Article pubs.acs.org/Organometallics
Oxo-Bridged Bimetallic Group 4 Complexes Bearing AmineBis(benzotriazole phenolate) Derivatives as Bifunctional Catalysts for Ring-Opening Polymerization of Lactide and Copolymerization of Carbon Dioxide with Cyclohexene Oxide Ching-Kai Su,†,‡ Hui-Ju Chuang,† Chen-Yu Li,‡ Chun-Yue Yu,† Bao-Tsan Ko,*,† Jhy-Der Chen,‡ and Ming-Jen Chen§ †
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan § Department of Applied Cosmetology and Graduate Institute of Cosmetic Science, Hungkuang University, Taichung City 43302, Taiwan ‡
S Supporting Information *
ABSTRACT: We report the synthesis, crystal structure, and catalytic studies for ringopening polymerization (ROP) of group 4 metal alkoxides based on amine-BiBTP derivatives (amine-BiBTP = amine-bis(benzotriazole phenolate)). Dinuclear group 4 metal alkoxides [{(amine-BiBTP)Mt(OiPr)}2(μ-O)] (4−7, Mt = Ti, Zr, Hf) resulted from treatment of amine-BiBTP-H2 as the ligand precursor with 1.0 molar equiv of metal precursor (Ti(OiPr)4, Zr(OiPr)4(iPrOH), or Hf(OiPr)4(iPrOH)), followed by the addition of H2O (0.5 equiv) in good yields. The solid-state structure of 4−7 reveals a bimetallic BiBTP-ligated metal(IV) alkoxide with an oxo ligand chelating two metal atoms, and the bonding mode of the metal-O(μ-oxo)-metal moiety assumes a linear type. Catalysis of lactide polymerization and carbon dioxide/cyclohexene oxide copolymerization of oxo-bridged bimetallic metal complexes was systematically examined. Zirconium alkoxide 6 shows an effective catalyst in the ROP of lactide with “living” and “immortal” fashions, yielding poly(lactide)s with the predicted molecular weights and narrow polydispersity indices (PDIs 95%).4 Activation of carbon dioxide (CO2) to produce useful chemicals, fuels, and materials has received considerable attention recently due to its inexpensive, nonflammable, abundant, and biorenewable character. The coupling of CO2 with epoxides by metal complexes provides one of the useful methods for transforming CO2 into important fine chemicals such as cyclic carbonates and aliphatic polycarbonates.5 Consequently, a number of groups have developed homogeneous catalysts using aluminum, cobalt, chromium, zinc, and titanium complexes incorporated by diverse ancillary ligands to demonstrate satisfactory catalytic activities that show good Received: July 30, 2014 Published: December 2, 2014 7091
dx.doi.org/10.1021/om500784a | Organometallics 2014, 33, 7091−7100
Organometallics
Article
turnover frequencies (TOFs).6 Based on mechanistic studies of copolymerization of CO2 and cyclohexene oxide (CHO) by the highly active zinc β-diiminate complexes, a cooperative ringopening copolymerization via a bimetallic mechanism was proposed.7 As a result, several catalytic systems were designed as dinuclear metal complexes and their performances of CO2/ epoxide coupling were evaluated.8−13 For example, Williams et al. developed a series of bimetallic zinc(II),9 cobalt(II/III),10 iron(III),11 magnesium(II),12 and zinc(II)−magnesium(II)13 acetate complexes bearing macrocyclic ligands to efficiently catalyze the CO2/CHO copolymerization at only 1 atm pressure. Most recently, Nozaki et al. reported iron-corrole complexes as catalysts for copolymerization of CO2 with a wide range of epoxides; the oxo-bridged Fe-corrole complexes were found to effectively copolymerize CHO and CO2 with high copolymer selectivity and high carbonate-linkage content (>90%).14 Considering the development of well-characterized metal complexes for bifunctional catalysis of ROP and CO2/epoxide copolymerization, functionalized-benzotriazole phenolate derivatives seem to be promising ligand candidates. Trimetallic magnesium alkoxide complexes coordinated by amine-bis(benzotriazole phenoxide) ligands were shown to be effective and bifunctional catalysts toward ROP of ε-caprolactone as well as cycloaddition of CO2 with CHO.15 This result prompted us to further investigate the potential utilization of tetravalent metal (Ti, Zr, Hf) complexes derived from amine-bis(benzotriazole phenolate) ligands and ancillary isopropoxide groups. Although many group 4 metal alkoxides of aminephenolate derivatives have been synthesized, structurally characterized, and demonstrated for ROP catalysis of lactide,2f well-defined oxo-bridged group 4 metal alkoxides denoted as [{(L)Mt(OR)}2(μ-O)] (L = ligand; Mt = Ti, Zr, Hf; OR = alkoxy) are rare,16 and no oxo-ligated zirconium or hafnium alkoxide complex of amine-bis(phenolate) ligands has been isolated to date. More importantly, their catalytic efficiencies of such bimetallic complexes for ROP of cyclic esters and CO2/ epoxide copolymerization have not been reported. In this study, we present the synthesis, crystal structure, and bifunctional catalysis for lactide polymerization and CO2/CHO copolymerization (Figure 1) of oxo-bridged group 4 metal alkoxides containing amine-bis(benzotriazole phenolate) derivatives.
