REGULAR PAPERS
Vibrational Spectra And Electrochemical Behavior Of Bispicolinate Copper (II)
Parajón-Costa, B. S.1; Wagner, C. C.2; Baran, E. J.,1
1Centro de Química Inorgánica
(CEQUINOR, CONICET/UNLP), Facultad de Ciencias Exactas, Universidad Nacional de
La Plata, C.Correo 962, 1900-La Plata, Argentina.
Fax:
+54 221 4259485*; e-mail: baran@quimica.unlp.edu.ar
2Departamento de Ingeniería Química,
Facultad de Ingeniería, Universidad Nacional del Centro de la Provincia de
Buenos Aires, 7400-Olavarría, Argentina.
Received February 12, 2003. In final form March 27,
2003
Dedicated to Prof. Pedro
J. Aymonino on the occasion of his 75th birthday
Abstract
The infrared and Raman
spectra of the Cu(II) complex of picolinic acid (Hpic), of stoichiometry [(pic)2].2H2O
were recorded and discussed by comparison with those of the free acid on the
basis of Cu(II) its known structural data. The electrochemical properties of the
complex were investigated in dimethylformamide solutions by cyclic voltammetry.
The results show that its reduction occurs through a quasi-reversible one
electron process, generating a Cu(I) species.
Resumen
Se registraron los espectros de
infrarrojo y Raman del complejo que forma el Cu(II) con el ácido picolínico
(Hpic), de estequiometría [Cu(pic)2].2H2O y se los discutió
por comparación con los del ácido libre y en base a sus datos estructurales,
que son conocidos. Las propiedades electroquímicas del complejo se investigaron
en soluciones de dimetilformamida, utilizando voltametría cíclica. Los
resultados muestran que su reducción ocurre a través de un proceso cuasi
reversible, involucrando un único electrón, y generando una especie de Cu(I).
Introduction
Picolinic acid (2-carboxypyridine, Figure 1), an isomer of nicotinic acid, is one of the most important chelating agents present in the human body.
Figure
1. Schematic structure of picolinic acid.
It
is generated in liver and kidneys during the catabolism of the essential amino
acid tryptophan and, subsequently, transported to the pancreas. During
digestion it is secreted to the intestine and is used as a complexing agent in
the absorption of essential metals [1,2].
In
recent years metallic complexes of picolinic acid have received increasing
attention from the pharmacological point of view, as useful systems to trace
element supplementation or for the design of new metallopharmaceuticals [3].
For example, at present the tris-picolinatochromium(III) complex constitutes
one of the most important and popular nutritional supplements, which worldwide
commercialization generates annually more than one hundred million dollars [4].
Also in the search for new drugs with insulinomimetic activity, oxovanadium
(IV) and oxovanadium (V) complexes of picolinic acid and of some of its
derivatives appear as very promising compounds [5-8].
In
order to contribute to a better characterization of this type of
metallopharmaceuticals, we have recently investigated the spectroscopic and
electrochemical behavior of some Cr(III) picolinate complexes [9]. As a continuation
of these studies, we have now performed a similar investigation of the Cu(II)
complex of picolinic acid of stoichiometry [Cu(pic)2].2H2O.
Experimental
Synthesis of the complex
The compound was obtained by interaction of hot diluted aqueous solutions of copper(II) acetate and potassium picolinate in a 1: 2 molar ratio. After mixing the solutions, the color changed from greenish-blue to deep blue and an abundant solid precipitate is produced. After cooling to room temperature, the solid was filtered off, washed with small portions of cold water, and finally dried in an oven at 50 °C [10]. Its purity was confirmed by elemental analysis and the crystal structure confirmed by an analysis of the x-ray powder diagram with the aid of the program POWDERCELL [11].
Spectroscopic studies
The IR spectra were recorded with a Bruker IFS 66 FTIR instrument, using the KBr pellet technique. Raman spectra were obtained with the same instrument, using the FRA 106 Raman accessory and the 1064 nm line of a Nd:YAG solid state laser for excitation.
