Heteroleptic lanthanide amide complexes bearing carbon-bridged bis(phenolate) ligands: synthesis, structure and their application in the polymerization of ε-caprolactone

Abstract

A convenient method for the synthesis of anionic lanthanide amide complexes bearing carbon-bridged bis(phenolate) ligands H₂L [L = (o-CH₃)PhCH(C₆H₂-3-^tBu-5-R-2-O)₂; R = Me, L = L₁ and R = ^tBu, L = L₂] is described. The bis(phenolato)lanthanide complexes LLnN(SiMe₃) [L = L₁, Ln = La (1), Ln = Gd (2) and L = L₂, Ln = La (3), Ln = Gd (4)] were synthesized in nearly quantitative yields by the reaction of Ln[N(SiMe₃)₂]₃ (Ln = La or Gd) with H₂L in a 1 : 1 molar ratio in THF at 60 °C. The bis(phenolato)lanthanide pyrazolato complexes LLnPzMe₂(THF)₃ [L = L₁, Ln = La (5), Ln = Gd (6) and L = L₂, Ln = La (7), Ln = Gd (8)] were obtained in high yields by further reacting the obtained LLnN(SiMe₃) with another 1 equiv. of 3,5-dimethylpyrazole (Me₂PzH). Meanwhile, the complexes LLnPzMe₂ can also be synthesized by the direct reaction of Ln[N(SiMe₃)₂]₃ with H₂L and Me₂PzH in 1 : 1 : 1 molar ratio in situ in THF. Complexes 5–8 have been characterized by X-ray crystal structural analysis. The central lanthanide metal atom is seven-coordinated by one bis(phenolate) ligand, one pyrazole ligand and three THF molecules in a distorted pentagonal bipyramid. The catalytic activity of compounds 1–8 in the ring-opening polymerization of ε-caprolactone was studied. The catalytic mechanism was studied and discussed as well.

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Introduction

Aliphatic polyesters are a preferred choice for potential environmentally friendly plastic materials because of their biodegradability, biocompatibility and potential availability from renewable resources.^1–3 The polymerization mechanism could be defined as enzymatic polymerization,^4–6 polycondensation,⁷ anionic polymerization,^8,9 cationic polymerization,^10,11 or coordination/insertion polymerization.^12,13 The ring opening polymerization of cyclic esters with metal complex-induced coordination/insertion type polymerization, however, is currently a topical research field. A wide range of metals have been explored for the ring-opening polymerization (ROP) of lactones through the coordination/insertion mechanism; typically alkali metals,¹⁴ alkaline earth metals,^15–17 transition metals,^18–20 and rare earth metals.^21–23 Among them, organolanthanide based systems seem to be active and suitable in preparing well-defined polymers with high molecular weights in narrow polydispersity due to their high activities and capabilities.^24,25 Recently, bulky carbon-bridged bis(phenolate) ligands have attracted increasing attention in organolanthanide chemistry.^26–28 They not only have attractive features, such as being easily available and tunable, but they can also provide O,O-bidentate chelating to stabilize the metal center, to realize the single active site complex and limit back-biting side reactions during polymerization. Carbon-bridged bis(phenolate) lanthanide derivatives showed high activity for the polymerization of cyclic esters. Shen and co-workers have demonstrated that bridged bis(phenolate) lanthanide complexes can serve as catalysts for the ROP of lactide²⁹ and lactones.³⁰ However, most studies paid attention to bis(phenolate)lanthanide alkoxides.^31,32 Meanwhile, the structural chemistry of anionic five-membered nitrogen heterocyclic ligands, especially pyrazolate and 1,2,4-triazolate ligands, has received much attention because of the structural diversity and variety of the coordination modes of these compounds.³³ Pyrazolate ligands are among the most versatile of ligands, with 20 different coordination modes identified so far.^34–36 To the best of our knowledge, the application of heteroleptic pyrazolato lanthanide complexes bearing carbon-bridged bis(phenolate) ligands in ring-opening polymerization has not been reported.

In order to develop these new lanthanide containing organometallic complexes which could be used as catalysts in the polymerization of polyesters, we turned our attention to related carbon-bis(phenolate) ligands with two O,O-bidentate ligands (H₂L₁–H₂L₂). We report here the synthesis of a series of bis(phenolato)lanthanide complexes (1–4) and heteroleptic pyrazolato lanthanide complexes (5–8) bearing carbon-bridged bis(phenolate) ligands for the first time. The complexes obtained were characterized by nuclear magnetic resonance (NMR) spectroscopy and elemental analysis, respectively. The single crystals of complexes 5–8 have been determined by X-ray diffraction. The catalytic properties of the complexes (1–8) have also been studied in the polymerization of ε-caprolactone under mild conditions.

