DFT insight into metals and ligands substitution effects on reactivity of phenoxy-imine catalysts for ethylene polymerization.

The reaction mechanism of ethylene (ET) polymerization catalyzed by the phenoxy-imine (FI) ligands using DFT calculations was studied. Among five possible isomers, isomer A which has an octahedral geometry and a (cis-N/trans-O/cis-Cl) arrangement is the most stable pre-reaction Ti-FI dichloride complex. The isomer A can be activated by MAO to form the active catalyst and the active form was used for the study of the mechanism for Ti-FI. The second ethylene insertion was found to be the rate-determining step of the catalyzed ethylene polymerization. To examine the effect of group IVB transition metals (M = Ti, Zr, Hf) substitutions, calculated activation energies at the rate-determining step (EaRDS) were compared, where values of EaRDS of Zr < Hf < Ti agree with experiments. Moreover, we examined the effect of substitution on (O, X) ligands of the Ti-phenoxy-imine (Ti-1) based catalyst. The results revealed that EaRDS of (O, N) > (O, O) > (O, P) > O, S). Hence, the (O, S) ligand has the highest potential to improve the catalytic activity of the Ti-FI catalyst. We also found the activation energy to be related to the Ti-X distance. In addition, a novel Ni-based FI catalyst was investigated. The results indicated that the nickel (II) complex based on the phenoxy-imine (O, N) ligand in the square-planar geometry is more active than in the octahedral geometry. This work provides fundamental insights into the reaction mechanism of M - FI catalysts which can be used for the design and development of M - FI catalysts for ET polymerization.

Highly Enantioselective Radical Cation [2 + 2] and [4 + 2] Cycloadditions by Chiral Iron(III) Photoredox Catalysis.

Radical cations show a unique reactivity that is fundamentally different from that of conventional cations and have thus attracted considerable attention as alternative cationic intermediates for novel types of organic reactions. However, asymmetric catalysis to promote enantioselective radical cation reactions remains a major challenge in contemporary organic synthesis. Here, we report that the judicious design of an ion pair consisting of a radical cation and a chiral counteranion induces an excellent level of enantioselectivity. This strategy was applied to enantio-, diastereo-, and regioselective [2 + 2] cycloadditions, as well as enantio-, diastereo-, and regioselective [4 + 2] cycloadditions, by using chiral iron(III) photoredox catalysis. We anticipate that this strategy has the potential to expand the use of several mature chiral anions to develop numerous unprecedented enantioselective radical cation reactions.

Asymmetric Dehydrative Cyclization of Allyl Alcohol to Cyclic Ether Using Chiral Brønsted Acid/CpRu(II) Hybrid Catalysts: A DFT Study of the Origin of Enantioselectivity.

To elucidate the reaction mechanism and the origin of the enantioselectivity of the asymmetric dehydrative cyclization of allyl alcohol to cyclic ether catalyzed by a Cp-ruthenium complex and a chiral pyridinecarboxylic acid, (R)-X-Naph-PyCOOH, density functional theory (DFT) calculations were performed. According to the DFT calculations, the rate-determining step is the dehydrative σ-allyl formation step with ΔG‡ = 18.1 kcal mol-1 at 80 °C. This agrees well with the experimental data (ΔG‡ = 19.01 kcal mol-1 at 80 °C). The DFT result showed that both hydrogen and halogen bonds play a key role in the high enantioselectivity by facilitating the major R,SRu-catalyzed reaction pathway via a σ-allyl Ru intermediate to generate the major (S)-product. In contrast, the reaction is sluggish in the presence of the diastereomeric R,RRu catalyst with an apparent activation energy of 33.1 kcal mol-1; the minor (R)-product is formed via a typical π-allyl Ru intermediate and via a minor pathway for the cyclization step. In addition, the calculated activation Gibbs free energies, 14.4 kcal mol-1 for I < 16.8 kcal mol-1 for Br < 18.1 kcal mol-1 for Cl, reproduced the observed halogen-dependent reactivity with the (R)-X-Naph-PyCOOH ligands. The origin of the halogen trend was clarified by a structural decomposition analysis.

Iron/Photosensitizer Hybrid System Enables the Synthesis of Polyaryl-Substituted Azafluoranthenes.

Photosensitization of organometallics is a privileged strategy that enables challenging transformations in transition-metal catalysis. However, the usefulness of such photocatalyst-induced energy transfer has remained opaque in iron-catalyzed reactions despite the intriguing prospects of iron catalysis in synthetic chemistry. Herein, we demonstrate the use of iron/photosensitizer-cocatalyzed cycloaddition to synthesize polyarylpyridines and azafluoranthenes, which have been scarcely accessible using the established iron-catalyzed protocols. Mechanistic studies indicate that triplet energy transfer from the photocatalyst to a ferracyclic intermediate facilitates the thermally demanding nitrile insertion and accounts for the distinct reactivity of the hybrid system. This study thus provides the first demonstration of the role of photosensitization in overcoming the limitations of iron catalysis.

Self-Assembled Multilayer Iron(0) Nanoparticle Catalyst for Ligand-Free Carbon-Carbon/Carbon-Nitrogen Bond-Forming Reactions.

