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Lanthanide complexes supported by phenolate-possessing aroylhydrazone: Synthesis, characterization and lactides polymerization
Accepted Manuscript Research paper Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie, Changan Wang, Yinfeng Han, Jianping Zhang, Yingming Yao PII: DOI: Reference:
S0020-1693(17)30367-5 http://dx.doi.org/10.1016/j.ica.2017.06.031 ICA 17680
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
11 March 2017 11 June 2017 12 June 2017
Please cite this article as: K. Nie, C. Wang, Y. Han, J. Zhang, Y. Yao, Lanthanide complexes supported by phenolatepossessing aroylhydrazone: synthesis, characterization and lactides polymerization, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.06.031
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Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie,a* Changan Wang,a Yinfeng Han,a Jianping Zhanga and Yingming Yaob
a
School of Chemistry and Chemical Engineering, Taishan University, Taian, 271021, China
b
College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, China *Corresponding author (email: [email protected])
1. Introduction As one of the most widely used aliphatic linear polyesters, polylactide (PLA), is an attractive alternative to petrochemicalbased polymers due to the desirable biodegradability and the fact that the monomer can be prepared from sustainable raw materials.[1] PLA has widely biomedical and pharmaceutical applications, such as controlled drug delivery, resorbable sutures, medical implants and scaffolds for tissue engineering.[2] Depending on the stereochemistry of the LA monomer, the reaction conditions, and the selectivity of initiators, an array of PLA with different microstructures can be obtained, and now control of the polymer microstructure has been demonstrated to significantly affect the polymers’ mechanical and physical properties.[3-5] Catalytic ring-opening polymerization (ROP) of lactide is a powerful synthetic methodology to produce PLA with good control regarding molecular weight, polydispersity (PDI), and tacticity. A number of stereoselective catalysts vary widely in terms of the metal centers and ancillary ligands employed have been developed.[3-20] Among them, lanthanide compounds have proved to be highly efficient catalysts for the living ROP of lactides, and notably, metal ionic radii and subtle modifications of the ligand framework have a strong influence on the stereoselective ROP of rac-LA. For example, in 2014, Williams and co-workers have shown that metal-size influence in iso-selective lactide polymerization for the phosphasalen lutetium ethoxide complex shows excellent iso-selectivity (Pi = 0.81-0.84) while the corresponding lanthanide derivative exhibits moderate heteroselectivity (Ps = 0.74).[10, 20] Carpentier et al. have prepared some
methoxy-amino bridged bis(phenolate) lanthanide compounds, which depending upon the nature of the ortho substituents of bis(phenol)s, produce ethier heterotactically or atactic PLA.[9] To further understand the metal ionic radii and ancillary ligand backbone effects and achieve rapid selective rac-LA polymerization, design and synthesis of new, well-defined rare-earth initiators is still attractive and meaningful. Schiff bases and their transition metal complexes possessing noteworthy biological activities such as antibacterial, antifungal and anticancer have been extensively studied in many fields including coordination chemistry, analytical chemistry and biochemistry.[21-37] Among them, aroylhydrazones have attracted much attention mainly because such type of polydentate ligands could meet the central metal ions’ charge-neutrality and stereochemical requirements via facile tautomerisation.[28-37] Furthermore, heteroatom-substituted aroylhydrazones could offer more possibilities of coordination mode. However, to the best of our knowledge, there is no literature concerning the utilization of phenolate-possessing aroylhydrazone ligands in lanthanide organometallic chemistry, neither the relationship between their structures and catalytic performance for the ROP of lactides. Therefore, in this contribution, an array of novel rare-earth bis(trimethylsilyl)amido compounds stabilized by phenolate-possessing aroylhydrazone ligands were prepared and fully characterized, and also their subsequent polymeric performance toward L-LA and rac-LA were reported.
2. Experimental Section 2.1. General procedures.
All syntheses and manipulations were performed using standard Schlenk techniques or in a N2-filled glovebox. The solvents were distilled over sodium benzophenone ketyl under argon prior to use. HN(SiMe3)2 and n-BuLi are purchased from Alfa. HN(SiMe3)2
was
distilled
after
dried
over
CaH2
for
5
days.
Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 (Ln = Y, Yb)[38, 39] and the ligands L1H2 [L1 = (2-O-C6H4)CH=NNHCOC6 H5], L2H2 [L2 = (2-O-C6H4)CH=NNHCOC6 H4CH3] and L3H2 [L3 = (2-O-C6H4)C(Me)=NNHCOC6 H4CH3] were prepared following the procedures reported.[40] L-LA and rac-LA were recrystallized with dry toluene, and then was sublimed at 50 ºC under vacuum. EDTA titration was used for lanthanide analysis with an xylenol orange indicator and a hexamine buffer.[41] Elemental analysis were performed by direct combustion with a Carlo-Erba EA-1110 instrument. 1
H and 13C NMR spectra were recorded in a C6D6 or THF-d 8 solution for complexes 1,
3 and 4 with a Unity Varian spectrometer at ambient temperature. No resolvable NMR spectrum of complex 2 was obtained due to paramagnetism. Gel permeation chromatography (GPC) analyses were carried out on a Waters 1515 Breeze instrument using narrowly distributed polystyrenes as standards and THF was the eluent at a flow rate of 1.0 mL/min at 40 ºC. Microstructures of PLAs was determined by homodecoupling 1H NMR spectroscopy at 25 ºC in CDCl3 on a Unity Varian AC-400 spectrometer. 2.2. Preparation of {L1Y[N(SiMe3)2](THF)}2 (1). A THF solution of Y[N(SiMe3)2](µ-Cl)Li(THF)3 (7.9 mL, 2.50 mmol) was added to a THF solution of L1 H2 (10 mL, 0.60 g, 2.50 mmol) at room temperature. A color
