Stereoselective Production of Poly(rac-lactide) by ROP with Highly

ARTICLE pubs.acs.org/Organometallics

Stereoselective Production of Poly(rac-lactide) by ROP with Highly Efficient Bulky Heteroscorpionate Alkylmagnesium Initiators Luis F. S
anchez-Barba,*,† Andr
es Garc
es,† Juan Fern
andez-Baeza,‡ Antonio Otero,*,‡ Carlos Alonso-Moreno,‡ Agustín Lara-S
anchez,‡ and Ana M. Rodríguez‡ † ‡

Departamento de Química Inorg
anica y Analítica, Universidad Rey Juan Carlos, M
ostoles-28933-Madrid, Spain Departamento de Química Inorg
anica, Org
anica y Bioquímica, Universidad de Castilla-La Mancha, Campus Universitario, 13071-Ciudad Real, Spain

bS Supporting Information ABSTRACT:

The reaction of the sterically hindered heteroscorpionate lithium acetamidinate [Li(κ3-pbptamd)(THF)] (pbptamd = N,N0 diisopropylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate) and [Li(κ3-tbptamd)(THF)] (tbptamd = Nethyl-N0 -tert-butylbis(3,5di-tert-butylpyrazol-1-yl)acetamidinate) with 1 equiv of RMgCl proceeds cleanly to give very high yields of the corresponding nonchiral monoalkyl magnesium complexes [Mg(R)(κ3-NNN)] (NNN = pbptamd, R = Me (1), Et (2), CH2SiMe3 (3), CH2Ph (4), C3H5 (5); NNN = tbptamd, R = Me (6), Et (7), CH2SiMe3 (8), CH2Ph (9), C3H5 (10)). Alternatively, these monoalkylmagnesium complexes can be obtained by protonolysis of the corresponding dialkylmagnesium reagents, R2Mg, with the proligands Hpbptamd and Htbptamd in very good yields. Interestingly, alkyls 610 are obtained as a mixture of structural isomers (a þ b) in a ratio that depends on the level of steric demand of the alkyl group. The single-crystal X-ray structures of derivatives 4, 6a 3 Et2O, and 7a confirm a four-coordinate structure in which the metal center is in a distorted-tetrahedral geometry and the bulky heteroscorpionate ligands have a κ3 coordination mode. Interestingly, the alkyl-containing magnesium complexes 13 and 8a can act as highly efficient single-component initiators for the very rapid ring-opening polymerization of ε-caprolactone and lactides even at 10 °C. While ε-caprolactone was polymerized within seconds to give high-molecular-weight polymers with narrow polydispersities (Mn > 105, Mw/Mn = 1.17), lactide afforded PLA materials with medium molecular weights in only a few minutes. The polymerizations are living, as evidenced by the narrow polydispersities (Mw/Mn = 1.01) of the isolated polymers and the linear nature of the number average molecular weight versus conversion plot. Inspection of the kinetic parameters for rac-LA showed that propagations present the usual first-order dependence on monomer and catalyst concentration and that the polymerization rates observed compare favorably with the highest rates reported to date for the ROP of lactides. Microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that the substituents on the amidinate fragment have a significant influence on both the degree of stereoselectivity and the rate of polymerization. Thus, propagations occur with enhanced levels of heteroselectivity, producing at low temperatures enriched-heterotactic PLAs with a Pr value of up to 0.79. ROP experiments with [Mg(κ3-pbptamd)(CH2SiMe3)] (3) revealed that this complex is extremely fast for rac-lactide, with 420 equiv polymerized in 93% yield in 2 min at 20 °C.

’ INTRODUCTION The production of polymeric biomaterials1a derived from lactic acid has been studied intensely over the past decade, due to their numerous potential applications in a variety of fields.1 One of the most representative examples concerns medical applications, in particular that related to biodegradable and resorbable surgical sutures. The resulting thermoplastic elastomers obtained from lactides (in combination in some cases with poly(trimethylene carbonates)) are prominently employed for heart tissue engineering,2 for drug delivery,3 and also as biodegradable internal fixation devices r 2011 American Chemical Society

for the repair of fractures to small bones and joints, such as feet/hands or ankles/wrists.4 These medicinal applications have promoted the use of biocompatible metals in the design of new catalysts, including magnesium5 and zinc6 systems. On the other hand, poly(lactide) (PLA)7,8 (and its co- and terpolymers) is rapidly emerging as a potential environmentally friendly replacement for the bioresistant poly(R-olefins).1b,9 Indeed, such materials have already Received: February 21, 2011 Published: April 29, 2011 2775

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Organometallics found ecological applications as bulk commodity materials10,11 that have many properties similar to and sometimes superior to those of traditional olefin-based polymers, with the added benefit of biodegradability.1214 The announcements of joint polymer ventures such as (i) Cargill-Dow LLC (today Cargill Inc.)7 for the production of the lactide monomer in quantities of 125 ktpa by 2001 (140 ktpa by 2009)7c and (ii) NatureWorks LLC-Teijen Limited,13 which opened a 136 ktpa polylactide production plant in 2007 (Ingeo, the first commodity biobased thermoplastic resin to be derived from corn instead of fossil fuel),7c represent two of the most notable recent achievements. There is a need for catalysts that can produce these biodegradable polymeric materials with novel and stereocontrolled microstructures, since stereochemistry plays a key role in PLAs, as it determines the mechanical properties, biodegradability, and, ultimately, the end use of the material.15 These new materials are obtained by ring-opening polymerization (ROP) of cyclic esters.16 In addition, zinc- and magnesium-based catalysts have been extensively employed in ROP and are among the most efficient initiators used to date for the well-controlled polymerization of cyclic esters such as ε-caprolactone and lactides.17 In this context, our research group has recently reported amidinatebased heteroscorpionates,18 which are related to the bis(pyrazol1-yl)methane system19 with different levels of steric congestion, as convenient ancillary ligands for the synthesis of well-defined alkylmagnesium20 and zinc alkyl21amide22 complexes of the type [M(R)(κ3-NNN)]. Moreover, heteroscorpionate alkylmagnesium20 and -zinc21 complexes proved to be highly effective singlecomponent living initiators20 for the well-controlled ROP of ε-caprolactone and lactides. Furthermore, heteroscorpionate zinc amide22 derivatives also behaved as efficient single-component initiators for the ROP of ε-caprolactone. Finally, very recently we employed a scorpionate/cyclopentadienyl hybrid ligand of the type NNCp, which was previously synthesized23 and intensely explored24,25 by our group for the preparation of highly productive single-component living initiators26 for ε-caprolactone and lactides. Nevertheless, in many cases the use of the bis(3,5dimethypyrazol-1-yl)methane system with low steric hindrance as a platform for the synthesis of the magnesium20,26 and zinc alkyl21,26 or amide22,26 initiators described above can lead to back-biting reactions/transesterifications as side reactions. These reactions can result in the formation of macrocycles with a wide range of molecular weight distributions. Additionally, given the lack of steric influence, methyl groups are insufficient to prevent metal complexes from undergoing the symmetrical (Schlenk)27 equilibrium in the magnesium complexes as an additional side reaction and, therefore, sandwich species can be formed in competition with catalytic performance. These undesired reactions can be minimized by using a sterically bulky ligand attached to an active metal center, which provides a steric barrier to prevent these side reactions. In fact, numerous coordinatively bulky metal complexes have been synthesized recently in order to enhance a mononuclear active site in ROP.28 Moreover, as a result of the low level of steric hindrance from the methyl groups, the resulting initiators are not capable of inducing high levels of stereoselectivity in the growing polymer microstructures. In contrast, more sterically hindered heteroscorpionate alkylzinc initiators,21 based on the bis(3,5-di-tert-butylpyrazol-1-yl)methane platform, offered good control of the microstructures. The resulting compounds promoted the formation of a heterotactic bias in the polymerization of rac-lactide (Pr = 0.68), but their catalytic activity was limited.

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The aim of the work described here was to prepare highly efficient initiators based on biocompatible metals that were also capable of exerting high levels of stereoselectivity in the ROP of lactides. These aims were achieved through the preparation of sterically hindered organomagnesium heteroscorpionates of the type [Mg(R)(κ3-NNN)]. The study of the different structural arrangements in these alkyl heteroscorpionates and the use of these new materials as good single-component living initiators for the polymerization of ε-caprolactone and L-/rac-lactide under well-controlled conditions are discussed in detail along with an analysis of the polymer microstructures.

’ RESULTS AND DISCUSSION Synthesis and Characterization of Heteroscorpionate Monoalkylmagnesium Complexes. In a manner similar to

the synthesis of analogous amidinate-based heteroscorpionate alkylmagnesium20 complexes, hexane suspensions of the sterically hindered lithium acetamidinates [Li(κ3-pbptamd)(THF)]21 (pbptamd = N,N0 -diisopropylbis(3,5-di-tert-butylpyrazol-1yl)acetamidinate) and [Li(κ3-tbptamd)(THF)]21 (tbptamd = Nethyl-N0 -tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate) were treated with a series of commercially available Grignard reagents RMgCl (R = Me, Et, CH2SiMe3, Bn, C3H5) in a 1:1 molar ratio. These reactions gave rise to the heteroscorpionate alkylmagnesium complexes [Mg(R)(κ3-NNN)] (NNN = pbptamd, R = Me (1), Et (2), CH2SiMe3 (3), CH2Ph (4), C3H5 (5); NNN = tbptamd, R = Me (6), Et (7), CH2SiMe3 (8), CH2Ph (9), C3H5 (10)) as white or pale yellow semicrystalline solids in good yields (ca. 85%) after the appropriate workup (see Scheme 1, method A). Alkyl derivatives 610 were obtained as mixtures of structural isomers (a:b) in different molar ratios. These isomers are not in equilibrium and actually can be separated by several recrystallization from the mother liquor (see Separation Procedure for the Mixture of Isomers 8a,b). The magnesium monoalkyls 110 can also be obtained by protonolysis of the magnesium dialkyls R2Mg with the proligands Hpbptamd21 and Htbptamd21 through alkane elimination (see Scheme 1, method B). This alternative method has been employed before in our group for the preparation of monoalkylzinc heteroscorpionates.21,26 Both methods A and B (Scheme 1) afford essentially the same proportion of isomers. The compounds are extremely air- and moisture-sensitive and decompose in dichloromethane. Qualitative halide tests showed the absence of halide through the formation of [MgCl(κ3-NNN)] derivatives. The 1H and 13C{1H} NMR spectra of 15 in benzene-d6 at room temperature show a single set of resonances for the pyrazole rings, indicating that both pyrazole rings are equivalent. The NMR signals due to the amidinate moiety in these complexes (in which R1 = R2 = iPr) show two sets of resonances for these substituents. This observation is indicative of a monodentate binding of the amidinate moiety to the magnesium atom (see Scheme 1), a situation previously observed in our group.2022 However, the 1H and 13C{1H} NMR spectra of 610 (where R1 = Et, R2 = tBu) present a double set of resonances for the pyrazole rings and the CH, revealing the presence of the two possible isomers in solution in different molar ratios, which depend on the binding nitrogen in the monodentate amidinate moiety. 1H NOESY-1D experiments were performed in order to assign the two isomers (Figure 1), and the response from the alkyl (MgR) protons on irradiating the methylene protons (CH2) of the ethyl group in the amidinate fragment in both isomers suggests that only in one isomer are both groups in a relative cis position 2776

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Scheme 1. Synthesis of the Sterically Hindered Heteroscorpionate Alkylmagnesiums 110

Figure 1. (a) 1H NMR spectrum of [Mg(Bn)(κ3-tbptamd)] (9) (molar ratio of isomers a and b 1:4). (b) NOESY-1D responses on irradiating the CH2a protons of the ethyl group in the amidinate fragment in isomer a. (c) NOESY-1D responses on irradiating CH2b protons of the ethyl group in the amidinate moiety in isomer b.

