Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water.

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

Benzimidazol-2-ylidene ruthenium complexes for C-N bond formation through alcohol dehydrogenation.

A low temperature hydrogen borrowing approach to generate secondary amines using benzimidazole-based N-heterocyclic carbene (BNHC) ruthenium complexes is reported. A series of the piano-stool complexes of the type [(η6-p-cymene)(BNHC)RuCl2] (1a-g) were synthesized via one-pot reaction of the NHC salt precursor, Ag2O, and [RuCl2(p-cymene)]2 and characterized using conventional spectroscopic techniques. The geometry of two precursors, [(η6-p-cymene)(Me4BnMe2BNHCCH2OxMe)RuCl2] (1f) and [(η6-p-cymene)(Me5BnMe2BNHCCH2OxMe)RuCl2] (1g), was studied by single crystal X-ray diffraction. These catalysts were found to dehydrogenate alcohols efficiently at temperatures as low as 50 °C to allow Schiff-base condensation and subsequent imine hydrogenation to afford secondary amines. Notably, this ruthenium-based procedure enables the N-alkylation of aromatic and heteroaromatic primary amines with a wide range of primary alcohols in excellent yields of up to 98%. The present methodology is green and water is liberated as the sole byproduct.

Unveiling the catalytic nature of palladium-N-heterocyclic carbene catalysts in the α-alkylation of ketones with primary alcohols.

We report herein the synthesis of four new Pd-PEPPSI complexes with backbone-modified N-heterocyclic carbene (NHC) ligands and their application as catalysts in the α-alkylation of ketones with primary alcohols using a borrowing hydrogen process and tandem Suzuki-Miyaura coupling/α-alkylation reactions. Among the synthesized Pd-PEPPSI complexes, complex 2c having 4-methoxyphenyl groups at the 4,5-positions and 4-methoxybenzyl substituents on the N-atoms of imidazole exhibited the highest catalytic activity in the α-alkylation of ketones with primary alcohols (18 examples) with yields reaching up to 95%. Additionally, complex 2c was demonstrated to be an effective catalyst for the tandem Suzuki-Miyaura-coupling/α-alkylation of ketones to give biaryl ketones with high yields. The heterogeneous nature of the present catalytic system was verified by mercury poisoning and hot filtration experiments. Moreover, the formation of NHC-stabilized Pd(0) nanoparticles during the α-alkylation reactions was identified by advanced analytical techniques.

Synthesis of α-Alkylated Ketones via Selective Epoxide Opening/Alkylation Reactions with Primary Alcohols.

A new method for converting terminal epoxides and primary alcohols into α-alkylated ketones under borrowing hydrogen conditions is reported. The procedure involves a one-pot epoxide ring opening and alkylation via primary alcohols in the presence of an N-heterocyclic carbene iridium(I) catalyst, under aerobic conditions, with water as the side product.

Iridium-Catalyzed Alkylation of Secondary Alcohols with Primary Alcohols: A Route to Access Branched Ketones and Alcohols.

Under borrowing hydrogen conditions, NHC-iridium(I) catalyzed the direct or one-pot sequential synthesis of α,α-disubstituted ketones via the alkylation of secondary alcohols with primary alcohols is reported. Notably, the present approach provides a new method for the facile synthesis of α,α-disubstituted ketones and featured with several characteristics, including a broad substrate scope, using easy-to-handle alcohols as starting materials, and performing the reactions under aerobic conditions. Moreover, the selective one-pot formation of ß,ß-disubstituted alcohols was achieved by the addition of an external hydrogen source to the reaction mixture.

Iridium(I)-Catalyzed C-C and C-N Bond Formation Reactions via the Borrowing Hydrogen Strategy.

Iridium(I) complexes having an imidazol-2-ylidene ligand with benzylic wingtips efficiently catalyzed the ß-alkylation of secondary alcohols with primary alcohols and acceptorless dehydrogenative cyclization of 2-aminobenzyl alcohol with ketones through a borrowing hydrogen pathway. The ß-alkylated alcohols, including cholesterol derivatives, and substituted quinolines were obtained in good yields by using a minute amount of the catalyst with a catalytic amount of NaOH or KOH under the air atmosphere, liberating water (and H2 in the case of quinoline synthesis) as the sole byproduct. Notably, this system demonstrated turnover numbers of 940 000 (for ß-alkylation of secondary alcohols with primary alcohols by using down to 0.0001 mol % = 1 ppm of the catalyst) and 9200 (acceptorless dehydrogenative cyclization of 2-aminobenzyl alcohol with ketones).

