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Oct 12, 2016 - aluminum complexes have received the greatest attention,2 in the past few ..... comparison, boronic Lewis acids studied by Beckett exhi...
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A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide Alexandre B. Kremer,† Ryan J. Andrews,† Matthew J. Milner,† Xu R. Zhang,† Tannaz Ebrahimi,† Brian O. Patrick,† Paula L. Diaconescu,‡ and Parisa Mehrkhodavandi*,† †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada Department of Chemistry & Biochemistry, University of California, Los Angeles, California 90095, United States

‡

S Supporting Information *

ABSTRACT: The impact of the metal size and Lewis acidity on the polymerization activity of group 13 metal complexes was studied, and it was shown that, within the same ligand family, indium complexes are far more reactive and selective than their gallium analogues. To this end, gallium and aluminum complexes supported by a tridentate diaminophenolate ligand, as well as gallium complexes supported by N,N′-ethylenebis(salicylimine)(salen) ligands, were synthesized and compared to their indium analogues. Using the tridentate ligand set, it was possible to isolate the gallium chloride complexes 3 and (±)-4 and the aluminum analogues 5 and (±)-6. The alkoxygallium complex (±)-2, supported by a salen ligand, was also prepared and characterized and, along with the three-component system GaCl3/ BnOH/NEt3, was tested for the ring-opening polymerization of lactide and ε-caprolactone. The polymerization rates and selectivities of both systems were significantly lower than those for the indium analogues. The reaction of (±)-2 with 1 equiv of lactide forms the first insertion product, which is stable in solution and can be characterized at room temperature. In order to understand the differences of the reactivity within the group 13 metal complexes, a Lewis acidity study using triethylphosphine oxide (the Gutmann−Beckett method) was undertaken for a series of aluminum, gallium, and indium halide complexes; this study shows that indium halide complexes are less Lewis acidic than their aluminum and gallium analogues. Density functional theory calculations show that the Mulliken charges for the indium complexes are higher than those for the gallium analogues. These data suggest that the impact of ligands on the reactivity is more significant than that of the metal Lewis acidity.

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INTRODUCTION In the last two decades, the development of active and stereoselective group 13 metal catalysts for the ring-opening polymerization (ROP) of cyclic esters has been a fruitful area of research with hundreds of complexes reported.1 Although aluminum complexes have received the greatest attention,2 in the past few years, interest in indium(III)3 and, to a lesser extent, gallium(III)3o,p,4 compounds has increased with respect to their application as versatile catalysts for polymer synthesis.5 Despite this interest, few groups have focused on a comparison between gallium,4b,c,g indium, and the widely used aluminum catalysts.3n−p,4d,5 For example, different mechanistic pathways have been postulated for the (κ2-N,O)iminophenolate alkyl complexes (Chart 1, A): coordination insertion for organoaluminum versus activated monomer for organoindium analogues.6 However, the use of different alkyl and alkoxide precursors, in addition to the different coordination modes obtained depending on the metal centers and the steric bulk of the ligand, made comparisons complicated for a related series of group 13 metal complexes (Chart 1, B).4f Finally, a comparison of organogallium and organoaluminum (κ3-N,O,N)-diaminophenolate complexes (Chart 1, C) also showed a different trend of the activity for © XXXX American Chemical Society

different cyclic esters, pointing out the complexity of understanding group 13 metal chemistry.4d As a general trend, aluminum complexes are usually the most stereoselective and indium complexes the most active. Interestingly, gallium complexes do not lie in between and often display poor control;4d,f,5,6 however, some have proven to be very active in ROP.3p,4b,c,e,g Although differences in the Lewis acidity are invoked in explaining these results and those involving other metals,7 no experimental assessment of the Lewis acidity of these complexes has been reported. We have investigated indium alkoxide complexes supported by tridentate aminophenolate and tetradentate salen ligands, for the controlled, stereoselective ROP of cyclic esters (Chart 1, D and E).8 In systems where a direct comparison was possible, indium alkoxides were far more active than aluminum analogues.9 They were also more robust (often inert toward alcohols) while offering the possibility of accessing a broader scope of monomers.10 Herein, we report the use of gallium alkoxides supported by diaminophenolate and salen derivatives as a direct comparison Received: October 12, 2016

A

DOI: 10.1021/acs.inorgchem.6b02433 Inorg. Chem. XXXX, XXX, XXX−XXX

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(8.39 and 8.20 ppm) and Ga-OCH2-CH3 signals (3.65 ppm, Figures S1 and S2). Single crystals of (±)-2, obtained from a saturated solution of tetrahydrofuran (THF) and diethyl ether, were evaluated by Xray diffraction (Figure 1). The solid-state structure of (±)-2

Chart 1. Group 13 Metal Complexes Reported for the Stereocontrolled ROP of Lactide

Figure 1. Solid-state structure of (±)-2 obtained by single-crystal Xray diffraction. Thermal ellipsoids are set at 50% probability; hydrogen and solvent atoms are removed for clarity.

