Chiral Mono(borohydride) Complexes of Scandium and Lutetium and

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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Chiral Mono(borohydride) Complexes of Scandium and Lutetium and Their Catalytic Activity in Ring-Opening Polymerization of DLLactide Tim P. Seifert,† Tobias S. Brunner,† Tobias S. Fischer,‡,§ Christopher Barner-Kowollik,‡,§,∥ and Peter W. Roesky*,† †

Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, 76131 Karlsruhe, Germany Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstraße 18, 76128 Karlsruhe, Germany § Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia ‡

S Supporting Information *

ABSTRACT: The enantiopure mono(borohydride) rare-earth complexes [Ln{(S)-PETA)}2(BH4)] (Ln = Sc (1), Lu (2); PETA = N,N′-bis((S)-1-phenylethyl)-tert-butylamidinate) are reported. The synthesis was achieved by salt metathesis reactions of the homoleoptic tris(borohydrides) [Ln(BH4)3(thf)n] and (S)-LiPETA. Complexes 1 and 2 were fully characterized, including single-crystal X-ray diffraction. Compounds 1 and 2 were evaluated as initiators for the ring-opening polymerization (ROP) of racemic lactide. Both 1 and 2 were found to be active in the ROP of DL-lactide. Polymerizations carried out in toluene show a higher activity for 2 but a lower selectivity in comparison to the scandium derivative.

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INTRODUCTION

(borohydride) complexes and their application as catalysts in the ring-opening polymerization of rac-lactide.29 Complexes of scandium, lanthanum, samarium, and lutetium were found to be active in the polymerization of rac-lactide; however they did not have a significant effect on the tacticity of the resulting polylactic acid (PLA).29 In general, PLA is attracting strong interest from both industrial and academic groups for several reasons. First of all, its precursor (lactic acid) is obtained from renewable sources, thus providing a “green” alternative to petroleum-based materials. Furthermore, these polymers are biodegradable and biocompatible, as they get hydrolyzed to lactic acid and metabolized to carbon dioxide and water.30 PLA is synthesized by ring-opening polymerization of lactide (LA). The polymer’s molecular mass and its tacticity strongly affect the chemical and physical properties of PLA, including its melting point, mechanical strength, degradation rate, and rheological properties.31−33 For example, atactic PLA is amorphous and has a

Amidinates play an important role in lanthanide chemistry and have been widely employed as spectator ligands since the pioneering work of Edelmann et al. in the 1990s.1,2 Amidinates [RC(NR′)2]− represent a class of N-chelating ligands that form stable complexes with almost every metal in the periodic table.3−8 The substituents R and R′ can be readily varied in numerous ways, which directly affects the steric and electronic properties of the ligand. In homogeneous catalysis rare-earthmetal amidinate complexes have been used for the polymerization of ethene9 and isoprene,10,11 ring-opening polymerization of polar monomers, e.g. ε-caprolactone and trimethylene carbonate, hydroboration, hydrosilylation, and intramolecular hydroamination/cyclization12 reactions.12−14 Recently, chiral amidinates, which are far less common, caught our attention.15−23 Enantiopure metal complexes derived from chiral amidinates are promising catalysts for asymmetric transformations, such as intramolecular hydroamination reactions.1,24,25 We reported the synthesis of the first rare-earthmetal complexes ligated by a chiral amidinate.26−28 Using N,N′bis(1-phenylethyl)benzamidinate as a ligand, a series of mono-, bis-, and tris(amidinate) complexes are accessible.27,28 We also disclosed the synthesis of chiral mono(amidinate) bis© XXXX American Chemical Society

Special Issue: Organometallic Complexes of Electrophilic Elements for Selective Synthesis Received: March 22, 2018

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DOI: 10.1021/acs.organomet.8b00172 Organometallics XXXX, XXX, XXX−XXX

