Readily Available Mesoionic Carbenes - American Chemical Society

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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1H‑1,2,3-Triazol-5-ylidenes: Readily Available Mesoionic Carbenes Gregorio Guisado-Barrios,*,† Michèle Soleilhavoup,‡ and Guy Bertrand*,‡ †

Institute of Advance Materials (INAM), Universitat Jaume I, Avenida Vicente Sos Baynat s/n, 12071 Castellon, Spain UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0343, United states

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‡

CONSPECTUS: Classical carbenes are usually described as neutral compounds featuring a divalent carbon with only six electrons in their valence shell. It was only in 1988 that our group prepared the first isolable example, in which the carbene center was stabilized by a push−pull effect, using a phosphino and a silyl substituent. In the last 30 years, a myriad of acyclic and cyclic push−pull and push−push carbenes, bearing different heteroatom substituents, have been isolated. Among them, the so-called N-heterocyclic carbenes (NHCs), which include cyclic (alkyl)(amino)carbenes (CAACs), are arguably the most popular. They have found a vast number of applications ranging from catalysis to material science, and even in medicine. In this Account, we focus on the synthesis, structure, electronic properties, coordination, and applications of a different class of stable cyclic carbenes, namely, 1H-1,2,3-triazol-5-ylidenes. In contrast with NHCs and CAACs, these compounds have no reasonable canonical resonance forms that can be drawn showing a carbene without additional charges. According to the IUPAC, they belong to the family of mesoionic compounds and thus they are named mesoionic carbenes (MICs). In 2010, we prepared the first stable 1,2,3-triazol-5-ylidene, via a CuAAC reaction, followed by alkylation of the resulting 1,2,3-triazole, and deprotonation. Later, we synthesized more robust N3-arylated counterparts from 1,3-diarylated-1H1,2,3-triazolium salts. Both synthetic routes can be carried out in multigram scales, making these MICS readily available. Importantly, MICs do not dimerize which contrasts with NHCs that can give the corresponding Wanzlick-type olefin. This property leads to relaxed steric requirements for their isolation; even C-unsubstituted MICs can be stored for months in the solid state at room temperature. The practicality and easily scalable syntheses of MICs allow for the preparation of polycarbenes, such as bis(1,2,3-triazol-5-ylidenes) (i-bitz), the analogues of the well-known 2,2′-bipyridines (bpy). MIC-transition metal complexes are excellent precatalysts for variety of chemical transformations, which include hydrohydrazination of alkynes, olefin metathesis, reductive formylation of amines with carbon dioxide and diphenylsilane, hydrogenation and dehydrogenation of N-heteroarenes in water, cycloisomerization of enynes, asymmetric Suzuki−Miyaura cross-coupling, and water oxidation (WO) reactions. Besides their catalytic applications, MIC−transition metal complexes have found applications in material sciences as exemplified by the preparation of the first iron(III) complex that is luminescent at room temperature. The peculiar properties of mesoionic triazolylidenes, combined with their enhanced stability, position them as excellent candidates to address some current challenges such as access to high-oxidation-state 3d metal complexes, the stabilization of highly reactive main group elements, the stabilization of nanoparticles, the preparation of efficient catalysts and photosensitizers based on earth-abundant transition metals, and the functionalization of self-assembled monolayers (SAMs) on gold.

1. INTRODUCTION In 1968, imidazol-2-ylidenes appeared as ligands for transition metals, when Wanzlick and Schö n herr 1 reported the preparation of the mercury complex A (Figure 1). Following this work, numerous reports involving carbene complexes of various transition metals were published; however, the field of stable carbenes remained stagnant until 1989 when our group reported the observation of the first stable divalent carbene II.2 Two years later, the group of Arduengo et al. succeeded in isolating and structurally characterizing a metal-free imidazol2-ylidene (Ia).3 Since that time, the so-called N-heterocyclic carbenes (NHCs) I have found a myriad of applications ranging from catalysis to material science, and even medicine.4 Interestingly, imidazol-2-ylidenes I have two five-membered © XXXX American Chemical Society

heterocyclic carbene isomers, namely, imidazol-5-ylidenes III and pyrazolin-4-ylidenes IV.5 Similar to imidazol-2-ylidenes I, both III and IV were first prepared in the coordination sphere of a metal. In 2001, Crabtree et al.6 discovered that a 2pyridylmethylimidazolium salt reacts with IrH5(PPh3)2 to give B which features the imidazole ring bound at C5 and not at C2. A few years later, Huynh et al.7 showed that a pyrazolium salt can serve as a precursor of metal complex C, featuring a pyrazol-4-ylidene ligand. These results prompted us to attempt the synthesis of the corresponding metal-free species, and in 2009 we isolated both the imidazol-5-ylidene IIIa8 and Received: September 22, 2018

