Ethylene Oligomerization Research Articles

Efficient conversion of syngas to linear α-olefins by phase-pure χ-Fe5C2.

Oil has long been the dominant feedstock for producing fuels and chemicals, but coal, natural gas and biomass are increasingly explored alternatives1-3. Their conversion first generates syngas, a mixture of CO and H2, which is then processed further using Fischer-Tropsch (FT) chemistry. However, although commercial FT technology for fuel production is established, using it to access valuable chemicals remains challenging. A case in point is linear α-olefins (LAOs), which are important chemical intermediates obtained by ethylene oligomerization at present4-8. The commercial high-temperature FT process and the FT-to-olefin process under development at present both convert syngas directly to LAOs, but also generate much CO2 waste that leads to a low carbon utilization efficiency9-14. The efficiency is further compromised by substantially fewer of the converted carbon atoms ending up as valuable C5-C10 LAOs than are found in the C2-C4 olefins that dominate the product mixtures9-14. Here we show that the use of the original phase-pure χ-iron carbide can minimize these syngas conversion problems: tailored and optimized for the process of FT to LAOs, this catalyst exhibits an activity at 290 °C that is 1-2 orders higher than dedicated FT-to-olefin catalysts can achieve above 320 °C (refs. 12-15), is stable for 200 h, and produces desired C2-C10 LAOs and unwanted CO2 with carbon-based selectivities of 51% and 9% under industrially relevant conditions. This higher catalytic performance, persisting over a wide temperature range (250-320 °C), demonstrates the potential of the system for developing a practically relevant technology.

Study on the structure and property of metallocene ethylene/1-heptene and ethylene/1-octene copolymers

Polyolefin elastomers are typically copolymers of ethylene with 1-butene, 1-hexene, or 1-octene. Other types of comonomers have not been widely applied industrially. The cost of 1-heptene obtained from Fischer-Tropsch synthesis is only half that of 1-octene, making it a viable economical substitute. This work utilized 1-heptene from Fischer-Tropsch synthesis and 1-octene from ethylene oligomerization as research subjects. Eight metallocene POEs based on 1-heptene and 1-octene were synthesized using Me2Si(C5Me4)(N t Bu)]TiCl2 (Ti-CGC) catalyst. The copolymers’ molecular structures and physical properties were characterized using HT-GPC, Temperature Rising Elution Fractionation (TREF),13C-NMR, DSC, Melt Index (MI), and Tensile tests. Compared to 1-heptene, 1-octene more effectively reduces the melting point and crystallinity, while 1-heptene’s stronger comonomer insertion capability facilitates the production of copolymers with higher comonomer content. Similar ternary sequence distributions and molecular dynamics indicate that ethylene/1-heptene and ethylene/1-octene copolymers possess comparable chain microstructures. Both ethylene/1-heptene copolymers and ethylene/1-octene copolymers exhibit excellent toughness and mechanical properties. Although the activity and insertion capacity of ethylene and 1-heptene decrease at low comonomer concentrations, they significantly improve at higher comonomer concentrations. Thus, Fischer-Tropsch synthesis of metallocene ethylene/1-heptene copolymers can effectively reduce raw material costs and be a cost-effective alternative to ethylene/1-octene copolymers.

Multisite-assisted design of asymmetric silicon-bridged diphosphine ligands for selective ethylene tri-/tetramerization

