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.


Introduction
The catalytic polymerization of conjugated dienes is one of the most promising methods for manufacturing synthetic rubber to make up for the deficiency of natural rubber. From a mechanistic point of view, 1,3-dienes can be polymerized in different ways, such as radical [1], cationic [2], or coordination polymerization [3,4], all yielding polymers with diverse structures and properties. Notably, coordination polymerization catalyzed by a transition metal has attracted much attention since the discovery of Ziegler-Natta catalysts in the 1950s [5][6][7]. From the early-1960s onwards, lanthanide metal-catalyzed polymerization of conjugated dienes came to be the focus of fundamental research [8]. Later, in 1995, Brookhart and coworkers applied cationic Ni and Pd complexes with bulky diimine ligands as new catalysts, marking a milestone in the research of olefin polymerization catalyzed by late transition metals [9]. On the basis of the good performance in terms of strength, flexibility and easy processing, recent progress in diene polymerization catalyzed by early transition metal complexes [8][9][10], rare earth complexes [11][12][13][14], and late transition metal complexes [15][16][17], has made it possible to tailor various properties of rubbers according to the type of catalyst used. complexes [11][12][13][14], and late transition metal complexes [15][16][17], has made it possible to tailor various properties of rubbers according to the type of catalyst used.
Catalysts using late transition metals are promising for 1,3-diene polymerization. Historically, iron, as an abundant, first-row, late transition metal, has attracted much less attention than Ni, Co and Pd compounds, despite the fact that it is less toxic and more sustainable than other metals. Early research on the iron-catalyzed polymerization of conjugated diene focused on (1) the addition of some nitrogenous or phosphorous compounds to stabilize the iron ions and improve the activity and selectivity, and (2) on the catalytic mechanism of these coordinating compounds [18][19][20][21]. Various well-defined iron complexes coordinated with bidentate or tridentate ligands, e.g., pyridine, imine, pyrazole, oxazoline, and phosphine moieties, etc. (Figure 1), have been systematically explored in olefin polymerization and have exhibited high levels of activity and the ability to control the molecular weight and molecular weight distribution of the resultant polymers [22][23][24][25][26][27][28][29][30][31][32][33]. As part of our work developing iron-based catalysts for isoprene polymerization, various pyridine-imine and pyridine-amine iron complexes have been applied to the polymerization of conjugated dienes to probe the ligand effects [34][35][36][37][38][39]. Therefore, the discovery of a novel catalytic system-with diverse steric hindrance and electronic circumstances that affect the activity and selectivity of isoprene polymerization-is particularly significant. Presently, we are trying to identify suitable ligands with different structures toward the iron center. Pyridine-oxime compounds are important organic ligands with unique coordination abilities and various coordination modes. They can be obtained from the Schiff base condensation of pyridine-2-aldehyde or ketone with hydroxylamine. Metal complexes chelating with pyridine-oxime ligands have long been attracted attention as bioinorganic model compounds and molecule-based magnetic materials [40,41]. However, the catalytic applications of these complexes have rarely been explored, especially in the field of olefin polymerization [42,43]. Here, we report the synthesis, structures, and activities of pyridine-oxime-ligated mononuclear iron complexes and investigate the novel catalytic performance in an iron-catalyzed isoprene polymerization process in which pyridineoxime is used as a neutral ligand. Additionally, the function of the H atom in oxime is determined by replacing it with a methyl group. Furthermore, we investigated isoprene polymerization with Fe/Al binary and Fe/Al/[Ph3C][B(C6F5)4] ternary catalytic systems, respectively, to demonstrate the wide application potential of these pyridine-oximeligated iron complexes. Pyridine-oxime compounds are important organic ligands with unique coordination abilities and various coordination modes. They can be obtained from the Schiff base condensation of pyridine-2-aldehyde or ketone with hydroxylamine. Metal complexes chelating with pyridine-oxime ligands have long been attracted attention as bioinorganic model compounds and molecule-based magnetic materials [40,41]. However, the catalytic applications of these complexes have rarely been explored, especially in the field of olefin polymerization [42,43]. Here, we report the synthesis, structures, and activities of pyridineoxime-ligated mononuclear iron complexes and investigate the novel catalytic performance in an iron-catalyzed isoprene polymerization process in which pyridine-oxime is used as a neutral ligand. Additionally, the function of the H atom in oxime is determined by replacing it with a methyl group. Furthermore, we investigated isoprene polymerization with Fe/Al binary and Fe/Al/[Ph 3 C][B(C 6 F 5 ) 4 ] ternary catalytic systems, respectively, to demonstrate the wide application potential of these pyridine-oxime-ligated iron complexes.

