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(PPh 3 ) 2 (Im(OR) 2 ) (Im(OR) 2 = C 3 H 2 (NCH 2 CH 2 OR) 2; R = Me ( 4a ), t- Bu ( 4b ), tert- hexyl ( 4c ), Ph ( 4d ), 2,6- i- Pr 2 C 6 H 3 ( 4e )). In an analogous fashion the species RuHCl(PPh 3 ) 2 (Y 2 Im(OMe) 2 ) (Y 2 Im(OMe) 2 = Y 2 C 3 (NCH 2 CH 2 OMe) 2; Y 2 = C 6 H 4 ( 4f ), Y = Cl ( 4g ), Me ( 4h )) were also synthesized. Similarly RuHCl(CO)(PPh 3 ) 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 )(PPh 3 ) ( 7 ), which reacts subsequently with Na[BPh 4 ] to give [RuH(Im(OMe) 2 )(SIMes)][(η 6 -Ph)BPh 3 ] ( 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.