Reductive coupling of [(RO)2Ti(L2)2] complexes containing a chelating bis(aryloxide) ligand with ketones (L 2 = bpy, dmbpy, or phen)

Article abstract of DOI:10.1021/om050231k

The reactivity of [(RO)₂Ti(L₂)₂] complexes (L₂ = bpy, dmbpy, or phen; (RO)₂ = DMSC or MBMP) with ketones was investigated (DMSC = 1,2-alternate dimethylsilyl-bridged p-tertbutylcalix[4]arene dianion; MBMP = 2,2′-methylenebis(6-tert-butyl-4- methylphenol) dianion). The molecular structures of [(DMSC)Ti(bpy)2] (6a) and [(DMSC)Ti(bpy)₂] (7a) were characterized by X-ray crystallography. Their structural data taken together with UV-visible and magnetic susceptibility data suggest some electron transfer into the LUMO (π* orbital) of the diimine ligands. [(RO)₂Ti(L₂)₂] complexes undergo light-assisted reaction with aromatic ketones in a reversible manner to afford Ti-η²-ketone complexes [(RO)₂Ti(η ²-OCArR)(L₂)] (Ar = aryl while R = aryl or alkyl), which undergo further reaction with ketone to give the corresponding 2-aza-5-oxa-titanacyclopentene. Qualitatively, the efficacy of the formation of 2-aza-5-oxa-titanacyclopentene [(RO)₂Ti(L₂)₂] complexes increased with increasing ketone concentration and in the order L ₂ = bpy < dmbpy < phen. This order is best explained in terms of the dependence of the equilibrium between [(RO)₂Ti(L ₂)₂] and [(RO)₂Ti(η²-OCArR) (L₂)] on relative abilities of the ketone and the diimine to accept π-electron density, as well as in terms of the order of the rate of reaction of [(RO)₂Ti(η²-OCArR)(L₂)] with ketone.