Free-radical ligand substitution reactions of alkylzirconium and alkylhafnium compounds
This study involved the preparation of several, previously unreported alkyl compounds of the Group IVA transition metals zirconium and hafnium. The majority of these 4-coordinate, air-sensitive compounds contained the bis([pi]-cyclopentadienyl)zirconium ("zirconocene", CP[sub]2Zr) moiety: namely, the dialkylzirconocenes Cp[sub]2Zr(CH[sub]2.C[sub]6H[sub]4.X)[sub]2 (X = o-CH[sub]3, m-CH[sub]3, p-CH[sub]3, m-Cl, p-Cl, and p-F), Cp[sub]2Zr(CH[sub]2.C[sub]10H[sub]7-1)[sub]2, Cp[sub]2Zr(CH[sub]2.C[sub]10H[sub]7-2)[sub]2, and (MeCp)[sub]2Zr(CH[sub]2Ph)[sub]2; and the monobenzyl compounds Cp[sub]2Zr(CH[sub]2Ph)X (X = CH[sub]3, Cl, Br, 0CH[sub]2Ph, and ONBu[sub]2[sup]t). In addition, the tribenzyl species CpZr(CH[sub]2Ph)[sub]3 and the hafnium compounds CP[sub]2Hf-(CH[sub]2.C[sub]6H[sub]4.CH[sub]3-p)[sub]2 and (Ph[sub]2N)[sub]2Hf(CH[sub]2Ph)[sub]2 were synthesised. Through kinetic experiments, dialkylzirconocenes were found to undergo solution-phase reactions with molecular oxygen which led to the corresponding dialkoxy compounds, proceeded by way of free-radical chain mechanisms, and involved S[sub]H2 displacement of an alkyl radical from the zirconium atom by an alkylperoxy radical. Rate constants and transition state data were measured for these S[sub]H2 processes. Evidence was obtained for intramolecular decomposition of the Cp[sub]2Zr(R)O[sub]2R intermediate, and for a direct radical substitution at zirconium by molecular oxygen. This work was extended to alkylzirconocene halides which showed similar behaviour. The effect of complexation with pyridine was observed for the rate of autoxidation of tetrabenzylhafnium at low temperatures. The stable free radical di-t-butyl nitroxide was found to bring about ligand substitution in alkyl compounds of both zirconium and hafnium, and these reactions were also interpreted on the basis of a rate-determining radical displacement at the metal. Rate constants for a series of benzyl compounds indicated (a) the deactivating effect of a halogen atom bonded to the metal, and (b) the low reactivity of a hafnium compound as compared to its zirconium counterpart. The reaction of dibenzylzirconocene and benzenethiol was investigated and evidence for a chain mechanism involving attack at zirconium by the phenylthiyl radical was obtained.