bis(benzotriazole phenol) pro-ligand C1NNBiBTP-H2 in equimolar proportion in toluene. The NMR spectra of Ti complex 1 displayed two sets of methine protons (δ 4.11 and 4.64 ppm), indicating that two chemically inequivalent −OiPr groups exist in solution. Similarly, treatment of the varying group 4 metal precursors (Zr(OiPr)4(iPrOH) and Hf(OiPr)4(iPrOH)) with the pincer pro-ligand containing the piperidinyl pendant (C1PPBiBTP-H2, 1.0 molar equiv) by following a similar synthetic pathway gave monometallic zirconium and hafnium alkoxides (2: [(C1PPBiBTP)Zr(OiPr)2]; 3: [(C1PPBiBTP)Hf(OiPr)2]) in >60% yield. Complex 1 further reacted with 0.5 molar equiv of water in the mixed solvent of THF/toluene at ambient temperature to give oxo-bridged dinuclear titanium complex [{(C1NNBiBTP)Ti(OiPr)}2(μ-O)] (4). Alternatively, 4 could also directly result from treatment of Ti(OiPr)4 with 1.0 molar equiv of C1NNBiBTP-H2, followed by the addition of H2O (0.5 equiv) in 75% yield. Due to facile synthesis of oxo-bridged metal complexes through the aforementioned one-pot reaction, we performed this approach to synthesize oxo-bridged bimetallic titanium, zirconium, and hafnium alkoxides incorporating the C1PPBiBTP ligand. The reaction of C1PPBiBTP-H2 with the corresponding group IV metal precursors (Ti(OiPr)4, Zr(OiPr)4(iPrOH), and Hf(OiPr)4(iPrOH)) by the same synthetic method afforded oxo-bridged dinuclear group 4 analogues (5: [{( C 1 P P BiBTP)Ti(O i Pr)} 2 (μ-O)]; 6: [{( C1PP BiBTP)Zr(O i Pr)} 2 (μ-O)]; 7: [{( C1PP BiBTP)Hf(OiPr)}2(μ-O)]) in >70% yield. The disappearance the O−H signal (∼11 ppm) of C1NNBiBTP-H2 or C1PPBiBTP-H2 ligand precursor and the resonance upshift for the methine protons of isopropoxide groups bound to metal centers proved the formation of the expected μ-oxo complexes 4−7. For instance, Zr complex 6 exhibited signals at δ 2.29 and 3.49 ppm for methyl protons of C1PPBiBTP ligands and methine protons of −OiPr groups with the integration ratio of 6:1 in the 1H NMR spectrum. All of these complexes were isolated as crystalline solids and fully characterized by spectroscopic studies as well as microanalyses. The molecular structures of complexes 1 and 4− 7 were further proved by single-crystal X-ray crystallography. Crystal Structural Studies of Complexes 1 and 4−7. Single crystals of complexes 1 and 4−7 suitable for X-ray structural determinations were obtained from their CH2Cl2/ hexane or saturated toluene solutions. Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawings including selected bond distances and angles of the molecular structures of 1, 4, and 6 are shown in Figures 2−4. Complex 1 crystallizes in the monoclinic space group P21/c, containing two independent molecules of [(C1NNBiBTP)Ti(OiPr)2] in the asymmetric unit. The crystal structure of 1 exhibits a monomeric titanium(IV) complex hexacoordinated by a C1NNBiBTP ligand and two ancillary isopropoxide (−OiPr) groups, and the geometry around the Ti center is distorted from octahedron. The average bond distances between the Ti(1) atom and O(phenoxy), N(amine), and O(isopropoxide) are respectively 1.9014(16), 2.325(2), and 1.8269(17) Å for 1, which are within the normal ranges in comparison with those previously published for titanium complexes incorporating amine-phenolate derivatives.16,17 It was also noted that two −OiPr ligands were bonded to the Ti(IV) center through the adjacent oxygen atoms in a distorted octahedral environment, adopting a cisgeometrical configuration. The molecular structure of 4 is a bimetallic Ti(IV) alkoxide that contains an oxo ligand bridging two titanium atoms. Each Ti center in 4 assumes a distorted octahedral geometry with each metal center six-coordinated by
