Chiral bisimidazolium salts derived from amino acids and their palladium(II)- and platinum(II)-biscarbene complexes

We report new chiral bisimidazolium salts synthesized from naturally occurring l-amino acids. They served as precursors for bidentate N-heterocyclic carbene metal complexes. The chiral imidazoles could be synthesized in good yields via a one-pot ring closing reaction, followed by esterification. The methylene bridged bisimidazolium iodide salts are accessible in moderate yields. Corresponding palladium(II)- and platinum(II)-NHC complexes could be synthesized and fully characterized, but do not show optical activity. We also report a solid state structure of one of the synthesized palladium(II) biscarbene compounds derived from alanine.     

Synthesis and structural chemistry of arene-ruthenium half-sandwich complexes bearing an oxazolinyl-carbene ligand

Reaction of a series of directly connected oxazoline-imidazolium salts with silver(I) oxide and subsequent transmetallation with [Ru(p-cymene)Cl₂]₂ and anion exchange with KPF₆ cleanly gave the corresponding 2-oxazolinyl-(N-mesityl)imidazolidene(chloro)ruthenium(II) half-sandwich complexes [RuCl(oxcarb)(p-cymene)]PF₆, two derivatives of which were characterized by X-ray diffraction. Abstraction of the chloro ligand furnished the dicationic aqua complexes [Ru(H₂O)(oxcarb)(p-cymene)](PF₆)₂ which possess a similar coordination geometry. The syntheses were found to be highly diastereoselective, since only one diastereoisomer could be observed in all ruthenium complexes upon reaction of the chiral enantiopure oxazoline-imidazolium salts. Their potential as transfer hydrogenation and Lewis acid catalysts has been probed.     

Butylene linked palladium N-heterocyclic carbene complexes: Synthesis and catalytic properties

New bis(NHC)-Pd complexes were synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR, and IR spectroscopy. The reaction of Pd(OAc)₂ and bis(benzimidazolium) salts in DMSO gave the monomeric palladium complex in which the N-heterocyclic carbene was bound to the metal centre. The crystal and molecular structure of the cis-dibromo{1,1′-di[2,3,4,5,6-pentamethylbenzyl]-3,3′-butylenedibenzimidazol-2,2′-diylidene}-palladium(II) complex was determined by single-crystal X-ray diffraction. The activity of the Pd(II) complexes in the direct arylation of benzothiazole with arylbromides was investigated. A preliminary catalytic study showed that these bis(NHC)-Pd complexes were highly active in the direct arylation of benzothiazole with arylbromides.     

Synthesis of pyrazine-bridged diimidazolium salts and their application in palladium catalyzed Heck-type coupling reactions

Reaction of imidazole derivatives with 2,3-di(bromomethyl)pyrazine results in the formation of the new pyrazine-bridged diimidazolium salts 1-8. These salts proved to be valuable precursors for dinuclear complexes with mixed NHC/pyrazine ligands. Two of the pyrazine-bridged diimidazolium salts 3·H₂O and 8·2H₂O have been characterized by X-ray diffraction. Furthermore, the first catalytic studies with mixtures of palladium acetate and the imidazolium salts have been carried out. The in situ prepared palladium complexes derived from the diimidazolium salts 1-8 exhibit a modest catalytic activity in Heck-type coupling reactions between 4-bromo benzaldehyde and styrene or n-butyl acrylate.     

Carbonylation of aryl- and vinyl-tellurium compounds with carbon monoxide in the presence of palladium(II) salts

Aryl- and vinyl-tellurium(II or IV) compounds react with carbon monoxide (CO) in suitable organic solvents to give the corresponding carboylic acids in moderate to quantitative yields in the presence of a stoichiometric amount of a palladium(II) salt. Treatment of (Z)-styrylphenyl telluride with atmospheric pressure of CO at room temperature in the presence of palladium(II) chloride or lithium chloropalladate(II) affords predominantly (E)-cinnamic acid, while in the presence of palladium(II) acetate similar reaction gives the (Z)-acid highly selectively. Under higher CO pressures (5–50 atm), however, the (Z)-acid becomes the major product, even when palladium(II) chloride is used. The following pathways are proposed for this carbonylation: (1) in the first step organotellurium compounds form the monomeric and/or dimeric palladium complexes such as [(R₂Te)PdCl₂]₂ and/or (R₂Te)₂PdCl₂ (R = aryl, vinyl), then (2) the migration of R moiety from tellurium to palladium (transmetallation) occurs to afford the reactive aryl- or alkenyl-palladium compounds, and (3) the compounds react with CO to give the corresponding acylpalladium compounds, after alkaline hydrolysis, the carboxylic acids are formed. The presence of an ionic carbene-like organopalladium complex is proposed for the formation of the (Z)-acid from (Z)-telluride.     

