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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Exploring Steric Effects of Zinc Complexes Bearing Achiral Benzoxazolyl Aminophenolate Ligands in Isoselective Polymerization of rac-Lactide Jianwen Hu, Chao Kan, and Haiyan Ma* Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China
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ABSTRACT: A series of tridentate achiral benzoxazolylbased aminophenolate zinc complexes, LZnN(SiMe3)2 (L = 2{[benzoxazoly-CH2N(R3)-]CH2}-6-R1-4-R2-C6H2O, R1 = R2 = Cl, R3 = Bn (1); R1 = R2 = tBu, R3 = Bn (2); R1 = trityl, R2 = Me: R3 = Bn (3); R3 = phenethyl (4); R3 = 3-methylbutyl (7); R3 = n-hexyl (8); R3 = cyclopentyl (9); R3 = cyclooctyl (11); R3 = 1-adamantyl (12)), was synthesized via the reactions of Zn[N(SiMe3)2]2 and 1 equiv of the corresponding aminophenol proligands. All of the complexes were obtained as racemates, and the X-ray diffraction studies confirmed the monomeric structures of typical complexes 11 and 12, where the metal center is tetra-coordinated by three donors of the aminophenolate ligand and one silylamido group. All of the complexes proved to be efficient initiators for the ring-opening polymerization of rac-lactide (rac-LA) at ambient temperature, and the polymerizations were better controlled in the presence of 2-propanol. The substituents on the orthoposition of the phenoxide unit of the ligand and the skeleton nitrogen atom show significant influences on the stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of heterotactic biased polylactide (PLA) by complexes 1 and 2 (Pm = 0.40−0.44) and moderately to highly isotactic PLA by complexes 3−12 (Pm = 0.74−0.89). Detailed mechanism studies and microstructure analysis of typical PLA samples revealed that these zinc initiators afforded isotactic stereoblock PLAs via a chain-end control mechanism, and there is no obvious polymer exchange process during the polymerization process.
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limited catalyst systems proved to be isoselective.12,13 Among them, aluminum complexes ligated by salen ligands or their derivatives have been extensively investigated.14,15 Although these complexes show impressive degrees of stereocontrol, they often suffer from low activities and thus generally require unacceptably high catalyst loadings and a prolonged reaction time (24 h or longer) at elevated temperatures (70−100 °C).16−18 Isoselective catalysts based on complexes of zinc,19−24 calcium,25,26 indium,27−29 potassium,30−36 gallium,37 group IV metals,38−40 copper,41 and rare-earth metals42−45 have been rarely reported. Among these nonaluminum-based isoselective catalysts, the o-(9-anthryl)phenoxide potassium complex bearing a crown ether ancillary ligand reported by Wu and coworkers afforded PLA with the highest Pm value of 0.94, but a less accessible reaction condition of −70 °C was required.33 Meanwhile, the potassium complex bearing a similar phenoxide ligand with an ortho-xanthenyl group reported by the same group displayed the highest activity, which could convert 400 equiv of rac-LA to a conversion of 76% within 5 min at an extremely low temperature of −60 °C
INTRODUCTION Polylactide (PLA) derived from renewable resources has attracted considerable interest due to its biodegradable and biocompatible advantages. Being regarded as a potential alternative of petroleum-based plastics, PLA possesses extensive applications in medicine, packaging, agriculture, and tissue engineering fields,1−6 which benefit mainly from the specific chain microstructures of PLA.7,8 Atactic PLA is an amorphous polymer with a glass transition temperature of 60 °C.6 On the other hand, isotactic PLA is crystalline, and the Tm value of homochiral poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) is 170 °C; while that of the stereocomplexed PLA derived from the equivalent mixture of PLLA and PDLA is around 230 °C.9−11 Stereocomplexed PLAs with elevated Tm (170−220 °C) can also be achieved from the highly isoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA)2 that is a much cheaper and accessible starting material than D-LA (D-LA) based on potential industrial applications; thus, development of new initiators capable of producing isotactic stereocomplexed PLA from rac-LA is very promising. Despite intensive recent research, it is still a fascinating challenge to prepare isotactic PLAs from rac-LA, and only © XXXX American Chemical Society
Received: July 3, 2018
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DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Synthetic Route of Aminophenol Proligands and Zinc Complexesa
a
The reported proligands L5H, L6H, and L10H and complexes 5, 6, and 10 are listed for comparison.47
and produce highly isotactic PLA (Pm = 0.87).30 However, these alkali-metal catalysts suffer from relatively low isoselectivities (Pm = 0.64−0.77) at ambient temperature. Generally, it still takes several hours or even days for those nonaluminum-based isoselective catalysts to reach high monomer conversions. In 2014, Du’s group prepared the amido-oxazolinate zinc complexes yielding highly isotactic PLA (Pm = 0.91), but the full conversion of monomer was reached after 44 h at 23 °C when adopting an initiator/monomer ratio of 1:100.22 In 2017, Xu and coworkers45 reported the yttrium bis(phenolate) ether complex which produced PLA with a Pm value of 0.90 and a significantly improved activity (1.5 h, 200 equiv. −15 °C); Williams’ group reported the phosphasalen indium complex capable of converting 500 equiv of rac-LA to highly isotacitc PLA (16 h, 500 equiv., −15 °C, Pm = 0.92),46 and phenylene-Salalen zirconium complex was also reported by Kol’s group to catalyze the highly isoselective ROP of rac-LA but with a relatively lower activity (24 h, 300 equiv., 50 °C, Pm = 0.91).40 Despite these exhilarating breakthroughs, the isoselectivities and activities of currently reported catalysts/ initiators are still not practical, and the mechanism on stereocontrol as well as the structure−performance relationship of given catalysts are far from well-understood and need to be further explored with the aim of discovering initiators combining both high rates and excellent stereocontrol for racLA polymerization. Our group is interested in developing zinc and magnesium complexes stabilized by achiral or chiral aminophenolate ligands for the ring-opening polymerization of rac-LA. In 2013, our group reported some zinc complexes supported by tridentate aminophenolate ligands containing a chiral pyrrolidinyl moiety for the ROP of rac-LA, which yielded isotactic PLAs with Pm values up to 0.84 at −38 °C.19 The
stereoselectivity of this series of zinc complexes possessing multiple stereogenic centers appears to be governed essentially by the steric effects of the aminophenolate ligand. Inspired by this work, we focused on constructing ligand skeletons with less stereogenic centers. In 2017, we reported several zinc complexes bearing chiral oxazolinyl- or achiral benzoxazolylbased aminophenolate ligands, which displayed a remarkable combination of high isoselectivity and high activity toward the ROP of rac-LA (1.5 h, 200 equiv., −20 °C, Pm = 0.92). A chain-end stereocontrol mechanism proved to be involved solely in the formation of multiblock isotactic PLAs by these complexes, regardless of the existence of the ligand chirality.47 Because complexes bearing achiral ligands can be synthesized from chemicals much cheaper than those for chiral ones and therefore are of benefit to industrial applications, in this work, we further expanded the achiral benzoxazolyl-aminophenolate ligand systems by varying substituents both on the orthoposition of the phenoxide ring and on the center skeleton nitrogen atom and studied in detail the effect of substituents on the performance of the corresponding zinc complexes on the stereoselective polymerization of rac-LA.