Results and discussion

Synthesis and characterization of lanthanide complexes 1–8

Ligands (H₂L₁ and H₂L₂) were prepared according to the literature procedure (Scheme 1).³⁷ A series of lanthanide complexes can be synthesized by amine elimination reactions. Bis(phenolato)lanthanide complexes (1–4) were prepared by the reaction of Ln[N(SiMe₃)₂]₃ (Ln = La or Gd) with H₂L (H₂L₁ and H₂L₂) in a 1 : 1 molar ratio in THF at 60 °C in nearly quantitative yields. Complexes 5–8 (LLnPzMe₂(THF)₃ [L = L₁, Ln = La (5), Ln = Gd (6), and L = L₂, Ln = La (7), Ln = Gd (8)]) could be synthesized in high yields by using complexes 1–4 and further reacting them with 1 equiv. of Me₂PzH in THF (Scheme 2). Complexes 5–8 can also be synthesized by the direct reaction of Ln[N(SiMe₃)₂]₃ with H₂L₁/H₂L₂ and Me₂PzH in a 1 : 1 : 1 molar ratio in situ in THF (Scheme 2).

Scheme 1 Synthesis of the ligands.

Scheme 2 Synthesis of the complexes 1–8.

Meanwhile, we also tried another route for the synthesis of complexes 5–8. First of all, Ln[N(SiMe₃)₂]₃ reacted with Me₂PzH in a 1 : 1 molar ratio in THF to produce complexes Me₂PzLn[N(TMS)₂]₂(THF)₂, which can be used as precursors to synthesize the corresponding heteroleptic pyrazolato lanthanide complexes 5–8 by a further reaction with 1 equiv. of H₂L_n (n = 1 or 2) (Scheme 3).

Scheme 3 Synthesis of the complexes 5–8 (Method C).

Due to the strong paramagnetism of the central Gd metal ions , complexes 2, 4, 6 and 8 did not provide any resolvable ¹H NMR spectra. In the ¹H NMR spectra of complexes 1 and 3, except for the resonances of the L²⁻ groups, Si(CH₃)⁻ group and THF molecules, no resonance of the hydroxyl proton of the bridged bis(phenol) was observed, indicating that the phenolate lanthanide amides were formed. Disappearance of the Si(CH₃)⁻ group signal (δ = 0.4–0.5) in the ¹H NMR spectra of lanthanide complexes 5 and 7 indicated that the substitution reaction had been achieved. In the FT-IR spectrogram, we find that the absorption peaks of complexes 1–8 are very similar to those of the corresponding ligands, and the absorption peaks of the active hydrogen of ligands H₂L and Me₂PzH are not present, which can also indicate that the substitution reaction had indeed occurred.

It was not possible to determine the definitive structures of complexes 1–4, due to solvent loss and the deterioration in crystal quality. By single-crystal structure analysis, the definitive structures of complexes 5–8 were determined (see below). The definitive molecular structures determined by X-ray crystal structural analysis are consistent with the ¹H NMR results.

Crystal structures

Crystals suitable for the X-ray structure determination of complexes 5–8 were obtained from a hexane/THF mixed solution at room temperature. The crystallographic data and experimental details of the data collection, as well as the structural refinement are given in Table 1. The molecular structures of molecules 5 and 6 are depicted in Fig. 1 and 2.

Table 1 Crystallographic data for 5–8

	5	6	7	8
a R1 = ∑(||Fo| − |Fc||)/∑|Fo|; wR2 = [∑(w(Fo^2 − Fc^2)^2)/∑(wFo^4)]^1/2.b w5 = 1/[σ^2(Fo^2) + (0.159P)^2], P = (Fo^2 + 2Fc^2)/3, w6 = 1/[σ^2(Fo^2) + (0.1414P)^2], P = (Fo^2 + 2Fc^2)/3, w7 = 1/[σ^2(Fo^2) + (0.0351P)^2], P = (Fo^2 + 2Fc^2)/3, w8 = 1/[σ^2(Fo^2) + (0.0405P)^2 + 1.2726P], P = (Fo^2 + 2Fc^2)/3.c S = [∑w(Fo^2 − Fc^2)^2]/(n − p)^1/2, n = number of reflections, p = parameters used.
Formula	C47H67LaN2O6	C47H67GdN2O6	C53H79LaN2O6	C53H79GdN2O6
Formula weight	894.94	913.28	979.09	997.43
Crystal system	Triclinic	Triclinic	Monoclinic	Monoclinic
Space group	P1̄	P1̄	P21/c	P21/c
a/Å	13.336 (2)	13.348 (2)	13.629 (3)	13.740 (2)
b/Å	14.166 (3)	14.064 (3)	19.851 (4)	20.389 (4)
c/Å	16.340 (3)	16.293 (3)	19.143 (4)	18.914 (3)
α/deg	75.288 (2)	75.129 (2)	90	90
β/deg	70.091 (2)	69.752 (2)	96.50 (3)	97.246 (2)
γ/deg	63.392 (2)	62.759 (2)	90	90
V/Å^−3	2576.1 (8)	2533.6 (8)	5145.8 (19)	5256.2 (15)
Z	2	2	4	4
Dc/Mg m^−3	1.154	1.197	1.264	1.260
F(000)	936	950	2064	2092
μ/mm^−1	0.87	1.35	0.88	1.31
Cryst size (mm)	0.20 × 0.20 × 0.20	0.20 × 0.20 × 0.20	0.20 × 0.20 × 0.20	0.20 × 0.20 × 0.20
Temp/K	296	296	173	296
θ range (°)	1.3–25	1.3–25	3.2–25	1.5–25
Reflections	8990	8764	9034	9257
R1^a	0.064	0.059	0.030	0.031
wR2^b	0.221	0.205	0.076	0.080
Δρmin, max/e Å^−3	−0.59, 1.82	−1.19, 1.89	−0.39, 0.60	−0.40, 0.54
Goodness of fit^c	1.12	1.05	1.05	1.03

