Flexible Binding of PNP Pincer Ligands to Monomeric Iron Complexes

Transition metal complexes supported by pincer ligands have many important applications. Here, the syntheses of five-coordinate PNP pincer-supported Fe complexes of the type (PNP)FeCl2 (PNP = HN{CH2CH2(PR2)}2, R = iPr ( iPrPNP), tBu ( tBuPNP), or cyclohexyl (CyPNP)) are reported. In the solid state, ( iPrPNP)FeCl2 was characterized in two different geometries by X-ray crystallography. In one form, the iPrPNP ligand binds to the Fe center in the typical meridional geometry for a pincer ligand, whereas in the other form, the iPrPNP ligand binds in a facial geometry. The electronic structures and geometries of all of the (PNP)FeCl2 complexes were further explored using 57Fe Mössbauer and magnetic circular dichroism spectroscopy. These measurements show that in some cases two isomers of the (PNP)FeCl2 complexes are present in solution and conclusively demonstrate that binding of the PNP ligand is flexible, which may have implications for the reactivity of this important class of compounds.