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Synthesis, characterization and l -lactide polymerization behavior of bis(amidinate) rare earth metal amide complexes
Inorganica Chimica Acta 363 (2010) 3597–3601
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and L-lactide polymerization behavior of bis(amidinate) rare earth metal amide complexes Yunjie Luo a,*, Ping Xu a,b, Yinlin Lei a, Yong Zhang b, Yaorong Wang b,** a b
Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China College of Chemistry, Chemical Engineering and Material Science, Suzhou University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 28 May 2010 Received in revised form 2 July 2010 Accepted 12 July 2010 Available online 21 August 2010 Keywords: Bis(amidinate) rare earth amide complex Synthesis Crystal structure L-lactide polymerization
a b s t r a c t A family of neutral and solvent-free bis(amidinate) rare earth metal amide complexes with a general formula [RC(N-2,6-Me2C6H3)2]2LnN(SiMe3)2 (R = phenyl (Ph), Ln = Y (1), Nd (2); R = cyclohexyl (Cy), Ln = Y (3), Nd (4)) were synthesized in high yields by one-pot salt metathesis reaction of anhydrous LnCl3, amidinate lithium salt [RC(N-2,6-Me2C6H3)2]Li, and NaN(SiMe3)2 in THF at room temperature. Single crystal structural determination of complexes 1, 2 and 4 revealed that the central metal adopts distorted pyramidal geometry. In the presence of 1 equivalent of iPr-OH, all these complexes were active for L-lactide polymerization in toluene at 70 °C to give high molecular weight (Mn > 104) polymers. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Amidinate anions [RC(NR0 )2] can be easily modified by tuning the C- and N-substituents for tailing the steric and electronic properties. They have been extensively employed as ancillary ligands to form complexes with a variety of metal centers across the periodic table [1]. Over the past decades, considerable attention has also been paid to the synthesis and reactivity of amidinate rare earth metal complexes, which could serve as reagents in organic synthesis and as initiators/catalysts for polymerization [2–6]. In contrast, there are only rare examples of amidinate-incorporated rare earth metal amide complexes [7,8]. Herein, we would like to report the synthesis and characterization of a novel family of bis(amidinate) rare earth metal amide complexes, as well as their activity towards L-lactide polymerization. 2. Results and discussion 2.1. Synthesis and characterization of bis(amidinate) rare earth metal complexes The one-pot salt metathesis reaction of anhydrous LnCl3 with 2 equivalents of amidinate lithium salt [RC(N-2,6-Me2C6H3)2]Li (formed in situ by treatment of [RC(N-2,6-Me2C6H3)2]H with 1
* Corresponding author. Tel.: +86 574 88130085; fax: +86 574 88130130. ** Corresponding author. Tel.: +86 512 65882806. E-mail addresses: [email protected] (Y. Luo), [email protected] (Y. Wang). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.07.036
equivalent of n-BuLi in THF), followed by addition of 1 equivalent of NaN(SiMe3)2 in THF at room temperature afforded the corresponding bis(amidinate) rare earth metal amide complexes [RC(N-2,6-Me2C6H3)2]2LnN(SiMe3)2 (R = Ph, Ln = Y (1), Nd (2); R = Cy, Ln = Y (3), Nd (4)) in 77–87% isolated yields (Scheme 1). Although salt metathesis strategy was employed as synthetic approach and THF was used as reaction solvent in these cases, complexes 1–4 were isolated as neutral, mononuclear, and solvent-free species. These complexes are air- and moisture-sensitive, soluble in THF, toluene, diethyl ether, but insoluble in aliphatic solvents such as hexane and pentane. They were characterized by elemental analysis, FT-IR, NMR spectroscopy (except for 2 and 4 for their strong paramagnetic property). Complexes 1, 2 and 4 were also subjected to X-ray single crystal structure determination. Room-temperature 1H NMR spectra of 1 and 3 in C6D6 showed a singlet peak for the methyl resonances of Y–N(SiMe3)2. Restricted rotation about the N–Caryl bonds was not observed because the methyl groups on the aryl rings showed only one singlet resonance. In 1 and 3, the main difference is the substituents at the carbon atom of ligating NCN moiety (phenyl group for 1 and cyclohexyl group for 3), however, the methyl resonances of Y–N(SiMe3)2 for 1 are found at d 0.24 ppm (1H NMR) and d 4.2 ppm (13C NMR), while those for 3 appear at d 0.31 ppm (1H NMR) and d 3.6 ppm (13C NMR), suggesting a similar electronic yttrium environment. Single crystals of complexes 1, 2 and 4 suitable for X-ray diffraction were grown from a mixture solution of hexane and THF at 30 °C. The molecular structures of 1 and 2 are shown in Fig. 1, and that of 4 is given in Fig. 2. The crystallographic data and the
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R Me Me Me
Me N R
1) n-BuLi, THF H
N Me
2) 0.5 LnCl3, THF, rt
N
0.5 NaN(SiMe3)2 THF, rt
Me
N
Me
Ln
N(SiMe3)2
MeMe
Me N
N
Me Me R R=
Ln = Y (1), Nd (2)
Ln = Y (3), Nd (4)
Scheme 1. Synthesis of bis(amidinate) rare earth metal amide complexes.
selected bond distances and angles are summarized in Table 1 and Table 2, respectively. The overall structural features of 1, 2 and 4, except for the difference of residue solvent molecule in the lattice (these solvent molecules could be removed under vacuum), are quite similar: the central metal is five-coordinated by two bidentate amidinate ligands through nitrogen atoms, and one amide group to form a distorted pyramidal geometry. Therefore, only the structure of 1 is discussed here. Complex 1 is a C2 symmetric molecule in the solid state. It is found that the phenyl group on the central carbon atom of NCN moiety provides steric protection in the plane of the ligand as well as above and below that plane (dihedral angle of N(2)C(1)N(1)Y(1) and the plane formed by phenyl group is 53.29°). The dihedral angle of N(2)C(1)N(1)Y(1) and the plane formed by Si(1)Y(1)N(3)Si(1A) is 66.03°), this orientation minimizes the interaction between the SiMe3 groups of the amide group and the 2,6-Me2C6H3 groups of the amidinate ligand. The Y– N(SiMe3)2 distance in 1 is 2.187(5) Å, which is slightly shorter than those in (dimb)Y[N(SiMe3)2]2 (dimb = N,N0 -diisopropyl(2,6-dimesityl)benzamidinate) (av. 2.24 Å) [7], and is identical to that in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av. 2.19 Å) [8], if the ionic radius is considered [9]. The distances between yttrium and the ligating nitrogen atoms (Y–N = 2.369(3) and 2.376(3) Å) in 1 are consistent with those found in (dimb)Y[N(SiMe3)2]2 (av. 2.34 Å) [7], but are somewhat longer than those in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av. 2.30 Å) [8] The bite angle of N–Y–N is 56.39°, which is smaller than those in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av. 59°) [8].
