N-heterocyclic carbene ligands bearing hydrophilic and/or hydrophobic chains: Rh(I) and Pd(II) complexes and their catalytic activity

A series of novel imidazolium salts bearing hydrophilic tetraethylene glycol (TEG) and/or hydrophobic long-chain alkyl (n-C12) functionalities, which are precursors for desired N-heterocyclic carbene (NHC) ligands, were synthesized and characterized. Rh(I)-NHC complexes were prepared in good yields by the silver carbene transfer method with NHC-Ag species derived from the imidazolium salts. The molecular structure of the Rh(I)-NHC complex having n-C12 chains has been determined by a single-crystal X-ray diffraction study and the complex is found to possess extended alkyl chains with anti conformations in the solid state. Hydrosilylation with the Rh(I) complexes and Suzuki-Miyaura coupling reactions with the Pd(II) complexes with these NHC ligands were carried out. In the latter case, the TEG moiety enhances catalytic activity considerably.     

Sulfur-functionalized N-heterocyclic carbene complexes of Pd(II): syntheses, structures and catalytic activities

N-heterocyclic carbenes (NHCs) can be easily modified by introducing functional groups at the nitrogen atoms, which leads to versatile coordination chemistry as well as diverse catalytic applications of the resulting complexes. This article summarizes our contributions to the field of NHCs bearing different types of sulfur functions, i.e., thioether, sulfoxide, thiophene, and thiolato. The experimental evidence for the truly hemilabile coordination behavior of a Pd(II) thioether-NHC complex has been reported as well. In addition, complexes bearing rigid CSC-pincer ligands have been synthesized and the reasons for pincer versus pseudo-pincer formation investigated. Incorporation of the electron-rich thiolato function resulted in the isolation of structurally diverse complexes. The catalytic activities of selected complexes have been tested in Suzuki-Miyaura, Mizoroki-Heck and hydroamination reactions.     

Redox noninnocence of carbene ligands: carbene radicals in (catalytic) C-C bond formation

In this Forum contribution, we highlight the radical-type reactivities of one-electron-reduced Fischer-type carbenes. Carbene complexes of group 6 transition metals (Cr, Mo, and W) can be relatively easily reduced by an external reducing agent, leading to one-electron reduction of the carbene ligand moiety. This leads to the formation of "carbene-radical" ligands, showing typical radical-type reactivities. Fischer-type carbene ligands are thus clearly redox-active and can behave as so-called "redox noninnocent ligands". The "redox noninnocence" of Fischer-type carbene ligands is most clearly illustrated at group 9 transition metals in the oxidation state II+ (Co(II), Rh(II), and Ir(II)). In such carbene complexes, the metal effectively reduces the carbene ligand by one electron in an intramolecular redox process. As a result, the thus formed "carbene radicals" undergo a variety of radical-type C-C and C-H bond formations. The redox noninnocence of Fischer-type carbene ligands is not just a chemical curiosity but, in fact, plays an essential role in catalytic cyclopropanation reactions by cobalt(II) porphyrins. This has led to the successful development of new chiral cobalt(II) porphyrins as highly effective catalysts for asymmetric cyclopropanation with unprecedented reactivity and stereocontrol. The redox noninnocence of the carbene intermediates results in the formation of carbene-radical ligands with nucleophilic character, which explains their effectiveness in the cyclopropanation of electron-deficient olefins and their reduced tendency to mediate carbene dimerization. To the best of our knowledge, this represents the first example in which the redox noninnocence of a reacting ligand plays a key role in a catalytic organometallic reaction. This Forum contribution ends with an outlook on further potential applications of one-electron-activated Fischer-type carbenes in new catalytic reactions.     

Pyridyl-supported pyrazolyl-N-heterocyclic carbene ligands and the catalytic activity of their palladium complexes in Suzuki-Miyaura reactions

Palladium complexes of two new types of unsymmetrical pyridyl-supported pyrazolyl-N-heterocyclic carbene ligands were synthesized and structurally characterized. A strategy to release the steric strain of the ligand was realized by the introduction of methylene linkers to the ligand molecule. All the palladium complexes exhibited good to excellent catalytic activity in Suzuki-Miyaura reactions of phenyl or p-tolylboronic acid with aryl halides including iodobenzene, aryl bromides, and activated aryl chlorides under mild conditions, revealing that the new ligands are promising for the construction of highly active transition-metal catalysts.     

Achiral banana-shaped mesogenic bidentate ligands and their Cu(II) and Pd(II) complexes

The first achiral bent-core banana-shaped bidentate ligands and their Cu(II) and Pd(II) metal complexes have been synthesized and investigated for mesomorphic behaviour. The bidentate ligands exhibit only one enantiotropic mesophase. The ligand having C 6 -alkoxy chains shows a mesophase that has been assigned as a two-dimensional B 1 phase while the C 8 and C 10 homologues stabilize the fluid B 2 mesophase showing antiferroelectric switching characteristics. In constrast, their corresponding Cu(II) and Pd(II) metal complexes are non-mesomorphic.     

