Solid nickel(II) complex with methionine sulfoxide

Article abstract of DOI:10.1016/S0925-8388(00)01022-7

Solid Ni(C₅H₁₀NO₃S)₂·2H₂O complex was prepared and characterized. Electronic absorption spectrum shows an octahedral geometry for the complex. Infrared spectroscopy analysis shows that the metal atom

Full text of DOI:10.1016/S0925-8388(00)01022-7

Journal of Alloys and Compounds 308 (2000) 153–157
L
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A solid nickel(II) complex with methionine sulfoxide
b
a
Pedro P. Corbia , , Petr Melnikov , Antonio C. Massabni
*
aInstitute of Chemistry — UNESP, Department of General and Inorganic Chemistry, Rua Prof. Francisco Degni s/n, CP 355, CEP 14801-970,
Araraquara, SP, Brazil
b
´
CCET, UFMS – Cidade Universitaria s/n, CEP 79070-900, Campo Grande, MS, Brazil
Received 21 February 2000; received in revised form 4 May 2000; accepted 12 May 2000
Abstract
Solid Ni(C5H10NO3S)2?2H2O complex was prepared and characterized. Electronic absorption spectrum shows an octahedral geometry
for the complex. Infrared spectroscopy analysis shows that the metal atom is coordinated to the ligand through (COO2) and (S5O)
groups. Thermal analysis confirmed the composition of the complex and suggests that the water molecules are not coordinated to the
metal ion. The complex shows extremely high solubility in water.
2000 Elsevier Science S.A. All rights reserved.
Keywords: Bioinorganic chemistry; Methionine sulfoxide; Amino acids; Nickel(II)
1. Introduction
the cases of acute poisonings. Metal(II) ion complexes of
the first transition series involving the amino acid D,L-
methionine (Met) have been reported in the literature
[11,12]. In these complexes, the central metal ions are
coordinated to the ligand through carboxylate and amino
groups, in an octahedral arrangement. These complexes are
also described as polymeric and insoluble in water com-
pounds, with formula [M(Met)2]n. However, no data has
been reported so far as to the MetSO compounds with
trace elements. This work deals with synthesis and charac-
terization of a solid nickel(II) metal complex involving
methionine sulfoxide as a ligand, in the form of anion.
Methionine sulfoxide (MetSO, C5H11NO3S, Fig. 1), a
product of vegetal origin, is an amino acid similar to
methionine. It is present in garlic [1], onion [2], nuts [3],
carrots [4], apples [5,6] and banana plants [7]. Methionine
sulfoxide was shown to be associated with Alzheimer
disease due to the glutamine synthetase inhibition. This
process is performed through the free radical oxidation
stress. So, the compound is considered a biological sensor
for this transformation [8,9]. This amino acid of easy
access and low cost was also shown to possess pronounced
anti-inflammatory properties [10]. In this context, the
ligand itself and transition metal complexes with MetSO
are of interest for obtaining virtual chemical vehicles for
safer drugs, pharmaceutical trappers or even antidotes for
2. Experimental
2.1. Materials and methods
D,L-Methionine sulfoxide, cysteine and nickel(II) chlo-
ride, NiCl2?6H2O, were Sigma and Merck products, ana-
lytical-grade purities. Elemental analysis for carbon, hy-
drogen and nitrogen was performed using a CHN-O
EA1110 Analyzer (CE Instruments), cysteine being used
as a reference substance. Nickel content was determined by
using the atomic absorption technique with a Perkin-Elmer
A Analyst 300. Infrared spectra were recorded on an FT-IR
Spectophotometer Spectrum 2000 (Perkin-Elmer); samples
were prepared in the form of KBr pellets. UV-Vis spectrum
Fig. 1. Schematic structure of methionine sulfoxide.
*Corresponding author.
E-mail address: pedrcorb@iq.unesp.br (P.P. Corbi).
0925-8388/00/$ – see front matter
PII: S0925-8388(00)01022-7
2000 Elsevier Science S.A. All rights reserved.