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DOI: 10.1021/acs.accounts.8b00480 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Synthesis of the first transition metal complexes A, B, and C featuring NHC I, aNHC III, and rNHC IV, respectively; II was the first carbene ever isolated, and Ia, IIIa, and IVa the first metal-free representative of their respective family, along with some of their resonance structures.

formation of MIC complexes 2a,b through C−H bond activation of triazolium salts 1a,b with Pd(OAc)2 (Figure 2). Our successful isolation of MICs III and IV prompted us to tackle the preparation of stable 1,3,4-substituted-1,2,3-triazol5-ylidenes VII,16 and this Account focuses on their synthesis, structure, electronic properties, coordination chemistry, and applications.

2. SYNTHESIS OF 1H-1,2,3-TRIAZOL-5-YLIDENES VII In 1975, Begtrup17 reported that deprotonation of the triazolium salt 1c with NaH at 0 °C, in the presence of alkyl Figure 2. 1,2,4-Triazol-5-ylidenes V and their 1,2,3-triazol-5-ylidenes MIC isomers VI and VII; synthesis of the first transition metal complexes (2a,b) with a MIC ligand of type VII.

Scheme 1. First Evidence for the Transient Existence of 1,2,3-Triazol-5-ylidenes VII

pyrazolin-4-ylidene IVa.9,10 Because of their lineage, compounds III and IV are sometimes referred to as abnormal NHCs (aNHCs) and remote NHCs (rNHCs), respectively. In contrast with NHCs I, there are no reasonable canonical resonance forms for III and IV that can be drawn showing a carbene without additional charges. Resonance forms IIIa″ and IVa′′′ are certainly the most accurate schematic representations of these compounds. They perfectly fit the IUPAC definition of mesoionic compounds, and therefore, we prefer to name compounds of types III and IV “mesoionic carbenes” (MICs). Another family of NHCs, the 1,2,4-triazol-5-ylidenes V, discovered by Enders et al. in 1995, 11 has attracted considerable interest as powerful organocatalysts12 (Figure 2). These carbenes also have two mesoionic carbene structural isomers, namely, the 1,2,4-substituted-1,2,3-triazolylidenes VI and 1,3,4-substituted-1,2,3-triazol-5-ylidenes VII. The former have not yet been isolated due to their low stability, although their ammonia adducts and related metal complexes were reported by Herrmann and Kühn et al.13 Just like NHCs I, aNHCs III, and rNHCs IV, 1,3,4-substituted-1,2,3-triazol-5ylidenes VII14 have first been described as ligands for transition metals. In 2008, Albrecht et al.15 reported the

halides or oxygen, gave rise to compounds 3 and 4, respectively (Scheme 1). This was the first indication that 1H-1,2,3-triazol5-ylidenes of type VII have a non-negligible lifetime in solution. Based on these results, we prepared 1,2,3-triazolium salts 1d,e bearing a bulky 2,6-diisopropylphenyl (Dipp) substituent at the nitrogen alpha to the ensuing carbene center.16 The onepot conversion of Dipp-NH2 to the desired Dipp-N3, followed by an in situ CuAAC (Copper Catalyzed Azide-Alkyne Cycloaddition) reaction with phenyl acetylene, was found to be very efficient for preparing 1,2,3-triazole 5. Then, simple alkylations with either methyl or isopropyl trifluoromethanesulfonate afforded the desired triazolium salts 1d,e, respectively, which were deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide to afford VII1 and VII2, respectively, in reasonable yields (Scheme 2). In B


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Accounts of Chemical Research Scheme 2. Synthesis of MICs VII1,2 and the Rearrangement of VII1 into 6

Scheme 3. Synthesis of 1,3-Diarylated MICs VII

Scheme 4. Synthesis of 1,3-Diarylated-1H-1,2,3-triazolium salts Using Diaryliodonium Salts

