DOI: 10.1002/chem.201304271
Full Paper
& Homogeneous Catalysis
Alkyne/Alkene/Allene-Induced Disproportionation of Cationic Gold(I) Catalyst Manish Kumar,[a] Jacek Jasinski,[b] Gerald B. Hammond,*[a] and Bo Xu*[a]
Abstract: The first detailed experimental study of the deactivation of cationic gold was conducted, and the influence of each component in the reaction system (substrate, counterion, solvent) on the decay process was examined. It was
found that a substrate (alkyne/allene/alkene)-induced disproportionation of gold(I) may play a key role in the decay process. Our mechanism is supported by kinetic, XPS, voltammetry studies, and high-resolution ESI-MS data.
Introduction Gold catalysis, a landmark addition to the field of organic synthesis,[1] is not without shortcomings, the main one being the relatively low turnover numbers observed in the majority of gold-catalyzed reactions. Indeed, high turnover numbers (or low catalyst loadings) are observed only for few types of goldcatalyzed reactions (e.g., hydration of alkyne).[2] In most goldcatalyzed reactions, loadings in the 1–5 % range are the norm.[3] One reason for the low turnover numbers (TON) is the relatively fast decay of cationic gold and its conversion into nonreactive species (e.g., Au0—gold mirror and/or gold particles and [L2 AuI] + .[4] Despite this drawback, there has been no comprehensive systematic investigations on the decay of homogeneous gold catalysis; a stark contrast with the extensive work reported on catalyst deactivation in heterogeneous catalysis.[5] Herein, we answer some fundamental and intriguing questions on gold(I) decay, such as the effects of starting material, nucleophile, solvent, counterion, and additives, and we suggest a plausible pathway for gold(I) decay.
Results and Discussion A simplified gold catalytic cycle is shown in Scheme 1. The cationic gold complex (L-Au + ) acts as carbophilic Lewis acid,[6] activating an unsaturated C C bond to form a p complex [L-Au + -S]. The interaction of [L-Au + -S] with a nucleophile (Nu) gives [a] M. Kumar, Prof. Dr. G. B. Hammond, Prof. Dr. B. Xu Department of Chemistry, University of Louisville Louisville, Kentucky 40292 (USA) E-mail: [email protected] [email protected] [b] Dr. J. Jasinski Materials Characterization Conn Center for Renewable Energy Research University of Louisville, Louisville, Kentucky 40292 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304271. Chem. Eur. J. 2014, 20, 3113 – 3119
Scheme 1. Simplified representation of the gold catalytic cycle.
a gold s complex ([L-Au-S-Nu]) that eventually undergoes protodeauration and the regeneration of cationic gold. Complex [L-AuI] + X (X = weakly coordinating counterion, such as OTf ) is relatively stable by itself, but in many reactions it decays at a fast rate (from few minutes to few hours). This behavior suggests that a cationic gold catalyst may decay rather rapidly in the presence of certain components in a reaction system. Because [Au + (Ph3P)OTf ] (1) is one of the most commonly used cationic gold catalysts,[3a, b, d] we chose it as our model catalyst to study the deactivation process. Compound 1 in CDCl3 is relatively stable (31P NMR peak at d = 27.4 ppm is the only peak)[7] and also catalytically active for 24 h at room temperature). First, we investigated the kinetic stability of [LAu + -S] (Scheme 1, S = alkyne/allene/alkene). After adding cyclohexene (10 equiv vs. 1) at room temperature, we noticed significant amounts of gold mirror/black particles (Au0) in the NMR tube, a telltale indication that 1 (0.01 m in CDCl3) had undergone significant decay. In the beginning, only a single 31 P NMR peak at 29.5 ppm was recorded, consistent with a gold-alkene p-complex or a fast equilibrium between free [Au + (Ph3P)OTf ] and [Au(Ph3P) alkene] + OTf (Figure 1). The signal at d = 29.5 ppm [gold(I)–alkene complex] decreased over time, and a new signal at d = 45.0 ppm increased over the same time period. We assigned this new signal the structure [Au + (Ph3P)2OTf ] (2). This assignment is in agreement with the 31 P NMR shift value of 2 reported in the literature[8] and also with the chemical shift recorded for our independently synthe-
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Full Paper vent, counterion, nucleophile, and additive. In all the solvents that we tested, 1 was stable for 24 h at room temperature. When cyclohexene (10 equiv vs. gold) was added, the decay process began in earnest. A highly coordinating solvent, such as CH3CN, showed good aptitude in preventing the deactivation of gold catalyst (Figure 2 c). As was mentioned above, [Au + (Ph3P)X ] alone is relatively stable, but when cyclohexene was added, different degrees of decay were observed in all cases (Figure 2 d). Compared to the commonly 31 Figure 1. Decay of 1 in the presence of cyclohexene, monitored by P NMR spectroscopy. used OTf counterion, NTf2 has a greater ability to stabilize the sized 2. This assignment was further confirmed by high-resolucationic gold catalyst (Figure 2 d).[9] Just as it was the case with tion ESI mass spectroscopy ([M-OTf] + at m/z = 721.1483; see solvents and counterions, most of the additives we screened did not induce the decay of cationic gold, but when we tested Figure S4 in the Supporting Information). We also recorded the 1 H NMR of the reaction mixture, and found that cyclohexene how they influenced the decay kinetics in the presence of cyhad remained unchanged during the decay process. However, clohexene, we found that the decay of cationic gold slowed we noticed that higher concentrations of cyclohexene sped up down differently depending on the type of additives used (Figthe decay (Figure 2 a). Other unsaturated substrates commonly ure 2 e). Many N-heterocycles and hexamethylphosphoramide used in gold catalysis, such as alkyne and allene (10 equiv vs. (HMPA) stabilized the gold catalyst significantly; similarly, other 1), also induced the decay of cationic gold 1 (Figure 2 b). additives (LiNTf2, N-oxide, sodium sufinimide, KCTf3, N,N’-DiNext, we investigated the influence of other common commethyl-N,N’-trimethyleneurea (DMPU)) slowed down the decay ponents on the kinetics of gold-catalyst decay, specifically, solof gold catalyst 1 to different extents.