Fig. 2. Molecular structure of 4 with thermal ellipsoids at 20% probability. Hydrogen atoms have been omitted for clarity.
2.2. L-lactide polymerization initiated by bis(amidinate) rare earth metal amide complexes Polylactides (PLAs) are the promising biodegradable and biocompatible synthetic macromolecules, and have been widely applied in medicine, pharmaceutics, as well as in tissue engineering [10]. The most effective method to prepare PLAs is the ring-opening polymerization (ROP) of lactides by metal-based catalysts/initiators, and it has been proven that metal alkoxides usually possess high efficiency [11]. Some rare earth metal complexes also exhibited good performance for lactide polymerization [5,8,12–16]. In order to understand the polymerization behavior of bis(amidinate) rare earth metal amide complexes, complexes 1–4 were tested for the ring-opening polymerization of L-lactide, and the preliminary polymerization results are listed in Table 3. All of these neutral complexes alone showed very poor activity towards L-lactide polymerization at 70 °C in toluene. However, when these complexes were treated with 1 equivalent of iPr-OH, they exhibited improved activity, albeit still with rather low activity compared with other amidinate rare earth metal initiators [5,8]. This might be contributed to that the steric demanding amidinate ligands in 1–4 hampered the coordination of monomer to central metal, sequentially resulted in decreasing the initiation capability
Fig. 1. Molecular structure of 1 (left) and 2 (right) with thermal ellipsoids at 10% probability. Hydrogen atoms have been omitted for clarity.
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Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601 Table 1 Details of the crystallographic date and refinements for 1, 2 and 4.
Formula FW T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F (0 0 0) Crystal size (mm) hmax (°) Reflections collected Independent reflections No. of variables Goodness-of-fit GOF on F2 R [I > 2r(I)] wR
1
2
4
C52H64N5Si2Y(C6H14) 990.34 293(2) monoclinic C 2/c 21.434(6) 11.627(3) 23.872(6) 90.00 106.272(8) 90.00 5711(2) 4 1.152 1.102 2112 0.80 0.60 0.50 25.35 269 48 5209 299 1.185 0.0775 0.1690
C52H64N5NdSi23(C4H8O) 1175.81 223(2) monoclinic P 21/n 17.6303(10) 16.7689(9) 21.5620(11) 90.00 92.6300(10) 90.00 6367.9(6) 4 1.226 0.899 2476 0.80 0.50 0.40 25.5 321 48 11760 649 1.076 0.0465 0.1232
C52H76N5NdSi2 971.60 293(2) monoclinic P 21/n 12.3407(12) 19.0488(17) 22.866(2) 90.00 96.766(3) 90.00 5337.8(9) 4 1.209 1.054 2044 0.80 0.80 0.45 25.35 497 40 9751 520 1.174 0.0611 0.1146
Table 2 Selected bond distances and bond angles for 1, 2, 4. 1
2
4
Bond distances (Å) Y1–N1 Y1–N1A Y1–N2 Y1–N2A Y1–N3 C1–N1 C1–N1A C1–N2 C1–N2A Y1–C1 Y1–C1A
2.369(3) 2.369(3) 2.376(3) 2.376(3) 2.187(5) 1.331(5) 1.331(5) 1.325(5) 1.325(5) 2.802(4) 2.802(4)
Nd1–N1 Nd1–N2 Nd1–N3 Nd1–N4 Nd1–N5 C1–N1 C1–N2 C24–N3 C24–N4 Nd1–C1 Nd1–C24
2.462(3) 2.484(3) 2.456(3) 2.473(3) 2.277(3) 1.346(5) 1.336(5) 1.342(5) 1.338(5) 2.921(4) 2.909(4)
Nd1–N1 Nd1–N2 Nd1–N3 Nd1–N4 Nd1–N5 C1–N1 C1–N2 C24–N3 C24–N4 Nd1–C1 Nd1–C24
2.454(4) 2.448(4) 2.472(4) 2.451(4) 2.284(4) 1.336(6) 1.330(6) 1.333(6) 1.339(6) 2.917(5) 2.921(5)
Bond angles (°) N2–Y1–N1 N2A–Y1–N1A N2–C1–N1 N2A–C1–N1A
56.39(11) 56.39(11) 115.2(4) 115.2(4)
N2–Nd1–N1 N4–Nd1–N3 N2–C1–N1 N4–C24–N3
54.38(10) 54.60(10) 114.9(3) 115.0(3)
N2–Nd1–N1 N4–Nd1–N3 N2–C1–N1 N4–C24–N3
53.96(14) 54.09(13) 113.1(4) 113.8(4)
Table 3 a L-lactide polymerization with amidinate rare earth metal amide complexes. O n
O Initiator/iPr-OH
O
O O
O
O
O L- Lactide
Run
Initiator
i
Pr-OH/Ln (molar ratio)
Yieldb (%)
1 2 3 4 5 6 7 8
1 2 3 4 1 2 3 4
0 0 0 0 1 1 1 1
trace trace trace trace 74 85 61 73
Mnc 104
n
Mw/Mnc
for L-lactide polymerization. The influence of organic substituents at the carbon atoms of the amidinate moiety on the polymerization activity was observed, and 1 and 2 showed a little bit higher activity than 3 and 4. This phenomenon could be reasoned that the cyclohexyl group is much more fluxional than the rigid phenyl ring. The influence of central metal on the polymerization activity is obvious, and the activity increases with the increase of the effective ionic radii (Nd3+ (1.123 Å) > Y3+ (1.040 Å)) [9].
