A T-shaped three-coordinate nickel(I) carbonyl complex and the geometric preferences of three-coordinate d9 complexes

A three-coordinate diketiminate-nickel(I) complex with a carbonyl ligand has been characterized using EPR and IR spectroscopies and X-ray crystallography. The T geometry (bending from the sterically favored C(2)(v)() structure) contrasts with that of isosteric d(9) copper(II) complexes. DFT calculations on a truncated model reproduce experimental geometries, implying that the geometric differences are electronic in nature. Analysis of the charge distribution in the complexes shows that the geometry of the three-coordinate d(9) complexes is affected by differential charge donation of the ligands to the metal center.     

New tetra- and pentacoordinate nickel(I) complexes

Summary The reduction of nickel(II) halides with NaBH₄ in ethanol has been studied in the presence of various tertiary phosphines and arsines. Complexes of the type XNiL₃ have been isolated in this way when X = Cl, Br, I and L = PPh₃, AsPh₃, no reaction being observed when L = PEt₃, PBu₃ and Ph₂P(CH₂)₂PPh₂.The reaction of XNiL₃ with CO gas at room temperature produces pentacoordinate carbonyl complexes XNi(CO)₂L₂ when L is triphenylphosphine. The lack of stability prevents the isolation of similar complexes when L is trip henylarsine.Structural data obtained by i.r. spectroscopy and susceptibility measurements as well as chemical behaviour of the new complexes are described.     

Four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with brightly luminescence properties

Three four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes, [Cu(Py-Im)(POP)](PF₆) (P1), [Cu(Py-BenIm)(POP)](PF₆) (P2), and [Cu(Py-c-BenIm)(POP)](PF₆) (P3) (Py-Im = 3-methyl-1-(pyridin-2-yl)-1H-imidazolylidene, Py-BenIm = 3-methyl-1-(pyridin-2-yl)-1H-benzo[d]imidazolylidene, Py-c-BenIm = 3-methyl-1-(pyridin-2-ylmethyl)-1H-benzo[d]imidazolylidene, POP = bis([2-diphenylphosphino]-phenyl)ether), have been synthesized without transmetalation of the NHC–Ag(I) complex for the first time. The photophysical properties of the resultant NHC–Cu(I) complexes have been systematically investigated via spectroscopic methods. All complexes exhibit good photoluminescence properties with long excited-state lifetimes and moderate quantum yields. Density functional theory and time dependent density functional theory calculations were employed to rationalize the photophysical properties of the NHC–Cu(I) complexes.     

Synthesis, isolation and characterization of cationic gold(I) N-heterocyclic carbene (NHC) complexes

A number of cationic gold(I) complexes have been synthesized and found to be stabilized by the use of N-heterocyclic carbene ligands. These species are often employed as in situ-generated reactive intermediates in gold catalyzed organic transformations. An isolated, well-defined species was tested in gold-mediated carbene transfer reactions from ethyl diazoacetate.     

Decarboxylation of aromatic carboxylic acids by gold(I)-N-heterocyclic carbene (NHC) complexes

A recently isolated gold(I) complex, [Au(IPr)(OH)], permits the transformation of carboxylic acids to the corresponding decarboxylated gold(I)-aryl complex without the use of silver co-catalyst under mild reaction conditions.     

Chiral iridium(I) bis(NHC) complexes as catalysts for asymmetric transfer hydrogenation

The common use of NHC complexes in transition‐metal mediated C–C coupling and metathesis reactions in recent decades has established N‐heterocyclic carbenes as a new class of ligand for catalysis. The field of asymmetric catalysis with complexes bearing NHC‐containing chiral ligands is dominated by mixed carbene/oxazoline or carbene/phosphane chelating ligands. In contrast, applications of complexes with chiral, chelating bis(NHC) ligands are rare. In the present work new chiral iridium(I) bis(NHC) complexes and their application in the asymmetric transfer hydrogenation of ketones are described. A series of chiral bis(azolium) salts have been prepared following a synthetic pathway, starting from L ‐valinol and the modular buildup allows the structural variation of the ligand precursors. The iridium complexes were formed via a one‐pot transmetallation procedure. The prepared complexes were applied as catalysts in the asymmetric transfer hydrogenation of various prochiral ketones, affording the corresponding chiral alcohols in high yields and moderate to good enantioselectivities of up to 68%. The enantioselectivities of the catalysts were strongly affected by the various, terminal N‐substituents of the chelating bis(NHC) ligands. The results presented in this work indicate the potential of bis‐carbenes as stereodirecting ligands for asymmetric catalysis and are offering a base for further developments. Copyright © 2010 John Wiley & Sons, Ltd.     

