Reactions between (E)-N-(2,6-diisopropylphenyl)-2-methyl-6,7-dihydroquinolin-8(5H)-imine nickel(II) dihalides and organoaluminum compounds AlMe3, (AlBu3)-Bu-i, MAO, and MMAO were monitored using EPR spectroscopy in situ. It was found that rapid one-electron reduction of the Ni(II) center results in formation of a variety of monovalent nickel species which are stable at room temperature in an oxygen-free atmosphere. On the basis of the EPR data obtained, these compounds were assigned to the heterobinuclear nickel-aluminum complexes of the type LNiI(mu-R)(2)AlX2, where L = N,N '-donor ligand, R = Me, Bu-i, H, Br, or Cl bridge, and X = Me, Bu-i, Br or, Cl, depending on conditions. Formation of the dihydride-bridged LNiI(mu-H)(2)AlX2 complex was confirmed using deuterium-labeled compounds. In the presence of an excess of ethylene or 1-hexene, LNiI(mu-R)(2)AlX2 eliminates RAlX2 to afford (LNiR)-R-I(C2H4) or (LNiR)-R-I(C6H12) species, respectively. Structural assignments of the Ni-I species are supported by DFT calculations of their g-tensor values.

Despite the Ni(II) α-diimine based ethylene polymerization catalysts were discovered almost 30 years ago, the mechanism of the ethylene polymerization over these catalysts still remains the subject of the numerous investigations. A significant progress in understanding the nature and role of the nickel compounds formed in real catalyst systems was made over the past 5–7 years. In present publication we summarized and analyzed the data on the nature and role of Ni(II) and Ni(I) species in the catalyst process.

A novel room-temperature stable diamagnetic nickel complex 2 was detected upon activation of Brookhart-type ethylene polymerization pre-catalyst LNiBr2 (1, L = 1,4-bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) with AlMe3. Using in situ 1H, 2H, and 13C NMR spectroscopy, as well as DFT calculations, this species has been identified as an antiferromagnetically coupled homodinuclear complex [LNiII(μ-Me)(μ-CH2)NiIIL]+Br−. Its behavior in the reaction solution is characteristic of the resting state of nickel catalyzed ethylene polymerization.

The decomposition of hydrogen peroxide (H2O2) is the main undesired side reaction in catalytic oxidation processes of industrial interest that make use of H2O2 as a terminal oxidant, such as the epoxidation of alkenes. However, the mechanism responsible for this reaction is still poorly understood, thus hindering the development of design rules to maximize the efficiency of catalytic oxidations in terms of product selectivity and oxidant utilization efficiency. Here, we thoroughly investigated the H2O2 decomposition mechanism using a Zr-monosubstituted dimeric Lindqvist tungstate, (Bu4N)6[{W5O18Zr(μ-OH)}2] ({ZrW5}2), which revealed high activity for this reaction in acetonitrile. The mechanism of the {ZrW5}2-catalyzed H2O2 degradation in the absence of an organic substrate was investigated using kinetic, spectroscopic, and computational tools. The reaction is first order in the Zr catalyst and shows saturation behavior with increasing H2O2 concentration. The apparent activation energy is 11.5 kcal·mol-1, which is significantly lower than the values previously found for Ti- and Nb-substituted Lindqvist tungstates (14.6 and 16.7 kcal·mol-1, respectively). EPR spectroscopic studies indicated the formation of superoxide radicals, while EPR with a specific singlet oxygen trap, 2,2,6,6-tetramethylpiperidone (4-oxo-TEMP), revealed the generation of 1O2. The interaction of test substrates, α-terpinene and tetramethylethylene, with H2O2 in the presence of {ZrW5}2 corroborated the formation of products typical of the oxidation processes that engage 1O2 (endoperoxide ascaridole and 2,3-dimethyl-3-butene-2-hydroperoxide, respectively). While radical scavengers tBuOH and p-benzoquinone produced no effect on the peroxide product yield, the addition of 4-oxo-TEMP significantly reduced it. After optimization of the reaction conditions, a 90% yield of ascaridole was attained. DFT calculations provided an atomistic description of the H2O2 decomposition mechanism by Zr-substituted Lindqvist tungstate catalysts. Calculations showed that the reaction proceeds through a Zr-trioxidane [Zr-η2-OO(OH)] key intermediate, whose formation is the rate-determining step. The Zr-substituted POM activates heterolytically a first H2O2 molecule to generate a Zr-peroxo species, which attacks nucleophilically to a second H2O2, causing its heterolytic O-O cleavage to yield the Zr-trioxidane complex. In agreement with spectroscopic and kinetic studies, the lowest-energy pathway involves dimeric Zr species and an inner-sphere mechanism. Still, we also found monomeric inner- and outer-sphere pathways that are close in energy and could coexist with the dimeric one. The highly reactive Zr-trioxidane intermediate can evolve heterolytically to release singlet oxygen and also decompose homolytically, producing superoxide as the predominant radical species. For H2O2 decomposition by Ti- and Nb-substituted POMs, we also propose the formation of the TM-trioxidane key intermediate, finding good agreement with the observed trends in apparent activation energies.

