Binary and mixed ligand complexes of nifuroxime, spectral, and thermal studies

Article abstract of DOI:10.1081/SIM-120023485

Complexes of nifuroxime (anti-5-nitro-2-furfuraldoxime, 5N-FDH) with Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), La(III) and UO₂(VI) have been prepared and characterized by elemental analyses, conductivity measurements, spectral and thermal studies. Also, some mixed nifuroximato-phenylalaninato, -tyrosinato or -methioninatonickel(II) complexes have been prepared and characterized by the previous methods. The complexes have the general formulae: [M(5N-FD)₂·mH₂O] _n; [La(5N-FD)(OOCCH₃)₂(H₂O) ₂]₂; [UO₂ (5N-FD)₂]·2H ₂O and [Ni(5N-FD)(L)]·H₂O where M = Mn(II) Co(II), Ni(II), Cu(II), Zn(II) or Cd(II); m = 0 for M = Mn(II), m = 1 for M = Co(II), Ni(II) or Cd(II) and m = 2 for M = Cu(II) or Zn(II); L = phenylalaninate, tyrosinate or methioninate. All complexes were isolated at pH 6 using NH ₄OH solution. Nifuroxime acts as a uninegative ligand in all complexes with various modes of coordination. Thermogravimetric studies show a rapid decomposition stage for all the complexes accompanied by other slow stages; the final product of all complexes is the metal oxide.

Full text of DOI:10.1081/SIM-120023485

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Binary and Mixed Ligand Complexes of Nifuroxime,
Spectral, and Thermal Studies
Asma I. El‐Said a
a Chemistry Department , Faculty of Science , Assiut University , Assiut, Egypt
Published online: 15 Feb 2007.
To cite this article: Asma I. El‐Said (2003) Binary and Mixed Ligand Complexes of Nifuroxime, Spectral, and Thermal Studies,
Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry, 33:7, 1171-1186, DOI: 10.1081/SIM-120023485
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 7, pp. 1171–1186, 2003
Binary and Mixed Ligand Complexes of
Nifuroxime, Spectral, and Thermal Studies
Asma I. El-Said*
Chemistry Department, Faculty of Science, Assiut University,
Assiut, Egypt
ABSTRACT
Complexes of nifuroxime (anti-5-nitro-2-furfuraldoxime, 5N-FDH) with
Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), La(III) and UO2(VI) have
been prepared and characterized by elemental analyses, conductivity
measurements, spectral and thermal studies. Also, some mixed
nifuroximato-phenylalaninato, -tyrosinato or -methioninatonickel(II)
complexes have been prepared and characterized by the previous
methods. The complexes have the general formulae: [M(5N-
FD)2ꢀmH2O]n; [La(5N-FD)(OOCCH3)2(H2O)2]2; [UO2(5N-FD)2]ꢀ2H2O
and [Ni(5N-FD)(L)]ꢀH2O where M = Mn(II) Co(II), Ni(II), Cu(II),
Zn(II) or Cd(II); m = 0 for M = Mn(II), m = 1 for M = Co(II), Ni(II) or
*
Correspondence: Asma I. El-Said, Chemistry Department, Faculty of Science,
Assiut University, Assiut, Egypt; E-mail: asmaa@acc.aun.edu.eg.
1171
DOI: 10.1081/SIM-120023485
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

1172
El-Said
Cd(II) and m = 2 for M = Cu(II) or Zn(II); L = phenylalaninate,
tyrosinate or methioninate. All complexes were isolated at pH 6 using
NH4OH solution. Nifuroxime acts as a uninegative ligand in all
complexes with various modes of coordination. Thermogravimetric
studies show a rapid decomposition stage for all the complexes
accompanied by other slow stages; the final product of all complexes
is the metal oxide.
Key Words: Nifuroxime; Metal oximate; Amino acids.
INTRODUCTION
Oximes are well known for their complexing properties.[1,2] They are
often used for analytical and biological purposes.[3,4] 2-Furfuraldoxime and
its complexes have been widely studied.[5–7] Furfuraldoxime acts as a
potential ligand towards many transition metal cations. Bouet et al.[6]
studied the influence of side chain substituents in position five of
furfuraldoxime on its coordinating behaviour. What has not been reported,
in the literature are any solid metal complexes containing the radiosensitizer
and bioreductive anti-5-nitro-2-furfuraldoxime [nifuroxime, Figure 1(a)].
Therefore, we have embarked on the synthesis and characterization of new
Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), La(III) and UO2(VI) binary
complexes of nifuroxime. Also, in view of the importance of metal-amino
acid complexes in the fields of biological studies and chemotherapy,[8–10]
we have synthesized and studied some mixed Ni(II) complexes containing
Figure 1. Structures of the ligands.

