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Review

Late Transition Metal Olefin Polymerization Catalysts Derived from 8-Arylnaphthylamines

by
Zonglin Qiu
1,†,
Wenyan Wang
2,†,
Handou Zheng
1,*,
Dengfei Wang
2,
Xinglong Zhao
2,
Guangshui Tu
1,
Jiahao Yang
1 and
Haiyang Gao
1,*
1
School of Materials Science and Engineering, PCFM Lab, GD HPPC Lab, Sun Yat-sen University, Guangzhou 510275, China
2
Daqing Petrochemical Research Center, Petrochemical Research Institute of PetroChina, Daqing 163714, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Inorganics 2024, 12(11), 277; https://doi.org/10.3390/inorganics12110277
Submission received: 30 September 2024 / Revised: 24 October 2024 / Accepted: 26 October 2024 / Published: 28 October 2024
(This article belongs to the Special Issue Metal Complexes with N-donor Ligands, 2nd Edition)
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Abstract

:
Late transition metal catalysts represent a significant class of olefin polymerization catalysts that have played an essential role in advancing the polyolefin industry owing to their highly tunable ligands and low oxophilicity. A key feature for the design of late transition metal catalysts lies in the steric bulk of the o-aryl substituents. Bulky 8-arylnaphthylamines have emerged as a promising aniline candidate for conducting high-performance catalysts by introducing axially steric hindrance around the metal center. This review focuses on late transition metal (Ni, Pd, Fe) catalysts derived from 8-arylnaphthylamines, surveying their synthesis, structural features, and catalytic applications in olefin (co)polymerizations. Additionally, the relationship between catalyst structure and catalytic performance is discussed, highlighting how these unique ligand systems influence polymerization activity, molecular weight, and polymer branching.

Graphical Abstract

1. Introduction

Olefin polymerization catalysts have played a dominant role in the development of the polyolefin industry, driving advancements in both materials and industrial processes [1,2]. In particular, late transition metal catalysts have facilitated significant breakthroughs in the synthesis of high-performance functional polyolefins with varied structures and functionalities due to the low oxophilicity of their metal centers and the easily tunable ligands [3,4,5,6]. Late transition metal catalysts are often formed by the activation of the main catalysts (late transition metal complexes) by cocatalysts (e.g., alkylaluminum compounds), which can initiate olefins polymerization. Late transition metal complexes are typically composed of ligands and late transition metals such as nickel (Ni), palladium (Pd), iron (Fe), and cobalt (Co). Ligand diversity makes these catalyst systems versatile for a range of olefin polymerizations [7]. Usually, ligands feature nitrogen donor atoms as primary ligating groups, although oxygen and phosphorus donors are also commonly employed [8,9,10,11,12,13,14,15,16,17].
Historically, late transition metal nickel catalysts were primarily employed for ethylene oligomerization in the early stages because of rapid chain transfer via β-H elimination [18]. However, a significant breakthrough occurred in 1995 when Brookhart discovered the α-diimine nickel and palladium catalysts [19]. These catalysts featured bulky ortho-isopropyl substituents on the aryl moieties of the α-diimine ligands, increasing steric hindrance around the metal center. This modification effectively shielded the axial coordination sites, significantly suppressing the chain transfer process (associative displacement or chain transfer to bound monomer) and enabling the synthesis of high molecular weight polyolefins [19,20].
Since the discovery of α-diimine nickel and palladium catalyst systems, extensive research has focused on further modifying the ligand structure to enhance catalyst performance [21]. A key strategy has been the introduction of bulky ortho-aryl substituents on the aryl moiety, including phenyl [22,23], 2,6-diarylhydryl [24,25,26], pentiptycenyl [27,28,29], and the particularly unique 8-arylnaphthyl substituents [30,31] (Figure 1). The 8-arylnaphthylamines are known for creating a “sandwich-like” structure around the metal center, and their use has gained significant attention because the bulky 8-arylnaphthylamines provide a unique chemical environment around the metal center, which is not realized by other aniline compounds [30,31].
Considering unique axially steric hindrance, 8-arylnaphthyl substituents have also been introduced into late transition metal catalysts with other chelate ligands, including pyridine-imine, bis(imino)pyridyl, salicylaldimine, and α-imino-ketone ligands (Figure 2) [9,32]. Catalysts featuring these various 8-arylnaphthyl substituted ligands have shown promise in olefin polymerizations and offer new pathways for tuning catalytic performance and polymer properties. In this review, we primarily survey the synthesis and structural characteristics of late transition metal catalysts featuring 8-arylnaphthyl substituents and their catalytic applications in olefin (co)polymerizations. The relationship between catalyst structure and catalytic performance will also be discussed, providing valuable insights for the future design of high-performance olefin polymerization catalysts.

