Complexes of polyvinylpyrrolidone and polyethylene glycol with palladium(II) ions: characterization and catalytic activity published by Ural Federal University eISSN 2411-1414 chimicatechnoacta.ru ARTICLE 2023, vol. 10(3), No. 202310301 DOI: 10.15826/chimtech.2023.10.3.01 1 of 6 Complexes of polyvinylpyrrolidone and polyethylene glycol with palladium(II) ions: characterization and catalytic activity Dina N. Akbayeva , Indira A. Smagulova , Azhar Timurkyzy, Botagoz S. Bakirova Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan * Corresponding author: dina.akbayeva@bk.ru This paper belongs to the RKFM'23 Special Issue: https://chem.conf.nstu.ru/. Guest Editors: Prof. N. Uvarov and Prof. E. Aubakirov. Abstract In this work, we obtained complexes by mixing aqueous solution of palla- dium(II) chloride with polyvinylpyrrolidone and polyethylene glycol. The composition of the complex compounds was determined by potentiometric and conductometric titration. IR spectroscopy and scanning electron mi- croscopy (SEM) confirmed the coordination of polymeric ligand to palladi- um and allowed evaluating the morphology and features of the complex surface. The catalytic activity of the synthesized compounds in the oxida- tion of octene-1 by inorganic oxidizers (NaBrO3, K2S2O8) in aqueous-organic media in dimethyl sulfoxide (DMSO) under mild conditions was calculated. The reaction product was octanone-2, obtained in good yield (62–98%). Quantitative analysis of octanone-2 was made by the gas-chromatographic method. Mass spectrometry confirms the formation of octanone-2. The complexes are able to participate in five consecutive catalytic cycles with- out significant loss of catalytic efficiency. Oxidation of octene-1 proceeds by the oxidation-reduction mechanism and consists of two key stages. Keywords palladium(II) ions polyvinylpyrrolidone polyethylene glycol catalysis oxidation octene-1 Received: 21.06.23 Revised: 13.07.23 Accepted: 19.07.23 Available online: 24.07.23 Key findings ● The composition of the complex compounds was determined by potentiometric and conductometric methods. ● IR spectroscopy and SEM confirmed the coordination of polymeric ligand to palladium and allowed evaluating the morphology of the complex surface. ● The catalytic activity of the complexes in the oxidation of octene-1 by inorganic oxidizers under mild conditions was evaluated. © 2023, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction The oxidation of unsaturated hydrocarbons with different oxidants leads to the formation of different oxygen- containing compounds, which are key materials for then synthesis of fine chemicals, pharmaceuticals, and im- portant intermediates in petroleum industrial chemistry [1]. The Pd(II)-catalyzed oxidation of alkenes to carbonyl compounds is one of the most well-known reactions medi- ated by palladium and has extensive synthetic applica- tions, usually referred to as the Wacker reactions [2–4]. The use of CuCl2, even in catalytic amount, resulted in the production of noxious copper waste, which is poten- tially very toxic to aquatic life. It leads to the formation of substantial amounts of ecologically hazardous chlorinated by-products. Elimination of CuCl2 as co-catalyst would ren- der the Wacker oxidation a completely green process [5]. The introduction of functional groups into the polymer during the development of new types of catalytic systems based on metal-polymer complexes plays an important role. Nowadays the palladium complexes with PEG and PVP ligands are widely used as catalysts for cross-coupling reactions and direct synthesis of hydrogen peroxide from hy- drogen and oxygen [6–8]. Both sodium bromate (NaBrO3) and potassium peroxy- disulfate (K2S2O8) are inexpensive, commercially available strong oxidizing agents. They have been used with differ- ent catalysts in various organic reactions as co-oxidants in stoichiometric quantities [9]. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2023.10.3.01 mailto:dina.akbayeva@bk.ru http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0001-9101-2418 http://orcid.org/0000-0002-8879-3134 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2023.10.3.01&domain=pdf&date_stamp=2023-07-24 Chimica Techno Acta 2023, vol. 10(3), No. 202310301 ARTICLE 2 of 6 DOI: 10.15826/chimtech.2023.10.3.01 Sodium bromate has been used for selective oxidation of vicinal diols to -hydroxy ketones, oxidation of alco- hols and alkyl arenes to the corresponding carbonyl compounds, and selective oxidation of sulfides to sulfox- ides [10–13]. The peroxydisulfate ion is one of the strongest known oxidizing agents in aqueous solution. Potassium peroxydi- sulfate has been used for the oxidation of 1,4- dihydropyridines to the pyridine derivatives, selective oxi- dation of alcohols to aldehydes and ketones, C–Se bond formation to construct -phenylseleno carbonyl compounds and ,-unsaturated carbonyl compounds [14–16]. This article describes the synthesis and characteriza- tion of complexes between palladium(II) chloride and hy- drophilic polymers (polyvinylpyrrolidone (PVP) and poly- ethylene glycol (PEG)). The complexes were characterized by conductometric and potentiometric titration. The ob- tained polymer-metal complexes Pd(II)–PVP and Pd(II)– PEG were tested as catalysts in the oxidation of octene-1 by inorganic oxidants (sodium bromate (NaBrO3) and po- tassium peroxydisulfate (K2S2O8)) in dimethyl sulfoxide (DMSO) under mild conditions (70 °C, PN2 = 1 atm). 2. Experimental Palladium chloride (content of metal 59.32%), polyvi- nylpyrrolidone (molecular mass 40000, AppliChem, Ger- many), polyethylene glycol (molecular weight 10,000, Sigma Aldrich, USA), hydrochloric acid (36%), dimethyl sulfoxide, DMSO (AppliChem, Germany), distilled water, inorganic salts NaBrO3, K2S2O8 (Sigma Aldrich, USA) were used without purification. 2.1. Synthesis of H2[PdCl4] The complex H2[PdCl4] was synthesized according to the literature [17]. Palladium(II) chloride (1.0 g, 5.6 mmol) was added to a solution of 30 mL water and 1.0 mL con- centrated HCl (36%) contained in a 100-mL round-bottom flask. The resulting mixture was heated under reflux until all the palladium(II) chloride had been dissolved. The dark-brown solution of H2[PdCl4] was then allowed to cool to room temperature. 2.2. Synthesis of PdCl2 – PVP 20 mL of H2[PdCl4] solution (0.5 mmol) was placed in a glass dish and mixed with 20 mL of an aqueous solution of PVP (0.167 g, 1.5 mmol). The mixture was stirred for 10– 20 minutes at ambient temperature in order to allow for complete linking of the Pd(II) ions to the polymer. All vol- atile components were removed in aspirator vacuum, and the complex was dried and stored in air at ambient tem- perature. Yield: 0.25 g (97%). 2.3. Synthesis of PdCl2 – PEG 20 mL of H2[PdCl4] solution (0.5 mmol) was placed in a glass dish and mixed with 20 mL of an aqueous solution of PEG (0.124 g, 2.0 mmol). The mixture was stirred for 10– 20 minutes at ambient temperature in order to allow for complete linking of the Pd(II) ions to the polymer. All vol- atile components were removed in aspirator vacuum, and the complex was dried and stored in air at ambient tem- perature. Yield: 0.17 g (95%). The process of complexation between palladium(II) ion and polymers was investigated by potentiometric and con- ductometric methods with several ionic strengths and temperatures. Potentiometric studies were carried out in a commercial device (PHYWE, Germany) under thermostati- cally controlled conditions. Conductometric studies were performed on a ConductivityMeter 13701/93 (PHYWE, Germany) under thermostated conditions. All experiments were carried out under temperature control with an accu- racy of ±0.2 °C. IR spectra were recorded on a FT IR-4100 type A JASCO instrument (USA) in the range of 4000–450 cm−1. SEM images were taken on a JSM-6490LA Jeol instrument equipped with an X-ray dispersive energy detector (EDX) for elemental analysis (Japan). Samples for the GС and mass spectroscopy were pre- pared according to the literature procedure [18] by extrac- tion with ethyl acetate. The GC analysis was carried out on Shimadzu GC-17A (Japan) and Varian 3900 (Canada) de- vices in the programmed mode from 70 to 280 °C with 5 °C/min heat rate using a capillary column CS- Chromatography Service of Type FS-OV-1-CB-0.25. Mass spectra were recorded on a Varian Saturn 2100T device (Canada). IR spectra, SEM images and mass spectra were ob- tained in analytical laboratories at the Rheinland-Pfalz Technical University of Kaiserslautern-Landau (RPTU, Germany). 