Metal-carbon Bond Research Articles

Regio- and stereochemistry of the first insertion step in the 1,3-butadiene polymerization catalyzed by CpTiCl 3–MAO

CpTiCl 3–MAO (methylaluminoxane) catalytic system promotes the butadiene polymerization leading to 85% of 4,1- cis polymer. The generally well accepted homopolymerization mechanism involves a diene insertion on a π-allylic terminal growing chain. In α-olefin–diene copolymerizations, diene monomer can also insert on a σ carbon–metal bond. In order to simulate an insertion of a conjugated diene on a σ chain, experimental and theoretical studies, relative to the first insertion step, are reported. Unexpectedly 56% of 1,2-primary and 44% of 4,1- trans first inserted units were obtained, by using as catalyst CpTiCl 3/MAO/Al( 13CH 3) 3. The experimental results were rationalized by DFT (Density Functional Theory) calculations, by including a solvent molecule coordinated to the catalytic site during the first insertion step.

Perspective on metal-mediated polar monomer/alkene copolymerization

A major unsolved problem in polymer synthesis is the design of efficient metal-mediated systems for the copolymerization of alkenes with polar vinyl monomers, such as acrylates and methacrylates. There are several reasons for the absence of efficient transition metal-based insertion copolymerization catalysts. First, following insertion, the ester group of the acrylate coordinates to the metal thereby hindering subsequent monomer coordination. A second reason stems from the preferred 2,1-insertion of acrylates into metal–carbon bonds resulting in the placement of the ester group on the α-carbon. This makes the metal–alkyl species particularly prone to homolysis because of the enhanced stability of the resultant alkyl radical, one that is essentially the same as the propagating species in radical-initiated acrylate polymerization. In this perspective we focus on this issue of facile metal–carbon bond homolysis, especially following acrylate insertion, using examples from our own work. We suggest ways to circumvent these issues, for example forcing 1,2-insertion by imposing steric crowding at the metal. Finally, we discuss the danger of relying on radical traps as probes for polymerization mechanism. Radical traps can react with metal-hydrides and attenuate metal-centered nonradical reactions. However, even when radical traps fail to stop an observed polymerization, it may be wrong to conclude that a nonradical mechanism is at work since the traps can be destroyed under certain reaction conditions.

Infrared study on the nature of active centers in anionic polymerization

The active species structure in alkali metal-initiated polymerization in different media has been investigated, living polydienes of low molecular weight being used as the main objects. An investigation of the IR spectra of living polydienes in the regions from 350 to 650 cm−1 and from 1500 cm−1 to 1700 cm−1 made it possible to find out the influence of the synthesis conditions on the metal-carbon and carbon-carbon double bond stretching frequencies in the active center. In alkali metal-initiated polymerization of hydrocarbon monomers the character of the polymerization process is greatly dependent on the nature of alkali metal and the solvent. By comparison of the spectral data with the microstructure of polymers formed under the same conditions the possible structures of different types of active centers are discussed.

Noble metals in carbon-rich metamorphic rocks of the Khanka terrane, Primorie

Carbon-rich metamorphic rocks of Riphean age in the northern part of the Khanka terrane were first analyzed for concentrations of noble metals (Au, Ag, Pt, Pd, Ir, Os, and Ru). According to the data of various physicochemical analytical techniques, the Au and Pt concentrations broadly vary: from 0.01 to 52 ppm for Pt and from 0.1 to 30 ppm for Au. Various techniques of sampling and analysis variably affect the losses of these metals because of difficulties in the decomposition of metal-carbon chemical bonds. The carbon isotopic composition (13C from −8.5 to −8.7‰) of the graphitized amphibolite-facies rocks widespread in the core of the Ruzhino paleodiapir suggest that their carbon is of mantle provenance. The Early Cambrian metaterrigenous rocks metamorphosed to the greenschist facies have 13C from −19.9 to −26.6‰, which testifies to its organic origin. The elevated concentrations of noble metals in these rocks suggest that the sources of carbon and metals were polygenetic and that the ore-forming system evolved over a long time span.

