N-Heterocyclic Carbenes in Materials Chemistry - Chemical Reviews

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

N‑Heterocyclic Carbenes in Materials Chemistry Christene A. Smith,† Mina R. Narouz,† Paul A. Lummis,† Ishwar Singh,† Ali Nazemi,†,§ Chien-Hung Li,† and Cathleen M. Crudden*,†,‡ †

Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada, K7L 3N6 Institute of Transformative Bio-Molecules, ITbM-WPI, Nagoya University, Nagoya, Chikusa 464-8601, Japan

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‡

ABSTRACT: N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

CONTENTS 1. Introduction 2. Hard Materials 2.1. NHC Ligands on Planar Metallic Surfaces 2.1.1. NHC SAMS on Planar Metallic Surfaces Computational Studies 2.1.2. NHC SAMS on Planar Metallic Surfaces− Demonstrated Applications 2.1.3. NHC SAMs on Planar Metalloid SurfacesSilicon(111) 2.2. NHCs on Nanorods 2.3. NHCs on Nanoparticles 2.3.1. Initial Reports of NHC-Stabilized Metal Nanoparticles 2.3.2. NHC-Stabilized Au Nanoparticles 2.3.3. Non-Au NHC-Stabilized Metal Nanoparticles 2.4. Atomically Precise Metallic Nanoclusters Stabilized by NHCs 2.4.1. Introduction to Ligands-Protected Atomically Precise Metallic Nanoclusters 2.4.2. Carbene-Stabilized Tris Gold Nanoclusters 2.5. NHC-Stabilized Coordination Clusters 2.5.1. NHC-Protected Metal Chalcogenide Cluster Complexes 2.5.2. Heterometallic Coinage Metal Coordination Clusters 2.5.3. NHC-Protected Coinage Metal-Phosphide Coordination Clusters 2.6. NHC-Stabilized Metal Carbonyl Clusters 3. Soft Materials

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3.1. NHC−Metal Complexes of Dendrimers and Hyperbranched Polymers 3.1.1. NHC-Transition Metal Complexes at the Periphery of Hyperbranched Polymers or Dendrimers 3.1.2. NHC-Transition Metal Complexes at Dendrimer Core 3.2. Polymeric NHCs and NHC−Metal Complexes 3.2.1. Polymeric NHCs Where the NHC Is Pendant to the Main Chain 3.2.2. Polymeric NHCs in which the NHC Is Included in the Main Chain 3.3. Polymerizations Catalyzed by NHCs 3.3.1. Step Growth Polymerization 3.3.2. Chain Growth Polymerization 3.4. Coordination Polymers and Metal Organic Frameworks 3.4.1. One-Dimensional Coordination Polymers 3.4.2. Two- and Three-Dimensional Coordination Polymers 3.5. Metal Organic Frameworks (MOFs) and Related Structures 3.6. NHC Liquid Crystals 3.7. Conclusions Associated Content Special Issue Paper Author Information Corresponding Author ORCID Present Address

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Received: August 15, 2018

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DOI: 10.1021/acs.chemrev.8b00514 Chem. Rev. XXXX, XXX, XXX−XXX

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estimated by calorimetric methods to be double that of classic R3P−M bonds.31,32 This higher bond strength inhibits decomposition pathways that involve metal−ligand cleavage, such as ligand oxidation or protonation, making NHCs ideal ligands for metal complexes employed in oxidation catalysis.33 These features encouraged our group to investigate whether NHC ligands would be able to stabilize metal surfaces as efficiently as they stabilize molecular complexes.34−37 For 2D films, the chemistry of self-assembled monolayers (SAMs) on metals is dominated by thiolate ligands, despite the fact that the kinetic, thermal, and oxidative instability of thiolate SAMs have been highlighted as drawbacks to these common ligands.38−46 The application of SAMs in areas such as biosensing, microelectronics and drug delivery would benefit significantly if a more stable ligand-to-surface bond could be discovered. In addition to their ability to form SAMs on planar surfaces, the use of NHCs to protect nanoparticles and nanoclusters is also an evolving and exciting area that will be covered in this review. The incorporation of NHCs into polymeric, dendrimeric, and MOF-type materials is facilitated by the strong C−metal bond in NHC−metal complexes and the ease of incorporation of NHCs into these soft materials using common organic chemistry reactions. These NHC-containing materials are often employed as catalysts where their recyclability is a significant advantage.47,48 Another application of NHCs in materials chemistry is as metal-containing liquid crystals (MLCs), which is an emerging materials application of NHCs that does not involve catalysis.49,50 Both of these types of NHC materials will be discussed in this review, as will the use of NHCs as catalysts for the production of polymeric materials. This review will focus on the use of NHC ligands in a variety of different applications in materials science. This will include the use of NHCs on planar metallic34,36,51,52 and metalloid surfaces,53 on nanoparticles,54−57 and on clusters,58−60 in addition to coordination polymers61−63 and metal−organic frameworks (MOFs).64,65 The use of NHCs in soft materials such as liquid crystals,49,66,67 dendrimers,68,69 and polymers,70,71 will also be described, including their use as catalysts for polymer synthesis.72,73 For previous reviews of NHC applications in materials chemistry, the reader is referred to several pertinent reviews, which include the topics of supported NHCs in catalytic applications, general immobilization strategies, NHCs in metal carbonyl clusters and metal organic frameworks, NHCs in medicinal and luminescent materials, NHCs on metallic surfaces and NHCs for the stabilization of elemental allotropes, planar surfaces, and nanoparticles.48,74−79 For a general review of NHC chemistry, the reader is directed to a comprehensive 2014 review by the Glorius group.2

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1. INTRODUCTION N-Heterocyclic carbenes (NHCs) have become one of the most investigated ligands for transition metal complexes in the past 20 years.1−4 These ligands received little attention after their initial description in 1968.5 However, after examples of the isolation and characterization of free carbenes by Bertrand and Arduengo,4 interest in these ligands increased exponentially.2 One of the key features of NHCs compared to other classes of carbenes is their stability. Simple carbenes, like the parent methylidene (“CH2”, 1, Chart 1), decompose above cryogenic Chart 1. Examples of Different Types of Carbenesa

a (1) Parent triplet methylidene, (2) N-heterocyclic carbene electronic structure, (3) first example of an isolated NHC.8

temperatures, cyclopropanate alkenes,6,7 dimerize,8−11 and insert into otherwise unreactive C−H bonds.10,12,13 These reactions can be violently exothermic.14 In contrast, NHCs can be stored, bottled, and even distilled.15,16 Although NHCs are far superior in stability to other types of carbenes, they still do not surpass the stability of classic phosphines such as PPh3. They require a strong base for generation and typically must be stored under inert atmosphere.17 The higher stability of NHCs compared to other carbenes is attributed to their electronic structure, including, but not limited to, the fact that they are singlet carbenes with the largest singlet− triplet gap of all divalent carbon species aside from CO.16,18 The two nitrogen substituents next to the carbenic carbon stabilize the empty orbital on the carbene through resonance16,19 and withdraw electron density from the carbene carbon through the C−N bonds (see 2, Chart 1). Reprotonation of the NHC does create instability in certain situations, but in situ generation of the NHC can alleviate this problem in most cases.20,21 LloydJones has reported that 6-membered NHCs react slowly but quantitatively by insertion into the C−H bond of toluene.22 The generality of decomposition via this route has not been well explored. Because of their interesting properties, NHCs have been extensively investigated as ligands for transition metal compounds. NHC−M bonds are better described as single bonds rather than double bonds, consistent with retention of singlet character even in complexation to a metal.23−27 Although metal−NHC bonds are more stable than metal−alkyl species, they can undergo decomposition by reductive elimination and other pathways similar to metal−alkyl bonds under certain conditions.1,28 The typically high kinetic stability of metal-NHC bonds, in addition to the thermodynamic strength of these bonds, is a critical feature in the use of these ligands in transition metal complexes.29,30 The NHC−M bond strength has been

2. HARD MATERIALS 2.1. NHC Ligands on Planar Metallic Surfaces

The first report of the use of NHCs to functionalize flat gold surfaces came from the Siemeling group in 2011.80 They exposed a clean polycrystalline gold surface with predominant (111) faces to enetetramine 5 (Scheme 1), which is known to be in equilibrium with the free carbene (4).81 After 24 h in THF, the resulting surface was analyzed by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). These data supported the presence of a monolayer of either the NHC or the enetetramine along with unknown impurities on the surface.80 A tilt angle to the surface B


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significant advance in the area of surface functionalization of gold since it showed that functional carbenes can be deposited on the surface and then modified through reaction with pendant functional groups. In addition, this work drew attention to the fact that the method of NHC generation can affect monolayer quality and result in surface impurities. In 2014, the Crudden and Horton groups reported the functionalization of gold surfaces with a variety of NHCs and demonstrated the high thermal and chemical stability of the NHC SAM.34 Drawing inspiration from molecular NHC complexes, the Crudden group showed that NHC-based SAMs are significantly more chemically, thermally, and electrochemically robust than thiolate SAMs.34 In this initial report, the free carbene was generated by deprotonation with KOtBu, filtration through Celite, and deposition on a variety of Au surfaces in solution under inert atmosphere (Scheme 3). The

Scheme 1. Possible Pathways for Deposition of NHCs on Au, from Either the Free Carbene (4) or Tetraazafulvalene (5)

Scheme 3. Deposition of Alkyl-Functionalized NHCs on an Au Surface and Stability of the Resulting Film

of 30 ± 6° was predicted from NEXAFS data, which the authors interpreted as NHC binding rather than enetetramine, since the latter was expected to lie flat on the surface. The next report of NHCs on gold surfaces came from the Johnson group in 2013.51 In this report, the free carbene was generated either from imidazolium salts 6 and 7, which were treated with strong base (KHMDS) followed by filtration to remove any salt, or thermally by heating a solution of the carbon dioxide adduct 6·CO2 prior to solution deposition in THF (Scheme 2). When the carbene was generated by deprotonation, Scheme 2. Deposition of Bulky NHCs on Au Surfacesa

resulting NHC SAMs proved to be resistant to a wide variety of conditions including 24 h exposure to boiling water, refluxing THF, aqueous acid, and aqueous base (pH 2 and pH 12 at room temperature and 100 °C). When the films were exposed to H2O2 (1% aq solution), 90% of the film remained on the surface, as assessed by XPS. These results provided the first indication that NHC-based SAMs would be robust alternatives to thiolatebased SAMs.82 In a subsequent report, Tang and co-workers also showed that SAMs of 10 (shown in Figure 1) were stable to ultrasonication in aqueous media.83

(i) KHMDS, THF; (ii) 100−110 °C, THF; (iii) THF.

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contamination of the surface by the amine resulting from deprotonation was problematic. However, when the NHC·CO2 adducts were employed, clean NHC deposition was observed. The authors also described the use of commercially available IMes (8) for direct deposition. The extent of surface functionalization was assessed by carrying out depositions on a quartz crystal microbalance (QCM), which gave information about areal mass density. Surface coverages of 63 ± 14 ng/cm2 (0.9 ± 0.2 molecules/nm2) were observed for functional carbene 7 and 56 ± 14 ng/cm2 (1.1 ± 0.3 molecules/nm2) for 6. Full coverage is predicted to be 85 ng/cm2, so these numbers are well within reason for good surface coverage. Functional NHCs 6 and 7 permitted the assessment of surface modification through the NHC−gold linker. Although modification of the bromide in 6 by cross-coupling was not effective, the addition of Grubbs catalyst to surfaces modified by NHC 7 led to incorporation of the active Ru alkylidene, which permitted polymerization from the surface.51 This report represented a

Figure 1. Electrochemical stability of 10@Au. After up to 150 cycles, the NHC-protected Au surface showed no evidence of decomposition. Reproduced with permission from ref 34. Copyright 2014 Nature Publishing Group. C


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The electrochemical stability of NHC SAMs was assessed by cycling the NHC on the gold film from −0.1 to 0.6 V (vs Ag/ AgCl) in a 5 mM Fe(CN)63−/5 mM Fe(CN)64− aqueous solution with 1 M NaClO4 as a supporting electrolyte.34 Electrochemical stability is an important criteria for SAMs on metal surfaces, since SAMs are often key components in electrochemical84 detection schemes, but thiolate-based SAMs are prone to decomposition after repeated cycling under oxidizing voltages.85,86 When a well-formed dodecanethiol SAM was cycled 150 times under the conditions described above, a 600% increase in current density was observed, indicative of film decomposition (Figure 1). In contrast, a SAM prepared from NHC 10 showed no change in the current density within error after 150 cycles. Density functional theory (DFT) studies were also performed to provide insight into the nature of the gold−NHC bond.34 Among various bonding modes for NHC 9 on an Au(111) surface, the most stable structure was found to be one in which the NHC occupies an “a-top” position, binding to only one gold atom (Figure 2).34 Thiol SAMs are historically predicted to bind

Figure 3. An STM image of 9@Au(111) shows the local order that comprises 5−10 stacked units of 9 on the surface. Inset, schematic representation of individual molecules overlaid to scale. Reproduced with permission from ref 34. Copyright 2014 Nature Publishing Group.

carbene 9. Examination of the surface by XPS confirmed that no sulfur peak remained, indicating complete displacement by the NHC within the limits of detection. NHC SAMs were exposed to thioether but resisted any incorporation (eq 1).34

Although NHC 9 was the primary focus of this report, other more complex NHCs such as 8 and 11−13 were also shown to functionalize gold surfaces (Chart 2).34 In general, lower

Figure 2. Various bonding modes for isolated 9@Au(111) and adsorption energies calculated by DFT. Bonding of the NHC to the surface via an a-top site provides the most stable complex with a bond length similar to those reported for molecular NHC−Au complexes. Reproduced with permission from ref 34. Copyright 2014 Nature Publishing Group.

Chart 2. NHCs Employed in the Preparation of NHC-Based SAMs on Au Surfaces by Crudden et al.

in a 3-fold hollow site, although other binding geometries are known, and the generality of this binding mode is under debate.87 The thermochemistry of thiol bonding to gold was measured experimentally by Bernasek et al. to be approximately 120 kJ mol−1.88 DFT calculations reported by Crudden et al. for the NHC−Au system predict the C−Au bond energy to be 150 ± 10 kJ mol−1, significantly stronger than the thiol−gold bond.89−91 Subsequent experimental studies have validated this number (see Figure 4).36 The packing of NHC 9 on the gold surface (Figure 3) was examined by surface tunneling microscopy (STM) under UHV at ambient temperature.34 Starting from an annealed Au(111) surface, solution deposition led to etch pits from restructuring of gold atoms, a phenomenon also seen in alkanethiol films.38,92 Areas of high order were characterized by units of 4.8 Å × 3.4 Å in size in STM, corresponding to the molecule standing perpendicular to the surface (Figure 3). These one-dimensional arrays are independent of the direction of the STM scan, indicating that any surface ordering is not a result of tip-induced surface restructuring. The ability of NHCs to displace existing ligands and to resist displacement themselves was also reported.34 Thioetherfunctionalized Au surfaces were prepared and exposed to

stability and higher pitting was observed with NHCs bearing larger wingtip groups such as IMes; however, the generality of this effect under different deposition conditions remains to be thoroughly tested. As described below, a recent report by Marder and co-workers suggests that sterically hindered NHCs do not bind to the surface by a chemical bond but rather via dispersive interactions.93 Functional versions of NHC 9 were also supported on gold, including those bearing a long alkyl chain (10, Figure 1) or an azide to permit postgrafting functionalization by click reactions. After surface modification with and azide-tagged NHC, propargyl alcohol and ethynylferrocene could be clicked onto the SAM. D


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Ferrocene was employed as an electrochemical tag to permit surface density measurements. The charge transfer measured during the second cycle of cyclic voltammetry studies was 3.5 ± 0.5 electrons nm−2, which equates to 3.5 ± 0.5 molecules nm−2. This value is in agreement with STM images of the unfunctionalized SAM 9@Au and represents a surface density close to but lower than thiol SAMs (ca. 4.5 molecules nm−2).34,94 In a subsequent paper, the Crudden and Horton groups described the preparation and use of a benzimidazolium hydrogen carbonate salt (9·H2CO3), which enables the formation of NHC films in air in protic solvents without any special precautions (Scheme 4).36 This bench-stable NHC Scheme 4. Preparation and Deposition Methodologies for the Preparation of 9@Au with Benzimidazolium Hydrogen Carbonates

precursor had been reported previously by Taton,95 but modifications to the synthetic route were found to be necessary to remove traces of iodide, which are a significant problem in the preparation of SAMs on Au. Starting from benzimidazolium iodide 9·HI, oxidative removal of iodide could be accomplished by treatment with H2O2 or through the use of a bicarbonate ionexchange resin.36 The ion-exchange resin is effective with a variety of functional groups on the NHC and with several different counterions.36 The deposition of these salts to give NHC SAMs was carried out simply by dissolving the salt in methanol in the presence of the Au film in air. NHC SAMs prepared by this method had the same characteristics as films made via the free carbene method in terms of density and chemical, thermal, and electrochemical stability.34,36 A significant advantage of these single component precursors is that NHC SAMs can be prepared in vacuo36 without the need for solvent. This opened the door for a host of surface science experiments that were employed to analyze NHC SAMs. Using a capillary attachment to an STM chamber, Crudden, Horton, Baddeley, and co-workers showed that heating the benzimidazolium hydrogen carbonate to 50 °C under UHV was sufficient to deposit a dense NHC monolayer.36 Within 5 min of dosing, 90% monolayer formation was observed. Temperature-programmed desorption (TPD) studies were employed to gauge the temperature of desorption by mass spectrometric identification of NHC fragments as they desorb from the surface (Figure 4). The desorption profile gave a Tmax of 605 K, and using the Redhead approximation, a bond energy of 158 ± 10 kJ/mol can be calculated, which is identical within error to the value previously predicted using DFT (150 ± 10 kJ/

Figure 4. (Top) TPD of signal of 9 at m/z = 41, stemming from the isopropyl substituent, showing a maximum desorption temperature of 605 K. (Bottom) HREELS study of 9 on Au(111) at 300 K (blue), after annealing to 475 K and cooling to 300 K (red), solution spectra of molecular analog of NHC−Au−Cl complex (orange) and a simulation showing the calculated vibrational modes, which have dipole components normal to the surface (black). Reproduced with permission from ref 36. Copyright 2016 Nature Publishing Group.

mol).34,36 Examination of the NHC SAM at sub-monolayer coverages by TPD led to the conclusion that no aggregation was occurring at low coverages and multilayer formation was not observed.36 High-resolution electron energy loss spectroscopy (HREELS) was also performed in order to investigate the orientation of the NHC on the surface.36 When obtained in the specular geometry, HREELS gives information about the orientation of the NHC in relation to the surface, since vibrational transitions perpendicular to the surface should increase in intensity and those parallel are expected to decrease. When performed on NHC SAMs derived from 9, aromatic C−H stretching modes at 3070 cm−1 were observed with increased intensity compared to solution phase measurements of the corresponding molecular complex 9·AuCl.36 Similarly, C−H vibrations in the isopropyl unit decreased on the gold SAM compared to solution. These observations both indicate that the NHC is bound upright on the surface. Some sharpening of signals upon annealing to 475 K was observed, implying an increase in ordering and possibly the introduction of a slight tilt upon annealing, although this phenomenon was not studied in detail.36 E


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For the significantly smaller NHC 14, with only methyl wingtip groups, several different binding geometries were observed.52 At full monolayer coverage, a close packed rectangular structure was observed, while at lower coverage, what appeared to be flat lying dimers and trimers were also observed by STM. This possible orientation of the NHC was subsequently observed in two additional studies for NHCs with small wingtip groups, as will be discussed in more detail below.96,97 A detailed report of the effect of the size of wingtip groups on NHC surface orientation was published by Baddeley, Crudden, Horton, et al., demonstrating that changing the wingtip group from methyl to ethyl and isopropyl on benzimidazolylidene carbenes results in different surface morphologies on Au(111) and Cu(111).96 Surface morphology and orientation were also probed by STM and by HREEL spectroscopy. The latter technique is significant since it is a bulk technique and thus provides characterization of the entire surface, including regions of low order. Consistent with previous studies on gold, the HREELs study showed clearly that isopropylated NHC 9 is oriented perpendicular to the surface of copper, while methyl- and ethyl-substituted NHCs (15 and 4) lie flat.96 As shown in Figure 6, methyl and ethyl NHCs 15 and 4 display a strong peak at 730

Fuchs, Glorius, and co-workers recently published an in-depth study of NHCs on Au(111) employing low-temperature STM.52 Three carbenes of varying size were the focus of this study: IMes (8), IPr (12), and IMe (14). IMes (8), one of the most commonly employed carbenes, did not give any ordered patterns, consistent with previous reports from Crudden at room temperature.34,36 However, a highly ordered hexagonal pattern was observed for NHC 12 on Au(111) (Figure 5), with

Figure 5. (a) STM image of an NHC directly bound to an Au(111) surface. (b) STM image of NHC 14 on Au(111). (c) The STM image of NHC 12 on Au(111) which shows a highly ordered and dense hexagonally packed full monolayer. (d) Submonolayer STM image that shows 12 preferentially occupying the fcc regions and elbow positions. Reproduced with permission from ref 52. Copyright 2017 Nature Publishing Group.

interesting differences at submonolayer coverages. At very low coverages (0.01 monolayers), 12 was observed to bind in the elbows of the herringbone structure, and at higher concentrations (0.05 of a monolayer), it bound in the FCC part of the herringbone structure.52 Unlike 8, the very sterically hindered NHC 12 showed high mobility on the surface, which the authors attributed to movement of the NHC with an attached Au atom. Height measurements supported the proposal that 12 binds to adatoms on the surface and/or extracts a gold atom from the surface in order to form the NHC−Au bond. Related measurements for IMes (8) suggest that this may be a steric phenomenon since similar increases in height were not observed for 8, suggesting that some of the characteristics observed may be restricted to bulky NHC 12. In concert with the work of Marder described below,93 it may be that this particular carbene binds to Au with a different mechanism compared to less bulky NHCs. However, it is notable that thermal treatments of 12 on Au(111) show the NHC being retained on the surface at relatively high temperatures, suggesting that regardless of the specifics of bonding, the interaction with the surface is strong.

Figure 6. NHC-based SAMs on Cu(111) and Au(111): (a) NHC SAMs examined in this study. (b) HREEL spectra of NHC monolayers derived from 4, 9, and 15 examined on Cu(111) and Au(111) at 300 K. (c) DFT-optimized binding geometry of isopropyl-substituted NHC 9 on Cu(111). Reproduced with permission from ref 96. Copyright 2017 Wiley-VCH

cm−1 in the HREEL spectra, assigned to the out-of-plane aromatic C−H bending mode. This signal would be significantly decreased in intensity if the NHCs were perpendicular to the surface.96 Peaks centered at ca. 3000 cm−1 (ascribed to aromatic ring C−H stretches) and at 1250−1600 cm−1 (C−N and C = C stretches, and C−H bending modes) were also very weak in the spectra of methyl and ethyl NHCs 15 and 4 (Figure 6). These data are all consistent with the NHCs lying flat on the surface. As F


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proposed to be an adsorbed enetetramine based on molecular geometry obtained from the STM images. Similar conclusions were arrived at in a detailed study by the Papageorgiou group,97 who described STM studies of NHC 14 (Figure 8) adsorbed on Au(111), Ag(111), and Cu(111) in a

shown in Figure 6, the same trend is seen for Cu surfaces as well as Au. When 9@Au and 9@Cu were subjected to the same analysis, all strong peaks for the methyl and ethyl NHCs were suppressed and weaker peaks were intensified, indicating a dramatic change in the surface orientation of these NHC SAMs.96 Consistent with previous results on Au(111), these data suggest that NHC 9 binds predominantly in an upright mode on the surface while NHCs 4 and 15 bind predominantly parallel to the surface. The binding energy of NHC 9 to Cu was experimentally determined to be 152 ± 10 kJ/mol based on thermal programmed desorption (TPD) studies, which is the same as that measured for 9@Au within experimental error.36,96 Thermal treatment of the various NHC SAMs was also instructive. Unlike 9@Cu and 9@Au (upright), in which the NHC desorbs cleanly from the surface upon heating, leaving no residue, SAMs prepared from methyl and ethyl NHCs 15 and 4 began to show changes at lower temperatures and seemed to decompose rather than simply desorb, leaving residues on the surface detectable by STM.96 These results are also consistent with NHCs 4 and 15 lying flat on the surface and 9 binding in an upright geometry and decomposing by simple M−C bond cleavage. These conclusions were supported by STM studies.96 As shown in Figure 7, for the smaller NHCs, images were observed

Figure 8. STM observations of 14 on Cu(111). (a) STM images of a dense layer of NHC−Cu−NHC complexes on Cu(111). (b) Line profiles of entetramine and NHC−Cu−NHC complexes on the surface. (c) Zoomed image of NHC−Cu−NHC. Reproduced with permission from ref 97. Copyright 2017 Royal Society of Chemistry.

report that appeared almost concurrently with the Baddeley report. Methylated NHC 14 was introduced via the CO2 adduct under UHV. Images ascribed to NHC2M species were observed on Au, Ag, and Cu. NHCs with secondary or larger substituents on nitrogen were not examined in this study. Using methylated NHC 14, surface species that could be attributed to enetetramine dimers were also observed in early imaging prior to dimer formation by metal abstraction. These studies are thus in agreement with the previously described experimental report,96 as well as a theoretical study by Tang and Jiang described in the next section.99 A subsequent publication from Amirjalayer, Glorius, Fuchs, and co-workers described the STM imaging of n-Bu2NHC (16) monolayers on Au(111).100 Like NHCs 4, 14, and 15, this compound also formed clearly discernible bis NHC complexes on the surface.100 In this case, several different surface structures were observed, sometimes concurrently, which coallesced into a single close packed structure at higher temperatures, mirroring the underlying herringbone structure of the Au(111) surface. Imaging of IMe NHC 14 on Au(111) suggested a different structure, although the small size of the Me substitutents made definitive assigment of this structure challenging. Previous work from this group and others showed that this NHC tends to take

Figure 7. Thermally induced formation of (15)2Cu complex on Cu(111): (a) High-resolution image of features comprising the SAM formed upon annealing to 365 K; unit cell denoted by overlaid grid. (b) Large-scale STM image of SAM. Domain boundaries and a Moiré pattern are visible. (c) Molecular structure of (15)2Cu (gas phase). (d) Proposed model of SAM shown in (a) comprised of two (15)2Cu complexes intercalated by Cu adatoms (orange dots). Reproduced with permission from ref 96. Copyright 2017 Wiley-VCH.

that seemed to correspond to the presence of NHC2M species on the surface that then form pairs of dimers. These would result from NHCs pulling a metal atom out of the surface. These structures are reminiscent of similar metal atom abstraction with isonitriles as reported by Tysoe and co-workers.98 An earlier, more mobile phase was seen to precede this structure that was G


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bis-NHC 17 between two gold tips (Figure 10).103 Tetrameric, pyramidal, chain-like structures, and adatom tip geometries were

up a variety of conformations on the surface depending on surface concentration.96,97,100 The Baddeley report, however, is unique in the use of HREEL spectroscopy as a bulk technique to show definitively that NHCs with small substituents take up flat lying geometries on metallic surfaces.96 In 2017, Marder and co-workers published a seminal report comparing the effect of NHC SAMs on the work function (WF) of gold, taking into account the effect of the wingtip group on the resulting monolayers.93 Bare gold has a high WF at 5.2 eV, which limits its use in the electronics industry. Thiol-based SAMs have been used to lower the WF of gold to approximately 4 eV; however, the instability of thiols under oxidative conditions renders them suboptimal surface modifiers.101 Using UV photoelectron spectroscopy (UPS), Marder and co-workers were able to measure the WF of Au(111) after modification with various NHC SAMs. Interestingly, the NHC-modified gold surface had a WF between 3.29 and 3.52 eV, even lower than that for thiol-modified gold. Additionally, it was proposed that some of the more bulky NHCs may not be bound to the surface in a covalent fashion but rather by strong van der Waals interactions between the NHC wingtip groups and the metallic surface.93 Taken together, these four studies illustrate the importance of careful choice of NHC wingtip groups to control the surface geometry of the resulting NHC film. Furthermore, it is clear that more needs to be done in this area to further elucidate these effects.93,96,97,100 2.1.1. NHC SAMS on Planar Metallic Surfaces Computational Studies. Richeter and co-workers recently explored the binding of NHC 15 to Au(111) through DFT calculations (Figure 9).102 By modeling the NHC geometry as

