Synthesis of Aluminum Complexes of Triaza Framework Ligands and

Article pubs.acs.org/Organometallics

Synthesis of Aluminum Complexes of Triaza Framework Ligands and Their Catalytic Activity toward Polymerization of ε‑Caprolactone K. Bakthavachalam and N. Dastagiri Reddy* Department of Chemistry, Pondicherry University, Pondicherry 605014, India S Supporting Information *

ABSTRACT: The synthesis and characterization of 1,5bis(2,6-diisopropylphenyl)-2,4-diphenyl-1,3,5-triazapenta-1,3diene (L1H), a triaza ligand, and Al complexes of its anion are reported. A neat condensation reaction between N-(Dipp)benzamidine (Dipp = 2,6-diisopropylphenyl) and N-(Dipp)benzimidoyl chloride affords L1H in good yield. The Al complexes [L1AlMe2] (1), [L1AlMe(Cl)] (2), and [L1AlCl2] (3) are prepared by treating L1H with a slight excess of AlMe3, AlMe2Cl, and AlMeCl2, respectively, in toluene. Further, the aluminum complexes [L2AlMe2] (5), [L2AlMe(Cl)] (6), and [L2AlCl2] (7) are obtained in good yields from 1,3-bis(2pyridylimino)isoindoline (L2H) in a similar fashion. The ligand L1H and complexes 1, 2, and 4−6 have been structurally characterized. All of the complexes have been explored for their catalytic activity toward the ring-opening polymerization (ROP) of ε-caprolactone. It has been found that [L1AlMe2], upon the addition of cocatalyst (benzyl alcohol), gives a tetranuclear Al alkoxide (8), which is highly efficient in catalyzing the ROP of ε-caprolactone. [L2AlMe2] has also been found to be a good catalyst. The crystal structure of 8 and the catalytic activities of all the complexes in detail are reported.

■

INTRODUCTION Poly(caprolactone) (PCL) is a synthetic biodegradable and biocompatible polymer which has a wide range of applications in agriculture and the biomedical and pharmaceutical industries.1 The most common route for the synthesis of PCL is ring-opening polymerization (ROP) of caprolactone using a catalyst. Several catalysts based on main group as well as transition metals have been reported to catalyze the ROP of caprolactone.2 Among them, Al-based catalysts have been promising because they are highly active and inexpensive. βDiketiminate ligands have been widely employed in developing efficient homogeneous catalysts for carrying out various transformations,3 including the ROP of caprolactone.4 However, triazapentadienate ligands, which closely resemble βdiketiminates (Figure 1), have been comparatively less explored.5 In our continuing efforts in exploring Al chemistry,6 especially the structural diversity in triazaalane networks, we have realized that there have been no reports on Al complexes of triazapentadienate ligands, though there have been a good number of Al complexes of triaza framework ligands, such as

bis(2-pyridyl)amine7 and phthalocyanin,8 reported in the literature. A report on cationic Al complexes of a quaternary 1,3,5-triazapenta-1,3-diene has also appeared.9 Consequently, we have synthesized a tertiary 1,3,5-triazapenta-1,3-diene ligand and its Al complexes. The catalytic activity of these complexes toward polymerization of caprolactone has also been explored and compared with that of Al complexes of another triaza framework ligand, 1,3-bis(2-pyridylimino)isoindolinate.10 The details of these reactions and observations are reported herein.

Figure 1.

Received: November 26, 2012

© XXXX American Chemical Society

■

RESULTS AND DISCUSSION Synthesis and Structural Characterization. Recently, Würthwein and his co-workers have synthesized tertiary 1,3,5triazapenta-1,3-dienes by modifying a literature procedure,5c which utilizes a condensation reaction between imidoyl chlorides (RC(Cl)NR) and N-substituted amidines (RC(NH)NHR) in diethyl ether. However, they have observed that this method is not effective when aromatic imidoyl chlorides are used. Subsequently, they have found an alternate route, which involves the synthesis of imidoylimidoates (RC(OEt)NC(R)NR)) and their reaction with primary amines. This two-step process suffers from low overall yield. Therefore, we decided to revisit the condensation reaction between imidoyl chlorides and N-substituted amidines, which are easily accessible. In a neat reaction, an equimolar

A

dx.doi.org/10.1021/om3011432 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

mixture of N-(Dipp)benzamidine (Dipp = 2,6-diisopropylphenyl) and N-(Dipp)benzimidoyl chloride was heated to 160 °C for 2.5 h to afford 1,5-bis(2,6-diisopropylphenyl)-2,4diphenyl-1,3,5-triazapenta-1,3-diene (L1H) (Scheme 1) in Scheme 1. Synthesis of 1,5-Bis(2,6-diisopropylphenyl)-2,4diphenyl-1,3,5-triazapenta-1,3-diene (L1H)

Figure 3. Single-crystal X-ray structure of 1. All hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level.

Figure 2. Single-crystal X-ray structure of L1H. All hydrogen atoms, except that on N1, are omitted for clarity. Thermal ellipsoids are drawn at the 40% probability level. Figure 4. Single-crystal X-ray structure of 2. All hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level.

Scheme 2. Synthesis of Aluminum and Lithium Complexes of L1

1,3-dienes similar to L1H have been reported in the literature,5a−e and interestingly, none of them show intramolecular hydrogen bonding. An ORTEP diagram along with selected bond distances and bond angles is given in Figure 2. The Al complexes [L1AlMe2] (1), [L1AlMe(Cl)] (2), and [L1AlCl2] (3) were prepared by refluxing L1H with a slight excess of AlMe3, AlMe2Cl, and AlMeCl2, respectively, in toluene. Our initial efforts to synthesize the complex [L1AlCl2] (3) by treating the lithiated ligand [(L1Li)2] or [L1Li(THF)3] with AlCl3 were unsuccessful. These reactions yielded large amounts of precipitate, which is presumably a polymeric substance, insoluble in common organic solvents. Using 2 equiv of AlMe3 and 1 equiv of L1H resulted in the formation of [L1AlMe2(AlMe3)] (4), in which AlMe3 is coordinated by the central N atom. An illustration of the synthesis of these complexes is given in Scheme 2. 4 is found to react with chloroform, and hence its NMR spectra have been recorded in toluene-d8. The 1H NMR spectrum at room temperature is not informative and contains very broad resonances (Supporting Information, Figure S1). As the temperature of the sample decreases the peaks resolve and at −70 °C, a well resolved spectrum, though quite complicated, has been obtained (Supporting Information, Figure S2). Interestingly, the peaks also resolve as the temperature increases above room temperature. A simple and well-resolved spectrum can be obtained at 80 °C (Supporting Information, Figure S3). As can be seen in the spectrum (80 °C), the AlMe3 group shows two

good yield (67%). L1H was characterized by NMR and single-crystal X-ray techniques. The molecular structure showed the triazapentadiene frame as a planar ring with terminal nitrogen atoms held together by intramolecular hydrogen bonding. Crystal structures of a few tertiary 1,3,5-triazapentaB


Organometallics

Article

Figure 7. Single-crystal X-ray structure of [L1Li(THF)3]. All hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å): N1−Li1 = 2.052(6), N1− C2 = 1.319(4), N2−C2 = 1.343(4), N2−C1 = 1.343(4), N3−C1 = 1.295(4), O1−Li1 2.011(7), O2−Li1 = 1.928(6), O3−Li1 2.004(6). Selected bond angles (deg): C2−N1−Li1 = 133.8(3), C2−N2−C1 = 120.9(3), N3−C1−N2 = 125.7(3), O1−Li1−N1 = 122.1(3). O2− Li1−N1 = 112.8(3), O3−Li1−N1 = 115.5(3), O2−Li1−O3 = 102.2(3), O3−Li1−O1 = 97.7(3).

Figure 5. Single-crystal X-ray structure of 4. All hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Labels of symmetry equivalent atoms have been changed for easy comparison of bond parameters with those of L1H, 1, and 2 (Table 1).

