Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

From Pincer to Paddlewheel: C-H and C-S Bond Activation at Bis(2-pyridylthio)methane by Palladium(II)

The bis(2-pyridylthio)methanidopalladium(II) pincer complex (1), containing a Pd-C bond, was obtained from the reaction of bis(2-pyridylthio)methane (H₂L) with palladium(II) acetate in toluene under reflux. When palladium(II) trifluoroacetate was used, H₂L reacted to generate the tetrakis(pyridine-2-thiol)palladium(II) complex (2). Complex 2 was converted to a heterobimetallic palladium(II)-iron(II) paddlewheel complex (3) upon treatment with iron(II) triflate in the presence of a base in acetonitrile at room temperature.

Synthesis of heterobimetallic gold(i) ferrocenyl-substituted 1,2,3-triazol-5-ylidene complexes as potential anticancer agents

1,2,3-Triazol-5-ylidene (trz) complexes of gold(i) containing a ferrocenyl substituent on the C4-position of the trz ring were synthesized to yield the neutral heterobimetallic gold(i) trz chlorido (2), gold(i) trz phenyl (3), and the cationic gold(i) trz triphenylphospine (5) complexes. In order to compare the effect of silver(i) as central metal vs. gold(i), [Ag(trz)2]+ (4) was also prepared, while variation of the C4-1,2,3-triazol-5-ylidene substituent from a ferrocenyl to a phenyl group was done to prepare the monometallic analogue of 5, namely the cationic Au(i) trz triphenylphosphine complex 6. The complexes were characterised with spectroscopic and electrochemical methods, and the single crystal X-ray structures of 2-6 were determined. NMR stability studies of 5 as a representative example of the series of complexes were performed to confirm the stability of the complexes in the solvent dimethylsulfoxide and in aqueous solution. The anti-cancer potential of 5 was evaluated against the lung cancer cell lines A549 and H1975, and the human embryonic kidney cell line (HEK-293) was used as a non-cancer model. IC50 values of 0.89, 0.23 and 5.43 μM, respectively, were obtained for A549, H1975 and HEK-293, respectively, indicating the activity and selectivity of 5 for cancer cells. Fluorescence microscopy experiments as a preliminary mode-of-action study evidenced an apoptotic cell death mechanism rather than necrotic cell death.

Carbohydrate-functionalized N-heterocyclic carbene Ru(ii) complexes: synthesis, characterization and catalytic transfer hydrogenation activity

Triazolylidene NHCs decorated with a carbohydrate wingtip group were complexed to a ruthenium(ii) center. Deprotection of the carbohydrate in the metal complex affords a carbohydrate–NHC hybrid system for use as a transfer hydrogenation catalyst.

A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry

A new chelating mesoionic carbene ligand, derived from 1,2,3-triazoles, with two redox-active tert-butyl-phenolate linkers has been synthesized and explored towards its reactivity and electrochemical properties in early transition metal chemistry.

(Di)triazolylidene manganese complexes in catalytic oxidation of alcohols to ketones and aldehydes

A robust bimetallic manganese complex with a mesoionic 1,2,3,-triazolylidene ligand for efficient oxidation of alcohols with tert-butylhydroperoxide.

Tandem transfer hydrogenation–epoxidation of ketone substrates catalysed by alkene-tethered Ru(ii)–NHC complexes

Eight novel alkene-tethered Ru(ii)–NHC complexes were employed as catalysts in tandem transfer hydrogenation–epoxidation reactions using phenacyl bromide derivatives as substrates.

Mechanistic insights and computational design of half-sandwich iridium and rhodium complexes for hydrogenation of quinoline

DFT calculations reveal a stepwise proton and hydride transfer mechanism for the hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline catalysed by half-sandwich Cp*Ir(NHC) complexes.

4-Halo-1,2,3-triazolylidenes: stable carbenes featuring halogen bonding

Synthesis and coordination of 4-halo-1,2,3-triazolylidenes have been developed. These novel ligands featured the character with σ-donation at carbon and a σ-hole at the halogen. Halogen bonding was observed by single crystal X-ray diffraction in their coinage metal complexes. The electronic properties of 4-iodo-1,2,3-triazolylidene were studied by both Ir-CO frequencies of the Tolman electronic parameter (TEP) and Huynh's electronic parameter (HEP) method, which suggested similar electronic properties to those of imidazolylidenes. During HEP tests, an interesting tunability was observed when different electron donors were employed.

Access to divalent lanthanide NHC complexes by redox-transmetallation from silver and CO2 insertion reactions

Through a redox-transmetallation procedure, divalent NHC-LnII (NHC = N-heterocyclic carbene; Ln = Eu, Yb) complexes were obtained from the corresponding NHC-AgI. The lability of the NHC-LnII bond was investigated and treatment with CO2 led to insertion reactions without oxidation of the metal centre. The EuII complex [EuI2(IMes)(THF)3] (IMes = 1,3-dimesitylimidazol-2-ylidene) exhibits photoluminescence with a quantum yield reaching 53%.

1 H-1,2,3-Triazol-5-ylidenes: Readily Available Mesoionic Carbenes

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, 1 H-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-1 H-1,2,3-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.

Highly Reactive Cyclic(alkyl)(amino) Carbene- and N-Heterocyclic Carbene-Bismuth(III) Complexes: Synthesis, Structure, and Computations

Cyclic(alkyl)(amino) carbene (CAAC)-stabilized complexes of phosphorus, one of the lightest group 15 elements, are well-established and can often be obtained in high yields. In contrast, analogous CAAC compounds of bismuth, the heaviest nonradioactive member of group 15, are unknown. Indeed, reactivity increases as you descend the group, and as a result there are only a few examples of N-heterocyclic carbene (NHC)-bismuth complexes. Moreover, activated bismuth compounds often readily extrude bismuth metal, making isolation of stable complexes highly challenging. We report that CAACs react with phenylbismuth dichloride (PhBiCl₂) to afford ^Et2CAAC-Bi(Ph)Cl₂ and ^CyCAAC-Bi(Ph)Cl₂. Significantly, these complexes represent the first structurally characterized examples of CAAC-coordination to bismuth. The CAAC-stabilized bismuth compounds can also be obtained from air-stable salts, [^Et2CAAC-H]₂²⁺ [Cl₂(Ph)Bi(μ-Cl₂)Bi(Ph)Cl₂]^2- and [^CyCAAC-H]₂²⁺ [Cl₂(Ph)Bi(μ-Cl₂)Bi(Ph)Cl₂]^2-, by deprotonation with potassium bis(trimethylsilyl)amide, K[N(SiMe₃)₂]. The electronic effects of the ligand on the bismuth center were investigated by comparing the CAAC-Bi(Ph)Cl₂ complexes to the NHC analogues, SIPr-Bi(Ph)Cl₂(THF) and IPr-Bi(Ph)Cl₂ (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Interestingly, the "normal" IPr-Bi(Ph)Cl₂ slowly isomerizes to the "abnormal" carbene complex, Cl₂(Ph)Bi-IPr-H, at -37 °C. In the solid-state, the CAAC-, NHC-, and abnormal NHC-bismuth compounds exhibit Bi atomic centers in unique coordination environments. The complexes were fully characterized by NMR, elemental analysis, and single crystal X-ray diffraction studies. In addition, the bonding was probed by natural bond orbital (NBO) calculations.