Au Complexes Research Articles

Selected Metal (Au, Ag, and Cu) Complexes of N-heterocyclic Ligands as Potential Anticancer Agents: A Review.

Nitrogen-based organic heterocyclic compounds are an important source of therapeutic agents. About 75% of drugs approved by the FDA and currently available in the market are N-heterocyclic organic compounds. The N-heterocyclic organic compounds like pyridine, indole, triazoles, triazine, imidazoles, benzimidazoles, quinazolines, pyrazoles, quinolines, pyrimidines, porphyrin, etc. have demonstrated significant biological activities. These heterocyclic organic compounds also coordinate with various metal ions and form coordination compounds. Most of them have shown improved biological activities. The research on the metal complexes of these compounds reported their significant biological activities. N-heterocyclic-based metal complexes showed outstanding anticancer activities against different cancer cell lines, including VEGFR-2, HT-29, MDA-MB-231, MCF-7 K562, A549, HepG2, HL60, A2780, WI-38, Colo-205, PC-3, and other cancer cell lines. Some of these compounds showed better anticancer activity than cisplatin. In this review, we summarized the anticancer properties of N-heterocyclic-based gold (Au), silver (Ag), and copper (Cu) complexes and explored the mechanisms of action and potential structure-activity relationships (SAR) of these complexes. Our goal is to assist researchers in designing highly potent N-heterocyclic-based Au, Ag, and Cu complexes for the potential treatment of various cancers.

Cyclometalated Au(III) complexes with alkynylphosphine oxide ligands: synthesis and photophysical properties.

A series of cyclometalated Au(III) complexes [Au(C^N^C)(C2-L-P(O)Ph2)] with C^N^C = 2,6-diphenylpyridine and alkynylphosphine oxide ligands (L = no linker, Au1; phenyl, Au2; biphenyl, Au3; naphthyl, Au4; anthracenyl, Au5) were synthesized and fully characterized by spectroscopic methods and single crystal XRD analysis. The complexes obtained exhibit triplet (Au1-Au3) and dual (Au4, Au5) emissions in solution, in the solid phase and in the PMMA film, whose characteristics depend on the linker's nature of the alkynylphosphine oxide ligand. The description of electronic transitions responsible for energy absorption and emission in Au(III) complexes was made on the basis of a detailed analysis of the results of DFT calculations and has shown to involve ILCT, LLCT and MLCT transitions of singlet and triplet nature. It was demonstrated that packing in the crystal affects the solid-state emission of Au(III) complexes, which differs from that in solution. Based on DFT calculations for the supramolecular dimer for Au1, it was shown that this phenomenon is the result of packing of the complex molecules in the crystal.

Nano onions based on an amphiphilic Au3(pyrazolate)3 complex.

Multilayer vesicles with an onion-like architecture can form by self-assembly of organic amphiphiles such as dendrimers, small-molecule surfactants, and block copolymers. Thus far, there are limited reports about multilayer vesicles based on coordination compounds. Herein, we show that nano onions are obtained by aggregation of an amphiphilic Au3(pyrazolate)3 complex in aqueous solution. The nanostructures were characterized by cryogenic and transition electron microscopy, dynamic light scattering, and energy-dispersive X-ray analysis. Control experiments with analogous Ag(I) and Cu(I) complexes revealed the importance of Au(I) for the formation of well-defined nano onions. A structurally related Au(I) complex without solubilizing polyethylene glycol side chains was analyzed by single-crystal X-ray diffraction. In the solid state, columns of offset, π-stacked Au3(pyrazolate)3 complexes are observed, but short intermolecular Au⋯Au contacts were not found. Molecular dynamics simulations provided further insights into the aggregation process in aqueous solution, supporting the formation of nano-onion structures through lateral interactions between stacked complexes.

Reversible Bimetallic Inhibition to Modulate Selectivity During Catalysis.

