Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands

Abstract

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.

Scheme 1. Reaction Chemistry Based on the Al(I/III) Couple, Which Includes Oxidative Addition Chemistry

dipp = 2,6-diisopropylphenyl (2,6-iPr₂Ph).

Scheme 2. NILs That Have Been Studied with Al(III) Include Iminepyridine (IP) and Diiminepyridine (I₂P), Substituted by R and Ar groups, Which Are Denoted in Blue

Ligands are shown here with the commonly observed charge states that are accessible with group 13 ions, Al(III) and Ga(III).

Scheme 3. NILs That Have Been Studied on Al(III) Include Dipyrazoylylpyridines (pz₂P), Acenaphthene-1,2-diimines (BIAN), and Bis(3,5-di-tert-butyl-2-phenol)amine (ONO)

cat and sq superscripts are catecholate and semiquinonate forms of the ONO ligand. Charge states shown are common when coordinated to group 13 ions, Al(III) and Ga(III).

Scheme 4. Commonly Observed Reactivity of Substituted _Ar^RI₂P Ligands upon One-Electron Reduction

Scheme 5. Synthetic Roadmap to _ArI₂P–Al Complexes Reported by Our Lab with Ligands in Various Charge States, M = Al, Ga, or In

For singly ligated Al complexes, Ar = dipp; for doubly ligated octahedral complexes, Ar = o-PhOMe, PhF₅, or p-PhNMe₂, as in Scheme 2.

Chart 1. Four-Coordinate Compounds (left) and the Five-Coordinate Solvated Analog (right)a

Chart 2. (top) Early Examples of Reduced Ligand Catecholate Complexesa and (bottom) Line Drawings of (IP^–)₂MX and (SQ^–)₂GaX with Possible Orbital Interactions Illustrated

Chart 3. Mixed-Valent Compounds of IP (left) and I₂P (right) Ligands

Scheme 6. Group Transfer Chemistry of Selected IP and I₂P Complexes

X = Cl, CCPh, N₃, SPh, and NHPh; TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical.

Scheme 7. (top) General MLCBA Activation of RE–H Bond by Noninnocent Al Complexes and (bottom) MLCBA Using De-/Re-aromatization Reactivity Demonstrated by Milstein

L is the multidentate ligand, R is an alkyl group, and E is typically a N or O atom whose bond E–H will be cleaved.

Scheme 8. Reactivity Summary of (I₂P^2–)AlH(THF) and (I₂P^2–)AlCl with Alcohols or Amines

H or H₂ is added to the label; this denotes the ligand is once or twice protonated, respectively.

Scheme 9. MLCBA Reactivity of [(c₄p^4–)Al]⁻ and (BIAN^2–)AlEt(Et₂O)

c₄p^4– = calix[4]pyrrolato. 12-c-4 = 12-crown-4. Below the c₄p^4– complexes on the left, the σ and 1,2 labels denote the binding mode of the aldehyde/ketone, which is dependent on the presence of lithium cation.

Scheme 10. (top) Comparison of Selected pz₂P Salts and (bottom) General Scheme for How Al Complexes Participate in HER and as HT Catalysts

Dashed blue box highlights dihydropyridinate (DHP^–).