Phenolate Substituent Effects on Ring-Opening Polymerization of ε

Dec 24, 2012 - Interaction of the 2-(phenyl-2-ol)-6-ketiminopyridines 2-(4′-R1-C6H3-2′-OH)-6-{CMe═N(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H (L1a-H), ...
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Phenolate Substituent Effects on Ring-Opening Polymerization of ε‑Caprolactone by Aluminum Complexes Bearing 2‑(Phenyl-2-olate)6-(1-amidoalkyl)pyridine Pincers Wafaa Alkarekshi,† Andrew P. Armitage,† Olivier Boyron,‡ Christopher J. Davies,† Matifadza Govere,† Andrew Gregory,† Kuldip Singh,† and Gregory A. Solan*,† †

Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, U.K. Laboratoire de Chimie, Catalyse, Polymères et Procédés, Université de Lyon, CNRS-UCBL, 43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne, France

‡

S Supporting Information *

ABSTRACT: Interaction of the 2-(phenyl-2-ol)-6-ketiminopyridines 2-(4′-R1-C6H3-2′-OH)6-{CMeN(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H (L1a-H), But (L1b-H), Cl (L1c-H), F (L1dH)) with AlMe3 at elevated temperature and subsequent crystallization from acetonitrile affords the five-coordinate 2-(phenyl-2-olate)-6-(2-amidoprop-2-yl)pyridine aluminum− methyl complexes [2-(4′-R1C6H3-2′-O)-6-{CMe2N(2″,6″-i-Pr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (1a), But (1b), Cl (1c), F (1d)), as their acetonitrile adducts, in good yield. In each case, complexation results in concomitant C−C bond formation via methyl migration from aluminum to the corresponding imino unit in L1-H. On the other hand, reactions of the aldimine-containing compounds 2-(4′-R1-C6H3-2′−OH)-6-{CHN(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H (L1e-H), But (L1f-H), Cl (L1g-H)) afford as the major crystallized products [2-(4′-R1C6H3-2′-O)-6-{CH(Me))N(2″,6″-i-Pr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (2a), But (2b), Cl (2c)), in which the migrated methyl group and aluminum−methyl are disposed mutually cis; evidence for the minor trans isomers 2a′−c′ is presented. The ring-opening polymerization of ε-caprolactone employing 1 and 2 in the presence of PhCH2OH proceeded efficiently, producing polymers of narrow molecular weight distribution with the catalytic activities highly dependent on the nature of the phenolate-containing 4R1 substituent with the F and But initiators showing the highest activities (1b ≈ 1d > 1a ≈ 1c and 2b > 2a > 2c); in general the CMe2-containing series 1 were more active than CH(Me)-containing 2 at 30 °C. The bimetallic complex [{2-(4′-But-C6H3-2′O)-6-{CHMeN(2″,6″-i-Pr2C6H3)}C5H3N}AlMe(μ-OMe)AlMe2] (3), the result of adventitious oxygenation, is also reported. The single-crystal X-ray structures are reported for L1d-H, L1f-H, 1a−d, 2a, 2b/2b′, 2c, and 3.

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INTRODUCTION The biodegradable and biocompatible properties of certain types of aliphatic polyesters has led to this class of polymer assuming considerable importance in the medical and pharmaceutical fields.1 Ring-opening polymerization (ROP) of cyclic esters (e.g., lactide, δ-valerolactone (δ-VL), and εcaprolactone (ε-CL)) has proved an effective route for generating such materials,2 and among the most widely employed metal-based initiators/catalysts have been aluminum alkoxides.3−8 Indeed, numerous articles have been dedicated to the catalytic evaluation of discrete Al−OR chelate complexes bearing a range of different multidentate stabilizing auxiliaries including N,N,3 N,O,4 O,O,5 O,N,N,O,6 N,N,N,O,7 and N,N,N,N frameworks.8 On the other hand, the application of nonsymmetric tridentate N,N,O chelates is less developed and limited to monoanionic frameworks9 related in structure to those employed to impart good catalytic activity on divalent metal catalysts (e.g., Zn, Sn, Ca, Mg) for ROP.10 Recently, we have been interested in the development of new families of pyridine-based N,Npy,O pincer ligands, e.g., L1 and L2 (Figure 1), for use in a variety of metal-based catalytic © 2012 American Chemical Society

Figure 1. 2-(Phenyl-2-olate)-6-iminopyridine (L1) and 2-(phenyl-2olate)-6-(1-amidoalkyl)pyridine (L2).

applications including olefin polymerization.11,12 In particular, we envisaged that dianionic L2 could act as a compatible ligand framework for well-defined aluminum(III) monoalkyl/alkoxide complexes that could be employed as initiators for ROP. Furthermore, the amenability of the ligand synthesis to systematic variation of the phenolate substituents (e.g., R1) Received: November 7, 2012 Published: December 24, 2012 249

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Scheme 1a.

a Reagents and conditions: (i) 2-Br-6-(CR2O)-C5H3N (R2 = H, Me), cat. Pd(PPh3)4, K2CO3 (aq), toluene−EtOH, heat; (ii) 2,6-i-Pr2C6H3NH2, cat. H+, MeOH, heat; (iii) BBr3, CH2Cl2, −78 °C.

view of L1d-H is depicted in Figure 2; selected bond distances and angles are given for both L1d-H and L1f-H in Table 1. The

offers considerable potential for probing the role played by electronic effects on catalytic performance. Indeed, if the mechanism of polymerization is considered to follow a coordination−insertion type pathway,2,13 electronic/steric perturbations within the Lewis acidic initiator would be expected to impact on the overall catalytic performance. In this article, we employ a series of protonated ketimine (R2 = Me) and aldimine (R2 = H) examples of L1 (R1 = H, But, Cl, F) as precursors to a new family of aluminum(III) methyl complexes bearing sterically encumbered (Ar = 2,6-i-Pr2C6H3) L2. In the presence of benzyl alcohol, these aluminum− monomethyl complexes exhibit high efficiency for the ringopening polymerization of ε-CL with their catalytic activity (propagation rate) influenced by both the nature of the phenolate R1 group and the 1-amidoalkyl R2 group. Herein we report a full account of the results.

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Figure 2. Molecular structure of L1d-H with the atom-labeling scheme and ellipsoids at the 30% probability level. All hydrogen atoms, apart from H1, have been omitted for clarity.

RESULTS AND DISCUSSION 1. Synthesis of 2-(Phenyl-2-ol)-6-iminopyridines (L1H). The 2-(phenyl-2-ol)-6-iminopyridines 2-(4′-R1-C6H3-2′− OH)-6-{CR2N(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H, R2 = Me (L1a-H); R1 = R2 = H (L1e-H); R1 = But, R2 = H (L1f-H); R1 = Cl, R2 = H (L1g-H)) could be prepared using methodologies previously described.11a Alternatively, a two-step procedure was developed involving Suzuki cross-coupling of the corresponding 2-hydroxyphenylboronic acid with 2-bromo-6-acetylpyridine followed by a condensation reaction with 2,6diisopropylaniline, affording 2-(4′-R1-C6H3-2′-OH)-6-{CR2 N(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H, R2 = Me (L1a-H); R1 = But, R2 = Me (L1b-H); R1 = Cl, R2 = Me (L1c-H); R1 = F, R2 = Me, (L1d-H)) in good yield (Scheme 1). The new compounds L1b-H, L1c-H, and L1d-H have been characterized by a combination of 1H, 19F{1H}, and 13C{1H} NMR and IR spectroscopy and ESI mass spectrometry (see the Experimental Section). In addition, single-crystal X-ray diffraction studies have been performed on L1d-H and L1f-H. Typically crystals suitable for the structural determination could be grown by slow evaporation of concentrated dichloromethane solutions. A

structures are similar and resemble those previously reported for L1a-H, L1e-H, and L1g-H,11a with a central pyridine ring substituted at its 2-position by a phenyl-2-ol group and at the 6Table 1. Selected Bond Distances (Å) and Angles (deg) for L1d-H and L1f-H L1d-H Bond Distances 1.359(4) 1.469(4) 1.488(5) 1.268(4) 1.429(4) 1.366(4) 1.505(5)

C(1)−O(1) C(6)−C(7) C(11)−C(12) C(12)−N(2) N(2)−C(13) C(4)−F(1) C(12)−C(12′) C(4)−C(25)

1.3591(15) 1.4764(17) 1.4668(17) 1.2571(15) 1.4230(15)

1.529(2)

C(12)−N(2)−C(13) C(11)−C(12)−N(2) 250

L1f-H

Bond Angles 122.5(3) 116.9(3)

