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Recently a huge chunk of the population has shifted towards shower gels. The biggest reason might be the fact that shower gels can provide a better care for the skin. They were made with the motive of providing gentle cleansing and moisturizing qualities contrary to that of a regular soap, so that the people can enjoy a better skin care experience. Shower gels generally use chemicals especially sodium lauryl sulfate and betaines. These are believed to cause skin irritation and may not be washed off properly especially in hard water. The recent shower gels have a combined scrubbing effect included along with the scented or flavored body wash. For the scrubbing activity microbeads are employed. These microbeads used as a scrubber are polymers, e.g. being polyethylene or nylon etc. We know how plastics are harmful to us and the environment. And this is a cause of major concern. And since it is small in size it makes it relatively dangerous. Microbeads, because of their small size, have a large surface area by volume which allows them to accumulate highly toxic materials which are toxic to the person and the environment. To overcome the menace of microbeads we propose an alternative

The study was conducted to assess the influence of different agro-ecological zones and cropping seasons on the incidence and severity of maize streak disease (MSD) in Ghana. Field surveys were conducted in two districts each of the coastal savannah, forest, and transitional agro-ecological zones of the Volta region to assess the incidence and severity of MSD in farmers’ fields, during the 2014 minor and 2015 major cropping seasons. The disease assessment was carried out in 12 fields from each district, and for each field on both maize plants growing under tree shade and in open parts of the field. The plants were scored for disease severity based on a 1-5 visual scale (1=no infection and 5=very severe infection). Percentage total N, available P, exchangeable K, organic matter levels and pH were determined from soil samples collected from each of the fields surveyed. Incidence and severity of MSD on maize plants growing under tree shade and in open parts of the fields were significantly higher (P

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.

The interaction of triethylaluminum (TEA) with a solution of the paramagnetic Cr(I) bis(phosphine) complex [Cr(CO)41][Al(OC(CF3)3)4] (1 = Ph2PN(iPr)PPh2) has been studied using EPR and DFT. It was found that the TEA is responsible for the complete removal of all CO groups from the [Cr(CO)41]+ complex, producing the [Cr(1-bis-η6-arene)]+, and this reaction occurs via a dominant pathway involving a series of [Cr(CO)x1]+ (x < 4) intermediates, consistently including the cis-[Cr(CO)31]+ complex (species A) and the “piano-stool”-type [Cr(CO)21]+ complex (species C). A further [Cr(CO)21]+ intermediate complex (labeled species D, which is a structural isomer of species C) was also identified experimentally, suggesting a second pathway for TEA activation may also be operative. All of these paramagnetic complexes have been characterized by CW EPR, and the spin Hamiltonian parameters were verified using DFT. The distribution and type of [Cr(CO)x1]+ intermediates formed were found to be very sensitive to the experimental conditions, including the quantity and manner of TEA addition, the temperature of activation, and the aging time of the solution.

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).

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO)(2)Cl] 15-22 are described. The complexes have been characterised by H-1 and C-13{H-1} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V-bur) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The sigma-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)(2)Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex 10, affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

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.

Activation of the catalytically relevant complex [Cr(CO)4(1)] (1 = Ph2P(C3H6)PPh2) by Et3Al (TEA) leads to formation of the Cr(I) bis-arene complex [Cr(1-bis- 6-arene)]+, as revealed by EPR and DFT calculations. This bis-arene complex is formed by intramolecular rearrangement and coordination of Cr(I) to the ligand phenyl groups in aliphatic solvents following loss of CO, preventing release of Cr(I) into solution. By comparison in aromatic solvents (toluene), the [Cr(bis-tolyl)]+ complex is preferentially formed.

