English

• Biocompatible polymers from lactides and lactones have received considerable attention and are extensively used in many fields owing to their environmentally friendly and biodegradable properties^[1]. The aluminum^[2-5] and zinc^[6-8] complexes have attracted much interest as initiators for the ring-opening polymerization (ROP) of lactides and lactones to produce biodegradable polymers^[9-10] due to their low toxicity, high reactivity, and inexpensiveness. Nevertheless, it has been well documented that the ligands govern the catalytic reactivity of the metal center in the complexes. Therefore, a variety of ligand systems (some are sophisticated) have been employed for aluminum and zinc complexes for ROP of cyclic esters, for example, half-salen^[11-12] and salen-type^[13-15], diphenol^[16-17], Schiff base^[18], ketiminate^[18-19] and β-diketiminate^{[7, 20-21]}, bidentate [N, O]^[22-23] and [N, N]^{[8, 24]} ligands, tridentate [N, N, N]^[25], [N, N, O]^[26-27] and [O, N, O]^[28], and [P, P, P]^[29] ligands. Our group has previously reported dimethylaluminum complexes based on bis(β-ketimininato) ligands as initiators for the ROP of lactones^[30]. For the sake of the simplicity of the ligand systems, our attention is turned to the 2-(N-arylamino)ethanol^[31-32] due to its readiness for synthesis, chelating nature, and variable steric hindrance of the ancillary aryl group. Herein, we report reactions of ArNHCH₂CH₂OH (HL, Ar=2,6-Me₂C₆H₃) with AlMe₃ and ZnEt₂ and the catalytic activity of the resulted complexes in the ROP of ε-caprolactone.

  1.   Experimental

  All manipulations of the moisture- and oxygen-sensitive organometallic compounds were carried out in a nitrogen atmosphere using the standard Schlenk technique or in a nitrogen-filled glove box. Hexane and toluene were dried by refluxing over sodium benzophenone ketyl and distilled in nitrogen. NMR spectra were obtained on a Bruker 400 MHz instrument and the chemical shifts were reported with respect to the reference (internal SiMe₄ for ¹H and ¹³C NMR spectra). Elemental analyses were carried out on an Elementar Vario EL analyzer. The molecular weights of the polymers were determined on a Waters GPC515-2410 instrument using THF as the solvent. M_n and M_w values were determined from calibration plots established with polystyrene standards. AlMe₃ and ZnEt₂ were purchased from Sigma Aldrich and used as received.

  1.1   Synthesis of HL

  The ligand HL was synthesized by a modified procedure^[32]. 2-Chloroethanol (8.9 g, 108 mmol), 2,6-dimethylaniline (13.2 g, 108 mmol), and potassium iodide (20 mg, catalytic amount) were added to a 100 mL round flask. The mixture was stirred for 3 h at 140 ℃ and then cooled to room temperature to form a solid residue, which was recrystallized from ethanol to give colorless crystals of HL·HCl (12 g). The crystals were dissolved in 100 mL of water and neutralized with saturated NaHCO₃ solution, and the solution was extracted with dichloromethane (50 mL×3). The organic layers were combined, dried over MgSO₄, and concentrated to give a colorless oil of HL (9.5 g, 53%). ¹H NMR (CDCl₃): δ 7.00 (d, J=6.6 Hz, 2H, C₆H₃), 6.85 (t, J=6.6 Hz, 1H, C₆H₃), 3.75 (t, J=5.4 Hz, 2H, CH₂O), 3.12 (t, J=5.4 Hz, 2H, CH₂NH), 2.75 (s, br, 2H, OH/NH), 2.30 (s, 6H, CH₃).

