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Review

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

by
Yu Pan
1,
Xinxin Jiang
1,
Yat-Ming So
2,*,
Ching Tat To
3 and
Gaohong He
1,*
1
State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China
2
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
3
Department of Chemistry, Tunghai University, No. 1727, Sec. 4, Taiwan Boulevard, Xitun District, Taichung 40704, Taiwan
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(1), 71; https://doi.org/10.3390/catal10010071
Submission received: 24 November 2019 / Revised: 20 December 2019 / Accepted: 31 December 2019 / Published: 3 January 2020
(This article belongs to the Special Issue Rare Earth Complexes as Polymerization Catalysts)
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Abstract

:
N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

Graphical Abstract

1. Introduction

N-heterocyclic carbenes (NHCs) are persistent carbenes and have been demonstrated to be versatile ancillary ligands for metal-based catalytic reactions [1,2,3]. Of particular significance is the use of NHCs in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization [4,5,6,7]. Despite the coordination chemistry and rich reactivity of NHCs complexes involving d-block and actinide elements, NHC-ligated rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) complexes (Ln-NHCs) remain under-developed [8,9,10]; this could be due partly to the thought of a hard–soft mismatch between the hard Ln(III) center and the soft carbon-donor ligand, and therefore the possible ease of NHCs dissociation during catalysis. Nonetheless, owing to their strong σ-donating ability, NHCs were shown to be able to form thermally and chemically stable complexes with Ln, and to confer good catalytic activity to the Ln centers [11,12,13,14,15].
The development of Ln catalysts for polymerization has attracted a considerable amount of attention in recent decades [16,17]. Compared with transition metal catalysts, Ln complexes are considered to have unique advantages in the coordination polymerization of styrene, the coordination polymerization of conjugated dienes, the ring-opening polymerization of cyclic esters [18,19,20,21,22,23,24,25,26], and the coordination polymerization of polar monomers [27,28,29,30,31,32,33,34]. Yet, the employment of Ln-NHCs in this area remains relatively unexplored. Given the success of both NHCs and Ln in polymerization chemistry, it is envisioned that NHC-ligated Ln complexes will become a promising class of catalysts in the area of polymerization. With the strong σ-donating ability and great structural and electronic varieties of NHCs, robust Ln–NHC catalysts with a diverse range of activities can be obtained by a judicious choice of central metals and NHCs. By using hetero-atom-tethered chelating NHCs and other structurally related NHCs, NHC-ligated Ln complexes have been demonstrated to be a fruitful candidate in polymerization. In this review, we describe the recent development of Ln complexes bearing NHCs ligands, covering their synthesis and reactivity, with an emphasis on their applications in polymerization [35]. For a comprehensive overview on the coordination chemistry and reactivity of Ln-NHCs, we refer the reader to a recent book chapter given by Kühn and coworkers [36].

