Ni(II) complexes with ligands resulted in condensation of N,N-dimethylbiguanide and pentane-2,4-dione: Synthesis, thermal behaviour and spectroscopic characterization

Article abstract of DOI:10.1007/s10973-005-0661-2

This paper deals with the first investigation concerning the thermal behaviour of Ni(II) complexes with ligands having biguanide moieties. The new complex [NiL(1)](ClO₄)₂·2.5H₂O (1) with the Schiff base resulted in [2+1] condensation of N,N-dimethylbiguanide with pentane-2,4-dione (L(1)) and the complex [Ni₂L(2)](ClO ₄)₄ (2), with the Schiff base, L(2), resulted through the oxidative condensation of L(1) have been synthesized and characterised. The bonding and stereochemistry of the complexes have been characterized by IR and electronic spectra. The cyclic voltammograms show the characteristic waves for mononuclear and respectively binuclear Ni(II) complexes and indicate that both complexes exhibit catalytic effects on the CO₂ electrochemical reduction. The thermal behaviour provided confirmation of the complexes composition as well as the number and the nature of water molecules and the intervals of thermal stability. The different nature of the ligands generates a different thermal behaviour for complexes.

Full text of DOI:10.1007/s10973-005-0661-2

Journal of Thermal Analysis and Calorimetry, Vol. 80 (2005) 363–367
Ni(II) COMPLEXES WITH LIGANDS RESULTED IN CONDENSATION
OF N,N-DIMETHYLBIGUANIDE AND PENTANE-2,4-DIONE
Synthesis, thermal behaviour and spectroscopic characterization
1
*
R. Olar , M. Badea , D. Marinescu , E. Iorgulescu and S. Stoleriu
1
1
2
3
1
University of Bucharest, Faculty of Chemistry, Department of Inorganic Chemistry, 90-92 Panduri Str., 050663 Bucharest, Romania
University of Bucharest, Faculty of Chemistry, Department of Analytical Chemistry, 90-92 Panduri Str., 050663 Bucharest, Romania
University ‘Politehnica’ Bucharest, Faculty of Industrial Chemistry, S.I.M.O. Department, P.O. Box 12-134, 011061 Bucharest,
2
3
Romania
This paper deals with the first investigation concerning the thermal behaviour of Ni(II) complexes with ligands having biguanide moi-
eties. The new complex [NiL(1)](ClO
with pentane-2,4-dione (L(1)) and the complex [Ni
4
)
2
⋅2.5H
2
O (1) with the Schiff base resulted in [2+1] condensation of N,N-dimethylbiguanide
L(2)](ClO (2), with the Schiff base, L(2), resulted through the oxidative conden-
2
4 4
)
sation of L(1) have been synthesized and characterised. The bonding and stereochemistry of the complexes have been characterized by
IR and electronic spectra. The cyclic voltammograms show the characteristic waves for mononuclear and respectively binuclear Ni(II)
complexes and indicate that both complexes exhibit catalytic effects on the CO electrochemical reduction. The thermal behaviour pro-
2
vided confirmation of the complexes composition as well as the number and the nature of water molecules and the intervals of thermal
stability. The different nature of the ligands generates a different thermal behaviour for complexes.
Keywords: biguanide moiety, carbon dioxide, electrochemical reduction, template, thermal stability
Introduction
tential by ca. 0.5 V. Moreover, in order to obtain the re-
duction products involving more than two electrons;
polynuclear complexes should be employed [9]. Such
type of compounds also presents a high thermodynamic
and kinetic stability and there are no studies concerning
its thermal behaviour. On the basis of these consider-
ations we have been synthesised complexes of Ni(II)
with ligands resulted by template condensation of
pentane-2,4-dione and N,N-dimethylbiguanide and re-
spectively by oxidative condensation of the ligand re-
sulted in this process. The complexes were formulated
on the basis of analytical and spectral data. The redox
behavior of these complexes in argon and respectively
carbon dioxide indicate that both complexes can cata-
lyze the electroreduction of carbon dioxide. The thermal
decomposition of these complexes was used in order to
confirm the composition and also in order to obtain in-
formation about the interval of the thermal stability of
these species.