120.83(11) 121.04(12)

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Scheme 2a

position by an imine unit (C(12)−N(2) = 1.268(4) Å (L1d-H), 1.257(2) Å (L1f-H)). The pyridine nitrogen adopts a trans configuration with respect to the neighboring imine nitrogen (torsion angles N(1)−C(11)−C(12)−N(2) = 174.6° (L1d-H), 167.8° (L1f-H)) while it is cis to the phenol oxygen; the latter configuration is the result of a hydrogen-bonding interaction between the phenol hydrogen atom and the neighboring pyridine nitrogen (O(1)···N(1) = 2.583 Å (L1d-H), 2.621 Å (L1f-H)). For both structures the 2,6-diisopropylphenyl rings are inclined essentially orthogonally (C(12)−N(2)−C(13)− C(18) = 89.3° (L1d-H), 88.4° (L1f-H)) to the plane of the adjacent pyridylimine unit. Compounds L1-H all display peaks corresponding to the protonated molecular ions in their ESI mass spectra, while their IR spectra reveal characteristic ν(CN)imine bands at ca. 1642 cm−1. Further support for imine formation is provided by the 1 H NMR spectra, which show signals for the ketimine protons at ca. δ 2.2 (L1a-H−L1d-H) and aldimine proton at ca. δ 8.27 (L1e-H−L1g-H); the imino carbons are seen at ca. δ 160 in their 13C NMR spectra. Fluorinated L1d-H displays a singlet in its 19F{1H} NMR spectrum at δ −125.09 with a typically large one-bond carbon−fluorine coupling constant (236 Hz) observable in its 13C{1H} NMR spectrum. 2. Formation of Aluminum−Methyl Complexes 1 and 2. Treatment of ketimine-containing L1a-H−L1d-H with 2 equiv of trimethylaluminum in toluene at elevated temperature, followed by workup in acetonitrile, results in complexation and methylation of the ketimine moiety to afford the air-sensitive 2(phenyl-2-olate)-6-(2-amido-prop-2-yl)pyridine aluminum− methyl complexes [2-(4′-R1-C6H3-2′-O)-,6-{CMe2N(2″,6″-iPr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (1a), But (1b), Cl (1c), F (1d)) as their acetonitrile adducts in good yield (Scheme 2). Conversely, interaction of aldimino L1e-H−L1g-H with AlMe3 followed by recrystallization from acetonitrile gave as the major complex [2-(4′-R1-C6H3O)-6-{CMeHN(2″,6″-iPr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (2a), But (2b), Cl (2c)) along with trace amounts of isomeric 2a′−c′, respectively (Scheme 2). Complexes 1 and 2 have been characterized by NMR and IR spectroscopy and mass spectrometry and give satisfactory microanalytical data (see the Experimental Section). In addition, single-crystal X-ray diffraction studies were performed on 1a−1d, 2a, 2b/2b′, and 2c. Single crystals of 1a−d suitable for X-ray determination were grown by slow cooling of acetonitrile solutions of each complex that had been previously brought to reflux. A perspective view of representative 1d is shown in Figure 3; selected bond distances and angles for all four structures are collected in Table 2. In each structure an aluminum center is bound by a dianionic N,N,O chelate along with a monoanionic methyl group and an N-bound neutral acetonitrile ligand to complete a fivecoordinate geometry. On the basis of the τ parameter (where τ = 0 implies ideal square-based pyramidal and τ = 1 ideal trigonal bipyramidal),14 the geometries of all four complexes display a modest tendency toward distorted trigonal bipyramidal (τ = 0.55 (1a), 0.57 (1b), 0.55 (1c), 0.52 (1d)). The presence of the gem-dimethyl group within the tridentate ligand backbone in 1a−d confirms the successful methyl migration. Perspective views of 2a and 2b/2b′ are shown in Figures 4 and 5; selected bond distances and angles for 2a, 2b/2b′, and 2c are given in Table 3. For 2c two “non-superimposable” mirror images (molecules A and B) are present within the asymmetric unit with slight variations in the relative inclinations of the N-aryl groups apparent between each molecule. The

a

Reagents and conditions: (i) 2 AlMe3, toluene, heat; (ii) MeCN, heat; (iii) MeCN, air/water, heat.

Figure 3. Molecular structure of 1d with atom-labeling scheme and ellipsoids at the 30% probability level. All hydrogen atoms have been omitted for clarity.

251

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Table 2. Selected Bond Distances (Å) and Angles (deg) for 1a−d 1a

1b

1c

1d

Al(1)−O(1) Al(1)−N(1) Al(1)−N(2) Al(1)−N(3) Al(1)−C(1) C(13)−C(14) C(13)−C(27) C(13)−N(2) C(28)−C(29) C(5)−R1

1.800(4) 1.995(5) 1.829(5) 2.083(6) 1.966(7) 1.515(8) 1.537(8) 1.487(7) 1.450(8)

C(1)−Al(1)− O(1) C(1)−Al(1)− N(1) C(1)−Al(1)− N(2) C(1)−Al(1)− N(3) N(2)− C(13)− C(14) N(2)− C(13)− C(27)

112.7(2)

Bond Distances 1.8008(15) 1.824(2) 2.0032(16) 2.021(3) 1.8555(16) 1.845(3) 2.1002(18) 2.088(3) 1.975(2) 1.972(4) 1.544(3) 1.533(4) 1.543(3) 1.536(4) 1.485(2) 1.486(4) 1.452(3) 1.452(5) 1.535(2) (R1 = 1.754(4) (R1 = C(30)) Cl(1)) Bond Angles 109.70(8) 112.02(13)

100.4(2)

100.04(8)

100.11(13)

102.47(8)

117.8(3)

120.77(8)

118.80(14)

120.34(9)

97.2(3)

95.91(8)

97.93(13)

95.79(9)

113.5(5)

113.59(15)

113.5(3)

113.13(15)

113.6(5)

113.10(15)

113.6(3)

113.36(16)

1.8077(17) 2.0212(19) 1.8502(19) 2.076(2) 1.970(2) 1.541(3) 1.524(3) 1.481(2) 1.449(3) 1.367(2) (R1 = F(1)) 109.02(9)

Figure 5. Molecular structure of 2b/2b′ with the atom-labeling scheme and ellipsoids at the 30% probability level. Dotted bonds refer to a 50:50 disorder of C(14).

Table 3. Selected Bond Distances (Å) and Angles (deg) for 2a, 2b/2b′, and 2c 2c 2a Al(1)−O(1) Al(1)−N(1) Al(1)−N(2) Al(1)−N(3) Al(1)−C(1) C(13)−C(14) C(13)−C(14′) C(27)−C(28) C(27)−N(3) C(1)−Al(1)− O(1) C(1)−Al(1)− N(1) C(1)−Al(1)− N(2) C(1)−Al(1)− N(3) N(1)−Al(1)− N(3) N(2)−C(13)− C(14)

Figure 4. Molecular structure of 2a with the atom-labeling scheme and ellipsoids at the 30% probability level. All hydrogen atoms, apart from H13, have been omitted for clarity.

2b/2b′

molecule A

molecule B

1.8104(17) 2.025(2) 1.863(2) 2.075(2) 1.961(3) 1.513(4)

1.8117(17) 2.019(2) 1.860(2) 2.073(2) 1.963(3) 1.502(4)

Bond Distances 1.8203(13) 1.7944(12) 2.0271(14) 2.0059(14) 1.8637(15) 1.8529(13) 2.0882(17) 2.0810(15) 1.9819(19) 1.9741(18) 1.517(3) 1.387(3) 1.3508 1.449(3) 1.445(2) 1.135(2) 1.129(2) Bond Angles (deg) 106.56(8) 113.65(7)

1.443(4) 1.132(3)

1.453(4) 1.123(3)

109.15(10)

108.54(10)

104.58(7)

100.01(7)

102.65(10)

102.89(10)

118.32(8)

121.12(7)

118.18(10)

120.16(10)

100.20(8)

95.11(7)

98.60(10)

96.80(10)

155.19(6)

164.86(6)

158.74(9)

160.31(9)

114.87(16)

121.34(19)

116.3(2)

