Iminoimidazole-based Co(II) and Fe(II) complexes: Syntheses, characterization, and catalytic behaviors for isoprene polymerization

Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine-Oxazoline-Ligated Cobalt Complexes

A family of pyridine-oxazoline-ligated cobalt complexes L2CoCl23a-h were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes 3a and 3d, ligated by two ligands, displayed a distorted tetrahedral coordination of a cobalt center. The X-ray structure indicated the pyridine-oxazoline ligands acted as unusual mono-dentate ligands by coordinating only to Noxazoline. Upon activation with AlEt2Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 106 g·molCo-1·h-1), affording cis-1,4-co-3,4-polyisoprene with molecular weights of 4.4-176 kg mol-1 and a narrow Ð of 1.79-3.42, suggesting a single-site nature of the active sites. The structure of cobalt catalysts and reaction parameters, especially co-catalysts and the reaction temperature, all have significant influence on the polymerization activity but not on the microstructure of polyisoprene.

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.

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.

Polymerisation of styrene using pincer type amine functionalized azo aromatic complexes of Co(II) as catalysts

In the present report, three mononuclear azo-aromatic complexes of Co(II), 1-3, and an imine-based Co(II) complex, 4, were synthesized through a reaction of respective amine-functionalized pincer-like ligands, HL1-4, with CoCl₂·6H₂O in the ligand-to-metal ratio of 1 : 1. All the complexes, 1-4, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory (DFT) calculations. Complexes 1-4 were explored for the catalytic styrene polymerisation reaction separately in the presence of modified methyl aluminoxane (MMAO). All the complexes, 1-4, are indeed active for the polymerisation of styrene under mild conditions at room temperature upon activation with MMAO. Among the azo-aromatic complexes 1-3, complex 3 is the most efficient. The activity of the imine complex 4 is poor compared to those of the azo-aromatic complexes 1-3. The weight average molecular weight (M_w) of the isolated polystyrene ranges from 32.9 to 144.0 kg mol^-1, with a polydispersity index (Đ) in the range of 1.1-1.8. Microstructural analysis of the isolated polymer from complexes 1-4 was carried out by ¹³C NMR spectroscopy, infrared spectroscopy, and powder X-ray diffraction studies. Their thermal properties were scrutinized by differential scanning calorimetry and thermogravimetric analysis. These studies have shown the atactic and amorphous nature of the polymers. The mechanical strength of the polymers was measured by a nanoindentation technique which has shown the good plastic/soft nature of the polymers.

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.

Isoprene Polymerization: Catalytic Performance of Iminopyridine Vanadium(III) Chloride versus Vanadium(III) Chloride

A series of vanadium complexes bearing iminopyridine bidentate ligands with various electronic and steric properties: V1 [CH₂Ph], V2 [CMe₂CH₂CMe₃], V3 [Ph] and V4 [2,6-ⁱPr₂Ph] were prepared and characterized by IR spectroscopy and microanalytical analysis. The catalytic capacity of all the complexes has been investigated for isoprene polymerization and was controlled by tuning the ligand structure with different N-alkyl and N-aryl groups. Activated by methylaluminoxane (MAO), the aryl-substituted complex V3 [Ph] exhibited high cis-1,4 selectivity (75%), and the resultant polymers had high molecular weights (M_n = 6.6 × 10⁴) and narrow molecular weight distributions (PDI = 2.3). This catalyst showed high activity up to 734.4 kg polymer (mol V)⁻¹ h⁻¹ with excellent thermostability even stable at 70 °C. Compared to the traditional VCl₃/MAO catalytic system, iminopyridine-supported V(III) catalysts displayed higher catalytic activities and changed the selectivity of monomer enchainment from trans-1,4 to cis-1,4.

Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: sterically controlled monomer enchainment

Aminopyridine iron(ii) chloride complexes coordinating a type of flexible ligand are reported as precursors for controlled polymerization of isoprenes.