Review pubs.acs.org/CR
Deactivation in Homogeneous Transition Metal Catalysis: Causes, Avoidance, and Cure Robert H. Crabtree* Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States 8.2. Other Methods 9. Solvent Inhibition 9.1. Solvation and Solvent Binding 9.2. Reactive Solvents 9.3. Solvent Impurities 10. Electrocatalysis 11. Photocatalysis 12. Conclusions Author Information Corresponding Author Notes Biography Acknowledgments References
CONTENTS 1. Introduction 1.1. Types of Deactivation 2. Counterion Inhibition 2.1. Late Metals 2.2. Early Metals 3. Ligand-Centered Processes 3.1. P-Donor Ligands 3.2. C-Donor Ligands 3.3. N-Donor Ligands 3.4. O-Donor Ligands 3.5. Chelates, Pincers, and Mixed Donor Ligands 3.6. Dissociative Ligand Loss 3.7. Designed Degradative Ligand Loss 3.8. Ligand Redistribution 3.9. Other Pathways 3.10. Self-Repair 4. Multimetallic Processes 4.1. Formation of Clusters and Multinuclear Complexes 4.2. Metal Deposition 4.3. Metal Reduction and Oxidation 4.4. Other Cases 5. Poisons 5.1. Unintended Poisoning 5.2. Deliberate Poisoning 6. Inhibition by Substrate or Coreactant 6.1. Noyori-type Hydrogenation 6.2. C−H Activation and Related Reactions 6.3. Palladium Coupling Chemistry 6.4. Alkene Metathesis. 6.5. Alkene and Alkyne Oligomerization and Polymerization 6.6. Other Cases 7. Product Inhibition 7.1. Attainment of Equilibrium 7.2. Kinetic Inhibition 7.3. pH Drift 8. Unconventional Methods 8.1. Microwave Heating
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1. INTRODUCTION Deactivation leads to loss of catalyst activity or selectivity with increasing reaction time, but has attracted less academic attention in homogeneous catalysis than is justified by its importance. Ultimate deactivation is inevitable, but catalyst performance can be greatly affected depending on the balance between the rates of deactivation and of productive catalysis. Even a small improvement in this rate ratio can have a big effect on performance in terms of turnover number (TON) and thus also increases the reaction yield for a given catalyst loading. Only a minority of reports in homogeneous catalysis tackles this problem in detail: not only is it sometimes difficult to identify the deactivation products involved, but workers may also prefer not to spend time studying an apparently failed catalyst or extending the life of a seemingly adequate one. Poater and Cavallo1 make this point persuasively: One of the reasons for this limited understanding [of catalyst deactivation] is that academic groups usually focus on the more rewarding improvement of activity and/or selectivity of a catalyst, since more or less rational strategies can be followed, rather than investing resources to follow catalyst deactivation along unexplored pathways. One aim of this review, however, is to point out that these deactivation pathways can sometimes be usefully explored and that the results can sometimes suggest rational strategies to counter deactivation. Ideally, finding out what went wrong in any given case should improve the catalyst in question. In commercial operation, where cost is a key driver, deactivation is a much more urgent issue than in academia, but the strategies adopted are much less likely to appear in the open literature,
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although some reports2,3 do deal with countering deactivation in commercial scale homogeneous work. A number of academic reviews4,5 have treated different aspects of the deactivation problem, including recent studies of stability under oxidative stress, either of organometallic6 or of coordination compounds.7 Reaction-specific information can include advice on deactivation. A notable review by Surry and Buchwald8 not only gives detailed and valuable advice on choosing the right ligand for Pd-catalyzed amination but also discusses the role of each component of the catalytic system and the likely effects of changing conditions; a troubleshooting guide gives suggestions for fixing common problems, some of which involve catalyst deactivation. The relatively restricted number of reviews on homogeneous catalyst deactivation contrasts with the much larger number to be found in the heterogeneous catalysis literature.9 The key monograph in the area,10 “Homogeneous Catalysts. ActivityStabilityDeactivation” by van Leeuwen and Chadwick, is an essential point of reference, particularly for olefin polymerization, asymmetric hydrogenation, carbonylation, cross coupling, and metathesis. Here we use the terms degradation, decomposition, and deactivation to refer to an irreversible process involving extensive breakup of bonds in a chemical structure; degradation usually refers to the ligand, decomposition to the metal complex as a whole, and deactivation to the resulting loss of catalytic activity. Inhibition refers to a reversible process that leads to partial loss of catalytic activity, typically involving binding of a poison to the metal. Arguably, exceptionally active catalysts, being so reactive, are also the ones most susceptible to deactivation. In an extreme case, an otherwise excellent catalyst might escape discovery if unsuitable catalytic conditions favor rapid deactivation. For example, the [(cod)IrLL′]X catalyst series11 shows no or low hydrogenation activity in the once-standard EtOH solvent but are extremely active in the much less coordinating solvent, CH2Cl2. These catalysts also exemplify another general principle: deactivation is usually more severe as we move to more “difficult” substrates, such as in going from cyclohexene to 1-methyl cyclohexene. If the deactivation rate stays constant but the rate of productive catalysis falls on moving to a more difficult substrate, the rate ratio is clearly affected, leading to deactivation taking place after fewer catalyst turnovers.11 Slow addition of catalyst during the reaction is a potential countermeasure for such substrates. One of the factors that can prevent practical application of homogeneous transition metal catalysis, particularly with precious metals, is the relatively high catalyst loadings that are sometimes needed to obtain good product yields. A catalyst requiring 1% loading implies a turnover number (TON) of ∼100, while a 1 ppm loading implies a TON of ∼106, clearly a much more attractive situation, all else being equal. Even a modest decrease in the ratio of the rate of deactivation versus the rate of productive catalysis can be expected to significantly improve catalyst performance. The relevance of the topic is not limited to catalyst chemists. Many researchers, not themselves specialists in the field, apply a homogeneous catalyst taken from the literature in the hope of obtaining a needed organic product or bringing about some desired transformation. These efforts sometimes go astray from neglect of some unstated assumption in the original work or from a change either in the substrate structure or in the conditions used in the prior reports. In an important review of
the Mizoroki−Heck reaction, Beletskaya and Cheprakov12 make this point in a colorful way: ...often a small variation of substrate structure, nature of base, ligands, temperature, pressure, etc., leads to unpredictable results. Trends in reactivity and selectivity are uneven and often break when nobody would expect. Brandname precious ligands which worked miracles for some sophisticated transformations often fail in the simplest cases. One aspect of the present review is therefore to gather together some leading examples that illustrate factors that can either promote or inhibit deactivation so that these can be taken into account in planning both exploratory and applied work with homogeneous catalysts. Active catalysts typically have labile sites that are in principle much more sensitive to all the species present in the reaction medium as well as to the ambient conditions than is a stable complex in solution. The effects of changing solvents, additives, and conditions may therefore have big effects. Avoiding impurities in the solvent or substrate can also be a help, big or small depending on the case. Counterions, even ones considered “noncoordinating”, can also play a role because ionpairing is significant in many of the organic solvents typically seen in the field. Research proposals in the catalysis field sometimes attract criticism from reviwers for lack of a rational basis for the choices of catalyst and conditions: the dismissive term “fishing expedition” is sometimes seen in their reports, for example. Another goal of this review is therefore to provide proposal writers with ammunition in the form of a set of deactivation countermeasures based on mechanistic understanding that could help justify those choices on rational grounds. These are tabulated in section 12. 1.1. Types of Deactivation
Deactivation takes many forms: degradation of the ligand; inhibition by the solvent; inadvertent admission of air or moisture; lability of metal−ligand bonds; inhibition by buildup of reaction products, from inappropriate substrate functionality, from undesired substrate reactions, or by deposition of bulk metal. Sometimes, reports simply invoke “catalyst decomposition” with no indication of the type of pathway involved.13 Since deactivation can often be reversed by additives or by a change in the ligands or reaction conditions, improvements should ideally be sought at an early stage of catalyst optimization, so the detailed work on the system is done with the improved catalyst. We therefore also consider ways in which the risk of catalyst deactivation can be minimized by careful design of procedures as well as ways in which deactivation can be slowed or reversed by altering the catalyst or conditions. Deactivation types differ in their effects. For example, the least dangerous is a one-time deactivation that might arise from a small impurity, say of peroxide in the substrate, that then decomposes 5% of the catalyst, but once the impurity is used up, no further deactivation need occur. In that case, the remaining 95% of the catalyst could survive through many subsequent cycles. Alternatively, a background deactivation reaction might continuously decompose the catalyst. In such a recurrent case, the extent of deactivation would build up with the number of cycles. In a stoichiometric reaction, a product yield of 95% is considered excellent. In a catalytic reaction, however, where the catalyst needs to be fully regenerated on completion of each cycle, a 95% regeneration yield in each cycle B
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undoubtedly very weakly, via a pair of Br atoms.24 In another similar case involving the phosphinooxazoline complex, [Ir(cod)(PHOX)]+, BArF4− also preserved the catalyst from deactivation.25 Pregosin, Kündig, and co-workers26 have shown by pulsed gradient spin−echo (PGSE) diffusion NMR measurements that BArF4− is less tightly ion-paired with a standard [CpRuL3]+ organometallic cation than BF4− or PF6−. Occasionally, however, coordinating anions prove advantageous, as in the Pd-catalyzed allylic amination of Togni and coworkers,27 where F− and BH4− were preferred.
is no longer excellent, because after 10 cycles only 63% of the catalyst would be left and after 100 cycles only 0.6%. Worst of all is the case where the initial decomposition product catalyzes subsequent decomposition, as might occur when a Pd complex deposited Pd nanoparticles that might then catalyze the deactivation of more catalyst. Even if only 5% of the catalyst were lost in the first cycle, 10% might be lost in the second and even more in each of the later cycles. Whatever the precise metal and ligand set, the catalytic performance is often very dependent on the exact reaction conditions: simply repeating the ones reported for a successful prior literature conditions is unlikely to be optimal. The reaction temperature, solvent, and catalyst loading may all need to be adjusted within bounds, broad or narrow, depending on the case. Quantitative optimization techniques have recently been suggested.14 Generally speaking, running the catalytic reaction of interest at a few different temperatures is often advantageous in part because deactivation can sometimes dominate at elevated temperatures15 or in other systems at low ones.16
2.2. Early Metals
Counterion effects are particularly marked in early metal alkene polymerization catalysts, such as the commercially useful catalyst, 2 (A = anion), where Ziegler and co-workers28 showed how counterion effects influence the mechanism, even to the extent of changing the turnover limiting step. The Lewis acid activator, MAO {(Me2AlO)n}, is a key promoter in alkene polymerization catalysis, where it abstracts a methyl group from a typical catalyst precursor such as Cp2TiMe2 (3) to form the active catalyst, in this case [Cp2TiMe]+[Me(Me2AlO)n]−, the anion being minimally coordinating. Alternative Lewis acids such as B(C6F5)3 follow the same pattern, but in one case, transfer of two C6F5 groups from the presumed intermediate boron counteranion to zirconium led to catalyst deactivation.
