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Organometallics 2009, 28, 4852–4867 DOI: 10.1021/om900280j
Ethylene Oligomerization Using First-Row Transition Metal Complexes Featuring Heterocyclic Variants of Bis(imino)pyridine Ligands Kenny Tenza,*,† Martin J. Hanton,† and Alexandra M. Z. Slawin‡ †
Sasol Technology (U.K.) Ltd, Purdie Building, North Haugh, St Andrews, KY16 9ST, U.K., and ‡School of Chemistry, University of St Andrews, North Haugh, St Andrews, KY16 9ST, U.K. Received April 13, 2009
This report examines the replacement of the imine and pyridine functionalities of the ubiquitous bis(imino)pyridine ligand with various heterocycles. The synthesis of a new class of ligand based around thiazole is described; 2,4-bis[1-(arylimino)ethyl]thiazole (aryl = Ph, 1a; Dipp, 1b) and 2,5bis[1-(arylimino)ethyl]thiazole (aryl=Ph, 1c; Dipp, 1d) have been prepared in good yield and fully characterized. The coordination chemistry of these ligands with chromium, iron, and cobalt is explored, and the potential of these complexes as ethylene oligomerization initiators is assessed. The chromium complex 2a shows an extremely unusual alternating distribution of higher R-olefin products, which has been previously observed on only one occasion. Both series of products, C4n and C4nþ2, show Schulz-Flory behavior but with distinctly different k values. The new ligand 2,5bis[1-(phenylimino)ethyl]-1-methylpyrrole (1e) is reported along with the attempted synthesis of some corresponding iron complexes. The complexation of chromium by 2,5-bis(phenyliminomethyl)thiophene (1f) is also described, and this material was screened for ethylene oligomerization activity, detailed studies indicating that the ligand may be labile under catalytic conditions. A number of other known heterocyclic ligands incorporating pyrazolyl and benzimidazole functionalities have also been explored with iron, and for the first time their potential to facilitate ethylene oligomerization was assessed. All complexes have been tested via activation with MMAO3A and AlEt3/[Ph3C][Al(OtBuF)4].
Introduction Non-metallocene-based oligomerization initiators have come of age over the past decade, with bis(imino)pyridine complexes being in the vanguard of this assault.1 Since the initial reports of ethylene polymerization with catalysts based upon ligands of this type,2 there has been a significant focus on developing alternative variants of the bis(imino)pyridine ligand with varying degrees of success.1c,3 One avenue of exploration has been the replacement of the central pyridine ring with alternative heterocycles (Scheme 1) *Corresponding author E-mail:
[email protected]. (1) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (c) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107, 1745. (2) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143. (b) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049. (c) Bennett, A. M. A. (Dupont) WO 98/27124, 1998; Chem. Abstr. 1998, 129, 122973x. (d) Bennett, A. M. A. CHEMTECH 1999 (July), 2428. (e) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849. (f) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Str€ omberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728. (g) Britovsek, G. J. P.; Dorer, B. A.; Gibson, V. C.; Kimberley, B. S.; Solan, G. A. WO patent 99/12981 (BP Chemicals Ltd); Chem. Abstr. 1999, 130, 252793. (3) (a) Small, B. L.; Rios, R.; Fernandez, E. R.; Carney, M. J. Organometallics 2007, 26, 1744. (b) Schmiege, B. M.; Carney, M. J.; Small, B. L.; Gerlach, D. L.; Halfen, J. A. Dalton Trans. 2007, 2547. pubs.acs.org/Organometallics
Published on Web 07/27/2009
including pyrimidine (A),4 pyrazine (B),5 triazine (C),4 pyrrole (D),6 carbazole (E),4,7 furan (F),4 or thiophene (G).4 While the six-membered N-heterocyclic permutations have generally still yielded complexes that initiated oligomerization catalysis with reasonable activity (A and B), the fivemembered heterocycle variants have been less successful.1,4 Indeed subtle changes to the ligand structure appear to have a dramatic effect upon catalysis, as well as the ability of the ligand to ligate iron or cobalt.4,6 It is noted that in the case of bis(imino)pyrrole ligands (D) a bidentate coordination mode is observed with zirconium, chromium, iron, and cobalt, rather