Ethylene Coordination Research Articles

Hydrostannylation of Olefins by a Hydridostannylene Tungsten Complex

Reaction of the aryltin(II) hydride {AriPr6Sn(μ-H)}2 (AriPr6 = −C6H3-2,6-(C6H2-2,4,6-iPr3)2) with two equivalents of the tungsten carbonyl THF complex, [W(CO)5(THF)], afforded the divalent tin hydride transition metal complex, W(CO)5{Sn(AriPr6)H}, (1). Complex 1 reacted rapidly with ethylene, or propylene under ambient conditions to yield the corresponding hydrostannylated organometallic species, W(CO)5{Sn(AriPr6)(Et)} (2), or W(CO)5{Sn(AriPr6)(nPr)} (3), via olefin insertion into the Sn–H bond. Treatment of 1 with the Lewis base dbu (dbu = 1,8-diazabicycloundec-7-ene) afforded the Lewis acid–base complex, W(CO)5{Sn(AriPr6)(dbu)H} (4), indicating that the Lewis acidity of the tin atom is preserved in 4. The complexes were characterized by X-ray crystallography, and by UV–visible, FT-IR, and multinuclear NMR spectroscopies. DFT calculations suggest hydrostannylation of ethylene with 1 proceeds via coordination of ethylene to the tin atom, then insertion into the Sn–H bond. Further computational study on the reactivity of 1 toward Ph3SiH indicated that the rate-determining step involves the metathesis reaction of a Sn–C/Si–H bond with a very high energy barrier of 71.3 kcal/mol. The calculated proton abstraction product of 1 with dbu, [W(CO)5{Sn(AriPr6)}]+[H(dbu)]−, is 18.2 kcal/mol less stable than the observed coordination product 4.

Mechanistic Insights into Ni(II)-Catalyzed Nonalternating Ethylene-Carbon Monoxide Copolymerization.

Polyethylene materials with in-chain-incorporated keto groups were recently enabled by nonalternating copolymerization of ethylene with carbon monoxide in the presence of Ni(II) phosphinephenolate catalysts. We elucidate the mechanism of this long-sought-for reaction by a combined theoretical DFT study of catalytically active species and the experimental study of polymer microstructures formed in pressure-reactor copolymerizations with different catalysts. The pathway leading to the desired nonalternating incorporation proceeds via the cis/trans isomerization of an alkyl-olefin intermediate as the rate-determining step. The formation of alternating motifs is determined by the barrier for the opening of the six-membered C,O-chelate by ethylene binding as the decisive step. An η2-coordination of a P-bound aromatic moiety axially oriented to the metal center is a crucial feature of these Ni(II) catalysts, which also modulates the competition between the two pathways. The conformational constraints imposed in a 2′,6′-dimethoxybiphenyl moiety overall result in a desirable combination of disfavoring ethylene coordination along the alternating incorporation pathway, which is primarily governed by electronics, while not overly penalizing the nonalternating chain growth, which is primarily governed by sterics.

The Role of Nickel and Brønsted Sites on Ethylene Oligomerization with Ni-H-Beta Catalysts

The present work studies the adsorption of ethylene on Ni-H-Beta particles to unravel the roles of nickel and Brønsted sites in the catalytic oligomerization of ethylene. Three models (i.e., two based on the Cossee–Arlman mechanism and one based on the metallacycle mechanism) are examined in terms of the nature of the active sites and the adsorption mechanism involved in the ethylene coordination step. The results are consistent with the participation of two active sites in the formation of [Ni(II)-H]+ Cossee–Arlman centers and also suggest that ethylene dissociates upon adsorption on [Ni(II)-H]+ sites. Further characterization of Ni-H-Beta catalysts prepared at different nickel loadings and silica-to-alumina ratios reveals that highly dispersed Ni2+ exists on the catalyst surface and interacts with the catalyst’s lattice oxygen and free NiO crystals. At the same time, the kinetic results indicate that Brønsted sites may form isolated nickel-hydride ([Ni(II)-H]+) centers on the catalyst surface. In addition, the presence of residual, noncoordinated Ni2+ and Brønsted sites (not involved in the formation of [Ni(II)-H]+ sites) shows a reduced probability of the formation of nickel-hydride sites, hindering the conversion rate of ethylene. A mechanism for forming [Ni(II)-H]+ centers is proposed, involving ethylene adsorption over Ni2+ and a Brønsted site. This research has important implications for improving ethylene oligomerization processes over nickel-based heterogeneous catalysts.

