Synthesis and Characterization of Neutral Ligand α-Diimine Complexes of Aluminum with Tunable Redox Energetics

Abstract

The synthesis and full characterization of a series of neutral ligand α-diimine complexes of aluminum are reported. The compounds [Al(L)Cl)][AlCl] [L = N, N'-bis(4-R-CH)-2,3-dimethyl-1,4-diazabutadiene] are structurally analogous, as determined by multinuclear NMR spectroscopy and solid-state X-ray diffraction, across a range of electron-donating [R = Me (2), Bu (3), OMe (4), and NMe (5)] and electron-withdrawing [R = Cl (6), CF (7), and NO (8)] substituents in the aryl side arm of the ligand. UV-vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties, respectively, of the complexes. Both sets of properties are shown to be dependent on the R substituent. Density functional theory calculations performed on the [Al(L)Cl)][AlCl] complex (1) indicate primarily ligand-based frontier orbitals and were used to help support our discussion of both the spectral and electrochemical data. We also report the reaction of the L ligand with both AlBr and AlI and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group.

Synthesis and Characterization of (pyNO−)2GaCl: A Redox-Active Gallium Complex

“…We are specifically interested in how coordination of Lewis acids with different stabilizing effects change the electronic structure of a common redox-active ligand. In this vein, we have been interested in the development of aluminum complexes of redox active-ligands [5][6][7][8][9] and have recently reported the synthesis and characterization of a series of Al-nitroxide complexes [7,8]. The nitroxide functional group is well known to exist over three oxidation states: the reduced aminoxyl anion (R 2 N-O − ), the neutral radical (R 2 N-O • ), and the oxidized oxoammonium cation (R 2 N=O + ) (Figure 1).…”

Unusually Strong Long-range Electronic Coupling Across Redox-Active Bridges in M3+/M4+ Mixed-Valence Complexes of Group 4 Congeners

“…37,[57][58][59][60][61] It is well established that bidentate exocyclic imines, such as αdiimine ligands, display electronic delocalization when coordinated to various metal centers. [62][63][64][65][66][67][68][69] Furthermore, the electronic properties of exocyclic imines can be easily tuned by varying the substituents on the imine via condensation of the parent ketone with an amine equipped with the desired substituents. 68,[70][71] Considering both the electronic behavior and tunability of this imine ligand class, it is quite surprising that there has been little progress towards the development of bridges based on their framework.…”

“…[62][63][64][65][66][67][68][69] Furthermore, the electronic properties of exocyclic imines can be easily tuned by varying the substituents on the imine via condensation of the parent ketone with an amine equipped with the desired substituents. 68,[70][71] Considering both the electronic behavior and tunability of this imine ligand class, it is quite surprising that there has been little progress towards the development of bridges based on their framework. One α-diimine framework that holds particular promise as a scaffold for a bridging ligand is bis(arylimino)acenaphtylene (BIAN).…”

A Ligand Protonation Series in Aluminum(III) Complexes of Tridentate Bis(enol)amine Ligand

“…To determine whether additional protonation states of ONO could be accessed in this report, we employed Al­(III). The stability of Al­(III) to both oxidation and reduction makes it ideal to support a wide range of charge and protonation states as has been demonstrated using ligands such as iminopyridine, bis­(imino)­pyridines, α-diimines, dpp-BIAN, bis­(pyrazolyl)­pyridine, and amidobis­(phenolate) . Here, Al­(III) enables us to introduce two additional electronic structures for the protonation states of ONO: HONO 2– and H 2 ONO – , along with interconversion of these compounds in acid–base reactions (Scheme ).…”