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(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis† Matthias Schmid,a,b,c Pascual Oña-Burgos,a,d Sophie M. Guillaume*b and Peter W. Roesky*a The (iminophosphoranyl)(thiophosphoranyl)methanide {CH(PPh2vNSiMe3)(PPh2vS)}− ligand has been used for the synthesis of divalent and trivalent rare-earth borohydride complexes. The salt metathesis of the potassium reagent [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 with [Yb(BH4)2(THF)2] resulted in the divalent monoborohydride ytterbium complex [{CH(PPh2vNSiMe3)(PPh2vS)}Yb(BH4)(THF)2]. The 2D

31

P/171Yb

HMQC-NMR spectrum clearly showed the coupling between both nuclei. The trivalent bisborohydrides [{CH(PPh2vNSiMe3)(PPh2vS)}Ln(BH4)2(THF)] (Ln = Y, Sm, Tb, Dy, Er, Yb and Lu) were obtained by reaction of [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 with [Ln(BH4)3(THF)3]. All new compounds were characterized by single X-ray diffraction. The divalent and trivalent compounds were next used as initiators in the ringopening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC). All complexes Received 30th December 2014, Accepted 27th January 2015 DOI: 10.1039/c4dt04034a www.rsc.org/dalton

afforded a generally well-controlled ROP of both of these cyclic esters. High molar mass poly(ε-caprolactone) diols (Mn,NMR < 101 300 g mol−1, ĐM = 1.44), and α,ω-dihydroxy and α-hydroxy,ω-formate telechelic poly(trimethylene carbonate)s (Mn,NMR < 20 000 g mol−1, ĐM = 1.61) were thus synthesized under mild operating conditions.

Introduction The deprotonated forms of bis( phosphinimino)methane [CH2(PPh2NR)2] (R = SiMe3, aryl), a bidentate P–N ligand, are the monoanionic {CH(PPh2NR)2}− bis( phosphinimino)methanide and dianionic {C(PPh2NR)2}2− bis( phosphinimino)methandiide. Both ligands have been extensively used in main group,1–16 transition16–34 and f-element5,13,15,19,35–49 chemistry.17–19,50 Both anions {CH(PPh2NR)2}− and {C(PPh2NR)2}2− show unusual coordination modes. Calculation studies revealed that the negative charge is mainly localized on the carbene carbon atom of the ligand backbone and that the charge delocalization from carbon to nitrogen is less important.7 The monoanionic {CH(PPh2NR)2}− species tends to adopt

a Institut für Anorganische Chemie, Karlsruher Institut für Technologie (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany. E-mail: [email protected] b Institut des Sciences Chimiques de Rennes, CNRS - Université de Rennes 1 (UMR 6226), Organometallics, Materials and Catalysis, Campus de Beaulieu, 35042 Rennes Cedex, France. E-mail: [email protected] c Institut für Ressourcenökologie, Helmholtz-Zentrum Dresden-Rossendorf e. V., Bautzner Landstraße 400, 01328 Dresden, Germany d Department of Chemistry and Physics, University of Almería, Carretera de Sacramento s/n, 04120 Almería, Spain † Cif files of compounds 2–9. CCDC 1040816–1040823. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt04034a

12338 | Dalton Trans., 2015, 44, 12338–12348

in most of the solid state structures a conformation, in which the methine carbon atom coordinates onto the metal center via a long interaction. Mostly six membered metallacycles (N–P–C– P–N–M) are formed by chelation of the two imine groups to the metal center, adopting a pseudo-boat conformation.51 During the last decade, some of us have focused on the use of the bis( phosphinimino)methanide {CH(PPh2NSiMe3)2}− ligand in rare-earth metal chemistry.6,14,16,20,36–49 In this context, some very efficient rare-earth metal initiators ligated by {CH(PPh2NSiMe3)2}− have been unveiled for polymerization reactions. Thus, the chloride complexes [{CH(PPh2NSiMe3)2}Ln{(Ph2P)2N}Cl] (Ln = Y, La, Nd, Yb) were shown to initiate the ring-opening polymerization (ROP) of ε-caprolactone (CL) and the polymerization of methyl methacrylate (MMA).37 The bisborohydride analogues, [{(CH(PPh2NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) were demonstrated as successful initiators for the ROP of CL49 and trimethylene carbonate (TMC),46 as well as for the polymerization of MMA.47 For the ROP of CL, the narrowest dispersity values ever obtained from a rare-earth metal borohydride initiator were then measured (ĐM = 1.06 1.11).49 Recently, we have focused on the analogous (iminophosphoranyl)(thiophosphoranyl)methanide ligand {CH(PPh2vNR)(Ph2PS)}− which is isoelectronic to the bis( phosphinimino)methanide ligand but has a hard and a soft donor atom.53,54

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We reported the potassium reagent [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 as well as its reactions with LnCl3, which led to the dichloro complexes [{CH(PPh2vNSiMe3)(PPh2vS)}LnCl2(THF)] (Ln = Dy, Er).53 The bisamido compounds [{CH(PPh2vNSiMe3)(PPh2vS)}Ln{N(SiHMe2)2}2] (Ln = Y, Sm, Er, Lu) were obtained by amine elimination from [CH2(PPh2vNSiMe3)(PPh2vS)] and [Ln{N(SiHMe2)2}3(THF)2]. We also reported the (iminophosphoranyl)(thiophosphoranyl)methane zinc halide complexes [{(PPh2vNSiMe3)(PPh2vS)CH2}ZnX2] (X = Cl, I) and the (iminophosphoranyl)(thiophosphoranyl)methanide zinc complexes of composition [{(PPh2vNSiMe3)(PPh2vS)CH}Zn{N(SiMe3)2}] and [{(PPh2v NSiMe3)(PPh2vS)CH}ZnPh].54 The aromatic derivatives of phosphine( phosphinimino) methane CH2(PPh2vNAr)(Ph2PS) (Ar = p-tolyl, p-tolyl) and some platinum compounds were reported in the 1990s by Elsevier.55 Lately, Cadierno and Gimeno reported on the aromatic derivatives CH2(PPh2vNAr)(Ph2PS) (Ar = 2,4,6-C6H2Me3, 4-C6F4CHO, 4-C6F4CN, and 4-C5F4N), which were coordinated as monoanionic ligands {CH(PPh2vNAr)(Ph2PS)}− to ruthenium.56 We were inspired by the work of So and coworkers, who recently reported {CH2(PPh2vNSiMe3)(PPh2vS)}, which was prepared by the reaction of the {Ph2PCH2(PPh2vNSiMe3)} with sulfur in refluxing toluene.57 Deprotonation of {CH2(PPh2vNSiMe3)(PPh2vS)} with nBuLi gave [Li{CH(PPh2vNSiMe3)(PPh2vS)}]. The corresponding dianion [Li2{(PPh2vNSiMe3)(PPh2vS)}] was obtained by the reaction of [CH2(PPh2vNSiMe3)(PPh2vS)] with tBuLi. So and coworkers also reported the derivatives of Mg,58 Al,58 and Sn.59,60 Herein, we now report the (iminophosphoranyl)(thiophosphoranyl)methanide borohydride complexes of divalent and trivalent lanthanides as well as their applications as initiator for the ROP of CL and TMC.

