Synthesis and Structural Characterization of Various N,O,N-Chelated

Article pubs.acs.org/Organometallics

Synthesis and Structural Characterization of Various N,O,N-Chelated Aluminum and Gallium Complexes for the Efficient ROP of Cyclic Esters and Carbonates: How Do Aluminum and Gallium Derivatives Compare ? Frédéric Hild,† Nirvana Neehaul,† Frédéric Bier,† Morgane Wirsum,† Christophe Gourlaouen,‡ and Samuel Dagorne*,† †

Institut de Chimie de Strasbourg, CNRS-Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France Laboratoire de Chimie Quantique, Institut de Chimie de Strasbourg, CNRS-Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France

‡

S Supporting Information *

ABSTRACT: The novel Al and Ga coordination compounds 4, 5, 7, and 8 of the type (NON)AlX (NON 2− = {RNC6H4}2O2−; 4, R = Cy, X = Me; 5, R = C5H9, X = OCH2Ph; 7, R = C6F5, X = Me) and (NON)GaX (NON2− = {R2NHC6H4}2O2−; 8, R = C6F5, X = Me) have been synthesized via, in the case of 4, 7, and 8, a methane elimination reaction between the corresponding protio ligands (NON)H2 and MMe3 precursors (M = Al, Ga). The Al alkoxide derivative 5 was prepared via an alcoholysis reaction between κ 3 -N,O,N-{(C 5 H 9 )NC 6 H 4 } 2 O} 2 AlNMe 2 and PhCH2OH. The tetracoordinate Al−THF adduct κ2-N,N′-{(C5H9)NC6H4}2O}2Al(Me)(THF) (6) was prepared via a methane elimination reaction between the protio ligand TfNONH2 (Tf = CF3SO2, 1c) and AlMe3 in a CH2Cl2/THF solvent mixture. As determined from X-ray studies, complexes 4, 7, and 8 are monomeric in the solid state and feature a central tetracoordinate metal center effectively κ3-N,O,N′ chelated and adopting a trigonal-monopyramidal geometry. The presence of Ga···F(o-C6F5) contacts in the solid-state structure of compound 8 are likely to reflect the Lewis acidity of the Ga center. The Al alkoxide derivative [κ3N,O,N-{(C5H9)NC6H4}2O}2AlOCH2Ph]2 (5) was isolated as a dimer containing two five-coordinate (trigonal-pyramidal) Al centers and retains its dimeric structure in solution, while the Al−THF adduct 6 crystallizes in a monomeric form with an Al center in a tetrahedral geometry. The catalytic performances of the Al and Ga species 4, 5, 7, and 8 as ROP initiators of raclactide (rac-LA), ε-caprolactone (ε-CL), and trimethylene carbonate (TMC) were estimated, and most of them efficiently mediate the controlled and immortal ROP of these three monomers in the presence of BnOH, such an alcohol acting as an effective chain transfer agent. Of particular interest, the Ga amido species κ3-N,O,N-{(C5H9)NC6H4}2O}2Ga-NMe2 (3) outperforms its Al counterpart κ3-N,O,N-{(C5H9)NC6H4}2O}2AlNMe2 (2) in the ROP of rac-LA, whether in terms of control or activity, to afford isotactically enriched PLA (Pm = 0.7). In contrast, all the Al derivatives are more efficient catalysts for the polymerization of ε-CL or TMC than the Ga analogues. For the ROP of TMC initiated by the Al and Ga complexes 2−5, an increased Lewis acidity of the metal center is clearly beneficial to both the activity and the ROP control. Notably, the C6F5N Ga species 8 was found to be inactive in the ROP of rac-LA, ε-CL, and TMC, which may be related to both electronic (C−F···Ga interactions) and steric factors.

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INTRODUCTION

as cyclic ester and carbonate ROP initiators, as they typically mediate such polymerization processes in a controlled (and possibly stereocontrolled) manner for the production of tailormade and narrowly disperse (and possibly stereoregular) polymeric materials.2 Among these metal-based ROP initiators, Al(III) alkoxide derivatives historically hold a special place as, for instance, they were the first metal species shown to initiate the controlled ROP of lactide (for the production of high-

The ring-opening polymerization (ROP) of cyclic esters, such as lactide (LA) and ε-caprolactone (ε-CL), and cyclic carbonates, such as trimethylene carbonate (TMC), initiated by metal alkoxide compounds has been the subject of numerous studies over the past 10−15 years, which is due to the biodegradability and biocompatibility of the resulting materials, such as polylactic acid (PLA) and poly(trimethylene carbonate) (PTMC).1,2 In this area, discrete ligand-supported complexes of Lewis acidic and oxophilic metals (M = Al(III), Sn(II), Zn(II), Mg(II), Ca(II), Ln(III)) have met great success © XXXX American Chemical Society

Received: November 19, 2012

A

dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

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(NON)H2 protio ligands bearing electron-withdrawing N ubstituents. To this end, aside from the (NON)H2 ligands {(C6H11)NHC6H4}2O (1a) and {(C5H9)NHC6H4}2O (1b),8a two novel (NON)H2 ligands {(Tf)NHC6H4}2O (1c;Tf = SO2CF3) and {(C6F5)NH-C6H4}2O (1d) were prepared in reasonable yields by adapting reported literature procedures, and their synthesis is depicted in Scheme 1.9 Also, the synthesis

molecular-weight PLA) via the now well-established ROP mechanism.3 Also, pioneering reports by Spassky and others on the possibility of using ligand-supported Al alkoxide initiators such as (salen)AlOR-type initiators for the stereoselective ROP of rac-lactide constituted important breakthroughs in the field.4 Yet, despite their relative low cost and ready availability, Al derivatives for use as ROP initiators of cyclic esters typically feature a lower ROP activity than other oxophilic metal complexes and are rather moisture sensitive.5 In addition, the controversial toxicity of Al(III) is likely to severely limit the potential usefulness of Al(III)-based ROP initiators for industrial applications. Contrasting with the numerous studies on Al-based ROP initiators of cyclic esters and carbonates, the use of metal initiators of the lower group 13 metals, i.e. Ga(III) and In(III), remains much less studied, with, in the case of Ga, only a couple of recent reports on lactide ROP.6,7 These investigations clearly evidenced that ligand-supported Ga(III) and In(III) alkoxides may initiate the controlled polymerization of raclactide with, in some cases, some degree of stereocontrol.6,7 Focusing on Ga, key attractive features include a better biocompatibility of Ga(III) vs Al(III) and a higher stability of Ga(III) alkyl/amido/alkoxide compounds in protic and polar media, which is of interest in polymerization catalysis. While the use of Ga(III) initiators for the ROP of cyclic esters/ carbonates is possibly promising, the potential usefulness and the particularities of Ga(III) species as ROP initiators deserve further study. In particular, it may be useful to assess how such derivatives perform in comparison to their Al(III) counterparts. We have become interested in the synthesis and potential use in catalysis of tetracoordinate Al(III) and Ga(III) species supported by adequate N,O,N diamido-ether dianionic ligands forcing the group 13 metal center into a trigonal-monopyramidal coordination geometry.8 Such distorted N,O,Nsupported Al and Ga complexes, whose increased reactivity arises from a destabilizing ligand-defined geometry rendering the metal center more Lewis acidic, appear as promising candidates for the efficient mediation of catalytic transformations, whether in polymerization catalysis or in the production of fine chemicals. On that matter, in preliminary studies, the Al amido derivative κ 3 -N,O,N-{(C 5 H 9 )NC6H4O}2AlNMe2 was observed to be highly active in the ROP of trimethylene carbonate in a controlled and immortal manner,8a while its Ga analogue was shown to be an effective catalyst for the hydroamination of terminal alkynes.8b The latter constitutes a rare instance of group 13 catalyzed hydroamination reactions, illustrating the reactivity potential of these N,O,N-bearing group 13 systems. To further probe and exploit the potential in polymerization catalysis of these reactive N,O,N-chelated group 13 systems and to gain insight into the relative performance in polymerization catalysis of Al vs Ga analogues, the present contribution reports on the synthesis and structural characterization of various N,O,N-supported Al and Ga derivatives and their use in the ROP of cyclic esters (rac-LA, ε-CL) and trimethylene carbonate.

