- Email: [email protected]
Infrared and Raman studies of palladium—nitrogen-containing polymers interactions
Journal of Molecular Structure 511–512 (1999) 205–215 www.elsevier.nl/locate/molstruc
Infrared and Raman studies of palladium—nitrogen-containing polymers interactions A. Drelinkiewicz a, M. Hasik b,*, S. Quillard c, C. Paluszkiewicz b,d b
a Department of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Crakow, Poland Department of Materials Science and Ceramics, Academy of Mining and Metallurgy, Al.Mickiewicza 30,30-059 Crakow, Poland c Institut des Materiaux,, rue de la Houssiniere, 44072 Nantes Cedex 03, France d Regional Laboratory, Jagiellonian University, ul. Ingardena 3, 30-060 Crakow, Poland
Received 12 December 1998; accepted 30 December 1998
Abstract In the present research the nature of interactions of poly(4-vinylpyridine) (PVP) and polyaniline (PANI) with various Pd 21 chlorocomplexes existing in PdCl2 –HCl–H2O solutions have been studied using IR (MIR, FIR) and Raman spectroscopies. It has been found that these interactions involve the nitrogen atoms of the polymers. In the PdCl2 solutions of high HCl concentration containing [PdCl4] 22 and [PdCl3(H2O)] 2 as the major species acid–base type reaction (protonation) with the formation of the polymer salt as well as the coordination of Pd 21 ions by the nitrogen atoms of the polymer take place. In the PdCl2 solutions of low HCl concentration containing [PdCl2(H2O) 2] as the major species protonation proceeds to a small extent. The main process in this case is most probably hydrolysis of the latter with the precipitation of hydrated palladium hydroxide or oxide on the polymer surface. In the case of PANI the oxidation–reduction between this precipitate and PANI takes place. It results in the oxidation of the polymer chain. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Poly(4-vinylpyridine); Palladium chlorocomplexes; FTIR spectroscopy; Raman spectroscopy
1. Introduction Palladium is known to react easily with a number of nitrogen-containing organic compounds, such as amines, imines, nitriles, etc. [1]. Such reactions result in the formation of complexes in which nitrogen atoms are coordinatively bonded to palladium ions. Complexes of palladium with nitrogen-containing polymers, such as polyethyleneimine, poly(Schiff bases) have been also described [2,3]. Vibrational spectroscopy (IR and Raman) is a suitable tool for characterization of polymer—transition metal ions systems. In the present studies this * Corresponding author.
method has been used to investigate the interactions between the solid polymer and various palladium chlorocomplexes present in the aqueous PdCl2 –HCl solutions. Two polymers have been studied, namely poly(4-vinylpyridine) (PVP) and polyaniline (PANI). Chemical compositions of PVP and PANI are represented by formulae (1) and (2) respectively. As can be seen PVP is a vinyl polymer containing nitrogen atoms in heteroaromatic ring substituents. PANI is a conjugated polymer whose chain contains two types of nitrogen atoms: amine (–NH–) and imine (yN–) ones. Nitrogen atoms present in the structure of both polymers constitute potential sites for polymer–palladium chlorocomplexes interactions in the systems studied.
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00161-1
206
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Table 1 PdCl2 solutions used for the preparation of the samples PVP–Pd and PANI–Pd studied in this research Polymer
PdCl2 solution
PdCl2 concentration (mol/dm 3)
HCl concentration (mol/dm 3)
PVP
A B C D E F G
2.3 × 10 23 2.3 × 10 23 0.02 4.9 × 10 23 0.12 0.12 0.24
2.00 0.66 × 10 23 – 0.02 0.15 0.24 2.30
PANI
The possibility for the formation of a coordination bond in e.g. PVP–Pd 21 systems is discussed in a number of articles. They show unequivocally that nitrogen atoms in PVP (polymer in the solid state or dissolved) form with Pd 21 (water–alcohol solutions of K2PdCl4 or PdCl2) complexes in which Pd 21 ions are coordinated by nitrogen atoms of the polymer [4– 6]. It has also been shown that PANI may play a role of the ligand in the complexes with Pd 21 [7] as well as with Ag 1 [8] and Cu 21 [9]. It should be noted that the systems selected for the present studies are of practical importance since they are potential candidates for catalysts in heterogeneous systems. In the literature catalytic activity of PANI– Pt and Pd in the hydrogenation of CxC bond has been reported [10]. Our preliminary results show that PANI–Pd system is also active in the liquid phase hydrogenation of 2-ethylanthraquinone (eAQ) [11]. The catalytic activity of PVP–Pd in the hydrogenation of allyl alcohol has also been studied [12]. However, it should be pointed out that catalysts of the transition
metal ion–polymer type are studied more often in the homogeneous media. It may be expected that the catalytic activity of the centers created in polymer–transition metal systems depends on the type of the compound formed. In the literature there are no systematic studies which would show how the type of the Pd 21 chlorocomplex interacting with the polymer influences the structure of the compound formed. Therefore studies of interactions between various Pd 21 chlorocomplexes and polymers (PANI, PVP) have been undertaken.
2. Experimental PANI was synthesized according to a standard procedure [13] by oxidation of aniline with ammonium peroxodisulfate in hydrochloric acid followed by deprotonation with aqueous ammonia. PVP was purchased from Aldrich. The reaction of PVP and PANI with palladium ions was carried out by stirring the appropriate solid
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 1. MIR spectra of PVP and the samples PVP–Pd containing 2 wt.% of Pd: (a) PVP; (b) PVP–Pd prepared in the conditions of low acidity (solution B); (c) deprotonated sample PVP–Pd prepared in the conditions of high acidity (solution A); (d) PVP–Pd prepared in the conditions of high acidity (solution A).
polymer at room temperature in the PdCl2 –HCl–H2O solutions. In the case of PVP two types of PdCl2 solutions have been used (Table 1): in the first one (denoted further on as A) the concentration of HCl was equal to 2 mol/dm 3, in the second one (denoted as B) it was equal to 0.66 × 10 23 mol/dm 3. The concentration of PdCl2 was the same in both types of the solutions and it was equal to 2.3 × 10 23 mol/dm 3. The reaction between PVP and palladium ions was carried out till the complete disappearance of Pd 21 from the solution which was checked colorimetrically by the reaction with KI. The sorption of Pd 21 was complete after 2 h in solution A (yellow-colored products) and in about 60 h in solution B (brown-colored products). Samples containing 2–10 wt.% of palladium were obtained by taking the appropriate volume of the PdCl2 solution. In the case of PANI several types of PdCl2 solutions were used (denoted as C, D, E, F, G, in Table 1). The concentration of PdCl2 in these solutions ranged from 0.02 to 0.24 mol/dm 3. The concentration of HCl varied from 0 (aqueous PdCl2, no HCl added) to 2.3 mol/dm 3. In all the reactions between PANI and palladium chlorocomplexes a constant molar ratio of PANI: PdCl2 equal to 3.5 was used. The reaction was carried for 3 h, i.e. up to complete sorption of Pd 21
207
from the solution (checked colorimetrically). Samples of PANI Pd containing from 8 to 18 wt.%. were prepared in this way. Additionaly samples of PANI– Pd were taken during synthesis at various time intervals and their MIR spectra recorded. FTIR spectroscopic studies in the middle infrared range (MIR) were carried out using a BioRad FTS 60v spectrometer and a standard KBr pellets technique in transmission geometry. The studies in the far infrared range (FIR) were performed on a BioRad FT60 spectrometer in polyethylene pellets. The resolution of the measurements was equal to 4 cm 21. Raman spectra were recorded using a Jobin–Yvon multichannel spectrometer at various excitation lines (l exc 457.9 and 676 nm). FT Raman spectra (l exc 1064 nm) were measured on a BioRad FT 6000 NIR spectrometer equipped with Raman attachment at a resolution of 4 cm 21.
