English

• 1   INTRODUCTION

  Aliphatic polyesters continue to attract intense attention, due to their promising permeable, biodegradable and biocompatible properties. These properties may provide advantages for their applications in packaging and textile industries, and IT and biomedical fields^{[1, 2]}. Among a series of aliphatic polyesters, polycaprolactones (PCL) have received continuous interest because such polymers are important building blocks for the production of segmented copolymers and polymer networks^[3]. Many efforts have been dedicated to the design of metallic catalysts enabling rapid and controlled ring-opening polymerization of ε-caprolactone to generate PCL^[4]. A plethora of organometallic complexes, e.g., magnesium^[5], aluminum^[6], titanium (zirconium and hafnium)^[7], zinc^[8], rare earth complexes^[9], etc., have been developed for ring-opening polymerization of ε-caprolactone. Lanthanide complexes have gathered particular attention owing to their good control over polymerization transformation[4a]. It is also reported in several recent reviews that the identities of the supporting ligands around the lanthanide ions have a profound influence on the molecular weight and molecular distribution of resultant polymers^[10]. Thus, the development of new ligand systems which could provoke novel reactivity and provide control of PDI remains one of the most intensively explored areas of organolanthanide chemistry.

  There is currently renewed interest in the employment of pyrrolide ligands in the organometallic chemistry^[11]. The utilization of pyrrolide ligands is driven by several considerations including: (i) pyrrolyl ring could act as a η¹ ligand by forming nitrogen-metal σ-bond. (ii) Pyrrolyl ring possesses a reasonable mimic of the cyclopentadienyl functions and could serve as potential η⁵ ligand^[12]. The flexibility of the coordination of the pyrrolide ligands provides tunable steric and electronic features required for compensating coordinative unsaturation of metal atoms and enhances catalytic activity toward polymerization. (iii) The hydrogen atom of the pyrrolyl ring is activated easily, which provides the possibility for being tailored as dif- ferent ligand sets.

  As a continuation of our previous study on the synthesis of organolanthanide complexes based on the pyrrolide ligands^[13], and investigation of the potential catalytic activities of these complexes, we studied the coordination chemistry of yttrium with a pyrrolide ligand H₃bptd (H₃bptd = 1, 9-bis (2- pyrrolyl)-2, 5, 8-triazanona-1, 8-diene; Scheme 1)^[13C]. This ligand has received scant attention in organometallic chemistry of both transition metals and lanthanides. No homometallic dinuclear lanthanide complexes of this ligand have been reported. Herein we present the synthesis, molecular structure and catalytic behavior of a dinuclear yttrium complex [Y (bptd)(THF)]₂ (1). For comparison purpose, the synthesis, molecular structure and catalytic behavior of a mononuclear yttrium compound [Y (tpa)(THF)₃] (2) (H₃tpa = tris (pyrrolyl-α-methyl) amine; Scheme 1^[14]) is also reported.

  Figure 1.  Syntheses of complexes 1 and 2

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  2   EXPERIMENTAL

  2.1   Materials and physical measurements

  All manipulations of air-sensitive compounds were carried out in a Mikrouna glovebox under a purified nitrogen atmosphere. Tetrahydrofuran, toluene, and hexane were heated to reflux over sodium benzophenoneketyl for at least 4 days. CDCl₃ was distilled from P₂O₅ under a nitrogen atmosphere. Elemental analyses (C, H, N) were performed with a Carlo-Erba EA 1110 CHNO-S microanalyser. ¹H and ¹³C spectra were recorded on an Agilent-400 spectrometer at ambient temperature using TMS as an internal standard, and chemical shifts were reported in ppm.

  2.2   Synthesis of [Y(bptd)(THF)]₂ (1)

  A solution of Y[N (SiMe₃)₂]₃ (0.2851 g, 0.5 mmol) in THF (5 mL) was added to a THF (5 mL) solution of H₃bptd (0.1286 g, 0.5 mmol). The reaction mixture was stirred at room temperature overnight, then the volatiles were removed under reduced pressure. The product crystallizes from THF/hexane as a light yellow block crystal. Yield: 0.3779 g (90%). ¹H NMR (300 MHz, CDCl₃): δ = 7.87～7.98 (m, 4H, -CHN), δ = 7.49 (s, 4H, -C₄H₃N), δ = 6.65 (s, 4H, -C₄H₃N), δ = 6.22 (s, 4H, -C₄H₃N), δ = 3.72 (s, 8H, OCH₂ of THF), δ = 3.48～3.46 (m, 8H, -CH^2-), δ = 2.84～2.98 (m, 8H, -CH₂NCH), δ = 1.83 (s, 8H, OCH₂ of THF); ¹³C NMR (75 MHz, CDCl₃): δ = 159.70, 139.38, 136.78, 118.28, 110.67, 68.09, 54.52, 50.85, 25.58. Elemental analysis: calcd. for C₃₆H₄₈N₁₀O₂Y₂: C, 52.05; H, 5.82; N, 16.86%. Found: C, 52.80; H, 6.30; N, 16.90%.