(isomer b, Figure 1c). Isomer b was observed to be the minor component (20%) for less sterically hindered alkyls such as 6 and 7 and the major component for more sterically hindered alkyls such as 8 (70%) and 9 (80%) (see Scheme 1). Additionally, the absence of a NOESY-1D response from the alkyl (MgR) protons on irradiating the methylene protons (CH2) of the ethyl group in the amidinate moiety in the other isomer provides evidence that these groups are not in a relative cis position (isomer a, Figure 1b).

Isomer a, in contrast to isomer b, was observed as the major component (80%) for the less sterically hindered alkyls 6 and 7 and as the minor component for the more sterically demanding alkyls 8 (30%) and 9 (20%), probably as a result of the higher level of steric hindrance of the tert-butyl group in the amidinate fragment (see Scheme 1). A doublet for protons Hanti and Hsyn, along with a multiplet for the central proton Hmethine, indicates σπ fluxional behavior of 2777

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Table 1. Selected Interatomic Bond Lengths (Å) and Angles (deg) for 4, 6a 3 Et2O, and 7a 7a 6a 3 Et2O

4

molecule 1

molecule 2

Bond Lengths Mg(1)N(1)

2.147(4)

Mg(1)N(1)

2.177(5)

Mg(1)N(1)

2.227(4)

Mg(2)N(7)

2.173(4)

Mg(1)N(3)

2.142(5)

Mg(1)N(3)

2.177(4)

Mg(1)N(3)

2.178(4)

Mg(2)N(9)

2.219(4)

Mg(1)N(5)

2.044(4)

Mg(1)N(5)

2.042(4)

Mg(1)N(5)

2.056(5)

Mg(2)N(11)

2.049(5)

Mg(1)C(31)

2.158(5)

Mg(1)C(31)

2.128(6)

Mg(1)C(31)

2.122(5)

Mg(2)C(63)

2.109(5)

N(2)C(23) N(4)C(23)

1.493(6) 1.481(6)

N(2)C(23) N(4)C(23)

1.472(6) 1.473(6)

N(2)C(23) N(4)C(23)

1.460(5) 1.468(6)

N(8)C(55) N(10)C(55)

1.473(6) 1.494(5)

N(5)C(24)

1.335(6)

N(5)C(24)

1.341(6)

N(5)C(24)

1.343(5)

N(11)C(56)

1.353(5)

N(5)C(25)

1.470(6)

N(5)C(27)

1.494(6)

N(5)C(25)

1.494(6)

N(11)C(57)

1.493(6)

N(6)C(24)

1.316(6)

N(6)C(24)

1.297(6)

N(6)C(24)

1.293(6)

N(12)C(56)

1.283(6)

N(6)C(28)

1.460(6)

N(6)C(25)

1.456(6)

N(6)C(29)

1.456(6)

N(12)C(61)

1.451(6)

C(23)C(24)

1.531(7)

C(23)C(24)

1.550(7)

C(23)C(24)

1.564(6)

C(55)C(56)

1.557(6)

N(5)Mg(1)N(1) N(5)Mg(1)N(3)

92.3(2) 92.5(2)

N(5)Mg(1)N(1) N(5)Mg(1)N(3)

90.8(2) 92.6(2)

91.9(1) 91.9(2)

N(11)Mg(2)N(9) N(11)Mg(2)N(7)

91.6(2) 93.6(2) 126.4(2)

Bond Angles N(5)Mg(1)N(1) N(5)Mg(1)N(3)

N(5)Mg(1)C(31)

131.6(2)

N(5)Mg(1)C(31)

129.9(2)

N(5)Mg(1)C(31)

129.4(2)

N(11)Mg(2)C(63)

N(1)Mg(1)N(3)

82.6(2)

N(1)Mg(1)N(3)

83.0(2)

N(1)Mg(1)N(3)

83.3(1)

N(7)Mg(2)N(9)

82.3(1)

C(24)N(5)Mg(1)

117.1(3)

C(24)N(5)Mg(1)

113.4(3)

C(24)N(5)Mg(1)

109.6(3)

C(56)N(11)Mg(2)

108.7(3)

C(24)N(5)C(25)

120.0(4)

C(24)N(5)C(27)

118.3(4)

C(24)N(5)C(25)

118.7(4)

C(56)N(11)C(57)

118.2(4)

C(24)N(6)C(28)

126.2(5)

C(24)N(6)C(25)

122.4(4)

C(24)N(6)C(29)

122.2(4)

C(56)N(12)C(61)

122.2(4)

N(6)C(24)N(5)

139.3(5)

N(6)C(24)N(5)

126.0(5)

N(6)C(24)N(5)

127.0(4)

N(12)C(56)N(11)

126.9(4)

N(6)C(24)C(23) N(5)C(24)C(23)

106.2(4) 114.5(4)

N(6)C(24)C(23) N(5)C(24)C(23)

120.9(4) 113.2(4)

N(6)C(24)C(23) N(5)C(24)C(23)

120.2(4) 112.8(4)

N(12)C(56)C(55) N(11)C(56)C(55)

120.7(4) 112.4(4)

the allyl ligand in solution for 5 and 10, even at 70 °C in toluene-d8. In this case, complex 10 shows an equal proportion of isomers a and b. Additional 1H NOESY-1D experiments were also carried out to confirm the assignment of the signals for the t Bu3 or 5 and H4 groups of the pyrazole rings in compounds 110. Finally, 1H13C heteronuclear correlation (g-HSQC) experiments allowed us to assign the resonances corresponding to C4, tBu3, and tBu5 of the pyrazole rings in all compounds (and the corresponding isomers). These data support a tetrahedral disposition for the magnesium atom with a κ3-NNN coordination of the heteroscorpionate ligand, a situation in which a plane of symmetry exists and contains the amidinate group, the magnesium metal center, and the alkyl ligand (Scheme 1). Single crystals of 4, 6a 3 Et2O, and 7a suitable for X-ray diffraction crystallography were readily grown from hexane at 26 °C. Selected bond lengths and angles are collected in Table 1, and crystallographic details are reported in Table S1 of the Supporting Information. Crystals of 6a 3 Et2O were obtained following method B (see Scheme 1 and the Experimental Section), and a molecule of diethyl ether is retained in the asymmetric unit. For compound 7a two independent molecules exist in the asymmetric unit. Comments are related only to molecule 1, as molecule 2 shows similar values (see Table 1). All attempts to crystallize isomers b proved fruitless. The molecular structures of 4, 6a 3 Et2O, and 7a are depicted in Figures 24, respectively, and these compounds consist of

Figure 2. ORTEP view of [Mg(Bn)(κ3-pbptamd)] (4). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.

a monomeric arrangement in the solid state. In all cases, the magnesium metal exhibits a distorted-tetrahedral geometry in which the pyrazolic nitrogens N(1) and N(3) occupy two positions and the amidinate nitrogen N(5) and the alkyl group occupy the other two positions. The N(1)Mg(1) and N(3)Mg(1) 2778

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Figure 3. ORTEP view of [Mg(Me)(κ3-tbptamd)] (6a 3 Et2O). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.

Figure 4. ORTEP view of [Mg(Et)(κ3-tbptamd)] (7a). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.

bond lengths of 2.147(4) and 2.142(5) Å for 4 are well-balanced and are comparable to those observed in the analogous heteroscorpionate ethyl zinc complex [Zn(Et)(κ3-pbptamd)]21 (N(1) Zn = 2.131(3) Å and N(3)Zn = 2.121(3) Å). However, these lengths are slightly shorter than the 2.177(5) and 2.177(4) Å for 6a 3 Et2O and 2.227(4) and 2.178(4) Å for 7a (Table 1), probably as a result of the greater steric demand of the tBu substituent in the amidinate fragment in comparison with the iPr group. Delocalization is also evidenced in the NCN core of the amidinate, with the bond lengths for C(24)N(5) and C(24)N(6) ranging on average from 1.343(5) to 1.293(6) Å. The N(5)Mg(1) bond lengths (2.044(4) Å for 4, 2.042(4) Å for 6a 3 Et2O, 2.056(5) Å for 7a) are considerably shorter than the average N(1)Mg and N(3)Mg(1) bond lengths (2.183(4) and 2.165(4) Å, respectively) and are comparable to that observed in the analogous heteroscorpionate ethylzinc complex [Zn(Et)(κ3-pbptamd)] (1.997(3) Å).21 Finally, the Mg(1)C(31) bond lengths (2.158(5) Å for 4, 2.128(6) Å for 6a 3 Et2O, 2.122(5) Å for 7a) decrease in the

