The sorptivity for ions of the Irving-Williams series (Zn2+, Cu2+, Co2+, Ni2+) was estimated for the chelate sorbent N-(2-carboxyethyl)chitosan cross-linked by nanosecond electron-beam irradiation. The maximum capacity reaches 3.7 mmol/g using a mixture of the appropriate sulfates at pH 4.5. The selectivity of copper sorption is 80%. The structure of the coordination sphere of the copper complexes with N-(2-carboxyethyl)chitosan was investigated by ESR spectroscopy

Molochnikov, L. S.

Pestov, A

Yatluk, Y.

Zabolotskaya, E

Journal of Applied Spectroscopy

English

Pestov, A.

Electronic archive of the Ural State Forest Engineering University

SOLID-STATE STRUCTURE OF COPPER COMPLEXESOF N-(2-CARBOXYETHYL)CHITOSANL. S. Molochnikov,a A. V. Pestov,b* E. V. Zabolotskaya,c and Yu. G. YatlukbUDC (541.6+541.183.12):543.422.27The sorptivity for ions of the Irving-Williams series (Zn2+, Cu2+, Co2+, Ni2+) was estimated for the chelatesorbent N-(2-carboxyethyl)chitosan cross-linked by nanosecond electron-beam irradiation. The maximum ca-pacity reaches 3.7 mmol/g using a mixture of the appropriate sulfates at pH 4.5. The selectivity of coppersorption is 80%. The structure of the coordination sphere of the copper complexes with N-(2-carboxyethyl)chi-tosan was investigated by ESR spectroscopy.Key words: N-(2-carboxyethyl)chitosan, copper complexes, spectroscopy, electron spin resonance.Introduction. Chitosan (poly-β-D-2-deoxy-2-aminoglucose) is a highly reactive aminopolymer that, in contrastwith its structural analog cellulose, can be more effectively functionalized in order to improve and adjust the sorptionproperties of the products [1–4]. A method for preparing N-(2-carboxyethyl)chitosan (CEC) from chitosan and acrylicacid in a gel was previously developed [5]. According to the established structure, CEC is a statistical copolymer ofβ-D-2-deoxy-2-aminoglucose and its N-derivatives:The sorbent produced by cross-linking CEC with lower dialdehydes possesses high sorption capacity for ions of theIrving-Williams series and is highly selective for copper(II) absorption [6]. Using electron beams for cross-linking hasan obvious advantage over any cross-linking by chemical reagents because in this instance the specific mass of thematrix ascribed to a single chelate group does not increase [7]. The structure of the coordination sphere of CEC com-plexes cannot be directly investigated by x-ray diffraction analysis because it is a polymeric substrate. Models of suchcomplexes for the free polymer have been proposed [8]. The dependence of the sorption properties of the material onthe structure of the resulting metal complexes must be understood in order to optimize its structure. These data havenot been reported for CEC. Herein an attempt is made to describe the structure of the copper complexes formed bysorption of copper in CEC cross-linked using an electron beam. The structure of the linking agent functional groupshas no effect because no linking agent is used.Experimental. CEC with various degrees of substitution (DS) was used as the sodium salt and was cross-linked beforehand by nanosecond electron-beam irradiation (80 kGy). An aqueous solution of a mixture of Co(II),Ni(II), Cu(II), and Zn(II) sulfates had the same concentration of each salt (0.025 M) and pH 4.5. The experiment wasaUral State Wood Technology University, Ekaterinburg, Russia; bI. Ya. Postovsky Institute of Organic Synthe-sis, Ural Branch of Russian Academy of Sciences, 22 S. Kovalevskaya St., Ekaterinburg, 620041, Russia; e-mail:yatluk@ios.uran.ru; cInstitute of Solid State Chemistry, Ural Branch of Russian Academy of Sciences, Ekaterinburg.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 635–639, September–October, 2008. Original ar-ticle submitted April 22, 2008.Journal of Applied Spectroscopy, Vol. 75, No. 5, 20080021-9037/08/7505-0648 2008 Springer Science+Business Media, Inc.648∗To whom correspondence should be addressed.carried out under static conditions for 24 h. The amount of sorbed metal ions was determined from the difference inthe concentrations of the starting and final solutions. The concentration was determined using atomic-emission analysison an Optima 4300 DV spectrometer.ESR spectra were recorded using a PS 100.X ESR spectrometer (Adani, Belarus) at 298 K. Samples for ESRstudies were prepared by storing for 4 days a weighed portion of CEC in a solution of the appropriate copper salt(acetate or nitrate) with subsequent