Achiral banana-shaped mesogenic bidentate ligands and their Cu(II) and Pd(II) complexes

The first achiral bent-core banana-shaped bidentate ligands and their Cu(II) and Pd(II) metal complexes have been synthesized and investigated for mesomorphic behaviour. The bidentate ligands exhibit only one enantiotropic mesophase. The ligand having C6 -alkoxy chains shows a mesophase that has been assigned as a two-dimensional B1 phase while the C8 and C10 homologues stabilize the fluid B2 mesophase showing antiferroelectric switching characteristics. In constrast, their corresponding Cu(II) and Pd(II) metal complexes are non-mesomorphic.     

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.     

Nickel(II) and copper(II) complexes with amidrazone-based ligands: Structure and catalytic activity

Nickel(II) and copper(II) complexes MeL, where H₂LL = [9-(2′-hydroxyphenyl)-6-methyl-3-acetyl-5,7,8-triazanona-3,6,8-trien-2-one], have been synthesized by template reaction of salicylaldehyde acetamidrazone with corresponding Me(acac)₂ and Hacac in the presence of the orthoformic ester at 110°C. The crystal structure of CuL has been determined by X-ray diffraction. The square-planar mode of coordination is realized in CuL. Comparison of geometrical parameters of CuL with those of the corresponding derivative based on S--allylisothiosemicarbazide showed that substitution of the -SR group by methyl in the quadridentate ligand does not affect the mode of binding nor the main interatomic distances and angles in the ligand. The data from magnetic measurements, ¹HH NMR and EI mass spectra indicate that NiL has a similarly structured coordination polyhedron. Epoxidation of norbornene can be performed efficiently with molecular oxygen (1 atm) in THF (or THF-EtOAc) in the presence of CuL at 70°C. The corresponding copper(II) derivatives based on S--substituted isothiosemicarbazides are much less active as catalysts.     

Copper(II) complexes of ligands derived from tryptamine

Three new ligands with an indole substituent tethered to a pyridylalkylamine or imidazolylalkylamine metal-binding domain have been prepared from tryptamine. Copper(II) complexes have been prepared and characterized, three by X-ray crystallography. Electrochemistry has been used to ascertain the mutual effects of the copper and indole redox centres upon each other.     

Non-symmetrically substituted di-N-heterocyclic carbene palladium(II) complexes: Synthesis,structure and catalytic activity

The synthesis and characterization of non-symmetric di-N-heterocyclic carbene (diNHC) silver(I) and palladium(II) complexes are described. The activity of the Pd(II) complexes in the Suzuki–Miyaura coupling reaction of bulky substrates was investigated. This synthetic route represents a possible general pathway into a wide variety of non-symmetrically substituted diNHC ligands.     

Some platinum(II) complexes derived from aromatic alkynes

Several trans‐platinum(II) complexes, of the type R′? {Pt(PBu₃)₂}? R″? {Pt(PR₃)₂}? R′, where R′ and R″ are groups derived from a series of aromatic alkynes and diynes, have been prepared and characterized. Extensive spectroscopic data for these and other known related complexes are presented. A more precise structural study of trans‐Pt(C≡CC₆H₄C≡CPh)₂(PBu₃)₂ (cf. Z. Kristallogr. 1998; 213: 483) is reported. Copyright © 2002 John Wiley & Sons, Ltd.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n -butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N * ) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC * ) mesophase. The metal complexes with n -butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N * phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C * phase of the two ligands.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n-butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N*) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC*) mesophase. The metal complexes with n-butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N* phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C* phase of the two ligands.     

Co-ligand effects in the catalytic activity of Pd(II)-NHC complexes

Three cis-chelating di-N-heterocyclic carbene palladium(II) complexes [PdX₂(diNHC)] (X = I, 1; X = SCN, 2; X = CF₃CO₂, 3) bearing different anionic co-ligands were synthesized and fully characterized. A comparison of their catalytic activities in the Mizoroki-Heck reaction and conjugate addition of arylboronic acids to cyclic enones revealed increasing efficiency in the order SCN⁻ < I⁻ < CF₃CO₂⁻. The di(trifluoroacetato) complex 3 showed the best activity in both transformations highlighting the importance of co-ligands effects in catalysis. In addition, the molecular structure of an unusual poly-heteronuclear complex salt 4 is reported, which has been isolated as a byproduct in the synthesis of complex 3.     

