Aluminum complexes of the redox-active [ONO] pincer ligand

A series of aluminum complexes containing the tridentate, redox-active ligand bis(3,5-di-tert-butyl-2-phenol)amine ([ONO]H-3) in three different oxidation states were synthesized. The aluminum halide salts AlCl3 and AlBr3 were reacted with the doubly deprotonated form of the ligand to afford five-coordinate [ONHOcat]AlX(solv) complexes (1a, X = Cl, solv = OEt2; 1b, X = Br, solv = THF), each having a trigonal bipyramidal coordination geometry at the aluminum and containing the [ONHOcat](2-) ligand with a protonated, sp(3)-hybridized nitrogen donor. The [ONO] ligand platform may also be added to aluminum through the use of the oxidized ligand salt [ONOq]K, which was reacted with AlCl3 in the presence of either diphenylacetylacetonate (acacPh(2)(-)) or 8-oxyquinoline (quinO(-)) to afford [ONOq]Al(acacPh(2)) Cl (2) or [ONOq]Al(quinO)Cl (3), respectively, with well-defined [ONOq](-) ligands. Quinonate complexes 2 and 3 were reduced by one electron to afford the corresponding complexes K{[ONOsq]Al(acacPh(2))(py)} (4) and K{[ONOsq]Al(quinO)(py)} (5), respectively, containing well-defined [ONOsq](2-) ligands. The addition of tetrachloro-1,2-quinone to 1a in the presence of pyridine resulted in the expulsion of HCl and the formation of an aluminum complex with two different redox active ligands, [ONO]Al(o-O2C6Cl4)(py) (6). Similar results were obtained when 1a was reacted with 9,10-phenanthrenequinone to afford [ONO]Al(o-O2C14H8)(py) (7) or with pyrene-4,5-dione to afford [ONO]Al(o-O2C16H8)(py) (8). Structural, spectroscopic and preliminary magnetic measurements on 6-8 suggest ligand non-innocent redox behavior in these complexes.