filtration and drying at 70oC. The copper content in the sample was determined asabove.Discussion. Table 1 lists the results for sorption of ions of the Irving–Williams series by cross-linked CECwith a high DS and various ratios of mono- and disubstituted glucosamine units. It can be seen that the capacity in-creased significantly initially with increasing DS whereas the selectivity for Cu(II) sorption decreased. This was easilyexplained within the framework of known sorption rules. The sorption capacity decreased at first and then increasedwith a further increase of the DS. The selectivity changed in the opposite direction. It increased at first and then de-creased. It should be noted that the number of absorbed ions exceeded the number of ion-binding carboxylate groups.This indicated that some of the sulfate ions were absorbed in addition to the cations.The molar amount of absorbed metal ions did not exceed the molar content of N in the sorbent. Acetamidegroups coordinate weakly as a result of the poor donor properties of the amide N. However, balance calculations con-firmed that some of the amino groups were not involved in complex formation, i.e., they were sterically unavailable.The use of sulfate solutions, which is advantageous from a practical viewpoint, introduced definite complications forthe sorption study. In contrast with the corresponding salts of monobasic acids, chitosan sulfate cross-linked by thedoubly charged anion was insoluble in water and swelled poorly. The possibility of the same CEC cross-linking withits larger basicity, which increased with increasing DS, complicated the process even further. It should also be consid-ered that the nature of the cross-linking by acetamide, amine, and mono- and disubstituted units of the polymeric chaindue to electron-beam irradiation cannot be entirely the same. The complicated interplay of these factors did not allowprevailing tendencies to be revealed. Nevertheless, it is certain that lower capacity always correlated with maximum se-lectivity; higher capacity, with minimum selectivity.Solutions of copper nitrate were used in order to avoid the effect of ionic cross-linking during the investiga-tion of the complex structure. Solutions of copper acetate were used to account for the possibility of complex forma-tion by low-molecular-weight ligands because the stability of copper sulfate complexes is about the same as that of theacetate complexes.Copper complexes of CEC were studied using ESR spectroscopy in order to establish the structure of thecompounds formed upon sorption. The corresponding complexes with chitosan and carboxymethylchitosan (CMC) werestudied for comparison. The results indicated that polymers were formed even in relatively acidic media (pH 3.1–4.3)by N-containing copper complexes despite the fact that the pKa of the amino group in the starting chitosan was 5.6[9]; in CEC (DS = 1), pKa = 6.42 [10]. This was in complete agreement with an analogous phenomenon that was ob-served earlier for cross-linked polymeric complexants with aliphatic amino or aminoacetate ligands [11, 12].TABLE 1. Sorption of Irving–Williams Series Ions at pH 4.5 by N-(2-Carboxyethyl)chitosan Cross-Linked by NanosecondElectron-Beam Irradiation (80 kGy)DS DS(m) DS(d) DS(m)/DS(d)Number of functionalgroups, mmol/g Number of sorbed metals, mmol/g (% of Σ)Ntot COOH Co2+ Ni2+ Cu2+ Zn2+ Σ0.96 0.71 0.12 6 4.44 3.57 0.08 0.20 1.65 (79) 0.15 2.081.06 0.39 0.33 1.18 4.32 3.84 0.33 0.44 1.64 (54 0.60 3.011.25 0.75 0.25 3 4.10 4.35 0.28 0.55 1.94 (52) 0.93 3.701.32 0.45 0.44 1 4.02 4.53 0.10 0.25 1.56 (78) 0.08 1.991.54 0.46 0.54 0.85 3.82 4.99 0.29 0.45 1.73 (58) 0.52 2.99Note: DS, total degree of substitution, DS = DS(m) + 2DS(d); DS(m), fraction of N-monosubstituted glucosamine units in the totalnumber of glucosamine units; DS(d), fraction of N,N-disubstituted glucosamine units in the total number of glucosamine units.649The observed ESR spectra were characteristic of Cu2+ ions in an axially symmetric field. They were anisot-ropic signals with well resolved hyperfine splitting (HFS) in the parallel orientation of the unpaired electron spin andexternal magnetic field (Fig. 1). This enabled ESR parameters gN and AN to be measured. The ESR parameters gM andAM were not determined because the HFS was not resolved for the perpendicular orientation. The ESR parameters forthe Cu2+ complexes of chitosan (gN = 2.255 and AN = 15.3 mT) (spectrum 5) agreed