Article pubs.acs.org/Organometallics
Aluminum Complexes Bearing N‑Protected 2‑Amino- or 2‑IminoFunctionalized Pyrrolyl Ligands: Synthesis, Structure, and Catalysis for Preparation of Pyrrolyl-End-Functionalized Polyesters Yun Wei,† Shaowu Wang,*,†,‡ Xiancui Zhu,† Shuangliu Zhou,† Xiaolong Mu,† Zeming Huang,† and Dongjing Hong† †
Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, People’s Republic of China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China S Supporting Information *
ABSTRACT: Reactivity of N-protected 2-amino- or 2-iminofunctionalized pyrroles with aluminum alkyls was investigated, resulting in the isolation of a series of aluminum alkyl complexes. Treatment of 2-imino-functionalized pyrrole with AlMe3 produced only imino-coordinated aluminum complex 1-Bn-2-(2,6-iPr2C6H3NCH)C4H3NAlMe3 (1), while reactions of N-protected 2-amino-functionalized pyrroles with aluminum alkyls produced the aluminum alkyl complexes {[η1-μ-η1:η1-1-R1-2-(2,6-iPr2C6H3NCH2)C4H2N]AlR}2 (R1 = Bn, R = Me (2); R1 = Bn, R = Et (3); R1 = R = Me (4); R1 = Me, R = Et (5)), bearing 3-carbon bonded pyrrolyl ligands via C−H σ-bond metathesis reaction. Further reactions of complexes 2−5 with a stoichiometric amount of isopropyl alcohol (iPrOH) afforded the corresponding aluminum alkoxide complexes [1-R1-2-(2,6-iPr2C6H3NCH2)C4H3NAlR(μ-OiPr)]2 (R1 = Bn, R = Me (6); R1 = Bn, R = Et (7); R1 = R = Me (8); R1 = Me, R = Et (9)) through selective cleavage of the Al−C (Pyr) bonds. The solid-state structures of the aluminum complexes 1−6 and 8 were confirmed by an X-ray diffraction study. These aluminum alkyl complexes exhibited notable activity toward the ring-opening polymerization of ε-caprolactone and L-lactide in the absence of alcohol. The end group analysis of the ε-CL oligomer gave strong support that the polymerization proceeded via a coordination−insertion mechanism involving a unique Al−C (Pyr) bond initiation, providing pyrrolyl-end-functionalized polyesters.
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INTRODUCTION Biodegradable and biocompatible polyesters, such as poly(εcaprolactone) (PCL) and polylactide (PLA), have attracted much attention due to their wide range of applications including environmentally friendly bulk packaging materials, implantable materials, sutures, and delivery media for controllable release of drugs.1 The major polymerization approach for synthesis of the polymers is ring-opening polymerization (ROP) of cyclic esters initiated by metal complexes. Among the metal-based initiators, aluminum complexes have attracted much attention because they have good control over the polymerization reaction and low toxicity.2 The co-initiator such as alcohols has to be adopted in combination with aluminum alkyl complexes, and several effective initiators that initiate ROP of lactones have been reported.3−5 However, the alcoholysis reaction of aluminum alkyl complexes is often not easily anticipated, for example, the dissociation of ancillary ligand from the metal center, and the products of the reaction can sometimes be difficult to identify.6 In the absence of alcohol, very few aluminum alkyl complexes can act as efficient initiators for the ROP of cyclic esters, because most of the aluminum alkyls showed no or low activity for the polymerization, and the processes were poorly controlled.7 Thus, a concept for the © XXXX American Chemical Society
design of chelating ligands incorporating different biting sites with different activity such that one active site can initiate the polymerization and the other site can still bond with the metal center displaying controllable function during the polymerization process comes to our mind, because synthesis of these kinds of aluminum alkyl catalysts that can initiate the ROP of cyclic esters directly providing polyesters with versatile functional end groups in a controllable manner would be meaningful in industrial and academic utilities.8 Different from the chemistry of some late transition metal complexes, the C−H bond can be oxidatively added to metal center, which provides a useful route to M−C bonds.9,10 However, construction of an M−C bond via C−H activation for main group metal and early transition metal complexes can only be achieved by an metathesis reaction of the C−H bond with M−R (R = H, alkyl, etc.). Many early transition metal complexes were prepared by this method.11 We hope that reactions of N-protected pyrrolyl amines with aluminum trialkyls could provide aluminum complexes having supporting Received: April 25, 2016
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DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics ligands with different biting sites (that is, C and N biting sites) with an aluminum center via C−H and N−H bond activation. Here, we wish to report the preparation of novel organoaluminum complexes from the reactions of N-protected 2imnio- or N-protected 2-amino-functionalized pyrroles with aluminum alkyls and their catalytic activity toward the polymerization of cyclic esters in the absence of a co-initiator. The catalytic mechanism of these aluminum catalysts for the polymerization of cyclic esters has also been probed, which represents a supporting ligand initiated coordination−insertion mechanism for the ROP of ε-CL, providing pyrrolyl-endfunctionalized polyesters.
Figure 1. Comparison of 1H NMR spectra of L1 (a) and complex 1 (b).
ring.13 The different reactivities between aluminum and transition metal complexes toward 2-imino-functionalized pyrrole ligands may be attributed to the electronic property of the central metals, which make the imino CN bond activated or the C−H bond of the pyrrole ring activated. The coordination mode of complex 1 was further confirmed by Xray single-crystal diffraction analysis. As shown in Figure 2,
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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes. The ligands were synthesized via a condensation reaction between pyrrole-2-carboxaldehyde and 2,6-diisopropylphenylaniline; then the resulting 2-imino-functionalized pyrroles can be alkylated by −CH2C6H5 (L1) or −CH3, followed by reduction to afford L2H and L3H (Scheme 1). These proligands were well characterized by spectroscopic methods and HR-MS analyses. Scheme 1. Preparation of the Proligands L1, L2H, and L3H
Scheme 2. Preparation of Complex 1
Figure 2. ORTEP of the X-ray structure of complex 1. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): Al(1)−C(1) 1.966(4), Al(1)−C(26) 1.968(3), Al(1)−C(27) 1.973(3), Al(1)−N(1) 2.023(2), C(1)−Al(1)−C(26) 114.51(2), C(1)−Al(1)−C(27) 112.66(2), C(26)−Al(1)−C(27) 113.98(2), C(1)−Al(1)−N(1) 102.78(1), C(26)−Al(1)−N(1) 105.16(1), C(27)−Al(1)−N(1) 106.41(1).
