Recent advancements in N-ligated group 4 molecular catalysts for the (co)polymerization of ethylene

Coordination Chemistry Reviews 411 (2020) 213254

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Review

Recent advancements in N-ligated group 4 molecular catalysts for the (co)polymerization of ethylene Shi-Fang Yuan a,b,⇑, Yi Yan a,c, Gregory A. Solan b,d,⇑, Yanping Ma b, Wen-Hua Sun b,e,⇑ a

Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, China Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China c The School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China d Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK e CAS Research/Education Center for Excellence in Molecular Sciences, University of Chinese Academy of Sciences, Beijing 100049, China b

a r t i c l e

i n f o

Article history: Received 3 January 2020 Received in revised form 9 February 2020 Accepted 14 February 2020

Keywords: Group 4 Non-metallocene catalysts Olefin (co)polymerization N-donor ligands Ultra-high molecular weight

a b s t r a c t Group 4 metal (Zr, Ti, Hf) catalysts for olefin polymerization and specifically those based on nonmetallocene complexes have continued to be a subject of intense study in homogeneous catalysis. With a view to forming new or improved polyolefinic materials, complexes bearing N-donor anionic ligands such as b-diketiminate, amidinate, guanidinate, amido, imido as well as mixed N-donor ligands including N,C,C-azaallyl and N,O-phenoxy-imine, have been central to many key developments; high catalytic activities for homo- and copolymerization of ethylene have been a highlight of their catalysis. The fine tuning of these nitrogen-containing ligands significantly controls the catalytic performances of their metal catalysts as well as the structural properties of the resulting polymers with high molecular weight or even ultra-high molecular weight materials accessible. In this review the focus is on more recent publications in the field, in which we correlate the influence of ligand structure with the catalytic performance and microstructure of the polyethylenes. Furthermore, we examine the effects of co-catalyst on activity and thermostability of the precatalyst while efforts directed towards the copolymerization of ethylene with 1-hexene are also summarized. Overall, this work presents an overview of current knowledge pertaining to catalyst design and especially with regard to how the modulation of steric and electronic properties impact on the (co)polymerization process. Ó 2020 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 N-donor-containing transition metal catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. b-Diketiminate and related complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Amidinate and guanidinate complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Amido-, imido- and amine-containing complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4. N,C,C-Azaallyl and related complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5. N,O-Phenoxy-imine and related complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

⇑ Corresponding authors. E-mail addresses: [email protected] (S.-F. Yuan), [email protected] (G.A. Solan), [email protected] (W.-H. Sun). https://doi.org/10.1016/j.ccr.2020.213254 0010-8545/Ó 2020 Elsevier B.V. All rights reserved.

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S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254

Nomenclature Bn Benzyl CGC Constrained geometry catalyst Cp Cyclopentadienyl Cy Cyclohexyl dipp diisopropylphenyl DSC Differential Scanning Calorimetry FI catalyst Bis(phenoxy-imine) group 4 catalyst GC–MS Gas chromatography-mass spectrometry GPC Gel Permeation chromatography g (mol metal)
1h
1 Grams of polyethylene per mole of metal precatalyst per hour HDPE High density polyethylene LLDPE Linear low density polyethylene Mw Weight averaged molecular weight

Mn Number averaged molecular weight Mv Viscosity averaged molecular weight MAO Methylaluminoxane MWD Molecular weight distribution (Mw/Mn) NMR Nuclear magnetic resonance PDI Polydispersity index (Mw/Mn) MMAO Modified methylaluminoxane THF Tetrahydrofuran TMA Trimethylaluminum TOF Catalyst turnover frequency Ph3C+B(C6F5)
4 Trityl tetrakis(pentafluorophenyl)borate UHMWPE Ultra-High Molecular Weight Polyethylene

1. Introduction Olefin polymerization promoted using transition metal catalysis represents one of the foremost processes in the chemical industry and moreover exemplifies one of the major successes of organometallic chemistry [1–6]. For polyethylene alone, its production accounts for more than one third of all the plastic manufactured worldwide, with about 77% of it being produced via transition metal catalyzed reactions [7–12]. Indeed, the wide range in ethylene-based polymers that constitute examples/grades of polyethylene are employed in countless products that benefit our society (e.g., food/specialty packaging, health/medical, pipes/fittings and consumer and durable goods etc.) [13–18]. As a result, a commercial olefin polymerization catalyst not only has to produce polymer products with the desired architecture and properties, it also must exhibit high catalytic activity under the particular polymerization conditions. Generally classical ZieglerNatta catalysts [19] are used to produce most polyolefins, although an increasing proportion is nowadays prepared using molecular catalysts [2–5]. The shift from Ziegler-Natta catalysts to molecular systems is motivated, in large measure, by the desire to manufacture products with superior properties which can be attributed to the better control over polymer microstructure that well-defined catalysts can offer. In addition, these types of catalysts have the ability to produce new products that are challenging or sometimes impossible to manufacture commercially using traditional ZieglerNatta catalysts. With particular regard to group 4 molecular catalysts, metallocenes, half-metallocenes [18,20–34] and ansa-bridged cyclopentadienyl-amido complexes have been at the forefront of catalyst design. In particular the latter class, often referred to as constrained geometry catalysts (CGC), has been thoroughly investigated at both academic and industrial level [18,28–34], and indeed in the case of the titanium derivative reached commercialization (see 1 in Fig. 1) [35,36]. Common to these cyclopentadienylcontaining catalysts is high efficiency and the ability to allow precise control on polymer properties such as tacticity and comonomer incorporation [37–41]. Notwithstanding the exceptional performance characteristics of the aforementioned systems, there has been a drive over the last twenty years or so to develop group 4 catalysts devoid of cyclopentadienyl ligands with a view to achieving not only high catalytic activity but also to produce polyolefins displaying unique properties and in turn broaden the range of end-uses. Of particular note has been the application of supporting ligands that incorporate one or more nitrogen-donor ancillary ligands [42–45]. For example, group 4 complexes bearing amidi-

Fig. 1. Constrained geometry titanium(IV) chloride (CGC) 1 [18,28–36].

nate and bridged bis(amidinates) have proved promising catalysts for not only olefin oligomerization and polymerization, but also hydroamination, intramolecular hydroamination/cyclization as well as hydrosilylation [46–59]. Elsewhere, group 4 compounds bearing silylamido, azaallyl, aminopyridinato and tridentate ligands based on mixed amidinate-amido groups, among others, have also seen some important developments [45,60–78]. In this review we focus on recent advances in non-metallocene N-ligated group 4 (Ti, Zr, Hf) catalysts for olefin (co)polymerization and in particular research reported by our group; for N-donor containing catalysts based on late transition metals the reader is directed elsewhere [79–85]. As a common theme, catalytic activity, thermal stability, molecular weight control and, where possible co-monomer incorporation, will form the basis of the discussion; the effects of ethylene pressure and co-catalyst will also be probed. While the polymerization performance of these complexes forms the main thrust of this work, the fundamental coordination chemistry of the (pre)catalyst will also be highlighted and related to previous studies. 2. N-donor-containing transition metal catalysts To summarize the information, the class of nitrogen-containing ligand is used as a means to divide each section with the performance characteristics of their metal complexes (titanium, zirconium and hafnium) discussed therein. Above all, we focus on anionic N,N-type ligands (e.g. b-diketiminato, amidinate, guanidinate, aminopyridonate) as well as their linked N,N,N and N,N,N,N [e.g. bis(amidinate)] derivatives; neutral N-donor and monodentate anionic examples (e.g. imido, amidine) will also be covered. Furthermore, recent developments using mixed donor ligands such as N,C, N,N,C, N,N,O and N,O-ligands will be described. The influence of substitution pattern of the ligand backbone will be, where possible, correlated with polymer microstructure and polymerization activity.

S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254

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2.1. b-Diketiminate and related complexes Ever since the 1990s, b-diketiminates have been recognized as effective N,N-chelating ligands for group 4 complexes due, in large measure, to their ability to form strong metal–ligand bonds and to their amenability to variation in steric and electronic properties. In 1991 the first b-diketiminato zirconium(IV) complex, 2 (Fig. 2), was synthesized by the insertion of acetonitrile into a 1-azaallylzirconium bond [86]. Subsequently Lappert’s group reported the zirconium(IV) complexes 3 and 4 (Fig. 2) [87–91], which displayed some activity for the polymerization of ethylene and propylene on treatment with methylaluminoxane (MAO). In 2010, our group reported the unsymmetrical bis(bdiketiminato) zirconium(IV) chloride precatalysts 5 and 6 [92] that could be readily prepared be reacting the corresponding lithium bdiketiminate with ZrCl4 (Fig. 3). On activation with MAO, both complexes delivered good activity for ethylene polymerization forming high molecular weight polyethylene when the polymerization runs were performed at 70°C [PC2H4 = 10 atm, Al:Zr molar ratio = 1000:1]. In terms of their relative performance, similar activities were displayed for each [1.09  106 gmol
1h
1 (6) and 1.03  106 gmol
1h
1 (5)]. On the other hand, 5 produced the higher molecular weight polyethylene [3.45  103 gmol
1] with the range in molecular weight distributions relatively narrow (Mw/Mn: 3.8–6.0). By comparison, the molecular weight of the polymer obtained using 6 reached a maximum value of 1.12  103 gmol
1, but with broader molecular weight distributions (Mw/Mn: 5.7–7.2); the presence of multiple active species was suggested as a reason to explain this less controlled polymerization [93,94]. The b-diketiminato M(IV) complexes 7 along with the related 1aza-1,3-butadienyl-imido complex 8 [95] (Fig. 4), both bearing extremely bulky Tbt substituents (Tbt = 2,4,6{(Me3Si)2CH}3C6H2), showed high activities for ethylene polymerization and ethylene/1-hexene copolymerization. Using Al(i-Bu)3/ [PhNHMe2][B(C6F5)4] as activator (temp = 40 °C, ethylene = 0.6 M Pa, Al(i-Bu)3:complex:B-compound = 400:1:3, time = 20 min), titanium-containing 8a showed the highest activity for both ethylene polymerization [7.2  106 gmol
1h
1] and ethylene/1hexene copolymerization [1.1  107 gmol
1h
1]. In addition, 8a generated ultra-high molecular-weight copolymer [Mw = 3.5  106gmol
1] with a fairly narrow molecular weight distribution (Mw/ Mn = 5.5). Elsewhere, complexes 9–13 [96] containing related multidentate ligands were recently reported by Mindiola and coworkers (Fig. 4). However, their catalytic activity for olefin polymerization was not mentioned. The very first report of a b-diketiminato titanium complex, [Ti {(N(Ph)C(Me))2CH}Cl2(THF)2], was actually disclosed in 1998, but only low activity in olefin polymerization was achievable [97– 99]. A structurally related series of heteroleptic 1,3,5-

