- Email: [email protected]
Dimethylaluminum complexes bearing a chiral diketiminate ligand: Synthesis, characterization and ring-opening polymerization of ε-caprolactone
Inorganic Chemistry Communications 22 (2012) 158–161
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Dimethylaluminum complexes bearing a chiral diketiminate ligand: Synthesis, characterization and ring-opening polymerization of ε-caprolactone Dexu Kong, Ying Peng ⁎, Di Li, Yang Li, Pingping Chen, Jingping Qu ⁎ State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, P.R. China
a r t i c l e
i n f o
Article history: Received 19 April 2012 Accepted 29 May 2012 Available online 4 June 2012 Keywords: Aluminum complexes Chiral β-diketiminate ligand ε-Caprolactone Initiator Ring-opening polymerization
a b s t r a c t Reaction of the chiral aliphatic N-substituted diketiminate ligand, N,N′-Di(R-phenylethyl)-2-amino-4iminopent-2-ene (R,R-nacnacCH(Me)PhH), with AlMe3 affords the monomeric dimethylaluminum complex R,RnacnacCH(Me)PhAlMe2 (R,R-1). Complex S,S-nacnacCH(i-Pr)PhAlMe2 (S,S-2) is prepared by S,S-nacnacCH(i-Pr)PhH and AlMe3. Complexes R,R-1 and S,S-2 are the first examples of chiral β-diketiminate ligands with main group metals. All these two complexes exhibit activity for the ring-opening polymerization of ε-caprolactone in the absence of alcohol, and complex R,R-1 shows a higher activity with a narrow molecular weight distribution (PDI= 1.17) in toluene. © 2012 Elsevier B.V. All rights reserved.
Polycaprolactone (PCL) is an important polymer due to its physical, thermal and mechanical properties, as well as its miscibility with other polymers and biodegradability [1]. It is widely applied in different fields such as scaffolds in tissue engineering [2], in long-term drug delivery systems [3], in microelectronics [4], as adhesives [5], and in packaging [6]. PCL is usually prepared by the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by metal complexes [1]. Among the reported initiators, organoaluminum complexes have attracted much attention because they have a good control over the polymerization reaction [7] and low toxicity [8]. However, few studies have involved the alkyl aluminum complexes as the initiator for the ROP of ε-CL without the presence of alcohol, because these complexes usually showed no or very low activity towards this polymerization [9], and most of them got a broad molecular weight distribution during the reaction [9,10]. Although plenty of various ligands are employed in the alkyl aluminum initiators [9–11], aluminum complexes bearing N,N′substituted β-diketiminate (“nacnac”) ligands and their application in ROP of cyclic esters are still less investigated [10c]. The β-diketiminate ligands have gained increasing interest due to their suitability as fine-tuned spectator ligands by variation of the steric and electronic properties of the N,N′-substituents [12,13]. Most of the current work with β-diketiminate ligands are focused on the aromatic N-substituents [10c,12,13], and metal complexes with aliphatic N-substituted ones are less reported [12,13]. Compared to the N-aryl β-diketiminates, the
⁎ Corresponding authors. Tel./fax: + 86 411 84986186. E-mail addresses: [email protected] (Y. Peng), [email protected] (J. Qu). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2012.05.050
aliphatic N-substituted ligands can not only afford the adjustable steric environment around the metal centers, but also can easily introduce chirality into the diketiminate ligands. Recently, Schaper et al. reported several instances of transition metals (copper, zinc, chromium and zirconium) supported by chiral βdiketiminate ligands with aliphatic N-substituents and invested their application in the polymerization of lactide [14], while no examples of main group metal bearing such chiral ligands are known. Herein we describe our work on the preparation of dimethylaluminum complexes with chiral diketiminate ligands (Scheme 1) and their application in the ring-opening polymerization of ε-caprolactone. The chiral dimethylaluminum complex R,R-1 [15] was prepared in 62% yield by the treatment of chiral ligand R,R-nacnac CH(Me)PhH [14c] with AlMe3 (1.07 molar equiv) in hexane at room temperature. The 1 H NMR of R,R-1 shows the characteristic resonance for Al–Me at −1.05 ppm in CDCl3, which is comparable to those for methylaluminum complexes [(4-MeC6H4NCMe)2HC]AlMe2 (δ −1.05 ppm), [(2,6-iPr2C6H3NCMe)2HC]AlMe2 (δ −1.00 ppm) with aryl-substituted nacnac ligands [16]. The most intense peak in the EI mass spectrum of R,R-1 appears at m/z 347.2060 [M+ − CH3]. Treatment of 1.16 molar equiv of AlMe3 with a more bulky chiral ligand S,S-nacnacCH(i-Pr)PhH [17] at 50 °C afforded complex S,S-2, and its 1H NMR shows Al–Me peak at δ −0.92 ppm [18]. When the reaction was carried out at room temperature, lots of unreacted ligands were detected from 1H NMR, which implies the more bulky chiral ligand S,S-nacnacCH(i-Pr)PhH is less reactive compared to ligand R,R-nacnacCH(Me)PhH due to the larger steric hindrance from isopropyl group instead of methyl group. In addition, the chiral nature of the optically active complexes R,R-1 and S,S-2 was
D. Kong et al. / Inorganic Chemistry Communications 22 (2012) 158–161
159
Scheme 1. Preparation of complexes R,R-1 and S,S-2.
