mixed-ligand peroxo complexes of vanadium containing 2-thiouracil and its 6-methyl derivative
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This article was downloaded by: [University of Wyoming Libraries]On: 16 September 2013, At: 19:09Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK
Synthesis and Reactivity inInorganic and Metal-OrganicChemistryPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lsrt19
Mixed-Ligand PeroxoComplexes of VanadiumContaining 2-Thiouracil and its6-methyl DerivativeAsh R. Sarkar a & Shipra Mandal aa Department of Chemistry, University of Kalyani,Kalyani, 741 235, West Bengal, IndiaPublished online: 23 Apr 2008.
To cite this article: Ash R. Sarkar & Shipra Mandal (2000) Mixed-Ligand PeroxoComplexes of Vanadium Containing 2-Thiouracil and its 6-methyl Derivative, Synthesisand Reactivity in Inorganic and Metal-Organic Chemistry, 30:8, 1477-1488, DOI:10.1080/00945710009351847
To link to this article: http://dx.doi.org/10.1080/00945710009351847
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SYNTlI. REACT. INORG. MET.-ORG. CHEM., 30(8), 1477 1488 (2000)
MIXED-LIGAND PEROXO COMPLEXES OF VANADIUM CONTAINING
2-THIOURACIL AND ITS 6-METHYL DERIVATIVE
Asit R. Sarkar* and Shipra M a n 4
Department of Chemistry, University of Kalyani, Kalyani - 741 235, West Bengal, India
Four novel complexes of vanadium have been isolated fiom aqueous solutions
containing vanadate, peroxide and the thiouracils. The complexes have been
characterized and formulated as [VO(O@IL)] and K[VO(O?)(L)] (H2L = 2-thiouracil
or 6-methyl-2-thiouracil) on the basis of their elemental analyses, conductances, TGA
studies, cyclic voltammograms, and lR, electronic and Nh4R spectra. The 1R spectra
indicate coordination through N(3) and the S atoms of the thiouracils, which is
further confirmed by the I3C NMR spectra of the complexes. The complexes are less
stable with respect to reduction than other VO; complexes as revealed fiom the
cyclic voltammetry studies.
lNTRODUCTION
The recognition of the biological role of vanadium as a trace element has
increased considerably over the past decade'*2. Besides, the potent biological activities
Copyright 0 2000 by Marcel Dekker, Inc.
1477
www.dekker.com
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1478 SARKAR AND MANDAL
of vanadium peroxo compounds in biological systems have increased the interest in
the coordination chemistry of v a n a d i ~ m ~ - ~ . The use of vanadium peroxo compounds as
antidiabetic agents in humans contributed to the interest in vanadium coordination
chemistry '. There have been extensive studies on mixed-ligand peroxo vanadium complexes
containing N.N-, N,O- and 0,O-chelating l igand~~.~ . Nucleic acid bases such as uracil,
cytosine and thiouracil possess N, 0 and S donor ligands. But there is no report on
mixed-ligand peroxo complexes of vanadium containing the said ligands though they
themselves are of biological interest'. The 2-thiouracil [2,3-dihydro-2-thioxo-( 1H)-
pyrimidine-4-oneI (H2tuc) is a nucleic acid constituent base possessing therapeutic
activitj. It exists in the thione form ( la) in Fig. 1, with two detachable protons". The
values of p k l and p& at 25O C are 7.5 and 12.7, respectively". The loss of the
proton kom N(1) or N(3) will lead to a structure with electron delocalisation12
between N(I)-C(2)-S (Ic) or N(3)-C(2)-S (le), respectively. The importance of the
tautomeric structures ( lb) and (Id) is also established from structural studies".". The
dianion of thiouracil coordinates to Co(ll1) through'' N(3) and S. But in a binuclear
mercury complex, thiouracil binds'6 one mercury unit through N( 1) and S and the other
through N(3). Thus, though thiouracil is a ligand of biochemical importance, the
stereochemistry of the thiouracilato complexes has not been W y characterized. Very
recently a mixed-ligand oxoperoxo tartarate complex has been reported17.
