Thermal behavior, spectroscopic and biological characterization of Co(II), Zn(II), Pd(II) and Pt(II) complexes with N,N-dimethylbiguanide

Article abstract of DOI:10.1007/s10973-005-0676-8

The complexes of the type [M(HDMBG)Cl₃] ((1) M:Co; (2) M:Zn;) and [M(DMBG)Cl₂] ((3) M:Pd; (4) M:Pt; DMBG: N,N-dimethylbiguanide) present in vitro antimicrobial activity. The modification evidenced in IR and 1H NMR spectra (in the case of complex (2)) was correlated with the presence of N,N-dimethylbiguanide ion as unidentate, coordinated through N³ and of N,N-dimethylbiguanide as chelate, coordinated through N¹ and N⁴ respectively. The electronic reflectance spectrum showed the d-d transition for complex (1) characteristic for the tetrahedral surrounding while the spectra for complexes (3) and (4) have the characteristic pattern for square-planar stereochemistry. The cyclic voltammetric data show the characteristic waves for mononuclear complexes of the metallic ions presented below. The thermal analysis has evidenced the thermal intervals of stability and also the thermodynamics effects that accompany them. The different nature of the ligands generates a different thermal behaviour for complexes.

Full text of DOI:10.1007/s10973-005-0676-8

Journal of Thermal Analysis and Calorimetry, Vol. 80 (2005) 451–455
THERMAL BEHAVIOR, SPECTROSCOPIC AND BIOLOGICAL
CHARACTERIZATION OF Co(II), Zn(II), Pd(II) AND Pt(II) COMPLEXES
WITH N,N-DIMETHYLBIGUANIDE
Rodica Olar1*, M. Badea1, E. Cristurean1, V. Lazar2, R. Cernat2 and C. Balotescu2
1University of Bucharest, Faculty of Chemistry, Department of Inorganic Chemistry, 90-92 Panduri Str., 050663 Sector 5,
Bucharest, Romania
2University of Bucharest, Faculty of Biology, Department of Microbiology, 1–3 Aleea Portocalilor Str., 060101 Sector 6,
Bucharest, Romania
The complexes of the type [M(HDMBG)Cl3] ((1) M:Co; (2) M:Zn;) and [M(DMBG)Cl2] ((3) M:Pd; (4) M:Pt;
DMBG: N,N-dimethylbiguanide) present in vitro antimicrobial activity. The modification evidenced in IR and 1H NMR spectra (in the
case of complex (2)) was correlated with the presence of N,N-dimethylbiguanide ion as unidentate, coordinated through N3 and of
N,N-dimethylbiguanide as chelate, coordinated through N1 and N4 respectively. The electronic reflectance spectrum showed the d–d
transition for complex (1) characteristic for the tetrahedral surrounding while the spectra for complexes (3) and (4) have the character-
istic pattern for square-planar stereochemistry. The cyclic voltammetric data show the characteristic waves for mononuclear com-
plexes of the metallic ions presented below. The thermal analysis has evidenced the thermal intervals of stability and also the thermo-
dynamics effects that accompany them. The different nature of the ligands generates a different thermal behaviour for complexes.
Keywords: complexes, Gram-negative strain, Gram-positive strain, N,N-dimethylbiguanide, thermal behaviour
Introduction
The in vitro antimicrobial activity of the investi-
gated compounds was tested against Gram-positive
and Gram-negative strains (Bacillus subtilis, Staphy-
lococcus aureus, Klebsiella pneumoniae, Pseudomo-
nas aeruginosa and Escherichia coli).
The most biological active compounds will be
incorporate in polymeric matrix. Having in view that
the polymerisation processes usually occur at higher
temperature it was also investigated the thermal be-
haviour of these derivatives in order to evidence the
thermal intervals of stability and also the thermody-
namics effects that accompany them.
There is a much interest in biguanides ligands and their
transition metal complexes with particular attention fo-
cused on structural studies [1–7] and to evidence the
ability of these species to generate hydrogen-bonded
supramolecular assemblies [8, 9]. It is also well-known
that the biguanides have biological properties. Among
this derivatives N,N-dimethylbiguanide is known as a
glucose lowering agent [10], analgesic, antimalarial
[11] and antimetabolite for organisms that inhibit the
metabolism of folic acid [12]. Recently it was shown
that vanadyl–biguanide complexes are potential syner-
gistic insulin mimics [13] while the complexes of tech-
netium isotope (99Tc) with biguanide derivatives could
be used as renal imaging agents [14]. Regarding the
thermal behaviour of these derivatives there is only one
study concerning the iron(III) complexes [15].
