1816
GEORGEB. HARES,W. CONARD FERNELIUS AND BODIEE. DOUGLAS [CONTRIBUTION FROM THECOLLEGE
OF
CHEWSTRY AND
PHYSICS,
Vol. 78
THEPENNSYLVANIA STATE UNIVERSITY]
Equilibrium Constants for the Formation of Complexes between Metal Ions and Polyamines'p2 BY GEORGEB.HARES,mr. CONARD FERNELIUS AND BODIEE. DOUGLAS3 RECEIVED NOVEMBER 4, 1955 In order to obtain a comparison of the relative stability of complex ions containing chelate rings of more than five members, an attempt was made to obtain the formation constants of the comdexes of copper(II), nickel, cobalt(I1) and zinc ions with a series of polyamines. No formation constants could be obtained in the case of 1,4-butanediamine, h7,hT-dimethyl-1,3and y-aminopropylmorpholine with any of the four metal ions studied propanediamine N-isopropyl-1,3-propanediamine because precipitates appeared before appreciable coordination had occurred. Data were obtained for all four metal ions and histamine. Incomplete data were obtained for the complexes of 3,3'only with 2,2-dimethyl-1,3-propanediamine diaminodipropylamine, 1,3-propanediamine and 8-aminoethylmorpholine because of precipitation with one or more of the metal ions. Where data were available a t two or more temperature AH and A S values were calculated for the reactions involved. In all cases the 6-membered rings were considerably less stable than the corresponding 5-membered rings and no complexes were formed with those amines which might form 7-membered chelate rings. Substitution on the nitrogen atom of the ligand caused a further decrease in stability except in the case of histamine. where a salt of the amine was used, this was weighed into Introduction the titration cell, 75 ml. of the metal ion solution added, and l h i s paper reports a study of the formation con- the resulting solution titrated with a standardized NaOH stants of complexes of copper, nickel, cobalt and solution, thereby generating the free amine during the zinc ions with 1,3-propanediamine, 3,3'-diaminodi- course of the titration. A nitrogen atmosphere was mainover the solutions during the course of the titrations. propylamine, 2,2-dimethyl-1,3-propanediamine, 1,- tained The pH measurements were made with the Beckman 4-butanediamine, N,KC'-dimethyl-1,3-propanedia- General Purpose Glass Electrode using a Beckman Calomel mine, N-isopropyl-1,3-~ropanediamine,P-aminoeth- Electrode as reference electrode. Titrations were carried ylmorpholine, y-aminopropylmorpholine and hista- out a t various temperatures obtained by use of a constantwater-bath. The 30 & 0.01' and 50 + 0.1" mine. The successive formation constants were temperature baths were controlled by mercury thermoregulators, while determined by the method of B j e r r ~ m . Compari~ the 0 i 0.1" bath was obtained by a water-crushed ice mixson of the values of the formation constants of the ture. The pH readings a t 0 and 50" were corrected using above complexes with data available for the com- the formula supplied by the Beckman Instrument Company. Aqueous solutions of the acids and metal salts used were plexes of the same metal ions with other amines4-" prepared and analyzed by means of generally accepted leads to a comparison of the stability of metal- methods. The pH-meter was standardized both before and amine complexes as a function of chelate ring size. after the titrations against buffer solutions prepared from National Bureau of Standards buffer salts. During the tiExperimental trations the systems reached equilibrium as soon as the heat 3,3'-Diaminodipropylamine, iY-isopropyl-1,3-propanediamine, N,?;-dimethyl-1,3-propanediaminewere gifts from the New Product Department of the American Cyanamid Company. 2,2 - Dimethyl - 1,3 - propanediamine dihydrochloride was prepared in this Laboratory by J . Rockett.'* 1,3-Propanediamine dihydrochloride and P-aminoethylmorpholine were gifts of Professor T. S. Oakwood. 1,4-Butanediamine dihydrochloride and histamine dihydrochloride were purchased from the Eastman Kodak Company. y Aminopropylmorpholine was a gift of the Carbide and Carbon Chemicals Company. All the amines and amine salts were carefully repurified before use. The general procedure was to pipet into the titration cell 75 ml. of a previously prepared solution containing a known concentration of metal salt, mineral acid and strong electrolyte (usually KSOa). This solution was titrated with a solution of amine of known concentration. In the cases (1) T h i s investigation was carried o u t under contract N6-onr 26913 between T h e Pennsylvania S t a t e University a n d T h e Office of Naval Research. (2) A portion of a thesis presented b y George B. Hares in partial fulfillment of t h e requirements for t h e degree of Doctor of Philosophy, January, 1952. (3) Chemistry Department: University of Pittsburgh, Pittsburgh 13, Pennsylvania. (4) J . Bjerrum, "hletal Ammine Formation in Aqueous Solution." P . Haase a n d Son, Copenhagen, 1941. ( 5 ) G. A. Carlson, J. P. McReynolds and F. H. Verhoek. THIS
67, 1334 (1945). (6) R. V. Bruehlman a n d F. H. Verhoek, i b i d . , 7 0 , 1401 (1948). (7) H. B. Jonassen, R . B. LeBlanc and R . M. Rogan. i b i d . , 72, 4968
JOURNAL.