than the expected tridentate coordination, rationalized as a result of the smaller reach of the pyrrolide-based ligand.1a,6,8 Further evidence to support this logic can be drawn from the ability of the reported bis(imino)carbazole ligands (E), which have an extended imine donor arm, to ligate iron,4,7 although the resulting complexes were inactive for ethylene oligomerization when combined with MAO.1c (4) Britovsek, G. J. P.; Gibson, V. C.; Hoarau, O. D.; Spitzmesser, S. K.; White, A. J. P.; Williams, D. J. Inorg. Chem. 2003, 42, 3454. (5) (a) Beaufort, L.; Benvenuti, F.; Noels, A. F. J. Mol. Catal. A: Chem. 2006, 260, 210. (b) Beaufort, L.; Benvenuti, F.; Noels, A. F. J. Mol. Catal. A: Chem. 2006, 260, 215. (6) Dawson, D. M.; Walker, D. A.; Thornton-Pett, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2000, 459. (7) Gibson, V. C.; Spitzmesser, S. K.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 2718. (8) Gibson, V. C.; Maddox, P. J.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 1651. r 2009 American Chemical Society
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Scheme 1. The Different Heterocycles and Their Complexes That Have Been Used to Replace Pyridine in Variations of the Bis(imino)pyridine Ligand
Furthermore, in the case of the triazine (C), furan (F), and thiophene (G) variants, the ligand fails to ligate iron or cobalt.4 An alternative strategy for modifying the bis(imino)pyridine scaffold has been to target the imine-donor arms, and again the use of heterocyclic moieties has been examined in the form of pyridine (H and I),9 phenanthroline (J),10 imidazolylidene (carbenes) (K),11 furan (L),12 thiophene (M),12 oxazoline (N),13 and benzimidazole (O þ P)14 functionalities (Scheme 2). The pyridyl (H þ I) and phenanthrolyl (J) derivatives, when bound to iron, serve as highly active oligomerization initiators when activated with methylalumoxane (MAO),9,10 contrasting with the imidazolylidene derivatives (K), which while exceptionally active in combination with chromium,11 have not been reported as successful oligomerization catalysts when bound to iron.1c Cobalt complexes of the furan (L) derivatives show low activity for the dimerization of ethylene to butenes, while the thiophenebased analogues (M) oligomerize ethylene with moderate activity.12 Oxazoline-based ligands (N) have been described in combination with iron and ruthenium for ethylene polymerization after activation with MAO, but low activities were observed.15,16 In the case of benzimidazole-substituted (9) Britovsek, G. J. P.; Baugh, S. P. D.; Hoarau, O.; Gibson, V. C.; Wass, D. F.; White, A. J. P.; Williams, D. J. Inorg. Chim. Acta 2003, 345, 279. (10) (a) Sun, W.-H.; Jie, S.; Zhang, S.; Zhang, W.; Song, Y.; Ma, H.; Chen, J.; Wedeking, K.; Fr€ ohlich, R. Organometallics 2006, 25, 666. (b) Pelletier, J. D. A.; Champouret, Y. D. M.; Cadarso, J.; Clowes, L.; Gaete, M.; Singh, K.; Thanarajasingham, V.; Solan, G. A. J. Organomet. Chem. 2006, 691, 4114. (11) (a) McGuinness, D. S.; Gibson, V. C.; Wass, D. F.; Steed, J. W. J. Am. Chem. Soc. 2003, 125, 12716. (b) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E. Organometallics 2004, 23, 166. (12) (a) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci, F.; Laschi, F. Organometallics 2003, 22, 2545. (b) Bianchini, C.; Giambastiani, G.; Mantovani, G.; Meli, A.; Mimeau, D. J. Organomet. Chem. 2004, 689, 1356. (c) Bianchini, C.; Gatteschi, D.; Giambastiani, G.; Rios, I. G.; Ienco, A.; Laschi, F.; Mealli, C.; Meli, A.; Sorace, L.; Toti, A.; Vizza, F. Organometallics 2007, 26, 726. (13) (a) Nomura, K.; Warit, S.; Imanishi, Y. Macromolecules 1999, 32, 4732. (b) Nomura, K.; Warit, S.; Imanishi, Y. Bull. Chem. Soc. Jpn. 2000, 73, 599. (14) (a) Zhang, W.; Sun, W.-H.; Zhang, S.; Hou, J.; Wedeking, K.; Schultz, S.; Fr€ ohlich, R.; Song, H. Organometallics 2006, 25, 1961. (b) Hao, P.; Zhang, S.; Sun, W.-H.; Shi, Q.; Adewuyi, S.; Lu, X.; Li, P. Organometallics 2007, 26, 2439. (c) Sun, W.-H.; Hao, P.; Zhang, S.; Shi, Q.; Zuo, W.; Tang, X. Organometallics 2007, 26, 2720. (d) Xiao, L.; Gao, R.; Zhang, M.; Li, Y.; Cao, X.; Sun, W.-H. Organometallics 2009, 28, 2225. (15) Nomura, K.; Warit, S.; Imanishi, Y. Bull. Chem. Soc. Jpn. 2000, 73, 599. (16) Nomura, K.; Warit, S.; Imanishi, Y. Macromolecules 1999, 32, 4732.