A variety of heterobinuclear Ni(I) complexes of the general formula [LNiI(μ-X1)(μ-X2)AlR2] formed upon the activation of the [LNiIIBr2] (1) ethylene polymerization catalyst (L = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with AlR3 (R = Me, Et, and iBu), AlR2Cl (R = Me, Et), and MMAO were detected and characterized in situ by EPR and 1H NMR spectroscopy. The nature of the bridging groups X1 and X2 (X1, X2 = R, Br, or Cl) depends on the nature and excess of the cocatalyst used. The catalyst systems displaying various [LNiI(μ-X1)(μ-X2)AlR2] species were compared in ethylene polymerization. Possible roles of Ni(I) species in the catalytic process are discussed.

A variety of heterobinuclear Ni(I) complexes of thegeneral formula [LNiI(mu-X1)(mu-X2)AlR2] formed upon the activationof the [LNiIIBr2](1) ethylene polymerization catalyst (L = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with AlR3(R =Me, Et, andiBu), AlR2Cl (R = Me, Et), and MMAO were detectedand characterized in situ by EPR and1H NMR spectroscopy. Thenature of the bridging groups X1and X2(X1,X2= R, Br, or Cl)depends on the nature and excess of the cocatalyst used. The catalystsystems displaying various [LNiI(mu-X1)(mu-X2)AlR2] species werecompared in ethylene polymerization. Possible roles of Ni(I) speciesin the catalytic process are discussed.

Oxacalix[6]arene vanadium complexes have been employed for the ROP of cyclic esters and ethylene polymerization.

Comparative data on the micro-structures and properties of branched polyethylenes (BPE) produced via ethylene homopolymerization over homogeneous N,N-alpha-diimine LNiBr2 complexes with different ligand composition (AlEt2Cl as a cocatalyst) and corresponding supported catalysts LNiBr2/SiO2(Al) (Al[iso-Bu](3) as a cocatalyst) are presented. Noticeable differences were observed between micro-structures of BPEs obtained using homo- and heterogeneous LNiBr2 complexes as catalysts. Supported catalysts produce BPEs with the majority of methyl branches (17-18 CH3(1000 C)(-1) characterized by different molecular masses (1800-210 kg mol(-1)) and molecular weight distributions (M-w[M-n](-)(1) = 5.9 and 2.6). Thermal and mechanical properties of these BPE samples obtained over supported Ni catalysts are similar to those of commercial LLDPE samples prepared with metallocene and Ziegler-Natta catalysts.

The nature of Ni(I) species formed upon the activation of the N,N-alpha-diimine Ni(II) ethylene polymerization precatalyst (LNiBr2)-Br-II (1) with AlMe3 and MMAO has been studied in detail by H-1, H-2 NMR, and electron paramagnetic resonance (EPR) spectroscopy (L = 1,4-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diazabuta-1,3-diene). It has been shown that neutral paramagnetic NMR and EPR active heterobinuclear complexes of types LNiI(mu-Me)(mu-Br)AlR2 and LNiI(mu-Me)(2)AlR2 (R = Me in the case of AlMe3 and R = Me or Bu-i in the case of MMAO) predominate in toluene solutions of 1/AlMe3 and 1/MMAO at room temperature. The effect of the cocatalyst/catalyst ratio on the relative concentrations of LNiI(mu-Me)(mu-Br)AlR2 and LNiI(mu-Me)(2)AlR2 has been studied and their role in ethylene polymerization is discussed.

The catalyst systems Fe2(PDPNR1R2)2/H2O2/RCOOH (PDP = N,N?-bis(pyridyl-2-methyl)-(S,S)-2,2?-bipyrrolidine, NR1R2 = NMe2, NEt2, NMeiPr or N(CH2)4 at the para-position in the pyridine rings, RCOOH = acetic or 2-ethylhexanoic acid) generate the high-spin (S = 3/2) oxoiron(V) oxidizing intermediates with proposed structure [(PDPNR1R2)FeV=O(OC(O)R)]2+ and the EPR spectrum g1, = 4.30, g2 = 3.69, g3 = 1.96. In contrast, the catalyst systems Fe2(PDPMe2OMe)2/H2O2/RCOOH and Fe2(PDPMeOCH2CF3)2/H2O2/RCOOH, based on similar iron complexes, containing 3,5-Me2-4-OMe and 3-Me-4-OCH2CF3 substituents at the pyridine rings, generate the low-spin (S = 1/2) oxoiron(V) intermediates with characteristic EPR spectrum g1 = 2.07, g2 = 2.01, g3 = 1.96. Both high and low-spin intermediates directly epoxidize cyclohexene, with the low-spin species being much more reactive than the high-spin congeners; the corresponding second-order rate constants have been evaluated. Crucially, the catalyst systems exhibiting the high-spin oxidizing intermediates demonstrate higher enantioselectivity in the epoxidation of prochiral olefinic substrates, compared with the catalyst systems featuring the low-spin intermediates. This enhanced enantioselectivity is explained in terms of attenuated reactivity of the high-spin intermediates, leading to more product-like transition state with tighter substrate-catalyst interactions.