154
P.P. Corbi et al. / Journal of Alloys and Compounds 308 (2000) 153–157
of the complex, measured in a 3.531022 M aqueous
solution of the complex, was registered on a Lambda 14 P
Spectrophotometer (Perkin-Elmer). Thermal analysis was
performed on a Thermoanalyzer TG/DTA simultaneous
SDT 2960 TA Instruments, under the following conditions:
synthetic air, 100 ml/min, and heating at 108C/min, from
40 to 12008C.
3. Results and discussion
3.1. Chemical analysis rendered the following values
Anal. calcd. for Ni(C5H10NO3S)2?2H2O (%): C, 28.52;
H, 5.76; N 6.65; Ni, 13.94. Found: C, 27.73; H, 5.20; N,
6.17; Ni, 14.55. In spite of a prolonged drying until
constant mass, water content in the samples may show
variations within the limits 0,n,2 for each mol of the
complex. Inner location of water molecules within the
structural units of the complex may be an explanation, as it
was observed for other similar metal complexes [13].
2.2. Preparation of the complex
The Ni(II) complex was synthesized by adding 0.002
mol of NiCl2?6H2O in alcoholic solution to an alcoholic
solution of the potassium salt of MetSO containing 0.004
mol of the ligand (molar proportion metal:ligand equal to
1:2), at room temperature and under stirring. The Ni(II)
complex was precipitated as a blue powder, after 18 h of
constant stirring, from the resulting blue intense solution at
pH 6.0. The compound was filtered, washed with ethanol
and maintained in a dessicator together with fresh P4O10
for 2 weeks. The yield of the synthesis was about 76%.
Crystals of the complex could not be obtained to perform
an X-ray study. Preparations in aqueous medium were also
tested but no solid product could be isolated probably due
to the high affinity of the complex to water. The potassium
salt solution of methionine sulfoxide used in the synthesis,
was previously prepared by adding 0.004 mol of potassium
hydroxide in alcoholic solution to a fine suspension
containing 0.004 mol of MetSO (molar proportion of 1:1)
in alcoholic medium. This reaction was carried out under
stirring and heating the mixture near its boiling point (ca.
708C).
3.2. Electronic absorption spectra
As shown in Fig. 2, the spectrum of Ni(C5H10NO3S)2?
2H2O is characterized by the presence of three well-
defined bands and a shoulder corresponding to the follow-
ing spectral bands: lmax. I 370 nm (27 016 cm21); lmax. II
619 nm (16 162 cm21); lmax. III 744 nm (13 439 cm21
,
shoulder); lmax. IV 1000 nm (9998 cm21). The analysis of
these bands show an octahedral geometry for the com-
pound. According to literature [14] the maxima are as-
signed to the following transitions: lmax. I –3T1g (P) ←
3A2g, lmax. II –3T1 g
← ←
3A2g and lmax. IV –3T2g 3A2g.
The shoulder (lmax. III) is assigned to the forbidden
1
3
transition Eg ← A2g. These results permitted to calculate
the 10 Dq and B values as 9998 and 884 cm21, respective-
ly, being similar to those obtained for the octahedral Ni(II)
complex of the amino acid methionine [11] and S,S9-
methylenebis(cysteine) [13], structurally related to
methionine sulfoxide.
Fig. 2. Electronic absorption spectrum for the Ni(II) complex with methionine sulfoxide.

P.P. Corbi et al. / Journal of Alloys and Compounds 308 (2000) 153–157
155
Table 1
Infrared absorption frequencies (cm21) of methionine sulfoxide, potassium methioninate-sulfoxide and Ni(II) complexa
Methionine
sulfoxide
C5H11NO3S
Potassium
Ni(C5H10NO3S)2?2H2O
Assignments
[13,15]
methioninate-
sulfoxide
K1(C5H10NO3S)2
3410 w
–
n (O–H)
nas. (NH2)
ns. (NH2)
nas. (CH2)
ns. (CH2)
nas. (COO2)
ds. (NH2)
d (CH2)
3665–2912 vs
2700–3200 s
3696–2839 vs
1589 vs
1516 s
1452 m
1414 s
1345 m
1276 m
1187 w
1156 w
1027 s
664 w
1589 vs
–
–
1419 s
1357 m
–
–
–
1587 s
–
–
1390 m
1337 w
–
–
–
ns. (COO2)
ds. (CH3)
d (C–H)
n (C–COO–)
nas. (C–C–N)
n (S5O)
1027 s
660 w
1000 m
654 m
n (C–S)
a vs, very strong; s, strong; m, medium; w, weak.
3.3. Infrared absorption spectroscopy
(COO2) and ns. (COO2) frequencies of that one found for
the Ni(II) complex with MetSO.
Ni(C5H10NO3S)2?2H2O infrared vibration frequencies
are shown in Table 1, in comparison to those of
methionine sulfoxide and its alkaline metal salt, the latter
compound being considered as an ionic approximation to
evaluate the free ligand frequencies (see infrared spectra in
Fig. 3).
The sulfoxide (S5O) group represents another possi-
bility for the Ni(II) ion coordination to the ligand. In order
to confirm or not the contribution of this site, electronic
structure of sulfoxides may be of value as represented by
According to these comparative data, one of the coordi-
nation sites is certainly the bidentate carboxylate group. It
is well-known that the difference between the vibrational
nas. (COO2) and ns. (COO2) frequencies, D, in amino
acids, generally increases with an increasing of the M–O
bond strength and with the coordination form, which can
be bidentade or monodentade. The latter coordination
entity gives rise to higher D values [16–18] while the
former leads to D values similar to those found in ionic
carboxylate compounds, as, for instance, the potassium
carboxylate salt. As one can see from Table 1, the
vibrational modes related to the nas. (COO2) and ns.
(COO2) in the potassium methioninate-sulfoxide occur at
1589 and 1419 cm21, while in the nickel complex they are
observed at 1587 and 1390 cm21. So, for potassium
methioninate-sulfoxide, D5170 cm21 and for nickel com-
plex, D5197 cm21. For the carboxylate group, these
relatively small differences indicate a bidentade coodina-
tion through metal ion. This inference is strongly con-
firmed by the results described to the relative first row
transition metal complexes with the amino acid methionine
[M(Met)2]n [11] in which the coordination of the metal
atoms to the ligand occur through the bidentade car-
boxylate and show the same difference between nas.
Fig. 3. Infrared absorption spectra for methionine sulfoxide, potassium
methioninate-sulfoxide and nickel(II) complex.

