Synthesis and spectrothermal studies on group 12 metals coordination with novel carboxyamide ligands

Article abstract of DOI:10.1016/j.saa.2007.04.007

Complexes of group 12 metals with new ligands N′,N″-bis(3-carboxy-1-oxopropanyl) 2-amino-N-arylbenzamidine (H₂L) and N′,N″-bis(3-carboxy-1-oxophenelenyl) 2-amino-N-arylbenzamidine (H₂L) have been synthesized and characterized by elemental analyses, vibrational spectra, electronic spectra, ¹H NMR spectra, TOF-mass spectra, thermal studies and molecular modeling studies. The infrared and ¹H NMR spectral data are in agreement with coordination of amide and carboxylate oxygen of the ligands giving a MO₄ tetrahedral chromophore. The elemental analyses and mass spectral data have justified the ML complexes. The thermal behaviour of complexes shows that water molecule is removed in first step-followed by decomposition of the rest of the molecule in the next steps. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm first order kinetics. Molecular structures of the complexes have been optimized by MM2 calculations and supported tetrahedral geometry around metal(II) ions.

Full text of DOI:10.1016/j.saa.2007.04.007

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 361–370
Synthesis and spectrothermal studies on group 12 metals
coordination with novel carboxyamide ligands
Bibhesh K. Singh, Parshuram Mishra, Bhagwan S. Garg∗
Department of Chemistry, University of Delhi, Delhi 110007, India
Received 30 December 2006; accepted 12 April 2007
Abstract
Complexes of group 12 metals with new ligands Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl) 2-amino-N-arylbenzamidine (H2L) and Nꢀ,Nꢀꢀ-bis(3-
carboxy-1-oxophenelenyl) 2-amino-N-arylbenzamidine (H2L) have been synthesized and characterized by elemental analyses, vibrational spectra,
1
1
electronic spectra, H NMR spectra, TOF-mass spectra, thermal studies and molecular modeling studies. The infrared and H NMR spectral
data are in agreement with coordination of amide and carboxylate oxygen of the ligands giving a MO4 tetrahedral chromophore. The elemental
analyses and mass spectral data have justified the ML complexes. The thermal behaviour of complexes shows that water molecule is removed in
first step—followed by decomposition of the rest of the molecule in the next steps. Kinetic and thermodynamic parameters were computed from
the thermal data using Coats and Redfern method, which confirm first order kinetics. Molecular structures of the complexes have been optimized
by MM2 calculations and supported tetrahedral geometry around metal(II) ions.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Amide; Group 12 metal complexes; Spectra; Thermal and molecular mechanics
1. Introduction
ing carboxylic anchors, which may act as biomimic to peptides
and certain enzymes. Ligands containing carboxyamide chains
having O4 donor atoms, and their complexes, have been cited
as possessing noteworthy pharmacological activity [11]. Thus,
we have designed amide ligands containing carboxylic anchors
condensed with amidines.
In recent years group 12 metal compounds, which have
a stable d10 electronic configuration, have received a lot of
attention in the fields of inorganic chemistry, biochemistry and
environmental chemistry [1–5]. We are interested to establish
spectroscopic model complexes of zinc(II), cadmium(II) and
mercury(II) ions with biomimitic polydentate amide ligands,
which have an amidine ring. The amidine unit is the key func-
tional group in a wide range of biologically active molecules
[6], including a variety of serine protease inhibitors [6–8] and
antimicrobial agents [9]. Magainin-2-amide, the bioactive pep-
toid exhibit selective, potent antibacterial activity against both
Gram-positive and Gram-negative bacteria [10]. These readily
synthesized, protease-resistant peptoids represents an important
advance in peptide biomimicry, the development of nonpep-
tide foldameric analogues of antibacterial peptides. Though,
much work has been done on amide ligands in general and
the amide ligands in combination with other functional groups,
but only less work has been done on amide system contain-
In general, the zinc(II) ion can coordinate to hard and soft
atoms, while the cadmium(II) ion and mercury(II) ion prefer
soft atoms and electron rich conditions [1]. Nitrogen donar lig-
ands are regarded as hard ligands. The reactivities of complexes
are affected by these differences in polarizability. In order to
increase reactivity, we introduced new polydentate ligands to
stabilize the complexes. In this context, the structure and mode
of bonding of amide bond in a complexes of amide ligands,
will be a great help in understanding the development in the
area of amide–metal interactions [12,13]. Our research inter-
est is centered on thermal stability and structure of metal ion
complexes of newly synthesized biomimic amides. In view
of the diverse coordinating behaviour of amide group, it was
considered worthwhile to prepare and study some of the com-
plexes. In the present manuscript, we report the preparation and
study of Zn(II), Cd(II) and Hg(II) complexes of the newly syn-
thesized ligands Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl) 2-amino-
N-arylbenzamidine and Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)
∗
Corresponding author. Tel.: +91 9868359781; fax: +91 27666250.
