Article pubs.acs.org/Organometallics
Well-Designed Unsymmetrical Salphen-Al Complexes: Synthesis, Characterization, and Ring-Opening Polymerization Catalysis Wenlong Luo,† Tong Shi,† Shaofeng Liu,*,†,‡ Weiwei Zuo,‡ and Zhibo Li*,† †
School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, P. R. China
‡
S Supporting Information *
ABSTRACT: The unsymmetrical salphen ligand (3,5-tBu-1OH-C 6 H 2 )CHN-C 6 H 4 -NCH(3-Ph-1-OH-C 6 H 3 ) ( tBu‑PhLH2) was designed and synthesized to support aluminum complexes. Its symmetric analogues, (3,5-tBu-1OH-C 6 H 2 )CHN-C 6 H 4 -NCH(3,5- t Bu-1-OH-C 6 H 2 ) (tBuLH2) and (3-Ph-1-OH-C6H3)CHN-C6H4-NCH(3Ph-1-OH-C6H3) (PhLH2), were also explored and compared. The methane elimination reactions between ligands and AlMe3 resulted in formation of tBu‑PhLAlMe (1), tBuLAlMe (2), and Ph LAlMe (3) in high yields, which were characterized by elemental analysis, 1H and 13C NMR. The coordination geometries of unsymmetrical and symmetric ligands in 1 and 2 were studied by X-ray diffractions, which revealed a fivecoordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1−3 reacted with BnOH at 70 °C to give tBu‑PhLAlOBn (4), tBuLAlOBn (5), and PhLAlOBn (6), respectively, which existed as monometallic species in solution as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reaction of tBu‑PhLH2 with 1 equiv of Al(OiPr)3 contained three species, one monometallic tBu‑PhLAlOiPr (7) and two bridged dimers μ-O2-(cis-tBu‑PhLAlOiPr)2 (8) and μ-O2-(trans-tBu‑PhLAlOiPr)2 (9). The catalytic performances of unsymmetrical 4 for the ring-opening polymerization of racemic lactide (rac-LA) were preliminarily studied and compared to those of symmetric 5 and 6.
1. INTRODUCTION The tetradentate Schiff bases, salen ligand and its derivatives, have more than a century of history. In the last two decades, they were used as one of the most important synthetic ligand systems in the areas of coordination chemistry and asymmetric catalysis.1−6 One major breakthrough is the discovery of Jacobsen’s catalyst in 1990s, which is enormously powerful and universally applied for catalyzing asymmetric epoxidations of unfunctionalized olefins.7 Since the first reports of manganesesalen complexes by Jacobsen8 and Katsuki,9 other transition and main group metals bearing the same ligand framework have been widely developed.10−13 The success of Jacobsen’s catalyst is apparently attributed to the coordinated chiral salen ligands, which create strongly stereogenic environments around the metal centers and lead to the remarkable discrimination to enantiomers.7 On the other hand, the controlled modifications of the catalytic properties are readily achieved by tuning of the substituents on two phenolate rings and the backbones. Inspired by these studies, Spassky reported an aluminum complex based on a chiral Schiff base and successfully synthesized isotactic PLA from rac-LA by a site control mechanism.14 Baker15 and Coates16,17 extended this research and synthesized a series of PLA with different tacticity depending on the nature of catalysts and feed stock. Nomura © XXXX American Chemical Society
reported aluminum complexes ligating an achiral salen ligand, exhibiting stereocontrol ROP of rac-LA by a chain-end control mechanism.18 Gibson systematically studied the relationship between the structures and the catalytic performances of this system for ROP of rac-LA.19 The current research mainly focuses on the symmetric chiral or achiral ligands having a C2-symmetric feature.20−25 In contrast, the unsymmetrical ligands might offer a much greater diversity of metal-salen species with more flexible and tunable electronic and steric properties, and thus lead to a possibility for better catalytic performances. However, the study of metal catalysts based on unsymmetrical salen-type ligands is far less investigated due to the challenge of synthesis.26 The most efficient methodology for synthesis of unsymmetrical salen-type ligands is a stepwise approach involving protection of one amino group by hydrochloric acid.27−29 Using this method, Carpentier synthesized an unsymmetrical salen-type ligand, which combined a fluorous alcohol and a phenol, and prepared corresponding Al complexes.30 Very recently, an unsymmetrical salphen-Cu complex was reported through the Cu2+-templating approach synthesis and showed unique catalytic properties in Received: February 9, 2017
A
DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 1. Synthesis of the Unsymmetrical Salphen Ligand
tBu‑Ph
LH2
Scheme 2. Synthesis of the Unsymmetrical and Symmetric Al Complexes 1−6
methyl methacrylate polymerization.31 Unfortunately, this convenient methodology could not be employed for the preparation of Al complexes due to the high sensitivity of Al precursor and led to an untraceable mixture. Herein, we reported the straightforward synthesis of the unsymmetrical salphen ligand with different substituents on the two phenolate rings without protection of an amino group. Aluminum complexes supported by this unsymmetrical ligand were prepared and characterized by NMR and X-ray diffractions. The Al complexes bearing symmetric salphen ligands were also synthesized for comparison. Preliminary investigations for ROP of rac-LA using unsymmetrical Al complexes were reported and compared to the symmetric analogues.