Scheme 1

and characterization of (NON)MX species incorporating various MX groups, i.e. X = Me, NMe2, OBn, were carried out for further use in polymerization catalysis. Earlier work on the coordination chemistry of ligands 1a,b with Al(III) and Ga(III) was restricted to the synthesis of the corresponding MNMe2 derivatives κ3-N,O,N{(C5H9NC6H4)2O}MNMe2 (2, M = Al; 3, M = Ga) via an amine elimination reaction between 1a,b and M2(NMe2)6 (M = Al, Ga), thus with both the Al and Ga amido precursors being suitable for the preparation of the desired (NON)MNMe2 derivatives.8 In contrast, we found in the present study that access to the corresponding MMe derivatives via a methane elimination route (reaction of (NON)H2 with MMe3) only to be successful for Al(III). Thus, while the Al methyl analogue κ3N,O,N-{(CyNC6H4)2O}AlMe (4) was prepared in good yield via reaction of ligand 1a with AlMe3 (120 °C, 5 days, toluene; Scheme 2), the formation of the Ga analogue (from a 1/1 mixture of GaMe3 and ligand 1b) was never observed even after a prolonged reaction time (120−150 °C, up to 7 days in Scheme 2

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RESULTS AND DISCUSSION Synthesis and Structure of a Variety of N,O,N-Bearing Al and Ga Complexes. A goal of the present study was the synthesis of various highly Lewis acidic tetracoordinate Al and Ga species of the type (NON)MX (for subsequent use as ROP initiators of cyclic esters/carbonates) through the use of B


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toluene). An NMR monitoring experiment of the latter reaction (C6D6, 120 °C, overnight) indicated the quantitative formation of a single species identified as the amido amino ether Ga dimethyl species κ 3 -N,O,N-{(C 5 H 9 )NC 6 H 4 OC 6 H 4 NH(C5H9)}GaMe2 (4′; Scheme 2) on the basis of 1H and 13C NMR data. In particular, the 1H NMR spectrum for species 4′ contains a characteristic NH-Cy doublet resonance and two CH-C5H9 signals, while the 13C NMR spectrum exhibits 2 GaMe resonances and 12 signals in the aromatic region. These NMR data are thus consistent with an overall C1-symmetric structure at room temperature. Compound 4′, which arises from a methane elimination reaction between GaMe3 and only one of the amino groups of ligand 1b, surprisingly yielded an intractable mixture of compounds upon further heating, which possibly arises from thermolysis reactions involving the ligand backbone (prior to a clean aminolysis of a Ga−C bond). The molecular structure of the Al methyl complex 4 was determined by X-ray crystallographic studies, confirming the formation of an effective κ3-N,O,N-(NON)Al chelate with an Al center in a trigonal-monopyramidal (tmp) geometry (Figure 1).

Scheme 3

lated as an analytically pure solid and formulated as a dimer on the basis of solid-state and solution data. In the solid state and as determined by X-ray crystallographic studies (Figure 2), compound 5 indeed crystallizes as a dimer

Figure 2. Molecular structure of the Al complex 5. Hydrogen atoms and the Ph groups of the PhCH2OAl groups are omitted for clarity. Selected bond lengths (Å) and angles (deg): Al(1)−N(1) = 1.870(3), Al(1)−N(2) = 1.846(3), Al(1)−O(1) = 2.027(2), Al(1)−O(4) = 1.878(2), Al(2)−N(3) = 1.858(3), Al(2)−N(4) = 1.836(3), Al(2)− O(2) = 1.858(2), Al(2)−O(3) = 2.016(2); O(1)−Al(1)−O(4) = 163.6(1), N(1)−Al(1)−N(2) = 120.4(1), N(3)−Al(2)−N(4) = 121.0(1), O(2)−Al(2)−O(3) = 166.4(1).

and may be seen as two κ3-N,O,N-{(C5H9NC6H4)2O}Al units being linked to one another via two μ-OBn alkoxide units, which results into a pseudo C2-symmetric compound (with a pseudo-C 2 axis of symmetry orthogonal to the Al 2 O 2 quadrilateral and going through its center; Figure 2), and with both five-coordinate Al centers adopting nearly ideal trigonal-bipyramidal (tbp) geometries (see, in particular, the angle values in Figure 2). The Al−N amido and Al−(μ-O) bond distances (average 1.852(5) and 1.868(4) Å, respectively) are within the expected range for these types of bonds.10,11 Overall, the bonding parameters for species 5 are rather normal and, with the exception of its dimeric nature, comparable to those in the Al methyl and amido analogues 2 and 4. It is noteworthy that the dimeric vs monomeric nature of the Al benzyloxide species 5 vs compounds 2 and 4 further illustrates the known propensity of group 13 alkoxide species to readily aggregate. In solution (C6D6, room temperature), the NMR data for species 5 agree with an effective C2h-symmetric structure (with an effective C2-symmetric axis and an orthogonal plane of symmetry). In particular, the 13C{1H} spectrum of 5 contains two singlet resonances (δ 69.1 and 57.1) assigned to the AlO-CH2Ph and the CH-C5H9 moieties, respectively, while two other 13C signals were attributed to the

Figure 1. Molecular structure of the Al complex 4. Hydrogens atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Al−N(1) = 1.837(2), Al−N(2) = 1.854(2), Al−C(1) = 1.947(3), Al− O = 1.937(2); N(1)−Al−-N(2) = 122.2(1), N(1)−Al−C(1) = 118.4(1), N(2)−Al−C(1) = 115.6(1), C(1)−Al−O = 117.0(1).

Overall, the structural and bonding parameters for species 4 are closely related to those for its AlNMe2 analogue 2 with, in particular, the presence of an apical vacant site well-disposed for substrate coordination to the Lewis acidic metal center. The solution NMR data for complex 4 agree with its solid-state structure being retained in solution and with an effective Cssymmetric structure (room temperature, C6D6). Due to the well-known suitability of discrete Al alkoxide species as ROP initiators of cyclic esters, the (NON)AlOR derivative 5 was also prepared for subsequent polymerization studies (Scheme 3). Thus, the amine elimination reaction of the AlNMe2 species 2 with 1 equiv of BnOH (80 °C, toluene, overnight) yielded the formation of the corresponding AlOBn complex [κ3-N,O,N-{(C5H9NC6H4)2O}AlOCH2Ph]2 (5), isoC


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CH2-C5H9 groups. Complex 5 appears to retain its solid-state dimeric nature in solution, as confirmed by DOSY (diffusion ordered spectroscopy) measurements (C6D6, room temperature). From the corresponding diffusion coefficient (D = 5.23 × 10−10 m2 s−1), a hydrodynamic radius of 6.6(6) Å was estimated for 5. This value is to be compared with the mean radius of 5 (7.37 Å) evaluated from its solid-state structure using a standard set of van der Waals radii.12 The DOSYestimated hydrodynamic radius and the calculated mean radius thus match reasonably well, taking into account experimental and calculations margins of error. Overall, the NMR and DOSY data are consistent with the solid-state structure of 5 being retained in solution. Going from N-cycloalkyl to N-Tf substituents in proligands (NON)H2 (1a,b vs 1c) was found to dramatically alter the reactivity/structure of the resulting Al(III) complexes. Thus, despite the screening of various temperature and solvent conditions, the reaction of 1c with the amido precursors M2(NMe2)6 (M = Al, Ga) consistently afforded complicated mixtures of unidentified products. In contrast and unlike ligands 1a,b, ligand 1c reacts quickly with 1 equiv of AlMe3 at low temperature (−35 to 0 °C, 10 min, 1/1 CH2Cl2/THF solvent mixture) to yield the quantitative formation of the Al− THF adduct chelate κ2-N,N-{(CF3SO2)NC6H4}2OAl(Me)(THF) (6; Scheme 4), which was isolated as a highly air-

Figure 3. Molecular structure of the Al complex 6. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Al−N(1) = 1.900(4), Al−N(2) = 1.890(4), Al−O(1) = 1.857(4), Al− C(1) = 1.926(5); N(1)−Al−N(2) = 108.1(2), N(1)−Al−C(1) = 110.3(2), N(2)−Al−C(1) = 121.8(2).