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Fig. 1 Molecular structure of complex 5 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Fig. 2 Molecular structure of complex 6 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Complexes 5 and 6 are isostructural and they have a monomeric structure; the lanthanide atoms are seven-coordinated by two nitrogen atoms from the pyrazolato group, two oxygen atoms from one L₁²⁻ group, and three oxygen atoms from three THF molecules. The coordination geometry at the lanthanide atom can be best described as a distorted pentagonal bipyramid. The average La–O(Ar) bond length of 5 (2.272 (6) Å) is comparable with the Gd–O(Ar) bond length of 6 (2.18 (3) Å) taking into account the difference in ionic radii. The average La–O(Ar) bond length (for 5) is comparable with those in (C₅H₅)La(MBMP)(THF)₃ (2.256 Å), (MBMP²⁻ = 2,2-methylene-bis(6-tert-butyl-4-methyl-phenoxo))³⁸ and [ONNO]LaCp (2.291 Å) [ONNO = Me₂NCH₂CH₂N{CH₂–(2-O–C₆H₂)–^tBu-3-Me-5}₂],³⁹ but slightly longer than those in [(MBMP)La(μ-OCH₂Ph)(THF)₂]₂ (2.226 Å)⁴⁰ and L′₂La(TMS)₂ (2.227 Å) (L′ = 3,5-But₂-2-(O)–C₆H₂CH=N-2,6-Prⁱ₂-C₆H₃).¹³ The average C–O bond lengths of the phenolate ligands in complexes 5 and 6 are 1.335(7) Å and 1.351(8) Å, respectively, which are apparently shorter than the single C–O bond length, reflecting a substantial electron delocalization from the oxygen into the aromatic rings.

In complexes 5 and 6, the two nitrogen atoms of the pyrazolato group are also coordinated to the lanthanide atom, the Ln–N (1) bond lengths are comparable with the Ln–N (2) bond lengths. The average La–N bond length of 5 (2.512(6) Å) is comparable with the Gd–N bond length of 6 (2.415 (7) Å) when the difference in ionic radii is considered.

Compared with the neutral pyrazole, the C–C bond lengths and C–N bond lengths of the anionic pyrazole have some obvious differences. In [La(Ph₂Pz)₃(Ph₂PzH)₂],⁴¹ the C–C bond lengths of the neutral pyrazole are 1.372 Å and 1.410 Å, and the C–N bond lengths are 1.343 Å and 1.354 Å. While in complex 5, the C–C bond lengths of the anionic pyrazole are 1.368 Å and 1.374 Å, and the C–N bond lengths are 1.333 Å and 1.339 Å. So, it’s very difficult to distinguish between C–C single bonds and C=C double bonds from the bond length, as it is also not possible to distinguish between a C–N single bond and a C=N double bond. The chemical shift of C–H protons for the neutral pyrazole ligand Me₂PzH is 5.73 ppm, however the chemical shift of C–H protons for the anionic pyrazole ligand Me₂Pz^¬ (complexes 5 and 7) is 5.76 ppm, so it is clear that there is a slight difference between neutral and anionic pyrazole ligands, which could be attributed to the anionic pyrazole ligand undergoing delocalization.

The molecular structures of 7 and 8 are depicted in Fig. 3 and 4. Complexes 7 and 8 are isostructural, and they have a monomeric structure; the central metal atom is seven-coordinated by one bis(phenolate) ligand, one pyrazole group, and three THF molecules in a distorted pentagonal bipyramid. The average La–O(Ar) bond length of 7 (2.304 (17) Å) is comparable with the Gd–O(Ar) bond length of 8 (2.195 (2) Å) considering the difference in ionic radii. The molecular structure analyses of complexes 7 and 8 are consistent with the complexes 5 and 6.