Figure 10. The four tip geometries with NHC 17 considered from left to right: a) tetramer, b) adatom, c) pyramid, and d) chainlike structure. In the case of the geometry shown in d), the Au atom above the pyramidal structure is shown in green. Reproduced with permission from ref 103. Copyright 2018 Hindawi Publishing Corporation.

examined for the carbene-gold junction. The LUMO level was predicted to be highly dependent on tip geometry, with a 0.8 eV range observed between all four tips analyzed. This factor is usually a good metric for predicting conductance, with low LUMO levels indicating high levels of conductance. The chaintype tip is predicted to have the lowest LUMO; however, as it has a decrease in electronic coupling, it is not predicted to have the highest conductance. The pyramidal tip had the highest estimated conductance of all four tips modeled; however, other tip geometries should be examined to predict the optimal tip geometry for these types of junctions. Foti and Vázquez also recently modeled the effect of surface NH2 adsorbates on current-induced heat transfer between the adsorbate and nearby theoretical carbene-based molecular circuits.104 Carbene circuits near NH2 groups, when compared to “clean” carbene-based junctions, are predicted to be cooled at all voltages and have an effect on the efficacy of elastic and electronic transport properties of carbene molecular junctions. Recently the Vázquez, Venkataraman, and Roy groups reported the synthesis of this class of NHC-based junctions and their analysis with a scanning tunneling microscope-based break junction (STM-BJ).105 They found that the identity of the metal involved in the junction directly affected the conductivity of the NHC-junction. This was confirmed both experimentally and theoretically using DFT. In 2016, Fyta et al. used quantum-mechanical simulations to study the use of adamantanyl-functionalized NHCs as diamandoid mimics.106 Their study investigated the stability, adsorption energies, and binding characteristics of the NHCs on metal surfaces. Au(111) and Pt(111) possessed the strongest predicted bond between the carbene-diamondoid SAMs. Work functions were calculated to be 3.26 eV for Ag(111), 3.80 eV for Au(111), and 4.18 eV for Pt(111). These NHC SAMs are expected to be more stable on these metal surfaces than thiolbased SAMs, as supported by experimental results. Interestingly, the group postulated that the NHC will be more reactive on Pt due to closeness of the isopropyl hydrogen to the Pt surface, a prediction later supported experimentally by McBreen and Crudden.107 The Pt-based SAM also showed a different charge

Figure 9. DFT-optimized geometries for different binding models of 15 on Au(111).

parallel, perpendicular, and at a 45° angle from the surface, they predicted that the perpendicular geometry with a raised gold atom had the lowest absorption energy of −63.55 kcal mol−1. Although this particular NHC has recently been shown to form NHC2M dimer arrays on the surface as described above,96 this report appeared after the computational study and so this geometry was not investigated computationally. The authors did surmise that partial extraction of a gold atom from the surface by NHC binding may facilitate the formation of molecular NHCgold complexes. Vázquez and co-workers recently used DFT and nonequilibrium Green’s functions to estimate the conductance of H


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phosphines, aryl radicals, less sterically hindered NHCs, and lastly alkylamines. The increased stability of arylated NHCs was predicted to arise from van der Waals interactions between the aryl groups and the gold surface. It should be noted that Marder et al. predict that smaller NHCs bind to the surface via strong covalent bonds while NHCs with larger aromatic groups have small/nonexistant covalent bonds to the surface but compensate for this by strong dispersive interactions.93 Interactions between pendant C−H substituents and the gold surface were also found to contribute to bonding energy. In all cases, the gold atom was found to protrude slightly from the surface, consistent with strong bonding to the surface and with previous calculations.96,102 Systems with adatom-decorated surfaces were also examined and in this case, thiolates, alkynyls, aryls, and less bulky NHCs were all predicted to form bis-NHC complexes with the adatom, with ligands lying flat on the surface.99 NHCs bearing methyl, mesityl, and phenyl groups were explored, including ring expanded and CAACs (cyclic amino alkyl carbenes). The use of NHCs as stabilizing ligands for gold clusters was also studied, and as will be discussed later in this review, an NHC-decorated Au13 icosahedral core was proposed to be stable with a high HOMO LUMO gap and superatom configuration. Lu and Cheng also recently used grand canonic quantum mechanics (GC-QM) to study NHC films on Au(111) and how they may react to changes in electrode potential.109 Their calculations predict that for carbenes with large singlet−triplet gaps, their respective adsorption energy on gold should increase with potential in the positive direction and decrease in the negative direction. Surfaces of Ag(111), Pt(111), and Cu(111) were also modeled, and the same trends were observed. 2.1.2. NHC SAMS on Planar Metallic Surfaces− Demonstrated Applications. Since NHC-based SAMs have only been described in the past few years, there have not been many published examples of their application to functional systems. However, NHC-based SAMs have been demonstrated in biosensing, using surface plasmon resonance (SPR)36,110,111 and surface enhanced Raman spectroscopy (SERS)112 as the detection schemes. In the case of SERS, specific biosensing was not reported but future applications in this area are anticipated. The evanescent wave phenomenon that occurs in SPR is used to achieve real-time analysis of different interactions between biomolecules and ligands on the device surface.113 SPR-based biosensing is an area that routinely employs thiol SAMs.114,115 To study how NHC-based SAMs perform in this application, Horton, Crudden, and co-workers prepared gold SPR chips functionalized with NHC 10 for comparison with commercial thiol-based chips called hydrophobic association (HPA) sensors (Figure 12a).36 HPA chips are typically employed for the preparation of supported hybrid bilayers that are then used to quantify receptor−analyte interactions analogous to those found in cell membranes. Exposing NHC chip 10@Au to phosphatidylcholine gave a clean supported hybrid bilayer. Under optimized conditions, the thiol analog is known to give multilayers and surface vesicles (Figure 12d), which then need to be removed by conditioning.113 When phosphatidylcholine vesicles are added to the surface at 600 s, for the NHC chip (blue) a clean first order adsorption results, and little to no change is observed after buffer treatment.36 In comparison, the thiol-based HPA (red) chip shows a huge adsorption of multilayers and vesicles, which are also observed by scanning electron microscopy (Figure 12e). Buffer treatment is then needed to remove this excess lipid.

distribution compared to Au and Ag, which was reflected in MO and PDOS calculations as well as predicted STM images. The Cheng group recently investigated surface binding of NHCs on Au(111) using computational methods to calculate adsorption energies (Figure 11).108 They found that the theoretical NHC 22 prefers to bind to a-top sites, with an energy of absorption (ΔEabs) of 2.05 eV.

Figure 11. Correlations of protonation energies with binding energies on Au(111). Reproduced with permission from ref 108. Copyright 2018 American Chemical Society.

The effects of steric bulk, ring size, benzannulation, and the presence of heteroatoms around the carbene carbon were evaluated. NHCs in which one nitrogen has been replaced by carbon, those with ring sizes larger than 6, and those with unsaturated backbones were predicted to have larger ΔEabs’s for binding to gold than the NHCs tested thus far experimentally.108 Interestingly, no correlation was found between the singlet/ triplet gap of a given carbene and its adsorption energy to gold. A strong correlation was, however, found between both the protonation energy of a given carbene and the Tolman electronic parameter with the adsorption energy to gold. This is consistent with computational predictions that the bonding energy arises predominantly from C to Au donation (Figure 11). Finally, it should be noted that only NHCs bearing hydrogens on nitrogen were considered for this study and thus the effect of the sterics of the wingtip group on bonding was not addressed. In 2017, Tang and Jiang compared six classes of ligands on Au(111) surfaces including NHCs via DFT.99 NHCs bearing aromatic wingtip groups were predicted to have the highest binding strength to gold, followed by terminal alkynes, thiolates, I


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Figure 12. (a) Structures of NHC and HPA coatings. (b) Schematic illustration of biotin sensing on NHC-based dextran-linked version of streptavidin (NHC-SA) chip surface. (c) Response of biotin as observed on the NHC-SA chip surface. (d) Planar supported hybrid bilayer formation on NHC and HPA chips as monitored by SPR. (e) Analysis of chips from d by scanning electron microscopy, indicating vesicle presence on HPA chips. (f) Run-torun variability in BSA adsorption on both chips as a function of buffer and pH. Average of four to eight runs in each case with standard deviation shown as a black bar. Reproduced with permission from ref 36. Copyright 2016 Nature Publishing Group.

The quality of the supported bilayer was assessed by treatment with bovine serum albumin (BSA), which is known to bind to flaws in the bilayer.116 As shown in Figure 12f, NHCbased films adsorbed less BSA, especially at pH extremes, which is likely an indication of the greater stability of the NHC-based films at more extreme pHs.36 Actual protein sensing117 was then tested with melittin, which is a membrane-bound protein. Under optimized conditions, the thiol-based HPA chip was not able to provide accurate sensing of melittin binding, while the NHC chip gave accurate and reproducible data. This initial study was expanded upon in 2017 by Horton and Crudden with the preparation of a more complex film comprised of carboxylated dextran with a surface streptavidin layer.111 Once prepared, this chip could be employed in the sensing of biotin.36,110,111 Since carboxylated dextran-functionalized SAMs are one of the most highly utilized matrices for biosensing on the market due to their stability and ease of modification,118,119 NHC derivatives of these chips were examined.110,111 SPR performance validation tests were conducted with antibody− antigen and drug−plasma protein interactions and found to yield comparable results to thiol-based SAM chips. The surfaces were seen to be homogeneous in nature by SPR and had a greater thermal stability than thiol films. Taken together, these publications provide promising results for the continued development of NHC based SPR biosensors. Recently the Horton and Crudden group showed that these materials could be modified to incorporate affinity capture biomolecules for biosensing applications.120 Another technique used frequently in biosensing applications is SERS.121 Camden and Jenkins reported the first example of the formation of NHC SAMs on a gold film coated on silver nanosphere substrates (AuFONs), which are designed for SERS.112 CO2 adducts of NHCs 9 and 31−33 were used for surface functionalization by thermal vacuum deposition (Figure 13). In the resulting Raman spectra, benzimidazolium peaks were seen to be enhanced by SERS. Ester and nitrile-

Figure 13. Surface-enhanced Raman scattering spectra of films functionalized with 9 (orange trace), 31 (blue trace), 32 (red trace), 33 (green trace). Asterisk (*) indicates contribution from polystyrene bands. Adapted with permission from ref 121. Copyright 2016 American Chemical Society.

functionalized versions of these NHCs were prepared, and their signals were clearly seen at 1720 and 2240 cm−1, respectively. Unlike other SERS substrates, which require coating of the metal particle to increase stability, the NHCfunctionalized surfaces were found to exhibit very high stability, including under conditions needed for deprotection and surface functionalization. As a demonstration, the team deprotected the ester tag on the NHC SAM and monitored the transformation by surface IR. This study illustrates the promising application of NHC-based SAMs for SERS techniques. In a seminal contribution in terms of applications of NHCbased materials, Ravoo and Glorius reported the use of carbene precursors: NHC·CO2 (15·CO2) adducts as well as NHC(H)· HCO3 salts (9·H2CO3), for the microcontact printing of NHC SAMs on gold (Figure 14).122 In order to create an NHC pattern, the researchers soaked an ozonized PDMS stamp in a solution of the NHC precursor in ethanol. The stamp was then J


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surface patterning. Throughout this work, the use of the carboxylate or bicarbonate precursors was critical, as these are single component precursors that generate only simple, volatile precursors upon surface functionalization and do not require inert atmospheres for handling. Thiol-based SAMs are extensively employed in the modification of electrode surfaces, an application where NHCs may be able to make significant impact. Chi and Glorius have illustrated this in their preparation of pentacene-based organic field-effect transistors (OFETs).123 Gold electrodes treated with IPr (12) survived annealing at 200 °C prior to pentacene deposition and performed significantly better than transistors prepared without an organic interlayer. Devices prepared with IPr under optimized conditions had hole mobilities higher than pentacene-based devices prepared on bare gold electrodes or thiol-modified electrodes. The IPrfunctionalized OFETs also had dramatically increased thermal stability, leading the authors to conclude that “N-heterocyclic carbenes turn out to be excellent substitutes as modifiers for Au electrodes in OFETs”.123 2.1.3. NHC SAMs on Planar Metalloid Surfaces Silicon(111). In addition to the increasing body of work describing the interaction of NHCs with metal surfaces, Johnson and co-workers published a seminal paper describing the reaction of a variety of NHCs with hydrogen terminated silicon surfaces.53 This transformation is relevant not only in terms of exploring the reactivity of NHCs with different surfaces but also in developing methods for precision functionalization of silicon surfaces. As a model for a hydrogen terminated silicon surface, Johnson’s team employed the model compound tristrimethylsilylsilane (HSi(SiMe3)3, 36), Figure 15,53 to probe the reactivity of various carbenes. Typical carbenes such as IMes (8), SIMes (11), and i-Pr2Im (37) were unsuitable, but cyclic amino alkyl carbenes (CAACs) and acyclic diamino carbenes (ADACs) could be employed to modify HSi(SiMe3)3 and eventually planar silicon surfaces. The researchers correctly postulated that based on the proposed mechanism of reaction with the surface, NHCs with both enhanced nucleophilicty and electrophilicity such as 38−41 were appropriate candidates. Controlling steric bulk and suppressing carbene dimerization were also found to be critical. Unlike NHC reactions with metal surfaces, in this case the carbene reacts by insertion into the Si−H bond (Figure 15 a).53 This reaction is predicted to occur by initial complexation of the nucleophilic NHC with a surface silicon atom, followed by subsequent migration of the hydride to the carbene carbon. This reaction profile parallels exactly that observed in the reaction of NHCs with isolated main group compounds containing Si−H or B−H bonds.124−127 Reaction on the surface of hydrogen terminated Si(111) proceeded to saturation with CAAC 40 but to lower levels with ADAC 41.53 The authors postulate that the tunable nature of the NHC sterics will be valuable in the controlled functionalization of different types of Si surfaces. Silicon nanoparticles were also investigated and will be described in section 2.3.

Figure 14. (a) Microcontact printing of NHCs through precursors 9· H2CO3 and 15·CO2. (b) In-filling of unfunctionalized spaces with azide-functionalized NHCs 34 and 35, followed by click reactions with alkynylated mannose or biotin. (c) TOF−SIMs imaging of patterned NHCs on Au. Adapted with permission from ref 122. Copyright 2018 Wiley-VCH.

brought into contact with the gold surface and the assembly heated to 55 °C in vacuo for 30 min to transfer the NHC to the surface. The process was monitored by XPS analysis to confirm NHC attachment to the surface (Figure 14). Since electronic devices need to withstand temperature extremes during operation, the NHC patterns were heated to 120 °C for 6 h.122 During this time, the NHC strips remained unchanged as determined by time-of-flight mass spectrometric (TOF-SIMS) imaging. Consistent with the results of Marder et al., who demonstrated that NHCs increase the work function of gold, the conductivity of the surface increased in areas where the NHC SAM was deposited, as determined by conductive probe atomic force microscopy (CP-AFM) measurements. To further illustrate the utility of the method, the Ravoo Glorius team decorated patterned surfaces orthogonally by filling in spaces with azide-functionalized NHCs such as 34 and 35 (Figure 14).122 These NHCs were then biofunctionalized by treatment with alkynylated mannose or biotin. Spatially resolved analysis of the surfaces by TOF-SIMS demonstrated that the NHC patterns were retained through multiple processing steps, highlighting the potentially high utility of this approach in

2.2. NHCs on Nanorods

Nanorods, which are intermediate species between planar surfaces and nanoparticles, have received little attention with regard to the use of NHCs as ligands. The only reported example thus far comes from the Johnson group, which employs a novel method in which the NHC ligand is introduced with an attached K


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complex was accomplished by treatment with tBuNH2·BH3. The resulting NHC-protected nanorods were shown to be highly resistant to temperature extremes, pH changes from 2 to 14, and long-term stability in high ionic strength media. Most importantly, these NHC-protected nanorods also have unprecedented stability in the presence of glutathione, which is a critical test for any species destined for biological activity. Nanorods prepared from the same NHC but without the attached gold atom were significantly less stable, illustrating the importance of the “adatom” strategy. Finally, the gold nanorods were able to affect changes in the temperature of their environment when treated with light, showing their potential for use in photothermal therapy. 2.3. NHCs on Nanoparticles

The use of NHCs as stabilizing ligands for metal nanoparticles is the most common application of these ligands in materials science to date.55,56 The development of robust ligands such as NHCs for gold nanoparticles is important fundamentally, and also for applications in biosensing and theranostics.129,130 NHCs have also been employed as ligands for nanoparticles of more reactive metals, including Pd, Ru, and Ir, where use in catalysis is the ultimate goal.131−134 This section will be organized by metal, starting with Au, followed by more reactive metals. Methods employed for the synthesis and characterization of the nanoparticles will be discussed in detail as this is still an emerging area of study. For reports in which NHC-functionalized nanoparticles of Au and more reactive metals are discussed, these will be presented separately in each respective section. 2.3.1. Initial Reports of NHC-Stabilized Metal Nanoparticles. The chemistry of NHCs as ligands on metal nanoparticles has its origins in investigations into the true nature of surface-bound species in imidazolium-based ionic liquid-stabilized nanoparticles.135−138 Finke was the first to demonstrate that NHCs bound to nanoparticles were likely key intermediates in imidazolium-stabilized ionic liquids (IL) based on the incorporation of deuterium at carbon 2 (eq 2) of the

Figure 15. (a) Model compounds for hydrogen-terminated Si surfaces, with planar and nanoparticle examples; (b) persistent carbenes employed in this study. Adapted with permission from ref 53. Copyright 2016 American Chemical Society.

gold atom in the form of an Au(I) complex (42-AuBr).128 The researchers hypothesized that this would negate the need to reorganize the underlying gold lattice of the nanorod. The presence of a pendant masked thiol in 42-AuBr, revealed by photoirradiation, provides an additional point of attachment to the surface and enables the researchers to carry out the functionalization in a two-step process: first, by revealing the thiol and, second, by reducing the gold(I) to gold(0) (Scheme 5). Commercial nanorods protected with cetyltrimethylammonium bromide were treated with NHC−Au(I) complex 42-Au under UV irradiation, which resulted in attachment of the NHC−Au−Br complex to the surface via a thiolate linkage (42Au@Au).128 Subsequent reduction of the surface-bound Au Scheme 5. Use of Au-Containing Complex to Introduce NHCs onto the Surface of Gold Nanorodsa

NHC upon exposure to D2, indicative of binding to the surface via the carbene carbon.138 Dupont and co-workers also reported the exposure of deuterated imidazolium ions to metal nanoparticles and H2. This resulted in deuterium loss at C-2, also suggesting that NHC formation occurs on the surface (eq 3).54 The intermediacy of surface-bound NHCs may then be responsible for the increased stability of nanoparticles dissolved in imidazolium ionic liquids. At approximately the same time as this work, Lin et al. described the formation of stable liquid crystals derived from functionalized silver NHC complexes and their reduction with NaBH4 to yield nanoparticles.139

a

Adapted with permission from ref 128. Copyright 2019 Nature Publishing Group. L


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Lin and co-workers later utilized gold NHC-based metallic liquid crystals (MLCs) as precursors for NHC-protected gold nanoparticles via both chemical reduction and thermal decomposition.140 Imidazolium and benzimidazolium liquid crystals have long been studied in the literature, but their use in NHC-protected metal nanoparticles is still in its infancy. NHCcontaining MLCs49,141,142 will be discussed in greater detail in section 3.6 of this review. 2.3.2. NHC-Stabilized Au Nanoparticles. The first intentional use of NHCs as protecting groups for nanoparticles came independently from the Fairlamb/Chechik and Tilley groups.55,56 Chechik, Fairlamb, and co-workers described the preparation of NHC-functionalized nanoparticles by treating preformed dodecylsulfide-protected gold nanoparticles with free NHCs.56 Incorporation of the NHC and loss of dodecylsulfide was confirmed by XPS analysis. The resulting nanoparticles were stable for long periods of time in the solid state; however, in solution they were only stable for 12 h. In this study, only t-butylfunctionalized NHC 44 was employed (Scheme 6). Fairlamb

It should be noted that the size regimes given in Table 1 are specific to the actual conditions reported. It is likely that similar starting materials and reducing agents can provide nanoparticles of different sizes under different conditions. Please note that water-soluble NHC-functionalized nanoparticles will be discussed in a separate section. 2.3.2.1. NHC-Stabilized Au Metal Nanoparticles by Bottom-up Methods. Pileni and Roland have described numerous examples of NHC-stabilized nanoparticles.144−147 In their first report, the bottom-up approach was employed by reduction of various NHC−M−Cl complexes (M = Au or Ag) (Table 1, Entries 3−4).144,145 In early methods (Table 1, Entry 3), thiol additives were included along with the NHC complex with the goal of improved stability.144 However, the same group later reported that this method leads to nanoparticles whose surfaces are predominanly covered with thiols, rather than NHCs (a maximum of 10% NHC was found on the surface as determined by XPS).145 The reduction of metal complexes without any added thiol gave NHC-protected nanoparticles, which were shown to have considerably higher stability in solution as well as under the highly oxidative environment of oxygen plasma (Table 1, Entry 3). Only 2.5% ligand loss was observed for 4@AuNP after oxygen plasma treatment compared to 17.5% for DDT protected NPs.145 In addition, Pileni and Roland have reported the self-assembly of 48@AuNP bearing long alkyl chains into 3D super lattices by the controlled evaporation of colloidal solutions of the nanoparticles (Table 1, Entry 4).147 Ultrasmall Au nanoparticles were prepared by Guari, Monge, and co-workers by thermal decomposition of NHC-ligated Au(C6F5) complexes.148 Various methods were employed to prepare 49·AuC6F5, which was heated to between 250 and 285 °C to produce ultrasmall nanoparticles (49@AuNP, Table 1, Entry 5). Analysis of the nanoparticles by MALDI-TOF (positive mode) gave multiple peaks of [Aun(C18H37−NHC)n+1]+ type, where, in each fragment, an additional “Au(NHC)″ component is added. This may indicate that the NHC remains on the nanoparticles after thermal decomposition but that cluster-like species are labile under mass spectrometric conditions.148 The intact NHCmodified nanoparticles were supported on silica and shown to be catalytically active for the reduction of 4-nitrophenol with NaBH4. Lee, Song, and co-workers used NHC 50 with bis-thiophene (BT) as the wingtip groups to prepare the Au complex 50·AuCl, Table 1, Entry 6.149 When the chloride ligand was removed from 50·AuCl with AgOTf, simultaneous polymerization of the BT units along with nanoparticle formation was observed. The authors postulated that the cationic Au(I) complexes disproportionated to Au(0) and Au(III) complexes, with the Au(III) species then causing oxidative polymerization of the BT units and the Au(0) species decomposing to form nanoparticles. While these nanoparticles were not of the highest quality, they were superior to those created without the NHC present, which underwent facile aggregation.149 The NHC-protected nanoparticles were shown to have catalytic activity in the NaBH4promoted reduction of 4-nitrophenol.149 Crudden and Nazemi recently described the use of amphiphilic NHC ligands for gold nanoparticles where one wingtip group contained a long alkyl dodecyl chain and the other had a triethylene glycol (TEG) group (51 Table 1, Entry 7).150 The NHC−Au−Br complexes were reduced with NaBH4 in a biphasic solvent system of water and methylene chloride to give highly uniform nanoparticles, which were then purified by

Scheme 6. Bottom-up and Top-down Approaches to the Preparation of Au Nanoparticles by Tilley55 and Fairlamb.56

and Chechik observed (NHC)2Au+ and NHC-Au-X species after nanoparticle degradation. This was confirmed by 1H NMR spectroscopic analysis, which showed no molecular species initially, but these species grew in as the nanoparticle degraded.56 The Tilley group took a different approach to the preparation of NHC-modified Au nanoparticles: reduction of molecular NHC−Au−Cl complexes with borohydrides or boranes143 to yield NHC-coated Au nanoparticles (Scheme 6).55 The nature of the wingtip groups was found to be very important. Complexes such as 12·AuCl with large wingtip groups did not yield nanoparticles but instead gave molecular Au hydrides when treated with borohydride reducing reagents. The use of 1,3-diisopropylimidazol-2-ylidene (ImiPr2, 37, Table 1, entry 1) did give nanoparticles, but these were small in size and difficult to purify from molecular species. Employing complex 45·AuCl (Table 1, Entry 2), with long alkyl chains as substituents on nitrogen gave NHC-protected nanoparticles that were stable for several months in the solid state and in solution and were redispersible in a variety of solvents, as the first example demonstrating the importance of the wingtip groups on nitrogen.55 Again, (NHC)2Au+-type species were observed when the nanoparticles were interrogated by mass spectrometry. Both Tilley’s “bottom-up” method and Fairlamb and Chechik’s ligand exchange or “top-down” approaches have been used extensively in the literature (Table 1), although few head-to-head comparisons of the two methods have appeared.57 M


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Table 1. Various NHCs Used in the Bottom-up Approach to Make Au NPs along with the Starting Ligands, Size, and Details

dialysis. The resulting nanoparticles had an average diameter of 4.1 ± 1.1 nm by TEM. Dynamic light scattering revealed a hydrodynamic radius of ∼9 nm in THF, a solvent in which they exist discretely. A plasmonic band was observed at 525 nm, and XPS confirmed the presence of the NHCs on the AuNP surface.