N(2) bite angle (108.1(2)°) of the Al chelate is significantly more acute than that in the related η3-N,O,N-(NON)Al chelate 4 (N(1)−Al−N(2) = 122.2(1)°). Such a smaller N−Al−N bond angle likely arises from the tetrahedral vs tmp geometry at Al, which somewhat pushes away the central Ph−O−Ph donor atom (as reflected by the long Al···O(2) distance of 2.694(2) Å) to minimize ring strain and thus certainly disfavors the formation of a κ3-N,O,N-(NON)Al chelate. As a comparison, the related tetracoordinate Al complex κ 3 -N,N′,N″{(TfNCHiPrCH2)2NBn}AlMe, in which the Al center adopts a tmp geometry and is κ3 supported by a diamido amino ligand, has been reported.14 In our case, the supporting NON2− dianionic ligand incorporates a central ether donor that is less σ-donating (to Al(III)) than an amine. In the case of species 6, the coordination of a THF molecule onto Al to yield a fourcoordinate tetrahedral Al center apparently provides a superior stability to the resulting Al methyl complex.15 Also, in compound 6, the Al−OTHF and Al−C(1) bond distances (1.857(4) and 1.926(5) Å, respectively) are a bit shorter than those observed in related neutral κ2-diamido Al−THF methyl complexes while being comparable to those in cationic Al methyl adducts,16,17 suggesting an important ionic character in these bonds and presumably reflecting a fairly electrophilic Al center in 6. The NMR data for complex 6 agree with a C1symmetric structure in solution under the studied conditions (CD2Cl2, room temperature) and the effective coordination of THF onto Al, as observed in the solid state. In particular, the 19 F NMR spectrum of 6 exhibits two rather sharp singlet CF3 resonances (δ −74.7, −77.4), indicating that the two CF3 groups are not equivalent (Figure S1, Supporting Information). Such a low-symmetry structure for 6 is most likely due to a slow ring inversion of the eight-membered Al metallacycle at room temperature on the NMR time scale. To probe this issue further, high-temperature 19F NMR studies of species 6 were performed, resulting in the observation of a coalescence of the two CF3 signals at 50 °C (C6D6) with a subsequent sharpening of the single CF3 resonance as the temperature is further raised (Figures S2 and S3, Supporting Information). These data also

Scheme 4

sensitive compound, soluble in CH2Cl2 and THF. Species 6 is thermally unstable even in the solid state and decomposes over weeks at low temperature (−35 °C) under N2. However, carrying out the reaction of 1c with AlMe3 in a noncoordinative solvent (CH2Cl2) afforded, in good yield after purification of the crude product, a colorless solid residue insoluble in all common solvents but THF and formulated as “{(CF3SO2)NC6H4}2OAl(Me)” on the basis of elemental analysis. The insolubility of the latter in nearly all solvents (even upon heating) and the fact that it solubilizes in THF to quantitatively yield the Al−THF adduct 6 suggest the formation of {(CF3SO2)NC6H4}2OAl(Me)-based Al polynuclear aggregates upon reacting a 1/1 1c/AlMe3 mixture in CH2Cl2. These observations presumably reflect the instability of the putative κ3-N,O,N-{(CF3SO2)NC6H4}2OAlMe under the studied conditions, this being likely due to the highly Lewis acidic Al metal center in such a derivative. The molecular structure of the Al−THF adduct 6 was determined by X-ray crystallographic analysis, and it is depicted in Figure 3. Unlike compounds 2, 4, and 5, which all feature a κ3-N,O,N-{(C5H9)NC6H4}2OAl chelate with a tmp geometry at Al, the four-coordinate Al center in species 6 adopts a distortedtetrahedral geometry and is supported by the dianionic diamido ligand [{(CF3SO2)NC6H4}2O]2− in a κ2-N,N′ fashion (with thus no coordination of the NON ligand central ether donor to Al) to yield the formation of an eight-membered-ring Al metallacycle in a boat−boat conformation.13 The N(1)−Al− D


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contacts between the Ga center and two o-fluorine atoms (one per C6F5 ring) constitutes a noteworthy structural feature, with the Ga(1)−F(1) and Ga(1)−F(6) distances (2.77 and 2.95 Å, respectively, Figure 4) well below the sum of the van der Waals radii for Ga and F (i.e., 3.34 Å).18 Such interactions clearly reflect the highly Lewis acidic nature of the Ga center in 8 and, in view of the robustness of 8, may play a key role (although not anticipated) in the stability of the Ga species 8. In addition, the room-temperature 19F NMR spectrum of species 8 contains a coalescence signal for the o-C6F5 group, while that at −30 °C features two decoalesced o-C6F5 resonances in a 1/1 ratio (each corresponding to 2F) indicating the presence of two types of oC6F5 fluorines (Figures S4 and S5, Supporting Information). These data are consistent with a restricted/slow rotation around the N−C6F5 bonds on the NMR time scale under the studied conditions. Although such a restrained rotation around the N−C6F5 bonds may be related to the persistence of Ga···F contacts in solution, steric factors may also be at play. As a comparison, the 19F NMR data for the Al analogue 7 from room temperature down to −30 °C only displays sharp resonances for the C6F5 groups. Despite numerous attempts, no suitable X-ray-quality single crystals of the Al compound 7 could be grown. Of relevance to the present C−F···Ga contacts, it may be noted that analogous C−F···M interactions have been observed and studied in group 4 complexes supported by tripodal amido ligands.19 The nature of the Ga···F interactions in species 8 was studied by theoretical methods (see the Experimental Section for further details). Figure 5 features the weak interactions in

allowed an estimation of the free enthalpy of activation of the dynamic process (ΔG⧧ = 14(1) kcal mol−1), a value in line with energy barriers observed for the ring inversion of heteroatomcontaining eight-membered rings.10 The poor stability of derivative 6 overall suggested that such NTf-ligated group 13 metal alkyl compounds may be of limited efficiency in polymerization catalysis and the coordination chemistry of ligand 1c was not extended to Ga(III). In contrast, the use of the C6F5NH-incorporating diamino ether protio ligand 1d provided access to robust Al(III) and Ga(III) Lewis acidic species. Thus, the methane elimination reaction of ligand 1d with 1 equiv of MMe3 (M = Al, Ga; 80 or 130 °C, toluene, overnight) readily afforded the corresponding (NON)-MMe chelates κ3-N,O,N-{(C6F5NC6H4)2O}MMe (7, M = Al; 8, M = Ga; Scheme 5), which were isolated as highly Scheme 5

air-sensitive yet thermally stable colorless solids. Aside from elemental analysis and NMR data, the proposed formulation was confirmed by a X-ray-determined solid-state structure in the case of the Ga derivative 8 (Figure 4). As was somewhat expected, species 8 crystallizes as discrete κ3-N,O,N-{(C6F5N− C6H4)2O}GaMe molecules in which the tetracoordinate Ga metal center adopts a tmp geometry imposed by the κ3coordinated NON2− ligand. The presence of very short Ga···F

Figure 5. Noncovalent interactions in complex 8. The strongly repulsive interactions are shown in red and the strongly attractive ones in blue. The green basins correspond to the purely van der Waals (dispersive) interactions.

complex 8. Around the gallium metal center, large repulsive annuluses represent the repulsive interactions between the lone pairs of the chelating NON ligand. Between the phenyls, there are large areas corresponding to van der Waals interactions between the rings. The NCI approach shows that the interaction between the gallium and fluorine atoms is weak and essentially of van der Waals type, though an ionic contribution could have been expected between the electrophilic Ga center and the negatively charged fluorines. A close inspection shows the presence of a small ionic contribution, highlighted by a small blue spot in the middle of the green area.