well with those of the complex[Cu(MEA)2(H2O)2]2+ in which two N,O rings from bidentate monoethanolamine ligands with protonated alcoholgroups are located in the equatorial plane of the copper complex [13]. The ESR parameters did not exclude the pos-sibility that a complex with an elongated octahedron of three N atoms in the Cu(II) coordination sphere and alcoholgroups situated along the apical axis was formed in chitosan the same as in some weakly basic anion exchangers [11].The fourth coordination site in the equatorial plane would probably be occupied by a water molecule. A choice be-tween these two interpretations was impossible based on the ESR data.The ESR parameters of the Cu2+ complexes with CMC (spectrum 1) were typical of the formation by Cuions of trans-complexes with two aminoacetate ligands in the equatorial plane. Complexes with analogous composi-tions and structures were observed in three-dimensional polymeric matrices of styrene and polyacrylate with complex-forming amino- and iminodiacetic and ethanolaminoacetic acids (Table 2).Various copper complexes were formed with CEC depending on the DS, initial form of the sorbent (Na- orH-form), and copper salt (acetate or nitrate II). Complexes of 1:2 composition with trans-location of amino groups ofβ-alaninate 6-membered rings and ESR parameters gN = 2.269 and AN = 15.8 mT (spectrum 3) were formed with theminimum DS (0.31) where only β-aminopropionic acid groups are found in the polymeric chain. We observed analo-gous complexes for CEC with DS = 1.0 upon reaction with copper nitrate solution. The ESR parameters changed togN = 2.288 and AN = 14.8 mT if copper acetate solution was used. This was consistent with a structural change of thecopper complex (spectrum 4). Iminodipropionate groups were also formed in the polymer molecules for DS = 1.0.Their involvement in the complexation to form at once two coordinated β-alaninate rings upon reaction with copperion generated in this instance complexes of 1:1 composition. Apparently the presence of acetate ions, which form cop-per complexes, stabilized this complex in the polymer by occupying the vacant fourth position in the equatorial plane.This conclusion agreed with those above about the sorption of inorganic anions. The Cu2+ complex with N-(2-hy-droxyethyl)iminodipropionic acid (diHe-ala) of 1:1 composition (CuL) has the same ESR parameters (Table 2). Com-plexes of 1:2 composition (CuL2) were not observed. The lack of a low-molecular-weight ligand in the solution(replacing acetate by nitrate) made existence of the 1:1 complex energetically unfavorable. In this instance incorpora-tion into the copper coordination sphere of a carboxylate from another iminopropionate group caused rearrangement ofthe copper complex, involvement in the complexation of the β-alaninate chelate ring of this group, and formation ofthe 1:2 complex, the structure of which was analogous to that described above for CEC with DS = 0.31.These conclusions were directly confirmed by an ESR study of CEC with DS = 1.0 in the mixed H+, Na+-form that had absorbed various amounts of Cu2+ from acetate solution. At first the ESR signal for the 1:1 complexFig. 1. ESR spectrum of Cu2+ in samples at 293 K at 9.3 GHz: carboxy-methylchitosan, 1.0 (Na+) (1), N-(2-carboxyethyl)chitosan, 1.0 (H+, Na+) (2),N-(2-carboxyethyl)chitosan, 0.31 (H+) (3), N-(2-carboxyethyl)chitosan, 1.0(Na+) (4), chitosan (5).650(at low copper contents) was observed. A signal for the 1:2 complex in addition to that for the 1:1 complex appearedin the ESR spectrum as the amount of absorbed copper increased. The fraction of the 1:2 complex increased rapidlyso that the ratio of copper ions bound in the different complexes equaled 1:1 upon reaching 0.8 mmol/g of absorbedcopper (spectrum 2). Further increasing the DS of CEC up to the maximum (DS = 1.62) resulted in formation of onlythe 1:1 copper complexes. The composition of the complexes remained the same regardless of whether the H+-formwas used or acetate ions were replaced by nitrate in the initial solution. Thus, it was established that 1:1 copper com-plexes were formed under all conditions with primarily iminodipropionic groups in the CEC. This distinguished thecross-linked polymer from the one-dimensional acid diHe-ala, for which 1:2 complexes formed in addition to the 1:1complexes [14].A comparison of our ESR parameters for Cu2+ with those in the literature [9, 15] showed that fundamentallydifferent