Leaching from Palladium Nanoparticles in an Ionic Liquid Leads to the

Letter pubs.acs.org/JPCL

Leaching from Palladium Nanoparticles in an Ionic Liquid Leads to the Formation of Ionic Monometallic Species Elena E. Zvereva,*,†,‡ Sergey A. Katsyuba,† Paul J. Dyson,§ and Alexey V. Aleksandrov*,∥ †

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre, Russian Academy of Sciences, Arbuzov street 8, 420088 Kazan, Russia ‡ Théorie-Modélisation-Simulation UMR CNRS UL 7565, Université de Lorraine, Boulevard des Aiguillettes 1, BP 70239 54506 Vandoeuvre-lès-Nancy, France § Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology (EPFL), CH-1015, Lausanne, Switzerland ∥ Laboratoire de Biochimie, UMR 7654, Ecole Polytechnique, CNRS, F-91128 Palaiseau cedex, France S Supporting Information *

ABSTRACT: Molecular dynamics simulations and DFT calculations suggest that leaching of palladium species from Pd nanoparticles in ionic liquids does not involve “naked” Pd(0) atoms or neutral ArPdX species formed by oxidative addition of arylhalides. Instead, the ionic liquid contributes largely to leaching of ionic PdX− or PdAr+ species.

D

the surrounding IL, which actively participates in the formation and stabilization of fragments, is essential to understand this process.21 Possible Leaching of “Naked” Pd Atoms f rom Pd Clusters. A series of small Pd clusters, i.e., Pd2, Pd4, Pd6, and Pd7, were used as models of Pd NPs to study leaching in [Mmim][BF4]. Larger species were not regarded, as at least the trends in the relative interaction strength between palladium atoms within Pdn particles or between Pdn species and various components of ILs were shown23−27 to be independent of n. Each Pd cluster was optimized in vacuum at the TPSS/def2-TZVP level of theory with inclusion of dispersion corrections as described in the Methods section of the Supporting Information (SI). In each case, the exiting Pd atom was displaced along the axis joining the center of mass of a cluster and this palladium atom. The displacement distance was varied from zero to ca. 6 Å. Computed free energies for the modeled leaching processes in vacuum and in the IL are shown in Figure 1 for Pd7 and in Figure 1S for the Pd2, Pd4, and Pd6 clusters. For all the studied clusters, the free energy in the IL to displace a Pd atom is higher than in vacuum. Furthermore, for all pathways and all clusters considered, we observe that the free energy to displace a single Pd atom significantly increases with the distance, both in vacuum and in IL. For example, removing a Pd atom from a Pd7 cluster to a distance of only 1 Å leads to an increase in the free energy by ∼35 kcal·mol−1. In general, as shown in Figure 1 and Figure 1S, the IL actually reinforces this tendency. This is

espite extensive investigations on palladium-catalyzed cross coupling reactions in ionic liquids (ILs), one of the most fundamental issues, i.e., whether the catalysis is homogeneous or heterogeneous, remains unclear.1−6 It is believed that catalytic cross coupling systems give rise to palladium nanoparticles (Pd NPs), which actually serve as the source of catalytically active Pd species.1,7,8 The nature of the active species, however, is still a matter of debate.1,9−17 The generation of molecular species via metal detachment from a NP, so-called leaching, can be described as an isolation of small fragments (single atoms or clusters) from insoluble species and their transfer to the liquid phase via a dissolution process. Leaching can be categorized into several main types according to the nature of the process taking place.3 In the present study we focus on leaching of monometallic species. In this case, it is generally assumed that either “naked” Pd atoms could leach from the NP, or the first step of the coupling reaction, namely, oxidative addition, could occur at the NP surface resulting in leaching of the oxidative addition product, i.e., ArPdX species. This latter species initiates a homogeneous catalytic cycle.7 The study of catalytic mechanisms poses many challenges that often cannot be solved solely by experiments.18−20 Computational chemistry is a valuable aid to complement experimental studies of catalytic systems.19,21,22 Recently, the combination of density functional theory (DFT) and molecular dynamics (MD) simulations with hybrid quantum mechanics/ molecular mechanical (QM/MM) potentials has provided insights into the mechanism by which Pd NPs are stabilized in the 1,3-dimethylimidazolium tetrafluoroborate ([Mmim][BF4]) IL.23 In the present study we use a similar approach to address the issue of palladium leaching in ILs. The explicit inclusion of © XXXX American Chemical Society