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order 4 > 6a 3 Et2O ≈ 7a, probably due to the higher level of congestion in the alkyl group for 4, and can be regarded as normal considering the average for MgC(alkyl) covalent bonds29a (ca. 2.142 Å) and the value found by our group for analogous alkylmagnesium derivatives such as [Mg(η1-C3H5)(κ3-tbpamd)]20 (2.133(4) Å). However, these bond lengths are considerably longer than the ZnC bond length previously observed in the analogous [Zn(Et)(κ3-pbptamd)]21 (ZnC(1) = 1.995(4) Å), and this is a result of the greater magnesium covalent radius (1.41 Å) in comparison with the value for zinc (1.22 Å).29b It is well-known that Grignard reagents exist in solution as a complex mixture of species due to facile ligand redistribution reactions: e.g., the Schlenk equilibrium.27 As a result, we considered it of interest to investigate the possibility of similar ligand redistribution reactions for the heteroscorpionate alkylmagnesium complexes [Mg(R)(κ3-NNN)] (110), which were synthesized in order to prepare the corresponding potential six-coordinate sandwich species of the type [Mg(κ3-NNN)2]. This type of behavior was previously observed in similar homoscorpionate alkylmagnesium systems by Parkin30 and, more recently, in less sterically hindered magnesium heteroscorpionates reported by our group.20 It is worth noting that although solutions of derivatives 110 proved stable for hours, after days at room temperature (or hours under reflux in toluene) only decomposition products were obtained and the ligand redistribution process to give the sixcoordinate sandwich complex [Mg(κ3-NNN)2] was not observed under any circumstances. Furthermore, when the reaction was carried out with 2 equiv of ligand versus Mg, the formation of the sandwich species was not detected. Similar behavior was previously observed by our group on employing alkylzinc21 complexes with these sterically hindered heteroscorpionates. The absence of ligand-exchange processes is undoubtedly a consequence of the high steric demand of the tert-butyl groups of the bis(3,5-di-tert-butylpyrazol-1-yl)methane fragment, which are sufficiently bulky to behave as “tetrahedral enforcer” units—a situation in contrast to that found when Me substituents are present in the pyrazole rings.20,27 Polymerization Studies. Complexes 13 and isomers 8a,b (see Separation Procedure for the Mixture of Isomers 8a,b) were assessed in the ring-opening polymerization of the polar monomers ε-caprolactone (CL) and L-/rac-lactide in toluene or tetrahydrofuran under a nitrogen atmosphere. These studies were carried out in order to probe the potential of these compounds as initiators for cyclic esters and to study kinetic parameters such as the reaction rate and the order dependence on monomer and catalyst concentration. These reactivity studies also allowed a comparison of these bulky Mg alkyls with the recently published analogous Zn21 alkyl and the less sterically hindered Mg20 alkyl moieties as initiating groups in these heteroscorpionate amidinate-based systems. ε-Caprolactone Polymerization Promoted by Sterically Hindered Heteroscorpionate Alkylmagnesium Complexes. Initiators 13 and 8a,b act as very rapid single-component catalysts for the polymerization of ε-caprolactone (CL) in toluene to give high-molecular-weight polymers without the need for an activator; the results of these experiments are collected in Table 2. A variety of polymerization conditions were explored. The magnesium alkyl derivatives 1 and 2 effectively polymerized CL at room temperature (entries 1 and 3), and 2 gave rise to 81% conversion of 500 equiv of CL after 3 min. The polymerization was well controlled and gave a high-molecular-weight polymer with a low polydispersity index (Mn = 43 500, Mw/Mn = 1.19). Not unexpectedly, the 2779

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Table 2. Polymerization of ε-Caprolactone Catalyzed by Complexes 13 and 8a,ba entry

initiator

temp (°C)

time

yield (g)

conversn (%)b

prodc

Mn (Da)d

Mw/Mnd 1.21

1

1

20

6 min

3.84

75

426

40 500

2

1

0

15 min

0.25

5

11

2 100

1.17

3

2

20

3 min

4.15

81

922

43 500

1.19

4

2

0

15 min

1.23

24

54

12 000

1.12

5

3

20

10 s

4.97

97

19 880

53 500

1.11

6

3

0

2 min

3.43

67

1 143

37 500

1.05

7

3e

20

10 min

10.48

92

3 144

181 000

1.47

8 9

8a 8b

20 20

20 s 15 s

4.89 4.94

95 96

9 780 13 173

52 000 52 300

1.05 1.08

10

[Mg(CH2SiMe3)(pbpamd)]f

20

1 min

4.98

97

3 320

52 000

1.41

11

[Mg(CH2SiMe3)(bpzcp)]f

20

1 min

4.71

92

3 140

51 800

1.19

12

[Zn(Et)(pbptamd)]f

20

19 h

1.08

21

19 500

1.14

0.63

Polymerization conditions: 90 μmol of initiator, 20 mL of toluene as solvent, [ε-CL]0/[initiator]0 = 500. b Percentage conversion of the monomer (weight of polymer recovered)/(weight of monomer)  100. c In units of kg of polymer (mol of Mg)1 h1. d Determined by GPC relative to polystyrene standards in tetrahydrofuran. The experimental Mn was calculated considering MarkHouwink’s corrections55 for Mn (Mn(obsd) = 0.56[Mn(GPC)]). e Polymerization conditions: 20 μmol of initiator, 30 mL of toluene as solvent, [ε-CL]0/[initiator]0 = 5000. f These data have been included for comparison in ROP with the alkylmagnesium20,26 and -zinc21 analogues. a

productivity decreased markedly on cooling (entries 2 and 4), and in the reaction at 0 °C, 24% of the polymer was recovered for 2 after 15 min, with very narrow polydispersity (Mw/Mn = 1.12, entry 4), whereas 1 had drastically reduced catalytic activity and only 5% of product was formed for the same reaction time (entry 2). Surprisingly, magnesium alkyls 3 and 8a,b (entries 5, 8, and 9) initiated extremely rapid polymerization at room temperature and this was accompanied by a marked increase in the viscosity of the solution. Indeed, catalyst 3 gave almost complete conversion in 10 s with a productivity of more than 19 800  103 g of PCL (mol of Mg)1 h1 (entry 5). This activity was retained at 0 °C, and after 2 min 67% of the monomer was converted with a narrow molecular weight distribution, a finding that suggests living behavior (Mw/Mn = 1.05, entry 6). The extraordinarily high activity shown by alkylmagnesium 3 is comparable to that for isomers 8a,b and that previously described for the analogous heteroscorpionate amidinate-based and hybrid scorpionate-/ cyclopentadienyl-based alkylmagnesium initiators [Mg(CH2SiMe3)(κ3-pbpamd)]20 (entry 10) and [Mg(CH2SiMe3)(bpzcp)]26 (entry 11), respectively. In these tests the polymer molecular weights were limited by the monomer to initiator ratio of 500:1. An increase in this ratio by a factor of 10 gave polymers with significantly higher molecular weight (Mn > 105) and a broader molecular weight distribution (entry 7, Mw/Mn = 1.47), possibly due to the higher levels of back-biting and transesterification side reactions, thus resulting in the formation of macrocycles with a wider range of molecular weight distributions. The close agreement between observed Mn values and the expected theoretical values in all cases (entries 16, 8, and 9) are also indicative of the low level of transfer reactions. In accordance with the aforementioned data, it is worth noting that the polymerization processes initiated by the analogous sterically hindered alkyl zinc initiators, previously described by our group21 (entry 12), are significantly slower than those initiated by their alkylmagnesium counterparts 13 and 8a,b. This marked difference between the two types of derivatives, which under the present polymerization conditions decrease in the order [Mg(R)(κ3-pbptamd)] . [Zn(R)(κ3-pbptamd)],21 is presumably a consequence of the MC bond strength; this trend is

consistent with an increase in the strength of the MC bond31 and may rationalize the delay in the initiation step observed on employing the analogous zinc21 initiators. The nature of the alkyl group also affects the catalytic activity, which decreases in the order CH2SiMe3 . Et > Me, and is also in agreement with the decrease in the lability of the MC bond.31 This behavior has also been observed in analogous amidinate-based and hybrid scorpionate-/cyclopentadienyl-based magnesium20,26 and zinc21,26 alkyl derivatives. The 1H NMR spectrum of poly(ε-caprolactone) oligomer, obtained by the reaction of 3 with 65 equiv of ε-CL, exhibits characteristic resonances that are consistent with the presence of one CH2SiMe3 end group per CH2OH hydroxyl chain end. This evidence confirms that the polymerization follows a nucleophilic route and is initiated by the transfer of an alkyl ligand to the monomer, with cleavage of the acyloxygen bond and formation of a metal alkoxide propagating species.32 Very few alkyl magnesium initiators33ce have been shown to polymerize εcaprolactone to medium-molecular-weight polymers. In contrast, catalysts 3 and 8a,b, as well as analogous alkylmagnesium20,26 initiators described by our group, proved to be highly efficient initiators for the production of medium-high-molecular-weight polymers with low molecular weight distributions, even with a catalyst loading of 0.107%, which yielded Mn values as high as 181 kg mol1 (entry 7). L-/rac-Lactide Polymerization Initiated by Sterically Hindered Heteroscorpionate Alkylmagnesium Complexes. Derivatives 3 and 8a,b were also systematically examined for the production of poly(lactides) (PLAs) (Table 3, entries 18 and 1120). Complex 3 proved to be a highly efficient catalyst for the well-controlled polymerization of L-lactide at room temperature in toluene and tetrahydrofuran without cocatalyst or activator (Table 3, entries 17). In all cases, the PLAs produced had molecular weights in close agreement with calculated values for one polymer chain per metal center (Mn(calcd, PLA200) = 28 800) (Table 3). The gel permeation chromatography (GPC) data for the resulting polyesters show a monomodal weight distribution, with polydispersities ranging from 1.01 to 1.06. At this point, it is worth mentioning that polymerization of the optically active (S,S)lactide (L-lactide) afforded 78% conversion of 200 equiv in just 2780

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Table 3. Polymerization of L-Lactide and rac-Lactide Catalyzed by Complexes 3 and 8a,ba conv entry

initiator

monomer

temp (°C)

time (min)

yield (g)

ersn (%)b

Mn(theor) (Da)c

Mn (Da)d

Mw/ Mnd

Tm (°C)e

[R]D22 (deg)f

Prg

1

3

L-LA

20

0.5

0.54

21

6 000

5 000

1.01

169

145

2

3

L-LA

20

1

0.98

38

10 900

10 100

1.02

168

147

3

3

L-LA

20

1.5

1.47

57

16 400

15 900

1.02

170

146

4

3

L-LA

20

2

2.02

78

22 400

21 100

1.04

171

146

5

3

L-LA

20

2.5

2.51

97

27 900

27 800

1.04

172

146

6 7

3 3h

L-LA L-LA

0 20

5 2

1.52 1.81

59 70

17 000 20 100

16 600 19 700

1.01 1.06

170 168

145 147

8

8a

L-LA

20

2.5

2.15

83

23 900

23 300

1.05

170

146

9

[Mg(CH2SiMe3)(pbpamd)]i

L-LA

70

96(h)

1.20

93

13 400

12 600

1.19

160

146

10

[Mg(CH2SiMe3)(bpzcp)]i

L-LA

90

2.5(h)

2.51

97

27 900

27 600

1.05

164

146

11

3j

rac-LA

20

6

0.85

66

9 500

9 100

1.04

0.70

12

3

rac-LA

20

1

0.89

69

9 900

9 400

1.03

0.74

13

3

rac-LA

20

2

1.18

91

13 100

12 800

1.09

0.72

14 15

3 3

rac-LA rac-LA

0 10

2 4

0.63 0.65

49 50

9 900 7 200

9 400 6 800

1.02 1.02

0.74 0.75

16

3k

rac-LA

20

2

5.06

93

56 200

50 300

1.12

0.71

17

8a

rac-LA

20

1

0.56

43

6 200

5 700

1.05

0.73

18

8a

rac-LA

20

2

1.07

83

11 900

11 400

1.05

0.71

19

8a

rac-LA

0

2.5

0.65

50

7 200

6 800

1.01

0.79

20

8b

rac-LA

0

2

0.66

51

7 300

6 800

1.03

0.70

Polymerization conditions: (a) 90 μmol of initiator, [L-lactide]0/[initiator]0 = 200, and 50 mL of toluene as solvent; (b) 90 μmol of initiator, [rac-LA]0/ [initiator]0 = 100, and 10 mL of tetrahydrofuran as solvent. b Percentage conversion of the monomer ((weight of polymer recovered)/(weight of monomer)  100). c Theoretical Mn = (monomer/initiator)  (% conversion)  (Mw of lactide). d Determined by GPC relative to polystyrene standards in tetrahydrofuran. The experimental Mn was calculated considering MarkHouwink’s corrections55 for Mn (Mn(obsd) = 0.56[Mn(GPC)]). e PLA melting temperature. f Optical rotation data of poly(L-lactide) obtained. [R]D22 values of L-lactide and poly(L-lactide) are 28° and 144°, respectively.56 g Pr is the probability of racemic linkages between monomer units and is determined from the relative intensity in the tetrads obtainied in the decoupled 1H NMR by Pr = 2I1/(I1 þ I2), with I1 = δ 5.205.25 ppm (sis, sii/iis) and I2 = δ 5.135.20 ppm (iis/sii, iii, isi).57 h With 15 mL of tetrahydrofuran as solvent. i These data have been included for comparison in ROP with alkylmagnesium analogues.20,26 For complex [Mg(CH2SiMe3)(pbpamd)], 90 μmol of initiator, [L-lactide]0/[initiator]0 = 100. j With 40 mL of toluene employed as solvent. k 90 μmol of 3, [rac-LA]0/[3]0 = 420 and 40 mL of tetrahydrofuran as solvent. a

