Chiral Rare Earth Borohydride Complexes Supported by Amidinate

M.K. thanks the Karlsruhe House of Young Scientists (KHYS). ...... Sami Fadlallah , Jashvini Jothieswaran , Frédéric Capet , Fanny Bonnet , Marc Vis...
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Chiral Rare Earth Borohydride Complexes Supported by Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of rac-Lactide Jochen Kratsch,† Magdalena Kuzdrowska,† Matthias Schmid,† Neda Kazeminejad,† Christoph Kaub,† Pascual Oña-Burgos,† Sophie M. Guillaume,*,‡ and Peter W. Roesky*,† †

Institut für Anorganische Chemie, Karlsruher Institut für Technologie (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany Organometallics: Materials and Catalysis, Institut des Sciences Chimiques de Rennes, CNRS−Université de Rennes 1 (UMR 6226), Campus de Beaulieu, 35042 Rennes Cedex, France

‡

S Supporting Information *

ABSTRACT: The monoamidinato bisborohydride rare earth complexes [Ln{(S)-PEBA}(BH4)2(THF)2] (Ln = Sc (1), La (2), Nd (3), Sm (4), Yb (5), Lu (6)) were isolated as crystalline materials upon treatment of potassium N,N′-bis((S)-1-phenylethyl)benzamidinate ((S)-KPEBA) with the homoleptic trisborohydrides [Sc(BH4)3(THF)2] and [Ln(BH4)3(THF)3] (Ln = La, Nd, Sm, Yb, Lu), respectively. Compounds 1−6 are unique examples of enantiopure borohydride complexes of the rare earth metals. Different ionic radii of the metal centers were selected to cover the whole range of these elements with respect to the extent of the coordination sphere. All new complexes were thoroughly characterized by 1H, 13C{1H}, 11B, and 15N NMR and IR spectroscopies, also including single-crystal X-ray diffraction structure determination of each compound. The scandium, lanthanum, samarium, and lutetium complexes 1, 2, 4, and 6 were found active in the ring-opening polymerization of rac-lactide under mild operating conditions, providing atactic α,ω-dihydroxytelechelic poly(lactic acid) (PLA; Mn,SEC up to 18 800 g·mol−1). Most of the polymerizations proceed with a certain degree of control that is directed by molar mass values and relatively narrow dispersities (1.10 < ĐM < 1.34), within a moderate monomer-to-initiator ratio.

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INTRODUCTION The trisborohydrides of the rare earth elements [Ln(BH4)3] were first claimed in the 1960s by Egon Zange. Back then, these complexes were prepared by the reaction of rare earth metal alkoxides [Ln(OCH3)3] with B2H6.1 Later on, Mirsaidov et al. reported in several publications a more convenient approach to the solvates [Ln(BH4)3(THF)3] starting from LnCl3 and NaBH4.2−8 In contrast to the other rare earth compounds, the (smaller) scandium borohydride [Sc(BH4)3(THF)2]9,10 features only two equivalents of THF coordinated to the metal center in the solid state. After that, Guillaume et al. optimized the operating conditions of their synthesis, in particular using a lower amount of NaBH4.11 Noteworthy, a convenient access to the divalent lanthanide borohydrides [Ln(BH4)2(THF)2] (Ln = Eu, Sm, Yb)12,13 and [Tm(BH4)2(DME)2]14 has been reported recently from the corresponding trisborohydride and Ln metal or from salt metathesis reactions of [EuI2(THF)2] or [TmI2(THF)3] with Na/KBH4. © XXXX American Chemical Society

Subsequently, [Ln(BH4)3(THF)n] have been used as starting materials to prepare a large number of lanthanide borohydride derivatives.15 Although some exceptions are known,16,17 the BH4− group usually reacts like a pseudohalide.11,18−32 Thus, derivatives of [Ln(BH4)3(THF)n] can generally be obtained from a salt metathesis reaction with alkali metal reagents of the corresponding ligand. In these reactions, the BH4− groups act as leaving moieties, forming MBH4 (M = Li, Na, K) byproducts.12,33 Although BH4− is isosteric with Cl−, it is much more electron-donating,34 and compared with halide analogues, the formation of ate complexes or bridged derivatives is rarely observed. Using this synthetic approach, cyclopentadienyl Special Issue: Recent Advances in Organo-f-Element Chemistry Received: October 26, 2012

A

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derivatives such as monocyclopentadienyl complexes11,18−22,31,32 and metallocenes17,18,31 could be prepared.23,24 Also postmetallocene borohydride complexes with a huge variety of ancillaries mostly based on O- and N-donor ligands have been synthesized over the recent years.14,18,25−29,35−38 However, to the best of our knowledge, rare earth metal borohydride complexes with chiral ligands have never been reported.15 Poly(lactic acid) (PLA) is attracting extensive interest from both industrial and academic research groups. As a renewable, biocompatible and biodegradable polymer, it provides a viable “green” alternative to petroleum-based materials, and it is also commonly investigated toward biomedical and microelectronics applications.39−44 The stereoselective ring-opening polymerization (ROP) of racemic-(D,L)-lactide (rac-LA) toward the production of isotactic PLA is the current challenge, and several metal-based catalysts have been sought to control the stereoregularity.45−48 Regarding borohydride complexes, they exhibit some hydridic character, through one, two, or three Ln(μ-H)B linkages.24,33,49,50 This property makes them valuable initiators for the polymerization of polar monomers, providing a direct access to the highly desirable α,ω-dihydroxytelechelic polymers.15,23,33,35−37,51−57 Such rare earth complexes have been reported as efficient catalysts in the ROP of cyclic esters, among which are lactides.15,18,26,27,37,38,51,54 Whereas the synthesis of isotactic PLA has been achieved from the ROP of L-lactide,18,51 either heterotactically enriched (Pr up to 87%) or atactic PLA is more commonly reported from the ROP of rac-LA.15,26,27,37,38,54 The stereoselective ROP of lactides has been achieved from other metal-based catalysts through judicious choice of metal−ligand systems. The nature of the metal, the ligand itself, and its more or less bulky substituents as well as chirality are parameters that impart, to some extent, stereocontrol.45,46,58−61 Recently, we reported the first chiral amidinate examples of the lanthanides.62,63 The chiral bisamidinato chloro [Ln{(S)PEBA}(Cl)2(THF)n] and amido complexes [Ln{(S)-PEBA}2{N(SiMe3)2}] ((S)-PEBA = N,N′-bis((S)-1-phenylethyl)benzamidinate) were isolated and structurally characterized. By using [Ln{(S)-PEBA}2{N(SiMe3)2}] as precatalysts, good catalytic activities and high enantioselectivities in the hydroamination/ cyclization reactions were reached.62,63 Herein, we report on the synthesis and structural features of the related chiral amidinato borohydride complexes of the rare earth elements64 and on their catalytic activity in the potentially stereocontrolled ROP of rac-LA.

