Pd and Pt Catalyst Poisoning in the Study of Reaction Mechanisms

Research Article Cite This: ACS Catal. 2019, 9, 2984−2995

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Pd and Pt Catalyst Poisoning in the Study of Reaction Mechanisms: What Does the Mercury Test Mean for Catalysis? Victor M. Chernyshev,† Alexander V. Astakhov,† Ilya E. Chikunov,† Roman V. Tyurin,† Dmitry B. Eremin,‡ Gleb S. Ranny,† Victor N. Khrustalev,§,∥ and Valentine P. Ananikov*,‡,† †

Platov South-Russian State Polytechnic University (NPI), Prosveschenya 132, Novocherkassk 346428, Russia Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 119991, Russia § National Research Center “Kurchatov Institute”, Acad. Kurchatov Sq. 1, Moscow 123182, Russia ∥ Peoples’ Friendship University of Russia, Miklukho-Maklay St. 6, Moscow 117198, Russia ACS Catal. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/08/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: The mercury test is a rapid and widely used method for distinguishing truly homogeneous molecular catalysis from nanoparticle metal catalysis. In the current work, using various M0 and MII complexes of palladium and platinum that are often used in homogeneous catalysis as examples, we demonstrated that the mercury test is generally inadequate as a method for distinguishing between homogeneous and cluster/nanoparticle catalysis mechanisms for the following reasons: (i) the general and facile reactivity of both molecular M0 and MII complexes toward metallic mercury and (ii) the very high and often unpredictable dependence of the test results on the operational conditions and the inability to develop universal quantitatively defined operational parameters. Two main types or mercury-induced transformations, the cleavage of M0 complexes and the oxidative−reductive transmetalation of MII complexes, including a reaction of highly popular MII/NHC complexes, were elucidated using NMR, ESI-MS, and EDXRF techniques. A mechanistic picture of the reactions involving metal complexes was revealed with mercury, and representative metal species were isolated and characterized. Even in an attempt to not overstate the results, one must note that the use of the mercury tests often leads to inaccurate conclusions and complicates the mechanistic studies of these catalytic systems. As a general concept, distinguishing reaction mechanisms (homogeneous vs cluster/nanoparticle) by using catalyst poisoning requires careful rethinking in the case of dynamic catalytic systems. KEYWORDS: homogeneous catalysis, heterogeneous catalysis, mechanisms, mercury test, Pd complexes, Pt complexes, nanoparticles

1. INTRODUCTION One of the main problems in studies of transition-metalcatalyzed reactions in solutions is the determination of the nature of active catalytic centersif they are molecular complexes or metal clusters/nanoparticles.1−15 This task is of great importance both for catalysis with well-defined molecular metal complexes, for example, Pd/NHC complexes, which are typically considered homogeneous catalysts, and for supported metal catalysts, such as Pd/C.15−19 Thus, well-defined molecular metal complexes can undergo decomposition during preactivation or in the course of a catalytic process to give metal clusters and nanoparticles possessing substantially higher catalytic activities, allowing them to become the predominant active centers.1−5,7−15,20−22 On the other hand, for heterogeneous catalysts, the leaching of supported metals into the solution to give active molecular complexes is common.4,12,15−18,21,23−25 Reliably distinguishing the nature of the active centers and the type of catalyst (homogeneous or cluster/nanoparticle) is of great significance for tuning the reaction conditions and © XXXX American Chemical Society

designing new generations of highly efficient, stable, and sustainable catalysts.1−5,7−15 Among the various methods, the mercury test (or “Hg test”, “mercury poisoning test”, “Hg poisoning test”, and “Hg drop test”) is one of the most frequently used rapid methods for distinguishing between truly homogeneous molecular catalysis and cluster/nanoparticle catalysis.1−5,7−9,11,15−18,20,21,26 The method is based on the assumption that metallic mercury will poison M0 clusters/nanoparticles that are acting as catalytically active centers and is inert toward molecular metal complexes.3,5,11 Inhibition of a catalytic reaction in the presence of metallic mercury is typically considered to be evidence of a cluster/nanoparticle in the catalytic mechanism. In contrast, the absence of a significant effect of mercury on a metal-catalyzed Received: September 13, 2018 Revised: January 28, 2019

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DOI: 10.1021/acscatal.8b03683 ACS Catal. 2019, 9, 2984−2995

Research Article

ACS Catalysis Scheme 1. Operating Principle of the Mercury Test in Mechanistic Studies of Catalytic Systems

mercury,37−39,41−50 numerous research groups have continued to apply the mercury test in catalytic studies35 even though the accuracy of the method is questionable. In the current work, we investigated the reactivity of a number of Pd and Pt compounds that are widely applied in catalysis toward metallic mercury and the effect of important operational parameters, such as mercury loading and agitation intensity, on the results of the mercury test for the Mizoroki−Heck reaction performed with various catalysts. Surprisingly, all of the studied compounds revealed facile reactivity toward metallic mercury that was unexpected with the mercury test (Scheme 2). For

