Cobalt(II) complexes of new biomimetic polydentate amide: Spectroscopic, kinetics of thermal decomposition and XRPD studies

Article abstract of DOI:10.1016/j.saa.2007.05.045

Complexes of Co(II) with new ligands N′,N″-bis(3-carboxy-1-oxoprop-2-enyl) 2-amino-N-arylbenzamidine (C₂₁H₁₇N₃O₆), N′,N″-bis(3-carboxy-1-oxopropanyl) 2-amino-N-arylbenzamidine (C₂₁H₂₁N₃O₆) and N′,N″-bis(3-carboxy-1-oxophenelenyl) 2-amino-N-arylbenzamidine (C₂₉H₂₁N₃O₆) have been synthesized and characterized by elemental analyses, vibrational spectra, electronic spectra, TOF-mass spectra, magnetic susceptibility measurements, thermal studies and X-ray powder diffraction studies. Vibrational spectra indicate coordination of amide and carboxylate oxygen of the ligands along with two water molecules giving a MO₆ weak field octahedral chromophore. Electronic spectra and magnetic susceptibility measurements reveal octahedral geometry for Co(II) complexes. The elemental analyses and mass spectral data have justified the ML complexes. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm first-order kinetics. Powder diffraction determines the cell parameters of the complexes.

Full text of DOI:10.1016/j.saa.2007.05.045

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 880–888
Cobalt(II) complexes of new biomimetic polydentate amide: Spectroscopic,
kinetics of thermal decomposition and XRPD studies
Bibhesh K. Singh, Parashuram Mishra, Bhagwan S. Garg∗
Department of Chemistry, University of Delhi, Delhi 110007, India
Received 16 November 2006; received in revised form 12 May 2007; accepted 17 May 2007
Abstract
Complexes of Co(II) with new ligands Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxoprop-2-enyl) 2-amino-N-arylbenzamidine (C21H17N3O6), Nꢀ,Nꢀꢀ-bis(3-
carboxy-1-oxopropanyl) 2-amino-N-arylbenzamidine (C21H21N3O6) and Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl) 2-amino-N-arylbenzamidine
(C29H21N3O6) have been synthesized and characterized by elemental analyses, vibrational spectra, electronic spectra, TOF-mass spectra, magnetic
susceptibility measurements, thermal studies and X-ray powder diffraction studies. Vibrational spectra indicate coordination of amide and car-
boxylate oxygen of the ligands along with two water molecules giving a MO6 weak field octahedral chromophore. Electronic spectra and magnetic
susceptibility measurements reveal octahedral geometry for Co(II) complexes. The elemental analyses and mass spectral data have justified the
ML complexes. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm
first-order kinetics. Powder diffraction determines the cell parameters of the complexes.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Amide; Cobalt(II); Spectra; Thermal and X-ray powder diffraction studies
1. Introduction
to peptides and certain enzymes. Thus, we have designed
amide ligands containing carboxylic anchors condensed with
amidines. The amidine unit is the key functional group in a
wide range of biologically active molecules [5], including a vari-
ety of serine protease inhibitors [6,7] and antimicrobial agents
[8].
The binding of metal ions to amide group(s) has been the
subject of research over the past three decades, precisely due to
ambidentate nature of amide bond and many of these reactions
provide simple models for much more complex metal–peptide
systems and enzymes [1]. There are numerous examples of the
in vivo interactions of transition metal ions with the ligands
containing amide group(s), and these interactions can be of
biological importance [2–4]. Magainin-2-amide, the bioactive
peptoid exhibit selective, potent antibacterial activity against
both Gram-positive and Gram-negative bacteria [2]. These
readily synthesized, protease-resistant peptoids represents
an important advance in peptide biomimicry, the develop-
ment of nonpeptide foldameric analogues of anti bacterial
peptides.
