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Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Reusable N‑Heterocyclic Carbene Complex Catalysts and Beyond: A Perspective on Recycling Strategies Wenlong Wang,‡ Lifeng Cui,‡ Peng Sun,† Lijun Shi,† Chengtao Yue,† and Fuwei Li*,† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, China ABSTRACT: With the continuous development of N-heterocyclic carbene (NHC) chemistry during the past decade, NHC metal complexes have gained wide applications in the research field of organometallic catalysis. The recycling and reuse of NHC metal complexes, which have undergone continuous expansion and diversification, can enhance their catalytic performance, extend their range of application, and afford new routes to green chemistry. Taking NHC metal complex catalysts as the main topic, this review intends to present a comprehensive study of recycling strategies of organometallic catalysts. By an elaborative summarization and classification of recycling strategies, a clear picture of all available of recycling strategies for organometallic catalysts is presented and the advantages and disadvantages of various recycling strategies for specific reactions are discussed in detail. This review is written with the hope of serving as a modest spur to induce other scientists’ further contributions in the fields of catalyst recycling and sustainable catalysis.

CONTENTS 1. Introduction 2. HOMOGENEOUS CATALYSIS AND HETEROGENEOUS SEPARATION 2.1. Ionic Salt Tagged NHC Complex Catalysts 2.1.1. “On Water” Catalysis 2.1.2. Ionic Lquid Phase Catalysis 2.1.3. PEG-400 Phase 2.1.4. Biphasic Catalysis 2.1.5. Supported Ionic Liquid Phase Catalyst 2.2. Soluble Polymer Supported NHC Complex Catalysts 2.2.1. Water Phase Catalysis 2.2.2. Polar Organic Solvent Phase Catalysis 2.2.3. Nonpolar Organic Solvent Phase Catalysis 2.3. Light-Controlled Phase Selective Strategy 2.4. pH-Controlled Phase-Selective Strategy 2.5. Natural Product Functionalized NHC Complex Catalysts 2.6. Fluorous NHC Complex Catalysts 2.6.1. Heavy Fluorous NHC Complex Catalysts 2.6.2. Light Fluorous NHC Complex Catalysts 2.7. Nanofiltration of Enlarged NHC Complex Catalysts 2.8. Simply Modified NHC Complex Catalysts 2.8.1. Column Chromatography Strategy 2.8.2. PEG-400 Phase 2.8.3. Precipitation Strategy 3. IMMOBILIZED (HETEROGENIZED) NHC COMPLEX CATALYST © XXXX American Chemical Society

3.1. Silica-Based Immobilization of NHC Complex Catalysts 3.1.1. Covalent Grafting Strategy 3.1.2. Solid Phase Synthesis Strategy 3.1.3. Sol−Gel Process Strategy 3.1.4. Noncovalent Interaction Strategy 3.2. Polymer-Based Immobilization of NHC Complex Catalysts 3.2.1. Covalent Grafting Strategy 3.2.2. Solid Phase Synthesis Strategy 3.2.3. Self-Supported Strategy 3.2.4. Noncovalent Interaction Strategy 3.3. Carbon-Based Immobilization of NHC Complex Catalysts 3.3.1. Covalent Grafting Strategy 3.3.2. Solid Phase Synthesis Strategy 3.3.3. Noncovalent Interaction Strategy 3.4. Magnetic-Nanoparticle-Based Immobilization of NHC Complex Catalysts 3.4.1. Covalent Grafting Strategy 3.4.2. Solid Phase Synthesis Strategy 3.4.3. Noncovalent Interaction Strategy 3.4.4. Encapsulation 3.5. Unconventional Immobilization of NHC Complex Catalysts 4. Conclusion and Perspective Author Information

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Special Issue: Carbene Chemistry Received: January 26, 2018

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DOI: 10.1021/acs.chemrev.8b00057 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations References

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Figure 1. General synthetic pathways and modification diversity (points 1−4) of NHC-based catalysts.

promising field of research, undergone sustained diversification of innovation, and become a current research hot spot. According to our statistics, about 300 research papers have been reported on the topic of recyclable NHC complexes. In this review, we will cover the core works of recyclable NHC complex catalysts from 2006 to 2017 (the works of the past decade), and the quantity of core articles of each year is displayed in Figure 2.

1. INTRODUCTION Homogeneous catalysis and heterogeneous catalysis are just like a pair of twins with different characteristics.1−5 With respect to efficiency and selectivity, homogeneous catalysis usually performs better than heterogeneous catalysis. However, most homogeneous catalysts have not been widely utilized, especially precious organometallic catalysts, on the one hand, due to depletable resources and, on the other hand, due to some specific technical details such as intricate recycling problems and metal contamination of the products. In contrast, heterogeneous catalysis facilitates the separation and recycling process, which represents the foremost advantage over homogeneous catalysis. In recent years, catalyst recycling has increasingly become more important in regard to resource conservation and environmental protection. Therefore, seeking an elegant balance between homogeneous catalysis and heterogeneous catalysis appears to be particularly significant.6,7 Homogeneous recyclable catalysts and heterogenized (immobilized) homogeneous catalysts have been two main solutions considering both sides of activity and recycling (or separation).8,9 Up to now, two main strategies, viz., (i) homogeneous catalysis and heterogeneous separationcatalysis under homogeneous conditions and separation in different phases10−13and (ii) immobilized (or heterogenized) homogeneous catalysts, have been extensively explored to recycle the catalysts.14−18 These are two central themes that we will discuss in detail in this review. N-Heterocyclic carbene (NHC) metal complexes rank among the most popular organometallic complexes that have been widely used in the field of homogeneous catalysis.19−34 Catalysts such as the famous second-generation Grubbs’ catalyst (NHCRu complex), whose invention was awarded the Nobel Prize in 2005, have been applied extensively in olefin metathesis reactions.35−47 Other NHC metal complexes also represent part of the latest major research findings in homogeneous catalysts. For example, Deng’s group found NHC-Co complexes are effective catalysts for the reductive silylation of N2 to provide N(SiMe3)3.48 NHC-Cu(I) complexes can catalyze the C−H amidation reaction under mild conditions.49 A highly efficient NHC-Au(I)-based catalytic system (as low as 10 ppm catalyst loadings) for alkyne hydration was developed by Nolan’s group.50 Li’s group reported NHC-Ag(I) complexes catalyzing “on water” the direct alkynylation of isatins.51 Recently, some novel abnormal NHCs,22,52−55 such as cyclic (alkyl)(amino)carbenes (CAACs),56−59 and pyrazoline60−62 or triazoline based carbenes63,64 (as well as their corresponding metal complexes65−70) have also emerged and demonstrated great vitality. The strong NHC−metal bonds, endowed by the unique σelectron-donating and weak π-accepting properties of the carbene centers,71,72 prevent (or delay) the decomposition process of the NHC metal catalysts. More importantly, the synthetic diversity of NHC precursors73 as well as their coordination diversity provides infinite possibilities with their modification and heterogenization (Figure 1). For this reason, the recycling of NHC metal complex catalysts has turned into a

Figure 2. Number of annually published core studies on recyclable NHC complex catalysts from 2006 to 2017.

For other representative historical works, please see the relevant references.74−99 It is evident from Figure 2 that there is a steady growth trend in this research area. Further statistics indicate 78% of the published research can be classified as immobilized homogeneous catalysts while the rest can be classified as recyclable homogeneous catalysts which work under homogeneous conditions and are separated in different phases (Figure 3). So far, four review articles14,18,100,101 on the subject of immobilized NHC compounds have been reported, whereas few reviews have been published on the topic of recyclable homogeneous NHC metal complexes (catalysis under homogeneous conditions and separation in different phases). Nevertheless, it is worth noting that the activity and selectivity of a

Figure 3. Pie graph of published reports on recyclable NHC complex catalysts from 2006 to 2017. B


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provides a new route to explore highly efficient reaction systems with simple phase separation procedures for catalyst recycling. “On water” reactions107,108 represent a group of organic reactions that take place as emulsions in water and that display an unusual reaction rate acceleration compared to the same reactions in organic solvents or compared to the corresponding dry media reactions. This interesting effect has been known for many years and could possibly be attributed to the enhancement of local concentration. “On water” catalysis not only inherits the advantage of rate acceleration but also facilitates catalyst recycling via simple phase separation procedures (Scheme 1). Therefore, the design and synthesis of water-soluble NHC metal complexes are of great importance.

homogeneous catalyst are often negatively affected by its immobilization which hinder diffusion and mass transfer. In contrast, soluble homogeneous recyclable catalysts can realize catalyst recycling without having to sacrifice the activity and selectivity. In this review, besides immobilized NHC metal catalysts, we will emphatically introduce the recyclable homogeneous NHC metal complex catalysts. In addition to traditional silica, insoluble polymers, magnetic nanoparticles (MNPs), carbon materials immobilized NHC metal complex catalysts, and various other innovative catalysts (such as soluble polymer supported catalysts, tagged switchable phase catalysts, microencapsulated catalysts, and nanofiltration-applicable catalysts) have been developed based on the strategy of homogeneous catalysis and heterogeneous separation (catalysis under homogeneous conditions and separation in different phases). In addition to NHC metal complex catalysts, other recyclable catalysts based on the strategy of homogeneous catalysis and heterogeneous separation will also be briefly discussed in this review. Widely unknown areas of this field are yet to be explored, providing enormous opportunities for future studies.

Scheme 1. “On Water” Catalysis and the Phase Separation Recycling Process

2. HOMOGENEOUS CATALYSIS AND HETEROGENEOUS SEPARATION In addition to high efficiency and selectivity, homogeneous catalysis presents further advantages to realize the objectives of green chemistry, such as high E factors and high atom economy. However, the intricate and costly catalyst separation and additional recycling process impede their applications. The homogeneous catalysis and heterogeneous separation strategy represents an advanced and elegant methodology in catalyst recycling which not only achieves the goal of catalyst recycling but also maintains a high catalytic activity and selectivity, and this is just the merit that the immobilized catalysts do not possess.102 In this section, we will take NHC metal complex catalysts as the main topic and introduce the catalyst recycling strategy of homogeneous catalysis and heterogeneous separation, as well as briefly mention other recyclable organometallic catalysts that have been recycled through this strategy have also been briefly introduced.

From 2006 to 2009, SanMartin et al. developed three hydrophilic NHC-Pd pincer complexes (1−3) as robust and recyclable precatalysts for Suzuki coupling reactions in water (Scheme 2).109,110 Very low loading (0.001 mmol of Pd) of Scheme 2. Hydrophilic Pincer NHC-Pd Complexes and Their Application in Suzuki Reactions

2.1. Ionic Salt Tagged NHC Complex Catalysts

Compared with neutral complexes, ionic salt tagged complexes show a very strong polarity that originates from their ionic nature, and this strong polarity facilitates their dissolving process with polar liquid media (such as water and ionic liquids).103 Once metal complex catalysts are ionic salt tagged, they are generally water or ionic liquid soluble, and this solubility makes recyclability possible by using water or ionic liquids as sustainable media. Homogeneously catalyzed recyclable industrial catalytic processes, which are performed in water or biphasic systems, have been already put into production on a plant scale, such as the famous rhodium-catalyzed Ruhrchemie/Rhône-Poulenc process104 for the hydroformylation of propylene, where the catalyst can easily be recycled through a biphasic separation process. No industrial application of water-soluble or ionicliquid-soluble NHC metal catalysts has been developed to date, and this area holds great potential for further industrial developments and academic studies. 2.1.1. “On Water” Catalysis. The utilization of waterthe cheapest, safest (nonflammable and nontoxic), environmentally benign (nonvolatile) and most abundant solvent in natureas a chemical reaction medium has attracted great interest.105,106 It

complex catalysts could be used to efficiently catalyze the coupling reactions. After the reaction, the aqueous phase was extracted with diethyl ether, and the flask was charged again with substrate for the next run. The catalysts were recycled up to five runs without observing any loss of the catalytic activity, as proved by the quantitative yields (>99%) obtained after every run. A select number of kinetic trials, poisoning experiments, and TEM studies were performed to gain a thoughtful insight into the real C


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role of the employed palladium pincer complexes, revealing that probably the latter palladacycles act as reservoirs of palladium nanoparticles. In 2010, Tu et al. reported a very similar hyrophilic pyridinebridged bis-benzimidazolylidene Pd pincer complex 4, and this complex was also used as a robust catalyst for Suzuki reactions in aqueous solvents (Scheme 3).111 The difference is this complex is

Scheme 5. Sulfonated NHC-Pd-PEPPSI Complexes and Their Application in Suzuki Couplings

Scheme 3. Hydrophilic Pincer NHC-Pd Complex

more robust than SanMartin’s catalyst, and no palladium black appeared during the reaction process according to the literature description. Through the method of filtration, the catalyst could be reused for three runs without a loss of its activity. The yield of the fourth run dropped dramatically, which may have been caused by catalyst loss during the filtration process, and how to avoid catalyst loss during the recycling process is an important issue to be considered. Türkmen et al. reported an NHC-PdBr2(TPPTS) complex and its applications in the Suzuki coupling of aryl chlorides in neat water (Scheme 4).112 The catalyst displayed good water solubility because of the sulfonated phosphine ligand (TPPTS), and very high activity was observed in the presence of 1 mol % catalyst at 100 °C. The author also investigated the recyclability of catalyst 6 for the Suzuki coupling of 4-chloroacetophenone and phenylboronic acid. After the reaction, the solid product was filtered, and the filtrate containing the catalyst was reloaded with substrate for the next run. The catalyst appeared to be reused for six runs, however, on the sixth cycle, the yield dropped to 54%. Zhong et al. described the facile preparation of four sulfonated water-soluble NHC-Pd-PEPPSI (pyridine, enhanced, precatalyst, preparation, stabilization, and initiation) complexes (Scheme 5, 7−10) for aerobic aqueous Suzuki coupling reactions.113 NHC-Pd-PEPPSI complexes, developed around 2005 by Michael G. Organ and co-workers at York University, have been proven to be efficient and stable in many crosscoupling reactions while being easy to synthesize.114,115 In this report, complex 10, bearing a 2,6-diisopropylphenyl substituent, exhibited the highest catalytic activity (with a catalyst loading of

0.1 mol % efficiently catalyzing the reaction), and this could be attributed to the steric and electronic effects of the 2,6diisopropylphenyl group. These water-soluble PEPPSI catalysts could be recycled and reused for at least four consecutive runs without an apparent loss in activity, and upon completion of the reaction, the mixture was extracted with ethyl acetate to separate the product and catalyst. A TEM analysis, as well as mercury poisoning experiments, indicated that Pd nanoparticles evolved during the catalytic process, and further experiments revealed that the isolated Pd nanoparticles were inactive toward Suzuki couplings and proved that the transformation was homogeneously catalyzed. Finding solutions to slow down the formation of Pd nanoparticles and prolong the catalyst life appears to be particularly important. In 2010, Peris’ group reported two sulfonate-functionalized NHC-Ru complexes as water-soluble catalysts for the isomerization of allylic alcohols in water (Scheme 6),116 and the Scheme 6. Water-Soluble NHC-Ru-Complex-Catalyzed Isomerization Reactions of Allylic Alcohols Running in H2O

Scheme 4. Synthesis of NHC-PdBr2(TPPTS) Catalyst and Its Applications in Suzuki Coupling of Aryl Chlorides in Neat Water

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Scheme 7. Ammonium Salt Tagged SIPr-Cu(I)X Complex Catalysts and Their Application in Three-Component Aqueous Click Reactions

recyclability of the catalysts was also investigated in detail. These two catalysts exhibited high activities (up to 99%) and excellent stabilities, and the catalyst was recovered and reused for 10 runs, obtaining a total TON of 828. When this “on water” catalytic reaction was finished, the catalyst could be easily recycled by simple liquid−liquid extractions (chloroform was added as an extraction agent in this case). Our group developed a series of reusable ammonium salt tagged NHC-Cu(I) complexes, and these complex catalysts were utilized in an aqueous three-component click reaction (Scheme 7).117 SIPr (SIPr = N,N′-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene) has been utilized in a variety of catalytic reactions as an effective ligand with various metals. However, Nolan et al. found that neutral [(SIPr)CuCl] showed a latent reactivity toward the standard two-component click reaction of benzyl azide and phenylacetylene, giving only a 26% conversion of the substrate after 1 week of reaction time with water as the solvent.118 In this case, the introduction of an ammonium electron-withdrawing group (EWG) to the ligand of the complex can adjust its physicochemical properties, such as solubility in aqueous media, and increase the catalyst’s activity. Complex 12 displayed the highest activity; a high yield of 98% with a 5 mol % loading was obtained after 3 h of reaction at room temperature. Complex 15 displayed a lower activity (76%) compared with that of complex 12, and we ascribed this discrepancy to the solubility difference of these two complexes, as the water solubility of the triethylamine-functionalized complex (12) is greater than that of the tributylamine-functionalized complex (15). The recyclability of ammonium-tagged SIPr-Cu(I) was validated; after the completion of reaction, ether was added to the reaction tube, the upper organic phase containing product was easily separated by simple liquid−liquid extractions to produce the final product triazole, and the residual aqueous catalyst phase was reused for the next run because of the obvious solubility difference between the ammonium salt functionalized [(SIPr)CuX] in ether and water. The water-soluble catalyst could be used at least four times with the fourth run giving an 84% isolated yield of triazole product. Krause et al. reported the modular synthesis of unsymmetrical ammonium salt tagged NHC-Au(I) complexes and their application as recyclable catalysts in the cyclization reactions of acetylenic carboxylic acids to lactones in water (Scheme 8).119 Water and a triethylammonium acetate buffer solution with pH 7 were used as two kinds of aqueous media. From the results of the recyclability test shown in Scheme 8, we can conclude that the buffer solution was a better medium than water in this case. The reason is probably due to the acidity of pentynoic acid (pKa =

Scheme 8. Unsymmetrical Ammonium Salt Tagged Gold(I) Complexes and Their Application in Cyclization Reactions of Acetylenic Carboxylic Acids to Lactones in Water

4.21), with the gold complex slowly decomposing in this acidic aqueous solution. The use of a buffer solution with pH 7 led to high conversions for over five cycles and enhanced yields of 89− 81%. The product was extracted with diethyl ether, and the substrate carboxylic acid substrate was reloaded for the next run. In 2015, ammonium salt tagged IMesAuCl complexes were reported by Krause’s group, and these complexes were applied as water-soluble catalysts for the cycloisomerization reaction of αhydroxyallenes in water (Scheme 9).120 Triethylamine-tagged Scheme 9. Ammonium Salt Tagged IMesAuCl Complexes and Their Application in Cycloisomerization Reactions

catalyst 24 displayed a better activity than did tributylaminetagged catalyst 25, and it is probably due to the solubility difference between these two complexes. Both catalysts could be recovered and reused five times through the separation method of liquid−liquid extraction; however, the yields of the fifth run dropped to 78% (24) and 71% (25). This yield decline was ascribed to the slow degradation of the catalyst (a purple solution containing gold nanoparticles was observed). Further study demonstrated the water-soluble gold catalysts could be stabilized by the addition of LiCl. E


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Scheme 10. Sulfonated NHC-Au(I)-Complex-Catalyzed Hydration Reaction of Phenylacetylene and Its Recyclability Test

Scheme 11. Iridium and Ruthenium NHC Catalysts for Transfer Hydrogenation from Glycerol

Fernández et al. described the sulfonated NHC-Au(I)complex-catalyzed hydration reaction of phenylacetylene in an aqueous medium and its recyclability test (Scheme 10).121 This sulfonated NHC-Au(I) complex exhibited a very high water solubility (up to 111 g/L). Control reactions revealed that a mixed solvent of H2O/CH3OH accelerated the activity compared with that of pure water as a solvent. The catalyst recycling study was carried out in a water/methanol medium, and the result showed a decline in catalytic activity, which was ascribed to degradation or loss of the catalyst in each process of extraction and reloading of reagent since only 0.5 mol % catalyst was used. As we can imagine, large-scale industrial production could effectively improve the loss problem. More recently, Voutchkova-Kostal’s group designed five sulfonate-functionalized NHC iridium and ruthenium complexes (Scheme 11).122 In the presence of glycerol in a KOH aqueous solution, these five complex catalysts were compared with respect to their activity and recyclability for the transfer hydrogenation of acetophenone. The fresh catalysts displayed good to excellent activities toward transfer hydrogenation of acetophenone; however, the use of platinum group metals as homogeneous catalysts can only be sustainable if these catalysts are recyclable. Given that the hydrophilic nature of these catalysts allowed for extraction of the organic substrate and products from the reaction mixture, the recyclability of the five catalysts were examined in the model reaction of the transfer hydrogenation of acetophenone. The preliminary experimental result indicated that the ruthenium NHC catalysts with chelating ligands

(complexes 27 and 30) were found to be more robust and recyclable relative to the iridium catalysts and the ruthenium mono-NHC catalyst. The reason was traced to the relative rate of degradation of the catalyst in the presence of glycerol and KOH but no substrate. The authors also found that the degradation took place only when glycerol was used, as replacing glycerol with isopropanol allowed the ruthenium catalyst to be fully recyclable. In addition to ionic salt tagged water-soluble NHC complexes, other representative ionic salt tagged ligands such as bipyridine,123−125 imidazole,126 Salen,127,128 and Schiff base,129 as well as phthalocyanine,130 have been developed by different groups to perform various “on water” catalytic reactions (Scheme 12). In general, imidazolium salt, ammonium salt, and sulfonate are the three most common substituents that have been used to modify ligands to realize their water solubility. This modifying strategy has universality and holds great application potential in the area of “on water” catalysis, as well as sustainable chemistry. 2.1.2. Ionic Lquid Phase Catalysis. Ionic liquids (ILs), especially room-temperature ionic liquids (RTILs), have attracted a great deal of attention as sustainable reaction media.131−133 On the one hand, RTILs have served as excellent solvents to dissolve compounds with medium to strong polarities; on the other hand, solvents with low polarities (such as ethers and alkanes) are immiscible with ionic liquids. Therefore, using ionic liquids as reaction solvents facilitates the separation of strong polar catalysts and low polar organic products by liquid−liquid extraction after reaction, and offers an opportunity to reuse the catalysts (Scheme 13). Furthermore, F


