Metal complexes of backbone-halogenated imidazol-2-ylidenes

In this manuscript, literature reports on mono- and di-halogen (F, Cl, Br, and I) substituted at positions 4 or/and 4,5 imidazol-2-ylidene (NHC) metal complexes are discussed: particularly, their structural diversity with various metals (groups 6-13), important physicochemical properties, catalytic and medicinal/biological applications are reviewed. To our knowledge, there are no literature reports on group 4 and 5 metal complexes with this type of NHC ligands. Halogenated imidazol-2-ylidene metal complexes deserve special attention because halogens are the classic electron donating groups (mesomerically) in conjugated aromatic/heteroaromatic ring systems, but electron withdrawing inductively. However, they exhibit a significant electron withdrawing inductive effect, thus providing unique electronic properties. This is important for fine tuning of σ-donor abilities of the "carbenic" carbon of imidazol-2-ylidenes, which directly affect catalytic performance of their metal complexes. Other applications, advantages, and disadvantages of halogenated vs. unsubstituted imidazol-2-ylidene metal complexes are critically analyzed and summarized in this review.

Diastereoselective Synthesis of Sulfoxide-Functionalized N-Heterocyclic Carbene Ruthenium Complexes: An Experimental and Computational Study

The synthesis of sulfoxide-functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6-p-cymene)(NHC-SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X-ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2-C,S) coordination mode of the NHC-SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β-alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC-functionalised S-Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β-ol-selectivities.

Ligand Rearrangement Leads to Tetrahydrothiophene-Functionalized N,S-Heterocyclic Carbene Palladium(II) Complexes

Tetrahydrothiophene-functionalized N,S-heterocyclic carbene palladium(II) complexes are synthesized through an unexpected rearrangement that proceeds with palladium(II) trifluoroacetate and not with palladium(II) acetate, palladium(II) bromide, or palladium(II) chloride. A series of these complexes were isolated and characterized by X-ray crystallography. The mechanism of formation of these [3.2.1]-palladabicycles was explored, and the catalytic capabilities of these complexes were demonstrated in representative C-C coupling reactions.

Pyridine Wingtip in [Pd(Py-tzNHC)2]2+ Complex Is a Proton Shuttle in the Catalytic Hydroamination of Alkynes

The cationic palladium(II) complex 1 of pyridyl-mesoionic carbene ligand catalyzes Markovnikov-selective intermolecular hydroamination between anilines and terminal alkynes into the corresponding imines. The reaction proceeds at room temperature, in the absence of additives, with exquisite selectivity and diverse functional group tolerance. The key intrinsic feature of the catalyst is the pyridine wingtip confined to the proximity of the alkynophilic metal active site, which mimics the function of enzyme-like architectures by assisting entropically favored proton transfers.

Fused azole-thiazolines via one-pot cyclization of functionalized N-heterocyclic carbene precursors

Various bi/tricyclic azole-thiazolines are obtained via a new one-pot protocol from cheap starting materials.

Donor Strengths Determination of Pnictogen and Chalcogen Ligands by the Huynh Electronic Parameter and Its Correlation to Sigma Hammett Constants

The suitability and accuracy of the Huynh electronic parameter (HEP) was further tested to reveal remote substituent effects in pyridines, which are located five or six bonds away from the reporter probe. These values show an excellent correlation to Hammett σ‐constants of the respective substituents with coefficients of R²=0.9856 (σ_m) and R²=0.9857 (σ_p). Based on this observation, a methodology for the re‐evaluation of certain Hammett constants with larger uncertainties has been proposed and demonstrated. Moreover, the scope of HEP was extended to various neutral pnictogen and chalcogen donors during which “transphobia effects” were revealed for mixed NHC complexes containing phosphites, arsine and stibine for the first time.

Palladium(II) Complexes with N-Phosphanyl-N-heterocyclic Carbenes as Catalysts for Intermolecular Alkyne Hydroaminations

The catalytic potential of palladium(II) complexes with chelating N-phosphanyl-N-heterocyclic carbenes featuring a saturated imidazolin-2-ylidene or tetrahydropyrimid-2-ylidene ring has been investigated in intermolecular alkyne hydroamination reactions. The complexes were found to be among the most active Pd-based catalysts for these processes and to enable the use of low reaction temperatures (40 °C) and of solventless conditions. The Pd complexes require activation by 2 equiv of a silver salt to remove chlorido ligands from the metal coordination sphere; they can however also be presynthesized in active form, which allows their use under silver-free conditions. The hydroamination reaction was found to efficiently proceed with terminal alkynes and different ring-substituted, primary arylamine substrates.

