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The synthesis and characterization of the new six- and seven-membered Pd–PEPPSI-type N-heterocyclic carbene (NHC) complexes 3 and 4 is described. Complexes of the general formula [Pd(NHC)(3ClPy)Cl2] (NHC = 6- or 7-Mes, 3ClPy = 3-chloropyridine) are accessed via the oxidation of the well-defined parent palladium(0) complexes 1 and 2. Complexes 3 and 4 have been employed in Suzuki–Miyaura cross-coupling and catalytic dehalogenation of a range of aryl halide substrates.

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d z 2〉 and ∣3d x 2-y 2〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 14≈6≫5).

The synthesis and characterization of novel six- and seven-membered expanded ring N-heterocyclic carbene (NHC) complexes of the general formula [Au(NHC)Cl] are described. The key structural parameters of complexes 1, 2, and 4 have been evaluated by solid-state analysis and by means of the percent buried volume (%Vbur) analysis. Complex 4 is found to demonstrate the largest %Vbur value of any gold(I) NHC complex reported to date, with a value of 52.6. All complexes 1–4 have been evaluated in a preliminary catalytic survey of the hydration of internal alkynes.

A series of three chiral, expanded six-membered NHC–palladium(II) complexes was prepared with successively increased sterical demand, while retaining natural d-(+)-camphor as a chiral motif. The catalysts showed different reaction profiles in the asymmetric, intramolecular α-arylation of amides. The molecular structure of two N-heterocyclic and one nitrogen acyclic carbene palladium isonitrile complex was unequivocally determined by X-ray crystallographic analysis. The results reported herein account for a correlation of catalytic activity and enantiodiscrimination in relation to the degree of chiral substitution and steric congestion at the metal center. The modular and convergent synthetic route of these air- and moisture-stable palladium isonitrile complexes underlines the usefulness of this approach.

The synthesis and characterisation of a series of new 6- and 7-membered NHC (N-heterocyclic carbene) complexes 7–12 of zero valent platinum dvtms (divinyltetramethyldisiloxane), [Pt(NHC)(dvtms)] are reported. A number of the complexes were investigated as catalyst precursors in the hydrosilylation of a range of unsaturated substrates, including alkynes, alkenes and a ketone with triethylsilane and bis(trimethylsiloxy)methylsilane (MD’M), at low catalyst loading (0.005 mol %); in general, the activity, and the selectivity for 1-functionalised product was found to be high.

The performances of a number of RhI and IrI complexes of type [M(NHC)(COD)Cl] in the transfer hydrogenation of ketones were tested under a variety of reaction conditions, and with a variety of substrates, allowing comparison of Rh- and Ir-NHC complexes, and also comparison of the influence of the NHC ligand on catalytic performance. Notably, of the RhI and IrI complexes with symmetrically substituted NHCs only those bearing cyclohexyl substituents were active, with RhI complexes of saturated 5-, 6- and 7-NHCs with N-Mes substituents, [Rh(5,6,7-Mes)(COD)Cl], showing no activity in transfer hydrogenation under the test conditions. RhI and IrI complexes of unsymmetrical o-methoxyphenyl donor-functionalised NHCs (df-NHC) with differing carbene ring sizes were also tested in transfer hydrogenations, with the RhI complexes displaying no catalytic activity. However, the corresponding df-NHC IrI complexes were found to be extremely effective catalysts. Catalyst tests also demonstrated the excellent stability of these complexes.

The chemistry of abnormally C4-bound imidazolylidenes as ligands for transition metals is reviewed with a special emphasis on the potential of these ligands in promoting metal-mediated catalytic and stoichiometric transformations.

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(eta(3)-C(3)H(5))(tmiy)(PR(3))]BF(4) (tmiy = tetramethylimidazolin-2-ylidene, PR(3) = phosphane), precursors to monoligated Pd(0) species, with aryl electrophiles yielding 2-arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd(IV) and sigma-bond metathesis pathways, and that transmetalation is the rate-determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd(II) complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin-2-ylidene).

Reaction of imidazole derivatives with 2,3-di(bromomethyl)pyrazine results in the formation of the new pyrazine-bridged diimidazolium salts 1–8. These salts proved to be valuable precursors for dinuclear complexes with mixed NHC/pyrazine ligands. Two of the pyrazine-bridged diimidazolium salts 3·H2O and 8·2H2O have been characterized by X-ray diffraction. Furthermore, the first catalytic studies with mixtures of palladium acetate and the imidazolium salts have been carried out. The in situ prepared palladium complexes derived from the diimidazolium salts 1–8 exhibit a modest catalytic activity in Heck-type coupling reactions between 4-bromo benzaldehyde and styrene or n-butyl acrylate.

