Solution and crystal structure of the dihydrogen complex [Ru(H2)(H)(PMe2Ph)4]PF6, an active alkyne hydrogenation catalyst

The complex [Ru(H-2)(H) (PMe2Ph)(4)]PF6 (1) has been prepared by reaction of [Ru(H) (PMe2Ph)(5)] PF6 (2) in THF with 1 atm H-2 and characterised by variable temperature P-31 and H-1 NMR. It undergoes four distinct fluxional processes listed in order of decreasing activation energy: (i) exchange of H-2 in solution with the dihydrogen Ligand above 273 K; (ii) isomerisation of cis and trans isomers of 1 above 230 K; (iii) exchange of H atoms between H-2 and hydride in trans-1 above 180 K; (iv) rapid H-2/hydride exchange in cis-1 to below 180 K. A. single crystal X-ray diffraction study dl at 173 K shows that the complex has the cis geometry in the solid state but does not clearly reveal the positions of the hydrogen ligands. Complex 1 starts out as a catalyst of high activity for the selective hydrogenation of 1-alkynes to 1-alkenes (RC=CH; R=Bu-n, Ph) but it is rapidly deactivated, possibly because of formation of the enynyl complex [Ru(eta(3)-RC3CHR)(PMe2Ph)(4)](+). Complex 1 efficiently catalyzes the hydrogenation of internal alkynes (3-hexyne, 2-pentyne) to internal cis-alkenes with little deactivation, although some isomerisation of the alkene produced is observed. These observations are consistent with those of Nkosi, Covine, Albers and Singleton who reported that complex 2 must dissociate one PMe,Ph ligand to produce the species active for alkyne hydrogenation. Complex 2 catalyses these hydrogenations with slower initial rates than complex 1 but deactivates less readily. In contrast to I, complex 2 does not appear to cause the isomerisation of internal alkenes. (C) 1998 Elsevier Science S.A.