crystalline complex 1 could be isolated in hexane. On the basis of the proton resonance integrations of the 1H NMR spectrum of this product, it could be found that the structure consists of three methyl groups bonded to the metal center and one ligand that displayed proton resonances distinguishable from those of L1. The proton signal of the benzyl methylene for the proligand L1 appears at 5.82 ppm, while the corresponding proton of complex 1 appears at 5.39 ppm. The proton of the imine shifts to the lower field region significantly (7.96 ppm in L1 vs 8.45 ppm in complex 1, Figure 1). All these features confirm that the N atom of the imine of the ligand coordinated with the aluminum atoms. However, neither C−H activation of the pyrrole ring nor alkyl addition to the imino CN bond was observed. This chemistry is different from our previous findings of alkyl addition to the C atom of imine;12 it is also different from the result of reaction of the N-methylated 2-iminofunctionalized pyrrole ligand with transition metals, which provides complexes via the C−H activation of the pyrrole
complex 1 can be interpreted as an imine adduct aluminum complex, and the central metal takes a distorted tetrahedral geometry with the angles of C1−Al1−N1 [102.78(2)°], C26− Al1−N1 [105.16(2)°], and C27−Al1−N1 [106.41(2)°] deviated from the regular tetrahedral angle of 109.28°. The coordination bond distance of Al1−N1 (imino) [2.023(2) Å] is shorter than that of Al−N(amino) [2.094(2) Å] reported by Ma.8d Treatment of N-benzylyated (L2H) or N-methylated (L3H) 2-amino-functionalized pyrroles with stoichiometric AlMe3 in toluene at 80 °C afforded the aluminum complexes {[η1-μη1:η1-1-R1-2-(2,6-iPr2C6H3NCH2)C4H2N]AlMe}2 (R1 = Bn (2); R1 = Me (4)) via C−H activation of the pyrrole ring, which were completely different from the above observation. The dimeric aluminum methyl complexes 2 and 4 incorporated bridged carbon σ-bonded pyrrolyl supporting ligands with different biting sites. When the proligands L2H or L3H were reacted with AlEt3 respectively at elevated temperature, similar results to those of 2 and 4 were observed with isolation of complexes 3 and 5 (Scheme 3). The 1H NMR spectrum of complex 2 displayed the absence of resonances of the amine proton and one proton of the pyrrole ring. Two doublets, each having an integration of one proton, are observed at δ 6.83 and 6.29 in the region expected for pyrrole ring protons, suggesting
Initially, the proligand L1 was treated with stoichiometric AlMe3 at 100 °C in toluene (Scheme 2), and a colorless
B
DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 3. Preparation of Complexes 2−5
formation of the C,N chelating product, confirming that C−H bond activation of the pyrrole ring occurred in the process. The proton resonances of each CH2 unit of the pyrrolyl moiety in complexes 2 split into two integrating sets, indicating the rotation of the methylene hydrogen atoms was restricted to some extent, and the protons of the four methyl groups of isopropyl are diastereotopic and display four distinct doublets, revealing a large rotation barrier of the N−CAr bonds. The differences in reactivities of 2-amino- and 2-imino-functionalized pyrroles with aluminum alkyls may be attributed to the C−N single bonds in 2-amino-functionalized pyrroles being more flexible than the CN double bond in 2-iminofunctionalized pyrroles, which may favor the C−H bond of the pyrrole ring to interact with the Al−C (alkyl) bond. Thus, the reaction pathway could be described as follows: the proligand L2H was first deprotonated by one alkyl of AlMe3 with the release of methane, 3-H of the pyrrolyl ring was then activated by the second metal alkyl to generate dianionic species (L2)2−, and (L2)2− coordinated to the Al3+ ion in a rare η1-μ-η1:η1 mode (Scheme 4).
Figure 3. ORTEP representation of the X-ray structure of complex 2. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
Scheme 4. Proposed Mechanism for the Preparation of Complexes 2
Figure 4. ORTEP representation of the X-ray structure of complex 5. Hydrogen atoms and solvated molecules have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
The dimeric structures of complexes 2−5 have also been confirmed by solid-state X-ray diffraction methods; they contained an Al2C2 core bridged by the 3-C atoms of the pyrrolyl rings. Each aluminum center possesses a distorted tetrahedral geometry (Figure 3 for 2, Figure 4 for 5). Selected bond lengths and angles are given in Table 1. The bond length of Al1−N1 [1.840 (2) Å for 2, 1.844(2) Å for 3] is significantly shorter than that of complex 1 [2.023(2) Å] mainly due to bond character (coordination bond in 1 vs covalent bond in 2 and 3). The bond distances of Al−C16 (3-C of the pyrrole ring) in complex 2 [2.051(2) Å] is shorter than that in complex 3 [2.070(2) Å], which is longer than the Al−C (Cp) [2.039(3) Å] found in THF·Li(μ-TMP)[μ-(C5H4)Fe(C5H5)]Al(iBu)2 (TMP = 2,2,6,6-tetramethylpiperidide)14 and Al−C (Mes*) [1.993 Å] found in Me2AlMes* (Mes* = 2,4,6-tBu3C6H2).15 The distances of Al−C1 (methyl-Al for 2, ethyl-Al for 3) in complex 2 [1.945(2) Å] are slightly shorter than that in complex 3 [1.953(3) Å]. Elemental analysis results of
