Synthesis, Structures, and Norbornene Polymerization Behavior of

Organometallics 2011, 30, 153–159 DOI: 10.1021/om100994s

153

Synthesis, Structures, and Norbornene Polymerization Behavior of Aryloxide-N-Heterocyclic Carbene Ligated Palladacycles Yong Kong,† Lifang Wen,‡ Haibin Song,† Shansheng Xu,† Min Yang,‡ Binyuan Liu,‡ and Baiquan Wang*,† †

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China, and ‡Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, People’s Republic of China Received October 19, 2010

Treatment of the o-hydroxyaryl imidazolinium pro-ligands (2-OH-3,5-tBu2C6H2)(R)(C3H3N2)þBr[R = Me (3a), iPr (3b), tBu (3c), Ph (3d), Mes (3e)] with the palladacycle [Pd(dmba)(μ-Cl)]2 (dmba = Me2NCH2C6H5) (2) afforded the corresponding aryloxide-NHC (NHC = N-heterocyclic carbene)ligated palladacycles (2-O-3,5-tBu2C6H2)(R)(NHC)Pd(dmba) [R = Me (4), iPr (5), Ph (6), Mes (7), t Bu (8)]. Notably, without isolating 2, complexes 4-8 could also be obtained by one-pot, threecomponent, sequential reaction of N,N-dimethylbenzylamine, PdCl2, and the pro-ligands in refluxing acetonitrile in the presence of K2CO3. When the N-functional group on the NHCs is tert-butyl, the NHC in 8 adopts an abnormal binding (C4-bonding). All these complexes were fully characterized by 1 H and 13C NMR, high-resolution mass spectrometry (HRMS), and elemental analysis. Singlecrystal X-ray diffraction analysis results further confirmed the molecular structures of 4-8 and the abnormal binding of NHC in 8. With methylaluminoxane (MAO) as cocatalyst these palladacycles showed excellent catalytic activities up to 107 g of PNB (mol of Pd) -1 h-1 in the addition polymerization of norbornene.

Introduction Since the discovery of the free N-heterocyclic carbenes (NHCs),1 NHCs and their transition metal complexes have found widespread uses in homogeneous catalysis.2 In particular, the Pd-NHC system has been extensively utilized in catalytic formation of carbon-carbon and carbon-heteroatom bonds.3 Although palladacycles have become one of the most

widely employed palladium catalysts because of their flexible framework and robustness,4 palladacycles with NHCs have been less studied.5 In addition, few palladacycles ligated by NHCs have been applied to olefin polymerization. Recently, much research efforts have been devoted to the study of transition metal complexes with anion-tethered NHCs ligands.6 As an anchor, the introduced anion group can enhance the bond between the NHCs and metal centers, reducing the tendency of ligand dissociation. More recently,

*To whom correspondence should be addressed. Tel and fax: þ86-2223504781. E-mail: [email protected]. (1) (a) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (b) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913. (2) For recent reviews, see: (a) Bourissou, D.; Guerret, O.; Gabbai, F.; Bertrand, G. Chem. Rev. 2000, 100, 39. (b) Herrmann, W. A.; K€ocher, C. Angew. Chem., Int. Ed. 2002, 41, 1290. (c) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239. (d) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247. (e) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978. (f) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (g) K€uhl, O. Chem. Soc. Rev. 2007, 36, 592. (h) Arnold, P. L.; Pearson, S. Coord. Chem. Rev. 2007, 251, 596. (i) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610. (j) Lin, I. J. B.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642. (k) Dragutan, V.; Dragutan, I.; Delaude, L.; Demonceau, A. Coord. Chem. Rev. 2007, 251, 765. (l) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251, 841. (m) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (n) de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862. (o) DíezGonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (3) Review articles: (a) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. Rev. 2000, 100, 2741. (b) Bedford, R. B.; Cazin, C. S. J.; Holder, D. Coord. Chem. Rev. 2004, 248, 2283. (c) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4442. (d) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366. (e) Beccalli, E. M.; Broggini, G.; Martinelli, M.; Sottocornola, S. Chem. Rev. 2007, 107, 5318.