Unprecedented Square-Planar α-Diimine Dibromonickel Complexes and Their Ethylene Polymerizations Modulated by Ni–Phenyl Interactions

Combined bulky 8-p-tolylnaphthylamine and bulky dibenzo-/dinaphthobarrelene backbones, unprecedented “opening box”-like α-diimine dibromonickel complexes were synthesized. Their square-planar geometries and the Ni–phenyl interactions in both solid and solution were identified by single-crystal X-ray diffraction analysis, NMR analysis, and density functional theory (DFT) simulations. Bulky α-diimine nickel complexes as ethylene polymerization precatalysts exhibited enhanced activity and thermal stability and produced unexpected lightly branched polyethylenes (PEs) (32–42/1000C) with melting temperatures of 81–93 °C. A combined experimental and theoretical study clearly showed how weak attractive Ni–phenyl interactions could decrease PE branching density in ethylene polymerization. The ethylene coordination as a crucial step was promoted by Ni–phenyl interactions, causing acceleration of linear chain growth.

Orchestration of ethylene and gibberellin signals determines primary root elongation in rice.

Primary root growth in cereal crops is fundamental for early establishment of the seedling and grain yield. In young rice (Oryza sativa) seedlings, the primary root grows rapidly for 7-10 days after germination and then stops; however, the underlying mechanism determining primary root growth is unclear. Here, we report that the interplay of ethylene and gibberellin (GA) controls the orchestrated development of the primary root in young rice seedlings. Our analyses advance the knowledge that primary root growth is maintained by higher ethylene production, which lowers bioactive GA contents. Further investigations unraveled that ethylene signaling transcription factor ETHYLENE INSENSITIVE3-LIKE 1 (OsEIL1) activates the expression of the GA metabolism genes GIBBERELLIN 2-OXIDASE 1 (OsGA2ox1), OsGA2ox2, OsGA2ox3, and OsGA2ox5, thereby deactivating GA activity, inhibiting cell proliferation in the root meristem, and ultimately gradually inhibiting primary root growth. Mutation in OsGA2ox3 weakened ethylene-induced GA inactivation and reduced the ethylene sensitivity of the root. Genetic analysis revealed that OsGA2ox3 functions downstream of OsEIL1. Taken together, we identify a molecular pathway impacted by ethylene during primary root elongation in rice and provide insight into the coordination of ethylene and GA signals during root development and seedling establishment.

Ethylene binding in mono- and binuclear CuI complexes with tetradentate pyridinophane ligands

Herein we report a series of CuI complexes supported by tetradentate RN4 pyridinophane ligands that coordinate to ethylene forming either mononuclear complexes with ethylene coordinated in an η2-mode or a binuclear complex where ethylene binds to two Cu atoms in a μ-η2-η2-mode, depending on the steric effects of the RN4 ligand and the reaction conditions. In the binuclear complex with bridging ethylene, the CC bond is significantly elongated, with a bond length of 1.444(8) Å according to X-ray diffraction analysis. This complex represents the only examination a μ-η2-η2-coordinated Cu-olefin complex reported to date, featuring one of the longest reported CC bonds. The spectroscopic characterization, structure, electrochemical properties and solution behavior are analyzed in this study. Coordination of ethylene was found to be reversible in these complexes and more favored in less sterically hindered RN4 ligands, so that ethylene binding is observed in a coordinating solvent (MeCN) environment. In the case of the MeN4 ligand, the ethylene complex is photoluminescent in the solid state. The ethylene binding modes in mono- and binuclear complexes are elucidated through Natural Bond Orbital and QTAIM analyses.

Efficient suppression of the chain transfer reaction in ethylene coordination polymerization with dibenzosuberyl substituents

Catalysts with dibenzosuberyl substituents possess a superior ability to suppress chain transfer in ethylene (co)polymerization, producing high molecular weight polyethylenes and functionalized polyethylenes (ethylene-MA copolymers).

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

The Coordination of Ethylene and Other Hormones in Primary Root Development.

The primary root is the basic component of root systems, initiates during embryogenesis and develops shortly after germination, and plays a key role in early seedling growth and survival. The phytohormone ethylene shows significant inhibition of the growth of primary roots. Recent findings have revealed that the inhibition of ethylene in primary root elongation is mediated via interactions with phytohormones, such as auxin, abscisic acid, gibberellin, cytokinins, jasmonic acid, and brassinosteroids. Considering that Arabidopsis and rice are the model plants of dicots and monocots, as well as the fact that hormonal crosstalk in primary root growth has been extensively investigated in Arabidopsis and rice, a better understanding of the mechanisms in Arabidopsis and rice will increase potential applications in other species. Therefore, we focus our interest on the emerging studies in the research of ethylene and hormone crosstalk in primary root development in Arabidopsis and rice.

Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts.

The most ubiquitous polymer, polyethylene (PE), is produced either through a radical-initiated process or, more commonly, through a coordination/insertion process employing early transition metal catalysts, particularly titanium- and chromium-based systems. These oxophilic early metal catalysts are not functional-group-tolerant and thus cannot be used to synthesize copolymers of ethylene and polar vinyl monomers such as alkyl acrylates and vinyl acetate. Such PE copolymers have enhanced properties relative to PE and are made through radical polymerization processes, requiring exceptionally high pressures and temperatures. Copolymerizations of polar vinyl monomers with ethylene using more functional group-tolerant late metal catalysts potentially offer an attractive alternative for generating such value-added copolymers since ligand variations may provide more control of polymer microstructures and milder reaction conditions would apply. This Account describes our efforts, particularly through detailed mechanistic studies, to probe and develop this potential using Pd(II) and Ni(II) α-diimine catalysts. To inform discussions of the copolymerizations, we briefly review key aspects of ethylene homopolymerizations using these diimine catalysts. These include ligand designs that incorporate axial blocking groups that retard chain transfer and promote production of a high polymer rather than an oligomer. These ligand designs also lead to unique branched polyethylenes via migration of the metal along the chain ("chain-walking") prior to insertion. Mechanistic investigations of copolymerizations of ethylene with polar vinyl monomers using the diimine complexes have revealed several impediments to developing practical catalysts: (1) The polar group of the comonomer can coordinate strongly to the metal center, blocking coordination of ethylene. (2) Weak binding affinity of the polar monomer relative to ethylene can result in very low levels of comonomer incorporation. (3) A metal alkyl chain bearing a heteroatom, X, on the β-carbon atom can undergo β-X elimination leading to deactivation of the catalyst. (4) Stable chelate formation following insertion of a polar comonomer can greatly retard the rate of chain growth. (5) A metal alkyl chain bearing an electron-withdrawing heteroatom at the □-carbon atom can result in a high insertion barrier. A patent disclosure by the DuPont Versipol group and our extensive mechanistic studies reveal that, remarkably, vinyl trialkoxysilanes are ideal comonomers and circumvent all of the impediments noted above. The Pd-catalyzed copolymerization of vinyl trialkoxysilanes with ethylene produces highly branched, low molecular weight copolymers with activities comparable to those of analgous ethylene homopolymerizations. A 1,2- insertion of the vinyl silane results in the formation of a five-membered Pd-O(R)Si chelate which is readily opened by ethylene and thus does not reduce the rate of chain growth. β-Silyl elimination results in chain transfer and accounts for the lower molecular weight polymer. The nickel α-diimine-catalyzed copolymerizations produce high molecular weight copolymers with structures that vary from nearly linear to moderately branched. Both four- and five-membered chelates are catalyst resting states but are rapidly opened by ethylene, and thus turnover frequencies are only slightly reduced relative to ethylene homopolymerizations. Finally, a convenient and practical nickel-based system has been developed for the efficient synthesis of this copolymer which can be cross-linked to form PEX- b, a commercial PE plastic used for hot water plumbing pipes and power cable coatings.

Mechanistic Insights into Chromium-Catalyzed Ethylene Trimerization

The kinetics of ethylene trimerization by a chromium N-phosphinoamidine (Cr-(P,N)) precatalyst activated by modified methylaluminoxane (MMAO) has been investigated by high-pressure NMR techniques. An in-depth kinetic analysis of this metallacyclic mechanism has been conducted. It was found that an intermediate in the trimerization catalytic cycle, proposed in this study as the chromium alkenyl hydride species, degrades into a polymer active site where this degradation step is independent of ethylene concentration and is first order in catalyst. Additionally, we report that at least one of the first two ethylene coordination steps must be reversible in order to predict the features of the monomer consumption profiles. The reaction order in ethylene is dependent on the reversibility of the ethylene coordination steps. The observation of these details of the mechanism explains many of the challenges inherent in the examination of this and similar catalyst systems and emphasizes the usefulness of operando hig...

Adaptation of root growth to increased ambient temperature requires auxin and ethylene coordination in Arabidopsis.

