Gregorio Guisado-Barrios

Crystalline 1H-1,2,3-triazol-5-ylidenes: new stable mesoionic carbenes (MICs).

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 [A] ... 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; νavg = 2019 cm−1) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (νavg = 2021 cm−1),[9a] and are indicative of donor properties that are superior to those of NHCs I and II (νavg = 2022–2031 cm−1),[22] but inferior to those of MICs III (νavg = 2003–2006 cm−1)[23] and IV (νavg = 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.

Pyracene-Linked Bis-Imidazolylidene Complexes of Palladium and Some Catalytic Benefits Produced by Bimetallic Catalysts

Two new palladium complexes with a pyracene-linked bis-imidazolylidene (pyrabim) group have been obtained and fully characterized. The related monometallic analogues were obtained from the coordination of an acetanaphthene-supported N-heterocyclic carbene (NHC). The catalytic properties of all complexes were studied in the acylation of aryl halides with hydrocinnamaldehyde, and in the Suzuki-Miyaura coupling of aryl halides and aryl boronic acids. The results show that the presence of a second metal in the dimetallic complexes induces some benefits in the catalytic behavior of the complexes. This effect is more pronounced in the Suzuki-Miyaura coupling, for which the dimetallic complexes exhibit significantly higher activity than their monometallic counterparts.

Isolation of a potassium bis(1,2,3-triazol-5-ylidene)carbazolide: a stabilizing pincer ligand for reactive late transition metal complexes

The synthesis and X-ray crystal structure of a potassium adduct of a monoanionic CNC-pincer ligand featuring two mesoionic carbenes is reported. Owing to the peculiar electronic and steric properties of this ligand, the first neutral stable Ni(II)-hydride, and an unusual Cu(II) complex displaying a seesaw geometry, have been isolated.

Anionic 1,2,3-Triazole-4,5-diylidene: A 1,2-Dihapto Ligand for the Construction of Bimetallic Complexes

A super pyrazolate: deprotonation of the flanking hydrogen of metal complexes of mesoionic carbenes (MICs) offers a simple and general route for the preparation of bimetallic complexes of a 1,2-dihapto anionic dicarbene ligand that is isoelectronic with widely used pyrazolate ligands, while conferring greater electron donation and stronger M-L bonds.

A hemilabile and cooperative N-donor functionalized 1,2,3-triazol- 5-ylidene ligand for selective and base-free rhodium(I) catalyzed alkyne hydrothiolation reactions

A series of novel cationic and neutral Rh complexes with an N-donor-functionalized 1,2,3-triazol-5-ylidene (TRZ) ligand (in which the pendant N donor is NHBoc, NH2 , or NMe2 ) is described. The catalytic activity of these complexes was evaluated in the hydrothiolation of alkynes. Among the catalysts, a neutral dicarbonyl complex featuring the tethered-NBoc amido-TRZ ligand proved very selective for alkyne hydrothiolation with an aryl thiol. Remarkably, the reaction could be carried out in the absence of pyridine or base additive. In addition, during the reaction, no evidence for oxidative addition of the thiol S-H bond was observed, strongly suggesting a reaction pathway in which a bifunctional ligand is involved. Experimental and theoretical mechanistic investigations suggest a ligand-assisted deprotonation of thiol, hemilabile dissociation of amine from the metal, and thiolate coordination, which is indicative of a different reaction mechanism to those previously reported for related alkyne hydrothiolation reactions.

Ruthenium complexes with an N-heterocyclic carbene NNC-pincer ligand: preparation and catalytic properties

1-Methyl-3-(2-((pyridin-2-ylmethylene)amino)ethyl)-1H-imidazol-3-ium bromide was prepared and used as an N-heterocyclic carbene NNC-pincer ligand precursor. Depending on the coordination strategy, a monometallic [Ru(NNC)(CO3)(PPh3)] complex, or the [Ru(μ-Cl)(NNC)]2(2Cl−) dimer, was obtained. A di-silver complex in which two ligands are monocoordinated to the metal center through the NHC groups was also obtained and characterised. The dimetallic ruthenium complex reacts with alcohols yielding a monohydride species. The preliminary studies on the catalytic activity of the ruthenium dimer indicate that the complex is active in the reduction of ketones and aldehydes under transfer hydrogenation conditions.

Rhodium, iridium and nickel complexes with a 1,3,5-triphenylbenzene tris-MIC ligand. Study of the electronic properties and catalytic activities

Summary The coordination versatility of a 1,3,5-triphenylbenzene-tris-mesoionic carbene ligand is illustrated by the preparation of complexes with three different metals: rhodium, iridium and nickel. The rhodium and iridium complexes contained the [MCl(COD)] fragments, while the nickel compound contained [NiCpCl]. The preparation of the tris-MIC (MIC = mesoionic carbene) complex with three [IrCl(CO)2] fragments, allowed the estimation of the Tolman electronic parameter (TEP) for the ligand, which was compared with the TEP value for a related 1,3,5-triphenylbenzene-tris-NHC ligand. The electronic properties of the tris-MIC ligand were studied by cyclic voltammetry measurements. In all cases, the tris-MIC ligand showed a stronger electron-donating character than the corresponding NHC-based ligands. The catalytic activity of the tri-rhodium complex was tested in the addition reaction of arylboronic acids to α,β-unsaturated ketones.

Self-Assembly of Di-N-Heterocyclic Carbene-Gold-Adorned Corannulenes on C60

The deprotonation of a corannulene-based bisazolium salt allowed the preparation of a corannulene-based NHC di-AuI complex. The prepared di-AuI -complex was tested in the recognition of fullerene-C60 , demonstrating good binding affinity in toluene solution, and producing a host-guest complex with 3:1 stoichiometry, as evidenced by a combination of NMR spectroscopy and ITC titrations. The experimental results are fully supported by DFT calculations. The binding of C60 with the di-AuI complex in toluene solution is enthalpically and entropically favoured. Remarkably, the entropic term is the dominant parameter in the binding process. The good complementarity that exists between the concave shape of the corannulene-di-gold complex and the convex surface of the fullerene, together with the presence of tBu groups and the AuCl fragment are key factors for the measured high affinity between host and guest. The obtained results indicate that fullerene may be acting as a template for the formation of a self-assembled aggregate involving up to three molecules of the di-AuI complex.