Iridium(I)-Catalyzed Alkylation Reactions To Form α-Alkylated Ketones.

A highly effective and green procedure for the formation of α-alkylated ketones has been disclosed via the reaction of primary alcohols with secondary alcohols and ketones by using [IrCl(COD)(NHC)] complexes as a catalyst. Various α-alkylated ketones were obtained in high yields from the alkylation of alcohol with alcohol and ketone with alcohol through a borrowing hydrogen reaction by using 0.05-0.5 mol % iridium(I) and a catalytic amount of KOH (5-10 mol %) as the base under air atmosphere and within very short reaction times.

Identification of radical structures on 1-pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide and 1,3-bis(pentamethylbenzyl)-4,5-dimethylbenzimidazoliumsilver(I)bromide exposed to gamma rays: an EPR study.

1-Pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide and 1,3-bis(pentamethylbenzyl)-4,5dimethylbenzimidazoliumsilver(I)bromide and their Ag+ complexes were synthesized and their polycrystal forms were produced by recrystallization in dichloromethane/Et2 O solvent system. Structural determinations were carried out by 1 H NMR and 13 C NMR with a Varian 400 NMR system using tetramethylsilane as internal standard and CDCl3 as solvent. The disappearance of acidic N-heterocyclic carbene proton showed the formation of Ag(I) complexes. Also, elemental analyses were carried out. Electron paramagnetic resonance (EPR) measurements were performed to determine the formed radical structure on the samples irradiated at the room temperature for 72 h by using 60 Co-source with dose rate of 0.680 kGy. The EPR measurements were carried out in the temperature range of 200 K-450 K. Identical radicals were determined on the irradiated compounds. It was observed that the shapes of the spectra of the samples were independent of the temperature but, the resonance line intensities changed linearly with the temperature. Also, it was detected that the free radical on the 1-pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide is not stable compared to that on the 1,3-bis(pentamethylbenzyl)-4,5dimethylbenzimidazoliumsilver(I)bromide. Copyright © 2016 John Wiley & Sons, Ltd.

Synthesis and characterization of 2,2'-(pyridine-2,6-diyl)bis(1H-benzo[d]imidazol-3-ium) 2,4,6-trimethylbenzenesulfonate chloride by experimental and theoretical methods.

The title molecular salt (2), 2,2'-(pyridine-2,6-diyl)bis(1H-benzo[d]imidazol-3-ium) 2,4,6-trimethylbenzenesulfonate chloride (C19H15N5(2+)·C9H11O3S(-)·Cl(-)), was synthesized unexpectedly from the reaction of 2,6-bis(benzimidazol-2-yl)pyridine (1) with 2-mesitylenesulfonyl chloride. Spectroscopic techniques (FT-IR, NMR and UV-vis.) were used to characterize compounds 1 and 2. Solid-state structure of compound 2 was identified by X-ray crystallography. Theoretical characterization of the spectroscopic properties of compounds 1 and 2 was achieved using the density functional theory (DFT) method with the 6-311G(d,p) basis set, and the results were checked against the experimental data. Electronic absorption spectra of the compounds have also been obtained.

1-Pentamethylbenzyl-3-(n)buthylbenzimidazolesilver(I)bromide complex: synthesis, characterization and DFT calculations.

A novel NHC complex of silver(I) ion, 1-pentamethylbenzyl-3-(n)buthylbenzimidazolesilver(I)bromide, was prepared and fully characterized by single crystal X-ray structure determination. FT-IR, NMR and UV-vis spectroscopies were employed to investigate the electronic transition behaviors of the complex. Additionally, the molecular geometry, vibrational frequencies, gauge including atomic orbital (GIAO) (1)H and (13)C chemical shift and electronic transition values of silver(I) complex were calculated by using density functional theory levels (B3LYP and PBE1PBE) with LANL2DZ basis set. Also, the vibrational frequencies were supported on the basis of the potential energy distribution (PED) analysis calculated for PBE1PBE level. We were also investigated total static dipole moment (µ), the mean polarizability (〈α〉), the anisotropy of the polarizability (Δα), the mean first-order hyperpolarizability (〈ß〉) of the title complex. Natural bond orbital (NBO) analysis was performed to determine the presence of hyperconjugative interactions, and charge distributions.

