Synthesis and Reactivity of Metal–Ligand Cooperative Bifunctional Ruthenium Hydride Complexes: Active Catalysts for β‑Alkylation of Secondary Alcohols with Primary Alcohols

Three unsymmetrical NNN ligands with a 2-hydroxypyridyl fragment were used to react with RuHCl­(PPh3)3(CO), affording the three bidentate ruthenium hydride complexes [(R1-C5H3N-CH2-C5H3N-C5H3N-R2)­RuH­(PPh3)2(CO)]­[Cl] (R1 = R2 = OH, 2a; R1 = OH, R2 = H, 2b; R1 = H, R2 = OH, 2c), respectively. When 2a,b were treated with t-BuOK, the two tridentate products [(O-C5H3N-CH2-C5H3N-C5H3N-R2)­RuH­(PPh3)­(CO)] (R2 = OH, 3a; R2 = H, 3b) were obtained, via selective deprotonation of the −OH group of PyCH2PyOH moiety, indicating that this −OH group is more acidic than that of the PyPyOH moiety. The reaction of 2c with t-BuOK generated the bidentate product [(C5H4N-CH2-C5H3N-C5H3N-O)­RuH­(PPh3)2(CO)] (3c) and the tridentate product [(C5H4N-CH2-C5H3N-C5H3N-O)­RuH­(PPh3)­(CO)] (3d). 3d could be further transformed to the diruthenium complex [(C5H4N-CH-C5H3N-C5H3N-O)­Ru­(PPh3)­(CO)]2 (3e) via C–H activation of the −CH2– group in boiling toluene. The catalytic activity for β-alkylation of secondary alcohols with primary alcohols of these eight ruthenium complexes was tested, and the bidentate complexes 2c and 3c exhibit the highest activity. Complex 3c can be regarded as the intermediate of 2c. These results are important for developing more efficient bifunctional catalysts for such reactions