Indian Journal of Chemistry Vol. 40A, April2001 , pp. 368-373

Synthesis and characterisation of platinum(II) complexes of 1-methyl-2-{ (arylazo )imidazoles} and their dioxolene derivatives

Goutam Kumar Rauth, Sanjib Pal, Debasis Das & Chittaranjan Sinha*

Department of Chemistry, The University of Burdwan, Burdwan, 713 104, India

Received 29 August 2000; revised 30 January 200 /

1-Methyl-2-(ary lazo)imidazoles, p-R-C6H4-N=N-C3H2 Nr 1-CI-13, (RaaiMe (1); R = H(a), Me(b), Cl (c) have been treated with K2PtCI4 to sy nthesise brown-red Pt(RaaiMe)CI2 (2) complexes. Addition of dioxolene in the presence of Et3N to CHClr MeOH so lution of Pt(RaaiMe)CI2 yield green coloured mixed complexes of the composition [Pt(RaaiMe)(O.O)] (0,0 = catecholate (cat) (3); 4-terr-butylcatecholate (tbcat), (4); 3,5-di-rert-butylcatecholate (dtbcat), (5): tetrac holorocate­cholate (tccat), (6) ). The complexes have been characterised by elementa l analyses. JR . UV-Vis-NIR and 1H NMR spectral data. The so lut ion electronic spectra ex hibit ligand-to- li gand charge transfer (LLCT) transition at red to NIR region: the po­sition and sy mmetry of the band depend on the substituent type on the di oxolene and arylazoimidazole frame s. This is qualitatively assigned as HOMO(dioxolene)~ LUMO(RaaiMe). Cyclic voltammogram shows consecutive ox idation of catechols to semiquinone and semiquinone to quinone and azo reductions. The difference in first ox idation and red ucti on potemial is linearl y related with LLCT transition energy. The comparison of the physical properties of present seri es of complexes with palladium(II )-analogues show the involvement of Pt(II )-dn orbita ls to stabili se the complexes.

R R

(i) (i i)

R

(iii)

We have focused our attention on functiona li sed tran­sition metal s coordinated to dioxolene moiety where functional group is n-acidic diimine/azoimine sys­tem1 ·5. Parallel development in platinum(ll) chemistry is scarce. The current research in the field of plati­num(ll) chemistry with N-donor ligands is in the de­sign of cisplatin analogues6

. We have been engaged for the last few years to design N,N'-donor ligands in azoimine family , the members in this group being ary lazopyridines (i)\ arylazoimidazoles (ii)4 and arylazopyrimidines (iii) 7

. Ary lazoimidazoles have drawn special attention because of the synthetic sim­plicity of the system and the biochemical ubiquity of imidazole8

. Platinum(Ir)-azoimidazoles are scarce9

and this has encouraged us to undertake this work . In this paper we wish to report the synthesis, spectral characterisation of dichloro-{ 1-methyl-2-(arylazo) imidazole }platinum(ll) complexes. The reactivity to­wards dioxolene of these complexes are also exam­ined and the ternary complexes are characterised by spectroscopic and electrochemical data.

Materials and Methods

The reagent H2PtC16, xH 20 was purchased from Arrora Matthey, Calcutta, India. K2PtC14 was pre­pared from the reported method 10. I -Methyl-2-(arylazo)imidazoles were synthesised according to published proecdure4·5. Pyrocatechols (H2cat), 3,5-di-

/ert-butylcatechol (H 2dtbcat) and tetrach lorocatechol (H2tccat) were obtained from Aldrich. 4-ter/­Butylcatechol (H 2tbcat) was purchased from Fluka. Catechols were purified before use by recrystallisa­tion from benzene. Dichloromethane and acetonitrile were further purified by distillation over P40 10 .

[Bu4N)[Cl04] was prepared according to the reported method4. Nitrogen gas was purified by bubbling through alkaline pyrogallol solution and cone. H2SO~.

Silica gel (60-120 mesh) for column chromatography was collected from SRL. Triethylamine and other chemicals and solvents used for the preparative work were of reagent grade and were used as received.