Trinuclear complexes of palladium(ii) with chalcogenated N-heterocyclic carbenes: catalysis of selective nitrile–primary amide interconversion and Sonogashira coupling

The interconversion of nitriles to amides and Sonogashira coupling were catalyzed with Pd(ii) complexes (0.5–2 mol%).

Development of (quinolinyl)amido-based pincer palladium complexes: a robust and phosphine-free catalyst system for C–H arylation of benzothiazoles

Well-defined (quinolinyl)amido-pincer palladium complexes are developed and employed for the catalytic C–H bond arylation of benzothiazoles with aryl iodides, which can be recycled and reused for several cycles.

Alkynyl-functionalized gold NHC complexes and their coinage metal clusters

Phenylpropynyl-functionalized N-heterocyclic carbenes as ligands for the synthesis of heterometallic hexanuclear coinagemetal clusters which exhibit mixed metallophillic interactions and intense white photoluminescence at low temperature.

Donor functionalized ruthenium N-heterocyclic carbene complexes in alcohol oxidation reactions

Ruthenium chelates bearing N^C^O-donors in bidentate or pincer coordination modes have been prepared. The ruthenium pincer complex catalyses the oxidation of alcohols to the corresponding aldehydes with yields as high as 99%.

Pincer versus pseudopincer: isomerism in palladium(ii) complexes bearing κ3C,S,C ligands

In NHC pincer complexes incorporating a hemilabile donor site, there exists an equilibrium between the true pincer form and a pseudopincer coordination isomer.

Sulfur-functionalized N-heterocyclic carbene complexes of Pd(II): syntheses, structures and catalytic activities

N-heterocyclic carbenes (NHCs) can be easily modified by introducing functional groups at the nitrogen atoms, which leads to versatile coordination chemistry as well as diverse catalytic applications of the resulting complexes. This article summarizes our contributions to the field of NHCs bearing different types of sulfur functions, i.e., thioether, sulfoxide, thiophene, and thiolato. The experimental evidence for the truly hemilabile coordination behavior of a Pd(II) thioether-NHC complex has been reported as well. In addition, complexes bearing rigid CSC-pincer ligands have been synthesized and the reasons for pincer versus pseudo-pincer formation investigated. Incorporation of the electron-rich thiolato function resulted in the isolation of structurally diverse complexes. The catalytic activities of selected complexes have been tested in Suzuki-Miyaura, Mizoroki-Heck and hydroamination reactions.

Synthesis, Solid-state Structures, Solution Behaviour and Catalysis Studies of Nickel Complexes of Bis(benzimidazolin-2-ylidene)pyridine Pincer Ligands

N-Heterocyclic carbene–nickel complexes with five- and four-coordinate geometries [(CNC)NiBr2] and [(CNC)NiBr]X (X = PF6 or BPh4) have been prepared with the pincer ligands 2,6-bis(N-octylbenzimidazolin-2-ylidene)pyridine and 2,6-bis(N-butyl-5,6-dimethoxybenzimidazolin-2-ylidene)pyridine. The addition of the n-octyl substituent significantly extends the solubility of the complexes and has allowed UV-vis solution studies of the complexes in dichloromethane and methanol. The four- and five-coordinate species exist in equilibrium in solution and this equilibrium has been explored by UV-vis studies. The complexes have also been characterized by NMR studies, and single crystal X-ray diffraction studies have been performed on [(CNC)NiBr2] (where CNC = 2,6-bis(N-octylbenzimidazolin-2-ylidene)pyridine) and [(CNC)NiBr]BPh4 (where CNC = 2,6-bis(N-butyl-5,6-dimethoxybenzimidazolin-2-ylidene)pyridine).

Nickel complexes of a bis(benzimidazolin-2-ylidene)pyridine pincer ligand with four- and five-coordinate geometries

The four- and five-coordinate complexes [(CNC)NiX(2)] (X = Cl, Br, I), [(CNC)NiX]PF(6) (X = Cl, Br) and [(CNC)NiCl]Cl·H(2)O have been isolated, where CNC is the bis(N-butylbenzimidazolin-2-ylidene)-2,6-pyridine pincer ligand. A five-coordinate geometry is rare for this class of complex. Where amenable, the complexes have been structurally characterised by single crystal X-ray diffraction studies and in solution by NMR, UV-vis and MS studies. The five-coordinate dibromo complex [(CNC)NiBr(2)] is readily prepared on the gram-scale from the benzimidazolium salt precursor and Ni(OAc)(2)·4H(2)O in DMSO without the exclusion of air. Halide exchange and salt metathesis reactions using [(CNC)NiBr(2)] afford the other four- and five-coordinate complexes. [(CNC)NiBr(2)] displays very low solubility, and upon dissolution affords solutions of the four-coordinate [(CNC)NiBr](+). Factors that influence the formation of four- or five-coordinate complexes with this ligand class are discussed.