Figure 6. Single-crystal X-ray structure of [(L1Li)2]. The diethyl ether molecule and all hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å): N6−C5 = 1.296(3), N6−Li1 = 1.991(5), N1−Li1 = 1.978(5), N1−C2 = 1.297(3), N2−C2 = 1.371(3), N2−Li1 2.164(5), N2−C3 = 1.370(3), N3−C3 = 1.296(3), N2−Li2 2.125(5), N3−Li2 = 1.996(5), N4−Li2 = 1.988(5), N4−C4 = 1.298(3), N5−Li2 2.150(5), N5−C5 = 1.368(3), N5−C4 = 1.376(3), N5−Li1 2.108(5). Selected bond angles (deg): C5−N6−Li1 = 92.6(2), C3−N3−Li2 = 92.8(2), C4−N4−Li2 = 93.7(2), C2−N1−Li1 = 94.6(2), C5−N5−C4 = 122.3(2), C5−N5− Li2 = 146.1(2), C5−N5−Li1 = 85.65(19), C4−N5−Li2 = 84.73(19), C4−N5−Li1 = 147.0(2), Li1−N5−Li2 = 77.86(18), C2−N2−Li2 = 147.4(2), C2−N2−Li1 = 84.63(18), C3−N2−C2 = 122.1(2), C3− N2−Li2 = 85.35(19), C3−N2−Li1 = 147.6(2), Li2−N2−Li1 = 77.20(18).

Figure 8. Single-crystal X-ray structure of 5. All hydrogens are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å): Al1−N1 = 2.1351(18), Al1−N5 = 2.1267(18), Al1−N3 = 1.9234(18), Al1−C1 = 1.987(2), Al1−C2 = 1.981(2), N1−C3 = 1.349(3), C3−N2 = 1.382(3), N2−C4 = 1.296(3), C4−N3 = 1.394(3), N3−C5 = 1.405(3), C5−N4 = 1.292(3), N4−C6 = 1.380(3), C6−N5 = 1.347(3). Selected bond angles (deg): C1−Al1−C2 = 122.27(10), C1−Al1−N1 = 91.74(8), C2−Al1−N1 = 90.81(8), N1−C3−N2 = 122.62(18), C3−N2−C4 = 123.64(18), N2−C4−N3 = 131.15(19), C4−N3−C5 = 106.81(16), N3−C5−N4 = 131.4(2), C5−N4−C6 = 124.14(18), N4−C6−N5 = 122.51(19), C6−N5−Al1 = 128.05(15), N5−Al1−C2 = 90.12(9), N5−Al1−C1 = 93.18(8), N5−Al1−N3 = 87.28(7), N3−Al1−N1 = 86.71(7), N3−Al1−C2 = 120.26(9), N3−Al1−C1 = 117.47(9), N5− Al1−N1 = 173.52(8).

peaks in a 1:2 ratio. While one of the methyl groups appears in the downfield region (δ 0.23 ppm, 3H), the other two appear in the upfield region (δ −0.83 ppm, 6H) in comparison to AlMe2 protons (δ −0.38 ppm). This behavior can be attributed to the orientation of methyl groups (AlMe3) with respect to the phenyl substituents, coupled with hindered rotation of the Me3Al−N bond. Though the NMR spectra of these complexes are quite conclusive of their formation, single-crystal X-ray structures of 1, 2, and 4 were obtained for further confirmation. Molecular structures are depicted in Figures 3−5. Important bond parameters are given in Table 1. The structures of 1, 2, and

4 contain a six-membered C2N3Al ring with similar bond lengths. In 1 and 2, except for Al, all the other atoms in the ring are almost in one plane. However, in 4, the central N atom also deviates significantly from the plane, presumably due to the steric repulsion between AlMe3, to which it is coordinated, and the phenyl substituents on the adjacent C atoms. The structural parameters of 1, 2, and 4 have been compared with those of similar β-diketiminate Al complexes reported, and it is found that Al in 1 has deviated from the mean plane of the rest of the ring atoms (0.825 Å) slightly more than that of a similar βC


Organometallics

Article

Table 1. Selected Bond Lengths and Bond Angles for L1H, 1, 2, and 4 L1H N1−C1 N2−C1 N2−C2 N3−C2 Al1−N3 Al1−N1 Al1−C4 Al1−C3 Al1−Cl1 Al2−N2 Al2−C5 Al2−C6

1.324(2) 1.341(2) 1.359(2) 1.316(2)

N1−C1−N2 C1−N2−C2 N3−C2−N2 N3−Al1−N1 N3−Al1−C4 N3−Al1−C3 N1−Al1−C4 N1−Al1−C3 C4−Al1−C3 C1−N1−Al1 C2−N3−Al1 N1−Al1−Cl1 N3−Al1−Cl1 C5−Al2−N2 C6−Al2−N2 C5−Al2−C6

123.03(17) 122.68(16) 122.54(17)

1

2

Bond Lengths (Å) 1.322(3) 1.352(3) 1.344(3) 1.329(3) 1.924(2) 1.938(2) 1.950(3) 1.958(3)

1.333(4) 1.339(4) 1.341(4) 1.338(4) 1.899(3) 1.893(3) 2.002(3) 2.126(13)

4 1.319(2) 1.3827(18) 1.3827(18) 1.319(2) 1.9415(14) 1.9415(14) 1.971(3) 1.955(3) 2.076(2) 1.975(2) 1.963(3)

Bond Angles (deg) 124.9(2) 126.1(2) 124.8(2) 93.05(8) 111.65(11) 110.92(11) 116.99(11) 106.78(11) 115.29(13) 118.13(16) 117.01(16)

124.3(3) 127.2(3) 124.3(3) 95.02(11) 119.02(12) 113.46(12) 117.7(2) 118.4(2) 108.72(9) 105.57(9)

123.04(15) 123.69(19) 123.04(15) 92.61(9) 107.00(7) 109.97(8) 107.00(7) 109.97(8) 125.34(13) 115.60(11) 115.60(11)

113.28(12) 103.93(7) 112.25(8)

diketiminate complex (0.721,11a 0.729 Å11b). It is also observed that the N−Al−N bond angle is somewhat smaller in 1 and 4 (∼93°) than in β-diketiminate complexes (∼96°).11 However, it is quite similar to those found in cationic Al complexes of a quaternary 1,3,5-triazapenta-1,3-diene ligand (N−Al−N = ∼92°) reported in the literature.9 While the Al−N bond distances in 1 and 4 are comparable with those in the symmetric [((Dipp)N(C(Me)N(Dipp))2)AlI2]+ complex, Al− C bond distances are like those found in [(Dipp)NC( CH2)(N(Dipp))(C(Me)N(Dipp)AlMe2]. A dried sample of the lithium complex made in ether medium showed no coordinated solvent, while that made in THF medium showed three THF molecules in 1H NMR. This observation generated curiosity about their structures, and therefore we obtained single-crystal X-ray structures of both. These structures reveal that the lithium complex obtained from ether, [(L1Li)2], exists as a dimer, as depicted in Figure 6. Though there are two ether molecules present in the asymmetric unit, they do not coordinate to Li ions. Each Li is coordinated by four N atoms, two from each ligand molecule and found in a highly distorted tetrahedral geometry. Basically the ligand molecules adopt a “W” shape and the Li ions are sandwiched between two ligand molecules. There are two articles on similar structures with triaza ligands having guanidine moieties reported in the literature.12 However, it has been reported that triaza ligands bearing perfluoroalkyl groups in α positions form six-membered chelate Li complexes, similar to the case for β-diketiminate ligands.13 In contrast, the lithium complex that has been obtained from THF, [L1Li-

(THF)3], is a monomer. It is a linear molecule with only one of the N atoms coordinated to the Li ion. The molecular structure of [L1Li(THF)3], along with selected bond parameters, is given in figure 7. 1,3-Bis(2-pyridylimino)isoindoline (L2H) was synthesized by following a literature procedure.10 The aluminum complexes of its anion, [L2AlMe2] (5), [L2AlMe(Cl)] (6), and [L2AlCl2] (7), were obtained in good yields by refluxing L2H for 4 h in toluene with an appropriate Al reagent as shown in Scheme 3. Its 1H Scheme 3. Synthesis of Aluminum Complexes of L2

NMR spectrum suggests a C2-symmetric structure, which can be envisaged by placing Al in a pentacoordinated geometry. This structure was further confirmed by undertaking singlecrystal X-ray studies for 5 and 6. Molecular structures along with important bond parameters of 5 and 6 are given in Figures 8 and 9, respectively. The crystal structures show Al in a slightly D


