- Email: [email protected]
Ring-opening polymerization of rac-lactide by mononuclear zinc complexes that contain chiral tetra-azane ligands
Polyhedron 121 (2017) 206–210
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Ring-opening polymerization of rac-lactide by mononuclear zinc complexes that contain chiral tetra-azane ligands Yancheng Han a,1, Qiyun Feng b,1, Yongfang Zhang a, Yunping Zhang a, Wei Yao a,⇑ a b
School of Resources and Environment, University of Jinan, Jinan 250022, PR China Department of Applied Chemistry, College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 23 June 2016 Accepted 3 October 2016 Available online 15 October 2016 Keywords: Catalyst Zinc complex Rac-lactide Ring-opening polymerization Chiral tera-azane ligand
a b s t r a c t A series of zinc complexes supported by chiral tetra-azane ligands, (1R,2R)-[(NHAr)C6H4CH@N]2C6H10 (L1H2 (Ar = 2,6-iPr2C6H3); L2H2 (Ar = 2,6-Et2C6H3); L3H2 (Ar = 2,6-Me2C6H3); L4H2 (Ar = 4-MeC6H4)), were synthesized via the reactions of the corresponding ligands with ZnMe2. These complexes were well characterized by NMR spectroscopy and elemental analysis. The molecular structures of L2Zn (2) and L3Zn (3) were further confirmed by X-ray diffraction studies. The zinc complexes are efficient initiators for the ring-opening polymerization of rac-lactide, yielding heterotactic-rich polylactide. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Biodegradable and biocompatible polyesters, especially polylactides (PLAs), have attracted much attention due to their wide range of applications, including scaffolds, antibodies, genes and delivery media for the controlled release of drugs [1]. The ring-opening polymerization (ROP) of lactide (LA) promoted by organometallic catalysts represents the most efficient method to prepare PLA with controlled microstructural properties, such as molecular weight, polydispersity and stereoregularity [2]. There are many judiciously designed catalysts that exhibit high activity and selectivity for the ROP of LA. The designing principles of such catalysts typically include (a) metal complexes containing the initiators of the formula LmMR where Lm is the ancillary ligand and R is the initiation group, usually being an alkoxide, phenoxide or amide [2], (b) metal complexes without normal initiators of the formula LmM [3]. The former strategy is commonly employed because of its flexibility for molecular design. On the other hand, the latter approach is relatively less reported even though excellent properties of the catalysts could be achieved from the appropriate combinations of Lm with M. On the basis of the anilido-aldimine backbone and the role of the zinc metal center in catalysis, a series of zinc complexes with anilido-aldimine ligands have been reported [4]. They showed a ⇑ Corresponding author. Fax: +86 531 82769233. 1
E-mail address: [email protected] (W. Yao). These authors contributed equally.
http://dx.doi.org/10.1016/j.poly.2016.10.010 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
high activity for the ROP of LA in the presence of an initiator. Considering the chiral nature of the cyclohexyl substituents on the imine, we recently revealed a number of chiral dinuclear zinc complexes with anilido-aldimine ligands [4c]. The dinuclear zinc complexes were efficient catalysts for cyclic esters in the presence of benzyl alcohol, but had no selectivity for rac-LA. In order to explore good catalysts with high activity and selectivity, we developed a family of mononuclear zinc complexes (1–4) with chiral tetradentate anilido-aldimine ligands. It was found that these zinc complexes were efficient catalysts for the ROP of rac-LA, giving a linear PLA with moderate selectivity. In this article, we report on the synthesis, structures and catalytic properties of 1–4 for the ROP of rac-LA. 2. Experimental 2.1. General considerations All reactions and manipulations which relate to the preparation of the metal complexes were carried out under nitrogen with standard Schlenk-line or glovebox techniques. All solvents were refluxed over the appropriate drying agent and distilled prior to use. The compound A (Scheme 1) was prepared according to a literature procedure [5]. 1H and 13C NMR spectra were measured using Varian Mercury-300 NMR, Bruker AVANCE-400 NMR and Bruker AVANCE-600 spectrometers. The elemental analyses were performed on a Perkin-Elmer 2400 analyzer. Gel permeation chromatography (GPC) measurement was performed on a TOSOH
207
Y. Han et al. / Polyhedron 121 (2017) 206–210
N
N
F
F
A
N
2.0 ArNHLi
N
N
1.0 ZnMe2
NH HN Ar Ar L1H2: L2H2: L3H2: L4H2:
Ar Ar Ar Ar
= = = =
2,6-iPr2C6H3 2,6-Et2C6H3 2,6-Me2C6H3 4-MeC6H4
Zn
N Ar L1Zn L2Zn L3Zn L4Zn
(1): (2): (3): (4):
Ar Ar Ar Ar
N N Ar
= = = =
2,6-iPr2C6H3 2,6-Et2C6H3 2,6-Me2C6H3 4-MeC6H4
Scheme 1. Synthesis of complexes 1–4.
HLC8220 GPC at 40 °C using THF as the eluent against polystyrene standards. 2.2. Synthesis of L4H2 A solution of n-BuLi (15 mL, 30 mmol, 2.0 M in hexane) was added to a solution of 4-methylaniline (3.2 g, 30 mmol) in THF (30 mL) at 20 °C. The mixture was allowed to warm to room temperature and stirred overnight. The resulting solution was transferred into a solution of A (4.9 g, 15 mmol) in THF (20 mL) at 25 °C. After stirring for 12 h, the reaction was quenched with H2O (5 mL), extracted with hexane, and the organic phase was evaporated to dryness in vacuum to give the crude product as a yellow solid. The pure product was obtained by recrystallization from MeOH at 20 °C as a white solid (4.6 g, 61%). Anal. Calc. for C34H36N4 (500.29): C, 81.56; H, 7.25; N, 11.19. Found: C, 81.47; H, 7.29; N, 11.24. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 10.91 (s, 2H, ArNH), 8.21 (s, 2H, ArCH@N), 7.41 (d, J = 7.8 Hz, 2H, Ar–H), 7.29 (d, J = 7.4 Hz, 2H, Ar–H), 7.20 (t, J = 7.6 Hz, 2H, Ar–H), 7.11–7.08 (m, 4H, Ar–H), 7.04 (t, J = 7.3 Hz, 2H, Ar–H), 6.94 (d, J = 7.4 Hz, 2H, Ar–H), 6.66–6.58 (m, 2H, Ar–H), 3.37–3.21 (m, 2H, C@NCH), 2.33 (s, 6H, ArCH3), 1.90 (m, 4H,CH2), 1.74 (m, 2H, CH2), 1.46 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 163.5, 146.0, 140.1, 133.9, 131.0, 126.5, 123.1, 121.9, 118.5, 116.6, 112.5, 75.0, 33.6, 24.6, 18.7. 2.3. Synthesis of L1Zn (1) ZnMe2 (1.2 mL, 1.2 mmol, 1.0 M in toluene) was added to a solution of L1H2 (0.77 g, 1.2 mmol) in toluene (10 mL) at 0 °C with stirring. The reaction mixture was gently heated to 80 °C for 12 h. After removal of the solvent, the product was recrystallized from hexane to give 1 as a yellow crystalline solid (0.75 g, 89%). Anal. Calc. for C44H54N4Zn (702.36): C, 75.03; H, 7.73; N, 7.95. Found: C, 75.14; H, 7.79; N, 7.78. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.52 (s, 2H, ArCH@N), 7.76 (t, J = 7.6 Hz, 2H, Ar–H), 7.35–7.32 (m, 2H, Ar–H), 7.22–7.14 (m, 4H, Ar–H), 7.12–6.94 (m, 2H, Ar–H), 6.38 (t, J = 7.1 Hz, 2H, Ar–H), 6.16 (d, J = 8.9 Hz, 2H, Ar–H), 3.64 (d, J = 7.8 Hz, 2H, C@NCH), 3.00 (m, 2H, CH(CH3)2), 2.74 (m, 2H, CH(CH3)2), 1.89 (m, 6H, CH2), 1.68–1.44 (m, 2H, CH2), 1.13 (d, J = 6.9 Hz, 6H, CH(CH3)2), 1.06 (d, J = 6.9 Hz, 6H, CH (CH3)2), 0.97 (d, J = 6.9 Hz, 6H, CH(CH3)2), 0.76 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 168.8, 157.3, 154.6, 143.7, 137.2, 133.5, 127.8, 124.8, 123.6, 115.8, 113.7, 112.3, 76.0, 33.8, 28.0, 27.8, 24.9, 24.3, 24.2, 24.1, 23.9.