Bimetallic complexes have demonstrated a great ability to enhance the activity of monometallic systems for bond activation and catalysis. In this work, we explore the opposite approach: using a second metal to passivate the activity of another by reversible bimetallic inhibition. To do so we have synthesized a family of nine electrophilic gold complexes of formula Au(PR3)(NTf2) ([NTf2]- = [N(SO2CF3)2]-) that can act as inhibitors in the semihydrogenation of terminal and internal alkynes catalyzed by the iconic iridium Vaska complex IrCl(CO)(PPh3)2. This behavior parallels the well-known passivation effect of lead over palladium in the heterogeneous Lindlard catalyst. Most gold fragments, except for the most hindered, form metal-only Lewis pairs upon combination with iridium, which have been fully characterized and exhibit distinct dative Ir → Au bonds. When applied to alkyne hydrogenation, these bimetallic structures have a clear tendency toward olefin formation, while the monometallic catalyst unselectively leads to overreduction products. Our computational studies not only provide a feasible mechanism for the Ir-only system, but also evince the active role of gold in passivating iridium by reversibly forming heterobimetallic structures that lead to enhanced selectivity.

Stoichiometry effect on the structure, coordination and anticancer activity of gold(I/III) bisphosphine complexes.

Rationalizing the impact of oxidation states of Au-based complexes on function require synthetic strategies that allow for conserved molecular formula in Au(I) and their Au(III) counterparts. Oftentimes achieving Au(I) and Au(III) coordination complexes with the same ligand system is challenging due to the reactivity and stability of the starting Au(I) or Au(III) starting materials. Thus, attempts to study the impact of oxidation state on biological function has been elusive. We posit that Au complexes with the same ligand framework but different oxidation states will affect complex geometry and hence elicit differences in biological function or mechanism. In this work, we reacted 1,2-bis(diphenylphosphino)benzene with respective Au starting materials in different mole ratios to facilitate the synthesis of structurally distinct Au(I) or Au(III) complexes. Briefly, by reacting two stoichiometric equivalents of HAuCl4·3H2O or AuCl3(tht) with one equivalent of 1,2-bis(diphenylphosphino)benzene, we obtained dicationic bis-[1,2-bis-(diphenylphosphino)benzene]gold(III) chloride whereas an equimolar ratio of HAuCl4·3H2O and 1,2-bis(diphenylphosphino)benzene gave the monocationic bis-[1,2-bis-(diphenylphosphino)benzene]gold(I) complex in moderate yield. The complexes were characterized spectroscopically by HRMS, RP-HPLC-MS, NMR and the purity ascertained by elemental analysis. The 31P NMR showed characteristic singlet peak at ∼22 ppm for the Au(I) complexes and ∼57 ppm for the Au(III) complexes. The structure of the Au(III) complexes was further confirmed by X-ray crystallography as a 5-coordinate Au(III) complex. Although both Au(I) and Au(III) complexes showed promising anticancer activity in MDA-MB-231 (breast cancer) and BT-333 (glioblastoma) cancer cell lines and inhibited maximal mitochondria respiration in MDA-MB-231 cells, the Au(III) complexes further induce ROS accumulation and facilitate depolarization of the mitochondria membrane potential in MDA-MB-231 cells. Taken together, the synthetic approach provides a way to elucidate the effect of Au(I)/Au(III) oxidation states on structure, activity, and potential mechanism with respect to the same ligand.

Supramolecular Assembly, Solvent-Induced, and Mechanochromic Luminescence of Au(I) Quinoline-8-thiolates.