2 min, with a very narrow molecular weight distribution (Mw/Mn = 1.04, entry 4). Interestingly, when the reaction mixture was cooled to 0 °C, 59% of the monomer was transformed after 5 min with a slight reduction in the molecular weight distribution (Mw/Mn = 1.01, entry 6). The polymerization occurred without observable epimerization reactions at the chiral centers, as evidenced by the homonuclear decoupled 1H NMR spectrum, which has only a single resonance at δ 5.16 ppm in the methine region. This reaction afforded highly crystalline, isotactic polymers with Tm values in the range 168172 °C.34,35 The low level of stereochemical imperfections ([iii] > 98%) is also evident in the poly(Llactide) with Mn > 27 000, for which the optical activity remained almost constant: [R]D22 = 146°. The high level of control afforded by this initiator in the polymerization of L-lactide is further exemplified by the narrow molecular weight distributions in conjunction with the linear correlations between Mn and percentage conversion (R2 = 0.996) (see Figure 5). These results are characteristic of well-controlled living propagations and the existence of a single type of reaction site, a situation similar to the living behavior previously observed in the amidinate-based20 and the hybrid scorpionate-/cyclopentadienyl-based26 alkylmagnesium derivatives. Surprisingly, when the polymerization was carried out in tetrahydrofuran (entry 7) the activity of 3 was maintained at room temperature and after 2 min 70% of the monomer was transformed. Competition

Figure 5. Plot of PLA Mn and molecular weight distribution values (PDI) as a function of monomer conversion (%) for the polymerization of L-LA initiated by 3 ([L-LA]0/[3]0 = 200, toluene, 20 °C).

between the coordinating solvent and the monomer moiety for the metal center may decrease the rate of polymerization, but the complexation of magnesium ions by coordinating solvents enhances in this case the nucleophilicity of the alkyl initiating group.36 Initiator 8a was also found to be very rapid for the polymerization L-LA and after 2.5 min 83% of the product was recovered (entry 8). The new sterically hindered amidinatebased alkylmagnesium derivatives 3 and 8a proved to be much more active than the analogous amidinate-based20 (entry 9) and 2781

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Organometallics hybrid scorpionate/cyclopentadienyl-based26 (entry 10) alkylmagnesium complexes in the L-LA ring-opening polymerization process, probably due to their inability to undergo possible symmetrical (Schlenk)27 equilibrium competition and, therefore, the absence of sandwich species that interfere with catalytic performance. Initiators 3 and 8a,b were also tested for the polymerization of rac-lactide, mainly using tetrahydrofuran as solvent at 20 °C (as a result of the lower solubility of rac-LA than L-LA in toluene at room temperature and in order to study the influence of solvent), and PLAs were again produced with molecular weights in good agreement with calculated values (Mn(calcd,PLA100) = 14 400) (Table 3, entries 1120). For instance, derivative 3 gave 69% conversion of 100 equiv after 1 min (entry 12) and produced a low-molecular-weight material with a very narrow polydispersity (Mn = 9 400, Mw/Mn = 1.03, entry 12). Extension of the reaction time to 2 min increased the productivity, with conversion rising to 91%, and led to a significant increase in the polymer molecular weight and a slight increase in the polydispersity index (Mn = 12 800, Mw/Mn = 1.09, entry 13). Furthermore, the activity of 3 was maintained at 0 °C, with 49% of the monomer transformed after 2 min to give a polymer with a very low molecular weight distribution (Mw/Mn = 1.02, entry 14). This compound retained its polymerization activity even at 10 °C (entry 15). At this point, it is also worth noting that catalyst 3 is capable of achieving almost complete transformation of 420 equiv in 2 min at 20 °C with narrow polydispersity index (Mw/Mn = 1.12, entry 16). Additionally, on using derivative 8a 43% of the monomer was transformed after 1 min, under otherwise identical conditions, with a very narrow molecular weight distribution obtained (Mw/Mn = 1.05, entry 17). The activity of 8a was also maintained at 0 °C (entry 19), although it was slightly lower than those of catalysts 3 (entry 14) and 8b (entry 20). These activity values compare favorably with those for the most heralded zinc catalyst reported to date for the ROP of lactides (see Chisholm,37 Coates,38 and Hillmyer and Tolman39), as well as other magnesium37,38 initiators described by Darensbourg36b and Lin.40 Furthermore, these initiators are significantly faster than aluminum salen complexes, which usually require several hours at elevated temperatures to convert LA under otherwise similar conditions.41,42 Very few examples of alkylmagnesium initiators20,26,32a,33,43 have been reported to act as active singlesite catalysts for the ROP of rac-lactide. However, to the best of our knowledge, these sterically hindered heteroscorpionate magnesium alkyls represent the first examples reported to date that show such excellent activity at very low temperatures and with short reaction time (entries 15 and 16). PLA end-group analysis showed that in all cases (entries 18 and 1120), as for PCL initiated by these alkylmagnesium initiators, the polymerizations of L-LA and rac-LA were initiated by nucleophilic attack of the alkyl group on lactide (see Supporting Information, Figure S1). Solution Kinetic Studies on the Ring-Opening Polymerization of rac-Lactide. Kinetic studies were conducted at 20 and 0 °C to establish the reaction order with respect to monomer and catalyst as well as the reaction constant rates at these temperatures. The polymerization kinetics were studied for catalysts [Mg(CH2SiMe3)(κ3-pbptamd)] (3) and [Mg(CH2SiMe3)(κ3tbptamd)] (8a) with [rac-LA]0/[Mg]0 = 93 and [rac-LA]0 = 0.8 M using tetrahydrofuran as solvent and without cocatalyst or activator. Polymerizations were monitored over time by regular manual sampling followed by 1H NMR analysis to determine the degree of monomer conversion. The semilogarithmic plot of ln([rac-LA]0/[rac-LA]t) versus reaction time for catalysts 3 and

ARTICLE

Figure 6. First-order kinetic plots for rac-LA polymerizations in tetrahydrofuran with [rac-LA]0 = 0.8 M, employing (a) [Mg(CH2SiMe3)(κ3pbptamd)] (3) and (b) [Mg(CH2SiMe3)(κ3-tbptamd)] (8a) as catalysts. (a) 3 as catalyst: (() [Mg]0 = 8.3 mM, 20 °C, kapp = 14.83  103 s1 (linear fit, R2 = 0.988); (2) [Mg]0 = 12.0 mM, 20 °C, kapp = 20.30  103 s1 (linear fit, R2 = 0.989); (9) [Mg]0 = 16.0 mM, 20 °C, kapp = 26.53  103 s1 (linear fit, R2 = 0.988); (b) [Mg]0 = 20.4 mM, 20 °C, kapp = 36.97  103 s1 (linear fit, R2 = 0.988); ()) [Mg]0 = 8.3 mM, 0 °C, kapp = 5.66  103 s1 (linear fit, R2 = 0.989); (4) [Mg]0 = 12.0 mM, 0 °C, kapp = 8.83  103 s1 (linear fit, R2 = 0.989); (0) [Mg]0 = 16.0 mM, 0 °C, kapp = 11.85  103 s1 (linear fit, R2 = 0.987); (O) [Mg]0 = 20.4 mM, 0 °C, kapp = 13.83  103 s1 (linear fit, R2 = 0.988). (b) 8a as catalyst: (() [Mg]0 = 8.3 mM, 20 °C, kapp = 12.96  103 s1 (linear fit, R2 = 0.989); (2) [Mg]0 = 12.0 mM, 20 °C, kapp = 17.46  103 s1 (linear fit, R2 = 0.989); (9) [Mg]0 = 16.0 mM, 20 °C, kapp = 26.00  103 s1 (linear fit, R2 = 0.985); (b) [Mg]0 = 20.4 mM, 20 °C, kapp = 33.33  103 s1 (linear fit, R2 = 0.989); ()) [Mg]0 = 8.3 mM, 0 °C, kapp = 3.01  103 s1 (linear fit, R2 = 0.985); (4) [Mg]0 = 12.0 mM, 0 °C, kapp = 4.00  103 s1 (linear fit, R2 = 0. 986); (0) [Mg]0 = 16.0 mM, 0 °C, kapp = 5.85  103 s1 (linear fit, R2 = 0.988); (O) [Mg]0 = 20.4 mM, 0 °C, kapp = 7.63  103 s1 (linear fit, R2 = 0.987).

