DANIELH. GOLDAND HARRY P. GREGOR
1464
Vol. 61
METAL-POLYELECTROLYTE COMPLEXES. VIII. THE POLY-NVINYLIMIDAZOLE-COPPER(I1) COXPLEX BY DANIELH. GOLD^
AND
HARRY P. GREGOR
Department of Chemistry of the Polytechnic Institute of Brooklyn, Brooklyn, h'ew Yorlc Received A p r i l 6,1960
Spectrophotometric studies of the poly-N-vinylimidazoleCu(I1) system showed that the complexes formed had an absorption maximum in the 550-650 mp region with the peak shifting to shorter wave lengths as the ratio of metal ion to polymer was increased. Continuous variation analysis showed a preferred coordination number of four. Potentiometric titrations of the polybase in the presence of the metal were performed. The method of Bjerrum, modified for polyelectrolyte behavior, was used to calculate over-all formation constants for the reaction Cu++ 41 = Cull++, in the presence of 0, 0.1 and 1.0 M salt at 0.01 and 0.1 M polymer concentrations. These values were dependent upon salt, and polymer concentrations and were comparable to those for the reaction of imidazole and copper under like conditions. Constants for the displacement reaction C u + + 4IH+ = CUI(+.. 4H+ were found to he nearly insensitive to the presence of salt and were considerably stronger than those with imidazole.
+
+
+
The imidazole group is found in many proteins and its complexes with metallic ions have been studied by potentiometric t i t r a t i ~ n , ~polarog-~ raphy5 and ~pectrophotometry.~In general, it was found that the binding constant for the reaction of a metal ion with the imidazole group was of the same order of magnitude whether the imidazole was free or part of a protein s t r ~ c t u r e . ~ *Pre~J vious papers in this series have reported on the binding of metallic ions by polyacrylic and polymethacrylic acids and have shown that significantly stronger metal ion-carboxylate binding constants are obtained when the carboxylic acid is part of a polymer than when it is free.' It was of interest to investigate the binding of metallic ions by poly-X-vinylimidazole in order to provide a firmer basis for evaluating the binding power of the imidazole group in more complex structures. This contribution details an investigation of the binding of copper(I1) by poly-N-vinylimidazole (PVI) as studied by potentiometric titration and spectrophotometry. Another communication8 will treat the binding of silver(1) by this polybase. Experimental Methods and Materials.-The poly-N-vinylimidazole (PVI) used was the same as that described earlier.gJ0 This polymer was ,essentially of high purity, being polymerized from the I-vinylimidazole monomer to yield
N HC' C 'H N-kH il (1) Taken in part from the Dissertation of Daniel H. Gold, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry at the Polytechnic Institute of Brooklyn, June, 1957. (2) r. R. X. Gurd, J. T. Edsall, G. Felsenfeld and D. S. Goodman, Federatzon Proc., 11, 224 (1952). (3) C. Tanford and hl. L. Wagner, . I . Am. Chem. Soc., 7 6 , 434 (1953). (1) J. T. Edssll, G. Folsenfeld, D. S. Goodman and F. R. N. Gurd, ibzd., 76, 3054 (1954). (5) N. C . Li, J. A I . White and E. Doody, {bid., 76,6219 (1954). (6) F. R. X Giird and D. S. Goodman, abid.. 74, 670 (19.52). (7) €1. P. Gregor, wlth L. B. Luttinger and E. bl. Loebl, THIE JOCRNAL, 59, 34,366,559, 990 (1955). (8) D. H. Gold and H. P. Gregor, ibid., 64, 1461 (1960). (9) H.P. Gregor and D. H. Gold, ibad., 61, 1347 (1557). (IO) D.H.Gold and H. P. Gregor, Z. physik. Cliem., Neue Folpe, 16, 53 (1958).