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Organometallics glass transition temperature (Tg) of close to 60 °C and heterotactic PLA can be available as a semicrystalline polymer with a melting temperature (Tm) of 120 °C. Isotactic PLA is crystalline and exhibits a melting point of Tm = 170 °C.31 Hence, research on new initiators with high activity and control of stereochemistry or tacticity starting from DL-lactide (raclactide) is being intensely pursued.34−39 So far, however, only a few initiators are able to produce isotactic polymers, including aluminum and yttrium alkoxides.40−46 Regarding borohydride complexes, their hydridic character makes them promising initiators for the polymerization of polar monomers.35,47−50 Such rare-earth complexes proved to be efficient catalysts in the ROP of cyclic esters, such as lactides and lactones.51−54 Herein, we report the synthesis and characterization of novel enantiopure bis(amidinate) mono(borohydride) complexes of scandium and lutetium. Their activity as initiators in ROP of DL-lactide was investigated with varied solvents, initiator/ monomer ratios, temperatures, and reaction times. As a chiral ligand we used the enantiopure amidine (S,S)-N,N′-bis(1phenylethyl)pivalamidine ((S)-HPETA). (S)-HPETA and the corresponding lithium salt (S)-LiPETA are accessible in facile procedures (Scheme 1). Using these species, a series of metal

(Scheme 2) in refluxing THF. Reactions in toluene were not successful. Single crystals of complexes 1 and 2 were obtained by recrystallization from hot toluene in moderate yields. In the solid state, compounds 1 and 2 crystallize isostructurally in the orthorhombic space group P212121 with four molecules in the unit cell (Figure 1). Each metal center is 7-fold coordinated by

Figure 1. Molecular structure of 1 in the solid state. Carbon-bound hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for compound 1 and isostructural 2: 1, Sc−N1 2.152(2), Sc−N2 2.198(2), Sc−N3 2.207(2), Sc−N4 2.155(2), Sc−B 2.367(4), N1−C1 1.359(4), N2−C1 1.332(4), C1−C2 1.568(4), N1−C6 1.464(3), N2−C14 1.463(4), N1−Sc−N2 61.15(9), N3−Sc−N4 60.90(9), N1−C1−N2 110.7(3), N3−C22−N4 110.6(2), C6−N1− Sc 137.30(18), C14- N2−Sc 137.9(2); 2, Lu−N1 2.288(5), Lu−N2 2.260(4), Lu−N3 2.293(5), Lu−N4 2.257(5), Lu−B 2.451(8), N1− C1 1.334(8), N2−C1 1.350(7), C1−C2 1.558(9), N1−C6 1.473(7), N2−C14 1.475(7), N1−Lu−N2 58.0(2), N3−Lu−N4 58.2(2), N1− C1−N2 110.4(5), N3−C22−N4 110.4(5), C6−N1−Lu 135.3(4), C14−N2−Lu 136.5(4).

Scheme 1. Synthesis of Lithium Salt (S)-LiPETA55

two amidinate ligands and three hydrogen atoms of the BH4− moiety. The H atoms of the BH4 groups were localized in the difference Fourier map and freely refined. In contrast to many dimeric bis(amidinate) chloro rare-earth complexes,55 the borohydride derivatives show a monomeric molecular structure in the solid state. This observation is consistent with the general trend of borohydrides to form monomers. The metal−boron bond lengths are in the expected range.58−62 Furthermore, the Lu−B bond distance (2.451(8) Å) is shorter than the Lu−Cl distance (2.698(2) Å) in the analogous chloro derivative [Lu{(S)-PETA}2(μ-Cl)]2. However, the Sc−B bond distance (2.367(4) Å) in 1 and the Sc−Cl bond distance in [Sc{(S)PETA}2Cl] (2.370(2) Å) are nearly equal. The amidinate ligands in complexes 1 and 2 are symmetrically bound to the metal center and are twisted against each other by an angle of 66°. As expected, due to the smaller ionic radius of Sc3+,63,64 the Sc−N bond lengths (2.18 Å in average) are shorter than the corresponding Lu−N distances (2.27 Å in average). This distinction also applies for the metal−boron distances (1, 2.367(4) Å; 2, 2.451(8) Å). The amidinate bite angles N1−C1−N2 (1, 110.7(3)°; 2, 110.5(5)°) are within the same range in both 1 and 2. In the 1H{11B}, 13C{1H}, and 11B