Organometallics

Article

the equatorial Al−N bond (5, Al1−N3 = 1.9234(18) Å; 6, Al1−N3 = 1.892(2) Å). The whole molecule, except Me and/ or Cl substituents on Al, is almost planar and hence a tight packing of the molecules (intermolecular shortest C···C contact is 3.358 Å in 5 and 3.380 Å in 6) has been observed. These kinds of short contacts, which are due to π−π interactions, are common in isoindoline complexes.14 Since these are the first structural reports on Al complexes of isoindolinate type ligands, the bond parameters were compared only with standard bonds and found to be in the same range. Polymerization Studies. Ring-opening polymerization (ROP) of ε-caprolactone (CL) using Al complexes 1−7 was carried out in the presence of benzyl alcohol as cocatalyst, and the results are given in Table 2. The polymer was characterized by NMR, and the Mn and Mw values were obtained from gel permeation chromatography (GPC). In this paper, the nomenclature for catalytic activities has been used as per the classification given in Redshaw’s review.2a Complex 1 was found to show high activity in initiating ROP of ε-caprolactone. It showed catalytic activity within 2 min of the addition of the monomer, and in 10 min, the conversion reached 97% for a [CL]/[Al] ratio of 200/1 (entry 1). Even when the [CL]/[Al] ratio was increased to 1000, a very good conversion (80%) was observed in 30 min (entry 5). The linear relationship between the [CL]/[Al] ratio and number-averaged molecular weight (Mn) that is shown in Figure 10 demonstrates the living nature of the polymerization. It is noteworthy that the neat reactions (entries 7−10) produced polymer with relatively low Mn and high Mw/Mn values, presumably because of the reaction mixture becoming highly viscous as the reaction progressed. Compounds 2 and 3 were found to be catalytically inactive toward ROP of ε-caprolactone. They yielded only traces of polymer even after several hours of stirring the reaction mixture at 80 °C (entries 12 and 13). Complex 4, which is an AlMe3 adduct of 1, showed a good activity (entries 14 and 15). The isoindolinate− AlMe2 complex 5 was quite slow in initiating ROP of ε-

Figure 9. Single-crystal X-ray structure of 6. All hydrogens are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å): Al1−N1 = 2.074(2), Al1−N5 = 2.081(2), Al1−N3 = 1.892(2), Al1−Cl1A= 2.141(2), Al1−Cl1B = 2.142(2), Al1−C1A= 2.187(9), Al1−C1B= 2.192(10), N1−C2 = 1.348(3), C2− N2 = 1.375(4), N2−C3 = 1.296(4), C3−N3 = 1.395(3), N3−C4 = 1.397(3), C4−N4 = 1.295(3), N4−C5 = 1.379(3), C5−N5 = 1.346(3). Selected bond angles (deg): Cl1A−Al1−ClA = 125.81(19), Cl1B−Al1−C1B = 125.0(2), N5−Al1−Cl1A = 90.54(7), N5−Al1−C1B = 90.78(8), N5−Al1−Cl1A = 90.78(8), N5−Al1−Cl1B = 90.54(7), N1−Al1−C1A = 90.0(2), N1−Al1−C1B = 89.5(2), N1−Al1−Cl1A = 90.16(8), N1−Al1−Cl1B = 91.35(7), N3−Al1−Cl1A = 122.10(8), N3−Al1−Cl1B = 120.22(7), N3−Al1− C1B = 114.8(2), Al1−N1−C2 = 126.97(18), N1−C2−N2 = 123.5(2), C3−N2−C2 = 107(2), N2−C3−N3 = 130.4(2), C3−N3−C4 = 107.4(2), N3−C4−N4 = 130.0(2), C4−N4−C5 = 123.8(2), N4− C5−N5 = 123.0(2), C5−N5−Al1 = 127.56(17), N5−Al1−N1 = 177.21(9), N5−Al1−N3 = 88.45(9), N3−Al1−N1 = 88.82(9).

distorted trigonal bipyramidal geometry with pyridine nitrogens occupying axial positions. In both cases, axial Al−N bonds (5, Al1−N1 = 2.1351(18) Å, Al1−N5 = 2.1267(18) Å; 6, Al1−N1 = 2.074(2) Å, Al1−N5 = 2.081(2) Å) are slightly longer than

Table 2. Ring-Opening Polymerization of ε-Caprolactone Initiated by 1−7/Benzyl Alcohola entry

complex

[CL]/[Al]/ [BnOH]

temp/°C

t/min

conversn (%)b

Mn(GPC)c

Mn(calcd)d

TOFe

PDI

1 2 3 4 5 6 7f 8f 9f 10f 11 12 13 14 15 16 17 18 19

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

200/1/1 500/1/1 500/1/1 1000/1/1 1000/1/1 2000/1/1 200/1/1 500/1/1 1000/1/1 1000/1/1 100/1/1 200/1/1 200/1/1 200/1/1 200/1/1 200/1/1 200/1/1 200/1/1 200/1/1

80 80 80 80 80 80 80 80 80 80 90 80 80 80 80 80 80 80 80

10 10 30 10 30 10 30 30 2 30 2 3840 3840 10 30 180 540 3840 3840

97 75 87 56 80 52 99 89 40 79 93 trace trace 91 97 trace 81 trace trace

29319 46685 49930 84544 92911 131800 45100 41478 49250 53127 15862

22224 42858 49698 63948 91308 118668 22680 50838 45708 90168 10710

1168 2259 870 3373 1600 6265 396 890 12121 1580 2818

1.61 1.44 1.52 1.58 1.51 1.56 2.09 2.53 1.32 1.49 1.73

21640 36996

20856 22224

1096 388

1.67 1.54

32060

18576

18

1.63

a

Conditions: 0.02 mmol of catalyst, 2 mL of toluene (wherever used). bDetermined by 1H NMR spectroscopy. cObtained from GPC analysis and calibrated by polystyrene standard (uncorrected). dTheoretical Mn = (monomer/initiator) × (% conversion) × (Mw of ε-CL) + (Mw of BnOH). e Moles of CL consumed per mole of catalyst per hour. fSolvent -free reaction. E


Organometallics

Article

of the alkoxide 8 was confirmed by single-crystal X-ray studies. The molecular structure and selected bond parameters are given in Figure 11. There are four Al atoms in the structure,

Figure 11. Single-crystal X-ray structure of 8. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å): Al1−O1 = 1.908(2), Al1−O2 = 1.900(2), Al2−O1 = 1.844(2), Al2−O2′ = 1.833(2), Al2−C1 = 1.954(3), Al2−C2 = 1.957(4), Selected bond angles (deg): O1−Al1− O2′ = 76.19(8), O1−Al2−O2′ = 79.44(9), C1−Al2−C2 = 114.34(17), O1−Al2−C1 = 114.72(14), O1−Al2−C2 = 114.82(15), O2−Al1−O2′ = 94.92(9), O1−Al1−O2″ = 166.97(8), Al1−O1−Al2 = 101.82(10), Al1−O2−Al2″ = 102.54(10), O1−Al1−O2 = 95.31(8), O1−Al1−O1″ = 94.94(9). Figure 10. (a) Plot of number-averaged molecular weights (Mn) and polydispersity (PDI) vs monomer conversion for the polymerization of ε-CL. Conditions: [CL]/[Al]/[BnOH] = 200/1/1 in 2 mL of toluene using complex 1 at 80 °C. (b) Plot of number-averaged molecular weights (Mn) and polydispersity (PDI) vs [CL]/[Al]/ [BnOH] for the polymerization of ε-CL using complex 1 at 80 °C (entries 1, 2, 4, and 6 in Table 2). In both plots, red squares (■) represent Mn (uncorrected) values and blue triangles (▲) represent PDI values.