2. Rare Earth Catalysts Containing a Monodentate NHC Ligand

In 1994, Schumann et al. synthesized the first NHC-ligated Ln complexes (C5Me4Et)2Ln(NHC) (Ln = Yb (1), Sm (2)) by reacting (C5Me4Et)2Ln(THF) with an in situ-generated NHC (Figure 1) [37,38]. Subsequently, Arduengo et al. reported the syntheses and molecular structures of the carbene-Sm(II) complex (C5Me5)2Sm(NHC) (3) and its bis(carbene) adduct (C5Me5)2Sm(NHC)2 (4), and also the mono-carbene Eu (5) and Y (6) complexes bearing acetylacetonate ligands (Figure 1) [39]. Owing to its strong nucleophilicity, substitution of a weakly coordinated ligand by NHC remains one of the most common methods for preparing Ln–NHC complexes [40,41].
Reactions of Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu) with an in situ-generated bulky NHC, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), afforded the corresponding four coordinated (IPr)Ln complexes (IPr)Ln(CH2SiMe3)3 (Ln = Y (7), Lu(8)) (Figure 2) [42]. The treatment of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with Lu(CH2SiMe3)3(THF)2 yielded the expected (IMes)Lu(CH2SiMe3)3 complex (9). However, an attempted synthesis of an yttrium analogue with Y(CH2SiMe3)3(THF)2 led to the formation of a cyclometalated product [(IMes’)Y(CH2SiMe3)2(THF)2] (IMes’ = IMes deprotonated at the ortho-methyl group of the mesityl substituent) (10), presumably via σ-bond metathesis. Prolonged reaction of Lu(CH2SiMe3)3(THF)2 with IMes also afforded the corresponding Lu cyclometalated product 11 [42,43]. Not surprisingly, the reactions of Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu) with excess IMes gave rise to the bis-cyclometalated complexes [(IMes’)2Ln(CH2SiMe3)] (Ln = Y (12), Lu (13)) [43].
Soon after, the analogous NHC-ligated Sc trialkyl complexes (14 and 15) were also reported by Lu and co-workers (Figure 3) [44]. The NHC–Sc complexes were found to be active catalysts for α-olefin polymerization with very high activities (up to 2520 kg·molSc−1·h−1) upon activation with two equivalents of [Ph3C][B(C6F5)4]. The polyolefin with high molecular weight (up to 63.1 × 104) could be produced at a low reaction temperature of 30 °C. In comparison, the analogous Y and Lu complexes showed very low activity or were inert for α-olefin polymerization, indicating the metal’s dependence on the catalytic activity. Furthermore, the 14/[Ph3C][B(C6F5)4] system was also efficient in catalyzing the copolymerization of 1-hexene with 1,5-hexadiene to give random copolymers (poly(hexene-co-hexadiene)s) with a wide range of 1,5-hexadiene contents (26.6–98.6 mol%) (Figure 3). It is noteworthy that the starting material Sc(CH2SiMe3)3(THF)2 showed poor polymerization controllability for α-olefin polymerization under the same conditions. This work nicely demonstrates the non-innocence of a monodentate NHC in Ln-catalyzed polymerization.