protection (Scheme 1). Thus, the reaction of 3-phenylsalicylaldehyde and a in EtOH selectively formed unsymmetrical tBu‑PhLH2 as a precipitate in 85% yield. However, the intermediate b reacted with 3,5-tBu-salicylaldehyde under a wide range of conditions to give a mixture of unsymmetrical tBu‑Ph LH2 and symmetric tBuLH2 and PhLH2. tBu‑PhLH2 was stable in the solid state and in H2O-free solution. Its structure was confirmed by elemental analysis and NMR spectroscopy. In the 1H NMR spectrum of tBu‑PhLH2, the resonances at 13.66 pm and 13.52 ppm for OH and 8.72 and 8.64 ppm for CHN clearly illustrated the unsymmetrical structure of the ligand. In sharp contrast, there was only one set of signals for OH and CHN in the 1H NMR spectra of symmetric tBuLH2 and Ph LH2, which were prepared according to the previous reports.19,32 We also tried to synthesize other similar unsymmetrical ligands using this method. When 3,5-dibromosalicylaldehyde was employed as the second sequential salicylaldehyde, the unsymmetrical tBu‑BrLH2 (Scheme S1) was obtained selectively as an orange solid. However, the synthesis of tBu‑H LH2 (Scheme S1) using this methodology was not successful. The details of synthesis and characterization of unsymmetrical tBu‑BrLH2 ligand and the corresponding Al compound are shown in the Supporting Information.
2. RESULTS AND DISCUSSION 2.1. Synthesis of the Unsymmetrical Salphen Ligand. The straightforward condensation of diamine with two different salicylaldehydes usually results in a statistical mixture of one unsymmetrical and two symmetric salen-type ligands.27−29 In order to prepare unsymmetrical salen-type ligands, the methodology that protecting one amino group and isolating the monoimine intermediate is generally employed. In this context, we found that it was possible to synthesize unsymmetrical salphen ligand via an intermediate a without the B
DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
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Organometallics 2.2. Synthesis of the Aluminum Complexes. The ligands reacted with 1 equiv of AlMe3 in toluene at room temperature to give Al methyl complexes tBu‑PhLAlMe (1), tBu LAlMe (2), and PhLAlMe (3) in high yields (Scheme 2). Again, the unsymmetrical nature of tBu‑PhLAlMe was suggested by nonequivalent two CHN and phenolate rings according to 1H NMR (see the Experimental Section). Crystals of unsymmetrical tBu‑PhLAlMe (1) and symmetric tBuLAlMe (2) were obtained by slow diffusion of hexane to their CH2Cl2 solutions. The thermal ellipsoid plots for 1 and 2 are given in Figures 1 and 2, both of which revealed a distorted square-
factor. These differences would lead to their different catalytic properties in the ROP of rac-LA (vide infra). Since aluminum alkoxides are better catalysts for ROP regarding the efficiency and molecular weight control,35−39 we tried to convert 1−3 into the corresponding Al-alkoxides 4−6 using 1 equiv of BnOH in toluene. The reactions were slow, but completed in 12 h at 70 °C (Scheme 2). Comparing the 1H NMR spectra of 1 and 4, the resonance at high field for Al-Me in 1 disappeared and a new resonance around 5.00 ppm for PhCH2-O in 4 was observed. Note that the 1H NMR spectrum of 4 had only one set of resonances (see the Experimental Section), indicating a monometallic isomer in solution. Otherwise, there should be two bridged dimers due to the unsymmetrical substituents on the phenolate rings (see the discussion below for AlOiPr compounds). Moreover, the27Al NMR of tBu‑PhLOBn (4) had a resonance at 38 ppm and no resonance around 0 ppm (Figure S1), which also indicated a five-coordinated Al center and a monometallic species.34 The aluminum isopropoxide complexes could be prepared by the alcohol elimination reaction of tBu‑PhLH2 and Al(OiPr)3 in toluene (Scheme 3). The reaction was found to proceed slowly and completed in 2 days at 70 °C. However, this reaction was not selective, but formed a mixture of monometallic and bridged dinuclear complexes in a ratio of 1:1 