  1.2   Synthesis of complex [(Me₂Al)(L)]₂ (1)

  HL (0.42 g, 2.55 mmol) and toluene (15 mL) were added to a 100 mL Schlenk flask. To this solution, AlMe₃ solution (2.06 g, 5.09 mmol, 17.8% in toluene) was syringed dropwise at room temperature. Bubbles were formed immediately. The reaction mixture was stirred at room temperature for 0.5 h. All the volatiles were removed in a vacuum and the residue was crystallized from a mixture of n-hexane/toluene (1∶1, V/V) to give complex 1 as colorless crystals. Yield: 0.83 g (74%). m.p. 124-126 ℃. ¹H NMR (C₆D₆): δ 7.19-6.89 (m, 6H, C₆H₃), 3.55-3.53 (m, 6H, CH₂O/NH), 2.77-2.75 (t, J=7.2 Hz, 4H, CH₂NH), 2.20 (s, 12H, C₆H₃(CH₃)₂), -0.40 (s, 12H, Al(CH₃)₂). ¹³C NMR (C₆D₆): δ 144.6, 129.3, 129.3, 122.8, 61.6, 49.0, 18.9, -10.0. Anal. Calcd. for C₂₄H₄₀Al₂N₂O₂(%): C 65.14, H 9.11, N 6.33; Found(%): C 64.86, H 8.78, N 6.05.

  1.3   Synthesis of complex [EtZn₂(L)₃]₂ (2)

  Complex 2 was similarly obtained as colorless crystals using ZnEt₂ solution (2 mol·L^-1 in toluene) instead of AlMe₃ solution as above-mentioned synthesis of 1. Yield: 62%. m.p. 115-117 ℃. ¹H NMR (CDCl₃): δ 7.03-6.98 (m, 12H, C₆H₃), 6.89-6.82 (m, 6H, C₆H₃), 3.94-3.92 (m, 12H, CH₂O), 3.58 (s, 4H, NH), 3.42 (s, 2H, NH), 3.22-3.18 (m, 12H, CH₂NH), 2.32 (s, 36H, C₆H₃(CH₃)₂), -0.67 to -0.77 (m, 10H, ZnC₂H₅). Anal. Calcd. for C₆₄H₉₄N₆O₆Zn₄(%): C 58.90, H 7.26, N 6.44; Found(%): C 58.67, H 6.94, N 6.19.

  1.4   Crystal structure determination

  Colorless crystals of 1 and 2 suitable for X-ray analysis were grown by slowly cooling their hot n-hexane/toluene solutions to room temperature under Ar. The N2 atom in 2 was found to be disordered. Satisfactory results were obtained when N2 was given an occupancy factor of 0.72 and N2A was given an occupancy factor of 0.28, respectively. All intensity data were collected on a Rigaku Saturn CCD detector using Mo Kα radiation (λ=0.071 073 nm) at 113 K. Semiempirical absorption corrections were applied using the CrystalClear program. The structures were solved by direct methods and difference Fourier map using SHELXS of the SHELXTL package and refined with SHELXL by full-matrix least-squares on F ². All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were added geometrically and refined with riding model position parameters. A summary of the fundamental crystal data for these complexes is listed in Table 1.

  Table 1

  

  Table 1.  Crystallographic data and refinement parameters for complexes 1 and 2

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  Parameter	1	2
  Formula	C24H40Al2N2O2	C64H94N6O6Zn4
  Formula weight	442.54	1 305.01
  Crystal size/mm	0.24×0.20×0.10	0.18×0.16×0.12
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a/nm	0.836 3(2)	1.175 7(2)
  b/nm	0.940 5(2)	1.236 9(3)
  c/nm	0.941 9(2)	1.377 1(3)
  α/(°)	62.76(3)	95.05(3)
  β/(°)	82.71(3)	111.48(3)
  γ/(°)	75.47(3)	116.33(3)
  V/nm3	0.637 5(2)	1.593 1(7)
  Z	1	1
  Dc/(g·cm-3)	1.153	1.360
  θ range/(°)	3.48-25.02	2.169-25.019
  F(000)	240	688
  μ/mm-1	0.135	1.541
  Measured reflection	4 671	11 783
  Unique reflection (Rint)	2 224 (0.058 7)	5 565 (0.043 0)
  Observed reflection with [I≥2σ(I)]	1 584	4 343
  Number of parameters	140	379
  GOF	0.975	1.053
  Residuals R1, wR2	0.043 9, 0.109 1	0.043 7, 0.092 1