reaction is accepted as characteristic of a homogeneous mechanism (Scheme 1). In previous mechanistic studies of several reactions catalyzed by complexes of Pd with N-heterocyclic carbenes (Pd/NHC), we performed control experiments to ascertain the inertness of the complexes toward metallic mercury. Surprisingly, solutions of Pd complexes, even highly stable PdII/NHC complexes, were decomposed by metallic mercury to give complex mixtures of products and dark metal precipitates on the surface of the mercury. This observation prompted us to thoroughly evaluate the applicability of the mercury test. The poisoning effect of mercury on the catalytic activity of palladium and platinum colloids was described 100 years ago by Paal and Hartmann27 and has been applied from time to time to suppress heterogeneous catalysis.28−34 Whitesides and coworkers demonstrated the successful use of this method to suppress unwanted nanoparticle catalysis in molecular transformations of organoplatinum compounds,31,34 since then the mercury test was widely used as a rapid tool for distinguishing between “homogeneous” and “heterogeneous” pathways. The application of the mercury test for distinguishing molecular from cluster/nanoparticle mechanisms of metal-catalyzed reactions has been mentioned in thousands of published papers.35 However, the mercury test has been the subject of criticism.3−5,11,18,36 Thus, Whitesides and coauthors warned about the nonuniversality of the method and the possibility of reactions between mercury and some organometallic compounds.34 Several articles have reported the incorrectness of the method for studying some organometallic compounds of Pd, Pt, Rh, and other metals due to their intrinsic reactivity toward metallic mercury.36−39 Notably, in the case of a molecular metal catalyst with sufficient reactivity toward mercury, its decomposition rate can be comparable to or higher than the rate of the catalytic reaction; therefore, mercury should inhibit the catalytic reaction even if only a truly homogeneous molecular catalyst is operational. Therefore, the mercury test can be inappropriate for studying reactions catalyzed by metal complexes possessing intrinsic reactivity toward metallic mercury. Moreover, several research groups have warned about the significant dependence of the results of the mercury test on the operational parameters, especially the mercury loading and agitation intensity.3,11,40 Surprisingly, despite the reports concerning problems with the mercury test36−38 and a substantial number of publications describing sufficiently high reactivity of various palladium, platinum, and other organometallic compounds toward metallic

Scheme 2. Typically Assumed Reactivity and the Real Reactivity (Described Herein) Involved in the Mercury Test

example, a reaction involving an oxidative−reductive transmetalation of MII/NHC complexes (M = Pd, Pt) with metallic mercury to give HgII/NHC complexes has been disclosed. Most of the studied complexes decomposed at rates higher than or comparable to those of the typical catalytic reactions. Moreover, the ease of obtaining false results by varying the Hg0 loading or agitation conditions has been demonstrated: for example, in the model Mizoroki−Heck reaction. The inaccuracy of the mercury test for studying reaction mechanisms is due to the following main factors: mercury induces facile decomposition of M0 and MII molecular complexes and “removes” the metals from the solution (Scheme 2985


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compounds was observed; only trace amounts of metals were detected by EDXRF and ESI-MS analyses. In the reacted solutions of the MII compounds, a significant decrease in Pd and Pt concentrations and the appearance of soluble Hg compounds in detectable concentrations were observed. ESI-MS experiments confirmed the presence of HgII compounds in the reacted solutions of the PdII and PtII complexes. Thus, the corresponding ions [Hg(PPh3)Cl]+, [Hg(PPh 3 ) 2 Cl] + , [HgPyX] + , [Hg(NHC)X] + , and [Hg(NHC)2X]+ were detected. These results support oxidative− reductive transmetalation reactions between metallic mercury and the MII complexes. The exceptions were PdCl2 and Pd(OAc)2, and in these cases, dissolved metal ions were almost completely absent in the solutions after reaction with mercury. Then, we quantitatively studied the reactions of the Pd and Pt compounds with mercury using NMR and EDXRF techniques (Table 1) and isolated the main reaction products (Scheme 3).

2), and the test results are highly and oftentimes unpredictably dependent on operational parameters such as Hg0 loading and agitation intensity. It appears that a substantial number of the mechanistic assessments reported to date in the literature in which the nature of the catalytic system was determined only on the basis of a single mercury test represent a kind of random noise rather than real insight. To the best of our knowledge, the applicability of the concept of catalyst trapping (catalyst poisoning)51−54 for mechanistic studies of dynamic catalytic systems is scrutinized for the first time in the present study.

2. RESULTS AND DISCUSSION 2.1. Reactions of M0 and MII Compounds with Mercury. We started reaction monitoring by studying the interaction of typical M0 and MII compounds (Chart 1) in Chart 1. Overview of M0 and MII Compounds Utilized in the Present Study

Table 1. Transformation of M0 and MII Compounds in the Presence of Metallic Mercury in THF-d8 within 2 h conversion (%)a compound

reaction temp (°C)

with Hg

without Hg

Pt(PPh3)4 Pd2(dba)3 Pt2(dba)3 PdCl2 Pd(OAc)2 PdPy2Cl2 PtPy2Cl2 Pd(PPh3)2Cl2 Pt(PPh3)2Cl2 1a 1b 1c