There is much interest in synthesis and characterisation
of nickel complexes with nitrogen donors ligands with
particular attention focused on species that could pres-
ent a similar behaviour with those of the natural nickel
containing species. One of the natural occurring species
F430, a Ni(II) hydrocorphinoid complex, is a prosthetic
group of S-methyl coenzyme M reductase which cata-
lyzes the reductive cleavage of S-methyl coenzyme M
to coenzyme M and methane in the final stage of carbon
dioxide reduction in the methanogenic bacteria [1]. The
design and synthesis of electrocatalyses that could per-
form the reduction of the CO may be important in less-
2
ening of the greenhouse effect and in developing new
energetic source. The reduction process involve two-,
four-, six- or eight-electron as a function of the reaction
products that could be CO, formic acid, formaldehyde,
methanol or methane. The high activation energy of
CO
2
reduction at the platinum electrode requires a more
negative potential than –2.24 V vs. SCE but this can be
lowered by using catalysts. So far, the catalytic effects Experimental
on the electrochemical reduction of CO were evidenced
2
All reagents were of commercial analytical quality and
for some transition metal complexes [2–4]. Some of
these, are mono-, di- or trinuclear Ni(II) complexes with
macrocyclic ligands [5–9]. It was shown that these com-
have been used without further purification. Chemical
analysis of carbon, nitrogen, sulphur and hydrogen has
been performed using an EA 1110 analyzer. Nickel was
determined gravimetrically using dimethylglyoxyme in
the laboratories of Inorganic Chemistry Department.
pounds perform the electrochemical reduction of CO
2
to
CO or formate anion by shifting the CO reduction po-
2
*
Author for correspondence: olar.marioara@unibuc.ro
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2005 Akadémiai Kiadó, Budapest

OLAR et al.
IR spectra were recorded in KBr pellets with a
Bio-Rad FTIR 135 spectrometer in the range
ous concentrated ammonia solution (5 mL) and iodine
(1 g). The reaction mixture was magnetically stirred
at 50°C for 4 h until a deep orange compound was
formed. The microcrystalline product was filtered off,
washed with ethanol, ether and air-dried. Analysis,
found: Ni, 11.71; C, 40.82; H, 6.38; N, 28.18%;
–1
400–4000 cm . Electronic spectra by diffuse reflec-
tance technique, with MgO as standard, were recorded
in the range 380–1100 nm, on a VSU 2P-Zeiss Jena
spectrometer. The voltammetric measurements were
accomplished with a polarographic and voltammetric
ensemble Trace Master 5 and POL 150 Polatrographic
Analyzer (Radiometer Copenhapen). Cyclic voltam-
Ni
N, 28.11%; IR (KBr pellet), cm : ν(NH), 3367s, 3269s,
3160s; νas(CH ), 2950m; ν (CH ), 2830m; ν(C=N),
1687vs; νas(COO), 1648vs; δ(NH ), 1514m; ν (COO),
1428m; ν(Ni–N), 424w.
2 34 62 20 8
C H N O requires: Ni, 11.78; C, 40.98; H, 6.27;
–1
3
s
3
metry was carried out in DMF/H O (1:9, v/v) contain-
2
2
s
ing 0.1 M NaClO . The working electrode was HMDE
4
(hanging mercury drop electrode), the auxiliary elec-
trode was a coiled platinum wire, and the reference
electrode was Ag/AgCl. The dissolved oxygen from Results and discussion
the analyzed solution was eliminated by bubble a pure
argon stream.
The heating curves (TG, T, DTA and DTG) were
Synthesis and physico-chemical characterisation of
the complexes
recorded in a static air atmosphere using a Shimadzu
DTG-TA-51H thermogravimetric analyzer with a
sample mass of 15 and 6 mg over the temperature
The template condensation reaction of polyamines with
polycarbonyl derivatives is the subject of many studies.