116.9(2)

disorder between cis (C(14)) and trans (C(14′)) occupations of the migrated methyl group. With regard to the N,N,O chelates in both 1 and 2, the donor−metal distances follow the order Al(1)−N(1)pyridyl (range 1.995(5)−2.0271(14) Å) > Al(1)−N(2)amide (range 1.829(5)−1.8637(15) Å) > Al(1)−O(1) phenolate (range 1.7944(12)−1.8203(13) Å), reflecting the anionic nature of the Al−Ophenolate and Al−Namide interactions and consistent with the greater oxophilicity of aluminum. When 1a (R1 = H) is compared with 2a (R1 = H), the corresponding three metal− ligand distances for 2a (2.0217(14), 1.8637(15), 1.8203(13) Å) fall at the top end of the ranges, while those for 1a (1.995(5), 1.829(5), 1.800(13) Å) are at the bottom end, highlighting the cumulative electronic or steric effect of exchanging a CMe2 for a

three structures resemble 1, with an aluminum center bound by a tridentate dianionic N,N,O chelate along with monodentate acetonitrile and methyl ligands to give five-coordinate geometries best described as distorted square-based pyramidal (τ = 0.33 (2a), 0.43, 0.48 (2c, molecules A and B)) or distorted trigonal bipyramidal (τ = 0.66 (2b/2b′)). This variation in structure is likely accommodated by the flexibility of the N,N,O chelate, which is best exemplified by the N(1)−C(8)−C(7)− C(2) torsion angles that differ by up to 10° within this series. In 2a,c the migrated methyl groups (C(14)) are located cis to the aluminum−methyl group, while in 2b/2b′ there is 50:50 252

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CHMe unit. The para substitution pattern on the phenolate group (R1) also appears to have a slight influence on the metal−oxygen distances for 2 (2a (1.8203(13) Å, R1 = H) > 2c (1.8104 (17), 1.8117(17) Å, R1 = Cl) > 2b (1.7944(12) Å, R1 = But)), although a related trend is absent for 1. The Al(1)−C(1) distances for 1 and 2 (range 1.961(3)−1.9819(19) Å) are typical in comparison with other five-coordinate mononuclear aluminum monomethyl complexes. 15 The Al−N(3) MeCN distances are in general the longest of the metal−ligand interactions (range: 2.073(2)−2.1002(18) Å),16 emphasizing the potential lability of this monodentate ligand during the intended polymerization process. The use of trimethylaluminum to facilitate the migration of a methyl group to an imino unit of a ligand precursor has been previously reported.17 No intermolecular interactions of note are apparent. In the 1H NMR spectra of 1a−d (recorded in C6D6 at ambient temperature) the peaks for the NCMe2 and Ar-oCHMe2 protons are all poorly resolved; attempts to improve the resolution by recording the spectra at lower and higher temperatures showed no appreciable differences. In contrast, the NCHMe and Ar-o-CHMe2 protons in 2a−c are sharp with clear multiplicities evident. Support for the solid-state structures being maintained in solution is given by the inequivalency of the CMeaMeb-N protons in 1a−d, which is accompanied by two different chemical shifts for the inequivalent Ar-o-CHMe2 protons; broad resonances are evident in each case. In 2a−c the Ar-o-CHMe2 protons are seen as four independent doublets, highlighting the restricted rotation about the Al− Namide bond. Unfortunately, attempts to confirm the cis configuration of Al−Me relative to the NCHMe methyl (as seen in the solid-state structures of 2a,c) using NOESY spectroscopy were unsuccessful. Notably for 2b, peaks consistent with the trans isomer 2b′ were also detected (2b:2b′ = 90:10), supporting the mixed occupancy found in the crystal structure for 2b (Figure 5). Closer inspection of the 1H NMR spectra for 2a and 2c also reveals trace amounts of their trans isomers 2a′,c′, respectively. In all cases the Al−Me protons are seen upfield (between δ −0.05 and −0.18), with the corresponding methyl carbon resonance occurring as quadrupolar broadened peaks between δ −8.6 and −14.3 in their 13C{1H} NMR spectra. The bound acetonitrile for both 1 and 2 can be seen as a singlet for the methyl group at ca. δ 0.7 in their 1H NMR spectra. In the FAB mass spectra of 1 and 2 (using nitrophenyl octyl ether as the matrix), fragmentation peaks corresponding to the loss of an acetonitrile molecule and a methyl/acetonitrile are evident in each case. The IR spectra confirm the presence of coordinated acetonitrile with weak bands visible between 2299 and 2319 cm−1. Complexes 1 and 2 proved to be extremely air and moisture sensitive, and on one occasion during the reaction of L1f-H with AlMe3 the bimetallic complex [{2-(4′-But-C6H3-2′-O),6{CHMeN(2″,6″-i-Pr2C 6H 3)}C5 H3 N}AlMe(μ-OMe)AlMe2 ] (3) was isolated. A single crystal of 3 was also grown during an attempted crystallization of 2b from a concentrated acetonitrile solution. A view of 3 is shown in Figure 6; selected bond distances and angles are collected in Table 4. The structure of 3 consists of two aluminum centers (Al(1) and Al(2)), each bridged by a methoxide group and by a phenolate oxygen atom from the dianionic N,N,O ligand. The N,N,O ligand, in addition, chelates to afford a more regular square-based pyramidal geometry at Al(1) (τ = 0.09), which is completed by an apical methyl group; at Al(2) two methyl groups complete a distorted-tetrahedral geometry. It is apparent that

Figure 6. Molecular structure of 3 with the atom-labeling scheme and ellipsoids at the 30% probability level. All hydrogen atoms, apart from H16, have been omitted for clarity.

Table 4. Selected Bond Distances (Å) and Angles (deg) for 3 Bond Distances Al(1)−O(1) Al(1)−O(2) Al(1)−N(1) Al(1)−N(2) Al(1)−C(1) C(1)−Al(1)−O(1) C(1)−Al(1)−O(2) C(1)−Al(1)−N(1) C(1)−Al(1)−N(2) C(2)−Al(2)−C(3)

1.9835(15) Al(2)−O(1) 1.9056(17) Al(2)−O(2) 2.0685(18) Al(2)−C(2) 1.8546(18) Al(2)−C(3) 1.966(2) C(16)−C(17) Bond Angles (deg) 98.82(9) 109.28(8) 107.26(8) 116.54(9) 118.86(12)

C(2)−Al(2)−O(1) C(2)−Al(2)−O(2) Al(1)−O(1)−Al(2) Al(1)−O(2)−Al(2) N(2)−C(16)−C(17)

1.9056(17) 1.8239(16) 1.960(2) 1.950(3) 1.534(3) 115.91(9) 109.51(10) 100.72(7) 104.35(7) 114.48(17)

the O atom of the “(MeO)AlMe2” unit in 3 has filled the site occupied by acetonitrile found in 2b with concomitant coordination of the phenolate oxygen atom to Al(2). This apparent substitution has the most significant effect on the Ophenolate−Al(1) distance with a ca. 0.189 Å elongation in comparison to the distance in 2b. It is likely that the MeO group arises from the inadvertent oxygenation of trimethylaluminum present in excess during the reaction. Indeed, related oxygenation of aluminum−alkyls has been previously reported as a means of forming aluminum alkoxides.9a,18 As with 2a,c the migrated methyl group on the −CHMe− unit adopts a cis configuration with respect to the apical methyl group on Al(1). There are no dominant intermolecular interactions of note. 3. Ligand Effects in 1 and 2 on the Ring-Opening Polymerization of ε-Caprolactone. Initially the gemdimethyl species 1a−d were screened as pre-initiators for the ring-opening polymerization of ε-CL. Typically, 1a−d were treated with 1.0 equiv of PhCH2OH in toluene prior to the addition of the ε-CL (250 equiv) and the commencement of the run at 30 °C (Scheme 3). The polymerization runs were monitored by 1H NMR spectroscopy, and the monomer conversion was also determined by 1H NMR spectroscopy. The results of the catalytic screening are collected in Table 5 (entries 1−16). Several points emerge from inspection of the data. All systems were found to display good activity for the polymerization of ε-CL, with p-But-containing 1b/PhCH2OH 253

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Scheme 3a.

a

Catalytic evaluation of 1 or 2/PhCH2OH for the ROP of ε-CL.