3. Results and discussion In the PdCl2 –HCl–H2O solutions different chlorocomplexes of Pd 21 may coexist. The concentration of individual species depends mainly on HCl concentration. With increasing HCl contents in the solution anionic complexes, i.e. [PdCl4] 22, [PdCl3(H2O)] 2 become dominant. This situation corresponds to our solutions of type A and G (Table 1). However in aqueous solution of PdCl2 or solution of very low acidity (0.66 × 10 23 M) electrically neutral species, i.e. [PdCl2(H2O)2] are the dominating ones (solutions B, C, Table 1). UV–Vis spectra registered for the solutions A, G and B, C have confirmed such solution composition [11]. In solution A the maximum absorption at 474 nm indicates the predominant presence of [PdCl4] 22 in accordance with Ref. [14] while in solution B this maximum appears at 420 nm which corresponds to the predominant presence of [PdCl2(H2O)2] [14,15]. In other solutions used in the studies in which the concentration of HCl was between 0.66 × 10 23 M and 1 M a number of chlorocomplexes coexist without the strong domination of an individual one. Because of the presence of nitrogen atoms in the structure of PANI and PVP both polymers can react with acids (be protonated) to give the corresponding salt. However, formation of Pd 21 –N coordination bond may also be expected. Therefore it seems to be
208
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 2. FT Raman spectra of PVP and the samples PVP–Pd containing 4 wt.% of Pd: (a) PVP; (b) PVP–Pd prepared in the conditions of low acidity (solution B); (c) PVP–Pd prepared in the conditions of high acidity (solution A).
of special interest to determine which reaction would take place in the acidic systems (A and G) where— besides HCl—anionic palladium chlorocomplexes ([PdCl4] 22, [PdCl3(H2O)] 2) are also present. Reactions between the polymers and Pd 21 chlorocomplexes were followed by the analysis of the IR and Raman spectra of the products. For the sake of clarity their results will be presented and discussed in separate chapters devoted to PVP and PANI.
4. Poly(4-vinylpyridine) (PVP) Data presented in the literature show that IR and Raman spectroscopies make it possible to distinguish easily between the protonated (salt) form and the metal-coordinated PVP similarly as in the case of
pyridine (Py) when either Py·HCl salt (e.g. (PyH)2CoCl4) or Py–MeXn bond (e.g. MnPy2Cl2) is formed [16]. Changes in the polymer chain (by analogy to Py) have been observed in the middle IR range (MIR) of the spectrum, whereas changes in the coordination sphere of Pd 21 chlorocomplexes—in the far IR range (FIR) where the bands due to Pd–Cl and Pd–N vibrations occur. Literature data indicate that FIR is also suitable to distinguish between the cis and trans complexes as well as to determine formation of Cl–Pd–Cl bridging structures [17–20]. MIR spectra of PVP and PVP–palladium systems prepared in PdCl2 solutions of type A and B are shown in Fig. 1. The band originating from the CyN vibrations in the spectrum of PVP appears at 1596 cm 21 (Fig. 1(a)). After treatment of PVP in the PdCl2 solutions of type A different spectrum is observed. In
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 3. FIR spectra of PVP and the samples PVP–Pd: (a) PVP; (b) PVP–Pd prepared in the conditions of high acidity (solution A), 4 wt.% of Pd; (c) PVP–Pd prepared in the conditions of low acidity (solution B), 6 wt.% of Pd.
particular, in the range of CyN vibrations a new band at 1635 cm 21 with a shoulder at 1612 cm 21 appears (Fig.1(d)). The band at 1635 cm 21 corresponds to the vibrations of the HCl-protonated polymer [16] whereas the shoulder at 1612 cm 21 may be attributed to the PVP–Pd bond vibrations [5] in the case of
Fig. 4. FIR spectra of the samples PVP–Pd: (a) PVP–Pd prepared in the conditions of high acidity (solution A) containing 1 wt.% of Pd; (b) PVP–Pd prepared in the conditions of low acidity (solution B) treated with 2 M HCl; (c) PVP protonated with HCl.
209
coordination of metal ions by nitrogen atoms of the polymer. When the Cl 2 ions are removed from the sample by washing it with water or NaOH the band at 1635 cm 21 due to Py ring-H 1 vibrations in the polymer becomes much less intensive while a shoulder at 1618 cm 21 assigned to coordination bond Pd 21 –N in PVP is still visible (Fig. 1(c)). Hence, MIR spectroscopy shows that the main reaction occurring in the PdCl2 solutions of type A containing predominantly anionic [PdCl4] 22 and [PdCl3(H2O)] 2 complexes as well as high concentration of HCl is the acid–base type reaction of PVP with HCl. However, the coordination of Pd 21 by nitrogen atoms in PVP also takes place. This conclusion is fully corroborated by the Raman (Fig. 2) and FIR (Fig. 3) spectra of the samples prepared in the PdCl2 solution of type A. Raman spectra of the samples prepared in the PdCl2 solutions of type A (Fig. 2(c)) indicate the formation of pyridinium salt: the bands at 1007 cm 21 (pyridine ring vibrations), 1636 cm 21 and 1612 cm 21 (CyN), 797 cm 21 [21]. Additionally in the spectra the band at 295 cm 21 is present. It can be assigned to the Pd–N vibrations since it has also been observed in the Raman spectra of [Pd(NH3)4]Cl2 [22]. FIR spectra of the samples of series A (Fig. 3(b)) show the bands at 340 cm 21 corresponding to Pd–Cl asymmetric stretching vibrations and the one at 280 cm 21 due to Pd–N vibrations. The band at 280 cm 21 is visible even in the spectrum of the sample containing as little as 1 wt.% of Pd (Fig. 4(a)). Intensities of the bands at 280 cm 21 and at 340 cm 21 in the spectra of the samples of series A depend on the amount of Pd incorporated into PVP matrix; they grow as the amount of Pd in the sample increases. Similar bands have been observed in the case when PVP interacted with K2PdCl4 alcohol solutions [5,12]. They have been assigned to the trans complexes in which Pd 21 ions are coordinated by the nitrogen atoms of two different chains; Pd 21 is incorporated between the polymer chains. This may be the case also in the PVP–PdCl2 –HCl–H2O system. However, it cannot be excluded that in this system simultaneously [HPdCl4] 2 act as counterions in the salt formed. FIR spectra give also information on the formation of PVP salt, i.e. its protonation. In the FIR spectra of protonated samples (Fig. 4(c)) the increasing background at around 200 cm 21 is observed whereas
210
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 5. MIR spectra of PANI, samples of PANI–Pd and PANI–HCl: (a) PANI; (b) PANI–Pd prepared in the PdCl2 solution C; (c) PANI–Pd prepared in the PdCl2 solution D; (d) PANI–Pd prepared in the PdCl2 solution E; (e) PANI–Pd prepared in the PdCl2 solution F; (f) PANI–Pd prepared in the PdCl2 solution G; (g) PANI protonated with HCl. (For Fig. 5(b)–(g) see Table 1.)
after deprotonation this increased background disappears. Thus, even though in the PdCl2 solution of type A HCl as well as [PdCl4] 22 were present the competition between coordination and protonation was so strong that both processes took place simultaneously. In the PdCl2 solutions of type B, the concentration of HCl was much lower and the major Pd 21 complex present in the solution was [PdCl2(H2O)2]. Fig. 1(b) shows the MIR spectrum of the sample containing
2 wt.%. of Pd, Fig. 2(b)-Raman spectrum of this sample. In the MIR spectrum—except for a band of low intensity at 1635 cm 21 —no changes with respect to the spectrum of the starting PVP (Fig.1(a)) are observed. This spectrum indicates that in the PdCl2 solution of type B only slight protonation of PVP occurs. This is confirmed by the Raman spectrum in which a slight broadening of the band corresponding to CyN vibrations at 1598 cm 21 with respect to the spectrum of PVP can be observed. Additionally in the
Sample
C–Cl str.