(4) Selected reviews and articles for palladacycles: (a) Dupont, J.; Pfeffer, M.; Spencer, J. Eur. J. Inorg. Chem. 2001, 1917. (b) Bedford, R. B. Chem. Commun. 2003, 1787. (c) Beletskaya, I. P.; Cheprakov, A. V. J. Organomet. Chem. 2004, 689, 4055. (d) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105, 2527. (e) Ruiz, J.; Cutillas, N.; Lopez, F.; Lopez, G.; Bautista, D. Organometallics 2006, 25, 5768. (f) Ruiz, J.; Villa, M. D.; Cutillas, N.; Lopez, G.; Haro, C.; Bautista, D.; Moreno, V.; Valencia, L. Inorg. Chem. 2008, 47, 4490. (g) Ruiz, J.; Rodríguez, V.; Cutillas, N.; Hoffmann, A.; Chamayou, A.; Kazmierczak, K.; Janiak, C. CrystEngComm. 2008, 10, 1928. (5) Selected reviews and articles: (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440. (b) Navarro, O.; Kelly, R. A., III; Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 16194. (c) Viciu, M. S.; Kelly, R. A., III; Stevens, E. D.; Naud, F.; Studer, M.; Nolan, S. P. Org. Lett. 2003, 5, 1479. (d) Frey, G. D.; Sch€utz, J.; Herdtweck, E.; Herrmann, W. A. Organometallics 2005, 24, 4416. (e) Bedford, R. B.; Betham, M.; Blake, M. E.; Frost, R. M.; Horton, P. N.; Hursthouse, M. B.; Lopez-Nicolas, R.-M. Dalton Trans. 2005, 2774. (f) Navarro, O.; Marion, N.; Oonishi, Y.; Kelly, R. A., III; Nolan, S. P. J. Org. Chem. 2006, 71, 685. (g) Broggi, J.; Clavier, H.; Nolan, S. P. Organometallics 2008, 27, 5525. (h) Kantchev, E. A. B.; Peh, G.; Zhang, C. Org. Lett. 2008, 10, 3949. (i) Kantchev, E. A. B.; Ying, J. Y. Organometallics 2009, 28, 289. (j) Peh, G. -R.; Kantchev, E. A. B.; Er, J.-C.; Ying, J. Y. Chem.;Eur. J. 2010, 16, 4010. (6) (a) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007, 36, 1732. (b) Arnold, P. L.; Casely, I. J. Chem. Rev. 2009, 109, 3599.

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Kong et al. Scheme 1

we succeeded in developing of a series of o-hydroxyarylsubstituted unsaturated NHC ligands and their palladium complexes.7 The o-hydroxyaryl-substituted NHC ligands possess an analogue of the salicylaldimine framework, a common motif in organometallic chemistry and extensively employed in catalytic organic reactions and olefin polymerization.8 We envisioned the o-hydroxyaryl-substituted NHC ligands would improve the activity of palladacycle complexes for olefin polymerization. The ligated palladacycle would also decrease the amounts of the cocatalyst methylaluminoxane (MAO) and improve the stability of the active species. In this study, we report the synthesis and structures of a series of aryloxide-NHC-ligated palladacycles. The addition polymerization of norbornene with these complexes is also investigated in the presence of methylaluminoxane (MAO). To the best of our knowledge, this is the first report of this kind of salicylaldimine-like NHC palladacycles for the addition polymerization of norbornene.

Results and Discussion Synthesis of Palladium Complexes. Following the synthetic route reported previously by our group,7 a series of o-hydroxyaryl imidazolinium pro-ligands can be facilely synthesized by the reactions of 4-bromo-2,4,6-tri-tert-butyl-2,5-cyclohexadien-1-one with different N-substituted imidazoles. The addition of N,N-dimethylbenzylamine (1) to PdCl2 in (7) (a) Ren, H.; Yao, P.; Xu, S.; Song, H.; Wang, B. J. Organomet. Chem. 2007, 692, 2092. (b) Kong, Y.; Ren, H.; Xu, S.; Song, H.; Liu, B.; Wang, B. Organometallics 2009, 28, 5934. (8) (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460. (b) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.; Grubbs, R. H. Chem. Commun. 2003, 2272. (c) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa, N.; Takagi, Y.; Kazutaka, T.; Nitabaru, M.; Nakano, T.; Tankaka, H.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2001, 123, 6847, and references therein. (d) Tian, J.; Hustad, P. D.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 5134. (e) Gibson, V. C.; Mastroianni, S.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2000, 1969. (f) Makio, H.; Fujita, T. Bull. Chem. Soc. Jpn. 2005, 78, 52. (9) (a) Cope, A. C.; Friedrich, E. C. J. Am. Chem. Soc. 1968, 90, 909. (b) Mentes, A.; Kemmitt, R. D. W.; Fawcett, J.; Russell, D. R. J. Mol. Struct. 2004, 693, 241. (10) (a) Sakaguchi, S.; Yoo, K. S.; O’Neill, J.; Lee, J. H.; Stewart, T.; Jung, K. W. Angew. Chem., Int. Ed. 2008, 47, 9326. (b) Yoo, K. S.; O'Neill, J.; Sakaguchi, S.; Giles, R.; Lee, J. H.; Jung, K. W. J. Org. Chem. 2010, 75, 95.