An alternative mechanism for intramolecular C−C coupling between heterocycles and alkenes with rhodium phosphine catalysts is presented involving oxidative addition, alkene insertion, and reductive elimination (route 2), as described previously for similar group 10 reactions by Cavell and McGuinness. Computational studies indicate that the rate-determining step is associated with reductive elimination of the product and overall barriers indicate this mechanism would be competitive with an alternative involving formation of a carbene complex derived from mechanistic work by Bergman, Ellman, and associates (route 1). Activation of the reacting azole through inclusion of an acid catalyst appears to support the route 2 mechanism. A much lower activation barrier is observed under acidic conditions, a result consistent with that found under experimental conditions.

This review focuses on the latest advances in homogeneous catalysis with donor-functionalised (donor = C, N, O, S or P) N-heterocyclic carbene complexes of group 9 and 10 metals. The applications of such complexes to C–C coupling reactions, alkyne and ketone hydrosilylation, olefin polymerisation, transfer hydrogenation, asymmetric hydrogenation and other catalysed reactions are discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

In situ-generated N-heterocyclic carbene (NHC) palladium catalysts and isolated NHC-palladium complexes have been tested for the telomerization reaction of 1,3-butadiene with primary and secondary amines. Superior catalyst activity (TON up to 400.000) and selectivity are obtained. Applying optimized conditions a variety of octa-2,7-dienylamines were prepared in high yield and excellent selectivity.

The coupling reaction between azolium salts (N-heterocyclic carbene precursors) and ethylene, catalyzed by zerovalent nickel complexes, has been investigated using a combination of experiment and density functional theory (DFT). The reaction proceeds via a redox mechanism involving the generation of Ni−carbene intermediates. The experimental studies employed an in situ catalyst system, derived from Ni(COD)2 and a variety of phosphine and N-heterocyclic carbene spectator ligands, to couple 1-propyl-3-methylimidazolium bromide (ionic liquid) and other azolium salts with ethylene. The DFT studies employed the simpler dimethylimidazolium salt to model the reaction.

The complexes NiBr2(L2) [L2 = (mim)2CO (7a), (bim)2CO (7b), py(mim)CO (8), PhN=CHbim (9)], NiBr(o-Tol){(mim)2CO} (10), Ni(L2)(o-Tol)(PPh3) [L2 = mimCO2- (11a); bmCO2- (11b), mimCPh2O- (12), mimC6H4O- (13)], and [Ni{(mim)2CO}3][BF4]2.0.25H2O (16) have been synthesised from nickel(II) salts (7–9, 16) or NiBr(o-Tol)(PPh3)2 [10–13], where mim = 1-methylimidazol-2-yl, bim = 1-benzylimidazol-2-yl, and py = pyridin-2-yl. The square-planar complexes (11a, 12) and octahedral 16 have been characterised by X-ray diffraction studies, together with octahedral cis-[Ni{(mim)2CO}2Br(OH2)]Br.CD3CN (14) and cis-[Ni{(mim)2CO}2(OH2)2]Br2.3CD3CN (15). Preliminary catalytic studies have been undertaken with a number of the nickel complexes. Complexes 11–13 form single component catalysts for the oligomerization or polymerization of ethylene and the bromo-complexes (7–9) form active catalysts for the oligomerization or polymerization of ethylene when activated with MMAO.

Novel palladium(II) complexes with oxazolin-2-ylidene ligands have been synthesized via direct reaction of palladium acetate and oxazolium salts, prepared in turn by alkylation of oxazole with methyl iodide or benzylic bromides. The resulting complexes have been characterized and used as catalysts in Heck coupling reactions of aryl bromides, where they exhibit remarkable catalytic activity, higher than that of the closely related bis-imidazolin-2-ylidene and bis-benzothiazolin-2-ylidene complexes.

The complete basis set method CBS-QB3 was used in conjunction with the CPCM solvation model to predict both the absolute and relative pKa's of 12 nucleophilic carbenes in dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and water. Average absolute pKa values in DMSO ranged from 14.4 +/- 0.16 for 3-methylthiazol-2-ylidene (12) to 27.9 +/- 0.23 in the case of bis(dimethylamino)carbene (11), while values in MeCN were determined to be between 25.7 +/- 0.16 (12) and 39.1 +/- 0.25 (11). Relative pKa calculations yielded similar results. Calculations in aqueous solution gave pKa's between 21.2 +/- 0.2 (12) and 34.0 +/- 0.3 (11). Excellent agreement between calculated and experimental pKa's was obtained for the few cases where experimental numbers are available, confirming that this theoretical approach may be used to calculate highly accurate pKa values.

