β-Diketiminate Rare Earth Borohydride Complexes - ACS Publications

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β‑Diketiminate Rare Earth Borohydride Complexes: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of ε‑Caprolactone and Trimethylene Carbonate Matthias Schmid,†,‡,§ Sophie M. Guillaume,*,§ and Peter W. Roesky*,† †

Institut für Anorganische Chemie, Karlsruher Institut für Technologie (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany Institut für Ressourcenökologie, Helmholtz-Zentrum Dresden-Rossendorf e. V., Bautzner Landstraße 400, 01328 Dresden, Germany § Institut des Sciences Chimiques de Rennes, CNRS - Université de Rennes 1 (UMR 6226), Organometallics: Materials and Catalysis, Campus de Beaulieu, 35042 Rennes Cedex, France ‡

S Supporting Information *

ABSTRACT: The synthesis of a series of divalent and trivalent β-diketiminate borohydrides [(dipp)2NacNacLn(BH4)(THF)2] ((dipp)2NacNac = (2,6-C6H3iPr2)NC(Me)CHC(Me)N(2,6-C6H3iPr2); Ln = Sm, Eu, Yb) and [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sc, Sm, Dy, Yb, Lu) is reported. All compounds were obtained by salt metathesis in THF from [(dipp)2NacNacK] and the corresponding homoleptic divalent and trivalent borohydrides [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb), [Sc(BH4)3(THF)2], and [Ln(BH4)3(THF)3] (Ln = Sm, Dy, Yb, Lu), respectively. The complexes were fully characterized, and their solid-state structures were established by single-crystal X-ray diffraction. In both the divalent and trivalent compounds, the BH4− groups coordinate in a κ3(H) mode to the metal. Only in the lutetium complex [(dipp)2NacNacLu(BH4)2(THF)] does one BH4− group coordinate in a κ3(H) mode, whereas the other one coordinates as κ2(H). This kind of mixed κ2/κ3(H) coordination mode is rare. The application of the divalent and trivalent compounds as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC) was investigated. All complexes afforded a generally well-controlled ROP of both of these cyclic esters. High molar mass poly(ε-caprolactone) diols (Mn,NMR < 92 700 g mol−1, ĐM = 1.51) and α-hydroxy,ω-formate telechelic poly(trimethylene carbonate)s (Mn,NMR < 16 000 g mol−1, ĐM = 1.59) were thus synthesized under mild operating conditions.

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scandium, [Sc(BH4)3(THF)2],12,13 features two equivalents of THF coordinated to the center metal in the solid state. The divalent bisborohydride complexes [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb) were originally prepared in 1999 from [NaLn(BH4)4(DME)4] (Ln = Sm, Eu, Yb) by thermal reduction at 150−200 °C in vacuum.14 Only lately has a convenient access to the divalent lanthanide borohydrides [Ln(BH4)2(THF)2] (Ln = Eu, Sm, Yb)15,16 and [Tm(BH4)2(DME)2]17 been reported from the corresponding trisborohydride and Ln metal or from salt metathesis reactions of [EuI2(THF)2] or [TmI2(THF)3] with NaBH4 or KBH4.

INTRODUCTION

The trisborohydrides of the rare earth elements [Ln(BH4)3] (Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were initially prepared in 1960 by Egon Zange from the reaction of rare earth metal alkoxides [Ln(OCH3)3] with B2H6.1 A few years later, Rossmanith investigated [LnCl(BH4)2],2 [Ln(BH4)3],2 and the divalent europium(II)-bromide-boronate [EuBr(BH4)].3 Later on, Mirsaidov et al. reported in several publications a salt metathesis of LnCl3 and NaBH4 to give the solvates [Ln(BH4)3(THF)3].4−10 More recently, Guillaume et al. optimized the reaction conditions of their synthesis, in particular using a lower amount of NaBH4.11 Usually, these solvates are coordinated by three molecules of THF. Only the trisborohydride complex of the smallest rare earth element © XXXX American Chemical Society

Received: July 9, 2014

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Scheme 1. Synthesis of [(dipp)2NacNacLn(BH4)(THF)2]

both homoleptic as well as heteroleptic ones, either from the initial metallocene or from the more recent postmetallocene generations, mono-, bis-, or tris-borohydrides, divalent or trivalent as well as neutral or cationic complexes, presynthesized or prepared in situ, have been used.42,50 Herein, we report the synthesis and structural characterization of a series of the divalent and trivalent mono-βdiketiminate borohydrides [(dipp)2NacNacLn(BH4)(THF)2] ((dipp)2NacNac = (2,6-C6H3iPr2)NC(Me)CHC(Me)N(2,6C6H3iPr2); Ln = Sm, Eu, Yb) and [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sc, Sm, Dy, Yb, Lu). Moreover, the application of these compounds as initiators in the ROP of CL and TMC is reported.

A large number of rare earth borohydride derivatives have been prepared by salt metathesis starting from the trivalent borohydrides of the rare earth elements [Ln(BH4)3(THF)n] and alkali metal reagents of the corresponding ligand. In these reactions, the BH4− ligand(s) act(s) as (a) leaving group(s), forming MBH4 (M = Li, Na, K) coproduct. Thus, numerous borohydride derivatives of the rare earth elements with cyclopentadienyl11,18−27 and other ligands11,28−48 have been reported. The general aspects of the synthesis of rare earth borohydride complexes and their application as initiators in the polymerization of polar monomers have been reviewed recently.49,50 However, to the best of our knowledge, only few structurally characterized rare earth metal borohydride complexes with β-diketiminates are known.49,51−54 These are the heteroleptic β-diketiminate lanthanide compounds [(Cp*Pr){(p-Tol)NN}Ln(BH4)] (Cp*Pr = C5Me4(nPr); (pTol)NN = (p-Tol)-NC(Me)CHC(Me)N(p-tol); Ln = Nd, Sm).52 The complexes were prepared by metathesis reaction of their monocyclopentadienyl precursors [Cp* P r Sm(BH4)2(THF)] and [Cp*PrNd(BH4)2(THF)2] with {(p-Tol)NN}K, respectively. The corresponding triphenyl derivative [CpPh3Sm{(p-Tol)NN}(BH4)] (CpPh3 = H2C5Ph3-1,2,4) was obtained by a one-pot reaction from [Sm(BH4)3(THF)3], KCpPh3, and {(p-Tol)NN}K.53 The bisborohydride [(2,6C6H3Me2)NC(Me)CHC(Me)N(2,6-C6H3Me2)Y(BH4)2(THF)] was also synthesized by salt metathesis from the lithiated ligand and yttrium trisborohydride.44,54 Regarding β-diketiminate borohydride complexes, their limited number is rather surprising since β-diketiminates, often denoted as “NacNac”, are very popular ancillary ligands for most elements of the periodic table.55−63 Even in rare earth chemistry, β-diketiminates have been widely used for the synthesis of trivalent compounds.53,54,64−78 In contrast, divalent rare earth β-diketiminates have been neglected, and to our knowledge, only a few structurally characterized complexes have been reported.51,79−83 Polar monomers and in particular cyclic esters such as lactones (β-butyrolactone, δ-valerolactone, ε-caprolactone, pentadecalactone), lactides, or carbonates (essentially trimethylene carbonate (TMC)) have been successfully polymerized by rare earth complexes.50 Among these, εcaprolactone (CL), which is a monomer that is rather easily polymerized, appears as the monomer of choice to first evaluate the efficiency of a catalytic system. More recently, much attention has been given to polycarbonates prepared by ringopening polymerization (ROP) of cyclic carbonates. Indeed, the ubiquitous TMC can be derived from the biomass through glycerol, which is produced as a byproduct during the generation of diesel.84 Both of the corresponding polymers, poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC), respectively, are biocompatible and biodegradable and, thus, highly desirable especially for biomedical applications or as alternatives to olefin-derived plastics.85 Rare earth borohydride derivatives used in the ROP of CL and TMC,