Figure 2. a) Decay of [Au + (Ph3P)OTf ] (1) in the presence of cyclohexene; b) decay of 1 in the presence of alkene/alkyne/allene; c) decay of 1 in various solvents in the presence of cyclohexene; d) decay of [Au + (Ph3P)X ] in the presence of cyclohexene; e) decay of 1 in the presence of cyclohexene and additives; and f) water effect on decay of 1. Chem. Eur. J. 2014, 20, 3113 – 3119
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Full Paper Because trace water is ubiquitous in the reaction system, we investigated the effect of water on catalyst deactivation. We found that water alone has little effect on the decay (Figure 2 f). Nevertheless, we observed a significantly faster deactivation of gold catalyst 1, when 1 (0.01 m) was mixed with both cyclohexene (10 equiv) and water (1 equiv) at room temperature. We postulated two possible pathways to explain the decay behavior (generation of Au0 from AuI). One pathway is disproportionation, and the other is reduction of gold(I) by certain reductants in the reaction system. Ceroni and co-workers[10] reported that gold nanoparticles (Au0) can be produced by disproportionation of Au + ions. Some research groups have also speculated that the deactivation of gold catalyst is due to disproportionation.[4b, c] However, no direct experimental evidence has been reported to support these statements. We decided to investigate the actual gold valence change in the decay process experimentally. The redox potential of cationic gold should provide us with quantitative information to determine the easiness of reduction of AuI in a reaction system. Laguna and co-workers have studied the redox chemistry of AuI and AuIII complexes by using cyclic voltammetry in CH2Cl2.[11] We also chose CH2Cl2 as solvent to have a frame of reference to measure the reductive potential of cationic AuI. The cyclic voltammogram of 1 revealed a single irreversible redox event at a very negative formal potential ( 2.5 V vs. ferrocenium/ferrocene; Figure 3). Compared with the formal potential of Ag + in CH2Cl2 (+ 0.65 V),[12] the highly negative potential of 1 denotes that a direct reduction of catalyst 1 could be very difficult. This ob-
Because 31P, 1H NMR, or voltammetry data do not reflect the change in the chemical state of gold itself, we used other more direct techniques to investigate the change in the chemical state of gold. X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, chemical state, and electronic state of the elements that exist within a material.[13] We[14] and others[15] have used XPS in the determination of the chemical state of gold catalysts. The Au 4 f7/2 peaks can be resolved into a doublet (spin–orbit splitting) and the binding energy of Au 4 f7/2 electrons in each gold oxidation state are usually sufficiently wide apart to be differentiated using this technology. First, we tested our gold-standard samples ([AuICl(PPh3)] and NaAuIIICl4) and found that the Au 4 f7/2 photoelectron peak was located at a binding energy (BE) value of 85.7 and 87.5 eV, respectively, which is quite consistent with literature reports (see the Supporting Information).[16] Then, we tested our cationic gold catalyst with various counterions. We prepared these cationic gold samples in chloroform; 24 h later, these solutions were dried in high vacuum and were subjected to XPS analysis (Figure 4 a–c). As was expected, the gold valence in these three samples is still AuI. Compound 1 (0.01 m in CDCl3) was treated with cyclohexene (10 equiv), and after 48 h the mixture was filtered, and the liquid portion (sample 1, Figure 4 d) and solid precipitate were collected (sample 2, Figure 4 e). We conducted XPS analyses of
Figure 3. Cyclic voltammogram (red line) of 1 (2 mm in CH2Cl2 with 0.1 m tetrabutylammonium hexafluorophosphate (TBAHFP)). Blue line is background CV of supporting electrolyte in CH2Cl2. Potential values are referenced versus the ferrocenium/ferrocene couple.
servation was further confirmed by adding a strong reductant to 1. When [CoCp*2] (reduction potential in CH2Cl2 is 1.94 V)[12] was added to a solution of 1, no reduction took place. [CoCp*2] is already a very strong reductant, most compounds in a reaction system are far less reductive. Also, in our decay-kinetic studies, we did not observe any change in the 1 H NMR of cyclohexene in the solution. This combined data suggests that the direct reduction of AuI is unlikely. Chem. Eur. J. 2014, 20, 3113 – 3119
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Figure 4. XPS spectra (AlKa = 1486.6 eV) corresponding to the Au 4 f7/2 photoelectron peaks.
