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Mechanistic Insights of the Initiation Process of the Ring-Opening Polymerization of ε-Caprolactone by Divalent Sm(BH4)2(THF)2 with DFT: Concerted or Oxidative Reaction? Christophe Iftner,† Fanny Bonnet,‡,§,|| Franc-ois Nief,^ Marc Visseaux,*,‡,§,|| and Laurent Maron*,† †
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Laboratoire de Physique et Chimie des Nano-objets (LPCNO), UMR 5215, Universite de Toulouse-CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France ‡ Universite Lille Nord de France, F-59000 Lille, France § USTL, UCCS, CNRS, F-59650 Villeneuve d’Ascq, France UMR 8181, F-59650 Villeneuve d’Ascq, France ^ Laboratoire Heteroelements et Coordination, CNRS, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau, France
bS Supporting Information ABSTRACT: Concerted and oxidative mechanisms for the initiation of the ROP of ε-caprolactone by divalent Sm(BH4)2(THF)2 have been computed at the DFT level and compared with experimental data. The concerted polymerization pathway is proposed to occur via divalent [Sm] (BH4) active species through classical O-acyl cleavage, leading to R,ω-dihydroxy telechelic polycaprolactone, which is fully compatible with chain end groups observed for low-Mn samples prepared with Sm(BH4)2(THF)2. Although it is calculated to be favorable, the oxidative route affording radical species is not in accordance with these experimental results, unless one considers transfer reactions with the solvent.
T
he demand for biodegradable polyesters is constantly increasing, owing to the numerous high-technology applications associated with these materials, particularly in the biomedical and pharmaceutical fields.1 Such polymers are generally produced by ring-opening polymerization (ROP),2 metal mediated in most cases,3,4 and over the last two decades, a growing number of studies involving rare-earth compounds as initiators, in homo- and copolymerization reactions, have been reported.5 9 Despite the fact that samarium diodide is a famously versatile reagent for organic chemistry, divalent samarium and, more generally, divalent lanthanide compounds have been much less studied in the framework of polymerization catalysis.10 Only a limited number of reports regarding the ROP of lactones can be found in the literature; they involve samarium halides,11,12 amides,11,13 phosphides,14 cyclopentadienyls,11,12,15 and related aromatic rings,16 19 as well as ytterbium and samarium phenolates.13,20 22 In most cases, the monomer conversion is complete within minutes with these initiators, but in contrast with their trivalent homologues, the process is generally not well controlled, with broad molecular weight distributions11 or high molecular weights connected to low initiation efficiencies.12,13,20,22 We recently synthesized and characterized unique examples of divalent borohydrido samarium compounds, and we found that they also efficiently initiated the polymerization of ε-caprolactone. Moreover, we observed that the molecular weights obtained r 2011 American Chemical Society
seemed to be connected to a polymerization process that was possibly not univocal: depending on the experimental conditions, putative divalent or trivalent active species could be formed and propagate the polymerization.23 Whereas the mechanism of ROP of lactones with trivalent rare-earth-based initiators, including borohydrides,24 has been thoroughly investigated,25 very little is known regarding the detailed steps when divalent initiators are utilized. This study intends to clarify these points and especially the competition between these two possible initiation pathways, by using DFT approaches. Over the past decade, DFT methods have been found to be reliable in explaining the reactivity of lanthanide-containing molecules.26 29 However, the number of theoretical studies dealing with lanthanide oxidation is complicated, because all problems that are solved by applying the computational methods defined by Maron and Eisenstein30 arise again. In this study, the methodology proposed by Labouille et al.31 was used to compute the oxidative pathway (see computational details in the Supporting Information). The Gibbs free energy profile has thus been computed for both the concerted (divalent) and the oxidative (trivalent species) ROP of ε-caprolactone (Figure 1). Sm(BH4)2(THF)2 (1) exhibits a tetrahedral-type geometry, with the two borohydride ligands η3-coordinated by their Received: June 14, 2011 Published: August 17, 2011 4482
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Figure 1. Gibbs free energy profile for the initiation process of the ROP of ε-caprolactone by Sm(BH4)2(THF)2: (black) concerted pathway; (red) oxidative pathway.