Scheme 2. Donor Arm-Modified Bis(imino)pyridine Complexes Previously Reported
pyridines, ligands of the type bis(benzimidazole)pyridine (O) have been applied only to chromium, where in combination with MAO or diethylaluminum chloride (DEAC) they acted as highly effective initiators for ethylene oligomerization.14a Variants of the bis(imino)pyridine ligand where only one imino arm has been replaced with benzimidazole (P) have been reported bound to iron, cobalt, and nickel, all of which successfully initiated ethylene oligomerization.14b-14d While the use of heterocycle-based variations of the bis(imino)pyridine ligand has clearly been widely explored, there appeared to us several rational alternatives that have not yet been considered. For example, while replacement of pyridine with a pyrrole ring allows a different ring size to be examined, it also introduces the complicating factor of a sp3hybridized-N as compared to the sp2-N in pyridine. Two options exist: the N-Me variant can be used to ensure a neutral N-donor, but the N-H offers the opportunity of an amide. The latter of these options has been the avenue chosen by others.6 In order to maintain the sp2-hybridized-N while moving to a five-membered heterocycle, we envisaged the replacement of the central pyridine ring with a thiazole. A further point of interest with such a motif is the option of N- or S-binding via the thiazole segment depending upon
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how the imine moieties are distributed (Scheme 3, 1a,b versus 1c,d). Herein we report the synthesis of ligands of this type with phenyl and 2,6-diisopropylphenyl (Dipp) substituents at the imine nitrogen, explore their coordination chemistry with chromium, iron, and cobalt centers, and study their use as initiators for ethylene oligomerization. The oligomerization catalysis has been performed using MMAO-3A17 as activator alongside the triethylaluminum (TEA)/trityl tetrakis(perfluoro-tert-butoxy)aluminate (TA) activator combination for comparison, which has been recently reported by us for the effective activation of bis(imino)pyridine complexes of the first-row transition metals.18 We also report the synthesis of a new 1-methylpyrrole-based bis(imine) ligand (Scheme 3, 1e) and describe the attempted formation of iron complexes. While the failure of bis(imino)thiophene ligands (Scheme 3, 1f) to bind iron centers has been previously reported (vide supra), we report here for the first time the ability of these compounds to ligate chromium and examine the resulting complexes as initiators for ethylene oligomerization. It is noteworthy that all of the most active iron initiators based upon the bis(imino)pyridine ligand scaffold and derivatives thereof feature at least one imine donor arm. Thus we choose to explore diaza-heterocycles further in the form of bis(pyrazole)pyridine ligands (Scheme 4, 1g and 1h) and bis(benzimidazole)pyridine ligands (Scheme 4, 1i and 1j), where the imine donor is incorporated into the heterocycle. While both of these ligand types are known,19-22 and 1g,23 1i,21a,21c and 1j21a,21c have been reported in combination with iron, the precise iron complexes prepared either have not been relevant precursors for this type of catalysis or their potential to initiate oligomerization catalysis has not been assessed. Herein we report the synthesis of the iron(II)bromide complexes of these ligands and probe the catalytic potential of these toward ethylene oligomerization.