2. COUNTERION INHIBITION Catalysts that carry an ionic charge require an appropriate counterion. In such a case, care needs to be taken to prevent catalyst inhibition by that counterion. This is a more severe problem for cationic than anionic catalysts because a variety of minimally coordinating counterions, Na+, K+, etc., are readily available for anions. The catalyst is somewhat more often a cation than an anion, however, for which the correspondingly simple monatomic counterions, Cl−, Br− etc., tend to be coordinating. 2.1. Late Metals
Numerous minimally coordinating anions have been developed such as BArF4− (1)17 and a suite of carboranes.18 Their large radius minimizes the electric field at the surface of the ion, disfavoring ion-pairing, and their halogenated exterior minimizes their tendency to bind to the metal. Choosing among the many possibilities has given rise to much discussion.19 Even if the counterion is “noncoordinating”, ion-pairing20 is often favored in a solvent such as dichloromethane; if so, some modification of catalyst properties is to be expected. Moving from a coordinating anion to an outer sphere counterion can have a much bigger effect. For example, the diastereoselectivity in cis-stilbene epoxidation and the product distribution in cyclohexene epoxidation were both very strongly affected by changing the X group of the MnIII(porphyrin)X catalyst from
Thus, from one particular precursor, CpZr(NP-t-Bu3)Me2, the deactivation product, CpZr(NP-t-Bu3)(C6F5)2, was obtained.29a Another case of counterion-dependent deactivation of early metal alkene polymerization catalysts was identified for a series of different Cp*2TiMe2/activator combinations. When the titanocene methyl cation 2 was generated from 3 in bromobenzene, by reaction with [Ph 3C][B(C 6 F 5) 4 ] as activator, only slow conversion to deactivation product 4 occurred. With [PhNMe2H][B(C6F5)4] in contrast, formation of 4 was faster and with [Et3NH][B(C6F5)4] deactivation to give 4 was almost instantaneous.29b Efficient proton shuttling by the liberated amine was thought to be the cause of the acceleration by the ammonium activators. For these early metals even such a weak donor as the bromine of the
Cl− to CF3SO3−.21 As a general rule, BArF4− has often proved to give the best catalytic performance.22,23 Counterion effects can in rare cases delay deactivation by binding to the catalyst active site strongly enough to inhibit deactivation without a corresponding inhibition of the catalytic cycle. Weller and co-workers have described such a case for [Ir(cod)(PPh3)2][closo-CB11H6Br6], which forms hydride bridged dinuclear clusters with conventional counterions such as PF6−. The crystal structure showed that the carborane binds,
bromoarene is able to bind; initial binding of the Br atom presumably helps the cyclometalation to give 4.
3. LIGAND-CENTERED PROCESSES Even a ligand that is only intended to be a spectator may undergo some undesired deleterious reaction that leads to deactivation. The typical deactivation pathways seen for each ligand type are discussed here, together with some strategies for countering these problems. Examples are taken from catalytic C
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systems wherever possible, but in other cases, potentially relevant ligand degradation processes are included even though these so far are only seen in stoichiometric reactions. The sequence of the discussion is first by ligand type, and then, within each subsection, by deactivation type. Not every ligand rearrangement leads to deactivation: on occasion, ligandcentered change is advantageous for catalysis, as in the Milstein catalyst30a or with redox-active ligands.30b A general point, so far only validated for N- and P-donors but plausibly applying to a much wider variety of ligands, is the bond-strengthening effect on specific intraligand bonds of ligand binding to a metal. The bonds affected are C−H bonds adjacent to the donor atom, X, as well as any XH bonds that may also be present. In the free ligand, the N or P ligand lone pair weakens any adjacent C−H bonds of alkyl substituents via overlap with the C−H σ* orbital; on lone pair binding to a metal, this effect is much reduced, strengthening the C−H bond. Any N−H bond is also strengthened on ligand binding. Computational work by Nova and Balcells31 shows this effect can be quite big, reaching 15 kcal/mol in some cases.
Cyclometalation can be reversible41 and so is not always as dangerous as P−C bond cleavage, which is most often irreversible. In one alkane dehydrogenation catalyst, for example, reversible cyclometalation did not permanently deactivate the catalyst, but P−C bond cleavage did do so.42 The latter pathway, treated theoretically by Hoffmann and coworkers,43 can lead to the formation of ArH and a bridging PAr2 unit that typically permanently deactivates the catalyst by forming a phosphido-bridged cluster, thus blocking labile sites, but other final P−C cleavage products are also possible,44,45 such as formation of the {(η6-ArH)M(PPh2OR)} substructure from ROH and {M(PPh2Ar)}. A less common P−C cleavage results in Ar3P conversion to an Ar2POMe phosphinite with intervention of the MeOH solvent.46 Of course, cyclometalated complexes are not always deactivation products but can sometimes also be good precursors in catalysis.47 In the Ni(cod)2/PhP(CH2CH2PPh2)2 catalyst for the isomerization of unsaturated nitriles, a second pathway was seen in addition to P−Ph bond cleavage: this was identified as a
3.1. P-Donor Ligands
Of all spectator ligand types, P-donors have attracted the most previous attention in homogeneous catalysis, and their leading decomposition pathways have been covered both in classic32 as well as in more recent reviews.33−35 These emphasize two of the main pathways for PAr3: cyclometalation (eq 1) and P−C
substrate C−CN bond cleavage, leading to formation of inactive cyanonickel phosphido species, such as 6.48 P−C bond cleavage does not always lead to deactivation; however, in another case, a phenyl group from a PPh3 ligand appeared in the final cross coupling product.49 The high level of phenyl incorporation was a function of the high catalyst loading (5%) and the large number of phenyl groups (12) per mole of the Pd(PPh3)4 catalyst. The authors suggested initial formation of [PPh4]Br by reductive elimination from [PhPd(PPh3)n]Br bond cleavage (eq 2) A more recent concern, oxidation to the phosphine oxide (eq 3), has not yet been specifically reviewed. Cyclometalation is particularly favored for bulky ligands,36 in compounds where the resulting ring is 5-membered,37 rather than 4- or 6-membered, and where an aryl−metal bond is formed. For example, all these favorable factors apply to triphenylphosphite, which undergoes double cyclometalation with [IrCl3(tht)3] (tht = tetrahydrothiophene) and bipy to give a facial tridentate PCC ligand in the [IrCl(dipy)(PCC)] product, 5.38 A point of departure in terms of countermeasures should clearly be avoidance of those factors known to favor deactivation. To identify specific aryl groups that resist or favor cyclometalation, Garrou and Dubois39 looked at various PAr3 ligands in the presence of Co2(CO)8 under the harsh hydroformylation conditions typical for these Co catalysts (190 °C, 140 atm CO/H2). PPh3 and P(o-tolyl)3 gave the most, and P(p-CF3C6H4)3 and P(p-FC6H4)3 the least, cyclometalation. Both cyclometalation and P−C cleavage have been identified as deactivation pathways in the Wilkinson hydroformylation catalyst, RhH(CO)(PPh3)3,40 where addition of excess PPh3 delays but does not altogether avoid deactivation.
led to Ph incorporation since [PPh4]Br itself was also a viable source of Ph groups for the same cross coupling (eq 4). Herrmann and co-workers50 found extensive P−C bond cleavage of Ar groups in Pd(OAc)2/PAr3 Mizoroki−Heck catalysts above 120 °C. P(o-tolyl)3 and P(mesityl)3 were resistant, however, presumably because the ortho substitution inhibits achievement of the transition state for the reaction,
D
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were made to eliminate dioxygen. Once all the phosphine was oxidized, [Rh4(CO)12] was formed. Dioxygen impurities in the CO2 were the suspected cause of the oxidation.57 The same pathway has been identified in Pd coupling reactions.58 An unusual deactivation pathway has been probed in detail for a model Grubbs metathesis catalyst, RuCl2(H2IMes)(PCy3)(CH2). First order decomposition gave the methylphosphonium salt and a carbide bridged diruthenium complex as shown in Scheme 1.59 After loss of PCy3, the related
believed to be 7. In any case, these ligands may be useful where P−C cleavage is troublesome. As early as 1980, a related side reaction was found in the hydroformylation of propene catalyzed by rhodium−PPh3 complexes; this resulted in the formation of coordinated diphenylpropylphosphine by transfer of a substrate-derived alkyl group from Rh to P.51a Other similar cases are also known.51b In another case, the bond cleaved was not the P−C
Scheme 1. Deactivation of Grubbs Metathesis Catalyst
RuCl2(PCy3)2(CHPh) catalyst deactivates via binuclear elimination of PhCHCHPh.60 In one case, a phosphine ligand became more active, not less, as a result of a ligand transformation that occurred during catalysis. Hartwig and co-workers61 discovered that 1,2-bis(di-tbutylphosphino)ferrocene underwent cyclometalation leading to pentaarylation of the (initially) unsubstituted Cp ring in an aryl halide etherification; this led to their subsequent adoption of the resulting pentaphenyl derivative, Q-Phos, as a preferred ligand for a wide variety coupling reactions.
but a C−CH3 bond also present in the ligand, resulting in weakly catalytic demethylation of the pincer ligand (eq 5).52 Grushin and Alper53 carried out one of the few extant studies on the mechanism of phosphine oxidation in which they showed how OH− brings about the disproportionation of a Pd(II) phosphine complex into Pd(0) and the phosphine oxide. This occurs by OH− attack on the phosphorus, leading to nucleophilic displacement of the reduced Pd. In an early example, the alkane dehydrogenation catalyst, [IrH2(CF3COO)(P{C6H11}3)2], decomposed at the reaction temperature of 150 °C with a half-life of 15 h to give cyclohexane and cyclohexene as P−C cleavage byproducts.54 Interestingly, the catalyst lifetime could be more than doubled by an apparently minor change in a non-PR3 ligand, in this case moving from CF3COO to CF3CF2COO. Further examples are cited by van Leeuwen.4,10 The third common decomposition pathway for phosphines is oxidation to the phosphine oxide. Since this is likely to be most troublesome in oxidation reactions, the whole PR3 ligand class can be considered less suitable for such cases. For example, [Pd(IPr)(PCy3)], a catalyst for the aerobic oxidation of alcohols, underwent complete loss of PCy3 as the oxide after 2 h.55 In contrast, a series of phosphines described by Barder and Buchwald56 (e.g., 8) strongly resist air oxidation even in the free state. The high steric bulk of biaryl structure is thought to be responsible by shielding the vulnerable phosphorus lone
3.2. C-Donor Ligands
In the last 15 years, N-heterocyclic carbene (NHCs, e.g., 9) ligands have risen to major significance in homogeneous catalysis,62 where they can sometimes usefully replace PR3 groups in an existing catalyst. Several decomposition pathways have been identified in this area, some similar to those seen for
PR3 but some new. Cyclometalation can take place where the NR group is a simple aryl (eq 6) and where a favorable 5membered ring is formed.63 To counter this pathway, mesityl substituents are widely adopted (10); this substitution pattern causes the arene ring to twist away from the conformation (11) that favors cyclometalation. IMes is not entirely exempt from cyclometalation, however, as shown by a report of double cyclometalation at the ortho-methyl groups resulting in a tris acetonitrile Ir(III) complex.64 This case is also unusual in that the conversion from the starting complex to the product (eq 7)
pair. It remains to be seen if these ligands will prove generally useful in catalysis beyond cross coupling. Phosphine oxidation can even occur in reductive reactions, for example, in a mixture of supercritical CO2 and ionic liquid, phosphine oxidation was identified as a deactivation pathway in an alkene hydroformylation study even though stringent efforts E
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of the ring may well be responsible for adjusting the electronic properties of the carbene in a favorable direction, but they may also make cyclometalation of the N substituents less favorable than in the analogous NHCs in which the carbene ring is expanded to 6 or 7 members, in which case cyclometalation apparently becomes much easier.71 No doubt because the Grubbs and Schrock metathesis catalysts72 are of such wide general use, their deactivation pathways have been studied more extensively than is the case for many other catalyst series. For example, the bis-N-phenyl substituted NHC version of Grubbs catalyst has a short lifetime (30 min) under standard conditions because it undergoes a
is driven by transfer of the resulting hydrides to propene acting as H2-acceptor, and so the cyclometalation is irreversible. A cyclometalation product can also be an unseen partner in an unfavorable reversible equilibrium, however, as shown by Leitner and co-workers65 from deuteration of the IMes when the arene deuteration catalyst, [Ru(H2)2H2(IMes)(PCy3)],
