Synthesis and structural characterisation of rhodium hydride complexes bearing N-heterocyclic carbene ligands

Addition of excesses of N-heterocyclic carbenes (NHCs) IEt₂Me₂, IⁱPr₂Me₂ or ICy (IEt₂Me₂ = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene; IⁱPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; ICy = 1,3-dicyclohexylimidazol-2-ylidene) to [HRh(PPh₃)₄] (1) affords an isomeric mixture of [HRh(NHC)(PPh₃)₂] (NHC = IEt₂Me₂ (cis-/trans-2), IⁱPr₂Me₂ (cis-/trans-3), ICy (cis-/trans-4) and [HRh(NHC)₂(PPh₃)] (IEt₂Me₂(cis-/trans-5), IⁱPr₂Me₂ (cis-/trans-6), ICy (cis-/trans-7)). Thermolysis of 1 with the aryl substituted NHC, 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH₂), affords the bridging hydrido phosphido dimer, [{(PPh₃)₂Rh}₂(μ-H)(μ-PPh₂)] (8), which is also the reaction product formed in the absence of carbene. When the rhodium precursor was changed from 1 to [HRh(CO)(PPh₃)₃] (9) and treated with either IMes (=1,3-dimesitylimidazol-2-ylidene) or ICy, the bis-NHC complexes trans-[HRh(CO)(IMes)₂] (10) and trans-[HRh(CO)(ICy)₂] (11) were formed. In contrast, the reaction of 9 with IⁱPr₂Me₂ gave [HRh(CO)(IⁱPr₂Me₂)₂] (cis-/trans-12) and the unusual unsymmetrical dimer, [(PPh₃)₂Rh(μ-CO)₂Rh(IⁱPr₂Me₂)₂] (13). The complexes trans-3, 8, 10 and 13 have been structurally characterised.     

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.     

Synthesis and X-ray crystallographic structures of [HGa(NMe2)2]2 and [PhGa(NHNMe2)2]2, and room-temperature conversions of [HGa(NMe2)2]2 to (HGaNH)n and (HGaNMe)n

The reactivity of bis(dimethylamido) complexes of phenyl- and hydridogallium with ammonia, dimethylamine and 1,1-dimethylhydrazine is described. Synthesis of the starting gallium hydride, [HGa(NMe₂)₂]₂, was achieved in nearly quantitative yield from the reaction of HGaCl₂(quinuclidine) with LiNMe₂. In neat ammonia or methylamine at room temperature both dimethylamido ligands in [HGa(NMe₂)₂]₂ were substituted by a single equivalent of NH₃ or MeNH₂ to produce amorphous (HGaNH)_n or (HGaNMe)_n, respectively. In contrast, the reaction of [PhGa(NMe₂)₂]₂ with neat Me₂NNH₂, at room temperature consumed two equivalents of the substituted hydrazine to form [PhGa(NHNMe₂)₂]₂ in a 73% yield. Single crystal X-ray crystallographic analyses of [HGa(NMe₂)₂]₂ and [PhGa(NHNMe₂)₂]₂ establish that in the solid state both compounds adopt a cyclic Ga-N-Ga-N structure with a crystallographic center of symmetry located at the center of the ring.     

N-heterocyclic carbene catalyzed domino reactions

While organocatalyzed domino reactions or "organocascade catalysis" developed into an important tool in synthetic chemistry during the past decade, the utility of N-heterocyclic carbenes (NHCs) as catalysts in domino reactions has only received growing attention in the past three years. Taking into account the unique activation modes of the substrates by NHC catalysts, it is often difficult to distinguish between a single chemical transformation and a sequential one-pot transformation. Therefore, herein we present a critical consideration of domino, cascade, and tandem catalysis in the case of NHC catalysts and highlight recent publications in this area.     

Oxidative addition of an imidazolium cation to an anionic gallium(I) N-heterocyclic carbene analogue: Synthesis and characterisation of novel gallium hydride complexes

The reactions of N-heterocyclic carbenes and imidazolium salts towards an anionic gallium(I) heterocycle, [:Ga{[N(Ar)C(H)]₂}]⁻, , have been studied. No reactions with N-heterocyclic carbenes were observed, though the reaction of the gallium heterocycle with the imidazolium salt, [HC{N(Mes)C(H)}₂]Cl, IMesHCl, Mes = C₆H₂Me₃-2,4,6, led to oxidative insertion of the Ga(I) centre into the imidazolium C-H bond and formation of the gallium hydride complex, [HGa{[N(Ar)C(H)]₂}(IMes)]. When this reaction was carried out in the presence of traces of water, partial hydrolysis of [HGa{[N(Ar)C(H)]₂}(IMes)] resulted in the formation of the hydroxy-bridged, anionic gallium hydride complex, [{HGa[N(Ar)C(H)]₂}₂OH][(IMes)₂H]. Both compounds have been spectroscopically and structurally characterised.     

