Nickel and Palladium Complexes of Pyridine-Phosphine Ligands

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Organometallics 2009, 28, 3264–3271

Nickel and Palladium Complexes of Pyridine-Phosphine Ligands Bearing Aromatic Substituents and Their Behavior as Catalysts in Ethene Oligomerization Jitte Flapper,†,‡ Piet W. N. M. van Leeuwen,† Cornelis J. Elsevier,† and Paul C. J. Kamer*,†,§ Van ’t Hoff Institute for Molecular Sciences, UniVersity of Amsterdam, Nieuwe Achtergracht 166, 1018WV Amsterdam, The Netherlands, Dutch Polymer Institute, PO Box 902, 5600 AX EindhoVen, The Netherlands, and School of Chemistry, UniVersity of St. Andrews, St. Andrews, Fife, Scotland KY16 9ST, U.K. ReceiVed January 16, 2009

Bidentate pyridine-phosphine ligands 1 of general structure 2-aryl-6-[2-(diphenylphosphino)ethyl]pyridine were developed, in which the aryl group is phenyl (a), 1-naphthyl (b), 9-phenanthryl (c), 9-anthracyl (d), and ferrocenyl (e). The influence of these substituents on the nickel and palladium complexes of the ligands and their ethene oligomerization behavior was studied. The largest influence was observed in species with a square-planar surrounded metal center, whereas the nickel dichloride complexes 5 appeared as monometallic species with a tetrahedrally surrounded metal center. A classical binding mode of the ligand was not possible for the methylpalladium chloride complexes coordinated in a square-planar fashion. Instead, binuclear species in which two ligands span two metal centers were observed for 6a-d, and an undefined mixture of complexes was obtained for 6e. In contrast to these neutral palladium complexes, the cationic methylpalladium complexes 7, lacking the chloride anion, appear as well-defined, monomeric complexes. When the nickel complexes 5a-d were activated with MAO, they catalyzed the oligomerization of ethene with a maximum turnover frequency of 11 × 103 mol ethene per mol nickel per hour, whereas 5e showed no activity. Selectivities for butenes were between 93 and 97 mol %, with a maximum 1-butene content of 93%. The catalytic behavior is different from that of the nickel complex lacking an aromatic group at the ligand, again showing the influence of these substituents. The palladium complexes 7 were hardly active in ethene oligomerization, giving very small amounts of oligomers. Introduction Industrially, R-olefins are produced on a scale of megatons per year. Oligomerization of ethene is the most important reaction for their production. Depending on their chain length, R-olefins find their most important applications in the production of linear low-density polyethylene (LLDPE) (C4-C10), polyR-olefins (C4, C10), plasticizers (C6-C10), lubricants (C8-C10), lube oil additives (C12-C18), and surfactants (C12-C20).1,2 Bidentate ligands with a nitrogen and a phosphorus donor atom (P,N ligands) have found considerable attention in the field of transition metal catalysis.3 Next to many other reactions, they * To whom correspondence should be addressed. E-mail: pcjk@ st-andrews.ac.uk. Fax: +44-(0)1334463808. Tel: +44-(0)1334467285. † University of Amsterdam. ‡ Dutch Polymer Institute. § University of St. Andrews. (1) (a) Skupinska, J. Chem. ReV. 1991, 91, 613–648. (b) Al-Jarallah, A. M.; Anabtawi, J. A.; Siddiqui, M. A. B.; Aitani, A. M.; Al-Sa’doun, A. W. Catal. Today 1992, 14, 1–124. (c) Vogt, D. Oligomerization of ethylene to higher linear R-olefins. In Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; WileyVCH: Weinheim, 1996; Vol. 1, pp 245-258. (2) Speiser, F.; Braunstein, P.; Saussine, L. Acc. Chem. Res. 2005, 38, 784–793. (3) (a) Braunstein, P. J. Organomet. Chem. 2004, 689, 3953–3967. (b) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680–699. (c) Chelucci, G.; Orru, G.; Pinna, G. A. Tetrahedron 2003, 59, 9471–9515. (d) Espinet, P.; Soulantica, K. Coord. Chem. ReV. 1999, 195, 499–556. (e) Guiry, P. J.; Saunders, C. P. AdV. Synth. Catal. 2004, 346, 497–537. (f) Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336–345. (g) Newkome, G. R. Chem. ReV. 1993, 93, 2067–2089. (h) Pfaltz, A.; Drury, W. J., III. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5723–5726.