RAUTH et a/. :SYNTHESIS OF Pt(II) COMPLEXES OF SUBSTITUTED IMIDAZOLES 369

Table 1-Analytical data

Empirical Formul ae Found (Cnlcd.), %

c H N

C10H10N4PtCI2 (2a ) 26.6 2.3 12.3 (26.5 ) (2.2) ( 12.4)

c II H 11N4PtCI2 (2b) 28 .5 2.5 12. 1 (28 .3) (2.6) ( 12.0)

C 10H9N4PtCI3 (2c) 24.5 1.9 11.4 (24.7) ( 1.8) ( II. I)

C1r,H1 4N402Pt , 0.5 H20 (3a) 39.0 3.0 11.5 (38 .6) (3 .0) ( 11.3)

C1 7H1 6N~02Pt , 0.5 H20 (3b) 41.0 3.2 10.8 (39.9) (3 .3) ( 10.9)

C1 6H uN~02CIPt , 0.5 H20 (3c) 37.0 2.6 10.8 (36. 1) (2.6) ( 10.5)

C2oHn N402Pt. 0.5 H20 (4a) 43.1 3.9 10.1 (43.2) (4. 1) ( 10.1)

C21 H 2~N~02 Pt. 0.5 H20 (4b) 45.0 4.3 9.9 ( 44.4) (4.4) (9.9)

C20H21 N~02C IPt , 0.5 H20 (4c) 41.0 3.7 9.6 (40.8) (3.7) (9.5)

C2~H,oN~02 Pt, 0.5 H20 (Sa) 48.0 5. 1 9.5 (47.3) (5. 1) (9.2)

C2, H ,2N~02Pt , 0.5 H20 (Sb) 49.0 5.3 8.9 (48. 1) (5.3) (8 .9)

C2~H29N~02CI Pt , 0.5 H20 (Sc) 45.8 4 .7 8.6 (44 .8) (4.7) (8.7)

C 16H 10N~02CI~Pt. 0.5 H20 (6a) 30.3 1.7 9.0 (30.2) ( 1.7) (8.8)

C 17H 1 2N402CI~ Pt. 0.5 H20 (6b) 32.0 2.0 8.5 (3 1.4) (2.0) (8.6)

C1r,H9N~02CI , Pt , 0.5 H20 (6c) 29.4 1.5 8.6 (28.7) ( 1.5) (8.4)

IR spectra (KBr di sk, 4000-200 cm-1) were re­

corded on FfiR JASCO model 420; UV-Yis-NIR spectra were recorded on JASCO model V-570 UV­Vi s-NIR spectrophotometer; 1H NMR spectra were recorded on Brucker AC(F)200 and 300 MHz Ff­NMR spectrometers . Thermal studies were carried out by Shimadzu-TG40/DT40 thermometric balance. Electrochemical studies were performed on a com­puter-controlled EG&G PAR model 270 VER­SASTAT electrochemical instruments with Pt-di sk and GC elec trodes . All measurements were carried out under dinitrogen environment at 298 K with refer­ence to saturated calo mel e lec trode (SCE) in acetoni­trile. [Bu4N][CI04] was used as the supporting elec­tro lyte. The reported potential s are uncorrected for junction potential. Microanalytical data were obtained from a Perkin-Elmer 2400 CHN elemental analyser.

Preparation of dicltloro{ 1-methyl- 2-(p -tolylazo) imi­dazole}plalin llln(11), Pt(MeaaiMe)Cl2 ( 2b)

To MeCN-H20 (I : I, v/v; 40 cm3) solution of

K2PtCI4 (0.85 g, 2.05 mmol) , 1-methy l-2-(p-toly lazo)

imidazole (0.42 g, 2.10 mmol) in the same solvent ( 15 cm3

) was added dropwise and the mixture refluxed for 30 h. The brown precipitate was obtained on slow evaporation of the solvent. It was then filtered and washed with cold MeCN-H20 ( I : I , v/v; 3 x 5 em\ The dried mass was dissolved in minimum volu me of CH2CI 2 and chromatographed over si lica ge l column. The desired compound was eluted as red-brown band by C6H6-MeCN (2 : I , v/v) . Yield, 0.48 g, 50%.

Other complexes were prepared under identical conditions and the yield varied in the range 45-50%.

