Reversible O–H bond activation by tripodal tris(nitroxide) aluminum and gallium complexes

Herein we report the preparation and characterization of the Group 13 metal complexes of a tripodal tris(nitroxide) based ligand, designated (TriNOx3−)M (M = Al (1), Ga (2), In (3)). Complexes 1 and 2 both activate the O–H bond of a range of alcohols spanning a ~13 pKa unit range via an element-ligand cooperative pathway to afford the zwitterionic complexes (HTriNOx2–)M–OR. Structures of these alcohol adduct products are discussed. We demonstrate that the thermodynamic and kinetic aspects of the reactions are both influenced by the identity of the metal, with 1 having higher reaction equilibrium constants and proceeding at a faster rate relative to 2 for any given alcohol. These parameters are also influenced by the pKa of the alcohol, with more acidic alcohols reacting both to more completion and faster than their less acidic counterparts. Possible mechanistic pathways are discussed.