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Scheme 12. Other Types of Ionic Salt Tagged Water-Soluble Cataytic Systems

Scheme 13. Ionic Liquid Phase Catalysis and the Phase Separation Recycling Process

Scheme 14. Imidazolium-Tagged Ruthenium Carbene Complexes and Their Reusability Test in Regard to Olefin Metathesis in Ionic Liquids

various catalytic reactions have proven feasible in a range of ionic liquids, with many reactions exhibiting improved activities and selectivities because of the unique physicochemical properties of ionic liquids, some of which are inactive in common organic solvents. From a practical standpoint, the use of specifically designed or modified catalysts with physicochemical properties more similar to those of ionic liquids, and the development of highly effective and recyclable NHC catalysts in ionic liquids for

homogeneous catalysis and heterogeneous separation, represents an important area in the future. In 2008, Wakamatsu et al. described the synthesis of imidazolium-tagged ruthenium carbene complexes and their application, as well as reusability, with regard to olefin metathesis in ionic liquids (Scheme 14).134 The ortho substitution of the benzylidene ligand was considered to have high reactivity due to the fast initiation by steric repulsion, and the imidazolium ion structure significantly enhanced the polarity of the metathesis G


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Scheme 15. Other Types of Ionic Phase Catalysis Using Ionic Salt Tagged Organometallic Catalytic System

Scheme 16. Aerobic Oxidation Catalyzed by an Ionic Palladium CNC Pincer Complex in a PEG Phase

summarized in Scheme 15. We can conclude from Scheme 15 that hydroformylation135−138 and hydroaminomethylation139 reactions have been a hot topic in this area because of the value of Rh complexes, as well as the cornerstone status of hydroformylation in the fine chemical industry, and various ionic salt tagged phosphine ligands have been designed and synthesized for use with the ionic liquids. Moreover, several ionic salt tagged phosphine palladium complexes/ionic liquids systems have been developed for classical cross-coupling reactions (Suzuki coupling,140 Heck coupling,141 and Sonogashira coupling142,143). ). Other reactions including asymmetric hydrogenation,144 cycloaddition of CO2 with epoxides,145 and the Diels−Alder reaction146 have also been reported in this area. It can be anticipated that a variety of ionic phase catalytic systems using

catalyst and caused it to be highly soluble in ionic liquid media. Catalyst 42 was examined with the RCM reaction in a miscible cosolvent of (CH2Cl2-[Bmim]PF6) at room temperature, and a continuous fast conversion was detected with reliable reusability being realized until the fifth run (diethyl ether was used as the extraction solvent). The catalyst was preserved by the strong electrostatic interaction with the ionic liquid, which prolonged its recycling life. Apart from the rare example of an ionic salt tagged NHC metal catalyst dissolved in the homogeneous phase of ionic liquids (or miscible cosolvent systems composed of an ionic liquid and organic solvent, such as CH2Cl2), other representative ionic liquid phase reactions using ionic salt tagged organometallic catalytic systems, which were reported after (or in) 2006 are H


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Scheme 17. Biphasic Catalysis and Its Natural Separation

new ionic salt tagged organometallic catalysts will be continually developed, as the contributions of new ligands by organic chemists proceed. 2.1.3. PEG-400 Phase. Although ionic liquids have been widely used as sustainable media for various chemical processes, toxicity and environmental burden data for most ionic liquids are still unidentified. Currently, the utilization of poly(ethylene glycol)s (PEGs),147−150 as green and environmentally benign solvents, has attracted considerable attention because of their nontoxicity, low vapor pressures, availability, low price, recyclability, and thermal stability, as well as their immiscibility with some organic solvents to facilitate phase separation after the reaction. In this sense, they are similar to ionic liquids to some extent and may be applied as important supplements to ionic liquids in the area of sustainable media. SanMartin’s group described aerobic oxidation reactions catalyzed by the ionic palladium CNC pincer complex 58 using PEG-400 as a sustainable medium (Scheme 16).151 After the reaction, a low-temperature extraction (diethyl ether was added to the reaction mixture, then the mixture was cooled to −78 °C, and the layers were decanted) was used to provide an effective catalyst separation from the final ketone products. By employing this workup procedure, six runs were performed with the PEG-400 phase containing the active catalytic species, and the yield of the sixth run was 87%. 2.1.4. Biphasic Catalysis. As stated above, an ionic salt tagged transition metal complex homogeneously catalyzed reaction in one single polar liquid phase, viz., aqueous phase or ionic liquid phase, not only makes catalyst recovery possible but also enhances the activities of certain reactions. However, a subsequent recycling method for the catalyst is required in terms of catalyst loss. Moreover, the utilization of an ionic liquid as the only solvent causes a rise of the production cost. Hence, the use of biphasic solvent systems (Scheme 17), which represent solvent systems made up of two immiscible solvents (in this review refers to the combination of a low polar organic solvent and water or an ionic liquid), to facilitate catalyst separation and recycling has become a very important strategy for both academic and industrial communities. One famous example is the Ruhrchemie/Rhône-Poulenc process that uses triphenylphosphine trisulfonate (TPPTS) as the polar ligand, and the product and catalyst can be easily separated and recycled by phase separation. It should be noted that the real mechanism of this biphasic catalysis is still homogeneous catalysis, which reacts at the interface of two phases. In 2007, Rix et al. developed a recoverable pyridinium-tagged Hoveyda−Grubbs precatalyst for biphasic olefin metathesis (Scheme 18).152 The introduction of an electron-withdrawing substituent (EWG-substituted catalyst) on the benzylidene ether

Scheme 18. Pyridinium-Tagged Hoveyda−Grubbs Catalyst for Olefin Metathesis and Its Biphasic Reusability in a Toluene−RTIL System

fragment may provide faster access to the key propagating 14 e− species, which not only improves the catalytic activity but also provides the tempting recyclability of the expensive catalyst via the change of its physicochemical properties, such as solubility in polar media. The author found that there was an optimal compromise between the activity and reusability of the pyridinium-tagged Hoveyda catalysts, with a precise adjustment of the length of the spacer allowed for the optimum balance to be attained between the activity and recoverability. Catalyst 60, whose pyridinium tag is directly attached to benzylidene, displayed poor reusability, while catalyst 59, with a methylene group between the pyridinium tag and benzylidene fragment, showed excellent reusability. The moderately activated catalyst 59 exhibited an outstanding activity for up to six runs (>98%), and the product was isolated by the evaporation of the toluene phase. Inductively coupled plasma mass spectroscopy (ICP-MS) indicated a very low level of Ru waste was detected in the product (cycle 1, 11.5 ppm; cycle 3, 1.6 ppm; and cycle 6, 9.5 ppm). Clearly, this “fine-tuned” catalyst could also find applications in metathesis reactions in the above-mentioned aqueous or RTIL media. Chen et al. synthesized novel Hoveyda-type Ru-carbene complexes tethering an imidazolium tag at the chelating isopropoxy group and examined their catalytic activities and recyclability in ring-closing metathesis (RCM) under biphasic conditions (Scheme 19).153 First, in a biphasic [bmin][PF6]/ toluene system, catalyst 61 could be reused for three cycles in the RCM reaction; however, in the third run, there was a dramatic decrease in activity (12 h to reach a conversion of 12%). When I


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system,154 and depending on the polar nature of the catalyst, after the biphasic reaction, the catalyst could be separated and recovered easily in the ionic liquid phase, while the polymer was dissolved in the toluene phase (Scheme 20). It is worth noting that, for a good separation of the ionic catalyst and the final polymer, the use of a chain transfer agent (CTA) to provoke the release of the catalyst from the polymer chain is necessary. The utilization of a CTA allowed for the synthesis of polymers with very low metal contents between 10 and 80 ppm, equal to a ruthenium elimination of 98−99.8% without any additional purification step. For catalyst 65 bearing a nitro-substituted benzylidene ligand, the recycling stability was evidently inferior to that of other catalysts, which is coincident with Rix and Chen’s report. Consorti et al. developed the new ionophilic secondgeneration Grubbs-type catalyst 68 with a phosphine ligand bearing an imidazolium fragment, and this catalyst displayed good catalytic activity and recyclability in RCM reactions under biphasic conditions (Scheme 21).155 Of note is that, for toluene/ ionic liquid biphasic systems, catalyst 68 exhibited an excellent IL phase affinity with undetectable Ru amounts (measured by atomic absorption for the BMI·NTf2/toluene biphasic system at room temperature). Conversely, for the same biphasic system, the classical neutral second-generation Grubbs’ catalyst had a very low IL phase affinity and was mainly present in the organic toluene phase. It is precisely because of the excellent affinity property of 68 for the IL, under the biphasic condition of BMI· PF6/toluene, that complex catalyst 68 could be recovered and reused for eight cycles without a significant loss of activity. In 2012, Cadierno’s group synthesized a pyridine-substituted and sulfonic acid substituted water-soluble NHC-Au(III) complex, and in a biphasic toluene/water system, this complex was successfully applied in the intramolecular cyclization of γalkynoic acids into eno-lactones (Scheme 22).156 Interestingly, an equilibrium was established between 69 and the zwitterionic derivative 70, the latter resulting from the coordination of the pyridyl unit to the gold center and the concomitant release of an

Scheme 19. Recycling of Catalysts 61−64 for RCM in Biphasic Systems

the ionic liquid was changed from [bmin][PF6] to [bdmin][PF6] (bdmin = 1-butyl-2,3-dimethylimidazolium), the recyclability was slightly improved. In contrast, the use of catalyst 62 in the biphasic system of [bdmin][PF6]/toluene significantly improved the recyclability; the catalyst could be reused for seven runs without a loss in activity. These results revealed that the structures of the imidazolium tag and ionic liquid used had an apparent effect on the recyclability. Interestingly, complexes 63 and 64, in which the imidazolium moieties are in the proximity of the Ru metal center, displayed high conversion rates compared with those of 61 and 62 in a biphasic reaction system of [bdmin][PF6]/toluene. However, the recyclabilities of complexes 63 and 64 were unsatisfactory. Compared with complexes 61 and 62, whose imidazolium moieties are far from the reaction metal center, these complexes with a short distance between the imidazolium moieties and the metal center showed a low stability, and this conclusion is consistent with Rix’s result. In summary, the structures of the imidazolium tag and the ionic liquid, as well as the distance of the imidazolium and metal center, are main factors in balancing activity and recyclability. Buchmeiser’s group described a biphasic ring opening metathesis polymerization (ROMP) catalyzed by an ionic Grubbs−Hoveyda complex catalyzed in an ionic liquid/toluene

Scheme 20. Biphasic Ring Opening Metathesis Polymerization (ROMP) Catalyzed by an Ionic Grubbs−Hoveyda Complex in an Ionic Liquid/Toluene System

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Scheme 21. Ionophilic Second-Generation Grubbs-Type Catalyst with Phosphine Ligands Bearing an Imidazolium Fragment and Its Recyclability Application in Biphasic System

Scheme 22. Sulfonated NHC-Au(III)-Complex-Catalyzed Intramolecular Cyclization of γ-Alkynoic Acids into Enolactones under a Biphasic Condition

Scheme 23. Sulfonated NHC-Au Complexes and Their Recyclability in the Cycloisomerization Reaction of γAlkynoic Acids into Eno-lactones under Biphasic Conditions [Adapted from ref 157. Copyright 2013 American Chemical Society.]

HCl molecule. The zwitterionic complex 70 was the dominant species present in aqueous solution, which was evidenced by Xray crystallography. Taking advantage of the separation of biphasic catalysis, using the 1,6-diyne compound as a model substrate, 10 consecutive reactions with quantitative yields could be performed (without the loss of any activity and selectivity). One year later in 2013, as a continuing work, Cadierno et al. comprehensively investigated the different activities and recyclabilities of a series of sulfonated NHC-Au complexes (71−74) in the cycloisomerization reaction of γ-alkynoic acids into eno-lactones in the biphasic toluene/water system (Scheme 23).157 These catalysts all exhibited excellent activity in the fresh run; however, in terms of recyclability, their performance differences emerged gradually. At the end of each cycle, after the phase separation process to obtain the toluene phase containing product, a fresh load of substrate dissolved in toluene was added to the aqueous phase in which the NHC-Au catalyst remained dissolved. Under these conditions, the Au(III) complexes (73 and 74) displayed excellent recyclabilities and could be recovered and reused at least 10 times without any loss of activity and selectivity (TOF remained 40 h−1 in each cycle). In sharp contrast, the recyclabilities of their Au(I) counterparts (71 and 72) began to decrease in the fifth run, and longer times were needed to finish the conversion. These differences are illustrated by histograms presented in Scheme 23. Further observations revealed that there was partial decomposition of the Au(I) complex into gold nanoparticles after the fourth run, and a mercury test discarded a heterogeneous catalysis mechanism involving gold nanoparticles as the active species, further validating the homogeneous mechanism. Except for NHC metal complex catalysts, other representative ionic salt tagged metal complex catalysts that have been used in biphasic systems (reported after 2006) are summarized in Scheme 24. In addition to the famous ligand of triphenylphosphine trisulfonate (TPPTS),158 ionic-liquid-modified phosphine

ligands159,160 and sulfonated salicylaldimine ligands161 have also been designed and synthesized for biphasic hydroformylation reactions. Sulfonated bipyridine palladium complexes were applied in the oxidative carbonylation reaction of amine under a biphasic condition of toluene/H2O.162 An ionic-tagged ferrocene−ruthenium catalyst was developed for the asymmetric hydrogenation of aromatic ketones in the biphasic toluene/H2O system.163 In some processes, supercritical carbon dioxide (scCO2) was employed as a clean phase to replace the traditional organic phase in the biphasic system, such as the In(OTf)3 catalyst was applied in the scCO2/ionic liquid biphasic system to catalyze the Friedel−Crafts acylation reaction.164 With the development of organic synthesis, as well as sustainable chemistry, ionic salt tagged complex-catalyst-based biphasic catalysis has wide application prospects in the future. 2.1.5. Supported Ionic Liquid Phase Catalyst. Although the capability of biphasic catalysis in ionic liquids has been successfully demonstrated, the chemical industry still prefers formal heterogeneous catalyst systems. Furthermore, the use of biphasic reaction systems requires a large amount of an ionic liquid. On account of economic criteria and possible toxicological concerns, it is desirable to reduce the amount of an ionic liquid in K


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Scheme 24. Other Types of Biphasic Catalysis Using an Ionic Salt Tagged Organometallic Catalytic System

a potential chemical process. On this occasion, supported ionic liquid phase (SILP) catalysts have emerged as a powerful supplement to biphasic and monophasic ionic liquid phase catalysts.165 The SILP concept involves the surface of a porous support material that is modified with a thin film of an ionic liquid (which has generally been realized by physical adsorption), and then the homogeneous catalyst (better for ionic salt tagged catalysts) is immobilized in the thin supported ionic liquid phase to form a flow homogeneous catalysis system on the surface of the porous supports. More interestingly, although the resulting material is an insoluble solid, the active species is dissolved in the ionic liquid phase and performs as a homogeneous catalyst. The concept of SILP represents an elegant balance between homogeneous catalysis and heterogeneous catalysis. Cole-Hamilton’s group developed a SILP catalyst for continuous-flow homogeneous alkene metathesis with compressed CO2 as a transport vector (Scheme 25).166 Through the ionic liquid modification of the classical Hoveyda-type metathesis catalyst, supported ionic liquid catalyst 82 was constructed, and this supported ionic liquid phase catalyst allowed the selfmetathesis of methyl oleate with only a slight loss in activity for at least 10 h. Such a system using compressed CO2 as the flowing medium offered advantages over ordinary systems where a liquid flows through a supported ionic liquid phase catalyst, and the reason is the loss of the ionic liquid and catalyst is greatly reduced, which is attributed to the low solubility of the ionic liquid and catalyst in the CO2 flow. In addition, the metathesis products obtained in this process exhibited very low metal contamination. More importantly, no solvent was present in the collected product, which meets the criteria of green chemistry perfectly. Buchmeiser et al. described the synthesis of dicationic ruthenium−alkylidene complex 83 and its application in continuous metathesis reactions by exploiting SILP technology (Scheme 26).167 In this case, the support was afforded through the ring opening metathesis polymerization of norborn-2-ene

Scheme 25. Supported Ionic Liquid Phase Process for the Self-Metathesis of Methyl Oleate [Adapted with permission from ref 166. Copyright 2011 Royal Society of Chemistry.]

and tris(norborn-5-ene-2-ylmethyloxy)methylsilane. The subsequent immobilization of the ionic liquid ([Bdmin][BF4]) containing the dicationic catalyst produced the SILP catalyst. It is worth noting that the ammonium salt modified support enhanced the electrostatic interaction between the support and the thin ionic phase, which is very beneficial to preventing the leaching of the active homogeneous catalyst species (catalyst leaching into the transport phase, 380 nm) for only 2 min, it was transformed back to the original neutral state 113, as proven by the UV−vis spectrum. By using the combined response effect of light and pH exhibited by this SP-tagged catalyst, this interesting homogeneous catalyst could be reused seven times at a low catalyst loading in Suzuki reactions. In conclusion, the utilization of lighta super clean and cheap resourceas the unique driving force to recover and recycle the homogeneous catalyst is on the rise and is increasingly attracting attention. This strategy can definitely be extended to various other homogeneous catalysts, although the report of other types of catalysts has so far been rare. Future works in this field should not only center on the development of new light-responsive homogeneous catalysts, as well as the related reaction/recycling systems, but also focus on the integration of light-controlled recycling systems with light-enhanced activity, with the latter objective being very challenging and requiring significant efforts of scientists in this area.

the SP tag to transform from the neutral state to the charged state, thus forming catalyst 110. The SP-tagged catalyst could simply be recycled by controlling the SP tag through the presence and absence of light. The schematic diagram of the catalyst recycling procedures is shown in Scheme 36B. After the reaction, the solvent was distilled under vacuum. Then, cyclohexane and a mixture of methanol and glycol were added to the residue. At the beginning, the catalyst and the products were dissolved in the cyclohexane (upper) layer. After irradiation, the ionic catalyst 110 was formed and shifted into the methanol/glycol (lower) layer completely, and the products remained in the cyclohexane layer (upper) layer. After the separation of the products, dichloromethane (or cyclohexane) was filled again to begin a new biphasic system. The biphasic system was placed in the dark for several minutes to allow the ionic state to convert back to the neutral state catalyst (109) and transferred back into the dichloromethane layer. Subsequently, the catalyst could be separated with excellent yield, or a new catalyst cycle could be started by the addition of fresh substrates. This light-responsive catalyst could be recycled for six runs in the RCM metathesis reaction without a significant loss of activity. Later, our group reported a light-sensitive phase-selective NHC-Cu complex for homogeneous catalysis (Scheme 37).188 The light-responsive tags, two SP units, were introduced by an azide-functionalized NHC-Cu complex self-catalyzed click chemistry. As anticipated, SP-modified NHC-Cu complex 111 exhibited a rapid and reversible phase-selective property in a biphasic system of ethyl acetate (EA) and glycol. By employing procedures similar to those of Wang’s work, taking the oxidative reaction of benzyl alcohol and click chemistry as model reactions, the light-responsive SP-functionalized complex 111 could be recycled several times without evident loss of activity. Recently, Wang’s group described the synthesis and application of the recoverable SP-tagged NHC-Pd complex 113, and interestingly, in addition to the approved lightresponsive property of this type of catalyst, the combined effect of light and pH on the SP-functionalized complex was also investigated (Scheme 38).189 In this case, when a solution of complex 113 in i-PrOH−H2O was solely irradiated with light, there was no absorption in bands from 400 to 800 nm in the

2.4. pH-Controlled Phase-Selective Strategy

Acid and base are two basic concepts in chemistry, and the acid− base reactions are among the most common chemical reactions in the areas of both organic chemistry and inorganic chemistry. Hence, there are thousands of pH-responsive molecules in nature and laboratories. For example, organic bases with a low to moderate polarity are very easily converted to high polar organic salts in the presence of acids. Making use of this favorable difference of polarity, a pH-controlled phase-selective strategy can be elegantly designed and realized as a novel method to recycle the homogeneous catalysts without sacrificing activity and selectivity. In 2013, our group described a series of pH-responsive NHCCu and NHC-Ag complexes that were functionalized with S


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Scheme 39. pH-Controlled Monophasic/Biphasic System Switching for Homogeneous Catalyst Recycling

morpholinean organic baseas pH-responsive tags (Scheme 39).190 Based on the pH-responsive NHC-Cu complexes, a pHcontrolled monophasic/biphasic switchable system was established as a novel and green strategy to realize the wonderful goal of homogeneous catalysis and heterogeneous separation. In this case, the pH-responsive NHC-Cu catalysts were applied in the carboxylation of arylboronic esters and benzoxazole with CO2. As illustrated in Scheme 39, the carboxylation reaction was performed under homogeneous conditions since the morpholine-functionalized NHC-Cu complex was soluble in the mixture and the product was in its carboxylate state after the reaction; When the acidic diethyl ether was added to the system, the RCOO− species was acidified to be RCOOH, and simultaneously, morpholine-functionalized NHC-Cu complex 117 was protonated to its ammonium salt tagged state (118) with a strong polarity and precipitated from the nonpolar diethyl ether solution. Hence, the catalytic system switched from monophasic to biphasic conditions and the precipitated ammonium salt tagged NHC-Cu catalyst 118 could be separated by centrifugation. The introduction of fresh substrate and base initiated a new catalytic cycle run, and the base could regenerate the protonated complex to its neutral state, which was soluble again in the reaction solvent. It is worth noting that HCl diethyl ether is a perfect acid and solvent in this recycling system because of its nonpolar character and anhydrous nature, as well as its excellent solvency for the product. This morpholine-functionalized pH-responsive catalyst could be recovered and reused at least four times without an evident loss of activity. Additionally, this strategy could be extended to the NHC-Ag-catalyzed carboxylation reaction of terminal alkynes. Wang’s group developed a carboxylate-tagged chelating Nheterocyclic dicarbene (NHDC) palladium complex 119, and this NHDC-Pd complex exhibited an excellent pH-responsive behavior (Scheme 40).191 The carboxylate-tagged complex dissolved in water to form a clear solution. When the solution was acidified with an HCl aqueous solution, the catalyst precipitated from the solution immediately due to the formation of carboxylic acid tagged complex 120, and more importantly, this process was reversible. Taking advantage of this pHresponsive behavior, the catalyst was applied as a recyclable catalyst in water phase Suzuki reactions. When the reaction was finished, the products were extracted with diethyl ether, and the aqueous phase was acidified with HCl to generate the carboxylic acid tagged catalyst. The solid catalyst was recovered after careful removal of the supernatant by decantation, the catalyst could be reused for at least four runs without a loss of activity.

