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Synthesis and Structure of Homo- and Heterometallic Lithium− Magnesium Complexes and Their Reactivity in the ROP of rac-Lactide ́ Carlos Gallegos, Vanessa Tabernero, Francisco M. Garcıa-Valle, Marta E. G. Mosquera, Tomás Cuenca,* and Jesús Cano* Dpto de Quı ́mica Inorgánica, Universidad de Alcalá, 28871 Alcalá de Henares, Spain S Supporting Information *
ABSTRACT: The homometallic azo complexes of lithium [Li2{(η2-O(C10H6)NN(C6H5)}2]n and magnesium [Mg{(η2O(C10H6)NN(C6H5)}2]n and the heterometallic lithium− magnesium derivative [Li2Mg2{(η2-O(C10H6)NN(C6H5)}6] have been synthesized. The heterometallic complex exhibits a novel structure described as two truncated cubes that share a Mg2O2 ring as one of the cube sides. These complexes have been studied in the polymerization of rac-lactide. In order to gain insight into the behavior of these compounds as catalysts, 1H NMR experiments, diffusion-ordered spectroscopy, and X-ray studies have been used. DOSY NMR experiments support the notion that the structure of the mixed-metal complex 3 persists in benzene solution, and 1H NMR studies show that it follows an activated monomer mechanism.
D
O(C10H6)NN(C6H5)}6] (3) and their reactivity in the ROP of lactide. Reactions between 1-phenylazo-2-naphthol and lithium bis(trimethylsilyl)amide or di-n-butylmagnesium in 1:1 or 2:1 molar ratio yield the homometallic compounds 1 and 2, respectively (Scheme 1). The heterometallic lithium−magnesium complex 3 has been prepared by the reaction of 3 equiv of 2 with 2 equiv of lithium bis(trimethylsilyl)amide or nbutyllithium. In this reaction 1 molar equivalent of the
uring the past two decades biodegradable polymers have received a great deal of attention as alternatives to petroleum-based materials. Polylactide (PLA) has a broad range of applications, due to its excellent mechanical and physical properties and its biocompatible and biorenewable nature. Ring-opening polymerization (ROP) of lactide is the most convenient method to prepare PLA with high molecular weight and low polydispersity.1 For this process, a large number of metal-based initiators have been described, including Sn, Al, or rare-earth-metal derivatives.2 Because of PLA’s use as a biomaterial for pharmaceutical and medical applications3 the development of new catalyst/initiators with nontoxic metals is an interesting goal for synthetic chemists. A wide variety of initiators with biocompatible metals such as Li, Na, K, Mg, Ca, and Zn have been synthesized,4 frequently stabilized with salen or hemisalen ligands.4,5 In contrast, no examples of main-groupmetal azo derivatives as catalysts for the polymerization of lactide or α-olefins have been reported, and with transitionmetal azo complexes only the polymerization of α-olefins6 has been studied. The azo group is less σ electron donating than the imine group and may increase the acidity and activity of the metal catalysts. Finally, heterometallic complexes are very useful reagents in many fields such as halogen−metal exchange7 and C−H activation,8 but only very few examples of these compounds have been used as polymerization catalysts,9 even though it has been demonstrated that cooperative effects in homobimetallic catalytic systems provoke different activities, or selectivities, from monometallic systems.10 Here we report the chelate homometallic complexes of lithium [Li2{(η2-O(C10H6)NN(C6H5)}2]n (1) and magnesium [Mg{(η2-O(C10H6)NN(C6H5)}2]n (2) and the heterometallic lithium and magnesium derivative [Li2Mg2{(η2© XXXX American Chemical Society
Scheme 1. Synthesis of Complexes 1−3a
a
Reaction conditions: (i) Li[N(SiMe3)2]; (ii) 1/2 MgnBu2; (iii) 2/3 Li[N(SiMe3)2]. Received: September 21, 2013
A
dx.doi.org/10.1021/om400940f | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Note