A series of novel silicon-bridged asymmetric diphosphine ligands (PNSiP-type L1-L3 and PCSiP-type L4) and their corresponding Cr(Ⅲ) catalysts have been synthesized and evaluated for ethylene tri-/tetramerization. When DMAO/AlEt3 was used as a co-catalyst, catalyst 1 based on ligand L1, with a cyclopentyl group on the N atom and two methyl groups on the Si atom, showed the highest catalytic activity of 8.21 kg g-1 Cr/h with 90.45 % combined (60.53 % 1-C6= + 29.92 % 1-C8=) selectivity. Moreover, the catalytic system based on catalyst 1 achieved a remarkable selectivity for 1-C8= of 62.58 % at 15 °C, even though polymers were the main products. Catalyst 2 based on ligand L2, bearing a cyclopentyl group at N and one methyl and one phenyl group on the Si moiety, exhibited a catalytic activity of 8.09 kg g-1 Cr/h accompanied with 71.61 % 1-C6= and 14.12 % 1-C8=product selectivity. Catalyst 3 based on ligand L3 having an isopropyl group on the N atom and two methyl groups on the Si atom, displayed 6.95 kg g-1 Cr/h catalytic activity with 69.06 % 1-C6= and 17.88 % 1-C8=product selectivity. Catalyst 4 based on PCSiP-type ligand L4, showed poor catalytic activity and 79.15 % of high 1-C6= selectivity. Compared with the previously reported long-chain Cr(Ⅲ) catalysts based on silicon-bridged diphosphine ligands (PNSiCP-/PCSiCP-type), Cr(Ⅲ) catalysts based on short-chain PNSiP-type and PCSiP-type ligands designed in this work exhibited higher ethylene oligomerization performance. Among them, PNSiP-type ligands are more inclined to generate 1-C8= than PCSiP-type ligand, and reducing the steric bulk of substituents on the Si atom in PNSiP-type ligands is also beneficial for 1-C8= generation. Density functional theory (DFT) calculations also revealed that the PNSiP-type catalysts could exhibit excellent ethylene trimerization performance. The DFT calculations thus provide a theoretical foundation for a deeper understanding of the ethylene tri-/tetramerization mechanism and designing of new ligand structures.

A Comparative Study of Ni-Based Catalysts Prepared by Various Sol-Gel Routes.

The use of heterogeneous catalysts to increase the development of green chemistry is a rapidly growing area of research to save industry money. In this paper, mesoporous SiO2-Al2O3 mixed oxide supports with various Si/Al ratios were prepared using two different sol-gel routes: hydrolytic sol-gel (HSG) and non-hydrolytic sol-gel (NHSG). The HSG route was investigated in both acidic and basic media, while the NHSG was explored in the presence of ethanol and diisopropyl ether as oxygen donors. The resulting SiO2-Al2O3 mixed oxide supports were characterized using EDX, N2 physisorption, powder XRD, 29Si, 27Al MAS-NMR and NH3-TPD. The mesoporous SiO2-Al2O3 supports prepared by NHSG seemed to be more regularly distributed and also more acidic. Consequently, a simple one-step NHSG (ether and alcohol routes) was selected to prepare mesoporous and acidic SiO2-Al2O3-NiO mixed oxide catalysts, which were then evaluated in ethylene oligomerization. The samples prepared by the NHSG ether route showed better activity than those prepared by the NHSG alcohol route in the oligomerization of ethylene at 150 °C.

Ethylene Dimerization, Isomerization and Trimerization: Mechanistic Insights into Competing Pathways on Metal–Organic Framework Supported Metal Hydrides

AbstractCatalytic ethylene oligomerization is important in the petrochemical industry, as it produces valuable products like linear alpha‐olefins (LAOs) for the synthesis of polymers and specialty chemicals. Catalysts, typically transition metal complexes or metal‐containing materials, play a crucial role in selective oligomerization by providing active sites for ethylene activation and controlling product distribution. Recently, defective metal‐organic frameworks (MOFs) with well‐defined structures and unsaturated active sites have emerged as potential catalysts for ethylene oligomerization. In this study, we employ density functional theory calculations to investigate the energetic selectivity of competing pathways (including dimerization, isomerization and trimerization) during ethylene oligomerization on defective HKUST‐1 supported metal hydrides (H−M–DHKUST‐1, M: Co, Ni, Ru, Rh and Pd). On all the five hydrides, ethylene dimerization is found to be more preferential than trimerization and isomerization. In the microenvironment of defective paddlewheel of HKUST‐1, isomerization is highly unfavorable compared with dimerization because of high energy required for chain walk, and 1‐butene is expected to be the major product on all the five hydrides. Analysis of NBO charges reveals a notable reduction in cationic nature at the metal site on H−Ru–DHKUST‐1, which facilitates β‐hydride elimination for dimerization. The microscopic insights from this computational study might facilitate the rational design of new MOFs and other MOF‐supported catalysts for selective ethylene oligomerization.