Received: May 16, 2017 Accepted: July 10, 2017 Published: July 10, 2017 3452

DOI: 10.1021/acs.jpclett.7b01212 J. Phys. Chem. Lett. 2017, 8, 3452−3456

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The Journal of Physical Chemistry Letters

Figure 1. Free energy required to displace a Pd atom from the Pd7 cluster in vacuum and IL. (A, B) Two possible displacement paths are shown. Distances are given relative to the position with the minimal energy.

Figure 2. Pd dissociation from PhPd2Br. (A) Structures of the species formed along the nudged elastic band (NEB)33 path: starting adduct (1), remaining part of the adduct (2) and leaving atom (3). (B) Distance between the Pd species as a function of the NEB path. (C) Free energy as a function of the NEB path in vacuum and IL.

likely due to the charged components of the IL that do not interact strongly with a single Pd atom,23 with the IL tending to stabilize larger Pd species via formation of ion layers around them.23 The latter tendency suggests that the IL stabilization of NPs comprising tens or hundreds of Pd atoms should be much more pronounced, and the free energy “cost” for extracting a neutral monometallic species into the IL would be extremely high. This situation contrasts with the results of classical MD simulations of Pd NPs containing up to 1055 atoms in aqueous solution, which demonstrated that the influence of water on the free energy required to abstract a Pd(0) atom from a NP was negligible,28 i.e., the abstraction energy amounted to ca. 44−65 kcal·mol−1, depending mainly on the NP shape and position of the abstracted atom. It should be noted that our simulations neglect another specific feature of ILs, namely−kinetic effects of NP stabilization in ILs, which are expected to be strong in dense layers of structured IL accumulated around large Pd species,23 and additionally retarding the leaching of a separate Pd(0) atom. Our results are in line with experimental findings, which show that in the absence of additional reactants leaching of Pd atoms from Pd NPs into liquid environment is negligible.29 Possible Leaching of Pd Species af ter Oxidative Addition. The possibility of leaching a Pd species from the reaction of the Pd2 cluster with bromobenzene, as one of the typical components of a cross-coupling reaction, was investigated. Such adducts (Figures 2 and 3) may form after oxidative addition of aryl halide to Pd during the first stage of the cross-coupling reaction.30,31 According to our QM/MM computations, formation of the PhPd2Br molecule from the Pd2 cluster and PhBr is barrierless and strongly exoenergic in the gas phase and in the IL, resulting in a dramatic lowering of the free energy of ca. −100 and −120 kcal·mol−1, respectively (Figure 2S, SI).32 We considered two possible scenarios; in the first a PhPdBr species is removed, whereas in the second proposed mechanism leaching of a PdBr− ion is considered. In the first mechanism the PhPdBr species departs from the remaining single palladium atom in the direction perpendicular to the plane of the phenyl ring. Figure 2 shows structures along this pathway

Figure 3. PdBr− dissociation from PhPd2Br. (A) Structures of the species formed along the NEB path: starting adduct (1), remaining part of the adduct (2) and leaving PdBr− (3). (B) Distance between the Pd species as a function of the NEB path. (C) Free energy as a function of the NEB path in vacuum and IL.

and free energies in vacuum and in IL computed for this reaction. A complex nonuniform behavior of the free energy may be explained by structural reorganization of the PhPdBr species, i.e., Pdb (Figure 2) shifts to the plane of the phenyl ring simultaneously with the departure from Pda. Mulliken atomic charges for the adduct PhPd2Br are given in Table 1S. As expected, the two “halves” of the dissociated adduct have 3453

