Synthesis and Catalytic Properties in Olefin Epoxidation of Novel Iron(II) Complexes with Pyridine-Containing Macrocycles Bearing an Aminopropyl Pendant Arm

Three novel iron(II) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm were
synthesized and characterized. Crystal structures of two of the complexes revealed high-spin iron(II) centers
coordinated to the five ligand nitrogen atoms with no coordination of either the solvent molecules or anions, resulting
in an unusual square-pyramidal geometry. Related tetradentate ligand CRH formed a low-spin iron(II) complex
(meso form was structurally characterized) with a planar arrangement of the four nitrogen atoms from the macrocycle
and two axial acetonitrile molecules. Similarly to the corresponding nickel and copper complexes of the pentadentate
ligands, the protonation of the amino group on the ligand arm in iron(II) complexes was found to be reversible.
Spectral changes and magnetic susceptibility measurements indicated that a change in the geometry and spin
state of the metal center is associated with this acid−base process. In the presence of noncoordinating acids (e.g.,
triflic acid), these complexes, as well as their nonmethylated analogue, can efficiently catalyze the epoxidation of
cyclooctene and 1-decene under mild conditions, using hydrogen peroxide as the oxidant. However, in the
deprotonated form or in the presence of coordinating acids like HCl, no epoxidation occurs.