3. Conclusion 2.17 1.19 2.59 1.08
1.88 2.10 1.98 1.85
a Polymerization conditions: [LA]/[Ln] = 100, [LA] = 0.5 M, in toluene, t = 10 h, 70 °C. b Isolated yields of PLA. c Determined by GPC at 40 °C in THF relative to polystyrene standards; corrected by the Mark–Houwink equation [Mn,obsd = 0.58Mn (GPC)] [17].
In summary, a family of neutral and solvent-free bis(amidinate) rare earth metal amide complexes were prepared by one-pot salt metathesis reaction. These complexes were active for L-lactide polymerization in the presence of one equivalent amount of iPrOH to give high molecular weight (Mn > 104) polymers. The polymerization activity is dependent on the ionic radii of rare earth metals, as well as the substituent at the carbon atom of the amidinate moiety.
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4. Experimental
934 (m), 841 (s), 777 (s). Anal. Calc. for C52H64N5NdSi2: C, 65.09; H, 6.72; N, 7.30. Found: C, 65.35; H, 6.56; N, 7.41%.
4.1. Materials and procedures 4.4. Synthesis of [CyC(N-2,6-Me2C6H3)2]2YN(SiMe3)2 (3) All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques and a argon-filled glovebox operating at less than 1 ppm oxygen and 1 ppm moisture. Solvents (toluene, hexane, and THF) were distilled from sodium/benzophenone ketyl, degassed by the freeze-pump-thaw method, and dried over fresh Na chips in the glovebox. Dichloromethane was died by stirring with CaH2, and distilled before use. Anhydrous LnCl3 were purchased from STREM. P2O5 was purchased from Sinopharm Chemical Reagent Co., Ltd. Hexamethyldisiloxane, 2,6-dimethylaniline, cyclohexanecarboxylic acid, benzoic acid, and n-BuLi (1.0 M in hexane solution) were purchased from Acros, and used as received. L-lactide was purchased from TCI, and recrystallized from hot toluene. Deuterated solvents (CDCl3, C6D6) were obtained from CIL. Polyphosphoric acid trimethylsilyl ester (PPSE) [18,19], [CyC(N-2,6-Me2C6H3)2]H [19], and [PhC(N-2,6-Me2C6H3)2]H [19] were prepared according to the literature. Samples of organo rare earth metal complexes for NMR spectroscopic measurements were prepared in the glovebox using J. Young valve NMR tubes. NMR (1H, 13C) spectra were recorded on a Bruker AVANCE III spectrometer at 25 °C, and referenced internally to residual solvent resonances unless otherwise stated. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion on a Carlo-Erba EA-1110 instrument, quoted data are the average of at least two independent determinations. FT-IR spectra were recorded on a Bruker TENSOR 27 spectrometer. Molecular weight and molecular weight distribution of the polymers were measured by PL GPC 50 at 40 °C using THF as eluent against polystyrene standards, flow rate: 1 mL/min, sample concentration: 1 mg/mL. 4.2. Synthesis of [PhC(N-2,6-Me2C6H3)2]2YN(SiMe3)2 (1) n-BuLi (1 mL, 1 mmol, 1 M in hexane)) was added drop by drop to a THF solution (30 mL) of [PhC(N-2,6-Me2C6H3)2]H (0.328 g, 1.0 mmol) at room temperature. After 15 min, the reaction mixture (containing [PhC(N-2,6-Me2C6H3)2]Li formed in situ) was added slowly to a THF slurry of YCl3 (0.098 g, 0.5 mmol). The mixture was stirred at room temperature for 2 h to afford a cloudy solution, to which NaN(SiMe3)2 (0.3 mL, 0.5 mmol) was added via a pipet. After the resulting reaction mixture was stirred at room temperature for 4 h, solvents were removed under reduced pressure to give brown oily product. The crude product was washed by hexane, and the residue was extracted by toluene (2 10 mL). Removal of volatiles yielded yellow powder. Recrystallization from a mixture solution of hexane and THF gave 1 as colorless block crystals (0.45 g, 85%). 1H NMR (400 MHz, C6D6): d 0.26 (s, 18H, SiMe3), 2.30 (s, 24H, CH3), 6.59–6.83 (m, 22H, Ar-H). 13C NMR (100 HMz, C6D6): d 4.2 (Si(CH3)3) 20.3 (CH3), 124.1, 127.1, 127.9, 128.5, 129.4, 132.6, 134.5, 146.7 (Ar-C), 177.6 (NCN). FT-IR (KBr, cm1): 3343 (m), 2953 (s), 1625 (s), 1589 (s), 1468 (s), 1374 (s), 1256 (m), 1095 (m), 979 (m), 840 (s), 764 (s). Anal. Calc. for C52H64N5Si2Y: C, 69.07; H, 7.13; N, 7.75. Found: C, 69.25; H, 7.41; N, 7.83%. 4.3. Synthesis of [PhC(N-2,6-Me2C6H3)2]2NdN(SiMe3)2 (2) Complex 2 was prepared by a procedure similar to that of 1. Using n-BuLi (1 mL, 1 mmol), [PhC(N-2,6-Me2C6H3)2]H (0.328 g, 1.0 mmol), NdCl3 (0.125 g, 0.5 mmol). 2 was produced as blue block crystals (0.48 g, 84%). FT-IR (KBr, cm1): 3344 (m), 2932 (s), 1638 (s), 1589 (s), 1473 (s), 1388 (s), 1252 (m), 1091 (w),