complexes 2−5 are also in agreement with NMR and the Xray analysis data. Reactivity of Aluminum Alkyl Complexes. Because the alcoholysis reaction of aluminum alkyl complexes is often not easily anticipated, and the products of the reaction are sometimes difficult to identify, for example, the dissociation of an ancillary ligand from the metal center or no reaction was found.16 Ma and co-workers reported that aluminum alkyl complexes bearing β-diketiminate ligands were unreactive toward isopropyl or benzyl alcohol even under harsh reaction conditions.16b To investigate the alcoholysis reactivity of complexes 2−5, the reactions of complexes 2−5 with a stoichiometric amount of isopropyl alcohol (iPrOH) were studied with isolation of well-defined corresponding aluminum alkoxide complexes formulated as [1-R1-2(2,6-iPr2C6H3NCH2)C4H3NAlR(μ-OiPr)]2 (R1 = Bn, R = Me (6); R1 = Bn, R = Et (7); R1 = R = Me (8); R1 = Me, R = Et (9)) in good yields (Scheme 5). Solution 1H NMR studies of complex 6 revealed that the resonances for the alkoxide group were observed, but the protons of methyl bonded to the aluminum center still resonated in the high-field region, and one more proton was observed in the low-field region. These results suggested that the selective cleavage of the Al−C (Pyr) bonds is preferred to the cleavage of the Al−C (alkyl) bonds. This observation could be attributed to strain force of the fourmembered C2Al2 ring, and alcoholysis of the Al−C (pyr) bond with release of the strain force would be preferred to alcoholysis of the Al−C (alkyl) bond. This phenomenon is different from the reactions of general aluminum alkyl complexes with C
DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 1−5 Al(1)−N(1) Al(1)−C(1) Al(1)−C(16) Al(1)−C(16A) N(1)−Al(1)−C(1) N(1)−Al(1)−C(16) N(1)−Al(1)−C(16A) C(1)−Al(1)−C(16) C(1)−Al(1)−C(16A) C(16)−Al(1)−C(16A)
1
2
3
4
5
2.023(2) 1.966(4)
1.840(2) 1.945(2) 2.051(2) 2.098(2) 124.63(9) 89.38(7) 108.63(7) 121.72(1) 108.47(9) 100.72(7)
1.844(2) 1.953(3) 2.070(2) 2.078(3) 127.10(2) 107.08(9) 89.27(9) 110.68(2) 117.18(2) 101.35(9)
1.859(2) 1.945(4) 2.074(2) 2.073(2) 129.14(2) 88.95(9) 106.50(1) 116.71(2) 109.60(2) 101.64(8)
1.837(2) 1.952(2) 2.067(2) 2.076(2) 126.22(1) 89.42(8) 108.69(9) 117.57(1) 109.83(1) 101.31(8)
102.78(1)
Scheme 5. Preparation of Complexes 6−9
alcohols and the formation of metal alkoxides through the alcoholysis of the Al−C (alkyl) with the corresponding alcohols.17 Complexes 6−9 were formulated on the basis of 1H and 13C NMR spectroscopy and elemental analyses. The structures of complexes 6 and 8 were further determined by X-ray crystallography study. The molecular structures of 6 and 8 are dimeric in the solid state, containing an Al2O2 core bridged by the oxygen atom of the isopropoxy groups, and the coordination geometry around the Al center is described as a distorted tetrahedron, as shown in Figure 5 for complex 6 (Figure 6 for 8). The bridged oxygen atoms are symmetrically bonded to two Al centers. Ring-Opening Polymerization of ε-Caprolactone. Catalytic ring-opening polymerization of ε-CL was carried out using 1−9 in the absence of alcohol as co-initiator (results
Figure 6. ORTEP representation of the X-ray structure of complex 8. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): Al(1)−N(1) 1.802 (2), Al(1)−O(1) 1.839(2), Al(1)− O(1)A 1.845(2), Al(1)−C(1) 1.939(2), N(1)−Al(1)−O(1) 113.81(6), N(1)−Al(1)−O(1)A 114.49(6), O(1)−Al(1)−O(1)A 79.72(5), N(1)−Al(1)−C(1) 115.97(8), O(1)−Al(1)−C(1) 115.15(8), O(1)A−Al(1)−C(1) 112.59(8).
are summarized in Table 2). The catalytic applications of trimethylaluminum adducts are less reported. Carpentier and Ma reported that aminobis(pyrazolyl) ligands and 2,6-dimethylN-[2-(1-piperidinyl)benzyl]aniline ligands could afford the adducts by reacting with trimethylaluminum,18,8d and Ma also reported that the trimethylaluminum adducts bearing 2,6dimethyl-N-[2-(1-piperidinyl)benzyl]aniline ligands were inactive toward the ring-opening polymerization of rac-lactide. The trimethylaluminum adducts 1 proved to be less active toward the ring-opening polymerization of ε-CL (Table 2, entry 15). The aluminum complexes 2−5 proved to be active initiators toward the polymerization of ε-CL when used as singlecomponent initiators under the conditions employed in this work. From the data compiled in Table 2, it could be observed that complex 5 shows higher catalytic activities than the other aluminum complexes, and high monomer conversions (up to 93%) could be reached within 20 min at 60 °C when the [CL]: [Al] ratio was 250:1 (entry 4). When the [CL]:[Al] ratio was increased to 1000, a very good conversion (99%) was observed in 60 min at 60 °C (entry 10) using complex 5 as initiator, and the linear relationship between the [CL]/[Al] ratio and the number-averaged molecular weight (Mn) that is shown in Figure 7 demonstrates the somewhat living nature of the polymerization (entries 4 and 7−10). It can be noticed that complexes 2−5 exhibited higher catalytic activity than the corresponding Al alkoxide complexes 6−9 (entries 11−14). For example, it requires a longer time (150 min) to reach 99% conversions of monomer using 6 and 8, respectively (entries 11
Figure 5. ORTEP representation of the X-ray structure of complex 6. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): Al(1)−N(1) 1.800 (2), Al(1)−O(1) 1.841(2), Al(1)− O(1)A 1.842(2), Al(1)−C(1) 1.938(2), N(1)−Al(1)−O(1) 114.95(7), N(1)−Al(1)−O(1)A 114.13(7), O(1)−Al(1)−O(1)A 79.33(6), N(1)−Al(1)−C(1) 115.63(9), O(1)−Al(1)−C(1) 114.88(9), O(1)A−Al(1)−C(1) 112.82(9). D
DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 2. Ring-Opening Polymerization of ε-CL Initiated by Complexes 1−9a entry
initiator
CL:Al
T (°C)
time (min)
convb (%)
Mn,calcdc × 10−4
Mnd × 10−4
Mw/Mnd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
2 3 4 5 5 5 5 5 5 5 6 7 8 9 1
250:1 250:1 250:1 250:1 250:1 250:1 500:1 600:1 750:1 1000:1 250:1 250:1 250:1 250:1 250:1
60 60 60 60 70 25 60 60 60 60 70 70 70 70 70
20 20 20 20 10 120 30 50 50 60 150 720 150 720 300
85 84 67 93 97 26 98 99 99 99 99 99 99 99 74
2.46 2.43 1.94 2.68 2.79 0.76 5.62 6.88 8.50 11.33 2.83 2.83 2.83 2.83 2.11
5.69 5.12 6.07 5.21 5.76 3.18 10.12 12.53 17.13 20.14 9.55 8.96 9.67 10.59 6.89
1.42 1.49 1.52 1.44 1.52 1.77 1.64 1.68 1.77 1.67 1.46 1.38 1.51 1.51 1.37
a
[ε-CL] = 1.0 M, in toluene. bDetermined by the integration ratio of the methylene protons in monomer and polymer in CDCl3. cMn,calcd = ([εCL]0/[Al]0) × 114.14 × conv (%) + mass of the free ligand. dMeasured by GPC calibrated with standard polystyrene samples.
