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Supramolecular Cluster Catalysis: Benzene Hydrogenation

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Supramolecular Cluster Catalysis: Benzene Hydrogenation View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Crossref Published in Angewandte Chemie International Edition, 41, issue 1, 99-101, 2002 1 which should be used for any reference to this work Supramolecular Cluster Catalysis: Benzene Hydrogenation Catalyzed by a Cationic Triruthenium Cluster under Biphasic Conditions** Georg S¸ss-Fink,* Matthieu Faure, and Thomas R. Ward + + Dedicated to Professor Lord Lewis of Newnham O O H 2 H2 + H2O H Organometallic catalysis most often proceeds through a Ru Ru Ru Ru catalytic cycle that involves the coordination of the substrate, H Ru H H Ru OH C6H5Et C6H11Et either by ligand substitution or by oxidative addition, trans- formation of the coordinated substrate, and liberation of the product, either by decoordination or by reductive elimina- 1 2 tion.[1] Classical examples that have been studied in great Scheme 1. Hydrolyis of the closed cluster cation 1 to give the open cluster detail are the hydrogenation of olefins with Wilkinson×s cation 2 during the hydrogenation of ethylbenzene to ethylcyclohexane catalyst[2] and the carbonylation of methanol with rhodium under biphasic conditions (no free C6H12 or C6Me6 detected). iodide (Monsanto Process).[3] The complete characterization of the intermediates of the latter process and the proposal of a Cation 2, dissolved in water, catalyzes the hydrogenation of well-established catalytic cycle represents one of the triumphs aromatic compounds with higher activity than cation 1:The of organometallic chemistry.[4] reaction proceeds with a catalyst/substrate ratio of 1/1000 In all these reactions, the elementary steps of the catalytic under hydrogen pressure (60 bar) at 1108C with vigorous process are believed to occur within the first coordination stirring of the biphasic system. For benzene, the reaction is sphere of the organometallic catalyst.[5] We now have reasons almost complete within 15 min, the catalytic turnover number to believe that organometallic catalysts may transform a (TON) being 911, corresponding to a catalytic turnover substrate without prior coordination, the interactions between frequency (TOF) of 3644 hÀ1. Cation 2 can be recovered À both partners entirely relying on weak intermolecular con- unaltered as the BF4 salt after a catalytic run from the tacts. Although hydrogen transfer from a catalyst molecule to aqueous phase and reused. Under identical conditions, cation a substrate via a merely hydrogen-bonded catalyst ± substrate 1 catalyzes the hydrogenation of benzene with a TOF of complex has already been considered as the mechanism of 289 hÀ1 (Scheme 2). ketone transfer hydrogenation reactions,[6] catalytic trans- formations by host ± guest interactions and molecular recog- H2O + 3 H2 nition are generally accepted only in enzymatic catalysis.[7] catalyst The water-soluble organometallic cluster cation 1 (see Scheme 2. Hydrogenation of benzene to cyclohexane catalyzed by 1-BF4 h6 2 h6 8 Scheme 1), accessible from [( -C6H6)Ru(H2O)3] with [( - or by 2-BF4 (catalyst/substrate 1:1000, 60 bar H2 , 110 C) in water (TOF m 289 hÀ1 for 1 and 3544 hÀ1 for 2, respectively). C6Me6)2Ru2( 2-H)3] in aqueous solution and isolated as the À [8] BF4 salt, was found to catalyze the hydrogenation of aromatic substrates under biphasic conditions. An unusually Although a large number of organometallic complexes high catalytic activity of 1 was observed for the hydrogenation are known to catalyze the hydrogenation of olefins or of ethylbenzene. From the reaction mixture the cluster cation acetylenes, only very few complexes are reported to catalyze À [9] 2 could be isolated as the BF4 salt (Scheme 1). the hydrogenation of aromatic compounds (TOF given in h6 m m À1 [10] parantheses): [( -C6Me6)2Ru2( 2-H)( 2-Cl)2]Cl2 (241 h ), h5 m À1 [11] h3 [( -C5Me5)2Rh2( 2-Cl)2](11h ), [( -C3H5)Co(P(OMe)3)3] À1 [12] h2 À1 [13] (0.7 h ), [RuH2( -H2)2(PCy3)2](1.6h), [Nb(OC6- À1 [14] HPh4-2,3,5,6)2Cl3] in combination (1:3) with BuLi (409 h ), h6 m À1 [15] [( -C6H6)4Ru4( 3-H)4]Cl2 in water (376 h ) or in ionic À1 [16] h6 m À1 liquids (364 h ), and [( -C6H6)2Ru2( 2-Cl2)Cl2](1998h ) [*] Prof. Dr. G. S¸ss-Fink, M. Faure, Prof. Dr. T. R. Ward in water.[17] The hydrogenation of arenes falls in general within Institut de Chimie, Universite¬ de Neucha√tel the domain of heterogeneous catalysis: Millions of tons of Case postale 2, 2007 Neucha√tel (Switzerland) benzene are hydrogenated per year to give cyclohexane using Fax : (41) 32-718-2511 E-mail: [email protected] Raney nickel as the heterogeneous catalyst; the so-called IFP [18] [**] This work was supported by the Fonds National Suisse de la (Institut FranÁais du Petrole) process using nickel and cobalt Recherche Scientifique. A genereous loan of ruthenium chloride by salts in combination with triethylaluminum seems destined to the Johnson Matthey Technology Centre is gratefully acknowledged. take over.