n-BuLi (1 mL, 1 mmol) was added drop by drop to a THF solution (30 mL) of [CyC(N-2,6-Me2C6H3)2]H (0.334 g, 1.0 mmol) at room temperature to give a clear yellow solution. After 15 min, the reaction mixture (containing [CyC(N-2,6-Me2C6H3)2]Li formed in situ) was added to a THF slurry of YCl3 (0.098 g, 0.5 mmol). The mixture was stirred at room temperature for 2 h to afford a cloudy solution, to which NaN(SiMe3)2 (0.3 mL, 0.5 mmol) was added via a pipet. After the resulting reaction mixture was stirred at room temperature for 4 h, the solvents were removed under reduced pressure to give brown oily product. The crude product was washed by hexane, and the residue was extracted by toluene (2 10 mL). Drying up the solvents afforded yellow powder. Recrystallization from a mixture solution of hexane and THF afforded 3 as colorless block crystals (0.35 g, 77%). 1H NMR d: 0.31 (s, 18H, SiMe3), 0.52–0.67 (m, 8H, Cy-CH2), 1.09 (d, 4H, Cy-CH2), 1.27 (d, 4H, Cy-CH2), 1.81 (d, 4H, Cy-CH2), 2.24 (m, 2H, Cy-CH), 2.47 (s, 24H, Ph-Me), 6.94–7.01 (12H, Ar-H). 13C NMR (100 HMz, C6D6) d: 3.6 (SiTMS), 20.6 (Ph-Me), 25.4, 25.9, 29.7 (Cy-CH2), 44.5 (CyCH), 123.4, 128.7, 133.3, 146.4 (Ar-C), 182.4 (NCN). FT-IR (KBr, cm1): 3344 (m), 2934 (s), 1637 (s), 1589 (s), 1473 (s), 1388 (s), 1275 (m), 1030 (m), 934 (m), 842 (s), 777 (s). Anal. Calc. for C52H76N5Si2Y: C, 68.16; H, 8.36; N, 7.64. Found: C, 68.43; H, 8.76; N, 7.71%. 4.5. Synthesis of [CyC(N-2,6-Me2C6H3)2]2NdN(SiMe3)2 (4) Complex 4 was prepared by a procedure similar to that of 3. Using n-BuLi (1 mL, 1 mmol), [CyC(N-2,6-Me2C6H3)2]H (0.334 g, 1.0 mmol), NdCl3 (0.125 g, 0.5 mmol). 4 was produced as blue block crystals (0.49 g, 87%). FT-IR (KBr, cm1): 3342 (m), 2954 (s), 1625 (s), 1589 (s), 1472 (s), 1360 (s), 1254 (m), 1093 (m), 931 (m), 840 (s), 776 (s). Anal. Calc. for C52H76N5NdSi2: C, 64.28; H, 7.88; N, 7.21. Found: C, 64.59; H, 7.56; N, 7.08%. 4.6. A typical procedure for L-lactide polymerization In a 50 mL Schleck flask, 1 (18 mg, 20 lmol), L-lactide (288 mg, 2.0 mmol), and toluene (4.0 mL) were stirred at 70 °C for 10 h (run 5, Table 3). The polymerization was terminated by quenching with excess ethanol containing 5% aq HCl. The polymer was collected by filtration, and dried under vacuum at 60 °C to constant weight (213 mg, 74%). 4.7. X-ray crystallographic study Suitable single crystals of complexes were sealed in a thinwalled glass capillary for determining the single-crystal structure. Intensity data were collected with a Rigaku Mercury CCD area detector in x scan mode using Mo Ka radiation (k = 0.71070 Å). The diffracted intensities were corrected for Lorentz polarization effects and empirical absorption corrections. The structures were solved by direct methods and refined by full-matrix least-squares procedures based on |F|2. All the non-hydrogen atoms were refined anisotropically. The structures were solved and refined using SHELEXL-97 program. 5. Supplementary material CCDC 763877, 763878 and 763879 contain the supplementary crystallographic data for 1, 2, and 4, respectively. These data can
Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601
be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Acknowledgments This work was partly financial supported by the National Natural Science Foundation of China (20604023), Zhejiang Provincial Natural Science Foundation (Y4090617), State Key Laboratory of Polymer Physics and Chemistry (200902), and Key Laboratory of Organic Synthesis of Jiangsu Province (KJS0907).
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
References [1] [2] [3] [4] [5]
F.T. Edelmann, Adv. Organomet. Chem. 57 (2008) 183. F.T. Edelmann, Chem. Soc. Rev. 38 (2009) 2253. Y. Yang, Q.Y. Wang, D.M. Cui, J. Polym. Sci. Part A 46 (2008) 5251. J.F. Wang, F. Xu, T. Cai, Q. Shen, Org. Lett. 10 (2008) 445. J.F. Wang, Y.M. Yao, Y. Zhang, Q. Shen, Inorg. Chem. 48 (2009) 744.
[17] [18] [19]
3601
S. Yao, H.S. Chan, C.K. Lam, H.K. Lee, Inorg. Chem. 48 (2009) 9936. J.A.R. Schmidt, J. Arnold, Chem. Commun. (1999) 2149. J.F. Wang, T. Cai, Y.M. Yao, Y. Zhang, Q. Shen, Dalton Trans. (2007) 5275. R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. O.D. Cabaret, B.M. Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147. N. Ajellal, D.M. Lyubov, M.A. Sinenkov, G.K. Flukin, A.V. Cherkasov, C.M. Thomas, J.F. Carpentier, A.A. Trifonov, Chem. Eur. J. 14 (2008) 5440. P.L. Arnold, J.C. Buffet, R.P. Blaudeck, S. Sujecki, A.J. Blake, C. Wilson, Angew. Chem., Int. Ed. 47 (2008) 6033. H.Y. Ma, T.P. Spaniol, J. Okuda, Inorg. Chem. 4 (2008) 3328. H.E. Dyer, S. Huijser, A.D. Schwarz, C. Wang, R. Duchateau, P. Mountford, Dalton Trans. (2008) 32. X.L. Liu, X.M. Shang, T. Tang, N.H. Hu, F.K. Pei, D.M. Cui, X.S. Chen, X.B. Jing, Organometallics 26 (2007) 2747. I. Barakat, P. Dubois, R. Jerome, P. Teyssie, J. Polym. Sci., Part A 31 (1993) 505. S.I. Ogata, A. Mochizuki, M.A. Kakimoto, Y. Imai, Bull. Chem. Soc. Jpn. 59 (1986) 2171. Y.J. Luo, X.L. Wang, J. Chen, C.C. Luo, Y. Zhang, Y.M. Yao, J. Organomet. Chem. 694 (2009) 1289.
