Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Dibenzofuranyl Ligands: Impact of Different Binding Modes on the Decay Time of the Excited State

Two C^C* cyclometalated platinum(II) N‐heterocyclic carbene (NHC) complexes with the general formula [(C^C*)Pt(O^O)] (C^C*=1‐dibenzofuranyl‐3‐methylbenzimidazolylidene; O^O=dimesitoylmethane) have been synthesized and extensively characterized, including solid‐state structure determination, ¹⁹⁵Pt NMR spectroscopy, and 2D NMR (COSY, HSQC, HMBC, NOESY) spectroscopy to elucidate the impact of their structural differences. The two regioisomers differ in the way the dibenzofuranyl (DBF) moiety of the NHC ligand is bound to the metal center, which induces significant changes in their physicochemical properties, especially on the decay time of the excited state. Quantum yields of over 80 % and blue emission colors were measured.     

Sulfonated N‐heterocyclic carbine‐silver (I) complexes: Synthesis,characterisation and biological evaluation

The synthesis, characterisation and biological activity of water‐soluble Ag(I)‐NHC complexes, general formula Na[(NHC)AgCl] where NHC is a sulfonated and sterically hindered N‐heterocyclic carbene, is reported. The Ag‐NHC complexes (2a–e) were synthesised by reacting the corresponding sulfonated NHC ligands with Ag₂O in the presence of NaCl or NaBr in methanol/water (1:1) solution. Synthesised silver (I)‐N‐heterocyclic carbene complexes have been characterised by NMR, micro‐analysis and HRMS spectroscopic methods. The IC₅₀ values of these complexes were determined by a proliferation BrdU enzyme‐linked immunosorbent assay (ELISA) against HeLa (human cervix carcinoma), HT29 (human adenocarcinoma) and L929 (mouse fibroblast) cell lines. These complexes have been highlighted as promising and original platforms for building new types of metalodrug. All new water‐soluble Ag(I) complexes demonstrated remarkable cytotoxic activity against HeLa, HT29 and L929 cell lines.     

Acyclic Palladium(II)‐N‐heterocyclic Carbene Metallacrown Ether Complexes: Synthesis,Structure and Catalytic Activity

Novel acyclic Pd(II)‐N‐heterocyclic carbene (NHC) metallacrown ethers 5a , 5b have been synthesized. Reaction of the imidazolium salts bearing a long polyether chain with Ag₂O afforded Ag‐NHC complexes, which then reacted as carbene transfer agent with PdCl₂(MeCN)₂ to give the desired acyclic Pd(II)‐NHC metallacrown ether complexes 5a and 5b . The ¹H NMR and ¹³C NMR spectra show 5a and 5b exist as mixtures of cis and trans isomers in solution. The trans isomer of 5a was characterized by X‐ray diffraction, which clearly demonstrated two pseudo‐crown ether cavities in trans‐ 5a . Pd(II)‐NHC complexes 5a and 5b have been shown to be highly effective in the Suzuki‐Miyaura reactions of a variety of aryl bromides in neat water without the need of inert gas protection.     

Direct C—H Bond Activation of Benzoxazole and Benzothiazole with Aryl Bromides Catalyzed by Palladium(II)‐N‐heterocyclic Carbene Complexes

Herein, we report that a series of novel palladium(II)‐NHC complexes (NHC=N‐heterocyclic carbene) were synthesized. The structures of all novel complexes were characterized by ¹H NMR, ¹³C NMR, FT‐IR spectroscopy and elemental analysis techniques. These palladium(II)‐NHC complexes were tested as efficient catalysts in the direct C—H bond activation of benzoxazole and benzothiazole with aryl bromides in the presence of 1 mol% catalyst loading at 150 °C for 4 h. Under the given conditions, various aryl bromides were successfully applied as the arylating reagents to achieve the 2‐arylbenzoxazoles and 2‐arylbenzothiazoles in acceptable to high yields.     

