Mechanism of the aerobic oxidation of alcohols by palladium complexes of N-heterocyclic carbenes

Quantum mechanics (B3LYP density functional theory) combined with solvation (Poisson-Boltzmann polarizable continuum solvent model) was used to investigate six mechanisms for the aerobic oxidation of alcohols catalyzed by (NHC)Pd(carboxylate)(2)(H(2)O) complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Of these, we find that "reductive beta-hydride elimination", in which the beta-hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provides the lowest-energy route and explains the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pK(a). The traditional beta-hydride elimination mechanism cannot be responsible for the experimentally observed kinetic parameters, which we find could result from the subsequent reductive elimination of acetic acid, which yields a slightly higher calculated activation barrier. Reversible beta-hydride elimination may provide a mechanism for the racemization of chiral alcohols, which would undermine attempts at an enantioselective oxidation. Competition among these pathways can be influenced by changing the electronic properties of the carboxylate and substrate.     

Rhenium(V) oxo complexes with N-heterocyclic carbenes

Air-stable rhenium(V) oxo complexes are formed when [ReOCl(3)(PPh(3))(2)] is treated with N-heterocyclic carbenes of the 1,3-dialkyl-4,5-dimethylimidazol-2-ylidene type, L(R) (R = Me, Et, i-Pr). Complexes of the compositions [ReO(2)(L(R))(4)](+), [ReOCl(L(R))(4)](2+), or [ReO(OMe)(L(R))(4)](2+) can be isolated depending on the alkyl substituents at the nitrogen atoms of the ligands and the reaction conditions applied. Despite the steric overcrowding of the equatorial coordination spheres of the metal atoms by each of the four carbene ligands, stable complexes with six-coordinate rhenium atoms are obtained. Steric demands of the alkyl groups allow control of the stability of the mono-oxo intermediates. Air-stable cationic complexes of the compositions [ReOCl(L(Me))(4)](2+), [ReOCl(L(Et))(4)](2+), and [ReO(OMe)(L(Me))(4)](2+) have been isolated, whereas reactions of [ReOCl(3)(PPh(3))(2)] or other rhenium(V) precursors with the more bulky 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (L(i)(-)(Pr)) directly yield the dioxo complex [ReO(2)(L(i)(-)(Pr))(4)](+). X-ray structures of [ReO(2)(L(i)(-)(Pr))(4)][ReO(4)], [ReO(2)(L(i)(-)(Pr))(4)][PF(6)], [ReO(2)(L(Me))(4)][ReO(4)](0.45)[PF(6)](0.55), [ReO(MeOH)(L(Me))(4)][PF(6)](2), and [ReOCl(L(Et))(4)][PF(6)](2) show that the equatorial coordination spheres of the rhenium atoms are essentially planar irrespective of the steric demands of the individual carbene ligands.     

Unusual reactivity of molecular oxygen with pi-allylnickel(N-heterocyclic carbene) chloride complexes

One-pot preparation and characterization of the first reported nickel(II) complexes containing a single imidazol-2-ylidene ligand are reported. Subsequent treatment of these complexes with molecular oxygen at room temperature results in rapid formation of a mu-hydroxo dimer and alpha,beta-unsaturated carbonyl compounds. Mechanistic investigations indicate that this reaction proceeds via reversible oxygen binding followed by rate-limiting decomposition of the resulting metal-oxygen species.     

pi-Face donor properties of N-heterocyclic carbenes in Grubbs II complexes

The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.     

Tuning N-heterocyclic carbenes in T-shaped Pt(II) complexes for intermolecular C-H bond activation of arenes

Small change matters: T-shaped Pt(II) complexes with less flexible substituents, than, for example, isopropyl or tert-butyl groups, on N-heterocyclic carbene (NHC) ligands allow for C-H bond activation reactions of aromatic compounds (see scheme; BAr(f)(4)(-) =tetrakis[(3,5-trifluoromethyl)phenyl]borate; F yellow, Pt red). NHC substituents that are not highly branched prevent agostic interactions and reduce the barriers to achieve the C-H bond cleavage.     

Synthesis of Ru(II) complexes of N-heterocyclic carbenes and their promising photoluminescence properties in water

Novel complexes 1 and 2 based on N-heterocyclic carbenes, which are analogous to Ru(bpy)(3)(2+) and Ru(terpy)(2)(2+), respectively, were synthesized. The complex, which is analogous to Ru(terpy)(2)(2+), exhibited promising photoluminescence properties with a long lifetime of 820 ns in acetonitrile and 3100 ns in water at room temperature, respectively. In addition, ab initio calculations were carried out.     