In order to modulate the biological activity of
N,N-dimethylbiguanide (DMBG) and to correlate this
activity with thermal stability, a series of complexes
with general formula [M(HDMBG)Cl3] ((1) M:Co;
(2) M:Zn) and respectively [M(DMBG)Cl2] ((3) M:Pd;
(4) M:Pt) have been characterised as mononuclear spe-
cies by elemental analysis, IR, 1H NMR and electronic
spectra. The redox behaviour of complexes has been
investigated by cyclic voltammetry.
Experimental
All reagents were of commercial analytical quality
and have been used without further purification.
Chemical analysis of carbon, nitrogen and hydrogen
has been performed using an EA 1110 analyzer. Co-
balt, zinc, palladium, platinum and chloride were de-
termined gravimetrically in the laboratories of
Inorganic Chemistry Department.
IR spectra were recorded in KBr pellets with a
Bio–Rad FTIR 135 spectrometer in the range
400–4000 cm–1. Electronic spectra by diffuse
reflectance technique, with MgO as standard, were re-
corded in the range 380–1100 nm, on a VSU 2P-Zeiss
*
Author for correspondence: rolar@chem.unibuc.ro
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

OLAR et al.
1
Jena spectrometer. The H NMR spectra (300 MHz)
were recorded on a Bruker AM 300 spectrometer with
tetramethylsilane as internal reference. The voltam-
metric measurements were accomplished with a
polarographic and voltammetric ensemble Trace Mas-
ter 5 and POL 150 Polarographic Analyzer (Radiome-
ter Copenhagen). Cyclic voltammetry was carried out
in DMF containing 0.1 M NaClO4. The working elec-
trode was HMDE (hanging mercury drop electrode),
the auxiliary electrode was a coiled platinum wire, and
the reference electrode was Ag/AgCl. The dissolved
oxygen from the analyzed solution was eliminated by
bubble a pure argon stream.
ethanol was added dropwise at 50ºC, under continu-
ous stirring, a solution of 2 mmol dimethylbiguanide
hydrochloride in 20 cm3 ethanol. The brown, spar-
ingly soluble compound formed after one hour was
filtered out and washed with ethanol and air-dried.
[Pd(DMBG)Cl2] (3): analysis found: Pd, 34.68;
C, 15.61; Cl, 23.16; H, 3.68; N, 22.86%; calculated
for PdC4Cl2H11N5: Pd, 34.72; C, 15.68; Cl, 23.14;
H, 3.62; N, 22.85%; yield (reported to palladium chlo-
ride) 95%; IR (KBr pellet), cm–1: νas(NH2), 3400vs;
ν(NH), 3361m; νs(NH2), 3297m; ν(C=N), 1654vs,
1622vs; δas(NH2), 1542vs; δs(NH2), 1466m; ν(C–N),
•
1250w; ρ(CH3), 723w.
The in vitro biological screening effects of the
complexes and dimethylbiguanide hydrochloride
[Pt(DMBG)Cl2] (4): analysis found: Pt, 49.30;
C, 12.14; Cl, 17.98; H, 2.71; N, 17.60%; calculated
for PtC4Cl2H11N5: Pt, 49.37; C, 12.16; Cl, 17.94; H,
2.80; N, 17.72%; yield (reported to platinum chlo-
ride) 91%; IR (KBr pellet), cm–1: νas(NH2), 3378vs;
ν(ΝΗ), 3309m; νs(NH2), 3200m; ν(C=N), 1652vs,
were tested against
a
bacterial inoculum
(1.5·108 UFC/cm3) represented by Gram-positive and
Gram-negative, reference and environmental (river
water) strains (Bacillus subtilis, Staphylococcus
aureus, Klebsiella pneumoniae, Pseudomonas
aeruginosa and Escherichia coli). The antibiogram
technique, namely the liquid medium dilution method
was used for determining minimum inhibitory con-
centration (M.I.C., µg cm–3). Stock solutions were
prepared by dissolving the compounds in DMF. The
standard inoculum of bacterial strain was inseminated
in a discontinuous gradient of concentration, repre-
sented by the tested compounds, in tubs containing
nutritive bullion Mueller Hinton. This mixture was
incubated at 37°C for 24 h.