corresponding magnesium bis-amide is formed. Complexes 1 and 2 are highly insoluble in toluene and were only characterized by elemental analysis, but they do coordinate Lewis base molecules such as C5D5N and THF to give the corresponding adducts 1a,b and 2a,b. The 1H NMR spectra of compounds 1a,b and 2a,b show resonances for the azo-naphtholate ligand shifted slightly downfield with respect to the free 1-phenylazo-2-naphthol molecule. In the 1H−15N HMBC spectra, only one 15N signal for the two nitrogen atoms of the azo group is observed because the other nitrogen is located too remote from any proton in the molecule. The 1H−15N HMBC spectrum in C6D6/C5D5N shows one resonance at δ 410 for 1a and δ 380 for 2a, also shifted downfield with respect to the corresponding signal observed in the free 1-phenylazo-2-naphthol at δ 295, suggesting the coordination of at least one nitrogen atom of the azo group to the metal center. The 7Li NMR spectrum for 1a shows one singlet at δ 3.02, while the signal for the lithium bis(trimethylsilyl)amide appears at δ 1.07. The 1H NMR spectrum of 3 shows two sets of signals in a 2:1 ratio due to the two different coordination modes of the naphtholate ligand. The resonance in the 7Li NMR changes from δ 3.02 for 1a to δ 2.58 for the mixed-metal compound 3. Figure 1 shows the molecular structures of compounds 1b and 2b. Interestingly, compounds with this kind of ligand have
Figure 2. ORTEP plots of 3 showing thermal ellipsoids at 30% probability: (left) core view with the phenyl rings omitted for clarity; (right) full view. Selected distances (Å): Mg1−O1 = 2.014(2), Mg1− O2 = 2.023(2), Mg1−O3 = 2.111(2), Mg1−O#3 = 2.120(3), Mg1− N2 = 2.207(3), Mg1−N3 = 2.219(3), Li1−O2 = 1.927(7), Li1−O1 = 1.933(6), Li1−O3 = 1.940(6), Li1−N5 = 2.096(7), N1−N2 = 1.277(4).
indicating that 3 retains its solid-state structure in a benzene solution. The polymerization of L- and rac-lactide using complexes 1− 3 in the presence of benzyl alcohol as co-initiator were tested, and the results obtained are summarized in Table 1. The influence of the selected solvent seems not to be significant, and similar polymerization behavior is observed for each metal derivative (entries 1−4), with only a slight decrease in the polymerization rate observed for THF. The L-lactide and raclactide polymerizations show that lithium derivative 1 is much more active than magnesium complex 2, which only reaches high conversions at 70 °C (entries 3 and 7). Nevertheless, the heterometallic complex 3 shows an activity intermediate between 1 and 2, reaching high conversions at 25 °C. The GPC analysis for the L-lactide polymerization in toluene shows a monomodal weight distribution ranging from 1.09 to 1.22. Similar values are observed in the rac-lactide polymerizations (entries 1−8). Polymer microstructures show a dependence of the metal center on the catalytic system. Under comparable conditions, the three compounds lead to different levels of tacticity. While lithium derivative 1 affords heterotactic-enriched PLA (Pr = 0.67, entry 6), magnesium compound 2 forms atactic PLA (Pr = 0.46, entry 7). However, we note that the heterometallic system 3 increases stereoselectivity (Pr = 0.75, entries 8, 9) versus its mononuclear counterparts, especially with respect to 2. Molecular weights, obtained by GPC, for the resulting polymers when using complexes 1 and 2 are lower than those expected assuming one growing chain per metal atom. However, rac-lactide polymers obtained with 3 in CH2Cl2 (entry 9) have molecular weights similar to those calculated, indicating the absence of side reactions observed in the polymerizations using 1 and 2. The polymerization kinetics was first order in lactide concentration when the polymerization with 3 was performed in the presence of BnOH in CH2Cl2 at 25 °C. The value of kapp obtained was 0.228 h−1. In all cases, terminal BnOC(O)R polymer groups were observed by 1H NMR spectroscopy. In order to get insight into the polymerization mechanism, the reaction of a 1:1:1 mixture of the precatalyst, BnOH, and L-lactide was monitored. We found with complexes 1 and 3 the product of ring-opening insertion of BnOH into the L-lactide while the metallic complexes remained intact, indicating an activated monomer
Figure 1. ORTEP plots of compounds 1b (left) and 2b (right), showing thermal ellipsoids at 30% probability. Selected distances (Å): 1b, Li1−O1 = 1.879(4), Li−O#1 = 1.899(4), Li1−O2 = 1.935(4), Li1−N2 = 2.016(4), N1−N2 = 1.284(2); 2b, Mg1−O1 = 1.943(2), Mg1−O2 = 2.125(2), Mg1−N2 = 2.240(2), N1−N2 = 1.286(3).
