Dialkylgallium Alkoxides Stabilized with N ... - ACS Publications

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Dialkylgallium Alkoxides Stabilized with N‑Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide Paweł Horeglad,*,† Martyna Cybularczyk,†,‡ Bartosz Trzaskowski,† Grazẏ na Zofia Ż ukowska,§ Maciej Dranka,§ and Janusz Zachara§ †

Centre of New Technologies, University of Warsaw, Banacha 2c, 02-097 Warsaw, Poland Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland § Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland ‡

S Supporting Information *

ABSTRACT: The structure of a series of Me2GaOR(NHC) complexes with N-heterocyclic carbenes (1,3-bis(2,4,6trimethylphenyl)imidazolin-2-ylidene (SIMes) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes)) have been characterized using spectroscopic and X-ray techniques and discussed in view of their reactivity in the polymerization of rac-lactide (rac-LA). Both structure studies and density functional theory (DFT) calculations show the significant influence of NHC and OR on the structure of investigated complexes and has indicated that the Ga−CNHC bond (32.6− 39.6 kcal mol−1) is strong enough to form stable Me2GaOR(NHC) complexes in the form of monomeric species. The reactivity of Me2Ga((S)-OCH(Me)CO2Me)(SIMes) (1) and Me2Ga((S)-OCH(Me)CO2Me)(IMes) (5) toward Lewis acids such as CO2 and GaMe3 has resulted in breaking of the Ga−CNHC bond with the formation of (NHC)CO2 and Me3Ga(NHC) (8 and 10) and [Me2Ga(μ-(S)-OCH(Me)CO2Me)]2. Different results have been obtained for l,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (SIPr), which coordinates more weakly to gallium, as demonstrated by the Ga−CNHC bond strength for model Me3GaSIMes, Me3GaIMes (8), and Me3GaSIPr (10) adducts. The reaction of SIPr with [Me2Ga(μ-OR)]2 has not allowed for the breaking of Ga2O2 bridges and the formation of monomeric Me2GaOR(SIPr) complexes, contrary to SIMes and IMes. In the case of the reaction with [Me2Ga(μ-(S)-OCH(Me)CO2Me)]2, the ionic compound [Me2Ga(OCH(Me)CO2)]−[SIPrH]+ (9) has been isolated. The investigated Me2GaOR(NHC) complexes are highly active and stereoselective in the ring-opening polymerization of rac-lactide from −20 °C to room temperature, due to the insertion of rac-LA exclusively into the Ga−Oalkoxide bond, leading to isotactically enriched polylactide (PLA) (Pm = 0.65−0.78). It has been shown that the polymerization of lactide at low temperature is influenced by the chelate interaction of (S)-OCH(Me)CO2Me or (OCH(Me)C(O))2OR resulting from the primary insertion of rac-LA into the Ga−Oalkoxide bond, with the Ga center, which can be responsible for the low control over the molecular weight of the obtained PLA. The latter effect can be eliminated by the initial synthesis of Me2Ga((PLA)nOR)(NHC) with short PLA chains, which allows for controlled polymerization. Although the adverse chelate effect can be also eliminated by the polymerization of rac-LA at room temperature, the stereoselectivity of rac-LA polymerization is strongly affected by transesterification reactions. Out of investigated Me2GaOR(SIMes) and Me2GaOR(IMes) complexes, only the latter allowed for the immortal ring opening polymerization of rac-LA in the presence of iPrOH.

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rac-LA,5 and syndiotactic PLA from meso-LA.4b,5a,i,6 Out of the isoselective catalysts, several have enabled the synthesis of isotactic stereoblock PLA1c,g,3,4b,d,e,7 or recently reported gradient isotactic multiblock PLA.8 Until recently, there have been no reports on heterotactic−isotactic block PLA. For the synthesis of the above, it is crucial to develop extraordinary catalysts, which can polymerize rac-LA in both isoselective and heteroselective fashions. Noteworthy in this regard, aluminum

INTRODUCTION Despite the fact that numerous metal based catalysts have been shown to polymerize lactide in controlled and stereoselective manner,1,2 there is still a need for the catalysts that offer the possibility for the synthesis of new microstructures of polylactide (PLA). Since the properties of PLA depend strongly on its tacticity, the stereoselective synthesis of new PLA microstructures is of great interest in order to tailor its properties and extend the scope of application.1c,f,3 Over the last two decades, a number of catalysts have allowed for the synthesis of isotactic PLA4 and heterotactic PLA obtained from © XXXX American Chemical Society

Received: January 25, 2015

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DOI: 10.1021/acs.organomet.5b00071 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

important for the design of new catalysts based on main group metal alkoxides for the controlled and stereoselective polymerization of PLA and other heterocyclic monomers. The latter is further supported, as Ti, Zr, and Y alkoxides,21 and very recently, Al alkoxides,22 stabilized with NHCs, have been shown to possess interesting catalytic properties in the ringopening polymerization of rac-LA. Out of the main group metal alkoxides with NHC, only zinc complexes have been reported to be highly active in the stereoselective polymerization of raclactide23 as well as ε-caprolactone and cyclohexyl oxide.24 Moreover, zinc and magnesium complexes with chelate alkoxide ligands bearing NHC were active in the polymerization of rac-LA due to insertion into Zn−OR and Mg−NHC, respectively.25 Although strongly basic properties of NHCs allow for their use as ligands for the synthesis of main group metal alkoxides, there is still paucity of data concerning synthesis, structure, and the catalytic activity of main group metal alkoxides stabilized with NHCs.26 As an extension of our initial studies on the Me2GaOR(NHC) complexes, which constitute a new class of group 13 complexes, we present here the results concerning the effect of NHC and the alkoxide group on their synthesis, structure, and catalytic properties in the polymerization of racLA.

and zirconium alkoxides showed iso- or heteroselectivity depending on the structure of supporting tetradentate phenoxide ligands.9 Moreover, the dependence of stereoselectivity (from isoselective to heteroselective) was reported to depend on the metal size (from Pm 0.89 − Pr 0.72) for lanthanide complexes,10 on metal for isoselective zinc (Pm 0.80) and heteroselective magnesium (Pr 0.81) complexes with chiral aminophenolate ligands,11 and on the ligand structure of yttrium phosphasalen complexes (from Pm 0.84 − Pr 0.87).12 Recently, a series of isotactic−heterotactic stereomultiblock PLA were synthesized with aluminum salen and salan complexes using cross-chain-transfer ROP of rac-LA.12 However, for the fully controlled synthesis of isotactic− heterotactic stereoblock PLA the main challenge is to switch easily from heterotactic to isotactic mode. In this context, noteworthy is the dependence of stereoselectivity on the solvent for indium complexes with dialkoxydiimino salen-like ligands (Pm 0.69−Pr 0.59)14 and for Mg calixarene complexes (Pm 0.69−Pr 0.59).15 We have recently developed a Me2GaOR/Lewis base catalytic system, which offered a stereoselectivity switch and therefore allowed for the synthesis of a stereo diblock PLA built of isotactically (Pm = 0.79) and heterotactically (Pr = 0.78) enriched blocks.16 We showed that such modification of the PLA structure considerably influences its physicochemical properties16 and gives opportunities for the modification of drug release properties of PLA-conjugates.17 For the development of the above catalytic system, crucial was our earlier finding, which had shown that the reaction of N-heterocyclic carbene (NHC) with dimeric dialkylgallium alkoxides which can polymerize rac-LA with heteroselectivity up to Pr = 0.7818 leads to highly active and isoselective (Pm = 0.78) monomeric Me2GaOR(NHC) complexes (1 and 2, Figure 1). It represents the first example of a facile switch of stereoselectivity in the polymerization of rac-LA.19 In the above case, Me2GaOR(NHC) complexes were also the very rare example of isoselective catalysts highly active at low temperatures.7a,20 Therefore, the detailed understanding of the influence of NHCs on the structure and activity of Me2GaOR(NHC) should be

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RESULTS AND DISCUSSION 1. Synthesis and Structure of Me2GaOR(NHC) (NHC = SIMes and IMes). Compounds 1 and 2 (Scheme 1), recently reported by us, were synthesized in essentially quantitative yield in the reaction of (S,S)-[Me2Ga(μ-OCH(Me)C(O)OMe)]2 and [Me 2 GaOMe]3 with 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) in 1:2 and 1:3 molar ratios, respectively.19 Both complexes were found to be monomeric with a four coordinate gallium center and a strongly bonded NHC, which was evidenced by NMR spectroscopy and X-ray diffraction. Interestingly, in the crystal structure complex 2 exists as an asymmetric molecule due to the orientation of the SIMes ligand to Ga−CMe bonds. The analogue’s structure was suggested for 1 (colorless oil) on the basis of two singlets of protons corresponding to the Ga−Me groups, present in the 1 H NMR spectrum. For this interpretation, the asymmetric orientation of the SIMes ligand and impeded rotation around the Ga−CNHC are required and initially seemed to be supported by slow rotation around the Ga−CNHC bond on an NMR time scale resulting in the broad signal corresponding to the GaMe2 protons of 2. However, for 1 with the (S)-methyl lactate ligand ((S)-melac) possessing a chiral center, the presence of two singlets could also result from the presence of diasterotopic Ga−Me groups due to impeded rotation around Ga− OR(S)‑melac. Similarly, the formation of configurationally stable diastereomers was shown for {L}SnOCH(CH3)CO2iPr as rotation around Sn−O was not possible due to the formation of the chelate Sn−OC bond.27 As the structure of Me2GaOR(SIMes) complexes was expected to have considerable effect on their rac-LA polymerization activity, as evidenced by the controlled nature of LA polymerization (Mn,theo ≈ Mn,exp, PDI < 1.36) for 1 and uncontrolled (Mn,theo < Mn,exp, 1.94 < PDI < 2.69) for 2,19 we decided to investigate closely the effect of NHC and OR groups on the structure of a series of Me2GaOR(NHC) complexes (Scheme 1). Compounds Me2Ga(OCH2CH2OMe)(SIMes) (3) (Figure 1) and Me2Ga(OCH(Me)CH2OMe)(SIMes) (4) (Figure 2) were synthesized, analogously to previously obtained 1 and 2,