2.4. Typical reaction procedure Oxidation of octene-1 by inorganic oxidizers was carried out in a temperature-controlled laboratory setup with in- tensive stirring in a glass temperature-controlled reactor under negligible temperature gradient (”a catalytic duck”), equipped by the potentiometric device and connected to the gas burette filled with nitrogen. Shaking the reactor with frequency of about 250–300 swingings/min minimiz- es both the mass transport and mass transfer resistance. The laboratory experiments were performed as follows. The reactor with an entire volume of 150 mL was charged with the catalyst (0.057 mmol) under nitrogen atmos- phere. The reactor and the gas burette were preheated to 70 °C. The temperature was maintained by the water cir- culating between the glass reactor and the heating devic- es. Then, in nitrogen flow, an oxidizer (3 mmol) and aque- ous solvent (8 mL, 4:1 by volume) were placed. Finally, octene-1 (1 mmol) was added, and an electric stirrer was switched on. The temperature was maintained with an accuracy of ±0.5 °C by means of the thermostat. After the experimental runs the reaction solutions were merged and analyzed on a gas chromatograph. https://doi.org/10.15826/chimtech.2023.10.3.01 Chimica Techno Acta 2023, vol. 10(3), No. 202310301 ARTICLE 3 of 6 DOI: 10.15826/chimtech.2023.10.3.01 3. Results and discussion 3.1. Potentiometric titration Figure 1 shows the potentiometric titration curves of PdCl2–PVP and PdCl2–PEG complexes. The mixing of solu- tions of polymer with salt is accompanied by a pH de- crease, which is explained by the deprotonation of initially protonated PVP and PEG during the complexation. From the titration curve (Figure 1a), the optimal molar ratio of the reacting components k (k=[Pd2+]/[PVP]=0.35) was found. It means that one central metal atom bonds with three mono-links of polymer ligands. From the titration curve (Figure 1b), the optimal molar ratio of the reacting components k (k=[Pd2+]/[PEG]=0.25) was found. It means that one central metal atom bonds with four mono-links of polymer ligands. 3.2. Conductometric titration In order to confirm the composition of the formed PVP– Pd2+ and PEG–Pd2+ complexes, the dependence of the con- ductivity corrected for the viscosity on the ratio of the initial component of the system was studied (Figure 2). The increase in electrical conductivity is due to the re- leased H+ ions during the reaction between PVP or PEG and palladium(II) ions. Figure 1 Potentiometriс titration curves of polymer (10−2 M) with palladium salt H2[PdCl4] (10 −2 M) (where V – titrant volume in mL, pH – pH of solution). PVP (a); PEG (b). As can be seen from Figure 2, the electrical conductivi- ty of the solution passes through the inflection point with an increase in the molar content of metal ions. Based on the data obtained as a result of conducted conductometric studies, it can be argued that the complexation process is accompanied by an increase in the electrical conductivity of the system at the ratios PVP–Pd2+=3:1 and PEG–Pd2+=4:1. The complexation process of PVP with Pd2+ ions is characterized by negative values of Gibbs energy that indicates the spontaneous process of polymer-metal complex formation. For the Pd2+–PVP complex in the temperature range 298–318 K the positive value of en- thalpy change (ΔrΗ0) indicates the endothermic character of complex formation. Therefore, the stability of the lat- ter increases with increasing temperature in this range. And in the temperature range of 318–343 K the process of complexation is accompanied by the release of heat (exothermic process). As a result, the strength of the polymer-metal complex decreases with increasing tem- perature. Figure 2 Potentiometriс titration curves of polymer (10−2 M) with palladium salt H2[PdCl4] (10 −2 M) (where V – titrant volume in mL, pH – pH of solution,  – specific electrical conductivity of solution in S cm−1). PVP(a); PEG (b). https://doi.org/10.15826/chimtech.2023.10.3.01 Chimica Techno Acta 2023, vol. 10(3), No. 202310301 ARTICLE 4 of 6 DOI: 10.15826/chimtech.2023.10.3.01 This ambiguous influence of temperature on the process of complexation is probably caused by conformational changes in the structure of PVP polymer, investigated by Liu et al. in [19]. It was found that conformational changes of PVP in aqueous medium are accompanied by exo effects that depend on temperature and molecular weight of the polymer. 