Scheme 5. Synthesis of Stable i-bitz (VII)2 and Rearrangement of bitz

More recently, another synthetic strategy for the preparation of 1,3-diarylated-1H-1,2,3-triazolium salts has been reported by Gao et al. (Scheme 4).20 The N3-arylated salt 1 was obtained in 90% yield from the reaction of 1-aryl-1H-1,2,3-triazole 9 and diaryliodonium salt 10 using a catalytic amount of a copper(II) acetate. The synthetic routes described in Schemes 2−4 are so far the only known methods that allow access to metal-free 1,2,3triazol-5-ylidenes and their immediate precursors. However, they are practical and easily scaled up, and they even allow for the preparation of polydentate ligands based on MICs. We first synthesized 1,4-bidentate ligands (VII)2 (i-bitz),21 which feature a topology similar to 2,2′-bipyridines (bpy) and their congeners, as well as bis(1,2,4-triazol-5-ylidenes) (bitz) introduced by Crabtree and Peris22 (Scheme 5). Interestingly, in marked contrast with bitz ligands, which spontaneously rearrange into bis(triazoles) 14, deprotonation of 12a−c with KHMDS cleanly afforded the free i-bitz (VII)2, which were isolated as stable solids in good yields. Other “metal-free” polydentate ligands based on MICs of type VII have been reported as exemplified by VII-carba-VII,23 VII-pyri-VII,24 and (VII)3-borate25 (Scheme 6).

the solid state, with the exclusion of oxygen and moisture, MIC VII1 remains stable for several days at −30 °C. However, upon heating in benzene solution for 12 h at 50 °C, VII 1 decomposes to give the triazole 6 among other products. MIC VII2 is significantly more stable, showing no signs of decomposition after 3 days at room temperature. To further improve the stability of MICs VII, we turned our attention to the preparation of 1,3-diarylated-1H-1,2,3triazolium salts.18 We used a synthetic route involving a formal 1,3-dipolar cycloaddition of an alkyne with a 1,3-diaza2-azoniaallene salt 8 (Scheme 3). When the alkynes are either expensive or not practically accessible, vinyl halides may be used as synthetic alkyne equivalents. The cycloaddition proceeds and spontaneous elimination of hydrogen halide occurs. A variety of free 1,3-diarylated MICs have been prepared using this synthetic sequence, and they can be stored in the solid state at room temperature under an inert atmosphere for several weeks.18,19 Additionally, in contrast to the case of VII1, no sign of rearrangement of 1,3-diarylated MICs was observed upon heating in a benzene solution for 12 h at 50 °C. C


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Accounts of Chemical Research Scheme 6. Synthesis of “Metal-Free” Polydentate Ligands Based on MICs VII

3. STRUCTURE AND ELECTRONIC PROPERTIES OF 1H-1,2,3-TRIAZOL-5-YLIDENES VII Although MICs VII and their classical NHC isomers V are quite different, both give a 13C NMR signal for the carbene

This was confirmed by the NICS (Nucleous-Independent Chemical Shifts) values for the parent MIC VII {NICS(0) = −14.93; NICS(1)zz= −36.06], which are comparable to those of other aromatic 5-membered heterocycles. From the ṽCO stretching frequencies of (VII)Ir(CO)2Cl complexes, it could be concluded that the overall donor properties of MICs VII are comparable to those of NHCs I and V, but significantly weaker than those of the other MICs such as the imidazol-5-ylidenes III and pyrazolin-4-ylidenes IV. High-level calculations (EDA-NOCV calculations at the BP86/TZ2P+ level)26 showed that the singlet−triplet gap of VII (≈60 kcal·mol−1) is comparable to that of aNHC III (≈60 kcal·mol−1), larger than that of rNHC IV (≈40 kcal·mol−1), but much smaller than those of classical NHCs I (≈92 kcal· mol−1) and V (≈83 kcal·mol−1). Analysis of the frontier orbitals shows that the HOMO of VII is a σ-lone pair at carbon as found in classical NHCs and other MICs, whereas the LUMO has a rather small orbital coefficient at C5 (Figure 3). In agreement with the assessment of the electronic properties derived from the CO stretching frequencies, the HOMO energy level of VII (−4.43 eV) is only slightly higher than that in classical NHCs I (−4.83 eV) and V (−5.22 eV) but lower than that in MICs III and IV (−4.03 and −3.47 eV). Along this line, the calculated proton affinity of VII (272−275 kcal·mol−1) is closer to that of imidazol-2-ylidenes I (≈270 kcal·mol−1) than to that of aNHC III (287.0 kcal·mol−1). Accordingly, just like I, MICs VII can be deprotonated with mild alkoxide bases, while the other MICs required stronger amide bases. Calculations also show that MICs VII are about 23 kcal· mol−1 higher in energy than the corresponding 1,2,4-triazol-5ylidene isomers V. In comparison, the aNHC III is only about 14 kcal·mol−1 higher in energy than its NHC isomer I. However, it is important to note that, in contrast with NHCs, which can dimerize to give the corresponding Wanzlick-type

Figure 3. Frontier orbitals of VII and orbital energies in eV at the BPI level of theory.