The structures,stability, and photoelectron spectroscopy of GaPX (X = C,Si, Ge; O,S; P and Ga) clusters

The equilibrium geometries and vibrational frequencies of GaPX⁻ and GaPX (X = C, Si, Ge; O, S; P and Ga) have been studied by hybrid B3LYP functional at cc-PVTZ and aug-cc-PVTZ levels. The results predict that the most stable structure of GaPC⁻ is linear while the others are trigonal. As for GaPX (X = C, Si, Ge; O, S; P and Ga), the ground structures of GaPC and GaPO are linear while the others are trigonal. The adiabatic electron affinities (AEAs) and vertical detachment energies (VDEs) of GaPX⁻ are calculated at B3LYP/aug-cc-PVTZ level. And the order of the AEAs and VDEs of GaPX⁻ are C < O < Ge ≈ Si < P < S < Ga and C < Ge ≈ Si < P < O < S < Ga, respectively. GaPC exhibits the lowest adiabatic electron affinities of all the clusters studied, indicating a particularly stable neutral species.     

Probing the aromaticity in 2,3-pyrido-annulated N-heterocyclic carbene and its heavier analogues

The aromaticity in 2,3-pyrido-annulated 1,3,2λ²-diazatetroles C₅H₃N(NR)₂E^II (E^II = C, Si, Ge, Sn, Pb) was studied using a set of experimental and calculated criteria: UV-VIS, Raman, ISE, NICS, GIMIC and EDDB. The data obtained indicate either a slight decrease in aromaticity (NICS, GIMIC, ISE methods) or equal aromaticity (UV-VIS, ISE methods) compared to benzo-annulated analogues C₆H₄(NR)₂E. The π-aromaticity increases down the group from Si to Pb.     

Synthesis and reactions of N-heterocyclic carbene boranes

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit "borane chemistry", but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.     

Toward N-heterocyclic carbene stabilized zinc sulfides

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Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: synthesis, structure, and catalytic activity

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy(3))(2)Cl(2)Ru==CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60 %. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.     

Comparative experimental and EXAFS studies in the Mizoroki-Heck reaction with heteroatom-functionalised N-heterocyclic carbene palladium catalysts

A study on the Mizoroki-Heck coupling of selected aryl bromides with acrylates catalysed by a series of Pd complexes of bidentate pyridyl-, picolyl-, diphenylphosphinoethyl- and diphenylphosphinomethyl-functionalised N-heterocyclic carbene (NHC) is reported. The observed activity is dependent on the type of solvent and base used and the nature of the "classical" donors of the mixed-donor bidentate ligand and its bite angle. A mechanistic model is presented for the pyridine-functionalised NHC complexes based on an in situ EXAFS study under dilute catalyst conditions (2 mM Pd). The model involves pre-dissociation of the pyridine functionality and oxidative addition of ArBr in the early stages of the reaction, as well as formation of monomeric and dimeric Pd species at the time of substrate conversion.     

Adsorption of pyridine on a polycrystalline gold electrode surface studied by infrared reflection absorption spectroscopy

The adsorption behavior of pyridine on a smooth polycrystalline gold electrode surface was investigated over a wide wavenumber region (2000–500 cm⁻¹) by in situ infrared reflection absorption spectroscopy (IRAS). The reversible adsorption/desorption of pyridine was observed upon the change in applied electrode potential, and the adsorption state at positive potentials was found to depend strongly on the kind of halide ion used as a supporting electrolyte. Symmetry analysis of absorption bands observed revealed that pyridine molecules adsorb with the molecular axis (C₂ axis) perpendicular to the electrode surface (vertical configuration) at positive potentials in 0.5 M KF, KCl and KBr solutions. A band due to the out-of-plane bending mode of the adsorbed pyridine molecule was observed at potentials more negative than ca. 0 V for 0.5 M KF solution containing 100 mM pyridine. We concluded that even in the 100 mM pyridine solution, adsorbed pyridine forms a monolayer and that the molecules reorient from a flat (parallel) to the vertical configuration as the potential becomes less negative. No bands due to adsorbed pyridine were detected for 0.5 M KI solution. The amount of adsorbed pyridine was found to depend strongly on the strength of specific adsorption of halide ions.     

Synthesis and reactivity of alkyl-palladium N-heterocyclic carbene complexes

The transamination of alkyl-palladium halide N-heterocyclic carbene complexes has enabled the isolation of products that reveal interesting insights into the factors which might be barriers to the development of a palladium-catalysed alkyl-amination reaction.     