  1.5   General procedure for ε-caprolactone polymerization

  Complex 2 (1.10 mL, 0.088 4 mol·L^-1 in toluene) was added to 15 mL of anhydrous toluene as an initiator. Then the calculated amount of ε-caprolactone was added by a syringe. The reaction mixture was stirred at 18 or 60 ℃ for a specified time and quenched with two drops of acetic acid/water solution (1∶1, V/V). Hexane (20 mL) was added to the reaction mixture to precipitate the polymers, which were then filtered, washed with hexane, dried in vacuo to a constant weight, and analyzed by gel permeation chromatography (GPC).

  2.   Results and discussion

  2.1   Synthesis of complexes 1 and 2

  Treatment of HL with two equivalents of AlMe₃ or ZnEt₂ in toluene at room temperature gave complexes 1 and 2 in good yields (Scheme 1). It is noteworthy that complex 2 was also isolated even when one equivalent of ZnEt₂ was used. These two complexes are sensitive to moisture and oxygen, and soluble in common solvents, such as toluene, chloroform, and THF, but only sparsely soluble in hexane. Compared with the ¹H NMR spectrum of HL, the considerable changes of complexes 1 and 2 in their ¹H NMR spectra were the disappearance of the signal of proton attributed to the hydroxyl group and the appearance of the signals of hydrogens attributed to metal-alkyls (δ=-0.40 in 1 and -0.67 to -0.77 in 2), which illustrates the formation of the complexes.

  Scheme 1

  

  Scheme 1.  Synthesis of complexes 1 and 2

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  2.2   Crystal structures of complexes 1 and 2

  The molecular structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction method, respectively. Their selected bond distances and angles are listed in Table 2. Fig. 1 shows that complex 1 is a crystallographically centrosymmetric dimer in the solid state with an inversion center located at the center of the parallelogram of O1-Al1-O1A-Al1A, and each aluminum atom is chelated by the aminoethanolate ligand and bridged by the two oxygen atoms of ligands, resulting in the aluminum atom with a five-coordinated environment. The Al1—O1 bond distance (0.184 6(2) nm) is shorter than Al1—O1A (0.192 0(2) nm). These values are similar to the Al—O distances in other five-coordinated aluminum compounds with an analogous Al₂O₂ core, such as {[OCMe₂CH(Me)=NAr]AlMe₂}₂ (0.184 9(1) nm and 0.196 2(1) nm, Ar=2,6-diethylphenyl)^[20]. The bond angles of O1—Al1—O1A and Al1—O1—Al1A are 76.76(7)° and 103.24(7)°, respectively. The O1, Al1, N1, and C2 atoms are almost coplanar with a mean deviation of only 0.001 42 nm, and the C1 atom is deviated from this plane at 0.059 19 nm. The dihedral angle of the O1—Al1—N1—C2 plane and the O1—Al1—O1A—Al1A plane is only 1.4°, which means that the Al₂O₂ core structure of complex 1 is almost planar. The two methyl groups connected to the Al atom are inequivalent because of the rigid structure. However, the two methyl groups display as a singlet in the ¹H NMR spectrum, suggesting that, in CDCl₃ solution, quick inversion of the configuration of nitrogen takes place at room temperature due to its weak coordination with the aluminum atom. This is evidenced by the Al1—N1 distance (0.237 8(2) nm), which is longer than the Al—N bond distance in {[OCMe₂CH(Me)=NAr]AlMe₂}₂ (0.220 7(1) nm)^[20].

