Bridge functionalized bis-N-heterocyclic carbene rhodium(I) complexes and their application in catalytic hydrosilylation

A series of chelating bridge functionalized bis-N-heterocyclic carbenes (NHC) complexes of rhodium (I) were prepared by reacting the corresponding imidazolium salts with [Rh(COD)Cl]₂ in an in-situ reaction. For the N-methyl substituted complex with a PF₆-anion an X-ray crystal structure was exemplary obtained. All complexes were spectroscopically characterized and tested for the hydrosilylation of acetophenone.     

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 of rhodium N-heterocyclic carbene complexes and their catalytic activity in the hydrosilylation of alkenes in ionic liquid medium

Rhodium complexes bearing N-heterocyclic carbene (NHC) ligands were prepared from bis(η⁴-1,5-cyclooctadiene) dichlorodirhodium and 1-alkyl-3-methylimidazolium-2-carboxylate, and the catalytic properties of rhodium complexes prepared in the hydrosilylation of alkenes in ionic liquid media were investigated. It was found that both the catalytic activity and selectivity of the rhodium complexes bearing NHC ligands were influenced by the attached substituents of the imidazolium cation. Additionally, rhodium complexes bearing NHC ligands in ionic liquid BMimPF₆ could be reused without noticeable loss of catalytic activity and selectivity.     

Synthesis of novel poly(ethylene glycol)‐containing imidazolium‐functionalized phosphine ligands and their application in the hydrosilylation of olefins

A series of polyethylene glycol‐containing imidazolium‐functionalized phosphine ligands (mPEG‐im‐PPh₂) were successfully synthesized and used in the rhodium‐catalyzed hydrosilylation of olefins. The results indicate that the RhCl₃/mPEG‐im‐PPh₂ catalytic system exhibits both excellent activity and selectivity for the β‐adduct. In addition, the catalytic system may be recycled at least six times.     

Chelating alkoxy NHC–Rh(I) complexes and their applications in the arylation of aldehydes

In this study, new rhodium(I) complexes (5 and 6) have been prepared by the reaction of [RhCl(COD)]₂ with a series of imidazolium salts (3 and 4), which were obtained from a chiral amino alcohol. The catalytic activities of these complexes were tested in the arylation of aldehydes. It was found that the synthesized rhodium complexes were highly effective catalysts for the arylation of aldehydes in short reaction times (5 min, TOF = 1193 h⁻¹). However, the obtained ee values (up to 32% ee) remained low. We have proposed a mechanism for the arylation reaction of aldehydes, which is confirmed via ¹⁹F NMR spectroscopy.     

Palladium complexes with ethylene-bridged bis(N-heterocyclic carbene) for C-C coupling reactions

A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)₂ produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs.     

Impact of Substituents Attached to N-Heterocyclic Carbenes on the Catalytic Activity of Copper Complexes in the Reduction of Carbonyl Compounds with Triethoxysilane

By using functionalized imidazolium salts such as 1‐allyl‐3‐alkylimidazolium or 1‐alkyl‐3‐vinylimidazolium salts as carbene ligand precursors, the reduction of aryl ketones with triethoxysilane may be catalyzed by copper salt/imidazolium salt/KO^tBu systems. The functional substituents attached to the N‐heterocyclic carbene (NHC) serve to enhance the catalytic activity. Different copper salts also have an effect on the catalytic activity, with copper(II) acetate monohydrate being superior to copper(I) chloride.     

N-heterocyclic carbene complexes of palladium and rhodium: cis/trans-isomers

Stable carbene complexes of palladium or rhodium are readily accessible by (i) reaction of imidazolium or triazolium salts with palladium complexes bearing basic ligands or rhodium alkoxide complexes, (ii) adduct formation of the free carbene, e.g. 1,3-dimethylimidazoline-2-ylidene, with metal compounds. In the case of palladium(II) and rhodium(I), the resulting complexes show cis/trans-isomerization and can be compared to analogous phosphine complexes.     

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.     

Tridentate, anionic tethered N-heterocyclic carbene of Pd(II) complexes

The synthesis of a series of azolium salts such as azolium iodides and chlorides having both N-anionic functional group and N-alkyl group have been developed. Reaction of azolium iodides or chlorides with Ag₂O gave the corresponding NHC-Ag complexes. It was found that the resulting NHC-Ag complexes derived from azolium iodides or chlorides differ in their physical properties. The azolium chlorides as well as azolium iodides were successfully converted into the NHC-Ag complexes, which subsequently reacted with PdCl₂(CH₃CN)₂ to give the anionic amidate/NHC-Pd complexes. Thus, a variety of the NHC-Pd complexes could be obtained from benzimidazolium and imidazolium salts.     

