Crystalline 1H-1,2,3-Triazol-5-ylidenes: New Stable Mesoionic Carbenes (MICs)

Abstract

Until recently, the availability of neutral carbon-based κ¹C ligands was limited to carbon monoxide, isocyanides, and carbenes. Compared to phosphorus-based ligands, carbenes tend to bind more strongly to metal centers, avoiding the necessity for the use of excess ligand in catalytic reactions. The corresponding complexes are often less sensitive to air and moisture, and are remarkably resistant to oxidation.^[1] As the robustness of carbene complexes is largely due to the presence of strong carbon–metal bonds, other types of carbon-based ligands are highly desirable. It is noteworthy that, although complexes between a carbene and a transition metal have been known for a long time,^[2] the recent developments in their application in catalysis^[3] have been greatly facilitated by the availability of carbenes that are stable enough to be bottled.^[4,5] Moreover, carbenes, especially imidazol-2-ylidenes I^[4c] and 1,2,4-triazol-5-ylidenes II,^[4e] are also excellent organocatalysts (Scheme 1).^[6]

In 2001, Crabtree and co-workers first reported complex A, which features an imidazole ring bound at the C5 position (III), and not at C2 as commonly observed.^[7] More recently, Huynh and co-workers^[8] and Albrecht and co-workers^[9a] showed that pyrazolium and 1,2,3-triazolium salts can serve as precursors to metal complexes of type B and C, which feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V as the ligand, respectively. As a consequence of their lineage, these have also been referred to as N-heterocyclic carbenes (NHCs). However, as no reasonable canonical resonance forms containing a carbene can be drawn for free ligands III–V without additional charges (see V′), these ligands have been described as abnormal or remote carbenes (aNHCs or rNHCs, respectively).^[10] As they are, in fact, mesoionic compounds,^[11] we suggest naming this family of compounds mesoionic carbenes (MICs). There have been no reported dimerizations of MICs III and IV, which suggests that the Wanzlick equilibrium pathway for classical carbenes is disfavored;^[12] this observation should lead to relaxed steric requirements for their isolation. Moreover, experimental and theoretical data suggest that MICs III–V are even stronger electron-donating species than NHCs I and II, which opens up interesting perspectives for their applications.^[10]

Our recent success in the isolation of a free imidazol-5-ylidene III^[13] and pyrazolin-4-ylidenes IV (cyclic bent allenes),^[14,15] prompted us to investigate the possibility of preparing new types of stable neutral compounds that feature a lone pair of electrons on the carbon atom.^[16] Preliminary calculations (B3LYP, 6–311G(d,p); for details, see the Supporting Information) predicted that the parent MIC V is located at an energy minimum, about 32 kcalmol⁻¹ above the regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore, parent V is predicted to exhibit an appreciably large singlet–triplet band gap (56 kcalmol⁻¹), which is a good predictor of carbene stability and thus of possible isolation. Herein, we report the preparation, isolation, and characterization of two free 1,2,3-triazol-5-ylidenes of type V.

By analogy with the synthetic route used for preparing NHCs and the related species III and IV, 1,2,3-triazolium salts (2a,b) were targeted as precursors for the desired 1,2,3-triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl substituent (2,6-diisopropylphenyl, Dipp) was selected to provide kinetic stabilization to the ensuing free ligand. 1,2,3-Triazole 1 was obtained in 83% yield from the copper-catalyzed azide–alkyne cycloaddition (CuAAC, click chemistry) of 2,6-diisopropylphenyl azide and phenylacetylene.^[17] The one-pot conversion of aniline into the desired aryl azide, followed in situ by CuAAC as reported by Moses and co-workers^[18] was found to be especially convenient for the synthesis of 1. Alkylation of 1 with methyl or isopropyl trifluoromethanesulfonate afforded the corresponding tri-azolium salts in moderate to excellent yields (2a and 2b, respectively; Scheme 2).

Scheme 2.

Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b.

Potassium bases have been identified as the reagents of choice for the depronation of carbene precursors, as they avoid the formation of stable carbene–alkali-metal adducts that are commonly encountered when lithium bases are used.^{[12,13,14a,19]} Gratifyingly, triazolium salts 2a,b were cleanly deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide in ethereal solvents to afford the corresponding MICs Va and Vb in 55 and 39% yield, respectively. Deprotonation was evidenced by the disappearance of the triazolium CH signal in their ¹H NMR spectra (2a: δ =8.62 ppm; 2b: δ =8.85 ppm) and the appearance of a signal at low field in the ¹³C NMR spectrum (Va: δ = 202.1 ppm; Vb: δ =198.3 ppm). The structure of Va was unambiguously confirmed by X-ray crystallography (Figure 1).^[20] In the solid state, Va contains a planar heterocycle, characterized by bond lengths that are intermediate between those of single and double bonds; both of these features are indicative of electronic delocalization. Upon deprotonation, the C5 carbon bond angle becomes more acute (2a: 106°; Va: 100°), which is consistent with an increased s character in the σ lone pair orbital of Va compared to the C–H bonding orbital of the precursor 2a. This is in agreement with the generally observed trend for carbenes and their conjugate acids.^[5]

Figure 1.