Results and Discussion Synthesis of Ligands. The thiazole precursors 2,4-diacetylthiazole and 2,5-diacetylthiazole were prepared according to the method of Aitken et al. via a three-step synthesis.24 (17) MMAO-3A is a modified alumoxane cocatalyst sourced from Akzo Nobel with the general formula [-Al(R)-O-]n, where R=Me (75%), Bu (25%). (18) (a) Hanton, M. J.; Tenza, K. Organometallics 2008, 27, 5712. (b) Tooze, R. P.; Hanton, M. J.; Tenza, K. (Sasol Technology) WO2008038173, 2008. (19) Jameson, D. L.; Goldsby, K. A. J. Org. Chem. 1990, 55, 4992. (20) Brien, K. A.; Garner, C. M.; Pinney, K. G. Tetrahedron 2006, 62, 3663. (21) (a) Addison, A. W.; Burman, S.; Wahlgren, C. G.; Rajan, O. A.; Rowe, T. M.; Sinn, E. J. Chem. Soc., Dalton Trans. 1987, 2621. (b) Piguet, C.; Bocquet, B.; Muler, E.; Williams, A. F. Helv. Chim. Acta 1989, 72, 323. (c) Wang, X.; Wang, S.; Li, L.; Sundberg, E. B.; Paolo Gacho, G. Inorg. Chem. 2003, 42, 7799. (22) Other examples of ethylene oligomerization using complexes with nitrogen-bridged bis(pyrazolyl) ligands have been reported, but are far less closely related to the bis(imino)pyridine scaffold and have not been demonstrated with iron, but instead chromium and nickel, metals with a much higher predisposition toward ethylene oligomerization. (a) Ajellal, N.; Kuhn, M. C. A.; Boff, A. D. G.; H€ orner, M.; Thomas, C. M.; Carpentier, J.-F.; Casagrande, O. L.Jr. Organometallics 2006, 25, 1212. (b) Junges, F.; Kuhn, M. C. A.; dos Santos, A. H. D. P.; Rabello, C. R. K.; Thomas, C. M.; Carpentier, J.-F.; Casagrande, O. L.Jr. Organometallics 2007, 26, 4010. (c) de Oliveira, L. L.; Campedelli, R. R.; Kuhn, M. C. A.; Carpentier, J.-F.; Casagrande, O. L.Jr. J. Mol. Catal. A 2008, 288, 58. (23) Calderazzo, F.; Englert, U.; Hu, C.; Marchetti, F.; Pampaloni, G.; Passarelli, V.; Romano, A.; Santi, R. Inorg. Chim. Acta 2003, 344, 197. (24) Aitken, K. M.; Aitken, R. A. Tetrahedron 2008, 64, 4384. i
Tenza et al. Scheme 3. Synthesis of Five-Membered Heterocycle-Based Bis(imine) Ligands
These materials were subsequently condensed with the relevant aniline in the presence of titanium tetrachloride to yield the corresponding bis(imino) products (Scheme 3) as yellow solids (1a, 96%; 1b, 40%; 1c, 81%; 1d, 33%).25 The 1 H and 13C{1H} NMR spectra of these materials were as expected, with resonances for the carbons of the imine moieties at ∼δ 160 ppm. The slight upfield shift observed for the resonances associated with the methyl groups, as compared to the ketone precursors, was as anticipated, whereas the resonances associated with the thiazole moiety remained largely unaffected by the conversion from diketone to diketimine. The preparation of 2,5-bis(acetyl)-1-methylpyrrole has been reported previously but requires the use of autoclaves at high temperature;26 thus an alternative method was employed here using trifluoroacetic anhydride to accelerate the reaction.27 Condensation of aniline with the diketone yielded the desired 2,5-bis[1-(phenylimino)ethyl]-1-methylpyrrole (1e) in good yield (Scheme 3). The synthesis of a number of bis(imino)thiophene compounds has been reported previously,4,28 including the phenylimino derivative used here.28e-28g The solvent-free mixture of the two precursors reacted rapidly to furnish the desired ligand (1f) in excellent yield (Scheme 3). The pyrazolyl ligand 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine (1g) has been widely reported and was prepared via a modification of the literature procedure,19 whereas the 2,6-bis(3,5-diphenylpyrazol-1-yl)pyridine ligand (1h) has been reported (25) The 2,6-diisopropylimine variants, 1b and 1d, were isolated in lower yield due to problems associated with their purification, specifically the removal of the DippNH3Cl byproduct. (26) Harsanyi, M. C.; Norris, R. K. J. Org. Chem. 1987, 52, 2209. (27) Song, C.; Knight, D. W.; Whatton (nee Fagan), M. A. Tetrahedron Lett. 2004, 10, 133. (28) (a) Skene, W. G.; Dufresne, S. Acta Crystallogr. E 2006, E62, 1116. (b) Skene, W. G.; Dufresne, S. Polym. Prepr. 2004, 45, 728. (c) Skene, W. G.; Dufresne, S. Org. Lett. 2004, 6, 2949. (d) Xe, W.; Wang, Q.; Wurz, R. P. (Nova Chemicals) EP1046647 A2, 2000. (e) Lumbroso, H.; Pastour, P. Compt. Rend. 1965, 261, 1279. (f) Vaysse, M.; Pastour, P. Compt. Rend. 1964, 259, 2657. (g) Sone, C. Bull. Chem. Soc. Jpn. 1964, 37, 1197.