Results and discussion As suitable transfer reagent for the (iminophosphoranyl)(thiophosphoranyl)methanide ligand, we used the corresponding potassium salt [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (1), obtained by treatment of {CH2(PPh2vNSiMe3)(PPh2vS)} with KH in THF at 60 °C (Scheme 1). Reaction of 1 with [Yb(BH4)2(THF)2] in THF resulted, after elimination of KBH4, in the heteroleptic divalent Yb complex [{CH(PPh2vNSiMe3)(PPh2vS)}Yb(BH4)(THF)2] (2) (Scheme 2).

Scheme 1 Synthesis of the dimeric compound [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (1).

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Scheme 2 Synthesis of the divalent Yb complex [{CH(PPh2vNSiMe3)(PPh2vS)}Yb(BH4)(THF)2] (2).

Fig. 1 2D 31P, 171Yb HMQC NMR of 2 (THF-d8 at 298 K) (cutout does not show signals of the decomposition products).

Compound 2 was fully characterized by analytic/spectroscopic techniques. Since compound 2 is diamagnetic 1H, 11B, 13 C{1H}, 29Si{1H} 31P{1H} and 171Yb{1H} NMR spectra could be recorded. The 1D 31P NMR spectrum shows two doublets at (δ 31.20 ppm, 2JP,P 14.8 Hz, 2JYb,P 44.7 Hz) and (δ 15.58 ppm, 2 JP,P 14.8 Hz, 2JYb,P 59.0 Hz), each signal shows satellites corresponding to the coupling with ytterbium-171 nuclei (14.3% natural abundance). As a result of the two non-equivalent P-atoms of the ligand, the 1D 171Yb spectrum should show a doublet of doublets. However, we could not resolve this coupling due to the signal width and the spectrum displaying a triplet resembled more that of [{(Me3SiNPPh2)2CH}Yb(BH4)(THF)2] (δ 744 ppm) (Fig. 1).61 Therefore, a 2D 31P, 171Yb HMQC NMR spectrum was recorded, then showing clearly the coupling, which was also observed in the 31P NMR. Although all NMR spectra were recorded from single crystalline material, significant amounts of decomposition product (mostly the free ligand [CH2(PPh2vNSiMe3)(PPh2vS)]57) were observed in solution, indicating some decomposition of 2. Besides NMR techniques, the complex was also characterized by EI-MS showing a molecular peak at m/z = 691.36 amu. The FT-IR spectrum of complex 2 unfortunately precluded an unambiguous assignment of the coordination mode of the BH4− group, which could however be gained by X-ray analysis (vide infra) (Fig. 2).

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Scheme 3 Synthesis of the trivalent lanthanide complexes [{CH(PPh2vNSiMe3)(PPh2vS)}Y(BH4)2(THF)] (Ln = Y (3), Sm (4), Tb (5), Dy (6), Er (7), Yb (8) and Lu (9)).

Fig. 2 Solid-state structure of 2, omitting carbon bonded hydrogen atoms. Selected bond lengths [Å] and angles [°]: Yb–B 2.611(7), Yb–C1 2.755(5), Yb–N 2.469(4), Yb–O1 2.438(4), Yb–O2 2.456(4), Yb–S 2.9155 (13), C1–P1 1.736(4), C1–P2 1.752(4), N–P2 1.579(4), N–Si 1.707(4), P1–S 2.000(2); B–Yb–C1 171.3(2), B–Yb–N 109.8(2); B–Yb–O1 94.9(2), B–Yb– O2 96.7(2), B–Yb–S 109.2(2), C1–Yb–N 63.13(12), C1–Yb–O1 92.27(13), C1–Yb–O2 88.53(13), C1–Yb–S 66.59(9), N–Yb–O1 155.35(13), N–Yb– O2 90.29(13), N–Yb–S 86.96(9), O1–Yb–O2 87.25(13), O1–Yb–S 84.35 (10), O2–Yb–S 153.30(10), C1–P1–S 112.9(2), C1–P2–N 111.0(2).

ˉ with Complex 2 crystallizes in the triclinic space group P1 two molecules of 2 in the unit cell. By considering the BH4− group as one ligand and the (iminophosphoranyl)(thiophosphoranyl)methanide ligand as tridentate, the coordination polyhedron can be viewed as a distorted octahedron, in which the apex is formed by the borohydride. As observed in [{CH(PPh2vNSiMe3)(PPh2vS)}LnCl2(THF)] (Ln = Dy, Er), the {CH(PPh2vNSiMe3)(PPh2vS)}− ligand coordinates through the nitrogen and the sulfur atoms to the ytterbium atom forming a six-membered metallacycle (Yb–S–P1–C1–P2–N). A twist boat conformation of the metallacycle with a long interaction between the methanide carbon atom (C1) and the lanthanide atom is observed. This kind of conformation is typical for most of the known bis( phosphinimino)methanide complexes of the early transition metals and the lanthanides.35,50 The Yb–C1 distance of 2.755(5) Å is longer than usual Yb–C distances but in the range of comparable bis( phosphinimino)methanide complexes such as [{(Me3SiNPPh2)2CH}YbI(THF)2]13 (Yb–C1 2.700(4) Å) and [{(Me3SiNPPh2)2CH}Yb(BH4)(THF)2]61 (Yb–C1 2.7403(7) Å). The hydrogen atoms of the BH4− group were localized from the difference Fourier map. As expected, the BH4− group binds in a κ3(H) coordination mode to the center metal with a Yb–B distance of 2.611(7) Å. This distance is significantly longer than in [{(Me3SiNPPh2)2CH}Yb(BH4)(THF)2] (2.3641 Å). The B–Yb–C1 angle of 171.3(2)° and the four angles within the O1–O2–N–S plane (O1–Yb–O2 87.25(13)°, O1–Yb–S 84.35(10)°, N–Yb–O2 90.29(13)°, and N–Yb–S 86.96(9)°) match with the interpretation of the coordination polyhedron as a distorted octahedron. The trivalent (iminophosphoranyl)(thiophosphoranyl)methanide lanthanide complexes [{CH(PPh2vNSiMe3)(PPh2vS)}Ln(BH4)2(THF)] (Ln = Y (3), Sm (4), Tb (5), Dy (6),