hydroxy end), two proton resonances belonging to the Nmethylated pyrrolyl ligand could be clearly identified, indicating the 3 position of the pyrrolyl ring was acylated. Thus, the resonances are assignable to the corresponding protons in the proposed oligomer structure.19 The 13C NMR spectra of the oligomer were also examined, and the resonance at 197.9 ppm (see Supporting Information, Figure S27), which is close to the ketone carbonyl peak of acetophenone (197 ppm), can be assigned as the resonance of the carbonyl carbon connected to the pyrrolyl ring, whereas the resonance of the carbon of the acetyl amide appeared at 174.3 ppm (see Supporting Information, Figure S31). Thus, it can be concluded that the Al−C (Al−Pyr) bond initiated the polymerization to produce the polymers having a 3-acylated pyrrolyl chain end, and other process initiated by Al−N or Al−C (alkyl) bonds can be ruled out. In order to understand the structure and composition of the oligomer obtained, a MALDI-TOF spectrum of the oligomer was performed (see Supporting Information); the mass spectrum of low molecular weight of the oligomer is not obvious, which may be attributed to the small molecular weight of the oligomer. However, the mass spectrum shows a cluster of homologous peaks separated by a molecular mass of 114 Da, and the ligand of L3H-capped oligomers was detected (1183). This result is further confirmed by the APCI (atmospheric pressure chemical ionization)-TOF20 mass spectrum of the same oligomer sample (Figure 8), where a series of peaks differing by 114 are systematically ended with the same group having an m/z of 271 of the molecular weight of the Nmethylated pyrrolyl amido ligand of L3H, providing linear polymers that are terminated by an intact pyrrolyl ligand on one end and a hydroxyl on the other end. On the basis of the above results, a coordination−insertion mechanism could be assumed for ε-CL polymerization (Scheme 6). The dimeric Al complexes may be changed to mononuclear form upon coordination of the ε-CL monomer. Subsequently, the 3-carbon of the pyrrolyl ring attacks the carbonyl carbon of the coordinated monomer to produce the acylated pyrrolyl end aluminum alkoxide species. This polymerization is propagated by continuous monomer insertion into the active metal−alkoxido bond until the polymerization is quenched.
Figure 7. Plot of number-averaged molecular weights (Mn) and molecular weight distribution (Mw/Mn) vs [CL]/[Al] for the polymerization of ε-CL using complex 4 at 60 °C (entries 4−7 in Table 2). In both plots, black squares (■) represent Mn values and black triangles (▲) represent Mw/Mn values.
and 14). Generally, the aluminum alkoxide complexes are superior to the aluminum alkyl complexes as the initiators for the ROP of ε-CL.6a,7c The new aluminum alkyl (Al−C (Pyr)) displayed higher activity than the corresponding alkoxides probably due to the strain force of the four-membered C2Al2 ring in complexes 2−5. From Table 2, it could be seen that the substituents on the nitrogen of pyrrolyl supporting ligands and the alkyl on the metal have different influences on the catalytic activities of these aluminum complexes. For example, ethyl aluminum complex 5, bearing an N-Me pyrrolyl ligand, exhibits higher catalytic activity than the methyl aluminum complex 4, bearing the same ligand (entries 4 and 3). Complex 5 also displayed higher catalytic activity than ethyl aluminum complex 3 (entries 4 and 2). This observation may be attributed to electronic and steric effects of the substituents. In order to understand the polymerization mechanism using complexes 2−5 as single-component catalysts, the polymerization using complex 4 as initiator was carried out at a molar ratio of 10:1 ([ε-CL]:[Al]) at room temperature. The oligomer was purified from THF/hexane and subjected to 1H NMR spectroscopy. Except for the typical resonances of the polymer chain (see Supporting Information, He for the protons of CH2 of the poly(ε-CL) next to the ketone connected to the pyrrole ring at the chain end and Hj for CH2 of the poly(ε-CL) at the E
DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
the 3-carbon pyrrolyl ring is more favorable than that via alkoxide initiation.
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CONCLUSIONS In conclusion, reactivity studies between 1-N-protected 2amino-functionalized pyrroles resulted in isolation and characterization of novel 1-alkyl-3-carbon σ-bonded pyrrolyl supported aluminum alkyl complexes via C−H activation. Reactions of these aluminum alkyl complexes with isopropyl alcohol produced the corresponding aluminum complexes with alkyl-alkoxide-amido mixed ligands. The 1-alkyl-3-carbon σbonded pyrrolyl supported aluminum alkyl complexes are excellent single-component initiators for the polymerization of ε-caprolactone (ε-CL) within the [ε-CL]:[cat] ratios of 250:1 to 1000:1 under the given conditions. It is found that the 1aklyl-3-carbon σ-bonded pyrrolyl supported aluminum alkyl complexes exhibited higher catalytic activity toward ε-CL polymerization than the corresponding aluminum complexes bearing alkyl-alkoxide-amido mixed ligands. The 1-alkyl-3carbon σ-bonded pyrrolyl supported aluminum alkyl complexes also displayed moderate catalytic activity toward polymerization of L-lactide. Experimental results supported the coordination− insertion mechanism for ε-CL polymerization with 1-alkyl-3carbon σ-bonded pyrrolyl supported aluminum alkyl complexes, producing pyrrolyl-end polyesters. Further works in this field are now in progress.
Figure 8. APCI-TOF mass spectrum of ε-CL oligomer obtained from ε-CL using 4 as initiator (ε-CL:Al = 10:1).