Because of the number and variety of coordination sites
in a biomimetic amide, its coordination chemistry is compli-
cated and the most definitive information has been obtained by
using results from combination of various techniques. Spectral
characterization and the kinetic calculations of thermal decom-
position reactions serve as important tools for the interpretation
of structures of molecules of biological and analytical impor-
tance. Powder diffraction data is especially useful to deduce
accurate cell parameters as well as particle size. The fact that the
three-dimensional reciprocal lattice information is condensed
onto a one-dimensional intensity profile with respect to 2θ in
powder diffraction is the main concern. This leads to overlapping
of the diffraction peaks, considerable and preferred orienta-
tion among the crystallites [9]. Therefore, we report herein the
synthesis, spectral characterization (infrared, electronic, mass
and 1H NMR), magnetic susceptibility measurements, thermal
and XRPD studies of the complexes of amide with cobalt(II)
ions.
Though, much work has been done on amide ligands in gen-
eral and the amide ligands in combination with other functional
groups, but only less work has been done on amide system
containing carboxylic anchors, which may act as biomimic
∗
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.2007.05.045

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
881
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 UV–vis 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.
Magnetic susceptibility measurements were carried out at room
temperature in powder form on a vibrating sample magnetome-
ter PAR 155 with 5000G-field strength, using Co [Hg(SCN)4] as
the calibrant (magnetic susceptibility ≈1.644 × 10−5 cm3 g−1).
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,
bothofwhichweresuppliedalongwiththeinstrument. Aflatbed
type aluminium crucible was used with ␣-alumina (99% pure) as
the reference material for DTA. The number of decomposition
steps was identified using TG. The activation energy and Arrhe-
nius constant of the degradation process was obtained by Coats
and Redfern method. The XRD powder pattern were recorded on
a vertical type Philips 1130/00 X-ray diffractometer, operated at
Fig. 1. L1.
acid, washed with ethanol and dried over P4O10 in vacuo. Ligand
is soluble in common polar solvents.
Light yellow crystal—Yield: 85%; Anal. Calc. for
C21H17N3O6 requires (%): C, 61.91; H, 4.18; N, 10.32.
Found: C, 61.48; H, 4.67; N, 10.41 (MW: 407.4); FTIR
(KBr, cm−1): ν(O–H) 2601(w), ν(N–H)Amide 3407(s),
ν(C O)Amide I 1645(w), [ν(C–N) + δ (N–H)]Amide II 1413(s),
ν[(N–H) + δ(C–H)]Amide III 1275, νas(CO2) 1568(s), νs(CO2)
1401(s); TOF-MS (m/z): 408 (12%, M]+), 348 (100%,
C19H15N3O4]+), 185 (11%, C12H14N2]+). H NMR (ppm):
1
11.20 (2H, s, –OH carboxylic acid), 8.25 (2H, s, sec. amide),
7.50 (4H, m, benzylidenimin), 7.20 (5H, m, H-benzene), 1.81
(4H, d, –CH methyne group); electronic spectral data (cm−1):
30,120 (␲ → ␲*, N-amide), 35,842 (n → n*, C O group).
2.2.3. Synthesis of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl)
2-amino-N-arylbenzamidine) (C21H21N3O6) (L2) (Fig. 2)
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.
˚
40 kVand 50 Ma generator using the Cu K␣ line at 1.54056 A as
the radiation sources. Sample was scanned between 10◦ and 70◦
(2θ) at 25 ◦C. The crystallographic data was analyzed by using
the CRYSFIRE-2000 powder indexing software package and the
space group was found by GSAS program. Debye–Scherer rela-
tion with the help of 100% peak width, determined the particle
size. The density was determined by Archimedes method.
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 (MW: 411.4); FTIR (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);TOF-MS (m/z):
412 (14%, M]+), 352 (100%, C19H19N3O4]+), 187 (11%,
2.2. Synthesis of the ligand
C12H14N2]+). H NMR (ppm): 11.30 (2H, s, –OH carboxylic
1
2.2.1. Synthesis of 2-amino-N-arylbenzamidine (C13H13N3)
2-Amino-N-arylbenzamidine (C13H13N3) as precursor was
synthesized by reported procedure [10]. This was characterized
1
by elemental analysis, infrared spectra, H NMR spectra and
mass spectra. Satisfactory results were obtained.
