A Pyridine Amine Extractant for Separation of La3+ and Ce3+ in HNO3 Solution

The separation of lanthanum and cerium is of great significance for improving the material properties and promoting scientific and technological progress. In this paper, a pyridine amine extractant, (1R,2S)-N1,N2-bis((5-methylpyridin-2-yl)methyl)cyclohexane-1,2-biamine (cis-BMPyMChBA), was designed for the separation of lanthanum and cerium. The aim is to use this extractant to catalyze the oxidation of tri- to tetravalent cerium and to increase the separation efficiency between lanthanum and cerium. The effects of the temperature, diluent, extractant concentration, and pH were evaluated to the extraction and separation performance of lanthanum and cerium. The experimental results demonstrate that cis-BMPyMChBA exhibits high selectivity for cerium, with a separation factor for lanthanum and cerium reaching 122.9. The coordination mechanism between cis-BMPyMChBA and cerium ion was clarified through the combined use of extraction isotherm, Job's curve, spectroscopic methods, and density functional theory (DFT) calculation. It involved the interaction between the secondary amine N atom and the pyridine N atom and the metal ion in which the cerium ion underwent a valence change from trivalent to tetravalent before extraction. This was found to be more conducive to the separation of lanthanum and cerium.

Achieving isopropenyl-enriched polyisoprene: unraveling the role of bidentate vs. tridentate iron precatalysts through ligand framework design

The coordination sphere and steric variations in iron catalysts present a fascinating strategy for adjusting monomer regio- and stereoselective enchainment, leading to the development of novel polymer structures in isoprene polymerization. This study investigates a range of iron complexes with variations in the coordination spheres (bidentate and tridentate) and steric/electronic properties of side arms to evaluate their impact on isoprene polymerization. X-ray analysis revealed that the tridentate Fe-NMe2 complex has a dinuclear structure with a μ2-O bridge, where each iron center is monoligated in an octahedral geometry. In contrast, the bidentate Fe-NHPh crystallizes as an ion pair, consisting of a trisligated [L3Fe]2+ cation and counteranions [FeCl4]- and [Cl]-. Toward isoprene polymerization, iron complexes with tridentate ligands bearing NMe2 or NiPr2 side arms exhibited high catalytic activity, whereas those with NH2 or OH side arms showed significantly lower activity. In addition, both bidentate iron complexes were also highly active precatalysts, with activities reaching up to 1.8 × 105 g (IP) per mol (Fe) per h. Variations in the ligand framework led to significant differences in polymer molecular weights, ranging from 37.5 × 103 g mol-1 to 272.8 × 103 g mol-1, with narrow to broad dispersities. Of significant note, tridentate iron complexes were particularly effective for 3,4 monomer addition (up to 63%), resulting in isopropenyl-enriched polyisoprene. In contrast, bidentate complexes showed a nearly equal preference for both 1,4 and 3,4 additions (1,4/3,4 ≈ 50/50). Polyisoprenes with a high number of isopropenyl pendant groups are highly sought after for post-functionalization and the production of high-performance resins.

Enhancing isoprene polymerization with high activity and adjustable monomer enchainment using cyclooctyl-fused iminopyridine iron precatalysts

In this study, a series of structurally rigid cyclooctyl-fused iminopyridine iron complexes, [L2FeCl][FeCl4] and [2L3Fe][Cl][3FeCl4], was synthesized via a one-pot method and investigated as precatalysts in conjunction with methylaluminoxane for isoprene (Ip) polymerization. Combined characterization through FTIR analysis, elemental analysis and single crystal XRD analysis fully verified the structure of these complexes. The most active iron complex, FeH, exhibited a trisligated nature, with its cation adopting an octahedral geometry around the metal center. In contrast, all the other iron complexes (Fe2Me, Fe2Et, Fe2iPr, Fe3Me, Fe2Et,Me) displayed bisligated configurations, with distorted trigonal bipyramidal geometry of cations. During isoprene polymerization, the extent of steric hindrance of the ligand framework exerted a significant impact on catalytic performance. The FeH precatalyst with less steric hindrance demonstrated excellent performance, producing high molecular weight polyisoprenes with conversions exceeding 99% for 4000 equiv. of monomer. Even at very low catalyst loadings, as low as 0.0025 mol% (Fe/Ip), the polymerization of isoprene could proceed smoothly with an exceptionally high activity of 4.0 × 106 gPI (molFe, h)-1. Moreover, this precatalyst exhibited good thermal stability, maintaining high activity levels (typically 105 gPI (molFe, h)-1) across a broad temperature range from -20 °C to 100 °C. Additionally, by adjusting steric substituents and the reaction temperature, the 1,4/3,4 regioselectivity could be modulated from 9/91 to 69/31 while maintaining a high stereoselectivity of cis-1,4 structures (cis/trans: >99/1).