Fig. 3 Molecular structure of complex 7 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Fig. 4 Molecular structure of complex 8 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Table 2 Polymerization of ε-CL initiated by 1–8^a

Entry	initiator	[M]0/[I]0	Time (min)	Yield^b (%)	Mn^c (10^4)	PDI^c
a General polymerization conditions: [M]0/[I]0: the ratio of monomer to Ln (Gd or La); solvent = toluene; T = 20 °C.b Yield: weight of polymer obtained/weight of monomer used.c Measured by GPC in THF calibrated with a polystyrene standard.
1	1	100	1	95	6.43	1.47
2	1	250	1	93	6.78	1.39
3	1	500	10	77	8.73	1.18
4	1	750	60	46	5.63	1.42
5	2	100	1	92	7.85	1.41
6	2	250	1	90	6.73	1.53
7	2	500	10	83	5.84	1.30
8	2	750	60	50	5.36	1.28
9	3	100	1	97	6.21	1.37
10	3	250	1	100	5.83	1.21
11	3	500	10	80	6.81	1.45
12	3	750	60	54	5.72	1.33
13	4	100	1	88	6.29	1.27
14	4	250	1	96	7.34	1.29
15	4	500	10	83	5.47	1.18
16	4	750	60	44	4.85	1.23
17	5	100	10	81	3.42	1.41
18	5	250	30	82	3.53	1.39
19	6	100	10	83	3.31	1.34
20	6	250	30	77	3.78	1.37
21	7	100	10	82	4.32	1.28
22	7	250	30	87	4.68	1.31
23	8	100	10	88	3.67	1.26
24	8	250	30	79	4.57	1.39
25	a	100	1	86	5.61	2.03
26	a	250	10	98	6.42	2.24
27	b	100	1	89	6.70	2.15
28	b	250	10	96	5.84	2.38

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Catalytic activity studies

The performance of complexes 1–8 as catalysts for the ROP of ε-CL was examined in a toluene solution at 20 °C, and the preliminary results are listed in Table 2. It can be seen that all of the complexes are efficient initiators of the polymerization of ε-CL in toluene. All polymerizations proceeded quickly and completed within 1 h to give polymers with relatively narrow molecular weight distributions (PDIs) (≤1.53). These initiators showed high activity (even for [M]₀/[I_1–4]₀ = 750, the polymerization still can proceed smoothly) and produced PCL with a molecular weight of 8.73 × 10⁴ and a PDI of 1.18. In comparison with the carbon-bridged bis(phenolato)lanthanide alkoxides reported by Yao and co-workers (time of polymerization 1 h, temperature 17 °C, yield 31–100%, M_n 0.68 × 10⁴–4.56 × 10⁴, PDI 1.14–1.29),⁴² complexes 1–4 exhibit higher catalytic activity.

The ionic radii of the lanthanide metals have a profound effect on the polymerization activity of the corresponding lanthanide complexes.³⁶ However, the lanthanum amides showed a similar activity for the polymerization in comparison with the corresponding gadolinium amide complexes. For example, using complex 1 as the initiator, the yields reach 95% (M_n: 6.43 × 10⁴) and 77% (M_n: 8.73 × 10⁴) when the molar ratios of monomer to initiator are respectively 100 and 500 (Table 2 entries 1 and 3); whereas the yields are 92% (M_n: 7.85 × 10⁴) and 83% (M_n: 5.84 × 10⁴) using the complex 2 as the initiator (Table 2 entries 5 and 7), which isn’t consistent with the activity trend for the polymerization of cyclic esters initiated by lanthanide-based complexes.^43,44 All of the same kind of amido lanthanide complexes show a similar catalytic behavior.

The amide groups have a significant effect on the polymerization. Complexes 1–4 showed a higher activity for polymerization than complexes 5–8. Using 1 as the initiator, the yield reaches 95% in 1 min at room temperature when the molar ratio of monomer to initiator ([M₀]/[I₀]) is 250, whereas the yield is 81% in 30 min using 5 as the initiator under the same polymerization conditions (Table 2 entries 1 and 17). Using 1–4 as the initiator, even for [M]₀/[I]₀ = 750, the polymerization still can proceed, while it wasn’t completed after several days when using complexes 5–8 as the initiator under the same polymerization conditions. This was attributed to the different molecular structures of the lanthanide amides, the bulky –PzMe₂ groups may have a bad effect on the polymerization. However, the effect of the bis(phenolato) groups in these complexes on the polymerization was not observed.

The microstructure of PCL was determined by ¹H NMR experiments using the initiator 1 and a 10 : 1 monomer to initiator ratio, as shown in Fig. 5.

Fig. 5 ¹H NMR spectrum of PCL (CDCl₃).

The ¹H NMR spectrum of the polymer clearly shows that only the –N(SiMe₃)₂ group was observed, according to the resonance at −0.01 ppm. No resonance signal was observed for the phenolate ligand. The signal at 3.64 ppm can be assigned to the methylene protons at the α-position to the terminal hydroxy group. One possible polymerization mechanism is via coordination/insertion (shown in Scheme 4), which is similar to the literature.^45,46

Scheme 4 Possible mechanism for the ROP of ε-CL.

