Article pubs.acs.org/IC
Comparative Study of Aluminum Complexes Bearing N,O- and N,SSchiff Base in Ring-Opening Polymerization of ε‑Caprolactone and L‑Lactide Meng-Chih Chang,†,‡ Wei-Yi Lu,† Heng-Yi Chang,† Yi-Chun Lai,† Michael Y. Chiang,*,†,‡ Hsing-Yin Chen,*,† and Hsuan-Ying Chen*,† †
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan 80708, R.O.C. Department of Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan 80424, R.O.C.
‡
S Supporting Information *
ABSTRACT: A series of Al complexes bearing Schiff base and thio-Schiff base ligands were synthesized, and their application for the ring-opening polymerization of εcaprolactone (CL) and L-lactide (LA) was studied. It was found that steric effects of the ligands caused higher polymerization rate and most importantly the Al complexes with N,S-Schiff base showed significantly higher polymerization rate than Al complexes with N,O-Schiff base (5−12-fold for CL polymerization and 2−7-fold for LA polymerization). The reaction mechanism of CL polymerization was investigated by density functional theory (DFT). The calculations predicted a lower activation energy for a process involved with an Al complex bearing an N,S-Schiff base ligand (17.6 kcal/mol) than for that of an Al complex bearing an N,O-Schiff base ligand (19.0 kcal/mol), and this magnitude of activation energy reduction is comparable to the magnitude of rate enhancement observed in the experiment. The reduction of activation energy was attributed to the catalyst−substrate destabilization effect. Using a sulfur-containing ligand to decrease the activation energy in the ring-opening polymerization process may be a new strategy to design a new Al complex with high catalytic activity.
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INTRODUCTION Poly(lactide) (PLA) and poly(ε-caprolactone) (PCL) are popular materials because of their biodegradability, biocompatibility, and permeability, and they have demonstrated extensive applicability in a variety of fields.1 Ring-opening polymerization (ROP) is the popular means for the synthesis of PCL and PLA. Many metal complexes have been used as catalysts for the ROP of cycloesters, and Al complexes are the most common choice as catalysts for ROP due to its ease in synthesis and low cost for precursors. There are various ligands including Schiff base,2 aminophenolate,3 ketiminate,4 Salen,5 enolic Salen,6 8-quinolinolate,7 and β-diketiminate8 ligands used to synthesize Al catalysts in ROP. Among these Al complexes, Al complexes2 bearing Schiff base ligands have attracted the most interest presumably because many forms of diverse substituents may be synthesized easily. The steric and electronic influences of phenolate or phenylimino group on these Al complexes bearing Schiff base ligands are thoroughly discussed in the literature in terms of their connection with catalytic activity in ROP. However, no literature has reported about the synthesis of Al complexes with thio-Schiff base ligands and their application in ROP. There was some research9a,b reported that Zr complexes with a thioether donor of ligands revealed higher catalytic activity than that of an ether donor in olefin polymerization. The ligands with a thioether donor were also used to associate with group 4 metals,9c−k Al,9l−n In,9o,p and group 3 metal9q−t in © XXXX American Chemical Society
ROP. The electronegativity of oxygen (3.44) is larger than that of sulfur (2.58), and the atomic radius of sulfur is approximately twice larger than oxygen. These differences between oxygen and sulfur should make their corresponding Al complexes exhibit different catalytic behavior. Herein, the syntheses of Al complexes with thio-Schiff base ligands together with their catalytic studies are reported. The effects on ROP catalysis due to the S/O swap in ligands on catalysts are discussed.
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RESULTS AND DISCUSSION Synthesis and Characterization of Al Complexes. Thiosalicylaldehyde3 was prepared through the deprotonation of benzenethiol by nBuLi and substitution reaction of DMF. The thio-Schiff base ligands were formed after condensation reactions with alkylamine. All ligands reacted with a stoichiometric quantity of alkylaluminum reagents in toluene to produce Al compounds (Figure 1). The comparative Al complexes bearing N,O-Schiff base were also synthesized through the above method. The formula and structure were confirmed by 1H and 13C NMR spectra, elemental analysis, and X-ray crystal analysis. The X-ray structure of S2AlMe2 (Figure 2, CCDC 1409254) illustrated the distorted tetrahedral geometry of the Al complex with the two methyl groups. The Al atom sits Received: August 17, 2015
A
DOI: 10.1021/acs.inorgchem.5b01858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Synthesis of ligands and their Al complexes.