Molecular views (thermal ellipsoids set at 50 % probability) of 2a (top) and Va (bottom) in the solid state. For clarity, counter ions, solvent molecules, and H atoms are omitted, except for the ring hydrogen of 2 a. Selected bond lengths [Å] and angles [°] for 2 a: N1–N2 1.3208(16), N2–N3 1.3183(16), N3–C4 1.3559(17), C4–C5 1.3647(19), C5–N1 1.3446(17); N1-C5-C4 105.85(12). Va: N1–N2 1.3439(12), N2–N3 1.3216(13), N3–C4 1.3682(13), C4–C5 1.4053(14), C5–N1 1.3662(13); N1-C5-C4 99.70(8).

In the solid state, with the exclusion of oxygen and moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–52°C decomp.) remained stable for several days at −30°C and for a few hours at room temperature. By contrast, Vb (m.p. 110–112°C) was significantly more stable, showing no sign of decomposition after three days at room temperature in the solid state. Upon heating in a benzene solution for 12 hours at 50°C, Va decomposed to give, among other products, triazole 3 (Scheme 3; for details, see the Supporting Information). We surmise that the latter product results from a nucleophilic attack of the carbon lone pair of Va on the methyl group of a second molecule of Va, giving rise to heterocycles 4 and 5, which react together to afford the observed product 3. This apparent rearrangement is reminiscent of that recently observed in the formation of imidazol-2-ylidenes of type I from imidazol-5-ylidenes of type III that contain an electro-philic Y group.^[21] In agreement with this hypothesis, MIC Vb, which contains the less-electrophilic isopropyl group at the N3 position, appears much more robust with respect to this decomposition pathway.

Scheme 3.

Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy with the rearrangement of III into I.

To evaluate the donor properties of 1,2,3-triazol-5-yli-denes, the [(Va)Ir(CO)₂Cl] complex was prepared by addition of Va to [{Ir(cod)Cl}₂] (cod = 1,5-cyclooctadiene), followed by treatment with an excess of carbon monoxide. The CO vibration frequencies (ν =2061 and 1977 cm⁻¹; ν_avg = 2019 cm⁻¹) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (ν_avg = 2021 cm⁻¹),^[9a] and are indicative of donor properties that are superior to those of NHCs I and II (ν_avg = 2022–2031 cm⁻¹),^[22] but inferior to those of MICs III (ν_avg = 2003–2006 cm⁻¹)^[23] and IV (ν_avg = 2002 cm⁻¹).^[14b]

Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by compounds Va,b, possess an ensemble of properties that portend to their utility. The synthesis of their precursors is short and efficient, from readily available starting materials, yet is modular and thus amenable to a wide variety of potential analogues. As with other mesoionic carbenes III and IV, the dimerization of MICs of type V has not been observed; therefore, the preparation of comparatively unhindered MICs is predicted to be viable. Their donor properties are greater than those of NHCs of type I and II, but they are nonetheless available by deprotonation using mild bases (e.g. alkoxides), thus signaling their potential for applications, such as nucleophilic organocatalysis. Free triazolylidenes V complement the rapidly growing numbers of neutral carbon-based κ¹C ligands that are now available. We predict that many other classes of MICs, that are derived from a variety of heteroaromatic scaffolds, can be isolated. This endeavor is currently the object of ongoing efforts in our laboratory.

Experimental Section

1H-1,2,3-triazol-5-ylidenes (Va,b): Anhydrous Et₂O (7 mL) was added to a Schlenk flask containing triazolium salt 2a (0.40 g, 0.85 mmol) and KN(SiMe₃)₂ (0.18 g, 0.9 mmol) that had been cooled to −78°C. The reaction mixture was stirred at −78°C for 10 min, and allowed to warm to room temperature for 50 min before the solvent was evaporated under reduced pressure. The residue was extracted twice by trituration in anhydrous hexanes (2 × 5 mL). After filtration and transfer into a second Schlenk flask using a filter cannula, the solvent was evaporated to give Va as a pale purplish-white solid (0.149 g, 55%). Using a similar procedure, Vb was obtained as pinkish-white solid (0.053 g, 39%).

Scheme 1.

Classical NHCs I and II, their mesoionic carbene isomers III–V, and the first complexes featuring the latter.