Synthesis And Characterization Of Aluminum-Alpha-Diimine Complexes Over Multiple Redox States

The aluminum complexes (L-Mes (2-))AlCl(THF) (3) and (L-Dipp(-))AlCl2 (4) (L-Mes = N,N'-bis[2,4,6-trimethylphenyl]-2,3-dimethyl-1,4-diazabutadiene, L-Dipp = 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 (L-Mes (-))AlCl2 (5) complex was prepared through one-electron oxidative functionalization of 3 with either AgCl or CuCl. Complex 3 was characterized using H-1 and C-13 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 L-Mes complexes 3 and 5 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 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.