8a at both 20 and 0 °C, employing different initial catalyst concentrations, are shown in Figure 6a for 3 and Figure 6b for 8a, where [rac-LA]0 is the initial lactide monomer concentration and [rac-LA]t the lactide concentration at a given reaction time t. For the rac-LA polymerization, the [rac-LA]0/[rac-LA]t ratio was determined by integration (I) of the peaks for rac-LA (4.35 ppm for the methine proton signal) and PLA (5.15 ppm for the methine proton signal in CDCl3). Therefore, according to the equation [rac-LA]0/[rac-LA]t is equal to (I5.15 þ I4.35)/I4.35. As expected, in all cases the linearity of the plots shows that the propagations were first order with respect to lactide monomer (square correlation coefficients g0.98) when polymerized at 20 and 0 °C in tetrahydrofuran. An induction period was not observed, and this indicates that initiators were reactive from the beginning; i.e. rearrangement of initiator aggregates was not necessary to produce active species. The linearity of the plots also illustrates that termination reactions did not occur during polymerization. Thus, the polymerization of rac-LA initiated by 2782

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3 and 8a presumably proceeds according to  d½rac-LA=dt ¼ kapp ½rac-LA1

ð1Þ

The fastest polymerization for rac-LA was observed for 3, which gave a pseudo-first-order rate constant of 36.97  103 s1 (2.2186 min1) at 20 °C when [Mg]0 was 20.4 mM. The kapp value for complex 8a at 20 °C (33.33  103 s1; 2.0 min1) was found to be as high as the kapp value found for 3, under otherwise identical conditions, as a result of the extraordinarily high catalytic activity observed for both catalysts. However, a similar trend was not observed when polymerizations were carried out at 0 °C at this catalyst concentration, with a kapp value of 13.83  103 s1 (0.83 min1) for 3: i.e., much higher (almost double) than that for 8a of 7.63  103 s1 (0.458 min1). The kapp values for the racLA polymerization at 50 °C with an enantiopure heteroscorpionate neodymium amide initiator were recently reported by our group. The values obtained were much lower than that reported here, even at a higher temperature (20 μmol of initiator, [rac-LA]0/ [Nd]0 = 200, 50 °C, kapp= 1.55  105 s1).44 In an attempt to determine the kinetic dependence on the catalyst, the initial concentrations of the initiators 3 and 8a were varied from [Mg]0 = 8.3 to 20.4 mM under otherwise similar conditions. As expected, the kapp values systematically increased with [Mg]0, and these parameters at 0 °C are considerably lower than those observed at 20 °C (parts a and b of Figure 6). As kapp = kp[Mg]n (eq 1), kp is the propagation rate constant and n the order in magnesium. In order to establish n and kp, we plotted ln kapp versus ln [Mg]0 (Figure 7a for 3 and Figure 7b for 8a). The slope of the regression lines, 1.0018 (linear fit, R2 = 0.988) and 1.0102 (linear fit, R2 = 0.985) for 3 at 20 and 0 °C, respectively, and 1.0792 (linear fit, R2 = 0.989) and 1.0557 (linear fit, R2 = 0.987) for 8a at identical temperatures, respectively, equals n, and thus the reaction is first order in catalysts 3 and 8a at both temperatures. Additionally, the y intercept of the regression lines equals ln kp, and thus the polymerization rate constants, kp, are 1.726 and 0.689 M1 s1 for 3 at 20 and 0 °C, respectively, and 1.274 and 0.310 M1 s1 for 8a (after the appropriate conversion of the kp units; Figure 7) at the same temperatures, respectively. These values are considerably lower at 0 °C. The kp values obtained compare well with the highest reported to date for the polymerization of rac-LA under similar experimental conditions: for instance, the β-diiminate zinc complexes in dichloromethane at 25 °C (0.9  103 M1 s1) reported by Coates,38 the zinc alkoxide complexes with tridentate NNO ligands in dichloromethane at 0 °C (0.3 M1 s1; however, 2.2 M1 s1 at 25 °C) reported by Tolman,39b the cationic zinc complexes in CD2Cl2 at 25 °C (0.17(1) M1 s1) reported by Hayes,45a the NNOMg2 alkoxide complexes in dichloromethane at 20 °C (3.9  103 M1 s1) reported by Tolman,39a the cyclohexylsalen aluminum initiators in toluene at 70 °C (1.50  104 M1 s1) reported by Feijen,34a and the recently reported magnesium silylamido complexes supported by monoanionic aminophenolato ligands in toluene at 20 °C (kapp = 5.74  103 s1; [racLA]0 = 0.25 M, [rac-LA]0:[catalyst]0:[iPrOH]0 = 2500:1:1) reported by Ma.45b Therefore, the polymerization of rac-LA mediated by [Mg(CH2SiMe3)(κ3-pbptamd)] 3 and [Mg(CH2SiMe3)(κ3-tbptamd)] 8a obeys an overall kinetic law of the form  d½rac-LA=dt ¼ kp ½Mg1 ½rac-LA1

ð2Þ

Typical rac-lactide ring-opening polymerizations are first order in monomer and first order in catatlyst,34a although

Figure 7. Plot of ln Kapp versus ln [Mg]0 for the polymerization of rac-LA employing initiators (a) 3 and (b) 8a in tetrahydrofuran at 20 and 0 °C, respectively, with [rac-LA]0 = 0.8 mol/L. (a) 3 as catalyst: ([) 20 °C, kp = 0.1035 mM1 min1 (linear fit, R2 = 0.988); (]) 0 °C, kp = 0.0413 mM1 min1 (linear fit, R2 = 0.985). (b) 8a as catalyst: ([) 20 °C, kp = 0.0764 mM1 min1 (linear fit, R2 = 0.989); (]) 0 °C, kp = 0.0186 mM1 min1 (linear fit, R2 = 0.987).

nonintegral orders with respect to catalyst below 1.0 and up to 1.5638—and even second order in catalyst46—have been reported. Such complex nonintegral kinetic orders have been attributed to aggregation of metal initiator or the growth of polymer chains,38,39,47 although an alternative interpretation was given by Tolman et al.,39b who cautioned against evaluating order in catalyst using a logarithmic analysis alone, due to the possibility of a deactivating impurity. This overall kinetic law could in principle be extrapolated to the other combinations of alkyl catalysts described above and other lactide enantiomers such as L-LA. Poly(rac-lactide) Microstructure Analysis. Microstructural analysis of the poly(rac-lactide) by 1H NMR spectroscopy revealed that initiator 3 exerts a moderate level of heterotacticity, while interestingly, initiator 8a shows significant preference for a heterotactic dyad enchainment. These preferences are in contrast to those of the less sterically hindered analogous magnesium and zinc alkyls, such as [(Mg/Zn)(Et)(κ3-pbptamd)]20 (Figure 8a), where amorphous atactic materials were obtained, and even the previous results obtained with alkylzinc21 initiators supported by these sterically hindered amidinate-based heteroscorpionates, such as [Zn(Et)(κ3-tbptamd)]. In the latter case, the levels of heterotacticity reached were significantly lower, with Pr = 0.68 (Figure 8b). This conclusion is based on the fact that the heterotactic tetrads isi and sis were significantly enhanced as a result of the preference of the consecutive alternate insertion of the L- and D-lactide units during the propagation through a heterospecific chain-end control mechanism in the polymerization of rac-lactide. It is also worth noting the notable influence of the solvent and temperature on the level of stereoselectivity. For instance, initiator 3 produced a heterotactic-enriched PLA with 2783

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essentially the same level of heteroactivity at room temperature as 3, with Pr = 0.73 (Figure 8e and Table 3, entry 17), probably as a result of the very high activity of both catalysts at this temperature. When the reaction mixture was cooled to 0 °C, initiator 8a afforded a substantially heterotactic PLA, with the probability value improved in this case to Pr = 0.79 (Figure 8f and Table 3, entry 19), a value higher than that at 20 °C for this catalyst (Figure 8e) and that at 0 °C for 3 (Pr = 0.74, Table 3, entry 14) and isomer 8b (Pr = 0.70, Table 3, entry 20). A decrease in the reaction temperature to 10 °C led to only a small increase in the Pr value in 3 to 0.75 (Table 3, entry 15), while for 8a Pr remained constant. In other words, the probability of an (R,R)lactide unit being enchained after an (S,S)-lactide unit (or vice versa) on employing initiator 8a at 0 or 10 °C in tetrahydrofuran is 0.79.38,48 This behavior during the propagation at this temperature is most probably the result of the high steric demand of the tert-butyl substituents in the two pyrazole rings. This leads to sterically more congested and less flexible (and therefore more selective) active centers to the incoming lactide and, furthermore, there is the permanent presence of the sterically hindered N-tert-butyl substituent in the amidinate fragment in isomer a, in the position cis to the alkyl leaving group. This evidence points out that during the polymerization the bulky ancillary hetereoscorpionate chelates the magnesium center in a defined geometry, which is believed to be the origin of the stereoselectivity.

Figure 8. 1H NMR spectra (500 MHz, 298 K, CDCl3) of the homodecoupled CH resonance of poly(rac-lactide) prepared employing (a) [Mg/Zn(Et)(κ3-pbpamd)], in toluene at 70 °C, (b) [Zn(Et)(κ3pbptamd)], in toluene at 90 °C, (c) [Mg(CH2SiMe3)(κ3-pbptamd)] (3), in toluene at 20 °C, (d) [Mg(CH2SiMe3)(κ3-pbptamd)] (3), in tetrahydrofuran at 20 °C, (e) [Mg(CH2SiMe3)(κ3-tbptamd)] (8a), in tetrahydrofuran at 20 °C, and (f) [Mg(CH2SiMe3)(κ3-tbptamd)] (8a), in tetrahydrofuran at 0 °C, as initiators. The tacticity of the polymer was assigned using the methine signals with homonuclear decoupling as described by Hillmyer and co-workers.49.

Pr = 0.74 in tetrahydrofuran at 20 °C (Figure 8d and Table 3, entry 12), whereas in toluene the Pr value was clearly lower (0.70) (Figure 8c and Table 3, entry 11). Initiator 8a exhibited

’ CONCLUSIONS AND PERSPECTIVES In conclusion, we report here two facile synthetic methods for the preparation of the nonchiral monoalkylmagnesium complexes [Mg(R)(κ3-NNN)] (110) through the reaction of a series of commercially available Grignard reagents and the sterically hindered lithium salts [Li(κ3-pbtpamd)(THF)] and [Li(κ3-tbptamd)(THF)] or, alternatively, through the protonolysis reaction of the dialkylmagnesium reagents R2Mg with the corresponding proligands Hpbptamd and Htbptamd. Interestingly, complexes 610 are obtained as a mixture of structural isomers (a and b) in different ratios depending on which nitrogen atom from the amidinate fragment is bonded to the magnesium metal center. The single-crystal X-ray diffraction studies on derivatives 4, 6a 3 Et2O, and 7a confirm a four-coordinate structure with the heteroscorpionate ligands arranged in a κ3coordination mode. More interestingly, the alkyl nucleophilicity of the magnesium complexes means that they can act as highly efficient single-site living initiators for the well-controlled polymerization of polar monomers such as ε-caprolactone and lactides at low temperatures (10 °C). ε-Caprolactone is polymerized within seconds to give medium-high-molecular-weight polymers with narrow polydispersity indexes. Surprisingly, the polymerization of LA also occurs very rapidly and offers very good control at room temperature, as evidenced by the living behavior. Regardless of the mechanism of polymerization, initial kinetic parameter studies for rac-LA revealed that propagations present the usual first-order dependence on monomer and catalyst concentration and that these complexes exhibit polymerization rates which compare favorably to the highest reported to date3740 for the ROP of lactides, without the need for a cocatalyst or activator.45b Additionally, the most sterically hindered initiator 8a promotes a heterotactic bias in the polymerization of rac-lactide in tetrahydrofuran at low temperature (0 °C), producing enhanced degrees of heteroactivity and resulting heterotactic-enriched PLAs with Pr up to 0.80. End group analysis suggests that 2784

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Organometallics magnesium catalysts initiate the polymerization process by alkyl transfer to the monomer. Work is continuing in our laboratories to synthesize new group 2 initiators, since these stereoselective catalysts now offer great potential for the preparation of new and distinct polymers from conventional monomers. Through rational tuning of the catalyst design, especially in positions 3 and 5 of the pyrazole rings, we believe that even more efficient catalyst systems with better stereo-/enantioselectivity might be developed and that these could be capable of affording new polyester architectures.

