Synthesis and Characterization of Amine-Bridged Bis(phenolate

Nov 19, 2014 - Qingbing Yuan , Shuangliu Zhou , Xiancui Zhu , Yun Wei , Shaowu Wang , Xiaolong Mu , Fangshi Yao , Guangchao Zhang , Zheng Chen...
0 downloads 0 Views 777KB Size
Download PDF
Article pubs.acs.org/Organometallics

Synthesis and Characterization of Amine-Bridged Bis(phenolate) Yttrium Guanidinates and Their Application in the Ring-Opening Polymerization of 1,4-Dioxan-2-one Tinghua Zeng,† Yaorong Wang,*,† Qi Shen,† Yingming Yao,*,†,‡ Yunjie Luo,*,§ and Dongmei Cui‡ †

Key Laboratory of Organic Synthesis of Jiangsu Province and Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China § Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, People’s Republic of China S Supporting Information *

ABSTRACT: A series of neutral yttrium guanidinates supported by an amine-bridged bis(phenolate) ligand were synthesized, and their catalytic behaviors for the ring-opening polymerization of 1,4-dioxan-2-one (p-dioxanone, PDO) were explored. Metathesis reactions of amine-bridged bis(phenolate) yttrium chlorides LLnCl(THF) [L = Me 2 NCH 2 CH 2 N{CH 2-(2-OC 6 H 2 -tBu 2 3,5)}2] with corresponding lithium guanidinates generated in situ in a 1:1 molar ratio in THF gave the neutral yttrium guanidinates LY[R2NC(NR1)2] [R1 = −Cy, R2N = −N(TMS)2 (1), −NiPr2 (2), −N(CH2)5 (3); R1 = −iPr, R2N = −NiPr2 (4) −NPh2 (5))]. These complexes were well characterized by elemental analyses, IR, and NMR spectroscopy. The definitive molecular structures of these complexes were determined by single-crystal X-ray analysis. It was found that these complexes can efficiently initiate the ring-opening polymerization (ROP) of PDO, and the catalytic activity is affected by the nature of the guanidinate groups with the active sequence of 1 > 2 ≈ 3 ≈ 4 > 5. The influences of reaction conditions such as polymerization time, polymerization temperature, and molar ratio of monomer to initiator on the polymerization were also investigated. The polymerization kinetics of PDO catalyzed by complex 1 is first-order with respect to monomer concentration, and the apparent activation energy amounts to 30.8 kJ mol−1. The mechanistic investigations showed that the ROP of PDO proceeded through a coordination−insertion mechanism with a rupture of the acyl− oxygen bond of the monomer. MALDI-TOF mass spectrum analysis of the oligomer revealed that there are two kinds of polymer chains in this catalytic system, e.g., the linear chains H−[OCH2CH2OCH2CO]n−OH and the PPDO macrocycles.

■

INTRODUCTION

the preparation of sutures with improved properties, as well as for the preparation of drug delivery systems.13−15 Generally, polyesters can be synthesized through ringopening polymerization (ROP) of the corresponding cyclic monomers in the presence of initiators. However, the ROP of PDO is sluggish since the structure of PDO is more stable compared with LA and CL, and the polymerization needs a long reaction time and has low monomer conversion. Furthermore, the easy precipitation of high molar mass PPDO from solvents at ambient temperature resulted in very low monomer conversion for a solution polymerization. Thus, the ROP of PDO is usually bulk polymerization. To date, the initiators used for the ROP of PDO usually were tin(II) bis(2ethylhexanoate), Sn(Oct)2,16−20 triethylaluminum, AlEt3,16,21 aluminum isopropoxide, Al(OiPr)3,22−25 zinc lactate, Zn-

Aliphatic polyesters, such as poly(lactide) (PLA) and poly(εcaprolactone) (PCL), have gained increasing interest for their excellent biodegradability, biocompatibility, and bioabsorbability and have found many applications as biomedical materials.1−8 Compared to PLA and PCL, poly(1,4-dioxan-2one) (PPDO), with ester and ether groups in the repeat units of the polymer chain, is found not only to possess outstanding flexibility, biodegradability, and biocompatibility but also to be tougher than polylactide and even high-density polyethylene (HDPE).9 In this respect, PPDO can be complementary to other biodegradable aliphatic polyesters by enlarging the available range of physical properties. Indeed, PPDO has attracted much attention as biomaterials for medical purposes and as films, molded products, laminates, foams, nonwoven materials, adhesives, and coatings.10−12 The copolymers of PDO with glycolide or trimethylene carbonate can be used for © 2014 American Chemical Society

