Syntheses, crystal structures, reactivity, and photochemistry of gold(iii) bromides bearing N-heterocyclic carbenes

Synthesis, Chemical Characterization, and Biological Evaluation of Hydrophilic Gold(I) and Silver(I) N-Heterocyclic Carbenes as Potential Anticancer Agents

A series of new gold(I) and silver(I) N-heterocyclic carbenes bearing a 1-thio-β-d-glucose tetraacetate moiety was synthesized and chemically characterized. The compounds' stability and solubility in physiological conditions were investigated employing a multitechnique approach. Interaction studies with biologically relevant proteins, such as superoxide dismutase (SOD) and human serum albumin (HSA), were conducted via UV-vis absorption spectroscopy and high-resolution ESI mass spectrometry. The biological activity of the compounds was evaluated in the A2780 and A2780R (cisplatin-resistant) ovarian cancer cell lines and the HSkMC (human skeletal muscle) healthy cell line. Inhibition studies of the selenoenzyme thioredoxin reductase (TrxR) were also carried out. The results highlighted that the gold complexes are more stable in aqueous environment and capable of interaction with SOD and HSA. Moreover, these carbenes strongly inhibited the TrxR activity. In contrast, the silver ones underwent structural alterations in the aqueous medium and showed greater antiproliferative activity.

Anticancer Activity of Imidazolyl Gold(I/III) Compounds in Non-Small Cell Lung Cancer Cell Lines

Lung cancer is a leading cause of cancer-related death worldwide that needs updated therapies to contrast both the serious side effects and the occurrence of drug resistance. A panel of non-small cell lung cancer (NSCLC) cells were herein employed as cancer models. Eight structurally related gold(I) and gold(III) complexes with NHC and halides or triphenylphosphane ligands were investigated as lung cancer cell growth inhibitors. As expected, gold compounds with PPh3 were found to be more cytotoxic than homoleptic [(NHC)2-Au(I)]X or heteroleptic NHC-Au(I)X or NHC-Au(III)X3 complexes. Mixed ligand gold(I) compounds exhibiting the linear NHC-AuPPh3 (compound 7) or the trigonal NHC-Au(Cl)PPh3 (compound 8) arrangements at the central metal were found to be the best lung cancer cytotoxic compounds. Analysis of the TrxR residual activity of the treated cells revealed that these compounds efficiently inhibit the most accredited molecular target for gold compounds, the TrxR, with compound 8 reaching more than 80% activity reduction in lung cells. Some of the current cancer lung therapy protocols consist of specific lung cancer cell cytotoxic agents combined with antifolate drugs; interestingly, the herein gold compounds are both TrxR and antifolate inhibitors. The human DHFR was inhibited with IC50 ranging between 10-21 µM, depending on substrate concentrations, proceeding by a likely allosteric mechanism only for compound 8.

Gold(i) and gold(iii) carbene complexes from the marine betaine norzooanemonin: inhibition of thioredoxin reductase, antiproliferative and antimicrobial activity

The natural marine betaine norzooanemonin (1,3-dimethylimidazolim-4-carboxylate) and its methyl and ethyl esters were used as ligand precursors to prepare a systematic series (12 members) of neutral monocarbene gold(i/iii) and cationic dicarbene gold(i/iii) complexes. The complexes were evaluated as inhibitors of bacterial thioredoxin reductase and for their antiproliferative and antimicrobial activities. While gold complexes with the parent norzooanemonin scaffold resulted in overall poor performance, the more lipophilic esters proved to be highly bioactive agents, related to their higher cellular uptake. The monocarbene gold(i/iii) complexes showed significant potency as inhibitors of bacterial thioredoxin reductase. In most assays, the efficacy of both gold(i) and gold(iii) analogues was found to be comparable. The cytotoxicity of dicarbene gold(i/iii) complexes against cancer cells was strong, in some cases exceeding that of the standard reference auranofin.