2. Synthesis of 8-Arylnaphthylamines

The 8-arylnaphthylamine compound 3 can be synthesized through the synthetic route shown in Scheme 1A [33,34]. N-(naphthalen-1-yl)picolinamide 1 is generated by the reaction of 1-naphthylamine and picolinic acid. The C-H bond functionalization reaction requires Pd(OAc)2 as a catalyst and stoichiometric AgOAc for halide removal and proceeds in the absence of solvent to yield N-(8-arylnaphthalen-1-yl)picolinamide 2. The hydrolysis of amide 2 results in the formation of 8-arylnaphthylamine. Each step of the synthesis demonstrates high yields, and the resulting amine is purified following chromatography. The resulting amines can be further modified through coupling reactions to obtain aromatic amines with substituents (X) exhibiting varying steric and electronic effects (Scheme 1B) [35]. The obtained 8-(X-phenyl)naphthylamines can be used to synthesize various ligands through condensation reactions.

3. α-Diimine Nickel and Palladium Catalysts

Brookhart and coworkers first synthesized “sandwich-like” α-diimine nickel complexes 4 (Figure 3) in 2013 [30]. The single crystal structures of these complexes show a smaller distorted tetrahedral geometry around the nickel center compared with traditional dihalide-nickel complexes [36,37,38,39], which is attributed to the axial 8-arylnaphthyl substituents. The bulky 8-arylnaphthyl substituents are oriented perpendicular to the naphthyl planes and shield the axial up and down sites of the metal center, effectively suppressing chain transfer processes. Consequently, catalysts featuring 8-arylnaphthylamines have demonstrated the ability to produce high molecular weight polyolefin. After activation with modified methylalumoxane (MMAO), these “sandwich-like” nickel catalysts were able to catalyze ethylene polymerization, producing high molecular weight (Mn > 106 g/mol) and highly branched (up to 85/1000C) polyethylene (PE) (Entry 1 and 2 in Table 1). However, turnover frequencies (TOFs) indicated that the polymerization activity of these nickel catalysts was lower than that of nickel catalysts with ortho-isopropyl substituents (TOF: 1.4 × 105 vs. 8.5 × 105 mol PE/(mol Ni·h)), but the branching density of the obtained PE was significantly higher (85 vs. 60/1000C). The corresponding “sandwich-like” palladium complexes were also synthesized [31]. The single crystal structures of these palladium complexes exhibit square planar geometries around the palladium center, and the two tolyl rings also effectively cap the axial sites of the metal. These palladium complexes were further treated with acetonitrile and sodium tetrakis (3,5-bis(trifluoromethyl)phenyl)borate (NaBArF) to yield cationic palladium catalysts 5 (Figure 3). The cationic palladium catalysts can directly catalyze ethylene polymerization to yield more highly branched PEs (up to 117/1000C) compared with the corresponding nickel system in a living polymerization fashion (Entry 3 and 4 in Table 1). Additionally, palladium catalysts 5 achieved the copolymerization of ethylene (E) and methyl acrylate (MA), which exhibited decreased activity compared with traditional palladium catalysts. However, under the same conditions, the molecular weight and MA incorporation of the resulting EMA copolymer were higher. Overall, palladium catalysts 5 produced highly branched EMA copolymers with both high molecular weight and MA incorporation (up to 14%) (Entry 1 in Table 2).
Coates and coworkers introduced fluorine and trifluoromethyl substituents on the “sandwich-like” ligand to form fluorinated nickel complexes 6 (Figure 3) [40,41,42,43]. The presence of these electron-withdrawing groups made the corresponding ligand synthesis more challenging and led to lower yields. In the presence of the cocatalyst methylaluminoxane (MAO), catalysts 6 with trifluoromethyl substituents were able to catalyze the “chain-walking” polymerization of α-olefins, producing semicrystalline polyethylene-type materials with distinct melting temperatures (Tm > 100 °C) (Entry 1 in Table 3) [40]. Moreover, catalysts 6 with fluorine substituents resulted in a switchable catalyst for ethylene polymerization in a living fashion and produced a well-defined tetrablock copolymer comprising both branched and highly linear PEs by varying polymerization temperatures and ethylene pressures (Entry 5 in Table 1) [43].