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Int.PLA/[Int.PLA + Int.LA], using the methine hydrogen (OCHCH3C(O), δLA 5.05 ppm, δPLA 5.18 ppm). Number average molar mass (Mn,SEC) and dispersity (ĐM = Mw/Mn) values were determined by size exclusion chromatography (SEC) in THF at 30 °C (flow rate = 1.0 mL·min−1) on a Polymer Laboratories PL50 apparatus equipped with a refractive index detector and a set of two ResiPore 300 × 7.5 mm columns. The polymer samples were dissolved in THF (2 mg·mL−1). All elution curves were calibrated with polystyrene standards. Mn,SEC values of PLA were calculated using the correcting factor previously reported (PLA: Mn,SEC = Mn,SEC raw data × 0.58),66,67 thus taking into account the difference in hydrodynamic radius of PLA vs polystyrene. The SEC traces of the polymers all exhibited a unimodal and symmetrical peak unless otherwise stated. [Ln{(S)-PEBA}(BH4)2(THF)2] (1−6) (ref 64). General Procedure. THF (20 mL) was condensed at −78 °C onto a mixture of [Ln(BH4)3(THF)n] (0.69 mmol) and (S)-KPEBA (0.69 mmol, 252 mg). The reaction solution was stirred for 18 h at 55 °C. The resulting suspension was filtered, and the solvent removed until a precipitate appeared. The mixture was heated until the solution became clear and was allowed to stand at room temperature to obtain the crystalline product. [Sc{(S)-PEBA}(BH4)2(THF)2] (1). [Sc(BH4)3(THF)2] (211 mg, 0.90 mmol): yield 105 mg, 0.19 mmol, 28% (colorless single crystals). 1 H{11B} NMR (THF-d8, 300 MHz, 25 °C): δ 7.60−7.51 (m, 3 H, Ph), 7.22−7.09 (m, 12 H, Ph), 4.20 (q, 2 H, 3JH,H = 6.9 Hz, CH), 1.53 (d, 6 H, 3JH,H = 6.9 Hz, CH3), 0.48 (s, 8 H, BH4) ppm. 13C{1H} NMR (THF-d8, 75 MHz, 25 °C): δ 179.8 (NCN), 146.0 (i-Ph), 132.7 (Ph), 128.9 (Ph), 128.6 (Ph), 127.6 (Ph), 127.5 (Ph), 126.8 (Ph), 126.0 (Ph), 57.4 (CH), 22.4 (CH3) ppm. 11B NMR (THF-d8, 96 MHz, 25 °C): δ −25.3 (qt, 1JH,B = 80 Hz) ppm. 15N NMR (THF-d8, 41 MHz, 25 °C): δ 199.4 ppm. IR (KBr): 3325 (w), 3219 (w), 3083 (w), 3062 (w), 3025 (m), 2970 (s), 2927 (s), 2874 (m), 2369 (s, br), 2287 (s), 2224 (s), 1612 (w), 1598 (m), 1579(w), 1494 (m), 1450 (s), 1375 (m), 1352 (m), 1335 (m), 1281 (w), 1258 (w), 1203(m), 1188 (m), 1115 (m), 1092 (m), 1063 (w), 1042 (w), 1018 (m), 961 (w), 916 (m), 863 (m), 805 (m), 774 (w), 761 (m), 739 (w), 726 (w), 701 (s), 632 (w), 615 (w), 582 (w), 544 (m) cm−1. Raman (neat solid): 3065 (s), 3059 (s), 3041 (m), 2971 (m), 2937 (m), 2905 (m), 2876 (m), 2486 (w), 2360 (w), 2240 (w), 1603 (m), 1582 (w), 1501 (w), 1431 (w), 1353 (w), 1337 (w), 1250 (w), 1187 (w), 1156 (w), 1135 (w), 1095 (w), 1061 (w), 1043 (w), 1028 (w), 1002 (s), 965 (w), 926 (w), 871 (w), 791 (w), 740 (w), 633 (w), 620 (w), 597 (w), 544 (w), 420 (w), 354 (w), 258 (w), 191 (w) cm−1. Anal. Calcd for C31H47B2N2O2Sc (546.29): C, 68.16, H, 8.67, N, 5.13. Found: C, 67.99, H, 8.73, N, 4.98. [La{(S)-PEBA}(BH4)2(THF)2] (2). [La(BH4)3(THF)3] (275 mg, 0.69 mmol): yield 145 mg, 0.23 mmol, 33% (colorless single crystals). 1 H{11B} NMR (THF-d8, 300 MHz, 25 C): δ 7.50−6.87 (m, 15 H, Ph), 4.04 (q, 2 H, 3JH,H = 6.8 Hz, CH), 1.44 (d, 6 H, 3JH,H = 6.8 Hz, CH3), 1.18 (s, 8 H, BH4) ppm. 13C{1H} NMR (THF-d8, 75 MHz, 25 C): δ 175.8 (NCN), 145.8 (i-Ph), 133.3 (Ph), 126.3 (Ph), 126.0 (Ph), 125.9 (Ph), 125.8 (Ph), 125.5 (Ph), 125.3 (Ph), 125.1 (Ph), 124.8 (Ph), 123.9 (Ph), 55.5 (CH), 43.1 (CH3) ppm. 11B NMR (THF-d8, 96 MHz, 25 C): δ −23.6 (qt, 1JH,B = 85 Hz) ppm. 15N NMR (THF-d8, 41 MHz, 25 C): δ 205.3 ppm. IR (KBr): 3407 (m), 3325 (w), 3060 (m), 3026 (m), 2974 (s), 2926 (s), 2391 (s), 2329 (m), 2284 (s), 1637 (m), 1612 (m), 1597 (s), 1579 (w), 1494 (s), 1452 (s), 1416 (m), 1359 (w), 1349 (w), 1328 (w), 1303 (w), 1278 (w), 1267 (w), 1163 (m), 1142 (w), 1119 (w), 1082 (m), 1027 (m), 927 (w), 914 (w), 864 (w), 801 (w), 763 (s), 700 (s), 625 (w), 600 (w), 583 (w), 573 (w), 544 (m) cm−1. Raman (neat solid): 3054 (s), 3037 (m), 2977 (m), 2966 (m), 2933 (m), 2899 (m), 2870 (m), 2424 (m), 2307 (w), 2220 (m), 2137 (w), 1602 (m), 1582 (w), 1494 (m), 1452 (w), 1422 (m), 1370 (w), 1350 (w), 1329 (w), 1277 (m), 1231 (w), 1205 (w), 1184 (w), 1157 (w), 1131 (w), 1091 (w), 1057 (w), 1039 (w), 1027 (m), 1000 (s), 958 (w), 924 (w), 873 (w), 787 (m), 774 (w), 736 (w), 631 (w), 619 (m), 595 (w), 541 (w), 416 (w), 342 (w), 255 (w), 244 (w), 180 (m) cm−1. Anal. Calcd for C25H35B2N2O0.5La (2 − 1.5 THF) (532.1) (539.30): C, 56.43, H, 6.63, N, 5.26. Found: C, 56.50, H, 5.69, N, 4.23.

