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Intramolecular Formation of a CrI(bis-arene) Species via TEA Activation of [Cr(CO)4(Ph2P(C3H6)PPh2)]+: An EPR and DFT Investigation Lucia McDyre,† Emma Carter,† Kingsley J. Cavell,*,† Damien M. Murphy,*,† James A. Platts,† Katharine Sampford,† Benjamin D. Ward,† William F. Gabrielli,‡ Martin J. Hanton,‡ and David M. Smith‡ † ‡
School of Chemistry, Cardiff University, Cardiff CF10 3AT, U.K. Sasol Technology (U.K.) Ltd, Purdie Building, North Haugh, St Andrews, U.K.
bS Supporting Information ABSTRACT: Activation of the catalytically relevant complex [Cr(CO)4(1)]+ (1 = Ph2P(C3H6)PPh2) by Et3Al (TEA) leads to formation of the Cr(I) bis-arene complex [Cr(1-bis-η6-arene)]+, as revealed by EPR and DFT calculations. This bis-arene complex is formed by intramolecular rearrangement and coordination of Cr(I) to the ligand phenyl groups in aliphatic solvents following loss of CO, preventing release of Cr(I) into solution. By comparison in aromatic solvents (toluene), the [Cr(bis-tolyl)]+ complex is preferentially formed.
C
r-based complexes are key catalytic species in the industrially important selective trimerization/tetramerization of ethylene.1 Although Cr(III) complexes are commonly used as catalyst precursors, lower oxidation state Cr species, e.g. Cr(I), are thought to be the active catalysts leading to production of predominantly 1-hexene and 1-octene.2 The bis(phosphine) family of ancillary ligands, such as [Cr(CO)4(1)]+ (Scheme 1), are frequently employed for this important process;2 the activity and selectivity of the catalytic reaction can be tuned by varying the properties of the ligand.2 It is also known that the choice of solvent is an important variable in the catalytic reactions; e.g., deactivation of the catalysts is known to occur in aromatic solvents compared to aliphatic solvents.3 Although the structures of these Cr precatalysts have been investigated by different techniques, the nature of the catalytically active complexes remains unclear. Furthermore, the oxidation state of the chromium center which exists during the catalytic reaction remains ambiguous, with Cr(I/III) and Cr(II/IV) redox cycles having been implicated, while possible changes to the spin state during the catalytic cycle adds further complexity to the problem.4 The Cr precatalysts are not reactive for the trimerization/tetramerization reaction and must therefore be activated before use. Activation of the Cr(III) or Cr(I) precursor complexes prior to catalysis is usually achieved by addition of a cocatalyst, such as triethylaluminium (Et3Al) or methylaluminoxane (MAO).5 One role of the cocatalyst is thought to be alkylation of the metal center following loss of the coordinating r 2011 American Chemical Society
Scheme 1. Structure of the CrI(CO)4 Bis(diphenylphosphino)propane and Bis(dicyclohexylphosphino)propane Complexes
carbonyl groups.2a,4 Hence, investigations into the structure of the Cr complex after activation are crucial for a further understanding of this industrially important reaction. Received: July 8, 2011 Published: August 15, 2011 4505
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Organometallics
Figure 1. X-band CW-EPR spectra of [Cr(CO)4(1)]+ dissolved in dichloromethane solvent after activation with Et3Al/toluene (1/10 Cr/ Et3Al (equiv)) at (a) 140, (b) 160, (c) 180, (d) 200, (e) 220, (f) 240, and (g) 260 K. The EPR spectrum of the starting [Cr(CO)4(1)]+ (140 K) is shown in the inset.