Materials
All experiments were carried out under argon atmosphere. FeCl 2 , L1, L3, 2-pyridinecarboxaldehyde, 6-methylpicolinaldehyde, hydroxylamine hydrochloride, methoxyamine hydrochloride and cocatalysts were purchased. Toluene was refluxed and distilled over sodium and stored over molecular sieves under nitrogen. Hexane, dichloromethane and isoprene were refluxed and distilled over calcium hydride and stored over molecular sieves under nitrogen. The molecular weights (M n ) and molecular weight distributions (M w /M n ) of polymers were measured by high temperature gel permeation chromatography (GPC) using a PL-GPC 220 chromatography and maintained at 150 • C, using trichlorobenzene as an eluent and polystyrene as a standard. The NMR spectra were measured on a Bruker Advance 400 spectrometer at 298 K. 1 H NMR and 13 C NMR spectra of polyisoprene were recorded in CDCl 3 and trimethylsilane as an internal reference. The polyisoprene microstructure of the 1,4 and 3,4 ratio was determined from 1 H NMR of the 1,4 =CH signals at 5.15 ppm and the 3,4 =CH 2 signal at 4.7 ppm. The trans/cis-1,4 stereoisomer ratio was determined from 13 C NMR of the -CH 3 signals of cis-1,4 at 23.8 ppm and trans-1,4 at 16.3 ppm. The mass spectra for iron complexes were detected using an ACQUITYTM UPLC & Q-TOF MS Premier. Elemental analyses were performed using a Vario EL III elemental analyzer. X-ray diffraction data were obtained using a Smart 1000 diffractometer with a Mo K-alpha X-ray source (λ = 0.71073 Å) at 298 K. Attenuated total reflection-infrared (ATR-IR) spectroscopy was conducted using a Thermo Scientific Nicolet iN10.

Preparation of Ligands L2 and L4
Ligand L2 was prepared using a procedure reported in [44] with a little change. Briefly, to a suspension of 6-methylpicolinaldehyde (2.0 g, 16.51 mmol) in MeOH (40 mL) was added hydroxylamine hydrochloride (1.15 g, 16.51 mmol) followed by Na 2 CO 3 (0.97 g, 9.08 mmol). The mixture was stirred at room temperature for 4 h. Then, the orange suspension was filtered and the solid was dissolved in ethyl acetate (40 mL). The solution was transferred to a separatory funnel, washed in water (3 × 20 mL), and dried over anhydrous Na 2 SO 4 . The solvent was removed in a rotary evaporator and the product was dried under vacuum. The compound was characterized as follows: a yellow solid, 1.9 g, 84% yield; 1  Ligand L4 was prepared similarly to ligand L2, i.e., by blending 2-pyridinecarboxaldehyde (5.30 g, 49.48 mmol) and methoxyamine hydrochloride (4.13 g, 49.48 mmol) in methanol (60 mL), followed by the addition of Na 2 CO 3 (2.91 g, 27.24 mmol). The compound was characterized as follows: light yellow oil, 5.8 g, 86% yield; 1

Preparation of Complexes Fe1-Fe4
Ligand (L1-L4) (2.0 equiv.) and FeCl 2 (1.0 equiv.) were added to anhydrous dichloromethane (10 mL) in a glovebox. The reaction mixture was stirred for 15 h at room temperature. Then, the solvent was concentrated to 2 mL and hexane (5 mL) was added, resulting in precipitation which was collected by filtration, washed with hexane (3 × 5 mL) and dried under vacuum to give the corresponding complex.

