J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3563-3566

Characteristic Behaviour of the Complexation of Copper(l1) with Polymer-bound Vinylimidazole Ligands

3563

Yoshimi Kurimura,* Toshiyuki Abe and Yoshiharu Usui Department of Chemistry, lbaraki University, Mito, lbaraki 3 10, Japan Eishun Tsuchida and Hiroyuki Nishlde Department of Polymer Chemistry, Waseda University, Ohkubo, Shinjuku-ku, Tokyo 169, Japan Ger Challa Laboratory of Polymer Chemistry, University of Groningen , Nijenborgh, AG Groningen, The Netherlands

Some polymer ligands of N-viny1imidazoleco-N-vinylpyrrolidone (Plm) having different mole fractions of N- vinylpyrrolidone have been prepared. Successive formation constants for the complexation of Cu" with these Plms in aqueous solutions have been determined by means of spectrum deconvolution at pH 3.5. The present results demonstrate that the successive formation constants are given by K , = [CuL]/[Cu][L], K, = [CuL,]/[CuL], K, = [CuL,]/[CuL,] and K, = [CuL,]/[CuL,] (charge omitted, L = imidazole residue on Plm). The results clearly indicate that the concentration ratio, [CuL] : [CuL,] : [CuL,] : [CuL,], is largely constant for a given Plm when the concentration of imidazole residues is varied. The values of K, , K, and K, decrease with increasing mole fraction of vinylpyrrolidone residues (VPRo) in the Plms. The successive formation constants increase in the order K, < K, < K, for poly(N-vinylimidazole), whereas the reverse (i.e. K, > K, > K4) is found for Plm with a VPRo mole fraction of 0.8. Above this VPRo mole fraction, CuL, species are rarely formed. These results also suggest that the distributions of CuL, species in Cull-Plm solutions can be controlled by adjusting the content of VPRo in the Plm.

A large number of investigations on the complexation of copper(I1) with polymer ligands have been reported. In these studies, the polymer ligands used were polypeptides, '-' poly-

vinylimidazole)' and others.16-19 However, very little quan- titative treatment of the equilibrium of the Cu"-polymer ligand system' has been undertaken.

It is considered that, in a metal ion-multidentate polymer ligand system, each polymer chain forms a micro- heterogeneous region, a so-called 'domain', such that the local concentration of imidazole residues in it is essentially constant when the total concentration of polymer ligand is varied under given experimental conditions. This means that the concentration of imidazole residues in the domain is not changed but the number of domains in the solution increases with increasing total concentration of the polymer ligand.

On this basis, a re-examination of the method of determi- nation of the successive formation constants for complex- ation of copper(1r) with a polymer ligand, poly(N- vinylimidazole) (PVIm), has been carried out using direct observation of the absorption spectra of the dissolved species by means of spectral deconvolution.20 The results strongly indicated that the successive formation constants for the complexation of Cu" with the multidentate polymer ligands are distinct from the usual definitions. For multidentate polymer ligands, the proposed successive formation constants are :

carboxylic poly(4-vinylpyridine),' 3 9 l4 P0lY"-

(3)

where [L] is the stoichiometric concentration of the non- protonated ligand residue in the polymer.

These equations show that, for a Cu"-multidentate polymer ligand system, the successive formation constant of the first-step ligation can be represented in the same form as that of the low-molecular-weight analogue, whereas defini- tions of the second-, third- and fourth-step ligations are clearly distinct from those of the usual formation constants.

It has also been established that, for complexation of PIm with Co(dop)(OH,)'+ (dop = planar Schiff base ligand) (Fig. l), the successive formation constants for the first-step liga- tion (complexation of an imidazole group on an axial site of the Co"' centre) and the second-step ligation (complexation of the second imidazole group on the opposite site of the axial ligand), ( K & and (K2)p, are expressed by eqn. ( 5 ) and (6), respectively.

In this system, the formation constant for the second step is also independent of the ligand concentration and this reac- tion proceeds through an intra-molecular mechanism.2 '

Determinations of the successive formation constants for the complexation of Cu" with PIms having different mole fractions of VPRo have been carried out by means of spec- trum deconvolution in order to verify eqn. (1)-(4) and to

(4) Fig. 1 Chemical structure of C0(dop)(0H,),~+

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3564 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90

examine the effect of the introduction of bulky groups into the polymer chains on the successive formation constants. Such fundamental quantitative information concerning the dissolved species in the transition-metal-multidentate polymer ligand systems is also quite important for kinetic analysis of the reactions of the polymer-metal complexes.