complexes including bis(amidinate) halogenidolanthanide, bis(amidinate) amidolanthanide, and bis(amidinate) alkyllanthanide complexes were disclosed by some of us.55 The rare-earth complexes were employed as catalysts in enantioselective intramolecular hydroamination reactions of nonactivated terminal amino olefins.55 It was shown by us that the substituent in the backbone of the amidinate has an influence on the bite angle of the ligand and thus also influences the catalytic activity.55

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RESULTS AND DISCUSSION (S)-LiPETA was utilized to synthesize novel enantiopure mono(borohydrido) complexes of the rare-earth metals scandium and lutetium. Typically, the homoleptic tris(borohydrides) of the rare-earth elements [Ln(BH4)3(thf)n] are used as starting materials, due to the pseudohalide character of the BH4− anion.29,36,51,56,57 Following this concept, we obtained the derivatives [Sc{(S)-PETA)}2(BH4)] (1) and [Lu{(S)-PETA)}2(BH4)] (2) by salt metathesis reactions from (S)-LiPETA and [Sc(BH4)3(thf)2] or [Lu(BH4)3(thf)3]

Scheme 2. Synthesis of the Monoborohydride Complexes 1 and 2

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izations, the reaction conditions for both catalysts were not changed equally. For the less active catalyst 1, initiator concentrations were varied between 1/200 and 1/300, the temperature was changed between 60 and 80 °C, and the reaction times were increased from 30 to 60 min. The numberaverage molar mass values were determined by size exclusion chromatography in THF using PS standards for calibration. Experimentally determined Mn values were corrected by a factor of 0.58, taking into account the difference in hydrodynamic radii between PLA and polystyrene.65,66 In all cases, the determined molecular masses (Mn,SEC) of the polymer were lower than the expected values (Mn,theo), suggesting a significant degree of chain-transfer reactions, as observed for other related rare-earth borohydride catalysts.29,36 However, for experiments with low conversions (Table 1, entry 1), the theoretical value is in close agreement with the experimental value. The resulting polymers show relatively narrow dispersity in the range of Đ = 1.34−1.43, which are comparable with values obtained from tris(borohydride) rare-earth complexes and linked bis(amide) bis(borohydride) and mono(amidinate) bis(borohydride) complexes of the rare-earth elements.29,44,52 For the more active lutetium catalyst 2, the initiator/monomer ratio was subsequently decreased to 1/500. Even with large monomer loadings, high conversions could be achieved in short reaction times. However, the experimentally determined molecular masses are significantly lower than the expected theoretical molecular masses. Note that polymerization of lactide in toluene should be performed at temperatures higher than 40 °C, due to insufficient monomer solubility, generating a lower starting concentration [LA]0 and, thus, a lower initiator/ monomer ratio (Table 1, entry 8). For catalyst 2, the polymer dispersity indices (Đ) increased to higher values of 1.52−1.75, suggesting, to some extent, transesterification reactions. Surprisingly, experiments carried out in THF instead of toluene as solvent show large inconsistencies in molar masses. Although polymer formation was detected in the 1H NMR, no PLA was detected in the SEC for catalyst 1. For catalyst 2, molar masses do not exceed 2200 Da, even for elongated reaction times and significant conversion. In the 1H NMR spectra of relatively low molar mass polymers (Table 1, entry 9), the characteristic quartet resonance at δ 4.35 ppm for the −CH(CH3)OH end group was detected in addition to the polymeric backbone multiplet