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Accessing Bioactive Hydrazones by the Hydrohydrazination of Terminal Alkynes Catalyzed by Gold(I) Acyclic Aminooxy Carbene Complexes and Their Gold(I) Arylthiolato and Gold(III) Tribromo Derivatives: A Combined Experimental and Computational Study

Hydrohydrazination of terminal alkynes with hydrazides yielding hydrazones 5-14 were successfully catalyzed by a series of gold(I) acyclic aminooxy carbene complexes of the type [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuCl, where R² = H, R¹ = Me (1b); R² = H, R¹ = Cy (2b); R² = t-Bu, R¹ = Me (3b); R² = t-Bu, R¹ = Cy (4b). The mass spectrometric evidence corroborated the existence of the catalytically active solvent-coordinated [(AAOC)Au(CH₃CN)]SbF₆ (1-4)A species and the acetylene-bound [(AAOC)Au(HC≡CPhMe)]SbF₆ (3B) species of the proposed catalysis cycle. The hydrohydrazination reaction was successfully employed in synthesizing several bioactive hydrazone compounds (15-18) with anticonvulsant properties using a representative precatalyst (2b). The DFT studies favored the 4-ethynyltoluene (HC≡CPhMe) coordination pathway over the p-toluenesulfonyl hydrazide (NH₂NHSO₂C₆H₄CH₃) coordination pathway, and that proceeded by a crucial intermolecular hydrazide-assisted proton transfer step. The gold(I) complexes (1-4)b were synthesized from the {[(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)]CH}⁺OTf^- (1-4)a by treatment with (Me₂S)AuCl in the presence of NaH as a base. The reactivity studies of (1-4)b yielded the gold(III) [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuBr₃ (1-4)c complexes upon reaction with molecular bromine and the gold(I) perfluorophenylthiolato derivatives, [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuSC₆F₅ (1-4)d, upon treatment with C₆F₅SH.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron(III) and iron(IV) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO₂ reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design.

A New Class of Task-Specific Imidazolium Salts and Ionic Liquids and Their Corresponding Transition-Metal Complexes for Immobilization on Electrochemically Active Surfaces

Adding to the versatile class of ionic liquids, we report the detailed structure and property analysis of a new class of asymmetrically substituted imidazolium salts, offering interesting thermal characteristics, such as liquid crystalline behavior, polymorphism or glass transitions. A scalable general synthetic procedure for N-polyaryl-N'-alkyl-functionalized imidazolium salts with para-substituted linker (L) moieties at the aryl chain, namely [LPhm ImH R]+ (L=Br, CN, SMe, CO2 Et, OH; m=2, 3; R=C12 , PEGn ; n=2, 3, 4), was developed. These imidazolium salts were studied by single-crystal X-ray diffraction (SC-XRD), NMR spectroscopy and thermochemical methods (DSC, TGA). Furthermore, these imidazolium salts were used as N-heterocyclic carbene (NHC) ligand precursors for mononuclear, first-row transition metal complexes (MnII , FeII , CoII , NiII , ZnII , CuI , AgI , AuI ) and for the dinuclear Ti-supported Fe-NHC complex [(OPy)2 Ti(OPh2 ImC12 )2 (FeI2 )] (OPy=pyridin-2-ylmethanolate). The complexes were studied concerning their structural and magnetic behavior via multi-nuclear NMR spectroscopy, SC-XRD analyses, variable temperature and field-dependent (VT-VF) SQUID magnetization methods, X-band EPR spectroscopy and, where appropriate, zero-field 57 Fe Mössbauer spectroscopy.

Synthesis and coordination chemistry of silver(I), gold(I) and gold(III) complexes with picoline-functionalized benzimidazolin-2-ylidene ligands

The reactions of N-alkyl-N′-picolyl-benzimidazolium bromides or N,N′-dipicolyl-benzimidazolium bromide with silver oxide yielded the silver dicarbene complexes of the type [Ag(NHC)2][AgBr2] 1–4 (NHC = picoline-functionalized benzimidazolin-2-ylidene). The silver complexes 1–4 have been used in carbene transfer reactions to yield the gold(I) complexes of the type [AuCl(NHC)] 5–8 in good yields. A halide exchange at the metal center of complexes 5–8 with lithium bromide yielded the gold bromide complexes 9–12. Finally, the oxidation of the gold(I) centers in complexes 9–12 with elemental bromine gave the gold(III) complexes of the type [AuBr3(NHC)] 13–16. Molecular structures of selected Au(I) and Au(III) complexes have been determined by X-ray diffraction studies.