50 50 50 50 50 50 50 50 50 50 50 50 100 50 100 50 50

100 100 64 100 100 97 18 100 25 75 73 trace 67 trace 35 16 9

traceb 29 17 0 0 0 0 13 0 0 0 0 0 0 0 0 0

1d

5 × 10−3 M solutions (poorly soluble PdCl2 was tested as a suspension) in degassed THF with a large excess (1000 mol equiv) of metallic mercury at 50 °C with stirring (1000 rpm) for 2 h under an argon atmosphere. When colored solutions (deep purple for Pd2dba3 and Pt2dba3, orange or yellowish for numerous other complexes) came into contact with mercury, they discolored or became gray within 30−120 min, and dark gray or black suspensions formed on the surface of the mercury during the reaction (see Section S9 in the Supporting Information). The exceptions were solutions of the Pd/NHC complexes 1c,d with bulky N-aryl substituents on the NHC ligands, which demonstrated no visible changes at 50 °C. However, increasing the temperature to 100 °C (a temperature often used in catalytic reactions) resulted in visible decomposition of compounds 1c,d and formation of gray suspensions after 2 h stirring with mercury. Control experiments showed no or insignificant changes in the complex solutions under the same conditions without mercury (THF, 2 h, 100 °C for complexes 1c,d, 50 °C for other compounds). After 2 h, the reacted solutions were centrifuged and analyzed by electrospray ionization mass spectrometry (ESI-MS, positive ion mode) and energy dispersive X-ray fluorescence spectrometry (EDXRF). These analyses revealed quite different pictures for the M0 and MII compounds. In the reacted solutions of the M0 complexes, almost complete disappearance of the dissolved palladium or platinum

2a 3

Conversion values were calculated as [(C0 − Ct)/C0] × 100%, where C0 and Ct are the initial concentration of a compound and the concentration after 2 h of reaction with mercury, respectively. Conversions of the organometallic compounds were determined by 1 H NMR, and conversions of PdCl2 and Pd(OAc)2 were evaluated on the basis of EDXRF analyses. The data were averaged from three parallel runs; the experimental errors did not exceed ±5%. The initial concentration of the compounds was 5 × 10−3 M, and 1000 mol equiv of metallic mercury relative to Pd or Pt compound was used. bNMR spectra changed very little over time and reflected the characteristic dissociation equilibrium Pt(PPh3)4 = Pt(PPh3)3 + PPh3.61 a

According to the 1H NMR and EDXRF data, all of the complexes reacted with mercury (Table 1 and Scheme 3). While M0 complexes (Pd2(dba)3, Pt2(dba)3, and Pt(PPh3)4) decomposed gradually in the control experiments, their decomposition in the presence of mercury was several times faster (Table 1). Overall, Pd complexes were more reactive than Pt complexes with similar structures (Table 1). This agrees well with the typical relative reactivity of Pd and Pt organometallic compounds.55−57 MII/NHC complexes demonstrated the highest stability toward mercury (Table 1), which is in 2986


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Scheme 3. Reactions of M0 and MII Complexes with Metallic Mercury and Isolated Yields of the Main Reaction Productsa

Conditions: (i) THF, 50 °C, 5 h; (ii) dioxane, 80 °C, 160 h; (iii) THF, 100 °C, 5 h.

a

the reaction mixtures (Scheme 3). Apparently, the dissociation is practically irreversible and accelerated due to the formation of M/Hg amalgams of variable compositions (structural features will be discussed below). Simple MII complexes, such as Pd(PPh3)2Cl2 and PtPy2Cl2, react selectively to give the corresponding HgIIL2Cl2 complexes, which were isolated from the reaction mixtures (Scheme 3). BisNHC complex 2a, according to NMR and ESI-MS data, reacted selectively and very slowly in THF at 50 °C (Table 1) to produce bis-NHC mercury complex 4a. Performing the reaction in 1,4dioxane at 80 °C over 160 h afforded compound 4a in 11% isolated yield (Scheme 3), while most of the starting compound (2a) remained unreacted and could be separated from 4a by manifold recrystallization. MII/NHC complexes 1a−d and 3, containing labile pyridine coligands, produced multicomponent reaction mixtures. NMR

agreement with the well-known general stability of Pd-NHC and Pt-NHC frameworks.58−60 According to the experimental data, the reactions of the studied complexes with mercury are depicted in Scheme 3. The scheme demonstrates the fundamental difference between the reactions of M0 and MII complexes: M0 complexes are decomposed by mercury to give free ligands and M0/Hg precipitates (usually regarded as amalgams in numerous catalytic studies),2,3,5,8,11,17,34 while MII complexes oxidize mercury to give HgII compounds and reduced metals in the form of M0/Hg precipitates (Scheme 3). Let us consider the features of the reactions of catalysts of different types and the structures of the main reaction products. The reactions of M0 complexes with metallic mercury can, in fact, involve simple dissociations to the corresponding ligands and metals M0. The corresponding ligands were isolated from 2987


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Figure 1. Molecular structures of compounds 4a,b according to single-crystal X-ray diffraction data. Thermal ellipsoids are shown at the 50% probability level.

