Synthesis and Characterization of Tetrahedral Ru 3 O Clusters with Intrinsic Framework Chirality: A Chiral Probe of the Intact Cluster Catalysis Concept † Ludovic Vieille-Petit, ‡ Georg Su ¨ ss-Fink,* ,‡ Bruno Therrien, ‡ Thomas R. Ward, ‡ Helen Stœckli-Evans, ‡,§ Gae ¨l Labat, ‡,§ Lydia Karmazin-Brelot, ‡,§ Antonia Neels, ‡,§ Thomas Bu ¨ rgi, ‡ Richard G. Finke, | and Collin M. Hagen | Institut de Chimie, Universite ´ de Neucha ˆ tel, Case Postale 2, CH-2007 Neucha ˆ tel, Switzerland, and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received July 28, 2005 To bring evidence for or against the hypothesis of catalytic hydrogenation by intact trinuclear arene ruthenium clusters containing an oxo cap, cationic Ru 3 O clusters with three different arene ligands (intrinsically chiral tetrahedra) have been synthesized as racemic mixtures. By introduction of a chiral auxiliary substituent at one of the three different arene ligands, the separation of the two diastereomers was possible. The chiral Ru 3 O framework was evidenced by X-ray crystallography, by circular dichroism in the UV and IR regions, and by chiral shift reagents in the NMR spectra. The catalytic hydrogenation of the prochiral substrate methyl 2-acetamidoacrylate using a chiral Ru 3 O cluster showed no asymmetric induction, suggesting that the catalytically active species is not the intact Ru 3 O cluster. Introduction In 1999 one of our groups (Su ¨ ss-Fink et al.) ob- served that the trinuclear cluster cation [H 3 Ru 3 (C 6 H 6 )- (C 6 Me 6 ) 2 (O)] + (1), accessible from the dinuclear percur- sor [H 3 Ru 2 (C 6 Me 6 ) 2 ] + and the mononuclear percursor [Ru(C 6 H 6 )(H 2 O) 3 ] 2+ and employed as the water-soluble tetrafluoroborate salt, efficiently catalyzes (or acts as an efficient catalyst precursor) the hydrogenation of benzene to cyclohexane under biphasic conditions. 1 From the mass spectroscopic observation of the host- guest complex [C 6 H 6 ⊂1] + , from molecular modeling studies, and on the basis of the fact that after the hydrogenation of C 6 D 6 to give C 6 D 6 H 6 catalyzed by 1, >95% of the cluster cation could be recovered unchanged without the C 6 H 6 ligand being exchanged by C 6 D 6 , it was hypothesized that the substrate molecule is incor- porated in the hydrophobic pocket spanned by the three arene ligands in 1, suggesting the catalytic reaction occurs within this host-guest complex, while the Ru 3 O cluster stays intact throughout the catalytic process. 2 However, for benzene hydrogenation catalysts derived from organometallic precursors, the true nature of the catalytic species remained a debatable point (“is it homogeneous or heterogeneous catalysis?”). 3 In the case of the putative homogeneous [(C 8 H 17 ) 3 Me][RhCl 4 ] ion pair catalyst, 4 Finke and co-workers were able to demonstrate that rhodium(0) nanoclusters are the true catalysts (“soluble analogs of heterogeneous catalysts”). 5 Homogeneous arene hydrogenation catalysts have been reviewed recently by Finke 6 and by Dyson, 7 and the question of the true nature of the catalytic species in arene hydrogenation with soluble metal complexes has been critically addressed by Finke et al. 8 The water-soluble cluster cation 1, reported by Su ¨ ss- Fink et al. to catalyze efficiently the hydrogenation of benzene and benzene derivatives, 1 seemed to be a molecular catalyst that works under biphasic conditions. The high substrate selectivity, 9 which can be tuned by * To whom correspondence should be addressed. E-mail: georg.suess-fink@unine.ch. Fax: 413 2718 2511. Tel: 413 2718 2405. † This paper is dedicated to Professor Brian F. G. Johnson, Univer- sity of Cambridge (Cambridge, U.K.), and to Prof. Heinrich Vahren- kamp, University of Freiburg (Freiburg, Germany), on the occasion of their retirement. ‡ Universite ´ de Neucha ˆ tel. § X-ray crystallography. | Colorado State University. (1) Faure, M.; Jahncke, M.; Neels, A.; Stœckli-Evans, H.; Su ¨ ss-Fink, G. Polyhedron 1999, 18, 2679. (2) Su ¨ ss-Fink, G.; Faure, M.; Ward, T. R. Angew. Chem. 2002, 114, 105; Angew. Chem., Int. Ed. 2002, 41, 99. (3) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891. (4) (a) Sasson, Y.; Zoran, A.; Blum, J. J. Mol. Catal. 1981, 11, 293. (b) Blum, J.; Amer, I.; Zoran, A.; Sasson, Y.; Tetrahedron Lett. 1983, 24, 4139. (c) Blum, J.; Amer, I.; Vollhardt, K. P. C.; Schwarz, H.; Hoehne, G. J. Org. Chem. 1987, 52, 2804. (5) Weddle, K. S.; Aiken, J. D., III; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 5653. (6) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 191, 187. (7) Dyson, P. J. Dalton Trans. 2003, 2964. (8) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198, 317. 6104 Organometallics 2005, 24, 6104-6119 10.1021/om050643t CCC: $30.25 © 2005 American Chemical Society Publication on Web 11/04/2005