[N,O] catalyst is obvious as it took only 1.5 h to reach 90% conversion of PCL for catalyst S2AlMe2 while it took 21 h for O2AlMe2. Moreover when t-butyl group (as in S2AlMe2 and O2AlMe2) was replaced by benzhydryl group (as in S1AlMe2 and O1AlMe2), enhanced catalytic activity was observed (entry 2 versus 1, and entry 4 versus 3). This implied that the steric effect by the bulkiness of the N-substituent also increased the catalytic activity. The result of LA polymerization study at 70 °C is showed in Table 2, and the catalytic trend is similar to CL polymerization. Both the results of CL and LA polymerization using these Al complexes as catalysts showed great control of polymer molecule weight with comparable value seen for MnNMR and MnGPC and narrow PDI. Obviously these [N,S] catalysts fulfill the requirements for good ROP catalysts. The controllable CL and LA polymerization using S2AlMe2 is evident by the linear relationship between Mn(GPC) and [monomer]0 × conversion/[BnOH]0 (Table 3 and Figures 3 and 4), as well as by the low PDIs (1.01−1.09 for PCL and 1.10−1.21 for PLA). Kinetic Study of ε-Caprolactone and L -Lactide Polymerization by [N,S] and [N,O] Al Complexes. The kinetic study on CL polymerization at room temperature and LA polymerization at 70 °C by [N,S] and [N,O] Al complexes is shown in Table 4 and Figure 5. The [N,S] Al catalysts showed significantly higher polymerization rate than [N,O] Al catalysts (5−12-fold for CL polymerization and 2−7-fold for LA polymerization). The increase of polymerization rate from t butyl to benzhydryl group in the N-substituent was 3-fold in CL polymerization and 6-fold in LA polymerization for the [N,O] Al catalysts, but no parallel result was found for [N,S] Al catalysts. Diethyl Al complex had greater activity than dimethyl Al complex. Some papers2x,11a−d reported that Al complexes
Figure 2. Molecular structures of S2AlMe2 with 30% probability ellipsoids (all of the hydrogen atoms were omitted for clarity) [d(Al− S) = 2.2690(7) Å, d(Al−N) = 1.98910(15) Å, d(Al−C12) = 1.968(2) Å, d(Al−C13) = 1.9655(18) Å; ∠S−Al−N = 97.66(4)°, ∠C13−Al− C12 = 119.25(9)°].
1.129 Å above the phenyl ring plane, which is higher than that in the N,O-Schiff base Al complexes2 (0.00 Å to 0.927 Å). This feature enlarged the space for potential substrate binding and might have contributed to the enhanced catalytic activity for N,S-Schiff base Al complexes (vide inf ra). Polymerization of ε-Caprolactone and L-Lactide. Polymerizations of ε-caprolactone (CL) and L-lactide (LA) were investigated by using Al complexes and two equivalents of BnOH as an initiator in toluene (Tables 1 and 2). As shown in entries 1−4 of Table 1, the positive effect of [N,S] catalyst over
Table 1. Polymerization of CL Catalyzed by [N,O] and [N,S] Al Complexesa
entry
cat.
time (min)
convnb (%)
MnCalc (g mol−1)
MnNMRb (g mol−1)
MnGPCd (g mol−1)
PDId
1 2 3 4 5
O2AlMe2 O1AlMe2 S2AlMe2 S1AlMe2 S1AlEt2
1260 360 90 80 80
94 89 90 92 97
5500 5200 5200 5400 5600
6700 4000 5800 4600 4400
5900 3100 5100 3300 3200
1.15 1.08 1.04 1.05 1.04
a
Reaction conditions: toluene (5 mL), [M]0/[Cat.]0/[BnOH]0 = 100:1:2, [CL] = 2.0 M, at room temperature. bObtained from 1H NMR analysis. Calculated from the molecular weight of monomer × [monomer]0/[BnOH]0 × conversion yield + Mw(BnOH). dObtained from GPC analysis and calibration based on the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.56. c
B
DOI: 10.1021/acs.inorgchem.5b01858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Polymerization of LA Catalyzed by [N,O] and [N,S] Al Complexesa
cat.