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RESULTS AND DISCUSSION Divalent Compounds. Recently, we and others reported an efficient synthesis of the divalent rare earth borohydrides [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb).15,16 The samarium and the ytterbium compounds were prepared by a comproportionation reaction from [Ln(BH4)3(THF)3] and Ln metal. The analogous europium bisborohydride compound was isolated either in a reductive pathway from EuCl3 and NaBH4 in high yield or by a nonreductive salt metathesis from [EuI2(THF)2] and NaBH4.16 The β-diketimine (dipp)2NacNacH and its corresponding potassium salt [(dipp)2NacNacK], which was obtained from the reaction of (dipp)2NacNacH with KH, have b e e n r e p o r t e d ea r l i e r . 8 4 , 8 6 , 8 7 T h u s , r e a c t i o n o f [(dipp)2NacNacK] with [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb) in THF afforded, at room temperature after workup, the heteroleptic complexes [(dipp)2NacNacLn(BH4)(THF)2] (Ln = Sm (1), Eu (2), Yb (3)) as black, yellow, or orange crystals, respectively, in moderate yield (27−53%; Scheme 1). The new products were fully characterized by standard analytical/ spectroscopic techniques and in particular by their solid-state structures established by single-crystal X-ray diffraction. The NMR spectroscopic characterization, including 1H, 11B, 13 C{1H}, and 171Yb{1H} NMR spectra, was carried out focusing on the diamagnetic compound 3 (see Supporting Information). The 1H NMR spectrum shows the expected series of signals for the (dipp)2NacNac− ligand (see Supporting Information). The two sets of resonances observed for the CH3 isopropyl groups at δ 1.19 and 1.27 ppm indicate a hindered rotation of the 2,6di(isopropyl)phenyl ligands. Moreover, a broad quartet characteristic of the BH4− group is recorded at δ 0.65 ppm (JBH = 80.7 Hz). This value is comparable to that of the divalent compound [(dipp) 2 pyrYb(BH 4 )(THF) 3 ] (δ 0.36 ppm; (dipp)2pyr− = 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl)).88 In the 13C{1H} NMR spectrum the expected signals are observed. The 11B NMR spectrum of 3 shows the expected quintet of the BH4− substituent at δ −33.0 ppm, which is in agreement with that recorded for [Yb(BH4)2(THF)2] (δ −33.6 ppm).16 The 171Yb NMR signal of 3 (δ 753.5 ppm) is significantly downfield shifted compared to [(dipp2pyr)Yb(BH4)(THF)3] (δ 618.6 ppm)88 and especially B

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to [Yb(BH4)2(THF)2] (δ 319.2 ppm),16 thus highlighting the (dipp)2NacNac− influence on the metal center. The FT-IR spectra of complexes 1−3 unfortunately preclude an unambiguous assignment of the coordination mode of the BH4− group, which could however be gained by X-ray analysis (vide inf ra). Compounds 1−3 are isostructural in the solid state and crystallize in the triclinic space group P1̅ with two molecules of the corresponding complex in the unit cell (Figure 1). The

the difference Fourier map in any of the three compounds. The base of the pyramid is formed by the oxygen atoms of the two THF molecules and the two nitrogen atoms of the ligand. The distortion of the coordination polyhedron is a result of the rigid nature and the larger steric demand of the (dipp)2NacNac− ligand. Thus, the apex of the pyramid is bent toward the THF molecules. The B−Ln−N angles (B−Ln−N1 115.82(11)° (1), 115.0(2)° (2), 112.7(2)° (3) and B−Ln−N2 115.36(11)° (1), 114.4(2)° (2), 111.9(2)° (3)) are significantly larger than the B−Ln−O angles (B−Ln−O1 104.33(11)° (1), 106.5(2)° (2), 105.0(2)° (3) and B−Ln−O2 105.73(11)° (1), 104.3(2)° (2), 104.6(2)° (3)), respectively. The bite angle of the ligand N1− Ln−N2 is 75.22(8)° (1), 76.37(15)° (2), and 79.7(2)° (3). As expected, the bond lengths, which are within the expected range, mainly depend on the ionic radius of the center metal. Trivalent Compounds. The reaction of [(dipp)2NacNacK] with the homoleptic trisborohydride complexes [Sc(BH4)3(THF)2]12,13 and [Ln(BH4)3(THF)3]10,11 (Ln = Sm, Dy, Yb, Lu) in THF resulted in the formation of the monosubstituted compounds [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sc (4), Sm (5), Dy (6) Yb (7), Lu (8)) (Scheme 2). Rare earth metals with different ionic radii were selected to cover the whole range of these elements, thereby providing a size-variable coordination sphere around the metal center. Compounds 4−8 were characterized by standard analytical and spectroscopic techniques, and their solid structures were determined by single-crystal X-ray diffraction analyses. The 1H, 11B, and 13C{1H} NMR spectra were recorded for the diamagnetic compounds 4 and 8 as well as for the paramagnetic samarium complex 5 (refer to the Supporting Information). All these spectra indicate a symmetrical coordination of the ligand in solution. However, as observed for 2, a hindered rotation of the 2,6-di(isopropyl)phenyl groups is always recorded. This results in two sets of signals for the isopropyl groups in the 1H NMR spectra (δ 1.17 and 1.29 ppm (4); −0.14 and 1.03 ppm (5); 1.18 and 1.30 ppm (8)). Moreover, characteristic singlets of the methyl groups of the βdiketiminate backbone are observed (δ 1.86 ppm (4), 3.09 (5), 1.82 (8)). The 11B NMR spectra display, for the BH4− group, one quintet for the diamagnetic compounds or a broad signal in the samarium complex (δ −19.3 (4), −34.4 (5), −23.6 (8) ppm). The resonances of the diamagnetic compounds 4 and 8 are in the range of those recorded previously with the bis(phosphinimino)methanide bisborohydride complexes [{CH(PPh 2 NSiMe 3 ) 2 }La(BH 4 ) 2 (THF)] 40 and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu; δ −22.4 ppm (La), −24.7 ppm (Y), and −25.8 ppm (Lu)).40 As observed for 1−3, the FT-IR spectra of complexes 4−8 preclude an unambiguous assignment of the coordination mode of the BH4− group, otherwise assessed by X-ray diffraction analyses (vide inf ra). Both complexes of the smaller rare earth elements 4 and 8 crystallize in the triclinic space group P1̅ with one molecule in

Figure 1. Solid-state structure of 3, omitting hydrogen atoms. Selected bond lengths [Å] and angles [deg] are given for 3 and the isostructural complexes 1 and 2. 3: Yb−B 2.576(8), Yb−N1 2.405(5), Yb−N2 2.413(5), Yb−O1 2.455(5), Yb−O2 2.448(4), C2−N1 1.329(7), C2− C3 1.403(8), C3−C4 1.412(9), C4−N2 1.325(7); B−Yb−N1 112.7(2), B−Yb−N2 111.9(2), B−Yb−O1 105.0(2), B−Yb−O2 104.6(2), C2−C3−C4 131.4(5), C3−C2−N1 124.6(5), C3−C4−N2 124.8(5), N1−Yb−N2 79.7(2), N1−Yb−O1 91.7(2), N1−Yb−O2 142.5(2), N2−Yb−O1 142.7(2), N2−Yb−O2 90.8(2), O1−Yb−O2 74.3(2), Yb−N1−C2 121.4(4), Yb−N2−C4 121.5(4). 1: Sm−B1 2.746(5), Sm−N1 2.512(2), Sm−N2 2.519(2), Sm−O1 2.571(2), Sm−O2 2.562(2), C2−C3 1.403(4), C2−N1 1.329(4), C3−C4 1.411(4), N2−C4 1.331(4); B−Sm−N1 115.82(11), B−Sm−N2 115.36(11), B−Sm−O1 104.33(11), B−Sm−O2 105.73(11), C2− C3−C4 130.8(3), C3−C2−N1 124.9(3), C3−C4−N2 124.9(3), N1− Sm−N2 75.22(8), N1−Sm−O1 139.70(8), N1−Sm−O2 91.78(8), N2−Sm−O1 91.34(8), N2−Sm−O2 138.62(8), O2−Sm−O1 73.38(8), Sm−N1−C2 122.1(2), Sm−N2−C4 122.3(2). 2: Eu−B 2.742(9), Eu−N1 2.515(4), Eu−N2 2.517(4), Eu−O1 2.571(4), Eu− O2 2.572(4), C2−C3 1.393(8), C2−N1 1.337(6), C3−C4 1.410(9), C4−N2 1.336(7); B−Eu−N1 115.0(2), B−Eu−N2 114.4(2), B−Eu− O1 106.5(2), B−Eu−O2 104.3(2), C2−C3−C4 131.8(5), C3−C2− N1 125.7(5), C3−C4−N2 124.7(5), N1−Eu−N2 76.37(15), N1− Eu−O1 91.03(14), N1−Eu−O2 140.57(15), N2−Eu−O1 138.80(15), N2−Eu−O2 91.35(14), O1−Eu−O2 73.7(2), Eu−N1−C2 120.2(3), Eu−N2−C4 121.2(4).