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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes of group 4 complexes (1−7) containing amine-bis(benzotriazole phenolate) ligands appear in Scheme 1. The monomeric titanium alkoxide [(C1NNBiBTP)Ti(OiPr)2] (1) was prepared in good yield via a direct alcoholysis of Ti(OiPr)4 with the amine-
Figure 1. (a) ROP of lactide; (b) copolymerization of CO2 and CHO. 7092
dx.doi.org/10.1021/om500784a | Organometallics 2014, 33, 7091−7100
Organometallics
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Scheme 1. Synthetic Routes for Complexes 1−7
Figure 2. ORTEP drawing of complex 1 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ti(1)−O(1) 1.9011(16), Ti(1)−O(2) 1.9017(16), Ti(1)−O(3) 1.8087(17), Ti(1)−O(4) 1.8451(16), Ti(1)−N(7) 2.3264(19), Ti(1)−N(8) 2.323(2), O(1)−Ti(1)−O(2) 166.75(7), O(1)−Ti(1)−O(3) 92.27(8), O(1)−Ti(1)−O(4) 92.42(7), O(2)−Ti(1)−O(3) 98.65(8), O(2)−Ti(1)−O(4) 91.72(7), O(3)−Ti(1)−O(4) 106.19(8), O(1)−Ti(1)−N(7) 82.49(7), O(1)−Ti(1)−N(8) 89.52(7), O(2)−Ti(1)− N(7) 84.96(7), O(2)−Ti(1)−N(8) 83.32(7), O(3)−Ti(1)−N(7) 163.62(8), O(3)−Ti(1)−N(8) 88.53(8), O(4)−Ti(1)−N(7) 89.58(7), O(4)− Ti(1)−N(8) 165.05(8), N(7)−Ti(1)−N(8) 75.97(7).
two oxygen atoms and two nitrogen atoms from the C1NNBiBTP ligand, an O atom from the −OiPr group, and one O atom of the oxide group. The average bond lengths of Ti−O(phenoxy) = 1.917(3) Å, Ti−N(amine) = 2.351(4) Å, and Ti−O(OiPr) = 1.800(3) Å in 4 are comparable to those observed for Ti complex 1, as shown in Table 1. It is worthy of note that the coordination mode of the Ti−O−Ti moiety from the doubly bridged oxo is attributed to a linear type, evidenced by the bond angle of Ti(1)−O(4)−Ti(1A) = 177.9(2)°. The average Ti−
O(μ2-oxo) bond length (1.8211(7) Å) in 4 is within a typical range previously reported for doubly oxo-bridged Ti−O bonds.16,18 Dinuclear titanium alkoxide 5 (Figure S1, Supporting Information (SI)) is isostructural to Ti analogue 4, except a piperidinyl (−NC5H10) substituent is attached at the pendant arm of the amine-bis(benzotriazole phenolate) ligands. As expected, the average six-coordinated Ti-containing bond distances of Ti−O(phenoxy) of 1.926(2) Å, Ti−N(amine) of 2.387(2) Å, Ti−O(OiPr) of 1.770(2) Å, and Ti−O(μ-oxo) of 7093
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Figure 3. ORTEP drawing of complex 4 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity. Ti(1)−O(1) 1.914(3), Ti(1)−O(2) 1.919(3), Ti(1)−O(3) 1.800(3), Ti(1)−O(4) 1.8211(7), Ti(1)−N(7) 2.323(4), Ti(1)−N(8) 2.379(4), Ti(1A)−O(4) 1.8211(7), O(1)−Ti(1)−O(2) 162.34(13), O(1)−Ti(1)−O(3) 92.52(13), O(3)−Ti(1)−O(4) 104.91(12), O(1)−Ti(1)−O(4) 97.15(13), O(2)− Ti(1)−O(3) 97.33(14), O(2)−Ti(1)−O(4) 94.44(14), O(1)−Ti(1)−N(7) 82.54(12), O(1)−Ti(1)−N(8) 84.74(13), O(2)−Ti(1)−N(7) 84.27(13), O(2)−Ti(1)−N(8) 80.65(13), O(3)−Ti(1)−N(7) 165.18(14), O(3)−Ti(1)−N(8) 90.00(14), O(4)−Ti(1)−N(7) 89.61(11), O(4)− Ti(1)−N(8) 164.83(13), N(7)−Ti(1)−N(8) 75.68(13), Ti(1)−O(4)−Ti(1A) 177.9(2).