A highly active and selective in situ formed platinum(N-heterocyclic carbene) catalyst for the hydrosilylation of styrene with triethylsilane is described, which unlike all other known hydrosilylation catalysts, selectively yields hydrosilylation products, but (almost) no dehydrogenative silylation products.

The potentially tridentate ligand 2,6-bis[(3-methylimidazolium-1-yl)methyl]pyridine dibromide reacts readily with silver(I) oxide in dichloromethane or dimethylsulfoxide to give a dinuclear silver(I)–carbene complex that was isolated as the tetrafluoroborate salt. Single crystal X-ray crystallography shows that each silver(I) ion is bridged by two ligands bonding through the carbene donors. Treatment of the silver(I) complex with suitable palladium(II) precursors gave the complexes PdCl[(CNC)]BF4 and [PdMe(CNC)]BF4 (CNC=2,6-bis[(3-methylimidazolin-2-yliden-1-yl)methyl]pyridine), in which the pyridyl and both carbene moieties are coordinated to a single palladium(II). The palladium(II) complexes have been fully characterised, including X-ray crystallography, and exhibit good activities in the Heck coupling reaction of 4-bromoacetophenone and n-butyl acrylate.

Nickel(II) heterocyclic carbene complexes of the formula NiI2(carbene)2 (carbene = 3-methyl-1-propylimidazolin-2-ylidene (1), 1-butyl-3-methylimidazolin-2-ylidene (2), 1-isopropyl-3-methylimidazolin-2-ylidene (3), 1,3-diisopropylimidazolin-2-ylidene (4)) have been prepared and fully characterized by both spectroscopic methods and single-crystal X-ray structure analysis. All complexes have “square-planar” nickel environments, with the donor atoms pairwise trans, all obligate, except for one of the two independent molecular types found in 2, in which, with different substituents at the nitrogen atoms, the two rotamers are observed as independent molecules in the one crystal. Complexes 1−4 were tested as catalysts for propene and 1-butene dimerization in both toluene and an imidazolium−chloroaluminate ionic liquid. While the complexes are inactive to sparingly active in toluene, highly active catalysts are produced in the ionic liquid. In all cases the complexes are more active toward 1-butene dimerization t...

Palladium complexes of functionalised heterocyclic carbene complexes have been synthesised. Treatment of imidazolium salts with Ag2O yields AgI(carbene)2 complexes, which act as carbene transfer agents when reacted with palladium salts. In this manner, [Pd(Me)(1-(2-ethylpyridyl)-3-methylimidazolin-2-ylidene)Cl]2 (4a) and Pd(Me)(1-benzyl-3-methylimidazolin-2-ylidene)2Cl (4c) have been prepared from PdMeCl(cod) (cod=1,5-cyclooctadiene) and the appropriate silver complex. Similarly, the reaction of a Ag(carbene)2 complex with PdCl2(MeCN)2 gives Pd(1-benzyl-3-methylimidazolin-2-ylidene)2Cl2 (4b). The tridentate carbene complex [Pd(Me)(1,3-di(2-picolyl)imidazolin-2-ylidene)]BF4 (6a) is synthesised via the in situ reaction of the imidazolium salt with Ag2O, followed by PdMeCl(cod) and AgBF4, whilst [PdCl{1,3-bis(diisopropyl-2-ethylamino)imidazolin-2-ylidene}]BF4 (6c) is synthesised in an identical manner from PdCl2(MeCN)2. The chelated complexes [1,1′-dimethyl-3,3′-(1,2-xylylene)diimidazolin-2,2′-diylidene]Pd(II) dibromide (5a), [1,1′-dimethyl-3,3′-(1,3-xylylene)diimidazolin-2,2′-diylidene]Pd(II) dibromide (5b) and Pd(imidazoliophane)Br2 (5c) have been synthesised via the reaction of the appropriate imidazolium salt with Pd(OAc)2. X-ray crystal structures of the imidazolium salt, 1,3-di(2-picolyl)imidazolium chloride (1f) and the complex [Pd(Me)(tetramethylimidazolin-2-ylidene)2Cl] (2) are reported. Complex 2 shows square planar coordination with the two carbene ligands trans to each other. The carbene ligands are inclined at 65.3° to the coordination plane. Several complexes proved to be highly stable and efficient catalysts for intermolecular Heck and Suzuki coupling reactions, giving turnover numbers of up to 980 000 (Heck) and 177 500 (Suzuki).