Silicon hydrides and nickel complexes : I. Phosphine-nickel(II) complexes as hydrosilylation catalysts

A variety of dihalodiphosphinenickel(II) complexes have been found to be effective catalysts for hydrosilylation of olefins and acetylenes. Reactions with nickel(II) catalysts, at least those containing a bidentate phosphine ligand, scarcely proceed below 90°. At 120° the reaction proceeds smoothly to form, in many cases, an unexpected (“abnormal”) adduct which has arisen from an interchange of hydrogen and chlorine on silicon in addition to the expected (“normal”) one. Both total yields, and the ratios of the “abnormal” to the “normal” adduct increase, in general, with increasing electron-donating ability of both the phosphine ligands and the substituents on the olefins. Isomerization of olefins occurs during the course of hydrosilylation. cis-Addition occurs preferentially. The reactivity of the silicon hydrides decreases in the order HSiCl₃ ? HSiMeCl₂ ? HSiMe₂Cl ? HSiMe₃.     

Stereoselective synthesis of cationic heterobidentate C(NHC)/SR rhodium(I) complexes using stereodirecting N,N-dialkylamino groups

The synthesis of two different types of chiral C/S ligands based upon N-(N,N-dialkylamino)-substituted N-heterocyclic carbenes and thioether functionalities, along with their neutral [RhCl(CNH)(COD)] and cationic [Rh(I)(NHC/S)(COD)]⁺ complexes, has been accomplished. (S)-2-[(Phenylthio)methyl]pyrrolidine, carrying the thioether moiety, and (2S,5S)-2,5-diphenylpyrrolidine, combined with a thioether functionalized side chain, were studied as potential stereodirecting groups. Only the latter provided high selectivity in the formation of the neutral complex, leading to a single atropoisomer (de >98%) of the newly formed, configurationally stable C(NHC)–Rh bond. The synthesis of the corresponding cationic [Rh(I)(NHC/S)(COD)]⁺ complexes, however, resulted in the formation of single (R_a,S_S) and (S_a,S_S) diastereomers, respectively, of the four possible complexes in each case [combinations of the (R_a/S_a) C(NHC)–Rh axis and the (S_s/R_s) stereogenic S center formed upon coordination]. For the proline derivative, the resolution of the mixture of (R_a/S_a)-[RhCl(CNH)(COD)] neutral complexes proceeds via dynamic kinetic resolution through coordinatively unsaturated Rh(I) intermediates formed after halide abstraction. The absolute configurations of both types of cationic complexes were unequivocally assigned on the basis of X-ray diffraction analysis.     

New cationic and anionic tetracoordinate nickel(I) complexes

Summary The preparation, structural study and chemical behaviour of new cationic, monoanionic and dianionic tetracoordinate nickel(I) complexes of the types: [NiL₄][BPh₄] (L=PPh₃, AsPh₃ or SbPh₃), [PR₄][NiX₂L₂] (X=Cl, Br or I; L=PPh₃, AsPh₃ or SbPh₃ and [PR₄]⁺=PPh₄, Ph₃PCH₂Ph or Ph₃PEt) and [PR₄]₂[NiX₃L] (X=Cl, Br or I; L=PPh₃ and [PR₄]⁺=PPh₄ or PPh₃CH₂Ph) are described.     

Paramagnetic complexes of nickel(I) stabilized by norbornadiene

Nickel(I) compounds whose concentration was 10^–4–10^–6 of the total concentration of nickel added to the system were identified by EPR in the reaction of 2,5-norbornadiene with nickel homoligand allyl complexes Niall₂ (all is C₃H₅, 1-CH₃C₃H₄, or 2-CH₃C₃H₄). The Ni(I) complexes were stable at room temperature under oxygen-free conditions. It was shown that the paramagnetic complexes were in equilibrium with diamagnetic forms. The temperature dependence of the concentration of the paramagnetic species was determined. The structure of the paramagnetic nickel(I) complexes and the possible routes of their formation are discussed on the basis of the obtained data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 490–493, July–August, 1990.     

