Catalytic site design: Impact of elongated bridges in open chain iron-NHC complexes on homogeneous epoxidation catalysis

To elucidate the relationship between ligand flexibility, coordination geometry and catalytic performance in homogeneous epoxidation catalysis, a series of four diacetonitrile[bis( o -imidazol-2-ylidenepyridine)alkane]iron(II) hexafluorophosphate complexes (Fe^II-NCCN) with two pyridine-imidazole units connected by aliphatic bridges of varying lengths (methylene to butylene, A – D ) are investigated. The increased steric bulk in the ligand scaffold switches the coordination geometry from an equatorial to a sawhorse-type with cis- labile coordination sites, corroborated by the butylene bridged complex ( D ). Structural characterization, DFT calculations, analyses of the buried volume and octahedral distortions are applied to compare the individual complexes. Catalytic evaluation in homogeneous epoxidation of cis -cyclooctene shows that the applied catalysts with extended bridges are less active due to their stability against hydrogen peroxide, corroborated by UV–Vis spectroscopy experiments. Exposure to a higher reaction temperature or adding a Lewis acid significantly increases their activity. This study shows that small modifications of the ligand topology have significant influence on the coordination behavior and result in pronounced catalytic performance changes of Fe^II-NCCN epoxidation systems. However, the coordination switch does not impact the catalytic performance in terms of activity, but the bridge elongation improves the stability against oxidative conditions, important for the rational design of epoxidation catalysts.