(1950). (8) J. E. P r u e and G. Schwarzenbach, Heiu. C h i m . Acta, 33, 985 (1950). (9) C,. Schwsrze~itiach,ibid., 33, 974 (1950). (IO) J. E. P r u e a n d G . Schwarzenbach, ibid., 33, 995 (1950). (11) J. E P u r e and G. Schwarzenbach, i b i d . , 3 3 , 963 (1950). (12) J. R o c k e t t , M . S . Thesis, T h e Pennsylvania S t a t e College, June, 1918.
of reaction could be dissipated to the surroundings and the original temperature regained. This was practically instantaneous a t 30 and 50°, but required about 10 min. a t 0". The acid dissociation constants of the amines were determined in similar titrations using barium ion, a non-coordinating ion, in place of the coordinating metal ions.
Data and Results Dissociation Constants of the Amines.-The sults are given in Table I.
re-
TABLE I ACID DISSOCIATION CONSTANTS OF SOMEAMINESIN SOLUTIONS 1.0 M IS KNO, A N D 0.01 M IN B a + + Amine
3,3'-Diaminodipropylamine
H N O Z , Temp., M OC. PKAH, P K A H ~ P K A E
0.0509 ,0483 ,0509 1,3-Propanediamine ,0509 ,0509 ,0509 2,Z-Dimethyl-1,3,0509 propanedialnine ,0242 ,0509 0-Aminoethylnior,0509 pholine Histamine ,0509 ,0509
0 30 50 0 30 50 0 30 50
8 . 9 8 10.77 11.70 8 . 0 2 9.70 10.70 7 . 6 2 9.19 10.12 9 . 9 5 11.60 8 . 9 5 10.54 8.38 9.32 9.07 11.22 8.18 10.22 7.55 9.65
30 30 50
4.84 6.38 6.01
9.45 9.86 9.27
Formation Constants of the Complexes.-The
pH measurements were used to calculate the a and A values for formation curves. A typical set of formation curves is shown in Fig. 1. From these
FORMATION OF COMPLEXES BETWEEN METALIONSAND POLYAMINES
May 5, 1956
curves the formation constants given in Tables IIIV were calculated. For certain combinations of amine and cation i t is not possible to determine the 2,0 formation curve because precipitates form before appreciable coordination takes place. This situation obtains for 1,4-butanediamine, N,N-dimethyl1.5 lI3-propanediamine, N-isopropyl-l,3-propanedi- ,i amine and y-aminopropylmorpholine with all four metal ions studied. Cobalt and zinc gave precipi1.0 tates at 0 and 30' with lI3-propanediamine and a t 30' with p-aminoethylmorpholine. Copper and o,5 nickel gave precipitates with lI3-propanediamine only at 50°, but the data for nickel a t 0' were not considered reliable. Precipitation occurred with 2,2-dimethyl-lI3-propanediamine only for zinc at
2.0
4.0
6.0
8.0
10.0
1817
12.0 14.0
50'.
