Synthesis of chiral imino- and amino-imidazolium salts and of chelating amino-N-heterocyclic carbene palladium(II) complexes

A new preparation of chiral imino-imidazolium salts has been developed by condensation of chiral primary amines with 1-(2-oxo-2-phenyl-ethyl)-imidazolium salts in chloroform. This reaction gave the (E)-imino-imidazolium salts with stereoselectivities superior to 95:5. The structure of the imines were determined by NMR analyses. Reduction of the chiral (E)-imino-imidazolium salts with NaBH₄ in MeOH led to amino-imidazolium salts as a mixture of diastereomers with selectivities ranging from 84:16 to 90:10. The major diastereoisomer could be purified in some cases by crystallization and the absolute configurations were determined by X-ray diffraction. Chelating amino-N-heterocyclic carbene dichloro palladium(II) complexes were obtained in two steps via formation of the corresponding silver(I) complexes and reaction of these latters with bis(acetonitrile)dichloropalladium. Crystal structure details of a cis-dichloro amino-imidazol-2-ylidene palladium complex are presented and confirmed the formation of a six-membered Pd-metallocycle.     

Synthesis and structural characterization of mixed carbene-carboxylate complexes of palladium(II)

Mixed carbene-carboxylate complexes of Palladium(II) have been prepared by reacting {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) diiodide (1) [Angew. Chem. 107 (1995) 2602; Angew. Chem. Int. Ed. Engl. 34 (1995) 2371; J. Organomet. Chem. 557 (1998) 93] with AgO₂CR, where R=CF₃, CF₂CF₃ and CF₂CF₂CF₃. In this manner, {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) bis(trifluo-roacetate) (2), {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) bis(pentafluoropropionate) (3) and {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) bis(heptafluorobutyrate) (4) were obtained. All three complexes were fully characterized by ¹H-, ¹³C- and ¹⁹F NMR spectroscopy as well as ESI mass spectrometry. X-ray crystal structure analyses of complexes 3 and 4 reveal mononuclear species with a square planar metal center coordinated by a cis-chelating dicarbene and two monodentate carboxylate ligands. The results show that the introduction of a cis-chelating N,N-heterocyclic carbene ligand stabilizes the palladium-carboxylate moiety effectively.     

A new class of o-hydroxyaryl-substituted N-heterocyclic carbene ligands and their complexes with palladium

A facile synthesis of o-hydroxyaryl-substituted N-heterocyclic carbene ligands and their complexes with palladium is presented. This kind of salicylaldimine-like NHC ligands expands the class of available NHC ligands for organometallic catalysts.     

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 and coordination properties of palladium(II) and platinum(II) complexes with phosphonated triphenylphosphine derivatives

Two triphenylphosphine derivatives, diethyl [4-(diphenylphosphanyl)benzyl]phosphonate (3a) and tetraethyl {[5-(diphenylphosphanyl)-1,3-phenylene]dimethylene}bis(phosphonate) (3b), and also the corresponding free acids 4a and 4b were prepared. These ligands were characterized by ¹H, ¹³C and ³¹P NMR spectroscopy and mass spectrometry. A full set of their Pd(II) and Pt(II) complexes of the general formula [MCl₂L₂] and one dinuclear complex trans-[Pd₂Cl₄(3a)₂] were synthesized and their isomerization behaviour in solution was studied. The complexes were characterized by ¹H, ¹³C, ³¹P and ¹⁹⁵Pt NMR spectroscopy, mass spectrometry and far-IR spectroscopy. The X-ray structures of all complexes with 3a or 3b have usual slightly distorted square-planar geometry on the metal ion. Salts of phosphonic acids 4a and 4b and their complexes are freely soluble in aqueous solution; therefore, they can be potentially useful in aqueous or biphasic catalysis.     

Novel Ag(I), Pd(II), Ni(II) complexes of N,N′-bis-(2,2-diethoxyethyl)imidazole-2-ylidene: Synthesis, structures, and their catalytic activity towards Heck reaction

Tetra-ether substituted imidazolium salts, LHX (where LH = N,N′-bis(2,2-diethoxyethyl)imidazolium cation and X = Br, BF₄, PF₆, BPh₄, NO₃ and NTf₂ anions) were derived from imidazole. Attempts to produce aldehyde functionalized imidazolium salt through acid hydrolysis of LHBr resulted an unexpected tetra-hydroxy compound L_AHBr and the dialdehyde compound L_BHBr. Reaction of LHBr with Ag₂O afforded [L₂Ag][AgBr₂] (1). Mononuclear Pd-complex trans-[L₂PdCl₂] (2) and dinuclear Pd-complex [(LPdCl₂)₂] (3) were obtained by 1:1 and 1:2 reaction of in situ generated Ag-carbene with Pd(CH₃CN)₂Cl₂. cis-[LPdPPh₃Cl₂] (4) was synthesized from reaction of PPh₃ with dinuclear complex 3. Hydrolysis of 3 under acidic conditions also generates a hydroxy derivative 3A and the aldehyde derivative 3B. Direct heating of LHBr with Ni(OAc)₂ · 4H₂O at 120 °C under vacuum generated trans-[L₂NiBr₂] (5). These complexes were characterized by NMR, mass, elemental analysis, and X-ray single crystal diffraction analysis. Pd--Pd interaction was observed in 3. All the Pd complexes exhibited excellent catalytic activity in Heck reaction.     

