Effect of the para-substituent of the tridentate pyridine-based Ru(II) complex upon the catalytic activity in transfer hydrogenation

Three stable 4-substituted pyridine-based ruthenium(II) complexes [RuCl₂(PPh₃)L] (L = 4-R-2,6-bis (diethylaminomethylene) pyridine, R = Br, H or allyloxy) were synthesized. The catalytic activities of the complexes toward transfer hydrogenation from alcohols to ketones were investigated. The electronic effects of the para-substituent in the pyridyl ring were probed and we found that the electron-donating group increased the catalytic activity. The result suggests that an electron-donating group is probably preferential for linking the catalytic ruthenium complex and the chemically inert supporting molecules such as a carbosilane dendrimer.