P[AI. TABLE I1 Fig. 1.-Formation curves for complexes of 1,3-propaneFORMATION CONSTANTS OF SEVERAL AMINE COMPLEXES diamine: 1, C u + + a t 0'; 11, C u + + a t 30"; 111, Ni+ a t OO. WITH VARIOUSBIVALENT METALIONS Amine
Temp,
OC.
3,3'-Diaminodipropylamine
0 30 50
log K1 log Ki log Ki log K 1
13-Propanediamine
fi-Aminoethylmorpholine
Cu++
Ni++
Co++
15.75 10.25 7.51 14.25 9.09 6 . 6 3 88 6 , 36 13.66
log K ,
8.05
30
log K I
30
log K1 log K P
9.62 7.00 6.60 3.96
log K P
7,00 4.92
...
.. ' '
...
..
3.78
...
..
..
TABLE I11 FORMATION CONSTANTS
Ni++
co++
Zn++
+
TABLE V THERMODYNAMIC QUANTITIES FOR THE STEPWISE FORMATION OF VARIOUSCOMPLEXES AT 30"
O F COMPLEXES O F 2,z-DIMETHYL-
1,3-PROPANEDIAMINE WITH VARIOUS
cu++
values in the table. The additional values for other complexes in Table V were calculated from constants determined a t two temperatures. The A F values are considered reliable to cu. 0.2 kcal. The average error in the A H values is probably ca. k 2 kcal., and for the A S values i t is probably ca. f 7 cal./degree. The errors are discussed more fully e1~ewhere.l~
log Ki log K P log K I log Kz log Ki log K I log Ki log K1
BIVALENT METALIONS
00
30°
10.95 8.25 7.22 4.99 5.41 3.52 5.58 5.58
9.94 7.45 6.59 4.41 4.88 3.07 5.21 5.20
9.41 6.86 6.23 4.15 4.38 PPt.
Ligand
3,3'-Diaminodipropylamine 1,3-Propanediamine 2,2-Dimethyl-1,3propanediamine
cu++ Ni++
co++
log KI log Kz log K I log Kz log K3 log K I log K P log K3
9.60 6.49 6.87 4.96 3.08 5.34 3.75 1.88
Cu++ Ni++ Co++ Cu++ Cu++ Ni++ Co++
TABLE IV FORMATION CONSTANTS OF COMPLEXES OF HISTAMINE WITH VARIOUSBIVALENT METALIONS 30'
Metal ion
50'
6.50 4.78 2.83 5.10 3.32 1.56
The plots of log K N v s . 1/T for the complexes for which data are available a t three temperatures were found to be linear within the limits of error. The thermodynamic quantities calculated from these data are presented in Table V. The A F values were calculated from the constants determined a t 30'. The A H values are the average of the values calculated for the intervals 0-30' and 30-50'. The A S values were calculated from the A F and A I 1
Zn++ Histamine
Xi++
Co++
n
value
1 1 1 1 2 1 2 1 2 1 2 1 2 1 2 3 1 2 3
-AF,
kcal./ mole
19.7 12.6 9.2 13.3 9.7 13.8 10.3 9.1 6.0 6.8 4.2 7 7 9.5 6.9 4.3 7.4 5.2 2.6
-AH, kcal./ mole
16 10 9 14 13 12 12 8 7 7 6
5 5 8
4 0 5 10 7
AS, cal./ degreemole
+12 9 1 - 3 -12 7
+ +
+ - 7 +3 -
3 0 - 6 + 8 8 5 t10 - 6 8 -10 -15
+
+ +
Discussion Although data could not be obtained in some cases because of the formation of hydroxides or basic precipitates, the appearance of such a precipitate is itself an indication of low stability. Precipitation occurred most often for the metal ions giving less stable complexes (Zn and Co) and for all four metal ions precipitation occurred more often a t the higher temperatures where the complexes are less stable. (13) E. Gonick, W. C. Fernelius and U . H . Douglas, THISJ O U R N A L , 7 6 , 4673 (1954).