reacts with D2; slow exchange into the cyclohexyl group was also noted at long reaction times. tert-Butyl substituents can also form a favorable 5-membered ring on cyclometalation,66 but the MC bond is expected to be somewhat less strong in this alkyl case than in the MC aryl bond formed from an N-phenyl group. Indeed, in one case,67a cyclometalation of a t-butyl NHC has been shown to be reversible. For alkyl substituted NHCs where the alkyl group can cyclometalate, a subsequent β-elimination step is possible if the cyclometalation followed by a benzylidene transfer from Ru to the ortho position of the N-aryl group (eq 9).73 Phosphine dissociation in 12, the initiation step in metathesis, is also thought to lead to a parallel parasitic pathway in which cyclometalation of the N-phenyl group takes place. This is followed by a 1,1-migratory insertion to give a benzyl ruthenium species that can then undergo reductive elimination to form the final product. Theoretical work has provided further insights into the mechanism.74 To counter this pathway, a modified Grubbs catalyst was developed in which the benzylidene was constrained by formation of a chelate ring (13); the lifetime was now extended to 8 days. Modification of the N-phenyl group was even more effective in that the N-mesityl version has a lifetime of 38 days. In a related study, the Hoveyda−Grubbs metathesis catalyst75 was decomposed by exposure to ethylene at elevated temperature in order to test a successful rescue protocol involving treatment with the carbene-precursor shown in eq 10.76 In aqueous solvents, loss of chloride ion can lead to
resulting alkyl has a β-H. Such is the case for the i-Pr substituted NHC in eq 8.67b One related reaction of potential importance that can follow cyclometalation is C−N cleavage with release of alkene, such as of propene from an N-iPr substituted NHC.68a In contrast, some catalytic reactions are helped, not hindered, by NHC cyclometalation.68b Most catalytic studies on NHCs use mesityl or similar bulky groups as substituents at N and N′ probably because bulk is required to stabilize the free carbene. Since there are now many methods to synthesize NHC complexes that do not require formation of the free carbene, this restriction is lifted. Consequently, another way to counter cyclometalation is to move to slim N-substituents. For example, in the N,N′dimethyl NHC, the methyl CH bonds are far removed in space from the metal center, and no example of cyclometalation has so far been reported for these ligands, at least in a mononuclear system. A cluster having a cyclometalated NHC-Me group is known, however, but this reaction required rather forcing conditions (110 °C) and the resulting ring is 5-membered because the cyclometalation takes place on an adjacent metal in the resulting triosmium clusters.69 Methyl substituted NHCs are still not widely seen in organometallic catalysis, although some useful catalysts have recently shown a specific preference for N-methyl substituted NHCs.70 Why do NHCs from 5-membered azoles appear to be so much more useful than analogues either from acyclic NHCs or from cyclic ones having larger rings? The geometric constraints F
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MeReO3 + H 2O2 ⇒ MeReO(O2 )2 1 O2 (13) 2 Cp* complexes are increasingly seen in homogeneous catalysis, so their decomposition pathways are of growing interest. Heiden and Rauchfuss83 found that complex 14 acts as a homogeneous dioxygen reduction catalyst by transfer hydrogenation. The deactivation pathway involved oxygenation of one Cp* methyl group and cyclometalation of the adjacent one to give 15. Activity can be recovered and the ring returned to its original Cp* structure by reduction of 15 with amine ⇒ MeOH + HReO4 +
deactivation, but a successful countermeasure proved to be addition of KCl.77 Among deactivation pathways not seen in PR3 chemistry, Hofmann elimination involving the β-H of any n-alkyl substituents is a potential problem, particularly in basic media. The counterstrategy here is either to move to aryl substituents or keep to the N,N′-dimethyl substituted NHCs,
borane. This example also illustrates the generally more destructive nature of oxidation chemistry, where better countermeasures against deactivation are more particularly needed. The ability of amine boranes to regenerate 14 meant that, in the catalytic dehydrogenation of amine boranes with the same catalyst, deactivation was not seen.83 This is an excellent example of catalyst rescue by the substrate itself. Cp* ligands can also be completely lost from a precatalyst, as in transfer hydrogenation with Cp*Ir(IMe)2Cl but in this case Cp* loss activates rather than deactivates the catalyst.70 The Cp*Ir(chelate)Cl water-oxidizing catalysts easily lose Cp*, either electrochemically or with chemical oxidants. Strictly speaking, this is not a deactivation since the products of the oxidation, sometimes homogeneous, sometimes heterogeneous, are still very active catalysts. Nevertheless, the oxidative loss of Cp* may be quite general and so represent an important caveat for work under oxidizing conditions. Macchioni and coworkers84 have done the most to trace the oxidation pathway, involving formation of CH3COOH and, depending on conditions, also HCOOH. The relevant steps involve both oxidation at the ring carbons and at the methyl groups. Computational work can be a help. A silica-supported {(≡SiO)M(≡NAr)(=CHtBu)(CH2tBu)} (M = Mo, W) unit proved to be an excellent alkene metathesis catalyst but with a limited life as a result of deactivation. The mechanism of deactivation was understood from DFT data to be an undesired β-H transfer at the metallacyclobutane stage of the reaction. Replacement of the CH2tBu by a pyrrolyl group, however, retarded the deactivation and thus greatly enhanced the catalyst performance.85 α-Hydrogen transfer has been suggested as a pathway for deactivation of the [Cp2ZrMe2]/[CPh3][B(C6F5)4] series of polymerization catalysts, leading to an inactive μ-methylene bridged dinuclear Zr complex.86 In related [Cp(R3PN)ZrMe2]/MAO catalysts, a methyl group was stripped of all its α-hydrogens to form a bridging carbide cluster as deactivation product.87
known to be stable in base at elevated temperature, as in the reaction illustrated in eq 11.78
Reductive elimination (RE) of an NHC with an adjacent alkyl or aryl (eq 12) is a pathway recognized by Yates and Cavell and co-workers.79,80 In general, RE tends to be fastest for cases involving a hydride, but an NHC- and hydride-rich but carbonyl-poor polyhydride cluster, [Ir6(IMe)8(CO)2H14]2+ (IMe = N,N′-dimethylimidazol-2-ylidene), can resist RE even at elevated temperature.81 This cluster is one of a series of similar deactivation products from [Cp*Ir(IMe)2Cl]BF4catalyzed conversion of glycerol to lactic acid (eq 11). The catalyst even works well with neat, unpurified dark brown crude glycerol coming straight from a biodiesel refinery. The CO ligands must arise from decarbonylation of the proposed glyceraldehyde intermediate in the conversion. This emphasizes the point that the viability of the RE pathway is very much dependent on the particular case. Catalysts that are designed to favor RE steps in their productive cycles, such as for Pd coupling,10 may be more subject to this deactivation pathway than other catalyst types. With H2O2, methylrhenium trioxide (MTO) gives a useful alkene epoxidation catalyst for which Herrmann and coworkers have looked into the deactivation pathways. Somewhat unexpectedly, hydrolysis of the MeRe bond is not the major route. Instead, oxidation of the methyl group occurs to give methanol (eq 13).82
3.3. N-Donor Ligands
Although rarer than for PR3, cyclometalation is also known for N-donors, as in a complex of Pd(II) with tribenzylamine active for Mizoroki-Heck coupling.88 N-donors are more common in G
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Figure 1. Successive steps in improving the degradation-resistance of the Collins TAML oxidation catalyst ligands.89,90 The figure is reproduced with permission from ref 89. Copyright 2002 American Chemical Society.
oxidation catalysis, where the typically harsh conditions can lead to deactivation. In rare cases, detailed studies have uncovered their main deactivation pathwaysthe resulting information can then be used to alter the ligand structure to make it more resilient. The leading example of the value of this approach is the development of the tetraamido-macrocyclic ligand (TAML) series of oxidation catalysts by Collins and coworkers.89,90 Over many decades, the N-donor ligand set was successively refined to eliminate weak points in the structure that proved susceptible to oxidative degradation. As the ligand set became more oxidatively robust, the performance of the catalyst improved (Figure 1). Thus, a CH bond α to N in the original 1980 ligand gave way to a phenylene group in 1982 and a phenol in the 1982 ligand gave way to a -CMe2O− group in 1985. The latest application, water oxidation,91 demonstrates this robustness to best effect because water is one of the most difficult of all substrates to oxidize and is thus the application in which ligand robustness is most severely tested. Berlinguette’s water oxidation catalyst, [Ru(terpy)(bpy)(OH2)]2+ (16; terpy = 2,2′:6′,2″-terpyridine, bpy = 2,2′bipyridine), driven by Ce(IV), undergoes deactivation by loss
identified that contains a CO group derived from oxidative attack on the ligand.93 This CO ligand proved to be stable to the oxidative conditions, rather than depart as CO2 as might have been expected. Along with the expected O2, an early water oxidation catalyst, the binuclear ion, [(NH3)3Ru(μ-Cl)3Ru(NH3)3]2+, also gave N2 from oxidation of the ammonia ligands, a path that also led to deactivation. Stabilization of the catalyst was possible by supporting it in Nafion, in which case the deactivation rate fell to 6% of its original value.94 Indeed, a wide variety of catalysts active for many different reactions have been supported on Nafion with protection from deactivation being one of the resulting advantages.95 Porphyrins are classic N-donor ligands often found in enzymes and their model systems. Shaik and co-workers96 have computationally identified a pathway for oxidative degradation of the porphyrin in a ferric hydroperoxide model of the active species in heme oxygenase. This involves oxidation of the mesoCH unit by an intermediate iron hydroperoxo species with porphyrin ring cleavage accompanied by formation of free CO. N-donors also occur in reductive catalysis, as in the hydrogenation and hydrosilylation catalyst reported by Chirik and co-workers.97 In this case the ligand does not degrade on deactivation: instead, it undergoes a rearrangement in which the aryl substituents on the ligand bind to the metal to give two different π-arene complexes (eq 14) that resist treatment with H2, silanes, or CO. The low turnover numbers encountered in N2 reduction to ammonia by the Schrock molybdenum catalyst are ascribed to
of bipy, transformed under the conditions into the bis-N-oxide. The presence of electron withdrawing groups on the bipy enhanced the stability,92 by slowing either ligand loss or Noxide formation. Åkermark and co-workers have developed a single site water oxidation catalyst, 17 (L = 4-picoline), driven by [Ru(bipy)3]3+ as primary oxidant. A deactivation product, 18 (L′ = OH2), was H
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One of the reactions that most severely tests the stability of the ligand is alkane dehydrogenation, where high temperatures are often needed and the substrate is also minimally coordinating and therefore not capable of stabilizing the catalytic intermediates very well. It is here that pincer complexes have proved their worth.107,108 For example, the PCP pincer complex, [(PCP)IrH4] (23), is exceptionally robust even at 200 °C,109 a temperature far beyond the usual 150 °C
degradation of the tetradentate trianionic N(CH2CH2NAr)3 ligand, with a number of alternative structures having been suggested on the basis of theoretical work.98,99 Tris(pyrazolyl)borates have been suggested as degradation-resistant ligands for alkane dehydrogenation catalysis.100 3.4. O-Donor Ligands
Rothwell and colleagues101 looked at bulky aryloxides on Ta(V) and found that 2,6-diphenylphenoxide readily cyclometalates (as in 19). This was successfully countered by introducing bulky substituents at the 3,5 positions that prevented the 2,6-diphenyl substituents from attaining the coplanar conformation 20 needed for cyclometalation. The related 3,5-di-t-butyl-2,6-diphenylphenoxide (21) proved to be completely cyclometalation-resistant. This conformational management technique seems to be well-suited to other ligand
upper limit commonly seen in homogeneous catalysis. The analogous complex with the more rigid anthraphos ligand, PCP′ (24), can even tolerate reaction temperatures up to an astonishing 250 °C.110 Phosphites, P(OR)3, are vulnerable to hydrolysis by nucleophilic attack at P, so these ligands are somewhat rare in catalysis. The pincer versions, such as POCOP (25),111 show enhanced stability and have a number of catalytic applications. For example, (POCOP)IrH2 (R = t-Bu) dehydrogenates ammonia−borane at room temperature, a reaction of interest for hydrogen storage.112
systems where bulk is required but undesired cyclometalation otherwise occurs; NHCs should fall into this category, for example. Martell and co-workers102 have identified a decomposition pathway for acetylacetonate (acac) in the case of aerial oxidation of H2S catalyzed by Fe(acac)3. The acac was converted to acetate and acetone, plausibly via attack by an intermediate OH radical at the C-3 position.