and 13C NMR spectroscopy (Section S5 in the Supporting Information). It should be noted that Hg/NHC complexes 4a− d demonstrated dynamic character, and their NMR spectra in DMSO-d6 solutions changed over time (Figure S27). Moreover, poor solubility in common NMR solvents complicated obtaining 13C NMR spectra of sufficient quality. Therefore, the structures of compounds 4a,b were unambiguously confirmed by single-crystal X-ray analyses (R1 = 0.081, wR2 = 0.189 (4a); R1 = 0.098, wR2 = 0.253 (4b); Figure 1 and Figures S33 and S34 and Section S6 in the Supporting Information). The structure of product 2a was confirmed by spectral data and single-crystal X-ray analysis (R1 = 0.052, wR2 = 0.128; Figure S32 and Section S6 in the Supporting Information). Isolated compound 4d′ afforded spectral data identical with those of the sample obtained by the procedure described in the literature.65 The structure of compound 4c was confirmed by NMR and ESIMS techniques. The structures of compounds 2c,d, 4c′, and 5 were assigned by comparison of their 1H NMR and ESI-MS spectra with the spectra of authentic samples obtained by the procedures described in the literature.56,65,66 Ionic structures of Hg/NHC complexes with N,N′-dialkylbenzimidazolylidene ligands have been described in the literature.67,68 The structures and compositions of metal precipitates formed during the reactions deserve particular attention. In the literature, it is usually assumed that metallic mercury deactivates metal nanoparticles by forming amalgams.2,3,5,8,11,17,34,69 It should be noted that the solubility of palladium and platinum in liquid mercury is very low (the solubility of Pd has been reported to be from ∼3.7 × 10−3 to 5.0 × 10−3 mol %, and the solubility of Pt has been reported to be from ∼5.0 × 10−6 to 5.0 × 10−4 mol % at 25 °C).70−72 Therefore, when the usual amount of mercury (50−300 mol %) relative to a catalyst is applied in the catalytic test, the quantity of mercury used may be insufficient to completely solubilize the M0 catalytic particles. However, Pd, Pt, and other catalytically active metals can form various solid intermetallic compounds, such as PdHg2,69 PdHg4, Pd2Hg5, and PtHg4.72−75 In our study, the compositions of the metal precipitates formed during the reactions were investigated by means of scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), EDXRF, X-ray powder diffraction (XRD), and optical microscopy. According to the optical microscopy and SEM studies (Figures S36−S39), the structures of the metal precipitates varied from amorphous pastes to microcrystalline powders depending on the starting compound and the reaction conditions. The compositions of the amorphous precipitates evaluated by SEM/EDS varied sub-

monitoring of the reaction between complex 1a and mercury in THF-d8 at 50 °C revealed that, during the initial 2 h, the reaction proceeds relatively quickly to give HgPy2I2 and bis-NHC palladium complex 2a as the main products. Deposition of the dark gray metal precipitate was also observed visually (Figure S42). After 5 h, both of the products, HgPy2I2 and 2a, can be isolated in good yields (Scheme 3, conditions i). At this stage, only traces of Hg/NHC complex 4a were observed in the reaction mixture. NMR monitoring over the course of an additional few hours demonstrated a slow decrease in the concentration of palladium bis-NHC complex 2a with a concomitant increase in the concentration of mercury bisNHC complex 4a and a slight decrease in the concentration of HgPy2I2. It should also be noted that complexes 2a and, especially, 4a are sparingly soluble in organic solvents and started to precipitate after 8−10 h. The separation of these complexes from solution complicated further NMR monitoring. According to the NMR data, similar trends were observed for the reactions of complexes 1b−d and 3 in THF at 50−100 °C (Scheme 3 and Figure S43). However, as they have the lowest reactivity, with 1c,d and 3 only trace amounts of the Hg/NHC complexes were observed by ESI-MS and NMR. The multicomponent character of the reaction mixtures and the dynamic nature of some of the mercury complexes complicated the isolation and purification of the reaction products. Nevertheless, the corresponding HgPy2X2 complexes were isolated from the reactions of compounds 1a,c,d. Crystalline bis-NHC palladium complex 2a was obtained from the reaction of mono-NHC complex 1a. Crystalline Hg/NHC complexes 4a−c,d’ were isolated after the reactions of 1a−d with mercury in dioxane at 80 °C for 160 h (Scheme 3). The formation of HgII/NHC complexes from compounds 1a−d, 2a, and 3 and Hg0 represents an oxidative−reductive transmetalation of PdII/NHC and PtII/NHC complexes. Typically, the transmetalation of HgII/NHC complexes with PdII62,63 or oxidative−reductive transmetalation with Pd064 precursors is used to generate PdII/NHC compounds. For the last case, the transmetalation described in the literature64 between HgII/NHC and Pd0(PPh3)4 complexes to give PdII/ NHC and metallic mercury demonstrates that the oxidative− reductive reactions disclosed in this article can be reversible. Apparently, a large excess of metallic mercury and deposition of amalgamated metal precipitates shifts the equilibrium under the mercury test conditions. The structures of the isolated products, dba, PPh3, Hg(PPh3)2Cl2, Hg(Py)2Cl2, and Hg(Py)2I2, were confirmed by 1H 2988


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ACS Catalysis stantially. Admixtures of halogens were usually observed in the insufficiently washed samples obtained from Pd(Pt)/NHC complexes (Figures S36 and S37), likely due to coprecipitation of organometallic compounds. However, more precise EDXRF analyses of carefully washed and dried samples demonstrated an approximately 1/4 molar ratio of Pd/Hg and Pt/Hg, but the ratios varied from sample to sample. XRD studies clearly revealed the presence of PtHg4 along with an amorphous phase (Figure S35). Unfortunately, the crystallinity of the Pd/Hg precipitates was very low and insufficient for structure determination, while the elemental compositions (EDXRF) suggested that PdHg4 and Pd2Hg5 were present in various samples. These data agree well with the anticipated compositions of the “amalgams” proposed by Whitesides and coauthors.34 2.2. Mechanistic Understanding of the Reactions of M0 and MII Complexes with Metallic Mercury. Reactions of M0 complexes with metallic mercury may be considered mercuryinduced dissociations of M0−L bonds to give free ligands (L) and metal amalgams as final products. We propose three main routes, A, B, and C, for this reaction (Scheme 4). Route A

