Solution and crystal structure of the dihydrogen complex [Ru(H 2)(H)(PMe 2Ph) 4]PF 6, an active alkyne hydrogenation catalyst

Abstract

The complex [Ru(H 2)(H)(PMe 2Ph) 4]PF 6 ( 1) has been prepared by reaction of [Ru(H)(PMe 2Ph) 5] FP 6 ( 2) in THF with 1 atm H 2 and characterised by variable temperature 31P and 1H 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 of 1 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= 11Bu, Ph) but it is rapidly deactivated, possibly because of formation of the enynyl complex [Ru( η 3RC 3CHR)(PMe 2Ph) 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, Coville, Albers and Singleton who reported that complex 2 must dissociate one PMe 2Ph 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 1, complex 2 does not appear to cause the isomerisation of internal alkenes.