Fig. 3. Bis(b-diketiminato) zirconium(IV) complexes 5 and 6 [92].

Fig. 4. M(IV) complexes 7–13 containing b-diketiminato and related multidentate N-donor ligands [95,96].

triazapentadienyl titanium(IV) complexes 14–16 [100] were later synthesized and identified (Fig. 5). These complexes contained both mono- and dianionic triazapentadienato N,N-ligands in which the Ti-N imido bond distances ranged from 1.737(2) Å to 1.779(3) Å; such values are shorter than in related titanium imido complexes such as half-titanocene silylquinolyl-based compounds [1.797(5) Å]. For 14/MAO, with the temperature at 70 °C (Al:Ti molar ratio at 1000:1), the maximum activity for the polymerization of ethylene reached a level of 5.66  104 gmol
1h
1. By com-

Fig. 2. b-Diketiminato zirconium(IV) complexes 2–4 [86–91].

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Fig. 5. 1,3,5-Triazapentadienyl titanium(IV) chloride complexes 14–16 [100].

parison, the catalytic activity of 15 attained a level of 4.50  104 gmol
1h
1 under similar conditions [101]. 2.2. Amidinate and guanidinate complexes Amidinate-containing group 4 metal complexes have been widely studied for polymerization applications [102] due to their amenability to electronic and steric tuning of the N-C-N ligand frame and their similarity to the well-studied cyclopentadienyl group. Our team reported the chiral bis(amidinato) zirconium(IV) chloride complexes 17 and 18 [103] derived from the widely utilized (1R,2R)-1,2-diaminocyclohexane precursor (Fig. 6). In 17, two chelating amidinate ligands are present with each containing one N-cyclohexylamine group, while in 18 the N,N-amidinate ligands are linked by a cyclohexyl group. This variation in the binding of the two amidinates resulted in some differences in coordination chemistry displayed by the two complexes. For 17, a distorted octahedral geometry based on the binding of two benzamidinates and two chlorides was evident; chair conformations for the two cyclohexyl rings was a further feature. By contrast, 18 assumed an approximately latitudinal coordinate model with a linear three atom Cl-Zr-OTHF axis. Furthermore, the two N–Zr–N–C chelate rings in 18 were approximately coplanar which was noticeably different to that in 17 (angle between the two mean planes = 80.9°). Variation in the tether linking the amidinates has also been examined. For example, the catalytic properties of the silyllinked bis(amidinato) titanium(IV) complexes 19 and 20 [104] have been investigated for ethylene polymerization (Fig. 7). In the presence of MAO, both 19 and 20 displayed high activities with chloride-containing 19 less active than methylated 20. Moreover, the catalytic activities were significantly affected by the polymerization parameters. At ambient pressure and at 30 °C, only trace amounts of polymer were observed. However, a sharp increase in the activities were noted by increasing the reaction temperature to 50 °C and by raising both the Al:Ti molar ratio and the ethylene pressure. Indeed, the catalytic efficiency of 19 climbed to 4.60  104 gmol
1h
1 at 50 °C and 10 atm ethylene while the corresponding value for 20 was almost doubled. Further increasing the reaction temperature and Al:Ti molar ratio saw their activities continue to rise, with values of 1.02  106 gmol
1h
1 observable at 100 °C for 19 and 1.61  106 gmol
1h
1 at 80 °C for 20. Notably, these activities are much higher than the Cp2ZrCl2 standard of

Fig. 6. Chiral bis(amidinato) zirconium(IV) chloride complexes 17 and 18 [103].

Fig. 7. Silyl-linked bis(amidinato) titanium(IV) chloride and methyl complexes 19 and 20 [104].

6.27  105 gmol
1h
1. Furthermore, it was noted that at higher ratios of MAO enhanced catalytic performance was observed [105]. The silyl-linked amidinate-amidine zirconium(IV) complex 21 [106] was synthesized and screened as a precatalyst for ethylene polymerization (Fig. 8). On activation with MAO and with the ethylene pressure at 10 atm, 21 exhibited an optimal activity of 8.08  105 gmol
1h
1 at 70 °C forming high molecular weight polyethylene (1.27  105 gmol
1) with a molecular weight distribution (Mw/Mn) of 5.76. Furthermore, the polyethylene displayed a melting temperature (Tm) in the range 133–136 °C, characteristic of high-density polyethylene (HDPE) [103,107–110]. The amidine-containing complexes 22–27 [111], each containing at least one N-tBu group, were developed for polymerization applications (Fig. 9). With MAO as the activator, all of these complexes were found to be moderately active for ethylene polymerization. For example, 22 displayed its highest activity of 2.25  104 gmol
1h
1 with the ethylene pressure at 10 atm [temp. = 50 °C; Al:Ti molar ratio = 1500:1]. Under the same conditions, 23–26 were less active, while 27 gave the highest activity of this series with a value of 2.53  105 gmol
1h
1. Both 22 and 27 can nevertheless be regarded as moderately active with the activity data being lower than the Cp2ZrCl2 standard (6.27  105 gmol
1h
1) [112]. The unsymmetrical guanidinato zirconium(IV) chloride complex 28 [113] was synthesized and its structure identified as a chloride-bridged dimer (Fig. 10). By varying the type of cocatalyst, 28 showed good catalytic activity for ethylene polymerization using MAO as activator. With the ethylene pressure at 1 atm, 28/MAO showed only a low activity of 4.48  103 gmol
1h
1 at 20 °C. However, on increasing the ethylene pressure to 10 atm, the activity of 28/MAO reached 4.98  105 gmol
1h
1 with an optimal Al:Zr molar ratio of 2000:1. Meanwhile, as the reaction temperature was raised from 20 °C to 80 °C, the activity initially decreased and then increased reaching a maximum of 6.48  105 gmol
1h
1 at 80 °C. In all cases, the polymers were highly linear as evidenced by the Tm values that fell between 136.7 °C and 134.3 °C. The tris(guanidinato) zirconium(IV) and hafnium(IV) chloride complexes 29 and 30 (R = SiMe3, R0 = Me2N, Fig. 11) as well as 31 and 32 (R = SiMe3, R0 = 1-piperidino, Fig. 11), were screened as catalysts for ethylene polymerization in combination with

Fig. 8. Silyl-linked amidinate-amidine zirconium(IV) chloride complex 21 [106].

S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254

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Fig. 9. Amidine-containing M(IV) complexes 22–27 [111].

Fig. 10. Unsymmetrical guanidinato zirconium(IV) chloride complex 28 [113].

MAO [114]. Under 10 atm of ethylene at room temperature (Al:M molar ratio of 1000:1), the highest activities for 29 and 30 were 1.05  104 gmol
1h
1 and 2.92  104 gmol
1h
1, respectively. On the other hand, 31 and 32 gave values of 1.52  104 gmol
1h
1 and 9.48  103 gmol
1h
1, respectively, in which zirconium 31 displayed a higher activity than its hafnium comparator 32. The most active of this series, 30/MAO, was then selected for a series of polymerizations at a variety of reaction temperatures between 20 °C and 80 °C with the other reaction parameters otherwise unchanged. It was found that its activity decreased from 2.92  104 gmol
1h
1 to 1.15  104 gmol
1h
1 as the temperature of the run was increased. Nonetheless, 30/MAO revealed good activity over a wide range of reaction temperatures with the higher reaction temperature leading to higher molecular weight polyethy-

lene (range: from 2.80  105 gmol
1 to 3.40  105 gmol
1). On the other hand, 29/MAO showed lower activity than the zirconium benzamidinate analogue Zr[j2-N(SiMe3)C(C6H4Me)NPh]3Cl [115]. It is likely that this finding can be attributed to the better donor properties of dimethylamido group (29) over its phenylcontaining analogue resulting in increased electron density on the zirconium and in-turn decreased catalytic activity. With MAO as the activator and under a range of conditions, the novel unsymmetrical aliphatic tris(benzamidinato) zirconium(IV) chloride complex 33 [116] was tested as a precatalyst for ethylene polymerization (Fig. 12). At a Al:Zr molar ratio of 1000:1, 33/MAO proved inactive at 30 °C with the ethylene pressure at 10 atm, but as the temperature was raised to 70 °C, a reasonable level of 1.87  103 gmol
1h
1 was observed. With an increase of the Al: Zr molar ratio to 2000:1, 33/MAO resulted in only a slightly higher activity of 4.13  103 gmol
1h
1 at 50 °C. However, on increasing the reaction temperature to 70 °C, 33/MAO gave the highest activity of 5.44  103 gmol
1h
1. Overall, 33/MAO was considered to display moderate activity which was little affected by the reaction temperature and the Al:Zr molar ratio. The bis(amidinato) zirconium(IV) chloride complex 34 [117] revealed a distorted pseudo-octahedral structure incorporating a C2 symmetry element (Fig. 13). With MAO activation and at 50 °C, 34 showed moderate activity for ethylene polymerization. With the temperature kept at 50 °C, the Al:Zr molar ratio was

Fig. 11. Tris(guanidinato) zirconium(IV) and hafnium(IV) chloride complexes 29–32 [114].