Scheme 7. Unlike NHCs, MICS Do Not Dimerize, Which Leads to Relaxed Steric Requirements for Their Isolation

carbon between 198 and 210 ppm. MICs VII have a planar ring with bond lengths halfway between those of single and double bonds, which is indicative of their aromatic character. D


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Accounts of Chemical Research Scheme 8. Coordination of Isolated 1,2,3-Triazol-5-ylidenes VII to Different Metal Fragments

Scheme 9. (a) MIC VII as Ligands for Gold Catalyzed Bis-Hydrohydrazination of Alkynes with Parent Hydrazine; (b) Sequential Preparation of Unsymmetrical Azines

olefin,27 MICs do not dimerize (Scheme 7). This property leads to relaxed steric requirements for their isolation, and indeed, the C-unsubstituted MIC VII3 can be stored for months in the solid state at room temperature.28

Suzuki−Miyaura cross-coupling,33 and water oxidation (WO).34 Their use in the latter process and other related oxidative catalysis that involves proton-coupled electron transfer stems from the idea that their mesoionic character endows a reservoir for charges and holes, prompting effective ligand−metal cooperativity strongly favoring these transformations.35 Recently, MICs VII have also been used for the stabilization of main group compounds. Crudden and coworkers have shown that MICs VII-stabilized borenium ions promote the reduction of imines at room temperature under 1 atm of hydrogen.36 Moreover, a recent study based on DFT calculations suggested that MICs VII are suitable candidates to stabilize B2 species.37 In our group, we have used isolated monodentate 1,2,3triazol-5-ylidenes VII to prepare iridium(I) 18a−c,16,18

4. COORDINATION CHEMISTRY OF FREE MIC VII AND APPLICATIONS The relaxed steric requirements for the isolation of free MICs VII, along with their straightforward synthesis, have facilitated their utilization as ancillary ligands for transition metals.29 The corresponding complexes have been used in important metal mediated catalytic transformations such as the reductive formylation of amines with carbon dioxide and diphenylsilane,30 hydrogenation and dehydrogenation of N-heteroarenes in water,31 cycloisomerization of enynes,32 asymmetric E


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Accounts of Chemical Research

Scheme 10. Complex 20 Is Not Catalytically Active for RCM, but Becomes Very Efficient by Addition of CF3SO3H (Blue) or HCl (Black), Even When Compared with the Second Generation Hoveyda−Grubbs Catalyst (White)

Scheme 11. Some examples of Metal Complexes Featuring Polydentate MICs, Including 30, the First Iron Complex Luminescent at Room Temperature

ruthenium(II) 19a−d18 and 20,28 copper(I) 21a−b,38 palladium(II) 22,38 and gold(I) 23, 24 complexes39 (Scheme 8). Previously, we had shown that (CAAC)40 and (anti-Bredt NHC)41 gold complexes promoted the hydrohydrazination of unactivated alkynes with the parent hydrazine.42 We found that, in the presence of a chloride abstractor, such as KB(C6F5)4, complex 24 allowed for the first transition metalcatalyzed bis-hydrohydrazination of alkynes with NH2NH2

(Scheme 9).39 In addition, unsymmetrical azines could also be produced using two different alkynes reacting sequentially. In collaboration with Grubbs et al., we had shown that replacing the NHC ligand of the second generation of Grubbs and Hoveyda−Grubbs ruthenium complexes by a small CAAC afforded the best catalysts known so far for the ethenolysis of methyl oleate.43 Encouraged by these results, we explored the performance of several N3-arylated MICs-VII based Ru(II) complexes for the ring closing metathesis.28 It has been known that because NHCs strongly bind to Ru, bis-NHC complexes F


