Reevaluation of the Additivity Relationship for Vanadyl−Imidazole Complexes:  Correlation of the EPR Hyperfine Constant with Ring Orientation

Five new vanadyl−imidazole complexes with the imidazole ring in different orientations to the vanadyl unit have been synthesized and characterized by X-ray crystallography and electron paramagnetic resonance (EPR). The hyperfine coupling constants (A_∥) from the EPR spectra of these complexes were analyzed using the additivity relationship. There is some discussion as to what contribution an “aromatic imine” such as imidazole bound in the equatorial plane of a vanadyl complex makes to the hyperfine coupling. This analysis, applied to the new complexes as well as those vanadyl−imidazole complexes previously published, reveals a trend in the contribution to the hyperfine coupling related to the orientation of the imidazole ring relative to the vanadyl unit; an imidazole parallel to the vanadium−oxygen double bond was found to have a contribution of 40 × 10^-4 cm^-1, while one perpendicular gave a contribution of 45 × 10^-4 cm^-1. A similar trend for imine values was also observed in the study of these complexes. These findings will significantly influence EPR studies of vanadium in biological systems, whether naturally occurring or substituted in as a spectroscopic probe. These data are also used to draw structural conclusions on molecules containing vanadyl ion whose structures are not published, including the reduced vanadium bromoperoxidase (VBrPO).