Oligomerization of ethylene by dinuclear chromium catalysts bearing phenyl-bridged bifunctional imine-pyrrolide/thiophene ligands

Dinuclear chromium(III) complexes [Cr(Z)-2-(HCN)-2-OCH3C6H4)(THF)Cl2]2 (Z= C4H3N, L1-Cr2) or [Cr(Z)-2-(HCN)-2-OCH3C6H4)(THF)Cl3]2 (Z = C4H3S, L2-Cr2; Z = 5-methyl-C4H2S, L3-Cr2; Z = 5-phenyl-C4H2S, L4-Cr2) were synthesized by reaction of the phenyl-bridged bifunctional imine-pyrrole/thiophene ligands with [CrCl3(THF)3]. DFT calculations for L2-Cr2 revealed that both isomers (anti and syn) are stable presumably due to the nonrigid skeleton of this class of ligands. On activation with MAO, all chromium complexes showed catalytic activity in ethylene oligomerization, providing a non-selective distribution of α-olefins (C4-C12+) together with varying amounts of polymer (3.6 – 45.0 wt%). The nature of the pendant heterocyclic moieties plays a substantial influence on the catalytic performances of these binuclear Cr catalysts. L1-Cr2 bearing pyrrolide unit yielded a significant quantity of polymer (45.0 wt%) while the binuclear chromium complexes based on thiophene unit showed higher productivities towards production of oligomers (up to 94.4 wt%). These latter Cr complexes showed higher productivities in comparison to their mononuclear analogues. DFT calculations, assuming the formation of cationic chromium (III) species after treatment with MAO in ethylene atmosphere, suggested the coordination of the thiophene group to the chromium metal center, and thus influencing the catalytic performance of L2Cr2– L4-Cr2. By adjusting the reaction conditions, L3-Cr2 showed TOF of 131,100 mol ethylene·mol Cr−1·h−1 producing predominantly oligomers (89.3 wt% of total products) with high selectivity for α-olefins (>91.5 wt%).

Synthesis of NiZn-based paddle-wheel metal-organic framework and its use as a catalytic precursor for ethylene dimerization

A bimetallic pillared-layered MOF based on Ni and Zn with improved catalytic activity was synthesized and applied in the ethylene oligomerization reaction. According to the XANES spectra, the MOF possesses Ni in the 2+ oxidation state, a well-known active catalytic precursor in oligomerization systems. In addition, Ni and Zn oxide or metallic species were not identified, indicating the absence of impurity phases. µ-XRF and SEM-EDS techniques showed the homogeneous distribution of Ni and Zn species across the SBUs of the bimetallic MOF. Ni/Zn-MOF was applied in the ethylene oligomerization reaction using EASC as the co-catalyst, and the results were compared to its monometallic counterpart Ni-MOF. The bimetallic material Ni/Zn-MOF obtained a TOF corresponding to 135 ×103 h−1, which accounts for a 60 % increase in the catalytic activity achieved by Ni-MOF (85 ×103 h−1) under 15 bar of ethylene in a Parr reactor. Moreover, the results obtained in this work are remarkable compared to literature reports for Ni-based MOFs, demonstrating that the co-catalyst employed plays an important role in the catalytic activity. However, Ni/Zn-MOF showed a lower selectivity to α-C4 oligomers (36 %) against 58 % obtained by Ni-MOF. According to the reuse tests conducted, the bimetallic MOF can be reused for up to two reactions (under 5 bar ethylene in a glass reactor), although presenting a considerable loss in activity due to the formation of metallic Ni.