Synthesis,structure and in vitro cytotoxicity of platinum(II) complexes containing eugenol and a quinolin‐8‐ol‐derived chelator

The synthesis of potassium (η²‐4‐allyl‐2‐methoxyphenol)trichloridoplatinate(II), K[PtCl₃(C₁₀H₁₂O₂)], ( 1 ), starting from Zeise's salt and Ocimum sanctum L. oil has been optimized. Starting from ( 1 ), three new platinum(II) complexes, namely (η²‐4‐allyl‐2‐methoxyphenol)chlorido(2‐methylquinolin‐8‐olato‐κ²N ,O )platinum(II), ( 2 ), (η²‐4‐allyl‐2‐methoxyphenol)chlorido(5‐nitroquinolin‐8‐olato‐κ²N ,O )platinum(II), ( 3 ), and (η²‐4‐allyl‐2‐methoxyphenol)chlorido(5,7‐dichloroquinolin‐8‐olato‐κ²N ,O )platinum(II), [Pt(C₉H₄Cl₂NO)Cl(C₁₀H₁₂O₂)], ( 4 ), containing eugenol and a quinolin‐8‐ol derivative (R‐OQ), have been synthesized and characterized by elemental analyses, MS, IR, ¹H NMR and NOESY spectra. For ( 1 ) and ( 4 ), single‐crystal X‐ray diffraction studies were also carried out. Complexes ( 2 )–( 4 ) show good inhibiting abilities on three human cancer cell lines, i.e. KB, Hep‐G2 and LU, with IC₅₀ values of 1.42–17.8 µM . Complex ( 3 ) gives an impressively high activity against KB, Hep‐G2, LU and MCF‐7, with IC₅₀ values of 1.42–4.91 µM , which are much lower than those of cisplatin and some other platinum(II) complexes.     

Synthesis and Analysis of the Structure,Diffusion and Cytotoxicity of Heterocyclic Platinum(IV) Complexes

We have developed six dihydroxidoplatinum(IV) compounds with cytotoxic potential. Each derived from active platinum(II) species, these complexes consist of a heterocyclic ligand (H_L) and ancillary ligand (A_L) in the form [Pt(H_L)(A_L)(OH)₂]²⁺, where H_L is a methyl‐functionalised variant of 1,10‐phenanthroline and A_L is the S,S or R,R isomer of 1,2‐diaminocyclohexane. NMR characterisation and X‐ray diffraction studies clearly confirmed the coordination geometry of the octahedral platinum(IV) complexes. The self‐stacking of these complexes was determined using pulsed gradient stimulated echo nuclear magnetic resonance. The self‐association behaviour of square planar platinum(II) complexes is largely dependent on concentration, whereas platinum(IV) complexes do not aggregate under the same conditions, possibly due to the presence of axial ligands. The cytotoxicity of the most active complex, exhibited in several cell lines, has been retained in the platinum(IV) form.     

Symmetric and dissymmetric N‐heterocyclic carbene rhodium(I) complexes: a comparative study of their catalytic activities in transfer hydrogenation reaction

Six new [RhBr(NHC)(cod)] (NHC = N‐heterocyclic carbene; cod = 1,5‐cyclooctadiene) type rhodium complexes ( 4–6 ) have been prepared by the reaction of [Rh(μ‐OMe)(cod)]₂ with a series of corresponding imidazoli(in)ium bromides ( 1–3 ) bearing mesityl (Mes) or 2,4,6‐trimethylbenzyl (CH₂Mes) substituents at N¹ and N³ positions. They have been fully characterized by ¹ H, ¹³ C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)₂] (NHC = imidazol‐2‐ylidene) ( 7b–9b ) were also synthesized to compare σ‐donor/π‐acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as catalysts. The symmetrically CH₂Mes‐substituted rhodium complex bearing a saturated NHC ligand ( 5a ) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

Highly Stable Water‐Soluble Platinum Nanoparticles Stabilized by Hydrophilic N‐Heterocyclic Carbenes

Controlling the synthesis of stable metal nanoparticles in water is a current challenge in nanochemistry. The strategy presented herein uses sulfonated N‐heterocyclic carbene (NHC) ligands to stabilize platinum nanoparticles (PtNPs) in water, under air, for an indefinite time period. The particles were prepared by thermal decomposition of a preformed molecular Pt complex containing the NHC ligand and were then purified by dialysis and characterized by TEM, high‐resolution TEM, and spectroscopic techniques. Solid‐state NMR studies showed coordination of the carbene ligands to the nanoparticle surface and allowed the determination of a ¹³C–¹⁹⁵Pt coupling constant for the first time in a nanosystem (940 Hz). Additionally, in one case a novel structure was formed in which platinum(II) NHC complexes form a second coordination sphere around the nanoparticle.     

