Reversible O–H Bond Activation by Tripodal tris(Nitroxide) Aluminum and Gallium Complexes

Abstract

Herein, we report the preparation and characterization of the Group 13 metal complexes of a tripodal tris(nitroxide)-based ligand, designated (TriNOx 3− )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 ∼10 pK a unit range via an element-ligand cooperative pathway to afford the zwitterionic complexes (HTriNOx 2− )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 pK a of the alcohol, with more acidic alcohols reacting both to more completion and faster than their less acidic counterparts. Possible mechanistic pathways for the O−H activation are discussed.