hydrogens and the two THF ligands coordinated through their oxygen to the Sm(II).23 In that sense, Sm is already eightcoordinated in 1. For both reaction pathways, the reaction begins by the coordination of the ε-caprolactone to the samarium center. It should be noticed that the coordination of the substrate induced the displacement of one THF molecule, with no haptotropic shift observed on the two borohydride ligands, leading to the formation of 2. It should also be noted that in 2 there is no oxidation of the Sm center and that the ε-caprolactone is coordinated through its exocyclic oxygen with a boat comformation. The lack of oxidation through the coordination of the substrate is different from uranium(III) chemistry where the coordination induces the oxidation.32 The replacement of one THF by ε-caprolactone is computed to be athermic. From 2, the two pathways are different and both will be described separately, starting with the concerted path. The concerted pathway implies formation of the “classical” BH4 nucleophilic attack transition state (TS) 3, with an activation barrier of 32.5 kcal mol 1 (similar to the barrier found for the Sm(BH4)3 system25). At the transition state, the geometry is trigonal planar around Sm, with the ancillary borohydride ligand still η3-coordinated. For the reactive part, the borane BH3 is fully formed (planar structure) and is no longer interacting with the metal center. At the same time, the hydrogen is already transferred to the carbon of the carbonyl and is no longer interacting with the boron. The endocyclic oxygen of ε-caprolactone is interacting with Sm (Sm O distance of 2.56 Å) with the boat conformation retained. From TS 3, the system evolves to the formation of the borate complex 4. Indeed, the BH3 is, as has already been reported, trapped by the exocyclic oygen to form a stable borate complex. The formation of 4 is exergonic by roughly 6 kcal mol 1.
In 4, the two oxygens are coordinated to the metal center in a distorted-trigonal-pyramidal fashion and residual interactions between two hydrogens of the BH3 and the metal center are observed (Sm H distances around 2.60 Å). From 4, the ring opening (RO) occurs through a classical second B H activation TS (5),25,33 with an activation barrier of ca. 40 kcal mol 1, leading to an accessible pathway, as solvent effects reduce the barrier by 9.4 kcal mol 1.40 The ring-opening process is thus the rate-determining step of the reaction. In 5, the ring is open and the carbonyl function has been reduced to a CH2 OBH2 end group, the precursor of R,ω-dihydroxy telechelic polycaprolactone (PCL). The former exocyclic oxygen is no longer interacting with the metal center, and it should be noted that there is pyramidalization at Sm. As for the TS 3, the barrier is similar to that reported for Sm(BH4)3.25 Following the intrinsic reaction coordinate, 5 is connected to the alkoxide complex 6. The geometry of 6 is very similar to that of 5, except that the former exocyclic oxygen has oriented one of its lone pairs toward the metal center. This induces an important stabilization of the complex, so that formation of 6 is athermic with respect to 4. Thus, the concerted pathway via divalent active species is computed as an accessible pathway through classical O-acyl cleavage, which is line with literature reports. For instance, according to Wakatsuki and co-workers, when the polymerization was initiated by a Sm(OAr)2 compound, propagation occurred via a growing [Sm] O (CH2)5CO [O (CH2)5CO]n OAr chain. Acylation of the quenching alcohol (ROH, R = Me, Et) by the electrophilic acyl end group was tentatively proposed to account for the final analyzed polymer: HO (CH2)5CO [O (CH2)5CO]n OR. In our case, careful examination of 1H and 13C NMR spectra of 4483
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Scheme 1. Possible Schematic Routes for an Oxidative Pathway
Figure 2. 1H NMR spectrum of poly(ε-caprolactone) in CDCl3 at 293 K: Mn(NMR) = 4440 g/mol (Supporting Information: Table S1, run 1).
