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

Abstract

AbstractA 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.