shows a five-coordinate mononuclear complex with a geometry between square-pyramidal and trigonal-bipyramidal (τ = 0.47).13 The structure of (±)-2 is comparable to that of the previously reported (±)-112 and analogous indium halide complexes14 (Table S1). The chloride complexes are more square-pyramidal (τ = 0.20 and 0.37 for the gallium and indium chloride complexes, respectively) than 2. This is in contrast to complex E8b (Chart 1), as well as related indium alkoxide complexes supported by salen ligands,3b,8d,14,15 which are dimeric with bridging alkoxide groups and a distorted octahedral geometry at the indium centers. Achiral and chiral diaminophenol proligands H(NNO) and (±)-H(NNO*) were prepared according to previously reported procedures.16 Salt metathesis reactions of the deprotonated analogues, K(NNO) and (±)-K(NNO*), with GaCl3 or AlCl3 form the respective gallium, (NNO)GaCl (3) and (±)-(NNO*)GaCl [(±)-4], and aluminum, (NNO)AlCl (5) and (±)-(NNO*)AlCl [(±)-6], complexes (Scheme 2). 1H NMR spectra of 3 and (±)-4 are similar to those of the analogous indium complexes reported previously.8c,17 Whereas the 1H NMR spectrum of (±)-4 features two singlets for the two inequivalent sets of N(CH3)2 protons, the analogous spectrum for complex 3 exhibits only one sharp singlet at 2.92 ppm, suggesting that these protons are equivalent on the NMR time scale. Similarly, the N(CH3)2 protons are equivalent for the aluminum complex 5 (Figure S17). Interestingly, the related indium complex (NNO)InCl shows two inequivalent signals for the N(CH3)2 protons,17 suggesting that the fluxionality observed for the corresponding aluminum and gallium compounds may be due to steric crowding due to the smaller ionic radii of these metals (see below). We explored the lability of the NMe2 group through the addition of a base as well as variable-temperature studies. The addition of up to 10 equiv of pyridine to complexes 3 and (±)-4 does not result in changes in the 1H NMR spectra at room temperature (Figures S11 and S15), while the addition of 1 equiv of 4-(dimethylamino)pyridine results in broadening of the N(CH3)2 signals for both complexes (Figure S15). Variable-temperature NMR spectra (CDCl3) of complexes 3

to our indium catalysts in order to understand the effect of the metal size on the catalyst activity. We also report an attempt to estimate the Lewis acidity of gallium halide complexes relative to the aluminum and indium analogues using the Gutmann− Beckett method11 as well as computational studies. We show that, in all cases, indium complexes are superior to their gallium analogues as catalysts for the ROP of cyclic esters. We also show that any impact of the metal Lewis acidity may be secondary to the role of the ligands in these catalytic systems.

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RESULTS AND DISCUSSION Synthesis of the Gallium Complexes. Reaction of the salen proligand (±)-H2(ONNO) with KO-t-Bu, followed by the addition of gallium trichloride, yields the previously reported (ONNO)GaCl, (±)-1 (Scheme 1).12 The further Scheme 1. Different Pathways for the Syntheses of (±)-112 and (±)-2

reaction of (±)-1 with KOEt yields (ONNO)Ga(OEt), (±)-2.8b,d Alternatively, complex (±)-2 can be obtained in a better overall yield through a one-pot synthesis by adding an excess of KOEt to (±)-H2(ONNO), followed by the addition of gallium trichloride. The 1H and 13C{1H} NMR spectra of (±)-2 confirm an asymmetric complex with characteristic imine B

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of the indium analogues obtained previously8c,17 (respectively (NNO)InCl2 (F) and (±)-(NNO*)InCl2 [(±)-G]), and the τ geometrical parameter was calculated for each complex (Table S3). The structures of these complexes are similar given the difference in the atomic radii of the respective metal ions (Ga3+ = 0.62 Å and In3+ = 0.80 Å).18 Whereas complexes 3, F, and (±)-4 are square-pyramidal with τ values between 0.11 and 0.20, complex (±)-G is highly distorted, with a τ value of 0.50. In contrast to their indium analogues,8a,c,10b,17,19 dichlorogallium complexes 3 and (±)-4 do not react cleanly with KOEt at room temperature (Figure S16). Upon the addition of potassium benzoxide, complexes 3 and (±)-4 form a mixture of starting material and other species regardless of the conditions used, most notably the equivalents of KOBn added (Scheme 3). These mixtures, likely consisting of both the mono- and

Scheme 2. Syntheses of Gallium and Aluminum Complexes Supported by Diaminophenolate Ligands

Scheme 3. Attempted Synthesis of (NNO)Ga(Cl)(OBn)

and (±)-4 show no major changes between −50 and +50 °C (Figures S10 and S14), although broadening of the signals for (±)-4 at 50 °C suggests that coalescence may occur at higher temperatures. The solid-state molecular structures of complexes 3 and (±)-4 were determined by single-crystal X-ray diffraction (Figure 2). The structures obtained were compared to those

bisalkoxide species, cannot be separated; however, single crystals of the monobenzoxide complex can be selectively isolated from a saturated solution of toluene and diethyl ether. Single-crystal X-ray diffraction reveals a mononuclear complex (Figure 3) with a square-pyramidal geometry around the

Figure 3. Solid-state structure of (NNO)Ga(Cl)(OBn) obtained by single-crystal X-ray diffraction. Thermal ellipsoids are set at 50% probability; hydrogen and solvent atoms are removed for clarity.

gallium center (τ = 0.18) and bond lengths similar to those in the related chloride complex (±)-4. This structure was surprising because the analogous indium complexes are invariably alkoxy-bridged dimers with octahedral centers.8a,c,17,19,20 The larger ionic radius of indium favors the aggregation and formation of dinuclear species. These results highlight the unusual stability of monoalkoxy-bridged indium complexes.21

Figure 2. Solid-state structures of complexes 3 and (±)-4 obtained by single-crystal X-ray diffraction. Thermal ellipsoids are set at 50% probability; hydrogen and solvent atoms are removed for clarity. The unit cell of complex 3 contains two crystallographically independent molecules; only one is shown here. C

