A class of two-coordinate thiazolyl-copper-carbazolyl complexes with substituents having different steric bulk demonstrate the relation between rotation barriers and geometric/photophysical properties.

Two-coordinate carbene Cu(Ι) amide complexes with sterically bulky groups such as the diisopropyl phenyl (dipp) on the carbenes have been shown to have comparable performance to the phosphorescent emitters bearing heavy atoms such as iridium and platinum. These bulky groups enforce a coplanar molecular structure and suppress the nonradiative decay rates. Here, three different two-coordinate Cu(Ι) complexes were investigated that bear a common thiazole carbene, 3-(2,6-diisopropylphenyl)-4,5-dimethylthiazol-2-ylidene, with only a single dipp group, and carbazolyl ligands with substituents of varying steric bulk ortho to N. These substituents have a negligible impact on luminescence energies of the complexes but serve to modulate the rotation barriers along the metal–ligand coordinate bond. The geometric arrangement of ligands (syn- or anti-conformer) in complexes with alkyl substituents were found to differ, being syn in the solid state versus anti in solution as revealed by crystallographic analysis and nuclear magnetic resonance spectroscopy. In addition, calculations were performed to determine potential energy surfaces for different conformations of the three complexes to provide a theoretical evaluation of rotation barriers around the metal–ligand bond axis. The relationship between rotation barriers and photophysical properties demonstrate that rates for nonradiative decay decrease with increasing bulk of the substituents on the carbazolyl ligand.

Palladium hetero-dicarbene complexes have been prepared and tested for their catalytic activities in the direct C-H arylation of thiophenes with aryl halides. Complexes containing more distinct NHC units give rise to superior catalysts due to an amplified “stereoelectronic asymmetry.” A preliminary photophysical study of some selected thiophene reaction products has been conducted as well. The ability to individually change each NHC donor in such dicarbene ligands allows for a better fine-tuning of complex properties in search for superior catalysts.

A library of neutral and cationic palladium complexes of cis-chelating hetero-dicarbene ligands have been prepared. These ligands contain two different NHC donors allowing for a wider degree of variation, and the impact of the distinct NHC units has been compared using various spectroscopic means. In addition, the catalytic activities of these complexes in the direct C-H arylation of thiophenes with aryl halides were studied leading to the finding that the neutral dibromido complexes of the type [PdBr₂(diNHC)] generally outperformed their cationic [Pd (NCMe)₂(diNHC)](OTf)₂ counterparts. More importantly, complexes containing more distinct NHC units give rise to superior catalysts due to an amplified “stereoelectronic asymmetry” within the complex. A preliminary photophysical study of some selected thiophene reaction products has been conducted as well. The ability to individually change each NHC donor in such dicarbene ligands allows for a better fine-tuning of complex properties in search for superior catalysts.

A water-soluble Ir-NHC complex is demonstrated as an efficient, durable, reusable, and sustainable alternative catalyst for transfer-hydrogenation of quinolines and related N-heteroarenes with (buffered) formic acid.

Abstract

Transfer hydrogenation of N-heteroarenes was successfully achieved using a water-soluble half-sandwich Cp*Ir-based catalyst containing a uracil-based bifunctional abnormal NHC ligand, using HCOOH/HCOONa buffer solution as the hydrogen source. Reduction of N-heteroarenes was shown to be highly pH-dependent, and an acidic pH = 3.0 was found to be suitable for the best activity. The catalyst showed excellent functional group compatibility and high turnover number (up to 10,400), with catalyst loadings as low as 0.005 mol%. Finally, we demonstrated the catalyst's efficacy and applicability toward reusable and repetitive transfer hydrogenation using liquid/liquid extraction methodology, which underscored the significance of sustainable usage of noble metal catalysts in such transformations.

N-heterocyclic carbene nickel complexes based on xanthines are active antifungal agents for Candida sp. A bis-N-heterocyclic carbene nickel complex based on caffeine shows a high selectivity for Candida glabrata.