Polymerization Procedure
Binary component-catalyzed polymerization: The iron complex was weighed in a glove box and transferred to a dried Schlenk tube (25 mL). Under argon, the quantitative solvent, isoprene, and the aluminum reagent were added. At the end of the reaction, HCl solution in methanol (methanol/HCl = 50/1) was added to quench the polymerization, and the mixture was poured into a large volume of methanol with 2,6-bis(1,1-dimethylethyl)-4-methylphenol as a stabilizing agent. The solid was collected and washed several times with methanol before being dried under vacuum at 50 • C for 15 h. The polymer yield was determined by gravimetry.
Ternary component-catalyzed polymerization: The iron complex was weighed in glove box and transferred to a dried Schlenk tube (25 mL). Under argon, the quantitative solvent and isoprene were added. Following that, a solution of [Ph 3 C][B(C 6 F 5 ) 4 ] in toluene (1.0 equiv. to iron catalyst) and cocatalyst were added. At the end of the reaction, HCl solution in methanol (methanol/HCl = 50/1) was added to quench the polymerization, and the mixture was poured into a large volume of methanol containing 2,6-bis(1,1-dimethylethyl)-4-methylphenol as a stabilizing agent. The solid was collected and washed several times with methanol before being dried under vacuum at 50 • C for 15 h. The polymer yield was determined by gravimetry.

Synthesis and Characterization of Ligands L2, L4 and Fe(II) Complexes
Following the reported procedure, ligands L2 and L4 were prepared by Schiff base condensation and were characterized by NMR and IR. Ligands L1 and L3 were used as received. After that, the former ligands were treated with FeCl 2 in methylene dichloride at room temperature to afford various of pyridine-oxime-ligated iron(II) complexes which were characterized by IR, high resolution mass spectroscopies and elemental analysis (Scheme 1). The IR spectra showed a shift in the characteristic v (C=N) bands (1650-1604 cm −1 ) of the iron complexes toward higher wave numbers than the v (C=N) bands (1586-1593 cm −1 ) observed for the corresponding free ligands, an indication of the coordination of ligands to the iron center. Krause et al. [47] postulated that transition metal complexes bearing pyridine-2-aldoxime and containing unionized oxime proton have v (CH=N) bands in the range of 1654-1614 cm −1 and v (N-O) bands in the range of 1069-1036 cm −1 , respectively. Furthermore, the v (OH) stretching frequency of metal complexes with unionized oxime proton displayed multiple bands between 3194 and 2791 cm −1 , instead of the broad band at 3250 cm −1 of v (OH) observed with free ligands [48]. Therefore, similar to Krause's findings, these IR data for present iron complexes (Fe1-Fe3) showed that oxime protons are unionized ( Figure S1).
In order to demonstrate the bonding and coordinating mode, a monocrystal of Fe2 that would be suitable for X-ray diffraction was grown by the slow diffusion of hexane into a dichloromethane solution at −30 • C in glovebox (The detailed data were displayed in Tables S1-S3). The molecular structure with selected bond distances and angles is described in Figure 2 (CCDC number: 2160530). These data also demonstrate the existence of two unionized oxime protons. The structure of Fe2 consists of two ligands and two chloride atoms and adopts a distorted octahedron geometry in which two N oxime atoms occupy the apical position [N(2) 1 -Fe-N(2) = 175.2 (2)] and N(1) pyridine , N(1) 1 pyridine , Cl(1) and Cl (1) [47,48]. To further study the electronic properties of the iron center, the 57 Fe Mössbauer spectrum of complex Fe1 was collected at 90 K, as displayed in Figure S2. However, three quadrupole doublets were noticed for the solid samples, which clearly indicated that the sample was composed of high-spin low-spin states. In order to demonstrate the bonding and coordinating mode, a monocrystal of Fe2 that would be suitable for X-ray diffraction was grown by the slow diffusion of hexane into a dichloromethane solution at −30 °C in glovebox (The detailed data were displayed in Tables S1-S3). The molecular structure with selected bond distances and angles is described in Figure [47,48]. To further study the electronic properties of the iron center, the 57 Fe Mössbauer spectrum of complex Fe1 was collected at 90 K, as displayed in Figure S2. However, three quadrupole doublets were noticed for the solid samples, which clearly indicated that the sample was composed of high-spin low-spin states.  In order to demonstrate the bonding and coordinating mode, a monocrystal of Fe2 that would be suitable for X-ray diffraction was grown by the slow diffusion of hexane into a dichloromethane solution at −30 °C in glovebox (The detailed data were displayed in Tables S1-S3). The molecular structure with selected bond distances and angles is described in Figure 2 (CCDC number: 2160530). These data also demonstrate the existence of two unionized oxime protons. The structure of Fe2 consists of two ligands and two chloride atoms and adopts a distorted octahedron geometry in which two Noxime atoms occupy the apical position [N(2) 1 -Fe-N(2) = 175.2 (2)] and N(1)pyridine, N(1) 1 pyridine, Cl(1) and Cl (1) [47,48]. To further study the electronic properties of the iron center, the 57 Fe Mössbauer spectrum of complex Fe1 was collected at 90 K, as displayed in Figure S2. However, three quadrupole doublets were noticed for the solid samples, which clearly indicated that the sample was composed of high-spin low-spin states.