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Scheme 4. Bis(diaza-heterocyclic)pyridine Ligands Employed in This Study
Scheme 5. Synthesis of Chromium Complexes
on only one previous occasion via a slightly different route from that employed here.20 The ligand 2,6-bis(1-methyl-2-benzimidazolyl)pyridine (1j) has also been reported previously, being prepared from the reaction of pyridine-2,6-dicarboxylic acid with 1-methyl-1,2-diaminobenzene,21 while the synthesis employed herein was via the N-methylation of 2,6-bis(2-benzimidazolyl)pyridine. Synthesis of Complexes. The chromium complexes, 2a-d, were isolated as green solids in near quantitative yield from the reaction between tris(tetrahydrofuran)chromium(III) chloride and the ligands (1a-d) at elevated temperature in toluene or chlorobenzene (Scheme 5). Elemental analysis was consistent with the proposed empirical formula, and FAB-MS analysis revealed the [M - Cl]þ fragment in all cases. The magnetic moments measured for 2a-d are consistent with an octahedral, high-spin chromium(III) environment (2a, μeff=3.83 μB; 2b, μeff=3.82 μB; 2c, μeff=3.59 μB; 2d, μeff=3.88 μB; OH, Cr(III), S=3/2, μeff(obs)=3.7-3.9 μB). The reaction of 1f under the same conditions as for 2a-d gave the corresponding chromium complex, 2f, as an orange powder in 93% yield (μeff = 3.45 μB). The ability of 1f to successfully bind chromium, given the failure to ligate iron, is surprising and remains without good explanation, the ionic radii of Cr(III) [0.62 A˚] and Fe(II) [0.64-0.78 A˚] being rather similar.29 Elemental analysis was as expected, but the FAB-MS analysis revealed only a very weak signal for the [M-Cl]þ fragment. Heating toluene solutions of the ligands, 1a and 1c, and iron(II) bromide at 60 °C furnished the iron complexes (3a (29) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
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and 3c) in high yields as dark green-brown solids (Scheme 6). The magnetic moments observed were lower than that typically seen for planar tridentate ligands bound to iron(II) (3a, μeff=3.96 μB; 3c, μeff=3.52 μB).4,30,31 FAB-MS analysis revealed a [M - Br]þ fragment for 3c, but no tractable signal for 3a. The ability of both forms of the bis(imino)thiazole ligand to bind iron is most interesting when considered in light of previous studies. In the case of 3a, where the thiazole moiety is bound via the N atom, the ligand is unequivocally tridentate (vide infra), contrasting with the bis(imino)pyrrole analogues and illustrating that replacement of pyridine with a five-membered N-heterocycle is possible but that maintenance of the N donor in a sp2-hybridized form is essential. Finally, the ability of 1c, with the thiazole moiety predisposed for S-binding, to complex iron when bis(imino)thiophene fails to do so was most unexpected. Crystals of 3a suitable for study by X-ray diffraction were grown from prolonged cooling of a DCM/toluene solution. The structure obtained clearly shows a tridentate binding mode via the three N donors for the 2,4-bis[1-(phenylimino)ethyl]thiazole ligand (Figure 1). The thiazole moiety of the ligand exhibits disorder as a consequence of the crystallographic C2 symmetry, and so no analysis of the metric parameters for this fragment can be made; similarly there is also a disordered toluene solvate molecule in the structure. The observation of a tridentate binding mode for ligand 1a with FeBr2 is particularly interesting when compared to the bidentate binding mode observed exclusively for the bis(imino)pyrrole ligands (vide supra),6 a difference we assign to the sp2-hybridized N in a thiazole as compared to the sp3-N in a pyrrole ring. This serves to illustrate that replacement of the central six-membered N-heterocycle in the bis(imino)pyridine ligand with a five-membered N-heterocycle is possible in a way that maintains the tridentate binding mode of the ligand. Interestingly the Fe-Ncentral heterocycle bond length observed in 3a of 2.050(3) A˚ is slightly shorter then that found for the examples of the bis(imino)pyridine ligand bound to FeX2 (X = Cl, Br): in these cases