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

The reactivity of [(RO) ₂Ti(L ₂) ₂] complexes (L ₂ = 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(L ₂) ₂] complexes undergo light-assisted reaction with aromatic ketones in a reversible manner to afford Ti-η ²-ketone complexes [(RO) ₂Ti(η ²-OCArR)(L ₂)] (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(L ₂) 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(η ²-OCArR) (L ₂)] 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(η ²-OCArR)(L ₂)] with ketone.