Isoprene Polymerization with Binary Catalytic System
As shown in Table 1, iron complexes Fe1-Fe4 were investigated for isoprene polymerization combined with methylaluminoxane (MAO) in toluene. They showed medium-tohigh activities at room temperature. In addition, the GPC analyses of polymers catalyzed by all iron complexes showed unimodal and narrow distributions (1.8-3.5), proving the occurrence of typical single site catalysis ( Figure 3A). Complexes Fe1, Fe3 and Fe4 revealed relatively high activities (8.2 × 10 5 g/mol·h) with about 50% of cis-1,4-selectivity, and high molecular weight polymers were obtained (Table 1, entries 1, 5, 6). However, complex Fe2, bearing one 6-methyl substituent at the pyridine group, showed lower activity because of steric hindrance. Notably, 98% conversion could be achieved in 5 h via catalysis with Fe2 (Table 1, entry 4). Interestingly, the substituent at 6-position of pyridine also slightly affected the selectivity to produce some trans-1,4-polyisoprene; the trans-1,4-unit gradually increased with an increase in the conversion rate. We attributed the variation in trans-1,4selectivity to the steric effect of the progressively larger polymer chains. Likewise, the PDI of polyisoprene obtained with complex Fe2 increased as the reaction time lengthened, albeit with decreasing molecular weight, suggesting fast chain transfer due to the large amount of MAO that was present ( Figure 3B). polymerization combined with methylaluminoxane (MAO) in toluene. They showed medium-to-high activities at room temperature. In addition, the GPC analyses of polymers catalyzed by all iron complexes showed unimodal and narrow distributions (1.8-3.5), proving the occurrence of typical single site catalysis ( Figure 3A). Complexes Fe1, Fe3 and Fe4 revealed relatively high activities (8.2 × 10 5 g/mol·h) with about 50% of cis-1,4-selectivity, and high molecular weight polymers were obtained (Table 1, entries 1, 5, 6). However, complex Fe2, bearing one 6-methyl substituent at the pyridine group, showed lower activity because of steric hindrance. Notably, 98% conversion could be achieved in 5 h via catalysis with Fe2 (Table 1, entry 4). Interestingly, the substituent at 6-position of pyridine also slightly affected the selectivity to produce some trans-1,4-polyisoprene; the trans-1,4-unit gradually increased with an increase in the conversion rate. We attributed the variation in trans-1,4selectivity to the steric effect of the progressively larger polymer chains. Likewise, the PDI of polyisoprene obtained with complex Fe2 increased as the reaction time lengthened, albeit with decreasing molecular weight, suggesting fast chain transfer due to the large amount of MAO that was present ( Figure 3B).  Subsequently, the effects of the reaction parameters on catalytic performance, e.g., the amount of MAO and reaction temperature, were systematically studied using Fe1 and Fe4. The results are summarized in Table 2. For precatalyst Fe1, the catalytic activities remained largely unchanged as the [Al]/[Fe] ratio decreased from 500 to 50, resulting Subsequently, the effects of the reaction parameters on catalytic performance, e.g., the amount of MAO and reaction temperature, were systematically studied using Fe1 and Fe4. The results are summarized in Table 2 ratio from 500 to 10 led to a decrease in the polymer molecular weight, probably due to the stability of the catalyst transition state maintaining chain propagation rather than causing chain termination/transfer in the presence of excessive MAO ( Figure 3C). Variation of the [Al]/[Fe] ratio did not have obvious effects on the molecular weight distribution or polymerization selectivity. Furthermore, in order to study the role of oxime hydroxyl groups, complex Fe4 was synthesized and catalytic isoprene polymerization was conducted. The results showed that complex Fe4 possessed higher catalytic activities than Fe1; this was ascribed to the electron-donating properties of the oxime methoxyl groups, which facilitated isoprene coordination to the Fe center. For example, when reactions were quenched after 1 min under the same conditions with an [Al]/[Fe] ratio of 100, complex Fe4 (Table 2, entry 9) exhibited the highest activity, i.e., 6.5 × 10 6 , compared to precatalyst Fe1, with 4.8 × 10 6 (Table 2, entry 2). These results may also indicate that there was an induction period of Fe1 to destroy hydrogen bonds between oxime hydroxyl groups and chloride atoms. To continue the comparison, temperature dependence tests were conducted with complexes Fe1 and Fe4. When the polymerization temperature was −30 • C, the catalyst systems of Fe1 (