Synthesis,characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

A series of monomeric palladacycle complexes bearing n‐butyl‐substituted N‐heterocyclic carbenes, namely [Pd(NHC)X(dmba)] (dmba: dimethylbenzylamine and [Pd(NHC)X(ppy)]; NHC: 1‐n‐butyl‐3‐substituted benzylimidazol‐2‐ylidene; ppy: 2‐phenylpyridine), were prepared either by transmetallation from the corresponding silver carbene complexes or by the reaction of the corresponding acetate‐bridged palladacycle dimer with N‐heterocyclic carbene ligands in high yields. The palladium(II) complexes were characterized using elemental analyses, APCI‐MS, ¹H NMR and ¹³C NMR spectroscopies. These complexes are efficient in the Suzuki–Miyaura coupling reaction between phenylboronic acid and aryl bromides.     

Synthesis of new iron–NHC complexes as catalysts for hydrosilylation reactions

A series of new piano‐stool iron(II) complexes comprising N‐heterocyclic carbene ligands [Fe(Cp)(CO)₂(NHC)]I (NHC = 1,3‐disubstituted imidazolidin‐2‐ylidene) have been synthesized and analyzed by ¹H NMR, ¹³C NMR, IR, elemental analysis and mass spectrometric techniques. These compounds were easily prepared from the reaction of disubstituted imidazolidin‐2‐ylidene with [FeI(Cp)(CO)₂] in toluene at room temperature. These complexes were tested in the catalytic hydrosilylation reaction of aldehydes and ketones with phenylsilane in solvent‐free conditions. After a basic hydrolysis step, the corresponding alcohols were obtained in good yields. Copyright © 2013 John Wiley & Sons, Ltd.     

Photophysical Properties and OLED Applications of Phosphorescent Platinum(II) Schiff Base Complexes

The syntheses, crystal structures, and detailed investigations of the photophysical properties of phosphorescent platinum(II) Schiff base complexes are presented. All of these complexes exhibit intense absorption bands with λ_max in the range 417–546 nm, which are assigned to states of metal‐to‐ligand charge‐transfer (¹MLCT) ¹[Pt(5d)→π*(Schiff base)] character mixed with ¹[lone pair(phenoxide)→π*(imine)] charge‐transfer character. The platinum(II) Schiff base complexes are thermally stable, with decomposition temperatures up to 495 °C, and show emission λ_max at 541–649 nm in acetonitrile, with emission quantum yields up to 0.27. Measurements of the emission decay times in the temperature range from 130 to 1.5 K give total zero‐field splitting parameters of the emitting triplet state of 14–28 cm^?1. High‐performance yellow to red organic light‐emitting devices (OLEDs) using these platinum(II) Schiff base complexes have been fabricated with the best efficiency up to 31 cd A^?1 and a device lifetime up to 77 000 h at 500 cd m^?2.     

Synthesis,Structural Characterisation and Stereochemical Investigation of Chiral Sulfur‐Functionalised N‐Heterocyclic Carbene Complexes of Palladium and Platinum

Palladium and platinum complexes containing a sulfur‐functionalised N‐heterocyclic carbene (S‐NHC) chelate ligand have been synthesised. The absolute conformations of these novel organometallic S‐NHC chelates were determined by X‐ray structural analyses and solution‐phase 2D ¹H–¹H ROESY NMR spectroscopy. The structural studies revealed that the phenyl substituents on the stereogenic carbon atoms invariably take up the axial positions on the Pd‐C‐S coordination plane to afford a skewed five‐membered ring structure. All of the chiral complexes are structurally rigid and stereochemically locked in a chiral ring conformation that is either (R_s,S,R)‐λ or (S_s,R,R)‐δ in both the solid state and solution.     

Effect of lipophilicity of wingtip groups on the anticancer potential of mono N‐heterocyclic carbene silver(I) complexes: Synthesis,crystal structures and in vitro anticancer study

