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′)­Br2]. In the solid state these complexes adopt a tetrahedral geometry with the PNP′ ligands coordinated in a κ2 P,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.