  2.3   Synthesis of Y (tpa)(THF)₃ (2)

  This compound was prepared as light yellow crystals from the reaction of H₃tpa (0.1271 g, 0.5 mmol) with Y[N (SiMe₃)₂]₃ (0.2851 g, 0.5 mmol) in THF (5 mL), and recrystallized from THF by a similar procedure as in the synthesis of 1. Yield: 0.2476 g (89%). ¹H NMR (400 MHz, CDCl₃): δ = 6.5 (s, 3H, -C₄H₃N), δ = 6.07 (s, 3H, -C₄H₃N), δ = 5.96 (s, 3H, -C₄H₃N), δ = 4.03 (s, 6H, -CH₂), δ = 3.84 (s, 12H, OCH₂ of THF), δ = 1.37～1.89 (s, 12H, OCH₂ of THF); ¹³C NMR (100 MHz, CDCl₃): δ = 138.76, 123.56, 107.14, 103.15, 68.94, 55.08, 25.61. Elemental analysis: calcd. for C₂₇H₃₉N₄O₃Y: C, 58.27; H, 7.06; N, 10.07%. Found: C, 57.63; H, 7.25; N, 10.86%.

  2.4   Single-crystal X-ray crystallography

  Data were collected at 293(2) K on a Bruker Smart Apex II diffractometer for 1 and 2 utilizing MoKα radiation (λ = 0.71073 Å); the ω-φ scan technique was applied. The structures were solved by direct methods using SHELXS-97^[15] and refined on F₂ using full-matrix least-squares with SHELXL-97^[16]. Selected bond lengths and bond angles of 1 and 2 are shown in Table 1.

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

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  1BondDist.BondDist.BondDist.Y(1)−N(5)2.390(5)Y(1)−N(3)2.425(5)Y(1)−O(1)2.521(4)Angle(°)Angle(°)Angle(°)N(5)−Y(1)−N(4)107.74(18)N(3)−Y(1)−N(5)#1133.17(18)N(2)−Y(1)−O(1)82.25(17)2BondDist.BondDist.BondDist.Y(1)−N(4)2.318(7)Y(1)−N(2)2.324(7)Y(1)−N(1)2.530(6)Angle(°)Angle(°)Angle(°)O(3)−Y(1)−N(1)133.7(2)N(4)−Y(1)−N(3)110.3(3)N(2)−Y(1)−N(1)69.7(2)N(3)−Y(1)−O(3)157.7(3)O(2)−Y(1)−O(3)81.5(3)N(4)−Y(1)−N(1)68.5(2)

  Y(1)−N(4)	2.403(5)	Y(1)−N(1)	2.424(6)	Y(1)−N(2)	2.418(5)
  Y(1)−N(5)#1	2.483(5)
  N(4)−Y(1)−N(3)	68.38(18)	N(1)−Y(1)−N(5)#1	136.99(18)	N(3)−Y(1)−O(1)	91.16(17)
  N(3)−Y(1)−N(1)	89.79(18)	N(5)−Y(1)−O(1)	176.84(16)	N(1)−Y(1)−O(1)	79.70(17)
  N(2)−Y(1)−N(1)	68.69(19)	N(4)−Y(1)−O(1)	72.43(17)	N(2)−Y(1)−N(5)#1	68.31(19)
  N(2)−Y(1)−N(5)	68.31(19)	N(5)−Y(1)−N(5)#1	82.03(17)	N(4)−Y(1)−N(5)#1	69.59(17)
  Y(1)−N(3)	2.339(7)	Y(1)−O(2)	2.370(8)	Y(1)−O(3)	2.446(6)
  Y(1)−O(1)	2.399(5)
  N(2)−Y(1)−N(3)	108.1(2)	N(4)−Y(1)−O(2)	85.7(3)	N(3)−Y(1)−N(1)	68.5(2)
  N(2)−Y(1)−O(2)	161.5(3)	N(3)−Y(1)−O(2)	81.5(3)	O(2)−Y(1)−N(1)	128.8(3)
  N(4)−Y(1)−O(1)	158.9(2)	N(2)−Y(1)−O(1)	83.6(2)	O(1)−Y(1)−N(1)	132.4(18)
  N(3)−Y(1)−O(1)	84.5(2)	O(2)−Y(1)−O(1)	81.6(3)	N(4)−Y(1)−N(2)	104.9(3)
  N(4)−Y(1)−O(3)	82.6(3)	N(2)−Y(1)−O(3)	84.8(2)	O(1)−Y(1)−O(3)	78.9(2)
  Symmetry codes: (1) #1 −x+1, −y, −z.