’ EXPERIMENTAL SECTION General Procedures. All manipulations were performed under nitrogen using standard Schlenk techniques. Solvents were predried over sodium wire (toluene, n-hexane, THF, n-pentane, diethyl ether) or calcium hydride (dichloromethane) and distilled under nitrogen from sodium (toluene), sodiumpotassium alloy (n-hexane, n-pentane), sodiumbenzophenone (THF, diethyl ether), or calcium hydride (dichloromethane). Deuterated solvents were stored over activated 4 Å molecular sieves and degassed by several freezethaw cycles. Microanalyses were carried out with a Perkin-Elmer 2400 CHN analyzer. 1H and 13 C NMR spectra were recorded on a Varian Mercury FT-400 spectrometer and referenced to the residual deuterated solvent. The 1H NMR homodecoupled and the NOESY-1D spectra were recorded on a Varian Inova FT-500 spectrometer with the following acquisition parameters: irradiation time 2 s and 256 scans, using standard VARIAN-FT software. Two-dimensional NMR spectra were acquired using standard VARIANFT software and processed using an IPC-Sun computer. The Grignard reagents RMgCl (R = Me, Et, CH2SiMe3, Bn, allyl) were used as purchased (Aldrich). The starting materials bdtbpzm50 (bdtbpzm = bis(3,5di-tert-butylpyrazol-1-yl)methane), [Li(κ3-pbptamd)(THF)],21 [Li(κ3tbptamd)(THF)],21 and R2Mg51 (R = Me, Et, CH2SiMe3, Bn, allyl) were prepared according to literature procedures. The R2Mg reagents were also subsequently dissolved in an exact volume of diethyl ether and conveniently standardized by titration prior to use. ε-Caprolactone was dried by stirring over fresh CaH2 for 48 h and then distilled under reduced pressure and stored over activated 4 Å molecular sieves. L-Lactide and rac-lactide were sublimed twice, recrystallized from THF, and finally sublimed again prior to use. Gel permeation chromatography (GPC) measurements were performed on a Polymer Laboratories PL-GPC-220 instrument equipped with a PLgel 5 Å Mixes-C column, a refractive index detector, and a PD2040 light-scattering detector. The GPC column was eluted with THF at 40 °C at 1 mL/min and was calibrated using eight monodisperse polystyrene standards in the range 580483 000 Da. PLA melting temperatures were measured using a melting point block (SMP 10). The sample was heated to 100 °C and then heated at a rate of 1 °C/min up to 165 °C. The specific rotation [R]D22 was measured at a concentration of 10 mg/mL in CHCl3 at 22 °C on a Perkin-Elmer 241 polarimeter equipped with a sodium lamp operating at 589 nm with a light path length of 10 cm. Preparation of Compounds 110. Synthesis of [MgMe(κ3pbptamd)] (1). The synthesis of compound 1 was carried out by two different procedures. Method A. In a 250 mL Schlenk tube, [Li(κ3-pbptamd)(THF)] (1.15 g, 2.00 mmol) was suspended in dry hexane (70 mL) and cooled to 70 °C. A solution of MeMgCl (3.0 M in THF) (0.67 mL, 2.00 mmol) was added, and the mixture was warmed to room temperature and stirred for 20 min. The suspension was filtered, and the resulting pale yellow solution was concentrated to ca. 10 mL. The solution was cooled to 26 °C to give compound 1 as a white semicrystalline solid. Yield: 0.89 g, 83%. Method B. In a 250 mL Schlenk tube, Hpbptamd (1.00 g, 2.00 mmol) was dissolved in dry hexane (70 mL) and cooled to 70 °C. A solution of Me2Mg (0.30 M in Et2O) (6.7 mL, 2.00 mmol) was added, and the mixture was warmed to room temperature and stirred for 20 min. The

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light suspension was filtered, and the resulting pale yellow solution was concentrated to ca. 10 mL. The solution was cooled to 26 °C to give compound 1 as a white semicrystalline solid. Yield: 0.81 g, 75%. Anal. Calcd for C31H56MgN6: C, 69.32; H, 10.51; N, 15.65. Found: C, 69.45; H, 10.62; N, 15.52. 1H NMR (C6D6, 297 K): δ 7.56 (s, 1 H, CH), 5.97 (s, 2 H, H4), 4.18 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 4.04 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.40 (s, 18 H, tBu5), 1.39 (s, 18 H, tBu3), 1.35 [d, 6 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.23 [d, 6 H, 3 JHH = 6.3 Hz, CH(CH3)2], 0.30 (s, 3 H, MgCH3). 13C{1H} NMR (C6D6, 297 K): δ 163.1, 155.3 (C3 or 5), 156.2 (NdCN), 102.3 (C4), 79.1 (CH), 47.0 [NCH(CH3)2], 46.4 [NCH(CH3)2], 32.6, 32.2 [C(CH3)3], 31.1 (tBu5), 30.5 (tBu3), 27.2 [NCH(CH3)2], 26.4 [NCH(CH3)2], 6.4 (MgCH3). Synthesis of [MgEt(κ3-pbptamd)] (2). The synthesis of 2 was carried out in a manner identical with that for 1. Method A: [Li(κ3-pbptamd)(THF)] (1.15 g, 2.00 mmol), EtMgCl (2.8 M in THF) (0.72 mL, 2.00 mmol); yield 0.89 g, 81%. Method B: Hpbptamd (1.00 g, 2.00 mmol), Et2Mg (0.34 M in Et2O) (5.9 mL, 2.00 mmol); yield 0.93 g, 84%. Anal. Calcd for C32H58MgN6: C, 69.73; H, 10.61; N, 15.25. Found: C, 69.85; H, 10.75; N, 15.31. 1H NMR (C6D6, 297 K): δ 7.53 (s, 1 H, CH), 5.96 (s, 2 H, H4), 4.18 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 4.01 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.98 (t, 3 H, 3JHH = 8.2 Hz, MgCH2CH3), 1.38 (s, 36 H, tBu5 and tBu3), 1.34 [d, 6 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.21 [d, 6 H, 3JHH = 6.3 Hz, CH(CH3)2], 0.39 (q, 2 H, 3JHH = 8.2 Hz, MgCH2CH3). 13C{1H} NMR (C6D6, 297 K): δ 163.2, 155.3 (C3 or 5), 156.4 (NdCN), 102.2 (C4), 78.9 (CH), 47.0 [NCH(CH3)2], 46.6 [NCH(CH3)2], 32.6, 32.2 [C(CH3)3], 31.1 (tBu5), 30.5 (tBu3), 27.1 [NCH(CH3)2], 26.4 [NCH(CH3)2], 13.9 (MgCH2CH3), 4.8 (MgCH2CH3). Synthesis of [Mg(CH2SiMe3)(κ3-pbptamd)] (3). The synthesis of 3 was carried out in a manner identical with that for 1. Method A: [Li(κ3pbptamd)(THF)] (1.15 g, 2.00 mmol), SiMe3CH2MgCl (1.0 M in Et2O) (2.00 mL, 2.00 mmol); yield 0.98 g, 80%. Method B: Hpbptamd (1.00 g, 2.00 mmol), (SiMe3CH2)2Mg (0.74 M in Et2O) (2.7 mL, 2.00 mmol); yield 0.92 g, 75%. Anal. Calcd for C34H64MgN6Si: C, 67.02; H, 10.59; N, 13.79. Found: C, 67.12; H, 10.68; N, 13.70. 1H NMR (C6D6, 297 K): δ 7.58 (s, 1 H, CH), 5.95 (s, 2 H, H4), 4.11 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 3.95 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.37 (s, 18 H, tBu5), 1.35 (s, 18 H, tBu3), 1.30 [d, 6 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.21 [d, 6 H, 3JHH = 6.3 Hz, CH(CH3)2], 0.52 (s, 9H, MgCH2SiMe3), 0.75 (s, 2H, MgCH2SiMe3). 13C{1H} NMR (C6D6, 297 K), δ 163.4, 155.3 (C3 or 5), 157.0 (NdCN), 102.4 (C4), 78.6 (CH), 47.0 [NCH(CH3)2], 46.8 [NCH(CH3)2], 32.6, 32.2 [C(CH3)3], 31.1 (tBu5), 30.9 (tBu3), 27.4 [NCH(CH3)2], 26.0 [NCH(CH3)2], 5.4 (MgCH2SiMe3), 1.5 (MgCH2SiMe3). Synthesis of [MgBn(κ3-pbptamd)] (4). The synthesis of 4 was carried out in a manner identical with that for 1. Method A: [Li(κ3-pbptamd)(THF)] (1.15 g, 2.00 mmol), BnMgCl (2.0 M in THF) (1.00 mL, 2.00 mmol); yield 1.06 g, 86%. Method B: Hpbptamd (1.00 g, 2.00 mmol), Bn2Mg (0.62 M in Et2O) (3.2 mL, 2.00 mmol); yield 0.98 g, 80%. Anal. Calcd for C37H60MgN6: C, 72.47; H, 9.86; N, 13.70. Found: C, 72.55; H, 9.94; N, 13.62. 1H NMR (C6D6, 297 K): δ 7.59 (s, 1 H, CH), 7.41 (m, 2H, MgCH2C6H5), 7.29 (m, 2H, MgCH2C6H5), 6.94 (m, 1 H, MgCH2C6H5), 5.93 (s, 2 H, H4), 4.16 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 3.93 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 2.32 (s, 2H, MgCH2C6H5), 1.34 (s, 18 H, tBu5), 1.29 (s, 18 H, tBu3), 1.29 [d, 6 H, 3 JHH = 6.3 Hz, CH(CH3)2], 1.19 [d, 6 H, 3JHH = 6.3 Hz, CH(CH3)2]. 13 C{1H} NMR (C6D6, 297 K): δ 163.4, 155.5 (C3 or 5), 156.6 (Nd CN), 138.0, 128.1, 127.9, 126.5 (MgCH2C6H5), 102.3 (C4), 78.7 (CH), 47.1 [NCH(CH3)2], 47.0 [NCH(CH3)2], 45.9 (MgCH2C6H5), 32.6, 32.1 [C(CH3)3], 31.0 (tBu5), 30.6 (tBu3), 27.3 [NCH(CH3)2], 26.4 [NCH(CH3)2]. Synthesis of [Mg(η3-C3H5)(κ3-pbptamd)] (5). The synthesis of 5 was carried out in a manner identical with that for 1. Method A: 2785

