Ruthenium Carbene–Diether Ligand Complexes: Catalysts for Hydrogenation of Olefins

A series
of carbene–diether ligands were prepared and the
corresponding Ag salts used to prepare the complexes RuHCl­(PPh3)2(Im­(OR)2) (Im­(OR)2 = C3H2(NCH2CH2OR)2; R = Me (4a), t-Bu (4b), tert-hexyl (4c), Ph (4d), 2,6-i-Pr2C6H3 (4e)). In an analogous fashion the species RuHCl­(PPh3)2(Y2Im­(OMe)2) (Y2Im­(OMe)2 = Y2C3(NCH2CH2OMe)2; Y2 = C6H4 (4f), Y = Cl (4g), Me (4h)) were also synthesized. Similarly RuHCl­(CO)­(PPh3)2(Im­(OMe)2) (5) was prepared and readily
converted to RuHCl­(CO)­(SIMes)­(Im­(OMe)2) (6) via treatment with SIMes. The reaction of 4a with
SIMes afforded RuHCl­(SIMes)­(Im­(OMe)2)­(PPh3)
(7), which reacts subsequently with Na­[BPh4] to give [RuH­(Im­(OMe)2)­(SIMes)]­[(η6-Ph)­BPh3] (8). In a series of tests, the species 4a–h, 5, 6,
and 8 were shown to catalyze the hydrogenations of 1-hexene,
cyclohexene, and dimethyl
itaconate. From the activity of 4a–h it is clear that the capability of the carbene–ether substituents
to coordinate to the metal as well as electron-donating substituents
on the carbene fragment enhances catalytic activity. Other variations
such as in 5, 6, and 8 resulted
in terminal-olefin-selective hydrogenation catalysts, although the
zwitterionic species 8 showed significantly enhanced
activity.