A series of symmetrically n ‐alkyl‐substituted mono benzimidazolium salts with steady increase in n ‐alkyl chain length have been prepared by stepwise N ‐alkylation resulting in salts ( 1 – 8 ). The mono N‐heterocyclic carbene (NHC)–Ag(I) complexes ( 9 – 16 ) derived from the respective salts were readily accessible by in situ deprotonation using Ag₂O. All the salts and the complexes were characterized using Fourier transform infrared, ¹H NMR, ¹³C NMR and elemental analyses. Furthermore, the structures of salts 3 and 7 and complex 16 were elucidated using X‐ray crystallography, which established that this mono NHC–Ag(I) complex has a linear bis‐carbene arrangement (C₂–Ag). The proligands and the respective Ag(I) complexes were studied for their in vitro anticancer potential against human colon cancer cell line (HCT‐116) using 5‐fluorouracil as a standard. From the IC₅₀ values of all the tested compounds, it can be postulated that there is an influential relationship between the increase in chain length of the wingtip n ‐alkyl groups and the anticancer potential. The proligands 4 – 8 and their respective complexes 12 – 16 with long n ‐alkyl chain lengths (n  = 6–10) showed better IC₅₀ values (0.3–3.9 μM) than the standard drug with the complexes displaying markedly better antiproliferation activity against HCT‐116 cell line than the respective proligands and the standard drug (IC₅₀ = 10.2 μM).     

N‐heterocyclic carbene complexes of Rh(I) and electronic effects on catalysts for 1,2‐addition of phenylboronic acid to aldehydes

1,3‐Diarylsubstituted imidazolinium salts, (NHC‐H)Cl, 3, containing hydrogen or alkyl groups at the 4,5‐positions of the imidazolidine ring, served as precursors to rhodium(I) complexes [RhCl(NHC)COD], 4, which were converted into cis‐[RhCl(NHC)(CO)₂] complexes, 5. All compounds prepared were characterized by elemental analyses, ¹H NMR and ¹³C NMR. The relative σ‐donor/π‐acceptor strength of the NHC ligands was determined by means of IR spectroscopy of 5. The ability of NHCs in 4 to enchance activity was explored in the 1,2‐addition of phenylboronic acid to aldehydes. A good correlation was observed between catalytic activity and the electron‐donating power of the NHC ligands. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis,Crystal Structures and Anticancer Activity of the New Chiral Mono‐ and Dinuclear Palladium(II) Complexes derived from (S)‐(‐)‐(1‐phenylethylimino)benzylphenylketone

The enantiopure ketoimine of benzil – the ( S )‐(‐)‐(1‐phenylethylimino)benzyl phenyl ketone ( 1 ) obtained under microwave irradiation in solvent‐free conditions – reacts with Na₂[PdCl₄] to give the new chiral mono‐ and dinuclear Pd‐complexes 2 and 3 , which have been partly characterized by IR, ¹H and ¹³C NMR spectroscopies along with MS‐FAB⁺ spectrometry. The crystal and molecular structures of both complexes has been fully confirmed by single‐crystal X‐ray studies. On the other hand, investigations in vitro of 2 and 3 have displayed growth inhibition against different classes of cancer: leukemia (K‐562 CML), colon cancer (HCT‐15), cancer breast (MCF‐7), central nervous system (U‐251 Glio) and prostate cancer (PC‐3) cell lines.     

Synthesis and antimicrobial activity of bulky 3,5‐di‐tert‐butyl substituent‐containing silver–N‐heterocyclic carbene complexes

A series of novel benzimidazolium bromides containing bulky 3,5‐di‐tert ‐butyl group were synthesized in high yields as N‐heterocyclic carbene (NHC) ligands. These NHC ligands were metallated with Ag₂O under moderate conditions to give novel silver–NHC complexes. The structures of all compounds were characterized using ¹H NMR, ¹³CNMR, infrared and elemental analysis techniques, which supported the proposed structures. The silver–NHC complexes were screened for their in vitro antimicrobial activities against the standard bacterial strains Enterococcus faecalis , Staphylococcus aureus , Escherichia coli and Pseudomonas aeruginosa and the fungal strains Candida albicans and C. tropicalis . The results showed that most of the silver–NHC complexes inhibited the growth of all bacterial strains and fungal strains and were found to display effective antimicrobial activity against different microorganisms.     