  Table 2

  

  Table 2.  Selected bond distances (nm) and angles (°) for complexes 1 and 2

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  1
  Al1—O1	0.184 6(2)	Al1—O1A	0.192 0(2)	Al1—C11	0.196 6(2)
  Al1—N1	0.237 8(2)	C2—N1	0.149 2(3)	Al1—C12	0.197 9(2)
  O1—Al1—O1A	76.76(7)	Al1—O1—Al1A	103.24(7)	C3—N1—Al1	124.6(1)
  C11—Al1—C12	118.9(1)	C2—N1—Al1	106.2(1)	C2—C1—O1	108.3(2)
  2
  Zn1—O1	0.200 3(3)	Zn1—O2	0.213 8(2)	Zn1—O3	0.195 2(3)
  Zn1—O2A	0.206 6(2)	Zn1—N1	0.223 9(3)	Zn2—O2	0.210 0(3)
  Zn2—O3	0.199 8(2)	Zn2—O1A	0.201 2(2)	Zn2—C31	0.197 5(3)
  C20—O2	0.143 5(4)	C1—N1	0.145 7(4)	C11—N2	0.144 9(6)
  O1—Zn1—O2	90.34(9)	O1—Zn1—O2A	83.08(9)	O2—Zn1—N1	139.83(9)
  O1—Zn1—O3	168.5(1)	O2—Zn1—O3	81.70(9)	O2—Zn1—O2A	87.21(8)
  O2—Zn2—O3	81.59(9)	O1A—Zn2—O2	82.01(9)	O3—Zn2—O1A	99.15(9)
  Zn1—O2—Zn1A	92.79(8)	Zn1A—O2—Zn2	94.99(9)	C1—N1—C9	112.0(3)
  C11—N2—C19	112.5(5)	C21—N3—C29	115.3(3)	C32—C31—Zn1	117.0(3)
  Symmetry codes: A: -x+1, -y+1, -z+1 for 1; A: -x+1, -y+1, -z+1 for 2.

  Figure 1

  

  Figure 1.  Molecular structure of complex 1 with 30% probability displacement ellipsoids

  Hydrogen atoms are omitted for clarity; Symmetry code: A: -x+1, -y+1, -z+1.

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  Fig. 2 displays that complex 2 is an interesting tetra-zinc aggregate with a centrosymmetric skeleton. The aminoethanolate ligands show three kinds of coordination modes in this complex. The N1 and O1 atoms from one ligand are coordinated to Zn1 to form a five-membered ring, meanwhile, the O1 atom is also coordinated to Zn2A. The O3 atom from another ligand is coordinated to Zn1 and Zn2 at the same time, and the N3 atom from this ligand is non-coordinated. In addition, the O2 atom from the third ligand is coordinated with Zn1, Zn2, and Zn1A atoms from the top of the plane of Zn1-Zn2-Zn1A-Zn2A, and the N2 atom from this ligand is also non-coordinated. Since the structure of 2 is complicated, the Zn₄O₆ core structure of 2 is presented in Fig. 3. The Zn₄O₆ core can be viewed as two side-by-side cubes, sharing the O2-Zn1-O2A-Zn1A face and missing a pair of opposite vertices. The coordination sphere of Zn1 is best described as pyramidal geometry with O1-O2-O3-N1 as the square base and O2A as the vertex, while that of Zn2 is tetrahedral geometry. The Zn—O bond distances vary from 0.195 2 to 0.213 8 nm, comparable to those found in other polynuclear zinc compounds, such as {[ArN= C(Me)COCHCO(Me)C(Me)=NAr][OCH(Me)C(Me)=NAr](ZnEt)₃} (0.196 7(3)-0.209 5(3) nm, Ar=2,6-ⁱPr₂C₆H₃) with a Zn₃O₃ core^[20] and (S)-diphenyl(pyrrolidin-2-yl)methanolate ethylzinc (B) with a Zn₂O₂ core (0.202 4(2)-0.203 8(2) nm)^[33]. The Zn1—N1 bond distance is 0.223 9(3) nm, slightly longer than those in B (0.213 6(2) and 0.215 6(2) nm)^[33]. The bond angles of Zn2-O2-Zn1A and Zn1—O2—Zn1A are 94.99(9)° and 92.79(8)°, respectively.