Table 5. Ring-Opening Polymerization of ε-CL Initiated by 1/PhCH2OH Catalyst Systemsa entry

proinitiator (R1)

time/ min

conversion/ %b

Mn(GPC)

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

1a (H) 1a (H) 1a (H) 1a (H) 1b (But) 1b (But) 1b (But) 1b (But) 1c (Cl) 1c (Cl) 1c (Cl) 1c (Cl) 1d (F) 1d (F) 1d (F) 1d (F)

30 60 90 120 30 60 90 120 30 60 90 120 30 60 90 120

54 67 72 74 94 99 100 100 50 61 67 73 98e 100 100 100

17290 19970 21334 22890 27400 28668 26359 28532 17775 20452 21171 24966 27867 29726 31951 32196

d

Mn(calcd)

Mw/ Mnd

15498 19203 21483 21198 26898 28328 28608 28608 14358 17395 19203 20913 27930 28608 28608 28608

1.23 1.29 1.26 1.30 1.22 1.20 1.26 1.30 1.25 1.22 1.18 1.48 1.15 1.17 1.23 1.25

c

Table 6. Ring-Opening Polymerization of ε-CL Initiated by 2/PhCH2OH Catalyst Systemsa entry

proinitiator (R1)

time/ min

conversion/ %b

Mn(GPC)d

Mn(calcd)c

Mw/ Mnd

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

2a (H) 2a (H) 2a (H) 2a (H) 2b (But) 2b (But) 2b (But) 2b (But) 2c (Cl) 2c (Cl) 2c (Cl) 2c (Cl)

30 60 90 120 30 60 90 120 30 60 90 120

75 85 88 92 96e 100 100 100 60 70 78 90

25507 29753 29614 30206 33409 34693 34697 34414 22402 25257 27919 28361

21483 24333 25188 26328 27408 28608 28608 28608 17208 20058 22338 25758

1.25 1.20 1.28 1.24 1.23 1.24 1.27 1.32 1.20 1.25 1.20 1.28

Conditions: 2 (0.04 mmol), PhCH2OH (0.04 mmol), ε-CL (10 mmol) ([CL]/[Al] = 250), toluene, 50 °C. bEstimated by 1H NMR spectroscopy; cCalculated from [(molecular weight of ε-CL) × [CL]/ [Al] × (conversion)] + (molecular weight of benzyl alcohol). dBy gel permeation chromatography (GPC) in THF vs polystyrene standards; values corrected according to the equation Mn(PCL) = 0.56Mn(GPC vs polystyrene standards).19 eTOF = 480 h−1. a

Conditions: 1 (0.04 mmol), PhCH2OH (0.04 mmol), ε-CL (10 mmol) ([CL]/[Al] = 250), toluene, 30 °C. bEstimated by 1H NMR spectroscopy. cCalculated from [(molecular weight of ε-CL) × [CL]/ [Al] × (conversion)] + (molecular weight of benzyl alcohol). dBy gel permeation chromatography (GPC) in THF vs polystyrene standards; values corrected according to the equation Mn(PCL) = 0.56Mn(GPC vs polystyrene standards).19 eTOF = 490 h−1. a

Nevertheless, similar trends are apparent, with the Butcontaining 2b displaying the highest catalytic activity and Clcontaining 2c the lowest. It is apparent from the above results that the more electron donating the phenolate p-R1 substituent and/or the 1-amido CMeR2N(2,6-i-Pr2C6H3) groups (R2 = Me vs H) are, the higher the conversions. Such electronic effects are similar to some literature reports,3d,6l,10b,20 with the activity orders herein being 1b (But)/PhCH2OH ≈ 1d (F)/PhCH2OH > 1a (H)/ PhCH2OH ≈ 1c (Cl) /PhCH2OH > 2b (But)/PhCH2OH > 2a (H)/PhCH2OH > 2c (Cl)/PhCH2OH. The high activity displayed by fluorine-substituted 1d (F)/PhCH2OH was unexpected, as aryl fluorides can display powerful negative inductive effects. However, an aryl fluoride para substituent can also exhibit a marked positive electron-donating mesomeric effect (via p−π F−Ar bonding),21 and it is this property that is viewed as responsible for the performance characteristics exhibited by 1d/PhCH2OH. It has been proposed previously that the overall rate of the polymerization depends on a combination of factors, including the Lewis acidity of the metal center and the alkoxide nucleophilicity.6l For these systems it would appear that the fluoride (1d)- and tert-butyl-containing systems (1b) enhance the alkoxide nucleophilicity while maintaining sufficient Lewis acidity to allow monomer binding. No doubt the rearrangements that ensue in the final ring opening of the monomer further contribute to the overall polymerization rate. Interestingly, a number of recent reports

and p-F-containing 1d/PhCH2OH being the most active (with TOFs as high as 490 h−1, 30 °C), allowing 100% conversion of the monomer to poly(CL) after 1 h (entries 6 and 14). The pCl-containing 1c/PhCH 2 OH and p-H-containing 1a/ PhCH2OH proved less active, reaching only 73% (entry 12) and 74% (entry 4) after 2 h, respectively. The molecular weight distributions (Mw/Mn) of the polyesters are narrow, ranging from 1.15 to 1.48 with the more active 1b,d showing maximum polydispersities of only 1.30 (1b) and 1.25 (1d), consistent with a single-site active species. With regard to the less active 1a, an approximately linear relationship between the number-average molecular weight (Mn) and the conversion exists which, coupled with the relatively low molecular weight variations, implies a controlled polymerization. In the absence of benzyl alcohol all four aluminum−methyl complexes showed much lower activity and required higher temperatures (ca. 70 °C) and the molecular weight of the polymers was much broader, suggesting partial decomposition of the catalyst. The CH(Me)-containing species 2a−c also proved, in the presence of benzyl alcohol, to be active catalysts for the ROP of ε-CL (entries 1−12, Table 6). However, in this case the polymerizations had to be conducted at 50 °C in order to achieve conversions comparable with those of 1/PhCH2OH. 254

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are expressed in hertz (Hz). The electrospray ionization (ESI) and fast atom bombardment (FAB) mass spectra were recorded using a micromass Quattro LC mass spectrometer and a Kratos Concept spectrometer with methanol and nitrophenyl octyl ether as the matrices, respectively. High-resolution FAB mass spectra were recorded on a Kratos Concept spectrometer (xenon gas, 7 kV) with NBA as the matrix. The infrared spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer on solid samples. Melting points (mp) were measured on a Gallenkamp melting point apparatus (Model MFB-595) in open capillary tubes and were uncorrected. Elemental analyses were performed on a Carlo Erba CE1108 instrument at the Department of Chemistry, London Metropolitan University. Size exclusion chromatography analyses were performed on an EcoSEC semimicro GPC system from Tosoh equipped with a dualflow refractive index detector and a UV detector. The samples were analyzed in THF at 30 °C using a flow rate of 1 mL min−1. All polymers were injected at a concentration of 1 mg mL−1 in THF, after filtration through a 0.45 μm pore size membrane. Separation was performed with a guard column and three PL gel 5 μm MIXED-C columns (7 μm, 300 × 7.5 mm). The average molar masses (numberaverage molar mass Mn and weight-average molar mass Mw) and the polydispersity index (PDI = Mw/Mn) were derived from the RI signal by a calibration curve based on poly(styrene) standards. The calibration was constructed with narrow molecular weight standards from 580 to 3053000 g/mol. A third-degree polynomial regression was applied. WinGPC software was used for data collection and calculation. The Mn values of the PCLs were corrected with a factor of 0.56 to account for the difference in hydrodynamic volumes with polystyrene.19 The MALDI-TOF mass spectrum of PCL was obtained with an ABI Voyager-DE STR, BioSpectrometry Workstation, Serial No. 4364, using a nitrogen laser source (337 nm, delay time 500 ns) in reflector mode with a positive acceleration voltage of 25 kV. The mass range (Da) analyzed was 1000−7000 Da and 10 shots/spectrum. The matrix was prepared as follows: a 2:1 mixture of a saturated solution of αcyano-4-hydroxycinnamic acid (∼10 mg/mL) (Fluka) in HPLCquality acetonitrile (ACN) and a 0.1% solution of trifluoroacetic acid in ultrapure water. The sample of PCL was prepared as follows: the sample (3.9 mg) was made to 10 mg/mL in THF and diluted 1/10, and 1 μL was used for analysis. A 1 μL portion of a 2/1 mixture of a saturated solution of α-cyano-4-hydroxycinnamic acid in HPLCquality ACN and a 0.1% solution of trifluoroacetic acid in ultrapure water was deposited on the sample plate. After total evaporation, 1 μL of the polymer in HPLC-quality THF was deposited. The peptide calibration standard was used for external calibration (Bradykinin Frag 1-7, Bradykinin Frag 2-9, Angiotensin I, Glu-Fibrinopeptide B, Adrenocorticotropic Hormone Clip 18-39, Insulin Chain B Oxidised (Sigma)). The reagents 2,6-diisopropylaniline and trimethylaluminum (2 M solution in toluene) were purchased from Aldrich Chemical Co. and used without further purification. Both benzyl alcohol and εcaprolactone were purchased from Aldrich and distilled prior to use. The compounds L1e-H−L1g-H,11a 2-bromo-6-acetylpyridine,27 and tetrakis(triphenylphosphine)palladium(0)28 were prepared according to previously reported procedures. 2-Hydroxyphenylboronic acid, 2hydroxy-4-chlorophenylboronic acid, 2-hydroxy-4-chlorophenylboronic acid, 2-hydroxy-4-tert-butylphenylboronic acid, and 2-hydroxy-4fluorophenylboronic acid were prepared by sequential lithiation of the corresponding bromophenol in diethyl ether at −78 °C with 2 equiv of n-BuLi, treatment with triisopropylborate, and hydrolysis with 2 M HCl.29 All other chemicals were obtained commercially and used without further purification. Synthesis of L1a-H. Formation of Ketone. An oven-dried Schlenk flask equipped with a stir bar was evacuated, back-filled with nitrogen, and charged with 2-bromo-6-acetylpyridine (0.894 g, 4.47 mmol), Pd(PPh3)4 (0.109 g, 0.094 mmol, 0.02 equiv), toluene (20 mL), and an aqueous 2 M solution of potassium carbonate (4.45 mL, 8.94 mmol, 2 equiv). The solution was stirred at room temperature for 15 min before the addition of 2-hydroxyphenylboronic acid (0.800 g, 5.80