Received: August 14, 2014 Published: November 19, 2014 6803

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

Synthesis of LY[(TMS)2NC(NCy)2] (1). A Schlenk flask was charged with HN(TMS)2 (TMS = SiMe3, 0.56 g, 3.5 mmol), THF (20 mL), and a stirring bar. The solution was cooled to 0 °C, and nBuLi (1.75 mL, 3.5 mmol, 2 M in hexane) was added by syringe. The solution was stirred for 1 h at 0 °C, and then N,N′-dicyclohexylcarbodiimide (0.72 g, 3.5 mmol) was added. The resulting solution was slowly warmed to room temperature, stirred for 1 h, and then added slowly to LYCl(THF) (2.52 g, 3.5 mmol) in 20 mL of THF. The mixture was stirred overnight at room temperature, and the solvent was removed in a vacuum. The residue was extracted with toluene, and LiCl was removed by centrifugation. Colorless crystals were obtained from a hexane (7 mL)/toluene (1 mL) solution at room temperature in a few days (1.78 g, 52%). Anal. Calcd for C53H94N5O2Si2Y: C, 65.06; H, 9.68; N, 7.16; Y, 9.09. Found: C, 64.77; H, 9.87; N, 7.31, Y, 8.99. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.60 (s, 2H, ArH), 7.11 (s, 2H, ArH), 4.26 (br, 2H, ArCH2), 3.54 (br, 1H, Cy−CH), 3.43 (br, 1H, Cy−CH), 2.99 (br, 2H, ArCH2), 2.39 (br, 2H, N(CH2)2N), 2.08 (br, 2H, N(CH2)2N), 1.88 (m, 2H, Cy−CH2), 1.84 (br, 6H, N(CH3)2), 1.76 (m, 4H, Cy−CH2), 1.73 (s, 18H, C(CH3)3), 1.57 (br, 6H, Cy− CH2) 1.47 (s, 18H, C(CH3)3), 1.35 (m, 8H, Cy−CH2), 0.43 (s, 18H, Si(CH3)3). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 167.2 (CN3), 162.4, 136.6, 135.8, 126.0, 124.9, 124.5 (Ar−C), 65.1 (ArCH2), 60.2 (N(CH2)2N), 55.2 (Cy−CH), 46.9 (N(CH3)2), 38.7 (Cy−CH2), 35.9 (C(CH3)3), 34.3 (C(CH3)3), 32.3 (C(CH3)3), 31.5 (C(CH3)3), 26.6 (Cy−CH2), 3.6. (Si(CH3)3). IR (KBr pellet, cm−1): 2950 (s), 2859 (s), 2362 (w), 1633 (s), 1555 (m), 1477 (s), 1414 (m), 1385 (m), 1360 (m), 1328 (m), 1305 (m), 1254 (m), 1238 (m), 1202 (m), 1166 (m), 1028 (m), 994 (m), 939 (m), 876 (m), 838 (m), 743 (m). Synthesis of LY[iPr2NC(NCy)2] (2). Following the procedure for the synthesis of complex 1, Li[iPr2NC(NCy)2] (3.8 mmol), which was formed in situ by the reaction of LiN(iPr)2 with N,N′-dicyclohexylcarbodiimide, reacted with LYCl(THF) (2.73 g, 3.8 mmol) in THF (40 mL), affording the final product as colorless crystals after crystallization in toluene solution (1.92 g, 55%). Anal. Calcd for C53H90N5O2Y: C, 69.33; H, 9.88; N, 7.63; Y, 9.68. Found: C, 69.05; H, 10.19; N, 7.78; Y, 9.55. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.60 (s, 2H, ArH), 7.13 (s, 2H, ArH), 4.24 (d, 2JHH = 13.5 Hz, 2H, ArCH2), 3.49 (m, 2H, CH(CH3)2), 3.40 (br, 2H, Cy−CH), 3.07 (d, 2JHH = 12.1 Hz, 2H, ArCH2), 2.41 (br, 2H, N(CH2)2N), 2.09 (br, 2H, N(CH2)2N), 1.83 (s, 6H, N(CH3)2), 1.79 (m, 4H, Cy−CH2), 1.72 (s, 18H, C(CH3)3), 1.60 (m, 6H, Cy−CH2), 1.47 (s, 18H, C(CH3)3), 1.34 (m, 6H, Cy−CH2), 1.30 (d, 3JHH = 5.6, 12H, CH(CH3)2), 1.19 (m, 4H, Cy−CH2). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 169.2 (CN3), 162.2, 136.5, 135.8, 125.8, 124.7, 124.4, 123.7 (Ar−C), 65.3 (ArCH2), 59.4 (N(CH2)2N), 55.3 (Cy−CH), 50.9 (Cy−CH), 48.7 (N(CH3)2), 46.6 (CH(CH3)2), 44.7 (CH(CH3)2), 38.7 (Cy−CH2), 35.6 (C(CH3)3), 34.1 (C(CH3)3), 32.4 (C(CH3)3), 32.2 (C(CH3)3), 30.8 (C(CH3)3), 30.1 (Cy−CH2), 26.8, 26.5 (Cy−CH2), 23.7 (CH(CH3)2). IR (KBr pellet, cm−1): 2951 (s), 2867 (s), 2837 (s), 2362 (w), 1770 (w), 1628 (m), 1604 (m), 1475 (s), 1442 (s), 1414 (s), 1381 (m), 1360 (m), 1348 (m), 1327 (m), 1302 (m), 1263 (m), 1235 (m), 1201 (m), 1165 (m), 879 (m), 838 (m), 523 (m), 459 (m). Synthesis of LY[(CH2)5NC(NCy)2] (3). The synthesis of complex 3 was carried out in the same way as that described for complex 1, but piperidine (0.34 g, 4 mmol) was used instead of HN(TMS)2. Colorless crystals were obtained from a concentrated toluene solution at room temperature in a few days (2.02 g, 56%). Anal. Calcd for C52H86N5O2Y: C, 69.23; H, 9.61; N, 7.76; Y, 9.85. Found: C, 68.98; H, 9.79; N, 7.88; Y, 9.73. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.60 (s, 2H, ArH), 7.36 (s, 2H, ArH), 4.15 (d, 2JHH = 12.7 Hz, 2H, ArCH2), 3.41 (m, 2H, Cy−CH), 3.25 (m, 4H, CH2−NC5H10), 3.14 (d, 2JHH = 13.6 Hz, 2H, ArCH2), 2.18 (br, 2H, N(CH2)2N), 2.08 (br, 2H, N(CH2)2N), 1.87 (m, 6H, Cy−CH2), 1.78 (br, 2H, Cy−CH2), 1.74 (s, 18H, C(CH3)3), 1.68 (s, 6H, N(CH3)2), 1.59 (m, 6H, CH2−NC5H10), 1.46 (s, 18H, C(CH3)3), 1.45 (m, 6H, Cy−CH2), 1.12 (m, 6H, Cy− CH2). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 171.0, (CN3), 161.9, 136.4, 135.9, 125.7, 124.7, 124.3 (Ar−C), 64.9 (ArCH2), 59.7 (N(CH2)2N), 55.9 (Cy−CH), 50.5, 49.6 (N(CH2)5), 46.7 (N(CH3)2), 38.3 (Cy−CH2), 35.6 (C(CH3)3), 34.3 (C(CH3)3), 32.4 (C(CH3)3), 30.7 (C(CH3)3), 26.9, 26.7 (N(CH2)5, Cy−CH2), 25.3 (N(CH2)5). IR