It is well-known that low-valent chromium has a strong tendency to form η6-arene complexes.6 Indeed, it was recently reported that a solvent-derived CrI(bis-arene) complex, [Cr(η6CH3C6H5)2]+ or [Cr(bis-tolyl)]+, was formed by activation of a Cr(acac)3/Ph2PN(i-Pr)PPh2 system via MAO addition in an aromatic solvent.7 Its formation was attributed to demetalation of the complex and interaction of Cr(I) with the aromatic (toluene) solvent.7 A range of substituted Cr(I) bis-arene complexes has been previously studied by electron paramagnetic resonance (EPR) spectroscopy, and all produce very characteristic and distinct EPR spectra.8 Furthermore, bridged bis-arene Cr(I) complexes have also been reported, including those formed with the bis(triphenylphosphino) ligands.9 Hence, one must consider the likelihood that analogous Cr(I) bis-arenes could conceivably be formed during the activation of the chromium bis(phosphine) complexes for selective trimerization/tetramerization of ethylene. To examine this, we applied EPR techniques coupled with DFT to explore the TEA activation of [Cr(CO)4(1)]+. The continuous-wave (CW) EPR/ENDOR spectra for a series of [Cr(CO)4PP]+ complexes (including [Cr(CO)4(1)]+) were recently reported by us;10 the EPR spectra were dominated by a pronounced g anisotropy and large PA couplings, typical for low-spin d5 Cr(I) with a SOMO of largely dxy character.10 A representative CW-EPR spectrum10 for the precursor [Cr(CO)4(1)]+ complex is shown in Figure 1 (inset). These bidentate diphosphine ligands containing chromium are highly effective for ethylene trimerization and tetramerization following activation.1 Upon addition of an Et3Al/toluene solution to the [Cr(CO)4(1)]+ complex at 298 K, the blue coloration of the precursor complex was immediately lost. The solutions were rapidly frozen to 77 K after activation. Full details on sample preparation, activation, and EPR measurements are given in the Supporting Information. The resulting EPR spectra of the activated system are shown in Figure.1. The low-temperature spectrum (140 K) was broad and poorly resolved, possessing a quasi-axial profile due to small amounts of unreacted [Cr(CO)4(1)]+ (Figure 1a). As the temperature was raised, a well-resolved isotropic signal progressively emerged (Figure 1b g). This new signal was consistently observed in these experiments following Et 3 Al
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Figure 2. X-band CW-EPR spectra (185 K) of [Cr(CO)4(1)]+ dissolved in dichloromethane, following activation with (a) Et3Al/toluene, (c) Me3Al/toluene, and (d) Et3Al/hexane. An EPR simulation is shown as a dashed line in (b).
Table 1. Computed vs. Experimental hfc Values (MHz, top) and g Values (bottom) for the [Cr(1-bis-η6-arene)]+ Complex H
aiso
ref 8ca
9.72
DFT
10.91
H
A(^, ||)
Cr
aiso
Cr
A(^, ||)
10.22 8.72
50.85
75.6 1.4
10.15
43.722
61.245
12.57
P
P
4.594
1.889
aiso
57.514
6.536
12.406 exptlb
9.65
nrd
50.5
73.0 nr
A(^, ||)
9.135 g^), in comparison to that for the precatalyst complex ([Cr(CO)4(1)]+; g^ = 2.063 > g|| = 1.987)10 and the g|| value of 2.0023, is also consistent with a bis-arene assignment possessing a SOMO of predominantly dz2 character.8a As stated earlier, activation of a CrIII system (Cr(acac)3/PNP) by MAO produces the [Cr(bis-tolyl)]+ complexes in toluene, as the liberated CrI ions form the bis-arene sandwich complex with the solvent.7 This interpretation can also account for the spectra shown in Figure 2a,c, since toluene was present. However, when [Cr(CO)4(1)]+ was activated using Et3Al/hexane, a virtually identical EPR spectrum was obtained (Figure 2d, labeled signal II). Clearly the EPR spectrum shown in Figure 2d is characteristic of a [Cr(bis-arene)]+ complex but cannot arise from coordination of Cr(I) by the hexane solvent and must therefore be ligand derived. In this case the complex is formed by an intramolecular rearrangement of Cr(I) in the activated and decarbonylated [Cr(CO)4 (1)]+ system (labeled [Cr(1-bis-η 6 -arene)]+ in Scheme 2). Coordination of Cr(I) by the two phenyl rings accounts for the 10 equivalent protons observed in the