Electrochemical measurements
Cyclic
voltammograms were obtained with a PAR potentiost/galvanostat model 263A
controlled by the 270/250 Sofware (EG&G Princeton Applied Research). A
printer and a standard electrochemical three electrode cell completed the
system. A glassy carbon disc was employed as working electrode. A platinum wire
was used as counter electrode, while an Ag/(10-3 M) AgNO3
in CH3CN/10-1 M (TBA)PF6 , was used as a
reference electrode. This reference electrode was calibrated against the [Fe(C5H5)2]/[Fe(C5H5)2]+ redox couple, for which a potential of + 0.4 V vs. NHE was assumed [12,13]. All
potentials reported here are referred to the NHE in volts.
Ohmic
drop in the solution was minimized by positive feedback [14].
The
measurements were performed in dimethylformamide (DMF) solutions containing 0.1
M tetrabutylammonium hexafluorophosphate, (TBA)PF6, as the
supporting electrolyte. The concentration of the complex was 10-3 M.
High purity DMF (Baker, spectroscopic grade) was dried over 4 Å molecular
sieves (Merck) and used without further purification. (TBA)PF6 was
purchased from Fluka (electrochemical grade) and used as received.
Before
each experiment, a cyclic voltammogram of the solution containing only the supporting
electrolyte was measured. Oxygen was removed by purging the solutions with
extra-pure nitrogen and a continuous gas stream was passed over the solutions
during the measurements.
Results And Discussion
Structure of the complex
The
complex crystallizes in the triclinic space group P with Z = 1. The Cu(II) cation is located at the center of a distorted N2O4
octahedron, axially elongated by Jahn-Teller distortion. The equatorial
positions are occupied by the two ligands coordinated through the nitrogen atom
and one of the carboxylate O-atoms, in trans geometry. Oxygen atoms from
adjacent complex units, and not involved in the primary coordination, occupy
the axial positions. The crystal is built up of polymeric chains of complex
molecules, joined by weak coordination bonds in the c direction and
H-bonded zig-zag chains of intercalated H2O molecules forming a
molecular layer extended parallel to the (100) plane involving H-bonds between
a carbonyl O-atom and a H2O molecule [15].
Vibrational spectra
In order to facilitate the analysis of the IR and Raman spectra of the complex, we have also recorded the corresponding spectra of the free ligand. These spectra have been recently investigated in detail including some interesting theoretical studies [16,17]. Therefore, these spectra are not reproduced here, but we have assigned our measured spectra on the basis of this analysis complemented with the information provided by some standard references [18,19].
Figure
2. Vibrational spectra of [Cu(pic)2].2H2O
in the spectral range between 2000 cm-1 and 200 cm-1 : infrared (top); Raman (bottom).
The IR and Raman spectra of [Cu(pic)2].2H2O, in the most interesting region between 2000 and 200 cm-1 are shown in Figure 2. The proposed assignment, together with that of the free ligand, is presented in Table 1, and is briefly commented as follows:
TABLE
1. Assignment of the vibrational spectra of picolinic acid and
of the
[Cu(pic)2].2H2O
complex. vs: very strong; s: strong; m: medium;
w:
weak; vw: very weak; sh: shoulder; br: broad
Picolinic acid |
[Cu(pic)2].2H2O |
Assignments |
||
Infrared / cm-1 |
Raman / cm-1 |
Infrared / cm-1 |
Raman /cm-1 |
|
3464 w,br |
|
|
|
n(O-H) acid |
|
|
3450 w,br |
|
n(O-H) water |
|
|
3170 w |
|
|
3118m/3056w |
3113s/3091w |
3074s/3036m |
|
n(CH) |
1719 s |
1692 m |
|
|
n(C=O) |
|
|
1642 vs |
1641 vw |
nas(COO-) |
1612vs/1600vs |
1614m/1600s |
1603s/1569w |
1612m/1588s |
|
1575 m |
1570 m |
1569 w |
|
n(ring) |
1459vs/1443m |
1443 w |
1475m/1448m |
1424 vw |
|
1347 s |
1358 m |
|
|
n(C-O) |
|
|
1389sh/1348vs |
1375w/1350s |
ns(COO-) |
1309 s |
|
|
|
|
1294 vs |
1300 w |
1285 s |
1284 w |
|
|
|
1266 m |
|
|
1251m/1203m |
1235 w |
1243 w |
1248 w |
|
|
|
1168 m |
|
d(CH) |
1158m/1088s |
1163m/1086w |
1152s/1096 m |
1122 m |
|
1048m/1009m |
1037m/1011vs |
1047s/1031m |
1044sh/1023vs |
|
995 m |
993 sh |
1015w/982w |
|
|
966 m |
|
|
|
d(O-H) acid |
890w |
914 w |
926 w |
911 w |
g(CH) |
837 m/825 m |
840 s |
849 s |
846 w |
|
801 s |
800 w |
826 m |
830 w |
scis.(COO-) |
767 s |
|
776 w |
|
g(CH) |
753vs/705s |
|
712s/693s |
708s |
f(CC) |
676 vs |
680 w |
660 m |
660 w |
wagg.(COO-) |
633s/616m |
628w/621vw |
|
|
f(CC) |
|
|
547 m |
536 vw |
f(CC) |
|
|
458 vs |
450 m |
r(H2O) |
539s/499s |
|
|
|
|
420vs/405s |
414 w |
418 s |
405 vw |
|
|
|
376 w |
|
n(Cu-O) |
|
|
338 s |
|
n(Cu-N) |
266 vs |
271 w |
257 s |
|
|
|
|
227m/186m |
|
|
In
the spectra of the free acid some characteristic bands could be clearly
identified:
a) the
O-H stretching vibration, seen only as a relatively weak and somewhat broadened
feature in the IR spectrum; b) The two stretching bands related to the
carboxylate moiety, n(C=O) and n(C-O);
c) The out-of-plane OH-bending motion, observable as a mean intensity IR band.