A fresh look at the roles of auxin, ethylene, and polar auxin transport during the plant root growth response to warmer ambient temperature (AT). The ambient temperature (AT) affects plant growth and development. Plants can sense changes in the AT, but how this change is transduced into a plant root growth response is still relatively unclear. Here, we found that the Arabidopsis ckrc1-1 mutant is sensitive to higher AT. At 27°C, the ckrc1-1 root length is significantly shortened and the root gravity defect is enhanced, changes that can be restored with addition of 1-naphthaleneacetic acid, but not indole-3-acetic acid (IAA). AUX1, PIN1, and PIN2 are involved in the ckrc1-1 root gravity response under increased AT. Furthermore, CKRC1-dependent auxin biosynthesis was critical for maintaining PIN1, PIN2, and AUX1 expression at elevated temperatures. Ethylene was also involved in this regulation through the ETR1 pathway. Higher AT can promote CKRC1-dependent auxin biosynthesis by enhancing ETR1-mediated ethylene signaling. Our research suggested that the interaction between auxin and ethylene and that the interaction-mediated polar auxin transport play important roles during the plant root growth response to higher AT.

Studies of the decomposition of the ethylene hydrophenylation catalyst TpRu(CO)(NCMe)Ph

TpRu(CO)(NCMe)Ph is a catalyst for the conversion of benzene and ethylene to ethylbenzene. Previously, the formation of ethylbenzene has been shown to occur through a pathway that involves ethylene coordination to Ru, insertion of ethylene into the Ru–phenyl bond and Ru–mediated benzene C–H activation. The effect of ethylene pressure and catalyst concentration (between 0.2 and 0.01 mol % based on benzene) on the decomposition of TpRu(CO)(NCMe)Ph was examined. Studies have shown that there are two competing catalyst deactivation pathways. At higher concentrations of TpRu(CO)(NCMe)Ph, the dominant deactivation pathway is likely initiated by a binuclear reaction of two Ru complexes that leads to formation of unidentified paramagnetic species. Kinetic studies reveal that this pathway for catalyst decomposition occurs with a second-order rate of 0.007 (1) M−1 s−1. At lower Ru concentrations, ethylene C–H activation to form the allyl complex TpRu(CO)(η3-C4H7) is the predominant deactivation pathway. The effect of ethylene pressure on catalyst decomposition was also examined. At higher ethylene pressure nearly quantitative formation of TpRu(CO)(η3-C4H7) was observed.

A density functional theory study of ethylene epoxidation catalyzed by niobium-doped silica

A mechanistic study of ethylene epoxidation by hydrogen peroxide catalyzed by niobium doped in a silica mesopore is reported. Density functional theory calculations at the M06-L/aug-cc-pVDZ level were used to investigate the catalytic pathway. A five-step cycle is proposed. The initial steps are the adsorption of H2O2 to the Nb center followed by coordination of ethylene to the hydrogen peroxide. The rate-limiting step is the subsequent epoxidation of ethylene via transfer of an oxygen atom, which has a calculated enthalpic barrier of 11.6kcal/mol relative to the preceding intermediate. This is followed by desorption of the ethylene oxide product and dehydration to regenerate the catalyst. The reaction barrier is lower than reported for other catalysts in the literature, consistent with recent experimental reports of the efficacy of Nb-doped mesoporous silica catalysts [Catal. Sci. Technol.,2014, 4, 4433–4439]. The mechanistic details elucidated in the present calculations may aid in the rational design of new epoxidation catalysts and thus the factors influencing the reaction, e.g., geometry and charge changes, are discussed.

Unraveling the Catalytic Synergy between Ti3+ and Al3+ Sites on a Chlorinated Al2O3: A Tandem Approach to Branched Polyethylene

AbstractAn original step‐by‐step approach to synthesize and characterize a bifunctional heterogeneous catalyst consisting of isolated Ti3+ centers and strong Lewis acid Al3+ sites on the surface of a chlorinated alumina has been devised. A wide range of physicochemical and spectroscopic techniques were employed to demonstrate that the two sites, in close proximity, act in a concerted fashion to synergistically boost the conversion of ethylene into branched polyethylene, using ethylene as the only feed and without any activator. The coordinatively unsaturated Al3+ ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti3+ sites for the traditional ethylene coordination polymerization.

Unraveling the Catalytic Synergy between Ti(3+) and Al(3+) Sites on a Chlorinated Al2 O3 : A Tandem Approach to Branched Polyethylene.

An original step-by-step approach to synthesize and characterize a bifunctional heterogeneous catalyst consisting of isolated Ti(3+) centers and strong Lewis acid Al(3+) sites on the surface of a chlorinated alumina has been devised. A wide range of physicochemical and spectroscopic techniques were employed to demonstrate that the two sites, in close proximity, act in a concerted fashion to synergistically boost the conversion of ethylene into branched polyethylene, using ethylene as the only feed and without any activator. The coordinatively unsaturated Al(3+) ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti(3+) sites for the traditional ethylene coordination polymerization.