High Activity Ethylene Oligomerization Using Asymmetric Alkyl P‐Substituted Bis(phosphanyl)amine Ni(II) complexes

ABSTRACTA series of novel asymmetric alkyl P‐substituted bis(phosphanyl)amine (PNP) Ph2PN (cyclopentyl) PR1R2‐type L1–L5 ligands and these corresponding Ni(II) precatalysts C1–C5 were synthesized and characterized. The structures of these complexes were confirmed by using 1H‐, 31P‐, 13C‐NMR, FT‐IR, and elemental analysis. Using ethylaluminum dichloride (EADC) as a cocatalyst, nickel complexes exhibited high activity in elective ethylene oligomerization, with the main products being dimers and a small number of trimers. Under optimized conditions of 60°C and 1.0 MPa ethylene pressure, C4, bearing a diisopropylphosphoryl group, exhibited highest catalytic activity of 1342.9 kg·g Ni−1·h−1 with 85.2% C4 and 14.8% C6 products selectivity, while C2, bearing a diethylphosphonyl group, showed catalytic activity of 596.4 kg·g Ni−1·h−1 with 88.2% C4 and 11.8% C6 product selectivity. Single crystal analysis offered a more comprehensive insight into the subtle effects of alkyl P‐substituted in the scaffold of C2 and C4 ligands on catalytic activity. Density functional theory (DFT) calculations indicated that lower energy of the LUMO in the C4A intermediate enhances the activity of ethylene oligomerization. It provides a new method for purposefully designing ligands for ethylene oligomerization with high catalytic activity.

Highly dispersed and stable Schiff base nickel catalyst on multi-walled carbon nanotubes promote ethylene oligomerization

α-Olefins are essential chemical raw materials in the synthesis of emulsifiers, plasticizers, and other value-added chemicals, and ethylene oligomerization is the main method used for preparing α-olefins. However, catalysts employed in ethylene oligomerization have drawbacks such as poor selection of target products, catalyst particle aggregation, and deactivation. To prevent the aggregation of the catalyst and expose a large number of active sites, we propose an anchoring strategy to stabilize and uniformly disperse catalysts on multi-walled carbon nanotubes (MWCNTs). Hybrid catalysts (SCn@MWCNTs) fabricated using the strategy exhibited considerably higher catalytic performance compared with homogeneous Schiff base nickel catalysts (SCn) owing to the combined effects of MWCNTs and active nickel centers. Under optimal catalytic conditions (temperature: 25 °C; Al/Ni molar ratio: 500 (SCn); Al/Ni molar ratio: 700 (SCn@MWCNTs); pressure: 0.7 MPa), the activities of SC1 and SC1@MWCNTs were 6.56 × 104 and 8.25 × 104 g/(mol Ni·h), respectively. In particular, SC1@MWCNTs showed remarkable recyclability and catalytic stability.

A Tandem Photovoltaic-Electrochemical Photothermal Process for CO2 Conversion to Butene

Decarbonizing the chemicals industry requires net carbon negative technologies capable of displacing current greenhouse gas emitting processes. In particular, the generation of valuable monomers, such as butene, are currently produced by significant greenhouse gas emitting processes, including ethylene oligomerization and crude oil refining, which require high temperatures and pressures. Integrated, tandem solar fuels devices for the conversion of CO2 to ethylene, using solar-driven CO2 reduction (CO2R), combined with a photothermal reactor for ethylene oligomerization, offers a potential path to producing clean butene from CO2, using solar energy as the only driving force. A key challenge for the successful operation of a tandem photo-driven electrochemical-photothermal system is the co-design of the two processes, which are typically optimized under different operating conditions. Specifically, photo-driven electrochemical CO2R (pCO2R) reactors operate under ambient temperature and pressure conditions while thermally-driven ethylene oligomerization reactors prefer elevated temperatures (80-140 °C) and pressures (30-150 atm) to promote high activity as well as pure ethylene in the inlet stream. Operation of the tandem process under solar-driven conditions imposes additional constraints on the maximum power output of the pCO2R reactor and the maximum temperature which can be obtained in the photothermal reactor. Herein, we present an unassisted solar-driven tandem system coupling pCO2R with photothermal ethylene oligomerization for the conversion of CO2 to butene in a single pass. This work presents a critical advancement in the development of solar fuels technologies for larger hydrocarbon generation and identifies the co-design principles which must be considered for implementation of tandem, solar-driven electrochemical-thermal processes.