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RESULTS AND DISCUSSION Synthesis and Characterization of Aminophenolate Zinc Complexes. According to the procedure we reported previously,47 a series of achiral benzoxazolyl-aminophenol proligands with various substituents was prepared via threestep reactions, as illustrated in Scheme 1. The corresponding zinc silylamido complexes were readily synthesized via the reactions of the aminophenol proligands with Zn[N(SiMe3)2]2 in a 1:1 molar ratio in toluene at ambient temperature and were isolated as white powders from toluene/n-hexane solutions in 41−66% yields. The previously reported B
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry complexes 5, 6, and 10 are also listed for comparison purposes.47 The reaction of Zn[N(SiMe3)2]2 with proligand L13H having a bulky trityl group on the skeleton nitrogen atom failed to afford the target zinc complex (L13)ZnN(SiMe3)2 even at 60 °C; likely, the steric bulkiness significantly hindered the reaction. All of the newly synthesized zinc silylamido complexes were characterized by 1H and 13C{1H} NMR spectroscopy as well as elemental analysis methods. In the 1H NMR spectra (C6D6) of these zinc complexes, the resonances were fully assigned, and only one set of signals accounting for the multidentate ligand and the silylamido group is shown. Therefore, all of these zinc complexes were obtained as a pair of racemates. The two protons of each methylene group in Ar−CH2−N and N− CH2−CN units of a given complex are inequivalent and give rise to two doublets, as compared to the singlets in the spectrum of the free ligand, indicating the participation of all three donors of the aminophenolate ligand in coordinating with the zinc center. The coupling constants of Ar−CH2−N and N−CH2−CN protons are normally large. For instance, four doublets at 4.36, 3.73, 2.83, and 2.77 ppm with coupling constants of 2J = 18.2 Hz or 2J = 12.0 Hz are displayed in the 1 H NMR spectrum of complex 7, suggesting that the difference in the surrounding environments of the two protons of each methylene group is distinct, which is also reflected by the significant separation of two doublets of each methylene group. All of these indicate strong coordination interactions of these N donors with the zinc center. Moreover, for complexes 1−3, the methylene protons of the pendant N-benzyl group display two doublets with very close chemical shifts in the 1H NMR spectra, which just look like one sharp peak. Whereas, in the case of complex 4, the two protons of each methylene group in the pendant NCH2CH2Ph unit become obviously inequivalent and give rise to four td resonances, which could be attributed to the restricted rotation of the NCH2CH2Ph unit. From the above description, it is suggested that the tetra-coordinating mode of these zinc complexes is maintained in solution. Molecular Structures of Complexes 11, 12, and 3d. Single crystals of complexes 11 and 12 were obtained from a mixture of toluene/n-hexane solution at room temperature. Surprisingly, homoleptic complex 3d with a bis-ligated structure was obtained when trying to get crystals of the corresponding silylamido complex 3, likely resulted from certain Schlenk balanced reactions. All of these complexes were then characterized by X-ray diffraction studies. Detailed crystal and refinement data are reported in Table S1. As shown in Figures 1 and 2, complexes 11 and 12 possess a monomeric structure in the solid state, where the zinc atom is four-coordinated by three donors of the aminophenolate ligand and one silylamido group, adopting a distorted tetrahedron geometry around the metal center. The bond distances of Zn− N1 and Zn−N2 range from 2.089(3) to 2.2620(12) Å, showing that both the skeleton N atom and the imine N atom of the benzoxazolyl ring have strong coordination interactions with the metal center, which is consistent with the structure in solution inferred from the 1H NMR spectroscopic studies. Due to the steric repulsion between the bis(trimethylsilyl)amido group and the aminophenolate ligand, the corresponding angles of N3−Zn1−O1 (116.99(5)−119.38(10)°), N3−Zn1− N skeleton (125.49(5)−126.53(11)°), and N3−Zn1−N benzoxazolyl (122.12(10)−125.50(6)° ) in these zinc complexes are all deviated from ideal 109.47°. It should be further noted that, in these two structures, the skeleton nitrogen atom of the
Figure 1. Molecular structure of 11 (ellipsoids drawn at the 30% probability, H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn1−O1 1.911(2), Zn1−N1 2.089(3), Zn1−N2 2.252(2) Zn1−N3 1.913(2), O1−Zn1−N3 119.38(10), O1−Zn1− N1 102.56(9), N3−Zn1−N1 126.53(11), O1−Zn1−N2 97.95(9), N3−Zn1−N2 122.12(10), N1−Zn1−N2 78.95(10).
Figure 2. Molecular structure of 12 (ellipsoids drawn at the 30% probability, H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn1−O1 1.9146(12), Zn1−N1 2.2620(12), Zn1− N2 2.1124(13) Zn1−N3 1.9172(13); O1−Zn1−N3 116.99(5), O1− Zn1−N1 94.97(5), N3−Zn1−N1 125.49(5), O1−Zn1−N2 105.51(5), N3−Zn1−N2 125.50(6), N1−Zn1−N2 79.90(5).
aminophenolate ligand becomes chiral upon complexation; meanwhile, the metal zinc center is also chiral because of the chelation of the tridentate ligand. However, only one pair of racemates is observed in the primitive cell of each complex, showing a close relation of these two stereogenic centers. As depicted in Figure 3, in the structure of the homoleptic complex 3d, the zinc atom is five-coordinated by two multidentate ligands with one benzoxazolyl ring located far away from the metal center. The two axial positions of the trigonal bipyramid are occupied by the skeleton N atoms of the two ligands, and both of the skeleton N atoms possess Sconfiguration. The equatorial positions are occupied by the two phenoxy oxygen atoms and one imine N atom of the two ligands. Being obtained as a pair of racemates, the structure C
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
2, these zinc silylamido complexes are efficient initiators for the ROP of rac-LA and the polymerization runs can reach completion within a few minutes or hours at 25 °C depending on the structures of the complexes, giving PLAs with high molecular weights and narrow to broad molecular weight distributions (Mw/Mn = 1.08−1.88).47,48 Moreover, most of these zinc complexes exhibit moderate to high isoselectivities (Pm = 0.74−0.89) toward the polymerization of rac-LA. The substituent at the ortho-position of the phenolate ring of the ligand exerts significant influence both on the catalytic activity and on the stereoselectivity of the corresponding zinc complex. When the polymerization was carried out in toluene, complex 1 with an o-chloro substituent on the phenolate ring required 65 min to convert 500 equiv of rac-LA to 94% monomer conversion in the presence of 2-propanol (Table 1, run 2), showing the lowest activity among complexes 1−3 bearing different ortho-substituents as indicated by the preliminary kinetic studies (Table 1 and Figure S19).49−51 Meanwhile, the stereoselectivity of this complex toward the ROP of rac-LA is also the lowest, and polymers with heterotactic preference were obtained (Pm = 0.40−0.41). The introduction of an o-tert-butyl group in complex 2 resulted in a surge of the activity, and a high conversion of 90% of the same amount of monomer could be reached just within several minutes under otherwise identical conditions (Table 1, run 4). Complex 2 is also the most active initiator among all the complexes in this work. In comparison to the significant increase in the catalytic activity, the stereoselectivity of complex 2 improved only slightly (Pm = 0.44). When the ortho-substituent of the phenolate ring was changed to a trityl group, an obvious decrease in activity was observed for complex 3 (33 min, 92%, run 6), but it is still higher than that of complex 1. The steric effect of o-trityl on the stereo-