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P.P. Corbi et al. / Journal of Alloys and Compounds 308 (2000) 153–157
Table 2
Thermogravimetric data for Ni(C5H10NO3S)2?2H2O
Temperature (8C)
Mass change (%)
40–180
180–940
950
Dm1
Dm2
Residual mass
4.69
78.62
16.69
Fig. 4. Schematic structures of the hybrids of sulfoxide group.
the resonance hybrids in Fig. 4 [19]. If the coordination
occurs through oxygen, giving rise to a partially ionized
bond, the frequency characteristic of n (S5O) in the
structure (II) would be shifted toward lower values. If, on
the contrary, the coordination occurs via sulfur, the (S1–
O2) frequency in the structure (I) should be shifted to
higher values. Indeed, as shown in Table 1, the n (S5O)
frequency of the free ligand and its closely related potas-
sium salt is observed at 1027 cm21, while in the nickel
complex this band is shifted to 1000 cm21. So, we can
conclude that the coordination of the metal ion to the
(S5O) group of the ligand occurs basically through
oxygen atom.
Possibility of coordination through the amino group is
discarded by analysing the infrared spectra (Fig. 3), in
comparison to other well-known complexes involving the
amino acid methionine [11,12]. For the Mn(II), Co(II),
Cu(II) and Ni(II) complexes with methionine, of general
formula [M(Met)2]n, the coordination of the metal ion by
the amino group was attested by the lowering of the
absorption range of the nas. (NH2) and ns. (NH2) group
(broad band) in the complexes in relation to the potassium
salt of the ligand. In the case of the Ni(II) complex with
methionine sulfoxide, there is no shifting of the frequen-
cies of the amino group in comparison to the potassium
salt of the ligand, which suggest that there is no metal ion
coordination through NH2 group.
3.4. Thermal analysis
3.4.1. Thermogravimetric analysis (TG)
Thermogravimetric analysis has been useful for con-
firmation of the complex compositions as well as for the
identification of the final product. Actually, the number of
water molecules of the complex occurs to be in the range
0–2 per mol of the complex. These water molecules are
lost at the beginning of the heating, at temperatures not
exceeding to 2008C (see Fig. 5a and Table 2). According
to this thermogravimetric behavior, we inferred that water
molecules do not participate in metal coordination. The
ligand oxidation in the Ni(II) complex starts simultaneous-
ly with the end of water molecules lost. Residue com-
Fig. 5. TG (a) and DTA (b) curves for Ni(C5H10NO3S)2?2H2O.

P.P. Corbi et al. / Journal of Alloys and Compounds 308 (2000) 153–157
157
4. Final conclusions
The composition of the Ni(II) complex with methionine
sulfoxide corresponds to the molar proportion 1:2 (metal:
ligand), with absorbed water molecules varying from
0,n,2 per mol of complex in each synthesis. The central
Ni(II) ion is coordinated to the ligand via carboxylate
(COO2) and sulfoxide (S5O) groups, in an octahedral
arrangement. Thermal decomposition leads to NiO as the
unique final product.
Acknowledgements
The authors are indebted to FAPESP, CNPq and CCET/
UFMS (financial support, Brazilian agencies).
Fig. 6. Structural scheme proposed for the coordination through metal ion
in the complex.
position after Ni(C5H10NO3S)2?2H2O decomposition was
identified by X-ray diffractometry as NiO [20]. No sulfide
or oxosulfite phases have been detected as could be
expected. Thermogravimetric data are listed in Table 2,
where Dm1 represents water loss, and Dm2 is inferred to
the ligand elimination.
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Differential thermal analysis has been useful to de-
termine the nature of the event occurring during the
constant heating of the compound from 40 to 12008C. The
DTA analysis shows two well-defined exothermic peaks
with their maxima at 257 and 5178C (Fig. 5b). These peaks
can be inferred to the ligand oxidation in the
Ni(C5H10NO3S)2?2H2O complex in two stages, leading to
the NiO as the final product at 9508C.
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3.5. Solubility
The Ni(II) complex with methionine sulfoxide shows
extremely high solubility in water, differing from the
Mn(II), Co(II), Cu(II) and Ni(II) complexes with
methionine [M(Met)2]n, which are insoluble.
Based on the chemical and spectroscopic results, the
following structural scheme (Fig. 6) is proposed to the
coordination of the metal ion to the ligand in the complex.
This proposed structure can give isomers, but their occur-
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