The chloro-substituent enhances performance of 2,4-bis (imino)pyridylchromium catalysts yielding highly linear polyethylene

The five unsymmetrical 2-[1-(2,4-dibenzhydryl-6-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine compounds (aryl: 2,6-Me₂Ph L1 , 2,6-Et₂Ph L2 , 2,6-ⁱPr₂Ph L3 , 2,4,6-Me₃Ph L4 and 2,6-Et₂–4-MePh L5 ) were prepared and characterized with FT-IR and ¹H/¹³C NMR spectroscopy as well as elemental analysis. The treatment of L1 – L5 with CrCl₃·3THF affords the corresponding chromium chloride complexes ( Cr1 – Cr5 ) in excellent yields. The molecular structures of Cr2 and Cr3 characterized by X-ray diffraction show a distorted octahedral geometry with three nitrogen atoms and three chlorine atoms around the metal center. On activation with either MAO or MMAO, Cr1 – Cr5 collectively display high activity (up to 14.96 × 10⁶ g (PE) mol⁻¹ (Cr) h⁻¹ at 60 °C) affording highly linear polyethylene with low molecular weight distribution (M_w/M_n) ranging from 1.06 to 2.81. An in-depth catalytic evaluation of Cr1 was conducted in order to investigate how the cocatalyst type and its amount, reaction temperature and polymerization time affect the catalytic activities and polymer properties.     

Synthesis of a Ni Complex Chelated by a [2.2]Paracyclophane-Functionalized Diimine Ligand and Its Catalytic Activity for Olefin Oligomerization

A diimine ligand having two [2.2]paracyclophanyl substituents at the N atoms (L1) was prepared from the reaction of amino[2.2]paracyclophane with acenaphtenequinone. The ligand reacts with NiBr₂(dme) (dme: 1,2-dimethoxyethane) to form the dibromonickel complex with (R,R) and (S,S) configuration, NiBr₂(L1). The structure of the complex was confirmed by X-ray crystallography. NiBr₂(L1) catalyzes oligomerization of ethylene in the presence of methylaluminoxane (MAO) co-catalyst at 10–50 °C to form a mixture of 1- and 2-butenes after 3 h. The reactions for 6 h and 8 h at 25 °C causes further increase of 2-butene formed via isomerization of 1-butene and formation of hexenes. Reaction of 1-hexene catalyzed by NiBr₂(L1)–MAO produces 2-hexene via isomerization and C12 and C18 hydrocarbons via oligomerization. Consumption of 1-hexene of the reaction obeys first-order kinetics. The kinetic parameters were obtained to be ΔG^‡ = 93.6 kJ mol⁻¹, ΔH^‡ = 63.0 kJ mol⁻¹, and ΔS^‡ = −112 J mol⁻¹deg⁻¹. NiBr₂(L1) catalyzes co-dimerization of ethylene and 1-hexene to form C8 hydrocarbons with higher rate and selectivity than the tetramerization of ethylene.     

Catalytic Performance of Cobalt(II) Polyethylene Catalysts with Sterically Hindered Dibenzopyranyl Substituents Studied by Experimental and MLR Methods

Given the great importance of cobalt catalysts supported by benchmark bis(imino)pyridine in the (oligo)polymerization, a series of dibenzopyran-incorporated symmetrical 2,6-bis(imino) pyridyl cobalt complexes (Co1–Co5) are designed and prepared using a one-pot template approach. The structures of the resulting complexes are well characterized by a number of techniques. After activation with either methylaluminoxane (MAO) or modified MAO (MMAO), the complexes Co1–Co4 are highly active for ethylene polymerization with a maximum activity of up to 7.36 × 10⁶ g (PE) mol⁻¹ (Co) h⁻¹ and produced highly linear polyethylene with narrow molecular weight distributions, while Co5 is completely inactive under the standard conditions. Particularly, complex Co3 affords polyethylene with high molecular weights of 85.02 and 79.85 kg mol⁻¹ in the presence of MAO and MMAO, respectively. The ¹H and ¹³C NMR spectroscopy revealed the existence of vinyl end groups in the resulting polyethylene, highlighting the predominant involvement of the β-H elimination reaction in the chain-termination process. To investigate the mechanism underlying the variation of catalytic activities as a function of substituents, multiple linear regression (MLR) analysis was performed, showing the key role of open cone angle (θ) and effective net charge (Q) on catalytic activity.     

