Stability constant of ethylenebisbiguanidenickel(II) ion

J. inorg.nucl.Chem.,1969,Vol.31,pp.3233to 3239. PergamonPress. Printedin Great Britain

STABILITY CONSTANT ETHYLENEBISBIGUANIDENICKEL

OF (II) I O N *

G E O R G E H U N G - Y I N L I N t and D A V I D J. M a c D O N A L D * Department of Chemistry, University of Nevada, Reno, Nevada 89507

(First received 3 February 1969; in revised form 12 March 1969) A l ~ t r a e t - T h e stability constant (/3) for formation of the 1 : 1 square-planar diamagnetic complex of nickel(II) with the tetradentate ligand ethylenebisbiguanide was measured at 25 ° in aqueous solution by a spectrophotometric method after prolonged equilibration. The acid dissociation constants of ethylenebisbiguanidinium ion were also measured at 25 ° in aqueous solution by a glass-electrode method. Results (corrected to zero ionic strength) are: log/3 = 19.5 ±0.2 (or/3 = 3 × 1019), pK1 = 12.075 ± 0-05 and pK2 = 13.08 ±0.2.

INTRODUCTION

A VALUE for the stability constant of the square-planar ethylenebisbiguanidenickel(II) ion, (abbreviation Ni(enbbg)2+), has been reported by Das Sarma and Ray [1], who found B = 1.27 x 1015at 32 ° in aqueous solution. They used a method involving glass-electrode pH measurements combined with values [2] for the acid dissociation constants of the conjugate acid form of the ligand, together with values of the concentration of Ni(enbbg) ~+ obtained by difference. The accuracy of their result is questionable for two reasons. First, it is difficult to measure the acid dissociation constants of the conjugate acid form of the ligand, i.e., enbbgH~ ~+, with high precision because it is an unusually weak acid and because its two successive acid dissociation constants have numerical values not greatly different from each other. The direct graphical method of data treatment used by Das Sarma in his measurement of those acid dissociation constants is incapable of surmounting these difficulties. Second, the ethylenebisbiguanidenickel(II) ion is kinetically inert. That is, a system comprising nickel(lI) ions and ethylenebisbiguanide in solution is slow to react when a change of pH is imposed on it. This means that one must take care to allow the system sufficient time to reach equilibrium before any meaningful equilibrium measurements can be made on it. Because it is now possible to obtain more accurate values for the acid dissociation constants of the conjugate acid of the ligand, and because we felt that a direct measurement of the concentration of Ni(enbbg) 2+ based on its optical absorbance *This work was sponsored partly by the Petroleum Research Fund administered by the American Chemical Society (Grant No. 39-G) and partly by the Air Force Office of Scientific Research, A F O S R (SRC)-OAR, U.S.A.F. (Grant No. AF-AFOSR-994-66). t Based on work done by G. H. -Y. Lin in partial satisfaction of requirements for the M.S. degree. ~Present address: U.S. Bureau of Mines, Reno Metallurgy Research Center, 1605 Evans Ave., Reno, Nevada 89505. Correspondence should he addressed to this author. 1. B. Das Sarma and P. Ray, J. Indian Chem. Soc. 33, 841 (1956). 2. B. Das Sarma, J. Indian Chem. Soc. 29, 217 (1952), reported pK~ = I 1.3 and pK~ = i 1.8 at 32 °. 3233

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G.H.-Y. LIN and D. J. MacDONALD

w o u l d b e m o r e a c c u r a t e t h a n the p r o c e d u r e u s e d b y D a s S r a m a a n d R a y , w e h a v e r e d e t e r m i n e d the stability c o n s t a n t , u s i n g a c o m b i n e d s p e c t r o p h o t o m e t r i c a n d glass-electrode method. EXPERIMENTAL