Experimental The preparation of PIms having different mole fractions of N-vinylpyrrolidone is reported elsewhere.,' Aqueous solu- tions of Cu" were prepared by dissolving G.R. reagent-grade Cu(NO,), in redistilled water. Absorption spectra were recorded on a Shimadzu Model 265 spectrophotometer. Spectrum deconvolutions were carried out using a personal computer employing a Gauss-Newton program.,, Determi- nations of concentrations of the dissolved Cu" species were carried out in a similar manner to that described elsewhere.,' Acid dissociation constants of the polymer ligands were determined by means of potentiometric titration. Hydrogen- ion concentrations were determined with a Horiba M-13 pHmeter. All the formation constants were determined at 0.10 mol dm-, NaNO, , pH 3.5 and 25 "C.

Results and Discussion The chemical structure of PIm is shown in Fig. 2 and the compositions of the PIms used in the present study are sum- marized in Table 1. The concentration of PIm is represented by that of the imidazole residues.

Determination of the Acid Dissociation Constants of PIm

Acid dissociation constants of PIms defined by [Im][H']/ [ImH+] (Im = unprotonted imidazole group, ImH' = protonated imidazole group), K,, and were deter- mined using the modified Henderson-Hasselbach equation :23

pH = pK, + n log 5 1 - a (7)

where M and n are the degree of proton dissociation and a measure of the effect of neighbouring groups on proton disso- ciation, respectively. The value of n normally ranges from 1 (no effect) to ca. 2 (large effect).

The results of determinations of pK, and n indicate that their values are markedly influenced by the introduction of pyrrolidone groups into the polymer backbone. The values of pK, and n are 5.07 and 1.94 for PVIm, 5.11 and 1.90 for PIm-24, 5.37 and 1.64 for PIm-53, and 5.98 and 1.15 for PIm-80, respectively. The variation of pK, and n with mole

Fig. 2 Chemical structure of N-vinylimidazole-co-N-vinylpyrrol- idone (PIm)

Table 1 Chemical compositions of PIms

mole fraction

ligand VIm(x) VPWY)

PIm-24 PIm-53 PIm-80

0.76 0.47 0.20

0.24 0.53 0.80

\ /

A

5.0 1 I I 1 I I 1.0 0.2 0.4 0.6 0.8 1.0 0

vinylpyrrolidone mole fraction

Fig. 3 Dependences of pK, and n on the mole fraction of N - vinylpyrrolidone residues in PIms at [NaNO,] = 0.10 mol dmP3 and 25 "C

fraction of the pyrrolidone residues is shown in Fig. 3. It can be seen that the introduction of pyrrolidone groups into the polymer backbone induces a depression of n, probably due to a decrease in the effect of the neighbouring positively charged groups. This causes a suppression of proton dissociation and, thus, an increase in pK,. Below a VPRo mole fraction of 0.2, only a small variation in the values of both n and pK, is seen. However, above a mole fraction of 0.3, the values of n and pK, change noticeably with increasing VPRo mole fraction.

Determination of the Successive Formation Constants

In the previous investigation, it was shown that the absorp- tion spectra of Cur'-imidazole and Cu"-poly(N-vinyl- imidazole) solutions can be resolved into their component spectral bands and, furthermore, all the resolved bands can be fitted reasonably well to a single Gaussian.

To avoid complications arising from formation of Cu" hydroxide species, spectrum deconvolutions were carried out at pH 3.5. For all cases, the absorption spectra were recorded at a constant concentration of Cu" (1.0 x mol dmP3) and five different total concentrations of the imidazole resi- dues, [Im],. Spectral deconvolutions were carried out by the use of known values of the halfwidth and maximum wavenumber (vma,.) for the corresponding CuL, species. An example of the deconvolution of the absorption spectrum is shown in Fig. 4.

Fig. 5 shows the absorption spectra in Cu"-PIm-24 (Fig. 5A), and their resolved spectral bands due to Cu, CuL, CuL, , CuL, and CuL, (Fig. 5B-D) at different [Im],. At a given pH, the following equation is applicable for the resolved bands :

C C U l T = A O / E O + A, /&, + A*/&, + A,/&, + A,/&, (8) where A , is the absorbance of the resolved band correspond- ing to CuL,.

Using the observed values of A,-A, and known values of EO-E~, the concentrations of all of the CuL, at given ligand concentrations were calculated. The details of the calculation were reported in a previous paper.,' The distribution of CuL, species plotted us. logarithmic total concentration of PIm, log[Im],, is shown in Fig. 6.