Jian reported a series of “sandwich-like” α-diimine nickel and palladium catalysts 7 with polyethylene glycol (PEG) units (Figure 3) [44]. The aryl rings with the PEG unit arms are sited perpendicular to the naphthyl planes, capping the metal center from two axial directions. However, the distances between the oxygen atoms in the PEG chains and the nickel center (>5.5 Å) and the palladium center (>5.1 Å) were far beyond the range of van der Waals radii of the oxygen atom and the nickel atom (3.55 Å) and the palladium atom (3.6 Å), suggesting that no weak interactions exist between the ligand and the metal centers [45]. However, the probable “ligand–cocatalyst effect” via the chelation of the oxygen atom in polyethylene glycol units with the aluminum atom in the activator was proposed as a unique secondary coordination interaction. The nickel catalysts 7 with varying polyethylene glycol units under the activation of an alkyl aluminum reagent drastically enhanced ethylene polymerization activities (~9 times) and notably demonstrated the ability to tune the branching densities of the resulting PEs (ranging from 19 to 106/1000C) (Entry 6 in Table 1). However, the corresponding palladium catalysts did not exhibit any activity for ethylene polymerization.
Recently, Brookhart and Daugulis reported a new series of “sandwich-like” nickel and palladium complexes 8 with perfluorinated aryl caps (Figure 3) [46]. Due to difficulties in introducing polyfluorinated aryl groups, the preparation of ligands follows procedures that are different from those for nonfluorinated sandwich catalysts, but these nickel and palladium complexes are all air stable. The single crystal structures of these complexes showed that the perfluorinated aryl caps were precisely centered over the nickel and palladium centers, with the distances between the ipso-carbons on the fluorinated caps and the palladium center (3.31 and 3.42 Å), as well as the nickel center (3.15 and 3.20 Å), being notably close. As a result, this led to higher steric hindrance around the metal centers compared with their nonfluorinated analogs. Polymer molecular weights can be controlled via hydrogen addition (hydrogenolysis) in these palladium-catalyzed systems (Entry 7 in Table 1), which is unusual for late-transition-metal-catalyzed olefin polymerizations with no catalyst deactivation occurring. Moreover, the branched ultrahigh-molecular-weight polyethylene (UHMWPE) with a very low-molecular-weight distribution (<1.1) was obtained (Entry 8 in Table 1).
Our group synthesized “opening box”-like α-diimine dibromonickel complexes 9 and 10, which combine bulky 8-(p-X-phenyl)naphthylamines (X = OMe, Me, CF3) and bulky dibenzo-/dinaphthobarrelene backbones (Figure 4) [47,48]. The significant steric hindrance between the bulky backbones and the bulky amines required a prolonged high-temperature reflux (~3 days) for the condensation reaction to proceed. In contrast to traditional α-diimine nickel complexes (~48 %VBur) [30], these “sandwich” structures of nickel complexes exhibit significantly more bulky steric hindrance (64.1~67.1 %VBur) around the metal center, which is quantitatively calculated through the space-filling capabilities of the metal center [49]. Consequently, the unique and bulky steric hindrance renders these nickel complexes as the first reported α-diimine nickel dihalide complexes with distorted square planar geometry rather than the conventional distorted tetrahedral geometry [30]. More strikingly, noncovalent Ni-phenyl π-interactions are also found in these nickel complexes and were confirmed through variable-temperature 1H NMR experiments due to the weakly paramagnetic of these nickel complexes with distorted square planar geometry.
Bulky α-diimine nickel catalysts exhibited enhanced activity and thermal stability and produced unexpected lightly branched PEs (32-42/1000C) with melting temperatures of 81–93 °C (Entry 9 and 10 in Table 1) [47]. Moreover, we also first reported efficient direct E/MA copolymerization using these nickel catalysts [48]. The produced lightly branched EMA copolymers with in-chain, pendant, and terminal MA units have a chain structure very similar to industrial EMA, which is not prepared using the previously reported nickel and palladium catalysts (Entry 2 in Table 2). DFT calculations illuminate the crucial role of Ni-phenyl interactions in ethylene polymerization and E/MA copolymerization.