methanol would lead to [Pd(dmba)(μ-Cl)]2 (dmba = Me2NCH2C6H5) (2),9 which is utilized in various reactions as a reactant or catalyst. Following the method by Jung et al.10 and our group7 for the synthesis of bis(alkoxide/aryloxideN-heterocyclic carbene) palladium complexes, reactions of the o-hydroxyaryl imidazolinium pro-ligands (2-OH-3,5-tBu2C6H2)(R)(C3H3N2)þBr- [R = Me (3a), iPr (3b), tBu (3c), Ph (3d), Mes (3e)] with 2 in refluxing acetonitrile were done, and the corresponding aryloxide-NHC-ligated palladacycle complexes (2-O-3,5-tBu2C6H2)(R)(NHC)Pd(dmba) [R =Me (4), i Pr (5), Ph (6), Mes (7), tBu (8)] were obtained in over 95% yields (Scheme 1). Complexes 4-8 are air and moisture stable. They are soluble in CH2Cl2, DME, THF, dioxane, acetone, and toluene, but insoluble in diethyl ether and hydrocarbon solvents. In their 1H NMR spectra the signals of the imidazole and phenol protons for the pro-ligands disappeared completely. The characteristic signals of the carbene carbons in the 13C NMR spectra (167.5 ppm for 4, 166.4 ppm for 5, 166.8 ppm for 6, and 168.4 ppm for 7) indicated that complexes 4-7 are close to the bis(aryloxide-NHC) palladium complexes.7b The signals of NMe2 and Ar(CH2)N protons are split into two groups of singlets and two groups of doublets for 4-6, but both are one singlet for 8. The presence of a signal at 8.13 ppm for an imidazole proton and the much larger chemical shift difference (1.26 ppm) between the two protons of an imidazole ring than those in complexes 4 (0.11 ppm), 5 (0 ppm), 6 (0.10 ppm), and 7 (0.24 ppm) support the abnormal binding (C4-bonding) of the NHC ligand in 8.2h,7b,11 In addition, in the 13C NMR spectrum of 8 no characteristic peak at about 167 ppm for the carbene carbon was found, but there were two peaks at 158.0 and 153.3 ppm for the imidazole carbons. High-resolution mass spectrometry (HRMS) analysis further confirmed the assignment of 4-8 by showing their molecular ion peaks. (11) (a) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5046. (b) Kluser, E.; Neels, A.; Albrecht, M. Chem. Commun. 2006, 4495. (c) Heckenroth, M.; Kluser, E.; Neels, A.; Albrecht, M. Angew. Chem., Int. Ed. 2007, 46, 6293. (d) Ellul, C. E.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Angew. Chem., Int. Ed. 2007, 46, 6343. (e) Albrecht, M. Chem. Commun. 2008, 3601. (f) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445. (g) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556.

Article

Figure 1. ORTEP diagram of 4. Thermal ellipsoids are shown at the 60% probability level. Hydrogen atoms have been omitted for clarity.

Figure 2. ORTEP diagram of 5 (showing one of two independent molecules in the unit cell). Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms have been omitted for clarity.

The molecular structures of 4-8 were established by single-crystal X-ray diffraction studies (Figures 1-5, Table 1). The carbene ligand’s five-membered ring topology varies slightly for different carbenes. The ligand retains the bond angles characteristic for a singlet carbene (∼104° for the imidazol-2-ylidenes). The Pd-C(carbene) bond lengths [1.964(2) A˚ for 4; 1.959(4), 1.962(4) A˚ for 5; 1.970(3) A˚ for 6; 1.973(3) A˚ for 7] are much shorter than that in the (aryloxide-NHC)Pd(Allyl) complexes [2.033(2) A˚]7a and between those in trans bis(aryloxide-NHC)Pd (2.007-2.023 A˚) and cis bis(aryloxide-NHC)Pd complexes (1.946-1.953 A˚).7b Different from Jung’s results,10a the NHC rings are not perpendicular to the square-planar Pd coordination plane, due to the bonding between oxygen and palladium atom. Notably, the Pd-O bonds are lengthened [2.0968(16) A˚ for 4; 2.114(2), 2.116(3) A˚ for 5; 2.107(2) A˚ for 6; 2.0817(19) A˚ for 7] relative to the bis(aryloxide-NHC)Pd complexes