2.2.2. Synthesis of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxoprop-2-enyl)
2-amino-N-arylbenzamidine (C21H17N3O6) (L1) (Fig. 1)
A finely ground mixture of 2-amino-N-arylbenzamidine
(0.5 mmol), maleic 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, filtrate on slow
evaporation forms fine crystals. This was recrystallized in acetic
Fig. 2. L2.

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B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
Fig. 4. Complex I.
Fig. 3. L3.
4
19,380 [4T1g(4F) → T1g(4P)(ν3)], 30,120 (␲ → ␲*, N-amide);
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);
electronic spectral data (cm−1): 30,488 (␲ → ␲*, N-amide),
35,842 (n → n*, C O group).
μeff (BM): 4.82; FTIR (KBr, cm−1): ν(N–H)Amide 3420(s),
ν(C O)Amide I 1635(w), [ν(C–N) + δ(N–H)]Amide II 1460(s),
ν[(N–H) + δ(C–H)]Amide III 1296, νas(CO2) 1574(s), νs(CO2)
1406(s), ν(M–O) 372(m); TOF–MS (m/z): 500 (12%, M]+),
464 (8%, C21H15N3O6Co]+), 407 (6%, C19H13N3O4Co]+),
364 (35%, C17H12N3O3 Co]+), 305 (50%, C17H11N3O3]+,
236 (100%, C14H10N3O]+, 222 (46%, C14H10N2O]+;TG [Step
I, Step II] E* (J mol−1): 68.81, 70.90; A (×105 s−1): 7.63,
5.23; ꢀS* (J K−1 mol−1): −133.64, −140.38; ꢀH* (J mol−1):
104.90, 140.3 and ꢀG* (kJ mol−1): 48.22, 78.19.
2.2.4. Synthesis of Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)
2-amino-N-arylbenzamidine (C29H21N3O6) (L3) (Fig. 3)
A finely ground mixture of 2-amino-N-arylbenzamidine
(0.5 mmol), phthalic 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.
2.3.2. Synthesis of cobalt(II)
Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl)
2-amino-N-arylbenzamidine complex (C21H23N3O8Co)
[Complex II] (Fig. 5)
Light yellow crystal—Yield: 84%; Anal. Calc. for
C29H21N3O6 requires (%): C, 68.64; H, 4.14; N, 8.28. Found: C,
68.60; H, 4.11; N, 8.25 (MW: 507); FTIR (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); TOF-MS (m/z):
507 (15%, M]+), 449 (100%, C27H19N3O4]+), 329 (12%,
To a methanolic solution of CoCl2·6H2O (0.5 mmol) was
added a methanolic solution of the ligand (L2) (0.5 mmol). The
resulting dark brown-colored solution was stirred for 3 h, after
which the volume was reduced on a warm water bath. The dark
brown 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 brown crystals of the complex.
Dark brown crystal—Yield: 85%; Anal. Calc. for
C21H23N3O8Co requires (%): C, 50.01; H, 4.56; N, 8.33;
Co, 11.69. Found: C, 50.23; H, 4.67; N, 8.39; Co, 11.71
(MW: 503.9); Electronic spectral data (cm−1): 15,600
1
C20H14N3O2]+), 236 (16%, C14H10N3O]+; H 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); Elec-
tronic spectral data (cm−1): 31,447 (␲ → ␲*, N-Amide), 35,842
(n → n*, C O group).
