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Microdetermination of aliphatic amines and polyamines with cupric ion: Relationship between pK1 and half-complexation potential
MICROCHEMICAL
JOURNAL
27, 200-209
(1982)
Microdetermination of Aliphatic Amines and Polyamines with Cupric Ion: Relationship between pK, and Half-Complexation Potential’,* WALTER
Lawrence
Livermore
Nationul Livermore, Received
SELIG
Laboratory, Californin June
University 94550
of California,
13, 1981
INTRODUCTION In previous publications (3, 8, 9) we have reported the potentiometric microdetermination of aliphatic amines and polyamines with cupric nitrate. Electromotive forces were monitored with a copper ion-selective electrode (ISE) and a single-junction reference electrode. In this paper we relate the first equilibrium constant, K 1, for the combination of ligand with cupric ion to the half-complexation potential, E,,, and discuss the limitations of our method. Some additional data are also presented. EXPERIMENTAL The experimental procedures have been described in detail previously (9). The titrant was 0.01 A4 cupric nitrate or sulfate. The amines were of at least 95% purity. Electromotive forces were monitored with an Orion cupric ISE and a single-junction reference electrode (0.1 N sodium nitrate filling solution). Titrations were performed at ambient temperature at a constant speed of 0.5 ml/min. The automated titration apparatus, as well as the method for endpoint calculation, have been previously described (6). ’ Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under Contract W-7405-Eng-48. ’ This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government thereof, and shall not be used for advertising or product endorsement purposes. 200 0026-265X/82/020200-10$01.00/0 Copyright All rights
@ 1982 by Academic Press, Inc. of reproduction in any form reserved.
DETERMINATION
RESULTS
OF
AMINES
201
AND DISCUSSION
In previous papers we described the microdetermination of some aliphatic amines with cupric ion (3), and of aliphatic diamines and polyamines using the same titrant (8, 9). We found that, in general, cupric ion reacted with 2 mol of aliphatic amines or vie-diamines. Polyamines (with more than two nitrogen atoms) reacted with cupric ion on an equimolar basis. Vie-diamines and polyamines could be determined in the presence of ammonia while monoamines could not. In Table 1 data are presented for some previously unreported amines which could not be titrated in the presence of ammonia. It is noteworthy that the new compounds examined yielded recoveries in excess of 100%. This may possibly be attributed to low-molecular-weight impurities. Titration curves in most cases were fairly shallow. Table 2 shows data for some a-amino acids and polyamines which could also be determined in the presence of ammonia. a-Amino acids have been previously determined with cupric ion at pH 9 (7). These compounds differ from aliphatic amines in that addition of ammonia for pH adjustment is necessary; in the absence of ammonia they cannot be determined. The buffer/pH conditions differ for each amino acid. The optimum pH for glycine was 6.75, while for L-serine it was between 6.1 and 6.9. As previously discussed, vie-diamines and polyamines yield upon titration with cupric ion a pink/violet color (9). This was not so for 1,4,8,1 ltetramethyl-1,4,8,1 I-tetraazacyclotetradecane, which yielded a deep blue color (9). The reason may be the full methylation of all available nitrogen atoms. We previously (9) stated that although 1,2-diaminoethane can be titrated in the presence of ammonia and yields a violet color, if any of the nitrogen atoms are methylated this is no longer true. This was based on experiments with di- or higher methyl-substituted 1,2-diaminoethanes. We have now found that N-methyl-1,2-diaminoethane can indeed be titrated in the presence of ammonia and also forms the characteristic violet color upon titration with cupric ion. We therefore modify our rule to state that monoalkyl-substituted vie-diamines, as well as unsubstituted vie-diamines, can be titrated in the presence of ammonia and yield a violet/pink color, while this is not true for vie-diamines which are more highly substituted. Meso-5,7,7,12,14,14-hexamethyl-l,4,8,1l-tetraazacyclotetradecane reacted with cupric ion on a 3:4 basis (1.33 mol of amine per Cu (II)), while all other polyamines reacted on an equimolar basis (some, however, showed several breaks, as reported previously (9)). This may be caused by the steric effect of the methyl groups attached to four carbon atoms in this ligand. In Table 3 we list some amines which could not be titrated vs cupric ion.