Figure 4. Molecular structure of the Ga complex 8. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−N(1) = 1.898(2), Ga(1)−N(2) = 1.923(2), Ga(1)−C(25) = 1.927(3), Ga(1)−O(1) = 2.104(1); N(1)−Ga(1)−N(2) = 110.2(7), N(1)−Ga(1)−C(25) = 125.6(9), C(25)−Ga(1)−N(2) = 123.5(9), C(25)−Ga(1)−O(1) = 111.9(8). E


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Table 1. ROP of rac-LA Initiated by the Al and Ga Complexes 2−5 and 7a entry

cat.

rac-LAb

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

i

100 100 100 100 100 100 100 100 20 100 100 100 100 100 100

2 2i 2j 3i 3k 3k 4l 4m 4m 5i 5i 5i 7m 7m 8m

BnOHb

5

1 1

1 5 5

t (h)c

conversn (%)d

Mn(theor)e

Mn(cor)f

PDIg

1 3 17 1 5 17 15 15 7 1 2 3 15 15 15

45 80 92 100 31 75 0 95 93 42 61 79 98 98 0

6480 11520 2650 14400 4464 10800

14200 12400 2700 12400 3800 8200

3.10 1.66 1.15 1.12 1.09 1.11

13680 2678 6048 8784 11376 14400 2822

8100 1700 6200 7700 7900 15600 3000

1.30 1.20 1.25 1.32 1.23 1.19 1.22

P mh

0.70

0.70

0.62 0.62 0.62

[rac-lactide]0 = 1 M. bIn equiv versus initiator. cReaction time. dMonomer conversion. eCalculated using Mn(theor) = [rac-lactide]0/[BnOH]0 × Mrac‑LA × conversn fMeasured by GPC in THF (30 °C) using PS standards and corrected by applying the appropriate correcting factor (0.58). g Measured by GPC in THF (30 °C). hDetermined by 1H NMR. iToluene, 80 °C. jToluene, 60 °C. kDichloromethane, 40 °C. lDichloromethane, 25 °C. mToluene, 90 °C. a

However, the dispersive forces mainly dominate the Ga···F interactions. Ring-Opening Polymerization of rac-Lactide by the (NON)M−X Complexes 2−5 and 7. The Al and Ga compounds 2−8 were all tested as ROP initiators of rac-lactide and, with the exception of derivatives 6 and 8, were essentially found to polymerize rac-lactide in a controlled manner. The lack of any ROP activity for species 6 is certainly to be ascribed to its thermal instability with a probable decomposition of 6 under polymerization conditions prior to any ROP activity. The polymerization results are presented in Table 1 for various reaction conditions (toluene or CH2Cl2, 25−90 °C, 1−17 h, with/without BnOH). As stated in the Introduction, a primary goal of the present investigations was aimed at comparing Al vs Ga catalytic performances. Thus, remarkably, while the GaNMe2 species 3 efficiently polymerizes rac-lactide (100 equiv of rac-LA, entries 4−6,Table 1) in a controlled manner, the Al amido analogue is less active under identical conditions and produces a broadly disperse PLA material (entries 1 and 2, Table 1), this being consistent with a poorly controlled ROP process. For the ROPs initiated by the Ga species, all polydispersities (PDI) are below 1.15, with SEC data featuring in all cases monomodal symmetric peaks (Figure S6, Supporting Information). This, together with a linear correlation between the molecular weight of the polymer chain and the monomer to polymer conversion, supports a well-controlled polymerization process (Figure 6; for additional kinetic data, see Figure S7, Supporting Information). For the rac-LA ROP carried out in CH2Cl2 at 40 °C (entry 5, Table 1), the resulting PLA material was also analyzed by MALDI-TOF spectrometry (Figure S8, Supporting Information), unambiguously establishing the presence of an NMe2 moiety at the ester end of the formed PLA and the absence of substantial transesterification reactions as the ROP proceeds. In addition, under these conditions, the production of isotactically enriched PLA (Pm = 0.70) was clearly evidenced from decoupled 1H NMR data (Figure S9, Supporting Information). Hence, all data suggest that the ROP of rac-LA by the Ga species 3 proceeds via a classical coordination−insertion mechanism.

Figure 6. Plot of Mn as a function of monomer conversion in the ROP of rac-lactide by the Ga species 3. Conditions: 100 equiv of rac-lactide (vs 3), [rac-lactide]0 = 1 M, CH2Cl2, 40 °C.

Going from the Al amido derivative 2 to its alkoxide analogue 5 dramatically improves rac-LA polymerization control (but with a ROP activity remaining lower than that of the Ga amido species 3), thereby illustrating that the nature of the AlX initiating moiety may be crucial for a well-behaved ROP process. Thus, reasonably narrowly disperse, chain-lengthcontrolled, and OBn-ester-ended PLA is produced upon using compound 5 as an initiator (entries 10−12, Table 1; Figures S10−S13, Supporting Information). While being transesterified (as evidenced by MALDI-TOF; Figure S13, Supporting Information), the resulting PLA features a slight isotactic bias (Pm = 0.62, Figure S14, Supporting Information). As for the (NON)MMe derivatives 4, 7, and 8, they expectedly were found to be inactive in the ROP of rac-LA as singlecomponent initiators, since the MMe moiety is typically a poor initiating group for the ROP of cyclic esters/carbonates. Performing the ROP of rac-LA using a 1/1 4/BnOH mixture (for an anticipated in situ generation of a ROP initiator of the type (NON)AlOBn, entries 8 and 9, Table 1) produced a mixture of PLA material consisting of OBn-ester-ended and NON-ligand functionalized PLA chains, as deduced from MALDI-TOF analysis (Figure S15, Supporting Information). This indicates that, under these ROP conditions (toluene, 90 F


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°C), the ROP of rac-LA may proceed either via an initial LA insertion into the Al−N amido bonds (of (NON)AlX) or into the in situ formed Al−OBn bond to form, after subsequent chain growth, the observed mixture of PLA chains. These results further illustrate that the reaction conditions frequently required for an in situ alcoholysis (of Al alkyl species) are such that competing reactions may also occur. The group 13 complexes 2, 7, and 8 were also tested for raclactide ROP activity in the presence of various amounts of an alcohol source such as BnOH so as to probe their suitability for the immortal ROP of rac-LA.20 As depicted in Table 1 (entries 3 and 13−15), though the Ga compound 8 is inactive, the Al species 2 and 7 were found to efficiently mediate the controlled and immortal ROP of rac-LA in the presence of BnOH (acting as a chain transfer agent) to afford narrowly disperse, chainlength-controlled and, in the case of initiator 2, isotacticenriched PLA (Pm = 0.70). In particular, for the 2/BnOH initiating system, the Mn(calcd) values match well the expected Mn(theor) values (based on the initial [rac-LA]0/[BnOH]0 ratio, Figure S16, Supporting Information). Overall, characterization and polymerization kinetic data for systems 2/BnOH and 7/BnOH (Figures S17−S21, Supporting Information) are consistent with controlled alcohol-mediated chain transfer reactions during these ROP reactions. One may note that the Al derivative 2 and the presumably more Lewis acidic analogue 7 (in the presence of 5 equiv of BnOH) exhibit comparable ROP activities under the studied (unoptimized) conditions with the nearly quantitative polymerization of 100 equiv of racLA within 15 h (60 °C, toluene; entries 3 and 14, Table 1). Also, the lack of ROP activity of the Ga species 8 in the presence of excess BnOH (entries 16 and 17, Table 1) obviously contrasts with the good activity of the other two compounds, which may be related to the presence of Ga···F close contacts (vide supra) that, along with steric factors, apparently severely hinder monomer approach and coordination. The higher ROP activity of the Ga analogue 3 versus the Al species 2 and 5 may be related to the less Lewis acidic character of Ga vs Al, which is expected to result in the formation of less robust κ2-(lactate)M adducts for Ga as the ROP proceeds (A, Chart 1) that are thus more susceptible to chain growth.