Complexes of Nifuroxime
1173
nifuroxime and amino acids (L-phenylalanine, L-tyrosine and L-methio-
nine) (Figure 1b).
EXPERIMENTAL
General
Micro analyses (C, H, N, S) were performed using an Analytischer
Funktionstest Vario E1 elemental analyzer. IR spectra were recorded on a
Shimadzu IR-470 spectrophotometer in the 200–4000 cm–1 range using
KBr discs. Electronic spectra in DMF solution and in the solid state were
obtained using a Shimadzu UV-2101 PC spectrophotometer. Conductivity
measurements (in DMF) were made using a LF Digi-550 conductance
bridge. Mass spectra were run at 70 eV on a Jeol JMS600 apparatus, ap-
plying the ionization method (EI+). The thermogravimetric analyses were
determined using a Sartorius 200 MP electrobalance converted to a thermo-
balance by the addition of a small furnace and sample holder. The
temperature was measured using a chromal-alumal thermocouple attached
to a Soar ME-550 digital multimeter, the heating rate was adjusted to be
8 °C min–1.
Starting Materials
Nifuroxime was BDH grade. MnCl2ꢀ4H2O, CoCl2ꢀ6H2O, NiCl2ꢀ6H2O,
CuCl2ꢀ2H2O, ZnCl2ꢀH2O, CdCl2ꢀ2.5H2O, La(OOCCH3)3ꢀ2H2O and UO2-
(NO3)2ꢀ6H2O were of analytical grade.
Preparation of Complexes
Binary Complexes
Since the preparation of the complexes followed essentially similar
procedures, a general method will be described. A solution of nifuroxime
(0.3 g, 1.9 mmol) in methanol (15 mL) was added to the metal salt solution
(3.8 mmol) in 10 mL aqueous methanolic solution (50:50 V/V). The
reaction mixture was stirred for 2 hours. Addition of NH4OH (0.2 M) to
adjust the pH at 6 gave the corresponding products: a light yellow Mn(II)
complex, yield 0.24 g; a dark brown Co(II) complex, yield 0.30 g; a
yellowish brown Ni(II) complex, yield 0.32 g; a dark green Cu(II) complex,

1174
El-Said
yield 0.31 g; a light yellow Zn(II) complex, yield 0.26 g; a yellow Cd(II)
complex, yield 0.25 g; a bright yellow La(III) complex, yield 0.95 g and a
yellow UO2(VI) complex, yield 0.37 g. The products were filtered, washed
with methanol (5 mL) and dried over P4O10.
Mixed-Ligand Complexes
Preparation of the Ni(II) mixed-ligand complexes followed the same
procedure. The preparation of (nifuroximato)(phenylalaninato)nickel(II)
is typical. A solution of nifuroxime (0.3 g, 1.9 mmol) in methanol (15
mL) was added to 0.45 g (1.9 mmol) of nickel chloride hexahydrate in
aqueous methanolic (50:50 V/V) solution (10 mL). The solution mixture
was treated with 0.31 g (1.9 mmol) phenylalanine in 20 mL methanol. The
resulting solution was stirred for 3 hours. A yellowish-brown product was
obtained by treating the above reaction mixture with NH4OH (0.2 M) to
adjust the pH at 6.5. The deposited solid was filtered, washed with
methanol and dried over P4O10; yield 0.65 g.
RESULTS AND DISCUSSION
The reaction between MCl2ꢀxH2O, La(OOCCH3)3ꢀ2H2O or UO2(NO3)2ꢀ
6H2O with nifuroxime (5N-FDH) in the molar ratio 1:2 or 1:1 and at pH 6
affords the corresponding complexes. The synthesis of the various complexes
may be represented as follows:
nMCl2ꢀxH2O þ 2nð5N-FDHÞ
ð1Þ
methanol=H2O
½Mð5N-FDÞ2ꢀmH2Oꢁn þ 2nHCl
NH4OH
M = Mn (1), Co (2), Ni (3), Cu (4), Zn (5) or Cd (6); x = 1–6 and m = 0
for Mn(II), m = 1 for Co(II), Ni(II), Cd(II) and m = 2 for Cu(II) and
Zn(II).
methanol=H2O
2LaðOOCCH3Þ3ꢀ2H2O þ 2ð5N-FDHÞ
NH4OH
ð2Þ
ð3Þ
½Lað5N-FDÞðOOCCH3Þ2ðH2OÞ2ꢁ2ð7Þ þ 2CH3COOH
UO2ðNO3Þ2ꢀ6H2O þ 2ð5N-FDHÞ
methanol=H2O
½UO2ð5N-FDÞ2ꢁꢀ2H2O ð8Þ þ 2HNO3
NH4OH