confirmed by the liquid circular dichroism (CD) spectra measured in methanol solution (Fig. 1). The CD spectrum of R,R-1 exhibits a positive Cotton effect at 294 and 336 nm, while S,S-2 shows Cotton effects of the opposite sign at 312 and 326 nm. The molecular structure (Fig. 2) of R,R-1 was confirmed by X-ray crystallography [19]. As shown in Fig. 2, the aluminum center in R,R-1 is surrounded by two nitrogen atoms of the chiral nacnac ligand and two methyl groups, exhibiting distorted tetrahedral geometry. The N(1)\Al(1)\N(2) angle (94.96(6)°) and C(22)\Al(1)\C(23) angle (114.55(11)°) are comparable respectively to those in alkylaluminum complexes with aryl-substituted nacnac ligands [(4-MeC6H4NCMe)2HC]AlMe2 (A) (94.72(14)°; 115.4(2)) [16], [(2,6-iPr2C6H3NCMe)2HC] AlMe2 (B) (96.18(9)°; 117.4(1)) [16,20], or [(2,6-iPr2C6H34-ClC6H4N2C2Me2)HC]AlEt2 (C) (94.50(7)°; 115.59(11)°) [10c]. The deviation of the aluminum center from the ligand backbone N1\C2\C3\C4\N2 is 0.85 Å, which is larger than those in the above mentioned complexes A (0.33 Å), B (0.72 Å) and C (0.56 Å) with aryl-substituted nacnac ligands. Thus we may conclude that the aliphatic substituents on the nitrogen atom increases the steric bulk of the ligand, and these phenomena were also found for the copper complex bearing this kind of chiral ligand [14c]. The chiral nacnac ligand in R,R-1 retains the delocalization with the very close bond distances of the C\C (1.396(3) and 1.398(3) Å) and C\N bonds (1.317(2) and 1.324(2)Å). The two Al\N bond distances (1.9223(15) and 1.9165(15) Å) are almost identical, which are close to those in a chiral complexes A, B and C. The Al\CH3 distances (1.952(2) and 1.967(2)Å) are also comparable to those in complexes A (1.955(4) and 1.961(3) Å) and B (1.958(3) and 1.970(3) Å). The polymerization of ε-CL was studied under different conditions using complexes R,R-1 and S,S-2 as the initiators (Table 1). From Table 1 (entries 1, 4) we can see that these two dialkylaluminum complexes both can initiate the ROP of ε-CL in the absence of alcohol, and the chiral complex R,R-1 exhibits a higher activity for the polymerization, producing a high conversion up to over 99% within 3 h at 80 °C
and a narrow molecular weight distribution of 1.22. Comparatively, the diethylaluminum complexes bearing N-aryl β-diketiminate ligands had broader molecular weight distributions (PDI=1.66–3.74) and longer reaction time (17 h vs. 96% conversion) under the same polymerization conditions [10]. The more bulky chiral complex S,S-2 had a lower activity compared to that of R,R-1 perhaps due to the larger isopropyl group on the chiral carbon atom instead of methyl group in R,R-1 which hinders the coordination of monomer to the initiator. The 1H NMR spectrum of the purified polymer indicates the existence of a tiny peak at 3.65 ppm which is assignable to the methene protons (\CH2\) adjacent to a terminal hydroxyl group, suggesting a linear structure of the oligomer instead of cyclic. In order to elucidate the effect of the structure of the initiators on the polymerization, the racemic complex rac-1 was prepared by the reaction of AlMe3 with racemic ligand nacnac CH(Me)PhH. Treatment of AlMe3 with chiral ligand S,S-nacnac CH(Me)PhH [14c] afforded complex S,S-1. Using complex rac-1 as the initiator, the polymerization was carried out in toluene (Table 1, entry 2) and a broader molecular weight distribution (PDI=1.38) was obtained. When using the mixture of equiv of R,R-1 and S,S-1 as the initiator for the polymerization, the similar PDI value (PDI=1.23, Table 1, entry 3) as that for R,R-1 (PDI=1.22, Table 1, entry 1) was attained. Although the isolation of mesomeric complex meso-1 from the reaction of AlMe3 with mesomeric ligand R, S-nacnacCH(Me)PhH was unsuccessful, we believe that the steric configuration of the initiators has a significant influence towards the polymerization based on the above mentioned results (Table 1, entries 1–3). Complex R,R-1 was chosen as the model initiator to investigate the influence of the reaction parameters on the ROP in detail. First, we examined the effect of temperature and solvent for the polymerization,
10
R,R-1 S,S-2
8 6 4
CD / mdeg
2 0 -2 -4 -6 -8 -10 -12 -14
260
280
300
320
340
360
380
Wavelength / nm Fig. 1. The liquid CD spectra of complexes R,R-1 (black) and S,S-2 (red).
Fig. 2. ORTEP drawing of complex R,R-1. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Al(1)\N(1) = 1.9223(15); Al(1)\N(2) = 1.9165(15); Al(1)\C(22) = 1.952(2); Al(1)\C(23) = 1.967(2); N(1)\C(2) = 1.317(2);C(2)\C(3) = 1.398(3); C(3)\C(4)= 1.396(3); C(4)\N(2) = 1.324(2); N(1)\Al(1)\N(2) = 94.96(6); N(1)\ Al(1)\C(22) = 114.47(8); N(1)\Al(1)\C(23) = 107.20(9); N(2)\Al(1)\C(22) = 116.19(9); N(2)\Al(1)\C(23) = 107.46(9); C(22)\Al(1)\C(23) = 114.55(11).
160
D. Kong et al. / Inorganic Chemistry Communications 22 (2012) 158–161
Table 1 Ring-opening polymerization of ε-CL by aluminum complexes 1 and 2. Entry Initiator
[ε-CL]/[Al]a T (°C) Time (h) Convb (%) Mn × 10− 4 PDI
1 2 3 4 5 6c 7 8 9 10 11 12 13 14
100 100 100 100 100 100 100 100 200 400 800 100 100 100
R,R-1 Rac-1 R,R-1 + S,S-1 S,S-2 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1
80 80 80 80 25 40 60 110 110 110 110 80 80 80
3 12 12 8 24 24 36 1 2 7 12 1 1.5 2
>99 76 >99 91 52 13 66 92 98 94 >99 19 39 76
6.19 6.18 8.46 3.37 2.66 1.70 4.25 5.92 6.02 6.46 6.79 1.27 2.97 4.96
1.22 1.38 1.23 2.03 1.21 1.46 1.32 1.17 1.21 1.17 1.22 1.13 1.13 1.14
In summary, we report herein the first examples of chiral β-diketiminate ligands with main group metals. The chiral dimethylaluminum complex R,R-1 is an efficient initiator for ring-opening polymerization of ε-caprolactone in the absence of alcohol with narrow molecular weight distribution. The activity of the chiral initiator R,R-1 for ROP of other cyclic esters is in progress in our laboratory. Acknowledgments We thank the financial supports from the Fundamental Research Funds for the Central Universities of China (DUT11LK27). We also gratefully thank Dr. Yuhan Zhou and Dr. Yuming Song for valuable discussions. Appendix A. Supplementary material
a
[ε-CL] = 1.0 M, the molecular weights (Mn) and molecular weight distributions (PDI) were determined by gel permeation chromatography (GPC). b Determined by 1H NMR spectroscopy. c Solvent: dichloromethane.