Figure 1. Molecular structure of 3 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−C(4) 1.985(2), Ga(1)− C(5) 1.986(3), Ga(1)−C(1) 2.106(2), Ga(1)−O(1) 1.8773(17), C(1)−Ga(1)−C(4) 108.33(9), C(1)−Ga(1)−C(5) 113.27(9), C(1)− Ga(1)−O(1) 100.72(8), N(1)−C(1)−N(2) 108.47(19), C(5)− Ga(1)−C(1)−N(1) 164.71(18), C(4)−Ga(1)−C(1)−N(1) 34.9(2), and O(1)−Ga(1)−C(1)−N(1) −79.33(19). B


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Organometallics Scheme 1. Synthesis of Me2GaOR(NHC) (NHC = SIMes and IMes) Complexes

in the reaction between 2 equiv of SIMes and [Me2Ga(μOCH2CH2OMe)]2, and [Me2Ga(μ-OCH(Me)CH2OMe)]2 (Scheme 1). In both cases, recrystallization from a toluene/ hexane solution gave colorless crystals suitable for X-ray diffraction analysis. Contrary to 2, in the case of 3 and 4 coordination of the SIMes ligand to gallium results in the formation of complexes with more symmetric alignment of SIMes ligand, which is evidenced by C−Ga−C(1)−N(1) torsion angles (Figures 1 and 2). For both 3 and 4, ether functionalities of methoxyethanolate and methoxy-2-propanolate do not interact with gallium, which adopts a distorted tetrahedral geometry. The strong coordination of the SIMes ligand to gallium in the solid state was reflected for Me2GaOR(SIMes) by the increase of the NCN angle 28 by almost four degrees for 3 (108.47(19)°), 4 (108.45(11)°), and 2 (108.57(12)°) in comparison with the free SIMes ligand (104.7(3)°).29 In solution, it was evidenced by the higher-field shift of signals corresponding to Ga−Me protons in 1H NMR and significantly shifted carbene carbon signals in 13C NMR (Table 1) in comparison with the free SIMes (244.4 ppm). Noteworthy, 1H NMR revealed the presence of a sharp singlet for 3 and two singlets for 4 corresponding to the Ga−Me protons. In the latter case, the two signals are not likely to originate from

Figure 2. Molecular structure of 4 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−C(4) 1.9818(15), Ga(1)− C(5) 1.9826(14), Ga(1)−C(1) 2.0928(13), Ga(1)−O(1) 1.873(3), C(1)−Ga(1)−C(4) 111.03(6), C(1)−Ga(1)−C(5) 112.55(6), C(1)− Ga(1)−O(1) 94.41(16), N(1)−C(1)−N(2) 108.45(11), C(5)− Ga(1)−C(1)−N(1) 163.21(11), C(4)−Ga(1)−C(1)−N(1) −30.33(13), and O(1)−Ga(1)−C(1)−N(1) 82.7(3).

Table 1. Selected X-ray, NMR, and FTIR Data for Compounds 1−7 1a Ga−CNHC Ga−Oalkoxy Ga···OC Ga···O(Me) NCNNHC ΔNCNNHCb 13

C NMR Ccarbene Δ 13C NMR Ccarbeneg νCO

2 2.1007(13) 1.8778(11)

108.57(12) 3.86 199.2 −45.2 1737, 1717d,f

3

4

5a

X-ray: Bond Distances (Å) and Angles (deg) 2.106(2) 2.0928(13) 2.094(2) 1.8773(17) 1.873(3) 1.8805(18) 3.3688(19) 108.47(19) 3.76 NMR 200.2 −44.2

7a

5b

6

2.085(2) 1.8919(17)

2.0888(11) 1.8669(8)

3.2520(19) 108.45(11) 104.2(2) 104.1(2) 3.74 2.78 2.68 (ppm) (Toluene-d8) 199.6 176.2 −44.8 −43.5 1747, 1723e, 1739, 1715d

103.83(9) 2.41 176.1c −43.6

176.0 −43.7 1766, 1753, 1741, 1723e,f

a

Compounds are oils. bIn comparison with respective NHC. cIn CD2Cl2. dIn CH2Cl2. eIn toluene. fAfter deconvolution. gIn comparison with SIMes, IMes or SIPr in toluene-d8. C


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Organometallics asymmetry caused by the orientation of NHC. This is evidenced by the formation of essentially symmetric species in solution for 3 and is further supported by our density functional theory (DFT) studies (see below). The presence of two signals should be explained, similarly to 1, by the presence of diastereotopic Ga−Me groups, which should be expected due to the high activation energy for the rotation of the Ga−O bond. On the basis of VT 1H NMR studies which revealed the coalescence of Ga−Me signals at elevated temperature, we estimated the free energy of activation of the rotation of Ga−O bond of 1 (ΔG⧧351 K = 85.3(10) kJ mol−1) and 4 (ΔG⧧345 K = 86.5(10) kJ mol−1). Both values for 1 and 4 may originate from the steric effect of the methyl group of (S)-methyl lactate ((S)melac) or methoxy-2-propanolate groups. The additional effect of a weak chelate interaction between (S)-melac or methoxy-2propanolate and Ga cannot, however, be excluded. For 1, the possibility of chelate interaction was demonstrated by FTIR in solution. Although initial FTIR studies showed one band at 1737 cm−1 corresponding to the free CO group,19 the deconvolution of FTIR spectra of 1, in CH2Cl2, showed two bands, a major band at 1737 cm−1 (79%) in line with the free CO of the (S)-melac ligand and a minor one at 1717 cm−1 (21%) corresponding to the CO coordinated to gallium. This observation indicates weak and labile coordination of the (S)-melac ligand to gallium due to the Ga···OC interaction. The use of IMes, instead of SIMes, for the synthesis of Me2Ga((S)-OCH(Me)CO2Me)(NHC) brought structural evidence for the chelate interaction between the (S)-melac ligand and Ga center in the solid state. It was demonstrated by X-ray (Figure 3) and Raman analysis of Me2Ga((S)-OCH(Me)CO2Me)(IMes) (5). Compound 5 was synthesized in the reaction between (S,S)-[Me2Ga(μ-OCH(Me)CO2Me)]2 and IMes (1:2) in an essentially quantitative yield and was isolated in the form of colorless crystals. In this case, the X-ray diffraction analysis revealed two unique molecules with weak chelate interactions: Ga···OC(OMe) (5a) and Ga···O(Me)C(O) (5b). The presence of 5a and 5b in the solid state was additionally confirmed by two CO bands at 1709 and 1734 cm−1 in the Raman spectrum. For both 5a and 5b, chelate interaction did not influence significantly the coordination sphere of gallium which can be described as distorted tetrahedral with essentially symmetrically arranged IMes ligand to Ga−CMe bonds (Figure 3). The chelate interactions observed for 5a and 5b are likely to result from a weaker coordination of IMes to gallium, in comparison with SIMes, which is in both cases in the trans position to Ga···OC or Ga···OMe interactions.30 Interestingly, the NCN angle (104.2(2)° (5a) and 104.1(2)° (5b)) could not indicate in this case the strength of Ga−CIMes in comparison to Ga−CSIMes, due to the structural difference between unsaturated IMes and saturated (SIMes) NHCs. The weaker Ga−CIMes bond was, however, evidenced by 13C NMR spectroscopy, which revealed for 5 a smaller carbene carbon shift (43.5 ppm) in comparison with that of 1 (45.1 ppm). 1H NMR was indicative of the presence of 5a and 5b in solution as revealed by the slightly shifted two sets of signals at room temperature (see the Supporting Information). Although the increase of temperature to about 40 °C resulted in one set of signals, the very small difference between both sets of signals did not allow for the precise observation of coalescence point and calculation of activation energy for the interconversion of 5a and 5b. However, similarly to 1 and 4, the coalescence of two singlets corresponding to Ga−Me groups at elevated

Figure 3. Molecular structure of 5 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 5a (a): Ga(1)−C(4) 1.986(3), Ga(1)−C(5) 1.975(3), Ga(1)−C(1) 2.094(2), Ga(1)−O(1) 1.8805(18), Ga(1)−O(3) 3.3688(19), C(1)−Ga(1)−C(4) 110.37(10), C(1)−Ga(1)−C(5) 111.11(10), C(1)−Ga(1)−O(1) 95.79(8), C(1)−Ga(1)−O(3) 152.83(8), N(1)−C(1)−N(2) 104.2(2), C(5)−Ga(1)−C(1)−N(1) −155.4(2), C(4)−Ga(1)− C(1)−N(1) −21.6(3), and O(1)−Ga(1)−C(1)−N(1) 92.9(2). Selected bond lengths (Å) and angles (deg) for 5b (b): Ga(2)− C(104) 1.985(2), Ga(2)−C(105) 1.983(3), Ga(2)−C(101) 2.085(2), Ga(2)−O(101) 1.8919(17), Ga(2)−O(102) 3.2520(19), C(101)− Ga(2)−C(104) 111.22(10), C(101)−Ga(2)−C(105) 110.30(10), C(101)−Ga(2)−O(101) 94.49(8), C(101)−Ga(2)−O(102) 147.50(8), N(101)−C(101)−N(102) 104.1(2), C(105)−Ga(2)− C(101)−N(101) −159.7(2), C(104)−Ga(2)−C(101)−N(101) −28.6(3), and O(101)−Ga(2)−C(101)−N(101) 85.9(2).

