English

• 1.   Introduction

  Over the last two decades, polyesters have attracted considerable attention due to its biocompatibility, biodegradability and good drug penetrability.^[1] The ring-opening polymerization (ROP) of lactones provides a convenient method for the synthesis of biodegradable polyesters.^[2] In this aspect, organometallic complexes with adjustment of ligands play an important role not only in molecular weight and molecular weight distribution, but also in stereoregularity of polyesters. Many metal complexes including alkali metals, ^[3] alkaline earth metals, ^[4] aluminum, ^[5] zinc, ^[6] tin, ^[7] early transition metals, ^[8] and lanthanides^[9] have been used as initiators for the ROP of ε-caprolactone.

  On the contrast, lanthanide catalysts have received great interest due to their high activity and capability of giving polymers with high molecular weights in narrow polydispersity without alcohol initiators. Additionally, a diversity of structurally well-defined lanthanide catalysts, such as lanthanide alkoxides or aryloxides, ^[10] amides, ^[11] alkylates, ^[12] and rare-earth borohydride^[13] is effective initiators for living polymerization of lactones. The homoleptic amidinate^[14] and amidate^[15] complexes of the rare-earth metals have been extensively investigated and have been modestly successful in ROP of ε-caprolactone. Recently, we reported^[16] that homoleptic tris(pyrrolyl) lanthanide complexes could be used as single-component catalysts for ε-caprolactone polymerization, indicating that lanthanide complexes without active group could also initiate the ε-caprolactone polymerization. We herein report the synthesis and characterization of dinuclear rare-earth metal complexes with nitrogen-containing bridged dipyrrolyl ligand and their catalytic activities toward ROP of ε-caprolactone.

  2.   Results and discussion

  2.1   Synthesis and characterization of complexes 2~4

  Treatment of RE(CH₂SiMe₃)₃(THF)₂ with 1.5 equiv. of (C₄H₃NHCH₂)₂NCH₃ (1) in toluene, after workup, afforded nitrogen-containing bridged dipyrrolyl dinuclear rare-earth metal complexes [η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃]RE{μ-η⁵: η¹:η⁵:η¹:η¹-(C₄H₃NCH₂)₂NCH₃}RE[η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃](THF) [RE＝Y (2), Er (3), Yb (4)] (Scheme 1). The complexes are sensitive to air and moisture, and they have good solubility in polar solvents such as THF, Et₂O, toluene, and slight solubility in n-hexane. All the complexes were fully characterized by spectroscopic and element analyses, and the structures of complexes 2 and 4 were furtherly determined by X-ray diffraction.

  Scheme 1

  

  Scheme 1.  Synthesis of complexes 2~4

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  2.2   Molecular structures of the complexes

  X-ray analyses showed that complexes 2 (Figure 1) and 4 (Figure 2) were isostructural dinuclear structure. Each of rare earth metals adopts a distorted octahedral geometry, and the two rare earth metals are coordinated by one dipyrrolyl ligand, which coordinated to one rare-earth metal in an η⁵:η¹:η¹ mode, and to another rare-earth metal in an η⁵:η¹ mode. Except for dipyrrolyl ligand of the linked two rare earth metals, each rare earth metal was coordinated to another dipyrrolyl ligand in an η¹:η¹:η¹ mode. Selected bond lengths and angles are listed in Table 1.

  Figure 1

  

  Figure 1.  Molecular structure of complex 2

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  Figure 2

  

  Figure 2.  Molecular structure of complex 4

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  Table 1

  

  Table 1.  Selected bond lengths ( ) and bond angles (°)