  Figure 2

  

  Figure 2.  Molecular structure of complex 2 with 30% probability displacement ellipsoids

  Hydrogen atoms except those on N atoms are omitted for clarity; Symmetry code: A: -x+1, -y+1, -z+1.

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  Figure 3

  

  Figure 3.  Core structure of complex 2 with 30% probability displacement ellipsoids

  Hydrogen atoms are omitted for clarity; Symmetry code: A: -x+1, -y+1, -z+1.

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  2.3   Catalytic activity of complexes 1 and 2

  The aluminum or zinc-catalyzed ROP of ε-caprolactone has been widely investigated in recent years^[2-3]. It′s known that the ligands significantly change the catalytic reactivity of the metal center. Herein, the ROP of ε-caprolactone catalyzed by complexes 1 and 2 have also been examined. Complex 1 showed negligible activity toward the ROP of ε-caprolactone. Similar results have been observed in other dimeric aluminum complexes with an Al₂O₂ core, which seems that the dimeric structures disfavor the initiation of the ROP of ε-caprolactone^[23]. In contrast, the zinc complex 2 showed good activity in this reaction. The results of the ROP of ε-caprolactone in toluene catalyzed by complex 2 at 18 and 60 ℃ are listed in Table 3. In general, higher temperature results in a shorter reaction time with higher M_n and yield (entries 3-6). When the molar ratio of the initial monomer (CL) to the initiator (Ⅰ), n_CL/n_I, was increased from 200 to 1 600, the M_n was increased from 3 721 to 28 617, whereas the yield of the poly(caprolactone) was decreased from 93% to 79% after the set reaction time. The measured M_w/M_n values ranged from 1.20 to 1.34 when n_CL/n_I exceeded 400, which were comparable to those of the recently reported Zn complexes^[34]. The M_n determined from GPC was slightly less than one-fourth of that calculated from M_n (Calcd.), suggesting that all of the four zinc atoms of 2 are active centers.

  Table 3

  

  Table 3.  ROP of ε-caprolactone catalyzed by complex 2

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  Entry	T/℃	t/h	nCL/nIa	Mw/Mnb	Mn (GPC)b	Mn (Calcd.)c	Yieldd/%
  1	18	12	200	2.14	3 721	21 396	93
  2	18	12	400	1.34	10 939	42 168	92
  3	18	12	800	1.27	15 732	91 480	88
  4	18	12	1 600	1.20	28 617	182 788	79
  5	60	2	800	1.34	17 873	91 480	90
  6	60	2	1 600	1.22	35 783	182 788	83
  a Molar ratio of initial ε-caprolactone (CL) to initiator 2 (Ⅰ); b Determined from GPC using standard polystyrene as a reference; c Calculated from Mn, CL×(nCL/nI)×Y, where Y represents the yield; d Isolated yield.

  3.   Conclusions

  In summary, the aluminum and zinc complexes chelated by 2-((2,6-dimethylphenyl)amino)ethanolate have been synthesized and characterized. Preliminary catalytic studies prove that the zinc complex 2 is highly active toward the ring-opening polymerization of ε-caprolactone, giving poly(caprolactones) with high molecular weights and narrow molecular weight distribution, while the aluminum complex 1 is inactive. This suggests that appropriately combining metal and simple ligands can also lead to a good catalyst.

  
  
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  Scheme 1  Synthesis of complexes 1 and 2

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  Figure 1  Molecular structure of complex 1 with 30% probability displacement ellipsoids

  Hydrogen atoms are omitted for clarity; Symmetry code: A: -x+1, -y+1, -z+1.

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  Figure 2  Molecular structure of complex 2 with 30% probability displacement ellipsoids

  Hydrogen atoms except those on N atoms are omitted for clarity; Symmetry code: A: -x+1, -y+1, -z+1.

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  Figure 3  Core structure of complex 2 with 30% probability displacement ellipsoids

  Hydrogen atoms are omitted for clarity; Symmetry code: A: -x+1, -y+1, -z+1.