Scheme 6. Proposed Mechanism for ε-CL Polymerization Initiated by Complex 4
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Ring-Opening Polymerization of L-Lactide. Experimental results indicated that complexes 2−5 are also efficient initiators for ring-opening polymerization of L-lactide. Polymerizations were carried out in toluene solution at 80 °C with a molar ratio of monomer to initiator of 100 ([LA]0:[Al]0). Aluminum alkyl complexes 2−5 proved to be moderately active toward the ROP of L-LA in the absence of alcohols under the conditions employed above for the polymerization of ε-CL. The conversion can reach 98% using 5 as an initiator in 14 h in refluxed toluene. From the data compiled in Table 3, it could be observed that Al alkoxide complexes 6 and 8 show lower activity than the corresponding Al alkyl complexes 2−5 for the ROP of L-LA, indicating that initiation of the polymerization via Table 3. Ring-Opening Polymerization of L-LA Initiated by Complexes 2−6 and 8a entry initiator 1 2 3 4 5 6 1
2 3 4 5 6 8 2
time (h)
convb (%)
Mn,calcdc × 10−4
Mnd × 104
Mw/ Mnd
14 16 16 14 25 25 14
97 96 97 98 78 trace 97
1.43 1.42 1.43 1.44 1.12
1.30 1.22 1.19 1.27 2.09
1.28 1.10 1.08 1.20 1.22
1.43
1.30
1.28
EXPERIMENTAL SECTION
General Methods. All syntheses and manipulations of air- and moisture-sensitive materials were performed under dry argon and in an oxygen-free atmosphere using standard Schlenk techniques or in a glovebox. All solvents were refluxed and distilled over sodium benzophenone ketyl under argon prior to use unless otherwise noted. Ligands were prepared according to a modified literature procedure.21 AlMe3 and AlEt3 were purchased from Acros and used as received. Elemental analyses were performed on a PerkinElmer 2400 CHN analyzer. Melting points were determined in sealed capillaries without correction. 1H NMR and 13C NMR spectra for analyses of compounds were recorded on a Bruker AV-300 NMR or AV-500 NMR spectrometer (in C6D6 or CDCl3, or THF-d8 for aluminum complexes). Organometallic samples for NMR spectroscopic measurements were prepared in the glovebox by use of J. Young valve NMR tubes. Chemical shifts (δ) are reported in ppm. J values are reported in Hz. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) analyses were carried out with a Shimadzu Axima Performance apparatus. HR-MS measurements were conducted with an Agilent model G6220 APCI-TOF mass spectrometer. IR spectra were recorded on a Shimadzu FTIR-8400S spectrometer (KBr pellet). Data for X-ray crystal structure determinations were obtained with a Bruker diffractometer equipped with a Smart CCD area detector. Preparation of 1-Bn-2-(2,6-iPr2C6H3NCH)C4H3N (L1). NaH (1.26 g, 60%, 31.5 mmol) was suspended in DMF (10 mL), and a solution of 2-(2,6-iPr2C6H3NCH)C4H3NH (7.63 g, 30.0 mmol) in DMF (30 mL) was added dropwisely at 0 °C. The reaction mixture was stirred for 1 h at room temperature; then C6H5CH2Cl (3.99 g, 31.5 mmol) was added dropwisely at 0 °C. After stirring for 4 h at room temperature, the reaction was quenched with H2O (20 mL), extracted with ethyl acetate (60 mL), and washed with water (3 × 10 mL). The organic layers were dried over anhydrous Na2SO4 followed by filtration, then were evaporated to dryness under reduced pressure to give the off-white solid (9.40 g) in 91% yield. Mp: 70−72 °C. 1H NMR (500 MHz, CDCl3): δ 7.96 (s, 1H), 7.29−7.26 (m, 2H), 7.23− 7.20 (m, 1H), 7.08−7.06 (m, 2H), 7.03−6.99 (m, 3H), 6.94 (s, 1H), 6.69−6.68 (m, 1H), 6.32−6.31 (m, 1H), 5.81 (s, 2H), 2.73−2.68 (m, 2H), 1.01 (d, J = 7.0 Hz, 12H). 13C NMR (125 MHz, CDCl3): δ
a Polymerization conditions: L-LA, 1.0 mol L−1 in toluene at 80 °C; LLA:Al = 100:1; Ar atmosphere. bDetermined by the integration ratio of the methine protons in the monomer and polymer. cMn,calcd = ([L − LA]0/[Al]0) × 144.14 × conv (%) + mass of the free ligand. d Measured by GPC calibrated with standard polystyrene samples.
F
DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
129.2, 128.1, 127.9, 126.7, 124.7, 124.3, 123.9, 119.6, 104.2, 54.0, 51.3, 27.7, 27.6, 26.9, 26.7, 25.1, 24.6, −8.31. IR (KBr pellet, cm−1): ν 2962 (s), 2926 (s), 2864 (s), 1606 (w), 1587 (w), 1495 (m), 1425 (m), 1361 (m), 1340 (m), 1213 (w), 1076 (m), 1049 (s), 964 (s), 800 (m), 729 (s), 711 (s). Anal. Calcd for C50H62Al2N4: C, 77.69; H, 8.08; N, 7.25. Found: C, 77.41; H, 8.12; N, 7.05. Preparation of {[η1-μ-η1:η1-1-Bn-2-(2,6-iPr2C6H3NCH2)C4H2N]AlEt}2 (3). Complex 3 was obtained like 1 from reaction of L2H (0.35 g, 1.0 mmol) and AlEt3 (1.0 mL, 1.0 M in toluene, 1.0 mmol). The reaction yielded white solid 3 (0.36 g, 89%). Single crystals of complex 3 were obtained from a mixture of hexane and toluene. Mp: 172−174 °C under Ar. 1H NMR (500 MHz, C6D6): δ 7.36−7.34 (m, 1H), 7.28 (d, J = 4.5 Hz, 2H), 6.9−6.96 (m, 2H), 6.92−6.90 (m, 1H), 6.63 (d, J = 2.5 Hz, 1H), 6.57 (d, J = 7.0 Hz, 2H), 6.19 (d, J = 3.0 Hz, 1H), 4.38 (d, J = 15.0 Hz, 1H), 4.15 (d, J = 15.0 Hz, 1H), 4.15−4.08 (m, 1H), 4.08 (s, 2H), 4.02−3.96 (m, 1H), 1.59 (d, J = 6.5 Hz, 3H), 1.39 (d, J = 6.5 Hz, 3H), 1.32 (d, J = 7.0 Hz, 3H), 1.22 (d, J = 7.0 Hz, 3H), 0.97 (t, J = 8.0 Hz, 3H), 0.38−0.34 (m, 1H), 0.30−0.24 (m, 1H). 