Catalysis of Organic..

16Supported Re Catalysts for Olefin MetathesisCapping. Capped silica-alumina was prepared by vapor phase transfer ofhexamethyldisilazane (≥ 99.5%, Aldrich) onto calcined silica-alumina until there wasno further uptake, as indicated by stabilization of the pressure. The reactor wasevacuated and the material heated to 350°C under dynamic vacuum for 4 h toremove ammonia produced during the capping reaction.Kinetics. The catalysts were loaded into a glass batch reactor (volume ca. 120mL) in a glovebox. The reactor was removed from the glovebox and evacuated. Thesection of the reactor containing the catalyst was immersed in an ice bath at 0°C inorder to control the rate of the reaction on a readily-monitored timescale, as well asto maintain isothermal reaction conditions. Propylene was introduced at the desiredpressure via a high vacuum manifold. Aliquots of 1.9 mL were expanded at timedintervals into an evacuated septum port that was separated from the reactor by astopcock. 50 μL samples of the aliquot were removed with a gas-tight syringe via aseptum. Gases were analyzed by FID on a Shimadzu GC 2010 equipped with a 30 mSupelco ® Alumina Sulfate PLOT capillary column (0.32 mm i.d.). Quantitation wasachieved using the peak area of the small propane contaminant present in thepropene as an internal standard.Computational analysis. Calculations were performed on an Intel Xeoncomputer running Linux, as well as the VRANA-5 and VRANA-8 clusters at theCenter for Molecular Modeling of the National Institute of Chemistry (Ljubljana,Slovenia), using the DFT implementation in the Gaussian03 code, Revision C.02 (8).The orbitals were described by a mixed basis set. A fully uncontracted basis set fromLANL2DZ was used for the valence electrons of Re (9), augmented by two ffunctions (ζ = 1.14 and 0.40) in the full optimization. Re core electrons were treatedby the Hay-Wadt relativistic effective core potential (ECP) given by the standardLANL2 parameter set (electron-electron and nucleus-electron). The 6-31G** basisset was used to describe the rest of the system. The B3PW91 density functional wasused in all calculations.Results and DiscussionReaction of silica-supported perrhenate with SnMe 4 . Computational analysis of thereaction of oxide-supported perrhenates with SnMe 4 was accomplished using cubemodels to represent the oxide surface. Cage-like structures, such as the partially andfully condensed silsesquioxanes (10), are good computational models for siliconbasedoxide surfaces because of their constrained Si-O-Si angles (11,12), andbecause of their oxygen-rich nature. Perrhenate was attached to a silsesquioxanemonosilanol cube, Scheme 2, to represent the grafted site ≡SiOReO 3 . The optimizedgeometry displays a single SiO-Re attachment. Transmetalation of the perrhenatecube by SnMe 4 , resulting in displacement of MeReO 3 and attachment of atrimethyltin fragment to the silsesquioxane framework, is slightly exothermic (by 4kJ/mol).