as a yellow crystalline solid (0.65 g, 91%). Anal. Calc. for C40H46N4Zn (646.30): C, 74.12; H, 7.15; N, 8.64. Found: C, 74.20; H, 7.19; N, 8.71. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.49 (s, 2H, ArCH@N), 7.20 (d, J = 7.8 Hz, 2H, Ar–H), 7.01 (d, J = 4.7 Hz, 4H, Ar–H), 6.94 (t, J = 4.6 Hz, 2H, Ar–H), 6.83 (d, J = 8.6, Hz, 2H, Ar–H), 6.34 (t, J = 7.1 Hz, 2H, Ar–H), 5.99 (d, J = 8.9 Hz, 2H, Ar–H), 3.29–3.12 (m, 2H, C@NCH), 2.58–2.50 (m, 4H, ArCH2CH3), 2.47–2.31 (m, 4H, ArCH2CH3), 2.09–2.07 (m, 2H, CH2), 1.88–1.70 (m, 2H, CH2), 1.62 (q, J = 8.3 Hz, 2H, CH2), 1.45 (q, J = 10.1 Hz, 2H, CH2), 0.95 (t, J = 7.5 Hz, 6H, CH2CH3), 0.78 (t, J = 7.0 Hz, 6H, CH2CH3). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 164.9, 156.7, 147.1, 141.2, 137.3, 137.2, 132.1, 126.3, 124.6, 123.9, 116.7, 114.7, 111.6, 67.5, 29.0, 25.1, 24.2, 21.0, 15.2, 12.6. 2.5. Synthesis of L3Zn (3) This compound was prepared in the same way as described above for 1 with L3H2 (0.63 g, 1.2 mmol) and ZnMe2 (1.2 mL, 1.2 mmol, 1.0 M in toluene) as starting materials. 3 was obtained as a yellow crystalline solid (0.62 g, 88%). Anal. Calc. for C36H38N4Zn (590.24): C, 73.03; H, 6.47; N, 9.46. Found: C, 73.10; H, 6.51; N, 9.53. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.49 (s, 2H, ArCH@N), 7.20 (d, J = 7.9 Hz, 2H, Ar–H), 7.06–6.91 (m, 2H, Ar–H), 6.91–6.81 (m, 6H, Ar–H), 6.35 (d, J = 7.2 Hz, 2H, Ar–H), 6.02 (d, J = 8.9 Hz, 2H, Ar–H), 3.30–3.17 (m, 2H, C@NCH), 2.45 (d, J = 12.7 Hz, 2H, CH2), 2.07 (m, 2H, CH2), 2.04 (s, 12H, PhCH3), 1.61 (m, 2H, CH2), 1.45 (t, J = 9.9 Hz, 2H, CH2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 164.7, 155.6, 147.8, 137.2, 136.6, 132.2, 131.6, 128.8, 128.1, 123.5, 116.2, 114.6, 111.6, 67.3, 28.8, 24.1, 18.9, 17.4. 2.6. Synthesis of L4Zn (4) This compound was prepared in the same way as described above for 1 with L4H2 (0.50 g, 1.0 mmol) and ZnMe2 (1.0 mL, 1.0 mmol, 1.0 M in toluene) as starting materials. 4 was obtained as a yellow solid (0.50 g, 90%). Anal. Calc. for C34H34N4Zn (562.21): C, 72.40; H, 6.08; N, 9.93. Found: C, 72.32; H, 6.10; N, 10.07%. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.50 (s, 2H, ArCH@N), 7.20 (d, J = 6.9 Hz, 2H, Ar–H), 7.17–6.99 (m, 2H, Ar–H), 6.81–6.74 (m, 2H, Ar–H), 6.58 (d, J = 6.3 Hz, 2H, Ar–H), 6.41 (d, J = 7.8 Hz, 2H, Ar–H), 6.33–6.30 (m, 2H, Ar–H), 5.99 (d, J = 6.9 Hz, 2H, Ar–H), 5.83 (d, J = 7.8 Hz, 2H, Ar–H), 3.31–3.16 (m, 2H, C@NCH), 2.51 (d, J = 12.7 Hz, 2H, CH2), 2.04 (s, 6H, PhCH3), 1.94 (m, 2H, CH2), 1.66 (m, 2H, CH2), 1.47 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 161.1, 155.7, 145.1, 138.7, 137.2, 134.8, 129.6, 129.0, 127.2, 124.3, 118.8, 114.3, 113.1, 65.8, 29.0, 24.9, 18.8, 18.5.