Dual reactivity of N-heterocyclic carbenes towards copper(II) salts

Complexes of N-heterocyclic carbenes (NHCs) with copper(II) halogenides are unstable. Upon formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted copper(II) NHC complexes are fairly stable. A series of new five-, six- and seven-membered ring NHC complexes of Cu(OAc)(2) have been synthesised and characterised in the solid state.     

Gold(I) complexes bearing mixed-donor ligands derived from N-heterocyclic carbenes

The new 2-phenylthiocarbamoyl-1,3-dimesitylimidazolium inner salt (IMes·CSNPh) reacts with [AuCl(L)] in the presence of NH(4)PF(6) to yield [(L)Au(SCNPh·IMes)](+) (L = PMe(3), PPh(3), PCy(3), CNBu(t)). The carbene-containing precursor [(IDip)AuCl] reacts with IMes·CSNPh under the same conditions to afford the complex [(IDip)Au(SCNPh·IMes)](+) (IDip = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Treatment of the diphosphine complex [(dppm)(AuCl)(2)] with one equivalent of IMes·CSNPh yields the digold metallacycle, [(dppm)Au(2)(SCNPh·IMes)](2+), while reaction of [L(2)(AuCl)(2)] with two equivalents of IMes·CSNPh results in [(L(2)){Au(SCNPh·IMes)}(2)](2+) (L(2) = dppb, dppf, or dppa; dppb = 1,4-bis(diphenylphosphino)butane, dppf = 1,1'-bis(diphenylphosphino)ferrocene, dppa = 1,4-bis(diphenylphosphino)acetylene). The homoleptic complex [Au(SCNPh·IMes)(2)](+) is formed on reaction of [AuCl(tht)] (tht = tetrahydrothiophene) with two equivalents of the imidazolium-2-phenylthiocarbamoyl ligand. This product reacts with AgOTf to yield the mixed metal compound [AuAg(SCNPh·IMes)(2)](2+). Over time, the unusual trimetallic complex [Au(AgOTf)(2)(SCNPh·IMes)(2)](+) is formed. The sulfur-oxygen mixed-donor ligands IMes·COS and SIMes·COS (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) were used to prepare [(L)Au(SOC·IMes)](+) and [(L)Au(SOC·SIMes)](+) from [(L)AuCl] (L = PPh(3), CN(t)Bu). The bimetallic examples [(dppf){Au(SOC·IMes)}(2)](2+) and [(dppf){Au(SOC·SIMes)}(2)](2+) were synthesized from the reaction of [(dppf)(AuCl)(2)] with the appropriate ligand. Reaction of [(tht)AuCl] with one equivalent of IMes·COS or SIMes·COS yields [Au(SOC·IMes)(2)](+) and [Au(SOC·SIMes)(2)](+), respectively. The compounds [(Ph(3)P)Au(SCNPh·IMes)]PF(6), [(Cy(3)P)Au(SCNPh·IMes)]PF(6) and [Au(AgOTf)(2)(SCNPh·IMes)(2)]OTf were characterized crystallographically.     

The solution stability of copper(I) and silver(I) complexes with N-heterocyclic carbenes

The tris-benzimidazolium cage LH(3)(3+), in MeCN solution, in the presence of OH(-), forms with Cu(I) and Ag(I) ions complexes of formula [M(I)(LH)](2+), in which each metal is linearly coordinated by two carbenes and one imidazolium N-H fragment remains intact. To achieve two-coordination, the two N-heterocyclic moieties of the cage make a saloon-door type motion, with a conformationally costless rotation of ca. 30° each. The two [M(I)(LH)](2+) complexes show high thermodynamic stability and are inert with respect to metal substitution, due to the mechanical constraints imposed by the ligating framework. Complexation with Cu(I) and Ag(I) with the reference unidentate carbene ligand Q, derived from the benzimidazolium precursor QH(+), was studied for comparison. Both metals in MeCN form 1:1 and 1:2 complexes with the carbene ligand Q according to two stepwise equilibria. Q complexes of both metals are labile with respect to metal substitution and those of Ag(I) are more stable than those of Cu(I). A significant cooperative effect has been observed with the formation of the [Ag(I)Q(2)](+) complex.     