Inspired by the successful employment of monodentate NHC ligands in the Ln-catalyzed polymerization reaction, Pan et al. prepared a series of bis(oxazoline)-derived N-heterocyclic carbene (IBiox)-supported Ln trialkyl complexes (IBiox)Ln(CH2SiMe3)3(THF)n (Ln = Sc (16), n = 0; Y (17), n = 1; Lu (18), n = 1) by the treatment of Ln(CH2SiMe3)3(THF)2 with one equivalent of the freshly prepared IBiox (Figure 4) [45]. A single crystal X-ray diffraction study revealed that the geometry around the metal center in 16 is pseudo tetrahedral, while those of 17 and 18 are distorted trigonal bipyramidal with a coordinated tetrahydrofuran (THF). Upon activation with two equivalents of [Ph3C][B(C6F5)4], 16 exhibited high activity for the polymerization of 1-hexene (up to 285 kg·molSc−1·h−1) with the resultant polymers dominated by vinylene end groups (ca. 95%). On the other hand, the analogous Y and Lu complexes were inactive for the polymerization. The copolymerization of 1-hexene with 1,7-octadiene could also be achieved by using the catalytic system of 16/[Ph3C][B(C6F5)4] to generate random poly(hexene-co-octadiene). Incomplete cyclization of 1,7-octadiene resulted in about 20% pendant vinyl groups in the copolymers. The hydrophilicity of the resulting copolymers could be enhanced by functionalizing the vinyl groups into the carboxylic acid groups via a thiol-ene reaction.
The monodentate NHC-supported Yb(II) amido complexes [(NHC)Yb{N(SiMe3)2}2] (NHC = 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (IiPr) (19), 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene (IMes) (20)) were prepared by the reactions of THF-coordinated Yb amides with one equivalent of the NHC ligand (Figure 5) [46]. The NHC–Yb amido complexes were effective catalysts for the dehydrogenative cross-coupling reactions of secondary amines (e.g., HN(SiMe3)2, HNiPr2, and HNEt2) with PhSiH3 to give various coupling products. The use of bulky 20 allowed for the selective formation of a single coupling product by changing the molar ratios of the two substrates. Followed by this work, the bis-NHC-Yb(II) amides (NHC)2Yb[N(SiMe3)2]2 (NHC = 1,3,4,5-tetramethylimidazo-2-ylidene (IMe4) (21), IiPr (22)) and the NHC-stabilized Yb(II) phosphide (IMe4)3Yb(PPh2)2 (23) were synthesized by Cui and coworkers (Figure 5) [47]. Complexes 21–23 were active precatalysts in the hydrophosphination of alkenes, alkynes, and dienes and were superior to the NHC-free amido complex (THF)2Yb[N(SiMe)2]2. This suggested the importance of an NHC ligand in boosting this catalysis. Furthermore, it was found that complex 23 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights (Mn = 0.85–18.2 × 104). Intermolecular Lewis pair complexes based on homoleptic Ln aryloxides, Ln(OAr)3 (Ln = La, Sm, Y), and NHCs were observed to activate dihydrogen under mild conditions [48]. In addition, the La(OAr)3/NHC pair also exhibited frustrated Lewis pair (FLP)-like reactivity towards carbon dioxide and phenylacetylene (Figure 6).
To enrich our understanding of bonding between an NHC ligand and Ln metals, the search for structurally related NHCs and the study of their structural and electronic properties have also drawn a considerable amount of attention [49,50,51]. In this regard, Liddle and coworkers reported a general route to prepare f-element complexes containing a neutral mesoionic carbene (MIC) [52]. The Ln–MIC complexes (MIC)M[N(SiMe3)2]3 (M = U (24), Y (25), La (26), Nd (27)) were synthesized by the reaction of Ln[N(SiMe3)2]3 with an MIC via a formal 1,4-proton migration (Figure 7). Computational studies showed that the Ln–C(MIC) bond is mostly composed of MIC→Ln σ donation, in contrast to the analogous actinide (An)–MIC complexes that exhibit both MIC→An σ donation and An→MIC π back-bonding.