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Er (7), Yb (8) and Lu (9)) were next synthesized by reaction of 1 with [Ln(BH4)3(THF)3] (Scheme 3). The 1H, 11B, 13C{1H}, 29Si{1H} and 31P{1H} NMR spectra were recorded for the diamagnetic compounds 3 and 9 as well as for the paramagnetic samarium complex 4. As expected, the spectra of 4 showed a paramagnetic broadening of the signals. The characteristic signals for the methine hydrogen atoms were observed at δ 2.49 ppm (3), δ 0.91 ppm (4), and δ 2.51 ppm (9). In comparison to 1 (δ 2.23 ppm) and [{CH(PPh2vNSiMe3)(PPh2vS)}Ln{N(SiHMe2)2}2] (Ln = Y (δ 2.14 ppm), Lu (δ 2.18 ppm)), this signal of the diamagnetic compounds was slightly shifted downfield and the expected coupling pattern was clearly evidenced. The other characteristic signal was observed for the SiMe3 group of the ligand. In the 31P{1H}NMR spectrum of 3 two doublets of doublets recorded at δ 20.3 ppm and δ 32.3 ppm resulted from a 2J(P,P) and a 2J(P,Y) coupling. For 9 the expected doublets were broadened in the 31P{1H} NMR spectrum. As observed for 2, all NMR spectra, which were recorded from single crystalline material, showed significant amounts of decomposition products (mostly the free ligand [CH2(PPh2vNSiMe3)(PPh2vS)]57) once again, indicating decomposition of the samples in solution. As observed for 2, the FT-IR spectra of complexes 3–9 precluded an unambiguous assignment of the coordination mode of the BH4− group, otherwise assessed by X-ray diffraction analyses (vide infra). Compounds 3–9 were also characterized by single crystal X-ray diffraction studies. Compounds 3 and 5–9 are isostructural and crystallize in the monoclinic space group P21/n with one molecule in the asymmetric unit (Fig. 3). Compound 4, which has the largest metal center in this series (Sm), crystalˉ, with one molecule of the lizes in the triclinic space group P1 complex and an additional molecule of THF in the asymmetric unit (Fig. 4). Very good X-ray data sets were collected for compounds 3, 7 and 8 and thus, the hydrogen atoms of the BH4− group were localized from the difference Fourier map. As observed for 2, the coordination polyhedron can be viewed as a distorted octahedron by considering the BH4− group as monodentate and the (iminophosphoranyl)(thiophosphoranyl)methanide ligand as a tridentate ligand. In comparison to 2, formally one THF molecule was exchanged by one BH4− group in 3–9. Thus, a similar six-membered metallacycle (Ln–S–P1–C1–P2–N) forming a twist boat conformation with a long

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Table 1 Comparison of the Ln–C1 bond distances with the ion radius of the metal center56,57

Fig. 3 Solid-state structure of 8, omitting carbon bonded hydrogen atoms. The isostructural complexes 3, 5–7 are not shown. Selected bond lengths [Å] and angles [°]: Yb–B1 2.494(5), Yb–B2 2.498(5), Yb–C1 2.614(3), Yb–N 2.317(2), Yb–O1 2.337(2), Yb–S 2.8321(10), C1–P1 1.746(3), C1–P2 1.756(3), N–P2 1.599(3), N–Si 1.745(3), P1–S 1.9975(14); B1– Yb–B2 103.8(2), B1–Yb–C1 155.0(2), B1–Yb–N 103.83(14), B1–Yb–O1 103.01(14), B1–Yb–S 89.55(15), B2–Yb–C1 100.2(2), B2–Yb–N 96.92(14), B2–Yb–O1 87.59(14), B2–Yb–S 164.57(14), C1–Yb–N 65.84(9), C1–Yb– O1 84.99(9), C1–Yb–P1 32.11(7), C1–Yb–P2 35.31(7), C1–Yb–S 67.90(7), N–Yb–O1 150.84(10), N–Yb–S 87.21(7), O1–Yb–S 81.77(7), P1–C1–P2 121.8(2).

Fig. 4 Solid-state structure of 4, omitting hydrogen atoms. Selected bond lengths [Å] and angles [°]: Sm–B1 2.593(5), Sm–B2 2.572(6), Sm– C1 2.751(4), Sm–N 2.407(3), Sm–O 2.456(3), Sm–S 2.8911(11), C1–P1 1.757(4), C1–P2 1.738(4), N–P1 1.596(3), N–Si 1.744(4), P2–S 2.0033(14); B1–Sm–B2 106.4(2), B1–Sm–C1 150.2(2), B1–Sm–N 104.3(2), B1–Sm–O 104.5(2), B1–Sm–S 88.62(14), B2–Sm–C1 102.2(2), B2–Sm–N 96.4(2), B2–Sm–O 84.3(2), B2–Sm–S 159.6(2), C1–Sm–N 63.20(11), C1–Sm–O 87.03(11), C1–Sm–S 66.40(9), N–Sm–O 149.76(11), N–Sm–S 93.14(8), O–Sm–S 78.55(8), P1–C1–P2 126.5(2), P1–N–Si 128.5(2), Sm–C1–P1 85.21(15), Sm–C1–P2 94.6(2), Sm–N–P1 101.3(2), C1–P1–N 108.2(2), C1–P2–S 111.26(14).