temperature allowed for the determination of activation energy for the rotation of the Ga−(S)-melac bond (ΔG⧧370 K = 90.1(10) kJ mol−1). The higher value obtained for 5 in comparison with 1 (ΔG⧧351 K = 85.3(10) kJ mol−1) and 4 (ΔG⧧346 K = 86.5(10) kJ mol−1), is in line with stronger chelate interaction between gallium and (S)-melac ligand in comparison D


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Organometallics

The structure studies of a series of Me2GaOR(NHC) (NHC = SIMes, IMes) complexes, supported by DFT calculations (see below) indicate that the chelate interaction of the (S)-melac ligand with Ga affects the structure and catalytic activity of Me2GaOR(NHC) to a greater extent than the orientation of NHC. The latter is confirmed by a low activity of 5 in the polymerization of rac-LA (see below). In regard to the latter case, we decided to synthesize Me2Ga((OCH(Me)C(O))2O(CH2)2OMe)(IMes) (7) with alkoxide ligands which mimics propagating species during LA polymerization with Me2GaOR(IMes) species. Although lactate ligands -OCH(Me)CO2R (R = Me, Et, iPr) have been regarded as mimicing propagating species for LA polymerization with aluminum,32 gallium,18 zinc5b and tin33 complexes, (OCH(Me)C(O))2OR ligands are expected to give more insight into the interaction of the PLA chain with the coordination center as demonstrated in the literature for (OCH(Me)C(O))2O(CH2)2OMe32 and OCH(Me)C(O))2O(CH2)2OiPr.34 In this regard, it is important to note the recently reported facile synthesis of L-LA ring-opened products HO[CH(CH 3 )CO]nOR (n = 2 or 3) with magnesium alkoxides.35 We synthesized (OCH(Me)C(O))2O(CH2)2OMe by the hydrolysis of previously reported [Me2Al(μ-OCH(Me)C(O))2O(CH2)2OMe]2,32 while the subsequent reaction with Me3Ga resulted in essentially quantitative formation of a mixture of dimeric (R,S)-[Me2Ga(OCH(Me)C(O))2O(CH2)2OMe]2 and (R*,R*)-[Me2Ga(OCH(Me)C(O))2O(CH2)2OMe]2 as evidenced by the presence of three singlets at −0.41, −0.34, and −0.26 (1:2:1) in the region of GaMe signals of the 1H NMR spectrum. Me2Ga((OCH(Me)C(O))2O(CH2)2OMe)(IMes) (7) was synthesized in the reaction between [Me2Ga(OCH(Me)C(O))2O(CH2)2OMe]2 and IMes (Ga/IMes = 1:1) (Scheme 1) and obtained in the form of a colorless oil. Although the 1H NMR spectrum of 7 was complex, the detailed analysis revealed, among others, that all signals corresponding to Ga-Me protons (−0.87 to −0.79 ppm) were significantly shifted in comparison with that of [Me2Ga(OCH(Me)C(O))2O(CH2)2OMe]2 (1H NMR and 13 C NMR) and the carbene carbon at 176.0 ppm, shifted by 43.7 ppm in comparison with that of IMes (13C NMR), which is in line with essentially the quantitative formation of 7. The complex character of the spectrum can therefore originate from different coordination modes of (OCH(Me)C(O)) 2 O(CH2)2OMe to gallium. As the 1H NMR spectrum of 7 changed in time at room temperature, the 1H NMR spectrum and the 13C NMR spectrum after 48 h revealed neither significant changes concerning proton and carbon shifts of GaMe groups (1H NMR and 13C NMR) nor changes of carbene carbon of IMes (171.2 ppm) (13C NMR). Furthermore, signals corresponding to protons of IMes remained essentially the same, while the most pronounced changes concerned the (OCH(Me)C(O))2 O(CH 2 ) 2 OMe ligand indicating that changes are associated with the chelate interaction of (OCH(Me)C(O))2O(CH2)2OMe with Ga. The latter is further supported by FTIR. One broad band corresponding to CO groups of 7 was found after dissolution and following 48 h. The deconvolution of FTIR spectra of 7, in toluene, showed four bands at 1723 cm−1 (5.2%), 1741 cm−1 (18.9%), 1753 cm−1 (67.7%), and 1766 cm−1 (8.2%). After 48 h, the intensities of CO bands changed significantly with minor changes concerning the following shifts: 1724 cm−1 (14.0%), 1744 cm−1 (36.0%), 1753 cm−1 (16.0%), and 1762 cm−1 (34.0%). The changes of the arrangement of the (OCH(Me)-

with 1. The strength of the Ga···OC interaction was estimated in solution by FTIR spectroscopy which revealed two strong CO bands in toluene (and CH2Cl2)) at 1747 (1739) cm−1 and 1723 (1715) cm−1 corresponding to free C O and CO groups coordinated to gallium, which is in line with the presence of 5a and 5b in solution. The carbonyl group in 5a is red-shifted by 24 cm−1 in comparison with νCO in 5b. It must be, however, noted that the CO band at 1739 cm−1 (in CH2Cl2) corresponding to 5b is slightly blue-shifted in comparison with that of 1, which is line with a weak Ga···OMe interaction. Therefore, the coordinated CO band of 5a is red-shifted by 22 cm−1 in comparison with that of free (S)melac ligand indicating a slightly stronger coordination of C O in comparison with that of 1, which is in accordance with slighly weaker coordination of IMes in comparison with the SIMes ligand. It must be noted that the shift of coordinated CO, in comparison with that of the free (S)-melac ligand, for both 1 (20 cm−1) and 5 (22 cm−1), is more significant in comparison with that of dimeric [Me2Ga((S)-melac)]2 (12 cm−1).18 The latter observation is, however, in contrast to a stronger Ga−CNHC bond for 5 (0.642 and 0.660 vu (valence units)) in comparison with Ga−Obridging (0.413 and 0.422 vu) in [Me2Ga((S)-melac)]2 (in trans position to Ga···OC interaction) as indicated by their valencies calculated according to Brown et al.31 using the following parameters: Ga−C, r1 = 1.931, b = 0.37; Ga−O, r1 = 1.708, b = 0.37.26g For methoxy derivative 6, the coordination strength of the IMes ligand to the gallium center was almost identical to that of 5 as was evidenced by 13C NMR, which revealed the carbene carbon signal significantly higherfield shifted by 43.6 ppm in comparison with that of free IMes. Similarly to 2, the presence of an asymmetrical tetrahedral gallium complex due to the orientation of the IMes ligand to Ga(1)−C(4) and Ga(1)− C(5) bonds and alignment of the OMe group essentially along the plane of the mesityl ring was, however, evidenced by X-ray analysis (Figure 4). In contrast to 2, for which the broad signal

Figure 4. Molecular structure of 6 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−C(4) 1.9827(12), Ga(1)− C(5) 1.9845(12), Ga(1)−C(1) 2.0888(11), Ga(1)−O(1) 1.8669(8), C(1)−Ga(1)−C(4) 110.36(5), C(1)−Ga(1)−C(5) 107.39(5), C(1)− Ga(1)−O(1) 103.14(4), N(1)−C(1)−N(2) 103.83(9), C(5)− Ga(1)−C(1)−N(1) −123.22(10), C(4)−Ga(1)−C(1)−N(1) 7.09(11), and O(1)−Ga(1)−C(1)−N(1) 118.87(10).

was observed for Ga-Me protons, 1H NMR revealed the presence of a sharp singlet corresponding to Ga-Me protons of 6. For 6, which forms an asymmetric complex in solid state, this clearly shows the fast rotation of Ga−CNHC on an NMR time scale at room temperature, which is in line with computational studies (see below). E


Article

Organometallics

C(1)−Ga−O(1) with different orientations of the NHC to the Ga−CMe bonds (Figure 5). Our calculations show that the global energy minimum is in agreement with the experimental structure, but also indicte that the difference in energy between the two observed minima is very small, between 1.1 and 1.3 kcal mol−1. Moreover, the energy barrier of the rotation is also relatively small, estimated at around 5.5−6.5 kcal mol−1. The solvation correction was found to have only a slight impact on obtained values (see the Supporting Information). These results suggest that the rotation of the Ga−CNHC bond for complexes 2 and 6 occurs freely in the solution at ambient temperature and that the asymmetric orientation of NHC to Ga−CMe bonds is barely favored over the essentially symmetric one. We can attribute the stability of the asymmetric conformation in the crystal structure to two phenomena. First, the crystal packing allows for an additional interaction of the OMe group with one of the mesityl rings of the adjacent molecule and intermolecular CHNHC...OMe hydrogen bond (see the Supporting Information). The OCH3···π interaction may stabilize this particular conformation by approximately 1.5 kcal mol−1.36 Additionally, this conformation may be stabilized by the subtle intramolecular π-face donation through the Cipso (C7) atom of the mesityl substituent of NHC.37 For 1 and 5, the results of the rotational energy scans along the N(2)− C(1)−Ga−O(1) dihedral angle are similar to each other but different from 2 and 6. Here, the global minimum was found for the conformation with the NHC oriented essentially symmetrically, while for the local minimum NHC is arranged asymmetrically to the Ga−CMe bonds. The difference in energy between these two minima is slightly larger than those for 2 and 6 and equal to approximately 3.5 kcal mol−1, while the rotation energy barriers are equal to 9.7 and 10.8 kcal mol−1 for 5 and 1, respectively. These results suggests that systems 1 and 5 with (S)-methyl lactate are substantially less flexible than 2 and 6 with OMe groups and that the rotation around the Ga−CNHC bond is still essentially free at room temperature. Additionally, for 1 and 5 we have performed the rotational scan along the O(1)−C(7)−C(8)−O(2) dihedral angle (Figure 6). For 5, the presence of 5a and 5b conformers is evidenced by the presence of two minima in the rotational scan. It is also not surprising that the energy difference between these two conformations is very small, below 0.5 kcal mol−1. Since the accuracy of the used DFT method is around 1 kcal mol−1, we can claim that both 5a and 5b have almost identical energies and are likely to occur with the same probability, which is in line with the structure of 5 in solution and solid state. Since the energy barrier between these two minima is approximately 8 kcal mol−1, both conformations are also likely to be present in the solution. The estimate of the energy barrier may be treated, in this case, also as an estimate of the strength of the Ga···O interaction. The strength of the Ga−CNHC bond was also estimated using the same DFT method by calculating the energy of systems 1, 2, 5, and 6 and subtracting the sum of the energies of NHC and gallium parts (with the counterpoise correction). Using this approach, we obtained the following values: for methoxy derivatives, 38.7 kcal mol−1 (2) and 39.6 kcal mol−1 (6); for (S)-melac derivatives, 32.6 kcal mol−1 (1) and 33.4 kcal mol−1 (5). Although DFT calculations do not clearly differentiate between SIMes and IMes complexes and cannot clearly support changes revealed by spectroscopic analysis, there are several interesting implications resulting from these results. According to calculations, it is clear that Ga−CNHC in the investigated