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Inorganic Chemistry Table 1. Polymerization of rac-LA Using Gallium Precursorsa 1 2 3 4 5 6 7 8

catalyst

solute

T (°C)

time

[LA]/[cat.]

convb (%)

Mntheo (Da)c

MnGPC (Da)d

Đc

Pre

InCl3/BnOH/NEt33c

CH2Cl2 CH2Cl2 toluene CH2Cl2 CH2Cl2 toluene

25 25 100 25 25 100 140 25

5h 6 days 4 days 30 min 10 days 7 days 6 days 24 h

200 200 200 200 200 200 200 205

96 In; Table 3, entries 1−3), whereas for the complexes bearing chiral ligands, the aluminum compound showed the most downfield shift (Al > Ga > In; Table 3, entries 5−7). A comparison of the halide complexes showed similar results (InCl2 ∼ InBr2 > InI2; Table 3, entries 7−9). Interpretation and assignment of the error values to these data were complicated by the observation of minor but significant signals in the low-temperature 31P{1H} NMR spectra of some complexes (Table 3 reports the most intense peak for each complex). These results were reproducible from different batches of pure complexes, and the additional signals

Chart 2. Aluminum, Gallium, and Indium Complexes Used for the Lewis Acidity Study

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optimized geometrical parameters agree well with the experimental values (Table S5). Mulliken charges support the observed reactivity trends, with higher values for the indium complexes than for the gallium complexes (Table 4).

may be attributed to different coordination isomers observable at lower temperatures. The Lewis acidity of the metal center may be intricately linked to the mechanism of polymerization for a given catalyst. Indeed, regardless of the mechanism (coordination insertion or activated monomer), after the first insertion product, a potential interaction with the adjacent carbonyl moiety can stabilize the lactate group formed (oligomer and polymer later on).4a,26 We hypothesize that increased Lewis acidity reinforces this interaction, leading to a more stable complex. Dissociation of this group is not favored, and thus coordination or activation of the next lactide monomer may become the rate-limiting step. To support this hypothesis, we attempted to identify the first insertion product from (±)-2, which is a very slow catalyst for the ROP lactide. A total of 1 equiv of lactide was added slowly to a solution of (±)-2 (Scheme 5). 1H NMR of the solution

Table 4. Mulliken Charges for Optimized Metal Complexes Using the PW91 Functional 1 2 3 4 5 6 7

Scheme 5. Proposed Reaction of (±)-2 and 1 equiv of Lactide

compound

metal Mulliken charge

(NNO)InCl2 (NNO*)InCl2 (NNO)GaCl2 (NNO*)GaCl2 [(ONNO)In(OEt)]2 (ONNO)In(OEt) (ONNO)Ga(OEt)

1.20 1.20 0.92 0.92 1.45 1.48 1.37

The differences between gallium and indium are more pronounced for the chloride complexes than the ethoxide complexes. Considering that these calculations were carried out in the gas phase, the small differences observed (Table 4) indicate that other factors can contribute greatly to the reactivity found in solution toward cyclic esters.

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CONCLUSIONS In this work, we attempt to compare gallium and indium complexes as catalysts for the ROP of lactide and explore the impact of the metal Lewis acidity on the reactivity. We report the syntheses of several gallium complexes bearing tetradentate salen and tridentate aminophenolate ligand backbones. The salen ethoxide complex (±)-2 can be synthesized easily through one- or two-step routes. The syntheses of gallium and aluminum chloride complexes 3, (±)-4, 5, and (±)-6 bearing tridentate ligands are reported; however, their alkoxylation formed mixtures of mono- and bisalkoxide species. Unlike their indium analogues, gallium complexes were poor catalysts for the ROP of rac-LA and were completely unreactive toward ε-caprolactone. We were able to take advantage of this poor reactivity to observe the first insertion product of lactide with (±)-2 in situ. The Lewis acidities of different precursors were analyzed to rationalize the differing polymerization results. We found using experimental (the Gutmann−Beckett method) methods that gallium and aluminum complexes are more Lewis acidic than the related indium complexes. However, computational methods showed higher Mulliken charges for indium alkoxide compared to its gallium analogue. The discrepancy between the computational and experimental results is likely due to the extent of solvation for the indium complexes in the experimental systems. Importantly, the experimental methods involve only metal halide complexes because studies of indium alkoxide complexes are not possible because of aggregation. Indeed, one of the most important comparisons between the gallium and indium catalysts is that the gallium alkoxide complexes are mononuclear, while the indium alkoxide complexes aggregate and are always dinuclear. Taken together, these results suggest that the most important factor in the reactivity of these systems may not be the Lewis acidiy of the metals. The smaller ionic radius of the gallium complexes may favor secondary interaction between the carbonyl moiety of the polymer and the metal center in the first insertion product. These interactions are likely not as