(Lac)2,26 titanium alkoxide, Ti(OiPr)4),27 enzyme,28−32 and so on. However, most of these initiators showed low activity for this polymerization. Organolanthanide complexes are known to catalyze the ROP of some cyclic esters, such as LA and CL, and cyclic carbonates, with high activity and high efficiency.33−53 However, lanthanide complexes have seldom been used for the ROP of PDO. It has been reported that the homoleptic lanthanum complexes stabilized by alkoxo, aryloxo, and Schiff base ligands can catalyze PDO polymerization.54−56 Among these lanthanumbased initiators, La(OiPr)3 showed the highest activity for PDO polymerization. It can polymerize 800 equiv of PDO at 60 °C in a few minutes to give about 80% conversion.56 These results demonstrated that lanthanide complexes have great potential for catalyzing PDO polymerization. However, no further improvement was achieved in recent years. The catalytic property of lanthanide complexes can be easily modified by tuning the coordination environments around the lanthanide metals. Thus, introduction of a proper ancillary ligand plays a key role in designing high-performance lanthanide-based catalysts/initiators. Recently, amine-bridged bis(phenol)s, as a type of dianionic chelate ligand, have received considerable attention in organolanthanide chemistry, because of their easily available and tunable features. It was found that some bis(phenolate) lanthanide complexes are efficient initiators for the ring-opening polymerization of cyclic esters, giving polymers both in high conversions and with high molecular weights.33,57−61 Especially, for rac-LA and rac-βhydroxybutyrate (BBL) polymerization, excellent stereoselectivity can be achieved in some cases.60,62−66 To further understand the catalytic property of amine-bridged bis(phenolate) lanthanide complexes, development of a new catalytic system is still required. In this article, several yttrium guanidinate complexes supported by amine-bridged bis(phenolate) ligands were prepared by simple metathesis reactions, and these complexes can be used as efficient initiators for the ROP of PDO under mild conditions. Moreover, the kinetics and mechanism of the ROP of PDO also have been described here.

■

EXPERIMENTAL SECTION

General Procedures. All the manipulations were performed under a purified argon atmosphere using standard Schlenk techniques or in a glovebox. All solvents were degassed and distilled from sodium benzophenone ketyl under argon prior to use. Crude PDO monomer was dried over calcium hydride (CaH2) at 50 °C for 48 h and then distilled under reduced pressure before use. The ligand LH2 [L = Me 2 NCH 2 CH 2 N{CH 2 -(2-O-C 6 H 2 -tBu 2 -3,5)} 2 ] 67 and LYCl(THF)57,58 were prepared according to the procedures reported in the literature. Yttrium analyses were performed by ethylenediaminetetraacetic acid titration with a xylenol orange indicator and a hexamine buffer. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion with a Carlo-Erba EA-1110 instrument. The IR spectra were recorded with a Nicolet-550 Fourier transform IR spectrometer as KBr pellets. The 1H and 13C NMR spectra were recorded in a C6D6 solution for complexes 1−5 with a Unity Varian spectrometer. 1H NMR spectra of the PPDO samples were recorded on a Varian Unity Plus 400 MHz spectrometer at 25 °C with tetramethylsilane as the internal reference in CDCl3. Molecular weight and molecular weight distribution (PDI) of the PPDO were determined against a polystyrene standard by gel permeation chromatography (GPC) on a PL 50 apparatus equipped with PL gel 10 μm MIXED-B columns (300 × 7.5 mm) and a refractive index detector, and chloroform (HPLC grade) was used as an eluent at a flow rate of 0.6 mL/min at 40 °C. 6804

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

Scheme 1. Synthesis of Complexes 1−5a

R = −Cy, R2N = −N(TMS)2 (1), −NiPr2 (2), −N(CH2)5 (3). R1 = −iPr, R2N = −NiPr2 (4), −NPh2 (5).

a 1

(KBr pellet, cm−1): 2946 (s), 2855 (s), 2362 (w), 1629 (s), 1477 (s), 1445 (s), 1414 (s), 1384 (s), 1360 (m), 1308 (s), 1256 (m), 1237 (m), 1202 (m), 1166 (m), 1029 (m), 876 (m), 837 (m), 743 (m), 527 (m), 446 (m). Synthesis of LY[iPr2NC(NiPr)2] (4). Following a procedure similar to that for the synthesis of complex 1, Li[iPr2NC(NiPr)2] (3.9 mmol), which was formed in situ by the reaction of LiN(iPr)2 with N,N′diisopropylcarbodiimide, reacted with LYCl(THF) (2.81g, 3.9 mmol) in THF (20 mL) to yield colorless crystals upon crystallization from a concentrated toluene solution (1.63 g, 50%). Anal. Calcd for C47H82N5O2Y: C, 67.36; H, 9.86; N, 8.36; Y, 10.61. Found: C, 67.12; H, 10.05; N, 8.54; Y, 10.51. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.60 (s, 2H, ArH), 7.11 (s, 2H, ArH), 4.10 (d, 2JHH = 12.6, 2H, ArCH2), 3.97 (br, 1H, CH(CH3)2), 3.79 (br, 1H, CH(CH3)2), 3.43 (m, 2H, CH(CH3)2) 2.98 (d, 2JHH = 12.5, 2H, ArCH2), 2.37 (br, 2H, N(CH2)2N), 1.75 (s, 6H, N(CH3)2), 1.70 (s, 18H, C(CH3)3), 1.48 (s, 18H, C(CH3)3), 1.45 (br, 2H, N(CH2)2N), 1.32−1.29 (m, 24H, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 168.6 (CN3), 162.1, 136.4, 135.7, 125.8, 124.7, 124.4 (Ar−C), 65.1 (ArCH2), 59.9 (N(CH2)2N), 48.4 (CH(CH3)2), 46.45 (N(CH3)2), 35.6 (C(CH3)3), 34.3 (C(CH3)3), 32.4 (C(CH3)3), 30.8 (C(CH3)3), 27.6 (CH(CH3)2), 23.7 (CH(CH3)2). IR (KBr pellet, cm−1): 2960 (s), 2867 (s), 2363 (w), 1628 (s), 1560 (w), 1477 (s), 1443 (s), 1415 (m), 1360 (m), 1308 (s), 1255 (m), 1237 (m), 1202 (m), 1165 (m), 1134 (m), 876 (m), 837 (m), 743 (m), 527 (m), 445 (m). Synthesis of LY[Ph2NC(NiPr)2] (5). The synthesis of complex 5 was carried out in the same way as that described for complex 4, but diphenylamine (0.59 g, 3.5 mmol) was used instead of diisopropylamine. Colorless crystals were obtained from a concentrated toluene solution at room temperature in a few days (1.52 g, 48%). Anal. Calcd for C53H78N5O2Y: C, 70.25; H, 8.68; N, 7.73; Y, 9.81. Found: C, 70.02; H, 8.83; N, 7.91; Y, 9.71. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.62 (s, 2H, ArH), 7.50 (d, 2JHH = 7.9 Hz, 4H, N(C6H5)2), 7.28, (m, 4H, N(C6H5)2), 7.09 (s, 2H, ArH), 6.92 (m, 2H, N(C6H5)2), 4.05 (d, 2 JHH = 12.7 Hz, 2H, ArCH2), 3.84 (m, 2H, CH(CH3)2), 2.93 (d, 2JHH = 12.7, 2H, ArCH2), 2.36 (br, 2H, N(CH2)2N), 1.80 (s, 18H, C(CH3)3), 1.76 (s, 2H, N(CH2)2N), 1.72 (s, 6H, N(CH3)2), 1.49 (s, 18H, C(CH3)3), 1.06 (br, 12H, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 163.3 (CN3), 161.9, 144.8, 136.8, 135.7, 129.6, 125.8, 124.7, 124.5, 122.7, 122.3, 121.1, 120.8 (Ar−C), 65.0 (ArCH2), 59.8 (N(CH2)2N), 48.6, 47.1 (CH(CH3)2), 46.3 (N(CH3)2), 35.7, 34.3 (C(CH3)3), 32.4, 30.8 (C(CH3)3), 30.44, 27.0 (CH(CH3)2). IR (KBr pellet, cm−1): 2956 (s), 2906 (s), 2869 (s), 1652 (s), 1594 (m), 1480 (s), 1303 (s), 1242 (m,), 1170 (m), 1126 (m), 1036 (m), 877 (m), 838 (m), 749 (m), 700 (m), 525 (m), 451 (m). Bulk Polymerization of 1,4-Dioxan-2-one (PDO). The bulk polymerization of PDO was carried out with magnetic stirring in ovendried and nitrogen-purged 20 mL vials. These vials were filled with ca.