Continuous-Wave and Pulse EPR Study of the Copper(II) Complex of N-Confused Tetraphenylporphyrin: Direct Observation of a σ Metal−Carbon Bond

N-confused or inverted porphyrins, a family of porphyrin isomers that contain a confused pyrrole ring connected through its alpha and beta' positions in the macrocycle, exhibit unique physical and chemical properties, like, for instance, the ability to stabilize unusual oxidation states of metals due to the reactivity of the inverted pyrrole. In this Article, a combined multifrequency continuous-wave and pulse electron paramagnetic resonance (EPR) study of the copper(II) complex of N-confused tetraphenylporphyrin (TPP) is presented. By use of pulse EPR methods like ENDOR and HYSCORE, the magnetic interactions between the unpaired electron of the compound and the surrounding nitrogen nuclei were revealed. Through 13C labeling of the macrocycle, a detailed study of the carbon hyperfine interaction became possible and provided further insight into the character of the metal-carbon bond. The observed hyperfine couplings of the ligand atoms in the first coordination sphere showed the presence of a remarkably strong sigma Cu-C bond and allowed for a detailed analysis of the spin delocalization over the porphyrin macrocycle. Interestingly, it was found that the observed delocalization is approximately 11% larger than the corresponding one for CuTPP.

Factors that affect the formation of metal-carbon bonds in metalorganic mixtures

Effect of the initial composition of the pulp, degree of evaporation, and temperature regime on the process of formation of metal-carbon bonds in tungsten-and molybdenum-containing metalorganic mixtures has been considered. An exothermic character of this process has been revealed and a mechanism of multistage polymerization of the mixture has been suggested. The effect of the degree of polymerization on the stability of metal-carbon bonds and structure of products formed upon the thermal decomposition of the mixture has been shown.

Formation of thin carbide films of titanium and tantalum by methane plasma immersion ion implantation

Titanium and tantalum samples were treated by high voltage pulses at −20 kV in an atmosphere of methane. The high voltage created a plasma, from which ions of methane and its fragments were accelerated towards the sample and were implanted. Process times between 0.5 and 2h at a pulse repetition rate of 1kHz were used. The gas pressure was 1Pa.X-ray diffraction results from the near-surface region of the implanted sample showed small peaks of the carbide phases next to dominant peaks of the metal target. Secondary ion mass spectrometry and X-ray photoelectron spectroscopy gave concentration versus depth profiles of the implanted carbon. The chemical shift proved that metal–carbon bonds had been formed. The profiles showed a thin carbon film on top, followed by a shallow implantation profile. The amount of implanted carbon increased with the process time.

Structurally characterized intermediates in the stepwise insertion of CO–ethylene or CO–methyl acrylate into the metal–carbon bond of Pd(ii) complexes stabilized by (phosphinomethyl)oxazoline ligands

The initial CO-ethylene or CO-methyl acrylate insertion steps into the Pd-Me bond of methylpalladium(II) complexes with (phosphinomethyl)oxazoline ligands, leading to metallacycles, have been fully characterized, including by X-ray diffraction.

Structural Characterization of the Active Site of the PduO-Type ATP:Co(I)rrinoid Adenosyltransferase from Lactobacillus reuteri

The three-dimensional crystal structure of the PduO-type corrinoid adenosyltransferase from Lactobacillus reuteri (LrPduO) has been solved to 1.68-A resolution. The functional assignment of LrPduO as a corrinoid adenosyltransferase was confirmed by in vivo and in vitro evidence. The enzyme has an apparent Km(ATP) of 2.2 microM and Km(Cobalamin) of 0.13 microM and a kcat of 0.025 s(-1). Co-crystallization of the enzyme with Mg-ATP resulted in well-defined electron density for an N-terminal loop that had been disordered in other PduO-type enzyme structures. This newly defined N-terminal loop makes up the lower portion of the enzyme active site with the other half being contributed from an adjacent subunit. These results provide the first detailed description of the enzyme active site for a PduO-type adenosyltransferase and identify a unique ATP binding motif at the protein N terminus. The molecular architecture at the active site offers valuable new insight into the role of various residues responsible for the human disease methylmalonic aciduria.

Selectivity Enhancement of Electroless Co Deposition for Cu Capping Process via Spontaneous Diazonium Ion Reduction

A one-step, room temperature route to form carbon-metal bonds via spontaneous reduction of diazonium ion in an acid solution has been developed. This property was later employed to design a new process for self-activated electroless Co deposition on Cu without Pd activation. X-ray photoelectron spectroscopy and IR analysis show the evidence of direct organic molecule attachments on Cu. From scanning electron microscopy, electrical resistance, and line-to-line leakage current observations, the selectivity of Co deposition on the modified Cu surface has been greatly improved. Impedance measurements indicated that the charge transfer of electroless reaction was partially blocked by a highly covalent bond to the Cu surface.