The corresponding in plane deformation apparently attains not enough intensity
in our spectra; it has been recently reported as a very weak IR feature at
1370 cm-1 [16]. Besides, also two deformation modes of the full
carboxylate group have been tentatively assigned in Table 1.
As
expected, after complex formation all the bands that involve OH motions
disappear and the two original carboxylate bands are transformed into the
symmetric and antisymmetric stretching vibrations of the carboxylate anion. The
difference between these two vibrations is equal to 294 cm-1,
confirming the essentially unidentate character of this group in bonding [20,21]
shown by the structural analysis [15]. However, interestingly, the respective ns(COO-)
vibration shows some splitting evidence probably originated from the
participation of one of the carboxylate O-atoms in the secondary bonding to
another complex unit. The two deformational vibrations of the COO-
-group show slight frequency shifts after complex formation as can also be seen
in Table 1.
The
water molecules, which are not involved in metal bonding, give rise to a weak
and broad IR band centered at about 3450 cm-1, which can be assigned
to the O-H stretching vibrations of this molecule. The position of this band
suggests the formation of hydrogen bonds of medium length [22]. The
corresponding deformational mode is probably overlapped by the strong ns(COO-)
IR band. In the lower spectral range, one deformational mode, of the hydrogen
bonded water molecules, similar to a rocking motion could also be identified.
Regarding the ring vibrational modes they
can roughly be assigned in the following spectral ranges [16-19]: n(CH)
vibrations in the highest frequency region (3500-3000 cm-1);
skeletal ring vibration in the region between 1650-1350 cm-1; in
plane d(CH) deformational modes between 1300 and
1000 cm-1; out of plane g(CH)
vibrations between 1000 and 700 cm-1 and out of plane f(CC)
ring deformation in the 700-550 cm-1 range.
Comparing
the spectra of the free acid and of the complex in the lower frequency range,
it was also possible to identify the stretching vibrations of the
ligand-to-metal bonds, with the n(Cu-O) mode at slightly
higher energy than the corresponding n(Cu-N)
mode, in a similar way as found in other picolinate complexes [9, 17, 23].
Besides, the n(Cu-N) mode is found in the same range as
in Cu(II) complexes of nicotinic acid and nicotinamide [24].
Electrochemical behavior
The electrochemical properties of the complex were investigated by
cyclic voltammetry in DMF solutions containing 0.1 M of TBAPF6 as
supporting electrolyte. Cyclic voltammograms were recorded at different scan
rates (v) between 1.5 V and –2.2 V vs. NHE. In this potential range only one
redox couple was observed whereas at more
positive or negative potential values only the solvent
decomposition current was evident. The effect of the scan rate on the
electrochemical response was investigated from 0.05 V/s to 5 V/s in the
potential range where the complex is electroactive, (+0.35 V to –1.3 V).
Selected voltammograms are shown in Figures 3 and 4. As
can be seen on the negative-going scan, at v =
0.1 V/s, a reduction peak is located at –0.69 V. Upon reversing the potential
at -1.3V the oxidative counterpart of this peak is observed at –0.59V.