and the results are listed in Table 1 (entries 1, 5–8). It was found that the higher the temperature was, the faster the polymerization proceeded (Table 1, entries 1, 5–8). A narrow molecular weight distribution of 1.17 was obtained when the temperature was elevated to 110 °C (Table 1, entry 8), and toluene was a better solvent than dichloromethane (Table 1, entries 5, 6). Then we investigated the influence of different monomer-to-initiator ratio ([ε-CL]/[Al]) towards the polymerization. As shown in Table 1 (entries 8–11) and Fig. 3a, there is a linear relationship between the number-average molecular weight (Mn) and the ratio ([ε-CL]/[Al]), indicating the classical feature of a living polymerization progress. The dynamic study about the relationship between Mn and monomer conversions is demonstrated in Table 1 (entries 12–14 and 1) and Fig. 3b. It was found in Fig. 3b that Mn increases linearly along with the monomer conversions, and the molecular weight distributions almost do not change.
Fig. 3. The relationship between Mn with a) [ε-CL]/[Al] and b) molecular weight distributions.
monomer conversion
Supporting information available: experimental details for synthesis. Supplementary data to this article can be found online at http://dx.doi. org/10.1016/j.inoche.2012.05.050. References [1] M. Labet, W. Thielemans, Synthesis of polycaprolactone: a review, Chem. Soc. Rev. 38 (2009) 3484–3504. [2] (a) C.X.F. Lam, S.H. Teoh, D.W. Hutmacher, Comparison of the degradation of polycaprolactone and polycaprolactone-(beta-tricalcium phosphate) scaffolds in alkaline medium, Polym. Int. 56 (2007) 718–728; (b) J. Peña, T. Corrales, I. Izquierdo-Barba, A.L. Doadrio, M. Vallet-Regí, Long term degradation of poly(epsilon-caprolactone) films in biologically related fluids, Polym. Degrad. Stab. 91 (2006) 1424–1432; (c) M.J. Jenkins, K.L. Harrison, M.M.C.G. Silva, M.J. Whitaker, K.M. Shakesheff, S.M. Howdle, Characterisation of microcellular foams produced from semi-crystalline PCL using supercritical carbon dioxide, Eur. Polym. J. 42 (2006) 3145–3151; (d) D.W. Hutmacher, T. Schantz, I. Zein, K.W. Ng, S. Hin, T. Kim, C. Tan, Mechanical properties and cell cultural response of polycaprolactone scaffolds designed and fabricated via fused deposition modeling, J. Biomed. Mater. Res. 55 (2001) 203–216. [3] V.R. Sinha, K. Bansal, R. Kaushik, R. Kumria, A. Trehan, Poly-ε-caprolactone microspheres and nanospheres: an overview, Int. J. Pharm. 278 (2004) 1–23. [4] J.L. Hedrick, T. Magbitang, E.F. Connor, T. Glauser, W. Volksen, C.J. Hawker, V.Y. Lee, R.D. Miller, Application of complex macromolecular architectures for advanced microelectronic materials, Chem. Eur. J. 8 (2002) 3308–3319. [5] P. Joshi, G. Madras, Degradation of polycaprolactone in supercritical fluids, Polym. Degrad. Stab. 93 (2008) 1901–1908. [6] Y. Ikada, H. Tsuji, Biodegradable polyesters for medical and ecological applications, Macromol. Rapid Commun. 21 (2000) 117–132. [7] R.H. Platel, L.M. Hodgson, C.K. Williams, Biocompatible initiators for lactide polymerization, Polym. Rev. 48 (2008) 11–63. [8] W. Yao, Y. Mu, A. Gao, W. Gao, L. Ye, Bimetallic anilido-aldimine Al or Zn complexes for efficient ring-opening polymerization of ε-caprolactone, Dalton Trans. (2008) 3199–3206. [9] W. Li, W. Wu, Y. Wang, Y. Yao, Y. Zhang, Q. Shen, Bimetallic aluminum alkyl complexes as highly active initiators for the polymerization of ε-caprolactone, Dalton Trans. 40 (2011) 11378–11381 (and references therein). [10] (a) A. Otero, A. Lara-Sánchez, J. Fernández-Baeza, C. Alonso-Moreno, J.A. Castro-Osma, I. Márquez-Segovia, L.F. Sánchez-Barba, A.M. Rodríguez, J.C. Garcia-Martinez, Neutral and cationic aluminum complexes supported by acetamidate and thioacetamidate heteroscorpionate ligands as initiators for ring-opening polymerization of cyclic esters, Organometallics 30 (2011) 1507–1522; (b) D.J. Darensbourg, O. Karroonnirun, S.J. Wilson, Ring-opening polymerization of cyclic esters and trimethylene carbonate catalyzed by aluminum half-salen complexes, Inorg. Chem. 50 (2011) 6775–6787; (c) S. Gong, H. Ma, β-Diketiminate aluminium complexes: synthesis, characterization and ring-opening polymerization of cyclic esters, Dalton Trans. (2008) 3345–3357; (d) D. Chakraborty, E.Y.-X. Chen, Neutral, three-coordinate, chelating diamide aluminum complexes: catalysts/initiators for synthesis of telechelic oligomers and high polymers, Organometallics 21 (2002) 1438–1442; (e) Y. Lei, F. Chen, Y. Luo, P. Xu, Y. Wang, Y. Zhang, Bimetallic amidinate aluminum methyl complexes: synthesis, crystal structure and activity for ε-caprolactone polymerization, Inorg. Chim. Acta 368 (2011) 179–186. [11] (a) W.-A. Ma, Z.-X. Wang, Synthesis and characterisation of aluminium(III) and tin(II) complexes bearing quinoline-based N, N, O-tridentate ligands and their catalysis in the ring-opening polymerisation of ε-caprolactone, Dalton Trans. 40 (2011) 1778–1786; (b) W.-H. Sun, M. Shen, W. Zhang, W. Huang, S. Liu, C. Redshaw, Methylaluminium 8-quinolinolates: synthesis, characterization and use in ring-opening polymerization (ROP) of ε-caprolactone, Dalton Trans. 40 (2011) 2645–2653;