NMR spectra, the expected resonances are present. For both compounds, only one set of resonances for the amidinate ligands is present, indicating a symmetric coordination in solution. However, the chemical shift of the BH4 moiety in the 1 H{11B} NMR is dependent on the rare-earth metal. The BH4 resonance in compound 2 (2.60 ppm) is shifted downfield in comparison to 1 (1.51 ppm). In the 11B NMR, both complexes show a quintet resonance (1, −22.85 ppm, 1JB,H = 82 Hz; 2, −23.57 ppm, 1JB,H = 79 Hz) due to a coupling to the adjacent hydrogen atoms. Ring-Opening Polymerization of DL-Lactide. Complexes 1 and 2, both featuring a potential initiating borohydride function, have been investigated in the ROP of DL-lactide (see Scheme 3). The polymerization conditions and results are Scheme 3. Synthesis of Polylactide (PLA): ROP of DLLactide Promoted by [Sc{(S)-PETA)}2(BH4)] (1) and [Lu{(S)-PETA)}2(BH4)] (2)

summarized in Table 1. Rare-earth borohydride catalyzed ringopening polymerization is typically carried out in either THF or toluene.29,36 These solvents differ not only in polarity but also in their ability to coordinate to a metal center. Thus, by the use of toluene, a competition of solvent against monomer coordination can be avoided. This allows for a faster polymerization rate of the ROP. Complexes 1 and 2 both show an activity in the polymerization of DL-lactide under mild conditions (refer to Table 1 for the conditions). The polymerization studies were started with a moderate initiator/monomer ratio of 1/200 for both [Sc{(S)-PETA)}2(BH4)] (1) and [Lu{(S)-PETA)}2(BH4)] (2) in toluene (Table 1, entries 1 and 6, and Figure 2). Under comparable reaction conditions, 2 was found to generate significantly higher monomer conversions. Thus, for the subsequent polymerTable 1. Ring-Opening Polymerization of

DL-Lactide

Promoted by Complexes 1 and 2

entry

solvent

catalyst

[LA]/[BH4]

T (°C)

time (min)

conversnb (%)

Mn,theoc (g/mol)

Mn,SECd (g/mol)

Pre

Đd

1 2 3 4 5 6 7 8 9 10 11

toluene toluene toluene toluene THF toluene toluene toluene toluene THF THF

1 1 1 1 1 2 2 2 2 2 2

200 200 300 300 200 200 300 300 500 200 200

60 60 60 80 60 60 60 40g 60 60 60

30 60 60 60 30 30 30 30 30 30 180

42.3 71.3 58.7 90.0 6.9 85.6 96.1 29.5 83.3 13.7 60.2

12200 20500 25400 38900 2000 24700 41500 12700 60000 3900 17300

10600 11400 14300 19000 n.d.f 21500 29600 18500g 5200 2000 2200

0.64 0.62 0.63 0.60

1.34 1.43 1.41 1.43 n.d.f 1.64 1.73 1.52 1.75 1.21 1.52

a

0.53 0.56 0.60 0.55 0.53 0.53

a

All reactions were carried out with a starting concentration of DL-lactide of [LA]0 = 0.5 M; the results are representative of duplicated experiments. Conversion was determined by integration of the methine resonances in the 1H NMR. cTheoretical molar mass values were calculated according to ([LA]/[BH4]) × conversion × MLA. dNumber-average molar mass values and dispersities were determined by SEC in THF (35 °C, 1 mL min−1) against polystyrene standards corrected by a factor of 0.58.65,66 eProbability of racemic enchainments was determined by homodecoupled 1H NMR. f Mn,SEC and Đ could not be determined. gDL-Lactide is only partly soluble in toluene at 40 °C; [LA]0 < 0.5 M. b

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Figure 2. SEC traces of PLAs obtained from catalyst 1 (left; Table 1, entries 1−4) and 2 (right; Table 1, entries 6−9) in toluene.

