Dual-mode EPR study of Mn(III) salen and the Mn(III) salen-catalyzed epoxidation of cis-beta-methylstyrene

Dual-mode electron paramagnetic resonance (EPR), in which an oscillating ma gnetic field is alternately applied parallel or perpendicular to the static magnetic field, is a valuable technique for studying both half-integer and integer electron spin systems and is particularly useful for studying tran sition metals involved in redox chemistry. We have applied this technique t o the characterization of the Mn(III) salen (salen = N,N'-ethylene bis(sali cylideneaminato)) complex [(R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene )-1,2-cyclohexanediaminomanganese(III)], with an S = 2 integer electron spi n system. Furthermore, we have used dual-mode EPR to study the Mn salen com plex during the Mn(III) salen-catalyzed epoxidation of cis-beta -methylstyr ene. Our study shows that the additives N-methylmorpholine N-oxide (NMO) an d 4-phenylpyridine-N-oxide (4-PPNO), which are used to improve epoxidation yields and enantioselection, bind to the Mn(III) center prior to the epoxid ation reaction, as evidenced by the alteration of the Mn(III) parallel mode EPR signal. With these additives as ligands, the axial zero-field splittin g values and Mn-55 hyperfine splitting of the parallel mode signal are indi cative of an axially elongated octahedral geometry about the Mn(III) center . Since the dual-mode EPR technique allows the observation of both integer and half-integer electron spin systems, Mn oxidation states of II, III, IV, and potentially V can be observed in the same sample as well as any radica l intermediates or Mn(III,IV) dinuclear clusters formed during the Mn(III) salen-catalyzed epoxidation reaction. Indeed, our study revealed the format ion of a Mn(III,IV) dinuclear cluster in direct correlation with expoxide f ormation. In addition to showing the possible reaction intermediates, dual- mode EPR offers insight into the mechanism of catalyst degradation and form ation of unwanted byproducts. The dual-mode technique may therefore prove v aluable for elucidating the mechanism of Mn(III) salen catalyzed reactions and ultimately for designing optimum catalytic conditions (solvents, oxidan ts, and additives such as NMO or 4-PPNO).