change from colorless to light yellow was observed instantly. The mixture was stirred for 5 h, and then volatiles were removed under reduced pressure. The residue was extracted twice by hot toluene (15 mL), and the precipitate formed was removed by centrifugation. Yellow crystals were obtained at ambient temperature in several days (0.99 g, 71%). Anal. Calcd. For: C48H72N6O6Si4Y2: C, 51.51; H, 6.48; N, 7.51; Y, 15.89. Found: C, 51.18; H, 6.33; N, 7.89; Y, 15.77%. 1H NMR (400 MHz, C6D6 +THF-d 8, 25 °C): δ 8.58 (s, 2H, N=CHAr), 8.46 (d, J = 8.2 Hz, 4H, ArH), 7.33-7.14 (m, 10H, ArH), 6.84 (d, J = 7.9 Hz, 2H, ArH), 6.59 (t, J = 7.8 Hz, 2H, ArH), 3.55 (s, 8H, α-CH2 THF), 1.50 (s, 8H, β-CH2 THF), 0.41 (s, 36H, SiMe3). 13C{1H} NMR (100 MHz, THF-d8, 25 °C): δ 170.2 (N=CHAr), 129.9, 129.8, 129.1, 128.2, 127.7 (Ar-C), 68.4 (α-CH2, THF), 25.5 (β-CH2, THF), 2.9 (SiMe3). 2.3. Preparation of {L1Yb[N(SiMe3)2](THF)}2 (2). Complex 2 was prepared in the similar method as that described for complex 1, but Yb[N(SiMe3)2](µ-Cl)Li(THF)3 (5.9 mL, 0.92 mmol) was used instead of Y[N(SiMe3)2](µ-Cl)Li(THF)3. Yellow crystals were obtained in a toluene (10 mL) solution (0.44 g, 75%). Anal. Calcd. For: C48H72N6O6Si4Yb2: C, 44.78; H, 5.64; N, 6.53; Yb, 26.88. Found: C, 45.13; H, 5.40; N, 6.79; Y, 26.53%. 2.4. Preparation of {L2Y[N(SiMe3)2](THF)}2 (3). Complex 3 was prepared in the similar method as that described for complex 1, but L2H2 (0.44 g, 1.74 mmol) was used instead of L1H2. Yellow crystals were obtained in a toluene (15 mL) solution (0.67 g, 67%). Anal. Calcd. For: C50H76N6O6Si4Y2: C, 52.34; H, 6.68; N, 7.32; Y, 15.50. Found: C, 52.04; H, 6.90; N, 7.61 Y, 15.78%. 1H
NMR (400 MHz, THF-d 8, 25 °C): δ 8.50 (s, 2H, N=CHAr), 8.10-6.51 (m, 16H, ArH), 3.55 (s, 8H, α-CH2 THF), 2.34 (s, 6H, ArCH3), 1.69 (s, 8H, β-CH2 THF), 0.20 (s, 36H, SiMe3). 13C{1H} NMR (100 MHz, THF-d 8, 25 °C): δ 170.7 (N=CHAr), 132.4, 131.6, 129.5, 129.0, 124.5 (Ar-C), 68.4 (α-CH2, THF), 25.6 (β-CH2, THF), 18.6 (ArCH3), 3.0 (SiMe3). 2.5. Preparation of {L3Y[N(SiMe3)2](THF)}2 (4). Complex 4 was prepared in the similar method as that described for complex 1, but L3H2 (0.27 g, 1.02 mmol) was used instead of L1H2. Yellow crystals were obtained in a toluene (10 mL) solution (0.39 g, 65%). Anal. Calcd. For: C52H80N6O6Si4Y2: C, 53.14; H, 6.86; N, 7.15; Y, 15.13. Found: C, 52.66; H, 6.70; N, 7.52; Y, 15.78%. 1H NMR (400 MHz, THF-d8, 25 °C): δ 8.15-6.54 (m, 16H, ArH), 3.54 (s, 8H, α-CH2 THF), 2.75 (s, 6H, N=CCH3), 2.36 (s, 6H, ArCH3), 1.69 (s, 8H, β-CH2 THF), 0.01 (s, 36H, SiMe3). 13C{1H} NMR (100 MHz, THF-d8, 25 °C): δ 171.3 (N=C), 131.7, 131.1, 129.5, 129.2, 124.0, 120.6 (Ar-C), 68.4 (α-CH2, THF), 25.6 (β-CH2, THF), 21.7 (N=CCH3), 18.6 (ArCH3), 2.9 (SiMe3). 2.6. General procedure for the homopolymerization of L-LA or rac-LA. All complexes were tested as initiators of the polymerization of L-LA or rac-LA. In the glovebox, a 20 mL Schlenk reaction tube, equipped with a magnetic stirrer bar, was charged with appropriate quantity of monomer and solvent. After the monomer was dissolved, a solution of the initiator was added to this solution by syringe. The mixture was immediately stirred vigorously for certain time, during which time an increase in the viscosity was observed. The reaction mixture was quenched with
ethanol and then poured into a large quantity of ethanol to afford the polymer which were further dried under vacuum and weighed. 2.7. X-Ray crystallographic structure determination. Suitable single crystals of complexes 1-4 were sealed in a thin-walled glass capillary for determination the single-crystal structures. Intensity data were collected with a Rigaku Mercury CCD area detector in ω scan mode using Mo-Kα radiation (λ = 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. 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 full-matrix least-squares refinement. The structures were solved and refined using SHELEXL-97 programs.
3. Results and discussion 3.1. Synthesis and characterization of lanthanide aroylhydrazone complexes. The most efficient and straightforward synthetic approach to lanthanide amides is amine elimination reaction, and Ln[N(SiMe3)2]3, Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 or Ln[N(SiHMe2)2]3(THF)2 are usually the typical precursors. However, in terms of Schiff bases ligands, this synthetic method is often hampered by side reactions and it is worth noting that the choose of appropriate starting materials for preparing the
corresponding lanthanide amides is crucial. For example, Y[N(SiMe3)2]3 reacted with the ligand precursor SalenH2 to afford an oligomeric, THF insoluble product while using the
less
steric
demanding
bis(dimethylsilyl)amido
yttrium
complex
Y[N(SiHMe2)2]3(THF)2 gave the expected heteroleptic monometallic yttrium salen-type complex.[42, 43] When the phenolate-possessing aroylhydrazone ligand precursors were prepared straightforwardly by condensation reactions between salicylaldehyde
or 2-acetylphenol and
the
appropriate
aroylhydrazide,
the
corresponding lanthanide amido complexes were tried to synthesize. An NMR-scale reaction
of
Y[N(SiMe3)2]3(µ-Cl)Li(THF)3
(2-O-C6H4)CH=NNHCOC6 H5]
was
L1 H2
with
conducted
initially.
[L1
After
=
equimolar
Y[N(SiMe3)2]3(µ-Cl)Li(THF)3 was added to a L1H2 solution in THF-d 8, resonances at 11.04 and 9.10 ppm belonging to the OH and NH protons respectively almost disappeared, indicating the reaction did occur. At a preparative scale, the desired neutral complex {L1Y[N(SiMe3)2](THF)}2 (1) was isolated with good yield (71%), followed by recrystallization from toluene solution. Similarly, the reaction between Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 (Ln = Yb, Y) and phenolate-possessing
aroylhydrazone
ligands
L1H2
[L1
=
(2-O-C6H4)CH=NNHCOC6 H5], L2H2 [L2 = (2-O-C6H4)CH=NNHCOC6 H4CH3] and L3H2 [L3
=
aroylhydrazone
(2-O-C6H4)C(Me)=NNHCOC6 H4 CH3 ] lanthanide
amido
complexes
generated
the
dimeric
{L1Yb[N(SiMe3)2](THF)}2
(2),
{L2Y[N(SiMe3)2](THF)}2 (3) and {L3Y[N(SiMe3)2](THF)}2 (4) respectively in good yields (Scheme 1).
R1 O
R1
R2
N NH
Ln{N(SiMe 3) 2} 3 (µ−Cl)Li(THF) 3 THF, RT.