Nickel(II) complexes with bidentate N,N-alpha-diimine ligands constitute a broad class of promising catalysts for the synthesis of branched polyethylenes via ethylene homopolymerization. Despite extensive studies devoted to the rational design of new Ni(II) alpha-diimines with desired catalytic properties, the polymerization mechanism has not been fully rationalized. In contrast to the well-characterized cationic Ni(II) active sites of ethylene polymerization and their precursors, the structure and role of Ni(I) species in the polymerization process continues to be a "black box ". This perspective discusses recent advances in the understanding of the nature and role of monovalent nickel complexes formed in Ni(II) alpha-diimine-based ethylene polymerization catalyst systems.

The nature of Ni(II) and Ni(I) species formed upon the activation of the N,N-alpha-diimine Ni(II) precatalyst (LNiBr2)-Br-II (1) with AlMe3, MMAO, and (AlBu3)-Bu-i has been studied by NMR and EPR spectroscopy (L = 1,4-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diazabuta-1,3-diene). It has been shown that the paramagnetic complex 3(AlMe3) with the proposed structure [LNiI(mu-Me)(2)AlMe2] or [(L-center dot(-))Ni-II(mu-Me)(2) AlMe2] predominates in the reaction solution of 1/AlMe3 at Al/Ni > 10. Upon activation of 1 with MMAO (Al/Ni > 20-30), nickel predominantly exists in the form of the structurally similar complex 3(MMAO) ([LNi1(mu-Me)(2)(AlBu2)-Bu-i] or [(L center dot-)Ni-II(mu-Me)(2)(AlBu2)-Bu-i]). The reaction of 1 with (AlBu3)-Bu-i leads to rapid ligand scrambling, affording the EPR-active complex 4 ([(L center dot-)(AlBu2)-Bu-i]). The latter process is accompanied by Ni reduction to the zerovalent state. Possible roles of the observed species in the catalytic systems are discussed.

α-Diimine and related complexes of late transition metals such as palladium and nickel have been attracting continuing interest as single-site catalysts of ethylene homopolymerization to branched polyolefins, having challenging mechanical properties. The state-of-the art catalysts demonstrate promising catalytic activities, and enhanced thermal stabilities, affording polyethylenes with a variable degree of branching and, in addition, are able to incorporate polar co-monomers into polyethylene structures. At the same time, fundamental understanding of the structure-reactivity relationships of such catalysts mostly remains at the phenomenological level, due to the lack of experimental data on the solution structures of intermediates that drive the polymerization process. In this perspective, we discuss recent advances of α-diimine nickel based catalysts of ethylene polymerization, focusing on the relationships between the catalyst structures on the one hand, and their thermal stabilities and properties of the resulting polyethylene, on the other hand. In addition, some intriguing novel mechanistic findings of these catalyst systems are presented.

>Introduction of polar comonomers into polyethylene is highly challenging, paving the way to tuning the copolymers’ mechanical properties, adhesion/compatibility, and biodegradability. Polar-group tolerant single-site palladium and, more tempting, nickel-based catalysts have demonstrated promising reactivities in the ethylene copolymerization with polar comonomers via coordination/insertion mechanism, providing tighter control of polymer micro-

The nature of nickel(II) species, formed upon the activation of alpha-diiminonickel pre-catalyst (LNNiBr2)-Br-II (L-N = 1,2-Bis[(2,4,6-trimethylphenyl)imino]acenaphthene) with Al2Et3Cl3 and Al2Me3Cl3, has been studied using H-1 and H-2 NMR spectroscopy. The paramagnetic heterobinuclear ion pairs of the type [LNNiII(mu-Cl)(2)AlRCl](+)[A](-) (where R = Et or Me, [A](-) = counter anion) have been observed in the catalyst systems (LNNiBr2)-Br-II/Al2Et3Cl3 and (LNNiBr2)-Br-II/Al2Me3Cl3. At room temperature, both ion pairs gradually transform into EPR-active complexes of Ni(I). Their possible role in ethylene polymerization is discussed. (c) 2019 Elsevier B.V. All rights reserved.