and all of them are found to be in one plane. Three Al atoms form a triangle, and the fourth Al atom occupies the center of the triangle. The terminal Al atoms are connected to the central Al via μ2-O benzyloxido bridges. The central Al, which is coordinated by six benzyloxido groups, is in a slightly distorted octahedral geometry. The terminal Al atoms, which possess two methyl groups each, exist in a tetrahedral geometry. Quite a few crystal structures of similar compounds have been reported in the literature, and the bond parameters of 8 are similar to those of the reported structures.16 Once the alkoxide 8 was thoroughly characterized, it was synthesized in high yield (85%) from AlMe3 and benzyl alcohol in toluene and explored for its catalytic activity toward ROP of ε-caprolactone. The results of these polymerization reactions are provided in Table 3. The reactions were carried out at 80 °C in toluene, without the addition of a cocatalyst. 8 is found to be highly active and polymerizes ε-caprolactone with >90% conversion within 10 min, even when the [CL]/[Al] ratio is as high as 2000/1 (entry 23). The controlled nature of the polymerization can be seen in plots of conversion vs Mn and [CL]/[Al] vs Mn (Figure 12). Kinetic studies performed on the reactions catalyzed by 1 and 8 reveal that these polymerization reactions proceed with first-order dependence on monomer concentration (Figure 13). These results suggest that, on treatment with benzyl alcohol, complex 1 readily forms 8, which initiates the polymerization reaction. Though 8 can be prepared from AlMe3 and benzyl alcohol, the AlMe3/benzyl alcohol mixtures are far less active15a in comparison to 1. The difference in the catalytic activities of AlMe3 and 1 can be attributed to the rate of formation of the active alkoxide, 8, when they are treated with benzyl alcohol. While 1 produces 8 within a few seconds, AlMe3 takes more than 1 h to form 8 in any significant quantities. Recently, the catalytic activity of Al complexes of β-diketiminate ligands

caprolactone, and it produced only traces of polymer even after 3 h of the addition of the monomer (entry 16). Nonetheless, by the end of 9 h, a very good conversion of the monomer (81%) was observed (entry 17). On the basis of TOF values, complex 5 can be considered as moderately active. Similar to the case for 2 and 3, complexes 6 and 7 also failed to initiate polymerization. In all those entries found in Table 2, benzyl alcohol was used as a cocatalyst, without which the reactions did not proceed. Recently, several articles have appeared in the literature on ring-opening polymerization of ε-caprolactone by Al catalysts.2a−i,15 Most of the catalysts contain alkyl or alkoxide moieties on aluminum. Most of the catalysts that do not possess Al−O bonds require a cocatalyst, which is mostly an alcohol. This envisages the presence of an alkoxide intermediate during the course of the reaction. In order to isolate the intermediate in case of 1, we treated it with 1 equiv of benzyl alcohol at 80 °C in toluene. To our surprise, we obtained only a simple alkoxide, [Al{(μ-OCH2Ph)2AlMe2}3] (8), which does not contain the ligand. The yield of crystals was good (71% based on BnOH), and the mother liquor was also found to contain a mixture of the free ligand and 8, along with some impurities (NMR). However, an NMR tube reaction between 1 and benzyl alcohol in a 4/6 ratio showed a clear and complete conversion of 1 into 8 even at room temperature. The structure F


Organometallics

Article

Table 3. Ring-Opening Polymerization of ε-Caprolactone Initiated by 8 and 9a entry

complex

[CL]/[Al]

time/min

conversn (%)b

Mn(GPC)c

Mn(calcd)d

TOFe

PDI

20 21 22 23 24 25 26 27

8 8 8 8 9 9 9 9

200/1 500/1 1000/1 2000/1 50/1 100/1 200/1 500/1

10 10 10 10 30 30 30 30

99 99 97 92 99 99 95 94

15393 22072 31342 45817 6837 8236 20752 61080

22680 56538 110688 209868 5751 11394 21768 53688

1192 2981 5843 11084 99 198 380 940

1.79 1.64 1.83 1.94 1.31 1.16 1.36 1.03

Conditions: 0.02 mmol of catalyst, 2 mL of toluene, 80 °C. bDetermined by 1H NMR spectroscopy. cObtained from GPC analysis and calibrated by polystyrene standard (uncorrected). dTheoretical Mn = (monomer/initiator) × (% conversion) × (Mw of ε-CL) + (Mw of BnOH). eMoles of CL consumed per mole of catalyst per hour. a

Figure 13. Plots of ln([CL]0/[CL]t) vs time for the polymerization of ε-CL catalyzed by 1 (red ■) and 8 (blue ▲). Conditions: for complex 1, [CL]/[Al]/[BnOH] = 200/1/1 in 2 mL of toluene at 80 °C; for complex 8, [CL]/[Al] = 200/1 in 2 mL of toluene at 80 °C.

do not decompose readily to form 8. This difference in the reactivity can be explained by comparing the donor abilities of both the types of ligands. β-Diketiminate ligands are strong σ donors, and replacing a carbon atom in the ligand backbone by nitrogen (as in triaza ligands) will decrease their donor ability. Hence, the triaza ligand is too weak a donor to retain Al in presence of BnOH. In order to find out the reason for the inactivity of complexes [L1AlMe(Cl)] (2) and [L1AlCl2] (3), we investigated their reactions with benzyl alcohol. Both 2 and 3 readily reacted with benzyl alcohol in toluene to give a white precipitate, which is found to be a mixture of [L1H2]Cl and some highly insoluble substance, presumably polymeric alkoxides of the type [AlClx(OCH2Ph)3−x]n. Formation of [L1H2]Cl was confirmed by 1H NMR and single crystal X-ray analysis (see the Supporting Information, Figures S8 and S9). Evaporation of volatiles from the filtrates of these reactions yielded no residue. These observations clearly explain why 2 and 3 cannot initiate polymerization. Unlike 1, complex 5 ([L2AlMe2]) does not react with benzyl alcohol at room temperature and even at 80 °C only a very slow reaction occurred, which explains the long hours of catalysis reaction time. It has been established that the formation of alkoxide involves nucleophilic attack by benzyl alcohol at the aluminum center and the rate of the reaction depends upon the acidity of aluminum. In complex 5, aluminum is fivecoordinated and moreover there are two pyridine donors. As a result, the Al center becomes less acidic and hence the reaction is sluggish. Complexes 6 and 7 did not react with benzyl alcohol even when the reaction mixtures were heated at

Figure 12. (a) Plot of number-averaged molecular weights (Mn) and polydispersity (PDI) vs monomer conversion for the polymerization of ε-CL. Conditions: [CL]/[Al] = 200/1 in 2 mL of toluene using complex 8 at 80 °C. (b) Plot of number-averaged molecular weights (Mn) and polydispersity (PDI) vs [CL]/[Al] for the polymerization of ε-CL using complex 8 at 80 °C (entries 20−23 in Table 3). In both plots, red squares (■) represent Mn (uncorrected) values and blue triangles (▲) represent PDI values.

toward ROP of ε-caprolactone has been reported.4 Since βdiketiminates have striking resemblance to 1,3,5-triazapentadienates, it would be interesting to compare the catalytic activities of their Al complexes with those of 1. Most of the βdiketiminate complexes reported took several hours (7−24 h) for a good conversion of ε-caprolactone into poly(εcaprolactone), and the polymer produced has a relatively high molecular weight distribution. Comparison of these results with those of 1 or 8 suggests that the β-diketiminate complexes G


Organometallics

Article

80 °C for 24 h. This may be due to the inert nature of the Al− Cl bond compounded by the five-coordinate aluminum center.2c The dibenzyloxidoaluminum complex [L2Al(OCH2Ph)2] (9) was synthesized in high yield by refluxing 5 with 2 equiv of benzyl alcohol in toluene for 4 h. Compound 9 was characterized by NMR. The results of the catalytic activity of 9 have been summarized in Table 3. As anticipated, 9 is found to be more efficient than the parent complex 5. It takes only 30 min for 9 to convert >90% of ε-caprolactone into poly(εcaprolactone), whereas 5 takes more than 6 h. Narrow molecular weight distributions and a linear relationship between the [CL]/[Al] ratio and Mn (Figure 14) suggest that the polymerization proceeds in a living fashion.