In a continuation of our work on the interaction of nucleic acid bases with metal
, we report here the preparation and characterization of four novel complexes,
[ VO( Oz)(Ht uc)], [ VO( 02)( 6-Me-Htuc)], K[ V0(02)(tuc)] and K[VO( 02)(6-Me-
ions 18. I9
I Ituc)] .
EXPERIMENTAL
Vanadium pentoxide was obtained &om Sigma, USA; 2-thiouracil and 6-methyl-
2-thiouracil were obtained fiom Aldrich Chemical Company, USA. Other chemicals
used were of analytical grade. 2-Thiouracil is resistant to oxidation by H202 under the experimental conditions.
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PEROXO COMPLEXES OF VANADIUM 1479
H ( 1 4 (Ib) (1c) ( 1 4 (le)
[R = H, 2-thiouracil (Hztuc); R = CH3, 6-Me-2-thiouracil(6-Me-H2tuc)]
Prevaration of the Comvlexes
IVO(OII(Htuc)l (1). A quantity of 0.90 g (4.9 mmol) of V2O5 was dissolved in
1.2 g (21.4 mmol) of KOH in 20 mL of water. A clear, pale-green solution was
obtained. The solution was cooled to 5" C. Previously cooled (5" C) 30 % HzOz (2.5
mL, 22.0 m o l ) was added to the above solution. The color of the resulting solution
changed to yellow. Then 1.3 g (10.1 mmol) of 2-thiouracil dissolved in water (650
mL) was added gradually to the above solution. The color of the resulting solution
became intensely yellow. The volume of the solution was reduced by slow evaporation
to almost half Yellow crystals which appeared on cooling were isolated by filtration
and dried in vucuo. The yield was 1.07 g (48 %). Decomposition temperature: 130" C.
Anal. Found: V, 22.81; C, 20.99; H, 1.71; N, 12.42; S, 14.02 %. Calculated for
VCdHsN20S(FW226.1)V,22.57;C,21.24;H, 1.33;N, 12.39;S, 14.16%.
IV0(02)16-Me-Htuc)l (2). The yellow compound was obtained in the same way
as adopted for the above compound (1). A quantity of 1.4 g (9.9 mmol) of 6-methyl-2-
thiouracil was used instead of 2-thiouracil. The yield was 1.25 g (52 %).
Decomposition temperature: 138" C. Anal. Found: V, 20.97; C, 24.68; H, 2.13; N,
11.51; S, 13.08 %. Calculated for V C ~ H ~ N Z O $ ~ (FW 240.l), V, 21.25; C, 25.00; H,
2.08;N, 11.66; S, 13.33 Yo.
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1480 SARKAR AND MANDAL
KTVO(Oz)(tuc)l (3). A quantity of 0.90 g (4.9 mmol) of V205 was dissolved in
1.2 g (21.4 mmol) of KOH in 20 mL of water. A clear, pale-green solution was
obtained. The solution was cooled to 5" C. Previously cooled (5" C) 30 % H202 (2.5
rnL, 22.0 m l ) was added to the above solution. The color of the resulting mixture
changed to yellow. Then 1.3 g (10.1 mmol) of 2-thiouracil dissolved in 0.60 g KOH
(10.7 mmol) in 75 mL of water was added gradually to the above yellow solution. The
resulting mixture was stirred for 30 minutes, and on cooling there appeared a yellow
precipitate which was separated and dried as above. The yield was 1.88 g (72 %).
Decomposition temperature: 168" C. Anal. Found: V, 19.60; C, 18.01; Ht 0.82; N,
10.67; S, 11.86 YO. Calculated for KVC4H2N204S (FW 264.1), V, 19.31; C, 18.17; H,
0.76; N, 10.60; S, 12.12 Yo.
KIVO(02)6-Me-tuc)l (4). The yellow compound was obtained in the same way
as the above compound (3). A quantity of 1.4 g (9.9 mmol) of 6-methyl-2-thiouracil
was used instead of 2-thiouracil; all other materials and condition remaining the same.
The yield was 1.79 g (65 %). Decomposition temperature: 180" C. Anal. Found: V,
18.16; C, 21.32; H, 1.71; N, 10.22; S, 11.63 %. Calculated for KVC5&N204S (FW
278.1):V, 18.34;C,21.57;H, 1.44;N, 10.07;S, 11.51 %.