resonances (−C(O)CH(CH3)O− at δ 5.14−5.32 ppm and the −C(O)CH(CH3)O− multiplet at δ 1.54−1.66 ppm). The −CH(CH3)OH end group is formed upon hydrolysis of the active rare-earth−alkoxide bond. Earlier studies found out that, in the ROP of LA promoted by rare-earth borohydrides, each BH4− group acts both as an initiating function and as a reducing agent, producing α,ω-dihydroxy telechelic PLAs.29,44,67 In our case, a second alcohol function at the other chain end could not be verified unambiguously. Additional resonances were observed, including δ 2.67 and 3.82 ppm, which could correspond to a −CH2OH end group.17 However, their low intensity and diversity suggest incomplete and/or multiple processes.53,57,67 Complexes 1 and 2 are enantiopure complexes, since the ligand (S)-PETA itself exhibits two stereocenters in S configuration. Thus, chirality can potentially be induced to the polymer chain, resulting in changes in the tacticity. Polylactic acids are biocompatible and biodegradable polymers. Their mechanical and physical properties (such as melting point and crystallinity) are determined by their chain length and tacticity.68 Thus, controlling these parameters is of specific interest for potential medical, agricultural, and packaging applications.69−72 In the field of lactide ROP, the degree of stereoregularity is expressed as the probability of racemic or meso enchainment and thus the probability of forming a new racemic (syndiotactic) or meso (isotactic) diad: Pr and Pm, respectively. For DL-lactide, Pr or Pm = 0.50 describes a completely atactic polymer, whereas Pr = 1.00 (Pm = 0.00) or Pm = 1.00 (Pr = 0.00) describe perfect syndiotactic and isotactic polymers, respectively.31 These parameters can be calculated directly from homodecoupled 1H NMR (see the Supporting Information).73,74 As demonstrated by the calculated Pr values (see Table 1), the polymers resulting from catalysis with complex 1 feature a slight syndiotactic enrichment, while catalyst 2 barely imparts any stereocontrol in the ROP of DLlactide.

to be active in the ROP of DL-lactide under mild conditions. The catalysts generated relatively narrow dispersities with low initiator/monomer ratios. The lutetium compound was observed to be the more active catalyst, while the scandium derivative is more selective in terms of polymer dispersity and chiral induction.

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EXPERIMENTAL SECTION

All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in Schlenk-type glassware, either on a dual-manifold Schlenk line interfaced to a high-vacuum (10−3 mbar) line or in an argon-filled MBraun glovebox. THF was distilled under a nitrogen atmosphere from potassium and benzophenone prior to use. Hydrocarbon solvents (toluene, npentane, and n-heptane) were dried using a MBraun solvent purification system (SPS-800). All solvents for vacuum-line manipulations were stored in vacuo over LiAlH4 in resealable flasks. Deuterated solvents were obtained from Aldrich GmbH (99 atom % D) and were degassed, dried, and stored in vacuo over Na/K alloy in resealable flasks. NMR spectra were recorded on Bruker Avance II 300 or 400 MHz NMR and Ascend 400 MHz FT-NMR spectrometers. Chemical shifts are referenced to internal solvent resonances and are reported relative to tetramethylsilane. IR spectra were obtained on a Bruker Tensor 37 instrument. EI mass spectra were recorded at 70 eV on a Finnigan MAT 8200 instrument. ESI mass spectra were recorded on a LTQ Orbitrap XL Q Exactive mass spectrometer (Thermo Fisher Scientific, San Jose, CA, USA) equipped with an HESI II probe. The instrument was calibrated in the m/z range 74−1822 using premixed calibration solutions (Thermo Scientific). A constant spray voltage of 4.6 kV, a dimensionless sheath gas of 8, and a dimensionless auxiliary gas flow rate of 2 were applied. The capillary temperature and the Slens rf level were set to 320 °C and 62.0, respectively. Elemental analysis was performed on an Elementar vario EL or microcube instrument. SEC was performed to obtain the dispersity of the synthesized polymers. The employed system was a PL-SEC 50 Plus instrument (Polymer Laboratories) running on tetrahydrofuran (THF, HPLC grade) featuring an autosampler and a PLgel Mixed C guard column (50 × 7.5 mm), followed by three PLgel Mixed C linear columns (300 × 7.5 mm, 5 mm bead size) and a differential refractive index (RI) detector. The device was operated at 35 °C column temperature with a flow rate of 1 mL min−1. The calibration was carried out with linear polystyrene standards ranging from 476 to 2.5 × 106 g mol−1. The injected samples were dissolved in THF (2 mg mL−1) and filtered through a 0.2 mm filter. DL-Lactide was purchased from Sigma-Aldrich, recrystallized from dry toluene, and sublimed three times prior to use. (S)-LiPETA55 and [Ln(BH4)3(thf)n]56 were prepared according to literature procedures. Synthesis of [Sc{(S)-PETA)}2(BH4)] (1).75 THF (10 mL) was condensed on a mixture of Sc(BH4)3(thf)2 (104 mg, 0.445 mmol, 1.00 equiv) and (S)-LiPETA (280 mg, 0.889 mmol, 2.00 equiv) and stirred at 65 °C for 18 h. All volatiles were removed under reduced pressure.