Experimental section

HN(TMS)₂ (TMS = SiMe₃), ε-caprolactone, and ⁿBuLi are commercially available. All manipulations were performed under pure nitrogen with a rigorous exclusion of air and moisture using Schlenk techniques and a glovebox. THF, toluene, and hexane were distilled from sodium benzophenone ketyl before use. ε-Caprolactone was dried over CaH₂ for 3 days, and distilled under reduced pressure. HN(TMS)₂ was dried with a small amount of sodium and distilled before use. The starting complexes La[N(TMS)₂]₃/Gd[N(TMS)₂]₃,⁴⁷ H₂L₁–H₂L₂ (ref. 38) and Me₂PzH⁴⁸ were synthesized according to published procedures. Lanthanide analyses were performed by EDTA titration with a xylenol orange indicator and hexamine buffer. NMR spectra were recorded on a Bruker Advance 400 spectrometer at resonant frequencies of 400 MHz for ¹H and 101 MHz for ¹³C nuclei using C₆D₆ or d₆-DMSO as the solvent. Elemental analysis was performed on a Perkin-Elmer 240C elemental analyzer. Melting points were observed in sealed capillaries and were uncorrected. The weight average molecular weight (M_w) and the number average molecular weight (M_n) were determined by GPC on a Water GPC system equipped with four Waters Ultrastyragel columns (300 × 7.5 mm, guarded and packed with 1 × 10⁵, 1 × 10⁴, 1 × 10³, and 500 Å gels respectively) in series. Tetrahydrofuran (THF, 1 mL min⁻¹) was used as the eluent and the signal was monitored by a differential refractive index detector. Monodisperse polystyrene was used as the molecular weight standard.

X-ray data collection and refinement of crystal structure

The crystals of the complexes were mounted on a glass fiber for X-ray measurement. Diffraction data were collected on a Rigaku Mercury CCD area detector in the ω scan mode using MoKα radiation (λ = 0.71073 Å) at 223(2) K. All of the measured independent reflections (I > 2σ(I)) were used in the structural analysis and semi-empirical absorption corrections were applied using SADABS.⁴⁹ The structure was solved and refined using SHELXL-97.⁵⁰ All of the hydrogen atoms were positioned geometrically and refined using a riding model. The non-hydrogen atoms were refined with anisotropic thermal parameters.

Ring-opening polymerization for ε-caprolactone

The procedures for the ring-opening polymerization of ε-caprolactone initiated by complexes 1–8 were similar, and a typical polymerization procedure is given below. In the glovebox, a 100 mL Schlenk flask, equipped with a magnetic stirring bar, was charged with a solution of initiator in toluene. To this solution was added the desired amount of ε-caprolactone by syringe. The mixture was stirred vigorously for the desired time. Then, a couple of drops of 1 M HCl–ethanol solution were added to fully quench the reaction and the polymer was precipitated, and the precipitate was dried under vacuum and weighed.

General procedure

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3-^tBu-5-Me-2-O)₂]La[N(TMS)₂], [L₁La[N(TMS)₂] (1). H₂L₁ (0.92 g, 2.06 mmol) dissolved in 15 mL of THF was slowly added to La[N(TMS)₂]₃ (1.28 g, 2.06 mmol) dissolved in 25 mL of THF. The reaction mixture was stirred for 3 h at 60 °C, and all volatiles were removed under oil pump vacuum. 10 mL of toluene was added to extract the product, and orange-yellow microcrystals were obtained in a nearly quantitative yield by cooling the toluene solution (1.41 g, 92% based on La). mp 236–238 °C. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ = 7.21 (s, ArH, 2H), 7.13 (s, ArH, 2H), 7.01 (d, J = 7.4 Hz, ArH, 2H), 6.74 (d, J = 7.9 Hz, ArH, 1H), 6.56 (d, J = 2.0 Hz, ArH, 1H), 5.84 (s, CH, 1H), 3.00 (s, OCH₃, 3H), 2.20 (s, CH₃, 6H), 1.29 (s, C(CH₃)₃, 18H), 0.48 (s, Si(CH₃)₃, 18H). ¹³C NMR (101 MHz, C₆D₆, 25 °C): δ = 158.7 (Ar), 158.1 (Ar), 136.8 (Ar), 132.5 (Ar), 130.2 (Ar), 129.4 (Ar), 129.3 (Ar), 127.3 (Ar), 125.8 (Ar), 124.1 (Ar), 121.5 (Ar), 113.1 (Ar), 55.9 (OCH₃), 41.7 (CH), 35.5 (C(CH₃)₃), 30.7 (C(CH₃)₃), 25.5 (CH₃), 5.4 (Si(CH₃)₃). Anal. calcd for C₃₆H₅₄LaNO₃Si₂: C, 58.12; H, 7.32; N, 1.88; La, 18.67. Found: C, 57.94; H, 7.54; N, 1.72; La, 18.51. IR (KBr, cm⁻¹): 2961 (s), 2899 (m), 2871 (m), 1601 (w), 1460 (m), 1437 (m), 1362 (m), 1248 (w), 1024 (w), 833 (w), 752 (w).

