Efficient transfer hydrogenation of ketones using molybdenum complexes by comprehensively verifying the auxiliary ligands†

Molybdenum complexes ligated with N¹,N¹-dialkyl-N²-(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines and auxiliary ligands, providing various structural features, were developed: [NN_H/NN_HN]Mo(CO)_4/3 (Mo1–Mo3), [NN_HN]Mo(CO)₂Br (Mo4–Mo5), [NN_H]Mo(CO)(η³-C₃H₅)Br (Mo6) and [NN_HN/S]Mo(CO)(PPh₃)₂ (Mo7–Mo8). All the complexes were highly active in the transfer hydrogenation (TH) of a model substrate (acetophenone), providing excellent yields of 1-phenylethanol. The structural variation in the ligand framework had a modest effect on the catalyst performance as compared to the changes in the auxiliary ligands Br, PPh₃ and CO. This structural evolution provided the complex [Mo(NN_H)(η³-C₃H₅)(CO)₂Br] (Mo6) as the most effective catalyst not only for the transfer hydrogenation of acetophenone but also for a wide range of diverse ketones (up to 43 examples). Moreover, easy purification of the products by only removing the acetone byproduct is another noteworthy feature of this environmentally friendly route.