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

The complex Ru(H₂)(H)(PMe₂Ph)₄]PF₆ (1) has been prepared by reaction of Ru(H)(PMe₂Ph)₅] FP₆ (2) in THF with 1 atm H₂ 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 H₂ 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 H₂ and hydride in trans-1 above 180 K; (iv) rapid H₂/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(η³RC₃CHR)(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, Coville, Albers and Singleton who reported that complex 2 must dissociate one PMe₂Ph 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.