toluene (10 mL). Experimental results indicated that 95% Llactide (L-LA) conversion was achieved for 24 h and the measured molecular weight Mn is very close to the theoretical value while the polymerization temperature was set at 80 °C. All group 4 complexes in this work were further used to polymerize 300 equiv of L-LA to compare their catalytic activities under the optimized conditions ([Init.]0 = 5 mM, 80 °C). It was found that Ti complexes 1, 4, and 5 were inactive for 24 h, whereas monometallic complexes 2 and 3 and bimetallic alkoxides 6 and 7 displayed efficient activity with good control of molecular weight under identical conditions (Table 2, entries 4−10). The activity trend of these group 4 bimetallic oxo derivatives could be attributed to the more open coordination sphere and the better nucleophilicity of the alkoxy group when replacing the titanium alkoxide with the zirconium or hafnium analogue. In comparison, the ROP activity of Zr complex 4 is superior to that of Hf complex 5, and this tendency agrees with the previous reports in the literature.4,21 Note that the catalytic performance of the oxo-bridged bimetallic Zr or Hf complex for LA polymerization is comparable to that of the monometallic Zr or Hf alkoxide (Table 2, entries 9 and 10 vs entries 5 and 6). Furthermore, ROP of L-LA promoted by Zr alkoxide 6 with different monomer-to-initiator ratios and the addition of excess alcohol (e.g., 2-propanol) demonstrated the “living” and “immortal” characters. As depicted by entries 2, 9, and 11−13 in Table 2, the conversion yields reached ≥92% within 24 h over a wide feeding ratio of [L-LA]0/[6]0; the observed molecular weight (Mn) of the isolated poly(L-lactide) (PLLA) nearly matched the theoretical value calculated from the molar ratio of L-LA to
1.8251(19) Å (Table 1) in 5 are similar to those found for complex 4. The solid structures of 6 and 7 (Figure S2, SI) also reveal a homologous bimetallic alkoxide complex with the Zr(IV) or Hf(IV) ion as the metal element. The average Zrincluding bond distances of Zr−O(phenoxy) = 2.0508(15) Å, Zr−N(amine) = 2.5146(19) Å, Zr−O(OiPr) = 1.9153(16) Å, and Zr−O(μ-oxo) = 1.9552(16) Å are >0.1 Å longer than the bond distances observed for bimetallic Ti analogue 5 as compared in Table 1. Such differences can be ascribed to the ionic radius of the zirconium atom being larger than that of the titanium atom. In comparison, the average bond lengths of M− O and M−N (Hf−O(phenoxy) = 2.039(2) Å, Hf−N(amine) = 2.487(3) Å, Hf−O(OiPr) = 1.905(2) Å, and Hf−O(μ-oxo) = 1.947(2) Å) for 7 are slightly shorter than those of 6 on altering the metal center from Zr to Hf, and the trend is consistent with the previous observations for other group 4 metal amino(phenolate) derivatives.17a,19 It is interesting to note that each amine-BiBTP ligand in complexes 1 and 4−7 assumes an O,O,N,N-tetradentate