immortal manner, allowing the production of narrowly disperse and chain-length-controlled PCL under rather mild conditions. All experimental data (SEC, MALDI-TOF and kinetic data) support controlled and immortal ROP processes in all cases, with BnOH behaving as an effective chain transfer agent (Figures S22−S28, Supporting Information). In particular, all data agree with PCLs bearing OBn-ester-end groups (as deduced from MALDI-TOF data), with the exception of the PCL produced via the ROP of ε-CL initiated by the Al species 2. For the latter, the MALDI-TOF data agree with the obtainment of two different chain-ended PCLs: OBn- and NMe2-ester-ended groups (Figure S24, Supporting Information). The presence of PCL chains bearing NMe2-ester-end groups reflects that the ROP of ε-CL initiated by 2 proceeds, at least to some extent, via a coordination−insertion mechanism. However, the presence of OBn-ester-ended groups along with the fact that no reaction is observed between species 2 and BnOH at room temperature (thus under polymerization conditions) suggest that an activated monomer mechanism is also at play.22 The Al compound 2 (in the presence of 5 equiv of BnOH) exhibits the best ROP performances with the quantitative conversion of 100 equiv of ε-CL to well-defined PCL within 10 min at room temperature (entry 1, Table 2) and lies among the most efficient group 13 based developed thus far for the polymerization of ε-CL.2g As for the Ga amido species 3, while displaying a good ROP activity and control (entries 3 and 4, Table 2), it is less active than its Al analogue 2 with a 41% conversion to PCL of 100 equiv of ε-CL within 10 min at room temperature. The (C6F5)N-based Al species 7, although less efficient than the AlNMe2 2, exhibits a good ROP activity, whereas the Ga species 8 is inactive under identical conditions (entry 6 vs 8, Table 2). The lower performances of the Ga vs Al analogues may be ascribed to the less Lewis acidic character of Ga vs that of Al, this being apparently the key factor determining an efficient ROP of ε-CL. In line with this and within the Al compounds, the more Lewis acidic AlMe species 7 performs significantly better than its analogue 4 (entry 6 vs 5, Table 2). For all metal complexes, the ROP of ε-CL proceeds in a highly controlled manner. Ring-Opening Polymerization of TMC by the (NON)MX Complexes 2−4 and 7. Well-defined metal initiators able to ring-open polymerize cyclic carbonates such as TMC to produce chain-length-controlled PTMC in an efficient/controlled manner and under mild reaction conditions remain surprisingly rare.23 In preliminary studies on group 13 (NON)MX-type complexes, the Al amido 2 was found to be highly active at room temperature in the ROP of TMC for the production of narrowly disperse and chain-length-controlled PTMC (entries 1−3, Table 3).8a In the present section, we detail our TMC polymerization studies carried out with the Ga analogues 3 and 8 and with the Al methyl/alkoxide species 4, 5, and 7. As summarized in Table 3, with the exception of the Ga species 8, all tested group 13 species were found to readily mediate the ROP of TMC at room temperature in the presence of the chain transfer agent BnOH. All available experimental data (SEC, MALDI-TOF, and kinetic data, Figures S29−S41, Supporting Information) support fairly well-controlled, immortal ROP processes with the formation of OBn-ester-end PTMC material, except for the PTMC resulting from the TMC ROP initiated by the Ga amido species 3. Thus, for the latter, the MALDI-TOF spectrum of the prepared PTMC contain peaks

Chart 1

Though attempts to cleanly generate κ2-(lactate)M-type adducts (M = Al, Ga) with our systems (by an equimolar reaction of 2 or 3 with (S)-methyl lactate) all were unsuccessful, several reactivity studies of Al complexes toward lactide have shown that the formation of robust η2-(lactate)Altype adducts may significantly hinder and even obstruct subsequent chain growth.21 Ring-Opening Polymerization of ε-CL by the (NON)M−X Complexes 2−4 and 7. As summarized in Table 2, the Al and Ga complexes 2−4 and 7 in the presence of BnOH readily mediate the ROP of ε-CL in a controlled and G


Organometallics

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Table 2. ROP of ε-CL Initiated by the Al and Ga Complexes 2−4 and 7a entry

cat.

ε-CLb

BnOHb

tc

conversn (%)d

Mn(theor)e

Mn(cor)f

PDIg

1 2 3 4 5 6 7 8

2 2 3 3 4 7 7 8

100 600 100 600 100 100 100 100

5 5 5 5 5 5 5 5

10 min 3h 10 min 3h 15 h 10 min 30 min 10 min

100 100 41 95 15 46 95 0

2280 13697 936 13012 432 1050 2166

2300 14400 700 12500 500 600 2400

1.26 1.29 1.15 1.05 1.09 1.06 1.40

a

Conditions: [ε-caprolactone]0 = 1 M, room temperature, CH2Cl2. bIn equiv versus initiator. cReaction time. dMonomer conversion. eCalculated using Mn(theor) = [ε-caprolactone]0/[BnOH]0 × Mε‑CL × conversn. fMeasured by GPC in THF (30 °C) using PS standards and corrected by applying the appropriate correcting factor (0.56). gMeasured by GPC in THF (30 °C).

Table 3. ROP of TMC Initiated by the Al and Ga Complexes 2−5, 7, and 8a entry

cat.

TMCb

BnOHb

t (min)c

conversn (%)d

Mn(theor)e

Mn(cor)f

PDIg

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

2 2 2 3 3 3 3 4 4 4 5 5 5 7 7 7 7 8

100 300 600 100 100 300 600 100 100 600 100 300 600 100 300 600 600 100

5 3 3 5 5 3 3 5 5 3 5 3 3 5 3 3 3 5

5 30h 30h 5 30 180 300 10h 1 30 30h 30h 30h 8 15 15 30 8

90 100 100 19 98 65 75 100 60 32 100 100 100 62 51 29 42 0

1944 10308 20508 495 2107 6738 15500 2148 1332 6642 1809 7760 15420 1373 5310 6024 8676 0

1500 9500 20700 400 2300 9600 22400 2500 1000 12800 1600 12800 25900 1100 1600 1900 3100 /

1.12 1.18 1.12 1.03 1.15 1.21 1.35 1.13 1.15 1.47 1.21 1.26 1.32 1.12 1.33 1.17 1.21 /

a

Polymerization conditions: [TMC]0 = 1 M, dichloromethane, room temperature. bIn equiv versus initiator. cReaction time. dMonomer conversion. Calculated using Mn(theor) = [TMC]0/[BnOH]0 × MTMC × conversn. fMeasured by GPC in THF (30 °C) using PS standards and corrected by applying the appropriate correcting factor (0.57, 0.76, or 0.88).24 gMeasured by GPC in THF (30 °C). hThe reaction time was not optimized.

e

for both OBn-ester-ended and NMe2-amido-end polymer chains (Figure S33, Supporting Information), suggesting, as discussed earlier, that two different mechanisms are involved in the ROP of TMC by species 3: the classical coordination/ insertion mechanism (yielding the NMe2-capped PTMC) and the “activated monomer” mechanism (yielding the OBn-capped PTMC). As a comparison, under identical ROP conditions, only OBn-capped PTMC is produced with the Al amido analogue 2. As deduced from Table 3 (entries 4−7), the activity of the Ga amido 3 is much lower than that of the Al analogue 2 (19% vs quantitative conversion to PTMC of 100 equiv of TMC, 5 equiv of BnOH, 5 min, room temperature) but the conversion of 100 equiv of TMC is nearly quantitative after 30 min with initiator 3. Kinetic studies performed with the Ga initiator 3 (300 equiv of TMC, 3 equiv of BnOH) are all consistent with a ROP proceeding in a controlled manner. In particular, a linear correlation between the Mn(cor) value of the formed PTMC and the monomer conversion during the polymerization reaction is observed (Figure S30, Supporting Information). Also, as depicted in Figure 7, despite Mn values higher than expected, varying the initial Ga initiator/BnOH ratio (for a given amount of monomer) was found to be linearly related to the molecular weight number (Mn) of the resulting polymers.