Figure 3. Molecular structure of 3d (ellipsoids drawn at the 30% probability, H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn1−O1 1.9576(16), Zn1−O3 1.9452(16), Zn1− N1 2.3027(19), Zn1−N2 2.1674(19), Zn1−N4 2.166(2); O1−Zn1− N1 128.34(7), O1−Zn1−N2 89.56(7), O1−Zn1−N4 92.87(7), O3− Zn1−O1 127.69(7), O3−Zn1−N1 102.69(7), O3−Zn1−N2 91.75(7), O3−Zn1−N4 91.57(7), N2−Zn1−N1 76.96(7), N4− Zn1−N1 96.80(7), N4−Zn1−N2 173.45(7).
with R-configuration on both the skeleton N atoms is also observed in the primitive cell. Polymerization of rac-LA. The newly synthesized zinc complexes were evaluated as initiators for the ROP of rac-LA in toluene and tetrahydrofuran (THF) at ambient temperature. The polymerization results of the reported complexes 5, 6, and 10 are cited herein for comparison.47 As shown in Tables 1 and
Table 1. Polymerization of rac-LA Initiated by Zinc Complexes 1−12 in Toluenea
[rac-LA]0 = 1.0 M, 25 °C. bDetermined by 1H NMR spectroscopy. cMn,calcd = ([rac-LA]0/[cat.]0) × 144.14 × conv %. dDetermined by GPC with polystyrenes as standards. ePm is probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR spectroscopy. fSee Figures S19−S21 for standard deviations. gThe polymerization results of complexes 5, 6, and 10 were cited for comparison.47 hThese kapp data were obtained with a molar ratio of [rac-LA]0/[iPrOH]0/[Zn]0 = 200:1:1, [rac-LA]0 = 1.0 M. a
D
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Polymerization of rac-LA Initiated by Zinc Complexes 1−12 in THFa
[rac-LA]0 = 1.0 M, 25 °C. bDetermined by 1H NMR spectroscopy. cMn,calcd = ([rac-LA]0/[cat.]0) × 144.14 × conv %. dDetermined by GPC with polystyrenes as standards. ePm is probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR spectroscopy. fThe polymerization results of complexes 5, 6 and 10 were cited for comparison.47 g−20 °C. a
activities of complexes 9−12, with complex 12 having a bulky 1-adamantyl group displaying an extremely low activity. A decreasing tendency of the activity was further observed in the order of 9 (cyclopentyl) > 10 (cyclohexyl) > 11 (cyclooctyl) > 12 (1-adamantyl) (Table 1, Figure S21). Obviously, the enhanced steric bulkiness of the substituent on the skeleton N brings remarkable steric hindrance to the coordination sphere of the zinc center and is thus unfavorable for the coordination/ insertion of the incoming monomer. Nevertheless, a different tendency was observed for the isoselectivity, wherein complex 10 with a medium cyclohexyl group exhibits the highest isoselectivity of Pm = 0.88−0.89. Neither decreasing nor increasing the ring size provides higher isoselectivities. Currently we have no convincing explanation on the effects of different types of substituents. We tentatively attribute these to the steric hindrance that these substituents can create either by their rigid structures or via free rotation, and substituents with medium steric hindrance are beneficial for the isoselectivity. Nevertheless, the electronic effects of N-benzyl and N-phenethyl might not be ruled out (vide post). As shown in Tables 1 and 2, except for complex 1, all the complexes exhibit higher activities in toluene than in THF. For example, a monomer conversion of 90% could be reached within 6 min in toluene when using 2 as the initiator, whereas 9 min was needed to achieve a similar conversion of 94% in THF (run 4 in Tables 1 and 2). The decrease in activity in THF is somewhat common for zinc complexes bearing bulky multidentate ligands and is usually attributed to the competed coordination of the solvent molecules with the monomer.52,53 A reversed trend is observed for complex 1 with chloro substituents, which shows higher activities in THF than in toluene. Probably the coordination of THF molecule(s) would reduce the electrophilicity of the zinc center that is an unfavorable factor in the case of complex 1 with electron
selectivity is even more profound. Complex 3 exhibited moderate isoselectivity toward the polymerization of rac-LA (Pm = 0.74), which is in a sharp contrast to those of complexes 1 and 2. By comparing the polymerization data of complexes 1−3, it is clear that the electron withdrawing nature of the chloro substituent exerts an effect only on the catalytic activity, and the stereoselectivity of complexes 1−3 is determined essentially by the steric bulkiness of the ortho-substituent. In parallel to the steric hindrance of the ortho substitution, a clear increasing tendency of the isoselectivity toward the ROP of rac-LA is observed in the order of 3 (CPh3, Pm = 0.74) > 2 (tBu, Pm = 0.44) > 1 (Cl, Pm = 0.41). Keeping the superior o-trityl group unchanged, the substituent on the skeleton N atom of the ligand was varied systematically, which proved to play an important role in influencing the polymerization activity and more significantly the isoselectivity of these zinc complexes. By replacing the Nbenzyl on the skeleton N atom to a N-phenethyl group, complex 4 showed a slightly lower activity but the same isoselectivity when compared to those of complex 3 (Table 1, run 6 vs 8). However, when either an acyclic or a cyclic aliphatic group was introduced, the isoselectivities of the corresponding zinc complexes 5−12 all improved significantly (Pm = 0.80−0.89), although accompanied with somewhat increased or decreased activities. Except for complex 6 with a tert-butyl group, complexes 5, 7, and 8 with an acyclic substituent on the skeleton N atom all exhibit enhanced activities toward the polymerization in comparison with complex 3 (Table 1, Figure S20). The flexible aliphatic chain shows benefits to not only the activity but also the isoselectivity. Among them, complexes 5 and 8 with a linear aliphatic chain exhibit slightly higher isoselectivities (Pm = 0.83−0.84). Introduction of a rigid cyclic substituent to the skeleton N atom led to considerable decrease in the catalytic E
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
about 1:1:1, which suggests the formation of predominantly isotactic stereoblocks. Although these features may imply the operation of a chain-end control mechanism, an enantiomorphic site control integrated with a polymer exchange process may be also applicable because racemic initiators are thoroughly involved. To have a further understanding of the mechanism of rac-LA polymerization catalyzed by this series of zinc complexes to afford isotactic stereoblock PLAs, we wanted to know whether there exists a polymer exchange process between different active centers during the polymerization. Then, the following controlled polymerization was performed initially. At the first stage, the highly isoselective complex 10 was chosen to catalyze the polymerization of 200 equiv of rac-LA in the presence of 2-propanol in toluene at 25 °C ([rac-LA]0:[10]0: [iPrOH]0 = 200:1:1), and the polymerization reaction was carried out for 30 min to ensure a complete conversion of the monomer.47 At the second stage, another 500 equiv of rac-LA and one equiv of 2-propanol, together with the slightly heteroselective complex 2, were added to the above polymerization mixture ([rac-LA]0:[2]0:[iPrOH]0 = 500:1:1). Because the activity of complex 2 is significantly higher than that of complex 10, the monomer added at the second stage would be mainly converted by complex 2. It is assumed that, if there exists a polymer exchange process during the polymerization catalyzed by these two complexes, the molecular weight of the resulting polymer will be more or less averaged, thus affording PLA with a broad molecular weight distribution but in unimodal type; however, if the polymer exchange process is absent or negligible, the high polymerization rate of complex 2 at the second stage will produce PLA with high molecular weight, which shows significant difference from that of PLA produced by the less active