  3   RESULTS AND DISCUSSION

  3.1   Syntheses and structures of the complexes

  Reaction of YCl₃(THF)₃ with 3 equivalents of NaN (SiMe₃)₂ in THF generated the known complex Y[N (SiMe₃)₂]₃ ^[17]. Treatment of H₃bptd with THF solution of Y[N (SiMe₃)₂]₃ afforded a new complex [Y (bptd)(THF)]2 (1, Scheme 1). The crystals of 1 were readily obtained in good yield after recrytallization in THF/hexane.

  Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic crystal system of the P2₁/c space group. The main structure data of complex 1 are as follows: a = 8.8924(18), b = 24.688(5), c = 9.1990(18) Å, β = 113.19(3)°, V = 1856.3(6) ų, Z = 2, F (000) = 856, D_c = 1.486 g·cm^-3, the final R = 0.0785 and wR = 0.1578 for 3375 observed reflections with I > 2σ(I). The structure of 1 consists of two Y³⁺ ions, two triply deprotonated H₃bptd ligands and two THF molecules (Fig. 1a). The overall structure is an aminebridged dinuclear yttrium complex. The metal ion is seven-coordinated, and the geometry of the Y³⁺ ion is well described as a distorted pentagonal bipyramid (Fig. 1b). The bptd^3- ligand chelates to the yttrium ion via all five nitrogen atoms, and the remaining two coordination positions of the yttrium atom are occupied by one oxygen atom of a THF molecule and one amine nitrogen atom from another bptd^3- ligand. The five nitrogen atoms of a bptd^3- ligand set, N (1), N (2), N (3), N (4) and N (5), form the equatorial plane, with the bond angles of 68.69(19) (N (1)-Y (1)-N (2)), 68.31(19) (N (2)- Y (1)-N (5)), 69.59(17) (N (5)-Y (1)-N (4)), 68.38(18) (N (4)-Y (1)-N (3)) and 89.79(18)° (N (3)-Y (1)-N (1)), respectively. The average Y-N (pyrrolyl) distance is 2.424(6) Å, which is comparable to those found in yttrium complexes supported by pyrrolide ligands^[18-31].

  Figure 1.  (a) Molecule structure of complex 1. (b) Coordination polyhedron of Y³⁺ ion

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  The most interesting feature regarding the structure of 1 is that the two yttrium ions are doubly bridged by two amine nitrogen atoms from two bptd^3- ligands to form a four-membered Y-N-Y-N ring. The employment of the H₃bptd ligand had previously given one SmIII compound^[13c] and one heterometallic Sm-Ti complex^[13c], but the fourmembered ring has never been observed in these complexes, revealing the structural novelty of complex 1.

  Encouraged by the observation that the coordination of the H₃bptd ligand to yttrium ions could give a complex with aesthetically pleasing structure, we are curious whether the reaction of Y[N (SiMe₃)₂]₃ with another pyrrolide ligand H₃tpa (H₃tpa = tris (pyrrolyl- α-methyl) amine) could also generate a new complex with intriguing structural feature. To this end, the reaction between Y[N (SiMe₃)₂]₃ and H₃tpa was examined. The 1:1 reaction (Scheme 1) generated a mononuclear complex [Y (tpa)(THF)₃] (2).