methanide carbon atom (C1) lanthanide atom interaction, was formed. Since, all compounds are isostructural (3 and 5–9) or isotype (4), the correlation of the Ln–C1 bond distance with the ion radius can be well documented within this series. The Ln–C1 bond distance is decreasing from 2.751(4) Å (4) to 2.595(8) Å (9) (Table 1). As expected, the BH4− group binds in a κ3(H) coordination mode to the metal center with Ln–B bond distances of Y–B1 2.527(5) Å and Y–B2 2.548(6) Å (3), Sm–B1 2.593(5) Å and Sm–B2 2.572(6) Å (4), Tb–B1 2.540(11) Å and

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Entry

Compound

Ln–C1 [Å]

Ion radius [Å]

1 2 3 4 5 6 7

Sm–C1 (4) Tb–C1 (5) Dy–C1 (6) Y–C1 (3) Er–C1 (7) Yb–C1 (8) Lu–C1 (9)

2.751(4) 2.668(8) 2.652(8) 2.646(4) 2.647(6) 2.614(3) 2.594(8)

1.10 1.06 1.05 1.04 1.03 1.01 1.00

Scheme 4 Synthesis of PCL diol by ROP of ε-caprolactone initiated by the rare-earth borohydride complexes 2–9.

Tb–B2 2.542(11) Å (5), Dy–B1 2.505(10) Å and Dy–B2 2.511(12) Å (6), Er–B1 2.512(9) Å and Er–B2 2.647(6) Å (7), Yb–B1 2.494(5) Å and Yb–B2 2.498(5) Å (8), and Lu–B1 2.479(10) Å and Lu–B2 2.467(13) Å (9). In general, the other bonding parameters are as expected and thus they will not be further discussed here in detail. A comparison of 6 and 7 with the analogous chloro compounds [{CH(PPh2vNSiMe3)(PPh2vS)}LnCl2(THF)] (Ln = Dy, Er)53 shows, that the bonding distances of the ligand to the metal are only slightly longer in 6 and 7, e.g. the Ln–S distances of 2.874(3) Å (6) and 2.852(2) Å (7) are longer than in [{CH(PPh2vNSiMe3)(PPh2vS)}DyCl2(THF)] (2.8469(8) Å) and [{CH(PPh2vNSiMe3)(PPh2vS)}ErCl2(THF)] (2.8224(9) Å). Obviously, as a result of the slightly increased steric demand of the BH4− group in comparison to the chloro atoms,62 the ligand is shifted away from the metal centers in 6 and 7. The divalent compound 2 and all trivalent compounds 3–9 have been evaluated in the ROP of ε-caprolactone (CL) (Scheme 4). Representative results are gathered in Table 2. Since all complexes slowly decompose in THF solution, the reactions were run over a short period of time (10 min) in toluene. Under these conditions, all complexes successfully ring-open polymerized CL at room temperature in toluene (Table 2). A quantitative monomer conversion was reached within this time frame (note that the polymerization times have not necessarily been optimized) for [CL]0/[BH4]0 ratios of 50 to 1000. Running the polymerization in a more diluted reaction medium remained successful (Table 2, entries 9, 10). The recovered poly(ε-caprolactone)s (PCLs) featured molar mass values as determined by SEC or by NMR (Mn,NMR) analyses (refer to the Experimental section), which generally remained in quite fair agreement with the expected values (Mn,theo) calculated from the monomer conversion assuming each BH4 group is active.

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Table 2

Dalton Transactions Characteristics of the ROP of CL initiated by compounds 2–9 in toluene at 23 °Ca

Entry

Initiator

[CL]0/ [BH4]0

Reaction time [min]

CL conv.b [%]

Mn,theoc [g mol−1]

Mn,NMRd [g mol−1]

Mn,SECe [g mol−1]

ĐM f (Mw/Mn)

1 2 3 4 5 6 7 8 9g 10h 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

2 3 3 3 3 3 4 4 4 4 4 4 4 4 5 5 5 5 6 6 6 7 7 7 7 8 8 8 8 8 9 9 9

200 50 100 200 500 1000 50 100 150 150 200 300 500 1000 50 100 200 500 50 200 500 50 100 200 500 50 100 200 500 1000 50 200 500

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

99 100 100 100 100 99 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 99 100 99 100 100 100

22.600 5.700 11.400 22.800 57.000 112.900 5.700 11.400 17.100 17.100 22.800 34.200 57.000 114.000 5.700 11.400 22.800 57.000 5.700 22.800 57.000 5.700 11.400 22.800 57.000 5.700 11.400 22.600 57.000 112.900 5.700 22.800 57.000

21.800 7.300 13.200 22.300 48.100 84.500 7.400 13.000 16.600 19.400 21.000 31.400 43.400 101.300 9.500 12.700 27.800 45.700 7.500 20.700 39.700 7.400 15.500 25.600 52.600 6.500 13.500 23.400 50.900 84.600 8.400 26.200 48.300

15.500 6.800 10.000 16.035 39.600 78.400 5.000 8.100 12.800 16.800 14.800 25.200 28.200 60.800 7.100 10.200 20.500 39.100 6.800 19.500 42.400 6.600 14.800 19.300 44.700 6.400 11.700 19.500 38.600 67.200 7.000 16.700 34.200

1.12 1.27 1.28 1.31 1.37 1.45 1.18 1.22 1.34 1.29 1.30 1.37 1.36 1.44 1.29 1.26 1.37 1.42 1.31 1.34 1.44 1.55 1.29 1.33 1.50 1.23 1.24 1.30 1.40 1.47 1.24 1.37 1.41

a All reactions were performed in 0.5 mL of toluene (unless otherwise stated) at 23 °C (reaction times were not necessarily optimized); results are representative of at least duplicated experiments. b Monomer conversion determined by 1H NMR spectroscopy of the crude reaction mixture (refer to Experimental section). c Theoretical molar mass value calculated assuming one (2) or two (3–9) growing polymer chains from the relation: [ε-CL]0/[BH4]0 × conv.ε-CL × Mε-CL, with [BH4]0 = [2]0, or 12 [3–9]0 and Mε-CL = 114 g mol−1. d Molar mass values determined by NMR analysis of the isolated polymer (refer to Experimental section). e Number-average molar mass values determined by SEC in THF at 30 °C vs. polystyrene standards and corrected by a factor of 0.56.52 f Dispersity (Mw/Mn) value calculated from SEC traces. g Reaction in 12 mL of toluene. h Reaction in 21 mL of toluene.