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SUMMARY In summary, the novel bis(amidinate) mono(borohydride) complexes [Sc{(S)-PETA)}2(BH4)] (1) and [Lu{(S)-PETA)}2(BH4)] (2) were reported. The synthesis was achieved through salt metathesis reactions of the homoleptic tris(borohydride) rare-earth complexes [Ln(BH4)3(thf)n] (Ln = Sc, Lu) with (S)-LiPETA. The compounds were fully characterized, including 1H NMR, 13C{1H} NMR, 11B NMR, and single-crystal X-ray diffraction. Both complexes were found D

DOI: 10.1021/acs.organomet.8b00172 Organometallics XXXX, XXX, XXX−XXX

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Organometallics n-Pentane (10 mL) was condensed onto the residue, and the mixture was stirred for 10 min and the solvent subsequently removed under reduced pressure. This washing step was repeated two times to remove the coordinating THF molecules in the metathesis salt Li(thf)n(BH4), leaving the insoluble LiBH4. The residue was extracted with toluene (10 mL) and concentrated under reduced pressure until the formation of a colorless solid. Recrystallization from hot toluene yielded in single-crystalline 1 (120 mg, 40%). 1 H{11B} NMR (300 MHz, C6D6, 298 K): δ (ppm) 7.46 (d, 3JH,H = 7.5 Hz, 8 H, o-Ph), 7.26 (t, 3JH,H = 7.5 Hz, 8 H, m-Ph), 7.09 (t, 3JH,H = 7.5 Hz, 4 H, p-Ph), 4.82 (q, 3JH,H = 6.6 Hz, 4 H, CH), 1.64 (d, 3JH,H = 6.6 Hz, 12 H, CH3), 1.51 (br s, 4 H, BH4), 1.08 (s, 18 H, C(CH3)3). 13 C{1H} NMR (75 MHz, C6D6, 298 K): δ (ppm) 186.6 (NCN), 148.1 (i-Ph), 128.5 (Ph), 127.0 (Ph), 126.4 (Ph), 55.8 (CH), 41.1 (C(CH3)3), 30.3 (C(CH3)3), 27.4 (CH3). 11B NMR (96 MHz, C6D6, 298 K): δ (ppm) −22.9 (qt, 1JB,H = 82 Hz). ESI-MS (positive): m/z [Sc{(S)-PETA)}2(THF)]+ calcd 731.4483, found 731.4495 (59%). IR (ATR): ν̃ (cm−1) 3084 (w), 3060 (w), 3026 (w), 2962 (m), 2928 (w), 2868 (w), 2815 (w), 2488 (w), 2362 (w), 2334 (w), 2282 (w), 1949 (w), 1873 (w), 1805 (w), 1729 (w), 1662 (w), 1633 (w), 1601 (w), 1492 (w), 1477 (w), 1448 (w), 1430 (w), 1400 (w), 1362 (w), 1310 (w), 1176 (w), 1155 (w), 1074 (w), 1027 (w), 967 (w), 943 (w), 909 (w), 840 (w), 758 (m), 698 (s), 670 (w), 612 (w), 588 (w), 536 (w), 509 (w), 477 (w), 416 (w). Anal. Calcd. for C42H58BN4Sc (674.704 g/mol): C, 74.77; H, 8.66; N, 8.30. Found: C, 73.25; H, 8.52; N, 8.17. Unfortunately, the elemental analyses frequently show low carbon values. Synthesis of [Lu{(S)-PETA)}2(BH4)] (2).75 THF (10 mL) was condensed on a mixture of Lu(BH4)3(thf)3 (162 mg, 0.373 mmol, 1.00 equiv) and (S)-LiPETA (234 mg, 0.746 mmol, 2.00 equiv) and