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Organometallics 2010, 29, 2292–2305 DOI: 10.1021/om100049n
Palladium Complexes of the Heterodiphosphine o-C6H4(CH2PtBu2)(CH2PPh2) Are Highly Selective and Robust Catalysts for the Hydromethoxycarbonylation of Ethene Tamara Fanjul,† Graham Eastham,‡ Natalie Fey,† Alex Hamilton,† A. Guy Orpen,† Paul G. Pringle,*,† and Mark Waugh‡ †
School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K., and Lucite International, Lucite International Technology Centre, PO Box 90, Wilton, Middlesbrough, Cleveland, TS6 8JE, U.K.
‡
Received January 21, 2010
The coordination chemistry and ethene hydromethoxycarbonylation catalysis with the diphosphine o-C6H4(CH2PtBu2)(CH2PPh2) (L3) is reported and the results compared with the analogous chemistry of the symmetrical diphosphines o-C6H4(CH2PtBu2)2 (L1) and o-C6H4(CH2PPh2)2 (L2). Palladium-catalyzed ethene hydromethoxycarbonylation studies under the commercial catalytic conditions are reported. The results obtained using L1-3 as supporting ligands show that the catalysts derived from L3 and L1 have similar activity and selectivity for methyl propanoate (MeP). In addition, the Pd-L3 catalyst has much greater longevity than the Pd-L1 catalyst. Treatment of the appropriate [Pt(X)(Y)(cod)] with L3 gave [PtCl2(L3)] (3), [Pt(CH3)2(L3)] (6), and [PtCl(CH3)(L3)] (9). At equilibrium, complex 9 is a 90:1 mixture of geometric isomers 9a (with CH3 trans to the tBu2P) and 9b (with Cl trans to the tBu2P). The fluxionality of complex 3, detected by 1H NMR, is interpreted in terms of the conformation of the seven-membered chelate. The complexes [Pt(CH3)(PMe3)(L3)]Cl (10b) and [PtH(PPh3)(L3)]Cl (12b) are formed as essentially single isomers with CH3/H trans to the Ph2P group. The palladium complexes [PdCl2(L3)] (13), [PdCl(CH3)(L3)] (14a/14b), and [PdH(PCy3)(L3)]BF4 (15b) have been made by similar methods to their platinum analogues. The factors controlling the relative isomer stabilities are explored experimentally and computationally. The complexes [PtCl2(L4)] (16) and [PtCl(CH3)(L4)] (17a/17b) where L4 = o-C6H4(CH2PnBu2)(CH2PPh2) are reported, and the geometric isomers of 17 are almost isoenergetic. The crystal structures of 3, 14a, 15b, and 16 have been determined by X-ray crystallography. DFT calculations on complexes of the type [Pt(X)(Y)(L3)] gave only small calculated differences in energy between the geometrical isomers (0-4 kcal mol-1), which are consistent with the experimental observations. It is suggested that repulsive intramolecular H 3 3 3 H interactions (between the Pt-CH3 and PC(CH3)3 groups) determine which isomer predominates. The reasons for the favorable catalytic properties of the Pd-L3 catalyst are probed by 13CO reactions with the model complexes 9a/9b and 14a/14b, and the structures of the resulting acyl complexes are assigned on the basis of 13C and 31P NMR and IR spectroscopy. From these studies, it is suggested that the reason for the Pd-L3 catalyst resembling the Pd-L1 catalyst in terms of selectivity is that the crucial acyl intermediates are similar.
Introduction The carbonylation of ethene (Scheme 1) in the presence of methanol is an excellent example of an atom-efficient homogeneous catalysis that has been commercialized for the production of two commodity chemicals: methyl propanoate (MeP) and polyketone (PK).1 The best catalysts for ethene carbonylations are palladium(II)-diphosphine complexes where the selectivity for MeP or PK is controlled by the ligand backbone and the substituents on the P atoms.2-7 *Corresponding author. E-mail:
[email protected]. (1) Drent, E.; Buzelaar, P. H. M. Chem. Rev. 1996, 96, 663, and references therein. (2) Clegg, W.; Eastham, G. R.; Elsegood, M. R. J.; Tooze, R. P.; Wang, X. L.; Whiston, K. Chem. Commun. 1999, 1877. pubs.acs.org/Organometallics
Published on Web 04/23/2010
For example, the highly active catalyst derived from bis(ditert-butylphosphino)xylene (L1) produces MeP in 99.9% selectivity,2 and in 2008, Lucite commercialized a process founded on this chemistry. By contrast, the catalyst derived from the diphenylphosphino analogue L2 has low activity and gives PK in ca. 90% selectivity (vide infra).