Palladium(II) complexes based on 1,8-naphthyridine functionalized N-heterocyclic carbenes (NHC) and their catalytic activity

Palladium complexes containing 2,7-bis(mesitylimidazolylidenyl)naphthyridine (NHC-NP) have been synthesized and characterized. Reaction of [{Ag(3)(NHC-NP)(2)}(PF(6))(3)] with [Pd(PhCN)(2)Cl(2)] provided an unusual dipalladium complex bridged by two NHC-NP units, forming a 20-membered dinuclear metallacycle [{Pd(2)(NHC-NP)(2)Cl(2)}(PF(6))] (2) in high yield. Treatment of 2 with KI in acetone yielded a neutral species [Pd(2)(NHC-NP)I(4)] (3). Meanwhile, the pyridinyl N-heterocyclic carbene (NHC-Py) precursor, 1-(2-pyridinyl)-3-mesitylimidazolium chloride, reacted with Pd(2)(dba)(3) directly to form the mononuclear palladium complex [Pd(NHC-Py)Cl(2)] (4). These complexes were characterized by elemental analyses as well as NMR spectroscopy, and the structures of 3 and 4 were further identified by X-ray diffraction analysis. The use of these palladium complexes for Suzuki-Miyaura and Kumada-Corriu coupling reactions has been examined. There is no significant difference in catalytic activities between 2 and 4 in Suzuki-Miyaura coupling reactions. However, the catalytic activity of 2 in the Kumada-Corriu coupling of ArBr with cyclohexylmagnesium bromide is quite different from that of 4. Thus complex 2 is active for the cross coupling, but complex 4 is active for the reduction of aryl halides.     

Nickel and palladium complexes of enolatefunctionalised N-heterocyclic carbenes

The reaction of chloroethyltrimethylsilylether with 1-methylimidazole furnishes an ionic liquid that undergoes methanolysis to crystalline 2-hydroxyethylimidazolium chloride (crystal structure presented). Conversion to defined hydroxyethylimidazol-2-ylidene nickel complexes failed, but was accomplished with 1-methyl-3-acetophenyl-imidazolium bromide. The bis(NHC^⋂O⁻) nickel(II) chelate is formed, rather than a methallylnickel monochelate, but with nickelocene a monochelate NiCp complex was detected. The bulky 1-(2,6-diisopropylphenyl)-3-(2’-phenyl-enolato)-imidazol-2-ylidene allylpalladium chloride was obtained in pure form. Attempts to generate catalysts for ethylene oligomerization by in situ techniques have failed so far whereas P^⋂O⁻ ligands, comparable by the P-C diagonal relationship, provide active catalysts.      

Rhenium and technetium complexes with N-heterocyclic carbenes – A review

The coordination chemistry of technetium and rhenium with N-heterocyclic carbenes of the dimethylimidazol-2-ylidene and 1,2,4-triazol-5-ylidene types is reviewed. Compounds containing the metals in the oxidation states “+7”, “+5” and “+1” are introduced. General trends and differences in the chemical behaviour of the complexes, particularly between the different metal cores (oxo, nitrido, imido) of Tc(V) and Re(V) compounds, are discussed. The influence of electronic and steric factors for the stabilisation of the metal complexes is explored.     

Steric and electronic properties of N-heterocyclic carbenes (NHC): a detailed study on their interaction with Ni(CO)4

N-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5) react with Ni(CO)(4) to give the saturated tricarbonyl complexes Ni(CO)(3)(IMes) (8), Ni(CO)(3)(SIMes) (9), Ni(CO)(3)(IPr) (10), Ni(CO)(3)(SIPr) (11), and Ni(CO)(3)(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)(3)(PR(3)) by recording their nu(CO) stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1-5 are surprisingly small. Novel, unsaturated Ni(CO)(2)(IAd) (13) and Ni(CO)(2)(I(t)()Bu) (14) compounds are obtained from the reaction of Ni(CO)(4) with IAd (6) and I(t)()Bu (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni-C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni(0) and Ni(II) complexes. Computational studies on compounds 8-14 are in line with the experimental findings and show that IAd (6) and I(t)()Bu (7) are more bulky than IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5). Furthermore, a method based on %V(bur) values has been developed for the direct comparison of steric requirements of NHCs and tertiary phosphines. Complexes 8-14, as well as NiCl(C(3)H(5))(I(t)()Bu) (16) and NiBr(C(3)H(5))(I(t)()Bu) (17), have been characterized by X-ray crystallography.     

Bis-ligated Ti and Zr complexes of chelating N-heterocyclic carbenes

In this communication we report the synthesis of novel titanium and zirconium complexes ligated by bidentate “salicylaldimine-like” N-heterocyclic carbenes (NHC). Double addition of the NHC chelate to either TiCl₄(thf)₂ or ZrCl₄ forms bis-ligated organometallic fragments with a distorted octahedral geometry. These complexes are rare examples of group IV transition-metal NHC adducts. Preliminary catalytic tests demonstrate that in the presence of methylaluminoxane (MAO) these complexes are useful initiators for the polymerization of ethylene and the copolymerization of ethylene with norbornene and 1-octene.     