D. Kong et al. / Inorganic Chemistry Communications 22 (2012) 158–161 (c) W.-A. Ma, L. Wang, Z.-X. Wang, Dimethylaluminium iminophosphoranylenamides and iminophosphoranylanilides: synthesis, characterisation, and their controlled ring-opening polymerisation of ε-caprolactone, Dalton Trans. 40 (2011) 4669–4677; (d) M. Shen, W. Huang, W. Zhang, X. Hao, W.-H. Sun, C. Redshaw, Synthesis and characterisation of alkylaluminium benzimidazolates and their use in the ring-opening polymerisation of ε-caprolactone, Dalton Trans. 39 (2010) 9912–9922; (e) C.-Y. Tsai, C.-Y. Li, C.-H. Lin, B.-H. Huang, B.-T. Ko, Reactions of 4-methylidene-bis(1-phenyl-3-methylpyrazol-5-one) with trimethylaluminum: synthesis, structure and catalysis for the ring-opening polymerization of ε-caprolactone, Inorg. Chem. Commun. 14 (2011) 271–275; (f) W. Zhang, Y. Wang, J. Cao, L. Wang, Y. Pan, C. Redshaw, W.-H. Sun, Synthesis and characterization of dialkylaluminum amidates and their ring-opening polymerization of ε-caprolactone, Organometallics 30 (2011) 6253–6261. [12] S.P. Sarish, S. Nembenna, S. Nagendran, H.W. Roesky, Chemistry of soluble β-diketiminatoalkaline-earth metal complexes with M\X bonds (M_Mg, Ca, Sr; X_OH, Halides, H), Acc. Chem. Res. 44 (2011) 157–170. [13] L. Bourget-Merle, M.F. Lappert, J.R. Severn, The chemistry of β-diketiminatometal complexes, Chem. Rev. 102 (2002) 3031–3065. [14] (a) F. Drouin, P.O. Oguadinma, T.J.J. Whitehorne, R.E. Prud'homme, F. Schaper, Lactide polymerization with chiral β-diketiminate zinc complexes, Organometallics 29 (2010) 2139–2147; (b) I. El-Zoghbi, S. Latreche, F. Schaper, Zirconium complexes of symmetric and of chiral diketiminate ligands: synthesis, crystal structures, and reactivities, Organometallics 29 (2010) 1551–1559; (c) P.O. Oguadinma, F. Schaper, Bis(2-phenylethyl)-nacnac: a chiral diketiminate ligand and its copper complexes, Organometallics 28 (2009) 4089–4097; (d) P.O. Oguadinma, A. Rodrigue-Witchel, C. Reber, F. Schaper, Intramolecular p-stacking in copper(I) diketiminate phenanthroline complexes, Dalton Trans. 39 (2010) 8759–8768; (e) S. Latreche, F. Schaper, Highly symmetrical chromium(II) bisdiketiminate complexes, Inorg. Chim. Acta 365 (2011) 49–53.
161
[15] R, R-nacnacCH(Me)PhAlMe2, (R, R-1): 1H NMR (400 MHz, CDCl3): δ −1.05 (s, 6H, AlCH3), 1.67 (d, J = 8 Hz, 6H, CH(Me)Ph), 1.92 (s, 6H, CH3), 4.64 (s, 1H, γ-CH), 4.94 (q, J = 8 Hz, 2H, CH(Me)Ph), 7.22–7.34 (m, 10H, Ph); 13C NMR (100 MHz, C6D6): δ − 5.85 (AlCH3), 21.07 (CH(Me)Ph), 22.91 (CMe), 56.04 (CH(Me)Ph), 98.43 (CH), 126.62 (Ph), 126.70, 128.33, 144.36 (ipsoPh), 167.88 (NCMe); MS (EI, m/z): 347.2060 (M + − CH3). [16] B. Qian, D.L. Ward, M.R. Smith, Synthesis, structure, and reactivity of β-diketiminato aluminum complexes, Organometallics 17 (1998) 3070–3076. [17] S, S-nacnacCH(i-Pr)PhH: 1H NMR (400 MHz, CDCl3): δ 0.87 (d, J = 8 Hz, 6H, CH(CH3)2), 1.07 (d, J = 8 Hz, 6H, CH(CH3)2), 1.76 (s, 6H, Me(C = N)),2.09–2.14 (m, J=8 Hz, 2H, CH(CH3)2), 4.18 (d, J=8 Hz, 2H, CH(iPr)Ph), 4.41 (s, 1H, CH(C= N)2), 7.17–7.31 (t, 2H, Ph), 11.60 (s, 1H, NH);MS (ESI, m/z): 363.2798 (M ++H). [18] S, S-nacnacCH(i-Pr)PhAlMe2 (S, S-2): 1H NMR (400 MHz, CDCl3): δ –0.92 (s, 6H, AlMe2), 0.81 (d, J = 8 Hz, 6H, CH(CH3)2), 0.86 (d, J = 8 Hz, 6H, CH(CH3)2), 2.07 (s, 6H, CH3), 2.49–2.55 (m, J = 8 Hz, 2H, CH(CH3)2), 4.19 (d, J = 8 Hz, 2H, CH(iPr)Ph), 4.72 (s, 1H, γ-CH), 7.17–7.29 (Ph). [19] Crystal data for R, R-1: C23H31AlN2, M = 362.48, orthorhombic, space group P212121, a = 8.5158(17), b = 9.0441(17), c = 28.909(6) Å, α = β = γ = 90° V = 2226.5(7) Å3, Z = 4, dc = 1.081 Mg m–3, F(000) = 784, λ = 0.71073 Å, T = 293(2) K, 11131 reflections collected, 3912 unique [R(int) = 0.0229], no. of observed reflections 3426 (I>2σ(I)); R1=0.0358, wR2=0.0946; GOF(F2)=1.043; max./min. residual electron density: 0.137∕-0.112 e Å–3. CCDC-855447 (R, R-1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)1223–336–033; or [email protected]). [20] C.E. Radzewich, M.P. Coles, R.F. Jordan, Reversible ethylene cycloaddition reactions of cationic aluminum β-diketiminate complexes, J. Am. Chem. Soc. 120 (1998) 9384–9385.
