Synthesis, Reactivity, and Catalytic Transfer Hydrogenation Activity of Ruthenium Complexes Bearing NNN Tridentate Ligands: Influence of the Secondary Coordination Sphere

By the introduction of -OH group(s) into different position(s) of 6-(pyridin-2-ylmethyl)-2,2'-bipyridine, several NNN-type ligands were synthesized and then introduced to ruthenium (Ru) centers by reactions with RuCl 2 (PPh 3 ) 3 . In the presence of PPh 3 or CO, these ruthenium complexes reacted with NH 4 PF 6 in CH 2 Cl 2 or CH 3 OH to give a series of ionic products 5-9 . The reaction of Ru(L 2 )(PPh 3 )Cl 2 ( 2 ) with CO generated a neutral complex [Ru(L 2 )(CO)Cl 2 ] ( 10 ). In the presence of CH 3 ONa, 10 was further converted into complex [Ru(L 2 )(HOCH 3 )(CO)Cl] ( 11 ), in which there was a methanol molecule coordinating with ruthenium, as suggested by density functional theory calculations. The catalytic transfer hydrogenation activity of all of these new bifunctional metal-ligand complexes was tested. Dichloride complex 2 exhibits best activity, whereas carbonyl complexes 10 and 11 are efficient for selectively reducing 5-hexen-2-one, suggesting different hydrogenation mechanisms. The results reveal the dramatic influence for the reactivity and catalytic activity of the secondary coordination sphere in transition metal complexes.