OH
- LiCl
R2
(Me 3 Si)2 N THF O Ln
N O
N
N
N
O Ln
R2
O THF N(SiMe 3 )2
R1 1
1
2
L H 2: R = H, R = H L 2H 2: R 1 = H, R2 = Me L 3H 2: R 1 = Me, R 2 = Me
1
R = H, R 2 = H, Ln = Y(1), Yb ( 2) R 1 = H, R 2 = Me, Ln = Y(3) R 1 = Me, R2 = Me, Ln = Y( 4)
Scheme 1 Synthesis of the complexes under investigation. All the resultant rare-earth compounds were completely characterized by elemental analysis and 1H,
13
C NMR spectroscopy except the ytterbium compound 2 due to
paramagnetism. 1H NMR spectra in THF-d8 showed a sharp single resonance in the field of δ 0-0.5 ppm assigned to the SiMe3 protons and resonances corresponding to the ligand were also observed. This suggested that the amide complex was indeed formed. The definitive solid-state molecular structures were authenticated by X-ray crystallography. Complexes 1-4 are sensitive to air and moisture whereas no signs of decomposition appear preserved in the glovebox at room temperature, and they are readily soluble in THF, but slightly soluble in toluene. 3.2. Crystal structures. Single crystals of compounds 1-4 suitable for the X-ray structure determination were grown from a toluene solution at ambient temperature. Crystallographic data and analysis results are listed in Table 1. Selected bond distances and angles of 1-4 are shown in Table 2, and their ORTEP drawings are depicted in Figures 1-4 respectively. X-Ray analyses revealed that compounds 1-4 are similar, and there is no difference between the two metal centers which were bridging through the phenolic oxygens. Obviously, the dimeric nature of these compounds arises from the insufficient
bulkiness of substituents on the ligands. Each metal center retains a ƙ1-amido, a ƙ1-THF and a ƙ3-dianionic aroylhydrazone ligand as well as an additional oxygen(aryloxide) from the other ligand that forms a µ bridge between the two metal centers. The coordination geometry around the metal center can be best described as a slightly distorted octahedron. The aroylhydrazone ligands have undergone keto-enol tautomerism and the electron delocalized within the anionic NNCO unit revealed by the N−N, C−N, C−O bond distances of anionic NNCO unit. In compound 1, the Y-O(phenolate) bond distance is 2.314(4) Å and the Y-O(aroylhydrazone) bond distance is 2.233(4) Å, which are a little longer than the corresponding bond distances of compound 2 [2.262(2), 2.182(2) Å]. This result is in accord with their metal ionic radii and similar consequences are also observed from the Ln-N, Ln-O(THF) bond lengths. The Y-O(phenolate) bond distance of 2.314(4) Å in complex 1 is also longer than 2.281(3), 2.286(1) Å found in complexes 3 and 4, this is probably because of different electronic and steric effects of the aroylhydrazone ligands. However, these effects influenced the other bond lengths little. The Ln-N(aroylhydrazone) bond distances [2.440(5) Å for 1, 2.382(2) Å for 2, 2.430(3) Å for 3, and 2.444(2) Å for 4] and the Ln-N(amide) bond distances (2.315(5) Å for 1, 2.205(2) Å for 2, 2.246(4) Å for 3, and 2.241(2) Å for 4) are comparable if the differences in ionic radius between the central lanthanide metals are considered. These values are also consistent with the corresponding bond lengths in some Schiff base complexes reported, such as L2Ln[N(SiMe3)2] [L = 3,5-Bu t2-2-O-C6H2CH=N-2,6-Pri2-C6H3, Ln = Y(2.467, 2.245 Å) and Yb(2.420, 2.219 Å)].[26]
3.3. ROP of L-LA and rac-LA by complexes 1-4. Performance of the new aroylhydrazone lanthanide amides in the ROP of L-LA and rac-LA were investigated, in order to understand the relationship between the catalysts’ structure and the ROP activity, controllability and stereoselectivity. The reactions were carried out in a 100:1 molecular ratio of LAs to catalysts 1-4 in tetrahydrofuran or toluene (from 25 °C to 70°C), and without the need for an activator. Representative data was compiled in Table 3. The results indicate that compounds 1-4 were active single-component initiators for the ROP of LAs, however, compared with some reported salen or salan rare-earth amides [8-20], they are relatively less active. This may be attributed to the dimeric structure which can hinder the proximity of monomer to the rare-earth center and the cleavage of dimeric structure is also slow. Reaction medium effect is important to polymerization behavior. For instance, complex 2 polymerized 68% of the monomer at 50 °C after 6 h in THF; while the yield is only 46% in toluene even if the temperature was elevated to 70°C and reaction time was prolonged to 24 h (Table 3, entries 1 and 2). This is possibly because that the cleavage of initiators’ dimeric structure is easier in coordinative solvent THF, so the optimum solvent was THF; all experiments were conducted using L-LA or rac-LA in an initial concentration of 1 M, so as to enable accurate comparison between different initiators. In comparison with the ytterbium complex 2, the yttrium complexes 1, 3 and 4 displayed higher activities under otherwise identical conditions (Table 3, entry 3-12). This result is in accordance with the order of metallic covalent radius, which has been observed in some other
rare-earth catalytic systems for LAs polymerization.[15, 18, 44, 45] The yttrium complex 1 was more reactive than complexes 3 and 4 bearing ligands L2H2 and L3H2 with methyl group (Table 3, entry 3-12). We ascribe that to the introduction of higher electron-donating group which decreases the metal center’s Lewis acidity. Polymers obtained with the complexes 1−4 gave PDI values varying from 1.23 to 1.57 determined by GPC (gel permeation chromatography) analysis. Furthermore, the number-average molecular weight (Mn) are slightly higher than calculated ones relying on the basis that one polymer chain is from a single metal core. These facts illustrate that the aroylhydrazone rare-earth metal amides initiated the ROP of LAs in a not well-controlled manner. In the MALDI-TOF mass spectrum (Fig. 5), oligomers prepared by the reaction of complex 2 with rac-LA in a 1:20 molar ratio with the N(SiMe3)2 end cap could be found. The result showed that the silylamido groups acted as the initiating group in the ring-opening polymerization. Thus, the polymerization proceeds via a common “coordination−insertion” mechanism. Remarkably, the MALDI-TOF mass spectrum also showed that the there is a great degree of intermolecular transesterification side reactions, giving rise to series of chains separated by the mass of a lactic acid unit (72). The polymerization control could be improved by addition of 1 equiv of BnOH (Table 3, entry 9). The result met our expectations, for the initiating group silylamide which is less nucleophilic than alkoxide always caused a relatively slow initiation.[9, 17, 46] The stereochemical microstructures of the obtained PLA samples was determined by the methine region homonuclear decoupled 1H NMR spectra. As shown in Table 3,
ROP of rac-LA initiated by compounds 1−4 affords PLA all with moderate heterotacticity (Pr = 0.67−0.71), so there is no appreciable effect of ionic radii and the introduced
electron-donating
group
of
aroylhydrazone
ligands
on
the
stereocontrolability of rac-LA polymerizations. Influence of temperature on hetero-selectivity is not notable as the Pr value was same when the polymerization was conducted at a lower temperature (0 °C) (Table 3, entry 10).
Conclusions In summary, as the first example, we have synthesized structurally defined neutral phenolate-possessing aroylhydrazone lanthanide amide complexes through amine elimination reaction of the ligands with commonly used bulky precursors bis(trimethylsilyl)amido rare-earth. These rare-earth compounds were efficient catalysts for the polymerization of LAs, and in the case of rac-LA, a moderate heteroselective (Pr = 0.67−0.70) behavior was also investigated. Ionic radii and the introduced electron-donating substituents of aroylhydrazone ligands could affect the catalytic activity but no significant effect was observed on the stereocontrolability of rac-LA polymerizations. Research of mechanism of the ROP of LAs and synthesis of more related complexes is under progress currently.
Acknowledgements Financial support from the National Natural Science Foundation of China (Grants 21402138, 21502136 and 21571137) is gratefully acknowledged.
Appendix A. Supplementary data CCDC <1417043-1417045, 1479686> contains the supplementary crystallographic
data for compounds 1-4. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44-1223-336033; or e-mail: [email protected].
