Chromium Catalysts Research Articles

Ring opening polymerization and copolymerization for polyester and polycarbonate formation by a diamino-bis(phenolate) chromium(iii) catalyst

Chromium catalyst performs controlled polymerization, copolymerization and terpolymerization of different monomers.

Highly efficient asymmetric (diphenylphosphino)benzene‐based chromium catalysts for selective ethylene trimerization

The corresponding Cr(III) catalysts (1–6) based on asymmetric (diphenylphosphino)benzene‐type L1–L6 ligands have been explored for ethylene selective trimerization. It is observed that the structure of the complexes has an obvious influence on their catalytic performance for ethylene selective oligomerization. All the 1–6 precatalysts may offer selective ethylene trimerization systems, and their catalytic performance is greatly influenced by the reaction conditions. The precatalyst 2, bearing diisopropylphosphanyl substituent, may exhibit the remarkably high catalytic activity of 9.83 × 107 g/(molCr·h) with 90.4% C6 selectivity under suitable conditions. Single crystal X‐ray analysis showed that the alkyl/alkylaminophosphanyl of the ligands may efficaciously tune the catalyst structure and thus influence the oligomerization activity and product selectivity.

Chromium Catalysts Based on Unsymmetrical PNP Ligands for Selective Ethylene Tri-/Tetramerization: Effect of Electron-Withdrawing/Donating Substituents on Catalytic Performance

The in situ formation and activation of Cr(III) catalysts based on unsymmetrical PNP ligands yield efficient catalytic systems for selective ethylene tri-/tetramerization. The electronic nature (electron-withdrawing or electron-donating) and position (para or meta) of the substituents over the phenyl rings of the PNP, the nature of cocatalyst (DMAO/AlEt3 and MMAO-3A), and reaction conditions have been observed to have a marked impact on catalytic performance, particularly catalytic activity. Ligand L2, bearing 4-(trifluoromethyl)phenyl substituents, yielded 33.6 kg(product).g(Cr)−1·h−1 catalytic activity with 57.7% C8 selectivity under optimal conditions. Ligand L4, having para-tolyl substituents, yielded 43.3 kg(product).g(Cr)−1·h−1 with 59.0% C8 selectivity under optimum conditions. Changing the positions of both the electron-withdrawing and electron-donating substituents from para to meta over the phenyls of the PNP may lead to both catalytic systems exhibiting poor performance.

Oligo- and polymerization of ethylene by pyrrolide-imine chromium catalysts bearing pendant O-, S- and N-donor groups. Synthesis, characterization and DFT studies

The synthesis and ethylene reactivity of a new set of chromium(III) complexes supported by pyrrolide-imine ligands bearing pendant O-, S- and N-donor groups [Cr{(C4H3N-2-CH=N)Z}(THF)Cl2] (Z = CH2Ph-2-OMe, Cr1; Z = Ph-2-OPh, Cr2; Z = Ph-2-OMe, Cr3; Z = Ph-2-SPh, Cr4; Z = Ph-2-SMe, Cr5; Z = CH2Py, Cr6; Z = C2H4Py, Cr7) are described. Activation of Cr1-Cr7 with MAO generates active systems affording a full range oligomerization of ethylene (C4-C12+) with turnover frequencies (TOFs) in the range of 13,900–94,200 mol(ethylene)·mol(Cr)–1·h–1 along with production of a varied amount of polymer (6.9 - 91.5 wt.%). The nature of the pendant group has a significant impact on the catalytic performances of these Cr complexes. Particularly, catalytic precursors containing pendant O-donor groups (Cr1-Cr3) produce mostly oligomers (up to 89.0 wt%) while those bearing sulfur-base donor groups (Cr4 and Cr5) are primarily polymerization systems. DFT calculations suggested the presence of two different CrIII catalytic species, the mononuclear species responsible for production of polymer and one dinuclear species that have been ascribed for production of oligomers. By tunning the reactional conditions, higher TOF and higher selectivities for production of α-olefins were achieved. Notably, the use of high MAO loading (1500 equiv.) enhanced the activity of Cr2 [TOF = 389,800 (mol ethylene)⋅(mol Cr)−1⋅h−1] producing mostly oligomers (97.7 wt% of total products) with high selectivity for α-olefins (>95.5 wt.%).

Hydrostabilization of Straight-Run Naphtha Pyrocondensate in the Presence of a Nickel–Chromium Catalyst

Hydrostabilization of Straight-Run Naphtha Pyrocondensate in the Presence of a Nickel–Chromium Catalyst

Cover Feature: [OSSO]‐Type Chromium(III) Complexes for the Reaction of CO2 with Epoxides (ChemPlusChem 8/2022)

The cover feature image shows a depiction of cars (representing different substrates) being directed in different paths at a junction and illustrates how the OSSO Chromium catalyst can produce cyclic carbonates or polycarbonate depending on the type of substrate and on the reaction conditions. More information can be found in the Research Article by C. Capacchione and co-workers.