lanthanide atom is surrounded by two THF molecules, the BH4− group, and the bidentate (dipp)2NacNac− ligand, which adopts a six-membered metallacycle boat conformation (Ln− N2−C4−C3−C2−N1). By considering the BH4− group as one ligand, the coordination polyhedron can be viewed as a distorted square pyramid, in which the apex is formed by the borohydride. The hydrogen atoms could not be localized from Scheme 2. Synthesis of [(dipp)2NacNacLn(BH4)2(THF)]

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and the composition could still be deduced. However, further bonding parameters of these two compounds are not discussed. Regarding compounds 5 and 8, very good data sets were collected and, thus, the hydrogen atoms of the BH4− group were localized from the difference Fourier map. Thus, the lutetium complex [(dipp)2NacNacLu(BH4)2(THF)] 8 features one BH4− group in a κ3(H) coordination mode, while the other one is κ2(H) coordinated (Figure 2). This mixed κ2/κ3(H) coordination mode is rather rarely observed. It was reported in the scandium complex [{(Me3SiNPPh2)2CH}Sc(BH4)2],39 but to our knowledge it is, prior to this present work, unknown for lutetium. In contrast, the two BH4− groups in 5 bind the metal in the expected κ3(H) mode (Figure 3). In all compounds, the (dipp)2NacNac− ligand coordinates as a bidentate ligand in a chelating mode, thereby forming a Ln−N2−C4−C3−C2−N1 metallacycle. Furthermore, the metal atoms are coordinated by two BH4− groups and one THF molecule. By considering the BH4− group as one ligand, each metal is thus five-coordinated and a distorted trigonal bipyramid is formed. The two BH4− groups and one nitrogen atom are localized on the base of this bipyramid, whereas the other nitrogen atom and the oxygen atom form the apexes. The deviation of the N2−Ln1−O1 angle (173.48(5)° (5), 173.63(13)° (7), 163.55(7)° (8)) from the ideal angle of 180° is small. The sum of the angles of the trigonal base (358.5° (5), 358.6° (7), 359.8 (8)) also shows a small deviation from the ideal angle (360°). A comparable scaffold is found in the bisborohydride [(2,6-C6H3Me2)NC(Me)CHC(Me)N(2,6-C6H3Me2)Y(BH4)2(THF)].54 Ring-Opening Polymerization of ε-Caprolactone and Trimethylene Carbonate. The divalent 2 and 3 as well as the trivalent 4−7 compounds have been evaluated in the ROP of εcaprolactone (Scheme 3). Representative results are summarized in Tables 1 and 2, respectively.

Figure 2. Solid-state structure of 8, omitting hydrogen atoms. Selected bond lengths [Å] and angles [deg]: Lu−B1 2.413(4), Lu−B2 2.649(4), Lu−N1 2.255(2), Lu−N2 2.305(2), Lu−O1 2.320(2), C2−N1 1.344(3), C2−C3 1.394(4), C3−C4 1.410(4), C4−N2 1.334(3); B1−Lu−B2 125.87(14), B1−Lu−N1 106.18(12), B1− Lu−N2 101.79(11), B1−Lu−O1 94.21(11), B2−Lu−N1 127.75(11), B2−Lu−N2 89.58(10), B2−Lu−O1 77.84(10), C2− C3−C4 130.8(2) C3−C2−N1 124.3(2), C3−C4−N2 124.6(2), N1− Lu−N2 83.67(8), N1−Lu−O1 95.60(7), N2−Lu−O1 163.55(7), Lu− N1−C2 120.8(2), Lu−N2−C4 118.6(2).

the asymmetric unit, as depicted in Figure 2 with the lutetium compound 8. Complexes 5−7 are isostructural in the solid state and crystallize in the monoclinic space group P21/c with two molecules of each complex in the asymmetric unit, as illustrated in Figure 3 for the samarium derivative 5. Unfortunately, only crystals of low quality were obtained for 4 and 6. Although the X-ray data collected from 4 and 6 were poor, the connectivity

Scheme 3. ROP of ε-Caprolactone Initiated by the Rare Earth Borohydride Complexes 2−7

The divalent monoborohydride complexes 2 and 3 successfully ring-open polymerized CL at room temperature in toluene (Table 1). A quantitative monomer conversion was reached within 10 min (note that the polymerization times have not necessarily been optimized) for [CL]0/[BH4]0 ratios of 200 or 500. The recovered PCLs featured molar mass values as determined by SEC (Mn,SEC corrected for the difference in hydrodynamic volume of PCL vs polystyrene standards) or NMR (Mn,NMR) analyses (refer to the Experimental Section), which were in fair agreement with the expected values (Mn,theo) calculated from the monomer conversion. PCLs of molar mass value as high as Mn,NMR = 56 600 g mol−1 were thus obtained (Table 1, entry 2). The dispersity values were slightly higher than those typically measured for PCL samples prepared from other divalent rare earth borohydride precursors (ĐM = ca. 1.34−1.59).50 Such Mn, and ĐM values suggest a possibly slow initiation compared to the propagation and/or the occurrence of undesirable side reactions such as inter- and intramolecular reshuffling and back-biting reactions. Such transesterification reactions are indeed often encountered in the ROP of cyclic

Figure 3. Solid-state structure of 5, omitting hydrogen atoms. Only one of two independent molecules is shown. Selected bond lengths [Å] and angles [deg] are also given for the isostructural complex 7. 5: Sm1−B1 2.596(3), Sm1−B2 2.578(3), Sm1−N1 2.354(2), Sm1−N2 2.3705(15), Sm1−O1 2.5012(14), C2−N1 1.325(2), C2−C3 1.411(3), C3−C4 1.397(3), C4−N2 1.342(3); B1−Sm1−B2 141.29(10), B1−Sm1−N1 103.49(7), B1−Sm1−N2 99.96(7), B1− Sm1−O1 85.39(7), B2−Sm1−N1 113.78(9), B2−Sm1−N2 96.58(7), B2−Sm1−O1 81.25(7), C2−C3−C4 130.6(2), C3−C2−N1 123.5(2), C3−C4−N2 124.5(2), N1−Sm1−N2 79.83(6), N1−Sm1−O1 95.37(5), N2−Sm1−O1 173.48(5), Sm1−N1−C2 121.28(12), Sm1−N2−C4 116.29(12). 7: Yb−B1 2.481(7), Yb−B2 2.487(8), Yb−N1 2.269(4), Yb−N2 2.284(4), Yb−O1 2.412(3), C2−C3 1.421(7), C2−N1 1.321(7), C3−C4 1.398(7), C4−N2 1.349(6); B1−Yb−B2 135.2(3), B1−Yb−N1 104.9(2), B1−Yb−N2 99.2(2), B1−Yb−O1 86.3(2), B2−Yb−N1 118.5(3), B2−Yb−N2 96.5(2), B2−Yb−O1 81.6(2), C2−C3−C4 130.6(5), C3−C2−N1 123.1(5), C3−C4−N2 124.2(4), N1−Yb−N2 83.29(14), N1−Yb−O1 92.27(13), N2−Yb−O1 173.63(13), Yb−N1−C2 120.8(3), Yb− N2−C4 115.6(3). D

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Table 1. Characteristics of the ROP of CL Initiated by the Divalent Compounds 2 and 3 in Toluene at 23 °Ca entry

initiator

[CL]0/[BH4]0

CL convb [%]

