Structure and pulsed EPR characterization of N,N′-bis(5-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide and its adducts with propylene oxide

The role of steric hindrance in controlling the binding mode of propylene oxide to a novel vanadyl salen-type complex N, N'-bis(5-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide, [VO(3)], has been investigated using CW/pulse EPR, ENDOR and HYSCORE spectroscopy and compared to that of the parent complex N, N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide, [VO(1)]. The single-crystal X-ray structure of [VO(3)]center dot HCCl3 has been determined by X-ray analysis and is complemented by DFT calculations and circular dichroism measurements. The structure of the complex in frozen solution, as revealed by the EPR methods, is in good agreement with the X-ray and DFT analyses. Removal of the 'inner' tert-butyl groups from the salicylidene rings reduces the steric hindrance between the ligand and epoxide substrate. As a result the selectivity for binding single enantiomers of propylene oxide in these complexes is reversed in [VO(3)] relative to [VO(1)].