Article pubs.acs.org/Organometallics
Ethylene Polymerization Catalysis by Vanadium-Based Systems Bearing Sulfur-Bridged Calixarenes Carl Redshaw,*,† Lucy Clowes,† David L. Hughes,† Mark R. J. Elsegood,‡ and Takehiko Yamato§ †
Energy Materials Laboratory, School of Chemistry, University of East Anglia, NR4 7TJ U.K. Chemistry Department, Loughborough University, Loughborough, Leicestershire, LE11 3TU U.K. § Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga-shi, Japan 840-8502 ‡
S Supporting Information *
ABSTRACT: Calixarenes bridged by thia (−S−), sulfinyl (−SO−), or sulfonyl (−SO2−) linkers have been employed as ancillary ligands in vanadium-based ethylene polymerization and in the ring-opening polymerization of ε-caprolactone. For ethylene polymerization, all pro-catalysts [PPh4][VOCl2(R-calix[4]areneH2)] (R = p-tert-butylthia (−S−) (1), sulfinyl (−SO−) (2), sulfonyl (−SO2−) (3)) were highly active, with the system bearing the sulfonyl (−SO2−)-bridged calixarene displaying enhanced activity. The molecular structures of pro-catalysts 2 and 3 are reported. Pro-catalysts 1−3 were inactive for the ROP of ε-caprolactone.
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recently deployed sulfur-bridged (−S−, −SO−, −SO2−) diphenolate ligation to mimic those parts of thiacalixarenes relevant to the binding at vanadium. The −SO2− bridged system was shown to be an efficient catalyst for the sulfoxidation of thioethers with t-BuOOH (haloperoxidase activity).7 A dinuclear titanium complex of the −S− bridged ligand set has been employed as a Lewis acid catalyst in the Mukaiyama-aldol reaction of aryl aldehydes with silyl enol ethers,8 as a catalyst in the cyclotrimerization of alkynes,9 and more recently in combination with MAO (methylaluminoxane) for ethylene polymerization, albeit with low observed activity (≤25 g/(mmol h bar)).10 By extending the synthetic methodology employed by Limberg et al. for the thiacalixarene derivative 1, whereby the parent calix[4]arene was treated with PPh 4[VO2Cl2] in tetrahydrofuran, the pro-catalyst family [PPh 4][VOCl2(Rcalix[4]areneH2)] (R = p-tert-butylthia (−S−) (1), sulfinyl (−SO−) (2), sulfonyl (−SO2−) (3)) (see Chart 1) was readily accessible as dark crystalline solids. Crystals of the sulfinyl procatalyst 2 suitable for single-crystal X-ray diffraction analysis were grown via diffusion of hexane into a dichloromethane solution of 2. The molecular structure is shown in Figure 1, with selected bond lengths and angles given in the caption. The vanadium center adopts a distorted-octahedral geometry with the oxygen of one of the sulfinyl bridges incorporated in the coordination sphere. This oxygen (O(5)) is trans to the oxo
ver the past decade or so, the search for new ligand sets capable of forming efficient catalytic systems for either αolefin polymerization or the ring opening of lactones has met with considerable success.1 Both early- and late-transition-metal complexes have been shown to be capable of impressive catalytic performance. In terms of the ligands employed, the use of the calixarene family has been somewhat limited.2 However, notable catalytic performance, particularly in vanadium-based systems, has been achieved when the more common methylene-bridged (−CH2−) calixarenes are replaced by those bearing dimethyleneoxa-type-bridged (−CH2OCH2−) calixarenes.3 The improved performance (near 100-fold increases in observed activity) was thought to arise partly via stabilization of the active species by the additional bridging ether groups of the calixarene linkers. There have also been a number of reports in the literature concerning the beneficial presence of sulfur in ligand backbones, particularly in diphenolate-type ligation.4 Given this and the availability of a range of other calixarenes which bear functionality at the linker, we have embarked upon a program to