Preparation of cateclwlato{ 1-methyl-2-(p-tolylazo) imidazole}platinum(ll),[ Pt( MeaaiMe)( cat)} (3b ), 0.5 H20

Pt(MeaaiMe)C)z (2b) (0.345 g, 0.74 mmol) was dissolved in CHCb-MeOH mixture (I: I , v/v; 30 em' ) and the solution was degassed by bubbl ing N2 th rough it. To this solution pyrocatechol (0.090 g, 0 .82 mmol) in MeOH ( I 0 cm3

) was added slowly followed by tri­ethylamine (2 mmo l) under N2. The solution was stirred for I h and the colour was changed from brown-red to green and then N2 gas was bubbled slowly for another I h to reduce the solution volume one-third to that of its original vo lume. Dark green precipitate was filtered , washed with cold MeOH and dried in vacuo. The dried mass was di ssolved in a minimum volume CH2C)z and chromatographed over silica ge l column and the desired green band was el uted by 3:2 (v/v) C6H6-MeCN mixture. Evaporation of the solvent in vacuo afforded the pure crystalline product. The yield was 0 .208 g ( 41 % ) .

All other complexes were prepared by an identical procedure and the yields were varied in the range 40-55%.

Results and Discussion 1-Methy 1-2-(ary lazo )imidazoles (Raai Me, I ) are

N,N1-chelating li gands. They react with K2PtCI~ in MeCN-H20 mixture under refluxing condition for 30 h and afford brown-red complex Pt(RaaiMe)C I2 (2). Upon addition of Et3N to the mixture of 2 and dioxolene in CHCb-MeOH (I: I, v/v) under N2 changes the colour immediately from brown-red to green and the product separates on s low bubbling of N2 for a period of I h or more . Th e re moval of sol­vents and chromatographic purification have yielded green catecholato complexes . The reaction strategy and abbreviation of the complexes are g iven in Scheme 1. The complexes are diamagnetic , ESR si­lent and non-conducting. The composition of the

370 INDIAN J. C HEM., SEC A, APRIL 2001

R 1

R = H (a ), CH 3 (b), Cl (c)

(ii)

p = x = y = z, [Pt(Raa i Me)(c at)] 0.5 H20 ( 3); p = y = z = H,

x = But, [Pt (RaaiMe)( tbcat)] 0.5 H20 (It); p = y = Bu', x = z = H,

[ PI( RaaiMe)(dtbcat)] 0.5 H20 (~); p:x:y : z:CI, [Pt(RaaiM<')

( t cc at ) ] 0.5 H20 ( 6)

Schemt 1 . (i) Ml'CN-water(l :l , v/v), ref lux , 30 h

(ii) Catechols, Et3N in CHCI3 - MeOH, stirr, under

N2 atmosphere

complexes are supported by C,H,N analyses and the data are given in Table 1.

The infrared spectra of Pt(RaaiMe)CI 2 (2) exhibit a sharp band at 1380 -1390 cm-1 for N=N vibration, which is lowered by 30 cm-1 from that of free li gand values (ca 1410 cm-1

). In the catecholato complexes (3) - (6), the N=N stretch appears at I 345 - I 350 cm-1

and the reduction of the frequency may be attributed to the ex tensive dn ( Pt)~ 7t* (RaaiM e) back bonding or 3b1 (cat) ~ 7t*(RaaiMe) charge transfer2

-4

• The complex (2) give a strong stretch at 345 cm- 1 with a weak shoulder at 330 cm-1

• They are corresponding to two Pt-CI bond vibrations 12 in cis-PtC12 motif. In the catecholato complexes, these stretches di sappear and new bands appear at ca. 540-550 cm- 1 corresponding to v(Pt - 0) which supports catecholato binding. The appearance of a broad band centred at 3450 cm-1 sup­ports the presence of coordinated H20. Upon heating t e complexes at above 140°C, the green colour changes to brown-green and the IR spectra shows the disappearance of v (H20).

The absorption spectra of the complexes were re­corded in CHCI3 solution. The complexes Pt(RaaiMe)CI 2 (2) exhibit three absorption bands in the vis ibl e region ca. 415, 480 and 545 nm. The tran­sitions <400 nm are due to intraligand charge-transfer transi tions. The absorption spectra of the catecholato complexes (3) - (6) are entirely different from that of the parent complexes (2). The most s ignificant feature of the spectra is the appearance of a strong band with

19.0

E ~ 9.0

"' 7.0

3.0

, - , ' \

. r · ,'

/ / . ,- i / \_.···

/) - , ' .. . , , ·... . . \

\ -··· ·· ... ... . .