A series of new diethyltin(IV) derivatives of substituted phenyl acrylates have been synthesized and characterized by elemental analysis, IR, multinuclear NMR (1H, 13C, 19F and 119Sn) and X-ray single crystal analysis. X-ray single crystal diffraction study of complex 8 has shown the formation of secondary O···Sn, F···H and H···O intermolecular interactions. These complexes proved to be good antitumor agents. The antitumor properties are presumably due to the intercalative mode of interaction of these complexes with the tumor cells' DNA. Copyright © 2011 John Wiley & Sons, Ltd.

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.

Azolium precursors to expanded-ring NHCs incorporating a chiral bicyclic skeleton derived from (1R)-camphor with N,N′-dibenzyl or N,N′-dipyridyl substituents have been prepared and used for the synthesis of a number of metal complexes of the resultant NHCs. The bicyclic framework contains two fused rings, one of which is seven-membered while the other is six-membered. Deprotonation of (1R,5S)-2,4-dibenzyl-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-2-ene hexafluorophosphate, 1a·HPF6, produces a solution of “free carbene”, 1a, that can be used for the preparation of [Ag(1a)2]OTf (2a), [Rh(1,5-COD)(1a)Cl] (3a), [Rh(CO)2(1a)Cl] (4a), and [Ir(1,5-COD)(1a)Cl] (5a) complexes. The fused expanded ring NHC ligands display properties and reactivity patterns more akin to those of five-membered NHCs rather than monocyclic six- or seven-membered NHCs. Consistent with this premise, the red Ni(II) pincer complex [Ni(η3-C8H13)(1b)]PF6, 6b, was obtained via oxidative addition of the azolium salt 1b·HPF6 to Ni(1,5-COD)2 in THF. This is the first unambiguous example of a saturated and/or expanded ring NHC oxidatively adding to a low-valent metal center. A single-crystal X-ray structure determination of 6b showed the NHC to be coordinated in a pincer fashion through the carbene and two pyridine nitrogens at a square-pyramidal nickel(II) center. One of the original 1,5-COD ligands has undergone a 1,5 to 1,3 isomerization before migratory insertion of one of the alkene groups into the Ni−H function, generated on oxidative addition of the azolium salt, to give the allylic η3-C8H13 ligand. When 6b was dissolved in CHCl3 or CH2Cl2 in air, the allyl ligand was lost and a chloride was abstracted from the solvent to give yellow square-planar [Ni(1b)Cl]PF6, 7b, the structure of which was confirmed by single-crystal X-ray techniques.

An unsaturated seven-membered amidinium salt 7 decomposes in the presence of metal salts under basic conditions. However, 7 readily forms a Diels–Alder cycloadduct with CpH from which the RhI complexes may be prepared. Thus, structurally elaborate, sterically crowded carbene ligand complexes bearing peripheral unsaturated functionality are available in a short versatile synthesis.

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).

The rhodium complexes [Rh(NHC)(COD)Cl] and cis-[Rh(NHC)(CO)2Cl] of seven-membered N-heterocyclic carbenes (NHC) bearing aromatic N-substituents (mesityl, xylyl and o-tolyl) were synthesised, and a comparison of their steric and electronic properties with those of the analogous five- and six-membered NHC complexes was made on the basis of infrared and solid-state data. The X-ray structures for the complexes Rh(6-Mes)(COD)Cl, Rh(7-Mes)(COD)Cl and Rh(7-oTol)(COD)Cl were determined.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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.

Compounds of general formula [Pd(L)(NHC)(L′)]BF4 (where L = allyl or crotyl, NHC = tetramethylimidazolin-2-ylidene (tmiy) or 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (dipdmiy), and L′ = tertiary phosphine) have been prepared from the parent [Pd(L)Cl(NHC)(L′)] complexes and fully characterized. A perpendicular orientation of the NHC is observed in the solid state (X-ray) and in solution (NOESY). The crystal structures of these complexes reveal the double-bond character of the allyl trans to the phosphine ligand; for example, [Pd(η3-C3H5)(tmiy)(PPh3)]BF4 (4d) displays C−C distances of 1.198 A (trans to P) and 1.374 A (trans to the NHC). The NHC-Pd-allyl precatalysts are thought to generate monoligated phosphine-Pd(0) active species by reductive elimination of 2-allylimidazolium following η3−η1 isomerization of the allyl group. This was observed in the case of [Pd(η3-C3H5)(tmiy)(PCy3)]BF4 (4b), which successfully catalyzed the coupling of imidazolium salts with ethylene. The intriguing reactivity of ...