3. Rare Earth Catalysts Containing a Heteroatom-Tethered NHC Ligand

Since the monodentate NHC ligand was thought to be labile in Ln-catalyzed reactions, much effort has been devoted to the development of chelating an NHC ligand with a heteroatom sidearm to facilitate complexation. Arnold et al. has pioneered the employment of an anionic heteroatom-tethered NHC to support Ln complexes [53]. The amido- and alkoxo-tethered NHC-supported Ln complexes were developed, some of which can induce addition–elimination reactions to activate small molecules and forge C–C and C–Si bonds [54,55,56,57,58,59]. The Y (28) and Ti (29) complexes with heteroatom-tethered NHCs are bifunctional catalysts for the ring-opening polymerization (ROP) of lactide, by using a combination of Lewis acid and base functionalities to initiate the ring-opening of the cyclic monomer (Figure 8) [54].
Furthermore, Arnold and coworkers have synthesized a homolytic Sc complex containing three equiv. of alkoxo-tethered NHCs (Alko-NHC), Sc(Alko-NHC)3 (30), via the treatment of ScCl3(THF)3 with the potassium salt of Alko-NHC (Figure 9) [60]. Complex 30 exhibited frustrated-Lewis-pair type behavior that could activate three equivalents of CO2 via the Sc-C(NHC) bond. On the other hand, activation of CS2 by 30 solely involved the NHC moiety. Another class of homoleptic Ln-tris(aryloxo-tethered NHC) complexes, Ln(Arylo-NHC)3 (Ln = Ce, Sm, and Eu) (31–35), were also synthesized by treatment of the corresponding ortho-aryloxide imidazolium salt proligand with KN(SiMe3)2 followed by salt metathesis with LnCl3(THF)n (Figure 9) [61]. The Ce complexes 31 and 34 were capable of activating CO2 and its isoelectronic heteroallenes using the Ce–C(NHC) bonds. It should be noted that the insertion of CO2 into the mesityl-substituted complex 35 is reversible. This led to the catalytic formation of propylene carbonate in the presence of propylene oxide under 1 atm of CO2 (Figure 10).
Shen and coworkers have also contributed to the synthesis and development of catalytic reactions with aryloxo-tethered NHC-supported Ln complexes [62,63,64]. Treatment of Li[Ln(NiPr2)]4 (Ln = Yb, Y) with aryloxo-tethered NHC precursors and nBuLi at a low temperature afforded the amido Yb complexes 36 and 37, or homoleptic Y complex 38. When the reaction of Li[Y(NiPr2)]4 with an aryloxo-tethered NHC precursor and nBuLi was carried out at room temperature, the Y complex 39 supported by a bridging bisphenoxo group was obtained, presumably derived from the cleavage of the NHC ligand (Figure 11) [63]. Cationic amido-phenoxo-functionalized NHC-ligated Ln bromides (Ln = Y (40), Lu (41), Er (42)) were obtained from the transmetalation reaction of LnCl3 with NHC-ligated Li in a molar ratio of 1:1 at room temperature, which could also be in situ formed by the salicylaldimino-functionalized imidazolium bromide with nBuLi (Figure 11) [64].
Treatment of Ln(CH2SiMe3)3(THF)2 with freshly prepared fluorenyl-tethered NHC proligand led to the formation of fluorenyl-NHC (Flu-NHC)-ligated Ln dialkyl complexes (Flu-NHC)Ln(CH2SiMe3)2 (Ln = Sc (43), Y (44), Lu (45), Ho (46), Dy (47), Er (48)), along with the elimination of SiMe4 and THF (Figure 12) [65,66]. Upon activation with AliBu3 and [Ph3C][B(C6F5)4], complexes 44–46 exhibited livingness in the polymerization of isoprene at various monomer-to-catalyst ratios under a broad range of temperatures (from 25 °C to 80 °C) [66]. The resultant polyisoprenes (PIPs) are of high 3,4-regioselectivity (up to 99%) and medium syndiotacticity, resulting in a kind of crystalline polymer. The catalytic activity and specific selectivity are strongly dependent on the steric hindrance of the ancillary ligand and central metal. Complexes 44, 45, and 46 bearing a fluorenyl moiety exhibited higher 3,4-regioselectivity than complexes 50, 51, and 52 bearing an indenyl moiety with the same central metal. The Sc complexes 43 and 49 were inert for the isoprene polymerization.