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Dimethylaluminum complexes bearing a chiral diketiminate ligand: Synthesis, characterization and ring-opening polymerization of ε-caprolactone Dexu Kong, Ying Peng ⁎, Di Li, Yang Li, Pingping Chen, Jingping Qu ⁎ State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, P.R. China
a r t i c l e
i n f o
Article history: Received 19 April 2012 Accepted 29 May 2012 Available online 4 June 2012 Keywords: Aluminum complexes Chiral β-diketiminate ligand ε-Caprolactone Initiator Ring-opening polymerization
a b s t r a c t Reaction of the chiral aliphatic N-substituted diketiminate ligand, N,N′-Di(R-phenylethyl)-2-amino-4iminopent-2-ene (R,R-nacnacCH(Me)PhH), with AlMe3 affords the monomeric dimethylaluminum complex R,RnacnacCH(Me)PhAlMe2 (R,R-1). Complex S,S-nacnacCH(i-Pr)PhAlMe2 (S,S-2) is prepared by S,S-nacnacCH(i-Pr)PhH and AlMe3. Complexes R,R-1 and S,S-2 are the first examples of chiral β-diketiminate ligands with main group metals. All these two complexes exhibit activity for the ring-opening polymerization of ε-caprolactone in the absence of alcohol, and complex R,R-1 shows a higher activity with a narrow molecular weight distribution (PDI= 1.17) in toluene. © 2012 Elsevier B.V. All rights reserved.
Polycaprolactone (PCL) is an important polymer due to its physical, thermal and mechanical properties, as well as its miscibility with other polymers and biodegradability [1]. It is widely applied in different fields such as scaffolds in tissue engineering [2], in long-term drug delivery systems [3], in microelectronics [4], as adhesives [5], and in packaging [6]. PCL is usually prepared by the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by metal complexes [1]. Among the reported initiators, organoaluminum complexes have attracted much attention because they have a good control over the polymerization reaction [7] and low toxicity [8]. However, few studies have involved the alkyl aluminum complexes as the initiator for the ROP of ε-CL without the presence of alcohol, because these complexes usually showed no or very low activity towards this polymerization [9], and most of them got a broad molecular weight distribution during the reaction [9,10]. Although plenty of various ligands are employed in the alkyl aluminum initiators [9–11], aluminum complexes bearing N,N′substituted β-diketiminate (“nacnac”) ligands and their application in ROP of cyclic esters are still less investigated [10c]. The β-diketiminate ligands have gained increasing interest due to their suitability as fine-tuned spectator ligands by variation of the steric and electronic properties of the N,N′-substituents [12,13]. Most of the current work with β-diketiminate ligands are focused on the aromatic N-substituents [10c,12,13], and metal complexes with aliphatic N-substituted ones are less reported [12,13]. Compared to the N-aryl β-diketiminates, the
⁎ Corresponding authors. Tel./fax: + 86 411 84986186. E-mail addresses: [email protected] (Y. Peng), [email protected] (J. Qu). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2012.05.050
aliphatic N-substituted ligands can not only afford the adjustable steric environment around the metal centers, but also can easily introduce chirality into the diketiminate ligands. Recently, Schaper et al. reported several instances of transition metals (copper, zinc, chromium and zirconium) supported by chiral βdiketiminate ligands with aliphatic N-substituents and invested their application in the polymerization of lactide [14], while no examples of main group metal bearing such chiral ligands are known. Herein we describe our work on the preparation of dimethylaluminum complexes with chiral diketiminate ligands (Scheme 1) and their application in the ring-opening polymerization of ε-caprolactone. The chiral dimethylaluminum complex R,R-1 [15] was prepared in 62% yield by the treatment of chiral ligand R,R-nacnac CH(Me)PhH [14c] with AlMe3 (1.07 molar equiv) in hexane at room temperature. The 1 H NMR of R,R-1 shows the characteristic resonance for Al–Me at −1.05 ppm in CDCl3, which is comparable to those for methylaluminum complexes [(4-MeC6H4NCMe)2HC]AlMe2 (δ −1.05 ppm), [(2,6-iPr2C6H3NCMe)2HC]AlMe2 (δ −1.00 ppm) with aryl-substituted nacnac ligands [16]. The most intense peak in the EI mass spectrum of R,R-1 appears at m/z 347.2060 [M+ − CH3]. Treatment of 1.16 molar equiv of AlMe3 with a more bulky chiral ligand S,S-nacnacCH(i-Pr)PhH [17] at 50 °C afforded complex S,S-2, and its 1H NMR shows Al–Me peak at δ −0.92 ppm [18]. When the reaction was carried out at room temperature, lots of unreacted ligands were detected from 1H NMR, which implies the more bulky chiral ligand S,S-nacnacCH(i-Pr)PhH is less reactive compared to ligand R,R-nacnacCH(Me)PhH due to the larger steric hindrance from isopropyl group instead of methyl group. In addition, the chiral nature of the optically active complexes R,R-1 and S,S-2 was
D. Kong et al. / Inorganic Chemistry Communications 22 (2012) 158–161
159
Scheme 1. Preparation of complexes R,R-1 and S,S-2.