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Organometallics [Li(κ3-pbptamd)(THF)] (1.15 g, 2.00 mmol), C3H5MgCl (1.7 M in THF) (1.18 mL, 2.00 mmol); yield 0.92 g, 83%. Method B: Hpbptamd (1.00 g, 2.00 mmol), (C3H5)2Mg (0.53 M in Et2O) (3.8 mL, 2.00 mmol); yield 0.96 g, 85%. Anal. Calcd for C33H58MgN6: C, 70.38; H, 10.38; N, 14.92. Found: C, 70.45; H, 10.44; N, 15.01. 1H NMR (C6D6, 297 K), δ 7.52 (s, 1 H, CH), 6.94 (m, 1 H, MgCH2CHCH2), 5.95 (s, 2 H, H4), 4.17 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 3.97 [sept, 1 H, 3JHH = 6.3 Hz, CH(CH3)2], 1.36 (s, 18 H, tBu5), 1.35 (s, 18 H, tBu3), 1.34 [d, 6 H, 3J HH = 6.3 Hz, CH(CH 3)2], 1.28 (d, 4 H, 3 JHH = 12.5 Hz, MgCH 2CHCH 2), 1.21 [d, 6 H, 3 JHH = 6.3 Hz, CH(CH 3)2]. 13 C{1 H} NMR (C6 D 6, 297 K): δ 163.4, 155.5 (C3 or 5 ), 156.3 (NdCN), 147.1(MgCH 2CHCH 2), 102.3 (C 4), 78.9 (CH), 47.1 [NCH(CH3 )2], 46.8 [NCH(CH3 )2 ], 32.6, 32.2 [C(CH 3)3], 31.0 (tBu5), 30.6 (MgCH 2CHCH 2), 30.5 (tBu3 ), 27.1 [NCH(CH 3)2], 26.4 [NCH(CH3 )2 ]. Synthesis of [MgMe(κ3-tbptamd)] (6a,b). The syntheses of 6a,b were carried out in a manner identical with that for 1. Method A: [Li(κ3tbptamd)(THF)] (1.15 g, 2.00 mmol), MeMgCl (3.0 M in THF), (0.67 mL, 2.00 mmol); yield 0.92 g, 85%. Anal. Calcd for C31H56MgN6: C, 69.32; H, 10.51; N, 15.65. Found: C, 69.39; H, 10.59; N, 15.56. Method B: Htbptamd (1.00 g, 2.00 mmol). Me2Mg (0.30 M in Et2O) (6.7 mL, 2.00 mmol); yield 0.97 g, 79%. Anal. Calcd for C31H56MgN6 3 Et2O: C, 68.77; H, 10.88; N, 13.75. Found: C, 68.85; H, 10.94; N, 13.64 (6a/6b ratio 4/1). Data for 6a are as follows. 1H NMR (C6D6, 297 K): δ 7.49 (s, 1 H, CH), 5.96 (s, 2 H, H4), 3.46 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 1.82 [s, 9H, NC(CH3)3], 1.34 (s, 18 H, tBu3), 1.28 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 1.17 (s, 18 H, tBu5), 0.42 (s, 3 H, MgCH3). 13C{1H} NMR (C6D6, 297 K): δ 164.3, 154.8 (C3 or 5), 155.8 (NdCN), 103.2 (C4), 69.6 (CH), 52.6 (NCH2CH3), 44.3 [N C(CH3)3], 32.3, 32.2 [C(CH3)3], 30.5 [NC(CH3)3], 30.4 (tBu3), 30.3 (tBu5), 19.1 (NCH2CH3), 5.6 (MgCH3). Data for 6b are as follows. 1 H NMR (C6D6, 297 K): δ 7.43 (s, 1 H, CH), 5.96 (s, 2 H, H4), 3.54 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 1.43 [s, 9H, NC(CH3)3], 1.38 (s, 18 H, tBu5), 1.37 (s, 18 H, tBu3), 0.91 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 0.43 (s, 3 H, MgCH3). 13C{1H} NMR (C6D6, 297 K): δ 163.0, 155.4 (C3 or 5), 159.3 (NdCN), 102.1 (C4), 80.1 (CH), 51.5 (NCH2CH3), 43.5 [NC(CH3)3], 33.0, 32.1 [C(CH3)3], 31.0 [NC(CH3)3], 30.5 (tBu5), 30.1 (tBu3), 20.0 (NCH2CH3), 5.8 (MgCH3). Synthesis of [MgEt(κ3-tbptamd)] (7a,b). The syntheses of 7a,b were carried out in a manner identical with that for 1. Method A: [Li(κ3tbptamd)(THF)] (1.15 g, 2.00 mmol), EtMgCl (2.8 M in THF) (0.72 mL, 2.00 mmol); yield 0.95 g, 86%. Method B: Htbptamd (1.00 g, 2.00 mmol), Et2Mg (0.34 M in Et2O) (5.9 mL, 2.00 mmol); yield 0.89 g, 81%. (7a/7b ratio 4/1). Anal. Calcd for C32H58MgN6: C, 69.73; H, 10.61; N, 15.25. Found: C, 69.79; H, 10.72; N, 15.19. Data for 7a are as follows. 1H NMR (C6D6, 297 K): 7.46 (s, 1 H, CH), 5.95 (s, 2 H, H4), 3.44 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 1.90 (t, 3 H, 3JHH = 8.2 Hz, MgCH2CH3), 1.82 [s, 9H, NC(CH3)3], 1.32 (s, 18 H, tBu3), 1.27 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 1.16 (s, 18 H, tBu5), 0.26 (q, 2 H, 3JHH = 8.2 Hz, MgCH2CH3). 13C{1H} NMR (C6D6, 297 K): δ 164.3, 154.8 (C3 or 5), 155.9 (NdCN), 103.2 (C4), 69.6 (CH), 52.5 (NCH2CH3), 44.2 [NC(CH3)3], 32.3, 32.2 [C(CH3)3], 30.5 [NC(CH3)3], 30.4 (tBu3), 30.3 (tBu5), 19.1 (NCH2CH3), 13.5 (MgCH2CH3), 5.2 (MgCH2CH3). Data for 7b are as follows. 1H NMR (C6D6, 297 K): δ 7.40 (s, 1 H, CH), 6.02 (s, 2 H, H4), 3.56 (q, 2 H, 3 JHH = 7.0 Hz, NCH2CH3), 1.92 (t, 3 H, 3JHH = 8.2 Hz, MgCH2CH3), 1.43 [s, 9H, NC(CH3)3], 1.37 (s, 18 H, tBu5), 1.36 (s, 18 H, tBu3), 1.06 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 0.35 (q, 2 H, 3JHH = 8.2 Hz, MgCH2CH3). 13C{1H} NMR (C6D6, 297 K): δ 162.9, 155.4 (C3 or 5), 159.2 (NdCN), 102.0 (C4), 80.2 (CH), 51.8 (NCH2CH3), 44.3 [NC(CH3)3], 33.0, 32.4 [C(CH3)3], 31.0 [NC(CH3)3], 30.5 (tBu5), 30.4 (tBu3), 20.0 (NCH2CH3), 13.7 (MgCH2CH3), 5.4 (MgCH2CH3).

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Synthesis of [Mg(CH2SiMe3)(κ3-tbptamd)] (8a,b). The syntheses of 8a,b were carried out in a manner identical with that for 1. Method A: [Li(κ3-tbptamd)(THF)] (1.15 g, 2.00 mmol), SiMe3CH2MgCl (1.0 M in Et2O) (2.00 mL, 2.00 mmol); yield 1.05 g, 86%. Method B: Htbptamd (1.00 g, 2.00 mmol), (SiMe3CH2)2Mg (0.74 M in Et2O) (2.7 mL, 2.00 mmol); yield 1.08 g, 89% (8a/8b ratio 3/7). Anal. Calcd for C34H64MgN6Si: C, 67.02; H, 10.59; N, 13.79. Found: C, 67.14; H, 10.65; N, 13.82. Data for 8a are as follows. 1H NMR (C6D6, 297 K): δ 7.44 (s, 1 H, CH), 5.95 (s, 2 H, H4), 3.43 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 1.85 [s, 9H, NC(CH3)3], 1.28 (s, 18 H, tBu3), 1.24 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 1.13 (s, 18 H, tBu5), 0.50 (s, 9H, MgCH2SiMe3), 0.83 (s, 2H, MgCH2SiMe3). 13C{1H} NMR (C6D6, 297 K): δ 164.0, 155.0 (C3 or 5), 155.9 (NdCN), 103.4 (C4), 69.5 (CH), 51.8 (NCH2CH3), 44.2 [NC(CH3)3], 32.4, 32.2 [C(CH3)3], 30.6 [NC(CH3)3], 30.4 (tBu3), 30.1 (tBu5), 19.0 (NCH2CH3), 5.2 (MgCH2SiMe3), 3.4 (MgCH2SiMe3). Data for 8b are as follows. 1H NMR (C6D6, 297 K): δ 7.41 (s, 1 H, CH), 5.94 (s, 2 H, H4), 3.60 (q, 2 H, 3 JHH = 7.0 Hz, NCH2CH3), 1.45 [s, 9H, NC(CH3)3], 1.36 (s, 18 H, t Bu5), 1.34 (s, 18 H, tBu3), 1.02 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 0.50 (s, 9H, MgCH2SiMe3), 0.76 (s, 2H, MgCH2SiMe3). 13C{1H} NMR (C6D6, 297 K): δ 163.1, 155.4 (C3 or 5), 158.5 (NdCN), 102.2 (C4), 79.9 (CH), 52.1 (NCH2CH3), 44.5 [NC(CH3)3], 32.4, 32.2 [C(CH3)3], 31.1 (tBu5), 30.8, (tBu3), 30.7 [NC(CH3)3], 20.3 (NCH2CH3), 4.9 (MgCH2SiMe3), 3.3 (MgCH2SiMe3). Synthesis of [MgBn(κ3-tbptamd)] (9a,b). The syntheses of 9a,b were carried out in a manner identical with that for 1. Method A: [Li(κ3tbptamd)(THF)] (1.15 g, 2.00 mmol), BnMgCl (2.0 M in THF) (1.00 mL, 2.00 mmol); yield 1.0 g, 81%. Method B: Htbptamd (1.00 g, 2.00 mmol), Bn2Mg (0.62 M in Et2O) (3.2 mL, 2.00 mmol); yield 0.94 g, 77% (9a/9b ratio 1/4). Anal. Calcd for C37H60MgN6: C, 72.47; H, 9.86; N, 13.70. Found: C, 72.59; H, 9.96; N, 13.66. Data for 9a are as follows. 1H NMR (C6D6, 297 K): δ 7.43 (s, 1 H, CH), 7.37 (m, 2H, MgCH2C6H5), 7.29 (m, 2H, MgCH2C6H5), 6.92 (m, 1 H, MgCH2C6H5), 5.92 (s, 2 H, H4), 3.40 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 2.26 (s, 2H, MgCH2C6H5), 1.77 [s, 9H, NC(CH3)3], 1.25 (s, 18 H, tBu3), 1.24 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 1.12 (s, 18 H, tBu5). 13C{1H} NMR (C6D6, 297 K): δ 164.2, 155.0 (C3 or 5), 156.0 (NdCN), 130.0 119.0 (MgCH2C6H5), 103.1 (C4), 69.6 (CH), 52.5 (NCH2CH3), 44.1 [NC(CH3)3], 32.7, 32.4 [C(CH3)3], 30.8 [NC(CH3)3], 30.7 (tBu3), 30.4, (tBu5), 25.5 (MgCH2C6H5), 18.9 (NCH2CH3). Data for 9b are as follows. 1H NMR (C6D6, 297 K): δ 7.38 (s, 1 H, CH), 7.36 (m, 2H, MgCH2C6H5), 7.25 (m, 2H, MgCH2C6H5), 6.89 (m, 1 H, MgCH2C6H5), 5.91 (s, 2 H, H4), 3.47 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 2.25 (s, 2H, MgCH2C6H5), 1.40 [s, 9H, NC(CH3)3], 1.32 (s, 18 H, tBu5), 1.28 (s, 18 H, tBu3), 0.87 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3). 13C{1H} NMR (C6D6, 297 K), δ 163.1, 155.5 (C3 or 5), 158.0 (NdCN), 130.0119.0 (MgCH2C6H5), 102.1 (C4), 80.0 (CH), 52.0 (NCH2CH3), 44.3 [NC(CH3)3], 32.7, 32.4 [C(CH3)3], 32.1 [NC(CH3)3], 31.0 (tBu5), 30.5, (tBu3), 25.5 (MgCH2C6H5), 20.4 (NCH2CH3). Synthesis of [Mg(η3-C3H5)(κ3-tbptamd)] (10a,b). The syntheses of 10a,b were carried out in a manner identical with that for 1. Method A: [Li(κ3-tbptamd)(THF)] (1.15 g, 2.00 mmol), C3H5MgCl (1.7 M in THF) (1.18 mL, 2.00 mmol); yield 0.92 g, 83%. Method B: Htbptamd (1.00 g, 2.00 mmol), (C3H5)2Mg (0.53 M in Et2O) (3.8 mL, 2.00 mmol); yield 0.95 g, 84%. (10a/10b ratio 1/1). Anal. Calcd for C33H58MgN6: C, 70.38; H, 10.38; N, 14.92. Found: C, 70.44; H, 10.45; N, 15.07. Data for 10a are as follows. 1H NMR (C6D6, 297 K): δ 7.44 (s, 1 H, CH), 6.90 (m, 1 H, MgCH2CHCH2), 5.94 (s, 2 H, H4), 3.41 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 1.80 [s, 9H, NC(CH3)3], 1.35 (s, 18 H, tBu5), 1.32 (d, 4 H, 3JHH = 12.5 Hz, MgCH2CHCH2), 1.27 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3), 1.14 (s, 18 H, tBu3). 13C{1H} NMR (C6D6, 297 K),: δ 164.6, 155.0 (C3 or 5), 157.7 (NdCN), 146.5 (MgCH2CHCH2), 103.3 (C4), 69.5 (CH), 52.6 (NCH2CH3), 44.2 [NC(CH3)3], 32.3, 32.1 [C(CH3)3], 30.7 (MgCH2CHCH2), 30.5 [NC(CH3)3], 30.4 (tBu5), 2786