Note that, for all cases, the concentration ratio, [CuL] : [CuL,] : [CuL,] : [CuL,], is essentially constant when the concentration of the ligand is varied. As mentioned above, a similar result was obtained in the Cu"-PVIm system. Again, such a characteristic feature of the multi-

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t

22 20 18 16 14 12 wavenumber/103 cm-'

Fig. 4 Example of the spectrum deconvolution for a solution con- taining 1.0 x mol dm-, Cu" and 3.0 x lo-, mol dm-3 PIm-24. Original absorption spectrum (a), resolved band for Cu (b), CuL (c), CuL, (d), CuL, (e) and CuL, (f).

dentate polymer ligand can be attributed to the fact that a polymer ligand forms a domain and the net concentration of imidazole groups in it is largely constant under the given conditions.

The concentration of the unprotonated imidazole residues, [L], can be obtained using [L] = a[L],, where [L], is the total concentration of the uncoordinated imidazole residues and the values of a can be calculated from the modified Henderson-Hasselbach equation. The value of [L], is given by eqn. (9).

i A

I ( f )

22 20 18 16 14 12

wavenumber/103 cm-'

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 log [ L I T

Fig. 6 Distribution of CuL, species against logarithmic total con- centration of PIm-24 calculated from the data shown in Fig. 5. (a) Cu, (b) CuL, (c) CuL, , (d) CuL, , (e) CuL,.

[L], = [LIT - [CUL] - 2[CUL,] - 3[CuL,] - 4[CUL,]

(9)

From the calculated values of the concentrations of Cu, CuL, CuL,, CuL,, CuL, and [L], distributions of the CuL, species against logarithmic concentration of the uncoordi- nated non-protonated imidazole residues, log[L], were obtained. The results are summarized in Fig. 7. The suc- cessive formation constants obtained shown in Table 2 reveal that those defined by eqn. (1)-(4) are applicable to the Cu"-

C

2 :I 0

1

( a : .*". . . . . . .

( b ) . -.. . .. \, . . . . . . . . . :

22 20 18 16 14 12

wavenumber/l O3 cm-'

Fig. 5 Deconvolutions of the absorption spectra of solutions containing 1.0 x lop3 mol dm-3 Cu" and PIm-24. [PIm]/10-3 mol drnw3: (a) 1.0, (b) 2.0, (c) 3.0, (d) 4.0, (e) 6.0, (f) 30.0. A, Original spectra. Deconvolutions: B, CuL, (-), CuL, (---); C, CuL,; D, CuL (-), Cu (---).

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3566

A \

J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90

B

Table 2 ation of Cu" with PIms"

Estimated successive formation constants for the complex-

~~

ligand K,/mol dm-, K 2 K , K4

PVImb 330 1.2 f 0.05 1.7 f 0.1 3.4 f 0.2 PIm-24' 650 0.94 0.04 1.2 0.1 2.4 & 0.1 PIm-53' 560 0.88 f 0.06 1.2 0.1 0.79 0.05 PIm-8V 620 0.63 & 0.07 0.54 f 0.02 0.38 & 0.05

" The errors in K for all PIms are relatively large (ca. 20%). K , , K , and K, are dimensionless. From ref. 20. The number represents the amount of VPRo (mol%) in the polymer.

PIm system in the same way as those for Cull-PVIm. Unfor- tunately, the errors in K , for all of the PIms are relatively large because of the relatively small absorption intensities of the resolved bands of Cu2+ and its molar absorptivity.

For all the PIm systems, it was confirmed that the calcu- lated spectra obtained from the successive formation con- stants, K , , and molar absorptivities, E , , were in satisfactory agreement with those of the corresponding observed absorp- tion spectra.

From a comparison of the successive formation constants for PVIm and PIms (Table 2), it is apparent that the values of K , , K , and K , decrease with increasing mole fraction of the pyrrolidone residues on the polymer backbone. This may be ascribed to (1) an increase in the steric hindrance of the pyr- rolidone groups for chelate formation, (2) an increase in the average distance between adjacent ligand groups, and/or a decrease in the chain flexibility in the second- and fourth-step complexations, with an increase in the mole fraction of pyr- rolidone residues.

Fig. 7 and 8 show the concentration distribution of the CuL, species in the Cull-polymer ligand (Fig. 7A-D) and Cu"-Im in the PIm and imidazole (Im) systems, respectively. As shown in these figures, the profile of the CuL, distribu- tions in the polymer system differs somewhat from that in the corresponding low-molecular-weight system. These figures also show that the fourth complexation for polymer ligands occurs at lower [L] than that for the low-molecular-weight system, probably owing to the high local concentration of the ligand groups in the polymer domains.