13C NMR (125 MHz, C6D6): δ 156.6, 148.4, 148.1, 147.9, 136.7, 128.8, 126.7, 125.0, 124.9, 124.2, 119.9, 102.8, 54.3, 50.5, 27.8, 27.7, 27.3, 26.8, 25.1, 24.9, 8.8, 3.7. IR (KBr pellet, cm−1): ν 2964 (s), 2926 (m), 2864 (m), 1560 (w), 1494 (m), 1452 (m), 1361 (w), 1301 9w), 1253 (w), 1076 (w), 1049 (m), 966(w), 800 (m), 746 (m), 729 (m), 711 (s). Anal. Calcd for C52H66Al2N4: C, 77.97; H, 8.30; N, 6.99. Found: C, 77.60; H, 8.48; N, 6.72. Preparation of {[η1-μ-η1:η1-1-Me-2-(2,6-iPr2C6H3NCH2)C4H2N]AlMe}2 (4). Complex 4 was obtained like 1 from reaction between L3H (0.27 g, 1.0 mmol) and AlMe3 (1.0 mL, 1.0 M in toluene, 1.0 mmol). The reaction yielded white solid 4 (0.28 g, 90% yield). Mp: 198−200 °C under Ar. 1H NMR (500 MHz, THF-d8): δ 7.00 (d, J = 8.0 Hz, 2H), 6.96−6.93 (m, 1H), 6.60 (s, 1H), 5.98 (s, 1H), 3.95 (s, 2H), 3.63−3.58 (m, 2H), 3.42 (s, 3H), 1.74 (s, 3H), 1.18 (d, J = 6.9 Hz, 6H), 1.14 (d, J = 7.0 Hz, 6H), −0.78 (s, 3H). 13C NMR (125 MHz, THF-d8): δ 148.7, 148.1, 147.2, 123.4, 123.1, 123.0, 112.0, 54.2, 31.9, 27.3, 25.8, −12.3. IR (KBr pellet, cm−1): ν 2960 (s), 2866 (s), 1625 (w), 1589 (w), 1495 (m), 1458 (s), 1444(s), 1330 (m), 1300 (m), 1251 (m), 1087 (m), 1048 (m), 962 (w), 804 (s), 709 (s). Anal. Calcd for C38H54Al2N4: C, 73.52; H, 8.77; N, 9.02. Found: C, 73.67; H, 8.94; N, 8.74. Preparation of {[η1-μ-η1:η1-1-Me-2-(2,6-iPr2C6H3NCH2)C4H2N]AlEt}2 (5). Complex 5 was obtained like 1 from reaction between L3H (0.27 g, 1.0 mmol) and AlEt3 (1.0 mL, 1.0 M in toluene, 1.0 mmol). The reaction yielded white solid 5 (0.27 g, 84%). Single crystals of complex 5 were obtained from C6D6 in J. Young valve NMR tubes. Mp: 202−204 °C under Ar. 1H NMR (500 MHz, C6D6): δ 7.40 (d, J = 6.0 Hz, 1H), 7.35−7.28 (m, 2H), 6.57 (s, 1H), 6.05 (s, 1H), 4.43 (d, J = 15.0 Hz, 1H), 4.21−4.17 (m, 1H), 4.14 (d, J = 15.0 Hz, 1H), 4.08− 4.02 (m, 1H), 2.40 (s, 3H), 1.61 (d, J = 6.5 Hz, 3H), 1.50 (d, J = 6.5 Hz, 3H), 1.40 (d, J = 6.5 Hz, 3H), 1.37 (d, J = 6.5 Hz, 3H), 0.88 (t, J = 8.0 Hz, 3H), 0.36−0.24 (m, 2H). 13C NMR (126 MHz, C6D6): δ 156.7, 148.4, 148.3, 148.0, 125.0, 124.3, 119.6, 102.1, 54.4, 32.4, 27.9, 27.4, 26.8, 25.1, 8.4, 3.8. IR (KBr pellet, cm−1): ν 2962 (s), 2868 (m), 1589 (w), 1496 (m), 1458 (m), 1444 (m), 1382 (w), 1336 (w), 1300 (m), 1255 (m), 1087 (m), 1047 (m), 962 (w), 804 (m), 709 (s). Anal. Calcd for C40H58Al2N4: C, 74.04; H, 9.01; N, 8.63. Found: C, 74.27; H, 8.92; N, 8.33. Preparation of [L2AlMe(μ-OiPr)]2 (6). To a rapidly stirred solution of 2 (0.77 g, 1.0 mmol) in toluene (30 mL) was added 2propanol (0.153 mL, 2.0 mmol), and the mixture was stirred at room temperature for 2 h. After removal of the solvent, the product was crystallized from hexane and toluene to give the desired complex as a colorless crystalline solid of 6 (0.82 g, 92%). Mp: 201−202 °C under Ar. 1H NMR (500 MHz, C6D6): δ 7.29−7.26 (m, 1H), 7.21 (d, J = 7.0 Hz, 2H), 6.97 (t, J = 7.5 Hz, 2H), 6.90 (t, J = 7.5 Hz, J = 7.0 Hz, 1H), 6.60 (d, J = 7.5 Hz, 2H), 6.50 (dd, J = 1.5 Hz, 1H), 6.31 (t, J = 3.0 Hz, 1H), 6.23 (t, J = 2.0 Hz, J = 2.5 Hz, 1H), 4.90−4.85 (m, 1H), 4.29 (s, 2H), 3.91 (s, 2H), 3.79−3.73 (m, 2H), 1.51 (d, J = 6.5 Hz, 3H), 1.39 (d, J = 6.5 Hz, 3H), 1.22 (d, J = 7.0 Hz, 6H), 0.99 (d, J = 6.5 Hz, 6H), −0.44 (s, 3H). 13C NMR (125 MHz, C6D6): δ 148.6, 146.8, 139.6, 134.8, 128.7, 128.3, 126.1, 125.4, 124.4, 121.7, 109.8, 108.3, 68.6, 49.4,
153.1, 150.2, 139.5, 138.4, 129.9, 129.3, 128.9, 127.5, 126.7, 124.0, 123.2, 119.5, 109.6, 52.5, 28.0, 24.0. HRMS (APCI) m/z: calcd for C24H29N2 (M + H+) 345.2325; found 345.2321. Preparation of 1-Bn-2-(2,6-iPr2C6H3NHCH2)C4H3N (L2H). The HL1 (10.33 g, 30.0 mmol) in 50 mL of THF solution was reduced with LiAlH4 (3.42 g, 90 mmol) at room temperature for 12 h; then it was hydrolyzed. The organic layer was separated, and the white residue was extracted with dichloromethane (3 × 20 mL). The combined organic layers were dried over anhydrous Na2SO4 followed by filtration. Removal of solvent under reduced pressure gave the objective compound as a white solid (9.56 g) in 92% yield. Mp: 62−64 °C. 1H NMR (300 MHz, CDCl3): δ 7.33−7.23 (m, 3H), 7.07 (s, 3H), 6.72 (d, J = 1.8 Hz, 2H), 6.73−6.71 (m, 1H), 6.22 (s, 1H), 6.19 (s, 1H), 5.25 (s, 2H), 3.84 (s, 2H), 3.19−3.10 (m, 2H), 2.98 (s, 1H), 1.14 (d, J = 6.9 Hz, 12H). 