Scheme 40. pH-Responsive NHDC Palladium Complex and Photographs of the Phase Switching [Adapted with permission from ref 191. Copyright 2011 Royal Society of Chemistry.]

Very recently, Liu and co-workers developed pH-controlled recyclable indolinooxazolidine-tagged NHC-Ru complex 121 for olefin metathesis (Scheme 41).192 Previously, indolinooxazolidine was used as a photochromic dye, and this paper extends its application as a useful pH-responsive tag. A two-phase solvent dispersion system consisting of ethylene glycol and cyclohexane Scheme 41. Indolinooxazolidine-Tagged pH-Responsive NHC-Ru Complex Catalyst [Adapted with permission from ref 192. Copyright 2017 Royal Society of Chemistry.]

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Scheme 42. NHC-Pd-Complex-Catalyzed Continuous Buchwald−Hartwig Reaction, Including Catalyst Recycling [Adapted from ref 193. Copyright 2016 American Chemical Society.]

phase-switchable surfactants that were induced by CO2.8,194 When CO2 was bubbled into the system, the hydrophobic longchain alkyl amidines could be protonated and turned watersoluble. When N2 was bubbled to remove the CO2, the amidinium ion could then be deprotonated and return to the organic phase. This CO2-induced phase switch process is a pHcontrolled phase-selective strategy in essence, and more interestingly, there is no inorganic salt byproduct produced! Inspired by this clean CO2-induced phase selective strategy, Cole-Hamilton and co-workers described a similar CO2-induced phase switch process to recycle a homogeneous catalyst (Scheme 43). By introducing amidine groups onto phosphorus ligands,

was applied for the phase-transfer process. When HCl was added to the system, the neutral NHC-Ru complex 121 was transformed to the ionic form 122 and transferred from the cyclohexane phase (upper layer) to the ethylene glycol phase (bottom layer); moreover, this process was reversible upon the addition of triethylamine. By varying the pH of the catalytic twophase solvent system, the homogeneous NHC-Ru catalyst could be recovered and reused at least six times without a slight loss of activity. Clearly, the pH-responsive indolinooxazolidine tag could be attached to other complex catalysts and applied as recyclable homogeneous catalysts for a number of catalytic reactions. In addition to the utilization of pH-responsive catalysts to realize the phase separation process, the employment of pHresponsive products to achieve the aim of catalyst recycling represents another smart strategy toward the very same goal. Recently, Chartoire et al. described a generalized approach to a continuous Buchwald−Hartwig reaction that was catalyzed by a recyclable NHC-Pd complex (Scheme 42).193 A designed bespoke flow reactor was employed to perform the reaction, which was then followed by a postreaction acidic aqueous treatment in a mixer−settler leading to two output streams: the aqueous stream containing the product salt, which could be ascribed to the basic nature of the product originating from the secondary and tertiary amines; and the organic stream containing the residual NHC-Pd catalyst, which could be recycled and reused following a defined workup procedure. Through systematic studies of various conditions, it was determined that this catalyst could be synthesized on a multihundred-gram scale and could be recycled and reused up to three times. The product of this Buchwald−Hartwig reaction was an important organic intermediate of a candidate drug that was previously developed at AstraZeneca for the treatment of central nervous systems disorders, receiving phase II clinical trials. Therefore, this case is of great importance for its practical utility and inspires us cultivate novel ideas to explore more practical processes in the future. Although the above-mentioned pH-controlled phase-selective strategies endowed the efficient catalyst separations and recycling, significant quantities of inorganic salt byproducts formed during pH adjusting processes. Jessop’s group developed

Scheme 43. Hydroformylation of Alkenes Using a CO2Triggered Phase Selective Strategy

the CO2-induced pH-controlled phase switch process was employed in the hydroformylation of 1-octene. The fresh catalytic system displayed an impressive activity (with an initial TOF of up to 11 046 h−1), and the catalyst was recycled several times without an apparent loss of activity and with very low leaching of rhodium. In summary, the pH-controlled phase-selective strategy holds great potential for large-scale industrial applications because of the easy handling such as the readily available acids and bases. However, there are still two problems and challenges that require further attention: (1) The pH-controlled phase-selective strategy relies on the elegant design of the whole process, including the reaction and workup procedures. (2) Generally, significant quantities of inorganic salt byproducts are formed during pH adjustment processes, and significant efforts are needed to search for cleaner and higher E-factor processes, such as the CO2triggered phase-selective strategy. It can be envisaged that more U


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2.6. Fluorous NHC Complex Catalysts

and more works, in both academic and industrial arenas, will emerge in this area in the future because of the practical utility of this strategy.

Fluorous technologies in catalysis and green chemistry, including fluorous biphasic catalysis and fluorous monophasic catalysis, have shown broad applications because of the facile separation of the system.196−199 It has been determined that the miscibility of perfluoroalkanes, perfluorotrialkylamines, and perfluorodialkyl ethers is very low even with frequently used organic solvents, such as THF, acetone, and toluene; hence, a general concept, viz., the f luorous biphase concept, has evolved, leading to the development of f luorous phase catalysis. The chemical term f luorous, which was introduced as an analogue to the word aqueous, is used to emphasize the concept that a particular chemical conversion is mainly controlled by a designed catalyst that is prone to dissolve in a fluorous phase. Therefore, according to the similar compatible principle, fluorous catalysts, which are tagged with fluorocarbon moieties, are preferentially soluble in fluorous phases. The most used and effective fluorocarbon moieties are perfluoroalkyl chains with a high carbon number, just like fluorous ponytails. According to the total number of fluorine atoms on the fluorous tag, fluorous catalysts can be divided into heavy fluorous catalysts and light fluorous catalysts. In addition, it is worth noting that the insertion of insulating groups between the main structure of the catalyst and the fluorous ponytail is necessary in many circumstances because of the strong electronwithdrawing properties of fluorous ponytails. 2.6.1. Heavy Fluorous NHC Complex Catalysts. Hošek et al. developed the four-fluorous ponytail-tagged ruthenium metathesis catalyst 135, which is classified as a heavy fluorous catalyst (Scheme 45).200 To avoid the strong electron-withdrawing effect of the four C6F13 groups, two −CH2CH2− groups as insulating groups were inserted between the backbone of the NHC ligand and the C6F13 groups. The polyfluoroalkyl ponytailmodified complex 135 matched the stability and activity of a commercial Hoveyda-type second-generation catalyst in the model RCM reaction of diethyl allylmethallylmalonate

2.5. Natural Product Functionalized NHC Complex Catalysts

Natural products, such as sugars, are very important molecules not only for their biological compatibility and activity but also for their excellent water solubility in some cases. Moreover, the Nsubstituents of NHC ligands generally enhance efficiency and stability. Therefore, NHC metal complexes modified by bulky sugar molecules have become an attractive class of candidates as NHC metal catalysts. Lin and co-workers developed glucopyranoside-modified NHC-Pd complexes (126 and 127) as efficient and recyclable water-soluble catalysts for aqueous Suzuki coupling reactions between phenylboronic acid and chlorobenzene derivatives (Scheme 44).195 After the reaction, the product Scheme 44. Glucopyranoside-Modified NHC-Pd Complexes and Their Application in Water Phase Suzuki Couplings

was separated by liquid−liquid extraction, the organic layer was collected and evaporated to afford the crude product, and the aqueous layer containing the catalyst was reloaded with substrate for a second cycle.

Scheme 45. Heavy Fluorous NHC-Ru Catalyst and Its Recycling Process in RCM Reaction

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Pd complex 130 and fluorous phosphine coordinated Pd complex202 131 (Scheme 47). These fluorous Pd complexes were applied to catalyze various cross-coupling reactions, such as the Suzuki coupling, Heck reactions, and Sonogashira reactions. After the reaction, the catalyst was separated and recycled through an FSPE strategy. Clearly, this recycling strategy can be expanded to a wide range of ligand-supported complex catalysts. The emphasis should be put on the development of the double functions of the attached ponytails, the catalytic activity enhancement effect and the fluorous recyclability.

(DEAMM). More importantly, this heavy fluorous NHC-Ru catalyst was highly soluble in HFE-7100 (perfluoroisobutyl ether/methyl perfluorobutyl mixture), whereas its solubility in common organic solvents was rather low (especially at temperatures below −15 °C). Thus, a combined solvent system of CH2Cl2/HFE-7100 was selected for the recyclable catalytic RCM reaction, which was characterized for its temperaturecontrolled homogeneous catalysis and heterogeneous separation. At a temperature of 30 °C, the whole system containing the heavy fluorous catalyst, CH2Cl2/HFE-7100, and substrate became a homogeneous phase and underwent homogeneous catalysis. When the reaction was finished and cooled to −25 °C, a two-phase system was formed. Catalyst 128 was recovered and recycled by the method of phase−phase separation. From here, we see that there is an intersection within the fluorous catalyst reaction and temperature-controlled phase separation strategy. Catalyst 128 could be recycled at least five times without a significant loss of activity, which represents the first reported example of a heavy fluorous separation technique in an alkene metathesis reaction. 2.6.2. Light Fluorous NHC Complex Catalysts. Catalysts bearing one small fluorous tag, such as C6F13 and C8F17, are defined as light fluorous catalysts. Light fluorous catalysts have attracted an abundance of attention, since they can typically catalyze reactions of organic substrate under the same conditions as other nonfluorous relatives; however, they are easily removed from the reaction products by fluorous solid phase extraction (FSPE) and can be routinely reused. Solid phase extraction (SPE) is a sample separation process, and it exploits the affinity difference of solutes dissolved or suspended in a liquid (defined as the mobile phase) as the mixture passes through a stationary phase to separate the desired from undesired components. FSPE is an extended concept of SPE, which means the stationary solid phase is constituted or modified with fluorous materials, such as a fluorous silica gel with abundant fluorocarbon bonds. Matsugi et al. developed a recyclable light fluorous Grubbs− Hoveyda metathesis catalyst 129 for RCM reactions (Scheme 46).94 The light fluorous tag was directly attached to the aromatic

2.7. Nanofiltration of Enlarged NHC Complex Catalysts

Nanofiltration represents an efficient separation process that uses a polymeric or (less often employed) ceramic membrane to separate different molecules with different molecular sizes dissolved in aqueous or organic solvents.203−205 The driving force of the separation process is the pressure difference (the socalled transmembrane pressure, TMP) between the two sides of the membrane, and the nanofiltration process of a real mixture is illustrated in Scheme 48. Since early in the 1960s, membrane processes have been gradually employed in industrial applications and have become feasible alternatives to some traditional separation and purification processes (such as extraction, evaporation, distillation, and chromatography). On the topic of homogeneous catalysis, nanofiltration becomes significantly more important in that it not only achieves the homogeneous efficiency but also realizes efficient heterogeneous separation without phase change. Although nanofiltration membranes can retain molecules with a small size on the order of 10−9 m, for better recycling of homogeneous molecular catalysts, the relatively small-sized catalysts should be enlarged through various modification methods. The enlarged catalysts can be retained at the nanofiltration membrane, while the relatively small-sized product molecules, as well as a small quantity of unreacted substrate, can pass through the membrane under pressure. The membrane technology offers greater advantages over conventional techniques in terms of safety, environment, and economy, and thus holds great potential in industrial applications. In 2008, Keraani et al. described the nanofiltration recovery of a series of gradually enlarged Grubbs−Hoveyda-type NHC-Ru catalysts in an RCM metathesis reaction (Scheme 49).206 Through an elegant chemical modification process, the molecular weight of these catalysts was gradually increased from 627 to 2195 g/mol, and the nanofiltration catalyst recovery was found to raise from approximately 70% to 90% in both toluene and dimethyl carbonate. Interestingly, the most retained catalyst was not the largest molecule; catalyst 134, with a medium molecular weight of 887, exhibited the best recovery. This indicated the recovery was not controlled by only the molecular weight, but also other properties of the catalyst molecule, such as the affinity to the membrane, also played a certain function. The most retained Ru catalyst 134 was then engaged in the RCM model reaction associated with a final nanofiltration process to realize the catalyst recovery and recycling. The catalyst functioned for up to five cycles before a decrease in catalytic activity happened. Almost during the same period, Plenio and co-workers developed a four Cy2NH group Grubbs−Hoveyda-type olefin metathesis catalyst 137, which was recovered and recycled by the method of solvent resistant nanofiltration (Scheme 50).207 The enlarged NHC-Ru metathesis catalyst displayed a slightly high activity compared to that of the related N-mesityl-based complex.

Scheme 46. Recyclable Light Fluorous Grubbs−Hoveyda Catalyst and Its Application in RCM Reactions

ring of the ligand, which not only served as an electronwithdrawing group but also used as a handle for the FSPE process. Interestingly, the directly attached light fluorous metathesis catalyst exhibited enhanced activity compared with those of the parent and a previously reported fluorous Grubbs− Hoveyda metathesis catalyst with an insulating group. The light fluorous catalyst could be routinely recovered for at least five cycles with a minimal loss in activity by using the FSPE process. In addition to fluorous NHC metal complex catalysts, other types of fluorous complexes were also reported as recyclable catalysts, such as the Schiff base201 (nitrogen ligand) coordinated W


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Scheme 47. Representative Recyclable Fluorous Pd Complexes

beneficial to the operation of the continuous nanofiltration membrane reactor. By employing the nanofiltration membrane reactor, this homogeneous dendrimer complex was applied in the double Michael reaction under continuous reaction conditions. Because of the macromolecular dimension of the dendrimer, the catalyst was easily retained in the reactor (99.5% retained). Moreover, the turnover number of the catalyst was greatly increased due to the long-time stability of the catalyst in the continuous membrane reactor. Vogt’s group described the synthesis of the molecular-weightenlarged monodentate phosphine ligands (141−143) via “click” chemistry (Scheme 54).211 These enlarged phosphine ligands together with Pd(OAc)2 salt could efficiently catalyze the Suzuki−Miyaura coupling reactions. The dendritic catalysts displayed similar activities to the unsupported analogues. Moreover, these catalysts could be recovered and reused through the method of nanofiltration using ceramic nanofiltration membranes. In summary, the nanofiltration technique is a versatile and green (energy-, waste-, and cost-efficient) separation method, with the potential to compete with traditional separation processes. It can efficiently realize the separation process without sacrificing the homogeneous catalytic activity. The enlarged molecular complex catalyst can be better retained in the nanofiltration membranes than can the parent small-sized catalyst, albeit a small-scale (2.9 mL total system volume) continuous-flow nanofiltration-mediated unmodified Grubbs− Hoveyda catalyst recycling system was reported very recently.212 Hence, the design and synthesis of enlarged molecular complex catalysts to realize efficient nanofiltration processes without sacrificing activity have become a feasible and promising strategy in the field of catalyst recovery and recycling.

Scheme 48. Conceptual Visualization of the Solvent-Resistant Nanofiltration Process

In batch reactions, the enlarged metathesis catalyst dissolved in toluene was efficiently retained (>99.8%) after a single nanofiltration process. Moreover, nearly equally efficient catalyst retention was observed in the continuous synthesis of an RCM product by using a membrane reactor. This enlarged NHC ligand holds great potential in the separation of other NHC-ligated transition metal complexes via the strategy of nanofiltration. Several months later, the same group of Plenio reported an enlarged NHC-Pd complex as an efficient catalyst for crosscoupling reactions and its solvent-resistant nanofiltration recycling applications (Scheme 51).208 The enlarged NHC-Pd complex 138 displayed a high activity in the homogeneous catalytic process. Furthermore, it could be efficiently separated from the product/catalyst mixture by means of a solvent-resistant nanofiltration by utilizing a polydimethylsiloxane (PDMS) membrane. A very high retention of between 97 and 99.9% for the enlarged NHC-Pd complex was observed. The residual Pd content in the products was in the range 3.5−25 ppm, which presented adequate security for the pharmaceutical chemistry. In 2013, Grela et al. developed the novel polyhedral oligomeric silsesquioxane (POSS) tagged molecular-weight-enlarged metathesis catalyst 139 for RCM reactions and their nanofiltration recycling process (Scheme 52).209 The activity of 139 was comparable to the parent Grubbs−Hoveyda catalyst. By employing the nanofiltration technique, the POSS-tagged enlarged catalyst could be recovered and recycled, and it was found that membranes Starmem 228 and PuraMem 280 were more suitable for the separation of the postreaction mixture. A very low catalyst residue (below 3 ppm) of the product could be realized after the nanofiltration process. The catalyst could be reused for at least three cycles without an apparent loss of activity. Next, we briefly introduce two other types of enlarged coordinated metal complexes, which are classified as nitrogenligand-coordinated and phosphine-ligand-coordinated complexes. Koten and co-workers developed the shape-persistent multi(pincer-Pd) complex 140, which can be classified as a homogeneous dendrimer complex (Scheme 53).210 The molecular size of this kind of dendrimer complex is significantly enlarged compared to the parent pincer-Pd complex, which is

2.8. Simply Modified NHC Complex Catalysts

Simply modified or unmodified NHC complexes, which can also be interpreted as “simple homogeneous complexes”, have several clear advantages such as easy preparation, low cost, and high efficiency. Generally, these complex catalysts are hard to recover because of the lack of modification that facilitates the recycling process. However, there are also some exceptions and some unmodified NHC complex catalysts have been recovered and reused after homogeneous catalytic reactions. The most conventional method of column chromatography,213 the utilization of PEG as a green reaction phase to facilitate biphasic separation, and the catalyst precipitation process have been exploited to recover the unmodified NHC complexes. 2.8.1. Column Chromatography Strategy. In 2006, Grela et al. developed a recycling process for a simply modified homogeneous metathesis catalyst by using a column chromatography strategy (Scheme 55).214 The used catalyst was very stable and easy to prepare from inexpensive α-asarone; even more X


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Scheme 49. Gradually Enlarged Grubbs−Hoveyda-Type NHC-Ru Catalysts

Scheme 50. Cy2NH Grubbs−Hoveyda-Type Olefin Metathesis Catalyst and Its Nanofiltration Recycling Application in RCM Reactions

Scheme 51. Cy2NH-NHC-Pd Complex as an Efficient Catalyst in Cross-Coupling Reactions and Its Nanofiltration Recycling Application

importantly, the catalyst displayed a high affinity for silica gel when CH2Cl2 was used as the eluent (Rf = 0.00 in CH2Cl2 for thin-layer chromatography), which can potentially ensure its

effective recovery by using silica gel column chromatography. The catalyst remained on the silica gel, whereas the product was collected after the elution of CH2Cl2. Next, elution with the more Y


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Scheme 52. POSS-Tagged Grubbs−Hoveyda-Type Olefin Metathesis Catalyst and Its RCM Nanofiltration Application

Scheme 54. “Click” Dendritic Phosphine Ligands and Their Palladium-Coordinated- Catalyst-Catalyzed Suzuki Reaction

Scheme 53. Multi(pincer-Pd)-Complex-Catalyzed Double Michael Reaction in a Continuous Nanofiltration Membrane Reactor

Scheme 55. Recycling Process of Unmodified Homogeneous Metathesis Catalyst by a Column Chromatography Strategy

polar solvent EtOAc afforded the recovery of the homogeneous metathesis catalyst. This process has been successfully applied to very small scaled reactions; hence, it may be found valuable for synthesizing small quantities of pure organic compounds for biological screening. 2.8.2. PEG-400 Phase. Poly(ethylene glycol)s (PEGs), environmentally benign green solvents, have attracted considerable attention because of their nontoxicity, low vapor pressures, availability, low price, recyclability and thermal stability, as well as their immiscibility with a number of organic solvents to facilitate phase separation after the reaction.147−150 A variety of unmodified homogeneous complex catalysts are very easily dissolved in the PEG phase, and by using the catalystcontaining homogeneous PEG phase, a high activity of homogeneous catalysis can be realized.215−221 Even more important, after the reaction, the product can be separated by simple liquid−liquid extraction, and the catalyst-dissolved PEG phase can be reused for the next run. SanMartin and co-workers

described the utilization of the NCN-type pincer palladium complex 145 as an exceptionally active catalyst for aerobic oxidation in a sustainable PEG-400 phase (Scheme 56).151 After the reaction, a low-temperature extraction (diethyl ether was Z


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oxygen. To summarize, oligomeric complexes with two or more units usually have an enhanced polarity compared with mononuclear analogues, and it is precisely because of this enhanced polarity that these complex catalysts can be easily recycled through the simple strategy of precipitation by using a nonpolar solvent. This simple and practical recycling strategy will definitely reduce the cost of valuable complex catalysts.

