Effect of Substituents in Functional Bipyridonate Ligands on Ruthenium-Catalyzed Dehydrogenative Oxidation of Alcohols: An Experimental and Computational Study

A series of hexamethylbenzene (HMB)-Ru complexes 2-5 bearing a 4,4'-functionalized 2,2'-bipyridine-6,6'-dionate (bpyO) ligand, which exhibits metal-ligand cooperative catalysis, was prepared with the aim of developing excellent catalyst for dehydrogenative oxidation of alcohols. Interestingly, the catalytic activity increased in the order 3 (CF3) < 4 (OMe) < 2 (H) < 5 (NMe2), where substituents at 4,4'-positions of bpyO ligand are in parentheses. This is different from the order of simple electron-donating ability. DFT calculations revealed that the rate-limiting step is the concerted proton/hydride transfer from the alcohol to the complex. The activation energy decreases as the interaction between the alcoholic proton and the O atom of the bpyO ligand becomes stronger; hence, the introduction of the NMe2 group decreases the activation energy, whereas that of the CF3 group increases it. The unexpectedly lower catalytic ability of 4 than that of 2 results from the enthalpy-entropy compensation effect.