AuCl(PMe3) and AuCl(PEt3) were used to react with quinoline-8-thiolate (8-QNS) to give three Au(I) complexes, i.e., [8-QNS(AuPMe3)2]ClO4 (1), [(8-QNS)2Ag(AuPMe3)2]ClO4 (2), and [8-QNS(AuPEt3)2]ClO4 (3), which have been structurally determined by X-ray diffraction to show various intra- and intermolecular metal···metal contacts (i.e., metal = Au(I) or Ag(I)). All complexes displayed solid-state luminescence at 535-568 nm, which is most likely attributed to an intraligand transition of 8-QNS and/or a S → Au(I) charge-transfer transition, possibly modified by Au(I)···Au(I) interactions. Notably, complex 1·CH2Cl2 exhibited remarkable mechanochromic luminescence from 535 to 601 nm upon grinding. While immersed in various solvents for the ground powder samples of complex 1·CH2Cl2, the luminescence at 601 nm mostly reverted to the original luminescence. Powder X-ray diffraction measurements indicated that complex 1·CH2Cl2 underwent structural transformation from a crystalline to an amorphous phase upon grinding, and it can be restored to the original crystalline phase upon immersion in various solvents. Additionally, the reversible experiments of complex 1·CH2Cl2 for the grinding and immersion actions were performed for up to five cycles. Moreover, the time-dependent luminescence spectra for the crystalline samples of complex 1·CH2Cl2 were measured, and they showed a significant luminescence change from 535 to 590 nm within 2 days.

Gold(I) complexation in chloride hydrothermal fluids

A critical assessment and processing of experimental data published in the literature on the stability of hydroxide and chloride complexes of Au(I) was carried out. Based on the obtained Gibbs energies of AuOH(aq), AuCl(aq) and AuCl2-, the standard thermodynamic properties and parameters of the HKF (Helgeson-Kirkham-Flowers) model equation of state were determined for these species. The resulting set of parameters makes it possible to calculate the solubility of Au in chloride fluids up to 1000 °C, 5000 bar with the possibility of extrapolation to higher PT parameters. As a geological application of the obtained data, a model calculation of the deposition of native gold by cooling chloride-sulfide fluid was carried out with an assessment of changes in the composition of the fluid, the sequence of formation of solid phases and changes in the fineness of gold.

Liposomal Formulation of an Organogold Complex Enhancing Its Activity as Antimelanoma Agent-In Vitro and In Vivo Studies.

Background/Objectives: The therapeutic management of melanoma, the most aggressive form of skin cancer, remains challenging. In the search for more effective therapeutic options, metal-based complexes are being investigated for their anticancer properties. Cisplatin was the first clinically approved platinum-based drug and, based on its success, other metals (e.g., gold) are being used to design novel compounds. Methods: the antimelanoma potential of a new organometallic cyclometalated Au(III) complex [[Au(CNOxN)Cl2] (CNOxN = 2-(phenyl-(2-pyridinylmethylene)aminoxy acetic acid))] (ST004) was evaluated in vitro and in vivo. Furthermore, the gold-based complex was incorporated in liposomes to overcome solubility and stability problems, to promote accumulation at melanoma sites and to maximize the therapeutic effect while controlling its reactivity. The antiproliferative activity of ST004 formulations was assessed in murine (B16F10) and human (A375 and MNT-1) melanoma cell lines after 24 and 48 h incubation periods. The proof-of-concept of the antimelanoma properties of ST004 formulations was carried out in subcutaneous and metastatic murine melanoma models. Results: the developed liposomal formulations showed a low mean size (around 100 nm), high homogeneity (with a low polydispersity index) and high incorporation efficiency (51 ± 15%). ST004 formulations exhibited antiproliferative activity with EC50 values in the μmolar range being cell-line- and incubation-period-dependent. On the opposite side, the benchmark antimelanoma compound, dacarbazine (DTIC), presented an EC50 > 100 μM. Cell cycle analysis revealed an arrest in G0/G1 phase for Free-ST004 in all cell lines. In turn, LIP-ST004 led to a G0/G1 halt in B16F10, and to an arrest in S phase in A375 and MNT-1 cells. Preliminary mechanistic studies in human red blood cells suggest that gold-based inhibition of glycerol permeation acts through aquaglyceroporin 3 (AQP3). In a metastatic murine melanoma, a significant reduction in lung metastases in animals receiving LIP-ST004, compared to free gold complex and DTIC, was observed. Conclusion: This study highlights the antimelanoma potential of a new gold-based complex. Additional studies, namely in vivo biodistribution profile and therapeutic validation of this organogold complex in other melanoma models, are expected to be performed in further investigations.