1 g of purified and dried PDO and then were thermostated in the IKA hot plate stirrer between 40 and 90 °C. The vial was thermostated at the desired temperature for 5 min, and a predetermined amount of catalyst solution was injected into the vials by a syringe. After a predetermined polymerization time, the vials were taken out rapidly and cooled in the cold alcohol (−20 °C). The contents were dissolved in hot chloroform, and PPDO was then selectively precipitated from methanol, which was dried under vacuum at 40 °C and weighed. Oligomer Preparation. Oligomerization of PDO was carried out with complex 1 as the initiator at 60 °C under the condition of a [monomer]/[initiator] ratio of 25. After the polymerization mixture was stirred for 5 min, the reaction vial was rapidly immersed in the cold alcohol (−20 °C) to quench the reaction or the reaction mixture was quenched by the addition of benzyl alcohol at room temperature. The oligomerization product was dissolved in CHCl3 and then precipitated from cold methanol (0 °C). The precipitated oligomers were collected, dried under vacuum, and used for 1H NMR and MALDI-TOF measurement. X-ray Crystallographic Structure Determination. Suitable single crystals of complexes 1−3 and 5 were sealed in a thin-walled glass capillary for determining the single-crystal structures. Intensity data were collected with a Rigaku Mercury CCD area detector in ωscan mode using Mo Kα radiation (λ = 0.710 70 Å). The diffracted intensities were corrected for Lorentz/polarization effects and empirical absorption corrections. Details of the intensity data collection and crystal data are given in Table S1. The structures were solved by direct methods and refined by fullmatrix least-squares procedures based on |F|2. The hydrogen atoms in these complexes were generated geometrically, assigned appropriate isotropic thermal parameters, and allowed to ride on their parent carbon atoms. All of the hydrogen atoms were held stationary and included in the structure factor calculation in the final stage of fullmatrix least-squares refinement. The structures were solved and refined using SHELEXL-97 programs.

■

RESULTS AND DISCUSSION Synthesis and Characterization of Amine-Bridged Bis(phenolate) Yttrium Guanidinate Complexes. Metathesis reaction is a simple and straightforward method for the synthesis of amine-bridged bis(phenolate) yttrium guanidinate complexes. Treatment of the amine-bridged bis(phenolate) yttrium chloride, LYCl(THF), with 1 equiv of lithium guanidinates, which were freshly prepared in situ by the reaction of lithium amides with carbodiimide in THF, after workup, afforded the corresponding amine-bridged bis(phenolate) yttrium guanidinate complexes as shown in Scheme 1. All of these complexes were characterized by 6805

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

elemental analysis, IR spectroscopy, and 1H and 13C NMR spectra. Elemental analysis revealed that each of the complexes is composed of one bis(phenolate) group, one guanidinate group, and one yttrium atom. The IR spectra of these complexes exhibit strong absorptions in the range 1628−1652 cm−1, which are consistent with the characteristic absorptions of partial CN double bonds, indicting the π-electrons being delocalized within the N−C−N linkage. The signals at 163− 171 ppm in the 13C NMR spectra also indicate the existence of guanidinate groups in these complexes. In the 1H NMR spectra, one set of characteristic peaks of amine-bridged bis(phenolate) ligand and guanidiante groups was observed, indicating the monomeric structure in solution of these complexes, which are consistent with their solid-state structures. Complexes 1−5 are extremely sensitive to air and moisture. The crystals decompose in a few minutes when they are exposed to air, but neither the crystals nor the solution showed any sign of decomposition after several months when they were stored under argon. Complexes 1−5 are freely soluble in THF, moderately soluble in toluene, and slightly soluble in hexane. Crystals suitable for an X-ray structure determination of complexes 1−3 and 5 were obtained from a concentrated toluene solution or a mixture solution of hexane and toluene at room temperature. X-ray diffraction analyses displayed that complexes 1−3 and 5 have monomeric structures, and their structure features are similar. Thus, only the ORTEP diagram of complex 1 is shown in Figure 1, and selected bond distances