DOI: 10.1021/acs.jpclett.7b01212 J. Phys. Chem. Lett. 2017, 8, 3452−3456

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The Journal of Physical Chemistry Letters

Table 1. Interaction Energies (IEs)a between Various Entities and IL Computed within the Framework of QM/MM MD Simulations and Formation Energies of the Species in Vacuum species b

PhPd2Br (1) PhPdBr (2)b Pd (3)b PhPd2Br (1)c PhPd+ (2)c PdBr− (3)c

IE of the species with the IL, kcal·mol−1

IE(1)−IE(2)−IE(3), kcal·mol−1

formation energy of (1) from (2) and (3) in vacuum, kcal·mol−1

−26.9 (±5.6) −31.3 (±5.4) −3.5 (±1.9) −26.9 (±5.6) −117.4 (±11.0) −99.5 (±13.2)

7.9 (±9.1)

−74.6

190.0 (±18.1)

−199.0

a

See SI for details. Standard deviations of IEs in MD simulations are given in parentheses. bNumbering according to Figure 2A. cNumbering according to Figure 3A.

Figure 4. Organization of the IL around the PhPd2Br species (A) and at a distance of 5.0 Å following dissociation of the Pda and Pdb (B). IL components closely interacting with PdaBr− and PdbPh+ species are marked by α and β respectively. Radial mass density distributions of the [BF4]− anion (C) and [Mmim]+ cation (D) around the PhPd2Br species (blue line) and after dissociation of Pda and Pdb (red line); the curves for the Pda and Pdb are shown in solid and dashed lines, respectively.

practically net zero charges at a large Pda−Pdb distance. As shown in Figure 2, at a large distance, the free energy to displace PhPdBr from Pda is higher by ca. 5 kcal·mol−1 in IL than in vacuum. This difference may be ascribed to stronger stabilization of the larger PhPd2Br species in the IL relative to smaller separate parts, Pda and PhPdbBr, similar to leaching of a single Pd atom from the Pdn clusters described above. Calculated interaction and formation energies are given in Table 1. The formation energy of PhPd2Br from the Pd and PhPdBr components in vacuum is −74.6 kcal·mol−1, strongly favoring the formation of the PhPd2Br species. Within the error of calculations the IL interacts with the PhPd2Br species more strongly than with the separated Pd atom and PhPdBr species, the difference in the interaction energies being 7.9 kcal·mol−1. Consequently, leaching of PhPdBr from PhPd2Br appears to be even less likely in IL than in vacuum. The IL stabilization effects discussed above are more pronounced for a neutral Pd atom leaching from the larger model oxidative addition product, PhPd4Br (Figure 3S, SI), in which case the increase of the free energy of the system in the IL relative to vacuum amounts to ca. 17 kcal·mol−1. This difference suggests that much larger Pd NPs functioning in real catalytic systems would be very strongly protected by IL from leaching of monometallic neutral species. An alternative leaching mechanism could involve the elimination of a PdBr− ion from the PhPd2Br species. In this case, at a large distance between the PdBr and PdPh species, the former has a charge of almost −1e, while the latter has a

charge of almost +1e. The NEB method33 was used to identify the minimum energy pathway for this reaction in vacuum. To model the final structure following departure of PdaBr from PdbPh, the structures of PdBr− and PdPh+ were optimized separately in vacuum using the DFT level of theory (see the Methods section, SI), and PdaBr was placed in the plane of the PdbPh moiety at a distance of 5.5 Å between the Pda and Pdb centers (Figure 3). Structures at intermediate displacements are shown in Figure 3, and Mulliken atomic charges for the corresponding species are given in Table 2S (SI). In this pathway the Pdb ion shifts to the plane of phenyl ring in the final, dissociated complex, while in the starting PhPd2Br adduct Pda and Pdb are located symmetrically relative to the plane of the phenyl ring and both interact with Br (Figures 2 and 3). The PdbPh species, following separation, carries a positive charge (Table 2S). By contrast, PdaBr has a negative charge. Figure 3 shows the free energies in vacuum and IL computed for this reaction. In this mechanism, the IL stabilizes the charged products influencing free energy profile for dissociation. The maximum on the curve in Figure 3 may be explained by a short distance between the Pda and Pdb ions that does not allow the IL to be positioned between the products to stabilize the charged PdaBr− and PdbPh+ species, in contrast to the case of longer distances between the products. Structures from QM/ MM MD simulations of the PhPd2Br dissociation reaction and corresponding ion distributions are shown in Figure 4 at different displacements of PdaBr−. As expected, at a large Pda− 3454

