Insulin-enhancing vanadium(III) complexes

Article abstract of DOI:10.1021/ic000984t

Simple, high-yield, large-scale syntheses of the V(III) complexes tris(maltolato)vanadium(III), V(ma)₃, tris(ethylmaltolato)vanadium(III), V(ema)₃, tris(kojato)vanadium(III) monohydrate, V(koj)₃·H₂O, and tris(1,

Full text of DOI:10.1021/ic000984t

4686
Inorg. Chem. 2001, 40, 4686-4690
Insulin-Enhancing Vanadium(III) Complexes
Marco Melchior,† Steven J. Rettig,† Barry D. Liboiron,† Katherine H. Thompson,†
Violet G. Yuen,‡ John H. McNeill,‡ and Chris Orvig*,†
Medicinal Inorganic Chemistry Group, Department of Chemistry and Faculty of Pharmaceutical
Sciences, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada
ReceiVed August 30, 2000
Simple, high-yield, large-scale syntheses of the V(III) complexes tris(maltolato)vanadium(III), V(ma)3, tris-
(ethylmaltolato)vanadium(III), V(ema)3, tris(kojato)vanadium(III) monohydrate, V(koj)3‚H2O, and tris(1,2-dimethyl-
3-hydroxy-4-pyridinonato)vanadium(III) dodecahydrate, V(dpp)3‚12H2O, are described; the characterization of
these complexes by various methods and, in the case of V(dpp)3‚12H2O, by an X-ray crystal structure determination,
is reported. The ability of these complexes to normalize glucose levels in the STZ-diabetic rat model has been
examined and compared with that of the benchmark compound BMOV (bis(maltolato)oxovanadium(IV)), an
established insulin-enhancing agent.
Introduction
oughly characterized chemically13,14 and tested biologically.15,16
BMOV has proven to be an excellent “benchmark” compound
Mononuclear coordination complexes of V(III) are generally
six-coordinate, hydrolytically stable, and axially symmetric with
an octahedral or pseudo-octahedral geometry.1 They are not
usually considered for therapeutic applications2,3 because of
rapid oxidation to V(IV/V) in aqueous solutions of pH >3.4,5
Physiologically relevant oxidation states of vanadium are
thought traditionally to be V(IV), as vanadyl, and V(V), often
as vanadate.2 There are examples of V(III) in biological systems;
however, these are not very common.6 To our knowledge, no
V(III) complex has been examined previously for insulin-
enhancing activity in vivo or in vitro.
againstwhichtotestmorerecentlysynthesizedcomplexes,10,11,17-19
as was done in this study as well. A new vanadium(V) insulin
mimetic coordination complex, dipicolinatooxovanadium(V),
VO2dipic-, has proven to have significant glucose-lowering
ability, though its rapid decomplexation and reduction in vivo
is unquestioned.20
Inorganic vanadyl and vanadate, as well as some vanadium-
(IV) and vanadium(V) complexes, enhance insulin action when
administered orally.7-12 The first chelated vanadium complex
designed and tested chronically in vivo as an insulin mimetic
agent (bis(maltolato)oxovanadium(IV), BMOV) has been thor-
The existence of strongly reductive V(III) compounds at high
concentrations (up to 1 M) in marine organisms, e.g., sea squirts
such as Ascidia ceratodes and A. nigra,21 has engendered
considerable speculation about the type of coordination environ-
ment that would be adequately protective of this oxidation state
in an aerobic, predominately neutral pH medium.22,23 Strongly
* To whom correspondence should be addressed. Tel: 604-822-4449.
Fax: 604-822-2847. E-mail: orvig@chem.ubc.ca.
† Department of Chemistry.
‡ Faculty of Pharmaceutical Sciences.
(1) Butler, A.; Carrano, C. J. Coord. Chem. ReV. 1991, 109, 61.
(2) Rehder, D. Angew. Chem., Int. Ed. Engl. 1991, 30, 148.
(3) Orvig, C.; Thompson, K. H.; Cam, M. C.; McNeill, J. H. In Uses of
Inorganic Chemistry in Medicine; Farrell, N. D., Ed.; Royal Society
of Chemistry: Cambridge, 1999; p 93.
(4) Mabbs, F. E.; Machin, D. J. Magnetism and Transition Metal
Complexes; Chapman and Hall: London, 1961; p 5.
(5) Soulti, K. D.; Troganis, A.; Papioannou, A.; Kabanos, T. A.;
Keramidas, A. D.; Deligiannakis, Y. G.; Raptopoulou, C. P.; Terzis,
A. Inorg. Chem. 1998, 37, 6785.
(6) Wever, R.; Kustin, K. AdV. Inorg. Chem. 1990, 35, 81.
(7) Heyliger, C. E.; Tahiliani, A. G.; McNeill, J. H. Science 1985, 227,
1474.
(8) Ramanadham, S.; Mongold, J. J.; Brownsey, R. W.; Cros, G. H.;
McNeill, J. H. Am. J. Physiol. 1989, 257, H904.