Scheme 56. Aerobic Oxidation by an Unmodified NCN-Pd Pincer Complex in a PEG Phase

3. IMMOBILIZED (HETEROGENIZED) NHC COMPLEX CATALYST The immobilization (heterogenization) of homogeneous smallmolecule catalysts represents a resurging research area in catalysis, which has shown constant progress in recent years. To immobilize NHC metal complex catalysts, it is the same situation. N-Heterocyclic carbenes, which are characterized with their unique σ-donating property, can form stronger bonds (carbon−metal bonds) with most transition metals than can classical phosphine ligands. Moreover, NHC metal complex catalysts have proven to be more effective than phosphine metal complex catalysts in many catalytic reactions. Therefore, the immobilization of homogeneous NHC metal complexes, which not only facilitates the separation process from the product mixture but also realizes catalyst recovery and recycling, has become a promising area of research both in academia and for industrially important processes. In this section, according to the different support materials, the recyclable immobilized NHC metal complex catalysts are divided into five categories: silicabased catalysts, polymer-based catalysts, carbon-based catalysts, MNP-based (MNPs, magnetic nanoparticles) catalysts, and other unconventional support catalysts (Figure 4). We can conclude from the pie chart in Figure 4 that silica-based catalysts account for half of all the reported studies, with the second being polymer-based catalysts accounting for 34%. Carbon-based catalysts and MNP-based catalysts account for 8 and 7% of the reported publications, respectively. Silica, polymer, carbon, and MNPs constitute the four main support materials (99%) for recyclable immobilized NHC complex catalysts. In the space below, we will briefly introduce the synthetic methods and recyclable catalytic applications of the four major categories of recyclable immobilized NHC metal complexes.

added to the reaction mixture, the mixture was cooled to −78 °C, and the layers were decanted) was used to provide an effective catalyst separation from the final ketone products. By employing this workup procedure, five runs (nearly quantitative yields) were performed with the PEG-400 phase containing the active catalytic species. 2.8.3. Precipitation Strategy. Complex catalysts with medium to strong polarity can precipitate from some nonpolar or low-polar solvents, such as diethyl ether and n-hexane. Therefore, the polar catalyst can be separated through a precipitation strategy, while the product is dissolved in the low polar solvent. This simple method can be very useful in some circumstances, especially for some polar unmodified complex catalysts. In 2013, Cao and co-workers described a series of simple and efficient NHC-Pd complexes as reusable homogeneous catalysts for the hydroamination reaction, which relies on the strategy of precipitation (Scheme 57).222 Among them, the propylene-bridged NHC-Pd bromide complex 146 displayed the best activity for the hydroamination reaction. After the reaction, diethyl ether was added to the mixture to precipitate the NHCPd complex, and the recovered catalyst could be reused the next run. The catalyst could be used for at least three times without the loss of its activity; however, further runs led to a decrease in activity, and this could probably be ascribed to the catalyst loss during the recovery process. Other representative neutral complex catalysts which were recycled through a simple precipitation strategy are shown in Scheme 58. Salen-metal complexes (147−151), especially the dimer, trimer, or tetramer Salen complexes, have often been recycled through a precipitation method because of the enhanced polarity of the oligomeric complexes. As a well-known type of chiral catalysts, these recyclable Salen complexes were used in the enantioselective epoxidation of alkenes223,224 and asymmetric synthesis of O-acetylcyanohydrins.225 The Salen-Zn trimer complex, as well as the tetraoxo-coordinated zinc catalyst, could be exploited as highly efficient Lewis acid catalysts for the cycloaddition reaction of CO2 to epoxides.226,227 The dimer phosphine-coordinated palladium complex was employed as an efficient and reusable catalyst for Suzuki reactions.228 In addition, a tetranuclear vanadium complex229 was applied as a recyclable catalyst for the oxidation of benzyl alcohols with molecular

3.1. Silica-Based Immobilization of NHC Complex Catalysts

Silica materials can be generally divided into amorphous silica and ordered silica. The term “amorphous silica” usually refers to silica gel, which does not possess long-range crystalline order. Silica gel has economic advantages such as low cost, accessibility, and mass production; moreover, beneficial physicochemical properties for heterogeneous catalysis such as a moderate specific surface area and average pore diameter are usually satisfied by silica gel. Therefore, silica gel can be frequently used as a catalyst support for various immobilized homogeneous molecular catalysts. Ordered silicas,230 which are characterized by a long-

Scheme 57. Recycling Study of Unmodified NHC-Pd Complex Catalyst for Hydroamination of Phenylacetylene with Aniline

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Scheme 58. Other Representative Complex Catalysts Recycled by Precipitation Strategy

strategy and the self-supported catalyst strategy (sol−gel process) account for 21 and 11% of the published literature, respectively (Figure 5).

Figure 4. Recyclable immobilized NHC metal complexes.

range crystalline order, such as the famous MCM-41 (Mobil Composition of Matter, No. 41) and SBA-15231 (Santa Barbara Amorphous, No. 15), usually possess a regular mesoporous structure and a tunable high specific surface area.232 This favorable pore structure and high surface area will certainly facilitate mass-transfer, adsorption, and activation processes in catalysis, especially for some relatively large sized molecules. In this section, according to the different immobilization strategies, recyclable silica-based immobilized NHC metal catalysts are further divided into four classes: the major class is the covalent grafting strategy, which accounts for 44%; the second class is the solid phase synthesis strategy (24%); the noncovalent interaction

Figure 5. Different immobilization strategies for recyclable silica-based heterogenized NHC complex catalysts.

3.1.1. Covalent Grafting Strategy. The grafting of metal complexes or catalytic ligand precursors to an insoluble support through a covalent bond has been by far one of the most frequently used strategies for the heterogenization (immobilization) of homogeneous catalysts. This traditional strategy provides the strongest chemical bond between a catalytically active species and the support with diversified synthetic routes. AB


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After 2006, with respect to NHC metal complex catalysts, the most reported grafting method is covalent grafting via trialkoxysilyl groups.233 In a typical grafting procedure, a trialkoxysilyl-functionalized NHC metal complex catalyst is grafted to a surface-modified (usually by hydroxy groups) solid support according to well-established organic chemical reactions between the functional groups to form stable Si−O bonds. The introduction of trialkoxysilyl functionalities to homogeneous catalysts offers an effective and widely applicable binding strategy that can be employed for their immobilization by covalent grafting onto insoluble solid supports, which are generally modified with surface hydroxyl groups. In this section, we will mainly introduce the covalent grafting strategy via trialkoxysilyl groups of recyclable NHC complex catalysts. Other grafting reactions, such as the ether bond formation reaction, will also be mentioned in passing. In 2006, Artok’s group developed an amorphous silicaimmobilized saturated NHC-Pd complex catalyst,234 and this heterogenized catalyst (152) exhibited an enhanced activity (TON up to 104−105) for Heck reactions, which was better than the original homogeneous analogue (Scheme 59). The

Scheme 60. Silica-Nanoparticle-Immobilized NHC-Pd Complex Catalyst and Its Catalytic Application in Suzuki Reactions

NHC-Pd complex catalyst. The catalyst could be recovered and reused for five cycles without a loss of productivity. However, the recycling experiment was conducted based on an isolated yield of >90%, which cannot accurately demonstrate the stability of the immobilized catalyst. The right method was to do the recycling experiment at a conversion rate of approximately 50%. In 2007, Jin and co-workers described a NHC-Pd complex immobilized on a commercially available amorphous silica gel (with a high surface area of 550 m2/g) (155),236 and this catalyst showed a high catalytic activity for Suzuki reactions in an aqueous medium (Scheme 61). As shown in Scheme 61, two routes were attempted to synthesize the immobilized catalyst, and method A for the immobilization of a triethoxysilylated ionic liquid onto silica gel, followed by coordination with Pd(OAc)2, was proven to be effective, while the method B synthesis of triethoxysilylated NHC-Pd complex 154 was first demonstrated as inefficient because of the low solubility of triethoxysilylated NHC-Pd complex. The catalyst could be simply recovered and reused six times without an apparent loss of activity. Similar to Sen’s work (Scheme 60), the problematic recycling experiment was conducted based on a gas chromatography yield of approximately 100%, which cannot prove the stability of the heterogenized catalyst. In 2009, Grubbs’ group described the synthesis of two silicagel-immobilized second-generation ruthenium olefin metathesis catalysts (156 and 157, Scheme 62).237 The two triethoxysilylfunctionalized (different triethoxysilyl group substitution positions) second-generation NHC-Ru olefin metathesis catalysts were synthesized and then grafted onto silica gel to afford the final heterogeneous silica-supported metathesis catalysts. These immobilized catalysts were shown to be competent catalysts for a number of olefin metathesis reactions, mimicking their homogeneous analogues. With regard to the RCM reactions, catalyst 156 could be recovered and reused for eight cycles without a significant loss of activity. More importantly, nearly no leaching of ruthenium under the standard reaction conditions was detected, as revealed by the ICP-MS analysis of the filtered reaction solutions (Ru concentration of filtrate, 90% at the beginning of the run, and clearly, this is not applicable and should be corrected. A yield of approximately 50% is suitable for testing the real stability of the catalyst. Oro and co-workers described the MCM-41-supported NHCRh(I)-complex-catalyzed hydrosilylation reaction of acetophenone and its derivatives (Scheme 74).255 The authors compared the activity of the supported catalyst 193 and the homogeneous counterpart. The homogeneous catalyst was relatively more active (TOF = 31.7 h−1) than the heterogeneous catalyst (TOF value = 27.5 h−1); however, the heterogeneous catalyst could be reused several cycles. Both catalysts yielded polymer products with a high molecular weight (Mw = 94 000 g mol−1) and a narrow molecular weight distribution (PDI = 1.5−1.7). In 2014, the same group of Oro reported new homogeneous and heterogeneous NHC-Rh(I) complexes and their catalytic hydrosilylation applications (Scheme 75).256 They found the 2,6diisopropylphenyl-substituted NHC-Rh(I) complex showed the highest activity toward hydrosilylation reactions compared with other substituted NHC-Rh(I) complexes. Therefore, the 2,6diisopropylphenyl-substituted NHC-Rh(I) complex was immobilized onto mesoporous MCM-41 and KIT-6 materials to afford the heterogeneous supported catalysts 194 and 195, respectively. The heterogeneous catalysts also exhibited excellent activities in the synthesis of poly(silyl ether). Regrettably, their catalytic efficiencies decreased after three cycles, and the authors claimed

Table 1. Miscellaneous Catalytic Reactions Mediated by Amorphous-Silica-Gel-Immobilized NHC Metal Catalysts reaction type

metal

ref

Suzuki coupling Sonogashira coupling Negishi coupling olefin metathesis alkene metathesis click reaction alkyne hydration cycloisomerization hydroarylation

Pd Pd Pd Ru W Cu Au Au Au

234−236, 242, 243 242 244 237, 238, 241 245 239, 240 246 246 246

From 2006 to 2012, Iglesias et al. reported a series of mesoporous MCM-41-supported NHC complex catalysts, including catalysts of NHC complexed with Ru, Au, Rh, and Pd and investigated their catalytic effects for reactions such as the asymmetric hydrogenation of alkenes and olefins, dehydrogenation reaction of a primary alcohol to an ester, and the multicomponent coupling reactions of aldehydes, terminal alkynes, and amines. Interestingly, some of the heterogenized NHC metal catalysts showed superior catalytic activities compared with those of their homogeneous analogues. Moreover, in most cases, when MCM-41 (with an ordered mesoporous structure) was used as a solid support, the catalytic activity was proven higher than that of an ordinary amorphoussilica-gel-supported complex catalyst, which demonstrated the superiority of ordered mesoporous materials in the field of heterogeneous catalysis. Next, let us introduce a typical example of Iglesias’ work. In 2012, they reported a series of multisite, MCM-41-supported solid (NHC)NN-Ru catalysts (184−187) and their catalytic application in the synthesis of secondary amines from nitro compounds (Scheme 72).253 The authors claimed the MCM-41-supported Ru complex catalysts functioned as bifunctional heterogeneous catalysts that chemoselectively transformed the nitro group to the amino group, which subsequently underwent condensation with the aldehyde at the acid sites of the hybrid catalyst, and, finally, partook in the catalytic hydrogenation of imine to yield the product amine. The bifunctional catalyst could be easily recovered and reused five

Scheme 72. Immobilization of Trialkoxysilyl-Ru Complexes on MCM-41, MCM-41/Al, and MCM-41/Sn and Their Catalytic Reductive Amination Applications

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Table 2. Recyclable Mesoporous-Silica-Supported NHC-Metal Catalysts Reported by Iglesias’ Group

typically ranging from 690 to 1040 m2/g. It was first prepared by Stucky et al. at the University of California, Santa Barbara. SBA15 has large pore sizes ranging from 4.6 to 30 nm compared to those of MCM-41 which possesses pore sizes from 1.5 to 6.5 nm. Furthermore, SBA-15 has pore walls (3.1−6.4 nm) that are thicker than those of MCM-41 (90%, which is improper from a professional catalysis perspective. In 2015, Pahlevanneshan et al. reported a solid phase synthesis of heterogeneous chelated NHC-Pd(II) catalyst 206, which started from 3-chloropropylated nanosilica (Scheme 83).265 The

Table 3. Recyclable Mesoporous Silica-Supported NHCMetal Catalysts reaction type reductive amination hydrogenation of alkenes dehydrogenation of primary alcohol to ester hydrogenation of olefins A3 coupling Suzuki coupling hydroformylation reaction hydrosilylation of acetophenone click reaction alkylation of amines and alcohols hydrogenation of olefin hydrothiolation of alkynes Hiyama reaction Suzuki coupling Heck coupling

support

ref

Ru Au, Rh Ru

metal

MCM-41 MCM-41 MCM-41

253 247, 248 249

Au, Rh, Pd, Ru Au Pd Rh Rh Cu Ir

MCM-41 MCM-41 MCM-41 MCM-41 MCM-41 MCM-41 SBA-15

250, 251 252 254 254 255, 256 257 258

Rh Cu Pd Pd Pd

SBA-15 SBA-15 SBA-15 SBA-16 SBA-16

259 260 261 263 263

Scheme 83. Nano-SiO2-Supported Chelating NHC-Pd(II) Complex Catalyst

immobilized catalyst was applied as a heterogeneous catalyst for Suzuki reactions. The heterogenized catalyst displayed a high activity compared with that of its homogeneous counterpart, and moreover, it could be recovered and reused five times without an appreciable loss of activity. Nevertheless, such an immobilization process is a little bit complicated, and the production cost also needs to be considered. Recently, Gogoi and co-workers described the preparation of silica-immobilized symmetric bis(NHC) palladium complex 207 through a solid synthesis strategy (Scheme 84),266 and this

functionalized NHC complex catalysts that are attached to the silica support by forming covalent ether groups through a dehydration reaction. It is believed that, in the near future, other simple and efficient reactions, such as click reactions, will be exploited in the covalent grafting strategy, keeping pace with the development of sophisticated surface modifications and NHC complex syntheses. 3.1.2. Solid Phase Synthesis Strategy. This context of solid phase synthesis generally refers to a synthetic process in which the formation of the NHC species or precursor is performed on the insoluble support by a quaternization or cyclization process. As is well-known, NHC precursors can be synthesized by a single-step quaternization of N-substituted imidazoles with alkyl halides in high efficiency, and the employment of solidified alkyl halides is an easy and feasible method. In 2008, Wang et al. described a solid phase synthesis of silica-immobilized NHC-Cu(I) catalyst 205 and its catalytic application in A3 (aldehyde−alkyne−amine) coupling and [3 + 2] (organic azides and terminal alkynes) cycloaddition reactions (Scheme 82).264 This heterogenized catalyst exhibited a high stability toward both reactions, and it could be quantitatively

Scheme 84. Silica-Immobilized Symmetric Bis(NHC) Palladium Complex Catalyst

Scheme 82. Solid Phase Synthesis of SiO2-Supported NHCCu(I) Complex Catalyst and Its Catalytic Application heterogeneous catalyst was applied as a highly efficient (down to 0.03 mol % Pd loading) and recyclable catalyst (reused up to six times without a loss of its activity) for Suzuki coupling reactions in an aqueous medium. Moreover, the authors performed some theoretical calculations to better understand the mechanism, which was a beneficial attempt for such a research work. The drawback of this work is the whole structure of the immobilized catalyst seems too complicated, which is unfavorable for cost control. In addition to common silica gel, mesoporous silica materials with a long-range order, such as SBA-15, have also been exploited as starting materials for solid phase synthesis. In 2013, Wu and co-workers developed a series of SBA-15-immobilized Grubbstype catalysts for heterogeneous olefin metathesis reactions (Scheme 85).267 Four SBA-15 materials with different pore sizes were synthesized and chosen as the starting material. Then, these materials were treated with 3-aminopropyl triethoxysilane to AM


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Scheme 85. SBA-15-Immobilized Grubbs-Type Catalysts

Scheme 86. MCF-Immobilized Grubbs−Hoveyda-Type Catalysts and Their Catalytic RCM Applications

In 2009, the same group described the solid phase synthesis of MCF-supported Grubbs−Hoveyda-type catalyst 212, and its continuous catalytic RCM application by use of a circulating flow reactor (Scheme 87).270 The authors found the in situ generated

afford the amino-functionalized SBA-15 materials. This was followed by reaction with 3-(3-vinyl-4-isopropoxyphenyl) propionic acid to afford Hoveyda-type-ligand-modified SBA-15 materials. Finally, in the presence of CuCl, the final SBA-15immobilized Grubbs-type catalysts were synthesized by reactions of the Hoveyda-type-ligand-modified SBA-15 and Grubbs’ first or second homogeneous catalyst. These immobilized Grubbs− Hoveyda-type catalysts showed good activities in RCM, as well as other metathesis reactions. The SBA-15-immobilized Grubbs− Hoveyda-type catalyst with the larger pore size displayed higher activity, and the authors ascribed this phenomenon to the better diffusion of reactants and products through the pore channels of the SBA materials. Furthermore, the authors claimed that the SBA-15 samples prepared at high temperature that displayed a high hydrophobicity and fewer silanol groups were favorable for metathesis reactions. These heterogeneous catalysts could be recovered and reused several times with a simple filtration procedure. Although the final heterogenized catalysts were successfully prepared, the solid phase synthetic steps in this case were too complicated to be useful in a real production. Siliceous mesocellular foam (MCF) is a kind of stable mesoporous silica with cell-like ultralarge pores of 24−42 nm that are interconnected by windows of 9−22 nm.268 This relatively new silica-based support material has a large specific surface area (SBET > 561 m2/g) and pore volume and, therefore, is well-suited for supporting bulky complex catalysts and facilitating diffusion of substrate and products. In 2007, Ying’s group developed MCF-supported Grubbs−Hoveyda-type catalysts (210 and 211) through a solid phase synthesis strategy (Scheme 86),269 and these heterogenized Grubbs−Hoveyda-type catalysts were applied in the RCM reaction. The authors claimed that the partial precapping of silanol groups with TMS groups and the postcapping of the residual silanol groups with TMS groups would improve the stability and activity of the MCF-supported catalysts. Furthermore, they found the introduction of additional MCF-supported free ligands to the reaction system significantly enhanced the reusability of the heterogeneous catalysts. These immobilized catalysts could be reused for more than 10 runs with only a slight loss of activity.