DOI: 10.1021/acs.jpclett.7b01212 J. Phys. Chem. Lett. 2017, 8, 3452−3456

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The Journal of Physical Chemistry Letters Pdb distance, the positively charged PdbPh+ species mostly interacts with BF4− anions (Figure 4B, 4C), and the PdaBr− ion is surrounded by a positive layer of [Mmim]+ cations of the IL (Figure 4B, 4D). Interaction energies between the IL and the adducts, as well as formation energies in vacuum, are given in Table 1. In vacuum, the energy to displace PdBr− from the PhPd2Br adduct is 199 kcal·mol−1. The computed free energy in vacuum shown in Figure 3 should approach this value at the infinite Pda−Pdb distance. In the IL, the charged PdBr− and PdPh+ species interact with the surrounding medium much more strongly than the initial neutral PhPd2Br adduct. The gain in the interaction energy with the IL (190 ± 18 kcal·mol−1) almost compensates the lost of energy (199 kcal·mol−1) caused by breaking the species into the two parts in vacuum. Thus, the free energy needed to completely remove PdBr− from PdPh+ in the IL amounts to ∼9 kcal·mol−1, and the red curve in Figure 3 should approach this value at infinite displacement. As the maximal free energy along the pathway of this reaction amounts to 50 kcal·mol−1 (Figure 3), the barrier for the extraction of PdBr− from PhPd2Br in IL is 41 kcal·mol−1. Modeling the separation of PdBr− from the larger oxidative addition product, PhPd4Br, demonstrated that the lowering of the free energy of the process in the IL relative to vacuum amounts to ca. 35 kcal·mol−1 (Figure 4S, SI). Very similar results shown in Figures 3 and 4S suggest that the estimates of the IL stabilization effects obtained could be extrapolated to larger systems of this kind. In summary, explicit IL MD simulations in combination with the DFT level of theory for free energy calculations were used to probe the formation of monometallic species from Pd clusters as a model for Pd leaching from Pd NPs. The calculations indicate that leaching of a “naked” Pd(0) atom from all clusters is very unlikely in vacuum and, in [Mmim][BF4] is even less likely, with the IL preferring to interact with larger palladium aggregates rather than with monometallic Pd(0) species. Presumably, the larger species exhibit a stronger induced polarization due to interactions with charged components of the IL and, thus, can interact more strongly with the IL compared to neutral, monometallic species.23 The calculations also indicate that local leaching,3,34 i.e., where a single Pd atom separates from the NP surface without leaving the near-surface area, cannot be achieved by solvation effects from the IL. To account for oxidative addition reactions of arylhalides, we also considered the possibility of dissociation of a Pd2bromobenzene and a Pd4-bromobenzene species. The C−C coupling reactions using Pd NPs involve the oxidative addition of an aryl halide Ar-X to a Pd atom on the NP surface as a first step. It is generally accepted that this Pd atom is then abstracted from the NP as an organometallic monometallic species “ArPdX”.10,12,15,28 The computations indicate that the formation of the neutral PhPdBr oxidative addition product is very unlikely in vacuum and IL. Instead, the calculations suggest that charged PdBr− and PdPh+ species, able to strongly interact with the IL, are more likely to form. The IL reduces the free energy needed to separate PdBr− from PdPh+, with cationic and anionic components of the IL strongly solvating these charged Pd species, thereby stabilizing them. Although the energy barrier obtained for this mechanism amounts to ca. 40 kcal· mol−1, we expect that for aryl-halide adducts with comparatively large Pd NPs containing surface disordered atoms, the energy barrier should be smaller.28,35