stirred at 65 °C for 18 h. All volatiles were removed under reduced pressure. n-Pentane (10 mL) was condensed onto the residue, stirred for 10 min, and subsequently removed under reduced pressure. This washing step was repeated two times to remove the coordinating THF molecules in the metathesis salt Li(thf)n(BH4), leaving the insoluble LiBH4. The residue was extracted with toluene (10 mL) and concentrated under reduced pressure until the formation of a colorless solid. Recrystallization from hot toluene yielded in single-crystalline 2 (95.0 mg, 32%). 1 H NMR (300 MHz, C6D6, 298 K): δ (ppm) 7.44 (d, 3JH,H = 7.5 Hz, 8 H, o-Ph), 7.26 (t, 3JH,H = 7.5 Hz, 8 H, m-Ph), 7.09 (t, 3JH,H = 7.5 Hz, 4 H, p-Ph), 5.00 (q, 3JH,H = 6.6 Hz, 4 H, CH), 2.60 (br s, 4 H, BH4), 1.51 (d, 3JH,H = 6.6 Hz, 12 H, CH3), 1.12 (s, 18 H, C(CH3)3). 13 C{1H} NMR (75 MHz, C6D6, 298 K): δ (ppm) 184.9 (NCN), 148.4 (i-Ph), 128.6 (Ph), 126.9 (Ph), 126.4 (Ph), 5.2 (CH), 41.8 (C(CH3)3), 30.4 (C(CH3)3), 27.8 (CH3). 11B NMR (96 MHz, C6D6, 298 K): δ (ppm) −23.6 (qt, 1JB,H = 79 Hz). EI-MS (70 eV): m/z [M]+ calcd 804.4162, found 804.4302 (3%); [M − PhEt]+ calcd 699.34580, found 699.3360 (40%); [M − PETA]+ calcd 497.19880, found 497.2031 (100%); [M − BH4]+ calcd 789.37562, found 789.3758 (7%). IR (ATR): ν̃ (cm−1) 3060 (w), 3027 (w), 2964 (m), 2926 (w), 2868 (w), 2815 (w), 2486 (w), 2361 (w), 2332 (w), 2249 (w), 1948 (w), 1874 (w), 1805 (w), 1751 (w), 1658 (w), 1632 (w), 1601 (w), 1585 (w), 1491 (w), 1448 (w), 1401 (w), 1363 (w), 1305 (w), 1252 (w), 1185 (w), 1156 (w), 1069 (w), 1025 (w), 907 (w), 841 (w), 756 (m), 697 (s), 670 (w), 610 (w), 586 (w), 537 (w). Anal. Calcd for C42H58BN4Lu (804.715 g/mol): C, 62.69; H, 7.26; N, 6.96. Found: C, 61.67; H, 6.89; N, 6.73. Unfortunately, the elemental analyses frequently show low carbon values. Typical Procedure for ROP of DL-Lactide. In the glovebox, a Schlenk tube was charged with DL-lactide (425 mg, 3.73 mmol, 300 equiv) and the rare-earth borohydride complex (for 2: 10.0 mg, 0.012 mmol, 1.00 equiv). Toluene or THF (5.90 mL) was added, producing a starting concentration of DL-lactide of 0.5 M. The flask was subsequently placed in a preheated oil bath at the appropriate temperature (e.g., 60 °C) and stirred for the desired reaction time. The reaction was quenched afterward with excess acetic acid (saturated solution of acetic acid in toluene). The solvent was evaporated under reduced pressure, and the crude material was analyzed by 1H NMR spectroscopy in CDCl3 to determine the conversion. The crude