Invasive fungal diseases affect more than two million people worldwide. The increasing incidence of invasive fungal infections is the result of many factors, including an increase in the resistance to current drugs. As such, there is an urgent need to obtain new drugs that are efficient, selective, and able to overcome existing resistance mechanisms. Candida yeasts are responsible for more than 70% of all nosocomial invasive fungal diseases. In this work, we describe the synthesis of nickel (II)(NHC) complexes based on xanthines, by direct metalation of xanthinium salts with nickelocene NiCp₂to yield [NiCpI(NHC)] complexes. For methyl caffeine, a biscarbene complex [NiCp(NHC)₂]⁺is also formed, resulting from carbene dissociation from the corresponding monocarbene. [NiCpI(NHC)] complexes are active as antifungals for Candida yeasts and show toxicity for human cells (HeLa) that is dependent on the substitution of N7 of the xanthine moiety. The biscarbene complex 5[NiCp(NHC)₂]⁺is highly selective for Candida glabrata and shows very low toxicity for human cells, being a promising candidate for selective treatment of C. glabrata infections.

N-Heterocyclic carbene complexes of gold bearing a thiocarboxylate ligand, (NHC)Au(SCOR), are stable and easily prepared. At elevated temperatures, they form equilibria with their disproportionation products [Au(NHC)₂]⁺ and [Au(SCOR)₂]⁻. (NHC)Au(SCOR) are active against a cisplatin-resistant ovarian cancer call line (OVCAR-8), exhibiting IC₅₀ < 10 μM.

Novel complexes of form (NHC)Au(SCOR) (NHC = N-heterocyclic carbene, SCOR⁻ = thiocarboxylate ligand) were synthesised and characterised by spectroscopic techniques and X-ray diffraction studies. The results of NMR and X-ray studies indicated that thiocarboxylate ligands are comparable with NHCs in their electron donor ability. The complexes were stable at room temperature in the solid state but in solution underwent disproportionation reactions to form equilibria with [Au(NHC)₂]⁺ and [Au(SCOR)₂]⁻. In solution, the thiocarboxylate ligand in (NHC)Au(SCOR) underwent rapid exchange with other thiocarboxylate or thiolate ligands. The (NHC)Au(SCOR) complexes showed toxicity against cisplatin-resistant ovarian cancer cells (OVCAR-8), with IC₅₀ < 10 μM, in the range exhibited by cationic [Au(NHC)₂]⁺ complexes well-known for their promising anticancer activity.

Heterometallic Re^I/Au^I complexes containing imidazo[4,5-f]-1,10-phenanthroline (N^N^C) as polytopic ligand were synthesised and their antiproliferative and emissive potential analysed. The gold complex bearing a thiolate sugar derivative is the most active complex.

Five heterobimetallic Re^I/Au^I and a tri-metallic Re^I/Au^I/Re^I species following the formulas fac-[ReCl (CO)₃(N^N^CAuR)]^0/+ and [(fac-[ReCl (CO)₃(N^N^C)])₂Au]⁺, where R is an iodide (1), phenylacetylene (2), dodecanethiol (3), 2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranose (4) and JohnPhos (5) and N^N^C is the fused imidazo[4,5-f]-1,10-phenanthroline heterotopic ligand, were synthesised and fully characterised by a variety of spectroscopic and analytical techniques. The resultant complexes are luminescent in the orange region, revealing classical metal-to-ligand charge transfer (³MLCT) ((Re (dπ) → (N^N^C)(π*)) emission in aerated DMSO solution. The red shifted emission observed on going from 3 to 4 suggests that the electronic properties of the gold ancillary ligand are implicated in the emissive properties. Antiproliferative activity in tumour cell lines, lung (A549) and cervix (HeLa) cells revealed that only complex 4 containing a 2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranose as gold ancillary ligand possesses certain cytotoxicity in both cell lines.

Three NHC silver(I) and mercury(II) complexes 1–3 were prepared and characterized, and the selective recognition of 1 for H₂PO₄ ⁻ was investigated.