N-Heterocyclic Carbene Gold(I) Complexes: Mechanism of the Ligand Scrambling Reaction and Their Oxidation to Gold(III) in Aqueous Solutions

N-Heterocyclic carbene (NHC) gold(I) complexes offer great prospects in medicinal chemistry as antiproliferative, anticancer, and antibacterial agents. However, further development requires a thorough understanding of their reaction behavior in aqueous media. Herein, we report the conversion of the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ((NHC)Au^IBr, 1) complex in acetonitrile/water mixtures to the bis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ([(NHC)₂Au^I]⁺, 7), which is subsequently oxidized to the dibromidobis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(III) ([(NHC)₂Au^IIIBr₂]⁺, 9). By combining experimental data from HPLC, NMR, and (LC-)/HR-MS with computational results from DFT calculations, we outline a detailed ligand scrambling reaction mechanism. The key step is the formation of the stacked ((NHC)Au^IBr)₂ dimer (2) that rearranges to the T-shaped intermediate Br(NHC)₂Au^I–Au^IBr (3). The dissociation of Br^– from 3 and recombination lead to (NHC)₂Au^I–Au^IBr₂ (5) followed by the separation into [(NHC)₂Au^I]⁺ (7) and [Au^IBr₂]⁻ (8). [Au^IBr₂]⁻ is not stable in an aqueous environment and degrades in an internal redox reaction to Au⁰ and Br₂. The latter in turn oxidizes 7 to the gold(III) species 9. The reported ligand rearrangement of the (NHC)Au^IBr complex differs from that found for related silver(I) analogous. A detailed understanding of this scrambling mechanism is of utmost importance for the interpretation of their biological activity and will help to further optimize them for biomedical and other applications.

By means of experimental data from HPLC and (LC-)MS in combination with DFT calculations, we present a detailed mechanism for the ligand scrambling reaction of (NHC)Au^IBr to the corresponding [(NHC)₂Au^I]⁺ complex and the oxidation to the [(NHC)₂Au^IIIBr₂]⁺ species in aqueous solutions.

Organometallic chemistry in aqua regia: metal and ligand based oxidations of (NHC)AuCl complexes

The synthesis and characterization of a series of N-heterocyclic carbene (NHC) complexes of Au(iii), (NHC)AuCl₃, is described. High yields are obtained when the corresponding Au(i) species (NHC)AuCl are oxidized with inexpensive aqua regia. The oxidation is in some cases accompanied by substitution and/or anti addition of Cl₂ across the backbone C[double bond, length as m-dash]C bond of unsaturated NHC ligands.

A Novel Ag-N-Heterocyclic Carbene Complex Bearing the Hydroxyethyl Ligand: Synthesis, Characterization, Crystal and Spectral Structures and Bioactivity Properties

In this study, a novel silver N-heterocyclic carbene (Ag-NHC) complex bearing hydroxyethyl substituent has been synthesized from the hydroxyethyl-substituted benzimidazolium salt and silver oxide by using in-situ deprotonation method. A structure of the Ag-NHC complex was characterized by using UV-Vis, FTIR, 1H-NMR and 13C-NMR spectroscopies and elemental analysis techniques. Also, the crystal structure of the novel complex was determined by single-crystal X-ray diffraction method. In this paper, compound 1 showed excellent inhibitory effects against some metabolic enzymes. This complex had Ki of 1.14 0.26 µM against human carbonic anhydrase I (hCA I), 1.88±0.20 µM against human carbonic anhydrase II (hCA I), and 10.75±2.47 µM against α-glycosidase, respectively. On the other hand, the Ki value was found as 25.32±3.76 µM against acetylcholinesterase (AChE) and 41.31±7.42 µM against butyrylcholinesterase (BChE), respectively. These results showed that the complex had drug potency against some diseases related to using metabolic enzymes.

Physicochemical Properties and Photochemical Reactions in Organic Crystals

Background:

Molecular organic photochemistry is concerned with the description of physical and chemical processes generated upon the absorption of photons by organic molecules. Recently, it has become an important part of many areas of science: chemistry, biology, biochemistry, medicine, biophysics, material science, analytical chemistry, among others. Many synthetic chemists are using photochemical reactions in crystals to generate different types of organic compounds since this methodology represents a green chemistry approach.