Analvsis
Carbon, hydrogen, nitrogen and sulfur microanalyses were obtained from the
Indian Association for Cultivation of Science, Calcutta. Vanadium was determined2' by
titration with Fe(1I) solution in the presence of sulphonated diphenylamine as indicator,
after decomposing the sample with sodium peroxide. The peroxo content was
determined by titration of a fieshly prepared solution with potassium perrnanganate in
2 N sulfuric acid solution.
Phvsical Measurements
'H and "C NMR spectra were obtained with a JEOL FT-100 spectrometer at
100 and 25 MHz, respectively in (CD3)2SO with TMS as internal standard. The W-
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PEROXO COMPLEXES OF VANADlUM 1481
Visible spectra were recorded either in a solid state mull or in aqueous solution using a
Karl-Zeiss DMR-21 spectrophotometer, and the IR spectra were recorded in KBr
pellets by means of a 1330 Perkin-Elmer spectrophotometer. Conductivity
measurements were carried out with a PR 9500 Philips conductivity bridge.
Thermogravimetric analyses were carried out with a Derivatograph (System: F. Paulik,
J. Paulik, L. Erday, MOM, Budapest). About 100 mg of the finely powdered
substances were heated at a rate of 2" C per minute. Electrochemical studies were
performed with a model CV-27 Bioanalytical system (BAS, USA) electrochemical
apparatus. The three-electrode measurements were carried out under a purified
dinitrogen atmosphere in a gas-tight cell by using a BAS planar glassy carbon inlay
working electrode, and a platinum wire auxhry electrode. The reference electrode
consisted of an AglAgCl electrode in aqueous tetramethylammonium chloride with the
concentration adjusted to make the electrode potential zero versus SCE.
RESULTS AND DISCUSSION
Reactions of KV03 and H202 with 2-thiouracil (H2tuc) or 6-methyl-2-thiouracil
(6-Me-Hztuc) gave the complexes (1) - (4) as shown in the following equations:
KVO3 + H202 + Hztuc 3 [V0(02)(Htuc)] (1) + H 2 0 + KOH
KVO3 + H202 + 6-Me-H2tuc 3 [V0(02)(6-Me-Htuc)] (2) + H20 + KOH
KV03 + H202 + Hztuc (aqueous KOH) 3 K[VO(Oz)(tuc)] (3) + 2H20
KVO3 + H202 + 6-Me-Hztuc (aqueous KOH) 3 K[VO(Oz)(6-Me-tuc)] (4) + 2H20
AU of the complexes are yellow, non-hygroscopic and are stable at room temperature.
AU of the complexes are appreciably soluble in DMSO and CHsCN. The compounds
(3) and (4) containing the tuc2- ligand are soluble in water showing molar conductance
values at molar concentration of around 115 o h - ' cm' mol-', indicating the
presence of 1 : 1 electrolytes. The themgrams of the complexes show that they begin
to decompose around 130 to 180" C, and the decomposition takes place gradually
without the formation of any stable intermediate compounds.
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1482 SARKAR AND MANDAL
Hztuc
3086 s 2928 s
1686 vs
1628 m
1566 vs, br
1451 m 1395 m
1240 s
1215 vs
1073 rn
Table I
Assignment of Important IR Bands (cm-l)
(1)
3275 vs 3160 s 2980 m 2910 m
1655 vs
1625 m 1585 m 1520 s
1445 m
1265 vs
1205 m
1065 rn
960 vs
890 vs 875 vs
630 vs
(2)
1660 vs
1625 m 1590 m 1525 s
1450 m
1270 vs
1210 w
1060 rn
965 vs
895 vs
635 vs
(3)
1600 vs
1585 s
1430 s
1305 vs
1200 w
1060 rn
965 vs
885 vs
630 vs
(4)
16.10 vs
I590 s
1440 s
1310 vs
1210 rn
1070 rn
970 vs
890 vs
630 vs
(1) = [VO(Oz)(Htuc)]; (2) = [V0(0~)(6-Me-Htuc)l; (3) = K[VO(Ot)(tuc)I; (4) = K[V0(02)(6-Me-tuc)].