High-temperature stable chromium complexes bearing fluorinated PNP ligands for selective ethylene tetramerization toward 1-octene

A series of new fluorinated PNP ligands of bis(ortho-fluorophenyl)-type (L1-L2) and ortho-fluorophenyl-type (L3-L4) have been synthesized and assessed for the selectivity of ethylene oligomerization. Compared to the non-fluorinated analogue ligand L6, the fluorinated ligand L1 exhibited approximately three times high catalytic activity. The bis(ortho-fluorophenyl)-type ligand L2 with a diethylphosphine substituent offered the highest catalytic performance of 3548 kg∙g−1∙h−1 and the 1-octene selectivity is 67.6 %. Both the L1 and L2 ligands exhibited excellent the activity of catalyst at a high temperature of 100 ℃, with ligand L2 providing the catalytic performance of 1587 kg∙g−1∙h−1 and good selectivity of 47.5 % for 1-C8=. The ortho-fluorophenyl substituent enhances the catalytic activity, while the para-fluorophenyl substituent reduces the catalytic activity. In addition, density functional theory (DFT) calculations and UV–vis spectroscopy have been used to rationalize the role of the fluorine effect in promoting catalytic performance. Our results indicated that the strong inductive effect of fluorine atoms and the non-covalent interaction between fluorine atoms and active centers jointly enhance the stability of the ortho-fluorophenyl-based catalyst and promote its overall activity.

Catalytic Synthesis of Butene-1 and Hexene-1 in the Homogeneous Oligomerization of Ethylene in the Presence of Nickel Complexes Based on N-Heteroaryl-Substituted α-Diphenylphosphinoglycines

It has been experimentally shown that N-heteroaryl-substituted α-diphenylphosphinoglycines N-(pyrazin-2-yl)-α-diphenylphosphinoglycine, N-(pyridin-2-yl)-α-diphenylphosphinoglycine and N-(pyrimidin-2-yl)-α-diphenylphosphinoglycine obtained by the reaction of three-component condensation of diphenylphosphine, the corresponding primary amine and glyoxylic acid monohydrate are capable in combination with Ni(COD)₂, where COD is cyclooctadiene-1,5, to form active forms of catalysts for selective homogeneous dimerization and trimerization of ethylene with the formation of butene-1 and hexene-1 as the main products. It has been established that the obtained organo-nickel catalytic systems provide a yield of short-chain (C₄–C₆) olefins at the level of 90% with a selectivity for linear α-olefins of 97%. The study of the influence of temperature on the process of homogeneous ethylene oligomerization using the obtained compounds made it possible to establish that the optimal temperature for ethylene oligomerization, providing the highest selectivity to butene-1 and hexene-1, is 80–105°C at the optimum pressure of ethylene is 20–35 atm. Under these conditions, the selectivity for butenes is 71.4–72.6% (selectivity for butene-1 – 69.3–71.1%), for hexenes 20.6–21.2% (selectivity for hexene-1 – 19.2–19.5%), and the optimal duration of the oligomerization process at a temperature 105°C is 1.5 h, which provides the rate of formation of butene-1 equal to 168.1 golig gNi⁻¹h⁻¹) and the rate of formation of hexene-1 – 47.3 golig gNi⁻¹h⁻¹).