have been applied in nickel- or palladium-catalyzed ethene oligomerization and polymerization.2,4 In a search to tune the performance of ethene oligomerization catalysts, different types of P,N-based catalysts have been tested. Among them are nickel and palladium complexes of pyridine-phosphines and -phosphinites,5-8 oxazoline-phosphines and -phosphinites,8–10 iminophosphoranes,11 amido- and imino-phosphines,12-14 imino-pyrrolylphosphines,15 pyrazole-phosphines,16 quinoline-phosphines,17,18 and pyridine-phospholes.19 (4) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100, 1169–1203. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283–316. (5) (a) Bluhm, M. E.; Folli, C.; Walter, O.; Doring, M. J. Mol. Catal. A: Chem. 2005, 229, 177–181. (b) Bonnet, M. C.; Dahan, F.; Ecke, A.; Keim, W.; Schulz, R. P.; Tkatchenko, I. J. Chem. Soc., Chem. Commun. 1994, 615–616. (c) Kermagoret, A.; Braunstein, P. Organometallics 2008, 27, 88–99. (d) Speiser, F.; Braunstein, P.; Saussine, L. Organometallics 2004, 23, 2625–2632. (e) Speiser, F.; Braunstein, P.; Saussine, L. Organometallics 2004, 23, 2633–2640. (6) Chen, H. P.; Liu, Y. H.; Peng, S. M.; Liu, S. T. Organometallics 2003, 22, 4893–4899. (7) Muller, G.; Klinga, M.; Osswald, P.; Leskela, M.; Rieger, B. Z. Naturforsch., B: Chem. Sci. 2002, 57, 803–809. (8) (a) Speiser, F.; Braunstein, P.; Saussine, L. Dalton Trans. 2004, 1539–1545. (b) Speiser, F.; Braunstein, P.; Saussine, L.; Welter, R. Inorg. Chem. 2004, 43, 1649–1658. (9) Doherty, M. D.; Trudeau, S.; White, P. S.; Morken, J. P.; Brookhart, M. Organometallics 2007, 26, 1261–1269. (10) (a) Speiser, F.; Braunstein, P.; Saussine, L.; Welter, R. Organometallics 2004, 23, 2613–2624. (b) Tang, X.; Zhang, D.; Jie, S.; Sun, W.H.; Chen, J. J. Organomet. Chem. 2005, 690, 3918–3928. (11) Buchard, A.; Auffrant, A.; Klemps, C.; Vu-Do, L.; Boubekeur, L.; Le Goff, X. F.; Le Floch, P. Chem. Commun. 2007, 1502–1504.

10.1021/om9000378 CCC: $40.75  2009 American Chemical Society Publication on Web 04/28/2009

Ni and Pd Complexes Bearing Aromatic Substituents

Herein, we present the development of new pyridine-phosphine ligands and nickel and palladium complexes thereof. Also, their behavior as catalyst precursors in ethene oligomerization was studied. The nickel dichloride complexes form active catalysts in this reaction, giving mainly butenes as the product, whereas the palladium complexes were inactive. The used ligands differ in the steric bulk of the aromatic substituent at the pyridine 6-position. We have shown that modification of 1,1′-bis(diarylphosphino)ferrocene ligands with large aromatic groups induces an increased enantioselectivity in palladium-catalyzed allylic substitution and rhodium-catalyzed hydrogenation.20,21 Furthermore, bulky aromatic substituents have a beneficial effect on the performance of nickel-salicylaldiminato-based ethene polymerization catalysts.22 Recently, we showed the unique coordination behavior of methylpalladium chloride complexes 6a-d of ligands 1a-d, which are modified with large aromatic substituents.23 In this article, we also report on the corresponding complex 6e of ligand 1e, the cationic methylpalladium acetonitrile complexes 7 obtained from the neutral palladium complexes, and the neutral nickel dichloride complexes 5 of the ligands. Also, complexes 5 (using MAO activation) and 7 were evaluated as catalyst precursors for ethene oligomerization.