Modification of (Poly)Siloxanes via Hydrosilylation Catalyzed by Rhodium Complex in Ionic Liquids

Summary. The hydrosilylation of 1-heptene, allyl glycidyl ether and, allyl polyether by heptamethylhydrotrisiloxane and poly(hydro, methyl)(dimethyl)siloxane catalyzed by rhodium(I) complexes (particularly [{Rh(μ–OSiMe₃)(cod)}₂]) in imidazolium ionic liquids (especially [TriMIM]MeSO₄) gives heptyl and glycidyloxy functional (poly)siloxanes and silicone polyethers with high yield and selectivity. The catalytic system based on rhodium siloxide can be easily separated from the product and successfully reused up to five times.     

Modification of (Poly)Siloxanes via Hydrosilylation Catalyzed by Rhodium Complex in Ionic Liquids

The hydrosilylation of 1-heptene, allyl glycidyl ether and, allyl polyether by heptamethylhydrotrisiloxane and poly(hydro, methyl)(dimethyl)siloxane catalyzed by rhodium(I) complexes (particularly [{Rh(μ–OSiMe₃)(cod)}₂]) in imidazolium ionic liquids (especially [TriMIM]MeSO₄) gives heptyl and glycidyloxy functional (poly)siloxanes and silicone polyethers with high yield and selectivity. The catalytic system based on rhodium siloxide can be easily separated from the product and successfully reused up to five times.     

A New Class of Task-Specific Imidazolium Salts and Ionic Liquids and Their Corresponding Transition-Metal Complexes for Immobilization on Electrochemically Active Surfaces

Adding to the versatile class of ionic liquids, we report the detailed structure and property analysis of a new class of asymmetrically substituted imidazolium salts, offering interesting thermal characteristics, such as liquid crystalline behavior, polymorphism or glass transitions. A scalable general synthetic procedure for N-polyaryl-N’-alkyl-functionalized imidazolium salts with para-substituted linker (L) moieties at the aryl chain, namely [LPh_mIm^HR]⁺ (L=Br, CN, SMe, CO₂Et, OH; m=2, 3; R=C₁₂, PEG_n; n=2, 3, 4), was developed. These imidazolium salts were studied by single-crystal X-ray diffraction (SC-XRD), NMR spectroscopy and thermochemical methods (DSC, TGA). Furthermore, these imidazolium salts were used as N-heterocyclic carbene (NHC) ligand precursors for mononuclear, first-row transition metal complexes (Mn^II, Fe^II, Co^II, Ni^II, Zn^II, Cu^I, Ag^I, Au^I) and for the dinuclear Ti-supported Fe-NHC complex [(OPy)₂Ti(OPh₂ImC₁₂)₂(FeI₂)] (OPy=pyridin-2-ylmethanolate). The complexes were studied concerning their structural and magnetic behavior via multi-nuclear NMR spectroscopy, SC-XRD analyses, variable temperature and field-dependent (VT-VF) SQUID magnetization methods, X-band EPR spectroscopy and, where appropriate, zero-field ⁵⁷Fe Mössbauer spectroscopy.     

Facile Synthesis of Triptycene-Azolium Salts and NHC-Metal Complexes

The cycloaddition reactions of eleven substituted anthracenes with nosylated quinone imines provides a convenient route to the respective triptycenes. Following re-aromatization, selective O-butylation and cleavage of the nosyl-group the respective triptycene anilines are obtained, which are converted into the respective imidazolium salts according to established procedures. Deprotonation of the imidazolium salts provide new triptycene-NHC-metal complexes (M=AuCl, RhCl(cod), IrCl(cod), RhCl(CO)₂, IrCl(CO)₂, PdI₂(py), PtCl₂(py), Pd(allyl)Cl) with unusual ligand sterics.     

乙酸功能化离子液体作为高效、可回收的催化剂用于2-萘酚与芳醛缩合反应(英文)

An efficient solvent‐free protocol for the synthesis of 14‐aryl‐14H‐dibenzo[a,j]xanthenes from the condensation of 2‐naphthol with arylaldehydes, using acetic acid functionalized imidazolium salts (1‐carboxymethyl‐3‐methylimidazolium bromide ([cmmim]Br) and 1‐carboxymethy1‐3-me-thylimidazolium tetrafluoroborate([cmmim]BF4) as reusable catalysts, has been developed. The turn over frequency on the catalysts is several times higher than the other previously reported catalysts. Also, thermal gravimetric analysis and powder X‐ray diffraction pattern of the catalysts have been studied.     

Synthesis of Functionalized (η5‐Indenyl)rhodium(III) Complexes and Their Application to C−H Bond Functionalization

It has been established that reductive complexation of functionalized benzofulvenes, which are readily prepared from commercially available indene and 2‐methylindene, with RhCl₃ in ethanol affords the corresponding indenyl–rhodium(III) dichlorides bearing substituents at the 1‐ (H or CO₂Et), 2‐ (H or Me), and 3‐ [CH₂Ph or CH₂(2‐MeOC₆H₄)] positions. The indenyl–rhodium(III) complexes bearing one ethoxycarbonyl group showed higher thermal stability and regioselectivity than our previously reported Cp^ERh^III complex toward the oxidative [3+2] annulation of acetanilides with internal alkynes.     