Coordination and insertion of alkenes and alkynes in Au(III) complexes: nature of the intermediates from a computational perspective.

The contribution of Au(III) species to catalysis is still debated due to the limited number of characterized intermediates with this oxidation state. In particular, the coordination of alkenes and alkynes to Au(III) followed by insertion into Au(III)-X bonds has been suggested but rarely proven experimentally. Here, these reactions are explored by means of DFT and CCSD(T) calculations considering [AuX3(L)] and [AuX2(L)2](+) complexes. In these complexes, L = ethylene and acetylene have been chosen as substrates of high interest and representative of any unsaturated organic substrate, whereas X is Cl, Me or H, as found in metal salts and as model for intermediates involved in catalysis. Isoelectronic Pt(II) complexes are also considered for comparison. Ethylene coordination occurs preferentially perpendicular for all X except H, whereas for acetylene, coordination takes place in-plane for all X except Cl. These coordination isomers can represent either minima (intermediates) or saddle points (transition states) on the potential energy surface, depending on X. NBO analysis shows how this variety of structures results from the combination of electronic (M-L donation and back-donation) and steric (cis L-X repulsion) effects. With the sole exception of [AuMe2(ethylene)2](+), rotation of the unsaturated ligand and insertion into a cis Au-X bond involve low to moderate energy barriers, ΔG(‡) = 2.5 to 23.5 kcal mol(-1), and are thermodynamically feasible, ΔG = 4.3 to -47.2 kcal mol(-1). The paucity of experimental observations for such reactions should thus be caused by other factors, like the participation of the intermediates and products in competitive side reactions including the reductive elimination of XCHnCHnX (n = 1 or 2).

Richard F. Heck (1931-2015).

Richard F. Heck (1931-2015).

Electronic Structure of Ti3+–Ethylene Complexes in Microporous Aluminophosphate Materials. A Combined EPR and DFT Study Elucidating the Role of SOMO Orbitals in Metal–Olefin π Complexes

The interaction of tetrahedrally coordinated Ti3+ ions generated in the framework of TiAlPO-5 microporous materials with 12,13C2H4 leads to the formation of side-on η2 {Ti3+—C2H4} complexes with a unique 5-fold coordination of titanium, supported by four oxygen donor ligands of the framework. The detailed electronic and magnetic structure of this adduct is obtained by the combination of advanced EPR techniques (HYSCORE and SMART-HYSCORE) in conjunction with periodic and cluster model DFT calculations. The binding of C2H4 results from the σ overlap of low lying C2H4 filled π orbitals with the 3dz2 empty orbital of titanium, enhanced by a small contribution due the π overlap between the semioccupied 3dyz orbital of titanium and the empty π* orbital of ethylene. The spin density repartition over the ethylene molecule, obtained experimentally, allows probing directly the entity of the metal-to-substrate π*-back-donation, highlighting an asymmetry in the spin density delocalization. This interesting feature is supported by parallel theoretical calculations, which cast the role of the oxygen donor ligands in driving this bonding asymmetry. As a consequence, the interesting structural feature of potential and actual inequality in the electronic spin states (α,β) on the two ethylene carbon atoms of the π coordinated ethylene molecule is produced. The underlying electronic effects associated with the π coordination of ethylene to an early transition metal in paramagnetic state are thus revealed with an unprecedented accuracy for the first time.

ω-Phenoxyalkyl substituted bis(indenyl)zirconium dichloride complexes as catalysts for homogeneous ethylene polymerization

Nine bis(indenyl)zirconium dichloride complexes of the type [C9H6-(CH2)n-O-Ar]2ZrCl2 (n=3–5; Ar=Ph, t-Bu-Ph) were synthesized, characterized, activated with methylalumoxane (MAO) and tested for ethylene polymerization. Structure–property-relationship studies showed that the activities of the catalysts depend on the length of the bridging chain between the indenyl and the phenoxy group as well as on the bulk at the phenoxy substituent. A t-Bu substituent at the ortho position of the phenoxy group (5a/MAO) gives a much higher catalyst activity (27,500kgPE/molcath) than the isomer 8a/MAO with a t-Bu substituent at the para position of the phenoxy group (16,700kgPE/molcath). Obviously substituents in the ortho position of the phenyl ring generate a bulkier catalyst cation and this can keep the MAO anion at a further distance to allow easier ethylene coordination and chain growth in the polymerization steps. The mono substituted bis(indenyl) complex (C9H7)[C9H6-(CH2)4-O-4-t-Bu]ZrCl2 shows lower activity (11,700kgPE/molcath) than 8a indicating that the electronic effect is dominating in this type of catalysts.