Catalytic Behavior of Cobalt Complexes Bearing Pyridine-Oxime Ligands in Isoprene Polymerization

Several cobalt(II) complexes Co1-Co3 bearing pyridine-oxime ligands (L1 = pyridine-2-aldoxime for Co1; L2 = 6-methylpyridine-2-aldoxime for Co2; L3 = phenyl-2-pyridylketoxime for Co3) and picolinaldehyde O-methyl oxime (L4)-supported Co4 were synthesized and well characterized by FT-IR, mass spectrum and elemental analysis. The single-crystal X-ray diffraction of complex Co2 reveals that the cobalt center of CoCl2 is coordinated with two 6-methylpyridine-2-aldoxime ligands binding with Npyridine and Noxime atoms, which feature a distorted octahedral structure. These Co complexes Co1-Co4 displayed extremely high activity toward isoprene polymerization upon activation with small amount of AlClEt2 in toluene, giving polyisoprene with high activity up to 16.3 × 105 (mol of Co)-1(h)-1. And, the generated polyisoprene displayed high molecular weights and narrow molecular distribution with a cis-1,4-enriched selectivity. The type of cobalt complexes, cocatalyst and reaction temperature all have effects on the polymerization activity but not on the microstructure of polymer.

Ultrahigh activity and broad temperature resistance of amine-imine cobalt precatalysts for butadiene polymerization

A series of amine-imine cobalt complexes (Co1-Co7) has been prepared and characterized. The complexes Co3, Co4, and Co6 have a distorted tetrahedral geometry, as determined by single crystal X-ray diffraction. In the presence of ethylaluminum sesquichloride (EASC), Co3 exhibited ultra-high activity toward butadiene (Bd) polymerization (up to 7813 kgpolymer mol-1 h-1). The activity is higher than any yet recorded for which yield high cis-1,4 polybutadiene by the well-defined late-transition metal catalytic system. The catalyst also exhibited excellent tolerance towards the ratio of Co/Bd and broad temperature stability. At a ratio of Bd/Co3 = 50 000, the complexes Co1-3 can afford polybutadiene with yields higher than 96% within 2 hours. At -20 °C to 100 °C, the complex Co3 afforded relatively high polymer yields at low catalyst concentrations (Bd/Co3 = 25 000). In addition, all polymers showed a relatively high molecular weight (up to 1.06 × 106 g mol-1).

3,4-Enhanced Polymerization of Isoprene Catalyzed by Side-Arm Tridentate Iminopyridine Iron Complex with High Activity: Optimization via Response Surface Methodology

3,4-Enhanced polymerization of isoprene catalyzed by late transition metal with high activity remains one of the great challenges in synthetic rubber chemistry. Herein, a library of [N, N, X] tridentate iminopyridine iron chloride pre-catalysts (Fe 1-4) with the side arm were synthesized and confirmed by the element analysis and HRMS. All the iron compounds served as highly efficient pre-catalysts for 3,4-enhanced (up to 62%) isoprene polymerization when 500 equivalent MAOs were utilized as co-catalysts, delivering the corresponding high-performance polyisoprenes. Furthermore, optimization via single factor and response surface method, it was observed that the highest activity was obtained by complex Fe 2 with 4.0889 × 10⁷ g·mol(Fe)^-1·h^-1 under the following conditions: Al/Fe = 683; IP/Fe = 7095; t = 0.52 min.

Synthesis, Characterization and Catalytic Property Studies for Isoprene Polymerization of Iron Complexes Bearing Unionized Pyridine-Oxime Ligands