(9) McNeill, J. H.; Yuen, V. G.; Hoveyda, H. R.; Orvig, C. J. Med. Chem.
1992, 35, 1489.
(13) Caravan, P.; Gelmini, L.; Glover, N.; Herring, F. G.; Li, H.; McNeill,
J. H.; Rettig, S. J.; Setyawati, I. A.; Shuter, E.; Sun, Y.; Tracey, A.
S.; Yuen, V. G.; Orvig, C. J. Am. Chem. Soc. 1995, 117, 12759.
(14) Hanson, G. R.; Sun, Y.; Orvig, C. Inorg. Chem. 1996, 35, 6507.
(15) Yuen, V. G.; Orvig, C.; McNeill, J. H. Can. J. Physiol. Pharmacol.
1993, 71, 263.
(16) Yuen, V. G.; Pederson, R. A.; Dai, S.; Orvig, C.; McNeill, J. H. Can.
J. Physiol. Pharmacol. 1996, 74, 1001.
(17) Reul, B. A.; Amin, S. S.; Buchet, J.-P.; Ongemba, L. N.; Crans, D.
C.; Brichard, S. M. Brit. J. Pharmacol. 1999, 126, 467.
(18) Amin, S. S.; Cryer, K.; Zhang, B.; Dutta, S. K.; Eaton, S. S.; Anderson,
O. P.; Miller, S. M.; Reul, B. A.; Brichard, S. M.; Crans, D. C. Inorg.
Chem. 2000, 39, 406.
(19) Yuen, V. G.; Caravan, P.; Gelmini, L.; Glover, N.; McNeill, J. H.;
Setyawati, I. A.; Shuter, E.; Zhou, Y.; Orvig, C. J. Inorg. Biochem.
1997, 68, 109.
(10) Woo, L. C. Y.; Yuen, V. G.; Thompson, K. H.; McNeill, J. H.; Orvig,
C. J. Inorg. Biochem. 1999, 76, 251.
(11) Melchior, M.; Thompson, K. H.; Jong, J. J.; Rettig, S. J.; Shuter, E.;
Yuen, V. G.; Zhou, Y.; McNeill, J. H.; Orvig, C. Inorg. Chem. 1999,
38, 2288.
(20) Crans, D. C.; Yang, L.; Jakusch, T.; Kiss, T. Inorg. Chem. 2000, 39,
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10.1021/ic000984t CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/25/2001

Insulin-Enhancing Vanadium(III) Complexes
Inorganic Chemistry, Vol. 40, No. 18, 2001 4687
chelating agents such as catecholates23 or Schiff bases such as
salicylideneiminate (SALEN)24,25 have been used successfully
in the synthesis of relatively stable V(III) complexes and may
yield useful clues to this conundrum.22,26
potential as insulin-enhancing agents. Oxovanadium(IV) and (V)
complexes of these ligands have been characterized chemically
and biologically previously; however, this is the first report of
V(III) maltol (and analogues) as candidate antidiabetic agents.
As with other metal chelates of these ligands, we anticipated
reasonable hydrolytic and thermodynamic stability; the relative
air stability of V(ma)3 was an unexpected advantage.
For the design of vanadium complexes appropriate for use
as insulin-enhancing agents,12 ligands of intermediate binding
strength are likely more appropriate. Possessing neither high
intrinsic bioactivity nor appreciable toxicity, maltol (Hma, a food
additive approved in many countries) is an excellent spectator
ligand in biological applications; Hma forms stable neutrally
charged metal complexes with an optimum combination of water
solubility, reasonable hydrolytic stability, and significant lipo-
philicity.27,28 Ligands related to maltol include ethylmaltol
(Hema), also a food additive, kojic acid (Hkoj), and Hdpp (1,2-
dimethyl-3-hydroxy-4-pyridinone); all can be used to alter
selectively the water solubility, hydrolytic stability, and lipo-
philicity of a metal complex.29,30
Experimental Section
Materials. All chemicals were reagent grade and were used as
received without further purification: VOSO4‚3H2O (Aldrich), ethyl-
maltol (Pfizer), maltol (Pfizer), kojic acid (Lancaster), sodium dithionite
(Fisher), 40% aqueous methylamine (Aldrich). Water was distilled
(Barnstead D8902 and D8904 Cartridges) and deionized (Corning MP-1
Megapure still) before use.
Instrumentation. Infrared spectra were recorded as KBr disks in
the range 4000-600 cm-1 on a Mattson Galaxy 5000 spectrophotometer
and referenced to polystyrene. Mass spectra were obtained with a Kratos
Concept II H32Q (Cs+, LSIMS) or a Kratos M50 (EIMS) spectrometer.
The UV-vis spectra of samples in 10 mm quartz cells were recorded
with a Shimadzu UV-2100 spectrophotometer equipped with a Julabo
UC circulating bath (25.0 ( 0.1 °C) or an HP 8453 spectrophotometer
connected to a Fisher ISOTEMP 1016D circulating bath (25.0 ( 0.1
°C). Analyses for C, H, and N were performed in this department by
Mr. Peter Borda. Room-temperature magnetic susceptibilities were
measured on a Johnson Matthey magnetic susceptibility balance;
diamagnetic corrections were based on Pascal’s constants.4
Syntheses of the Complexes. Tris(maltolato)vanadium(III), V(ma)3.