C–H 1,4aromatic out of plane
B–NH 1 –B. QyNH 1 –B str. a
NyQyN str.
C–H 1,4aromatic in plane
B–N–B–N–B str.
QyN–B–NyB
QyN–B–NyQ
C–N str.
CyN str.
PANI–HCl G F E D C PANI
792 795 799 799 799 799 –
809 822 822 823 828 831 828
1106 1119 1139 1139 1141 1143 –
– – – – – – 1164
– – – – 1215 1215 1215
1236 1238 1241 1241 1242 1242 1240
1293 1295 1302 1302 1306 1307 1306
– – – – 1374 1376 1378
1472 1471 1481 1483 1486 1492 1494
1554 1553 1565 1568 1576 1580 1584
a
Q denotes quinoid units in PANI, i.e.
. B denotes benzenoid units in PANI, i.e.
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Table 2 Band positions (in cm 21) and their assignments in the MIR spectra of PANI, PANI–Pd systems and PANI protonated with HCl
211
212
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
PVP, low volume of PdCl2 solution, small amount of HCl) cannot be excluded. Upon treatment of sample B with HCl the precipitate does not dissolve but the sample changes its color from brown to yellow (color characteristic of samples A). This is accompanied by the change in the FIR spectrum of the sample (Fig. 4(b)) in which a new band at 320 cm 21 appears while the band at 380 cm 21 disappears. According to [18] the band at 320 cm 21 is due to the vibrations of Pd–Cl bond in the bridging structures of PdCl2.
5. Polyaniline (PANI)
Fig. 6. MIR spectra of PANI, samples of PANI–Pd taken during the synthesis in PdCl2 solution F (Table 1) and PANI–HCl: (a) PANI; (b) sample of PANI–Pd taken immediately after the beginning of the synthesis; (c) final sample of PANI–Pd; (d) PANI protonated with HCl.
Raman spectra of PVP–Pd systems of type B changes in the range of C–H vibrations (1170–1220 cm 21) as well as a new intensive band at 1370 cm 21 corresponding to –CH2 –, –CH3 vibrations appears. FIR spectra do not indicate protonation of PVP in the PdCl2 solutions of type B (Fig. 3(c)). They show the intensive band at 340 cm 21 characteristic of Pd– Cl vibrations as well as a new one at 378 cm 21. The band corresponding to Pd–N vibrations has not been observed in this case. Hence, the FIR spectra indicate that coordination of Pd 21 by nitrogen atoms does not take place upon interactions of PVP with [PdCl2(H2O)2]. Using other methods (UV–Vis, XPS, SEM) it has been established that in the system of type B on the surface of the polymer matrix hydrated palladium oxide or hydroxide precipitates [23]. Therefore it can be concluded that in this system due to basic properties of the polymer hydrolysis of [PdCl2(H2O)2] takes place resulting in the formation of these type of compounds. It cannot be excluded that the band at 380 cm 21 corresponds to these compounds. However, the band at 380 cm 21 was also observed in the spectrum of the sample A containing 1%weight of Pd (Fig. 4(a)). Thus, partial hydrolysis of [PdCl2(H2O)2] also in this case (high amount of
In the case of PANI–Pd 21 systems the effects observed in the IR and Raman spectra have not been as evident as in the case of PVP. However, these studies have made it possible to draw some general conclusions on the interactions taking place between PANI and palladium chlorocomplexes present in the PdCl2 –HCl–H2O solutions. MIR spectra of PANI–Pd systems prepared in various PdCl2 solutions are presented in Fig. 5. The band assignments and positions are given in Table 2. The spectrum of PANI (Fig. 5(a)) is consistent with the ones recorded by other authors [24]. This spectrum confirms the chemical composition of the synthesized polymer chain given by the formula (2). It contains two intensive bands at 1584 and 1494 cm 21 corresponding to the stretching vibrations of the CyN in the quinoid (Q) unit of PANI and C–N in benzenoid (B) unit of PANI, respectively. The band at 1164 cm 21 corresponding to the stretching vibrations of NyQyN is also clearly visible. After treatment of PANI in the PdCl2 solutions of various HCl concentrations the band at 1164 cm 21 disappears and a new band at ca. 1140 cm 21 evolves (Fig. 5, Table 2). It shifts to ca. 1119 cm 21 and significantly broadens at higher HCl contents. This band can be assigned to the B–NH 1 –B or QyNH 1 –B stretching vibrations indicating therefore protonated form of PANI. Additionally the shift towards lower wave numbers in the CyN (1584 cm 21) and C–N (1494 cm 21) stretching vibrations band positions is observed (Table 2, Fig. 5). It should be noted that this shift depends on HCl contents in the PdCl2 solution used for the synthesis of the sample (Table 1).
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
213
Table 3 Band positions and their assignments in the Raman spectra of PANI, sample of PANI–Pd prepared in the PdCl2 solution of low acidity (solution D, Table 1) and sample of PANI–Pd prepared in the PdCl2 solution of high acidity (solution G, Table 1) Sample
PANI
PANI–Pd (D)
PANI–Pd (G)
a
Band position, cm 21
Band assignment a
l exc 457.9 nm
l exc 676 nm
1617 1480 – 1170 1623 1494 1329 1214 1189 1169 1623 1339 1258 1189
1585 1475 1217 1162 1616 1463 1334 (weak) 1221 1164 1622 1340 1253 1193
CC str. CC str. 1 CH bending CN str. CH bending CC str. CC str. 1 CH bending CN protonated str. CN str. CH bending CC str. CN protonated str. CN str. CH bending
In accordance with Ref. [25].
