572a91

Synthesis and Characterization of Some Cobalt(III) Complexes
Containing Heterocyclic Nitrogen Donor Ligands

a
S. C. NAYAK, b P. K. DAS, and c K. K. SAHOO

a Department of Chemistry, B. N. M. A. College, Paliabindha, Bhadrak, Orissa,
India-756167
e-mail: saratnayak1@rediﬀmail.com

b Department of Chemistry, F. M. College Balasore, Orissa, India-756001

c P. G. Department of Chemistry, Bhadrak College, Bhadrak, Orissa, India-756100

Received 27 November 2001
Accepted for publication 3 December 2002

A number of mixed-ligand complexes of the type trans-[Co(DH)2 LCl] where DH = dimethylgly-
oximato anion, L = heterocyclic nitrogen donor ligand (pyridine, piperidine, quinoline, isoquinoline,
triazole, indole, benzimidazole, benzotriazole, carbazole) have been synthesized and characterized.
These complexes are not soluble in ordinary organic solvent but highly soluble in dimethylsulfoxide.
The conductivity data show the nonelectrolytic nature of the complexes. The IR spectra reveal the
coordination from nitrogen of ligand to central cobalt atom and intramolecular hydrogen bonding.
1
H NMR data of dimethylglyoxime and its Co(III) complexes are in agreement with the trans struc-
ture. The 13 C NMR spectra of the complexes show a sharp signal for carbon atom in the methyl
group. So the four methyl groups in the Co(III) complexes are equivalent and are situated in a plane.
The thermal decomposition studies indicate the absence of water of crystallization or association
of solvent molecules. This fact is supported by IR measurement. The loss of heterocyclic nitrogen
donor ligands does not take place up to 215 ◦C. The ligand and chlorine atom are bonded strongly.
The last product of decomposition is Co3 O4 or CoO.

Recent literature on the chemistry of cobaloximes EXPERIMENTAL
indicates that not much attention has been paid to the
possibility of studying pyridine, piperidine, quinoline, All chemicals, including ligands, were anal. grade.
isoquinoline, triazole, indole, benzimidazole, benzotri- Cobalt [4] and chlorine [5] were determined by re-
azole, carbazole-containing cobaloximes. Cobaloximes ported methods. Elemental analyses were carried out
[1] contain a bisdimethylglyoximatocobalt(III) moiety by using an Erba instrument. The UV spectra (λ =
– Co(DH)+ (DH = monoanion of dimethylglyoxime).
2 200 to 500 nm) were taken by Hitachi-320 Perkin—
They are known to stimulate the reactions of vitamin Elmer Lambda-15 instrument in DMSO solution and
B12 and are important in vitamin B12 model chemistry IR spectra in the ν region 400—4000 cm−1 were
˜
[2]. The X-ray crystal structure of alkylcobaloxime [3] recorded by Perkin—Elmer 783 spectrophotometer by
shows a higher positive charge on cobalt, thus devel- the KBr pellet technique. 1 H NMR and 13 C NMR
oping a strong bond from a base or from an anion spectra in DMSO-d6 were obtained on a Bruker DRX-
along the axial direction. The above-mentioned het- 300 (300 MHz) FT NMR spectrometer using TMS
erocyclic nitrogen donor ligands are of considerable as internal reference. The conductances of the com-
interest, as they play important roles in many biologi- plexes in DMSO solution were measured at room tem-
cal systems. These ligands provide a potential binding perature by employing a Systronic 321 conductivity
site for metal ions. bridge.
For proper understanding of interactions between Thermal decompositions of the complexes were
the biologically important cobaloxime and hetero- carried out by using NETZSCH thermal analyzer.
cyclic nitrogen donor ligand, we have initiated a pro- Measurements were carried out in an atmosphere of
gram of synthesizing and characterizing the complexes nitrogen by using a platinum crucible with a sample
of cobaloxime containing chloride as an axial lig- mass of 100 mg in the temperature range of 25—
and. 1000 ◦C. The rate of temperature increase of 10 ◦C