EXPERIMENTAL SECTION

General Procedures. All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in flame-dried Schlenk-type glassware either on a dual-manifold Schlenk line, interfaced to a high vacuum (10−3 Torr) line, or in an argon-filled MBraun or Jacomex glovebox. Tetrahydrofuran was distilled under nitrogen from Na/K alloy benzophenone ketyl prior to use and stored in vacuo over LiAlH4 in a resealable flask. Deuterated tetrahydrofuran was obtained from Aldrich Inc. (all 99 atom % D) and was degassed, dried, and stored in vacuo over Na/K alloy in a resealable flask. NMR spectra were recorded on a Bruker Avance 400 MHz, Avance II NMR 300 MHz, or AC-500 spectrometer. Chemical shifts are referenced to internal solvent resonances and are reported relative to tetramethylsilane (1H and 13C NMR), 15% BF3·Et2O (11B NMR), and NH3 (15N NMR), respectively. IR spectra were obtained on a FTIR spectrometer Bruker Tensor 37, and Raman spectra on a Bruker MultiRAM spectrometer. Mass spectra were recorded at 70 eV on a DFS Thermo Scientific. Elemental analyses were carried out with an Elementar Vario Micro Cube. [Ln(BH4)3(THF)n]11 and (S)-KPEBA65 were prepared according to literature procedures. Monomer conversions were determined from 1H NMR spectra of the crude polymer sample, from the integration (Int.) ratio B

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B NMR (THF-d8, 96 MHz, 25 °C): δ −28.2 (qt, 1JH,B = 85 Hz) ppm. N NMR (THF-d8, 41 MHz, 25 °C): δ 190.5 ppm. IR (KBr): 3325 (w), 3214 (w), 3082 (w), 3062 (w), 3024 (m), 2967 (s), 2934 (s), 2602 (s), 2872 (m), 2429 (s), 2338 (m), 2281 (s), 2230 (s), 1612 (w), 1598 (m), 1578 (w), 1548 (m), 1493 (s), 1452 (m), 1373 (m), 1352 (m), 1334 (m), 1306 (w), 1280 (w), 1252 (w), 1201 (m), 1177 (m), 1133 (w), 1117 (w), 1091 (m), 1063 (w), 1017 (m), 959 (w), 915 (m), 866 (m), 804 (m), 762 (m), 738 (m), 702 (s), 614 (w), 583 (w), 545 (m) cm−1. Raman (neat solid): 3064 (s), 3056 (s), 3040 (m), 2984 (m), 2968 (m), 2936 (m), 2902 (m), 2871 (m), 2431 (m), 2360 (w), 2254 (m), 2127 (w), 1603 (m), 1582 (w), 1498 (w), 1462 (m), 1449 (w), 1433 (w), 1367 (w), 1354 (w), 1336 (m), 1253 (w), 1210 (w), 1196 (w), 1186 (w), 1156 (w), 1135 (w), 1093 (w), 1060 (w), 1043 (w), 1028 (m), 1001 (s), 963 (w), 923 (w), 876 (w), 790 (m), 775 (w), 739 (w), 634 (w), 620 (m), 596 (w), 544 (w), 417 (w), 348 (w), 250 (w), 186 (m), 160 (w) cm−1. EI-MS: m/z 531 ([M − 2 THF]+, 1), 517 ([M − BH4 − 2 THF]+, 24), 503 ([M − 2 BH4 − 2 THF]+, 6), 413 (13), 328 ([PEBA]+, 3), 277 (6), 262 (5), 247 (4), 223 ([PEBA-PhEt]+, 3), 196 (4), 120 ([PhEtN]+, 9), 105 ([PhEt]+, 33), 72 (93), 42 ([C2H4N]+, 100). Anal. Calcd for C31H47B2LuN2O2 (676.31): C, 55.05, H, 7.00, N 4.14. Found: C, 54.67, H, 6.76, N, 4.03. X-ray Crystallographic Studies of 1−6. A suitable crystal was covered in mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to a −73 °C cold stream of a STOE IPDS 2 diffractometer. All structures were solved by the direct method (SHELXS-97).68 The remaining non-hydrogen atoms were located from successive difference in Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F, minimizing the function (Fo − Fc)2, where the weight is defined as 4Fo2/2(Fo2) and Fo and Fc are the observed and calculated structure factor amplitudes, respectively, using the program SHELXL-97.68 Carbon-bound hydrogen atom positions were calculated. The final values of refinement parameters are given in the Supporting Information. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities in each case were of no chemical significance. Positional parameters, hydrogen atom parameters, thermal parameters, and bond distances and angles have been deposited as Supporting Information. Typical Procedure for ROP of rac-Lactide. In the glovebox, a Schlenk tube was charged with rac-LA (0.466 g, 3.24 mmol, 200 equiv) and THF (2.6 mL). That was transferred onto a solution of the lanthanide complex (for 6: 11 mg, 16.2 μmol, 1.0 equiv) in THF (0.62 mL), thus producing a starting concentration of rac-Lactide of 1 M. The mixture was then stirred at 60 °C over the appropriate reaction time (reaction times were not systematically optimized). The reaction was then quenched with excess acetic acid (ca. 0.1 mL of a 1.6 M solution in toluene). The solvent was evaporated under vacuum, and the crude material was analyzed by 1H NMR spectroscopy in CDCl3. The crude polymer was then dissolved in CH2Cl2 and purified upon precipitation in cold methanol or pentane, filtered, and dried under vacuum. The final polymer was then analyzed by NMR and SEC. The probability of racemic enchainments between monomer units, Pr, i.e., the ratio of the mrm and rmr units toward all of the other types of units, was determined by examination of the methine region of the homonuclear CH3-decoupled 1H NMR spectrum of PLA at 20 °C in CDCl3 on a Bruker AC-500 spectrometer.46,69−72 11