The spectrum of the sample prepared in the PdCl2 solution G in which the HCl concentration was equal to 2.3 mol/dm 3 (Table 1), is similar to the spectrum of HCl protonated PANI (Fig. 5(g)). In contrast, the band at 1140 cm 21 indicating protonation of PANI is present even in spectrum of the sample prepared in the PdCl2 solution C (Fig. 5(b)) which was aqueous solution of PdCl2 with no HCl added. In such solution [PdCl2(H2O)2] was the predominant complex. Owing to the basic properties of PANI it hydrolyzed similarly to the case of PVP. HCl resulting from this hydrolysis can protonate nitrogen atoms in PANI. It is interesting to compare the spectra of the samples of PANI –Pd (PdCl2 solution F, Table 1) taken from the reaction medium during synthesis (Fig. 6). They show the change in the positions of the bands corresponding to CyN and C–N vibrations. In the spectrum of the sample taken immediately after the beginning of the synthesis (small amount of Pd 21 ions in the sample) the band due to to CyN vibrations is located at 1560 cm 21 (Fig. 6(b)) whereas in the spectrum of the final sample at 1572 cm 21 (Fig. 6(c)). The position of the band originating from the C–N vibrations (1580 cm 21) does not depend on the reaction time (Fig. 6(b) and (c)). Thus, MIR spectra prove immediate protonation of PANI. The shifts in the bands positions suggest that PANI was protonated with HCl first (Cl 2 acting as counterions) and only
after some time Cl 2 ions were replaced by [HPdCl4] 2. This replacement involved only imine nitrogen atoms present in PANI. Therefore the shift in the bands positions with respect to the spectrum of PANI (Fig. 6(a)) is smaller in the spectra of the final reaction product. Since this shift should depend on the type of the counterion such explanation seems to be possible. Hence, MIR spectra of PANI–Pd 21 systems prepared in various PdCl2 –HCl–H2O solutions show that the main process is protonation of PANI. In the resulting PANI–Pd 21 products, Cl 2 as well as anionic chlorocomplexes of Pd 21, i.e. [HPdCl4] 2, [PdCl3(H2O)] 2 present in the PdCl2 solution of high HCl contents can play the role of counterions. In the case of PANI–Pd system the competition between protonation ([HPdCl4] 2 as counterions) and coordination (formation of Pd 21 –N coordination bond) is not so strong as in the case of PVP–Pd system. In some cases coordination of Pd 21 ions by –Ny groups has been confirmed by XPS as well as UV–Vis spectroscopy [11]. The conclusions concerning the processes taking place in the PdCl2 solutions containing predominantly electrically neutral [PdCl2(H2O)2] species in the PVP– palladium chlorocomplexes system seem to be valid in the case of PANI–Pd systems as well. Thus, it seems probable that on the surface of the polymer hydrated palladium oxide or hydroxide precipitate.
214
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
However, spectroscopic features characteristic of the products of this complex hydrolysis have not been found. The plausible explanation for this is that to the redox properties of PANI (connected with the presence of two types of nitrogen groups, –Ny and –NH–) Pd 21 ions have been immediately reduced to Pd o and the polymer chain has been oxidized (–NH– groups transform to –Ny). Reduction of Pd 21 to Pd 0 has been established by XRD and XPS, whereas the oxidation of the polymer by UV–Vis spectroscopy [11]. Raman results for PANI–Pd systems are collected in Table 3. Raman spectra of the samples prepared in the PdCl2 solutions of low acidity (solution D, Table 1) and in the highly acidic PdCl2 solution (solution G, Table 1) give information which agrees well with the MIR results. They prove high protonation of PANI in the PdCl2 solutions of high HCl concentration and low protonation in the PdCl2 solution of low HCl content. In particular, the spectrum of sample D measured at l exc 457.9 nm (Table 3) shows some vibrational features of PANI salt (1623 and 1189 cm 21) but also some bands of unprotonated PANI (1494, 1169 and 1214 cm 21). Only the bands corresponding to the protonated form of PANI (1623 and 1189 cm 21) are present in the spectrum of sample G. Additionally in the spectra recorded at l exc 676 nm (Table 3) a sign of protonation (a splitted band at 1330 cm 21) is clearly visible only for the sample G, whereas the spectrum of the sample D is very close to the one of starting PANI. 6. Conclusions IR and Raman studies of the interactions of PVP and PANI with various palladium chlorocomplexes present in the PdCl2 –HCl–H2O solutions allow to draw the following conclusions: 1. In the case of both polymers the type of polymer– Pd 21 interactions in the PdCl2 –HCl–H2O solution depends on the types of palladium chlorocomplexes present in the reaction medium. 2. In the PdCl2 solutions containing predominantly anionic palladium chlorocomplexes ([PdCl4l 22, [PdCl3(H2O)] 2) a competition between formation of the salt (protonation) and coordination of Pd 21 ions by the nitrogen atoms of the polymer exists for
both polymers. This competition is strong in PVP while in the case of PANI protonation is stronger. 3. In the PdCl2 solutions containing predominantly electrically neutral palladium chlorocomplex ([PdCl3(H2O)2]) in the case of both polymers hydrolysis of this complex takes place resulting in the formation of hydrated palladium oxide or hydroxide precipitate. This is due to the basic properties of both polymers. However, owing to the redox properties of PANI, Pd 21 in this precipitate reduces to Pd 0. In the case of PVP this reduction does not take place and therefore the presence of the hydrolysis products has been determined spectroscopically.
References [1] F.R. Hartley, The Chemistry of Platinum and Palladium, Applied Science Publishers Ltd, London, 1973. [2] A.D. Pomogajlo, Kataliz Immobilizowannymi Kompleksami, Nauka, Moscow, 1991. [3] J.A. Biekturow, S. Kudajbiergienow, Chamzamulina, Polimery kationowe, PWN, Warsaw, 1991. [4] O.S. Roszukina, M.W. Kliujev, Koord. Khim. 8 (1982) 188. [5] L.N. Karklin, M.W. Kliujev, A.D. Pomogajlo, Kin. Kat. 24 (1983) 408. [6] W.N. Pierczenko, J.C. Mirskova, Dokl. Akad. Nauk SSSR 251 (2) (1980) 1437. [7] H.S. Li, M. Josowicz, D.R. Baer, M.H. Engelhard, J. Janata, J. Electrochem. Soc. 142 (1995) 798. [8] A.Q. Zhang, C.Q. Cui, J.Y. Lee, F.C. Loh, J. Electrochem. Soc. 142 (1995) 1097. [9] D. Higuchi, T. Imoda, Hirao, Macromolecules 29 (1996) 8277. [10] J.W. Sobczak, B. Lesiak, A. Jablonski, A. Kosinski, W. Palczewska, Polish , J. Chem. 69 (1995) 1732. [11] A. Drelinkiewicz, M. Hasik, M. Choczynski, Mater. Res. Bull. 33 (1998) 739. [12] D.W. Sokolski, A.K. Zharmagambetowa, S.G. Mukhamedzhanowa, E.A. Biekturov, S.E. Kudajbiergienov, React. Kinet. Catal. Lett. 33 (1987) 387. [13] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 230. [14] H.A. Droll, B.P. Block, W.C. Fernelius, J. Phys. Chem. 61 (1957) 1000. [15] L. Rasmusen, Chr. K. Jorgensen, Acta Chem. Scand. 22 (1968) 2313. [16] N.S. Gill, R.H. Nutall, D.E. Scaife, D.W.A. Sharp, J. Inorg. Nucl. Chem. 18 (1961) 71. [17] M. Pfeffer, P. Braunstein, J. Dehaud, Spectrochimica Acta, 30A (1974) 331. [18] C.F.J. Barnard, M.J.H. Russel, Comprehensive Coordination Chemistry, vol. 5, p. 1115.
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215 [19] [20] [21] [22]
P.L. Goggin, J. Chem. Soc., Dalton Trans. 10 (1974) 1483. R.J.H. Clark, Ch. Williams, Inorg. Chem. 4 (1965) 350. E. Spinner, J. Catal. 89 (1984) 433. S.S. Chau, A.T. Bell, J. Catal. 89 (1984) 433.
215
[23] W.J. Kazakowa, B.W. Pticyn, Zh. Nieorg. Khim. 12 (1967) 620. [24] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231. [25] I. Harada, Y. Furukawa, F. Ueda, Synth. Met. 29 (1989) E303.