155



(2.017-2.058 A˚).7b The Pd-C(Ph) (1.986-1.999 A˚) and PdN (2.132-2.152 A˚) bond lengths are within the expected range, relative to other palladacycle complexes.11 The ranges for the C(carbene)-Pd-O, C(carbene)-Pd-C(Ph), C(carbene)Pd-N, O-Pd-C(Ph), N-Pd-C(Ph), and N-Pd-O angles are 86.12-87.55°, 97.38-99.75°, 167.49-178.40°, 171.32175.18°, 81.06-82.0°, and 92.63-95.03°, respectively. The dihedral angles between the imidazole ring and phenyl ring of the aryloxide are 35.1°, 44.3°, 40.0°, and 33.3° for 4-7, respectively. Similar to the previous results,5h-j the fivemembered palladacycle moiety is puckered due to the presence of a sp3 carbon atom and a nitrogen atom. The solid-state structure of 8 shows that the NHC is bonded through an abnormal binding mode (C4 bonding), which is similar to previous reports.2h,7b,11 The Pd-C(carbene) [1.967(2) A˚], Pd-C(Ph) [1.985(2) A˚], Pd-N [2.146(2) A˚], and

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Pd-O [2.1089(17) A˚] bond lengths are close to those in the normal binding complexes 4-7. The C(carbene)-Pd-O [89.31(9)°] and N-Pd-C(Ph) [82.27(10)°] angles are slightly larger, but the C(carbene)-Pd-C(Ph) angle (95.66(10)°) is slightly smaller than those in complexes 4-7. The dihedral angle between the imidazole ring and phenyl ring of the aryloxide is 35.7°. The five-membered palladacycle moiety is also puckered. Similar to the bis(arylxoide-NHC)Pd complex,7b the abnormal binding in 8 may be also due to the steric effect of the tert-butyl group at the NHC. Ying developed a novel, practical synthesis of NHC-ligated palladacycles by one-pot, three-component, sequential reaction of N,N-dimethylbenzylamine, PdCl2 or PdBr2 and a wide range of imidazolium salts and their 4,5-dihydro- or benzofused


counterparts in refluxing acetonitrile in the presence of mild bases (K2CO3 or Cs2CO3) in air.5h-j Herein, we utilized this method for synthesis of the aryloxide-NHC-ligated palladacycles. Without isolating 2, the mixture of N,N-dimethylbenzylamine, PdCl2, the pro-ligands, and K2CO3 was refluxed in acetonitrile for 24 h to give the corresponding aryloxideNHC-ligated palladacycles in over 95% yield (Scheme 2). The reactions proceeded well with all the aryloxide-NHC proligands with different substituents at the NHCs. Norbornene Polymerization. Vinyl polynorbornene has received considerable attention owing to its dielectric and mechanical properties for technical application as an interlevel dielectric in microelectronics applications.12 Recently, some N-heterocyclic carbene nickel and palladium complexes have been used in the addition polymerization of norbornene with excellent activities.13 The nickel and palladium complexes with a salicylaldiminato ligand have been extensively applied in catalytic olefin polymerization.8a,b,12,14 As an analogue of the salicylaldimino metal complexes, this kind of salicylaldimine-like NHC palladium complex would have potential uses in catalysis. Recently, we reported the addition polymerization of norbornene with the bis(aryloxide-NHC) palladium complexes in the presence of MAO.7b We envisioned the structure of palladacycles could reduce the amount of MAO and improve the activities of the norbornene polymerization. So we studied the addition polymerization of norbornene with these palladacycles in the presence of MAO. The results are listed in Table 2. Complex 4 was chosen as the precatalyst for the study of the polymerization in detail. It was found that the activity decreased with increasing temperature from 40 to 80 °C (entries 2-4). The color of the PNB obtained at 80 °C is slightly black, indicating that the active species is unstable at high temperature and decomposes. The activity at 40 °C is higher than that at 20 °C (entries 1, 2), showing the polymerization needed an appropriate temperature to generate the

Table 1. Selected Bond Lengths (A˚) and Angles (deg) for 4-8

parameter

4

5

6

7

8

C1-N1 C1-N2 N1-C3 C3-C4 C4-N2 Pd-C1 Pd-O Pd-C7 Pd-N3 C1-Pd-O C1-Pd-C7 C1-Pd-N3 C7-Pd-N3 C7-Pd-O N3-Pd-O N1-C1-N2

1.363(3) 1.355(3) 1.391(3) 1.334(3) 1.384(3) 1.964(2) 2.0968(16) 1.999(2) 2.1340(19) 86.12(8) 98.40(9) 178.40(8) 81.94(8) 171.32(8) 93.74(7) 104.69(19)

1.365(5), 1.363(4) 1.365(4), 1.363(4) 1.384(5), 1.384(4) 1.342(5), 1.337(5) 1.384(5), 1.379(5) 1.959(4), 1.962(4) 2.114(2), 2.116(3) 1.988(4), 1.986(5) 2.143(3), 2.152(4) 86.68(12), 86.26(12) 98.37(17), 97.78(16) 172.08(13), 171.01(15) 81.77(17), 82.0(2) 171.40(12), 172.23(13) 94.19(10), 95.03(16) 103.9(3), 104.3(3)