It was water soluble a t all dcgrees of neutralization at concentration up to 0.4 M (base moles) in water; a t concentration as high as 0.1 M i t was not salted out by 1 M solutions
of alkali metal chlorides and nitrates. Imidazole ( I ) was obtained from Eastman Kodak Co. (m.p. 90°, lit. 90') and was used without further purification after drying a t 60" and 10 mm. for several days. Stock solutions were prepared by weight and the titer checked potentiometrically with standard acid. Copper(I1) nitrate (hereafter Cu) solution was prepared from an analytical grade reagent, with about 10% nitric acid added t o prevent hydrolysis. The amount of acid added was determined potentiometrically and taken into considem tion in calculting degrees of neutralization. The titer of the Cu solution was determined both iodornetricallyl1 and electrolytically; both procedures agreed t o within 0.3%. All complexometric titrations were carried out by starting with a PVI solution to which an amount of nitric acid in excess of that required for neutralization had been added; where used, the Cu-nitric acid solution mas added and then the titration performed with standard COz-free sodium hydroxide. Because gels formed in the presence of Cu, stepwise titrations similar to those described by Gregor, et al.,' were employed. Solutions were prepared in polyethylene containers and equilibrated for 24 hours a t 25O between measurements, within which period of time a constant pH was attained. This stepwise procedure gave the same results as continuous titrations in those experiments where no gel phase was observed. Hydrogen ion activities a-ere determined with a Beckman Model G pH meter in the manner described previously.10 When a neutral electrolyte was present, a sleeve-type saturated calomel electrode was employed; %*henno neutral salt was present a fiber tip electrode was used, which allowed a total salt contamination amounting to less than 1 0 - 6 M. The point of incipient precipitation of copper( 11) hydroxide was determined by titrating solutions of Cu in the different salt concentrations used and in the absence of PVI, employing the stepwise procedure and observing the first appparance of turbidity. This point was in the pH range 4.5 to 5.0; no data obtained above this p H level were used in subsequent calculations. Absorption spectra measurements were made with a Beckman Model DU spectrophotometer against water blanks; 10 cm. quartz fare cells were used. A preliminary study using 1 cm. cells was facilitated by the use of an automatic recorder. All quantitative measurements were taken manually. Spectrophotometric Analysis.-Preliminary absorption spectra measurements with PVI showed a peak at 310 mp. As Cu N ~ Sadded the absorbancy (log I 0 / E at this nave lengfh increased v-ith little shift in the peak while a new maxlmum appeared a t 550-650 mp. .4s Cu w i s further increased, the absorbancv at the longer wave lengths increased with the peak shifting to shorttir wave lengths. The absorbancy of Cii, PT'I and mixtures of the two are shown in Fig. 1 over the region \\-here the PVI-CII complex had an absorptivity considerably greater than either of the reactants. (11) I. M. Kolthoff and E. €3. Sandell. "Textbook of Quantitative Inorganic Analysis," The .\Iacmill~nCo., Kern York, N.Y.. 1947.
POLY-N-VINYLIMIDAZOLE-COPPER(II) COMPLEX
Oct., 1960
1465
6
0.08
5 550 590 630 670 710 750 790 830 870 910 Wave length, mp. Fig. 1.-Absorbancy as a function of wave length a t pH AI 4.20 for 9.86 X lo-' M copper(I1) nitrate, 1.8 X M in polypolyvinylimidazole and a solution 1.6 X vinylinlidmole and 3.94 X lop4M in copper(I1) nitrate.
~
R
4
I 3
i 0
9
0.2 0.4 0.6 0.8 1.0 Mole fraction Cu(I1). Fig. 2.-Continuous variations analysis of the poyvinvlimidazole-Cu(I1) complex measured a t 620 mp and pH 4.20 using a 0.002 M total solution concentration.