Mn,theoc [g mol−1]

Mn,NMRd [g mol−1]

Mn,SECe [g mol−1]

ĐMf (Mw/Mn)

1 2 3 4

2 2 3 3

200 500 200 500

99 99 100 100

22 300 57 100 22 900 57 100

19 900 56 600 38 800 52 800

40 100 61 100 37 400 51 100

1.70 1.85 1.46 1.49

All reactions were performed in 0.5 mL of toluene at 23 °C in 10 min (reaction times were not necessarily optimized); results are representative of at least duplicated experiments. bMonomer conversion determined by 1H NMR spectroscopy of the crude reaction mixture (refer to Experimental Section). cTheoretical molar mass value calculated from the relation [ε-CL]0/[BH4]0 × convε‑CL × Mε‑CL, with [BH4]0 = [(dipp)2NacNacLn(BH4)(THF)2]0 and Mε‑CL = 114 g·mol−1. dMolar mass value determined by NMR analysis of the isolated polymer (refer to Experimental Section). e Number-average molar mass value determined by SEC in THF at 30 °C vs polystyrene standards and corrected by a factor of 0.56.89 fDispersity (Mw/Mn) value calculated from SEC traces. a

Table 2. Characteristics of the ROP of CL Initiated by the Trivalent Compounds 4−7 in Toluene at 23 °Ca entry

initiator

[CL]0/[BH4]0

reaction time [min]

CL convb [%]

Mn,theoc [g mol−1]

Mn,NMRd [g mol−1]

Mn,SECe [g mol−1]

ĐMf (Mw/Mn)

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

4 4 4 5 5 5 6 6 7 7 7 7 7 7 7 8 8

200 500 1000 200 500 1000 200 500 50 100 200 350 500 1000 2000 200 500

10 10 10 10 10 10 10 10 15 15 15 15 15 15 15 10 10

99 93 81 100 100 100 100 100 100 100 100 100 100 100 99 99 100

22 500 52 200 92 700 22 700 57 000 135 700 22 300 55 600 5800 11 300 22 900 40 100 57 500 114 000 225 000 22 600 55 800

21 800 35 300 73 500 21 500 59 900 92 700 36 500 40 700 8300 15 000 26 900 31 200 55 100 nd nd 25 000 41 900

15 500 26 900 46 200 27 300 44 800 75 400 26 000 38 300 nd 14 500 20 400 34 000 40 900 80 300 138 800 23 200 48 900

1.12 1.14 1.26 1.34 1.44 1.51 1.38 1.49 nd 1.27 1.37 1.45 1.47 1.55 1.53 1.42 1.50

a All reactions were performed in 0.5 mL of toluene at 23 °C in 10 min (reaction times were not necessarily optimized); results are representative of at least duplicated experiments. bMonomer conversion determined by 1H NMR spectroscopy of the crude reaction mixture (refer to Experimental Section). cTheoretical molar mass value calculated from the relation [ε-CL]0/[BH4]0 × convε‑CL × Mε‑CL, with [BH4]0 = 1/2[(dipp)2NacNacLn(BH4)2(THF)]0, and Mε‑CL = 114 g·mol−1. dMolar mass values determined by NMR analysis of the isolated polymer (refer to Experimental Section). eNumber-average molar mass values determined by SEC in THF at 30 °C vs polystyrene standards and corrected by a factor of 0.56.89 f Dispersity (Mw/Mn) value calculated from SEC traces. nd: not determined.

esters.50 Furthermore, these two examples represent, along with the thulium derivative [(TptBu,Me)Tm(BH4)(THF)]17 (TptBu,Me = tris(2-tert-butyl-4-methyl)pyrazolylborate), and [(dipp)2pyrEu(BH4)(THF)3]88 the only divalent rare earth monoborohydride complex active in the ROP of CL. 1H NMR spectroscopic characterization of the polymers recovered after precipitation evidenced the unambiguous formation of α,ωdihydroxy telechelic PCLs, as expected (refer to the Supporting Information for a typical PCL spectrum). Indeed, rare earth borohydride complexes are now well known to directly afford, upon ROP of CL, the corresponding PCL diol. This arises, after the coordination−insertion of the first incoming lactone unit, from the reduction of the carbonyl group adjacent to the BH4− ligand. The resulting rare earth alkoxide [(dipp)2NacNac]Ln− O(CH2)5C(O)−HBH3 thus gives [(dipp)2NacNac]Ln−O(CH2)6−OBH2 as the active species, from which further propagation proceeds to generate [(dipp)2NacNac]Ln−{O(CH2)5C(O)}nO(CH2)6OBH2. Upon termination/deactivation by hydrolysis, both the rare earth−oxygen and the terminal borate group lead to the formation of a hydroxyl end-group, thereby affording α,ω-dihydroxy telechelic PCL, HO−PCL− OH.19,50

The trivalent bisborohydride complexes 4−7 were also found as effective initiators for the ROP of CL in toluene at room temperature. Up to 2000 equiv of CL per Ln−BH4 active site were successfully consumed within a few minutes (≤15 min; Table 2). Noteworthy, given that 4−7 are bisborohydride compounds, this corresponded to the full conversion of nearly 4000 equiv of CL per rare earth metal center, a ratio rarely achieved in the ROP of CL mediated by rare earth borohydride derivatives.50 To our knowledge, only one successful experiment was reported at such a high [CL]0/[BH4]0 value, namely, 5000, with the monoborohydride neodymium complex [(iPr(SiMe3)NC(NiPr)N(CH2)3NC(NiPr)N(SiMe3)iPr)Nd(BH4)(DME)] (Mn,NMR = 228 000 g·mol−1, ĐM = 1.45).90 The control of the polymerization obtained from 4−7 in terms of experimental/theoretical molar mass values agreement (Mn,SEC and Mn,NMR vs Mn,theo) and of rather narrow dispersity values was generally observed throughout the rare earth series. The PCL molar mass values fall in the range Mn,NMR = 8300−92 700 g·mol−1, while the ĐM = 1.12−1.55 values fall in the lower range of typical data reported for trivalent rare earth borohydride initiators (ĐM = 1.16−1.83).50 Again, well-defined α,ω-dihydroxy telechelic PCLs were thus rather easily E

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Table 3. Characteristics of the ROP of TMC Initiated by 3, 5, or 7 in Toluene at 23 °Ca entry

initiator

[TMC]0/[BH4]0

teaction timeb (min)

TMC convc (%)

Mn,theod (g/mol)

Mn,NMRc (g/mol)

Mn,SECe (g/mol)

ĐMf (Mw/Mn)

1 2 3 4 5 6 7 8 9 10

3 3 3 5 5 5 7 7 7 7

50 150 250 50 150 250 50 150 200 250

10 10 30 10 10 30 10 10 120 30

88 79 90 72 73 98 82 44 94 73

4600 12 200 23 300 3700 11 200 25 000 4200 6700 19 300 18 500

7600 8600 nd 4700 nd 16 000 6000 6100 14 900 11 300

nd 23 400 31 000 7400 14 600 34 600 10 700 14 100 28 300 24 000

nd 1.48 1.71 1.56 1.37 1.59 1.60 1.38 1.51 1.47

All reactions were performed in 0.5 mL of toluene at 23 °C in 10 min; results are representative of at least duplicated experiments. bReaction times were not necessarily optimized. cMonomer conversion determined by 1H NMR spectroscopy of the crude reaction mixture (refer to Experimental Section). dTheoretical molar mass value calculated from [TMC]0/[BH4]0 × convTMC × MTMC, with [BH4]0 = [(dipp)2NacNacYb(BH4)(THF)2]0, or 1/2[(dipp)2NacNacLn(BH4)2(THF)]0 with Ln = Sm, Yb, and MTMC = 102 g·mol−1. eNumber-average molar mass value determined by SEC in THF at 30 °C vs polystyrene standards and corrected by a factor of 0.73.91 fDispersity (Mw/Mn) value calculated from SEC traces. nd: not determined. a