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Full Paper both samples. The majority of the solid sample (sample 2) was Au0 (Figure 4 e), and the liquid fraction was AuI (Figure 4 d). Because we detected none or very little of AuIII component, it was highly possible that a reductive agent, such as the phosphine ligand, reduced AuIII (because of the high oxidation power of cationic AuIII itself). Next, we investigated the disproportionation of gold(I) in the absence of phosphine ligand. First, we tested a commercial AuICl sample; although the majority of it corresponded to AuI, there was also a small amount of AuIII present (Figure 4 f). We wanted to investigate the behavior of cationic gold(I) (AuIOTf) under these conditions. AuICl and AgOTf (1 equiv) were mixed in dry dichloromethane with and without cyclohexene. Thus, after stirring for 30 min under dark and anhydrous conditions, we observed the formation of black particles. We collected the black particles (sample 3) and conducted a XPS analysis of sample 3 (Figure 4 g), which denoted the presence of Au0, AuI, and AuIII. We conducted the same experiment but in the company of cyclohexene and observed a similar result. 1H NMR analysis indicated no change in the structure of cyclohexene in the decay solution. This result suggested that cyclohexene did not play a role as reductant. All together, the experimental data supported the disproportionation of cationic gold(I) in the absence of phosphine ligand. Because XPS was conducted under high vacuum, and the samples were tested in the solid state, there was some concern that the gold catalyst might have changed under these conditions. We wanted to further confirm the existence of gold(III) by investigating the gold species directly in solution phase. Electrospray ionization mass spectroscopy (ESI-MS) is a soft ionization technique,[17] and is an especially suitable technique, because cationic gold intermediates are charged species. In a previous work, we succeeded in detecting the existence of gold(III) species by using high-resolution (HR) ESI-MS.[18] First, we checked the HR ESI-MS spectra of sample 1 (Figure 4). We only detected the presence of various AuI species (e.g., [AuI(Ph3P)] + (m/z 459.0571), [AuI(Ph3P)H2O] + (m/z 477.0677), [AuI(Ph3P)2] + (m/z 721.1483)). This observation is consistent with our previous XPS analysis (Figure 4 d). According to our previous studies of gold valence by using ESI-MS,[14] AuIII species are difficult to detect under standard condition, but we have been able to detect AuIII species by adding a bidentate ligand and a halide (bipyridine and chloride).[14] Because AuIII complexes have square-planar geometry, a bidentate ligand should greatly stabilize the AuIII cation during the ionization process, and chloride will further stabilize cationic AuIII.[14] Therefore, we added bipyridine and chloride to sample 1. Again, only AuI species (e.g., [AuI(Ph3P)bipyridine] + (m/z 615.1266), [AuI(Ph3P)2] + (m/z 721.1483)) were detected. This result indicated that there was no AuIII in the presence of phosphine ligand (PPh3). We used the same method to investigate the ESI-MS of sample 3. Various AuIII species were detected (Figure 5). Our HR ESI-MS studies are consistent with our previous XPS studies, that is, there was no AuIII species present (only AuI species) in sample 1, but various AuIII species were detected in sample 3 (for more details, see the Supporting Information, section 5).
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Figure 5. ESI-MS spectrum of sample 3.
Based on the combination of all the above described experimental data, we are now in a good position to understand the decay mechanism, which can be summed up as follows: 1) Cationic AuI without a suitable ligand (e.g., AuIOTf) disproportionates readily compared to its noncationic form (e.g., AuICl; Figure 4, sample 3). 2) In the presence of a suitable ligand, cationic gold(I) decays (or disproportionates) at a much slower pace. Complex [Au + (Ph3P)OTf ] is stable for at least 24 h (Figure 2 a), and AuOTf disproportionates in less than 0.5 h (Figure 4). This conclusion explains why ligands are so ubiquitous in gold catalysis. 3) Cationic AuI p complexes with p donors (C C unsaturated compounds, such as alkyne/allene/alkene) undergo relative fast decay, whereas cationic AuI complexes with most s donors (e.g., N-heterocycles and acetonitrile) are very stable.
The detailed role of substrates (alkyne/allene/alkene) in substrate-induced cationic gold decay is not very clear yet. Considering the great stabilization effect of the phosphine ligand (or NHC ligand) against disproportionation, our hypothesis is that an alkyne/allene/alkene may help to temporarily displace the phosphine ligand from the gold(I) center by a trans effect. The trans effect is the labilization (destabilization) of ligands that are trans to certain other ligands in a metal complex. Although most examples of trans effect are observed in square-planar complexes (e.g., Pd, Pt), we propose that it may also operate in linear metal complexes, such as gold(I) complexes.[19] And alkenes/alkynes are indeed the ligands that have shown the strongest trans effect. Odell and co-workers demonstrated that alkene and alkyne showed highest trans effect than any other compounds in platinum complexes (trans effect: C2H4 ~ iPrCH= CHMe ~ Me2C(OH)C C-C(OH)Me2 @ Et3Sb > Ph3Sb > Me3P > Et3P > Ph3P).[20] Our proposed decay mechanism is shown in Scheme 2. First, cyclohexene will complex with cationic gold to form a gold– alkene p complex, then, because of the trans effect of the
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Full Paper co-workers[26] have found that the gold resting state is the gold–allene complex 5 in this reaction (Scheme 3 a). Intermolecular hydroamination of alkene[3a] is a similar example (Scheme 3 b). In these reactions, high loadings of gold catalyst (5 % or higher) are typically used. And in these reactions, we did observe the formation of decay gold products 2 and Au0 in the reaction mixture, especially
Scheme 2. Proposed mechanism for substrate-induced cationic gold decay.