2.4. Synthesis of L2Zn (2) 2.7. Ring-opening polymerization of rac-LA by 1–4 This compound was prepared in the same way as described above for 1 with L2H2 (0.64 g, 1.1 mmol) and ZnMe2 (1.1 mL, 1.1 mmol, 1.0 M in toluene) as starting materials. 2 was obtained
In a typical polymerization experiment, the zinc complex and the required amount of rac-LA in toluene were loaded in a
208
Y. Han et al. / Polyhedron 121 (2017) 206–210
flame-dried vessel containing a magnetic bar. The vessel was placed in an oil bath thermostated at the proposed temperature. After a certain reaction time, the polymer was isolated by precipitation with cold ethanol. The precipitate was dried under vacuum at 40 °C for 24 h. For some polymerization reactions, samples were taken for determining the monomer conversion by 1H NMR spectroscopy during the reaction. 2.8. X-ray crystallography Diffraction data of 2 were collected at 120 K with an Xcalibur, Eos, Gemini diffractometer equipped with graphite-monochromated Mo-Ka radiation (k = 0.71073 Å). Diffraction data of 3 were collected at 293 K with a Bruker SMART-CCD diffractometer equipped with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). Using OLEX2 [6], the structures were solved with the SHELXS [7] structure solution program using direct methods and refined with the SHELXL [7] refinement package using least squares minimization. 3. Results and discussion 3.1. Synthesis and characterization of the zinc complexes The free chiral tetra-azane chelating ligands o-(C6H4(NHAr)CH@N)2C6H10 ((1R,2R)-L1H2 (Ar = 2,6-iPr2C6H3); (1R,2R)-L2H2 (Ar = 2,6-Et2C6H3); (1R,2R)-L3H2 (Ar = 2,6-Me2C6H3); L4H2 (Ar = 4-MeC6H4)) were synthesized according to the literature procedure [8]. Complexes 1–4 were synthesized through alkane elimination reactions using one equivalent of the appropriate ligand with ZnMe2 in dry toluene, as summarized in Scheme 1. All the complexes were isolated as yellow crystalline solids and characterized by elemental analyses, along with 1H and 13C NMR spectroscopies. The 1H NMR spectra of 1–4 confirmed that the complexes were monometallic with one coordinated tetra-azane ligand. Crystals of 2 and 3 suitable for X-ray crystal structure determination were grown from a mixture of CH2Cl2/hexane at 20 °C. Their molecular structures with selected bond lengths and angles are shown in Figs. 1 and 2 respectively. The structural refinement parameters and crystallographic data are given in Table S1. The X-ray structural analyses of 2 and 3 revealed that the complexes adopt a distorted square planar geometry around the Zn center with the N(2)–Zn(1)–N(3) and N(1)–Zn(1)–N(4) planes inclined by 50.7° for 2, and 51.1° for 3. The dihedral angles of N(2)–Zn (1)–N(3)–C(32) and N(1)–Zn(1)–N(4)–C(27) are 14.1 and 7.0° for 2, and 5.3° and 17.6° for 3, respectively. The torsion angles between the Zn(1)–N(3)–C(32) and Zn(1)–N(4)–C(27) planes for 2 and 3 are 67.5° and 69.6° respectively. The N(1)–Zn(1)–N(2) bond angle in the five-membered chelating ring is 80.14(1)° for 2 and 80.45(1)° for 3. This is slightly larger in comparison to the corresponding bond angle in salen Zn(II) complexes (78.95(1)°) [9]. The imino C@N bonds retained their double bond character, being 1.285(4) and 1.297(5) Å for 2, and 1.282(4) and 1.286(4) Å for 3. The Zn–N (amido) distances (2.043(3) and 2.033(3) Å in 2, and 2.029(2) and 2.035(2) Å in 3) are longer than the Zn–N (imine) distances (1.969(3) and 1.959(3) Å in 2 and 1.966(2) and 1.961(2) Å in 3), which is contrary to the case of Zn(II) complexes with tridentate anilido-imine ligands [4]. It is obvious that the Zn–N (amido) distances are elongated due to the tension of the ligand. 3.2. Ring-opening polymerization of rac-LA Complexes 1–4 were tested as catalysts for the ROP of rac-LA without any initiator. The results are listed in Table 1. Complexes 1–4 all exhibit high reactivity for the ROP of rac-LA (entries 1–4).
Fig. 1. The molecular structure of 2. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–N(1) 2.043(3), Zn(1)–N(2) 2.033(3), Zn(1)–N(3) 1.969(3), Zn (1)–N(4) 1.959(3), N(1)–C(9) 1.285(4), N(2)–C(21) 1.297(5), N(1)–Zn(1)–N(2) 80.14 (1), N(2)–Zn(1)–N(3) 93.36(1), N(4)–Zn(1)–N(3) 116.80(1), N(4)–Zn(1)–N(1) 93.17 (1).
Fig. 2. The molecular structure of 3. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms and solvent are omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–N(1) 2.029(2), Zn(1)–N(2) 2.035(2), Zn(1)–N(3) 1.966(2), Zn(1)–N(4) 1.961(2), N(1)–C(9) 1.282(4), N(2)–C(21) 1.286(4), N(1)–Zn (1)–N(2) 80.45(1), N(2)–Zn(1)–N(3) 93.12(1), N(4)–Zn(1)–N(3) 117.05(1), N(4)–Zn (1)–N(1) 93.17(1).
The reactivities of 1–4 as catalysts for the ROP of rac-LA under the same conditions is in the order 4 > 3 > 2 > 1, which is in reverse order of the size of the substituents in their ligands. The results suggest that the zinc center in a complex with a less bulky substituent is more easily approached by the LA monomer. A similar observation has been reported in the polymerization of e-caprolactone using
209
Y. Han et al. / Polyhedron 121 (2017) 206–210 Table 1 ROP of rac-LA catalyzed by complexes 1–4.a
a b c d e f g h i
Entry
Cat.
[LA]0/[Zn]0
T (°C)
Time (min)
Yield (%)b
TOFc
Mn.calcdd(103)
Mn e(103)
PDIe
Prf
1 2 3 4 5 6 7 8g 9h 10 11 12 13i 14i 15i 16i
1 2 3 4 4 4 4 4 4 4 4 4 4 1 2 3
100:1 100:1 100:1 100:1 100:1 100:1 100:1 100:1 100:1 150:1 200:1 250:1 100:1 100:1 100:1 100:1
70 70 70 70 50 30 0 30 30 70 70 70 70 70 70 70
1.5 1.2 1 1 6 15 1440 20 35 2 3 4 20 30 30 30
82 80 87 97 92 93 94 90 92 94 95 96 90 82 86 92
3280 4000 5220 5820 920 372 4 270 158 4230 3800 3600 270 164 172 184
11.8 11.5 12.5 14.0 13.2 13.4 13.5 13.0 13.2 20.3 27.4 34.6 13.1 11.9 12.5 13.4
7.8 8.9 9.0 12.9 13.2 12.9 13.1 12.4 12.7 18.5 26.2 31.8 13.3 7.6 8.3 11.2
1.13 1.09 1.17 1.21 1.18 1.14 1.11 1.18 1.24 1.17 1.23 1.20 1.47 1.53 1.40 1.43
0.58 0.58 0.59 0.61 0.67 0.71 0.77 0.65 0.60 – – – 0.56 0.54 0.54 0.55
Polymerization conditions: catalyst, 30 lmol, [rac-LA]0 = 0.25 M in toluene, a N2 atmosphere. Determined by 1H NMR. Mole of LA consumed per mol of catalyst per hour. Calculated from ([rac-LA]0/[Zn]0) conversion 144.13 or ([rac-LA]0/[BnOH]0) conversion 144.13 + 108. Determined by GPC against polystyrene standards in THF, multiplied by 0.58 [10]. Pr is the probability of racemic linkages between monomer units determined by homonuclear decoupled 1H NMR spectroscopy [11]. DCM was used instead of toluene. THF was used instead of toluene. 1.0 equiv. BnOH was added.
anilido-imine aluminium catalysts [12]. In comparison with the literature results, the reactivity of 1–4 is higher than that of the anilido-aldimine zinc complexes [4]. Complexes 1–4 are also more reactive than the zinc complexes without a normal initiator [3h]. The reactivity of 1–4 is lower as compared to zinc b-diiminate complexes [13]. The polymerization initiated by 4 was completed within 1 min and the value of the turnover frequency (TOF) reached 5820 h 1 when 100 equiv. of rac-LA were added (entry 4). Complexes 1–4 showed moderate selectivity for the ROP of rac-LA, giving a heterotactic-rich polymer (entries 1–4). The results suggest that the substituents on the ligands slightly affect the selectivity of 1– 4. Complex 4 was used to examine the effects of different reaction conditions in detail. When the polymerization was carried out at a lower temperature, the selectivity increased up to Pr = 0.77 (entries 4–7, Fig. S1). The solvents influenced the catalytic activity and selectivity of 4 with the increasing trend of THF < DCM < toluene (entries 6–8). This is probably due to the THF solvent acting as a donor agent which easily coordinates to the active metal center such that the complex is more discriminatory to the incoming LA. It was also found that the polydispersity indexes (PDIs) of the PLA catalyzed by 4 range from 1.11 to 1.24. To further explore the behavior of 4, polymerization experiments for different times with a monomer/4 = 300:1 system were performed. A linear relationship between the number-averaged molecular weight (Mn) and the monomer conversion exists, as shown in Fig. 3. These results demonstrate the ‘‘living” characterization of the catalyst. The end group of the PLA was characterized by 1H NMR spectra and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy. 1H NMR spectrum of a typical polymer sample (Fig. S2) showed prominent peaks of the PLA main chain at d 1.48–1.60 and 5.00–5.25 ppm. In addition, signals relating to the hydroxyl and ethyl groups could also be identified at d 2.72, 1.26 and 4.18 ppm respectively. The results demonstrated that the PLA chain was capped by one acetoxyl and one hydroxyl end. The MALDI-TOF spectra further confirmed such an observation with molecular weights of the polymer molecules relatively close to M = n MLA/2 + MEtOH + MNa.