1

H NMR characterisation of the polymer samples revealed the formation of α,ω-dihydroxy telechelic PCLs in agreement with literature data on the ROP of CL promoted by rare-earth borohydride complexes.52,63 Molar mass values were thus determined from NMR spectra using the methylene signals in α-position of the end-capping hydroxyl (δCH2OH, δ 3.65 ppm; refer to the Experimental section). The control of the polymerization obtained from 2–9 in terms of experimental/theoretical molar mass values agreement (Mn,SEC and Mn,NMR vs. Mn,theo) and of rather narrow dispersity values, was thus generally observed throughout the rare-earth series. The PCL molar mass values fall in the range Mn,NMR = 7400–101 300 g mol−1, while ĐM = 1.12 (2) and 1.18–1.55 (3–9) values fit with the lower range of typical data reported for trivalent rare-earth borohydride initiators (ĐM = 1.16–1.83).63 These dispersity values suggested a possible mismatch between the rates of initiation and propagation, and/or the presence of some, yet rather limited, transesterification side reactions (intermolecular (reshuffling) and intramolecular (backbiting) reactions)

12342 | Dalton Trans., 2015, 44, 12338–12348

typically encountered in the ROP of cyclic esters.64 Thus, PCL diols were rather easily synthesized with a reasonable control. Further evidence of some living character of the polymerization of CL was gained from a second-feed experiment. A PCL sample was first synthesized from the ROP of 150 equiv. of CL in toluene with the samarium initiator 4 (Table 2, entry 10). After 15 min, an aliquot of the mixture was removed and analyzed (quantitative CL consumption, Mn,NMR = 16 600 g mol−1, ĐM = 1.34), and the polymerization was then resumed by the subsequent addition of 150 equiv. of CL. After 15 min, the polymer recovered from this mixture featured a Mn,NMR = 31 400 g mol−1 with ĐM = 1.37 (Table 2, entry 12). The ROP of trimethylene carbonate (TMC) was then investigated from a few selected initiators (Scheme 5). The divalent and trivalent complexes 2, 5, and 8, respectively, enabled the ROP of TMC in toluene at room temperature with a good efficiency (Table 3). Medium to high TMC conversions were generally obtained using [TMC]0/[initiator]0 ratios in the range 50–250. In contrast

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Scheme 5 ROP of trimethylene carbonate initiated by the rare-earth borohydrides 3, 5, 7.

to the polymerization of CL (Table 2), quantitative TMC consumption was not reached within 10–30 min (Table 3), but rather in 120 min (entry 5). Despite this lower activity, the polymerization of TMC with complexes 2, 5, and 8 showed a 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 all poly(trimethylene carbonate) (PTMC) samples have 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 studies on the ROP of TMC using [Sm(BH4)3(THF)3]65 and [(dipp)2NacNacLn(BH4)2(THF)] ((dipp)2NacNac = (2,6-C6H3iPr2)NC(Me)CHC(Me)N(2,6-C6H3iPr2); Ln = Sm, Yb) (Scheme 5).66 As previously reported,66 Mn,NMR values were 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).66 This data agreed in most cases with the expected molar mass value (Mn,theo) as well as with the experimental data determined by SEC. Considering the methylene chain-end signal ((CH2OC(O)H; δ = 8.05 ppm) which integrated in some cases (Table 3, entries 1, 7, 10) to ca. 0.4–0.6 vs. 2 for the chain-end hydroxyl group (CH2OH; δ = 3.73 ppm), suggested the formation of both the α,ω-dihydroxy telechelic PTMC (contributing to only this latter resonance) along with the α-hydroxy,ω-formate telechelic PTMC (twice as much as the PTMC diol in these cases). Such a

Table 3

behavior was previously observed experimentally and computationally predicted as well in the ROP of TMC from the related borohydride complexes [{CH(PPh2NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu).46,63 PTMCs of molar mass up to Mn,NMR = 21 900 g mol−1 were thus prepared. The dispersity values of the recovered PTMC samples (ĐM = 1.34–1.82) were slightly higher than those measured for PCL (Table 2), a trend common in the ROP of these two classes of cyclic esters, namely lactones and carbonates63 and suggesting undesirable transcarbonatation reactions, which are also often encountered in the ROP of cyclic carbonates.67–77 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], [{(Me3SiNPPh2)2CH}Lu(BH4)2], and [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sm, Yb).46,63,66

Experimental section78 General All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in flamedried 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 potassium 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), [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),79

Characteristics of the ROP of TMC initiated by 2, 3, or 8 in toluene at 23 °Ca

Entry

Initiator

[TMC]0/ [BH4]0

Reaction timeb (min)

TMC conv.c (%)

Mn,theod (g mol−1)

Mn,NMR c (g mol−1)

Mn,SECe (g mol−1)

ĐM f (Mw/Mn)

1 2 3 4 5 6 7 8 9 10

2 2 2 4 4 4 4 8 8 8

50 150 250 50 100 150 250 50 150 250

10 10 30 10 120 10 30 10 10 30

73 50 50 72 100 31 86 84 45 78

3.700 7.700 12.750 3.700 10.200 4.750 21.950 4.300 6.900 19.900

11.450 7.900 11.200 5.100 9.700 5.000 16.750 4.700 5.500 12.400

14.500 24.800 41.700 7.100 13.000 8.400 26.500 6.600 10.300 20.800

1.67 1.61 1.82 1.49 1.37 1.40 1.61 1.40 1.34 1.46

a

All reactions were performed in 0.5 mL of toluene at 23 °C; results are representative of at least duplicated experiments. b Reaction times were not necessarily optimized. c Monomer conversion determined by 1H NMR spectroscopy of the crude reaction mixture (refer to Experimental section). d Theoretical molar mass value calculated assuming one (2) or two (4, 8) growing polymer chains from [TMC]0/[BH4]0 × conv.TMC × MTMC, with [BH4]0 = [2]0, or 12 [3–9]0, and MTMC = 102 g mol−1. e Number-average molar mass value determined by SEC in THF at 30 °C vs. polystyrene standards and corrected by a factor of 0.73.65 f Dispersity (Mw/Mn) value calculated from SEC traces.