The presence of ionic monometallic Pd species described here in many cross-coupling reactions and other transformations catalyzed by ligand-free palladium species was sometimes postulated on the basis of indirect experimental observations, e.g., ref 15, but to the best of our knowledge never proved, and our results suggest that studies aiming at elucidating of mechanism of such catalytic reactions should take into account leaching of ionic PdX− or PdAr+.

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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.jpclett.7b01212. Computational details; free energy required to displace an individual palladium atom from the Pd2, Pd4, or Pd6 clusters in vacuum and IL (Figure 1S); formation of PhPd2Br from the Pd2 cluster and PhBr (Figure 2S); Mulliken atomic charges of the PhPd2Br species in IL at different position of the displaced palladium within mechanisms 1 and 2 of the leaching (Tables 1S and 2S); Pd dissociation from PhPd4Br (Figure 3S); PdBr− dissociation from PhPd4Br (Figure 4S) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Elena E. Zvereva: 0000-0002-9374-2323 Paul J. Dyson: 0000-0003-3117-3249 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Russian Foundation for Basic Research (Grant 15-0301058 A) is gratefully acknowledged for financial support. REFERENCES

(1) Pérez-Lorenzo, M. Palladium Nanoparticles as Efficient Catalysts for Suzuki Cross-Coupling Reactions. J. Phys. Chem. Lett. 2012, 3, 167−174. (2) Yan, N. The Nature of Metal Catalysts in Ionic Liquids: Homogeneous vs Heterogeneous Reactions. In Ionic Liquids (ILs) in Organometallic Catalysis; Dupont, J.; Kollár, L., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2015; pp 1−15. (3) Eremin, D. B.; Ananikov, V. P. Understanding Active Species in Catalytic Transformations: From Molecular Catalysis to Nanoparticles, Leaching, “Cocktails” of Catalysts and Dynamic Systems. Coord. Chem. Rev. 2017, 346, 2−19. (4) Ayvali, T.; Philippot, K. On the Use of Organometallic Chemistry Concepts for the Synthesis of Nanocatalysts. In New Materials for Catalytic Applications; Elsevier: Amsterdam, 2016; Chapter 3, pp 41− 79. (5) Youquan, D.; Shimin, L. Catalysis with Room Temperature Ionic Liquids Mediated Metal Nanoparticles. In Petrochemical Catalyst Materials, Processes, and Emerging Technologies, Hamid, A.-M.; Tiancun, X., Eds.; IGI Global: Hershey, PA, 2016; pp 285−329. (6) Mastrorilli, P.; Monopoli, A.; Dell’Anna, M. M.; Latronico, M.; Cotugno, P.; Nacci, A. Ionic Liquids in Palladium-Catalyzed CrossCoupling Reactions. In Ionic Liquids (ILs) in Organometallic Catalysis; Dupont, J., Kollár, L., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2015; pp 237−285. (7) Balanta, A.; Godard, C.; Claver, C. Pd Nanoparticles for C-C Coupling Reactions. Chem. Soc. Rev. 2011, 40, 4973−4985. 3455