have also highlighted the beneficial effects of introducing fluorine into the ligand framework of an aluminum-based ROP initiator.4a,f,6m,22,23 Attempted reaction of 1b with 1.0 equiv of PhCH2OH alone in C6D6 at ambient temperature afforded a mixture of products, as evidenced by a series of the peaks in the 1H NMR spectrum in the region characteristic of the Al-OCH2Ph protons (between δ 4.9 and 5.4).24,6n This observation would suggest that the presence of ε-CL is vital for generating the single active Al−benzyloxide species. Some support that the initiation could occur through the insertion of ε-CL into an Al−OCH2Ph bond was achieved by end group analysis (by 1H NMR spectroscopy) of the PCL obtained using a reduced [CL]/[Al] ratio of 25:1 (with 1b/PhCH2OH as the catalyst system), revealing an approximately 1:1 ratio between the −CH2OH chain end and the PhCH2O ester end; nevertheless an activated-monomer mechanism cannot be ruled out.25 Furthermore, a MALDITOF mass spectrum of a PCL sample generated from 1a/ PhCH2OH showed a series of major peaks separated by a caprolactone unit (114 g mol−1) corresponding to linear [H(CL)n-OBn]·Na+ cations (see Figure S21 in the Supporting Information). In addition, minor peaks assignable to linear [H(CL)n-OBn]·K+, [H-(CL)n-OH]·Na+, and [H-(CL)n-OH]·K+ cations were also observable, the latter OH-capped polymers being the likely products of hydrolysis under ionization conditions. No evidence for cyclic isomers could be detected, suggesting transesterification/back-biting is not significant in the polymerization. These data also demonstrate that the aryloxide, associated with the N,N,O ligand, acts purely as a spectator functionality during the polymerization.

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CONCLUDING REMARKS A series of five-coordinate monoalkylaluminum complexes, [2(4′-R1C6H3-2′-O)-6-{CMe2N(2″,6″-i-Pr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (1a), But (1b), Cl (1c), F (1d)) and [2-(4′R 1 C 6 H 3 -2′-O)-6-{CH(Me))N(2″,6″-i-Pr 2 C 6 H 3 )}C 5 H 3 N]AlMe(NCMe) (R1 = H (2a), R1 = But (2b), R1 = Cl (2c)), bearing 2-(phenyl-2-olate)-6-(1-amidoalkyl)pyridine pincer ligands have been synthesized and characterized on the basis of spectroscopic and analytical data, while their structures were determined by single-crystal X-ray diffraction. The ROPs of εCL using 1a−d, in the presence of PhCH2OH, proceeded efficiently in a controlled fashion with the propagation rates influenced by the nature of the phenolate R1 substituents. Notably, the tert-butyl (1b/PhCH2OH) and fluorine containing systems (1d/PhCH2OH) proved to be the most efficient catalyst systems and possess good activities (with TOFs ≈ 490 h−1, 30 °C).2h The importance of electron-donating substituents on the catalytic performance was further highlighted when the amido CMe2N(2,6-i-Pr2C6H3) arm of the pincer ligand in 1 was replaced with a less donating CH(Me)N(2,6-iPr2C6H3) unit (in 2), resulting in a less active system.

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EXPERIMENTAL SECTION

General Procedures. All reactions, unless otherwise stated, were carried out under an atmosphere of dry, oxygen-free nitrogen, using standard Schlenk techniques. Solvents were distilled under nitrogen from appropriate drying agents and degassed prior to use.26 NMR spectra were recorded on a Bruker DPX 300 spectrometer operating at 300.03 (1H) and 75.4 MHz (13C) or a Bruker DRX400 spectrometer at 400.13 (1H), 100.61 (13C), and 188 MHz (19F) at ambient temperature unless otherwise stated; chemical shifts (ppm) are referred to the residual protic solvent peaks, and coupling constants 255

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mmol) in ethanol (11 mL). After it was heated to 90 °C for 42 h, the mixture was cooled to room temperature and 30% hydrogen peroxide (0.4 mL) was added. The mixture was stirred at room temperature for 30 min, the product was extracted using diethyl ether (2 × 100 mL), and the extract was washed with saturated sodium chloride solution (1 × 30 mL) and water (3 × 30 mL) to give an orange organic layer. Following drying with magnesium sulfate, the volatiles were removed under reduced pressure to give an orange-brown residue. The catalyst residues were removed using a short silica gel column employing dichloromethane/petroleum ether (80/20) as eluting solvent. After the solvent was removed under reduced pressure, the product 2(C6H4-2-OH)-6-(CMeO)-C5H3N was obtained as a pale yellow solid (0.762 g, 80%). 1H NMR (300 MHz, CDCl3): δ 2.66 (s, 3H, CMeO), 6.84−6.98 (m, 1H, Ar-H), 6.96 (dd, J = 8.1, 1.2, 1H, ArH), 7.24−7.29 (m, 1H, Ar-H), 7.73 (dd, J = 8.1, 1.1, 1H, Ar-H), 7.88− 7.90 (m, 2H, Py-H), 8.00 (dd, J = 6.4, 2.6, 1H, Py-H), 13.42 (s, 1H, OH). ESIMS: m/z 214 [M + H]+. These data are consistent with those previously reported.11a Formation of Imine. To a solution of 2-(C6H4-2-OH)-6-(CMe O)-C5H3N (0.716 g, 3.36 mmol) in methanol (10 mL) was added 2,6diisopropylaniline (0.892 g, 5.04 mmol, 1.5 equiv). The solution was stirred at reflux for 5 min before the addition of 2 drops of formic acid. After it was refluxed for 2 days, the reaction mixture was cooled to room temperature and the suspension filtered, washed with cold methanol, and dried to give L1a-H as a yellow solid (1.00 g, 80%). 1H NMR (300 MHz, CDCl3): δ 1.08 (d, J = 6.9, 12H, CH(Me)2), 2.18 (s, 3H, CMeN), 2.65 (sept, J = 6.9, 2H, CH(Me)2), 6.88 (dt, J = 8.8, 0.7, 1H, Ar-H), 6.97 (dd, J = 6.5, 0.8, 1H, Ar-H), 7.11−7.15 (m, 3H, Ar-H), 7.28 (dt, J = 6.6, 1.0, 1H, Ar-H), 7.79 (dd, J = 6.9, 1.4, 1H, ArH), 7.85−8.00 (m, 2H, Py-H), 8.23 (dd, J = 7.1, 1.2, 1H, Py-H), 14.2 (s, OH). ESIMS: m/z 373 [M + H]+. These data are consistent with those previously reported.11a Synthesis of L1b-H. Formation of Ketone. Following a procedure similar to that described for L1a-H, using 2-bromo-6-acetylpyridine (1.31 g, 6.55 mmol), 2-hydroxy-4-tert-butylphenylboronic acid (1.50 g, 7.73 mmol), tetrakis(triphenylphosphine)palladium(0) (0.302 g, 0.262 mmol), and aqueous 2 M potassium carbonate (6.55 mL, 13.10 mmol) gave 2-(4-ButC6H3-2-OH)-6-(CMeO)C5H3N as a pale yellow solid (1.146 g, 65%). Mp: 111−113 °C. 1H NMR (400 MHz, CDCl3): δ 1.30 (s, 9H, C(CH3)3), 2.70 (s, 3H, CCH3O), 6.95 (d, J = 8.5, 1H, Ar-H), 7.35 (dd, J = 8.5, 2.3, 1H, Ar-H), 7.75 (d, J = 2.3, 1H, Ar-H), 7.89−7.96 (m, 2H, Py-H), 8.07 (dd, J = 7.3, 1.8, 1H, Py-H), 13.38 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 25.1 (CMeO), 30.5 (C(CH3)3), 33.2 (C(CH3)3), 116.5 (C), 117.1, 118.6, 121.8, 121.9, 128.6, 137.7 (CH), 140.9, 149.1, 156.1, 156.7 (C), 196.6 (CMeO). IR (cm−1): 1702 (CO), 1588 (CNpyridine). ESIMS: m/z 270 [M + H]+. HRMS (FAB): calcd for C17H20NO [M + H]+ 270.1505, found 270.1505. Formation of Imine. Following a procedure similar to that described for L1 a -H, using 2-(4-Bu t C 6 H 3 -2-OH)-6-(CMe O)C5H3N (0.985 g, 3.66 mmol), 2,6-diisopropylaniline (0.927 g, 5.49 mmol, 1.5 equiv), and methanol (10 mL) gave L1b-H as a yellow solid (1.013 g, 65%). Mp: 120−122 °C. 1H NMR (400 MHz, CDCl3): δ 1.08 (d, J = 6.7, 12H, CH(CH3)2), 1.31 (s, 9H, C(CH3)3), 2.18 (s, 3H, CCH3N), 2.65 (sept, J = 6.7, 2H, CH(CH3)2), 6.92 (d, J = 8.8, 1H, Ar-H), 7.02−7.06 (m, 1H, Ar-H), 7.10 (s, 1H, Ar-H), 7.11 (d, J = 1.6, 1H, Ar-H), 7.34 (dd, J = 8.8, 2.6, 1H, Ar-H), 7.77 (d, J = 2.4, 1H, Ar-H), 7.92 (t, J = 7.6, 1H, Py-H), 7.98 (d, J = 7.6, 1H, Py-H), 8.22 (dd, J = 7.6, 0.8, 1H, Py-H), 13.85 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 17.4 (CMeN), 21.9, 22.2 (CH(CH3)2), 27.3 (CH(CH3)2), 30.5 (C(CH3)3), 33.2 (C(CH3)3), 116.9, 118.2, 120.2, 121.8, 122.0, 122.9, 128.1 (CH), 134.7, 134.8 (C), 137.3 (CH), 140.7, 145.0, 152.2, 156.1, 156.3 (C), 163.9 (CMeN). IR (cm−1): 1640 (CNimine), 1566 (CNpyridine). ESIMS: m/z 429 [M + H]+. HRMS (FAB): calcd for C29H37NO2 [M + H]+ 429.2906, found 429.2917. Synthesis of L1c-H. Formation of Ketone. Following a procedure similar to that described for L1a-H, using 2-bromo-6-acetylpyridine (1.44 g, 7.23 mmol), 2-hydroxy-4-chlorophenylboronic acid (1.50 g, 8.72 mmol), tetrakis(triphenylphosphine)palladium(0) (0.334 g, 0.289 mmol), and aqueous 2 M potassium carbonate (7.20 mL, 14.40 mmol)