conclusions were made about the structure of the copper complexes in modified chitosans from similar ESRspectra. The previous interpretation [9, 15] was based on the hypothesis that Cu2+ can react with only one aminogroup of chitosan and only at rather high pH values (pH ≥ 8) where it is expected to be deprotonated. Attempts werenot even made to show that the polymeric chain is flexible enough to twist and form di- and even higher amine cop-per complexes. However, it was proved experimentally [8] that this is possible. ESR investigations of amine, aminoal-cohol, and aminocarboxylate polyelectrolytes (including cross-linked) and their monomeric analogs confirmed thatdiamine (and higher) copper complexes are formed even in acidic media [11–13, 16, 17]. It should be noted that thereis a general tendency for the ESR parameters of Cu2+ complexes of similar structure with aminoacetic groups of poly-mers to have large AN values (and in several instances gN too) compared with Cu2+ complexes with aminopropionicgroups.A comparison of the results with those in the literature showed that the nature of the polymeric backbone(natural or synthetic polymer) had no effect on the structure of the resulting copper complexes. The structure of thefunctional group had the main influence. It should be noted that the presence of a low-molecular-weight ligand, evenone so simple as acetate, in the solution from which copper is extracted caused formation of mixed complexes.TABLE 2. ESR Parameters of Various Chelating PolymersSample, DS Form gN ± 0.005 AN ± 0.03, mTChitosan –NH2, acetate 2.255 15.3CEC, 0.31 H+, acetate 2.269 15.8CEC, 1.0 Na+, acetate 2.271 15.6CEC, 1.0 Na+, acetate 2.288 14.8CEC, 1.0 H+, Na+, acetate 2.293 14.92.273 15.6CEC, 1.62 Na+, acetate 2.291 14.8Na+, nitrateCMC, 1.0 Na+, acetate 2.263 16.7AMP-95 [12] H+, sulfate 2303 14.92.266 16.5ANKB-10/35 [12] H+, sulfate 2.303 14.92.265 16.7ANKB-50 [12] H+, sulfate 2.298 15.32.264 17.2AMP-15 [12] H+, sulfate 2.316 14.82.301 15.7CuL 2.295 15.2diHe-ala [14] CuL2 2.261 13.4651Conclusion. The DS of N-(2-carboxyethyl)chitosan corresponding to the highest capacity and selectivity forCu(II) in Irving–Williams series metals (maximum capacity up to 3.7 mmol/g; selectivity, up to 80%) was found. Itwas shown that anions of the absorbed solution are involved in the sorption. The structure of copper complexes in-volving mono- and disubstituted units was studied using ESR spectroscopy. Bis- and monoligand complexes wereformed for both fragments. It should also be noted that bisligand complexes were not formed with disubstituted units,in contrast with low-molecular-weight (model) compounds.REFERENCES1. G. Cárdenas, P. Orlando, and T. Edelio, Int. J. Biol. Macromol., 28, 167–174 (2001).2. E. Guibal, N. Von Offenberg Sweeney, M. C. Zikan, et al., Int. J. Biol. Macromol., 28, 401–408 (2001).3. Y. Gao, K. Oshita, K.-H. Lee, et al., Analyst, 127, 1713–1719 (2002).4. Y. Baba, K. Masaaki, and Y. Kawano, React. Funct. Polym., 36, 167–172 (1998).5. A. V. Pestov, N. A. Zhuravlev, and Yu. G. Yatluk, Zh. Prikl. Khim., 80, 1184–1189 (2007).6. Yu. A. Skorik, A. V. Pestov, and Yu. G. Yatluk, Vestn. UGTU-UPI, 37, 53–55 (2004).7. S. Yu. Sokovnin, M. E. Balezin, I. S. Puzyrev, et al., Izv. Ross. Akad. Nauk, Ser. Khim., in press (2008).8. Yu. A. Skorik, C. A. R. Gomes, N. V. Podberezskaya, et al., Biomacromolecules, 6, 189–195 (2005).9. E. Chiessi, M. Branca, A. Palleschi, and B. Pispisa, Inorg. Chem., 34, 2600–2609 (1995).10. Yu. A. Skorik, C. A. R. Gomes, T. M. S. D. Vasconcelos, and Yu. G. Yatluk, Carbohydr. Res., 338, 271–276(2003).11. L. S. Molochnikov, B. K. Radionov, and E. G. Kovaleva, Zh. Fiz. Khim., 69, 855–862 (1995).12. B. K. Radionov and L. S. Molochnikov, Zh. Fiz. Khim., 67, 1063–1068 (1993).13. L. S. Molochnikov and B. K. Radionov, Zh. Fiz. Khim., 69, 849–854 (1995).14. L. S. Molochnikov, A. V. Pestov, A. P. Slepukhin, et al., Zh. Obshch. Khim., 78, in press (2008).15. E. Chiessi, G. Paradossi, M. Venanzi, and B. Pispisa, Int. J. Biol. Macromol., 15, 145–151 (1993).16. L. S. Molochnikov and B. K. Radionov, Zh. Fiz. Khim., 66, 2537–2542 (1992).17. L. S. Molochnikov and B. K. Radionov, Zh. Fiz. Khim., 67, 466–471 (1993).652

Solid-state structure of copper complexes of N-(2-carboxyethyl)chitosan

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Solid-state structure of copper complexes of N-(2-carboxyethyl)chitosan

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