2.3. Synthesis of cobalt(II) complexes
[4T1g(4F) → A2g(4F)(ν2)], 20,408 [4T1g(4F) → T1g(4P)(ν3)],
30,488 (␲ → ␲*, N-amide), 34,965 (n → n*, C O group);
μeff (BM): 5.01; FTIR (KBr, cm−1): ν(N–H)Amide 3434(s),
ν(C O)Amide I 1640(w), [ν(C–N) + δ(N–H)]Amide II 1425(s),
ν[(N–H) + δ(C–H)]Amide III 1320, νas(CO2) 1563(s), νs(CO2)
4
4
2.3.1. Synthesis of cobalt(II)
Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxoprop-2-enyl)
2-amino-N-arylbenzamidine complex[C21H19N3O8Co]
[Complex I] (Fig. 4)
To a methanolic solution of CoCl2·6H2O (0.5 mmol) was
added a methanolic solution of the ligand (L1) (0.5 mmol). The
resulting dark pink-colored solution was stirred for 3 h, after
which the volume was reduced on a warm water bath. The
dark pink 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 pink crystals of the complex.
Dark pink crystal—Yield: 80%; Anal. Calc. for
C21H19N3O8Co requires (%): C, 50.41; H, 3.80; N, 8.40;
Co, 11.78. Found: C, 50.64; H, 4.00; N, 8.34 (MW: 499.9); elec-
4
tronic spectral data (cm−1): 16,393 [4T1g(4F) → A2g(4F)(ν2)],
Fig. 5. Complex II.

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
883
posed formula. All three ligands have similar tendency of initial
lossoftwomoleculesofHC O]+ with100%m/zvalues. Ligands
(L1 and L2) seem to be almost of similar trend of degradation and
finally forms two molecules of C6H5NH2]+, whereas ligand (L3)
finally degrades to amidine]+. In the spectra of Co(L1)2H2O,
Co(L2)2H2O, and Co(L3)2H2O initial fragmentation pattern is
again similar (loss of two water molecules), a mononuclear
nature for these complexes can be deduced. The selected range
of m/z for the spectrum of complexes allows the observation
of [Co(L)]+ and 100% 236 m/z species [C14H10N3O]+. A con-
clusive proof of a strict attribution of these signals is shown in
(Fig. 7: TOF-MS spectra of Complex I), where the isotopic pro-
file observed at 236/238 in the spectrum is in good agreement
with that calculated for [C14H12N3O]+ species, which could
be the result of demetallation and subsequently a partial intra
molecular hydrogen bonding. Likewise peaks attributable to
unstable monomeric species as [Co(L)]+ and [H2L]+ are usually
present in the mass spectra of these systems [13,14].
Fig. 6. Complex III.
1408(s), ν(M–O) 365 (m); TOF-MS (m/z): 504 (10%, M]+),
468 (6%, C21H19N3O6Co]+), 292 (100%, C16H10N3O3]+), 246
(56%, C15H8N3O]+), 220 (40%, C14H8N2O]+ TG [Step I, Step
II] E* (J mol−1): 35.04, 38.23; A (×105 s−1): 1.74, 1.32; ꢀS*
(J K−1 mol−1): −146.34, −152.27; ꢀH* (J mol−1): 100.75,
142.78 and ꢀG* (kJ mol−1): 55.56, 90.80.
2.3.3. Synthesis of cobalt(II)
3.2. IR spectra and mode of bonding
Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)
2-amino-N-arylbenzamidine complex (C29H23N3O8Co)
[Complex III] (Fig. 6)
An ambidentate nature of the amide can coordinate with
cobalt(II) either through nitrogen of the-NH2 or the oxygen
of the C O group. The bands due to the amide ν(N–H) mode
at 3200–3400 cm−1 for the free ligands L1, L2 and L3 are
shifted towards higher frequencies, while the ν(C O) (amide
I) frequency (1630–1650 cm−1) undergoes shift towards lower
frequency (1600–1640 cm−1) in the cobalt complexes indicat-
ing non-involvement of the amide nitrogen and involvement of
the carbonyl oxygen atom of amide group in coordination with
the metal ion [15] (Fig. 8a and b). This result is explained by
the decrease in the double-bond character of C O and the sub-
sequent increase of C–N double-bond character [16,17]. 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 (1410–1480 cm−1 in ligands) and Amide
III (1275–1305 cm−1 in ligands) bands arising out of ν(C–N) as
well as δ(N–H) modes (coupled to one another), shift towards
To a methanolic solution of CoCl2·6H2O (0.5 mmol) was
added a methanolic solution of the ligand (L3) (0.5 mmol). The
resulting dark brown-colored solution was stirred for 3 h, after
which the volume was reduced on a warm water bath. The dark
brown 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 brown crystals of the complex.
