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

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.


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].

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 heteroatomtethered 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].   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(N i Pr2)]4 (Ln = Yb, Y) with aryloxo-tethered NHC precursors and n BuLi at a low temperature afforded the amido Yb complexes 36 and 37, or homoleptic Y complex 38. When the reaction of Li[Y(N i Pr2)]4 with an aryloxo-tethered NHC precursor and n BuLi 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 n BuLi (Figure 11) [64].    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/Al i Bu3/[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/Al i Bu3/[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-methyleneγ-butyrolactone (MBL), γ-methyl-α-methylene-γ-butyrolactone (γMMBL), and β-methyl-αmethylene-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.

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.

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.