AUTOXIDATION OF AN ELECTRON-RICH ORGANOMETALLIC - MECHANISM OF THE REACTION OF A SUBSTITUTED NIOBOCENE COMPOUND WITH DIOXYGEN

The subsituted niobocene compound [Cp'2NbCl](x) (1, Cp' = eta(5)-C(5)H (4)SiMe(3)) is an electron-rich Nb(III) system that reacts readily wit h oxygen and other potential oxygen atom donors. The ultimate product of these processes is the known Nb(V) oxo compound Cp'Nb-2(=O)(Cl) (2) , which obviously arises by way of a multistep mechanism. We have inve stigated the mechanism of the autoxidation using a series of alternate syntheses and/or modified reaction conditions and provide direct evid ence for a sequence involving the sequential formation of the eta(2)-p eroxide Cp'Nb-2(O-2)Cl, its reaction with excess 1 to generate 2, the subsequent conproportionation of 2 and 1 to make the mu-oxo compound [ Cp'2NbCl](2)(O) (3), and the further oxidation of this compound to 2. The reaction of 1 and 2 is postulated to involve the intermediacy of a peroxide-bridged system [Cp'2NbCl](2)(O-2) (5); although this compoun d has not been observed directly, attempted synthesis of 5 using super oxide lead directly to 3. These individual reactions are kinetically c ompetent, and the results thus constitute a four-step mechanism for th e production of 3. In addition, studies on the reactions of 3 with PMe Ph(2) illustrate that 3 disproportionates to 2 and 1 and that the latt er is trapped rapidly. Conversely, reaction of 3 with additional dioxy gen does not require dissociation and may proceed via a direct redox p athway. Finally, the oxygen atom transfer process involved in dioxygen reduction is compared to reactions involving nitrosoarenes and carbon dioxide, processes that yield azoxyarene (or azoarene) and carbon mon oxide, respectively; mechanistic features differ, even though the over all process involves X=O scission in all cases.