N-benzyl substituted N-heterocyclic carbene complexes of iridium(I): assessment in transfer hydrogenation catalyst.

Iridium(I) complexes of N-heterocyclic carbenes (NHCs) (1a-3a) were obtained by transmetalation reactions from the corresponding Ag(I)-NHC complexes. These complexes have been fully characterized by (1)H, (13)C, heteronuclear multiple-bond correlation NMR spectroscopies, and elemental analysis. X-ray diffraction studies on single crystals of 1a and 2a confirm the square planar geometry at the metal center. [IrCl(CO)2(NHC)] complexes 1b-3b were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various ketones and imines have been studied using complexes 1a-3a as precatalysts. N-Benzyl substituted NHC complexes of Ir(I) proved to be highly efficient precatalysts in the reduction of aromatic and aliphatic ketones to afford the corresponding alcohol products with turnover frequencies values up to 24,000 h(-1).

Iridium(I) N-heterocyclic carbene complexes of benzimidazol-2-ylidene: effect of electron donating groups on the catalytic transfer hydrogenation reaction.

Two new [Ir(NHC)(COD)Cl] (NHC = N-heterocyclic carbene; COD = 1,5-cyclooctadiene) iridium complexes (2a,b) have been prepared by the reaction of [Ir(COD)Cl]2 with in situ prepared NHC-Ag carbene transfer agents in dichloromethane at ambient temperature. They have been fully characterized by (1)H, (13)C NMR, and elemental analysis. X-ray diffraction studies on single crystals of 2a and 2b confirm the square planar geometry. Complexes of type [Ir(NHC)(CO)2Cl] [NHC = 1,3-diisopropyl(5,6-dimethyl)benzimidazol-2-ylidene] 3 were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various aldehydes and ketones have been studied using complexes 2a and 2b as catalysts. The 5,6-dimethyl substituted iridium complex (2b) showed the highest catalytic activity for the TH reaction.

Synthesis, spectroscopic characterization, X-ray structure and DFT studies on 2,6-bis(1-benzyl-1H-benzo[d]imidazol-2-yl)pyridine.

The title molecule, 2,6-bis(1-benzyl-1H-benzo[d]imidazol-2-yl)pyridine (C(33)H(25)N(5)), was synthesized and characterized by elemental analysis, FT-IR spectroscopy, one- and two-dimensional NMR spectroscopies, and single-crystal X-ray diffraction. In addition, the molecular geometry, vibrational frequencies and gauge-independent atomic orbital (GIAO) (1)H and (13)C NMR chemical shift values of the title compound in the ground state have been calculated using the density functional theory at the B3LYP/6-311G(d,p) level, and compared with the experimental data. The complete assignments of all vibrational modes were performed by potential energy distributions using VEDA 4 program. The geometrical parameters of the optimized structure are in good agreement with the X-ray crystallographic data, and the theoretical vibrational frequencies and GIAO (1)H and (13)C NMR chemical shifts show good agreement with experimental values. Besides, molecular electrostatic potential (MEP) distribution, frontier molecular orbitals (FMO) and non-linear optical properties of the title compound were investigated by theoretical calculations at the B3LYP/6-311G(d,p) level. The linear polarizabilities and first hyper polarizabilities of the molecule indicate that the compound is a good candidate of nonlinear optical materials. The thermodynamic properties of the compound at different temperatures were calculated, revealing the correlations between standard heat capacity, standard entropy, standard enthalpy changes and temperatures.

Chitosan-ferrocene film as a platform for flow injection analysis applications of glucose oxidase and Gluconobacter oxydans biosensors.