Iron complexes of the types [Fe(HL)2Cl2] (Fe1: HL1 = pyridine-2-aldoxime; Fe2: HL2 = 6-methylpyridine-2-aldoxime; Fe3: HL3 = phenyl-2-pyridylketoxime; Fe4: HL4 = picolinaldehyde O-methyl oxime) were prepared and characterized by elemental analysis and IR spectroscopy. The crystal structure of Fe2, determined by single-crystal X-ray diffraction, featured a distorted octahedral coordination of the iron center binding with two ligands of HL2. The X-ray structure and infrared spectral data indicated that pyridine-oxime ligands act as unionized bidentate ligand by coordinating with Npyridine and Noxime. The catalytic performance for isoprene polymerization, catalyzed by these pyridine-oxime-ligated iron complexes, was examined. For a binary catalytic system combined with MAO, complexes Fe1, Fe3 and Fe4 were found to be highly active (up to 6.5 × 106 g/mol·h) in cis-1,4-alt-3,4 enchained polymerization, with average molecular weights in the range of 60–653 kg/mol and narrow PDI values of 1.7–3.5, even with very low amounts of MAO (Al/Fe = 5). Upon activation with [Ph3C][B(C6F5)4]/AlR3 for the ternary catalytic system, theses complexes showed extremely high activities, as well about 98% yield after 2 min, to afford cis-1,4-alt-3,4-polyisoprene with a molecular weight of 140–420 kg/mol.

Late Transition Metal Catalysts with Chelating Amines for Olefin Polymerization

Polyolefins are the most consumed polymeric materials extensively used in our daily life and are usually generated by coordination polymerization in the polyolefin industry. Olefin polymerization catalysts containing transition metal–organic compound combinations are undoubtedly crucial for the development of the polyolefin industry. The nitrogen donor atom has attracted considerable interest and is widely used in combination with the transition metal for the fine-tuning of the chemical environment around the metal center. In addition to widely reported olefin polymerization catalysts with imine and amide donors (sp2 hybrid N), late transition metal catalysts with chelating amine donors (sp3 hybrid N) for olefin polymerization have never been reviewed. In this review paper, we focus on late transition metal (Ni, Pd, Fe, and Co) catalysts with chelating amines for olefin polymerization. A variety of late transition metal catalysts bearing different neutral amine donors are surveyed for olefin polymerization, including amine–imine, amine–pyridine, α-diamine, and [N, N, N] tridentate ligands with amine donors. The relationship between catalyst structure and catalytic performance is also encompassed. This review aims to promote the design of late transition metal catalysts with unique chelating amine donors for the development of high-performance polyolefin materials.

Crystallographic Evidence of η1-Coordination of Bulky Aminopyridine in Halide-Containing Iron (II) Complexes

Reaction of N-(2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)-pyridine-2-yl]-amine (ApH) in equimolar ratio with anhydrous FeBr2 and FeI2 in tetrahydrofuran (THF) afforded, after workup in toluene, the first examples of mono(aminopyridine) Fe(II) complexes, [ApHFeBr(µ-Br)]2 (1) and [ApHFeI2(thf)] (2), respectively. X-ray analysis shows 1 to be dimeric, whereas compound 2 is monomeric. In both cases, aminopyridine ligands show rare η1-coordination to Fe through pyridine nitrogen atom. Compound 1 exhibits intramolecular N–H⋯Br hydrogen bonds [3.363 Å] with an N–H⋯Br angle of 158.84°. Hirshfeld surface and fingerprint plots identify the significant intermolecular interactions in the crystal network. Both compounds crystallized in the monoclinic space group. For compound 1, C2/c, the cell parameters are: a  =  25.5750(5) Å, b  =  10.5150(5) Å, c = 18.9610(8) Å, β  =  97.892(5)°, V  =  5050.7(3) A3, Z  =  4. For compound 2, P21/c, the cell parameters are: a  =  10.3180(7) Å, b  =  16.1080(10) Å, c  =  18.6580(11) Å, β  =  102.038(5)°, V  = 3032.8(3) A3, Z  =  4.

One-pot synthesis of cobalt complexes with 2,6-bis(arylimino)phenoxyl/phenthioxyl ligands and catalysis on isoprene polymerization