0
The method of Job12 can be used to detect the predominant complex species present. Solutions were prepared of varying ligand to metal ion ratios, keeping the total concentration constant a t 0.002 M . Absorbancy measurements were made a t wave lengths where the complex absorbed strongly compared to either of the reactants. The solution absorbancy was corrected for reactant absorbancy assuming no reaction had occurred and assuming Beer's law. Figure 2 shows a sharp maximum when the Cu(I1) mole fraction is 0.2 indicating that the predominant species formed under these experimental conditions was a complex of four imidazole groups with one cupric ion. Assuming complete reaction, the molar absorptivity (l/bc) log IO/I of the complex was 63 1. mole-' cm.-l a t 620 mp. Other measurements performed a t higher total concentrations also led to a sharp maximum at a mole fraction of 0.2. Edsall, el al. , 4 studied the imidazole-Cu system spectrophotometrically in the visible region and found that as the ratio of imidazole to Cu was increased the molar absorptivity increased and the nhsorption maximum shifted to lower wave lengths. Also, while the ultraviolet absorption of imidazole was virtually negligible above 240 mp, Cu-I complexes showed marked absorption between 240 and 290 mp. Here, as the ratio of imidazole to Cu was increased the absorptivity (12) P. Jab Ann. chim., [IO]9, 113 (1928); 1111 6, 97 (1936).
1 0.2
I 0.4
_I I
0.6
0.8
1.0
LY.
Fig. 3.-Titration of 0.01 J4 polyvinylimidaaole in 1 sodium nitrate. Copper(I1) nitrate concentrations in millimoles liter-' are: 0 (0); 0.0493 ( A ) ; 0.198 ( 0 ) ; 0.493 ( 0 ) ; 0.986 (v); 2.96 (B); 9.86 (A); 29.6 (v). increased and the absorption maximum shifted toward longer wave lengths. Fourfold coordination of Cu with imidazole was found; the molar absorptivity of the c u b + + complex was estimated as 53 zk 2 1. mole-' em.-' a t 590 mp. I n general, these results are similar to those obtained for the Cu-PVI complexes. Potentiometric Titrations.-Figure 3 shows titration data for 0.01 AI PVI in the presenre of 1 Jf sodium nitrate and several concentrations of Cu(I1) varying from 5 X 10-6 to 3 X 10-2 M . In each case the polybase was overneutralized by a small excess of nitric acid and then titrated with standard base. Comparable titrations were performed in thepresence of 0.0 and 0.1 M neutral salt and a t a PVI concentration of 0.1 ill. As the coordinating metal ion concentration was increased it was observed that the titration curves were displaced downward, typical with complex-forming systems. The decrease in pH a t a givcn degree of ncutrali7,a t'ion, or the larger amount of base required to reach the same pH, are measures of the hydrogen ions displaced by the metal; approximate formation constants sometimes are calculated directly from these displacements. When neutral salt is added t o a polybase, the so-called shielding effect makes i t stronger as a base and weaker as an acid, as discussed previously.1D Figure 4 shows titration curves of 0.01 M PVI solutions carried out in the presence of no salt, in 1 M sodium nitrate and in the absence and presence of 0.000986 M copper. Data for additional titrations, those of 0.01 &I imidazole in 1 M sodium nitrate in the absence and presence of 0.000986 M copper are also given in Fig. 4.