Scheme 87. MCF-Immobilized Grubbs−Hoveyda-Type Catalyst and Its Catalytic Application Using a Circulating Flow Reactor [Adapted with permission from ref 270. Copyright 2009 Royal Society of Chemistry.]

ethylene during the RCM reaction played a role in catalyst deactivation, and ethylene solubility in the reaction system adversely affected the catalytic conversion. However, the specially designed circulating flow reactor could expel the gaseous ethylene and, thus, minimize the deactivation of the catalyst. The MCF-immobilized Grubbs−Hoveyda-type catalyst AN


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nanoparticles formed after the first run. TEM characterization is a good technique to answer this question and will help us to find the facts directly. In 2012, Pleixats and co-workers introduced the porous-silicaimmobilized Hoveyda−Grubbs catalyst 214 (Scheme 89).275

was demonstrated as a long-life catalyst by employing the circulating flow reactor. This work provides a new perspective, i.e., the improvement of the stability of a catalyst through elegant process design, for the development of a sustainable catalytic process. To conclude, solid phase syntheses allow the facile separation of the solid support-bound functional products from the mixture, which is a powerful supplement to the covalent grafting strategy. However, it should be pointed out, the characterization of solid products is not easy; moreover, the existence of incompletely generated ligands cannot be ignored. Up to now, the most prominent starting silica supports for solid phase synthesis have been those functionalized with alkyl halides, azide groups, and amine groups, which rely on the quaternization reaction, click chemistry, and amidation reaction, respectively. It is foreseeable that, with the continuing development of organic chemistry, various new starting silica supports for solid phase synthesis will emerge in the future. 3.1.3. Sol−Gel Process Strategy. In addition to immobilizing NHC metal complexes on the surface of silica supports, the “bottom-up” synthesis through a sol−gel process 271,272 represents an attractive strategy to attain NHC complexes immobilized on various silica materials. The more cutting-edge sol−gel process is based on the polycondensation of silylmodified NHC complexes with different alkoxysilanes. Compared to other traditional immobilizing processes, it leads to a more homogeneous distribution of active metal species throughout the organic−inorganic hybrid material. Furthermore, the introduction of selected functionalities proceeds in a more controllable approach, and the structure of the resulting materials, can be tailored through the fine-tuning of the reaction conditions of the sol−gel process (e.g., the surfactants selected, the ratio of reactants, reaction solvent, and aging).14 Apparently, the different synthetic parameters can greatly influence the catalytic performance in some cases. In 2007, Polshettiwar et al. reported the NHC-Pd-complexbased hybrid material 213, which was synthesized through the sol−gel process (Scheme 88).273 This material displayed a high

Scheme 89. Hybrid Silica-Immobilized Grubbs−HoveydaType Catalyst

The bis-silylated NHC-Ru complex and tetraethylorthosilicate (TEOS) were self-supported via a sol−gel co-gelification process to afford catalyst 214. The synthesized hybrid material showed a medium specific surface area (SBET = 404 m2/g) and a total pore volume of 0.23 cm3/g. Good activities and recyclability have been realized in RCM reactions by using the heterogeneous catalyst 214. However, the bis-silylated NHC-Ru complex is not readily synthesized, and −78 °C is needed to handle the Grignard reagent, which is not easy to perform under industrial conditions. Later, the same group reported NHC-Pd complex incorporated porous silica materials (215 and 216, Scheme 90).276 The Scheme 90. Hybrid Silica-Immobilized NHC-Pd Catalysts

Scheme 88. NHC-Pd-Incorporated Silica Hybrid Material and Its Catalytic Heck Reactions

monosilylated NHC-Pd complex and TEOS were self-supported through a sol−gel co-gelification process. The afforded hybrid silica materials displayed high surface areas (SBET up to 810 m2/ g) and medium to large mesopores (4.2−12 nm). Both materials could be applied in the catalytic Suzuki, Heck, and Sonogashira reactions. Moreover, these heterogeneous catalysts could be recovered and reused several times without a significant loss of its activity. Nevertheless, the authors also claimed that, at least in part, the molecular Pd0 species and Pd nanoparticles might have also catalyzed these C−C couplings in this case. In 2014, Pleixats et al. reported the preparation of hybrid NHC-Rh catalysts immobilized on silica materials (217−220) by either sol−gel or traditional grafting processes (Scheme 91).277 Through tuning the structures of the NHC-Rh monomers and the amount of TEOS used, three different hybrid silica materials

activity as a heterogeneous catalyst in Heck reactions and could be recovered and reused for at least five cycles with an unchanged catalytic activity. One year later, Polshettiwar et al. reported the hybrid material could also be used as a recyclable heterogeneous catalyst in Suzuki coupling reactions.274 Although the hybrid silica-supported NHC-Pd catalyst displayed a high activity for C−C coupling reactions, it is unclear whether there were any Pd AO


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pentynoic acid (Scheme 92). Interestingly, the heterogeneous hybrid catalyst displayed a significantly better activity than its homogeneous analogue with regard to the rearrangement reaction of allylic esters, and the authors explained that extensive oligomerization was avoided by using the supported NHC-Au complex catalyst. The heterogeneous catalyst could be recovered and reused several times without an appreciable loss of activity. The use of structure-directing agents (SDAs) as a template in the sol−gel process can afford periodic mesoporous hybrid organic−inorganic silica materials, which have long-rangeordered structures with tunable pore sizes and surface areas, and, thus, facilitate the diffusions of substrate and products to enhance the catalytic activity. In 2011, Han’s group developed a family of periodic mesoporous hybrid silica-immobilized NHCPd catalysts (223) through the sol−gel process,279 which mimics the synthetic procedure of MCM-41 (Scheme 93). In the sol−gel process, the SDA of cetyltrimethylammonium bromide (CTAB) was utilized as a template to induce the formation of a periodic mesoporous structure. In principle, this type of material displays a two-dimensional (2D) channel-like hexagonal (p6mm) structure. These hybrid silica-immobilized NHC-Pd complex catalysts showed a high activity and selectivity for the hydrogenation of alkenes and allyl alcohol. The catalyst could be reused three times without the loss of its activity, and the TEM image showed there were no visible Pd nanoparticles on the wall of the silica after the reaction. In addition to 2D channel-like hexagonal mesoporous silica materials, such as SBA-15 and MCM-41, the three-dimensional (3D) cubic mesostructure of SBA-16 (Im3m, body-centered cubic) displays more attractive properties in catalysis because of its 3D cage-like mesoporous network (each spherical cage is connected with eight neighboring cages through entrances). Such a distinctive structure in SBA-16 was demonstrated to facilitate the efficient diffusion of substrate and products. In 2010, Yang et al. developed a family of SBA-16 type hybrid silica catalysts (224) with built-in NHC-Pd species through the sol− gel process in the presence of a mixture of P123 and F127 as a template (Scheme 94).280 A well-ordered 3D mesostructure could be obtained when the NHC loading on the whole material

Scheme 91. NHC-Rh Complex Catalysts Immobilized on Hybrid Silica Materials and Their Catalytic [2 + 2 + 2] Cycloaddition Application

(217−219) were synthesized via the sol−gel process. Another hybrid silica material (220) was prepared by the direct covalent grafting of a triethoxysilane-modified NHC-Rh complex onto the surface of mesoporous SBA-15. These catalysts were exploited to catalyze the [2 + 2 + 2] cycloaddition reactions of a triyne substrate, and there was no pronounced difference with regard to activities. The heterogeneous catalyst could be reused up to six runs; however, the reaction times increased gradually after each cycle, which indicated a gradual decomposition and leaching of these silica-material-immobilized catalysts. In 2016, Pleixats’ group reported the hybrid silica-supported NHC-Au(I) catalyst 221, which was prepared via the same sol− gel process as in their previous works,278 and this heterogeneous catalyst was employed in a series of organic reactions, such as the rearrangement of allylic esters and cycloisomerization of 4-

Scheme 92. Hybrid Silica-Immobilized NHC-Au Complex Catalyst and Its Catalytic Applications

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catalysts displayed improved catalytic activities in Suzuki coupling reactions. A variety of aryl chlorides, even those with deactivated and sterically hindered groups, were successfully coupled under relatively mild conditions. The recyclability test showed the hybrid catalyst could be recovered and reused for up to 10 runs without a significant loss of activity. The combined utilization of the sol−gel process and solid phase synthesis strategies has also been employed to prepare recyclable immobilized NHC metal complex catalyst. In 2008, Thieuleux and co-workers described a synthetic route that combined the sol−gel process and solid phase synthesis to prepare the tailored organometallic−inorganic hybrid mesostructured silica-material-immobilized NHC-Ir catalyst 231 (Scheme 96).282 The cohydrolysis and copolycondensation (sol−gel process) of p-(chloromethyl)phenyltrimethoxysilane and TEOS with the aid of Pluronic P123, as a structure-directing agent, afforded the chloromethylbenzene-functionalized hybrid silica material with a long-range-ordered structure. This was followed by a solid phase synthesis process affording the imidazolium salt modified silica materials, and after the metalation procedure, the final hybrid mesoporous silicaimmobilized NHC-Ir catalyst was obtained. The heterogeneous catalyst was tested in the H/D exchange reaction of acetophenone, and it exhibited a comparable activity with that of the homogeneous analogue. Moreover, the heterogeneous catalyst could be recovered and reused three times without an apparent loss in activity. This kind of composite strategy holds great potential in exploring diverse heterogeneous silica-based catalysts. In 2009, once again, Thieuleux et al. described a combined synthetic strategy of the sol−gel process and solid phase synthesis to afford mesoporous hybrid silica-immobilized Grubbs’ second-type catalyst (232 and 233, Scheme 97).283 First, p-chlorobenzyltrimethoxysilane or 3-iodopropyltriethoxysilane and TEOS in the presence of the structure-directing agent Pluronic P123 were cohydrolyzed and copolycondensed via the sol−gel process to afford the alkyl halide modified mesoporous hybrid materials. Second, through the solid phase synthesis strategy, the alkyl halide modified mesoporous hybrid materials were treated with mesitylimidazole to afford the corresponding imidazolium salt functionalities. Finally, the imidazolium salt functionalized porous hybrid silica material was reacted with the first-generation Grubbs’ catalyst to afford the mesoporous hybrid silica-immobilized Grubbs-type catalysts. These immobilized NHC-Ru catalysts were tested in the metathesis of ethyl oleate, and a high initial TOF of 65 min−1 was achieved for the M-RuPr catalyst. Furthermore, the solid catalyst could be reused many times to afford a high TON value. Two years later, in 2011, Thieuleux et al. reported the hybrid silica-immobilized NHC-Ru catalysts (234 and 235) again via the combined synthetic strategy of the sol−gel process and solid phase synthesis (Scheme 98).284 The imidazolium salt functionalized porous hybrid silica material was synthesized through the sol−gel process and solid phase synthesis strategy, which was similar to their previous work. The subsequent treatment with KHMDS and [RuCl2(p-cymene)]2 afforded the mesoporous silica-immobilized NHC-Rucym catalyst 234. The addition of PMe3 to coordinate and replace the p-cymene ligand produced the immobilized NHC-RuPMe3 catalyst 235. However, there is another possibility; i.e., the addition of PMe3 is expected to displace the chlorido ligand instead of the p-cymene to maintain the 18 electron count. These two immobilized NHC-Ru complex

Scheme 93. Synthesis of Periodic Mesoporous Hybrid SilicaImmobilized NHC-Pd Complex Catalysts via the Sol−Gel Process [Adapted with permission from ref 279. Copyright 2011 Royal Society of Chemistry.]

Scheme 94. SBA-16-Type Hybrid Silica Materials with Builtin NHC-Pd Species Prepared through the Sol−Gel Process [Adapted with permission from ref 280. Copyright 2010 Elsevier.]

was lower than 0.64 mmol/g. These catalysts showed medium to good activity in Suzuki coupling reactions. The recyclability test showed the heterogeneous catalyst could be reused eight times without obvious loss of activity. One year later in 2011, Yang’s group reported another family of 3D hybrid silica materials (225−230) with built-in NHC-Pd complexes via the sol−gel process (Scheme 95).281 NHC species were homogeneously incorporated in the framework of the hybrid materials. These materials exhibited high surface areas (up to 743 m2/g) and relatively large pore volumes (up to 1.15 cm3/ g). The XRD results indicated a less-ordered structure was formed, which may be explained by the presence of a high molar fraction of the bulky NHC ligand in the initial gel dramatically affecting the assembly between the silicon templates and oligomers. Compared with their previous report, this series of AQ


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Scheme 95. Hybrid Silica Materials with Built-in NHC-Pd Catalysts [Adapted with permission from ref 281. Copyright 2011 Royal Society of Chemistry.]

Scheme 96. Hybrid Mesoporous Silica-Immobilized NHC-Ir Catalyst Prepared through the Combined Strategy of the Sol−Gel Process and Solid Phase Synthesis, and Its Catalytic H/D Exchange Reaction

series of modern nuclear magnetic resonance (NMR) technologies, the authors demonstrated that secondary interactions were responsible for the stability of the heterogeneous hybrid silica-immobilized catalyst. The secondary interactions, which could be described as interactions between the active site (Ru metal center) and the surface functionality of the silica support, were deemed to supply key stabilization to the metal center. However, this kind of stabilization effect was present only when the Ru metal center was attached to the silica surface via a flexible linker (a propyl group, 237), which allowed the metal center to either react with the alkene substrate or coordinate to the surface groups of the silica support, consequently providing stability. In sharp contrast, the utilization of a rigid linker (in this

catalysts were applied in the catalytic hydrogenation of CO2 in the presence of amine. The immobilized NHC-RuPMe3 catalyst displayed a higher catalytic activity than that of the immobilized NHC-Rucym catalyst, and the author ascribed this excellent activity to the beneficial effect of the basic trimethylphosphine ligands. Unfortunately, these heterogeneous catalysts suffered from metal leaching in the catalytic process, and further studies to improve the stability of the catalyst are needed with regard to this reaction. In 2013, Thieuleux et al. compared the activities of two hybrid silica-supported NHC-Ru metathesis catalysts (236 and 237), which were prepared from a combined strategy of the sol−gel process and solid phase synthesis (Scheme 99).285 By using a AR


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Scheme 97. Hybrid Silica-Immobilized Grubbs’ Second-Type Catalyst

recycled for up to seven runs without the loss of activity. The supported NHC-Ir catalyst 238 was used to catalyze the H/D exchange of acetophenone with AgOTf as an activation agent for the catalyst. The heterogeneous catalyst 238 exhibited comparable activity with the homogeneous analogue, and the recycling experiment showed that the yield decreased after three cycles. Furthermore, the supported ruthenium catalysts 241 and 242 were applied in the hydrogenation reaction of CO2 in the presence of H2 and pyrrolidine. Compared with catalyst 241, the phosphine-ligand-coordinated ruthenium catalyst 242 displayed an improved stability and allowed a 2900 TON to be obtained. Furthermore, the heterogeneous hybrid silica-supported NHCPd catalyst 243 was employed in the Z-selective hydrogenation reaction of alkynes, and the heterogeneous catalyst displayed a substantially better activity and selectivity than the homogeneous NHC-Pd complex catalyst. The authors claimed that the combined synthetic strategy of the sol−gel process and solid phase synthesis allowed the tuning of metal binding fragments that can mimic the catalytic behavior of homogeneous complex catalysts. The reported synthetic strategy together with DNPSENS may provide a new perspective for the construction of quantitative structure−activity relationships of heterogeneous molecular catalysts. In 2015, Thieuleux et al. described the long-range-ordered hybrid silica-immobilized NHC-Ir catalyst 245 via the combined strategy of the sol−gel process and solid phase synthesis followed by modification via surface organometallic chemistry (Scheme 101).287 This synthetic methodology provided the isolated, homogeneously distributed active NHC-Ir sites, which was in a fashion consistent with their previous research works. The advanced analysis of DNPSENS demonstrated that the Ir sites were stabilized by the silica surface via secondary interactions. Astonishingly, the catalytic activity (TOF or TON) of the hydrogenation reaction of the supported catalysts was 1−2 orders magnitude higher than those of their homogeneous analogues, and the authors claimed that this enhanced activity and stability could be ascribed to the inhibition of the bimolecular deactivation of Ir complexes commonly observed in homogeneous solutions.288 Evidently, this kind of heterogeneous catalyst could be recovered and reused, albeit the authors did not study the recyclability property in this work. These recyclable silica-supported NHC metal catalysts are classified and listed in Table 4 in the interest of a ready reference. Concluding the results reviewed, hybrid silica materials, which can be synthesized via the sol−gel process, allow the incorporation of the well-defined metal complex catalysts with a homogeneous distribution. Furthermore, the use of a SDA as a

Scheme 98. Hybrid Silica-Immobilized NHC-Ru Catalysts and Their Catalytic Applications

case, mesityl phenyl, 237) led to the active metal center far from the surface of the support; therefore, the stabilization effect was absent from the surface support, and the stabilization effect from the phosphine ligand was not sufficient to stabilize the Ru center, thus leading to the faster decomposition of the catalysts. This research opens new perspectives in the design of immobilized catalysts with excellent stability and high activity by exploiting secondary interactions between the active metal center and the surface of the support. One year later in 2014, Thieuleux et al. reported a series of NHC metal complexes immobilized on mesostructured hybrid organic-silica materials (238−243) through the combined strategy of sol−gel synthesis and solid phase synthesis (Scheme 100).286 Because of the introduction of a SDA to the sol−gel process, these synthesized hybrid silica materials displayed a long-range-ordered structure, which was characterized by TEM and small-angle XRD techniques. With the aid of dynamic nuclear polarization surface-enhanced solid-state NMR spectroscopy (DNPSENS), the surface metal interactions were observed for the hybrid silica-supported NHC metal catalysts with flexible linkers, and this suggested that surface interactions could be a general phenomenon for these materials that have flexible linkers. The catalysts 239 and 240 were tested for the metathesis reaction of ethyl oleate, and these catalysts could be AS


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Scheme 99. Two Types of Hybrid Silica-Supported NHC-Ru Metathesis Catalysts and a Representative Synthetic Route of the Rigid Linked Catalyst

workers described the synthesis of pyridinium-tagged NHC-Ru complex 246, which was further immobilized onto the surface of sulfonated silica to afford the supported metathesis catalyst 247 via electrostatic interactions (Scheme 102).289 In the immobilization process, a solution of complex 247 was mixed with a silica-based resin functionalized with the lithium p-toluenesulfonate and stirred at room temperature to conduct a cationic exchange reaction. The final supported catalyst was tested, and a result of 0.09 mmol of Ru/g was obtained. This value was higher than the typical loading of those immobilized catalytic materials obtained from the physisorption method; thereby electrostatic force was considered as the main interaction response for this immobilization. The heterogeneous catalyst could be applied in both batch and continuous flow RCM reactions as a recyclable catalyst, albeit a slight loss of activity was observed after each run. In 2014, Balcar and co-workers reported a series of mesoporous silica-supported Grubbs-type catalysts, in which there were strong electrostatic interactions between the ionic complex catalysts and silanol groups on the surface of the silica support (Scheme 103).290 The immobilization process was very simple: the ammonium-tagged Grubbs−Hoveyda Ru catalyst and the mesoporous silica materials were mixed in CH2Cl2 and stirred for hours to afford the final supported catalysts. The catalytic metathesis activity of these supported materials was found to increase with the pore size of the silica support employed, and the activity order of the support was SBA-15 > MCM-41 > ordinary silica gel, with the best results observed for SBA-15 catalyst 248. An influence of the counterion present in the ruthenium complex on the activity of the supported catalysts was also discovered: the chloride-containing ion displayed the highest activity. Additionally, the authors indicated that this may have been attributed to the high electronegativity of Cl compared with those other elements, such as I. Moreover, in some cases,

template (usually a block copolymer containing hydrophilic units and hydrophobic units, such as Pluronic P123 and F127), can afford periodic mesoporous structures with high surface areas and large pore volumes, which facilitate molecular diffusion. The exploitation of the combined strategy of the sol−gel process and solid phase synthesis have further expanded the scope of hybrid silica-based catalytic materials. These research works have prompted us to approach heterogeneous catalysis by means of molecular principles. 3.1.4. Noncovalent Interaction Strategy. In addition to the immobilization strategies mentioned above, other heterogenization methods with relatively weak interaction modes between the solid support and the NHC metal species such as electrostatic effects, entrapment, and adsorption have also been reported. Although the strategy of noncovalent interaction is less frequently employed because of the relatively weak interactions, it can still be a practical method for the heterogenization of NHC metal complexes under some circumstances. Compared with the above-mentioned strategies, the advantages of the noncovalent interaction strategy, especially for the methods of entrapment and adsorption, can be elucidated in two aspects. First, a particular modification or design of the target homogeneous complex or organic ligand may not be needed, which implies a direct immobilization of homogeneous catalysts is feasible. Second, the implementation of this strategy is relatively simple and effective, which considerably smooths the progress of the immobilization process, thus leading to its easy adaptation for large-scale industrial applications. Among noncovalent interaction strategies, immobilization through electrostatic interaction is a frequently used noncovalent heterogenization approach, where charged catalysts are, generally, deposited on solid supporting materials of opposite charge through an exchange reaction.14 In 2012, Mauduit and coAT


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Scheme 100. Hybrid Silica-Immobilized NHC-Metal Complex Catalyst

Scheme 101. Hybrid Silica-Immobilized NHC-Ir Catalysts

this kind of immobilized catalyst showed a higher conversion

weaker than covalent bonding. Similar catalysts were reported later by the same group. One year later, in 2015, Thiel and co-workers developed periodic mesoporous organosilica (PMO) immobilized NHCPd catalyst 249 via the electrostatic interaction strategy (Scheme

than did the corresponding homogeneous system. Evidently, these immobilized catalysts could be recovered and reused several times, albeit the electrostatic interaction is relatively AU


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Table 4. Recyclable Silica-Supported NHC-Metal Catalysts Prepared through the Sol−Gel Process and Their Catalytic Applications support

metal

reaction type

amorphous silica amorphous silica amorphous silica amorphous silica amorphous silica

Pd Pd Pd Ru Rh

273, 276 274, 276 276 275 277

amorphous silica

Au

amorphous silica long-range-ordered silica long-range-ordered silica long-range-ordered silica long-range-ordered silica long-range-ordered silica

Au Pd

Heck coupling Suzuki coupling Sonogashira coupling metathesis (RCM) [2 + 2 + 2] cycloaddition of triyne rearrangement of allylic esters cycloisomerization hydrogenation reactions

Pd

Suzuki coupling

280, 281

Ir

H/D exchange reaction

282, 286

Ru

metathesis reaction

283, 285, 286

Ru

hydrogenation of CO2

284, 286

Scheme 104. PMO-Immobilized NHC-Pd Complex Catalyst

ref

278 278 279, 287

could be reused for at least six runs without a loss of its activity. However, the aminophenyl-functionalized NHC-Pd complex is rather complicated to synthesize, and its significance is limited to academic value. In 2014, Schulz and co-workers described the immobilization of an NHC-Ru metathesis catalyst by the charge-transfer interaction between 2,4,7-trinitrofluoren-9-one (TNF) and anthracene (Scheme 105).292 In the immobilization process, Scheme 105. Silica-Supported NHC-Ru Metathesis Catalyst Immobilized through Charge-Transfer Interactions