Fig. 3. Linear relationship between Mn and rac-LA conversion initiated by complex 4 ([rac-LA]0/[4]0 = 300/1, T = 30 °C, toluene).
All the results suggest that the tetra-azane ligands in the zinc complexes act as initiating groups to initiate the ROP of LA in a living manner. Based on the results and the generally accepted mechanisms for the ROP of LA mediated by metal complexes in the absence of alcohol, a coordination-insertion mechanism was proposed for the rac-LA polymerization initiated by the zinc complexes (Scheme 2) [14]. Firstly, the LA monomer coordinates to the zinc center through the carbonyl oxygen atom. Subsequently, the monomer ring is cleaved at the acyl-oxygen bond and inserts into one of the Zn–N (amido) bonds of the zinc complex to form a new alkoxyl zinc inthermediate. Thereafter, repetition of the same procedure forms the PLA chain on the Zn center. Finally, the polymerization reaction is quenched by an ethanol molecule. In the meantime, the ligand is substituted by an acetoxy group. A similar phenomenon was found when benzyl alcohol (BnOH) was added (Entries 13–16). It was found that the activities of 1–4 in the presence of BnOH were lower as compared to those of 1–4 alone. The 1H NMR spectrum of the polymer is shown in Fig. S4. The polymer was found to be capped with a benzyl ester and hydroxyl chain end.
210
Y. Han et al. / Polyhedron 121 (2017) 206–210
N
N Zn
N
N
LA
N
N
N
N
Zn N
N
O
LA
Zn
O
N
O
O
O
Zn
N
N
O O
O
O
N
N
N
O
O
O O
O
2n
O
EtOH
O
HO O
O
O 2n
O
Scheme 2. Proposed polymerization mechanism of LA initiated by the zinc complexes.
4. Conclusion In conclusion, a series of zinc complexes (1–4) supported by chiral tetra-azane ligands was synthesized and structurally characterized. It was found that the zinc complexes were active toward the ROP of rac-LA to provide polymers with controllable molecular weights and narrow molecular weight distributions, with moderate heterotacticity up to Pr = 0.77. The substituents of the ligands affect the activities and selectivities of these complexes. Acknowledgments
[4]
We are grateful for financial support from the National Natural Science Foundation of China (No. 21104026), China Scholarship Council (No. 2015020000004). Appendix A. Supplementary data CCDC 909691 and 901268 contains the supplementary crystallographic data for 2 and 3. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.poly.2016.10.010.
[5] [6] [7] [8] [9] [10] [11] [12] [13]
References [1] (a) (b) [2] (a) (b) (c) (d) [3] (a) (b)
G. Swift, Acc. Chem. Res. 26 (1993) 105; R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 250 (2006) 602; C.M. Thomas, Chem. Soc. Rev. 39 (2010) 165; M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486; C. Wheaton, P.G. Hayes, B.J. Ireland, Dalton Trans. (2009) 4832. Z. Dai, Y. Sun, J. Xiong, X. Pan, J. Wu, ACS Macro Lett. 4 (2015) 556; J. Xiong, J. Zhang, Y. Sun, Z. Dai, X. Pan, J. Wu, Inorg. Chem. 54 (2015) 1737;
[14]
(c) J. Zhang, J. Xiong, Y. Sun, N. Tang, J. Wu, Macromolecules 47 (2014) 7789; (d) Z. Dai, Y. Sun, J. Xiong, X. Pan, N. Tang, J. Wu, Catal. Sci. Technol. 6 (2016) 515; (e) P.K. Eckert, I.S. Vieira, V.H. Gessner, J. Börner, C. Strohmann, S. HerresPawlis, Polyhedron 49 (2013) 151; (f) Y.Y. Kang, H.-R. Park, M.H. Lee, J. An, Y. Kim, J. Lee, Polyhedron 95 (2015) 24; (g) A. John, V. Katiyar, K. Pang, M.M. Shaikh, H. Nanavati, P. Ghosh, Polyhedron 26 (2007) 4033; (h) E.D. Akpan, S.O. Ojwach, B. Omondi, V.O. Nyamori, Polyhedron 110 (2016) 63; (i) D. Appavoo, B. Omondi, I.A. Guzei, J.L. van Wyk, O. Zinyemba, J. Darkwa, Polyhedron 69 (2014) 55; (j) S. Bhunora, J. Mugo, A. Bhaw-Luximon, S. Mapolie, J.L. van Wyk, J. Darkwa, E. Nordlander, Appl. Organomet. Chem. 25 (2011) 133. (a) Y.-H. Tsai, C.-H. Lin, C.-C. Lin, B.-T. Ko, J. Polym. Sci., Part A: Polym. Chem. 47 (2009) 4927; (b) N. Yang, L. Xin, W. Gao, J. Zhang, X. Luo, X. Liu, Y. Mu, Dalton Trans. 41 (2012) 11454; (c) A. Gao, W. Yao, Y. Xiao, M. Zhang, G. Zhu, N. Zhang, S. Wang, D. Wang, Y. Zhang, Y. Gao, Z. Xu, P. Lu, Z. Zhang, Polyhedron 85 (2015) 537; (d) W. Yao, N. Cui, Y. Xue, T. Liu, Y. Wang, Chem. Res. Chin. Univ. 29 (2013) 48; (e) C.-H. Wang, C.-Y. Li, B.-H. Huang, C.-C. Lin, B.-T. Ko, Dalton Trans. 42 (2013) 10875; (f) A. Gao, W. Yao, Y. Mu, W. Gao, M. Sun, Q. Su, Polyhedron 28 (2009) 2605; (g) W. Yao, Y. Mu, A. Gao, W. Gao, L. Ye, Dalton Trans. 24 (2008) 3199. Z. Li, K.R. Conser, E.N. Jacobsen, J. Am. Chem. Soc. 115 (1993) 5326. O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl. Cryst. 42 (2009) 339. G.M. Sheldrick, Acta. Cryst. A64 (2008) 112. Y. Zhang, W. Yao, H. Li, Y. Mu, Organometallics 31 (2012) 4670. G.A. Morris, H. Zhou, C.L. Stern, S.T. Nguyen, Inorg. Chem. 40 (2001) 3222. A. Kowalski, A. Duda, S. Penczek, Macromolecules 31 (1988) 2114. T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc. 124 (2002) 1316. W. Yao, Y. Mu, A. Gao, Q. Su, Y. Liu, Y. Zhang, Polymer 49 (2008) 2486. (a) M. Cheng, A.B. Attygalle, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 121 (1999) 11583; (b) B.M. Chamberlain, M. Cheng, D.R. Moore, T.M. Ovitt, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 123 (2001) 3229; (c) M.H. Chisholm, J.C. Huffman, K. Phomphrai, J. Chem. Soc., Dalton Trans. (2001) 222; (d) M.H. Chisholm, J. Gallucci, K. Phomphrai, Inorg. Chem. 41 (2002) 2785. (a) J. Liu, H. Ma, Dalton Trans. 43 (2014) 9098; (b) L. Postigo, M.C. Maestre, M.E.G. Mosquera, T. Cuenca, G. Jiménez, Organometallics 32 (2013) 2618.