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S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254

ratio of 1000:1, 39/MAO showed its highest activity of 1.85  106 gmol
1h
1. The molecular weights of the resulting polymers were <2.0  105 gmol
1, while the molecular weight distributions were fairly broad (Mw/Mn: 3.3–8.1). 2.3. Amido-, imido- and amine-containing complexes

Fig. 12. Unsymmetrical aliphatic tris(benzamidinato) zirconium(IV) chloride complex 33 [116].

varied from 500:1 to 3000:1 resulting in 34/MAO achieving its highest activity of 6.66  104 gmol
1h
1 with a molar ratio of 2500:1. However, further increasing the reaction temperature reduced the catalytic activity. In comparison to previously reported six-coordinate bis(amidinato) zirconium(IV) chloride complexes [118] and analogous metallocene systems [119], 34/MAO displayed lower activity. The 1,3,5-triazapentadienyl zirconium(IV) and titanium(IV) complexes 35 and 36 along with the titanocene guanidinate complexes 37 and 38 [120] have been structurally characterized and have been the subject of a catalytic study (Fig. 14). The structure of 35 revealed that three 1,3,5-triazapentadienyl ligands coordinated to the zirconium center as N,N-bidentate ligands resulting in a propeller-like arrangement. In complexes 36 and 37, the ligands also adopted N,N-chelation modes but in addition bridge to a second titanium center. By contrast in 38, the guanidinate assumed an unusual bonding mode by acting as an N-bound monodentate ligand leading to the pentahapto Cp-Ti complex adopting a piano-stool type geometry. Under 10 atm pressure of ethylene, 35/MAO gave a reasonable activity of 1.02  104 gmol
1h
1 [Al: Zr molar ratio of 2500:1] at 70 °C. In comparison with other analogues of the L3ZrCl type [114,115], the catalytic activity of 35/ MAO was slightly higher. For the guanidinate-containing 37/ MAO, the optimal activity of 1.52  106 gmol
1h
1 was evident at 50 °C and an Al:Ti molar ratio at 2000:1. On increasing the Al: Ti molar ratio from 500:1 to 2500:1, the molecular weight of the resulting polymer reached a maximum (8.78  105 gmol
1) with the Al:Ti molar ratio at 2000:1; the Tm of the resulting polyethylenes reduced from 133.1 °C to 130.5 °C. In addition, by elevating the reaction temperature from 30 °C to 70 °C, the activity showed an initial increase before some loss in activity was evident. The dizirconium(IV) complex 39 [121] incorporating a bridging and chelating N-acylamidine ligand was reported by Rojas and coworkers (Fig. 15). At a reaction temperature of 72 °C and an Al:Zr

Fig. 13. Bis(amidinato) zirconium(IV) chloride complex 34 [117].

Our group has synthesized and characterized a wide range of silyl-linked anilido-amine zirconium and hafnium precatalysts, 40–44 (Fig. 16) [122]. On activation with MAO and with the ethylene pressure at 10 atm, zirconium complexes 40, 41, 42 and 44 revealed good activity for ethylene polymerization (up to 1.73  105 gmol
1h
1) and formed polyethylenes with a range in molecular weight distributions. For example, 40 bearing two N,N-ligands generated polymers with comparatively narrow molecular weight distributions (Mw/Mn: 4.2–4.9) due to the likely improved stability of its active species, while 41 and 42 containing one N,N-ligand provided high activity and generated polymers with high molecular weights but with broader molecular weight distributions (Mw/Mn: 8.7–56). Dimeric 44 showed evidence for multiple active species by producing polymer with noticeably broader dispersities (Mw/Mn: 11–65). In the ethylene and 1hexene copolymerization evaluation, 44/MAO showed evidence for about 3 mol% incorporation with butyl-branched polyethylene identifiable in the 13C NMR spectrum, findings that warrant further investigation. More importantly, 44 indicated excellent thermal stability at an industrially relevant 70 °C. Similarly, hafnium 43/ MAO also revealed a reasonable activity of 2.40  104 gmol
1h
1 at 70 °C, while 43 exhibited lower activity than its zirconium analogue 42. In general, the polymers possessed melting temperatures in the range 133–136 °C, typical for a highly linear polyethylene. The zirconium(IV)-lithium chloride complex 45 incorporating a tridentate aminopyridinato-amine and dinuclear titanium(IV) chloride 46 bearing a bidentate and bridging aminopyridinate, have been investigated as part of an ethylene polymerization study (Fig. 17) [123]. In the presence of MAO, 45 was evaluated with the reaction temperature varied from 50 °C to 90 °C. The highest catalytic activity of 1.24  105 gmol
1h
1 was obtained at 70 °C. On increasing the molar ratio of Al:Zr from 1000:1 to 2500:1, with the temperature kept at 70 °C, an increase in the polymerization activity from 6.80  104 gmol
1h
1 to 1.52  105 gmol
1h
1 was observed. However, a further increase of the Al:Zr molar ratio to 3500:1 led to a reduction in the catalytic activity. On raising the Al:Ti molar ratio from 1000:1 to 3000:1 at 60 °C, 46/MAO showed its highest activity of 1.24  105 gmol
1h
1 at a Al:Ti molar ratio of 2000:1. Without an in-depth consideration of the polymerization conditions and types of co-catalyst, the polymerization activities of 45 and 46 were equivalent or higher than that of less sterically crowded aminopyridinate zirconium [124,125] or titanium [126–129] complexes, but lower than that of the more sterically demanding zirconium [124,130] or titanium [126,131] comparators. In comparison to previously reported aminopyridinate zirconium [124] or titanium [126–128,131] complexes, 45 and 46 afforded polyethylenes with much higher molecular weights, the exception being that observed using more bulky electron-rich amino-pyridinate titanium complexes [129]. In addition, both 45/MAO and 46/MAO gave polyethylenes with broad molecular weight distributions (Mw/Mn: 10.4–55.3). All the melting temperatures recorded for the resulting polyethylenes fell within relatively small ranges and were higher than 132 °C, consistent with the formation of HDPE [105]. The tris(aminopyridinato) zirconium(IV) chloride complex 47 [132] has been structurally characterized revealing a sevencoordinate zirconium center surrounded by a chloride and three strained N,N-coordinated aminopyridinato ligands that ordered themselves in a propeller-like configuration (Fig. 18). At 50 °C

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Fig. 14. Guanidinato zirconium(IV) and titanium(IV) complexes 35–38 [120].

Fig. 15. N-acylamidine-coordinated zirconium(IV) complex 39 [121].

and the ethylene pressure at 10 atm, 47/MAO reached a maximum activity of 1.47  105 gmol
1h
1 with an Al:Zr molar ratio of 2000:1. Fontaine’s group developed a series of amidoquinolinecontaining group 4 trialkyl complexes, namely hafnium 48, zirconium 49 and titanium 50 (Fig. 19) [133]. Using [HNMe(C18H37)2] [B(C6F5)4] as the activator for ethylene/1-octene copolymerization with a 2:1 M ratio of 1-octene to ethylene at 140 °C, 48 exhibited excellent activity 4.67  107 gmol
1h
1 and produced high molecular weight copolymers (up to 4.61  105 gmol
1). The arylimido zirconium-lithium complex 51 and arylimido titanium complex 52 were synthesized and characterized and showed some differences in nuclearity, coordination and geometry (Fig. 20) [134,135]. Notably in 51, each zirconium center in the dimer was six-coordinate while the single titanium center in 52 was five-coordinate. Furthermore, 52 indicated crystallographic mirror symmetry in which the mirror plane contained the central Ti=N vector. Both complexes displayed good activities for ethylene

polymerization when activated with MAO. With the Al:M molar ratio fixed at 1200:1, the highest activity for 51 of 1.09  106 gmol
1h
1 was observed at 70 °C. By comparison, the highest activity of 52 at the same temperature was 7.20  105 gmol
1h
1. The polyethylene products possessed melting temperatures in the range of 133–134 °C, supportive of a linear backbone for the polymer. Furthermore, the polymer exhibited high molecular weight (9.76  105 gmol
1) with a molecular weight distribution (Mw/ Mn) of 3.8. The g2-hydrazonide zirconium(IV) chloride complexes 53 and 54 [136] adopted distorted tetragonal bipyramidal geometries, with the three chlorides and an oxygen atom from the tetrahydrofuran forming the equatorial belt and the axial coordination sites by a second tetrahydrofuran as well as the g2-coordinated 1-(fur an-2-ylmethylene)-2-phenylhydrazonide (Fig. 21). Upon activation with MAO, both 53 and 54 delivered low catalytic activities for ethylene polymerization. With the Al:Zr molar ratio fixed at 1000:1 and by varying the reaction temperature from 20 °C to 50 °C, the highest activity of 7.6  104 gmol
1h
1 was obtained using 53 at 40 °C. With the temperature kept at 40 °C, 54 achieved an activity of 4.0  104 gmol
1h
1 at PC2H4 = 10 atm and an Al:Zr molar ratio of 1000:1. Interestingly, the GPC data showed that the polyethylene displayed a molecular weight (Mw) of over 1.0  106 gmol
1 which falls within the ultra-high molecular weight window. Furthermore, the resulting polyethylenes possessed melting temperatures in the range of 133.5–140.5 °C in accordance with the formation of HDPE [137].