an atmosphere of dry nitrogen. Trimethylaluminum, methylaluminum dichloride, dimethylaluminum chloride, n-butyllithium, benzoyl chloride, pyridine, benzyl alcohol, and 2,6-diisopropylaniline were procured from Aldrich and used as received. ε-Caprolactone, purchased from Acros Organics, was stirred over CaH2 for 24 h and distilled under vacuum. N-(Dipp)benzamidine6a was prepared by following the literature procedure. Hexane, benzene, diethyl ether, and toluene (from Na/benzophenone ketyl) were distilled fresh when required. CDCl3 was distilled from calcium hydride. C6D6 and toluened8 were dried over sodium metal. 1H and 13C spectra were recorded on a Bruker 400 MHz instrument. Elemental analyses were performed using an Elementar Vario EL III analyzer. Structural Determinations for L1H, 1, 2, 4, [(L1Li)2]·2Et2O, [L1Li(THF)3], 5, 6, and 8. Single crystals of 1, 2, 4, [(L1Li)2]·2Et2O, [L1Li(THF)3], 5, 6, and 8 were mounted on glass fibers in paraffin oil and then brought into the cold nitrogen stream of a low-temperature device so that the oil solidified. Data collections were performed on an OXFORD XCALIBUR diffractometer, equipped with a CCD area detector, using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation and a low-temperature device. Data collection for L1H was done at room temperature. All calculations were performed using SHELXS-97 and SHELXL-97.17 The structures were solved by direct methods and successive interpretation of the difference Fourier maps, followed by full-matrix least-squares refinement (against F2). All nonhydrogen atoms, except the disordered phenyl carbons in 1, were refined anisotropically. In 6, Me and Cl groups on Al exchange their positions, leading to substitutional disorder. They were refined by using the PART instruction together with free variables. Disorders in the phenyl group in 1, indoline moiety in 6, and methylene group in 8 were also treated similarly. The contribution of the hydrogen atoms, in their calculated positions, was included in the refinement using a riding model. Upon convergence, the final Fourier difference map of the Xray structures showed no significant peaks. All the data sets were collected to 2θ values >50°, but they were cut off to 2θ = 50° during the refinement. Relevant data concerning crystallographic data, data collection, and refinement details are summarized in Table S4 (see Supporting Information). Crystallographic information files (CIF) for the structures reported in this paper have also been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 910941− 910949 and 933359. Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44) 1223-336-033; e-mail, [email protected]). Preparation of N-(Dipp)benzamide (PhC(O)NH(2,6-iPr2C6H3)). A literature method was modified.18 Freshly distilled benzoyl chloride (3.95 g, 28.24 mmol) was added dropwise to a solution of 2,6-diisopropylaniline (5.00 g, 28.24 mmol) and pyridine (6.70 g, 84.74 mmol) in 100 mL of toluene. A white precipitate was formed, and the mixture was refluxed for 2.5 h. The mixture was brought to room temperature and washed with NaHCO3 solution, and the organic layer was dried over Na2SO4. A white solid was obtained after removal of the volatiles using a rotary evaporator. Recrystallization from a toluene/hexane mixture gave colorless crystals. Yield: 6.50 g (82%). Mp: 135−136 °C. 1H NMR (CDCl3, 400 MHz): δ 1.11 (d, 12H, CH(CH3)2), 3.03 (m, 2H, CH(CH3)2), 7.13 (d, 2H, Ar H), 7.26 (t, 1H, Ar H), 7.37 (m, 2H, Ph H), 7.46 (m, 1H, Ph H), 7.51 (1H, s, NH), 7.82 (m, 2H, Ph H). 13C NMR (CDCl3, 100 MHz): δ 23.66, 28.89, 123.55, 127.25, 128.48, 128.78, 131.75, 134.48, 146.48, 146.48, 167.04. Synthesis of N-(Dipp)benzimidoyl Chloride (2,6-iPr2C6H3NC(Ph)Cl). A literature method was modified.9,18 PCl5 (5.29 g, 25.44 mmol) was dissolved in benzene (30 mL) with heating under N2, and the mixture was cooled before adding N-(Dipp)benzamide (6.50 g, 23.13 mmol). The mixture was refluxed for 1 h, until gas evolution ceased. Volatiles were removed under high vacuum, and the residue was subjected to vacuum distillation to give the product as a yellow oil, which solidified after 1 day. Yield: 95% (6.56 g). Mp: 104− 105 °C. 1H NMR (CDCl3, 400 MHz): δ 1.07 (d, 6H, CH(CH3)2), 1.13 (d, 6H, CH(CH3)2), 2.73 (m, 2H, CH(CH3)2), 7.41 (m, 3H, Ar H), 7.46 (m, 3H, Ph H), 8.13 (m, 2H, Ph H). 13C NMR (CDCl3, 100

Figure 14. Plot of number-averaged molecular weights (Mn) and polydispersity (PDI) vs [CL]/[Al] for the polymerization of ε-CL using complex 9 at 80 °C (entries 24−27 in Table 3). Red squares (■) represent Mn (uncorrected) values, and blue triangles (▲) represent PDI values.

In addition to the above studies, we also examined complexes 1, 5, and 8 for initiating ROP of rac-lactide. Reactions were carried out with and without BnOH for 1 and 5 and without BnOH for 8. Even after 36 h of stirring in toluene at 80 °C, surprisingly, none of these complexes displayed activity and no polymer was produced in these reactions.

■

CONCLUSION Al complexes of a Dipp-substituted tertiary 1,3,5-triazapenta1,3-dienate ligand (L 1 ) and 1,3-bis(2-pyridylimino)isoindolinate ligand (L2) have been synthesized, and some of them have been characterized structurally. The catalytic activity of all of the complexes toward ROP of ε-caprolactone has been explored, and it was found that the complex [L1AlMe2], when treated with the cocatalyst benzyl alcohol, readily forms a tetranuclear alkoxide (8), which is highly active in initiating the polymerization reaction. In the case of [L2AlMe2], the dibenzyloxido intermediate [L2Al(OCH2C6H5)2] was isolated. This complex showed a good catalytic activity toward ROP of ε-caprolactone. Compound 8, which can be readily prepared from AlMe3 and benzyl alcohol, is the first simple aluminum alkoxide (without any supporting ligand) reported to initiate εcaprolactone polymerization.

■

EXPERIMENTAL SECTION

General Methods and Instrumentation. All manipulations were carried out using standard Schlenk line and glovebox techniques under H