typical low-Mn samples prepared with the initiator 1 (see Table S1 in the Supporting Information) gave evidence for CH2 OH as unique end groups (Figures 2 and S2), which corresponds well to the calculations done for a concerted pathway. Regarding the occurrence of a polymerization initiated by an oxidative Sm(II) Sm(III) process, we considered it of interest, given the lack of clear and unequivocal information available in the literature. Indeed, Wakatsuki and co-workers acknowledged that the details of the initial step of their polymerization study with the bis-phenoxide samarium complex were not clear. Monoelectronic oxidizing initiation could be envisaged ,but the polymer end groups did not account for such a process.20 The mechanism of ROP with divalent Sm(PPh2)2 was also investigated, and a similar mechanism was postulated.14 Agarwal claimed that initiation and propagation of lactones by samarium diodide progress in different ways, O-alkyl and O-acyl cleavage, respectively, but details of the oxidation stage of the samarium moiety are not represented.12 Identical reactivity was advanced with SmBr2 and Cp2Sm (Cp = C5H5) in the same paper, but actually, it is not clear whether the initiation of the polymerization proceeds via monoelectronic oxidation or simply by insertion into a Sm(II) X active bond.41 In general also, most authors observed that the dark color of the divalent samarium species instantaneously disappeared at the addition of the monomer to a divalent (Sm or Yb) initiator, suggesting that oxidation was taking place. However, it is noteworthy that SmI2/Sm, SmBr2, and Cp2Sm (already reported by Evans for isoprene polymerization with SmI234) behaved differently: polymerization experiments conducted with these complexes led to a blue color, typical of Sm(II), which was maintained all along the process. With Sm(BH4)2(THF)2 as initiator, we also noticed a change of coloration, but after an induction period.23 Very recently, Delbridge et al. presumed that the instant color change they observed using ytterbium and samarium(II) bisphenolate initiators was indicative of an initial lanthanide oxidation (and concomitant monomer reduction) preceding the ringopening polymerization of ε-caprolactone.14,21,22 According to them, only trivalent species would be the active ones, and the divalent compounds would necessarily be oxidized to subsequently propagate the polymerization.35 Thus, in order to consider whether an initiation process that would involve Sm(II) f Sm(III) oxidation would be in our case a reasonable alternative to the concerted mechanism, we computed oxidation
pathways (Figure 1, in red). Both pathways have a similar intermediate where the oxidation of the Sm center has been achieved. Since in 2, the oxidation of Sm was not achieved, the search for the oxidized state of Sm led to us to find complex 7 (Scheme 1). In 7, the ring is already open with the former endocyclic oxygen ensuring the coordination to the metal center (Figure 1). The occurrence of formation of a [Sm(III)] ketyl complex36 without ring opening of the monomer was discarded, considering the lack of a cyclic ether end group signal by NMR (Figures 2 and S2). A radical character is developed on the carbonyl complex in 7, making this complex reactive. The formation of this complex is predicted to be endergonic by 35.4 kcal mol 1, in line with the kinetic cost of the ring opening (rate-determining step) already discussed for the concerted pathway. Thus, complex 7 is a transient radical complex that may either dimerize (pathway 2a, Scheme 1), as proposed by Jaroschik et al.,23 or react with another Sm(BH4)2 (pathway 2b, Scheme 1) to form a dinuclear complex, as proposed for the reaction of SmI2 with acid dihalides leading to R-diketones.37 The coupling of two radicals at the initiation stage was also proposed in a publication devoted to the polymerization of vinyl butyrolactones, this hypothesis being supported by kinetic studies.38 Also, one must keep in mind that such possible pathways had also to be compatible with the CH2 OH end groups observed by NMR. To further test these possibilities, calculations on both the dimer (complex 8) and the dinuclear complex (complex 9) were carried out (see the Supporting Information for a full representation of the oxidative pathway). First of all, and as expected, the formation of the C C bond from