1818
GEORGE B. HARES,W. CONARD FERNELIUS AND BODIEE. DOUGLAS
Vol. 78
The considerable decrease in the stability of the The heats of formation of the 2,2-dimethyl-1,3-procomplex as the size of the chelate ring increases from panediamine complexes are slightly less than those five to six members is shown by the comparison of of the 1,3-propanediamine complexes. the data in Table I1 for 1,3-propanediamine with Although the differences in the AH values are not data for eth~lenediamine.~The decreased stabil- outside the average experimental error given, the ity is shown by the smaller formation constants for difference is probably significant for two closely copper and nickel and the fact that cobalt and zinc related complexes. Heats of formation and free gave precipitates. Only two molecules of 1,3-pro- energy changes are usually lower for ligands of panediamine were coordinated while three mole- lower basicity, but the AF values are slightly, but cules of ethylenediamine and 1,2-propanediamine significantly, greater for the 2,2-dimethyl-1,3-proare coordinated by the same metal ions. The panediamine complexes. The reversal in the value lower coordination number with 1,3-propanedia- of A F is due to the entropy effects. Since the ligmine is probably due partly to the lower affinity for ands are very similar and the R value is the same in each molecule of ligand, but probably to a greater each case, it would seem that the difference in enextent to spatial limitations. tropy might lie in the differences in the interaction The same relative stability is found for the com- of the complexes with the solvent. Frank and parison of the complexes of terdentate amines which EvansI5have proposed an explanation which can be form 5- and 6-membered chelate rings. The forma- extended to lead one to expect that the entropy tion constants of 3,3’-diaminodipropylaminecom- change accompanying the formation of the more plexes (Table 11) are smaller than those of the cor- “hydrocarbon-like” complexes such as those of 2,2responding 2,2’-diaminodiethylamine c ~ m p l e x e s . ~dimethyl-l,3-propanediaminemight be considerThe complexes of 3,3‘-diaminodipropylaminecon- ably more favorable due to the lesser tendency to tain only one molecule of amine in comparison with create an ice-like structure in the solvent. The only N-substituted-l,3-diamine studied two molecules of 2,2-diaminodiethylamine. The separation of the amino groups by four car- which would form complexes under the conditions bon atoms in 1,4-butanediamine and y-aminopro- used was histamine. Histamine is peculiar not pylmorpholine resulted in the precipitation of each only in that the formation constants of its complexes of the four metal ions before appreciable coordina- are greater, except for Cu++, than those of any tion occurred. Solid coordination compounds of other 1,a-diamine studied (Table IV), but also in copper with 1,4-butanediamine and 1,6-hexanedi- that the complexes of histamine with Ni++ and amine were obtained by Pfeiffer and co-workersl* Co++ contain thrce molecules of amine.16 This in alcoholic solution, but the compounds were un- phenomenon probably is due to the steric effect of the pyrazole ring. stable in water. The formation curve of Ni++ with P-aminoethylThe copper and nickel complexes of 2,2-dimethyl1,3-propanediamine were more stable than those of morpholine and the blue-green color of the complex 1,3-propanediamine and the cobalt and zinc com- in solution indicate that only one molecule of the plexes of the former ligand were stable in water be- amine per metal ion was coordinated. It is unulow 50’. Cobalt and zinc gave precipitates with sual that although only one molecule of diamine 1,3-propanediamine even at the lower temperatures. was coordinated no precipitate formed during the The increased stability of the 2,2-dimethyl-1,3-pro- titration. PARK,PA. panediamine complexes cannot be explained on the UNIVERSITY basis of the inductive effects of the methyl groups (15) H. S.Frank and M. W.Evans, J . Chcm. Phys., 13, 531 (1054). (16) Since this work was completed additional formation constants because the basicity of the ligand toward the proton been published by B. L. Mickel and A. C. Andrews, THISJOURis actually less than that of 1,3-propanediamine. have N A L , ??, 323, 5291 (1955), for histamine complexes of N i + + , C u + + (14) P. Pfeiffer, A. Bohm and E. Schmitz, Nafurluissenschaflen, 36, 190 (1948).
and C o + + at 0 and 2.5’. The values reported agree very well with those given in Table I V although the temperature is different.