  The crystal structure determination indicates that complex 2 crystallizes in the orthorhombic crystal system of space group P2₁2₁2₁. The main structure data of complex 2 are as below: a = 9.871(2), b = 16.683(3), c = 16.824(3) Å, V = 2770.6(10) ų, Z = 4, F (000) = 1168, D_c = 1.334 g·cm^-3, the final R = 0.0693 and wR = 0.1703 for 6119 observed reflections with I > 2(I). The molecule of 2 possesses a seven-coordinated Y³⁺ centre, one tpa^3- ligand and three coordinated THF molecules (Fig. 2a). The Y³⁺ ion exhibits a capped octahedral geometry, with the amine nitrogen atom capping on the face formed by three pyrrolyl nitrogen atoms (Fig. 2b). The three Y-N (pyrrolyl) bond distances are 2.318(7), 2.324(7), and 2.339(7) Å, respectively, which are shorter than the Y-N (amine) bond distance (2.530(6) Å). Complex 2 joins a very small family of complexes supported by trispyrrole-containing pyrrolide ligands^[32-35]. However, the organometallic compounds incorporating the H₃tpa ligand are very rare^{[35, 13a]}.

  Figure 2.  (a) Molecule structure of complex 2. (b) Coordination polyhedron of Y³⁺ ion

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  3.2   Polymerization of ε-caprolactone initiated by 1 and 2

  The catalytic activities of 1 and 2 towards ringopening polymerization of ε-caprolactone were investigated and the results are summarized in Table 2. It is found that both 1 and 2 can effectively initiate ε-caprolactone polymerization, and all of the obtained polymers have high molecular weights and relatively narrower molecular weight distributions (PDIs). The PDIs of polymers generated by utilizing 2 as an initiator are smaller than those employing 1 as the initiator.

  Table 2.  Polymerization of ε-Caprolactone Initiated by 1 and 2

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  Entry	Initiator	Solvent	[M/I]	T(min)	Yielda(%)	Temp(℃)	Mnb(calc)(104)	Mnc(104)	Mnd(104)	PDI
  1	2	Tol	200	0.5	93	25	2.12	2.17	1.22	1.13
  2	2	THF	200	0.5	91	25	2.07	2.12	1.19	1.15
  3	2	DME	200	0.5	89	25	2.03	2.07	1.16	1.19
  4	1	Tol	200	3	85	25	1.94	2.39	1.34	1.38
  5	1	Tol	200	2	61	60	1.39	1.4	0.78	1.26
  6	1	Tol	200	2	63	80	1.44	1.45	0.81	1.23
  7	1	THF	200	5	81	25	1.85	2.19	1.23	1.37
  8	1	THF	200	4	63	60	1.44	1.5	0.84	1.26
  9	1	THF	200	3	60	80	1.37	1.48	0.83	1.22
  10	1	DME	200	7	95	25	2.17	2.53	1.42	1.33
  11	1	DME	200	5	61	60	1.4	1.57	0.88	1.23
  12	1	DME	200	3	55	80	1.25	1.53	0.86	1.22
  a Yield: weight of polymer obtained /weight of monomer used. bMn (calc) = M mono x [M]/[I] x Conv. c Measured by GPC relative to polystyrene standards. Measured by GPC relative to polystyrene standards with Mark-Houwink corrections[36]for Mn (obsd) = 0.56 Mn (GPC) for ε-caprolactone.

  Complex 2 showed higher activity for polymerization than compound 1. Using 2 as the initiator, the yield reached 93%, 91% and 89% in Tol, THF and DME, respectively, only in 30 second at room temperature. In contrast, the yield was 85% by employing 1 as the initiator in Tol when the reaction time was prolonged to 3 min. The solvent has no obvious effect on the catalytic activity of 1 and 2 because the yields and PDIs of the polymers did not vary greatly.

  For complex 1, it seems that the extention of the reaction time and increase of temperature did not result in the increase of the yields. It is possible that 1 was partially decomposed after catalyzing the reaction for several minutes.

  It is reasonable that complex 2 exhibited higher activity. Compound 1 is dinuclear and has more rigid structure than 2, rendering 2 more facile to be attacked by the ε-caprolactone.

  4   CONCLUSION

  In conclusion, towards our goal of synthesis of organoyttrium complexes supported by pyrrolide ligands with potential catalytic activities, we have prepared two yttrium complexes. The ring-opening polymerization reactions of ε-caprolactone initiated by 1 and 2, respectively，were conducted. The results of the catalytic reactions indicated that 1 and 2 exhibited good catalytic activity toward the polymerization reaction of ε-caprolactone. The high catalytic activities of 1 and 2 reveal their potential applications in polymer industry.