Designing novel Au(III) complexes based on the structure of diazepam: Achieving a multiaction mechanism against glioma.

Metal-based drugs have been used in the clinical treatment of tumors for over 30 years. However, no metal-based drugs have been clinically approved to treat glioma. Although metal complexes have excellent cytotoxicity, their most critical problem is crossing the blood-brain barrier. Therefore, to enable metal complexes to cross blood-brain barrier and target glioma therapy, herein, we propose to rationally used the basic structure of diazepam (5-chlorobenzophenone) and thiosemicarbazide to synthesize gold (Au) complexes C1, C2 and C3 with antiglioma activity. The C3 complex with two methyl groups attached to the N3 of thiosemicarbazone exhibited excellent cytotoxicity to glioma cells through its multiaction mechanism against glioma, inducing apoptosis, autophagy death, and deoxyribonucleic acid damage. In addition, the synthesized C3 complex can effectively cross the blood-brain barrier and accumulate in glioma, considerably decreasing the untoward reaction in vivo. Our findings provide a novel strategy for designing metal-based complexes for the treatment of glioma.

Silver- and gold-catalyzed azide−alkyne cycloaddition by functionalized NHC-based polynuclear catalysts: Computational investigation and mechanistic insights

This study evaluated the catalytic efficiency of Au(I), Ag(I), and Cu(I) complexes in the azide‒alkyne cycloaddition (AAC) reaction through density functional theory (DFT) calculations. Cu(I) complexes exhibit superior catalytic performance, with lower energy barriers (8.8 kcal/mol) and a favorable Gibbs free energy of -0.9 kcal/mol in the key cycloaddition step, significantly outperforming Ag and Au complexes. Structural analysis revealed that shorter M−C bond lengths in the Cu complex contributed to increased stability. Additionally, the copper complex has a more negative Gibbs free energy for the formed metallacycle, indicating a thermodynamically favorable reaction pathway. Noncovalent interaction (NCI) and reduced density gradient (RDG) analyses of the Cu, Ag, and Au systems highlighted distinct interaction patterns influencing the reactivity. Furthermore, electron localization function (ELF) and localized orbital locator (LOL) analyses revealed bonding characteristics in those complexes. This study offers valuable insights into the mechanistic differences among Au(I), Ag(I), and Cu(I) complexes, paving the way for future research on enhancing the catalytic activity of copper, silver and gold complexes through ligand modification.

(Invited) Organic Transformations with Strongly Photoreducing Catalysts

The recent advances of synthetic methodologies benefit from molecular photoredoxcatalysis. Molecular photoredox catalysts offer particular advantages in steering reactions that involve key radical intermediates, as they enable large exoergicity for generating radical species under ambient conditions through heterobimolecular photoinduced electron transfer. To expand the catalytic potential, my group investigated photoinduced electron transfer of doublet molecules and electron-rich, low-valent metal complexes. We found that radical anions of organic molecules exhibited strong photoreducing power capable of generating synthetically important aryl radical intermediates from aryl halides which reacted with aryl boronate esters to produce C-C cross-coupled products. Our studies provided convincing evidence that the active catalyst species was the excited-state radical anion which could be generated through consecutive absorption of a photon, an electron, and another photon. In particular, femto- and nanosecond transient absorption spectroscopic investigations provided direct evidence for the generation and oxidative quenching of the excited-state radical anion. As an alternative strategy to utilize the strong photoreducing power of a molecular catalyst, we seek electron-rich, low-valent transition metal complexes as photoredoxcatalysts. Our exploration enabled us to identify heteroleptic, linear Au(I) complexes as potent photoreducing catalysts with an excited-state oxidation potential as cathodic as -2.23 V vs standard calomel electrode. These Au(I) complexes can catalyse heteroaryl C-C cross-coupling reactions of redox-resistant aryl chlorides. Our investigations demonstrate that the Au(I) complexes offer several benefits, including strong visible-light absorption, a long excited-state lifetime, and the capacity of a 91% yield in the production of free-radical intermediates.