alkene, the phosphine ligand will be temporarily replaced to form complex 2 and free AuOTf. The disproportionation of AuOTf generates Au0 and AuIII(OTf)3. This disproportionation may be enhanced by the presence of trace water in the system. This statement is supported by a literature report that stated that a AuI complex underwent disproportionation in aqueous solution, whereas AuI was stable in organic solvents.[21] The disproportionation of AuOTf or its alkene/alkyne complex can be a relatively fast process (Figure 4, sample 3). This statement also has literature support: Dias and co-workers[22] reported that AuI alkene or alkyne complexes without ligands are only stable at very low temperature, turning into black particles at room temperature. It is known that AuIII will oxidize the phosphine ligand,[23] and itself will be reduced to AuI again. We propose that Ph3P is oxidized to Ph3P(OTf)2,[24] which could then be hydrolyzed to give Ph3P=O in the presence of water. The presence of Ph3P=O has been confirmed by ESI-MS and TLC (by comparison with an authentic sample). Furthermore, when Au(OTf)3 (generated from AuCl3/AgOTf) was mixed with PPh3, [Au + (Ph3P)2OTf ] (2) and Ph3P=O were obtained. This behavior could explain why we were not able to detect AuIII in gold solutions using XPS and ESI-MS (sample 1). We now can apply our decay mechanistic study to explain some unanswered questions: 1) What causes the decay of cationic gold? The C C unsaturated compounds (alkyne/allene/alkene)-induced cationic gold decay is the major reason for cationic gold decay. It should be noted that the starting material is not the only source of C C unsaturated compounds; the products often contain C C unsaturated compounds as well. 2) Why is it that a high turnover can be achieved in some reactions but not in others? The decay process is highly dependent on the resting state of the gold catalyst. Based on our most recent study,[25] we have classified gold-catalyzed reactions according to their turnover limiting stage: in type I, the formation of gold–s complex ([L-Au-S-Nu] in Scheme 1) is the turnover limiting stage; in type II, the regeneration of cationic gold catalyst (e.g., protodeauration) from [L-Au-S-Nu] is the turnover-limiting stage. For type I reactions, the resting state is often [L-Au + -S] (Scheme 1); thus, a relatively fast decay is to be expected, and so, turnover numbers will be low in general. On the other hand, in type II reactions, the resting state is usually not [L-Au + -S], so the decay is relatively slow and high TON are to be expected. According to our recent study,[25] the hydroamination of allene is a type I reaction (Scheme 3 a). Toste and Chem. Eur. J. 2014, 20, 3113 – 3119
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Scheme 3. Catalyst decay in different gold catalyzed reactions.
when triflate was used as counterion. A typical example of type II reactions is the cyclization of propargyl amide (7) (Scheme 3 c); its catalyst resting state is a [L-Au-S-Nu] type intermediate 9.[25, 27] Thus, the decay of active catalyst is expected to be slow. Indeed, low catalyst loadings (0.1 %) have been achieved for this reaction.[28] Similarly, the alkyne hydration (Scheme 3 d) reported by Nolan and co-workers[2c] and the intramolecular addition of a diol to an alkyne reported by Hashmi and co-workers[2d, 29] have been achieved with very high TONs. In these reactions, the turnoverlimiting determining stage is most likely the protodeauration of [L-Au-S-Nu]-type complex (Scheme 1).[25] 3) What can we do to reduce the decay of an active gold catalyst? Our studies indicate that the use of s donors, such as N-heterocycles and CH3CN, will enhance the stability of the cationic gold,[30] but s donors also have a significant detrimental effect on reactivity, so their use is normally not a good option. But it is still possible to disrupt the gold– alkyne/alkene interaction and therefore reduce the decay by using suitable ligands and counterions. Recently, we and others have reported that the use of a ligand containing a o-biphenyl steric handle can reduce the decay of cationic gold.[25, 31] As shown in Scheme 3 e, the o-biphenyl motif will clearly interrupt the bonding of the gold center with an alkene/alkyne, which may be the reason it slowed down its decay. A steric handle, such as o-biphenyl will not reduce the reactivity though; actually, for many types of gold-cata-
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Full Paper lyzed reactions, ligands with biphenyl motif often give faster reactions.[1c, 32] In Figure 2 d, we demonstrated that some counterions are capable of stabilizing the cationic gold. NTf2 is a notable example. The use of NTf2 will not reduce reactivity; in many reactions, NTf2 actually enables faster reactions compared with a commonly used counterion, such as OTf .[9]
[3]
[4]
It should be noted that the alkyne/alkene/allene-induced disproportionation of cationic gold(I) species may generate small gold clusters or gold nanoparticles under proper conditions; in turn, these clusters exhibit high reactivity in some gold-catalyzed reactions.[2e, 33] Therefore, disproportionation may not be the end of the story for gold catalysts. We believe that if relatively large gold particles or a gold mirror are generated through disproportionation, the reactivity of the gold catalyst will be lost, but if we can control this disproportionation process so as to generate small gold clusters or gold nanoparticles, we may then be able to engender new reactivities.
Conclusion The substrate (alkyne/allene/alkene)-induced cationic gold decay (disproportionation) may be the main reason behind cationic gold deactivation, but it should be noted that there could be other decay pathways. Further investigations on how to reduce gold-catalyst decay in particular and homogeneous metal-catalyst decay in general are ongoing in our laboratory.
[5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15]
Acknowledgements We are grateful to the NSF for financial support (CHE-111316). We acknowledge Professor Craig A. Grapperhaus (University of Louisville) for allowing us the use of his electrochemistry facility and for helpful discussions. We also acknowledge the useful electrochemistry discussions held with Professor Richard M. Baldwin (University of Louisville) and the use of the CREAM Mass Spectrometry Facility (University of Louisville), funded by NSF/EPSCoR (EPS-0447479).