Reagents. All chemicals were of reagent grade or Eastman 'White Label' grade. Ethylenebisbiguanidenickel(lI)chloride, CsHleN10CI2Ni, was prepared by heating together for 2 hr at 145° a mixture of cyanoguanidine (0-6mole), ethylenediamine dihydrochloride (0.3 mole) and NiCI~. 6H20 (0-3mole). As heating was begun the mixture melted to a blue-violet syrupy liquid, frothed for a few minutes as steam was evolved, then gradually became semisolid. The resulting pasty brown mass was removed from the reaction vessel while still hot and before it could solidify. After cooling to room temperature it was crushed and dissolved in boiling water, ca. 6 ml of water per gram of solid, then cooled to 5°. Columnar to needle-like orange-red crystals of [Ni(enbbg)]C12. ?H.20 formed promptly. Yield was about 25 per cent of theoretical. A flocculent white precipitate also formed which could not be eliminated by recrystallization from water. However, it was found that the flocculent white material could be decanted away from the [Ni (enbbg)]C12 by repeated rinsing with ethanol. The crude [Ni(enbbg)]Cl2 was recrystallized and rinsed in this manner twice, to produce material which had the following composition after two days of drying in vacuum at 95* over Anhydrone: Anal. Calcd. for CeH,~N10CI~Ni: C, 20.14; H, 4"508; N, 39.14; CI, 19.82; Ni, 16.40. Found: C, 20.17; H, 5.19; N, 39.40; CI, 19"50; Ni, 16-39. Analyses for C, H, and N were performed by Drs. Weiler and Strauss, Microanalytical Laboratory, 168 Banbury Rd, Oxford, England. Analysis for C1 was done by potentiometric titration with std AgNO3 solution, and Ni was determined volumetrically by use of std EDTA solution with murexide as indicator according to the method described by Harris and Sweet [3]. Ethylenebisbiguanidinium chloride (i.e., the chloride salt of enbbgH~2+) was prepared from Eastman 'White Label' Ethylenebisbiguanide Sulfate (sic) by treating an aqueous solution of that substance with sufficient Dowex 1 anion-exchange resin in the OH- form to bring the solution to pH 7, then passing the solution through a column containing an excess of Dowex 1 anion-exchange resin in the C1- form, after which the effluent solution was evaporated to dryness. The resulting solid chloride salt was recrystallized twice from a boiling solution in 50% ethanol-water, yielding colorless plate-like crystals. Anal. Calcd. for C6H~sN,0CI2: C, 23.92; H, 6.024; C1, 23-54; N, 46.51. Found: C, 24.04; H, 6.17; C1, 23.6; N, 46.80. Instruments. Absorbance measurements were made with a Cary Model 14 spectrophotometer using l-cm standard silica cells at 478 nm, the wavelength of maximum absorbance for Ni(enbbg)2+ in aqueous solution at 25°. Measurements of pH were made with a Radiometer TTTIc pH Meter equipped with a temperature compensator, and using a Type K401 saturated calomel reference electrode and a Type G202 B glass electrode. The pH meter was calibrated against reference solutions having pH values at 25° of 1.68, 4.01,9.18 [4] and 12.45[5]. During measurement of the pK values of ethylenebisbiguanidinium ion, test solutions and electrodes were held to 25.0" --+0.2* by use of water jackets carrying water circulated from a temperatureregulated bath. Experiments to measure the stability constant of ethylenebisbiguanidenickel(lI)ion were conducted at room temperature, 25°--_2°, but with no special thermostatting. Class A volumetric ware was used throughout. RESULTS

p K values o f ethylenebisbiguanidinium ion T i t r a t i o n s o f 20.00-ml p o r t i o n s o f 0-100-F e t h y l e n e b i s b i g u a n i d i n i u m c h l o r i d e with s u c c e s s i v e i n c r e m e n t s o f 2.000-N K O H (P. H. T a m m Co., CO2-free) d e l i v e r e d f r o m a m i c r o m e t e r s y r i n g e b u r e t into a n i t r o g e n - b l a n k e t e d v e s s e l g a v e r e s u l t s as s h o w n i n T a b l e 1. I n the t i t r a t i o n u s i n g a d d e d K C I , t h e 20.00-ml s o l u t i o n w a s 0.100 F in K C I b e f o r e a d d i t i o n of K O H was b e g u n . B e c a u s e the s u c c e s s i v e 3. w. F. Harris and T. R. Sweet, Analyt. Chem. 24, 1062 (1952). 4. R. G. Bates, Electrometric pH Determinations p. 74. Wiley, New York (1954). 5. Saturated calcium hydroxide (Beckman Part No. 14532).