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3-^tBu-5-Me-2-O)₂]Gd[N(TMS)₂], [L₁Gd[N(TMS)₂] (2). The synthesis of complex 2 was carried out in the same way as is described for the synthesis of complex 1, but Gd[N(TMS)₂]₃ (1.32 g, 2.07 mmol) was used instead of La[N(TMS)₂]₃. Orange-yellow microcrystals were obtained from subsequent work (1.47 g, 93% based on Gd). mp 242–245 °C. Anal. calcd for C₃₆H₅₄GdNO₃Si₂: C, 56.73; H, 7.14; N, 1.84; Gd, 20.63. Found: C, 56.58; H, 7.37; N, 1.75; Gd, 20.49. IR (KBr, cm⁻¹): 2960 (s), 2902 (m), 2870 (m), 1599 (w), 1460 (m), 1439 (m), 1361 (m), 1245 (w), 1018 (w), 834 (w), 750 (w).

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3,5-^tBu₂-2-O)₂]La[N(TMS)₂], [L₂La[N(TMS)₂] (3). The synthesis of complex 3 was carried out in the same way as is described for the synthesis of complex 1, but H₂L₂ (0.99 g, 1.86 mmol) was used instead of H₂L₁. The following procedure is similar to that described for complex 1, and complex 3 was isolated as orange-yellow crystals (1.45 g, 92%). mp 241–243 °C. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ = 7.43 (d, J = 7.6 Hz, ArH, 2H), 7.40 (d, J = 2.0 Hz, ArH, 2H), 7.12 (d, J = 7.9 Hz, ArH, 1H), 6.76 (t, J = 7.4 Hz, ArH, 1H), 6.59 (d, J = 2.4 Hz, ArH, 1H), 5.79 (s, CH, 1H), 3.00 (s, OCH₃, 3H), 2.20 (s, CH₃, 6H), 1.36 (s, C(CH₃)₃, 18H), 1.29 (s, C(CH₃)₃, 18H), 0.47 (s, Si(CH₃)₃, 18H). ¹³C NMR (101 MHz, C₆D₆, 25 °C): δ = 158.6 (Ar), 158.3 (Ar), 137.8 (Ar), 137.3 (Ar), 135.9 (Ar), 132.1 (Ar), 129.8 (Ar), 127.2 (Ar), 126.0 (Ar), 121.6 (Ar), 121.1 (Ar), 113.5 (Ar), 56.3 (OCH₃), 42.1 (CH), 35.8 (C(CH₃)₃), 34.4 (C(CH₃)₃), 32.1 (C(CH₃)₃), 30.7 (C(CH₃)₃), 5.4 (Si(CH₃)₃). Anal. calcd for C₄₂H₆₆LaNO₃Si₂: C, 60.92; H, 8.03; N, 1.69; La, 16.77. Found: C, 60.98; H, 8.14; N, 1.52; La, 16.84. IR (KBr, cm⁻¹): 2957 (s), 2905 (m), 2871 (m), 1595 (w), 1462 (m), 1440 (m), 1359 (m), 1246 (w), 1022 (w), 831 (w), 751 (w).

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3,5-^tBu₂-2-O)₂]Gd[N(TMS)₂], [L₂Gd[N(TMS)₂] (4). Synthesis of complex 4 was carried out in the same way as is described for complex 1, but H₂L₂ (0.99 g, 1.86 mmol) and Gd[N(TMS)₂]₃ (1.19 g, 1.86 mmol) were used and the subsequent work afforded 4 as orange-yellow microcrystals (1.46 g, 93% based on Gd). mp 248–251 °C. Anal. calcd for C₄₂H₆₆GdNO₃Si₂: C, 59.60; H, 7.86; N, 1.65; Gd, 18.58. Found: C, 59.54; H, 7.94; N, 1.53; Gd, 18.64. IR (KBr, cm⁻¹): 2960 (s), 2895 (m), 2868 (m), 1603 (w), 1458 (m), 1442 (m), 1360 (m), 1251 (w), 1025 (w), 829 (w), 754 (w).

