The Importance of Ligand Kinetic Basicity on the Preparation of Surface-Supported Zirconium Complexes by Proton Transfer from Hydroxylated Aluminum or Silicon

Surface hydroxylation of Al or Si powders or thin films was readily accomplished under various conditions of oxidation vigor. In the first, the metal was exposed to ambient water; in the second, it was immersed in liquid water; in the third, it was treated with steam. Hydroxylation using D₂O steam was also accomplished. Metal surfaces thus activated were examined using various spectroscopic techniques including IR and Auger spectroscopies. The reaction between tetraneopentylzirconium or tetra-tert-butoxyzirconium was studied with variously hydroxylated Si or Al. Rates for reaction between the organometallic and surface hydroxyls were studied gravimetrically using the quartz crystal microbalance. Measured kinetic isotope effects suggest a mechanism for organometallic complex surface hydroxyl reactivity involving reversible coordination followed by rate-determining proton transfer. Deposition rates and kinetic isotope effects for the two complexes are discussed in terms of kinetically likely sites for protonation of the two complexes. A simplistic approach, based solely on relative thermodynamic basicities of the two complexes, fails to correctly predict relative reactivities for the two species: tetra-tert-butoxy zirconium is ca. an order of magnitude more reactive, ratewise, than is the more thermodynamically basic tetraneopentylzirconium.