Ethylene Oligomerization into Linear α-Olefins Catalyzed by VSB-5 without Alkyl-Aluminum Cocatalysts

Ethylene Oligomerization into Linear α-Olefins Catalyzed by VSB-5 without Alkyl-Aluminum Cocatalysts

Tuning a Cr-Catalyzed Ethylene Oligomerization Product Profile via a Rational Design of the N-aryl PNP Ligands

An approach towards incorporating varied degrees of steric profiles around the ligand’s backbone, which were envisaged to alter the catalytic paths leading to targeted 1-C8/1-C6 olefin products, were explored. Cr-pre-catalysts designed with PNP ligands comprising a fused aryl moiety were delivered at a relatively higher C8 olefin selectivity (up to 74.6 wt% and C8/C6 of 3.4) when the N-connection to the aromatic unit was placed at the 2-position. A relatively higher C6 olefin selectivity (up to 33.7 wt% and C8/C6 of 1.9) was achieved with the PNP unit anchored at the 1- or 6-position. Based on detailed catalytic studies, we confirm the fact that by introducing a controlled degree of bulkiness on the N-site through a judicious selection of the N-aryl moiety of different sizes, the selectivity of the targeted olefin product could be tuned in a rational manner.

Stable CAAC-Triazenes: A New Nitrogen Ligand System With Donor and Conformational Flexibility, and With Application in Olefin Activation Catalysis.

N-heterocyclic imines such as pyridylidene amines impart high catalytic activity when coordinated to a transition metal, largely imposed by their electronic flexibility. Here, this donor flexibility has been applied for the first time to CAAC-based systems through the synthesis of CAAC-triazenes. These new ligands offer a larger π-conjugation that extends from the N-heterocyclic carbene through three nitrogens rather than just one, as observed in N-heterocyclic imines. We demonstrate the straightforward synthesis of three new CAAC-triazenes containing different substituents on the terminal triazene nitrogen. These compounds are remarkably stable up to 120 °C without loss of N2 as typically observed with similar triazenes. E-to-Z isomerization within the triazene is instigated by UV light and is partially reversible dependent on the triazene substituent. The quinoline-substituted CAAC-triazene 1-Q has been applied as an L,L'-type ligand in the synthesis of [PdCl2(1-Q)], [PdCl(Me)(1-Q)] and [Pd(Me)(H2O(1-Q)]+. E-to-Z ligand isomerization also occurs when coordinated to PdCl2, providing access to on-metal manipulation. The cationic complex [PdMe(H2O)(1-Q)]+ is a precatalyst for oligomerization of ethylene to form initially 2-butene and subsequently linear and branched C8-C12 products from butene activation. Moreover, isomerization of 1-hexene takes place efficiently with exceptionally low catalyst loading (10 ppm) and up to 74,000 turnover numbers.

Catalytic ethylene oligomerization reactions of terpyridine (NNN)-based nickel(II) complexes – Studies on catalytical intermediates

The present work demonstrates the synthesis of four new terpyridine (tpy) ligands (4′-(4-(pent-4-en-1-yloxy)phenyl)-2,2′:6′,2″-terpyridine (3), 4-((3-([2,2′:6′,2″-terpyridin]-4′-yl)phenoxy)methyl)benzonitrile (4), 4′-(3-(pent-4-en-1-yloxy)phenyl)-2,2′:6′,2″-terpyridine (5), and 4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl 5-chlorothiophene-2-carboxylate (6)), the corresponding Ni(II) complexes (3C–6C), and their usage as catalysts in ethylene oligomerization reactions. The purity of all the ligands and the complexes employed in the present study was assessed by various analytical and spectroscopic techniques. One of the tpy-Ni(II) complexes (6C) was structurally characterized by single-crystal XRD analysis. Notably, the nature of ligands as well as temperature strongly influence the organic product distribution, i.e. elevated temperatures facilitate C6 product in significant amounts. Among all the complexes, the Ni(II)-tpy (6C) substituted with 5-chloro-thiophenyl group at 4′ position showed high selectivity for 1-hexene (C6, 80%) at room temperature and 1-octene (C8, 91%) at elevated temperatures. Though both mechanistic pathways were proved to be feasible, the present study is also intended to identify and analyze the key intermediate species of the reactions. In order to shed more light on mechanistic aspects, ethylene insertion reactions were compared with selected platinacycloalkane complexes in a high-pressure autoclave reactor, and the formation of large metallacycloalkane species was observed in the mass spectral data.