., 1.0 l.,---2::..:.:.:;-;S--~---;8;7;00.,--:~-~100JoO__o_.._.·-"' ... _ ;z(1Z00

.oo }..(nm) __..

Fig. 1- Eiectronic absorption spectra of [Pt(HaaiMe) (cat )] (- ): [Pt(HaaiMe) (tbcat)] (- - -); [Pt(HaaiMe) (dtbcat)] ( . . . . . ): and [Pt(HaaiMe) (tccat)] (- • -) in CH2CI2 at 298 K

a higher energy shoulder in the red to near-in frared region (Table 2, Fig. I) . The position of the band and its symmetry is dependent on the nature of the sub­stituent(s) in catechol and RaaiMe. In [Pt(MeaaiMe)(O,O)] (3b - 6b) the band moves to the shorter wavelength region while the reverse move­ment is observed in [Pt(CiaaiMe)(O,O)] (3c- 6c) with respect to [Pt(HaaiMe)(O,O)] (3a - 6a). Thi s is cer­tainly due to the effect of the substi tuent in RaaiMe; e lectron-donating-Me enhances the energy difference between the two states involved in the charge trans­ference process while electron-wi thdrawing -CI re­duces the gap compared to Haai Me. The wavelength movement of the absorption maxi ma fo r a particular RaaiM e in [Pt(RaaiMe)(O,O)] follows the order tccat < cat < tbcat < dtbcat. The transi tion is ass igned to ligand-to-ligand charge-transfer transi tion (LLCT) : HOMO(cat) ~ LUMO(RaaiM e). The electron­releasing effect of Bu' group expectedly increases the energy of HOMO. In the tccat, HOMO has lowest energy in the series because of electron-withdraw ing character of -CI groups. The LUMO is dominated by azoimidazole function. Thus the band originates from the charge transi tion from HOMO characteri sed by catechols to LUMO of hybrid functi on o f platinum(ll) and RaaiMe 3.4 .

The 1H NMR spectra of the complexes were re­corded in CDCI3 at 298 K. The signals are ass igned

RAUTH eta/. :SYNTHESIS OF Pt(II) COMPLEXES OF SUBSTITUTED IMIDAZOLES 371

Table 2-UV-vis spectralb and cyclic voltammetric datad

Compound" Amax(nm)(E, dm3 mor'cm' 1) Rcq/Rsq Rsq!Rq azo· /azo azo·2 I azo·

E1

112• E2

112• -E3

112• -E4

112. VLLcrg eY f>E h l /2t

V(f>Ep) V(E>Er) V(f>Ep) V(f>Ep) v (2a) 542(493}", 480( 1216), 412(3680) 0.411 1.05[

(65)

(2b) 543(308)<, 412(727), 420(2270) 0.423 1.03[

(70)

(2c) 549(600f. 483(1273), 412(4136) 0.396 0.945[ (75)

(3a) 81 0(6769), 630(3338)<, 405( 18703) 0.432 0.822c 0.795 l . li 1.532 1.227 ( 120) (90)

(4a) 853(7159), 635(3543)<, 405( 12265) 0.377 0.651c 0.835 1.29 1.455 1.212 (100) ( 120) (140)

(Sa) 929(6370), 665(2240)c. 466(260W 0.290 0.548e 1.042 1.34 1.336 1.332 ( 100) (100) ( 120)

(6a) 725(5644 ), 590{3512)<, 407{ 11390) 0.682c 0.564 0.889 1.71 2 ( 120) ( 140)

(3b) 780(9483), 618{4352)<, 417(18766) 0.424 0.804c 0.842 1.19[ 1.591 1.266 (90) ( 120)

(4b) 830(5843), 630(2782)<, 417( 11280) 0.342 0.629 0.888 1.33 1.495 1.230 (95) ( 100) (110) (140)

(Sb) 896(3798). 650(2476)c,421 (II 079) 0.264 0.526 0.892 1.40[ 1.385 1. 156 ( 100) (95) (120)

(6b) 71 8(3608), 585(2308)<, 421 (8416) 0.616c 0.644 1.032 1.728 ( 130) ( 120)

(3c) 827(4132), 640(2893)c, 411(11781) 0.451 0.750e 0.628 1.061 1.501 1.079 ( 100) ( 125) (115)