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  Compd.	2	4
  Bond length
  RE(1)—N(1)	0.2315(4)	0.2276(6)
  RE(1)—N(2)	0.2527(4)	0.2506(6)
  RE(1)—N(3)	0.2297(4)	0.2249(6)
  RE(1)—N(4)	0.2584(3)	0.2359(5)
  RE(1)—N(5)	0.2620(3)	0.2604(5)
  RE(1)—N(6)	0.2405(3)	0.2569(5)
  RE(1)—C(12)	0.2707(4)	0.2583(7)
  RE(1)—C(13)	0.2819(4)	0.2722(7)
  RE(1)—C(14)	0.2763(4)	0.2786(7)
  RE(1)—C(15)	0.2598(4)	0.2675(6)
  Bond angle
  RE(1)—N(4)—RE(2)	110.48(10)	108.69(17)
  RE(1)—N(6)—RE(2)	107.71(11)	111.04(18)
  Bond length
  RE(2)—N(4)	0.2457(3)	0.2691(5)
  RE(2)—N(6)	0.2720(3)	0.2413(5)
  RE(2)—N(7)	0.2318(3)	0.2277(5)
  RE(2)—N(8)	0.2524(3)	0.2505(5)
  RE(2)—N(9)	0.2291(3)	0.2280(5)
  RE(2)—O(1)	0.2369(3)	0.2364(4)
  RE(2)—C(19)	0.2752(4)	0.2666(6)
  RE(2)—C(20)	0.2765(4)	0.2684(6)
  RE(2)—C(21)	0.2747(4)	0.2711(6)
  RE(2)—C(22)	0.2702(4)	0.2675(6)
  Bond angle
  N(4)—RE(1)—N(6)	71.96(9)	71.28(16)
  N(4)—RE(2)—N(6)	68.94(9)	68.35(15)

  From Table 1, the distances between rare-earth ions with the linked nitrogens [RE(1)—N(2), RE(1)—N(5) and RE(2)—N(8)] are longer than the distances of the other σ- RE—N bonds. The average distance between lanthanide ions with the five membered pyrrolyl ring of 0.2716(5) nm in complex 2 is slightly longer than the corresponding value of 0.2683(7) nm found in complex 4, due to reflection of ionic radius from Y³⁺ and Yb³⁺. The another feacture of the complexes 2 and 4 is that the sum of the four bond angles of RE(1)—N(4)—RE(2), RE(1)—N(6)—RE(2), N(4)— RE(1)—N(6) and N(4)—RE(2)—N(6) is about 360°, indicating that RE(1), RE(2), N(4), and N(6) atoms are coplanar.

  2.3   Catalytic activities of the complexes

  In general, rare-earth complexes catalyzed the ε-caprol- actone polymerization to require the active group or additional alcohol as initiator. In our previous study, we reported the ring-opening polymerization of ε-caprolactone catalyzed by tris-iminopyrrolyl rare-earth metal complexes without additional alcohol as initiator.^[16] Herein, the catalytic activities of complexes 2~4 towards ROP of ε-caprolactone were examined, and results are listed in Table 3. From Table 2, it can be seen that the solvents have a slight influence on polymerization, and complex 2 exhibited good catalytic activities on ring-opening polymerization of ε-caprolactone in toluene and THF. The M_n obtained by complex 2 in THF, Et₂O and CH₂Cl₂ are much larger than those calculated (Entries 4~6, Table 2), and the M_n obtained by complexes 3, and 4 are also larger than those calculated (Entries 7, 8, Table 2), suggesting slow initiation followed by rapid polymerization by small quantities of catalytically active species. Furtherly, complex 2 as an initator for ROP of ε-caprolactone was examined at 10 ℃ in the monomer-to-initiator ratio ([M]/[Cat]) range of 100~500. Results revealed that the linear relationship was associated with the M_n and [M]/[Cat] ratio and the narrow polydispersity index (PDI) ranging from 1.38 to 1.62 of the polymers ([M]/[Cat.] ratio of 100~500 (Entries 2, 9~12, Table 2). The results also represented rare examples of the ring-opening polymerization of ε-caprolactone catalyzed by nitrogen-containing bridged dipyrrolyl rare-earth complexes 2~4 without additional alcohol as initiator, which was similar to our previous example of the ring-opening polymerization of ε-caprolactone catalyzed by tris-imino- pyrrolyl rare-earth metal complexes.^[16] Mechanism of the ring-opening polymerization of ε-caprolactone was proposed that the ε-caprolactone firstly coordinated to the rare-earth metal, and followed by insertion of a pyrrolyl group of rare-earth metal complex into ε-caprolactone.^[16]

  Table 2

  

  Table 2.  Ring-opening polymerization of ε-caprolactone catalyzed by complexes 2~4^a