E-mail address: bibheshksingh@yahoo.co.in (B.K. Singh).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.04.007

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B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
2-amino-N-arylbenzamidine. The molecular modeling studies
for energy minimization and optimization of geometry of the
metal complexes have also been carried out.
2. Experimental
2.1. Material and methodology
All the chemicals used in this study were of analytical grade
and used as procured. Solvents used were of analytical grade
and were purified by standard procedures. The stoichiomet-
ric analyses(C, H and N) of the complexes were performed
using Elementar vario EL III (Germany) model. Metal con-
tents were estimated on an AA-640-13 Shimadzu flame atomic
absorption spectrophotometer in solution prepared by decom-
posing the respective complex in hot concentrated HNO3. Their
IR spectra were recorded on Perkin-Elmer FT-IR spectropho-
tometer in KBr and polyethylene pellets. The electronic spectra
were recorded in water on Beckman DU-64 spectrophotometer
with quartz cells of 1 cm path length and mass spectra (TOF-
MS) were recorded on Waters (USA) KC-455 model with ES+
mode in D2O. 1H NMR spectra were recorded in D2O solvent
(solvent peak 4.8 ppm) on a Bruker Advance 400 instrument.
Rigaku model 8150 thermoanalyser (Thermaflex) was used for
simultaneous recording of TG–DTA curves at a heating rate of
10◦ min−1. For TG, the instrument was calibrated using calcium
oxalate while for DTA, calibration was done using indium metal,
both of which were supplied along with the instrument. A flat
bed type aluminium crucible was used with ␣-alumina (99%
pure) as the reference material for DTA. The number of decom-
position steps was identified using TG. The activation energy
and Arrhenius constant of the degradation process was obtained
by Coats and Redfern method.
Fig. 1. Ligand L1.
2.3.2. Synthesis of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl)
2-amino-N-arylbenzamidine (C21H21N3O6) (L1) (Fig. 1)
A finely ground mixture of 2-amino-N-arylbenzamidine
(0.5 mmol), succinic anhydride (1.0 mmol) and fused sodium
acetate (0.5 mmol) in glacial acetic acid (25 ml) was refluxed on
water bath for 3 h. After filtering the residue, filterate on slow
evaporation forms fine crystals. This was recrystallized in acetic
acid, washed with ethanol and dried over P4O10 in vacuo. Ligand
is soluble in common polar solvents.
Off white crystal; Yield: 83%; Anal. Calc. for C21H21N3O6
requires (%): C, 61.31; H, 5.11; N, 10.21; Found: C, 61.18;
H, 5.37; N, 10.29 (mol. wt.: 411.4); FT-IR (KBr, cm−1):
ν(O–H) 2600(w), ν(N–H)amide 3369(s), ν(C O)amide I 1650(w),
[ν(C–N) + δ(N–H)]amide II 1410(s), ν[(N–H) + δ(C–H)]amide III
1300, νas(CO2) 1558(s), νs(CO2) 1402(s); 1H NMR (ppm):
11.30(2H, s, –OH carboxylic acid), 8.15(2H, s, sec. amide),
7.45(4H, m, benzylidenimin), 7.35(5H, m, H-benzene),
1.43(8H, s, –CH2 methylene group); TOF-MS (m/z): 412(14%,
[M]+), 352(100%, [C19H19N3O4]+), 187(11%, [C12H14N2]+).
