Design of nickel chelates of tetradentate N-heterocyclic carbenes with subdued cytotoxicity

One-Pot Tandem Hiyama Alkynylation/Cyclizations by Palladium(II) Acyclic Diaminocarbene (ADC) Complexes Yielding Biologically Relevant Benzofuran Scaffolds

A series of palladium acyclic diaminocarbene (ADC) complexes of the type cis-[(R¹NH)(R²)methylidene]PdCl₂(CNR¹) [R¹ = 2,4,6-(CH₃)₃C₆H₂: R² = NC₅H₁₀ (2); NC₄H₈ (3); NC₄H₈O (4)] were used not only to perform the C_sp² -C_sp Hiyama coupling between aryl iodide and triethoxysilylalkynes but also to subsequently carry out the one-pot tandem Hiyama alkynylation/cyclization reaction between 2-iodophenol and triethoxysilylalkynes, giving a convenient time-efficient access to the biologically relevant benzofuran compounds. The palladium ADC complexes (2-4) were conveniently synthesized by the nucleophilic addition of secondary amines, namely, piperidine, pyrrolidine, and morpholine on the cis-{(2,4,6-(CH₃)₃C₆H₂)NC}₂PdCl₂ in moderate yields (ca. 61-66%).

Cyanosilylation of Aromatic Aldehydes by Cationic Ruthenium(II) Complexes of Benzimidazole-Derived O-Functionalized N-Heterocyclic Carbenes at Ambient Temperature under Solvent-Free Conditions

A series of ruthenium complexes, namely, [{1-(N-R₁-2-acetamido)-3-(R₂)-benzimidazol-2-ylidine}Ru(p-cymene)Cl]Cl, where {R₁ = 2,6-(i-Pr)₂C₆H₃, R₂ = i-Pr (1c); R₁ = 2,6-(i-Pr)₂C₆H₃, R₂ = Et (2c); R₁ = 2,4,6-(CH₃)₃C₆H₂, R₂ = Et (3c)}, of benzimidazole-derived N/O-functionalized N-heterocyclic carbene ligands successfully carried out the cyanosilylation reaction of aromatic aldehydes and heteroaryl aldehydes with trimethylsilyl cyanide, providing good to excellent yields (ca. 60-95%) at room temperature under solvent-free condition. The ruthenium (1-3)c complexes were synthesized from the silver (1-3)b analogues in ca. 67-80% yields. The silver (1-3)b complexes exhibited an argentophilic d ¹⁰···d ¹⁰ interaction in its dinuclear macrometallacyclic motif, as observed by a short Ag···Ag contact of 3.1894(3) Å in single-crystal X-ray diffraction studies for a representative silver complex 2b and also in photoluminescence studies that showed characteristic emission band(s) at ca. 534-536 nm in the CHCl₃ solution and at ca. 482-487 and 530-533 nm in the solid state.

Computational Insight Into the Hydroamination of an Activated Olefin, As Catalyzed by a 1,2,4-Triazole-Derived Nickel(II) N-Heterocyclic Carbene Complex

A density functional theory (DFT) investigation performed at the B3LYP/TZVP//B3LYP/6-31G(d)-LANL2DZ level of theory on the hydroamination of dimethylamine (Me₂NH) on an activated olefin (namely, acrylonitrile (CH₂═CHCN)), as catalyzed by a 1,2,4-triazol based nickel(II) N-heterocyclic carbene complex (namely, [1,4-dimethyl-1,2,4-triazole-5-ylidene]₂ nickel dichloride) revealed that the olefin coordination pathway is favorable over the amine coordination pathway, although the initial olefin coordination step is higher in energy than the initial amine coordination step. Significantly enough, the reaction involved a crucial 1,3-proton transfer step between the resonance intermediates, i.e., the C-bound [(NHC)₂Ni(CH(CN)CH₂NHMe₂)]⁺ (D) species or N-bound [(NHC)₂Ni(NCCHCH₂NHMe₂)]⁺ (E) species and the intermediate [(NHC)₂Ni(NCCH₂CH₂NMe₂)]⁺ (F), depicting the cleavage of a N-H bond and the formation of a C-H bond facilitated by a water-assisted/amine-assisted proton shuttle. Overall, among the various pathways explored, the lowest energy pathway involved alkene coordination, followed by an amine-assisted 1,3-proton transfer step.