Vanadyl sulfate trihydrate (1.03 g, 4.7 mmol) and maltol (1.80 g, 14.3
mmol) were added to a small flask, which was subsequently purged
with Ar. Sodium dithionite (2.62 g, 15.0 mmol) was dissolved in 25
mL of degassed (Ar stream) water. This solution was filtered anaero-
bically such that the filtrate was collected in the reaction flask containing
the vanadyl sulfate and maltol. The black-colored reaction mixture was
stirred magnetically under a positive Ar pressure at 40 °C for 2 h, during
which time the color of the mixture changed to a dark red. The dark
red precipitate was collected by suction filtration and washed with a 3
× 20 mL portion of water followed by 3 × 20 mL portions of diethyl
ether. The powder was dried overnight in vacuo over P2O5 and stored
in an inert atmosphere glovebox. Yield: 1.39 g, 71% based on V. Anal.
Calcd (found) for C18H15O9V (426.55): C, 50.72 (50.43); H, 3.55 (3.63).
Mass spectrum (EI): m/z 426 (M+), 301 ([V(ma)2]+). IR (cm-1): 1606,
1571, 1507, 1464 (pyrone ring vibrations), 722 (νV-O). Magnetic
moment: µeff ) 2.7 BM.
The biological chemistry of ML3 complexes of these ligands
is not new. Many years ago, we ourselves introduced Al(ma)3
as an aluminum source for neurotoxicological studies.31,32 The
coordination chemistry of Fe(III) with 3-hydroxy-4-pyrones and
3-hydroxy-4-pyridinones has been explored,33 with the former
complexes having potential as iron supplementation agents34-36
and the latter ligands as orally active scavenging agents for iron
overload, particularly Hdpp (also known as L1 or defer-
iprone).37,38 VO(koj)2 has also been the subject of solution and
insulin-enhancing biological studies; it is insulin-mimetic but
is somewhat less potent than VO(ma)2.19 By contrast, cis-[VO2-
(ma)2]-, a dioxovanadium(V) analogue of BMOV was phar-
macologically less active than BMOV (each at 0.55 mmol
kg-1).19
In this paper, we examine V(III) ligation to several repre-
sentative hydroxypyrones and pyridinones and assess their
Tris(ethylmaltolato)vanadium(III), V(ema)3. a. [VO(ema)2(Hema)]‚
H2O. Vanadyl sulfate trihydrate (19.70 g, 90.72 mmol) and ethylmaltol
(25.43 g, 182.0 mmol) were suspended in 300 mL of water at 45 °C.
Following an induction period of 15-20 min, a fine yellow powder
precipitated rapidly, and the suspension (pH ∼1) was cooled to room
temperature. The solid was collected by filtration and air-dried (21.90
g, 43.51 mmol, 72% yield based on Hema). Anal. Calcd (found) for
C21H24O11V (503.36): C, 50.11 (50.09); H, 4.73 (4.81). Mass spectrum
(LSIMS): m/z 329 ([V(ema)2]+, 100%). IR (cm-1): 1634, 1581 (Hema
ring vibrations) 1596, 1562 (ema- ring vibrations), 971 (νVdO).
Magnetic moment: µeff ) 1.71 BM.
(24) Bonadies, J. A.; Butler, W. M.; Pecoraro, V. L.; Carrano, C. J. Inorg.
Chem. 1987, 26, 1218.
(25) Choudhary, N.; Hughes, D. L.; Klienkes, U.; Larkworthy, L. F.; Leigh,
G. J.; Maiwald, M.; Marmion, C. J.; Sanders, J. R.; Smith, G. W.;
Sudbrake, C. Polyhedron 1997, 16, 1517.
(26) Bonadies, J. A.; Pecoraro, V. L.; Carrano, C. J. J. Chem. Soc., Chem.
Commun. 1986, 1218.
(27) Clevette, D. J.; Orvig, C. Polyhedron 1990, 9, 151.
(28) Lord, S. J.; Epstein, N. A.; Paddock, R. L.; Vogels, C. M.; Hennigar,
T. L.; Zaworotko, M. J.; Taylor, N. J.; Driedzic, W. R.; Broderick, T.
L.; Westcott, S. A. Can. J. Chem. 1999, 77, 1249.
(29) Nelson, W. O.; Karpishin, T. B.; Rettig, S. J.; Orvig, C. Can. J. Chem.
1988, 66, 123.
b. Reduction of [VO(ema)2(Hema)]‚H2O. Yellow [VO(ema)2-
(Hema)]‚H2O (0.287 g, 0.573 mmol) was dissolved in 50 mL of water
at 65 °C, and excess sodium dithionite (0.200 g) was added under Ar.
