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Towards a Rational Design of Ruthenium CO 2 Hydrogenation Catalysts by Ab Initio Metadynamics
Author(s) -
Urakawa Atsushi,
Iannuzzi Marcella,
Hutter Jürg,
Baiker Alfons
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700254
Subject(s) - ruthenium , metadynamics , chemistry , formate , catalysis , formic acid , activation energy , photochemistry , ab initio , rate determining step , reaction mechanism , hydride , ab initio quantum chemistry methods , hydrogen , computational chemistry , molecule , molecular dynamics , organic chemistry
Complete reaction pathways relevant to CO 2 hydrogenation by using a homogeneous ruthenium dihydride catalyst ([Ru(dmpe) 2 H 2 ], dmpe=Me 2 PCH 2 CH 2 PMe 2 ) have been investigated by ab initio metadynamics. This approach has allowed reaction intermediates to be identified and free‐energy profiles to be calculated, which provide new insights into the experimentally observed reaction pathway. Our simulations indicate that CO 2 insertion, which leads to the formation of formate complexes, proceeds by a concerted insertion mechanism. It is a rapid and direct process with a relatively low activation barrier, which is in agreement with experimental observations. Subsequent H 2 insertion into the formateRu complex, which leads to the formation of formic acid, instead occurs via an intermediate [Ru(η 2 ‐H 2 )] complex in which the molecular hydrogen coordinates to the ruthenium center and interacts weakly with the formate group. This step has been identified as the rate‐limiting step. The reaction completes by hydrogen transfer from the [Ru(η 2 ‐H 2 )] complex to the formate oxygen atom, which forms a dihydrogen‐bonded RuH⋅⋅⋅HO(CHO) complex. The activation energy for the H 2 insertion step is lower for the trans isomer than for the cis isomer. A simple measure of the catalytic activity was proposed based on the structure of the transition state of the identified rate‐limiting step. From this measure, the relationship between catalysts with different ligands and their experimental catalytic activities can be explained.