Scheme 102. Sulfonated Silica-Immobilized Ru Metathesis Catalyst and Its Catalytic RCM Reaction

the anthracenyl-functionalized NHC-Ru complex was mixed with a TNF-modified silica support in a mixed solvent system of hexane and dichloromethane (molar ratio of 1/3) and stirred for 10 min to afford the final supported catalyst (250). The asprepared solid catalyst was analyzed with 77% of the introduced NHC-Ru catalyst immobilized on the silica support. This heterogeneous catalyst could be recovered and reused at least three times without an apparent loss in activity. It should be pointed out that this kind of charge-transfer interaction is a reversible process; when increasing the molar ratio of the NHCRu complex and TNF-modified silica support to 1/9, the recyclability was substantially improved due to the easier formation of the charge-transfer complex. In addition to the evident electrostatic interactions, the simple adsorption of small-molecular catalysts onto the surface of a solid support, which is commonly unmodified and commercially available, through weak physical interactions has also been used as a method for the immobilization of homogeneous catalysts. Apparently, the procedure for this method is just as simple as the traditional impregnation method in heterogeneous catalysis, which makes these catalysts more accessible for industrial applications. In 2008, Jacobs and co-workers reported a silicapellet-immobilized second-generation Hoveyda−Grubbs catalyst via a simple adsorption method (Scheme 106).293 Supported catalyst 251 was efficient in a variety of metathesis reactions and stable for a TON value of at least 4000. The split test confirmed the heterogeneous nature of the supported catalyst. Finally, the authors proposed an anchoring mechanism, whereby there was

Scheme 103. Immobilized Ammonium Tagged Grubbs− Hoveyda-Type Complex

104).291 The authors claimed that the electrostatic grafting method allowed the catalyst to interact more flexibly on the surface of the support compared with the covalent anchorage. The immobilized catalyst displayed a high activity in Suzuki coupling reactions under mild conditions. The leaching test confirmed the heterogeneity of the catalytic system, and the authors assumed that the active species were stabilized by the basic surface sites. The recyclability test showed the catalyst AV


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Scheme 106. Preparation of Silica-Pellet-Supported Hoveyda−Grubbs Catalyst [Adapted with permission from ref 293. Copyright 2008 John Wiley & Sons.]

the direct attachment of the Ru center to the surface silanols of the silica support via ligand exchange. In 2012, Limbach et al. reported the heterogenization of a series of new NHC-Ru complexes (252−255) and tested their catalytic activities and reusability with regard to metathesis reactions (Scheme 107).294 These catalysts showed excellent

Scheme 108. Recycling Results of an RCM Reaction over Different Supported Catalysts

Scheme 107. Preparation of Silica-Immobilized NHC-Ru Catalysts via Physisorption

of the mesoporous silica materials. The authors claimed that the high stability of the SBA-1-immobilized catalyst could be attributed to the confinement effect of the isolated cage of SBA-1 in preventing the formation of an inactive dinuclear Ru complex. Apparently, this method for stabilizing the mononuclear metal complex by the isolated nanocage can be transferred to other catalytic systems. In 2015, Yang’s group described the encapsulation of a Hoveyda−Grubbs catalyst within a yolk−shell-structured silica for heterogeneous olefin metathesis reactions (Scheme 109).297 Yolk−shell-structured mesoporous materials, possessing a voided space between the outer shell and the core of the material, are emerging as an appealing class of hollow nanoarchitectures.298 The enclosed space can be used to accommodate various guest catalytic molecules and nanoparticles, while the shell with mesopores provides channels for the substrate to diffuse into the voided space. In this work, the authors employed a two-step method, namely, molecular catalyst permeation and a subsequent reduction of shell pores through the silylation of dichlorodiphenylsilane, to afford encapsulated Grubbs−Hoveyda catalyst 256. Because of the highly flexible nature of the molecular catalyst in the voided space, the obtained heterogeneous catalyst displayed excellent activity and recyclability in RCM and CM reactions. Apparently, this kind of yolk− shell mesoporous-structured material can be transferred for use in other catalytic systems for wider applications. Hydrogen bonding between homogeneous molecular catalysts and the silanol groups on the surface of silica supports is considered a weak interaction that is between physisorption and traditional chemical bonding. Under some circumstances, it is

activity in the ring-opening/ring-closing metathesis reaction of cyclooctene, as well as in the self-metathesis of methyl oleate. ICP-MS analysis suggested that ruthenium leaching was low, and the reactions could be considered as truly heterogeneous catalysis, which was further verified by the continuous-flow process. Moreover, the immobilization of NHC-Ru complexes on mesoporous silicas (such as MCM-41 and SBA-15) as heterogeneous metathesis catalysts via the same adsorption method was reported by the same group in 2013.295 Yang’s group described the immobilization of a Hoveyda− Grubbs catalyst on a series of mesoporous silica materials, such as SBA-1, SBA-15, SBA-16, MCM-41, and FDU-1, through the simple method of adsorption.296 These immobilized catalysts exhibited good to excellent recycling abilities for the RCM reaction. As is shown in Scheme 108, the 3D cage-like mesostructured SBA-1 showed the best recyclability (reused at least seven times without a loss of activity) compared with those AW


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Scheme 109. Encapsulation of a Hoveyda−Grubbs Catalyst within a Yolk−Shell Structured Silica Material and Its Catalytic Application [Adapted from ref 297. Copyright 2015 American Chemical Society.]

Scheme 111. Preparation of a Fluorous-Silica-Gel-Supported NHC-Pd Complex Catalyst via Fluorous−Fluorous Interactions

To conclude, because of the significantly easier immobilization process, immobilized catalysts prepared through a noncovalent bonding strategy play an important role, both now and later, in the field of catalyst immobilization. Moreover, under some circumstances, this type of catalyst can display a high activity compared with other immobilized catalysts that are linked through covalent bonds because of the beneficial flexibility afforded to them. Through the appropriate tuning of the reaction systems, these catalysts can also be recycled and reused for many runs. Therefore, similar to the impregnation method in traditional heterogeneous catalysts, the strategy of noncovalent interaction will open up new opportunities for the future.

sufficient enough to minimize the catalyst leaching. In 2015, Schulz et al. reported the direct immobilization of a NHC-Ru complex on silica via hydrogen bonds (Scheme 110).299 This Scheme 110. NHC-Ru Complex Immobilized on Silica via Hydrogen Bonds

3.2. Polymer-Based Immobilization of NHC Complex Catalysts

In addition to various silica materials, insoluble macromolecular polymers are considered to be another major support for catalyst immobilization.74,301−305 Compared with silica materials, polymeric materials are far more versatile because of tunable monomer structures and various polyreactions. Furthermore, it is foreseeable that, with the development of modern organic chemistry and materials chemistry, more and more novel polymeric materials will emerge in the future. Polymersupported NHC complex catalysts can also be divided into four classes, i.e., self-supported, covalent grafting, solid phase synthesis, and noncovalent interaction. Of note, different from silica materials, the self-supported polymeric catalyst is a major class that accounts for 42% (Figure 6), and the second class is solid phase synthesis (31%). The covalent grafting and the noncovalent interaction strategies account for 21 and 6% of the published literature, respectively. 3.2.1. Covalent Grafting Strategy. Because of the flourishing of the surface functionalization of various polymer materials, the immobilization of homogeneous catalysts onto the surface of a polymer support via the covalent grafting strategy has become one of the most important strategies in this area. Various covalent bond formation reactions, such as ether bond formation via a strong base, ester or acid amide formation reaction, Cucatalyzed azide−alkyne cycloaddition reaction,306,307 and radical polymerization, have been exploited to immobilize a variety of NHC metal complex catalysts. In 2008, Buchmeiser et al.

hydrogen bond effect was further evidenced by diffuse reflectance infrared Fourier transform infrared spectroscopy (DRIFTS-FTIR) analysis. More interestingly, the supported catalyst 257 displayed a better activity than its homogeneous congener, and the authors claimed that the formation of hydrogen bonds rendered the facile release of the electrondeficient isopropoxybenzylidene moiety and, thus, speeded the initiation phase. The heterogeneous catalyst could be easily recovered by simple filtration and reused in high yield for five runs. The fluorous−fluorous interaction is another weak interaction that can be used to immobilize homogeneous catalysts.198,199 In 2012, Cai’s group developed fluorous-silica-gel-immobilized perfluoro-tagged molecular NHC-Pd catalyst 259 via fluorous−fluorous interactions (Scheme 111).300 The heterogeneous catalyst was applied in the Suzuki reaction, and through a filtration process, catalyst 259 could be easily recovered and reused three times without an apparent loss in activity. However, metal leaching did exist in the recycling process because of the relatively weak interactions. This fluorous−fluorous interaction provides another option in the field of catalyst immobilization. AX


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Scheme 113. PS-Supported NHC-Rh Complex Catalyst and Its Catalytic Application for Addition of Arylboronic Acids to Aldehydes

Figure 6. Different immobilization strategies for recyclable polymerbased heterogenized NHC complex catalysts.

Scheme 114. PS-Supported NHC-Ag Complex Catalysts and Their Catalytic Applications in A3 Coupling Reactions

developed a (meth)acrylate-based porous polymeric monolithsupported NHC-Cu(I) catalyst 260 through the method of electron-beam-triggered radical graft polymerization (Scheme 112).308 ICP-OES analysis showed the Cu loadings were Scheme 112. Polymeric Monolith-Immobilized NHC-Cu(I) Complexes and Their Catalytic Applications

In 2014, Kühn’s group described the immobilization of a methylene-bridged bis(NHC)-Pd complex to a polystyrene support via the ether bond formation reaction between a hydroxyl group and acyl chloride (Scheme 115).311 Of note, as shown in route 1 of Scheme 115, the direct immobilization of bis(NHC)-Pd complex 265 to a polystyrene support was ineffective (the Pd content was less than 0.1% after the immobilization process) because of the poor solubility of complex 265. However, another route (route 2, Scheme 115), which was through the postcomplexation step, provided a Pd content of 5.6%, as evidenced by AAS analysis. The PSimmobilized NHC-Pd complex 266 showed good activity in the Suzuki reactions, and the heterogeneous catalyst could be recycled and reused for at least four runs without a loss of activity. However, the recycling experiment was conducted based on an NMR yield of approximately 100% in the beginning run, and from a catalytic professional point of view, this is not a rigorous scientific procedure. Moreover, TEM characterization should be performed to detect whether there were any Pd nanoparticles formed after the reactions, for as we know Pd nanoparticles may also catalyze Suzuki reactions. Recently, biopolymers such as cellulose, chitosan, starch, and wool have been employed as supports for catalytic applications because of their biodegradability, their environmental safety, and their chemical versatility. In 2015, Pourjavadi and Habibi described the immobilization of a NHC-Cu(I) complex catalyst onto cellulose via covalent Si−O bonds (Scheme 116).312 The heterogeneous catalyst 267 was active in the synthesis of 1,2,3triazoles through a one-pot reaction of benzyl/alkyl halides or tosylates and terminal alkynes, with sodium azide in water. The

between 1.3 and 1.5 mg/g. The immobilized catalyst was used to catalyze the carbonyl cyanosilylation and hydrosilylation reactions by applying a continuous flow setup. The activity of catalyst 260 remained constant over at least 6 days for the cyanosilylation reaction while the Cu contamination was 2 nm) hydration reactions than that of microporous Au-NHC@POPs3 with a higher surface area (798 m2/g); however, they showed comparable activities for the smaller substrate, i.e., phenylacetylene. Au-NHC@POPs1 was recycled and reused six times with only a slight loss of activity. In 2016, Lu’s group also used the Sonogashira coupling to synthesize a NHC-Cu(I)-based conjugated microporous polymer (CMP) (Scheme 129).340 The afforded CMP-NHCScheme 129. NHC-Cu(I)-Based Conjugated Microporous Polymer and Its Catalytic Hydrosilylation Application

BE


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Scheme 130. NHC-Pd Polymeric-Based Network and Its Catalytic Suzuki Coupling Reaction

Scheme 131. Preparation of PIS and PIS-Pd Catalysts

Scheme 132. Preparation of Fluoro-Functionalized Polymeric NHC-Zn Complex Catalyst and Its Catalytic Methylation of Amines with CO2

was easily recovered and reused without an apparent loss of catalytic activity. The above two reports demonstrated that the direct knitting strategy via the Lewis acid catalyzed Friedel− Crafts reaction of NHC precursors is effective and low cost for

such as the oxidative condensation of indoles, 1,3-dicarbonyl compounds, and phenylglyoxal monohydrate; click reactions; and Ullmann coupling and Glaser coupling reaction (Scheme 134). In all these transformations, the HCP-NHC-Cu catalyst BF


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Scheme 133. Direct Knitting of NHC Precursors via FeCl3-Catalyzed Friedel−Crafts Reaction and Subsequent Metalation with Pd(OAc)2

Scheme 134. Direct Knitting of NHC Precursors via FeCl3-Catalyzed Friedel−Crafts Reaction and Subsequent Metalation with CuCl

synthesized an organometallic microporous polymer (MOMP, 297) with a chemically similar main-chain structure with OMHS via the same strategy of coordination-directed assembly (Scheme 136).347 However, the MOMP exhibited the irregular morphology of an impact porous material (with a low BET surface area of 54 m2/g), which was totally different from the spherical structure of OMHS. The MOMP material was employed as a heterogeneous catalyst for the Suzuki coupling reaction and provided the products with moderate to excellent yields. Furthermore, the MOMP catalyst could be easily recovered and reused for at least four cycles without an apparent loss in activity. Apart from amorphous porous organic polymers, NHC-metalcomplex-incorporated metal−organic frameworks (MOFs) with crystalline structures have also been used as a class of selfsupported heterogeneous catalysts. In general, MOFs have larger

the development of chemically homogeneous but physically heterogeneous POP catalysts. In 2010, Son’s group reported organometallic hollow spheres (OMHS, 296) bearing a bis(N-heterocyclic carbene)-Pd species (Scheme 135).346 These unique OMHS were afforded through the coordination-directed assembly of molecular NHC building blocks, which are something like metal−organic frameworks. Interestingly, uniformly dispersed micrometer-scale hollow spheres were formed, which were evidenced by SEM and timedependent TEM investigations. More importantly, the OMHS were applied as a recyclable heterogeneous catalyst for cascade Strecker reactions, and excellent activity that was higher than that of the homogeneous NHC-Pd complex catalyst was obtained. The catalyst could be reused at least three times without a loss of activity. In 2016, Nie and co-workers applied 1,3,5,7-tetrakis(4(imidazole-1-yl)phenyl)adamantane as a starting molecule and BG


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Scheme 135. Preparation of OMHS and Its Catalytic Cascade Strecker Reactions

Pd-MOFs presented excellent activity and recyclability toward the Suzuki reaction; however, a remarkable effect of catalytic activity dependency on the framework was also revealed. One year later, the same group employed a similar ligand and developed a novel metal−organic nanotube (MONT),353 and single-crystal X-ray diffraction analysis revealed that the MONT had an exterior wall diameter of 4.91 nm and an interior channel diameter of 3.32 nm. Importantly, the MONT integrated the multiple functionalities of MOFs, nanotubes, and NHCs with a high catalytic activity for a variety of reactions, such as Suzuki− Miyaura and Heck cross-coupling reactions, hydrogenation of olefins, and the reduction of nitrobenzene. Moreover, the MONT could be recovered and reused for at least six successive runs without a loss of activity. In 2014, Sumby et al. described the synthesis of a Zncoordinated MOF material comprising a Cu(I) bis-NHC ligand catalytic site (299, Scheme 138).354 The MOF entity was afforded in situ from an azolium ligand and [Cu(CH3CN)4]PF6 source. Interestingly, the MOF material displayed a structurally flexible diamondoid 3D network. In addition, with respect to the catalytic hydroboration of CO2, the MOF showed catalytic activity similar to that of the corresponding homogeneous Cu(I)NHC species, yet possessed the advantage of being a heterogeneous catalyst that can easily be recovered after reaction. ́ In 2015, Martin-Matute and co-workers synthesized a dicarboxy group functionalized NHC-Ir complex as a new metallolinker, and then the NHC-Ir-complex-based zirconium MOF material 300 was generated via two different routes: direct synthesis and postsynthetic exchange (Scheme 139).355 It is worth noting that the postsynthetic exchange is a powerful strategy for introducing metal complexes into MOFs, which, specifically in this case, led to high metal loadings compared with the direct synthesis strategy. The MOFs synthesized through the postsynthetic exchange route displayed excellent activity for the

Scheme 136. Main-Chain Organometallic Microporous Polymer (MOMP)

specific surface areas and more regularly arranged pore environments than do POP materials, and thus, under some circumstances, MOFs should be a powerful supplement to POP materials in heterogeneous catalysis.331,348−351 In 2011, Wu’s group synthesized a double-azolium-derivative-based MOF that was bridged by a copper paddle-wheel subunit (Scheme 137),352 and a subsequent metalation process with Pd(OAc)2 provided the final heterogeneous NHC-Pd-MOF catalyst 298. The NHCBH


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Scheme 137. Double-Azolium-Derivative-Based MOFs Materiala

a

(A) View of the lamellar network of the azolium based MOF. [Adapted with permission from ref 352. Copyright 2011 Royal Society of Chemistry.] (B) Schematic representation of the ligand. (C) Suzuki reaction catalyzed by a heterogeneous NHC-Pd-MOF catalyst.

class of heterogenized NHC metal catalysts. The key advantages of these materials include rapid tunability and adaptability, due to a diverse array of synthetic outlays with modular features, as well as compliance with a variety of transition metals. In 2009, Karimi and co-workers employed a self-supported main-chain NHC-Pd polymer as a recyclable heterogeneous catalyst for Suzuki reactions in water (Scheme 140).356 Importantly, deactivated

Scheme 138. Three-Dimensional Diamondoid MOF Catalyst and Its Catalytic Hydroboration of CO2 [Adapted with permission from ref 354. Copyright 2014 Royal Society of Chemistry.]

Scheme 140. Main-Chain NHC-Pd Polymer

substrates of aryl chlorides and aryl fluorides could be efficiently converted. The mercury poisoning experiment and hot-filtration test, as well as TEM analysis, were conducted to exclude the effects of Pd nanoparticles and dissociated homogeneous analogues. This concept of catalyst design opens a new gate for this class of self-supported polymers as efficient recyclable heterogeneous catalysts. In continuation of this work, in 2011, the same group further developed a series of similar linear mainchain NHC-Pd polymeric catalysts bearing different alkyl chain functionalities, and the effect of alkyl chains on catalyst activity

isomerization of 1-octen-3-ol. Moreover, in the absence of base, the catalyst could be recycled three times without a loss of activity. In addition to the above-mentioned 3D self-supported porous materials, self-supported main-chain NHC-metal-complex-based one-dimensional (1D) linear polymers are another interesting

Scheme 139. Different Synthetic Routes of an NHC-Ir-Complex-Based Zirconium MOF and Its Catalytic Isomerization of Allylic Alcohol [Adapted with permission from ref 355. Copyright 2015 Royal Society of Chemistry.]

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excellent catalytic activity and recyclability, as well as the scalability of these self-supported polymeric catalysts, hold potential practical implementation in the transformation of glycerol to value-added chemicals. Based on the above work on self-supported NHC-Ir coordination polymers, very recently in 2017, Tu’s group developed another NHC-Ru coordination polymer (304) through the same self-supported strategy (Scheme 143).360

and the nature of catalytic active sites during the Suzuki coupling reaction were fully discussed.357 Although these catalysts were recyclable, a slight loss of activity was observed after each run. In 2015, Karimi’s group further developed a self-supported main-chain NHC-Pd polymer with an N-dodecyl-substituted group (Scheme 141).358 The polymeric catalyst was found to be Scheme 141. Main-Chain NHC-Pd Polymer with an NDodecyl-Substituted Group and Its Catalytic Applications

Scheme 143. Linear Polymeric NHC-Ru Complex Catalyst and Its Catalytic Reductive Amination of Levulinic Acid

a highly efficient precatalyst for the cyanation of aryl halides by using the nontoxic cyanic source [K4Fe(CN)6] with the aid of DMF as a solvent. Interestingly, when DMF was changed to PEG-200, the Ullmann coupling product became the dominant product. The authors ascribed this solvent-dependent phenomenon to the trapping (or scavenging) of CN− ions through the formation of hydrogen bonds with PEG; thus, the dominant reaction in the solvent of PEG was the Ullmann homocoupling. Recycling experiments suggested that there was a gradual decomposition under the reaction conditions. The mercury poisoning experiment and the GPC test of the recovered polymeric catalyst further proved the formation of active Pd species in the form of either fragmented NHC-Pd complexes or Pd nanoparticles. These detailed investigations of deactivation mechanism will help us to understand the deactivation process and further design more robust heterogenized catalysts. Tu and co-workers described the preparation of a series of selfsupported, linear main-chain organometallic polymeric NHC-Ir complex catalysts,359 which displayed high activity (parts per million level loading of the catalysts) and selectivity toward the oxidative conversion of glycerol to potassium lactate (Scheme 142). Of note, an ultrahigh TON value of 1.24 × 105 was obtained for multigram-scale reactions by using the heterogeneous polymeric catalyst. Remarkably, these self-supported polymeric catalysts were very robust and could be recycled and reused up to 31 times without a significant loss of activity. This

This insoluble solid polymeric molecular catalyst displayed high activity (a TON value up to 67 000) for the reductive amination of the biomass levulinic acid (LA) to provide the challenging unprotected 5-methyl-2-pyrrolidone product with the aid of ammonium formate as both the hydrogen transfer agent and nitrogen source. Surprisingly, the heterogeneous catalyst could be recovered and reused for 37 runs with only a slight loss of activity, which further demonstrated the high stability of this class of self-supported polymeric catalysts. Moreover, highly chemoselective tandem reductive amination reactions from different aldehydes/ketones to access a variety of structurally intriguing Nsubstituted-5-methyl-2-pyrrolidones were also smoothly catalyzed by catalyst 304. These recyclable self-supported polymeric NHC metal catalysts are classified and listed in Table 5 in the interest of a ready reference. In summary, the above discussions clearly show that self-supported polymeric catalysts, constructed from organic secondary building blocks, can provide wide chemical and structural diversity. Taking advantage of the modular nature of the catalyst synthesis, both the microstructure and the catalytic properties of the self-supported polymeric catalysts are, in principle, tunable through the tailoring of the molecular secondary building blocks. Some bifunctional (or even multifunctional) heterogeneous self-supported catalysts can readily be realized by integrating specifically designed molecular synthons. Self-supported multifunctional heterogeneous catalysts with cooperative (or synergetic) catalytic effects, which are comparable to (or even beyond) the corresponding homogeneous catalytic system, are the eternal objective in this area. Although successful examples of practical applications are still scarce, the potential for future development is anticipated to be great given the extensive chemical and structural diversity of these catalysts. 3.2.4. Noncovalent Interaction Strategy. Polymersupported NHC metal complex catalysts that are immobilized through the noncovalent interaction strategy have rarely been reported. The main reason may be attributed to the flourishing of

Scheme 142. Linear Polymeric NHC-Ir Complex Catalysts and Their Catalytic Conversion of Glycerol to Potassium Lactate

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polymeric bead is typically porous, providing a large surface area on and inside them, and is thus quite suitable as a catalyst support. In 2014, Verpoort and co-workers reported an anionexchange-resin-supported heterogeneous NHC-Cu catalyst (305), which was prepared through the anion-exchange method. The immobilized catalyst was applied to the three-component click reaction of benzyl bromide, sodium azide, and phenylacetylene, and interestingly, acetylene gas as the source of alkyne was also transformed to the triazole product. The heterogeneous catalyst could be reused four times; however, after the fourth run, the yield decreased dramatically. The authors claimed there was a leaching of the Cu(I) species, which was proven by an XRF test of the organic phase. Therefore, it can be seen that noncovalent bonds are still not robust enough to prevent the decomposition of the active sites compared with covalent bonds.