A fine air-sensitive red powder precipitated; the reaction mixture was
left stirring overnight and allowed to cool to room temperature over
that period. The precipitate was collected by filtration and dried in vacuo
to yield 0.246 g (92% based on V). Anal. Calcd (found) for C21H21O9V
(468.33): C, 53.86 (53.31); H, 4.56 (4.46). IR (cm-1): 1600, 1567,
1502, 1470 (pyrone ring vibrations), 715 (νV-O). Mass spectrum
(LSIMS): m/z 329 ([V(ema)2]+, 100%). Magnetic moment: µeff ) 2.6
BM.
(30) Clevette, D. J.; Nelson, W. O.; Nordin, A.; Orvig, C.; Sjo¨berg, S.
Inorg. Chem. 1989, 28, 2079.
(31) Finnegan, M. M.; Rettig, S. J.; Orvig, C. J. Am. Chem. Soc. 1986,
108, 5033.
(32) Finnegan, M.; Lutz, T. G.; Nelson, W. O.; Smith, A.; Orvig, C. Inorg.
Chem. 1987, 26, 2171.
(33) Hider, R. C.; Hall, A. D. Prog. Med. Chem. 1991, 28, 42.
(34) Scarrow, R. C.; Riley, P. E.; Abu-Dari, K.; White, D. L.; Raymond,
K. N. Inorg. Chem. 1985, 24, 954.
(35) Taylor, D. M.. Kontoghiorghes, G. J. Inorg. Chim. Acta 1986, 125,
L35.
(36) Kontoghiorghes, G. J. Inorg. Chim. Acta 1987, 135, 145.
(37) Molenda, J. J.; Jones, M. M.; Cecil, K. M.; Basinger, M. A. Chem.
Res. Toxicol. 1994, 7, 815.
(38) Olivieri, N. F.; Brittenham, G. M.; McLaren, C. E.; Templeton, D.
M.; Cameron, R. G.; McClelland, R. A.; Burt, A. D.; Fleming, K. A.
N. Engl. J. Med. 1998, 339, 417.
Tris(kojato)vanadium(III) Monohydrate, V(koj)3‚H2O. Vanadyl
sulfate trihydrate (2.13 g, 9.81 mmol) and kojic acid (4.26 g, 29.9 mmol)
were dissolved with stirring in 50 mL of water at 55 °C under Ar.
Addition of excess sodium dithionite (5.40 g) yielded, upon cooling, a

4688 Inorganic Chemistry, Vol. 40, No. 18, 2001
Table 1. Selected Crystallographic Data for V(dpp)3‚12H2O
Melchior et al.
Table 2. Bond Lengths (Å) and Angles (deg) in V(dpp)3‚12H2O
with Estimated Standard Deviations in Parentheses
empirical formula
C21H48N3O18V
681.56
fw
Bond Lengths (Å)
space group
a, Å
P3h (No. 147)
16.6167(9)
6.8101(2)
1628.45(11)
2
1.390
3.82
15 267
3038
0.058
0.064
V(1)-O(1)
O(1)-C(2)
N(1)-C(1)
N(1)-C(6)
C(1)-C(7)
C(3)-C(4)
2.007(1)
1.355(2)
1.363(2)
1.470(3)
1.482(3)
1.412(3)
V(1)-O(2)
O(2)-C(3)
N(1)-C(5)
C(1)-C(2)
C(2)-C(3)
C(4)-C(5)
2.035(1)
1.294(2)
1.370(3)
1.390(3)
1.405(3)
1.359(3)
c, Å
V, Å3
Z
F
calc, g cm-3
µ(Mo KR), cm-1
total reflections
unique reflections
Ra
Bond Anglesa (deg)
91.30(6) O(1)-V(1)-O(2)
167.94(6) O(1)-V(1)-O(2)′′
91.64(6) V(1)-O(1)-C(2)
113.0(1)
121.8(9)
118.8(2)
122.5(2)
116.9(2)
117.4(2)
117.5(2)
121.8(2)
O(1)-V(1)-O(1)′
O(1)-V(1)-O(2)′
O(2)-V(1)-O(2)′
V(1)-O(2)-C(3)
C(1)-N(1)-C(6)
N(1)-C(1)-C(2)
C(2)-C(1)-C(7)
O(1)-C(2)-C(3)
O(2)-C(3)-C(2)
C(2)-C(3)-C(4)
N(1)-C(5)-C(4)
80.57(6)
97.74(6)
111.8(1)
120.8(2)
117.3(2)
118.7(2)
121.7(2)
121.4(2)
125.1(2)
119.6(2)
a
Rw
C(1)-N(1)-C(5)
C(5)-N(1)-C(6)
N(1)-C(1)-C(7)
O(1)-C(2)-C(1)
C(1)-C(2)-C(3)
O(2)-C(3)-C(4)
C(3)-C(4)-C(5)
a R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) (∑[w(Fo - Fc2)2]/∑[wFo ])1/2
.
