Organometallics 2010, 29, 6569–6577 DOI: 10.1021/om100908q
6569
Bis(dimethylsilyl)amide Complexes of the Alkaline-Earth Metals Stabilized by β-Si-H Agostic Interactions: Synthesis, Characterization, and Catalytic Activity Yann Sarazin,*,† Dragos- Ros-ca,‡ Valentin Poirier,† Thierry Roisnel,† Anca Silvestru,‡ Laurent Maron,§ and Jean-Franc-ois Carpentier*,† †
Catalysis and Organometallics, UMR CNRS 6226-Universit e de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France, ‡Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, RO-400028, Cluj-Napoca, Romania, and §LPCNO, UMR 5215, Universit e de Toulouse-CNRS, UPS135 Avenue de Rangueil, 31077 Toulouse, France Received September 21, 2010
The syntheses of the homoleptic compounds Ae[N(SiMe2H)2]2(THF)x (Ae=Ca, x=1, 2; Sr, x=2/3, 3; Ba, x=0, 4) are reported. They can be prepared by salt metathesis involving the alkaline-earth metal iodides and KN(SiMe3)2 (1) or by transamination between Ae[N(SiMe3)2]2(THF)2 and HN(SiMe2H)2. These precursors constitute convenient starting materials for the subsequent preparation of {LnO}AeN(SiMe2H)2 heteroleptic complexes of the large alkaline-earth metals, as exemplified by the syntheses of {LO3}AeN(SiMe2H)2 ({LO3}- = 2-{(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl}-4,6-di-tert-butylphenolate; Ae=Ca, 5; Sr, 6; Ba, 7). Both homo- and heteroleptic complexes are stabilized in the solid state by secondary β-Si-H agostic interactions. The structures of the kinetically stable {LO3}BaN(SiMe2H)2 (7) and those of its potassium synthetic precursors 1 and {LO3}K 3 KN(SiMe2H)2 (8) are described, and the catalytic activity of the heteroleptic complexes in the ring-opening polymerization of lactide is presented.
Introduction Mostly used until recently for sol-gel and metal-organic chemical vapor deposition purposes,1 d0 complexes of the
large alkaline-earth metals (Ae) calcium (rionic = 1.14 A˚), strontium (1.32 A˚), and barium (1.49 A˚)2 have now found new applications as potent and versatile catalysts for a variety of transformations involving σ-bond metathesis.3 Well-defined heteroleptic Ae complexes have for instance displayed remarkable activities in the ring-opening polymerization (ROP) of cyclic esters,4,5 the polymerization of styrene and dienes,6 terminal alkyne coupling,7 hydrogenation,8 and especially internal hydroelementation of alkenes,3 and the accurate knowledge of the structure of the catalytic precursors has opened up access to detailed mechanistic studies. However, suitable {LnX}- ancillary ligands able to stabilize {LnX}Ae-Nu (Nu = nucleophilic group) heteroleptic complexes are still very scarce, and the selection is essentially restricted to tris(pyrazolyl)borates,4c,d,9 aminotrop(on)iminates,10 β-diketiminates,11 and bis- or tris(imidazolin-2-ylidene-1-yl)borate.12
*To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Bradley, D. C. Chem. Rev. 1989, 89, 1317–1322. (b) Deacon, G. B.; Junk, P. C.; Moxey, G. J.; Guino-o, M.; Ruhlandt-Senge, K. Dalton Trans. 2009, 4878–4887. (c) Saly, M. J.; Heeg, M. J.; Winter, C. H. Inorg. Chem. 2009, 48, 5303–5312. (2) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767. (3) (a) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Procopiou, P. A. Proc. R. Soc. A 2010, 466, 927–963. (b) Harder, S. Chem. Rev. 2010, 110, 3852–3876. (4) (a) Westerhausen, M.; Schneiderbauer, S.; Kneifel, A. N.; S€ oltl, Y.; Mayer, P.; N€ oth, H.; Zhong, Z.; Dijkstra, P. J.; Feijen, J. Eur. J. Inorg. Chem. 2003, 3432–3439. (b) Hill, M. S.; Hitchcock, P. B. Chem. Commun. 2003, 1758–1759. (c) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Chem. Commun. 