Abstract

The preparation of three metal (Ag(I) and Hg(II)) complexes based on bis-carbene ligands and correlative three precursors H ₂ L ⁿ ·(PF ₆ ) ₂ (n = 1–3) were reported. In each cationic moiety of complexes 1 and 2 and the monomer of complex 3, two Ag⁺ (or Hg²⁺) ions and two L ⁿ (n = 1–3) formed one 36-membered macrometallocycle. There existed π–π interactions between two naphthalenes in each cationic moiety of 1 and 2. The monomers of 3 were connected together through HgO bonds to form 1D chain. In the crystal packings of 1–3, two-dimensional and three-dimensional supramolecular structures were formed through CH˙˙˙π contacts and CH˙˙˙F H bonds. Additionally, the recognition capacity of 1 for H₂PO₄ ⁻ was studied via fluorescence spectra, UV/Vis spectra, HRMS, ¹H NMR titrations, and infrared (IR) spectra.

A straightforward procedure to gold-steroidyl complexes bearing saturated and unsaturated NHC ligands is reported. These derivatives display good cytotoxicity against a panel of cancer cell lines with IC₅₀ values in the low micromolar range. Cellular uptake of the most active complex into MCF-7 breast cancer cells was facilitated by the coordinated ethisterone ligand.

A straightforward synthetic route to new N-heterocyclic carbene (NHC)-gold-steroidyl complexes is reported. The desired complexes were obtained using a weak base (such as K₂CO₃) through a concerted-metallation-deprotonation (CMD) reaction mechanism occurring between [Au(NHC)Cl] and ethisterone as a model steroid-based alkyne. Most complexes displayed good cytotoxicity against a panel of cancer cell lines with IC₅₀ values in the low micromolar range. Cellular uptake of the most active complex 2a into MCF-7 breast cancer cells was facilitated by the coordinated ethisterone ligand.

A Ru catalyst 1 converts primary amines to nitriles using NaIO₄ in EtOAc/H₂O mixture, whereas imines are obtained under O₂ balloon pressure in toluene. These catalytic reactions illustrate a subtle dependency on the choice of oxidants and solvents in the oxidation of primary amines.

Abstract

The catalytic activity of a Ru complex 1, bearing a fused π-conjugated imidazo[1,2–a][1,8]naphthyridine-based mesoionic carbene (MIC) ligand, is examined for the oxidation of primary amines. Complex 1 affords nitrile or imine depending on the nature of the terminal oxidants and solvents used in the reactions. Primary amines are converted to nitriles using NaIO₄ in EtOAc/H₂O mixture, whereas imines are obtained under O₂ balloon pressure in toluene. A variety of nitriles and imines are accessed with high yields and selectivity. A set of control experiments, reaction profiles, and kinetic studies are undertaken to disclose the mechanistic details for nitrile and imine formation. The catalytic reactions illustrate a subtle dependency on the choice of oxidants and solvents in the oxidation of primary amines.

In this work, four novel 4,5-dihydro-1H-imidazolium salts were synthesized as saturated ring carbene precursors. After that, well-defined air- and moisture-stable four novel PEPPSI-type palladium complexes with 4,5-dihydro-imidazol-2-ylidene ligands were prepared. The palladium-complexes were tested in the direct arylation of azoles.

Recently, PEPPSI-type palladium-complexes bearing N-heterocyclic carbene (NHC) ligand have commonly been used as the effective catalysts in the direct arylation of heteroaromatic compounds. In most of previous studies catalyzed by such complexes, unsaturated ring carbene ligands such as benzo[d]imidazol-2-ylidene and imidazol-2-ylidene were used. However, the use of saturated ring carbene ligands such as 4,5-dihydro-imidazol-2-ylidene has been highly limited. Therefore, in this study, four novel 4,5-dihydro-1H-imidazolium salts were synthesized as saturated ring carbene precursors. Then, well-defined air- and moisture-stable four novel PEPPSI-type palladium-complexes with 4,5-dihydro-imidazol-2-ylidene ligands were prepared. All synthesized carbene precursors and palladium-complexes were structurally characterized by different spectroscopic and analytical techniques. Further structural characterization of two of the palladium-complexes was performed by single-crystal X-ray diffraction. Next, the palladium-complexes were tested in the direct arylation of azoles such as 4,5-dimethylthiazole and 1-methyl-1H-imidazole with (hetero)aryl halides in presence of 1 mol% catalyst loading at 120°C. The results showed that these novel palladium complexes are effective catalysts.