The self-assembly of these nanoparticles was carried out in polar solvents such as deionized water and ethanol, which was employed to preferentially solubilize the TEG wingtip group. While some small aggregates were observed in water, much larger, higher order aggregates were observed in ethanol. N


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Table 2. Various NHCs Used in the Top-Down Approach to Make Au NPs

studies illustrated that the method of preparation was more important than denticity. Thus the most stable nanoparticles were prepared by the top-down method employing NHCs 52− 54, which have long alkyl tails. Large differences in stability were not observed for nanoparticles prepared in this way using monodentate or bidentate NHCs, as long as they were functionalized with a long alkyl chain on the backbone. This remains one of the few reports to study the effect of the method of preparation of the nanoparticles. A recent example of optically active NHC-protected gold nanoparticles comes from the Chin and Reithofer groups.151 Land D-histidine were used as the base for the preparation of the NHC ligand, which was then complexed with silver and reacted with Au(SMe2)Cl to give NHC−Au−Cl complexes 55·AuCl (Table 1, Entry 10). These were then reduced with t-BuNH2· BH3 to give chiral NHC-stabilized Au NPs. The resulting nanoparticles displayed mirror image CD spectra for L- and Dhistidine−functionalized NHCs; however, the starting complexes were CD-inactive. This implies that surface-stacking may be important in the chirality of these materials. The nanoparticles were stable in aprotic organic solvents such as CH2Cl2 and THF for 48 h. In a recent study by Toste and co-workers, Au NPs were prepared by reduction of NHC−Au−Cl complex 56·AuCl with either t-BuNH2BH3 or NaBH4 (Table 1, Entry 11).152 These particles were stabilized by incorporation within a dendrimer, which prevented aggregation of the particles. This stabilization resulted in an increase of catalytic activity for lactonization at 20 °C versus the ligandless Au NPs, which were inactive below 80 °C for this reaction. Aggregation was further prevented through

Consistent with the changes in size, the plasmonic band shifted from 525 to 555 and 580 nm, respectively, for water and ethanol solutions.150 The first example of the use of ditopic-NHC ligands for the stabilization of gold nanoparticles was reported recently by Crudden et al. (Table 1, Entry 8, and Table 2, Entry 5, Figure 16).57 The NHC-protected nanoparticles were prepared by both the bottom-up and top-down methods, and the stabilities of the nanoparticles resulting from both methods were compared. The influence of both denticity and the presence or absence of long alkyl chains on the backbone of the NHC was examined. The presence of the NHC on the surface was confirmed by XPS analysis for all nanoparticles prepared. For the top-down approach, nanoparticles protected with dodecyl sulfide were used as starting materials, and the bidentate NHCs were introduced as the free carbenes. TEM analysis showed that the resulting nanoparticles were of a similar size as the starting nanoparticles, indicating little to no etching took place during the reaction (Figure 16). In the bottom-up approach, preformed NHC-Au-X complexes were reduced with NaBH4 in ethanol yielding NHC-protected nanoparticles that were approximately ∼2.0 nm in diameter.57 The bottom-up method produced smaller nanoparticles when compared to the top-down method with the same ligand. All nanoparticles prepared with NHC 9, or its ditopic analog, which do not have long alkyl tails, aggregated in solution within 1 week.57 By comparison, nanoparticles protected with alkylated NHCs 52−54 or their Au complexes showed very high stability. These nanoparticles were stable upon heating to 130 °C in xylene, although slight sharpening of the surface plasmon peak suggested some ripening of the particles. Thiol-degradation O


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quantities of molecular (NHC)2Au+ species were observed to contaminate the nanoparticles, which may also be a consequence of solubility issues preventing efficient removal of molecular contaminants by washing. Consistent with this, when the group employed NHCs substituted with long alkyl chains, the resulting nanoparticles were more soluble and the (NHC)2Au+ contaminant could be removed. In addition to identifying an important possible surface contaminant species, this work presented the first theoretical evidence for the restructuring of the Au surface upon binding of the NHC.153 The prediction of a slight lifting of the gold atom bound to the NHC at the surface has been supported by other work as will be described below and may be a precursor to etching of Au from the surface by NHC ligands. NHC-protected nanoparticles prepared via ligand exchange with thioether-protected Au nanoparticles have also been reported by Glorius et al.154 In this case, the goal was to prepare NHC-protected nanoparticles of more reactive metals such as Pd for applications in catalysis, which will be discussed in section 2.3.3.2, but Au-nanoparticles were examined as an initial benchmark. Methyl wingtip groups were employed in order to limit steric interactions with the surface of the nanoparticle, and the carbene and long alkyl chains were incorporated on the backbone of the carbene to inhibit aggregation (NHC 58, Table 2, Entry 3). t-BuONa was used to deprotonate the imidazolium salt in situ to give the free carbene ligand. This solution was then exposed to the nanoparticles in a biphasic system of acetonitrile and hexanes.154 The resulting nanoparticles were analyzed by TEM and UV−vis spectroscopy. Their stability and catalytic activity were not reported, but the catalytic activity of related Pd nanoparticles was studiedsee section 2.3.3.2. Yang, Chang, and co-workers recently described NHCprotected Au nanoparticles made using the ligand exchange method as catalysts for the electrochemical reduction of CO2.155 Ligand exchange was carried out on oleylamine-protected Au nanoparticles employing the free carbene (IMes, 8, Table 2, Entry 4). Solid-state 13C NMR spectroscopy and FT infrared spectroscopy (FT-IR) were employed to verify ligand exchange; however, XPS analysis of the surface was not reported. The resulting nanoparticles showed catalytic activity for CO2 reduction, which is a process that is attracting considerable interest in an attempt to valorize this high-volume green-house gas. NHC-protected nanoparticles showed higher catalytic activity compared to bare Au nanoparticles along with improved Faradaic efficiency (selectivity for CO production),155 making these species interesting for electrocatalysis. 2.3.2.3. NHC-Stabilized Au Metal Nanoparticles from Ionic Liquids. Building on much of the work proposing NHCs as incidental surface-bound ligands in ionic liquid-stabilized nanoparticles, in 2009 Lin et al. disclosed the use of liquid crystalline NHC-Au MLC complexes as precursors for NHCprotected Au-NPs.140 Alkyl chains of varying lengths were used as wingtip groups on imidazolylidene ligands coordinated to gold. These complexes were reduced with NaBH4 in a biphasic solution with H2O and methylene chloride to produce nanoparticles. TEM analysis showed that the nanoparticles had diameters between 10 and 20 nm in size. In this initial study, nanoparticle stability was not assessed. The Beer group has described the deliberate use of goldcontaining ionic liquids as precursors to NHC-functionalized nanoparticles (eq 4).156 In initial attempts, the team employed Im+AuCl4− ionic liquids (59) that were reduced with NaBH4 to produce Au nanoparticles.156 Although this method failed to

Figure 16. TEM images and particle size distributions of Au NPs stabilized by DDS (dodecylsulfide) and of NHC-stabilized Au NPs prepared by the top-down and bottom-up approaches. (a) DDS-Au NP (2.4 ± 0.8 nm), (b) 54@AuNP TD (3.0 ± 0.8 nm), (c) 52@AuNP TD (3.4 ± 1.1 nm), (d) 53@AuNP TD (3.5 ± 1.9 nm), (e) 54@AuNP BU (2.5 ± 0.4 nm), (f) 52@AuNP BU (2.0 ± 0.4 nm), (g) 53@AuNP BU (1.9 ± 0.7 nm). Adapted with permission from ref 57. Copyright 2018 American Chemical Society.

incorporation of the aforementioned particles on a solid silica support. 2.3.2.2. NHC-Stabilized Au Metal Nanoparticles Prepared by Ligand Exchange. The preparation of nanoparticles by ligand exchange (top-down) has been a popular method since it was first described in 2009 by Fairlamb and co-workers (Table 2, Entry 1).56 In 2014, Richeter prepared NHC-protected nanoparticles by a top-down ligand exchange method starting from dodecylsulfide-protected nanoparticles (Table 2, Entry 2).153 As is often the case in nanoparticle work, concerns about the instability of the free carbene prompted its generation in situ from the corresponding imidazolium or benzimidazolium salts along with strong base (in this case t-BuONa or LiBEt3H). Similar to the work of Chechik and Fairlamb, Richeter observed precipitation of the nanoparticles, likely a consequence of the lack of solubilizing groups on the NHC. In addition, significant P


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Johnson and co-workers reported an effective strategy to create water-soluble, NHC-protected nanoparticles by employing imidazolylidene carbene 62, with polar poly(ethylene glycol) (PEG) functional groups on the backbone.159 An alkynefunctionalized imidazolium salt was reacted with 2 kDa N3− PEG−OMe using click chemistry, resulting in the desired PEGylated salt (Scheme 7). Metalation of this imidazolium ion Scheme 7. Synthesis of Water-Soluble AuNPs Reported by Johnson et al.159 create high quality nanoparticles, when this ionic liquid was reacted with NaH prior to reduction, nanoparticles were obtained.156 This was attributed to the ability of NaH to deprotonate the imidazolium cation, which likely then reacted with the AuCl4− anion to form some type of NHC-Au-X complex as a precursor to reduction. Evidence for surface functionalization was presented through 1 H NMR spectroscopy, in which the C-1 imidazolium proton was absent. Although this would not distinguish between the formation of molecular complexes and surface-bound NHCs, the method did provide stable nanoparticles as evidenced by TEM studies. As with previous reports, the naure of the wingtip groups was crucial to nanoparticle stability: NHCs with hexyl groups (59) gave stable nanoparticle whereas both tBu and nPrsubstituted NHC salts provided unstable nanoparticles. Pd nanoparticle preparation and catalysis was also described, which will be discussed in section 2.3.3.2.156 In 2015, the Pileni group employed this method to prepare supracrystals of NHC-modified AuNPs,145 as noted in the previous section. In 2018, the Fensterbank and Ribot groups reported a similar method for the preparation of NHC-protected gold nanoparticles with diameters from 3 to 12 nm (Chart 3).157

and treatment with Me2S−Au−Cl gives Au complexes NHC· AuCl (62·AuCl) and NHC·AuCl3 (62·AuCl3) in equal amounts.159 This mixture of complexes was then reduced with t BuNH2·BH3 to give the desired water-stable nanoparticles. The hydrodynamic diameter of the resulting nanoparticles was observed to be 16.2 ± 0.1 nm, and the nanoparticles were stable for up to 3 months in water at room temperature. Stability in a variety of pHs, ionic strengths, and buffers was tested, and the nanoparticles performed extremely well, being stable between pH 3 and 14, to aqueous NaCl concentrations up to 250 mM for 6 h and to buffers at pH 3.2, 7.4, and 9.1 for up to 3 days. At high or low extremes of temperature (−78 or 95 °C), the nanoparticles did degrade in 5 h; however, this would not be relevant for biological applications. Exposure to exogenous thiols did cause degradation of the nanoparticles, but the NHCmodified nanoparticles were stable in thiol-free solutions for upward of 3 months. Another technique employed to create water-soluble nanoparticles is to place charged functionality, such as sulfonate or carboxylate groups, onto the carbene either at the wingtip group or onto the backbone. Glorius and co-workers utilized these functional groups on the wingtip group to give the imidazolylidene carbenes 63 and 64 (Table 3, Entry 2).160 The precursor salts were deprotonated in situ and then introduced onto preformed Au nanoparticles protected with thioethers by an exchange reaction. The presence of the NHC on the surface was assessed by NMR spectroscopy, showing broadened signals indicative of surface effects of the metal nanoparticle. Thermal gravimetric analysis (TGA) gave a metal content of 90 ± 2%, which corresponds to a metal:ligand ratio of ca. 12:1. TEM analysis showed that the exchange process was accompanied by changes in the size of the nanoparticle from 8.5 ± 1.7 nm to 4.1 ± 1.5 nm, which is attributed to etching by the NHC producing molecular complexes. Consistent with this, molecular species were observed after purification by dialysis. Interestingly, etching did not appear to be an issue when Pd nanoparticles were exposed to the same conditions showing the importance of testing different metals rather than extrapolating directly. In both cases, the NHC-modified nanoparticles were stable in solution for upward of 3 months.

Chart 3. Liquid Crystalline Imidazolium Salts Used as Precursors for NHC-Protected AuNPs

Nanoparticles were prepared by reducing azolium salts 59−61 directly, without prior deprotonation. The diameter of the nanoparticles was controlled by introducing (benz)imidazolium bromide to the reaction or by changing the reductant to NaBH4 (4 to 6 nm nanoparticles) or tBuNH2BH3 (6 to 12 nm nanoparticles). Nanoparticles were also synthesized via a one pot synthesis with AuCl, (benz)imidazolium bromide and NaBH4, which afforded nanoparticles with diameters of ∼4 nm. 2.3.2.4. Water-Soluble NHC-Stabilized Au Nanoparticle Syntheses. For nanoparticles to be employed in biological applications, water solubility is a critical parameter. Several different approaches have been described in the literature to prepare water-soluble NHC-modified gold nanoparticles. Gold is often the metal of choice for these studies due to its nontoxicity and biocompatibility. Another key factor in nanoparticles destined for biological applications is resistance to thiols, which are common functional groups in biological systems and are known to cause decomposition of nanoparticles.158 Q


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Table 3. Use of NHCs Used to Make Water-Soluble Au NPs along with Preparative Method, Size, and Stability

Cortes-Llamas and co-workers described the formation of Au nanoparticles when NHC−Au−X complexes derived from amino acids such as glycine, alanine, methionine, and phenylalanine underwent decomposition in the solid state in the absence of any reducing agent or heat.162 In a 2016 follow-up report on these nanoparticles, their stability in aqueous solutions was described.161 Similar to Glorius’ work,160 these ligands contain pH-sensitive carboxylic acids. The most stable nanoparticles were those functionalized with amino acid-derived NHC ligands based on rac-methionine as the wingtip groups (65, Table 3, Entry 3). This may be due to the presence of sulfur in the ligand, which may bind to the surface in a multidentate mode, although this possibility was not explored. Interestingly, for the [NHC·H][AuCl4] salt employed as the nanoparticle precursor, the structure predicted by NMR spectroscopy was consistent with the sulfur, not the NHC, being coordinated to gold.161 As seen with other carboxylic acid-functionalized nanoparticles,37 dispersion was observed in base, and aggregation in acid. However, again, the effect of the thioether in the methionine-derived system was important, since aggregation was irreversible for all nanoparticles other than those derived from methionine, which could be reversibly precipitated and redissolved up to 15 times. The methionine−NHC-functionalized nanoparticles were treated with various amino acids and could recognize different enantiomers, but a simple relationship between ee and chiroptial response could not be determined. The Crudden group recently reported another method for the preparation of water-soluble/pH tunable NHC-functionalized nanoparticles in which carboxylate-functionalized benzimidazolylidenes were employed as the ligand (66, Table 3, Entry 4).37 In this case, the carboxylate group is attached to the backbone of the ligand and thus is prevented from reacting with the NHC surface. To prepare the nanoparticles, a carboxylate-functionalized benzimidazolium salt was synthesized and then reacted with Me2S−Au−Cl to produce 66·AuCl (Figure 18). The prepara-

The behavior of the nanoparticles was tested under a range of pH conditions, with aggregation observed at lower pH, likely because protonation of the sulfonate and carboxylate groups removes electrostatic repulsion between the nanoparticles.160 However, this is a reversible process, such that increasing the pH to 10 redisperses the nanoparticles. This effect could be repeated for up to three cycles without any observable decomposition of the nanoparticles. This reversible aggregation was observable in UV−visible spectroscopy as seen in Figure 17.

Figure 17. UV−vis measurements of 64@AuNPs in aqueous solution showing the reversible aggregation and redispersion of the NPs depending on the pH. Adapted with permission from ref 160. Copyright 2015 American Chemical Society. R


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while the [(NHC)2Au]+ complex took over 24 h to achieve comparable nanoparticles. To circumvent this problem, HAuCl4 was added to the reduction of [662·Au]+X−, ostensibly to provide a source of the nonligated gold atoms that would be needed at the nanoparticle core. In this case, reaction times for nanoparticle formation could be reduced to 5 h from 24 h.37 Thermogravimetric analysis (TGA) of particles prepared by the two different methods showed that they had the same ratio of NHC to Au, suggesting that the surfaces of both were densely covered with NHC. XPS analysis also confirmed the presence of the NHC on surfaces.37 This is the first report to describe nanoparticles prepared by reduction of a mixture of functionalized and nonfunctionalized Au complexes. Polyacrylamide gel electrophoresis (PAGE) and transmission electron microscopy (TEM) were employed to visualize the nanoparticles (Figure 18b) and showed that nanoparticles made from a mixture of HAuCl4 and complex [662·Au]+ were in the same size regime as those made with NHC−Au−Cl alone.37 The stability and pH response of the two types of nanoparticles also supported the presence of NHC on the surface of both nanoparticles. PAGE analysis of the nanoparticles also showed a high level of monodispersity (Figure 18c).37 By altering the reaction time, nanoparticle size could be controlled, with longer times giving larger nanoparticles and shorter times giving smaller nanoparticles. Importantly, both conditions gave nanoparticles with narrow size distributions. TEM analysis of the nanoparticles gave core sizes of 3.3 ± 0.4 nm when the biscarbene complex alone was reduced and 2.4 ± 0.3 nm when the same complex was reduced in the presence of HAuCl4 for 24 h.37 UV/vis spectroscopy was employed to assess nanoparticle stability. As expected, nanoparticles were stable when the carboxylate was deprotonated (pH 8 and 10) for up to 2 months. Stability to high ionic strength media (e.g., 150 mM NaCl solutions) was also observed for up to 7 days. Small nanoparticles without a surface plasmon signal were more sensitive, degrading after 24 h in pH 8 solutions, while larger SPR-active nanoparticles were resistant to these conditions, implying an increased stability at this size.37 Although these stability parameters bode well for in vivo applications, in all cases the nanoparticles were found to decompose upon exposure to thiols.37,159 Photoacoustic imaging, which gives excellent spatial resolution and deep tissue penetration for bioimaging,163,164 was examined by treatment of the water-soluble nanoparticles with a pulsed laser beam. Acoustic wave responses that were linear with respect to nanoparticle concentration were observed, which is promising for eventual imaging applications.37 2.3.3. Non-Au NHC-Stabilized Metal Nanoparticles. In addition to the NHC-protected Au nanoparticles described above, there has been considerable interest in the use of NHCs to stabilize nanoparticles of reactive metals, including Ru, 133,165−169 Pd, 56,131,132,154,156,160,170−173 Ir, 137,138 Ag,139,144,174,175 and Pt.176 These metal nanoparticles are typically but not exclusively examined for catalytic applications. 2.3.3.1. NHC-Stabilized Ruthenium Nanoparticles. The first paper describing NHC-protected Ru nanoparticles came from the Chaudret group in 2011 (NHCs 44 and 12, Chart 4a).165 In order to create these NHC-functionalized nanoparticles, NHCs were introduced as the free carbenes during the decomposition of [(Ru(cod)(cot)]. Imidazolium salts were avoided in order to decrease the chance of simple ionic liquid-type adsorption on

Figure 18. (a) Synthesis of water-soluble NHC−Au nanoparticles by direct reduction of molecular NHC−Au complexes 66·AuCl or [662· Au]+. (b) PAGE (Tris-HCl/glycine) analysis of the purified products. (c) Synthesis of water-soluble NHC−Au nanoparticles by direct reduction of molecular NHC−Au complexes 66·AuCl or [662·Au]+, or a mixture of [662·Au]+ and HAuCl4. PAGE (Tris-HCl/glycine) analysis of the purified products. Scale bar: 20 nm in all TEM images. Adapted with permission from ref 37. Copyright 2017 Wiley-VCH.

tion of NHC−Au−Cl (66·AuCl) was always accompanied by [(NHC)2Au]+X− ([662·Au]+X−, X = Cl or OTf, with the triflate originating from the imidazolium ion and halide from gold salt), although 66·AuCl could be separated from the mixture. Considering the ease of preparation of [662·Au]+X−, this species was also employed as a precursor for NHC-functionalized nanoparticles (Figure 18a).37 As expected, [662· Au]+X−reacted at a much slower rate than the NHC−Au−Cl complex, since the formation of nanoparticles would require cleavage of a strong Au−C bond. The NHC−Au−Cl complex with NaBH4 produced monodispersed nanoparticles after 5 h, S


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Chart 4. Ru Nanoparticles in Catalysisa

In order to observe the carbenic carbon, 13C-labeled derivatives were prepared and analyzed by 13C NMR spectroscopy. Nanoparticle 44@RuNP was characterized by a broad peak at 190 ppm attributed to the carbene carbon coordinated to the Ru surface, and IPr analogue 12@RuNP displayed a similar signal at 205 ppm.165 Nanoparticles 44@RuNP required 0.5 equiv of ligand relative to the starting ruthenium complex in order to form stable nanoparticles, whereas both 0.2 and 0.5 equiv of NHC 12 (IPr) produced stable nanoparticles (Figure 19). Free sites on the nanoparticles were probed by the addition

Figure 19. Space-filling model of an NHC-stabilized Ru nanoparticle with 1.5 hydrides per surface Ru. Reproduced with permission from ref 165. Copyright 2011 Wiley-VCH.

of 13CO, which showed that accessible Ru atoms are present on the surface, with differences in terms of bridging vs terminal CO ligands being observed depending on the nature of the NHC. The catalytic activity of these nanoparticles for pure arene hydrogenation was further examined in collaboration with the van Leeuwen group (Chart 4c).133 Using o-methyl anisole as the substrate, the effect of nanoparticle ligand was examined. Ru nanoparticles protected with the bulkier iPr ligand (12) were found to be considerably more active than those protected with ItBu (44). In the latter case, excess NHC is needed to stabilize the nanoparticles, which may inhibit catalysis. In general, the authors observed a strong relationship between the nature of the ligand and the catalytic activity.169 This same group, in collaboration with Glorius, also investigated the activity of Ru nanoparticles functionalized by chiral NHC ligands.166 Chiral NHC 67 was able to protect and stabilize Ru nanoparticles, and treatment of the surface with 13 CO showed the presence of acccessible sites on the nanoparticle. Consistent with this, the NHC-protected nanoparticles were active for the hydrogenation of various substrates, however no enantiomeric enrichment was observed in any cases for the hydrogenation of ethyl-2-oxocyclohexanecarboxylate (Chart 4d).166 Fogg et al. reported that Ru nanoparticles are formed by decomposition of the NHC-containing second-generation Grubbs metathesis catalyst during olefin metathesis.167 The presence of NHC on the surface of the nanoparticles was not described. Interestingly, these nanoparticles were highly active for the isomerization of estragole (Chart 4e), a reaction that could be halted by the addition of phosphine or phosphite ligands.167 These observations have important implications for the use of isomerization-sensitive olefins in metathesis.

a

(a) Basic structure of RuNP. (b) Various NHC-Ru NPs.133,166−169 (c) Hydrogenation of methylanisole by 12 or [email protected] (d) Selective hydrogenation of ethyl-2-oxocyclohexanecarboxylate by [email protected] (e) Isomerization of estragole by [email protected] (f) Oxidation of (E)-3,7-dimethylocta-2,6-dien-1-ol with 68 or 58@ RuNP.168 (g) Transformation of phenylmethanol to N-benzylpropan2-amine using 68 or [email protected] (h) Hydrogenation of phenylacetylene with 69 or [email protected]

the surface. The nanoparticles had good monodispersity and were highly crystalline as determined by high-resolution transmission electron microscopy (HRTEM) analysis. When the nanoparticles were analyzed by 1H NMR spectroscopy, only the protons on the wing-tip groups could be observed, which the authors attributed to the increase in T2 from slow tumbling of the particles in solution as well as surface heterogeneities, which may inhibit detection of the other protons on the ligand.165,177 T


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Chart 5. Pd Nanoparticles in Catalysisa

a

(a) Various structures of NHCs employed in the preparation of Pd nanoparticle catalysts, either self-supported or on various inorganic supports. (b) Reactions catalyzed by the NHC-functionalized nanoparticles described in section a, along with key results for each system

In a collaborative effort between Chaudret, Glorius, and Philippot, the effect of employing NHCs decorated with long alkyl chains on Ru nanoparticles was examined (Chart 4f and g).168 NHCs with small (dimethyl) and large (2,6-diisopropylphenyl) wingtip substituents were examined (68 and 58, Chart 4f and g).168 CO complexion studies indicated that nanoparticles derived from the bulkier NHC 68 had more open sites, as would be expected. Different reactivity was also observed, such that nanoparticles protected by the less bulky NHC showed no activity for the hydrogenation of ketones or aromatic rings, while nanoparticle 68 resulted in over-reduction, for example nonselective reduction of both olefins of geranial alcohol (Chart 4f) or concomitant arene reduction occurring alongside desired reductive amination (Chart 4g).

NHC-protected Ru nanoparticles supported on inorganic oxides have been reported by Tada and Glorius.169 In order to prepare these species, Ru nanoparticles supported on Al2O3 were prepared and then treated with NHCs 8 and 69 (Chart 4h). When 13C-labeled NHCs were employed, signals were observed at 165 ppm (for 8@RuNP) and 164 ppm (69@ RuNP). These spectra were compared with NHCs prepared on pure potassium-doped alumina (K-Al2O3), without Ru, which led to a low intensity signal in the CP-MAS NMR spectrum at 169 ppm (for 8/K-Al2O3) and 168 ppm (for 68/K-Al2O3). XPS analysis was also employed to support the presence of Ncontaining species on the surface. The resulting nanoparticles were then used in the hydrogenation of a number of different substrates (Chart 4f).169 U


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prior addition of NaH, in the case of Pd, stable nanoparticles were prepared by simple treatment with NaBH4. This may be due to the more active nature of Pd, as noted above, which is capable of inserting into C−H bonds. The effect of modification of the wingtip groups on the NHC has rarely been studied systematically; however, in a 2014 publication, Ravoo and Glorius explored this important parameter in the context of Pd nanoparticles.154 They employed NHCs functionalized by small methyl substituents on nitrogen and long alkyl chains on the backbone of the carbene in order to stabilize the nanoparticles (58@Pd NP) against aggregation.154 NHCs with benzyl substituents on the NHC nitrogens were also examined in order to introduce potential π interactions between the ligands on the surface (73@Pd NP, Chart 5a). The top-down approach was utilized to introduce the NHC via ligand exchange on preformed Pd nanoparticles. The long alkyl chains on the backbone were shown to protect the nanoparticles, which were stable against aggregation in solution for over four months. When the bulky ligand 12 was employed, unstable nanoparticles resulted, although this ligand did appear capable of stabilizing smaller nanoparticles in the 1−2 nm regime. 13C NMR spectroscopy was employed to illustrate the formation of a Pd-carbene bond after exchange. While the NHCs protected the nanoparticles against incorporation of thioether, thiol was able to penetrate and displace the NHC. As these were intended for catalytic applications, thiol instability is not a complicating issue. Nanoparticle 58@PdNP was tested for catalytic activity in the chemoselective hydrogenation of 3,7-dimethyloct-6-en-1-yl acrylate as seen in Chart 5b.154 Selectivity was only observed when the nanoparticles were protected with an NHC ligand. Thioether-protected nanoparticles showed lower activity, and Pd/C gave no selectivity, highlighting the significant influence of the NHC on catalytic activity and selectivity for the Pd nanoparticle. In a comprehensive study, Ravoo and Glorius also prepared catalytically active, water-soluble Pd nanoparticles prepared with charged substituents on the NHC nitrogens.160 Carboxylate or sulfonate groups were incorporated on the nitrogen wingtip groups to impart water solubility and introduce electrostatic stabilization of the resulting nanoparticles (64, 74, 75, and 78 Chart 5a). A top-down method was employed in which the NHC displaced thioethers from an independently prepared thioether-protected nanoparticle. The charged nature of the NHCs enabled an elegant synthesis, in which a biphasic mixture of DMF and hexanes could be employed to prepare the NHCstabilized nanoparticle and simultaneously separate the nonpolar thioether byproduct. Functionalized nanoparticles could then be isolated by dialysis in water (Figure 20). Thermogravimetric analysis was employed to determine the ligand to metal ratio. With a metal content of 63−70%, the M:L ratio ranged from 3.5:1 to 7:1 and correlated with ligand size. Unlike related gold nanoparticles,160 the Pd nanoparticles showed no detectable change in size or dispersity during this process. Examination of size changes by TEM after pH-induced precipitation/dissolution cycles showed that the NHCprotected NPs were able to retain their size distribution after repeated precipitation/dissolution cycles. These NHC-protected Pd nanoparticles were shown to be active in the aqueous hydrogenation of olefins even under low pressures of hydrogen, with differences observed based on the nature of the NHC. Under conditions of low loading and short times, 78@PdNP was the most active among those tested (74, 75, 64, and 78@

The catalytic activity of the various supported catalysts, ranging from those without any NHC to those prepared with excesses of NHC to Ru was measured.169 In all cases, increasing the amount of NHC on the surface led to a decrease in the catalytic activity, explained by a reduction in the number of active sites on the surface. The authors were able to take advantage of the increased reactivity by surface-bound NHCs to carry out the selective hydrogenation of the alkyne but not the arene in phenyl acetylene (Chart 4h). Higher selectivity for alkyne hydrogenation producing ethylbenzene relative to full hydrogenation producing ethylcyclohexane was observed in catalysts with larger amounts of NHC on the surface (Chart 4h).169 2.3.3.2. NHC-Stabilized Palladium Nanoparticles. NHCmodified Pd nanoparticles have also been extensively studied, with early reports appearing in 2009 by the Chechik group along with their study of NHC-functionalized Au nanoparticles (56, Table 2, Entry 1).56 For both metals examined in this early study, aggregation in solution was observed after 12−15 h. The particles were only soluble in solvents such as DMSO, which is highly polar, and would still aggregate within 15 h.56 Most of the initial work on NHC-functionalized nanoparticles following up on the Chechik report was focused on tuning the reactivity of Pd nanoparticles on various inorganic supports, as will be described subsequently. However, in early work on unsupported nanoparticles, Shafir and co-workers described the use of tris-carbenes (70) designed around a central xylene framework as ligands to protect Pd nanoparticles (Chart 5a).132 Nanoparticles were prepared by reaction of trisimidazolium salt 70·3HX with Pd(dba)2 under hydrogen at room temperature. It should be noted that because of the higher catalytic activity of metals such as Pd for insertion into C−H bonds including that at C2 of imidazolium salts, NHC-functionalized nanoparticles can be prepared without the need for deprotonation of the imidazolium salt. In the 13C NMR spectrum of the NHC-functionalized nanoparticles, a broad signal at about 165 ppm was attributed to the carbene-Pd bond, which is similar to the shift of Pd-NHC bonds in molecular complexes. Interestingly this peak was only observed when the counterion of the starting imidazolium salts was iodide. When BF4− was used as the counterion, this peak did not appear, which the authors attribute to the formation of NHC-protected nanoparticles in the presence of the iodide but not tetrafluoroborate salts.132 Catalytic activity was tested in the Suzuki−Miyaura cross coupling reaction at 0.2 mol % Pd (Chart 5b). The nanoparticles showed good activity. TEM analysis of recovered nanoparticles showed that they were smaller and more aggregated clusters of these small nanoparticles than the starting nanoparticles, which implies ripening of the nanoparticles, complicating interpretation of the catalytically active species.132 Closely related NHC 71 was employed by Chang and Wen for the ligation of Pd foil in the electrocatalytic reduction of CO2 to formate and carbon monoxide (Chart 5b).178 Beer et al. used halopalladate imidazolium salts as precursors for Pd nanoparticles.156 Similar to their work generating Au complexes, the Beer group prepared Pd nanoparticles from imidazolium palladate salts, which were reacted with a strong base (NaH). The base is proposed to deprotonate the imidazolium salt, generating the free carbene and, hence, the Pd complex in situ. Subsequent addition of a reducing agent (NaBH4) then gives nanoparticles 44, 59, and 72@Pd NP, which have different substitutents on nitrogen (Chart 5a). Unlike the formation of gold nanoparticles, which requires the V


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Figure 20. Method of preparation of NHC-protected Pd and Au nanoparticles by Ravoo and Glorius. Adapted with permission from ref 160. Copyright 2015 American Chemical Society.

