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

The synthesis and full characterization of a series of neutral ligand α-diimine complexes of aluminum are reported. The compounds [Al­(L_Ar)₂Cl₂)]­[AlCl₄] [L_Ar = <i>N</i>,<i>N</i>′-bis­(4-R-C₆H₄)-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 (<b>2</b>), ^<i>t</i>Bu (<b>3</b>), OMe (<b>4</b>), and NMe₂ (<b>5</b>)] and electron-withdrawing [R = Cl (<b>6</b>), CF₃ (<b>7</b>), and NO₂ (<b>8</b>)] 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_Ph)₂Cl₂)]­[AlCl₄] complex (<b>1</b>) 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_Ph ligand with both AlBr₃ and AlI₃ and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group