2.4. N,C,C-Azaallyl and related complexes Constrained geometry catalysts (CGCs, Fig. 1) [28,138–143] are capable of producing a wide variety of copolymers such as ethylene-propylene and ethylene-1-octene copolymers and exhibiting very high activities at elevated reactor temperatures.

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Fig. 16. Anilido-amine zirconium(IV) and hafnium(IV) chloride complexes 40–44 [122].

Fig. 17. Aminopyridinato-amine-zirconium(IV) 45 and aminopyridinato-titanium (IV) 46 [123].

Fig. 19. Amidoquinoline M(IV) trialkyl complexes 48–50 [133].

Fig. 18. Tris(aminopyridinato) zirconium(IV) chloride complex 47 [132].

Importantly, the high reactivity toward a-olefins of CGCs has surpassed classical Ziegler–Natta catalysts. In an attempt, to develop alternatives to the g5-g1-CGC-type catalysts [35,144–147], we have been interested in replacing the monoanionic g5cyclopentadienyl unit with a monoanionic g3-azaallyl unit and the g1-amide with an g1-amine. Hence, the two azaallyl zirconium compounds 55 and 56 [144,145], differing in their coordination modes (Fig. 22), were prepared by reacting the lithium salt of the ligand with either half or one equivalent of zirconium(IV) chloride, respectively. Bis(azaallyl) 55 and azaallylamine 56, on activation with MAO, were both employed as catalysts for ethylene polymerization and copolymerization of ethylene with 1-hexene, affording polymers with high molecular weights. In addition, these polymers

showed broad distributions in their GPC traces and a single Tm value in their DSC chromatograms. At 10 atm of ethylene pressure, with the reaction temperature varied between 30 and 70 °C (with an Al:Zr molar ratio of 1000:1), the catalytic activity of 55 exhibited a peak value of 3.65  106 gmol
1h
1 at 70 °C. Notably, the resulting polyethylene was of ultra-high molecular weight (6.58  106 gmol
1). More significantly, 56 revealed one order of magnitude higher in activity than 55, which was attributed the g3:g1-binding mode exhibited by the azaallyl-amine. Moreover, in the copolymerization of ethylene and 1-hexene, 56/MAO showed better catalytic activity than 55/MAO and higher incorporation of 1-hexene, which is indicated by the lower melting temperature of the polymer (126–133 °C) when compared with that of linear polyethylene. Indeed, a maximum in activity of 3.25  106 gmol
1h
1 was obtained at an industrially relevant operating temperature of 70 °C. The resulting polymer, nevertheless, revealed a broad molecular weight distribution (Mw/ Mn = 7.6) while the values of Mw for the copolymers were again high at around 2.77  106 gmol
1. The 13C NMR spectroscopic analysis of the resulting poly(ethylene-co-1-hexene) exhibited a 13 C NMR spectrum consistent with linear low density polyethylene (LLDPE) [36,147–151]. In particular, the 1-hexene incorporation achieved a value of 1.92 mol% when employing 55/MAO and 3.08 mol% with 56/MAO. The oxo-bridged azaallyl-amine zirconium(IV) chloride complexes 57 and 58 [152,153] were screened as precatalysts for ethylene polymerization (Fig. 23). With the Al:Zr molar ratio of MAO to precatalyst fixed at 1000:1 and the ethylene pressure at 10 atm,

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9

Fig. 20. Arylimido zirconium and titanium complexes 51 and 52 [134].

Fig. 21. g2-Hydrazonide zirconium(IV) chloride complexes 53 and 54 [136].

the highest activity achievable using 57 was 6.41  105 gmol
1h
1 at 70 °C. The resulting polyethylene showed melting temperatures of between 133 and 136 °C while the molecular weight was high and the dispersity broad (9.58  105 gmol
1, Mw/Mn = 13.6). By comparison, 58/MAO was more active at 70 °C (7.23  105 gmol
1h
1) with the polyethylene again possessing high molecular weight with a broad unimodal molecular weight distribution (7.96  105 gmol
1, Mw/Mn = 15.4). However, as the observed activities for 57 and 58 were lower than their reported homologues [36], it was considered that the presence of the oxo-bridge had a detrimental effect on catalytic activity. Overall, this finding highlights the importance of preventing air and moisture in such catalysis. The zirconium(IV) complexes 59–61 [154] containing either N, N-benzamidinate (59, 60) or N-benzamidine (61) units that were each tethered by ethyl amide or ethyl amine donors were synthesized and characterized (Fig. 24). In 59, two tridentate dianionic nitrogen donor ligands coordinate to the metal center to complete distorted octahedral geometries. By contrast, in 60 and 61, bimetallic (ZrLi 60, Zr2 61) arrangements exist in which one monoanionic multidentate nitrogen donor is bound to each zirco-

Fig. 23. Oxo-bridged azaallyl-amine zirconium(IV) complexes 57 and 58 [152].

nium center along with three chloride ligands. The catalytic behavior of complexes 59–61 were screened with MAO as co-catalyst. Precatalysts 60 and 61, incorporating monoanionic N,N,N and N, N-ligands, showed catalytic activities of 4.86  105 gmol
1h
1 and 3.32  105 gmol
1h
1, respectively. On the other hand, the dianionic bis(N,N,N) precatalyst 59 was less active (6.96  104 gmol
1h
1). In terms of the polyethylene, 59 generated material with a molecular weight of 1.35  105 gmol
1 and with a relatively narrow polydispersity (Mw/Mn = 2.14). By comparison, 60

Fig. 22. Azaallyl zirconium(IV) chloride complexes 55 and 56 [144,145].

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Fig. 25. Anilido-imine metal(IV) complexes 65–67 [170].

Fig. 24. Tethered [154,155].

benzamidinate/benzamidine-metal(IV)

complexes

59–64

gave polyethylene with the highest molecular weight of 1.42  105 gmol
1 but with a broader molecular weight distribution (Mw/ Mn = 8.47). Meanwhile, 61 produced lower molecular weight polymer (1.24  105 gmol
1) and an even broader molecular weight distribution (Mw/Mn = 10.96) than that seen using 60. All the melting temperatures of the polyethylenes were within the range 131.8–136.2 °C as is typical of a HDPE. More importantly, this class of zirconium precatalysts showed not only excellent thermal stability at 70 °C but also formed higher molecular weight polymer. Zirconium(IV) complex 63, the bis(tridentate) analogue of 59 differing in the length of the linker to the phenylamide N-donor, was also reported as were the titanium 62 and hafnium 64 counterparts [155]. All three complexes were investigated for ethylene polymerization with titanium-containing 62/MAO showing good activity (up to 1.78  105 gmol
1h
1) with an Al:Ti molar ratio of 2500:1 and a temperature of 30 °C. Moreover, high molecular weight to ultra-high molecular weight polyethylenes (up to 1.07  106 gmol
1) with broad molecular weight distributions (Mw/Mn = 6.21–16.3) were obtained; the melting temperatures of the polyethylene (132.4–134.9 °C) were in line with highly linear materials. More importantly, 62 exhibited excellent thermal stability at an industrially relevant 80 °C. By comparison, the hafnium catalyst 64 generally showed lower polymerization activity [156– 166] than the corresponding titanium and zirconium counterparts. This finding was attributed to either an increased barrier to monomer insertion with hafnium [167] or to less efficient activation by MAO [168,169]. Mu and coworkers disclosed a series of titanium, zirconium and hafnium complexes 65–67 [170], bearing dianionic tetradentate anilido-imine ligands (Fig. 25). In the presence of MAO and/or AliBu3/Ph3C+B(C6F5)
4 , these complexes showed moderate catalytic activities and good thermostability for ethylene polymerization. Moreover, they generated high molecular weight to ultra-high molecular weight polyethylenes (up to 4.29  106 gmol
1). By comparison to the zirconium and hafnium analogues, the titanium complexes exhibited higher catalytic activities.