according to 1H NMR (Figure 3A and C, blue). In the region of 1.65−1.30 ppm (Figure 3D), there were six singlets for tBu groups. The two bigger singlets (1.59 and 1.36 ppm) belonged to the monometallic 7, while the other four were attributed to two bridged dimers μ-O2-(cis-tBu‑PhLAlOiPr)2 (8) and μ-O2(trans-tBu‑PhLAlOiPr)2 (9) as shown in Scheme 3. To the above mixture was added 8 equiv of Al(OiPr)3 at room temperature to selectively form monometallic compound 7 (Figure 3B, red and E). After removing the excess Al(OiPr)3 through washing with hexane, the mixture of 7, 8, and 9 was recurrent in solution according to NMR experiments. It was reported that the Al metal center could be stabilized by the formation of Al···O interactions in solution.40 The excess Al(OiPr)3 in solution could compete with the other tBu‑PhLOiPr molecule and interact with the Al center in 7. This would stabilize the monometallic species and shift the equilibrium to form 7 selectively (Figure 3). However, this interaction was so weak and once Al(OiPr)3 was removed from the solution, the equilibrium of monomer and dimers was recurrent. To get a better understanding of this reaction, we ran the reaction of tBu‑Ph LH2 and Al(OiPr)3 in THF at 70 °C. It was found that the reaction in THF was very slow, and after 48 h, there was over 50% of free ligand unreacted. Moreover, this reaction resulted in a mixture of untraceable compounds (Figure S3 in the SI), probably due to the partial coordination of THF. The reaction of tBu‑PhLAlMe (1) and iPrOH in toluene was also investigated, and a mixture of monomer and dimers was obtained similar as the reaction of tBu‑PhLH2 and Al(OiPr)3 in toluene. With the comparison of the cases of benzyloxide and isopropoxide Al compounds, it was believed that the bulky OBn group led to the formation of monomer due to steric hindrance, while the Al compound with a relatively small isopropoxide group existed as a mixture of monometallic and bridged dinuclear in solution. To the best of our knowledge, this is the first time to systematically study the relationship of monomer and dimer for this Al system, and it is of great importance to better understand the catalytic species of ROP for aluminum alkoxides.
Figure 1. ORTEP of the molecular structure of 1. Ellipsoids at 50% probability level. Hydrogen atoms and uncoordinated CH2Cl2 are omitted for clarity.
Figure 2. ORTEP of the molecular structure of 2. Ellipsoids at 50% probability level. Hydrogen atoms are omitted for clarity.
pyramidal geometry. The Me groups located in the axial position, which was common for salen-Al alkyl complex. The selected bond distances and angles for 1 and 2 are summarized in Tables S1 and S2 in the Supporting Information, respectively. The bond distances and angles in both of 1 and 2 were like those of previously reported salen-Al compounds.33,34 However, due to the unsymmetrical feature in 1 as compared to 2, the geometry around the Al center in the structure of 1 was more distorted than 2, which could be quantified by the torsion angle between the two iminophenolate mean planes (42.25° in 1 and 22.50° in 2). This value in 1 was even larger than that observed in unsymmetrical Ar ONCyNOCF3Al complex (36.47°) reported by Carpentier.30 Thus, the upper side of the coordination plane in unsymmetrical 1 (Figure 1) was more open comparing to that in symmetric 2 (Figure 2). The mechanism of ROP by Al complexes is usually believed as a coordination−insertion one. In both of complexes 1 and 2, the coordination and insertion of monomer would occur at the Me (or resulted alkoxide) located side, namely, the upper side, which is the main considered C
DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 3. Synthesis of the Monometallic 7 and Bridged Dinuclear 8 and 9
Figure 3. 1H NMR spectra: (A) tBu‑PhLH2 with 1 equiv of Al(OiPr)3; (B) mixture of 7, 8, and 9 with 8 equiv of Al(OiPr)3; (C) zoom in A from 3.5 to 4.5 ppm; (D) zoom in A from 0.9 to 1.7 ppm; (E) zoom in stacked A and B from 6.6 to 8.2 ppm.
