This graphical abstract describes how a new type of trans-dichloro-Ir(III) complexes derived from non-symmetrical NacNac-type ligands were unexpectedly obtained as the main reaction product from the former ligands and the dimeric species [Ir(COD)Cl₂]. Finally, an evaluation was made of the catalytic activity of all complexes in the transfer hydrogenation reaction of ketones and imines.

A new type of trans-dichloro-Ir(III) complexes derived from non-symmetrical NacNac-type ligands was unexpectedly obtained as the main reaction product from the former ligands and the dimeric species [Ir(COD)Cl₂]. One equivalent of LH was reacted with an excess of [Ir(COD)Cl]₂ in dichloromethane or toluene as solvent and at room temperature. The general formula of the product is [IrCl₂(COD)L] and was isolated as a sole product instead of the expected Ir(I) compound [Ir(COD)L]; all the new Ir(III) were prepared in high yields as microcrystalline solids. They were all stable under laboratory atmosphere, lasting for weeks in solution and for months in solid state. The structure of each compound was examined by 1D and 2D nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS). Complex 1k was selected for an X-ray diffraction study. Finally, an evaluation was made of the catalytic activity of all complexes in the transfer hydrogenation reaction of ketones and imines.

N-(benzo[d]thiazol-2-ylcarbamothioyl)benzamide (H₂L₂) was synthesized as a novel thiosemicarbazide containing benzothiazole moiety, by the reaction of benzoyl isothiocyanate with 2-aminobenzothiazole in benzene. The (H₂L₂) thiosemicarbazide and its isolated Cr(III), Fe(III), and Co(II) complexes were fully analyzed by different spectroscopic techniques. The thermal fragmentation of [Cr (HL₂)Cl₂(H₂O)₂].10H₂O, [Co (HL₂)Cl(H₂O)₂].6H₂O, and [Fe(L₂)Cl(H₂O)₂] were analyzed. We have also calculated the thermal parameters with using two different methods. The ligand undergoes tranamidation in the presence of PdCl₂. Crystal structure of the product obtained by tranamidation is measured. Biological activities of those compounds were assessed as antioxidants, anticancer, and antimicrobial.

The design and research of luminescent and volatile organic compound (VOC) fluorescent sensing materials are of great significance and challenge. We report herein the ligand substitution reaction and VOC sensing of new heteroleptic [Cu(P^˄P)N₂]⁺ type copper(I) complexes. Firstly, three new complexes 1–3 were designed by utilizing a chelate diphosphine ligand 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) and synthesized by the substitution reaction of different N-containing ligands of 4-PBO (1), 3-PBO (2), and 4,4′-Bipy (3) (4-PBO = 2-(4′-pyridyl)-benzoxazole, 3-PBO = 2-(3′-pyridyl)-benzoxazole, 4,4′-Bipy = 4,4′-bipyridine), respectively. Three complexes were characterized by elemental analysis, spectroscopic analysis (IR, UV–Vis), single-crystal X-ray diffraction (SCXRD), and photoluminescence study. The SCXRD study revealed that complexes 1 and 2 both exhibit a molecular structure with tetrahedral copper(I) complex cation and hexafluorophosphate anion, while complex 3 differs in that its cation is a binuclear copper(I) structure bridged by 4,4′-bipyridine. Three complexes 1–3 present supramolecular ribbon, supramolecular dimer, and supramolecular framework structure, respectively. Some differences of their UV–Vis absorption spectra were explained by TD-DFT calculation and wavefunction analysis. It is found that 1 has an abnormal luminescence blue shift, and a luminescence mechanism through the high-energy T₂ excited state is proposed by TD-DFT calculation. Based on 3, a fluorescent test strip was developed, and its fast and selective fluorescent sensing of pyridine vapor through quenching mechanism was successfully realized. The fluorescent quenching mechanism of the material was also studied, and it was proposed that the quenching should be attributed to the photoinduced electron transfer (PET) mechanism.

Mn(I) complexes bearing NHC ligands with picolyl wingtips substituents are efficient catalysts for the visible light-induced hydrosilylation of ketones at ambient temperature.

Manganese catalysis has attracted significant attention in the last few years. In comparison with the extensively studied Mn complexes bearing phosphine-based pincer ligands, catalysis with Mn containing N-heterocyclic carbenes (NHC) is poorly developed. In this work, new manganese(I) complexes bearing an NHC ligand with picolyl wingtip substituents (an N,C,N ligand) have been synthesized and fully characterized. The structure of fac-[Mn(CO)₃(κ² N,C,N)Br] and fac-[Mn(CO)₃(κ³ N,C,N)](OTf) complexes with a bidentate and tridentate coordination of the N,C,N ligand, respectively, has been elucidated by single crystal X-ray diffraction studies. These Mn complexes were applied as catalysts for the visible light-induced hydrosilylation of ketones. The mild protocol operated efficiently at room temperature under visible light irradiation (blue LED), enabling the conversion of a wide variety of aromatic ketones to their respective alcohols in high yields. Evidence for radical intermediates and the participation of Mn-H species has been demonstrated by a series of trapping experiments.

The field of asymmetric catalysis is rapidly developing, and the electron-rich chiral NHC ligands play a key role as stereodirecting ancillary ligands in enantioselective transition-metal catalysis. Several well-defined novel palladium(II), iridium(I)/(III), and rhodium(I)/(III) complexes possessing easily accessible tailor-made chiral NHC ligands for asymmetric catalysis have been described.