Electronic spectral data (cm−1): 30,488 (␲ → ␲*, N-amide),
35,842 (n → n*, C O group).
2.2. 3D molecular modeling
3D molecular modeling of the proposed structure of the
complexes was performed using CsChem3D Ultra 10-program
package. The correct stereochemistry was assured through the
manipulation and modification of the molecular coordinates to
obtain reasonable low energy molecular geometries. The poten-
tial energy of the molecule was the sum of the following terms:
E = Estr + Eang + Etor + Evdw + Eoop + Eele, where all Es represent
the energy values corresponding to the given types of inter-
action (kcal/mol). The subscripts str, ang, tor, vdw, oop and
ele denote bond stretching, angle bonding, torsion deformation,
Van der Waals interactions, out of plain bending and electronic
interaction, respectively.
2.3.3. Synthesis of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)
2-amino-N-arylbenzamidine (C29H21N3O6) (L2) (Fig. 2)
A finely ground mixture of 2-amino-N-arylbenzamidine
(0.5 mmol), phthalic anhydride (1.0 mmol) and fused sodium
2.3. Synthesis of the ligands
2.3.1. Synthesis of 2-amino-N-arylbenzamidine (C13H13N3)
2-Amino-N-arylbenzamidine (C13H13N3) as precursor was
synthesized by reported procedure [14]. This was characterized
1
by elemental analysis, infrared spectra, H NMR spectra and
Fig. 2. Ligand L2.
mass spectra. Satisfactory results were obtained.

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
363
acetate (0.5 mmol) in glacial acetic acid (25 ml) was refluxed
on water bath for 3 h. After filtering the residue, filterate
on slow evaporation forms fine crystals. This was recrys-
tallized in acetic acid, washed with ethanol and dried
over P4O10 in vacuo. Ligand is soluble in common polar
solvents.
Light yellow crystal; Yield: 84%; Anal. Calc. for
C29H21N3O6 requires (%): C, 68.64; H, 4.14; N, 8.28; Found:
C, 68.64; H, 4.14; N, 8.28 (mol. wt.: 507); FT-IR (KBr, cm−1):
ν(O–H) 2602(w), ν(N–H)amide 3370(s), ν(C O)amide I 1630(w),
[ν(C–N) + δ(N–H)]amide II 1485(s), ν[(N–H) + δ(C–H)]amide III
1296, νas(CO2) 1563(s), νs(CO2) 1400(s); 1H NMR (ppm):
11.25(2H, s, –OH carboxylic acid), 8.10(2H, s, sec. amide),
7.35(4H, m, benzylidenimin), 7.25(13H, m, H-benzene); TOF-
MS (m/z): 507(15%, [M]+), 449(100%, [C27H19N3O4]+),
329(12%, [C20H14N3O2]+), 236(16%, [C14H10N3O]+). Elec-
tronic spectral data (cm−1): 31,447 (␲ → ␲*, N-amide), 35,842
(n → n*, C O group).
2.4. Synthesis of complexes
2.4.1. Synthesis of zinc(II), cadmium(II) and mercury(II)
complexes of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl)
2-amino-N-arylbenzamidine
To a methanolic solution of ZnCl2, Cd(CH3CO2)2·2H2O and
Hg(CH3CO2)2 (0.5 mmol) each in a separate flask was added a
methanolic solution of the ligand (L1) (0.5 mmol). The solu-
tion was stirred for 3 h, after which the volume was reduced on
a warm water bath. The product obtained was washed with a
small amount of methanol and air-dried. The above product was
redissolved in excess warm methanol, and clear solution was
left undisturbed for weeks to give beautiful white crystals of the
complex.
Fig. 3. (a) Electronic spectra of complexes of ligand L1. (b) Electronic spectra
of complexes of ligand L2.
2.4.2. Synthesis of zinc(II), cadmium(II) and mercury(II)
complexes of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)
2-amino-N-arylbenzamidine
undisturbed for weeks to give beautiful white/off white crystals
of the complex.