N-heterocyclic carbene stabilized copper- and silver-phenylchalcogenolate ring complexes

The ligation of a N-heterocyclic carbene (NHC) to group 11 metal salts (Cu, Ag) was explored as an alternative to PR(3) ligands for the formation of copper- and silver-chalcogenolate cluster complexes. AgOAc and CuCl salts ligate with the NHC 1,3-di-isopropylbenzimidazole-2-ylidene ((i)Pr(2)-bimy) forming [Ag(OAc)((i)Pr(2)-bimy)] 1, [Ag(OAc)((i)Pr(2)-bimy)(2)] 2, [CuCl((i)Pr(2)-bimy)](2)3 and [CuCl((i)Pr(2)-bimy)(2)] 4 depending on the ratio of ligand to metal used. These have been characterized via spectroscopic and crystallographic methods. Complexes 1 and 3 were reacted with S(Ph)SiMe(3) and Se(Ph)SiMe(3) to form the polynuclear metal-chalcogenolates [Ag(4)(μ-EPh)(4)((i)Pr(2)-bimy)(4)] (5, E = S; 6, E = Se) and [Cu(3)(μ-EPh)(3)((i)Pr(2)-bimy)(3)] (7, E = S; 8, E = Se) in good yields. The structures of 5-8, as determined by single crystal X-ray crystallography, are described.     

N-heterocyclic carbene enabled synthesis of conjugated polymers

The Suzuki polycondensation of a dihalogenated 4,5-diphenylimidazole (2) with a fluorenyl diboronic acid diester followed by methylation afforded a conjugated poly(imidazolium) copolymer (P2) in 93% yield. Upon exposure to strong base, P2 was converted in situ to the corresponding poly(N-heterocyclic carbene) P3, as evidenced by ¹H NMR spectroscopy and a trapping experiment involving sulfur that afforded the corresponding poly(thiourea) P4. Similarly, treating a solution of P2 with KOtBu and [Ir(1,5-cyclooctadiene)Cl]₂ afforded a conjugated polymer bearing pendant Ir complexes (P5) in 63% yield. Thermal and photophysical analyses of the aforementioned polymers revealed that they were thermally stable with tunable fluorescence properties, features which poise them for use in various electronic and sensing applications. The presented methodology is expected to facilitate the synthesis of a broad range conjugated organometallic polymers from a common and readily accessible precursor.     

Hybrid ligands with N-heterocyclic carbene and chiral phosphaferrocene components

N-Heterocyclic carbenes (NHCs) possessing one or two 3,4-dimethylphosphaferrocenyl substituents and either methylene or ethylene alkyl bridges have been prepared. These carbenes turned out to be remarkably stable and were characterized by NMR methods and partly by mass spectrometry. Their molybdenum and ruthenium complexes were examined in order to determine the electronic properties and the coordination behaviour of these chiral PC- and PCP-chelate ligands, which combine a NHC unit as a strong sigma-donor with pi-accepting phosphaferrocene moieties. Crystal structures of one ligand precursor and of three complexes have been determined.     

A stable anionic N-heterocyclic carbene and its zwitterionic complexes

Pyrimidinium beta?nes (1), readily accessible via a straightforward modular synthesis from a formamidine and a monosubstituted malonic acid, are readily deprotonated by nBuLi (or KHMDS) to give the stable carbene species [2]Li+ (abbreviated as maloNHC). The latter represents the archetype of a subgroup of N-heterocyclic carbenes incorporating a malonate as remote anionic functional group within their heterocyclic backbone. While playing the dual role of monodentate 2 e- L type donor and noncoordinating charge carrier X, such ligands are seen to provide a rational route to zwitterionic complexes, as illustrated here by three examples (Rh, Fe, Ag). In particular, the reaction of [2]Li+ with [RhCl(1,5-COD)]2 produces the neutral 14 e- complex Rh(maloNHC)(COD) (3) in which coordinative unsaturation at the metal is relieved in the solid state by an uncommon labile bonding interaction between the Cipso of one of the mesityl arms and the Rh center.     

N-heterocyclic carbene tethered amido complexes of palladium and platinum

With a view to applications in bifunctional catalysis, a modular cross-coupling strategy has been used to prepare amine bis(imidazolium) salts (3a and 3b) and an amine mono(imidazolium) salt (6) as precursors to chelating amido-NHC ligands. Treating the pro-ligands 3 with 3 equivalents of the bulky base KHMDS and Pd(OAc)(2) or PtCl(2)(COD) gave the four amido bis(N-heterocyclic carbene) pincer complexes [CNC-R]M-I [M = Pd (7) or Pt (8); R = i-Pr (a) or n-Bu (b)], including the first examples of platinum complexes of a CNC ligand. The reaction of 7a with AgOTf in pyridine gave the cationic complex {[CNC-i-Pr]Pd-py}OTf (9a). Heating a mixture of amine mono(imidazolium) salt 6 with PdCl(2) or K(2)PtCl(4), K(2)CO(3) and KI in pyridine at 100 °C gave the complexes [C,NH]MI(2)py [M = Pd (10) or Pt (11)], in which the amine arm of the NHC ligand is not deprotonated and does not coordinate to the metal. For a solution of 10 in 1,4-dioxane, deprotonation of the amine occurred in a biphasic reaction with aqueous KOH at 40 °C, giving the dimeric amido complex {[C,N]Pd(μ-OH)}(2) (12). The more inert Pt analogue 11 was unreactive under the same conditions. Solid-state structures of the complexes 7a, 7b, 9a, 10, 11 and 12 have been determined by single crystal X-ray diffraction.