On the other hand, half-sandwich Sc complexes bearing a CGC-type NHC (43, 49, and 53) exhibited medium to high catalytic activity in the polymerization of ethylene, while the Y and Lu analogues were inactive. In addition, the copolymerization of ethylene with norbornene, 1-hexene, or 1-octene could be achieved with high activity (ca. 3600–5000 kg·molSc−1·h−1·atm−1) by using the 43/AliBu3/[Ph3C][B(C6F5)4] catalytic system (Figure 13) [67,68]. The thermal properties of the copolymers could be conveniently adjusted by changing the feed ratio of ethylene and co-monomer. The copolymerization of ethylene and styrene or other styrene derivatives could be successfully achieved by the 43/AliBu3/[Ph3C][B(C6F5)4] system to produce a series of pseudo-random copolymers. The incorporation content of the styrene monomer could be tuned swiftly from 16.1 mol% to 43.2 mol% through varying the monomer’s feeding mole ratio and polymerization time, and no consecutive styrene sequences were observed in the copolymers [69]. According to the NMR spectra of the copolymers, there were no consecutive styrene sequences and the copolymers showed only one glass transition temperature, which could vary from −21.2 °C to 18.2 °C by changing the styrene content. Remarkably, the direct copolymerization of ethylene with a series of unmasked polar styrenes and nonpolar styrenes could be achieved by using an NHC side-armed fluorenyl Sc complex. Complex 55 with a methyl-substituted NHC showed the highest activity (319 kg·molSc−1·h−1·atm−1), which was 10 times higher than the sterically bulky complex 43 [70]. All the resultant copolymers are composed of pseudo-alternating microstructures. The copolymer with a perfect alternating structure could be generated when para-(N,N-dimethylamino)styrene was used as a co-monomer.
In addition to the coordination polymerization, the Y and Lu complexes (Flu-NHC)Ln(CH2SiMe3)2 (Ln = Y (44), Lu (45)) were also employed as the catalysts for rapid and stereoselective polymerization of renewable methylene methylbutyrolactones, such as α-methylene-γ-butyrolactone (MBL), γ-methyl-α-methylene-γ-butyrolactone (γMMBL), and β-methyl-α-methylene-γ-butyrolactone (βMMBL) (Figure 13) [71]. Both 44 and 45 were highly active for the polymerization of γMMBL at room temperature, achieving an exceptionally high turnover frequency (TOF) of 24,000 h−1, which was 2400 times higher than that of homoleptic Ln-trialkyls. Moreover, 44 and 45 were also found to be active in the homopolymerization of βMMBL and its copolymerization with γMMBL, which represents the first example of metal-catalyzed coordination polymerization of βMMBL. The homopolymer resulting from βMMBL displays high stereoregularity (91% mm) with a high Tg of 290 °C.
Admidinate-tethered NHCs (Amidino-NHC) were also employed by Cui et al. for the synthesis of amidino-NHC-supported Ln complexes (56–59) (Figure 14) [72]. Under the activation with [Ph3C][B(C6F5)4], the Lu complexes (57–59) exhibited high activities and 3,4-selectivities (96.9%) in the polymerization of 2-phenyl-1,3-butadiene in a living mode to afford new plastic polydienes with high glass transition temperatures (Tg = 44.3–56.1 °C). However, the analogous Sc complex 56 was inert under the same reaction conditions. The different catalytic performance of 56 and 57 on isoprene polymerization is similar to that of 43 and 45 with a fluorenyl-tethered NHC ligand, showing the importance of the type of central metal.

