A DFT study of aldehyde hydrogenation by an octahedral PONOP iron catalyst where the bifunctional mechanism is out of the question -

Abstract

Selective hydrogenation of aldehydes and ketones with H2 is commonly done using Noyori-type ruthenium amino hydride catalysts. These catalysts are generally accepted to work by an outer-sphere bifunctional mechanism involving a six-membered transition state for transfer of a hydride and a proton from the metal and the amino centers, respectively, to a carbonyl group. Hu and co-workers have recently reported that complex [iPrPONOP)Fe(CO)(H)Br] (PONOP= 2,6-bis(phosphinito)pyridine) (4-Br) in methanol is an active catalyst for aldehyde hydrogenation. There are two important and interesting features in this system; first, it has the economic and environmental advantages of using iron instead of ruthenium and second, this catalyst is based on a pincer ligand that lacks the amino functionality, making Noyori’s bifunctional mechanism inapplicable. Elucidating the mechanism in this system is of interest from a fundamental perspective, and can be crucial in the rational design of new more effective iron catalysts. We use density functional theory to conduct a comprehensive investigation of the reaction mechanism in the given system. Our study considers the possibility of a mechanism following two stages: (i) insertion of the aldehyde into an Fe-H bond taking place in an outer sphere mode to generate an octahedral iron alkoxide intermediate; and (ii) hydrogenolysis of the iron-alkoxide bond with H2. The study also compares the reactivity of 4-Br with the octahedral trans-dihydride as well as the reactivity of aldehydes compared to that of ketones. The bulk of the study involves using the M06L density functional locat and characterizes minima and transition states on the potential energy surfaces. The critical transition states are subjected to detailed intrinsic reaction coordinate analyses. Our result support a mechanism in which the intact 4-Br is the active species in catalysis.