Calculation of Equilibrium Constants Formation constants were calculated in the manner introduced by Bjerrum,13 as modified for the
DANIELH. GOLDAXD HARRY P. GREGOR
1466 !f
1
I
I
I
,
I
r
I
\'d.6-1
binding of metals by polyelectrolytes by Gregor, et aL17the method will only be summarized herein. In Bjerrum's method, the average number of ligands bound per metal ion, a, is determined as a function of the free ligand Concentration to give the formation function of the system. For the imidazolecopper(I1) spstem rt is expressed as [It1 - 111 - [IS+! t/ = [Cut++I
6
where [It]is the total base molar concentration of imidazole, [I]and [IH+] the concentration of imidazole and imidazolium groups, respectively, and 5 [Cut++] the total copper(I1) concentration. From material balance equations and the electroneutrality requirement it can easily be shown 1 that the imidazolium ion concentration is given by measurable quantities.8 For a polybase, the relation between [PVIH+], [PVI] and [H+] in the 3 absence of a coordinating metal is given by the modified Henderson-Hasselbach expression.* Since spectrophotometric analysis had shown 1 I I I J that the predominant complex species was Cu0 0.2 0.4 0.6 0.8 1.0 (PVI)4++,it was assumed that other possible species ff. were present in negligible amounts only. The final Fig. 1.-Titration of 0.01 Jf imidazole in the presence of expression here is identical with equation 9 of the 1 JI sodium nitrate ( 0 )and with 0.986 X 10-8 copper(I1) nitrate (a). Comparable titrations of 0.01 M polyvinyl- previous paper,8 because in the complex the ratio imidazole in no salt ( 0 )and with the same concentration of of charge to number of ligands was the same (1:2). copper (a) and also in the presence of 1 M sodium nitrate The hydroxyl ion concentration was generally ( A )and with copper id. negligible in these experiments compared to the other terms in the final expression and was neglected. Although it has been established experimentally that activity coefficients of counterions (at low ionic strengths) are depressed considerably below corresponding values in simple electrolytic solutions, for lack of a better approximation it was assumed that the hydrogen ion activjtp coefficient was the same as the mean activity Coefficient of the supporting 1-1 electrolyte; ~ * E I was taken as unity in the absence of salt, 0.76 in 0.1 ill sodium nitrate and 0.55 in 1 M sodium nitrate.14 The final expression may then be solved for the only unknown, [PVI] through an iterative proredure and hence fi obtained. ResuIts and Discussion The formation curves for 0.01 M PVI-Cu in the absence of salt, 0.1 and 1 M sodium nitrate are shown in Fig. 5 . This figure also shows the formation curve of 0.1 M PVI-Cu in l M sodium nitrate. For comparison, the formation curve of imidazoleCu at 23" at ionic strength 0.16 determined by Edsall, et u Z . , ~ is shown. The PVI-Cu formation curves mould appear to extrapolate to a maximum coordination number N = 4. If two- or sixfold PVI-Cu coordination were assumed in the calculation of It and p(PVI), the formation curves still approached coordination number four; these 0 1 3 3 1 5 polymeric formation curve plots tend to indicate p [PVII. the maximum value of f i even if the assumed value Fig. 5.-Formation curves for the Cu-PVI complexes in 0 01 Jf polyvinylimidazole in the presence of no salt (o), of the maximum coordination number is incorrect. 0.1 JI sodium nitrate ( A ) and 1 M sodium nitrate (a). The job plot indication that S = 4 is more definiCurve for the Cu-imidazole complexes4 are shown (m). tive. Curve for 0.1 M polyvinylimidazole in 1 ill sodium nitrate is All the PVI-Cu formation rurws n-ere conshown (V). structed from titration data with, in most cases, __ five different copper ion concentrations. The gen+
(13) J. Bierrum, "Metal Ammine Formation i n Aqiieolis Solution.."
P. Haas and Son, Copenhagen, 1941.
(14)
R.A . Robinson, J . A m .
Che7n SOL. 6 1 , 1165 (1935).
