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

Abstract

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.

Selective Formation of an FeIVO or an FeIIIOOH Intermediate From Iron(II) and H2O2: Controlled Heterolytic versus Homolytic Oxygen–Oxygen Bond Cleavage by the Second Coordination Sphere

“…Unlike the work by Taktak et al., the NH Bn‐tpen ligand was designed in such a way that the secondary amine could not bind the metal through a stable 5‐ or 6‐membered metallocycle. The X‐ray structure of [Fe II ( NH Bn‐tpen)(OH 2 )] 2+ ( 1 II ) is displayed in Figure .…”

Selective Formation of an FeIVO or an FeIIIOOH Intermediate From Iron(II) and H2O2: Controlled Heterolytic versus Homolytic Oxygen–Oxygen Bond Cleavage by the Second Coordination Sphere

“…Unlike the work by Taktak et al, [24] the NH Bn-tpen ligand was designed in such aw ay that the secondary amine could not bind the metal through as table 5-or 6-membered metallocycle.T he X-ray structure of [Fe II ( NH Bn-tpen)-(OH 2 )] 2+ (1 II )i sd isplayed in Figure 1. It shows that the first coordination sphere of Fe II is constituted of the typical N 5 environment provided by Bn-tpen and related ligands [23,25,26] and confirms that the secondary amine (N6) is not coordinated to the metal center.T he coordination sphere is completed by aw ater ligand at 1.92 ,avalue consistent for an aqua engaged in aH -bond [27] ( Figure S1 in the Supporting Information).…”

Catalytic Applications of Pyridine‐Containing Macrocyclic Complexes

“…The introduction of a 3‐aminopropyl pendant arm onto a 14‐membered pyridine‐containing macrocycle allowed for that goal (Figure ). The corresponding iron(II) complexes in fact proved to be competent catalysts for alkene epoxidation by H 2 O 2 . The 14‐membered macrocycle was perfectly suited for the equatorial coordination of its four nitrogen atoms to an iron(II) ion, whereas the flexible arm could be tuned for axial coordination through acid‐base reactions.…”