Highly Efficient Ethylene Tetramerization Using Cr Catalysts Constructed with Trifluoromethyl-Substituted N-Aryl PNP Ligands.

Tetramerization of ethylene by chromium catalysts stabilized with functionalized N-aryl phosphineamine ligands C6H4(m-CF3)N(PPh2)2 (1), C6H4(p-CF3)N(PPh2)2 (2), C6H4(o-CF3)N=PPh2-PPh2 (3), and C6H3(3,5-bis(CF3))N(PPh2)2 (4) was evaluated. The parameter optimization includes temperature, co-catalyst, and solvent. Upon activation with MMAO-3A, the new catalyst system especially with m-functional PNP ligand (1) exhibited high 1-octene selectivity and productivity while giving minimum undesirable polyethylene and C10+ olefin by-products. Using PhCl as a solvent at 75 °C led to a remarkable α-olefin (1-C6 + 1-C8) selectivity (>90 wt %) at a reaction rate of 2000 kg·gCr–1·h–1. Under identical conditions, analogous PNP ligands bearing −CH3, −Et, and −Cl functional moieties at the meta position of the N-phenyl ring displayed significantly lower reactivity. The catalyst with p-functional ligand (2) exhibited lower activity and comparable selectivities, while the Cr/PPN (with ligand 3) system gave no noticeable reactivity. The molecular structure of the precatalyst (1-Cr), exhibiting a monomeric structural feature, was elucidated with the aid of single-crystal X-ray diffraction study.

Isomerization of Aziridines to Allyl Amines via Titanium and Chromium Cooperative Catalysis.

A Ti/Cr cooperative catalyst isomerizes aziridines to allyl amines under mild conditions. The reaction tolerates a broad range of aziridines with various nitrogen substituents. The titanium catalyst is most successful in opening 1,2-disubstituted aziridines, forming radical intermediates in a highly regioselective manner. The chromium catalyst appears to abstract an H• from these radical intermediates and then return the H• to the titanium system in the form of an H+ and an electron. The reaction is complementary to previous reports on the isomerization of aziridines to allyl amines.

Bis(pyrazolyl)thioether/amine‐chromium(III) catalysts bearing pendantO‐ andN‐donor group for oligomerization and polymerization of ethylene

In this work, a new family of chromium complexes supported by bis(pyrazolyl)thioether/amine ligands containing pendantO‐ andN‐donor groups CrCl3{S‐bis[(3,5‐DMPz)methyl]sulfide} (2a), CrCl3{N‐bis[(3,5‐DMPz)benzylamine]} (2b), CrCl3{N‐bis[(3,5‐DMPz)butylamine]} (2c), [CrCl2{N‐bis[(3,5‐DMPz)methyl][(2‐pyridinyl)methyl]amine}]Cl (2d), [CrCl2{N‐bis[(3,5‐DMPz)methyl][quinoline]amine}]Cl (2e), [CrCl2{N‐bis[3,5‐DMPz‐methyl][EtNMe2]amine}]Cl (2f), CrCl3{N‐bis[(3,5‐DMPz)methyl][2‐methoxyphenyl]amine} (2g), and CrCl3{N‐bis[2‐(3,5‐DMPz)methyl][(2‐pyridyl)ethyl]amine} (2h) were synthetized and characterized by elemental analysis and Fourier transform infrared spectroscopy (FTIR) spectroscopy. Density functional theory (DFT) calculations demonstrated that the pendant group as well as the spacer between theN‐donor group and the central amine nitrogen have strong influence on the coordination motif of the ligand. Thus,1d–1fact as tetradentate ligands while1gand1hpreferably coordinated to the chromium metal center in a tridentate [N,N,N] mode fashion. The catalytic performance of2a–2his strongly influenced by the nature of the central bridging atom (S, N) as well as the pendantO‐ andN‐donor group. Upon activation with methylaluminoxane (MAO), chromium precatalysts2a–2c,2e, and2fgenerated active systems producing oligomers ranging from C4to C12+with a high selectivity for α‐olefins (>82.4%). On the other hand, precatalysts bearing pyridine (2dand2h) and methoxy (2g) functionalized amine bis (pyrazolyl) ligands exclusively produce polyethylene (PE) with activities in the range of 107.2–372.0 kg of PE·mol[Cr]−1 h−1.

Photoactive Metal Carbonyl Complexes Bearing N-Heterocyclic Carbene Ligands: Synthesis, Characterization, and Viability as Photoredox Catalysts.