1.363(3) 1.370(4) 1.392(3) 1.336(4) 1.396(4) 1.970(3) 2.107(2) 1.986(3) 2.143(2) 86.71(9) 97.38(11) 171.44(10) 81.50(11) 175.18(9) 94.83(8) 104.1(2)

1.366(3) 1.369(3) 1.394(3) 1.338(4) 1.383(3) 1.973(3) 2.0817(19) 1.988(3) 2.132(2) 87.55(9) 99.75(11) 167.49(10) 81.06(10) 171.79(9) 92.63(8) 104.0(2)

1.341(3) 1.326(3) 1.407(3) 1.368(3) 1.392(3) 1.967(2)a 2.1089(17) 1.985(2) 2.146(2) 89.31(9)a 95.66(10) 173.11(9)a 82.27(10) 174.86(9) 92.95(8) 103.7(2)a

a

For the abnormal NHC ligand.

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Table 2. Addition Polymerization of Norbornene with Palladacycles Activated by MAOa entry

catalyst

R

T (°C)

t (min)

[cat] (μmol)

MAO (Al/Pd)

PNB (g)

activityb

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

4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 6 7 8

Me Me Me Me Me Me Me Me Me Me Me Me Me Me i Pr Ph Mes t Bu

20 40 60 80 40 40 40 40 40 40 40 40 40 40 40 40 40 40

10 10 10 10 10 10 10 10 10 5 3 2 10 5 3 3 3 3

0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.20 0.20 0.20 0.10 0.10 0.20 0.20 0.20 0.20

1500 1500 1500 1500 1250 1750 2000 2250 2500 1750 1750 1750 1750 1750 1750 1750 1750 1750

0.6009 0.7707 0.4311 0.2372 0.2916 0.9706 0.9711 0.9697 0.9707 0.7296 0.6582 0.3780 0.9711 0.1128 0.4963 0.3527 0.5182 0.4444

7.51 11.56 6.47 3.56 4.37 14.56 14.57 14.55 14.56 43.78 65.82 56.70 58.27 11.28 49.63 35.27 51.82 44.44

a

Polymerization conditions: in 15.5 mL of toluene; norbornene 1.0 g; MAO (1.4 M) in toluene. b In units of (106 g of PNB) (mol of Pd)-1 h-1.

Scheme 2

activity species in situ. The activity increased with increasing the Al/Pd ratio from 1250 to 1750 (entries 2, 5, 6). But the activity would not be improved when the Al/Pd ratio further increased (entries 7-9). When the amount of catalyst was reduced to 0.20 μmol and the reaction time was shortened to 3 min, the catalyst exhibited the highest polymerization activity (entries 10-14). The optimal polymerization conditions for the catalytic system are at 40 °C and with an Al/Pd molar ratio of 1750 in 3 min. Similarly, complexes 5-8 also exhibited excellent catalytic activities [107 g of PNB (mol of Pd) -1 h-1] in the polymerization of norbornene (entries 15-18). For the different substituted group, the steric effect resulted in different activities for the precatalysts (4 > 5 ∼ 7 > 8 > 6). Although the phenomena of the polymerization are similar to those with the bis(aryloxide-NHC)Pd complexes, the palladacycles show better activities and lower Al/ Pd molar ratio. The coordination of N,N-dimethylbenzylamine to the metal atom is much weaker than that of

(12) (a) Janiak, C.; Lassahn, P. G. J. Mol. Catal. A: Chem. 2001, 166, 193. (b) Blank, F.; Janiak, C. Coord. Chem. Rev. 2009, 253, 827. (13) (a) Wang, X.; Liu, S.; Jin, G.-X. Organometallics 2004, 23, 6002. (b) Wang, X.; Liu, S.; Weng, L.-H.; Jin, G.-X. Organometallics 2006, 25, 3565. (c) Jung, I. G.; Seo, J.; Chung, Y. K.; Shin, D. M.; Chun, S. H.; Son, S. U. J. Polym. Sci. A: Polym. Chem. 2007, 45, 3042. (d) Crosbie, D.; Stubbs, J.; Sundberg, D. Macromolecules 2007, 40, 5743. (e) Crosbie, D.; Stubbs, J.; Sundberg, D. Macromolecules 2007, 40, 8947. (f) Crosbie, D.; Stubbs, J.; Sundberg, D. Macromolecules 2008, 41, 2445. (g) Sujith, S.; Noh, E. K.; Lee, B. Y.; Han, J. W. J. Organomet. Chem. 2008, 693, 2171. (h) Jung, I. G.; Lee, Y. T.; Choi, S. Y.; Choi, D. S.; Kang, Y. K.; Chung, Y. K. J. Organomet. Chem. 2009, 694, 297. (14) (a) Liang, H.; Liu, J.; Li, X.; Li, Y. Polyhedron 2004, 23, 1619. (b) Hu, T.; Li, Y.-G.; Li, Y.-S.; Hu, N.-H. J. Mol. Catal. A: Chem. 2006, 253, 155.

