Thermal studies on some biologically active organomercury(II) complexes

Article abstract of DOI:10.1023/A:1011505408406

Thermal behaviour of a number of organomercury(II) complexes of the type, p-XC₆H₄HgCl(L¹) (I), p-XC₆H₄HgCl₃(L²) (II), p-XC₆H₄HgL³ (III) and p-XC₆H₄HgL⁴ (IV) [L¹ =isoniazid, L² =theobromine, L³=phenyldithiocarbamate, L⁴=p-nitrophenyldithiocarbamate; X=Me, MeO, NO₂] has been investigated. From TG curves, the order and activation energy of the thermal decomposition reaction have been elucidated. The variation of the activation energy has been correlated with the nature of the substituent on the phenyl ring. The heat of reaction has been elucidated from DSC or DTA studies. The fragmentation pattern has been analysed on the basis of mass spectra.

Full text of DOI:10.1023/A:1011505408406

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 249–255
THERMAL STUDIES ON SOME BIOLOGICALLY
ACTIVE ORGANOMERCURY(II) COMPLEXES*
J. Kaur1 and G. S. Sodhi2
1Department of Chemistry, University of Delhi, Delhi-110007, India
2Department of Chemistry, S. G. T. B. Khalsa College, University of Delhi, Delhi-110007, India
(Received March 20, 1999; in revised form July 7, 2000)
Abstract
Thermal behaviour of a number of organomercury(II) complexes of the type, p-XC6H4HgCl(L1) (I),
p-XC6H4HgCl3(L2) (II), p-XC6H4HgL3 (III) and p-XC6H4HgL4 (IV) [L1=isoniazid, L2=theobromine,
L3=phenyldithiocarbamate, L4=p-nitrophenyldithiocarbamate; X=Me, MeO, NO2] has been investi-
gated. From TG curves, the order and activation energy of the thermal decomposition reaction have
been elucidated. The variation of the activation energy has been correlated with the nature of the
substituent on the phenyl ring. The heat of reaction has been elucidated from DSC or DTA studies. The
fragmentation pattern has been analysed on the basis of mass spectra.
Keywords: biologically active material, kinetics, organomercury complexes
Introduction
Organomercury complexes are known to be potential diuretics, antiseptics, fungicides
and bactericides [1]. We report here the thermal behaviour of a few organomercury(II)
complexes with biologically active ligands (structures I–IV).
*
A part of this work was carried out at Institute of Microbial Technology, Chandigarh, under the
Visiting Associateship Scheme (1992–95) of the Council of Scientific and Industrial Research, New
Delhi.
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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KAUR, SODHI: ORGANOMERCURY(II) COMPLEXES
This follows from our interest to investigate the thermal characteristics of orga-
nomercury-biomolecule interactions [2–5].
Experimental
Isoniazid and theobromine (Sigma) were used without further purification. Sodium
dithiocarbamates were prepared by the method of Gilman and Blatt [6]. Aryl-
mercury(II) chlorides (p-XC6H4HgCl, X= Me, MeO, NO2) were synthesized by the
literature method [7].
The isoniazid complexes were prepared by adding slowly a solution of
p-XC6H4HgCl (0.01 mol) in 25 ml DMF to a stirred solution of isoniazid (0.01 mol)
in 25 ml DMF. The contents were stirred for 6 h at 50°C and filtered. The filtrate was
slowly poured over crushed ice and stirred vigorously. The precipitates so obtained
were washed successively with hot water and benzene. The resulting product was
dried and recrystallized from THF. The theobromine complexes were prepared by the
same procedure. The dithiocarbamate complexes too were prepared similarly by the
reaction of p-XC6H4HgCl and sodium dithiocarbamates.
The IR and UV spectra were recorded on Shimadzu model IR-435 and Perkin
Elmer UV-Vis spectrometer model 554 respectively. The TG curves upto 1273 K and
DSC/DTA curves upto 673 K were recorded on RIGAKU 8150 instrument in air at-
mosphere, at a heating rate of 15°C min–1. Mass spectra were recorded on a Jeol JMX
DX-303 spectrometer.
Results and discussion
In the IR spectra of isoniazid, the amide I (νCO) band occurred at 1660 cm–1. In the
complexes, the band shifted to ca. 1600 cm–1. The ν(N–H) vibrations also shifted
from 3300 cm–1 in the free ligand to ca. 3150 cm–1 in the complexes. The observations
reveal that isoniazid ligand was coordinated to mercury(II) ion through carbonyl and
hydrazinic groups [8]. In the UV spectra the π–π* absorptions of the chromopheric
C=O group absorbed at 266 nm (logε 6.8) in isoniazid and at ca. 278 nm (logε ca. 5.2)
in the complexes, confirming the involvement of this group in complexation. The ab-
sorption due to the ν(C=N) stretching frequency was shifted from 1540 cm–1 in free
theobromine to ca. 1525 cm–1 in the complexes. The ν(C=O) stretching frequency,
however, remained unaltered. It was, therefore, concluded that theobromine coordi-
nated to mercury(II) ion through N(9) [9]. The monodentate or bidentate behaviour of
the dithiocarbamates is reflected in the ν(C–S) stretching frequency [10]. The pres-
&&&
ence of one band at ca. 1000 cm–1 reveals bidentate behaviour, a doublet being ex-
pected in case of monodentate behaviour. In the present complexes, only one absorp-
tion band at ca. 1000 cm–1 was observed in dithiocarbamate complexes, supporting
the bidentate nature of the ligands. In the UV spectra, the ligand displayed a band at
ca. 340 nm, attributed to π–π* transition of S–C–S chromophore. This band tended to
&&& &&&
J. Therm. Anal. Cal., 65, 2001