Preparation and characterization of PEPPSI-palladium N-heterocyclic carbene complexes using benzimidazolium salts catalyzed Suzuki–Miyaura cross coupling reaction and their antitumor and antimicrobial activities

New palladium complexes were efficiently synthesized from the reaction of benzimidazolium salts 2a–e, potassium carbonate (K₂CO₃) and palladium chloride (PdCl₂) in pyridine (for 3a–e). The catalytic activity of these complexes in a catalytic system including palladium complexes and K₂CO₃ in DMF-H₂O was evaluated in Suzuki–Miyaura cross-coupling reactions of aryl bromides and chlorides with phenylboronic acid. Our novel complexes show excellent catalytic activities with high turnover numbers (TON) and high turnover frequencies (TOF) (e.g. for the Suzuki–Miyaura reaction: TON up to 370 and TOF up to 123.3?h^?1). Both benzimidazolium salts 2a–e and complexes 3 have been characterized using spectroscopic data and elemental analysis. The antimicrobial activity of the N-heterocyclic carbene palladium complexes 3a–e varies with the nature of the ligands. Also, the IC₅₀ values of both, complexes (3a–e) and benzimidazoles 2a–e, have been determined. In addition, the new palladium complexes were screened for their antitumor activity. Complexes 3e and 3d exhibited the highest antitumor effect with IC₅₀ values 6.85?μg/mL against MCF-7 and 10.75?μg/mL against T47D, respectively.     

Synthesis of binuclear palladium complexes with a rigid phenylene bridge linker

The complexes [PdX₂Py]₂(di-NHC) (X = Br or Cl) in which di-NHC represents a di-N-heterocyclic carbene, featuring a rigid phenylene spacer between the carbene units, have been prepared from reactions of the corresponding diimidazolium halide salts with PdCl₂ in pyridine. The molecular structures of three of the complexes were determined by X-ray diffraction studies. The influences of different substitutions and of the halide ligand (Br or Cl) on the structure and reactivity of the complexes have been studied. The catalytic activity of the binuclear palladium complexes was tested in the Mizoroki–Heck reaction of styrene with bromobenzene.     

Dibenzyltin(IV) complexes of the 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olates: Synthesis and an investigation of structures by X-ray diffraction, solution and solid-state tin NMR, Sn Mössbauer and electrospray ionization MS

A series of cis-bis{5-[(E)-2-(aryl)-1-diazenyl]quinolinolato}dibenzyltin(IV) complexes have been synthesized by reacting sodium salts of 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-ol (LH) and dibenzyltin dichloride. These complexes have been characterized by ¹H, ¹³C, ¹¹⁹Sn NMR, ESI-MS in solution and by IR and ^119mSn Mössbauer, ¹¹⁷Sn CP-MAS NMR spectroscopy in solid state. In addition, the structures of three of the dibenzyltin(IV) complexes, viz., Bz₂Sn(L²)₂ (2), Bz₂Sn(L³)₂ (3), and Bz₂Sn(L⁵)₂ (5) (L = 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-ol: aryl = 4′-methylphenyl- (L²H), 4′-methoxylphenyl- (L³H) and 4′-bromophenyl- (L⁵H)) were determined by single-crystal X-ray diffraction. In general, the complexes were found to adopt a distorted cis-octahedral arrangement around the tin atom in both solution and solid state.     