aryloxide-NHC and may be beneficial for the coordination and insertion of norbornene. The polymers obtained are insoluble in most organic solvents, such as cyclohexane, chloroform, benzene, chlorobenzene, acetone, dioxane, methanol, 1,2,4-trichlorobenzene, and tetrachloroethane. Therefore, we cannot measure the molecular weights of the polymers. The missing absorption of a double bond at 1600-1700 cm-1 in the IR spectra of the polymers (see the Supporting Information) indicates that the polymerization initiated by the palladacycles/MAO system adopts a vinyl-type addition manner. DSC analysis shows multiple transitions of the PNB derived from the palladacycles, and it is impossible to determine the glass transition temperatures. The difficulty of determining the glass transition temperature of vinyl homopolynorbornene has been attributed to the fact that it is located close to the temperature range where decomposition tends to set in.15 According to the TGA study (see the Supporting Information), the polymers are thermally stable up to 400 °C.

Conclusion In summary, we have successfully synthesized a series of ohydroxyaryl-substituted NHC-ligated palladacycles. Moreover, without isolating [Pd(dmba)(μ-Cl)]2 (2), the aryloxideNHC-ligated palladacycles could also be obtained by one-pot, three-component, sequential reaction of N,N-dimethylbenzylamine, PdCl2, and the pro-ligands in refluxing acetonitrile in (15) Haselwander, T. F. A.; Heitz, W.; Kr€ ugel, S. A.; Wendorff, J. H. Macromol. Chem. Phys. 1996, 197, 3435.

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the presence of K2CO3. The aryloxide-NHC adopts an abnormal binding (C4-bonding) when the N-functional group on the NHCs is changed to tert-butyl. By treatment with MAO, all the aryloxide-NHC-ligated palladacycles show excellent catalytic activities [107 g of PNB (mol of Pd)-1 h-1] in the addition polymerization of norbornene. The palladacycles show better activities and lower Al/Pd molar ratios compared with the bis(aryloxide-NHC) Pd complexes. The steric effect of N-substituents at the NHCs influences the activities of the polymerizations.