1623vs; δas(NH2), 1542vs; δs(NH2), 1467m; ν(C–N),
•
1253w; ρ(CH3), 722w.
DMBG·HCl: IR (KBr pellet), cm–1: νas(NH2),
3375vs; ν(NH), 3297m; νs(NH2), 3175vs; ν(C=N),
1629vs; δas(NH2), 1578vs; δs(NH2), 1478m; ν(C–N),
•
1280w, 1240w; ρ(CH3), 729w; 1H NMR (DMSO–d6,
300 MHz): δ 2.923 (s, 6H, CH3), 6.803 (s, 4H, NH2),
7.219 (s, 2H, NH).
Results and discussion
The heating curves (TG, T, DTA and DTG) were
recorded in a static air atmosphere using a Shimadzu
DTG-TA-51H thermogravimetric analyzer with a
sample mass between 6–13 mg over the temperature
range of 20–1000°C, using a heating rate of 10 K min–1.
Physico-chemical characterisation of complexes
In this paper, we report the preparation, physico-
chemical and biological characterisation of some
complexes with N,N-dimethylbiguanide of type
[M(HDMBG)Cl3] ((1) M:Co; (2) M:Zn) and respec-
tively [M(DMBG)Cl2] ((3) M:Pd; (4) M:Pt). The ma-
jor goal of this paper was to evidence the thermal be-
haviour of these complexes that also present in vitro
an antibacterial activity.
It is to be mentioned that several study have evi-
denced that the coordination mode of N,N-dimethyl-
biguanide strongly depends by the preference of the
metallic ion for tetrahedral or square planar stereo-
chemistry, that influences the bond strength and, con-
sequently, the thermal behaviour. The cation that
forms easily tetrahedral species leads to species that
contain the unidentate N,N-dimethylbiguanidium ion
[2, 3] while those that prefer the square planar stereo-
chemistry generated species with chelate N,N-di-
methylbiguanide [1, 5].
Synthesis of the complexes
The syntheses and structural data for complexes
[M(HDMBG)Cl3] ((1) M:Co; (2) M:Zn)) were re-
ported elsewhere [2, 3]. The composition of com-
plexes has been confirmed by chemical analyses.
[Co(DMBG)Cl3] (1): IR (KBr pellet), cm–1:
νs(NH2), 3414vs; νs(NH2), 3322vs; ν(ΝΗ), 3222m;
ν(C=N), 1639vs; δas(NH2), 1520vs; δs(NH2), 1459m;
ν(C–N), 1277w, 1235w; ρ(CH3), 720w.
•
[Zn(DMBG)Cl3] (2): IR (KBr pellet), cm–1:
νas(ΝΗ2), 3412vs; νs(NH2), 3320vs; ν(NH), 3225m;
ν(C=N), 1638vs; δas(NH2), 1526vs; δs(NH2), 1460m;
ν(C–N), 1275w, 1239w; ρ(CH3), 721w; 1H NMR
•
(DMSO-d6, 300 MHz): δ 2.923 (s, 6H, CH3), 6.554 (s,
4H, NH2), 7.192 (s, 2H, NH).
Moreover, the coordination mode of the
N,N-dimethylbiguanide in complexes it is possible
now to achieve without no doubt, on the basis of the
structural X-ray determination. We have been corre-
The complexes, [M(DMBG)Cl2] ((3) M:Pd; (4)
M:Pt)), were obtained following the general proce-
dure: to a suspension of 2 mmole of MCl2 in 50 cm3
452
J. Therm. Anal. Cal., 80, 2005

Co(II), Zn(II), Pd(II) AND Pt(II) COMPLEXES WITH N,N-DIMETHYLBIGUANIDE
lated the spectral data with the known structural data
for the complexes (1) and (2) and for similar species.
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 moiety [16].