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  Entry	Cat.	[M]:[catal.]	Solvent	Temp./℃	Time/min	Mnb/(×10-4)	Mnc/(×10-4)	PDIc	Conv./%
  1	2	100:1	Toluene	0	3	1.08	2.41 (1.35)	1.94	95
  2	2	100:1	Toluene	10	1	1.13	2.81 (1.57)	1.38	99
  3	2	100:1	Toluene	20	1	1.13	3.23 (1.81)	1.78	99
  4	2	100:1	THF	10	1	1.12	5.98 (3.34)	1.54	98
  5	2	100:1	Et2O	10	3	0.99	3.81 (2.13)	1.41	87
  6	2	100:1	CH2Cl2	10	5	0.83	3.96 (2.22)	1.45	73
  7	3	100:1	Toluene	10	1	0.68	2.27 (1.27)	1.41	60
  8	4	100:1	Toluene	10	1	1.03	3.23 (1.81)	1.67	90
  9	2	200:1	Toluene	10	1	2.26	4.90 (2.74)	1.61	99
  10	2	300:1	Toluene	10	1	3.39	7.89 (4.41)	1.62	99
  11	2	400:1	Toluene	10	1	4.52	10.60 (5.93)	1.49	99
  12	2	500:1	Toluene	10	2	5.59	12.96 (7.25)	1.54	98
  a Solvent (5.0 mL). b Calculated from [M(ε-capralactone)×[M]/[catal.]×conversion. c Obtained from GPC analysis and calibrated by polystyrene standard, and values in parentheses are the values obtained from GPC times 0.56.

  3.   Conclusions

  In summary, nitrogen-containing bridged dipyrrolyl rare-earth metal complexes were synthesized via reaction of RE(CH₂SiMe₃)₃(THF)₂ with (C₄H₃NHCH₂)₂NCH₃ in toluene in good yields. The X-ray diffraction analyses of the complexes indicated that the complexes were a dimeric structure and two central metals were linked through one nitrogen-containing bridged dipyrrolyl ligand in an η⁵:η¹: η¹:η⁵:η¹ mode. The dimeric rare-earth complexes showed high catalytic activities towards the ROP of ε-caprolactone.

  4.   Experimental

  4.1   General procedure

  All syntheses and manipulations of air- and moisture- sensitive materials were carried out under an atmosphere of argon using standard Schlenk techniques or in a glovebox. THF, toluene and hexane were refluxed and distilled over sodium benzophenone ketyl under argon prior to use unless otherwise noted. ε-Caprolactone (ε-CL) was dried over finely divided CaH₂, distilled before use. (C₄H₃NHCH₂)₂N- CH₃^[17] and RE(CH₂SiMe₃)₃(THF)₂^[18] were prepared according to literature methods. ¹H NMR and ¹³C NMR spectra for analyses of compounds were recorded on a Bruker AV-500 NMR spectrometer (500 MHz for ¹H NMR, 125 MHz for ¹³C NMR) in d₈-THF for rare-earth metal complex. Elemental analyses data were obtained on a Perkin-Elmer Model 2400 Series II elemental analyzer. IR spectra were recorded on a Shimadzu Model FTIR-8400s spectrometer (KBr pellet). Gel permeation chromatography (GPC) analyses of the polymer samples were carried out at 30 ℃ using THF as an eluent on a Waters-2414 instrument and calibrated using monodispersed polystyrene standards at a flow rate of 1.0 mL•min^-1.

  4.2   General synthesis of complexes 2~4

  To a THF (20.0 mL) solution of RE(CH₂SiMe₃)₃(THF)₂ (1.0 mmol) was added a toluene (10.0 mL) solution of (C₄H₃NHCH₂)₂NCH₃ (0.29 g, 1.5 mmol) at 0 ℃. After the reaction mixture was stirred at room temperature for 3 h, the solvent was evaporated under reduced pressure. The residue was extracted with toluene/n-hexane (V:V＝1:1, 20 mL). The extractions were combined and concentrated to about 10.0 mL. The complex crystals were obtained by cooling the concentrated solution at 0 ℃ for several days