C(O))2O(CH2)2OMe ligand were also reflected by the activity of 7 in the polymerization of rac-LA (see below). 2. Computational Studies of 1, 2, 5, and 6. In order to confirm our observation concerning the Ga−CNHC bond strength as well as the influence of the rotation of Ga−CNHC and Ga−OR on the structure of Me2GaOR(NHC) complexes, we performed calculations for complexes 1, 2, 5, and 6. Despite the fact that calculations were done for isolated molecules, the optimized structures of 2, 5, and 6 are very similar to the experimental crystal structures. The only small discrepancy can be observed for the C(17)−N(2)−N(1)−C(11) dihedral angle, which is very close to 0° in all calculated structures, but between 5° and 11° for the experimental ones. As expected, the predicted structure of 1 is very similar to the experimental structure of 5 due to the orientation of the SIMes ligand to the Ga−CMe bonds. In order to gain more insight into the energetic features of Ga complexes and, in particular, to answer the question about the arrangement of NHC and OR groups, we decided to perform rotational scans along the N(2)−C(1)− Ga−O(1) dihedral angle of 1, 2, 5, and 6 (Figure 5), and the

Figure 5. Rotational scan along the N(2)−C(1)−Ga−O(1) dihedral angle for 1, 2, 5, and 6.

O(1)−C(7)−C(8)−O(2) dihedral angle of 1 and 5 (Figure 6) (see the Supporting Information). For 2 and 6, the rotational energy scan along N(2)−C(1)−Ga−O(1) gives almost identical results. In both cases, there are two minima on the energy plot corresponding to two conformations (N(2)−

Figure 6. Rotational scan along the O(1)−C(7)−C(8)−O(2) dihedral angle for 1 and 5. F


Article

Organometallics Scheme 2. Reactivity of 1 and 5 toward Me3Ga and CO2

shows that Me2GaOR(NHC) complexes are not suitable candidates for the copolymerization reactions including CO2. However, the easy removal of NHC from the Ga center with gaseous CO2 could offer an easy switch between isoselective Me 2 GaOR(NHC) 16,19 and nonselective/heteroselective [Me2Ga(μ-OR)]2 catalytic centers16,18 in the polymerization of rac-LA. 4. Reaction of Dimethylgallium Alkoxides with SIPr. The effect of strong bases on the formation of Ga−CNHC could be well demonstrated by the competition between strongly basic NHC and Me2GaOR centers in the formation of Me2GaOR(NHC) and [Me2Ga(μ-OR)]2, respectively. In order to demonstrate that the change of relative basicity may have a substantial effect on the formation of mentioned adducts, we attempted to show the effect of 1,3-bis(2,6diisopropylphenyl)imidazolin-2-ylidene (SIPr), of weaker coordinating properties in comparison with those of SIMes or IMes,28 on the synthesis of Me2GaOR(NHC). Weaker coordination properties of SIPr result mainly from steric hindrances, which are reflected by its buried volume parameter.41 Contrary to the synthesis of 1 and 5, the reaction of (S,S)-[Me2Ga(μ-OCH(Me)C(O)OMe)]2 with the SIPr ligand resulted in the formation of a complex mixture of products, as evidenced by NMR. However, the slow crystallization from toluene solution gave colorless crystals of Me2Ga(OCH(Me)CO2)]−[SIPrH]+ (9) (Figure 8), an ionic compound consisting of an anionic dimethylgallium derivative

complexes is fairly strong; however, it is at least twice as weak as a standard single covalent bond (80−100 kcal mol−1). For instance, it is much weaker than the Ru−Ccarbene bond in a commonly used Hoveyda ruthenium catalyst (81.7 kcal mol−1).38 On the basis of these results, we may expect that Lewis bases, which are able to coordinate stronger to the Me2GaOR moiety than NHC, and acids, which are able to form more stable compounds with NHC than Me2GaOR(NHC), are both able to break the Ga−CNHC bond. 3. Reaction of 1 and 5 with Me3Ga and CO2. We addressed the problem of the stability for Me2GaOR(NHC) complexes in the presence of Lewis acids by investigating the reactivity of 1 and 5 with Me3Ga and CO2. Previously, we have shown that 1 reacts with weak Brönsted acid CDCl3 to form (S,S)-[Me2Ga(μ-OCH(Me)C(O)OMe)]2 and SIMesD(CCl3) adducts.19 Moreover, the equimolar reaction between 1 and Me3Ga resulted in the formation of Me3Ga(SIMes) and (S,S)[Me2Ga(μ-OCH(Me)C(O)OMe)]2.26g The analogous reaction for 5 allowed for the isolation of Me3Ga(IMes) (8) (Scheme 2). The structure of 8 was determined both in the solid state by X-ray analysis (Figure 7) and in solution, and is

Figure 7. Molecular structure of 8 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) are presented in Table 2.

discussed later in the text along with the coordination properties of NHC toward akylgallium complexes. Prior to the potential catalytic activity of Me2GaOR(NHC), e.g., for the copolymerization of olephine oxides with CO2,39 the reactivity of 1 and 5 toward CO2 was particularily interesting. Therefore, we performed the reaction of 1 and 5 with CO2, which resulted in the formation of (S,S)-[Me2Ga(μ-OCH(Me)C(O)OMe)]2 dimers and (NHC)CO2 (NHC = SIMes, IMes) adducts (Scheme 2), and no insertion of CO2 into the Ga−Oalkoxide bond was observed. (SIMes)CO2 and (IMes)CO2 adducts have been already reported in the literature.40 Interestingly, 13C NMR revealed that the Ga−CNHC bond is stronger in the case of 1 and 5 in comparison with that of Me3Ga(NHC), while the formation of the latter in the reaction between 1 or 5 and Me3Ga is most probably driven by the formation of (S,S)[Me2Ga(μ-OCH(Me)C(O)OMe)]2 dimer. The latter results

Figure 8. Molecular structure of 9 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−C(4) 1.976(3), Ga(1)− C(5) 1.967(2), Ga(1)−O(1) 1.8931(15), Ga(1)−O(2) 1.9560(18), O(2)−C(8) 1.292(3), O(3)−C(8) 1.235(3), O(1)...C(1) 2.917, and N(1)−C(1)−N(2) 112.90(18). G


Article

Organometallics of lactic acid. The gallium center in [Me2Ga(OCH(Me)CO2)]− is four coordinate with a distorted tetrahedral coordination sphere, and the [SIPrH]+ forms a strong charge assisted C−H··· O hydrogen bond with the alkoxide oxygen of anion. Interestingly, the observed C NHC −H···O interaction (CNHC...O 2.917(2) Å) is the shortest among only a few known analogues interactions with metal alkoxides or aryloxides.42 Another strong hydrogen bond is formed by the interaction of terminal carboxylate oxygen with the CH moiety of the imidazolinium ring of an adjacent molecule (C(2)...O(3) 3.047(3) Å). The formation of 9 can be explained by the reaction of SIPr with the (S)-melac ligand of (S,S)-[Me2Ga(μOCH(Me)C(O)OMe)]2 due to the limited coordination ability of SIPr to the gallium center. The formation of 9 indicates that the reaction of SIPr with (S,S)-[Me2Ga(μ-OCH(Me)C(O)OMe)]2 leads to the abstraction of the methine proton and CH3+, which is evidenced by the presence of SIPrH+ and [Me2Ga(OCH(Me)CO2)]− anion, respectively. The lack of formation of Me2GaOR(SIPr) due to the weaker coordinating properties of SIPr in comparison with those of SIMes and IMes was confirmed by the reaction between SIPr and [Me2Ga(OCH2CH2OMe)]2. In this case, free SIPr was observed in solution for 10 days as evidenced by 13C NMR spectroscopy (see the Supporting Information). In this regard, the comparison of the coordination properties of SIMes, IMes, and SIPr toward Ga were further investigated with the use of model gallium complexes. 5. Determination of the Coordination Properties of SIMes, IMes, and SIPr with the Use of Model Organogallium Compounds. In order to compare the coordination properties of SIPr with SIMes and IMes for the formation of a Ga−CNHC bond, we analyzed the structures of Me3Ga(SIMes),38 Me3Ga(IMes) (8), and Me3Ga(SIPr) (10) (Scheme 3).

Figure 9. Molecular structure of 10 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) are presented in Table 2.