Scheme 5. Proposed Mechanism for the Reactions between MII Complexes and Metallic Mercury

reactions include the initial dissociation of one of the ligands (L) with the sequential formation of Ln−1MII−Hg0 intermediates and reduction of Ln‑1MII to M0/L via electron transfer from Hg0. Apparently, the HgII ions are stabilized by complexation with solvated ligands L. Complexes M0/L are then decomposed by mercury via pathways A−C, as depicted in Scheme 4. The formation of M0xHg0y/Ln and [MxHgy]m+/Ln clusters as intermediates could not be excluded if the literature data is taken into account.76−83 The higher reactivity of MIIPy2X2 and MII(PPh3)2X2 complexes containing weakly bound pyridine and phosphine ligands toward mercury in comparison with the lower reactivity of complexes 1a−d, 2, and 3 containing strongly bonded NHC ligands may be indirect support of this simplified scheme. In addition, the initial formation of HgPy2X2 rather than Hg/NHC complexes from compounds 1a−d and 3 is additional presumptive evidence for this system because pyridine ligands should dissociate preferably from complexes 1a−d and 3 in the first stage. Accumulated pyridine is bound by HgII ions. Overall, the reactions of MII/NHC complexes with mercury are similar to their reactions with other reductants. For example, the formation of bis-NHC complexes 2 as intermediates was also observed in the reactions of complexes 1 and 3 with aliphatic amines.56 Schemes 4 and 5 outline the different pathways for metal capture under the mercury test conditions. It should be noted that numerous catalytic systems are dynamic in nature and include various types of metal species such as molecular complexes, metal clusters, and metal nanoparticles in dynamic equilibrium.15 Therefore, the capture of any type of metal species by mercury to give an amalgam or insoluble intermetallic compound can shift the dynamic equilibrium via any of the channels and deactivate the whole catalytic system regardless of the nature of the dominant active centers. 2.3. Effect of the Operational Conditions on the Mercury Test Results. The procedure for the mercury test represents a typical heterogeneous process that is usually performed in complex multiphase media and depends substantially on the involvement of the liquid mercury surface. The reaction system, in addition to the liquid mercury and reagent solution, can include metal clusters/nanoparticles, porous heterogeneous catalysts, and reaction vessels and stirrers bearing deposited metal precipitates on their surfaces as well as bubbling gaseous reagents. It is well-known that the effectiveness of heterogeneous processes is typically highly dependent on the contact area and hydrodynamic conditions. Therefore, among other issues, the results of the mercury test can change significantly depending on the contact area between phases and the mass-transfer coefficients, which are dependent on the quantity of added mercury and the agitation intensity. Whitesides and coauthors, and then Finke and coauthors,

Scheme 4. Proposed Mechanisms for the Reactions between M0 Complexes and Metallic Mercury

includes initial reversible dissociation of the M0−L bond to release solvated metal atoms and ligands (L). Then, solvated atoms M0 are adsorbed at the surface of the mercury to give solutions (up to saturation) and intermetallic compounds. The last stage (formation of the amalgam) is essentially irreversible because a large excess of mercury is usually used. Notably, the considerable negative Gibbs energy of formation of intermetallic compounds (for PdHg4, ΔG298 = −84 kJ mol−1)73 can significantly shift the equilibrium. Route B consists of the reversible or quasi-reversible formation of metal clusters and/or nanoparticles, most likely stabilized by ligands or other molecules or ions, via decomposition of molecular complexes and/or agglomeration of solvated metal atoms and subsequent adsorption of the formed metal clusters/nanoparticles at the mercury surface to give amalgams. Route C involves an initial reversible adsorption of M0/L on the surface of the mercury to form unstable compounds that then undergo dissociation of the M−L bonds to form solvated ligands (L) and amalgamated metals (M/Hg). The last stage is practically irreversible for the same reasons as seen in routes A and B. In our opinion, all three routes, A, B, and C, can operate in the reaction systems, and the contribution of each route should depend on the specific conditions. Each of the routes can include the formation of M0x−Hg0y/Ln clusters as intermediates as described in the literature for reactions of various metal complexes with mercury or its amalgams.76−78 Reactions of MII/L complexes with mercury, in fact, represent a type of oxidative−reductive transmetalation (Scheme 5). While diverse mechanisms can be proposed, it is likely that the 2989


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Table 2. Effect of Hg0 Loading on the Yield of (E)-n-Butyl Cinnamate (8) in the Mizoroki−Heck Reaction Catalyzed by Various Pd Compounds yielda of 8 (%) in the presence of [Pd] catalyst entry 1 2 3 4 5 6 7 8

