Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands.

ConspectusAluminum 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.

Group 13 ion coordination to pyridyl breaks the reduction potential hydricity scaling relationship for dihydropyridinates.

The relationship Δ correlates the applied potential () needed to drive organohydride formation with the strength of the hydride donor that is formed: in the absence of kinetic effects Δ should be linear but it would be more energy efficient if could be shifted anodically using kinetic effects. Biological hydride transfers (HT) performed by cofactors including NADH and lactate racemase do occur at low potentials and functional modeling of those processes could lead to low energy HT reactions in electrosynthesis and to accurate models for cofactor chemistry. Herein we probe the influence of -alkylation or -metallation on Δ for dihydropyridinates (DHP) and on of the DHP precursors.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis.

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHPs). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH.

Group 13 ion coordination to pyridyl models NAD reduction potentials.

-alkylation and -metallation of pyridine are explored herein to understand how metal-ligand complexes can model NAD redox chemistry. Syntheses of substituted dipyrazolylpyridine (pzP) compounds (pzP)Me (1) and (pzP)GaCl (2) are reported, and compared with (pzP)AlCl(THF) and transition element pzP complexes from previous reports. Cyclic voltammetry measurements of cationic 1 and 2 show irreversible reduction events ∼900 mV anodic those for neutral pzP complexes of divalent metals.

Ligand Conjugation Directs the Formation of a 1,3-Dihydropyridinate Regioisomer.

The selective formation of the 1,4-dihydropyridine isomer of NAD(P)H is mirrored by the selective formation of 1,4-dihydropyridinate ligand-metal complexes in synthetic systems. Here we demonstrate that ligand conjugation can be used to promote selective 1,3-dihydropyridinate formation. This represents an advance toward controlling and tuning the selectivity in dihydropyridinate formation chemistry.