C(106) angles (Table 2). The strains resulting from steric repulsion between the SIPr ligand and methyl groups were also demonstrated by a resultant bond valence vector (BVV), calculated according to the bond valence vector model (BVVM).44 In the case of 10, the resultant BVVs (0.051 and 0.062 vu) for gallium centers are aligned next to the Ga(1)− C(6) and Ga(2)−C(106) bond lines. In order to compare the strength of the Ga−CNHC bond for Me3Ga(SIMes), 8, and 10 in solution, we analyzed the differences between 13C NMR carbene carbon shifts of coordinated and free NHC (Table 2).28 Accordingly, the coordination strength of NHC in solution decreases in a series: Me3Ga(SIMes) > Me3Ga(IMes) (8) > Me3Ga(SIPr) (10). 6. Ring Opening Polymerization of rac-LA with 1−7. Compounds 1−7 were used as catalysts for the ring-opening polymerization (ROP) of rac-LA, and the most important finding concerns their stereoselectivity. As previously communicated by us for 1 and 2,19 the formation of highly active isoselective Me2GaOR(NHC) catalysts, resulting from a simple reaction of NHC with nonselective/heteroselective dialkylgallium alkoxides,18 causes a facile stereoselectivity switch in the polymerization of rac-LA. Noteworthy is the fact that stereoselectivity was essentially not dependent on the structure of NHC (NHC = SIMes, IMes) in the studied Me2GaOR(NHC) complexes. Polymerization of rac-LA at mild conditions led to isotactically enriched polylactide (PLA) (Pm up to 0.78 at −20 °C) (Table 3). The patterns of methine protons of PLA obtained with 1−7, revealed by homodecoupled 1H NMR spectra (see the Supporting Information), were in all cases essentially the same and not indicative, as we had previously observed, of either enantiomorphic site control or a chain end mechanism.19 Interestingly, they were similar to the spectra of PLA obtained by the polymerization of rac-LA with NHC carbenes.48 Besides the high stereoselectivity of the investigated complexes, another important aspect, in light of rac-LA polymerization with 1 and 2,19 concerns control of the polymerization. Previously, we demonstrated that compound 1 allows for the controlled polymerization of L-LA and rac-LA at r.t. and −20 °C with high isoselectivity (Pm up to 0.78 at −20 °C). Although the polymerization was controlled, the Mn vs rac-LA conv. graph (see the Supporting Information) revealed slow polymerization over first 15 min, which showed that the

Scheme 3. Me3Ga(NHC) Complexes (NHC = SIMes, IMes, and SIPr)

Compound 10 was synthesized in the equimolar reaction between Me3Ga and SIPr and isolated in the form of colorless crystals. Similarly to Me3Ga(SIMes)38 and Me3Ga adduct of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene,43 in the case of 8 and 10 (Figure 9) X-ray analysis revealed the presence of four coordinated gallium species with the coordination sphere adopting distorted tetrahedral geometry. Out of the mentioned adducts, Ga−CSIPr bonds in 10 (2.132(3), 2.138(3) Å) are the longest (Table 2). However, Ga−CNHC bond distances for Me3Ga(SIMes) and Me3Ga(IMes) (8) indicate a slightly stronger Ga−CNHC bond in the latter case. This is in line with a slightly shorter Ga−CNHC bond for 6 in comparison with that for 2. The differences in the coordination of SIMes, IMes, and SIPr to gallium should be also seen by the deformations around carbene and gallium centers. It is well reflected by the considerable pitch angles (out of NHC plane tilting) for 10 in comparison with that of Me3Ga(SIMes) and 8, and significant lowering of the C(1)−Ga(1)−C(6) and C(101)−Ga(2)− H


Article

Organometallics Table 2. Selected X-ray and 13C NMR Data for Me3Ga(SIMes), 8, and 10 Me3Ga(SIMes)26g Ga(1)−C(1)NHC Ga(1)−C(4) Ga(1)−C(5) Ga(1)−C(6) C(4)−Ga(1)−C(1) C(5)−Ga(1)−C(1) C(6)−Ga(1)−C(1) N(1)−C(1)−N(2) NHC tilte pitch angle (deg) 13

a

8

2.121(3) 2.119(3)d 2.001(3) 2.008(3)d 1.994(3) 2.000(3)d 2.002(3) 2.007(3)d 106.02(12) 106.15(11)d 108.70(11) 110.69(11)d 107.41(12) 104.76(11)d 107.89 (Δ = 3.18)a 107.82 (Δ = 3.11)a,d 1.7 2.1d 0.7 0.9d 205.8 (Δ = −38.6)b

C NMR Ccarbene (ppm)

2.1047(16) 2.0029(13) 2.0029(13)c 1.9928(18) c 105.63(4) 105.63(4) 108.55(7) c 103.78 (Δ = 2.36)a 1.6 0.0 182.1 (Δ = −37.6)b

10 2.132(3) 2.138(3)d 2.009(3) 1.991(3)d 1.989(3) 2.003(3)d 2.006(3) 2.000(3)d 110.88(10) 107.51(11)d 106.71(11) 110.80(10)d 100.47(11) 100.20(12)d 107.77 (Δ = 2.80)a 108.28 (Δ = 3.31)a,d 13.5 12.6d 13.5 12.6d 209.3 (Δ = −35.1)b

In comparison with SIMes, IMes and SIPr. In comparison with SIMes, IMes and SIPr in toluene-d8. cTwo methyl groups are related by a crystallographic symmetry plane passing through the molecule. dFor the second independent molecule of the asymmetric unit. eThe tilt is quantified by the offset angle of the M−C bond to the C2 axis of the NHC, which is split into pitch and yaw angles.47 29

45

46 b

suggested by us for 2.19 It should be rather assigned to the reduction of polymerization rate due chelate interaction between the O(CH(Me)C(O))2OR chain formed after the primary insertion of rac-LA into the Ga−Oalkoxide bond (Scheme 5). In this case, only part of the gallium centers would be engaged in polymerization, which is shown by the sequential polymerization of 25 equiv of rac-LA with 2, followed by the next 25 equiv of rac-LA after 24 h (Table 3, entry 19). In this case, we observed a bimodal weight distribution of obtained PLA, which shows that gallium centers initially blocked by the chelate interaction of (OCH(Me)C(O))2OR resulting from a primary insertion of rac-LA into the Ga−Oalkoxide bond (Scheme 5) are able to further polymerize rac-LA. The engagement of almost all gallium centers, after two rac-LA polymerization steps, was further evidenced by Mn determined from NMR, which was close to the expected value. The chelate interaction between (OCH(Me)C(O))2OR and Ga center, leading to uncontrolled polymerization at low temperature, had essentially no effect on the polymerization of rac-LA with 2−4 at room temperature, which supports our earlier reasoning. In all cases, GPC revealed the monomodal weight distribution of PLA and the Mn value close to that expected from the initial rac-LA:Ga ratio (Table 3, entries 6, 8, and 10). In this case, MALDI-TOF was, however, indicative of the presence of intermolecular and minor intramolecular transesterification reactions (Figure 11). As can be expected, transesterification had adverse impact on isoselectivity as evidenced by Pm between 0.65 and 0.67. With regard to uncontrolled polymerization with 2−4 at −20 °C, due to the chelate interaction between gallium and (OCH(Me)C(O))2OR formed after the primary insertion of PLA into the Ga−OR bond (Scheme 5), it must be noted that the latter effect could be avoided by the formation of short PLA blocks at r.t.. Therefore, the sequential polymerization of 25 equiv of racLA at r.t with 2 or 3, leading to Me2GaO-(PLA)25OR(SIMes)

initiation rate (kini) was lower in comparison with the propagation rate (kprop) due to the weak chelate interaction of (S)-melac ligand with gallium (Scheme 4a). This is in agreement with, e.g., lower PDI for PLA obtained with 1:racLA = 320 (1.20, Table 3 entry 3) than with 1:rac-LA = 90 (1.36, Table 3, entry 2). For 2, the uncontrolled nature of polymerization at −20 °C was evidenced by significantly increased PDI and Mn of obtained PLA, in comparison with the expected value of Mn, while high isoselectivity was retained (Table 3). As insertion of rac-LA occurred solely into the Ga−OR bond and any side reactions were not observed (see MALDI TOF in the Supporting Information), the results for 2 indicated that only a fraction of catalytic centers was engaged in the polymerization. In order to explain observations for 1 and 2, we further investigated the activity of 3−4 in the polymerization of rac-LA. Compounds 3 and 4 initiated rac-LA rapidly at −20 °C (Table 3, entries 7 and 9) leading to almost full conversion of 50 equiv of lactide within 30 min. Similarly to 1 and 2, 1H NMR revealed the presence of PLA with OH and OR (OR = OCH2CH2OMe or OCH(CH3)CH2OMe) end groups, resulting from the insertion of rac-LA exclusively into Ga−Oalkoxide bond. The latter was further confirmed for the polymerization of rac-LA with 1−4 at −20 °C as MALDI-TOF of obtained PLA revealed in each case only PLA chains with OH and a corresponding OR group and showed that only minor transesterification may occur at low temperatures (Figure 10). Despite the latter and high isoselectivity observed for 3 and 4 at −20 °C, Mn values of obtained PLA were much higher than expected, and PDI values were equal to 1.89 and 1.95, respectively, therefore indicating the uncontrolled nature of polymerization. On the basis of structural studies of 1−4, the uncontrolled nature of the polymerization in the case of 2, 3, and 4 at −20 °C cannot be the result of the initial arrangement of NHC, resulting from a slow rotation of Ga−CNHC bond, as initially I


Article

Organometallics Table 3. Polymerization of rac-LA in CH2Cl2 at −20 °C no. 19

cat.