2003, 48–49. (d) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg. Chem. 2004, 43, 6717–6725. (e) Darensbourg, D. J.; Choi, W.; Richers, C. P. Macromolecules 2007, 40, 3521–3523. (f) Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh, N. Macromolecules 2008, 41, 3493–3502. (g) Poirier, V.; Roisnel, T.; Carpentier, J.-F.; Sarazin, Y. Dalton Trans. 2009, 9820–9827. (h) Xu, X.; Chen, Y.; Zou, G.; Mac, Z.; Li, G. J. Organomet. Chem. 2010, 695, 1155–1162. (i) Sarazin, Y.; Poirier, V.; Roisnel, T.; Carpentier, J.-F. Eur. J. Inorg. Chem. 2010, 3423– 3428. (5) For examples of molecular Ae initiators for ROP, see for instance: (a) Zhong, Z.; Dijkstra, P. J.; Birg, C.; Westerhausen, M.; Feijen, J. Macromolecules 2001, 34, 3863–3868. (b) Darensbourg, D. J.; Choi, W.; Ganguly, P.; Richers, C. P. Macromolecules 2006, 39, 4374–4379. (c) Sarazin, Y.; Howard, R. H.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Dalton Trans. 2006, 340–350. (d) Davidson, M. G.; O0 Hara, C. T.; Jones, M. D.; Keir, C. G.; Mahon, M. F.; Kociok-K€ohn, G. Inorg. Chem. 2007, 46, 7686–7688.
(6) (a) Harder, S.; Feil, F.; Knoll, K. Angew. Chem., Int. Ed. 2001, 40, 4261–4264. (b) Harder, S.; Feil, F. Organometallics 2002, 21, 2268–2274. (c) Jochmann, P.; Dols, T. S.; Spaniol, T. P.; Perrin, L.; Maron, L.; Okuda, J. Angew. Chem., Int. Ed. 2009, 48, 5715–5719. (7) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Lomas, S. L.; Procopiou, P. A.; Suntharalingam, K. Chem. Commun. 2009, 2299–2301. (8) Spielmann, J.; Buch, F.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 9434–9438. (9) (a) Chisholm, M. H.; Gallucci, J. C.; Yaman, G.; Young, T. Chem. Commun. 2009, 1828–1830. (b) Chisholm, M. H.; Gallucci, J. C.; Yaman, G. Dalton Trans. 2009, 368–374. (10) (a) Datta, S.; Roesky, P. W.; Blechert, S. Organometallics 2007, 26, 4392–4394. (b) Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2008, 27, 1207–1213.
r 2010 American Chemical Society
Published on Web 11/09/2010
pubs.acs.org/Organometallics
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Organometallics, Vol. 29, No. 23, 2010
Sarazin et al.
Scheme 1. Schlenk-Type Equilibrium with Heteroleptic Alkaline-Earth Complexes
Scheme 2. Synthesis of Homoleptic Alkaline-Earth Precursors 2-4
Our interest in this field stems from the desire to take advantage of the high reactivity of the large, oxophilic Ae metals (Ae=Ca, Sr, Ba) in order to develop new families of catalysts for the ROP of cyclic esters.4g,i Recently, our group has shown that well-defined {LnO}M-N(SiMe3)2 (where M=Zn, Mg, Ca) precatalysts stabilized by bulky, chelating {LnO}- phenolate ligands could be successfully isolated.4g However, we, like others, have found that the preparation of these heteroleptic amido complexes is often impeded by the presence of detrimental Schlenk-type equilibria (a propensity that increases on descending from Mg to Ba with the size of the metal and its electropositive nature), which lead in solution to the formation of a mixture of two homoleptic species (Scheme 1). The inability of the easily accessed and routinely employed Ae[N(SiMe3)2]2(THF)2 precursors13 to guarantee the synthesis of kinetically stable heteroleptic complexes prompted us to consider the use of different amido precursors. A variety of sterically encumbered Ae[N(SiMexR3-x)(SiMexR0 3-x)]2(L)2 (R, R0 = alkyl, aryl; L = donor)14 have been disclosed,15 but they require considerable synthetic efforts, and owing to its bulkiness, the nucleophilicity of the amido group (an essential feature for ROP purposes) is comparatively poor. On the other hand, it occurred to us that, unlike in the related chemistry of the rare-earth metals, the simpler amide N(SiMe2H)2- had to date never been used for the preparation of complexes of the large Ae metals.16