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[Sc(BH4)3(THF)2],79 [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb),80,81 [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (1)53 were prepared according to literature procedures. CL (Aldrich) was dried over CaH2 (at least one week) prior to distillation. Trimethylene carbonate (TMC, 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 recrystallized from cold THF. Size-exclusion chromatography (SEC) giving numberaverage 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 PCL52,82 and 0.73 for PTMC65 (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 Int.PCL/[Int.PCL + Int.ε-CL], using the δCH2OC(O) methylene triplet for PCL and CL (δPCL 4.04 ppm, δCL 4.19 ppm), or from the integration (Int.) ratio Int.PTMC/ [Int.PTMC + Int.TMC], using the δCH2OC(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 hydrogen atoms (δCH2OC(O), δ 4.04 ppm), and those of the PCL chain-end methylene hydrogen atoms (δCH2OH, δ 3.65 ppm), or of the PTMC main-chain methylene hydrogen atoms (δCH2OC(O), δ 4.23 ppm), and those of the PTMC chain-end methylene protons (δCH2OH, δ 3.73 ppm). [{CH(PPh2vNSiMe3)(PPh2vS)}Yb(BH4)(THF)2] (2). THF (25 ml) was condensed at −78 °C onto a mixture of [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (261 mg, 0.24 mmol) and [Yb(BH4)2(THF)2] (167 mg, 0.48 mmol) and was allowed to warm to room temperature. The resulting deep orange-red suspension was stirred over 20 h at room temperature. The solution was filtered off and then concentrated to 10 mL. Storage at −20 °C for five days afforded the product as orange crystals suitable for X-ray analysis. Yield: 85 mg (21%) of orange-red crystals. 1 H NMR (THF-d8, 300.13 MHz, 25 °C): δ −0.08 (s, 9 H, SiCH3), 0.29–1.26 (q, br, 4 H, BH4, 1JH–B = 81.2 Hz), 2.21 (br, 1 H, P-CH-P), 7.07–7.31 (m, 12 H, o-, p-PPh), 7.66–7.83 (m, 8 H, m-PPh) ppm. 11B NMR (THF-d8, 128.38 MHz, 25 °C): δ −34.4 (qt, 1JH–B = 82.4 Hz) ppm. 13C{1H} NMR (THF-d8, 75.48 MHz, 25 °C): δ 3.1 (d, 3JC–P = 4.2 Hz, SiCH3), 25.4 (P-CH-P), 127.2, 127.3, 129.4, 129.8, 131.3 (Ph), 137.2 (dd, i-PPh, 1JP–C = 91.4 Hz,

12344 | Dalton Trans., 2015, 44, 12338–12348

Dalton Transactions 3

JP–C = 5.8 Hz) ppm. 29Si{1H} NMR (THF-d8, 59.63 MHz, 25 °C): δ −7.3 (d, 2JSi–P(N) = 4.6 Hz) ppm. 31P{1H} NMR (THF-d8, 121.49 MHz, 25 °C): δ 15.6 (d, PN, 2JP(N)–P(S) = 14.8 Hz), 31.2 (d, PS, 2JP(S)–P(N) = 14.8 Hz) ppm. 171Yb{1H} NMR (THF-d8, 70.02 MHz, 25 °C): δ 757.3 (dd, br) ppm. IR (ATR, cm−1): 3053 (vw), 2948 (m), 2387 (w), 2219 (br), 1481(w), 1436 (s), 1305 (br), 1242 (m), 1153 (s), 1100 (s), 1068 (w), 1035 (w), 999 (w), 932 (w), 829 (s), 772 (m), 737 (s), 706 (m), 691 (vs), 659 (m), 632 (m), 603 (m), 588 (m), 549 (w). EI-MS (70 eV, 220 °C): m/z (%) = 692 ([M + H]+, 6), 676 ([M − (BH4)]+, 4), 488 ([(Me3SiNPPh2) (SPPh2)CH − CH3]+, 34), 471 ([(Me3SiNPPh2)(SPPh2)CH − 2CH3 − 2H]+, 18), 456 ([(Me3SiNPPh2)(SPPh2)CH − 3CH3 − 2H]+, 100). HR-MS (EI, 70 eV, 220 °C): m/z = 691.355 (calc. for C28H34P211B32SN28Si174Yb: 691.113). Element. Anal. C36H50BNO2P2SSiYb (834.74): calc. C, 51.80; H, 6.04; N, 1.68; S, 3.84; found C, 51.99; H, 6.33; N, 1.65; S, 3.88. [{CH(PPh2vNSiMe3)(PPh2vS)}Ln(BH4)2(THF)] (3–9). General procedure: THF (30 ml) was condensed at −78 °C onto a mixture of [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 and [Ln(BH4)3(THF)3] and was allowed to warm to room temperature. The resulting suspension was stirred at 60 °C for two days, then cooled down to room temperature, filtered off and concentrated to 10 mL. Storage at −20 °C for three days afforded the product as colourless crystals suitable for X-ray analysis. [{CH(PPh2vNSiMe3)(PPh2vS)}Y(BH4)2(THF)] (3). [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (525 mg, 0.48 mmol) and [Y(BH4)3(THF)3] (339 mg, 0.97 mmol). Yield: 354 mg (54%) of colourless crystals. 1 H NMR (THF-d8, 300.13 MHz, 25 °C): δ 0.00 (s, 9 H, SiCH3), 0.21–1.23 (q, br, 4 H, BH4), 2.49 (br, 1 H, P-CH-P), 6.85–7.13 (m, 4 H, Ph), 7.28–7.71 (m, 12 H, Ph), 7.84–8.12 (m, 4 H, Ph) ppm. 11B NMR (THF-d8, 128.38 MHz, 25 °C): δ −24.4 (qt, 1JH–B = 84.8 Hz) ppm. 13C{1H} NMR (THF-d8, 75.48 MHz, 25 °C): δ 2.8 (d, 3JC–P = 3.9 Hz, SiCH3), 25.4 (P-CH-P), 127.2–128.6 (m, Ph), 129.1–130.6 (m, Ph), 131.0–132.3 (m, Ph) ppm. 29Si{1H} NMR (THF-d8, 59.63 MHz, 25 °C): δ −0.5 (d, br) ppm. 31P{1H} NMR (THF-d8, 121.49 MHz): δ 19.7 20.3 (dd, PNSi(CH3)3, 2J (P,Y) = 4.5 Hz, 2J (P,P) = 6.4 Hz), 33.2 (dd, PS, 2J (P,Y) = 4.4 Hz, 2J (P,P) = 7.1 Hz) ppm. IR (ATR, cm−1): 3055 (vw), 2924 (m), 2853 (w), 2436 (w), 2323 (m), 2287 (m), 2237 (m), 2166 (m), 1482 (w), 1436 (s), 1308 (br), 1259 (m), 1248 (m), 1099 (s), 1068 (s), 1026 (m), 999 (m), 913 (m), 839 (vs), 799 (s), 768 (s), 739 (vs), 727 (vs), 707 (m), 690 (vs), 632 (m), 617 (s), 597 (s), 542 (m). Element. Anal. C36H54B2NO2P2SSiY (3 THF) (765.46): calc. C, 56.59; H, 7.11; N, 1.83; S, 4.19; found C, 57.24; H, 6.94; N, 1.77; S, 3.67. [{CH(PPh2vNSiMe3)(PPh2vS)}Sm(BH4)2(THF)] (4). [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (285 mg, 0.265 mmol) and [Sm (BH4)3(THF)3] (216 mg, 0.53 mmol). Yield: 137 mg (35%) of colourless crystals. 1 H NMR (THF-d8, 300.13 MHz, 25 °C): δ −10.3 (br, 4 H, BH4), −1.13 (s, 9 H, SiCH3), 0.91 (br, 1 H, P-CH-P), 7.18–7.46 (m, 8 H, m-PPh), 7.49–7.91 (br, 12 H, o-, p-PPh) ppm. 11B NMR (THF-d8, 128.38 MHz, 25 °C): δ −35.5 (br) ppm. 13C{1H} NMR (THF-d8, 75.48 MHz, 25 °C): δ 3.0 (d, 3JC–P = 3.1 Hz, SiCH3), 25.4 (P-CH-P), 127.0–128.6 (m, Ph), 129.4–131.8 (m, Ph),