Chem. Pap. 57 (2) 91—96 (2003) 91

S. C. NAYAK, P. K. DAS, K. K. SAHOO

Table 1. Analytical Data and other Physical Properties of [Co(DH)2 LCl] Complexes I—IX

wi (calc.)/%
wi (found)/% M.p.
Compound L Formula Mr Λ/(S cm2 mol−1 )
Co Cl C H N ◦C

I py CoC13 H19 N5 O4 Cl 403.5 14.60 8.73 38.51 4.63 17.32 7.21 >240
14.62 8.79 38.66 4.7 17.34
II pi CoC13 H23 N5 O4 Cl 409.5 14.35 8.64 38.12 6.18 17.08 11.34 >240
14.40 8.66 38.09 6.10 17.09
III qui CoC17 H21 N5 O4 Cl 453.5 13.12 7.80 44.72 5.53 15.44 5.25 >240
13.00 7.82 44.98 5.63 15.43
IV isoqui CoC17 H21 N5 O4 Cl 453.5 13.12 7.80 44.72 5.53 15.44 6.51 >240
13.00 7.82 44.98 5.63 15.43
V tria CoC10 H17 N7 O4 Cl 393.5 14.54 8.75 30.01 4.32 24.23 21.31 >240
14.62 8.79 29.73 4.21 24.28
VI ind CoC16 H21 N5 O4 Cl 441.5 13.26 8.71 43.47 4.71 15.75 14.51 >240
13.36 8.04 43.48 4.75 15.8
VII bmz CoC15 H20 N6 O4 Cl 442.5 13.41 8.06 40.51 4.55 18.78 17.01 >240
13.33 8.02 40.67 4.51 18.98
VIII btz CoC14 H19 N7 O4 Cl 443.5 13.25 8.13 38.07 4.31 22.21 15.31 >240
13.30 8.00 37.88 4.28 22.09
IX car CoC20 H22 N5 O4 Cl 491.5 12.06 7.14 48.72 4.63 14.18 13.32 >240
12.01 7.23 48.87 4.68 14.25

min−1 was chosen for the measurement. The equip- Table 2. Prominent IR Bands of trans-[Co(DH)2 LCl] Com-
ment records TG and DTA simultaneously. plexes

νi /cm−1
˜
Chloro Cobaloximes [Co(DH)2 LCl] I—IX
L ν(N—O) ν(Co—N)
— ν(Co—N)
To a warm EtOH solution (25 cm3 ) of CoCl2 ·
6H2 O (0.5 g; 2 mmol) dimethylglyoxime (DH) (0.5 py 1245, 1090 1570 530, 425
g; 4.2 mmol) was added. The solution was warmed pi 1247, 1092 1575 532, 430
qui 1247, 1095 1577 520, 430
and stirred for 2—3 min. Then it was cooled and a
isoqui 1245, 1090 1582 525, 432
heterocyclic ligand (2 mmol) (pyridine (py), piperi- tria 1235, 1090 1565 525, 430
dine (pi), quinoline (qui), isoquinoline (isoqui), tria- ind 1224, 1108 1607 507, 446
zole (tria), indole (ind), benzimidazole (bmz), benzo- bmz 1230, 1095 1580 525, 445
triazole (btz), carbazole (car)) was added. It was again btz 1241, 1091 1562 513, 430
warmed and stirred for 2 min. It was then cooled and car 1248, 1095 1585 528, 430
a steady stream of air was passed through it for 1—3
h (depending on base and substituent). A dark brown
solid deposited at the bottom of the flask. The mix-
ture was set aside for 1 h, after which the product was 1
A1g → 1 T1g transition. The 1A1g → 1 T2g of the trans-
collected on a sintered glass and washed successively [Co(DH)2 LCl] complexes is marked by the intense
with small amounts of EtOH, Et2 O and finally dried charge-transfer bands. The band occurring at 28000
in vacuo. cm−1 is due to heterocyclic ligand to cobalt metal
transition (LMCT). The band [7] at 33.000 cm−1 is
RESULTS AND DISCUSSION due to dπ(Co)—π* (DH) (MLCT) transition. A band
at 40000 cm−1 is due to intraligand π—π* transition
All the compounds reported in this work are pre- of the coordinated dimethylglyoxime. The σ DH to
sented in Table 1 along with their analytical data, σ* Co(III) (LMCT) is marked by the intense short-
melting point and conductivity values Λ at room wavelength bands of chloro(ligand)cobaloximes.
temperature. The chloro(ligand)cobaloximes are sta- The free uncoordinated dimethylglyoxime has no
ble and show sign of loss of indole or any other ligand band at 1240 cm−1 . In the Ni(DH)2 complex, a band
and nonelectrolyte in the different solution (10−3 mol appears [8] at 1240 cm−1 due to (N—O) of the ion-
dm−3 ) of dimethylsulfoxide, conductances being in the ized (N—OH) group of dimethylglyoxime [9]. Here in
range of 7—21 S cm2 mol−1 . all cobalt complexes, this band [10] is seen in the re-
The ligand field spectra of complexes in MeOH gion of 1224 to 1248 cm−1 and additional band (N—
show a peak of weak to moderate intensity [6] at O) appears at ν = 1090 to 1108 cm−1 (Table 2, see
˜
ν ≈ 18000—20000 cm−1 . This is the spin-allowed
˜ Fig. 1). From this it is evident that the N—O bonds of

