Abstract:
One important challenge in green chemistry is the development of safe and
sustainable catalytic systems for molecular transformations that do not rely on precious
metals. In this vein, I explore how one can expand the utility of aluminum complexes.
Aluminum is a prime choice for such catalytic development as it is relatively inexpensive
(less than $2/kg), readily available, and non-toxic. Although there are ample examples of
aluminum complexes acting as Lewis-acid catalysts, the lack of readily accessible multielectron
redox states leaves aluminum complexes inept in redox-based transformations.
In order for aluminum complexes to be used in redox reactions, I prepared complexes
that contain redox-active ligands.
Specifically, I focused on the synthesis of aluminum complexes implementing a
multidentate ligand incorporating three nitroxide (N-O) functional groups, which is
known to exist across three oxidation states. It is hypothesized that the Al-nitroxide
complex will similarly span several oxidation states. Herein, I report the synthesis
(TriNOx3-)Al-py, which uses a multidentate redox-active tripodal tris(nitroxide) ligand.
Moreover, I report the synthesis of analogous gallium and indium complexes, providing
an array of complexes that vary in electronic parameters, steric profiles, and metal ionic
radius. The alNo uminum complex exhibits multi-electron electrochemical behavior, and has
the ability to participate in metal-ligand cooperative catalysis due to the Lewis acidity of
the aluminum center and the Lewis basicity of the nitroxide nitrogens. These
characteristics have been exploited in a catalytic system for the hydroboration of
carbonyls. Such use of the aluminum complex represents new transition-metal-like
reactivity for an earth-abundant metal.