The Adsorption Kinetics of CTA+, Br-, and Ag+ Determining Anisotropic Growth of Gold Nanorods

Gold nanorods (AuNRs) have received significant attention due to their tunable plasmonic properties across the visible and near-infrared spectrum, large surface area to volume ratio, and chemical inertness, making them valuable in biomedical engineering and photochemistry. The physical properties of anisotropic AuNRs strongly depend on their aspect ratio, defined as the ratio of length to width. Previous studies have primarily focused on synthetic methods to precisely control the aspect ratio of AuNRs. Among various synthetic routes, the seed-mediated growth method in aqueous solution, comprising nucleation and further growth steps, is considered the best approach to obtain highly uniform AuNRs with a high yield. Notably, the method utilizing Ag+ with cetyltrimethylammonium cation (CTA+) and Br- during the growth step from Au nanoparticle seeds to NRs is the most convenient for controlling the aspect ratio of AuNRs. In this method, Ag+ acts as a key additive for symmetry breaking, ultimately modulating the aspect ratio of AuNRs. Despite advancements in synthetic methods, the underlying mechanism governing the cooperative interaction between additives (CTA+, Br-, and Ag+) and the anisotropic growth of AuNRs remains incompletely understood. Although computational simulations shed light on the detailed chemistry regarding the adsorption of additives, further investigation into the adsorption kinetics and their impact on the reduction of Au complexes is necessary. Due to the lack of information on the detailed chemistry during AuNR growth, synthetic results are sometimes inadequately explained by existing mechanisms. Therefore, this study aims to investigate the effect of CTA+, Br-, and Ag+ on the anisotropic growth of AuNRs. Electrochemical analyses will explore how Ag+ affects the adsorption behavior of CTA+ and Br-. This presentation will introduce the influence of each additive on Au reduction and the formation of suppression layers on AuNR surfaces across varying adsorption and growth durations.

Biosourced Au(III) Complexes from D-Xylose: Synthesis and Biological Evaluation.

A series of xylose-based ligands was obtained using a convenient approach, in a few steps from D-xylose. The complexation properties of these ligands towards Au3+ cations have been studied through different methods (multinuclear NMR, mass spectrometry, elemental analysis). The biological properties (antibacterial and anti-tumoral) of all the isolated xyloside Au(III) complexes were investigated in vitro. The xyloside Au(III) complexes gave the highest activities against E. coli (vs P. aeruginosa, S. aureus and S. epidermidis). The study also revealed that the nature of the sugar may play an important role in determining the selectivity of the antibacterial effect. Preliminary anti-tumoral evaluations showed that one complex containing a polyamine chain, exhibited interesting anti-proliferative activities on breast tumor cell lines MDA-MB-231 and BT-20. The anti-migratory effect of this complex also showed an average 35 % reduction in cell migration on the same two cancer cell lines.

Xylose Incorporated in Unsymmetrical Gold(I) N-Heterocyclic Carbene Complexes: Synthesis, Characterization and Antibacterial Activity.

A series of Au(I) complexes containing unsymmetrical N-heterocyclic carbene (imidazolylidene and benzimidazolylidene) functionalized with a xyloside group and an alkyl moiety (methyl and mesityl) was prepared using efficient procedures from D-xylose. Their characterization was carried out in solution by multinuclear NMR, HR-MS spectrometry and cyclic voltammetry, as well as in the solid state by means of single crystal X-ray diffraction analysis for two of them. Evaluation of their ability to inhibit bacterial growth showed a preference for a Gram-positive strain, Staphylococcus aureus, over a Gram-negative strain, Pseudomonas aeruginosa.