Chitosan-ferrocene (CHIT-Fc) hybrid was synthesized through covalent modification and its electrochemical properties in immobilized form were studied by using cyclic voltammetry. The hybrid film exhibited reversible electrochemistry with a formal potential of +0.35 V (vs. Ag/AgCl) at pH 5.5. The Fc in CHIT matrix retained its electrocatalytic activity and did not diffuse from the matrix. This redox-active hybrid was further employed as a support for immobilization of glucose oxidase (GOx) and whole cells of Gluconobacter oxydans using glutaraldehyde on a glassy carbon electrode (GCE). The experimental conditions were optimized and the analytical characteristics of enzyme and microbial biosensors were evaluated for glucose in flow injection analysis (FIA) system. Under optimized conditions, both enzyme and microbial biosensors exhibited wide linear ranges for glucose from 2.0 to 16.0 mM and from 1.5 to 25.0 mM, respectively. Moreover, the biosensors have the advantages of relatively fast response times, good reproducibility and stability in FI mode. It was demonstrated that CHIT-Fc provides a biocompatible microenvironment for both bioctalysts and an electron transfer pathway. Additionally, integration of the enzyme and microbial biosensors into the FIA system has several advantages including capability of automation and high throughput at low cost. This promising redox hybrid can be utilized as an immobilization matrix for biomolecules in biosensor systems.

1-Ferrocenylmethyl-3-(2,4,6-trimethylbenzyl)-1H-imidazolidin-3-ium iodide and trans-bis(3-benzyl-1-ferrocenylmethyl-1H-imidazolidin-2-ylidene)diiodidopalladium(II).

Owing to increasing interest in the use of N-heterocyclic carbenes (NHCs) based on imidazolidinium ions as ligands in the design of highly efficient transition-metal-based homogeneous catalysts, the characterizations of the 1-ferrocenylmethyl-3-(2,4,6-trimethylbenzyl)imidazolidin-3-ium iodide salt, [Fe(C(5)H(5))(C(19)H(24)N(2))]I, (I), and the palladium complex trans-bis(3-benzyl-1-ferrocenylmethyl-1H-imidazolidin-2-ylidene)diiodidopalladium(II), [Fe(2)Pd(C(5)H(5))(2)(C(16)H(17)N(2))(2)I(2)], (II), are reported. Compound (I) has two iodide anions and two imidazolidinium cations within the asymmetric unit (Z' = 2). The two cations have distinctly different conformations, with the ferrocene groups orientated exo and endo with respect to the N-heterocyclic carbene. Weak C-H donor hydrogen bonds to both the iodide anions and the π system of the mesitylene group combine to form two-dimensional layers perpendicular to the crystallographic c direction. Only one of the formally charged imidazolidinium rings forms a near-linear hydrogen bond with an iodide anion. Complex (II) shows square-planar coordination around the Pd(II) metal, which is located on an inversion centre (Z' = 0.5). The ferrocene and benzyl substituents are in a trans-anti arrangement. The Pd-C bond distance between the N-heterocyclic carbene ligands and the metal atom is 2.036 (7) Å. A survey of related structures shows that the lengthening of the N-C bonds and the closure of the N-C-N angle seen here on metal complexation is typical of similar NHCs and their complexes.

Preparation of a series of Ru(II) complexes with N-heterocyclic carbene ligands for the catalytic transfer hydrogenation of aromatic ketones.

The reaction of [RuCl(2)(p-cymene](2) with Ag-N-heterocyclic carbene (NHC) complexes yields a series of [(p-cymene)Ru(NHC)] complexes (2a-f). All synthesised compounds were characterized by elemental analysis, NMR spectroscopy and the molecular structure of 2a was determined by X-ray crystallography. All complexes have been tested as catalysts for the transfer hydrogenation of aromatic ketones, showing excellent activity in this reaction.

Concomitant polymorphism of a pyridine-2,6-dicarboxamide derivative in a single space group: experimental and molecular modeling study.