Results and Discussion We prepared the ligands 1 as depicted in Scheme 1. They are all bidentate, with a pyridine and a diphenylphosphine donor group, which are connected through an 1,2-ethanediyl bridge. The ligands differ in the bulky substituents at the 6-position of the pyridine group. The substituents are phenyl (for a), 1-naphthyl (b), 9-phenanthryl (c), 9-anthracyl (d), and ferrocenyl (e). Ligand Synthesis. The ligands were synthesized according to Scheme 1. Hydrophosphination of 2-bromo-6-vinylpyridine (2) with diphenylphosphine and subsequent oxidation using household bleach gave 2-bromo-6-[2-(diphenylphosphinoyl)ethyl]pyridine (3), which is a suitable precursor for differently (12) (a) Keim, W.; Killat, S.; Nobile, C. F.; Suranna, G. P.; Englert, U.; Wang, R. M.; Mecking, S.; Schroder, D. L. J. Organomet. Chem. 2002, 662, 150–171. (b) Van den Beuken, E. K.; Smeets, W. J. J.; Spek, A. L.; Feringa, B. L. Chem. Commun. 1998, 223–224. (13) Shi, P. Y.; Liu, Y. H.; Peng, S. M.; Liu, S. T. Organometallics 2002, 21, 3203–3207. (14) (a) Weng, Z.; Teo, S.; Hor, T. S. A. Organometallics 2006, 25, 4878–4882. (b) Weng, Z.; Teo, S.; Koh, L. L.; Hor, T. S. A. Angew. Chem., Int. Ed. 2005, 44, 7560–7564. (c) Kwon, H. Y.; Lee, S. Y.; Lee, B. Y.; Shin, D. M.; Chung, Y. K. Dalton Trans. 2004, 921–928. (15) (a) Anderson, C. E.; Batsanov, A. S.; Dyer, P. W.; Fawcett, J.; Howard, J. A. K. Dalton Trans. 2006, 5362–5378. (b) Dyer, P. W.; Fawcett, J.; Hanton, M. J. Organometallics 2008, 27, 5082–5087. (16) Mukherjee, A.; Subramanyam, U.; Puranik, V. G.; Mohandas, T. P.; Sarkar, A. Eur. J. Inorg. Chem. 2005, 125, 4–1263. (17) Sirbu, D.; Consiglio, G.; Gischig, S. J. Organomet. Chem. 2006, 691, 1143–1150. (18) Sun, W.-H.; Li, Z.; Hu, H.; Wu, B.; Yang, H.; Zhu, N.; Leng, X.; Wang, H. New J. Chem. 2002, 26, 1474–1478. (19) de Souza, R. F.; Bernardo-Gusma˜o, K.; Cunha, G. A.; Loup, C.; Leca, F.; Re´au, R. J. Catal. 2004, 226, 235–239. (20) Nettekoven, U.; Widhalm, M.; Kalchhauser, H.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M.; Lutz, M.; Spek, A. L. J. Org. Chem. 2001, 66, 759–770. (21) Nettekoven, U.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M.; Widhalm, M.; Spek, A. L.; Lutz, M. J. Org. Chem. 1999, 64, 3996–4004. (22) (a) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149– 3151. (b) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460–462. (23) Flapper, J.; Wormald, P.; Lutz, M.; Spek, A. L.; Van Leeuwen, P. W. N. M.; Elsevier, C. J.; Kamer, P. C. J. Eur. J. Inorg. Chem. 2008, 4968–4976.

Organometallics, Vol. 28, No. 11, 2009 3265 Scheme 1. Synthesis of Ligands 1a

a i: (1) HPPh , KOtBu, THF, 60 °C, 16 h; (2) NaClO , H O, CH Cl , 2 4 2 2 2 rt, 1 h; ii (for a, b, and c): RB(OH)2, Pd(PPh3)4, K2CO3, toluene, water, reflux, 16 h; iii (for d and e): RZnCl, Pd(PPh3)4, THF, 60 °C, 24h.; iV: PhSiH3, reflux, 16 h.