LA/Ga

t [min]

conv. [%]

10−3 Mnb

10−3 Mnc

10−3 Mnd

Mw/Mn

P me

P mf

6.6 12.6 45.1 5.3 21.7 6.9 32.8 6.4 49.0 7.4

6.8 12.9 46.1 5.3 14.1 6.4 17.2 5.9 21.7 5.7

6.6 13.0 46.3 5.7 7.3 6.6 7.3 5.1 7.3 5.9

1.18 1.36 1.20 1.06 1.94 1.19 1.89 1.32 1.95 1.49

0.77 0.78 0.78 0.65 0.78 0.67 0.79 0.67 0.78 0.66

0.67 0.77 0.66 0.78 0.65

45.4 36.1 5.0 63.1 6.9

19.1 9.8 6.3 53.0 8.4

7.3 7.3 6.3 7.2 7.3

1.55 1.65 1.85 1.32 1.19

0.78 0.78 0.67 0.78 0.68

0.77 0.65 0.78 0.67

17.0 39.9 5.9 11.4 12.6 96.6 17.9 6.5 10.0 31.5 12.7 4.5 124.2 13.2

6.8 8.0

4.6 7.3

0.76 0.78

0.77

14.7 15.8 16.2

13.8 14.2 14.2

8.6 13.9 33.9 10.0 6.7 15.6

7.1 13.8 33.0 7.3 2.9 12.9

1.1 1.42 1.07 1.77 1.60 1.18 1.30 2.57 2.53 1.66 1.70 3.69 1.22 1.26

1 219 319 419 519 6 7 8 9 10 11 12 13 14 15 16 17 18 19

1 1 1 1a 2 2a 3 3a 4 4a 5 5 5 5a 6 6a 7 7 2

50 90 320 40 50 50 50 50 50 50 50 50 50 50 50 50 50 50 25 + 25

30 240 960 30 30 10 60 10 30 10 30 180 1260 10 10 10 30 30 1440/60

90 99 >99 97 >99 90 >99 70 99 81 5 99 >99 87 98 95 10 62 >99

20 21 22

2 3 5

25a + 75 25a + 75 100

10/360 10/360 1440/60

95 98 98

23 24 25 26 27 28

5 5 5 5/iPrOH (1:1) 5/iPrOH (1:4) 6

25a + 25 25a + 75 25a + 275 100 100 50 + 50

10/180 10/360 10/650 330 330 1440/60

97 95 76 98 98 89

0,74 0.74 0.78 0.70 0,72 0,74 0,73 0.73 0.78

a

At ambient temperature. bDetermined by gel permeation chromatography (GPC) in THF. cDetermined by 1H NMR. dExpected according to the LA/Ga ratio, conversion and cat:iPrOH ratio. eCalculated on the basis of homonuclear decoupled 1H NMR spectra according to Chemberlain et al.5a f Calculated on the basis of 13C NMR.7a,49

from the fact that insertion of rac-LA into the Ga−Oalkoxide bond of 5 is difficult and leads to more active dialkylgallium species (Me2GaO-PLA(IMes)), which are expected to interact with gallium much more weakly in comparison with that of the (S)-melac ligand (Scheme 4b). The latter was additionally confirmed by the activity of 7 which should be expected to mimic Me2Ga(OCH(Me)C(O))2OR(IMes) species formed after the primary insertion of rac-LA into the Ga−Oalkoxide bond of Me2GaOR(IMes) (Scheme 5). Freshly prepared 7 showed indeed slightly higher activity than 5 (Table 3, entry 17). Interestingly, when compound 7 was kept in toluene solution at r.t. for 48 h and was used as a catalyst, the polymerization of rac-LA was observed immediately (Table 3, entry 18). This result shows that, although the chelate interaction of (OCH(Me)C(O))2OR to Ga still strongly affects the polymerization of rac-LA, the insertion of rac-LA to the Ga−Oalkoxide bond of 5 may facilitate further polymerization. This experiment additionally suggests that the longer PLA chain should increase the polymerization rate. In order to confirm the above and prove that living polymerization is possible with Me2GaOR(IMes), the following experiment was performed. First, the sequential polymerization of 25 equiv of rac-LA at r.t with 5 was performed in order to synthesize Me2GaO-(PLA)25(S)-melac(IMes), including short PLA chains instead of (S)-melac ligand. Subsequent polymerization of 25

(OR = OMe (2), OCH2CH2OMe (3)), followed by the next 75 equiv of rac-LA at −20 °C was performed (Table 3, entries 20 and 21) and revealed in each case PLA with monomodal weight distribution and Mn close to the expected value calculated on the basis of the rac-LA/catalyst ratio. This confirms our reasoning and shows that Me2GaOR(SIMes) can polymerize rac-LA in a controlled manner. Notably, in this case stereo diblock (isotactic PLA Pm-0.65)25-b-(isotactic PLA Pm0.78)75 was formed, which was supported in this case by Pm = 0.74. The influence of the chelate effect of the (S)-melac ligand (described above for 1 and presented in Scheme 4a) and (OCH(Me)C(O))2OR resulting from the primary insertion of rac-LA into the Ga−Oalkoxide bond (described above for 2−4 and presented in Scheme 5)) was even more pronounced for Me2GaOR(IMes) complexes 5−7. The former is confirmed by essentially no activity of Me2Ga((S)-OCH(Me)CO2Me)(IMes) (5) during the first 30 min (Table 3, entry 11) due to the stronger coordination of (S)-melac to the Ga center in comparison with that of 1 (for the Mn:rac-LA conversion graph, see the Supporting Information). The full conversion was reached only after a prolonged reaction time (Table 3, entries 12 and 13). In this case, the Mn of the obtained PLA was much higher than expected, which shows that only a small part of Ga centers are active in the polymerization of rac-LA. This results J


Article

Organometallics

Scheme 5. Propagating Species in the Polymerization of racLA with 2−4 and 6

Scheme 4. Initiation and Propagation Steps in the Polymerization of rac-LA with 1 and 5

Figure 11. MALDI-TOF spectrum of PLA obtained with 3 at room temperature. The distributions refer to PLA with OH and OCH2CH2OMe end groups, with Na+ (black dots) and cyclic PLA with Na+ (black squares).

also much higher in comparison with that of the PLA obtained with 2−4. The latter result indicates much fewer active centers for 6, which is caused by a stronger chelate effect of (OCH(Me)C(O))2OR formed after the primary insertion of rac-LA into the Ga−Oalkoxide bond and therefore leading to a higher difference between kprop1 and kprop2 (Scheme 5). In this case, a stronger chelate effect resulted in slower polymerization rate, in comparison with that of 2, as evidenced by Mn:rac-LA conversion graphs (see the Supporting Information). Finally, other results concerning the activity of 5 and 6 support our earlier reasoning. For 5 and 6, the insertion of rac-LA into the Ga−Oalkoxide bond and the lack of initiation by free dissociated IMes was confirmed by 1H NMR and MALDI-TOF analysis of PLA obtained at either −20 °C or r.t. In this case, PLA with OH and OR (OR = (S)-OCH(Me)C(O)OMe or OMe, respectively) end groups was observed (see the Supporting Information). Similarly to Me2GaOR(SIMes) (2−4), the Mn value of PLA obtained at room temperature (Table 3, entries 14 and 16) was essentially as expected from the rac-LA/Ga ratio. 7. Synthesis of Stereo Diblock PLA. In the case of 1, the controlled nature of polymerization may lead to (atactic PLA)50-b-(isotacticPLA-Pm-0.78)50. The polymerization was run with [Me2Ga((S)-melac)]2 at 40° for 1 week in order to ensure the complete conversion of rac-LA. Then, SIMes was added to [Me2Ga(O-PLA-(S)-melac]2 to generate Me2Ga(OPLA-(S)-melac)(SIMes), which was followed by the subsequent addition of rac-LA at −20 °C. After 3 h, the polymerization was

Figure 10. MALDI-TOF spectrum of PLA obtained with 3 at −20 °C. The two distributions refer to PLA with OH and OCH2CH2OMe end groups, with Na+ (black dots) and K+ (white dots).

equiv, 75 equiv, or 275 equiv of rac-LA at −20 °C was performed (Table 3, entries 23−25) and revealed in each case PLA with monomodal weight distribution and Mn close to the expected value calculated from the rac-LA/catalyst ratio. This clearly shows that under polymerization conditions there is essentially no interaction between the growing PLA chain and Ga center, in contrast to (S)-melac or (OCH(Me)C(O))2OR (Scheme 5). The observed chelate effect of (OCH(Me)C(O))2OR formed after the primary insertion of rac-LA into the Ga−Oalkoxide bond for 2−4 (Scheme 5) was even more pronounced in the case of 6. Although full conversion of 50 equiv of rac-LA was achieved at −20 °C after 10 min, the Mn value was much higher than expected (Table 3, entry 15) and K


Article

Organometallics

of the metal center is required for the polymerization of ε-CL in comparison with that of lactide.54 It must be therefore noted that, although low Lewis acidity of Me2GaOR(NHC) is essential for high activity in the polymerization of rac-LA in order to avoid a strong chelate effect of the ligand or growing polymer chain of PLA, it significantly reduces the activity toward lactones.