97.37
in
97.09 97.9t 82.78 93.73
Mean
vs CUPRIC
recovery mo)
113.2 113.2 110.24 110.3 108.13 108.97
Mrso-5.7,7,12,14,14-hexamethyl-1,4,8,11tetraazacyclotetradecane dihydrate 20% methanol/80?& water
1 ,Zdiaminoethane
vs CUPRIC
recovery (74
FOR AMIKES
Mean
FOR AMINES
1,4,8,11-Tetramethyl-1,4,8,11tetraazacyclotetradecane
N-Methyl-
r-Serine
Glycine
STATISTICS
in 20% methanol
Jpropanediol
Compound
2-Amino-Z-methyl-I Diisopropanolamine 2-Methylaminoethanol 2-Dimethylaminoethanol Z-Aminoethanol 1.3.Diphenylguanidine
Compound
STATISTICS
1 FEASIXL~
2 (FEASIBLE
0.49
0.27
0.81 0.19 0.18
Standard deviation
ION
TABLE
0.3 0.65 0.14 0.27 0.16 0.31
Standard deviation
(NOT
TABLE ION
OF AMMONIA)
5
5
4 6 5
Number of replicates
IN PRESENCE
3 4 6 5 5 4
(as
9.25 10.25 10.35 10.25 9.3 10.5
Initial PH
pH
is)
10.6 (as is)
10.15
6.75 6.1-6.9 10.6 (as is)
Optimum
OF AMMONIA)
Number of replicates
IN PRESENCE
1.33
1
2 2 2
Mole amjne per Cu(II)
Shallow Shallow
Remarks
Violet
Blue
Blue Blue Violet
Color
curve curve
+ F
DETERMINATION
OF
TABLE COMPOUNDS
203
AMINES
3 vs Cull)
NOT TITRATABLE
Compound
loti
I
2.41-2.86 -0.34 3.98 3.9-4.44
Pyridine Hexamethylenetetramine Tris(hydroxymethyl)aminomethane Triethanolamine Diethanolamine
(initial
3.8
pH 8)
(20 ")
This table also gives the logarithm of the first equilibrium constant, K,, for the combination of ligand with cupric ion obtained from the literature (4, 5, 9) according to
ML1 K1=
[M][L]
.
We have attempted to relate log K 1 to the potential of half-complexation, E HC, of the various amines examined in this and the previous studies by using a polynomial regression for 10gKl
= a + h[EH,].
(2)
When all available data points were used the correlation coefficient, Y, was -0.73; when the meuns of all values were used, Y was -0.84. The data was then separated into 2 sets: (1) amines with n s 2 (where n is the number of nitrogen atoms) and (2) amines with n B 2. A summary of the values for r is presented in Table 4. We consider the correlation quite good inasmuch as log K, values reported in the literature (4, 5, 9) were obtained under varying conditions and over many years. Although the difference between r using the means of the values and using all values was quite small, we used the latter. The results are presented graphically TABLE COEFFICIENTS
Data
FOR
4
LocK,vsE,,
Correlation coefficient.
All values Means of all values
-0.729 -0.840
n 4 2, all values n G 2. means of all values
-0.953 -0.935
n 3 2, all values
-0.979 -0.975
n 2 2, means
of values
Y
1.27669 ~2.58351
-0.03357 -0.07746
Amines and diamines 2-Aminoethanol 2-Dimethylaminoethanol 2-Methylaminoethanol 2-Amino-2-methyl1,3-propanediol Diisopropanolamine Triethylenediamine (DABCO) Piperazine 1,7-Diaminoheptane 1 ,PDiaminononane I, 12-Diaminododecane 1 ,I-Diaminooctane 1, IO-Diaminodecane N,N,N’,N’-Tetramethyl-1,2-diaminoethane lJDiamino-2-propanol N.N,N’-Trimethyl-1,2-diaminoethane N, N, N’N’-Tetramethyl-l,2-diaminocyclohexane
Compound
EH,> LOGK,(LITERKTURE
TABLE
5
-230.5
-86.2 -106.2 ~ 110.5 -110.5 - 113.3 -117.6 - 149.2 - 147.3 -170.1 ~ 172.7 - 177.1 - 179.7 - 188.9 -214.6 -220
E,,c
3.32
7.27 9.70
3.32
7.19, 7.20, 9.70 (30”)
7.38
5.22 4.7 5.0
Log K,, average
9.01
4.17 4.84 4.99 4.99 5.08 5.22 6.29 6.22 6.99 7.07 7.22 7.31 7.62 8.48 8.66
Log K,, calculated
POLYAMINES
5.7, 4.73 4.7 5.0
7.30,
log K,
ANDCALCULATEDVALUES)FORVARIOUSAMINESAND
+4.8 -12.6
+89.5
-20.1 +3.0 -0.2
Log K, (lit.) log K 1 (calcd) (SE)
-
F i;
r; E
Polyuminc.7 Diethylenetriamine I ,4,8,1 I-Tetramethyl-1.4.8, I Itetraazacyclotetradecane 1,5,9,13-Tetraazatridecane I ,5,8,12-Tetraazadodecane Triethylenetetramine Tetraethylenepentamine 1,4,8,1 I-Tetraazaundecane I ,4,8,1 I-Tetraazacyclotetradecane 1,4,8,12-Tetraazacyclotetradecane
-279.4 -281.9 -293.3
1 JDiaminopropane 1,2-Diamino-2-methylpropane 1,2-Diaminocyclohexane
17.7 16.63,
-259.8 -260.0 -309.6 -314.3 22.80. 23.9 24.4
-331.7 -334.3 -349.2 -350.5
21.3, 20.1
15.91,
-240.1
22.9,
21.69,
17.3
16.02,
25.25
21.84,
16.17,
21.8
16.7
24.4
23.32 23.9
17.7 16.97 21.66 20.1
16.20
10.78
10.94
10.61.