Allylic alkylation and amination using mixed (NHC)(phosphine) palladium complexes under biphasic conditions

A dramatic improvement of the catalytic activity was observed when a phosphine was added in allylic alkylation reactions catalyzed by (NHC)Pd(η³-C₃H₅)Cl complexes. Consequently, several palladium complexes, generated in situ from different NHC-silver complexes, [Pd(η³-C₃H₅)Cl]₂ and PPh₃, were tested in this reaction to evaluate their potential. High reaction rates and conversions could be obtained with this catalytic system in the alkylation of allylic acetates with dimethylmalonate, particularly under biphasic conditions using water/dichloromethane and KOH 1 M as the base. These conditions are experimentally more convenient and gave higher reaction rates than the classical anhydrous conditions (NaH/THF). In this system, the phosphine is essential since no conversion was obtained when it is not present. The steric hindrance of the carbene ligand has a great influence on the activity and the stability of the catalytic system. The best NHC ligands for this reaction are either 1-mesityl-3-methyl-imidazol-2-ylidene or 1-(2,6-diisopropylphenyl)-3-methyl-imidazol-2-ylidene which are less bulky among the NHC tested. These two ligands led in 5 min to a complete conversion at 20 °C. The Pd-catalyzed allylic amination reaction using (E)-1,3-diphenylprop-3-en-yl acetate and benzylamine was also tested with (NHC)(PPh₃)Pd complexes and under the biphasic conditions. This reaction was found to be slower than the alkylation with dimethylmalonate but a complete conversion could be reached in 6 h at 20 °C using K₂CO₃ 1 M as the base. NMR experiments indicated that mixed (NHC)(PPh₃)Pd complexes are formed in situ but their structure could not be established exactly.     

Synthesis and X-ray structural characterization of the triphenylphosphine derivative of the closo-dodecaborate anion, closo-[B12H11P(C6H5)3][N(n-C4H9)4]

The reaction of a molar excess of closo-[B₁₂H₁₁I][N(n-C₄H₉)₄]₂ (1) with tetrakis(triphenylphosphine)palladium (0), Pd(0)L₄, yields to the formation of the title monoanionic compound, closo-[1-B₁₂H₁₁P(C₆H₅)₃][N(n-C₄H₉)₄] (2). The structure of 2 was determined by X-ray diffraction analysis performed on a single crystal. The mechanism of formation of 2 is also discussed. We suggested a two-step mechanism for the formation of 2 consisting in a oxidative addition of the palladium complex followed by a reductive elimination involving P(C₆H₅)₃ and assisted by Na₂CO₃. To our knowledge, this is the first example of monosubstitution of B₁₂ with formation of boron-phosphorus bond.     

Silver(I) complexes based on novel tripodal thioglycosides: synthesis, structure and antimicrobial activity

The reaction of tris(2-bromoethyl)amine hydrobromide with sugar thiols or thioacetates leads to the formation of novel carbohydrate substituted tripodal NS₃ ligands. Complexation with silver(I) ions gives stable complexes. NMR, X-ray, MS and EXAFS studies indicate their mononuclear C₃-symmetric structure. The highly water soluble complexes formed from the unprotected ligands show a wide spectrum of effective antimicrobial activities and their use lowers the cytotoxic and antiproliferative activities compared to the free silver salts.     

The first example of hydrozirconation of alkynes in room temperature ionic liquids

Hydrozirconation of terminal alkynes in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) at 30 °C gave highly regio- and stereoselectively (E)-vinylzirconium complexes, which underwent a cross-coupling reaction with aryl halides in the presence of Pd(PPh₃)₄ to afford (E)-1,2-disubstituted ethenes in good to high yields. Our system not only avoids the use of easily volatile THF or CH₂Cl₂ as solvent but also solves the basic problem of palladium catalyst reuse.     

Imidazolium salicylaldimine frameworks for the preparation of tridentate N-heterocyclic carbene ligands

Sterically hindered salicylaldimine functionalized imidazolium salts 2 have been prepared. The structures of the synthesized compounds were determined by spectroscopic techniques. The reaction of these salts containing arylmethyl-N chain (aryl: phenyl (2a), 2,4,6-trimethylphenyl (2b), 2,3,4,5,6-pentamethylphenyl (2c)) with Pd(OAc)₂ in boiling toluene afforded Pd(II) complexes 3 in high yields. The X-ray structure of 1-[3-(3,5-di-tert-butyl-2-oxophenyl)propyliminato]-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidenebromopalladium(II) (3b) has been determined. The Suzuki-Miyaura reaction was used to investigate their activity as catalysts either prepared in situ or from well-defined complexes. They are efficient when activated arylbromides are used as substrates.     