time (min)
convnb (%)
MnCalc (g mol−1)
MnNMRb (g mol−1)
MnGPCd (g mol−1)
PDId
O AlMe2 O1AlMe2 S2AlMe2 S1AlMe2 S1AlEt2
1080 230 200 80 40
93 92 94 90 91
6800 6700 6900 6600 6700
4500 5000 6400 6100 6900
3600 4900 5700 5900 5600
1.12 1.04 1.04 1.07 1.01
entry 1 2 3 4 5
2
Reaction conditions: toluene (5 mL), [M]0/[Cat.]0/[BnOH]0 = 100:1:2, [LA] = 2.0 M, at 70 °C. bObtained from 1H NMR analysis. cCalculated from the molecular weight of monomer × [monomer]0/[BnOH]0 × conversion yield + Mw(BnOH). dObtained from GPC analysis and calibration based on the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.58. a
Table 3. Results of Controlled CL and LA Polymerizations Using S2AlMe2 as Catalysta entry e
1 2e 3e 4e 5f 6f 7f 8f
[M]/[Al]/ [BnOH]
MnCalc (g mol−1)
MnNMRb (g mol−1)
MnGPCd (g mol−1)
PDId
50:1:2 150:1:2 200:1:2 300:1:2 50:1:2 75:1:2 125:1:2 150:1:2
3000 8600 11500 17200 3700 5500 9100 11000
3500 8400 12900 21500 2500 4400 4800 6800
3600 8000 11300 19600 2300 3700 5200 7100
1.08 1.01 1.05 1.09 1.10 1.20 1.21 1.13
a
Conversions of all polymerizations are >99%. bObtained from 1H NMR analysis. cCalculated from the molecular weight of monomer × [monomer]0/[BnOH]0 × conversion yield + Mw(BnOH). dObtained from GPC analysis and calibration based on the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.56 for PCL and 0.58 for PLA. eReaction condition: toluene (5 mL), [Al] = 0.02 M, at room temperature for CL polymerization. fReaction condition: toluene (5 mL), [Al] = 0.02 M, at 70 °C for LA polymerization.
Figure 4. Linear plot of Mn(GPC) vs [LA]0 × conversion/[BnOH], with polydispersity indexes (PDI) indicated by closed circles (Table 3, entries 5−8).
Table 4. Observed Rate Constant (kobs) for Polymerization of CL and LA Catalyzed by [N,O] and [N,S] Al Complexes with 2 equiv of BnOH in Toluene (at Room Temperature for CL and 70 °C for LA)a kobs (error) 10−3 min−1 entry 1 2 3 4 5
catalyst 2
O AlMe2 O1AlMe2 S2AlMe2 S1AlMe2 S1AlEt2
CL 2.09 6.58 27.83 37.19 40.64
LA
(1) (24) (175) (225) (134)
2.51 12.49 15.28 26.93 59.04
(6) (48) (96) (166) (478)
a Observed kobs was the slope of first-order kinetic plot of εcaprolactone and L-lactide polymerization with time. Conversion of ε-caprolactone and L-lactide with time was monitored by 1H NMR.
maybe one of the alkyl groups is replaced by BnOH and the other remains as a ligand during the polymerization process.11h−j To see if this happens in our case, the 1H NMR spectrum of the mixture of S1AlMe2 and BnOH (1:2) was studied (Figure S24). The results revealed that the methyl groups disappeared. It means that both alkyl groups on Al atom are replaced by BnOHs and they have no influence on the polymerization rate. It is unclear so far why Al complexes with ethyl groups had higher catalytic activity than those with methyl groups.
