Conspectus
Aluminum is the most abundant
metal in the earth’s crust
at 8%, and it is also widely available domestically in many countries
worldwide, which ensures a stable supply chain. To further the applications
of aluminum (Al), such as in catalysis and electronic and energy storage
materials, there has been significant interest in the synthesis and
characterization of new Al coordination compounds that can support
electron transfer (ET) and proton transfer (PT) chemistry. This has
been achieved using redox and chemically noninnocent ligands (NILs)
combined with the highly stable M(III) oxidation state of Al and in
some cases the heavier group 13 ions, Ga and In.
When ligands
participate in redox chemistry or facilitate the breaking
or making of new bonds, they are often termed redox or chemically
noninnocent, respectively. Al(III) in particular supports rich ligand-based
redox chemistry because it is so redox inert and will support the
ligand across many charge and protonation states without entering
into the reaction chemistry. To a lesser extent, we have reported
on the heavier group 13 elements Ga and In, and this chemistry will
also be included in this Account, where available.
This Account
is arranged into two technical sections, which are
(1) Structures of Al–NIL complexes and (2) Reactivity of Al–NIL
complexes. Highlights of the research work include reversible redox
chemistry that has been enabled by ligand design to shut down radical
coupling pathways and to prevent loss of H2 from unsaturated
ligand sites. These reversible redox properties have in turn enabled
the characterization of Class III electron delocalization through
Al when two NIL are bound to the Al(III) in different charge states.
Characterization of the metalloaromatic character of square planar
Al and Ga complexes has been achieved, and characterization of the
delocalized electronic structures has provided a model within which
to understand and predict the ET and PT chemistry of the NIL group
13 compounds. The capacity of Al–NIL complexes to perform ET
and PT has been employed in reactions that use ET or PT reactivity
only or in reactions where coupled ET/PT affords hydride transfer
chemistry. As an example, ligand-based PT reactions initiate metal–ligand
cooperative bond activation pathways for catalysis: this includes
acceptorless dehydrogenation of formic acid and anilines and transfer
hydrogenation chemistry. In a complementary approach, ligand based
ET/PT chemistry has been used in the study of dihydropyridinate (DHP–) chemistry where it was shown that N-coordination
of group 13 ions lowers kinetic barriers to DHP– formation. Taken together, the discussion presented herein illustrates
that the NIL chemistry of Al(III), and also of Ga(III) and In(III)
holds promise for further developments in catalysis and energy storage.