spectrum (Figure 2d). We also observed similar intramolecular bis-arene complexes for other members of the Cr(I) bis(phosphine) complexes (see the Supporting Information, Figure S3). This ligand-based [Cr(1-bis-η6-arene)]+ assignment and the role of the ligand phenyl groups in its formation were further confirmed by synthesizing the cyclohexyl derivative bearing no phenyl groups (i.e., [Cr(CO)4bis(dicyclohexylphosphine)]+, [Cr(CO)4(2)]+ in Scheme 1). [Cr(CO)4(2)]+ was dissolved in dichloromethane and reacted with Et3Al/hexane; in this case no [Cr(bis-arene)]+ signal was observed (see the Supporting Information, Figure S5) in the absence of any arene ring to coordinate the Cr(I). However, when this [Cr(CO)4(2)]+ complex was dissolved in dichloromethane/toluene (50/50) and activated using Et3Al/hexane, a strong [Cr(bis-arene)]+ signal was subsequently detected by EPR (see the Supporting Information, Figure S5); in this case the bis-arene can only arise by reaction of Cr(I) ions with the toluene in solution: i.e., producing the solvent-based [Cr(bis-tolyl)]+ species I. The paramagnetic [Cr(1-bis-η6-arene)]+ complex represents a small component of the total [Cr(CO)4(1)]+ content,11 indicating that a large fraction of the Cr(I) centers were converted to EPR-silent or diamagnetic species following Et3Al addition. This mixture of chromium centers hampered any
Figure 3. DFT structure of the [Cr(1-bis-η6-arene)]+ complex.
attempts to isolate this complex. As a result, DFT calculations were carried out and the optimized structure of [Cr(1-bis-η6arene)]+ is shown in Figure 3; the calculated spin Hamiltonian parameters are given in Table 1. The g, CrA, and HA values were in good agreement with the EPR data. However, the calculated DFT value of Paiso = 4.594 MHz was not observed experimentally, where the intrinsic line width of the CW-EPR spectrum was 7.6 MHz, implying a maximum experimental Paiso of only 3.78 MHz (assuming a triplet from two equivalent 31P nuclei).12 We attempted to resolve the 31P using CW-ENDOR (see the Supporting Information, Figure S7). Unfortunately, the spectra were dominated by large anisotropic hfc’s to the arene ring protons; thus, the 31P hfc’s could not be detected. The above results indicate that the nature of the observed [Cr(bis-arene)]+ complex depends on which solvent is used (solvent-based [Cr(bis-tolyl)]+ in aromatic solvents or ligandbased [Cr(1-bis-η6-arene)]+ in aliphatic solvents; Scheme 2). To investigate which one is preferentially formed under competitive conditions, a further experiment was performed in which [Cr(CO)4(1)]+ was dissolved in dichloromethane/perdeuterated-toluene (50/50) and activated with Et3Al/hexane. The spectral resolution disappeared completely, producing a broad unresolved line (as expected, 10 equivalent 2H compared to 1H, with giso = 1.987 and 2Haiso ≈ 2.8 MHz) due to the deuterated bis-tolyl [Cr(η6-CD3C6D5)2]+ (see the Supporting Information, Figure S5). In other words, the solvent-based [Cr(η6CD3C6D5)2]+ complex forms preferentially over the ligandbased [Cr(1-bis-η6-arene)]+. In conclusion, we have shown that intramolecular coordination of the Cr(I) center to the ligand phenyl groups occurred when [Cr(CO)4(1)]+ was activated using Et3Al in the absence of aromatic solvents. The resulting [Cr(1-bis-η6-arene)]+ complex was characterized by EPR and DFT. However, in aromatic solvents (such as toluene), solvent-based [Cr(bis-tolyl)]+ complexes are preferentially formed. These results are fully consistent with the enhancement in catalytic performance observed when Cr(I)bis(phosphine) complexes are used in aliphatic solvents, since studies have shown deactivation occurs more rapidly in aromatic solvents compared to the longer catalyst lifetimes observed in alkane solvents.3 While [Cr(bis-tolyl)]+ is catalytically inactive, the facile formation of [Cr(1-bis-η6-arene)]+ simply by CO loss implies reversible phosphine binding could 4507
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Organometallics occur under high pressure catalytic conditions, suggesting an indirect role for this complex in the catalysis.