[Nd{(S)-PEBA}(BH4)2(THF)2] (3). [Nd(BH4)3(THF)3] (280 mg, 0.69 mmol): yield 110 mg, 0.17 mmol, 25% (blue single crystals). IR (KBr): 3407 (m), 3325 (m), 3081 (m), 3060 (m), 3028 (m), 2955 (m), 2924 (s), 2854 (s), 2396 (m), 2330 (m), 2284 (m), 1638 (s), 1612 (m), 1598 (m), 1578 (w), 1494 (s), 1451 (s), 1373 (w), 1360 (w), 1350 (w), 1328 (w), 1309 (w), 1267 (m), 1208 (w), 1167 (m), 1142 (m), 1119 (w), 1073 (m), 1027 (m), 928 (m), 764 (m), 699 (s), 601 (w), 572 (w), 544 (m) cm−1. Raman (neat solid): 3055 (s), 3038 (m), 2979 (m), 2967 (m), 2934 (m), 2900 (m), 2870 (m), 2427 (m), 2318 (w), 2227 (m), 2147 (w), 1602 (m), 1583 (w), 1495 (w), 1454 (m), 1424 (w), 1351 (w), 1332 (m), 1250 (w), 1231 (w), 1206 (w), 1185 (w), 1156 (w), 1092 (w), 1057 (w), 1041 (w), 1027 (m), 1001 (s), 959 (w), 924 (w), 874 (w), 788 (m), 775 (w), 737 (w), 632 (w), 620 (m), 595 (w), 542 (w), 416 (w), 343 (w), 256 (w), 246 (w), 182 (m) cm−1. Anal. Calcd for C31H47B2N2NdO2 (645.58): C, 57.67, H, 7.34, N, 4.34. Found: C, 57.52, H, 7.55, N, 4.25. [Sm{(S)-PEBA}(BH4)2(THF)2] (4). [Sm(BH4)3(THF)3] (283 mg, 0.69 mmol): yield 138 mg, 0.21 mmol, 30% (yellow single crystals). 1 H{11B} NMR (THF-d8, 300 MHz, 25 C): δ 8.24 (d, 2 H, 3JH,H = 7.0 Hz, Ph), 8.02−7.89 (m, 3 H, Ph), 7.35−7.15 (m, 2 H, Ph), 6.77 (s, 8 H, Ph), 3.81 (q, 2 H, 3JH,H = 6.4 Hz, CH), 1.99 (d, 6 H, 3JH,H = 6.7 Hz, 6 H, CH3), −5.48 (s, 8 H, BH4) ppm. 13C{1H} NMR (THF-d8, 75 MHz, 25 °C): δ 204.2 (NCN), 147.4 (i-Ph), 138.5 (Ph), 129.3 (Ph), 128.7 (Ph), 128.0 (Ph), 127.9 (Ph), 127.8 (Ph), 127.2 (Ph), 127.0 (Ph), 126.1 (Ph), 125.5 (Ph), 55.7 (CH), 25.4 (CH3) ppm. 11B NMR (THF-d8, 96 MHz, 25 C): δ −31.4 (qt, 1JH,B = 76 Hz) ppm. 15N NMR (THF-d8, 41 MHz, 25 C): δ 75.5 ppm. IR (KBr): 3315 (w), 3257 (w), 3208 (w), 3081 (m), 3061 (m), 3023 (m), 2979 (s), 2966 (s), 2932 (s), 2900 (s), 2428 (s), 2329 (s), 2214 (s), 2162 (s), 1676 (w), 1613 (m), 1600 (m), 1581 (m), 1493 (s), 1443 (s), 1372 (m), 1351 (m), 1331 (m), 1306 (w), 1279 (w), 1251 (w), 1201 (m), 1168 (m), 1131 (m), 1089 (s), 1062 (w), 1020 (s), 957 (w), 915 (m), 867 (s), 802 (m), 774 (w), 762 (s), 735 (m), 702 (s), 660 (w), 631 (w), 582 (m), 545 (m) cm−1. Raman (neat solid): 3058 (s), 3039 (m), 3004 (m), 2979 (m), 2968 (m), 2954 (m), 2932 (m), 2901 (m), 2875 (m), 2427 (m), 2228 (m), 1601 (m), 1581 (w), 1454 (w), 1422 (w), 1332 (w), 1184 (w), 1156 (w), 1027 (m), 1001 (s), 959 (w), 920 (w), 911 (w), 874 (w), 787 (w), 736 (w), 620 (m), 596 (w), 543 (w), 344 (w), 256 (w), 243 (w), 180 (m) cm−1. Anal. Calcd for C31H47B2N2O2Sm (651.70): C, 57.13, H, 7.27, N, 4.30. Found: C, 56.70, H, 7.02, N, 4.30. [Yb{(S)-PEBA}(BH4)2(THF)2] (5). [Yb(BH4)3(THF)3] (299 mg, 0.69 mmol): yield 95 mg, 0.14 mmol, 20% (orange single crystals). IR (KBr): 3317 (w), 3212 (w), 3082 (w), 3062 (w), 3024 (m), 2982 (s), 2967 (s), 2931 (s), 2092 (s), 2872 (m), 2431 (s), 2339 (m), 2282 (m), 2232 (s), 1613 (w), 1599 (w), 1579 (w), 1493 (m), 1448 (s), 1372 (m), 1353 (m), 1334 (m), 1306 (w), 1280 (w), 1253 (w), 1202 (m), 1178 (m), 1133 (m), 1091 (s), 1063 (w), 1018 (s), 959 (w), 916 (m), 868 (s), 804 (m), 774 (w), 762 (m), 738 (w), 702 (s), 633 (w), 583 (w), 545 (m) cm−1. Raman (neat solid): 3060 (s), 3039 (m), 2971 (m), 2935 (m), 2904 (m), 2873 (m), 2431 (m), 2252 (m), 1643 (w), 1601 (m), 1581 (w), 1458 (m), 1370 (w), 1354 (w), 1336 (w), 1252 (w), 1185 (w), 1157 (w), 1093 (w), 1027 (w), 1001 (w), 983 (w), 963 (w), 925 (w), 877 (w), 790 (m), 738 (w), 639 (m), 595 (w), 545 (w), 455 (m), 400 (m), 345 (w), 303 (w), 251 (w), 227 (w), 170 (m) cm−1. EI-MS: m/z 529 ([M − 2 THF]+, 4), 515 ([M − BH4 − 2 THF]+, 5), 501 ([M − 2 BH4 − 2 THF]+, 8), 410 (6), 394 (92), 341 (18), 328 ([PEBA]+, 83), 313 ([PEBA − Me]+, 17), 290 (4), 273 (5), 262 (5), 257 (6), 235 (20), 223 ([PEBA − PhEt]+, 83), 209 ([PEBA − PhEtN]+, 23), 196 (24), 186 (28), 180 (37), 149 (32), 137 (76), 120 ([PhEtN]+, 98), 105 ([PhEt]+, 100), 91 ([Bz]+, 79), 80 (98), 77 ([Ph]+, 78), 70 (100), 57 (88). Anal. Calcd for C33H51B2N2O2Yb (5 + 0.5 THF): C, 56.43, H, 7.43, N, 3.99. Found: C, 56.76, H, 7.46, N, 3.40. [Lu{(S)-PEBA}(BH4)2(THF)2] (6). [Lu(BH4)3(THF)3] (300 mg, 0.69 mmol): yield 198 mg, 0.29 mmol, 42% (colorless single crystals). 1 H{11B} NMR (THF-d8, 300 MHz, 25 C): δ 7.60−7.52 (m, 3 H, Ph), 7.20−7.08 (m, 12 H, Ph), 4.20 (q, 2 H, 3JH,H = 6.8 Hz, CH), 1.48 (s, 8 H, BH4), 1.44 (d, 6 H, 3JH,H = 6.9 Hz, CH3) ppm. 13C{1H} NMR (THF-d8, 75 MHz, 25 °C): δ 146.7 (i-Ph), 134.6 (i-Ph), 128.5 (Ph), 128.5 (Ph), 127.6 (Ph), 127.4 (Ph), 126.8 (Ph), 125.9 (Ph), 57.3 (CH), 44.9 (CH3) ppm (the NCN carbon atom was not detected).

