Hydrogen activation and aldehyde elimination promoted by homogeneous Pt–Sn catalyst: a theoretical study

Abstract

Quantum mechanical calculations at the MP4(SDQ)//MP2 level of theory were carried out to evaluate the energies and reaction mechanisms involved in the hydrogenolysis process (hydrogen activation followed by aldehyde reductive elimination) of the olefin hydroformylation promoted by homogeneous Pt–Sn catalyst, which is the last step of the catalytic cycle. The H 2 oxidative addition has a very small energy barrier of 0.7 kcal/mol, relative to the η 2-H 2 adduct and the dihydride compound formed is 12.1 kcal/mol more stable than the η 2-H 2 adduct. Starting from the dihydride complex, there are two possible reaction pathways for the aldehyde reductive elimination, regenerating the original catalyst [Pt(H)(PH 3) 2(SnCl 3)] in different stereochemistry. As a result of the trans labilizing effect of the SnCl 3 ligand, which weakens the Pt–H bond trans to it, the computed energy barrier for the pathway which regenerates the cis isomer of the original catalyst, has a small energy barrier. The hydrogenolysis process has total energy barrier of 33.9 kcal/mol, with the aldehyde elimination being the rate-determining step due to the large stability of the dihydride compound, formed in the H 2, oxidative addition step. Comparisons with previous theoretical results for the other elementary steps of the catalytic cycle were made, leading to the conclusion that the hydrogenolysis and carbonylation processes, are the slowest steps of the olefin hydroformylation cycle promoted by Pt–Sn catalyst, which is consistent with the experimental findings. Comparisons with other results obtained for cobalt and rhodium carbonyls and rhodium modified catalysts were also made and are discussed as well as the study of the nature of the bonds on the η 2-H 2 adduct and transition state structures found in the H 2 oxidative addition pathway.