Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH2R)], through DFT periodic calculations: silica is just a large siloxy ligand

Abstract

DFT plane-wave periodic calculations using the VASP code have been carried out to model the silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH₂R)]. The structure, spectroscopic and electronic properties of this highly active catalyst have been compared with those of non-efficient molecular analogues, [(X₃SiO)Re(CR)(CHR)(CH₂R)] (X₃SiO is triphenylsiloxy or polyoligomeric silsesquioxane (POSS)). The silica surface was modelled using cristobalite and edingtonite ideal polymorph surfaces, and the organometallic fragment has been represented with the experimental (R = tBu) and simplified (R = Me) ligands, [(SiO)Re(CR)(CHR)(CH₂R)]. The calculated structures, alkylidene J_C–H coupling constants and ν_C–H stretching frequencies agree with experimental data. The syn and anti isomers of the Re complexes are close in energy, the former being always more stable. A secondary Re⋯O interaction experimentally detected by EXAFS is found to have no stabilizing influence, but is possible because of the facile distortion of (SiO)Re(CR)(CHR)(CH₂R). More importantly, the geometry and electronic structure of the Re fragment is essentially the same for the triphenylsiloxy, the POSS and the silica surface, which shows that the siloxy group of the first coordination sphere of Re determines the metal properties. The silica surface is thus electronically equivalent to the other siloxy groups, and should be viewed as a large bulky ligand.