Synthesis of new cyclic aromatic carbene ligands bearing remote amino groups and their palladium(ii) complexes

New cyclic aromatic carbene ligands bearing remote amino groups were developed. The oxidative addition of Pd(0) into the chloride precursors yielded the corresponding Pd(ii) complexes whose carbene ligand was demonstrated to have stronger donor ability than classical NHCs.     

Pd(II) complexes of N,S-heterocyclic carbenes with pendant and coordinated allyl function and their Suzuki coupling activities

3-(2-Propenyl)benzothiazolium bromide () provides a direct and simple entry to Pd(ii) complexes with N,S-heterocyclic carbene (NSHC) ligands functionalized with an allyl pendant with hemilabile potential. Addition of salt to Pd(OAc)(2) eliminates HOAc and affords the bis(carbene) complexes cis-[PdBr(2)(NHSC)(2)] (cis-, NSHC = 3-(2-propenyl)benzothiazolin-2-ylidene) and trans-[PdBr(2)(NHSC)(2)] (trans-) along with the monocarbene complexes [PdBr(2)(NSHC)] () and trans-[PdBr(2)(benzothiazole-kappaN)(NSHC)] () as minor side products. Salt-metathesis of cis- with AgO(2)CCF(3) yields the mixed dicarboxylato-bis(carbene) complex cis-[Pd(O(2)CCF(3))(2)(NSHC)(2)] (). Complexes cis-, trans- and were characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and elemental analysis. The molecular structures of complexes cis-, and have been determined by X-ray single crystal diffraction. Complexes cis- and as well as an in situ mixture of Pd(OAc)(2) and salt are active toward Suzuki-Miyaura coupling of aryl bromides and activated aryl chlorides giving good conversions.     

Metal chelates of benzeneazo-<Emphasis Type="Italic">N</Emphasis>-tosyl-2-naphthylamine

New Cu(II), Co(II), and Pd(II) complexes with benzeneazo-N-tosyl-2-naphthylamine are synthesized by chemical and electrochemical methods and characterized by IR, ¹H NMR, and XRD. XRD was used to determine that two six-membered metal rings are formed in transplanar Ni(II) and Pd(II) complexes.     

Chemical bonding and properties in [Ni(N-heterocylic carbene)(NO)(R)] (R = H, Me, HC=CH2, and C≡CH) complexes: Theoretical insights

A series of N-heterocyclic carbene nickel complexes of the type [Ni(N-heterocylic carbene)(NO)(R)] (R = H, Me, HC=CH₂, and C≡CH) are examined to study the influence of a substituent on the molecular structure and bonding of these complexes. Geometrical and AIM analyses of the interaction between Ni and the carbene fragment reveal that for the metal-carbene bond donation is more important than back-donation. The NICS values suggest that aromaticity in the heterocyclic ring is less than in the free heterocycle.     

Synthesis,characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

A series of monomeric palladacycle complexes bearing n‐butyl‐substituted N‐heterocyclic carbenes, namely [Pd(NHC)X(dmba)] (dmba: dimethylbenzylamine and [Pd(NHC)X(ppy)]; NHC: 1‐n‐butyl‐3‐substituted benzylimidazol‐2‐ylidene; ppy: 2‐phenylpyridine), were prepared either by transmetallation from the corresponding silver carbene complexes or by the reaction of the corresponding acetate‐bridged palladacycle dimer with N‐heterocyclic carbene ligands in high yields. The palladium(II) complexes were characterized using elemental analyses, APCI‐MS, ¹H NMR and ¹³C NMR spectroscopies. These complexes are efficient in the Suzuki–Miyaura coupling reaction between phenylboronic acid and aryl bromides.     

Influence of the chelate effect on the electronic structure of one-electron oxidized group 10 metal(II)-(disalicylidene)diamine complexes