2
4
moderately air-stable orange powder that was collected by filtration,
air-dried, and subsequently dried in vacuo to yield 3.60 g (75% based
on V). Anal. Calcd (found) for C18H15O12V‚H2O (492.27): C, 43.97
(43.92); H, 3.57 (3.48). IR (cm-1): 1613, 1556, 1514, 1471 (pyrone
ring vibrations), 758 (νV-O). Mass spectrum (LSIMS): m/z 333
([V(koj)2]+, 100%). Magnetic moment: µeff ) 2.7 BM.
a Symmetry operations: (′) -x + y, 1 - x, z; (′′) 1 - y, 1 + x - y,
z.
Tris(1,2-dimethyl-3-hydroxy-4-pyridinonato)vanadium(III) Dodeca-
hydrate, V(dpp)3‚12H2O. Vanadyl sulfate trihydrate (10.8 g, 49.9
mmol) and maltol (18.8 g, 149 mmol) were dissolved with stirring at
65 °C in 300 mL of water under Ar. Aqueous 40% methylamine (40
mL, 573 mmol) was added, at which time all reagents dissolved. Sodium
dithionite (10.0 g) was added as a solid, and the reaction mixture was
stirred overnight. The reaction was cooled to room temperature, and
the resulting brown crystalline solid was isolated by filtration and dried
in a stream of Ar gas to yield 15.6 g (45% based on V). Anal. Calcd
(found) for C21H24N3O6V‚12H2O (681.56): C, 37.01 (37.19); H, 7.10
(6.98); N, 6.17 (6.03). IR (cm-1): 1606, 1550, 1501, 1461 (pyridinone
ring vibrations), 706 (νV-O). Mass spectrum (LSIMS): m/z 327
([V(dpp)2]+, 100%). Magnetic moment: µeff ) 2.5 BM.
X-ray Crystallographic Analysis of V(dpp)3‚12H2O. All measure-
ments were performed on a Rigaku/ADSC CCD area detector with
graphite-monochromated Mo KR radiation. The data were collected at
a temperature of 180 ( 1 K to a maximum 2θ value of 60.1° in 0.50°
oscillations with 70.0 s exposures. A sweep of data was performed
using φ oscillations from 0.0 to 190.0° at ø ) -90°, and a second
sweep was performed using ω oscillations between -23.0 and 18.0°
at ø ) -90°. The structure was solved by direct methods39 and
expanded using Fourier techniques.40 The non-hydrogen atoms were
refined anisotropically; the hydrogen atoms were refined isotropically.
Neutral atom scattering factors, anomalous dispersion corrections, values
of ∆f and ∆f′′, and those for the mass attenuation coefficients were
taken from International Tables for X-ray Crystallography.41 All
calculations were performed using the teXsan crystallographic software
package.42 Selected crystallographic data are shown in Table 1 with
distances and angles displayed in Table 2.
water and allowed to oxidize under ambient air and moisture for 1
month, resulting in a mixture of yellow and blue-green solids. Allowing
the mixture to sit for a further 3 months gave only a blue-green solid
(VO(ema)2) which was collected by filtration and air-dried (0.920 g,
2.66 mmol, 93% yield based on V). The filtrate was strongly acidic
(pH ∼1).
For solution studies, maltol and either NaCl or KCl were weighed
and placed into a two-neck 25 mL flask. Both necks were sealed with
septa. The flask was thoroughly flushed with Ar. V(ma)3 was added
into the flask while under constant Ar flux to minimize any solid-state
oxidation of the complex. An 0.05 M buffer solution (either (1) pH
4.0-potassium biphthalate or (2) pH 7.4-N-(2-hydroxyethyl)pipera-
zine-N′-2-ethanesulfonic acid, Hepes) was prepared and vigorously
degassed with Ar. An aliquot (25.0 mL) of the buffer solution was
injected into the flask by syringe, and the resulting red-orange solution
was stirred magnetically until all reactants had dissolved. A quartz
cuvette was fitted with a septum and flushed thoroughly with Ar. To
the cuvette was added an aliquot of the reaction solution by syringe.
The cuvette was kept under Ar until the experiment was initiated. The
cuvette was inserted into a thermostated cell holder, and at experiment
initiation, the atmosphere in the cuvette was changed to 1 atm of O2.
The reaction was followed by monitoring the absorbance of the solution
at 400 nm.
Animal Studies. Male Wistar rats (Animal Care Unit, University
of British Columbia), weighing between 190 and 220 g, were cared
for in accordance with the principles and guidelines of the Canadian
Council on Animal Care. Experimental animals were randomly divided
into four groups: control (C), control-treated (CT), diabetic (D), and
diabetic-treated (DT) (n ) 5 for each group). Housing and feeding of
the animals, the induction of a hyperglycemic state through strepto-
zotocin (STZ) injection, and the monitoring of glucose levels were all
conducted as described previously.10,11,13 In experiments involving
administration of the appropriate complex by intraperitoneal (i.p.)
injection, D rats were given saline (0.9% w/w NaCl), whereas DT
animals were given a single i.p. dose of 0.1 mmol kg-1 of the
appropriate compound in saline. In the oral gavage experiments, D rats
were given 3% w/w gum arabic; DT were given a single dose of 0.6
mmol kg-1 of the appropriate compound suspended in 3% w/w gum
arabic. Glucose levels were determined immediately prior to admin-
istration of the compound and at 2, 4, 6, 8, 12, 16, 20, 24, 48, and 72
h following compound administration. A positive response to testing
was taken as blood glucose lowering to <9 mM within the first 24 h
following compound administration. Values are presented as means (
SEM. Data from the acute studies were analyzed using GLM repeated
measures ANOVA, with a significance level taken as p < 0.05.