(4c) 888( 4993), 653(2 136)<, 408( I 0930) 0.434 0.668 0.795 1.142 1.397 1.229 ( 100) ( 100) (130) ( 140)

(Sc) 957(6826), 670(2479)<. 474(4996), 0.323 0.567 0.833 1.295 1.297 1.156 406( 1 I 123) (100) ( 100) ( 120) ( 140)

(6c) 741(1215), 598( 1001 f , 407(3273) 0.740c 0.740e 0.593 0.593 1.675 ( 140) ( 140)

" All the compounds gave sati sfactory C, H, N analyses; " solvent is dichloromethane, c shoulder, d working electrode Pt-di sk, auxil -iary electrode Pt-wire, reference electrode SCE, supporting electrolyte [Bu4 N][CI04](0. 1 M), so lute concentration - I o·' M, pot en-tials are expressed in V, E 112 = 0.5(E.,a + Epc). f>EP = (Epa- Epc), mY c Epa is anodic peak potential , V; r Epc is cathodic peak potential. Y; ~LLCT = 1241 /A. (n m) eV; ht,E 112 = (E 1

11z- E3112) Y.

on the basis of spin-spin interaction and changes there-of on substitution. The proton numbering pat­tern is shown in Scheme I . The 1-Me signal in the complexes 2 appears at 4.1 - 4.2 ppm. Imidazole 4-and 5- H appear as singlet at 7.3 and 7.2 ppm, respec­ti vely and are downfi eld shifted by >0.2 ppm com­pared to the free ligand values5

. Thi s is in support to the binding of imidazole-N to Pt(ll) . Thi s is again supported by the coupling of 4-H with 195 Pt and the coupling constant is J= 20-22 Hz. The aryl protons (8-H - 12-H) are also downfield shifted on coordination to metal centre relative to the ligand values and are moved in usual manner4

• In the catecholato complexes (3) - (6) the RaaiMe protons, in general , suffer sig­nificant upfield shifting compared to that of Pt(RaaiMe)Ch by 0.1 - 0 .2 ppm. The upfield shift is

owing to the charge delocali sation from HOMO(cat) to LUMO(RaaiMe)4

• Only exception is the presence of a doublet at down fie ld, 7.9 - 8.4 ppm. This is as­signed to 8-H and may be accounted by considering the asymmetric orientation of the pendant aryl ring of the coordinated RaaiMe around the metal ion. This magnetically differentiates the 8-H and 12-H, whereas 8-H is considered stereochemically nearer to the metal centre5

. The coordination of RaaiMe to Pt(II) is again supported by the coupling of 4-H with 195 Pt (I = Y2, abundance 33 .7%). The coupling constant 3Jr,.H lies between 20 - 25 Hz9

·13

. The catecholato ring protons appear in the upfield side in the spectra and is ex­pected due to electron releasing effect of catecholato oxygens. The Bu'-group appears at ca 1.30 ppm 111

[Pt(RaaiMe)(tbcat)] and ca 1.3, 1.4 ppm m

372 INDI AN 1. CHEM., SEC A, APRIL 200 1

[Pt(RaaiMe)(dtbcat)] complexes . Other protons in catechols ring appear in the usual manner3

.4 .

Thermal study of five complexes, Pt(RaaiMe)Ch (2b) and Pt(RaaiMe)(O ,O) (3b - 6b) were carried out in air under noni sothermal conditions. The complex 2b starts mass loss above 250°C and was not studied fu rther, while 3b- 6b exhibit weioht loss in the ranoe b b

120 - 140°C and the weight loss is equi valent to 0.5 mol of H20 followed by second mass loss above 250°C. This supports that the catecholato complexes include one mo l of H20 per couple of Pt(RaaiM e)(O,O) in their coordination zone.

Redox studies The catecholato complexes (3) - (6) exhibit three/

fo ur successive redox responses within the potential range + 1.5 to -1 .5 V versus SCE using g lassy carbon working electrode in acetonitrile under dinitrooen at-e mosphere in the presence of [Bu4N][CI04 ) as sup-port ing electrolyte. The redox data are g iven in Table 2. Two couples negative to SCE refer to azo reduc­ti ons and correspond to [N=N] I [N-N]" and [N-N]" I [N-N]"2

, peak-to-peak separati on (~Er > 90 mY) sup­ports quas ireversible character of the redox process. The redox responses positive to SCE are due to cate­chol oxidation (Eqn . I , Fi g. 2).