Synthesis and characterization of bidentate NHC-Pd complexes and their role in amination reactions

The new well-defined and air-stable ortho-xylyl-linked N-heterocyclic carbene (NHC) Pd complexes (2a-d) have been synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR, IR spectroscopy, and single crystal X-ray diffraction studies. The palladium atom in the complex 2a lies on a crystallographic mirror plane and can be described as having a square-planar coordination environment with the carbene atoms of the benzimidazole rings of the ligand occupying two coordination sites in cis positions. Two further coordination sites are occupied by chloride ligands. The benzimidazole rings are connected to each other by an ortho-xylyl bridge. The catalytic activity of these palladium complexes has been tested in the coupling reactions of various N-containing substrates with bromobenzene. A preliminary catalytic study shows that the bis(NHC)-Pd complexes are highly active in the Buchwald-Hartwig amination reaction.     

Palladium(II)-N-heterocyclic carbene complexes derived from proline: synthesis and characterization

Pd(II)-N-heterocyclic carbene complexes derived from proline have been successfully synthesized in good yields and their structures have been characterized by X-ray single crystal diffraction. It was found that the substituents on the N-atom of the pyrrolidine skeleton dramatically affect on the coordination pattern of the palladium complexes. In a word, when an electron-rich group as benzyl group was attached on the N-atom, both of the N-atom and NHC were coordinated to the Pd(II) center; while when an electron-poor group as Ts group was attached, a dimeric mono-coordinated Pd(II)-NHC was obtained exclusively.     

Formation and structures of Pd(II) N,S-heterocyclic carbene-pyridyl mixed-ligand complexes

Mononuclear mixed-ligand complexes of Pd(II) containing a N,S-heterocyclic carbene (NSHC) with a secondary alkyl N-substituent and pyridyl ligand, with the general formula [PdI₂(C₁₀H₁₁NS)L] (C₁₀H₁₁NS = 3-isopropylbenzothiazolin-2-ylidene; L = pyridine, 2-aminopyridine, 3-iodopyridine and 4-tert-butyl-pyridine) have been synthesized and characterized by X-ray single-crystal crystallography. Both solution and solid-state structures, as evident from their ¹H NMR spectra and X-ray structures, show anagostic γ-hydrogen interactions of metal with methine of the substituent on the carbene or pyridyl ligand giving 5-membered-chelate-like structures.     

Ag(I) and Pd(II) complexes of a 1,3-dibenzhydryl substituted benzannulated N-heterocyclic carbene: Unexpected rearrangement, structures and catalytic studies

Reaction of the sterically bulky 1,3-dibenzhydrylbenzimidazolium bromide (Bh₂-bimyH⁺Br⁻) (A) with Pd(OAc)₂ in DMSO yielded a mono(carbene) Pd(II) complex 1 with a N-bound benzimidazole derivative, which resulted from an unusual NHC rearrangement reaction. Reaction of A with Ag₂O, on the other hand, cleanly gave the Ag(I) carbene complex [AgBr(Bh₂-bimy)] (2), which has been used as a carbene-transfer agent to prepare the acetonitrile complex trans-[PdBr₂(CH₃CN)(Bh₂-bimy)] (3). Dissociation of acetonitrile from complex 3 and subsequent dimerization afforded the dinuclear Pd(II) complex [PdBr₂(Bh₂-bimy)]₂ (4) in quantitative yield. All complexes were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Furthermore, the catalytic activity of complex 4 in aqueous Suzuki-Miyaura cross-coupling reactions was studied and compared with that of its previously reported less bulky analogue [PdBr₂(ⁱPr₂-bimy)]₂.     

Synthesis of a new bidentate ferrocenyl N-heterocyclic carbene ligand precursor and the palladium (II) complex trans-[PdCl2(CfcC)], where (CfcC) = 1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene

A new ferrocenyl-N-heterocyclic carbene ligand precursor 1,1′-bis[(1-tert-butylimidazolium)-3-methyl]ferrocene dichloride has been synthesised and structurally characterised. The imidazolium salt was readily deprotonated in situ with KN(SiMe₃)₂ and reacted with [PdCl₂ (cod)] to afford the structurally characterised palladium (II) complex trans-[PdCl₂(C^∧fc^∧C)], where (cod) = 1,5-cyclooctadiene and (C^∧fc^∧C) = 1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene.     