Catalytic dimerisation of ethene by [Pd(1,10-phen)2]2+ supported in three forms of Nafion has confirmed that diffusion of reactants and/or products through Nafion is rate-limiting. However, a combination of low catalyst loading and high dispersion of Nafion has resulted in extraordinary increases in ethene dimerisation activities when compared with the homogeneous analogue. Initial activities as high as 760,000 turnovers h−1 have been obtained for the supported complex compared with a homogeneous activity of 32,000 turnovers h−1. These increases in activity most likely result from the very polar, strongly acidic environment in Nafion which ensures effective anion–cation separation and promotes labilisation of the nitrogen donor ligands. Enhancements in activities for co-polymerisation of ethene and carbon monoxide, and carbomethoxylation of ethene have also been observed using Nafion supported Pd(II) catalysts in methanol.

Highly effective catalysts for the conversion of ethylene into linear α-olefins may be generated in situ from zirconium tetrachloride, a β- aminoketone or β-aminothioketone ligand and an alkylaluminium chloride Lewis acid cocatalyst. Catalysts may also be generated from ZrCl4 · 2HL adducts (where HL = monodentate, oxygen-bound β-aminoketones), and from bis-ligand complexes of the type ZrCl2L2 (where L = monovalent, bidentate β- aminoketones or β-aminothioketones) on treatment with an alkylaluminium chloride cocatalyst. Product distribution and catalyst activity can be adjusted by ligand substituent variation and/or by the method of catalyst formation. Catalyst systems generated from preformed complexes, ZrCl2L2, were in general significantly more active than those formed in situ or from the bis-ligand adducts, ZrCl4 · 2HL. Activities of up to 60,000 turnovers/h were obtained with selected complexes. However, in situ mixtures and bis- ligand adducts generally gave a much narrower oligomer distribution, with up to 95% of the oligomers occurring in the C4-C10 range for a number of the systems tested. In situ NMR (nuclear magnetic resonance) tests indicate that complex ligand/cocatalyst interactions are present and that these new oligomerisation systems are active even under very mild conditions, 30°C and 1 atm ethylene.

Cationic η 3 -2-methylallylpalladium(II) complexes with potentially hemilabile ligands XCH 2 COOEt (X = NPh 2 , PPh 2 , OPh, SPh) or methylpicolinate have been prepared and their catalytic properties in the codimerisation of styrene with ethylene have been investigated. The catalytic activity decreases in the order of decreasing trans influence of X: P ⪢ N ≈ O > S > pyr. The relationship between catalytic activity and spectroscopic properties of the complexes is discussed.

A series of arylnickel phosphine complexes containing chelating Nz.sbnd;O ligands, of the type [NiAr(N-O)L] [NO = pyridine carboxylate (pyca); Ar = o-tolyl; L = PPh3, P(CH2Ph)3, PMePh2, PMe2Ph, PCy3: R = p-tolyl; L = PPh3: Ar = phenyl; L = PPh3: R = mesityl; L = PMePh2: NO = pyridine acetate (pyac); At = mesityl; L = PMePh2] have been prepared, providing complexes with chelate ring sizes of five and six. Crystal structures for the complexes indicate that both complexes have “square planar” coordination about the nickel centre, with the nitrogen of the pyridine being trans to the phosphine in each case. Whereas significant bending and buckling of the six-membered chelate ring is evident for [Ni(mesityl)(pyac)PMePh2], the five-membered ring of [Ni(mesityl)(pyca)PMePh2] is essentially planar. On warming the complexes readily insert ethylene into the Ni-aryl bond, forming single component catalysts for the conversion of ethylene into higher oligomers with low to moderate activity. Products are linear with generally 60–80% having the double bond in the a-position. Catalyst activities and product distributions are markedly dependent on the phosphine present. Addition of excess PPh3 to the catalyst [Ni(o-tolyl)(pyca)PPh3] leads to a marked change in the product distribution with a less dramatic change in catalyst activity. A possible mechanism in which ethylene insertion occurs via an associative pathway from a five coordinate intermediate, [NiAr(CH2=CH2(pyca)PR3], is suggested.

