Metal Hydride and Ligand Proton Transfer Mechanism for the Hydrogenation of Dimethyl Carbonate to Methanol Catalyzed by a Pincer Ruthenium Complex

The hydrogenation of dimethyl carbonate to methanol catalyzed by a PNN-ligated ruthenium complex (PNN)­Ru­(CO)­(H) was studied computationally using the density functional theory at the range-separated and dispersion-corrected ωB97X-D functional level in conjunction with an all-electron 6-31++G­(d,p) basis set (Stuttgart ECP28MWB basis set for Ru). A direct metal hydride and ligand proton transfer mechanism with three cascade catalytic cycles for the hydrogenation of dimethyl carbonate, methyl formate, and formaldehyde to methanol is proposed. The resting state in the catalytic reaction is the trans dihydride complex <i>trans-</i>(PNN)­Ru­(H)₂(CO). Calculation results indicate that the rate-determining step in the whole reaction is the formation of the second methanol molecule through simultaneous breaking of a C–OCH₃ bond and transferring a ligand methylene proton to the dissociated CH₃O^– in the catalytic cycle for hydrogenation of methyl formate. The essential role of the noninnocent PNN pincer ligand is to split H₂ and assist methanol formation through the aromatization and dearomatization of the pyridine ring in the ligand. A new iron pincer complex, <i>trans-</i>(PNN)­Fe­(H)₂(CO), is proposed and evaluated as a promising low-cost and high efficiency catalyst for this reaction