The present results demonstrate that the concentration ratio of CuL, species, [CuL] : [CuL,] : [CuL,] : [CuL,], is greatly influenced by varing the content of the bulky group of vinylpyrrolidone. At given concentrations of Cu" and L, the

1 .o 0.8

In 0.6

0.4 a .-

In

=> 0.2

5 O 0

S

3 .o 4- 0.8

0.6

0.4

0.2

0

4-

\\ c l D

-6 -4 -2 -6 -4 -2 0 log [LI log[Ll

Fig. 7 Distribution of CuL, species against log[L] ([L] = concentration of unprotonated free ligand residues) in solu- tions containing Cu'', PVIm and PIm at pH 3.50. A, PVIm (data from ref. 20); B, PIm-24; C, PIm-53; D, PIm-80. (a) Cu, (b) CuL, (c) CuL, , (d) CuL, , (e) CuL,.

1 .o

3 0.8 .- 0

Q

= 0.6 0

$ 0.4

0 2 0.2

y.

.- c. 3

4 4

U

0

Fig. 8 Distribution of CuL, species against logarithmic concentra- tion of unprotonated free ligands in solution containing Cu" and imidazole. The successive formation constants used were log K , = 4.33, log K, = 3.54, log K , = 2.82, log K, = 2.02 (data from ref. 24). (a) Cu, (b) CuL, ( c ) CuL,, (d) CuL,, (e) CuL,.

proportions of CuL, and CuL, decrease with increasing mole fraction of the VPRo moieties. For example, the concentra- tions of CuL, increase in the order [CuL] < [CuL,] < [CuL,] < [CuL,] in Cu"-PVIm, whereas this order is com- pletely reversed in Cu"-PIm-80; i.e. in Cu"-PVIm, the most predominant complex species is CuL, whereas, in Cu'I-PIm- 80, it is CuL.

This work was partially financially supported by Nippon Sanso Co. Ltd.

References 1

2

3 4

5

6 7

8 9

10 1 1

12

13 14

15

16 17

18

19

20

21

22

23 24

M. Hatano, T. Nozawa, S. Ikeda and T. Yamamoto, Makromol. Chem., 1971,141, 1. A. Levitzki, I. Pechat and A. Berger, J. Am. Chem. SOC., 1972,84, 6844. M. Branca and M. E. Marini, Biopolymers, 1976, 15,2219. M. Palumbo, A. Cosani, M. Terbojevich and E. Peggin, J. Am. Chem. SOC., 1977,99,939. M. Koide, E. Tsuchida and Y. Kurimura, Makromol. Chem., 1981,182,359. M. Mandel and J. C. Leyte, J. Polym. Sci. A , 1978,2,2883. T. Pecht, A. Levitzki and J. Anber, J. Am. Chem. SOC., 1967,89, 1578. L. Toshi and A. Garnier, Biophys. Res. Commun., 1974,58,427. A. Kekchiri, M. Brighli, J. Methenitis, J. Morecellet and M. Morcellet, J. Inorg. Biochem., 1991,44, 229. A. Garnier and L. Toshi, Biopolymer, 1975,14,2247. E. M. Loebl, L. B. Luttinger and H. P. Gregor, J. Phys. Chem., 1955,59, 559. A. M. Kotliar and H. Morawetz, J. Am. Chem. SOC., 1955, 77, 3692. H. Nishikawa and E. Tsuchida, J. Phys. Chem., 1975,79,2072. Yu. E. Kirsh, V. Ya. Kovner, A. I. Kokorin, K. 1. Zamaraev, V. Y. Chernyak and V. A. Kabanow, Eur. Polym. J., 1974,10,671. D. H. Gold and H. P. Gregor, J. Phys. Chem., 1960, 64, 1461; 1464. M. Teyssie and P. J. Teyssie, J. Polym. Sci., 1961,28,253. Y. Kurimura, K. Takato, M. Takeda and N. Ohtsuka, J. Phys. Chem., 1985,89,1023. Y. Kurimura and K. Takato, J. Chem. SOC., Faraday Trans. 1, 1988,84,841. D. V. Subotic, J. Ferguson and B. C. Warren, Eur. Polym. J., 1991,27, 65. K. Seki, M. Isobe, K. Yanagita, T. Abe, Y. Kurimura and T. Kimijima, J. Phys. Chem., 1994,98, 1288. Y. Kurimura and K. Hasegawa, J. Chem. SOC., Faraday Trans. 1 , 1990,86,3537. J. Kowalik and M. R. Osborne, Method for Unconstrained Opti- mization Program, Elsevier, New York, 1986. A. Katchalsky and P. Spitnnik, Polym. Sci., 194.7, 2,432. y. Nozaki, F. R. N. Gurd, R. F. Chen and J, T. Edsall, J. Am. Chem. SOC., 1957,79,2123.

Paper 4/03873H; Received 27th June, 1994

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