PdNP), giving turnover numbers of 2000 h−1 for the hydrogenation of styrene (Chart 5b). Important control experiments were performed with Pd/C, which was effective at high catalyst loadings; however, at lower loadings (0.041 mg of Pd/0.2 mmol substrate), Pd/C was found to be inferior to the NHC-protected Pd nanoparticles (16% yield compared with quantitative yield).160 Molecular species Pd−NHC complexes were also inferior to NHC-protected Pd nanoparticles. An interesting class of hybrid bidentate NHC ligands was also reported by Ravoo and Glorius, in which the ligand contains an NHC and a pendant thioether on one of the wingtip groups (NHCs 79−82, Chart 5).172 Nanoparticles were prepared using the top-down approach of ligand exchange with preformed Pd nanoparticles. The solubility of the resulting nanoparticles was controlled by substituents introduced at the thioether, including alkyl chains, PEG groups, and carboxylates.172 As with gold nanoparticles, the Pd nanoparticles could be cycled between pH extremes to precipitate and redisperse the material. Chemoselective catalytic hydrogenation was carried out, with the carboxylated nanoparticles displaying a solvent dependent catalytic activity and high selectivity for terminal olefins in the presence of internal olefins (Chart 5b). Control experiments carried out with Pd/C showed no such selectivity. Chaudret and co-workers reported the use of Knight shifts in 13 C NMR spectra as a powerful method to study NHCprotected Pd nanoparticles (77@PdNP, Figure 21).170 Knight shifts occur when nuclei couple to conducting electrons present in their immediate environment and are commonly observed for ligands attached to metal nanoparticles.179 As these shifts are not observed for the molecular complexes, they provide conclusive proof that a ligand is indeed coordinated on the metal nanoparticle surface. These studies are important since the CNHC-M carbon may not be easily distinguishable from the corresponding adsorbed molecular complex. Thus, the observation of a Knight shift provides definitive proof of high proximity to the nanoparticle surface.154 For NHC-functionalized Pd nanoparticles, Knight shifts were observed at 600 ppm in the 13C NMR spectra when the carbenic carbon was 13Clabeled (77@PdNP Figure 21).170,180 Chaudret and de Jesús also reported Pd nanoparticles protected with sulfonate-functionalized NHCs.181 NHC−Pd complexes were decomposed either thermally, under 13CO or under H2 to give NHC-protected nanoparticles that had average diameters ranging from 1.5 to 7 nm depending on the decomposition conditions. Initial studies carried out using spin echo techniques are shown in Figures 21a−d. Application of the Carr−Purcell−

Figure 21. Effect of nanoparticle size in the solid-state 13C NMR spectra of 77@Pd after 13CO adsorption. (a−d) Spin−echo spectra for 77@Pd NP (4.8 nm), (3.8 nm), and (3.1 nm), and 77@Pd NP synthesized with CO (1.3 nm). (e,f) Spectra for 77@Pd NP (4.8 nm) and 77@Pd NP synthesized with CO (1.3 nm) recorded using the CPMG technique. Adapted with permission from ref 170. Copyright 2017 Wiley-VCH.

Meiboom−Gill (CPMG) sequence (Figures 21e−f) resulted in a strong reduction in line width and signal-to-noise. Knight shift frequencies were observed at 600 ppm in the 13C NMR spectra, confirming the presence of the NHC 77 on the surface of the nanoparticle as discussed above.170,48 In addition to providing definitive confirmation of the presence of the NHC on Pd nanoparticles, this detailed study also showed that despite multiple purifications, the NHC-protected nanoparticles could not be isolated without significant contamination by the molecular complex. It was estimated that with ligand 77 (Chart 5b), only 3% of the total NHC content is bound to the nanoparticle by a metal−C bond and the remainder is in the form of molecular species. NOESY experiments revealed that there are significant dynamic interactions between adsorbed molecular species and the Pd surface, illustrating the complexity of this system. The nanoparticles were also useful as recyclable catalysts for the chemoselective hydrogenation of styrene, although the dynamic nature of the surface makes it challenging to ascribe catalytic activity to a single species. Chaudret and de Jesús also reported Pd nanoparticles protected with sulfonate-functionalized NHCs including 77.181 NHC−Pd complexes were decomposed either thermally, under 13CO or under H2 to give NHC-protected nanoparticles W


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addition tests, hot filtration studies, as well as a three-phase test where one substrate was immobilized on a solid support (Chart 5b).182 All of these tests led to a shutdown of reactivity, implying that the active catalyst is heterogeneous in nature. Consistent with this result, higher loadings of NHC led to lower catalytic activity, suggesting that high surface coverage is counterproductive in terms of catalysis. 2.3.3.3. NHC-Stabilized Iridium Nanoparticles. As previously noted, the first example of a metal nanoparticle protected with an NHC ligand came from the Finke group, with the metal being iridium.138 In a follow-up paper, Finke showed that the presence of NHCs on the surfaces of Ir nanoparticles is detrimental to catalytic activity in the case of acetone hydrogenation reactions, with the implication being that NHCs block the active surface of Ir.137 As shown above, however, NHC-protected nanoparticles of Ru and Pd are often highly active, and thus, it is not clear whether this effect is metalspecific.132,133,168 2.3.3.4. NHC-Stabilized Silver Nanoparticles. One metal that has thus far been difficult to functionalize with NHCs is silver,139,174 with the exception of a handful of reports from Pileni and co-workers,144 who synthesized both gold and silver nanoparticles using the bottom-up approach with NHC 4 (Table 1, Entry 3). However, only gold nanoparticles were found to be stable, and the silver analogues gave insoluble precipitates. Using DDT as an additive allowed for nanoparticles of both metals to be isolated; however, as described previously, the thiol may be the true ligand in this case. In the organometallic precursor to the nanoparticle, the silver-carbene bond was shown to be sensitive to thiol exchange, which suggests loss of the NHC during any syntheses involving thiols.144 Even in thiol-free systems, low stability is expected to some degree since silver NHC complexes have long been used as NHC transfer agents because of their weaker M−C bonds when compared to other complexes.183 Prezhdo and Brutchey recently examined the ability of ditetradecylbenzimidazolylidene (85) to protect nanoparticles of either Ag or Ag2E (E = S, Se).175 A bottom-up approach was employed to synthesize the nanoparticles by reducing the corresponding 85·AgBr complex with NaBH4 as the reducing agent (Figure 22). A low concentration of the molecular complex was employed specifically to limit aggregation. The diameter of the nanoparticles was easily controlled by altering the concentration of the precursor and NaBH4.175 When dispersed in nonpolar solvents, the nanoparticles were stable for up to 1 week. These nanoparticles were treated with a solution of Se or S at 120 °C in 1-octadecene to give [email protected] The Ag2E nanoparticles were larger in size than their Ag counterparts with diameters of 12.4−15.2 nm depending on whether S or Se was used. They were considerably more stable than the 85@ AgNP and did not aggregate in solution after 6 months at room temperature. Larger binding affinities were calculated by DFT for the 85@Ag2ENPs compared to the Ag nanoparticles, which supports why there is greater stability in solution for the Ag2E type nanoparticles.175 When analyzed by 1H NMR spectroscopy, both Ag and Ag2E derivatives showed NHC signals with surface broadening effects as expected.184 In order to further confirm the presence of NHCs on the surface, diffusion-ordered NMR spectroscopy (DOSY) studies were conducted. The diffusion coefficients imply that the ligands are labile on the surface and that an equilibrium between unbound and bound ligands exists.175,184 All Ag-based nanoparticles prepared had a significantly lower

that had average diameters ranging from 1.5 to 7 nm depending on the decomposition conditions. The activity of 77@PdNP in the hydrogenation of styrene was examined, and it gave high conversion after 3 h without evidence for metallic Pd, although analysis after reaction by TEM showed substantial nanoparticle reorganization giving worm-like particles. 2.3.3.2.1. Supported NHC-Stabilized Pd Nanoparticles. Glorius and co-workers have prepared NHC-protected Pd nanoparticles supported on magnetite (Fe3O4) in order to facilitate catalyst removal from solution.131 Since enantioselective catalysis was the goal of this study, chiral NHC 83 was used as the protecting ligand (Chart 5a). As in the Glorius/Tada report of supported Ru nanoparticles,169 the NHC was introduced after the Fe3O4 nanoparticles were impregnated with Pd nanoparticles. Moderate to high levels of enantioselectivity were observed when these catalysts were employed for the asymmetric α-arylation of ketones with aryl halides (Chart 5b). Mercury poisoning tests and filtration test results led the authors to attribute catalytic activity to supported species. Interestingly, racemic arylation products were observed when Pd(OAc)2 was employed as a homogeneous catalyst with ligand 83, demonstrating a remarkable effect of the nanoparticle-based catalyst. Glorius and co-workers also employed siloxane ligands to tether chiral NHCs such as 84 to magnetite nanoparticles, followed by introduction of Pd and reduction to nanoparticles.173 The supported NHC was also examined as an immobilized molecular Pd catalyst and on its own as an organocatalyst, where it was shown to be highly effective for the allylation of aldehydes. Although recyclability was problematic in some cases for the supported Pd nanoparticles, the supported nanoparticles were found to catalyze the allylation of 4nitrobenzaldehyde with similar yields and selectivities as the molecular catalyst (Chart 5b). Interestingly, the solvent played a significant role, with reasonable yields but no enantioselectivity obtained for the supported catalyst in any solvent other than THF. This system had other unique aspects, such as a reversal in the absolute sense of enantioselectivity depending on whether the Pd was in the form of a supported complex or nanoparticles. When allyltrichlorosilane was employed instead of allyltributylstannane, the tethered NHC itself was able to catalyze the allylation in 74% ee.173 Glorius, Doltsinis, and Muratsugu recently reported another example of the effect of NHCs on supported nanoparticles.182 Using Pd/Al2O3 as the catalyst, modified by imidazolium salts added alongside KOtBu as a base, the team compared the catalytic activities of various formulations for the hydrogenolysis of bromobenzene. The researchers observed that the addition of bulky NHC precursors (8 and 12) gave the best results, with smaller NHCs showing results similar to but slightly better than unmodified Pd/Al2O3. XPS, 13C NMR spectroscopy, FT-IR, and TGA studies were used to confirm the presence of the NHC on the surface. The use of 13C-labeled NHCs facilitated analysis by 13 C CP-MAS NMR spectroscopy, showing two distinct carbenic signals at 163 and 156 ppm. Both signals are attributed by the authors to the binding of the NHC to the Pd nanoparticle, presumably in different coordination environments. Electronic modification of the Pd nanoparticles was inferred by the observation of 0.3−0.4 eV shifts in the X-ray photoelectron spectra. The nanoparticles were used to catalyze Buchwald−Hartwig aminations of aryl halides, a reaction that was proposed to proceed via heterogeneous catalysis based on the results of Hg X


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protected platinum nanoparticles.186 When the ligand/metal ratio was optimized to 0.2 equiv of NHC 12 (IPr) to Pt(dba)2, nanoparticles with 2.0 ± 0.2 nm diameters were obtained and displayed high chemoselectivity and catalytic activity for the mild hydrogenation of nitroaromatics (1 bar H2, 30 °C). The Chaudret group continued their investigation into NHC ligands for platinum nanoparticle stabilization with a 2014 report of NHC-modified water-stable platinum nanoparticles.187 As discussed in previous sections, the synthesis of water-stable metal nanoparticles is highly desirable for many applications.37,181 Similar to the approach taken by the Glorius group for water-soluble gold and palladium nanoparticles, sulfonate groups were incorporated on the wingtip groups of the NHCs 76, 77, 87, and 88 (Chart 6).160 The resulting nanoparticles Chart 6. Water-Soluble Pt NPs and Their Application in the Hydrogenation of Styrene

Figure 22. (a) UV−vis absorption spectra of 85@AgNP, 85@Ag2SNP, and 85@Ag2SeNP suspensions in toluene. The inset is a photograph of dilute toluene suspensions of the NCs. (b) Powder XRD patterns of 85@AgNP, 85@Ag2SNP, and 85@Ag2SeNPs. (c−e) TEM micrographs of (c) 85@AgNPs, (d) 85@Ag2SNPs, and (e) Ag2SeNPs. The insets are the size distributions of the corresponding NPs (n = 500 counts). Reproduced with permission from ref 175. Copyright 2016 American Chemical Society.

concentration of NHCs on the surface when compared to related NHC-Au nanoparticles.159 The authors also synthesized NHC-protected Cu nanoparticles; however, these possessed limited stability. The corresponding NHC-Cu2−xE nanoparticles were stable, consistent with the results obtained for NHC-Ag2E particles.175 2.3.3.4.1. NHC-Stabilized Silver Nanoparticles from Ionic Liquids. In 2006, the Lin group reported the appearance of Ag nanoparticles when excess Ag2O was employed in the synthesis of Ag-NHC MLCs (Figure 23A).139 The starting metal−

were synthesized by the thermal decomposition of Pt-NHC complexes, which promotes reductive elimination of alkyl ligands. The resulting Pt(0) nanoparticles could be dispersed in aqueous media for months without degradation. Solution NMR spectroscopy experiments confirmed the presence of the NHC on the surface of the nanoparticles by the observation of 13 C−195Pt coupling in the solid state NMR spectra (J = 940 Hz), performed with a 13C-labeled NHC ligand. These nanoparticles were shown to be active catalysts for the chemoselective hydrogenation of styrene to ethylbenzene at room temperature in water (1 bar H2, 0.4−0.6 mol %, Chart 6) and were more active than the starting Pt-NHC complexes. The nanoparticles could be recycled nine times with only 0.44% loss of platinum observed and no change in morphology (ICP-MS and TEM).187 Gross and Toste described the use of synchrotron-radiationbased infrared nanospectroscopy (SINS) for the analysis of transformations of NHC-protected Pt nanoparticles in situ.188,176 In an intriguing new approach, NHC 89 was employed with hydroxyl groups integrated into the wingtip substituents so that their oxidation and reduction could be followed by SINS when on Pt nanoparticles (Scheme 8). SINS

Figure 23. (a) SEM images of AgNPs. Reproduced with permission from ref 139. Copyright 2006 American Chemical Society. (b) Betaineester-derived NHCs.

Scheme 8. Reversible Oxidation and Reduction between 89a@Pt NP and 89b@Pt NP

carbene complexes could also be reduced using NaBH4 in a biphasic system with CH2Cl2 and water, with the resulting nanoparticles displaying a band at ∼409 nm in the UV−vis spectra. In a subsequent study, they produced Ag NPs in the presence of a betaine-ester analog of imidazolium salts (86·HX, Figure 23b), as betaine-ester-containing salts often have excellent liquid crystalline properties. However, the resulting nanoparticles were quite unstable and characterization of surface species was impossible; thus, the presence of an Ag-NHC bond on the surface was never confirmed.185 2.3.3.5. NHC-Stabilized Platinum Nanoparticles. In 2014, Chaudret and co-workers reported the first example of NHCY


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subnanometer to ∼2.2 nm, or the equivalent of 10−300 gold atoms.193 NCs fill the gap between isolated metal atoms and plasmonic metal nanoparticles (Figure 24).194 Plasmonic

permitted the analysis of different Pt sites on the nanoparticle, such as edge sites, which are lower in coordination number than other sites on the nanoparticle. In this case, the strength of the NHC−Pt bond was invoked to ensure immobilization of the NHC on the Pt throughout the process. SINS analysis of centers and edges of nanoparticles showed deviations in both reduction and oxidation of the wingtip groups based on the location examined. Step edge sites were found to be more active for oxidation and reduction when compared to flatter areas of the Pt nanoparticles. NHCs containing nitro-tagged wingtip groups were also placed on Au nanoparticles and shown to behave similarly in surface oxidation/reduction reactions of the nitro substituents.176 2.3.3.6. NHC-Stabilized Nickel Nanoparticles. The bulk of nanoparticles of the platinum group metals are based on palladium and less so platinum as described in the previous two sections. However, Kobayashi and co-workers have also described the preparation of polymer-incarcerated nickel nanoparticles that are held within the polymer with the additional aid of a polymer-bound NHC.189 The resulting nanoparticles were shown to be active catalysts for the Kumada coupling. More recently, Godard, de los Bernardos, and co-workers demonstrated that NHC-functionalized, well-defined nickel nanoparticles could be prepared by the decomposition of Ni(COD)2 in the presence of imidazolium carboxylate 14·CO2. The resulting nanoparticles were employed in the semihydrogenation of alkynes to alkenes.190 2.3.3.7. NHC-Stabilized Silicon Nanoparticles. In addition to the functionalization of metal nanoparticles, the reaction of NHCs with silicon nanoparticles has also been described.53 Johnson and co-workers prepared hydride-terminated silicon nanoparticles by a sol−gel/HF exthing method. Attenuated total reflectance IR (ATIR-FTIR) was used to characterize the nanoparticles, whose surfaces were terminated with mono-, di-, and trihydrides. Reaction with CAAC NHC 40 and ADAC carbene 41 occurring via insertion into the Si−H bonds was predicted based on extensive spectroscopic studies, including NMR spectroscopy and X-ray photoelectron spectroscopy.53 2.3.3.8. NHC-Stabilized Upconversion Nanoparticles. Although most of the examples described in this section relate to metals in their zerovalent state, surface functionalization of highly oxidized nanoparticles has also been reported.191 “Upconversion” nanoparticles with the composition NaTF4:Yb,Tm were protected by NHC 58, previously described as an effective ligand for protection of Pd surfaces (Chart 5b).154 The NHC ligand was introduced via ligand exchange onto nanoparticles coated with oleic acid. The NHC-protected nanoparticles were stable under acidic conditions and showed long-term stability in solution. These materials are photoactive and can activate photoresponsive azobenzene-type molecules using light in the near-infrared region (λ = 980 nm). The rate of the switching reaction was 3 times as fast in a 2 h time frame when the nanoparticles were covered with NHC ligands compared to the starting nanoparticles coated with oleic acid.

Figure 24. Representation of the common differences between a nanoparticle (top) and nanocluster (bottom). Reproduced with permission from ref 194. Copyright 2014 American Chemical Society.

nanoparticles are typically larger than 3 nm, and although they can be prepared with precision on the nanometer scale, e.g. (6 ± 0.3 nm), they lack atomic precision and they cannot be described with exact molecular formulas. Unlike NCs, the energy levels of nanoparticles can be described as a continuous electronic band characterized by a distinctive surface plasmon resonance.193 While a complete understanding of the interfacial binding of organic ligands and surface metals in conventional nanoparticles is still missing, the X-ray structures of many nanoclusters have been determined by analysis of single crystals, providing a clear understanding of their precise structures.193,195 A critical component of nanoclusters is the presence of surface ligands that stabilize the structure and prevent agglomeration. X-type ligands, such as thiolates, alkynyls, and selenolates, and L-type ligands, such as phosphines, are commonly employed as capping agents to prepare atomically precise metal nanoclusters.192,193,195 The wide majority of nanoclusters are prepared from gold as the metal, stabilized by thiolate ligands. In 2007, the first X-ray crystal structure of the Au102(SR)44 nanocluster was revealed, providing critical information about thiolate bonding.196 Instead of simple covalent bonds, thiolates stabilize the central Au(0) atoms by surrounding them with an oxidized layer of Au(I) thiolate “staple” structures (Figure 25). L-type ligand-protected nanoclusters are predominantly represented by phosphine-stabilized Au nanoclusters.193,197,198 A seminal report by Hutchison describes the synthesis of two structurally similar Au11 clusters, protected by triphenylphosphine ligands.198 In this, both Au11(PPh3)7Cl3 (left, Figure 26) and [Au11(PPh3)8Cl2]Cl (right, Figure 26) were prepared via the reduction of Ph 3 P·AuCl with NaBH 4 . Of these, [Au11(PPh3)8Cl2]Cl was found to be more stable, with Au11(PPh3)7Cl3 decomposing into Au nanoparticles over time. The presence of 8 Au(0) centers means that both of these clusters are examples of small Au superatom clusters. Simon and co-workers reported the structure of an Au14 cluster, [Au14(PPh3)8(NO3)4], with 2 neighboring central gold atoms.199 Teo et al. revealed the synthesis and the crystal structure of a large Au39 cluster, [Au39(PPh3)14Cl6]Cl2, with an hcp layered structure (1:9:9:1:9:9:1).200 Schmid et al. reported the synthesis a gold cluster with an exceptional size, Au55[P-

2.4. Atomically Precise Metallic Nanoclusters Stabilized by NHCs

2.4.1. Introduction to Ligands-Protected Atomically Precise Metallic Nanoclusters. Metal nanoclusters (NCs) are atomically precise ultrasmall nanoparticles in which surface metal atoms are protected by organic ligands.192 Gold NCs are by far the most common, with diameters ranging from Z


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the position of the carbene carbon bound to AuI in the corresponding mono NHC−Au−Cl and bis−NHC−Au complexes, which appears at 175 and 187 ppm, respectively.212 The cluster was stable in d8-THF, CD2Cl2, and CD3CN solutions for days at ambient temperature under inert atmosphere. Also, no decomposition was detected by 1H NMR spectroscopy upon exposing CD2Cl2 solutions to humid air for several hours or adding excess water or methanol. The authors attributed the high stability of this cluster to the large HOMO−LUMO gap, calculated to be 5.42 eV. On the other hand, this cluster was easily oxidized by iodine, a source of HI, or a thiol, eventually forming three equivalents of NHC−Au−X (X = I or SR). The Bertrand group described the preparation of a cationic trigold clusters [(CAAC-Au)3]+ (92 and 93) stabilized by three cyclic (alkyl)(amino)carbene (CAAC) ligands (97) (Scheme 9).60 One of the synthetic routes involved the use of the CAAC− AuCl complex (94) to prepare the μ3-oxo complex [(CAACAu)3O]+BF4− (95) followed by carbon monoxide reduction (Scheme 9). The analogous μ3-oxo cluster bearing simple NHC ligands in place of the CAAC ligands could not be prepared by this method.211 The other synthetic route employed involved the exchange of Ph3P ligands in the μ3-oxo-[(PPh3Au)3O]+ BF4− complex (96) with free CAAC ligands, followed by carbon monoxide reduction (Scheme 9). X-ray diffraction studies of the CAACbased clusters revealed that the Au3 cluster adopts a planar ring with Au−Au distances ranging from 2.6324 to 2.6706 Å with a similar structure and bond distances found in [(NHC-Au)3]+ (91).211 In the 13C NMR spectra, the carbene-carbon appeared at 265 ppm whereas the mono CAAC-AuICl and μ3-oxo AuI3 CAAC3 appeared at 237 and 230 ppm.213 Notably, the carbene carbon appeared at 287 ppm in the neutral (CAACmenthyl)-Au0Au0(CAACmenthyl) compound, highlighting the large shifts that can occur in Au(0) compared to Au(I) species.213 The 13C NMR spectroscopic values are in accordance with this species being a trinuclear mixed-valence AuI/Au0 cluster. The [(CAAC-Au)3]+X− cluster (92) was reported to be airand moisture-tolerant in the solid state and in solution. It was employed in the carbonylation reaction of amines, where it showed good catalytic activity. Interestingly, the reported catalytic process appears to represent a rare example where the CAAC-stabilized Au0 atoms reversibly change their oxidation state from Au0 to Au1, unlike the common use of gold catalysts as soft Lewis acids without changes in oxidation state.212,214 Evidence to support this comes from the isolation of proposed intermediates with these oxidation state changes and with Au−Au bond distances consistent with AuI−AuI distances. It is worth noting that in this example and the previous Sadighi report, the number of valence electrons is two, making these the smallest NHC-protected gold superatom structures.89,215−217 In addition to these two synthetic examples, theoretical studies have been performed on NHC-protected clusters and

Figure 25. Au102(SR)44 nanocluster, with representation of “staple” bond, where two thiol ligands stabilize an external Au(I) atom. Adapted with permission from ref 89. Copyright 2012 Nature Publishing Group.

Figure 26. Molecular structure of Au11(PPh3)7Cl3 (left) and [Au11(PPh3)8Cl2]+ (right). Adapted with permission from ref 198. Copyright 2014 American Chemical Society.

(C6H5)3]12Cl6.201 Based on Mössbauer and X-ray absorption spectroscopy studies, Schmid et al. proposed a cuboctahedral structure of Au55.202−204 However, a quasi-icosahedral structure of Au55 was later suggested based on X-ray diffraction studies205,206 and predicted by DFT calculations.207 To date, the Au55 structure has not been confirmed by mass spectrometry or by single-crystal X-ray structure determination.193 Gold clusters protected with multidentate phosphine ligands are also known, for instance Au20 clusters protected with di-208 or tri-209 phosphine ligands and Au22 clusters210 protected with diphosphines have been reported. In terms of L-type ligandprotected nanoclusters, the number of known examples is limited and comprised mainly of phosphine-stabilized Au nanoclusters. Bonding does not involve staples as in thiolstabilized clusters (Figure 25), but rather simple dative bonds to the surface gold atoms (Figure 26).192,193 Examples of gold clusters protected with mixed thiolate-phosphine, seleonolate (SeR), and alkynyl (CCR) have been reported.192,193 2.4.2. Carbene-Stabilized Tris Gold Nanoclusters. Until recently, the use of NHCs to stabilize metallic Au nanoclusters was limited to only two examples, all of which involve mixed valence metals.60,211 The Sadighi group were pioneers in this area, reducing a cationic trigold(I) carbonate (90) with carbon monoxide (eq 5), which gave a trinuclear mixed-valence AuI/ Au0 cluster [(NHC-Au)3]+ (91).211 In this cluster, stabilized by three imidazol-2-ylidene molecules with bulky N-2,6-diisopropylphenyl substituents (12), the three gold atoms form a nearly equilateral Au3 ring with all carbene-carbon atoms coplanar. Au−Au distances range from 2.6438 to 2.6633 Å. Interestingly, for the first time, 13C NMR spectroscopy could be employed to observe the resonance for the carbene-carbon bound to 2 Au0/ AuI, which occurred at 205 ppm. This is a significant shift from AA


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Scheme 9. Preparation of Homoleptic CAAC Protected tris-Au Nanoclusters, and Mixed Phosphine/CAAC tris-Au Nanoclusters

atom in the simulation. Among all metal clusters, the abnormal NHC examined is predicted to have the highest binding energy to the cluster, followed by normal NHC and then imidazole. The order of the binding strength for abnormal and normal NHCs is Au > Cu > Ag whereas for imidazole it is Cu > Au > Ag. Although M20 metal clusters are not magic-number clusters, this study sheds light on the differences expected for abnormal and normal NHCs bound to coinage metals. As part of a comprehensive study of the bonding of NHCs to surfaces, Tang and Jiang examined the bonding of several NHCs to hypothetical Au13 nanoclusters (Figure 28).99 They hypothesized that an Au13 icosahedral core capped by NHCs would result in magic-number clusters with an eight freeelectron count. As an Au13 icosahedral core contains 12 surface Au atoms, 12 ligands (NHCs and Cl) were employed to protect the surface and maintain an eight-electron count. Using the less bulky 1,3-dimethylbenzimidazol-2-ylidene NHC (15), Tang and Jiang estimated that a maximum of 12 ligands would be allowed on the icosahedral cluster. Thus, the following cluster compositions were proposed: [Au 1 3 (NHC15) 1 2 ] 5 + , [Au 1 3 (NHC15) 1 0 Cl 2 ] 3 + , [Au 1 3 (NHC15) 8 Cl 4 ] 1 + , and [Au13(NHC15)7Cl5]0, and [Au13(NHC15)4Cl8]3− (Figure 28). By optimizing these Au13 clusters with DFT, they found that they all possess a large HOMO−LUMO gap from 1 to 2 eV making them viable structures. Interestingly, when the bulkier IMes NHC (8) was employed, a maximum of only four NHC ligands are predicted to fit around the Au13 cluster whereas the remaining eight Au are protected by chlorides to satisfy the need

other closely related systems. Using density functional theory (DFT), Chattaraj and co-workers studied cyclic trinuclear clusters (M3+ where M = Au, Ag, Cu) stabilized with different ligands such as NHC (dimethyl imidazol-2-ylidene), isoxazole, pyridine, furan, noble gases (Ng = Ar − Rn), and carbon monoxide (Chart 7).218 Their study showed that the NHC formed the strongest bond with all M3+ ions examined and that among these ions, Au3+ formed the strongest bonds, followed by Cu3+ and then Ag3+. This ordering is consistent with the detailed experimental and computational studies of Bertrand on NHC2M2 species.213 For these L3M3+ species, electrostatic attraction was predicted to contribute more than covalent bonding. In another theoretical study, Wang, and co-workers employed DFT to study the nature of binding of imidazole and carbenebased ligands in M20 metal nanoclusters (Figure 27).219 They investigated three metal clusters, Au20, Ag20 and Cu20 with three ligands: imidazole (bound through N(3)), imidazol-2-ylidene (normal NHC, bound through C(2)) and imidazol-4-ylidene (abnormal NHC, bound through C(4)).219 The tetrahedral M20 structures are highly symmetrical and represent a fragment of FCC bulk metal having (111) faces for each four surface atoms (Figure 27). The interactions of the ligands and the metals were considered on the apex and face center positions. Calculations of the binding energies indicated that all ligands bind more strongly to the apex-A position, which is not surprising considering that this is the most unsaturated metal AB


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Chart 7. Pictorial Depiction of M3+, L, and M3L3+ Complexesa

a

M−M and M−L bond distances in Å are also provided at the M06-2X/def2-TZVP level. Reproduced with permission from ref 218. Copyright 2016 Royal Society of Chemistry.

phosphine clusters [Au11(PPh3)8Cl2]Cl and treated these with benzimidazolium hydrogen carbonates 9·H2CO3, 4·H2CO3, 15· H2CO3, and 101·H2CO3 (Figure 29). Depending on the conditions and the nature of the wingtip groups, clusters with varying levels of NHC incorporation were obtained. Treatment of the phopshine cluster with 9·H2CO3 gave single, clean substitution giving a cluster with molecular formula [Au11(PPh3)7(NHC9)Cl2]Cl (102). However, the free carbene 9 gave a mixture of clusters bearing from 1 to 5 NHCs. The structure of cluster 102 was determined by X-ray crystallographic analysis of single crystals and is shown in Figure 29. As predicted by DFT analysis, the preferred site of exchange is at the least hindered gold atom. The introduction of a 13C label at carbon 2 of 9·H2CO3 permitted the observation of the carbon−gold bond in the 13C NMR spectrum at 209 ppm as an octet due to coupling to the 7 phosphines in the cluster. Since these phosphines are not all equivalent, this implies the presence of exchange processes, a supposition supported by analysis of 31P NMR spectra. The NHC-containing clusters were tested for stability and activity in the electrocatalytic reduction of CO2 to CO. It was found that the presence of the NHC increased the cluster stability, although not uniformly, with cluster 104 ([Au11(PPh3)7(NHC15)Cl2]Cl) being the most stable. This cluster was also the most active for the electrocatalytic reduction, suggesting that catalytic activity may be tied to cluster stability.