Synthesis of new cyclic aromatic carbene ligands bearing remote amino groups and their palladium(ii) complexes

New cyclic aromatic carbene ligands bearing remote amino groups were developed. The oxidative addition of Pd(0) into the chloride precursors yielded the corresponding Pd(ii) complexes whose carbene ligand was demonstrated to have stronger donor ability than classical NHCs.     

Synthesis,structural characterization and catalytic activity of benzimidazole‐functionalized Pd(II) N‐heterocyclic carbene complexes

Two Pd(II)–NHC complexes bearing benzimidazole and pyridine groups have been successfully prepared and fully characterized by NMR and X‐ray diffraction analysis. The structure of palladium complexes are a typical square‐planar with palladium surrounded by two pairs of trans‐arranged benzimidazole and carbene ligands. The Pd–NHC complexes have been proved to be a highly efficient catalyst for the Mizoroki–Heck coupling reaction of aryl halides with various substituted acrylates under mild conditions in excellent yields. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis, structure and catalytic activity of thioether-phosphane complexes of Pd(II) and Pt(II)

New thioether-phosphanes 2-RSC6H4CH2PPh2(RS-PPh2: R = Me, tBu, Ph) and the corresponding complexes [PdCl2(MeS-PPh2)], [PdCl2(tBuS-PPh2)], [PdCl2(PhS-PPh2)], [PdClMe(MeS-PPh2)] and [PtMe2(MeS-PPh2)] have been prepared, characterized and the X-ray crystal structures of all complexes determined. Whilst Pd(II) complexes of RS-PPh2 show low activity for CO/ethene copolymerisation, the complexes [PdCl2(RS-PPh2)] have been found to be very efficient for the Heck arylation of n-butylacrylate with bromobenzene under aerobic conditions.     

Pd(II) complexes of a sterically bulky, benzannulated N-heterocyclic carbene and their catalytic activities in the Mizoroki-Heck reaction

The bis(N,N′-diisopropylbenzimidazolin-2-ylidene)Pd(II) complexes trans-[PdBr₂(ⁱPr₂-bimy)₂] (trans-1) and trans-[PdI₂(ⁱPr₂-bimy)₂] (trans-2) have been prepared in good yields by in situ deprotonation of the corresponding N,N′-diisopropylbenzimidazolium salt (ⁱPr₂-bimyH⁺X⁻) (A: X = Br, B: X = I) with Pd(OAc)₂ in DMSO at elevated temperature. Salt metathesis of trans-1 or trans-2 with AgO₂CCF₃ in refluxing CH₃CN afforded the novel mixed carbene-carboxylato complex cis-[Pd(O₂CCF₃)₂(ⁱPr₂-bimy)₂] (cis-3). This halo/trifluorocarboxylato ligand substitution can be regarded as a selective method for the synthesis of cis-configured bis(carbene) complexes. All compounds have been fully characterized by multinuclei NMR spectroscopies and ESI mass spectrometry. X-ray diffraction studies on single crystals of trans-1, trans-2 and cis-3 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H protons pointing to the metal center to maximize rare C-H?Pd preagostic interactions. These interactions are also retained in solution as indicated by the large downfield shift of the isopropyl C-H protons in the ¹H NMR spectrum compared to those in precursor salts A or B. A preliminary catalytic study revealed that all complexes are highly active in the Mizoroki-Heck coupling of aryl bromides and chlorides. However, these complexes gave slower conversions as compared to catalysts with less bulky benzimidazolin-2-ylidenes. This is most likely due to the steric bulk of the ligands, which hamper a fast reductive formation of catalytically active Pd(0) species.     

Anticonvulsant activity and toxicity evaluation of Cu(II) and Zn(II) metal complexes derived from triazole-quinoline ligands

A novel Cu(II) and Zn(II) complexes of triazolo-quinoline derivatives were synthesized by in situ method and were characterized by the spectro-analytical methods and their pharmacological properties were evaluated. The compound C3 has exhibited promising anticonvulsant activity towards the electroshock induced seizures in Wistar rats and possesses low toxicity, providing a high safety profile.     

Pd(II) complexes with mixed benzothiazolin-2-ylidene and phosphine ligands and their catalytic activities toward C-C coupling reactions

Novel Pd(II) mixed N,S-heterocyclic carbene (NSHC)-phosphine complexes of the general formula [PdBr(2)(NSHC)(PR(3))] were obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr(2)(NSHC)](2) [NSHC = 3-benzylbenzothiazolin-2-ylidene and 3-propylbenzothiazolin-2-ylidene] with triphenylphosphine, tricyclohexylphosphine and 2-diphenylphosphanyl-pyridine. All complexes have been fully characterized by (1)H and (13)C NMR spectroscopy, ESI mass spectrometry and elemental analysis. The X-ray crystal structures of complexes 3-8 are reported. The complexes exhibit moderate to good catalytic activity in the Suzuki-Miyaura coupling reaction of aryl bromides and chlorides.