Scheme 2. Synthesis of Nickel Complexes 5

substituted phosphine oxides 4. These were obtained via palladium-catalyzed Suzuki coupling with arylboronic acids (for a, b, and c) or Negishi-Takahashi coupling with arylzinc chlorides (for d and e). The thus obtained phosphine oxides 4 were reduced using phenylsilane to give the free phosphines 1 in good yields. Synthesis and Characterization of Nickel Complexes. The nickel dichloride complexes 5 were obtained by reaction of the ligands with the precursor (DME)NiCl2 [DME ) 1,2-dimethoxyethane]; see Scheme 2. The reaction mixture was stirred at room temperature and then filtered through a pad of Celite to remove residual (DME)NiCl2. After evaporation of the solvent, the solid was washed with diethyl ether to remove remaining free ligand. The complexes were paramagnetic species, as evidenced by their magnetic moment in solution. They were further characterized by elemental analyses, which were in agreement with the proposed structures, and high-resolution mass spectrometry. Using fast atom bombardment (FAB) ionization, the [(ligand)NiCl]+ species were observed in the mass spectrum for all complexes. The loss of one chlorine atom is a consequence of the ionization technique employed, as the ionization of the complex by proton addition is immediately followed by loss of HCl. The magnetic moments in CD2Cl2 were 2.50 (5a), 2.24 (5b), 2.62 (5c), 2.75 (5d), and 2.28 µB (5e). These values are indicative for a distorted tetrahedral geometry of the nickel center.24 No useful NMR signals were observed, and all complexes were EPR-silent, as can be expected for this type of complexes.25 Synthesis and Characterization of Neutral Palladium Complexes. We recently reported on the methylpalladium chloride complexes 6a-d of ligands 1a-d.23 They appear as

3266 Organometallics, Vol. 28, No. 11, 2009 Scheme 3. Synthesis of Dimeric Palladium Complexes 6a-d

dimeric species, with two ligands spanning two palladium centers, as shown in Scheme 3. The complexes are insoluble and were characterized by different solid state techniques. This revealed that the methyl and chloride anions can be in the trans or cis configuration at both metal centers, but also complexes were obtained in which one palladium center has a cis and one a trans configuration of the anions within one complex, like the structure depicted in Figure 1. We tried to obtain the methylpalladium chloride complex of ferrocenyl-substituted ligand 1e by the same procedure we obtained 6a-d, viz., via reaction with (COD)Pd(CH3)Cl [COD ) 1,5-cyclooctadiene] in dichloromethane; see Scheme 4. To our surprise, this gave rise to a complicated mixture of products. The products were soluble in dichloromethane and chloroform, and after workup no signals corresponding to free or coordinated COD were detected in the 1H NMR spectrum. We performed the reaction several times, and identical 1H and 31P NMR spectra were obtained for the products of the individual reactions, showing that the product formation was reproducible. As a consequence of the stoichiometry of the reactants, a 1:1 mixture of methylpalladium chloride and the ligand was obtained. This was confirmed by elemental analysis. The mixture was used directly in the next step (see below). The steric bulk of the ferrocenyl group might prevent the ligand from acting as a classical bidentate ligand. We have shown before that the bulk of a ferrocenyl substituent can be more demanding than that of large aromatic substituents such as 9-phenanthryl.20 Synthesis and Characterization of Cationic Palladium Complexes. The cationic palladium species 7 were obtained by reaction of their precursors with NaBAr′4 [Ar′ ) 3,5di(trifluoromethyl)phenyl] and acetonitrile; see Scheme 5. Cannula filtration separated the product from the sodium chloride precipitate, and evaporation of the solvent yielded the pure products in high yields.