polymer was then dissolved in CH2Cl2 , purified upon precipitation into cold pentane, filtered, and dried under vacuum. The final polymer was subsequently 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 homonunclear CH3-decoupled 1H NMR spectrum of PLA at 20 °C in CDCl3. X-ray Crystallographic Studies of 1 and 2. A suitable crystal was covered in mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to a−173 °C cold stream of a STOE STADIVARI diffractometer. All structures were solved by using the program SHELXS/T76,77 using Olex2.78 The remaining non-hydrogen atoms were located from successive difference Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F2 by using the program SHELXL.76,77 In each case, the locations of the largest peaks in the final difference Fourier map calculations and the magnitude of the residual electron densities were of no chemical significance. Positional parameters, hydrogen atom parameters, thermal parameters, and bond distances and angles have been deposited as Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. 1831795−1831796. Crystal data for 1: C42H58BN4Sc, Mr = 674.69, orthorhombic, P212121 (No. 19), a = 12.357(3) Å, b = 16.691(3) Å, c = 18.862(4) Å, α = β = γ = 90°, V = 3890.1(14) Å3, T = 100 K, Z = 4, Z′ = 1, μ(Mo Kα) = 0.22, 16792 reflections measured, 7652 unique reflections (Rint = 0.0539) which were used in all calculations. The final wR2 value was 0.0660 (all data) and R1 value was 0.0360 (I > 2σ(I)). Crystal data for 2: C42H58BLuN4, Mr = 804.70, orthorhombic, P212121 (No. 19), a = 12.3938(3) Å, b = 16.8392(5) Å, c = 18.8514(5) Å, α = β = γ = 90°, V = 3934.32(18) Å3, T = 100 K, Z = 4, Z′ = 1, μ(Mo Kα) = 2.55, 18021 reflections measured, 7730 unique reflections (Rint = 0.0408) which were used in all calculations. The final wR2 value was 0.0645 (all data) and R1 value was 0.0278 (I > 2σ(I)).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00172. Details of the syntheses and IR, NMR, MS, and XRD data (PDF) Accession Codes

CCDC 1831795−1831796 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.W.R.: [email protected]. ORCID

Christopher Barner-Kowollik: 0000-0002-6745-0570 Peter W. Roesky: 0000-0002-0915-3893 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.S.B. thanks the Helmholtz Research School Energy-Related Catalysis (VH-KO-403) and the Fonds der Chemischen E

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Industrie (190481) for support. Ms. S. Schneider and Dr. M. T. Gamer are acknowledged for help in collecting and refining single-crystal X-ray diffraction data. T.P.S. thanks the Karlsruhe House of Young Scientists (KHYS) for a Research Travel Grant Scholarship. T.S.F., C.B.-K., and P.W.R. acknowledge support from the SFB 1176, Project A1 and A2, funded by the German Research Council (DFG), and C.B-K. acknowledges continued support by the Queensland University of Technology (QUT).

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