Several cobalt complexes supported by 2,6-bis(arylimino)phenoxyl/phenthioxyl ligands κ2N,X-Ar[NXN]CoCl(THF) (1a, X = O, Ar = 2,6-Me2C6H3; 1b, X = O, Ar = 2,6-iPr2C6H3; 2a, X = S, Ar = 2,6-Me2C6H3; 2b, X = S, Ar = 2,6-iPr2C6H3) were synthesized by direct oxygen(sulfur) insertion into the C-Co bond of the mixed-valence cobalt complexes {κ2C,N,η6-Ar[NCN]Co-κN-CoCl(μ-Cl)}2. Crystallization of 1b in the presence of water gave the hydrolysis product 1b'. Treatment of Ar[NCN]Li with dioxygen followed by the addition of CoCl2 afforded the heteroatomic complexes {κ2N,O-Ar[NON]Co(μ-Cl)2Li}2 (3a, Ar = 2,6-Me2C6H3; 3b, Ar = 2,6-iPr2C6H3) or κ2N,O-Ar[NON]Co2Cl2(μ-Cl)2Li(THF)2 (4a, Ar = 2,6-Me2C6H3; 4b, Ar = 2,6-iPr2C6H3) depending on the amount of CoCl2 used. The Co(iii)/Li heterometallic complex 3b' with imino-phenoxyl-amino ligands was formed probably via a redox reaction of 3b. The reactions of Ar[NCN]Li with elemental sulfur and CoCl2 gave κ2N,S-Ar[NSN]Co2Cl2(μ-Cl)2Li(THF)2 (5a, Ar = 2,6-Me2C6H3; 5b, Ar = 2,6-iPr2C6H3) respectively. These complexes were well characterized by FT-IR and elemental analyses, and the molecular structures of 1b', 3b', 4a, and 4b were confirmed by X-ray crystallography. Upon activation with Al2Et3Cl3 in toluene, these complexes showed high activities in isoprene polymerization affording cis-1,4 enriched polymers with a moderate molecular weight (0.85-4.72 × 104 Da).

An unsymmetrical binuclear iminopyridine-iron complex and its catalytic isoprene polymerization

A series of chloride-bridged unsymmetrical mixed Fe(ii)-HS/Fe(ii)-LS binuclear structures has been prepared and characterized. Upon activation with MAO, highly efficient catalytic polymerization of isoprene was achieved, delivering an ultra-high molecular weight (catalyst loading = 2.5 ppm, Mn = 1.8 × 106 g mol-1, Mw/Mn = 1.4).

Synthesis of characteristic polyisoprenes using rationally designed iminopyridyl metal (Fe and Co) precatalysts: investigation of co-catalysts and steric influence on their catalytic activity

A series of iminopyridyl ligand-based iron and cobalt complexes are utilized as catalysts in isoprene polymerization, in which the microstructure of resultant polyisoprenes was investigated.

Highly Active Iminopyridyl Iron-Based Catalysts for the Polymerization of Isoprene

A series of iminopyridyl-based ligands, 6-[(Ar)N=C(R)]-2-C₆H₅N [(Ar = 2,6-Me₂-C₆H₃, R = Me (L1); Ar = 2,6-ⁱPr₂-C₆H₃, R = Me (L2); Ar = 2,6-Me₂-C₆H₃, R = H (L3); Ar = 2,6-ⁱPr₂-C₆H₃, R = H (L4); Ar = 3,5-(CF₃)₂-C₆H₃, R = Me (L5); Ar = C₆F₅, R = Me (L6)], and their corresponding iron (II) complexes were developed to investigate their application in the controlled coordinative polymerization of isoprene. The modulation of steric and electronic properties within this family of ligands/pre-catalysts has shown to influence the stereo-selectivity and activity of the polymerization of isoprene after activation. Upon activation with various co-catalysts such as AlⁱBu₃/[Ph₃C][B(C₆F₅)₄], AlEt₃/[Ph₃C][B(C₆F₅)₄] or MAO, the resulting catalysts produced polyisoprenes with an excellent conversion (>99% of 500–5000 equiv.) within less than 1 h (TOF > 500 h⁻¹) and having a variety of stereo-/regio-regularities. The presence of electron-donating and withdrawing groups drastically impacted the activity and the stereoselectivity of the catalysts during the course of the polymerization of isoprene. When activated with AlⁱBu₃/[Ph₃C][B(C₆F₅)₄], the complexes {6-[(2,6-Me₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C1) and {6-[(2,6-ⁱPr₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C2) exhibited moderate trans-1,4 selectivity (>67%) while the iron-based systems bearing related aldiminopyridyl ligands {6-[(2,6-Me₂-C₆H₃)N=C(H)]-2-C₆H₅N}FeCl₂ (C3) and {6-[(2,6-ⁱPr₂-C₆H₃)N=C(H)]-2-C₆H₅N}FeCl₂ (C4) were found to afford significant cis-1,4 selectivity at low temperature (>86% at −40 °C). On the other hand, the ternary {6-[(3,5-(CF₃)₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C5) or {6-[(C₆F₅)N=C(Me)]-2-C₆H₅N}FeCl₂ (C6)/AlⁱBu₃/[Ph₃C][B(C₆F₅)₄] catalytic combinations showed exceptional activity for the polymerization of isoprene (TOF > 1,000,000 h⁻¹), albeit providing less stereoselectivity.