confirmed by the liquid circular dichroism (CD) spectra measured in methanol solution (Fig. 1). The CD spectrum of R,R-1 exhibits a positive Cotton effect at 294 and 336 nm, while S,S-2 shows Cotton effects of the opposite sign at 312 and 326 nm. The molecular structure (Fig. 2) of R,R-1 was confirmed by X-ray crystallography [19]. As shown in Fig. 2, the aluminum center in R,R-1 is surrounded by two nitrogen atoms of the chiral nacnac ligand and two methyl groups, exhibiting distorted tetrahedral geometry. The N(1)\Al(1)\N(2) angle (94.96(6)°) and C(22)\Al(1)\C(23) angle (114.55(11)°) are comparable respectively to those in alkylaluminum complexes with aryl-substituted nacnac ligands [(4-MeC6H4NCMe)2HC]AlMe2 (A) (94.72(14)°; 115.4(2)) [16], [(2,6-iPr2C6H3NCMe)2HC] AlMe2 (B) (96.18(9)°; 117.4(1)) [16,20], or [(2,6-iPr2C6H34-ClC6H4N2C2Me2)HC]AlEt2 (C) (94.50(7)°; 115.59(11)°) [10c]. The deviation of the aluminum center from the ligand backbone N1\C2\C3\C4\N2 is 0.85 Å, which is larger than those in the above mentioned complexes A (0.33 Å), B (0.72 Å) and C (0.56 Å) with aryl-substituted nacnac ligands. Thus we may conclude that the aliphatic substituents on the nitrogen atom increases the steric bulk of the ligand, and these phenomena were also found for the copper complex bearing this kind of chiral ligand [14c]. The chiral nacnac ligand in R,R-1 retains the delocalization with the very close bond distances of the C\C (1.396(3) and 1.398(3) Å) and C\N bonds (1.317(2) and 1.324(2)Å). The two Al\N bond distances (1.9223(15) and 1.9165(15) Å) are almost identical, which are close to those in a chiral complexes A, B and C. The Al\CH3 distances (1.952(2) and 1.967(2)Å) are also comparable to those in complexes A (1.955(4) and 1.961(3) Å) and B (1.958(3) and 1.970(3) Å). The polymerization of ε-CL was studied under different conditions using complexes R,R-1 and S,S-2 as the initiators (Table 1). From Table 1 (entries 1, 4) we can see that these two dialkylaluminum complexes both can initiate the ROP of ε-CL in the absence of alcohol, and the chiral complex R,R-1 exhibits a higher activity for the polymerization, producing a high conversion up to over 99% within 3 h at 80 °C
and a narrow molecular weight distribution of 1.22. Comparatively, the diethylaluminum complexes bearing N-aryl β-diketiminate ligands had broader molecular weight distributions (PDI=1.66–3.74) and longer reaction time (17 h vs. 96% conversion) under the same polymerization conditions [10]. The more bulky chiral complex S,S-2 had a lower activity compared to that of R,R-1 perhaps due to the larger isopropyl group on the chiral carbon atom instead of methyl group in R,R-1 which hinders the coordination of monomer to the initiator. The 1H NMR spectrum of the purified polymer indicates the existence of a tiny peak at 3.65 ppm which is assignable to the methene protons (\CH2\) adjacent to a terminal hydroxyl group, suggesting a linear structure of the oligomer instead of cyclic. In order to elucidate the effect of the structure of the initiators on the polymerization, the racemic complex rac-1 was prepared by the reaction of AlMe3 with racemic ligand nacnac CH(Me)PhH. Treatment of AlMe3 with chiral ligand S,S-nacnac CH(Me)PhH [14c] afforded complex S,S-1. Using complex rac-1 as the initiator, the polymerization was carried out in toluene (Table 1, entry 2) and a broader molecular weight distribution (PDI=1.38) was obtained. When using the mixture of equiv of R,R-1 and S,S-1 as the initiator for the polymerization, the similar PDI value (PDI=1.23, Table 1, entry 3) as that for R,R-1 (PDI=1.22, Table 1, entry 1) was attained. Although the isolation of mesomeric complex meso-1 from the reaction of AlMe3 with mesomeric ligand R, S-nacnacCH(Me)PhH was unsuccessful, we believe that the steric configuration of the initiators has a significant influence towards the polymerization based on the above mentioned results (Table 1, entries 1–3). Complex R,R-1 was chosen as the model initiator to investigate the influence of the reaction parameters on the ROP in detail. First, we examined the effect of temperature and solvent for the polymerization,
10
R,R-1 S,S-2
8 6 4
CD / mdeg
2 0 -2 -4 -6 -8 -10 -12 -14
260
280
300
320
340
360
380
Wavelength / nm Fig. 1. The liquid CD spectra of complexes R,R-1 (black) and S,S-2 (red).
Fig. 2. ORTEP drawing of complex R,R-1. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Al(1)\N(1) = 1.9223(15); Al(1)\N(2) = 1.9165(15); Al(1)\C(22) = 1.952(2); Al(1)\C(23) = 1.967(2); N(1)\C(2) = 1.317(2);C(2)\C(3) = 1.398(3); C(3)\C(4)= 1.396(3); C(4)\N(2) = 1.324(2); N(1)\Al(1)\N(2) = 94.96(6); N(1)\ Al(1)\C(22) = 114.47(8); N(1)\Al(1)\C(23) = 107.20(9); N(2)\Al(1)\C(22) = 116.19(9); N(2)\Al(1)\C(23) = 107.46(9); C(22)\Al(1)\C(23) = 114.55(11).
160
D. Kong et al. / Inorganic Chemistry Communications 22 (2012) 158–161
Table 1 Ring-opening polymerization of ε-CL by aluminum complexes 1 and 2. Entry Initiator
[ε-CL]/[Al]a T (°C) Time (h) Convb (%) Mn × 10− 4 PDI
1 2 3 4 5 6c 7 8 9 10 11 12 13 14
100 100 100 100 100 100 100 100 200 400 800 100 100 100
R,R-1 Rac-1 R,R-1 + S,S-1 S,S-2 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1 R,R-1
80 80 80 80 25 40 60 110 110 110 110 80 80 80
3 12 12 8 24 24 36 1 2 7 12 1 1.5 2
>99 76 >99 91 52 13 66 92 98 94 >99 19 39 76
6.19 6.18 8.46 3.37 2.66 1.70 4.25 5.92 6.02 6.46 6.79 1.27 2.97 4.96
1.22 1.38 1.23 2.03 1.21 1.46 1.32 1.17 1.21 1.17 1.22 1.13 1.13 1.14
In summary, we report herein the first examples of chiral β-diketiminate ligands with main group metals. The chiral dimethylaluminum complex R,R-1 is an efficient initiator for ring-opening polymerization of ε-caprolactone in the absence of alcohol with narrow molecular weight distribution. The activity of the chiral initiator R,R-1 for ROP of other cyclic esters is in progress in our laboratory. Acknowledgments We thank the financial supports from the Fundamental Research Funds for the Central Universities of China (DUT11LK27). We also gratefully thank Dr. Yuhan Zhou and Dr. Yuming Song for valuable discussions. Appendix A. Supplementary material
a
[ε-CL] = 1.0 M, the molecular weights (Mn) and molecular weight distributions (PDI) were determined by gel permeation chromatography (GPC). b Determined by 1H NMR spectroscopy. c Solvent: dichloromethane.