On the Mechanism of an Asymmetric α,β-Unsaturated Carboxylic Acid Hydrogenation: Application to the Synthesis of a PGD2 Receptor Antagonist

Ruthenium complexes employing axially chiral ligands were found to be effective asymmetric hydrogenation catalysts for the reduction of alpha,beta-unsaturated ene acid 1-E to give 2, a prostaglandin D2 (PGD2) receptor antagonist. With [(S-BINAP)Ru(p-cymene)Cl2]2 (3, S-BINAP = (S)-(+)-2,2'-bis(diphenylphospino)-1,1'-binapthyl), it was discovered that low hydrogen pressures (<30 psi) were essential to achieve high enantioselectivities (92% ee). A detailed mechanistic study was undertaken to elucidate this pressure dependence. It was determined that compound 1-E is in a ruthenium-catalyzed equilibrium with endocylic isomer 1-Endo and in photochemical equilibrium with Z isomer 1-Z. Each isomer could be hydrogenated to give 2, albeit with different rates and enantioselectivities. Hydrogenation of 1-Endo with 3 was found to give 2 in high enantiomeric excess, regardless of pressure and at a rate substantially faster than that of hydrogenation of 1-E and 1-Z. In contrast, isomers 1-E and 1-Z exhibited pressure-dependent enantioselectivities, with higher enantiomeric excesses obtained at lower pressures. A rationale for this pressure dependence is described. Deuterium labeling studies with 1-Endo and tiglic acid were used to elucidate the mechanism of hydride insertion and product release from ruthenium. Under neutral conditions, protonolysis was the major pathway for metal-carbon cleavage, while under basic conditions, hydrogenolysis of the metal-carbon bond was predominant.

Infrared Spectra and Density Functional Calculations of CH2MHX and CH⋮MH2X Complexes Prepared in Reactions of Methyl Halides with Mo and W Atoms

The simple methylidene and methylidyne complexes (CH2=MHX and CH[triple bond]MH2X; X = F, Cl, Br, and I) are prepared in reactions of laser-ablated Mo and W atoms with the methyl halides and investigated by matrix infrared spectroscopy and density functional theory calculations. These complex structures are photoreversible: visible irradiation converts the methylidene complex to the methylidyne complex, and UV irradiation reverses this effect via alpha-hydrogen migration. While the higher oxidation state complexes are readily formed regardless of halogen size, the Mo methylidyne complex is relatively less favored with increasing halogen size, and the W complex shows the opposite tendency. The group 6 metal methylidenes are predicted to have the most agostically distorted structures among the early transition-metal methylidenes. The computed carbon-metal bond shortens with increasing halogen size for both the methylidene and methylidyne complexes. Harmonic and anharmonic frequencies computed by DFT converge on the experimental values and thus provide support for the identification of these new Mo and W complexes.

Relationship Between One‐Electron Transition‐Metal Reactivity and Radical Polymerization Processes

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Synthesis and Characterization of trans-[Co((BA)2en)(amine)(CN)] and trans-K[Co((BA)2en)(CN)2] Complexes:Indication of Cobalt-cyanid π -Back Bonding Interaction

The structure of vitamin B12 (cyano cobalamine) was determined in the laboratory of Hodgkin and co-workers in the mid-1960s. In the equatorial plane the central six coordinate cobalt (III) ion is surrounded by four pyrole nitrogen atoms of a corrin ligand. While one of the axial sites is occupied by a 5,6-dimethyl benzimidazol-α-Dribofuranose-3-phosphate, the second axial ligand position is occupied by a cyanide group. The vitamin can be converted in vivo to active alkylcobalamin coenzymes. These alkylcobalamin are know naturally occurring compounds with a biologically active carbon-metal bond. The macrocyclic tetradentate ligands with four nitrogen donor or Schiff base ligands (N2O2) which coordinated to four equatorial position octahedral structure of cobalt (III) center are considered as models for vitamin B12. 6-10 The effect of axial ligation (amine) on properties of cyano group with an organic monoanion ligand has played an important role in our knowledge of the properties of vitamin B12 and of model compounds. The coordination of the amine ligand in trans position can lead to cyano activation barrier of dissociation. A variety of physical methods such as infrared and NMR have been used to study of π-back bonding from the cobalt (III) metal center into cyano ligand. Infrared and NMR spectroscopy allows one to investigation of the effect amines on the cyano ligand π-back bonding in complexes. These results were indicative of the existence of dπ-pπ back bonding in cyano cobalt species (Scheme 1). The effect of trans axial ligands on the cyanide chemical shifts and stretching properties can influence the relative stabilization of I and II species by the axial ligand. In this paper, we report result of H NMR and IR study of a series of trans-(Schiff base)CN(amine)cobalt(III) complexes with different axial ligands (Scheme 2), which permits a more detailed analysis of these properties.