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RESULTS AND DISCUSSION Recently, we described the syntheses of the chiral proligand N,N′-bis((S)-1-phenylethyl)benzamidine ((S)-HPEBA) and its potassium salt ((S)-KPEBA), which can be obtained in one step by deprotonation of the amidine with potassium hydride (Scheme 1).65 While HPEBA was first reported about 30 years ago by H. Brunner,73,74 we recently established a significantly modified and improved three-step procedure for the preparation of (S)-HPEBA (Scheme 1).65 With the objective to obtain enantiopure borohydride complexes of the rare earth elements, the homoleptic trisborohydrides C

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spectra were recorded for all diamagnetic compounds as well as for the paramagnetic samarium compound 4. All these NMR spectra indicate a symmetrical coordination of the ligand in solution. Thus, only one set of signals is observed for each (S)1-phenylethyl moiety of the ligand. The 1H{11B} NMR spectra of 1, 2, 4, and 6 show one doublet for the methyl group (δ = 1.53 ppm (1), 1.44 ppm (2), 1.99 ppm (4), 1.44 ppm (6)), as well as one quartet for the methine proton (δ = 4.20 ppm (1), 4.04 ppm (2), 3.81 ppm (4), 4.20 ppm (6)), respectively. These signals fall in the range of those of the earlier reported lutetium compounds [Lu{(S)-PEBA}2Cl]2 and [Lu{(S)-PEBA}2{N(SiMe3)2}].62,63 In addition, for the protons of the borohydride groups, one broad singlet (δ = 0.48 ppm (1), 1.18 ppm (2), and 1.48 ppm (6)) could be detected in compounds 1−6. This signal is strongly shifted to higher field in compound 4 (δ = −5.48 ppm) as a result of the weak paramagnetic nature of samarium. The 11B NMR spectra display one quintet for the BH4 group (δ = −25.3 ppm (1), −23.6 ppm (2), −31.4 ppm (4), 28.2 ppm (6)) for each compound. The signals of the diamagnetic compounds are in the range of those recorded with the bis(phosphinimino)methanide bisborohydride complexes [{CH(PPh2NSiMe3)2}La(BH4)2(THF)]35 and [{CH(PPh2NSiMe3)2}Ln(BH4)2]35 (Ln = Y, Lu) (δ = −22.4 ppm (La), −24.7 ppm (Y), and −25.8 ppm (Lu)). Furthermore, the chemical shifts of the 15N atoms of the

Scheme 1. Synthesis of (S)-KPEBA from KH and (S)-HPEBA65

[Sc(BH4)3(THF)2]9,75 and [Ln(BH4)3(THF)3]4,11 (Ln = La, Nd, Sm, Yb, Lu) were reacted via a salt metathesis with (S)-KPEBA in THF at 60 °C (Scheme 2). The desired monoamidinato compounds [Ln{(S)-PEBA}(BH4)2(THF)2] (Ln = Sc (1), La (2), Nd (3), Sm (4), Yb (5), Lu (6)) were then recovered as crystalline material in a reasonable yield.64 Rare earth metals with different ionic radii were selected so as to cover the whole range of these elements with respect to the ion radius, thereby providing a sizevariable coordination sphere around the metal center. Compounds 1−6 were characterized by standard analytical and spectroscopic techniques, and their solid structures were determined by single-crystal X-ray diffraction analyses. NMR Scheme 2. Synthesis of [Ln{(S)-PEBA}(BH4)2(THF)2]64

Figure 1. 1H/15N gHMQC NMR spectrum of [Sc{(S)-PEBA}(BH4)2(THF)2] (1). D

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Table 1. Crystallographic Details of 1−6 1

2

3

chemical formula formula mass a/Å b/Å c/Å β/deg unit cell volume/Å3 temperature/K space group no. of formula units per unit cell, Z radiation type absorp coeff, μ/mm−1 Nno. of reflns measd no. of indep reflns Rint final R1 values (I > 2σ(I)) final wR(F2) values (all data) goodness of fit on F2 Flack parameter

C31H47B2N2O2Sc 534.28 13.282(3) 12.463(3) 10.964(2) 118.85(3) 1589.8(6) 200(2) C2 2 Mo Kα 0.258 6323 3366 0.0758 0.0594 0.1153 0.883 −0.05(6) 4

C31H47B2N2O2La 640.24 13.297(3) 12.881(3) 10.822(2) 117.00(3) 1651.6(6) 200(2) C2 2 Mo Kα 1.320 11 051 4111 0.0852 0.0294 0.0681 1.035 −0.01(2) 5

C31H47B2N2O2Nd 645.57 13.223(3) 12.679(3) 10.765(2) 116.91(3) 1609.3(6) 150(2) C2 2 Mo Kα 1.641 23 264 3424 0.1488 0.0338 0.0714 1.015 −0.01(2) 6

chemical formula formula mass a/Å b/Å c/Å β/deg unit cell volume/Å3 temperature/K space group no. of formula units per unit cell, Z radiation type absorp coeff, μ/mm−1 no. of reflns measd no. of indep reflns Rint final R1 values (I > 2σ(I)) final wR(F2) values (all data) goodness of fit on F2 Flack parameter