Infrared and Raman studies of palladium—nitrogen-containing polymers interactions A. Drelinkiewicz a, M. Hasik b,*, S. Quillard c, C. Paluszkiewicz b,d b
a Department of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Crakow, Poland Department of Materials Science and Ceramics, Academy of Mining and Metallurgy, Al.Mickiewicza 30,30-059 Crakow, Poland c Institut des Materiaux,, rue de la Houssiniere, 44072 Nantes Cedex 03, France d Regional Laboratory, Jagiellonian University, ul. Ingardena 3, 30-060 Crakow, Poland
Received 12 December 1998; accepted 30 December 1998
Abstract In the present research the nature of interactions of poly(4-vinylpyridine) (PVP) and polyaniline (PANI) with various Pd 21 chlorocomplexes existing in PdCl2 –HCl–H2O solutions have been studied using IR (MIR, FIR) and Raman spectroscopies. It has been found that these interactions involve the nitrogen atoms of the polymers. In the PdCl2 solutions of high HCl concentration containing [PdCl4] 22 and [PdCl3(H2O)] 2 as the major species acid–base type reaction (protonation) with the formation of the polymer salt as well as the coordination of Pd 21 ions by the nitrogen atoms of the polymer take place. In the PdCl2 solutions of low HCl concentration containing [PdCl2(H2O) 2] as the major species protonation proceeds to a small extent. The main process in this case is most probably hydrolysis of the latter with the precipitation of hydrated palladium hydroxide or oxide on the polymer surface. In the case of PANI the oxidation–reduction between this precipitate and PANI takes place. It results in the oxidation of the polymer chain. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Poly(4-vinylpyridine); Palladium chlorocomplexes; FTIR spectroscopy; Raman spectroscopy
1. Introduction Palladium is known to react easily with a number of nitrogen-containing organic compounds, such as amines, imines, nitriles, etc. [1]. Such reactions result in the formation of complexes in which nitrogen atoms are coordinatively bonded to palladium ions. Complexes of palladium with nitrogen-containing polymers, such as polyethyleneimine, poly(Schiff bases) have been also described [2,3]. Vibrational spectroscopy (IR and Raman) is a suitable tool for characterization of polymer—transition metal ions systems. In the present studies this * Corresponding author.
method has been used to investigate the interactions between the solid polymer and various palladium chlorocomplexes present in the aqueous PdCl2 –HCl solutions. Two polymers have been studied, namely poly(4-vinylpyridine) (PVP) and polyaniline (PANI). Chemical compositions of PVP and PANI are represented by formulae (1) and (2) respectively. As can be seen PVP is a vinyl polymer containing nitrogen atoms in heteroaromatic ring substituents. PANI is a conjugated polymer whose chain contains two types of nitrogen atoms: amine (–NH–) and imine (yN–) ones. Nitrogen atoms present in the structure of both polymers constitute potential sites for polymer–palladium chlorocomplexes interactions in the systems studied.
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00161-1
206
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Table 1 PdCl2 solutions used for the preparation of the samples PVP–Pd and PANI–Pd studied in this research Polymer
PdCl2 solution
PdCl2 concentration (mol/dm 3)
HCl concentration (mol/dm 3)
PVP
A B C D E F G
2.3 × 10 23 2.3 × 10 23 0.02 4.9 × 10 23 0.12 0.12 0.24
2.00 0.66 × 10 23 – 0.02 0.15 0.24 2.30
PANI
The possibility for the formation of a coordination bond in e.g. PVP–Pd 21 systems is discussed in a number of articles. They show unequivocally that nitrogen atoms in PVP (polymer in the solid state or dissolved) form with Pd 21 (water–alcohol solutions of K2PdCl4 or PdCl2) complexes in which Pd 21 ions are coordinated by nitrogen atoms of the polymer [4– 6]. It has also been shown that PANI may play a role of the ligand in the complexes with Pd 21 [7] as well as with Ag 1 [8] and Cu 21 [9]. It should be noted that the systems selected for the present studies are of practical importance since they are potential candidates for catalysts in heterogeneous systems. In the literature catalytic activity of PANI– Pt and Pd in the hydrogenation of CxC bond has been reported [10]. Our preliminary results show that PANI–Pd system is also active in the liquid phase hydrogenation of 2-ethylanthraquinone (eAQ) [11]. The catalytic activity of PVP–Pd in the hydrogenation of allyl alcohol has also been studied [12]. However, it should be pointed out that catalysts of the transition
metal ion–polymer type are studied more often in the homogeneous media. It may be expected that the catalytic activity of the centers created in polymer–transition metal systems depends on the type of the compound formed. In the literature there are no systematic studies which would show how the type of the Pd 21 chlorocomplex interacting with the polymer influences the structure of the compound formed. Therefore studies of interactions between various Pd 21 chlorocomplexes and polymers (PANI, PVP) have been undertaken.
2. Experimental PANI was synthesized according to a standard procedure [13] by oxidation of aniline with ammonium peroxodisulfate in hydrochloric acid followed by deprotonation with aqueous ammonia. PVP was purchased from Aldrich. The reaction of PVP and PANI with palladium ions was carried out by stirring the appropriate solid
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 1. MIR spectra of PVP and the samples PVP–Pd containing 2 wt.% of Pd: (a) PVP; (b) PVP–Pd prepared in the conditions of low acidity (solution B); (c) deprotonated sample PVP–Pd prepared in the conditions of high acidity (solution A); (d) PVP–Pd prepared in the conditions of high acidity (solution A).
polymer at room temperature in the PdCl2 –HCl–H2O solutions. In the case of PVP two types of PdCl2 solutions have been used (Table 1): in the first one (denoted further on as A) the concentration of HCl was equal to 2 mol/dm 3, in the second one (denoted as B) it was equal to 0.66 × 10 23 mol/dm 3. The concentration of PdCl2 was the same in both types of the solutions and it was equal to 2.3 × 10 23 mol/dm 3. The reaction between PVP and palladium ions was carried out till the complete disappearance of Pd 21 from the solution which was checked colorimetrically by the reaction with KI. The sorption of Pd 21 was complete after 2 h in solution A (yellow-colored products) and in about 60 h in solution B (brown-colored products). Samples containing 2–10 wt.% of palladium were obtained by taking the appropriate volume of the PdCl2 solution. In the case of PANI several types of PdCl2 solutions were used (denoted as C, D, E, F, G, in Table 1). The concentration of PdCl2 in these solutions ranged from 0.02 to 0.24 mol/dm 3. The concentration of HCl varied from 0 (aqueous PdCl2, no HCl added) to 2.3 mol/dm 3. In all the reactions between PANI and palladium chlorocomplexes a constant molar ratio of PANI: PdCl2 equal to 3.5 was used. The reaction was carried for 3 h, i.e. up to complete sorption of Pd 21
207
from the solution (checked colorimetrically). Samples of PANI Pd containing from 8 to 18 wt.%. were prepared in this way. Additionaly samples of PANI– Pd were taken during synthesis at various time intervals and their MIR spectra recorded. FTIR spectroscopic studies in the middle infrared range (MIR) were carried out using a BioRad FTS 60v spectrometer and a standard KBr pellets technique in transmission geometry. The studies in the far infrared range (FIR) were performed on a BioRad FT60 spectrometer in polyethylene pellets. The resolution of the measurements was equal to 4 cm 21. Raman spectra were recorded using a Jobin–Yvon multichannel spectrometer at various excitation lines (l exc 457.9 and 676 nm). FT Raman spectra (l exc 1064 nm) were measured on a BioRad FT 6000 NIR spectrometer equipped with Raman attachment at a resolution of 4 cm 21.