Table 1. Ring-Opening Polymerization of rac-LA using 1 and 4−6a
2.3. Preliminary Studies in the Ring-Opening Polymerization of rac-LA. Salen-type aluminum complexes are well-known as initiators for the ring-opening polymerization (ROP) of lactide in a living and stereoselective manner. Therefore, we are interested in evaluating the performances of these new complexes in the ROP of rac-LA. Complex 1 was first tested with 1 equiv of BnOH (Table 1, entry 1). The polymerization was slow, and the produced PLA had a much higher Mn as compared to the calculated one, which was consistent with the previous report using an Al-alkyl compound as initiator.18 This was probably because the reaction of 1 and BnOH was slow and only part of the Al compound was activated (vide supra). Generally, the metal alkoxides catalyze ROP of lactide in a better controlled way than the alkyl
entry
cat.
t/h
conv. (%)b
Mnc
Mn (cal.)d
PDIc
P ie
1 2 3 4
1f 4 5 6
24 12 24 12
75 95 85 95
9200 6700 6800 6800
5500 6900 6200 6900
1.24 1.12 1.13 1.12
78 77 79 68
a Conditions: 20 μmol of Al, LA/Al = 50, 2 mL of toluene solution, 110 °C. bDetermined by 1H NMR. cGPC data in THF vs polystyrene standards, using a correcting factor of 0.58.44 dMn (cal.) = MLA × ([LA]: [Al]) × conversion + MBnOH. eDetermined by homodecoupled 1H NMR. f20 μmol of BnOH was added.
D
DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
(3,5-tBu-1-OH-C6H2) (tBuLH2)19 and (3-Ph-1-OH-C6H3)CHNC6H4-NCH(3-Ph-1-OH-C6H3) (PhLH2),32 were synthesized according to the methods reported in the literatures. 4.2. Synthesis of the Unsymmetrical Ligand. 3,5-tBu-2(OH)C6H2CHN-C6H4-NH2. To an EtOH solution of o-phenylenediamine (2.16 g, 20 mmol) were added 3,5-tBu-salicylaldehyde (2.34 g, 10 mmol) and p-TsOH (20 mg) at room temperature. The mixture was then refluxed for 12 h, cooled down to room temperature, and filtered. The filter cake was washed with 20 mL of EtOH to give a yellow solid (2.85 g, 8.8 mmol, 88%). 1H NMR (CDCl3): δ 13.40 (s, 1 H, OH), 8.64 (s, 1 H, CHN), 7.45 (s, 1 H), 7.23 (s, 1 H), 7.09 (t, 1 H, J = 7.5 Hz), 7.04 (d, 1 H, J = 7.5 Hz), 6.83−6.75 (m, 2 H), 4.41− 3.39 (br, 2 H, NH2), 1.50 (s, 9 H, CMe3), 1.32 (s, 9 H, CMe3). Anal. Calcd for C21H28N2O: C, 77.74; H, 8.70; N, 8.63. Found: C, 77.75; H, 8.62; N, 8.55. (3,5- t Bu-1-OH-C 6 H 2 )CHN-C 6 H 4 -NCH(3-Ph-1-OH-C 6 H 3 ) (tBu‑PhLH2). To an EtOH solution of 3,5-tBu-2-(OH)C6H2CHNC6H4-NH2 (2.85 g, 8.8 mmol) were added 3-Ph-salicylaldehyde (1.74 g, 8.8 mmol) and p-TsOH (20 mg) at room temperature, and then the mixture was stirred for 12 h. The mixture was concentrated under vacuum to 10 mL and filtered. The yellow solid obtained was purified by washing with 10 mL of EtOH (3.76 g, 7.5 mmol, 85%). 1H NMR (CDCl3): δ 13.66 (s, 1 H, OH), 13.52 (s, 1 H, OH), 8.72 (s, 1 H, CHN), 8.64 (s, 1 H, CHN), 7.66 (d, 2 H, J = 7.5 Hz), 7.49−7.43 (m, 2 H), 7.41−7.38 (m, 3 H), 7.36−7.31 (m, 3 H), 7.26−7.18 (m, 3 H), 7.00 (t, 1 H, J = 7.8 Hz), 1.45 (s, 9 H, CMe3), 1.32 (s, 9 H, CMe3). 13C NMR (CDCl3): δ 164.89, 164.16, 158.86, 158.71, 142.84, 142.41, 140.58, 137.72, 137.30, 134.37, 132.00, 130.14, 129.58, 128.40, 128.15, 127.82, 127.47, 127.14, 126.95, 120.14, 119.89, 119.59, 118.98, 118.49, 35.26, 34.30, 31.60, 29.52. Anal. Calcd for C34H36N2O2: C, 80.92; H, 7.19; N, 5.55. Found: C, 80.81; H, 7.33; N, 5.55. 