Oxidative Stability Analysis. Red V(ma)3 (10.0 g, 23.5 mmol) was
suspended in 10 mL of water and allowed to oxidize under ambient
air and moisture for 1 month, with the resulting gray precipitate
collected by filtration and air-dried (0.359 g, 1.13 mmol, (VO(ma)2)
97% yield). The filtrate was strongly acidic (pH ∼1). Similarly,
intensely red V(ema)3 (1.34 g, 2.86 mmol) was suspended in 10 mL of
(39) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A. J. Appl.
Crystallogr. 1993, 26, 343.
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Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-94 program system;
University of Nijmegen: The Netherlands, 1994.
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lography; Wilson, A. J. C., Ed., Vol. IV and C; Kynoch Press:
Birmingham, U.K.; present distributor Kluwer Academic Pulishers:
Dordrecht: The Netherlands, 1974 and 1992; pp 99-102, 200 and
219.
(42) teXsan Crystal Structure Analysis Package; Molecular Structure
Corporation, 1985 and 1992.

Insulin-Enhancing Vanadium(III) Complexes
Inorganic Chemistry, Vol. 40, No. 18, 2001 4689
not O2, with V(V) the major oxidant accounting for the nonlinear
nature of the reaction kinetics.
Oxidations of V(ma)3 and V(ema)3 are slow under acidic
conditions, each having a lifetime of approximately 16 h at pH
4.0 under 1 atm of p(O2). These lifetimes decrease with
increasing basicity; at pH 7.4, V(ma)3 was shown to have a
lifetime of 2 h. Lifetimes are also greatly reduced in organic
solvents such as methanol, in which V(ma)3 and V(ema)3 are
least stable, possessing lifetimes on the order of only 10 s at
p(O2) ) 1 atm. Intermediate lifetimes are seen in increasingly
basic conditions. The V(III) complexes V(ma)3, V(ema)3,
V(koj)3‚H2O, and V(dpp)3‚12H2O are all ultimately sensitive
to oxidation in the solid state. Because the oxidation of V(ma)3,
the most oxidatively stable of the vanadium(III) complexes, is
relatively slow (particularly at the acidic pH range of the
stomach) and is characterized by an induction period in which
no appreciable oxidation occurs, it was deemed most suitable
for biological testing (vide infra).
Significant lowering of the pyrone (∼40 cm-1) and pyridinone
(∼20 cm-1) ring stretching frequencies, inseparable combina-
tions of the ligand carbonyl and high energy ring vibrations,46
were observed when the infrared spectra of the uncomplexed
heterocycles, and their analogous V(III) complexes were
compared. The V(IV) precursor of V(ema)3, formulated as [VO-
(ema)2(Hema)]‚H2O, had pyrone ring vibrations attributable to
deprotonated ema- (with a low energy shift of 40 cm-1 from
Hema) as well as pyrone ring vibrations intermediate in energy
between those of Hema and V(ema)3, suggestive of neutral
pyrone coordination (with a 10 cm-1 reduction from values for
Hema). The infrared spectra of the V(III) complexes, as
expected, were devoid of any peaks attributable to vanadyl νVdO
stretching frequencies.
The positive ion detection mode LSIMS spectra of all the
V(III) complexes were dominated by a 100% relative intensity
ion with m/z characteristic of [VL2]+, with the parent molecular
mass, the [VL3]+ ion, appearing as a much less intense peak.
Higher molecular-weight ions, including [V2L5]+, were par-
ticularly intense and diagnostic; the [V2L5]+ ion has been shown
in previous work to be characteristic of trivalent metal ion tris-
(oxypyridinonato) and -(oxypyronato) complexes.30,31,47-49 Subtle
distinctions were found between the mass spectra of the
V-pyrone complexes in different oxidation states. The mass
spectra of V(ema)3 and its V(IV) precursor differ only in the
intensity of key ions: for example, the [HVO(ema)2]+ ion is
significantly less intense in the mass spectrum of V(ema)3 than
in the spectrum of [VO(ema)2(Hema)]‚H2O.
Figure 1. Oxidation of V(ma)3 in the presence of excess Hma: (a)
[V(ma)3]total ) 1.77 mM, [Hma]total ) 35.5 mM, p(O2) ) 1 atm, [KCl]
) 1 M, [KPTH] ) 0.05 M, pH 4.0, 298 K, λ ) 400 nm; (b) [V(ma)3]total
) 1.87 mM, [Hma]total ) 36.9 mM, p(O2) ) 1 atm, [NaCl] ) 1 M,
[Hepes] ) 0.05 M, pH 7.35, 298 K, λ ) 400 nm.