4. Rare Earth Catalysts Containing a Tridentate NHC Ligand

Tridentate ligands, such as pincer-type ligands (pincer phosphines, amines, and NHC ligands), have widely been used to support transition metal catalysts and usually show high thermal stability and well-defined reactivity [73,74,75,76,77,78]. Therefore, efforts have also been devoted to develop Ln catalysts supported by a tridentate NHC ligand.
A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated Ln dibromides (60–69) were synthesized via the treatment of imidazolium dibromide proligands and LnCl3 with dropwise addition of nBuLi (Figure 15) [79,80]. The Y, Nd, Gd, Dy, and Ho complexes exhibited high activity and cis-1,4 selectivity (99.6%, 25 °C) in the polymerization of isoprene under activation with AlR3 (R = Me, Et, iBu) and [Ph3C][B(C6F5)4], while the others were inactive. Moreover, this system maintained a high cis-1,4 selectivity (97.4%) at 80 °C, indicating its tolerance to elevated temperature conditions. According to the analysis of in situ NMR spectra, the yttrium hydrido aluminate cation was speculated to be the active species.
Wang and coworkers have reported the synthesis of a number of Ln amides supported by a CNC-pincer ligand (denoted as diarylamido-linked biscarbene (DLB)) (Figure 15) [81,82]. Reaction of the in situ-generated DLB with [(Me3Si)2N]3Ln(μ-Cl)Li(THF)3 afforded the CNC-pincer complexes (DLB)Ln[N(SiMe3)2]2 (Ln = Er (70), Y (71), Sm (72), Eu (73)). Alternatively, complexes 70–73 could also be prepared by one-pot reaction of the DLB proligand (H3DLB-Cl2) with five equivalents of NaN(SiMe3)2, followed by the treatment with LnCl3. These complexes displayed good catalytic activity in the addition of terminal alkynes to carbodiimides to give propiolimidines, which was the first example in Ln chemistry to catalyze such an organic transformation at room temperature (Figure 15).
Shen, Ni, and their coworkers have designed and synthesized two chelating bis(phenolate)-NHC (BisP-NHC) ligands for Ln complexation [83,84,85]. The BisP-NHC proligands featuring a five-membered (denoted as BisP-NHC-5 hereafter) and a six-membered (denoted as BisP-NHC-6 hereafter) imidazolinium rings were prepared using the corresponding H2Salen. The treatments of the BisP-NHC-5 proligand with KN(SiMe3)2 and [Ln{N(SiMe3)2}3] afforded the heterobimetallic Ln/K complexes Ln(BisP-NHC-5)K(THF) (Ln = Nd (74), Sm (75), Y (76), La (77)), while that with KN(SiMe3)2 and [(Me3Si)2N]3Nd(μ-Cl)Li(THF)3 led to the isolation of a Nd/Li complex Nd(BisP-NHC-5)Li(THF) (78) (Figure 16). Similarly, the reaction of the BisP-NHC-6 proligand with KN(SiMe3)2 and [Ln{N(SiMe3)2}3] resulted in the formation of heterobimetallic Ln/K complexes Ln(BisP-NHC-6)K(THF) (Ln = Nd (79), Y (80)). In the absence of KN(SiMe3)2, the reactions of BisP-NHC-5 or BisP-NHC-6 proligands with [Ln{N(SiMe3)2}3] or [(Me3Si)2N]3Ln(μ-Cl)Li(THF)3 only gave rise to dinculear Ln complexes (81–85) containing bridging chlorides and imidazolinium-bridged bis(phenolate) ligands (Figure 16).
The complexes 74–78 were effective catalysts in the ROP of L-lactide (L-LA) to obtain polylactides with moderate molecular weight (Mn = 7−21 × 103), while the dinuclear 81–83 were found to be inactive for this polymerization [84]. Similarly, the heterobimetallic Ln/K complexes 74–76, 79, and 80 containing an Ln–NHC moiety were highly active in catalyzing the polymerization of n-hexyl isocyanate to give polymers with a very high molecular weight (up to 106) and a narrow molecular weight distribution (Mw/Mn = 1.7−2.3), whereas the dinuclear complexes 81, 84, and 85 were inert for this transformation [85]. These results indicate the importance of an Ln–NHC moiety for initiating the ROP of L-LA and polymerization of n-hexyl isocyanate.
Two similar phenol-tethered NHC proligands featuring six-membered imidazolinium rings (PhenoH-NHC-1 and PhenoH-NHC-2) were also prepared by Ni and coworkers. However, unlike the bis(phenolate)-NHC analogue, the reactions of PhenoH-NHC-1 or PhenoH-NHC-2 with KN(SiMe3)2 and Ln[N(SiMe3)2]3 only led to the isolations of heterobimetallic Ln/K complexes containing a bridging Pheno-NHC-1 or Pheno-NHC-2, which binds to the Ln(III) and K(I) centers via oxygen and carbon atoms, respectively [86].

5. Conclusions

In summary, considerable efforts have been devoted to the development of Ln complexes containing NHCs. Both monodentate NHCs and heteroatom-tethered NHCs have been shown to be a good supporting ligand for Ln catalysts and their applications in polymerization reactions. Given each previous success of Ln and NHCs in the area of polymerization, one can envision that the development of NHC-ligated Ln catalysts in polymerization chemistry will flourish.

Author Contributions

Conceptualization, Y.P. and Y.-M.S.; writing—original draft preparation, Y.P., X.J., and Y.-M.S.; writing—review and editing, Y.P., Y.-M.S., and C.T.T.; supervision, G.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (21971029) and the State Key Laboratory of Fine Chemicals (KF1711). And the APC was funded by the Fundamental Research Funds for the Central Universities (DUT19LK27) and the Natural Science Foundation of Liaoning Province, China (20180550505).