Organometallics

Article

MHz): δ 22.95, 23.38, 28.73, 123.13, 124.94, 128.61, 129.47, 132.14, 135.07, 136.75, 143.59, 143.8. Synthesis of 1,5-Bis(2,6-diisopropylphenyl)-2,4-diphenyl1,3,5-triazapenta-1,3-diene (L1H). A Schlenk flask was charged with N-(Dipp)benzamidine (14.00 g, 50.00 mmol) and N-(Dipp)benzimidoyl chloride (14.97 g, 50.00 mmol) and heated to 160 °C for 2.5 h. The reaction mixture was cooled to room temperature, 150 mL of CH2Cl2 and 100 mL of saturated NaHCO3 were added, and the organic layer was separated. The aqueous layer was extracted two times with CH2Cl2. All of the organic layers were combined and dried over Na2SO4. After removal of volatiles using a rotary evaporator, a viscous yellow oil was obtained. This was dissolved in 100 mL of hexane and kept in the refrigerator to give a yellow crystalline material (18.41 g, 67%). Mp: 159−160 °C. 1H NMR (CDCl3, 400 MHz): δ 0.98 (d, 12H, CH(CH3)2), 1.15 (d, 12H, CH(CH3)2), 3.27 (m, 4H, CH(CH3)2), 7.11 (m, 6H, Ar H), 7.25 (m, 6H, Ph H), 7.64 (m, 4H, Ph H), 14.61 (s, 1H, NH). 13C NMR (CDCl3, 100 MHz): δ 22.34, 24.76, 28.59, 123.51, 125.63, 127.62, 129.68, 129.74, 136.75,139.25, 140.94, 164.40. Anal. Calcd for C38H45N3: C, 83.93; H, 8.34; N, 7.73. Found: C, 83.66; H, 8.40; N, 7.71. General Procedure for the Synthesis of [L1AlRR′] and [L1AlMe2(AlMe3)] (1−4). A solution of L1H in toluene (20 mL) was cooled to −78 °C in a 100 mL Schlenk flask, and AlMeRR′ was added. The reaction mixture was warmed to room temperature and stirred for an additional 1 h. The mixture was refluxed for 2 h and cooled to room temperature. The volume of the reaction mixture was reduced to 10 mL and kept at ambient temperature to afford colorless crystals of [L1AlRR′]. [L1AlMe2], (1). L1H (0.45 g, 0.82 mmol), AlMe3 (0.5 mL, 2 M in toluene, 1.00 mmol). Yield: 97% (0.48 g). Mp: 213−214 °C. 1H NMR (CDCl3, 400 MHz): δ −0.80 (s, 6H, AlMe), 0.76 (d, 12H, CH(CH3)2), 1.23 (d, 12H, CH(CH3)2), 3.29 (m, 4H, CH(CH3)2), 7.10 (m, 6H, Ar H), 7.21 (m, 6H, Ph H), 7.38 (m, 4H, Ph H). 13C NMR (CDCl3, 100 MHz): δ −10.13, 23.79, 22.58, 28.61, 124.65, 127.31, 127.42, 129.77, 130.43, 138.20, 139.67, 143.58, 169.50. Anal. Calcd for C40H50AlN3: C, 80.09; H, 8.40; N, 7.01. Found: C, 79.48; H, 8.55; N, 6.66. [L1AlMe(Cl)] (2). L1H (0.50 g, 0.92 mmol), AlMe2Cl (1 mL, 1 M in hexanes, 1.00 mmol). Yield: 84% (0.48 g). Mp: 175 °C dec. 1H NMR (CDCl3, 400 MHz): δ −0.79 (s, 3H, AlMe), 0.33 (d, 6H, CH(CH3)2), 1.15 (m, 12H, CH(CH3)2), 1.38 (d, 6H, CH(CH3)2), 3.27 (m, 2H, CH(CH3)2), 3.44 (m, 2H, CH(CH3)2), 7.10 (m, 6H, Ar H), 7.26 (m, 6H, Ph H), 7.39 (m, 4H, Ph H). 13C NMR (CDCl3, 100 MHz): δ −7.12, 22.63, 24.10, 24.67, 26.98, 28.32, 29.72, 123.93, 125.88, 127.55, 128.03, 130.22, 130.48, 137.55, 138.23, 142.99, 144.60, 170.07. Anal. Calcd for C39H47AlClN3: C, 75.52; H, 7.64; N, 6.77. Found: C, 75.47; H, 7.69; N, 6.67. [L1AlCl2] (3). L1H (0.50 g, 0.92 mmol), AlMeCl2 (1 mL, 1 M in hexanes, 1.00 mmol). Yield: 88% (0.52 g). Mp: 199 °C dec. 1H NMR (CDCl3, 400 MHz): δ 0.73 (d, 12H, CH(CH3)2), 1.24 (d, 12H, CH(CH3)2), 3.29 (m, 4H, CH(CH3)2), 7.08 (m, 6H, Ar H), 7.22 (m, 6H, Ph H), 7.37 (m, 4H, Ph H). 13C NMR (CDCl3, 100 MHz): δ 23.75, 25.57, 29.09, 125.08, 127.64, 128.51, 130.75, 130.83, 136.89, 137.04, 144.06, 171.25. Anal. Calcd for C38H44AlCl2N3: C, 71.24; H, 6.92; N, 6.56. Found: C, 71.17; H, 6.95; N, 6.51. [L1AlMe2(AlMe3)] (4). L1H (0.45 g, 0.82 mmol), AlMe3 (0.9 mL, 2 M in toluene, 1.65 mmol). Yield 83% (0.46 g) Mp: 193 °C dec. 1H NMR (toluene-d8, 400 MHz at 80 °C): δ −0.73 (s, 6H, AlMe3), −0.39 (s, 6H, AlMe2), 0.21 (s, 3H, AlMe3), 0.94 (d, 12H, CH(CH3)2), 1.21 (d, 12H, CH(CH3)2), 3.41 (m, 4H, CH(CH3)2), 6.58 (s, 1H, Ph H), 6.92 (m, 5H, Ph H), 6.99 (m, 6H, Ar H) 7.64 (m, 4H, Ph H). 13C NMR (toluene-d8, 100 MHz at 80 °C): δ −8.97, −4.84, 1.42, 24.12, 26.82, 28.99, 127.59, 131.50, 132.86, 136.13, 137.37, 137.94, 139.62, 144.13. Anal. Calcd for C43H59Al2N3: C, 76.86; H, 8.85; N, 6.25. Found: C, 76.76; H, 8.91; N, 6.20. Synthesis of [(L1Li)2]. A solution of L1H (0.34 g, 0.62 mmol) in diethyl ether (20 mL) was cooled to −78 °C in a 100 mL Schlenk flask, and n-BuLi (0.4 mL, 1.6 M in hexane, 0.64 mmol) was added. The reaction mixture was warmed to room temperature and stirred for an additional 1 h, during which time the solution changed from yellow

to colorless. The volume of the reaction mixture was reduced to 10 mL and kept at 0 °C to afford colorless crystals of [(L1Li)2] (0.32 g, 93%). Mp: 266 °C dec. 1H NMR (C6D6, 400 MHz): δ 0.92, (d, 12H, CH(CH3)2), 1.04 (d, 12H, CH(CH3)2), 1.41 (d, 12H,CH(CH3)2), 3.35 (m, 4H, CH(CH3)2), 3.68 (m, 4H, CH(CH3)2), 6.58 (m, 8H, Ar H), 6.94 (m, 4H, Ar H), 7.01 (m, 5H, Ph H), 7.09 (m, 5H, Ph H), 7.158 (m, 10H, Ph H). 13C NMR (C6D6, 100 MHz): δ 22.01, 22.69, 23.97, 24.53, 28.61, 29.33, 122.73, 123.18, 124.14, 137.99, 138.70, 141.36, 144.21, 172.73. Anal. Calcd for C76H88Li2N6: C, 83.03; H, 8.07; N, 7.64. Found: C, 83.09; H, 8.02; N, 7.63. Synthesis of [L1Li(THF)3]. A solution of L1H (0.50 g, 0.92 mmol) in tetrahydrofuran (25 mL) was cooled to −78 °C in a 100 mL Schlenk flask, and n-BuLi (0.6 mL, 1.6 M in hexane, 0.96 mmol) was added. The reaction mixture was stirred for 1 h at this temperature and warmed to room temperature. The volume of the reaction mixture was reduced to 10 mL and kept at 0 °C to afford colorless crystals of [L1Li(THF)3] (0.61 g, 87%). Mp: 235 °C dec. 1H NMR (CDCl3, 400 MHz): δ 0.71 (m, 12H, CH(CH3)2), 1.25 (d, 6H, CH(CH3)2), 1.31 (d, 6H, CH(CH3)2), 1.85 (m, 12H, THF), 3.03 (m, 2H, CH(CH3)2), 3.36 (m, 2H, CH(CH3)2), 3.74 (m, 12H, THF), 6.75 (m, 6H, Ar H), 6.87 (m, 10H, Ph H). 13C NMR (CDCl3, 100 MHz): δ 21.69, 22.38, 23.49, 24.13, 25.77, 28.23, 28.95, 68.13, 122.19, 122.57, 123.25, 127.04, 127.92, 128.21, 137.83, 138.40, 141.16, 143.87. Anal. Calcd for C50H68LiN3O3: C, 78.40; H, 8.95; N, 5.49. Found: C, 78.37; H, 8.92; N, 5.53. General Procedure for the Synthesis of [L2AlRR′] (5−7). To a stirred solution of L2H in toluene (15 mL) was added MeAlRR′ at −78 °C. The reaction mixture was warmed to room temperature, and the mixture was refluxed for 4 h. Volatiles were removed under vacuum to give yellow crystalline material. Recrystallization from toluene afforded yellow solid/crystals. [L2AlMe2] (5). L2H (0.45 g, 1.50 mmol), AlMe3 (0.8 mL, 2 M in toluene, 1.60 mmol). Yield: 97% (0.52 g). Mp: 245 °C dec. 1H NMR (CDCl3, 400 MHz): δ −0.66 (s, 6H, AlMe), 7.13 (m, 2H, Py H), 7.44 (m, 2H, Py H), 7.59 (m, 2H, Ar H), 7.77 (m, 2H, Py H), 8.00 (m, 2H, Ar H), 8.43 (m, 2H, Py H). 13C NMR (CDCl3, 100 MHz): δ −0.80, 119.91, 122.51, 124.92, 131.59, 138.44, 139.92, 146.48, 156.77, 164.03.Anal. Calcd for C20H18AlN5: C, 67.60; H, 5.11; N, 19.71. Found: C, 67.52; H, 5.07; N, 19.64. [L2AlMe(Cl)] (6). L2H (0.33 g, 1.10 mmol), AlMe2Cl (1.2 mL, 1 M in hexanes, 1.20 mmol). Yield: 97% (0.40 g). Mp: 282 °C dec. 1H NMR (CDCl3, 400 MHz): δ −0.44 (s, 3H, AlMe), 7.20 (m, 2H, Py H), 7.51 (m, 2H Py H), 7.63 (m, 2H, Ar H), 7.85 (m, 2H, Py H), 8.03 (m, 2H, Ar H), 8.77 (m, 2H, Py H). 13C NMR (CDCl3, 100 MHz): δ −7.56, 120.27, 120.44, 122.87, 123.14, 125.50, 125.95, 132.11, 132.53, 137.47, 137.92, 140.76, 141.48, 147.62, 148.78, 156.14, 156.29, 163.16.. Anal. Calcd for C19H15AlClN5: C, 60.73; H, 4.02; N, 18.64. Found: C, 60.66; H, 4.16; N, 18.56. [L2AlCl2] (7). L2H (1.00 g, 3.34 mmol), AlMeCl2 (3.6 mL, 1 M in hexanes, 3.60 mmol). Yield: 96% (1.27 g). Mp: 285 °C dec. 1H NMR (CDCl3, 400 MHz): δ 7.13 (m, 2H, Py H), 7.44 (m, 2H, Py H), 7.59 (m, 2H, Ar H), 7.77 (m, 2H, Py H), 8.00 (m, 2H, Ar H), 8.43 (m, 2H, Py H). 13C NMR (CDCl3, 100 MHz): δ 120.61, 123.35, 125.83, 132.69, 137.25, 141.58, 148.81, 155.81, 162.39.Anal. Calcd for C18H12AlCl2N5: C, 54.57; H, 3.05; N, 17.68. Found: C, 54.50; H, 3.11; N, 17.62. Formation of [Al{(μ-OCH2Ph)2AlMe2}3] (8) in a Reaction between 1 and Benzyl Alcohol. To a stirred solution of 1 (0.30 g, 0.50 mmol) in toluene (5 mL) was added benzyl alcohol (0.05 g, 0.50 mmol) at room temperature. The mixture was stirred for 15 min at room temperature before volatiles were removed under vacuum. The residue was extracted with hexane to give colorless crystals of 8 at room temperature (0.05 g, 71%, based on BnOH). Mp: 176−177 °C. 1 H NMR (C6D6, 400 MHz): δ −0.60 (s, 18H, AlMe), 5.30 (d, 6H, CH2), 5.38 (d, 6H, CH2), 7.16 (t, 6H, Ph H), 7.22 (t, 12H, Ph H), 7.65 (d, 12H, Ph H). 13C NMR (C6D6, 100 MHz): δ −6.75, 67.77, 129.06, 129.14, 130.45, 138.09. Anal. Calcd for C48H60Al4O6: C, 68.56; H, 7.19. Found: C, 68.49; H, 7.23. Synthesis of 8 from AlMe3 and Benzyl Alcohol. To a stirred solution of AlMe3 (3.0 mL, 2 M in toluene, 6.00 mmol) in hexane (40 I