Beyond phosphines, pincer NHCs113 have been widely adopted for enhanced stability in catalysis, as in the CNC pincer complex 26 that acts as a robust homogeneous Mizoroki−Heck catalyst.114 Terpyridine has also long been a favorite pincer in oxidation chemistry.115 Berkessel and co-workers116 have traced two decomposition pathways of a Ti salalen complex (27) that is active for the asymmetric epoxidation of reactive alkenes when driven by H2O2. In the first pathway, hydrolysis deposits TiO2 and liberates free ligand, but in the second, the NH group is hydroxylated to NOH, followed by loss of water and formation of the inactive salen complex (eq 15). Pincers are not the only ligand types to benefit from a reduced tendency for deactivation. Wolfe and Buchwald117 consider chelate ligands such as BINAP to be superior to monodentate PAr3 in this respect. However, Hartwig and coworkers118 found that P−C cleavage of BINAP was a source of catalyst deactivation in their Pd-catalyzed amination of aryl halides. With certain substrates, Carpentier and colleagues saw catalyst deactivation in their Noyori-type Ru(II)-catalyzed asymmetric transfer hydrogenation of carbonyl compounds
3.5. Chelates, Pincers, and Mixed Donor Ligands
A strategy that goes a long way to counter deactivation problems in many cases is moving to chelates and particularly to pincer ligands. Indeed, there is now a vast literature on these tridentate ligands, a factor that provides a rationale for the recent sharp expansion of the field.103−106 Beyond the effect of their double chelation, a number of factors may contribute to pincer stability. The conformational rigidity imposed by the fused ring system, particularly if each is 5-membered as in a PCP ligand, e.g., 22, limits the freedom of the R groups on phosphorus to approach the metal as would be needed to attain the transition states either for cyclometalation or for P−C cleavage. In PCP pincers, the central MC bond is likely to be very stable so even if a wingtip PR2 group should temporarily dissociate, the ligand as a whole is not lost.
I
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confirmation of this idea, 2-picolinic acid caused demetalation while the 3- and 4-picolinic acids were unproblematic, presumably because they cannot deliver the proton in the same way with the pyridyl N bound to Fe. The ligand loss category can be extended to cases in which binding of a catalyst promoter occurs at a distal site in the coordination sphere. Such is the case for a rhenium nitrosyl complex, [ReH(NO)2(PCy3)2], in alkene hydrogenation. A Lewis acid such as B(C6F5)3 can bind to an NO oxygen, inducing isomerization from the 3-electron linear to the 1electron bent forms of this NO, thus generating a 2-electron vacancy at the metal at which substrate can then bind. Deactivation was traced to dissociation of the Lewis acid, followed by return of that NO to the linear binding mode. The appropriate countermeasure in this case was simply to add an excess of the Lewis acid.123 3.7. Designed Degradative Ligand Loss
with i-PrOH as reductant. This they ascribed to displacement of the asymmetric β-amino alcoholate ligand by the substrate. As a result, the reaction rates did not correspond to the intrinsic activity of the catalyst, but rather to the amount of Ru species that remained active.119
Degradative ligand loss is not always deactivating. It can be designed into precatalysts to generate needed open sites for substrate binding. As long as some accompanying spectator ligands remain bound, the catalyst can maintain activity as a homogeneous catalyst. A classic early example was loss of 1,5cyclooctadiene (cod) via hydrogenation to cyclooctane from the Schrock−Osborn124 hydrogenation catalysts of the type [(cod)RhL2]BF4. A number of recent cases125,126 involve Cp* loss from Ir(III) and Rh(III) as a catalyst activating step in transfer hydrogenation and water oxidation. Amatore, Jutand, and colleagues have evidence of phosphine oxidation as a means of reducing Pd(II) precatalysts to the active Pd(0) form.127
3.6. Dissociative Ligand Loss
If spectator ligands undergo undesired dissociation, catalyst deactivation can result. Alternatively, metal particles can form, in which case catalysis may continue, no longer in the intended homogeneous mode, but as a covert heterogeneous catalyst instead.120 In an early case, O2 evolution from aqueous NaOCl was catalyzed by [(H2O)(terpy)Mn(μ-O)2Mn(H2O)(terpy)]+, as reported by Brudvig and colleagues.115,121 Loss of terpy leads to the formation of permanganate, itself inactive in the catalytic reaction. Ligand loss is not usually expected for a pincer ligand such as terpy, but here we have high-spin d5 Mn(II) as part of the cycle, this being one of the more labile metal oxidation states since it lacks any ligand field stabilization. The low solubility of the ligand in the aqueous solvent helped drive further ligand loss by precipitation from the solution. Making a ligand more soluble by incorporating solubilizing substituents may therefore be a helpful counter to deactivation in such cases. Anionic ligands can be useful in oxidation catalysis since they favor the attainment of the high oxidation states often required for activity. Their potential disadvantage is hydrolytic instability if the medium can provide protons to cap the anionic groups
2Cp*IrCl2(PMe3) ⇌ ⎡⎣Cp*IrCl(PMe3)2 ]Cl + 0.5[Cp*IrCl 2]2 (17)
3.8. Ligand Redistribution
Ligand redistribution is a good indication of ligand dissociation. Such is the case for Cp*(PMe3)IrCl2 (28) which is active for the isotopic labeling of a variety of organic compounds, with
D2O being both solvent and deuterium source. Partial deactivation of the catalyst followed from the ligand redistribution of eq 17. Attempts were therefore made to counter this deactivation by covalently linking the Cp group to the P-donor ligand.128 The main deactivation route for the ethylene polymerization10 activity of L2ZrCl2 (29)/[Me2AlO]x (MAO) is somewhat similar, going via transfer of L from Zr to Al, with LAlMe2 being the deactivation product.129 Ligand redistribution in the alkene isomerization catalyst, [Ru(bpy)(solv)4]2+, to give inactive bis- and tris-bpy complexes, led to catalyst deactivation, but on introducing chloride ligands as a way to prevent redistribution, the activity was raised by a factor of ten.130 Oxidation state also plays a role; in the isomerization of 3-buten-2-ol to methyl isobutyl ketone, the
and facilitate their departure as the protonated ligand. Collins and co-workers122 have looked at ways to control this type of hydrolytic instability of the tetra-amido ligand of their TAML oxidation catalysts. This hydrolysis was particularly promoted by phosphate buffer, leading to the mechanistic proposal shown in eq 16, which in turn suggested a strategy for avoiding ligand loss: avoidance of buffers that can bind in this way. Indeed, in J
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4. MULTIMETALLIC PROCESSES Undesired reactions at the metal can divert the catalytic cycle from its intended course. Catalysts usually have labile sites for substrate binding as well as contain potentially bridging ligands such as CO or hydride. This means that the potential very often exists for reactions such as the formation of clusters with M M bonds or multinuclear complexes with bridging ligands.
soft Ru(II) catalyst was inhibited by the isoprene also present in the reaction medium, but a hard Ru(III) catalyst, formed from RuCl3 and phen, resisted inhibition.131 A plausible counterstrategy would be to adopt a chelate or ansa ligand modification to hinder redistribution. 3.9. Other Pathways
As reviewed by Bonnet,7 multiple deactivation paths were seen for the important “blue dimer” water-oxidizing catalyst of Meyer and co-workers,132 [(bpy)2(OH2)Ru−O−Ru(bpy)2(OH2)]4+: oligomerization to give multinuclear complexes; anation, even with apparently innocuous anions such as sulfate; photoisomerization from the active cis to the inactive trans forms; and ligand oxidation, ultimately to form CO2, were all documented. The latter process was studied in a simple model system, [Ru(bpy)3]3+. In aqueous solution, this releases large amounts of CO2 and gives more than 10 different ruthenium(II) complexes, as yet unidentified but believed to contain partially hydroxylated ligands.133
4.1. Formation of Clusters and Multinuclear Complexes
Sometimes the complexes formed in catalyst deactivation were previously known. For example, in the hydroacylation of vinylsilanes catalyzed by [Cp*Co(CH2CHSiMe3)2], one of the deactivation products is the dicobalt complex [Cp*Co(CO)]2, with the CO deriving from the formyl group of the aldehyde coreactant.140 In another case, the known hydridebridged complex, [Cp*Ir(μ-H)3IrCp*]+, was formed in the deactivation of the asymmetric hydrogenation10 catalyst, [Cp*Ir(TsDPEN)]+, after protonation and subsequent loss of the tosylamido TsDPEN ligand.141 Multinuclear complex formation was implicated in the deactivation of a Ru-based water oxidation electrocatalyst. The deactivation was countered in this case by stabilizing the cationic catalyst in a Nafion membrane, presumably as a result of limiting the mobility of the catalyst through ion-pairing in the membrane.142a A similar situation applies to the known [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)]3+ water oxidation catalysts that deactivate by dimerization and by ligand loss. Two ways to counter this problem have been successful: supporting the catalyst either on clay compounds or in the cavities of metal organic frameworks (MOFs), where mobility is limited either by ion-pairing or by confinement in the cage.142b,143 In other cases, clusters discovered in catalyst deactivation studies proved to be new. Early examples144 came from work on hydrogenation catalysts of type [(cod)IrLL′]PF6 (cod = 1,5cyclooctadiene; L and L′ = various N- and P-donor ligands). Once the cod is irreversibly removed by hydrogenation to cyclooctane, the resulting {IrH2LL′}+ unit is stabilized by
3.10. Self-Repair
Although not a homogeneous system and therefore not strictly relevant here, the Co phosphate water-oxidizing catalyst of Nocera and co-workers134 does illustrate the topic of rescue by self-repair, in which a deactivation step is reversed under the reaction conditions. This is a possible strategy for future work in homogeneous catalysis and so merits discussion. Electrolysis at 1.3 V versus NHE of Co2+ salts in pH 7 phosphate buffer results in deposition of a dark green film on an ITO electrode. The deposit, highly active for electrolytic water oxidation at ∼1.3 V versus NHE, was characterized as an amorphous Co oxide. The incorporated phosphate ion may help with proton transfer in the PCET mechanism. The point of special interest here is the fact that loss of Co from the deposit, a process that would normally lead to catalyst deactivation, is reversed by the presence of the phosphate in the buffer, leading to redeposition of the catalytic layer in a self-healing process.135 This redeposition was successfully authenticated with radioactive 57 Co and 32P isotopes. Hill has not only shown how homogeneous polyoxometalate clusters (POMs) can act as oxidatively stable catalysts in oxidation catalysis but also how such systems can undergo selfrepair. This is because, under the right conditions, POMs easily equilibrate in aqueous solution. For example, on heating in the presence of air, 1 equiv of aluminum(III), 1 equiv of vanadium(V), and 11 equiv of tungsten(VI) produce an equilibrating mixture of POMs including the catalytically active cluster [AlVW11O40]6−. This was employed as a self-repairing lignin oxidation catalyst.136 A recent review137 of water oxidation catalysts stresses the self-repairing ability of a number of Ru and Mn cubane cluster catalysts. “Ligand-free”, more properly phosphine-free, Pd Mizoroki− Heck catalysis may deactivate as a result of buildup of Pd(II) salts and of Pd(0) deposits. Shmidt and Smirnov have shown how excess aryl halide substrate and addition of formate as a reducing agent can extend the life of these catalysts. Formate is believed to reduce the Pd(II) salts to the reactive Pd(0) state, and the aryl halide then intercepts the Pd(0) before it has a chance to deposit metal.138 A deactivation product that can be revived by suitable treatments is sometimes called a “dormant state”.139