Coates and coworkers also synthesized the corresponding palladium complex 11 (Figure 4) with the same ligand structure [50]. The complex exhibits a slightly distorted square planar geometry, with the axial sites occupied by the 8-tolylnaphthyl groups. This palladium catalyst 11 was able to catalyze the copolymerization of long-chain α-olefins and polar monomers to produce functionalized and semicrystalline polyethylene materials (Entry 2 in Table 3).
Table 1. Representative data for the ethylene polymerizations with α-diimine nickel and palladium catalysts a.
Table 1. Representative data for the ethylene polymerizations with α-diimine nickel and palladium catalysts a.
EntryCatalystT
(°C)		P
(atm)		Time
(h)		Act. bMn
(105 g/mol)		PDIBD
(/1000C)		Tm
(°C)		Ref.
14 (R = Me)2580.516.04.61.39344[30]
24 (R = An)2580.538.710.21.88547[30]
3 c5 (R = Me)258160.044.91.1115- d[31]
4 c5 (R = An)258160.054.81.1115- d[31]
5 e6 (R = F)−3560.170.6 0.11.29128[43]
6 f7 (R = O, x = 2)257.90.527.04.21.619107[44]
7 g8 (ArF = 4-CF3C6F4)2517.6160.11.01.2123- d[46]
8 h8 (ArF = 4-CF3C6F4)2513.60.17225.9631.130104[46]
9 i9 (X = Me)80100.56.31.71.63685[47]
10 i10 (X = Me)80100.55.61.91.83983[47]
11 j12 (R1 = p-Tol)3050.54.84.41.26168- d[51]
a Conditions: 1.6 μmol of Ni, MMAO, Al/Ni = 1000, toluene 200 mL; b Activity in 105 g mol−1 h−1; c 10 μmol of Pd, CH2Cl2 40 mL; d Not determined; e 5 μmol of Ni, Et2AlCl, Al/Ni = 300, toluene 100 mL; f 2 μmol of Ni, MMAO, Al/Ni = 1000, toluene/CH2Cl2 (48/2 mL); g 10 μmol of Pd, CH2Cl2 50 mL, [H2] 0.3 atm; h 0.5 μmol of Ni, Et2AlCl, Al/Ni = 1000, toluene/CH2Cl2 (150/2 mL); i 2 μmol of Ni, Et2AlCl, Al/Ni = 600, toluene/CH2Cl2 (48/2 mL); j 2 μmol of Ni, MAO, Al/Ni = 500, toluene/CH2Cl2 (20/1 mL).
Table 2. Representative data for copolymerization of ethylene and MA with nickel and palladium catalysts.
Table 2. Representative data for copolymerization of ethylene and MA with nickel and palladium catalysts.
EntryCatalystT
(°C)		P
(atm)		Conc.
(mol/L)		Time
(h)		Act. aMn
(104 g/mol)		PDIBD
(/1000C)		X b
(%)		Tm
(°C)		Ref.
1 c5 (R = Me)2565.016.50.60.51.512113.8- d[31]
2 e9 (X = Me)80102.061.20.61.9352.997[48]
3 f12 (R1 = p-Tol)3042.0121.11.01.31203.9- d[51]
4 g15 (R = OMe)4042.0100.60.61.213422.7- d[52]
5 g164042.0100.91.31.412115.3- d[53]
a Activity in 103 g mol−1 h−1; b Incorporation of MA determined by 1H NMR spectroscopy; c Conditions: 10 μmol of Pd, 0.05 mmol galvinoxyl, CH2Cl2 and MA 45 mL; d Not determined; e 5 μmol of Ni, MAO, Al/Ni = 200, toluene/chlorobenzene (37/2 mL); f 10 μmol of Pd, CH2Cl2 and MA 20 mL; g 20 μmol of Pd, CH2Cl2 and MA 20 mL.
Although the 8-arylnaphthyl substituents provide exceptionally effective axial shielding, they may suffer from reduced catalytic activity due to their bulky steric hindrance. The asymmetric substituents approach for the catalyst design may help to solve this problem. Building on this strategy, Dai and coworkers synthesized a series of asymmetric nickel and palladium complexes 12, which simultaneously feature a bulky 8-arylnaphthyl group and a low steric hindrance aliphatic imine moiety (Figure 5) [51,54]. However, the synthesis of ligands containing aliphatic imine moieties is more challenging due to the poor stability of aliphatic imines, which are easy to decompose during purification processes. These palladium complexes were confirmed by 1H and 13C NMR analyses to be a mixture of two isomers with different ratios due to the asymmetry of the ligands. According to the single crystal analysis, both nickel and palladium complexes showed “half sandwich-like” structures. For the “half-sandwich-like” α-diimine nickel and palladium catalysts, the 8-arylnaphthyl substituent is also effective in suppressing chain transfer and facilitating chain walking to obtain high molecular weight PEs and copolymers with highly branched structures (up to 168/1000C) (Entry 11 in Table 1 and Entry 3 in Table 2).
Table 3. Representative data for polymerization of α-olefins with nickel and palladium catalysts a.
Table 3. Representative data for polymerization of α-olefins with nickel and palladium catalysts a.
EntryCatalystMonomerConc.
(mol/L)		Act. bMn
(104 g/mol)		PDIBD
(/1000C)		Tm
(°C)		Ref.
1 c6 (R = CF3)1-decene0.10.53.21.2- d106[40]
2 e111-hexene5.01.00.91.644106[50]
3 f14 (R = Ph)1-decene0.10.21.31.526105[55]
a Conditions: 24 h, 22 °C; b Activity in 103 g mol−1 h−1; c 5 μmol of Ni, MAO, Al/Ni = 200, toluene/chlorobenzene (37/2 mL); d Not determined; e 5 μmol of Pd, CH2Cl2 25 mL; f 10 μmol of Ni, Et2AlCl, Al/Ni = 200, 2 mL CHCl3, total volume 20 mL, 20 °C.