Figure 1. Structure of complex with cis- and trans-surrounded metal centers. Scheme 4. Synthesis of a Mixture of Palladium Complexes 6e

Flapper et al. Scheme 5. Synthesis of Cationic Palladium Complexes 7

In contrast to their precursors (the dimeric complexes 6a-d and the undefined mixture 6e), all complexes 7 are well-defined, monometallic, soluble complexes. The 1H, 13C, and 31P NMR spectra showed that in all cases one species was present, and the possibility of an unsymmetrical dimer was excluded. The presence of the coordinated acetonitrile molecule was evidenced by a singlet in the proton spectrum. The Pd-CH3 protons gave rise to a singlet or a doublet. For 7a-d, the methyl signals for these two groups were at remarkably low ppm values, compared to similar complexes lacking the bulky aryl substituents at the ligands.26 For the methylpalladium group, this value became lower with increasing size of the aryl substituent, and the signal appeared at 0.36, -0.22, -0.63, and -0.67 ppm for 7a-d, respectively. The CH3CN singlet was also observed at lower ppm value than expected: 1.95, 1.51, 1.55, and 1.62 ppm, respectively. These chemical shifts can be explained by an interaction of the ring current of the aryl substituents with the methyl protons, thus causing an increased shielding of these nuclei. In 7e, the ferrocenyl substituent does not have an interaction with the methylpalladium group and the signal for the Pd-CH3 protons appears at 0.73 ppm in the 1H NMR spectrum. The chemical shift of the CH3CN protons of 1.98 ppm does show an interaction of this group with the ferrocenyl substituent. This is an indication of a cis orientation of the acetonitrile with respect to the pyridine, as this places the acetonitrile-methyl in closer proximity to the ferrocenyl than the palladium-methyl. This configuration around palladium can be expected on the basis of the larger trans effect of the phosphorus donor atom compared to the nitrogen donor. Indeed, the cis configuration of the palladium-methyl and the phosphine was shown by the 3JPH coupling constant of the Pd-CH3 signals. This coupling with the phosphorus nucleus is small or not observed, indicating that the methyl group is orientated cis with respect to the phosphorus. The 31P NMR spectra show one singlet between 36 and 39 ppm, and the 19F signal appears at -63.0 ppm for all complexes. The high-resolution mass spectra show the peak for the [(ligand)PdCH3]+ species. Apparently, acetonitrile dissociates under the conditions used for FAB ionization, as was observed before for similar complexes.26 When the milder field desorption (FD) ionization technique is used, the [(ligand)Pd(CH3)(CH3CN)Na]+ ions were observed for 7a-d, but as a consequence of FD ionization, not in high resolution. For 7e, the same species is observed as when FAB is used. The ions observed in MS have an overall charge of +1, as evidenced by the distance of 1 amu between the peaks of different isotopes. This shows that the cationic complexes (24) Hayter, R. G.; Humiec, F. S. Inorg. Chem. 1965, 4, 1701–1706. (25) Desrochers, P. J.; Telser, J.; Zvyagin, S. A.; Ozarowski, A.; Krzystek, J.; Vicic, D. A. Inorg. Chem. 2006, 45, 8930–8941. (26) (a) Flapper, J.; Kooijman, H.; Lutz, M.; Spek, A. L.; Van Leeuwen, P. W. N. M.; Elsevier, C. J.; Kamer, P. C. J. Organometallics 2009, 28, 1180–1192. (b) Flapper, J.; Kooijman, H.; Lutz, M.; Spek, A. L.; Van Leeuwen, P. W. N. M.; Elsevier, C. J.; Kamer, P. C. J. Organometallics 2009, http://dx.doi.org/10.1021/om900038u.

Ni and Pd Complexes Bearing Aromatic Substituents

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Table 1. Ethene Oligomerization Using Complexes 5 as Catalyst Precursorsa productivity (g C2H4/ complex (mol Ni · h)) 5a 5b 5c 5d 5e

8 × 104 18 × 104 16 × 104 30 × 104 0

product distribution (%)c TOFb

C4

C6

C8

C10

2.8 × 103 6.5 × 103 5.7 × 103 11 × 103 0

93 96 93 97

6 4 7 3

1