In complex 1, the plane defined by Si1N5Si2 (the N(SiMe3)2 group) and the plane of N3C35N4Y are nearly perpendicular (the dihedral angle is 87.96°), which militates against the πoverlap between these two moieties. The dihedral angle of the plane of N3C35N4Y and the plane defined by O1YO2N1 is 87.17°. This orientation minimizes the steric congestion between the bis(phenolate) ligand and the guanidinate group. The average Y−O(Ar) distances in these complexes range from 2.128(3) to 2.162(4) Å, which are comparable with the corresponding values in the amine-bridged bis(phenolate) yttrium derivatives.58,66 ROP of PDO in Bulk by Complexes 1−5. The catalytic property of these amine-bridged bis(phenolate) yttrium guanidinates for the ring-opening polymerization of 1,4dioxan-2-one was explored. The ROP of PDO with complexes 1−5 may be an equilibrium reaction just as that initiated by metal alkoxides such as La(OiPr)3,56 Al(OiPr)3,23 and Ti(OiPr)4.27 Thus, the influence of polymerization time, polymerization temperature, and the molar ratio of monomer to initiator on the ROP of PDO was investigated at first using complex 1 as an initiator, and the results are summarized in Table 1. Obviously, the polymerization rate was very fast in this system, and the polymers obtained have high molecular weights. For example, the monomer conversion reached 60% in 1 min and 80% in 5 min at 60 °C when the molar ratio of monomer to initiator was 1200 (entries 1 and 3). The molecular weights of PPDO obtained exceed 7.0 × 104 g/mol. However, the monomer conversion and molecular weight did not increase anymore after the polymerization time exceeded 5 min (entries 3−6), indicating that polymerization reaction attained thermodynamic equilibrium in 5 min. Therefore, the influence of temperature on the ROP of PDO was investigated in 5 min at a temperature ranging from 40 to 90 °C with a constant initial molar ratio of 1200 (Table 1, entries 7−9). The results showed that the equilibrium conversions and molecular weight of PPDO decreased with an increase of polymerization temperature. When the polymerization temperature was raised from 60 to 90 °C, the equilibrium conversion and the Mn of PPDO dropped to 54% and 5.2 × 104 g/mol, respectively. The temperature dependence of the equilibrium monomer conversion is in agreement with previously reported data on the ROP of PDO initiated by La(OiPr)3,56 Al(OiPr)3,23 and AlEt316 even though the time required to attain the equilibrium conversion is different. To further prove the equilibrium polymerization, a depolymerization experiment was conducted. A PPDO with a molecular weight of 9.6 × 104 g/mol and molecular weight distribution of 1.75 was heated at 130 °C for 3 h using complex 1 as an initiator with a molar ratio of 1200. After workup, only 30% polymer was recovered and the molecular weight and molecular weight distribution changed to 3.1 × 104 g/mol and 1.85, respectively. This result indicates the occurrence of depolymerization for PPDO, which is quite different from that of poly(lactide).68 On the other hand, polymerization of PDO initiated by complex 1 with a molar ratio of 1200 at 130 °C for 5 min gave the polymer with 32% yield. However, when the polymerization temperature decreased to 40 °C, the monomer conversion of PDO rose to 95% (Table 1, entry 7). These results indicate that the depolymerization rate constant is very small under the low temperature in this polymerization system. Considering the melting temperature of PDO is 28 °C, 40 °C should be the

Figure 1. ORTEP diagram of complex 1 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.

and angles are listed in Table S2. The metal ions in these complexes are six-coordinated to two oxygen and two nitrogen atoms from the dianionic amine-bridged bis(phenolate) ligand and two nitrogen atoms from the guanidinate group. The coordination geometry around the metal center can be best described as a distorted octahedron, in which O1, O2, N1, and N4 can be considered to occupy equatorial positions. N3 and N2 occupy axial positions, and the N3−Y−N2 angles are slightly distorted away from the idealized position of 180° to 160−165° for these complexes. 6806

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

Table 1. Polymerization of PDO in Bulk by Complexes 1−5

a

entry

initiator

temp (°C)

[M]0/[I]0

time (min)

conva (%)

Mnb (×104)

PDI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 3 3 4 4 5 5 LY(OCH2Ph)(THF)

60 60 60 60 60 60 40 80 90 40 40 40 40 40 40 40 40 60 40 60 40 60 40 60 40 60 40

1200 1200 1200 1200 1200 1200 1200 1200 1200 1500 1800 2000 2500 3000 3000 3000 3000 2400 3000 2400 3000 2400 3000 2400 3000 2400 3000

1 3 5 10 60 120 5 5 5 5 5 5 5 5 10 15 20 15 20 15 20 15 20 15 20 15 20

60 70 80 78 77 77 95 68 54 93 91 84 79 77 87 92 91 72 82 61 83 58 86 57 74 43 89

7.0 7.5 9.0 8.0 8.0 8.0 8.1 5.9 5.2 9.2 10.3 14.0 16.5 17.0 17.5 17.2 17.8 13.5 11.2 10.6 10.1 9.7 9.0 8.8 8.3 6.2 18.4

2.01 1.88 1.93 1.88 1.95 1.98 1.98 2.11 1.95 2.41 1.98 1.80 1.74 1.92 1.61 1.86 1.83 1.97 1.60 1.78 1.59 1.88 1.97 1.89 2.04 1.95 1.76

Conv (%): weight of polymer obtained/weight of monomer used. bMeasured by GPC calibrated with standard PSt samples.

optimum reaction temperature for ROP of PDO initiated by complex 1. It was also found that the molar ratio of monomer to initiator ([M]0/[I]0) has a slight effect on the catalytic activity of complex 1 for PDO polymerization (Table 1, entries 7, 10− 14). The monomer conversion of PDO decreased slowly as the [M]0/[I]0 increased. For example, the monomer conversion is 93% when the [M]0/[I]0 is 1500 at 40 °C in 5 min, whereas the monomer conversion is 77% when [M]0/[I]0 is 3000 under the same polymerization conditions. However, for a ratio of 3000, the monomer conversion of 92% is obtained when the reaction time is extended to 15 min (Table 1, entry 16), indicating that longer time was needed to reach the equilibrium monomer conversion at high molar ratio of monomer to initiator. On the other hand, the number-average molecular weights of PPDO (Mn) increased with the increase in molar ratio of monomer to initiator ([M]0/[I]0). For example, the number-average molecular weights were 8.1 × 104, 14.0 × 104, and 17.0 × 104 g/mol, respectively, when the [M]0/[I]0 were 1200, 2000, and 3000. This polymerization property corresponds well to that reported by Wang et al. with Ti(OiPr)4 as the initiator.27 However, the observed molecular weights for all the polymers are apparently lower than the theoretical ones (Mn) (calculated for one polymer chain per yttrium atom), the molecular weight distributions are relatively broad (1.61−2.41), and there is no linear relationship between the molecular weights and the molar ratios of monomer to initiator (Figure 2), indicating this polymerization is not well controlled. This could be attributed to the occurrence of a transesterification reaction during the PDO polymerization (vide infra).