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  Table 1.  Crystallographic data and refinement parameters for complexes 1 and 2

  Parameter	1	2
  Formula	C24H40Al2N2O2	C64H94N6O6Zn4
  Formula weight	442.54	1 305.01
  Crystal size/mm	0.24×0.20×0.10	0.18×0.16×0.12
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a/nm	0.836 3(2)	1.175 7(2)
  b/nm	0.940 5(2)	1.236 9(3)
  c/nm	0.941 9(2)	1.377 1(3)
  α/(°)	62.76(3)	95.05(3)
  β/(°)	82.71(3)	111.48(3)
  γ/(°)	75.47(3)	116.33(3)
  V/nm3	0.637 5(2)	1.593 1(7)
  Z	1	1
  Dc/(g·cm-3)	1.153	1.360
  θ range/(°)	3.48-25.02	2.169-25.019
  F(000)	240	688
  μ/mm-1	0.135	1.541
  Measured reflection	4 671	11 783
  Unique reflection (Rint)	2 224 (0.058 7)	5 565 (0.043 0)
  Observed reflection with [I≥2σ(I)]	1 584	4 343
  Number of parameters	140	379
  GOF	0.975	1.053
  Residuals R1, wR2	0.043 9, 0.109 1	0.043 7, 0.092 1

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  Table 2.  Selected bond distances (nm) and angles (°) for complexes 1 and 2

  1
  Al1—O1	0.184 6(2)	Al1—O1A	0.192 0(2)	Al1—C11	0.196 6(2)
  Al1—N1	0.237 8(2)	C2—N1	0.149 2(3)	Al1—C12	0.197 9(2)
  O1—Al1—O1A	76.76(7)	Al1—O1—Al1A	103.24(7)	C3—N1—Al1	124.6(1)
  C11—Al1—C12	118.9(1)	C2—N1—Al1	106.2(1)	C2—C1—O1	108.3(2)
  2
  Zn1—O1	0.200 3(3)	Zn1—O2	0.213 8(2)	Zn1—O3	0.195 2(3)
  Zn1—O2A	0.206 6(2)	Zn1—N1	0.223 9(3)	Zn2—O2	0.210 0(3)
  Zn2—O3	0.199 8(2)	Zn2—O1A	0.201 2(2)	Zn2—C31	0.197 5(3)
  C20—O2	0.143 5(4)	C1—N1	0.145 7(4)	C11—N2	0.144 9(6)
  O1—Zn1—O2	90.34(9)	O1—Zn1—O2A	83.08(9)	O2—Zn1—N1	139.83(9)
  O1—Zn1—O3	168.5(1)	O2—Zn1—O3	81.70(9)	O2—Zn1—O2A	87.21(8)
  O2—Zn2—O3	81.59(9)	O1A—Zn2—O2	82.01(9)	O3—Zn2—O1A	99.15(9)
  Zn1—O2—Zn1A	92.79(8)	Zn1A—O2—Zn2	94.99(9)	C1—N1—C9	112.0(3)
  C11—N2—C19	112.5(5)	C21—N3—C29	115.3(3)	C32—C31—Zn1	117.0(3)
  Symmetry codes: A: -x+1, -y+1, -z+1 for 1; A: -x+1, -y+1, -z+1 for 2.

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  Table 3.  ROP of ε-caprolactone catalyzed by complex 2

  Entry	T/℃	t/h	nCL/nIa	Mw/Mnb	Mn (GPC)b	Mn (Calcd.)c	Yieldd/%
  1	18	12	200	2.14	3 721	21 396	93
  2	18	12	400	1.34	10 939	42 168	92
  3	18	12	800	1.27	15 732	91 480	88
  4	18	12	1 600	1.20	28 617	182 788	79
  5	60	2	800	1.34	17 873	91 480	90
  6	60	2	1 600	1.22	35 783	182 788	83
  a Molar ratio of initial ε-caprolactone (CL) to initiator 2 (Ⅰ); b Determined from GPC using standard polystyrene as a reference; c Calculated from Mn, CL×(nCL/nI)×Y, where Y represents the yield; d Isolated yield.

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