Figure 27. Structure and bond distances in theoretical M20 clusters (M = Au, Ag, and Cu) at the CAM-B3LYP/LANL2DZ level of theory. Reproduced with permission from ref 219. Copyright 2015 Royal Society of Chemistry.

for an eight electron count. These results provide valuable guidance in the design of the NHC-protected magic-number Au nanoclusters. In recent work, the groups of Crudden, Tsukuda, and Hakkinen described the first example of Au(0) nanoclusters functionalized by an NHC ligand.220 To introduce the NHC, the team began with the previously described undecagold AC


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2.5. NHC-Stabilized Coordination Clusters

Unlike metallic nanoclusters, coordination cluster complexes, which can be referred to in the literature as metal clusters,221 are polynuclear metal complexes in which all the metals are in nonzero oxidation states.222 A limited number of examples of NHC-stabilized cluster complexes have been reported in which phosphides,221 chalcogenides,222,223 halides,224,225 or carbonyls75 are typically also present, along with NHCs as ancillary ligands, although there are examples of pure NHC-stabilized coordination clusters. Although low in number, these examples still outweigh reported examples of fully metallic clusters. One example of such a cluster was detailed in a recent report from Shionoya and co-workers, in which they described the synthesis of an Au6 cluster (Scheme 10).226 This cluster contains Scheme 10. Synthesis of [(37·Au)6(μ6-C)]2+ Clustera

a

Adapted with permission from ref 226. Copyright 2018 American Chemical Society.

an endohedral carbon atom, which is formally C4−, and thus all the gold atoms are in the +1 oxidation state. [(NHC·AuI)6(μ6C)][BF4]2, 106 (where NHC = 1,3-diisopropylimidazol-2ylidene), was prepared by the reaction of 37·AuCl with NaBF4 and AgBF4 to form [(NHC37·AuI)(μ3-O)][BF4] and subsequent treatment of this intermediate with Me3SiC(H)N2 in the presence of Et3N, to give the hexagold cluster in 25% yield. The product was air- and moisture-stable, and the solid state structure revealed a compound with D3h symmetry and varying Au−Au bond lengths. Only one set of resonances for the ligands were observed by both 1H and 13C NMR spectroscopy, however, consistent with rapid ligand exchange between all Au(I) sites.226 2.5.1. NHC-Protected Metal Chalcogenide Cluster Complexes. Generally, the synthesis of NHC-protected metal chalcogenide cluster complexes involves either a direct self-assembly between the NHC−metal complex and a chalcogenide,58,227 or exchange reactions of ligand-capped cluster complex with free NHCs.223 Examples of both types of synthetic approaches are given, organized by metal, in the following sections. 2.5.1.1. Iron. In a seminal paper, Deng and Holm reported the synthesis and detailed analysis of the first examples of all-ferrous iron−sulfur cluster complexes with Fe−C σ bonds, an edgebridged double cubane [Fe8S8(Pri2NHCMe2)6] (107, Scheme 11), and a cubane [Fe4S4(Pri2NHCMe2)4] (108, Scheme 11).58 Interest in these clusters stemmed from their use as models of the cysteinate-protected [Fe4S4]0 core of the iron protein of nitrogenase from Azotobacter vinelandii (Av2).228 In this case, NHCs were employed as ligands specifically to provide stability to the iron−sulfur clusters. The polynuclear complexes were

Figure 28. Examples of predicted structures for NHC-containing Au(0) clusters. Reproduced with permission from ref 99. Copyright 2017 American Chemical Society.

Figure 29. Synthesis of NHC-functionalized magic number clusters and X-ray crystal structure of cluster 102.

AD


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Scheme 11. Synthesis of NHC-Stabilized Fe−S Cubane Clusters and Their Subsequent Transformations

prepared either by self-assembly or phosphine displacement on FenSn (n = 4 and 8) clusters. In the self-assembly approach, starting molecular phosphineFe(II) precursors such as (i-Pr3P)2FeCl2 (109) were treated with (Me3Si)2S as a source of sulfur and scavenger of chloride, and the carbene Pri2NHCMe2 (110). Cluster formation and introduction of NHC could be performed sequentially or simultaneously, as shown in Scheme 11. NHC stoichiometry could be controlled to yield different clusters. Although the NHC-ligated cluster was still sensitive to oxidation, the addition of NHC ligands provided sufficient stability to enable manipulation under ordinary anaerobic conditions. By comparison, previously isolated CN-protected [Fe4S4(CN)4]4− was highly unstable.229 Similarly, phosphine-protected complexes [Fe4S4(PR3)4] (R = iPr, tBu, C6H11)230,231 were not stable toward isolation due to dissociation of the phosphines and aggregation of the cores into the dicubanes [Fe8S8(PR3)6] and tetracubanes [Fe16S16(PR3)8]. Similar decomposition pathways were not observed for the NHC-protected clusters, and this study provided the first opportunity for the detailed examination of the reactivity of [Fe4S4]0-type clusters. In subsequent work, Holm, Münck, and Bominaar performed 57 Fe Mössbauer and electron paramagnetic resonance studies of the NHC-capped cubane, [Fe4S4(NHC110)4] (108).232,233

Interestingly, the paramagnetic ground spin state (S = 4) and the magnetic hyperfine interactions of this NHC-capped cluster complex are very similar to those of the cysteinate-bound protein cluster. In another report (Figure 30), Bominaar and co-workers examined the geometry and spin states (S) of edge-bridged

Figure 30. Representation of the coupled spin states in [Fe4S4(NHC110)4] (107) that result in an overall S = 0 ground state. Adapted with permission from ref 234. Copyright 2010 American Chemical Society.

double cubanes,234 NHC- and R3P-substituted clusters, [Fe8S8(NHC110)6] (107) and [Fe8S8(PPri3)6] (113), by Mössbauer spectroscopy. They found that these clusters have diamagnetic ground spin states (S = 0) with similar quadrupole patterns to that of the cubane [Fe4S4(NHC110)4] (108). They suggested that the diamagnetic ground state of the Fe8S8 clusters AE


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of the complex results from the magnetic exchange interactions, which in turn depend mainly on the core geometry and are not significantly affected by ancillary ligands. 2.5.1.2. Cobalt. After the successful synthesis of NHCstabilized all-ferrous iron−sulfur cluster complexes, Holm and co-workers extended that chemistry to prepare isostructural cubane-type Co4S4 cluster complexes stabilized by phosphines and NHCs, [Co4S4(PPri3)4] (114) and [Co4S4(NHC110)4] (115) (Scheme 12).236 Analogous to the iron−sulfur cubane synthesis, the NHC-protected Co4S4 complex was prepared by the self-assembly of CoCl2, Pri3P, and (Me3Si)2S in THF forming the phosphine-stabilized complex [Co4S4(PPri3)4] followed by an in situ substitution of phosphines by an excess of free carbene 110 (Scheme 12). Alternatively, the same cluster could be synthesized by treating the preisolated phosphine cluster complex with a small excess of the free NHC. Again, single crystal X-ray structures of all the cluster complexes were determined and revealed the expected cubane-type structures. Cyclic voltammetry experiments showed that the oxidation potential of the carbene cluster was lower than that of the related phosphine cluster, Ecarbene0/1+;1+/2+ < Ephosphine0/1+;1+/2+, representing interesting quantitative evidence of the stronger σdonating ability of carbenes to the core, which in turn facilitates electron removal.236 Furthermore, the phosphine and carbene cubanes were paramagnetic with ground spin states of 3 (S = 3), which was attributed to exchange coupling among three parallel and one antiparallel S = 3/2 spins of the CoII centers in the cluster.

originates from antiferromagnetic exchange coupling of two identical Fe4S4 modules. Notably, the structural and electronic properties of iron− sulfur double cubane cores were weakly affected by the capping ligands.234 DFT studies by Boominar and co-workers on the [Fe4S4(imidazol-2-ylidene)4]0 complex led to a predicted optimized geometry of a broken-symmetry configuration with one unique and three equivalent iron sites (Figure 31).235

Figure 31. Optimized structure of [Fe4S4(imidazol-2-ylidene)4]0 with the unique iron site located at the bottom right. Carbon (gray), nitrogen (blue), iron (red), and sulfur (yellow). Hydrogen atoms are not shown for the sake of clarity. Reproduced with permission from ref 235. Copyright 2011 American Chemical Society.

Simulation of the experimental Mössbauer spectral pattern supported the postulate that the observed high spin state (S = 4)

Scheme 12. Synthesis and Transformations of NHC Stabilized Co−S Cubane Clusters

AF


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2.5.1.3. Rhenium. Szczepura and co-workers described the preparation of the first examples of hexanuclear rhenium(III)sulfide and selenide cluster complexes containing SIMes (11) and IMes (8), namely [Re6Se8(PEt3)5(SIMes)](OTs)2 (122) and [Re6S8(PEt3)2(IMes)2Cl2 (124) (Scheme 13).223 The

Scheme 14. Synthesis of NHC Stabilized Mn−Te Clusters

Scheme 13. Synthesis of NHC-Stabilized Re Clustersa

a Adapted with permission from ref 223. Copyright 2015 Royal Society of Chemistry.

situ-formed Mn(allyl)2Cl2 can be reacted with iPr2NHC-Te to give Mn4Te4(iPr2NHC)4. Single-crystal X-ray diffraction studies showed that the phosphine and NHC-capped clusters had similar core structures with comparable Mn−Te bond distances, although the Mn−Mn distances were slightly longer in the NHC-capped clusters. Notably, 127a had an increased solution-phase stability compared to that of Mn4Te4(PR3)4, which was attributed to the stronger binding of the NHC to the Mn(II). It is worth noting that after several weeks at −35 °C, 127a dimerized to the dicubane cluster 128a indicating that the NHC−M bonds are not completely inert. 2.5.1.5. Silver/Copper. Corrigan and co-workers have made substantial contributions to the area of metal chalcogenolate clusters stabilized by phosphines or NHCs, with the latter tending to give more stable complexes. In 2012, the Corrigan group described the synthesis of NHC-stabilized copper- and silver-phenylchalcogenolate ring complexes by the preparation of Cu or Ag NHC complexes 129 and 132 and their treatment with Me3SiEPh, where E = S or Se (Scheme 15a).238 These reactions lead to the preparation of multinuclear clusters 131 and 133 (Scheme 15a). Related structures were prepared by the treatment of polymeric [MStBu] species (M = Ag, Cu) with NHCs 9 or 110. In the case of copper, clusters with the same basic structure as 131a (with tBu in place of Ph) were always observed, while with silver, different clusters were observed depending on the nature of the ligand. For example, treating [AgStBu] with NHC 9 led to a cluster related to 131a, but NHC 110 gave [Ag5(StBu)6][Ag(NHC110)2].239 2.5.2. Heterometallic Coinage Metal Coordination Clusters. 2.5.2.1. Copper−Mercury. Corrigan and co-workers described the formation of the high nuclearity NHC-protected copper−mercury-sulfide complex [(NHC9)6Cu10S8Hg3] (135, Scheme 15b).240 The synthesis of this species began by reacting (iPr2-bimy)CuOAc (129b) with S(SiMe3)2 yielding [(iPr2bimy)Cu-SSiMe3]2 (134). The reaction of 134 with 0.5 equiv of Hg(OAc)2 produces the desired Cu10 cluster 135 via S−Si bond activation (Scheme 15b).

mono-SIMes Re−Se complex 122 was prepared by substituting the weakly coordinating tosylate ligand in the starting phosphine-modified complex, [Re6Se8(PEt3)5(OTs)](OTs), with 1.4 equiv of free SIMes (11) in THF at room temperature for 1 h (Scheme 13). Interestingly, attempts to replace more phosphine ligands by heating the starting complex, [Re6Se8(PEt3)5(OTs)](OTs) (121), with six equivalents of SIMes (11) for 6 h in THF, only resulted in the incorporation of two SIMes units in the cluster complex, [Re6Se8(PEt3)4(SIMes)2](OTs)2, as the major product along with the mono-SIMes complex as the minor product (ca. 10%). Cluster complexes with more than two coordinating NHCs were not detected, even with excess SIMes or prolonged amounts of time at reflux. The authors also pointed out that the separation of the bisfrom the mono-SIMes cluster complexes by chromatography was difficult, although solubility differences in acetone were sufficient to obtain enough pure samples for analysis. The bis-IMes Re-sulfur complex, [Re6S8(PEt3)2(IMes)2Cl2] (124), was obtained by refluxing the chlorine-containing complex, (Bu4N)2[Re6S8(PEt3)2Cl4] (123), with two equivalents of Ag(IMes)2PF6 in chlorobenzene for 19 h (Scheme 13). Notably, the incorporation of NHC ligands in those rhenium(III) chalcogenide cluster complexes led to attenuation of their luminescent properties compared to the starting complexes. 2.5.1.4. Manganese. Roy and co-workers described the synthesis of various trialkylphosphine- and NHC-capped manganese telluride cluster complexes (Scheme 14).237 They prepared the cubane-type core Mn4Te4(NHC)4 complex (127) by exchanging the labile phosphine-capped clusters with 4.5 equiv of free NHCs 110 or 125 in THF at room temperature. Mn 4 Te 4 (NHC110) 4 (127a) and Mn 4 Te 4 (NHC125) 4 (127b) could also be prepared using a direct self-assembly approach in which the low-valent Mn(η4-butadiene)2PMe3 was reacted with the corresponding NHC-Te. Alternatively, the in AG


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Scheme 15. NHC-Stabilized Bimetallic Chalcogenide Clustersa

a (a) Preparation and structure of Cu and Ag−chalcogenide clusters stabilized by NHCs. (b) Synthesis and structure of Cu−Hg heterobimetallic clusters. Adapted with permission from ref 238, Copyright 2012 Royal Society of Chemistry; ref 239, Copyright 2014 Royal Society of Chemistry; and ref 240, Copyright 2016 Wiley-VCH.

Under similar conditions, the reaction of [(IPr)Cu−SSiMe3], IPr (NHC 12), with Hg(OAc)2 afforded the heterometallic cluster complex [((IPr)CuS)2Hg].241 Unlike related biphosphine-stabilized cluster complexes with similar sizes and sharp emission bands at about 700 nm,242 NHC-stabilized Cu10 cluster 135 displayed a broad emission band at about 800 nm. Although compound 135 was stable under an inert atmosphere at room temperature in the solid state, it decomposed quickly in solution. 2.5.2.2. Copper−Iron. Shieh and co-workers prepared [TeFe 3 (CO) 9 Cu 2 (Me 2 Im) 2 ] (136) and the related [TeFe3(CO)9Cu2(MeIm(CH2)nImMe)2], (138a−c) n = 1−3 (Scheme 16),243 in which the small 1,3-dimethylimidazol-2ylidene NHC (14) was employed, in addition to bidentate versions 138a−c. The synthesis was achieved by reacting [TeFe3(CO)9Cu2(MeCN)2]244 with NHCs or NHC precursors (Scheme 16). These heteronuclear complexes were employed as catalysts for the homocoupling of aryl boronic acids. 2.5.2.3. Gold−Silver. Corrigan and co-workers reported the synthesis of a novel class of luminescent NHC-stabilized coinage metal chalcogenide octanuclear complexes with the molecular formulas: [Au4M4(μ3-E)4(IPr)4] (M = Ag, Au; E = S, Se, Te) (139a−c).222 These cluster complexes were synthesized by reacting an equimolar amount of [(IPr)AuESiMe3] with phosphine-ligated metal acetates, L-MOAc (Scheme 17). In contrast to many phosphine-stabilized polynuclear clusters,245 the NHC-based clusters exhibited enhanced stability, with no

Scheme 16. Synthesis of Heterobimetallic Cu−Fe Clusters Stabilized by Monodentate and Ditopic NHCs

obvious dissociation of the NHC ligands in solution, which enabled detailed NMR spectroscopic characterization. By changing the composition of the metal and chalcogen ions, the researchers were able to show that these clusters have tunable luminescent properties in 2-methyl tetrahydrofuran and in the solid state at 77 K, emitting colors that ranged from yellow-green to red.222 Time-dependent DFT calculations suggested that the tunable emission may be attributed to electronic transitions of mixed ligand to metal−metal charge transfer (IPr → AuAg2) with an interligand (IPr → M) AH


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groups prevented the aggregation of the cluster complexes, the SiMe3 moieties rendered them sensitive to air and moisture. 2.5.3.1.1. NHC-Protected Silver Halide Cluster Complexes. Few examples of silver-halide cluster complexes with the formula Ag4X4, stabilized by hybrid phosphorus-NHC,224,225,247−249 sulfur-NHC,59 and bis-NHCs63,250 bidentate ligands have been reported. In general, these types of silver complexes are mainly used as air-stable carbene transfer reagents to other catalytically active metals such as nickel or palladium. It is noteworthy that monodentate NHC-protected Ag4X4 complexes are unknown. 2.5.3.1.2. Hybrid Phosphorus-NHC Protected Silver Complexes. Braunstein and co-workers described the preparation of the first cubane-type silver-iodide cluster complex stabilized by hybrid NHC-phosphinite bidentate ligands, [Ag4 I4(PONHC)2], PO-NHC = 142a (Scheme 18).224 The aim of this

Scheme 17. NHC-Stabilized Au−Au (139a) and Au−Ag Clusters (139c)a

a

Reproduced with permission from ref 222. Copyright 2017 American Chemical Society.

Scheme 18. Application of NHC-P Ditopic Ligands in the Preparation of Ag Clusters, and the Relationship between the Size of Wingtip Groups, the Number of Atoms between the Tether, and the Structure of the Resulting Cluster/Molecules

phosphorescence character, demonstrating the role of the NHC ligands in the emission process. 2.5.2.4. Gold−Iron. Bortoluzzi et al. described the synthesis of trinuclear Fe(CO)4(AuNHC)2 complexes, where NHC = IMes, IPr, or ItBu (8, 12, or 44, respectively), by reaction of Na2Fe(CO)4 with two equivalents of Au(NHC)Cl in THF (eq 6).246 Interestingly, thermal decomposition of Fe(CO)4(Au-

(NHC8))2 (140) in DMF yields the pentanuclear cluster complex [Au(NHC8)2][Au3Fe2(CO)8(NHC8)2] (141) (eq 6). The authors carried out gas-phase DFT calculations on the coordination clusters to study the metal−metal and metal− ligand interactions in terms of molecular orbital energies, bond lengths and angles without the influence of packing forces. Theoretical data afforded computed structures that are close to those reported by X-ray diffraction analysis and indicated that the Au−C bonds are essentially sigma in nature with a low degree of π-interaction. 2.5.3. NHC-Protected Coinage Metal-Phosphide Coordination Clusters. 2.5.3.1. Silver. Najafabadi and Corrigan employed NHC 9 to stabilize the silver−phosphide polynuclear complexes [Ag12(PSiMe3)6(NHC9)6] and [Ag26P2(PSiMe3)10(NHC9)8].221 The authors noted that the ratio of the NHC-Ag complex to the phosphorus source determined the nuclearity of the complex produced. The reaction of NHC-AgOAc with P(SiMe3)3 in a 2:1 ratio produced the Ag12 complex (ca. 10%) whereas a ratio of 2:0.9 yielded the Ag26 complex (ca. 5%) via activation of P−Si bonds. NMR spectroscopy and single crystal X-ray diffraction studies suggested that NHCs are ligated to silver atoms in the +1 oxidation state while the phosphides are in the −3 oxidation state.221 Although the presence of surface NHC and SiMe3

study was to prepare clusters with hemilabile ligands to take advantage of the different bond strengths of the NHCs and less tightly bound ligands. The authors showed that treating the imidazolium salt with 0.5 equiv of Ag2O and AgI led to cluster compound 143a in high yield (95%) (Scheme 18). Singlecrystal X-ray diffraction studies illustrated the pseudocubane geometry with two PO-NHC ligands bridging opposite Ag(I) centers of the Ag4 core. The N-Me NHC 142b was employed in the synthesis of cluster 143b.225 Attempts to transmetalate the NHC from 143 to Ir(I) in [Ir(COE)2(μ-Cl)]2 were not AI


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bipyramidal geometry with the four silver atoms lying in a square planar arrangement. The silver cluster complexes were remarkably stable toward heat, light, air, and moisture, and they were used to transfer the bis-NHCs to Ni and Pd. The bis(diimidazolylidine)nickel(II) complexes, ([(diNHC)2Ni]2+) were active precatalysts in Mizoroki−Heck and Suzuki−Miyaura coupling reactions.250 Hollis and co-workers obtained NHC-capped tetrasilver(I)halide cubane-type cluster complexes by reacting different bisNHC imidazolium (151a−c) or triazolium salts (151d,e) having a 1,3-phenylene bridge with Ag2O. By comparison with the aforementioned tetrasilver complexes,251 the 1,3-phenylene spacer length allowed the NHC moieties to bridge two opposite Ag(I) centers forming a cubane-type Ag4 core (eq 8).

successful. Similar cubane clusters were prepared with phosphine-NHC ligands 142c. A cubane-type silver-bromide cluster complex stabilized by hybrid NHC-trialkylphosphine ligands, [Ag4Br4(P-NHC)2] was also prepared by treating imidazolium/phosphonium bromide salt 142c·H+ with Ag2O.247 Notably, by modifying the structures of the hybrid NHC-trialkylphosphine ligands such that the distance between the N and P atoms becomes not long enough to bridge 2 silver centers, silver complexes with structures other than the cubane-like tetrasilver geometries were obtained upon the reaction with Ag2O. For instance, NHC 144a with an NtBu wingtip group afforded a tetragonal bipyramidal structure with four silver atoms in a square planar geometry (145) while with the N-Mes derivative 144b gave a diuclear metallacycle (146, Scheme 18).248 These and related Ag−Br cubane complexes could be used in the synthesis of Ag-based cluster complexes.249 2.5.3.1.3. Hybrid Sulfur-NHC Protected Silver Complexes. Fliedel and Braunstein reported the synthesis of a planar tetrasilver complex, [Ag4I4(S-NHC)2] (148), capped with hybrid NHC-thioether bidentate ligands (S-NHC = N-aryl-Nthioether imidazol-2-ylidene, (147)).59 Analysis by X-ray crystallography showed that the silver atom formed a square planar geometry with two bridging S-NHC ligands.59 The tetranuclear complex was synthesized by treating the corresponding thioether imidazolium salts with Ag2O in the presence or absence of AgI (Scheme 19), and the resulting Ag(I)S-NHC complexes were used to generate [PdCl2(S-NHC)2] by transmetalation to Pd.59 Scheme 19. Synthesis of Silver Clusters Featuring Ditopic Mixed NHC/Thioether Donors

Crossover and variable temperature 13C NMR spectroscopic experiments were consistent with intramolecular 107/109Ag−13C exchange in these different cubane-type complexes rather than intermolecular exchange suggesting a molecular rotor-type mechanism for Ag−C interchange.251 Charra et al. tested the effect of different anions of bis-imidazolium salts on the formation of silver complexes. Again, only bromides or iodides of bis-NHCs with 1,3-phenylene bridges gave tetrasilver-halide cube-type complexes upon reaction with Ag2O.63 2.6. NHC-Stabilized Metal Carbonyl Clusters

As part of his in-depth studies examining the reactivity of Nheterocyclic carbenes with transition metals, Lappert reported pioneering work on the preparation of NHC-functionalized Ru clusters in 1977.252 Unexpectedly, the reaction of enetetramine 153 with Ru3(CO)12 resulted in a single substitution of one carbonyl ligand to yield [Ru3(CO)11(NHC)] (154) rather than disruption of the cluster (eq 9). The field lay dormant for some

2.5.3.1.4. Bis-NHC-Protected Silver Complexes. Foley and co-workers prepared tetrasilver(I) cluster complexes [Ag 4 Br 4 (bis-NHC) 2 ] (150a,b) capped with methylene bridged-bis-NHCs by reacting equimolar amounts of Ag2O and the corresponding imidazolium salts 149a/b (eq 7).250

time until the work of Whittlesey,253 Cole,254 Cabeza,255 and Wang.256 As these studies were critically reviewed in 2011 by ́ ́ lvarez, this section will focus on reports Cabeza and Garcia-A subsequent to that review.75 While early studies of NHC-substituted clusters highlighted the high propensity for secondary reaction at the NHC, typically C−H activation processes,253,255,257,258 Cabeza and co-workers investigated the reactivity of bidentate NHC-phosphine ligands (155a and 155b) with Ru3(CO)12 (Scheme 20).259 Both ligands

Single crystal X-ray crystallographic analysis showed that the cluster is a tetrasilver complex that adopts a tetragonal AJ


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Scheme 20. Synthesis of Bridging NHC/Phosphine Clusters and Their Transformation into Doubly C−H Activated Clusters

Scheme 21. Reaction of Ditopic NHCs with [Ru4(μH)4(CO)12] Leading to NHC-Functionalized Ru Clusters

reacted with Ru3(CO)12 to give edge-bridged disubstituted products (156a and b), which reacted further either at room temperature (156b) or upon heating (156a) to give products 157a and b resulting from oxidative addition of both C−H bonds of the bridging methylene, followed by rearrangement to the dihydrido derivative (Scheme 20). The more facile C−H activation in cluster 156b was attributed to the higher basicity of PCy2-containing ligand. Complete breakdown of the cluster leading to mononuclear ruthenium(0) complexes [Ru(CO)3(L)] was observed when Ru3(CO)12 was reacted with 3 equiv of the ligands.259 DFT studies performed by Zhang and co-workers on the clusters prepared by Cabeza et al. suggested that the process takes place via phosphine dissociation, sequential CH activation, and recoordination of the phosphine, with the entire process being endothermic.260 The same group studied C−H activation processes on simple monodentate NHC−Ru clusters such as [Ru3(CO)11(NHC)], where NHC = 14, which also undergoes two sequential C−H activations. This study predicted that the first C−H activation would be rate limiting and that the entire process is endergonic by ca. 15 kcal/mol.261 NHC-tethered thioether ligand 158 reacted in a different manner from NHC phosphines 155a/b, leading to a change in cluster size to tetranuclear derivative 159, which is the first example of a polynuclear derivative with an NHC-thioether ligand (eq 10).259

Notably reactions with NHC-tethered thioether (158) and phosphine (160) proceed via initial protonation of the carbene to generate [NHC·H]+[Ru4(μ-H)3(CO)12]− intermediates 161 and 162, which, upon heating, release CO and return a proton to the clusters to form NHC-ligated tetranuclear clusters (163/ 164). For pyridyl-based ligands 165a/165b and bis-NHC 166, no reaction was observed at room temperature; however, at higher temperatures, the corresponding tetranuclear complexes [Ru 4 (μ-H) 4 (κ 2 -L n )(CO) 10 ] (167−170) were observed (Scheme 21). Unlike the ligands described in Scheme 21, ditopic NHC/ quinoline ligands reacted with [Ru3(CO)12] to give simple monosubstitution rather than a bridged cluster species.263 Eventually heating to 70 °C resulted in C−H activation and binding of the quinoline to the cluster. Whittlesey and co-workers studied the reaction of ring expanded NHCs (171−173) with [Ru3(CO)12] (Scheme 22). Interestingly, these NHCs were previously shown by the Scheme 22. Reaction of Ring-Expanded NHCs 171−173 with Ru3(CO)12 Leading to Simple Substitution Accompanied by Ru Cluster Reorganization

In 2012, Cabeza and co-workers studied the reaction of tetraruthenium carbonyl cluster [Ru4(μ-H)4(CO)12] with ditopic NHC ligands (155a, 158, and 160), Scheme 21.262 AK


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Chart 8. Ruthenium Clusters 178, 179, and 181 Formed by Reaction of IPr-Au-Ph with Ruthenium Carbide Carbonyl Clusters and Their Subsequent Transformations Leading to Au−C Bond Cleavage and Rearrangementsa

a

Adapted with permission from ref 266. Copyright 2015 Elsevier.

stoichiometries, molecular (NHC)M(CO)3 species are obtained. In the Os series, small NHCs also give [Os(CO)11(NHC)] clusters, but larger NHCs remain completely unreactive at reasonable temperatures,255 and when they do react, mesoionic species form, indicating a likely greater sensitivity to steric effects in the Os series.257,269 As a further indication of sensitivity to steric effects, Hyunh and co-workers were able to demonstrate that [Os3(CO)12] reacts with NHCs via initial attack at one of the carbonyl carbons.269 The product of direct attack at the mesoionic position (183) was isolated along with acyl species 182, resulting from attack at CO. Both compounds were characterized crystallographically. Although there had been suggestions of anionic nucleophiles attacking CO in clusters to form η1C(O)Nu-substituted trinuclear clusters,270 the Hyunh study provided the first concrete evidence. DFT studies suggested that a migratory deinsertion of the acyl ligand is the pathway to cluster 183 (Scheme 23). Notably products resulting from binding via the C2 of the NHC (normal mode) were seen in small amounts and only for NHCs 8 and 12. A separate report by Huynh and Leong described the preparation of mixed Os/Pd clusters.271 By employing bulky ligands such as IPr (12), the authors were able to produce different cluster complexes under mild conditions. In all cases they obtained clusters where metal cores were “raftlike”, with CO ligands bridging all the Os−Pd edges.