Figure 2. Plot of Mn vs [M]0/[I]0 for bulk polymerization of PDO initiated by complex 1.

To understand the effect of the guanidinate groups in these yttrium complexes on the polymerization activity, the catalytic behavior of complexes 1−5 for the ROP of PDO was also examined. The representative polymerization data under different [M]0/[I]0 ratios and different reaction temperatures are summarized in Table 1 (entries 17−26). It can be seen that all of the monomer conversions initiated by complex 1 {LY[(TMS)2NC(NCy)2]} under different conditions are slightly higher than those initiated by complexes 2 {LY[iPr2NC(NCy)2]}, 3 {LY[(CH2)5NC(NCy)2]}, and 4 {LY[iPr2NC(NiPr)2]}, whereas the monomer conversion initiated by complex 5 {LY[Ph2NC(NiPr)2]} is the lowest. The decreasing activity order is 1 > 2 ≈ 3 ≈ 4 > 5, which revealed 6807

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

that the structure of the guanidinate groups in these complexes has some influence on the activity for PDO polymerization. On the basis of these results, we postulated that the electrondonating group on nitrogen, such as the SiMe3 group,69 could benefit the polymerization, whereas the electron-withdrawing phenyl group may slow down the polymerization. This influence of initiating groups on the catalytic activity was also observed in other polymerization systems, although the real reason is still unclear.70−73 To get more insight into the PDO polymerization by aminebridged bis(phenolate) yttrium guanidinates, the kinetics of PDO polymerization has been studied at 50−70 °C using complex 1 as an initiator. The first-order kinetics of propagation for an equilibrium polymerization system can be expressed simply as follows: ln{([M]0 − [M]e )/([M]t − [M]e )} = kappt

Figure 4. Time dependence of ln{([M]0 − [M]e)/([M]t − [M]e)} in the bulk ROP of PDO catalyzed by complex 1 at 50−70 °C.

where [M]0, [M]e, and [M]t are initial monomer concentration, equilibrium monomer concentration, and actual monomer concentration at time t, respectively, and kapp is the apparent rate constant. We assumed that the polymerization reactions were homogeneous because the bulk polymerization of PDO proceeded very quickly and little crystallization of PPDO took place during the 5 min polymerization time. Therefore, [M]0 and [M]t can be calculated from the monomer conversion and the densities of the monomers and molten polymer according to the literature methods.16 [M]e can be calculated from the equilibrium monomer conversion and the densities of the monomers and molten polymer. However, [M]e can also be calculated according to Dainton’s equation (R ln[M]e = ΔHp/T − ΔSp°),74 the linear relationship between R ln[M]e and 1/T shown in Figure 3 at temperatures in the range 40−90 °C.

the slope of the plot at each temperature. The temperature dependence of kapp is depicted with an Arrhenius plot in Figure 5. From the slope of the plot, the apparent activation energy

Figure 5. Temperature dependence of the apparent rate constant of PDO with complex 1.

(Eapp) was calculated as 30.8 kJ mol−1, which is lower than that of PDO polymerization catalyzed by Sn(Oct)2 (Eapp = 71.8 kJ mol−1)16 and La(OiPr)3 (Eapp = 50.5 kJ mol−1).56 The lower Eapp value demonstrates that the ROP process catalyzed by complex 1 is easier to carry out. Mechanism. To gain some insights into the ROP mechanism of PDO with these yttrium guanidinate complexes, oligomerization of PDO by complex 1 in a [M]0/[I]0 ratio of 25 at 60 °C was carried out. However, no guanidinate or bis(phenolate) group could be identified from the 1H NMR spectrum of the oligomer (Figure S1). In the MALDI TOF mass spectrum (Figure 6), peaks accounting for oligomers with the guanidinate or bis(phenolate) end groups also could not be found; only two series of periodically repeating peaks, namely, those corresponding to the linear chains H− [OCH2CH2OCH2CO]n−OH and to the PPDO macrocyclics, dominate in the spectrum. Formation of cyclic oligomers resulted from the intramolecular transesterification in the last stage of the ROP, whereas the linear oligomer might stem from the hydrolysis of oligomer with the guanidinate end-cap considering that the oligomer of rac-lactide with the amide

Figure 3. Plot of R ln[M]e vs 1/T for bulk polymerization of PDO initiated by complex 1.

Therefore, the representative kinetics plots are shown in Figure 4. Linear relationships between ln{([M]0 − [M]e)/([M]t − [M]e)} and reaction time were observed from 50 to 70 °C at the [M]0/[I]0 ratio of 3000, which confirmed that the polymerization kinetics of PDO catalyzed by complex 1 is first-order with respect to monomer concentration in the full range of monomer conversion investigated. The apparent rate constants (kapp) of the polymerization are 0.094, 0.126, and 0.175 min−1, respectively, which was determined according to 6808

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

Figure 6. MALDI-TOF mass spectrum of PDO oligomer initiated by complex 1 (doped with NaI).

Figure 7. 1H NMR spectrum (400 MHz, CDCl3) of PDO oligomer initiated by complex 1 after quenching by the addition of benzyl alcohol.

end group is easily hydrolyzable.53,75,76 In order to verify the instability of the oligomer with the guanidinate end-cap, a new oligomer was prepared when the oligomerization was quenched by benzyl alcohol. The 1H NMR spectrum of the new oligomer clearly showed that only the benzyloxy group was observed as shown in Figure 7, according to the resonances at 5.17 and 7.35 ppm. Obviously, the benzyloxy group existing in the oligomer must come from benzyl alcohol, which was introduced by the exchange reaction of benzyl alcohol with the guanidinate group. These results indicated that the guanidinate group in these yttrium guanidinate complexes acted as the initiating group in the ROP of PDO, and the polymerization proceeds via a coordination−insertion mechanism by acyl−oxygen bond cleavage, as illustrated in Scheme 2. According to the proposed mechanism, the corresponding alkoxides should be efficient initiators for the PDO polymerization as well. Actually, it was found that LY(OCH2Ph)(THF)66 could initiate the ring-

opening polymerization of PDO with high activity (Table 1, entry 27).