4.3. Synthesis of the Unsymmetrical and Symmetric Aluminum Complexes. Synthesis of tBu‑PhLAlMe (1). To a stirred toluene solution (30 mL) of tBu‑PhLH2 (1.01 g, 2.00 mmol) was added Me3Al (1 mL, 2 M in toluene). The mixture was allowed to stir at 25 °C for 12 h and concentrated to 5 mL under reduced pressure. 10 mL n-hexane was added and filtered to obtain a yellow powder (1.03 g, 1.89 mmol, 95%). 1H NMR (C6D6): δ 8.02 (s, 1 H, CHN), 7.98 (s, 1 H, CHN), 7.92 (d, 2 H, J = 7.2 Hz), 7.76 (s, 1 H), 7.50 (d, 1 H, J = 7.0 Hz), 7.41 (t, 2 H, J = 7.4 Hz), 7.28 (t, 1 H, J = 7.4 Hz), 6.99 (s, 1 H), 6.93 (d, 1 H, J = 7.6 Hz), 6.90−6.84 (m, 2 H), 6.75−6.68 (m, 3 H), 1.54 (s, 9 H, CMe3), 1.38 (s, 9 H, CMe3), −0.39 (s, 3 H, AlMe). 13 C NMR (C6D6): δ 164.67, 162.35, 161.86, 141.80, 139.55, 139.11, 138.93, 138.38, 137.71, 134.74, 133.69, 132.03, 129.79, 128.75, 128.31, 128.17, 127.41, 126.55, 119.62, 118.17, 116.82, 116.30, 116.20, 35.21, 34.17, 31.45, 29.10, −10.08. Anal. Calcd for C35H37AlN2O2: C, 77.18; H, 6.85; N, 5.14. Found: C, 77.32; H, 6.80; N, 5.01. Synthesis of tBuLAlMe (2). Using the similar method for 1, 2 was obtained as a yellow solid (1.01 g, 1.74 mmol, 87%). 1H NMR (C6D6): δ 8.13 (s, 2 H, CHN), 7.85 (d, 2 H, J = 2.1 Hz), 7.03 (d, 2 H, J = 1.9 Hz), 6.88 (dd, 2 H, J = 5.9 Hz, J = 3.2 Hz), 6.75 (dd, 2 H, J = 5.8 Hz, J = 3.3 Hz), 1.88 (s, 18 H, CMe3), 1.41 (s, 18 H, CMe3), −0.47 (s, 3 H, AlMe). 13C NMR (C6D6): δ 165.67, 162.14, 142.05, 139.01, 138.33, 132.17, 128.25, 127.87, 119.22, 115.91, 36.19, 34.24, 31.62, 30.35, −7.96. Anal. Calcd for C37H49AlN2O2: C, 76.52; H, 8.50; N, 4.82. Found: C, 76.32; H, 8.39; N, 4.85. Synthesis of PhLAlMe (3). Using the similar method for 1, 3 was obtained as a yellow solid (0.934 g, 1.84 mmol, 92%). 1H NMR (CDCl3): δ 8.78 (s, 2 H, CHN), 7.71 (d, 4 H, J = 7.5 Hz), 7.68 (dd, 2 H, J = 5.9 Hz, J = 3.4 Hz), 7.59 (d, 2 H, J = 7.3 Hz), 7.45 (dd, 2 H, J = 6.0 Hz, J = 3.3 Hz), 7.33 (d, 2 H, J = 7.7 Hz), 7.20 (t, 2 H, J = 7.4 Hz), 7.06 (t, 4 H, J = 7.7 Hz), 6.85 (t, 2 H, J = 7.5 Hz), −1.00 (s, 3 H, AlMe). 13C NMR (CDCl3): δ 164.42, 161.66, 138.89, 138.15, 137.43, 133.30, 129.24, 128.48, 127.79, 126.64, 119.80, 117.07, 116.15, 114.81, −6.67. Anal. Calcd for C33H25AlN2O2: C, 77.94; H, 4.96; N, 5.51. Found: C, 77.68; H, 4.72; N, 5.52. Synthesis of tBu‑PhLAlOBn (4). To a stirred toluene solution (30 mL) of 1 (0.544 g, 1.00 mmol) was added BnOH (0.108 g, 1.00 mmol) at room temperature. The mixture was allowed to stir at 70 °C for 12 h. After cooling down to room temperature, the solvent was
analogues.36,41,42 Thus, the benzyloxide aluminum complexes 4−6 were next used for the evaluations of catalysis. With 4−6 as the initiators, the PLAs with Mn’s matching the calculated values and narrow PDIs were synthesized (Table 1, entries 2− 4), suggesting a controlled polymerization. According to Gibson’s research, the steric effect of ortho-phenoxy substituents dramatically influences the catalytic activity and selectivity in a reversed manner, in which bulky substituents accelerate the polymerization rate but