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3-^tBu-5-Me-2-O)₂]La(PzMe₂)(THF)₃, [L₁Gd(Me₂Pz)(THF)₃] (5).
Method A. Complex 1 (1.16 g, 1.56 mmol) dissolved in 10 mL of THF was added to Me₂PzH (0.15 g, 1.56 mmol) dissolved in 10 mL of THF. The reaction mixture was stirred for 6 h at 60 °C, and then all of the volatiles were removed under oil pump vacuum. Toluene was added to extract the product, and pale-blue microcrystals were obtained from the concentrated toluene solution at −30 °C (1.20 g, 86% based on La). mp 256–258 °C. ¹H NMR (400 MHz, d₆-DMSO, 25 °C): δ = 7.25 (t, J = 7.5 Hz, ArH, 1H), 7.16 (s, ArH, 1H), 7.08 (m, ArH, 1H), 6.77 (d, J = 7.9 Hz, ArH, 1H), 6.60 (s, ArH, 2H), 6.51 (s, ArH, 2H), 6.39 (s, CH, 1H), 5.75 (s, PzH, 1H), 3.60 (t, J = 5.8 Hz, THF, 12H), 3.34 (s, OCH₃, 3H), 2.12 (s, Pz(CH₃)₂, 6H), 2.02 (s, ArCH₃, 6H), 1.76 (t, J = 5.9 Hz, THF, 12H), 1.37 (s, C(CH₃)₃, 18H). ¹³C NMR (101 MHz, d₆-DMSO, 25 °C): δ = 162.7 (Ar), 161.2 (Ar), 157.9 (C(Pz)), 137.1 (Ar), 134.2 (Ar), 133.9 (Ar), 129.7 (Ar), 128.2 (Ar), 127.3 (Ar), 125.3 (Ar), 122.8 (Ar), 121.5 (Ar), 112.2 (Ar), 103.1 (C(Pz)), 67.5 (THF), 56.4 (OCH₃), 55.5 (CH), 35.0 (C(CH₃)₃), 31.4 (C(CH₃)₃), 31.2 (ArCH₃), 25.6 (THF), 21.5 (CH₃). Anal. calcd for C₄₇H₆₇LaN₂O₆: C, 63.08; H, 7.55; N, 3.13; La, 15.22. Found: C, 62.92; H, 7.67; N, 3.04; La, 15.13. IR (KBr, cm⁻¹): 2960 (s), 2920 (m), 2868 (m), 1590 (w), 1465 (m), 1442 (m), 1397 (w), 1285 (w), 1250 (m), 1178 (w), 1034 (w), 864 (w), 754 (m), 692 (w).
Method B. H₂L₁ (0.92 g, 2.06 mmol) dissolved in 15 mL of THF was slowly added to La[N(TMS)₂]₃ (1.28 g, 2.06 mmol) dissolved in 25 mL of THF at 60 °C. After the solution was stirred for 3 h at 60 °C, Me₂PzH (0.20 g, 2.06 mmol) was added. The reaction mixture was stirred for 6 h at 60 °C, and then all of the volatiles were removed under oil pump vacuum. Toluene was added to extract the product, and pale-blue microcrystals were obtained by cooling the toluene solution (1.56 g, 85% based on La).
Method C. Me₂PzH (0.84 g, 1.35 mmol) dissolved in 15 mL of THF was slowly added to La[N(TMS)₂]₃ (0.13 g, 1.35 mmol) dissolved in 25 mL of THF at −30 °C. After the solution was stirred for 12 h at 60 °C, H₂L₁ (0.60 g, 1.35 mmol) was added. The reaction mixture was stirred for 6 h at 60 °C, and then all of the volatiles were removed under oil pump vacuum. Toluene was added to extract the product, and pale-blue microcrystals were obtained by cooling the toluene solution (0.86 g, 85% based on La).

Complex c {Me₂PzLa[N(TMS)₂]₂(THF)₂}: mp 187–189 °C. ¹H NMR (400 MHz, C₆D₆): δ 6.17 (s, 1H, Pz-H), 3.68 (m, 8H, THF), 2.29 (s, 6H, CH₃), 1.34 (m, 8H, THF), 0.34 (s, 36H, SiMe₃). ¹³C NMR (101 MHz, C₆D₆): δ 145.2(C=N), 109.4(C=C), 68.5(THF), 24.9(THF), 13.0(CH₃), 3.6(Si(CH₃)₃). Anal. calcd for C₂₅H₅₇LaN₄O₂Si₄: La, 19.93; C, 43.08; H, 8.24; N, 8.04. Found: La, 19.73; C, 43.24; H, 8.02; N, 8.19.

Complex d {Me₂PzGd[N(TMS)₂]₂(THF)₂}: mp 194–196 °C. Anal. calcd for C₂₅H₅₇GdN₄O₂Si₄: Gd, 21.92; C, 41.86; H, 8.29; N, 7.81. Found: La, 21.71; C, 42.03; H, 8.13; N, 8.02.

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3-^tBu-5-Me-2-O)₂]Gd(PzMe₂)(THF)₃, [L₁Gd(Me₂Pz)(THF)₃] (6). The synthesis of complex 6 was carried out in a similar way to that described for complex 5 (Method B), but Gd[N(TMS)₂]₃ (1.12 g, 1.75 mmol) was used instead of La[N(TMS)₂]₃. An amount of pale-blue microcrystals was obtained from the concentrated toluene solution (1.39 g, 87%, based on Gd). mp 267–269 °C. Anal. calcd for C₄₇H₆₇GdN₂O₆: C, 61.81; H, 7.39; N, 3.07; Gd, 17.22. Found: C, 61.72; H, 7.51; N, 3.14; Gd, 17.11. IR (KBr, cm⁻¹): 2958 (s), 2919 (m), 2871 (m), 1587 (w), 1467 (m), 1442 (m), 1394 (w), 1288 (w), 1246 (m), 1180 (w), 1030 (w), 866 (w), 756 (m), 691 (w).