quenched. The monomodal character of obtained PLA, revealed by GPC, indicated that in this case essentially all centers were active (Mn = 16.5 × 103 Da, PDI = 1.23). As a result, we obtained the structure of PLA which is in agreement with (atacticPLA)50-b-(isotacticPLA-Pm-0.78)50. The above result encouraged us to investigate whether it is possible to use the same strategy for the synthesis of PLA built of heterotactically and isotactically enriched blocks, as we have recently demonstrated for Me2GaOR/organosuperbase systems.16 Therefore, we performed an analogous experiment to the one described above initially using the [Me2Ga((S)melac)]2/γ-picoline system for the synthesis of heterotactically enriched PLA.18 After the subsequent addition of SIMes, the resulting catalyst was, however, inactive in the polymerization of rac-LA at −20 °C. Therefore, we may conclude that although the Me2GaOR(NHC) system is interesting both as a model system for the stereoselective polymerization of lactide and the potential application for the synthesis of stereoblock PLA, it is fragile. However, as the sequential polymerization of rac-LA with 2, 3, and 5 at different temperatures allowed for the synthesis of stereo diblock PLA (isotactic PLA Pm-0.65)25-b(isotactic PLA Pm-0.78)n (n = 25−275), it showed the utility of Me2GaOR(NHC) for the synthesis of stereo block PLA with isotactically enriched PLA blocks (Pm = 0.65−0.78). 8. Immortal ROP of rac-LA. We have also investigated the utility of Me2GaOR(NHC) for the “immortal” ring opening polymerization of rac-LA,50 in the presence of iPrOH. With regard to the above, the 1H NMR revealed that the SIMes bonded to gallium in 1 reacts with iPrOH to give a complex mixture of products (see the Supporting Information). On the contrary, in the case of 5 IMes remained bonded to gallium in the presence of iPrOH (see the Supporting Information), making 5 a suitable catalyst for “immortal” polymerization of rac-LA. The ROP of rac-LA with 5 at −20 °C in the presence of 1 and 4 equivs of iPrOH (Table 3, entries 26 and 27) supports the “immortal polymerization” with Me 2 GaOR(IMes), although Mn was not indicative of the complete exchange of alkoxide groups and iPrOH. In this case, MALDI-TOF was indicative of the presence of PLA chains with OH and (S)OCH(Me)C(O)OMe) as well as OH and OiPr end groups. Moreover, it was indicative of the presence of transesterification, which is agreement with the Pm of PLA equal to 0.73. Interestingly, the addition of iPrOH eliminated the chelate effect for 5 on rac-LA polymerization which is in line with the interaction of iPrOH with gallium and in line with the literature.51 9. Activity of Me2GaOR(NHC) toward ε-Caprolactone. The high activity of dialkylgallium alkoxides with Nheterocyclic carbenes encouraged us to investigate the activity of Me2GaOR(NHC) in the polymerization of lactones. The latter could enable the easy synthesis of PLA copolymers, important to tailor PLA properties.52 For this purpose, εcaprolactone (ε-CL) served as an exemplary lactone. However, both 1 and 3 were essentially inactive in the polymerization of ε-CL up to 40 °C. Moreover, the reaction between 1 and ε-CL (1:6) revealed no insertion of ε-CL into the Ga−O bond up to 80 °C. The NMR spectroscopy revealed signals in agreement with unreacted 1 in a whole temperature range (see the Supporting Information). The lack of activity can be associated with the low Lewis acidity of the gallium center of Me2GaOR(NHC).53 It is supported by a recent report on the ROP activity of Ti and Zr complexes supported by ferrocenebased ligands, which shows that a more Lewis acidic character

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CONCLUSIONS In summary, we have focused on the role of NHC and alkoxide ligands on the synthesis, structure, and catalytic properties of Me2GaOR(NHC) complexes in the polymerization of rac-LA. We have shown that the formation and structure of monomeric Me2GaOR(NHC) is strongly dependent on the coordination properties of NHC toward the Ga center. Despite the high stability of Me2GaOR(NHC) (NHC = SIMes and IMes) complexes, the reactivity with Lewis acids results in breaking of the Ga−CNHC bond. Both structure and reactivity have been associated with the Ga−CNHC bond strength demonstrated by both experimental data concerning Me2GaOR(NHC) and model Me3Ga(NHC) complexes, and DFT studies. Additionally, the influence of chelate bond formation with the functionality of the alkoxide ligand of Me2Ga(O,O′)(NHC) complexes was investigated in relation to the structure and reactivity in the polymerization of rac-LA. Me2GaOR(NHC) are highly active and isoselective (Pm = 0.65−0.78) catalysts for the polymerization of rac-LA at mild conditions and most active among a few examples of gallium catalysts for the polymerization of lactide.2,53,55 However, the uncontrolled nature of polymerization at −20 °C results from the interaction of the (S)-melac ligand or (OCH(Me)C(O))2OR formed after the primary insertion of rac-LA into the Ga−Oalkoxide bond, which was essentially not observed for the growing PLA chain. The latter shows for the first time that the weak chelate effect can influence the polymerization at low temperatures, even for highly active catalysts with low Lewis acidity of the coordination center. Interestingly, the chelate effect was not observed for the immortal polymerization of rac-LA, which could be achieved with Me2GaOR(IMes)/iPrOH. Although, Me2GaOR(NHC) complexes give new possibilities for the synthesis of PLA and PLA copolymers, their high reactivity due to the presence of the Ga−NHC moiety and low activity toward lactones currently reduces the scope of their applications. Our present research focuses on structure/activity studies of gallium complexes with N-heterocyclic carbenes in order to tailor their catalytic properties and their application in the synthesis of stereo block copolymers of PLA.

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EXPERIMENTAL SECTION

General Procedures. All operations were carried out under dry argon using standard Schlenk techniques. Solvents and reagents were purified and dried prior to use. Solvents were either dried over potassium (toluene and hexane) or calcium hydride (CH2Cl2) or purified using MBRAUN Solvent Purification Systems (MB-SPS-800). rac-Lactide were purchased from Aldrich and further purified by crystallization from anhydrous toluene. (S)-Methyl lactate, 2methoxyethanol, and 2-methoxypropanol were purchased from Aldrich, dried over molecular sieves, and distilled under argon. Anhydrous iPrOH was purchased from Aldrich and used as received. Me3Ga and Me3Al were purchased from STREM Chemicals, Inc. and used as received. (S,S)-[Me 2 Ga(μ-(S)OCH(Me)CO 2 Me)] 2 , 18 [Me 2 GaOMe] 3 , 1 9 Me 2 Ga((S)OCH(Me)CO 2 Me)(SIMes), 1 9 Me 2GaOMe(SIMes), 19 [Me 2 AlOCH(Me)C(O)OCH(Me)C(O)OCH2CH2OMe]2,32 IMes+Cl−, and SIPr+Cl−56 were synthesized L