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Ring-opening polymerization of rac-lactide by mononuclear zinc complexes that contain chiral tetra-azane ligands Yancheng Han a,1, Qiyun Feng b,1, Yongfang Zhang a, Yunping Zhang a, Wei Yao a,⇑ a b
School of Resources and Environment, University of Jinan, Jinan 250022, PR China Department of Applied Chemistry, College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 23 June 2016 Accepted 3 October 2016 Available online 15 October 2016 Keywords: Catalyst Zinc complex Rac-lactide Ring-opening polymerization Chiral tera-azane ligand
a b s t r a c t A series of zinc complexes supported by chiral tetra-azane ligands, (1R,2R)-[(NHAr)C6H4CH@N]2C6H10 (L1H2 (Ar = 2,6-iPr2C6H3); L2H2 (Ar = 2,6-Et2C6H3); L3H2 (Ar = 2,6-Me2C6H3); L4H2 (Ar = 4-MeC6H4)), were synthesized via the reactions of the corresponding ligands with ZnMe2. These complexes were well characterized by NMR spectroscopy and elemental analysis. The molecular structures of L2Zn (2) and L3Zn (3) were further confirmed by X-ray diffraction studies. The zinc complexes are efficient initiators for the ring-opening polymerization of rac-lactide, yielding heterotactic-rich polylactide. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Biodegradable and biocompatible polyesters, especially polylactides (PLAs), have attracted much attention due to their wide range of applications, including scaffolds, antibodies, genes and delivery media for the controlled release of drugs [1]. The ring-opening polymerization (ROP) of lactide (LA) promoted by organometallic catalysts represents the most efficient method to prepare PLA with controlled microstructural properties, such as molecular weight, polydispersity and stereoregularity [2]. There are many judiciously designed catalysts that exhibit high activity and selectivity for the ROP of LA. The designing principles of such catalysts typically include (a) metal complexes containing the initiators of the formula LmMR where Lm is the ancillary ligand and R is the initiation group, usually being an alkoxide, phenoxide or amide [2], (b) metal complexes without normal initiators of the formula LmM [3]. The former strategy is commonly employed because of its flexibility for molecular design. On the other hand, the latter approach is relatively less reported even though excellent properties of the catalysts could be achieved from the appropriate combinations of Lm with M. On the basis of the anilido-aldimine backbone and the role of the zinc metal center in catalysis, a series of zinc complexes with anilido-aldimine ligands have been reported [4]. They showed a ⇑ Corresponding author. Fax: +86 531 82769233. 1
E-mail address: [email protected] (W. Yao). These authors contributed equally.
http://dx.doi.org/10.1016/j.poly.2016.10.010 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
high activity for the ROP of LA in the presence of an initiator. Considering the chiral nature of the cyclohexyl substituents on the imine, we recently revealed a number of chiral dinuclear zinc complexes with anilido-aldimine ligands [4c]. The dinuclear zinc complexes were efficient catalysts for cyclic esters in the presence of benzyl alcohol, but had no selectivity for rac-LA. In order to explore good catalysts with high activity and selectivity, we developed a family of mononuclear zinc complexes (1–4) with chiral tetradentate anilido-aldimine ligands. It was found that these zinc complexes were efficient catalysts for the ROP of rac-LA, giving a linear PLA with moderate selectivity. In this article, we report on the synthesis, structures and catalytic properties of 1–4 for the ROP of rac-LA. 2. Experimental 2.1. General considerations All reactions and manipulations which relate to the preparation of the metal complexes were carried out under nitrogen with standard Schlenk-line or glovebox techniques. All solvents were refluxed over the appropriate drying agent and distilled prior to use. The compound A (Scheme 1) was prepared according to a literature procedure [5]. 1H and 13C NMR spectra were measured using Varian Mercury-300 NMR, Bruker AVANCE-400 NMR and Bruker AVANCE-600 spectrometers. The elemental analyses were performed on a Perkin-Elmer 2400 analyzer. Gel permeation chromatography (GPC) measurement was performed on a TOSOH
207
Y. Han et al. / Polyhedron 121 (2017) 206–210
N
N
F
F
A
N
2.0 ArNHLi
N
N
1.0 ZnMe2
NH HN Ar Ar L1H2: L2H2: L3H2: L4H2:
Ar Ar Ar Ar
= = = =
2,6-iPr2C6H3 2,6-Et2C6H3 2,6-Me2C6H3 4-MeC6H4
Zn
N Ar L1Zn L2Zn L3Zn L4Zn
(1): (2): (3): (4):
Ar Ar Ar Ar
N N Ar
= = = =
2,6-iPr2C6H3 2,6-Et2C6H3 2,6-Me2C6H3 4-MeC6H4
Scheme 1. Synthesis of complexes 1–4.
HLC8220 GPC at 40 °C using THF as the eluent against polystyrene standards. 2.2. Synthesis of L4H2 A solution of n-BuLi (15 mL, 30 mmol, 2.0 M in hexane) was added to a solution of 4-methylaniline (3.2 g, 30 mmol) in THF (30 mL) at 20 °C. The mixture was allowed to warm to room temperature and stirred overnight. The resulting solution was transferred into a solution of A (4.9 g, 15 mmol) in THF (20 mL) at 25 °C. After stirring for 12 h, the reaction was quenched with H2O (5 mL), extracted with hexane, and the organic phase was evaporated to dryness in vacuum to give the crude product as a yellow solid. The pure product was obtained by recrystallization from MeOH at 20 °C as a white solid (4.6 g, 61%). Anal. Calc. for C34H36N4 (500.29): C, 81.56; H, 7.25; N, 11.19. Found: C, 81.47; H, 7.29; N, 11.24. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 10.91 (s, 2H, ArNH), 8.21 (s, 2H, ArCH@N), 7.41 (d, J = 7.8 Hz, 2H, Ar–H), 7.29 (d, J = 7.4 Hz, 2H, Ar–H), 7.20 (t, J = 7.6 Hz, 2H, Ar–H), 7.11–7.08 (m, 4H, Ar–H), 7.04 (t, J = 7.3 Hz, 2H, Ar–H), 6.94 (d, J = 7.4 Hz, 2H, Ar–H), 6.66–6.58 (m, 2H, Ar–H), 3.37–3.21 (m, 2H, C@NCH), 2.33 (s, 6H, ArCH3), 1.90 (m, 4H,CH2), 1.74 (m, 2H, CH2), 1.46 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 163.5, 146.0, 140.1, 133.9, 131.0, 126.5, 123.1, 121.9, 118.5, 116.6, 112.5, 75.0, 33.6, 24.6, 18.7. 2.3. Synthesis of L1Zn (1) ZnMe2 (1.2 mL, 1.2 mmol, 1.0 M in toluene) was added to a solution of L1H2 (0.77 g, 1.2 mmol) in toluene (10 mL) at 0 °C with stirring. The reaction mixture was gently heated to 80 °C for 12 h. After removal of the solvent, the product was recrystallized from hexane to give 1 as a yellow crystalline solid (0.75 g, 89%). Anal. Calc. for C44H54N4Zn (702.36): C, 75.03; H, 7.73; N, 7.95. Found: C, 75.14; H, 7.79; N, 7.78. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.52 (s, 2H, ArCH@N), 7.76 (t, J = 7.6 Hz, 2H, Ar–H), 7.35–7.32 (m, 2H, Ar–H), 7.22–7.14 (m, 4H, Ar–H), 7.12–6.94 (m, 2H, Ar–H), 6.38 (t, J = 7.1 Hz, 2H, Ar–H), 6.16 (d, J = 8.9 Hz, 2H, Ar–H), 3.64 (d, J = 7.8 Hz, 2H, C@NCH), 3.00 (m, 2H, CH(CH3)2), 2.74 (m, 2H, CH(CH3)2), 1.89 (m, 6H, CH2), 1.68–1.44 (m, 2H, CH2), 1.13 (d, J = 6.9 Hz, 6H, CH(CH3)2), 1.06 (d, J = 6.9 Hz, 6H, CH (CH3)2), 0.97 (d, J = 6.9 Hz, 6H, CH(CH3)2), 0.76 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 168.8, 157.3, 154.6, 143.7, 137.2, 133.5, 127.8, 124.8, 123.6, 115.8, 113.7, 112.3, 76.0, 33.8, 28.0, 27.8, 24.9, 24.3, 24.2, 24.1, 23.9.
