Reductive Coupling of [(RO)2Ti(L2)2] Complexes Containing a Chelating Bis(aryloxide) Ligand with Ketones (L2 = bpy, dmbpy, or phen)

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-tert-butylcalix[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(η2-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 L2 = bpy < dmbpy < phen. This order is best explained in terms of the dependence of the equilibrium between [(RO)2Ti(L2)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.