10.61
10.75 10.4
10.56
10.40,
10.61
10.44,
10.44,
10.66
9.94 10.40
10.50,
10.76,
10.47
10.72,
10.75,
10.09,
9.19 9.69
10.44, 10.64, 10.50 10.71, 10.78 10.4
11.02,
10.76,
10.6,
9.26
-277.8
9.23,
~256.2 ~261.4
9.08, 9.18, 9.77, 9.82, 9.98 9.69, 9.69, 9.72, 9.996, 10.26, 10.40, 10.55
-237.1 -246
N, N-DimethylI ,2-diaminoethane 1,3-Diaminopropane N, N’-Dimethyl-1,2-diaminoethane N-Methyl1,2-diaminoethane 1.2-Diaminoethane
24.57
21.76 23.11 23.31 24.47
17.54 17.56 21.40
16.01
11.12
10.66 10.74
+0.7
+8.3 -0.9 -2.5
~ 1.0
+3.5
1.2 -0.9
~
-0.8 +3.3 +3.2
0
-3.4
10.05 10.61
+0.5 - 1.7 -0.6
9.53 9.88
9.24
cc zm vl
%
0 2.
u z z3 z=i
206
WALTER 12-
I
10 -
SELIG
I
I
I
I
:>,: % .
*
S:
J
;
6\
I
< 4. 2I -250
I -300
I -200
Half-complexation
FIG.
1. Relation
between
I -150 potential,
I -100
-50
mV
Epjc and log K, for amines
with n s 2.
in Fig. 1 for II c 2 and in Fig. 2 for n 3 2. The coefficients a and m for Eq. (2) are also given in Table 4. Finally, Table 5 summarizes the experimental values for EHC,the literature values for log K,, and the calculated values obtained from Eq. (2) using the coefficients given in Table 4, as well as the difference between them in percent. Blanks in Table 5 indicate that no literature values were available. For amines with n G 2, if the high calculated log K, for 2-aminoethanol and piperazine are disregarded, the mean difference between the average log K, reported in the literature and the calculated value was -0.4%; for II 2 2 it was +0.8%. Table 5 shows that only a single value was available for piperazine, while the two values for 2-aminoethanol differed by more than one order of magnitude. Omitting these points and replotting the data yielded a better correlation with Y = -0.98 for the mean, and coefficients
14’
-350
-300
Half-complexation
FIG.
2. Relation
between
E,,
-250 potential,
-200
mV
and log K, for amines
with
n 3 2.
DETERMINATION
OF
AMINES
207
208
WALTER
SELIG
CL= 1.3733 and b = -0.03440. The equations for calculating the known E,, are, for IZ < 2, log K, = 1.3733 - 0.03340 [EHC],
log K, from (3)
and, for n > 2, log K, = -2.5835
- 0.07746 [EHC].