as a yellow crystalline solid (0.65 g, 91%). Anal. Calc. for C40H46N4Zn (646.30): C, 74.12; H, 7.15; N, 8.64. Found: C, 74.20; H, 7.19; N, 8.71. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.49 (s, 2H, ArCH@N), 7.20 (d, J = 7.8 Hz, 2H, Ar–H), 7.01 (d, J = 4.7 Hz, 4H, Ar–H), 6.94 (t, J = 4.6 Hz, 2H, Ar–H), 6.83 (d, J = 8.6, Hz, 2H, Ar–H), 6.34 (t, J = 7.1 Hz, 2H, Ar–H), 5.99 (d, J = 8.9 Hz, 2H, Ar–H), 3.29–3.12 (m, 2H, C@NCH), 2.58–2.50 (m, 4H, ArCH2CH3), 2.47–2.31 (m, 4H, ArCH2CH3), 2.09–2.07 (m, 2H, CH2), 1.88–1.70 (m, 2H, CH2), 1.62 (q, J = 8.3 Hz, 2H, CH2), 1.45 (q, J = 10.1 Hz, 2H, CH2), 0.95 (t, J = 7.5 Hz, 6H, CH2CH3), 0.78 (t, J = 7.0 Hz, 6H, CH2CH3). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 164.9, 156.7, 147.1, 141.2, 137.3, 137.2, 132.1, 126.3, 124.6, 123.9, 116.7, 114.7, 111.6, 67.5, 29.0, 25.1, 24.2, 21.0, 15.2, 12.6. 2.5. Synthesis of L3Zn (3) This compound was prepared in the same way as described above for 1 with L3H2 (0.63 g, 1.2 mmol) and ZnMe2 (1.2 mL, 1.2 mmol, 1.0 M in toluene) as starting materials. 3 was obtained as a yellow crystalline solid (0.62 g, 88%). Anal. Calc. for C36H38N4Zn (590.24): C, 73.03; H, 6.47; N, 9.46. Found: C, 73.10; H, 6.51; N, 9.53. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.49 (s, 2H, ArCH@N), 7.20 (d, J = 7.9 Hz, 2H, Ar–H), 7.06–6.91 (m, 2H, Ar–H), 6.91–6.81 (m, 6H, Ar–H), 6.35 (d, J = 7.2 Hz, 2H, Ar–H), 6.02 (d, J = 8.9 Hz, 2H, Ar–H), 3.30–3.17 (m, 2H, C@NCH), 2.45 (d, J = 12.7 Hz, 2H, CH2), 2.07 (m, 2H, CH2), 2.04 (s, 12H, PhCH3), 1.61 (m, 2H, CH2), 1.45 (t, J = 9.9 Hz, 2H, CH2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 164.7, 155.6, 147.8, 137.2, 136.6, 132.2, 131.6, 128.8, 128.1, 123.5, 116.2, 114.6, 111.6, 67.3, 28.8, 24.1, 18.9, 17.4. 2.6. Synthesis of L4Zn (4) This compound was prepared in the same way as described above for 1 with L4H2 (0.50 g, 1.0 mmol) and ZnMe2 (1.0 mL, 1.0 mmol, 1.0 M in toluene) as starting materials. 4 was obtained as a yellow solid (0.50 g, 90%). Anal. Calc. for C34H34N4Zn (562.21): C, 72.40; H, 6.08; N, 9.93. Found: C, 72.32; H, 6.10; N, 10.07%. 1H NMR (400 MHz, CDCl3, 293 K) d, ppm: 8.50 (s, 2H, ArCH@N), 7.20 (d, J = 6.9 Hz, 2H, Ar–H), 7.17–6.99 (m, 2H, Ar–H), 6.81–6.74 (m, 2H, Ar–H), 6.58 (d, J = 6.3 Hz, 2H, Ar–H), 6.41 (d, J = 7.8 Hz, 2H, Ar–H), 6.33–6.30 (m, 2H, Ar–H), 5.99 (d, J = 6.9 Hz, 2H, Ar–H), 5.83 (d, J = 7.8 Hz, 2H, Ar–H), 3.31–3.16 (m, 2H, C@NCH), 2.51 (d, J = 12.7 Hz, 2H, CH2), 2.04 (s, 6H, PhCH3), 1.94 (m, 2H, CH2), 1.66 (m, 2H, CH2), 1.47 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3, 293 K) d, ppm: 161.1, 155.7, 145.1, 138.7, 137.2, 134.8, 129.6, 129.0, 127.2, 124.3, 118.8, 114.3, 113.1, 65.8, 29.0, 24.9, 18.8, 18.5.
2.4. Synthesis of L2Zn (2) 2.7. Ring-opening polymerization of rac-LA by 1–4 This compound was prepared in the same way as described above for 1 with L2H2 (0.64 g, 1.1 mmol) and ZnMe2 (1.1 mL, 1.1 mmol, 1.0 M in toluene) as starting materials. 2 was obtained
In a typical polymerization experiment, the zinc complex and the required amount of rac-LA in toluene were loaded in a
208
Y. Han et al. / Polyhedron 121 (2017) 206–210
flame-dried vessel containing a magnetic bar. The vessel was placed in an oil bath thermostated at the proposed temperature. After a certain reaction time, the polymer was isolated by precipitation with cold ethanol. The precipitate was dried under vacuum at 40 °C for 24 h. For some polymerization reactions, samples were taken for determining the monomer conversion by 1H NMR spectroscopy during the reaction. 2.8. X-ray crystallography Diffraction data of 2 were collected at 120 K with an Xcalibur, Eos, Gemini diffractometer equipped with graphite-monochromated Mo-Ka radiation (k = 0.71073 Å). Diffraction data of 3 were collected at 293 K with a Bruker SMART-CCD diffractometer equipped with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). Using OLEX2 [6], the structures were solved with the SHELXS [7] structure solution program using direct methods and refined with the SHELXL [7] refinement package using least squares minimization. 3. Results and discussion 3.1. Synthesis and characterization of the zinc complexes The free chiral tetra-azane chelating ligands o-(C6H4(NHAr)CH@N)2C6H10 ((1R,2R)-L1H2 (Ar = 2,6-iPr2C6H3); (1R,2R)-L2H2 (Ar = 2,6-Et2C6H3); (1R,2R)-L3H2 (Ar = 2,6-Me2C6H3); L4H2 (Ar = 4-MeC6H4)) were synthesized according to the literature procedure [8]. Complexes 1–4 were synthesized through alkane elimination reactions using one equivalent of the appropriate ligand with ZnMe2 in dry toluene, as summarized in Scheme 1. All the complexes were isolated as yellow crystalline solids and characterized by elemental analyses, along with 1H and 13C NMR spectroscopies. The 1H NMR spectra of 1–4 confirmed that the complexes were monometallic with one coordinated tetra-azane ligand. Crystals of 2 and 3 suitable for X-ray crystal structure determination were grown from a mixture of CH2Cl2/hexane at 20 °C. Their molecular structures with selected bond lengths and angles are shown in Figs. 1 and 2 respectively. The structural refinement parameters and crystallographic data are given in Table S1. The X-ray structural analyses of 2 and 3 revealed that the complexes adopt a distorted square planar geometry around the Zn center with the N(2)–Zn(1)–N(3) and N(1)–Zn(1)–N(4) planes inclined by 50.7° for 2, and 51.1° for 3. The dihedral angles of N(2)–Zn (1)–N(3)–C(32) and N(1)–Zn(1)–N(4)–C(27) are 14.1 and 7.0° for 2, and 5.3° and 17.6° for 3, respectively. The torsion angles between the Zn(1)–N(3)–C(32) and Zn(1)–N(4)–C(27) planes for 2 and 3 are 67.5° and 69.6° respectively. The N(1)–Zn(1)–N(2) bond angle in the five-membered chelating ring is 80.14(1)° for 2 and 80.45(1)° for 3. This is slightly larger in comparison to the corresponding bond angle in salen Zn(II) complexes (78.95(1)°) [9]. The imino C@N bonds retained their double bond character, being 1.285(4) and 1.297(5) Å for 2, and 1.282(4) and 1.286(4) Å for 3. The Zn–N (amido) distances (2.043(3) and 2.033(3) Å in 2, and 2.029(2) and 2.035(2) Å in 3) are longer than the Zn–N (imine) distances (1.969(3) and 1.959(3) Å in 2 and 1.966(2) and 1.961(2) Å in 3), which is contrary to the case of Zn(II) complexes with tridentate anilido-imine ligands [4]. It is obvious that the Zn–N (amido) distances are elongated due to the tension of the ligand. 3.2. Ring-opening polymerization of rac-LA Complexes 1–4 were tested as catalysts for the ROP of rac-LA without any initiator. The results are listed in Table 1. Complexes 1–4 all exhibit high reactivity for the ROP of rac-LA (entries 1–4).
