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

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.