■

CONCLUSION In summary, a series of neutral yttrium guanidinates supported by an amine-bridged bis(phenolate) ligand were successfully synthesized via metathesis reaction using the easily available amine bis(phenolate) yttrium chloride as the precursors, and their structural features have been provided by an X-ray diffraction study. It was found that all these yttrium guanidinates can initiate the ROP of PDO with high equilibrium conversion at 40 °C, and the nature of the guanidinate groups in these complexes has some influence on the activity of the PDO polymerization. The kinetics investigation revealed that the polymerization is first-order with respect to the monomer concentration, and the apparent activation energy (Eapp) in this system is lower than those catalytic systems reported in the literature, which demonstrates 6809

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

Scheme 2. Proposed Mechanism for the Ring-Opening Polymerization of PDO

(2) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (3) Tschan, M. J.-L.; Brule, E.; Haquette, P.; Thomas, C. M. Polym. Chem. 2012, 3, 836. (4) Ni, Q.; Yu, L. J. Am. Chem. Soc. 1998, 120, 1645. (5) Yasuda, H.; Aludin, M. S.; Kitamura, N.; Tanabe, M.; Sirahama, H. Macromolecules 1999, 32, 6047. (6) Dobrzynski, P.; Li, S. M.; Kasperczyk, J.; Bero, M.; Gasc, F.; Vert, M. Biomacromolecules 2005, 6, 483. (7) Li, S.; Dobrzynski, P.; Kasperczyk, J.; Bero, M.; Braud, C.; Vert, M. Biomacromolecules 2005, 6, 489. (8) She, H.; Xiao, X.; Liu, R. J. Mater. Sci. 2007, 42, 8113. (9) Doddi, N.; Versfelt, C. C.; Wasserman, D. U.S. Pat. 4,052,988. 1977. (10) von Fraunhofer, J. A.; Storey, R. S.; Stone, I. K.; Masterson, B. J. J. Biomed. Mater. Res. 1985, 19, 595. (11) Lin, H. L.; Chu, C. C.; Grubb, D. J. Biomed. Mater. Res. 1993, 27, 153. (12) Tomihata, K.; Suzuki, M.; Oka, T.; Ikada, Y. Polym. Degrad. Stab. 1998, 59, 13. (13) Bennett, S. L.; Liu, C.-K.; Roby, M. S. Eur. Pat. 626,404. 1994. (14) Liu, C. K.; Stevenson, R. P. Eur. Pat. 654,549. 1995. (15) Wang, H.; Dong, J. H.; Qiu, K. Y.; Gu, Z. W. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1301. (16) Nishida, H.; Yamashita, M.; Endo, T.; Tokiwa, Y. Macromolecules 2000, 33, 6982. (17) Libiszowski, J.; Kowalski, A.; Szymanski, R.; Duda, A.; Raquez, J.-M.; Degée, P.; Dubois, P. Macromolecules 2003, 37, 52. (18) Yoon, K. R.; Chi, Y. S.; Lee, K. B.; Lee, J. K.; Kim, D. J.; Koh, Y. J.; Joo, S. W.; Yun, W. S.; Choi, I. S. J. Mater. Chem. 2003, 13, 2910. (19) Yu, X.; Feng, J.; Zhuo, R. Macromolecules 2005, 38, 6244. (20) Chen, S.; Wang, X.; Yang, K.; Wu, G.; Wang, Y. J. Polym. Sci., Polym. Chem. 2006, 44, 3083. (21) Yang, K.; Guo, Y.; Wang, Y.; Wang, X.; Zhou, Q. Polym. Bull. 2005, 54, 187. (22) Raquez, J.-M.; Degée, P.; Narayan, R.; Dubois, P. Macromol. Rapid Commun. 2000, 21, 1063.

that the ROP of PDO catalyzed by amine bis(phenolate) yttrium guanidinates is easier. End group analysis of the oligomer revealed that the ROP of PDO proceeded through a coordination−insertion mechanism with a rupture of the acyl− oxygen bond of the monomer. The existence of linear and macrocyclic PPDO, which was observed from MALDI-TOF mass spectrum analysis, revealed that there is intramolecular transesterification during polymerization.

■

ASSOCIATED CONTENT

S Supporting Information *

CIF files and crystallographic data for complexes 1−3 and 5 are available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 21174095, 21132002, and 21372172), the PAPD, the Major Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (Project 14KJA150007), and the Qing Lan Project is gratefully acknowledged.