binding of the substrate alkene. Once the substrate is used up or its concentration falls below a certain value, the only stabilizing agent that remains is the very weakly binding ligand solvent, CH2Cl2. The result is cluster formation, leading to irreversible catalyst deactivation to give 30 (L = PCy3, L′ = pyridine); similar reactions are also known for other choices of L and L′,145,146 including a recent case of tetranuclear 31, where L = PCy3 and L′ = tetrahydroquinoline.147 In the deactivation of a series of bis-NHC iridium complexes active for glycerol conversion to lactic acid, a hexanuclear cluster, [Ir6(IMe)8(CO)2(H)14][BAr4F]2, was identified as one product.81 The Schrock−Osborn catalysts [(nbd)Rh(PPh3)2]+ (nbd = norbornadiene) give rather similar deactivation products such as [{(PPh3)2RhH}2(μ-H)(μ-Cl)2]+, the μ-Cl groups coming from the CH2Cl2 solvent.148 An anion dependence on the K
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catalytic cycle for Pd than for other metals; loss of one L is thus enough to release Pd(0). The Wacker process is striking in being developed from a stoichiometric reaction in which Pd deposition was successfully countered by simple measures. It had been known for decades that [PdCl4]2− ion stoichiometrically oxidizes ethylene to acetaldehyde, but since an irreversible deposition of the Pd(0) reduction product always followed, no turnover beyond the first was ever possible until Smidt at Wacker Chemie came to study the problem. By adding Cu(II), he was able to intercept the Pd(0) and reoxidize it to the active Pd(II) form before the Pd(0) had a chance to agglomerate and thus deposit the metal. The resulting Cu(I) reduction product was then readily reoxidized by air to regenerate the Cu(II) ion, leading to robust catalytic turnover.155,156 All components being mutually compatible, the reaction can be run commercially in a one-pot procedure. The organic synthetic version of this reaction, the Wacker−Tsuji oxidation, converts a variety of RCHCH2 derivatives to RCOCH3; other more elaborate organic applications of the process are now also available.157 A number of other countermeasures have been suggested to suppress deposition of metallic Pd. In the Mizoroki−Heck arylation, addition of excess P-donor can be useful.12 In methyl acrylate arylation by PhI, supporting the catalyst on polystyrene functionalized with PPh2 groups delayed deactivation, perhaps by interfering with the Pd(0) agglomeration step.158 In Pd(OAc)2-catalyzed ethylene methoxycarbonylation, Brønsted acid ionic liquids such as [Et3N(CH2)4SO3H]OTs, acting also as acidic promoters, had the effect of avoiding the deposition of Pd black that otherwise occurred in conventional ionic liquids.159 Imidazolium ionic liquids were also useful to prevent metal deposition in NiI2(NHC)2-catalyzed olefin dimerization.160 In the air-oxidation of alcohols catalyzed by Pd(OAc)2, increasing p(O2) and lowering catalyst concentration both help suppress Pd deposition, but ligand effects have also been seen. For example, the Pd(OAc)2 system is more stable with 3(2,3,4,5-tetraphenylphenyl)pyridine as ligand versus other smaller pyridine derivatives, presumably for steric reasons, but the steric effect is unusual in operating somewhat remotely from the metal, leaving a pocket near the metal for reaction to occur.161 Ligands that incorporate such remote steric bulk may prove generally useful for combating deactivation by preventing deleterious bimolecular processes while still permitting the approach of small molecules that enter an active site pocket close to the metal. Other countermeasures against Pd deposition are reviewed by Vries and colleagues.162 As discussed by Beletskaya,163 homeopathic amounts of Pd in the form of 0.01−0.05% loading of palladium acetate, without any added phosphine, seem to maximize productive C−C coupling while delaying Pd deposition, with TOFs of up to 30 000 h−1. On completion of the reaction, the Pd can precipitate, but recycling may then be possible with I2 as reoxidant. A useful additive for stabilizing homeopathic Pd is DABCO, in which case even 0.0001% palladium acetate is effective for the favorable case of ArI/ Ar’B(OH)2 coupling, where TOFs can approach 106 per hour. Another such stabilizer is t-butylammonium bromide.164 Both stabilizers may act to prevent aggregation of Pd nanoparticles, the true catalysts. This area is related to the problem of high turnover Pd cross coupling catalysis, reviewed in detail by Farina.165 High turnover is usually taken to refer to reactions with TONs of 103−106 that contrast with the TONs of ∼100
deactivation rate was noted, with the minimally coordinating BArF4− anion providing longer life in some cases.149,150 Countering this deactivation became a major aspect in developing the system to make it suitable for commercial application at the >10 000 tons/year scale required for the production of (S)-metolachlor, an agrochemical.3 A move from Rh to Ir was the first step in the process. Deactivation was then
severely repressed with L2 = xyliphos (32, R = Ph, R′ = 3,5dimethylphenyl), but performance was further improved when acetic acid and NBu4I were identified by empirical methods as additives that enhance the catalyst rate and lifetime. Unlike the situation in academic work, the best process from a commercial viewpoint was not the one with the highest ee, because changes that improved the ee tended to reduce the more important factors of TON and TOF. The current process, the largest scale asymmetric process in commercial operation, uses conditions that give a modest ee of 79−80% but with an exceptionally high TON of 106 and TOF of more than 200 000 h−1. The transition from cluster formation to metal deposition has been followed by combined UV−vis ED-XAFS studies in the case of allylic amination catalyzed by [Pd(1,1-dimethylallyl)(P−P)]OTf (P−P = dpe or xantphos), where successively higher nuclearity [(1,1-dimethylallyl){(P−P)Pd}n] clusters (n = 2, 3...) were found with the passage of time.151 In aqueous solution, pH change can bring about cluster buildup, as in the case of a Mn-based water oxidation catalyst that rearranges from a dinuclear to a trinuclear state with acid;152 under basic conditions, on the other hand, MnO2 can deposit. The NMR technique, parahydrogen induced polarization (PHIP), can detect species present at low concentration if the reaction to form them proceeds in a spin correlated manner starting from a sample of hydrogen enriched in parahydrogen by cooling in the presence of a catalyst such as Fe2O3. In the case of the hydrogenation catalyst, [RhCl(PPh3)2]2, PHIP enabled detection of deactivation products such as [RhClH(PPh3)2(μ-Cl)(μ-H)RhClH(PPh3)2] that are normally hard to detect because they are only present in a small quantity.153 In other cases where deactivation is associated with bimolecular processes, supporting the catalyst on a polymer can prolong its life by up to 100 times, as in the case for hydrogenation by CpTiCl3 supported on styrene-divinylbenzene copolymer.154 4.2. Metal Deposition
A catalytic solution can form nanoparticles or deposit the metal; these may be active or inactive depending on the case: the high surface area/volume ratio nanoparticles initially formed may be very active, but when Pd black has formed, the area/volume ratio is no longer favorable and deactivation can occur. A widely recognized problem that has received much attention,12 metal deposition is particularly acute for Pd, an element that is very commonly encountered in homogeneous catalysis. The reason Pd is so prominent may be that low coordinate LM(0) intermediates are more common in the L
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4.4. Other Cases
that are usually seen. For a number of catalysts that have shown ultrahigh TOFs and TONs, gradual release of Pd at very low concentration has been proposed as the key parameter, and
Locking the metal into one oxidation state can lead to catalyst deactivation where the cycle involves two or more oxidation states. For example, a Pd(II)(xantphos) complex catalyzes azidocarbonylation of iodoarenes with CO and NaN3, to give aroyl azides by a Pd(0)/Pd(II) Heck-like mechanism. MacGregor, Grushin, and co-workers have shown that the catalyst deactivation product, [(xantphos)PdI2], cannot cycle on its own, but can be reduced in situ with the polymeric silane, PMHS, to form an active Pd(0) state, resulting in reactivation of the catalyst.173 In a similar vein, vanadium-based catalysts for the Ziegler−Natta polymerization of α-olefins deactivate by reduction to lower valent V(II) species, a topic reviewed by van Koten and colleagues. Oxidation of the V(II) deactivation product to V(III) can reactivate the catalyst, leading to the adoption of a number of additives. Hexachlorocyclopentadiene was one of the first additives to be employed in this way, but trichloroacetic acid is now preferred.174a Attracting much attention recently, catalysis by Au(I) can suffer from low turnovers. In one case disproportionation into Au(0) and Au(III) was identified as the deactivation pathway.174b
similar results have been obtained simply by using Pd(OAc)2 at very low concentration. In competition experiments between different substrates, the same Heck selectivity patterns are obtained whatever the starting catalyst, suggesting that all are simply releasing “ligand-free” Pd into solution.166 Some of the best catalyst types currently known, however, involve PdL complexes or their precursors, where L is a bulky, strong-donor phosphine, such as X-Phos (33). The bulky L keeps the Pd:L
5. POISONS A catalyst poison is an impurity, generally present in the reactants or solvent or produced as a byproduct during the reaction that deactivates the catalyst, often by binding to the active site or altering the catalyst structure in some way. In contrast, the term inhibition tends to be applied when the additive causes partial reduction in the rate or selectivity of the reaction. The substrates or products may themselves act as poisons or inhibitors, but those cases are considered under substrate inhibition and product inhibition (sections 7 and 8).
ratio at 1:1, the donor power favors the oxidative addition, and the reductive elimination is favored by the 3-coordinate character of the key LPd(A)(B) unit, with A and B being the coupling partners.167 In direct opposition to the ligand-free approach, Doucet, Santinelli, and co-workers168 took the view that the Pd should be maximally coordinated to avoid deposition. Their tetraphosphine 34 indeed showed remarkable performance in the amination of allyl acetates, with a TON of 680 000 and a TOF of 8125 h−1. Amination could even be successfully performed with as little as 0.0001% catalyst loading. Mechanistic studies suggested that the best 34/Pd ratio is 1:1. In a detailed FT-NIR spectroscopic study of Mizoroki−Heck catalysis with a Pd phosphoramidite complex, a number of deactivation processes, including metal deposition, were modeled as a first order process.169 Platinum is also subject to similar deactivation pathways. Poli and co-workers looked at the deactivation process in the PtBr2/ Br− ethylene hydroamination catalyst; with basic amines this involves deposition of Pt(0).170 The lifetime of the Pd(phen)Cl2 catalysts for CO/vinyl arene copolymerization to give polyketones was greatly increased by running the reaction in CF3CH2OH, a solvent with strong proton-donor H-bonding capability that prevents Pd deposition,171 perhaps by enhancing the rates of the productive steps in the catalysis.