Complexes of Nifuroxime
1175
Also, the reaction between NiCl2ꢀ6H2O, nifuroxime and various amino
acids in the molar ratio 1:1:1 yields the corresponding mixed-ligand com-
plex according to the following general equation:
NiCl2ꢀ6H2O þ 5N-FDH þ HL
methanol=H2O
ð4Þ
½Nið5N-FDÞLꢁꢀH2O þ 2HCl
NH4OH
where L = phenylalaninate (phala) (9), tyrosinate (tyr) (10) or methioninate
(met) (11).
The present compounds are sufficiently stable under normal atmo-
spheric conditions. They are insoluble in many common polar or non-polar
solvents but soluble in DMF and DMSO giving rise to non-conducting
solutions. Physical characterisation, microanalytical, and molar conductance
data of the complexes are given in Table 1. The complexes were identified
by elemental analyses, spectral data (IR, UV-Vis, MS) and thermal studies.
IR Spectra
The infrared spectra in the region 4000–400 cm–1 provide some
information regarding the mode of coordination in the complexes in
comparison with that of the free ligand. Generally, furfuraldoxime has three
potential coordinating sites[5] which are the furanic O-ring, O-oxime and N-
oxime. In the case of our ligand (nifuroxime) there is the NO2 group
besides the above mentioned three coordinating sites. Thus, on complexa-
tion of 5N-FDH it has been expected that the main perturbed vibrations in
its spectrum are O–H stretching, N–O stretching, C=N stretching, NO2
stretching and ring breathing. In addition, new bands arise from the
following new bonds: metal-N and metal-O (ring or oxime).[11] The most
relevant bands and proposed assignments for the free ligand and the
complexes are listed in Table 2. The spectra of the binary complexes are
very similar to one another except for [La(5N-FD)(OOCCH3)2(H2O)2]2 and
[UO2(5N-FD)2]ꢀ2H2O. However, all are different from the spectrum of the
free ligand. The anionic character of the nifuroxime ligand and its
coordination mode through the nitrogen and oxygen atoms or the ring-
oxygen and oxime-oxygen are inferred from the positions and the shifts in
the IR bands of the studied compounds, compared to the bands of the free
ligand. Bands at 3010 and 3160 cm–1 of the free ligand are attributable to
the n(OH) vibration of the N-OH group.[5] On the other hand, all complexes
show no IR bands in this region (3010-3160 cm–1), indicating that the
oxime proton is fully removed.[12]