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Cheng, Z.-X. Huang, Synthesis, structure, and catalytic activity of tetracoordinate lanthanide amides [(Me3Si)2N]3Ln(µ-Cl)Li(THF)3 (Ln=Nd, Sm, Eu), Polyhedron 22 (2003) 1019-1024. [40] R.M. Moriarty, B.A. Berglund, M.S.C. Rao, Synthesis of 1,2-Diacylbenzenes from o-Hydroxyaryl Ketone Acylhydrazones Using [(Diacetoxy)iodo]benzene, Synthesis 1993 (1993) 318-321. [41] J.L. Atwood, W.E. Hunter, A.L. Wayda, W.J. Evans, Synthesis and crystallographic characterization of a dimeric alkynide-bridged organolanthanide: [(C5H5)2ErC.ident.CC(CH3)3]2, Inorg. Chem. 20 (1981) 4115-4119. [42] W.J. Evans, C.H. Fujimoto, J.W. Ziller, Synthesis and structure of inorganic and organometallic N,N′-bis(3, 5-di-tert-butylsalicylidene)ethylenediamine yttrium complexes, Polyhedron 21 (2002) 1683-1688. [43] O. Runte, T. Priermeier, R. Anwander, Synthesis of highly soluble yttrium-salen complexes and the X-ray structure of N,N[prime or minute]-bis(3,5-di-tert-butylsalicylidene)ethylenediamine-[bis(dimethylsily)amid o]yttrium, Chem. Commun. (1996) 1385-1386. [44] W. Li, Z. Zhang, Y. Yao, Y. Zhang, Q. Shen, Control of Conformations of Piperazidine-Bridged Bis(phenolato) Groups: Syntheses and Structures of Bimetallic and Monometallic Lanthanide Amides and Their Application in the Polymerization of Lactides, Organometallics 31 (2012) 3499-3511. [45] J. Zhang, J. Qiu, Y. Yao, Y. Zhang, Y. Wang, Q. Shen, Synthesis and Characterization of Lanthanide Amides Bearing Aminophenoxy Ligands and Their Catalytic Activity for the Polymerization of Lactides, Organometallics 31 (2012) 3138-3148. [46] R.H. Platel, A.J.P. White, C.K. Williams, Bis(phosphinic)diamido Yttrium Amide, Alkoxide, and Aryloxide Complexes: An Evaluation of Lactide Ring-Opening Polymerization Initiator Efficiency, Inorg. Chem. 50 (2011) 7718-7728.
Table 1 Crystallographic data for complexes 1-4 Compound
2(1·0.5Toluene)
2
3
4·2Toluene
Formula
C 103H151N 12O 12Si8 Y4
C48H72N6O6Si4 Yb2
C50H76N6O6Si4 Y2
C 66H 96N 6O6Si4 Y2
fw
2329.71
1287.55
1147.34
1359.66
T/K
273(2)
273(2)
293(2)
293(2)
Crystal system
monoclinic
monoclinic
monoclinic
triclinic
Crystal size/mm
0.60 x 0.50 x 0.30
0.30 x 0.23 x 0.20
0.30 x 0.26 x 0.20
0.25 x 0.15 x 0.13
Space group
C2
P2(1)/n
P2(1)/n
Pī
a/Å
20.3662(12)
15.4269(7)
10.7796(8)
11.9510(8)
b/Å
23.7320(14)
10.4331(4)
15.8791(10)
12.7391(8)
c/Å
12.6880(8)
17.3716(8)
17.0702(12)
13.9929(9) 84.879(2)
α/deg β/deg
99.347(2)
99.5740(10)
96.362(7)
71.068(2) 63.230(2)
γ/deg V/Å3
6051.1(6)
2757.0(2)
2903.9(4)
1795.0(2)
Z
4
2
2
1
Dcalcd/g cm-3
1.279
1.551
1.312
1.258
µ/mm-1
2.035
3.507
2.119
1.725
F(000)
2434
1292
1200
716
θmax/deg
25.500
27.555
29.629
27.508
Collected
53600
69527
18699
66026
Unique reflns
11259
6342
7094
8237
Obsd reflns [I >2.0σ(I)]
11687
5822
4385
6666
No. of variables
775
308
293
428
GOF
1.053
1.294
1.021
1.042
R
0.0425
0.0214
0.0681
0.0355
wR
0.1136
0.0641
0.1422
0.0719
Rint
0.0658
0.0388
0.0794
0.0703
Largest diff. peak, hole/e Å-3
0.690, -0.609
1.105, -1.468
1.262, -1.492
0.820, -0.485
Table 2 Selected bond lengths (Å) and bond angles (deg) for complexes 1-4 Bond lengths
1
2
3
4
Ln1-O1
2.314(4)
2.262(2)
2.281(3)
2.286(1)
Ln1-O2
2.233(4)
2.182(2)
2.192(3)
2.195(1)
Ln1-O3
2.432(4)
2.375(2)
2.423(3)
2.413(1)
Ln1-O4(O1A)
2.302(4)
2.252(2)
2.302(3)
2.284(1)
Ln1-N1
2.440(5)
2.382(2)
2.430(3)
2.444(2)
Ln1-N3
2.315(5)
2.205(2)
2.246(4)
2.241(2)
Bond angles
1
2
3
4
O1-Ln1-O3
146.53(14)
143.88(7)
141.29(10)
143.78(5)
O2-Ln1-N1
66.92(16)
66.88(8)
66.07(11)
66.61(5)
N1-Ln1-O1A
113.42(15)
114.43(7)
108.62(11)
114.62(5)
O1A-Ln1-N3
136.54(15)
138.29(8)
146.02(13)
136.29(6)
N3-Ln1-O2
121.35(16)
120.33(9)
113.18(13)
122.57(6)
Table 3 Polymerization of L-LA and rac-LA initiated by complexes 1-4a Entry
Monomer
Cat.
[M]0/[I]0
t
Yield (%)b
Mcc (×104)
Mn d (×104)
PDId
Pre
1f
L-LA
2
100
24 h
46
0.66
1.29
1.23
-
2
L-LA
2
100
6h
68
0.98
1.69
1.30
-
3
L-LA
2
100
17 h
80
1.15
2.13
1.46
-
4
L-LA
1
100
17 h
89
1.28
2.66
1.51
-
5
L-LA
3
100
17 h
86
1.25
3.33
1.56
-
6
L-LA
4
100
17 h
80
1.15
2.18
1.45
-
7
rac-LA
1
100
17 h
99
1.43
2.31
1.49
0.68
8
rac-LA
2
100
17 h
83
1.20
3.54
1.57
0.70
9g
rac-LA
2
100
41 h
66
0.95
1.08
1.37
0.71
10h
rac-LA
2
100
24 h
39
0.56
1.33
1.50
0.70
11
rac-LA
3
100
17 h
93
1.34
2.37
1.48
0.67
12
rac-LA
4
100
17 h
87
1.25
2.15
1.55
0.67
General polymerization conditions: THF as the solvent, [L-LA] = 1 mol/L, at 50 °C; [rac-LA] = 1 mol/L, at 25 °C. b Yield: weight of polymer obtained/weight of monomer used. c Mc = (144.13) ×[M]0/[I]0 × (polymer yield) (%). d Measured by GPC calibrated with standard polystyrene samples. a
Measured by homodecoupling 1H NMR spectroscopy at 25 °C. f In toluene, at 70 °C. g 1 equiv (vs entry 8) of BnOH was added. h At 0 °C. e
Fig. 1 ORTEP diagram of complex 1 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
Fig. 2 ORTEP diagram of complex 2 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
Fig. 3 ORTEP diagram of complex 3 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
Fig. 4 ORTEP diagram of complex 4 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
I nt ens. [ a. u. ]
x104 2. 0
1534. 1 1678. 2 1389. 9 1822. 3 1966. 4 1462. 0
1750. 3 1894. 4
2110. 5 2254. 6
1. 5
2182. 5
2398. 6 2542. 6
2686. 7
2614. 7
2830. 7 2902. 7 3046. 7 3118. 7
1. 0
3190. 6 3334. 6 3478. 6 3622. 5 3694. 5
3838. 5
3982. 4 4126. 3 4270. 34414. 2
0. 5
4558. 1
4702. 1 4846. 0
0. 0
1500
2000
2500
3000
3500
4000
4500
m/ z
Fig. 5 MALDI-TOF mass spectrum of rac-LA oligomer initiated by complex 2 ([rac-LA]0/2 = 20:1, in THF, 25 °C).
