Dinuclear silver and gold bisNHC complexes as drug candidates for cancer therapy.

We report four dinuclear silver(I) and gold(I) complexes containing two different bidentate N-heterocyclic carbene ligands (bisNHC). One of these complexes 4, shows strong and selective anticancer activity against the human ovarian cancer cell line A2780. Mechanistically, 4 enhances the oxidative stress by stimulating reactive oxygen species production and inhibiting the scavenging activity of thioredoxin reductase. Our findings provide evidence that tuning ligand and electronic properties of metal-NHC complexes can modulate their reactivity and selectivity and it may result in potential novel anticancer drugs.

Enhanced Metallophilicity in Metal-Carbene Systems: Stronger Character of Aurophilic Interactions in Solution.

Metallophilicity is an essential concept that builds upon the attraction between closed shell metal ions. We report on the [M2 (bisNHC)2 ]2+ (M=AuI , AgI ; NHC=N-heterocyclic carbene) systems, which display almost identical features in the solid state. However, in solution the Au2 cation exhibits a significantly higher degree of rigidity owed to the stronger character of the aurophilic interactions. Both Au2 and Ag2 cationic constructs are able to accommodate Ag+ ions via M-M interactions, despite their inherent Coulombic repulsion. When electrostatic repulsion between host and guest is partially diminished, M-M distances are substantially shortened. Quantum chemical calculations estimate intermetallic bond orders up to 0.2. Although at the limit of (or beyond) the van der Waals radii, metallophilic interactions are responsible for their behavior in solution.

Steroid-AuI -NHC Complexes: Synthesis and Antibacterial Activity.

A series of gold(I) pioneer complexes bearing N-heterocyclic carbenes and steroid derivatives (ethynylestradiol and ethisterone) with the generic formula [Au(R2 -imidazol-2-ylidene)(steroid)] (where R=CH3 or CH2 CH2 OCH3 ) were synthesized, and the X-ray structure of a rare of gold(I)-estradiol derivative is discussed. Toxicity studies reveal notable antibacterial activity of the gold-based compounds, which is significantly increased in vivo by the presence of the estradiol unit. Toxicity profiling was estimated in vitro versus Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, and in vivo on Galleria mellonella larvae against E. coli.

From Imidazole toward Imidazolium Salts and N-Heterocyclic Carbene Ligands: Electronic and Geometrical Redistribution.

Investigations dealing with N-heterocyclic carbenes and their derivatives are usually centered on the influence that they exert by acting as catalysts, ionic liquids, or metallodrugs and consequently on their capabilities to tune the properties and reactivity of these systems. In this context, we aimed to focus on the internal molecular changes undergone by imidazole derivatives, from electronic and geometrical points of view. This work represents an empirical evidence of the molecular modifications that an imidazole skeleton undergoes upon protonation, alkylation, and metalation.

Hirshfeld and DFT analysis of the N-heterocyclic carbene proligand methylenebis(N-butylimidazolium) as the acetonitrile-solvated diiodide salt.

N-Heterocyclic carbene (NHC) based systems are usually exploited in the exploration of catalytic mechanisms and processes in organocatalysis, and homo- and heterogeneous catalysis. However, their molecular structures have not received adequate attention. The NHC proligand methylenebis(N-butylimidazolium) has been synthesized as the acetonitrile solvate of the diiodide salt, C15H26N4(2+)·2I(-)·CH3CN [1,1'-methylenebis(3-butylimidazolium) diiodide acetonitrile monosolvate], and fully characterized. An interesting cation-anion connection pattern has been identified in the crystal lattice, in which three iodide anions interact simultaneously with the cisoid-oriented cation. A Hirshfeld surface analysis reveals the predominance of hydrogen bonding over anion-π interactions. This particular arrangement is observed in different methylene-bridged bis(imidazolium) cations bearing chloride or bromide counter-anions. Density functional theory (DFT) calculations with acetonitrile as solvent reproduce the geometry of the title cation.

Direct X-Ray Scattering Evidence for Metal-Metal Interactions in Solution at the Molecular Level.

The study of the aggregation of small molecules in solution induced by metallophilic interactions has been traditionally performed by spectroscopic methods through identification of chemical changes in the system. Herein we demonstrate the use of SAXS (small-angle X-ray scattering) to identify structures in solution, taking advantage of the excellent scattering intensity of heavy metals which have undergone association by metallophilic interactions. An analysis of the close relationship between solid-state and solution arrangements of a dynamic [Ag2 (bisNHC)2 ](2+) (NHC=N-heterocyclic carbene) system, and how they are complementary to each other, is reported.

Argentophilicity as essential driving force for a dynamic cation-cation host-guest system: [Ag(acetonitrile)2]⁺⊂[Ag2(bis-NHC)2]²âº (NHC = N-heterocyclic carbene).

A cationic [Ag2(bis-NHC)2](2+) system behaves as an excellent host for Ag(+). In the solid-state variation, Ag···Ag are the only bonding interactions between host and guest, overcoming their inherent electrostatic repulsion. It represents a clear example of ligand-unsupported ("pure") argentophilicity. In solution, we also found evidence for this kind of Ag···Ag approximation, which might be recognized as an initial step of transmetalation mechanisms involving formally closed-shell metal centers as transferring agents.