gave 2-(4-ClC6H3-2-OH)-6-(CMeO)C5H3N as a pale yellow solid (84%, 1.496 g). Mp: 98−100 °C. 1H NMR (400 MHz, CDCl3): δ 2.70 (s, 3H, CCH3O), 6.94 (d, J = 8.8, 1H, Ar-H), 7.23 (dd, J = 8.8, 2.4, 1H, Ar-H), 7.73−7.74 (d, J = 2.4, 1H, Ar-H), 7.95−8.02 (m, 3H, PyH), 13.64 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 25.1 (CMeO), 118.2 (C), 119.0, 119.4, 121.8, 123.0, 125.1, 130.8, 138.0 (CH), 149.0, 154.9, 157.1 (C), 196.0 (CMeO). IR (cm−1): 1698 (CO), 1586 (CNpyridine). ESIMS: m/z 248 [M + H]+. HRMS (FAB): calcd for C13H11NO2Cl [M + H]+ 248.0478, found 248.0473. Formation of Imine. Following a procedure similar to that described for L1a-H, using 2-(4-ClC6H3-2-OH)-6-(CMeO)C5H3N (1.38 g, 5.58 mmol), 2,6-diisopropylaniline (1.480 g, 8.37 mmol, 1.5 equiv), and methanol (10 mL) gave L1c-H as a yellow solid (1.290 g, 57%). Mp: 189−191 °C. 1H NMR (400 MHz, CDCl3): δ 1.08 (d, J = 7.0, 12H, CH(CH3)2), 2.17 (s, 3H, CCH3N), 2.62 (sept, J = 6.9, 2H, CH(CH3)2), 6.91 (d, J = 8.8, 1H, Ar-H), 7.03−7.07 (m, 1H, ArH), 7.10 (s, 1H, Ar-H), 7.11 (dd, 1H, Ar-H, J 8.8, J 2.0), 7.22 (dd, J = 8.8, 2.6, 1H, Ar-H), 7.83 (d, J = 2.6, 1H, Ar-H), 7.92−7.07 (m, 2H, PyH), 8.28 (dd, J = 6.1, 2.6, 1H, Py-H), 14.17 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 17.3 (CCH3N), 22.9, 22.2 (CH(CH3)2), 28.4 (CH(CH3)2), 119.8, 119.9, 112.0 (CH), 120.4 (C), 123.1, 123.0, (CH), 124.0 (C), 126.0, 131.4 (CH), 135.6 (C), 138.7 (CH), 145.9, 153.3, 155.6, 158.2 (C), 164.5 (CMeN). IR (cm−1): 1641 (CNimine), 1586 (CNpyridine). ESIMS: m/z 407 [M + H]+. HRMS (FAB): calcd for C25H28NO2Cl [M + H]+ 407.1890, found 407.1895. Synthesis of L1d-H. Formation of Ketone. Following a procedure similar to that described for L1a-H, using 2-bromo-6-acetylpyridine (1.57 g, 7.85 mmol), 2-hydroxy-4-fluorophenylboronic acid (1.50 g, 9.62 mmol), tetrakis(triphenylphosphine)palladium(0) (0.362 g, 0.314 mmol), and aqueous 2 M potassium carbonate (7.85 mL, 15.70 mmol) gave 2-(4-FC6H3-2-OH)-6-(CMeO)C5H3N as a pale yellow solid (85%, 1.52 g). Mp: 188−189 °C. 1H NMR (400 MHz, CDCl3): δ 2.76 (s, 3H, CMeO), 6.92−6.99 (m, 1H, Ar-H), 7.0−7.07 (m, 1H, ArH), 7.45 (dd, J = 9.7, 2.9, 1H, Ar-H), 8.03−8.13 (m, 3H, Py-H), 13.37 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 25.1 (CMe=O), 111.1 (d, JCF = 24, CH), 117.4 (d, JCF = 7, C), 118.1 (d, JCF = 24, CH), 118.6 (d, JCF = 7, CH), 119.3, 121.9, 138.0 (CH), 149.2, 154.6 (C), 154.9 (d, JCF = 236, CF), 155.2 (C), 196.2 (CMeO). 19F{1H} (NMR (188 MHz, CDCl3): δ −124.54 (Ar-F). IR (cm−1): 1698 (C O), 1590 (CNpyridine). ESIMS: m/z 232 [M + H]+. HRMS (FAB): calcd for C13H11NO2F [M + H]+ 232.0774, found 232.0766. Formation of Imine. Following a procedure similar to that described for L1a-H, using 2-(4-FC6H3-2-OH)-6-(CMeO)C5H3N (1.31 g, 5.67 mmol), 2,6-diisopropylaniline (1.51 g, 8.51 mmol, 1.5 equiv), and methanol (10 mL) gave L1d-H as a yellow solid (0.994 g, 45%). Mp: 85−89 °C. 1H NMR (400 MHz, CDCl3): δ 1.07 (d, J = 6.9, 12H, CH(CH3)2,), 2.17 (s, 3H, NCCH3), 2.58−2.69 (sept, J = 6.9, 2H, CH(CH3)2), 6.89−6.92 (m, 1H, Ar-H), 6.95−6.99 (m, 1H, Ar-H), 7.02−7.06 (m, 1H, Ar-H), 7.09 (s, 1H, Ar-H), 7.10 (d, J = 8.4, 1H, Ar-H), 7.45 (dd, J = 9.8, 3.0, 1H, Ar-H), 7.86 (d, J = 8.0, 1H, PyH), 7.92 (t, J = 7.8, 1H, Py-H), 8.27 (dd, J = 7.6, 0.8, 1H, Py-H), 13.98 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 16.3 (NCCH3), 21.8, 22.2, 27.4, 111.1 (d, JCF = 23.5, CH), 117.6 (d, JCF = 23.5, CH), 117.8 (d, JCF 7.5, C), 118.3 (d, JCF = 7.5, CH), 118.9 (CH), 122.1 (CH), 123.0 (CH), 134.6 (C), 137.6 (CH), 144.9 (C), 152.3 (C), 154.8 (d, JCF = 234.0, C), 154.7 (C), 154.8 (C), 156.0 (C), 163.5 (C N). 19F{1H} NMR (188 MHz, CDCl3): δ −125.1 (Ar-F). IR (cm−1): 1645 (CNimine), 1587 (CNpyridine). ESIMS: m/z 391 [M + H]+. HRMS (FAB): calcd for C25H28NO2F [M + H]+ 391.2186, found 391.2190. Synthesis of Complexes 1a−d. Preparation of 1a. An ovendried Schlenk vessel equipped with a magnetic stir bar was evacuated and back-filled with nitrogen. The vessel was charged with L1a-H (0.220 g, 0.591 mmol) and dissolved in dry toluene (15 mL). Trimethylaluminum (0.60 mL, 1.18 mmol, 2 equiv) was introduced and the reaction mixture stirred and heated to 110 °C overnight. After the mixture was cooled to room temperature, all volatiles were removed under reduced pressure. Acetonitrile (15 mL) was introduced, the suspension was heated until dissolution, and the 256