Synthesis of Pd complexes bearing an enantiomerically resolved seven-membered N-heterocyclic carbene ligand and initial studies of their use in asymmetric Wacker-type oxidative cyclization reactions

The development of enantiomerically resolved, axially-chiral seven-membered N-heterocyclic carbene (⁷NHC) ligands for palladium is described. These ⁷NHC ligands are derived from enatiomerically pure 2,2′-diamino-6,6′-dimethylbiphenyl, which is transformed via a synthetic sequence consisting of ortho-arylation, N-alkylation, and cyclization to afford seven-membered-ring amidinium salts. Synthesis of the seven-membered amidinium salts benefits from microwave irradiation, and in-situ metalation of the amidinium salts yields ⁷NHC-Pd^II complexes. The chiral ⁷NHC-Pd complexes were examined as chiral catalysts under aerobic conditions in two intramolecular oxidative amination reactions of alkenes. In one case, enantioselectivities up to 63% ee were obtained, while the other substrate underwent cyclization to afford essentially racemic products. The catalytic data compare favorably to results obtained with a Pd^II catalyst bearing a chiral five-membered-ring NHC ligand and, thereby, highlight the potential significance of this new class of chiral NHC ligands.     

Palladium catalyzed transformations of monoterpenes: stereoselective deuteriation and oxidative dimerization of camphene

Camphene undergoes a highly regio and stereoselective palladium catalyzed deuteriation in deuteriated acetic acid solutions of Pd(OAc)₂. NMR reveals that an outward oriented vinylic hydrogen is selectively exchanged for ²H, resulting in 90% camphene-d₁ (ca. 100% stereoselectivity) and 10% camphene-d₂ at 75% conversion of camphene (6 h, 25 °C). Neither π-allyl nor π-olefin palladium complexes are formed in detectable concentrations during the reaction, whereas palladium hydride (singlet at −6.86 ppm) and palladium deuteride (singlet at −6.78 ppm) intermediates have been detected by ¹H and ²H NMR, respectively. At higher temperature, oxidative coupling of camphene readily occurs giving the (E,E)-diene, i.e., bis(3,3-dimethyl-2-norbornylidene)ethane, which formally originates by abstracting the outward oriented vinylic hydrogens and coupling the resulting fragments of two camphene molecules. The reaction is catalytic at palladium in the Pd(OAc)₂-LiNO₃(cat)-O₂ and Pd(OAc)₂-benzoquinone systems. Similar mechanisms for the deuteriation and oxidative coupling of camphene are proposed, which involve the formation of σ-vinyl palladium hydride intermediates. No deuteriation neither oxidative coupling of limonene, myrcene and β-pinene were observed under the same conditions.     

Reaction of ZnCl2 and HgCl2 metal salts with a bidentate Schiff base ligand in methanol solution, X-ray crystal structure, and theoretical studies

From the reaction of ZnCl₂ and HgCl₂ metal salts with (E)-4-chloro-N-(pyridine-2-ylmethylene)benzeneamine (L) in methanol solution, two binuclear Schiff base complexes were prepared. Both complexes were characterized by elemental analysis, UV–Vis, and IR spectrophotometry. X-ray crystal structure analysis showed that metal ion in the resulting centrosymmetric dinuclear ML₂Cl₄ complexes is in a distorted trigonal bipyramidal and a distorted square pyramidal coordination environment in the case of Zn(II) and Hg(II) metal ions, respectively. Three types of reaction between MCl₂ salts and ligand L producing three different types of products—ML₂ ²⁺, MLCl₂, and M₂L₂Cl₄—were simulated in both the gas phase and solution. The gas phase calculations at DFT (B3LYP) level of theory using SDD, CEP-121G, and LanL2DZ basis sets showed that the binuclear M₂L₂Cl₄ complexes are more stable than corresponding mononuclear MLCl₂ complexes. Furthermore, both the gas phase and solution studies showed that the formation of M₂L₂Cl₄ complexes from the metal cations, chloride anion, and ligand molecule is energetically more favored than that of MLCl₂ and ML₂ ²⁺ complexes.     