Objective & Method:

Chemical reactions in crystals are quite different from reactions in solution. The range of organic solid state reactions and the degree of control which could be achieved under these conditions are quite wider and subtle. Therefore, for a large number of molecular crystals, the photochemical outcome is not the expected product based on topochemical principles. To explain these experimental results, several physicochemical factors in crystal structure have been proposed such as defects, reaction cavity, dynamic preformation or photoinduced lattice instability and steric compression control. In addition, several crystal engineering strategies have been developed to bring molecules into adequate orientations with reactive groups in good proximity to synthesize complex molecules that in many cases are not available by conventional methods. Some strategies involve structural modifications like intramolecular substitution with different functional groups to modify intermolecular interactions. Other strategies involve chemical techniques such as mixed crystal formation, charge transfer complexes, ionic and organometallic interactions. Furthermore, some examples of the single crystal to single crystal transformations have also been developed showing an elegant method to achieve regio and stereoselectivity in a photochemical reaction.

Conclusion:

The several examples given in this review paper have shown the wide scope of photochemical reactions in organic molecular crystals. There are several advantages of carrying photochemical reaction in the solid state. Production of materials unobtainable by the traditional solution phase reactions, improved specificity, reduction of impurities, and enhancement in the yields by the reduction of side reactions. These advantages and the multidisciplinary nature of solid-state photochemistry make this discipline quite likely to develop a lot in the future.

Synthesis and study of Au(iii)-indolizine derivatives: turn-on luminescence by photo-induced controlled release

The photo- and structural properties of a series of Au(iii) indolizine complexes were determined. Controlled release of halogenated indolizine derivatives from the corresponding Au(iii) complexes was achieved by photoinduced C-X bond formation, which provided turn-on luminescence with an increase in emission intensity of up to 67 times.

Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(i) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC)₂]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%V_B) values of 40-50% with the less flexible six-membered analogues having %V_B values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %V_B values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent.

Stable AuIII complexes with four N-heterocyclic carbene groups can be prepared in high yield directly from KAuCl4

Au^III complexes bearing four N-heterocyclic carbene groups are surprisingly stable and can be synthesized directly from KAuCl₄ and azolium salts.

Effects of histidin-2-ylidene vs. imidazol-2-ylidene ligands on the anticancer and antivascular activity of complexes of ruthenium, iridium, platinum, and gold

Couples of N-heterocyclic carbene complexes of ruthenium, iridium, platinum, and gold, each differing only in the carbene ligand being either 1,3-dimethylimidazol-2-ylidene (IM) or 1,3-dimethyl-N-boc-O-methylhistidin-2-ylidene (HIS), were assessed for their antiproliferative effect on seven cancer cell lines, their interaction with DNA, their cell cycle interference, and their vascular disrupting properties. In MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays only the platinum complexes were cytotoxic at single-digit micromolar IC₅₀ concentrations with the (HIS)Pt complex being on average twice as active as the (IM)Pt complex. The former was highly efficacious against cisplatin-resistant HT-29 colon carcinoma cells where the latter had no effect. Both Pt complexes were accumulated by cancer cells and bound to double-helical DNA equally well. Only the (HIS)Pt complex modified the electrophoretic mobility of circular DNA in vitro due to the HIS ligand causing greater morphological changes to the DNA. Both platinum complexes induced accumulation of 518A2 melanoma cells in G2/M and S phase of the cell cycle. A disruption of blood vessels in the chorioallantoic membrane of fertilized chicken eggs was observed for both platinum complexes and the (IM)gold complex. The (HIS)platinum complex was as active as cisplatin in tumor xenografted mice while being tolerated better. We found that the HIS ligand may augment the cytotoxicity of certain antitumoral metal fragments in two ways: by acting as a transmembrane carrier increasing the cellular accumulation of the complex, and by initiating a pronounced distortion and unwinding of DNA. We identified a new (HIS)platinum complex which was highly cytotoxic against cancer cells including cisplatin-resistant ones.