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.

Silver and palladium complexes of bis-NHC ligands bearing a weakly coordinating ether functionality have been prepared as counterparts to the well-studied pyridine-functionalized bis-NHC pincer ligands. The Ag(I) complex of bis[2-(3-methylimidazolin-2-yliden-1-yl)ethyl] ether (MeCOC) was prepared through the reaction of the corresponding bis-imidazolium salt with Ag2O and crystallographically characterized as the triflate salt. The structure reveals a dinuclear [Ag2(MeCOC)2](OTf)2 formulation and the presence of two separate forms in the crystal lattice arising from varying conformations of the ether bridging groups. The related pentamethylene-bridged bis-NHC ligand 1,5-bis(3-methylimidazolin-2-yliden-1-yl)pentane was analogously complexed to Ag(I) and crystallized as the [AgBr2]- salt. A single-crystal X-ray study of the pentamethylene-bridged complex revealed continuous ladders of quasi-planar dinuclear cations linked by [AgBr2]- anions; the divergent metal−ligand associations in the two Ag(I) compounds...

Novel, well-defined, coordinatively unsaturated NHC palladium(0) and nickel(0) complexes have been synthesized and fully characterized. Mixed N-heterocyclic carbene−olefin complexes, M0(NHC)n(DMFU)m (where M = Ni, Pd, NHC = 1,3-dimesitylimidazolin-2-ylidene, and DMFU = dimethyl fumarate; for Pd n = 1, m = 2 and n = 2, m = 1; for Ni n = 1, m = 1, 2 and n = 2, m = 1) have been prepared, including a dimeric Ni complex in which the DMFU acts as a bridging ligand, via the double bond and one carbonyl oxygen. The bis-NHC complexes of Ni0 and Pd0 undergo facile ligand exchange equilibria in solution in the presence of excess olefin (DMFU), leading to mono-NHC−M0−(olefin) complexes. This exchange is driven by the formation of an NHC−olefin coupling product. The dimeric Ni complex is labile in solution and can be used in situ, to provide a source of the L−Ni0 moiety. The complexes M0(NHC)n(DMFU)m (M = Pd, n = 1, m = 2; M = Ni, n = 1, m = 1, 2 and n = 2, m = 1) could be isolated as pure crystalline compounds in the...

A series of discrete, mononuclear palladium(II)–methyl complexes, together with several palladium(II)–chloro analogues, of pyridine-functionalised bis-NHC ligands have been prepared via ligand transmetallation from the silver(I)-NHC complexes. The reported complexes comprise examples with both the methylene-bridged 2,6-bis[(3-R-imidazolin-2-yliden-1-yl)methyl]pyridine (RC∧N∧C; R = Mes, dipp, tBu) and planar 2,6-bis(3-R-imidazolin-2-yliden-1-yl)pyridine (RCNC; R = Mes, dipp) ligands and, when combined with the previously reported MeC∧N∧C/MeCNC examples, cover a broad spectrum of ligand substituent steric and electronic properties, including the bulky Mes and dipp groups frequently used in catalytic applications. The palladium(II) complexes have been characterised by a variety of methods, including single crystal X-ray crystallography, with the shielding of the Pd–Me groups in the proton NMR spectra of some of the N-aryl substituted examples correlated with the proximity of the aryl rings to the methyl group in the solid state structures. The [PdMe(RC∧N∧C/RCNC)]+ complexes undergo thermal degradation via reductive methyl-NHC coupling to give 2-methyl-3-R-imidazolium-1-yl species with relative stabilities in the order of [PdMe(MesC∧N∧C)]BF4 > [PdMe(MeC∧N∧C)]BF4 ≈ [PdMe(MesCNC)]BF4 > [PdMe(MeCNC)]BF4 > [PdMe(tBuC∧N∧C)]BF4 ≫ [PdMe(tBuCNC)]BF4 (not isolable). A comparison of the activity of the complexes as precatalysts in a model Heck coupling reaction shows greatest activity in those species bearing bulkier N-substituents, with complexes bearing RC∧N∧C ligands generally more efficient precatalysts than those bearing RCNC ligands.