Reactive Couples: Imidazolium salts react with alkenes under mild conditions in the presence of a Ni0-based catalyst to produce 2-alkyl imidazolium salts (see scheme, cod=1,5-cyclooctadiene). The mechanism involves oxidative addition of an imidazolium CH bond to M0 to give a (carbene)MH intermediate and finally bond formation between the carbene and an alkyl ligand through reductive elimination.

A recently developed, highly selective, CuSiO2 catalyst for the hydrogenation of unsaturated hydrocarbon contaminants in C2 and C3 alkene streams has been employed to remove alkynes from an industrial C4 cracker stream. At a temperature of 320°C and with approximately equal concentrations of H2 and C4 feed effectively all the contaminating alkynes and all the butadienes were hydrogenated selectively to butenes. No further hydrogenation of butenes to butanes was detected. The purified C4-stream was successfully employed in a butene dimerization reaction catalyzed by a homogeneous, Ziegler-type nickel catalyst system [Ni (dithioacetylacetonato) (PnBu3) Cl + diethylaluminiumchloride]. Prior to selective hydrogenation, alkynes present in the C4 stream had poisoned the nickel catalyst. After removing the contaminants dimerization proceeded with an activity (1480 turnovers/h) and product distribution in line with that observed using pure 1-butene feeds.

An unusual example of diastereoselectivity has been observed in Cu(κ(3)-P,C,P'-1)X complexes where 1 is an asymmetric tridentate ligand containing a bicyclic NHC framework and X is a halide. When X is Cl(-), the S(Cu) isomer is formed selectively whereas when X = I(-) the R(Cu) diastereomer is preferred.

A comprehensive investigation of the ability of Naƒon to influence catalytic reactions has been undertaken using the [Pd( 1,10-phen) 2 ] 2+ species immobilized in Naƒon—H + . The effects of catalyst loading, solvent, substrate, temperature and pressure on catalytic activity have been studied. We have found that whilst the homogeneous [Pd(1,10-phen) 2 ][ p -tolyl-SO 3 ] 2 model complex dimerizes ethene and propene, and isomerizes 1-butene, Naƒon supported [Pd(1,10-phen) 2 ] 2+ effectively dimerizes ethene only. The ethene dimerization activity of the supported catalyst has been found to be highly dependent on catalyst loading and solvent. Rate limitations are overcome at very low catalyst loadings and activities as high or higher than homogeneous activities were obtained. It appears likely that ethene solubility in the reaction solvent, ethene diffusion through Naƒon, Naƒon morphology, and possible steric constraints on the catalyst at high loadings are important factors. The use of water as the reaction solvent resulted in a dramatic increase in the activity of the supported catalyst and adds weight to the argument that cation-anion separation and a non-coordinating anion are important aspects of catalysis using cationic Pd(II) species. Tests with propene and 1 -butene as the feed indicate that the activity of the supported catalyst is limited by the diffusion of these substrates in Naƒon.

The cationic organonickel species, [Ni (o-tolyl) (py)2PPh3]+, has been immobilized within the perfluorinated ion-exchange polymer Nafion. Immobilization was achieved by replacement of the halide and a PPh3 ligand, in Ni (o-tolyl) Br(PPh3)2, with pyridine. The strong interaction between the Nafion and the cationic species was the driving force for the reaction, the Nafion thus acting in the dual role of firstly forming and then supporting the catalyst precursor. The structure of the Nafion immobilized species was confirmed by comparison of FTIR and 31P NMR spectra of the Nafion supported species with spectra for the [Ni(o-tolyl) (py)2PPh3]PF6 complex. Both the unsupported and supported species were shown to be catalytically active for ethene dimerization. The inability of the solvent to significantly swell the Nafion limited the catalytic activity of the supported species. Immobilization in Nafion did however extend the lifetime of the supported catalyst.