(Fluoroalkyl)phosphine complexes of nickel(0) and cobalt(I)

The synthesis of a series of (fluoroalkyl)phosphine complexes of nickel is reported. Treatment of (cod)₂Ni with dfepe (dfepe=(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂) yields (dfepe)Ni(cod) (1), which has been structurally characterized. Treatment of 1 with CO or bipy results in the formation of (dfepe)Ni(CO)₂ (2) and (dfepe)Ni(bipy) (3), respectively. Addition of excess dfepe to 1 results in incomplete cod displacement to form (dfepe)₂Ni (4). The homoleptic complex 4 may be independently prepared in high yield by reduction of (acac)₂Ni with (ⁱBu)₃Al in the presence of butadiene and excess dfepe. Solvation of (dfepe)Ni(cod) in acetonitrile gives a new complex tentatively identified as (dfepe)Ni(MeCN)₂ (6), whereas dissolution of (dfepe)₂Ni in acetonitrile leads to a mixture of 6 and the partial displacement product (dfepe)(η¹-dfepe)Ni(MeCN) (5). In contrast to (R₃P)₄Ni(0) phosphine and phosphite complexes, which undergo protonation by strong anhydrous acids such as HCl, H₂SO₄ and CF₃CO₂H to give (R₃P)₄Ni(H)⁺ products, Treatment of (dfepe)₂Ni with neat CF₃CO₂H or excess HOTf in dichloromethane gave no spectroscopic evidence for (dfepe)₂Ni(H)⁺. Exposure for extended periods leads to dfepe loss and decomposition to Ni(II) products. The synthesis of the first cobalt complex of dfepe, (dfepe)Co(CO)₂H, is also reported.     

Efficient synthesis of fumaric amides through cross-metathesis of acrylic amides with the NHC Grubbs ruthenium catalyst

Application of the second generation Grubbs metathesis catalyst for the homo-cross-metathesis of acroyl amides from chiral amines is reported. This efficient and high-yielding reaction provides a side-product free synthesis of fumaric acid diamides which are formed with complete (E)-selectivity under the reaction conditions. In particular, products which cannot be synthesized from the corresponding fumaroyl chloride via classic condensation route can now be provided in excellent yields (88–98%) with a catalyst loading from 2.5 mol% to even 0.375 mol%.     

Bulky guanidinato nickel(I) complexes: synthesis, characterization, isomerization, and reactivity studies

Reactions of lithium complexes of the bulky guanidinates [{(Dip)N}(2)CNR(2)](-) (Dip=C(6)H(3)iPr(2)-2,6; R=C(6)H(11) (Giso(-)) or iPr (Priso(-)), with NiBr(2) have afforded the nickel(II) complexes [{Ni(L)(μ-Br)}(2)] (L=Giso(-) or Priso(-)), the latter of which was crystallographically characterized. Reduction of [{Ni(Priso)(μ-Br)}(2)] with elemental potassium in benzene or toluene afforded the diamagnetic species [{Ni(Priso)}(2)(μ-C(6)H(5)R)] (R=H or Me), which were shown, by X-ray crystallographic studies, to possess nonplanar bridging arene ligands that are partially reduced. A similar reduction of [{Ni(Priso)(μ-Br)}(2)] in cyclohexane yielded a mixture of the isomeric complexes [{Ni(μ-κ(1)-N-,η(2)-Dip-Priso)}(2)] and [{Ni(μ-κ(2)-N,N'-Priso)}(2)], both of which were structurally characterized. These complexes were also formed through arene elimination processes if [{Ni(Priso)}(2)(μ-C(6)H(5)R)] (R=H or Me) were dissolved in hexane. In that solvent, diamagnetic [{Ni(μ-κ(1)-N-,η(2)-Dip-Priso)}(2)] was found to slowly convert to paramagnetic [{Ni(μ-κ(2)-N,N'-Priso)}(2)], suggesting that the latter is the thermodynamic isomer. Computational analysis of a model of [{Ni(μ-κ(2)-N,N'-Priso)}(2)] showed it to have a Ni-Ni bond that has a multiconfigurational electronic structure. An analogous copper(I) complex [{Cu(μ-κ(2)-N,N'-Giso)}(2)] was prepared, structurally authenticated, and found, by a theoretical study, to have a negligible Cu···Cu bonding interaction. The reactivity of [{Ni(Priso)}(2)(μ-C(6)H(5)Me)] and [{Ni(μ-κ(2)-N,N'-Priso)}(2)] towards a range of small molecules was examined and this gave rise to diamagnetic complexes [{Ni(Priso)(μ-CO)}(2)] and [{Ni(Priso)(μ-N(3))}(2)]. Taken as a whole, this study highlights similarities between bulky guanidinate ligands and the β-diketiminate ligand class, but shows the former to have greater coordinative flexibility.     