complex 10, therefore leading to PLAs in bimodal distribution. The resultant polymer sample was subjected to GPC measurement, which shows a bimodal distribution with molecular weights of 2.04 × 104 and 6.83 × 104 g/mol just corresponding to the calculated values (Figure S32), indicating that there was no obvious polymer exchange process during the polymerization. Moreover, a melting point at 174 °C could be observed in the DSC curve of this sample (Figure S33), which should be attributed to the long isotactic chains produced by complex 10. To further verify the presence of a polymer exchange process or not, we chose a combination of complexes 1 and 10 with similar catalytic activities but significantly different stereoselectivities to catalyze the polymerization of 400 equiv of racLA in toluene at 25 °C ([rac-LA]0:[1]0:[10]0:[iPrOH]0 = 400:1:1:2, with both complexes added at the same time). Due to the similar activities of these two complexes, it is expected that a PLA sample with a unimodal distributed molecular weight will be obtained. It is also assumed that, if there is a polymer exchange process between the active centers generated from complexes 1 and 10, respectively, the resulting polymer chains will contain both isotactic and heterotacticbiased blocks, and the isotactic blocks would tend to be shorter when compared with those produced solely by complex 10, thus likely leading to the vanishing of the melting point peak. However, if the polymer exchange process is absent or negligible, a mixture of isotactic-block polymer chains and approximately atactic polymer chains will be obtained, which will still have a melting point. The result of this controlled polymerization shows that a PLA sample with a molecular weight of 2.76 × 104 g/mol and a unimodal molecular weight
withdrawing substituents and hence enhance the nucleophilicity of (Zn)−OR group, favoring the insertion of lactide monomer. It is further noticed that the influence of solvent on stereoselectivity depends on the type of the substituent on the skeleton N atom in these complexes. Complexes 1−4 either with a benzyl group or a phenethyl group show slightly enhanced Pm values in THF, whereas complexes 5−12 with an aliphatic group display in principle the same isoselectivities in both solvents. It seems that the pendant phenyl group of the skeleton N-substituent in complexes 1−4 might exert certain unfavorable electronic effect on the stereoselectivity and the presence of coordinative THF molecules would diminish such effect. Upon addition of 1 equiv of 2-propanol, the activities of all zinc silylamido complexes increased significantly, giving PLAs in a more controlled manner as indicated by the matched molecular weights with the calculated ones as well as narrower molecular weight distributions (Mw/Mn = 1.07−1.44). This phenomenon is in line with the cases reported by us and other groups utilizing silylamido complexes/2-propanol catalytic systems during the ROP of LA.19,24,47,52 The relative broad distributions in some cases should be attributed to the transesterification side reactions occurred at the late stage of the polymerization when high monomer conversions were reached.47,48 The polymerization process becomes even better controlled at a lower reaction temperature. For instance, the polymerization run carried at −20 °C using complex 11 as the initiator in the presence of 2-propanol afforded PLA with improved isoselectivity of Pm = 0.89 and a narrower distribution (Mw/Mn = 1.08) when compared with those carried out at ambient temperature (Table 2, runs 22 and 23). DSC measurement of this sample further indicates the formation of a stereocomplexed structure (Tm = 183 °C, Figure S29). From the 1H NMR spectra of the resultant polymer samples obtained by these zinc silylamido complexes, no clear endgroups could be distinguished. However, in the MALDI-TOF mass spectrum of a typical oligomer sample obtained by complex 10 with [rac-LA]0: [Zn]0 = 20:1, a series of signals end-capped with N(SiMe3)2 and a hydroxyl group are dominant (Figure S30), indicative of the initiation with the N(SiMe3)2 group and therefore a coordination−insertion polymerization by a single-site catalyst. Still, the intra- and intermolecular transesterifications are inevitable, as evident from the MALDI-TOF mass spectrum. To acquire some information about PLA initiated by [Zn]/iPrOH system, polymerization with [rac-LA]0: [10]0: [iPrOH]0 = 20:1:1 was conducted. The 1H NMR spectrum of the isolated oligomer indicated the existence of oligomer chains capped by −CH(CH3)OH and iPrOCC(O)− termini (Figure S31). Isoselective Polymerization Mechanism Studies. To have some insight into the isoselective polymerization mechanism conducted by most of these zinc complexes, the kinetics of rac-, D-, and L-lactide polymerizations catalyzed by complex 11 were further investigated in this work. It is found that, as previously reported for complex 6,47 as a racemic mixture complex 11 exerts no difference in the polymerization rates of D-lactide (D-LA) and L-lactide (L-LA) but a nearly 2fold rate preference for D-LA/L-LA polymerization relative to that of rac-LA in THF at 25 °C (Figure S22). Moreover, the homonuclear-decoupled 1H NMR spectrum of a typical polymer sample produced by complex 11 (Figure S27) shows that the rmm:mmr:mrm tetrad signal intensity ratio is F
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
polymerization were dried in an oven at 120 °C overnight and exposed to a vacuum−argon cycle three times. NMR spectra were recorded on a Bruker AVANCE-400 spectrometer at 25 °C (1H, 400 MHz; 13C, 100 MHz) unless otherwise stated. Chemical shifts for 1H and 13C{1H} NMR spectra were referenced internally using the residual solvent resonances and reported relative to TMS. Elemental analyses were performed on an EA-1106 instrument. Spectroscopic analyses of polymers were performed in CDCl3. Gel permeation chromatography (GPC) analyses were carried out on a Waters instrument (M1515 pump, Optilab Rex injector) in THF at 35 °C, at a flow rate of 1 mL/min. Calibration standards were commercially available narrowly distributed linear polystyrene samples that cover a broad range of molar masses (4 × 103 g/mol < Mn < 4.4 × 105 g/mol). Syntheses. 2-{N-Benzyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4,6- dichlorophenol (L 1 H). 2-(Chloromethyl)benzoxazole (1.51 g, 9.00 mmol) was dissolved in 20 mL of DMF, and then the solution was slowly added to a mixture of benzylamine (9.64 g, 90.0 mmol) and K2CO3 (1.37 g, 9.90 mmol) within about 2 h. The mixture was poured into water and extracted with ethyl acetate three times, and the organic phase was dried over anhydrous MgSO4. Evaporation of the solvent gave the target secondary amine as a viscous oil with a purity of about 80%, which was used directly for the next step. The viscous oil (2.04 g, ca. 8.6 mmol) was dissolved in 10 mL of DMF, and K2CO3 (1.30 g, 9.41 mmol) was added. Then, 2bromomethyl-4, 6-dichlorophenol (2.19 g, 8.55 mmol) was added slowly in 10 min to the above mixture. The mixture was stirred for 2 h and then poured into water and extracted with ethyl acetate three times. The organic phase was dried over anhydrous MgSO4. Evaporation of the solvent gave a viscous oil, which was purified by column chromatography (silica gel 200−300 Merck, petroleum ether/ ethyl acetate/triethylamine = 50:1:0.01) to provide white solids (2.16 g, 61%) after removal of all the volatiles. 1H NMR (CDCl3, 400 MHz, 298 K): δ 11.03 (s, 1H, OH), 7.80−7.74 (m, 1H, ArH), 7.57−7.52 (m, 1H, ArH), 7.39−7.34 (m, 6H, ArH), 7.32−7.26 (m, 1H, ArH), 7.27 (d, 1H, 4J = 2.5 Hz, ArH), 6.96 (d, 1H, 4J = 2.5 Hz, ArH), 3.96 (s, 2H, ArCH2), 3.95 (s, 2H, NCH2CN), 3.80 (s, 2H, PhCH2). 