Dark brown crystal—Yield: 79%; Anal. Calc. for
C29H23N3O8Co requires (%): C, 58.01; H, 3.91; N,
7.00; Co, 9.82. Found: C, 58.13; H, 3.99; N, 7.06; Co,
9.92 (MW: 599.9); electronic spectral data (cm−1): 15,898
4
4
[4T1g(4F) → A2g(4F)(ν2)], 19,646 [4T1g(4F) → T1g(4P)(ν3)],
31,447 (␲ → ␲*, N-amide); μeff (BM): 4.95; FTIR (KBr,
cm−1): ν(N–H)Amide 3396(s), ν(C O)Amide I 1610(w),
[ν(C–N) + δ(N–H)]Amide II 1493(s), ν[(N–H) + δ(C–H)]Amide III
1338, νas(CO2) 1559(s), νs(CO2) 1409(s), ν(M–O) 380 (m);
TOF-MS (m/z): 601 (12%, M]+), 564 (8%, C29H19N3O6Co]+),
292 (89%, C16H10N3O3]+), 236 (100%, C14H10N3O]+), 222
(47%, C14H10N2O]+. TG [Step I, Step II]: E* (J mol−1): 32.77,
35.04; A (×105 s−1): 1.81, 1.39; ꢀS* (J K−1 mol−1): −146.10,
−151.62; ꢀH* (J mol−1): 148.80, 76.54 and ꢀG* (kJ mol−1):
56.03, 86.80.
higher frequencies 1430–1530 cm−1 and 1295–1340 cm−1
,
respectively, in complexes further confirming the coordination
through carbonyl oxygen. The ν(C O) and ν(C–O) stretching
frequencies in the 1540–1590 and 1420–1400 cm−1 region in
the uncomplexed ligands have been assigned to νas(CO2−) and
νs(CO2−)modesofthecarboxylategroupandthesebandsshows
considerable shift in all the complexes. A positive and negative
shift in complexes indicates a ꢀν enhancement upon complex-
ation and suggests coordination of carboxylate groups in all
the complexes of the three ligands in a monodentate fashion
[18,19]. 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-
ging modes of water and metal–oxygen stretching modes are
observed in the 800–350 cm−1 region for all the complexes of
all the ligands and may be attributed to coordinated water in
3. Results and discussion
Satisfactory results of elemental analysis and spectral studies
revealed that the ligands and complexes were of good purity.
3.1. TOF-MS
Mass spectrometry has been successfully used to investigate
molecular species in solution [11,12]. The molecular ion peaks
of the ligands and complexes have been used to confirm the pro-

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B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
Fig. 7. TOF-MS spectrum of Complex I.
4
transition 4T1g(4F) → T2g(4F) could not be observed in all the
complexes. The second transition is two-electronic process and
therefore its bands are with low-intensity. The broad band with
maximum at 21,250 cm−1 corresponds to the highest energy
all the complexes [18]. This can be confirmed with the help of
thermograms. A new band in the 390–360 cm−1 regions in the
spectra of the complexes is assignable to ν(Co–O) [20].
4T1g(4F) → T1g(4P) (ν3) transition.