Monodentate Phosphinoamine Nickel Complex Supported on a Metal-Organic Framework for High-Performance Ethylene Dimerization.

Ethylene dimerization is an efficient industrial chemical process to produce 1-butene, with demanding selectivity and activity requirements on new catalytic systems. Herein, a series of monodentate phosphinoamine-nickel complexes immobilized on UiO-66 are described for ethylene dimerization. These catalysts display extensive molecular tunability of the ligand similar to organometallic catalysis, while maintaining the high stability attributed to the metal-organic framework (MOF)scaffold. The highly flexible postsynthetic modification method enables this study to prepare MOFs functionalized with five different substituted phosphines and 3 N-containing ligands and identify the optimal catalyst UiO-66-L5-NiCl2 with isopropyl substituted nickel mono-phosphinoamine complex. This catalyst shows a remarkable activity and selectivity with a TOF of 29000 (molethyl/molNi/h) and 99% selectivity for 1-butene under ethylene pressure of 15bar. The catalyst is also applicable for continuous production in the packed column micro-reactor with a TON of 72000 (molethyl/molNi). The mechanistic insight for the ethylene oligomerization has been examined by density functional theory (DFT) calculations. The calculated energy profiles for homogeneous complexes and truncated MOF models reveal varying rate-determining step as β-hydrogen elimination and migratory insertion, respectively. The activation barrier of UiO-66-L5-NiCl2 is lower than other systems, possibly due to the restriction effect caused by clusters and ligands. A comprehensive analysis of the structural parameters of catalysts shows that the cone angle as steric descriptor and butene desorption energy as thermodynamic descriptor can be applied to estimate the reactivity turnover frequency (TOF) with the optimum for UiO-66-L5-NiCl2. This work represents the systematic optimization of ligand effect through combination of experimental and theoretical data and presents a proof-of-concept for ethylene dimerization catalyst through simple heterogenization of organometallic catalyst on MOF.

New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes.

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5 a) and [CrIIIBr2.3Cl0.7(CNC)] (5 b), where CNC=3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5 a and 5 b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5 a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH)=3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

Supported Organochromium Ethylene Oligomerization Enabled by Surface Lithiation

Supported Organochromium Ethylene Oligomerization Enabled by Surface Lithiation

New Data on the Reactions of Zirconium and Hafnium Tetrachlorides with Aliphatic Acids

The reaction of ZrCl4 or HfCl4 with excess 2-methylpropanoic acid when boiling under reflux has been studied. The formation of polynuclear Zr and Hf complexes of the composition M2O(i-C3H7CO2)6 during prolonged reflux of the reaction mixtures was found. The complexes are very sensitive to hydrolysis, forming hexanuclear [M6(O)4(OH)4(i-C3H7CO2)12]. The reactions have a general character for aliphatic acids and can be used as an alternative to the known methods for the synthesis of polynuclear carboxylate clusters of Group 4 metals. The crystal and molecular structures of previously undescribed {[Hf6(μ3-O)4(μ3-OH)4(i-C3H7CO2)12(H2O)]·3i-C3H7COOH} have been determined. The molecular structure is a completely asymmetric hexanuclear cluster containing six Hf(IV) atoms united by a 4:4 μ3-O/OH system of bridges, and stabilized by twelve 2-methylpropanoate ligands, eight of which are bidentate bridging, three are chelating, and one is monodentate. The crystal structure of the complex includes three independent solvating 2-methylpropanoic acid molecules. The obtained IR spectroscopy data make it possible to determine the type of complexes in the reaction mixture. The results of the study may be useful for improving the catalytic systems for ethylene oligomerization.