To a methanolic solution of ZnCl2, Cd(CH3CO2)2·2H2O and
Hg(CH3CO2)2 (0.5 mmol) each in a separate flask was added a
methanolic solution of the ligand (L2) (0.5 mmol). The solu-
tion was stirred for 3 h, after which the volume was reduced on
a warm water bath. The product obtained was washed with a
small amount of methanol and air-dried. The above product was
redissolved in excess warm methanol, and clear solution was left
3. Results and discussion
Satisfactory results of elemental analysis (Table 1) and spec-
tral studies (IR, UV, 1H NMR, TOF-MS) explained the bonding
modes of ligands with metal ions. The complexes were of good
purity and are of crystalline nature.
Table 1
Color reaction yield and elemental analysis of complexes
Complex
Empirical formula
Color
Yield (%)
Analysis:found (calculated) (%)
C
H
N
M
[Zn(L1)]
C21H19N3O6Zn
C21H21N3O7Cd
C21H21N3O7Hg
C29H19N3O6Zn
C29H23N3O8Cd
C29H21N3O7Hg
White
Silky white
White
White
White
78
81
75
80
82
77
53.04 (53.12)
46.48 (46.72)
40.10 (40.15)
60.93 (60.96)
53.17 (53.23)
48.00 (48.09)
3.96 (4.01)
3.81 (3.89)
3.30 (3.35)
3.30 (3.33)
3.45 (3.52)
2.79 (2.90)
8.80 (8.85)
7.72 (7.79)
6.62 (6.69)
7.31 (7.36)
6.39 (6.42)
5.73 (5.80)
13.70 (13.79)
20.78 (20.83)
31.93 (31.96)
11.41 (11.46)
17.21 (17.19)
27.69 (27.72)
[Cd(L1)]·H2O
[Hg(L1)]·H2O
[Zn(L2)]
[Cd(L2)]·2H2O
[Hg(L2)]·H2O
Off white

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B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
Table 2
Important mass peaks for the complexes studied
Mass assignments
%
m/z
Complex I
[C21H19N3O6Zn]+
[C18H12N3O3]+
12
16
21
26
100
19
476
318
294
236
224
207
[C16H
12N3O3]+
[C14H10N3O]+
[C14H12N2O]+
[C14H11N2]+
Complex II
[C21H21N3O7Cd]+
[C21H21N3O6]+
[C19H15N3O2]+
[C15H15N3O2]+
[C14H12N2O]+
[C14H11N2]+
42
100
100
100
60
540/542
414
317
269
224
60
207
Complex III
[C21H21N3O7Hg]+
[C21H19N3O6Hg]+
22
20
45
51
100
68
628/629
610
346
294
276
[C20
[C16
H
H
16N3O3]+
12N3O3]+
[C16H10N3O2]+
[C14H10N3O]+
[C14H10N2O]+
236
222
67
Complex IV
[C29H19N3O6Zn]+
[C27H13N3O4]+
[C22H11N3O2]+
[C20H11N3O2]+
[C14H10N3O]+
26
30
24
100
34
571/572
443
347/349
324
236
Complex V
[C29H23N3O8Cd]+
[C22H17N3O6]+
[C19H13N3O2]+
[C14H7N3O]+
15
72
43
654/655
417/419
315
100
233
Complex VI
[C29H21N3O7Hg]+
[C27H13N3O4]+
[C19H13N3O2]+
[C14H6N3O]+
20
100
47
724/725
442/443
317
90
232
3.1. Electronic spectra
The electronic spectra of the ligands and its metal complexes
have been studied in the range 190–400 nm (Fig. 3(a) and (b)).
Theshoulderbandobservedat279 nminwatersolventinligands
may be assigned to n → n* transition within the C O group of
the amide moiety in the free ligands. This band is red shifted to
299 nm in case of complex IV and 284 nm in case of complex
V, while it disappeared in other complexes of ligand L1 and
ligand L2 revealing the involvement of the C O of amide group
or carboxylate oxygen in chelate formation. The band observed
at 328 and 318 nm in ligands L1 and L2, respectively, may be
assignedto␲ → ␲*transitionoftheN-aminogroupoftheamide
ligand, remain at the same position in all the respective metal
complexes, indicating they are not affected by metal chelation
[15]. The result supports the mode of interaction of amide with
group 12 metal ions.
Fig. 4. (a) TOF-MS spectra of complex I. (b) TOF-MS spectra of complex III.