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resulting solution was transferred by cannular filtration into a second oven-dried Schlenk vessel. On standing at room temperature pale yellow crystals of 1a formed (0.189 g, 68%). 1H NMR (300 MHz, C6D6): δ −0.17 (s, 3H, Al-CH3), 0.67 (s, 3H, Al-NCCH3), 1.37−1.55 (m, 12H, CH(CH3)2), 1.63 (br s, 3H, NC(CH3)2), 1.88 (br s, 3H, NC(CH3)2), 3.80 (br s, 1H, Ar-CH(CH3)2), 4.47 (br s, 1H, ArCH(CH3)2), 6.93 (dd, J = 8.0, 0.8, 1H, Ar-H), 7.00 (t, J = 8.2, 1H, ArH), 7.20 (d, J = 8.0, 1H, Ar-H), 7.25−7.31 (m, 4H, Ar-H), 7.49 (t, J = 8.0, 1H, Py-H), 7.58 (dd, J = 8.0, 1.2, 1H, Py-H), 7.74 (dd, J = 8.0, 1.8, 1H, Py-H). 13C{1H} NMR (100 MHz, CDCl3): δ −9.2 (Al-CH3), 1.2 (Al-NCCH3), 23.5, 24.3, 25.9, 26.2, 26.9, 27.5, 28.3, 34.9, 61.5 (NC(CH3)2), 115.3 (C), 115.9 (CH), 117.3 (CH), 118.6 (CH), 122.5 (C), 123.4 (CH), 123.6 (CH), 123.9 (CH), 127.1 (C), 127.9 (CH), 128.0 (CH), 137.7 (C), 139.1 (C), 146.6 (C), 154.1 (C), 163.6 (C), 169.1 (C). FABMS: m/z 429 [M − NCMe]+, 413 [M − NCMe − Me]+. IR (cm−1): 2322 ν(CN)MeCN. Anal. Calcd for C29H36AlN3O: C, 74.17; H, 7.73; N, 8.95. Found: C, 74.55; H, 7.81; N, 9.01. Preparation of 1b. The synthesis was carried out by using the same procedure as for 1a, except L1b-H (0.253 g, 0.591 mmol) was used, forming 1b as pale yellow crystals (0.116 g, 40%). 1H NMR (400 MHz, C6D6): −0.07 (s, 3H, Al-CH3), 0.60 (s, 3H, Al-NCCH3), 1.42− 1.62 (m, 12H, CH(CH3)2), 1.63 (s, 9H, C(CH3)3), 1.74 (br s, 3H, NC(CH3)2), 1.99 (br s, 3H, NC(CH3)2), 3.88 (br s, 1H, ArCH(CH3)2), 4.61 (br s, 1H, Ar-CH(CH3)2), 7.07 (dd, J = 7.8, 0.8, 1H, Ar-H), 7.36 (d, J = 7.8, 1H, Ar-H), 7.39 (d, J = 7.0, 1H, Py-H), 7.42− 7.51 (m, 5H, ArH/PyH), 8.03 (s, 1H, Ar-H). 13C{1H} NMR (100 MHz, CDCl3): δ −8.6 (Al−CH3), 1.4 (Al-NCCH3), 24.0, 25.9, 26.2, 26.5, 27.3, 27.9, 28.7, 31.6, 34.3, 35.5, 60.1 (N-C(CH3)2), 115.7 (C), 118.9 (C), 121.9 (C), 123.5, 123.6, 123.7, 130.6, 139.4, 139.5 (CH), 147.4, 151.2, 154.09, 161.9, 169.6 (C). IR (cm−1): 2324 ν(CN)MeCN. FABMS: m/z 484 [M − NCMe]+, 469 [M − NCMe − Me]+. Anal. Calcd for C33H44AlN3O: C, 75.39; H, 8.44; N, 7.99. Found: C, 75.27; H, 8.40; N, 7.87. Preparation of 1c. The synthesis was carried out by using the same procedure as for 1a, except L1c-H (0.241 g, 0.591 mmol) was used, forming 1c as pale yellow crystals (0.260 g, 86%). 1H NMR (400 MHz, C6D6): δ −0.12 (s, 3H, Al-CH3), 0.81 (s, 3H, Al-NCCH3), 1.42−1.62 (m, 12H, CH(CH3)2), 1.72 (br s, 3H, NC(CH3)2), 1.96 (br s, 3H, NC(CH3)2), 3.84 (br s, 1H, Ar-CH(CH3)2), 4.56 (br s, 1H, ArCH(CH3)2), 6.99 (d, J = 8.0, 1H, Ar-H), 7.04 (d, J = 7.6, 1H, Ar-H), 7.28 (app q, J = 7.0, 2H, Py-H/Ar-H), 7.38−7.50 (m, 4H, ArH/PyH), 7.89 (d, J = 2.9, 1H, Ar-H). 13C{1H} NMR (100 MHz, CDCl3, limited solubility): δ −14.3 (Al-CH3), −1.39 (Al-NCCH3), 60.1 (NC(CH3)2), 114.4, 115.3, 117.4, 122.1, 122.6, 125.5, 131.0, 137.8. IR (cm−1): 2322 ν(CN)MeCN. FABMS: m/z 463 [M − NCMe]+, 445 [M − NCMe − Me]+. Anal. Calcd for C29H35AlClN3O: C, 69.10; H, 7.00; N, 8.34. Found: C, 68.97; H, 7.05; N, 8.28. Preparation of 1d. The synthesis was carried out by using the same procedure as for 1a, except L1d-H (0.231 g, 0.591 mmol) was used, forming 1d as orange-yellow crystals (0.210 g, 73%). 1H NMR (400 MHz, C6D6): δ −0.18 (s, br, 3H, Al-Me), 0.78 (s, br, 3H, NCMe), 1.30−1.51 (m, 12H, CHMe2), 1.68 (br s, 3H, NC(CH3)2), 1.90 (br s, 3H, NC(CH3)2), 3.70 (br s, 1H, CHMe2), 4.46 (br s, 1H, CHMe2), 6.91 (d, J = 8.0, 2H, Ar-H), 7.14−7.30 (m, 4H, Ar-H/Py-H), 7.33− 7.38 (m, 1H, Ar-H/Py-H), 7.39−7.44 (m, 1H, Ar-H/Py-H), 7.53 (dd, J = 9.6, 3.0, 1H, Ar-H/Py-H). 13C{1H} NMR (100 MHz, CDCl3): δ −9.3 (Al-CH3), 1.2 (Al-NCCH3), 23.5, 24.2, 25.9, 26.1, 26.9, 27.5, 28.3, 34.8, 61.7 (N-C(CH3)2), 112.4 (d, JCF = 23, CH), 115.4, 118.7 (C), 119.5 (d, JCF = 24, CH), 122.1 (d, JCF = 8, C), 123.4, 124.0 (CH), 124.3 (d, JCF = 8, CH), 128.0, 128.1, 128.3 (CH), 129.1 (C), 139.2 (CH), 146.5, 152.8 (C), 156.1 (d, JCF = 234, C), 159.8, 169.1 (C). 19 1 F{ H} NMR (188 MHz, CDCl3): δ −127.9 (Ar-F). IR (cm−1): 2323 ν(CN)MeCN. FABMS: m/z 447 [M − NCMe]+, 432 [M − NCMe − Me]+. Anal. Calcd for C29H35AlFN3O: C, 71.44; H, 7.24; N, 8.62. Found: C, 71.21; H, 7.37; N, 8.50. Synthesis of 2a−c. Preparation of 2a. An oven-dried Schlenk vessel equipped with a magnetic stir bar was evacuated and back-filled with nitrogen. The vessel was charged with L1e-H (0.212 g, 0.591 mmol) and dissolved in dry toluene (15 mL). Trimethylaluminum (0.60 mL, 1.18 mmol, 2 equiv) was introduced and the reaction