3. Results and discussion In the PdCl2 –HCl–H2O solutions different chlorocomplexes of Pd 21 may coexist. The concentration of individual species depends mainly on HCl concentration. With increasing HCl contents in the solution anionic complexes, i.e. [PdCl4] 22, [PdCl3(H2O)] 2 become dominant. This situation corresponds to our solutions of type A and G (Table 1). However in aqueous solution of PdCl2 or solution of very low acidity (0.66 × 10 23 M) electrically neutral species, i.e. [PdCl2(H2O)2] are the dominating ones (solutions B, C, Table 1). UV–Vis spectra registered for the solutions A, G and B, C have confirmed such solution composition [11]. In solution A the maximum absorption at 474 nm indicates the predominant presence of [PdCl4] 22 in accordance with Ref. [14] while in solution B this maximum appears at 420 nm which corresponds to the predominant presence of [PdCl2(H2O)2] [14,15]. In other solutions used in the studies in which the concentration of HCl was between 0.66 × 10 23 M and 1 M a number of chlorocomplexes coexist without the strong domination of an individual one. Because of the presence of nitrogen atoms in the structure of PANI and PVP both polymers can react with acids (be protonated) to give the corresponding salt. However, formation of Pd 21 –N coordination bond may also be expected. Therefore it seems to be
208
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 2. FT Raman spectra of PVP and the samples PVP–Pd containing 4 wt.% of Pd: (a) PVP; (b) PVP–Pd prepared in the conditions of low acidity (solution B); (c) PVP–Pd prepared in the conditions of high acidity (solution A).
of special interest to determine which reaction would take place in the acidic systems (A and G) where— besides HCl—anionic palladium chlorocomplexes ([PdCl4] 22, [PdCl3(H2O)] 2) are also present. Reactions between the polymers and Pd 21 chlorocomplexes were followed by the analysis of the IR and Raman spectra of the products. For the sake of clarity their results will be presented and discussed in separate chapters devoted to PVP and PANI.
4. Poly(4-vinylpyridine) (PVP) Data presented in the literature show that IR and Raman spectroscopies make it possible to distinguish easily between the protonated (salt) form and the metal-coordinated PVP similarly as in the case of
pyridine (Py) when either Py·HCl salt (e.g. (PyH)2CoCl4) or Py–MeXn bond (e.g. MnPy2Cl2) is formed [16]. Changes in the polymer chain (by analogy to Py) have been observed in the middle IR range (MIR) of the spectrum, whereas changes in the coordination sphere of Pd 21 chlorocomplexes—in the far IR range (FIR) where the bands due to Pd–Cl and Pd–N vibrations occur. Literature data indicate that FIR is also suitable to distinguish between the cis and trans complexes as well as to determine formation of Cl–Pd–Cl bridging structures [17–20]. MIR spectra of PVP and PVP–palladium systems prepared in PdCl2 solutions of type A and B are shown in Fig. 1. The band originating from the CyN vibrations in the spectrum of PVP appears at 1596 cm 21 (Fig. 1(a)). After treatment of PVP in the PdCl2 solutions of type A different spectrum is observed. In
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 3. FIR spectra of PVP and the samples PVP–Pd: (a) PVP; (b) PVP–Pd prepared in the conditions of high acidity (solution A), 4 wt.% of Pd; (c) PVP–Pd prepared in the conditions of low acidity (solution B), 6 wt.% of Pd.
particular, in the range of CyN vibrations a new band at 1635 cm 21 with a shoulder at 1612 cm 21 appears (Fig.1(d)). The band at 1635 cm 21 corresponds to the vibrations of the HCl-protonated polymer [16] whereas the shoulder at 1612 cm 21 may be attributed to the PVP–Pd bond vibrations [5] in the case of
Fig. 4. FIR spectra of the samples PVP–Pd: (a) PVP–Pd prepared in the conditions of high acidity (solution A) containing 1 wt.% of Pd; (b) PVP–Pd prepared in the conditions of low acidity (solution B) treated with 2 M HCl; (c) PVP protonated with HCl.
209
coordination of metal ions by nitrogen atoms of the polymer. When the Cl 2 ions are removed from the sample by washing it with water or NaOH the band at 1635 cm 21 due to Py ring-H 1 vibrations in the polymer becomes much less intensive while a shoulder at 1618 cm 21 assigned to coordination bond Pd 21 –N in PVP is still visible (Fig. 1(c)). Hence, MIR spectroscopy shows that the main reaction occurring in the PdCl2 solutions of type A containing predominantly anionic [PdCl4] 22 and [PdCl3(H2O)] 2 complexes as well as high concentration of HCl is the acid–base type reaction of PVP with HCl. However, the coordination of Pd 21 by nitrogen atoms in PVP also takes place. This conclusion is fully corroborated by the Raman (Fig. 2) and FIR (Fig. 3) spectra of the samples prepared in the PdCl2 solution of type A. Raman spectra of the samples prepared in the PdCl2 solutions of type A (Fig. 2(c)) indicate the formation of pyridinium salt: the bands at 1007 cm 21 (pyridine ring vibrations), 1636 cm 21 and 1612 cm 21 (CyN), 797 cm 21 [21]. Additionally in the spectra the band at 295 cm 21 is present. It can be assigned to the Pd–N vibrations since it has also been observed in the Raman spectra of [Pd(NH3)4]Cl2 [22]. FIR spectra of the samples of series A (Fig. 3(b)) show the bands at 340 cm 21 corresponding to Pd–Cl asymmetric stretching vibrations and the one at 280 cm 21 due to Pd–N vibrations. The band at 280 cm 21 is visible even in the spectrum of the sample containing as little as 1 wt.% of Pd (Fig. 4(a)). Intensities of the bands at 280 cm 21 and at 340 cm 21 in the spectra of the samples of series A depend on the amount of Pd incorporated into PVP matrix; they grow as the amount of Pd in the sample increases. Similar bands have been observed in the case when PVP interacted with K2PdCl4 alcohol solutions [5,12]. They have been assigned to the trans complexes in which Pd 21 ions are coordinated by the nitrogen atoms of two different chains; Pd 21 is incorporated between the polymer chains. This may be the case also in the PVP–PdCl2 –HCl–H2O system. However, it cannot be excluded that in this system simultaneously [HPdCl4] 2 act as counterions in the salt formed. FIR spectra give also information on the formation of PVP salt, i.e. its protonation. In the FIR spectra of protonated samples (Fig. 4(c)) the increasing background at around 200 cm 21 is observed whereas
210
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Fig. 5. MIR spectra of PANI, samples of PANI–Pd and PANI–HCl: (a) PANI; (b) PANI–Pd prepared in the PdCl2 solution C; (c) PANI–Pd prepared in the PdCl2 solution D; (d) PANI–Pd prepared in the PdCl2 solution E; (e) PANI–Pd prepared in the PdCl2 solution F; (f) PANI–Pd prepared in the PdCl2 solution G; (g) PANI protonated with HCl. (For Fig. 5(b)–(g) see Table 1.)
after deprotonation this increased background disappears. Thus, even though in the PdCl2 solution of type A HCl as well as [PdCl4] 22 were present the competition between coordination and protonation was so strong that both processes took place simultaneously. In the PdCl2 solutions of type B, the concentration of HCl was much lower and the major Pd 21 complex present in the solution was [PdCl2(H2O)2]. Fig. 1(b) shows the MIR spectrum of the sample containing
2 wt.%. of Pd, Fig. 2(b)-Raman spectrum of this sample. In the MIR spectrum—except for a band of low intensity at 1635 cm 21 —no changes with respect to the spectrum of the starting PVP (Fig.1(a)) are observed. This spectrum indicates that in the PdCl2 solution of type B only slight protonation of PVP occurs. This is confirmed by the Raman spectrum in which a slight broadening of the band corresponding to CyN vibrations at 1598 cm 21 with respect to the spectrum of PVP can be observed. Additionally in the
Sample
C–Cl str.