the two radicals (leading to complex 8) is predicted to be highly exergonic, leading to an overall oxidation mechanism which is exergonic by 24.2 kcal mol 1. Thus, this oxidation route appears to be competitive with the concerted one, in excellent agreement with the experimental observation of color change of the reaction. Complex 8 is a bis-alkoxide system that will easily insert ε-caprolactone, leading also, after hydrolysis, to R,ω-dihydroxy telechelic PCL with a diketone bridge. However, the formation of such a polymer was ruled out experimentally, since we did not observe the presence of R-diketone moieties in the NMR (1H, δ(C(O)CH2) ca. 2.8 ppm; 13C, δ(C(O)) ca. 200 ppm) spectra of low-Mn polymers. The reaction between complex 7 and another molecule of Sm(BH4)2 (path 2b) was computed to be highly favorable, with the whole oxidation process being exergonic by 9.1 kcal mol 1. The geometry of complex 9 is interesting, since the ketone is bonded to the Sm center in an η2 fashion, indicating possible subsequent reduction of the ketone. Complex 9 can thus isomerize to the “π-enolate”-type complex 10 (Scheme S1, Supporting Information), as already proposed.39 This isomerization is computed to be highly favorable (reaction 4484
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Organometallics exergonic by 19.8 kcal mol 1), making 10 the most stable complex of all pathways. The PCL issued from 10 after propagation and hydrolysis would bear one secondary CH2 CHOH CH3 alcohol end group coming from keto enolic rearrangement, followed by reduction with borane, similar to the process proposed at the extremity of the PCL chain initiated by Ln(BH4)3 complexes.24 However, since no trace of any chain end of that sort was observed experimentally, pathway 2b must be discarded. Finally, one could propose that CH2 OH end- groups originate from the borane reduction—similarly as in the concerted pathway—of a terminal aldehyde 14 simply issued from H abstraction of the solvent by complex 7.35 This aldehyde formation was computed to be favorable by 24.3 kcal mol 1 with respect to the reactants if one involves the coupling reaction of two THF radicals. The occurrence of simultaneous concerted and oxidative processes can be thus advanced to match with the observed Mn values and with the slight broadening of the molecular weight distribution in the presence of high loadings of monomer (see the Supporting Information), but the relative contributions of the two types of mechanisms are still under investigation and will be the subject of a forthcoming paper. In this contribution, concerted and oxidative mechanisms for the initiation of the ROP of ε-caprolactone catalyzed by Sm(BH4)2(THF)2 have been computed at the DFT level. The interplay between experiment and theory prompts us to report that, although it is computed to be favorable, the oxidative route does not occur experimentally unless one considers transfer reactions with the solvent. To the best of our knowledge, this study is the only one reporting any DFT investigation of the reactivity of Sm(II) complexes that takes into account both concerted and oxidative pathways. This study encourages us to investigate in more detail the reactivity of Ln(II) complexes by theoretical methods and to continuously examine the redox properties of divalent borohydrido lanthanide complexes.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text, tables, and figures giving experimental polymerization procedures, polymer characterization data, polymerization experiments, and Cartesian coordinates and energies of all the stationary points optimized in this study. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (M.V.); laurent.maron@ irsamc.ups-tlse.fr (L.M.).
’ ACKNOWLEDGMENT We thank the Institut Universitaire de France and CALMIP, CINES, for grants of computing time and Prof. P. L. Arnold for fruitful discussions. We also thank Dr. Franc-ois Stoffelbach for helpful discussions and Aurelie Malfait for GPC analysis. ’ REFERENCES (1) Woodruff, M. A.; Hutmacher, D. W. Prog. Polym. Sci. 2010, 35, 1217–1256. (2) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147–6176. (3) Arbaoui, A.; Redshaw, C. Polym. Chem. 2010, 1, 801–826. (4) Ajellal, N.; Carpentier, J. F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363–8376. (5) Visseaux, M.; Bonnet, F. Coord. Chem. Rev. 2011, 255, 374–420.