4
3.3. Magnetic susceptibility measurement and electronic
spectra
3.4. Kinetics of thermal decomposition
The magnetic susceptibility measurements were carried out
in the solid state at 298 K. The magnetic moment (μeff) value
of the complexes lie in the range of 4.7–5.2 BM, which is in
agreement with high-spin state of Co(II) and middle in strength
crystal field [21]. The electronic spectra of the ligands and its
cobalt complexes have been studied in the range 200–900 nm.
In the UV region, the shoulder band observed at 279 nm in water
solvent in ligands 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 286 nm in case of Complex II, while it dis-
appeared in the Complexes I and III revealing the involvement
of the C O of amide group or carboxylate oxygen in chelate for-
mation. The band observed at 332, 328 and 318 nm in ligands L1,
L2 and L3, respectively, may be assigned to ␲ → ␲* transition
of the N-amino group of the amide ligand, remain at the same
position in all the respective cobalt complexes, indicating they
are not affected by metal chelation [22]. In the visible absorption
spectra of complexes (Fig. 9a and b), two bands in the range of
15,475–16,500 and 20,900–21,300 cm−1 were observed. The
first one is attributed to d–d transition and the second one to
O (σ) → Co(II) transition. Assuming an octahedral coordina-
tion for Co(II) complexes, three spin allowed transitions are
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 [23,24]. Ther-
mogravimetric (TG) and differential thermogravimetric (DTA)
analyses were carried out for Co(II)-amide complexes in ambi-
ent conditions (Fig. 10). The correlations between the different
decomposition steps of the complexes with the corresponding
weight losses are discussed in terms of the proposed formula of
the complexes.
The Complex I with the molecular formula [C21H19N3O8Co]
is thermally decomposed in two successive decomposition
steps. The first estimated mass loss of 7.40% (calculated mass
loss = 7.20%) within the temperature range 349–413 K may be
attributed to the loss of two water molecules. The DTA curve
gives an endothermic peak at 360 K (the maximum peak tem-
perature). The second step occurs within the temperature range
540–593 K with the estimated mass loss 45.01% (calculated
mass loss = 44.65%) which corresponds to the loss of organic
moiety leaving CoO and Co3O4 as residue. The DTA curve
gives an exothermic peak at 556 K (the maximum peak tem-
perature). Total estimated mass loss is 52.41% (calculated mass
loss = 51.85%).
4
4
expected: 4T1g(4F) → T2g(4F) (ν1), 4T1g(4F) → A2g(4F) (ν2),
4
4T1g(4F) → T1g(4P) (ν3). The first band appeared in the region
The thermal decomposition of Complex II with the molecular
formula [C21H23N3O8Co] proceeds with two main degrada-
8080–8650 cm−1 usually in the IR region, so ν1 transition due to

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
885
Fig. 8. (a) IR spectrum of Complex II. (b) IR spectrum of Complex III.
tion steps. The first step occurs within the temperature range
365–421 K with an estimated mass loss 7.10% (calculated mass
loss = 7.14%) which is reasonably accounted for the loss of two
water molecules. The DTA curve gives an endothermic peak at
379 K (the maximum peak temperature). The second step occurs
within the temperature range 570–684 K with an estimated mass
loss 44.40% (calculated mass loss = 45.10%), which is reason-
ably accounted for the loss of organic moiety, leaving CoO and
Co3O4 as residue. The DTA curve gives an exothermic peak at
595 K (the maximum peak temperature). Total estimated mass
loss is 51.50% (calculated mass loss = 52.24%).