13C NMR (75 MHz, CDCl3): δ 143.0, 138.7, 131.4, 128.9, 127.5, 126.1, 124.3, 123.7, 122.8, 108.7, 107.5, 50.8, 47.8, 27.9, 24.3. HRMS (APCI) m/z: calcd for C24H31N2 (M + H+) 347.2482; found 347.2476. Preparation of 1-Me-2-(2,6-iPr2C6H3NHCH2)C4H3N (L3H). NaH (1.32 g, 60%, 33.0 mmol) was suspended in DMF (10 mL), and a solution of 2-(2,6-iPr2C6H3NCH)C4H3NH (7.63 g, 30.0 mmol) in DMF (30 mL) was added dropwisely at 0 °C. The reaction mixture was stirred for 1 h at room temperature; then CH3I (4.68 g, 33.0 mmol) was added dropwisely at 0 °C. After stirring for 4 h at room temperature, the reaction was quenched with H2O (20 mL), extracted with ethyl acetate (60 mL), and washed with water (3 × 10 mL). The organic layers were dried over anhydrous Na2SO4 followed by filtration. The solution was then evaporated to dryness under reduced pressure to give a yellow, oily product. The yellow, oily compound in CH3OH (80 mL) solution was reduced with NaBH4 (6.8 g, 0.18 mol) at room temperature for 12 h and then hydrolyzed. The mixture was extracted with 50 mL of ether, and the aqueous layers were extracted with ether (3 × 20 mL). The organic fractions were combined and dried over anhydrous Na2SO4, filtered, and evaporated to dryness. Recrystallization of crude product from hexane gave the objective compound as an off-white solid (7.22 g) in 89% yield. Mp: 48−50 °C. 1 H NMR (500 MHz, CDCl3): δ 7.15−7.09 (m, 3H), 6.63 (t, J = 1.9 Hz, 1H), 6.15 (d, J = 1.6 Hz, 1H), 6.11 (t, J = 3.0 Hz, 1H), 3.95 (s, 2H, CH2), 3.66 (s, 3H), 3.34−3.29 (m, 2H), 3.01 (s, 1H), 1.26 (s, J = 1.26 Hz, 6H), 1.25 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 143.1, 131.5, 124.4, 123.7, 122.9, 108.0, 107.0, 47.8, 34.0, 28.0, 24.4. HR-MS (APCI) m/z: calcd for C18H27N2 (M + H+) 271.2169; found 271.2161. Preparation of L1AlMe3 (1). L1 (0.34 g, 1 mmol) in 10 mL of toluene was added slowly to a solution of AlMe3 (1.0 mL, 1.0 M in toluene, 1.0 mmol) in 10 mL of toluene. The mixture wad gently heated at 100 °C for 12 h, during which a pale yellow solution was formed. After removal of the solvent, the product was crystallized from hexane to give the desired complex as a colorless crystalline solid (0.36 g, 86% yield). Mp: 152−154 °C under Ar. 1H NMR (500 MHz, CDCl3): δ 8.44 (s, 1H), 7.39−7.35 (m, 3H), 7.31−7.39 (m, 1H), 7.22 (d, J = 7.5 Hz, 2H), 7.04 (d, J = 6.5 Hz, 3H), 6.08−6.07 (m, 1H), 5.39 (s, 2H), 5.36 (d, J = 4.5 Hz, 1H), 2.75−2.69 (m, 2H), 1.14 (d, J = 7.0 Hz, 6H), 0.84 (d, J = 6.5 Hz, 6H), −1.09 (s, 9H). 13C NMR (125 MHz, CDCl3): δ 155.9, 141.9, 140.07, 136.7, 131.7, 129.6, 129.0, 127.7, 126.3, 125.2, 125.1, 122.7, 111.7, 52.6, 28.4, 24.9, 24.4, −7.8. IR (KBr pellet, cm−1): ν 2916 (m), 2864 (m), 1633 (s), 1589 (w), 1477 (w), 1458 (w), 1450(w), 1311 (w), 1176 (w), 1080 (w), 1028 (w), 787 (m), 690 (m). Anal. Calcd for C27H37AlN2: C, 77.85; H, 8.95; N, 6.72. Found: C, 77.52; H, 8.64; N, 6.47. Preparation of {[η1-μ-η1:η1-1-Bn-2-(2,6-iPr2C6H3NCH2)C4H2N]AlMe}2 (2). Complex 2 was formed like 1 from reaction between L2H (0.35 g, 1.0 mmol) and AlMe3 (1.0 mL, 1.0 M in toluene, 1.0 mmol). The reaction yielded white solid 2 (0.35 g, 92%). Mp: 162−164 °C. 1 H NMR (500 MHz, CDCl3): δ 7.28−7.25 (m, 2H), 7.23−7.21 (m, 1H), 7.15 (s, 3H), 6.92 (d, J = 7.5 Hz, 2H), 6.83 (s, 1H), 6.29 (s, 1H), 5.05 (d, J = 16.0 Hz, 1H), 4.93 (d, J = 16.5 Hz, 1H), 4.11 (q, J = 15.5 Hz, 2H), 3.70−3.62 (m, 2H), 1.23 (d, J = 6.5 Hz, 3H), 1.11 (d, J = 6.5 Hz, 3H), 1.08 (d, J = 7.0 Hz, 3H), 0.98 (d, J = 7.0 Hz, 3H), −0.87 (s, 3H). 13C NMR (125 MHz, CDCl3): δ 157.7, 148.5, 147.9, 136.8, G
DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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Organometallics 47.9, 28.3, 26.6, 25.6, 25.1, 24.1, −10.5. IR (KBr pellet, cm−1): ν 2962 (s), 2924 (s), 2886 (s), 2827 (m), 1604 (w), 1479 (m), 1436 (m), 1377 (m), 1354 (m), 1301 (m), 1242 (w), 1199 (m), 1116 (m), 1031 (s), 945 (s), 858 (m), 827 (m), 725 (s), 669 (s). Anal. Calcd for C56H78Al2N4O2: C, 75.30; H, 8.80; N, 6.27. Found: C, 75.23; H, 8.99; N, 6.11. Preparation of [L2AlEt(μ-OiPr)]2 (7). Complex 7 was obtained like 6 from reaction between 3 (0.80 g, 1.0 mmol) and 2-propanol (0.153 mL, 2.0 mmol). The reaction yielded white solid 7 (0.81 g, 88%). Mp: 216−218 °C under Ar. 1H NMR (500 MHz, CDCl3): δ 7.21−7.15 (m, 4H), 7.13 (d, J = 7.5 Hz, 2H), 6.62 (d, J = 7.0 Hz, 2H), 6.17−6.16 (m, 2H), 6.03 (t, J = 3.0 Hz, 1H), 4.87−4.81 (m, 1H), 4.17 (s, 2H), 3.93 (s, 2H), 3.66−3.61 (m, 2H), 1.69 (d, J = 6.5 Hz, 3H), 1.59 (d, J = 6.5 Hz, 3H), 1.19 (d, J = 7.0 Hz, 6H), 0.81 (d, J = 6.5 Hz, 6H), 0.70 (t, J = 8.0 Hz, 3H), 0.07−0.02 (m, 2H). 13C NMR (125 MHz, CDCl3): δ 148.5, 147.0, 138.9, 134.9, 128.7, 127.2, 126.9, 125.2, 124.4, 121.1, 109.4, 107.8, 68.4, 49.5, 47.5, 28.3, 26.1, 26.0, 25.7, 24.6, 9.5, 0.60. IR (KBr pellet, cm−1): ν 2976 (s), 2933 (m), 2866 (w), 1631 (w), 1460 (w), 1440 (w), 1199 (w), 1184 (w), 1035 (s), 950 (s), 831 (m), 665 (s). Anal. Calcd for C58H82Al2N4O2: C, 75.62; H, 8.97; N, 6.08. Found: C, 75.33; H, 8.79; N, 6.31. Preparation of [L3AlMe(μ-OiPr)]2 (8). Complex 8 was obtained like 6 from reaction between 4 (0.62 g, 1.0 mmol) and 2-propanol (0.153 mL, 2.0 mmol). The reaction yielded white solid 8 (0.68 g, 91%). Mp: 220−222 °C under Ar. 1H NMR (500 MHz, CDCl3): δ 7.16−7.13 (m, 1H), 7.10 (d, J = 8.5 Hz, 2H), 6.26 (t, 1H), 6.06−6.05 (m, 1H), 5.98 (t, J = 3.5 Hz, 1H), 4.91−4.86 (m, 1H), 4.15 (s, 2H), 3.70−3.75 (m, 2H), 2.58 (s, 3H), 1.69 (d, J = 6.4 Hz, 3H), 1.59 (d, J = 6.4 Hz, 3H), 1.16 (d, J = 7.0 Hz, 6H), 0.96 (d, J = 6.5 Hz, 6H), −0.74 (s, 3H). 