C31H47B2N2O2Sm 651.68 13.223(3) 12.678(3) 10.773(2) 117.34(3) 1604.2(6) 150 C2 2 Mo Kα 1.86 13 545 3250 0.0570 0.0223 0.0523 1.074 −0.040(13)

C31H47B2N2O2Yb 674.37 13.256(3) 12.600(3) 10.874(2) 118.16(3) 1601.2(6) 200 C2 2 Mo Kα 2.948 9473 4318 0.0576 0.0235 0.0590 1.052 0.018(10)

C31H47B2N2O2Lu 676.30 13.250(3) 12.600(3) 10.877(2) 118.22(3) 1600.0(6) 200 C2 2 Mo Kα 3.113 12 073 4312 0.0759 0.0357 0.0789 1.083 0.02(2)

the stabilizing contribution of Cl− vs BH4−.15 As expected, the bond lengths between the ligands and the metal center increase with the metal radius, whereas the N−Ln−N and O−Ln−O decrease (Tables 1 and 2). In accordance with the observations in solution, the amidinate ligand coordinates to the metal center in a symmetric fashion. For compounds 3 and 4, the hydrogen BH4− group could be localized in the difference Fourier map atoms, showing a η3bonding mode of this group to the metal center. This is further supported by FT-IR and Raman data. For tridentate complexes, a sharp band at 2420 cm−1 and a strong broad band centered at 2230 cm−1 are expected.33,50 Especially, these bands were clearly observed in the Raman spectra of complexes 2−6 (2: 2424 and 2220 cm−1; 3: 2427 and 2227 cm−1; 4: 2428 and 2214 cm−1; 5: 2431 and 2252 cm−1; 6: 2431 and 2254 cm−1). However, in the FTIR and Raman spectra of complex 1, the broadness of the bands precluded a proper assignment of the coordination mode. Ring-Opening Polymerization of rac-(D,L)-Lactide. The N,N′-bis((S)-1-phenylethyl)benzamidinato scandium, lanthanum, samarium, and lutetium complexes 1, 2, 4, and 6, respectively,

ligand were determined by a two-dimensional 1H/15N gHMQC NMR experiment (δ = 199.4 ppm (1), 205.3 ppm (2), 75.5 ppm (4), and 190.5 ppm (6)). The nitrogen atoms were coupled with the methyl and also with the methine groups of the ligand (Figure 1). Complexes 1−6 are isostructural in the solid state and crystallize in the monoclinic space group C2 with two molecules of the complex in the unit cell. The asymmetric unit contain only half of the complex; the other atoms are generated by a crystallographic C2 axis running through the lanthanide atom and the carbon atom of the N−C−N unit. Each metal center in the complexes is coordinated by the two nitrogen atoms of the amidinate ligand, as well by two borohydrides and two THF molecules, leading to a distorted octahedral coordination polyhedron by considering the two borohydrides as monodentate. In contrast to the corresponding bischloro complexes [Sm{(rac)PEBA}Cl2(THF)3] and [Ln{(S)-PEBA}Cl2(THF)2] (Ln =Yb, Lu),63 in which the number of coordinated solvent molecules depends on the ion radius, complexes 1−6 contain two additional coordinating THF molecules regardless of the metal radius, thus highlighting as previously observed the difference in E

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Table 2. Ring-Opening Polymerization of rac-Lactide in THF Promoted by Complexes 1, 2, 4, and 6 entry

catalyst

[rac-LA]0/ [BH4]0a

temp (°C)

timeb (min)

convc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20I

1 1 1 1 1 2 2 2 2 4 4 4 4 6 6 6 6 6 6 6

100 100 300 300 500 50 150 300 500 50 150 300 500 50 150 150 150 250 500 100

60 80 60 80 80 60 60 60 60 60 60 60 60 60 23 40 60 60 60 60

15 60 45 120 180 7 21 45 75 12 12 45 75 27 180 20 7 23 45 25

0 65 0 50 11 63 51 75 34 84 17 53 10 84 40 67 83 60 25 91

Mn,theod (g·mol−1)

Mn,SECe (g·mol−1)

9350

6700

21 600 7900 4500 11 000 32 500 24 500 6000 3700 22 900 7200 6050 8650 14 500 17 900 21 600 18 000 13 350

8100 7400 5000 5700 18 800 10 000 5400 3000 9750 8000 6500 5700 11 300 11 900 9500 8200 10 200

ĐMf 1.10 1.23g 1.11h 1.34 1.26 1.27 1.18g 1.31 1.30 1.24 1.29 1.27 1.22 1.27 1.34 1.34 1.13g 1.25

a All reactions were performed in THF with [rac-LA]0 = 1.0 mol L1; results are representative of at least duplicated experiments. bReaction times were not necessarily optimized. crac-LA conversion determined by 1H NMR analysis. dTheoretical molar mass value calculated from the relation ([rac-LA]0/[BH4]0) × convrac‑LA × Mrac‑LA, with [BH4]0 = 2[catalyst]0 and Mrac‑LA = 144 g·mol−1. eNumber average molar mass values (corrected by a factor of 0.58; refer to Experimental Section) determined by SEC in THF vs polystyrene standards. fMolar mass distribution values determined by SEC in THF. gSEC trace featuring a slight shoulder. hBimodal SEC trace with Mn = 32 200 g·mol−1, ĐM = 1.07. IExperiment ran with L-lactide.

rare earth center, i.e., via initiation of the polymerization through both borohydride groups. However, with larger monomer loadings

featuring two potential initiating borohydride functions, have been evaluated in the ROP of rac-LA (Scheme 3). Representative results are summarized in Table 2. ROP of cyclic esters promoted by rare earth borohydride complexes are typically investigated in either THF (a polar solvent) or toluene (a nonpolar solvent).15 Indeed, use of toluene enables avoiding solvent/monomer competition for coordination to the oxophilic metal and may thus allow a faster rate of ROP as compared to THF.27,38,76 However, because of the poor solubility of the complexes 1−6 in toluene, THF was preferred.77 Preliminary investigations of the ROP of rac-LA catalyzed by the lutetium complex 6 in THF revealed an activity at room temperature, giving polylactide (Table 2, entry 15). Raising the temperature from 23 °C up to 60 °C resulted in an improved rate of polymerization for 6 (Table 2, entries 15, 16, and 17, respectively). The living nature of the polymerization of raclactide ([rac-LA]0 = 1.0 M) promoted by 6 ([rac-LA]0/[6]0 = 100) in THF at 60 °C was demonstrated by the linear increase of the molar mass values determined by SEC with monomer conversion and by the close match with calculated values (Figure 3). The corresponding evolution of the molar mass is depicted in Figure 4 (see also Table S1 in the Supporting Information). All subsequent polymerizations were then run at 60 °C with [rac-LA]0/[catalyst]0 ranging from 100 to 1000 and [rac-LA]0 = 1.0 mol·L−1. Under these operating conditions, all complexes except the scandium one then proved to be active; a higher reaction temperature of 80 °C enabled the ROP of racLA from the smaller scandium complex 1 to proceed (Table 2, entries 1−5). The number average molar mass values determined experimentally by size exclusion chromatography (Mn,SEC; values corrected for the difference in hydrodynamic radii between PLAs and the polystyrene standards used for calibration)66,78 were generally in close agreement with the theoretical ones (Mn,theo) calculated on the assumption of two growing polyester chains per