Alcohol‐Induced C−N Bond Cleavage of Cyclometalated N‐Heterocyclic Carbene Ligands with a Methylene‐Linked Pendant Imidazolium Ring

Reaction of the pentamethylcyclopentadienyl rhodium iodide dimer [Cp*RhI₂]₂ with 1,1′‐diphenyl‐3,3′‐methylenediimidazolium diiodide in non‐alcohol solvents, in the presence of base, led to the formation of bis‐carbene complex [Cp*Rh(bis‐NHC)I]I (bis‐NHC=1,1′‐diphenyl‐4,4′‐methylenediimidazoline‐5,5′‐diylidene). In contrast, when employing alcohols as the solvent in the same reaction, cleavage of a methylene C?N bond is observed, affording ether‐functionalized (cyclometalated) carbene ligands coordinated to the metal center and the concomitant formation of complexes with a coordinated imidazole ligand. Studies employing other 1,1′‐diimidazolium salts indicate that the cyclometalation step is a prerequisite for the activation/scission of the C?N bond and, based on additional experimental data, a S_N2 mechanism for the reaction is tentatively proposed.     

Click Ionic Liquids: A Family of Promising Tunable Solvents and Application in Suzuki–Miyaura Cross‐Coupling

A series of click ionic salts 4 a – 4 n was prepared through click reaction of organic azides with alkyne‐functionalized imidazolium or 2‐methylimidazolium salts, followed by metathesis with lithium bis(trifluoromethanesulfonyl)amide or potassium hexafluorophosphate. All salts were characterized by IR, NMR, TGA, and DSC, and most of them can be classified as ionic liquids. Their steric and electronic properties can be easily tuned and modified through variation of the aromatic or aliphatic substituents at the imidazolium and/or triazolyl rings. The effect of anions and substituents at the two rings on the physicochemical properties was investigated. The charge and orbital distributions based on the optimized structures of cations in the salts were calculated. Reaction of 4 a with PdCl₂ produced mononuclear click complex 4 a‐Pd , the structure of which was confirmed by single‐crystal X‐ray diffraction analysis. Suzuki–Miyaura cross‐coupling shows good catalytic stability and high recyclability in the presence of PdCl₂ in 4 a . TEM and XPS analyses show formation of palladium nanoparticles after the reaction. The palladium NPs in 4 a are immobilized by the synergetic effect of coordination and electrostatic interactions with 1,2,3‐triazolyl and imidazolium, respectively.     

Grignard allylic substitution catalyzed by imidazol-2-ylidene- and imidazol-4-ylidene-magnesium complexes

In the presence of a catalytic amount of 1,2-disubstituted or 1,2,3-trisubstituted imidazolium salts, γ-substituted allyl chlorides reacted with alkyl Grignard reagents to undergo substitution reactions in an S_N2′-selective fashion, where the magnesium ate complexes [(N-heterocyclic carbene-MgR₃)⁻(MgX)⁺] of imidazol-2-ylidenes or imidazol-4-ylidenes, generated in situ, were postulated as the active species. It was observed that the reactions with imidazol-4-ylidene catalysts were faster than those with imidazol-2-ylidenes. Enantioselective catalysis using a chiral imidazolium salt was preliminarily investigated.     

Direct in situ synthesis of cationic N-heterocyclic carbene iridium and rhodium complexes from neat ionic liquid: Application in catalytic dehydrogenation of cyclooctadiene

A direct synthetic route to cationic N-heterocyclic carbene (NHC) complexes of rhodium and iridium from neat dialkyl-imidazolium ionic liquids (ILs) has been found. The method uses complexes bearing basic anionic ligands, [M(COD)(PPh₃)X], X = OEt, MeCO₂, which react with the inactivated imidazolium cation in the absence of external bases yielding one M-NHC moiety and the free protonated base. This new one-pot synthesis leaving pure, catalytically active IL solutions is faster, cleaner and more efficient than traditional syntheses of such NHC complexes. The observed reactivity also gives insight into NHC incorporation of rhodium and iridium catalyzed reactions performed in common dialkyl-imidazolium ILs.The complexes synthesised in this manner are compared with their bis-phosphine analogues in terms of activity for catalytic dehydrogenation of 1,5-cyclooctadiene and 1,3-cyclooctadiene in neat [BMIM][NTf₂] as solvent. Even at high temperature, no ligand exchange reaction is observed with [(COD)M(PPh₃)₂] [NTf₂] catalysts. As expected, the yields of all the reactions were low, iridium was much more active in C-H activation than rhodium and the NHC ligands were more stable than triphenylphosphine. For all catalysts, the isomerisation of 1,5-cyclooctadiene is the major reaction. However, the phosphine-NHC complex of iridium seems to be more selective for dehydrogenation than its bis-phosphine counterpart, which is more active in transfer-hydrogenation and less stable under the applied conditions. Different reaction conditions were tried in order to optimise selectivity for dehydrogenation over isomerisation and transfer-hydrogenation. Surprisingly, with 1,3-cyclooctadiene as substrate selectivity for dehydrogenation is much higher than with 1,5-cyclooctadiene for all catalysts.