synthesized from 4−7 with a fair control, as evidenced by 1H NMR analysis of the isolated PCLs. The ROP of trimethylene carbonate (TMC) was investigated from a few selected initiators. The divalent and trivalent complexes 3 and 5/7, respectively, enabled the ROP of TMC in toluene at room temperature with a similar efficiency (Table 3). High TMC conversion was generally obtained using [TMC]0/ [initiator]0 ratios in the range 50−250. In contrast to the polymerization of CL (Tables 1 and 2), quantitative TMC consumption was not always reached within 10−30 min (Table 3). Despite this lower activity, the polymerization of TMC with complexes 3, 5, and 7 showed a rather fair control. In these experiments, the theoretical molar mass values have been calculated from the initial concentration in rare earth complexes assuming one growing polymer chain for each BH4− unit, i.e., one from each of the one or two borohydride active ligands. Mn,NMR values of low molar mass PTMC samples have also been determined from 1H NMR analyses, assuming the formation of α-hydroxy,ω-formate telechelic PTMC, namely, HO−PTMC−O(CH2)3OC(O)H, as hinted by the previous study on the ROP of TMC using [Sm(BH4)3(THF)3].91 Indeed, NMR spectra of the isolated PTMCs showed the formation of PTMC end-capped by a hydroxyl and a formate group (refer to the Supporting Information for a typical PTMC spectrum). PTMCs of molar mass up to Mn,NMR= 16 000 g mol−1 were thus prepared. In the case of low molar mass polycarbonates (Mn,theo ≤ 10 000 g·mol−1), Mn,NMR values, calculated from the integration of the hydroxyl chain-end group (HO−CH2, δ 3.73 ppm) and of the resonance of the hydrogens of the main chain methylene (−CH2OC(O), δ 4.23 ppm), generally agreed with the expected molar mass value (Mn,theo) as well as with the experimental data determined by SEC (Mn,SEC value corrected for the difference in hydrodynamic volume of PTMC vs polystyrene standards). For higher molar mass PTMCs, the hydroxyl chain-end signal was then too small for the NMR integral value to remain reliable. The dispersity values of the recovered PTMC samples (ĐM = 1.37−1.71) are slightly higher than those measured for PCL (Tables 1 and 2), a trend common to the ROP of these two classes of cyclic esters, namely, lactones and carbonates.50 These results also suggested the occurrence of undesirable transcarbonatation reactions often encountered in the ROP of cyclic carbonates.85,92−101 The activity of these new initiators is within the range of that of

other bisborohydride complexes of the rare earth elements, e.g., [{(Me3SiNPPh2)2CH}La(BH4)2(THF)], [{(Me3SiNPPh2)2CH}Y(BH4)2], and [{(Me3SiNPPh2)2CH}Lu(BH4)2].102 Scheme 4. ROP of Trimethylene Carbonate Initiated by the Rare Earth Borohydrides 3, 5, and 7

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SUMMARY In summary, the β-diketiminate borohydrides of both divalent and trivalent rare earth elements have been synthesized. Reaction of [(dipp)2NacNacK] with [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb) resulted in the novel heteroleptic monoborohydride complexes [(dipp)2NacNacLn(BH4)(THF)2] (Ln = Sm (1), Eu (2), Yb (3)), whereas the reaction of [(dipp) 2 NacNacK] with [Sc(BH 4 ) 3 (THF) 2 ] and [Ln(BH4)3(THF)3] (Ln = Sm, Dy, Yb, Lu) afforded the monosubstituted bisborohydride compounds [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sc (4), Sm (5), Dy (6) Yb (7), Lu (8)). The diamagetic complexes and the trivalent samarium compound 5 were fully characterized by 1H, 13 C{1H}, and 11B and, when possible, by 171Yb NMR spectroscopy. For each compound, the solid-state structure was established by single-crystal X-ray diffraction structure, highlighting, as previously observed,49 the difference in the stabilizing contribution of Cl− vs BH4−. In the divalent and trivalent compounds, the BH4− groups coordinate in a κ3(H) mode to the metal. Only the BH4− groups of the lutetium complex [(dipp)2NacNacLu(BH4)2(THF)] exhibit the unidentical κ3(H) and κ2(H) coordination modes. This mixed κ2/κ3(H) is singular for Lu. The application of the divalent and trivalent compounds as initiators in the ROP of ε-caprolactone and trimethylene carbonate was investigated. All borohydride complexes 2−7 were effective, affording a generally wellcontrolled ROP of both of these cyclic esters under mild operating conditions. In particular, well-defined PCL diols of rather high molar mass values (8300 < Mn,NMR(PCL) < 92 700 g· F

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mol−1) as compared to the state of the art on the ROP of cyclic esters mediated by rare earth metal initiators, regardless of the metal oxidation state or the monomer considered,50 were thus prepared. Also, α-hydroxy,ω-formate telechelic PTMCs were synthesized with 4700 < Mn,NMR(PTMC) < 16 000 g·mol−1. Rather high productivities in the ROP of CL (up to 3960 turnover numbers, expressed in molCL·molytterbium−1, in 15 min from the ytterbium bisborohydride complex 7; Table 2, entry 15) and more moderate ones in the ROP of TMC (up to 245 turnover numbers, expressed in molTMC·molsamarium−1, in 30 min from the samarium bisborohydride complex 5; Table 3, entry 6) were thus reached in the present ROP study.

