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

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.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.

Redox-active ligands - a viable route to reactive main group metal compounds

Anionic redox-active ligands such as o-amidophenolates, catecholates, dithiolenes, 1,2-benzendithiolates, 2-amidobenzenethiolates, reduced α-diimines, ferrocenyl and porphyrinates are capable of reversible oxidation and thus have the ability to act as sources of electrons for metal centres. These and other non-innocent ligands have been employed in coordination complexes of base transition metals to influence their redox chemistry and afford compounds with useful catalytic, optical, magnetic and conducting properties. Despite the focus in contemporary main group chemistry on designing reactive compounds with potential catalytic activity, comparatively few studies exploring the chemistry of main group metal complexes incorporating redox-active ligands have been reported. This article highlights relevant chemical reactivity and electrochemical studies that probe the oxidation/reduction of main group metal compounds possessing redox-active ligands and comments on the prospects for this relatively untapped avenue of research.

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 (DHP-s). 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. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.