5.1. Unintended Poisoning
Low valent catalysts are most likely to be poisoned by impurities such as CN− that can act as soft ligands, or by oxidants such as trace peroxide in substrate or solvent; high valent catalysts, in contrast, tend to be more affected by hard ligands such as F− or by hydrolytic reaction with trace water. Electropositive early metals, having more ionic ML bonds, are likely to be more sensitive to hydrolysis by trace water than late metals; as an extreme example of hydrolytic resistance, MeHgOSO3H is even stable in concentrated sulfuric acid at 180 °C.175 Farina has pointed out that TON values in Pd cross coupling can rise with the purity of the substrate, presumably because the trace impurities in the substrate can poison the catalyst.165 Excess chloride can poison Suzuki reactions, particularly in ionic liquids (ILs); alcohol cosolvents mitigate the problem, presumably by solvating the halide, but Dyson and co-workers showed that an N-hydroxyethyl substituted IL gives a poisonresistant Suzuki catalyst system without the need for a cosolvent.176 Air is a classic poison for organometallic catalysts,177 but in large scale work it is hard to eliminate entirely. In an unusual counterstrategy, Wang and co-workers found that i-PrOH addition greatly enhanced the air-stability of a Pd(OAc)2-based catalyst for the cyanation of iodoarenes.178 The mechanism of protection involves reduction by the i-PrOH additive of the inactive Pd(II) state, formed on air oxidation, to the active Pd(0) state. Admission of air can deactivate a catalyst if any of the intermediates in the catalytic cycle are air-sensitive. For
4.3. Metal Reduction and Oxidation
In the tetramerization of ethylene to octene catalyzed by a Cr(II) complex of a bidentate diphosphinoamine ligand with a modified MAO species as promoter, undesired reduction to Cr(I) brings the catalysis to a halt, as shown by combined EPR and XAS studies under operating conditions.172a In contrast, in Pd-catalyzed aromatic azidocarbonylation of iodoarenes, the Pd0L2 active form is oxidized to a I2PdIIL2 deactivation product which could be successfully rescued by reduction with Zn dust or with the cheap hydrosilane, polymethylhydrosiloxane.172b
M
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having to run the catalytic step in the presence of the inhibiting functionality. 6.1. Noyori-type Hydrogenation
Substrates commonly bind to the catalyst in order to undergo the catalytic reaction, but this is not always the case. For example, Noyori-type catalysts deliver H+ and H− to the substrate double bond, typically a polar one such as CO or CN, but the substrate itself never binds but always remains in the outer sphere. In a recent series of N-heteroarene hydrogenation catalysts, a problem with relevance to energy storage,194 the Ir-based catalysts bind H2 in the key first step. The Ir−H2 complex then delivers H+ to the N atom of the heterocyclic substrate, typically acridine or quinaldine, followed by delivery of H− to the protonated heterocycle to complete the outer sphere hydrogenation. In such a case, direct substrate binding to Ir is unhelpful because, in that case, H2 is excluded from the binding site and the catalyst is inhibited. Quinoline itself therefore proved to be an unsatisfactory substrate because it is unhindered enough to bind directly to the site required for H2 binding; bulky quinaldine and acridine were satisfactory substrates because they could not bind directly to Ir and thus were easily reduced by the outer sphere (H+ + H+) transfer mechanism. This reverses the usual situation in inner sphere catalysis in which sterically bulky substrates are less reactive substrates than slim ones.195 In styrene hydrogenation by a related [(cod)Ir(PPh3)2]BF4 catalyst, the phenyl group adjacent to the substrate CC bond undergoing reduction ended up irreversibly π-bound to the metal in the [(η6-PhEt)Ir(PPh3)2]BF4 deactivation product.195 The analogous Rh catalysts do not suffer from this problem, perhaps because the η6-PhEt complex is labile in this case.
example, air interferes with the Rh(I)/(III) cycle of the Wilkinson hydrogenation catalyst, the Rh(I) state being particularly vulnerable. In contrast, the [(cod)IrLL′]PF6 catalysts (L = P-donor; L′ = N-donor) still operate in the presence of air, probably because the Ir(III)/(V) cycle they adopt179,180 avoids Ir(I). Unlike the Rh catalysts, the Ir catalysts also tolerate oxidizing functional groups and solvents (e.g., reactive halides or CH2Cl2). Lactic acid is a poison in lactide polymerization catalyzed by LZr(OEt)2 (L = 35, Ar = 2,6-dimethylphenyl). Partial repair is achieved by addition of up to 1 equiv of water or of ptoluenesulfonic acid, but the reasons are yet to be clarified.181 5.2. Deliberate Poisoning
Deliberate addition of selective poisons is often seen in attempts to distinguish authentic homogeneous catalysis from adventitious heterogeneous catalysis by metal deposits or nanoparticles.120,182 For example metallic Hg is often considered a selective poison for heterogeneous catalysis, as illustrated here by some recent examples.183−187 Ideally, the mercury test is just one step in a thorough kinetic analysis of the catalyst to distinguish heterogeneous from homogeneous catalysis.188 Poisons can also help enhance catalyst selectivity. A suitable selective poison can occasionally be found that suppresses an undesired catalyst activity. Such is the case for hydrosilylation catalyzed by [LRu3(CO)7] (L = acenaphthylene). The unmodified catalyst is active for ketones, esters, and amides, but addition of amines such as NEt3 suppresses activity for ketones and esters but does not affect the activity for amides, normally the least reactive substrate of the three.189 In the case of a complex organic molecule, this finding suggests the possibility that the activity and selectivity of reaction at one functional group might be greatly altered by the presence of a remote functional group that does not itself undergo any change during the reaction and thus would not normally be suspected of affecting the outcome. As detailed in a review by Faller and co-workers,190 a homochiral additive can be deliberately added to a racemic catalyst with the ideal aim of poisoning only one enantiomer of the catalyst so as to obtain asymmetric induction in the subsequent catalytic reaction. Rational design protocols have been suggested to facilitate choice of the right poison/catalyst combination.191−193 In particular, the effectiveness of any given catalyst−poison combination depends on the thermodynamics of binding, something that is easier to predict than the relative heights of diastereomeric transition states in classical asymmetric catalysis.190 Poisoning by substrate or products of a catalytic reaction are dealt with in the following sections.
6.2. C−H Activation and Related Reactions
Heteroarenes such as benzimidazole also gave undesired competitive binding to the catalyst active site in the [Ir(cod)(OMe)]2/4,4′-di-t-butyl bipyridine-catalyzed C−H borylation of heteroarenes in an extension of the prior borylation of simple arenes to the case of heteroarenes as recently reported by Larsen and Hartwig.196 The silane σ complex, [CpCO(PPh3)Fe(HSiEt3)]+, acts as a silane alcoholysis catalyst, but the alcohol slowly deactivates the catalyst by displacing H2 from the intermediate dihydrogen complex, [CpCO(PPh3)Fe(H2)]+ to form [CpCO(PPh3)Fe(HOEt)]+. The σ H2 complex is also extremely acidic, and a second mode of deactivation proved to be proton transfer to EtOH to give inactive [CpCO(PPh3)FeH] and [EtOH2]+.197 6.3. Palladium Coupling Chemistry
In a Pd cross coupling study by Fleckenstein and Plenio,198 the same type of inhibiting effect caused by N-heterocycle binding to Pd was seen, but in this case an effective countermeasure proved to be introduction of water into the solvent mixture. The proposed explanation involved hydrogen bonding to the N lone pair making it less available for binding to Pd, although a contributing factor may well have been the very bulky chelate ligands employed in this work. Thiols also bind strongly to low valent metals, yet Pd cross coupling involving a thiol cosubstrate is a valuable reaction. Hartwig found that chelation is a useful countermeasure against this type of deactivation and identified Josiphos, 32 (R = R′ = C6H11), as particularly useful; this catalyst even gave satisfactory results at a loading of just 0.01%.199
6. INHIBITION BY SUBSTRATE OR COREACTANT Substrates with strongly coordinating substituents may interfere with the catalysis. It is sometimes possible to redesign the synthesis or protect the problematic functionality to avoid N
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transfer of a propene methyl proton to the Zr−Me group with loss of methane.204 A similar η3-allyl was seen as deactivation product in a Ti-based, Ziegler−Natta propene polymerization system, where reactivation proved possible by treatment either with AlEt3 or H2.205 A series of Ni(II) precatalysts for ethylene dimerization were shown to undergo bimolecular reductive elimination of ethane as the major deactivation pathway, rather than the hydrolysis of
Coreagents required for the catalytic transformation can pose serious problems too, and in this case they are hard to counter. Cyanation of haloarenes catalyzed by Pd(PPh3)4 is a useful cross coupling reaction. One deactivation path proves to be formation of polycyano palladium complexes such as [Pd(CN)4]2−, where loss of the phosphine ligands leads to deactivation. Water also proved to be particularly deleterious because the resulting hydrolysis of cyanide ion produced HCN, a troublesome competitor with the haloarene for oxidative addition to the (PPh3)2Pd(0) intermediate implicated in the cycle.200 Reactive intermediates are necessarily present in catalytic cycles, and for long catalyst life, the rates of the productive pathways must greatly outcompete the deactivation step(s). Reductive elimination is a common last step in a variety of cycles such as Pd coupling, and thus, the complex must be predisposed to favor this reaction. It is therefore understandable that alternate modes of reductive elimination with undesired partners are common modes of deactivation. For example, Yates, Cavell, and co-workers called early attention to the relatively easy reductive elimination of an NHC with an adjacent methyl group in Pd(II) complexes such as [(cod)Pd(NHC)Me]+ (NHC = IMe), the products being the 2-methylimidazolium ion and Pd(0).80 Likewise, Ni(NHC)2 (NHC = IiPr) is active for homocoupling of ArBr to give Ar2. An intermediate NiBr2(NHC)2 was found to react with ArBr to give 2-aryl-imidazolium ion, presumably by a similar reductive elimination.201
the Ni−C bond that was expected.206 Pincer complexes are not exempt from undesired substrate reactions. For example, a PCP pincer nickel hydride, 36, reacts with ethylene to substitute the central NHC unit with an ethyl group at the 2-position (eq 18) for which the authors suggest an insertion/reductive elimination sequence.207 The CpCo(PPh3)-catalyzed cyclotrimerization of alkynes goes via an oxidative coupling to give intermediate 37. The formation of the deactivation product, 38, the kinetics and ́ mechanism of which have been studied by Martinez and coworkers,208 involves reductive elimination, followed by binding of the cyclobutadiene product to the cobalt. The deactivation rate proved to be very dependent on the solvent, emphasizing the importance of solvent variation to counter deactivation.
