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.     

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.     

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.     

Coordination polymers of silver(I) with bis(N-heterocyclic carbene): Structural characterization and carbene transfer

Reactions of the ethylene- and methylene-bridged bis(imidazolium) salts with an equivalent amount of silver oxide in dichloromethane at room temperature produced readily the silver NHC compounds [Ag₂LBr₂]. These compounds are partially soluble in DMF. The X-ray structure determination on 3d (L = 1,1′-dibenzyl-3,3′-ethylenediimidazolin-2,2′-diylidene) reveals the formation of bromide bibridged (Ag₂LBr₂)_n chains and a unique supramolecular motif with weak Ag?Ag interactions of 3.429 Å. Similar to monomeric silver(I) NHC complexes, the silver coordination polymers can also act as carbene transfer reagents for the formation of chelating palladium NHC complexes in excellent yields.     

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.     

Palladium(II) complexes bearing methylene and ethylene bridged pyrido-annelated N-heterocyclic carbene ligands as active catalysts for Heck and Suzuki-Miyaura cross-coupling reactions

The synthesis and characterization of new bidentate N-heterocyclic carbene ligands is described. The ligands are derived from methylene (me) and ethylene (et) bridged imidazo[1,5-a]pyridine-3-ylidenes (impy) and can be synthesized readily from imidazo[1,5-a]pyridine and the respective dihaloalkanes. Palladium(II) dihalide complexes bearing these et(impy)₂ or me(impy)₂ ligands were prepared and also structurally characterized. The angle of the impy plane vs. the C-Pd-C plane is depending on the bridging unit as well as the halide ligands. In the solid state the me(impy)₂PdBr₂ complex forms a dimer by weak intermolecular Pd-hydrogen bridges. The activity of the Pd-complexes as catalysts for the Heck and Suzuki-Miyaura reactions was tested under various conditions. The catalysts show good activity at 120 °C in Heck and at 85 °C in Suzuki-Miyaura reactions.     

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.     

Polyisobutylene-supported N-heterocyclic carbene palladium catalysts

This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable when such solvent systems are perturbed to be biphasic. The results here show that such PIB-bound NHC ligands can be used to synthesize useful palladium catalysts. In this paper, both PIB-bound analogs of an N,N′-bis(2,6-diisopropylphenyl) heterocyclic carbene and simpler N,N′-dialkyl heterocyclic carbene ligand were prepared and were successfully used to form palladium cross-coupling catalysts. The reactivity, recycling and reusability of these catalysts has been examined.     

cis-Fashioned palladium (II) complexes of 2-phenylbenzimidazole ligands: Synthesis, characterization, and catalytic behavior towards Suzuki-Miyaura reaction

Reaction of 2-arylbenzimidazole with PdCl₂(CH₃CN)₂ in CH₂Cl₂ affords benzimidazole palladium (II) complexes in high yields. The structure of complexes C1, C2, and C3 has been confirmed by X-ray structure analysis. The configuration of complexes depends on the substituent on the 2-position of benzimidazole. Phenyl affords the complexes in cis-fashion due to π-π stacking of phenyl and benzimidazole. Tolyl affords the complex in trans-fashion. The catalytic studies show that cis-configured 2-phenylbenzimidazole palladium (II) complexes are highly efficient catalysts in the Suzuki-Miyaura reaction.     

Synthesis, molecular structures and solution NMR studies of N-heterocyclic carbene-amine silver complexes

The synthesis of the Boc-protected 1-(2-aminoethyl)-3-methylimidazolium salts [BocNHCH₂CH₂ImMe]X [2]X (X = I, PF₆) and their straightforward transformation into [NH₂CH₂CH₂ImMe]X [3]X is reported. The reaction between [2]X and Ag₂O leads to the formation in the solid state of three different bonding motifs: a biscarbene salt [(NHC-NHBoc)₂Ag]PF₆ ([4]PF₆, NHC-NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene), a tetranuclear complex [Ag(NHC-NHBoc)₂]₂[Ag₂I₄], (5), and a polymeric silver “staircase” [(NHC-NHBoc)₂-Ag₄-I₄]_n, (6) composed of Ag₄I₄ clusters. The same reaction carried out with [3]I showed that a primary silver mono-NHC-NH₂ carbene complex of the type [(NHC-NH₂)AgI] (7) is likely to form but it is unstable in solution. The solid state molecular structures of [4]PF₆, 5 and 6 were determined by X-ray diffraction analysis, whereas PGSE NMR experiments were employed to investigate the hydrodynamic dimension of the imidazolium salts and silver complexes and, consequently, to gain information on the level of aggregation in solution. PGSE NMR studies were complemented by NOE NMR investigations in order to obtain information on anion-cation relative orientation within aggregates.     

Preparation of polymer-supported palladium/N-heterocyclic carbene complex for Suzuki cross-coupling reactions

A novel polymer-supported N-heterocyclic carbene (NHC) was prepared from chloromethyl polystyrene (CM PS) resin using a simple procedure, and was used as the ligand for palladium (Pd) catalysts. The polymer-supported Pd-NHC complexes efficiently catalyzed the Suzuki cross-coupling of aryl halides and phenylboronic acid in good yields and excellent purities under aqueous conditions.     

Salen and half-salen palladium(II) complexes: synthesis, characteriztion and catalytic activity toward Suzuki-Miyaura reaction

Salen and half-salen palladium(II) complexes (salden)Pd (1, salden=N,N′-bis(3,5-di- tert-butylsalicylidene)-1,2-dimethylethylenediamine), (hsalph)PdCl (2, hsalph=3,5-di-tert- butylsalicylidene-1-iminophenylene-2-amine), and (salph)Pd (4, salph=N,N′-bis(3,5-di-tert- butylsalicylidene)-1,2-phenylenediamine) were prepared and structurally characterized by X-ray crystallography. Complex 2 proved to exhibit high catalytic activity toward Suzuki-Miyaura reaction. Polyaromatic C₃-symmetric derivatives and various fluorinated biphenyl derivatives were readily achieved in good yields using Suzuki-Miyaura reaction catalyzed by complex 2.     