and the results are listed in Table 1 (entries 1, 5–8). It was found that the higher the temperature was, the faster the polymerization proceeded (Table 1, entries 1, 5–8). A narrow molecular weight distribution of 1.17 was obtained when the temperature was elevated to 110 °C (Table 1, entry 8), and toluene was a better solvent than dichloromethane (Table 1, entries 5, 6). Then we investigated the influence of different monomer-to-initiator ratio ([ε-CL]/[Al]) towards the polymerization. As shown in Table 1 (entries 8–11) and Fig. 3a, there is a linear relationship between the number-average molecular weight (Mn) and the ratio ([ε-CL]/[Al]), indicating the classical feature of a living polymerization progress. The dynamic study about the relationship between Mn and monomer conversions is demonstrated in Table 1 (entries 12–14 and 1) and Fig. 3b. It was found in Fig. 3b that Mn increases linearly along with the monomer conversions, and the molecular weight distributions almost do not change.
Fig. 3. The relationship between Mn with a) [ε-CL]/[Al] and b) molecular weight distributions.
monomer conversion
Supporting information available: experimental details for synthesis. Supplementary data to this article can be found online at http://dx.doi. org/10.1016/j.inoche.2012.05.050. References [1] M. Labet, W. Thielemans, Synthesis of polycaprolactone: a review, Chem. Soc. Rev. 38 (2009) 3484–3504. [2] (a) C.X.F. Lam, S.H. Teoh, D.W. Hutmacher, Comparison of the degradation of polycaprolactone and polycaprolactone-(beta-tricalcium phosphate) scaffolds in alkaline medium, Polym. Int. 56 (2007) 718–728; (b) J. Peña, T. Corrales, I. Izquierdo-Barba, A.L. Doadrio, M. Vallet-Regí, Long term degradation of poly(epsilon-caprolactone) films in biologically related fluids, Polym. Degrad. Stab. 91 (2006) 1424–1432; (c) M.J. Jenkins, K.L. Harrison, M.M.C.G. Silva, M.J. Whitaker, K.M. Shakesheff, S.M. Howdle, Characterisation of microcellular foams produced from semi-crystalline PCL using supercritical carbon dioxide, Eur. Polym. J. 42 (2006) 3145–3151; (d) D.W. Hutmacher, T. Schantz, I. Zein, K.W. Ng, S. Hin, T. Kim, C. Tan, Mechanical properties and cell cultural response of polycaprolactone scaffolds designed and fabricated via fused deposition modeling, J. Biomed. Mater. Res. 55 (2001) 203–216. [3] V.R. Sinha, K. Bansal, R. Kaushik, R. Kumria, A. Trehan, Poly-ε-caprolactone microspheres and nanospheres: an overview, Int. J. Pharm. 278 (2004) 1–23. [4] J.L. Hedrick, T. Magbitang, E.F. Connor, T. Glauser, W. Volksen, C.J. Hawker, V.Y. Lee, R.D. Miller, Application of complex macromolecular architectures for advanced microelectronic materials, Chem. Eur. J. 8 (2002) 3308–3319. [5] P. Joshi, G. Madras, Degradation of polycaprolactone in supercritical fluids, Polym. Degrad. Stab. 93 (2008) 1901–1908. [6] Y. Ikada, H. Tsuji, Biodegradable polyesters for medical and ecological applications, Macromol. Rapid Commun. 21 (2000) 117–132. [7] R.H. Platel, L.M. Hodgson, C.K. Williams, Biocompatible initiators for lactide polymerization, Polym. Rev. 48 (2008) 11–63. [8] W. Yao, Y. Mu, A. Gao, W. Gao, L. Ye, Bimetallic anilido-aldimine Al or Zn complexes for efficient ring-opening polymerization of ε-caprolactone, Dalton Trans. (2008) 3199–3206. [9] W. Li, W. Wu, Y. Wang, Y. Yao, Y. Zhang, Q. Shen, Bimetallic aluminum alkyl complexes as highly active initiators for the polymerization of ε-caprolactone, Dalton Trans. 40 (2011) 11378–11381 (and references therein). [10] (a) A. Otero, A. Lara-Sánchez, J. Fernández-Baeza, C. Alonso-Moreno, J.A. Castro-Osma, I. Márquez-Segovia, L.F. Sánchez-Barba, A.M. Rodríguez, J.C. Garcia-Martinez, Neutral and cationic aluminum complexes supported by acetamidate and thioacetamidate heteroscorpionate ligands as initiators for ring-opening polymerization of cyclic esters, Organometallics 30 (2011) 1507–1522; (b) D.J. Darensbourg, O. Karroonnirun, S.J. Wilson, Ring-opening polymerization of cyclic esters and trimethylene carbonate catalyzed by aluminum half-salen complexes, Inorg. Chem. 50 (2011) 6775–6787; (c) S. Gong, H. Ma, β-Diketiminate aluminium complexes: synthesis, characterization and ring-opening polymerization of cyclic esters, Dalton Trans. (2008) 3345–3357; (d) D. Chakraborty, E.Y.-X. Chen, Neutral, three-coordinate, chelating diamide aluminum complexes: catalysts/initiators for synthesis of telechelic oligomers and high polymers, Organometallics 21 (2002) 1438–1442; (e) Y. Lei, F. Chen, Y. Luo, P. Xu, Y. Wang, Y. Zhang, Bimetallic amidinate aluminum methyl complexes: synthesis, crystal structure and activity for ε-caprolactone polymerization, Inorg. Chim. Acta 368 (2011) 179–186. [11] (a) W.-A. Ma, Z.-X. Wang, Synthesis and characterisation of aluminium(III) and tin(II) complexes bearing quinoline-based N, N, O-tridentate ligands and their catalysis in the ring-opening polymerisation of ε-caprolactone, Dalton Trans. 40 (2011) 1778–1786; (b) W.-H. Sun, M. Shen, W. Zhang, W. Huang, S. Liu, C. Redshaw, Methylaluminium 8-quinolinolates: synthesis, characterization and use in ring-opening polymerization (ROP) of ε-caprolactone, Dalton Trans. 40 (2011) 2645–2653;