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3,5-^tBu₂-2-O)₂]La(PzMe₂)(THF)₃, [L₂La(PzMe₂)(THF)₃] (7). The synthesis of complex 7 was carried out in a similar way to that described for complex 5 (Method B), but H₂L₂ (1.15 g, 2.16 mmol) was used instead of H₂L₁. Pale-blue microcrystals were obtained from the concentrated toluene solution (1.84 g, 87%, based on La). mp 264–266 °C. ¹H NMR (400 MHz, d₆-DMSO, 25 °C): δ = 7.25 (m, ArH, 1H), 7.18 (d, J = 7.1 Hz, ArH, 1H), 7.06 (m, ArH, 1H), 6.99 (d, J = 6.3 Hz, ArH, 2H), 6.76 (d, J = 5.8 Hz, ArH, 2H), 6.65 (m, ArH, 1H), 6.27 (s, CH, 1H), 5.76 (s, PzH, 1H), 3.60 (t, J = 5.8 Hz, THF, 12H), 3.19 (s, OCH₃, 3H), 2.13 (s, Pz(CH₃)₂, 6H), 1.76 (t, J = 5.9 Hz, THF, 12H), 1.39 (s, C(CH₃)₃, 18H), 1.13 (s, C(CH₃)₃, 18H). ¹³C NMR (101 MHz, d₆-DMSO, 25 °C): δ = 162.71 (Ar), 161.9 (Ar), 159.1 (C(Pz)), 139.8 (Ar), 133.6 (Ar), 133.5 (Ar), 133.0 (Ar), 132.6 (Ar), 130.4 (Ar), 129.4 (Ar), 128.7 (Ar), 124.5 (Ar), 112.9 (Ar), 103.8 (C(Pz)), 67.5 (THF), 56.8 (OCH₃), 54.9 (CH), 35.0 (C(CH₃)₃), 33.9 (C(CH₃)₃), 32.5 (C(CH₃)₃), 31.5 (C(CH₃)₃), 25.6 (THF), 21.5 (CH₃). Anal. calcd for C₅₃H₇₉LaN₂O₆: C, 65.01; H, 8.13; N, 2.86; La, 14.19. Found: C, 64.88; H, 8.26; N, 2.77; La, 14.28. IR (KBr, cm⁻¹): 2958 (s), 2905 (m), 2870 (m), 1594 (w), 1460 (m), 1438 (m), 1360 (m), 1288 (w), 1251 (w), 1049 (w), 1030 (w), 835 (w), 756 (m), 691 (w).

Synthesis of [(o-OCH₃)PhCH(C₆H₂-3,5-^tBu₂-2-O)₂]Gd(PzMe₂)(THF)₃, [L₂Gd(PzMe₂)(THF)₃] (8). The synthesis of complex 8 was carried out in a similar way to that described for complex 5 (Method B), but H₂L₂ (1.04 g, 1.95 mmol) and Gd[N(TMS)₂]₃ (1.24 g, 1.95 mmol) were used and the subsequent work afforded 8 as pale-blue microcrystals (1.64 g, 84%, based on Gd). mp 272–274 °C. Anal. calcd for C₅₃H₇₉GdN₂O₆: C, 63.82; H, 7.98; N, 2.81; Gd, 15.77. Found: C, 63.68; H, 7.86; N, 2.97; Gd, 15.63. IR (KBr, cm⁻¹): 2960 (s), 2904 (m), 2866 (m), 1600 (w), 1461 (m), 1436 (m), 1361 (m), 1292 (w), 1254 (w), 1054 (w), 1029 (w), 833 (w), 756 (m), 692 (w).

Conclusion

In summary, a series of new heteroleptic lanthanide amide complexes bearing carbon-bridged bis(phenolate) ligands have been synthesized via simple protonolysis exchange reactions using Ln[N(TMS)₂]₃ (Ln = La or Gd) as the starting material. At first, Ln[N(TMS)₂]₃ reacted with H₂L_n (n = 1 or 2) to produce complexes 1–4, which can further react with 1 equiv. of Me₂PzH in THF to yield heteroleptic pyrazolato lanthanide complexes 5–8 in high yield. Meanwhile, Ln[N(TMS)₂]₃ (Ln = La or Gd) reacted with Me₂PzH to form the complex Me₂PzLn[N(TMS)₂]₂(THF)₂, which can be used as a precursor to synthesize the corresponding heteroleptic pyrazolato lanthanide complexes by a further reaction with 1 equiv. of H₂L_n (n = 1 or 2). Moreover, the complexes LLnPzMe₂ can also be synthesized by the direct reaction of Ln[N(SiMe₃)₂]₃ with H₂L and Me₂PzH in 1 : 1 : 1 molar ratio in situ in THF, all of these syntheses are quite straight forward and easy to access. These aminolanthanide complexes are well-characterized, and complexes 5–8 are structurally characterized. The coordination geometries around the central metals in these complexes are similar. Furthermore, it was found that these complexes can catalyze the controlled polymerization of ε-CL via a coordination-insertion mode, and the amide groups have a significant effect on the polymerization.

Acknowledgements

This work was supported by State Key Laboratory of ASIC & System, Fudan University (12KF007), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1013330, 1013332, 1013334, 1024515. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra23520k

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