13 C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.3 (OCN), 152.3, 150.9, 140.6, 135.9, 129.6, 129.3, 129.0, 128.2, 127.8, 125.6, 124.8, 124.0, 123.8, 122.0, 120.3, 110.9 (all Ar-C), 57.9 (ArCH2), 57.0 (PhCH2), 49.3 (NCH2CN). Anal. Calcd for C22H18Cl2N2O2: C, 63.93; H, 4.39; N, 6.78. Found: C, 63.75; H, 4.47; N, 6.53%. 2-{N-Benzyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4,6-ditert-butylphenol (L2H). The procedure was the same as that of L1H, except that 2-bromomethyl-4, 6-di-tert-butylphenol (2.54 g, 8.49 mmol), N-(2-benzoxazolylmethyl)benzylamine (2.02 g, ca. 8.5 mmol), and K2CO3 (1.29 g, 9.34 mmol) were used in the second step. White solids (2.45 g, 63%) were obtained through column chromatography (silica gel 200−300 Merck, petroleum ether/ethyl acetate/triethylamine = 20:1:0.01) after removal of all the volatiles. 1 H NMR (CDCl3, 400 MHz, 298 K): δ 10.13 (s, 1H, OH), 7.78−7.72 (m, 1H, ArH), 7.58−7.52 (m, 1H, ArH), 7.43−7.31 (m, 6H, ArH), 7.31−7.27 (m, 1H, ArH), 7.25 (d, 1H, 4J = 2.0 Hz, ArH), 6.90 (d, 1H, 4 J = 2.0 Hz, ArH), 3.98 (s, 2H, ArCH2), 3.97 (s, 2H, NCH2CN), 3.81 (s, 2H, PhCH2), 1.49 (s, 9H, C(CH3)3), 1.28 (s, 9H, C(CH3)3). 13 C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.6 (OCN), 154.0, 151.0, 141.0, 136.6, 136.0, 129.9, 128.7, 127.9, 125.3, 124.6, 124.4, 123.5, 120.9, 120.3, 110.9 (all Ar-C), 58.9 (ArCH2), 57.2 (PhCH2), 49.1 (NCH2CN), 35.1 (C(CH3)3), 34.3 (C(CH3)3), 31.8 (C(CH3)3), 29.8 (C(CH3)3). Anal. Calcd for C30H36N2O2: C, 78.91; H, 7.95; N, 6.13. Found: C, 78.66; H, 7.87; N, 5.99%. 2-{N-Benzyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-methyl-6-(triphenylmethyl)phenol (L3H). The procedure was the same as that of L 1 H, except that 2-bromo methyl-4-methyl-6(triphenylmethyl)phenol (3.91 g, 8.83 mmol), N-(2benzoxazolylmethyl)benzylamine (2.23 g, ca. 8.8 mmol), and K2CO3 (1.34 g, 9.71 mmol) were used in the second step. Light yellow solids (2.94 g, 56%) were obtained through column chromatography (silica gel 200−300 Merck, petroleum ether/ethyl acetate/triethylamine = 100:1:0.01) after removal of all the volatiles.
distribution (Mw/Mn = 1.12) was obtained (Figure S34). The melting point of this sample measured by DSC is 172 °C, further indicating the existence of long isotactic chains (Figure S35). To verify whether a polymeric mixture was obtained, this polymer sample was washed with a large amount of methanol (20 mg with 80 mL of methanol at rt) on the basis of different solubilities of isotactic PLA and atactic PLA in methanol. Both the soluble and the insoluble fractions were collected and subjected to the homonuclear decoupled 1H NMR measurements. It is found that the spectrum of the original polymer sample indicates a Pm value of 0.66 (Figure S36). The soluble fraction proved to consist of most atactic PLA and a small amount of isotactic PLA, and its Pm value is 0.51 (Figure S37), while the insoluble fraction consists mainly of the isotactic PLA and a small amount of atactic PLA with the Pm value being 0.80 (Figure S38). Thus, it is clear that a polymeric mixture was obtained in this controlled polymerization, and there is no obvious polymer exchange process during the polymerization. On the basis of kinetic studies and homonuclear-decoupled 1 H NMR spectra of typical polymers as well as the results of the above controlled polymerizations, we believe that a chainend control mechanism is involved in producing isotactic stereoblock PLAs by this series of racemic zinc complexes, and there is no obvious polymer exchange process during the polymerization.
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CONCLUSION A series of zinc complexes supported by achiral benzoxazolyl aminophenolate ligands was synthesized and structurally characterized. Most of these complexes can serve as highly isoselective and active initiators for the ring-opening polymerization of rac-LA. The substituents of the ancillary ligand, both at the ortho-position of the phenolate ring and on the skeleton N atom of the ligand framework, have a profound influence on the catalytic activity and stereoselectivity. In general, a less bulky group at the ortho-position of the phenoxy moiety and a less bulky group on the skeleton N atom of the ligand would benefit the catalytic activity of the corresponding zinc complexes. High isoselectivities were observed for complexes with a trityl group at the ortho-position of the phenolate ring and a rigid cyclic substituent on the skeleton N atom of the ligand. Detailed mechanism studies and microstructure analysis of typical PLA samples revealed that these zinc initiators afforded isotactic stereoblock PLAs via a chain-end control mechanism, and there is no obvious polymer exchange process during the polymerization.
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EXPERIMENTAL SECTION
Materials and Methods. All manipulations were carried out under a dry argon atmosphere using standard Schlenk-line or glovebox techniques. Toluene and n-hexane were refluxed over sodium benzophenone ketyl prior to use. Benzene-d6, chloroform-d, and other reagents were carefully dried and stored in the glovebox. rac-Lactide from Jinan Daigang Biomaterial Co., Ltd. was recrystallized with dry toluene and then sublimed twice under vacuum at 100 °C. 2-Propanol was dried over calcium hydride under argon prior to distillation. K[N(SiMe3)2] and Zn[N(SiMe3)2]2 were synthesized according to the literature method.54 2-Bromomethyl-4,6-dichlorophenol,55 2-bromomethyl-4,6-di-tert-butylphenol,56 2-bromomethyl4-methyl-6- (triphenylmethyl)phenol,57,58 and 2-chloromethylbenzoxazole59 were synthesized according to the reported literature procedures. All other chemicals were commercially available and used after appropriate purification. Glassware and vials used in the G
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry H NMR (CDCl3, 400 MHz, 298 K): δ 9.83 (s, 1H, OH), 7.72−7.70 (m, 1H, ArH), 7.50−7.48 (m, 1H, ArH), 7.37−7.32 (m, 2H, ArH), 7.26−7.19 (m, 15H, ArH), 7.16−7.12 (m, 3H, ArH), 6.93−6.91 (m, 3H, ArH), 6.82 (br s, 1H, ArH), 3.93 (s, 2H, ArCH2), 3.75 (s, 2H, PhCH2), 3.55 (s, 2H, NCH2CN), 2.16 (s, 3H, ArCH3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.0 (OCN), 153.8, 150.7, 146.0, 140.9, 136.0, 134.2, 131.2, 130.0, 129.3, 128.5, 127.6, 127.1, 125.5, 125.1, 124.4, 121.7, 120.1, 110.8 (all Ar-C), 63.3 (Ph3C), 58.3 (ArCH2), 55.9 (PhCH2), 48.2 (NCH2CN), 21.0 (ArCH3). Anal. Calcd for C42H36N2O2: C, 83.97; H, 6.04; N, 4.66. Found: C, 83.93; H, 6.19; N, 4.69%. 2-{N-Phenethyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4methyl-6-(triphenylmethyl)phenol (L4H). The procedure was the same as that of L1H, except that phenethylamine (10.91 g, 90.0 mmol) was used in the first step, and 2-bromomethyl-4-methyl-6(triphenylmethyl)phenol (3.91 g, 8.83 mmol), N-(2benzoxazolylmethyl)phenethylamine (2.23 g, ca. 8.8 mmol), and K2CO3 (1.34 g, 9.71 mmol) were used in the second step to provide L4H as colorless crystals via recrystallization from dichloromethane and petroleum ether (3.14 g, 58%). 1H NMR (400 MHz, CDCl3, 298 K): δ 9.69 (s, 1H, ArOH), 7.75−7.69 (m, 1H, ArH), 7.53−7.47 (m, 1H, ArH), 7.39−7.33 (m, 2H, ArH), 7.25−7.18 (m, 13H, ArH), 7.17−7.09 (m, 5H, ArH), 7.03−6.98 (m, 2H, ArH), 6.95 (d, 1H, 4J = 1.7 Hz, ArH), 6.82 (d, 1H, 4J = 1.7 Hz, ArH), 3.94 (s, 2H, ArCH2N), 3.92 (s, 2H, NCH2CN), 2.74−2.67 (m, 2H, NCH2CH2), 2.67− 2.60 (m, 2H, NCH2CH2), 2.18 (s, 3H, ArCH3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.2 (OCN), 154.0, 150.9, 146.1, 141.0, 139.1, 134.3, 131.2, 129.2, 128.8, 128.5, 127.3, 127.1, 126.4, 125.5, 125.3, 124.6, 121.9, 120.2, 111.0 (all Ar-C), 63.4 (Ph3C), 58.5 (ArCH2), 54.4 (NCH2CH2), 49.1 (NCH2CN), 33.3 (NCH2CH2), 21.1 (ArCH3). Anal. Calcd for C43H38N2O2: C, 84.01; H, 6.23; N, 4.56. Found: C, 84.10; H, 6.24; N, 4.43%. 