screen the catalytic potential of such systems. Herein, we report vanadium-based systems bearing thia (−S−)-, sulfinyl (−SO−)-, or sulfonyl (−SO2−)-bridged calixarenes and compare their performance in ethylene polymerization catalysis and in the ring opening of ε-caprolactone. We note that the coordination chemistry of such “sulfur calixarene” ligands is limited,5 and their use in catalysis is restricted to reports by Limberg et al., in which the thia-bridged ligand set was employed in vanadium-catalyzed alcohol oxidations.6 Furthermore, the same group has also © 2011 American Chemical Society
Received: May 14, 2011 Published: October 14, 2011 5620
dx.doi.org/10.1021/om200402u | Organometallics 2011, 30, 5620−5624
Organometallics
Article
Chart 1. Pro-Catalysts 1−4
O(1), O(2), O(5) plane. Oxygen atoms O(6), O(7), and O(8) all exhibit conformational disorder with the O atoms predominantly in the “out” position. The minor component lies “up”, as does O(5), which does not exhibit disorder. The structure contains two dichloromethane molecules of crystallization; one is well-defined within the calixarene cone, and the other, substantially disordered, lies exo to the cone. The two phenol groups make intramolecular H bonds. Crystals of the sulfonyl pro-catalyst 3 suitable for singlecrystal X-ray diffraction were grown via diffusion of hexane into a dichloromethane solution of 3. The molecular structure is shown in Figure 2, with selected bond lengths and angles given
Figure 1. View of the anion [VOCl2(p-tert-butylcalix[4,SO]areneH2)]− (2), indicating the atom-numbering scheme. The [PPh4]+ cation, hydrogen atoms except OH, and two dichloromethane molecules of crystallization have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): V(1)−O(1) = 1.933(4), V(1)−O(2) = 1.919(4), V(1)−O(9) = 1.574(4), V(1)−O(5) = 2.191(4), V(1)− Cl(1) = 2.293(2), V(1)−Cl(2) = 2.2858(18); O(1)−V(1)−O(9) = 95.45(18), O(1)−V(1)−Cl(1) = 87.46(13), O(1)−V(1)−O(5) = 84.87(15), Cl(1)−V(1)−Cl(2) = 92.55(7). H-bond lengths (Å): O(3)−H(3)···O(4), H(3)···O(4) = 2.41; O(4)−H(4)···O(1), H(4)···O(1) = 2.34.
group (O(9)), and the bond to vanadium is somewhat elongated (V(1)−O(5) = 2.191(4) Å) in comparison with the other V−O bonds (see caption to Figure 1) but is similar to other reported V−O(S) bonds.4 This is in contrast with the situation observed by Limberg et al. for the dimeric sulfinylbridged diphenolate complex (PPh4)2[LV(O)(μ 2-O)2V(O)L] (L = 2,2′-sulfinylbis(2,4-di-tert-butylphenol)), for which the nonbonded V···S distance is 3.2437(5) Å, and the sulfinyl oxygen points away from the vanadium center. This results in a distorted-square-pyramidal geometry at the metal; the related thia and sulfonyl biphenolate complexes adopt distortedoctahedral geometries with V−S and V−O bridging interactions, respectively.7 The calixarene ligand retains the cone conformation, and the vanadium center sits 1.24 Å above the
Figure 2. View of the anion [VOCl2(p-tert-butylcalix[4,SO 2]areneH2)]− (3), indicating the atom-numbering scheme. The [PPh4]+ cation, hydrogen atoms not involving H bonds, minor disorder components, and three molecules of CH2Cl2 have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): V(1)−O(1) = 1.895(3), V(1)−O(2) = 1.937(3), V(1)−O(5) = 2.357(3), V(1)− O(13) = 1.577(3), V(1)−Cl(1) = 2.2771(14), V(1)−Cl(2) = 2.2904(12); O(1)−V(1)−O(2) = 86.62(12), O(5)−V(1)−O(13) = 177.53(13).
in the caption. In 3, the coordination of the VOCl2 unit to the calixarene is very similar to that adopted in 2. However, the calixarene in 3 adopts an “up−up−up−down” conformation 5621
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Figure 3. View of 3 showing intra- and intermolecular H bonding: O(3)−H(3)···O(7), H(3)···O(7) = 1.89 Å; O(4)−H(4)···O(10), H(4)···O(10) = 2.24 Å; O(4)−H(4)···O(11A), H(4)···O(11A) = 2.30 Å (symmetry operator A: −x + 2, −y, −z.