Figure 7. Plot of the molecular weight number (Mn) of the formed PTMC as a function of [3]0/[BnOH]0 in the ROP of TMC by a 3/ BnOH initiating system. Conditions: 300 equiv of TMC (vs 3), [TMC]0 = 1 M, CH2Cl2, room temperature, 5 h, quantitative conversion.

For a given set of conditions (100 equiv of TMC, 5 equiv of BnOH), an apparent first-order kinetics in monomer was established for the ROP of TMC by 3/BnOH (Figure S31, H


Organometallics

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of ε-CL or TMC. Nevertheless, the results obtained here with the Ga initiator 3 first shows that discrete Ga derivatives are suitable candidates for the controlled and immortal ROP of εCL and TMC. Interestingly, in the case of the ROP of TMC and with these (NON)MX systems, an increased Lewis acidity of the metal center is clearly beneficial to both the activity and the ROP control. Though the present results do not allow any broad conclusion to be drawn, they suggest that, in addition to their biocompatibility, Ga(III) initiators may, in some instances, complement their Al(III) congeners.

Supporting Information) with a rate constant (kobs = 0.006 min−1) consistent with a ROP process being much slower in comparison to that initiated by the Al system 2/BnOH system (kobs = 0.152 min−1; Figure S32, Supporting Information): thus, kobs(Al)/kobs(Ga) ≈ 25. In addition, in the case of 3 and unlike what is observed for species 2, a loss of chain-length control for the resulting PTMC is observed with increasing monomer loadings (entry 3 vs 7, Table 3). Altogether, in the present systems, the obtained results in the ROP of TMC suggest that going from the Al to the less Lewis acidic Ga counterpart is detrimental both to the catalytic activity and, to a lesser extent, to the chain-length control of the resulting material. Within the Al series, the AlMe and AlOCH2Ph compounds 4 and 5 both efficiently polymerize TMC at room temperature in the presence of the chain-transfer agent BnOH to afford chainlength-controlled PTMC with, for the most part, molecular numbers (Mn) in good agreement with the monomer/BnOH initial ratios (entries 8−13, Table 3). While exhibiting TMC ROP performances comparable to those of 2, the Al alkoxide is significantly more active than its Al methyl analogue 4, with a quantitative vs 32% conversion of 600 equiv of TMC to PTMC within 30 min at room temperature (entry 13 vs 10, Table 3). Overall, these data likely reflect the key importance of Lewis acidity for high activity and control in the ROP of TMC. The more Lewis acidic Al center in the amido and alkoxide complexes 2 and 5 vs that in species 4 may rationalize the better ROP activity and control.

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EXPERIMENTAL SECTION

General Procedures. All experiments were carried out under N2 using standard Schlenk techniques or in a MBraun Unilab glovebox. Toluene, pentane, and dichloromethane were collected after going through drying columns (SPS apparatus, MBraun) and stored over activated molecular sieves (4 Å) for 24 h in a glovebox prior to use. Tetrahydrofuran was distilled over Na/benzophenone and stored over activated molecular sieves (4 Å) for 24 h in a glovebox prior to use. CH2Cl2, CD2Cl2, and C6D6 were distilled from CaH2, degassed under a N2 flow, and stored over activated molecular sieves (4 Å) in a glovebox prior to use. All deuterated solvents were obtained from Eurisotop (CEA, Saclay, France). All other chemicals were purchased from Aldrich and were used as received, with the exception of trimethylene carbonate purchased from either TCI Europe Corporation or Boehringer: it was dried over CaH2 in THF for 24 h and precipitated with pentane prior to use. The NMR spectra were recorded on Bruker AC 300, 400, and 500 MHz NMR spectrometers in Teflon-valved J. Young NMR tubes at ambient temperature. 1H and 13 C chemical shifts are reported vs SiMe4 and were determined by reference to the residual 1H and 13C solvent peaks. Elemental analyses for all compounds were performed at the Service de Microanalyse of the Université de Strasbourg (Strasbourg, France). GPC analyses were performed on a system equipped with a Shimadzu RID10A refractometer detector using HPLC-grade THF as the eluent. Molecular weights and polydispersity indices (PDIs) were calculated using polystyrene standards. In the case of molecular weight number (Mn), these were corrected with appropriate correcting factors for the Mn values. MALDI-TOF mass spectroscopic analyses were performed at the Service de Spectrométrie de Masse de l’Institut de Chimie de Strasbourg and run in a positive mode: samples were prepared by mixing a solution of the polymers in CH2Cl2 with a 0.5 mg/100 mL concentration; 2,5-dihydroxybenzoic acid (DHB) was used as the matrix in a 5:1 volume ratio. The compounds Al2(NMe2)6, Ga2(NMe2)6, bis-amino-ether protio ligands {(C6H11)NHC6H4}2O (1a) and {(C5H9)NH-C6H4}2O (1b), the Al complex κ3-N,O,N{(C5H9)NC6H4O}2AlNMe2 (2), and the Ga complex κ3-N,O,N{(C5H9)NC6H4O}2GaNMe2 (3) were all synthesized according to reported literature procedures.8,25 Computational Details. DFT calculations on experimental geometries were performed using the B3LYP functional as implemented in the GAUSSIAN 09 package26 witha 6-31G basis set for all atoms except for gallium. For the latter, the small core pseudopotential from Dolg27 and the associated basis set were used.28 Weak interactions were studied by analysis of the obtained wave function by the NCIPLOT package.29

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SUMMARY AND CONCLUSIONS Through the use of different N-substituted (NON)H2 protio ligands and studies of their coordination chemistry toward Al(III) and Ga(III) precursors, a family of structurally diverse Al and Ga complexes of the type (NON)MX has been synthesized and characterized. Four of these group 13 derivatives were isolated as mononuclear species featuring a central tetracoordinate metal center adopting a tmp geometry (species 3, 4, 7, and 8), whereas the Al alkoxide derivative 5 was isolated as a dimer containing two five-coordinate and tbp Al centers. Remarkably, the reactivity/stability balance is quite subtle in such species and may greatly be impacted by the nature of the N substituents. Thus, while the C6F5N-based species 7 and 8 were initially thought of and anticipated to afford highly Lewis acidic and thus very reactive Al and Ga (NON)MX species, the presence of M···F(o-C6F5) interactions in these species is proposed to significantly quench the Lewis acidity of the metal center, as deduced from structural and catalytic studies. Using the Tf-based (NON)H2 protio ligand 1c (reacted with AlMe3) allowed the isolation of the Al−THF adduct 7, in which the Al center is four-coordinate and adopts a preferred tetrahedral geometry. Due to its presumed instability, the sought after TfN-based species η3-N,O,N-(NON)Al-Me was never observed. In addition to the structural patterns and diversity within the (NON)MX series, the catalytic performances of Al and Ga analogues in the ROP of rac-LA, ε-CL, and TMC were estimated and the vast majority of these derivatives efficiently mediate the controlled and immortal ROP of these three monomers in the presence of BnOH acting an effective chain transfer agent. Of particular interest, the Ga amido species 3 was found to exhibit performance superior to that of its Al counterpart 3 in the ROP of rac-LA, whether in terms of control or activity, to afford isotactically enriched PLA. In contrast, the Al derivatives perform better in the polymerization

{(CF3SO2)NH-C6H4}2O (1c). In a glovebox, 2,2′-oxydianiline (940.0 mg, 4.700 mmol) was charged into a 250 mL roundbottom one-necked flask containing a Teflon-sealed stir bar and was dissolved in dry dichloromethane (10 mL). In another Schlenk flask, NaHCO3 (4 equiv, 1,580 g, 18.80 mmol) was charged and dissolved in dry dichloromethane (10 mL). The sodium carbonate CH2Cl2 solution was then added via cannula to the diamine solution. The resulting mixture was cooled to 0 °C, and triflic anhydride (O(CF3SO2)2; 2.1 equiv, 2.820 g, 1,680 mL, 10.00 mmol) was then added dropwise via a syringe at 0 °C. Upon addition, the solution turned brown and then I