Figure 3. Linear plot of Mn(GPC) vs [CL]0 × conversion/[BnOH], with polydispersity indexes (PDI) indicated by closed circles (Table 3, entries 1−4).
with ethyl groups had higher catalytic activity than those with methyl groups, but the opposite phenomenon was also reported without expanation.2f,11e−g A possible reason for the influence of alkyl groups on the polymerization rate is that C
DOI: 10.1021/acs.inorgchem.5b01858 Inorg. Chem. XXXX, XXX, XXX−XXX
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is indeed a transition state in electronic energy surface, free energy corrections make it lower in energy than the intermediate c. From the calculated free energy profile, it can be clearly seen that the overall free energy of activation is determined by the highest-energy transition state TSbc and the lowest-energy reactant adduct a. The overall activation free energy for the S2Al(OMe)2 system was estimated to be 17.6 kcal/mol, smaller than the 19.0 kcal/mol for the O2Al(OMe)2 system. With the DFT predicted free energies of activation, and the experimental conditions that the polymerization reaction was conducted at room temperature and [catalyst] = 0.02 M, the theoretical kobs was roughly estimated to be 1.6 × 10−2 and 1.5 × 10−3 s−1 for S2Al(OMe)2 and O2Al(OMe)2, respectively, by using transition-state theory. Although the theoretical values of kobs are somewhat larger compared to the experimental measurements (about 1 order of magnitude), the theoretical ratio of kobs of S2Al(OMe)2 to O2Al(OMe)2 is 11, agreeing well with the experimental ratio of 13. The calculated free energy profiles indicate that while the stabilities of the transition state TSbc for both systems are very close (difference of 1 kcal/mol), the lactone binds more loosely with S2Al(OMe)2 than with O2Al(OMe)2 in reactant adduct a (difference in 2.4 kcal/mol), which in turn leads to the reduction of 1.4 kcal/mol in the activation free energy. In other words, it is a catalyst−substrate destabilization effect that renders S2Al(OMe)2 to display a relatively higher reactivity. The weaker binding of reactant adduct a for [N,S] than [N,O] catalyst can be simply rationalized by chemical intuition. Since the electronegativity of sulfur is smaller than that of oxygen, the Al center of S2Al(OMe)2 should possess less positive charge and, thus, form a weaker bonding with lactone in comparison with O2Al(OMe)2. This inference is supported by the natural population analysis (NPA); the NPA charge of Al center is 1.841 au for S2Al(OMe)2 and 2.072 au for O2Al(OMe)2, respectively. However, the reason that gives rise to the reduction of the stability difference in the transition state is not so clear. One of the possible factors should be the steric effect. In the TSbc, the lactone becomes bidentate bonding with Al and, therefore, has a more crowded Al environment compared to the reactant adduct a. From the X-ray analysis we know that Al is further away from the ligand plane in [N,S] complex. This gives its Al center a larger room to accommodate lactone substrate, which in turn reduces the steric hindrance to its transition state TSbc. To verify this inference, we performed the natural steric analysis12 using NBO 6.0 program.13 The results showed that for the [N,O] system the total steric exchange energy is going from 1484.36 for the reactant adduct a to 1642.85 kcal/mol for TSbc with increment of 158.49 kcal/ mol. In contrast, for the [N,S] system the total steric exchange energy is going from 1532.29 for the reactant adduct a to 1684.56 kcal/mol for TSbc with increment of 152.27 kcal/mol. Further analysis of pairwise steric exchange interaction revealed that the smaller increment of steric repulsion for the [N,S] system mainly originates from the interligand interaction rather than from intraligand interaction (see Supporting Information).
Figure 5. First-order kinetic plots for (a) CL and (b) LA polymerizations by various Al complexes (green ▲, S1AlEt2; blue ◆, S1AlMe2; purple ×, S2AlMe2; red ■, O1AlMe2; brown *, O2AlMe2).