C–H 1,4aromatic out of plane
B–NH 1 –B. QyNH 1 –B str. a
NyQyN str.
C–H 1,4aromatic in plane
B–N–B–N–B str.
QyN–B–NyB
QyN–B–NyQ
C–N str.
CyN str.
PANI–HCl G F E D C PANI
792 795 799 799 799 799 –
809 822 822 823 828 831 828
1106 1119 1139 1139 1141 1143 –
– – – – – – 1164
– – – – 1215 1215 1215
1236 1238 1241 1241 1242 1242 1240
1293 1295 1302 1302 1306 1307 1306
– – – – 1374 1376 1378
1472 1471 1481 1483 1486 1492 1494
1554 1553 1565 1568 1576 1580 1584
a
Q denotes quinoid units in PANI, i.e.
. B denotes benzenoid units in PANI, i.e.
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
Table 2 Band positions (in cm 21) and their assignments in the MIR spectra of PANI, PANI–Pd systems and PANI protonated with HCl
211
212
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
PVP, low volume of PdCl2 solution, small amount of HCl) cannot be excluded. Upon treatment of sample B with HCl the precipitate does not dissolve but the sample changes its color from brown to yellow (color characteristic of samples A). This is accompanied by the change in the FIR spectrum of the sample (Fig. 4(b)) in which a new band at 320 cm 21 appears while the band at 380 cm 21 disappears. According to [18] the band at 320 cm 21 is due to the vibrations of Pd–Cl bond in the bridging structures of PdCl2.
5. Polyaniline (PANI)
Fig. 6. MIR spectra of PANI, samples of PANI–Pd taken during the synthesis in PdCl2 solution F (Table 1) and PANI–HCl: (a) PANI; (b) sample of PANI–Pd taken immediately after the beginning of the synthesis; (c) final sample of PANI–Pd; (d) PANI protonated with HCl.
Raman spectra of PVP–Pd systems of type B changes in the range of C–H vibrations (1170–1220 cm 21) as well as a new intensive band at 1370 cm 21 corresponding to –CH2 –, –CH3 vibrations appears. FIR spectra do not indicate protonation of PVP in the PdCl2 solutions of type B (Fig. 3(c)). They show the intensive band at 340 cm 21 characteristic of Pd– Cl vibrations as well as a new one at 378 cm 21. The band corresponding to Pd–N vibrations has not been observed in this case. Hence, the FIR spectra indicate that coordination of Pd 21 by nitrogen atoms does not take place upon interactions of PVP with [PdCl2(H2O)2]. Using other methods (UV–Vis, XPS, SEM) it has been established that in the system of type B on the surface of the polymer matrix hydrated palladium oxide or hydroxide precipitates [23]. Therefore it can be concluded that in this system due to basic properties of the polymer hydrolysis of [PdCl2(H2O)2] takes place resulting in the formation of these type of compounds. It cannot be excluded that the band at 380 cm 21 corresponds to these compounds. However, the band at 380 cm 21 was also observed in the spectrum of the sample A containing 1%weight of Pd (Fig. 4(a)). Thus, partial hydrolysis of [PdCl2(H2O)2] also in this case (high amount of
In the case of PANI–Pd 21 systems the effects observed in the IR and Raman spectra have not been as evident as in the case of PVP. However, these studies have made it possible to draw some general conclusions on the interactions taking place between PANI and palladium chlorocomplexes present in the PdCl2 –HCl–H2O solutions. MIR spectra of PANI–Pd systems prepared in various PdCl2 solutions are presented in Fig. 5. The band assignments and positions are given in Table 2. The spectrum of PANI (Fig. 5(a)) is consistent with the ones recorded by other authors [24]. This spectrum confirms the chemical composition of the synthesized polymer chain given by the formula (2). It contains two intensive bands at 1584 and 1494 cm 21 corresponding to the stretching vibrations of the CyN in the quinoid (Q) unit of PANI and C–N in benzenoid (B) unit of PANI, respectively. The band at 1164 cm 21 corresponding to the stretching vibrations of NyQyN is also clearly visible. After treatment of PANI in the PdCl2 solutions of various HCl concentrations the band at 1164 cm 21 disappears and a new band at ca. 1140 cm 21 evolves (Fig. 5, Table 2). It shifts to ca. 1119 cm 21 and significantly broadens at higher HCl contents. This band can be assigned to the B–NH 1 –B or QyNH 1 –B stretching vibrations indicating therefore protonated form of PANI. Additionally the shift towards lower wave numbers in the CyN (1584 cm 21) and C–N (1494 cm 21) stretching vibrations band positions is observed (Table 2, Fig. 5). It should be noted that this shift depends on HCl contents in the PdCl2 solution used for the synthesis of the sample (Table 1).
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
213
Table 3 Band positions and their assignments in the Raman spectra of PANI, sample of PANI–Pd prepared in the PdCl2 solution of low acidity (solution D, Table 1) and sample of PANI–Pd prepared in the PdCl2 solution of high acidity (solution G, Table 1) Sample
PANI
PANI–Pd (D)
PANI–Pd (G)
a
Band position, cm 21
Band assignment a
l exc 457.9 nm
l exc 676 nm
1617 1480 – 1170 1623 1494 1329 1214 1189 1169 1623 1339 1258 1189
1585 1475 1217 1162 1616 1463 1334 (weak) 1221 1164 1622 1340 1253 1193
CC str. CC str. 1 CH bending CN str. CH bending CC str. CC str. 1 CH bending CN protonated str. CN str. CH bending CC str. CN protonated str. CN str. CH bending
In accordance with Ref. [25].