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132.7–133.2 (m, Ph) ppm. 29Si{1H} NMR (THF-d8, 59.63 MHz, 25 °C): δ −4.25 (br) ppm. 31P{1H} NMR (THF-d8, 121.49 MHz, 25 °C): δ 45.1 (br, PN), 52.6 (br, PS) ppm. IR (ATR, cm−1): 3057 (vw), 2924 (m), 2853 (w), 2445 (m), 2205 (br), 1483 (w), 1457 (w), 1436 (s), 1308 (w), 1259 (m), 1247 (m), 1158 (s), 1107 (s), 1076 (s), 1017 (w), 999 (m), 932 (m), 837 (vs), 784 (m), 764 (m), 742 (s), 725 (s), 705 (s), 690 (vs), 664 (m), 614 (m), 594 (m), 542 (m), 506 (s). No matching elemental analysis could be obtained. [{CH(PPh2vNSiMe3)(PPh2vS)}Tb(BH4)2(THF)] (5). [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (284 mg, 0.26 mmol) and [Tb (BH4)3(THF)3] (220 mg, 0.52 mmol). Yield: 168 mg (42%) of colourless crystals. IR (ATR, cm−1): 3056 (vw), 2922 (s), 2853 (m), 2221 (br), 1459 (m), 1436 (m), 1376 (m), 1303 (m), 1247 (m), 1149 (m), 1101 (m), 1068 (m), 1026 (m), 998 (m), 914 (m), 830 (s), 769 (m), 769 (m), 740 (vs), 727 (vs), 707 (m), 689 (m), 632 (m), 615 (m), 596 (m), 541 (m). Element. Anal. C36H54B2NO2P2SSiTb (5·THF) (835.47): calc. C, 51.75; H, 6.51; N, 1.68; S, 3.84; found C, 52.47; H, 6.75; N, 1.71; S, 3.43. [{CH(PPh2vNSiMe3)(PPh2vS)}Dy(BH4)2(THF)] (6). [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (767 mg, 0.655 mmol) and [Dy (BH4)3(THF)3] (552 mg, 1.31 mmol). Yield: 336 mg (34%) of colourless crystals. IR (ATR, cm−1): 3056 (vw), 2922 (s), 2853 (m), 2435 (w), 2223 (br), 1458 (w), 1437 (m), 1375 (m), 1306 (m), 1259 (m), 1247 (m), 1149 (m), 1116 (m), 1099 (s), 1083 (s), 1068 (s), 1027 (m), 998 (m), 917 (m), 841 (vs), 782 (m), 768 (m), 739 (vs), 727 (vs), 707 (s), 690 (vs), 666 (m), 631 (m), 619 (m), 596 (m), 541 (m). Element. Anal. C36H54B2NO2P2SSiDy (6·THF) (839.04): calc. C, 51.53; H, 6.49; N, 1.67; S, 3.82; found C, 51.00; H, 6.59; N, 1.69; S, 4.08. [{CH(PPh2vNSiMe3)(PPh2vS)}Er(BH4)2(THF)] (7). [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (463 mg, 0.425 mmol) and [Er (BH4)3(THF)3] (366 mg, 0.85 mmol). Yield: 378 mg (57%) of pale pink crystals. IR (ATR, cm−1): 3056 (vw), 2924 (m), 2853 (w), 2438 (w), 2224 (br), 1481 (w), 1457 (w), 1436 (m), 1307 (m), 1259 (m), 1246 (m), 1172 (m), 1148 (m), 1117 (s), 1099 (s), 1084 (s), 1068 (m), 1026 (m), 1014 (m), 999 (m), 914 (m), 837 (vs), 782 (m), 768 (s), 739 (s), 727 (vs), 707 (m), 691 (vs), 665 (m), 619 (m), 598 (s), 542 (m), 513 (m). EI-MS (70 eV, 140 °C): m/z (%) = 698 ([M]+, 2σ(I)). The final R1 values were 0.0475 (all data). The final wR(F2) values were 0.0869 (all data). Crystal data for 3: C32H46B2NOP2SSiY·C4H8O, M = 765.42, a = 9.843(2) Å, b = 11.425(2) Å, c = 36.050(7) Å, β = 97.48(3)°, V = 4019.6(14) Å3, T = 150 K, space group P21/n, Z = 4, 37 089 reflections measured, 8503 independent reflections (Rint = 0.1139). The final R1 values were 0.0523 (I > 2σ(I)). The final wR(F2) values were 0.0688 (I > 2σ(I)). The final R1 values were 0.1074 (all data). The final wR(F2) values were 0.0795 (all data). Crystal data for 4: C32H46B2NOP2SSiSm, M = 754.76, a = 12.8630(5) Å, b = 14.1648(5) Å, c = 15.3321(6) Å, α = 107.559(3)°, β = 106.975(3)°, γ = 96.833(3)°, V = 2480.80(17) Å3, T = 150 K, ˉ, Z = 2, 20 469 reflections measured, 9205 indespace group P1 pendent reflections (Rint = 0.0648). The final R1 values were 0.0431 (I > 2σ(I)). The final wR(F2) values were 0.1096 (I > 2σ(I)). The final R1 values were 0.0514 (all data). The final wR(F2) values were 0.1135 (all data). Crystal data for 5: C32H46B2NOP2SSiTb·C4H8O, M = 835.43, a = 9.8355(3) Å, b = 11.4119(4) Å, c = 36.1580(11) Å, β = 97.657 (2)°, V = 4022.2(2) Å3, T = 150 K, space group P21/n, Z = 4, 30 568 reflections measured, 7935 independent reflections (Rint = 0.1463). The final R1 values were 0.0531 (I > 2σ(I)). The final wR(F2) values were 0.1162 (I > 2σ(I)). The final R1 values were 0.1072 (all data). The final wR(F2) values were 0.1384 (all data). Crystal data for 6: C32H46B2DyNOP2SSi·C4H8O, M = 839.01, a = 9.8348(7) Å, b = 11.4038(6) Å, c = 36.103(3) Å, β = 97.516(6)°, V = 4014.3(5) Å3, T = 150 K, space group P21/n, Z = 4, 30 469 reflections measured, 7838 independent reflections (Rint = 0.1482). The final R1 values were 0.0638 (I > 2σ(I)). The final wR(F2) values were 0.1304 (I > 2σ(I)). The final R1 values were 0.1069 (all data). The final wR(F2) values were 0.1448 (all data). Crystal data for 7: C32H46B2ErNOP2SSi·C4H8O, M = 843.77, a = 9.835(2) Å, b = 11.424(2) Å, c = 36.010(7) Å, β = 97.21(3)°, V = 4013.7(14) Å3, T = 150 K, space group P21/n, Z = 4, 36 768 reflections measured, 10 832 independent reflections (Rint = 0.1001). The final R1 values were 0.0659 (I > 2σ(I)). The final wR(F2) values were 0.1551 (I > 2σ(I)). The final R1 values were 0.1049 (all data). The final wR(F2) values were 0.1810 (all data). Crystal data for 8: C32H46B2NOP2SSiYb·C4H8O, M = 849.55, a = 9.8319(2) Å, b = 11.5940(3) Å, c = 35.9470(9) Å, β = 97.227 (2)°, V = 4065.08(17) Å3, T = 150 K, space group P21/n, Z = 4, 24 215 reflections measured, 7540 independent reflections (Rint = 0.0502). The final R1 values were 0.0278 (I > 2σ(I)). The final wR(F2) values were 0.0595 (I > 2σ(I)). The final R1 values were 0.0411 (all data). The final wR(F2) values were 0.0624 (all data). Crystal data for 9: C32H46B2LuNOP2SSi·C4H8O, M = 851.48, a = 9.7913(4) Å, b = 11.4667(4) Å, c = 35.8275(19) Å, β = 97.003 (4)°, V = 3992.5(3) Å3, T = 150 K, space group P21/n, Z = 4, 31 573 reflections measured, 8389 independent reflections (Rint = 0.1301). The final R1 values were 0.0652 (I > 2σ(I)). The final wR(F2) values were 0.1283 (I > 2σ(I)). The final R1 values were 0.1125 (all data). The final wR(F2) values were 0.1441 (all data).