4. Pyridine-Imine Nickel and Palladium Catalysts

Pyridine-imine, as another type of [N,N] ligand, has also found numerous applications in late transition metal catalysts. Compared with the asymmetric “half sandwich-like” α-diimine ligand, the pyridine-imine ligands exhibit superior reactivity, ease of preparation, and enhanced chemical stability. Brookhart and Daugulis first synthesized the “half sandwich-like” pyridine-imine nickel complexes 13 (Figure 6) [56]. The single crystal structures of these nickel complexes show that these nickel complexes with smaller steric hindrances crystallize as centrosymmetric dimers in which each nickel atom is coordinated to a pyridine-imine ligand and two bridging halogen atoms. A terminal halide ion completes the square cone coordination layer. The phenyl substituents on the naphthyl moiety are nearly parallel to the five-membered chelate ring, effectively blocking an axial coordination site in the formation of mononuclear cationic complexes. When MMAO is used as a cocatalyst, these nickel catalysts 13 can catalyze ethylene polymerization and produce higher molecular weight PEs (Mn up to 2.6 × 104 g/mol) compared with pyridine-imine catalysts bearing a single ortho-disubstituted aryl group (Entry 1 in Table 4) [56].
Chen developed a series of pyridine-imine nickel complexes 14 containing both the dibenzhydryl and the 8-arylnaphthyl substituents (Figure 6) [55]. Due to the instability of the nickel complexes, multiple attempts to determine their molecular structure were unsuccessful. However, X-ray diffraction analysis of the ligand revealed that the 8-arylnaphthyl substituents were expected to effectively block the axial position of the metal center. These pyridine-imine nickel catalysts 14 exhibited high thermal stability and were capable of catalyzing ethylene polymerization with high activity to produce ultra-high molecular weight polyethylenes. Additionally, these nickel catalysts also effectively catalyzed the polymerization of α-olefins, yielding polymers with obvious melting temperature (Tm up to 105.5 °C) through significant chain straightening through a combination of 2,1-monomer insertion and precision chain walking (Entry 3 in Table 3).
Furthermore, Dai synthesized a series of pyridine-imine nickel and palladium complexes 15 (Figure 6) with the electronic effect of remote non-conjugated substituents (H, OMe, Me, F) [52]. Due to the asymmetry of the ligands, the palladium complexes in 15 were mixtures of two isomers with different ratios. The aryl rings are positioned directly over the metal center, indicating an effective blockage and shielding the axial positions of the metal center. These nickel catalysts exhibited moderate activities and generated highly branched (57~90/1000 C) PEs with high molecular weights (~105 g/mol) in ethylene polymerization. Compared to classical Brookhart-type α-diimine catalysts, these palladium catalysts showed great advantages in the copolymerization of ethylene with polar monomers and produced EMA copolymers with significantly higher MA incorporation (8.4~10.9 times higher) and higher molecular weights (Entry 4 in Table 2).
The pyridine-imine nickel and palladium catalysts 16 (Figure 7) bearing dibenzosuberyl groups and 8-arylnaphthyl substituents were synthesized [53,57]. The 8-arylnaphthyl substituents and the phenyl rings of the dibenzosuberyl substituents are both nearly parallel to the five-membered coordination plane, effectively blocking the axial coordination sites of the metal complexes and creating a highly congested environment around the metal center (~51 %VBur). These nickel catalysts containing both 8-arylnaphthyl and dibenzosuberyl substituents produced higher molecular weight PEs with higher branching density compared to those derived from “half-sandwich” nickel catalysts containing 8-arylnaphthyl and diarylmethyl groups (Entry 2 in Table 4).
These pyridine-imine palladium catalysts were highly efficient for the copolymerization of ethylene with various polar monomers, such as methyl acrylate (MA) (Entry 5 in Table 2), acrylic acid (AA), n-butyl acrylate (BA), and the fluorinated acrylate 2,2,3,4,4,4-hexafluorobutyl acrylate (6FA), enabling access to high-molecular-weight functionalized polyethylene (Mn up to 32 kg/mol) with high polar monomer incorporation (up to 24 mol%).
A series of pyridine-imine nickel and palladium complexes 17 with a flexible backbone were synthesized by Dai and coworkers (Figure 8) [58]. Although 1H NMR analysis shows that the ligand with a flexible backbone has a certain proportion of enamine and imine interconversion isomers, these isomers are completely converted to imine structures during coordination with metal catalyst precursors. The nickel catalyst generated PEs with high branching densities (76/1000C) and high molecular weights (up to 35.3 kg/mol) (Entry 3 in Table 4). The palladium catalyst generated PEs similar to those of the nickel catalyst. Moreover, highly branched (86~109/1000C) EMA copolymers with high molecular weights (up to 15.4 kg/mol) and high incorporation (up to 17.4 mol%) were achieved.

