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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 n_s(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 n_s(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 (v^1/2) but is not proportional to it (cf. Figure 5). The ratio ipc/ v^1/2 diminishes slightly from 68 to 61 mA s^1/2 V^-1/2 [25-27].

Figure 5. Dependence of the cathodic current peak on the square root of the scan rate (v^1/2).

     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:

 [Cu^II(pic)₂]    [Cu^I(pic)₂]^-                               E_1/2  @ -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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