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Dalton Transactions

Polymerization ROP of CL or TMC initiated by 2–9. In a typical experiment (Table 2, entry 8), 4 (2.5 mg, 12.3 μmol, 1 equiv.) was dissolved in toluene (0.5 mL) prior to the addition of CL (0.38 g, 1.84 mmol, 100 equiv.). The mixture was then stirred at 23 °C over the appropriate reaction time (reaction times were not necessarily optimized). The polymerization was then quenched by addition of acetic acid (ca. 10 μL of a 1.6 mol L−1 solution in toluene). The resulting mixture was concentrated to dryness under vacuum and the conversion was determined by 1H NMR analysis of the residue in CDCl3. The crude polymer was then dissolved in CH2Cl2 (2 mL) and precipitated in pentane (10 mL), filtered and dried under vacuum (typical isolated yield 90–95%). The final polymer was then analyzed by NMR and SEC analyses (Tables 2 and 3). NMR analyses of the PCL and PTMC samples agreed with literature data.52,65

Conclusions In summary, (iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes of divalent ytterbium and trivalent rare-earth elements have been synthesized and structurally characterized by single crystal X-ray diffraction. Reaction of [K{CH(PPh2vNSiMe3)(PPh2vS)}]2 (1) with [Yb(BH4)2(THF)2] in THF resulted in the divalent Yb complex [{CH (PPh2vNSiMe3)(PPh2vS)}Yb(BH4)(THF)2] (2). The trivalent compounds [{CH(PPh2vNSiMe3)(PPh2vS)}Ln(BH4)2(THF)] (Ln = Y (3), Sm (4), Tb (5), Dy (6), Er (7), Yb (8) and Lu (9)) were obtained by treatment of 1 with [Ln(BH4)3(THF)3]. The diamagnetic complexes and the trivalent samarium compound 4 were fully characterized by 1H, 13C{1H}, 11B, and when possible by 171 Yb NMR spectroscopy. The application of the divalent and trivalent compounds as initiators in the ROP of CL and TMC was investigated using short reaction times to minimize the effect of decomposition. All borohydride complexes 2–9 revealed effective, affording a generally quite well-controlled ROP of both these cyclic esters under mild operating conditions. Well-defined PCL diols, and α-hydroxy,ω-formate telechelic PTMC along with PTMC diols were thus prepared.

Acknowledgements 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 de la Matière” (SDLM) of the University of Rennes 1 (fellowship to M.S.). P.O.-B thanks the grant PIOF-GA-2011299571 (7th FP, People Marie Curie Actions) for funding and BITAL (Research Centre for Agricultural and Food Biotechnology). Dr Michael Gamer (KIT) is acknowledged for support in refining the single crystal X-ray structures. A. T. Wagner is acknowledged for designing the cover image.

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