5. Bis(imino)pyridyl Iron Catalysts

Although the 8-arylnaphthylamine compounds are attractive anilines for the design of late transition metal Ni/Pd olefin polymerization catalysts, “sandwich” bis(imino)pyridyl iron catalysts had never been reported before our recent work [67,68,69], possibly because researchers are worried about their inactivity caused by bulky steric hindrance [70,71].
Our group synthesized a series of bis(imino)pyridyl iron complexes 18 (Figure 9) bearing substituted 8-(p-X-phenyl)naphthylamines (X = OMe, Me, H, CF3) with different electron-donating/withdrawing groups [59]. The capping aryl substituents are positioned above and below and nearly parallel to the pyridyl ring. The buried volumes (%VBur) of 18 vary from 57.0 to 59.5. A buried volume of 59.5 for 18 (X = OMe) represents the largest value of the reported bis(imino)pyridyl iron complexes. Surprisingly, the existence of intramolecular π-π stacking interactions was clearly confirmed by single-crystal X-ray diffraction analysis, UV−vis, and photoluminescence spectra. Despite the bulky nature of our bis(imino)pyridyl iron catalysts, the intra-molecular π-π interactions cause the naphthyl rings to tilt away from the iron center in the horizontal direction. This results in a more open horizontal space within the iron complexes, facilitating ethylene coordination. Thus, ethylene polymerization results show that π-π interactions are a crucial driving force rather than the steric and electronic effects of ligands. Unprecedentedly, bulky “sandwich” bis(imino)pyridyl iron catalyst (X = CF3) produces low-molecular-weight PE with a bimodal distribution (Entry 4 in Table 4). Two chain transfer pathways, including β-H transfer to ethylene monomer and chain transfer to aluminum, have been identified by clean separation of the two fractions. The β-H transfer to the ethylene monomer is the dominant chain transfer pathway in iron-catalyzed ethylene polymerization, which leads to a large proportion of low-molecular-weight unsaturated polyethylene. These unusual ethylene polymerization behaviors are ascribed to weak noncovalent π-π interactions.

6. Salicylaldimine Nickel Catalysts

Brookhart and Daugulis reported the synthesis of neutral salicylaldimine nickel catalysts 19 and 20 (Figure 10) containing 8-arylnaphthyl substituents [60]. Unfortunately, nickel catalyst 19 derived from less sterically hindered amines could not be obtained due to its instability. The nickel catalysts 20 show high stability without decomposition over 2 weeks in a solvent. These nickel catalysts adopt a nearly square planar coordination geometry, and the aryl rings effectively shield the axial sites above and below the square coordination plane. These active neutral nickel single-component catalysts generated lightly branched ultrahigh-molecular-weight polyethylene (UHMWPE) and showed a “quasi-living” polymerization behavior (Entry 5 in Table 4). Guo and Li prepared a series of salicylaldimine ligands with different substituents (X = tBu, Me, H, CF3) and the corresponding neutral salicylaldimine nickel complexes 21 (Figure 10) [61]. These nickel complexes exhibited high catalytic activity for ethylene polymerization (6.88 × 105 g·mol−1·h−1) (Entry 6 in Table 4), producing high molecular weight polyethylenes (up to 8.1 × 105 g/mol).
Mecking synthesized a series of neutral salicylaldimine nickel catalysts 22 (Figure 10) containing 8-arylnaphthyl substituents (X = trifluoromethyl group and longer perfluoroalkyl groups) [62,72]. Unlike the previously performed multistep reaction based on C-H activation, naphthylamine derivatives were synthesized in a one-pot reaction, beginning with a selective, quantitative lithiation reaction, followed by the introduction of a boronic acid group. Suzuki coupling bromobenzene gave naphthylamine products in an 80% yield. These nickel catalysts exhibited enhanced activity and demonstrated living/controlled polymerization when using THF and diethyl ether as reaction media, allowing the production of ultra-high molecular weight polyethylene (UHMWPE) (Entry 7 in Table 4).
Jian and coworkers synthesized a neutral salicylaldimine nickel catalyst 23 (Figure 10) by the combination of 8-arylnaphthyl and dibenzosuberyl groups [63]. Verified by X-ray diffraction analysis, the aryl rings effectively shield the axial sites above and below the square coordination plane. Ethylene polymerization in a living fashion was achieved, enabling the production of virtually linear UHMWPE (Mw = 6020 kg/mol). More importantly, catalyst 23 was able to produce UHMWPE (Mw = 1002 kg/mol) even at elevated temperatures of up to 90 °C and in polar solvents (Entry 8 in Table 4).