This report details the synthesis and characterization of a small family of previously unreported, structurally related chromium, molybdenum, tungsten, manganese, and iron complexes bearing N-heterocyclic carbene and carbonyl supporting ligands. These complexes have the general form [ML(CO)3X] or [ML(CO)3], where X = CO or Br and L = 1-phenyl-3-(2-pyridyl)imidazolin-2-ylidene. Where possible, the solid-state, spectroscopic, electrochemical, and photophysical properties of these molecules were studied using a combination of experiment and theory. Photophysical studies reveal that decarbonylation occurs when these complexes are exposed to ultraviolet light, with the CO ligand being replaced with a labile acetonitrile solvent molecule. To obtain insights into the potential utility, scope, and applications of these complexes in visible-light-mediated photoredox catalysis, their capacity to facilitate a range of photoinduced reactions via the reductive or oxidative functionalization of organic molecules was investigated. These chromium, molybdenum, and manganese catalysts efficiently facilitated atom-transfer radical addition processes. In light of their photolability, these types of catalysts may potentially allow for the development of photoinduced reactions involving less conventional inner-sphere electron-transfer pathways.

Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation

Chromium catalysts with diazaphospholane ligands have shown good activities and selectivities for ethylene tri- and tetramerisation. Oligomerisations with a doubly N-bridged cyclodiphosphazane result in a Schulz–Flory distribution of α-olefins.

Polyoxomolybdate-Based Chromium(III) Catalyst for the Synthesis of Disulfides from Boronic Acids

Polyoxomolybdate-Based Chromium(III) Catalyst for the Synthesis of Disulfides from Boronic Acids

Application of polymer-coated Macadamiaintegrifolia nutshell biomass impregnated with palladium for chromium(VI) remediation

Freely suspended and porous basket restrained granules of palladium nanoparticles supported on polymer-grafted Macadamia nutshell biomass (Pd@Polym-MNS) composite were used for the treatment chromium(VI)-containing water. In the presence of formic acid, the Pd@Polym-MNS demonstrated its activity in the adsorption-reduction-based conversion of noxious chromium(VI) to less toxic chromium(III) with a low activation energy of 13.4 kJ mol–1, ΔH0 (+ 10.8 kJ mol–1), ΔS0 (−270.0 J mol–1 K–1), and ΔG0 (+ 91.3 to + 98.0 kJ mol–1) indicated the exothermic, endergonic and non-spontaneous nature of the catalytic redox reaction. In addition to facilitating easy recovery, rinsing, and reuse, restraining the Pd@Polym-MNS in the basket reactor helped maintain the integrity of the catalysts by preventing violent collisions of suspended granules with the mixing apparatus and the walls of the reaction vessel. Whereas the pseudo-first-order rate constant was recorded as 0.157 min–1 upon initial use, values of the mean and relative standard deviation for the second, third and fourth consecutive uses were found to be 0.219 min–1 and 1.3%, respectively. According to a response surface methodological approach to batch experimentation, the initial concentration of chromium(VI) and catalyst dosage had the greatest impact on the redox reaction rate, accounting for 85.7% and 11.6% of the variability in the value of the pseudo-first-order rate constant, respectively. Mutually beneficial effects of the combinations of high formic acid and low chromium(VI) concentration, high temperature and catalyst dosage as well as high formic acid and catalyst dosage were recorded.

Chromium catalysts supported on carbon nanotubes and graphene nanoflakes for CO2-assisted oxidative dehydrogenation of propane

CO2-assisted oxidative dehydrogenation of propane (ODP) is a promising approach for addressing two great modern challenges: effective CO2 utilization and propylene synthesis. In this work different carbon materials were for the first time used as supports for Cr-based catalysts of CO2-assisted ODP. Carbon nanotubes (CNTs), jellyfish-like graphene nanoflakes (GNFs), and their oxidized and N-doped derivatives were tested and compared with a commercial activated carbon. The best activities and propylene yield up to 25% were observed for the oxidized CNT- and pristine GNF-supported catalysts. The stability of these two catalysts against destruction and particle sintering during tests was confirmed by Raman spectroscopy. High activity and stability of the oxidized CNT- and pristine GNF-supported catalysts was explained by their macro- and mesoporosity that enhance diffusion of reagents and products and by the highest surface graphitization degree confirmed by XPS. These catalysts showed much better performance than the activated carbon-supported catalyst. Therefore, GNFs and CNTs can be considered as effective supports of the CO2-ODP catalysts.

Activation of Chromium Catalysts by Photoexcited Hantzsch Ester for Decarboxylative Allylation of Aldehydes with Butadiene.

