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Energy & Fuels 1996, 10, 520-523
Hemilabile Phosphonate-Phosphane-Rh Catalysts for Homogeneous and Heterogeneous Carbonylation S. Bischoff,*,† A. Weigt,† H. Miessner,‡ and B. Lu¨cke† Institute for Applied Chemistry Berlin-Adlershof, Rudower Chaussee 5, 12489 Berlin, Germany, and Centre for Heterogeneous Catalysis (KAI e.V.), Rudower Chaussee 5, 12489 Berlin, Germany Received August 23, 1995X
New rhodium complexes with phosphonate-phosphane ligands (RO)2P(O)-X-PPh2 (1a-d), a, X ) CH2; b, X ) (CH2)2; c, X ) (CH2)3; d, X ) p-C6H4, have been synthesized and characterized. Both, open-chain structures [ClRh(cod)PPh2-X-P(O)(OR)2] (2a-d) and cyclic complexes [(cod)Rh(PPh2-X-P(O)(OR)2)]A (3a-c, A ) BF4, PF6) were isolated after stoichiometric reactions at mild conditions. FTIR investigations of 2b supported on silica at temperatures between 150 and 250 °C suggest that the phosphonate-phosphane ligand stabilizes rhodium monocarbonyl species and allows also the formation of free coordination sites to form dicarbonyl species, which is in accord with the proposed hemilabile behavior of the complexes in methanol carbonylation. During the catalytic cycle, the phosphane group is assumed to be strongly coordinated to the rhodium, while the phosphoryl oxygen of the phosphonate group is supposed to change between a free and a coordinated state, thus vacating or occupying a coordination site. This is supported by the findings that the activation enthalpies for the open-chained catalyst precursors 2a-d increased significantly with growing distance between the phosphonate and phosphane groups. The new rhodium-phosphonate-phosphane complexes 2a and 2b afforded also heterogeneous catalysts with enhanced activity in vapor-phase carbonylation. Activated carbon has been found to be a suitable support for hemilabile rhodium complexes, but normal diffusion of reactants begins to limit the high intrinsic carbonylation rate over the supported catalysts.
Introduction Mixed bidentate phosphane ligands such as etherphosphanes,1-3 phosphane oxide-phosphanes,4 phosphanopyridines, and phosphanoamines5 containing weakly coordinating O- or N-functional groups and a strongly coordinating phosphane group are known to enhance activities or selectivities of Rh-catalyzed carbonylations. Also other transition metals form hemilabile O,P-chelate complexes, which have been extensively reviewed by Lindner.6 It has been suggested that the weakly donating oxygen site of the bidentate ligand changes between a coordinated and an uncoordinated state during the catalytic cycle, thus forming chelate and open-chain metal complex structures.1,4 The intramolecular generation and occupance of free coordina* Address correspondence to this author at Institut fu¨r Angewandte Chemie (Institute for Applied Chemistry), Abteilung Katalyse, Rudower Chaussee 5, D-12489 Berlin, F. R. Germany. † Institute for Applied Chemistry. ‡ Centre for Heterogeneous Catalysis. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Lindner, E.; Mayer, H. A.; Wegner, P. Chem. Ber. 1986, 119, 2616-2630. (2) Lindner, E.; Sickinger, A.; Wegener, P. J. Organomet. Chem. 1988, 349, 75-94. (3) Lindner, E.; Bader, A.; Bra¨unling, H.; Jira, R. J. Mol. Catal. 1990, 57, 291-300. (4) Wegmann, R. W.; Abatjoglou, A. G.; Harrison, A. M. J. Chem. Soc., Chem. Commun. 1987, 1891-1892. Wegmann, R. W.; Abatjoglou, A. G. PCT Int. Appl. WO 8600888 (1986); C.A. 1987, 105, 174788. (5) Abu-Gnim, C.; Amer, I. J. Mol. Catal. 1993, 85, L275-L278. AbuGnim, C.; Amer, I. J. Chem. Soc., Chem. Commun. 1994, 115-117. (6) Bader, A.; Lindner, E. Coord. Chem. Rev. 1991, 108, 27-110. (7) Knowles, W. S. Acc. Chem. Res. 1983, 16, 106-112. (8) Michalska, Z. M. J. Mol. Catal. 1983, 19, 345-358. (9) Ostoja Starzewski, K. A.; Witte, J. Angew. Chem. 1985, 97, 610612.
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tion sites is assumed to accelerate rate-determining steps in the methanol carbonylation route. Heterobidentate complexes catalyzed not only carbonylations effectively but also hydrogenation,7 hydrosilylation,8 and ethylene polymerization.9 The phosphane sulfide-phosphane Ph2PCH2P(S)Ph2 was reported to form a catalyst that is 8 times more active in methanol carbonylation than the conventional Monsanto catalyst,10 but no evidence for hemilabile behavior was found for this new catalyst. Rhodium complexes with phosphonate-phosphanes11 appeared as promising catalyst precursors because they form hemilabile complexes, which should enhance the necessary formation of free coordination sites in RhI intermediates by ring opening of chelate structures and facilitate the generation of the more oxophilic RhIII intermediates by O,P-chelate formation (Figure 1). The concept of hemilabile catalysts with chelate structures involved in rate-determining steps implies that the distance and structure between the phosphane and the phosphonate group should affect the carbonylation activity. To examine this hypothesis, we studied the activity of new rhodium complex catalysts derived from ligands 1a-d (Figure 2) in homogeneous and vaporphase methanol carbonylation. Experimental Section Materials. Methanol, methyl iodide, and methyl acetate (Merck) were used as received. Dichloromethane was dried (10) Baker, M. J.; Giles, M. F.; Orpen, A. G.; Taylor, M. J.; Watt, R. J. J. Chem. Soc., Chem. Commun. 1995, 197-198. (11) Freiberg, J.; Weigt, A.; Dilcher, H. J. Prakt. Chem. 1993, 335, 337-344.
© 1996 American Chemical Society
Hemilabile Phosphonate-Phosphane-Rh Catalysts for Carbonylation
Figure 1. Suggested catalytic cycle for methanol carbonylation catalyzed by phosphonate-phosphane-rhodium complexes (R ) Me, i-Pr; L ) CO, I, or phosphane; ki ) rate constants).
Figure 2. Ligands (1a-d), open-chained (2a-d), and chelate (3a-c) complexes; a: X ) CH2, R ) i-Pr; b: X ) (CH2)2, R ) Me; c: X ) (CH2)3, R ) i-Pr; d: X ) p-C6H4, R ) i-Pr; A ) BF4, PF6. over molecular sieve, type 3A (Alfa Products). Activated carbon (Desorex ED 47, Lurgi, specific surface area 800 m2/g) and silica (large-pore silica, Alfa Products, specific surface area 270 m2/g) were dried at 250 °C in vacuum (