after 20 min shows a new product in addition to unreacted lactide and (±)-2 (Figure S5). Full conversion to this new species is observed after 24 h. After the addition of a second 1 equiv of lactide, no reaction occurs and free lactide is observed even after an additional 24 h. Oligomers are observed only when the sample is heated to 65 °C. The new product was consistent with the formation of a metallic complex with a pendant ring-opened lactide group according to COSY and NOESY NMR spectroscopy (Figures S6 and S7). An important piece of evidence is the through-space interaction of the three −CH3− groups in the new species with the O-t-Bu groups of the ligand, which is strong evidence for the formation of the chelated fist insertion product. The observation of this species is strong evidence that the formation of the first insertion product for lactide is not the rate-limiting step. Rather, it is subsequent insertions that limit its polymerization rate. In contrast, the complete lack of reactivity of (±)-2 with ε-caprolactone suggests that the stabilizing chelating interaction is not an important factor in the lower reactivity of the gallium complex and that the Lewis acidity may not be directly correlated with the reactivity. Computational Studies. Geometry optimizations on the full molecules of (NNO)MCl2, (NNO*)MCl2 (M = In, Ga), [(ONNO)In(OEt)]2, and (ONNO)M(OEt) (M = In, Ga) were performed for the present study employing ADF2013.01,27 using the PW9128 and BP functionals.29 These compounds were chosen because molecular solid-state structures are available, except for (ONNO)In(OEt), which was calculated in order to have a comparison with (ONNO)Ga(OEt). Also, most complexes exist as monomers for both indium and gallium. Two functionals were tested because Mulliken charges are known to be functional-dependent. The F

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temperature for 47 h, forming a cloudy brown mixture. Toluene was then removed in vacuo, and the resultant brown solids were dissolved in dichloromethane (5 mL). The dichloromethane solution was filtered twice through Celite to obtain a clear brown solution. Dichloromethane was removed in vacuo to again afford a brown solid as the crude product. The crude product was washed three times with hexane and then dried in vacuo to afford the final product as a darkyellow powder (0.039 g, 46%). Alternative Synthesis of (±)-2. Proligand (±)-H2(ONNO) (0.100 g, 0.182 mmol) was dissolved in ether (8 mL). Potassium ethoxide (0.081 g, 0.96 mmol) was added to the ligand solution. The reaction was stirred at room temperature for 2 h. Gallium(III) chloride (0.032 g, 0.18 mmol) was dissolved in ether and added to the mixture. The reaction was stirred at room temperature overnight. The ether was then removed in vacuo, and the resultant solids redissolved in dichloromethane (5 mL). The dichloromethane solution was filtered through Celite until a clear solution was obtained. The solvent was removed again in vacuo to afford the crude product. The final product was obtained as a yellow powder after washing with hexane and drying (0.059 g, 49%). Single crystals suitable for X-ray crystallographic analysis were grown from a saturated solution of THF and diethyl ether. 1H NMR (400 MHz, CDCl3, 25 °C): δ 8.39 (1H, d, J = 1.3 Hz, NCH), 8.20 (1H, d, J = 1.3 Hz, NCH), 7.51 (2H, t, J = 2.5 Hz, ArH), 7.02 (1H, d, J = 2.5 Hz, ArH), 6.97 (1H, d, J = 2.5 Hz, ArH), 3.79 (1H, m, NCH), 3.65 (2H, m, OCH2CH3), 3.11 (1H, m, NCH), 2.63 (1H, m, CH2), 2.48 (1H, m, CH2), 2.10 (2H, m, CH2), 1.58 (9H, s, ArC(CH3)3), 1.57 (9H, s, ArC(CH3)3), 1.55−1.40 (4H, m, CH2), 1.34 (9H, s, ArC(CH3)3), 1.33 (9H, s, ArC(CH3)3), 0.96 (3H, t, J = 6.5 Hz, OCH2CH3). 13C{1H} NMR (150 MHz, CDCl3, 25 °C): δ 168.23, 167.05, 165.44, 163.93, 141.50, 141.29, 137.49, 137.16, 130.66, 129.68, 128.39, 126.00, 116.87, 116.77, 64.29, 62.35, 59.23, 35.73, 35.64, 33.92, 33.89, 31.38, 31.32, 29.78, 29.67, 28.14, 26.88, 24.12, 23.54, 20.76. Anal. Calcd (found) for C38H57GaN2O3: C, 69.20 (67.62); H, 8.71 (8.41); N, 3.78 (4.25). Synthesis of Complex 3. Proligand H(NNO) (1.334 g, 4.16 mmol) was dissolved in toluene and then potassium tert-butoxide (0.458 g, 4.08 mmol) was added. The reaction was stirred at room temperature overnight, resulting in a muddy beige mixture. Toluene was removed in vacuo to yield an off-white powder. The resulting powder (0.257 g, 0.716 mmol) was dissolved in toluene (8 mL). Gallium(III) chloride (0.126 g, 0.714 mmol) was dissolved in toluene (2 mL) and added to the stirring proligand solution. The reaction mixture was stirred at room temperature for 91 h to give a cloudy white mixture. Extra toluene (7 mL) was added to maximize dissolution of the product. The mixture was filtered through Celite to obtain a clear, nearly colorless solution. Toluene was removed in vacuo to afford the crude product as a beige solid. The solids were washed with hexane (2 × 5 mL) and dried in vacuo to finally afford the product as a white powder (0.229 g, 69%). Single crystals suitable for X-ray crystallographic analysis were grown from a saturated solution of the complex in toluene. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.31 (1H, d, J = 1.9 Hz, ArH), 6.81 (1H, d, J = 1.9 Hz, ArH), 4.30 (1H, d, J = 12.1 Hz, NCH2Ar), 3.41 (1H, d, J = 12.1 Hz, NCH2Ar), 3.15−3.01 (2H, m, NCH2CH2N), 2.92 (8H, overlapping signals, N(CH3)2 and NCH2CH2N), 2.40 (3H, s, NCH3), 1.49 (9H, s, ArC(CH3)3), 1.28 (9H, s, ArC(CH3)3). 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 156.84, 139.75, 139.46, 124.49, 123.80, 120.15, 62.52, 55.92, 53.34, 47.94, 46.17, 35.02, 34.05, 31.67, 29.78. Anal. Calcd (found) for C20H35Cl2GaN2O: C, 52.21 (52.27); H, 7.67 (7.58); N, 6.09 (6.08). Synthesis of Complex (±)-4. Proligand (±)-(NNO*) (0.534 g, 1.48 mmol) was dissolved in toluene (15 mL). Potassium tert-butoxide (0.163 g, 1.45 mmol) was suspended in toluene and added to the ligand solution. The reaction was stirred at room temperature overnight. Toluene was then removed in vacuo to yield a white powder. The resulting powder (0.394 g, 0.99 mmol) was redissolved in toluene (8 mL) and then added to a solution of gallium(III) chloride (0.174 g, 0.99 mmol) in toluene (8 mL). The reaction was stirred at room temperature overnight. Toluene was then removed in vacuo, and the resultant solids were redissolved in dichloromethane. The dichloromethane solution was filtered through Celite and the solvent