Fig. 1. The molecular structure of 2. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–N(1) 2.043(3), Zn(1)–N(2) 2.033(3), Zn(1)–N(3) 1.969(3), Zn (1)–N(4) 1.959(3), N(1)–C(9) 1.285(4), N(2)–C(21) 1.297(5), N(1)–Zn(1)–N(2) 80.14 (1), N(2)–Zn(1)–N(3) 93.36(1), N(4)–Zn(1)–N(3) 116.80(1), N(4)–Zn(1)–N(1) 93.17 (1).
Fig. 2. The molecular structure of 3. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms and solvent are omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–N(1) 2.029(2), Zn(1)–N(2) 2.035(2), Zn(1)–N(3) 1.966(2), Zn(1)–N(4) 1.961(2), N(1)–C(9) 1.282(4), N(2)–C(21) 1.286(4), N(1)–Zn (1)–N(2) 80.45(1), N(2)–Zn(1)–N(3) 93.12(1), N(4)–Zn(1)–N(3) 117.05(1), N(4)–Zn (1)–N(1) 93.17(1).
The reactivities of 1–4 as catalysts for the ROP of rac-LA under the same conditions is in the order 4 > 3 > 2 > 1, which is in reverse order of the size of the substituents in their ligands. The results suggest that the zinc center in a complex with a less bulky substituent is more easily approached by the LA monomer. A similar observation has been reported in the polymerization of e-caprolactone using
209
Y. Han et al. / Polyhedron 121 (2017) 206–210 Table 1 ROP of rac-LA catalyzed by complexes 1–4.a
a b c d e f g h i
Entry
Cat.
[LA]0/[Zn]0
T (°C)
Time (min)
Yield (%)b
TOFc
Mn.calcdd(103)
Mn e(103)
PDIe
Prf
1 2 3 4 5 6 7 8g 9h 10 11 12 13i 14i 15i 16i
1 2 3 4 4 4 4 4 4 4 4 4 4 1 2 3
100:1 100:1 100:1 100:1 100:1 100:1 100:1 100:1 100:1 150:1 200:1 250:1 100:1 100:1 100:1 100:1
70 70 70 70 50 30 0 30 30 70 70 70 70 70 70 70
1.5 1.2 1 1 6 15 1440 20 35 2 3 4 20 30 30 30
82 80 87 97 92 93 94 90 92 94 95 96 90 82 86 92
3280 4000 5220 5820 920 372 4 270 158 4230 3800 3600 270 164 172 184
11.8 11.5 12.5 14.0 13.2 13.4 13.5 13.0 13.2 20.3 27.4 34.6 13.1 11.9 12.5 13.4
7.8 8.9 9.0 12.9 13.2 12.9 13.1 12.4 12.7 18.5 26.2 31.8 13.3 7.6 8.3 11.2
1.13 1.09 1.17 1.21 1.18 1.14 1.11 1.18 1.24 1.17 1.23 1.20 1.47 1.53 1.40 1.43
0.58 0.58 0.59 0.61 0.67 0.71 0.77 0.65 0.60 – – – 0.56 0.54 0.54 0.55
Polymerization conditions: catalyst, 30 lmol, [rac-LA]0 = 0.25 M in toluene, a N2 atmosphere. Determined by 1H NMR. Mole of LA consumed per mol of catalyst per hour. Calculated from ([rac-LA]0/[Zn]0) conversion 144.13 or ([rac-LA]0/[BnOH]0) conversion 144.13 + 108. Determined by GPC against polystyrene standards in THF, multiplied by 0.58 [10]. Pr is the probability of racemic linkages between monomer units determined by homonuclear decoupled 1H NMR spectroscopy [11]. DCM was used instead of toluene. THF was used instead of toluene. 1.0 equiv. BnOH was added.
anilido-imine aluminium catalysts [12]. In comparison with the literature results, the reactivity of 1–4 is higher than that of the anilido-aldimine zinc complexes [4]. Complexes 1–4 are also more reactive than the zinc complexes without a normal initiator [3h]. The reactivity of 1–4 is lower as compared to zinc b-diiminate complexes [13]. The polymerization initiated by 4 was completed within 1 min and the value of the turnover frequency (TOF) reached 5820 h 1 when 100 equiv. of rac-LA were added (entry 4). Complexes 1–4 showed moderate selectivity for the ROP of rac-LA, giving a heterotactic-rich polymer (entries 1–4). The results suggest that the substituents on the ligands slightly affect the selectivity of 1– 4. Complex 4 was used to examine the effects of different reaction conditions in detail. When the polymerization was carried out at a lower temperature, the selectivity increased up to Pr = 0.77 (entries 4–7, Fig. S1). The solvents influenced the catalytic activity and selectivity of 4 with the increasing trend of THF < DCM < toluene (entries 6–8). This is probably due to the THF solvent acting as a donor agent which easily coordinates to the active metal center such that the complex is more discriminatory to the incoming LA. It was also found that the polydispersity indexes (PDIs) of the PLA catalyzed by 4 range from 1.11 to 1.24. To further explore the behavior of 4, polymerization experiments for different times with a monomer/4 = 300:1 system were performed. A linear relationship between the number-averaged molecular weight (Mn) and the monomer conversion exists, as shown in Fig. 3. These results demonstrate the ‘‘living” characterization of the catalyst. The end group of the PLA was characterized by 1H NMR spectra and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy. 1H NMR spectrum of a typical polymer sample (Fig. S2) showed prominent peaks of the PLA main chain at d 1.48–1.60 and 5.00–5.25 ppm. In addition, signals relating to the hydroxyl and ethyl groups could also be identified at d 2.72, 1.26 and 4.18 ppm respectively. The results demonstrated that the PLA chain was capped by one acetoxyl and one hydroxyl end. The MALDI-TOF spectra further confirmed such an observation with molecular weights of the polymer molecules relatively close to M = n MLA/2 + MEtOH + MNa.