A~ ~ ... (l )

(Rcq) (Asq) (Aq)

First couple is ass igned to the redox pair catecho­lato (Rcq)/semiquinone (Rsq) and second couple cor­responds to semiquinone (Rsq)/quinone (Rq) . The degree of reversibility of the Rcq /Rsq couple vari es with the substituent(s) present in the catechol ring3

.4 .

The data in Table 2 reveal that E pa (anodic peak po­tential) is shi fted to more pos iti ve values due to the presence of electron withdrawing -CI g roups in Pt(RaaiM e)(tccat). Reversibility of tbcat and dtbcat compl exes may be due to the stabili sation of cation radical produced after oxidation via electron density fl ow fro m Bu1 group(s) into the catechol ring 11

• The di fference in potential fo r first reducti on and first ox i­dation is correlated with the LLCT transitions (Eqn. 2). Thi s supports the involvement o f same redox or­bi ta ls in charge transfer transition and redox reac tions.

VrT= 2.20 ~E 112 - 1.22 .. . (2)

Co111parison with pa//adiwn(/1) analogues The spectroscopic and redox property of

[Pt(RaaiX)(O,O)] are comparable with previously

---' ___.__,__ · 1.3 - I .1 -0 .9 -0.7 - 0.5 0.0 0.2 O.t. 0.6 0.8

E/V

Fig. 2-Cyclic Yo ltammogram of [Pt(HaaiM e) (Cat)] ( - ); (Pt(HaaiMe) (tbcat)] (-- -); [Pt( HaaiMe)(dtbcat)] (- · -); and Pt(Haa iMe) (tc cat)] ( . .. . ) in MeCN using GC-disk e lectrode

Table 3--Spectral and redox potenti al comparison of M(Meaai Me)(O,O), ( M = Pd , Pt )

Complex

Pd(MeaaiMe)(cat)

Pd(MeaaiMe)( tbcat)

Pd(MeaaiMe)(dtbcat)

Pt(MeaaiM e)(cat)

Pt(MeaaiMe)( tbcat)

Pt(MeaaiMe)(dtbcat)

ALLCT

(nm)

960

988

11 20

780

830

896

Rcq /Rsq (N::Ny I (N = N) E ,n _Y E112_Y (t.EP,mV)

(t.Er,mY)

0.5 1 (80) - 0.40 (80)

0.43 (70) - 0.46 (80)

0.34 (80) - 0.40 (70)

0.42 (90) -0.84 ( 120)

0.34 (95) - 0.88 ( 11 0)

0. 26 ( 100) - 0.89 ( 120)

reported Pd(RaaiX)(O,O) system4. Although the

spectral pattern of both the seri es of complexes is similar, the magnitude of the spectral transition en­ergy of platinum(II) complexes are higher than that of pall adium(ll) complexes. A represemati ve case is shown in Table 3. Thi s is also supported by the dec­rement o f Rcq/Rsq and increment of (N~Nr I (N = N) and (N- N f 2

/ (N~Nr redox potenti al data. Thi s may be due to the participation of Pt(ll ) drr-orbitals with the rr* (RaaiX) levels whi ch is relati vistically less effi-. . p ( 14 ctent 111 d II )-system . Thus metallo-li gand orbitals

are better stabili sed in bonding and equi valently de­stabili sed in antibonding molecular orbitals in Pt(lf)­complexes compared to Pd(ll)-complexes. The LLCT band is assigned as HOM0--7 LUMO charge shi ft ing and the energy difference is increased in [Pt(Raai X)(O,O)] than that of Pd(l l)-analogues. Similarly, electron ex tracti on from HO MO and ac­commodati on to LUMO are equally high energy de­manding in pl atinum(ll)-compl exes relati ve to analo­gous pall adium(Il ) complexes .

RAUTH eta/. :SYNTHESIS OF Pt(ll) COMPLEXES OF SUBSTITUTED IMIDAZOLES 373

Acknowledgement We thank the CSIR, New Delhi for financial

assistance. Our sincere thanks are due to Prof. G N Mukherjee, Calcutta University for recording UY­Yis-NIR spectra and Prof. N Roy Choudhury, lACS, Calcutta, India, for thermal studies.

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