it is 0.04-0.07 A˚ shorter, while the Fe-Nimine bond length of 2.3046(18) A˚ is 0.05-0.10 A˚ longer.30,32 These comparisons indicate that the iron center sits closer to the central heterocycle but that the distance to the imine arms is longer, presumably due to an inherently larger exocyclic angle {C(7)-C(8)-S(8) or C(7A)-C(8A)-C(10A)} for ligands of this type, as compared to those with a six-membered central ring.6 The imine bond length observed of 1.290(3) A˚ is equipollent with that seen in the bis(imino)pyridine analogues and is indicative of conjugation between the imine and the central heterocycle.30 Attempts to ligate iron with ligand 1e and 1f all failed, specifically, heating toluene and THF solutions of the ligand with iron(II) bromide and bis(acetonitrile)iron(II) triflate. (30) Britovsek, G. J. P.; Gibson, V. C. G.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849. (31) (a) Bianchini, C.; Giambastiani, G.; Guerrero, I. R.; Meli, A.; Passaglia, E.; Gragnoli, T. Organometallics 2004, 23, 6087. (b) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci, F.; Zanobini, F.; Laschi, F.; Sommazzi, A. Eur. J. Inorg. Chem. 2003, 1620. (c) Barbaro, P.; Bianchini, C.; Giambastiani, G.; Guerreo Rios, I.; Meli, A.; Oberhauser, W.; Segarra A. M.; Sorace, L.; Toti, A. Organometallics 2007, 26, 4639. (d) Bianchini, C.; Giambastiani, G.; Guerreo Rios, I.; Meli, A.; Oberhauser, W.; Sorace, L.; Toti, A. Organometallics 2007, 26, 5066. (32) Tellmann, K. P.; Gibson, V. C. G.; White, A. J. P.; Williams D. J. Organometallics 2005, 24, 280.
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Scheme 6. Synthesis of Iron and Cobalt Complexes
In the case of 1e, presumably in-plane coordination of the ligand is blocked due to the presence of the N-methyl group, precluding even the bidentate coordination mode seen for the pyrrolide derivatives.6 In the case 1f, the failure to ligate iron corresponds with previous reports where the 2,5bis[(2,6-diisopropylphenyl)iminomethyl]thiophene variant failed to ligate iron.4 Heating THF solutions of the ligands 1g-j and iron(II) bromide at 60 °C furnished the iron complexes in high yields as tan (3g), ochre (3h), and purple (3i-j) colored solids. Crystals of 3g grown from a MeOH solution layered with petroleum ether 40-60 facilitated an X-ray diffraction study. The geometry at iron can be considered as a strongly distorted trigonal bipyramid, with the ligand 1g bound in a tridentate fashion, as expected, occupying the two axial and one equatorial position (Figure 2). An examination of the metric parameters for 3g reveals it to be essentially isostructural with the molecular structure determined for the corresponding FeCl2 complex.23 The cobalt complexes, 4a and 4c, were isolated as bright green solids in excellent yield from the reaction of the ligands 1a and 1c and cobalt(II)chloride in THF at 60 °C. Elemental
analysis and FAB-MS gave results consistent with the proposed formula, and magnetic moments consistent with a high-spin cobalt(II) five-coordinate center were observed (4a, μeff =4.44 μB; 4c, μeff =4.81 μB),33 corresponding with that expected for a planar tridentate ligand bound to a cobalt dichloride moiety.4,31b,32 Crystals of 4a grown from a DCM solution layered with hexane facilitated an X-ray diffraction analysis. As can be seen from Figure 3, ligand 1a is again bound in a planar tridentate coordination mode via the two imine arms and the sp2-hybridized thiazole N (sum of angles at N = 359.9°). This structure does not exhibit the C2 crystallographic symmetry observed for 3a, and so the various metric parameters of the thiazole ring may be meaningfully examined. A comparison of the N(8)-C(8) and N(8)-C(18) bond lengths reveals that the first, formally a double bond, is shorter than the latter, which is formally a single bond; however, the difference in these bond lengths is (33) (a) Morassi, R.; Bertini, I.; Sacconi, L. Coord. Chem. Rev. 1973, 11, 343. (b) Sacconi, L.; Morassi, R.; Midollini, S. J. Chem. Soc. A 1968, 1510. (c) Sacconi, L. J. Chem. Soc. A 1970, 248. (d) Morassi, R; Sacconi, L. J. Chem. Soc. A 1971, 492.