In the range characteristic for the NH vibrations,
the spectrum of the N,N-dimethylbiguanide hydro-
chloride display three components, which could be
associated with the presence of primary amine and
imine groups. The bands corresponding to the vibra-
tion modes of the NH2 group are significantly shifted
towards higher wavenumbers in the spectra of the
complexes (1) and (2), moreover those assigned to
NH vibration mode is shifted towards lower
wavenumbers with 80 cm–1. This information sug-
gests that the coordination through N3 decreases the
charge density on this atom and also generates the de-
struction of the hydrogen bonds network, evidenced
in the lattice of the dimethylbiguanide hydrochloride
by the X-ray diffraction [17]. For these complexes in
the range characteristic for the ν(C=N), δas(NH2), and
respectively δs(NH2), vibrations [18], the spectra of the
complexes display three very strong bands. The pres-
ence of unidentate N,N-dimethylbiguanidium leads to
the shift of the band associated with ν(C=N) towards
higher wavenumbers while the bands assigned to
δ(NH2) are both shifted at lower wavenumbers.
Fig. 1 Cyclic voltammogram of complex (2) at a platinum
electrode in DMF measured under argon (HMDE,
working electrode; concentration of complex, 2·10–4 M;
electrolyte, 0.1 M NaClO4; scan rate, 50 mV/s –1).
chemistry of d8 ions. The intense bands about
1
26315 cm–1 are assigned to A1→1E transition while
the shoulder at 20 000 and the band at 16 950 cm–1 are
associated with the spin forbidden transition 1A1→3E.
Cyclic voltammogram of (1) shows a catodic
peak at –1.260 V vs. Ag/AgCl that is assigned to
Co(II) reduction process. For complex (2) it was ob-
served a well defined reversible peak at E0' = –0.933 V
(E1/2red= –1.008 V and E1/2ox= –0.858 V) assigned to
redox couple Zn(II)/Zn (Fig. 1). The voltammogram
for complex (3) shows a weak-defined peak at
–1.220 V characteristic for Pd(II) reduction in 0.1 M
ammoniacal buffer. For the complex (4) the
voltammogram exhibits two weak-defined peaks at
0.172 and 0.372 V that can be assigned to the irrevers-
ible oxidations Pt(II)→Pt(III)→Pt(IV).
For complexes (3) and (4) the different spectral
features indicate a chelate coordination mode for the
ligand. In the spectra of these compounds, the bands
corresponding to NH stretching vibrations preserve the
aspect and intensity ratio as in the spectrum of
dimethylbiguanide hydrochloride but all are shifted
significantly toward higher wavenumbers. Moreover
in range characteristic for ν(C=N) vibration there ap-
pear two bands, one being shifted to higher wave-
numbers. At 1384 cm–1 appears for both complexes a
new band. This band was observed in the spectra of
some complexes with biguanide derivatives [15, 16] and
can be associated either with the formation of the che-
late ring or with the activation of an IR band as a result
of the ligand symmetry decrease upon coordination.
Biological activity
Antibacterial activity of the dimethylbiguanide hydro-
chloride and complexes have been carried out against
Gram positive and Gram negative, reference and clini-
cal strains (Bacillus subtilis, Staphylococcus aureus,
Klebsiella pneumoniae, Pseudomonas aeruginosa and
Escherichia coli) using liquid medium dilution
method. The values of minimum inhibitory concentra-
tion (M.I.C., µg cm–3) are presented in Table 1.
It was observed that the DMF does not influence
the antimicrobial activity of the tested compounds at the
working concentrations. The M.I.C. values indicate a
better activity against the Gram-negative strains except
that of Pseudomonas aeruginosa that usually is very re-
sistant at antibiotics, including carbapenems that are
β-lactame derivatives. Beside this, the microorganism
expresses an antibiotic resistance mechanism of efflux
pump type, known to be involved in the resistance to
heavy metals [20]. The most effective antibacterial ac-
tivity was manifested against Bacillus subtilis. In this
case, except the compound (3), the activity of the com-
plexes is higher than the ligand. Moreover, the com-
plex (2) presents constantly the most effective activity
against all bacterial strains tested being so a potential
antimicrobial agent with a large spectrum of action. It is
1
The H NMR spectrum of complex (2) shows
modification in the amine region, namely both signals
characteristic to DMBG·HCl are shifted to lower values.
The electronic spectrum of (1) is characterized
by the presence of a broad band of medium intensity
at 14705 cm–1 and a second one situated in the near IR
range, which exceeds the spectrometer recording lim-
its. These bands can be associated with the spin al-
4
4
lowed d–d transitions A2→4T1(P) and A2→4T1(F)
which are characteristic for Co(II) in tetrahedral sur-
rounding. The spectra of compounds (3) and (4) show
the characteristic pattern of the square-planar stereo-
J. Therm. Anal. Cal., 80, 2005
453

OLAR et al.