Organometallics

Article

mL) was added benzyl alcohol (1.00 g, 9.25 mmol) dropwise at −78 °C. The resultant slurry was allowed to come to room temperature slowly and stirred overnight. The precipitate was separated by filtration, washed with hexane, and dried under high vacuum. Yield: 85% (1.10 g). Synthesis of [L2Al(OCH2Ph)2] (9). A solution of L2H (0.55 g, 1.83 mmol) in toluene (20 mL) was cooled to −78 °C in a 100 mL Schlenk flask, and AlMe3 (1 mL, 2 M in toluene, 2.00 mmol) was added. The reaction mixture was warmed to room temperature, and the mixture was refluxed for 4 h. The reaction mixture was cooled to room temperature, and benzyl alcohol (0.40 g, 3.67 mmol) was added. The mixture was refluxed again for 8 h. Volatiles were removed under high vacuum, and the residue was washed with hexane and dried under vacuum, to give a yellow powder. Yield: 80% (0.79 g). Mp: 214 °C dec. 1H NMR (CDCl3, 400 MHz): δ 4.34 (s, 4H, CH2), 6.98 (m, 8H, Ph H), 7.16 (m, 2H, Ph H), 7.52 (m, 2H, Py H), 7.58 (m, 2H, Ar H), 7.85 (m, 2H, Py H), 7.96 (m, 2H, Ar H), 8.95 (m, 2H, Py H). 13C NMR (CDCl3, 100 MHz): δ 65.27, 120.03, 122.67, 125.36, 125.84, 127.81, 131.81, 138.06, 140.45, 145.22, 147.75, 157.14, 163.12. Anal. Calcd for C32H26AlN5O2: C, 71.23; H, 4.86; N, 12.98. Found: C, 71.01; H, 4.90; N, 12.98. Ring-Opening Polymerization (ROP) of ε-Caprolactone Catalyzed by 1−7 in the Presence of Benzyl Alcohol. Toluene (2 mL) was added to a Schlenk flask containing complex 1 (0.014 g, 0.023 mmol). Then the flask was placed in an oil bath at 80 °C and benzyl alcohol (0.23 mL, 0.1 M in toluene, 0.023 mmol) was added. The mixture was stirred for 10 min, and ε-caprolactone (0.53 g, 4.68 mmol) was added into the Schlenk flask. After the solution was stirred for 30 min, an increase in viscosity was observed. The polymerization was terminated by addition of several drops of glacial acetic acid (∼0.2 mL) into the reaction mixture. The resulting viscous solution was diluted with dichloromethane and then transferred into a flask containing cold methanol (70 mL) with stirring. The resultant polymer was then collected by filtration, washed once with cold methanol, and dried under vacuum, to give a white solid (0.52 g, 99%). Ring-Opening Polymerization (ROP) of ε-Caprolactone Catalyzed by 8 and 9 in the Absence of Benzyl Alcohol. A mixture of complex 8 (0.015 g, 0.027 mmol) and ε-caprolactone (0.63 g, 5.56 mmol) in toluene (2 mL) was stirred at 80 °C for 30 min, during which time an increase in viscosity was observed. The polymerization was terminated by addition of several drops of glacial acetic acid (∼0.2 mL) into the reaction mixture. The resulting viscous solution was diluted with dichloromethane and then transferred into a flask containing cold methanol (70 mL) with stirring. The resultant polymer was then collected by filtration, washed once with cold methanol and dried under vacuum, to give a white solid (0.60 g, 95%).

■

of a glovebox. We thank the Central Instrumentation Facility, Pondicherry University, for NMR spectra and DST-FIST for the single-crystal X-ray diffraction facility.