Results and Discussion
New V(III) complexessV(ma)3, V(ema)3, V(koj)3‚H2O, and
V(dpp)3‚12H2Oswere synthesized on a large scale by dithionite
reduction of an aqueous vanadyl complex: a convenient method
to access V(III) complexes in aqueous solutions that has been
employed previously in the facile synthesis of vanadium(III)
diketonates.43 The “one-pot” synthesis of the 3-hydroxy-4-
pyridinone complex V(dpp)3‚12H2O from maltol, methylamine,
dithionite, and vanadyl was adapted from the previously reported
syntheses of Al(dpp)3‚12H2O and Ga(dpp)3‚12H2O.44 The
synthesis of V(ema)3 was accomplished through the dithionite
reduction of an insoluble V(IV) precursor formulated, based
on the available data, as [VO(ema)2(Hema)]‚H2O. Unlike
V(pic)3‚H2O but similar to V(acac)3,43,45 V(ma)3, V(ema)3,
V(koj)3‚H2O, and V(dpp)3‚12H2O are all air-sensitive; however,
the degree of air sensitivity varies considerably from that of
V(ema)3 which is readily decomposed in air, to V(ma)3, the
most air-stable, which only slowly generates VO(ma)2 in
solution over extended periods.
V(dpp)3‚12H2O crystallizes from saturated aqueous solutions
at 4 °C as large brown crystals which are robust, but not
indefinitely stable to solvent loss and oxidation. Key bond
lengths and angles are summarized in Table 2. V(dpp)3‚12H2O
is isomorphous with the previously reported exoclathrate
dodecahydrates of other metal ions: Al(dpp)3‚12H2O,48 Ga-
(dpp)3‚12H2O,48 In(dpp)3‚12H2O,49 and Fe(dpp)3‚12H2O.50,51
The structure consists of V(dpp)3, a distorted octahedron of three
planar 1,2-dimethyl-3-oxy-4-pyridinonato ligands (Figure 2),
In aqueous solution, the oxidations of two of the vanadium-
(III) complexes were probed spectrophotometrically and proved
to be complicated and nonlinear. Although the oxidation could
not be modeled using any simple mathematical treatment, the
lifetimes of the V(III) complexes could be estimated from
inflection points in the absorbance vs time graphs, as the
oxidations are typically biphasic (Figure 1). The first phase
corresponds to the oxidation of V(III) to V(IV); the second phase
corresponds to the oxidation of V(IV) to V(V). V(III), a strong
reductant, and V(V), a moderate oxidant, react together rapidly.
It is likely that the major oxidant of V(III) is in fact V(V) and
(46) Cox, H. R.; Bothner-By, A. A. J. Phys. Chem. 1969, 73, 2465.
(47) Nelson, W. O.; Rettig, S. J.; Orvig, C. J. Am. Chem. Soc. 1987, 109,
4121.
(48) Nelson, W. O.; Karphishin, T. B.; Rettig, S. J.; Orvig, C. Inorg. Chem.
1988, 27, 1045.
(49) Matsuba, C. A.; Nelson, W. O.; Rettig, S. J.; Orvig, C. Inorg. Chem.
1988 27, 3935.
(50) Charalambous, J.; Dodd, A.; McPartlin, M.; Matondo, S. O. C.;
Pathirana, N. D.; Powell, H. R. Polyhedron 1988, 7, 2235.
(51) Clarke, E. T.; Martell, A. E.; Reibenspies, J. Inorg. Chim. Acta 1992,
196, 177.
(43) Dilli, S.; Patsalides, E. Aust. J. Chem. 1976, 29, 2389.
(44) Zhang, Z.; Hui, T. L. H.; Orvig, C. Can. J. Chem. 1989, 67, 1708.
(45) Morgan, G. T.; Moss, H. W. J. Chem. Soc. 1913, 78.

4690 Inorganic Chemistry, Vol. 40, No. 18, 2001
Melchior et al.
mM; for V(ema)3,10.5 ( 1.8 mM; and for VO(ma)2, 8.2 ( 3.0
mM, which were all significantly lower than untreated diabetic
rat (D) plasma glucose levels (>20 mM). Positive response
(defined as [glucose] < 9 mM within 24 h of injection) was
seen in 3 of 5 rats treated with V(ma)3, none of the rats treated
with V(ema)3, and 4 of 5 rats treated with VO(ma)2. Neither
V(koj)3‚H2O ([glucose]24h ) 19.7 ( 0.4 mM) nor V(dpp)3‚
12H2O ([glucose]24h ) 19.9 ( 0.9 mM) proved active as insulin-
enhancing agents; animals treated with these compounds had
no appreciable glucose-lowering compared to D ([glucose]24h
) 20.9 ( 0.7 mM). There were no fatalities, and the only
observable toxicity manifested as mild gastrointestinal distress;
however V(dpp)3‚12H2O produced severe diarrhea.