(c) TOF-MS spectra of complex V.

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
365
3.2. TOF-MS spectra
corresponding to the various fragments. Their intensity gives
an idea of stability of fragments. The ligands have similar
tendency of initial loss of two molecules of [HC O]+ with
100% m/z values. Ligand (L1) starts degradation and it finally
forms two molecules of [C6H5NH2]+, whereas Ligand (L2)
finally degrades to [amidine]+. In the TOF-mass spectra of
metal complexes initial fragmentation pattern is again similar
(loss of one or two water molecules), a mononuclear nature
Mass spectrometry has been successfully used to investi-
gate molecular species in solution [16–18]. The molecular ion
peaks of the ligands and complexes have been used to con-
firm the proposed formula (Table 2). The multi-peaks pattern
of the mass spectrum gives an impression of the successive
degradation of the target compound with the series of peaks
Fig. 5. (a) IR spectrum of complex I. (b) IR spectrum of complex II. (c) IR spectrum of complex III. (d) IR spectrum of complex IV. (e) IR spectrum of complex V.
(f) IR spectrum of complex VI.

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B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
Table 3
IR spectral data (cm−1) of ligand and metal complexes
Frequency
νNH
Amide I
Amide II
Amide III
νas(CO2)
νsy(CO2)
M–O
[Zn(L1)]
3424(s)
1634(w)
1638(m)
1644(m)
1614(w)
1626(w)
1626(w)
1415(s)
1416(s)
1415(m)
1489(s)
1488(m)
1487(m)
1325(m)
1302(m)
1319(w)
1305(w)
1375(m)
1385(m)
1596(s)
1560(s)
1562(s)
1580(s)
1556(s)
1567(s)
1376(m)
1341(s)
1385(m)
1395(s)
1410(s)
1409(m)
489(m)
[Cd(L1)]·H2O
[Hg(L1)]·H2O
[Zn(L2)]
3419(s,b)
3446(s,b)
3421(s)
3417(s,b)
3437(s,b)
510(m), 372(s), 306(s)
644(s), 524(s)
482(m)
519(m), 376(s), 300(s)
526(m), 480(m)
[Cd(L2)]·2H2O
[Hg(L2)]·H2O
b, broad; s, strong; m, medium; w, weak.
Table 4
1H NMR δ values (ppm) of metal complexes
Compounds
Sec. amide
H-benzylidenimin
H-benzene
–CH2 methylene group
Proton of H2O
[Zn(L1)]
8.00(2H,s)
8.04(2H,s)
8.09(2H,s)
8.08(2H,s)
8.00(2H,s)
8.02(2H,s)
7.44(4H,m)
7.43(4H,m)
7.42(4H,m)
7.32(4H,m)
7.30(4H,m)
7.28(4H,m)
7.34(5H,m)
7.32(5H,m)
7.30(5H,m)
7.23(13H,m)
7.22(13H,m)
7.21(13H,m)
1.42(8H,s)
1.37(8H,s)
1.41(8H,s)
–
–
–
–
[Cd(L1)]·H2O
[Hg(L1)]·H2O
[Zn(L2)]
2.38
2.27
–
2.50
2.45
[Cd(L2)]·2H2O
[Hg(L2)]·H2O
for these complexes [M(L)]+ can be deduced (Fig. 4(a)–(c):
TOF-MS spectrum of complexes I, III and V). The last two
fragments appears in nearly all the complexes at positions (m/z
values) 233/236 (26% complex I, 68% complex III, 34% com-
plex IV, 100% complex V and 90% complex VI) and 222/224
(100% complex I, 60% complex II and 67% complex III) cor-
responds to [C14H10N3O]+ and [C14H10N2O]+, respectively,
which could be the result of demetallation and subsequently
a partial intra-molecular hydrogen bonding. Another prominent
peak in the middle of the degradation pathway of the complexes
of ligand (L2) at position (m/z) 443 (30% complex IV, 100%
complex VI) and 324 (100% complex IV) are [C27H13N3O4]+
and [C20H11N3O2]+, respectively. Complex II (23%) and com-
plex III (19%) have peak at 185 (m/z value) correspond to
the formation of two molecules of C6H5NH2 like a degrada-
tion of the ligand (L1). Likewise peaks attributable to unstable
monomeric species as [M(L)]+ and [H2L]+ are usually present
in the mass spectra of these systems [19,20].