■

(1) (a) Jiang, H.; Hu, Y.; Zhao, P.; Li, Y.; Zhu, K. J. Biomed. Mater. Res.: Part B 2006, 79B, 50−57. (b) Ikada, Y.; Tsuji, H. Macromol. Rapid Commun. 2000, 21, 117−132. (c) Avérous, L.; Pollet, E. Biodegradable Polymers. In Environmental Silicate Nano-Biocomposites; Avérous, L., Pollet, E., Eds.; Springer-Verlag: London, 2012; pp 13− 39. (d) Mitrus, M.; Wojtowicz, A.; Moscicki, L. Biodegradable Polymers and Their Practical Utility. In Thermoplastic Starch; Janssen, L. P. B. M., Moscicki, L., Eds. Wiley-VCH: Weinheim, Germany, 2009; pp 1−33. (2) (a) Arbaoui, A.; Redshaw, C. Polym. Chem. 2010, 1, 801−826. (b) Ajellal, N.; Carpentier, J. F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363−8376. (c) Yu, R.-C.; Hung, C.-H.; Huang, J.-H.; Lee, H.-Y.; Chen, J.-T. Inorg. Chem. 2002, 41, 6450−6455. (d) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006, 250, 602−626. (e) Huang, W. Y.; Chuang, S. J.; Chunag, N. T.; Hsiao, C. S.; Datta, A.; Chen, S. J.; Hu, C. H.; Huang, J. H.; Lee, T. Y.; Lin, C. H. Dalton Trans. 2011, 40, 7423−7433. (f) Ma, W.-A.; Wang, Z.-X. Organometallics 2011, 30, 4364−4373. (g) Yao, W.; Mu, Y.; Gao, A.; Su, Q.; Liu, Y.; Zhang, Y. Polymer 2008, 49, 2486−2491. (h) Otero, A.; LaraSánchez, A. n.; Fernández-Baeza, J.; Alonso-Moreno, C.; Castro-Osma, J. A.; Márquez-Segovia, I.; Sánchez-Barba, L. F.; Rodríguez, A. M.; Garcia-Martinez, J. C. Organometallics 2011, 30, 1507−1522. (i) Koller, J. R.; Bergman, R. G. Organometallics 2011, 30, 3217−3224. (j) Lee, J.I.; Lee, T.-Y.; Chang, L.-C.; Lin, C.-Y.; Lee, H. M.; Hung, L.; Datta, A.; Huang, J.-H. J. Mol. Struct. 2009, 929, 207−212. (k) Wu, J.-C.; Huang, B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-C. Polymer 2005, 46, 9784− 9792. (3) (a) Rieth, L. R.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 15239−15248. (b) Marshall, E. L.; Gibson, V. C.; Rzepa, H. S. J. Am. Chem. Soc. 2005, 127, 6048−6051. (c) Kong, D.; Peng, Y.; Li, D.; Li, Y.; Chen, P.; Qu, J. Inorg. Chem. Commun. 2012, 22, 158−161. (d) Shen, X.; Xue, M.; Jiao, R.; Ma, Y.; Zhang, Y.; Shen, Q. Organometallics 2012, 31, 6222−6230. (e) Chen, H.; Liu, P.; Yao, H.; Zhang, Y.; Yao, Y.; Shen, Q. Dalton Trans. 2010, 39, 6877− 6885. (f) Xue, M.; Jiao, R.; Zhang, Y.; Yao, Y.; Shen, Q. Eur. J. Inorg. Chem. 2009, 27, 4110−4118. (g) Champouret, Y.; MacLeod, K. C.; Baisch, U.; Patrick, B. O.; Smith, K. M.; Poli, R. Organometallics 2009, 29, 167−176. (h) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; Rzepa, H. S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834−9843. (4) Gong, S.; Ma, H. Dalton Trans. 2008, 25, 3345−3357. (5) (a) Heße, N.; Fröhlich, R.; Humelnicu, I.; Würthwein, E.-U. Eur. J. Inorg. Chem. 2005, 2189−2197. (b) Heße, N.; Fröhlich, R.; Wibbeling, B.; Würthwein, E.-U. Eur. J. Org. Chem. 2006, 3923−3937. (c) Häger, I.; Fröhlich, R.; Würthwein, E.-U. Eur. J. Inorg. Chem. 2009, 2415−2428. (d) Dias, H. V. R.; Singh, S. Dalton Trans. 2006, 16, 1995−2000. (e) Dias, H. V. R.; Singh, S.; Cundari, T. R. Angew. Chem., Int. Ed. 2005, 44, 4907−4910. (f) Siedle, A. R.; Webb, R. J.; Behr, F. E.; Newmark, R. A.; Weil, D. A.; Erickson, K.; Naujok, R.; Brostrom, M.; Mueller, M.; Chou, S.-H.; Young, V. G. Inorg. Chem. 2003, 42, 932−934. (g) Dias, H. V. R.; Singh, S. Inorg. Chem. 2004, 43, 5786− 5788. (h) Dias, H. V. R.; Singh, S. Inorg. Chem. 2004, 43, 7396−7402. (i) Dias, H. V. R.; Singh, S.; Flores, J. A. Inorg. Chem. 2006, 45, 8859− 8861. (j) Flores, J. A.; Dias, H. V. R. Inorg. Chem. 2008, 47, 4448− 4450. (6) (a) Maheswari, K.; Rajendran, N. M.; Meyer, J.; Reddy, N. D. Organometallics 2010, 29, 3799−3807. (b) Maheswari, K.; Reddy, N. D. Organometallics 2012, 31, 197−206. (7) (a) Pfeiffer, M.; Murso, A.; Mahalakshmi, L.; Moigno, D.; Kiefer, W.; Stalke, D. Eur. J. Inorg. Chem. 2002, 12, 3222−3234. (b) Gornitzka, H.; Stalke, D. Eur. J. Inorg. Chem. 1998, 3, 311−317. (c) Ashenhurst, J.;

ASSOCIATED CONTENT

S Supporting Information *

CIF files and a table giving crystallographic data for L1H, 1, 2, 4, [(L1Li)2], [L1Li(THF)3], 5, 6, 8, and [L1H2]Cl, figures giving 1H NMR spectra of 4 and [L1H2]Cl and 1H and 13C NMR spectra of N-(Dipp)benzamide and N-(Dipp)benzimidoyl chloride, and a figure giving the single-crystal Xray structure of [L1H2]Cl. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for N.D.R.: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge the Department of Science and Technology, New Delhi, for financial support and the Alexander von Humboldt Stiftung of Germany for donation J


Organometallics

Article

Brancaleon, L.; Gao, S.; Liu, W.; Schmider, H.; Wang, S.; Wu, G.; Wu, Q. G. Organometallics 1998, 17, 5334−5341. (8) (a) Lu, X.-B.; Wang, H.; He, R. J. Mol. Catal. A: Chem. 2002, 186, 33−42. (b) Rajagopal, G.; Kim, S. S; Kwak, J. M. Bull. Korean Chem. Soc. 2006, 27, 1907. (9) Masuda, J. D.; Stephan, D. W. Dalton Trans. 2006, 17, 2089− 2097. (10) (a) Elvidge, J. A.; Linstead, R. P. J .Chem. Soc. 1952, 5000−5007. (b) Clark, P. F.; Elvidge, J. A.; Linstead, R. P. J. Chem. Soc. 1953, 3593−3601. (11) (a) Qian, B.; Ward, D. L.; Smith, M. R. Organometallics 1998, 17, 3070−3076. (b) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 9384−9385. (12) (a) Zhou, M.; Song, Y.; Gong, T.; Tong, H.; Guo, J.; Weng, L.; Liu, D. Inorg. Chem. 2008, 47, 6692−6700. (b) Zhou, M.; Li, P.; Tong, H.; Song, Y.; Gong, T.; Guo, J.; Weng, L.; Liu, D. Inorg. Chem. 2008, 47, 1886−1888. (13) (a) Dias, H. V. R.; Singh, S. Dalton Trans. 2006, 16, 1995−2000. (b) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M. M.; Roesky, H. W.; Power, P. P. Dalton Trans. 2001, 23, 3465−3469. (14) (a) Bröring, M.; Kleeberg, C.; Cónsul Tejero, E. Eur. J. Inorg. Chem. 2007, 20, 3208−3216. (b) Bröring, M.; Kleeberg, C. Inorg. Chim. Acta 2009, 362, 1065−1070. (c) Baird, D. M.; Maehlmann, W. P.; Bereman, R. D.; Singh, P. J. Coord. Chem. 1997, 42, 107−126. (d) Siggelkow, B.; Meder, M. B.; Galka, C. H.; Gade, L. H. Eur. J. Inorg. Chem. 2004, 3424−3435. (e) Bröring, M.; Kleeberg, C. Dalton Trans. 2007, 1101−1103. (f) Anderson, O. P.; la Cour, A.; Dodd, A.; Garrett, A. D.; Wicholas, M. Inorg. Chem. 2003, 42, 122−127. (g) Meder, M. B.; Gade, L. H. Eur. J. Inorg. Chem. 2004, 2716−2722. (h) Balogh-Hergovich, E.; Kaizer, J.; Speier, G.; Huttner, G.; Jacobi, A. Inorg. Chem. 2000, 39, 4224−4229. (15) (a) Zhang, W.; Wang, Y.; Cao, J.; Wang, L.; Pan, Y.; Redshaw, C.; Sun, W.-H. Organometallics 2011, 30, 6253−6261. (b) Chen, H.-L.; Dutta, S.; Huang, P.-Y.; Lin, C.-C. Organometallics 2012, 31, 2016− 2025. (c) Lewiński, J.; Horeglad, P.; Wójcik, K.; Justyniak, I. Organometallics 2005, 24, 4588−4593. (d) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Dalton Trans. 2001, 15, 2215−2224. (e) Shen, M.; Zhang, W.; Nomura, K.; Sun, W. H. Dalton Trans. 2009, 41, 9000−9009. (f) Lamberti, M.; D’Auria, I.; Mazzeo, M.; Milione, S.; Bertolasi, V.; Pappalardo, D. Organometallics 2012, 31, 5551−5560. (16) (a) Atwood, D. A.; Jegier, J. A.; Liu, S.; Rutherford, D.; Wei, P.; Tucker, R. C. Organometallics 1999, 18, 976−981. (b) Oliver, J. G.; Worrall, I. J. J. Chem. Soc. A 1970, 1389−1391. (c) Neumuller, B. Chem. Soc. Rev. 2003, 32, 50−55. (d) Szumacher, S.; Kunicki, A. R.; Madura, I.; Zachara, J. J. Organomet. Chem. 2003, 682, 196−203. (17) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122. (18) Boeré, R. T.; Klassen, V.; Wolmershäuser, G. J. Chem. Soc., Dalton Trans. 1998, 24, 4147−4154.

K


Synthesis of Aluminum Complexes of Triaza Framework Ligands and

Recommend Documents