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.

The title compound, K[bimCO2]·2/3H2O (C11H19N2O2K· 2/3H2O), has been synthesized, and characterized by a single crystal X-ray study; it has the grouping [K3(bimCO2)3(OH2)2] cross-linked into a polymeric sheet in which potassium environments are diverse and unsymmetrical, containing six-coordinate and seven-coordinate geometries.

New furan- and thiophene-functionalised nucleophilic heterocyclic carbene (NHC) complexes of Ag(I) were prepared via the reaction of novel furan- and thiophene-functionalised bis-imidazolium salts with Ag 2 O. Samples of both the N -methyl substituted furan- and thiophene-functionalised Ag(I) complexes suitable for single crystal X-ray studies were obtained following anion metathesis to the tetrafluoroborate salts. The structural characterisations revealed dinuclear [Ag 2 ( Me CEC) 2 ](BF 4 ) 2 (E = O, S) formulations with discrete twenty-membered dimetallacycles present in both instances; however, the overall molecular conformation varies considerably, notably in the orientations of the two bridging furan or thiophene heterocycles to the silver coordination plane. The functionalised bis-imidazolium salts were tested as in situ additives in a Pd(0)-catalysed aryl amination coupling reaction, with the best observed activities around 20% of those seen with 1,3-bis(2,6-di- iso -propylphenyl)imidazolium chloride under identical conditions. The bulkier N - t Bu and N -mesityl substituted salts were found to be more active than the N -methyl substituted analogues.

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 dimerization reactions of a series of heterocyclic carbenes based on the 1,3-dimethylimidazol-2-ylidene template were studied extensively using molecular orbital calculations. The carbenes studied were the 1,3-XY five-membered ring heterocyclic carbenes with all 10 possible combinations of X,Y = NCH3, PCH3, O and S. Two different mechanisms for dimerization were studied: a direct carbene plus carbene dimerization reaction and a proton-catalysed dimerization. The parent carbene with XY = NN is found to be kinetically and thermodynamically stable under both mechanisms, whereas XY = NS is predicted to be kinetically stable under aprotic conditions. All remaining carbenes were predicted to be not particularly stable towards dimerization. Indirect schemes for calculating the stability of carbenes towards dimerization were found to give good estimates of the enthalpy of dimerization, but do not take into account the activation barriers. Copyright © 2004 John Wiley & Sons, Ltd.

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.

The first of the title compounds, [Fe(C9H10N4O)3]Cl2·5.5H2O, is the first structurally characterized homoleptic tris­[bis­(imidazol-2-yl) ­ketone]–metal complex to be structurally defined. In the second of the title compounds, [FeCl2(C9H10N4O)2], a pair of monodentate donors supplants one of the bidentate ligands. In the third complex, [Fe2Cl4(C9H10N4O)2]·2CH3OH, a centrosymmetric binuclear compound with a pair of bridging chloride ligands, the introduction of a third chloro ligand results in five-coordination about each of the metal atoms; the environments of the latter are square pyramidal, with a chloro ligand at the apex and the two bidentate arrays about the base of the pyramid.