Research Highlights New neutral lanthanide complexes supported by phenolate-possessing aroylhydrazone were synthesized. The crystal structures of the lanthanide complexes were determined. The lanthanide complexes are efficient initiators for the ROP of rac-lactide. Heterotactic-rich polylactides are obtained.
Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie,a* Changan Wang,a Yinfeng Han,a Jianping Zhang a and Yingming Yao b
Graphical Abstract
Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie,a* Changan Wang,a Yinfeng Han,a Jianping Zhang a and Yingming Yao b
Graphical Abstract Phenolate-possessing
aroylhydrazone-type
ligand
was
introduced
in
organolanthanide chemistry for the first time, and a series of neutral lanthanide amido complexes were synthesized. It was found that these lanthanide complexes are efficient initiators for the ring-opening polymerization of rac-lactide, giving polylactides with moderate heterotacticity.
S0020-1693(17)30367-5 http://dx.doi.org/10.1016/j.ica.2017.06.031 ICA 17680
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
11 March 2017 11 June 2017 12 June 2017
Please cite this article as: K. Nie, C. Wang, Y. Han, J. Zhang, Y. Yao, Lanthanide complexes supported by phenolatepossessing aroylhydrazone: synthesis, characterization and lactides polymerization, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.06.031
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Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie,a* Changan Wang,a Yinfeng Han,a Jianping Zhanga and Yingming Yaob
a
School of Chemistry and Chemical Engineering, Taishan University, Taian, 271021, China
b
College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, China *Corresponding author (email: [email protected])
1. Introduction As one of the most widely used aliphatic linear polyesters, polylactide (PLA), is an attractive alternative to petrochemicalbased polymers due to the desirable biodegradability and the fact that the monomer can be prepared from sustainable raw materials.[1] PLA has widely biomedical and pharmaceutical applications, such as controlled drug delivery, resorbable sutures, medical implants and scaffolds for tissue engineering.[2] Depending on the stereochemistry of the LA monomer, the reaction conditions, and the selectivity of initiators, an array of PLA with different microstructures can be obtained, and now control of the polymer microstructure has been demonstrated to significantly affect the polymers’ mechanical and physical properties.[3-5] Catalytic ring-opening polymerization (ROP) of lactide is a powerful synthetic methodology to produce PLA with good control regarding molecular weight, polydispersity (PDI), and tacticity. A number of stereoselective catalysts vary widely in terms of the metal centers and ancillary ligands employed have been developed.[3-20] Among them, lanthanide compounds have proved to be highly efficient catalysts for the living ROP of lactides, and notably, metal ionic radii and subtle modifications of the ligand framework have a strong influence on the stereoselective ROP of rac-LA. For example, in 2014, Williams and co-workers have shown that metal-size influence in iso-selective lactide polymerization for the phosphasalen lutetium ethoxide complex shows excellent iso-selectivity (Pi = 0.81-0.84) while the corresponding lanthanide derivative exhibits moderate heteroselectivity (Ps = 0.74).[10, 20] Carpentier et al. have prepared some
methoxy-amino bridged bis(phenolate) lanthanide compounds, which depending upon the nature of the ortho substituents of bis(phenol)s, produce ethier heterotactically or atactic PLA.[9] To further understand the metal ionic radii and ancillary ligand backbone effects and achieve rapid selective rac-LA polymerization, design and synthesis of new, well-defined rare-earth initiators is still attractive and meaningful. Schiff bases and their transition metal complexes possessing noteworthy biological activities such as antibacterial, antifungal and anticancer have been extensively studied in many fields including coordination chemistry, analytical chemistry and biochemistry.[21-37] Among them, aroylhydrazones have attracted much attention mainly because such type of polydentate ligands could meet the central metal ions’ charge-neutrality and stereochemical requirements via facile tautomerisation.[28-37] Furthermore, heteroatom-substituted aroylhydrazones could offer more possibilities of coordination mode. However, to the best of our knowledge, there is no literature concerning the utilization of phenolate-possessing aroylhydrazone ligands in lanthanide organometallic chemistry, neither the relationship between their structures and catalytic performance for the ROP of lactides. Therefore, in this contribution, an array of novel rare-earth bis(trimethylsilyl)amido compounds stabilized by phenolate-possessing aroylhydrazone ligands were prepared and fully characterized, and also their subsequent polymeric performance toward L-LA and rac-LA were reported.
2. Experimental Section 2.1. General procedures.
All syntheses and manipulations were performed using standard Schlenk techniques or in a N2-filled glovebox. The solvents were distilled over sodium benzophenone ketyl under argon prior to use. HN(SiMe3)2 and n-BuLi are purchased from Alfa. HN(SiMe3)2
was
distilled
after
dried
over
CaH2
for
5
days.
Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 (Ln = Y, Yb)[38, 39] and the ligands L1H2 [L1 = (2-O-C6H4)CH=NNHCOC6 H5], L2H2 [L2 = (2-O-C6H4)CH=NNHCOC6 H4CH3] and L3H2 [L3 = (2-O-C6H4)C(Me)=NNHCOC6 H4CH3] were prepared following the procedures reported.[40] L-LA and rac-LA were recrystallized with dry toluene, and then was sublimed at 50 ºC under vacuum. EDTA titration was used for lanthanide analysis with an xylenol orange indicator and a hexamine buffer.[41] Elemental analysis were performed by direct combustion with a Carlo-Erba EA-1110 instrument. 1
H and 13C NMR spectra were recorded in a C6D6 or THF-d 8 solution for complexes 1,
3 and 4 with a Unity Varian spectrometer at ambient temperature. No resolvable NMR spectrum of complex 2 was obtained due to paramagnetism. Gel permeation chromatography (GPC) analyses were carried out on a Waters 1515 Breeze instrument using narrowly distributed polystyrenes as standards and THF was the eluent at a flow rate of 1.0 mL/min at 40 ºC. Microstructures of PLAs was determined by homodecoupling 1H NMR spectroscopy at 25 ºC in CDCl3 on a Unity Varian AC-400 spectrometer. 2.2. Preparation of {L1Y[N(SiMe3)2](THF)}2 (1). A THF solution of Y[N(SiMe3)2](µ-Cl)Li(THF)3 (7.9 mL, 2.50 mmol) was added to a THF solution of L1 H2 (10 mL, 0.60 g, 2.50 mmol) at room temperature. A color