[16] [17]
[18] [19] [20] [21]
Keywords: catalyst deactivation · catalyst disproportionation · homogeneous catalysis · gold
decay
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Received: November 1, 2013 Revised: December 31, 2013 Published online on February 12, 2014
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& Homogeneous Catalysis
Alkyne/Alkene/Allene-Induced Disproportionation of Cationic Gold(I) Catalyst Manish Kumar,[a] Jacek Jasinski,[b] Gerald B. Hammond,*[a] and Bo Xu*[a]
Abstract: The first detailed experimental study of the deactivation of cationic gold was conducted, and the influence of each component in the reaction system (substrate, counterion, solvent) on the decay process was examined. It was
found that a substrate (alkyne/allene/alkene)-induced disproportionation of gold(I) may play a key role in the decay process. Our mechanism is supported by kinetic, XPS, voltammetry studies, and high-resolution ESI-MS data.
Introduction Gold catalysis, a landmark addition to the field of organic synthesis,[1] is not without shortcomings, the main one being the relatively low turnover numbers observed in the majority of gold-catalyzed reactions. Indeed, high turnover numbers (or low catalyst loadings) are observed only for few types of goldcatalyzed reactions (e.g., hydration of alkyne).[2] In most goldcatalyzed reactions, loadings in the 1–5 % range are the norm.[3] One reason for the low turnover numbers (TON) is the relatively fast decay of cationic gold and its conversion into nonreactive species (e.g., Au0—gold mirror and/or gold particles and [L2 AuI] + .[4] Despite this drawback, there has been no comprehensive systematic investigations on the decay of homogeneous gold catalysis; a stark contrast with the extensive work reported on catalyst deactivation in heterogeneous catalysis.[5] Herein, we answer some fundamental and intriguing questions on gold(I) decay, such as the effects of starting material, nucleophile, solvent, counterion, and additives, and we suggest a plausible pathway for gold(I) decay.
Results and Discussion A simplified gold catalytic cycle is shown in Scheme 1. The cationic gold complex (L-Au + ) acts as carbophilic Lewis acid,[6] activating an unsaturated C C bond to form a p complex [L-Au + -S]. The interaction of [L-Au + -S] with a nucleophile (Nu) gives [a] M. Kumar, Prof. Dr. G. B. Hammond, Prof. Dr. B. Xu Department of Chemistry, University of Louisville Louisville, Kentucky 40292 (USA) E-mail: [email protected] [email protected] [b] Dr. J. Jasinski Materials Characterization Conn Center for Renewable Energy Research University of Louisville, Louisville, Kentucky 40292 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304271. Chem. Eur. J. 2014, 20, 3113 – 3119
Scheme 1. Simplified representation of the gold catalytic cycle.
a gold s complex ([L-Au-S-Nu]) that eventually undergoes protodeauration and the regeneration of cationic gold. Complex [L-AuI] + X (X = weakly coordinating counterion, such as OTf ) is relatively stable by itself, but in many reactions it decays at a fast rate (from few minutes to few hours). This behavior suggests that a cationic gold catalyst may decay rather rapidly in the presence of certain components in a reaction system. Because [Au + (Ph3P)OTf ] (1) is one of the most commonly used cationic gold catalysts,[3a, b, d] we chose it as our model catalyst to study the deactivation process. Compound 1 in CDCl3 is relatively stable (31P NMR peak at d = 27.4 ppm is the only peak)[7] and also catalytically active for 24 h at room temperature). First, we investigated the kinetic stability of [LAu + -S] (Scheme 1, S = alkyne/allene/alkene). After adding cyclohexene (10 equiv vs. 1) at room temperature, we noticed significant amounts of gold mirror/black particles (Au0) in the NMR tube, a telltale indication that 1 (0.01 m in CDCl3) had undergone significant decay. In the beginning, only a single 31 P NMR peak at 29.5 ppm was recorded, consistent with a gold-alkene p-complex or a fast equilibrium between free [Au + (Ph3P)OTf ] and [Au(Ph3P) alkene] + OTf (Figure 1). The signal at d = 29.5 ppm [gold(I)–alkene complex] decreased over time, and a new signal at d = 45.0 ppm increased over the same time period. We assigned this new signal the structure [Au + (Ph3P)2OTf ] (2). This assignment is in agreement with the 31 P NMR shift value of 2 reported in the literature[8] and also with the chemical shift recorded for our independently synthe-
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Full Paper vent, counterion, nucleophile, and additive. In all the solvents that we tested, 1 was stable for 24 h at room temperature. When cyclohexene (10 equiv vs. gold) was added, the decay process began in earnest. A highly coordinating solvent, such as CH3CN, showed good aptitude in preventing the deactivation of gold catalyst (Figure 2 c). As was mentioned above, [Au + (Ph3P)X ] alone is relatively stable, but when cyclohexene was added, different degrees of decay were observed in all cases (Figure 2 d). Compared to the commonly 31 Figure 1. Decay of 1 in the presence of cyclohexene, monitored by P NMR spectroscopy. used OTf counterion, NTf2 has a greater ability to stabilize the sized 2. This assignment was further confirmed by high-resolucationic gold catalyst (Figure 2 d).[9] Just as it was the case with tion ESI mass spectroscopy ([M-OTf] + at m/z = 721.1483; see solvents and counterions, most of the additives we screened did not induce the decay of cationic gold, but when we tested Figure S4 in the Supporting Information). We also recorded the 1 H NMR of the reaction mixture, and found that cyclohexene how they influenced the decay kinetics in the presence of cyhad remained unchanged during the decay process. However, clohexene, we found that the decay of cationic gold slowed we noticed that higher concentrations of cyclohexene sped up down differently depending on the type of additives used (Figthe decay (Figure 2 a). Other unsaturated substrates commonly ure 2 e). Many N-heterocycles and hexamethylphosphoramide used in gold catalysis, such as alkyne and allene (10 equiv vs. (HMPA) stabilized the gold catalyst significantly; similarly, other 1), also induced the decay of cationic gold 1 (Figure 2 b). additives (LiNTf2, N-oxide, sodium sufinimide, KCTf3, N,N’-DiNext, we investigated the influence of other common commethyl-N,N’-trimethyleneurea (DMPU)) slowed down the decay ponents on the kinetics of gold-catalyst decay, specifically, solof gold catalyst 1 to different extents.