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Typical Synthesis of [(dipp)2NacNacLn(BH4)(THF)2] (1−3). THF (20 mL) was condensed at −78 °C onto a mixture of [(dipp)2NacNacK] and [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb) and next allowed to warm to room temperature. The resulting green-black (1), yellow (2), or red (3) suspension was stirred for 20 h (1) or 16 h (2, 3), at room temperature. The solution was filtered off and then concentrated to ca. 10 mL. Storage at −20 °C for three (1) or two (2, 3) days afforded the product as nearly black (1), yellow (2), or orange (3) prismatic crystals suitable for X-ray analysis. [(dipp)2NacNacSm(BH4)(THF)2] (1): [(dipp)2NacNacK] (142 mg, 0.31 mmol) and [Sm(BH4)2(THF)2] (101 mg, 0.31 mmol). Yield: 89 mg (43%) of black crystals. FT-IR (ATR, cm−1): 2959 (s), 2925 (m), 2867 (m), 2313 (br), 1661 (m), 1621 (m), 1590 (w), 1549 (vs), 1486 (m), 1460 (m), 1439 (s), 1380 (m), 1362 (s), 1324 (m), 1276 (m), 1254 (m), 1221 (m), 1175 (s), 1140 (m), 1101 (m), 1058 (m), 1032 (m), 935 (m), 882 (m), 823 (s), 799 (m), 785 (s), 757 (vs), 727 (m), 702 (m), 556 (m). Anal. Calcd for C37H61BN2O2Sm (727.07): C, 61.12; H, 8.46; N, 3.85. Found: C, 61.82; H, 8.24; N, 4.16. [(dipp)2NacNacEu(BH4)(THF)2] (2): [(dipp)2NacNacK] (152 mg, 0.33 mmol) and [Eu(BH4)2(THF)2] (108 mg, 0.33 mmol). Yield: 124 mg (51%) of yellow crystals. FT-IR (ATR, cm−1): 2960 (s), 2926 (m), 2868 (m), 2298 (br), 1661 (m), 1621 (m), 1590 (w), 1549 (s), 1486 (m), 1460 (m), 1438 (m), 1400 (m), 1381 (m), 1363 (s), 1312 (m), 1258 (s), 1221 (m), 1173 (s), 1142 (m), 1099 (vs), 1057 (m), 1033 (vs), 934 (m), 920 (m), 877 (s), 799 (s), 784 (vs), 756 (vs), 702 (m), 670 (m), 614 (m), 596 (m), 505 (m). Anal. Calcd for C37H61BN2O2Eu (728.67): C, 60.99; H, 8.44; N, 3.84. Found: C, 61.79; H, 8.52; N, 3.82. [(dipp)2NacNacYb(BH4)(THF)2] (3): [(dipp)2NacNacK] (140 mg, 0.31 mmol) and [Yb(BH4)2(THF)2] (106 mg, 0.31 mmol). Yield: 123 mg (53%) of orange crystals. 1H NMR (d8-THF, 300.13 MHz, 25 °C): δ 0.65 (q, br, 4 H, BH4, 1JH−B = 80.7 Hz), 1.18 (d, 12 H, CHMeMe′, 3 JH−H = 6.6 Hz), 1.27 (d, 12 H, CHMeMe′, 3JH−H = 6.7 Hz), 1.61 (s, 6 H, Mebackbone), 3.4 (sept, 4 H, CHMe2, 3JH−H = 6.3 Hz), 4.72 (s, 1 H, Hbackbone), 7.00−7.07 (m, 2 H, p-Ph), 7.09−7.15 (m, 4 H, m-Ph) ppm. 11 B NMR (d8-THF, 128.38 MHz, 25 °C): δ −33.0 (qt, 1JH−B = 81.1 Hz) ppm. 13C{1H} NMR (d8-THF, 75.48 MHz, 25 °C): δ 22.5 (Mebackbone), 25.4 (CHMeMe′), 27.6 (CHMeMe′), 28.1 (CHMe2), 93.9 (Cbackbone), 123.2 (Cm), 123.5 (Cp), 141.9 (Co), 147.9 (Cipso), 164,3 (CN) ppm. 171Yb{1H} NMR (d8-THF, 70.02 MHz, 25 °C): δ 752.5 (br) ppm. FT-IR (ATR, cm−1): 3059 (vw), 2960 (s), 2927 (m), 2868 (m), 2237 (br), 1661 (m), 1622 (m), 1590 (w), 1550 (s), 1513 (m), 1488 (m), 1459 (m), 1436 (s), 1402 (m), 1382 (s), 1363 (s), 1325 (m), 1311 (m), 1275 (m), 1254 (m), 1221 (m), 1171 (s), 1100 (m), 1057 (m), 1043 (m), 1008 (m), 925 (m), 899 (m), 828 (m), 783 (s), 756 (vs), 703 (m), 615 (m), 598 (m), 507 (m). Anal. Calcd for C37H61BN2O2Yb (749.74): C, 59.27; H, 8.20; N, 3.74. Found: C, 58.94; H, 8.52; N, 3.68. Typical Synthesis of [(dipp)2NacNacLn(BH4)2(THF)] (4−8). THF (20 mL) was condensed at −78 °C onto a mixture of [(dipp)2NacNacK] and [Ln(BH4)3(THF)2] (Ln = Sc, Sm, Dy, Yb, Lu) and next allowed to warm to room temperature. The resulting orange (4, 5, 8), deep yellow (6), or deep purple (7) suspension was stirred for 20 h at 60 °C (4), 20 h room temperature (5), 24 h at room temperature (6, 7), or 3 days at room temperature (8). The solution was filtered off and then concentrated to ca. 10 mL (4) or 5 mL (6, 7, 8). Storage at −20 °C for 2 days (4) or 3 days (8), at ambient temperature for 3 days (6), or at 5 °C for 3 days (7) afforded the product as colorless (4), yellow prismatic (6), deep purple prismatic (7), or colorless (8) crystals suitable for X-ray analysis. Alternatively, the solution (5) was filtered off and then the solvent removed under vacuum. Toluene (5 mL) was added and the product (5) was obtained by storage at ambient temperature for 2 days as orange needles suitable for X-ray analysis. [(dipp)2NacNacSc(BH4)2(THF)] (4): [(dipp)2NacNacK] (236 mg, 0.52 mmol) and [Sc(BH4)3(THF)2] (121 mg, 0.52 mmol). Yield: 125 mg (43%) of colorless crystals. 1H NMR (d8-THF, 300.13 MHz, 25 °C): δ 0.02−1.03 (q, br, 8 H, BH4), 1.17 (d, 12 H, CHMeMe′, 3JH−H = 6.8 Hz), 1.29 (d, 12 H, CHMeMe′, 3JH−H = 6.8 Hz), 1.86 (s, 6 H, Mebackbone), 3.33 (sept, 4 H, CHMe2, 3JH−H = 6.8 Hz), 5.55 (s, 1 H,

EXPERIMENTAL SECTION103

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 Bruker 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 [Yb(C5Me5)2(THF)2] (171Yb NMR), respectively. FT-IR spectra were obtained on a Bruker Tensor 37 spectrometer. Elemental analyses were carried out with an Elementar Vario Micro Cube. [Ln(BH4)3(THF)3] (Ln = Sm, Eu, Yb, Lu),11 [Sc(BH4)3(THF)2],11 [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb),16,104 (dipp)2NacNacH,84,86,87 and (dipp)2NacNacK84,86,87 were prepared according to literature procedures. CL (Aldrich) was dried over CaH2 (at least 1 week) prior to distillation. Trimethylene carbonate (1,3-dioxane-2-one, Labso Chimie Fine, Blanquefort, France) was purified by first dissolving it in THF, stirring over CaH2 for 2 days before being filtered and dried in vacuo, and finally recrystallizing it from cold THF. Size-exclusion chromatography (SEC) giving number-average molar mass (Mn,SEC) and dispersity (ĐM = Mw/Mn) values of the PCLs and PTMCs was carried out 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 Mixed E 300 × 7.5 mm columns. All elution curves were calibrated with 11 monodisperse polystyrene standards in the range Mn = 580−380 000 g·mol−1. The recovered polymer samples were dissolved in THF (2 mg·mL−1) and filtered; only the soluble fraction was analyzed. The Mn,SEC values of the PCLs and PTMCs were corrected for the difference in hydrodynamic radius vs polystyrene standards used for calibration using the reported correcting factors, namely, 0.56 for PCL89,105 and 0.73 for PTMC91 (Mn,SEC = Mn,SEC raw data × correcting factor). The SEC traces of the polymers all exhibited a unimodal and symmetrical peak. Monomer conversions were determined from 1H NMR spectra of the crude polymer sample, from the integration (Int) ratio IntPCL/ [IntPCL + Intε‑CL], using the −CH2OC(O) methylene triplet for PCL and ε-CL (δPCL 4.04 ppm, δε‑CL 4.19 ppm), or from the integration (Int) ratio IntPTMC/[IntPTMC + IntTMC], using the −CH2OC(O) methylene triplet for PTMC and TMC (δPTMC 4.23 ppm, δTMC 4.45 ppm). The molar mass values of short-chain PCLs and PTMCs were determined by 1H NMR analysis in CDCl3 of the crude polymer samples from the relative intensities of the signals of the PCL mainchain methylene protons (−CH2OC(O), δ 4.04 ppm) and those of the PCL chain-end methylene protons (−CH2OH, δ 3.65 ppm), or of the PTMC main-chain methylene protons (−CH2OC(O), δ 4.23 ppm) and those of the PTMC chain-end methylene protons (−CH2OH, δ 3.73 ppm) (refer to the Supporting Information). G