2-{N-3-Methylbutyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4methyl-6-(triphenylmethyl)phenol (L7H). The procedure was the same as that of L1H, except that 3-methylbutylamine (7.84 g, 90.0 mmol) was used in the first step, and 2-bromomethyl-4-methyl-6(triphenylmethyl)phenol (3.81 g, 8.59 mmol), N-(2-benzoxazolylmethyl)-3-methylbutylamine (1.87 g, ca. 8.6 mmol), and K2CO3 (1.31 g, 9.45 mmol) were used in the second step to provide L7H as colorless crystals via recrystallization from dichloromethane and petroleum ether (3.05 g, 61%). 1H NMR (400 MHz, CDCl3, 298 K): δ 9.88 (s, 1H, OH), 7.74−7.67 (m, 1H, ArH), 7.52−7.46 (m, 1H, ArH), 7.39−7.31 (m, 2H, ArH), 7.25−7.16 (m, 12H, ArH), 7.15− 7.08 (m, 3H, ArH), 6.91 (d, 1H, 4J = 1.6 Hz, ArH), 6.81 (d, 1H, 4J = 1.6 Hz, ArH), 3.88 (s, 2H, ArCH2), 3.84 (s, 2H, NCH2CN), 2.46− 2.38 (m, 2H, NCH2CH2), 2.17 (s, 3H, ArCH3), 1.42−1.33 (m, 1H, NCH2CH2CH(CH3)2), 1.32−1.24 (m, 2H, CH2 of NCH2CH2CH(CH3)2), 0.78 (d, 6H, 3J = 6.5 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.4 (OCN), 154.1, 150.9, 146.2, 140.9, 134.2, 131.2, 131.1, 129.1, 127.1, 125.5, 125.2, 124.5, 121.9, 120.2, 110.9 (all Ar-C), 63.3 (Ph3 C), 58.4 (ArCH 2), 51.1 (NCH 2 CH 2 ), 48.8 (NCH 2 CN), 35.3 (NCH 2 CH 2 ), 26.7 (CHCH 3), 22.7 (CHCH 3 ), 21.0 (ArCH3 ). Anal. Calcd for C40H40N2O2: C, 82.72; H, 6.94; N, 4.82. Found: C, 82.85; H, 6.93; N, 4.63%. 2-{N-nHexyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-methyl6-(triphenylmethyl)phenol (L8H). The procedure was the same as that of L1H, except that hexylamine (9.11 g, 90.0 mmol) was used in the first step, and 2-bromomethyl-4-methyl-6-(triphenylmethyl)phenol (3.87 g, 8.73 mmol), N-(2-benzoxazolylmethyl)hexylamine (2.03 g, ca. 8.7 mmol), and K2CO3 (1.33 g, 9.60 mmol) were used in the second step to provide L 8 H as colorless crystals via recrystallization from dichloromethane and petroleum ether (3.15 g, 61%). 1H NMR (CDCl3, 400 MHz, 298 K): δ 9.88 (s, 1H, OH), 7.74−7.68 (m, 1H, ArH), 7.52−7.46 (m, 1H, ArH), 7.39−7.32 (m, 2H, ArH), 7.24−7.17 (m, 12H, ArH), 7.15−7.09 (m, 3H, ArH), 6.91 (d, 1H, 4J = 1.6 Hz, ArH), 6.80 (d, 1H, 4J = 1.6 Hz, ArH), 3.89 (s, 2H, ArCH2), 3.86 (s, 2H, NCH2CN), 2.42−2.34 (m, 2H, NCH2CH2), 2.17 (s, 3H, ArCH3), 1.41−1.31 (m, 2H, CH2 of n-hexyl), 1.28−1.13 (m, 4H, CH2 of n-hexyl), 1.12−1.02 (m, 2H, CH2 of n-hexyl), 0.85 (t,
3H, 3J = 8.0 Hz, CH2CH3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.3 (OCN), 154.1, 150.9, 146.2, 141.0, 134.2, 131.2, 131.1, 129.1, 127.1, 125.5, 125.2, 124.5, 122.0, 120.2, 111.0 (all Ar-C), 63.3 (Ph3C), 58.5 (ArCH2), 52.6 (NCH2CH2), 48.7 (NCH2CN), 31.7 (CH2 of n-hexyl), 26.9 (CH2 of n-hexyl), 26.6 (CH2 of n-hexyl), 22.6 (CH2 of n-hexyl), 21.1 (ArCH3), 14.2 (CH2CH3). Anal. Calcd for C41H42N2O2: C, 82.79; H, 7.12; N, 4.71. Found: C, 82.59; H, 7.27; 4.75%. 2-{N-Cyclopentyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4methyl-6-(triphenylmethyl)phenol (L9H). The procedure was the same as that of L1H, except that cyclopentylamine (7.66 g, 90.0 mmol) was used in the first step, and 2-bromomethyl-4-methyl-6(triphenylmethyl)phenol (3.84 g, 8.67 mmol), N-(2benzoxazolylmethyl)cyclopentylamine (1.88 g, ca. 8.7 mmol), and K2CO3 (1.32 g, 9.54 mmol) were used in the second step to provide L9H as colorless crystals via recrystallization from dichloromethane and petroleum ether (3.21 g, 64%). 1H NMR (400 MHz, CDCl3, 298 K): δ 9.95 (s, 1H, OH), 7.73−7.67 (m, 1H, ArH), 7.52−7.46 (m, 1H, ArH), 7.38−7.31 (m, 2H, ArH), 7.26−7.17 (m, 12H, ArH), 7.16− 7.10 (m, 3H, ArH), 6.89 (d, 1H, 4J = 1.6 Hz, ArH), 6.85 (d, 1H, 4J = 1.6 Hz, ArH), 3.98 (s, 2H, ArCH2), 3.81 (s, 2H, NCH2CN), 3.02− 2.91 (m, 1H, NCH of cyclopentyl), 2.18 (s, 3H, ArCH3), 1.82−1.71 (m, 2H, CH2 of cyclopentyl), 1.61−1.55 (m, 2H, CH2 of cyclopentyl), 1.49−1.37 (m, 2H, CH2 of cyclopentyl), 1.36−1.24 (m, 2H, CH2 of cyclopentyl). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.9 (OCN), 154.1, 150.8, 146.2, 140.9, 134.3, 131.2, 131.0, 128.9, 127.12, 127.08, 125.4, 125.1, 124.5, 122.3, 120.1, 110.9 (all ArC), 63.4 (Ph3C), 63.1 (ArCH2), 55.9 (NCH), 47.4 (NCH2CN), 29.6 (CH2), 23.8 (CH2), 21.1 (ArCH3). Anal. Calcd for C40H38N2O2: C, 83.01; H, 6.62; N, 4.84. Found: C, 82.91; H, 6.56; N, 4.73%. 2-{N-Cyclooctyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4methyl-6-(triphenylmethyl)phenol (L11H). The procedure was the same as that of L1H, except that cyclooctylamine (11.45 g, 90.0 mmol) was used in the first step, and 2-bromomethyl-4-methyl-6(triphenylmethyl)phenol (3.94 g, 8.89 mmol), N-(2benzoxazolylmethyl)cyclooctylamine (2.30 g, ca. 8.9 mmol), and K2CO3 (1.35 g, 9.78 mmol) were used in the second step to provide L11H as colorless crystals via recrystallization from dichloromethane and petroleum ether (3.82 g, 62%). 1H NMR (CDCl3, 400 MHz, 298 K): δ 9.91 (s, 1H, ArOH), 7.67−7.60 (m, 1H, ArH), 7.42−7.37 (m, 1H, ArH), 7.35−7.29 (m, 2H, ArH), 7.24−7.13 (m, 12H, ArH), 7.11−7.05 (m, 3H, ArH), 6.89 (d, 1H, 4J = 1.6 Hz, ArH), 6.78 (d, 1H, 4 J = 1.6 Hz, ArH), 3.83 (s, 2H, ArCH2), 3.78 (s, 2H, NCH2CN), 2.80−2.71 (m, 1H, NCH of cyclooctyl), 2.16 (s, 3H, ArCH3), 1.73− 1.57 (m, 5H, CH2 of cyclooctyl), 1.50−1.36 (m, 6H, CH2 of cyclooctyl), 1.36−1.27 (m, 2H, CH2 of cyclooctyl), 1.25−1.16 (m, 1H, CH2 of cyclooctyl). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 163.8 (OCN), 154.3, 151.0, 146.2, 141.0, 133.9, 131.3, 131.1, 129.1, 127.0, 126.8, 125.4, 125.1, 124.4, 121.6, 120.0, 111.1 (all Ar-C), 63.4 (Ph3C), 58.5 (ArCH2), 54.3 (NCH), 46.4 (NCH2CN), 29.0 (CH2 of cyclooctyl), 26.4 (CH2 of cyclooctyl), 26.3 (CH2 of cyclooctyl), 25.5 (CH2 of cyclooctyl). Anal. Calcd for C43H44N2O2: C, 83.19; H, 7.14; N, 4.51. Found: C, 83.26; H, 7.14; N, 4.46%. 2-{N-1-Adamantyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4methyl-6-(triphenylmethyl)phenol (L12H). The procedure was the same as that of L1H, except that 1-adamantylamine (13.61 g, 90.0 mmol) was used in the first step, and 2-bromomethyl-4-methyl-6(triphenylmethyl)phenol (3.83 g, 8.64 mmol), N-(2-benzoxazolylmethyl)-1-adamantylamine (2.44 g, ca. 8.6 mmol), and K2CO3 (1.31 g, 9.50 mmol) were used in the second step to provide L12H as colorless crystals via recrystallization from dichloromethane and petroleum ether (4.06 g, 73%). 1H NMR (CDCl3, 400 MHz, 298 K): δ 10.19 (br s, 1H, OH), 7.69−7.67 (m, 1H, ArH), 7.49−7.47 (m, 1H, ArH), 7.37−7.32 (m, 2H, ArH), 7.20−7.11 (m, 15H, ArH), 6.86 (s, 1H, ArH), 6.83 (s, 1H, ArH), 4.20 (s, 2H, ArCH2), 3.85 (s, 2H, NCH2CN), 2.16 (s, 3H, ArCH3), 1.97 (br s, 3H, CH(CH2)3), 1.60−1.46 (m, 12H, CHCH2). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 164.9 (OCN), 154.3, 150.7, 146.2, 140.9, 134.2, 131.34, 131.26, 130.1, 128.4, 127.0, 126.9, 125.4, 125.1, 124.5, 123.3, 120.1, 111.1 (all Ar-C), 63.4 (Ph3C), 56.8 (ArCH2), 50.2 (NC(CH2)3), 43.3
1
H
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