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Organometallics 30.3 (tBu3), 19.0 (NCH2CH3). Data for 10b are as follows. 1H NMR (C6D6, 297 K): δ 7.39 (s, 1 H, CH), 6.90 (m, 1 H, MgCH2CHCH2), 5.93 (s, 2 H, H4), 3.57 (q, 2 H, 3JHH = 7.0 Hz, NCH2CH3), 1.42 [s, 9H, NC(CH3)3], 1.33 (s, 18 H, tBu5), 1.32 (d, 4 H, 3JHH = 12.5 Hz, MgCH2CHCH2), 1.29 (s, 18 H, tBu3), 1.01 (t, 3 H, 3JHH = 7.0 Hz, NCH2CH3). 13C{1H} NMR (C6D6, 297 K): δ 163.1, 155.6 (C3 or 5), 159.3 (NdCN), 146.7 (MgCH2CHCH2), 102.1 (C4), 80.1 (CH), 51.9 (NCH2CH3), 44.2 [NC(CH3)3], 32.8, 32.4 [C(CH3)3], 31.0 [NC(CH3)3], 30.7 (MgCH2CHCH2), 30.5 (tBu5), 30.3 (tBu3), 20.2 (NCH2CH3). Separation Procedure for the Mixture of Isomers 8a,b. The synthesis of the complex mixture 8a,b (which is the only one employed in the ring-opening polymerization of L-/rac-lactide) was carried out by increasing the scale of reagents described above by a factor of 3, with no appreciable changes in the yield. The following quantities and conditions were employed: [Li(κ3-tbptamd)(THF)] (3.45 g, 6.00 mmol), SiMe3CH2MgCl (1.0 M in Et2O) (6 mL, 6.00 mmol). Yield: 2.85 g, 78% (8a/8b ratio 3/7). Subsequent recrystallization from pentane of the whole solid obtained (ca. 15 mL) at 26 °C during 16 h afforded 1.150 g of the 8a,b mixture, now in a 3/2 ratio. Additionally, removal of the volatiles from the remaining mother liquor yielded 1.700 g of a semicrystalline 8a,b mixture, now in a 1/9.3 ratio. Both enriched semicrystalline fractions were dissolved separately again in the minimum amount of pentane (ca. 10 mL) and cooled to 26 °C for an additional 16 h, affording independently isomers 8a,b in spectroscopically pure form as white crystalline solids. Separation yields with respect to the amount of both isomers in the initial mixture: 0.345 g, 40.3% for 8a; 1.050 g, 52.6% for 8b. Typical Polymerization Procedures. Polymerizations of εcaprolactone (CL) were carried out on a Schlenk line in a flame-dried round-bottomed flask equipped with a magnetic stirrer. In a typical procedure, the initiator was dissolved in the appropriate amount of solvent and temperature equilibration was ensured by stirring the solution for 15 min in a temperature-controlled bath. ε-CL was injected, and polymerization times were measured from that point. Polymerizations were terminated by addition of acetic acid (5 vol %) in water. Polymers were precipitated in methanol, filtered, dissolved in THF, reprecipitated in methanol, and dried in vacuo to constant weight. Polymerizations of L-lactide and rac-lactide (LA) were performed on a Schlenk line in a flame-dried round-bottomed flask equipped with a magnetic stirrer. The Schlenk tubes were charged in the glovebox with the required amount of L-/rac-lactide and initiator, separately, and then attached to the vacuum line. The initiator and monomer were dissolved in the appropriate amount of solvent, and temperature equilibration was ensured in both Schlenk flasks by stirring the solutions for 15 min in a bath. The appropriate amount of initiator was added by syringe, and polymerization times were measured from that point. Polymerizations were stopped by injecting a solution of acetic acid (5 vol %) in water. Polymers were precipitated in methanol, filtered, dissolved in THF, reprecipitated in methanol, and dried in vacuo to constant weight. General Kinetic Procedures. A solution of catalyst in tetrahydrofuran (2.5 mL) was added to a tetrahydrofuran solution of monomer (10 mL) to give [rac-LA]0 = 0.8 M. The mixture was then stirred at 20 °C under N2. At appropriate time intervals, 0.5 mL aliquots were removed using a syringe and quickly quenched into 5 mL vials with methanol (3 drops). The aliquots were then dried to constant weight in vacuo and analyzed by 1H NMR spectroscopy.

X-ray Crystallographic Structure Determination for Complexes 4, 6a 3 Et2O, and 7a. A summary of crystal data collection and

refinement parameters for compounds 4, 6a 3 Et2O, and 7a is given in Table S1 in the Supporting Information. The single crystals for 4, 6a 3 Et2O, and 7a were mounted on glass fibers and transferred to a Bruker X8 APEX II CCD-based diffractometer equipped with a graphite-monochromated Mo KR radiation source (λ = 0.710 73 Å). Data

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were integrated using SAINT,52 and an absorption correction was performed with the program SADABS.53 The software package SHELXTL version 6.1054 was used for space group determination, structure solution, and refinement. A successful solution by direct methods provided most non-hydrogen atoms from the E map. The remaining nonhydrogen atoms were located in an alternating series of least-squares cycles and difference Fourier maps. All non-hydrogen atoms were refined with anisotropic displacement coefficients, and all hydrogen atoms were included in the structure factor calculations at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients by full-matrix least-squares methods based on F2.

’ ASSOCIATED CONTENT A figure giving the 1H NMR spectrum of PLA, showing resonances for the CH2SiMe3 chain termini, and a table and CIF files giving full crystallographic data for 4, 6a 3 Et2O, and 7a. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

Supporting Information.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (L.F.S.-B.); antonio. [email protected] (A.O.).

’ ACKNOWLEDGMENT We gratefully acknowledge financial support from the Ministerio de Educaci
on y Ciencia (Direcci
on General de Investigaci
on) of Spain (Grant Nos. CTQ2008-05892/BQU, CTQ2008-00318/ BQU, and Consolider-Ingenio 2010 ORFEO CSD2007-00006) and the Junta de Comunidades de Castilla-La Mancha (Grant No. PCI08-0010). ’ REFERENCES (1) (a) Shoichet, M. S. Macromolecules 2010, 43, 581–591. (b) Coates, G. W.; Hillmyer, M. A. Macromolecules 2009, 42, 7987– 7989. (c) Goodstein, D. Out of Gas: The End of the Age of Oil; W. W. Norton: New York, 2004. (d) Seal, B. L.; Otero, T. C.; Panitch, A. Mater. Sci. Eng., R. 2001, 34, 147–230. (2) (a) P^ego, A. P.; Siebum, B.; VanLuyn, M. J. A.; Gallego, X. J.; Van Seijen, Y.; Poot, A. A.; Grijpma, D. W.; Feijen, J. Tissue Eng. 2003, 9, 981–994. (b) Marler, J. J.; Upton, J.; Langer, R.; Vacanti, J. P. Adv. Drug Delivery Rev. 1998, 33, 165–182. (c) Frazza, E. J.; Schmitt, E. E. J. Biomed. Mater. Res. Symp. 1971, 1, 43–58. (d) Dexon and Vicryl are products of Davis & Geek Corp., Wayne, NJ, and Ethicon, Inc., Somerville, NJ, respectively. (3) (a) Penco, M.; Donetti, R.; Mendichi, R.; Ferruti, P. Macromol. Chem. Phys. 1998, 199, 1737–1745. (b) Leupron Depot is a product of Takeda Chemical Industries, Ltd., Japan, for drug delivery purposes. (c) Taehan Hwakakhoe Chi 1990, 34, 203; Chem. Abst. 1990, 113, 98014g. (d) Smith, A.; Hunneyball, I. M. Int. J. Pharm. 1986, 30, 215–220. (4) (a) Darensbourg, D. J.; Choi, W.; Richers, C. P. Macromolecules 2007, 40, 3521–3523. (b) Beiser, I. H.; Konat, I. O. J. Am. Podiatric Med. Assoc. 1990, 80, 272–275. (5) (a) Cowan, J. A., Ed. The Biological Chemistry of Magnesium; VCH: New York, 1995. (b) Campbell, N. A. Biology, 3rd ed.; Benjamin/ Cummings: Redwood City, CA, 1993; pp 718, 811. (6) (a) Parkin, G. Chem. Commun. 2000, 1971–1985. (b) Mills, C. F. Zinc in Human Biology; Springer-Verlag: New York, 1989. (7) (a) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polym. Rev. 2008, 48, 11–63. (b) Chisholm, M. H.; Zhou, Z. J. Mater. Chem. 2004, 2787

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