DOI: 10.1021/acs.jpclett.7b01212 J. Phys. Chem. Lett. 2017, 8, 3452−3456

Letter

The Journal of Physical Chemistry Letters

(27) Yuan, X.; Yan, N.; Katsyuba, S. A.; Zvereva, E. E.; Kou, Y.; Dyson, P. J. A Remarkable Anion Effect on Palladium Nanoparticle Formation and Stabilization in Hydroxyl-Functionalized Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 6026−6033. (28) Ramezani-Dakhel, H.; Mirau, P. A.; Naik, R. R.; Knecht, M. R.; Heinz, H. Stability, Surface Features, and Atom Leaching of Palladium Nanoparticles: Toward Prediction of Catalytic Functionality. Phys. Chem. Chem. Phys. 2013, 15, 5488−5492. (29) Shmidt, A. F.; Mametova, L. V. Main Features of Catalysis in the Styrene Phenylation Reaction. Kinet. Catal. 1996, 37, 406−408. (30) McMullin, C. L.; Fey, N.; Harvey, J. N. Computed Ligand Effects on the Oxidative Addition of Phenyl Halides to Phosphine Supported Palladium(0) Catalysts. Dalton Trans. 2014, 43, 13545− 13556. (31) Hirakawa, T.; Uramoto, Y.; Mimura, D.; Takeda, A.; Yanagisawa, S.; Ikeda, T.; Inagaki, K.; Morikawa, Y. First-Principles Molecular Dynamics Analysis of Ligand-Free Suzuki−Miyaura CrossCoupling in Water Solvent: Oxidative Addition Step. J. Phys. Chem. B 2017, 121, 164−173. (32) According to ab initio MD simulations of oxidative addition of PhBr to Pd(0) atom in water (ref 31) the activation-free-energy barrier for the oxidative additon of PhBr to H2O-ligated Pd (3.4 kcal·mol−1) is slightly larger than that for its addition to the monoligated PdPPh3 (2.4 kcal·mol−1). The barrier calculated for the same process in the gas phase is smaller by ca. 10%. The reaction results in free energy release of ca. 15 kcal·mol−1. A comparison of these estimates with our computations suggests that the oxidative addition to Pd clusters/NPs is energetically much more favourable than the addition to Pd monometallic species. (33) Aleksandrov, A.; Field, M. A Hybrid Elastic Band String Algorithm for Studies of Enzymatic Reactions. Phys. Chem. Chem. Phys. 2012, 14, 12544−12553. (34) Ananikov, V. P.; Beletskaya, I. P. Toward the Ideal Catalyst: From Atomic Centers to a “Cocktail” of Catalysts. Organometallics 2012, 31, 1595−1604. (35) Briggs, B. D.; Bedford, N. M.; Seifert, S.; Koerner, H.; Ramezani-Dakhel, H.; Heinz, H.; Naik, R. R.; Frenkel, A. I.; Knecht, M. R. Atomic-Sscale Identification of Pd Leaching in Nanoparticle Catalyzed C-C Coupling: Effects of Particle Surface Disorder. Chem. Sci. 2015, 6, 6413−6419.