3.3. Oxidation of octene-1 catalysed by Pd(PVP)3 Cl2 and Pd(PEG)4Cl2 complexes The synthesized complexes were used as catalysts in the process of oxidation of octene-1 by inorganic oxidizers (NaBrO3, K2S2O8) in aqueous-organic media in dimethyl sulfoxide under mild conditions (equation 1). Reaction conditions and conversion rates of liquid- phase oxidation of octene-1 by oxidizers in water-organic solutions are given in Table 1. The yield of octanone-1 (or n-hexyl methyl ketone) in the experiments in the presence of Pd(II)–PEG complex was determined by gas- chromatographic analysis and was 62–81%, that is lower than the product yield in the presence of Pd(II)–PVP com- plex. According to GC analysis and mass spectrometry of reaction solutions, when the yield was less than 100%, the formation of products other than octanone-2 was not ob- served. The yield of octanone-2 did not exceed 5% after experiments in the presence of inorganic oxidizers (Na- BrO3, K2S2O8). (1) The high catalytic activity of the monometallic poly- meric palladium complexes relates to their solubility in the water-organic reaction solution. As a polarity index of DMSO is equal to 7.2, the components of the reaction solution would be expected to dissolve completely in DMSO [20]. The mass spectra of octanone-2 obtained from the re- action solution contain identical intensive peaks with m/z: 41, 43, 58, 85, 129, which confirms formation of octanone-2 (Figure 3). The catalysts could be recycled as shown in Figure 4. The catalytic activity of the PdCl2 declines strongly, while it is demonstrated that Pd(PVP)3Cl2 and Pd(PEG)4Cl2 can be cycled for at least five times. Table 1 Wacker process of octene-1 oxidation in the presence of Pd(PVP)3Cl2 and Pd(PEG)4Cl2 complexes [a]. Entry Oxidizer Conversion, % Octanone-2 yield, %b Cat Pd(PVP)3Cl2 1 NaBrO3 97 97 2 K2S2O8 98 98 Cat Pd(PEG)4Cl2 3 NaBrO3 81 81 4 K2S2O8 62 62 а Reaction conditions: octene-1 (1 mmol), Cat (0.057 mmol), oxidizer (3 mmol), solvent/water (4:1, 10 mL), 120 min, 70 °C, PN2 = 1 atm. b Yields were determined by GC analysis of samples. The activity of the catalyst in DMSO correlates best with the level of Pd(II) immobilized on the PVP, and this Pd(II) complex, therefore, seems to be the primary catalyt- ic species. Thus, when a solution of the yellow complex was heat- ed at 70 °C it became brown-black, indicative of cluster or colloidal palladium formation. The yield of the product reached 12–30% in DMSO after the fifth cycle. A noticeable problem in homogeneous Pd-mediated oxidation is catalyst deactivation by aggregation into inactive metallic Pd. The selectivity does not change, and octanone-2 was observed as the only product, as in the previous experiments. According to the known values of redox potentials, Na- BrO3 and K2S2O8 were chosen as oxidizing agents. The oxi- dation of Pd(0) and reduction of BrO3−, S2O82− are thermo- dynamically resolved, they proceed quite easily and are characterized by negative values of Gо for these processes (–88 and –197 kJ, respectively). Considering the transitions BrO3− + 6H+ + 6e → Br− + 3H2O and S2O82− + 2e → 2SO42−, it should be noted that the oxidation-reduction potential is higher for K2S2O8 (2.010 V) than for NaBrO3 (1.440 V) [21, 22]. Figure 3 Mass spectra of the reaction solution. Reaction condi- tions: Pd(PVP)3Cl2 (0.057 mmol) – C8H16 (1 mmol) – KIO4 (3 mmol) – DMSO/H2O (4:1, 10 mL) at 70 °C and PN2 = 1 atm. Figure 4 Reusing the catalysts: Pd(PVP)3Cl2 (а) and Pd(PEG)4Cl2 (b). https://doi.org/10.15826/chimtech.2023.10.3.01 Chimica Techno Acta 2023, vol. 10(3), No. 