Using this method there were synthesised a great num-
ber of macrocyclic compounds containing azomethynic
groups [10]. We have synthesised ligands containing
biguanide residues by condensation of N,N-dimethyl-
biguanide (DMBG) and pentane-2,4-dione (Scheme 1).
The synthesis has been performed in order to obtain
azamacrocyclic complexes bearing biguanide moieties
as part of the ligand superstructure. However it is to be
mentioned that no matter of the molar ratio between the
organic components, the final ligand is acyclic and it
corresponds to a [2+1] condensation of N,N-dimethyl-
biguanide and pentane-2,4-dione. Considering that the
molecular mechanics simulation of N,N-dimethyl-
biguanide indicate that the lower charge density appear
–
1
range 20–1000°C, using a heating rate of 10 K min .
Synthesis of the complexes
Compound [NiL(1)](CH COO) ⋅2.5H O (1)
3
2
2
To a solution of nickel(II) acetate (5 mmol, 1.244 g),
dimethylbiguanide chlorhydrate (10 mmol, 1.655 g)
in 100 mL ethanol was added dropwise (5 mmol,
–1
0
.51 mL (d=0.975 g mL )) pentane-2,4-dione. The re-
action mixture was refluxed 48 h until an orange com-
pound was formed. The microcrystalline product was
filtered off, washed with EtOH, Et O and air-dried.
2
Analysis, found: Ni, 10.68; C, 37.48; H, 6.88;
2
on the N we have been considered that this group is the
most susceptible of the nucleophilic attack of the car-
bonyl groups of acetylacetone.
The oxidative condensation of complex (1) in ex-
cess iodine in an alkaline medium produced the species
N, 25.78%; NiC H N O requires: Ni, 10.79;
17
37 10 6.5
–1
C, 37.52; H, 6.85; N, 25.74%; IR (KBr pellet), cm :
ν(OH), 3430m, sh; ν(NH), 3369s, 3304m, 3262m;
ν (CH ), 2930m; ν (CH ), 2840m; ν(C=N), 1683vs;
as
3
s
3
ν (COO), 1640vs; δ(NH ), 1519m; ν (COO), 1436m;
as
2
s
[
Ni L(2)](ClO ) (2) as it is shown in Scheme 1. This
ν(Ni–N), 475w.
2 4 4
method was successfully performed on the S-alkyliso-
thiosemicarbazide derivatives [11] that suffer the con-
densation process not only in the presence of iodine but
also with oxygen. Our study has evidenced that the
Compound [Ni L(2)](CH COO) (2)
2
3
4
To a suspension of [NiL(1)](CH COO) ⋅2.5H O
3
2
2
(2 mmol, 1.088 g) in 50 mL ethanol was added an aque-
Scheme 1
364
J. Therm. Anal. Cal., 80, 2005

CHARACTERIZATION OF Ni(II) COMPLEXES
biguanide derivatives suffer the oxidative condensa-
tion only in the presence of the iodine.
The two complexes are polycrystalline solids
soluble in N,N-dimethylformamide while the com-
plex (2) is water soluble too.
The major IR spectral features of complexes pre-
sented at experimental part indicate that in the spectra of
complexes appear the characteristic bands of biguanide
moieties [12]. The bands corresponding to NH and C=N
vibration modes are significantly shifted toward higher
wavenumbers in the spectra of the complexes. This be-
haviour could arise from the breaking of the hydrogen
bonds network, from the condensation process and also
Fig. 2 Cyclic voltammogram of a – [Ni(DMBG)
b – complex (1) and c – (2) at a platinum electrode in
DMF/H O (1:9, v/v) measured under argon (HMDE,
working electrode; concentration of complex, 10 M;
2 3 2
](CH COO) ,
2
–4
–1
4
electrolyte, 0.1 M NaClO ; scan rate, 200 mV s )
–1
from the coordination. At 1345 cm appears for both
complexes a new band. This band was observed in the
spectra of some complexes with biguanide derivatives
aspect of the spectra. The absorption at 395, 505
and 610 nm could be assigned to the spin allowed
1
1
1
1
1
1
transition A → A , A → E, A → B .