Table 1 The minimum inhibitory concentration (M.I.C., µg cm–3) for dimethylbiguanide hydrochloride and complexes (1)–(4)
Klebsiella
pneumoniae
Pseudomonas
aeruginosa
Staphylococcus
aureus
Compounds
Escherichia coli
Bacillus subtilis
DMBG·HCl
128
512
32
256
256
32
128
512
128
512
512
128
64
>1024
128
(1)
(2)
(3)
(4)
32
64
512
1024
512
512
512
64
256
1024
also to be pointed that, complexes (1) and (2) inhibit at a
very low M.I.C. the Staphylococcus aureus grow, a bac-
teria resistant at methicillin. This indicates that the two
compounds could be used in the therapy of MRSA
(methicillin-resistant Staphylococcus aureus) infec-
tions.
and 240ºC) (Fig. 2, Table 2). The melting is immedi-
ately followed by the thermal decomposition that
starts in the first step with HCl elimination and partial
oxidative degradation of DMBG with generation of a
stable intermediate until 430ºC. According to the
mass loss this compound is a complex of the metallic
ions with paracyanide. The nature of this compound
has been confirmed by chemical analysis and IR spec-
tra where appears a strong band at 1623 cm–1 that
could be assigned to ν(C=N) vibration. This step is an
overlapping of at least four processes as DTG indi-
cates. The second step corresponds to the transforma-
tion of the paracyanide complex into the most stable
oxides, respectively Co3O4 and ZnO, as XRD indi-
cate. Two processes corresponding to the stepwise
depolymerisation of the paracyanide and chlorine
elimination form this last step.
Thermal behaviour of complexes
The results concerning the thermal decomposition/deg-
radation of the complexes are presented in the following.
Complexes (1) and (2) have the same thermal be-
haviour and the general aspect of the DTG, DTA and
TG curves as those corresponding to DMBG·HCl.
Unlike DMBG·HCl, which melts at 180ºC and starts
decomposition itself at 200ºC, the complexes are
more stable. The decomposition of the complexes
starts at a higher temperature that being a clue of the
ligand stabilisation upon coordination. Before the
thermal decomposition it was observed the melting of
complexes about at the same temperatures (246
The different nature of the ligand and coordina-
tion mode generate the different thermal behaviour
for complexes type [M(DMBG)Cl2] (Fig. 3). Except-
ing this, the two complexes have a similar behaviour.
Table 2 Thermal behaviour of the complexes
Complex
Step
Thermal effect
Temperature interval/°C
∆mexp/%
∆mcalc/%
1
2
3
Endothermic
Exothermic
Exothermic
246 (m. p.)*
250–426
0
0
39.06
33.75
27.19
38.41
34.41
27.18
[Co(HDMBG)Cl3] (1)
426–650
Residue (Co3O4)
1
2
3
Endothermic
Exothermic
Exothermic
240 (m. p.)*
250–430
38.10
35.23
26.67
42.75
11.42
11.10
34.73
32.17
8.81
37.72
35.56
26.72
42.10
11.59
11.59
34.72
32.66
8.99
[Zn(HDMBG)Cl3] (2)
[Pd(DMBG)Cl2] (3)
430–600
Residue (ZnO)
1
2
3
Exothermic
Exothermic
Exothermic
225–400
400–486
486–630
Residue (Pd)
1
2
3
Exothermic
Exothermic
Exothermic
190–399
399–505
505–653
[Pt(DMBG)Cl2] (4)
9.06
8.99
Residue (Pt)
49.96
49.36
* melting point
454
J. Therm. Anal. Cal., 80, 2005

Co(II), Zn(II), Pd(II) AND Pt(II) COMPLEXES WITH N,N-DIMETHYLBIGUANIDE
bial active and present higher activity than free ligand
against Bacillus subtilis.
The compounds with low molecular masses melt
before decomposition, leading in two steps to metallic
oxides. This behaviour could arise from lower lattice
energy and a higher degree of covalence. The com-
plexes with the higher molecular masses display a
thermal behaviour more complex, the steps of the
degradation being not well delimited. In these cases
the final products being the metals.
Acknowledgements
Fig. 2 TG, DTG and DTA curves of [Co(HDMBG)Cl3]
The CERES National Romanian Research Program finan-
cially supported this study.
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455