Metallaphotocatalysis often needs light-absorbing metal-polypyridyl complexes, semiconductors, or organic dyes, which can modify the oxidation state of metal catalysts. Here, we first report that photoexcitation of Hantzsch ester can directly activate chromium reagents through a single-electron transfer process. The synthetic application was demonstrated through a photoredox decarboxylative allylation of aldehydes with feedstock butadiene without exogenous photocatalysts, metallic reductants, or additives.

Chromium catalysts stabilized by alkylphosphanyl PNP ligands for selective ethylene tri-/tetramerization

The alkyl substituent(s) in the scaffold of the alkylphosphanyl PNP ligands of the form Ph2PN(cyclopentyl)PPhR, Ph2PN(cyclopentyl)PR2, and RPhPN(cyclopentyl)PPhR have been observed to have a marked impact on the catalytic performance of Cr(III) catalysts in ethylene oligomerization reactions. Precatalyst 6, bearing diisopropylphosphanyl substituent, afforded the highest catalytic activity of 4490 kg(product)·g(Cr)−1·h−1 and 51.0% C8 selectivity with good product purity under suitable conditions. Precatalyst 7, having diethylphosphanyl substituent in the PNP scaffold, exhibited 72.0% high C8 selectivity with 1717 kg(product)·g(Cr)−1·h−1 improved catalytic activity. Precatalysts based on monoalkylphosphanyl PNP (2–4) and symmetrical alkylphosphanyl PNP (9–10) exhibited poor catalytic performance toward selective ethylene oligomerization. Single crystal analysis suggested that the alkyl substituents in the scaffold of the alkylphosphanyl PNP ligands may effectively induce the complex structure and consequently the catalytic performance. Density functional theory (DFT) calculations revealed that in comparison to the trimerization pathway, the rate-determining states in the tetramerization pathways of precatalysts 6–7 may face low energy barriers and therefore may offer high C8 selectivity.

Chromium catalysts based on PNP(NR2)2 ligands for selective ethylene oligomerization

High active and selective 1–3 chromium catalysts based on the corresponding Ph2PN(cyclopentyl)P (NR2)2‐type L1–L3 ligands (L1: R = methyl, L2: R = ethyl, L3: R = isopropyl) have been explored for selective ethylene tri‐/tetramerization. We found that the ligand substituents and the experimental conditions considerably influenced the catalytic performance of 1–3 based catalysts. Combining a higher ethylene pressure can elevate the selectivity of 1‐octene (67.98%) of precatalyst 1 with high catalytic activity (9.3 × 106 g/[molCr h]). Furthermore, precatalysts 2–3 efficiently offer selective ethylene trimerization and afforded 97.76% 1‐hexene selectivity with 5.8 × 106 g/(molCr h) catalytic activity (in case precatalyst 3). A relatively low catalyst mass, high Al/Cr molar ratio, and high ethylene pressure display excellent catalytic activity and selectivity for ethylene oligomerization.

Oxidative Esterification of Alcohols with a Polyoxometalate-Containing Chromium Catalyst

Oxidative Esterification of Alcohols with a Polyoxometalate-Containing Chromium Catalyst

Comparison of Support Effects on Phillips and Metallocene Catalysts

Both metallocene and Phillips chromium catalysts are used in the commercial manufacture of polyethylene. Unlike most other commercial metallocene systems, the Chevron Phillips Chemical (CPC) platform does not use methylaluminoxane or fluoroorganic boranes. Instead, the support itself serves to activate (ionize) the metallocenes, which then polymerize ethylene at high activity. Most of these solid acid supports can also be used to anchor Cr to make a Phillips catalyst. This provides an interesting opportunity to compare the polymerization responses by these two disparate systems, Phillips Cr and CPC metallocene, when supported on the same solid acid carriers. In this study, both chromium oxide and several metallocenes were deposited onto a variety of solid oxides, under a variety of conditions, and the resulting support effects were observed and compared. Although using seemingly different chemistries, the two catalyst systems exhibited a surprising number of similarities, which can be attributed to the acidity and porosity of these diverse supports.

Cover Feature: The Stronger the Better: Donor Substituents Push Catalytic Activity of Molecular Chromium Olefin Polymerization Catalysts (Chem. Eur. J. 43/2021)

Chromium based single-site catalysts allow the production of ultra-high molecular weight polyethylene with very high productivities. The picture shows a donor-functionalised indenyl chromium catalyst in action where a strong electron-donating substituent leads to an increase in catalytic activity and co-monomer incorporation. The correlation between donor strength, electronic and catalytic properties was studied systematically. More information can be found in the Full Paper by M. Enders et al. (DOI: 10.1002/chem.202101586).