Non-Redox Assisted Oxygen-Oxygen Bond Homolysis in Titanocene Alkylperoxide Complexes: [Cp(2)Ti(eta-OOBu)L], L = Cl, OTf, Br, OEt(2), Et(3)P.
The titanium(IV) alkylperoxide complex Cp(2)Ti(OO(t)Bu)Cl (1) is formed on treatment of Cp(2)TiCl(2) with NaOO(t)Bu in THF at -20 degrees C. Treatment of 1 with AgOTf at -20 degrees C gives the triflate complex Cp(2)Ti(OO(t)Bu)OTf (2), which is rapidly converted to the bromide Cp(2)Ti(OO(t)Bu)Br (3) on addition of (n)Bu(4)NBr. The X-ray crystal structures of 1 and 3 both show eta(1)-OO(t)Bu ligands. Complex 2 is stable only below -20 degrees C; (1)H, (13)C, and (19)F NMR spectra suggest that it also contains an eta(1)-OO(t)Bu ligand. Removal of the chloride from 1 with [Ag(Et(2)O)(2)]BAr'(4) (Ar' = 3,5-(CF(3))(2)C(6)H(3))) yields the etherate complex [Cp(2)Ti(OO(t)Bu)(OEt(2))]BAr'(4) (4). Again, coordination of a fourth ligand to the Ti center indicates an eta(1)-OO(t)Bu ligand in 4. These peroxide complexes do not directly oxidize olefins or phosphines. For instance, the cationic etherate complex 4 reacts with excess Et(3)P simply by displacement of the ether to form [Cp(2)Ti(eta(1)-OO(t)Bu)(Et(3)P)]BAr'(4) (5). Compounds 1-5 all decompose by O-O bond homolysis, based on trapping and computational studies. The lack of direct oxygen atom transfer reactivity is likely due to the eta(1) coordination of the peroxide and the inability to adopt more reactive eta(2) geometry. DFT calculations indicate that the steric bulk of the (t)Bu group inhibits formation of the hypothetical [Cp(2)Ti(eta(2)-OO(t)Bu)](+) species.
Dipasquale
Antonio G
AG
Department of Chemistry, Campus Box 351700, University of Washington, Seattle, Washington 98195-1700.
Hrovat
David A
DA
Mayer
James M
JM