(4)
Inspection of the data in Tables 3 and 5 shows that aliphatic amines with log K, < 4 cannot be determined by titration vs cupric ion. Again piperazine is an exception; our findings indicate that the literature value of 3.32 is probably too low. Another requirement, already stated previously, is that the initial pH of a 10e3 M solution of the amine should be at least 9 for titration with Cu(I1) to succeed. Hall and Amma (2) have discussed the macrocyclic effect in which the stability of a copper complex is additionally enhanced by coordination to a tetraamine macrocyclic ligand, as compared to a similar noncyclic tetraamine ligand. They reported log K, = 28 for the mes~ isomer of .5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane with cupric ion in aqueous solution. We have found this compound not sufficiently soluble in water and titrated it in 20% methanol. The log K, calculated from Eq. (4) was 15.6. Anichini et al. (1) also pointed out the striking changes that cyclization of a linear polyamine ligand produces in the properties of the metal complexes when compared to the equivalent complexes of linear ligands. One of these properties is the high thermodynamic stability which is reflected in the stability constants, which can be several orders of magnitude larger than that for the corresponding linear ligands. These compounds are of particular interest because they can be regarded as simple models for naturally occurring metal-macrocycle centers found in proteins. We present the literature and calculated values of log K 1 for some of these compounds in Table 6. It is evident that Eqs. (3) and (4) are useful for estimating log K, for aliphatic amines. SUMMARY The first equilibrium to the half-complexation
constant for the combination potential, E,,c, by log K,
of ligand
with
cupric
ion, K,, is related
= a + h[E,,,..
The coeffkients differ for n G 2 and n 3 2, where n is the number of nitrogen atoms in the molecule. The correlation coefficients, I, based on data available from the literature were -0.98. The equations can be used for estimating log K, of aliphatic amines. Successful titration of aliphatic amines vs cupric ion requires log K , Z= 4 and an initial pH of a lo-” A4 solution of 29.
DETERMINATION
OF
209
AMINES
ACKNOWLEDGMENT The author evaluation.
wishes
to thank
Hal R. Brand
for help with
computer
programs
for the data
REFERENCES I.
2. 3.
4.
5. 6. 7.
8.
Y. 10.
Anichini, A., Fabbrizzi, L., Paoletti, P.. and Clay. R. M., A microcalorimetric study of the macrocyclic effect. Enthalpies of formation of copper(I1) and zinc(H) complexes with some tetra-aza macrocyclic ligands in aqueous solution. J.C.S. L)ulfon 1978, 577-583. Hall, E. A., and Amma, E. L.. Macrocyclic effect on the stability of copper(I1) tetramine complexes. J. Avw. Chem. SW. 91, 6540-6541 (1969). Hassan, S. S. M., Tadros, F.. and Selig, W., Microdetermination and pK, measurement of some aliphatic amines using the copper-ion-selective electrode. Microc.hr~n. J. 26, 426643.5 (1981). “Stability Constants of Metal-Ion Complexes,” Suppl. 1. Mat-tell. A. E. (compiler), Part II. “Organic. Including Macromolecule, Ligands.” The Chemical Society. London, 1971. Perrin, D. D. (compiler). “‘Stability Constants of Metal-Ion Complexes,” Part B, “Organic Ligands.” Pergamon, Oxford, 1979. Selig, W., Evaluation of toluene as a replacement for benzene in tetrabutylammonium hydroxide titrant. Microchr,rr. .I. 23, 4666468 (1978). Selig, W., “Ion-Selective Electrodes in Organic Elemental and Functional Group Analysis: A Review (1975 to 1978),” pp. 166 18. Lawrence Livermore National Laboratory Report UCRL-52393, Suppl. I, 1978. Selig, W., The Analysis of Composites Containing Ethylenediamine Dinitrate, Ammonium Nitrate, and Hexahydro-1,3,5-trinitro-l,3,5-triazine (RDX).” Lawrence Livermore National Laboratory Report UCID-18934, 1981. Selig, W., Microdetermination of aliphatic diamines and polyamines using cupric ion. Microchrm. J. 27, 102% 111 (1982). Sillen, L. G. (compiler), “Stability Constants of Metal-Ion Complexes,” Section 11, “Organic Ligands.” The Chemical Society, London, 1964.
JOURNAL
27, 200-209
(1982)
Microdetermination of Aliphatic Amines and Polyamines with Cupric Ion: Relationship between pK, and Half-Complexation Potential’,* WALTER
Lawrence
Livermore
Nationul Livermore, Received
SELIG
Laboratory, Californin June
University 94550
of California,
13, 1981
INTRODUCTION In previous publications (3, 8, 9) we have reported the potentiometric microdetermination of aliphatic amines and polyamines with cupric nitrate. Electromotive forces were monitored with a copper ion-selective electrode (ISE) and a single-junction reference electrode. In this paper we relate the first equilibrium constant, K 1, for the combination of ligand with cupric ion to the half-complexation potential, E,,, and discuss the limitations of our method. Some additional data are also presented. EXPERIMENTAL The experimental procedures have been described in detail previously (9). The titrant was 0.01 A4 cupric nitrate or sulfate. The amines were of at least 95% purity. Electromotive forces were monitored with an Orion cupric ISE and a single-junction reference electrode (0.1 N sodium nitrate filling solution). Titrations were performed at ambient temperature at a constant speed of 0.5 ml/min. The automated titration apparatus, as well as the method for endpoint calculation, have been previously described (6). ’ Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under Contract W-7405-Eng-48. ’ This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government thereof, and shall not be used for advertising or product endorsement purposes. 200 0026-265X/82/020200-10$01.00/0 Copyright All rights
@ 1982 by Academic Press, Inc. of reproduction in any form reserved.
