| Abstract
| - A series of five- and seven-coordinate substituted dipicolinate complexes of vanadium(V) were synthesized and compared. NMR spectroscopy indicated that the hydroxylamido complexes are more hydrolytically stable and have greater electron density localized on the V(V) ion than in the dipicolinate ligand. Isomers of the hydroxylamido complexes were observed in solution, and new V(V) species were observed under nonstoichiometric conditions. Enzyme inhibition by five-coordinate complexes likely arises from the stability gained in an enzyme complex.
- A number of 4-substituted, dipicolinatodioxovanadium(V) complexes and their hydroxylamido derivatives weresynthesized to characterize the solid state and solution properties of five- and seven-coordinate vanadium(V)complexes. The X-ray crystal structures of Na[VO2dipic-NH2]·2H2O (2) and K[VO2dipic-NO2] (3) show the vanadiumadopting a distorted, trigonal-bipyramidal coordination environment similar to the parent coordination complex,[VO2dipic]- (1), reported previously as the Cs+ salt. The observed differences in the chemical shifts of the complexesboth in the 1H (ca. 0.7−1.4 ppm) and 51V (ca. 1−11 ppm) NMR spectra were consistent with the electron-donatingor electron-withdrawing properties of the substituent groups, respectively. Stoichiometric addition of a series ofhydroxylamine ligands (H2NOH, MeHNOH, Me2NOH, and Et2NOH) to complexes 1−3 led to the formation of seven-coordinate vanadium(V) complexes. The X-ray crystal structure of [VO(dipic)(Me2NO)(H2O)]·0.5H2O (1c) was foundto be similar to the previously characterized complexes [VO(dipic)(H2NO)(H2O)] (1a) and [VO(dipic)(OO-tBu)(H2O)].While only slight differences in the 1H NMR spectra were observed upon addition of the hydroxylamido ligand, thesignals in the 51V NMR spectra change by up to 100 ppm. The addition of the hydroxylamido ligand increased thecomplex stability of complexes 2 and 3. Evidence for a nonstoichiometric redox reaction was found for the monoalkylhydroxylamine ligand. The reaction of an unsaturated five-coordinate species with a hydroxylamine to form a seven-coordinate vanadium complex will, in general, dramatically increase the amounts of the vanadium compound thatremain intact at pH values near neutral.
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