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(6) Oshimura, M.; Takasu, A. Macromolecules 2010, 43, 2283–2290. (7) Dyer, H. E.; Huijser, S.; Susperregui, N.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford, P. Organometallics 2010, 29, 3602–3621. (8) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. J. Am. Chem. Soc. 2009, 131, 16042–16044. (9) Xue, M.; Jiao, R.; Zhang, Y.; Yao, Y.; Shen, Q. Eur. J. Inorg. Chem. 2009, 4110–4118. (10) For a review of SmI2 for polymerization: Nomura, R.; Endo, T. Chem. Eur. J. 1998, 4, 1605–1610. (11) Evans, W. J.; Katsumata, H. Macromolecules 1994, 27, 2330–2332. (12) Agarwal, S.; Brandukova-Szmikowski, N. E.; Greiner, A. Macromol. Rapid Commun. 1999, 20, 274–278. (13) Binda, P. I.; Delbridge, E. E.; Abrahamson, H. B.; Skelton, B. W. Dalton Trans. 2009, 2777–2787. (14) Jiang, L.; Lou, L.; Sun, W.; Xu, L.; Shen, Z. J. Appl. Polym. Sci. 2005, 98, 1558–1564. (15) Visseaux, M.; Barbier-Baudry, D.; Blacque, O.; Hafid, A.; Richard, P.; Weber, F. New J. Chem. 2000, 24, 939–942. (16) Wei, Y.; Yu, Z.; Wang, S.; Zhou, S.; Yang, G.; Zhang, L.; Chen, G.; Qian, H.; Fan, J. J. Organomet. Chem. 2008, 693, 2263–2270. (17) Zhou, S.; Wang, S.; Sheng, E.; Zhang, L.; Yu, Z.; Xi, X.; Chen, G.; Luo, W.; Li, Y. Eur. J. Inorg. Chem. 2007, 1519–1528. (18) Wang, S.; Wang, S.; Zhou, S.; Yang, G.; Luo, W.; Hu, N.; Zhou, Z.; Song, H. B. J. Organomet. Chem. 2007, 692, 2099–2106. (19) Wu, Y.; Wang, S.; Qian, C.; Sheng, E.; Xie, M.; Yang, G.; Feng, Q.; Zhang, L.; Tang, X. J. Organomet. Chem. 2005, 690, 4139–4149. (20) Nishiura, M.; Hou, Z.; Koizumi, T.; Imamoto, T.; Wakatsuki, Y. Macromolecules 1999, 32, 8245–8251. (21) Delbridge, E. E.; Dugah, D. T.; Nelson, C. R.; Skelton, B. W.; White, A. H. Dalton Trans. 2007, 1, 143–153. (22) Dugah, D. T.; Skelton, B. W.; Delbridge, E. E. Dalton Trans. 2009, 8, 1436–1445. (23) Jaroschik, F.; Bonnet, F.; Le Goff, X. F.; Ricard, L.; Nief, F.; Visseaux, M. Dalton Trans. 2010, 39, 6761–6766. (24) Guillaume, S. M.; Schappacher, M.; Soum, A. Macromolecules 2003, 36, 54–60. (25) Barros, N.; Mountford, P.; Guillaume, S. M.; Maron, L. Chem. Eur. J. 2008, 14, 5507–5518. (26) Maron, L.; Eisenstein, O. J. Am. Chem. Soc. 2001, 123, 1036–1039. (27) Perrin, L.; Maron, L.; Eisenstein, O. Inorg. Chem. 2002, 41, 4355–4362. (28) Eisenstein, O.; Maron, L. J. Organomet. Chem. 2002, 647, 190–197. (29) Schinzel, S.; Bindl, M.; Visseaux, M.; Chermette, H. J. Phys. Chem. A 2006, 110, 11324–11331. (30) Maron, L.; Eisenstein, O. J. Phys. Chem. A 2000, 104, 7140–7143. (31) Labouille, S.; Nief, F.; Maron, L. J. Phys Chem. A 2011, 115, 8295–8301. (32) Castro, L.; Bart, S. C.; Lam, O. P.; Mayer, K.; Maron, L. Organometallics 2010, 29, 5504–5510. (33) Jenter, J.; Roesky, P. W.; Ajellal, N.; Guillaume, S. M.; Susperregui, N.; Maron, L. Chem. Eur. J. 2010, 16, 4629–4638. (34) Evans, W. J.; Giarikos, D. G.; Allen, N. T. Macromolecules 2003, 36, 4256–4257. (35) Some divalent samarocenes, unlikely to polymerize ε-caprolactone other than by an oxidative mechanism, have been described.11,12,15 (36) Hou, Z.; Fujita, A.; Zhang, Y.; Miyano, T.; Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 1998, 120, 754–766. (37) Agarwal, S.; Greiner, A J. Chem. Soc. Perkin Trans. 1 2002, 2033–2042 and references therein. (38) Miyake, G. M.; Newton, S. E.; Mariott, W. R.; Chen, E. Y. X. Dalton Trans. 2010, 39, 6710–6718. (39) Marks, T. J. Science 1982, 217, 989–997. (40) CPCM calculations were carried out on the gas-phase geometry, leading to a decrease of the barrier. (41) We carried out calculations on the O-alkyl cleavage; the results indicate that this process is much higher in energy than the O-acyl process; therefore, it has not been reported herein.
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