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.     

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.     

Heteroleptic platinum(II) and palladium(II) complexes with thiacrown and diimine ligands

We wish to report the synthesis, crystal structures, spectroscopic and electrochemical properties of several new Pt(II) heteroleptic complexes containing the thiacrown, 9S3 (1,4,7-trithiacyclononane) with a series of substituted phenanthroline ligands and related diimine systems. These five ligands are 5,6-dimethyl-1,10-phenanthroline(5,6-Me₂-phen), 4,7-dimethyl-1,10-phenanthroline(4,7-Me₂-phen), 4,7-diphenyl-1,10-phenanthroline(4,7-Ph₂-phen), 2,2′-bipyrimidine(bpm), and pyrazino[2,3-f]quinoxaline or 1,4,5,8-tetraazaphenanthrene(tap). All complexes have the general formula [Pt(9S3)(N₂)](PF₆)₂ (N₂ = diimine ligand) and form similar structures in which the Pt(II) center is surrounded by a cis arrangement of the two N donors from the diimine chelate and two sulfur atoms from the 9S3 ligand. The third 9S3 sulfur in each structure forms a longer interaction with the platinum resulting in an elongated square pyramidal structure, and this distance is sensitive to the identity of the diimine ligand. In addition, we report the synthesis, structural, electrochemical, and spectroscopic properties of related Pd(II) 9S3 complex with tap. The ¹⁹⁵Pt NMR chemical shifts for the six Pt(II) complexes show a value near −3290 ppm, consistent with a cis-PtS₂N₂ coordination sphere although more electron-withdrawing ligands such as tap show resonances shifted by almost 100 ppm downfield. The physicochemical properties of the complexes generally follow the electron-donating or withdrawing properties of the phenanthroline substituents.     

N(4)-tolyl-2-benzoylpyridine-derived thiosemicarbazones and their palladium(II) and platinum(II) complexes: Cytotoxicity against human solid tumor cells

Complexes [Pt(2Bz4oT)Cl], [Pt(2Bz4mT)Cl], and [Pt(2Bz4pT)Cl] were prepared with N(4)-ortho-(H2Bz4oT), N(4)-meta-(H2Bz4mT), and N(4)-para-(H2Bz4pT) tolyl-2-benzoylpyridine-derived thiosemicarbazones. The thiosemicarbazones exhibited moderate anti-proliferative activity against HepG2 (hepatoma) and UACC-62 (melanoma) cancer cell lines, but showed high anti-proliferative effect against A431 (epithelial carcinoma) cancer cell lines. Upon coordination to platinum(II) the anti-proliferative activity decreases in all cases. The cytotoxicity of the previously prepared palladium(II) analogues [Pd(2Bz4oT)Cl], [Pd(2Bz4mT)Cl], and [Pd(2Bz4pT)Cl] was also investigated. As in the case of the platinum(II) complexes, coordination to palladium(II) did not lead to activity improvement. Investigations on the mechanism of cytotoxic action against A431 cells revealed that [Pd(2Bz4oT)Cl] induced DNA fragmentation and apoptosis while H2Bz4oT did not present this effect. The high anti-proliferative effect of the thiosemicarbazones and [Pd(2Bz4oT)Cl] against A431 cells, together with the pro-apoptotic effect of [Pd(2Bz4oT)Cl] suggests that these compounds have potential as chemotherapeutic drug candidates.     

Synthesis, spectroscopy and structures of palladium(II) and platinum(II) complexes containing mercapto-o-carborane

The reactions of [M₂Cl₂(μ-Cl)₂(PMe₂Ph)₂] with mercapto-o-carboranes in the presence of pyridine afforded mono-nuclear complexes of composition, [MCl(SCb°R)(py)(PMe₂Ph)] (M = Pd or Pt; Cb° = o-C₂B₁₀H₁₀; R = H or Ph). The treatment of [PdCl₂(PEt₃)₂] with PhCb°SH yielded trans-[Pd(SCb°Ph)₂(PEt₃)₂] (4) which when left in solution in the presence of pyridine gave another substitution product, [Pd(SCb°Ph)₂(py)(PEt₃)] (5). The structures of [PdCl(SCb°Ph)(py)(PMe₂Ph)] (1), [Pd(SCb°Ph)₂(PEt₃)₂] (4) and [Pd(SCb^oPh)₂(py)(PEt₃)] (5) were established unambiguously by X-ray crystallography. The palladium atom in these complexes adopts a distorted square-planar configuration with neutral donor atoms occupying the trans positions. Thermolysis of [PdCl(SCb°)(py)(PMe₂Ph)] (2) in TOPO (trioctylphosphine oxide) at 200 °C gave nanocrystals of TOPO capped Pd₄S which were characterized by XRD pattern and SEM.     