Thanks to the pioneering work of Anwander, it has been known for over a decade that the N(SiMe2H)2- substituent allows the synthesis of lanthanide complexes with unusual structure and reactivity.17-19 Thorough experimental and theoretical studies have been conducted on (L)xM[N(SiMe2H)2]3 (M=Sc, Y, lanthanide; L=donor) and {LnX}xM[N(SiMe2H)2]3-x (x=0-3; LnX=indenyl, (pentamethyl)cyclopentadienyl) species where the metal center is stabilized by internal β-Si-H agostic interactions. Besides the stabilizing effect imparted by the N(SiMe2H)2- group with respect to the standard N(SiMe3)2-, complexes incorporating the former type of amide often exhibit significantly improved selectivity in the ROP of cyclic esters, most likely as a result of their relatively higher nucleophilicity.20 In this contribution, we describe the preparation of the first Ae[N(SiMe2H)2]2(THF)x precursors (Ae = Ca, Sr, Ba) and their valuable use for the synthesis of stable {LnO}AeN(SiMe2H)2 heteroleptic complexes. The presence of stabilizing β-Si-H agostic interactions has been identified by spectroscopic methods and X-ray diffraction crystallography. Finally, their catalytic activity in the immortal ROP (iROP)21 of lactide is briefly presented.
(11) (a) Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc. 2005, 127, 2042–2043. (b) Avent, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B. Dalton Trans. 2005, 278–284. (c) Harder, S.; Brettar, J. Angew. Chem., Int. Ed. 2006, 45, 3474–3478. (d) Ruspic, C.; Nembenna, S.; Hofmeister, A.; Magull, J.; Harder, S.; Roesky, H. W. J. Am. Chem. Soc. 2006, 128, 15000–15004. (e) Pillai Sarish, S.; Roesky, H. W.; John, M.; Ringe, A.; Magull, J. Chem. Commun. 2009, 2390–2392. (12) (a) Arrowsmith, M.; Hill, M. S.; Kociok-K€ ohn, G. Organometallics 2009, 28, 1730–1738. (b) Arrowsmith, M.; Heath, A.; Hill, M. S.; Hitchcock, P. B.; Kociok-K€ohn, G. Organometallics 2009, 28, 4550– 4559. (13) (a) Westerhausen, M. Inorg. Chem. 1991, 30, 96–101. (b) Westerhausen, M. Coord. Chem. Rev. 1998, 176, 157–210. (c) Buchanan, W. D.; Allis, D. G.; Ruhlandt-Senge, K. Chem. Commun. 2010, 46, 4449–4465. (14) (a) Vargas, W.; Englich, U.; Ruhlandt-Senge, K. Inorg. Chem. 2002, 41, 5602–5608. (b) Gillett-Kunnath, M.; Teng, W.; Vargas, W.; Ruhlandt-Senge, K. Inorg. Chem. 2005, 44, 4862–4870. (c) Torvisco, A.; Decker, K.; Uhlig, F.; Ruhlandt-Senge, K. Inorg. Chem. 2009, 48, 11459– 11465. (15) For nonsilylated Ae-amide precursors with reduced basicity, see: (a) G€ artner, M.; G€ orls, H.; Westerhausen, M. Dalton Trans. 2008, 1574– 1582. (b) Glock, C.; G€orls, H.; Westerhausen, M. Inorg. Chem. 2009, 48, 394–399. (16) The use of the -N(SiMe2H)2- amido group with magnesium (a small metal with a reactivity very different from that of the much larger Ca, Sr, and Ba) has already been described; see for instance: (a) Zapilko, C.; Liang, Y.; Anwander, R. Chem. Mater. 2007, 19, 3171– 3176. (b) Schofield, A. D.; Barros, M. L.; Cushion, M. G.; Schwarz, A. D.; Mountford, P. Dalton Trans. 2009, 85–96. (c) Cushion, M. G.; Meyer, J.; Heath, A.; Schwarz, A. D.; Fernandez, I.; Breher, F.; Mountford, P. Organometallics 2010, 29, 1174–1190. (17) (a) Herrmann, W. A.; Eppinger, J.; Spiegler, M.; Runte, O.; Anwander, R. Organometallics 1997, 16, 1813–1815. (b) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847–858. (c) Hieringer, W.; Eppinger, J.; Anwander, R.; Herrmann, W. A. J. Am. Chem. Soc. 2000, 122, 11983– 11994. (d) Meermann, C.; Gerstberger, G.; Spiegler, M.; T€ornroos, K. W.; Anwander, R. Eur. J. Inorg. Chem. 2008, 2014–2023. (e) Yuen, H. J.; Marks, T. J. Organometallics 2008, 27, 155–158.
