Electron spin echo envelope modulation spectroscopy reveals and distinguishes equatorial and axial nitrogen ligands bound to VO²⁺

ESEEM (electron spin echo envelope modulation) spectra of a series of vanadyl complexes have for the first time revealed the spectral features that arise from amine nitrogen donors trans to the vanadyl oxo moiety. The complexes [VO(H₂O)ada] (1) and [VO(H₂O)Hheida] (2) (H₂ada, N-(2- acetamido)iminodiacetic acid, H₃heida, N-(2-hydroxyethyl)iminodiacetic acid) have recently been synthesized and crystallographically characterized (Hamstra, B. J., Houseman, A. L. P.; Colpas, G. J.; Kampf, J. W.; LoBrutto, R.; Frasch, W.D.; Pecoraro, V. L. Inorg. Chem. 1997, 36, 4866-4874). In each structure, the ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo. In solution, the structural integrity of these compounds is maintained as observed by UV/vis and EPR spectroscopies. ESEEM spectra of 1, 2, and [VO(H₂O)nta]^- (3) (H₃nta, nitrilotriacetic acid) reveal that the superhyperfine coupling constants for axial amines are ≃ 1.3 MHz as opposed to the ≃ 5.0 MHz superhyperfine couplings typically observed for equatorially coordinated amines. The data indicate that not only can axial nitrogen donors to VO²⁺ be detected, but they may also be distinguished from nitrogen donors in the equatorial plane of the vanadyl ion. The implications of this study with respect to understanding the coordination environment of VO²⁺ in the vanadyl-substituted chloroplast F₁-ATPase (CF₁) and the reduced, inactive form of vanadium bromoperoxidase (VBrPO are discussed.