Naı¨ vely, it might be predicted that catalysts derived from the ditopic ligand L3 would give mixtures of MeP, oligomers, and PK. However, we show here that the L3-Pd catalyst r 2010 American Chemical Society
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Organometallics, Vol. 29, No. 10, 2010 Scheme 1
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Scheme 2
Chart 1
gives MeP as selectively as the commercial Pd-L1 catalyst and that the Pd-L3 catalyst is significantly more stable than the Pd-L1 catalyst.8 In order to rationalize the excellent catalytic results obtained with Pd-L3 complexes, it was considered important to understand the stereoelectronic factors governing the relative stabilities of the geometric isomers that would be involved in every putative intermediate in the mechanism. We report here a combined spectroscopic, structural, and computational investigation of isomeric model complexes of the type denoted a and b in Chart 1 where M = Pt or Pd and R = H, CH3, or COCH3. (3) (a) Eastham, G. R.; Tooze, R. P.; Wang, X. L.; Whiston K. (ICI) World Patent WO 96/19434, 1996. (b) Drent, E.; Kragtwijk, E. (Shell) Eur. Patent EP495,548, 1992. (c) Pugh, R. I.; Drent, E.; Pringle, P. G. Chem. Commun. 2001, 1476. (d) Drent, E.; Kragtwijk, E.; Pello, D. H. L. (Shell) Eur. Patent EP495,547, 1992. (e) Drent, E.; Pello, D. H. L.; Suykerbuyk, J. C. L. J.; van Gogh, J. B. (Shell) World Patent WO5354, 1994. (f) Doherty, S.; Robins, E. G.; Knight, J. G.; Newman, C. T.; Rhodes, B.; Champkin, P. A.; Clegg, W. J. Organomet. Chem. 2001, 640, 182. (g) Knight, J. G.; Doherty, S.; Harriman, A.; Robins, E. G.; Betham, M.; Eastham, G. R.; Tooze, R. P.; Elsegood, M. R. J.; Champkin, P.; Clegg, W. Organometallics 2000, 19, 4957. (h) Gusev, O. V.; Kalsin, A. M.; Peterleitner, M. G.; Petrovskii, P. V.; Lyssenko, K. A.; Akhmedov, N. G.; Bianchini, C.; Meli, A.; Oberhauser, W. Organometallics 2002, 21, 3637. (i) Bianchini, C.; Meli, A.; Oberhauser, W.; van Leeuwen, P. W. N. M.; Zuideveld, M. A.; Freixa, Z.; Kamer, P. C. J.; Spek, A. L.; Gusev, O. V.; Kalsin, A. M. Organometallics 2003, 22, 2409. (j) Gusev, O. V.; Kalsin, A. M.; Petrovskii, P. V.; Lyssenko, K. A.; Oprunenko, Y. F.; Bianchini, C.; Meli, A.; Oberhauser, W. Organometallics 2003, 22, 913. (k) Vautravers, N. R.; Cole-Hamilton, D. J. Dalton Trans. 2009, 2130. (4) Drent, E.; van Broekhoven, J. A. M.; Doyle, M. J. J. Organomet. Chem. 1991, 417, 235. (5) (a) Dossett, S. J.; Gillon, A.; Orpen, A. G.; Fleming, J. S.; Pringle, P. G.; Wass, D. F.; Jones, M. D. Chem. Commun. 2001, 699. (b) Bianchini, C.; Lee, H. M.; Meli, A.; Moneti, S.; Vizza, F.; Fontani, M.; Zanello, P. Macromolecules 1999, 32, 4183. (c) Bianchini, C.; Lee, H. M.; Barbaro, P.; Meli, A.; Moneti, S.; Vizza, F. New J. Chem. 1999, 23, 929. (d) Lindner, E.; Schmid, M.; Wald, J.; Queisser, J. A.; Gepr€ags, M.; Wegner, P.; Nachtigal, C. J. Organomet. Chem. 2000, 602, 173. (e) Jiang, Z.; Sen, A. Macromolecules 1996, 27, 7215. (f) Verspui, G.; Papadogianakis, G.; Sheldon, R. A. Chem. Commun. 