1176
El-Said

Complexes of Nifuroxime
1177

1178
El-Said
For the complexes (1)–(7) and (9)–(11) bands in the region 1650–
1680 cmꢂ1 are assigned to n(C=N) which is observed in the free ligand at
1620 cmꢂ1. This blue-shift may indicate that the bond order of the carbon-
nitrogen link is increased upon coordination to the various metal ions.[13]
However, this band appears at the same position as in the free ligand (1620
cm–1) for the complex [UO2(5N-FD)2]ꢀ2H2O, indicating that the C=N
group is not taking part in coordination. From the shift in the N–O
vibration to higher frequency (970–990 cmꢂ1) for all complexes (1)–(11)
one can conclude that the ligand does not exist in the oxime form. Taking
the C–O–C stretching frequency of furan as a criterion for the denticity of
this ligand, there is no shift of this band in all complexes (Table 2), except
for complex (8) in which there is a red-shift by 20 cm–1 compared to that of
the free ligand. This may mean that the furan oxygen does not participate in
coordination for all complexes but is taking part in coordination in the
uranyl complex.[13] The strong withdrawing effect of the NO2 group may
alter the furan oxygen to participate in coordination. In the spectra of the
complexes additional bands appear in the regions 1420–1445 cm–1 and
1560–1565 cm–1. For complexes (7) and (9)–(11), these are assigned to
ns(COO) and nas(COO), respectively. This defines bidentate chelating
acetato groups (Dn = nas ꢂ ns = 1560–1420 = 140 cm–1)[14] for complex
(7) and suggests the presence of an ionized and coordinated carboxylate
group of the amino acid in complexes (9)–(11).
For the latter three complexes (9)–(11), the n(N–H) band appears in the
3340–3350 cm–1 region.[15] The presence of water of crystallisation in the
complexes (2)–(6) and (8)–(11) is shown by a broad band in the 3450–3580
cm–1 region, while the intense band at 3350 cm–1 is assigned to n(OH), the
frequency of coordinated water for complex (7). The new band in the 610–
680 cm–1 range is attributable to the M–O bond. For the UO2 complex (8),
the band appearing at 880 cm–1 corresponds to n(O=U=O). For all com-
plexes, except (8), the M–N bond is observed in the 405–460 cm–1 re-
gion. The band which appears at 1350–1355 cm–1 for all complexes is
assigned to ns(NO2), which demonstrates that no important spectral
changes are found for this band compared with that in the free ligand
(1350 cm–1), indicating that the NO2 group in 5N-FDH does not participate
in coordination.
Electronic Absorption Spectra
The electronic spectra of the complexes were recorded in solution
(DMF) and in the solid state (Nujol mull) (Table 3). The Ni(II) binary
complex (3) shows two broad bands at 613 and 544 nm; these bands could

Complexes of Nifuroxime
1179
Table 3. Electronic spectral data (nm) (emax, M–1cm–1) of the complexes in DMF
solution and in the solid state.
Complex
d–d
Charge transfer
Intraligand transition
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
–
880 (98), 530 (324)
405a
377a, 261a
426 (2301)
427 (1017)
444 (2033)
406a
417a
433a
547a, 433 (1015)
424 (1813)
421 (2050)
454 (2076)
391 (31220), 277 (10262)
340 (27550), 264 (12371)
352 (32662), 268 (11135)
355a, 257a
338a, 254a
358a, 250a
613 (106), 544 (266)
625 (178), 477 (822)
–
–
–
–
356 (15232), 249 (11452)
335 (21613), 277 (18658)
358 (21613), 279 (18340)
332 (22615), 275 (19918)
633 (117), 888 (92)
642 (123), 875 (85)
630 (136), 890 (77)
aNujol mull.
1
1
1
1
be assigned to the transitions A1g ! A2g (n1), A1g ! B1g (n2), res-
pectively, as seen in other Ni(II) square-planar complexes.[16] The broad
shoulder at 427 nm in solution and shoulders at 416 and 377 nm in the solid
state can be assigned to the d–p* transition. The UV-Vis spectra of the
cobalt(II) complex (2) in DMF and/or Nujol are identical and show a
shoulder at 530 nm. The existence of this shoulder suggests either octahedral
or square-planar structure around the cobalt(II) ion.[17] The observation of a
narrow band at 880 nm supports the existence of a square-planar structure
around cobalt(II) ion.[17] The intense band at 426 nm, could be assigned to a
charge transfer transition (M ! L). The electronic spectrum of the Cu(II)
complex (4) in solution displayed two weak and broad bands around 625 and
2
2
2
2
477 nm which may be assigned to B1g ! Eg and B1g ! A1g transitions,
respectively, in an idealized square-planar symmetry. This complex in the
Nujol mull spectrum displays a band at 557 nm which could be taken as
evidence for distortion from the planar configuration in the solid state. A
charge transfer band appears at 444 nm. Bands appearing in the 340–391 and
264–277 nm regions for the above complexes (2), (3) and (4) may be
assigned to the p–p* and n–p* intraligand transitions, respectively.
The Nujol mull spectra of the Mn(II), Zn(II) and Cd(II) complexes (1),
(5) and (6) (Table 3) show three bands at 405, 377, 261 nm; 406, 355, 257
nm and 417, 338, 254 nm, respectively. The first intense band of all is as-
signed to charge transfer (L !M), while the latter two bands are less intense
and are attributed to p–p* and n–p* intraligand transitions, respectively.