0

Hg loaded (equiv) b

0 50b 100b 300b 500b 1000b 1000c 2000c

Pd(PPh3)4

Pd(OAc)2

Pd(PPh3)2Cl2

1a

1d

Pd/Cd

Pd/MWCNTd

Pd/graphited

colloidal Pde

100 6 3 2 0 0 0 0

100 100 90 87 86 67 10 0

100 100 90 48 10 0 0 0

100 8 5 3 0 0 0 0

100 100 96 73 62 5 0 0

100 100 98 94 81 65 11 11

100 98 88 73 69 64 50 10

100 100 99 93 93 90 16 8

100 97 94 78 47 43 4 3

a An average of three runs is reported. The experimental errors did not exceed ±7%. bThe magnetic stirrer rotation rate was 300 rpm. cThe magnetic stirrer rotation rate was 1000 rpm. dThe Pd/C catalyst was a commercial 1 wt % palladium on carbon (Sigma-Aldrich, SKU: 205672). The palladium catalysts supported on multiwalled carbon nanotubes (Pd/MWCNT, 1 wt % of Pd) and on graphite (Pd/graphite, 1 wt % of Pd) were obtained according to the described procedure.88 eColloidal palladium (1 wt %) in ionic liquid. The catalyst was obtained by heating Pd(OAc)2 in the mixture of tetrabutylammonium bromide and tetrabutylammonium acetate (molar ratio 1.5/1) at 130 °C according to the described procedure.89

the Hg0 inhibition effect (Table S1 and Figures S1 and S2); however, the magnitude of the effect varied for different catalysts (Table S1). This is likely due to different relationships between the “rate of catalyst activation” and the “rate of activated catalyst reaction with mercury” for each specific catalyst. For example, well-known Pd nanoparticle precursors, such as Pd(OAc)2, as well as colloidal Pd nanoparticles and palladium nanoparticles deposited on various supports (Pd/C, Pd/MWCNT, Pd/ graphite), which were anticipated to be deactivated by Hg0 more readily, demonstrated high catalytic activities even at relatively high Hg0 loadings (500−1000 equiv). In contrast, phosphine complexes (Pd(PPh3)4 and Pd(PPh3)2Cl2) and Pd/NHC complex 1a were deactivated almost completely by relatively small amounts of mercury (50−100 equiv). Interestingly, even very large amounts of Hg0 and high agitation intensities could not completely deactivate Pd nanoparticles under the studied conditions (Table 2, last four columns, entries 6−8). Apparently, in these cases the rate of the Mizoroki−Heck reaction is sufficiently high for the product 8 to form in small amounts even at large mercury loadings and agitation intensity. It should also be noted that, in the experiments shown in Table 2 and Tables S1 and S2 and Figures S1 and S2, mercury and the other reagents were loaded into the reaction mixture simultaneously, while some authors recommended adding mercury only after the catalyst has been activated.5,11,40 In separate experiments in which the mercury was added after 10 min and the catalytic experiments had started, we observed similar results with larger scatter between parallel runs.

highlighted the acute importance of intimate contact between mercury and the entire reaction system, which requires a large excess of Hg0 in a well-stirred solution.3,34 Finke’s group, using an example of a Rh-catalyzed hydrogenation of benzene, demonstrated the insufficiency of low mercury loadings and recommended use of ≥300 equiv of Hg0 relative to the loaded catalyst.40 Notably, despite the apparent importance of the mercury loading and agitation efficiency, these factors have not been studied comprehensively and are often disregarded. According to the literature data, the quantities of mercury used in mercury test experiments vary over a wide range.84 Indeed, the loadings of Hg0 varied from 1 to 9000 equiv relative to the amount of metal catalyst, while the loadings were more often between 100 and 500 equiv. To evaluate the effects of the mercury loadings (Table 2) and agitation intensity (Table S1 and Figures S1 and S2), we selected a well-studied Pd-catalyzed Mizoroki−Heck reaction between iodobenzene (6) and n-butyl acrylate (7) as an appropriate model (Scheme 6 and Table 2).85 It was recently demonstrated Scheme 6. Mizoroki−Heck Reaction between Compounds 6 and 7 Used as a Model for Studying the Influence of Hg0 Loading and Agitation Intensity on the Mercury Test Results

that, under the reaction conditions (Scheme 6), the catalytic system involves a “cocktail of catalysts” in which “ligandless” palladium species are the dominant active centers.56,57,86,87 The data presented in Table 2 and Table S1 clearly demonstrate the inaccuracy of the “mercury test”. On the one hand, the test results depend significantly on the Hg0 loading (Table 2) and agitation intensity (Table S1). Opposite conclusions can be made for the same catalyst. On the other hand, the effect of the mercury loading is different for different catalysts (Table 2). Therefore, a universal recommendation for the required amount of Hg0 to obtain reliable results cannot be made (for example, compare the results for Pd(PPh3)4, Pd(OAc)2, and 1a). A similar rather complicated influence of Hg0 loadings was also observed for the Suzuki−Miyaura reaction (Table S2). Increasing agitation intensity predictably increased

3. CONCLUSIONS Over the past few decades, the mercury test has been widely applied in catalytic studies to distinguish between truly homogeneous molecular catalysis and so-called “heterogeneous” nanoparticle catalysis. The general inaccuracy of the mercury test as a rapid method for mechanistic evaluation is stipulated by the following points: (1) Metallic mercury reacts easily with M0 and MII molecular complexes and “removes” the metals from the solutions and gives amalgams or intermetallic compounds. (2) The results of the test depend heavily on the mercury loading and agitation intensity. Slight changes in these operational parameters can often bring unpredictability to the opposite test results. 2990