Rhodium(I) complexes with N-heterocyclic carbenes bearing a 2,3,4,5-tetraphenylphenyl and its higher dendritic frameworks

Novel rhodium(I) complexes with N-heterocyclic carbenes bearing a 2,3,4,5-tetraphenylphenyl and its higher dendritic frameworks were synthesized and the complexes were used as catalysts in rhodium-catalyzed hydrosilylation of alpha,beta-unsaturated ketones to show high 1,4-adduct selectivity.     

Mild and rational synthesis of palladium complexes comprising C(4)-bound N-heterocyclic carbenes

Oxidative addition of pyridyl-functionalised 4-iodoimidazolium salts to palladium(0) gives catalytically active complexes in which the N-heterocyclic carbene is bound to the palladium(II) centre in a non-classical bonding mode via C(4).     

Unprecedented long-range 1,7-bromination in gold complexes of N-(aryl)imino functionalized N-heterocyclic carbenes

An unique long-range 1,7-bromination reaction is observed in gold(iii) complexes of N-(aryl)imino functionalized N-heterocyclic carbene with the bromination occurring at two different carbon (sp(2) and sp(3)) centers spatially separated by ca. 6.4 A but existing in extended conjugation to each other. In particular, the unusual distant 1,7-brominated gold(iii) complexes [1-R-3-{N-(p-bromo-2,6-di-i-propylphenylimino)-2-phenyl-1-bromoethyl}imidazol-2-ylidene]AuBr(3) [R = Me (), i-Pr (), t-Bu (), -CH(2)Ph ()] were synthesized cleanly at room temperature under ambient conditions from the reactions of molecular bromine with the gold(i) complexes [1-R-3-{N-(2,6-di-i-propylphenylimino)-2-phenylethyl}imidazol-2-ylidene]AuCl [R = Me (), i-Pr (), t-Bu (), -CH(2)Ph ()]. All of the 1,7-bromination products (, , and ) have been structurally verified by X-ray diffraction studies.     

Synthesis, structural characterization, and catalytic behaviour in Heck coupling of palladium(II) complexes containing pyrimidine-functionalized N-heterocyclic carbenes

[Pd(L1)2(CH3CN)](PF6)2 (L1 = 1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)2](PF6)2 (L2 = 1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), prepared via carbene transfer reactions of [Ag(L1)2]PF6 (1) and [Ag2(L2)2](PF6)2 (2) with palladium salts, respectively, have been fully characterized by 1H and 13C NMR spectroscopy and elemental analysis. The X-ray crystal structures of complexes 1-4 are reported. Complex 3 is an unusual pentacoordinated palladium complex, in which the palladium is coordinated by two imidazolylidene, two pyridine, and one acetonitrile molecule in a square-pyramidal geometry. The apical position is occupied by a pyrimidine nitrogen atom with a relatively long Pd-N distance (2.762(6) angstroms). Complex is a typical square-planar palladium complex with palladium surrounded by two pairs of cis-arranged pyridine and imidazolylidene ligands. The complexes exhibit good catalytic activities in the Heck coupling reaction of aryl bromides and activated aryl chlorides under mild conditions.     

Synthesis of bis N-heterocyclic carbenes,derivatives and metal complexes

A method for the synthesis and isolation of 1,1′-methylene-bis-(3-aryl-imidazol-2-ylidene) ligands, aryl = 2,6-diisopropyl-phenyl (DiPP), L^DiPP, mesityl (mes), L^mes, is reported, which provides synthetically useful quantities of high purity. Derivatisation of L^DiPP with chalcogenides gave the adducts L^DiPPE₂, E = S, Se, Te. Reaction of L^DiPP with [Pd(tmeda)Me₂], [Pt(μ-SMe₂)Me₂]₂, [Ir(1,5-COD)(μ-Cl)]₂/KPF₆ and [NiBr₂(dme)] gave [Pd(L^DiPP)Me₂] (1), [Pt(L^DiPP)Me₂] (2), [Ir(L^DiPP)(1,5-COD)](PF₆) (3) and [Ni(L^DiPP)Br₂] (4), respectively. The latter was reduced in the presence of CO to [Ni(L^DiPP)(CO)₂] (5). The structures of L^mes, L^DiPPTe₂, and 1–5 are also reported.     

N-杂环卡宾及其金属络合物的性质和合成方法

由于N-杂环卡宾结构的多样性，以及它们的金属络合物良好的稳定性和催化活性，近年来受到了人们的广泛关注。本文对N-杂环卡宾及其金属络合物的性质和合成方法进行了综述。参考文献30篇。