2-Acetyloxymethyl-substituted 5,6,7-trihydroquinolinyl-8-ylideneamine-Ni(II) chlorides and their application in ethylene dimerization/trimerization

Six examples of 2-acetyloxymethyl-substituted 5,6,7-trihydroquinolinyl-8-ylideneamine-nickel(II) chlorides, [2-(CH₂OC(O)Me)-8-{N(Ar)}C₉H₈N]NiCl₂ (Ar = 2,6-Me₂C₆H₃ Ni1 , 2,6-Et₂C₆H₃ Ni2 , 2,6-i-Pr₂C₆H₃ Ni3 , 2,4,6-Me₃C₆H₂ Ni4 , 2,6-Et₂–4-MeC₆H₂ Ni5 , 2,4,6-t-Bu₃C₆H₂ Ni6 ), have been prepared by a one-pot template reaction of 2-(hydroxymethyl)-6,7-dihydroquinolin-8(5H)-one with nickel dichloride hexahydrate, the corresponding aniline and acetic acid. All complexes were characterized by elemental analysis and IR spectroscopy, while dinuclear Ni2 and mononuclear Ni3 ·OH₂ have additionally been the subject of single crystal X-ray diffraction studies; in both structures the acetyloxymethyl group remained uncoordinated. On activationof Ni1 – Ni6 with MMAO, hexenes (C6: ca. 48% 1-hexene) formed the major product of ethylene oligomerization along with minor quantities of butenes (C4); high overall activities of up to 1.33 × 10⁶ g·mol–1 (Ni)h–1 (for mesityl-containing Ni4 ) were achieved at 30 ^oCand 10 atm C₂H₄. By comparison with MAO as co-catalyst, Ni1 – Ni6 exhibited lower activities but displayed a specificity towards ethylene dimerization (C4: 64–99% 1-butene). Furthermore, sizable induction periods were a feature of the MAO runs with Ni1 /MAO reaching peak catalytic activity only after 45 mins.     

Thermochemistry of Li, Na, K, Rb and Cs alkylated phenoxides

Lattice energies and thermochemical radii of the anions OR⁻ (R = 2-Me; 2,6-Me₂; 2,4,6-Me₃; 2,6-t-Bu₂) in alkali metal phenoxides, MOR (M = Li, Na, K, Rb and Cs) were investigated based on the corresponding standard molar enthalpies of formation determined by reaction-solution calorimetry. The results obtained at 298.15 K were as follows: (MOR, cr)/kJ mol⁻¹ = −398.4 ± 1.1 (LiO-2-MePh), −423.4 ± 1.6 (LiO-2,6-Me₂Ph), −457.3 ± 7.1 (LiO-2,4,6-Me₃Ph), −346.6 ± 1.4 (NaO-2-MePh), −399.1 ± 1.5 (NaO-2,6-Me₂Ph), −422.4 ± 7.1 (NaO-2,4,6-Me₃Ph), −496.6 ± 7.1(NaO-2,6-t-Bu₂Ph), −367.8 ± 1.2 (KO-2-MePh), −399.1 ± 1.4 (KO-2,6-Me₂Ph), −368.8 ± 1.2 (RbO-2-MePh), −403.6 ± 1.3 (RbO-2,6-Me₂Ph), −387.0 ± 1.6 (CsO-2-MePh) and −413.6 ± 1.3 (CsO-2,6-Me₂Ph). Estimates of thermochemical raddi, lattice energies and standard enthalpies of formation of not experimentally measured alkali metal phenoxides was successfully done with a model based on the Kapustinskii equation and the set of values experimentally determined.     

Investigating the Effects of Para-methoxy Substitution in Sterically Enhanced Unsymmetrical Bis(arylimino)pyridine-cobalt Ethylene Polymerization Catalysts