Thiocoumarin-based Au(I) Complexes and Au(0) Systems over TiO2 as Hybrid Photocatalysts for Hydrogen Generation under UV-Vis Light.

This work focuses on the photocatalytic production of hydrogen from the photodehydrogenation of ethanol using several gold(I) complexes and gold(0) systems over titanium dioxide (P90 TiO2) as hybrid photocatalysts. The photocatalytic systems are composed of at least one coumarin-based ligand, which can enhance the photocatalytic activity by its photon-absorbing capacity due to its chromophore properties. The photocatalytic behavior for hydrogen generation of the studied samples is compared under UV-vis light setting the total gold-based co-catalyst loading at 1 wt% onto the TiO2 photocatalysts and when the gold content is maintained at 0.25 wt%. The incorporation of gold co-catalysts results in an enhancement of hydrogen production up to 2.7 times compared to a conventional Au/TiO2 reference sample. The results show an increase in the total hydrogen production under UV-vis light due to the combined presence of coumarin chromophore, gold-based co-catalysts, and gold plasmonic nanoparticles. A deep characterization of the samples from each group is performed by UV-vis spectroscopy, XPS, HRTEM, and HAADF-STEM, observing the presence of plasmonic gold nanoparticles for sample "AuL1NPs" and the reduction of the gold present in sample "AuL1a," which explains the highest observed hydrogen production rates of this study.

Ligand-Based Radical Reactivity of Metal Thiosemicarbazones Prompts the Identification of Platinum(II)-Based Cytoprotectants.

CuATSM, a copper(II) complex of a bis(thiosemicarbazone) of diacetyl, prevents oxidative cell death and acts as a neuroprotectant in vivo, prompting its evaluation to treat amyotrophic lateral sclerosis and other neurodegenerative conditions in the clinic. We recently demonstrated that CuATSM functions as a potent radical-trapping antioxidant (RTA), inhibiting lipid peroxidation and associated ferroptotic cell death by a noncanonical mechanism based on radical addition to the ligand backbone. Herein we report our investigations of the generality of this reactivity, which include studies of corresponding complexes of various other metals, including Co, Ru, Ni, Pd, Pt, and Au. Inhibited autoxidations of styrene and dioxane reveal that most of these complexes exhibit RTA activity, consistent with ligand-based reactivity, but the identity of the metal atom nevertheless plays a role. In particular, analyses of the electronic structures of the complexes of metals within the same group (i.e., the group 10 metals Ni, Pd and Pt) highlight how the metal atom can modulate the ligand-based reactivity by enabling spin delocalization to the other thiosemicarbazone moiety. The RTA activity determined in organic solution largely translates to phospholipid bilayers and mammalian cells, where most complexes inhibited lipid peroxidation and associated ferroptotic cell death. A preliminary structure-activity study revealed Pt complexes with potencies eclipsing those of archetype ferroptosis inhibitors ferrostatin-1 and liproxstatin-1, suggesting that Pt (and to a lesser extent Ni) bis(thiosemicarbazone)s may be better suited to optimization for therapeutic development than those based on Cu.

A bio scientific study of the new gold complex as an anti-breast cancer agent, as well as its preparation , Molar Conductivity and Spectral identification of some other metal complexes

