N-Tetradentate SPANamine Derivatives and Their MnII-Complexes as Catalysts for Epoxidation of Alkenes

Five new tetradentate N-donor ligands with the spirobi(chroman) skeleton (either chiral or racemic) have been synthesized and fully characterized by NMR, ESI-MS, and optical polarimetry. These ligands have been coordinated to a series of manganese salts, affording a family of new chiral and racemic Mn^II complexes, which have also been characterized through analytical and spectroscopic techniques. The crystallographic structures of five of them have been obtained by X-ray diffraction; all show a distorted octahedral geometry with the ligand adopting a cis-α conformation over the Mn ion. All the complexes have been tested in the catalytic epoxidation of styrene, showing that both conversion and selectivity values increase in the presence of additives in the catalytic media. Experimental results and computational calculations explain the stereospecificity for the cis epoxide in the epoxidation of cis-β-methylstyrene mediated by one of the complexes synthesized.