Bis(diphenylphosphino)ethane nickel polychloridophenylthiolate complexes: synthesis and characterization

Transition Metal Chemistry - Reactions of (dppe)NiCl2 (dppe?=?1,2-bis(diphenylphosphino)ethane) with chlorido-substituted phenyl thiolates (RClS?) produced the corresponding...     

Modified (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes for room-temperature Suzuki-Miyaura and Buchwald-Hartwig reactions

A series of (NHC)Pd(R-allyl)Cl complexes [NHC: IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr = N,N'-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene; R = H, Me, gem-Me2, Ph] have been synthesized and fully characterized. When compared to (NHC)Pd(allyl)Cl, substitution at the terminal position of the allyl scaffold favors a more facile activation step. This translates into higher catalytic activity in the Suzuki-Miyaura and Buchwald-Hartwig reactions, allowing for the coupling of unactivated aryl chlorides at room temperature in minutes. In the Suzuki-Miyaura reaction, aryl triflates, bromides, and chlorides react with boronic acids using very low catalyst loading. In the N-aryl amination reaction, a wide range of substrates has been coupled efficiently; primary-, secondary-, alkyl-, or aryl-amines react in high yields with unactivated, neutral, and activated aryl chlorides and bromides. In both reactions, extremely hindered substrates such as tri-ortho-substituted biaryls and tetra-ortho-substituted diarylamines can be produced without loss of activity. Finally, the present catalytic system has proven to be efficient with as low as 10 parts-per-million (ppm) of precatalyst in the Buchwald-Hartwig reaction and 50 ppm in the Suzuki-Miyaura reaction.     

Synthesis and characterization of nickel inverse 9-metallacrown-3, palladium-silver, and dinuclear platinum complexes containing pyrazole-functionalized NHC ligands

Three metallacrown nickel complexes [Ni(3)(μ-OH)(L1)(3)](PF(6))(2) (1, L1 = 3-((N-methylimidazolylidenyl)methyl)-5-methylpyrazolate), [Ni(3)(μ-OH)(L2)(3)](PF(6))(2) (2, L2 = 3-((N-mesitylimidazolylidenyl)methyl)-5-methylpyrazolate), and [Ni(3)(μ-OH)(L3)(3)](PF(6))(2) (3, L3 = 3-((N-pyrimidin-2-ylimidazolylidenyl)methyl)-5-methylpyrazolate) were obtained by the reactions of corresponding silver-NHC complexes with Raney nickel powder at 45 °C. The same reaction at 80 °C afforded [Ni(3)(L2)(4)](PF(6))(2) (4). The carbene-transfer reaction of the silver-carbene complex with [(η(3)-C(3)H(5))PdCl](2) yielded the heterotrimetallic complex [AgPd(2)(η(3)-C(3)H(5))(2)(L2)(2)](PF(6)) (5), whereas the carbene-transfer reaction with Pt(cod)Cl(2) gave [Pt(2)(L3)(2)](PF(6))(2) (6). All of these complexes have been fully characterized by ESI-MS, NMR spectroscopy, and elemental analysis. The molecular structures of 1-6 were also studied by X-ray diffraction analysis. In 1-3, three nickel centers are bridged together by three pyrazole-NHC ligands and a hydroxide group, forming a 9-metallacrown-3 topology. Complex 4 is paramagnetic, consisting of two square-planar nickel(II) ions and one tetrahedral nickel ion in which three Ni ions are bridged by four pyrazolate units. In the mixed Pd-Ag complex 5, two palladium and one silver centers are bridged by two pyrazole-NHC ligands. Complex 5 showed good catalytic activity in the Sonogashira coupling reaction of aryl bromides and phenylacetylene under mild conditions typically catalyzed by Pd-Cu systems.     