1180
El-Said
For the La(III) complex (7), the solid state spectrum shows peaks at
433, 358 and 250 nm. The first band was assigned to charge transfer and the
latter two bands to intraligand transitions (p–p* and n–p*). The f–f
transition of the lanthanum ions in the visible region may be obscured due to
the broad charge-transfer band. The UO2(VI) complex (8) exhibits bands in
DMF due to the p-p* and n-p* transitions at 356 and 249 nm, respectively.
The broad band at 433 nm can be assigned to charge-transfer from the
uranyl oxygen to the uranium f orbitals.[18] The ligand-to-uranium charge
transfer transition band appears in the Nujol mull spectrum at 547 nm.
The electronic spectra of the mixed-ligand Ni(II) complexes (9)–(11)
in DMF show a broad band in the 630–642 nm region which is assigned to
1A2g transition in square-planar Ni(II) complexes.[16] Bands at
1
the A1g
!
421–454 nm can be assigned to charge transfer transitions. The strong
absorption band appearing in the 332–335 nm region could be assigned to
the p–p* transition of the nifuroximate moiety, while the intense broad
band located in the 275–279 nm region could be ascribed to the intraligand
transition of the amino acid moiety. The n–p* transition of the nifuroximate
moiety is probably obscured by the relatively strong absorption band of the
amino acid moiety.
Mass Spectra
The mass spectra of some of the complexes were recorded to help
determine their stoichiometric composition. For the Cu(II) binary complex
(4), the molecular ion peak (C10H14CuN4O12) appears at m/z = 409.5 (calc.
409.8). Other low-intensity peaks appear at m/z = 415.5 and 432.3; and
may support a bridged structure of the complex to form a polynuclear
compound (Figure 3) through the deprotonated oximate oxygen.[19]
However, the Ni(II) binary complex (3) shows the highest mass number
at m/z = 386 (calc. 386.9) which corresponds to the molecular ion
C10H8N4NiO9. This confirms the stoichiometry of these complexes as being
of the ML2 type.
On the other hand, for the UO2(VI) complex (8) and [Ni(5N-FD)-
(phala)]ꢀH2O (9) the molecular ion peaks were not detected, probably due
to the high thermal instability of these complexes in the ionization beam.
From the foregoing results we may suggest that nifuroxime coordinates
through the N (oxime) atom and is bridged by the deprotonated oximate to
another metal ion in complexes (1)–(7) and (9)–(11). In view of the
effective bridging function of the oximate group, many workers have
studied oximates or dioximates as bridging[12,19–23] ligands. The X-ray crys-
tal structure of a binulcear Cu(II)Cu(II) complex doubly bridged by oximate
oxygens has been reported.[22] However, nifuroxime acts as a monoanionic