Whittlesey group to provide stabilization to coordinatively and electronically unsaturated metal complexes, and in the case of clusters, they did the same.264 Thus, even though unsaturated (46 electron instead of 48 electron) Ru3 clusters are highly unusual, the reaction of NHCs 171 and 173 resulted in monosubstituted clusters 174 and 176 [Ru3(6-NHC)(CO)10]. The Et derivative 172 produced two different sizes of clusters (175 and 177).264 In addition to these clusters prepared with ring expanded NHCs, clusters featuring pyridylidenes have also been reported by the Cabeza group.75,265 Bimetallic clusters containing NHCs have also received considerable attention. The Adams group studied the reactions of [Ru6(C)(CO)17] and [Ru5(C)(CO)15] with (NHC)AuPh (NHC = IPr, 12).266 Disparate reactivity was observed as reaction of [Ru6(C)(CO)17] with 12·AuPh resulted in the formation of π-arene cluster complex 178, while [Ru5(C)(CO)15] gave a mixture of complexes 179 and 180 (179 shown), both resulting from Au−Ph cleavage/oxidative addition (Chart 8). Further heating led to a new cluster where the phenyl ring bridges two Ru atoms (181). Similar reactivity had been observed in 1983 by Lewis and co-workers where reaction of Ru5C(CO)15 with [Au(PPh3)Cl] leads to formation of open cluster complex with loss of one equivalent of CO.267 Bimetallic clusters were also reported by Captain and Saha, who reacted [Ru3(CO)12] with Pt(NHC8)2 in benzene at room temperature, yielding a monoplatinum−triruthenium cluster complex and a bisplatinum−triruthenium cluster complex.268 The authors noted that compared with related reactions between [Ru3(CO)12] and Pd(P(t-Bu)3)2, in which three Pd/ P(t-Bu) 3 units were incorporated, a maximum of two Pt(NHC8)2 units could be added to the Ru cluster. These clusters were found to react with hydrogen yielding hydridecontaining clusters, but with significant reorganization of the cluster framework. Osmium clusters share many features with ruthenium clusters in terms of their reaction with NHC ligands, with some important differences.75 As reported above, the quintessential Ru cluster [Ru3(CO)12] reacts with small NHCs to give simple substitution of one NHC for one CO, while bulkier NHCs promote cluster rearrangement. At higher NHC-to-cluster

Scheme 23. Reactivity of [Os3(CO)12] with NHCs Yielding Mesoionic Carbene Complexes along with the Observation of Clusters Resulting from Attack of the Carbene on a Carbonyl Ligand

AL


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Chart 9. Various Dendrimers with NHC−Pd(II) Complexes at Their Termini

their backbone, where each layer typically results from one stage of growth (termed a “generation”), and (c) end groups on the peripheral layer. When dendrimers are prepared from a monovalent core moiety, named a focal point, a wedge-like structure typically called a “dendron” results.275 Owing to their well-defined architecture, nanosized dimensions at high generations, existence of numerous peripheral groups, and presence of internal cavities in their 3D structure, they have attracted interest.278−280 The application of dendronized transition metal complexes in catalysis has been the subject of considerable interest in the past few decades.69,281−283 Factors such as exceptional control over size, structure, and shape as well as the ability to precisely locate the catalytic sites either at the core or on the periphery of the dendrimers have made them exciting materials.284 Several excellent reviews describing advances in the field of dendritic transition metal catalysis exist.284−289 In this section, we limit our attention to the progress made in the chemistry of dendritic NHC-transition metal-based macromolecules. In these structures, the NHC−metal complexes are either on the periphery or at the core of dendrimers. 3.1.1. NHC-Transition Metal Complexes at the Periphery of Hyperbranched Polymers or Dendrimers. In 2002, ́ Diez-Barra and co-workers described the synthesis of a hexacarbene Pd complex based on 1,1′-methylenebis(4-butyl1H-1,2,4-triazolium) diiodide (184, Chart 9).290 Compound 184 was prepared as a yellow solid in a few steps by first coupling 4-[bis(1,2,4-triazol-1-yl)methyl]phenol, as an AB2-type monomer, with 1,3,5-trischlorocarbonylbenzene followed by quater-

Work has also been carried out on Ir carbonyl clusters, with recent studies including computational work describing the structures, relative energies, natural charges, and ligand dissociation energies of serval [Irx(CO)z(NHC)y] clusters.272 The effects of the NHC on the cluster and the CO ligands were investigated. The use of NHCs in polyhydride clusters of Ir has also been described in two publications from Balcells, Campos, and Crabtree. 2 7 3 , 2 7 4 In this case, polyhydride clusters [Ir4(NHC14)7(CO)H10]2+ and [Ir4(NHC14)8H9]3+ were isolated from iridium-catalyzed glyceral dehydrogenation reactions.273 Crystallographic identification of the cluster structures was possible after crystallization when the anions were exchanged for polyoxometalates, and the resulting salts crystallized in a gel matrix. In a subsequent paper, the related cluster [Ir4(NHC14)8H10]2+ was characterized by neutron diffraction, which permitted observation of the hydride ligands.274 For other work in the area of metal cluster complexes, the reader is referred to a pertinent review.75

3. SOFT MATERIALS 3.1. NHC−Metal Complexes of Dendrimers and Hyperbranched Polymers

Dendrimers are an important class of macromolecules that are mainly characterized by three-dimensional (3D) globular architectures.275−277 These materials are highly branched, structurally perfect molecules with single or very narrow molar weight distributions. They are comprised of three structural regions: (a) a core, (b) layers of branching repeat units forming AM


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nization with butyl tosylate, exchange of the tosylates by iodide counterions, and finally reaction of the resulting hexacarbene precursor with palladium(II) acetate.290 The authors then investigated the catalytic activity of this dendritic NHC-Pd(II) molecule in the Mizoroki−Heck reaction using p-bromobenzaldehyde and n-butyl acrylate. It was shown that the use of 0.16 mol % of 184 resulted in 74% conversion after 5 h. The use of a related monomeric catalyst with a similar structure led to 50% conversion. In addition, both the dendritic and monomeric molecules catalyzed the reaction to 100% conversion after 8 and 50 h, respectively.290 The same group also reported the synthesis and catalytic activity of dinuclear and tetranuclear dendritic molecules (185 shown) with a different backbone but similar terminal Pd(II) moieties (Chart 9).291 Although only modest catalytic activities with these dendritic NHC−Pd(II) complexes were observed in the Mizoroki−Heck reaction, under the same reaction conditions, it was shown that tetranuclear species 185 was more reactive than the analogous dinuclear catalyst, which was in turn more reactive than a mononuclear model derivative. In all cases, the addition of mercury to the reaction mixture halted catalytic activity, suggesting the presence of heterogeneous catalysis by Pd nanoparticles. Consistent with this, the authors observed the positive effect of added tetrabutyl ammonium bromide, which is known to stabilize Pd nanoparticles. Darkening of the reaction mixtures was often observed, which also likely indicates nanoparticle formation.291 In another study, Meise and co-workers employed hyperbranched water-soluble polyglycerol macromolecules such as 186 as scaffolds for introduction of triazoles and subsequent preparation of Pd(II)−NHC complexes (Chart 9).292 The surface hydroxyl groups of the starting hyperbranched polyglycerol were first converted to azides, which were then converted to triazoles by a Huisgen cycloaddition reaction. Rather than employing the triazoles as precursors to mesoionic NHCs, the authors chose to introduce the NHC as a substituent on the alkyne. Metalation was then accomplished by reaction of the dendritic NHC precursor with palladium(II) acetate. Using inductively coupled plasma mass spectroscopy, the authors determined the metal loading to be 65 Pd centers per each hyperbranched molecule which corresponds perfectly to the expected 2:1 ratio of NHC to Pd.292 Investigation of the catalytic activity of this dendrimer in Suzuki−Miyaura crosscoupling reactions in aqueous media gave turnover frequencies and turnover numbers of up to 2586 h−1 and 59000, respectively. Interestingly, these values are comparable to those of related polymer-supported catalyst in water; however, the dendridic catalyst was more effective with challenging pyridine-based substrates. As an added advantage, the recovery of the dendritic catalyst and its reuse in five consecutive reactions was reported with no apparent loss in activity. The need for dialysis to recover the catalyst was cited as one of the problematic features in recycling of this species.292 In addition to the dendritic NHC−Pd(II) examples discussed above, dendritic systems with Ru,293,294 Rh,295 and Au295 have also been reported. The first example of an NHC−metal complex presented on a dendritic scaffold dates back to a 2000 report by Hoveyda and co-workers, who prepared tetranuclear NHC−Ru(II) dendrimer 187 with a tetraalkylsilyl dendrimer backbone (Chart 10).293 This macromolecule was found to be an efficient catalyst for ring-closing metathesis, ring-opening metathesis, and cross metathesis reactions. Interestingly, the dendritic NHC−Ru(II) catalyst showed higher catalytic activity

Chart 10. NHC-Terminated Dendrimers Based on Tetrahedral Silicon Cores. (Top) Chemical Structure of Dendritic Tetranuclear NHC−Ru(II) in Ref 293. (Bottom) Dendritic Rh(I) and Au(I) Macromolecules Prepared by Noncovalent Interactions

compared to the analogous dendritic structure in which the NHCs were replaced by tricyclohexylphosphines. Owing to their larger size compared to the corresponding monomeric analogs, the dendritic catalyst system was reported to be recyclable, although 5−13% loss of Ru was observed upon catalysis and recovery processes.293 Gebbink and co-workers reported the synthesis of a dendritic Grubbs II macromolecule based on a G0 carbosilane dendrimer with four NHC−Ru(II) centers.294 This catalyst was examined in the ring-closing metathesis of diethyl diallylmalonate. Although the catalyst was found to be significantly less efficient than commercially available olefin metathesis catalysts, its reactivity was still higher than its monomeric analog.294 In a different approach, the same group described the immobilization of negatively charged NHC complexes onto a polycationic dendrimer via noncovalent interactions (188, Chart 10).295 To achieve this, a zwitterionic imidazolium salt was first synthesized by reacting mesityl imidazole with 1,4-butane sultone. The resulting molecule was then used to prepare the corresponding bis-NHC silver complex by reaction with excess Ag2O. The octacationic dendrimer, the bis-NHC-Ag(I) complex, and the appropriate Rh and Au precursors were then used to access the target dendritic NHC-Rh(I) and NHC-Au(I) compounds, respectively, via a one-pot transmetalation−immobilization approach. In this strategy, the dendrimer acts as both the macromolecular scaffold as well as the halide salt source by means of its eight halide counterions.295 The use of the resulting dendritic systems in catalysis was not described. 3.1.2. NHC-Transition Metal Complexes at Dendrimer Core. In addition to dendrimers where the metal complexes are located on the periphery, the formation and catalytic activities of various dendritic NHC-metal-containing macromolecules where the metal is located at the core have also been reported. In these systems, the dendron wedges are typically anchored to AN


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Chart 11. Chemical Structures of NHC−Rh(I) Complexes with (a) Flexible and (b) Rigid Dendrons. (c) A Bidentate Dendritic Rh(III) Macromolecule

Chart 12. Preparation of Dendrimers with NHC-Based Dendrimers with the NHC Metal Species at the Core

either one or both of the nitrogen atoms adjacent to the carbenic carbon to form dendritic systems in which the NHC-metal center is precisely at the core. Examples of such structures with Rh,68,296 Pd,297,298 Au,299,300 Ag,61 Ir,301 and Ru302 as the central metal will be discussed in this section. Tsuji and co-workers described the synthesis and catalytic activity of a series of dendritic NHC−Rh(I) complexes in which the metal center was located at the core of flexible Fréchet-type poly(benzyl ether) dendrimer (Chart 11a).68 The synthesis started with the construction of imidazolium salts of the G0-G3 dendrimers. The corresponding dendritic NHC−Ag(I) derivatives were then prepared and used in carbene-transfer reactions with [RhCl(COD)]2 to obtain the final products. The authors then investigated the catalytic activities of the resulting dendritic NHC-Rh(I) species (189G0-G3) in the hydrosilylation of ketones using diphenylsilane. It was observed that catalytic activities increased with increasing the dendrimer generation number. The effect was more dramatic as the overall concentration of the reaction decreased. Considering that there is only one active metal center per dendrimer regardless of their generation numbers, this observation was rationalized by a positive dendrimer effect resulting from the interactions of aromatic rings within the dendrimers with Rh.68

The same group also reported a dendritic NHC−Rh(I) system in which the rigidity of the dendrimer backbone was significantly increased by using 2,3,4,5-tetraphenylphenyl as the building block (190G0/G1, Chart 11b).296 It was shown that these molecules were efficient catalysts for the hydrosilylation of α,β-unsaturated ketones using diphenylsilane. However, an unexpected pattern was observed in which these rigid dendritic NHC-Rh(I) catalysts led to the formation of 1,4-adducts as the major product rather than the expected 1,2-adducts. This reversal in regioselectivity was attributed to the rigidity of the dendrimer backbone,296 which was further confirmed by using more flexible dendrimers, which gave the expected 1,2-adducts. Following a similar synthetic procedure, the Tsuji group also reported bidentate NHC−Rh(III) complexes with flexible Fréchet-type dendritic backbones (191G0-G2, Chart 11c).296 Subjecting these flexible dendritic NHC−Rh(III) macromolecules to the same hydrosilylation reaction resulted in the formation of mixtures of both 1,4- and 1,2-adducts with the former being the major product. Changes in the dendrimer generation number had no effect on selectivity.296 In addition to the Rh-based systems discussed above, Lukowiak and co-workers reported a dendritic (G1-G3) glycerol-based system with NHC−Pd(II) complexes at the cores.297 When tested as catalyst in Suzuki−Miyaura crossAO


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catalysts imparted a moderate level of substrate specificity in competitive reactions based on the size of the ketones, with the largest differences being observed with the most sterically bulky substrate and catalyst, as expected.301

coupling reactions in neat water, it was found that although the dendritic architecture imparts a positive catalytic effect, a generation number is often reached where the steric contributions from the dendrimers hamper catalytic efficacy.297 NHC-centered dendrimers with Pd at the core were developed by de Jesús, Flores, et al., in which the imidazolium moieties were unsymmetrically functionalized with a bulky aryl group such as 2,6-diisopropylphenyl or mesityl on one nitrogen atom and poly(benzyl ether) dendrons (193) on the other nitrogen (Chart 12).298 These macromolecules were synthesized by transmetalation of the corresponding Ag derivatives using commercially available Pd(II) precursors. When tested as catalysts under Mizoroki−Heck reaction conditions, the dendridic catalysts did show different reaction rates depending on generation number; however, the ordering was unusual with G2 < G0 < G1 < PdCl2 < G3. Interestingly, simple PdCl2 had identical initial rates to the most active dendridic catalyst (G3), but its rate tapered off, likely due to formation of Pd black. Examining data obtained by X-ray diffraction, DOSY-NMR spectroscopy, fluorescence spectroscopy, and simulations, the authors suggested that the semiflexible backbone of the poly(benzyl ether) dendron and the benzylic linkages between the dendrons and the NHCs facilitate conformational changes of the dendrons at the metal center to accommodate the incoming substrates.298 The same poly(benzyl ether) dendrons (193 G0G2) were also employed to develop dendritic macromolecular structures with an NHC−Ru(II) active catalytic site at their core.302 Interestingly, no dendron generation-dependent change in the catalytic behavior of these dendrimers was observed in olefin ring-closing metathesis reactions.302 Fujita and co-workers have reported a series of hydrophilic dendritic NHC-Au(I) complexes (NHC194·AuCl, Chart 12) based on the oligo-benzylic ether motif, which were made soluble in polar organic solvents and in aqueous media by virtue of the penta(ethylene glycol) units at their periphery.299,300 These dendritic complexes were used as catalysts for carboxylative cyclization of propargylic amines under an atmospheric pressure of carbon dioxide at room temperature and both in methanol and in water.300 Chianese and co-workers prepared the rigid dendron 195 with Ir at its core. The dendron structure resulted in an organometallic entity with a bowl-like internal cavity (Chart 13).301 These dendritic NHC−Ir(I) complexes were catalytically active for the hydrosilylation of aryl methyl ketones. Interestingly, unlike their nondendritic analogs, the dendritic

3.2. Polymeric NHCs and NHC−Metal Complexes

Unlike dendronized NHCs and NHC−metal complexes that are obtained via a stepwise synthetic approach, polymeric analogs are often synthesized by a single polymerization reaction of a monomeric unit. This approach effectively reduces the number of steps, and thus the cost, required to access this class of materials, although it also introduces an element of uncertainty in terms of precise structure. Polyionic liquids are obvious precursors to polymeric NHCs (polyNHCs), which employ derivatives of, for instance, imidazolium,303−309 benzimidazolium,310 and triazolium311 salts as their backbones. Advances in living polymerization techniques, including ionic,312 radical,313−316 and metathesis317 polymerization, have made it possible to synthesize well-defined poly(ionic liquid)s with tunable chain lengths, compositions, and low molecular weight distributions. Intriguing properties of polyionic liquids such as their tunable ionic conductivity, chemical and electrochemical stability, solubility in a wide of range of solvents, and thermodynamic stability have resulted in their use in various areas of science and technology.318−326 However, in this section we focus on polymeric materials in which ionic liquid repeat units on their backbone are transformed to either free NHCs or NHC−metal complexes and their corresponding applications. For more information on the synthesis, properties, and applications of poly(ionic liquid)s, readers are referred to several pertinent reviews.327−332 NHC−metal complexes immobilized on different types of solid supports, such as resins, constitute an important class of catalytically active materials for which readers are referred to other review articles.74,333,334 3.2.1. Polymeric NHCs Where the NHC Is Pendant to the Main Chain. Lu and co-workers employed postpolymerization modification to synthesize a polymer bearing NHC precursors 196·CO2 (Chart 14).335 To accomplish this, a random copolymer was prepared by copolymerization of styrene and 4-vinylbenzyl chloride. This copolymer was then reacted with 1-(2,6-diisopropylphenyl) imidazole to afford the target polyionic polymer by SN2 displacement of the benzylic chloride. Deprotonation of the imidazolium groups followed by reaction with CO2 resulted in the formation of NHC−CO2 polymers (196·CO2, Chart 14). This transformation was confirmed by the appearance of a broad absorption peak at ∼1640 cm−1 corresponding to asymmetric CO2 vibrations. Although free polymeric NHCs were not isolated in this study, the formation of the zwitterionic CO2 adduct is good evidence for their intermediacy. The authors showed that prior to generation of the CO2 adduct, the polyimidazolium polymer can be used as a highly effective CO2-selective adsorbent capable of capturing CO2 under the wide temperature range of 20−100 °C. CO2 could then be quantitatively released under N2 flow at 140 °C.335 In a different approach by Buchmeiser and Pawar, a norbornene-substituted tetrahydropyrimidinium salt was prepared, reacted with base and CO2, and then polymerized via ring opening metathesis polymerization (ROMP) with Schrock’s Mo-based catalyst (Chart 14).336 Using a cross-linker at the final stages of polymerization enabled the preparation of polymeric imidazolium carboxylates (197·CO2). It was shown that these resins were highly efficient precatalysts for the metal-free cyanosilylation of carbonyl compounds with high turnover

Chart 13. Iridium−NHC Complexes with Bulky, Dendrimerlike Wingtip Groups

AP


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Chart 14. Chemical Structure of the CO2-Masked polyNHC 196·CO2 and Structures of CO2-Masked polyNHC 197 Prepared by ROMP and Polymerized NHC−Metal Complexes 198a−c (metal = Rh, Ir, Pd) Immobilized on a Resin Support

Scheme 24. Synthesis of CO2-Masked Polymeric NHCs as Organocatalysts Reported by Taton and Co-workers

carbenic proton signal and the subsequent upfield shift of the aromatic proton signals in the 1H NMR spectra. The free NHCs were then reversibly masked with CO2 under a pressure of 1 atm to yield their corresponding zwitterionic polymeric adducts (201a−c·CO2). The authors note that this strategy substantially enhances the air- and moisture-stability compared to the parent polymeric free NHCs.337 These materials were examined as organocatalysts for transesterification and benzoin condensation reactions; however, degradation upon reuse was problematic in many cases. In a subsequent study by the same group,338 imidazoliumbased poly(ionic liquid)s with hydrogen carbonate counterions were reported as alternatives to imidazolium carboxylates, with the advantage being that the free polymeric NHC did not need to be prepared. The imidazolium hydrogen carbonate polymers were synthesized by either a direct free radical polymerization of vinyl-functionalized imidazolium salts with hydrogen carbonate counterions or via a postpolymerization anion exchange of the presynthesized poly(ionic liquid)s with hydrogen carbonate salts. The generation of polymeric free NHCs by the loss of carbonic acid (H2O + CO2) was demonstrated indirectly using these polymers as precatalysts in organic transformations, such as benzoin condensation, transesterification, and cyanosilylation of aldehydes, in which molecular NHCs are known to be efficient catalysts. Although loss of activity upon recycling was still observed, these polymers faired significantly better than did the CO2 adducts.338 Taton and co-workers also developed polymeric NHCs based on poly(styrene)-co-poly(4-vinylbenzyl-butylimidazolium) copolymer with Cl− and HCO3− anions, which can be converted into polymeric NHC−metal complexes via the postpolymerization modification approach (202·M, Scheme 25).339 The protonated polymer (202·HCl) was reacted with Ag2O to form the corresponding polymeric NHC−Ag copolymer (202· AgCl). Formation of this metallopolymer was confirmed by the appearance of a peak in its 13C NMR spectrum corresponding to the carbon attached to Ag. Alternatively, the copolymer containing hydrogen carbonate anions was used to prepare polymeric NHC−Pd (202·Pd(allyl)Cl)and NHC−Au complexes (202·AuCl), which is advantageous in that the metalation could be carried out without exclusion of air and moisture.339 Chung and co-workers used histamine as the starting material for the synthesis of vinyl-functionalized imidazolium salt 205 (Figure 32a), which was was employed as a monomer in free radical polymerization to give poly(ionic liquid) 206·HI.340 Using this polymer as the precatalyst in the presence of a strong base, the authors generated polymeric NHC 206 and demonstrated its superior catalytic activity in the benzoin condensation reaction and the tandem formation of γ-

numbers. The trimerization of isocyanates was also affected by these polymers after CO2 release.336 The authors also prepared immobilized Rh, Ir, and Pd catalysts (198a−c) by direct reaction of the resin with the appropriate metal precursors at high enough temperatures to promote CO2 loss (Chart 14). In the Mizoroki−Heck coupling reaction, TONs of up to 100,000 were reached using catalyst 198c. The homogeneous analog reacted with higher rates (up to a factor of 3) although the same eventual TONs were reached in both systems. The Ir-catalyzed hydrogenation of benzaldehyde and Rh-catalyzed polymerization of phenylacetylene were also carried out with catalysts 198a and b.336 Different levels of metal leaching were observed depending on which catalyst was employed, with Pd leaching in the Mizoroki−Heck reaction being below detection but on the order of 5% leaching being found in phenyl acetylene polymerization or transfer hydrogenations catalyzed by Rh and Ir systems, respectively. It is not clear whether these differences result from the type of reaction or type of catalyst. In a study by Taton, Gnanou, and co-workers,337 polymeric NHCs and their CO2 adducts were synthesized and investigated as recyclable organocatalysts for a number of organic transformations (Scheme 24). Vinyl-functionalized alkylimidazolium bromide salts were used as monomers for free radical polymerization, yielding the corresponding polyionic liquids (199a−c). Anion exchange using bis(trifluoromethanesulfonyl)imide was performed to enhance the solubility of the polymers in common organic solvents. Free polymeric NHCs were then synthesized by the deprotonation of the imidazolium ions using potassium bis(trimethylsilyl)amide, potassium tert-butoxide, or sodium hydride.337 The authors followed the formation of free NHCs on the polymer backbone by 1H NMR spectroscopy, looking for the disappearance of the AQ


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controlled, with C60 domain sizes much smaller than those obtained from typical bulk heterojunctions formed using C60 with the same donor/acceptor ratios.341 Yuan and co-workers have reported a novel porous membrane actuator based on polycarbenes that can act as an ultrasensitive acid sensor and a real-time chemical reaction monitor.342 In their design, a poly(ionic liquid) based on a poly(1,2,4triazolium) backbone was selected as the in situ carbene source. The porous free-standing membrane was synthesized by dropcasting a mixture of this polymer and trimesic acid on a glass plate and peeling it off after drying at elevated temperature. Using 1H NMR spectrocopy experiments, the authors then showed that NH3 can act as a base and deprotonate the proton at the C5 position of the 1,2,4-triazolium ring, followed by insertion of the carbene into one of the N−H bonds of NH3 forming an adduct.342 The basis for weak acid sensing is then illustrated by demonstrating that this adduct reverts back to its triazolium cation in the presence of acetic acid. On this basis, acetic acid at concentrations as low as 3.7 × 10−6 M could be detected by this actuator membrane.342 It is further demonstrated that two structural gradients, namely degree of electrostatic complexation and density distribution of the carbene−NH3 adduct along the membrane, can be used to monitor the entire process of a proton-involved chemical reaction. Weck and co-workers synthesized NHC-Pd-based copolymers starting from a norbornene-based monomer containing an NHC ligand (Chart 15).343 This monomer was then reacted

Scheme 25. Facile Synthesis of Polymeric NHC−Metal Complexes (metal = Ag, Au, and Pd) Developed by Taton and Co-workers

Chart 15. Chemical Structure of NHC−Pd Copolymer 209 Developed by Weck and Co-workers343