Gold(I) and Gold(III) Complexes of Cyclic (Alkyl)(amino)carbenes

The chemistry of Au(I) complexes with two types of cyclic (alkyl)(amino)carbene (CAAC) ligands has been explored, using the sterically less demanding dimethyl derivative Me2CAAC and the 2-adamantyl ligand AdCAAC. The conversion of (AdCAAC)AuCl into (AdCAAC)AuOH by treatment with KOH is significantly accelerated by the addition of tBuOH. (AdCAAC)AuOH is a convenient starting material for the high-yield syntheses of (AdCAAC)AuX complexes by acid/base and C-H activation reactions (X = OAryl, CF3CO2, N(Tf)2, C2Ph, C6F5, C6HF4, C6H2F3, CH2C(O)C6H4OMe, CH(Ph)C(O)Ph, CH2SO2Ph), while the cationic complexes [(AdCAAC)AuL]+ (L = CO, CN t Bu) and (AdCAAC)AuCN were obtained by chloride substitution from (AdCAAC)AuCl. The reactivity toward variously substituted fluoroarenes suggests that (AdCAAC)AuOH is able to react with C-H bonds with pKa values lower than about 31.5. This, together with the spectroscopic data, confirm the somewhat stronger electron-donor properties of CAAC ligands in comparison to imidazolylidene-type N-heterocyclic carbenes (NHCs). In spite of this, the oxidation of Me2CAAC and AdCAAC gold compounds is much less facile. Oxidations proceed with C-Au cleavage by halogens unless light is strictly excluded. The oxidation of (AdCAAC)AuCl with PhICl2 in the dark gives near-quantitative yields of (AdCAAC)AuCl3, while [Au(Me2CAAC)2]Cl leads to trans-[AuCl2(Me2CAAC)2]Cl. In contrast to the chemistry of imidazolylidene-type gold NHC complexes, oxidation products containing Au-Br or Au-I bonds could not be obtained; whereas the reaction with CsBr3 cleaves the Au-C bond to give mixtures of [AdCAAC-Br]+[AuBr2]- and [(AdCAAC-Br)]+ [AuBr4]-, the oxidation of (AdCAAC)AuI with I2 leads to the adduct (AdCAAC)AuI·I2. Irrespective of the steric demands of the CAAC ligands, their gold complexes proved more resistant to oxidation and more prone to halogen cleavage of the Au-C bonds than gold(I) complexes of imidazole-based NHC ligands.

Non-covalent interactions in coinage metal complexes of 1,2,4-triazole-based N-heterocyclic carbenes

Seven coinage metal(I) complexes bearing two different triazole-based N-heterocyclic carbene (NHC) ligands, [1-tert-butyl-4-{2-[(N,N-dimethylamino)methyl]phenyl}-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide and 1-tert-butyl-4-(4-methylphenyl)-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide], were synthesized and fully characterized in solution by NMR spectroscopy as well as in the solid state by X-ray diffraction techniques. Furthermore, the XRD analysis showed that the bidentate coordination of the amino group substituted NHC ligand, previously observed for rhodium and palladium complexes, does not take place in the solid state structure of Au(I) complexes with various halide ligands. Nevertheless, the formation of sets of two head-to-tail oriented monomers aggregated via a weak metallophilic contact was revealed for both NHC ligands as well as for all three coinage metals with different halides. These experimental data correlate quite well with the previously published theoretical study on related complexes.

Gold(iii) compounds containing a chelating, dicarbanionic ligand derived from 4,4′-di-tert-butylbiphenyl

Various gold(iii)heterocycles were prepared and their structures and photophysical properties were examined.

N-Heterocyclic carbene rhodium(i) complexes containing an axis of chirality: dynamics and catalysis

Novel rhodium(i) complexes [RhCl(NBD)(NHC)] [NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me, Bz, Tr, ^tBu]: determination of the rotation barriers about the Rh-carbene and catalytic activity in the hydrosilylation of terminal alkynes.

Facile oxidation of NHC-Au(i) to NHC-Au(iii) complexes by CsBr3

CsBr₃was investigated as a new and convenient oxidant for NHC-Au(i) complexes for the preparation of the respective Au(iii) complexes.

Synthesis, characterization and luminescence studies of gold(I)-NHC amide complexes

A flexible, efficient and straightforward methodology for the synthesis of N-heterocyclic carbene gold(I)–amide complexes is reported. Reaction of the versatile building block [Au(OH)(IPr)] (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with a series of commercially available (hetero)aromatic amines leads to the synthesis of several [Au(NRR’)(IPr)] complexes in good yields and with water as the sole byproduct. Interestingly, these complexes present luminescence properties. UV–vis and fluorescence measurements have allowed the identification of their excitation and emission wavelengths (λ_max). These studies revealed that by selecting the appropriate amine ligand the emission can be easily tuned to achieve a variety of colors, from violet to green.