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Synthesis, characterization and L-lactide polymerization behavior of bis(amidinate) rare earth metal amide complexes Yunjie Luo a,*, Ping Xu a,b, Yinlin Lei a, Yong Zhang b, Yaorong Wang b,** a b
Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China College of Chemistry, Chemical Engineering and Material Science, Suzhou University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 28 May 2010 Received in revised form 2 July 2010 Accepted 12 July 2010 Available online 21 August 2010 Keywords: Bis(amidinate) rare earth amide complex Synthesis Crystal structure L-lactide polymerization
a b s t r a c t A family of neutral and solvent-free bis(amidinate) rare earth metal amide complexes with a general formula [RC(N-2,6-Me2C6H3)2]2LnN(SiMe3)2 (R = phenyl (Ph), Ln = Y (1), Nd (2); R = cyclohexyl (Cy), Ln = Y (3), Nd (4)) were synthesized in high yields by one-pot salt metathesis reaction of anhydrous LnCl3, amidinate lithium salt [RC(N-2,6-Me2C6H3)2]Li, and NaN(SiMe3)2 in THF at room temperature. Single crystal structural determination of complexes 1, 2 and 4 revealed that the central metal adopts distorted pyramidal geometry. In the presence of 1 equivalent of iPr-OH, all these complexes were active for L-lactide polymerization in toluene at 70 °C to give high molecular weight (Mn > 104) polymers. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Amidinate anions [RC(NR0 )2] can be easily modified by tuning the C- and N-substituents for tailing the steric and electronic properties. They have been extensively employed as ancillary ligands to form complexes with a variety of metal centers across the periodic table [1]. Over the past decades, considerable attention has also been paid to the synthesis and reactivity of amidinate rare earth metal complexes, which could serve as reagents in organic synthesis and as initiators/catalysts for polymerization [2–6]. In contrast, there are only rare examples of amidinate-incorporated rare earth metal amide complexes [7,8]. Herein, we would like to report the synthesis and characterization of a novel family of bis(amidinate) rare earth metal amide complexes, as well as their activity towards L-lactide polymerization. 2. Results and discussion 2.1. Synthesis and characterization of bis(amidinate) rare earth metal complexes The one-pot salt metathesis reaction of anhydrous LnCl3 with 2 equivalents of amidinate lithium salt [RC(N-2,6-Me2C6H3)2]Li (formed in situ by treatment of [RC(N-2,6-Me2C6H3)2]H with 1
* Corresponding author. Tel.: +86 574 88130085; fax: +86 574 88130130. ** Corresponding author. Tel.: +86 512 65882806. E-mail addresses: [email protected] (Y. Luo), [email protected] (Y. Wang). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.07.036
equivalent of n-BuLi in THF), followed by addition of 1 equivalent of NaN(SiMe3)2 in THF at room temperature afforded the corresponding bis(amidinate) rare earth metal amide complexes [RC(N-2,6-Me2C6H3)2]2LnN(SiMe3)2 (R = Ph, Ln = Y (1), Nd (2); R = Cy, Ln = Y (3), Nd (4)) in 77–87% isolated yields (Scheme 1). Although salt metathesis strategy was employed as synthetic approach and THF was used as reaction solvent in these cases, complexes 1–4 were isolated as neutral, mononuclear, and solvent-free species. These complexes are air- and moisture-sensitive, soluble in THF, toluene, diethyl ether, but insoluble in aliphatic solvents such as hexane and pentane. They were characterized by elemental analysis, FT-IR, NMR spectroscopy (except for 2 and 4 for their strong paramagnetic property). Complexes 1, 2 and 4 were also subjected to X-ray single crystal structure determination. Room-temperature 1H NMR spectra of 1 and 3 in C6D6 showed a singlet peak for the methyl resonances of Y–N(SiMe3)2. Restricted rotation about the N–Caryl bonds was not observed because the methyl groups on the aryl rings showed only one singlet resonance. In 1 and 3, the main difference is the substituents at the carbon atom of ligating NCN moiety (phenyl group for 1 and cyclohexyl group for 3), however, the methyl resonances of Y–N(SiMe3)2 for 1 are found at d 0.24 ppm (1H NMR) and d 4.2 ppm (13C NMR), while those for 3 appear at d 0.31 ppm (1H NMR) and d 3.6 ppm (13C NMR), suggesting a similar electronic yttrium environment. Single crystals of complexes 1, 2 and 4 suitable for X-ray diffraction were grown from a mixture solution of hexane and THF at 30 °C. The molecular structures of 1 and 2 are shown in Fig. 1, and that of 4 is given in Fig. 2. The crystallographic data and the
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R Me Me Me
Me N R
1) n-BuLi, THF H
N Me
2) 0.5 LnCl3, THF, rt
N
0.5 NaN(SiMe3)2 THF, rt
Me
N
Me
Ln
N(SiMe3)2
MeMe
Me N
N
Me Me R R=
Ln = Y (1), Nd (2)
Ln = Y (3), Nd (4)
Scheme 1. Synthesis of bis(amidinate) rare earth metal amide complexes.
selected bond distances and angles are summarized in Table 1 and Table 2, respectively. The overall structural features of 1, 2 and 4, except for the difference of residue solvent molecule in the lattice (these solvent molecules could be removed under vacuum), are quite similar: the central metal is five-coordinated by two bidentate amidinate ligands through nitrogen atoms, and one amide group to form a distorted pyramidal geometry. Therefore, only the structure of 1 is discussed here. Complex 1 is a C2 symmetric molecule in the solid state. It is found that the phenyl group on the central carbon atom of NCN moiety provides steric protection in the plane of the ligand as well as above and below that plane (dihedral angle of N(2)C(1)N(1)Y(1) and the plane formed by phenyl group is 53.29°). The dihedral angle of N(2)C(1)N(1)Y(1) and the plane formed by Si(1)Y(1)N(3)Si(1A) is 66.03°), this orientation minimizes the interaction between the SiMe3 groups of the amide group and the 2,6-Me2C6H3 groups of the amidinate ligand. The Y– N(SiMe3)2 distance in 1 is 2.187(5) Å, which is slightly shorter than those in (dimb)Y[N(SiMe3)2]2 (dimb = N,N0 -diisopropyl(2,6-dimesityl)benzamidinate) (av. 2.24 Å) [7], and is identical to that in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av. 2.19 Å) [8], if the ionic radius is considered [9]. The distances between yttrium and the ligating nitrogen atoms (Y–N = 2.369(3) and 2.376(3) Å) in 1 are consistent with those found in (dimb)Y[N(SiMe3)2]2 (av. 2.34 Å) [7], but are somewhat longer than those in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av. 2.30 Å) [8] The bite angle of N–Y–N is 56.39°, which is smaller than those in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av. 59°) [8].