7. Nickel and Palladium Catalysts Bearing Other [N,O] Chelate Ligands

Chen and coworkers developed a 2-iminopyridine-N-oxide ligand and the corresponding nickel catalyst 24 (Figure 11) by replacing the phenyl moiety of the salicylaldimine with a pyridyl moiety [64]. The nickel catalyst 24, featuring diarylhydryl and 8-arylnaphthyl substituents, was able to catalyze ethylene polymerization, producing high molecular weight polyethylene with low branch density (Entry 9 in Table 4). In addition, it could also catalyze the copolymerization with methyl 10-undecenoate.
Brookhart and Daugulis prepared a series of neutral β-ketoamine nickel and palladium catalysts 25 (Figure 11) [65]. The nickel complex was isolated as a mixture of two isomers in a 2.3:1 ratio after recrystallization and showed approximately square planar geometry at the nickel center. Efficient blocking of the nickel axial sites was characterized by X-ray crystallography. The arrangement of the aryl caps relative to the metal center is similar to that observed for the palladium complexes. In contrast to the nickel complex, these palladium complexes exist as single isomers in solution, and the nitrile ligand is positioned trans to the nitrogen in the solid state. The nickel catalyst produced lightly branched ultra-high molecular weight polyethylene (Mn up to 4.1 × 106 g/mol) (Entry 10 in Table 4), while the palladium catalysts generated relatively lower molecular weight polyethylene (only ~104 g/mol) with moderate branching (~50/1000C).
Chen synthesized α-imino-ketone nickel catalysts 26 (Figure 12) bearing a combination of 8-arylnaphthyl and diarylmethyl groups [66]. The nickel center exhibited a square planar geometry, and these nickel catalysts were found to be diamagnetic. The sterically open configuration leaves one side of the nickel center fully exposed. These nickel catalysts produced branched ultra-high molecular weight PEs and also facilitated the copolymerization of ethylene with a series of polar comonomers (Entry 11 in Table 4).

8. Conclusions and Outlook

The introduction of 8-arylnaphthylamines in late-transition metal catalysts represents a significant advancement in the field of olefin polymerization. These bulky ligands create “sandwich-like” or “half-sandwich-like” structures around the metal center, leading to increased steric hindrance and influencing both catalyst stability and reactivity. A variety of late transition metal (Ni, Pd, Fe) catalysts bearing 8-arylnaphthyl-based ligands, including α-diimine, pyridine-imine, bis(imino)pyridyl, salicylaldimine, and other [N,O] chelate ligands, have been explored for ethylene (co)polymerizations. Generally, the introduction of 8-arylnaphthylamines shields the axial sites of the metal center, effectively suppressing chain transfer and tuning the chain-walking processes. This allows for the production of high molecular weight polymers with finely tunable branching. However, we also note that the polymerization activity usually decreases because bulky steric hindrance prohibits the coordination of monomers. For these issues, further modification of the 8-arylnaphthylamines combining electronic effect substituents can enhance the performance of the catalyst, particularly in terms of polymerization activity. For example, 8-halonaphthyl and 8-alkylnaphthyl Ni(II)/Pd(II) catalysts developed by Brookhart and Dai exhibit higher ethylene polymerization activity but produce bimodal-distribution PE because of equilibrating syn/anti catalyst diastereomers [73,74]. Based on a flexible axial shielding strategy, pyridine-imine Ni(II) and Pd(II) catalysts developed by Dai can synthesize high-molecular-weight PE and polar functionalized PE [75]. The continued development of novel ligand architectures incorporating 8-arylnaphthylamines holds significant promise for the design of next-generation catalysts in the polyolefin industry. Late transition metal olefin polymerization catalysts derived from 8-arylnaphthylamines would show significant potential for industrial applications, although they are currently employed in industrial production. Besides, copper catalysts featuring various 8-arylnaphthyl substituted α-diimine ligands also show improved performance in C-H functionalization [76,77,78,79,80], also showing great potential in catalytic small-molecule reactions.

Author Contributions

Writing—original draft preparation, Z.Q. and W.W.; writing—review, D.W., X.Z., G.T. and J.Y.; writing—review and editing, H.Z. and H.G.; funding acquisition, H.Z. and H.G. supervision, H.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by National Natural Science Foundation of China (NSFC) (52173016), the State Key Research Development Program of China (Grant No. 2021YFB3800701), Guangdong Basic and Applied Basic Research Foundation (2024A1515012784, 2024A1515011102, and 2023A1515110549), Fundamental Research Funds for the Central Universities, Sun Yat-sen University (24qnpy047), and PetroChina Projects (2022DJ6308, 2021DJ5902, and 2020-CB-02-13).