Article

Organometallics

Hz, CHCH3), 1.50 (d, 3H, JHH = 7.0 Hz, CHCH3), 3.34 (s, 3H, OCH3), 3.55 (m, 2H, CH2), 4.20−4.30 (m, 2H, CH2), 4.46 (m, 1H, CHCH3), 5.13 (dq, 1H, JHH = 1.6, 7.0, CHCH3). 13C{1H} NMR (CD2Cl2, 100 MHz): −6.1, −48, −4.1 (GaMe2), 17.2, 22.0, 22.1, 59.2, 65.0, 67.8, 67.9, 70.3, 70.4, 70.6, 170.4 (CO), 178.0 (CO). Synthesis of 3. To the solution of [Me2Ga(μ-OCH2CH2OMe)]2 (147 mg, 0.42 mmol) in toluene (6 mL), 2 mL of a toluene solution of SIMes (257 mg, 0.84 mmol) was added at room temperature, and the resulting solution was stirred for 0.5 h. Then, toluene was removed under vacuum to give a white crystalline solid which was subsequently crystallized from toluene (2 mL) after the addition of hexane (10 mL) at room temperature to give colorless crystals. Then, the solvents were removed, and the obtained crystals were washed twice with 5 mL of hexane and dried under vacuum (242 mg, 60%). Element. anal. calcd for C26H39GaO2N2: C, 64.88; H, 8.17; N, 5.82. Found: C, 64.76; H, 8.27; N, 5.85. 1H NMR(toluene-d8, 400 MHz): −0.86 (s, 6H, GaCH3), 2.09 (s, 6H, CH3), 2.28 (s, 12H, CH3), 3.11 (s, 4H, CH2), 3.26 (s, 3H, OCH3), 3.47 (t, 2H, 3J(H,H) = 6.3 Hz, CHCH3), 3.47 (t, 2H, 3J(H,H) = 6.3 Hz, CHCH3), 6.74 (s, 4H, CHAr). 13C{1H} NMR (toluene-d8, 100 MHz): −7.8 (GaMe2), 17.9, 21.0, 50.8, 58.6, 64.2, 77.8, 129.6, 135.2, 136.4, 138.5, 200.2 (carbene). Synthesis of 4. To the solution of [Me2Ga(μ-OCH(CH3)CH2OMe)]2 (132 mg, 0.35 mmol) in toluene (6 mL), 2 mL of a toluene solution of SIMes (214 mg, 0.70 mmol) was added at room temperature, and the resulting solution was stirred for 0.5 h. Then, toluene was removed under vacuum to give a white colorless oil. The obtained oil was dissolved in hexane and cooled to −18 °C for 24 h, which resulted in the formation of colorless crystals. Then, hexane was removed, and the obtained crystals were washed twice with 5 mL of hexane and dried under vacuum (132 mg, 38%). Element. anal. calcd for C27H41GaO2N2: C, 65.47; H, 8.34; N, 5.66. Found: C, 65.32; H, 8.21; N, 5.62 1H NMR(toluene-d8, 400 MHz): −0.88−0.89 (s, 6H, GaCH3), 1.30 (d, 3H, 3J(H,H) = 6.0 Hz, CHCH3), 2.09 (s, 6H, CH3), 2.30, 2.31 (s, 12H, CH3), 3.09 (s, 4H, CH2), 3.25 (s, 3H, OCH3), 3.93 (m, 1H, CHCH3), 6.75 (s, 4H, CHAr). 13C{1H} NMR (toluene-d8, 100 MHz): −7.0, −7.0 (GaMe2), 18.1, 21.0, 24.3, 50.7, 58.5, 67.4, 82.2, 129.4, 135.1, 136.3, 138.4, 199.6 (carbene). Synthesis of 5. To the solution of (S,S)-[Me2Ga(μ-OCH(CH3)C(O)OMe)]2 (226 mg, 0.56 mmol) in toluene (8 mL), 4 mL of a toluene solution of IMes (339 mg, 1.11 mmol) was added at room temperature, and the solution was stirred for 0.5 h. Then, toluene was removed under vacuum to give a pale yellow solid. The obtained solid was dissolved in 2 mL of toluene, and after the addition of 15 mL of hexane, colorless crystals were obtained. Then, the solvents were removed, and the obtained crystals were washed twice with 5 mL of hexane and dried under vacuum (450 mg, 80%). Anal. Calcd for C27H37GaO3N2: C, 63.92; H, 7.35; N, 5.52. Found: C, 63.90; H, 7.48; N, 5.32. 1H NMR(toluene-d8, 400 MHz): −0.83 (s, 3H, GaCH3), −0.81 (s, 3H, GaCH3), 1.37 (d, 3H, 3J(H,H) = 6.7 Hz, CHCH3), 2.08 (s, 6H, CH3), 2.11 (s, 12H, CH3), 3.40 (s, 4H, CH2), 3.42 (s, 3H, OCH3), 4.26 (q, 1H, 3J(H,H) = 6.7 Hz, CHCH3), 6.12 (s, 4H, CH), 6.75 (s, 4H, CHAr). 13C{1H} NMR (toluene-d8, 100 MHz): −7.9, −7.5 (GaMe2), 17.6, 17.7, 21.0, 24.0, 50.1, 70.2, 122.5, 129.1, 129.2, 135.2, 135.7, 135.8, 139.3, 176.2 (carbene), 178.2 (carbonyl). FTIR (CH2Cl2, cm−1): νCO = 1739, 1715; (toluene, cm−1), νCO = 1747, 1723. Synthesis of 6. To the solution of [Me2Ga(μ-OMe)]3 (163 mg, 0.42 mmol) in toluene (6 mL), 4 mL of a toluene solution of IMes (378 mg, 1.24 mmol) was added at room temperature resulting in the formation of colorless crystals. After 0.5 h, toluene was removed, and the obtained crystals were washed with 5 mL of toluene and 5 mL of hexane and dried under vacuum (410 mg, 76%). Element. anal. calcd for C24H33GaN2O: C, 66.23; H, 7.64; N, 6.44. Found: C, 66.19; H, 7.66; N, 6.31. 1H NMR(CD2Cl2, 400 MHz): −1.26 (s, 6H, GaCH3), 2.11 (s, 12H, CH3), 2.36 (s, 6H, CH3), 2.98 (s, 3H, OCH3), 7.02 (s, 4H, CHAr), 7.09 (s, 2H, CH). 3C{1H} NMR (CD2Cl2, 100 MHz): −9.0 (GaMe2), 17.8, 21.4, 52.8, 123.7, 129.4, 135.4, 136.0, 140.0, 176.1 (carbene). Synthesis of 7. To the solution of [Me2Ga(OCH(Me)C(O))2O(CH2)2OMe]2 (229 mg, 0.72 mmol) in toluene (6 mL), 2 mL of a toluene solution of IMes (217 mg, 0.72 mmol) was added at room

according to the literature. [Me2Ga(μ-OCH2CH2OMe)]2 and [Me2Ga(μ-OCH(Me)CH2OMe)]2 were synthesized in the reaction of corresponding alcohol with trimethyl gallium, a generally used method for the synthesis of dialkylgallium alkoxides,30a and their structure confirmed on the basis of NMR spectroscopy in comparison with literature data.57 SIMes was synthesized as described previously.19 IMes and SIPr were synthesized by the modification of the method described in the literature.29 1 H and 13C NMR spectra were recorded on Agilent 400-MR DD2 400 MHz and Varian UnityPlus 200 MHz spectrometers with shifts given in ppm according to the deuterated solvent shift. FTIR spectra were recorded on a FTIR PerkinElmer System 2000. GPC measurements were recorded on a Spectra-Physics chromatograph equipped with two high performance Plgel 5 μm MIXED-C columns and detectors: RI (VE3580 Viscotek) and viscometer (270 Dual Detector Array Viscotek) with universal calibration according to a polystyrene standard. MALDI-TOF spectra were recorded on a Bruker model UltrafleXtreme. Elemental analysis was performed on a Vario EL III instrument (Heraeus). The Raman spectrum was recorded on a Nicolet Almega Dispersive Raman spectrometer, with an excitation line at 532 nm and exposure time of 15 s, and each spectrum averaged from 2 accumulations. Synthesis of Gallium Complexes. Synthesis of IMes. (IMesH)+Cl− (1.90 g (5.58 mmol)) was added to the suspension of KH (289 mg, 7.21 mmol) in toluene (70 mL) at room temperature followed by the addition of a catalytic amount of KOtBu ( 2σ(I)) were 0.0218 and 0.0528, respectively. The final R1 and wR(F2) values for all data were 0.0239 and 0.0540, respectively. Crystal Data for 4. C27H41GaN2O2, M = 495.34, monoclinic, a = 15.49578(13) Å, b = 8.82820(6) Å, c = 20.11413(17) Å, α = 90°, β = 102.9238(8)°, γ = 90°, V = 2681.90(4) Å3, T = 100.0(2) K, space group P21/c, Z = 4, 273917 reflections measured, and 7384 independent reflections (Rint = 0.0401). The final R1 and wR(F2) values (I > 2σ(I)) were 0.0314 and 0.0768, respectively. The final R1 and wR(F2) values for all data were 0.0340 and 0.0780, respectively. Crystal Data for 5. C27H37GaN2O3, M = 507.30, monoclinic, a = 8.06764(9) Å, b = 34.1655(3) Å, c = 10.37453(11) Å, α = 90°, β = 111.3800(12)°, γ = 90°, V = 2662.80(5) Å3, T = 100.0(2) K, space group P21, Z = 4, 222899 reflections measured, and 12695 independent reflections (Rint = 0.0436). The final R1 and wR(F2) values (I > 2σ(I)) were 0.0235 and 0.0569, respectively. The final R1 and wR(F2) values for all data were 0.0237 and 0.0570, respectively. Crystal Data for 6. C24H33GaN2O·C7H8, M = 527.38, monoclinic, a = 11.18740(14) Å, b = 13.73487(18) Å, c = 18.7348(2) Å, α = 90°, β = 96.2667(12)°, γ = 90°, V = 2861.54(6) Å3, T = 100.0(2) K, space group P21/c, Z = 4, 156710 reflections measured, and 6811 independent reflections (Rint = 0.0371). The final R1 and wR(F2) values (I > 2σ(I)) were 0.0225 and 0.0593, respectively. The final R1 and wR(F2) values for all data were 0.0241 and 0.0605, respectively. Crystal Data for 8. C24H33GaN2, M = 419.24, orthorhombic, a = 22.9304(3) Å, b = 12.19709(14) Å, c = 8.07487(8) Å, α = 90°, β = 90°, γ = 90°, V = 2258.42(4) Å3, T = 100.0(2) K, space group Pnma, Z = 4, 106314 reflections measured, and 2677 independent reflections (Rint = 0.0458). The final R1 and wR(F2) values (I > 2σ(I)) were 0.0220 and 0.0566, respectively. The final R1 and wR(F2) values for all data were 0.0226 and 0.0569, respectively. Crystal Data for 9. [C5H10GaO3]−[C27H39N2]+, M = 579.45, orthorhombic, a = 10.80578(17) Å, b = 16.9075(3) Å, c = 17.9200(2) Å, α = 90°, β = 90°, γ = 90°, V = 3273.95(8) Å3, T = 100.0(2) K, space group P212121, Z = 4, 39507 reflections measured, and 8602 independent reflections (Rint = 0.0405). The final R1 and wR(F2) values (I > 2σ(I)) were 0.0292 and 0.0646, respectively. The final R1 and wR(F2) values for all data were 0.0334 and 0.0665, respectively. Crystal Data for 10. C30H47GaN2, M = 505.41, triclinic, a = 9.93095(14) Å, b = 16.6032(4) Å, c = 17.9090(4) Å, α = 83.4035(18)°, β = 75.2194(16)°, γ = 89.9091(15)°, V =

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data of 3−6 and 8−10 in CIF format; NMR spectra of H(OCH(Me)C(O))2O(CH2)2OMe, [Me2Ga(OCH(Me)C(O))2O(CH2)2OMe], 5, 7, reaction mixtures of (S,S)[Me 2 Ga(μ-OCH(CH 3 )C(O)OMe)] 2 , and [Me 2 Ga(μOCH2CH2OMe)]2 with SIPr and 1 with ε-CL; VT NMR spectra for 4 and 5; FTIR spectra of 5, 6, 7, and 8; Raman spectrum of 5; DFT data for rotational energy scans of 1, 2, 5, and 6; and selected regions of homonuclear decoupled 1H NMR, 13C NMR, and MALDI-TOF spectra of PLA. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00071.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +48 22 5543670. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Centre of Poland (SONATA BIS2 Programme, Grant No. DEC-2012/07/E/ST5/02860). P.H. also thanks the Foundation for Polish Science “Team Programme” cofinanced by the EU European Regional Development Fund, Operational Program Innovative Economy 2007/2013 for financial support. The artwork for the table of contents graphic was done by Anna Maria Dab̨ rowska ([email protected]).

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REFERENCES

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