Isoprene Polymerization with Ternary Catalytic System
The catalytic performance of a ternary catalytic system using Fe1-Fe4, together with [Ph 3 C][B(C 6 F 5 ) 4 ] and trialkylaluminium, is summarized in Table 3. The polymerization results showed that the substituent at the 6-position of pyridine group also significantly affected activity and selectivity of isoprene polymerization because of the crowded iron center. In the case of complex Fe2, when AlEt 3 was used as a cocatalyst, no polyisoprene was produced (Table 3, entry 3), and a fairly low yield was obtained by activating with Al(i-Bu) 3 ( Table 3, entry 4). Free 6-substitute catalysts (Fe1, Fe3 and Fe4) showed higher activity than complex Fe2 or previously reported imine ligated iron catalysts [31,35], regardless of cocatalyst type. Interestingly, when the Ph-substituent was added at the C=N position, complex Fe3 showed the highest activity, i.e., up to 4.0 × 10 6 g/mol·h (  4 ].
In addition, it was clearly seen that the pyridine-oxime-ligated Fe1 had big advantages in terms of these activities over complex Fe4, further emphasizing the particularity of oxime hydroxyl group. Otherwise, the ternary catalytic system provided polyisoprene with higher PDI than the binary one, but possessed similar selectivity and molecular weight. The results of previous works on iron-catalyzed isoprene polymerization are compared with those presented in this work in Table S4. Notably, the pyridine-oxime-ligated iron complexes showed higher activities in a ternary catalytic system.

Conclusions
Iron complexes chelated with bidentate pyridine-oxime ligands were developed and examined for use in isoprene polymerization. For the binary catalytic system activated by MAO, the iron catalysts exhibited activities up to 6.5 × 10 6 g/mol·h, and produced polyisoprene with high molecular weight (0.6-6.5 × 10 5 g/mol) and narrow PDI (1.7-3.5), even at very low temperatures. The catalytic performance of precatalysts Fe1-Fe4 was further studied by combing with [Ph 3 C][B(C 6 F 5 ) 4 ] and trialkylaluminium, yielding polymers at up to 98% yield in 2 min and suggesting the high catalytic capacity of these compounds. Steric influences on the catalytic activity and selectivity of complex Fe2 were also clearly represented both in the binary and ternary catalytic methods. Moreover, these Fe complexes demonstrated high thermal stability regarding both activity and molecular weight. In summary, the newly developed, unionized, pyridine-oxime-ligated iron complexes are very promising compounds for isoprene polymerization applications. The development of other metal complexes suitable for pyridine-oxime ligand-catalyzed olefin polymerization is ongoing.