1
2
1
1
1
[12, 13] and can be associated either with the formation
The cyclic voltammogram of complex (1) under
of the chelate ring or with the activation of an IR band as
a result of the ligand symmetry decrease upon coordina-
tion. In the range characteristic for the carboxylic vibra-
tions, the spectra of the complexes display two bands.
The position of these bands is in accord with the pres-
ence of the free acetate [14]. The spectrum of com-
plex (1) in the characteristic ranges for water shows a
argon atmosphere (Fig. 2) shows a cathodic peak at
E
1/2= –1.046 V vs. Ag/AgCl corresponding to a
unielectronic reduction for Ni(II)/Ni(I) couple. The
reduction peak for complex (2) (E1/2= –1.138 V) is
wider than corresponding to compound (1) indicates
that the electrochemical process involved two elec-
trons in this case. As it is shown in Fig. 2 the two cy-
clic voltammograms are different one from the other
and it is also different comparing with the one pre-
sented by [Ni(DMBG) ](CH COO) indicating that
–1
shoulder about 3430 cm assigned to ν(OH) stretching
vibrations [15].
The diffuse-reflectance spectrum of complex (2)
2
3
2
(
Fig. 1) is characteristic for Ni(II) complexes with
the processes of condensation occurred. Thus the new
obtained ligands generated bonds different as strength
that results in different redox behaviour.
square-planar stereochemistry showing a narrow and
symmetrical band centred at 470 nm. In the spectrum
of complex (1) it can be seen one broad band, as it is
usually observed for Ni(II) complexes with ligands
that generate a strong field and moreover are dis-
symmetric [16].
The absence of the symmetry centre of the com-
plexes allows the appearance of the three compo-
nents, very closely as energy, that generates the broad
Fig. 3 Cyclic voltammogram of complex (1) at a platinum
electrode in DMF/H
gon and b, c – under CO
concentration of complex, 2⋅10 M; electrolyte,
2
O (1:9, v/v) measured a – under ar-
2
(HMDE, working electrode;
–4
–1
0
4
.1 M NaClO ; scan rate, 200 mV s )
Fig. 4 Cyclic voltammogram of complex (2) at a platinum
electrode in DMF/H O (1:9, v/v) measured a – under ar-
gon and b,c – under CO (HMDE, working electrode;
concentration of complex, 2⋅10 M; electrolyte,
2
2
–4
Fig. 1 Electronic spectra of complexes (1) and (2)
–1
0
4
.1 M NaClO ; scan rate, 200 mV s )
J. Therm. Anal. Cal., 80, 2005
365

OLAR et al.
The cyclic voltammograms recorded under CO2
oxidative degradation of the ligand. In the next two
steps the ligand suffers some oxidative transforma-
tions. The chemical composition of the intermediates
cannot be determined due to multidirectional course of
the Schiff base thermal degradation. Most probably,
demethylation and deammination reactions occur (ac-
cording to the mass loss). Such behaviour was also ob-
served during the thermal transformation of other com-
plexes with ligands having a complex structure [17] as
well as for multifunctional organic derivatives [18].
The resulted species of these thermal transfor-
mation are Ni(II) complexes with paracyanide (hav-
ing different polymerisation degree). The paracyanide
formation was confirmed by the chemical analysis
and IR spectra (ν(C=N) 1630 cm ) of intermediates.
For complex (2) the thermal behaviour is accord-
ing to the proposed formulation for (1) on the basis of
the chemical analysis, spectral data and voltammetric
studies. This compound is even more stable than (1), the
thermal decomposition starting at 257ºC. The first exo-
thermic step corresponds to a partial oxidative degrada-
tion of the ligand in two processes (as TG indicates).
The second one consists in transformation of acetate in
carbonate according to the mass loss (Table 1). In the
next step occurs the oxidative degradation of the resid-
ual ligand and the carbonate decomposition.
atmosphere indicate for both complexes the decrease
of the current intensity and the shifting of the E1/2 to-
ward more negative potentials. When the solution was
saturated with CO the cathodic peak disappeared as it
2
can be seen in Figs 3 and 4. This behaviour indicates
that the complexes catalysed the CO electrochemical
2
reduction.