cral validity of the approach used is confirmed by the presence of points corresponding to copper ion to 3 X concentrations ranging from 5 X M on the same line. The steep slopes evinced by the PVI-Cu formation curves indicate spreading factors of less than unity13 between the stepwise formation constants. This effect appears to be quite general with polymeric complexing systems; once the coordinating metal ion is attached to one group on the polyelectrolyte coil the high, local concentration of available ligands triggers the completion of binding and make the apparent successive formation constants larger than the first. Formation and displacement constants for the association of copper with PVI under several experimental conditions are summarized in Table I. For comparison, constants for copper with imidazole, 1-methylimidazole and serum albumin imidazole are also given. The formation constants are expressed in two ways: K4 refers to the chelation reaction, Cutr 41 = C U ( I ) ~ + +B4; refers to the displacement reaction, Cu++ 4IH+ = Cu(I)4++ 4H+. The K constant characterizes the formation process itself while the B constant is more descriptive of the actual extent of complex formation, particularly at lower pH levels. With the Cu-PVI formation process, Table I and Fig. 5 shorn thnt as the ionic strength increases, the value of K4 increases also. This effect appears to be analogous to our usual experience with chelate acids where K increases as the acidity of the acid decreases, for the strength of the acid PVIH+ does decrease with the ionic strength.'O However, since the chemical nature of the acid is not being altered, it is seen that the effecr, here is primarily one of electrostatic shielding. This formation process is accompanied by a successive charging of the polymer chain. h s each successive mole of Cu(I1) binds to four imidazole groups on the chain, the charge on the latter is increased by + 2 . The standard free energy change for any of these processes may be written
+
+
+
AFo = AH0
-
TASO
- AFe
where AFerefers to the electrical work which is positive (by convention) for processes which involve electrostatic attraction and negative for those which involve repulsion. It has been shown previously7 that AHo is nearly the same for the bindina of metals by polymeric and monomeric ligands. k t h polyacrylic acid-Cu(I1) and glutaric acid-Cu(I1) binding, the entropy change would similarly be experted to i l e nearly the same because the metal chelate rings differ only by the two side chains (n-ith polyaerylic acid), and the addition of side chains to ligands does not make for profound differences in formation constants. Accordingly, for the carboxyl-Cu(I1) system the diflereiwe in the magnitude of the electrical free energy terms for polymeric and nionomwic binding is A F e ,>l\I.,pT
-A F e monomer = RT In lo6in the absence of salt; in concentrated ( 2 M ) salt solutions the difference would be RT In lo4.' TABLE I FORMATIOX
CONSTANTS
O F COPPER( 11) K I T H IXID.4ZOLE-
CONTAINING Ligand
LIGANDS AT 2.3" Ionic Htrengtb
log
K1
log
B4
0 10.64 - 6 40 'PVI, 0.01 M - 7.12 ~ ~ 7 0.01 1 , -I!I 1) 10 12 76 - 9 08 PVI, 0.01 M 1 0 14 72 PVI, 0.1 Jf IO 11 00 -10.96 Imidazole" 0.16 12 7" -15.72 Imidazoleb 15 12 6 -15.84 l-Methylimidazoleb .03 12 86 -15 94 Serum albumin imidazolec . 1 5 -14.8 -( - 9 . 60d) Extrapolated from data of Edsall, et d . , 4 * From Li, et aZ.5 C. Tanford, J. A m . Chem. SOC.,74, 211 (1952). Value of p R , = 6.10 for serum alhumin imidazole from C. Tanford, ibid., 72,441 (1950).
I n the case of the PVI-Cu complexes, one would similarly expect that the AHo terms are equal. Fourfold coordination along the same polymer chain is indicated here because the spreading factors are so small. This would g k e rise to considerable steric hindrance, but would not make for a large difference in the corresponding ASo values when comparing imidazole and PT'I binding. Accordingly, the difference in the AF? terms here is equal to -RT In lo2 a t lorn ionic strengths and +RT In lo? at high levels, referring to the fourfold co6rdinatioii process. true basis for comparing the intrinsic biiiding process is to be found in the use of the first coordination coiistant k,. With polymers the high, localized (.oncentration of ligands in the coil makes kz, kj antl liqlarger than kl, so that K 4 (equal to klk2k3k4) becomes considerably larger than K 4 for the process with monomeric ligands. With imidazole, log h.1 = 4.3, while for PVI the value is log kl = 3.9 in 1 M salt and log kl = 2.8 in the absence of salt. Here it is seen that the intrinsic binding of imidazole with copper is the strongest , that the first imidazole group on PVI which binds copper even in the presence of concentrated salt is weaker, and that in the absence of salt, where higher chain potentials must be o\Terrome, polymeric binding is even weaker. In the case of displacement reactions where the c,hain potential is lowered from +4 to +2, B, is nearly independent of ionic strength a t constant polybase concentrations. The displacement constant is much larger with the polymeric reaction, antl it is interesting to note that only serum albumin imidazole is coniparable with the polymer in this respect. Acknowledgment.-This investigatioii was supported in part by the Piiblic Health Service Reqearch Grant RG-2934 from the division of General \\Tedicnl Scienoes, Public Health Service