Fig. 2. Molecular structure of 4 with thermal ellipsoids at 20% probability. Hydrogen atoms have been omitted for clarity.
2.2. L-lactide polymerization initiated by bis(amidinate) rare earth metal amide complexes Polylactides (PLAs) are the promising biodegradable and biocompatible synthetic macromolecules, and have been widely applied in medicine, pharmaceutics, as well as in tissue engineering [10]. The most effective method to prepare PLAs is the ring-opening polymerization (ROP) of lactides by metal-based catalysts/initiators, and it has been proven that metal alkoxides usually possess high efficiency [11]. Some rare earth metal complexes also exhibited good performance for lactide polymerization [5,8,12–16]. In order to understand the polymerization behavior of bis(amidinate) rare earth metal amide complexes, complexes 1–4 were tested for the ring-opening polymerization of L-lactide, and the preliminary polymerization results are listed in Table 3. All of these neutral complexes alone showed very poor activity towards L-lactide polymerization at 70 °C in toluene. However, when these complexes were treated with 1 equivalent of iPr-OH, they exhibited improved activity, albeit still with rather low activity compared with other amidinate rare earth metal initiators [5,8]. This might be contributed to that the steric demanding amidinate ligands in 1–4 hampered the coordination of monomer to central metal, sequentially resulted in decreasing the initiation capability
Fig. 1. Molecular structure of 1 (left) and 2 (right) with thermal ellipsoids at 10% probability. Hydrogen atoms have been omitted for clarity.
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Formula FW T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F (0 0 0) Crystal size (mm) hmax (°) Reflections collected Independent reflections No. of variables Goodness-of-fit GOF on F2 R [I > 2r(I)] wR
1
2
4
C52H64N5Si2Y(C6H14) 990.34 293(2) monoclinic C 2/c 21.434(6) 11.627(3) 23.872(6) 90.00 106.272(8) 90.00 5711(2) 4 1.152 1.102 2112 0.80 0.60 0.50 25.35 269 48 5209 299 1.185 0.0775 0.1690
C52H64N5NdSi23(C4H8O) 1175.81 223(2) monoclinic P 21/n 17.6303(10) 16.7689(9) 21.5620(11) 90.00 92.6300(10) 90.00 6367.9(6) 4 1.226 0.899 2476 0.80 0.50 0.40 25.5 321 48 11760 649 1.076 0.0465 0.1232
C52H76N5NdSi2 971.60 293(2) monoclinic P 21/n 12.3407(12) 19.0488(17) 22.866(2) 90.00 96.766(3) 90.00 5337.8(9) 4 1.209 1.054 2044 0.80 0.80 0.45 25.35 497 40 9751 520 1.174 0.0611 0.1146
Table 2 Selected bond distances and bond angles for 1, 2, 4. 1
2
4
Bond distances (Å) Y1–N1 Y1–N1A Y1–N2 Y1–N2A Y1–N3 C1–N1 C1–N1A C1–N2 C1–N2A Y1–C1 Y1–C1A
2.369(3) 2.369(3) 2.376(3) 2.376(3) 2.187(5) 1.331(5) 1.331(5) 1.325(5) 1.325(5) 2.802(4) 2.802(4)
Nd1–N1 Nd1–N2 Nd1–N3 Nd1–N4 Nd1–N5 C1–N1 C1–N2 C24–N3 C24–N4 Nd1–C1 Nd1–C24
2.462(3) 2.484(3) 2.456(3) 2.473(3) 2.277(3) 1.346(5) 1.336(5) 1.342(5) 1.338(5) 2.921(4) 2.909(4)
Nd1–N1 Nd1–N2 Nd1–N3 Nd1–N4 Nd1–N5 C1–N1 C1–N2 C24–N3 C24–N4 Nd1–C1 Nd1–C24
2.454(4) 2.448(4) 2.472(4) 2.451(4) 2.284(4) 1.336(6) 1.330(6) 1.333(6) 1.339(6) 2.917(5) 2.921(5)
Bond angles (°) N2–Y1–N1 N2A–Y1–N1A N2–C1–N1 N2A–C1–N1A
56.39(11) 56.39(11) 115.2(4) 115.2(4)
N2–Nd1–N1 N4–Nd1–N3 N2–C1–N1 N4–C24–N3
54.38(10) 54.60(10) 114.9(3) 115.0(3)
N2–Nd1–N1 N4–Nd1–N3 N2–C1–N1 N4–C24–N3
53.96(14) 54.09(13) 113.1(4) 113.8(4)
Table 3 a L-lactide polymerization with amidinate rare earth metal amide complexes. O n
O Initiator/iPr-OH
O
O O
O
O
O L- Lactide
Run
Initiator
i
Pr-OH/Ln (molar ratio)
Yieldb (%)
1 2 3 4 5 6 7 8
1 2 3 4 1 2 3 4
0 0 0 0 1 1 1 1
trace trace trace trace 74 85 61 73
Mnc 104
n
Mw/Mnc
for L-lactide polymerization. The influence of organic substituents at the carbon atoms of the amidinate moiety on the polymerization activity was observed, and 1 and 2 showed a little bit higher activity than 3 and 4. This phenomenon could be reasoned that the cyclohexyl group is much more fluxional than the rigid phenyl ring. The influence of central metal on the polymerization activity is obvious, and the activity increases with the increase of the effective ionic radii (Nd3+ (1.123 Å) > Y3+ (1.040 Å)) [9].