Synthesis and properties of new Mo(II) complexes with N-heterocyclic and ferrocenyl ligands

New Mo(II) complexes with 2,2′-dipyridylamine (L1), [Mo(CH₃CN)(η³-C₃H₅)(CO)₂(L1)]OTf (C1a) and [{MoBr(η³-C₃H₅)(CO)₂(L1)}₂(4,4′-bipy)](PF₆)₂ (C1b), with {[bis(2-pyridyl)amino]carbonyl}ferrocene (L2), [MoBr(η³-C₃H₅)(CO)₂(L2)] (C2), and with the new ligand N,N-bis(ferrocenecarbonyl)-2-aminopyridine (L3), [MoBr(η³-C₃H₅)(CO)₂(L3)] (C3), were prepared and characterized by FTIR and ¹H and ¹³C NMR spectroscopy. C1a, C1b, L3, and C2 were also structurally characterized by single crystal X-ray diffraction. The Mo(II) coordination sphere in all complexes features the facial arrangement of allyl and carbonyl ligands, with the axial isomer present in C1a and C2, and the equatorial in the binuclear C1b. In both C1a and C1b complexes, the L1 ligand is bonded to Mo(II) through the nitrogen atoms and the NH group is involved in hydrogen bonds. The X-ray single crystal structure of C2 shows that L2 is coordinated in a κ²-N,N-bidentate chelating fashion. Complex C3 was characterized as [MoBr(η³-C₃H₅)(CO)₂(L3)] with L3 acting as a κ²-N,O-bidentate ligand, based on the spectroscopic data, complemented by DFT calculations.The electrochemical behavior of the monoferrocenyl and diferrocenyl ligands L2 and L3 has been studied together with that of their Mo(II) complexes C2 and C3. As much as possible, the nature of the different redox changes has been confirmed by spectrophotometric measurements. The nature of the frontier orbitals, namely the localization of the HOMO in Mo for both in C2 and C3, was determined by DFT studies.     

Synthesis and structures of cyclopentadienyl N-heterocyclic carbene copper(I) complexes

A series of cyclopentadienyl N-heterocyclic carbene copper complexes CpCu(NHC) were synthesized and structurally characterized. The effect of the substituents at the nitrogen atom of the NHC ligands on the structures and thermally stability was discussed.     

Pincer-type bis(carbene)-derived complexes of nickel(II): Synthesis, structure, and catalytic activity

Air- and moisture-stable NHC (N-heterocyclic carbene)-derived CNC-type pincer complexes of nickel(II) 4a-d were successfully synthesized, and their structures were fully characterized using X-ray crystallography and analytical and spectroscopic methods. These complexes exhibit a high catalytic activity for the Suzuki-Miyaura coupling reaction of aryl bromides and chlorides with aryl- and alkenylboronic acids, providing an array of biphenyls and stilbenes generally in high yields.     

Copper-free Sonogashira cross-coupling reaction catalyzed by polymer-supported N-heterocyclic carbene palladium complex

A core-shell type of polymer-supported N-heterocyclic carbene (NHC) palladium catalyst was applied to Sonogashira cross-coupling reactions without copper cocatalyst under ambient atmosphere. This supported NHC-palladium complex efficiently catalyzed the copper-free Sonogashira reaction of various aryl iodides and bromides with terminal alkynes; the reaction exhibited high dependency on the temperature and the amount of base as well as its nature. In addition, this heterogeneous catalyst exhibited good reusability for the copper-free Sonogashira reaction.     

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.     

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 of chromium N-heterocyclic carbene complexes using chromium Fischer carbenes as a source of chromium carbonyls

Chromium Fischer carbene complexes, [Cr{OMe(R)}(CO)₅] have been utilized as a source of chromium carbonyls in the synthesis of chromium NHC complexes. Using the synthetic method, chromium complexes of various NHC ligands were isolated in reasonable yields. Moreover, the method can be employed for the synthesis of molybdenum and tungsten NHC compounds.     

Synthesis and characterization of Cu(I) and Cu(II) complexes containing ketiminate ligands

A series of Cu(I) and Cu(II) complexes containing substituted ketiminate ligands was synthesized. Reaction of CuCl₂ with 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] in toluene generated dark green solid of Cu[OC(Me)CHC(Me)N(Ar)]₂ (1). Similarly, Cu(I) complex, {Cu[OC(Me)CHC(Me)N(Ar)]Li[OC(Me)CHC(Me)N(Ar)]}₂ (2) was synthesized by reacting 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] with CuCl in toluene at room temperature for 12 h. While the reaction of CuCl with Li[OC(Me)CHC(Me)N(Ar)] in the presence of triphenylphosphine in THF solution at room temperature, a three-coordinated Cu[OC(Me)CHC(Me)N(Ar)](PPh₃) (3) can be isolated in high yield. Replacing the PPh₃ of 3 with N-heterocarbene (NHC) generates Cu[OC(Me)CHC(Me)N(Ar)](NHC) (4) in low yield. Complexes 2, 3, and 4 were characterized by ¹H and ¹³C NMR spectroscopies and all molecules were structurally characterized by X-ray diffractometry. Two coordination modes of ketiminate ligands were found in the molecular structure of 2, one of which is copper-coordinated terminal ketiminates and the other is lithium-copper-coordinated bridging ketiminates.