A series of complexes of the type [Ni(L) (PR3)X] and [Ni(L) (Ph2P(CH2)nPPh2)]BPH4 (where L=4-thioxo-2-pentanoate (sacac); R=ethyl (Et), R=n-butyl (nBu); X=Cl, Br; n=1 or 2; where L=2,4-pentanedithionate (sacsac); R=methyl (Me), ethyl (Et), cyclohexyl (Cy) and phenyl (Ph); X=Cl, Br) has been prepared. The complex [Ni (sacac) (PnBu3)Cl] is extremely labile. However, it could be prepared using ultrasound and identified by NMR. Attempts to recrystallize the complex lead to disproportionation. Several of the complexes were found to be active oligomerisation catalysts when tested for the oligomerisation of propene and 1-hexene. Factors that have been investigated are the effect of changing the Lewis acid co-catalyst, phosphine, chelating ligand (L) and halogen on the oligomerisation reaction. It is found that the phosphine ligand influences both the activity and selectivity of the oligomerisation reaction, whilst changing the thio ligand has a marked effect on catalyst activity only. In addition, the crystal structure of [Ni(C5H7OS) (P(C2H5)3)Cl] has been determined. Crystals of [Ni(C5H7OS)(P(C2H5)3)Cl] are monoclinic, P21/n with a=7.644(4), b=18.005(8), c=11.021(3) A, β=96.17(3)° V=1508(5) A3, Z=4. The structure was solved using a Patterson map and refined to R=0.056 for 2227 reflections and 162 parameters. The Ni atom has squareplanar coordination with Cl trans to the sulphur atom of the sacac ligand.

The three cationic palladium species; Pd(1,10-phen)2+2, Pd(2,2′-bipy)2+2 and Pd(1,10-phen) (py)2+2 have been immobilized in Nafion-H+ and Nafion-Na+ by ion-exchange. Characterization of the Nafion supported species has been achieved by UV, IR and 13C-NMR spectroscopies. Both Pd(1,10-phen)2+2 and Pd(2,2′-bipy)2+2 are unchanged when immobilized in Nafion, however, it appears that partial decomposition of Pd(1,10-phen) (py)2+2 to give 1,10-phen and pyridine occurs in Nafion-H+. The ability of the immobilized species to dimerize ethene has been tested and the dimerization activity was found to be dependent upon the basicity of the nitrogen donor ligand and the counterion in Nafion.

A series of novel quasi-scorpionate CNC donor ligands, MeC(2-C5H4N){CH2(imidazole-R)} (R = methyl, n-butyl, n-propenyl), in which a chelating bis(NHC) core is supplemented by a hemi-labile pyridyl donor, were prepared. The coordination chemistry of these ligands was investigated with silver, palladium, rhodium and iridium. The single crystal X-ray structures of [Rh(NC2Me)(COD)]Cl 8a and [Ir(NC2Pr)(COD)]Br 9b were determined. The catalytic potential of the rhodium and iridium complexes was assessed in the transfer hydrogenation of ketones; the iridium complexes, which show superior performance, form very effective and stable catalysts. Copyright (C) 2011 John Wiley & Sons, Ltd.

Nafion tubes that function both as a reaction catalyst and a pervaporation membrane have been used to increase the yield in the esterification of acetic acid with methanol and n -butanol by selectively removing products, mainly water, from the reaction mixture. The experiments were performed at room temperature in a batchwise reactor using acidic protons in Nafion pellets and the Nafion membrane as the reaction catalyst. In some experiments the acidic protons in the membrane were partially or fully exchanged with caesium ions, which increased the intrinsic selectivity of the membrane for water but lowered its permeability. In the methanol reaction, the yield of methyl acetate was increased from the usual equilibrium value of 73% to a maximum of 77%. In the n -butanol reaction, the yield of n -butyl acetate increased from 70% to a maximum of 95%. The effect of the membrane's catalytic activity on its permselectivity was investigated. The catalytically active membranes showed significantly higher permselectivities for water at the same or higher flux, compared to when no reaction was taking place within the membrane phase.

The complexes [Ni{R1C(S)CR2C(S)R3}2]1 and [Ni{R1C(S)CR2C(S)R3}(PL1L2L3)X]2(R, L = alkyl, aryl or CF3; X = Cl, Br or I) have been prepared. The complexes 2 can be chemically anchored to polystyrene or silica–alumina supports via pendant phosphine ligands to form the derivatives 3. The species 2 and 3 in combination with a suitable co-catalyst form extremely active catalysts for the oligomerization of ethylene, propylene or butenes. The immobilised systems show no significant signs of leaching after 24 h operation. In general the oligomerization is highly selective to dimers, however, selectivity and catalyst activity are markedly affected by changes in L. Changes in R affect catalyst activity only.

The catalytic double-bond shift isomerization of α-olefins by dithio-β-diketonate palladium complexes Pd{CH3C(S)CHC(S)CH3}(PL1L2L3)X(1) and [Pd{CH3C(S)CHC(S)CH3}{Ph2PCH2CH2PPh2}]Y(2)(X = 1, Br; Y = halide, PF6, BPh4) is described. Catalysts are formed on activation of the complexes by a variety of alkylaluminium cocatalysts . Several of the catalysts are highly active for isomerization of oct-1-ene and but-1-ene, activities being higher than normally obtained for palladium systems. In contrast to the majority of palladium-based catalysts product distributions rapidly approached thermodynamic equilibrium, with a strong preference for trans products at each stage of the reaction. Products from the isomerization of oct-1-ene were analysed by g.l.c.-F.t.i.r . in conjunction with Kovats retention indices.