Liquid–liquid extraction of palladium(II) and gold(III) with N-benzoyl-N′,N′-diethylthiourea and the synthesis of a palladium benzoylthiourea complex

N-Benzoylthioureas have been reported to form complexes with gold (III) and palladium (II) and other transition metals. In this study, an N-benzoyl-N′,N′-diethylthiourea (3f) ligand was used in the solvent extraction of palladium(II) and gold(III) from aqueous chloride media (0.1 mol l−1 NaCl). The distribution coefficient was determined as a function of both metal concentration in the aqueous phase and extractant concentration in the organic phase. The experimental distribution data were numerically analysed by letagrop-distr software in order to obtain the thermodynamic model corresponding to the metal extraction. It is found that pH does not affect the metal extraction process in the 1–2 pH range. Synthesis of the palladium benzoyl thiourea complexes was carried out by mixing quantities of metal and ligand solutions in methanol in a 1:2 ratio stoichiometric. Yields of 74 and 80.9% were obtained for the Pd-3c and Pd-3f complexes. In order to confirm the formation of the palladium complexes, NMR, FTIR and MS analyses were performed. From MS analyses a complex stoichiometry 1:2 (metal:ligand) was confirmed. The formation of crystals of palladium N-benzoyl-N′,N′-diethylthiourea complex (Pd-3f) in the methanolic solution allows the characterisation of the complex structure by XRD. The resulting structure is described and discussed. Bis(1,1,-diheptadecyl-3-benzoyl-thioureate)palladium(II) (Pd-3c) and bis(1,1,-diheptadecyl-3-benzoyl-thioureate)palladium(II) (Pd-3f) were used as ionophores in polymeric membrane electrodes. Their potentiometric responses to different anionic metal chlorocomplexes are evaluated and discussed taking into consideration the results obtained in the liquid–liquid distribution studies. A nernstian response was only obtained for AuCl4 − (PDL=8.8×10−8) and PdCl4 2− (PDL=1.5×10−4 M) with a selectivity coefficient of KAuCl4-, PdCl42−pot=−3.4, calculated taking AuCl4 − as being the primary anion.     

Synthesis of iridium complexes with new planar chiral chelating phosphinyl-imidazolylidene ligands and their application in asymmetric hydrogenation

The synthesis of planar chiral phosphinoimidazolium salts such as (R_p)-3-(4-diphenyl-phosphino[2.2]paracyclophan-12-ylmethyl)-1-(2,6-diisopropylphenyl)imidazolium bromide (R_p)-11c starting from enantiopure 4,12-dibromo[2.2]paracyclophane (R_p)-6 is reported. After deprotonation of these salts and a subsequent reaction with [Ir(COD)Cl]₂, chelating iridium imidazolylidene complexes (R_p)-5a–c are obtained. These complexes catalyze the asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee.     

Synthesis and characterization of cyclometallated palladium(II) and platinum(II) complexes with amide-thiolate ligands

The redox reaction of bis(2-benzamidophenyl) disulfide (H₂L-LH₂) with [Pd(PPh₃)₄] in a 1:1 ratio gave mononuclear and dinuclear palladium(II) complexes with 2-benzamidobenzenethiolate (H₂L⁻), [Pd(H₂L-S)₂(PPh₃)₂] (1) and [Pd₂(H₂L-S)₂ (μ-H₂L-S)₂(PPh₃)₂] (2). A similar reaction with [Pt(PPh₃)₄] produced only the corresponding mononuclear platinum(II) complex, [Pt(H₂L-S)₂(PPh₃)₂] (3). Treatment of these complexes with KOH led to the formation of cyclometallated palladium(II) and platinum(II) complexes, [Pd(L-C,N,S)(PPh₃)]⁻ ([4]⁻) and [Pt(L-C,N,S) (PPh₃)]⁻ ([5]⁻). The molecular structures of 2, 3 and [4]⁻ were determined by X-ray crystallography.     

New binuclear Pd(II) thioamide complexes for the Heck reaction of aryl bromides

The reaction of binucleating thioamide ligands (L₁-L₃) with [PdCl₂(PPh₃)₂] in 1:2 molar ratio in methanol medium afforded a series of binuclear palladium(II) complexes. The synthesized complexes were characterized by elemental analysis and ¹H NMR. The molecular structure of one of the complexes was established by X-ray diffraction method. The binuclear palladium(II) thioamide complex has been shown to be an active catalyst for the Heck reaction of aryl bromides with alkenes.