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(12) The Fermi contact term (Paiso) for 31P is very large: 13 291 MHz. Hence, the small difference between the calculated Paiso value of 4.594 MHz vs the estimated experimental value of Paiso = 3.78 MHz is likely due to the limitations of the calculations.
bS
Supporting Information. Text, figures, and a table giving details of the synthesis and characterization data for 1 and 2 and full details for the EPR/ENDOR experiments and computational DFT studies. This material is available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT L.M. acknowledges funding from Sasol. EPSRC funding (EP/H023879) is also acknowledged. ’ REFERENCES (1) (a) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641. (b) McGuinness, D. S. Chem. Rev. 2011, 111, 2321. (c) McGuinness, D. S. Chem. Rev. 2011, 111, 2321–2341. (2) (a) Rucklidge, A. J.; McGuinness, D. S.; Tooze, R. P.; Slawin, A. M. Z.; Pelletier, J. D. A.; Hanton, M. J.; Webb, P. B. Organometallics 2007, 26, 2782. (b) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J. T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272. (c) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S. Chem. Commun. 2005, 622. (3) Blann, K.; Bollmann, A.; de Bod, H.; Dixon, J. T.; Killian, E.; Nongodlwana, P.; Maumela, M. C.; Maumela, H.; McConnell, A. E.; Morgan, D. H.; Overett, M. J.; Pretorius, M.; Kuhlmann, S.; Wasserscheid, P. J. Catal. 2007, 249, 244. (4) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 4337. (5) (a) Crewdson, J. C.; Gambarotta, S.; Kokobkov, I.; Duchateau, R. Organometallics 2006, 25, 715. (b) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2007, 129, 14281. (c) Schofer, S. J.; Day, M. W.; Henling, M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2006, 25, 2743. (d) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 5122. (6) Kundig, E. P.; Pache, S. H. In Organometallics: Compounds of Group 7-3; Imamoto, T., Ed.; Thieme: Stuttgart, Germany, 2003; Vol. 2, pp 155 228. (7) Bruckner, A.; Jabor, J. K.; McConnell, A. E. C.; Webb, P. B. Organometallics 2008, 27, 3849. (8) (a) Rieger, A. L.; Rieger, P. H. Organometallics 2002, 21, 5868. (b) Li, T. T.; Kung, W.; Ward, D. L.; McCulloch, B.; Brubaker, C. H., Jr. Organometallics 1982, 1, 1229. (c) Prins, R.; Reinders, F. J. Chem. Phys. Lett. 1969, 3, 45. (9) (a) Song, L.-C.; Yu, G.-A.; Hu, Q.-M.; Che, C.-M.; Zhu, N.; Huang, J.-S. J. Organomet. Chem. 2006, 691, 787. (b) Nesmeyanov, A. N.; Yur’eva, L. P.; Zaitseva, N. N.; Domrachev, G. A.; Zinov’ev, V. D.; Shevelev, Yu. A.; Vasyukova, N. I. J. Organomet. Chem. 1976, 121, C52. (c) Elschenbroich, C.; Heck, J. Angew. Chem. 1977, 16, 479. (d) Elschenbroich, C.; Kroker, J.; Massa, W.; Wunsch, M.; Ashe, A. J. Angew. Chem. 1986, 25, 571. (e) Elschenbroich, C.; Heikenfeld, G.; Wunsch, M.; Massa, W.; Baum, G. Angew. Chem. 1988, 27, 414. (f) Elschenbroich, C.; Sebbach, J.; Metz, B.; Heikenfeld, G. J. Organomet. Chem. 1992, 426, 173. (10) McDyre, L. E.; Hamilton, T.; Murphy, D. M.; Cavell, K. J.; Gabrielli, W. F.; Hanton, M. J.; Smith, D. M. Dalton Trans. 2010, 39, 7792. (11) Upon activation ca. 80% of the original [Cr(CO)4(1)]+ paramagnetic EPR signal is lost. However, there was no evidence of any Cr(III) ions in the EPR spectra. 4508
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