The Pd 4 (dppm) 4 (H) 2 2+ Cluster: A Precatalyst for the Homogeneous Hydrogenation of Alkynes David Evrard, 1a,b Katherine Groison, 1b Yves Mugnier, 1a and Pierre D. Harvey* ,1b Contribution from the Laboratoire de Synthe ` se et d’E Ä lectrosynthe ` se Organome ´ talliques, (LSEO-CNRS UMR 2595), Faculte ´ des Sciences Gabriel, UniVersite ´ de Bourgogne, 6 BouleVard Gabriel, 21000 Dijon, France, and the De ´ partement de Chimie, UniVersite ´ de Sherbrooke, Sherbrooke, Que ´ bec, Canada J1K 2R1 Received July 10, 2003 The catalytic properties of the title cluster toward the homogeneous hydrogenation of phenylacetylene, diphenylethyne and phenyl-1-propyne have been investigated as a function of temperature, pressure, solvents, substrate and cluster concentrations, and counterions. The title cluster is a precatalyst that exhibits a good catalytic activity under mild conditions (1 atm of H 2 at 20 °C) for the hydrogenation of alkynes and alkenes. For the alkyne substrates, the turnover frequencies (tof’s) range between 200 and 500 h -1 , and the product distribution varies as: cis-products, 75-90%; trans-products; 0-8% after 3 h of reaction. Based on the graphs -d[substrate]/dt vs [Pd 4 ] 1/2 , the mechanism indicates a cluster dissociation into two dimers (presumably of the type Pd 2 (dppm) 2 (H)(solvent) + ). The variations of tof (or -d[substrate]/dt) as a function of [substrate] and pressure of H 2 are linear. At 1600 psi of H 2 , the tof can reach 2500-3000 h -1 (in THF). The tof also increases with temperature reaching a maximum at ∼35 °C (tof: 1000-1300 h -1 ), but at higher temperatures cluster decomposition begins to occur, leading to a rapid decrease in rates of catalysis. At 50 °C, no catalysis is observed. The hydrogenation reaction can be stopped at the corresponding cis-alkenes with ∼95% yields, depending on the substrate and experimental conditions used. The tof’s also vary with the solvent, where stronger coordinating solvent molecules give higher tof’s. In addition, the tof’s do not change with the nature of the counterion, which acts as “spectator” in the catalysis. Introduction The synthesis of a purple compound active for the catalytic hydrogenation of alkynes and alkenes was reported by Kirss and Eisenberg over 10 years ago 2 but was of an unknown composition for several years until it was confidently identified as the title cluster as characterized by our teams using various spectroscopic tools. 3 The structure consists of a cyclic Pd 4 frame supported by four bridging dppm ligands (dppm ) bis(diphenylphosphino)methane), in which the two hydrides are bonded to the Pd centers; in solution the complex exhibits a rapid fluxional process at room temper- ature, keeping a C 4 -symmetry of the molecule. A closer look at the structure suggests that two cavities described by the 16 dppm-phenyl groups are formed above and below the Pd 4 plane, resembling picket fences. 4 This cyclic structure has been confirmed from X-ray crystallography for a close derivative issued from the redox reaction between the title cluster and the reducing tetraphenylborate anion to form the diamagnetic [Pd 4 (dppm) 4 (H)]BPh 4 species. 5 The title cluster is also a precursor of the electrocatalytic reduction of H + into H 2 and oxidative formate decomposition. 6 Homogeneous catalyzed hydrogenations are clearly important processes, 7 but for Pd-catalyzed reactions, the number of published works appears to be relatively smaller than for other active transition metal complexes. 8 A close look at the known * To whom correspondence should be addressed: Telephone: (819) 821- 2005. Fax: (819) 821-8017. E-mail: pharvey@courrier.usherb.ca. (1) (a) Universite ´ de Bourgogne. (b) Universite ´ de Sherbrooke. (2) Kirss, R. U.; Eisenberg, R. Inorg. Chem. 1989, 28, 3372. (3) Gauthron, I.; Gagnon, J.; Zhang, T.; Rivard, D.; Lucas, D.; Mugnier, Y.; Harvey, P. D. Inorg. Chem. 1998, 37, 1112. (4) Meilleur, D.; Harvey, P. D. Can. J. Chem. 2001, 79, 552. (5) Evrard, D.; Meilleur, D.; Drouin, M.; Mugnier, Y.; Harvey, P. D. Z. Anorg. Allg. Chem. 2002, 628, 2286. (6) Meilleur, D.; Rivard, D.; Harvey, P. D.; Gauthron, I.; Lucas, D.; Mugnier, Y. Inorg. Chem. 2000, 39, 2909. (7) (a) James, B. R. Homogeneous Hydrogenation; Wiley: New York, 1973. (b) James, B. R. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 8, Chapter 51. (c) Chaloner, P. A.; Esteruelas, M. A.; Jo ´o, F.; Oro, L. A. Homogeneous Hydrogenation; Kluwer Academic: Dordrecht, Netherlands, 1994. Inorg. Chem. 2004, 43, 790-796 790 Inorganic Chemistry, Vol. 43, No. 2, 2004 10.1021/ic0347992 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/25/2003