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(LAr)2Cl2)][AlCl4] [LAr = N, N'-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 (2), tBu (3), OMe (4), and NMe2 (5)] and electron-withdrawing [R = Cl (6), CF3 (7), and NO2 (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(LPh)2Cl2)][AlCl4] 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 LPh ligand with both AlBr3 and AlI3 and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group.