change from colorless to light yellow was observed instantly. The mixture was stirred for 5 h, and then volatiles were removed under reduced pressure. The residue was extracted twice by hot toluene (15 mL), and the precipitate formed was removed by centrifugation. Yellow crystals were obtained at ambient temperature in several days (0.99 g, 71%). Anal. Calcd. For: C48H72N6O6Si4Y2: C, 51.51; H, 6.48; N, 7.51; Y, 15.89. Found: C, 51.18; H, 6.33; N, 7.89; Y, 15.77%. 1H NMR (400 MHz, C6D6 +THF-d 8, 25 °C): δ 8.58 (s, 2H, N=CHAr), 8.46 (d, J = 8.2 Hz, 4H, ArH), 7.33-7.14 (m, 10H, ArH), 6.84 (d, J = 7.9 Hz, 2H, ArH), 6.59 (t, J = 7.8 Hz, 2H, ArH), 3.55 (s, 8H, α-CH2 THF), 1.50 (s, 8H, β-CH2 THF), 0.41 (s, 36H, SiMe3). 13C{1H} NMR (100 MHz, THF-d8, 25 °C): δ 170.2 (N=CHAr), 129.9, 129.8, 129.1, 128.2, 127.7 (Ar-C), 68.4 (α-CH2, THF), 25.5 (β-CH2, THF), 2.9 (SiMe3). 2.3. Preparation of {L1Yb[N(SiMe3)2](THF)}2 (2). Complex 2 was prepared in the similar method as that described for complex 1, but Yb[N(SiMe3)2](µ-Cl)Li(THF)3 (5.9 mL, 0.92 mmol) was used instead of Y[N(SiMe3)2](µ-Cl)Li(THF)3. Yellow crystals were obtained in a toluene (10 mL) solution (0.44 g, 75%). Anal. Calcd. For: C48H72N6O6Si4Yb2: C, 44.78; H, 5.64; N, 6.53; Yb, 26.88. Found: C, 45.13; H, 5.40; N, 6.79; Y, 26.53%. 2.4. Preparation of {L2Y[N(SiMe3)2](THF)}2 (3). Complex 3 was prepared in the similar method as that described for complex 1, but L2H2 (0.44 g, 1.74 mmol) was used instead of L1H2. Yellow crystals were obtained in a toluene (15 mL) solution (0.67 g, 67%). Anal. Calcd. For: C50H76N6O6Si4Y2: C, 52.34; H, 6.68; N, 7.32; Y, 15.50. Found: C, 52.04; H, 6.90; N, 7.61 Y, 15.78%. 1H
NMR (400 MHz, THF-d 8, 25 °C): δ 8.50 (s, 2H, N=CHAr), 8.10-6.51 (m, 16H, ArH), 3.55 (s, 8H, α-CH2 THF), 2.34 (s, 6H, ArCH3), 1.69 (s, 8H, β-CH2 THF), 0.20 (s, 36H, SiMe3). 13C{1H} NMR (100 MHz, THF-d 8, 25 °C): δ 170.7 (N=CHAr), 132.4, 131.6, 129.5, 129.0, 124.5 (Ar-C), 68.4 (α-CH2, THF), 25.6 (β-CH2, THF), 18.6 (ArCH3), 3.0 (SiMe3). 2.5. Preparation of {L3Y[N(SiMe3)2](THF)}2 (4). Complex 4 was prepared in the similar method as that described for complex 1, but L3H2 (0.27 g, 1.02 mmol) was used instead of L1H2. Yellow crystals were obtained in a toluene (10 mL) solution (0.39 g, 65%). Anal. Calcd. For: C52H80N6O6Si4Y2: C, 53.14; H, 6.86; N, 7.15; Y, 15.13. Found: C, 52.66; H, 6.70; N, 7.52; Y, 15.78%. 1H NMR (400 MHz, THF-d8, 25 °C): δ 8.15-6.54 (m, 16H, ArH), 3.54 (s, 8H, α-CH2 THF), 2.75 (s, 6H, N=CCH3), 2.36 (s, 6H, ArCH3), 1.69 (s, 8H, β-CH2 THF), 0.01 (s, 36H, SiMe3). 13C{1H} NMR (100 MHz, THF-d8, 25 °C): δ 171.3 (N=C), 131.7, 131.1, 129.5, 129.2, 124.0, 120.6 (Ar-C), 68.4 (α-CH2, THF), 25.6 (β-CH2, THF), 21.7 (N=CCH3), 18.6 (ArCH3), 2.9 (SiMe3). 2.6. General procedure for the homopolymerization of L-LA or rac-LA. All complexes were tested as initiators of the polymerization of L-LA or rac-LA. In the glovebox, a 20 mL Schlenk reaction tube, equipped with a magnetic stirrer bar, was charged with appropriate quantity of monomer and solvent. After the monomer was dissolved, a solution of the initiator was added to this solution by syringe. The mixture was immediately stirred vigorously for certain time, during which time an increase in the viscosity was observed. The reaction mixture was quenched with
ethanol and then poured into a large quantity of ethanol to afford the polymer which were further dried under vacuum and weighed. 2.7. X-Ray crystallographic structure determination. Suitable single crystals of complexes 1-4 were sealed in a thin-walled glass capillary for determination the single-crystal structures. Intensity data were collected with a Rigaku Mercury CCD area detector in ω scan mode using Mo-Kα radiation (λ = 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. 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 full-matrix least-squares refinement. The structures were solved and refined using SHELEXL-97 programs.
3. Results and discussion 3.1. Synthesis and characterization of lanthanide aroylhydrazone complexes. The most efficient and straightforward synthetic approach to lanthanide amides is amine elimination reaction, and Ln[N(SiMe3)2]3, Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 or Ln[N(SiHMe2)2]3(THF)2 are usually the typical precursors. However, in terms of Schiff bases ligands, this synthetic method is often hampered by side reactions and it is worth noting that the choose of appropriate starting materials for preparing the
corresponding lanthanide amides is crucial. For example, Y[N(SiMe3)2]3 reacted with the ligand precursor SalenH2 to afford an oligomeric, THF insoluble product while using the
less
steric
demanding
bis(dimethylsilyl)amido
yttrium
complex
Y[N(SiHMe2)2]3(THF)2 gave the expected heteroleptic monometallic yttrium salen-type complex.[42, 43] When the phenolate-possessing aroylhydrazone ligand precursors were prepared straightforwardly by condensation reactions between salicylaldehyde
or 2-acetylphenol and
the
appropriate
aroylhydrazide,
the
corresponding lanthanide amido complexes were tried to synthesize. An NMR-scale reaction
of
Y[N(SiMe3)2]3(µ-Cl)Li(THF)3
(2-O-C6H4)CH=NNHCOC6 H5]
was
L1 H2
with
conducted
initially.
[L1
After
=
equimolar
Y[N(SiMe3)2]3(µ-Cl)Li(THF)3 was added to a L1H2 solution in THF-d 8, resonances at 11.04 and 9.10 ppm belonging to the OH and NH protons respectively almost disappeared, indicating the reaction did occur. At a preparative scale, the desired neutral complex {L1Y[N(SiMe3)2](THF)}2 (1) was isolated with good yield (71%), followed by recrystallization from toluene solution. Similarly, the reaction between Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 (Ln = Yb, Y) and phenolate-possessing
aroylhydrazone
ligands
L1H2
[L1
=
(2-O-C6H4)CH=NNHCOC6 H5], L2H2 [L2 = (2-O-C6H4)CH=NNHCOC6 H4CH3] and L3H2 [L3
=
aroylhydrazone
(2-O-C6H4)C(Me)=NNHCOC6 H4 CH3 ] lanthanide
amido
complexes
generated
the
dimeric
{L1Yb[N(SiMe3)2](THF)}2
(2),
{L2Y[N(SiMe3)2](THF)}2 (3) and {L3Y[N(SiMe3)2](THF)}2 (4) respectively in good yields (Scheme 1).
R1 O
R1
R2
N NH
Ln{N(SiMe 3) 2} 3 (µ−Cl)Li(THF) 3 THF, RT.