observed activity (run 28). Use of an [Al]:[V] ratio of 6000:1 at various temperatures gave an optimum activities in excess of about 44 000 g/(mmol h bar) at 60 °C (run 36). A comparative screening study of pro-catalysts 1−4 under the same conditions (60 °C and 6000 equiv of MADC, runs 34−37) led to the observed activity order 3 > 4 > 2 > 1. The presence of the donor oxygen atoms for the pro-catalysts herein appears to be somewhat more beneficial than was observed recently for the non-oxo vanadium(IV) pro-catalysts [V(LS)2] and [V(LSO2)2] (where LS and LSO2 are the deprotonated forms of 2,2′thiobis(4,6-di-tert -butylphenol) and 2,2′-sulfonylbis(4,6-di-tertbutylphenol), respectively): viz. 20 500 versus 21 600 g/(mmol h bar), respectively.13 While the exact nature of the active species in these systems remains unknown, treatment of these pro-catalysts on a preparative scale with an excess of the cocatalysts (5 equiv) involved initial reaction of the free phenolic groups (shown by IR spectroscopy). EPR spectra were also consistent with reduction to the V(IV) state; more indepth EPR studies have been conducted on a number of related systems.14 Selected polymer data are presented in Table 2. All procatalysts afforded high-molecular-weight linear polyethylene; typical melting points by DSC were in the range of 138−140 °C for runs 33 and 36, respectively. The range of PDIs measured was in the range 2.7−6.5, with the lower value associated with higher temperature (80 °C). At this higher temperature, the polymer molecular weight was substantially reduced. Why is MADC more favorable than DMAC? We tentatively propose that this is due to the nature of the active species, which is thought to contain multiple V−Cl−Al bridges, and would be more readily formed via the multiple chlorides available in MADC as compared to DMAC. This is consistent with the work of Gambarotta,15 who postulated the catalytically active species I (Chart 2). Complexes 1−3 were also screened for their ability to polymerize ε-caprolactone, in the presence or absence of benzyl alcohol. However, these systems, under the conditions employed (monomer:metal 400:1; 25−80 °C; 20 mL of toluene; 0−5 equiv of benzyl alcohol), were inactive. In conclusion, the use of sulfur-containing calixarenes as ancillary ligands in vanadium-based ethylene polymerization catalysis affords highly active, thermally stable catalytic systems.
rather than the cone observed in 2. As in 2, there are two intramolecular H bonds, but in this case the calixarene conformation facilitates an additional centrosymmetric pair of intermolecular H bonds between phenol group O(4)−H(4) and one oxygen of an SO2 group on a neighboring molecule (Figure 3). Previous NMR studies (1H/13C) were consistent with the adoption of the cone conformation for the parent calixarene ligands in 2 and 3 (as for the parent ligand of 1, which shows rapid interconversion among stable conformers such as the cone). However, in the case of the sulfonyl ligand, there was evidence for a pair of distal SO groups in the axial position with another equatorial group.11a,12 Subsequent studies were consistent with the adoption of “all trans” 1,3-alternate conformations in solution.11b 1H NMR data for 2 and 3 at 254 K were consistent with a Cs-symmetric structure in solution: one signal for the two OH groups and two singlets for the protons of the tert-butyl groups.
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ETHYLENE POLYMERIZATION For Schlenk-line screening, pro-catalysts 1−3 have been screened using either DMAC (dimethylaluminum chloride) or MADC (methylaluminum dichloride) as a cocatalyst in the presence of the reactivator ETA (ethyl trichloroacetate), and as a benchmark, screening was also conducted using the known pro-catalyst [V(O()p-tert-butylhexahomotrioxacalix[3]arene)] (4).3 In the case of pro-catalyst 1/DMAC, varying the [Al]: [V] ratio at 20 °C led to an increase from about 3000 g/(mmol h bar) (Table 1, run 1) at 2000:1 to ca. 16 000 g/(mmol h bar) (run 3) at 6000:1. Further increases in the [Al]:[V] ratio were detrimental to the observed activity. Using an [Al]:[V] ratio of 6000:1 and varying the temperature revealed optimized activity results at 60 °C (run 11). In the case of MADC, similar trends were observed, with somewhat higher optimized observed activities (runs 7 and 14). In the case of pro-catalyst 2, activities using DMAC were unimpressive ( 2σ(I)) = 0.065, and wR2 (all data) = 0.173. One of the CH2Cl2 molecules was modeled as a diffuse region of electron density with the Platon Squeeze procedure. 19 One t-Bu group, one Ph ring in the cation, and sulfinyl oxygen atoms O(6), O(7), and O(8) were modeled as disordered. Crystal Data for 3·3CH2Cl2: C64H66Cl2O13PS4V·3CH2Cl2, M = 1579.1, triclinic, space group P1̅, a = 13.7043(16) Å, b = 16.4065(19) Å, c = 17.809(2) Å, α = 85.5859(16)°, β = 76.1046(16)°, γ = 74.8501(16)°, V = 3751.5(7) Å3, T = 150(2) K, Z = 2, μ(Mo Kα) = 0.608 mm−1. A total of 31 727 reflections were measured on a Bruker APEX II CCD diffractometer using 0.3° narrow frames.20 Data were integrated to θ max = 26.42°. Data were corrected for Lorentz and polarization effects and for absorption. A total of 15 209 unique data (Rint = 0.029) were obtained, 10 393 of which were “observed”. The structure was solved by direct methods and refined with anisotropic non-H atoms on F 2.18 H atoms were constrained. R(F 2) (I > 2σ(I)) = 0.068, and wR2 (all data) = 0.219. Three t-Bu groups, two Ph rings in the cation, and one CH2Cl2 molecule were modeled as disordered.