13C NMR (125 MHz, CDCl3): δ 148.6, 146.1, 134.5, 125.0, 124.2, 122.0, 109.3, 107.2, 68.6, 47.3, 33.4, 28.3, 26.5, 62.0, 25.5, 24.1, −11.0. IR (KBr pellet, cm−1): ν 2962 (s), 2866 (m), 1494 (w), 1460 (m), 1444 (m), 1384 (w), 1197 (m), 1118 (m), 950 (m), 854 (m), 785 (m), 725 (s), 667 (s). Anal. Calcd for C44H70Al2N4O2: C, 71.32; H, 9.52; N, 7.56. Found: C, 71.36; H, 9.18; N, 7.94. Preparation of [L3AlEt(μ-OiPr)]2 (9). Complex 9 was obtained like 6 from reaction between 5 (0.65 g, 1.0 mmol) and 2-propanol (0.153 mL, 2.0 mmol). The reaction yielded white solid 9 (0.67 g, 87%). Mp: 219−221 °C under Ar. 1H NMR (500 MHz, CDCl3): δ 7.16−7.11 (m, 1H, Ar-H), 7.07 (d, J = 7.0 Hz, 2H), 6.25 (s, 1H), 6.09 (s, 1H), 5.98 (t, J = 3.0 Hz, 1H), 4.89−4.84 (m, 1H), 4.19 (s, 2H), 3.71−3.64 (m, 2H), 2.59 (s, 3H), 1.72 (d, J = 6.5 Hz, 3H), 1.63 (d, J = 6.5 Hz, 3H), 1.20 (d, J = 7.0 Hz, 6H), 0.93 (d, J = 7.0 Hz, 6H), 0.71 (t, J = 8.0 Hz, 3H), 0.04−0.01 (m, 2H). 13C NMR (125 MHz, CDCl3): δ 148.4, 146.6, 134.5, 125.0, 124.3, 122.0, 109.5, 107.2, 68.4, 47.4, 33.6, 28.3, 26.1, 26.0, 25.7, 24.7, 9.5, 0.5. IR (KBr pellet, cm−1): ν 2964 (s), 2867 (m), 1496 (w), 1462 (m), 1448 (m), 1382 (w), 1195 (m), 956 (m), 855 (m), 782 (s), 725 (s). Anal. Calcd for C46H74Al2N4O2: C, 71.84; H, 9.70; N, 7.29. Found: C, 71.62; H, 9.47; N, 7.65. Ring-Opening Polymerization of ε-CL Using 1−9 as SingleComponent Catalysts. A mixture of complex 4 (4.9 mg, 0.016 mmol) in 2.0 mL of toluene and ε-CL (2 mL, 2.0 M in toluene, 4.0 mmol) was stirred at 70 °C for 20 min, during which an increase in viscosity was observed. After stirring for the prescribed time, the reaction mixture was quenched with wet n-hexane. After removal of the volatiles, the residue was subjected to 1H NMR analysis. Monomer conversion was determined by calculation of the integration of monomer vs polymer methylene resonance in the 1H NMR (CDCl3, 500 MHz) spectrum. The polymer was purified by dissolving the crude samples in THF and precipitating into methanol. The obtained polymers were dried to a constant weight, and the dry polymer samples were analyzed by GPC. Ring-Opening Polymerization of L-LA Using 2−6 and 8 as Single-Component Catalysts. A mixture of complex 4 (6.2 mg, 0.02 mmol) and L-LA (288 mg, 2.0 mmol) in 2 mL of toluene was stirred and then immersed into an oil bath at 80 °C for polymerization. The polymerization was quenched by the addition of wet n-hexane. After removal of the volatiles, the residue was subjected to 1H NMR analysis. Monomer conversion was determined by calculation of the methine resonance integration of monomer vs polymer in the 1H
NMR (CDCl3, 500 MHz) spectrum. The purification of the polymer in each case was managed by dissolving the crude samples in THF and precipitating the polymer solution with methanol. The obtained polymers were further dried in a vacuum oven at 60 °C for 24 h. The polymer samples were analyzed by GPC. X-ray Crystallographic Analyses of Aluminum Complexes. Suitable crystals of complexes 1−6 and 8 were each mounted in a sealed capillary. Diffraction was performed on a Bruker SMART APEXII CCD area detector diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) at 293(2) K, with φ and ω scan techniques. An empirical absorption correction was applied using the SADABS program.22 All structures were solved by direct methods, completed by subsequent difference Fourier syntheses, and refined anisotropically for all non-hydrogen atoms by full-matrix least-squares calculations based on F2 using the SHELXTL program package.23 The hydrogen atom coordinates were calculated with SHELXTL by using an appropriate riding model with varied thermal parameters. The residual electron densities were of no chemical significance. Selected bond lengths and angles are compiled in Table 1, and crystal data and details of the data collection and structure refinements are given in the Supporting Information.
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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.organomet.6b00327. Crystallographic and data processing parameters for 1−6 and 8; NMR spectra data for L1, L2H, L3H, and 1−9 (PDF) Crystallographic data for 1−6 and 8 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S. Wang). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation (21432001, 21372010, 21202002), the National Basic Research Program of China (2012CB821600), and Special and Excellent Research Fund of Anhui Normal University.
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DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.6b00327 Organometallics XXXX, XXX, XXX−XXX