detrimental for the larger indium cation. A more important factor may be that small ligand variations have an important impact on the Lewis acidity and reactivity, making the aminophenolate ligand sets unsuitable as supports for active gallium catalysts.

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EXPERIMENTAL SECTION

General Procedures. All air- and/or water-sensitive reactions were carried out under a nitrogen atmosphere in an MBraun glovebox. Bruker Avance 600, 400, or 300 MHz spectrometers were used to record the 1H, 13C{1H}, and 1H NMR spectra. 1H NMR chemical shifts are given in ppm versus residual protons in deuterated solvents as follows: δ 7.27 for CDCl3. 13C{1H} NMR chemical shifts are given in ppm versus residual 13C in solvents as follows: δ 77.00 for CDCl3. 31 1 P{ H} NMR chemical shifts are given in ppm versus 31P{1H} NMR of free OPEt3: δ 48.30 ppm measured using a capillary placed inside the NMR tub. Diffraction measurements for X-ray crystallography were made on Bruker X8 APEX II and Bruker APEX DUO diffractometers with graphite-monochromated Mo Kα radiation. The structures were solved by direct methods and refined by full-matrix least squares using the SHELXTL crystallographic software of Bruker AXS. Unless specified, all non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were constrained to geometrically calculated positions but were not refined. Elemental CHN analysis was performed using a Carlo Erba EA1108 elemental analyzer. The elemental composition of an unknown sample was determined using a calibration factor. The calibration factor was determined by analyzing a suitable certified organic standard (OAS) of a known elemental composition. Molecular weights were determined by gel permeation chromatography (GPC)−laser light scattering (LLS) using an Agilent liquid chromatograph equipped with an Agilent 1200 series pump and autosampler, three Phenogel 5 μm narrow-bore columns (4.6 × 300 mm with 500, 103, and 104 Å pore sizes), a Wyatt Optilab differential refractometer, a Wyatt tristar miniDAWN (LLS detector), and a Wyatt ViscoStar viscometer. The column temperature was set at 40 °C. A flow rate of 0.5 mL/min was used, samples were dissolved in tetrahydrofuran (THF; ca. 2 mg/mL), and a dn/dc value of 0.042 mL/g was used. Narrow-molecular-weight polystyrene standards were used for system calibration purposes. Materials. Toluene, diethyl ether, hexane, and THF were degassed and dried using alumina columns in a solvent purification system. In addition, CHCl3 and CH2Cl2 were dried over CaH2 and vacuumtransferred to a Straus flask, where they were degassed prior to use. Deuterated chloroform (CDCl3) and toluene (toluene-d8) were dried over CaH2, vacuum-transferred to a Straus flask, and then degassed through a series of freeze−pump−thaw cycles. InCl3 was obtained from Strem Chemicals and GaCl3 from Alfa Aesar, and both were used without further purification. N,N,N′-Trimethylethylenediamine, potassium tert-butoxide, (±)-trans-diaminocyclohexane, and 2,4-di-tertbutylphenol were obtained from Alfa Aesar and used without further purification. NaOEt and KOEt were obtained from Alfa Aesar, dissolved in dry ethanol, stirred for 16 h, precipitated from solution with hexanes, filtered, washed with hexanes, and dried at 50 °C under vacuum. Lactide samples were obtained from Purac Biomaterials, recrystallized several times from hot, dry toluene, and dried under vacuum prior to use. The synthesis of (±)-N,N-dimethyl-trans-1,2diaminocyclohexane was performed according to literature procedures30 from (±)-trans-diaminocyclohexane and distilled at 70 °C under reduced pressure prior to use. Syntheses of proligands were performed according to literature procedures: H(NNO),16a H(NNO*),16b and (ONNO)31 The salen gallium chloride complex (±)-1 was synthesized according to a literature procedure.12 Syntheses of indium complexes used for the Lewis acidity study were performed according to literature procedures.8a−c,19a Synthesis of Complex (±)-2. (±)-(ONNO)GaCl (0.084 g, 0.13 mmol) was dissolved in toluene (3 mL). Potassium ethoxide (0.033 g, 0.39 mmol) was suspended in toluene (4 mL), and the suspension was added to the chloride complex solution with two toluene quantitative transfers (0.5 mL). The reaction mixture was stirred at room G