Chromium(0)-rhodium(I) metal exchange: Synthesis and X-ray structure of new Fischer (NHC)carbene complexes of rhodium(I)

An easy approach to Fischer (NHC)carbene complexes of rhodium(I) 3 from methoxy- and aminocarbene complexes of chromium 1 and (NHC)(cod)RhCl (2) is described. The process involves the transfer of the carbene unit and a CO ligand from chromium to rhodium. The X-ray analysis is provided for 3d and the preliminary results on their thermal stability and reactivity toward alkynes and allenes are also reported.     

Solvent-free mechanochemical synthesis of dithiophosphonic acids and corresponding nickel(II) complexes

We report a green chemistry route for dithiophosphonic acids of the type [HS₂P(OR)(4-MeOC₆H₄)] [R = H, (1); Me (2); Et (3); ⁱPr (4)]. The different dithiophosphonic acids formed through the stoichiometric addition of water or alcohols to Lawesson's Reagent (molar ratio 2:1), followed by an intimate grinding of the mixture (mechanochemistry). The products formed without the use of solvent or external heat in less than 5 minutes. The acids are formed with 100% atom economy, and because they form in essentially quantitative yield, are also formed with >98% atom efficiency and an E-factor = 0, because no waste is produced. Of importance is that this methodology is different from conventional methods in forming dithiophosphonic acids where the use of organic solvents, added heat, long reaction times and lower yields are commonplace. We further demonstrate that nickel(II) complexes can form directly from the in situ generated acids. Thus, the reaction between 1–4 and NiCl₂ ? 6 H₂O (molar ratio 2:1) lead to complexes of the type [Ni{S₂P(OR)(4-MeOC₆H₄)}₂] [R = H, (5); Me (6); Et (7); ⁱPr (8)] with no use of organic solvent. All compounds were characterized or verified by a combination of ¹H, ³¹P NMR, elemental analysis (solids), and FT-IR.     

A method for the synthesis of nickel(0) bis(carbene) complexes

A new method leading to Ni(NHC)2 (NHC = IMes, IPri, SIPr(i), SIBu(t)) complexes in moderate to good yields, involves the reaction of NHC (pre-formed or generated in situ) with Ni(CH3)2(tmed), tmed = N,N'-tetramethylethylenediamine; in one case, the intermediate Ni[I(Me2)Pr(i)]2(CH3)2, I(Me2)Pr(i) = N,N'-diisopropyl-4,5-dimethylimidazol-2-ylidene, has been isolated and structurally characterised.     

Homoleptic complexes of cobalt(0) and nickel(0,I) with 1,1'-bis(diphenylphosphino)ferrocene (dppf): synthesis and characterization

Reduction of Co(dppf)Cl2 with 2 equiv of sodium naphthalenide in THF, in the presence of dppf, affords the homoleptic complex Co(dppf)2, 1, isolated in 65% yield as a brick red solid, extremely air sensitive. In solution, under inert atmosphere, 1 slowly decomposes into Co and dppf, following a first-order kinetic law (t1/2 = 21 h at 22 degrees C). Similarly to the Rh and Ir congeners, 1 undergoes a one-electron reversible reduction to [Co(dppf)2]-. Attempts to obtain this d10 species by chemical as well as electrochemical reduction of 1 lead to the hydride HCo(dppf)2, 2, as the only product that can be isolated. Reduction of Ni(dppf)Cl2 with sodium in the presence of dppf and catalytic amounts of naphthalene affords Ni(dppf)2, 3, isolated in 60% yield as a yellow air stable solid. The stoichiometric oxidation of 3 with [FeCp2]PF6 forms the d9 complex [Ni(dppf)2]PF6, 4, which represents the second example of a structurally characterized Ni(I) complex stabilized by phosphines. A single-crystal X-ray analysis shows for the metal a distorted tetrahedral environment with a dihedral angle defined by the planes containing the atoms P(1), Ni, P(2) and P(3), Ni, P(4) of 78.2 degrees and remarkably long Ni-P bond distances (2.342(3)-2.394(3) A). The EPR spectroscopic properties of 1 (at 106 K in THF) and 4 (at 7 K in 2-methyl-THF) have been examined and g tensor values measured (1, gx = 2.008, gy = 2.182, gz = 2.326; 4, gx = 2.098, gy = 2.113, gz = 2.332). A linear dependence between the hyperfine constants and the Ni-P bond distances has been evidenced. Finally, the change with time of the EPR spectrum of 4 indicates that it very slowly releases dppf.