3.3. IR spectra and mode of bonding
An ambidentate nature of the amide can coordinate with
zinc(II), cadmium(II) and mercury(II) either through nitrogen
of the –NH2 or the oxygen of the C O group. The bands due to
the amide νNH mode at 3200–3400 cm−1 for the free ligands L1
and L2 are shifted towards higher frequencies, while the νC O
(amide I) frequency (1630–1650 cm−1) undergoes shift towards
lowerfrequency(1610–1645 cm−1)inthemetalcomplexesindi-
cating non-involvement of the amide nitrogen and involvement
of the carbonyl oxygen atom of amide group in coordination
with the metal ion [21] (Fig. 5(a)–(f)). This result is explained
by the decrease in the double bond character of C O and the
subsequent increase of C–N double bond character [22,23]. It
is the lengthening of the C O bond and shortening of the C–N
bond which cause the decrease and increase of the frequencies,
respectively. Amide II (1410–1485 cm−1 in ligands) and amide
III (1295–1301 cm−1 in ligands) bands arising out of νC–N as
well as δN–H modes (coupled to one another), shift towards
higher frequencies 1415–1490 and 1302–1385 cm−1, respec-
tively, in complexes further confirming the coordination through
carbonyl oxygen (Table 3). The ν(C O) and ν(C–O) stretch-
ing frequencies in the 1540–1590 and 1420–1400 cm−1 region
in the uncomplexed ligands have been assigned to νas(CO2
−
)
and νs(CO2−) modes of the carboxylate group and these bands
shows considerable shift in all the complexes. A positive and
negative shift in complexes indicates a ꢀν enhancement upon
complexation and suggests coordination of carboxylate groups
in all the complexes of the three ligands in a monodentate fashion
[24,25]. Here ꢀν implies the difference between νas(CO2−) and
νs(CO2−) (i.e., asymmetric and symmetric stretching modes,
respectively). The ν(O–H) due to carboxylic group appearing
at 2600 cm−1 in the free ligands disappears in the spectra of
their complexes, indicates both carboxylic groups are involved
in complexation. The vibrational bands due to rocking and wag-
Fig. 6. 1H NMR spectra of complex II.

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
367
ging modes of water and metal–oxygen stretching modes are
observed in the 800–350 cm−1 region for all the complexes
(except complexes I and IV) of both the ligands and may be
attributed to the coordinated water [24]. This can be confirmed
with the help of thermograms. A new band in the 650–300 cm−1
regions in the spectra of the complexes is assignable to ν(M–O)
[24].
3.4. 1H NMR spectra
The 1H NMR spectra of a D2O solution of the amide ligands
and its Zn(II), Cd(II) and Hg(II) complexes show well-resolved
1
signals as expected (Fig. 6: H NMR spectra of complex II).
1
The solvent resonates near 4.8 ppm. The H NMR resonances
are summarized in Table 4, with the complexes slightly upfield
compared to the free ligands, indicative of the presence of free
carboxylate (absence of signals for COOH and OH–COOH
group (11.2–11.3)). A similar trend has been observed in the
1H NMR spectra of metal–acid complexes [26,27], in agree-
ment with coordination by COO−, as also suggested by IR data.
In all complexes, peaks in the range 2.50–2.20 ppm are from
H2O (water of crystallization).
3.5. Thermal studies
Recently, there has been increasing interest in determin-
ing the rate-dependent parameters of solid-state non-isothermal
decomposition reactions by analysis of TG curves [28–30].
Thermogravimetric (TG) and differential thermogravimetric
analyses (DTA) were carried out for group 12 metal–amide
complexes in ambient conditions. In thermogram of complexes
(Fig. 7(a) and (b)), we have found that on heating at a rate
of 10 ◦C/min it loses nearly 2.5–5.6% of its weight between
75 and 110 ◦C (except in complexes I and IV). This corre-
sponds to the loss of water molecule(s). This has been supported
by endothermic peak in DTA. In another steps, decompo-
sition of organic moiety takes place with the formation of
metal oxide as residue. In final steps an exothermic peak
in the DTA suggests that conformational changes occurred
in the amide chelate rings (Table 5). In the temperature
range 320–530 ◦C, the complexes finally forms metal oxide
that has been confirmed with the help of X-ray diffraction
studies.