Figure 2. a) Decay of [Au + (Ph3P)OTf ] (1) in the presence of cyclohexene; b) decay of 1 in the presence of alkene/alkyne/allene; c) decay of 1 in various solvents in the presence of cyclohexene; d) decay of [Au + (Ph3P)X ] in the presence of cyclohexene; e) decay of 1 in the presence of cyclohexene and additives; and f) water effect on decay of 1. Chem. Eur. J. 2014, 20, 3113 – 3119
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Full Paper Because trace water is ubiquitous in the reaction system, we investigated the effect of water on catalyst deactivation. We found that water alone has little effect on the decay (Figure 2 f). Nevertheless, we observed a significantly faster deactivation of gold catalyst 1, when 1 (0.01 m) was mixed with both cyclohexene (10 equiv) and water (1 equiv) at room temperature. We postulated two possible pathways to explain the decay behavior (generation of Au0 from AuI). One pathway is disproportionation, and the other is reduction of gold(I) by certain reductants in the reaction system. Ceroni and co-workers[10] reported that gold nanoparticles (Au0) can be produced by disproportionation of Au + ions. Some research groups have also speculated that the deactivation of gold catalyst is due to disproportionation.[4b, c] However, no direct experimental evidence has been reported to support these statements. We decided to investigate the actual gold valence change in the decay process experimentally. The redox potential of cationic gold should provide us with quantitative information to determine the easiness of reduction of AuI in a reaction system. Laguna and co-workers have studied the redox chemistry of AuI and AuIII complexes by using cyclic voltammetry in CH2Cl2.[11] We also chose CH2Cl2 as solvent to have a frame of reference to measure the reductive potential of cationic AuI. The cyclic voltammogram of 1 revealed a single irreversible redox event at a very negative formal potential ( 2.5 V vs. ferrocenium/ferrocene; Figure 3). Compared with the formal potential of Ag + in CH2Cl2 (+ 0.65 V),[12] the highly negative potential of 1 denotes that a direct reduction of catalyst 1 could be very difficult. This ob-
Because 31P, 1H NMR, or voltammetry data do not reflect the change in the chemical state of gold itself, we used other more direct techniques to investigate the change in the chemical state of gold. X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, chemical state, and electronic state of the elements that exist within a material.[13] We[14] and others[15] have used XPS in the determination of the chemical state of gold catalysts. The Au 4 f7/2 peaks can be resolved into a doublet (spin–orbit splitting) and the binding energy of Au 4 f7/2 electrons in each gold oxidation state are usually sufficiently wide apart to be differentiated using this technology. First, we tested our gold-standard samples ([AuICl(PPh3)] and NaAuIIICl4) and found that the Au 4 f7/2 photoelectron peak was located at a binding energy (BE) value of 85.7 and 87.5 eV, respectively, which is quite consistent with literature reports (see the Supporting Information).[16] Then, we tested our cationic gold catalyst with various counterions. We prepared these cationic gold samples in chloroform; 24 h later, these solutions were dried in high vacuum and were subjected to XPS analysis (Figure 4 a–c). As was expected, the gold valence in these three samples is still AuI. Compound 1 (0.01 m in CDCl3) was treated with cyclohexene (10 equiv), and after 48 h the mixture was filtered, and the liquid portion (sample 1, Figure 4 d) and solid precipitate were collected (sample 2, Figure 4 e). We conducted XPS analyses of
Figure 3. Cyclic voltammogram (red line) of 1 (2 mm in CH2Cl2 with 0.1 m tetrabutylammonium hexafluorophosphate (TBAHFP)). Blue line is background CV of supporting electrolyte in CH2Cl2. Potential values are referenced versus the ferrocenium/ferrocene couple.
servation was further confirmed by adding a strong reductant to 1. When [CoCp*2] (reduction potential in CH2Cl2 is 1.94 V)[12] was added to a solution of 1, no reduction took place. [CoCp*2] is already a very strong reductant, most compounds in a reaction system are far less reductive. Also, in our decay-kinetic studies, we did not observe any change in the 1 H NMR of cyclohexene in the solution. This combined data suggests that the direct reduction of AuI is unlikely. Chem. Eur. J. 2014, 20, 3113 – 3119
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Figure 4. XPS spectra (AlKa = 1486.6 eV) corresponding to the Au 4 f7/2 photoelectron peaks.