Synthesis of Heterocyclic Organic Ligand involved the preparation of non-homogeneous ring ligand derived from ortho-aminobenzylamine. This was achieved by the azo compound prepared in the first step. The second step included the reaction of the azo compound with 4-(benzyloxy)benzaldehyde .ligand was characterized using various spectroscopic and diagnostic techniques, including mass spectrometry, proton nuclear magnetic resonance, Fourier-transform infrared spectroscopy , and ultraviolet-visible spectroscopy . In addition, precise analysis of C, H, N elements and melting point measurements were conducted. the molar ratio (1:2) (M:L) was found for the ligand (BPDIM ) to prepare metal complexes for cobalt, and copper (divalent) while maintaining a molar ratio of (1:1) for gold complexes (trivalent). While supporting the values of accurate analysis of elements (C.H.N) and the results of flame atomic absorption spectroscopy, the proposed molecular formulas for all studied complexes were investigated. In addition, infrared spectroscopy, Electronic spectra of metal complexes, molar conductance.The new complexes showed variations in conductance depending on the metal ion and oxidation state. This study aimed to conduct an in vitro cytotoxicity comparative study of BPDIM and its Au(III) complex on human breast cancer cells (MCF-7) and other normal cells. The Au(III) complex was found to be highly selective in targeting cancer cells without affecting normal healthy cells, compared to the ligand. Thus, this complex can be considered as a new drug for treating breast cancer cells (MCF-7), and an attempt in the future to study its effect on other types of cancer.

Synthesis, characterization and anticancer activity of new benzimidazolium salts with their Ag(I) and Au(I) bis-NHC complexes

The research shows how to the synthesis and characterization of new substituted benzimidazolium salts, identify their Ag(I), and Au(I) bis-NHC complexes, and assess their anticancer activity of these compounds. Benzimidazolium salts have been derived from various substitution such as aliphatic chains and phenylacetamide chloride to produce asymmetrically substituted salts. New Ag(I) bis N-heterocyclic carbene complexes were synthesized from the in situ deprotonation process of asymmetrically di substituted benzimidazolium salts with Ag2O. Afterwards, Ag(I) bis-NHC complexes were used as transfer reagents in the transmetallation process to create Au(I) bis-NHC. The prepared complexes were characterized using (1H-NMR, 13C-NMR, and FT-IR) spectro techniques. The anticancer activity of benzimidazolium salts and Ag(I) and Au(I) complexes showed good activity compared to cisplatin as standard substance.

Au(I) complexes installed on a self-assembled peptide efficiently catalyze intramolecular cyclization reactions.

Self-assembled peptides are used for diverse applications in the biomedical and technological fields. The morphology and function of the assembled systems are dictated by the peptide sequence and length. In this work, a supramolecular catalyst was obtained upon self-assembly of the diphenylalanine peptide conjugated to a triphenylphosphine Au(I) complex in acetonitrile. The assembled molecules were characterized by spectroscopic techniques and by scanning electron microscopy. The activity of the catalyst was tested on two substrates in cyclization reactions. The morphology and the dimensions of the assembled systems vary depending on the presence of a carboxyl versus an amide C-terminal end. The catalyst efficiently promotes intramolecular cyclization reactions. Results obtained encourage the use of self-assembled peptides for the obtainment of new and efficient catalysts.

Azobenzene-Attached (NHC)Gold(I) and (NHC)Copper(I) Complexes as Photoswitchable Catalysts.

Photoswitchable (pre)catalysts, N,N'-bis-azobenzene-based (NHC)gold(I) and N,N'-bis-azobenzene-derived (NHC)copper(I) complexes are reported. Trans to cis isomerization of the attached photoswitchable moieties in the Au(I) complex enables four-fold decrement in the rate of oxazoline formation reaction. Whereas the progress of the copper(I) catalyzed, azide-alkyne cycloaddition reaction gets reduced by at least threefold. Alternate exposure to UV and blue light could easily toggle the rate of reactions remotely. The catalytic activity of thermodynamically stable trans-trans isomers is found to be similar to the common N-aryl substituted NHC-Au/Cu(I) complexes. NHC-Au(I) and -Cu(I) compounds bearing (trans)azobenzene moieties were characterized by X-ray diffraction. Photoswitching, recyclability studies, and the metastable isomer's thermal half-life in both complexes were studied via UV-visible spectroscopy. Whereas the extent of photoswitching and concomitant formation of geometrical isomers were investigated by using 1H-NMR spectroscopic study. Calculated percentage buried volumes of the three geometrical isomers show the trend trans-trans<trans-cis<cis-cis.