Self-assembled multilayer iron(0) nanoparticles (NPs, 6-10 nm), namely, sulfur-modified Au-supported Fe(0) [SAFe(0)], were developed for ligand-free one-pot carbon-carbon/carbon-nitrogen bond-forming reactions. SAFe(0) was successfully prepared using a well-established metal-nanoparticle catalyst preparative protocol by simultaneous in situ metal NP and nanospace organization (PSSO) with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (Si-DHP) as a strong reducing agent. SAFe(0) was easy to handle in air and could be recycled with a low iron-leaching rate in reaction cycles.

Aluminum porphyrins with quaternary ammonium halides as catalysts for copolymerization of cyclohexene oxide and CO2: metal-ligand cooperative catalysis.

Bifunctional AlIII porphyrins with quaternary ammonium halides, 2-Cl and 2-Br, worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2 at 120 °C. Turnover frequency (TOF) and turnover number (TON) reached 10 000 h-1 and 55 000, respectively, and poly(cyclohexene carbonate) (PCHC) with molecular weight of up to 281 000 was obtained with a catalyst loading of 0.001 mol%. In contrast, bifunctional MgII and ZnII counterparts, 3-Cl and 4-Cl, as well as a binary catalyst system, 1-Cl with bis(triphenylphosphine)iminium chloride (PPNCl), showed poor catalytic performances. Kinetic studies revealed that the reaction rate was first-order in [CHO] and [2-Br] and zero-order in [CO2], and the activation parameters were determined: ΔH ‡ = 12.4 kcal mol-1, ΔS ‡ = -26.1 cal mol-1 K-1, and ΔG ‡ = 21.6 kcal mol-1 at 80 °C. Comparative DFT calculations on two model catalysts, AlIII complex 2' and MgII complex 3', allowed us to extract key factors in the catalytic behavior of the bifunctional AlIII catalyst. The high polymerization activity and carbonate-linkage selectivity originate from the cooperative actions of the metal center and the quaternary ammonium cation, both of which facilitate the epoxide-ring opening by the carbonate anion to form the carbonate linkage in the key transition state such as TS3b (ΔH ‡ = 13.3 kcal mol-1, ΔS ‡ = -3.1 cal mol-1 K-1, and ΔG ‡ = 14.4 kcal mol-1 at 80 °C).

Zeolite-supported ultra-small nickel as catalyst for selective oxidation of methane to syngas.

The development of simple catalysts with high performance in the selective oxidation of methane to syngas at low temperature has attracted much attention. Here we report a nickel-based solid catalyst for the oxidation of methane, synthesised by a facile impregnation method. Highly dispersed ultra-small NiO particles of 1.6 nm in size are successfully formed on the MOR-type zeolite. The zeolite-supported nickel catalyst gives continuously 97-98% methane conversion, 91-92% of CO yield with a H2/CO ratio of 2.0, and high durability without serious carbon deposition onto the catalyst at 973 K. DFT calculations demonstrate the effect of NiO particle size on the C-H dissociation process of CH4. A decrease in the NiO particle size enhances the production of oxygen originating from the NiO nanoparticles, which contributes to the oxidation of methane under a reductive environment, effectively producing syngas.

l-Cysteine-Modified Acacia Gum as a Multifunctional Binder for Lithium-Sulfur Batteries.

A binder plays an important role in stabilizing the electrode structure and improving the cyclic stability of batteries. However, the traditional binders are no longer satisfactory in lithium-sulfur (Li-S) batteries because of their failure in accommodating the large volume changes of sulfur and trapping soluble intermediate polysulfides, thus causing severe capacity decay. In this work, we prepared a multifunctional binder for Li-S batteries by merely modifying the acacia gum (AG), a low-cost biomass polymer, with l-cysteine under mild conditions. Owing to the introduced amino and carboxyl branches by the l-cysteine, the modified AG shows enhanced polysulfide trapping ability and can effectively restrain the shuttling of polysulfides. In addition, the introduction of branches can help form a cross-linked 3D network with better mechanical strength and flexibility for adhering sulfur and accommodating the volume changes of cathode materials. As a result, compared with the normally used polyvinylidene fluoride binder and the unmodified AG binder, the l-cysteine-modified AG binder effectively enhanced the rate capability and cycling stability of the Li-S batteries directly using sulfur as the cathode, showing a promising way to prompt the practical use of Li-S batteries.

Catalytic Cyclopropanation by Myoglobin Reconstituted with Iron Porphycene: Acceleration of Catalysis due to Rapid Formation of the Carbene Species.

Myoglobin reconstituted with iron porphycene catalyzes the cyclopropanation of styrene with ethyl diazoacetate. Compared to native myoglobin, the reconstituted protein significantly accelerates the catalytic reaction and the kcat/Km value is 26-fold enhanced. Mechanistic studies indicate that the reaction of the reconstituted protein with ethyl diazoacetate is 615-fold faster than that of native myoglobin. The metallocarbene species reacts with styrene with the apparent second-order kinetic constant of 28 mM-1 s-1 at 25 °C. Complementary theoretical studies support efficient carbene formation by the reconstituted protein that results from the strong ligand field of the porphycene and fewer intersystem crossing steps relative to the native protein. From these findings, the substitution of the cofactor with an appropriate metal complex serves as an effective way to generate a new biocatalyst.