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).

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A wide range of asymmetric Schiff base ligands containing primary amine functions can be prepared in one step and in high yield by the reaction of VO(OEt)3 with salicylaldehyde and the appropriate diamine. The resulting VOZL complexes contain tridentate ligands with phenolate, imine, and amine coordination to the dioxovanadium(V) complex ion. An X-ray structure of one compound ([V02(1,2-pnSAL)], SA, where 1,ZpnSAL = 1(Nsalicylideneamino)-2-aminopropane) demonstrates that it is a dimer in the solid state with a V2O4 core. However, when dissolved in DMSO, this material dissociates into monomers. Proton NMR spectroscopy reveals that the second isomer, 5B [V02( 1-amino-2(salicylideneamino)propane], is prepared in the process, but to a much lesser extent than SA (85:15). The reactivity of V02(enSAL) [enSAL = l-(N-salicylideneamino)-2-aminoethane] with peroxides in neutral, basic, and acidic media is discussed. Significantly, the reported synthetic methodology d m not yield successful VO2( 1 -(N-salicylideneamino)-3-aminopropane) [VO2( 1,3-pnSAL)]. Instead, the V(IV) complex VO(SALPN)DMSO (8) is isolated and has been characterized structurally (SALPN = 1,3-bis-(N-salicylidencamino)propane). Although VO(SALPN)DMSO and VO(SALEN) have markedly different solution spectra, their reactivity with coordinating and noncoordinating acids are very similar. X-ray parameters: [V02( 1,2-pnSAL)] (5A), C I O H~~N~O~V , 260.1 g/mol, triclinic crystal system, Pi, (No. 2), a = 7.270(2) A, b = 7.686(2) A, c = 11.082(4) A, CY = 72.16(2)", / 3 = 73.23(2)", 7 = 85.95(2)", V = 564.2(3) A3, 2 = 2,2244 data collected with 5"< 28 < 50°, 1786 data with (Fo) h 0.6a(F), R = 0.072, R,= 0.097; [VO(SALPN).DMSO] (8), C I~H~~N~O~V S , 425.4 g/mol, monoclinic crystal system, P21, (No. 14), a = 11.436(3) A, b = 7.775(2) A, c = 11.661(2) A, fl= 109.47(2)", V = 9 7 7 3 3 ) A3, 2 = 2,6224 data collected in the range 5" < 28 < 55", with 4240 data with (Fo) f 0.6u(F), R = 0.054, R , = 0.062.

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