X-ray crystal structures of novel platinum(II) and palladium(II) complexes of dialkyl phosphonated phosphines

Two diethyl phosphonated phosphine ligands of formula Ph₂P(CH₂)₃PO₃Et₂ (ligand L) and Ph₂P(4-C₆H₄PO₃Et₂) (ligand L′) were used to prepare different complexes of platinum(II) (1, cis-PtCl₂L₂; 2, trans-PtCl₂L₂·H₂O; 3A and 3B, cis- and trans-PtCl₂L′₂) and palladium(II) (4, [PdCl₂L]₂; 5, trans-PdCl₂L₂·H₂O; 6, trans-PdCl₂L′₂·CH₂Cl₂). The single-crystal X-ray structure analyses of complexes 1, 2, 4-6 indicate that complexation involved only the phosphine end, whereas the strong polarization of the PO bond was highlighted by the formation of hydrogen bonds with a water molecule in 2 and 5, and with a dichloromethane molecule in 6, with an exceptionally short CH?O hydrogen bond length (C?O separation 3.094(3) Å).     

Rotamers of palladium complexes bearing IR active N-heterocyclic carbene ligands: Synthesis, structural characterization and catalytic activities

The preparation and properties of mono- versus bis(carbene) Pd(II) complexes bearing unsymmetrical cyano- and ester-functionalized NHC ligands as potential IR probes were studied in detail. Direct reaction of Pd(OAc)₂ with functionalized imidazolium salts afforded either bis(carbene) (3a, c) or monocarbene complexes (5, 6) with a N-coordinated imidazole co-ligand. The latter were exclusively obtained with N-ethylene substituted salts, which were found to undergo N-C cleavage reaction. The milder Ag-carbene transfer reaction on the other hand was tolerable to the length of the substituents and the nature of the functional groups. All bis(carbene) complexes (3a-c, 4a-c) were obtained as a inseparable mixture of square-planar trans-anti and trans-syn rotamers. The identity, ratio and dynamic equilibrium of these rotamers have been investigated and the relatively high rotational barrier for rotamers of 3a was estimated to be about 74 kJ mol⁻¹ at 380 K. All eight complexes were fully characterized by NMR and IR spectroscopies, ESI mass spectrometry and X-ray single crystal and powder diffraction. A preliminary catalytic study showed that ester-functionalized complexes 4a and 4b gave rise to highly active catalyst in the double Mizoroki-Heck coupling of aryl dibromides, while the in situ ester-hydrolyzed complexes were also active in the coupling of activated aryl chlorides.     

New platinum(II) and palladium(II) quinoline-imine-pyridine, quinoline-imine-thiazole and quinoline-imine-imidazole complexes by metal-assisted condensation reactions

Pt(II) and Pd(II) methyl- and chloro-complexes with the tridentate N-donor ligands ((pyridin-2-yl)methylene)quinolin-8-amine (NNPy), ((pyridin-2-yl)ethylidene)quinolin-8-yl-amine (NNMePy), (phenyl(pyridin-2-yl)methylene)quinolin-8-yl-amine (NNPhPy), ((thiazol-2-yl)methylene)quinolin-8-amine (NNTh) and ((imidazol-4-yl)methylene)quinolin-8-amine (NNImH) were prepared by metal-assisted condensation of 8-aminoquinoline and an ortho-substituted aldehydo- or keto- N-heterocycle. Preliminary reactivity studies involving the coordinated tridentate N-donors, the chloro-ligand and the M-CH₃ bond were carried out, leading to the synthesis of several new complexes. During these studies, the formation of a novel five-coordinate Pt(II) carbonyl-complex was observed.     

Synthesis and characterization of novel water-soluble zinc(II) Schiff-base complexes derived from amino acids and salicylaldehyde-5-sulfonates

New water-soluble zinc(II) Schiff-base complexes derived from amino acids (glycine, L-phenylalanine, and L-valine) and salicylaldehyde-5-sulfonates (sodium salicylaldehyde-5-sulfonate and sodium 3-methoxy-salicylaldehyde-5-sulfonate) have been synthesized. The complexes were characterized by elemental analysis, IR, electronic, ¹H?NMR, and ¹³C?NMR spectra. In the IR spectra of the complexes, the large difference between the asymmetric ν_as(COO) and symmetric ν_s(COO) carboxylate stretch, Δν(ν_as(COO)–ν_s(COO)) of 199–247?cm^?1, indicates monodentate coordination of the carboxylate group. Spectral data showed that in these complexes the ligand is a tridentate ONO moiety, coordinating to the metal through its phenolic oxygen, imine nitrogen, and carboxyl oxygen.