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  Figure 1  Syntheses of complexes 1 and 2

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  Figure 1  (a) Molecule structure of complex 1. (b) Coordination polyhedron of Y³⁺ ion

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  Figure 2  (a) Molecule structure of complex 2. (b) Coordination polyhedron of Y³⁺ ion

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

  1BondDist.BondDist.BondDist.Y(1)−N(5)2.390(5)Y(1)−N(3)2.425(5)Y(1)−O(1)2.521(4)Angle(°)Angle(°)Angle(°)N(5)−Y(1)−N(4)107.74(18)N(3)−Y(1)−N(5)#1133.17(18)N(2)−Y(1)−O(1)82.25(17)2BondDist.BondDist.BondDist.Y(1)−N(4)2.318(7)Y(1)−N(2)2.324(7)Y(1)−N(1)2.530(6)Angle(°)Angle(°)Angle(°)O(3)−Y(1)−N(1)133.7(2)N(4)−Y(1)−N(3)110.3(3)N(2)−Y(1)−N(1)69.7(2)N(3)−Y(1)−O(3)157.7(3)O(2)−Y(1)−O(3)81.5(3)N(4)−Y(1)−N(1)68.5(2)

  Y(1)−N(4)	2.403(5)	Y(1)−N(1)	2.424(6)	Y(1)−N(2)	2.418(5)
  Y(1)−N(5)#1	2.483(5)
  N(4)−Y(1)−N(3)	68.38(18)	N(1)−Y(1)−N(5)#1	136.99(18)	N(3)−Y(1)−O(1)	91.16(17)
  N(3)−Y(1)−N(1)	89.79(18)	N(5)−Y(1)−O(1)	176.84(16)	N(1)−Y(1)−O(1)	79.70(17)
  N(2)−Y(1)−N(1)	68.69(19)	N(4)−Y(1)−O(1)	72.43(17)	N(2)−Y(1)−N(5)#1	68.31(19)
  N(2)−Y(1)−N(5)	68.31(19)	N(5)−Y(1)−N(5)#1	82.03(17)	N(4)−Y(1)−N(5)#1	69.59(17)
  Y(1)−N(3)	2.339(7)	Y(1)−O(2)	2.370(8)	Y(1)−O(3)	2.446(6)
  Y(1)−O(1)	2.399(5)
  N(2)−Y(1)−N(3)	108.1(2)	N(4)−Y(1)−O(2)	85.7(3)	N(3)−Y(1)−N(1)	68.5(2)
  N(2)−Y(1)−O(2)	161.5(3)	N(3)−Y(1)−O(2)	81.5(3)	O(2)−Y(1)−N(1)	128.8(3)
  N(4)−Y(1)−O(1)	158.9(2)	N(2)−Y(1)−O(1)	83.6(2)	O(1)−Y(1)−N(1)	132.4(18)
  N(3)−Y(1)−O(1)	84.5(2)	O(2)−Y(1)−O(1)	81.6(3)	N(4)−Y(1)−N(2)	104.9(3)
  N(4)−Y(1)−O(3)	82.6(3)	N(2)−Y(1)−O(3)	84.8(2)	O(1)−Y(1)−O(3)	78.9(2)
  Symmetry codes: (1) #1 −x+1, −y, −z.

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  Table 2.  Polymerization of ε-Caprolactone Initiated by 1 and 2

  Entry	Initiator	Solvent	[M/I]	T(min)	Yielda(%)	Temp(℃)	Mnb(calc)(104)	Mnc(104)	Mnd(104)	PDI
  1	2	Tol	200	0.5	93	25	2.12	2.17	1.22	1.13
  2	2	THF	200	0.5	91	25	2.07	2.12	1.19	1.15
  3	2	DME	200	0.5	89	25	2.03	2.07	1.16	1.19
  4	1	Tol	200	3	85	25	1.94	2.39	1.34	1.38
  5	1	Tol	200	2	61	60	1.39	1.4	0.78	1.26
  6	1	Tol	200	2	63	80	1.44	1.45	0.81	1.23
  7	1	THF	200	5	81	25	1.85	2.19	1.23	1.37
  8	1	THF	200	4	63	60	1.44	1.5	0.84	1.26
  9	1	THF	200	3	60	80	1.37	1.48	0.83	1.22
  10	1	DME	200	7	95	25	2.17	2.53	1.42	1.33
  11	1	DME	200	5	61	60	1.4	1.57	0.88	1.23
  12	1	DME	200	3	55	80	1.25	1.53	0.86	1.22
  a Yield: weight of polymer obtained /weight of monomer used. bMn (calc) = M mono x [M]/[I] x Conv. c Measured by GPC relative to polystyrene standards. Measured by GPC relative to polystyrene standards with Mark-Houwink corrections[36]for Mn (obsd) = 0.56 Mn (GPC) for ε-caprolactone.

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