not been structurally characterized for s-block metals and within main-group metals only for Sn and Ge.11 Of particular interest is the heterometallic structure of 3 (Figure 2). The mixed Mg−Li core could be described as two truncated cubes that share a Mg2O2 ring as one of the cube sides. This central Mg 2O2 is surrounded by four MgLiO2 rings. In this centrosymmetric structure, the lithium is tetracoordinated and shows unusual trigonal-pyramidal geometry. The study of the molecular structure and the aggregation degree in solution is crucial to gain insight into the behavior of these compounds as catalysts. Diffusion-ordered spectroscopy (1H-DOSY) is a powerful tool to obtain molecular parameters such as molecular weight (FW)12 and hydrodynamic radii.13 This method has been successfully applied to ascertain the structures of heterobimetallic and mixed-metal complexes in solution.14 An NMR tube with C6D6 as solvent was loaded with complex 3 and three internal reference patrons in a 1:1:1 ratio. Diffusion (D) and FW values of patrons and 3 are given in the Supporting Information. A diffusion coefficient of 3.80 × 10−10 m2 s−1 and a molecular weight of 1528.15 are obtained, which are in agreement with the X-ray studies (1.06% error), B
dx.doi.org/10.1021/om400940f | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Note
Table 1. Polymerization of L- and rac-Lactidea entry
init.
1 2 3 4 5 6 7 8 9 10
1 1 2 2 3 1 2 3 3 3b
monomer
solvent
time (h)
temp (°C)
conversn (%)
Mn(calcd)
Mn(exptl)c
PDI
Pr
L-lactide
tol THF tol THF tol tol tol tol CH2Cl2 tol
0.5 1.5 7 7 8 0.5 7 8 8 10
25 25 70 70 25 25 70 25 25 70
88 92 81 77 82 90 83 94 84 88
12780 13356 11772 11196 11916 13068 12060 13644 12204 12780
7945 8553 6400 4701 6499 5589 5132 7340 11139 28225
1.22 1.54 1.09 1.08 1.11 1.16 1.29 1.26 1.83 1.74
0.67 0.46 0.75 0.75 0.68
L-lactide L-lactide L-lactide L-lactide rac-lactide rac-lactide rac-lactide rac-lactide rac-lactide
a
Polymerization conditions: 1:1:100 mixture of the [M], BnOH, and lactide. bAbsence of benzyl alcohol. cDetermined by gel permeation chromatography, calibrated with polystyrene standards in tetrahydrofuran and corrected by the Mark−Houwink factor of 0.58.
mechanism.15 Nevertheless, when this reaction was monitored with 2, the protonated 1-phenylazo-2-naphthoxo ligand (1phenylazo-2-naphthol) was detected, assuming a possible insertion coordination mechanism assisted by the ligand.16 Moreover, in the absence of benzyl alcohol, when the polymerization of rac-lactide with 3 was conducted at 70 °C (entry 10), high-molecular-weight PLA was observed, with slightly lower stereoselectivity, indicating that the nature of the propagating species is probably not the same. No terminal polymer groups were identified by 1H NMR, indicating the cyclic nature of the polymers in the absence of benzyl alcohol. This was confirmed by MALDI-TOF experiments of oligomer samples (2% catalyst loading, Supporting Information). In conclusion, new homo- and heterometallic lithium− magnesium complexes have been synthesized and characterized by NMR and X-ray diffraction studies. Compounds 1 and 2 are insoluble in toluene. Their solid structures in donor solvents exhibit a bimetallic and monometallic disposition. 1H-DOSY NMR experiments support that the structure of 3 obtained by X-ray diffraction studies remains in benzene solution. Under comparable conditions, lactide polymerization behavior of the three compounds was studied. The activities are variable, and the mixed heterometallic compound features an activity intermediate between the two homometallic compounds. 1H NMR experiments show that the heterometallic system 3 follows an activated monomer mechanism instead of the typical coordination−insertion pathway observed with phenolate− alcohol systems.