Thermal behaviour of the complexes
Thermal behaviour of complexes was investigated by
thermogravimetry and the final residues were examined
by X-ray diffraction on powder. The majority of
intermediate products formed during thermal analysis
were not possible to identify because the steps were not
distinct. The excepts are the anhydrous complex formed
after the first decomposition step of complex (1) and the
intermediates formed at 350ºC for both complexes.
Also, it was observed that the two complexes have a
different thermal behaviour (Figs 5 and 6).
For complex (1) the water loss occurs in a single
step, until 150°C (Table 1). This indicates the uncoor-
dinated nature of the water molecules. The anhydrous
species (square-planar, too) is very stable, its oxidative
degradation begins at 240ºC. This step, exothermic
one, results from at least two processes corresponding
to acetate transformation into carbonate and to partial
–
1
The last degradation steps lead to NiO as final
product in the both cases, as XRD indicates.
Fig. 5 The TG, DTG and DTA curves of (1)
Fig. 6 The TG, DTG and DTA curves of (2)
Table 1 Thermal behaviour data (in static air atmosphere) for the complexes
Complex
Step
Thermal effect
Temperature interval/°C
∆mexp/%
∆mcalc/%
1
2
3
4
endo
exo
exo
exo
56–150
8.22
29.66
10.03
24.64
13.36
14.09
8.27
29.21
10.02
24.50
13.23
13.79
240–352
352–476
476–597
[
NiL(1)](CH
3
COO)
2
2
⋅2.5H O (1)
597–706
residue (NiO)
1
2
3
endo
exo
exo
257–348
348–452
452–682
residue (NiO)
36.63
11.38
37.13
14.86
36.93
11.64
37.33
14.10
2 3 4
[Ni L(2)](CH COO) (2)
366
J. Therm. Anal. Cal., 80, 2005

CHARACTERIZATION OF Ni(II) COMPLEXES
Conclusions
4 R. W. Hay, J. A. Crayston, T. J. Cromie, P. Lightfoot and
D. C. L. de Alwis, Polyhedron, 16 (1997) 3557.
M. Beley, J. P. Collin, R. Rupert and J. P. Sauvage,
J. Chem Soc. Chem. Commun., (1984) 1315.
M. Beley, J. P. Collin, R. Rupert and J. P. Sauvage,
J. Am. Chem Soc., 108 (1986) 7461.
5
6
7
8
The nickel(II) complexes with ligand resulted in
[
2
2+1] condensation of N,N-dimethylbiguanide and
,4-pentanedione and respectively in their oxidative
condensation have been synthesized.
J. P. Collin, A. Jouaiti and J. P. Sauvage, Inorg. Chem.,
The bond and stereochemistry were character-
ised by means of IR and electronic spectroscopy. Ac-
cording to the electronic spectra the proposed stereo-
chemistry is square planar for both complexes.
The cyclic voltammograms are characteristic for
mononuclear and binuclear Ni(II) complexes respec-
tively and indicate that both complexes exhibit cata-
27 (1988) 1315.
E. Fujita, J. Haff, R. Sanzenbacher and H. Elias,
Inorg. Chem., 33 (1994) 4627.
9 E. Y. Lee, D. Hong, H. W. Park and M. P. Suh,
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5 K. Nakamoto, ‘Infrared and Raman Spectra of Inorganic
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1
1
lytic effects on the CO electroreduction.
2
1
1
Thermal decomposition of complexes gave the
possibility to establish the number and nature of water
molecule, the composition of complexes and also the
intervals of thermal stability. In order to confirm the
nature of final residues and some intermediates the
chemical analysis, IR spectra and X-ray diffracto-
grams were performed.
1
1
6 A. B. P. Lever, ‘Inorganic Electronic Spectroscopy’,
Elsevier, Amsterdam, London, New York 1986, p. 507.
7 F. C. D. Lemos, M. Muraro, J. Zukerman-Schpector,
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