Comparative Study of the Polymerization Mechanism between [N,O] and [N,S] Al Complexes by DFT Calculation. To understand the reason why N,S-Schiff base Al complexes had greater catalytic activity than N,O-Schiff base Al complexes, DFT calculations were employed to investigate the reaction mechanism of CL polymerization catalyzed by O2AlMe2 and S2AlMe2. It is well accepted that the Al complexes react with benzyl alcohols to form the real catalysts before polymerization takes place.2l,t,4b The exchange between the methyl groups and benzyl alcohols is relatively fast because of fast acid−base reaction, indicating that the activation of the catalysts should not play a dominating role in kinetics. We therefore directly started from the reaction of lactone with activated catalysts O2Al(OMe)2 and S2Al(OMe)2 where the benzyl oxide was replaced by methoxide in order to reduce computational time. The proposed reaction pathway and the calculated free energy profiles of ring-opening polymerization from DFT calculations are illustrated in Figure 6. The first step is the association of lactone and Al catalyst. The calculations revealed that the lactone can be coordinated to Al via carbonyl oxygen from the opposite site of imine nitrogen, resulting in the formation of a trigonal bipyramidal five-coordinated Al complex a. One of the methoxide groups then undergoes a nucleophilic attack on carbonyl carbon to form the intermediate b. Subsequently, the lactone rearranges to switch from monodentate bonding mode in b to bidentate bonding mode in c, which is immediately followed by the ring opening and the recovery of the active state d. The tetrahedral four-coordinated complex d can accommodate another lactone like structure a and follow the aforementioned steps to repeat the ring-opening polymerization. It should be mentioned that although the TScd
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CONCLUSIONS A series of Al complexes bearing N,O- and N,S-Schiff base ligands were synthesized, and their comparison in CL and LA polymerization was studied. The Al complexes with N,S-Schiff base showed significantly higher polymerization rate than Al complexes with N,O-Schiff base in both the CL polymerization D
DOI: 10.1021/acs.inorgchem.5b01858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 6. B3LYP-D3/6-311+G**//B3LYP-D3/6-31G* free energy profile for ring-opening polymerization of ε-caprolactone catalyzed by N,OSchiff base (black) and N,S-Schiff base (red) Al complexes. The relative free energies are in kcal/mol. The selected bond lengths (in Å) are also shown. recorded on a Varian Gemini 2000-200 (200 MHz for 1H and 50 MHz for 13C) spectrometer with chemical shifts given in ppm from the internal TMS or center line of CDCl3. Microanalyses were performed using a Heraeus CHN-O-RAPID instrument. GPC measurements were performed on a Jasco PU-2080 PLUS HPLC pump system equipped with a differential Jasco RI-2031 PLUS refractive index detector using THF (HPLC grade) as an eluent (flow rate 1.0 mL/ min, at 40 °C). The chromatographic column was JORDI Gel DVB 103 Å, and the calibration curve was made by primary polystyrene standards to calculate Mn(GPC). Thiosalicylaldehyde10,14 and ligands of O1−H,15 O2−H,16 S2−H17 and O2AlMe22d were prepared following literature procedures. Synthesis of S1-H. A mixture of thiosalicylaldehyde (2.76 g, 20 mmol) and benzhydrylamine (3.66 g, 20 mmol) was stirred for 2 h in THF (30 mL). THF of the solution was removed under vacuum to give a red powder, and then hexane (50 mL) was transferred to the washed red powder 3 times to give the red powders. Yield: 1.78 g (27%). 1H NMR (CDCl3, 200 MHz): 8.19 (1H, d, J = 9 Hz, NCH), 7.68−6.86(14 H, m, Ar), 5.98 (1H, s, CHPh2) ppm. 13C NMR (CDCl3, 50 MHz): δ 165.07 (NC), 138.89 (C-CH-C), 137.38 (CCN), 136.53 (o-C-ArSH), 130.87 (o-C-Ar-CN), 129.01 (m-CPhCH), 128.25 (C-SH), 127.19 (o-C-PhCH), 126.93 (p-C-PhCH), 124.48 (m-C-ArSH), 120.30 (p-C-ArSH), 71.05 (CHPh2) ppm. Elemental anal. found (calcd) for S1-H, C20H17NS: N, 3.77 (4.62); C, 79.10 (79.17); H, 5.20 (5.65) %. Synthesis of O1AlMe2. A mixture of O1−H (2.87 g, 10 mmol) and AlMe3 (6 mL, 2.0 M, 11 mmol) in the mixed solvent of toluene (10 mL) and THF (3 mL) was stirred for 1 h at 0 °C. Volatile materials were removed under vacuum to give yellow powder, and then hexane
and LA polymerization. Mechanistic study by DFT calculation indicated that the reaction rate is determined by the key step of switching from monodentate bonding mode to bidentate bonding mode between lactone and Al. The activation energies were calculated to be 17.6 and 19.0 kcal/mol for N,S- and N,OSchiff base catalysts, respectively. When converted to kobs values, they agree well with the rate enhancement observed by experiment (vide supra). Replacement of N,O-Schiff base ligand by N,S-Schiff base ligand causes a relatively larger destabilization for the reactant adduct than for the transition state and hence reduces the activation energy. This is a consequence of complicated interplay between electronic and steric effects. Our new approach on swapping oxygen donor for sulfur donor in Schiff base Al catalysis has proven to be successful. We hope this work may intrigue other scientists to design new Al catalysts bearing sulfur donor atom for ringopening polymerization.