The perfluorinated ion-exchange polymer, Nafion-H+, has been used as a Bronsted acid to activate [Ni{P(OEt)3}4] and as a support for the resulting cationic species. Spectroscopic evidence has confirmed that the species present within the Nafion-H+ film are the same as those found in a homogeneous solution using H2SO4 as the Bronsted acid. Activity for oct-1-ene isomerization in the first 30 min was approximately one third of the activity obtained in homogeneous solution using H2SO4 as co-catalyst with a similar [Ni{P(OEt)3}4]: H+ ratio, and the activity varied linearly with the amount of Nafion-H+ present. Comparative tests with but-1-ene employing homogeneous and supported catalyst systems have demonstrated a significant increase in isomerization rate on changing from oct-1-ene to but-1-ene when using the supported catalyst. This indicates that diffusion of oct-1-ene into, and/or products out of, Nafion-H+ are controlling factors. Diffusion coefficients of oct-1-ene and but-1-ene in Nafion have been determined and support this conclusion. It has also been demonstrated that very little, if any, of the catalyst species is leached from the Nafion support during catalysis.

The complexes [Ni{R1C(S)CR2C(S)R3}2]1 and [Ni{R1C(S)CR2C(S)R3}(PL1L2L3)X]2(R, L = alkyl, aryl or CF3; X = Cl, Br or I) have been prepared. The complexes 2 can be chemically anchored to polystyrene or silica–alumina supports via pendant phosphine ligands to form the derivatives 3. The species 2 and 3 in combination with a suitable co-catalyst form extremely active catalysts for the oligomerization of ethylene, propylene or butenes. The immobilised systems show no significant signs of leaching after 24 h operation. In general the oligomerization is highly selective to dimers, however, selectivity and catalyst activity are markedly affected by changes in L. Changes in R affect catalyst activity only.

The application of non-phosphine-based chelating ligands in homogeneous catalyst systems is a surprisingly recent and relatively unexplored area of research. Chelating ligands can concurrently stabilize intermediates, control catalyst activity and direct the product distribution far more effectively than monodentate ligands. In this review our studies with catalyst systems containing chelate ligands primarily of the P-diketonate type [dithio-P-diketonate (sacsac); monothio-P-diketonate (sacac); and imino P-diketonate (nacac) ligands] is discussed. Examples of the catalyst systems show enzyme-like superactivity. Studies modelling these catalyst systems have provided valuable information relating the effects of ligand modifications to reaction pathways and to activities. Our most recent investigations of simple chelating ligands based on picolinic acid are also discussed. Studies modelling CO/ethene insertion/elimination with extremely labile alkylplatinum picolinate complexes led to the development of new single-component nickel oligomerization catalysts.

Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in α-olefin polymerisation and oligomerisation, CO/olefin copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discussed in subsequent sections.

N-heterocyclic carbenes, and their derivatives, are powerful donor ligands that form strong M–NHC bonds, particularly with late transition metals. Interest in these species has largely focused on their application as ligands in catalysis, in which they have shown some promise. However, they have not had the impact originally anticipated; NHCs undergo a number of processes that lead to catalyst deactivation and in the following chapter the range of deactivation reactions that have been reported will be discussed. By far the most detrimental deactivation process uncovered to date is reductive elimination, in which the NHC is lost as an imidazolium salt and the metal centre is reduced. This decomposition pathway is discussed in some detail and its impact on catalysis is put in context. Nevertheless, as apparent from some of the following discussion, the opportunity exists for an imaginative approach to the problem of catalyst decomposition, and it is possible to develop novel synthetic chemistry utilising these reaction pathways.

N-Heterocyclic carbenes (NHCs) are commonly regarded as strong donor ligands that are valuable in coordination chemistry and catalysis. Many reports describing this aspect of their chemistry have been published. The alternative view of NHCs as reaction intermediates has been little explored, and yet excellent examples exist and will be reviewed in this perspective. Group 10 hydrocarbyl complexes of NHCs [(R)(NHC)ML2 where R = H, alkyl, aryl, acyl; M = Ni, Pd, Pt] undergo a facile reductive elimination to generate M(0) and R-substituted-azolium salt as products. On the other hand, 2-Hazolium salts will oxidatively add to M(0) complexes to afford reactive NHC–M–H compounds, suitable as catalysts for selected reactions. Combining the oxidative addition and reductive elimination steps into a single cycle, in the presence of an alkene, provides of a novel and potentially exciting, atom efficient catalytic C–C bond forming process for the substitution, and annulation of heterocyclic rings.