The neutral and one-electron oxidized group 10 metal, Ni(II), Pd(II) and Pt(II), six-membered chelate Salpn (Salpn = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,3-propanediamine) complexes have been investigated and compared to the five-membered chelate Salen (N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethanediamine) and Salcn (N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) complexes. Reaction of the Salpn complexes with 1 equivalent of AgSbF(6) affords the oxidized complexes which exist as ligand radical species in solution and in the solid state. The solid state structures of the oxidized complexes have been determined by X-ray crystal structure analysis. While the Ni and Pt analogues exhibit an essentially symmetric coordination sphere contraction upon oxidation, the oxidized Pd derivative exhibits an asymmetric metal binding environment demonstrating at least partial ligand radical localization. In comparison to the oxidized Salen and Salcn complexes, the propyl backbone of the Salpn complexes leads to a larger deviation from a planar geometry in the solid state. The electronic structure of the oxidized Salpn complexes was further probed by UV-vis-NIR measurements, electrochemistry, EPR spectroscopy, and theoretical calculations. The intense NIR band for the one-electron oxidized Salpn complexes shifts to lower energy in comparison to the 5-membered chelate analogues, which is attributed to lower metal d(xz) character in the β-LUMO for the Salpn series. The reactivity of the one-electron oxidized Salpn complexes with exogenous ligands was also studied. In the presence of pyridine, the oxidized Ni analogue exhibits a shift in the locus of oxidation to a Ni(III) species. The oxidized PtSalpn complex rapidly decomposes in the presence of pyridine, even at low temperature. Interestingly, electronic and EPR spectroscopy suggests that the addition of pyridine to the oxidized Pd analogue results in initial dissociation of the phenoxyl radical ligand, likely due to the increased flexibility of the propyl backbone.     

Bonding Analysis of N‐Heterocyclic Carbene Tautomers and Phosphine Ligands in Transition‐Metal Complexes: A Theoretical Study

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal–ligand bonding in transition‐metal complexes that contain imidazole (IMID), imidazol‐2‐ylidene (nNHC), or imidazol‐4‐ylidene (aNHC). The calculated complexes are [Cl₄TM(L)] (TM=Ti, Zr, Hf), [(CO)₅TM(L)] (TM=Cr, Mo, W), [(CO)₄TM(L)] (TM=Fe, Ru, Os), and [ClTM(L)] (TM=Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID<nNHC<aNHC. The energy levels of the carbon σ lone‐pair orbitals suggest the trend aNHC>nNHC>IMID for the donor strength, which is in agreement with the progression of the metal–ligand bond‐dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand–metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)₅TM(L)] (L=nNHC, aNHC, IMID) with phosphine complexes (L=PMe₃ and PCl₃) shows that the phosphine ligands are weaker σ donors and better π acceptors than the NHC tautomers nNHC, aNHC, and IMID.     

Carbene complexes : IV. Far infrared and 31P NMR spectra of palladium and platinum carbene complexes

Infrared bands mainly associated with v(M—X₂) stretching modes (M = Pd or Pt, and X = Cl, Br, or P) have been identified in the spectra of 35 carbene complexes. Based on these results and on |¹J| (³¹ P—¹⁹⁵ Pt) the trans-influence of the carbene ligands is assessed.     

Synthesis of N-heterocyclic carbene palladium(II) bis-phosphine complexes and their decomposition in the presence of aryl halides

Methylpalladium(II) carbene complexes of the type [Pd(NHC)Me(P-P)]BF(4) (NHC = N-heterocyclic carbene, P-P = chelating phosphine) have been synthesised, the complex [Pd(tmiy)Me(dcype)]BF(4) (tmiy = 1,3,4,5-tetramethylimidazol-2-ylidene, dcype = 1,2-bis(dicyclohexylphosphino)ethane) being characterised crystallographically. Complexes bearing the tmiy ligands were shown to decompose in an analogous manner to complexes bearing monodentate phosphine ligands, with the rate of decomposition being nominally linked to the size of the chelate ring. The decomposition of these complexes in the presence of aryl halides-expected to yield Pd(Ar)X(P-P)-was studied and shown instead to yield PdX(2)(P-P) and [Pd(tmiy)X(P-P)]BF(4). Additionally, Pd(Me)X(P-P) and Pd(Ar)X(P-P) were observed in some cases. Intermolecular cross-over reactions between the starting complex and Pd(Ar)X(P-P) were found to be the source of these unexpected products.     

Kinetic and density functional studies on alkyl-carbene elimination from Pd(II) heterocylic carbene complexes: a new type of reductive elimination with clear implications for catalysis.