Fig. 3. Linear relationship between Mn and rac-LA conversion initiated by complex 4 ([rac-LA]0/[4]0 = 300/1, T = 30 °C, toluene).
All the results suggest that the tetra-azane ligands in the zinc complexes act as initiating groups to initiate the ROP of LA in a living manner. Based on the results and the generally accepted mechanisms for the ROP of LA mediated by metal complexes in the absence of alcohol, a coordination-insertion mechanism was proposed for the rac-LA polymerization initiated by the zinc complexes (Scheme 2) [14]. Firstly, the LA monomer coordinates to the zinc center through the carbonyl oxygen atom. Subsequently, the monomer ring is cleaved at the acyl-oxygen bond and inserts into one of the Zn–N (amido) bonds of the zinc complex to form a new alkoxyl zinc inthermediate. Thereafter, repetition of the same procedure forms the PLA chain on the Zn center. Finally, the polymerization reaction is quenched by an ethanol molecule. In the meantime, the ligand is substituted by an acetoxy group. A similar phenomenon was found when benzyl alcohol (BnOH) was added (Entries 13–16). It was found that the activities of 1–4 in the presence of BnOH were lower as compared to those of 1–4 alone. The 1H NMR spectrum of the polymer is shown in Fig. S4. The polymer was found to be capped with a benzyl ester and hydroxyl chain end.
210
Y. Han et al. / Polyhedron 121 (2017) 206–210
N
N Zn
N
N
LA
N
N
N
N
Zn N
N
O
LA
Zn
O
N
O
O
O
Zn
N
N
O O
O
O
N
N
N
O
O
O O
O
2n
O
EtOH
O
HO O
O
O 2n
O
Scheme 2. Proposed polymerization mechanism of LA initiated by the zinc complexes.
4. Conclusion In conclusion, a series of zinc complexes (1–4) supported by chiral tetra-azane ligands was synthesized and structurally characterized. It was found that the zinc complexes were active toward the ROP of rac-LA to provide polymers with controllable molecular weights and narrow molecular weight distributions, with moderate heterotacticity up to Pr = 0.77. The substituents of the ligands affect the activities and selectivities of these complexes. Acknowledgments
[4]
We are grateful for financial support from the National Natural Science Foundation of China (No. 21104026), China Scholarship Council (No. 2015020000004). Appendix A. Supplementary data CCDC 909691 and 901268 contains the supplementary crystallographic data for 2 and 3. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.poly.2016.10.010.
[5] [6] [7] [8] [9] [10] [11] [12] [13]
References [1] (a) (b) [2] (a) (b) (c) (d) [3] (a) (b)
G. Swift, Acc. Chem. Res. 26 (1993) 105; R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 250 (2006) 602; C.M. Thomas, Chem. Soc. Rev. 39 (2010) 165; M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486; C. Wheaton, P.G. Hayes, B.J. Ireland, Dalton Trans. (2009) 4832. Z. Dai, Y. Sun, J. Xiong, X. Pan, J. Wu, ACS Macro Lett. 4 (2015) 556; J. Xiong, J. Zhang, Y. Sun, Z. Dai, X. Pan, J. Wu, Inorg. Chem. 54 (2015) 1737;
[14]
(c) J. Zhang, J. Xiong, Y. Sun, N. Tang, J. Wu, Macromolecules 47 (2014) 7789; (d) Z. Dai, Y. Sun, J. Xiong, X. Pan, N. Tang, J. Wu, Catal. Sci. Technol. 6 (2016) 515; (e) P.K. Eckert, I.S. Vieira, V.H. Gessner, J. Börner, C. Strohmann, S. HerresPawlis, Polyhedron 49 (2013) 151; (f) Y.Y. Kang, H.-R. Park, M.H. Lee, J. An, Y. Kim, J. Lee, Polyhedron 95 (2015) 24; (g) A. John, V. Katiyar, K. Pang, M.M. Shaikh, H. Nanavati, P. Ghosh, Polyhedron 26 (2007) 4033; (h) E.D. Akpan, S.O. Ojwach, B. Omondi, V.O. Nyamori, Polyhedron 110 (2016) 63; (i) D. Appavoo, B. Omondi, I.A. Guzei, J.L. van Wyk, O. Zinyemba, J. Darkwa, Polyhedron 69 (2014) 55; (j) S. Bhunora, J. Mugo, A. Bhaw-Luximon, S. Mapolie, J.L. van Wyk, J. Darkwa, E. Nordlander, Appl. Organomet. Chem. 25 (2011) 133. (a) Y.-H. Tsai, C.-H. Lin, C.-C. Lin, B.-T. Ko, J. Polym. Sci., Part A: Polym. Chem. 47 (2009) 4927; (b) N. Yang, L. Xin, W. Gao, J. Zhang, X. Luo, X. Liu, Y. Mu, Dalton Trans. 41 (2012) 11454; (c) A. Gao, W. Yao, Y. Xiao, M. Zhang, G. Zhu, N. Zhang, S. Wang, D. Wang, Y. Zhang, Y. Gao, Z. Xu, P. Lu, Z. Zhang, Polyhedron 85 (2015) 537; (d) W. Yao, N. Cui, Y. Xue, T. Liu, Y. Wang, Chem. Res. Chin. Univ. 29 (2013) 48; (e) C.-H. Wang, C.-Y. Li, B.-H. Huang, C.-C. Lin, B.-T. Ko, Dalton Trans. 42 (2013) 10875; (f) A. Gao, W. Yao, Y. Mu, W. Gao, M. Sun, Q. Su, Polyhedron 28 (2009) 2605; (g) W. Yao, Y. Mu, A. Gao, W. Gao, L. Ye, Dalton Trans. 24 (2008) 3199. Z. Li, K.R. Conser, E.N. Jacobsen, J. Am. Chem. Soc. 115 (1993) 5326. O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl. Cryst. 42 (2009) 339. G.M. Sheldrick, Acta. Cryst. A64 (2008) 112. Y. Zhang, W. Yao, H. Li, Y. Mu, Organometallics 31 (2012) 4670. G.A. Morris, H. Zhou, C.L. Stern, S.T. Nguyen, Inorg. Chem. 40 (2001) 3222. A. Kowalski, A. Duda, S. Penczek, Macromolecules 31 (1988) 2114. T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc. 124 (2002) 1316. W. Yao, Y. Mu, A. Gao, Q. Su, Y. Liu, Y. Zhang, Polymer 49 (2008) 2486. (a) M. Cheng, A.B. Attygalle, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 121 (1999) 11583; (b) B.M. Chamberlain, M. Cheng, D.R. Moore, T.M. Ovitt, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 123 (2001) 3229; (c) M.H. Chisholm, J.C. Huffman, K. Phomphrai, J. Chem. Soc., Dalton Trans. (2001) 222; (d) M.H. Chisholm, J. Gallucci, K. Phomphrai, Inorg. Chem. 41 (2002) 2785. (a) J. Liu, H. Ma, Dalton Trans. 43 (2014) 9098; (b) L. Postigo, M.C. Maestre, M.E.G. Mosquera, T. Cuenca, G. Jiménez, Organometallics 32 (2013) 2618.