6.4. Alkene Metathesis.10
6.6. Other Cases
Grubbs metathesis catalyst RuCl2(PCy3)2(CHPh) is rapidly deactivated by substrates containing −NH2 groups that typically cause displacement of both PCy3 ligands. The countermeasure proved to be a move to the second generation RuCl2(IMes)(PCy3)(CHPh) version, where the single remaining PCy3 ligand is still displaced but the NHC remains firmly bound and the resulting complex retains catalytic activity.202 In the ring-closing enyne metathesis of certain sterically unhindered substrates, the Grubbs metathesis catalysts tend to deactivate, but this problem is avoided by introduction of ethylene into the headspace, because ethylene outcompetes the deleterious binding of a second alkyne group by the catalyst.203
Byproducts formed from the substrate can be effective poisons: in nitrile hydration with [Ru(η6-p-cymene)Cl2{PR2(OH)}], free cyanide ion proved to be a poison in the case of cyanhydrin precursors, which spontaneously liberate CN− . Simple protection of the cyanhydrin OH group prevented CN− release.209 The same authors also showed how moving from [Ru(η6-arene)Cl2(PAr3)] to [Ru(η6-arene)Cl2(P{NMe2}3)]
6.5. Alkene and Alkyne Oligomerization and Polymerization
Polymerization catalysts must cycle particularly efficiently to produce high molecular weight polymer from an alkene such as propene. The [Cp2ZrMe]+ class of catalysts has been subjected to close scrutiny in this respect in view of their commercial importance. This has led to identification of one pathway to give [Cp2Zr(η3-allyl)]+ as deactivation product after undesired
improved both cyanide-resistance and thus also catalyst efficiency.210 Bonnet7 points out that the strong primary oxidant needed in water oxidation catalysis is typically present in high concentration and thus poses a risk that the catalyst will be oxidatively destroyed. This is the reason that ligands chosen for oxidation catalysis typically incorporate structural units designed to resist oxidation. A recent example is the pyCMe2OH (39) chosen for the Cp*Ir series of water-oxidizing catalysts, where the CMe2 group protects the otherwise sensitive benzylic position and the OH deprotonates to give a strongly donating alkoxide group to stabilize the higher oxidation states that are expected to be needed in the catalytic cycle. The Cp* unit, however, is unstable under the conditions and is oxidatively removed from the metal.127,211 Combining enzymes with organometallic catalysis can lead to mutual interference. For example in the hydroxylation of 2O
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another case, when Shvo’s catalyst was applied to ammonia− borane dehydrogenation, the borazine product coupled with the OH substituent on the Cp group via a Cp−O−B bridge (eq 20). Since this OH plays a key role in catalysis, its loss led to deactivation.215 Finally, [CpRu(NCMe)3]PF6 is an alkyne trimerization catalyst that deactivates to form [CpRu(η6arene)]PF6, for which a nitrile-loss mechanism was suggested from DFT calculations.216 In the asymmetric hydrogenation of enamine amides and esters by [(cod)RhCl]2/Josiphos, product inhibition was noted by a team at Merck, but was successfully countered by protecting the amino group of the substrate with a Boc group by in situ addition of (Boc)2O. The improved activity caused by lifting the inhibition in this way meant that the high pressures of hydrogen originally needed were no longer required, considerably simplifying the procedure.217 In the catalytic conversion of RCH2NH2 to RCN with release of H2, catalyzed by a Ru(II) pincer complex, the product nitrile was shown to inhibit the catalytic reaction by binding to the metal and so hindering binding of the substrate amine.218 In the intramolecular Ti(IV)-catalyzed Schmidt reaction of alkyl azides and ketones to form lactams, the product is necessarily a better ligand than the reactants, and product inhibition is the result. The countermeasure involved the use of hexafluoro-2-propanol, a strong H-bond donor that was shown
hydroxybiphenyl to the corresponding catechol with 2hydroxybiphenyl 3-monooxygenase, an NADH biomimic was required and had to be rereduced after each catalytic cycle. This was accomplished with formate as reductant and [Cp*Rh(bpy)H]+ as catalyst, but deactivation occurred by undesired binding of the enzyme to the Rh. The appropriate countermeasure proved to be derivatization of the peripheral −SH and −NH2 groups of the enzyme with an epoxide-containing polymer.212
7. PRODUCT INHIBITION 7.1. Attainment of Equilibrium
In the most benign form of product inhibition, a reaction can come to a halt when equilibrium is attained, with the catalyst retaining full activity, as in the Haber−Bosch process (eq 19). This can give the appearance that deactivation has occurred, hence its relevance here. This situation can be identified by showing that the same equilibrium mixture is formed whether we start from the product or the reactant side of the equation; thus, the Haber catalyst also decomposes ammonia. To increase the yield of the reaction, the product must be removed from the system in some way. For example, in the Haber process, the equilibrium is perturbed by selective removal of ammonia from the cooled exit gases as liquid NH3 while maintaining the high reaction pressure of ∼200 atm. N2 + 3H 2 = 2NH3 (19) In a homogeneous example, Waddell and Goldman213a find their pincer iridium alkane dehydrogenation catalysts108 bring numerous cycloalkane/cycloalkene + H2 mixtures to equilibrium. Alkane dehydrogenation can also be assisted by subsequent alkene metathesis and rehydrogenation removing the product.213b Heteroarenes such as 1,2,4-triazoles cannot be hydrogenated at or above ambient temperature, not because the N lone pairs bind to and poison the catalyst, as once thought, but because the arene/perhydroarene equilibrium very greatly favors the arene at room temperature and above.194
to bind to the lactam carbonyl and thus minimize inhibitory binding to the catalyst.219 One of the most important practical uses of homogeneous catalysis is Pd cross coupling as applied to pharmaceuticals. Since N-heterocyclic units are often present in the relevant substrates and products, about one-third of the time according to a recent estimate,162 and their N lone pairs are capable of unproductive binding to the Pd, substrate and product inhibition are common. Of course, this binding is reversible, but a high concentration of substrate is typically present, so a substantial fraction of the catalyst can be tied up in an unproductive form. A useful countermeasure proved to be adoption of the bulky phosphine ligand, S-Phos (40), that is presumably effective220a for steric reasons. Ionic liquid solvents have also proved effective in minimizing product inhibition in the case of [Pd(PBu3)2(solv)2]2+ catalysts active for the dimerization of methyl acrylate.220b
7.2. Kinetic Inhibition
7.3. pH Drift
Two cases of inhibition are most commonly seen. In the first, the key intermediate in the cycle usually goes on to the desired product but occasionally misbehaves with the irreversible formation of a deactivation product. Such is the case for the CpCo(PPh3)-based alkyne trimerization catalysts discussed in section 6.5. An unrelated deactivation route in the same complex, reversible this time, involves reassociation of the PPh3 originally lost from the precatalyst.208 Alternatively, a product of the catalytic reaction may react with the catalyst in an unproductive way. For example, in an ester hydrogenation catalyst, the product alcohol competitively binds to the metal, slowing the productive catalysis.214 In
Many catalytic reactions give protons or hydroxide ions as reaction products: water oxidation and reduction fall into these categories, for example. If a reaction is sensitive to pH, either for thermodynamic or kinetic reasons, inhibition can arise from the pH drift consequent to the production of protons or hydroxide ions as the reaction proceeds. Of course buffers may be able to moderate the pH drift, but some reactions only proceed in strong acid or strong base. In such a case production of any compound that buffers the acid or base by solvent leveling could halt the reaction. For example, Periana and coworkers developed a series of methane partial oxidation catalysts that operate in oleum. One role of this strong acid P
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is protonating potential competitor ligands such as the MeOH product of the partial oxidation; by conversion to MeOH2+, it is protected against overoxidation, a path that would ultimately lead to CO2 in a common failure mode of most intended partial methane oxidation reactions. As the MeOH builds up, however, the acidity of the medium falls, and the conversion halts, albeit at a substantial ∼1 M MeOH concentration.221
8. UNCONVENTIONAL METHODS A number of unconventional methods have recently been successfully applied to catalysis, some of which may be effective by delaying deactivation.
9. SOLVENT INHIBITION Solvent choice needs to be carefully considered. Strongly binding solvents, e.g., MeCN, may deactivate the catalyst by blocking active sites or may prolong catalyst life by stabilizing either the catalyst resting state or transient intermediates in the cycle. Alternatively, if the key transition states for productive catalysis versus deactivation differ in dipole moment, change of solvent polarity could in principle also alter their rate ratio although no one seems to have yet examined this point.
8.1. Microwave Heating
Microwave (MW) heating deserves a special mention because it can dramatically shorten reaction times, sometimes even from tens of hours to minutes. Accelerations of up to 200-fold for microwave over thermal conditions have been seen in Heck arylation of methyl acrylate, for example.222 The reasons for the acceleration have been the subject of debate, notably over the question of whether there is a special microwave effect beyond simple heating.223 Yamada, Dudley, and their co-workers have obtained apparently nonthermal accelerations in a number of reactions.224 Yamada looked at asymmetric catalytic Agcatalyzed Claisen rearrangement, where acceleration takes place without loss of enantioselectivity. If the acceleration were to be ascribed purely to an excess temperature buildup at hot spots or other purely thermal explanation, the ee might be expected to fall, but it does not. Dudley looked at a Friedel− Crafts reaction where the solvent was transparent to MWs but the ionic reactant absorbed MW. The acceleration versus a purely thermal control reaction was thought to be the result of nonequilibrium effects by which the reactant was specifically excited. An improvement in the rate ratio of productive catalysis to deactivation under microwave conditions is thought to be one contributing factor to the improved performance, as in the most recent results by Holland and co-workers,225 who saw microwave acceleration in Buchwald−Hartwig coupling. These authors favor the operation of pure thermal effects over any special microwave effect, however. One aspect of the Holland study points the way to a possible strategy for gaining much of the benefit of microwave heating in a purely thermal process by preheating the reactants to reaction temperature before injecting the catalyst to start the reaction. Since microwave work is not easily adapted to commercial production, if this preheating procedure proves general, it could provide a broadly useful procedure, even for large scale application, but more work is clearly required. Whatever the mechanism, the faster reaction times are of great practical value and should encourage wider use of microwave heating in homogeneous catalysis.
9.1. Solvation and Solvent Binding
Solvent selection227 can also influence the rate of deactivation. For example, the cyclometalation of eq 21 took place with CD3CN but not with CD2Cl2 or DMSO.67 An unusual feature of this reaction is that coordinated cod acts as an internal H acceptor. Solvents can also directly intervene in the catalytic cycle. For example, a Pd(II)-diimine Brookhart catalyst for alkene polymerization was deactivated in aqueous emulsions by the water present, which gave an undesired Wacker-like nucleophilic attack at coordinated ethylene, followed by formation of acetaldehyde and deposition of Pd(0).228 Likewise, Steinhoff and Stahl229 found Pd(0) deposition deactivated the Pd(OAc)2/DMSO-catalyzed aerobic alcohol oxidation. Improved results were obtained by Sheldon and co-workers at 30 atm O2 pressure, suggesting that the O2 is capable of intercepting and reoxidizing the Pd(0) to Pd(II) before deposition of metal occurs, thus delaying deactivation.230 9.2. Reactive Solvents
Albeniz and co-workers have looked at the effect of a series of solvents on a model Pd(II) complex at 100 °C. [Pd2(μBr)2Br2(C6F5)2]2− was chosen because of its relation to known intermediates in catalysis. In a number of cases, an H atom was transferred from the solvent to the fluoroaryl group to give C6F5H, presumably formed by reductive elimination of an intermediate hydride complex. Toluene, thf, water, and MeCN were all classified as safe solvents in this respect, but DMF, NMP, DMA, 1,4-dioxane, and DME were shown to give H transfer.231 This risk was much enhanced for the “ligandless catalysts”, where the solvent necessarily has an even greater involvement with the catalyst. The role of temperature was also investigated with ≤50 °C being considered “safe” while more elevated temperatures of 80 and 100 °C were considered progressively more risky, at least in the model system chosen. Grubbs classical metathesis catalyst, RuCl2(PCy3)2( CHPh), and its variant, RuCl2(IPr)(PCy3)(CHPh), if employed in reducing alcohols under basic conditions, can give rise to hydride complexes active for alkene isomerization, an undesired reaction for most metathesis reactions. Work by Banti and Mol suggests that the product of the reaction of EtOH with the second of these two catalysts is RuHCl(IPr)(PCy3)(CO). Although this is not deactivation in the sense of
8.2. Other Methods
Imidazolium-based ionic liquids are useful solvents that prolong the life of Pd coupling catalysts with conventional heating, perhaps via formation of imidazol-2-ylidene complexes.12 More exotic procedures of mechanistic interest, but that are not readily to hand in a standard synthetic lab, include high pressure work. For example, high pressure (8 kbar) greatly enhances yields and lifetimes in Pd-catalyzed arylation of 2,3dihydrofuran with 1:2 Pd(OAc)2/PPh3.226
Q
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standard procedure in homogeneous, solution-based electrocatalysis. It is always necessary to check that the catalysis indeed arises from the solution and not from a deposit that has formed on the electrode. The simplest way to check is to take the electrode out of the solution, rinse it, and introduce it into a cell identical in all respects to the first except that it lacks the soluble catalyst. If the reaction still proceeds, deposition of a catalytic layer on the electrode is strongly indicated, although in the Fox system, this test proved negative. It is not unusual for a catalytic deposit to form and act as the true catalyst, however, as shown by a recent example involving water oxidation by [Cp*Ir(OH2)3]SO4 as precatalyst. The vigorous electrocatalytic water oxidation observed in this case proved to be predominantly due to a deposit of IrO x nanoclusters, as shown by the same test mentioned in connection with the Fox system, as well as by electrochemical quartz crystal nanobalance data. In this technique a quartz crystal nanobalance is fitted with a Au electrode and the mass buildup on the electrode followed with time. An (IrO2)5 core based on the rutile structure of bulk IrO2 was proposed for the nanoclusters in the amorphous deposit from the pair distribution function obtained by X-ray spectroscopy.241 This is a deceptive situation in the sense that we have simultaneous deactivation of the homogeneous form but activation of the heterogeneous form of the catalyst. In this case, introduction of a chelate ligand into the complex proved sufficient to stop the deposition without suppressing catalysis. One of the best such precatalysts, truly homogeneous this time, proved to be [Cp*IrCl(OCMe2-2-pyridyl)].242 The organometallic ligand is lost on activation of the precatalyst, but the N,O-donor chelate ligand (39) is retained in the resting state of the water-oxidizing catalyst, as shown by extensive physicochemical data, including O-17 NMR.211 Metal deposition can also lead to catalyst deactivation. Such is the case for a series of Co complexes of pentadentate polypyridyl ligands that are electrocatalysts for proton reduction, where dynamic light scattering showed evidence of nanoparticle formation concomitant with the deactivation.243 The deactivation pathway of the Kubiak precatalyst, Re(bpy)(CO)3Cl, best known for the electrocatalytic reduction of CO2 to CO, has been closely investigated by spectroelectrochemistry, a particularly incisive technique in this area. Formation of Re−Re bonded dimer, [Re(bpy)(CO)3]2, was implicated, but successfully countered by introducing bulky, strongly donor tBu substituents at the 4 and 4′ positions of the bpy.244 A water-soluble Ir PCP pincer catalyst for CO2 reduction to formate, developed by Meyer, Brookhart, and co-worker, required the presence of MeCN to counter deactivation. Acting as a ligand, it shifted the reduction potential of the catalyst so as to avoid undesired side reactions, and the binding also facilitated dissociation of the formate product from the metal.245 Demetalation and ring reduction were the main routes for deactivation in the electrocatalytic reduction of NO by watersoluble manganese porphyrins reported by Yu and Su.246
forming a complex that is catalytically inactive, it is a deactivation as far as metathesis activity is concerned.232 Water is also deleterious, cleaving the benzylidene to give benzaldehyde.233 Decarbonylation of alcoholic solvents give [RhCl(CO)L2] as deactivation products from the [Rh(cod)Cl]2/phosphine series of hydrogenation catalysts.234 In early work on solvent scope in Wilkinson’s catalyst, acetonitrile was shown to poison the system and certain alcohols led to a deactivation ascribed to CO abstraction from the solvent;235 this can occur by dehydrogenation to the aldehyde, RCHO, that can transfer CO to the metal with release of RH.236 Early metals can be much more solvent sensitive; for example, the [W(OAr)2Cl4]/SnR4 olefin metathesis system is deactivated by HOAc, BuOH, and H2O. A great merit of the Ru-based metathesis catalysts is their much greater resistance to polar groups and most coordinating solvents.237 An otherwise useful solvent, dichloromethane, can give undesired oxidation of organometallic catalysts. For example, in a study of the outer sphere hydrogenation of heteroarenes by [IrH2(PPh3)2(NHC)] (NHC = N,N′-dimethylbenzimidazole2-ylidene), this complex was found to react slowly with CD2Cl2 over several days to form [(NHC)(PPh3)2IrHCl]PF6, possibly accounting for the poor catalytic conversions seen in that solvent.195 The solvent scope of this catalyst is very limited; among all the solvents tried, only toluene and PhCF3 proved suitable. This may be because the H2 ligand in the intermediate dihydrogen complex is easily displaced even in solvents of weak coordinating power. 9.3. Solvent Impurities
Peroxides sometimes present in ethereal solvents can cause problems by oxidizing organometallic catalysts. Of course, use of pure solvents is obviously desirable, and there are good ways to remove peroxides from ethers,238 but ethers that have a low tendency to form peroxides are sometimes preferred. Some such ethers suggested for Suzuki cross coupling include methyl THF, diethoxymethane, and t-butyl methyl ether.239 Beyond catalyst deactivation, improved safety of operation is another desirable feature of these solvents, specially in large scale work.