The field of asymmetric catalysis is rapidly developing and the chiral ligands play a key role in enantioselective transition-metal catalysis. The electron-rich chiral N-heterocyclic carbenes (NHCs) have established themselves as a popular class of stereodirecting ancillary ligands to catalyze enantioselective organic transformations in more efficient ways. Several novel transition-metal complexes in combination with tailored ligand design have emerged during last few decades in asymmetric catalysis. The tailor-made NHCs can easily be accessed due to the modular synthesis of their parent azolium salt precursors. Their donor capability and the molecular shape can easily be tuned by changing substituent at N-atom or by changing the cyclic backbone framework. This review article aims to describe the recent advances in this rapidly evolving research area of enantioselective catalysis using well-defined transition-metal complexes possessing chiral NHC donor ligands.

Solution and solid-state properties of novel dinuclear and trinuclear Ag(I) and Au(I) complexes based on a di-(N-heterocyclic carbene) ligand are studied.

Abstract

The coordinating properties of the bis(1,4-dimethyl-1,2,4-triazol-5-ylidene) ligand (abbreviated as bitz) towards silver(I) and gold(I) centres have been explored. The bitz behaves as bridging ligand giving both dinuclear [M₂(bitz)₂](PF₆)₂ and trinuclear [M₃(bitz)₃](PF₆)₃ species, whose formation has been supported by X-ray diffraction analysis, NMR spectroscopy and mass spectrometry evidences. The dinuclear and trinuclear gold(I) complexes present very similar luminescence properties, emitting in the blue region, at ~450 nm, when excited at 350 nm. Selected experimental aspects dealing with the synthesis of the dinuclear and trinuclear species, as well as with the absorption properties of the gold(I) complexes, have been investigated also through relativistic DFT calculations.

An air stable CCC-NHC Re pincer complex was developed for the borrowing hydrogen reaction between ketones and primary alcohols.

To date, no CCC-NHC pincer complexes of Re have been reported in the literature. The first CCC-NHC pincer complex of Re is reported. It was fully characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, elemental analysis, and X-ray crystallographic methods to determine the molecular structure. It was synthesized via transmetallation from an isolated Zr precursor and was found to be air stable. The catalytic activity of the CCC-NHC Re(I) pincer complex was demonstrated for the borrowing hydrogen coupling reaction between benzylic ketones and primary alcohols to generate a new C–C bond in an environmentally friendly catalysis requiring no activating groups for the alcohol functionality. This borrowing hydrogen coupling reaction produced a stoichiometric amount of water as the only by-product and did not require the conversion of the primary alcohol to a leaving group. A broad range of substrates was examined, and isolated yields from 53% to 92% were obtained. A catalytic cycle for the CCC-NHC Re(I) pincer complex catalyzed borrowing hydrogen coupling reaction is proposed.

In the current study, we obtained three new catalysts containing two unsymmetrical NHC ligand moieties increasing stability of the resulted complexes, making them ideal candidates for use in reactions of demanding substrates. They have been successfully used in ring-closing metathesis leading to products with a tetrasubstituted double bond, as well as in a selective ethenolysis reaction.

The activity profile of the new indenylidene complexes containing two unsymmetrical N-heterocyclic carbene (NHC) ligands was investigated. They turned out to be effective in both straightforward olefin metathesis reactions, such as ring-closing metathesis (RCM) of diethyl diallylmalonate, as well as in more demanding processes, such as RCM of 2,2-di-(2-methylallyl)tosylate leading to a product bearing a tetrasubstituted double bond, and in selective ethenolysis of methyl and ethyl oleate.

Catalytic application of ruthenium(II)-CNC pincer complexes is explored for acceptorless alcohol dehydrogenation reactions under conventional and microwave heating. The effect of different ancillary ligands has been observed during catalysis matching with the trans effect of these ligands.

The application of cationic Ru(II)-CNC pincer complexes [Ru(CNC)(CO)(PPh₃)Cl]PF₆ (1), [Ru(CNC)(PPh₃)₂Cl]PF₆ (2), [Ru(CNC)(η²:η²-COD)Cl]PF₆ (4), and [Ru(CNC)(DMSO)₂Cl]PF₆ (5) in acceptorless dehydrogenation of alcohols is reported under conventional “oil-bath” and microwave heating conditions. The effect of different ancillary ligands (CO, PPh₃, COD, and DMSO) have been demonstrated during catalysis. Complexes with more π-acid ligands CO and COD perform better compared with those with PPh₃ and DMSO ligands. A plausible mechanism is proposed based on NMR and mass investigation during the catalysis. The catalytic performance matches with the order of trans effect among the four ancillary ligands studied in these complexes. Further, a significant reduction in reaction time, as well as reaction temperature, was observed for catalytic reactions under the microwave-assisted reaction compared with the oil-bath heating reaction.

The unique photophysical properties of photoactive NHC transition metal complexes have enabled their promising emergence as potential photosensitisers and photocatalysts in a wide range of catalytic processes. In this Minireview, we summarise key structural motifs and fundamental photophysical characteristics of photoactive NHC transition metal complexes and highlight representative bond-forming reactions.

Abstract

This Minireview presents and discusses recent developments in photoactive/photoluminescent N-heterocyclic carbene (NHC) transition metal complexes and their viability as catalysts in bond-forming photocatalysis. Specifically, we summarise key structural motifs adopted by photoactive NHC transition metal complexes and highlight some of their photophysical properties. Representative examples of their applications as catalysts in bond-forming photochemical reactions are also showcased presenting the state-of-the-art and future opportunities for this exciting class of NHC transition metal complexes.