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Full Paper both samples. The majority of the solid sample (sample 2) was Au0 (Figure 4 e), and the liquid fraction was AuI (Figure 4 d). Because we detected none or very little of AuIII component, it was highly possible that a reductive agent, such as the phosphine ligand, reduced AuIII (because of the high oxidation power of cationic AuIII itself). Next, we investigated the disproportionation of gold(I) in the absence of phosphine ligand. First, we tested a commercial AuICl sample; although the majority of it corresponded to AuI, there was also a small amount of AuIII present (Figure 4 f). We wanted to investigate the behavior of cationic gold(I) (AuIOTf) under these conditions. AuICl and AgOTf (1 equiv) were mixed in dry dichloromethane with and without cyclohexene. Thus, after stirring for 30 min under dark and anhydrous conditions, we observed the formation of black particles. We collected the black particles (sample 3) and conducted a XPS analysis of sample 3 (Figure 4 g), which denoted the presence of Au0, AuI, and AuIII. We conducted the same experiment but in the company of cyclohexene and observed a similar result. 1H NMR analysis indicated no change in the structure of cyclohexene in the decay solution. This result suggested that cyclohexene did not play a role as reductant. All together, the experimental data supported the disproportionation of cationic gold(I) in the absence of phosphine ligand. Because XPS was conducted under high vacuum, and the samples were tested in the solid state, there was some concern that the gold catalyst might have changed under these conditions. We wanted to further confirm the existence of gold(III) by investigating the gold species directly in solution phase. Electrospray ionization mass spectroscopy (ESI-MS) is a soft ionization technique,[17] and is an especially suitable technique, because cationic gold intermediates are charged species. In a previous work, we succeeded in detecting the existence of gold(III) species by using high-resolution (HR) ESI-MS.[18] First, we checked the HR ESI-MS spectra of sample 1 (Figure 4). We only detected the presence of various AuI species (e.g., [AuI(Ph3P)] + (m/z 459.0571), [AuI(Ph3P)H2O] + (m/z 477.0677), [AuI(Ph3P)2] + (m/z 721.1483)). This observation is consistent with our previous XPS analysis (Figure 4 d). According to our previous studies of gold valence by using ESI-MS,[14] AuIII species are difficult to detect under standard condition, but we have been able to detect AuIII species by adding a bidentate ligand and a halide (bipyridine and chloride).[14] Because AuIII complexes have square-planar geometry, a bidentate ligand should greatly stabilize the AuIII cation during the ionization process, and chloride will further stabilize cationic AuIII.[14] Therefore, we added bipyridine and chloride to sample 1. Again, only AuI species (e.g., [AuI(Ph3P)bipyridine] + (m/z 615.1266), [AuI(Ph3P)2] + (m/z 721.1483)) were detected. This result indicated that there was no AuIII in the presence of phosphine ligand (PPh3). We used the same method to investigate the ESI-MS of sample 3. Various AuIII species were detected (Figure 5). Our HR ESI-MS studies are consistent with our previous XPS studies, that is, there was no AuIII species present (only AuI species) in sample 1, but various AuIII species were detected in sample 3 (for more details, see the Supporting Information, section 5).
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Figure 5. ESI-MS spectrum of sample 3.
Based on the combination of all the above described experimental data, we are now in a good position to understand the decay mechanism, which can be summed up as follows: 1) Cationic AuI without a suitable ligand (e.g., AuIOTf) disproportionates readily compared to its noncationic form (e.g., AuICl; Figure 4, sample 3). 2) In the presence of a suitable ligand, cationic gold(I) decays (or disproportionates) at a much slower pace. Complex [Au + (Ph3P)OTf ] is stable for at least 24 h (Figure 2 a), and AuOTf disproportionates in less than 0.5 h (Figure 4). This conclusion explains why ligands are so ubiquitous in gold catalysis. 3) Cationic AuI p complexes with p donors (C C unsaturated compounds, such as alkyne/allene/alkene) undergo relative fast decay, whereas cationic AuI complexes with most s donors (e.g., N-heterocycles and acetonitrile) are very stable.
The detailed role of substrates (alkyne/allene/alkene) in substrate-induced cationic gold decay is not very clear yet. Considering the great stabilization effect of the phosphine ligand (or NHC ligand) against disproportionation, our hypothesis is that an alkyne/allene/alkene may help to temporarily displace the phosphine ligand from the gold(I) center by a trans effect. The trans effect is the labilization (destabilization) of ligands that are trans to certain other ligands in a metal complex. Although most examples of trans effect are observed in square-planar complexes (e.g., Pd, Pt), we propose that it may also operate in linear metal complexes, such as gold(I) complexes.[19] And alkenes/alkynes are indeed the ligands that have shown the strongest trans effect. Odell and co-workers demonstrated that alkene and alkyne showed highest trans effect than any other compounds in platinum complexes (trans effect: C2H4 ~ iPrCH= CHMe ~ Me2C(OH)C C-C(OH)Me2 @ Et3Sb > Ph3Sb > Me3P > Et3P > Ph3P).[20] Our proposed decay mechanism is shown in Scheme 2. First, cyclohexene will complex with cationic gold to form a gold– alkene p complex, then, because of the trans effect of the
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Full Paper co-workers[26] have found that the gold resting state is the gold–allene complex 5 in this reaction (Scheme 3 a). Intermolecular hydroamination of alkene[3a] is a similar example (Scheme 3 b). In these reactions, high loadings of gold catalyst (5 % or higher) are typically used. And in these reactions, we did observe the formation of decay gold products 2 and Au0 in the reaction mixture, especially
Scheme 2. Proposed mechanism for substrate-induced cationic gold decay.