A group of five bis(arylimino)pyridine-cobalt(Ⅱ)chloride complexes,[2-{(2,6-(Ph₂CH)₂-4-MeOC₆H₂)N=CMe}-6-(ArN=CMe)C_5 H₃N]CoCl₂(Ar=2,6-Me₂C₆H₃Co1,2,6-Et₂C₆H₃Co2,2,6-iPr₂C₆H₃Co3,2,4,6-Me₃C₆H₂Co4,2,6-Et₂-4-MeC₆H₂Co5),each containing one N-4-methoxy-2,6-dibenzhydrylphenyl group and one smaller sterically/electronically variable N-aryl group,have been synthesized in good yield(>71%)from the corresponding neutral terdentate nitrogen-donor precursor,L1-L5.All complexes have been characterized by¹H-NMR and FTIR spectroscopy with the former highlighting the paramagnetic nature of these cobaltous species and the unsymmetrical nature of the chelating ligand.The molecular structures of Co3 and Co4 emphasize the steric differences of the two inequivalent N-aryl groups and the distorted square pyramidal geometry about the metal centers.In the presence of MAO or MMAO,Co1-Co5 collectively displayed high activities for ethylene polymerization producing high molecular weight polyethylenes that,in general,exhibited narrow dispersities(M_w/M_n values:2.12-4.07).Notably,the least sterically hindered Co1 when activated with MAO was the most productive(6.92×10⁶g_PE·mol^-1_(Co)·h^-1)at an operating temperature of60℃.Conversely,the most sterically hindered Co3/MMAO produced the highest molecular weight polyethylene(M_w=6.29×10⁵g·mol^-1).All the polymers displayed high linearity as demonstrated by their melting temperatures(>130℃)and their~1 H-and¹³C-NMR spectra.By comparison of Co1 with its para-methyl,-chloro and-nitro counterparts,the presence of the para-methoxy substituent showed the most noticeable effect of enhancing the thermal stability of the catalyst.     

Synthesis,characterization and ethylene polymerization of 1-(2,6-dimethyl-4-fluorenylphenylimino)-2-aryliminoacenaphthylnickel bromides

A series of 1-(2,6-dimethyl-4-fluorenylphenylimino)-2-aryliminoacenaphthylene compounds (aryl = 2,6-di(Me)Ph (L1), 2,6-di(Et)Ph (L2), 2,6-di(i-Pr)Ph (L3), 2,4,6-tri(Me)Ph (L4), 2,6-di(Et)-4-MePh (L5)) was prepared and used to form their corresponding dibromonickel complexes (D1–D5). Both L1–L5 and D1–D5 were fully characterized by FT-IR and elemental analysis as well as NMR measurements in the case of ligands L1–L5. The molecular structure of the representative complex D5 was confirmed by single crystal X-ray diffraction revealing a distorted trigonal bipyramidal geometry around the nickel center. On activation with either ethylaluminium sesquichloride (Et₃Al₂Cl₃, EASC) or methylaluminoxane (MAO), all nickel complexes exhibited high activities up to 9.82 × 10⁶ g of PE (mol of Ni)⁻¹ h⁻¹ for ethylene polymerization. In comparison with the polyethylenes obtained with related Ni pre-catalysts, the polyethylenes obtained in this work possessed relatively higher molecular weights and lower levels of branching, highlighting the significant influence of the remote fluorenyl substituent.     

Determination of 2-Acetyl-1-pyrroline via a Color-Change Reaction Using Chromium Hexacarbonyl

At present, there is no colorimetric method for the quantitation of the aroma compound 2-acetyl-1-pyrroline (2AP). A novel colorimetric method was developed for the determination of 2AP content using chromium hexacarbonyl (Cr(CO)₆) as a reagent. The reaction of synthetic 2AP with chromium hexacarbonyl reagent solution in the presence of light produced a green product with an absorption maximum (λ_max) at 623 nm. GC–MS was used to confirm the color-change reaction, which showed the loss of 2AP after the addition of Cr(CO)₆. This novel method enables facile and cost-effective determination of 2AP in fragrant rice. A comparative analysis of fragrant and nonfragrant rice grain extracts showed that no color-change reaction occurred with the nonfragrant rice sample. A limit of detection (LOD) of 2.00 mg L⁻¹ was determined by method validation with an effective linear concentration ranging from 5.00 to 60.00 mg L⁻¹ of 2AP. The results obtained using the developed colorimetric method were consistent with those obtained by automated static headspace gas chromatography with nitrogen-phosphorus detection (SHS-GC–NPD).     