The comparison of nucleophilic heterocyclic carbenes with phosphines and the premise that they are phosphine analogues is a useful but limited concept. Electronically and sterically NHC’s are different. These differences lead to unique chemical behavior where the NHC can act as a spectator ligand or as a participative ligand—finishing up in the products in altered form. Here, we review the redox processes undergone by the NHC/imidazolium salt couple in conjunction with Group 10 metals. The often facile reductive elimination reaction of hydrocarbylM-carbene complexes, yielding 2-substituted imidazolium salt and M(0) is examined, with factors influencing the reaction reviewed in some detail. Conversely, the related imidazolium salts (the important imidazolium based ionic liquids (IL’s)) may generate carbene complexes by oxidative addition to low valent metal centres. The significance of these reactions for the application of carbene complexes in catalysis, and for the use of imidazolium based ionic liquids as solvents is manifest.

Tetrafluoroborate anion boronfluorine bond activation, yielding a boron trifluoride-nucleophilic heterocyclic carbene (NHC) adduct, is reported. Bis-[2-(2-(trifluoroborane)-3-methylimidazolin-2-yliden-1-yl)ethyl]ether (4) was obtained as a by-product from the reaction of di-silver-bis-[bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether] di-tetrafluoroborate (3) and ZrCl4. The AgI(NHC)(BF4) complex was prepared from bis-(2-(3-methylimidazolium-1-yl)ethyl)ether diiodide (2) and silver(I) oxide; spectroscopic data suggests that it is a dinuclear species. Compounds 2–4 were spectroscopically characterised, the structures of the NHC:BF3 adduct and parent bis-imidazolium salt being confirmed by single crystal X-ray studies. The geometry of 4 is similar to that of previously reported NHC:BF3 adducts. Heating the NHC:BF3 adduct in dimethyl-d6 sulfoxide (d6-DMSO) in the presence of CsF leads to complete hydrogen/deuterium substitution of the imC4,5 positions. It was found that the H/D substitution was readily effected at room temperature on the free carbene bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether. Prolonged heating was required to observe a similar H/D substitution in 3, preliminary to its decomposition, and (2-(3-methylimidazolium-1-yl)ethyl)ether tetrafluoroborate was found to be resistant to substitution.

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.

A theoretical study of methyl-Pd heterocyclic silylene and germylene complexes has been carried out and reveals a very low activation barrier for methyl migration to the silylene or germylene ligand, but indicates that, in the absence of solvent or counterion effects, reductive elimination of silicenium or germacenium cations is less likely. The results have been compared with those for the more common N-heterocyclic carbenes, and the activation barrier for coupling of the methyl group with C/Si/Ge was found to follow the order Si < Ge < C. The effect of solvent on the reaction has been modeled by incorporation of THF into the calculations on the silylene, which lowers the energy of all stationary points on the potential energy surface. Furthermore, with THF present reductive elimination of the silicenium cation becomes feasible. The results are discussed in terms of the implications they have on the potential application of these ligands in homogeneous catalysis.

The syntheses and characterisation of tri- and bi-dentate coordinated chromium(III) and vanadium(III) complexes of the general composition [MCl3(N⁁D⁁N)], [MCl3(N⁁N)]2 {M = Cr, (N⁁D⁁N) = (mim)3COCH32a [mim = 1-methylimidazolyl], (mim)2CHCH2PPh22b, (mim)2CHCH2C(O)But2c, (tBupim)3P 2d [tBupim = 1-isopropyl-4-tert-butylimidazolyl], (N⁁N) = (mim)2CH23e, (mim)2CH2PPh23f, (mim)2CO 3g, (Bzmim)2CO 3h [Bzmim = 1-methybenzimidazolyl], (tBupim)2CO 3i, (mim)2CNPh 3j; M = V: (N⁁D⁁N) = (mim)3COCH35a; (N⁁N) = (mim)2CO 5g}, [CrCl3(mim)3] 4k, and [CrCl3(MeTAM)] 6 (MeTAM = 1-methyltriacetylmethane) is described. Crystallisation of 3g from CH3CN/Et2O gave the mononuclear complex [CrCl3{(mim)2CO}(CH3CN)] 3g′. The molecular structure of 3g′ shows the chromium atom is quasi-octahedral, six-coordinate, the three coordinated chlorine atoms disposed mer in the coordination sphere. The electronic spectra of the chromium complexes exhibit d–d transitions typical of a pseudo-octahedral coordinated d3 ion, falling into the region ν14A2g → 4T2g 600–700 nm and ν24A2g → 4T1g(F) 430–470 nm, and 10Dq values between 14 400 and 16 700 cm−1. In the presence of MMAO the complexes give active catalyst systems for the conversion of ethylene into 1-alkenes or polymers, with activities and selectivities depending on the electronic and steric factors of the ligand system and the metal centre, respectively.

