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

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Abstract
The aluminum complexes (LMes2–)AlCl(THF) (3) and (LDipp–)AlCl2 (4) (LMes = N,N′-bis[2,4,6-trimethylphenyl]-2,3-dimethyl-1,4-diazabutadiene, LDipp = N,N′-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 (LMes–)AlCl2 (5) complex was prepared through one-electron oxidative functionalization of 3 with either AgCl or CuCl. Complex 3 was characterized using 1H and 13C NMR spectoscopies. Single-crystal X-ray diffraction analysis of the complexes revealed that 3–5 are all four-coordinate, with 3 exhibiting a trigonal pyramidal geometry, while 4 and 5 exist between trigonal pyramidal and tetrahedral. Notable in the LMes complexes 3 and 5 is a systematic lengthening of the C–Nimido 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 3 and 5 were optimized using DFT, which showed primarily ligand-based frontier orbitals, supporting the analysis of the solid-state structural data. The complexes 3–5 were also characterized by electrochemistry. The cyclic voltamogram of complex 3 showed an oxidation processes at −0.94 and −0.03 V versus ferrocene, while complexes 4 and 5 exhibit both reduction (−1.37 and −1.34 V, respectively) and oxidation (−0.62 and −0.73 V, respectively) features.

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