[(L3)ZnN(SiMe3)2] (3). The procedure was the same as that of complex 1, except that L3H (0.601 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 3 as white solids (0.563 g, 68.2%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.51 (d, 6H, 3J = 7.6 Hz, ArH), 7.28 (d, 1H, 3J = 7.6 Hz, ArH), 7.24 (d, 1H, 4J = 2.0 Hz, ArH), 7.10−7.02 (m, 3H, ArH), 6.98 (t, 6H, 3J = 7.6 Hz, ArH), 6.93−6.89 (m, 1H, ArH), 6.82−6.77 (m, 7H, ArH), 6.24 (d, 1H, 4J = 2.0 Hz, ArH), 4.60 (d, 1H, 2J = 12.0 Hz, ArCH2), 4.27 (d, 1H, 2J = 14.0 Hz, NCH2CN), 3.90 (d, 1H, 2J = 14.0 Hz, NCH2CN), 3.50 (s, 2H, PhCH2), 3.19 (d, 1H, 2J = 12.0 Hz, ArCH2), 1.99 (s, 3H, ArCH3), 1.28−1.19 (m, 8H × 0.25, hexane), 0.88 (t, 6H × 0.25, 3J = 6.8 Hz, hexane), 0.28 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 168.0 (OCN), 151.2, 147.9, 136.8, 136.0, 133.7, 131.90, 131.87, 131.7, 131.54, 131.47, 129.0, 127.06, 127.03, 126.4, 125.7, 125.2, 120.91, 120.85, 120.3, 110.8 (all Ar-C), 64.1 (Ph3C), 61.2 (ArCH2), 60.1 (PhCH2), 45.8 (NCH2CN), 20.8 (ArCH3), 6.42 (N(Si(CH3)3)2). Anal. Calcd for C48H53N3O2Si2Zn·0.25 C6H14: C, 70.19; H, 6.72; N, 4.96. Found: C, 69.75; H, 6.56; N, 5.00%. [(L4)ZnN(SiMe3)2] (4). The procedure was the similar to that of complex 1, except that L4H (0.615 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 4 as white solids (0.379 g, 45.4%) after recrystallization with a mixture of THF and n-hexane. 1H NMR (C6D6, 400 MHz, 298 K): δ 7.48 (d, 6H, 3J = 7.6 Hz, ArH), 7.35 (d, 1H, 3J = 7.6 Hz, ArH), 7.32 (d, 1H, 4J = 2.0 Hz, ArH), 7.11−7.02 (m, 5H, ArH), 6.96 (t, 6H, 3J = 7.6 Hz, ArH), 6.94−6.88 (m, 1H, ArH), 6.87−6.80 (m, 2H, ArH), 6.77−6.71 (m, 3H, ArH), 6.61 (d, 1H, 4J = 2.0 Hz, ArH), 4.50 (d, 1H, 2J = 12.0 Hz, ArCH2), 3.79 (d, 1H, 2J = 18.0 Hz, NCH2CN), 3.59−3.54 (m, 4H × 0.1, THF), 3.08−2.98 (m, 1H, NCH2CH2), 2.93−2.86 (m, 1H, NCH2CH2), 2.84 (d, 1H, 2J = 12.0 Hz, ArCH2), 2.81−2.77 (m, 1H, NCH2CH2), 2.75 (d, 1H, 2J = 18.0 Hz, NCH2CN), 2.60−2.51 (m, 1H, NCH2CH2), 2.18 (s, 3H, ArCH3), 1.41−1.38 (m, 4H × 0.1, THF), 0.23 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 167.8 (OCN), 164.4, 151.1, 147.8, 138.4, 137.3, 136.2, 133.8, 131.7, 131.6, 129.01, 128.96, 127.0, 126.4, 125.6, 125.2, 121.1, 120.8, 120.4, 110.8 (all Ar-C), 67.8 (THF), 64.1 (Ph3C), 61.6 (ArCH2), 61.2 (NCH2CH2), 49.0 (NCH2CN), 31.7 (PhCH2), 25.8 (THF), 21.0 (ArCH3), 6.36 (N(Si(CH3)3)2). Anal. Calcd for: C49H55N3O2Si2Zn·0.1 C4H8O: C, 70.07; H, 6.64; N, 4.96. Found: C, 69.94; H, 6.75; N, 4.72%. [(L7)ZnN(SiMe3)2] (7). The procedure was the same as that of complex 1, except that L7H (0.581 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 7 as white solids (0.463 g, 57.4%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.48 (d, 6H, 3J = 7.2 Hz, ArH), 7.34−7.28 (m, 2H, ArH), 7.13−7.02 (m, 5H × 0.8, toluene), 6.96 (t, 6H, 3J = 7.2 Hz, ArH), 6.94−6.89 (m, 1H, ArH), 6.85−6.80 (m, 2H, ArH), 6.75 (t, 3H, 3J = 7.2 Hz, ArH), 6.60 (d, 1H, 4J = 2.0 Hz, ArH), 4.36 (d, 1H, 2J = 12.0 Hz, ArCH2), 3.73 (d, 1H, 2J = 18.2 Hz, NCH2CN), 2.83 (d, 1H, 2J = 12.0 Hz, ArCH2), 2.77 (d, 1H, 2J = 18.2 Hz, NCH2CN), 2.71−2.61 (m, 1H, NCH2CH2), 2.45−2.35 (m, 1H, NCH2CH2), 2.18 (s, 3H, ArCH3), 2.10 (s, 3H × 0.8, toluene), 1.69−1.56 (m, 1H, CH2 of NCH2CH2CH), 1.35−1.24 (m, 2H, CH2 of NCH2CH2CH), 0.84 (d, 3H, 3J = 6.2 Hz, CHCH3), 0.77 (d, 3H, 3J = 6.2 Hz, CHCH3), 0.22 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 167.9 (OCN), 164.6, 151.1, 147.8, 137.1, 136.1, 133.7, 131.7, 131.5, 129.3 (toluene), 128.6 (toluene), 127.0, 126.4, 125.7 (toluene), 125.6, 125.2, 121.1, 120.7, 120.6, 110.8 (all Ar-C), 64.1 (Ph3C), 60.8 (ArCH2), 58.5 (NCH2CH2), 48.6 (NCH2CN), 32.9 (NCH2CH2), 27.1 (CHCH3), 22.9 (CHCH3), 22.4 (CHCH3), 21.5 (ArCH3), 21.1 (toluene), 6.33 (N(Si(CH3)3)2). Anal. Calcd for: C46H57N3O2Si2Zn·0.8 C7H8: C, 70.49; H, 7.27; N, 4.78. Found: C,70.32; H, 7.18; N, 4.71%. [(L8)ZnN(SiMe3)2] (8). The procedure was the same as that of complex 1, except that L8H (0.595 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 8 as white solids (0.382 g, 46.5%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.47 (d, 6H, 3J = 7.2 Hz, ArH), 7.34 (d, 1H, 3J = 8.0 Hz, ArH), 7.31 (d, 1H, 4J = 2.4 Hz, ArH), 7.13−7.03 (m, 5H × 0.2, toluene), 6.95 (t,
(NCH 2 CN), 38.8 (CH(CH 2 ) 3 ), 36.3 (C(CH 2 ) 3 ), 29.6 (CHCH2CH), 21.1 (ArCH3). Anal. Calcd for C45H44N2O2·0.1 CH2Cl2: C, 82.91; H, 6.82; N, 4.29. Found: C, 82.73; H, 6.77; N, 4.13%. 2-{N-Triphenylmethyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-methyl-6- (triphenylmethyl)phenol (L13H). The procedure was the same as that of L1H, except that triphenylmethylmine (23.34 g, 90.0 mmol) was used in the first step, and 2bromomethyl-4-methyl-6-(triphenylmethyl)phenol (2.95 g, 6.66 mmol), N-(2-benzoxazolylmethyl)triphenylmethylamine (2.60 g, ca. 6.7 mmol), and K2CO3 (1.01 g, 7.33 mmol) were used in the second step to provide L13H as colorless crystals via recrystallization from dichloromethane and petroleum ether (3.44 g, 69%). 1H NMR (CDCl3, 400 MHz, 298 K): δ 7.32 (d, 6H, 3J = 7.3 Hz, ArH), 7.24− 7.16 (m, 10H, ArH), 7.02−6.95 (m, 14H, ArH), 6.84 (td, 1H, 3J = 7.2 Hz, 4J = 1.2 Hz, ArH), 6.80 (br s, 1H, ArH), 6.76 (br s, 1H, ArH), 6.73 (td, 1H, 3J = 7.2 Hz, 4J = 1.2 Hz, ArH), 6.67 (d, 1H, 3J = 7.2 Hz, ArH), 6.60 (d, 1H, 3J = 7.2 Hz, ArH), 4.58 (d, 1H, 2J = 16.0 Hz, ArCH2), 4.40 (d, 1H, 2J = 16.0 Hz, ArCH2), 2.31−2.26 (m, 1H, NCH2CN), 2.14 (s, 3H, ArCH3), 2.13−2.08 (m, 1H, NCH2C N). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 149.8 (OCN), 149.5, 146.3, 146.1, 137.6, 135.4, 131.8, 131.6, 130.8, 130.4, 129.6, 129.1, 128.2, 127.5, 126.6, 126.3, 125.9, 122.0, 121.3, 120.7, 118.1, 109.6, 109.2 (all Ar-C), 70.7 (NCPh3), 63.5 (CPh3), 48.7 (ArCH2), 43.9 (NCH2CN), 21.0 (ArCH3). Anal. Calcd for C42H42N2O2·0.3 CH2Cl2: C, 83.78; H, 5.85; N, 3.34. Found: C, 83.85; H, 5.79; N, 3.37%. [(L1)ZnN(SiMe3)2] (1). In a glovebox, the aminophenol L1H (0.413 g, 1.00 mmol) was dissolved in toluene (5 mL) and was added dropwise to a solution of Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) in toluene (3 mL). The reaction mixture was stirred at room temperature overnight, and all the volatiles were removed under vacuum to afford a white solid which was then recrystallized with a mixture of n-hexane and toluene. Colorless crystals of complex 1 were obtained in 67.7% yield (0.223 g). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.56 (d, 1H, 3J = 8.0 Hz, ArH), 7.15−7.12 (m, 3H, ArH), 6.97− 6.93 (m, 2H, ArH), 6.85−6.80 (m, 2H, ArH), 6.71−6.66 (m, 2H, ArH), 6.13 (d, 1H, 4J = 2.6 Hz, ArH), 4.01 (d, 1H, 3J = 14.2 Hz, PhCH2), 3.97 (d, 1H, 3J = 14.2 Hz, PhCH2), 3.81 (d, 1H, 2J = 11.8 Hz, ArCH2), 3.51 (d, 1H, 2J = 16.9 Hz, NCH2CN), 2.74 (d, 1H, 2J = 16.9 Hz, NCH2CN), 2.59 (d, 1H, 2J = 11.8 Hz, ArCH2), 1.27− 1.19 (m, 8H × 0.1, hexane), 0.88 (t, 6H × 0.1, 3J = 6.8 Hz, hexane), 0.50 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 166.1 (OCN), 162.1, 151.0, 135.2, 132.1, 131.5, 130.0, 129.3, 129.1, 128.6, 126.7, 126.4, 125.8, 123.2, 119.5, 117.3, 110.8 (all Ar-C), 63.1 (ArCH2), 59.8 (PhCH2), 50.7 (NCH2CN), 32.0 (hexane), 23.1 (hexane), 14.4 (hexane), 6.18 (N(Si(CH3)3)2). Anal. Calcd for: C28H35Cl2N3O2Si2Zn·0.1 C6H14: C, 53.12; H, 5.67; N, 6.50. Found: C, 52.80; H, 5.61; N, 6.43%. [(L2)ZnN(SiMe3)2] (2). The procedure was the same as that of complex 1, except that L2H (0.457 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 2 as white solids (0.253 g, 55.4%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.53 (d, 1H, 3J = 8.0 Hz, ArH), 7.10−7.02 (m, 5H, ArH), 7.04 (d, 1H, 4 J = 2.0 Hz, ArH), 6.80−6.74 (m, 1H, ArH), 6.68−6.57 (m, 2H, ArH), 6.53 (d, 1H, 4J = 2.0 Hz, ArH), 4.22 (d, 1H, 2J = 11.2 Hz, ArCH2), 4.15 (d, 1H, 2J = 13.8 Hz, PhCH2), 4.11 (d, 1H, 2J = 13.8 Hz, PhCH2), 3.70 (d, 1H, 2J = 17.2 Hz, NCH2CN), 3.30 (d, 1H, 2J = 17.2 Hz, NCH2CN), 3.01 (d, 1H, 2J = 11.2 Hz, ArCH2), 1.56 (s, 9H, C(CH3)3), 1.29−1.23 (m, 8H × 0.1, hexane), 1.17 (s, 9H, C(CH3)3), 0.88 (t, 6H × 0.1, 3J = 7.2 Hz, hexane), 0.55 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 166.8 (OCN), 164.7, 151.1, 138.0, 135.5, 135.3, 132.7, 131.6, 129.1, 126.3, 125.6, 125.1, 124.4, 120.2, 119.9, 110.8 (all Ar-C), 63.3 (ArCH2), 62.1 (PhCH2), 50.5 (NCH2CN), 35.5 (C(CH3)3), 33.9 (C(CH3)3), 32.2 (hexane), 32.0 (C(CH3)3), 30.2 (C(CH3)3), 23.0 (hexane), 14.3 (hexane), 6.36 (N(Si(CH3)3)2). Anal. Calcd for C36H53N3O2Si2Zn·0.1 C6H14: C, 63.71; H, 7.95; N, 6.09. Found: C, 63.15; H, 7.63; N, 5.93%. I