Stability constant of ethylenebisbiguanidenickel(I I) ion

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Table 1. Titration of ethylenebisbiguanidinium chloride Volume of 2.000 N KOH (ml) 0-00 0"10 0"20 0"30 0"40 0"50 0"60 0"70 0"80 0"90 1.00 1"10 1"20 1"30 1"40 1"50 1"60 1"70 1"80 1"90 2"00 2"10 2"20 2"30 2"40 2"50

pH No added salts /~ = 0-3 7"04 11"25 11.60 11"82 11"98 12-09 12" 19 12"28 12"34 12"39 12"46 12"50 12"55 12"59 12"63 12"68 12"70 12"74 12"77 12"79 12"81 12.84 12"88 12"89 12.90 12'91

Added KC1 /z = 0.4 7"36 11"28 11"65 I 1"87 12.00 12" 12 12"21 12-30 12"38 12"43 12"49 12"53 12"59 12"62 12"67 12"70 12"73 12"77 12"79 12"81 12"85 12"88 12"90 12"91 12"93 12"96

pK values are close together and because the basicity of ethylenebisbiguanide is not much less than the basicity of hydroxide, these titration data are not amenable to the usual graphical treatment. For example, the data do not yield any inflection points when volume of titrant is plotted vs. pH. Therefore, the experimental data were analyzed by means of a digital computer program[6] designed for this purpose. The computer program embodies provision for the increase of volume due to addition of titrant to the system; it accounts for the accumulation of unreacted O H - at high pH; and it includes corrections for the change of activity coefficients with ionic strength (/.~) so that the pK values calculated in this way represent estimated or apparent thermodynamic equilibrium constants based on activities rather than on concentrations. Activity coefficients were estimated by means of the semi-empirical Davies equation, which has been said to be in error by not more than 8 per cent even at ionic strength as high as 0.5 [7]. The results are shown in Table 2. The errors indicated there are standard deviations based on 6. A copy of the program (in Fortran II) and an explanation of how it works can be obtained on request. 7. J. N. Butler, Ionic Equilibrium: A Mathematical Approach, p. 437. Addison-Wesley, Massachussetts (1964).

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G . H . - Y . LIN and D. J. M a c D O N A L D Table 2. pK values for ethylenebisbiguanidinium ion at 25*, corrected to zero ionic strength PK1 No added salts, ~, = 0.3 * Added KCI, p, = 0.4* Average

12.075 +- 0.047 12"076---0"043 12.075 - 0.050

PK2 13.001 ___0.092 13"170---0"108 13.080 -+ 0.200

*Measurements were made at this ionic strength, but the pK values have been corrected to zero ionic strength.

the random errors inherent in the input data. The difference between results at = 0.3 and/z = 0.4 is probably not significant in terms of an ionic strength effect.

Stability constant o f ethylenebisbiguanidenickei( l l ) Ion A major difficulty in the measurement of the stability constant of this complex is the compound's kinetic inertness. It is necessary to make measurements on solutions which are at equilibrium, but their approach to equilibrium is quite slow. In an earlier investigation[8] it was found that the reaction Ni (enbbg)~++ 2H + --->Ni 2++ enbbgH22+ follows the rate law - d[complex]/dt = 0.27 M -1 sec -1 [complex][H +] + 128 M -2 sec -1 [complex][H+] 2 at 25°. Since the rate of this reaction is known (at least in the forward direction), it is possible to estimate the time required for the system to approach equilibrium after addition of acid to a solution of Ni (enbbg) 2+. For approach to within 99-9 per cent of equilibrium at pH 5 and 25 °, the time required is about 4 months (assuming that the reverse reaction can be neglected). For mixtures more acidic than pH 5, approach to equilibrium would be considerably faster. In practice, the test solutions, prepared by adding varying amounts of hydrochloric acid to solutions of ethylenebisbiguanidenickel(lI) chloride, were allowed to stand for at least 10 months at room temperature (about 25°) before the resulting equilibrium concentrations were measured. Approach to equilibrium could have been speeded by holding the mixtures at an elevated temperature, then cooling them to 25° prior to making the concentration measurements, but the position of equilibrium thus achieved would probably be different from the position of equilibrium at 25 °. The absence of any definite trend in the calculated values of/3 as a function o f p H , i.e., a trend such as would result from incomplete approach to equilibrium in samples at higher pH, suggests that a sufficiently close approach to equilibrium was achieved in each mixture. Values of/3 were calculated for each set of measured pH and [Ni(enbbg) 2+] values by use of the equations 8. D.J. MacDonald, J. inorg, nucl. Chem. 29, 1271 (1967).