On the basis of above thermal decomposition, the kinetic
analysis parameters such as activation energy (E*), enthalpy
of activation (ꢀH*), entropy of activation (ꢀS*), free energy
change of decomposition (ꢀG*) were evaluated graphically by
employing the Coats–Redfern relation [31]. The Coats and Red-
fern linearization plots (Fig. 7(a) and (b)), confirms the first order
kinetics for the decomposition process. The calculated values
Fig. 7. (a) TG/DTA and Coats and Redfern linearization plot of complexes of
ligand L1. (b) TG/DTA and Coats and Redfern linearization plot of complexes
of ligand L2.
of E* (J mol−1), A (s−1), ꢀS* (J K−1 mol−1), ꢀH* (J mol−1
)
and ꢀG* (J mol−1) for the decomposition steps of the com-
plexes are reported in Table 6. According to the kinetic data
obtained from the TG curves, the activation energy relates the
thermal stability of the metal complexes. Among metal com-
plexes, activation energy increases as complex II ∼ complex
V < complex IV < complex VI < complex III < complex I, same

368
B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
Table 5
Thermoanalytical data (TG, DTA) of group 12 metal–amide complexes
Complex
[Zn(L1)]
Step
TGrange (K)
DTAmax (K)
Thermal effect
Mass loss: obs. (cal.) (%)
Assignment
Metalic residue
ZnO
I
II
493–598
693–773
528
728
Endo
Exo
9.15 (9.26)
72.95 (73.60)
CO2
Organic moiety
[Cd(L1)]·H2O
[Hg(L1)]·H2O
I
II
III
338–393
473–573
663–738
355
527
717
Endo
Endo
Exo
3.40 (3.33)
8.20 (8.15)
64.35 (64.37)
H2O
CO2
Organic moiety
CdO
I
II
III
353–440
453–530
593–683
372
510
655
Endo
Endo
Exo
2.60 (2.87)
7.32 (7.00)
55.31 (55.67)
H2O
CO2
Organic moiety
HgO
ZnO
[Zn(L2)]
I
II
413–493
698–798
445
775
Endo
Exo
8.00 (7.71)
77.60 (78.03)
CO2
Organic moiety
[Cd(L2)]·2H2O
I
II
III
IV
348–423
498–588
623–663
723–793
393
551
648
781
Endo
Endo
Exo
5.60 (5.51)
6.70 (6.73)
17.90 (17.89)
50.10 (50.24)
H2O
CO2
C9H9
Organic moiety
CdO
HgO
Exo
[Hg(L2)]·H2O
I
II
III
350–410
448–538
615–673
366
513
637
Endo
Endo
Exo
2.50 (2.49)
12.20 (12.15)
55.50 (55.47)
H2O
2CO2
Organic moiety
trends happens with thermal stability of metal complexes. It is
probably because of increase in bulkiness in ligand molecule
from L1 to L2. Cadmium complexes are thermally less stable
than zinc and mercury complexes. All the complexes have neg-
ative entropy, which indicates that the complexes are formed
spontaneously. The negative value of entropy also indicates a
more ordered activated state that may be possible through the
chemisorption of oxygen and other decomposition products.
The negative values of the entropies of activation are com-
pensated by the values of the enthalpies of activation, leading
to almost the same values for the free energy of activation
[32].
3.6. Molecular structures and analysis of bonding modes
In order to ascertain the structural preferences and coordina-
tion behaviour of polydentate amide to metal ions, molecular
mechanics calculations on the [ML]n+ species were under-
taken. We have optimized the molecular structure of complexes.