Article
Figure 1. X-ray molecular structure of 3a. The complex has a crystallographic 2-fold axis running through the Fe-N(8) axis; the C3NS ring is thus disordered, and only one of the 50% occupancy models is illustrated. The toluene solvate molecule has been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Fe(1)-Br(1) 2.4303(4), Fe(1)-Br(1A) 2.4303(4), Fe(1)N(8) 2.050(3), Fe(1)-N(1) 2.3046(18), Fe(1)-N(1A) 2.3047(18), N(1)-C(7) 1.290(3); N(8)-Fe(1)-N(1) 71.46(5), N(8)-Fe(1)N(1A) 71.46(5), N(1)-Fe(1)-N(1A) 142.93(9), N(8)-Fe(1)-Br(1A) 121.805(12), N(1)-Fe(1)-Br(1A) 103.05(5), N(1A)-Fe(1)-Br(1A) 96.27(5), N(8)-Fe(1)-Br(1) 121.803(12), N(1)Fe(1)-Br(1) 96.27(5), N(1A)-Fe(1)-Br(1) 103.05(5), Br(1A)Fe(1)-Br(1) 116.39(2), C(7)-N(1)-Fe(1) 117.23(15).
Figure 2. X-ray molecular structure of 3g. The half-weight methanol solvate molecule has been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Fe(1)-N(9) 2.147(5), Fe(1)N(15) 2.174(5), Fe(1)-N(1) 2.178(5), Fe(1)-Br(2) 2.4317(11), Fe(1)-Br(1) 2.4935(12), N(1)-N(2) 1.377(6), N(14)-N(15) 1.371(7); N(9)-Fe(1)-N(15) 72.84(18), N(9)-Fe(1)-N(1) 72.68(17), N(15)-Fe(1)-N(1) 145.06(19), N(9)-Fe(1)-Br(2) 121.32(12), N(9)-Fe(1)-Br(1) 122.19(12), Br(2)-Fe(1)-Br(1) 116.48(4).
only 0.03 A˚, while the C(18)-C(10) bond length is very long for a C(sp2)-C(sp2) double bond. Furthermore, as with the structure of 3a, the imine bond lengths [N(1)-C(7) and N(11)-C(17)] and [C(7)-C(8) and C(17)-C(18)] can be
Organometallics, Vol. 28, No. 16, 2009
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Figure 3. X-ray molecular structure of 4a. The molecule is disordered, and the major (75%) orientation is illustrated. The DCM solvate molecule has been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Co(1)-Cl(1) 2.246(2), Co(1)Cl(2) 2.233(2), Co(1)-N(8) 2.001(5), Co(1)-N(1) 2.395(5), Co(1)-N(11) 2.305(5), N(8)-C(8) 1.311(9), N(8)-C(18) 1.344(8), S(8)-C(8) 1.680(7), S(8)-C(10) 1.598(5), C(10)-C(18) 1.576(8), N(1)-C(7) 1.289(8), (N11)-C(17) 1.277(9); N(8)-Co(1)-N(1) 71.31(19), N(8)-Co(1)-N(11) 74.1(2), N(1)-Co(1)-N(11) 145.18(19), N(8)-Co(1)-Cl(1) 125.44(17), N(8)-Co(1)-Cl(2) 118.25(17), Co(1)-N(8)-C(8) 124.6(5), Co(1)-N(8)-C(18) 121.2(4), C(8)-N(8)-C(18) 114.1(6), Cl(2)-Co(1)-Cl(1) 115.78(8).