  [η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃]Y{μ-η⁵:η¹:η⁵:η¹:η¹-(C₄H₃- NCH₂)₂NCH₃}Y[η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃](THF) (2): Colorless crystals (0.68 g, 75% yield). m.p. 173~174 ℃; ¹H NMR (d₈-THF, 500 MHz, 25 ℃) δ: 7.20~7.08 (m, 5H), 6.60 (s, 3H), 5.94~5.92 (m, 3H), 5.89~5.88 (m, 3H), 3.62 (t, J＝3.6 Hz, 4H), 3.42 (s, 12H), 2.31 (s, 3H), 2.08 (s, 9H), 1.79 (t, J＝3.6 Hz, 4H); ¹³C NMR (d₈-THF, 500 MHz, 25 ℃) δ: 137.8, 129.3, 129.1, 128.3, 125.4, 117.0, 107.4, 107.1, 67.6, 54.3, 41.5, 25.0, 20.9; IR (KBr) ν: 3176 (m), 3122 (m), 2989 (m), 2943 (m), 2916 (s), 2835 (m), 2791 (s), 1642 (m), 1639 (s), 1516 (m), 1462 (m), 1357 (m), 1300 (m), 1176 (s), 1101 (m), 968 (m), 846 (m), 713 (m) cm^-1. Anal. calcd for C₃₇H₄₇N₉OY₂•2C₇H₈: C 61.51, H 6.38, N 12.66; found C 61.25, H 6.73, N 12.42.

  [η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃]Er{μ-η⁵:η¹:η⁵:η¹:η¹-(C₄H₃- NCH₂)₂NCH₃}Er[η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃](THF) (3):Pinkcrystals, 0.85 g, 81% yield. m.p. 186~187 ℃; IR (KBr) ν: 3176 (m), 3122 (m), 3103 (m), 2989 (m), 2943 (m), 2835 (m), 2791 (s), 1462 (m), 1357 (m), 1300 (s), 1253 (s), 1176 (m), 1028 (m), 968 (m), 848 (m), 800 (m), 713 (m), 609 (m) cm^-1. NMR (d₈-THF) spectra of the compound were not informative due to lack of locking signals for the paramagnetic property of the erbium(III) complex 3. Anal. calcd for C₃₇H₄₇N₉OEr₂•C₇H₈: C 49.83, H 5.23, N 11.89; found C 49.84, H 5.33, N 11.92.

  [η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃]Yb{μ-η⁵:η¹:η⁵:η¹:η¹-(C₄H₃- NCH₂)₂NCH₃}Yb[η¹:η¹:η¹-(C₄H₃NCH₂)₂NCH₃](THF) (4):Yellowcrystals, 0.83 g, 79% yield. m.p. 195~196 ℃; IR (KBr) ν: 3174 (m), 3103 (m), 2989 (m), 2943 (m), 2835 (m), 2791 (m), 1637 (s), 1562 (s), 1516 (s), 1462 (m), 1537 (m), 1300 (s), 1253 (m) 1232 (m), 1176 (m), 1039 (m), 968 (m), 887 (m), 848 (m), 800 (m), 732 (m), 609 (m) cm^-1. NMR (d₈-THF) spectra of the compound were not informative due to lack of locking signals for the paramagnetic property of the ytterbium(III) complex 4. Anal. calcd for C₃₇H₄₇N₉O- Yb₂•OC₄H₈: C 46.81, H 5.27, N 11.98. found C 46.30, H 5.36, N 11.94.

  4.3   X-ray crystallographic analyses

  Single crystals of complexes 2 and 4 suitable for X-ray diffraction studies were sealed in thin-walled glass capillaries under argon. Diffraction was performed on a Bruker SMART CCD area detector diffractometer using graphite- monochromated Mo Kα radiation (λ＝0.071073 nm). An empirical absorption correction was applied using the SADABS program. All structures were solved by direct methods, completed by subsequent difference Fourier syntheses, refined anisotropically for all nonhydrogen atoms by full-matrix least-squares calculations on F² using the SHELXTL program package. All hydrogen atoms were refined using a riding model. Crystal data and details of the data collection and structure refinements are given in Table S1 in supporting information. CCDC-1502991-1502992 for complexes 2 and 4 contain the supplementary crystallo- graphic data of this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

  4.4   ε-Caprolactone polymerization

  ε-CL polymerization reactions were performed in a 50.0 mL Schlenk flask, placed in an external temperature-controlled bath on a Schlenk line or in a glovebox. In a typical procedure, the catalyst (20~50 mg) was loaded into the Schlenk flask and the solvent was added. The ε-CL was added through a gastight syringe after the external bath temperature was stabilized. The polymer product was precipitated into hydrochloric acid (0.1 mol/L, 50.0 mL), washed with 0.1 mol/L hydrochloric acid, and then dried to a constant weight in a vacuum oven at 50 ℃. The molecular weights of the polymers were analyzed by GPC techniques.