Table 1 Thermal data
TG
DSC or DTA*
Tmax/K
Compound
∆H/
Trange
Ea/kcal mol–1
n
Thermal effect
(muva min g–1) or cal g–1*
endothermic
exothermic
501
512
664.72
647.28
p-MeC6H4HgCl(L1)
p-MeOC6H4HgCl(L1)
p-NO2C6H4HgCl(L1)
449–528
435–498
470–573
1
1
1
28.76
30.50
27.44
endothermic
490
529.89
exothermic
exothermic
537
595
469.61
3307.57
exothermic
endothermic
441
481
483.78
247.67
p-MeC6H4HgCl(L2)
p-MeOC6H4HgCl(L2)
436–673
498–643
1
1
21.34
22.88
exothermic
endothermic
exothermic
409
446
623
75.49
244.32
190.74
p-NO2C6H4HgCl(L2)
p-MeC6H4HgL3
p-MeC6H4HgL3
p-NO2C6H4HgL3
p-MeC6H4HgL4
p-MeOC6H4HgL4
p-NO2OC6H4HgL4
amuv=micro unit volt
443–698
382–614
499–565
388–613
458–578
528–590
510–623
1
1
1
1
1
1
1
18.30
12.20
21.73
6.30
exothermic
endothermic
endothermic
endothermic
endothermic
endothermic
endothermic
598
530
513
540
502
568
532
110.64
50.28*
45.83*
39.86*
51.26*
43.61*
33.34*
9.15
36.60
8.69

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KAUR, SODHI: ORGANOMERCURY(II) COMPLEXES
Fig. 1 Linearisation curves of isoniazid complexes: A – p-MeC6H4HgCl(L1);
B – p-MeOC6H4HgCl(L1); C – p-NO2C6H4HgCl(L1)
Fig. 2 Linearisation curves of theobromine complexes: A – p-MeC6H4HgCl(L2);
B – p-MeOC6H4HgCl(L2); C – p-NO2C6H4HgCl(L2)
J. Therm. Anal. Cal., 65, 2001

KAUR, SODHI: ORGANOMERCURY(II) COMPLEXES
253
Fig. 3 Linearisation curves of phenyldithiocarbamate complexes: A – p-MeC6H4HgL3;
B – p-MeOC6H4HgL3; (C) – p-NO2C6H4HgL3
Fig. 4 Linearisation curves of p-nitrophenyldithiocarbamate complexes:
A – p-MeC6H4HgL4; B – p-MeOC6H4HgL4; C – p-NO2C6H4HgL4
J. Therm. Anal. Cal., 65, 2001

254
KAUR, SODHI: ORGANOMERCURY(II) COMPLEXES
disappear in the complexes, confirming the conclusions of IR spectra that dithio-
carbamates behave as bidentate ligands [11].
From TG curves, the mass loss accompanying the thermal degradation of the
complexes has been calculated. In each case, the theoretical as well as calculated
mass change corresponded to the formation of mercury(II) oxide. Beyond ca. 750 K,
HgO slowly volatilised, leaving the crucible of the thermobalance vacant. The
DSC/DTA profile showed, in each case, thermal effects, corresponding to the decom-
position of the complex to HgO. Thermal data are presented in Table 1.
From the TG curves, the order (n) and activation energy (Ea) of the thermal de-
composition reaction have been calculated by the method of Coats and Redfern [12].
The linearisation curves of the complexes I, II, III and IV are shown in Figs 1, 2, 3
and 4, respectively.
The order of reaction in each case is unity, similar to the lanthanide complexes
reported by us recently [13]. A comparison of the activation energy data for each se-
ries of complexes revealed that the p-nitrophenylmercury(II) derivative had the low-
est value of Ea. This may be explained on the basis of the electron withdrawing effect
of the nitro group, which causes the weakening of the metal-ligand bond, conse-
quently making thermal degradation relatively easy. In case of p-methylphenyl-
mercury(II) analogue, the methyl group was of electron releasing type and the metal-
ligand bond was strengthened, which subsequently resulted in higher Ea value for
thermal reaction. As the methoxy group had a greater electron donating character
than the methyl group, the p-methoxyphenylmercury(II) compound had the highest
value of Ea. Thus for any one of the ligand, the activation energy varied in the order,
X=MeO>O>Me>NO2.
That the XC6H4–Hg bond cleavage was involved in the pyrolysis of the com-
plexes was also evidenced from the mass spectra. The carbonium ion, XC6 H+4 , consti-
tuted the base peak in each case. Thus the peaks at m/e 91, 107 and 122 corresponded
to the formation of MeC6 H4+ , MeOC6 H4+ , and NO2C6 H4+ fragments respectively. The
mass spectra also showed peaks corresponding to C46 H4+ (m/e 76), HgCl+ (m/e 236)
and Hg+ (m/e 201).
The TG data were supplemented by DSC/DTA studies. The heat of reaction
(∆H) was enumerated from DSC/DTA curves. For dithiocarbamate complexes,
p-XC6H4HgL3 and p-XC6H4HgL4, the thermal effects on DTA curves were of endo-
thermic nature. At least one exothermic thermal effect on DSC curves was recorded
for the theobromine complexes, p-XC6H4HgCl(L2) and isoniazied complexes,
p-XC6H4Hg(L1). However, no exothermic thermal effect was observed for the
p-MeOC6H4HgCl(L1) complex. As these complexes had a chloro group in addition to
the ligand moiety, it may be possible that the exothermic step in the thermal reaction
involved the release of hydrogen chloride gas.
* * *
One of us (J. K.) is thankful to the Council of Scientific and Industrial Research, New Delhi for the
award of a Research Associateship.
J. Therm. Anal. Cal., 65, 2001

KAUR, SODHI: ORGANOMERCURY(II) COMPLEXES
255
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