D. Kong et al. / Inorganic Chemistry Communications 22 (2012) 158–161 (c) W.-A. Ma, L. Wang, Z.-X. Wang, Dimethylaluminium iminophosphoranylenamides and iminophosphoranylanilides: synthesis, characterisation, and their controlled ring-opening polymerisation of ε-caprolactone, Dalton Trans. 40 (2011) 4669–4677; (d) M. Shen, W. Huang, W. Zhang, X. Hao, W.-H. Sun, C. Redshaw, Synthesis and characterisation of alkylaluminium benzimidazolates and their use in the ring-opening polymerisation of ε-caprolactone, Dalton Trans. 39 (2010) 9912–9922; (e) C.-Y. Tsai, C.-Y. Li, C.-H. Lin, B.-H. Huang, B.-T. Ko, Reactions of 4-methylidene-bis(1-phenyl-3-methylpyrazol-5-one) with trimethylaluminum: synthesis, structure and catalysis for the ring-opening polymerization of ε-caprolactone, Inorg. Chem. Commun. 14 (2011) 271–275; (f) W. Zhang, Y. Wang, J. Cao, L. Wang, Y. Pan, C. Redshaw, W.-H. Sun, Synthesis and characterization of dialkylaluminum amidates and their ring-opening polymerization of ε-caprolactone, Organometallics 30 (2011) 6253–6261. [12] S.P. Sarish, S. Nembenna, S. Nagendran, H.W. Roesky, Chemistry of soluble β-diketiminatoalkaline-earth metal complexes with M\X bonds (M_Mg, Ca, Sr; X_OH, Halides, H), Acc. Chem. Res. 44 (2011) 157–170. [13] L. Bourget-Merle, M.F. Lappert, J.R. Severn, The chemistry of β-diketiminatometal complexes, Chem. Rev. 102 (2002) 3031–3065. [14] (a) F. Drouin, P.O. Oguadinma, T.J.J. Whitehorne, R.E. Prud'homme, F. Schaper, Lactide polymerization with chiral β-diketiminate zinc complexes, Organometallics 29 (2010) 2139–2147; (b) I. El-Zoghbi, S. Latreche, F. Schaper, Zirconium complexes of symmetric and of chiral diketiminate ligands: synthesis, crystal structures, and reactivities, Organometallics 29 (2010) 1551–1559; (c) P.O. Oguadinma, F. Schaper, Bis(2-phenylethyl)-nacnac: a chiral diketiminate ligand and its copper complexes, Organometallics 28 (2009) 4089–4097; (d) P.O. Oguadinma, A. Rodrigue-Witchel, C. Reber, F. Schaper, Intramolecular p-stacking in copper(I) diketiminate phenanthroline complexes, Dalton Trans. 39 (2010) 8759–8768; (e) S. Latreche, F. Schaper, Highly symmetrical chromium(II) bisdiketiminate complexes, Inorg. Chim. Acta 365 (2011) 49–53.
161
[15] R, R-nacnacCH(Me)PhAlMe2, (R, R-1): 1H NMR (400 MHz, CDCl3): δ −1.05 (s, 6H, AlCH3), 1.67 (d, J = 8 Hz, 6H, CH(Me)Ph), 1.92 (s, 6H, CH3), 4.64 (s, 1H, γ-CH), 4.94 (q, J = 8 Hz, 2H, CH(Me)Ph), 7.22–7.34 (m, 10H, Ph); 13C NMR (100 MHz, C6D6): δ − 5.85 (AlCH3), 21.07 (CH(Me)Ph), 22.91 (CMe), 56.04 (CH(Me)Ph), 98.43 (CH), 126.62 (Ph), 126.70, 128.33, 144.36 (ipsoPh), 167.88 (NCMe); MS (EI, m/z): 347.2060 (M + − CH3). [16] B. Qian, D.L. Ward, M.R. Smith, Synthesis, structure, and reactivity of β-diketiminato aluminum complexes, Organometallics 17 (1998) 3070–3076. [17] S, S-nacnacCH(i-Pr)PhH: 1H NMR (400 MHz, CDCl3): δ 0.87 (d, J = 8 Hz, 6H, CH(CH3)2), 1.07 (d, J = 8 Hz, 6H, CH(CH3)2), 1.76 (s, 6H, Me(C = N)),2.09–2.14 (m, J=8 Hz, 2H, CH(CH3)2), 4.18 (d, J=8 Hz, 2H, CH(iPr)Ph), 4.41 (s, 1H, CH(C= N)2), 7.17–7.31 (t, 2H, Ph), 11.60 (s, 1H, NH);MS (ESI, m/z): 363.2798 (M ++H). [18] S, S-nacnacCH(i-Pr)PhAlMe2 (S, S-2): 1H NMR (400 MHz, CDCl3): δ –0.92 (s, 6H, AlMe2), 0.81 (d, J = 8 Hz, 6H, CH(CH3)2), 0.86 (d, J = 8 Hz, 6H, CH(CH3)2), 2.07 (s, 6H, CH3), 2.49–2.55 (m, J = 8 Hz, 2H, CH(CH3)2), 4.19 (d, J = 8 Hz, 2H, CH(iPr)Ph), 4.72 (s, 1H, γ-CH), 7.17–7.29 (Ph). [19] Crystal data for R, R-1: C23H31AlN2, M = 362.48, orthorhombic, space group P212121, a = 8.5158(17), b = 9.0441(17), c = 28.909(6) Å, α = β = γ = 90° V = 2226.5(7) Å3, Z = 4, dc = 1.081 Mg m–3, F(000) = 784, λ = 0.71073 Å, T = 293(2) K, 11131 reflections collected, 3912 unique [R(int) = 0.0229], no. of observed reflections 3426 (I>2σ(I)); R1=0.0358, wR2=0.0946; GOF(F2)=1.043; max./min. residual electron density: 0.137∕-0.112 e Å–3. CCDC-855447 (R, R-1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)1223–336–033; or [email protected]). [20] C.E. Radzewich, M.P. Coles, R.F. Jordan, Reversible ethylene cycloaddition reactions of cationic aluminum β-diketiminate complexes, J. Am. Chem. Soc. 120 (1998) 9384–9385.