The thermal decomposition of Complex III with the molec-
ular formula [C29H23N3O8Co] also proceeds with two main
degradation steps. The first estimated mass loss of 6.05%
(calculated mass loss = 6.00%) within the temperature range
360–403 K could be attributed to the liberation of two water
molecules. The DTA curve gives an endothermic peak at 383 K
(the maximum peak temperature). The second step occurs within

886
B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
Fig. 10. Thermograms of complexes.
where α is the mass loss up to the temperature T, R the gas con-
stant, E*theactivationenergyinJ mol−1, θ thelinearheatingrate
∼
and the term (1 − 2RT/E*) 1. A straight line plot of left hand
=
side of Eq. (1) against 1/T gives the value of E* while its intercept
corresponds to A (Arrhenius constant). The linearization plots
(Fig. 11a–c), confirms the first-order kinetics for the decompo-
sition process. The calculated values 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 complexes are reported in experi-
Fig. 9. (a) Electronic spectra of Complexes I and II. (b) Electronic spectra of
Complex III.
the temperature range 563–691 K with an estimated mass loss
53.55% (calculated mass loss = 53.88%), which is reasonably
accounted for the decomposition of organic moiety leaving CoO
and Co3O4 as residue. The DTA curve gives an exothermic peak
at 572 K (the maximum peak temperature). Total estimated mass
loss is 59.60% (calculated mass loss = 59.88%).
The final product of decomposition at 878 K corresponds to
the formation of CoO and Co3O4 as the end product, which
was confirmed by comparing the observed/estimated and the
calculated mass of the pyrolysis product [25].
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 [26] (1):
ꢀ
ꢁ
ꢀ
ꢂ
ꢃꢁ
−log(1 − α)
AR 1 − 2RT
log
= log
T2
θE∗
E
E∗
−
(1)
2.303RT
Fig. 11. Linearization plot of complexes.

B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
887
Table 1
Crystallographic data of ligands
Empirical formula
C21
H
17N3O6 (L1)
C21H21N3O6 (L2)
C29H21N3O6 (L3)
Formula weight
Temperature (K)
407.4
298
411.4
298
507.5
298
˚
Wavelength (A)
1.54056
Tetragonal
P422
1.54056
Triclinic
P1
1.54056
Tetragonal
P422
Crystal system
Space group
˚
Unit cell dimensions (A)
a = 10.253400, b = 10.253400,
a = 7.089918, b = 8.763797, c = 12.380810,
a = 12.240700, b = 12.240700,
c = 12.78373, α = β = γ = 90◦
α = 69.31414◦, β = 72.2477◦, γ = 104.1482◦
c = 20.354620, α = β = γ = 90◦
Volume (A3)
2θ range
1343.98
10–70◦
628.12
10–70◦
3049.86
10–70◦
Limiting indices
0 ≤ h ≤ 5
0 ≤ k ≤ 4
0 ≤ l ≤ 7
1.005
−4 ≤ h ≤ 4
−4 ≤ k ≤ 4
0 ≤ ≤ 7
0 ≤ h ≤ 4
0 ≤ k ≤ 4
0 ≤ l ≤ 9
1.104
Density (g/cm3)
Z
1.086
2
1
4
Avs. Eps
Merit of fitness
Particle size (nm)
0.0000537
M (7) = 15, F(7) = 4
0.932
0.0000685
M (22) = 4, F (22) = 3
0.8036
0.000089
M (20) = 6, F (20) = 8
13.873
Table 2
Crystallographic data of cobalt(II) complexes
Empirical formula
C21H19N3O8 Co (Complex I)
C21H23N3O8Co (Complex II)
C29H23N3O8Co (Complex III)
Formula weight
Temperature (K)
499.9
298
503.9
298
599.9
298
˚
Wavelength (A)
1.54056
1.54056
1.54056
Crystal system
Space group
Unit cell dimensions (A)
Cubic
P432
Hexagonal
P6/mmm
Tetragonal
P422
a = 9.419596, b = 9.419596,
c = 19.160880, α = β = γ = 90◦
1698.32
10–70◦
˚
a = 4.861712, b = 4.861712,
c = 9.499986, α = β = γ = 90◦
224.54
10–70◦
a = 17.445040, b = 17.445040, c = 9.833171,
α = 90◦, β = 90◦, γ = 120◦
2591.59
10–70◦
Volume (A3)
2θ range
Limiting indices
Density (g/cm3)
Z
1 ≤ h ≤ 3, 0 ≤ k ≤ 2, 0 ≤ l ≤ 3
0 ≤ h ≤ 10, 0 ≤ k ≤ 3, 0 ≤ l ≤ 6
0 ≤ h ≤ 5, 0 ≤ k ≤ 4, 0 ≤ l ≤ 9
1.66
1.29
1.173
1
4
2
Avs. Eps
0.0000101
0.0000512
0.0000723
Merit of fitness
Particle size (nm)
M (6) = 37, F(6) = 10
2.684
M (10) = 13, F (10) = 4
10.292
M (8) = 10, F (8) = 3
13.78
mental section. According to the kinetic data obtained from the
TG curves, the activation energy relates the thermal stability
of the metal complexes. Among metal complexes, activation
energy increases as Complex III < Complex II < Complex I,
same trends happens with thermal stability of metal complexes.
