Grafting and thermolysis of tetraneopentylzirconium on silica

Abstract

The gas-solid reaction of tetra(neopentyl)zirconium with the surface of Aerosil 200 pretreated thermally at 200°C has been investigated. Consistent with previous investigations of a similar reaction involving deposition of tetra(neopentyl)zirconium from solution, a unique supported organometallic fragment is formed. It was characterized by IR, 13C CP/MAS NMR, XAS and analytical techniques as (≡SiO)2Zr(CH2 C(CH3)3)2. Thermolysis of silica-supported bis(neopentyl)zirconium(IV) at ca. 120°C results in the extrusion of one equiv. of neopentane. The nature of the remaining surface organometallic fragment was inferred, by its reactivity in olefin metathesis as well as by curve-fitting of its EXAFS spectrum, to be neopentylidenezirconium(IV), presumably formed by alpha-H abstraction. This result is analogous to previously established transformations of bis(alkyl) complexes of titanium, vanadium and chromium supported on silica, but was unexpected since earlier reports of the thermolysis of Zr(CH2C(CH3)3) 4 supported gamma-H abstraction and the formation of a zirconacyclobutane intermediate. The alpha-H abstraction occurs with pseudo-first-order kinetics and activation parameters DeltaH‡ = 24(5) kJ/mol and DeltaS ‡ = -19(7) J/K•mol. Both the bis(neopentyl)- and neopentylidenezirconium(IV) fragments were found to catalyze polymerization of ethylene, at room temperature without any added cocatalyst. The reactions are pseudo-first-order in P(C2 H4).