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 12 00277 g001
Figure 1. α-Diimine nickel and palladium catalysts with different ortho-aryl substituents.
Figure 1. α-Diimine nickel and palladium catalysts with different ortho-aryl substituents.
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Figure 2. Late transition metal catalysts with 8-arylnaphthyl substituents.
Figure 2. Late transition metal catalysts with 8-arylnaphthyl substituents.
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Scheme 1. Synthetic route of 8-arylnaphthylamines (A) and the corresponding 8-(X-phenyl)naphthylamines (B).
Scheme 1. Synthetic route of 8-arylnaphthylamines (A) and the corresponding 8-(X-phenyl)naphthylamines (B).
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Figure 3. “Sandwich-like” α-Diimine nickel complexes with different substituents.
Figure 3. “Sandwich-like” α-Diimine nickel complexes with different substituents.
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Figure 4. “Sandwich-like” α-dimine nickel and palladium complexes with bulky dibenzo-/dinaphthobarrelene backbones.
Figure 4. “Sandwich-like” α-dimine nickel and palladium complexes with bulky dibenzo-/dinaphthobarrelene backbones.
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Figure 5. “Half-sandwich-like” α-diimine nickel and palladium catalysts.
Figure 5. “Half-sandwich-like” α-diimine nickel and palladium catalysts.
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Figure 6. Unsymmetrical pyridine-imine nickel and pallidum catalysts with different substituents.
Figure 6. Unsymmetrical pyridine-imine nickel and pallidum catalysts with different substituents.
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Figure 7. Unsymmetrical pyridine-imine nickel and pallidum catalysts with dibenzosuberyl substituents.
Figure 7. Unsymmetrical pyridine-imine nickel and pallidum catalysts with dibenzosuberyl substituents.
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Figure 8. Pyridine-imine nickel and palladium catalysts with flexible backbone.
Figure 8. Pyridine-imine nickel and palladium catalysts with flexible backbone.
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Figure 9. Bis(imino)pyridyl iron catalysts.
Figure 9. Bis(imino)pyridyl iron catalysts.
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Figure 10. Salicylaldimine nickel catalysts with different substituents.
Figure 10. Salicylaldimine nickel catalysts with different substituents.
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Figure 11. Nickel and palladium catalysts with other [N,O] chelate ligands.
Figure 11. Nickel and palladium catalysts with other [N,O] chelate ligands.
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Figure 12. α-Imino-ketone nickel catalysts.
Figure 12. α-Imino-ketone nickel catalysts.
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Table 4. Representative data for the ethylene polymerizations with iron, nickel, and palladium catalysts.
Table 4. Representative data for the ethylene polymerizations with iron, nickel, and palladium catalysts.
EntryCatalystT
(°C)		P
(atm)		Time
(h)		Act. aMn
(105 g/mol)		PDIBD
(/1000C)		Tm
(°C)		Ref.
1 b13 (R1 = CH3)2527.2112.30.31.84885[56]
2 c163060.53.421.987−4[57]
3 c173060.52.40.41.67354[58]
4 d18 (X = CF3)30100.512.70.0215.1- e124[59]
5 f20 (R = CF3)5040.80.59.512.62.46130[60]
6 g21 (X = tBu)2090.56.95.71.423103[61]
7 h22 (R1 = R2 = CF3)6039.40.580.711.11.23.5128[62]
8 i239039.50.532.56.61.515108[63]
9 j244080.510.08.42.626116[64]
10 k252527.242.7411.219ND[65]
11 l26 (R = tBu)80816.012.32.07063.4[66]
a Activity in 105 g mol−1 h−1; b 5.4 μmol of Ni, MMAO Al/Ni = 1000, toluene 200 mL; c 2 μmol of Ni, Et2AlCl, Al/Ni = 200, toluene/CH2Cl2 (20/1 mL); d 2.4 μmol of Fe, MAO, Al/Ni = 1000, toluene/CH2Cl2 (68/2 mL; e Not determined; f 5 μmol of Ni, toluene 200 mL; g 10 μmol of Ni, 20 μmol of Ni(COD)2, toluene 50 mL; h 5 μmol of Ni, THF 100 mL; i 3 μmol of Ni, 1,4-dioxane 100 mL; j 1 μmol of Ni, MAO, Al/Ni = 80, toluene/CH2Cl2 (18/2 mL); k 2 μmol of Ni, toluene 50 mL, catalyst added as a CH2Cl2 solution; l 1 μmol of Ni, toluene/CH2Cl2 (28/2 mL).
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Qiu, Z.; Wang, W.; Zheng, H.; Wang, D.; Zhao, X.; Tu, G.; Yang, J.; Gao, H. Late Transition Metal Olefin Polymerization Catalysts Derived from 8-Arylnaphthylamines. Inorganics 2024, 12, 277. https://doi.org/10.3390/inorganics12110277

AMA Style

Qiu Z, Wang W, Zheng H, Wang D, Zhao X, Tu G, Yang J, Gao H. Late Transition Metal Olefin Polymerization Catalysts Derived from 8-Arylnaphthylamines. Inorganics. 2024; 12(11):277. https://doi.org/10.3390/inorganics12110277

Chicago/Turabian Style

Qiu, Zonglin, Wenyan Wang, Handou Zheng, Dengfei Wang, Xinglong Zhao, Guangshui Tu, Jiahao Yang, and Haiyang Gao. 2024. "Late Transition Metal Olefin Polymerization Catalysts Derived from 8-Arylnaphthylamines" Inorganics 12, no. 11: 277. https://doi.org/10.3390/inorganics12110277

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