Synthesis and Characterization of Aluminum Complexes of Redox-Active Pyridyl Nitroxide Ligands

The
aluminum complexes (RpyNO–)2AlCl (RpyNO– = N-tert-butyl-N-(2-pyridyl)­nitroxyl; R = H
(1), CH3 (2), CF3 (3)) were prepared in 80–98% yield through the protonolysis
reaction between the pyridyl hydroxylamine ligand precursors RpyNOH and dimethylaluminum chloride. Complex 1 was also prepared using a salt metathesis route in 92% yield. Complexes 1–3 were characterized using 1H and 13C NMR spectroscopies. Single-crystal X-ray diffraction
analysis of the complexes revealed that 1–3 are isostructural, with the AlIII cation in all
cases being five coordinate with distorted square pyramidal geometries.
The geometry of complex 1 was studied using DFT, which
showed primarily ligand-based frontier molecular orbitals. Reaction
of 1 with NaOt-Bu gave (pyNO–)2AlOt-Bu (4), while reaction
of 1 with AgBPh4 gave [(pyNO–)2Al­(THF)2]­[BPh4] (5) in 54% and 87% yields, respectively. Compounds 4 and 5 were both characterized using 1H and 13C NMR spectroscopies and compound 5 by X-ray diffraction.
Complexes 1–5 were also characterized
by UV–vis electronic absorption spectroscopy and electrochemistry.
The cyclic voltammograms of the complexes show two separate oxidation
process, the potentials of which are dependent on both the substitution
pattern of the RpyNO– ligands and the
anion that completes the aluminum coordination sphere. A correlation
was determined between the chemical shift of the t-Bu of the RpyNO– ligand in the 1H NMR spectroscopy and the potentials of the redox events
for complexes 1–4.