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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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