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 RuCl₂(PPh₃)₃. In the presence of PPh₃ or CO, these ruthenium complexes reacted with NH₄PF₆ in CH₂Cl₂ or CH₃OH to give a series of ionic products <b>5–9</b>. The reaction of Ru­(L₂)­(PPh₃)­Cl₂ (<b>2</b>) with CO generated a neutral complex [Ru­(L₂)­(CO)­Cl₂] (<b>10</b>). In the presence of CH₃ONa, <b>10</b> was further converted into complex [Ru­(L₂)­(HOCH₃)­(CO)­Cl] (<b>11</b>), 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 <b>2</b> exhibits best activity, whereas carbonyl complexes <b>10</b> and <b>11</b> 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