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EXPERIMENTAL SECTION
Standard Schlenk techniques and a N2-filled glovebox were used throughout the isolation and handling of all the compounds. Solvents, ε-caprolactone, and deuterated solvents were purified prior to use. Thiophenol was purchased from Showa. Deuterated chloroform, εcaprolactone, and dimethyl aluminum cholide were purchased from Acros. Benzyl alcohol, N,N,N′,N′-tetramethylethylenediamine, tertbutylamine, benzhydrylamine, trimethylaluminum, and triethylaluminum were purchased from Alfa Aesar. 1H and 13C NMR spectra were E
DOI: 10.1021/acs.inorgchem.5b01858 Inorg. Chem. XXXX, XXX, XXX−XXX
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calculations21 were performed for the transition state structures TSab and TScd to confirm that they connect to the corresponding intermediates. However, the IRC calculations for the transition state TSbc encountered a convergence problem. In this case, we used the visualization program GaussView to change the TSbc structure a little bit along the imaginary vibrational vector and reoptimized the geometry to confirm its proper connectivity. The information on charge distribution and steric exchange repulsion was obtained by NBO 6.0 program.13
(30 mL) was transferred to be the suspension. The yellow powder was obtained after filtering. Yield: 1.37 g (60%). 1H NMR (CDCl3, 200 MHz): 7.98 (1H, s, NCH), 7.47−6.82 (14 H, m, Ar), 6.30 (1H, s, CHPh2), −0.94 (s, 6H, Al(CH3)2) ppm. 13C NMR (CDCl3, 50 MHz): δ 171.08 (NCH), 146.60 (C-CH-C), 137.16 (C-CN), 136.92 (oC-ArS), 136.32 (o-C-Ar-CN), 133.07 (m-C-PhCH), 129.90 (C-S), 129.29 (o-C-PhCH), 129.05 (p-C-PhCH), 128.72 (m-C-ArS), 123.32 (p-C-ArS), −10.08 (Al(CH3)2) ppm. Elemental anal. found (calcd) for O1AlMe2·THF, C26H30AlNO2: N, 3.77 (3.37); C, 75.03 (75.16); H, 7.60 (7.28) %. Mp: 240 °C. Synthesis of S1AlMe2. A method similar to that used for O1AlMe2 was employed. Yield: 1.40 g (39%). 1H NMR (CDCl3, 200 MHz): 8.02 (1H, s, NCH), 7.66−7.05 (14 H, m, Ar), 6.51 (1H, s, CHPh2), −0.86 (s, 6H, Al(CH3)2) ppm. 13C NMR (CDCl3, 50 MHz): δ 172.29 (NCH), 146.60 (C-CH-C), 137.16 (C-CN), 136.92 (o-C-ArS), 136.32 (o-C-Ar-CN), 133.07 (m-C-PhCH), 129.90 (C-S), 129.29 (o-C-PhCH), 129.05 (p-C-PhCH), 128.72 (m-C-ArS), 123.32 (p-CArS), 71.95 (CHPh2), −9.31 (Al(CH3)2) ppm. Elemental anal. found (calcd) for S1AlMe2, C22H22AlNS: N, 3.53 (3.90); C, 73.71 (73.51); H, 6.07 (6.17) %. Mp: 258 °C. Synthesis of S2AlMe2. A method similar to O1AlMe2 was employed. Yield: 1.85 g (40%). 1H NMR (CDCl3, 200 MHz): 8.33 (1H, s, NCH), 7.61 (1H, d, J = 6 Hz, Ar), 7.24−7.35 (2 H, m, Ar), 7.12 (1 H, t, J = 12 Hz), 1.58 (9 H, s, C(CH3)3), −0.65 (s, 6H, Al(CH3)2) ppm. 13C NMR (CDCl3, 50 MHz) δ 167.46 (NCH), 145.66 (C-CN), 136.50 (o-C-ArS), 135.82 (o-C-Ar-CN), 132.55 (m-C-ArS), 130.34 (p-C-ArS), 123.33 (C-S), −6.98 (Al(CH3)2) ppm. Elemental anal. found (calcd) for S2AlMe2, C13H20AlNS: N, 5.64 (5.62); C, 62.39 (62.62); H, 7.80 (8.08) %. Mp: 220 °C. The X-ray structure of a suitable single crystal of S2AlMe2 (Figure 2), grown via slow evaporation of a CH2Cl2 solution of the compound, illustrates the distorted tetrahedral geometry of the Al complex with the two methyl complex. Synthesis of S1AlEt2. A method similar to O1AlMe2 was employed, but triethylaluminum in place of trimethylaluminum was used as the Al reagent. However, S1AlEt2 is oil and cannot be purified with hexane. Yield: 1.98 g (51%). 