N-heterocyclic carbene metal complexes as bio-organometallic antimicrobial and anticancer drugs

Late transition metal complexes that bear N-heterocyclic carbene (NHC) ligands have seen a speedy growth in their use as both, metal-based drug candidates and potentially active homogeneous catalysts in a plethora of C-C and C-N bond forming reactions. This review article focuses on the recent developments and advances in preparation and characterization of NHC-metal complexes (metal: silver, gold, copper, palladium, nickel and ruthenium) and their biomedical applications. Their design, syntheses and characterization have been reviewed and correlated to their antimicrobial and anticancer efficacies. All these initial discoveries help validate the great potential of NHC-metal derivatives as a class of effective antimicrobial and anticancer agents.

Luminescent ruthenium(II) complex bearing bipyridine and N-heterocyclic carbene-based C∧N∧C pincer ligand for live-cell imaging of endocytosis

Luminescent ruthenium(II)-cyanide complex with N-heterocyclic carbene pincer ligand C(∧)N(∧)C = 2,6-bis(1-butylimidazol-2-ylidene)pyridine and 2,2'-bipyridine (bpy) shows minimal cytotoxicity to both human breast carcinoma cell (MCF-7) and human retinal pigmented epithelium cell (RPE) in a wide range of concentration (0.1-500 μM), and can be used for the luminescent imaging of endocytosis of the complex in these cells.

Recent developments of metal N-heterocyclic carbenes as anticancer agents

Metal based anticancer drugs have demonstrated their crucial role in preventing all types of cancers whereas their effectiveness is selective with respect to the cancer cells rather than the normal cells. Recently metal N-heterocyclic carbenes have established their selective performance for cancer cells excluding normal healthy cells based on which they are widely utilised for targeting cancer cells specifically which leads to cell death or cell growth inhibition. This is mainly due to their ionic character which helps them to localise in cancer cells with the help of enhanced expression of Organic Cation Transporters (OCT). Also their unique mechanism of action involving DNA binding, less recognizable by DNA repair machinery, mitochondria targeting gives them a new area for anticancer drug development. This review summarises the medicinal as well as pharmacological approach to the anticancer properties of metal NHC complexes.

Study on the interaction between a water-soluble dinuclear nickel complex and bovine serum albumin by spectroscopic techniques

The interaction of a water-soluble dinuclear nickel(II) complex, [Ni2(EGTB)(CH3CN)4](ClO4)4·4H2O (EGTB = ethylene glycol-bis(β-aminoethyl ether) N,N,N',N'-tetrakis(2-benzimidazoyl)) (1), and bovine serum albumin (BSA) was investigated under physiological conditions using fluorescence, synchronous fluorescence, UV-vis absorption and circular dichroism (CD). The experimental results suggested that the nickel(II) complex could bind to BSA with binding constant (K) ~ 10(4) M(-1) and quench the intrinsic fluorescence of BSA through a static quenching mechanism. The thermodynamic parameters, ΔG°, ΔH°, and ΔS°, calculated at different temperatures, indicated that the binding reaction was spontaneous and electrostatic interactions played a major role in this association. Based on the number of binding sites, it was considered that one molecule of complex 1 could bind to a single site or two sites of the BSA molecule or the two binding modes coexisted. In view of the results of site marker competition experiments, the reactive sites of BSA to complex 1 mainly located in subdomain IIA (site I) and subdomain IIIA (site II) of BSA. Moreover, the binding distance, r, between donor (BSA) and acceptor (complex 1) was 5.13 nm according to Förster nonradiation energy transfer theory. Finally, as shown by the UV-vis absorption, synchronous fluorescence and CD, complex 1 could induce conformation and microenvironmental changes of BSA. The results obtained herein will be of biological significance in toxicology investigation and anticancer metallodrug design.

N-Heterocyclic carbene metal complexes in medicinal chemistry

Metal complexes with N-heterocyclic carbene (NHC) ligands are widely used in chemistry due to their catalytic properties and applied for olefin metathesis among other reactions. The enhanced application of this type of organometallics has over the last few years also triggered a steadily increasing number of studies in the fields of medicinal chemistry, which take advantage of the fascinating chemical properties of these complexes. In fact it has been demonstrated that metal NHC complexes can be used to develop highly efficient metal based drugs with possible applications in the treatment of cancer or infectious diseases. Complexes of silver and gold have been biologically evaluated most frequently but also platinum or other transition metals have demonstrated promising biological properties.