92 Chem. Pap. 57 (2) 91—96 (2003)

COBALT(III) COMPLEXES

Fig. 1. Infrared spectrum of [Co(DH)2 indCl].
11.20

8.09
8.07
7.47
7.44
7.38
7.35
7.32
7.15
7.13
7.10

3.51

2.39

14 12 10 8 6 4 2 0
δ
Fig. 2. Part of P NMR spectra of [Co(DH)2 carCl].

coordinated dimethylglyoxime are not entirely equiv- complexes shifted from 1562 to 1607 cm−1 . The char-
alent. This observation is consistent with the result acteristic band of (Co—NDH ) appears at 513 cm−1
of X-ray diffraction analysis of Cu(DH)2 complexes in and (Co—Nind ) band [14] appears at 446 cm−1 .
which two different Cu—N distances are observed [11]. The band involving the metal and chlorine ν(M—
The stretching vibration of C—N in the pyridine ap- Cl) has been reported to appear around ν = 200 to
˜
pears at 1590 cm−1 . Upon complex formation the peak 300 cm−1 [15, 16]. Due to limitation of our IR in-
shifts to higher frequencies, which is confirmed by the strument this particular band could not be observed.
bond formation of metal with the nitrogen atom of Many examples of such bonding between cobalt and
the respective heterocyclic ring. The weak broad band chlorine are known [17, 18]. The 1 H NMR spectra of
appearing at around 3410 cm−1 belongs to the in- cobaloxime complexes clearly show that on coordina-
tramolecular hydrogen bonding and suggests the trans tion, the δ due to the CH3 group of DH2 at 1.91 un-
configuration [12, 13]. The oxime part (C—N) takes dergoes a downfield shift by 0.43 to 2.33—2.41 (see
part in the chelate ring formation in cobalt complexes. Fig. 2). In all complexes δ appears as a singlet in-
The normal stretching vibration (C—N) of free DH2 dicating the equivalent of four methyl groups and the
is at ν = 1620 cm−1 . In the present case this band in all
˜ presence of trans structure. The δ due to OH proton in

Chem. Pap. 57 (2) 91—96 (2003) 93


11.33
dimethylglyoxime appears at 11.33 (Fig. 3). This sig-

3.40

1.91
nal shifted upfield in the complexes appears at around
11.2. The indole proton or other ligand protons also
undergo a considerable downfield shift.