1H NMR (CDCl3, 200 MHz): 8.02 (1H, s, NCH), 7.41−6.99 (14 H, m, Ar), 6.48 (1H, s, CHPh2), 0.83 (6 H, t, Al(CH2CH3)2), −0.20 (q, 4H, Al(CH2CH3)2) ppm. 13C NMR (CDCl3, 50 MHz): δ 172.39 (NCH), 146.85 (C-CH-C), 137.07 (CCN), 136.91 (o-C-ArS), 136.22 (o-C−Ar-CN), 132.97 (m-CArS), 129.87 (m-C-PhCH), 129.13 (o-C-PhCH), 128.68 (p-C-PhCH), 127.61 (p-C-ArS), 123.17 (C-S), 71.78 (CHPh 2), 8.99 (Al(CH2CH3)2), 0.54 (Al(CH2CH3)2) ppm. Elemental anal. found (calcd) for S1AlEt2, C24H26AlNS: N, 3.63 (3.77); C, 77.71 (77.60); H, 6.97 (7.06) %. General Procedures for the Polymerization of ε-Caprolactone. A typical polymerization procedure was exemplified by the synthesis of entry 4 (Table 1) using complex S1AlMe2 as a catalyst. The polymerization conversion was analyzed by 1 H NMR spectroscopic studies. Toluene (5.0 mL) was added to a mixture of complex S1AlMe2 (0.1 mmol), BnOH (0.2 mmol), and ε-caprolactone (10 mmol) at room temperature. At indicated time intervals, 0.05 mL aliquots were removed, trapped with CDCl3 (1 mL), and analyzed by 1 H NMR. After the solution was stirred for 80 min, the reaction was then quenched by adding a drop of isopropanol, and the polymer precipitated as a white solid when poured into n-hexane (30.0 mL). The isolated white solid was dissolved in CH2Cl2 (5.0 mL), and then n-hexane (70.0 mL) was added to give purified crystalline solid. Yield: 0.80 g (70%). Computational Methods. All the DFT calculations were accomplished by the Gaussian 09 package.18 The B3LYP hybrid functional19 with the D3 version of Grimme’s dispersion correction20 was employed in the present DFT calculations. Geometry optimizations and vibrational frequency calculations were performed at the B3LYP-D3/6-31G* level. The single point energy calculations with a larger basis set 6-311+G** were used to obtain more accurate energies. Numerical integrations were carried out using the setting of ultrafine grid. Free energy corrections were performed at the standard conditions of 298.15 K and 1 atm. Intrinsic reaction coordinate (IRC)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01858. Crystallographic data (CIF) Polymer characterization data, details of the kinetic study, and natural steric analysis data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Michael Y. Chiang. E-mail:
[email protected]. Tel: +886-7-5252000-3949. *Hsing-Yin Chen. E-mail:
[email protected]. Tel: +886-73121101-2807. Fax: +886-7-3125339. *Hsuan-Ying Chen. E-mail:
[email protected]. Tel: +886-73121101-2585. Fax: +886-7-3125339. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is supported by Kaohsiung Medical University “Aim for the top 500 universities grant” under Grant No. KMUDT103007, NSYSU-KMU JOINT RESEARCH PROJECT, (#NSYSUKMU 104-P006), and the Ministry of Science and Technology (Grant MOST 104-2113-M-037-010). We thank Center for Research Resources and Development at Kaohsiung Medical University for the instrumentation and equipment supportf.
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DOI: 10.1021/acs.inorgchem.5b01858 Inorg. Chem. XXXX, XXX, XXX−XXX
