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.