The dioxovanadium(IV) com
plexes with
pida
4- ligands (H
4pida) =
N-(
phos
phonomethyl)iminodiacetic acid),Na
4[V
2O
2{(O)
2P(O)CH
2N(CH
2COO)
2}
2]·10H
2O (
1) and Na
8[V
2O
2{(O)
2P(O)CH
2N(CH
2COO)
2}
2]
2·16H
2O (
2), wereisolated from reactions of H
4pida with either oxovanadium(V) (i.e., NaVO
3) or oxovanadium(IV)
precursors withinthe
pH range of 2-8. The structures of com
plexes
1 and
2 were investigated by X-ray diffraction methods andin contrast to ex
pectation were both found to be dinuclear. Com
plex
1 crystallized in the monoclinic system:
P2
1/
n,
a = 10.5632(1) Å,
b = 11.1868(1) Å,
c = 12.6921(1) Å,
= 106.45
![](/images/entities/deg.gif)
,
V = 1438.44(2) Å
3,
Z = 4, and
R(w
R2) = 0.0781 (0.2017). Com
plex
2 crystallized in the monoclinic system
P2
1/
c:
a = 13.9822(2) Å,
b = 11.1888(2) Å,
c = 18.6519(3) Å,
= 100.88
![](/images/entities/deg.gif)
,
V = 2865.51(8) Å
3,
Z = 4, and
R (w
R2) = 0.046 (0.125). Both com
plexes
1 and
2 have similar dimeric frameworks where two vanadium centers are linked by two
phos
phonate grou
ps oftwo
pida
4- ligands (quadridentate binucleating), bridging through their four oxygen atoms to form a V
2O
4P
2eight-membered ring which
possesses a crystallogra
phic inversion center. In contrast to their solid-state features,in aqueous solution both dinuclear crystalline com
pounds immediately dissociate to monomeric s
pecies, as observedby EPR and UV/vis s
pectrosco
py. Both solution-state EPR and NMR s
pectrosco
py confirmed that redox chemistryis involved in the reaction between vanadate and H
4pida. Studies in mixed solvent systems showed that the dinuclearcom
plex would remain intact in the
presence of sufficient organic solvent. In the absence of oxygen the mononuclearand the dinuclear com
plexes will reversibly interconvert, whereas, in the
presence of oxygen, the com
plexes willoxidize. These studies document the existence of higher oligomeric vanadium com
pounds and sur
prisingly, ingeneral, lend credibility to several emerging mechanistic
pro
posals involving oligomeric s
pecies of vanadiumcom
pounds in catalytic
processes.