It is probably because of increase in bulkiness in ligand molecule
from L1 to L3. All the complexes have negative entropy, which
indicates that the complexes are formed spontaneously. The neg-
ative value of entropy also indicates a more ordered activated
state that may be possible through the chemisorption of oxy-
gen and other decomposition products. The negative values of
the entropies of activation are compensated by the values of the
enthalpies of activation, leading to almost the same values for
the free energy of activation [27].
of the complex. The indexing procedures were performed using
(CCP4, UK) Crysfire programme [28] giving different crystal
system with varying space group. The merit of fitness and parti-
cle size of ligands and its cobalt complexes has been calculated.
The cell parameters of the ligands and complexes are shown in
Tables 1 and 2, respectively.
4. Conclusion
Cobalt(II) complexes were found to be monomer and
involved coordination through carboxylate oxygen of the amide
ligands along with two water molecules giving octahedral geom-
etry. Vibrational spectra indicate the presence of H2O molecule
in the complexes that has been supported by TG/DTA. Kinetic
parameter shows that the decomposition follows first-order
kinetics and proceeds in two-step decomposition. The thermal
behaviour of complexes shows that water molecule is removed
in first step—followed by decomposition of the ligand molecule
in the second step. Magnetic susceptibility measurements and
3.5. X-ray powder diffraction studies
X-ray powder data is especially useful to deduce accurate cell
parameters. The diffraction pattern reveals the crystalline nature

888
B.K. Singh et al. / Spectrochimica Acta Part A 69 (2008) 880–888
electronic spectra also support octahedral coordination geome-
try around the Cobalt(II) ions. The complex crystallizes in the
cubic/hexagonal/tetragonal crystal system.
[9] B.K. Singh, P. Mishra, B.S. Garg, Spectrochim. Acta A 67 (2007)
719.
[10] W. Szczepankiewicz, J. Suwinski, Tetrahedron Lett. 39 (1998) 1785.
[11] J. Sanmartin, F. Novio, A.M. Garcia-Deibe, M. Fondo, N. Ocampo, M.R.
Bermejo, Polyhedron 25 (2006) 1714.
Acknowledgement
[12] I. Beloso, J. Castro, J.A. Garcia–Vazquez, P. Perez–Lourido, J. Romero, A.
Sousa, Polyhedron 22 (2003) 1099.
[13] N. Yoshida, K. Ichikawa, M. Shiro, J. Chem. Soc., Perkin Trans. 2 (2000)
17.
B. K. Singh is thankful to University Grants Commission,
New Delhi, India for financial assistance under research award
scheme.
[14] N. Yoshida, H. Oshio, T. Ito, J. Chem. Soc., Perkin Trans. 2 (1999) 975.
[15] T.A. Kabanos, J.M. Tsangaris, J. Coord. Chem. 13 (1984) 89.
[16] M. Nonoyama, K. Yamasaki, Inorg. Chim. Acta 3 (1969) 585.
[17] D.J. Barnes, R.L. Chapman, F.S. Stephens, R.S. Vagg, Inorg. Chim. Acta
51 (1981) 155.
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