2.5. N,O-Phenoxy-imine and related complexes Bis(phenoxy-imine) early transition metal complexes (Fig. 26), developed by Fujita and co-workers [171–174], have afforded unprecedented catalytic activities for the polymerization of ethylene. The research based on a ‘‘ligand oriented catalyst design”

[171,173–176] has seen the discovery of not only FI catalysts but also a number of other highly active group 4 metal precatalysts for the polymerization of ethylene. These have included, but not limited to, those based on N,N-indolide-imine (II, Fig. 26) [177], N,N-pyrrolide-imine (PI, Fig. 26) [178–182], N,O-imine-phenoxy (IF, Fig. 26) [183] and O,O-phenoxy-ether (FE, Fig. 26) [184]. When compared to group 4 metallocene catalysts under analogous conditions, MAO-activated FI catalysts have shown remarkably higher activities for ethylene polymerization [176]. For example, zirconium-containing 68 achieved a strikingly high value of 6.55  109 gmol
1h
1 at one bar ethylene pressure and at 25 °C (Fig. 27). This activity corresponded to a catalyst turnover frequency (TOF) of 64900 s
1atm
1, which is two orders of magnitude greater than that observed with metallocene catalysts [171,173,185–188]. Furthermore, 69 exhibited significant activity at 0 °C [1.93  108 gmol
1h
1] which far exceeds that obtained using Cp2ZrCl2 under similar conditions (Fig. 24), while at 25 °C its activity was up to 5.19  108 gmol
1h
1 and 5.87  108 gmol
1h
1 at 40 °C [189]. The bis(indolide-imine) titanium-containing 70a–70d (II, Fig. 28), on treatment with MAO, afforded highly linear polyethylene with moderate to high molecular weights (Mw = 1.38–32.30  104 gmol
1) [177]. It was found that an increase in the number of fluorine substituents appended to the N-aryl unit resulted in increased catalytic activity. Moreover, these were rare examples of catalysts that were able to promote living ethylene polymerization at lower temperatures [190,191]. For example, 70d/MAO displayed a catalytic activity of 1.94  106 gmol
1h
1 at
10 °C, which was higher than that observed at 25 °C (1.14  106 gmol
1h
1); notably the resulting polyethylenes formed at the lower temperature showed extremely narrow molecular weight distributions (Mw/Mn = 1.12–1.25). Evidently, higher temperature deactivated the catalytic activity of 70d/MAO. The bis(pyrrolide-imine) family of early transition metal complexes (PI, Fig. 26) have been reported to give very high molecular weight polymers in ethylene polymerization [178–180]. On activation with MAO and at ambient temperature, 71a–71f [181,182] showed very high levels of activity and afforded highly linear polyethylenes with molecular weights (Mv) as high as 3.17  106 gmol
1. In particular, 71f/MAO achieved a catalytic activity of 3.32  107 gmol
1h
1, which surpassed the optimal activities seen for group 4 metallocenes (Cp2MCl2: M = Ti, 1.67  107 gmol
1h
1; M = Zr 2.00  107 gmol
1h
1). On activation with i-Bu3Al/Ph3CB(C6F5)4, 71a–71f converted ethylene into ultra-high molecular weight polyethylenes (Mv = 4.40  107 gmol
1) with the high catalytic activity maintained (>1.5  106 gmol
1h
1). In addition, the zirconium–PI catalyst 72a and hafnium–PI catalyst 72b afforded superior activities of 2.29  107 gmol
1h
1 and 2.10  106 gmol
1h
1, respectively, when activated with MAO at 25 °C (Fig. 29) [192]. Bis(imine-phenoxy) early transition metal complexes (IF catalysts), in particular those based on titanium as the metal center, have shown high activities in ethylene polymerization. For example titanium-containing 73b, on treatment with MAO at 25 °C,

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Fig. 26. N,O-containing FI precatalysts along with their II, PI, IF and FE comparators [171–184].

Fig. 30. Titanium and zirconium IF precatalysts 73 [183].

Fig. 27. N,O-containing zirconium FI catalysts 68 and 69 [176].

Fig. 31. Titanium-containing FE catalysts 74 [184].

Fig. 28. Titanium-containing II precatalysts 70 [177].

exhibited an activity of 1.99  107 gmol
1h
1, a value comparable or better to those seen with Cp2MCl2/MAO (M = Ti; 1.67  107 gmol
1h
1, M = Zr; 2.00  107 gmol
1h
1) (Fig. 30). In this system, the electron-withdrawing C6F5 N-aryl group was considered as the cause for higher activity. On the other hand, in the presence of i-Bu3Al/Ph3CB(C6F5)4, 73a exhibited higher activity when the temperature was raised from 25 °C to 75 °C reaching a peak value of 1.87  107 gmol
1h
1 at 75 °C over a 60 min run time. By contrast, zirconium analogues 73c and 73d, on activation with MAO or i-Bu3Al/Ph3CB(C6F5)4, displayed inferior activity for ethylene polymerization (Fig. 30) [183]. Bis(phenoxy-ether) titanium complexes (FE catalysts) have allowed the transformation of ethylene to highly linear polyethylenes displaying exceptionally high molecular weights. In the case of the titanium examples 74a and 74b (Fig. 31), 74a/MAO afforded polyethylene with ultra-high molecular weight (Mv = 5.42  106

Fig. 29. Titanium, zirconium and hafnium PI precatalysts 71 [181,182] and 72 [192].

gmol
1) while 74b/i-Bu3Al/Ph3CB(C6F5)4 gave extremely high activity (3.47  106 gmol
1h
1) at 25 °C [184]. Development work around the FI catalysts has seen some further disclosures [193]. For example, Marks’ team reported a series of phenoxy-imine zirconium(IV) dimethylamido precatalysts [194] for the copolymerization ethylene with 1-octene. On treatment with trimethylaluminum (TMA), 75 was found to show very high ethylene polymerization activity (2.85  106 gmol
1h
1) and a relatively high 1-octene co-enchainment selectivity (up to 7.2 mol%). Meanwhile 76a gave a higher activity of 7.10  106 gmol
1h
1 but the 1-octene co-enchainment selectivity (1.5 mol %) was lower. Our group has also synthesized and characterized zirconium complexes 77 and 78 [195] which upon activation with MAO, revealed moderate catalytic activities and good thermal stability for ethylene polymerization (Fig. 32). Chan and coworkers developed a series of tridentate O,N,Cligand frameworks for titanium(IV) benzyl complexes 79–81 (Fig. 33) [196], in which the phenoxy and pyridine donors are linked by a methylene group; the fluorinated substitution pattern of the r-aryl was the focus of the study. All complexes adopted C1 symmetry in solution. In combination with dried MAO, complexes 79–81 were investigated as ethylene polymerization catalysts at 7 atm of ethylene pressure; the zirconium derivatives were also screened but these were less active. Complex 79 gave the highest activity of the series (6.80  105 gmol
1h
1) at a reaction temperature of 75 °C, while the polymer displayed a molecular weight of 3.40  105 gmol
1 and a narrow molecular weight distribution (Mw/Mn = 2.6). Significantly, using 80/MAO at 50 °C good activity was also observed with the polymer displaying exceptionally high molecular-weight (up to Mw = 1.34  106 gmol
1) and a narrow molecular weight distribution (Mw/Mn = 2.4).

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Fig. 32. Phenoxy-amine zirconium(IV) complexes 75–78 [194,195].

Fig. 33. O,N,C-Titanium(IV) benzyl complexes 79–81 [196].

Solan et al. reported the related N,N,O-titanium precatalysts as their chloride (82) and fluoride derivatives (83) (Fig. 34). On activation with MAO (at PC2H4 = 1 atm and room temperature), the phenyl-substituted 82b proved the most active catalyst of the study. Notably 83b/MAO represented the most active nonmetallocene metal-fluoride precatalyst reported to date and produced ultra-high molecular weight polyethylene albeit with a very broad molecular weight distribution (Mw/Mn = 25–48) [197,198]. Magna and coworkers reported a series of tridentate aryloxybased titanium complexes 84–87 (Fig. 35) [199]. When activated with MAO for ethylene oligomerization and polymerization, these precatalysts exhibited very low activities (~103 gmol
1h
1) with preferential formation of polyethylene which suggested the

Fig. 34. O,N,N-Titanium(IV) halide complexes 82 and 83 [197,198].

Fig. 35. Tridentate aryloxy- and pyrrolide-based titanium(IV) chloride complexes 84–88 [199,200].

existence of several catalytic centers in the reaction mixture. Furthermore, Casagrande Jr group developed a series of tridentate PI zirconium complexes 88a–88d (Fig. 35) [200]. Upon activation with MAO, all precatalysts showed high thermal stability and high activities (up to 4.53  105 gmol
1h
1 at 100 °C in ethylene polymerization. 3. Conclusions and outlook The pioneering reports by Lappert and co-workers in the 1960’s of complexes bearing b-diketiminate ligands have, over the course of time, proved an important signpost in the development of nonmetallocene transition metal catalysts for the (co)polymerization of ethylene and other olefins. With the prospect of forming new polyolefinic materials with advanced properties, the integration of such N-donor units and others into a cyclopentadienyl-free ligand framework of a group 4 (Ti, Zr, Hf) metal catalyst, has remained a challenging research area with great potential. In this review, we have documented more recent disclosures pertaining to N-based ligand sets such as amidinate, guanidinate, amido, imido, N,C,C-azaallyl, N,O-phenoxy-imine and so on. The fine tun-

S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254

ing of the electronic and steric characteristics of these ligands and their influence on the effectiveness of the group 4 catalyst has presented a valuable means of affecting catalytic performance and modulating the resulting polymer properties. In addition, a rich coordination chemistry has emerged through structural studies of their complexes. By imaginative design, these catalysts have not only achieved high activities towards (co)polymerization of ethylene (in some cases rivalling or exceeding metallocenes) but have also addressed, to some degree, the demand for more thermally stable catalysts. Furthermore, their effects on polymer properties such as co-monomer incorporation, polydispersity and molecular weight control (from low molecular weight to ultrahigh molecular weight) have further underlined their controllability and tunability. While the N,O-containing FI catalysts have been commercialized by Mitsui Chemicals, it would seem likely that other group 4 metal catalysts based on N-based ligands will emerge in the industrial arena. We hope the research developments gathered in this work will both guide further advances and provide alternative catalysts for industrial consideration. Moreover, it is hoped that the content of this review will provide a useful source for industrial scientists with an interest in scaling-up molecular catalysts according to industrial engineering requirements. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 21871275). GAS thanks the Chinese Academy of Sciences for a President’s International Fellowship for Visiting Scientists. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccr.2020.213254. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