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry 6H, 3J = 8.0 Hz, ArH), 6.86−6.81 (m, 3H, ArH), 6.71 (t, 3H, 3J = 7.2 Hz, ArH), 6.62 (d, 1H, 4J = 2.4 Hz, ArH), 4.44 (d, 1H, 2J = 12.0 Hz, ArCH2), 3.77 (d, 1H, 2J = 18.0 Hz, NCH2CN), 2.82 (d, 1H, 2J = 12.0 Hz, ArCH2), 2.70 (d, 1H, 2J = 18.0 Hz, NCH2CN), 2.56 (td, 1H, 3J = 12.5 Hz, 4J = 4.0 Hz, NCH2CH2), 2.27−2.20 (m, 1H, NCH2CH2), 2.19 (s, 3H, ArCH3), 2.10 (s, 3H × 0.2, toluene), 1.89− 1.76 (m, 1H, CH2 of n-hexyl), 1.50−1.37 (m, 1H, CH2 of n-hexyl), 1.32−1.25 (m, 2H, CH2 of n-hexyl), 1.17−1.06 (m, 2H, CH2 of nhexyl), 1.05−0.96 (m, 2H, CH2 of n-hexyl), 0.90 (t, 3H, 3J = 6.9 Hz, CH2CH3), 0.22 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 168.0 (OCN), 164.4, 151.2, 147.8, 137.3 (toluene), 136.2, 133.7, 131.7, 131.6, 129.3 (toluene), 128.6 (toluene), 127.0, 126.4, 125.6 (toluene), 125.1, 121.1, 120.7, 120.6, 110.8 (all Ar-C), 64.1 (Ph3C), 60.6 (ArCH2), 60.2 (NCH2CH2), 48.8 (NCH2CN), 31.9 (CH2 of n-hexyl), 27.4 (CH2 of n-hexyl), 24.9 (CH2 of n-hexyl), 23.0 (CH2 of n-hexyl), 21.0 (ArCH3), 14.3 (CH2CH3), 6.26 (N(Si(CH3)3)2). Anal. Calcd for: C47H59N3O2Si2Zn· 0.2 C7H8: C, 69.37; H, 7.29; N, 5.01. Found: C, 68.88; H, 7.25; N, 4.98%. [(L9)ZnN(SiMe3)2] (9). The procedure was the same as that of complex 1, except that L9H (0.579 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 9 as white solids (0.449 g, 55.9%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.48 (d, 6H, 3J = 7.6 Hz, ArH), 7.33 (d, 1H, 3J = 7.6 Hz, ArH), 7.29 (d, 1H, 4J = 2.0 Hz, ArH), 6.97 (t, 6H, 3J = 7.6 Hz, ArH), 6.94−6.89 (m, 1H, ArH), 6.86−6.80 (m, 2H, ArH), 6.76 (t, 3H, 3J = 7.6 Hz, ArH), 6.59 (d, 4J = 2.0 Hz, 1H, ArH), 4.45 (d, 1H, 2J = 12.0 Hz, ArCH2), 3.74 (d, 1H, 2J = 18.2 Hz, NCH2CN), 2.97−2.84 (m, 3H, 1H of ArCH2, 1H of NCH2CN, 1H of NCH), 2.16 (s, 3H, ArCH3), 1.77−1.67 (m, 1H, CH2 of cyclopentyl), 1.59−1.50 (m, 1H, CH2 of cyclopentyl), 1.48−1.31 (m, 4H, CH2 of cyclopentyl), 1.28−1.16 (m, 2H, CH2 of cyclopentyl), 0.27 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 168.4 (OCN), 164.6, 151.0, 147.9, 136.8, 136.2, 133.6, 131.7, 131.4, 127.1, 126.3, 125.6, 125.1, 121.2, 120.6, 120.5, 110.7 (All Ar-C), 69.2 (ArCH2), 64.1 (Ph3C), 59.7 (NCH), 47.3 (NCH2CN), 28.3 (CH2), 28.1 (CH2), 23.8 (CH2), 23.6 (CH2), 21.0 (ArCH3), 6.43 (N(Si(CH3)3)2). Anal. Calcd for: C46H55N3O2Si2Zn: C, 68.76; H, 6.90; N, 5.23. Found: C, 68.59; H, 6.91; N, 5.07%. [(L11)ZnN(SiMe3)2] (11). The procedure was the same as that of complex 1, except that L11H (0.621 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 11 as white solids (0.423 g, 50.1%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.51 (d, 6H, 3J = 7.6 Hz, ArH), 7.25 (d, 1H, 4J = 2.0 Hz, ArH), 7.21 (d, 1H, 3J = 7.6 Hz, ArH), 7.03 (t, 6H, 3J = 7.6 Hz, ArH), 6.92−6.87 (m, 1H, ArH), 6.84 (t, 3H, 3J = 7.6 Hz, ArH), 6.77−6.74 (m, 2H, ArH), 6.57 (d, 4J = 2.0 Hz, 1H, ArH), 4.25 (d, 1H, 2J = 12.0 Hz, ArCH2), 3.55 (d, 1H, 2J = 18.2 Hz, NCH2CN), 3.09 (d, 1H, 2J = 18.2 Hz, NCH2CN), 3.03 (d, 1H, 2J = 12.0 Hz, ArCH2), 2.12 (s, 3H, ArCH3), 2.07−1.97 (m, 1H, NCH of cyclooctyl), 1.64−1.38 (m, 6H, CH2 of cyclooctyl), 1.37−1.18 (m, 7H, CH2 of cyclooctyl), 1.06− 0.94 (m, 1H, CH2 of cyclooctyl), 0.27 (s, 18H, N(Si(CH3)3)2). 13 C{1H} NMR (C6D6, 100 MHz, 298 K): δ 168.3 (OCN), 165.3, 150.9, 147.9, 135.9, 135.8, 133.7, 131.6, 131.2, 127.2, 126.2, 125.7, 125.1, 121.0, 120.62, 120.60, 110.6 (All Ar-C), 65.2 (Ph3C), 64.1 (ArCH2), 57.6 (NCH), 46.0 (NCH2CN), 30.8 (CH2 of cyclooctyl), 30.3 (CH2 of cyclooctyl), 29.5 (CH2 of cyclooctyl), 26.7 (CH2 of cyclooctyl), 26.1 (CH2 of cyclooctyl), 26.0 (CH2 of cyclooctyl), 25.7 (CH2 of cyclooctyl), 21.0 (ArCH3), 6.32 (N(Si(CH3)3)2). Anal. Calcd for C49H61N3O2Si2Zn: C, 69.60; H, 7.27; N, 4.97. Found: C, 69.65; H, 7.43; N, 4.74%. [(L12)ZnN(SiMe3)2] (12). The procedure was the same as that of complex 1, except that L12H (0.645 g, 1.00 mmol) and Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 12 as white solids (0.479 g, 55.1%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.50−7.45 (m, 7H, ArH), 7.33 (d, 1H, 4J = 2.0 Hz, ArH), 7.14−7.11 (m, 2H × 0.2, toluene), 7.08−7.00 (m, 3H × 0.2, toluene), 6.99−6.91 (m, 7H, ArH), 6.85−6.78 (m, 5H, ArH), 6.63 (d, 1H, 4J = 2.0 Hz, ArH), 4.52 (d, 1H, 2J = 11.7 Hz, ArCH2), 3.88 (d, 1H, 2J = 18.7 Hz, NCH2CN), 3.58−3.55 (m, 4H × 0.4, THF), 3.41 (d, 1H, 2J = 18.7
Hz, NCH2CN), 3.04 (d, 1H, 2J = 11.7 Hz, ArCH2), 2.19 (s, 3H, ArCH3), 2.11 (s, 3H × 0.2, toluene), 1.83 (br s, 3H, CH(CH2)3), 1.69−1.61 (m, 6H, NC(CH2)3), 1.43−1.39 (m, 7.6 H, 6H of CHCH2CH and 4H × 0.4 of THF), 1.29−1.23 (m, 8H × 0.2, hexane), 0.88 (t, 6H × 0.2, 3J = 7.2 Hz, hexane), 0.31 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 169.6 (OCN), 164.6, 150.8, 147.9, 137.9 (toluene), 136.5, 136.2, 133.6, 131.9, 131.6, 129.3 (toluene), 128.6 (toluene), 127.4, 127.2, 127.1, 126.2, 125.7 (toluene), 125.5, 125.1, 121.4, 121.0, 120.7, 110.7 (all Ar-C), 67.8 (THF), 64.1 (Ph3C), 60.4 (ArCH2), 53.2 (NCH2CN), 42.7 (NC(CH2)3), 37.7 (CH(CH2)3), 36.1 (C(CH2)3), 32.0 (hexane), 29.8 (CHCH2CH), 25.7 (THF), 23.0 (hexane), 21.1 (ArCH3 and toluene), 14.3 (hexane), 6.74 (N(Si(CH3)3)2). Anal. Calcd for C51H61N3O2Si2Zn·0.2C7H8, ·0.4C4H8O, and ·0.2C6H14: C, 70.98; H, 7.40; N, 4.50. Found: C,70.72; H, 7.25; N, 4.54%. X-ray Crystallography. The X-ray diffraction measurements of single crystals of complexes 11, 12, and 3d were performed on a Bruker SMART APEX II diffractometer with graphite-monochromated Mo Kα (λ= 0.71073 Å) radiation. All data were collected at 293 K using the ω-scan techniques. All structures were solved by direct methods and refined using Fourier techniques. An absorption correction based on SADABS was applied.60 All non-hydrogen atoms were refined by full-matrix least-squares on F2 using the SHELXTL program package.61 Hydrogen atoms were located and refined by the geometry method. The cell refinement and data collection and reduction were done by Bruker SAINT.62 The structure solution and refinement were performed by SHELXS-9763 and SHELXL-97,64 respectively. For further crystal data and details of measurements, see Table S1 in the Supporting Information. Molecular structures were generated using the ORTEP program.65 Typical Polymerization Procedure. In a glovebox, an initiator solution (0.5 mL, 10 mmol/mL) from a stock solution in toluene or THF was injected sequentially into a series of 10 mL vials loaded with rac-LA (0.144 g, 1.00 mmol) and a suitable amount (0.5 mL) of the same dry solvent. The mixture was stirred at room temperature and quenched at specific time intervals by adding an excess amount of normal light petroleum ether. After being dissolved with dichloromethane, a small amount of an aliquot of the bulk solution was withdrawn and dried under reduced pressure for monomer conversion determination via 1H NMR spectroscopy. The bulk solution was slightly concentrated and the polymer was precipitated from dichloromethane via the addition of excess methanol. The collected polymer sample was further dried in a vacuum oven at 60 °C for 16 h to a constant weight for GPC and 1H and homonuclear-decoupled 1H NMR analyses. In the cases where 2-propanol was used, the monomer solution was treated at first with a solution of 2-propanol for 5 min, and then a solution of the initiator was injected into the mixture. Otherwise, the procedures were the same.
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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.8b01839. 1 H and 13C NMR spectra of all complexes and related NMR reactions, homonuclear decoupled 1H NMR spectra of typical polymer samples, plots of kinetic studies of rac-LA polymerization by complexes 1−12, GPC trace of the PLA samples, and DSC measurement of the typical samples (PDF) Accession Codes
CCDC 1852679−1852681 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. J
DOI: 10.1021/acs.inorgchem.8b01839 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; Fax/Tel.: +86 21 64253519. ORCID
Haiyan Ma: 0000-0003-0810-2493 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was subsidized by the National Natural Science Foundation of China (NNSFC, Grant 21474028) and the Fundamental Research Funds for the Central Universities (Grant WD1113011). All financial support is gratefully acknowledged. H.M. also thanks the very kind donation of a Braun glovebox by AvH foundation.
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