mixture stirred and heated to 110 °C overnight. After the mixture was cooled to room temperature, all volatiles were removed under reduced pressure. Acetonitrile (15 mL) was introduced, the suspension was heated until dissolution, and the resulting solution was transferred by cannular filtration into a second oven-dried Schlenk vessel. On standing at room temperature pale yellow crystals of 2a formed (0.140 g, 52%). 1H NMR (400 MHz, C6D6): δ −0.06 (s, 3H, Al-CH3), 0.87 (m, 6H, Al-NCCH3/Ar-CHMeaMea′), 1.44 (d, J = 6.5, 3H, ArCHMeaMea′), 1.53 (d, J = 6.6, 3H, Ar-CHMebMeb′), 1.64 (d, J = 7.0, 3H, NCHCH3), 1.86 (d, J = 6.6, 3H, Ar-CHMebMeb′), 4.09 (sept, J = 6.7, 1H, Ar-CHMe2), 4.34 (sept, J = 6.7, 1H, Ar-CHMe2), 5.26 (q, J = 6.5, 1H, NCHCH3), 6.94−7.03 (m, 2H, Ar-H/Py-H), 7.09−7.20 (m, 2H, Py-H/Ar-H), 7.25 (dd, J = 7.4,1.9, 1H, Py-H), 7.32−7.42 (m, 3H, Ar-H/Py-H), 7.52 (dd, J = 7.7,1.8, 1H, Ar-H), 7.85 (d, J = 7.8, 1H, ArH). IR (cm−1): 2327 ν(CN)MeCN. FABMS: m/z 415 [M − NCMe]+, 374 [M − NCMe − Me]+. Anal. Calcd for C28H34AlN3O: C, 73.82; H, 7.52; N, 9.22. Found: C, 74.01; H, 7.62; N, 9.15. Preparation of 2b. The synthesis was carried out by using the same procedure as for 2a, except L1f-H (0.245 g, 0.591 mmol) was used, forming 2b as pale yellow crystals (0.121 g, 40%). 1H NMR (400 MHz, C6D6): δ −0.05 (s, 3H, Al-CH3), 0.81 (s, 3H, Al-NCCH3), 0.83 (d, J = 7.1, 3H, Ar-CHMeaMea′), 1.44 (d, J = 6.9, 3H, Ar-CHMeaMea′), 1.50 (s, 9H, C(CH3)3), 1.52 (d, J = 6.8, 3H, Ar-CHMebMeb′), 1.54 (d, J = 5.6, 3H, NCHCH3), 1.87 (d, J = 6.4, 3H, Ar-CHMebMeb′), 4.11 (sept, J = 6.9, 1H, Ar-CHMe2), 4.30 (sept, J = 7.1, 1H, Ar-CHMe2), 5.32 (q, J = 6.5, 1H, NCHCH3), 6.95−7.04 (m, 3H, Ar-H/Py-H), 7.11 (dd, J = 6.4, 2.3, 1H, Py-H/Ar-H), 7.30−7.41 (m, 2H, Ar-H/Py-H), 7.48 (d, J = 7.8, 1H, Ar-H), 7.62 (d, J = 2.4, 1H, Ar-H), 7.77 (d, J = 8.0, 1H, Ar-H). IR (cm−1): 2296 ν(CN)MeCN. FABMS: m/z 471 [M − NCMe]+, 455 [M − NCMe − Me]+. Anal. Calcd for C32H42AlN3O: C, 75.11; H, 8.27; N, 8.21. Found: C, 75.32; H, 8.41; N, 8.03. Preparation of 2c. The synthesis was carried out by using the same procedure as for 2a, except L1g-H (0.232 g, 0.591 mmol) was used, forming 2c as pale yellow crystals (0.191 g, 66%). 1H NMR (400 MHz, C6D6): δ −0.10 (s, 3H, Al-CH3), 0.85 (d, J = 7.8, 3H, ArCHMeaMea′), 0.89 (s, 3H, Al-NCCH3), 1.42 (d, J = 7.8, 3H, ArCHMeaMea′), 1.50 (d, J = 7.7, 3H, NCHCH3), 1.58 (d, J = 7.7, 3H, ArCHMebMeb′), 1.77 (d, J = 7.7, 3H, Ar-CHMebMeb′), 4.05 (sept, J = 7.6, 1H, Ar-CHMe2), 4.29 (sept, J = 7.7, 1H, Ar-CHMe2), 5.25 (q, J = 7.6, 1H, NCHCH3), 6.98 (d, J = 7.2, 1H, Ar-H/Py-H), 7.00 (d, J = 7.3, 1H, Py-H/Ar-H), 7.09 (d, J = 6.8, 1H, Ar-H/Py-H), 7.18−7.23 (m, 2H, Ar-H), 7.28 (t, J = 6.9, 1H, Ar-H/Py-H), 7.35 (d, J = 6.7, 1H, ArH), 7.65 (d, J = 1.8, 1H, Ar-H), 7.75 (d, J = 6.8, 1H, Ar-H). IR (cm−1): 2292 ν(CN)MeCN. FABMS: m/z 449 [M − NCMe]+, 433 [M − NCMe − Me]+. Anal. Calcd for C28H33AlClN3O: C, 68.63; H, 6.79; N, 8.58. Found: C, 68.64; H, 6.62; N, 8.37. Ring-Opening Polymerization (ROP) of ε-CL. Typical polymerization procedures (Tables 5 and 6) are as follows. An oven-dried Schlenk vessel equipped with a stirrer bar was evacuated and loaded in the glovebox. The aluminum pro-initiator 1 or 2 (0.04 mmol) was introduced, followed by 15 mL of a 0.0027 M solution of benzyl alcohol (0.04 mmol, 1 equiv) in toluene. The mixture was stirred for 10 min at room temperature and then placed in an oil bath preheated to the desired temperature. CL (1.1 mL, 10.0 mmol, 250 equiv) was added and the mixture stirred for the designated time period. A small aliquot (0.2 mL) of the reaction mixture was removed at selected time intervals and treated with a few drops of methanol and the residue analyzed by 1H NMR spectroscopy (to determine monomer conversion) and by GPC (to determine Mn and Mw/Mn). Crystallographic Studies. Data for L1d-H, L1g-H, 1a−d, 2a, 2b/ 2b′, 2c, and 3 were collected on a Bruker APEX 2000 CCD diffractometer. Details of data collection, refinement and crystal data are given in Table S1 in the Supporting Information. The data were corrected for Lorentz and polarization effects and empirical absorption corrections applied. Structure solution by direct methods and structure refinement based on full-matrix least squares on F2 employed SHELXTL version 6.10.30 Hydrogen atoms were included in calculated positions (C−H = 0.95−1.00 Å) riding on the bonded atom with isotropic displacement parameters set to 1.5[Ueq(C)] for methyl H atoms and 1.2[Ueq(C)] for all other H atoms. All non-H 257

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atoms were refined with anisotropic displacement parameters. 2b/2b′ shows disorder at C(14) and has been split (50:50), while the tertbutyl methyl groups are also disordered. CCDC reference numbers 905972−905981.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 and CIF files giving X-ray crystallographic data for L1d-H, L1g-H, 1a−d, 2a, 2b/2b′, 2c, and 3 along with figures giving representative 1H, 13C{1H}, and 19F{1H} NMR, MALDI-ToF, and FAB mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +44(0)116 2522096. Fax: +44 (0)116 2523789. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC (to C.J.D. and A.P.A.) and the University of Leicester for financial assistance. We are also grateful to Johnson-Matthey for the loan of the palladium salts.

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REFERENCES

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Organometallics

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dx.doi.org/10.1021/om301057d | Organometallics 2013, 32, 249−259