alkene, the phosphine ligand will be temporarily replaced to form complex 2 and free AuOTf. The disproportionation of AuOTf generates Au0 and AuIII(OTf)3. This disproportionation may be enhanced by the presence of trace water in the system. This statement is supported by a literature report that stated that a AuI complex underwent disproportionation in aqueous solution, whereas AuI was stable in organic solvents.[21] The disproportionation of AuOTf or its alkene/alkyne complex can be a relatively fast process (Figure 4, sample 3). This statement also has literature support: Dias and co-workers[22] reported that AuI alkene or alkyne complexes without ligands are only stable at very low temperature, turning into black particles at room temperature. It is known that AuIII will oxidize the phosphine ligand,[23] and itself will be reduced to AuI again. We propose that Ph3P is oxidized to Ph3P(OTf)2,[24] which could then be hydrolyzed to give Ph3P=O in the presence of water. The presence of Ph3P=O has been confirmed by ESI-MS and TLC (by comparison with an authentic sample). Furthermore, when Au(OTf)3 (generated from AuCl3/AgOTf) was mixed with PPh3, [Au + (Ph3P)2OTf ] (2) and Ph3P=O were obtained. This behavior could explain why we were not able to detect AuIII in gold solutions using XPS and ESI-MS (sample 1). We now can apply our decay mechanistic study to explain some unanswered questions: 1) What causes the decay of cationic gold? The C C unsaturated compounds (alkyne/allene/alkene)-induced cationic gold decay is the major reason for cationic gold decay. It should be noted that the starting material is not the only source of C C unsaturated compounds; the products often contain C C unsaturated compounds as well. 2) Why is it that a high turnover can be achieved in some reactions but not in others? The decay process is highly dependent on the resting state of the gold catalyst. Based on our most recent study,[25] we have classified gold-catalyzed reactions according to their turnover limiting stage: in type I, the formation of gold–s complex ([L-Au-S-Nu] in Scheme 1) is the turnover limiting stage; in type II, the regeneration of cationic gold catalyst (e.g., protodeauration) from [L-Au-S-Nu] is the turnover-limiting stage. For type I reactions, the resting state is often [L-Au + -S] (Scheme 1); thus, a relatively fast decay is to be expected, and so, turnover numbers will be low in general. On the other hand, in type II reactions, the resting state is usually not [L-Au + -S], so the decay is relatively slow and high TON are to be expected. According to our recent study,[25] the hydroamination of allene is a type I reaction (Scheme 3 a). Toste and Chem. Eur. J. 2014, 20, 3113 – 3119
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Scheme 3. Catalyst decay in different gold catalyzed reactions.
when triflate was used as counterion. A typical example of type II reactions is the cyclization of propargyl amide (7) (Scheme 3 c); its catalyst resting state is a [L-Au-S-Nu] type intermediate 9.[25, 27] Thus, the decay of active catalyst is expected to be slow. Indeed, low catalyst loadings (0.1 %) have been achieved for this reaction.[28] Similarly, the alkyne hydration (Scheme 3 d) reported by Nolan and co-workers[2c] and the intramolecular addition of a diol to an alkyne reported by Hashmi and co-workers[2d, 29] have been achieved with very high TONs. In these reactions, the turnoverlimiting determining stage is most likely the protodeauration of [L-Au-S-Nu]-type complex (Scheme 1).[25] 3) What can we do to reduce the decay of an active gold catalyst? Our studies indicate that the use of s donors, such as N-heterocycles and CH3CN, will enhance the stability of the cationic gold,[30] but s donors also have a significant detrimental effect on reactivity, so their use is normally not a good option. But it is still possible to disrupt the gold– alkyne/alkene interaction and therefore reduce the decay by using suitable ligands and counterions. Recently, we and others have reported that the use of a ligand containing a o-biphenyl steric handle can reduce the decay of cationic gold.[25, 31] As shown in Scheme 3 e, the o-biphenyl motif will clearly interrupt the bonding of the gold center with an alkene/alkyne, which may be the reason it slowed down its decay. A steric handle, such as o-biphenyl will not reduce the reactivity though; actually, for many types of gold-cata-
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Full Paper lyzed reactions, ligands with biphenyl motif often give faster reactions.[1c, 32] In Figure 2 d, we demonstrated that some counterions are capable of stabilizing the cationic gold. NTf2 is a notable example. The use of NTf2 will not reduce reactivity; in many reactions, NTf2 actually enables faster reactions compared with a commonly used counterion, such as OTf .[9]
[3]
[4]
It should be noted that the alkyne/alkene/allene-induced disproportionation of cationic gold(I) species may generate small gold clusters or gold nanoparticles under proper conditions; in turn, these clusters exhibit high reactivity in some gold-catalyzed reactions.[2e, 33] Therefore, disproportionation may not be the end of the story for gold catalysts. We believe that if relatively large gold particles or a gold mirror are generated through disproportionation, the reactivity of the gold catalyst will be lost, but if we can control this disproportionation process so as to generate small gold clusters or gold nanoparticles, we may then be able to engender new reactivities.
Conclusion The substrate (alkyne/allene/alkene)-induced cationic gold decay (disproportionation) may be the main reason behind cationic gold deactivation, but it should be noted that there could be other decay pathways. Further investigations on how to reduce gold-catalyst decay in particular and homogeneous metal-catalyst decay in general are ongoing in our laboratory.
[5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15]
Acknowledgements We are grateful to the NSF for financial support (CHE-111316). We acknowledge Professor Craig A. Grapperhaus (University of Louisville) for allowing us the use of his electrochemistry facility and for helpful discussions. We also acknowledge the useful electrochemistry discussions held with Professor Richard M. Baldwin (University of Louisville) and the use of the CREAM Mass Spectrometry Facility (University of Louisville), funded by NSF/EPSCoR (EPS-0447479).
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Keywords: catalyst deactivation · catalyst disproportionation · homogeneous catalysis · gold
decay
·
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Received: November 1, 2013 Revised: December 31, 2013 Published online on February 12, 2014
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