C. Redshaw, Y. Tang, Chem. Soc. Rev. 41 (2012) 4484. J.P. McInnis, M. Delferro, T.J. Marks, Acc. Chem. Res. 47 (2014) 2545. H.Y. Suo, G.A. Solan, Y.P. Ma, W.-H. Sun, Coord. Chem. Rev. 372 (2018) 101. Z. Wang, G.A. Solan, W. Zhang, W.-H. Sun, Coord. Chem. Rev. 363 (2018) 92. Z. Wang, Q. Liu, G.A. Solan, W.-H. Sun, Coord. Chem. Rev. 350 (2017) 68. S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000) 1169. H. Makio, H. Terao, A. Iwashita, T. Fujita, Chem. Rev. 111 (2011) 2363. K. Nomura, W. Zhang, Chem. Rev. 111 (2011) 2342. J.Q. Wu, Y.S. Li, Coord. Chem. Rev. 255 (2011) 2303. Z. Jonas, Coord. Chem. Rev. 254 (2010) 2227. W. Huang, W.-H. Sun, C. Redshaw, Dalton Trans. 40 (2011) 6802. W. Huang, W. Zhang, S. Liu, T. Liang, W.-H. Sun, J. Polym. Sci., Part A: Polym. Chem. 49 (2011) 1887. S.F. Yuan, L.J. Wang, Q.Y. Zhang, W.-H. Sun, Prog. Chem. 29 (2017) 1462. S.F. Yuan, C.X. Niu, X.H. Wei, W.-H. Sun, Prog. Chem. 28 (2016) 1070. W. Huang, W.J. Zhang, W.-H. Sun, Chin. Polym. Bull. 52 (2011) 3732. W.W. Zuo, W.-H. Sun, Chin. Polym. Bull. 04 (2009) 1. V.C. Gibson, C. Redshaw, G.A. Solan, Chem. Rev. 107 (2007) 1745. V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2003) 283. H.H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, R.M. Waymouth, Angew. Chem., Int. Ed. Engl. 34 (1995) 1143. K. Ziegler, H.G. Gellert, K. Zosel, W. Lehmkuhl, W. Pfohl, Angew. Chem. 67 (1955) 424. K. Ziegler, E. Holzkamp, H. Breil, H. Martin, Angew. Chem. 67 (1955) 541. G. Natta, J. Polym. Sci. 16 (1955) 143. G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, G. Moraglio, J. Am. Chem. Soc. 77 (1955) 1708.

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[24] E. Groppo, C. Lamberti, S. Bordiga, G. Spoto, A. Zecchina, Chem. Rev. 105 (2005) 115. [25] H. Sinn, W. Kaminsky, H.J. Vollmer, R. Woldt, Angew. Chem. Int. Ed. Engl. 19 (1980) 390. [26] W. Kaminsky, Macromol. Chem. Phys. 197 (1996) 3907. [27] H.G. Alt, A. KVoppl, Chem. Rev. 100 (2000) 1205. [28] A.L. McKnight, R.M. Waymouth, Chem. Rev. 98 (1998) 2587. [29] G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. 111 (1999) 448. [30] G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem., Int. Ed. Engl. 38 (1999) 428. [31] Y. Imanishi, N. Naga, Prog. Polym. Sci. 26 (2001) 1147. [32] S. Arndt, J. Okuda, Chem. Rev. 102 (2002) 1953. [33] J. Okuda, Dalton Trans. (2003) 2367. [34] Y. Qian, J. Huang, M.D. Bala, B. Lian, H. Zhang, Chem. Rev. 103 (2003) 2633. [35] P.J. Shapiro, E. Bunel, W.P. Schaefer, J.E. Bercaw, Organometallics 9 (1990) 867. [36] P.J. Shapiro, W.D. Cotter, W.P. Schaefer, J.A. Labinger, J.E. Bercaw, J. Am. Chem. Soc. 116 (1994) 4623. [37] S. Liu, J. Yi, W. Zuo, K. Wang, D. Wang, W.-H. Sun, J. Polym. Sci., Part A: Polym. Chem. 47 (2009) 3154. [38] W. Zuo, S. Zhang, S. Liu, X. Liu, W.-H. Sun, J. Polym. Sci., Part A: Polym. Chem. 46 (2008) 3396. [39] S. Liu, W. Zuo, S. Zhang, P. Hao, D. Wang, W.-H. Sun, J. Polym. Sci., Part A: Polym. Chem. 46 (2008) 3411. [40] K. Nomura, Dalton Trans. 41 (2009) 8811. [41] K. Nomura, J. Liu, S. Padmanabhan, B. Kitiyanan, J. Mol. Catal. A: Chem. 267 (2007) 1. [42] J. Barker, M. Kilner, Coord. Chem. Rev. 133 (1994) 219. [43] A.A. Mohamed, Coord. Chem. Rev. 254 (2010) 1918. [44] F.T. Edelmann, Chem. Soc. Rev. 28 (2009) 2253. [45] H. Braunschweig, F.M. Breitling, Coord. Chem. Rev. 250 (2006) 2691. [46] D.G. Dick, R. Duchateau, J.H. Edema, S. Gambarotta, Inorg. Chem. 32 (1993) 1959. [47] B.D. Ward, H. Risler, K. Weitershaus, L.S. Bellemin, H. Wadepohl, L.H. Gade, Inorg. Chem. 45 (2006) 7777. [48] S. Aharonovich, M. Botoshansky, R.M. Waymouth, M.S. Eisen, Inorg. Chem. 49 (2010) 9217. [49] S. Aharonovich, M. Kapon, M. Botoshansky, M.S. Eisen, Organometallics 27 (2008) 1869. [50] E. Smolensky, M.S. Eisen, Dalton Trans. (2007) 5623. [51] V. Volkis, C. Averbuj, M.S. Eisen, J. Organomet. Chem. 692 (2007) 1940. [52] J.R. Hagadorn, J. Arnold, Angew. Chem., Int. Ed. 37 (1998) 1729. [53] J.R. Hagadorn, J. Arnold, Organometallics 17 (1998) 1355. [54] J.R. Hagadorn, J. Arnold, Inorg. Chem. 36 (1997) 2928. [55] S. Bambirra, A. Meetsma, B. Hessen, J.H. Teuben, Organometallics 20 (2001) 782. [56] S. Ge, A. Meetsma, B. Hessen, Organometallics 27 (2008) 3131. [57] T.J.J. Sciarone, C.A. Nijhuis, A. Meetsma, B. Hessen, Dalton Trans. (2006) 4896. [58] W.J. Meerendonk, K. Schröder, E.A.C. Brussee, A. Meetsma, B. Hessen, J.H. Teuben, Eur. J. Inorg. Chem. 3 (2003) 427. [59] W. Zhang, L.R. Sita, Adv. Synth. Catal. 350 (2008) 439. [60] A. Epshteyn, E.F. Trunkely, D.A. Kissounko, J.C. Fettinger, L.R. Sita, Organometallics 28 (2009) 2520. [61] W. Zhang, L.R. Sita, J. Am. Chem. Soc. 130 (2008) 442. [62] W.A. Chomitz, J. Arnold, Chem. Commun. 45 (2007) 4797. [63] H. Ferreira, A.R. Dias, M.T. Duarte, J.R. Ascenso, A.M. Martins, Inorg. Chem. 46 (2007) 750. [64] V. Passarelli, F. Benetollo, P. Zanella, Eur. J. Inorg. Chem. 8 (2004) 1714. [65] S. Bambirra, A. Meetsma, B. Hessen, Organometallics 25 (2006) 3486. [66] L. Morello, P.H. Yu, C.D. Carmichael, B.O. Patrick, M.D. Fryzuk, J. Am. Chem. Soc. 127 (2005) 12796. [67] S.J. Kim, I.N. Jung, B.R. Yoo, S. Cho, J. Ko, S.H. Kim, S.O. Kang, Organometallics 20 (2001) 1501. [68] S.J. Kim, I.N. Jung, B.R. Yoo, S.H. Kim, J. Ko, D. Byun, S.O. Kang, Organometallics 20 (2001) 2136. [69] S.J. Kim, Y.J. Lee, S.H. Kim, J. Ko, S. Cho, S.O. Kang, Organometallics 21 (2002) 5358. [70] S. Barroso, J.L. Cui, A.R. Dias, M.T. Duarte, H. Ferreira, R.T. Henriques, M.C. Oliveira, J.R. Ascenso, A.M. Martins, Inorg. Chem. 45 (2006) 3532. [71] V. Passarelli, F. Benetollo, P. Zanella, G. Carta, G. Rossetto, Dalton Trans. 7 (2003) 1411. [72] W.A. Chomitz, S.G. Minasian, A.D. Sutton, J. Arnold, Inorg. Chem. 46 (2007) 7199. [73] D.J. Brauer, H.B. Vurger, G.R. Liewald, J. Wilke, J. Organomet. Chem. 310 (1986) 317. [74] M. Veith, W. Frank, F. Tvollner, H. Lange, J. Organomet. Chem. 326 (1987) 315. [75] A.D. Horton, J. de With, Organometallics 16 (1997) 5424. [76] F.J. Shattenmann, R.R. Schrock, W.M. Davis, Organometallics 17 (1998) 989. [77] H. Schumann, J. Gottfriedsen, S. Dechert, F. Girgsdies, Z. Anorg, Allg. Chem. 626 (2000) 747. [78] M.S. Hill, P.B. Hitchcock, Organometallics 21 (2002) 3258. [79] S.F. Yuan, T. Duan, R.D. Zhang, G.A. Solan, Y.P. Ma, T.L. Liang, W.-H. Sun, Appl. Organomet. Chem. 33 (2019) e4785. [80] S.F. Yuan, E.L. Yue, C.Y. Wen, W.-H. Sun, RSC Adv. 6 (2016) 7431. [81] C.Y. Wen, S.F. Yuan, Q.S. Shi, E.L. Yue, D.S. Liu, W.-H. Sun, Organometallics 33 (2014) 7223.