A number of new methyl-Pd(II) complexes of heterocyclic carbenes of the form [PdMe(tmiy)L(2)]BF(4) have been prepared, and their reaction behavior has been studied (tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, L = cyclooctadiene (8), methyldiphenylphosphine (9), triphenyl phosphite (10), triphenylphosphine (11)). In common with other hydrocarbyl-M carbene complexes (M = Pd, Ni) the complexes are predisposed to a facile decomposition process. A detailed mechanism for the process and of the decomposition pathway followed is presented herein. All complexes decompose with first-order kinetics to yield 1,2,3,4,5-pentamethylimidazolium tetrafluoroborate and Pd(0) species. The kinetic investigations combined with density functional studies show that the complexes decompose via a mechanism of concerted reductive elimination of the methyl group and carbene. The reaction represents a new type of reductive elimination from transition metals and also represents a low-energy pathway to catalyst deactivation for catalysts based on heterocyclic carbenes. The theoretical studies indicate extensive involvement of the p(pi) orbital on the carbene carbon in the transition structure. Methods of stabilizing catalysts based on heterocyclic carbene complexes are suggested, and the possibility of involvement of carbene species during catalysis in ionic liquids is discussed.     

Palladium catalysts for aerobic oxidative kinetic resolution of secondary alcohols based on mechanistic insight

The oxidative kinetic resolution of secondary alcohols has been accomplished using 1:1 complexes of PdCl(2) and N-heterocyclic carbenes. In these reactions, both achiral and chiral carbene ligands are used in conjunction with the chiral base (-)-sparteine. A general synthesis of 1:1 PdCl(2)-carbene complexes has been developed and is amenable to a wide range of carbene ligands. The potential of these complexes in aerobic oxidations is highlighted by the use of a chiral Pd(II) complex and the chiral base (-)-sparteine to enhance the kinetic resolution of a racemic alcohol. [reaction--see text]     

Diaminocarbene- and Fischer-carbene complexes of palladium and nickel by oxidative insertion: preparation, structure, and catalytic activity

Oxidative insertion of [Pd(PPh3)4] or [Ni(cod)2]/PPh3 into the C-Cl bond of various 2-chloroimidazolinium- and other -amidinium salts affords metal-diaminocarbene complexes in good to excellent yields. This procedure is complementary to existing methodology in which the central metal does not change its oxidation state, and therefore allows to incorporate carbene fragments that are difficult to access otherwise. The preparation of a variety of achiral as well as enantiomerically pure, chiral metal-NHC complexes (NHC = N-heterocyclic carbene) and metal complexes with acyclic diaminocarbene ligands illustrates this aspect. Furthermore it is shown that oxidative insertion also paves a way to prototype Fischer carbenes of Pd(II). Since the required starting materials are readily available from urea- or thiourea derivatives, this novel approach allows for substantial structural variations of the ligand backbone. The catalytic performance of the resulting library of nickel- and palladium-carbene complexes has been evaluated by applications to prototype Suzuki-, Heck-, and Kumada-Corriu cross-coupling reactions as well as Buchwald-Hartwig aminations. It was found that even Fischer carbenes show appreciable catalytic activity. Moreover, representative examples of all types of neutral and cationic metal-carbene complexes formed in this study have been characterized by X-ray crystallography.     

Porphyrins Fused to N‐Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N‐heterocyclic carbene ligands 1 M . Rhodium(I) complexes 6 M – 9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni^II, Zn^II, Mn^III, Al^III, 2H). The electronic properties of the corresponding N‐heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [( 1 M )Rh(cod)Cl] and [( 1 M )Rh(CO)₂Cl] complexes (cod=1,5‐cyclooctadiene). Porphyrin–NHC ligands 1 M with a trivalent metal cation such as Mn^III and Al^III are overall poorer electron donors than porphyrin–NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni^II and Zn^II. Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring‐opening polymerization of L ‐lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.     

Characterization of Copper(II), nickel(II), cobalt(II) and palladium(II) complexes of vicinal oxime-imine ligands; induced chelate isomerism in the same molecule of the nickel(II) complex