(8) Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J.-M.; Polshettiwar, V. Nanocatalysts for Suzuki Cross-Coupling Reactions. Chem. Soc. Rev. 2011, 40, 5181−5203. (9) Djakovitch, L.; Köhler, K.; de Vries, J. G. The Role of Palladium Nanoparticles as Catalysts for Carbon−Carbon Coupling Reactions. In Nanoparticles and Catalysis; Wiley-VCH Verlag GmbH & Co. KGaA:Weinheim, Germany, 2008; pp 303−348. (10) Reetz, M. T.; de Vries, J. G. Ligand-Free Heck Reactions Using Low Pd-Loading. Chem. Commun. 2004, 1559−1563. (11) Durand, J.; Teuma, E.; Gómez, M. An Overview of Palladium Nanocatalysts: Surface and Molecular Reactivity. Eur. J. Inorg. Chem. 2008, 2008, 3577−3586. (12) Astruc, D. Palladium Nanoparticles as Efficient Green Homogeneous and Heterogeneous Carbon−Carbon Coupling Precatalysts: A Unifying View. Inorg. Chem. 2007, 46, 1884−1894. (13) Corma, A.; Das, D.; García, H.; Leyva, A. A Periodic Mesoporous Organosilica Containing a Carbapalladacycle Complex as Heterogeneous Catalyst for Suzuki Cross-Coupling. J. Catal. 2005, 229, 322−331. (14) Sanhes, D.; Raluy, E.; Retory, S.; Saffon, N.; Teuma, E.; Gomez, M. Unexpected Activation of Carbon-Bromide Bond Promoted by Palladium Nanoparticles in Suzuki C-C Couplings. Dalton Trans. 2010, 39, 9719−9726. (15) de Vries, J. G. A Unifying Mechanism for All High-Temperature Heck Reactions. The Role of Palladium Colloids and Anionic Species. Dalton Trans. 2006, 421−429. (16) de Vries, A. H. M.; Parlevliet, F. J.; Schmieder-van de Vondervoort, L.; Mommers, J. H. M.; Henderickx, H. J. W.; Walet, M. A. M.; de Vries, J. G. A Practical Recycle of a Ligand-Free Palladium Catalyst for Heck Reactions. Adv. Synth. Catal. 2002, 344, 996−1002. (17) Pacardo, D. B.; Slocik, J. M.; Kirk, K. C.; Naik, R. R.; Knecht, M. R. Interrogating the Catalytic Mechanism of Nanoparticle Mediated Stille Coupling Reactions Employing Bio-Inspired Pd Nanocatalysts. Nanoscale 2011, 3, 2194−2201. (18) Schmidt, A. F.; Kurokhtina, A. A. Distinguishing Between the Homogeneous and Heterogeneous Mechanisms of Catalysis in the Mizoroki-Heck and Suzuki-Miyaura Reactions: Problems and Prospects. Kinet. Catal. 2012, 53, 714−730. (19) Sperger, T.; Fisher, H. C.; Schoenebeck, F. Computationally Deciphering Palladium-Catalyzed Reaction Mechanisms. Wiley Interdisciplinary Reviews: Computational Molecular Science 2016, 6, 226− 242. (20) Janusson, E.; Zijlstra, H. S.; Nguyen, P. P. T.; MacGillivray, L.; Martelino, J.; McIndoe, J. S. Real-Time Analysis of Pd2(dba)3 Activation by Phosphine Ligands. Chem. Commun. 2017, 53, 854−856. (21) Vidossich, P.; Lledós, A.; Ujaque, G. First-Principles Molecular Dynamics Studies of Organometallic Complexes and Homogeneous Catalytic Processes. Acc. Chem. Res. 2016, 49, 1271−1278. (22) Vidossich, P.; Ujaque, G.; Lledos, A. Palladium Monophosphine Pd(PPh3): is it Really Accessible in Solution? Chem. Commun. 2014, 50, 661−663. (23) Zvereva, E. E.; Katsyuba, S. A.; Dyson, P. J.; Aleksandrov, A. V. Solvation of Palladium Clusters in an Ionic Liquid: A QM/MM Molecular Dynamics Study. J. Phys. Chem. C 2016, 120, 4596−4604. (24) Katsyuba, S. A.; Zvereva, E. E.; Yan, N.; Yuan, X.; Kou, Y.; Dyson, P. J. Rationalization of Solvation and Stabilization of Palladium Nanoparticles in Imidazolium-Based Ionic Liquids by DFT and Vibrational Spectroscopy. ChemPhysChem 2012, 13, 1781−1790. (25) Zvereva, E. E.; Grimme, S.; Katsyuba, S. A.; Ermolaev, V. V.; Arkhipova, D. A.; Yan, N.; Miluykov, V. A.; Sinyashin, O. G.; Aleksandrov, A. Solvation and Stabilization of Palladium Nanoparticles in Phosphonium-Based Ionic Liquids: a Combined Infrared Spectroscopic and Density Functional Theory Study. Phys. Chem. Chem. Phys. 2014, 16, 20672−20680. (26) Zhao, S.; Ren, Y.; Lu, W.; Wang, J.; Yin, W. A Theoretical Investigation of the Interaction Between Small Pd Particles and 1Butyl-3-Methyl Imidazolium Ionic Liquids with Cl−, BF4− and PF6− Anions. Phys. Chem. Chem. Phys. 2012, 14, 13444−13451. 3456

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