Results and Discussion The reaction of KH and HN(SiMe2H)2 in toluene gave KN(SiMe2H)2 (1) in quantitative yields.18b Compound 1 could be recrystallized from a concentrated toluene solution stored overnight at -30 °C, and X-ray diffraction studies indicated that it consisted of one-dimensional coordination polymers (vide infra). The analogous Ae-homoleptic precursors Ca[N(SiMe2H)2]2(THF) (2), Sr[N(SiMe2H)2]2(THF)2/3 (3), and Ba[N(SiMe2H)2]2 (4) were obtained upon treatment of the corresponding metal iodide with a stoichiometric amount of 1 in THF (Scheme 2, route A). Although the most direct, this route typically ensured moderate yields in the range 60-75%, and we found that the resulting materials were somewhat waxy, a phenomenon tentatively attributed to the presence of various amounts of KAe[N(SiMe2H)2]3 ate complexes in the final products, in a similar way to that observed recently for the synthesis of Ca[N(SiMe3)2]2.22 By contrast, compounds 2-4 could be quantitatively obtained (18) For β-Si-H agostic interactions with other metals, see: (a) Procopio, L. J.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 1994, 116, 177–185. (b) Eppinger, J.; Herdtweck, E.; Anwander, R. Polyhedron 1998, 17, 1195– 1201. (c) Schneider, J.; Popowski, E.; Reinke, H. Z. Anorg. Allg. Chem. 2003, 629, 55–64. (19) For β-Si-H agostic interactions with divalent samarium, see: Nagl, I.; Scherer, W.; Tafipolsky, M.; Anwander, R. Eur. J. Inorg. Chem. 1999, 1405–1407. (20) For examples in ROP of lactide, see: (a) Cai, C.-X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J.-F. Chem. Commun. 2004, 330–331. (b) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Chem.;Eur. J. 2006, 12, 169–179. (21) (a) Asano, S.; Aida, T.; Inoue, S. J. Chem. Soc., Chem. Commun. 1985, 1148–1149. (b) Inoue, S. J. Polym. Sci. A: Polym. Chem. 2000, 38, 2861–2871. (c) Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363–8376.
Article
Organometallics, Vol. 29, No. 23, 2010 Table 1. Spectroscopic Data for Complexes 1-8 a
compound
IR νs(Si-H)
HN(SiMe2H)2 1 8 2 3 4 5 6 7
2122 1972 1993, 1977 (sh) 2028, 1959 (sh) 1959, 1925 (sh) 1985, 1935 (sh) 1989, 1941 (sh) 2005, 1918 1977, 1905
1
b
H NMR δ(SiH) 4.70 4.98 5.31 4.92 4.84 4.65d 5.20 5.27 5.27
29
6571
Scheme 3. Synthesis of Heteroleptic Complexes 5-8
b
Si NMR δ(SiH) -11.5 -30.7 -31.2 -20.5 -20.7 -30.6d -25.9 -27.5 -30.5
1
J(Si-H)c 194 162 165 154 148 163d 165 161 160
a Recorded at room temperature as Nujol mulls in KBr plates (cm-1). Recorded in C6D6 unless stated otherwise (ppm). c Frequencies in Hz. d Recorded in THF-d8 (ppm). b
free of impurity by transamination (pKa(THF): HN(SiMe3)2, 25.8; HN(SiMe2H), 22.6)18b between HN(SiMe2H)2 and the corresponding Ae[N(SiMe3)2]2(THF)2 reagent (Scheme 2, route B). Considering that Sr2þ and Sm2þ have nearly identical ionic radii (respectively 1.35 and 1.36 A˚ for CN =7),2 the THF/Sr stoichiometry of 2:3 observed in 3 was not unexpected: indeed, it matches that already reported in the solid state for Sm[N(SiMe2H)2]2(THF)x 2σ(I)] data/restraints/params goodness-of-fit on F2 R1 [I > 2σ(I)] (all data) wR2 [I > 2σ (I)] (all data) largest diff peak and hole, e A-3