Experimental Section General Considerations. All experimental manipulations were carried out under an atmosphere of dry argon using standard Schlenk techniques. All solvents were distilled from appropriate drying agents under argon before use. MAO (10% solution in toluene) was purchased from Arbemarle Co. 1H and 13C NMR spectra were recorded on a Bruker AV400 or Varian AS-400, while ESI mass spectra and HRMS were done on Thermo Finnigan LCQ Advantag and Varian 7.0 T FTICR-mass spectrometers, respectively. IR spectra were recorded as KBr disks on a Nicolet 380 FT-IR spectrometer. Elemental analyses were performed on a Perkin-Elmer 240C analyzer. TG and DSC data were obtained from TA Instrument SDT-2960 and SC-2910 thermal analyzers, respectively. The aryloxide-NHC preligands 3a-3e7 and [Pd(dmba)(μ-Cl)]2 (2)9 were prepared according to the literature procedures. General Procedures for Preparation of the Aryloxide-NHCLigated Palladacycles 4-8. Method A. A mixture of o-hydroxyaryl-substituted imidazolium salts 3 (1 mmol), 2 (0.5 mmol, 0.28 g), K2CO3 (6 mmol, 0.84 g), and acetonitrile (20 mL) was heated under reflux for 24 h. It was then allowed to cool to room temperature. After removal of solvent, CH2Cl2 (40 mL) was added to the residue. Then the mixture was filtered on a pad of Celite. After the volatiles were removed, the corresponding palladacycles were obtained as an orange solid. Method B. A mixture of PdCl2 (1.00 mmol, 0.18 g), acetonitrile (20 mL), and N,N-dimethylbenzylamine (1.05 mmol, 0.18 g) was refluxed until PdCl2 dissolved completely, forming a clear dark orange solution. Then K2CO3 (6 mmol, 0.84 g) was added, and the mixture was refluxed until the palladacycle formation was complete, as indicated by the change in the color of the mixture to canary yellow. Immediately, o-hydroxyaryl-substituted imidazolium salts 3 (1 mmol) was added, and the mixture was refluxed with vigorous stirring for 24 h. After removal of solvent, CH2Cl2 (40 mL) was added to the residue. Then the mixture was filtered on a pad of Celite. After the volatiles were removed, the corresponding aryloxide-NHC-ligated palladacycles were obtained as an orange solid. Compound 4 (R = Me): yield 98%, mp 235-237 °C. Anal. Calcd for C27H37N3OPd: C, 61.65; H, 7.09; N, 7.99. Found: C, 61.81; H, 7.30; N, 7.90. 1H NMR (CDCl3): δ 7.30 (d, J=1.85 Hz, 1H, Ar-H), 7.22 (d, J=2.55 Hz, 1H, im-H), 7.11 (d, J=2.50 Hz, 1H, im-H), 7.00 (d, J=1.74 Hz, 1H, Ar-H), 6.90-6.99 (m, 3H, Ar-H), 6.81-6.86 (m, 1H, Ar-H), 4.53 (d, J = 12.9 Hz, 1H, Ar(CH2)N), 3.65 (s, 3H, NCH3), 3.37 (d, J = 12.9 Hz, 1H, Ar(CH2)N), 3.06 (s, 3H, NCH3), 2.47 (s, 3H, NCH3), 1.53 (s, 9H, C(CH3)3), 1.35 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 167.5, 157.1, 151.2, 148.4, 140.0, 138.7, 133.7, 129.3, 125.4, 123.0, 122.3, 122.3, 122.2, 120.4, 115.9, 72.6, 49.5, 48.4, 39.8, 35.7, 33.9, 31.8, 30.1, 29.7 ppm. HRMS (MALDI, m/z): calcd for C27H37N3OPd (M þ H): 526.2053, found 526.2056. Compound 5 (R = iPr): yield 97%, mp 220-222 °C. Anal. Calcd for C29H41N3OPd: C, 62.86; H, 7.46; N, 7.58. Found: C, 63.05; H, 7.70; N, 7.32. 1H NMR (CDCl3): δ 7.31 (d, J=1.99 Hz, 1H, Ar-H), 7.22 (d, J=2.62 Hz, 1H, Ar-H), 7.09 (d, J=2.26 Hz, 2H, im-H), 6.81-7.01 (m, 4H, Ar-H), 4.90 (m, 1H, CH(CH3)2), 4.54 (d, J=12.7 Hz, 1H, Ar(CH2)N), 3.34 (d, J=12.7 Hz, 1H,