Chromium Complexes Supported by Phenyl Ether-Pyrazolyl [N,O] Ligands as Catalysts for the Oligo- and Polymerization of Ethylene

A new series of Cr (III) complexes [Cr{1-(3-phenoxypropyl)-1H-pyrazole}Cl₃]₂ (Cr1), [Cr{1-(3-phenoxypropyl)-3,5-dimethyl-1H-pyrazole}Cl₃]₂ (Cr2 ), and [Cr{1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole}Cl₃]₂ (Cr3) have been synthesized and characterized by elemental analysis, high-resolution mass spectrometry (HRMS) and IR spectroscopy. Upon activation with methylaluminoxane (MAO), chromium precatalysts Cr2 and Cr3 showed moderate activity in ethylene oligomerization [TOF = 17,900–29,200 mol (ethylene)·mol (Cr)⁻¹·h⁻¹ at 80 °C] with Schultz-Flory distribution of oligomers (K = 0.54–0.66) and production of polymer varying from 2.8 to 6.7 wt.%. On the other hand, under identical oligomerization conditions, Cr1 /MAO behaved as a polymerization catalyst generating predominantly polyethylene (63.7 wt%). The amount of 1-butene is the largest component in the liquid fraction suggesting that these precatalysts operate via a Cossee-Arlman mechanism. The catalytic activities, selectivity and product distribution are quite sensitive to the R-group at the 3- and 5-position of the pyrazolyl ring. Based on the electronic and steric effects of R- substituents, it is possible to stablish a trend of activity: Cr2 (Pz^Me2) > Cr3 (Pz^Ph) > Cr1 (Pz). Moreover, the effect of oligomerization parameters (cocatalyst, temperature, [Al]/[Cr] molar ratio, time) on the activity and on the product distribution were examined.     

Vinyl addition polymerization of norbornene catalyzed by β-iminoamine Ni(II) complexes/methylaluminoxane systems

Two nickel(II) complexes (A and B) bearing β-iminoamine ligands, [2-(ArNCH)-C₆H₄-NMe₂] (La, Ar = 2,6-i-Pr₂C₆H₃; Lb, Ar = 2,6-Me₂C₆H₃), were synthesized and characterized by elemental analyses and ¹H NMR. X-ray crystal structure of complex B reveals that the six-membered chelate ring adopts a envelope conformation, with nickel(II) atom deviating from the plane of backbone aromatic ring by 1.164 Å. In the presence of methylaluminoxane (MAO), both complexes showed moderate activities of 10⁵ g mol_Ni⁻¹ h⁻¹ for norbornene polymerization. β-iminoamine Ni(II)/MAO catalysts gave unimodal polymers (M_w, 3.16-8.02 × 10⁵ g/mol) with a relatively narrow MWD (M_w/M_n, 1.59-2.14), indicative of single-site catalyst behavior. The obtained polymers are vinyl-type polynorbornenes (PNBs), which are soluble in common solvents such as toluene, cyclohexane and dichlorobenzene.     

Synthesis, structure and norbornene polymerization behavior of nickel complexes bearing two β-ketoiminato chelate ligands

A series of nickel(II) complexes bearing two nonsymmetric bidentate β-ketoiminato chelate ligands have been prepared, and the structures of complexes [(2,6-Me₂C₆H₃)NC(CH₃)C(H)C(Ph)O]₂Ni (4a) and [(2,6-Me₂C₆H₃)NC(CH₃)C(H)C(CF₃)O]₂Ni (4c) have been confirmed by X-ray crystallographic analysis. These nickel(II) complexes were investigated as catalysts for the vinylic polymerization of norbornene. Using modified methylaluminoxane (MMAO) as a cocatalyst, these complexes display very high activities and produce high molecular weight polymers. Catalytic activity of up to 1.16 × 10⁴ kg/mol_Ni · h and the viscosity-average molecular weight of polymer of up to 870 kg/mol were observed. Catalyst activity, polymer yield, and polymer molecular weight could be controlled over a wide range by the variation of the reaction parameters such as Al/Ni molar ratio, norbornene/catalyst molar ratio, monomer concentration, polymerization reaction temperature and time.     

Chromium(III) complexes bearing 2-benzoxazolyl-6-arylimino-pyridines: Synthesis and their ethylene reactivity

A series of chromium(III) complexes bearing 2-benzoxazolyl-6-aryliminopyridines was synthesized and characterized by IR spectroscopic and CHN analysis. The X-ray crystallographic analysis of complex Cr3 revealed a distorted octahedral geometry. When activated by Et₂AlCl, MAO or MMAO, these chromium complexes exhibited activities towards ethylene reactivity. High activities of ethylene oligomerization (up to 9.19 × 10⁶ g mol⁻¹ (Cr) h⁻¹) were observed in the catalytic system using MMAO as a cocatalyst, meanwhile good activities of ethylene polymerization were achieved (up to 5.20 × 10⁵ g mol⁻¹ (Cr) h⁻¹) by using MAO as a cocatalyst. Various reaction parameters were investigated in detail, and the steric and electronic effects of ligands were discussed.     