Figure 2. Perspective ORTEP view of the molecular structure of 3. Thermal ellipsoids are drawn to encompass 50% probability. Hydrogen atoms at carbon are omitted for clarity. Selected distances [Å] and angles [deg] (data for the isostructural compounds 1, 2, 4−6 are also given): 3: Nd−N 2.443(4), Nd−O 2.450(3), Nd−B 2.662(6), N−C1 1.340(5), N− C6 1.478(5); N−Nd−N 55.0(2), N−C1−N 114.7(2); 1: Sc−N 2.207(3), Sc−O 2.226(3), Sc−B 2.526(6), N−C1 1.330(4), N−C6 1.470(5); N− Sc−N 60.8(2), N−C1−N 114.2(5), O−Sc−O 173.3(2); 2: La−N 2.512(2), La−O 2.518(2), La−B 2.747(4), N−C1 1.334(3), N−C6 1.465(3); N−La−N 53.62(11), N−C1−N 116.2(3), O−La−O 166.56(9); 4: Sm−N 2.421(3), Sm−O 2.430(2), Sm−B 2.631(5), N− C1 1.332(4), N−C6 1.467(4); N−Sm−N 55.45(12), N−C1−N 115.5(4), O−Sm−O 167.53(12); 5: Yb−N 2.323(3), Yb−O 2.340(3), Yb−B 2.550(5), N−C1 1.338(3), N−C6 1.474(4); N−Yb−N 57.77(13), N− C1−N 114.0(3), O−Yb−O 169.16(14); 6: Lu−N 2.312(4), Lu−O 2.325(3), Lu−B 2.527(7), N−C1 1.350(5), N−C6 1.476(6); N−Lu−N 58.2(2), N−C1−N 112.7(5), O−Lu−O 169.0(2). F

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Scheme 3. ROP of rac-Lactide Promoted by the Bisamidinato Rare Earth Complexes 1, 2, 4, and 6: Synthesis of PLA Diol

typically affording similarly atactic PLAs with high conversion (up to 98%) from 200 equiv of rac-LA per BH4 group being reached within one hour at room temperature (in THF or toluene).15,18,26,27,37,38,51,54 Also, to the best of our knowledge, the amidinate complex 1 is the first scandium borohydride compound reported active in the ROP of a cyclic ester.15 The 1H NMR spectra of relatively low molar mass PLA samples analyzed in CDCl3 displayed, besides the main polymer chain signals (−C(O)CH(Me)O multiplet at δ = 5.16 ppm, −C(O)CH(CH3)O doublet at δ = 1.56 ppm), the quartet characteristic of the −CH(Me)OH terminal group (δ = 4.37 ppm), which is formed upon hydrolysis of the active rare earth− oxygen (alkoxide) bond (Figure S1 in the Supporting Information). The other chain-end signals integrating for one and two hydrogen atoms are observed at δ = 2.72 ppm and 3.74 ppm, corresponding to −CH(Me)CH2OH, respectively. Indeed, Shiono and Nakayama et al. previously reported that in the ROP of LA initiated from rare earth borohydride complexes [Ln(BH4)3(THF)3], each BH4− group acts both as an initiating function and as a reducing agent,51 thereby affording α,ω-dihydroxy telechelic PLAs in a similar coordination/insertion mechanism as that described by Guillaume et al. for the ROP of the related ε-caprolactone.23,52,53,55,79 PLAs featuring both −CH(Me)CH2OH and −CH(Me)OH chain ends were similarly claimed from other postmetallocene rare earth borohydride complexes.27,37,54 The PLAs obtained from 1, 2, 4, and 6 showed a rather atactic microstructure, as judged by the Pr values, which generally lie in the range 0.50−0.61, as determined by NMR analysis.80 These results show that the chiral (S)-PEBA ligand did not impart any stereocontrol in the ROP of rac-LA. A control experiment of the ROP of L-lactide with complex 6 (Table 2, entry 20) showed the formation of pure isotactic poly(L-lactide), as evidenced by a single sharp resonance recorded in the methine region of the decoupled 1H NMR spectrum. This observation supported the absence of base-promoted epimerization of L-lactide or poly(L-lactide) and argued in favor of a coordination/insertion polymerization mechanism rather than a purely anionic one.81

Figure 3. Evolution of the Mn (■, experimental values determined by SEC; ⧫, calculated values) and dispersity (Δ) values of PLA as a function of monomer conversion in the ROP of rac-LA promoted by [Lu{(S)-PEBA}(BH4)2(THF)2] (6) in THF at 60 °C ([rac-LA]0 = 1.0 M; [rac-LA]0/[6]0 = 100; see Supporting Information, Table S1).

Figure 4. Evolution of the monomer conversion as a function of time in the ROP of rac-LA promoted by [Lu{(S)-PEBA}(BH4)2(THF)2] (6) in THF at 60 °C ([rac-LA]0 = 1.0 M; [rac-LA]0/[6]0 = 100; see Supporting Information, Table S1).

■

([rac-LA]0/[BH4]0 = 250−500), the molar mass measured by SEC tends to become lower than expected (Mn,SEC < Mn,theo), suggesting a significant degree of chain-transfer reactions, as previously observed with related rare earth borohydride initiating complexes.15 The PLAs had a relatively narrow dispersity, in the range ĐM = 1.10−1.34, suggesting, to some extent, some transesterification reactions. Indeed these values fall in the range of the lowest ones obtained in the ROP of rac-LA from rare earth trisborohydride or linked bis(amide) bisborohydride derivatives, as reported by Nakayama et al.54 or Mahrova et al.,26 respectively.15 No significant influence of the metal could be evidenced throughout the present series of complexes investigated. The behavior of 1, 2, 4, and 6 in promoting the ROP of rac-lactide is mimicking that reported in the literature for related rare earth borohydride initiators.15,18,26,27,37,38,51,54 Indeed, homoleptic as well as postmetallocene rare earth borohydride complexes have demonstrated a comparable activity,

