Ad. Keramidas et al., VANADIUM(V) HYDROXYLAMIDO COMPLEXES - SOLID-STATE AND SOLUTION PROPERTIES, Journal of the American Chemical Society, 119(38), 1997, pp. 8901-8915
A novel series of vanadium(V) hydroxylamido complexes with weak ligand
s including glycine, [VO(NH2O)(2)(Glycine)]. H2O (1); serine, [VO(NH2O
)(2)(Serine)]. H2O (2); glycylglycine, [VO(NH2O)(2)(GlyGly)]. H2O (3);
and imidazole, [VO(NH2O)(2)(imidazole)(2)]Cl (4) were prepared and ch
aracterized both in solution and in the solid state. All complexes wer
e prepared in aqueous solution at neutral pH at ambient temperature an
d as crystalline solids. The vanadium atom in these four complexes is
seven-coordinate with pentagonal bipyramidal geometry. In complexes 1-
3 the hydroxylamido groups are coordinated side-on with the hydroxylam
ido nitrogen cia to the organic ligand in the equatorial plane. In com
plex 4, the hydroxylamido groups are coordinated side-on with the hydr
oxylamido nitrogen trans to the imidazole ligand in the equatorial pla
ne. The UV/vis spectra of these complexes were also examined, and the
absorbance peaks show similarities between the properties of the vanad
ium(V) hydroxylamido complexes and known side-on peroxovanadium comple
xes. Multinuclear NMR studies were conducted to characterize the solut
ion structure and properties of compounds 1-4. A particularly detailed
study of compound 4 was carried out since the analogous vanadium(V) p
eroxo complex could-also be prepared. Complex 4 was less labile and mo
re stable than the corresponding diperoxovanadium(V)-imidazole complex
, H[VO(O-2)(2)(imidazole)] (5). In solution the inherent asymmetry of
the hydroxylamido ligand has facilitated an in-depth study of ligand e
xchange. Upon dissolution, compound 4 forms three isomeric complexes,
all of which have one of the original two-coordinated imidazole groups
in the complex dissociated. ID and 2D EXSY and multinuclear NMR spect
roscopies were used to examine the stoichiometry of the isomers, their
structures, and the dynamics of their ligand exchanges. Specifically;
both inter- and intramolecular exchanges were observed for the dihydr
oxylamine-vanadium(V)-imidazole involving both the coordinated imidazo
le and the coordinated hydroxylamido groups. The intramolecular exchan
ge of-the coordinated imidazole in 5 was compared to the exchange in t
he hydroxylamido complex, and the hydroxylamido compounds were found t
o have some properties that may be advantageous over those of the dipe
roxovanadium(V) complexes. In summary, evidence was generated to suppo
rt the existence of a novel and unprecedented asymmetric hydroxylamido
-metal complex as well as the first isolation and characterization of
a vanadium(V)-imidazole complex not enjoying stabilization by other or
ganic ligands.