Nakayama, Y.; Hirao, T.; Yasuda, H.; Harada, A. J. Organomet. Chem. 2004, 689, 612. (f) Janas, Z.; Wisniewska, D.; Jerzykiewicz, L. B.; Sobota, P.; Drabent, K.; Szczegot, K. Dalton Trans. 2007, 2065. (g) Janas, Z.; Godbole, D.; Nerkowski, T.; Szczegot, K. Dalton Trans. 2009, 8846. (h) Lian, B.; Ma, H.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2009, 9033. (5) (a) Iki, N.; Miyano, S. J. Inclusion Phenom. Macrocyclic Chem. 2001, 41, 99. (b) Kajiwara, T.; Yokozawa, S.; Ito, T.; Iki, N.; Morohashi, N.; Miyano, S. Angew. Chem., Int. Ed. 2002, 41, 2076. (c) Hirata, K.; Suzuki, T.; Noya, A.; Takei, I.; Hidai, M. Chem. Commun. 2005, 3718. (d) Bilyk, A.; Dunlop, J. W.; Fuller, R. O.; Hall, A. K.; Harrowfield, J. M.; Hosseini, M. W.; Koutsantonis, G. A.; Murray, I. W.; Skelton, B. W.; Stamps, R. L.; White, A. H. Eur. J. Inorg. Chem. 2010, 2106 and references therein. (6) Hoppe, E.; Limberg, C. Chem. Eur. J. 2007, 13, 7006. (7) Werncke, C. G.; Limberg, C.; Knispel, C.; Metzinger, R.; Braun, B. Chem. Eur. J. 2011, 17, 2931. (8) Morohashi, N.; Hattori, T.; Yokomakura, K.; Kabuto, C.; Miyano, S. Tetrahedron Lett. 2002, 43, 7769. (9) Morohashi, N.; Yokomakura, K.; Hattori, T.; Miyano, S. Tetrahedron Lett. 2006, 47, 1157. (10) Proto, A.; Giugliano, F.; Capacchione, C. Eur. Polym. J. 2009, 45, 2138. (11) (a) Iki, N.; Kumagai, H.; Morohashi, N.; Ejima, K.; Hasegawa, M.; Miyanari, S.; Miyano, S. Tetrahedron Lett. 1998, 39, 7559. (b) Morohashi, N.; Iki, N.; Sugawara, A.; Miyano, S. Tetrahedron 2001, 57, 5557. (12) Morohashi, N.; Narumi, F.; Iki, N.; Hattori, T.; Miyano, S. Chem. Rev. 2006, 106, 5291. (13) Homden, D. H.; Redshaw, C; Warford, L.; Hughes, D. L.; Wright, J; Dale, S. H.; Elsegood, M. R. J. Dalton Trans. 2009, 8900. (14) (a) Soshnikov, I. E.; Semikolenova, N. V.; Bryliakov, K. P.; Shubin, A. A.; Zakharov, V. A.; Redshaw, C; Talsi, E. P. Macromol. Chem. Phys. 2009, 210, 542. (b) Soshnikov, I. E.; Semikolenova, N. V.; Bryliakov, K. P.; Zakharov, V. A.; Redshaw, C; Talsi, E. P. J. Mol. Catal. A: Chem. 2009, 303, 23. (c) Soshnikov, I. E.; Semikolenova, N. V.; Shubin, A. A.; Bryliakov, K. P.; Zakharov, V. A.; Redshaw, C; Talsi, E. P. Organometallics 2009, 28, 6714. (15) Gambarotta, S. Coord. Chem. Rev. 2003, 237, 229. (16) The coordination sphere of the active catalyst species may be notably different than those observed in 2 and 3 prior to the addition of Me2AlCl or MeAlCl2 and ethyl chloroacetate. (17) Programs CrysAlis-CCD and -RED; Oxford Diffraction Ltd., Abingdon, U.K., 2005. (18) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (19) Spek, A. L. Acta Crystallogr. 1990, A46, C34. (20) APEX II and SAINT software for CCD diffractometers; Bruker AXS Inc., Madison, WI, 2008.
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ASSOCIATED CONTENT * Supporting Information CIF files giving X-ray crystallographic data for 2·2CH2Cl2 and 3·3CH2Cl2 and figures giving alternative views of 2 and 3. This material is available free of charge via the Internet at http:// pubs.acs.org. S
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. ACKNOWLEDGMENTS
We thank the EPSRC and The University of East Anglia for support. The EPSRC Mass Spectrometry Service Centre Swansea and Rapra Smithers Ltd (GPC) are thanked for data.
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REFERENCES
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