A Mechanistic Study: O–H Bond Activation by Tripodal Tris(nitroxide) Aluminum and Gallium Complexes

The development of sustainable catalytic systems is a major effort in green chemistry. Expanding main group chemistry into new catalytic applications offers relatively non-toxic, sustainable, and environmentally conscious alternatives to their heavy metal counterparts. We have focused on the group 13 elements, aluminum and gallium. Stable primarily in the 3⁺ oxidation state, both lack the ability to readily access multiple redox states. One way to overcome this challenge is through the addition of non-innocent ligands which can enable reactivity in their metal complexes in many ways, including actively participating in the forming or breaking of substrate covalent bonds. We report the O–H bond activation of various alcohols via element-ligand cooperative chemistry with aluminum and gallium tripodal tris(nitroxide) [{(2- ᵗBuNO)C₆H₄CH₂}₃N]³⁻ (TriNOx³⁻) complexes. To understand the effect of the different metals on the mechanistic pathway of the O–H bond activation HOR (R = tBu, Ph) by (TriNOx)M (M = Al, Ga), we explored the pathways through DFT calculations. The mechanism of (HTriNOx)M–OR formation was found to be through neutral compounds as opposed to a stepwise deprotonation resulting in a charged intermediate. The strength of the acid, not the difference in catalyst by metal, was shown to drive changes in the mechanism of each reaction pathway. Understanding the mechanistic difference of these O–H bond activations gives us insight into utilizing aluminum and gallium catalysts in similar reactivity as transition metal catalysts