14

S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254

[82] Z. Wang, G.A. Solan, Q. Mahmood, Q. Liu, Y. Ma, X. Hao, W.-H. Sun, Organometallics 37 (2018) 380. [83] S. Zhang, Q. Xing, W.-H. Sun, RSC Adv. 6 (2016) 16624. [84] Y.N. Zeng, Q.F. Xing, Y.P. Ma, W.-H. Sun, Chin. J. Polym. Sci. 36 (2018) 207. [85] F. Huang, W.J. Zhang, Y. Sun, X.Q. Hu, G.A. Solan, W.-H. Sun, New J. Chem. 40 (2016) 8012. [86] L. Bourget-Merle, M.F. Lappert, J.R. Severn, Chem. Rev. 102 (2002) 3031. [87] P.B. Hitchcock, M.F. Lappert, D.S. Liu, J. Chem. Soc., Chem. Commun. 22 (1994) 2637. [88] B.J. Deelman, M.F. Lappert, H.K. Lee, T.C.W. Mak, W.P. Leung, P.R. Wei, Organometallics 16 (1997) 1247. [89] P.B. Hitchcock, M.F. Lappert, S. Tian, J. Chem. Soc., Dalton Trans. (1997) 1945. [90] B.J. Deelman, P.B. Hitchcock, M.F. Lappert, H.K. Lee, W.P. Leung, J. Organomet. Chem. 513 (1996) 281. [91] B.J. Deelman, M.F. Lappert, W.P. Leung, H.K. Lee, T.C.W. Mak, Organometallics 18 (1999) 1444. [92] S.F. Yuan, L.P. Zhang, D.S. Liu, W.-H. Sun, Macromol. Res. 18 (2010) 690. [93] S.F. Yuan, S.D. Bai, H.B. Tong, D.S. Liu, W.-H. Sun, Inorg. Chim. Acta 370 (2011) 215. [94] S.F. Yuan, J. Li, W.-H. Sun, China Pat. (2017), 105482007A. [95] H. Hamaki, N. Takeda, M. Nabika, Macromolecules 45 (2012) 1758. [96] T. Kurogi, J.X. Chu, Y.F. Chen, Chem. Asian J. 14 (2019) 2629. [97] W.K. Kim, M.J. Fevola, L.M. Liable-Sands, A.L. Rheingold, K.H. Theopold, Organometallics 17 (1998) 4541. [98] P.H.M. Budzelaar, A.B.V. Oort, A.G. Orpen, Eur. J. Inorg. Chem. 1998 (1998) 1485. [99] L. Kakaliou, W.J. Scanlon, B. Qian, S.W. Baek, M.R. Smith, D.H. Motry, Inorg. Chem. 38 (1999) 5964. [100] X.Y. Wang, Z.L. Guo, H.B. Tong, M.S. Zhou, J Organomet Chem. 828 (2017) 10. [101] P. Wang, H.F. Li, X.Y. Xue, X. Chen, Dalton Trans. 44 (2015) 4718. [102] S. Collins, Coord. Chem. Rev. 255 (2011) 118. [103] J.F. Li, S.P. Huang, L.H. Weng, D.S. Liu, J. Organomet. Chem. 691 (2006) 3003. [104] S.D. Bai, F. Guan, M. Hu, S.F. Yuan, J.P. Guo, D.S. Liu, Dalton Trans. 40 (2011) 7686. [105] E.Y. Chen, T.J. Marks, Chem. Rev. 100 (2000) 1391. [106] H.B. Tong, M. Li, S.D. Bai, S.F. Yuan, J.B. Chao, S.P. Huang, D.S. Liu, Dalton Trans. 40 (2011) 4236. [107] S.D. Bai, J.P. Guo, D.S. Liu, Dalton Trans. 250 (2006) 2244. [108] S.D. Bai, H.B. Tong, J.P. Guo, M.S. Zhou, D.S. Liu, Inorg. Chim. Acta 362 (2009) 1143. [109] S.D. Bai, J.P. Guo, D.S. Liu, W.Y. Wong, Eur. J. Inorg. Chem. (2006) 4903. [110] S.D. Bai, H.B. Tong, J.P. Guo, D.S. Liu, Inorg. Chim. Acta 362 (2009) 114. [111] S.D. Bai, R.Q. Liu, T. Wang, F. Guan, Y.B. Wu, J.B. Chao, H.B. Tong, D.S. Liu, Polyhedron 65 (2013) 161. [112] K. Soga, T. Shiono, Prog. Polym. Sci. 22 (1997) 1503. [113] M.S. Zhou, S. Zhang, H.B. Tong, W.-H. Sun, D.S. Liu, Inorg. Chem. Commun. 10 (2007) 1262. [114] M.S. Zhou, H.B. Tong, X.H. Wei, D.S. Liu, J. Organomet. Chem. 692 (2007) 5195. [115] P.B. Hitchcock, M.F. Lappert, P.G. Merle, Dalton Trans. 5 (2007) 585. [116] M. Fan, Q.K. Yang, H.B. Tong, S.F. Yuan, B. Jia, D.L. Guo, M.S. Zhou, D.S. Liu, RSC. Adv. 2 (2012) 6599. [117] Q.K. Yang, M.S. Zhou, H.B. Tong, D.L. Guo, D.S. Liu, Inorg. Chem. Commun. 29 (2013) 1. [118] D. Herskovics-Korine, M. Eisen, J. Organomet. Chem. 503 (1995) 307. [119] X. Yang, C.L. Stern, T.J. Marks, J. Am. Chem. Soc. 116 (1994) 10015. [120] M.S. Zhou, Q.K. Yang, H.B. Tong, L. Yan, X.Y. Wang, RSC Adv. 5 (2015) 105292. [121] T. Holtrichter-Roessmann, I. Haeger, C.G. Daniliuc, Organometallics 35 (2016) 1906. [122] S.F. Yuan, X.H. Wei, H.B. Tong, L.P. Zhang, D.S. Liu, W.-H. Sun, Organometallics 29 (2010) 2085. [123] X.E. Duan, S.F. Yuan, H.B. Tong, S.D. Bai, X.H. Wei, D.S. Liu, Dalton Trans. 41 (2012) 9460. [124] A. Noor, W.P. Kretschmer, G. Glatz, A. Meetsma, R. Kempe, Eur. J. Inorg. Chem. 2008 (2008) 5088. [125] C. Morton, P.O. Shaughnessy, P. Scott, Chem. Commun. 21 (2000) 2099. [126] M. Talja, T. Luhtanen, M. Polamo, M. Klinga, T. Pakkanen, M. Leskela, Inorg. Chim. Acta 361 (2008) 2195. [127] M. Talja, M. Polamo, M. Leskelä, J. Mol. Catal. A: Chem. 280 (2008) 102. [128] M. Talja, M. Klinga, M. Polamo, E. Aitola, M. Leskela, Inorg. Chim. Acta 358 (2005) 1061. [129] M. Hafeez, W.P. Kretschmer, R. Kempe, Eur. J. Inorg. Chem. 2011 (2011) 5512. [130] W.P. Kretschmer, B. Hessen, A. Noor, N.M. Scott, R. Kempe, J. Organomet. Chem. 692 (2007) 4569. [131] A. Noor, W.P. Kretschmer, G. Glatz, R. Kempe, Inorg. Chem. 50 (2011) 4598. [132] L. Yan, X.Y. Wang, M.S. Zhou, Inorg. Chem. Commun. 65 (2016) 32. [133] P.P. Fontaine, S. Ueligger, J. Klosin, Organometallics 34 (2015) 1354. [134] S.F. Yuan, L.J. Wang, Y.P. Hua, J. Zhang, W.-H. Sun, Z. Naturforsch. 71 (2016) 1019. [135] S.F. Yuan, W. Cao, China Pat. (2017), 105754020A. [136] X.E. Duan, Q.F. Xing, Z.M. Dong, J.B. Chao, S.D. Bai, D.S. Liu, W.-H. Sun, Chin. J. Polym. Sci. 34 (2016) 390. [137] R.G. Alamo, J.D. Londono, L. Mandelkern, F.C. Stehling, G.D. Wignall, Macromolecules 27 (1994) 411. [138] W.J. Wang, E. Kolodka, S. Zhu, A.E. Hamielec, J. Polym. Sci., Part A: Polym. Chem. 37 (1999) 2949.