IR Data
All of the complexes show similar h i k e d spectra (Table I). Hztuc does not
absorb strongly2' below 1000 cm", and so the characteristic V=O and other peroxide
vibrations could be assigned without any difficulty. The characterktic strong
absorption bands around 960 cm-' and 890 cm-' may be assigned to V=O and 0-0 bondsz2. The very strong band around 630 cm-' may be assigned to asymetric V-02
vibrations. The bands of the V(O)(Oz) group are in the range characteristic f~?'*~'
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PEROXO COMPLEXES OF VANADIUM 1483
monoperoxo complexes of vanadium. The absorption due to the V=O vibration around
960 cm-' indicates a high A bond order of the vanadium-oxygen bond and also the
presence of a monomeric 0x0-vanadium species25. The absorption bands due to the
thouracil ligand have been assigned by comparing them with those of free 2-
thiouracil". The position of the band due to v(C=O) is shifted to lower fiequency in
all the complexes compared to ffee I12tuc. The strong bands at 1520 cm-' in the
complexes containing mono-deprotonated anions Htuc' or 6-Me-Htuc- are assigned
to p(N-H) vibrations. The absence of both stretching and bending N-I1 vibrations in
the complexes containing the dianion tuc2- or 6-Me-tuc2- suggests the absence of fiee
N-H bonds in the complexes as expected for the dianions. The position of the v(C=S)
band in the compkxes around 1210 cm-' changes insignificantly and indicates the
coordination of the ligands through S. The shifting of the strong band in 2-thiouracil at
1240 cm-' to both higher fiequency and intensity in all of the complexes suggests
chelation of the ligands through N( 1 or 3) and S; the ring electron density in the Htuc'
or tuc2-anions will increase due to chelation by N( 1 or 3) and S. The shift in the v(C0)
fiequency to lower wave numbers will be mainly due to N(3) coordination, and this
will be more pronounced in the case of the tuc2- ligand than the Htuc' ligand. Since the
shift of the v(C0) band at 1686 cm-' in Hztuc towards lower wave numbers is larger
for tuc2- than for Htuc- complexes, it can be concluded that the actual coordination in
both cases occurs through N(3) and S. The diflierence in the vibrational absorption
frequency due to the V=O band is not due to any structural changes of the
corresponding complexeJ6 but may be due to a packing effect in the solid state,
charges on the ligand, or the substituents. Hence both thiouracils act as chelates.
UV-Visible Spectra
The UV-visible spectra of the complexes show two bands having a similar pattern
characteristic of peroxo complexes2728. The low-intensity band around 325 nm (E =
5900 dm3 moT' cm") and the high-intensity band around 205 nm (E = 12800 dm' mol-'
cm-') may be assigned to the electron transition fiom the filled II' orbital of the peroxo
group to the vacant d orbitals of vanadium*'. The high-intensity peak around 205 nm
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1484 SARKAR AND MANDAL
?'able I1
'H and I3C Nh4R Spectral Data (6, ppm, in DMSO-4)
'H - 13c
Compound NH C(5)H C(6)H m H ~ ~ u c 12.36(br, s) 5.81(d, 7.6b,1H) 7.40(d, 7.6b, 1H) 141.12
1V0(02)(H~~C)l (~) 5.34(d, 6.4b, 1H) 7.28(d, 7.6b, 1H) 139.98
[V0(02)(6-Me-Htuc)] (2) 12.62(s, 1H) 5.42(s, 1H) 2.08(s, 3H') 150.5 I
K[vo(oz)(tuc)1(3) 5.88(d, 7.2b, 1H) 7.34(d, 7.4b, I H ) 151.36
K[VO(O,)(6-Me-tuc)](4) 5.26(s, 1H) 1.95(s, 3H") 163.70
(a = for CH2; s = singlet; d = doublet; b = J values in Hz)
is also expected to contain the n - ~ * transition of the thiouracil ligands and the n-d and
n'-d transitions fiom the thiouracil ligands to the metal ion2'. The s u b to vanadium
LMCT transition is exhibited at 325 nm30- The absence of d-d transitions in the visible
region also indicates the presence of vanadium in the complexes in its +5 oxidation
state.