202310301 ARTICLE 5 of 6 DOI: 10.15826/chimtech.2023.10.3.01 The IR spectra of green PVP–PdCl2 complex and the used one are given in Figure 5. Both IR spectra contain bands at 3402 cm−1 characteristic of PVP. The carbonyl group in PVP–PdCl2 complex is characterized by the peak at 1646 сm−1. It is slightly shifted to 1652 cm−1 in the PVP– PdCl2 complex after the experiment. To evaluate the structure and morphology of the cata- lysts after the experiment and to identify the arrangement and nature of the phases present on the catalyst surface, SEM and EDX techniques were employed. Figure 6 shows the SEM micrograph of a selected region of the Pd catalyst used for the octene-1 oxidation. As shown in Figure 6b, the SEM images of the Pd(PVP)3Cl2 after the fifth recycling experiment did not show any significant morphological change compared to those of the original sample. One of the reasons in decrease of the catalytic reaction rate is an excess of surface species such as the bromine-, oxygen-, and carbon-containing surface compounds, which could block the Pd active sites [23]. The oxidation-reduction mechanism consists of two key stages [24]: reduction of Pd(II)(Pol) by octene-1 to Pd(0)(Pol) with formation of octanone-2 and oxidation of Pd(0)(Pol) to Pd(II)(Pol) by oxidizer. In the hypothetical oxidation reaction equations (equations 2–4), the PVP or PEG mono link denotes one polymer ligand bound to the polymer coordinated with palladium. Activation [Pd(II)(Pol)3–4Cl]+ + C8H16 + H2O → [Pd(0)(Pol)3–4] + C8H16O + 2H+ + Cl− (2) Bromate oxidation, protonation: [Pd(0)(Pol)3–4] + NaBrO3 + 6H+ → [HOPd(II)(Pol)3–4]+ + NaBr + 3H+ + 2OH− (3) Dissociation of one polymer ligand: [HOPd(II)(Pol)3–4]+ ⇌ [HOPd(II)(Pol)3–4]+ + (Pol)dangling (4) 4. Limitations The main problem that appeared during this research is the high hygroscopicity of Pd(II)–PEG complex in compari- son with Pd(II)–PVP one. A possible solution of this prob- lem is the further immobilization of the complex on the supports. 5. Conclusions Palladium–polymer complexes [Pd(PVP)3Cl]+ and [Pd(PEG)4Cl]+ were obtained from palladium(II) chloride, polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG). Using potentiometric and conductometric titration as well as IR spectroscopy, the structure of the polymeric com- plexes was established. The polymer complexes were test- ed for catalytic activity in octene-1 oxidation by the inor- ganic oxidizers NaBrO3 and K2S2O8 in dimethyl sulfoxide under mild conditions (70 °C, PN2 = 1 atm). Conversion of octene-1 was achieved in 62–98% yield. The final reaction product was octanone-2 (or n-hexyl methyl ketone). The catalysts can be reused at least up to five times. The SEM and EDX techniques were employed for evaluating the structure and morphology of the catalysts after experi- ment, and for the identification of the arrangement and nature of the phases present on the catalyst surface. The oxidation-reduction mechanism consisting of two key stages was proposed. ● Supplementary materials No supplementary materials are available. Figure 5 IR spectra of green PVP–PdCl2 and the used catalyst. Figure 6 Electronic microphotographs of the Pd(PVP)3Cl2 complex before (a) and after (b) experiment. https://doi.org/10.15826/chimtech.2023.10.3.01 Chimica Techno Acta 2023, vol. 10(3), No. 202310301 ARTICLE 6 of 6 DOI: 10.15826/chimtech.2023.10.3.01 ● Funding This research had no external funding. ● Acknowledgments None. ● Author contributions Conceptualization: D.N.A. Data curation: B.S.B., I.A.S. Investigation: I.A.S., A.T. Methodology: D.N.A., B.S.B. Supervision: D.N.A. Writing – original draft: D.N.A., B.S.B. Writing – review & editing: D.N.A. ● Conflict of interest The authors declare no conflict of interest. ● Additional information Author IDs: Dina N. Akbayeva, Scopus ID 6505789588; Botagoz S. Bakirova, Scopus ID 57204585748; Indira A. Smagulova, Scopus ID57220043414. Website: Al-Farabi Kazakh National University, https://www.kaznu.kz/en. References 1. Weissermel K, Arpe HJ. Industrial Organic Chemistry. 4th ed. Weinheim: Wiley-VCH; 2003. 491 p. 2. Tsuji J. 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