Summary Metal complexes of the title ligands were characterized in order to determine the factors influencing the stability of chelate isomerism in the same molecule. The ligands were prepared by 1:1 condensation of isonitrosoacetylacetone (Hiso) with eithero-aminophenol (H₂ isoaph),p-aminophenol (H₂ isopph), or aniline (Hisoanil). The following complexes have been synthesized: [(isoaph)Cu]₄, (Hisoaph)₂Co, (Hisopph)₂ M·nH₂O (M=Ni(II), n=2;M=Pd(II), n=0;M=Co(II), n=2), [(isopph) Cu·H₂O]₂, and (isoanil)₂ M (M=Ni(II), Cu(II), Co(II), or Pd(II)). Both chelate rings in these metal complexes are five-membered. Transimination of one –C=N–C₆H₅ group to –C=NH in (isoanil)₂Ni produced a six-membered chelate ring in (isoim)Ni(isoanil). The induced chelate isomerism is ascribed to intermolecular hydrogen bonding of the imino-hydrogen and the basic nitrogen of the same six-membered chelate ring of an adjacent square planar molecule. Other types of hydrogen bondings with the oximato oxygen (intra- or intermolecular) favour the formation of five-membered chelate rings. Analytical, spectroscopic, and magnetic moment data are in accordance with the suggested formulations.Part of the Ph.D. thesis of Sana M. Imam     

Synthesis and characterization of new (pyrazolyl)aryl phosphinite PCN pincer palladium(II) complexes

Two unsymmetrical PCN pincer Pd(II) complexes 3a–3b which are based on (pyrazolyl)aryl phosphinite ligands and contain two fused six-membered palladacycles have been synthesized from 3-(3,5-dimethylpyrazol-1-ylmethyl)benzyl alcohol (2) by one-pot phosphorylation/palladation reaction via C–H bond activation of the related ligands. The pyrazole-coordinated phosphine-free Pd(II) complex (4) was also isolated in the preparation of pincer complex 3a. The new complexes were characterized by elemental analysis, ¹H NMR, ¹³C NMR, ³¹P {¹H} NMR (for pincer complexes) and IR spectra. And the molecular structures of 3b and 4 have been further determined by X-ray single-crystal diffraction. The pincer Pd complexes 3a and 3b exhibited rather low activity in the allylation of benzaldehyde.     

Nickel‐Triad Complexes of a Side‐on Coordinating Distannene

NHC adducts of the stannylene Trip₂Sn (Trip=2,4,6‐triisopropylphenyl) were reacted with zero‐valent Ni, Pd, and Pt precursor complexes to cleanly yield the respective metal complexes featuring a three‐membered ring moiety Sn‐Sn‐M along with carbene transfer onto the metal and complete substitution of the starting ligands. Thus the easily accessible NHC adducts to stannylenes are shown to be valuable precursors for transition‐metal complexes with an unexpected Sn? Sn bond. The complexes have been studied by X‐ray diffraction and NMR spectroscopy as well as DFT calculations. The compounds featuring the structural motif of a distannametallacycle comprised of a [(NHC)₂M⁰] fragment and Sn₂Trip₄ represent rare higher congeners of the well‐known olefin complexes. DFT calculations indicate the presence of a π‐type Sn–Sn interaction in these first examples for acyclic distannenes symmetrically coordinating to a zero‐valent transition metal.     

Ketenimine Complexes from Carbene Complexes and Isocyanides: Versatile Building Blocks for Carbocycles and N-Heterocycles [New Synthetic Methods (74)]

Ketenimine complexes are readily available in great variety by reaction of isocyanides with carbene complexes. They have proven to be useful building blocks in new synthetic approaches to carbocyclic and N-heterocyclic four-, five-, and six-membered rings. The reactions involve new metal-induced bond formation patterns of the ketenimine ligands, which can be influenced across a wide range by varying the following five parameters: the metal, the ligands, and the three substituents on the N?C?C unit.     

Reactions of vinyl acetate and vinyl trifluoroacetate with cationic diimine Pd(II) and Ni(II) alkyl complexes: identification of problems connected with copolymerizations of these monomers with ethylene

