPr is indeed the best one, with tBu having increased bulkiness and Ph gaining some conjugation between the phenyls and the pyridine, thus, requiring more energy to overcome the upper barrier of the whole mechanism. Further studies are planned to in silico predict a modified PNP pincer Ru-based catalyst that does not need the presence of any water molecule, together with the control of the potential Ru− P bond cleavage that here has been demonstrated to lead toward the decomposition of the catalyst.

Figure 3. Alternative pathway through intermediate I′: (a) transition state I → I′, (b) intermediate I′ and (c) transition state I′ → II (most of hydrogens omitted for clarity, selected distances given in Å).

the six-membered cycle of the water assisted transition state I → II displayed in Figure 2b is fundamental to explaining its relative high stability. To understand the nature of the pincer ligand and its role in the catalytic studied reaction here, we have also performed an analysis of catalyst 1 replacing the iPr ligands bonded to the phosphorus atoms of the pincer ligand by tert-butyl (tBu) groups (1-tBu) and phenyl (Ph) ligands (1-Ph). The results for both latter catalysts, with poorer catalytic performance with respect to 1, are compared in Figure 1, as well. According to our data, we have found that, just as Milstein et al. found, the isopropyl ligands are better than the tert-butyl and phenyl ligands. Looking into the energy data in Figure 1, we can see that 1-tBu and 1-Ph are both higher in energy in all cases (except III for 1-Ph, vide infra) going up to a maximum energy difference of 12.9 kcal (I−II) for 1-tBu and 7.7 kcal/ mol (I) for 1-Ph. Even though the iPr and the Ph ligands quite resemble one another, for the tBu there are increased differences with respect to the other ligands, which is caused by the higher steric hindrance of the latter ligand. As we can see in Figure 4, the steric hindrance of the tBu groups is remarkable compared to the Ph and the iPr ones;

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02630. Computational details and all XYZ coordinates, energies and 3D structures of all species (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Albert Poater: 0000-0002-8997-2599 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.A.L.-U. thanks Universitat de Girona for a IFUdG2017 predoctoral fellowship. A.P. thanks the Spanish MINECO for a project CTQ2014-59832-JIN, and the EU for a FEDER fund (UNGI08-4E-003)). We thank Profs. David Milstein and Rong Zheng for helpful comments.

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REFERENCES

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Figure 4. Steric maps for the pincer ligands of the catalysts 1, 1-tBu, 1Ph. Ru metal center and CO ligand omitted for clarity (tBu center is out of scale). C

DOI: 10.1021/acs.inorgchem.7b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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