Gamma-alumina: An active support to obtain immobilized electron poor Zr complexes

By grafting well-defined metallic complexes on a solid support, surface organometallic chemistry provides a bridge between homogeneous and heterogeneous catalysis. This innovative approach has already proven its efficiency for several reactions: polymerization, olefin and alkane metathesis... However several fundamental questions remain open on the chemical role of the oxide surface, on the grafting mechanism and on the reactivity of the grafted complex. Is a single well-defined surface/metal/ligand complex formed? Is the solid surface just an inert support, or does it play an active chemical role? We combine on the one hand reactivity and spectroscopic experiments with on the other hand total energy calculations and spectra simulations (NMR, IR) in order to propose insights for these open questions, using the example of [Zr(CH ₂tBu)₄] complex grafted on partially dehydroxylated gamma-alumina. A molecular level approach to the grafting mechanism is proposed. Alumina plays a dual role, both as support and Lewis acid co-catalyst. The hydroxyl groups permit the covalent grafting through Al-O-Zr linkages, while the neighboring Al Lewis sites allow the formation of an ion pair, by transfer of an alkyl ligand from the Zr toward the Al. The formed cationic Zr center shows an enhanced electrophilic character. The mechanism of the formation of the Zr hydride supported on alumina, upon treatment under H2 gas is described, with the formation of a mixture of mono-hydride (Zr⁺-H; Al ^--CH₂tBu) and bis-hydride (Zr⁺-H; Al ^--H). This Zr hydride supported on alumina is active for butane hydrogenolysis. A mechanism with reasonable energy barriers is proposed, the rate limiting step being the C-C scission.