SUMMARY In summary, the monoamidinato bisborohydride rare earth complexes [Ln{(S)-PEBA}(BH4)2(THF)2] (Ln = Sc (1), La (2), Nd (3), Sm (4), Yb (5), Lu (6)) were reported. They have been obtained as crystalline materials from the reaction of (S)KPEBA with the homoleptic trisborohydrides [Sc(BH4)3(THF)2] and [Ln(BH4)3(THF)3] (Ln = La, Nd, Sm, Yb, Lu), respectively. Compounds 1−6 are unique examples of enantiopure borohydride complexes of the rare earth metals. Most of the new complexes were completely characterized by 1 H, 13C{1H}, and 11B NMR. Furthermore, the chemical shifts of the 15N atoms of the ligand were determined by a twodimensional 1H/15N gHMQC NMR experiment. For each compound the solid-state structure was established by singlecrystal X-ray diffraction, highlighting as previously observed the G

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difference in the stabilizing contribution of Cl− vs BH4−. All four complexes 1, 2, 4, and 6 were found to be active in the ROP of rac-lactide under mild operating conditions, providing atactic α,ω-dihydroxytelechelic PLA. Most of the polymerizations generally proceed with a certain degree of control that is directed by molar mass values and relatively narrow dispersities, within a moderate monomer-to-initiator ratio.

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(17) Meyer, N.; Jenter, J.; Roesky, P. W.; Eickerling, G.; Scherer, W. Chem. Commun. 2009, 4693−4695. (18) Bonnet, F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44, 9046−9055. (19) Zinck, P.; Valente, A.; Mortreux, A.; Visseaux, M. Polymer 2007, 48, 4609−4614. (20) Bonnet, F.; Violante, C. D. C.; Roussel, P.; Mortreux, A.; Visseaux, M. Chem. Commun. 2009, 3380−3382. (21) Visseaux, M.; Terrier, M.; Mortreux, A.; Roussel, P. C. R. Chim. 2007, 10, 1195−1199. (22) Barbier-Baudry, D.; Blacque, O.; Hafid, A.; Nyassi, A.; Sitzmann, H.; Visseaux, M. Eur. J. Inorg. Chem. 2000, 2333−2336. (23) Palard, I.; Soum, A.; Guillaume, S. M. Chem.Eur. J. 2004, 10, 4054−4062. (24) Visseaux, M.; Chenal, T.; Roussel, P.; Mortreux, A. J. Organomet. Chem. 2006, 691, 86−92. (25) Yuan, F.; Yang, J.; Xiong, L. J. Organomet. Chem. 2006, 691, 2534−2539. (26) Mahrova, T. V.; Fukin, G. K.; Cherkasov, A. V.; Trifonov, A. A.; Ajellal, N.; Carpentier, J.-F. Inorg. Chem. 2009, 48, 4258−4266. (27) Skvortsov, G. G.; Yakovenko, M. V.; Castro, P. M.; Fukin, G. K.; Cherkasov, A. V.; Carpentier, J.-F.; Trifonov, A. A. Eur. J. Inorg. Chem. 2007, 3260−3267. (28) Yuan, F.; Zhu, Y.; Xiong, L. J. Organomet. Chem. 2006, 691, 3377−3382. (29) Skvortsov, G. G.; Yakovenko, M. V.; Fukin, G. K.; Cherkasov, A. V.; Trifonov, A. A. Russ. Chem. Bull. 2007, 56, 1742−1748. (30) Cendrowski-Guillaume, S.; Nierlich, M.; Lance, M.; Ephritikhine, M. Organometallics 1998, 17, 786−788. (31) Barbier-Baudry, D.; Bouyer, F.; Madureira Bruno, A. S.; Visseaux, M. Appl. Organomet. Chem. 2006, 20, 24−31. (32) Jian, Z.; Zhao, W.; Liu, X.; Chen, X.; Tang, T.; Cui, D. Dalton Trans. 2010, 39, 6871−6876. (33) Ephritikhine, M. Chem. Rev. 1997, 97, 2193−2242. (34) Xu, Z.; Lin, Z. Coord. Chem. Rev. 1996, 156, 139−162. (35) Jenter, J.; Roesky, P. W.; Ajellal, N.; Guillaume, S. M.; Susperregui, N.; Maron, L. Chem.Eur. J. 2010, 16, 4629−4638. (36) Bonnet, F.; Hillier, A. C.; Collins, A.; Dubberley, S. R.; Mountford, P. Dalton Trans. 2005, 421−423. (37) Dyer, H. E.; Huijser, S.; Susperregui, N.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford, P. Organometallics 2010, 29, 3602−3621. (38) Sinenkov, M. A.; Fukin, G. K.; Cherkasov, A. V.; Ajellal, N.; Roisnel, T.; Kerton, F. M.; Carpentier, J.-F.; Trifonov, A. A. New J. Chem. 2011, 35, 204−212. (39) Lim, L.-T.; Cink, K.; Vanyo, T. In Poly(Lactic Acid): Synthesis, Structures, Properties, Processing, and Applications; Auras, R., Lim, L.-T., Selke, S. E. M., Tsuji, H., Eds.; John Wiley & Sons, Inc.: New York, 2010; pp 189−215. (40) Becker, J. M.; Pounder, R. J.; Dove, A. P. Macromol. Rapid Commun. 2010, 31, 1923−1937. (41) Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 1078−1085. (42) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147−6176. (43) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12, 1841−1846. (44) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Chem. Rev. 1999, 99, 3181−3198. (45) Dijkstra, P. J.; Du, H.; Feijen, J. Polym. Chem. 2011, 2, 520−527. (46) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486−494. (47) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem.Eur. J. 2007, 13, 4433−4451. (48) Nakano, K.; Kosaka, N.; Hiyama, T.; Nozaki, K. Dalton Trans. 2003, 4039−4050. (49) Makhaev, V. D. Russ. Chem. Commun. 2000, 69, 727−746. (50) Marks, T. J.; Kolb, J. R. Chem. Rev. 1977, 77, 263−293. (51) Nakayama, Y.; Okuda, S.; Yasuda, H.; Shiono, T. React. Funct. Polym. 2007, 67, 798−806.

ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for the structure determinations of 1−6, monitoring of ROP reaction promoted by 6, and 1H NMR spectrum of the α,ω-dihydroxytelechelic PLA are available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.K. thanks the Landesgraduiertenförderung for support. P.W.R. and M.K. thank the Helmholtz Research School: Energy-Related Catalysis for financial support. M.K. thanks the Karlsruhe House of Young Scientists (KHYS). M.S. thanks the Cusanuswerk for support. P.O.-B. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. SG thanks the CNRS for financial support.

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REFERENCES

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Organometallics

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dx.doi.org/10.1021/om301011v | Organometallics XXXX, XXX, XXX−XXX