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Organometallics

Article

Hbackbone), 7.21−7.24 (m, 6 H, Ph) ppm. 11B NMR (d8-THF, 128.38 MHz, 25 °C): δ −19.3 (qt, 1JH−B = 78.1 Hz) ppm. 13C{1H} NMR (d8THF, 75.48 MHz, 25 °C): δ 23.9 (Mebackbone), 24.2 (CHMeMe′, CHMeMe′), 27.9 (CHMe2), 99.2 (Cbackbone), 124.1 (Cm), 126.5 (Cp), 143 (Co), 143.8 (Cipso), 168.8 (CN) ppm. FT-IR (ATR, cm−1): 3057 (vw), 2962 (s), 2926 (m), 2868 (m), 2508 (w), 2494 (w), 2146 (br), 1623 (w), 1576 (m), 1551 (m), 1522 (m), 1457 (m), 1436 (m), 1374 (vs), 1359 (vs), 1313 (s), 1255 (s), 1217 (m), 1169 (m), 1134 (m), 1100 (m), 1057 (m), 1042 (m), 1021 (m), 930 (m), 858 (m), 797 (s), 786 (s), 758 (s), 704 (m), 668 (m), 639 (m), 623 (m), 595 (m), 504 (s). EI-MS (70 eV, 140 °C): m/z (%) = 492 ([M]+, 2), 477 ([M − (BH4)]+, 4), 462 ([M − 2(BH4)]+, 1), 418 ([L]+, 18), 403 ([L − CH3]+, 58), 375 ([L − CH(CH3)2]+, 15). HR-MS (EI, 70 eV, 140 °C): m/z = 492.363 (calcd for C29H49N211B245Sc1: 492.364). Anal. Calcd for C33H57B2N2OSc (564.4): C, 70.23; H, 10.18; N, 4.96. Found: C, 70.21; H, 10.84; N, 4.75. [(dipp)2NacNacSm(BH4)2(THF)] (5): [(dipp)2NacNacK] (188 mg, 0.41 mmol) and [Sm(BH4)3(THF)3] (169 mg, 0.41 mmol). Yield: 138 mg (50%) of orange crystals. 1H NMR (d8-THF, 300.13 MHz, 25 °C): δ −8.5 to −6.5 (br, 8 H, BH4), −0.14 (d, 12 H, CHMeMe′, 3JH−H = 6.5 Hz), 1.03 (d, 12 H, CHMeMe′, 3JH−H = 6.5 Hz), 3.09 (s, 6 H, Mebackbone), 5.87 (m, 4 H, CHMe2), 6.25 (m, 2 H, p-Ph), 6.95−7.25 (m, 4 H, m-Ph), 9.07 (s, 1 H, Hbackbone) ppm. 11B NMR (d8-THF, 128.38 MHz, 25 °C): δ −34.4 (br) ppm. 13C{1H} NMR (d8-THF, 75.48 MHz, 25 °C): δ 22 (Mebackbone), 22.2 (CHMeMe′), 25.4 (CHMeMe′), 27 (CHMe2), 103.6 (Cbackbone), 122.3 (Cm), 123.1 (Cp), 138.7 (Co), 142.4 (Cipso), 177.6 (CN) ppm. FT-IR (ATR, cm−1): 3061 (vw), 2961 (s), 2926 (m), 2868 (m), 2456 (w), 1664 (w), 1624 (w), 1590 (w), 1551 (m), 1507 (w), 1459 (s), 1438 (s), 1382 (s), 1363 (s), 1331 (m), 1310 (m), 1256 (s), 1164 (s), 1140 (s), 1101 (s), 1056 (s), 1041 (s), 1019 (s), 926 (m), 871 (m), 840 (m), 797 (vs), 788 (vs), 756 (vs), 798 (m), 668 (m), 623 (m), 600 (m), 523 (m), 508 (m). Anal. Calcd for C33H57B2N2OSm (669.8): C, 59.18; H, 8.58; N, 4.18. Found: C, 58.22; H, 8.37; N, 3.82. [(dipp)2NacNacDy(BH4)2(THF)] (6). [(dipp)2NacNacK] (332 mg, 0.73 mmol) and [Dy(BH4)3(THF)3] (308 mg, 0.73 mmol). Yield: 126 mg (36%) of yellow crystals. FT-IR (ATR, cm−1): 3060 (vw), 2961 (s), 2926 (m), 2868 (m), 2309 (w), 1663 (w), 1622 (m), 1590 (w), 1550 (m), 1521 (m), 1460 (m), 1437 (m), 1382 (m), 1363 (m), 1312 (m), 1260 (s), 1175 (m), 1098 (s), 1056 (s), 1018 (s), 929 (m), 864 (m), 842 (m), 796 (vs), 757 (s), 728 (s), 701 (m), 624 (m), 599 (m), 514 (m). Anal. Calcd for C37H65B2N2O2Dy (6·THF) (754.05): C, 58.93; H, 8.69; N, 3.72. Found: C, 59.27; H, 8.82; N, 3.44. [(dipp)2NacNacYb(BH4)2(THF)] (7). [(dipp)2NacNacK] (207 mg, 0.45 mmol) and [Yb(BH4)3(THF)3] (197 mg, 0.45 mmol). Yield: 106 mg (35%) of purple crystals. FT-IR (ATR, cm−1): 3055 (vw), 2962 (s), 2925 (m), 2868 (m), 2477 (w), 2239 (br), 2126 (br), 1623 (w), 1551 (m), 1532 (m), 1509 (m), 1458 (m), 1435 (s), 1383 (vs), 1362 (vs), 1309 (vs), 1259 (s), 1167 (s), 1101 (s), 1055 (m), 1040 (m), 1014 (s), 929 (m), 867 (m), 845 (m), 798 (vs), 769 (vs), 758 (s), 701 (m), 688 (m), 624 (m), 601 (m), 513 (m). EI-MS (70 eV, 180 °C): m/z (%) = 606 ([M − (BH4)]+, 10), 590 ([M − 2(BH4) − H]+, 2), 418 ([L]+, 10), 403 ([L − CH3]+, 39), 375 ([L − CH(CH3)2]+, 14). Anal. Calcd for C37H65B2N2O2Yb (7·THF) (764.59): C, 58.12; H, 8.57; N, 3.66. Found: C, 58.39; H, 8.35; N, 3.75. [(dipp)2NacNacLu(BH4)2(THF)] (8). [(dipp)2NacNacK] (203 mg, 0.45 mmol) and [Lu(BH4)3(THF)3] (194 mg, 0.45 mmol). Yield: 84 mg (27%) of colorless crystals. 1H NMR (d8-THF, 300.13 MHz, 25 °C): δ 1.18 (d, 12 H, CHMeMe′, 3JH−H = 6.8 Hz), 1.30 (d, 12 H, CHMeMe′, 3JH−H = 6.8 Hz), 1.82 (s, 6 H, Mebackbone), 3.35 (sept, 4 H, CHMe2, 3JH−H = 6.8 Hz), 5.34 (s, 1 H, Hbackbone), 7.18−7.23 (m, 6 H, Ph) ppm; the signal of the BH4 group was not detected). 11B NMR (d8-THF, 128.38 MHz, 25 °C): δ −23.6 (qt, 1JH−B = 82.3 Hz) ppm. 13 C{1H} NMR (d8-THF, 75.48 MHz, 25 °C): δ 23.9 (Mebackbone), 24.3 (CHMeMe′), 25.4 (CHMeMe′), 28.0 (CHMe2), 98.2 (Cbackbone), 123.9 (Cm), 125.9 (Cp), 143 (Co), 144.7 (Cipso), 168.8 (CN) ppm. FT-IR (ATR, cm−1): 3057 (vw), 2960 (s), 2922 (vs), 2852 (m), 2511 (vw), 2392 (w), 2349 (w), 2304 (br), 2240 (br), 2134 (br), 2052 (vw), 1694 (w), 1624 (m), 1580 (m), 1554 (m), 1514 (m), 1460 (s), 1446 (s), 1382 (vs), 1363 (vs), 1335 (m), 1311 (s), 1256 (m), 1169 (s), 1127

