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

The aluminum complexes (^RpyNO^–)₂AlCl (^RpyNO^– = <i>N</i>-<i>tert</i>-butyl-<i>N</i>-(2-pyridyl)­nitroxyl; R = H (<b>1</b>), CH₃ (<b>2</b>), CF₃ (<b>3</b>)) were prepared in 80–98% yield through the protonolysis reaction between the pyridyl hydroxylamine ligand precursors ^RpyNOH and dimethylaluminum chloride. Complex <b>1</b> was also prepared using a salt metathesis route in 92% yield. Complexes <b>1</b>–<b>3</b> were characterized using ¹H and ¹³C NMR spectroscopies. Single-crystal X-ray diffraction analysis of the complexes revealed that <b>1</b>–<b>3</b> are isostructural, with the Al^III cation in all cases being five coordinate with distorted square pyramidal geometries. The geometry of complex <b>1</b> was studied using DFT, which showed primarily ligand-based frontier molecular orbitals. Reaction of <b>1</b> with NaO<i>t</i>-Bu gave (pyNO^–)₂AlO<i>t</i>-Bu (<b>4</b>), while reaction of <b>1</b> with AgBPh₄ gave [(pyNO^–)₂Al­(THF)₂]­[BPh₄] (<b>5</b>) in 54% and 87% yields, respectively. Compounds <b>4</b> and <b>5</b> were both characterized using ¹H and ¹³C NMR spectroscopies and compound <b>5</b> by X-ray diffraction. Complexes <b>1</b>–<b>5</b> 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 <i>t</i>-Bu of the ^RpyNO^– ligand in the ¹H NMR spectroscopy and the potentials of the redox events for complexes <b>1</b>–<b>4