Vinyl acetate (VA) and vinyl trifluoroacetate (VA(f)) react with [(NwedgeN)Pd(Me)(L)][X] (M = Pd, Ni, (NwedgeN) = N,N'-1,2-acenaphthylenediylidene bis(2,6-dimethyl aniline), Ar(f) = 3,5-trifluoromethyl phenyl, L = Ar(f)CN, Et2O; X = B(Ar(f))4-, SbF6-) to form pi-adducts 3 and 5 at -40 degrees C. Binding affinities relative to ethylene have been determined. Migratory insertion occurs in a 2,1 fashion (DeltaG++ = 19.4 kcal/mol, 0 degrees C for VA, and 17.4 kcal/mol, -40 degrees C for VA(f)) to yield five-membered chelate complexes [(NwedgeN)Pd(kappa2-CH(Et)(OC(O)-CH3))]+, 4, and [(NwedgeN)Pd(kappa2-CH(Et)(OC(O)CF3))]+, 6. When VA is added to [(NwedgeN)Ni(CH3)]+, an equilibrium mixture of an eta2 olefin complex, 8c, and a kappa-oxygen complex, 8o, results. Insertion occurs from the eta2 olefin complex, 8c (DeltaG++ = 15.5 kcal/mol, -51 degrees C), in both a 2,1 and a 1,2 fashion to generate a mixture of five- and six-membered chelates, 9(2,1) and 9(1,2). VA(f) inserts into the Ni-CH3 bond (-80 degrees C) to form a five-membered chelate with no detectable intermediate. Thermolysis of the Pd chelates results in beta-acetate elimination from 4 (DeltaG++ = 25.5 kcal/mol, 60 degrees C) and beta-trifluoroacetate elimination from 6 (DeltaG = 20.5 kcal/mol, 10 degrees C). The five-membered Ni chelate, 9(2,1), is quite stable at room temperature, but the six-membered chelate, 9(1,2), undergoes beta-elimination at -34 degrees C. Treatment of the OAc(f) containing Pd chelate 6 with ethylene results in complete opening to the pi-complex [(NwedgeN)Pd(kappa2-CH(Et)(OAc(f)))(CH2CH2)]+ (OAc(f) = OC(O)CF3), 18, while reaction of the OAc containing Pd chelate 4 with ethylene establishes an equilibrium between 4 and the open form 16, strongly favoring the closed chelate 4 (DeltaH = -4.1 kcal/mol, DeltaS = -23 eu, K = 0.009 M(-1) at 25 degrees C). The open chelates undergo migratory insertion at much slower rates relative to those of the simple (NwedgeN)Pd(CH3)(CH2CH2)+ analogue. These quantitative studies provide an explanation for the behavior of VA and VA(f) in attempted copolymerizations with ethylene.     

Contributions to the chemistry of carbenemetal chemistry

This paper provides a brief review of our researches on carbenemetal complexes. The main emphasis is on saturated heterocyclic bis(amino)carbene ligands, referred to here as electron-rich olefin- or ERO-derived ligands, as outlined in Sections 2 and 3. The following Section deals with Fischer-type carbene complexes, based on reactions of a transition metal substrate with an imidoyl chloride, Vilsmeyer reagent or Eschenmoser salt. The researches of Sections 2, 3, 4 have resulted in the synthesis, characterisation and selected reactions (including as catalysts) of carbene complexes of V, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt and Au in various oxidation states. A final Section describes some more recent work on C[(NCH₂Bu^t)₂C₆H₄–1,2], its adducts with M[(NCH₂Bu^t)₂C₆H₄–1,2] (M = Si, Ge, Sn, or Pb) and the biphenyl- bis(carbene), [C{NCH₂Bu^t}₂C₆H₃–3,4]₂.     

Bulky iminophosphine-based nickel and palladium catalysts bearing 2,6-dibenzhydryl groups for ethylene oligo-/polymerization

A series of 2,6-dibenzhydryl substituted bulky Ni and Pd complexes containing P,N-chelating ligands, {[2,6-(Ph₂CH)₂-4-R-C₆H₂-N=CH-C₆H₄-2-PPh₂]MX₂; MX₂ =NiBr₂; R = Me ( Ni1 ); R = F ( Ni2 ); MX₂ =PdCl₂, R = Me ( Pd1 )}, have been prepared and used as catalyst precursors for ethylene oligo-/polymerization. Compared to the corresponding 2,6-diisopropyl Ni catalyst, these bulky Ni precatalysts activated by Et₂AlCl exhibited excellent catalytic performance toward ethylene polymerization with activity of up to 1.90 × 10⁵ g PE (mol Ni)⁻¹ h⁻¹, and result in semicrystalline PEs with high molecular weight. The catalytic performance of these bulky P,N-type complexes was significantly improved by introducing two ortho-dibenzhydryl on the N-aryl substituents. However, the formation of C₁₀–C₂₄ oligomers were generated using their palladium catalysts through ethylene oligomerization at high temperatures.