■

REFERENCES

(1) Jeong, B.; Bae, Y. H.; Lee, D. S.; Kim, S. W. Nature 1997, 388, 860. 6810

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811

Organometallics

Article

(23) Raquez, J.-M.; Degée, P.; Narayan, R.; Dubois, P. Macromolecules 2001, 34, 8419. (24) Raquez, J.-M.; Degée, P.; Narayan, R.; Dubois, P. Macromol. Chem. Phys. 2004, 205, 1764. (25) Lochee, Y.; Bhaw-Luximon, A.; Jhurry, D.; Kalangos, A. Macromolecules 2009, 42, 7285. (26) Kricheldorf, H. R.; Damrau, D.-O. Macromol. Chem. Phys. 1998, 1089. (27) Zeng, J.; Srinivansan, M.; Li, Y.; Narayan, R.; Wang, Y. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 5885. (28) Nishida, H.; Yamashita, M.; Nagashima, M.; Endo, T.; Tokiwa, Y. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1560. (29) He, F.; Jia, H.; Liu, G.; Wang, Y.; Feng, J.; Zhuo, R. Biomacromolecules 2006, 7, 2269. (30) Chen, R.; Zhang, Y.; Wang, Y. J. Mol. Catal. B: Enzym. 2009, 57, 224. (31) Dong, F.; Zhang, L.; Tong, X.; Chen, H.; Wang, X.; Wang, Y. J. Mol. Catal. B: Enzym. 2012, 77, 46. (32) Dong, F.; Xu, C.; Tong, X.; Wang, X.; Song, F.; Wang, Y. J. Mol. Catal. B: Enzym. 2013, 96, 40. (33) Bonnet, F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44, 9046. (34) Xue, M.; Yao, Y.; Shen, Q.; Zhang, Y. J. Organomet. Chem. 2005, 690, 4685. (35) Yao, Y.; Xu, X.; Liu, B.; Zhang, Y.; Shen, Q.; Wong, W.-T. Inorg. Chem. 2005, 44, 5133. (36) Yao, Y.; Zhang, Z.; Peng, H.; Zhang, Y.; Shen, Q.; Lin, J. Inorg. Chem. 2006, 45, 2175. (37) Gamer, M. T.; Roesky, P. W.; Palard, I.; Le Hellaye, M.; Guillaume, S. M. Organometallics 2007, 26, 651. (38) Xu, X.; Zhang, Z.; Yao, Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 9379. (39) Zhou, H.; Guo, H.; Yao, Y.; Zhou, L.; Sun, H.; Sheng, H.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 958. (40) Gao, W.; Cui, D.; Liu, X.; Zhang, Y.; Mu, Y. Organometallics 2008, 27, 5889. (41) Otero, A.; Fernández-Baeza, J.; Antiñolo, A.; Lara-Sánchez, A.; Martínez-Caballero, E.; Tejeda, J.; Sánchez-Barba, L. F.; AlonsoMoreno, C.; López-Solera, I. Organometallics 2008, 27, 976. (42) Stanlake, L. J. E.; Beard, J. D.; Schafer, L. L. Inorg. Chem. 2008, 47, 8062. (43) Wang, J.; Yao, Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2008, 48, 744. (44) Zhang, Z.; Xu, X.; Li, W.; Yao, Y.; Zhang, Y.; Shen, Q.; Luo, Y. Inorg. Chem. 2009, 48, 5715. (45) Luo, Y.; Li, W.; Lin, D.; Yao, Y.; Zhang, Y.; Shen, Q. Organometallics 2010, 29, 3507. (46) Susperregui, N.; Kramer, M. U.; Okuda, J.; Maron, L. Organometallics 2011, 30, 1326. (47) Shen, X.; Xue, M.; Jiao, R.; Ma, Y.; Zhang, Y.; Shen, Q. Organometallics 2012, 31, 6222. (48) Zhang, W.; Liu, S.; Yang, W.; Hao, X.; Glaser, R.; Sun, W.-H. Organometallics 2012, 31, 8178. (49) Yang, S.; Nie, K.; Zhang, Y.; Xue, M.; Yao, Y.; Shen, Q. Inorg. Chem. 2013, 53, 105. (50) Zhou, L.; Yao, Y.; Zhang, Y.; Xue, M.; Chen, J.; Shen, Q. Eur. J. Inorg. Chem. 2004, 2004, 2167. (51) Sheng, H.; Xu, F.; Yao, Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 7722. (52) Yasuda, H. J. Organomet. Chem. 2002, 647, 128. (53) Ma, H.; Okuda, J. Macromolecules 2005, 38, 2665. (54) Fang, Q.; Zheng, H.; Shen, G.; Shen, Z. Chin. J. Polym. Sci. 2007, 25, 427. (55) Zheng, H.; Shen, G.; Ni, X.; Shen, Z. Chin. J. Polym. Sci. 2008, 26, 799. (56) Zhu, X.; Wu, G.; Qiu, Z.; Zhou, Y.; Gong, J.; Yang, K.; Wang, Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5214. (57) Yao, Y.; Ma, M.; Xu, X.; Zhang, Y.; Shen, Q.; Wong, W.-T. Organometallics 2005, 24, 4014.

(58) Willans, C. E.; Sinenkov, M. A.; Fukin, G. K.; Sheridan, K.; Lynam, J. M.; Trifonov, A. A.; Kerton, F. M. Dalton Trans. 2008, 3592. (59) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J. F. Chem.−Eur. J. 2006, 12, 169. (60) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen, X.; Jing, X. Organometallics 2007, 26, 2747. (61) Dyer, H. E.; Huijser, S.; Susperregui, N.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford, P. Organometallics 2010, 29, 3602. (62) Amgoune, A.; Thomas, C. M.; Ilinca, S.; Roisnel, T.; Carpentier, J. F. Angew. Chem., Int. Ed. 2006, 45, 2782. (63) Ajellal, N.; Bouyahyi, M.; Amgoune, A.; Thomas, C. M.; Bondon, A.; Pillin, I.; Grohens, Y.; Carpentier, J. F. Macromolecules 2009, 42, 987. (64) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. J. Am. Chem. Soc. 2009, 131, 16042. (65) Nie, K.; Gu, X.; Yao, Y.; Zhang, Y.; Shen, Q. Dalton Trans. 2010, 39, 6832. (66) Nie, K.; Fang, L.; Yao, Y.; Zhang, Y.; Shen, Q.; Wang, Y. Inorg. Chem. 2012, 51, 11133. (67) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2001, 20, 3017. (68) The depolymerization of PLA proceeded at 130 °C for 3 h with a molar ratio of 1200 using complex 1 as an initiator. It was found that 98% polymer was recovered and the molecular weight and molecular weight distribution varied slightly from 37.1 × 104 g/mol and 1.48 to 36.4 × 104 g/mol and 1.53, respectively. (69) Gassman, P. C.; Deck, P. A.; Winter, C. H.; Dobbs, D. A.; Cao, D. H. Organometallics 1992, 11, 959. (70) Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2014, 53, 9226. (71) Ma, H.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2008, 47, 3328. (72) Chapurina, Y.; Klitzke, J.; Casagrande, O. d. L., Jr.; Awada, M.; Dorcet, V.; Kirillov, E.; Carpentier, J.-F. Dalton Trans. 2014, 43, 14322. (73) Nie, K.; Gu, W.; Yao, Y.; Zhang, Y.; Shen, Q. Organometallics 2013, 32, 2068. (74) Dainton, F. S.; Ivin, K. J. Q. Rev. 1958, 12, 61. (75) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F.; Spassky, N.; LeBorgne, A.; Wisniewski, M. Macromolecules 1996, 29, 6461. (76) Sun, S.; Nie, K.; Tan, Y. F.; Zhao, B.; Zhang, Y.; Shen, Q.; Yao, Y. M. Dalton Trans. 2013, 42, 2870.

6811

dx.doi.org/10.1021/om5008242 | Organometallics 2014, 33, 6803−6811