The optimized molecular structure of complex I is given in
Fig. 8. Energy minimization was repeated several times to find
the global minimum. The energy minimization values for the
optimized structure for the complex I is 62.12 kcal/mol. This
supports tetrahedral geometry of the metal complexes. The
selected bond length and bond angles of the complex I are rep-
Table 6
Thermodynamic activation parameters of group 12 metal–amide complexes
Complex
[Zn(L1)]
Order (n)
1
Step
E* (J mol−1
)
A (s−1
)
ꢀS* (J K−1 mol−1
)
ꢀH* (J mol−1
)
ꢀG* (kJ mol−1
)
I
II
47.78
44.11
4.99 × 104
−159.46
−166.13
−160.93
−165.91
−170.89
−166.92
−172.19
−179.34
02.91
51.75
84.20
120.99
3.08 × 104
[Cd(L1)]·H2O
[Hg(L1)]·H2O
1
1
I
II
III
16.56
18.20
18.35
2.81 × 104
2.29 × 104
1.71 × 104
12.72
35.26
185.98
57.14
87.47
122.71
I
II
III
38.23
38.23
31.86
1.43 × 104
1.04 × 104
5.64 × 103
13.23
95.74
656.42
62.11
87.91
118.12
[Zn(L2)]
1
1
I
II
19.77
20.23
2.66 × 104
−163.26
20.61
62.75
72.67
133.47
1.59 × 104
−172.14
[Cd(L2)]·2H2O
I
II
III
IV
15.29
35.35
19.11
15.93
1.25 × 104
4.75 × 104
1.18 × 104
6.79 × 103
−168.49
−160.22
−173.13
−179.26
−159.58
−166.02
−169.62
46.76
59.55
31.94
89.72
66.26
88.34
112.22
140.09
[Hg(L2)]·H2O
1
I
II
III
21.25
20.23
20.23
3.41 × 104
2.20 × 104
1.77 × 104
78.35
117.72
439.47
58.48
85.29
108.49

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
369
Fig. 8. The optimized molecular structure of complex I.
Table 7
resented in Table 7, is comparable with tetrahedral geometry.
The all bond lengths of Zn–O bond and bond angles O–Zn–O
are same.
Data for selected bond lengths and bond angles of complex I
˚
Bond length (A)
Zn(1)–O(5)
Zn(1)–O(4)
Zn(1)–O(3)
Zn(1)–O(2)
1.8900
On the basis of above physiochemical data with considera-
tion of ring strain, a tetrahedral geometry (Fig. 9(a) and (b)) is
proposed for the group 12 metal–amide complexes.
1.8900
1.8900
1.8900
Bond angle (◦)
O(5)–Zn(1)–O(4)
O(5)–Zn(1)–O(3)
O(5)–Zn(1)–O(2)
O(4)–Zn(1)–O(3)
O(4)–Zn(1)–O(2)
O(3)–Zn(1)–O(2)
C(14)–O(5)–Zn(1)
C(6)–O(4)–Zn(1)
C(7)–O(3)–Zn(1)
C(15)–O(2)–Zn(1)
109.3272
109.5000
109.5000
109.5000
109.5000
109.5000
109.5000
109.5000
109.5000
109.5000
4. Conclusion
It can be concluded that metal(II) complexes were found
to be monomer and involved coordination through carboxy-
late oxygen of the amide ligands giving tetrahedral geometry.
Vibrational spectra indicate the presence of H2O molecule in
the complexes (except zinc complexes) that has been sup-
ported by TG/DTA. Spectroscopic data in solution showed that
the polydentate amide adopts similar coordination behaviour
as suggested by vibrational spectra. Kinetic parameter shows
that the decomposition follows first order kinetics and pro-
ceeds in two/three-step decomposition. The thermal behaviour
of complexes shows that water molecule is removed in first
step—followed by decomposition of the organic moiety in the
second step. Thermal stability of the complexes has been com-
pared. The molecular modeling studies for energy minimization
Fig. 9. (a) Proposed structure of complexes. I: M = Zn(II), n = 0; II: M = Cd(II),
n = 1; III: M = Hg(II), n = 1. (b) Proposed structure of complexes. IV: M = Zn(II),
n = 0; V: M = Cd(II), n = 2, VI; M = Hg(II), n = 1.

370
B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 361–370
and optimization of geometry of the metal complexes have also
been carried out.
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Acknowledgement
B.K. Singh is thankful to University Grants Commission,
New Delhi, India for financial assistance under research award
scheme.
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