3. Conclusion 2.17 1.19 2.59 1.08
1.88 2.10 1.98 1.85
a Polymerization conditions: [LA]/[Ln] = 100, [LA] = 0.5 M, in toluene, t = 10 h, 70 °C. b Isolated yields of PLA. c Determined by GPC at 40 °C in THF relative to polystyrene standards; corrected by the Mark–Houwink equation [Mn,obsd = 0.58Mn (GPC)] [17].
In summary, a family of neutral and solvent-free bis(amidinate) rare earth metal amide complexes were prepared by one-pot salt metathesis reaction. These complexes were active for L-lactide polymerization in the presence of one equivalent amount of iPrOH to give high molecular weight (Mn > 104) polymers. The polymerization activity is dependent on the ionic radii of rare earth metals, as well as the substituent at the carbon atom of the amidinate moiety.
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4. Experimental
934 (m), 841 (s), 777 (s). Anal. Calc. for C52H64N5NdSi2: C, 65.09; H, 6.72; N, 7.30. Found: C, 65.35; H, 6.56; N, 7.41%.
4.1. Materials and procedures 4.4. Synthesis of [CyC(N-2,6-Me2C6H3)2]2YN(SiMe3)2 (3) All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques and a argon-filled glovebox operating at less than 1 ppm oxygen and 1 ppm moisture. Solvents (toluene, hexane, and THF) were distilled from sodium/benzophenone ketyl, degassed by the freeze-pump-thaw method, and dried over fresh Na chips in the glovebox. Dichloromethane was died by stirring with CaH2, and distilled before use. Anhydrous LnCl3 were purchased from STREM. P2O5 was purchased from Sinopharm Chemical Reagent Co., Ltd. Hexamethyldisiloxane, 2,6-dimethylaniline, cyclohexanecarboxylic acid, benzoic acid, and n-BuLi (1.0 M in hexane solution) were purchased from Acros, and used as received. L-lactide was purchased from TCI, and recrystallized from hot toluene. Deuterated solvents (CDCl3, C6D6) were obtained from CIL. Polyphosphoric acid trimethylsilyl ester (PPSE) [18,19], [CyC(N-2,6-Me2C6H3)2]H [19], and [PhC(N-2,6-Me2C6H3)2]H [19] were prepared according to the literature. Samples of organo rare earth metal complexes for NMR spectroscopic measurements were prepared in the glovebox using J. Young valve NMR tubes. NMR (1H, 13C) spectra were recorded on a Bruker AVANCE III spectrometer at 25 °C, and referenced internally to residual solvent resonances unless otherwise stated. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion on a Carlo-Erba EA-1110 instrument, quoted data are the average of at least two independent determinations. FT-IR spectra were recorded on a Bruker TENSOR 27 spectrometer. Molecular weight and molecular weight distribution of the polymers were measured by PL GPC 50 at 40 °C using THF as eluent against polystyrene standards, flow rate: 1 mL/min, sample concentration: 1 mg/mL. 4.2. Synthesis of [PhC(N-2,6-Me2C6H3)2]2YN(SiMe3)2 (1) n-BuLi (1 mL, 1 mmol, 1 M in hexane)) was added drop by drop to a THF solution (30 mL) of [PhC(N-2,6-Me2C6H3)2]H (0.328 g, 1.0 mmol) at room temperature. After 15 min, the reaction mixture (containing [PhC(N-2,6-Me2C6H3)2]Li formed in situ) was added slowly to a THF slurry of YCl3 (0.098 g, 0.5 mmol). The mixture was stirred at room temperature for 2 h to afford a cloudy solution, to which NaN(SiMe3)2 (0.3 mL, 0.5 mmol) was added via a pipet. After the resulting reaction mixture was stirred at room temperature for 4 h, solvents were removed under reduced pressure to give brown oily product. The crude product was washed by hexane, and the residue was extracted by toluene (2 10 mL). Removal of volatiles yielded yellow powder. Recrystallization from a mixture solution of hexane and THF gave 1 as colorless block crystals (0.45 g, 85%). 1H NMR (400 MHz, C6D6): d 0.26 (s, 18H, SiMe3), 2.30 (s, 24H, CH3), 6.59–6.83 (m, 22H, Ar-H). 13C NMR (100 HMz, C6D6): d 4.2 (Si(CH3)3) 20.3 (CH3), 124.1, 127.1, 127.9, 128.5, 129.4, 132.6, 134.5, 146.7 (Ar-C), 177.6 (NCN). FT-IR (KBr, cm1): 3343 (m), 2953 (s), 1625 (s), 1589 (s), 1468 (s), 1374 (s), 1256 (m), 1095 (m), 979 (m), 840 (s), 764 (s). Anal. Calc. for C52H64N5Si2Y: C, 69.07; H, 7.13; N, 7.75. Found: C, 69.25; H, 7.41; N, 7.83%. 4.3. Synthesis of [PhC(N-2,6-Me2C6H3)2]2NdN(SiMe3)2 (2) Complex 2 was prepared by a procedure similar to that of 1. Using n-BuLi (1 mL, 1 mmol), [PhC(N-2,6-Me2C6H3)2]H (0.328 g, 1.0 mmol), NdCl3 (0.125 g, 0.5 mmol). 2 was produced as blue block crystals (0.48 g, 84%). FT-IR (KBr, cm1): 3344 (m), 2932 (s), 1638 (s), 1589 (s), 1473 (s), 1388 (s), 1252 (m), 1091 (w),