The spectrum of the sample prepared in the PdCl2 solution G in which the HCl concentration was equal to 2.3 mol/dm 3 (Table 1), is similar to the spectrum of HCl protonated PANI (Fig. 5(g)). In contrast, the band at 1140 cm 21 indicating protonation of PANI is present even in spectrum of the sample prepared in the PdCl2 solution C (Fig. 5(b)) which was aqueous solution of PdCl2 with no HCl added. In such solution [PdCl2(H2O)2] was the predominant complex. Owing to the basic properties of PANI it hydrolyzed similarly to the case of PVP. HCl resulting from this hydrolysis can protonate nitrogen atoms in PANI. It is interesting to compare the spectra of the samples of PANI –Pd (PdCl2 solution F, Table 1) taken from the reaction medium during synthesis (Fig. 6). They show the change in the positions of the bands corresponding to CyN and C–N vibrations. In the spectrum of the sample taken immediately after the beginning of the synthesis (small amount of Pd 21 ions in the sample) the band due to to CyN vibrations is located at 1560 cm 21 (Fig. 6(b)) whereas in the spectrum of the final sample at 1572 cm 21 (Fig. 6(c)). The position of the band originating from the C–N vibrations (1580 cm 21) does not depend on the reaction time (Fig. 6(b) and (c)). Thus, MIR spectra prove immediate protonation of PANI. The shifts in the bands positions suggest that PANI was protonated with HCl first (Cl 2 acting as counterions) and only
after some time Cl 2 ions were replaced by [HPdCl4] 2. This replacement involved only imine nitrogen atoms present in PANI. Therefore the shift in the bands positions with respect to the spectrum of PANI (Fig. 6(a)) is smaller in the spectra of the final reaction product. Since this shift should depend on the type of the counterion such explanation seems to be possible. Hence, MIR spectra of PANI–Pd 21 systems prepared in various PdCl2 –HCl–H2O solutions show that the main process is protonation of PANI. In the resulting PANI–Pd 21 products, Cl 2 as well as anionic chlorocomplexes of Pd 21, i.e. [HPdCl4] 2, [PdCl3(H2O)] 2 present in the PdCl2 solution of high HCl contents can play the role of counterions. In the case of PANI–Pd system the competition between protonation ([HPdCl4] 2 as counterions) and coordination (formation of Pd 21 –N coordination bond) is not so strong as in the case of PVP–Pd system. In some cases coordination of Pd 21 ions by –Ny groups has been confirmed by XPS as well as UV–Vis spectroscopy [11]. The conclusions concerning the processes taking place in the PdCl2 solutions containing predominantly electrically neutral [PdCl2(H2O)2] species in the PVP– palladium chlorocomplexes system seem to be valid in the case of PANI–Pd systems as well. Thus, it seems probable that on the surface of the polymer hydrated palladium oxide or hydroxide precipitate.
214
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215
However, spectroscopic features characteristic of the products of this complex hydrolysis have not been found. The plausible explanation for this is that to the redox properties of PANI (connected with the presence of two types of nitrogen groups, –Ny and –NH–) Pd 21 ions have been immediately reduced to Pd o and the polymer chain has been oxidized (–NH– groups transform to –Ny). Reduction of Pd 21 to Pd 0 has been established by XRD and XPS, whereas the oxidation of the polymer by UV–Vis spectroscopy [11]. Raman results for PANI–Pd systems are collected in Table 3. Raman spectra of the samples prepared in the PdCl2 solutions of low acidity (solution D, Table 1) and in the highly acidic PdCl2 solution (solution G, Table 1) give information which agrees well with the MIR results. They prove high protonation of PANI in the PdCl2 solutions of high HCl concentration and low protonation in the PdCl2 solution of low HCl content. In particular, the spectrum of sample D measured at l exc 457.9 nm (Table 3) shows some vibrational features of PANI salt (1623 and 1189 cm 21) but also some bands of unprotonated PANI (1494, 1169 and 1214 cm 21). Only the bands corresponding to the protonated form of PANI (1623 and 1189 cm 21) are present in the spectrum of sample G. Additionally in the spectra recorded at l exc 676 nm (Table 3) a sign of protonation (a splitted band at 1330 cm 21) is clearly visible only for the sample G, whereas the spectrum of the sample D is very close to the one of starting PANI. 6. Conclusions IR and Raman studies of the interactions of PVP and PANI with various palladium chlorocomplexes present in the PdCl2 –HCl–H2O solutions allow to draw the following conclusions: 1. In the case of both polymers the type of polymer– Pd 21 interactions in the PdCl2 –HCl–H2O solution depends on the types of palladium chlorocomplexes present in the reaction medium. 2. In the PdCl2 solutions containing predominantly anionic palladium chlorocomplexes ([PdCl4l 22, [PdCl3(H2O)] 2) a competition between formation of the salt (protonation) and coordination of Pd 21 ions by the nitrogen atoms of the polymer exists for
both polymers. This competition is strong in PVP while in the case of PANI protonation is stronger. 3. In the PdCl2 solutions containing predominantly electrically neutral palladium chlorocomplex ([PdCl3(H2O)2]) in the case of both polymers hydrolysis of this complex takes place resulting in the formation of hydrated palladium oxide or hydroxide precipitate. This is due to the basic properties of both polymers. However, owing to the redox properties of PANI, Pd 21 in this precipitate reduces to Pd 0. In the case of PVP this reduction does not take place and therefore the presence of the hydrolysis products has been determined spectroscopically.
References [1] F.R. Hartley, The Chemistry of Platinum and Palladium, Applied Science Publishers Ltd, London, 1973. [2] A.D. Pomogajlo, Kataliz Immobilizowannymi Kompleksami, Nauka, Moscow, 1991. [3] J.A. Biekturow, S. Kudajbiergienow, Chamzamulina, Polimery kationowe, PWN, Warsaw, 1991. [4] O.S. Roszukina, M.W. Kliujev, Koord. Khim. 8 (1982) 188. [5] L.N. Karklin, M.W. Kliujev, A.D. Pomogajlo, Kin. Kat. 24 (1983) 408. [6] W.N. Pierczenko, J.C. Mirskova, Dokl. Akad. Nauk SSSR 251 (2) (1980) 1437. [7] H.S. Li, M. Josowicz, D.R. Baer, M.H. Engelhard, J. Janata, J. Electrochem. Soc. 142 (1995) 798. [8] A.Q. Zhang, C.Q. Cui, J.Y. Lee, F.C. Loh, J. Electrochem. Soc. 142 (1995) 1097. [9] D. Higuchi, T. Imoda, Hirao, Macromolecules 29 (1996) 8277. [10] J.W. Sobczak, B. Lesiak, A. Jablonski, A. Kosinski, W. Palczewska, Polish , J. Chem. 69 (1995) 1732. [11] A. Drelinkiewicz, M. Hasik, M. Choczynski, Mater. Res. Bull. 33 (1998) 739. [12] D.W. Sokolski, A.K. Zharmagambetowa, S.G. Mukhamedzhanowa, E.A. Biekturov, S.E. Kudajbiergienov, React. Kinet. Catal. Lett. 33 (1987) 387. [13] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 230. [14] H.A. Droll, B.P. Block, W.C. Fernelius, J. Phys. Chem. 61 (1957) 1000. [15] L. Rasmusen, Chr. K. Jorgensen, Acta Chem. Scand. 22 (1968) 2313. [16] N.S. Gill, R.H. Nutall, D.E. Scaife, D.W.A. Sharp, J. Inorg. Nucl. Chem. 18 (1961) 71. [17] M. Pfeffer, P. Braunstein, J. Dehaud, Spectrochimica Acta, 30A (1974) 331. [18] C.F.J. Barnard, M.J.H. Russel, Comprehensive Coordination Chemistry, vol. 5, p. 1115.
A. Drelinkiewicz et al. / Journal of Molecular Structure 511–512 (1999) 205–215 [19] [20] [21] [22]
P.L. Goggin, J. Chem. Soc., Dalton Trans. 10 (1974) 1483. R.J.H. Clark, Ch. Williams, Inorg. Chem. 4 (1965) 350. E. Spinner, J. Catal. 89 (1984) 433. S.S. Chau, A.T. Bell, J. Catal. 89 (1984) 433.
215
[23] W.J. Kazakowa, B.W. Pticyn, Zh. Nieorg. Khim. 12 (1967) 620. [24] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231. [25] I. Harada, Y. Furukawa, F. Ueda, Synth. Met. 29 (1989) E303.