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ACS Catalysis These findings are critical to understanding reaction mechanisms. In fact, the prerequisite of the method, the assumption of the inertness of mercury toward molecular metal complexes, is violated. It should be noted that the decomposition of several M0 and MII organometallic compounds by metallic mercury has been reported previously, and several publications have reported the inappropriateness of the mercury test for the study of specific catalysts.36−39 This article emphasizes the importance of the problem for transition-metal catalysis considering the examples of Pd and Pt catalysts. M0/L complexes can undergo mercury-induced dissociation to give free ligands L and intermetallic compounds MxHgy (for example, PtHg4) or amalgams as the main reaction products. Apparently, the formation of a metal−mercury bond is the “driving force” of this process. MII/L complexes react with metallic mercury via oxidative−reductive transmetalation forming HgII/L complexes, intermetallic MxHgy compounds, and amalgams. In these types of reactions, mercury serves as a reductant for the more electronegative MII cations. Moreover, complexation of Hg2+ cations with ligands L and deposition of the amalgams shift the oxidative−reductive equilibrium. Although we have investigated the reactions of typical and, in some cases, relatively stable organometallic compounds (M/NHC), there is no reason to doubt that similar reactivity would be observed with thermodynamically less stable active catalysts and M0 and MII catalytic intermediates. In other words, Hg was found to be an unselective poison and therefore was not suitable for distinguishing the reaction mechanisms in Pd and Pt catalytic systems. In the present study, a number of experiments were performed, and numerous opportunities were considered for identifying useful conditions for the mercury test or for developing another alternative selective poisoning technique to determine the nature of catalytic systems. Although the development of a truly selective poisoning tool is always a deserving goal, careful consideration of the results obtained and the role of dynamic processes of active species interconversions in numerous catalytic systems15 raise a potential problem of fundamental limitations in poisoning approaches for studying the mechanisms of catalytic reactions. In these regards, two possible active species (homogeneous vs heterogeneous) can be considered, and two types of selective poisoning tests can be analyzed (poisoning 1 for soluble metal complexes and poisoning 2 for metal nanoparticles; Scheme 7). As soon as a real catalytic system is assembled by the addition of the necessary components (starting materials, reagents, bases, etc.), a number of dynamic processes immediately occur (Scheme 7). As representative, well-known dynamic processesincluding leaching, agglomeration/aggregation, Ostwald ripening, redeposition, and capture/releaseshould be mentioned.15,21,22,90−95 Indeed, a number of synthetically useful catalytic systems were found to be dynamic with a quasiequilibrium among the different types of metal species involved.15,19,21,22,90−98 It is quite possible that, for many cases, the rate of reaching equilibrium (req) between different catalytic species would be comparable to (or higher than) the rate of poisoning (rp1, rp2) and the rate of the catalytic reaction (rkat) (Scheme 7). In these cases, a quasi-equilibrium among the different catalyst species (M/Ln and M NPs; Scheme 7) is present in the catalytic system and, therefore, even selective poisoning of a specific catalytic species will deactivate the whole system. Indeed, in the case of homogeneous catalysis with soluble metal complexes as active species (Scheme 7A), selective

Scheme 7. Fundamental Difficulties of the Concept of Selective Poisoning for Distinguishing Reaction Mechanisms in Dynamic Catalytic Systems

poisoning of nanoparticles can deactivate the whole system and can give a false positive result in a dynamic catalytic system (if metal species do agglomerate, trapping of nanoparticles removes the metal from solution). As far as nanoparticle active species are concerned (Scheme 7B), selective poisoning of soluble complexes can give false positive outcomes in the case of leaching. Leaching of metal from the nanoparticles following by trapping of the leached species will deactivate the whole catalytic system. Thus, without careful considerations even selective poisoning reagents may show an overall unselective poisoning effect in the dynamic catalytic systems. Concerning the application of the mercury test, we can consider the following mandatory conditions required to get reliable results. The first requirement is ensuring a sufficient contact area and hydrodynamic conditions to remove diffusion limitations for mercury interaction with metal species. As seen from Table 2 and Tables S1 and S2, various catalysts and reaction systems require different Hg loadings and agitation intensities. Therefore, a number of dedicated experiments are needed for each catalyst and reaction system to optimize the reaction conditions. The second requirement concerns the fundamental properties of the studied system. For the mercury test to be valid, the rate of the reaction between Hg and metal nanoparticles (rP2) should be sufficiently higher than the rates of the reaction with metal complexes (rP1) and the rate of the equilibrium establishment (req): rP2 ≫ rP1, rP2 ≫ req (Scheme 7). The relative rates rP1, rP2, and req are usually unknown. Therefore, special kinetic studies are required to check the fulfillment of this requirement for each catalytic system under the conditions being studied. Without verification of the fulfillment of both requirements, the mercury test is “guesswork”. However, experimental checking of these requirements is 2991


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The authors also thank the Shared Research Center “Nanotechnologies” of the Platov South-Russian State Polytechnic University and the Department of Structural Studies of Zelinsky Institute of Organic Chemistry for analytical services. Synchrotron radiation-based single-crystal X-ray diffraction measurements were performed at the unique scientific facility Kurchatov Synchrotron Radiation Source supported by the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007).

usually a rather complex and very laborious task. For the aforementioned reasons, we believe that simplified implementations of the mercury test should be avoided as a standard method for mechanistic studies in catalysis. Without careful considerations, the mercury test should not be considered as a reliable method for assessing the nature of catalytic systems.99,100 In general, more powerful concepts should be considered for developing reliable tests on the nature of catalytic systems. As a promising step in this regard, a fruitful approach based on quantitative kinetic poisoning experiments, which include quantitative poisoning combined with thorough kinetic experiments for different catalytic species, can be mentioned.36,101−104 Another useful approach for distinguishing between “homogeneous” and “heterogeneous” catalysis mechanisms is based on differential selectivity measurements and competitive reaction methods.12,105,106 Obviously, the universal reliable approach to the problem can include determining the complete speciation of a precatalyst under the reaction conditions, catalyst dynamics (possible interconversions of alternative active species), and the kinetic contribution of each catalytic species.13 However, this is quite a difficult task in practice. Therefore, comprehensive studies of the effects of catalyst dynamics on the catalyst poisoning and further works to find reliable universal tests for distinguishing the nature of catalytic systems are highly welcome. The present study provides a thorough examination of Pd and Pt catalytic systems. Unselective poisoning behavior of the mercury test gives a strong warning message for other catalytic systems. Additional studies involving different metals are highly welcome for further development of this area.

