Modelling of Grubbs type precatalysts with bidentate hemilabile ligands
Raymakers, Maria de Fatima Marques
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Metathesis is a valuable reaction for the production of new alkenes. In the last 50 years, heterogeneous as well as homogeneous catalysts have been used for this reaction. In the homogeneous category are the very successful catalysts designed by the Grubbs group. The first generation Grubbs precatalyst (Gr1) bearing two phosphine ligands was followed after extensive studies by the more active second generation Grubbs precatalyst (Gr2). In Gr2, one of the phosphine ligands is replaced with an N-heterocyclic carbene. Grubbs type precatalysts bearing pyridynyl-alcoholato chelating ligands are pertinent to this study. Scheme 1: The synthesis of Grubbs type precatalysts bearing a pyridynyl-alcoholato ligand. In two previous studies, both supported by computational methods, Grubbs type precatalysts with N^O chelating ligands were synthesised. These investigations were motivated by the fact that chelating ligands bearing different donor atoms can display hemilability. The loosely bound donor atom can de-coordinate to make available a coordination site to an incoming substrate “on demand”, whilst occupying the site otherwise and hence preventing decomposition via open coordination sites. In the first investigation, the incorporation of an O,N-ligand with both R1 and R2 being phenyl groups into the Gr2 precatalyst, resulted in an increase in activity, selectivity and lifetime of the precatalyst in comparison to Gr2 in the metathesis reaction with 1- octene. In the second study, three synthesised complexes were found to be active for the metathesis of 1-octene. This computational study sought to better understand the structural differences and thermodynamic properties of these Grubbs type precatalysts with bidentate/hemilabile ligands. A large number of structures were constructed in Materials Studio by varying the R groups of the bidentate/hemilabile ligand attached to both the Gr1 and Gr2 catalysts. The majority of structures were Gr1-type complexes. For each ligand selected, a group of structures consisting of closed precatalyst, open precatalyst, and where applicable a precatalyst less PCy3, closed metallacycle, open metallacycle and where applicable a metallacycle less PCy3, was constructed and optimised using DMol3. Bond lengths, bond angles, HOMO and LUMO energies and Hirshveld charges of structures were compared with one another. PES scans were performed on the metallacycles of four groups. The purpose of the PES scans was to ascertain whether these bidentate ligands were hemilabile and to illuminate the preferred reaction mechanism for these types of precatalysts. The major finding of this study was that the possibility of an associative mechanism cannot be ruled out for some Gr2-type precatalysts with bidentate ligand. For some precatalysts hemilability is energetically expensive and possibly not viable. No evidence of a concerted mechanism was found. The dissociative mechanism was found to be the preferred mechanism for most of the structures that were subjected to PES scans. The HOMO-LUMO energies of a complex can be used, as a predictive tool, to assess the reactivity and stability of a complex, as well as its preference for substrates.