Exceptionally high molecular weight linear polyethylene by using N,N,N′‐Co catalysts appended with a N′‐2,6‐bis{di(4‐fluorophenyl)methyl}‐4‐nitrophenyl group

The bis(arylimino)pyridines, 2‐[CMeN{2,6‐{(4‐FC₆H₄)₂CH}₂–4‐NO₂}]‐6‐(CMeNAr)C₅H₃N (Ar = 2,6‐Me₂C₆H₃ L1 , 2,6‐Et₂C₆H₃ L2 , 2,6‐i‐Pr₂C₆H₃ L3 , 2,4,6‐Me₃C₆H₂ L4 , 2,6‐Et₂–4‐MeC₆H₂ L5 ), each containing one N′‐2,6‐bis{di(4‐fluorophenyl)methyl}‐4‐nitrophenyl group, have been synthesized by two successive condensation reactions from 2,6‐diacetylpyridine. Their subsequent treatment with anhydrous cobalt (II) chloride gave the corresponding N,N,N′‐CoCl₂ chelates, Co1 – Co5 , in excellent yield. All five complexes have been characterized by ¹H/¹⁹F NMR and IR spectroscopy as well as by elemental analysis. In addition, the molecular structures of Co1 and Co3 have been determined and help to emphasize the differences in steric properties imposed by the inequivalent N‐aryl groups; distorted square pyramidal geometries are adopted by each complex. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), precatalyts Co1 – Co5 collectively exhibited very high activities for ethylene polymerization with 2,6‐dimethyl‐substituted Co1 the most active (up to 1.1 × 10⁷ g (PE) mol^?1 (Co) h^?1); the MAO systems were generally more productive. Linear polyethylenes of exceptionally high molecular weight (M_w up to 1.3 × 10⁶ g mol^?1) were obtained in all cases with the range in dispersities exhibited using MAO as co‐catalyst noticeably narrower than with MMAO [M_w/M_n: 3.55–4.77 ( Co1 – Co5 /MAO) vs. 2.85–12.85 ( Co1 – Co5 /MMAO)]. Significantly, the molecular weights of the polymers generated using this class of cobalt catalyst are higher than any literature values reported to date using related N,N,N‐bis (arylimino)pyridine‐cobalt catalysts.     

Real-time analysis of methylalumoxane formation

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H₂O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12–15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)₁₆(Me₃Al)₆Me]⁻, and we have computationally located a new sheet structure for this ion 94 kJ mol⁻¹ lower in Gibbs free energy than any previously calculated.

The activator methylaluminoxane is made by hydrolysis of trimethylaluminum. Analysis using ESI-MS reveals rapid formation of small oligomers is followed by slower aggregation to the larger precursors most capable of releasing [Me₂Al]⁺.     

Hyperbranched bisphosphinoamine ligands: synthesis,characterization and application in ethylene oligomerization

Two hyperbranched bisphosphinoamine (PNP) ligands and chromium complexes were synthesized in good yield with 1.0 generation (1.0 G) hyperbranched macromolecules, chlorodiphenylphosphine (Ph₂PCl) and CrCl₃(THF)₃ as raw materials. The hyperbranched PNP ligands and chromium complexes were characterized by FT-IR, ¹H NMR, ³¹P NMR, UV and ESI-MS. Comparing with the chromium complexes, the hyperbranched PNP ligands, in combination with Cr(III), and activation by methylaluminoxane (MAO) in situ generated species with better catalytic performance for ethylene oligomerization. The effect of solvent, chromium source, ligand/Cr molar ratio, reaction temperature, Al/Cr molar ratio and reaction pressure on the catalytic activity and product selectivity were studied. The results showed that with increase of ligand/Cr molar ratio, reaction temperature and Al/Cr molar ratio, the catalytic activity increased at first and then decreased. However, the catalytic activity continuously increased with increase of reaction pressure. Under the optimized conditions, the catalytic system of hyperbranched PNP/Cr(III)/MAO led to catalytic activity of 2.68 × 10⁵ g/(mol Cr·h) and 37.71% selectivity for C₆ and C₈.     