Theoretical investigations into trioxo group 7 compounds LRO 3 with perfluorated ligands

Group 7 trioxides LRO 3 with perfluorated ligands L (L = CF 3, C 5F 5 and C 6F 5) are investigated using density based (BP86) and wave-function based (MP2) methods together with energy-adjusted scalar relativistic pseudopotentials for the metal atoms Mn, Tc and Re. The C 6F 5 compounds have short metal-carbon bond distances and are more stable than the η 1-C 5F 5 and the CF 3 compounds. The perfluorinated cyclopentadienyl compounds, C 5F 5MO 3, all are η 1 coordinated in contrast to the C 5Me 5MO 3 homologues. Our calculations indicate that C 6F 5MnO 3 might be a stable complex and therefore a promising target for future synthesis.

Multiple Metal—Carbon Bonds for Catalytic Metathesis Reactins (Nobel Lecture).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

β-Elimination in the Reactions of ·CR1R2CR3R4X Radicals with Metal Powders Immersed in Aqueous Solutions

The reactions of several radicals of the type .CR1R2CR3R4X (where X = OH or NH3+) with metal powders that have been immersed in aqueous solutions were studied. The radicals were formed by radiation chemical techniques. One of the products in all these reactions is the corresponding alkene, R1R2C=CR3R4. The results are in accord with a mechanism in which the radicals react with the metals that are forming transients with metal-carbon sigma bonds. The latter transients decompose via two competing reactions: (a) heterolysis of the metal-carbon sigma bond and (b) beta-elimination of X-. Moreover, the dehalogenation of BrCH2CH2NH3+ and ClCH2(CH3)2COH by metal powders was studied. Also in these reactions, the corresponding alkene is one of the products. This result is consistent with the suggestion that, in the dehalogenation reaction, an alkyl radical is formed in the first step. This radical then reacts with the metal. Alternatively, the transients with metal-carbon sigma bonds in the dehalogenation processes might be formed via a concerted mechanism.

Relationship between One‐Electron Transition‐Metal Reactivity and Radical Polymerization Processes

Controlled radical polymerization has come along in leaps and bounds following the development of efficient transition-metal catalysts for atom-transfer radical polymerization. Another type of controlled radical polymerization process, namely organometallic radical polymerization, uses the reversible formation of metal-carbon bonds. Metals are also implicated in catalytic chain transfer, a process that involves the abstraction of hydrogen atoms. This Minireview discusses the importance of one-electron transition-metal reactivity in metal-mediated controlled radical polymerization processes.

Diffusion of limonene in polyethylene

Diffusion coefficients of limonene in various linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) resins have been determined from sorption data using a thermogravimetric methodology. From these data, one can determine whether polymer synthesis parameters such as the choice of catalytic process or co-monomer result in substantial differences in how much food packaging additives might migrate to food. For example, LLDPE is currently manufactured using either one of two distinct catalytic processes: Ziegler–Natta (ZN) and metallocene, a single-site catalyst. ZN catalysis is a heterogeneous process that has dominated polyolefin synthesis over the last half-century. It involves a transition metal compound containing a metal-carbon bond that can handle repeated insertion of olefin units. In contrast, metallocene catalysis has fewer than 20 years of history, but has generated much interest due to its ability to produce highly stereospecific polymers at a very high yield. In addition to high stereospecificity, metallocene-catalysed polymers are significantly lower in polydispersity than traditional ZN counterparts. Absorption and desorption testing of heat-pressed films made from LLDPE and LDPE resins of varying processing parameters indicates that diffusion coefficients of limonene in these resins do not change substantially.

Facile Functionalization of a Metal Carbon Bond by O-Atom Transfer

The facile conversion of M-R to M-OR that could be useful for the functionalization of electron-rich metal alkyl intermediates is shown to proceed via a Baeyer-Villiger-type pathway involving a nonredox, electrophilic, O-atom insertion in reactions with non-peroxo O-donors.