Iron(II) Complexes Containing Chiral Unsymmetrical PNP′ Pincer Ligands: Synthesis and Application in Asymmetric Hydrogenations

Four new chiral PNP' pincer ligands with a scaffold consisting of a planar chiral ferrocene and a centro chiral aliphatic unit were synthesized and characterized. Treatment of anhydrous FeBr2(THF)(2) with 1 equiv of the unsymmetrical chiral PNP' pincer ligands afforded complexes of the general formula [Fe(PNP')Br-2]. In the solid state these complexes adopt a tetrahedral geometry with the PNP' ligands coordinated in a kappa P-2,N-fashion, as shown by X-ray crystallography. These complexes react with CO in the presence of NaBH4 to yield hydride complexes of the type [Fe(PNP')(H)(Br)(CO)], which were isolated and tested as catalysts in the asymmetric hydrogenation of ketones. Enantioselectivities of up to 81% ee were obtained.