Stability constant of ethylenebisbiguanidcnickel(lI)ion

logB = log

[Ni(enbbg) ~+] [Ni,+]

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2pH + pK1 + pK2 - log [enbbgHz z+] - log3,z,

where [enbbgH~ 2+] = [Ni 2+] (to a close approximation),

/ and ~ = 3 [Ni (enbbg) 2+] + 6[Ni 2+] + [KCI]. The ions Ni(enbbg) 2+ and Ni 2+ have the same charge and approximately the same hydrated radius: therefore they probably have nearly the same activity coefficients. In the equation for log/3, the activities of Ni (enbbg) ~+ and Ni z÷ occur as a ratio in which the activity coefficients cancel out, leaving a ratio of molar concentrations. The pH and pK terms refer directly to activities rather than to concentrations, and so the resulting value for 13 is a true thermodyanic equilibrium constant, corresponding to the stoichiometric equilibrium quotient at zero ionic strength. The experimental data are shown in Tables 3 and 4. Because the results do not reveal any clear trend related either to ionic strength or to chloride concentration, it must be assumed that the difference between the average values for/3 shown in Table 3 and Table 4 is the result of random errors rather than of any identifiable systematic error. Thus, the final average result, at 25 ° and corrected to zero ionic strength, (with a standard deviation including the errors contributed by pK1 and pKz) is l o g / / = 19-5___0.2 (orb = 3 × 1019). Table 3. Data for measurement of /3 without added KCI Total

Nickel concentration = 0.0180 F;

Temp. = 25°_+ 2 °.

/z = 0.056 to 0.095

pH

[Ni(enbbg) z+]

4"53 4"41 4-23 4.19 4.19 4.09 4-01 4.01 3.98 3.93 3-90 3-84 3 "76

0-01415 0.01309 0"01166 0.01098 0"01016 0.00940 0.00861 0.00857 0-00785 0"00710 0.00643 0.00565 0.00431

[Ni 2+] 0.00385 0"00491 0.00634 0.00702 0.00784 0.00860 0.00939 0.00943 0-01015 0-01090 0.01147 0"01235 0.01369

log fl 19.464 19.465 19.560 19.530 19.405 19.494 19.456 19.540 19.501 19.500 19.473 19.479 19.437

Average log B = 19.492 ___0-045 *; fl = 3" 10 x 10 TM. *Standard deviation based only on data in this table, excluding the error contributed by uncertainty in pKI and pKz.

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G . H . - Y . LIN and D. J. M a c D O N A L D Table 4. Data for measurement ofl3 with added KCI Total

nickel concentration=0.01285 F; [KCI] = 0.0715; Temp. = 25 ° ± 2*;/~ = 0.117-0.140

pH

[Ni(enbbg) ~+]

4.79 4.54 4.39 4"32 4.28 4"21 4.17 4" 11 4" 10 4"08 4.00 3.99 3.87

0"01070 0.00965 0.00871 0"00825 0.00745 0"00715 0.00651 0-00597 0"00562 0.00493 0-00447 0.00403 0.00302

[Ni ~+] 0.00215 0.00320 0.00414 0.00460 0.00540 0.00570 0.00634 0"00688 0.00723 0.00792 0.00838 0.00882 0.00983

log/3 19.410 19.523 19.558 19"585 19.485 19"562 19.510 19.492 19.475 19.380 19.457 19-385 19.407

Average log/3 = 19.479±0.069";/3 = 3"01 x 10TM. *Standard deviation based only on data in this table, excluding the error contributed by uncertainty in PK1 and pK2. DISCUSSION