reduce the isoselectivity.19 Fujita reported that, in the bis(phenoxy−imine) titanium complexes, the ortho-phenyl group provided a more open active site than did the ortho-tBu group due to its rotation.43 Therefore, introduction of phenyl groups instead of tBu groups onto the phenolate rings would accelerate the polymerization process but reduce the isotacticity of PLAs (Table 1, entry 3 vs 4). Note that the unsymmetrical 4 was more active than symmetric 5 and comparable to 6. In regard to the isoselectivity, the value of 4 was higher than that of 6 (77 versus 68), but similar as that of 5 (77 versus 79). This result indicated the unsymmetrical 4 took the advantage of combining t Bu and Ph groups and showed interesting catalytic properties in activity and selectivity as compared to its symmetric analogues (Table 1, entry 2 vs entries 3 and 4).
3. CONCLUSION In summary, we designed and synthesized unsymmetrical salphen Al complex tBu‑PhLAlMe (1) and its symmetric analogues tBuLAlMe (2) and PhLAlMe (3). The methyl complexes 1−3 slowly reacted with 1 equiv of BnOH at 70 °C in toluene to form the Al benzyloxide complexes tBu‑Ph LAlOBn (4), tBuLAlOBn (5), and PhLAlOBn (6), which were monometallic compounds in solution as indicated by NMR studies. In contrast, the Al isopropoxide complexes prepared from the reaction of tBu‑PhLH2 with 1 equiv of Al(O i Pr) 3 contained three species, one monometallic tBu‑Ph LAlO i Pr (7) and two bridged dimers μ-O 2 tBu‑Ph LAlOiPr)2 (8) and μ-O2-(trans-tBu‑PhLAlOiPr)2 (9). (cisAccording to the NMR experiments, the bridged dimers 8 and 9 would convert to monometallic 7 in the presence of excess Al(OiPr)3, and this transformation was reversed by removing the Al(OiPr)3. The preliminary study for ROP of rac-LA revealed that the unsymmetrical 4 took the advantage of combining tBu and Ph groups and exhibited intriguing catalytic properties in activity and selectivity as compared to its symmetric analogues 5 and 6. Further studies including copolymerization and using the unsymmetrical ligands to support rare earth metals are in progress in our group. 4. EXPERIMENTAL SECTION 4.1. General Considerations. All moisture/oxygen-sensitive reactions/compounds were performed using standard Schlenk techniques or glovebox techniques in an atmosphere of high-purity nitrogen. CH2Cl2 was dried over CaH2, distilled, and stored over 4A MS. THF, toluene, and n-hexane were dried by refluxing over sodium and benzophenone. CDCl3 dried over CaH2 and C6D6 dried over Na/ K were vacuum transferred prior to use. rac-LA, anhydrous BnOH, and Al(OiPr)3 were purchased from TCI and used as received. FT-IR, elemental analyses, and NMR were performed on a Bruker Tensor 27 instrument, PE2400II Series and Bruker DMX-500 (500 MHz for 1H, 125 MHz for 13C) instrument, respectively. The GPC measurements were collected on a Wyatt OPTILAB rEX refractive index detector using THF as the eluent (flow rate: 1 mL min−1, at 40 °C) and polystyrenes as standard with a correcting factor of 0.58.44 The symmetric ligands, (3,5-tBu-1-OH-C6H2)CHN-C6H4-NCHE
DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics removed under vacuum and the residue was washed with 20 mL of hexane (0.591 g, 0.93 mmol, 93%). 1H NMR (CDCl3): δ 8.11 (s, 1 H, CHN), 8.08 (s, 1 H, CHN), 7.88 (s, 1 H), 7.87 (s, 1 H), 7.78 (d, 1 H, J = 2.2 Hz), 7.52−7.47 (m, 1 H), 7.42 (d, 2 H, J = 7.6 Hz), 7.30 (t, 1 H, J = 7.4 Hz), 7.24 (s, 1 H), 7.22 (s, 1 H), 7.04−6.87 (m, 7 H), 6.79 (s, 1 H), 6.77 (s, 1 H), 6.71 (t, 1 H, J = 7.5 Hz), 5.03 (d, 1 H, J = 14.0 Hz, OCHH), 4.99 (d, 1 H, J = 14.0 Hz, OCHH), 1.56 (s, 9 H, CMe3), 1.37 (s, 9 H, CMe3). 13C NMR (CDCl3): δ 165.78, 163.63, 163.35, 146.59, 142.74, 140.28, 139.96, 139.71, 139.03, 134.74, 133.26, 130.87, 129.59, 129.37, 129.01, 128.47, 127.63, 127.18, 126.57, 120.49, 119.28, 118.30, 117.15, 117.01, 66.25, 36.25, 35.22, 32.45, 30.27. Anal. Calcd for C41H41AlN2O3: C, 77.34; H, 6.49; N, 4.40. Found: C, 77.21; H, 6.32; N, 4.39. Synthesis of tBuLAlOBn (5). Using the similar method for 4, 5 was obtained as a yellow solid (0.646 g, 0.96 mmol, 96%). 1H NMR (C6D6): δ 8.20 (s, 2 H, CHN), 7.87 (d, 2 H, J = 2.4 Hz), 7.06 (d, 2 H, J = 6.8 Hz), 7.03 (d, 2 H, J = 2.3 Hz), 6.94−6.87 (m, 5 H), 6.77 (dd, 2 H, J = 6.0 Hz, J = 3.4 Hz), 4.89 (s, 2 H, OCH2), 1.90 (s, 18 H, CMe3), 1.41 (s, 18 H, CMe3). 13C NMR (C6D6): δ 165.64, 162.19, 146.75, 141.88, 138.84, 138.72, 132.21, 129.62, 127.47, 126.90, 126.29, 125.53, 119.22, 115.61, 65.54, 36.25, 34.27, 31.60, 30.42. Anal. Calcd for C43H53AlN2O3: C, 76.75; H, 7.94; N, 4.16. Found: C, 76.55; H, 7.78; N, 4.30. Synthesis of PhLAlOBn (6). Using the similar method for 4, 6 was obtained as a yellow solid (0.582 g, 0.97 mmol, 97%). 1H NMR (CDCl3): δ 8.81 (s, 2 H, CHN), 7.69 (d, 4 H, J = 7.5 Hz), 7.64− 7.60 (m, 4 H), 7.47−7.42 (m, 2 H), 7.32 (dd, 2 H, J = 7.7, 1.4 Hz), 7.21 (t, 2 H, J = 7.4 Hz), 7.04 (t, 4 H, J = 7.7 Hz), 6.97−6.95 (m, 1 H), 6.93 (d, 4 H, J = 4.2 Hz), 6.89 (t, 2 H, J = 7.6 Hz), 4.60 (s, 2 H, OCH2Ph). 13C NMR (CDCl3): δ 164.19, 162.07, 145.11, 138.57, 137.77, 133.58, 129.41, 128.47, 127.95, 127.44, 126.78, 126.05, 125.51, 119.50, 117.51, 115.79, 64.96. Anal. Calcd for C39H29AlN2O3: C, 77.99; H, 4.87; N, 4.66. Found: C, 77.88; H, 4.59; N, 4.80. Synthesis of tBu‑PhLAlOiPr (7), μ-O2-(cis-tBu‑PhLAlOiPr)2 (8), and μO2-(trans-tBu‑PhLAlOiPr)2 (9). To a stirred toluene solution (30 mL) of tBu‑Ph LH2 (0.504 g, 1.00 mmol) was added Al(OiPr)3 (0.204 g, 1.00 mmol) at room temperature. The mixture was allowed to stir at 70 °C for 2 days. After cooling down to room temperature, the solvent was removed under vacuum and the residue was washed with 20 mL of hexane. 1H NMR of the product indicated three isomers: one monomer 7 and two dimers 8 and 9 in the ratio of 1.0:0.5:0.5. To the above mixture was added 8 equiv of Al(OiPr)3, and the mixture was stirred overnight. All the solvent was removed under vacuum to give 7 + Al(OiPr)3: 1H NMR (C6D6): δ 8.17 (s, 1 H, CHN), 8.14 (s, 1 H, CHN), 7.98 (d, 2 H, J = 7.5 Hz), 7.78 (d, 1 H, J = 2.4 Hz), 7.51 (d, 1 H, J = 7.2 Hz), 7.47 (t, 2 H, J = 7.7 Hz), 7.32 (t, 1 H, J = 7.4 Hz), 7.02 (d, 1 H, J = 2.2 Hz), 6.98 (d, 1 H, J = 7.8 Hz), 6.93−6.84 (m, 4 H), 6.71 (t, 1 H, J = 7.5 Hz), 4.70 (dt, 12 H, J = 12.4 Hz, J = 6.2 Hz, excess of Al(OiPr)3), 4.48−4.36 (m, 12 H, excess of Al(OiPr)3), 4.25 (dt, 1 H, J = 11.9 Hz, J = 6.0 Hz, CH(CH3)2), 1.69 (d, 36 H, J = 6.2 Hz, excess of Al(OiPr)3), 1.59 (s, 9 H, CMe3), 1.39 (d, 36 H, J = 6.2 Hz, excess of Al(OiPr)3), 1.36 (s, 9 H, CMe3), 1.32 (d, 72 H, J = 5.8 Hz, excess of Al(OiPr)3), 1.18 (d, 3 H, J = 2.6 Hz, CHCH3CH3), 1.17 (d, 3 H, J = 2.6 Hz, CHCH3CH3). 