DOI: 10.1021/acs.inorgchem.6b02433 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

temperature in a vial or at higher temperature in a vacuum-sealed bomb. The solvent was then removed in vacuo, and a small portion of the crude polymer was tested for conversion and tacticity via 1H and 1 H{1H} NMR spectroscopy (25 °C, CDCl3, 600 MHz), respectively. The remaining crude polymer was redissolved in a minimum of dichloromethane (1−2 mL). Methanol (2−5 mL) was then added to this solution, causing precipitation of the polymer only for the polymer samples obtained in CH2Cl2. The solution was allowed to settle, and the supernatant solution was removed. This process was repeated two more times, and the resulting polymer was dried under vacuum. The polymer was tested for the presence of the remaining catalyst or monomer using 1H NMR spectroscopy before being tested for molecular weight and polydispersity index (PDI) using GPC in THF. Representative Polymerization of rac-LA Using Gallium Complexes. rac-LA (200 equiv) in CH2Cl2 or toluene was added to a solution of the complex (5 mg) in CH2Cl2 or toluene to obtain a 2 mM concentration of the catalyst. The mixture was allowed to stir at room temperature in a vial or at higher temperature in a vacuumsealed bomb. The solvent was then removed in vacuo and a small portion of the crude polymer was tested for conversion and tacticity via 1H and 1H{1H} NMR spectroscopy (25 °C, CDCl3, 600 MHz), respectively. The remaining crude polymer was redissolved in a minimum of dichloromethane (1−2 mL). Methanol (2−5 mL) was then added to this solution, causing precipitation of the polymer only for the polymer samples obtained in CH2Cl2. The solution was allowed to settle, and the supernatant solution was removed. This process was repeated two more times, and the resulting polymer was dried under vacuum. The polymer was tested for the presence of the remaining catalyst or monomer using 1H NMR spectroscopy before being tested for molecular weight and PDI using GPC in THF. Representative Polymerization of ε-Caprolactone Using the Gallium Complex 2. A solution of ε-caprolactone (0.167 mL, 1.52 mmol) in CH2Cl2 or toluene was added to a solution of the complex 2 (5.00 mg, 0.00785 mmol) in CH2Cl2 or toluene to obtain a 2 mM concentration of the catalyst. The mixture was allowed to stir at room temperature in a vial (for over 1 week) and at higher temperature (100 °C, overnight) in a vacuum-sealed J. Young Tube. A small portion of the crude polymer solutions were then tested for monomer conversion using 1H NMR spectroscopy (25 °C, CDCl3, 400 MHz). Synthesis of the First Insertion Product. (±)-(ONNO)Ga(OEt) (0.0490 g, 0.0743 mmol) was dissolved in 0.7 mL of CDCl3 with rac-LA (0.0108 g, 0.0749 mmol). The resultant clear yellow solution was transferred to a J. Young NMR sample tube and sealed under a nitrogen atmosphere. The tube was then shaken, and a 1H NMR sample was immediately obtained. The clear yellow reaction mixture was left within the J. Young NMR sample tube for 24 h, before subsequent 1H and 13C{1H} NMR samples were obtained. 1H NMR (400 MHz, CDCl3, 25 °C): δ 8.35 (1H, dd, J1 = 22.2 Hz, J2 = 1.9 Hz, NCH), 8.17 (1H, t, J = 1.9 Hz, NCH), 7.48(2H, m, ArH), 7.00 (1H, d, J = 2.45 Hz, ArH), 6.94 (1H, dd, J1 = 7.0 Hz, J2 = 2.54 Hz, ArH), 4.58 (1H, sex, J = 7.0 Hz, CH(CH3)), 4.46 (1H, quin, J = 7.0 Hz, CH(CH3)), 4.20−3.90 (3H, overlapping signals, NCH and OCH2CH3), 3.07 (1H, m, NCH), 2.47 (2H, m, CH2), 2.07 (2H, t, J = 12.7 Hz, CH2), 1.54 (18H, m, C(CH3)3), 1.30 (18H, m, C(CH3)3), 1.26−1.14 (9H, overlapping signals, OCH2CH3 and CH(CH3)). 13 C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 177.16, 171.07, 168.57, 168.32, 164.39, 164.01, 141.37, 141.17, 139.94, 136.16, 130.67, 130.62, 129.49, 128.10, 126.64, 126.06, 67.91, 67.65, 64.69, 64.10, 62.01, 61.67, 60.93, 35.72, 34.91, 34.01, 33.25, 31.38, 29.38, 27.85, 26.87, 24.32, 24.19, 23.38, 16.92. Representative Procedure for the Lewis Acidity Study Using OP(Et)3. The complex was dissolved in 0.7 mL of CDCl3 to obtain a 20 mM concentration. The mixture was transferred to a flask containing 0.8 equiv of OP(Et)3 and then transferred to an NMR tube. A capillary containing a solution of OP(Et)3 (20 mM in CDCl3) was added to the NMR tube, and a sample was taken for 31P NMR (162 MHz) at the required temperature.