The unusually large stability constant for ethylenebisbiguanidenickel (II) ion reflects the exceptional chelating ability and the extreme basicity of the tetradentate ethylenebisbiguanide ligand. By contrast, triethylenetetramine (abbreviation trien), a representative tetradentate nitrogen-donor ligand, forms a nickel(II) complex whose log/3 value is only 14.019]. The value of log/3 for Ni(enbbg) 2+ exceeds that for Ni(trien) 2+ by 5.5. A major part of this difference, i.e., 4.16, can be ascribed to the greater basicity of ethylenebisbiguanide. (For trienH +, PKa = 9"92; while for enbbgH +, pKa = 13"08). The remainder of that difference, amounting to 1.34, may be accounted for by a combination of several factors, described as follows. A hypothetical conversion of Ni(trien)2+; (actually trans-Ni(trien)(H20)22+) to Ni(enbbg) 2÷ would entail the following free-energy changes: loss of crystal field stabilization energy for Ni(trien) 2÷ to the extent of 12 Dq[10], which equals 12,500 cm-l[ 11]; gain of crystal field stabilization energy for Ni (enbbg) 2÷ to the extent of 24.56 Dq or 51,200 cm-1; loss of the bond energy between Ni z+ and two water molecules, which amounts to 163.8 kcal/mole or 57,300 cm -x if we assume that the change in Ni-OH2 bond energy between Ni(trien)(H20)2 ~+ and Ni(enbbg) 2+ equals one-third of the free energy of hydration of Ni2+[ 12]; gain of 9. G. Schwarzenbach, Heir. chim. A cta 33,974 (1950). 10. For values of C F S E in terms of Dq, cf. F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions, 2nd Edn, p. 70. Wiley, New York (1967). 11. For Ni(trien) 2+, 10 Dq ---- 10,400 cm -1. C. K. JCrgensen,Acta chem. scand. 11,399 (1957). 12. A Gh~d value taken from K. B. Harvey and G. B. Porter, Introduction to Physical Inorganic Chemistry, p. 326. Addison-Wesley, Massachussetts (1963).

Stability constant of ethylenebisbiguanidenickel(l I) ion

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792 cm -1 corresponding to the difference of stability constants expressed as 2.303 R T log 1-34; and loss of the electron spin pairing energy, P, occasioned by the change from spin-free Ni(trien) 2+ to spin-paired Ni(enbbg) 2+. Since the sum of these energy changes must total zero, the pairing energy, P, must equal 17,800 cm -1. Direct spectroscopic measurement of pairing energy in a transition-metal complex is probably not possible because the required transition, e.g., 3F-1D for Ni 2+, is spin-forbidden as well as LaPorte forbidden. Even for a gaseous Ni 2+ ion, the only available published value, namely P = 14,704 cm -~, refers to aF-~D in. the Ni ÷1 ion having electronic configuration 3 d S 4 s ~ [13]. The value of pairing energy in Ni (II) complexes reported here can be used to predict whether a given nickel complex of known crystal field splitting will be paramagnetic or diamagnetic. The complex should be spin-free and paramagnetic if 10 Dq < 17,800 cm -1 or spin-paired and diamagnetic if 10 Dq > 17,800 cm -1. In this connection, it is interesting to note that a recently reported complex of Ni(II) with the tetradentate nitrogen-donor ligand, 2,6-bis(2-pyridylmethyliminothio)pyridine [14], having 10 Dq = 19,650 cm -x, was found to be diamagnetic just as we would predict. Likewise, the macrocyclic ligand hexamethyl1,4,8,11-tetraazacyclotetradecadiene was found to form a diamagnetic nickel(II) complex having 1 0 D q in the range 22,400cm-~-23,150cm-l[15, 16]. On the other hand, all known complexes of nickel(II) which are definitely paramagnetic appear to have 10 Dq values considerably smaller than 17,800 cm -1. A c k n o w l e d g e m e n t - H e l p f u l discussions with Dr. Bernard Porter, U.S. Bureau of Mines, are gratefully acknowledged. 13. C. E. Moore, Atomic Energy Levels, Vol. II, Nat. Bur. Stds. Circular 467, Washington, D.C. (1952). 14. C.W. Kaufman and M. A. Robinson, J. inorg, nucl. chem. 30, 2475 (1968). 15. Y. M. Curtis and N. F. Curtis, Aust. J. Chem. 18, 1933 (1965). 16. L. G. Warner, N. J. Rose and D. H. Busch, J. A m. chem. Soc. 90, 6938 (1968).