Revised Mechanisms of the Catalytic Alcohol Dehydrogenation and Ester Reduction with the Milstein PNN Complex of Ruthenium

The combined experimental/DFT computational study of RuH2(CO) [Et2NCH2PyCH2Pt-Bu-2] (2) suggests that this dihydride is the catalyst of the acceptorless alcohol dehydrogenation and ester hydrogenation reactions developed in the group of Milstein, whereas the corresponding alkoxide RuH(OR)(CO)[Et2NCH2PyCH2Pt-Bu-2] (4) is an important reaction intermediate. A relatively fast equilibrium of dihydride 2 and ethanol with ethoxide 4 and H-2 was demonstrated by NMR experiments, as well as the proton exchanges occurring between the OH of ethanol, RuH, and the CH2 groups of the PNN ligand backbone of 2 and 4. A detailed critical discussion of the previously proposed mechanisms with the Milstein catalyst is presented. This paper also reports the preparation of the osmium dihydride OsH2(CO)[Et2NCH2PyCH2Pt-Bu-2] (2-O-s) and a comparative study of 2, 2-Os, and the Noyori-type osmium catalyst OsH2(CO) [PyCH2NHCH2CH2NHPt-Bu-2].