considered as indicative of conjugation between the imine functionalities and the central thiazole ring. Indeed, an examination of the structure reveals that the entire ligand scaffold (thiazole and imine moieties) and iron center are coplanar, with the imine substituents sitting out of plane by 72° and 51°. Ethylene Oligomerization Results. All of the complexes (10 μmol of precatalyst) were screened for ethylene oligomerization with modified methylalumoxane (MMAO-3A)17 and AlEt3/[Ph3C][Al(OtBuF)4] (TEA/TA). The chromium complexes of the bis(imino)thiazole, 2a-d, and bis(imino)thiophene, 2f, ligands facilitated ethylene oligomerization with moderate activity with either activation method (Table 1, entries 1-25) and were thus selected for further study. In contrast the iron and cobalt complexes of the bis(imino)thiazole ligand were less successful (Table 1, entries 26-33), showing low productivities and short lifetimes, with the heaviest oligomers observed being C8; in all but one case there was no concomitant polymer formation. The bis(pyrazole)pyridine and bis(benzimidazole)pyridine complexes of iron, 3g-j, all initiated ethylene oligomerization (Table 2), but the productivities were exceptionally low, the catalyst lifetime was very short, and no material heavier than C6 was produced. In all cases only liquid product was formed, no polymer being evident. Some of the catalysts 3g-j were totally selective toward dimerization, and the selectivity to R-olefins was high in all cases, but this is often seen with poorly active catalysts and is somewhat artificial. A closer examination of the chromium complexes beginning with 2a reveals that this complex, when activated with MMAO-3A, is primarily a polymerization system (Table 1, entry 1). A repeat of this test (Table 1, entry 2), but under more concentrated conditions and higher temperature (30 °C vs 20 °C), induces a further shift toward polymerization and achieves a higher productivity and longer catalyst lifetime,
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Tenza et al.
Table 1. Ethylene Oligomerization Results Obtained with Bis(imino)thiazole- and Bis(imino)thiophene-Based Catalystsa based on Al
cat entry {μmol} 1
2a (10)
2
h,i
2a
3
2a (10)
cocat b (equiv) MMAO (500)
T {min} prodc
actd
based on transition metal
prode
actf
PE {%}
total liq product {%} {g}
kg
C4 (1-C4) {wt %}
C6 (1-C6) {wt %}
C8 (1-C8) {wt %}
C10 (1-C10) {wt %}
6.6 (37.8) 9.2
1.1 (59.8) 4.0
0.7 (46.1) 3.1
0.4 1.5 (63.7) 3.4 14.5
C12 (1-C12) {wt %}
C14þ {wt %}
16
110
430
56 700 212 610
68.8
31.2
15.91
0.63
54
140
160
70 670 78 840
94.3
5.7
19.83
0.73
89.7 (99.1) 65.8
TEA/TA (100/1.5)
16
510 1920
51 320 192 430
63.1
36.9
14.40
0.64, 0.90
(98.4) 8.3
(62.2) 7.4
(89.0) 9.3
(88.5) 7.7
(72.3) 8.4 59.0
4
2ai (2.5) TEA/TA (100/1.5)
29
830 1750
83 110 174 980
52.5
47.5
5.83
(99.6) 0.56, 0.96 34.0
(83.5) 13.2
(84.7) 9.7
(84.2) 5.9
(90.8) 5.2 32.1
5
2ai (2.5) TEA/TA (100/1.5)
41
600
880
120 400 175 770
80.2
19.8
8.44
(99.5) 0.57, 0.95 42.1
(87.2) 6.7
(83.5) 6.4
(83.7) 4.7
(89.4) 5.1 35.1
6
2ai,j (2.5) TEA/TA (100/1.5)
12
1460 7320
146 410 732 070
77.2
22.8
10.26
(99.6) 0.65, 0.81 13.9
(79.1) 12.7
(79.6) 12.4
(82.8) 9.4
(90.6) 8.8 42.8
7
2ai (2.5) TEA/TB (100/1.5) 35
800 1370
79 980 136 720
54.1
45.9
5.61
8
2ai (2.5) TEA/ B(C6F5)3 (100/1.5)
550 2870
54 930 286 570
69.3
30.7
3.85
(98.8) 0.53, 0.56 76.0 (99.5) 0.53, 0.59 95.8
(97.5) 11.2 (92.4) 2.7
(97.2) 5.4 (96.0) 0.3
(98.0) (98.6) 2.7 1.4 (95.9) (>99.9) 0.2 0.2
9
2b (10)
MMAO (500)
3
150
3790 75 760
98.1
1.9
1.06
-
10
2b (10)
TEA/TA (100/1.5)
3
80 1650
7770 164 510
77.8
22.2
2.18
0.70
11
2c (10)
MMAO (500)
6
20
180
8920 89 170
89.7
10.4
2.50
0.57
12
2c (10)
TEA/TA (100/1.5) 11
210 1120
20 590 112 290
64.3
35.7
3.72
0.51
TEA/TA (100/1.5) 30
280
570
28 440 56 890
76.0
24.0
3.99
0.54
450 1260
45 250 125 690
84.1
15.9
6.35
0.55
(10) MMAO (500)
i
12