10. ELECTROCATALYSIS A field of rising importance in the context of energy research, electrocatalysis can involve homogeneous catalysts that may suffer from deactivation. The first substantial discussion of catalyst deactivation, by Fox and co-workers in 1991,240 involved NiCl2(o-C6H4{PiPr2}2) as an electrocatalyst for the reductive coupling of aryl halides to give biaryls. Deactivation proved to be related to the instability of the reduced Ni(0) form of the catalyst. The exact mechanism remained unclear, but P−C bond cleavage was suggested as the most likely route. Consequently, use of the more reactive aryl halides such as PhBr over PhCl proved beneficial in more quickly intercepting the reactive Ni(0) species, returning it to the Ni(II) state via oxidative addition. The catalyst also proved more stable in more coordinating solvents, presumably by better stabilization of the Ni(0) state: deactivation was seen in thf but not in dmso, for example. Higher temperature favored productive catalysis over deactivation: catalytic turnovers doubled on going from 25 to 65 °C. A comparison between the PiPr2 and PEt2 derivatives showed that the steric effect of the iPr group was beneficial, a finding that was ascribed to inhibition of catalyst dimerization, considered to be a prime deactivation pathway in this case. One aspect of the Fox work specific to electrocatalysis is now a
11. PHOTOCATALYSIS Beyond the classic reasons for deactivation in photochemical processes in general, such as buildup of light-absorbing impurities, some factors specific to photocatalysis have been identified. Interest in solar fuel production from water led Brewer and co-workers to construct a molecular triad consisting R
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CO binding to Co was considered a possible cause of deactivation, but this fortunately proved to be reversible on replenishing the gas phase with fresh CO2.248 Thorp and co-workers reported on deactivation of [Pt2(pop)4]4− (pop = P2O5H24−) in the photooxidation of 1phenylethanol to acetophenone. At high pH, the catalyst is deactivated by deprotonation of the pop ligand, and at low pH, the catalyst is deactivated by protonation of the intermediate hydride complex, [Pt2(pop)4H2]4−, to form the inactive aqua complex, [Pt2(pop)4(H2O)2]4−. Adjustment to a pH of 3 gave the best compromise for maximizing turnovers.249 Photolysis can cause isomerization in metal complexes, which can potentially lead to deactivation. For example, when the good water oxidation catalyst, 41, was irradiated with visible light (>420 nm), the isomerization of eq 22 took place to give the much less good catalyst, 42.250 Clearly if this catalyst were part of a photooxidation system, the accompanying isomerization would lead to partial deactivation. Likewise, Holland, Eisenberg, and co-workers identified ligand exchange on the [CoIII(dmgH)2(py)Cl] hydrogen production catalyst during the photoreduction of protons as deactivation pathway.251
of two Ru polypyridyl wingtips for light capture around a Rh polypyridyl core for water reduction; indeed, the system liberates H2 in the presence of visible light and a sacrificial electron donor, DMA. This achieves 280 TONs over 19.5 h or 420 TONs over 50 h with a maximum Φ = 0.023. Oxidative addition to Rh(I) was considered to be the deactivation pathway, a process partly countered by reducing the H2 partial pressure by expanding the reaction head space and by periodic purging with Ar. The system passed a Hg(0) poison test that disfavored the possibility of deposited metal contributing to the catalysis.247
12. CONCLUSIONS Although the field is still developing and many points remain to be clarified, enough is now known for general trends to be identified. Application of some of these should enable catalysts
In a study by Hirosa and co-workers of photocatalytic carbon dioxide photoreduction to CO by solution phase [Co(bpy)3]2+ sensitized by [Ru(bpy)3]2+ fixed to a cation exchange polymer,
Table 1. Some Strategies That Have Proved Useful To Counter Catalyst Deactivationa strategy dilution of catalyst slow catalyst addition252 protecting a problematic functional group purifying substrates and solvents vary solvent
useful for
mechanism
section
M deposition, cluster formation countering deactivation
slows any bimolecular deactivation processes
4.2, 9.3
slows bimolecular deactivation processesb
1
substrate with reactive substituent
prevents that functionality from poisoning the catalyst
5.2, 6.1, 6.6, 7.2
impure components
removes problematic impurities
1, 5.1
various
suppress solvent binding to M; ILs or neat reactants may help; replace reactive solvents such as CH2Cl2 with PhCl or PhCF3 rate ratios of productive catalysis and deactivation may differ with T adjusts level of binding or ion-pairing to M, e.g., BArF4− (1)
3.2, 4.2, 5.1, 6.5, 7.2, 8.2, 9 1.1, 3.1 2, 3.8−3.10, 4.1, 5.1
vary T vary anion
various cationic catalysts
redesign synthesis redesign ligand
multistep syntheses where L reactivity or low solubility leads to deactivation to counter ligand redistribution various various oxidations, reductions
e.g., running the catalytic step earlier or later in the synthesis prefer chelates, pincers and degradation-resistant ligands; modify ligands as in Figure 1 3.1−3.6, 4.2, 5.1, and 21; incorporate remote steric bulk; halogenate ligand, e.g., porphyrins253 6.4, 6.6, 7.2, 10
various
move to a chelate or ansa ligand redesign ligand set support catalyst pH control with buffers or otherwise conformational control control oxidation state of M microwave heating additives catalyst rescue or self-repair computational
where undesired redox changes affect the catalyst slow reactions various various various
hinders dissociation
3.8
e.g., replacing PR3 by an NHC or a bulky PR3 can slow bimolecular deactivation; try Nafion, clay, or MOF occurs when pH change during the reaction affects reactivity
3.2, 7.2 3.2, 3.3, 4.1 4.1, 7.3, 11
introduce substituents on L to disfavor the conformation of L that leads to fast deactivation adding a reductant or oxidant can disfavor the oxidation state that leads to deactivation
3.4
mechanism not fully understood numerous pathways involving binding to M or catalyst rescue by oxidizing or reducing the metal reconversion to active form under suitable conditions or by addition of an appropriate reagent helps understand and avoid factors that lead to fast deactivation
8.1 4.2, 4.4, 5.1−5.2, 6.3−6.4, 10 3.2, 3.10, 4.3, 4.4, 6.5 3, 3.2-3.3, 7.2
3.8, 3.10, 5.1
a
IL = ionic liquid, T = temperature, NHC = N-heterocyclic carbene, L = ligand, esp = especially. bNot often reported but could be a more generally useful strategy. S
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to be optimized, not just for activity but also for lifetime enhancement. Table 1 identifies some strategies that have proved generally useful. In many cases, kinetic and mechanistic analysis has proved helpful in finding out what went wrong in the deactivation. Easiest to avoid are unsuitable conditions and solvents; harder are cases where a ligand is reactive, the substrate is an inhibitor, or metal is deposited, but no situation should be considered hopeless given the many examples of deactivation pathways successfully identified and countermeasures successfully adopted.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biography
Robert H. Crabtree, educated at New College Oxford with Malcolm Green, did his Ph.D. studies with Joseph Chatt at Sussex University and spent four years in Paris in Hugh Felkin’s lab at the CNRS Natural Products Institute, then headed by Derek Barton. In 1977 he became Assistant Professor at Yale, where he is now Whitehead Professor of Chemistry. He has received the ACS and Royal Chemical Society prizes for organometallic chemistry and is a Fellow of the RSC, ACS, and the American Academy. He has also been Dow lecturer (Berkeley), Williams lecturer (Oxford), Centenary lecturer (RSC), and Osborn lecturer (Strasbourg). He is the author of The Organometallic Chemistry of the Transition Metals and has long been involved in organometallic and coordination catalysis.
ACKNOWLEDGMENTS I thank the referees for making numerous valuable suggestions as well as the US Department of Energy, Office of Science, Office of Basic Energy Sciences, for their support of different aspects of our catalysis work under award numbers DESC0001059 and DE-FG02-84ER13297. REFERENCES (1) Poater, A.; Cavallo, L. Theor. Chem. Acc. 2012, 131, 1155. (2) Ager, D. J.; de Vries, A. H. M.; de Vries, J. G. Chem. Soc. Rev. 2012, 41, 3340. (3) Blaser, H.-U.; Pugin, B.; Spindler, F.; Thommen, M. Acc. Chem. Res. 2007, 40, 1240. (4) van Leeuwen, P. W. N. M. Appl. Catal., A 2001, 212, 61. van Leeuwen, P. W. N. M. Homogeneous Catalysis; Kluwer: Dordrecht, 2004. T
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