Acknowledgments

We are grateful to Xiaohui Kang in Dalian Medical University and Gen Luo in Anhui University for their helpful discussion and suggestion.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Ln complexes with N-heterocyclic carbene (NHC) ligands.
Figure 1. Ln complexes with N-heterocyclic carbene (NHC) ligands.
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Figure 2. Tri-, dialkyl, or monoalkyl Ln complexes with sterically hindered NHC ligands.
Figure 2. Tri-, dialkyl, or monoalkyl Ln complexes with sterically hindered NHC ligands.
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Figure 3. Trialkyl Sc complexes bearing an NHC used for α-olefin (co-)polymerization.
Figure 3. Trialkyl Sc complexes bearing an NHC used for α-olefin (co-)polymerization.
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Figure 4. The trialkyl Sc complex bearing a bis(oxazoline)-derived NHC (IBiox) used for copolymerization of olefins.
Figure 4. The trialkyl Sc complex bearing a bis(oxazoline)-derived NHC (IBiox) used for copolymerization of olefins.
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Figure 5. NHC-ligated Ln amides and phosphide used as catalysts for organic synthesis.
Figure 5. NHC-ligated Ln amides and phosphide used as catalysts for organic synthesis.
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Figure 6. NHC-ligated Ln amides and phosphide used as catalysts for organic synthesis.
Figure 6. NHC-ligated Ln amides and phosphide used as catalysts for organic synthesis.
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Figure 7. U and Ln–mesoionic carbene (MIC) complexes and two of the principal resonance forms for the MIC complexes.
Figure 7. U and Ln–mesoionic carbene (MIC) complexes and two of the principal resonance forms for the MIC complexes.
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Figure 8. The Y and Ti complexes bearing heteroatom-tethered NHC ligands used for ring-opening polymerization (ROP) of lactide.
Figure 8. The Y and Ti complexes bearing heteroatom-tethered NHC ligands used for ring-opening polymerization (ROP) of lactide.
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Figure 9. Homoleptic Ln complexes with alkoxo/aryloxo-tethered NHC ligands.
Figure 9. Homoleptic Ln complexes with alkoxo/aryloxo-tethered NHC ligands.
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Figure 10. Reactivity of Ce(Arylo-NHC)3 with CO2 and their catalytic performance on the formation of propylene carbonate [59].
Figure 10. Reactivity of Ce(Arylo-NHC)3 with CO2 and their catalytic performance on the formation of propylene carbonate [59].
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Figure 11. Ln complexes bearing alkoxy-NHC ligands.
Figure 11. Ln complexes bearing alkoxy-NHC ligands.
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Figure 12. NHC side-armed half-sandwich Ln complexes.
Figure 12. NHC side-armed half-sandwich Ln complexes.
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Figure 13. NHC side-armed half-sandwich Ln complexes used for coordination polymerization.
Figure 13. NHC side-armed half-sandwich Ln complexes used for coordination polymerization.
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Figure 14. The amidino-NHC ligand-supported Ln complexes used for polymerization of 2-phenyl-1,3-butadiene.
Figure 14. The amidino-NHC ligand-supported Ln complexes used for polymerization of 2-phenyl-1,3-butadiene.
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Figure 15. CCC- and CNC-pincer Ln complexes.
Figure 15. CCC- and CNC-pincer Ln complexes.
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Figure 16. The Ln complexes bearing chelating bis(phenolate)-NHC ligands (BisP-NHC-5 and BisP-NHC-6) [81,82].
Figure 16. The Ln complexes bearing chelating bis(phenolate)-NHC ligands (BisP-NHC-5 and BisP-NHC-6) [81,82].
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Pan, Y.; Jiang, X.; So, Y.-M.; To, C.T.; He, G. Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization. Catalysts 2020, 10, 71. https://doi.org/10.3390/catal10010071

AMA Style

Pan Y, Jiang X, So Y-M, To CT, He G. Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization. Catalysts. 2020; 10(1):71. https://doi.org/10.3390/catal10010071

Chicago/Turabian Style

Pan, Yu, Xinxin Jiang, Yat-Ming So, Ching Tat To, and Gaohong He. 2020. "Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization" Catalysts 10, no. 1: 71. https://doi.org/10.3390/catal10010071

APA Style

Pan, Y., Jiang, X., So, Y. -M., To, C. T., & He, G. (2020). Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization. Catalysts, 10(1), 71. https://doi.org/10.3390/catal10010071

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