OH
- LiCl
R2
(Me 3 Si)2 N THF O Ln
N O
N
N
N
O Ln
R2
O THF N(SiMe 3 )2
R1 1
1
2
L H 2: R = H, R = H L 2H 2: R 1 = H, R2 = Me L 3H 2: R 1 = Me, R 2 = Me
1
R = H, R 2 = H, Ln = Y(1), Yb ( 2) R 1 = H, R 2 = Me, Ln = Y(3) R 1 = Me, R2 = Me, Ln = Y( 4)
Scheme 1 Synthesis of the complexes under investigation. All the resultant rare-earth compounds were completely characterized by elemental analysis and 1H,
13
C NMR spectroscopy except the ytterbium compound 2 due to
paramagnetism. 1H NMR spectra in THF-d8 showed a sharp single resonance in the field of δ 0-0.5 ppm assigned to the SiMe3 protons and resonances corresponding to the ligand were also observed. This suggested that the amide complex was indeed formed. The definitive solid-state molecular structures were authenticated by X-ray crystallography. Complexes 1-4 are sensitive to air and moisture whereas no signs of decomposition appear preserved in the glovebox at room temperature, and they are readily soluble in THF, but slightly soluble in toluene. 3.2. Crystal structures. Single crystals of compounds 1-4 suitable for the X-ray structure determination were grown from a toluene solution at ambient temperature. Crystallographic data and analysis results are listed in Table 1. Selected bond distances and angles of 1-4 are shown in Table 2, and their ORTEP drawings are depicted in Figures 1-4 respectively. X-Ray analyses revealed that compounds 1-4 are similar, and there is no difference between the two metal centers which were bridging through the phenolic oxygens. Obviously, the dimeric nature of these compounds arises from the insufficient
bulkiness of substituents on the ligands. Each metal center retains a ƙ1-amido, a ƙ1-THF and a ƙ3-dianionic aroylhydrazone ligand as well as an additional oxygen(aryloxide) from the other ligand that forms a µ bridge between the two metal centers. The coordination geometry around the metal center can be best described as a slightly distorted octahedron. The aroylhydrazone ligands have undergone keto-enol tautomerism and the electron delocalized within the anionic NNCO unit revealed by the N−N, C−N, C−O bond distances of anionic NNCO unit. In compound 1, the Y-O(phenolate) bond distance is 2.314(4) Å and the Y-O(aroylhydrazone) bond distance is 2.233(4) Å, which are a little longer than the corresponding bond distances of compound 2 [2.262(2), 2.182(2) Å]. This result is in accord with their metal ionic radii and similar consequences are also observed from the Ln-N, Ln-O(THF) bond lengths. The Y-O(phenolate) bond distance of 2.314(4) Å in complex 1 is also longer than 2.281(3), 2.286(1) Å found in complexes 3 and 4, this is probably because of different electronic and steric effects of the aroylhydrazone ligands. However, these effects influenced the other bond lengths little. The Ln-N(aroylhydrazone) bond distances [2.440(5) Å for 1, 2.382(2) Å for 2, 2.430(3) Å for 3, and 2.444(2) Å for 4] and the Ln-N(amide) bond distances (2.315(5) Å for 1, 2.205(2) Å for 2, 2.246(4) Å for 3, and 2.241(2) Å for 4) are comparable if the differences in ionic radius between the central lanthanide metals are considered. These values are also consistent with the corresponding bond lengths in some Schiff base complexes reported, such as L2Ln[N(SiMe3)2] [L = 3,5-Bu t2-2-O-C6H2CH=N-2,6-Pri2-C6H3, Ln = Y(2.467, 2.245 Å) and Yb(2.420, 2.219 Å)].[26]
3.3. ROP of L-LA and rac-LA by complexes 1-4. Performance of the new aroylhydrazone lanthanide amides in the ROP of L-LA and rac-LA were investigated, in order to understand the relationship between the catalysts’ structure and the ROP activity, controllability and stereoselectivity. The reactions were carried out in a 100:1 molecular ratio of LAs to catalysts 1-4 in tetrahydrofuran or toluene (from 25 °C to 70°C), and without the need for an activator. Representative data was compiled in Table 3. The results indicate that compounds 1-4 were active single-component initiators for the ROP of LAs, however, compared with some reported salen or salan rare-earth amides [8-20], they are relatively less active. This may be attributed to the dimeric structure which can hinder the proximity of monomer to the rare-earth center and the cleavage of dimeric structure is also slow. Reaction medium effect is important to polymerization behavior. For instance, complex 2 polymerized 68% of the monomer at 50 °C after 6 h in THF; while the yield is only 46% in toluene even if the temperature was elevated to 70°C and reaction time was prolonged to 24 h (Table 3, entries 1 and 2). This is possibly because that the cleavage of initiators’ dimeric structure is easier in coordinative solvent THF, so the optimum solvent was THF; all experiments were conducted using L-LA or rac-LA in an initial concentration of 1 M, so as to enable accurate comparison between different initiators. In comparison with the ytterbium complex 2, the yttrium complexes 1, 3 and 4 displayed higher activities under otherwise identical conditions (Table 3, entry 3-12). This result is in accordance with the order of metallic covalent radius, which has been observed in some other
rare-earth catalytic systems for LAs polymerization.[15, 18, 44, 45] The yttrium complex 1 was more reactive than complexes 3 and 4 bearing ligands L2H2 and L3H2 with methyl group (Table 3, entry 3-12). We ascribe that to the introduction of higher electron-donating group which decreases the metal center’s Lewis acidity. Polymers obtained with the complexes 1−4 gave PDI values varying from 1.23 to 1.57 determined by GPC (gel permeation chromatography) analysis. Furthermore, the number-average molecular weight (Mn) are slightly higher than calculated ones relying on the basis that one polymer chain is from a single metal core. These facts illustrate that the aroylhydrazone rare-earth metal amides initiated the ROP of LAs in a not well-controlled manner. In the MALDI-TOF mass spectrum (Fig. 5), oligomers prepared by the reaction of complex 2 with rac-LA in a 1:20 molar ratio with the N(SiMe3)2 end cap could be found. The result showed that the silylamido groups acted as the initiating group in the ring-opening polymerization. Thus, the polymerization proceeds via a common “coordination−insertion” mechanism. Remarkably, the MALDI-TOF mass spectrum also showed that the there is a great degree of intermolecular transesterification side reactions, giving rise to series of chains separated by the mass of a lactic acid unit (72). The polymerization control could be improved by addition of 1 equiv of BnOH (Table 3, entry 9). The result met our expectations, for the initiating group silylamide which is less nucleophilic than alkoxide always caused a relatively slow initiation.[9, 17, 46] The stereochemical microstructures of the obtained PLA samples was determined by the methine region homonuclear decoupled 1H NMR spectra. As shown in Table 3,
ROP of rac-LA initiated by compounds 1−4 affords PLA all with moderate heterotacticity (Pr = 0.67−0.71), so there is no appreciable effect of ionic radii and the introduced
electron-donating
group
of
aroylhydrazone
ligands
on
the
stereocontrolability of rac-LA polymerizations. Influence of temperature on hetero-selectivity is not notable as the Pr value was same when the polymerization was conducted at a lower temperature (0 °C) (Table 3, entry 10).
Conclusions In summary, as the first example, we have synthesized structurally defined neutral phenolate-possessing aroylhydrazone lanthanide amide complexes through amine elimination reaction of the ligands with commonly used bulky precursors bis(trimethylsilyl)amido rare-earth. These rare-earth compounds were efficient catalysts for the polymerization of LAs, and in the case of rac-LA, a moderate heteroselective (Pr = 0.67−0.70) behavior was also investigated. Ionic radii and the introduced electron-donating substituents of aroylhydrazone ligands could affect the catalytic activity but no significant effect was observed on the stereocontrolability of rac-LA polymerizations. Research of mechanism of the ROP of LAs and synthesis of more related complexes is under progress currently.