  Supporting Information    Copies of NMR of complex 2, crystallographic data and refinements for complexes 2 and 4. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

  
  
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  Scheme 1  Synthesis of complexes 2~4

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  Figure 1  Molecular structure of complex 2

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  Figure 2  Molecular structure of complex 4

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  Table 1.  Selected bond lengths ( ) and bond angles (°)

  Compd.	2	4
  Bond length
  RE(1)—N(1)	0.2315(4)	0.2276(6)
  RE(1)—N(2)	0.2527(4)	0.2506(6)
  RE(1)—N(3)	0.2297(4)	0.2249(6)
  RE(1)—N(4)	0.2584(3)	0.2359(5)
  RE(1)—N(5)	0.2620(3)	0.2604(5)
  RE(1)—N(6)	0.2405(3)	0.2569(5)
  RE(1)—C(12)	0.2707(4)	0.2583(7)
  RE(1)—C(13)	0.2819(4)	0.2722(7)
  RE(1)—C(14)	0.2763(4)	0.2786(7)
  RE(1)—C(15)	0.2598(4)	0.2675(6)
  Bond angle
  RE(1)—N(4)—RE(2)	110.48(10)	108.69(17)
  RE(1)—N(6)—RE(2)	107.71(11)	111.04(18)
  Bond length
  RE(2)—N(4)	0.2457(3)	0.2691(5)
  RE(2)—N(6)	0.2720(3)	0.2413(5)
  RE(2)—N(7)	0.2318(3)	0.2277(5)
  RE(2)—N(8)	0.2524(3)	0.2505(5)
  RE(2)—N(9)	0.2291(3)	0.2280(5)
  RE(2)—O(1)	0.2369(3)	0.2364(4)
  RE(2)—C(19)	0.2752(4)	0.2666(6)
  RE(2)—C(20)	0.2765(4)	0.2684(6)
  RE(2)—C(21)	0.2747(4)	0.2711(6)
  RE(2)—C(22)	0.2702(4)	0.2675(6)
  Bond angle
  N(4)—RE(1)—N(6)	71.96(9)	71.28(16)
  N(4)—RE(2)—N(6)	68.94(9)	68.35(15)

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  Table 2.  Ring-opening polymerization of ε-caprolactone catalyzed by complexes 2~4^a

  Entry	Cat.	[M]:[catal.]	Solvent	Temp./℃	Time/min	Mnb/(×10-4)	Mnc/(×10-4)	PDIc	Conv./%
  1	2	100:1	Toluene	0	3	1.08	2.41 (1.35)	1.94	95
  2	2	100:1	Toluene	10	1	1.13	2.81 (1.57)	1.38	99
  3	2	100:1	Toluene	20	1	1.13	3.23 (1.81)	1.78	99
  4	2	100:1	THF	10	1	1.12	5.98 (3.34)	1.54	98
  5	2	100:1	Et2O	10	3	0.99	3.81 (2.13)	1.41	87
  6	2	100:1	CH2Cl2	10	5	0.83	3.96 (2.22)	1.45	73
  7	3	100:1	Toluene	10	1	0.68	2.27 (1.27)	1.41	60
  8	4	100:1	Toluene	10	1	1.03	3.23 (1.81)	1.67	90
  9	2	200:1	Toluene	10	1	2.26	4.90 (2.74)	1.61	99
  10	2	300:1	Toluene	10	1	3.39	7.89 (4.41)	1.62	99
  11	2	400:1	Toluene	10	1	4.52	10.60 (5.93)	1.49	99
  12	2	500:1	Toluene	10	2	5.59	12.96 (7.25)	1.54	98
  a Solvent (5.0 mL). b Calculated from [M(ε-capralactone)×[M]/[catal.]×conversion. c Obtained from GPC analysis and calibrated by polystyrene standard, and values in parentheses are the values obtained from GPC times 0.56.

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