Figure 32. Synthesis of (a) histamine-based polymeric NHCs (Reproduced with permission from ref 340. Copyright 2014 Royal Society of Chemistry.); and (b) a poly(NHC-C60) hybrid material (Reproduced with permission from ref 341. Copyright 2015 Nature Publishing Group.).

with Ag2O to yield its corresponding silver complex, which was reacted with a number of Pd precursors such as Pd(OAC)2, Pd2(dba)3, and [Pd(η3- C3H3)2Cl]2 to obtain the target NHC− Pd monomers. This monomer was copolymerized via ROMP along with an unfunctionalized norbornene monomer in various ratios yielding the desired copolymers (209, Chart 15). The authors investigated these copolymers in a range of reactions including Suzuki−Miyaura, Sonogashira, and Mizoroki−Heck cross coupling reactions. Interestingly, catalytic activities observed were the same as those corresponding to analogous discrete NHC−metal complexes. It is worth noting that similar NHC−Ru polymers were also synthesized via a postpolymerization modification approach in this study, and their catalytic activities in the ring-closing metathesis were also similar to analogous discrete NHC−metal complexes.343 3.2.1.1. Pendant Polymeric NHCs Applied to the Immobilization of Metathesis Catalysts. Buchmeiser and co-workers have published a series of papers on the synthesis of polymeric NHC−metal complexes and their applications in catalysis.344−346 In one of their early reports, they used monolithic

butyrolactone compared to the corresponding monomeric NHC. In a study by Chen and co-workers, polymeric free NHCs (207) derived from polymeric imidazolium salts were used as Lewis bases for the formation of adducts with C60 as the Lewis acid component (208, Figure 32b).341 Interestingly, the addition of C60 caused an apparent increase in molecular weight, due to either aggregation or multiple additions of different NHCs onto the same molecule of C60. When heated, further cross-linking was observed to yield thermally robust polymer networks. This method proved to be a robust strategy to access C60-containing materials with unprecedented levels of C60 incorporation (up to ∼70 wt %). Furthermore, increasing the C60 content in the resulting polymers could be used to tune their thermal stability. Polymers 208 were investigated as potential candidates for application in organic photovoltaic devices, which are often based on bulk heterojunctions formed by blends of donor and acceptor constituents. The morphology of the bulk heterojunction obtained from these materials was found to be well AR


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materials as platforms for the synthesis of such materials.344 One of the important advantages of monolithic materials over other commonly used solid supports is the fast mass transfer between the liquid phase and the support that results from the unique structure and pore size distribution of the polymer monolith (Figure 33). To construct such materials, monolithic structures

from” ROMP. Subsequent conversion of the polyanhydrides to their corresponding silver salts and their reaction with Grubbs− Herrmann catalyst [RuCl2(CHPh)(IMesH2)(PCy3)] afforded the target supported polymeric NHC-Ru materials. These heterogeneous catalysts were used in ring-closing metathesis of olefins with turnover numbers of up to 1000.346 In addition to NHC-metal complexes attached to solid supports, Blechert and co-workers prepared polymers containing an isopropoxy styrene moiety for use in metathesis reactions that were converted into Ru catalysts by alkylidene formation at pendant styrene units (213, Chart 16).347,348 Attachment of the Chart 16. Structures of Homogeneous Polymeric NHC−Ru Catalysts 213 and 214, with Various Spacer Groups

Figure 33. Synthesis of polymeric NHC−Ru complexes 210 and 212 by ROMP on a monolithic support. Adapted with permission from ref 344. Copyright 2001 Wiley-VCH.

with microglobule sizes of 1.5 ± 0.5 μm and interparticle porosity of 40% were prepared via ring-opening metathesis polymerization (ROMP) of norbornene with the first generation Grubbs Ru ROMP catalyst. Without end-capping, the Ru alkylidene catalyst remained active on the polymer terminus (210, Figure 33). The remaining active ruthenium units were then reacted with a mixture of norbornene-functionalized imidazolium salt 211 and norbornene itself to prepare a random copolymer containing NHC precursors on the exterior of the monolithic materials. In the last step, any active Ru centers are removed by end-capping with ethyl vinyl ether. The active catalyst is then formed by deprotonation of the imidazolium unit with 4-dimethylaminopyridine and reaction with the ruthenium complex [Cl2Ru(CHPh)(PCy3)2]. This results in a monolith tethered with polymeric tentacles that have Grubbs second generation catalyst units on their ends (212, Figure 33). These monoliths were shown to be highly active catalysts for ring-closing metathesis reactions and ROMP with turnover frequencies of up to 25 min−1 for the former, faster than even homogeneous analogs tested by the researchers.344 The authors noted that the use of NHCs as ligands suppressed loss of Ru resulting in organic products with Ru content below 70 ppm. Using a different approach, the Buchmeiser group also supported Grubbs catalyst on nonporous silica and employed the catalyst in ring-closing metathesis reactions.345 The same group together with Nuyken and co-workers reported the monolith- and silica-supported polymeric NHC-Ru Grubbs− Herrmann metathesis catalyst.346 In this case, an anhydridecontaining norbornene monomer was employed for the “graft-

Ru catalyst to the polymer backbone using CuCl as phosphine scavenger yielded a random copolymer with NHC-Ru/ unfunctionalized alkene ratio of 1:99, achieved by employing 1% of the second generation Grubbs catalyst relative to the styrene unit. When used for ring-closing metathesis reactions, catalyst 213 needed long reaction times for complete conversions. The authors attributed this to the presence of excess free vinyl groups in the polymer backbone that could potentially impose difficulties in forming the catalytically active 14-electron metal center.347 To circumvent this issue, a higher ratio of catalyst to backbone vinyl groups was employed and an unfunctionalized monomer (oxanorbornene benzoate) was employed as a spacer between the active metal centers in the polymer backbone. The new polymer (214) was found to have impressively high activity in ring-closing metathesis, ring-opening metathesis−cross metathesis, and tandem metathesis reactions. The presence of some residual isopropoxy-adjacent olefins was found to be important for efficient recovery of the Ru such that for this polymer, a maximum of 0.004% Ru was found to be present in each of the four cycles, which was considerably better than known systems at the time and also better than a polymer with all the isopropoxy groups saturated with Ru. The supported catalyst could be recycled up to 5 times before loss of activity was noted. In a study by Yao and co-workers, a Hoveyda-type isopropoxy styrene was employed on the backbone of a simple polymer in the same fashion as described above, except that fluorinated alkyl esters were used as the spacer chains (215, Chart 17).349 AS


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tris(bromomethyl)mesitylene (218) at high temperatures was employed to generate imidazolium-based polymers as spherical particles of ∼1.2 μm in diameter (Scheme 26). Unlike previous

Chart 17. Recyclable Polymerized NHC−Ru Complexes Supported on a Fluorinated Backbone (215); and the Amphiphilic Polymer Tethered to the Hoveyda−Grubbs Catalyst (216)

Scheme 26. Poly(imidazolium)-Based Spherical Particles Prepared by Condensation Polymerization and Their Free NHC Analoguesa

Treatment of the copolymer with Grubbs second generation Ru catalyst in the presence of CuCl resulted in the incorporation of the active metathesis catalyst onto the fluorinated polymer backbone. In the presence of minimal amounts of fluorinated solvent systems, this fluorinated polymeric NHC−Ru system was shown to be highly active in ring-closing metathesis reactions of a wide range of diene and enyne substrates. Furthermore, the presence of the fluorinated segments in the polymer backbone made it possible to recover and reuse the catalyst by a simple extraction with fluorinated solvents.349 To enable ring-closing metathesis reactions in aqueous media, Weberskirch and co-workers developed an amphiphilic polymer with catalytically active Hoveyda−Grubbs catalysts on its backbone (216, Chart 17).350 The polymer was obtained by first reacting carboxylic acid-decorated poly(2-oxazoline) statistical copolymer with 2-isopropoxy-5-hydroxystyrene. Reaction of the polymer with Grubbs second generation catalyst resulted in its immobilization on the backbone of the amphiphilic polymer. Using diethyl diallylmalonate as substrate for the ring-closing metathesis reaction, the authors observed turnover numbers as high as 390 in water, which was even higher than those obtained using the same catalyst in common organic solvents such as dichloromethane. The high catalytic activity was attributed to the possible formation of micellar structures in water, which could accelerate the conversion of hydrophobic substrates in their hydrophobic compartment and at the same time stabilize the active alkylidene species.350 Using the same strategy and same amphiphilic polymer backbone, the Weberskirch group immobilized NHC−Rh complexes onto these polymers and demonstrated high catalytic activity (turnover frequencies up to 2360 h−1) in the hydroformylation of 1-octene in aqueous media.350 3.2.2. Polymeric NHCs in which the NHC Is Included in the Main Chain. Although less common, there are also reports of polymeric materials in which the NHC is incorporated into the main chain of the polymer. In this section, we will restrict the discussion to examples where the NHC remains accessible for reaction. Examples of NHC-based polymers where the NHC is part of a one-dimensional coordination network will be covered later in this review. For examples of where the NHC is used as a nucleophile to prepare main-chain zwitterionic polymers in which the NHC is converted to an imidazolium ion, the reader is referred to the work of Johnson351 and Bielawski.352 In a study by Zhang, Ying, and Tan,353 condensation polymerization between bisimidazole derivative 217 and 2,4,6-

a

Adapted with permission from ref 353. Copyright 2009 Wiley-VCH.

examples described thus far, in this case the monomers are in the main chain of the resulting polymers rather than the side chain. By reacting these particles with sodium tert-butoxide, polymeric free NHCs were generated within the particle backbone and characterized by elemental analysis, FT-IR, and 13C NMR spectroscopy. It was shown that such free NHC-containing particles can catalyze a range of organic reactions including ketone/imine hydrosilylation, silane alcohol condensation, and ketone hydrosilylation reactions with, in this case, good catalytic activity upon recycling.353 The Ying/Zhang group reacted the same NHC-containing polymeric particles (220) with CO2, using solid-state 13C NMR spectroscopy to confirm the formation of free and CO2protected NHC particles, which were then used as heterogeneous catalysts for the reduction of carbon dioxide to methanol with hydrosilane.354 Although the catalysts were recyclable and even showed an increase in rate after reuse, the overall yield was inferior compared to the use of IMes (8) as a homogeneous catalyst. The formylation of N−H bonds with carbon dioxide and hydrosilanes was also described. In this case, similar reactivities to polymeric or monomeric carbenes were observed as long as simple silanes were employed, but with larger polymeric silanes such as polymethylhydrosiloxane, the polymeric NHC showed lower reactivity. Despite this fact, the ease of recycling and reuse of these polymeric NHCs is a significant advantage.354 In a study by Hahn, Li, and co-workers, copper-catalyzed alkyne−azide click polymerization was employed to construct a series of alternating heterobimetallic polymers bearing NHC− metal monomeric units (Figure 34).355 In their approach, NHC−metal monomers (221·M−223·M) with either flexible p(azidomethylene)phenyl or rigid p-azidophenyl and p-ethynylphenyl wingtip groups were used to obtain the strictly alternating polymers (Figure 34). An interesting aspect of this work is the use of the azide-functionalized NHC−Cu complexes 221/222 as both the monomeric units and the catalyst for “click” polymerization. This means that no additional additives were required for the polymerization reaction as the same monomer that gets incorporated in the polymer backbone also catalyzes the polymerization reaction. AT


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Figure 34. (Top) Chemical structures of monomeric NHC−metal complexes used for the synthesis of heterobimetallic metallopolymers. (Bottom) Example synthesis of heterobimetallic polymeric NHC−metal complexes, prepared with flexible and rigid linkers.

indium tin oxide glass slide. Owing to the presence of bithiophene moieties, the resulting film exhibited good conductivity. Cowley also employed this approach for the synthesis of conductive polymers having NHC−metal repeat units comprised of Ag, Au, and Ir−NHC complexes (Figure 35).357 In these polymers, the transition metals are positioned orthogonal to the polymer main chain. It was shown that such polymers exhibit electrochromic properties dependent on the type of the metal in their backbone. For instance, the Ircontaining film was shown to have an absorbance in the near-IR region at 1100 nm in which the metal centers were proposed to function as electron sinks.357

Employing this elegant strategy, alternating polymers with NHC-Ag/NHC-Cu units (223·Ag/221·Cu or 223·Ag/222· Cu), NHC-Au/NHC-Au (223·Ag/222·Cu), and NHC-Cu/ NHC-Pd (223·Pd/222·Cu, 223·Pd/222·Cu) constituents with molecular weights in the range of 20−30 kDa and relatively narrow molecular weight distributions were prepared (Figure 34).355 Pd-containing polymers were prepared by transmetalling the silver NHC with allylpalladium(II) chloride dimer. The Cuand Ag-based polymers could be used for the catalytic alkylation of trifluoromethyl ketones to the corresponding propargylic alcohols. This reaction proceeds via the formation of Cu-alkyne intermediates, and the authors reasoned that the presence of the NHC−Ag repeat units, as Lewis acids, increased reactivity in the catalytic alkynylation by activating the carbonyl group of the starting material in a cooperative Cu/Ag catalysis mechanism for this transformation. In this context, the authors showed that the polymer with a flexible methylene-triazole bridge had the highest catalytic activity. Such flexibility in the polymer backbone might be important to bring the neighboring Cu and Ag centers in closer proximity and enhance their cooperative catalytic activity.355 Electrochemical polymerization techniques have also been used to access polymeric NHC−metal complexes.356−358 In this context, Cowley and co-workers employed an electropolymerization method to synthesize polymeric NHC−metal complexes (225, Figure 35).356 For this purpose, they prepared an NHC− Ag monomer bearing two bithiophene groups in the backbone, which was then electropolymerized onto a platinum disk or an

3.3. Polymerizations Catalyzed by NHCs

N-Heterocyclic carbenes (NHCs) have a long history as organocatalysts dating back to the original Breslow proposal for the mechanism of action of thiamine.359 In this section, we will focus on the use of NHCs as catalysts for polymerization reactions. The reader is referred to earlier reviews that cover specific aspects of NHC-catalyzed polymerizations360−370 and to an excellent review by Rovis and co-workers in the area of NHCs as catalysts for small molecule transformations.371 Despite the significant advantages of metal catalysts for polymerization reactions,372 the presence of metal impurities in polymeric products is problematic, especially when these polymers are employed in electronic devices where metal contamination can result in electrical shorts.372,373 Organocatalytic polymerization strategies provide significant advantages in these situations.360 There have been various reports of NHCs being employed for both step-growth and ring opening polymerizations (ROP) in the past decade.365−367,374 Early research in the field focused on using free NHCs360 derived from corresponding salts for catalysis; however, there have been efforts by various groups in using protected NHCs which are more tolerant to air and moisture. Fairly recently, there have been reports describing the use of NHCs for polymerizing methacrylates360 as well. The field of NHC-mediated polymerization has been thoroughly reviewed in 2013 and 2016 by Taton and co-workers360,375 and in 2016 by Naumann and

Figure 35. Synthesis of a series of main chain polymeric NHC−metal complexes 225 (metal = Ag, Au, and Ir) by electropolymerization. AU


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Dove.367 Thus, this section will be employed as a brief overview of the use of NHCs in polymerization catalysis, and the reader is referred to these excellent reviews for a more in-depth treatment. 3.3.1. Step Growth Polymerization. The first reports of polymer synthesis employing NHCs as organocatalysts came from Jones and co-workers in 1996.376 They applied the Stetter reaction for the synthesis of simple 2-(pyrrol-2-yl)pyridenes and oligomers of alternating pyrrole/pyridene, furyl/pyridene, and thienyl/pyridene systems.376,377 The field lay dormant for a few years until Hedrick, Waymouth, and co-workers378 showed that NHCs could catalyze transesterifications,378 providing an organocatalyzed method for depolymerization. The same group showed that NHCs could catalyze polymerizations by reacting ethyl glycolate (226) with 14 at 60 °C under vacuum (Scheme 27).378 Aliphatic polyesters such as 227,

proton exchange of the corresponding azolium salt (eq 12). The reactions proceeded under mild conditions in THF with gentle

heating to 40 °C. The proposed reaction pathway invokes formation of the Breslow intermediate, with polybenzoin condensation being predominant in the initial phase and formation of macrocycles in later phases of reactions. The use of 44 was also described by Taton and co-workers in 2012 for the synthesis of linear polyurethane.382 This report was one of the first to demonstrate the replacement of tertiary amine catalysts (such as DABCO) with NHCs for the synthesis of polyurethanes. Masked NHC 233·CO2 was employed by Plasseraud and coworkers for the step-growth polymerization (eq 13) of crude

Scheme 27. Use of NHCs as Organocatalysts for Transesterification and the Preparation of Polyesters

glycerol and dimethyl carbonate to produce glycerol carbonates.383 This reaction was extended to the transformation of α,ω-diols to produce cyclic carbonates. The removal of methanol is essential to drive the polycondensation reaction toward the polymerization of low molecular weight fragments to high molecular weight polymers. Burel and co-workers extended the scope of this transformation to the preparation of telechelic polycarbonates bearing novel functionalities.384 Buchmeiser and co-workers385 used 8·CO2, 38·CO2, 234· CO2, and 235·CO2 (eq 14) for the polymerization of polyols and triisocyanates using thermal activation of catalyst at 65 °C in CH2Cl2. The authors suggested that polymerization is initiated through deprotonation of the alcohol by the highly basic NHC.386 The thermal latency of the catalyst negates the need for phenylmercury neodecannoate, which is a common additive when using bases such as DABCO. 3.3.2. Chain Growth Polymerization. Another type of NHC mediated polymerization that has been widely studied is chain growth polymerization. In the following sections, ring opening polymerization and group transfer polymerizations will be described. 3.3.2.1. Ring Opening Polymerization. Ring opening polymerization (ROP) is perhaps the most widely studied NHC-mediated organopolymerization.360,366,367,375 Hedrick and Waymouth’s pioneering 2002 report72 sparked considerable interest in the field.360,366,367,375 The ROP of cyclic esters369,375 and lactams367,369 has been extensively studied in the past decade;72 however, substrate scope has been recently extended to the ROP of carboxysilanes,387 cyclic phosphate monomers,388

obtained using this procedure, are structural analogues of high molecular weight poly(glycolide). NHC 14 or its enetetramine derivative could similarly catalyze the transesterification of 228 with ethylene glycol to give ester 229 in quantitative yield in 1 h. This is significant since 229 is a precursor to the high volume commercial polymer polyethyleneterthalate (230), which is produced by self-condensation of 229.378 NHCs 8, 37, 44, and 69 were shown (eq 11) by Baceiredo and co-workers to catalyze the polymerization of α,ω-dihydroxy

disilanol oligomers to form polydimethylsiloxanes (PDMS).379 In a subsequent report in 2008, it was found that the molecular weight of the resulting silicone polymer was dependent on catalyst loading.380 Lower catalyst loadings led to longer chain length polymers, and in the presence of water, active NHCs could depolymerize PDMS. Gnanou and Taton have described the use of NHCs to replace cyanide for the polymerization of terephthaldehyde (231).381 NHCs 3, 13, 37, and 44 were employed in the polybenzoin condensation of terephthaldehyde, and it was found that 13 afforded the highest conversions, attributed to a faster kinetic AV


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izations are also known as group transfer polymerizations.366 Two separate reports by Hedrick and Waymouth et al., and Gnanou and Taton et al. illustrated use of NHC as organocatalyst for group transfer polymerizations of methacrylates in the late 2000s.73,395,396 Hedrick and Waymouth proposed a mechanism where the NHC activates the silyl ketene acetal (236) to generate a free enolate, favoring a dissociative type mechanism for more electron rich NHCs such as 110, while Taton et al. provided evidence that an associative mechanism prevailed when NHCs such as 37 were used (Scheme 28).73,395,396 Scheme 28. Group Transfer Polymerization by Activation of Silyl Ketene Acetal 236 and Potential Intermediate

In 2014, Chen and co-workers illustrated the first example of a new technique termed “proton (H)-transfer polymerization” (HTP), for the conversion of dimethyl acrylates (DMAs) to unsaturated polyesters using NHC 44.397 In 2016, they published another report expanding the scope of monomer and catalyst for HTP, while providing mechanistic details through combined theoretical and experimental investigations.398 One of the main conclusions of these reports is that NHCs 8, 44, and 37 were ineffective at promoting HTP of DMAs while triazolylidenes (13, 239, and 240) were highly effective (eq 16).397 The authors reported that reactivity toward

propylene oxide,389 cyclic carbonates,390 and carboxyanhydrides.391 In 2016, Taton and co-workers demonstrated the first organic ROP of N-activated aziridines using 110 (eq 15).392 It is known that aziridines and oxiranes are isoelectronic, but they behave very differently in ROP processes.

It was reported in 2005 that the anionic ROP of aziridines could be controlled through activation by N-tosylation.393 The polymerization requires N-alkyl-methanesulfonamide and KHMDS in stoichiometric amounts to obtain a narrow molar mass distribution of polyaziridines. In this report, excellent control could be obtained over molar masses up to 20 kDa and low polydispersities (280 °C). In a completely metal-free approach, the Bielawski group took advantage of carbene dimerization to design a reversible covalent polymerization that did not require metal catalysts (251).415 Deprotonation of the ditopic imidazolium and benzimidazolium salts described above to generate the free carbene was found to lead spontaneously to polymerization via the Wanzlick equilibrium (Chart 20c). As expected, monomers that have smaller substituents on nitrogen have a higher equilibrium concentration of the enetetramine and thus give high molecular weight insoluble polymers, while those with intermediate sized substituents gave polymers that could be more easily characterized by NMR spectroscopy, UV−vis spectroscopy, and viscosity measurements. Heating the polymer was shown to result in the generation of monomer in a reversible process that permitted the incorporation of functionalized NHCs as end-capping agents. In addition, Pd can be incorporated by the addition of PdCl2.415 These main-chain NHC-based organometallic polymers incorporating Pd (Chart 20a, 245·Pd) were employed by Akhavan and co-workers as self-supporting catalysts for Suzuki− Miyaura coupling reactions at relatively low catalyst loadings (5 × 10−3 to 5 × 10−4 mol %).416 These polymers were shown to

Chart 18. First Example of One-Dimensional AgTriazolylidene Polymer (241)a

a


solid state and was not stable in solution. In all of these cases, dissolution of the polymer in solvent resulted in return to discrete bimetallic complexes. Chiu and co-workers reported a detailed example of onedimensional NHC coordination polymers in 2005.412 Bisimidazolium salts with ethylene or methylene linkages were reacted with silver oxide to form (Ag2bis-NHCBr2)n polymers such as 242 with weak Ag−Ag interactions (Chart 19). These polymers were subsequently used as transfer reagents for bis-NHCs to form chelated palladium bis-NHC complexes. Similar coordination polymers were also reported previously by Simons and co-workers,413 however without full characterization. In 2005, Bielawski and co-workers described NHC-based organometallic polymers employing bisimidazolium salts with three different types of linkers (243−245) between the imidazolium units (Chart 20a). The imidazolium salts were AX


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Chart 20. First Three Examples of NHC-Based Organic Microporous Polymersa

a

(a) Tetraphenylmethane-based tetrakis-imidazolylidene Pd coordination polymer; (b) triptycene-based tris-imidazolylidene Pd polymer; (c) triphenylene-based tris NHC-based Au polymer.

developed another new class of coordination polymers with Ir and Ru based on p-phenylene-bridged bis-benzimidazolium salts. The NHC-Ru polymer was used as the catalyst for the solvent free reductive amination of levulinic acid by ammonium formate and showed the high efficiency and reusability (37 runs).421 The authors employed these robust polymers as catalyst for a one-pot reductive amination to generate a variety of N-substituted pyrrolidones from ketones or aldehydes. 3.4.2. Two- and Three-Dimensional Coordination Polymers. Recently, knowledge from one-dimensional NHC coordination polymers has been extended to two- and threedimensional structures. Son and co-workers reported the first preparation of hollow organometallic spheres by quaternizing tetrakis[4-(1-imidazolyl)phenyl]methane with methyl iodide and treating the resulting salt with palladium acetate to form an interpenetrating network of Pd NHC polymers (see Chart 21a, 254).422 Interestingly, the morphology of this interpenetrating polymer was highly regular, made up of hollow spheres approximately 2 μm in diameter. Elemental mapping showed even distributions of Pd and I across the sphere. These NHCbased hollow Pd spheres were employed as catalysts in the onepot three-component Strecker reactions of ketones, where they displayed high activity and little to no change in morphology. In 2013, Xu and co-workers reported three-dimensional mainchain organometallic microporous polymers (MOMPs) with NHC−Pd complexes as the key organometallic units with a triptycene-type framework (Chart 21b, 255).423 The NHC MOMP was synthesized by an Ullman coupling reaction between triiodotriptycene and imidazole. After exhaustive methylation, polymerization was induced by reaction with palladium acetate to form the MOMP. This NHC MOMP was employed as a catalyst for the Suzuki−Miyaura coupling reactions of aryl halides with boronic acids. Peris and co-workers have prepared numerous examples of multidentate NHC ligands and, in 2014, described the preparation of triphenylene-based tris-NHC gold microporous polymers (Chart 21c, 256).62 Triphenylene−tris−NHC gold complexes formed the core and were coupled together by the addition of 1,4-diethynylbenzene or 1,3,5-triethynylbenezene to form microporous polymers with two different pore sizes. These polymers were tested in the catalytic reductions of nitroarenes with NaBH4 and in the three-component Strecker reaction for

catalyze the reaction between related inactived aryl chlorides and aryl fluorides in water and could be used for up to six cycles. Heterogeneity tests were presented that suggested the catalyst was operating in a homogeneous fashion, but the fact that the catalyst is in the Pd(II) oxidation state and the reported unreactivity of bis-NHC Pd complexes previously417 suggest that the Pd polymers are likely not the catalytically active species. The same group investigated the effect of the N-substituent on catalytic activity and showed that an n-dodecyl substituent on nitrogen leads to better catalytic activity in the Suzuki−Miyaura coupling reaction in comparison with benzyl and hexyl groups.418 Moreover, they also introduced triethylene glycol groups (TEG) on the central core, resulting in water-soluble organometallic polymers that were catalytically active.418 In a creative approach, Albrecht and co-workers developed dicarbene-bridged bimetallic complexes employing diimine ligands as the polymerization points to link bis FeCp(CO)2NHC units (252, eq 17). By combining two dimeric

ligands, the Albrecht group was able to prepare a highly interesting redox active polymer. These unique organometallic polymers showed substantially improved stability compared with related bimetallic model complexes.419 Recently, Tu and co-workers reported the syntheses of a series of bis-benzimidazolylidene Ir coordination polymers and applied the polymers as self-supporting catalysts in a variety of environmentally relevant reactions.420,421 In the conversion of glycerol to lactate, the self-supported Ir polymers had exceptional activity, with up to 105 turnovers obtained and virtually unchanged activity after >25 runs. Similarly high activity, selectivity, and reusability were observed for the catalytic monomethylation of anilines, and high turnover numbers but slightly lower reuse efficiency was found in the hydrogenation of biomass oxoacids to lactones.421 In addition, the same team AY


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Chart 21. First Three Examples of NHC-Based Organic Microporous Polymersa

a

(a) Tetraphenylmethane-based tetrakis-imidazolylidene Pd coordination polymer; (b) triptycene-based tris-imidazolylidene Pd polymer; (c) triphenylene-based tris NHC-based Au polymer.

prior to introduction of copper. Li and Tan described a different strategy to prepare less well-defined NHC-based microporous polymers using Friedel−Crafts reactions to generate multiple attachment points between benzannulated imidazolium ions and added benzene. Pd is then introduced by addition of Pd(OAc)2. The resulting polymers were employed as catalysts for Suzuki−Miyaura cross-coupling reactions and related Cusystems employed in click reactions, Ullmann couplings, and Glaser couplings.426 Jia and co-workers recently described NHC-copper hypercross-linked polymers (HCPs) in 2016, prepared by uncontrolled polymerization through Friedel−Crafts arylation of benzimidazolium monomer (110·HCl) (Scheme 29).427 The resulting materials (257) had large BET surface areas (∼500 m2 g−1) and pore volumes (∼0.10 m2 g−1), as well as excellent thermal and chemical stability. These materials were then reacted with CuCl2 and used as a catalyst for many chemical transformations such as three-component “click” reactions,

the synthesis of aminonitriles and showed excellent activity in both reactions. Li and co-workers prepared porous organometallic polymers with rigid trigonal or tetrahedral units as the central cores.424 Aromatic alkynes introduced around the periphery of central adamantane, benzene, or triaryl benzene cores were crosscoupled with gold NHC complexes that had iodine atoms installed at the para positions of the aromatic wingtip groups. The resulting porous polymers were then tested in a variety of catalytic reactions and were shown to have activities similar to homogeneous catalysts, depending on the total porosity of the material. Thiel and co-workers described a porous Cu-based NHC network, which was applied in the activation of CO2 and the deoxygenation of sulfoxides to sulfides.425 In this case, the central core was based on a tetraaryl methane bearing amino groups on the periphery, which were condensed with glyoxal and converted into imidazolium units with chloromethyl ethyl ether AZ


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3.5. Metal Organic Frameworks (MOFs) and Related Structures

Scheme 29. Preparation of Cross-Linked Aromatic Polymer by Friedel−Crafts Arylation of Benzyl Benzimidazolium Salt 101·HCl, and its Conversion into a Supported NHC and Cu Complex

MOFs are an exciting class of organic/inorganic hybrid materials prepared by condensing metal-containing-units with rigid organic molecules to create crystalline frameworks with persistent porosity.428 These materials have been applied in gas storage, separation, sensing, and catalysis.429−432 The reader is referred to a recent review in which azolium and NHCcontaining MOFs are discussed.76 In this section, we will focus only on MOFs in which azolium ions are employed as precursors to NHCs. Generally, there are three synthetic methods for the preparation of NHC-containing MOFs: (1) examples in which the formation of the NHC−metal bond takes place during MOF formation; (2) the incorporation of preformed NHC−metal complexes into MOFs; (3) routes that use postmodification of azolium salt-containing MOFs to generate NHCs or their complexes.

