Synthesis and Characterization of Aluminum-α-diimine Complexes over Multiple Redox States

The aluminum complexes (L_Mes^2–)­AlCl­(THF) (<b>3</b>) and (L_Dipp^–)­AlCl₂ (<b>4</b>) (L_Mes = <i>N</i>,<i>N</i>′-bis­[2,4,6-trimethylphenyl]-2,3-dimethyl-1,4-diazabutadiene, L_Dipp = <i>N</i>,<i>N</i>′-bis­[2,6-diisopropylphenyl]-2,3-dimethyl-1,4-diazabutadiene) were prepared by direct reduction of the ligands with sodium metal followed by salt metathesis with AlCl₃. The (L_Mes^–)­AlCl₂ (<b>5</b>) complex was prepared through one-electron oxidative functionalization of <b>3</b> with either AgCl or CuCl. Complex <b>3</b> was characterized using ¹H and ¹³C NMR spectoscopies. Single-crystal X-ray diffraction analysis of the complexes revealed that <b>3</b>–<b>5</b> are all four-coordinate, with <b>3</b> exhibiting a trigonal pyramidal geometry, while <b>4</b> and <b>5</b> exist between trigonal pyramidal and tetrahedral. Notable in the L_Mes complexes <b>3</b> and <b>5</b> is a systematic lengthening of the C–N_imido bonds and shortening of the C–C bond in the N–C–C–N backbone with increased electron density on the ligand. The geometries of the complexes <b>3</b> and <b>5</b> were optimized using DFT, which showed primarily ligand-based frontier orbitals, supporting the analysis of the solid-state structural data. The complexes <b>3</b>–<b>5</b> were also characterized by electrochemistry. The cyclic voltamogram of complex <b>3</b> showed an oxidation processes at −0.94 and −0.03 V versus ferrocene, while complexes <b>4</b> and <b>5</b> exhibit both reduction (−1.37 and −1.34 V, respectively) and oxidation (−0.62 and −0.73 V, respectively) features