Acknowledgements Financial support from the National Natural Science Foundation of China (Grants 21402138, 21502136 and 21571137) is gratefully acknowledged.
Appendix A. Supplementary data CCDC <1417043-1417045, 1479686> contains the supplementary crystallographic
data for compounds 1-4. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44-1223-336033; or e-mail: [email protected].
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Table 1 Crystallographic data for complexes 1-4 Compound
2(1·0.5Toluene)
2
3
4·2Toluene
Formula
C 103H151N 12O 12Si8 Y4
C48H72N6O6Si4 Yb2
C50H76N6O6Si4 Y2
C 66H 96N 6O6Si4 Y2
fw
2329.71
1287.55
1147.34
1359.66
T/K
273(2)
273(2)
293(2)
293(2)
Crystal system
monoclinic
monoclinic
monoclinic
triclinic
Crystal size/mm
0.60 x 0.50 x 0.30
0.30 x 0.23 x 0.20
0.30 x 0.26 x 0.20
0.25 x 0.15 x 0.13
Space group
C2
P2(1)/n
P2(1)/n
Pī
a/Å
20.3662(12)
15.4269(7)
10.7796(8)
11.9510(8)
b/Å
23.7320(14)
10.4331(4)
15.8791(10)
12.7391(8)
c/Å
12.6880(8)
17.3716(8)
17.0702(12)
13.9929(9) 84.879(2)
α/deg β/deg
99.347(2)
99.5740(10)
96.362(7)
71.068(2) 63.230(2)
γ/deg V/Å3
6051.1(6)
2757.0(2)
2903.9(4)
1795.0(2)
Z
4
2
2
1
Dcalcd/g cm-3
1.279
1.551
1.312
1.258
µ/mm-1
2.035
3.507
2.119
1.725
F(000)
2434
1292
1200
716
θmax/deg
25.500
27.555
29.629
27.508
Collected
53600
69527
18699
66026
Unique reflns
11259
6342
7094
8237
Obsd reflns [I >2.0σ(I)]
11687
5822
4385
6666
No. of variables
775
308
293
428
GOF
1.053
1.294
1.021
1.042
R
0.0425
0.0214
0.0681
0.0355
wR
0.1136
0.0641
0.1422
0.0719
Rint
0.0658
0.0388
0.0794
0.0703
Largest diff. peak, hole/e Å-3
0.690, -0.609
1.105, -1.468
1.262, -1.492
0.820, -0.485
Table 2 Selected bond lengths (Å) and bond angles (deg) for complexes 1-4 Bond lengths
1
2
3
4
Ln1-O1
2.314(4)
2.262(2)
2.281(3)
2.286(1)
Ln1-O2
2.233(4)
2.182(2)
2.192(3)
2.195(1)
Ln1-O3
2.432(4)
2.375(2)
2.423(3)
2.413(1)
Ln1-O4(O1A)
2.302(4)
2.252(2)
2.302(3)
2.284(1)
Ln1-N1
2.440(5)
2.382(2)
2.430(3)
2.444(2)
Ln1-N3
2.315(5)
2.205(2)
2.246(4)
2.241(2)
Bond angles
1
2
3
4
O1-Ln1-O3
146.53(14)
143.88(7)
141.29(10)
143.78(5)
O2-Ln1-N1
66.92(16)
66.88(8)
66.07(11)
66.61(5)
N1-Ln1-O1A
113.42(15)
114.43(7)
108.62(11)
114.62(5)
O1A-Ln1-N3
136.54(15)
138.29(8)
146.02(13)
136.29(6)
N3-Ln1-O2
121.35(16)
120.33(9)
113.18(13)
122.57(6)
Table 3 Polymerization of L-LA and rac-LA initiated by complexes 1-4a Entry
Monomer
Cat.
[M]0/[I]0
t
Yield (%)b
Mcc (×104)
Mn d (×104)
PDId
Pre
1f
L-LA
2
100
24 h
46
0.66
1.29
1.23
-
2
L-LA
2
100
6h
68
0.98
1.69
1.30
-
3
L-LA
2
100
17 h
80
1.15
2.13
1.46
-
4
L-LA
1
100
17 h
89
1.28
2.66
1.51
-
5
L-LA
3
100
17 h
86
1.25
3.33
1.56
-
6
L-LA
4
100
17 h
80
1.15
2.18
1.45
-
7
rac-LA
1
100
17 h
99
1.43
2.31
1.49
0.68
8
rac-LA
2
100
17 h
83
1.20
3.54
1.57
0.70
9g
rac-LA
2
100
41 h
66
0.95
1.08
1.37
0.71
10h
rac-LA
2
100
24 h
39
0.56
1.33
1.50
0.70
11
rac-LA
3
100
17 h
93
1.34
2.37
1.48
0.67
12
rac-LA
4
100
17 h
87
1.25
2.15
1.55
0.67
General polymerization conditions: THF as the solvent, [L-LA] = 1 mol/L, at 50 °C; [rac-LA] = 1 mol/L, at 25 °C. b Yield: weight of polymer obtained/weight of monomer used. c Mc = (144.13) ×[M]0/[I]0 × (polymer yield) (%). d Measured by GPC calibrated with standard polystyrene samples. a
Measured by homodecoupling 1H NMR spectroscopy at 25 °C. f In toluene, at 70 °C. g 1 equiv (vs entry 8) of BnOH was added. h At 0 °C. e
Fig. 1 ORTEP diagram of complex 1 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
Fig. 2 ORTEP diagram of complex 2 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
Fig. 3 ORTEP diagram of complex 3 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
Fig. 4 ORTEP diagram of complex 4 showing an atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity.
I nt ens. [ a. u. ]
x104 2. 0
1534. 1 1678. 2 1389. 9 1822. 3 1966. 4 1462. 0
1750. 3 1894. 4
2110. 5 2254. 6
1. 5
2182. 5
2398. 6 2542. 6
2686. 7
2614. 7
2830. 7 2902. 7 3046. 7 3118. 7
1. 0
3190. 6 3334. 6 3478. 6 3622. 5 3694. 5
3838. 5
3982. 4 4126. 3 4270. 34414. 2
0. 5
4558. 1
4702. 1 4846. 0
0. 0
1500
2000
2500
3000
3500
4000
4500
m/ z
Fig. 5 MALDI-TOF mass spectrum of rac-LA oligomer initiated by complex 2 ([rac-LA]0/2 = 20:1, in THF, 25 °C).
Research Highlights New neutral lanthanide complexes supported by phenolate-possessing aroylhydrazone were synthesized. The crystal structures of the lanthanide complexes were determined. The lanthanide complexes are efficient initiators for the ROP of rac-lactide. Heterotactic-rich polylactides are obtained.
Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie,a* Changan Wang,a Yinfeng Han,a Jianping Zhang a and Yingming Yao b
Graphical Abstract
Lanthanide complexes supported by phenolate-possessing aroylhydrazone: synthesis, characterization and lactides polymerization Kun Nie,a* Changan Wang,a Yinfeng Han,a Jianping Zhang a and Yingming Yao b
Graphical Abstract Phenolate-possessing
aroylhydrazone-type
ligand
was
introduced
in
organolanthanide chemistry for the first time, and a series of neutral lanthanide amido complexes were synthesized. It was found that these lanthanide complexes are efficient initiators for the ring-opening polymerization of rac-lactide, giving polylactides with moderate heterotacticity.