[139] J. Klosin, W.J. Kruper, P.N. Nickias, G.R. Roof, P. DeWaele, K.A. Abboud, Organometallics 20 (2001) 2663. [140] J. Klosin, W.J. Kruper, J.T. Patton, K.A. Abboud, J. Organomet. Chem. 694 (2009) 2581. [141] R. Leino, H. Luttikhedde, C.E. Wilén, R. Sillanpaa, J.H. Nasman, Organometallics 15 (1996) 2450. [142] Y. He, X. Qiu, J. Klosin, R. Cong, G.R. Roof, Macromolecules 47 (2014) 3782. [143] J. Klosin, P.P. Fontaine, R. Figueroa, Acc. Chem. Res. 48 (2015) 2004. [144] S.F. Yuan, S.D. Bai, D.S. Liu, W.-H. Sun, Organometallics 29 (2010) 2132. [145] S.F. Yuan, S.D. Bai, D.S. Liu, H.B. Tong, W.J. Zhang, W.-H. Sun, China Pat. (2013) 101786018A. [146] J.C. Stevens, F.J. Timmers, D.R. Wilson, G.F. Schmidt, P.N. Nickias, R.K. Rosen, G.W. McKnight, S. Lai, Eur. Pat. Appl., (1991) 0416815A2. [147] A. Yano, S. Gasegawa, T. Kaneko, M. Sone, M. Sato, A. Akimoto, Macromol. Chem. Phys. 200 (1999) 1542. [148] R. Quijiada, G.B. Galland, R.S. Mauler, S.C. Menezes, Macromol. Rapid Commun. 17 (1996) 607. [149] K.J. Chu, J.B.P. Soares, A. Penlidis, Macromol. Chem. Phys. 201 (2000) 340. [150] Y. Zhang, X. Yang, Y. Yu, Y. Qian, J. Huang, Chin. J. Chem. 23 (2005) 1081. [151] K. Czaja, M. Bialek, Polymer 42 (2001) 2289. [152] S.F. Yuan, T. Duan, L.J. Wang, X.H. Wei, X.X. Wang, W.-H. Sun, Inorg. Chim. Acta 466 (2017) 497. [153] S.F. Yuan, J. Zhang, L.J. Wang, China Pat. (2019), 106519088A. [154] W. Li, S.D. Bai, S.F. Yuan, X.E. Duan, D.S. Liu, New J. Chem. 41 (2017) 661. [155] W. Li, F. Su, S.F. Yuan, X.E. Duan, S.D. Bai, D.S. Liu, RSC Adv. 6 (2016) 40741. [156] Y. Takii, A. Inagaki, K. Nomura, Dalton Trans. 42 (2013) 11632. [157] I.E. Nifant’ev, P.V. Ivchenko, V.V. Bagrov, S.M. Nagy, L.N. Winslow, J.A. Merrick-Mack, S. Mihan, A.V. Churakov, Dalton Trans. 42 (2013) 1501. [158] G. Li, M. Lamberti, G. Roviello, C. Pellecchia, Organometallics 31 (2012) 6772. [159] N. Marquet, E. Kirillov, T. Roisnel, A. Razavi, J.F. Carpentier, Organometallics 28 (2009) 606. [160] S.R. Golisz, J.E. Bercaw, Macromolecules 42 (2009) 8751. [161] Z. Janas, D. Godbole, T. Nerkowski, K. Szczegot, Dalton Trans. (2009) 8846. [162] M. Lamberti, M. Mazzeo, C. Pellecchia, Dalton Trans. (2009) 8831. [163] S. Gong, H. Ma, J. Huang, Dalton Trans. (2009) 8237. [164] J. Niemeyer, G. Kehr, R. Froehlich, G. Erker, Dalton Trans. (2009) 3731. [165] R.J. Long, V.C. Gibson, A.J.P. White, Organometallics 27 (2008) 235. [166] C. Ramos, P. Royo, M. Lanfranchi, M.A. Pellinghelli, A. Tiripicchio, Organometallics 26 (2007) 445. [167] L. Fan, D. Harrison, T.K. Woo, T. Ziegler, Organometallics 14 (1995) 2018. [168] V. Busico, R. Cipullo, R. Pellecchia, G. Talarico, A. Razavi, Macromolecules 42 (2009) 1789. [169] K.P. Bryliakov, E.P. Talsi, A.Z. Voskoboynikov, S.J. Lancaster, M. Bochmann, Organometallics 27 (2008) 6333. [170] X. Ji, W. Yao, X. Luo, W. Gao, Y. Mu, New J. Chem. 40 (2016) 2071. [171] S. Matsui, T. Fujita, Catal. Today 66 (2001) 63. [172] R. Furuyama, T. Fujita, S.F. Funaki, Catal. Surv. Asia 8 (2004) 61. [173] M. Mitani, J. Saito, S. Ishii, Y. Nakayama, H. Makio, N. Matsukawa, S. Matsui, J. Mohri, R. Furuyama, H. Terao, H. Bando, H. Tanaka, T. Fujita, Chem. Rec. 4 (2004) 137. [174] H. Makio, T. Fujita, Acc. Chem. Res. 42 (2009) 1532. [175] R. Furuyama, J. Saito, S. Ishii, H. Makio, M. Mitani, H. Tanaka, T. Fujita, J. Organomet. Chem. 690 (2005) 4398. [176] T. Matsugi, T. Fujita, Chem. Soc. Rev. 37 (2008) 1264. [177] T. Matsugi, S. Matsui, S. Kojoh, Y. Takagi, Y. Inoue, T. Nakano, T. Fujita, N. Kashiwa, Macromolecules 35 (2002) 4880. [178] Y. Yoshida, S. Matsui, Y. Takagi, M. Mitani, T. Nakano, H. Tanaka, N. Kashiwa, T. Fujita, Organometallics 20 (2001) 4793. [179] Y. Yoshida, J. Saito, M. Mitani, Y. Takagi, S. Matsui, S. Ishii, T. Nakano, N. Kashiwa, T. Fujita, Chem. Commun. (2002) 1298. [180] S. Matsui, T.P. Spaniol, Y. Takagi, Y. Yoshida, J.J. Okuda, Dalton Trans. (2002) 4529. [181] Y. Yoshida, J. Mohri, S. Ishii, M. Mitani, J. Saito, S. Matsui, H. Makio, T. Nakano, H. Tanaka, M. Onda, Y. Yamamoto, A. Mizuno, T. Fujita, J. Am. Chem. Soc. 126 (2004) 12023. [182] Y. Yoshida, S. Matsui, T. Fujita, J. Organomet. Chem. 690 (2005) 4382. [183] Y. Suzuki, H. Tanaka, T. Oshiki, K. Takai, T. Fujita, Chem. Asian J. 1 (2006) 878. [184] Y. Suzuki, Y. Inoue, H. Tanaka, T. Fujita, Macromol. Rapid Commun. 25 (2004) 493. [185] H. Makio, N. Kashiwa, T. Fujita, Adv. Synth. Catal. 344 (2002) 477. [186] S. Ishii, J. Saito, M. Mitani, J. Mohri, N. Matsukawa, Y. Tohi, S. Matsui, N. Kashiwa, T. Fujita, J. Mol. Catal. A: Chem. 179 (2002) 11. [187] S. Ishii, R. Furuyama, N. Matsukawa, J. Saito, M. Mitani, H. Tanaka, T. Fujita, Macromol. Rapid Commun. 24 (2003) 452. [188] N. Matsukawa, S. Matsui, M. Mitani, J. Saito, K. Tsuru, N. Kashiwa, T. Fujita, J. Mol. Catal. A: Chem. 169 (2001) 99. [189] S. Matsui, M. Mitani, J. Saito, Y. Tohi, H. Makio, N. Matsukawa, Y. Takagi, K. Tsuru, M. Nitabaru, T. Nakano, H. Tanaka, N. Kashiwa, T. Fujita, J. Am. Chem. Soc. 123 (2001) 6847. [190] A. Sakuma, M.S. Weiser, T. Fujita, Polym. J. 39 (2007) 193. [191] G.J. Domski, J.M. Rose, G.W. Coates, A.D. Bolig, M. Brookhart, Prog. Polym. Sci. 32 (2007) 30. [192] S. Matsui, Y. Yoshida, Y. Takagi, T.P. Spaniol, J. Okuda, J. Organoment. Chem. 689 (2004) 1155. [193] H. Makio, H. Terao, A. Iwashita, Chem. Rev. 111 (2011) 2363.

S.-F. Yuan et al. / Coordination Chemistry Reviews 411 (2020) 213254 [194] Y.S. Gao, M.D. Christianson, Y. Wang, J.Z. Chen, S. Marshall, J. Klosin, T.L. Lohr, T.J. Marks, J. Am. Chem. Soc. 141 (2019) 7822. [195] Y.B. Zhang, F.Q. Zhang, J.B. Chao, H.B. Tong, S.F. Yuan, S.D. Bai, D.S. Liu, China Pat. (2015), 2727335. [196] C.C. Liu, Q. Liu, S.M. Yiu, M.C.W. Chan, Organometallics 38 (2019) 2963.

15

[197] L.A. Wright, E.G. Hope, G.A. Solan, W.B. Cross, K. Singh, Organometallics 35 (2016) 1183. [198] G.R. Giesbrecht, G.A Solan, C. J. Davies, PCT Int. Appl. (2009), WO 2009082556 A1 20090702. [199] H. Audouin, R. Bellini, L. Magna, Eur. J. Inorg. Chem. 31 (2015) 5272. [200] A.C. Pinheiro, S.M. da Silva, T. Roisnel, New J. Chem. 42 (2018) 1477.