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.

The first example of a tandem reaction involving double-bond migration in combination with telomerization is reported. Homogeneous and heterogeneous Ru catalysts were employed as isomerization catalysts, and telomerization was realized using a homogeneous Pd(0) precursor complex with a N-heterocyclic carbene (IMes) ligand. Overall conversions approaching 60% were achieved with the best selectivity to telomerization products of 91% attained at 11% conversion. Conversion was markedly higher in the presence of longer-chain alcohol (1-butanol) as the nucleophile (telogen).

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...

Novel methylpalladium(II) complexes of the general composition [PdCl(CH3){N∧D∧N}] ({N∧D∧N} = (mim)3COCH3 (1a), (mim)2CHCH2C(O)Bu-t (3a)) were synthesized from [PdCl(CH3)(COD)] and the respective N-methylimidazole (mim) ligands. The cationic derivatives [Pd2(CH3)2{N∧D∧N}2](BF4)2 (1b; {N∧D∧N} = (mim)3COCH3) and [Pd(CH3){N∧D∧N)(CH3CN)](BF4) (3b; {N∧D∧N} = (mim)2CHCH2C(O)Bu-t) were obtained by halide abstraction with AgBF4. The effect of varying the third donor moiety (D = “O”, “N”, “P”) in the structurally very similar ligands on the coordination chemistry and reactivity of 1 and 3 and the previously reported [PdCl(CH3){N∧D∧N}] (2a; {N∧D∧N} = (mim)2CHCH2PPh2) was investigated in detail. Thus, a neutral monomeric and an ionic dimeric isomer were identified for 1a, with the ligand adopting a σ2-N∧N coordination mode in the former and a σ3-N∧N∧N coordination mode in the latter. Exclusive formation of the ionic dimer occurs in methanol. Crystal structures are recorded for 1b and [Pd2Cl(CH3){N∧N∧N}2]Cl2 (1c). Eac...

Reaction of (THF)3RCrCl2 (R = Cl, Me) with tri- and bidentate bis(N-methylimidazole-2-yl) ligands gave the new Cr(III) complexes [Cr(N∧N∧Y)Cl3] (3a−c), [Cr(N∧N)Cl3]2 (4a−c), and [Cr(Me)(N∧N∧Y)Cl2] (5a,b) in ≥90% yield. Toluene solutions of 3,4/MMAO convert ethylene into linear 1-alkenes with selectivities of up to 79%.

The reaction of the functionalized bis(imidazolium) salt 1,3-bis(1-methylimidazolium-3-yl)propan-2-one diiodide with palladium(II) acetate gave as the sole product the unexpected complex cis-I 2Pd[(CHR) 2C(O)] (R = 1-methylimidazolium-3-yl). Single-crystal X-ray crystallography and infrared spectroscopy revealed a highly puckered palladacyclobutan-3-one ring with an η 3-allylic (η 3-oxodimethylenemethane) bonding disposition of Pd(II) to the [C-C(O)-C] 2- fragment. No imidazolin-2-ylidene formation was evident. The neutral complex may be viewed as zwitterionic, with the negatively charged ligands at the Pd(II) center balanced by the positive charges on the pendant imidazolium groups.

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).