n-BuLi (1 mL, 1 mmol) was added drop by drop to a THF solution (30 mL) of [CyC(N-2,6-Me2C6H3)2]H (0.334 g, 1.0 mmol) at room temperature to give a clear yellow solution. After 15 min, the reaction mixture (containing [CyC(N-2,6-Me2C6H3)2]Li formed in situ) was added to a THF slurry of YCl3 (0.098 g, 0.5 mmol). The mixture was stirred at room temperature for 2 h to afford a cloudy solution, to which NaN(SiMe3)2 (0.3 mL, 0.5 mmol) was added via a pipet. After the resulting reaction mixture was stirred at room temperature for 4 h, the solvents were removed under reduced pressure to give brown oily product. The crude product was washed by hexane, and the residue was extracted by toluene (2 10 mL). Drying up the solvents afforded yellow powder. Recrystallization from a mixture solution of hexane and THF afforded 3 as colorless block crystals (0.35 g, 77%). 1H NMR d: 0.31 (s, 18H, SiMe3), 0.52–0.67 (m, 8H, Cy-CH2), 1.09 (d, 4H, Cy-CH2), 1.27 (d, 4H, Cy-CH2), 1.81 (d, 4H, Cy-CH2), 2.24 (m, 2H, Cy-CH), 2.47 (s, 24H, Ph-Me), 6.94–7.01 (12H, Ar-H). 13C NMR (100 HMz, C6D6) d: 3.6 (SiTMS), 20.6 (Ph-Me), 25.4, 25.9, 29.7 (Cy-CH2), 44.5 (CyCH), 123.4, 128.7, 133.3, 146.4 (Ar-C), 182.4 (NCN). FT-IR (KBr, cm1): 3344 (m), 2934 (s), 1637 (s), 1589 (s), 1473 (s), 1388 (s), 1275 (m), 1030 (m), 934 (m), 842 (s), 777 (s). Anal. Calc. for C52H76N5Si2Y: C, 68.16; H, 8.36; N, 7.64. Found: C, 68.43; H, 8.76; N, 7.71%. 4.5. Synthesis of [CyC(N-2,6-Me2C6H3)2]2NdN(SiMe3)2 (4) Complex 4 was prepared by a procedure similar to that of 3. Using n-BuLi (1 mL, 1 mmol), [CyC(N-2,6-Me2C6H3)2]H (0.334 g, 1.0 mmol), NdCl3 (0.125 g, 0.5 mmol). 4 was produced as blue block crystals (0.49 g, 87%). FT-IR (KBr, cm1): 3342 (m), 2954 (s), 1625 (s), 1589 (s), 1472 (s), 1360 (s), 1254 (m), 1093 (m), 931 (m), 840 (s), 776 (s). Anal. Calc. for C52H76N5NdSi2: C, 64.28; H, 7.88; N, 7.21. Found: C, 64.59; H, 7.56; N, 7.08%. 4.6. A typical procedure for L-lactide polymerization In a 50 mL Schleck flask, 1 (18 mg, 20 lmol), L-lactide (288 mg, 2.0 mmol), and toluene (4.0 mL) were stirred at 70 °C for 10 h (run 5, Table 3). The polymerization was terminated by quenching with excess ethanol containing 5% aq HCl. The polymer was collected by filtration, and dried under vacuum at 60 °C to constant weight (213 mg, 74%). 4.7. X-ray crystallographic study Suitable single crystals of complexes were sealed in a thinwalled glass capillary for determining the single-crystal structure. Intensity data were collected with a Rigaku Mercury CCD area detector in x scan mode using Mo Ka radiation (k = 0.71070 Å). The diffracted intensities were corrected for Lorentz polarization effects and empirical absorption corrections. The structures were solved by direct methods and refined by full-matrix least-squares procedures based on |F|2. All the non-hydrogen atoms were refined anisotropically. The structures were solved and refined using SHELEXL-97 program. 5. Supplementary material CCDC 763877, 763878 and 763879 contain the supplementary crystallographic data for 1, 2, and 4, respectively. These data can
Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601
be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Acknowledgments This work was partly financial supported by the National Natural Science Foundation of China (20604023), Zhejiang Provincial Natural Science Foundation (Y4090617), State Key Laboratory of Polymer Physics and Chemistry (200902), and Key Laboratory of Organic Synthesis of Jiangsu Province (KJS0907).
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
References [1] [2] [3] [4] [5]
F.T. Edelmann, Adv. Organomet. Chem. 57 (2008) 183. F.T. Edelmann, Chem. Soc. Rev. 38 (2009) 2253. Y. Yang, Q.Y. Wang, D.M. Cui, J. Polym. Sci. Part A 46 (2008) 5251. J.F. Wang, F. Xu, T. Cai, Q. Shen, Org. Lett. 10 (2008) 445. J.F. Wang, Y.M. Yao, Y. Zhang, Q. Shen, Inorg. Chem. 48 (2009) 744.
[17] [18] [19]
3601
S. Yao, H.S. Chan, C.K. Lam, H.K. Lee, Inorg. Chem. 48 (2009) 9936. J.A.R. Schmidt, J. Arnold, Chem. Commun. (1999) 2149. J.F. Wang, T. Cai, Y.M. Yao, Y. Zhang, Q. Shen, Dalton Trans. (2007) 5275. R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. O.D. Cabaret, B.M. Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147. N. Ajellal, D.M. Lyubov, M.A. Sinenkov, G.K. Flukin, A.V. Cherkasov, C.M. Thomas, J.F. Carpentier, A.A. Trifonov, Chem. Eur. J. 14 (2008) 5440. P.L. Arnold, J.C. Buffet, R.P. Blaudeck, S. Sujecki, A.J. Blake, C. Wilson, Angew. Chem., Int. Ed. 47 (2008) 6033. H.Y. Ma, T.P. Spaniol, J. Okuda, Inorg. Chem. 4 (2008) 3328. H.E. Dyer, S. Huijser, A.D. Schwarz, C. Wang, R. Duchateau, P. Mountford, Dalton Trans. (2008) 32. X.L. Liu, X.M. Shang, T. Tang, N.H. Hu, F.K. Pei, D.M. Cui, X.S. Chen, X.B. Jing, Organometallics 26 (2007) 2747. I. Barakat, P. Dubois, R. Jerome, P. Teyssie, J. Polym. Sci., Part A 31 (1993) 505. S.I. Ogata, A. Mochizuki, M.A. Kakimoto, Y. Imai, Bull. Chem. Soc. Jpn. 59 (1986) 2171. Y.J. Luo, X.L. Wang, J. Chen, C.C. Luo, Y. Zhang, Y.M. Yao, J. Organomet. Chem. 694 (2009) 1289.