removed again in vacuo to afford the crude product. Washing with hexane and ether gave a white powder (0.299 g, 60%). Single crystals suitable for X-ray crystallographic analysis were grown from a saturated solution of the complex in a mixture of THF and diethyl ether and separately from a saturated solution in pyridine and diethyl ether. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.29 (1H, d, J = 2.5 Hz, ArH), 6.79 (1H, d, J = 2.5 Hz, ArH), 4.42 (1H, d, J = 12.8 Hz, ArCH2N), 3.88 (1H, dd, J1 = 12.8 Hz, J2 = 5.3 Hz, ArCH2N), 2.89 (3H, s, N(CH3)2), 2.75−2.63 (1H, m, NCH), 2.49 (5H, overlapping signals, N(CH3)2, NCH, CH2), 2.27−2.41 (1H, m, CH2), 2.75−1.84 (3H, m, CH2), 1.48 (9H, s, ArC(CH3)3), 1.36−1.24 (13H, overlapping signals, ArC(CH3)3 and CH2). 13C{1H} NMR (150 MHz, CDCl3, 25 °C): δ 157.58, 139.94, 139.26, 124.87, 124.03, 118.60, 67.73, 53.02, 48.68, 45.39, 38.48, 35.14, 34.09, 31.64, 29.87, 29.78, 24.21, 23.79, 22.89. Anal. Calcd (found) for C23H39Cl2GaN2O: C, 55.23 (54.26); H, 7.86 (7.47); N, 5.60 (5.40). Synthesis of Complex 5. Proligand H(NNO) (0.288 g, 0.898 mmol) was mixed with potassium tert-butoxide (0.099 g, 0.88 mmol) and dissolved in10 mL of toluene. The resultant clear pale-yellow solution was stirred at room temperature for 25 h, remaining as a clear yellow solution. Toluene was removed in vacuo, and the resulting paleyellow powder was washed twice with hexanes (1 mL) and dried in vacuo. The powder was then dissolved in toluene (5 mL). Aluminum chloride (0.120 g, 0.900 mmol) was suspended in toluene (3 mL) and was transferred dropwise to the reaction mixture with two additional toluene transfers (1 mL). The clear pale-yellow reaction mixture was stirred at room temperature for 18 h. Toluene was removed in vacuo, and the resultant pale-yellow powder was dissolved in 5 mL of dichloromethane and filtered over Celite until a clear solution was obtained. Dichloromethane was removed in vacuo to afford a paleyellow powder. The mixture was washed six times with hexane (1 mL) and dried in vacuo to afford a crystalline pale-yellow powder (0.305 g, 93%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.30 (1H, s, ArH), 6.87 (1H, s, ArH), 4.05 (1H, d, J = 11.4 Hz, ArCH2N), 3.63 (1H, d, J = 11.1 Hz, ArCH2N), 3.25−3.12 (2H, s, NCH2CH2N), 2.98−2.85 (8H, overlapping signals, N(CH3)2 and NCH2CH2N), 2.49 (3H, s, NCH3), 1.46 (9H, s, ArC(CH3)3), 1.29 (9H, s, ArC(CH3)3). 13C{1H} NMR (100 MHz, CDCl3, 25 °C): 153.90, 140.28, 137.84, 124.38, 123.42, 119.85, 61.40, 58.85, 52.38, 49.19, 48.86, 34.88, 34.11, 31.85, 29.58. Anal. Calcd (found) for C20H35Cl2AlN2O: C, 57.55 (55.29); H, 8.45 (8.65); N, 6.71 (5.74). Synthesis of Complex (±)-6. Proligand (±)-H(NNO*) (0.166 g, 0.460 mmol) was dissolved in toluene (8 mL). Potassium tert-butoxide (0.051 g, 0.45 mmol) was suspended in toluene and added to the ligand solution. The reaction was stirred at room temperature overnight. Toluene was then removed in vacuo to yield a white powder. The resulting powder was washed with hexanes (1 mL) and then dried in vacuo. The resulting powder was redissolved in toluene (8 mL) and then added to a solution of aluminum chloride (0.060 g, 0.45 mmol). The reaction was stirred at room temperature overnight. Toluene was then removed in vacuo and the resultant solids redissolved in dichloromethane (5 mL). The dichloromethane solution was filtered through Celite, and the solvent removed again in vacuo to afford the crude product. Washing with hexane and ether yielded a white powder (0.110 g, 52%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.30 (1H, d, J = 2.3 Hz, ArH), 6.83 (1H, d, J = 2.3 Hz, ArH), 4.51 (1H, d, J = 13.5 Hz, ArCH2N), 3.80 (1H, dd, J1 = 13.7 Hz, J2 = 3.3 Hz, ArCH2N), 2.92 (3H, s, N(CH3)2), 2.72−2.60 (2H, m, NCH), 2.53 (3H, s, N(CH3)2), 2.49−2.38 (2H, m, CH2), 2.12−1.81 (3H, m, CH2), 1.46 (9H, s, ArC(CH3)3), 1.31−1.20 (12H, overlapping signals, ArC(CH3)3 and CH2). 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 155.27, 139.77, 138.88, 124.36, 123.17, 118.49, 69.13, 51.20, 46.67, 45.73, 38.75, 34.98, 34.11, 31.69, 29.91, 29.68, 24.47, 23.61, 23.11. Anal. Calcd (found) for C23H39Cl2AlN2O: C, 60.39 (60.58); H, 8.59 (9.38); N, 6.12 (5.93). Representative Polymerization of rac-LA Using GaCl3/ BnOH/NEt3. rac-LA (200 equiv) in CH2Cl2 or toluene was added to a solution of GaCl3 (10 mg) in CH2Cl2 or toluene to obtain a 10 mM concentration. BnOH followed by NEt3 was then added to the mixture using a micropipette. The mixture was allowed to stir at room H

DOI: 10.1021/acs.inorgchem.6b02433 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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COMPUTATIONAL STUDIES