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deduced from 1H NMR data. 1H NMR (C6D6, 400 MHz): δ 7.63 (d, 3JHH = 8.4 Hz, Ar, 1H), 7.00 (t, 3JHH = 8.0 Hz, Ar, 1H), 6.95 (d, 3JHH = 8.0 Hz, Ar, 1H), 6.91 (d, 3JHH = 8.0 Hz, Ar, 1H), 6.86 (t, 3JHH = 8.4 Hz, Ar, 1H), 6.52 (t, 3JHH = 8.8 Hz, Ar, 2H), 6.45 (t, 3JHH = 7.2 Hz, Ar, 1H), 4.57 (d, 3JHH = 8.2 Hz, NH, 1H), 3.73 (dq, 3JHH = 8.5 Hz, 3JHH = 6.5 Hz, CH, 1H), 3.68 (q, 3JHH = 6.5 Hz, CH, 1H), 1.08−2.08 (m, C5H9, 16H), 0.29 (s, CH3, 6H). 13C{1H} NMR (C6D6, 100 MHz): δ 160.8 (Cipso), 151.0 (Cipso), 137.0 (Cipso), 135.7 (Cipso), 124.8 (Ar), 123.2 (Ar), 122.0 (Ar), 119.1 (Ar), 118.2 (Ar), 117.4 (Ar), 113.1 (Ar), 112.0 (Ar), 69.3 (CH-C5H9), 57.4 (CH-C5H9), 34.9 (C5H9), 33.5 (C5H9), 31.5 (C5H9), 30.6 (C5H9), 24.4 (C5H9), 24.2 (C5H9), 24.1 (C5H9), 23.9 (C5H9), −3.4 (Me), −4.4 (Me). [κ3-N,O,N-{(C5H9)NC6H4}2O}2AlOCH2Ph]2 (5). Under a nitrogen atmosphere, the Al amido complex 2 (420.0 mg, 0.519 mmol) was dissolved in anhydrous toluene. Benzylic alcohol (1 equiv, 109 μL) was then added and the mixture heated to 80 °C (oil bath) overnight with stirring. The volatiles were subsequently evaporated to yield a brown oily residue. Recrystallization of the latter from a 1/1 Et2O/pentane mixture (cooled to −35 °C) afforded compound 5 as an analytically pure white solid (200.0 mg, 0.428 mmol, 44% yield). Anal. Calcd for C29H33AlN2O2: C, 74.34; H, 7.10; N, 5.98. Found: C, 74.56; H, 7.01; N, 5.54. 1H NMR (300 MHz, CD2Cl2): δ 7.62 (dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 2H), 7.28 (dt, 3JHH = 6.9 Hz, 4 JHH = 1.5 Hz, 2H), 7.25 (m, 5H), 6.82 (dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H), 6.44 (dt, 3JHH = 7.2 Hz, 4JHH = 1.5 Hz, 2H), 5.21 (s, CH2Ph), 4.18 (q, 3JHH = 8.7 Hz, 2H), 1.36−2.30 (m, 16H). 13 C{1H} NMR (75 MHz, CD2Cl2): δ 145.6 (Ar), 144.8 (Ar), 139.8 (Ar), 128.1 (Ar), 126.5 (Ar), 126.1 (Ar), 124.5 (Ar), 117.0 (Ar), 115.5 (Ar), 112.6 (Ar), 69.1 (OCH2), 57.1 (CHC5H9), 28.4 (C5H9), 25.3 (C5H9). κ2-N,N-{(CF3SO2)NC6H4}2OAlMe (6). Under a nitrogen atmosphere, a precooled pentane solution of AlMe3 (88.5 mg, 1.23 mmol) was quickly syringed into a precooled (−35 °C) CH2Cl2 solution of the bis-triflate diamino-ether ligand 1c (570.0 mg, 1.23 mmol) with vigorous stirring. Upon addition of AlMe3, immediate bubbling took place (formation of methane) and the colorless solution was warmed to room temperature and stirred overnight. After this time, it was evaporated to dryness to yield a colorless crude residue found to be insoluble in aromatic solvents and CH2Cl2. Dissolution of the latter crude product in a minimal amount of a 1/1 THF/Et2O solvent mixture and subsequent storage at −35 °C afforded analytically pure compound 6 as colorless crystals (577 mg, 72% yield). Anal. Calcd for C19H19AlF6N2O6S2 (576.04): C, 39.59; H 3.32; N, 4.86. Found: C, 40.17; H, 3.56; N, 5.32. 1H NMR (400 MHz, C6D6): δ 8.00 (br, 1H, Ar), 7.55 (m, 1H, Ar), 6.96 (m, 6H, Ar), 3.94 (br, 4H, THF), 1.16 (br, 4H, THF), −0.43 (s, 3H, Al-Me). 19F{1H} NMR (376 MHz, C6D6): δ -74.7 (s, CF3), −77.40 (s, CF3). 19F{1H} NMR (376 MHz, C6D6, 333 K): δ − 76.0 (s, CF3). 13C{1H} NMR (75 MHz, C6D6): δ 150.9 (br, Cipso-O), 133.9 (br, Ar), 126.3 (Ar), 125.6 (Ar), 123.2 (Ar), 121.8 (br, CF3), 117.7 (Ar), 74.7 (THF), 24.6 (THF), −12.3 (Al-Me). κ3-N,O,N-{(C6F5)NC6H4}2OAlMe (7). In a glovebox, AlMe3 (27.0 mg, 0.380 mmol) was added to a toluene solution of the bis-amino ligand 1d (200.0 mg, 0.380 mmol) and the resulting greenish yellow solution was stirred at 80 °C overnight. After evaporation to dryness, the resulting green solid was dissolved in a minimum amount of dichloromethane and then precipitated in pentane. The mixture was filtered and further dried in vacuo to afford compound 7 in a pure form as a white