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(1) Lewis, L. N. Chemical Catalysis by Colloids and Clusters. Chem. Rev. 1993, 93, 2693−2730. (2) Lin, Y.; Finke, R. G. A More General Approach to Distinguishing ″Homogeneous″ from ″Heterogeneous″ Catalysis: Discovery of Polyoxoanion- and Bu4N+-Stabilized, Isolable and Redissolvable, High-Reactivity Ir∼190−450 Nanocluster Catalysts. Inorg. Chem. 1994, 33, 4891−4910. (3) Widegren, J. A.; Finke, R. G. A Review of the Problem of Distinguishing True Homogeneous Catalysis from Soluble or Other Metal-Particle Heterogeneous Catalysis under Reducing Conditions. J. Mol. Catal. A: Chem. 2003, 198, 317−341. (4) Phan, N. T. S.; Sluys, M. V. D.; Jones, C. W. On the Nature of the Active Species in Palladium Catalyzed Mizoroki−Heck and Suzuki− Miyaura Couplings − Homogeneous or Heterogeneous Catalysis, A Critical Review. Adv. Synth. Catal. 2006, 348, 609−679. (5) Crabtree, R. H. Resolving Heterogeneity Problems and Impurity Artifacts in Operationally Homogeneous Transition Metal Catalysts. Chem. Rev. 2012, 112, 1536−1554. (6) Barreiro, E. M.; Hao, Z.; Adrio, L. A.; van Ommen, J. R.; Hellgardt, K.; Hii, K. K. Spatial, Temporal and Quantitative Assessment of Catalyst Leaching in Continuous Flow. Catal. Today 2018, 308, 64−70. (7) 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. (8) Artero, V.; Fontecave, M. Solar Fuels Generation and Molecular Systems: Is It Homogeneous or Heterogeneous Catalysis? Chem. Soc. Rev. 2013, 42, 2338−2356. (9) Yan, N.; Yuan, Y.; Dyson, P. J. Nanometallic Chemistry: Deciphering Nanoparticle Catalysis from the Perspective of Organometallic Chemistry and Homogeneous Catalysis. Dalton Trans. 2013, 42, 13294−13304. (10) Fukuzumi, S.; Hong, D. Homogeneous versus Heterogeneous Catalysts in Water Oxidation. Eur. J. Inorg. Chem. 2014, 2014, 645−659. (11) Sonnenberg, J. F.; Morris, R. H. Distinguishing Homogeneous from Nanoparticle Asymmetric Iron Catalysis. Catal. Sci. Technol. 2014, 4, 3426−3438. (12) Schmidt, A. F.; Kurokhtina, A. A.; Larina, E. V. Differential Selectivity Measurements and Competitive Reaction Methods as Effective Means for Mechanistic Studies of Complex Catalytic Reactions. Catal. Sci. Technol. 2014, 4, 3439−3457. (13) Stracke, J. J.; Finke, R. G. Distinguishing Homogeneous from Heterogeneous Water Oxidation Catalysis when Beginning with Polyoxometalates. ACS Catal. 2014, 4, 909−933. (14) Asraf, M. A.; Younus, H. A.; Yusubov, M.; Verpoort, F. EarthAbundant Metal Complexes as Catalysts for Water Oxidation; Is It Homogeneous or Heterogeneous? Catal. Sci. Technol. 2015, 5, 4901− 4925. (15) 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. (16) Del Zotto, A.; Zuccaccia, D. Metallic Palladium, PdO, and Palladium Supported on Metal Oxides for the Suzuki-Miyaura CrossCoupling Reaction: A Unified View of the Process of Formation of the Catalytically Active Species in Solution. Catal. Sci. Technol. 2017, 7, 3934−3951.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b03683. Description of experimental procedures, NMR spectra, ESI-MS and XRD data, and details of the X-ray structure determinations (PDF) Crystallographic data for 2a (CIF) Crystallographic data for 4a (CIF) Crystallographic data for 4b (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for V.P.A.: [email protected]. ORCID

Victor M. Chernyshev: 0000-0001-9182-8564 Dmitry B. Eremin: 0000-0003-2946-5293 Victor N. Khrustalev: 0000-0001-8806-2975 Valentine P. Ananikov: 0000-0002-6447-557X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present study was supported by the Russian Foundation for Basic Research RFBR grant 16-29-10786 (synthesis and transformations of metal complexes), the Russian Science Foundation RSF grants no. 14-13-01030 (mass-spectrometry and NMR study), 14-23-00078 (mechanistic studies and investigation of the catalytic reactions), and the RUDN University Program “5-100” (some of X-ray structural studies). 2992


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ACS Catalysis

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Pd and Pt Catalyst Poisoning in the Study of Reaction Mechanisms

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