1 C4H14K1N1Si2 171.44 orthorhombic Pnam 5.8129(9) 11.2427(19) 14.979(2) 90 90 90 978.9(3) 4 1.163 0.713 368 0.51 0.28 0.2 3.62 to 27.48 -7 e h e 4 -14 e k e 10 -19 e l e 12 0.04 3944 1157 1157/0/35 1.071 0.0327 (0.0394) 0.0791 (0.0822) 0.453 and -0.319
ppm; 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): δ 166.4 (i-C), 136.4 (o-C), 131.4 (p-C), 126.7 (m-C), 124.0 (m-C), 123.4 (o-C) 69.2, 68.9, 68.1, 67.4 (all O-CH2), 64.1 (Ar-CH2-N), 54.3 (NCH2-CH2), 35.9 (o-C(CH3)3), 34.2 (p-C(CH3)3), 32.7 (p-C(CH3)3), 30.7 (o-C(CH3)3), 5.2 (Si-CH3) ppm. 29Si{1H} NMR (C6D6, 298 K, 79.49 MHz): δ -30.5 ppm. IR (Nujol in KBr plates): νh 1977 (s), 1905 (s), 1602 (w), 1461 (s), 1418 (m), 1377 (s), 1356 (m), 1343 (m), 1327 (s), 1301 (m), 1262 (m), 1252 (m), 1233 (s), 1097 (s), 1077 (s), 1052 (m), 1037 (m), 987 (m), 964 (m), 938 (m), 925 (m), 900 (s), 881 (s), 853 (m), 828 (m), 795 (m), 765 (m), 751 (m), 739 (m), 722 (m), 703(w), 668 (w), 637 (w), 614 (w), 584 (w), 516 (w) cm-1. Anal. Calcd for C29H56BaN2O5Si2 (706.29 g mol-1): C 49.32, H 7.99, N 3.97. Found: C 49.2, H 7.9, N 3.8. {LO3}K 3 KN(SiMe2H)2 (8). The salt 1 (3.37 g, 19.6 mmol) was added at room temperature in portions to a solution of {LO3}H (4.30 g, 9.8 mmol) in Et2O (150 mL). The solution turned pale orange, and a white precipitate formed rapidly. Stirring was maintained overnight, after which the solution was concentrated to ca. 50 mL. Complete precipitation was ensured by addition of pentane (120 mL), and the precipitate was isolated by filtration. Washing with pentane (2 40 mL) and drying in vacuo afforded 8 as a fine white powder. Yield: 5.30 g (84%). X-ray quality crystals were grown by recrystallization from C6D6 at room temperature. 1H NMR (C6D6, 298 K, 500.13 MHz): δ 7.52 (d, 4JHH=3.0 Hz, 1H, m-H), 7.12 (d, 4JHH=3.0 Hz, 1H, mH), 5.31 (m, 1JSiH=165 Hz, 2H, Si-H), 3.67 (br, 2H, Ar-CH2-N), 3.25-2.92 (br m, 8H, O-CH2), 2.90-2.60 (m, 10H, O-CH2 and N-C(H)H-CH2), 2.26-2.20 (dd, 2JHH =13.5 Hz, 3JHH =5.5 Hz, 2H, N-CH(H)-CH2), 1.80 (s, 9H, o-C(CH3)), 1.51 (s, 9H, pC(CH3)), 0.46 (d, 3JHH = 3.0 Hz, 12H, Si-CH3) ppm. 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): δ 168.1 (i-C), 136.1 (o-C), 128.8 (p-C), 127.5 (m-C), 125.1 (o-C), 123.2 (m-C), 69.6, 69.2, 68.8, 67.5 (all O-CH2), 63.7 (Ar-CH2-N), 55.7 (N-CH2-CH2), 35.7 (o-C(CH3)3), 34.0 (p-C(CH3)3), 32.7 (p-C(CH3)3), 30.8 (oC(CH3)3), 5.8 (Si-CH3) ppm. 29Si{1H} NMR (C6D6, 298 K, 79.49 MHz): δ -31.2 ppm. IR (Nujol in KBr plates): νh 1993 (m), 1977 (sh), 1598 (w), 1463 (s), 1377 (s), 1360 (s), 1331(s), 1291 (m), 1253 (m), 1233 (m), 1134 (s), 1113 (s), 1085 (sh), 1042 (m), 992 (w), 936 (m), 880 (s), 810 (m), 756 (m), 735 (m), 658 (w), 613 (w) cm-1. Anal. Calcd for C29H56K2N2O5Si2 (647.14 g mol-1): C 53.82, H 8.72, N 4.33. Found: C 53.7, H 8.6, N 4.1.