Complexes of Nifuroxime
1181
bidentate ligand through the furan-oxygen and oxime-oxygen atoms in
complex (8), which may be attributable to the stronger U–O bond than U–
N bond.[7]
Thermal Analysis
All the prepared complexes decompose with explosive force. This
difficulty of obtaining the thermograms of these complexes has been
Table 4. Characterized thermoanalytical data (TG/DTG) for the complexes.
Mass loss, %
Temperature
DTG
Evolved moiety
(residue)
Complex Stage
range (°C) max (°C) Found Calcd.
(1)
1
2
50–155
160–175
100
165
32.7
43.1
24.1
5.0
32.6 2NO2 + HCN
43.6 rest of ligand
23.8 (MnO2)
4.7 H2O
Residue >175
(3)
1
2
50–90
110–130
85
125
58.2
58.9 5N–FD +
NO2 + HCN
3
Residue >295
130–295
240
17.4
19.4
8.5
17.1 rest of ligand
19.3 (NiO)
8.8 2H2O
(4)
(6)
(7)
1
2
3
50–90
90–175
175–185
75
140
180
13.5
58.9
19.1
14.9
16.5
39.5
29.0
8.0
13.2 2HCN
58.6 rest of ligand
19.4 (CuO)
Residue >185
1
2
3
45–80
80–125
25–255
75
14.5 H2O + N2O
16.6 NO2 + HCN
39.8 rest of ligand
29.1 (CdO)
115
205
Residue >255
1
2
75–175
140
180
8.0 4H2O
30.8 2(C5H2N2O3)
175–185
185–275
275–450
30.5
plateau
375
3
10.0
51.4
10.1 rest of ligand
51.1 (La2(CO3)3)
Residue >450
> 800 °C (La2O3)
(8)
(9)
1
2
70–165
165–205
100, 145 30.6
31.0 2H2O + 5N-FD
23.5 rest of ligand
45.5 (U3O8)
4.5 H2O
175
23.8
45.5
4.9
Residue >205
1
2
3
65–105
150–230
230–245
95
190
235
41.3
35.4
18.5
41.5 phala
35.1 5N-FD
18.9 (NiO)
Residue >245

1182
El-Said
Figure 2. TG-DTG curves of (a) [Ni(5N-FD)2ꢀH2O]n (3), (b) [Cu(5N-FD)2ꢀ2H2O]n
(4), and (c) [La(5N-FD)(OOCCH3)2(H2O)2]2 (7).

Complexes of Nifuroxime
1183
overcome by using a sample of Al2O3 pre-calcined at 1000 °C as a diluent.
Each complex was admixed with 20% by weight Al2O3. The thermal
behaviour of some the complexes were studied using TG/DTG technique in
air over the temperature range from ambient to 600 °C. Thermogravimetric
data are shown in Table 4 and thermograms in Figure 2. All of the studied
complexes decompose in two or three stages, the second or third stage of
each is a very rapid one. Complexes (3), (4) and (9) lose water at 85, 75
and 95 °C corresponding to 5% (calc. 4.7%), 8.5% (calc. 8.8%) and 4.9%
(calc. 4.5%) weight loss, respectively. This suggests that these complexes
contain water of crystallization (one molecule for complexes (3) and (9)
and two for complex (4)). For complex (6) the water molecule is evolved
with NO2 at 75 °C (Table 4). However, for complex (7) two water
molecules are evolved at 140 °C indicating the presence of coordinated
water. The first decomposition step for complex (8) is a composite one
including loss of two water molecules and the oxime moiety. Depending on
the percentage weight loss, it was obvious that the decomposition in these
complexes proceeds mostly with rupture of the bonds inside the oxime
Figure 3. Proposed structures of the complexes.

1184
El-Said
ligand, resulting in the elimination of NO2 and HCN molecules[24] (Figure 2
and Table 4). Depending upon the percentage weight loss and the IR
spectra, the end product of the complexes (3), (4), (6) and (9) is NiO, CuO,
CdO and NiO, respectively. For complex (8) it is U3O8 and for complex (7)
it is La(CO3)3, which upon heating in an external oven at 800 °C changes
to La2O3. The proposed residue for complex (1) is MnO2. If the final stage
in the decomposition of the complexes is the most rapid one, the residue
was obtained at lower temperatures. This is examplified for complexes (1),
(4), and (8) (Table 4, Figure 2b). This may be due to burning-off of the
decomposition products to form the metal oxide residue rapidly.[25] In all
cases, the intermediates formed are unstable and undergo further
decomposition at higher temperatures.
Based on the above studies, tentative structures for the complexes
(Figure 3) have been proposed.
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Received May 1, 2002
Accepted March 20, 2003
Referee I: R. D. Pike
Referee II: S. C. Sendlinger