Catalyst control of selectivity in the C–O bond alumination of biomass derived furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)₂CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furans proceeded between 25 and 80 °C leading to dearomatised products due to the net transformation of a sp² C–O bond into a sp² C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH^‡ = +19.7 (±2.7) kcal mol⁻¹, ΔS^‡ = −18.8 (±7.8) cal K⁻¹ mol⁻¹ and ΔG^‡_{298 K} = +25.3 (±0.5) kcal mol⁻¹ and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4 + 1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an α-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp² C–O bond through population of the σ*-orbital. Inclusion of [Pd(PCy₃)₂] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans and improves selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to the sp²C–H bond. The aluminium(iii) dihydride reagent [{(MesNCMe)₂CH}AlH₂] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination occurs with a KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to the palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy₃)₂]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to important platform chemicals from biomass.

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle.     

Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended Temperature Range (273–837 K)

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CH₃O)₂CH₃PO) (reaction 1) OH + DMMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 295–837 K temperature range. Hydroxyl radicals were produced in the fast reaction of electronically excited oxygen atoms O(¹D) with H₂O. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge H₂O/Ar lamp. The reaction rate constant exhibits a pronounced V-shaped temperature dependence, negative in the low temperature range, 295–530 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 530–837 K (the rate constant increases with temperature), with a turning point at 530 ± 10 K. The rate constant could not be adequately fitted with a standard 3-parameter modified Arrhenius expression. The data were fitted with a 5-parameter expression as: k₁ = 2.19 × 10⁻¹⁴(T/298)^2.43exp(15.02 kJ mol⁻¹/RT) + 1.71 × 10⁻¹⁰exp(−26.51 kJ mol⁻¹/RT) cm³molecule⁻¹s⁻¹ (295–837 K). In addition, a theoretically predicted pressure dependence for such reactions was experimentally observed for the first time.     

Synthesis,characterization and ethylene oligomerization and polymerization of 2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridylchromium chlorides

A series of N^N^N tridentate chromium complexes (C1–C6) bearing 2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridine derivatives was synthesized and characterized by elemental and spectroscopic analysis along with single-crystal X-ray crystallography. X-ray crystallographic analyses reveal chromium complex C1 as a distorted six-coordinated octahedral geometry. On treatment with modified methylaluminoxane (MMAO), the chromium complexes exhibited high activities for ethylene oligomerization (up to 1.50 × 10⁶ g mol⁻¹ (Cr) h⁻¹) and polymerization (up to 2.06 × 10⁶ g mol⁻¹ (Cr) h⁻¹) at 10 atm ethylene pressure. Various reaction parameters were investigated in detail, and less steric hindrance and electron-withdrawing substituents of ligands enhance the catalytic activities of their chromium complexes.     

Atmospheric reactivity of ethylene catalyzed by β‐diketiminato Ni(II)/methylaluminoxane system

Atmospheric ethylene reactions were studied with backbone fluorinated β‐diketiminato Ni(II) complexes CH{C(CF₃)NAr}₂NiBr (1, Ar = 2,6‐Me₂C₆H₃, and 2 2,6‐ⁱPr₂C₆H₃) activated by methylaluminoxane (MAO). The catalytic systems exhibit the characteristics of catalyzing simultaneously polymerization and oligomerization of ethylene, indicating different active species involved in the reaction system. In an effort to investigate the alkylation species involved in the β‐diketiminato nickel (II)/MAO system, the reaction of 1 with methylaluminoxane were studied. With ¹⁹F{¹H NMR} spectra, two sets of new signals different from 1 were presented. Two alkylation products were proposed precursors of active species for producing oligomer and polymer of ethylene in the β‐diketiminato Ni(II)/MAO system. Copyright © 2014 John Wiley & Sons, Ltd.     

Chromium(III) complexes ligated by 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines: Synthesis, characterization and their ethylene oligomerization and polymerization

A series of chromium(III) complexes bearing 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines were synthesized and characterized by IR spectroscopic and elemental analysis. The X-ray crystallographic analysis revealed a distorted octahedral geometry of the chromium complexes. When activated by Et₂AlCl, MAO or MMAO, these chromium complexes exhibited catalytic activities for ethylene oligomerization and polymerization; while the good to high activities (up to 3.95 × 10⁶ g mol⁻¹ (Cr) h⁻¹) were observed in the catalytic systems with MMAO. Therefore, various reaction parameters of the catalytic system with MMAO were investigated in detail. The steric and electronic effects of ligands affected the catalytic activities and the distribution of the products predominantly. Interestingly, sometimes their distributions of oligomers did not resemble the rules of Schulz-Flory or Poisson due to the hexenes produced in low yield.