The glucose-lowering abilities of V(ma)3, V(ema)3, and VO-
(ma)2, when administered by oral gavage, were also examined.
V(koj)3‚H2O and V(dpp)3‚12H2O were excluded from the oral
gavage studies because of their poor performance in the i.p.
experiments. Both V(ma)3 ([glucose] ) 14.5 ( 3.2 mM) and
VO(ma)2 ([glucose] ) 15.7 ( 2.2 mM) resulted in comparable
glucose-lowering 24 h after administration by oral gavage; the
glucose levels were significantly different from D ([glucose]
) 19.0 ( 0.3). V(ema)3 (n ) 5, 19.0 ( 3.2 mM) proved
inactive. There were also no fatalities in these groups.
Figure 2. Structure of the metal complex in V(dpp)3‚12H2O showing
the crystallographic numbering; thermal ellipsoids for non-hydrogen
atoms are drawn at 33% probability.
Previous comparisons of biodistribution53 and pharmacology54
of V(III), V(IV), and V(V) compounds in vivo suggest that all
are handled similarly in terms of tissue uptake and anticipated
toxicity. Nonetheless, because V(III) complexes are known to
be notoriously unstable to oxidation at pH >3, further pursuit
of these compounds as pharmaceutical agents was generally
considered likely to be futile. This did not turn out to be the
case. Treatment with either V(ma)3 and VO(ma)2 resulted in
significant glucose lowering in STZ-diabetic rats of comparable
magnitudes, both when administered by i.p. injection or by oral
gavage, with no overt toxicity other than mild gastrointestinal
distress and no fatalities.
Concluding Remarks
V(ma)3, V(koj)3, V(ema)3, and V(dpp)3‚12H2O have been
synthesized in high-yield large-scale preparations. All were
screened for insulin-enhancing potential by i.p. injection in STZ-
diabetic rats, and biologically effective compounds were then
also tested for oral availability. Despite the fact that prior art
teaches against it, V(III) compounds proved as equally effective
as many V(IV) complexes in terms of insulin-enhancing
behavior. Speciation and biodistribution studies are currently
underway for these novel insulin-enhancing V(III) complexes.
Figure 3. Packing diagram for V(dpp)3‚12H2O showing the hexagonal
channels of water molecules.
enclosed in an exoclathrate hydrate structure,47,49 in which the
complex is excluded from a hydrogen-bonded network provided
by 12 water molecules forming hexagonal channels (Figure 3).
In V(dpp)3‚12H2O, there is a compression of the octahedron
along the trigonal axis; the compression is seen in a large
exocyclic angle O(1)-V(1)-O(2) of 97.74(6)° as well as in a
relatively small bite angle O(1)-V(1)-O(2) of 80.57(6)°,
characteristics which are shared by homologues such as
Fe(dpp)3‚12H2O.50.51 The bond lengths, 2.007(1) Å for V(1)-
O(1) and 2.035(1) Å for V(1)-O(2), and the bite angle, 80.57-
(6)°, in this complex are well within the range of those in the
Acknowledgment. This work was supported by the Medical
Research Council (now CIHR) of Canada and by the Science
Council of British Columbia (GREAT program, B.D.L.). M.M.
acknowledges NSERC (Natural Sciences and Engineering
Research Council) of Canada and the University of British
Columbia for postgraduate fellowships. C.O. acknowledges the
Alexander von Humboldt Foundation for a Research Award and
Prof. Dr. F. E. Hahn for his hospitality in 1999-2000.
V(III) catecholates, including [V(cat)3]3- 23 and [V(trencam)]3-
,
which contains a macrocyclic tricatecholate.52 [V(cat)3]3- and
[V(trencam)]3- possess, respectively, mean V-O bond lengths
of 2.013(9) Å and 1.996(7) Å as well as mean bite angles of
81.3(9)° and 80.75(8)°.23,52
Supporting Information Available: An X-ray crystallographic file
in CIF format for the structure determination of V(dpp)3‚12H2O. This
material is available free of charge via the Internet at http://pubs.acs.org.
The glucose lowering characteristics of V(ma)3, V(ema)3,
V(koj)3‚H2O, and V(dpp)3‚12H2O as well as VO(ma)2 (BMOV,
for purposes of comparison) were determined following intra-
peritoneal (i.p.) administration to STZ-diabetic rats. The glucose
levels at 24 h after i.p. injection were for V(ma)3, 12.3 ( 3.7
IC000984T
(53) Sabbioni, E.; Marafante, E.; Amantini, L.; Ubertalli, L.; Birattari, C.;
Bioinorg. Chem. 1978, 8, 503.
(52) Bulls, A. R.; Pippin, C. G.; Hahn, F. E.; Raymond, K. N. J. Am. Chem.
Soc. 1990, 112, 2627.
(54) Venugopal, B.; Luckey, T. D. Metal Toxicity in Mammals, Vol. 2;
Plenum Press: New York, 1978; p 220.