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Phenolate, and Directing Ligand-Free Indium Initiators for the RingOpening Polymerization of rac-Lactide. Organometallics 2011, 30, 1202−1214. (h) Bompart, M.; Vergnaud, J.; Strub, H.; Carpentier, J. F. Indium(III) halides as exceptionally active, water-tolerant catalysts for cationic polymerization of styrenics. Polym. Chem. 2011, 2, 1638− 1640. (i) Broderick, E. M.; Guo, N.; Vogel, C. S.; Xu, C.; Sutter, J.; Miller, J. T.; Meyer, K.; Mehrkhodavandi, P.; Diaconescu, P. L. Redox Control of a Ring-Opening Polymerization Catalyst. J. Am. Chem. Soc. 2011, 133, 9278−9281. (j) Kalita, L.; Walawalkar, M. G.; Murugavel, R. Synthesis and structural characterization of dinuclear complexes of trivalent aluminum, gallium, indium and chromium derived from pyrazole-2-ethanol. Inorg. Chim. Acta 2011, 377, 105−110. (k) Normand, M.; Kirillov, E.; Roisnel, T.; Carpentier, J. F. Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands for Ring-Opening Polymerization of rac-Lactide: How Does Indium Compare to Aluminum? Organometallics 2012, 31, 1448−1457. (l) Allan, L. E. N.; Briand, G. G.; Decken, A.; Marks, J. D.; Shaver, M. P.; Wareham, R. G. Synthesis and structural characterization of cyclic indium thiolate complexes and their utility as initiators for the ring-opening polymerization of cyclic esters. J. Organomet. Chem. 2013, 736, 55− 62. (m) Kapelski, A.; Okuda, J. Ring-Opening Polymerization of racand meso-Lactide Initiated by Indium Bis(phenolate) Isopropoxy Complexes. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4983−4991. (n) Normand, M.; Dorcet, V.; Kirillov, E.; Carpentier, J. F. {Phenoxyimine}aluminum versus -indium Complexes for the Immortal ROP of Lactide: Different Stereocontrol, Different Mechanisms. Organometallics 2013, 32, 1694−1709. (o) Pal, M. K.; Kushwah, N. P.; Manna, D.; Wadawale, A. P.; Sudarsan, V.; Ghanty, T. K.; Jain, V. K. Diorgano-Gallium and -Indium Complexes Derived from Benzoazole Ligands: Synthesis, Characterization, Photoluminescence, and Computational Studies. Organometallics 2013, 32, 104−111. (p) Ghosh, S.; Gowda, R. R.; Jagan, R.; Chakraborty, D. Gallium and indium complexes containing the bis(imino)phenoxide ligand: synthesis, structural characterization and polymerization studies. Dalton Trans. 2015, 44, 10410−22. (q) Quan, S. M.; Diaconescu, P. L. High activity of an indium alkoxide complex toward ring opening polymerization of cyclic esters. Chem. Commun. 2015, 51, 9643−9646. (4) (a) Horeglad, P.; Kruk, P.; Pecaut, J. Heteroselective Polymerization of rac-Lactide in the Presence of Dialkylgallium Alkoxides: The Effect of Lewis Base on Polymerization Stereoselectivity. Organometallics 2010, 29, 3729−3734. (b) Horeglad, P.; Szczepaniak, G.; Dranka, M.; Zachara, J. The first facile stereoselectivity switch in the polymerization of rac-lactide-from heteroselective to isoselective dialkylgallium alkoxides with the help of N-heterocyclic carbenes. Chem. Commun. 2012, 48, 1171−1173. (c) Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. 8-Quinolinolato Gallium Complexes: Iso-selective Initiators for rac-Lactide Polymerization. Inorg. Chem. 2013, 52, 12561−12567. (d) Hild, F.; Neehaul, N.; Bier, F.; Wirsum, M.; Gourlaouen, C.; Dagorne, S. 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 ? Organometallics 2013, 32, 587−598. (e) Horeglad, P.; Litwinska, A.; Zukowska, G. Z.; Kubicki, D.; Szczepaniak, G.; Dranka, M.; Zachara, J. The influence of organosuperbases on the structure and activity of dialkylgallium alkoxides in the polymerization of rac-lactide: the road to stereo diblock PLA copolymers. Appl. Organomet. Chem. 2013, 27, 328−336. (f) Maudoux, N.; Roisnel, T.; Dorcet, V.; Carpentier, J. F.; Sarazin, Y. Chiral (1,2)-Diphenylethylene-Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide. Chem. - Eur. J. 2014, 20, 6131−6147. (g) Horeglad, P.; Cybularczyk, M.; Trzaskowski, B.; Ż ukowska, G. y. Z.; Dranka, M.; Zachara, J. Dialkylgallium Alkoxides Stabilized with NHeterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide. Organometallics 2015, 34, 3480−3496. (5) Dagorne, S.; Normand, M.; Kirillov, E.; Carpentier, J. F. Gallium and indium complexes for ring-opening polymerization of cyclic

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All structures were optimized using the ADF2013.01 software suite. Full molecules were used for calculations. Optimizations were performed at the PW9128 or BP functional29 theory level, with frozen cores and triple-ζ-potential (TZP) basis sets and using the relativistic scalar ZORA.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02433. Details of solution and solid-state characterization, mass spectrometry, Lewis acidity studies, and DFT calculations (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paula L. Diaconescu: 0000-0003-2732-4155 Parisa Mehrkhodavandi: 0000-0002-3879-5131 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.M. thanks the NSERC for financial support. R.J.A. thanks the NSERC for an Undergraduate Summer Research Award. P.L.D. thanks the NSF (Grant 1362999) for financial support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02433 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02433 Inorg. Chem. XXXX, XXX, XXX−XXX