13C NMR (CDCl3): δ 164.93, 162.51, 162.3, 141.44, 139.32, 138.91, 138.69, 137.64, 134.47, 133.34, 132.04, 129.76, 128.50, 128.20, 127.93, 127.32, 126.54, 119.13, 117.93, 117.05, 116.16, 115.95, 62.56, 35.01, 33.96, 31.27, 28.86, 27.07. Currently, we could not separate or selectively synthesize the dimers 8 and 9 from the monomer 7. However, comparing the 1H NMR spectra of a mixture of 7 + 8 + 9 and 7 + Al(OiPr)3, the assignments for the two dimers 8 and 9 are given as follows: 7.92 + 7.90 + 7.89 + 7.85 (s, 8 H, CHN), 7.72−7.68 (m, 8 H), 7.62 (d, 2 H, J = 2.5 Hz) + 7.60 (d, 2 H, J = 2.5 Hz), 7.43−7.28 (m, 16 H), 6.93−6.72 (m, 24 H), 6.64− 6.59 (m, 4 H), 3.70−3.62 (m, 4 H), 1.42 + 1.40 + 1.39 + 1.33 (s, 72 H, t Bu), 0.95 (d, 24 H, iPr). 4.4. The ROP of rac-LA. A general polymerization procedure (Table 1, entry 2) is given as follows: 4 (0.020 mmol), rac-LA (1.00 mmol), and 2.00 mL of toluene were loaded into a 25 mL tube and sealed with a Teflon cap in the glovebox at room temperature. The tube was brought out and placed into the oil bath (110 °C). After a
desired time, the reaction was quenched by drops of glacial acetic acid. A small part of the solution was taken and dried for 1H NMR characterization to determine the conversion. The other solution was poured into methanol (200 mL) to precipitate polymer. 4.5. Crystal Structure Determinations. Single crystals of tBu‑Ph LAlMe (1) and tBuLAlMe (2) suitable for X-ray structural analysis were obtained by diffusing hexane into their CH2Cl2 solutions at room temperature. The data were collected on a Rigaku RAXIS Rapid IP diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 100(2) K. Using Olex2,45 the structures were solved and refined by XS46 and SHELXL,47 respectively. The H atoms were placed into the geometrically calculated positions and refined riding on the atoms to which they should be attached. Crystal data and processing parameters for 1 and 2 are given in Table S3.
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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.7b00106. Synthesis of unsymmetrical tBu‑BrLH2 and corresponding Al complexes. Selected distances (Å), angles (deg), crystal data, and structure refinement for 1 and 2 (PDF) Crystallographic details for 1 and 2 (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S.L.). *E-mail:
[email protected] (Z.L.). ORCID
Shaofeng Liu: 0000-0002-1230-0946 Zhibo Li: 0000-0001-9512-1507 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The project was supported by NSFC No. B040102, the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (Donghua University) No, LK1501, and the Department of Science and Technology of Qingdao and Shandong Province Nos. 159181jch and 2015GGX107015.
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REFERENCES
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DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.7b00106 Organometallics XXXX, XXX, XXX−XXX