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in a Varian HPL apparatus in THF at room temperature calibrated with respect to polystyrene standards and corrected with a factor of 0.58. MALDI-TOF MAS analysis was performed using an Agilent MALDI-TOF LC/MS; the ionization source was Masstech AP/ MALDI. The mass spectrum was recorded in positive mode. 1,8,9Anthracenetriol was used as matrix, and trifluoroacetic acid was added as a cationization agent. Synthesis of 1 and Formation of 1a. A solution of Li[N(SiMe3)2] (350 mg, 2.1 mmol) in 15 mL of toluene was added to a stirred solution of 1-phenylazo-2-naphthol (511 mg, 2.1 mmol) in 30 mL of toluene at −78 °C. The reaction mixture was warmed to room temperature and was stirred overnight. After filtration the solid was washed with hexane (2 × 10 mL) and an orange solid was obtained (299 mg, 57%). 1 was insoluble in the absence of nondonor solvents. Anal. Calcd for C16H11N2OLi (1): C, 75.57; H, 4.36; N, 11.02. Found: C, 75.52; H, 4.46; N, 10.81. 1H NMR of 1a (400 MHz, 293 K, C6D6/C5D5N): δ 9.27 (d, 1H, C6H5), 8.18 (m, 2H, C6H5), 7.52 (m, 3H, C10H6, C6H5), 7.30 (m, 1H, C10H6), 7.23 (m, 1H, C10H6), 7.16 (m, 2H, C10H6), 6.97 (m, 1H, C10H6).13C NMR (100.6 MHz, 293 K, C6D6/C5D5N): δ 161.5, 155.5, 137.4, 135.8, 133.2, 129.2, 127.4, 126.9, 122.4, 121.6. 15N NMR (40.5 MHz, 293 K, C6D6/C5D5N): δ 410. 7Li (156 MHz, 293 K, C6D6/C5D5N): δ 3.02. Synthesis of 1b. Compound 1b was obtained by slow diffusion of hexane into a saturated solution of 1 in THF at −20 °C. Anal. Calcd for C40H38N4O4Li2 (1b): C, 73.61; H, 5.87; N, 8.58. Found: C, 73.29; H, 5.59; N, 8.36. 1H NMR of 1b (400 MHz, 293 K, THF-d8): δ 8.77 (m, 2H, C6H5), 7.87 (m, 3H, C6H5), 7.67 (m, 2H, C10H6), 7.61 (m, 2H, C10H6), 7.42 (m, 7H, C10H6 + C6H5), 7.22 (m, 4H, C10H6), 7.00 (m, 2H, C10H6), 3.65 (m, 8H, OCH2CH2), 1.82 (m, 8H, OCH2CH2). 13 C NMR (100.6 MHz, 293 K, THF-d8): δ 160.3, 153.6, 135.3, 133.1, 130.4, 126.9, 126.1, 125.6, 124.9, 119.5, 119.0, 65.4 (OCH2CH2), 23.5 (OCH2CH2). 15N NMR (40.5 MHz, 293 K, THF-d8): δ 394. 7Li (156 MHz, 293 K, THF-d8): δ 1.35. Synthesis of 2 and Formation of 2a. Following the same procedure described for 1 but using MgnBu2 (4 mL, 4 mmol 1 M in toluene) and 1-phenylazo-2-naphthol (2 g, 8.1 mmol) complex 2 (1.66 g, 80%) was obtained as a red solid. 2 was insoluble in the absence of nondonor solvents. Anal. Calcd for C32H22N4O2Mg (2): C, 74.10; H, 4.24; N, 10.79. Found: C, 73.96; H, 4.07; N, 10.58. 1H NMR of 2a (400 MHz, 293 K, C6D6/C5D5N): δ 8.88 (m, 2H, C6H5), 7.89 (m, 4H, C6H5), 7.56 (m, 2H, C10H6), 7.47 (m, 2H, C10H6) 7.32 (m, 2H, C10H6), 7.14 (m, 2H, C10H6), 7.09 (m, 4H, C6H5), 7.00 (m, 2H, C10H6), 6.91 (m, 2H, C10H6). 13C NMR (100.6 MHz, 293 K, C6D6/ C5D5N): δ 162.2, 156.6, 137.1, 136.7, 132.7, 129.5, 127.3, 126.8, 125.3, 123.1, 121.9. 15N NMR (40.5 MHz, 293 K, C6D6/C5D5N): δ 380. Synthesis of 2b. Compound 2b was dissolved in THF, and the solution was filtered and stored at −20 °C overnight. Red block crystals for suitable for X-ray diffraction were obtained. Anal. Calcd for C40H38N4O4Mg (2b): C, 72.49; H, 5.73; N, 8.45. Found: C, 72.14; H, 5.61; N, 8.28. 1H NMR (400 MHz, 293 K, THF-d8): δ 8.42 (d, 2H, C6H5), 7.90 (d, 4H, C6H5), 7.51 (d, 2H, C10H6), 7.44 (d, 2H, C10H6), 7.24 (m, 6H, C10H6 + C6H5), 7.08 (m, 4H, C10H6), 6.71 (d, 2H,
EXPERIMENTAL SECTION
All manipulations were carried out under an argon atmosphere using Schlenk techniques (O2