(m), 1099 (m), 1056 (m), 1042 (m), 1020 (m), 1006 (m), 959 (m), 926 (m), 861 (m), 836 (m), 789 (s), 779 (s), 755 (vs), 718 (m), 700 (m), 673 (m), 663 (m), 636 (m), 622 (m), 595 (m), 515 (m), 443 (m). EI-MS (70 eV, 200 °C): m/z (%) = 622 ([M]+, 10), 607 ([M − (BH 4 )] + , 9), 592 ([M − 2(BH 4 )] + , 7). Anal. Calcd for C33H57B2N2OLu (694.4): C, 57.08; H, 8.27; N, 4.03. Found: C, 57.43; H, 8.27; N, 4.36. X-ray Crystallographic Studies of 1−8. A suitable crystal was covered with mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to a cold stream of a STOE IPDS 2 diffractometer. All structures were solved by using SHELXS-97.106 The remaining non-hydrogen atoms were located from difference 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 using the program SHELXL-97.106 Carbon-bound hydrogen atom positions were calculated. 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, bond lengths, and angles have been deposited as Supporting Information. Crystal data for 1: C37H61BN2O2Sm, M = 727.03, a = 8.7602(3) Å, b = 12.3497(4) Å, c = 17.8330(6) Å, α = 76.521(3)°, β = 84.648(3)°, γ = 85.551(3)°, V = 1864.85(11) Å3, T = 150 K, space group P1̅, Z = 2, μ(Mo Kα) = 1.606 mm−1, 19 975 reflections measured, 6032 independent reflections (Rint = 0.0429). The final R1 values were 0.0268 (I > 2σ(I)). The final wR(F2) values were 0.0636 (I > 2σ(I)). The final R1 values were 0.0341 (all data). The final wR(F2) values were 0.0656 (all data). The goodness of fit on F2 was 0.992. Crystal data for 2: C37H61BEuN2O2, M = 728.64, a = 8.8497(4) Å, b = 12.4662(6) Å, c = 17.8524(9) Å, α = 76.231(4)°, β = 84.663(4)°, γ = 85.552(4)°, V = 1901.5(2) Å3, T = 200 K, space group P1̅, Z = 2, μ(Mo Kα) = 1.680 mm−1, 16 335 reflections measured, 7347 independent reflections (Rint = 0.0944). The final R1 values were 0.0475 (I > 2σ(I)). The final wR(F2) values were 0.1043 (I > 2σ(I)). The final R1 values were 0.0731 (all data). The final wR(F2) values were 0.1150 (all data). The goodness of fit on F2 was 0.919. Crystal data for 3: C37H61BN2O2Yb, M = 749.74, triclinic, a = 8.7800(3) Å, b = 12.3442(4) Å, c = 17.6886(5) Å, α = 75.771(3)°, β = 84.403(3)°, γ = 85.904(3)°, V = 1847.22(10) Å3, T = 150(2) K, space group P1,̅ Z = 2, μ(Mo Kα) = 2.563 mm−1, 22 654 reflections measured, 6703 independent reflections (Rint = 0.0789). The final R1 values were 0.0406 (I > 2σ(I)). The final wR(F2) values were 0.1173 (I > 2σ(I)). The final R1 values were 0.0414 (all data). The final wR(F2) values were 0.1177 (all data). The goodness of fit on F2 was 1.218. Crystal data for 4: C33H57B2N2OSc, M = 564.38, triclinic, a = 10.3710(12) Å, b = 12.7653(13) Å, c = 16.7374(17) Å, α = 78.818(8)°, β = 77.406(9)°, γ = 73.426(9)°, V = 2052.0(4) Å3, T = 150(2) K, space group P1̅, Z = 2. Crystal data for 5: C33H57B2N2OSm, M = 669.77, a = 30.579(6) Å, b = 10.445(2) Å, c = 23.642(5) Å, β = 110.19(3)°, V = 7087(3) Å3, T = 150 K, space group P21/c, Z = 8, μ(Mo Kα) = 1.682 mm−1, 60 353 reflections measured, 13 353 independent reflections (Rint = 0.0401). The final R1 values were 0.0220 (I > 2σ(I)). The final wR(F2) values were 0.0461 (I > 2σ(I)). The final R1 values were 0.0287 (all data). The final wR(F2) values were 0.0476 (all data). The goodness of fit on F2 was 1.031. Crystal data for 6: C33H57B2DyN2O, M = 681.92, monoclinic, a = 30.5057(11) Å, b = 10.4522(3) Å, c = 23.6859(8) Å, β = 109.867(3)°, V = 7102.8(4) Å3, T = 150(2) K, space group P21/c, Z = 8. Crystal data for 7: C33H57B2N2OYb, M = 692.48, a = 30.4451(9) Å, b = 10.4128(3) Å, c = 23.6196(7) Å, β = 109.663(2)°, V = 7051.2(4) Å3, T = 173 K, space group P21/c, Z = 8, μ(Mo Kα) = 2.678 mm−1, 46 519 reflections measured, 13 018 independent reflections (Rint = 0.0634). The final R1 values were 0.0312 (I > 2σ(I)). The final wR(F2) values were 0.0549 (I > 2σ(I)). The final R1 values were H

dx.doi.org/10.1021/om500708x | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

0.0684 (all data). The final wR(F2) values were 0.0739 (all data). The goodness of fit on F2 was 0.894. Crystal data for 8: C33H57B2LuN2O·C4H8O, M = 766.50, a = 11.7844(4) Å, b = 12.8442(4) Å, c = 14.2905(4) Å, α = 65.951(2)°, β = 86.995(3)°, γ = 82.341(3)°, V = 1957.65(11) Å3, T = 200 K, space group P1̅, Z = 2, μ(Mo Kα) = 2.552 mm−1, 56 498 reflections measured, 7695 independent reflections (Rint = 0.0698). The final R1 values were 0.0227 (I > 2σ(I)). The final wR(F2) values were 0.0459 (I > 2σ(I)). The final R1 values were 0.0288 (all data). The final wR(F2) values were 0.0464 (all data). The goodness of fit on F2 was 0.900. Polymerization of ε-Caprolactone and Trimethylene Carbonate. Typical Procedure for the ROP of ε-CL. In a typical experiment, a solution of the initiator (4, 5.8 mg, 10.30 μmol) in toluene (0.5 mL) and ε-CL (2.354 g, 20.62 mmol, 2000 equiv) were charged in a Schlenk flask in the glovebox (Table 2, entry 3). The mixture was then immediately stirred at 23 °C over the appropriate time (reaction times have not been systematically optimized). The reaction was quenched with an excess of acidic toluene (ca. 1 mL of a 1.6 mmol·L−1 acetic acid solution in toluene). The resulting mixture was concentrated under vacuum, and the conversion determined by 1H NMR analysis of the residue. This crude polymer was then dissolved in CH2Cl2 and purified upon precipitation in cold methanol, filtration, and drying under vacuum. The final polymer was then analyzed by NMR and SEC. PCL: 1H NMR (400 MHz, CDCl3, 23 °C): δ 4.06 (t, JH−H = 6.7 Hz, (2n + 2)H, C(O)OCH2), 3.64 (t, JH−H = 6.7 Hz, 4H, HOCH2), 2.30 (t, JH−H = 7.5 Hz, (2n + 2)H, CH2C(O)), 1.64 (m, (4n + 8H), CH2CH2CH2), 1.37 (m, (2n + 4)H), CH2CH2CH2) ppm (see Figure 22 in Supporting Information). Typical Procedure for ROP of TMC. In a typical experiment, a solution of initiator (3, 9.5 mg, 12.67 μmol) in toluene (5 mL) and TMC (194 mg, 1.90 mmol, 150 equiv) were charged in a Schlenk flask in the glovebox (Table 3, entry 2). The reaction mixture was stirred at 23 °C over the appropriate time (note that reaction times have not been systematically optimized). The reaction was then quenched by adding an excess of an acetic acid solution (ca. 0.1 mL of a 1.6 mmol· L−1 solution in toluene). The resulting mixture was concentrated under vacuum, and the conversion determined by 1H NMR analysis of the residue. 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. PTMC: 1H NMR (400 MHz, CDCl3, 23 °C): δ 8.05 (s, 1H, OC(O)H), 4.23 (t, J(H,H) = 6.2 Hz, (4n + 6)H, CH2CH2CH2, CH2CH2CH2OC(O)H, HOCH2CH2CH2OC(O)), 3.73 (t, 2H, HOCH2), 2.04 ppm (quintuplet, J(H,H) = 6.2 Hz, (2n+2)H, CH2CH2CH2, CH2CH2CH2OC(O)H), 1.89 (quintuplet, J(H,H) = 6.7 Hz, 2H, HOCH2CH2CH2OC(O)) (see Figure 22 in Supporting Information).

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de la Matière” (SDLM) of the University of Rennes 1 (fellowship to M.S.). Dr. Michael Gamer (KIT) is acknowledged for support in refining the single-crystal X-ray structures.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for the structure determinations of 1−3, 5, 7, and 8 and NMR spectra of 1−8 and of a PCL and PTMC samples are available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.W.R. thanks the Helmholtz Research School: Energy-Related Catalysis for financial support. M.S. thanks the Cusanuswerk for support. S.M.G. thanks the CNRS. This research has also been financially supported in part by the Ecole Doctorale “Sciences I

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Organometallics

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dx.doi.org/10.1021/om500708x | Organometallics XXXX, XXX, XXX−XXX