7 C27H56Ba1N2O5Si2 706.28 monoclinic P21/c 13.1638(4) 15.5426(4) 17.7016(5) 90 97.1090(10) 90 3593.90(18) 4 1.305 1.207 1472 0.4 0.1 0.08 1.56 to 27.48 -15 e h e 17 -20 e k e 14 -21 e l e 22 0.0269 29 052 8065 8065/0/362 1.137 0.0241 (0.0311) 0.0606 (0.072) 0.602 and -0.462
8 C58H112K4N4O10Si4, C6H6 1372.38 triclinic P1 9.4506(3) 14.7067(4) 16.7604(6) 107.9280(10) 102.538(2) 107.5100(10) 1986.74(11) 1 1.147 0.335 742 0.37 0.17 0.13 2.95 to 27.43 -12 e h e 12 -19 e k e 19 -21 e l e 21 0.0318 33 195 8928 8928/0/416 1.034 0.0419 (0.0486) 0.116 (0.1221) 0.88 and -0.739
Synthesis of {LO3}2Ca. Ca[N(SiMe3)2]2(THF)2 (0.25 g, 0.49 mmol) was added in portions to a solution of {LO3}H (0.43 g, 0.98 mmol) in Et2O (30 mL). A white precipitate formed instantaneously. The solution was stirred overnight, and the white precipitate was isolated by filtration. It was washed with pentane (220 mL) and dried under vacuum to give analytically pure {LO3}2Ca as a colorless solid. Yield: 0.30 g (67%). 1 H NMR (C6D6, 298 K, 400.13 MHz): δ 7.57 (d, 4JHH=2.4 Hz, 2H, m-H), 7.31 (br s, 2H, m-H), 3.92 (br s, 4H, Ar-CH2-N), 3.54-3.25 (m, 32H, all O-CH2), 2.87, (br s, 8H, N-CH2-CH2), 1.80 (s, 18H, o-C(CH3)), 1.57 (s, 18H, p-C(CH3)) ppm. 13C{1H} NMR (C6D6, 298 K, 100.03 MHz): δ 165.9 (i-C), 135.3 (o-C), 134.7 (p-C), 125.3 (m-C), 122.8 (m-C), 70.0 (br), 69.6, 69.2 (all O-CH2), 61.8 (Ar-CH2-N), 54.7 (N-CH2-CH2), 35.9 (o-C(CH3)3, 34.4 (p-C(CH3)3, 32.9 (p-C(CH3)3, 31.1 (o-C(CH3)3 ppm. Anal. Calcd for C50H84CaN2O10 (913.31 g mol-1): C 65.76, H 9.27, N 3.07. Found: C 65.7, H 9.2, N 2.9. Typical Procedure for the Polymerization of Lactide. In the glovebox, the metallic initiator was placed in a Schlenk tube, while the monomer was loaded in a bent glass finger. The Schlenk tube and bent finger were sealed and removed from the glovebox. All subsequent operations were carried out on a vacuum line using standard Schlenk techniques. The required amount of solvent was added with a syringe to the Schlenk tube containing the initiator. The co-initiator (iPrOH) was then added, and the resulting mixture was stirred at the desired temperature until complete dissolution of the solids was ensured. The monomer was added with the bent finger, and the polymerization time was measured from this point. The reaction was terminated by addition of acidified MeOH (HCl, 10 wt %), and the polymer was precipitated in methanol and washed thoroughly. The polymer was then dried to constant weight by heating at 60 °C under dynamic vacuum (