purple. After the addition was complete, the solution was stirred at 0 °C for 15 min and then at room temperature for 2 days. The reaction mixture was quenched with water (200 mL) and diluted with ethyl acetate (400 mL), and brine (200 mL) was added. The phases were separated, the organic phase was dried over MgSO4, and the solvent was evaporated. A green oil was obtained. Ether and pentane were added, and the mixture was kept at −35 °C. The precipitate was filtered, and a white solid was obtained (400.0 mg, 18% yield). Anal. Calcd for C14H10F6N2O5S2 (464.36): C, 36.21; H, 2.17. Found: C, 36.56; H, 2.05. 1H NMR (300 MHz, CDCl3): δ 7.32 (m, 2H, Ar), 6.60 (m, 4H, Ar), 6.38 (m, 2H, Ar). 19F{1H} NMR (282.4 MHz, CDCl3): δ −76.7 (s, CF3). 13C{1H} NMR (75 MHz, C6D6): δ 149.7 (Cipso-O), 129.0 (Ar), 126.2 (Ar), 125.8 (Ar), 125.4 (Ar), 122.8 (q, 1JCF = 290 Hz, CF3), 118.6 (Ar). {(C6F5)NH-C6H4}2O (1d). Under a dry nitrogen atmosphere, 2,2′-oxydianiline (1.08 g, 5.38 mmol) was dissolved in dry THF (15 mL) and added to a hexamethyldisilazane lithium salt suspension (4.050 g, 24.00 mmol, 4.5 equiv) in THF (10 mL) at −78 °C. The mixture was stirred until room temperature was reached and then cooled again to −78 °C. Hexafluorobenzene (2.5 equiv, 2.500 g, 13.45 mmol) was then slowly added, and the reaction mixture was stirred at room temperature for 3 days. The reaction mixture was quenched with water (100 mL), and the product was extracted with diethyl ether (3 × 100 mL). The combined organic layers were dried over K2CO3, and the solvents were evaporated. The resulting brown oil was purified by column chromatography (SiO2, pentane/dichloromethane 7/3 as eluent) to afford analytically pure 1d (1.320 g, 2.480 mmol, 46% yield). Anal. Calcd for C24H10F10N2O: C, 54.15; H, 1.89; N, 5.26. Found: C, 54.14; H, 2.07; N, 5.27. 1H NMR (300 MHz, C6D6): δ 6.83 (dt, 3JHH = 8.7 Hz, 4JHH = 2.1 Hz, 2H), 6.70 (dt, 3JHH = 8.1 Hz, 4JHH = 2.1 Hz, 2H), 6.66 (dd, 3JHH = 7.5 Hz, 4JHH = 1.8 Hz, 2H), 6.47 (dd, 3JHH = 8.1 Hz, 4JHH = 2.1 Hz, 2H), 5.35 (s, NH, 2H). 19F{1H} NMR (282.4 MHz, C6D6): δ −148.7 (dd, 3JFF = 25.2 Hz, 4JFF = 6.9 Hz, 4F), −161.6 (t, 3JFF = 22.8 Hz, 2F), −163.1 (dt, 3JFF = 24.0 Hz, 4JFF = 6.9 Hz, 4F). 13C{1H} NMR (75 MHz, C6D6): δ 144.7 (Cipso), 142.6 (d, 1JCF = 249 Hz, C6F5), 141.8 (d, 1JCF = 242 Hz, C6F5), 138.0 (d, 1JCF = 256 Hz, C6F5), 134.4 (Cipso), 124.6 (CH), 121.4 (CH), 118.4 (CH), 114.7 (CH), 67.5 (Cipso), 25.5 (C6F5). κ3-N,O,N-{(C6H11)NC6H4}2OAlMe (4). In a glovebox, AlMe3 (130.0 g, 1.76 mmol) was added to a precooled (−35 °C) toluene solution of the bis-amino ligand 1a (640.0 mg, 1.76 mmol). The resulting mixture was stirred at 120 °C for 5 days. After evaporation to dryness, the crude product was precipitated in pentane to give a white powder (0.42 g, 1.03 × 10−3 mol, 63% yield). Anal. Calcd for C25H33AlN2O: C, 74.23; H, 8.22; N, 6.93. Found: C, 73.94; H, 8.01; N, 6.57. 1H NMR (400 MHz, C6D6): δ 7.22 (dd, 3JHH = 8.2 Hz, 4JHH = 1.3 Hz, 2H), 7.02 (dt, 3JHH = 6.6 Hz, 4JHH = 1 Hz, 2H), 6.63 (dd, 3 JHH = 8.2 Hz, 4JHH = 1.3 Hz, 2H), 6.36 (dt, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 2H), 3.18 (m, 2H), 2.23 (m, 4H), 1.69−1.57 (m, 6H), 1.39−1.12 (m, 10H), −0.20 (s, 3H). 13C{1H} NMR (100 MHz, C6D6): δ 147.8 (Ar), 145.0 (Ar), 128.2 (Ar), 118.3 (Ar), 112.8 (Ar), 112.1 (Ar), 53.9 (CH-Cy), 35.4 (Cy), 35.2 (Cy), 26.7 (Cy), 26.3 (Cy), 26.1 (Cy), −7.6 (AlMe). κ2-N,O-{(CyNC6H4)O(C6H4NHCy)}GaMe2 (4′). In a glovebox, GaMe3 (0.07 g, 0.59 mmol) was added to a precooled (−35 °C) toluene solution of the bis-amino ligand 1b (0.20 g, 0.59 mmol). The resulting mixture was stirred at 130 °C for 6 days to yield the quantitative conversion to complex 4′, as J


Organometallics

Article

F.H. acknowledges the ADEME and the Région Alsace for a Ph.D. fellowship.

solid (200.0 mg, 0.350 mmol, 92% yield). Anal. Calcd for C25H11AlF10N2O (572.05): C, 52.46; H, 1.94; N, 4.89. Found: C, 51.07; H, 2.25; N, 4.77. 1H NMR (300 MHz, C6D6): δ 6.98 (d, 3JHH = 7.8 Hz, 2H), 6.85 (t, 3JHH = 7.9 Hz, 2H), 6.45 (dd, 3 JHH = 8.6 Hz, 4JHH = 7.5 Hz, 4H), 0.31 (s, 3H, Al-Me). 19F{1H} NMR (282.4 MHz, C6D6): δ −148.6 (dd, 3JFF = 16.5 Hz, 4JFF = 2.1 Hz, 4F), −163.2 (td, 3JFF = 25.5 Hz, 4JFF = 6.3 Hz, 4F), −164.4 (t, 3JFF = 23 Hz, 2F). 13C{1H} NMR (125 MHz, C6D6): δ 146.7 (Cipso-O), 143.5 (d, 1JCF = 252 Hz, C6F5), 139.0 (d, 1JCF = 251 Hz, C6F5), 140.0 (Cipso-N), 137.5 (d, 1JCF = 251 Hz, C6F5), 125.2 (CH-Ar), 122.1 (CH-Ar), 120.7 (Cipso-C6F5), 116.1 (CH-Ar), 115.4 (CH−Ar), −15.0 (Al-Me). κ3-N,O,N-{(C6F5)NC6H4}2OGaMe (8). In a glovebox, GaMe3 (68.2 mg, 0.590 mmol) was added to a toluene solution of the bis-amino ligand 1d (158.0 mg, 0.300 mmol) and the resulting solution was stirred at 130 °C for 3 days. After evaporation to dryness, the resulting black solid was washed with pentane and dried in vacuo to afford compound 8 in a pure form as a white solid (100.0 mg, 0.160 mmol, 53% yield). Anal. Calcd for C25H11GaF10N2O (615.07): C, 48.82; H, 1.80; N, 4.55. Found: C, 48.54; H, 1.97; N, 4.62. 1H NMR (300 MHz, C6D6): δ 7.06 (d, 3JHH = 8.1 Hz, 2H), 6.86 (t, 3JHH = 7.9 Hz, 4JHH = 1.5 Hz, 2H), 6.49 (m, 4H), 0.01 (s, 3H, Ga-Me). 19F{1H} NMR (282.4 MHz, C6D6): δ −148.4 (dd, 3JFF = 23.5 Hz, 4JFF = 3.7 Hz, 4F), −163.0 (td, 3JFF = 22.3 Hz, 4JFF = 4.2 Hz, 4F), −164.4 (t, 3JFF = 22.6 Hz, 2F). 13C{1H} NMR (125 MHz, C6D6): δ 146.6 (CipsoO), 143.6 (d, 1JCF = 238 Hz, C6F5), 139.3 (Cipso-N), 137.5 (d, 1 JCF = 239 Hz, C6F5), 127.2 (CH-Ar), 119.2 (CH-Ar), 118.5 (CH-Ar), 115.6 (CH-Ar), −11.3 (Ga-Me). Typical Procedure for the ROP of rac-LA, ε-CL, and TMC. In a glovebox, the catalyst (6.11 mg, 1.04 × 10−5 mol) was charged in a vial equipped with a Teflon screw cap and a dichloromethane or toluene solution (1 M) of the appropriate amount of monomer was added via a syringe all at once. The resulting solution was vigorously stirred (at room temperature or heated) for the appropriate time. The vial was then removed from the glovebox, and the reaction mixture was quenched with MeOH, provoking the precipitation of the polymer, which was washed several times with MeOH, dried in vacuo until constant weight, and subsequently analyzed by 1H NMR, SEC, and MALDI-TOF spectrometry.

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving characterization data for all polymers, including MALDI-TOF SEC data, selected kinetic data for the carried ROP reactions, and crystal and refinement data for compounds 4−6 and 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the CNRS and The University of Strasbourg for financial support. Lydia Brelot and Corinne Bailly (Service de Crystallographie, Institut de Chimie de Strasbourg) are gratefully acknowledged for the X-ray analysis of all complexes. K


Organometallics

Article

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