Table 5. Recyclable Self-Supported Polymeric NHC-Metal Catalysts and Their Catalytic Applications polymer type POPs POPs POPs POPs POPs POPs POPs POPs OMHS OMHS MOFs MOFs MOFs MONT MONT MONT linear polymer linear polymer linear polymer linear polymer

synthetic method

metal

reaction type

ref

Sonogashira coupling Sonogashira coupling quaternization quaternization

Au

alkyne hydration

339

Cu

340

Friedel−Crafts reaction Friedel−Crafts reaction Friedel−Crafts reaction Friedel−Crafts reaction coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds coordination bonds

Pd

hydrosilylation of alkynes Suzuki coupling formylation and methylation of amine Suzuki coupling oxidative condensation reaction click reaction

345

345

Pd

Ullmann coupling and Glaser coupling cascade Strecker reaction Suzuki coupling

Pd

Suzuki coupling

352

Cu

hydroboration of CO2

354

Ir

isomerization of enol

355

Pd

Suzuki coupling and Heck coupling hydrogenation of olefins reduction of nitrobenzene Suzuki coupling

353

cyanation of aryl halides oxidative conversion of glycerol reductive amination of levulinic acid

358

Pd Zn

Cu Cu Cu Pd

Pd Pd Pd Pd Ir Ru

341, 342 343 344

345

3.3. Carbon-Based Immobilization of NHC Complex Catalysts

346

Apart from silica and polymeric materials, carbon materials constitute the third class of important solid support materials for the immobilization of various homogeneous molecular catalysts, owing to their fascinating properties including a high surface area, tunable porosity, and functionality, as well as a nonswelling nature. As a particular class within carbon-based materials, carbon nanotubes (CNTs) possess a number of inherent properties that make them attractive for catalyst supports.362−364 For instance, they have graphite-like walls with adequate surface areas, being more stable toward oxidation than activated carbon and more reactive than graphite. Furthermore, they display an outstanding chemical stability, due to their inert nature, in the majority of reaction solvents. Furthermore, the controlled porosity and the chemical regularity of CNTs provide an ideal model for the deep investigation of catalytic mechanisms through combined strategies of computational chemistry and experimental demonstration. Graphene oxide (GO) is a type of layered carbon nanomaterial that is exfoliated by the oxidation of graphite.365 GO has several characteristic oxygen-containing functional groups, such as hydroxyl and epoxy groups on the basal planes and carboxyl and carbonyl groups at the edges, which provide easy functionality via stable covalent bonds. Moreover, GO has a significantly larger specific surface area than do activated carbons and carbon black. These favorable properties make GO a promising candidate as a support for heterogeneous catalysis. In this section, we will discuss NHC metal complex catalysts supported by the carbon material (mainly for CNTs and GO) reported after 2006. Carbonmaterial-supported NHC complex catalysts can be divided into three classes, i.e., covalent grafting, solid phase synthesis, and noncovalent interaction strategies. Interestingly, noncovalent interaction is the most used strategy and accounts for 42% (Figure 7) of all the reported literature, owning to the easy immobilization process via noncovalent π−π stacking interactions (vide infra).366 3.3.1. Covalent Grafting Strategy. Covalent functionalization of CNTs has been achieved mainly on the basis of oxidative treatments.367 As a general method, CNT oxidation produces opened tubes with oxygen-containing functional groups (mainly carboxylic acid) at both the tube endings and the sidewall. These functional groups can then be applied as chemical anchors for further derivatization. In 2013, Á lvarez and co-workers grafted hydroxyl-ending imidazolium salts to oxidized multiwall CNTs through the covalent bonding strategy and, following a metalation process, provided the final CNT-supported NHC-Ir

347

353 353 356, 357

359 360

other strategies such as covalent grafting and solid phase synthesis, as well as the self-supported strategy. However, there is one example of an anion-exchange-resin-supported NHC-Cu complex catalyst (Scheme 144).361 An ion-exchange resin is a resin that acts as a medium for ion exchange. It is an insoluble matrix normally in the form of small (0.25−0.5 mm radius) microbeads, fabricated from an organic polymer substrate. The Scheme 144. Anion-Exchange-Resin-Supported NHC-Cu Complex Catalyst and Its Catalytic Click Reaction

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which suggested that the imidazolium played an important role in improving the dispersibility of the Pd nanoparticles. One year later, in 2014, Kim’s group developed a GO-supported NHC-Pd complex catalyst by using the stable covalent amido bond (Scheme 147).371 This heterogeneous GO-NHC-Pd catalyst displayed high activity for the aqueous Suzuki reaction under relatively mild conditions. The catalyst could be reused six times without loss of its activity. Bazgir’s group exploited a relatively new silylation modification technique to immobilize NHC-Pd complexes and ionic liquids on the surface of GO (Scheme 148).372 The active molecular species were linked through covalent Si−O bonds. The obtained heterogeneous catalyst displayed a high activity toward the Suzuki reaction in a mixture of EtOH−H2O (1:1). The immobilized catalyst could be recovered and reused for five successive runs with a slight loss of activity. However, welldistributed Pd nanoparticles with a size of less than 5 nm dispersed on the GO-IL sheets could be detected by TEM for the recycled GO-supported catalyst. This good dispersibility of Pd nanoparticles on the GO-IL sheets should be ascribed to the positive role of imidazolium ionic liquids. 3.3.2. Solid Phase Synthesis Strategy. In principle, the anchoring bonds of solid phase synthesis can also be classified as covalent bonds. The difference between solid phase synthesis and the covalent grafting strategy is the tethered NHC carbene precursors are formed step by step from a modified solid phase support for solid phase synthesis, while the covalent grafting strategy represents the direct immobilization of NHC precursors or NHC metal complexes onto solid supports via covalent bonds. In 2013, Lee’s group developed a dual-functional CNTsupported Grubbs’ catalyst plus Pd nanoparticles (311) through the solid phase synthesis strategy (Scheme 149).373 Taking advantage of the metathesis catalytic activity of Grubbs’ catalyst and the catalytic hydrogenation activity of Pd nanoparticles, the dual-functional CNT-supported catalyst was applied for tandem metathesis/hydrogenation reactions in an ionic liquid. Excellent activity and good recyclability were realized. Recyclable heterogeneous organic−inorganic hybrid catalysts for tandem reactions have not been widely reported and represent a promising research area in green catalysis because of their efficiency and sustainability. Recently, Hajipour and co-workers

Figure 7. Different immobilization strategies for recyclable carbonmaterial-based heterogenized NHC complex catalysts.

catalysts (306 and 307, Scheme 145).368 Of note, the nanotubesupported NHC-Ir catalysts showed an enhanced catalytic activity compared with the corresponding homogeneous analogues toward the hydrogen-transfer reduction of cyclohexanone to cyclohexanol, and the authors proposed a supportassisted key H-transfer process as the explanation of this positive support effect. Additionally, a good recyclability of the supported catalysts, without a loss of activity, and stability in air were observed. In 2016, using the same hydrogen-transfer reduction reaction as a model reaction, the same group further performed detailed catalytic studies based on CNT-supported NHC-Ir catalysts with a gradual oxidation level of the surface of the CNT supports.369 It is noticeable that the most oxidized hybrid catalyst exhibited a superior catalytic activity, observing a gradual increase in the catalytic activity with the oxidation degree of the corresponding parent CNTs. Wang et al. immobilized an amine-group-functionalized imidazolium salt via the nucleophilic addition of epoxy groups on the graphene oxide, and following a metalation process with H2PdCl4 yielded the GO-supported NHC-Pd catalyst 308 (GONHC-Pd2+, Scheme 146).370 GO-NHC-Pd2+ was demonstrated as an efficient heterogeneous catalyst for Suzuki reactions. In addition, the catalyst could be easily recovered and reused for at least six consecutive runs without an apparent loss in activity; however, after several runs, the Pd2+ species aggregated and accumulated into Pd nanoparticles on the surface of GO. The Pd nanoparticles were homogeneously dispersed on the GO sheets,

Scheme 145. CNT-Supported NHC-Ir Complex Catalysts and Their Catalytic Hydrogen-Transfer Reduction of Cyclohexanone to Cyclohexanol

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Scheme 146. Schematic Representation of Preparation of GO-NHC-Pd2+

Scheme 147. GO-Supported NHC-Pd Complex

Scheme 148. Preparation of GO-Supported NHC-Pd Complex Catalyst via a Silylation Modification Technique

thermodynamics. CNTs and graphene (or graphene oxide) have a delocalized π-electronic system that permits the immobilization of polyaromatic rings through π-stacking interactions. This kind of immobilization does not need the previous covalent functionalization of CNTs or graphene material and only requires the attachment of a pyrene group to the target complex catalysts. Compared with the covalent grafting strategy and solid phase synthesis, the strategy of noncovalent π−π interactions represents a substantially more convenient approach for the immobilization of homogeneous molecular catalysts onto solid materials via delocalized π-electronic systems. In 2008, Wang’s group described the reversible immobilization of an NHC-Ru catalyst on CNTs through π−π interactions (Scheme 151).375

described the solid phase synthesis of CNT-supported NHC-Co catalyst 312, which starts from the phenylamine (Scheme 150).374 The obtained supported NHC-Co catalyst displayed a high catalytic activity toward the Mizoroki−Heck reaction. The system presented an environmentally friendly and economical means for the Heck reaction. Moreover, the heterogeneous catalyst could be recycled and reused six times without a significant loss of activity. 3.3.3. Noncovalent Interaction Strategy. In chemistry, π−π stacking (also called π stacking) refers to the attractive, noncovalent interactions between aromatic rings since they all contain π bonds.366 The essence of π−π stacking could be ranked with strong van der Waals forces, and its nature is controlled by BM


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Scheme 149. Solid Phase Synthesis of Dual-Functional CNT-Supported Grubbs’ Catalyst plus Pd Nanoparticles

Scheme 150. Solid Phase Synthesis of CNT-Supported NHC-Co Complex Catalyst

tion and purified by washing methods as a recycling support material. Recently, Reek et al. developed a CNT-supported pyrene-tagged NHC-Ir complex catalyst for the electrocatalytic oxidation of water in aqueous solutions.376 Clearly, this kind of carbon-material-supported molecular catalyst holds large potential in other fields, such as in the popular electrocatalytic field.377 Apart from CNTs, graphene, and graphene oxide (GO), reduced graphene oxide (rGO) is also characterized with a delocalized π-electronic system that permits the immobilization of polyaromatic rings through π-stacking interactions.378,379 In 2014, Peris and co-workers reported the immobilization of pyrene-tagged NHC-Pd and NHC-Ru complexes onto the surface of rGO to afford hybrid catalytic materials (designated as NHC-Pd-rGO (314) and NHC-Ru-rGO (315), Scheme 152).380 The NHC-Pd-rGO catalyst was applied in the hydrogenation of alkenes, while NHC-Ru-rGO was employed for the oxidant-free oxidation of alcohols. Importantly, both of the hybrid heterogenized catalysts exhibited enhanced activity compared with the parent homogeneous molecular catalysts, which could probably be attributed to the positive role of rGO, although the definitive mechanism is not clear. Recyclability tests

Scheme 151. Reversible Immobilization of NHC-Ru Complex onto CNTs through Noncovalent π−π Interaction

Under certain conditions of solvent and temperature, the pyrenetagged Grubbs−Hoveyda-type catalyst could be desorbed from the CNTs reversibly. Of note, although some amount of catalyst was desorbed from the CNTs at high temperature, the desorbed catalyst could be reanchored onto the CNTs when stirred for approximately 30 min at 0 °C. A good recyclability of the supported NHC-Ru complex catalyst was realized. In addition, when the NHC-Ru part of the immobilized catalyst was totally deactivated, the CNTs could be easily separated by centrifugaBN


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catalysts were applied for the synthesis of carboxylic acids from the dehydrogenation of alcohols in aqueous media. The results showed that the catalytic activities were enhanced when the molecular NHC-Ru catalysts were immobilized on the surface of rGO, and this enhancement could be explained as a stabilization of the active species provided by the rGO support. The hot filtration tests demonstrated that the rGO-supported NHC-Ru catalysts with pentafluorophenyl and anthracene tags formed weak π-stacking interactions with graphene, which underwent a homogeneous “boomerang” mechanism. However, the rGOsupported NHC-Ru complex with the pyrene tag displayed a high stability, and no desorption of the molecular catalyst was observed even at high temperatures. Moreover, this heterogenized catalyst could be reused for at least 10 runs without an observable loss of activity. In summary, up to now, carbon-material-supported NHC metal catalysts have not been frequently reported. Nevertheless, a few examples of very favorable activity enhancement effects have been realized among the carbon-material-supported molecular catalysts, and these effects could probably be attributed to the unique structure of the delocalized π system of carbon materials. Of note, the noncovalent interactions of π−π stacking represent a distinct immobilization strategy related to graphitized carbon materials (such as CNTs, GO, and rGO). In some cases, the π−π stacking process is reversible and is capable of promoting reactions in a homogeneous solution, thus catalyzing the reactions with higher rates than other solidsupport catalysts under otherwise identical conditions. Moreover, when the physicochemical parameters (such as temperature and polarity) of the reaction systems are finely tuned, the molecular catalysts can be reimmobilized onto the surface of carbon materials via π−π stacking and recovered by filtration or centrifugation. This interesting “boomerang” recycle model deserves further investigation in the field of sustainable catalysis.

Scheme 152. Preparation of rGO-Immobilized NHC-Pd and NHC-Ru Hybrid Catalysts via π−π Stacking

and hot filtration, as well as large-scale experiments, were performed to investigate the stability of the hybrid catalysts. Although the true active species of NHC-Pd-rGO could be explained as the formation of Pd nanoparticles, the molecular catalytic nature of NHC-Ru-rGO remained unchanged. Both catalysts could be recycled and reused for up to 10 runs without an apparent loss in activity. Later, the same group reported the coimmobilization of palladium and ruthenium complexes with pyrene-tagged NHC ligands onto the rGO support, which was used to catalyze the hydrodefluorination (HDF) reaction (Scheme 153).381 Strikingly, the molecular counterpart of the mixture of the NHC-Pd and NHC-Ru complexes only worked when it was immobilized onto the graphene surface, which showed an “abnormal” enhancement in the activity after the immobilization. An in-depth analysis of the hybrid catalyst before and after the catalytic reaction indicated the true catalytic system consisted of the in situ formed Pd nanoparticles and molecular NHC-Ru species, and the rGO support probably stabilized the active Pd nanoparticles and, thus, enhanced the synergistic effect of the catalytic process. Moreover, the hybrid catalyst could be recycled for up to 12 runs without a detectable loss of activity, representing one of the most stable immobilized catalysts generated via the noncovalent π−π stacking strategy. In 2016, Mata et al. reported rGO-supported NHC-Ru complexes (317−319) functionalized with different polyaromatic groups (pentafluorophenyl, anthracene, and pyrene) via π−π stacking (Scheme 154).382 These heterogeneous hybrid

3.4. Magnetic-Nanoparticle-Based Immobilization of NHC Complex Catalysts

The above-mentioned bulk solid supports including silica materials, polymers, and carbon materials facilitate the separation of the heterogenized catalyst and products. However, customarily, this convenience is at the cost of sacrificing considerable catalytic activity because of the contact problem, as well as mass transfer limitation, in these relatively large sized solid supports. Exceptionally, nanoparticles are recognized as a semiheterogeneous support since they are easily dispersed and display an intrinsically large surface area, which provides excellent

Scheme 153. Schematic Representation of rGO-Immobilized Polymetallic NHC Metal Catalyst and Its Catalytic Hydrodefluorination Reaction

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Scheme 154. Preparation of rGO-Supported NHC-Ru Complex Catalysts with Different Aromatic Tags via π−π Stacking

Suzuki reactions under mild conditions. Noticeably, the MNPsupported bis(NHC) palladium complex 321 displayed better activity than did the corresponding homogeneous catalyst. Moreover, the magnetic catalyst could be straightforwardly retrieved by magnetic separation from the catalytic solution containing the product. By using the magnetic separation approach, the MNP-supported bis(NHC)-Pd catalyst could be recycled for more than 12 runs without a significant loss of activity. However, the MNP-immobilized mono(NHC) complex 320 showed poor stability, which decomposed quickly and formed Pd black. This means the microcoordination environment severely impacted the stability of the MNP-supported complex catalysts. Abu-Reziq et al. also reported MNPsupported semiheterogeneous bis-NHC-Rh complex389 and bis-NHC-Pd catalysts,390 in which the NHC part was attached to the surface of Fe3O4 nanoparticles via covalent Si−O bonds. The semiheterogeneous bis-NHC-Rh catalyst displayed excellent activity and recyclability toward selective hydroaminomethylation reactions, while the supported bis-NHC-Pd catalyst exhibited good activity in catalytic carbonylative Sonogashira reactions. ́ Recently, Diez-Gonzá lez and co-workers reported the SiO2coated MNP-supported NHC-Pd catalyst 322 that was linked with covalent ether bonds (Scheme 156).391 This heterogenized

accessibility to the surface-bound catalytic active sites. Furthermore, magnetic nanoparticles (MNPs)383−385 are amenable to magnetic separation, therefore circumventing the need for catalyst separation through traditional separation methods, such as filtration, that frequently results in the blocking of filters and valves.386,387 These magnetic nanoparticle supports have shown great potential for bridging heterogeneous catalysis and homogeneous catalysis. Similar to other traditional supports, the immobilization strategies related to magnetic nanoparticles can also be divided into four groups, i.e., covalent grafting (accounting for 50%), solid phase synthesis, noncovalent, and encapsulation strategies (Figure 8).

Scheme 156. SiO2-Coated MNPs-Supported NHC-Pd Complex Catalyst

Figure 8. Different immobilization strategies for recyclable magneticnanoparticle-based heterogenized NHC complex catalysts.

3.4.1. Covalent Grafting Strategy. Materials scientists have developed various functionalized MNPs, such as aminefunctionalized MNPs, silica-coated MNPs, and hydroxyfunctionalized MNPs. These functionalized MNPs have been identified as excellent support materials for semiheterogeneous catalysis. In 2016, Andrés and co-workers described the immobilization of mono- and bis(NHC) complexes onto core−shell γ-Fe2O3/silica MNPs via covalent Si−O bonds (Scheme 155).388 The heterogenized catalysts were applied for Scheme 155. Core−Shell γ-Fe2O3/Silica MNP-Supported NHC Complex Catalysts with Covalent Si−O Bonds

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Scheme 157. Solid Phase Synthesis of MNP-Supported Grubbs−Hoveyda NHC-Ru Catalyst

Scheme 158. Solid Phase Synthesis of Magnetically Separable NHC-Pd Catalyst [Adapted with permission from ref 393. Copyright 2014 Royal Society of Chemistry.]

NHC ligand for Suzuki reactions (Scheme 158).393 Interestingly, in this case, a magnetic polymer carrier with a chloromethyl group was utilized as the starting material for the solid phase synthesis, followed by the nucleophilic addition of a bulky 2,6diisopropyl group, and the subsequent metalation process afforded the final magnetically separable NHC-Pd catalyst. Remarkably, by using phenylboronic acid and aryl bromide as substrates, the heterogeneous catalyst could be recycled and reused for at least 21 cycles without a significant loss of activity. Moreover, when PdCl2 and 3-Cl-pyridinyl were applied as the Pd source to synthesize Pd complex 324, the stability of the magnetic catalyst was further improved and showed no apparent leaching in Suzuki reactions with aryl chlorides at high temperature. 3.4.3. Noncovalent Interaction Strategy. Because of the many merits of the noncovalent π−π stacking strategy, this strategy was also transferred to magnetically separable catalysts. In 2010, Reiser’s group developed MNP-supported Pd catalyst 326 via the noncovalent π−π stacking strategy (Scheme 159).394 In this case, graphene-coated Co/C nanoparticles were applied as a support material, and the pyrene-tagged NHC-Pd complex 325 was immobilized onto the surface of Co/C nanoparticles

catalyst displayed good activity toward the Suzuki reaction under mild conditions; however, it is a pity that the recyclability of the heterogeneous catalyst was very poor, as the yield declined to 20% in the second run. The reason for this instability may be ascribed to the allyl coordination mode of the Pd center. 3.4.2. Solid Phase Synthesis Strategy. The solid phase synthesis strategy is similar to the covalent grafting strategy; at least, the two approaches rely on covalent bonds to realize the immobilization process. Therefore, in terms of their chemical nature, the strategies are all covalent-bond-oriented heterogenization methods. In 2014, Lee and co-workers reported MNPsupported Grubbs−Hoveyda NHC-Ru complex 323, which was synthesized through the strategy of solid phase synthesis (Scheme 157).392 The Grubbs−Hoveyda catalyst was covalently grafted onto ionic magnetic nanoparticles skillfully by utilizing an imidazolium salt linker. The heterogenized catalyst displayed a high activity for RCM and CM reactions with a catalyst loading of less than 1 mol % ruthenium. In addition, the magnetic catalyst could be readily recovered magnetically and reused up to seven times without a loss of activity. In 2015, Lin and co-workers described the synthesis of a magnetically separable NHC-Pd complex containing a bulky BQ


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of the conjugated triphenylbenzene and triphenyltriazine severely influenced the catalytic performance, as well as the recyclability, of the heterogeneous MNPs. The core−shell MNPs expressing tripodal imidazolium salt with triphenylbenzene showed better recyclability (six runs without a loss of activity) than the triphenyltriazine-expressed MNPs. Moreover, the spherical morphology of the triphenylbenzene-expressed MNPs (TEM images) remained after six cycles, which further demonstrated the stability of the MNPs.