[19] 2 In the case of 2, catalyzing the hydrogenation of ethyl- benzene, no substitution of a h6-bonded arene ligand (benzene or hexamethylbenzene) in 2 by ethylbenzene is observed. The recovered material shows the identical 1H NMR spectrum and the same catalytic activity for further catalytic runs as the original 2-BF4 . This striking observation rules out the conven- tional organometallic mechanism for the catalytic hydro- genation of aromatic substrates by 2, in which the substrate is coordinated to the metal of the active complex, because in this Figure 1. a) Superposition of the molecular structure of the eleven h5 m h2 h2 h2 n case one of the three arene ligands (either benzene or structurally characterized clusters [( -C5R5)3M3( 3- : : -arene)] (M Co, Ru, Rh); hydrogen atoms and arene substituents are omitted hexamethylbenzene) in 2 should be replaced by ethylbenzene. for clarity. b) Molecular structure of 2, emphasizing the hydrophobic We therefore conclude that the aromatic substrate molecule is pocket and the preorganization to accomodate a benzene guest (compare not coordinated to ruthenium throughout the catalytic cycle with Figure 1a; C: green; H: white; Ru: magenta; O: red). c) CPK model of the optimized structure of C H 2; the {Ru} -C H distance fixed at 3 ä, but interacts with the Ru3 surface in 2 only through weak 6 6 3 6 6 h6 intermolecular interactions. This may be possible thanks to -bound arenes green and white, docked benzene blue and white. the three h6-arene ligands in 2 which form a hydrophobic pocket capable of accommodating the aromatic substrate and benzene host in the hydroxo-bridged cluster 2 (shortest HÀH in placing it in a perfect position underneath the Ru3 face contact 1.60 ä; Figure 1 c). With the catalytically less active opposite to the oxo cap of the cluster. In the spirit of the cluster 1, a similar procedure yields shorter HÀH contacts triterpene cyclases[7] and due to the lipophilic character of (1.28 ä) in the energy-minimized structure, suggesting that both substrate and catalyst binding pocket, the host ± guest the cluster must relax to accomodate the benzene guest. interaction is very favorable, as the aromatic substrate tries to We believe the hydrogenation of the aromatic substrate to escape from the aqueous medium. occur within the hydrophobic pocket of the catalyst in a To verify this hypothesis, we carried out a mass spectro- stepwise manner, that is to say via cyclohexadiene and metric study of mixtures of hexadeuterobenzene with 1 and cyclohexene intermediates. Thus, hydrogenation of the di- À with 2 (as BF4 salts) in acetone. The adducts C6D6 1 and substituted o-, m-, and p-xylenes afford both cis and trans C6D6 2 ¥H2O can be clearly identified in the electrospray isomers of the corresponding dimethylcyclohexanes. This mass spectrum. The intensity of these peaks, which can be implies that after the dihydride transfer, the (partially) assigned unambiguously on the basis of their characteristic reduced substrate leaves the hydrophobic pocket, allowing Ru3 isotope pattern, increases with the concentration of C6D6 regeneration of the active catalyst by H2 oxidative addition. in the acetone solution. These results strongly support the Accordingly, in the hydrogenation of benzene with 2-BF4 hypothesis of host ± guest interactions between the substrate under biphasic conditions (catalyst/substrate ratio of 1/1000, 8 and the catalyst. H2 60 bar, 20 C, 1 h), small amounts of 1,3-cyclohexadiene Facial coordination of an arene on the triangular face of a and of cyclohexene can indeed be detected by GC analysis. A cluster has been known since the characterization of [Os3- mechanism consistent with these observations is outlined in m h2 h2 h2 [20] (CO)9( 3- : : -C6H6)] by Lewis et al. These structures Scheme 3. have been invoked as models for the chemisorption of arene The reaction of cyclohexene with D2 catalyzed by 2-BF4 in 8 derivatives at a threefold site of close-packed metal surfaces. H2O (catalyst/substrate ratio of 1/1000, D2 40 bar, 110 C, Incorporating arene derivatives as ancillary ligands consti- 15 min) affords exclusively 1,2-dideuterocyclohexane, sug- tutes a good model for the interaction between a hydrophobic gesting that the hydrogenation mechanism involves indeed a triangular face and an arene derivative. A search of the Cambridge Structural Database reveals eleven such structures composed of three {(h5-C R )M} moieties capped by an arene, 5 5 Ru Ru h5 m h2 h2 h2 n HH HH yielding [( -C5R5)3M3( 3- : : -arene)] (M Co, Ru, [21] Rh, R H, Me; Figure 1a). Comparing these structures Ru Ru Ru Ru to the molecular structure of clusters 1 and 2 (Figure 1b) 2 suggests that these are preorganized to accommodate a benzene host.
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