bonding mode to coordinate the metal center in the solid state. Ring-Opening Polymerization of Lactides. On the basis of the good catalytic performances for lactone polymerizations mediated by well-defined group 4 complexes [LxMt(OR)y] (L = ligand; Mt = Ti, Zr, Hf; OR = alkoxy; x, y = integer),2f,20 we used group 4 alkoxides 1−7 as initiators to explore their catalytic efficiency for lactide polymerization. ROP catalysis in solution and bulk polymerizations was systematically studied, and representative results are tabulated in Table 2. Zr complex 6 was first employed to examine the temperature effect of ROP under the condition of an initiator concentration of 0.005 M in 7094
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Figure 4. ORTEP drawing of complex 6 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity. Zr(1)−O(1) 2.0646(15), Zr(1)−O(2) 2.0428(15), Zr(1)−O(3) 1.9128(16), Zr(1)−O(4) 1.9552(16), Zr(1)−N(7) 2.4765(19), Zr(1)−N(8) 2.550(2), Zr(2)− O(4) 1.9552(16), Zr(2)−O(5) 2.0498(15), Zr(2)−O(6) 2.0461(16), Zr(2)−O(7) 1.9177(16), Zr(2)−N(15) 2.4594(19), Zr(2)−N(16) 2.5725(19), O(3)−Zr(1)−O(4) 102.17(7), O(3)−Zr(1)−O(2) 100.22(7), O(4)−Zr(1)−O(2) 96.49(6), O(3)−Zr(1)−O(1) 99.46(7), O(4)− Zr(1)−O(1) 92.80(6), O(2)−Zr(1)−O(1) 155.91(6), O(3)−Zr(1)−N(7) 166.24(7), O(4)−Zr(1)−N(7) 91.50(6), O(2)−Zr(1)−N(7) 79.54(6), O(1)−Zr(1)−N(7) 78.04(6), O(3)−Zr(1)−N(8) 93.79(7), O(4)−Zr(1)−N(8) 163.99(7), O(2)−Zr(1)−N(8) 81.67(7), O(1)−Zr(1)−N(8) 83.29(6), N(7)−Zr(1)−N(8) 72.51(6), Zr(1)−O(4)−Zr(2) 166.25(9), O(7)−Zr(2)−O(4) 104.06(7), O(7)−Zr(2)−O(6) 99.74(7), O(4)− Zr(2)−O(6) 91.56(7), O(7)−Zr(2)−O(5) 99.29(7), O(4)−Zr(2)−O(5) 98.43(6), O(6)−Zr(2)−O(5) 155.64(6), O(7)−Zr(2)−N(15) 167.31(7), O(4)−Zr(2)−N(15) 88.51(6), O(6)−Zr(2)−N(15) 77.87(6), O(5)−Zr(2)−N(15) 80.22(6), O(7)−Zr(2)−N(16) 94.33(7), O(4)− Zr(2)−N(16) 161.03(6), O(6)−Zr(2)−N(16) 80.58(6), O(5)−Zr(2)−N(16) 82.95(6), N(15)−Zr(2)−N(16) 73.01(6).
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1 and 4−7 M(1)−O(1) M(1)−O(2) M(1)−O(3) M(1)−O(4) M(1)−N(7) M(1)−N(8) M(2)−O(4) O(1)−M(1)−O(2) O(3)−M(1)−N(7) O(4)−M(1)−N(8) M(1)−O(4)−M(2)
1 (M = Ti)
4 (M = Ti)
5 (M = Ti)
6 (M = Zr)
7 (M = Hf)
1.9011(16) 1.9017(16) 1.8087(17) 1.8451(16) 2.3264(19) 2.323(2)
1.914(3) 1.919(3) 1.800(3) 1.8211(7) 2.323(4) 2.379(4) 1.8211(7)1 162.34(13) 165.18(14) 164.83(13) 177.9(2)2
1.928(2) 1.929(2) 1.764(2) 1.8280(19) 2.325(2) 2.421(2) 1.8222(19) 161.69(9) 171.71(9) 164.89(9) 166.83(13)
2.0646(15) 2.0428(15) 1.9128(16) 1.9552(16) 2.4765(19) 2.550(2) 1.9552(16) 155.91(6) 166.24(7) 163.99(7) 166.25(9)
2.030(2) 2.053(2) 1.900(2) 1.946(2) 2.444(3) 2.528(3) 1.947(2) 156.87(9) 165.88(10) 165.02(10) 166.73(15)
166.75(7) 163.62(8) 165.05(8)
1
The bond length of Ti(1A)−O(4). 2The bond angle of Ti(1)−O(4)−Ti(1A).
monomer-to-initiator as well as very narrow polydispersity indices (PDIs