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Accounts of Chemical Research are poor olefin metathesis catalysts.44 Similarly, since MICs VII strongly bind metals, the mixed NHC−Ru−MIC complex 20 is not catalytically active. However, upon addition of a catalytic amount of Brønsted acid, the MIC ligand is protonated and becomes a leaving group, which gives rise to the active species of the Grubbs second-generation catalyst. In the presence of acid, 20 is superior to the latest commercial catalysts and can complete RCM reactions within a matter of minutes at 30 °C (Scheme 10). Complex 20 combines the shelf-stability of bis-NHC complexes while maintaining low activation temperatures, properties that are critical in materials science applications.28 Isolated polydentate MICs have also been used in coordination chemistry as exemplified by the preparation of the homoleptic bis(1,2,3-triazol-5-ylidene)pyridine iron(II) complex 2524 and the (VII)3-borate nickel(II) complex 2625 (Scheme 11). In collaboration with Bezuidenhout et al., we explored the coordination versatility of the T-Shaped (LXL) VII-carba-VII ligand.23 We first found that the rhodium(I)− oxygen complex 27 catalyzes the homodimerization and hydrothiolation of alkynes with excellent selectivity to give rise to the gem-enyne and α-vinyl sulfide isomers, respectively,45 (Scheme 11). Then, we described the synthesis of the Au(I)−pincer complex 28,46 by treatment of VII-carbaVII with [AuCl(tht)]; and in contrast with most Au(I) complexes, it reacts with electrophiles. Protonation of 28 with excess trifluoroacetic acid afforded the cationic Au(III) hydride complex 29.46 Surprisingly, although 29 gives a 1H NMR signal at −8.3 ppm, it reacts with NaH, and not HCl, with elimination of H2 and formation of the gold(I) complex 28, arguing for a protic behavior. The homoleptic [(i-bitz)3]iron(III) complex 30 is certainly the most striking illustration of the potential applications of MICs VII.47 Indeed, as demonstrated by Wärnmark and coworkers, the superior σ-donor and π-acceptor electron properties of the MIC ligands is crucial for accessing iron complexes with long charge-transfer lifetime of 100 ps and exceptional room-temperature photoluminescence.

applications in the stabilization of nanoparticles as well as for the functionalization of self-assembled monolayers on gold.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Gregorio Guisado-Barrios: 0000-0002-0154-9682 Guy Bertrand: 0000-0003-2623-2363 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Gregorio Guisado-Barrios received his Ph.D. from the University of St Andrews in 2010 under the supervision of Dr. David T. Richens. He then joined the group of Guy Bertrand at the University of California, Riverside before moving in 2012 to the Universitat Jaume I, Castellón where he is currently working as “Juan de la CiervaIncorporación” Fellow at the Institute of Advance Materials (INAM). Michèle Soleilhavoup received her Ph.D. in 1993 from the University of Toulouse under the supervision of Guy Bertrand. After 2 years at BASF AG at Ludwigshafen, she became Chargèe de Recherche CNRS at University Paris VI. From 2000 to 2001, she worked in the Laboratoire de Chimie de Coordination in Toulouse, before joining the UCR/CNRS Laboratory and in 2012 the UCSD/CNRS Joint Research Laboratory at the University of California, San Diego. Guy Bertrand obtained his Ph.D. from the University of Toulouse. From 1988 to 1998, he was a “Director of Research” at the Laboratoire de Chimie de Coordination du CNRS, and from 1998 to 2005 the Director of the Laboratoire d’Hétérochimie Fondamentale et Appliqueé at the University Paul Sabatier. In 2001, he moved to the University of California at Riverside, and since 2012 he is the Director of the UCSD/CNRS Joint Research Laboratory at the University of California, San Diego.

5. CONCLUSIONS AND PERSPECTIVES Although MIC VII ligands have inevitably been compared to NHCs I and V, they have quickly stood out due to their unique electronic properties. Importantly, MICs do not dimerize, which is in contrast to NHCs that can give the corresponding Wanzlick-type olefin. This property leads to relaxed steric requirements for their isolation; even C-nonsubstituted MIC, such as VII3, can be stored for months in the solid state at room temperature. They are readily available and can be prepared in multigram quantities. The ease of their synthesis makes them attractive scaffolds for polydentate ligands. We foresee a promising future for MICs of type VII in catalysis where they have already demonstrated their great potential as ligands in olefin metathesis, hydrohydrazination, hydrothiolation and dimerization of alkynes, water oxidation (WO) reactions, reductive formylation of amines with carbon dioxide and diphenylsilane, cycloisomerization of enynes, asymmetric Suzuki−Miyaura cross-coupling, and metal-free hydrogenations. Their ability to generate high-valent late transition metals and room temperature iron-based luminescent compounds may open new avenues in metallaphotocatalysis involving high-oxidation states especially for late 3d transition metals, and enable the use of photosensitizers based on earth-abundant transition metals. Likewise, they may find

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, under Award # DE-SC0009376, and the NSF (CHE1661518). G.G.-B thanks MINECO for a “Juan de la Cierva Fellowship” (GGB, IJCI-2015-23407).

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REFERENCES

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