The first water‐soluble copper(I) complexes bearing sulfonated imidazole‐ and benzimidazole‐derived N‐heterocyclic carbenes: Synthesis and anticancer studies

The first water‐soluble bis(NHC^SO3)CuCl complexes (NHC^SO3 = NaIm^Bn,PrSO3, Na₂(4‐Me)Im^PrSO3 and Na₂BzIm^PrSO3) derived from the sulfonated N‐heterocyclic carbene precursors HIm^Bn,PrSO3 (3‐(1‐benzyl‐1H‐imidazol‐3‐ium‐3‐yl)propane‐1‐sulfonate), Na(4‐Me)HIm^PrSO3 (sodium 3,3′‐(4‐methyl‐1H‐imidazole‐3‐ium‐1,3‐diyl)dipropane‐1‐sulfonate) and NaHBzIm^PrSO3 (sodium 3,3′‐(1H‐benzoimidazole‐3‐ium‐1,3‐diyl)dipropane‐1‐sulfonate) have been synthesized. These compounds have been characterized using infrared and NMR spectroscopy and electrospray ionization mass spectrometry. The in vitro anti‐tumour effects of the bis(NHC^SO3)CuCl complexes and the corresponding free ligands were evaluated for a panel of various human tumour cell lines, including examples of lung, colon, ovarian and cervical carcinoma as well as of melanoma. Their cytotoxic properties were also evaluated against non‐transformed human cells and on a cellular model of cisplatin resistance. NHC–copper complexes induced cell killing effects preferentially against tumour cells, with IC₅₀ values in the micromolar range. Additionally, they were found able to overcome acquired cisplatin resistance.     

Coinage metal complexes of N‐heterocyclic carbene bearing nitrile functionalization: Synthesis and photophysical properties

A new series of N‐heterocyclic carbene (NHC) ligand precursors ( 1 and 2 ) with their [Ag(I)(NHC)₂]PF₆ complexes ( 3 and 4 ) and [ClAu(I)(NHC)] complexes ( 5 and 6 ) are reported. Complexes 5 and 6 were synthesized via transmetalation reaction using either 3 or 4 and AuCl(SMe₂) as reactants, respectively. All the synthesized compounds were fully characterized using elemental analyses and Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopies. In the crystal structures of 3 , 5 and 6 , the Ag(I) and Au(I) ions are in a linear geometry. The entire structure of 3 is stabilized by significant π–π interactions, while the structures of 5 and 6 are stabilized with the presence of aurophilic interactions between the adjacent Au(I) ions as well as CH–π or π–π interactions. From photoluminescence studies, complexes 5 and 6 show dual‐emission characteristics. The higher‐energy fluorescence originates from ¹XLCT mixed with ¹MLCT, while the lower‐energy phosphorescence is ascribed to ³XLCT and ³MLCT with small contribution of ³ILCT, as evidenced by density functional theory (DFT) and time‐dependent DFT calculations of the modelled molecules.     

Photofunctional Platinum Complexes Featuring N‐heterocyclic Carbene‐Based Pincer Ligands

Photoactive platinum complexes of stoichiometry [Pt(^RCCC^R)L]^0/+ (R=Me, nBu and L=? CN, ? C≡CPh, ? N≡CCH₃, ? Py, ? CO) featuring pincer‐type bis N‐heterocyclic carbene (NHC) ligands (^RCCC^R) were synthesized. Organometallic syntheses of these complexes are facile and achievable through standard laboratory procedures. Control of intermolecular Pt???Pt interaction, π–π stacking, and emission tuning is achieved through suitable choice of the NHC‐wingtip substituent (R) and the auxiliary ligand (L). Exposure to specific volatile organic compounds (VOCs) or mechanical grinding triggers changes in emission colors, which render these complexes photofunctional. Solid‐state structures and photoluminescence results are described herein.     

Synthesis and Structures of Arene Ruthenium (II)–NHC Complexes: Efficient Catalytic α‐alkylation of ketones via Hydrogen Auto Transfer Reaction

A panel of six new arene Ru (II)‐NHC complexes 2a‐f , (NHC = 1,3‐diethyl‐(5,6‐dimethyl)benzimidazolin‐2‐ylidene 1a , 1,3‐dicyclohexylmethyl‐(5,6‐dimethyl)benzimidazolin‐2‐ylidene 1b and 1,3‐dibenzyl‐(5,6‐dimethyl)benzimidazolin‐2‐ylidene 1c ) were synthesized from the transmetallation reaction of Ag‐NHC with [(η⁶‐arene)RuCl₂]₂ and characterized. The ruthenium (II)‐NHC complexes 2a‐f were developed as effective catalysts for α‐alkylation of ketones and synthesis of bioactive quinoline using primary/amino alcohols as coupling partners respectively. The reactions were performed with 0.5 mol% catalyst load in 8 h under aerobic condition and the maximum yield was up to 96%. Besides, the different alkyl wingtips on NHC and arene moieties were studied to differentiate the catalytic robustness of the complexes in the transformations.