Scheme 159. Magnetically Separable NHC-Pd Complex and Its Catalytic Hydroxycarbonylation

3.5. Unconventional Immobilization of NHC Complex Catalysts

In addition to the above four general categories of support materials, some unconventional materials have also been applied as special-interest supports for molecular catalysts. In 2009, Wright and co-workers applied α-zirconium phosphonates as versatile supports for NHC-based catalysts (Scheme 161).397 αZirconium phosphonate (ZrP) materials, α-[Zr(O3PR)2]∞, are easily accessible hybrid organic−inorganic materials with welldefined structures and high degrees of crystallinity. The welldefined structure with a high degree of crystallinity not only facilitates the characterization of the supported catalysts but also allows for the accurate simulation of the catalytic process by computational methods, which helps us to understand the catalytic mechanism, as well as the support effect. First, imidazolium salt functionalized zirconium phosphonates were synthesized through the acid hydrolysis of 4-bromobutyl phosphonate ester, followed hydrothermal reaction with ZrOCl2 and the aid of HF as a crystallization acid. Then, various metalation processes afforded the ZrP-supported NHC-Rh, NHC-Ir, NHC-Ru, and NHC-Au complexes (329−332). These ZrP-supported NHC metal complexes and ZrP-supported NHC organocatalyst were utilized as heterogeneous catalysts in various reactions such as hydroformylation, metathesis, hydroamination, and ring-opening-polymerization reaction. Interestingly, the ZrP-supported NHC organocatalyst displayed considerable stability in the ring-opening polymerization of ε-caprolactone,

through π−π stacking. Such a noncovalent interaction is strongly temperature-dependent; hence a “catch−release” concept was employed for effective catalyst recycling. At high temperatures, the catalyst exhibited homogeneous nature for efficient catalysis, and at room temperature, the pyrene-tagged NHC-Pd complex 325 was returned to the carbon surface via π−π stacking. The feasibility of this “boomerang”-type catalyst was proven in the hydroxycarbonylation of aryl halides in water under carbon monoxide at atmospheric pressure. The immobilized NHC-Pd complex was highly active in more than 16 iterative runs and was easily recovered through magnetic decantation. 3.4.4. Encapsulation. The encapsulation of a core structure by a functional polymeric species is a facile method for the preparation of classical core−shell materials.395 In 2015, Wang’s group developed two core−shell MNP-encapsulated NHC-Pd complexes (327 and 328, Scheme 160).396 The direct assembly of tripodal imidazolium salts and Pd(OAc)2 on the surface of Fe3O4 nanoparticles afforded functionalized spherical core−shell MNPs. These core−shell-structured NHC-Pd catalysts exhibited a high activity for Suzuki reactions. Of note, the different effects

Scheme 160. Spherical Core−Shell MNPs Prepared through Encapsulation Strategy [Adapted with permission from ref 396. Copyright 2015 Royal Society of Chemistry.]

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Scheme 161. Synthesis of Functionalized Zirconium Phosphonates, Their Metalation, and Their Catalytic Applications

supported NHC-Fe(III) catalysts (333). Both catalysts showed good performance for the dehydration of fructose to HMF (with a TOF value up to 206 h−1). However, the Starbon-supported catalyst displayed relatively better stability compared to that of HACS-supported catalyst, which could probably be ascribed to the inert and relatively hydrophobic nature of the porous carbon material. The Starbon-supported NHC-Fe catalyst could be recycled and reused at least five times without a loss of its activity. At the end of section 3, we wish to briefly mention the characterization of the NHC moiety on the solid support, which can be divided into two aspects. One is the assessment of the real loading of the supported NHC compounds; the other is the identification of the NHC compound within the classification of heterogeneous catalysts. To determine the amount of imidazolium tether (NHC precursor) on the solid support, elemental analysis is the main tool to provide the elemental contents of the immobilized compound. Utilizing the nitrogen content obtained from elemental analysis, the loading of NHC precursors can be calculated assuming that all nitrogen sources are correctly defined. It is worth noting that X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) can also be used as tools to analyze the loading of NHC moieties on the surface of heterogeneous catalyst by the nitrogen content or according to the weight loss at a certain temperature, respectively. However, the accuracies of XPS and TGA are dubious, and these two tools should be at best used as supplementary tools. To confirm the fixation of the desired NHC moieties, the most common strategy is to compare the analytic information on the immobilized compound with its homogeneous analogue. A number of analytical tools, such as IR or NMR spectroscopy, are powerful methods for such characterizations.14

whereas other heterogenized NHC metal catalysts showed poor recyclability. To summarize, some unconventional support materials possess desirable properties in some particular catalytic reactions, and we have to choose the applicable support materials according to the specific requirements. Very recently, Matharu and co-workers reported novel Starbon- and HACS (high amylase cornstarch)-supported NHC-Fe(III) catalysts for the efficient conversion of fructose to 5-(hydroxymethyl) furfural (HMF) (Scheme 162).398 Starbon is a trademark name of a patented porous carbon material derived from porous starch,399 and these porous carbonaceous materials from renewable resources have gradually been accepted as catalyst supports. The NHC precursor was covalently grafted onto Starbon and HACS via a stable amide bond, followed by metalation with FeCl3, yielding the final Starbon- and HACSScheme 162. Synthetic Routes to Starbon- and HACSSupported NHC-Fe(III) Catalysts

4. CONCLUSION AND PERSPECTIVE All the recyclable molecular catalysts can be divided into two classes: recyclable homogeneous catalysts and heterogenized molecular catalysts. First, NHC metal complex catalysts as the main line, recyclable metal complexes that work under BS


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Table 6. Various Reactions Catalyzed by NHC-Metal Catalysts and Related Recycling Strategies reaction type Suzuki coupling

olefin metathesis

click reaction

Heck coupling

recycling strategy

section

star level

water phase catalysis light-controlled phase selective strategy pH-controlled phase selective strategy natural-product-modified strategy nanofiltration strategy immobilized NHC complex catalysts IL phase catalysis biphasic catalysis SILP catalysis soluble polymer supported Grubbs’ catalysts light-controlled phase selective strategy pH-controlled phase selective strategy fluorous NHC complex catalysts nanofiltration strategy column chromatography strategy immobilized NHC complex catalysts water phase catalysis light-controlled phase selective strategy immobilized NHC complex catalysts soluble polymer supported Pd catalysts immobilized NHC complex catalysts

2.1.1 and 2.2.1 2.3

★★★ ★

2.4

★★★

2.5

★★

2.7 3

★★ ★★

2.1.2 2.1.4 2.1.5 2.2

★★ ★★★ ★★★ ★★

2.3

★★★

2.4

★★

2.6

★★

2.7 2.8.1

★★★ ★★

3

★★

2.1.1 2.3

★★★ ★★

3.1 and 3.2

★★

2.2.2 and 2.2.3

★★

3

★★

reaction type cycloisomerization reactions

alkyne hydration

isomerization of allylic alcohols transfer hydrogenation

aerobic oxidation of alcohols oxidant-free dehydrogenation of alcohols alkynylation of trifluoromethyl ketones carboxylation of arylboronic esters or benzoxazole Buchwald−Hartwig reaction hydroamination of phenylacetylene with aniline Sonogashira coupling A3 coupling alkene hydroformylation

recycling strategy

section

star level

water phase catalysis

2.1.1

★★★

biphasic catalysis immobilized NHC complex catalysts water phase catalysis immobilized NHC complex catalysts water phase catalysis

2.1.4 3

★★★ ★★★

2.1.1 3.1.1 and 3.2.3

★★★ ★★

2.1.1

★★★

water phase catalysis immobilized NHC complex catalysts PEG-400 phase

2.1.1 3.3.1

★★★ ★★★

2.1.3 and 2.8.2

★★★

immobilized NHC complex catalysts soluble polymer supported catalysts pH-controlled phase selective strategy

3.3.1 and 3.3.3

★★★

2.2.2

★★

2.4

★★★

pH-controlled phase selective strategy nanofiltration strategy precipitation strategy

2.4

★★★

2.7 2.8.3

★★ ★★

immobilized NHC complex catalysts immobilized NHC complex catalysts immobilized NHC complex catalysts

3

★★

3.1 and 3.2

★★

3.1 and 3.2

★★

metal catalysts onto solid supports. Most of these heterogenized molecular NHC metal complex catalysts display good recyclability, although some of them partially lose activity. Enhanced activity and excellent recyclability are the sought-after goals for heterogenized molecular catalysts when compared with their homogeneous analogues. Although silica materials represent the most popular support for the immobilization of NHC metal complexes, polymers, especially self-supported polymers, have become a class of increasingly more attractive and desirable supports, as they lead to immobilized NHC metal complexes with a homogeneous distribution of active metal centers in a controllable way. In particular, self-supporting functional POP polymers, which combine numerous secondary synthons into a microporous 3D rigid structure using very robust organic covalent bonds, definitely can generate stable polymeric materials with exceptional chemical and thermal stabilities, high surface areas, and versatile chemical functionalities. The integration of favorable structural properties and chemical functionalities into polymeric materials holds great potential in designing intriguing catalytic materials with attractive synergistic effects.400−409 Benefiting from the quick development of materials chemistry, carbonaceous materials, such as CNTs, grapheme, and MNPs, have also been employed to immobilize NHC metal complexes, albeit they have been less explored compared to traditional silica and polymer materials. Noncovalent π−π stacking is one of the most popular immobilization strategies with regard to carbonaceous materials, due to their delocalized π-electron system. More

homogeneous conditions and are separated in heterogenized systems have been reviewed in detail. NHC metal complexes exhibit better stability compared to other ligand-coordinated complexes because of the strong σ-electron donating properties of NHC ligands, providing the important theoretical basis to effectively recycle the homogeneous NHC metal catalysts. Under the appropriate stimulus, these catalysts can be readily recycled through various methods, such as precipitation, phase transfer, and column chromatography, without having to sacrifice activity and selectivity because of their homogeneous nature during the catalytic process. It is worth noting that the recycling of homogeneous catalysts greatly increases total TON values without depressing TOF values and meets the general principles of green catalysis. However, the elegant modification of the parent catalyst, and the properly designed technological process of recycling, together with cost accounting in large-scale industrial catalysis, should be carefully considered according to specific reaction systems. Second, with the development of organic syntheses and material chemistry, a plethora of immobilized NHC metal complex catalysts have been and continue to be developed. Silicabased materials, polymers, carbon materials, and MNPs are the four major types of support materials for the immobilization of NHC metal complex catalysts; other types of materials, such as αzirconium phosphonates and HACS, have also been reported, albeit less frequently. The covalent grafting, solid phase synthesis, self-supporting, and noncovalent interaction strategies are the four main approaches to the immobilization of molecular NHC BT


Chemical Reviews

Review

interestingly, the “boomerang” mechanism of NHC metal catalysts supported on carbonaceous material, which is caused by the temperature-dependent noncovalent π−π stacking, has become an effective recycling method without sacrificing activity. A few carbon-supported NHC metal catalysts, immobilized through π−π stacking, display enhanced activities compared to their homogeneous counterparts, and this interesting and favorable support effect requires thorough clarification of the theoretical mechanism. Because of their nanosize, MNPs are considered as semiheterogeneous catalysts that bridge homogeneous catalysis and heterogeneous catalysis. Enhanced catalytic activities plus a convenient magnetic recyclability are expected for MNP-supported catalysts, which hold the potential for largescale industrial applications. A summarization with the major and most studied NHC metal complex catalyzed reactions and various recycling strategies is listed in Table 6 for easy retrieval. This table includes our appreciation of which strategies appear most satisfactory, which are marked in the form of a star level (from one to three stars). Regarding the catalytic reactions of recyclable NHC metal complex catalysts, the Nobel-Prize-awarded catalytic reactions NHC-Ru-complex-catalyzed olefin metathesis reaction and NHC-Pd-complex-catalyzed catalyzed cross-couplingare the two most intensively examined reactions. However, in our opinion, these two reactions, especially cross-coupling reactions (such as Suzuki reactions), are overly concerned today, and recyclable catalytic applications of other important NHC metal complex catalysts, such as Cu,410−418 Ag,51,419−423 Au,424−431 Rh,432−440 and Ir,441−447 are treasure troves that are worth exploiting further. Recently, the immobilization of NHCCo,448−452 NHC-Ni,453−459 and NHC-Fe398,460−466 complexes has attracted great attention and NHC complexes containing these low-priced and environmentally benign metals have been a research hot spot, albeit the corresponding applications remain scarce. It is important to note that, so far, there are three general serious problems in the research field of catalyst recovery. First, most works have performed recycling experiments based on yields of >90% since the beginning run. This is unscientific and unacceptable, because in such high yields a slight deactivation or decomposition of the recovered catalyst cannot be detected from the angle of product yields. The scientific way is to perform the recycling experiments based on a yield of approximately 50%. Second, deep and detailed characterizations of the recovered catalysts are often missing, especially with Pd complexes, where a part of them converted in Pd0 nanoparticles after the first run. Therefore, here are the questions: Which is the actual catalyst? Do they all have catalytic activities? If it is the latter, then such an immobilization is certainly not very suitable, as the heterogenized NHC-Pd catalyst simply serves as an expensive source for palladium. Last but not least, extensively complicated syntheses for the preparation of recyclable NHC catalysts are not encouraged. It is evident that some works have gone awry in this aspect. In consideration of the current applications of recyclable NHC metal complex catalysts, there is still a long way to large-scale industrial applications.467 Although their recyclability, activity, and stability have been greatly improved, their high manufacturing cost remains a major issue. Applying a fixed-bed reactor loaded with solid-supported NHC metal catalysts to continuous productions might be a feasible and promising direction in the field of industrial catalysis. As the most important nonphosphine ligand, NHC-coordinated metal complexes have exerted and will

continue to demonstrate their functionality in both academia and industry. Recyclable NHC metal complexes will certainly extend their range of catalytic applications and solve real problems of industrial catalysis in the future.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Fuwei Li: 0000-0003-2895-1185 Notes

The authors declare no competing financial interest. Biographies Wenlong Wang received his Ph.D. degree in physical chemistry from Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS), in 2014 under the guidance of Prof. Fuwei Li. Then he became an assistant professor at LICP. Two years later, he joined the group of Prof. Yunjie Ding at Dalian Institute of Chemical Physics as a postdoctoral fellow. Now he is an associate professor in the Department of Materials Science and Engineering of Dongguan University of Technology, China. His research interest is focus on organometallic complex based green catalysis. Lifeng Cui finished his undergraduate studies in electrical engineering at Xi’an University of Posts & Telecommunications in 2001. In 2007 he received his Ph.D. degree in materials science under the supervision of Prof. Lai-Sheng Wang at Washington State University. After postdoctoral studies in materials science at Stanford University with Prof. Yi Cui, he joined Amazon Inc. in 2010 and worked as a research and development engineer. He joined Shanghai University for Science & Technology as a professor in 2013 and moved to his present position as Professor of Materials Science and Engineering. His current research interests are catalysis for biomass conversion, MOF-based catalysts for environmental applications, and recoverable and recyclable catalysts for green chemistry. Peng Sun earned her Ph.D. degree in chemical technology at Nanjing Tech University under the supervision of Prof. He Huang. In 2012, she became a postdoctoral fellow under the guidance of Prof. Fuwei Li. Presently she is working as a research associate at LICP. Her goal is to develop new catalytic materials for heterogeneous catalysis. Lijun Shi received his master’s degree in polymeric chemistry from Northwest Normal University, China, in 2015. He joined Prof. Fuwei Li’s research group in the next year for his Ph.D. studies. His research interest is organometallic chemistry and homogeneous catalysis. Chengtao Yue received his master’s degree in industrial catalysis from Heilongjiang University, China, in 2014. He joined the group of Prof. Fuwei Li in 2015 for his Ph.D. studies. His research interest is focused on the immobilization of N-heterocyclic carbene catalysts and the development of novel materials for various catalytic organic reactions. Fuwei Li received his Ph.D. degree in physical chemistry at Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS), in 2005. Then he became a research assistant at the Institute of Process Engineering, Chinese Academy of Sciences, in 2005. One year later, he moved to the Department of Chemistry of the National University of Singapore in 2006 as a postdoctoral fellow. Since 2010, he has been a full professor under the “hundred talents” program at LICP. He has received “The Catalysis Rising Star Award” presented by The Catalysis Society of China in 2012 and “The Outstanding Young Scientist Foundation” from the National Natural Science Foundation in 2015. His research is devoted to catalyst design and investigations on BU


Chemical Reviews

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MOF = metal−organic framework MOMP = main-chain organometallic microporous polymer MONT = metal−organic nanotube MPP = mesoporous polymer particles MPS = microporous polystyrene Mw = molecular weight NHC = N-heterocyclic carbene NHDC = N-heterocyclic dicarbene NMR = nuclear magnetic resonance NPs = nanoparticles OMHS = organometallic hollow spheres PDI = polymer dispersity index PDMS = polydimethylsiloxane PE = polyethylene PEG = polyethylene glycol PEGA = polyethylene glycol adipate PEPPSI = pyridine, enhanced, precatalyst, preparation, stabilization, and initiation PIB = polyisobutylene PIS = polyimidazolium salts POPs = porous organic polymers POSS = polyhedral oligomeric silsesquioxane PMO = periodic mesoporous organosilica PS = polystyrene P123 = Pluronic P123 RCM = ring-closing metathesis rGO = reduced graphene oxide ROMP = ring opening metathesis polymerization RTILs = room temperature ionic liquids SBA = Santa Barbara Airport SBUs = secondary building units scCO2 = supercritical CO2 SDA = structure-directing agent SILP = supported ionic liquid phase SP = spiropyran SPE = solid-phase extraction TBAF = tetrabutylammonium fluoride TEOS = tetraethoxysilane TEM = transmission electron microscopy TMP = transmembrane pressure TNF = 2,4,7-trinitrofluoren-9-one TON = turnover number TOF = turnover frequency TPPTS = trisodium salt of tri(m-sulfonatophenyl)-phosphine UV−vis = ultraviolet−visible XRD = X-ray diffraction XRF = X-ray fluorescence ZrP = α-zirconium phosphonates

their preparative chemistry to construct catalytic active centers for the activation of specific chemical bonds (such as C−H and C−O) and gas molecules, to develop efficient and stable catalysts for the syntheses of valuable fine chemicals containing N and O atoms. He has published 80 peer-reviewed papers (H factor is 30).

ACKNOWLEDGMENTS We gratefully acknowledge support and funding from the National Natural Science Foundation of China (21522609 and 21403258), and the Science & Technology Innovation Institute of Dongguan University of Technology. This work was also supported by the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDJ-SSWSLH051). ABBREVIATIONS A3 = aldehyde−alkyne−amine AAS = atomic absorption spectroscopy anh = anhydrous BET = Brunauer−Emmett−Teller BIAN = bis(arylimino)acenaphthene BIM = bis(imidazole-1-yl)methane BMT = 2,4,6-tris(4-(bromomethyl)phenyl)-1,3,5-triazine CAACs = cyclic (alkyl)(amino)carbenes Cat = catalyst CM = cross-metathesis CMP = conjugated microporous polymer CMPS = chloromethylated polystyrene CNTs = carbon nanotubes Cp = cyclopentadienyl CTA = chain transfer agent CTAB = cetyltrimethylammonium bromide CuAAC = copper-catalyzed azide−alkyne cycloaddition DEAMM = diethyl allylmethallylmalonate DMSO = dimethyl sulfoxide DMF = dimethylformamide DNPSENS = dynamic nuclear polarization surface-enhanced solid-state NMR spectroscopy DRIFTS-FTIR = diffuse reflectance infrared Fourier transform infrared spectroscopy DVB = divinylbenzene EA = ethyl acetate EB = electron beam EWG = electron-withdrawing group FDU = Fudan University FSPE = fluorous solid-phase extraction F127 = Pluronic F-127 GO = graphene oxide GPC = gel permeation chromatography HACS = high amylase cornstarch HCP = hyper-cross-linked polymers HDF = hydrodefluorination HMF = hydroxymethyl-2-furfural ICP-MS = inductively coupled plasma mass spectrometry ICP-OES = inductively coupled plasma optical emission spectrometer IL = ionic liquid KHMDS = potassium bis(trimethylsilyl)amide LA = levulinic acid MCF = mesocellular foam MCM = Mobil Composition of Matter MMO = monooxygenase MNPs = magnetic nanoparticles

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