Integral
1.0000

0.5976

4.0746
10 8 6 4 2 0
δ
Fig. 3. 1H NMR spectra of dimethylglyoxime in DMSO-d6 so-
The 13 C NMR shifts of the chloro cobalt complexes lution using TMS as internal reference.
containing indole or other related axial ligand were as-
signed. The δ due to oxime CH3 carbon in dimethyl-
glyoxime appears at 9.2. This signal shifted downfield ments. The loss of indole, pyridine, etc. ligands does
appears at 12.98 on coordination. The δ for imine car- not take place up to 215 ◦C. Both the ligand and chlo-
bon appears at 154 in free dimethylglyoxime. On co- rine are bonded strongly. The final product of decom-
ordination this signal shifted upfield appears at 151.5. position is Co3 O4 or CoO.
The δ value of axial ligands in indole complexes gen- Thermal decomposition data on the compound VI
erally shifted downfield appears at 39 to 40 (Fig. 4). are collected in Table 3. The thermal decomposition
All of the observations show the trans structure of the compounds is a multistage process, the final
of these indole complexes of the chlorocobaloxime product of each case was Co3 O4 . The results of the
[Co(DH)2 indCl]. The indole or the other related lig- thermal decomposition of the complexes are compared
ands are bonded to the cobalt atom through their het- with the literature results [19].
erocyclic nitrogen. The TG and DTA curves for the decomposition
Thermal decomposition studies indicate the ab- of [Co(DH)2 indCl] are shown in Fig. 5 and other re-
sence of water of crystallization or association of sol- lated thermal decomposition data are collected in Ta-
vent molecules. This fact is supported by IR measure- ble 3. The TG curve indicates that it is thermally
150.627

40.387
40.112
39.834
39.555
39.274

12.770

200 150 100 50 0
δ
Fig. 4. Part of 13 C NMR spectra of [Co(DH)2 indCl].

94 Chem. Pap. 57 (2) 91—96 (2003)

COBALT(III) COMPLEXES

Table 3. Thermal Decomposition Data of Compound [Co(DH)2 indCl]

DTA TG Mass loss/% Loss of Composition
θpeak/◦C θrange/◦C found (calc.) Cl, indole,1/4DH of residue

216 endo 200—300 41.1 (41.5) [Co(DH)1.75 ]
467.3 endo 300—600 13.14 (13.26) 0.5 (DH) [Co(DH)1.25 ]
947 exo 600—1000 19.71 (19.81) 0.75 (DH) [Co(DH)0.5 ]
1239.9 endo 1000—1250 8.85 (8.48) Decomposition of DH 1/3 Co3 O4

TG DTA
100 DTA 1 exo
90
0
80

σ /(µV mg-1)
70
w/mass %

Co(DH)2 -1
60 Co(DH)
TG -2
50
40 -3
30 Co3O4

20 -4
endo

200 400 600 800 1000 1200
θ / °C
Fig. 5. TG and DTA curves of [Co(DH)2 indCl], sample mass 100 mg, heating rate 10 ◦C min−1 , atmosphere: nitrogen.

stable up to 210 ◦C showing the absence of water three endothermic peaks at 200—300 ◦C, 300—600 ◦C,
in crystal lattice. Afterwards, the TG curve shows and 1000—1250 ◦C corresponding to removal of chlo-
four mass loss steps. The first step between 200 to rine atom of the indole molecule and decomposi-
300 ◦C is accompanied by 41.1 % mass loss, attributed tion of dimethylglyoxime with simultaneous formation
to the loss of chlorine atom, indole molecule and of Co3 O4 . There is an exothermic process at 600—
25 % of dimethylglyoxime molecules. The second step 1000oC corresponding to decomposition reaction of
(300 ◦C—600 ◦C) and the third step (600 ◦C—1000 ◦C) dimethylglyoxime.
are accompanied by mass loss 13.26 % and 19.71 %, re- Acknowledgements. The authors thank the authorities of
spectively. These two steps may be due to the decom- the Regional Sophisticated Instrumentation Centre (RSIC) at
position of dimethylglyoxime molecules. The last step IIT (Indian Institute of Technology), Madras for UV and IR
(1000 ◦C—1200 ◦C) is accompanied by 8.85 % mass spectra and thermogravimetric analysis. Thanks are due to the
Regional Sophisticated Instrumentation Centre (RSIC), Luc-
loss, at which the complex is converted to metallic ox-
know for providing NMR and 13 C NMR spectra. One of us
ide. The thermal decomposition reaction of the com- (S. C. Nayak) is thankful to the University Grants Commis-
plex II [Co(DH)2 indCl] can be represented as sion (UGC), Kolkata Unit for financial support under a minor
research project.
[Co(DH)2 indCl] 200—300 ◦C endo [Co(DH)1.75 ] + Cl
+ indole + (DMG)0.25
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96 Chem. Pap. 57 (2) 91—96 (2003)

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