Inspection of the voltammetric data shows that at increasing scan
rates, the reduction(Epc) and oxidation (Epa) peaks are shifted to more
negative and positive
Figure
3. Cyclic voltammogram of [Cu(pic)2].2H2O
in DMF solution, at scan rates v £ 0.5 V/s.
values respectively. The separation between them, DEp, exceeds the Nernstian requirement of 59 mV expected for a reversible one-electron process. This value increases from DEp = 0.11V at 0.05 V/s to DEp = 0.33 V at 5 V/s indicating a kinetic inhibition of the electron transfer process [14, 25-27]. This behavior cannot be attributed to some uncompensated solution resistance, as the internal standard Fc/Fc+ couple, that shows rapid heterogeneous electron transfer in most of the solvents, gave a DEp value of 73 mV (measured at v = 0.2 V/s). Thus, any possible uncompensated resistance is sufficiently small so that the resulting voltage drops are negligible compared to the DEp values attributable to kinetic effects.
Figure
4. Cyclic voltammogram of [Cu(pic)2].2H2O
in DMF solution, at scan rates v ³ 0.5 V/s.
On the other hand, and for diagnostic purposes, the ratio of anodic and cathodic current (ipa/ipc) was determined using the well-known empirical relation derived by Nicholson 14,27]. This ratio remains close to unity independent of the sweep rate used, giving, for this quasi-reversible process, a value of 0.5 for the charge transfer coefficient, a. The cathodic peak increases with the square root of the scan rate (v1/2) but is not proportional to it (cf. Figure 5). The ratio ipc/ v1/2 diminishes slightly from 68 to 61 mA s1/2 V-1/2 [25-27].
Figure
5. Dependence of the cathodic current peak on the square root
of the scan rate (v1/2).
Moreover,
comparison of the cyclic voltagramms of the complex with the corresponding
ligand response, under identical experimental conditions, indicates that the
redox process involves only the metal ion. The cyclic voltagramm of picolinic
acid at v = 0.1 V/s exhibits one reduction peak at –1.1 V and two oxidation
peaks located at –0.75 and –0.2 V.
Important
differences were also detected when the redox behavior of the complex was
compared with those obtained in the presence of Cu(ClO4)2.
The cyclic voltagramm of this compound, at v > 0.2 V/s exhibits one
reduction and one oxidation peak, situated at more positive potentials than
those obtained with the picolinate complex, and the process is followed by a
disproportionation and the adsorption of metallic copper on the electrode (cf.
also [28]). In contrast, in the case of the complex, and for all investigated
scan rates, neither disproportionation nor Cu0 adsorption on the
electrode surface could be detected. Therefore, the reduction peak observed in
this case can only be attributed to a one electron charge transfer from Cu(II)
to Cu(I).
To
conclude, the experimental results prove that the reduction of [Cu(pic)2].2H2O
proceed through a quasi-reversible one electron process, generating the Cu(I)
species, which has a life time long enough to be detected by cyclic
voltammetry. The redox process can be formulated as follows:
[CuII(pic)2]
[CuI(pic)2]- E1/2
@ -0.62
V
The quasi-reversibility associated with the reduction process probably arises as a consequence of a geometry change from the originally distorted octahedral towards a distorted tetrahedral environment around the Cu(I) species.
Conclusions
The
infrared and Raman spectra of the Cu(II) of picolinic acid of stoichiometry
[Cu(pic)2].2H2O were analyzed on the basis of its known
crystal structure and by comparison with the spectroscopic behavior of the free
ligand. The vibrational characteristics of the metal-to-ligand linkage could be
fully interpreted. The electrochemical behavior of the complex was investigated
by cyclic voltammetry in dimethylformamide solutions, showing that its
reduction occurs through a quasi-reversible one-electron process. A value of
about –0.62 V was determined for the redox couple. No evidence of
disproportionation of the generated Cu(I) or for the adsorption of metallic
copper on the electrode surface could be found.
Besides,
a comparison with the previously investigated Cr(III) picolinate, of
stoichiometry Cr(pic)3.H2O, shows that the reduction of
this species is more difficult and occur at a more negative potential (-1.23V) [9].
Acknowledgements
This study has been supported by CONICET and ANPCyT (PICT 06-06148). B.S.P.C. and E.J.B. are members of the Research Career of CONICET.
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