The perfluorinated polymer Nafion and porous PTFE/Nafion composite membranes have been employed as supports for nickel complexes or for platinum and palladium metal particles. The resultant materials have been employed as catalysts in various olefin conversion processes. Supported platinum and palladium metal systems were evaluated as catalysts for the hydrogenation of cyclohexene. Rates of reaction are better than those of commercially available catalysts; turnover numbers in excess of 6000 have been obtained with no poisoning apparent. Catalysts may be regenerated many times. The reduction rate approaches a limit at high pressures of hydrogen and has an activation energy of 13 kJ mol−1 in neat cyclohexene. Nafion was employed as a strong acid cocatalyst to activate and then support a nickel complex catalyst. The resultant catalyst was active for double-bond-shift isomerization.

The reductive elimination of 2-hydrocarbyl-imidazolium salts from hydrocarbyl–palladium complexes bearing N-heterocyclic carbene (NHC) ligands represents an important deactivation route for catalysts of this type. We have explored the influence that carbene N-substituents have on both the activation energy and the overall thermodynamics of the reductive elimination reaction using density functional theory (DFT). Given the proximity of the N-substituent to the three-centred transition structure, steric bulk has little influence on the activation barrier and it is electronic factors that dominate the barriers' magnitude. Increased electron donation from the departing NHC ligand acts to stabilise the associated complex against reductive elimination, with stability following the trend: Cl < H < Ph < Me < Cy < iPr < neopentyl < tBu. The intimate involvement of the carbene pπ-orbital in determining the barrier to reductive elimination means N-substituents that are capable of removing π-density (e.g. phenyl) act to promote a more facile reductive elimination.

Novel palladium(II) complexes with oxazolin-2-ylidene ligands have been synthesized via direct reaction of palladium acetate and oxazolium salts, prepared in turn by alkylation of oxazole with methyl iodide or benzylic bromides. The resulting complexes have been characterized and used as catalysts in Heck coupling reactions of aryl bromides, where they exhibit remarkable catalytic activity, higher than that of the closely related bis-imidazolin- 2-ylidene and bis-benzothiazolin-2-ylidene complexes.

Palladium complexes of a series of functionalised phosphines bearing the OPN donor set [2-pyCH(2)P(Ph)CH(2)(CHOCH(2)CH(2)CH(2)), 1; 2-py CH(2)P(Ph)-CH(2)CH(2)(CHOCH(2)CH(2)O), 2; 2-pyCH(2)P(Ph)CH(2)CH(2)CO(2)Me, 3; 2-pyP(Ph)CH(2)(CHOCH(2)CH(2)CH(2)), 4; 2-py = 2-pyridyl] have been prepared and characterised. Ligands 1-3 form five membered P-N chelates which is confirmed for PdCl(2) complexes of and by X-ray crystallography. O-coordination appears to be generally disfavoured although there is evidence of transient O-coordination for selected Pd complexes of 4. Palladium methyl and acetate complexes of all four ligands have been tested for catalytic activity in ethene/CO copolymerisation as well as alkoxy-carbonylation of propyne. Complexes of 1 and 4 show some activity in the copolymerisation reaction and complexes of 4 are active in the methoxy carbonylation of propyne. Unlike related pyridyl(diaryl)phosphines, 4 produces a much more stable catalyst system that does not require large excesses of ligand to maintain activity.

The telomerization reaction of 1,3-butadiene with alcohols to give alkyl octadienyl ethers in the presence of palladium-carbene catalysts has been studied in detail. Unprecedented catalyst efficiency with turnover numbers (TON) up to 1,500,000 and turnover frequencies (TOF) up to 100,000 h(-1) have been obtained after optimization for the reaction of methanol in the presence of an excess of in situ generated carbene ligands. High yields (75-97 %) and catalyst productivities (TON 15,000-100,000) are observed for other aliphatic alcohols and phenols. For comparison five carbene-palladium(0) complexes have been synthesized and characterized by X-ray crystallography. Both electronic and steric effects on the stability and reactivity of the catalysts have been discussed on the basis of density functional theory calculations.