Ullmann C−N coupling, and Glaser couplings. Catalyst recovery was easily affected, and the catalytic activity was tested after several cycles with no substantial loss in activity observed for all reactions.

Figure 36. Strategies for the synthesis of NHC-containing MOFs: (a) the use of preformed NHC−metal complex prior to formation of MOFs structures; (b) formation of the NHC−metal bond during MOF formation; (c) Formation of Pd-NHC-based MOFs using preformed Pd complexes. Adapted with permission from refs 433, Copyright 2009 American Chemical Society; 434, Copyright 2010 American Chemical Society; 435, Copyright 2014 Royal Society of Chemistry; and 436, Copyright 2010 American Chemical Society. BA


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Figure 37. Two strategies for the incorporation of Ir-metallolinker into the Zr MOF. Adapted with permission from ref 438. Copyright 2015 Royal Society of Chemistry.

Wu and co-workers described the preparation of bisimidazolium bis-carboxylates, where the carboxylates would bind to metal nodes during the MOF synthesis and the imidazolium ions would be used to install other, catalytically competent metals after MOF synthesis.64 The structural metal employed was Cu, which did not react in this case with the imidazolium moieties. After the MOF synthesis, Pd(OAc)2 was added to form a MOF-included Pd−NHC complex and the material was used to catalyzed the Suzuki−Miyaura crosscoupling reaction. Despite the fact that the need for base and high temperature has been challenging for catalysis with MOFs,439 this MOF was employed as a catalyst for the Suzuki−Miyaura reaction using K2CO3 at 70 °C, and pXRD studies of the recovered material showed that it had retained its structure. However, it should be noted that it is a serious challenge to assess MOF stability in this way since any amorphous or molecular material would be undetectable by pXRD. The Suzuki−Miyaura reaction is a popular reaction employed to test catalytic activity of many Pd-based materials including MOFs, but relatively harsh conditions can be required for this reaction (heat and aqueous base or fluoride). In nonNHC-containing MOFs, these conditions have been shown to lead to degradation of the MOF and thus optimization of conditions and careful analysis of the MOFs before and after catalysis are crucial.439 Using bis-imidazolium bis-carboxylates (267a and b), Wu and co-workers prepared NHC-containing metal organic nanotubes (MONTs) with Zn2+ serving as the metal nodes (268, Figure 38).440 These unique materials have a large exterior wall with 4.9 nm in diameter and an interior channel diameter of 3.3 nm (see Figure 38b). Introduction of Pd by treatment of the MOF with Pd(OAc)2 in the absence of base led to metalation of about 50% of the imidazolium salts, as determined by 1 H NMR spectroscopic analysis and ICPMS to determine Pd content. Interestingly, MONT 268 was found to be more stable than the starting azolium MOF. The Pd-containing MONTs were examined for catalytic activity in a series of reactions, including the Suzuki−Miyaura and Mizoroki−Heck cross-coupling reactions, the hydrogenation of olefins, and the reduction of nitrobenzene. Higher yields were obtained for these reactions by comparison to unligated Pd species such as Pd(OAc)2 or Pd/C, but the effect of the NHC ligand, i.e. by comparison to molecular Pd−NHC complexes, was not reported. Hupp and co-workers described the conversion of the imidazoles present in the TIF-1 MOF to abnormal NHC 269 by treatment with n-butyl lithium (see Figure 38c).441 Using

Son and co-workers reported the formation of NHC−Cu complexes within a supramolecular structure during selfassembly between carboxylic acid-terminated imidazolium salts (259) and metal salts (see Figure 36a).433,434 Cu was used both as a structural element, bringing the MOF together through carboxylate bridges, and to simultaneously form a carbon−metal bond to the NHC. In subsequent work, Ce3+ was also employed as a structural element along with Cu2O.434 In this case, only some of the NHCs were metalated to yield organometallic complexes and the rest remained in the imidazolium state to balance charge. Using a related bis-carboxylated NHC (260), Sumby and coworkers prepared a MOF in which the NHC was bound to Cu and the metal nodes were based on Zn4O.435 In this case, MOFs with a diamond-like 3D structure were formed with pore channels lined by the Cu-linked NHCs and Zn4O nodes (See Figure 36b). Catalytic activity for the hydroboration of CO2 employing this MOF in 5 mol % loading was demonstrated, and the MOF was also employed as a reagent for the N-formylation of benzylamine. Catalytic results were found to be similar to related homogeneous catalysts, but the MOFs had the advantage that they were able to be recovered and reused. Yaghi and co-workers also prepared NHC-based MOFs with a carboxylated NHC derived from imidazolium salt 262 (Figure 36c). In this case, Pd complex 263 was preformed such that Zn could then be employed as the structural element. Even though the MOF synthesis was performed at 100 °C for 36 h, the NHC−Pd complex remained intact, a testament to the strength of the metal−carbon bond (see Figure 36c).436 The strength of the metal−carbon bond is one of the advantages of the employment of NHCs in MOFs. This motif, where the NHC is on the central ring of a paradicarboxylated triphenyl unit, has been frequently employed in NHC-based MOF chemistry following this report. For example, Zou, Martin-Matute, and co-workers employed NHC-Ir complex 265, which is closely related to 259, for the preparation of an iridium-containing MOF prepared via two different synthetic routes: direct synthesis and postsynthetic modification, otherwise known as “SALE”, solvent assisted linker exchange 437 (see Figure 37). 438 In this report, postsynthetic modification methods result in higher metal loading compared to the direct synthesis method. MOFs obtained by this method showed good catalytic activity and recyclability for the isomerization of allylic alcohol compared to MOFs without NHCs. BB


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Figure 38. (a) Bifunctional azolium carboxylate ligands for the preparation of NHC-containing MOFs; (b) perspective view of the NHC-containing nanotube (MONT), and (c) schematic structure of TIF-1 (red prisms represent the tetrahedral cobalt nodes, chloride ions are in green, and the arrows indicated the potential sites for NHC formation). Adapted with permission from refs 440 and 441. Copyright 2012 American Chemical Society.

Figure 39. Use of the MIL-101 MOF to support Ru-based metathesis catalysts based on simple adsorption. Adapted with permission from ref 442. Copyright 2016 American Chemical Society.

Imidazolium-containing linker 272 was prepared by the introduction of a methylene bromide substituent on the typical biphenyl dicarboxyate unit and then SN2 reaction with an arylated imidazole (Scheme 30). When mixed with Zn(NO3)·

these self-supported abnormal NHCs, faster rates and higher yields were obtained for the catalytic Michael addition of alcohols to α,β-unsaturated ketones compared to a molecular normal NHC catalyst, although a comparison to abnormal NHCs as molecular versions of the MOF was not presented. Multiple control experiments led the authors to conclude that catalysis was happening on the exterior of the MOFs, which was potentially attributed to the inability of the organolithium species to form stable aggregates on the interior due to size constraints. In addition to the design of MOFs with NHCs incorporated into their backbone, Grela, Chmielewski and co-workers showed that Ru-based olefin metathesis catalysts can be incorporated by simple absorption within the benzene-dicarboxylate-based MIL101 MOF (270, Figure 39).442 High loadings of NHCfunctionalized Ru complexes could be absorbed into the pores at loadings even greater than one molecule per pore, although for catalytic applications, lower loadings were preferred to prevent bimolecular decomposition pathways. Noncharged catalysts could be removed by washing the catalysts with toluene or dichloromethane; however, only alcohol solvents were able to desorb the charged catalysts. Although the “as-prepared” catalysts were inactive for olefin metathesis, simple activation by HCl treatment results in high activity without loss of order. The authors postulate that added chloride prevents the MOF from removing chloride from Ru and opening it up to decomposition by reaction with hydroxide or fluoride. Under these optimized conditions, the encapsulated Ru catalysts were able to catalyze the ring closing metathesis of a variety of terminal alkenes with high activity (TONs up to 8900 in ring-closing olefin metathesis and a TON of 4700 under continuous flow conditions), and under mild conditions (room temperature 0.1% loading). The development of NHC-doped MOFs as organocatalysts for the transesterification of vinyl acetate with benzyl alcohol was described by Beauvais, Perry, and co-workers in 2016.443

Scheme 30. Preparation of Imidazolium-Containing MOF by Condensation of Simple and Imidazolium-Containing Biphenyl Bis Carboxylic Acid Monomers 271 and 272 with Zr Salts

6H2O and Cu(CH3CN)4PF6 along with equimolar amounts of the unfunctionalized biphenyl dicarboxylate, well ordered MOF (273) was obtained. The NHC could then be revealed by deprotonation with LDA, which did not appear to affect the structure of the MOF (Scheme 30).443 Targeting noncatalytic applications, three-dimensional carbene metal−organic frameworks have been prepared from the combination of Cu(OAc)2, 1,2,4-triazole, and the Keggin polyoxoanion [PW12O40]3−/[SiW12O40]4−.65 In the extended framework that results from the hydrothermal combination of these species, Cu is seen to bind to the nitrogen atoms and carbons of the triazole units. Like noncarbene-containing polyoxometalate-based MOFs, these materials showed good characteristics as electrode materials in lithium-ion batteries. Silver-containing polyoxometalate-based metal−organic frameworks were also studied for applications in lithium-ion batteries.65 BC


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NHC]+. Typically, long alkyl chains are employed on the wingtip groups of the benzimidazolylidene (as in 275, Figure 40) to impart liquid crystalline properties. Compound

Investigating other noncatalytic applications, Doonan, Sumby, and co-workers prepared NHC-containing MOFs and studied their hydrogen adsorption properties.444 MOFs were prepared by mixing a bis-carboxylated imidazolium precursor (in which the carboxyl groups are in the meta position), with Zn(NO3)·6H2O and Cu(CH3CN)4PF6. This generated a tethered NHC-Cu(I)-containing MOF with Zn units as the nodes. Optimization studies indicated that the best materials contained only 40% metalation of the NHC. These MOFs showed stronger adsorption of H2 compared to previously reported imidazolium-containing MOFs. In addition to MOFs, COFs, and CTFs (covalent triazene frameworks) are other three-dimensional porous materials with the advantage that they are comprised of chemical rather than ionic bonds and are thus more chemically inert.444 This is likely to be especially important for the development of porous self-supporting catalysts for challenging reactions such as the Suzuki−Miyaura reaction. By employing pyridyl-functionalized imidazolium units that have nitriles in the para position of the wingtip groups, Yoon and co-workers were able to prepare NHC-based CTFs (Chart 22,

Figure 40. Liquid crystal and molecular structure of [(275)2Au]+Br−. Adapted with permission from ref 49. Copyright 1997 Wiley-VCH.

[(275)2Au]+Br− represents one of the latter types of metallomesogens. Figure 40 shows the bilayer-type structure this compound takes up in the solid state, which was confirmed by analysis of the crystals by XRD. This particular MLC was shown to have improved properties vs existing Au-based MLCs at the time, which typically required high temperatures for conversion to the LC phase, narrow temperature windows for mesophase stability, and limited thermal stability of the metal−ligand bond.49 The presence of the metal was shown to be important since compound [(275)2Au]+Br− had lower temperatures for the crystal to liquid crystal transitions and wider temperature ranges for the mesotropic phase compared to starting benzimidazolium salt 275·HBr.49,66 Simple imidazolylidene-based NHCs (Scheme 31) were also explored as ligands for gold MLCs, again with long alkyl chains as wingtip groups.140 Metals bearing one or two NHC ligands were explored, as were metals with one NHC ligand and one alkylated imidazole.140 Of these strucutres, those bearing

Chart 22. Covalent Triazene Framework (CTF) Appropriately Substituted to Enable the Preparation of Ir Catalyst 274a

a


Scheme 31. NHC-Containing Au Complexes 276−279 for Examination of Liquid Crystal Behavior

Ir-NHC-CTF 274).445 The pyridine units adjacent to the imidazolium unit were then able to offer a bidentate ligand environment to Ir(III) complexes after preparation of the CTF. The resulting Ir-containing CTFs can be used as catalysts for the hydrogenation of CO2 to formate with extremely high TOF (16,000 h−1) and TON of 24,300.445 Despite the unique catalyst design and high activity, recyclability of the Ir-supported catalyst was low and Ir leaching was observed by analysis of the filtrate solutions. 3.6. NHC Liquid Crystals

The first report of an NHC in a liquid crystal was by the Lin group in 1997.49 In this report, the authors took advantage of the strong NHC−metal bond to create liquid crystalline materials based on Au−NHC organometallic compounds.49 These species are called metal-containing liquid crystals (MLCs) or metallomesogens. Prior to the Lin work, many MLCs were unstable, decomposing before the clearing point, but the strong NHC−Au bond provided MLCs of sufficient thermal stability. Two basic structures were employed, NHC-Au-X or [NHC-AuBD


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incorporate aluminum-based activators for the proposed application, the catalytic dimerization of olefins. ̂ et al.449 Dimeric silver MLCs have been produced by Circu These systems contain a ditopic NHC ligand bridged through the nitrogen by a simple 10 carbon-long alkyl chain (280−282 Scheme 32).449 In order to decrease temperatures required to

imidazole ligands 278−279 had lower decomposition temperatures (150−160 °C) compared with complexes (276 and 277) (280 °C); however, only the former species (278 and 279) were liquid crystalline.140 Mono-NHC complexes 278 and 279 were found to have smectic A mesophases, with a wider variety of complexes of type 278 displaying this property. This is theorized to be due to the imidazole ligands being more effective at achieving a balance between intermolecular interactions such as hydrogen bonding and dipole−dipole interactions. A proper balance of these effects is known to be important in liquid crystals.140 This study helped build the foundation for understanding the properties of NHCcontaining metallomesogens. Lin and co-workers also studied the liquid crystalline properties of NHC-containing palladium141 and silver142,446 complexes. In palladium MLCs, ligands with varying alkyl chain lengths on the nitrogen substituents were examined.141 All complexes were four-coordinate, with the NHC ligands trans to each other, with either Br or Cl as X-type ligands on the metal center.141 Alkyl chain lengths on the wingtip nitrogens of 10, 12, 14, and 16 carbons were employed for both systems. In the case of molecules of type 276, all complexes studied showed mesomeric properties with a smectic mesophase, except for compounds with two methyl groups on the backbone of the ligand.141 In the case of (NHC)2PdCl2-type compounds, where NHC = 276 mesophases were observed for alkyl chains that are 12, 14, and 16 methylene units long, but not for the 10 carbon derivative. Silver NHC complexes were initially quite hard to obtain as liquid crystals. Early attempts employed Ag2O, but only tetranuclear complexes with a Ag2Br2 center were obtained.142 Although the starting imidazolium salts showed liquid crystalline properties, the resulting complexes did not.142 However, if the complex and the matching mesomeric imidazolium salt are mixed in a 1:1 ratio, liquid crystalline properties are observed.142 The Lin group, who at that point dominated the field of NHCcontaining MLCs, made the first thermotropic silver−NHC complexes in 2012.446 This was achieved by modifying the wingtip groups such that one side of the ligand contained the traditional unfunctionalized alkyl chain, while the other contained an N-acetamido group,446 which improved the hydrogen bonding capacity of the material. Gold analogs with the same ligands were also synthesized and showed similar mesotropic characteristics when compared to their silver counterparts.446 These complexes were later used to make different xerogel structures with a variety of morphologies such as fibers, bunches of belts shaped like paper lanterns, and helical fibers. Meyer et al. have also prepared metal mesogens, in this case based on nickel with implications for catalysis.447 However, unlike the work of the Lin group, the Ni-NHC complexes themselves were not employed as liquid crystals but instead were combined in up to 10 wt % with the corresponding imidazolium BF4 salt, and the resulting mixture was supported on a solid silica support. Interestingly the mixture retained its liquid crystallinity even after being supported on silica, which means that it will be thermally switchable.447 The concept of employing supported ionic liquid phases (SILPs) as media to immobilized catalysts has been previously described,448 with the use of a mesogenictype ligand for the metal catalyzed being an additional selling feature. No catalytic studies were reported in this publication, possibly due to complications in finding efficient ways to

Scheme 32. Synthesis of Bis-NHC Silver Complexes from NHCs 280−282

access liquid crystal phases relative to past Ag-NHC MLCs,446 cyanobiphenyl and cholesteryl units were incorporated (Scheme 32).449 Smectic A mesophases were observed for all complexes synthesized, and when compared to the starting imidiazolium salts, the complexes had slightly lower transition temperatures and wider mesophase ranges.449 All smectic phases for these materials were below 100 °C, which is appreciably lower than observed in earlier systems reported by Lin.446,449 Another example of dinuclear MLCs was reported by Accorsi et al. (Scheme 33).450 A propylene linker was used to bridge two NHCs, and the wingtip groups consisted of functionalized benzyl groups containing long alkyl chain substituents connected via an ether linkage (Scheme 33).450 In this case, complexes were prepared by reaction with acetate to generate the free carbene in situ, followed by direct reaction with AuCl(SMe2), where previous examples employed transmetalation with Ag2O.140,141,446,450 The imidazolium precursor salts all have liquid crystal properties, but only compound [(284)2Au2]2X (R1 = OC12H25, R2 = H, Scheme 33), with four alkyl chains, had a stable mesomorphic phase.450 Other complexes prepared do not melt up to their decomposition temperatures approaching 200 °C.450 However, it should be noted that only a limited type of NHC was examined in this study, and therefore, many possibilities exist for the preparation of liquid crystalline materials from NHC−Au complexes. Syu, Lin et al. recently reported photoluminescent liquid crystalline complexes of Ag and Au.451 These materials contain hydroxyl groups at the 2-position on one wingtip group, designed to increase hydrogen bonding interactions. This interaction helped to facilitate the formation of a stable liquid crystal phase for the tetranuclear silver complexes. These complexes displayed photoluminescence signals at 569 and 607 nm for the tetranucleaur silver complex and the mononuclear gold complex, respectively. This property is attributed to BE


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Scheme 33. Synthesis of the Gold(I) Bis-NHC Complexes [(NHC)2Au2]2X−, Where NHC = a Symmetrical Ditopic NHC with Liquid Crystalline Properties by Virue of Muliple Long Alkyl Chains on the N Wingtip Groups

metallophilic interactions arising from metal−metal-to-ligand charge transfer interactions (MMLCT).452 The other two compounds prepared did not display any photoluminescence, most likely due to the lack of Au−Au interactions. For the mononuclear gold complexes, different photoluminescent properties were observed depending on whether the material was in the crystalline or material phases, unlike the tetranuclear silver complex. The state of the tetranuclear silver complex had a Smectic A phase from 92.6−97.6 °C upon soft material, and thin film format. The Polarz group has described the formation of bis-NHC chelating ligands (289a,b, Scheme 34) attached to Pd, Fe, Cu,

and platinum.454 The majority of complexes synthesized exhibit liquid crystalline columnar mesophases, melting points between 30 and 60 °C, and clearing points between 57 and 112 °C. Only the gold complexes and one copper complex displayed liquid crystalline properties. Small-angle X-ray scattering was used to determine the mesophase structures of the complexes. Nanosegregation of NHC-M moieties and triphenylene columns, with alkoxy chains separating them, was predicted to be the source of multicolumnar systems present. This indicates that π-stacking of the triphenylene moieties is the driving force for this segregation and thus for the liquid crystalline nature of these complexes.

Scheme 34. Synthesis of NHC-Based Metal Mesogens 289a/b

As has been demonstrated in this review, the use of Nheterocyclic carbenes as stabilizing ligands for all types of metals has opened the door to a variety of new materials, from welldefined discrete molecules and clusters to nanoparticles, functionalized surfaces, metal-containing liquid crystals and beyond. One of the most appealing aspects of these ligands is the manner in which, through the judicious choice of functional group(s), their properties can be tuned to best match various specific applications. Through chemical modification, the donor and acceptor properties of these ligands can be altered and the degree of steric protection altered to fit an individual need. In the area of NHCs on planar surfaces, the future will likely hold a wealth of new possibilities as the properties of NHCbased films are further tested in applications previously reserved only for thiol-based SAMs. Structure−property relationships need considerably more exploration, and the structural features that control ordering on the surface remain to be fully elucidated. In this area, we predict a large number of new applications of NHC-based SAMs in the near future. In the realm of nanoparticles, again work remains to be done to explore further the relationship between NHC structure, bonding orientation, and NP stability. How these properties are intertwined with catalytic activity will be a key feature going forward. Chiral NHCs are relatively underexplored here and provide an interesting opportunity for future work. Many metal classes have not been explored at all, or to a limited extent by comparison to gold and palladium. In the area of clusters, much remains to be elucidated, particularly whether NHCs are able to support larger clusters and again what the catalytic properties of these species will be. NHCs in polymer and dendrimer chemistry are more well explored, with the challenge in this area being elucidating the true active species when these species are used in catalysis. In the

3.7. Conclusions

and Ag, which display liquid crystalline phases.453 Of particular interest is their MLC of Pd, which is also catalytically active in both Suzuki and Heck cross coupling reactions. A hexagonal phase (P6/mm) was observed for the MLCs, in comparison with lamellar phases for their precursors. The goal of this research was to utilize these compounds as surfactants and catalysts in H2O, and specifically to cross couple organic compounds that have amphiphillic properties. When compared to a Pd complex with the same framework without LC character, an increase in catalytic activity by 1 order of magnitude was observed for the more lipophilic Pd complex. Catalytic activity was only tested using the Pd derivative; however, other types of catalysis with other metals are conceivable. Recently Miguel-Coello et al. reported the synthesis of NHCs containing pentadodecyloxytriphenylene unit(s) and their use in the formation of NHC−M complexes of copper, gold, silver, BF


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area of liquid crystal chemistry, much remains to be done to explore different structures and metals.

Ishwar Singh is a Ph.D. candidate in Professor Cathleen Crudden’s group at Queen’s University. Prior to joining her lab, he was an undergraduate research student with Professor Heinz-Bernhard Kraatz at University of Toronto, Scarborough (UTSC) between 2015−2016, where he studied the self-assembly of small ferrocene peptide conjugates to form gels. His research interests include exploring selfassembly of NHCs on planar surfaces and nanoparticles, with the goal of understanding the underlying chemistry and processes.

ASSOCIATED CONTENT Special Issue Paper

This paper is an additional review for Chem. Rev. 2018, volume 118, issue 19, “Carbene Chemistry”.

Ali Nazemi grew up in Mianeh, Iran, and obtained his B.Sc. in Chemistry from K. N. Toosi University of Technology (Iran) in 2005. He then moved to Canada and completed his M.Sc. in Inorganic Chemistry at the University of Toronto with Prof. Datong Song (2009) and his Ph.D. at The University of Western Ontario with Prof. Elizabeth R. Gillies (2013). After his graduate studies, Ali first spent two years (2014−2016) in England as a Marie Curie Postdoctoral Research Fellow with Prof. Ian Manners at the University of Bristol and then came back to Canada at Queen’s University (2016−2017) to work with Prof. Cathleen M. Crudden as a postdoctoral fellow. In January 2018, Ali started his independent career as an assistant professor at Université du Québec à Montréal. His group focuses on design, synthesis, and selfassembly of novel polymeric and dendritic systems to fabricate nanomaterials for applications in nanomedicine.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Christene A. Smith: 0000-0003-2631-5783 Ali Nazemi: 0000-0002-6504-582X Cathleen M. Crudden: 0000-0003-2154-8107 Present Address §

(A.N.) Department of Chemistry, University of Quebec at Montreal. Notes

The authors declare no competing financial interest.

Chien-Hung (Henry) Li was born in Taipei, Taiwan. He earned a B.S. degree in Chemistry from National Dong Hwa University in 2008. After one year of military service, he joined Prof. T. Randall Lee’s group at University of Houston and received his Ph.D. in 2015. His Ph.D. research focused on the developments of photonic nanomaterials in different applications. In 2016, he joined Prof. Cathleen Crudden’s group at Queen’s University as a postdoctoral fellow and worked on self-assembled monolayer of N-heterocyclic carbenes on Au surfaces and nanoparticles. In 2018, he returned to Taiwan and joined Kaohsiung Medical University as an Assistant Professor.

Biographies Christene Anne Smith studied chemistry at Saint Francis Xavier University and recently received her Ph.D. in chemistry at Queen’s University under the supervision of Prof. Cathleen Crudden. Currently she is a postdoctoral fellow in Dr. Guterman’s group under Director Prof. Markus Antonietti at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany. She has also studied under Prof. Martin Albrecht during a research visit at the University of Bern. She has held numerous awards including an NSERC Doctoral Fellowship and a Queen Elizabeth II Graduate Scholarship in Science in Technology. Her research interests include the application of NHCs for materials science, catalysis, and materials for energy applications. While being an enthusiastic chemist, she is also a passionate cook and adequate rock climber.

Cathleen Crudden was born in Belfast, Northern Ireland, and grew up in Toronto. She graduated from the University of Toronto with a B.Sc. and M.Sc., studying with Professor Mark Lautens. She carried out her Ph.D. at the University of Ottawa with Professor Howard Alper. During this time, she carried out a Ph.D. with Professor Shinji Murai. After an NSERC postdoctoral fellowship with Professor Scott Denmark at the University of Illinois at Urbana−Champaign, she took up an independent career at the University of New Brunswick. In 2002, she moved to Queen’s University as a Queen’s National Scholar and was promoted to full Professor in 2007. She is currently a Canada Research Chair (Tier 1) in metal organic chemistry and Associate Editor for ACS Catalysis. She holds a cross appointment at the Institute of Transformative Bio-Molecules (ITbM-WPI) at Nagoya University, where she runs a satellite lab. She is lucky to have a wonderful family, who tolerate her many travels, and outstanding students, who tolerate her many editorial requests.

Mina R. Narouz was born in Alexandria, Egypt. He obtained his B.Sc. in chemistry in 2008 and his M.Sc. in carbohydrate chemistry under the mentorship of Professor Mina A. Nashed in 2013 at Alexandria University. He is currently conducting his Ph.D. in chemistry at Queen’s University under the supervision of Professor Cathleen Crudden. Mina’s graduate research is focused on employing Nheterocyclic carbenes as novel ligands for gold surfaces, nanoparticles, and nanoclusters. Paul A. Lummis was born in England and spent his formative years mostly in the town of Swadlincote, Derbyshire. He completed his undergraduate studies at the University of Leeds with an exchange year at McMaster University in Hamilton, Canada. He then worked in industry in both the UK and Canada for 3 years, before returning to school to complete his Ph.D. under the supervision of Prof. Eric Rivard at the University of Alberta in Edmonton, Canada, where he investigated the stabilization and reactivity of main-group and transition metal hydride compounds. After this, he spent one year working under the direction of Prof. Philip P. Power at the University of California, Davis. He is currently a postdoctoral fellow in the group of Prof. Cathleen M. Crudden at Queen’s University, Canada, where he investigates the stabilization of metal clusters and surfaces with a wide variety of carbene-based ligands. In his free time, Paul can often be found in the kitchen or shouting about sport and politics.

ACKNOWLEDGMENTS The Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation of Innovation (CFI), and the Japan Society for the Promotion of Science are thanked for funding this research through various grants. Queen’s University, Nagoya University, and GreenCentre Canada are thanked for support. Megan Bruce is thanked for help with the preparation of this manuscript. BG


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