Kong et al. Ar(CH2)N), 3.07 (s, 3H, NCH3), 2.44 (s, 3H, NCH3), 1.54 (s, 9H, C(CH3)3), 1.35 (s, 9H, C(CH3)3), 1.35 (d, J=12.5 Hz, 6H, CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3): δ 166.4, 157.4, 151.2, 148.0, 139.8, 138.8, 133.6, 129.6, 125.0, 123.0, 122.3, 120.6, 117.1, 116.3, 72.8, 51.8, 49.4, 48.3, 35.7, 33.9, 31.8, 30.2, 24.8, 22.6 ppm. HRMS (MALDI, m/z): calcd for C29H41N3OPd (M þ H): 554.2367, found 554.2361. Compound 6 (R = Ph): yield 96%, mp 234-235 °C. Anal. Calcd for C32H39N3OPd: C, 65.35; H, 6.68; N, 7.15. Found: C, 65.25; H, 6.85; N, 6.93. 1H NMR (CDCl3): δ 7.78 (d, J=7.89 Hz, 2H, Ar-H), 7.44 (s, 1H, Ar-H), 7.23 (d, J=2.48 Hz, 1H, im-H), 7.13 (d, J=2.15 Hz, 1H, im-H), 6.94-7.06 (m, 4H, Ar-H), 6.65 (d, J=7.26 Hz, 1H, Ar-H), 6.50 (t, J=7.28 Hz, 1H, Ar-H), 6.34 (d, J=7.44 Hz, 1H, Ar-H), 6.22 (t, J=7.36 Hz, 1H, Ar-H), 4.42 (d, J = 12.7 Hz, 1H, Ar(CH2)N), 3.27 (d, J = 12.6 Hz, 1H, Ar(CH2)N), 3.10 (s, 3H, NCH3), 2.39 (s, 3H, NCH3), 1.55 (s, 9H, C(CH3)3), 1.35 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 166.8, 157.3, 150.8, 145.7, 140.2, 140.2, 138.0, 133.9, 129.2, 127.6, 127.4, 125.9, 124.4, 122.6, 122.5, 121.8, 121.0, 120.6, 116.3, 72.7, 49.7, 49.2, 48.2, 35.7, 34.0, 31.8, 30.1, 29.7 ppm. HRMS (MALDI, m/z): calcd for C32H39N3OPd (M þ H): 588.2212, found 588.2219. Compound 7 (R=Mes): yield 98%, mp 249-250 °C. Anal. Calcd for C35H45N3OPd: C, 66.71; H, 7.20; N, 6.67. Found: C, 66.61; H, 7.26; N, 6.43. 1H NMR (CDCl3): δ 7.46 (s, 1H, Ar-H), 7.27 (s, 1H, Ar-H), 7.19 (d, J=1.88 Hz, 1H, im-H), 6.95 (s, 1H, im-H), 6.56-6.66 (m, 5H, Ar-H), 6.36 (t, J=7.27 Hz, 1H, Ar-H), 2.16-2.84 (m, 11H, Ar-CH3, NCH3, Ar(CH2)N), 2.06 (s, 3H, Ar-CH3), 1.59 (s, 9H, C(CH3)3), 1.40 (s, 9H, C(CH3)3), 1.30 (s, 3H, Ar-CH3) ppm. 13C NMR (100 MHz, CDCl3): δ 168.4, 157.7, 150.3, 146.0, 139.6, 138.3, 137.6, 136.3, 133.4, 129.0, 128.5, 123.6, 122.9, 122.4, 121.3, 120.8, 120.0, 116.6, 72.7, 35.8, 33.9, 31.8, 29.8, 29.7, 20.6 ppm. HRMS (MALDI, m/z): calcd for C35H45N3OPd (M þ H): 630.2682, found 630.2687. Compound 8 (R = tBu): yield 96%, mp 185-186 °C. Anal. Calcd for C30H43N3OPd: C, 63.43; H, 7.63; N, 7.40. Found: C, 63.38; H, 7.39; N, 7.51. 1H NMR (CDCl3): δ 8.13 (d, J = 1.44 Hz, 1H, N(CH)N), 7.33 (t, J = 4.81, 3.81 Hz, 1H, Ar-H), 7.24 (d, J = 2.36 Hz, 1H, Ar-H), 7.02 (t, J = 3.92, 4.76 Hz, 1H, Ar-H), 6.94-6.97 (m, 3H, Ar-H), 6.87 (d, J = 1.44 Hz, 1H, im-H), 3.93 (s, 2H, Ar(CH2)N), 2.78 (s, 6H, NCH3), 1.63 (s, 9H, C(CH3)3), 1.55 (s, 9H, C(CH3)3), 1.33 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 158.0, 153.3, 149.6, 148.9, 140.7, 137.6, 133.4, 127.6, 125.2, 125.2, 122.9, 122.7, 122.5, 122.0, 115.7, 72.1, 57.1, 49.1, 35.8, 33.9, 31.8, 30.2, 29.7, 29.7 ppm. HRMS (MALDI, m/z): calcd for C30H43N3OPd (M þ H): 568.2524, found 568.2516. Crystallographic Studies. Single crystals suitable for X-ray diffraction were obtained from CH2Cl2/hexane for 4-8. Data collections were performed on a Rigaku Saturn 70 diffractometer equipped with a rotating anode system at 113(2) K by using graphite-monochromated Mo KR radiation (ω-2θ scans, λ = 0.71073 A˚). Semiempirical absorption corrections were applied for all complexes. The structures were solved by direct methods and refined by full-matrix least-squares. All calculations were performed by using the SHELXL-97 program system. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned idealized positions and were included in structure factor calculations. The molecular structure of 4 contains one CH2Cl2 molecule of solvation. Selected bond lengths and angles are listed in Table 1. Norbornene Polymerization. In a typical procedure, 1.00 g of norbornene in 13.1 mL of toluene and 0.25 mL of MAO (1.4 N) were added into a flask (100 mL) with stirring under an Ar atmosphere. After the mixture was kept at the desired temperature for 2 min, 0.20 μmol of the palladium complex in 2 mL of toluene was injected into the flask via syringe, and the reaction was started. Three minutes later, the polymerization was terminated by addition of 10% HCl in ethanol. The precipitated polymer was washed with ethanol and water and dried at 60 °C

Article in vacuo to a constant weight. For all the polymerization procedures, the total reaction volume was 15.5 mL, which can be achieved by variation of the added toluene when necessary.

Acknowledgment. We are grateful to the National Natural Science Foundation of China (Nos. 20874051, 20672058, and 20721062) and the Research Fund for the


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Doctoral Program of Higher Education of China (No. 20070055020) for financial support. Supporting Information Available: CIF files giving X-ray structural information for 4-8, crystal data and a summary of the X-ray data collection, 1H and 13C NMR spectra of complexes 4-8, and IR and TGA spectra of the PNB obtained. This material is available free of charge via the Internet at http:// pubs.acs.org.

Synthesis, Structures, and Norbornene Polymerization Behavior of

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