1998, 401. (6) (a) Nozaki, K. US Patent 3689460, 1972. (b) Nozaki, K. US Patent 3694412, 1972. (c) Sen, A.; Lai, T. J. Am. Chem. Soc. 1982, 104, 3520. (d) Sen, A.; Lai, T. Organometallics 1984, 3, 866. (e) Drent, E. US Patent, 4835250, 1989. (f) Keim, W.; Mass, H.; Mecking, S. Z. Naturforsch., B: Chem. Sci. 1995, 50b, 430. (g) Smith, G.; Vautravers, N. R.; Cole-Hamilton, D. J. Dalton Trans. 2009, 872. (7) van Leeuwen, P. W. N. M.; Zuideveld, M. A.; Swennenhuis, B. H. G.; Freixa, Z.; Kamer, P. C. J.; Goubitz, K.; Fraanje, J.; Lutz, M.; Spek, A. L. J. Am. Chem. Soc. 2003, 125, 5523. (8) Eastham, G. R.; Waugh, M.; Pringle, P. G.; Fanjul Solares, T. World Patent WO2010001174, 2010.
The results provide insights into the role of stereoelectronic effects in unsymmetrical chelate complexes that may have important consequences for the design of bidentate ligands for catalysis.
Results and Discussion Ligand Synthesis. The synthetic route to the unsymmetrical diphosphine L3 (Scheme 2) is a modification of literature procedures for the synthesis of unsymmetrical o-xylenyl diphosphines.9 Ligand L3 is a white crystalline solid that has been fully characterized (see Experimental Section).10 The 31P NMR spectrum of L3 showed two singlets at -14.9 (PPh2) and 26.3 (PtBu2) ppm, and so, not surprisingly, 5JPP is less than the line width of ca. 4 Hz. The two P-donors in ligand L3 would be expected to have considerably different coordination properties: RPtBu2 are more bulky and better σ-donors than the corresponding RPPh2.11 It has been shown12 that the JPSe for R3PdSe is a reliable measure of the donor properties of R3P. The diselenides of L1-3 were generated by treatment of the diphosphines with KSeCN in MeOH (see Experimental Section), and the values of 1JPSe (in CDCl3) for L3(Se)2 of 694 and 724 Hz are the same as those determined for the corresponding symmetrical L1(Se)2 and L2(Se)2, showing that, in nonchelated L3, the donors do not appear to influence each other significantly. Hydromethoxycarbonylation Catalysis. Ethene carbonylation was carried out with palladium catalysts derived from L1-3 under the same conditions (see Experimental Section), and the results are given in Table 1. As expected from previous reports,2 the catalyst derived from ligand L1 produces MeP at high rate and with very high selectivity. Under the conditions used here, the catalyst derived from ligand L2 had very low activity and produced a mixture of solid polyketone, liquid oligomers, and traces of MeP. Strikingly, when ligand L3 was employed, MeP was formed in >99.5% (9) (a) Leone, A.; Consiglio, G. Helv. Chim. Acta 2005, 88, 210. (b) Rucklidge, A. J.; Morris, G. E.; Slawin, A. M. Z.; Cole-Hamilton, D. J. Helv. Chim. Acta 2006, 89, 1783. (10) Ligand L3 appeared in a table of data on the hydromethoxycarbonylation of styrene. It was reported to give a catalyst of low activity and selectivity. The synthesis and characterization of L3 were not reported. See: Ooka, H.; Inoue, T.; Itsuna, S.; Masato, M. Chem. Commun. 2005, 1173. (11) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313. (b) Gusev, D. G. Organometallics 2009, 28, 763. (12) Muller, A.; Otto, S.; Roodt, A. Dalton Trans. 2008, 650.
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Table 1. Catalyst Selectivity and Productivitya ligand L1 L2 L3
MeP
oligomers
>99.9% 10% >99.5%