The title compound, N(2),N(6)-bis{2-[(Z)-2-hydroxybenzylideneamino]phenyl}pyridine-2,6-dicarboxamide (3), has been synthesized by the reaction of 2-{(2-aminophenylimino)methyl}phenol (1) with pyridine-2,6-dicarbonyl dichloride (2), and characterized by elemental analysis, FT-IR and NMR spectroscopies and thermal analysis. Compounds 1 and 3 were evaluated for their antibacterial activities against Gram-positive and Gram-negative bacteria. The catalytic activity of 3 was also studied, and as a result, the in situ prepared three component system Ru(II)/3/KOH is shown to be an efficient catalyst for the transfer hydrogenation reaction of various ketones under mild conditions. Compound 3 has been crystallized in two polymorphic forms under the same conditions, and their crystal structures have been determined using single crystal X-ray diffraction technique. The molecular geometry, vibrational frequencies and gauge-independent atomic orbital (GIAO) (1)H and (13)C NMR chemical shift values of 3 in the ground state have been calculated using the density functional theory (DFT/B3LYP) method with the 6-31G(d) basis set, and compared with the experimental data. The results are in good agreement with experimental data. The effect of different solvents on the geometry, vibrational frequencies, total energies and dipole moments was also studied using the same method by applying the Onsager Model. There are subtle differences in the conformations and packing of the two polymorphs as a consequence of intermolecular hydrogen bonding interactions. Therefore, DFT calculations for the hydrogen bond interactions in the polymorphs were carried out using same basis set. The changes of thermodynamic properties from the monomers to 3 with the temperature ranging from 200 K to 400 K have been obtained using the statistical thermodynamic method.

Synthesis and antimicrobial activities of hexahydroimidazo[1,5-a]pyridinium bromides with varying benzyl substituents.

Variously substituted benzyl bromides were employed to quaternize hexahydrobenzylimidazo[1,5-a]pyridine (A) and the resulting bromides (1-11) were evaluated for their in vitro antimicrobial activity against 10 pathogenic microorganisms: Staphylococcus aureus, Staphylococcus epidermidis, Bacillus cereus, Micrococcus luteus, Proteus vulgaris, Escherichia coli, Salmonella typhimurium, Klebsiella pneumonia, Candida albicans and Candida krusei. Antimicrobial activities were surprisingly high (MIC: 0.78-400 µg/mL) and the sensitivity of the salts tested has been found to depend strongly both on the benzyl substituents and the microorganisms used. However, the correlation observed between antimicrobial activity and calculated partition coefficient (ClogP) was poor. Acute toxicity assessment of these salts showed LD(50) of 757-2000 mg/kg, after oral administration in mice in 24h.

Palladium-catalyzed heck coupling reaction of aryl bromides in aqueous media using tetrahydropyrimidinium salts as carbene ligands.

An efficient and stereoselective catalytic system for the Heck cross coupling reaction using novel 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (1, LHX) and Pd(OAc)2 loading has been reported. The palladium complexes derived from the salts 1a-f prepared in situ exhibit good catalytic activity in the Heck coupling reaction of aryl bromides under mild conditions.

A mediated polyphenol oxidase biosensor immobilized by electropolymerization of 1,2-diamino benzene.

A biosensor based on a partially purified polyphenol oxidase (PPO) enzyme was developed by using electropolymerization of [(2,2'-bipyridine)(chloro)(p-cymene)rutenium(II)]chloride] mediator complex and 1,2-diamino benzene (DAB) on a screen printing Pt electrode (1mm diameter). The electropolymerization was carried out at +0.7V for 45min in phosphate buffer (50mM, pH 7.0) which contained 14.0U/10mL polyphenole oxidase, 200mM DAB and 2.5mM Ru-mediator complex solutions. Measurement is based on the detection of the oxidation current of the Ru-mediator complex that related to the enzymatic reaction catalyzed by PPO at +0.65V. The phosphate buffer (50mM, pH 7.0 containing 0.1M KCl) and 30 degrees C were established as being the optimum working conditions. Under the optimum experimental conditions a linear calibration curve was obtained between 5 and 100microM catechol concentration. The detection limit of the biosensor is 2.385microM. In the characterization studies of the biosensor some parameters such as effect of Ru-mediator types on the biosensor response, substrate specificity, reproducibility and storage stability were studied. From the experiments, the average value (x), standard deviation (SD) and coefficient of variation (CV%) were found to be 48.75microM,+/-1.56microM, and 3.2% respectively for 50microM catechol standard.