'H and "C Nh4R Data
Selected 'H and "C NMR signals are given in Table 11. The assignments have been
made as reported in the l i t e r a t ~ e ~ ' . ~ ~ . The NH signal for the complex (1) containing
Htuc' is not observed, probably due to rapid H-D exchange. As expected, the complexes
containing tuc2- do not give any NH signal. The C(6) "C NMR signal in complex (3)
containing the tuc2- ligand (6 = 151.36 ppm) shifts downfield by about 11 ppm
compared to complex (1) containing the Htuc- ligand (6 = 139.98 ppm). Similar is the
case for complex (4) containing the 6-Me-tuc2- ligand (6 = 163.70 ppm) which shows
a downfield shift compared to that of the complex (2) (6 = 150.51 ppm) containing
the 6-Me-Htuc- ligand. Thus, the position of the C(6) NMR signal arises at the higher
field for the complexes containing the Htuc- or 6-Me-Htuc- ligands compared to
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PEROXO COMPLEXES OF VANADIUM
Table 111
1485
Anodic and Cathodic Peak Potentials of the Complexes
Complexes E,.(VI !L?m Eirr Wl
[V(O)(OzXHtuc)l(1) 0.84 -0.6 1 0.115
[ V( 0)( 02)( 6-Me-Htuc)] (2) 0.8 8 -0.68 0.100
K[VO(Oz)(tuc)l(3) 0.75 -0.67 0.040
K[VO(Oz)(6-Me-tuc)] (4) 0.78 -0.72 0.030
.....................................................................................................
complexes containing the tuc2- or 6-Me-tuc2- ligands. This may be explained by the
presence of the N(1)-H group which shields" the a-carbon atom at the C(6) position
in the complexes containing the Htuc' or the 6-Me-Htuc- ligands. Hence, the proton
must be present at the nitrogen atom at the N( 1) position and this conclusively proves
that both 2-thiouracil and 6-Me-2-thiouracil in their mono or dianionic forms act as
chelates coordinating through their S and N(3) donor atoms.
Electrochemical Studies
The anodic peak potential (EpJ and the cathodic peak potential (&) and the El, z
values for the complexes are given in Table 111. The scan rate was 100 mVs-'. The
complexes in dichloromethane solution show quasi-reversible redox behaviour
corresponding to the VvNN couple. The E I ~ values of the Htuc' complexes are in
general lower compared to the tucZ- complexes. These differences arise due to charge
differences and the effect of the substituents on the ligands. The potentials of the
VOgC-VOZt couple in the complexes containing tuc2- are significantly more positive
than those observed for vanadium(V) complexesg4, indicating that the complexes are
relatively unstable.
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1486 SARKAR AND MANDAL
0 0
K+
R = H or CHs
Fig. 2. Proposed Structure of the Complexes.
Conclusion
We can definitely conclude fiom the above studies that all of the complexes are
monomeric and the thiouracil ligand, irrespective of either mono deprotonated or di
deprotonated forms acts as a chelate coordinating through the sulfur on C(2) and
nitrogen on N(3). The methyl substituent at the C(6) position does not appreciably
change the properties of the complexes. The presence of V=O is certain and both the
peroxide and the thiouracil ligand occupy two coordination positions. The
coordination environment of the vanadium atom approximates a square-pyramidal
configuration with the ligand occupying the apical position and the four donor atoms,
two each from the peroxide and the thiouracil ligand, occupying the basal plane, as
proposed in Fig. 2.
ACKNOWLEDGEMENT
The authors are gratehl to Dr. P. K. Bhattacharya of the Indian lnstitute of
Chemical Biology, Calcutta, Dr. J. N. Bera and Dr. R. K. Biswas of the Indian lnstitute
of Science, Bangalore, and to Dr. P. S. Roy of North Bengal University for recording
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PEROXO COMPLEXES OF VANhDllJM 1487
the NMR spectra and cyclic voltammograms. The authors are also gratefid to the
Kalyani University Authorities for providing a fellowship to one of us (S. M.),
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Received 25August 1999 Referee I: D. E. Pennington Accepted: 10 May 2000 Referee 11: L. J. Boucher
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