Chirality and diastereoselection in the mu-oxo diiron complexes L(2)Fe-O-FeL(2) (L = bidentate salicylaldiminato)

A number of bis(salicylaldiminato) (Schiff base) complexes of Fe(II) i.e. L(2)Fe with a variety of ligand substituents and functionality are synthesised, usually by salt metathesis but on one occasion via the (sal)(2)Fe complex, and characterised by standard methods. Two molecular structure determinations indicate the expected tetrahedral geometry. Treatment with dry oxygen yielded the Fe(III) complexes [(FeL(2))(2)-mu-O], generally in high yield and in the absence of hydroxylated impurities; one bulky complex L(2)Fe gave no oxidation product for steric reasons, and one hydroxyl-substituted compound gave an insoluble product. Five molecular structures of these [(FeL(2))(2)-mu-O] compounds were determined. All were homochiral in the sense that the absolute configuration at Fe was the same for both atoms. Three of the structures - those based on optically pure salicylaldimine ligands - were diastereomeric. For two, the diastereomers observed are those predicted on the basis of steric considerations, while a third highly distorted and congested structure represents the unexpected diastereomer. Powder X-ray diffraction confirms high phase purity of the bulk samples (i.e. they are diastereomerically pure to the level detectable). These investigations, coupled with molecular structure and NMR studies on model gallium complexes, indicate that the diastereoselection arises from cooperative interactions across the oxo bridge.