In this work, we report a series of novel nickel(II) dibromide and dichloride complexes with 2-iminopyridine and 2-iminoquinoline ligands bearing electron-withdrawing substituents (F, Cl, CF3), that have demonstrated high ethylene dimerization activity [up to 19.2 × 106 g of oligomers·(mol Ni)−1 h−1] in the presence of MAO or Et2AlCl, affording predominantly a mixture of 1-butene and cis- and trans-2-butene (C4 selectivity varies from 89 to 100%; 2-butenes selectivity in the C4 fraction approaches 96%). The effects of ligand substituents and the cocatalyst nature on the activity and selectivity of the nickel(II) complexes in ethylene dimerization have been established. Several nickel complexes were supported on silica-alumina and the resulting heterogeneous catalysts were probed towards ethylene dimerization. These catalysts also afford 2-butenes, with the activity and C4 selectivity being comparable to that of homogeneous catalysts.

The nature of Ni(ii) species formed upon the activation of the Brookhart's α-diimine polymerization pre-catalyst LNiBr2 with MAO and MMAO (L = 1,4-bis-2,4,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) has been established using 1H and 13C NMR spectroscopy. The heterobinuclear ion pair [LNiII(μ-Me)2AlMe2]+[MeMAO]- is observed at the initial stage of the reaction of LNiBr2 with MAO at -40 °C, whereas the ion pair [LNiII-tBu]+[MeMMAO]- predominates at the initial stage of the reaction of LNiBr2 with MMAO under the same conditions. At higher temperatures, both ion pairs transform into a Ni(i) species displaying an axially anisotropic EPR spectrum (g‖ = 2.21, g⊥ = 2.06, A⊥ = 1.06 mT).

Using EPR spectroscopy, nickel(I) species formed in the catalyst system 1/MMAO for ethylene polymerization has been investigated (1 = LNiBr2, L = 1,4-bis-2,4,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-dien). It has been shown that diamagnetic cationic complex [LNiII-Bu-t](+)[MeMMAO](-) (2) persists in system 1/MMAO at low temperatures, but rapidly decays at 25 degrees C to give new EPR active species 3 (major product) and 4 (minor product). Species 3 is a nickel(I) complex with the proposed structure [LNiI(S)](+)[MeMMAO](-) (S = solvent), whereas 4 is L(center dot-)AlMe2 species. Upon the addition of ethylene, complex 3 partially reversibly converts into the adduct [LNiI(C2H4)](+)[MeMMAO](-) (5). After ethylene consumption, species 3 restores its original concentration. (c) 2018 Elsevier B.V. All rights reserved.

Vanadium(III) species formed upon reacting alpha-diimine (1) and bis(imino) pyridine (5) vananadium(III) pre-catalysts with MAO, AlMe2Cl, AlMe2Cl/[Ph3C][B(C6F5)(4)], and AlMe3/[Ph3C][B(C6F5)(4)] have been characterized in detail by H-1, H-2, and F-19 NMR spectroscopy; the VIII-CH3 moiety has been observed by H-1 and H-2 NMR spectroscopy. For complex 1, zwitterion-like species [(LVR2III)-R-III MeMAO(-)] and ion pairs [(LVR2+)-R-III(THF)(2)](+) [A]-(L' = 1,4-bis-3,5-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene; [A] = [MeMAO](-) or [B(C6F5)(4)](-)) have been identified. The outer-sphere ion pairs of the type [L(Cl) V-III(mCl)(2)AlMe2] + [A](-) [L(Me)V-III(m-Cl)(2)AlMe2](+)[A](-), [(LVCl2)-Cl-III(THF)](+)[A](-) and [LVIII(Cl) Me(THF)](+)[A](-) (L = 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridine; [A](-) = [AlMe3Cl](-) or [B(C6F5)(4)](-)) have been found in the systems based on the complex 5. The nature of the vanadium species active in ethylene polymerization and the catalyst deactivation pathways are discussed. (c) 2017 Published by Elsevier B. V.

The ion pairs formed upon the activation of bis(imino)pyridine complex LVCl3 (L=2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridine) with AlMe2Cl and AlMe3/[Ph3C][B(C6F5)(4)] were characterized in detail by H-1 and (HNMR)-H-2 spectroscopy, including the first NMR observation of the V-III-CH3 group. The following ion pairs were identified: [L(Cl)V-III(-Cl)(2)AlMe2](+)[A](-), [L(Me)V-III(-Cl)(2)AlMe2](+)[A](-), [(LVCl2)-Cl-III(THF)](+)[A](-), and [LVIII(Cl)Me(THF)](+)[A](-) ([A](-)=[AlMe3Cl](-) or [B(C6F5)(4)](-), THF=tetrahydrofuran). The nature of the ethylene polymerization active species is discussed.