DETERMINATION
RESULTS
OF
AMINES
201
AND DISCUSSION
In previous papers we described the microdetermination of some aliphatic amines with cupric ion (3), and of aliphatic diamines and polyamines using the same titrant (8, 9). We found that, in general, cupric ion reacted with 2 mol of aliphatic amines or vie-diamines. Polyamines (with more than two nitrogen atoms) reacted with cupric ion on an equimolar basis. Vie-diamines and polyamines could be determined in the presence of ammonia while monoamines could not. In Table 1 data are presented for some previously unreported amines which could not be titrated in the presence of ammonia. It is noteworthy that the new compounds examined yielded recoveries in excess of 100%. This may possibly be attributed to low-molecular-weight impurities. Titration curves in most cases were fairly shallow. Table 2 shows data for some a-amino acids and polyamines which could also be determined in the presence of ammonia. a-Amino acids have been previously determined with cupric ion at pH 9 (7). These compounds differ from aliphatic amines in that addition of ammonia for pH adjustment is necessary; in the absence of ammonia they cannot be determined. The buffer/pH conditions differ for each amino acid. The optimum pH for glycine was 6.75, while for L-serine it was between 6.1 and 6.9. As previously discussed, vie-diamines and polyamines yield upon titration with cupric ion a pink/violet color (9). This was not so for 1,4,8,1 ltetramethyl-1,4,8,1 I-tetraazacyclotetradecane, which yielded a deep blue color (9). The reason may be the full methylation of all available nitrogen atoms. We previously (9) stated that although 1,2-diaminoethane can be titrated in the presence of ammonia and yields a violet color, if any of the nitrogen atoms are methylated this is no longer true. This was based on experiments with di- or higher methyl-substituted 1,2-diaminoethanes. We have now found that N-methyl-1,2-diaminoethane can indeed be titrated in the presence of ammonia and also forms the characteristic violet color upon titration with cupric ion. We therefore modify our rule to state that monoalkyl-substituted vie-diamines, as well as unsubstituted vie-diamines, can be titrated in the presence of ammonia and yield a violet/pink color, while this is not true for vie-diamines which are more highly substituted. Meso-5,7,7,12,14,14-hexamethyl-l,4,8,1l-tetraazacyclotetradecane reacted with cupric ion on a 3:4 basis (1.33 mol of amine per Cu (II)), while all other polyamines reacted on an equimolar basis (some, however, showed several breaks, as reported previously (9)). This may be caused by the steric effect of the methyl groups attached to four carbon atoms in this ligand. In Table 3 we list some amines which could not be titrated vs cupric ion.
97.37
in
97.09 97.9t 82.78 93.73
Mean
vs CUPRIC
recovery mo)
113.2 113.2 110.24 110.3 108.13 108.97
Mrso-5.7,7,12,14,14-hexamethyl-1,4,8,11tetraazacyclotetradecane dihydrate 20% methanol/80?& water
1 ,Zdiaminoethane
vs CUPRIC
recovery (74
FOR AMIKES
Mean
FOR AMINES
1,4,8,11-Tetramethyl-1,4,8,11tetraazacyclotetradecane
N-Methyl-
r-Serine
Glycine
STATISTICS
in 20% methanol
Jpropanediol
Compound
2-Amino-Z-methyl-I Diisopropanolamine 2-Methylaminoethanol 2-Dimethylaminoethanol Z-Aminoethanol 1.3.Diphenylguanidine
Compound
STATISTICS
1 FEASIXL~
2 (FEASIBLE
0.49
0.27
0.81 0.19 0.18
Standard deviation
ION
TABLE
0.3 0.65 0.14 0.27 0.16 0.31
Standard deviation
(NOT
TABLE ION
OF AMMONIA)
5
5
4 6 5
Number of replicates
IN PRESENCE
3 4 6 5 5 4
(as
9.25 10.25 10.35 10.25 9.3 10.5
Initial PH
pH
is)
10.6 (as is)
10.15
6.75 6.1-6.9 10.6 (as is)
Optimum
OF AMMONIA)
Number of replicates
IN PRESENCE
1.33
1
2 2 2
Mole amjne per Cu(II)
Shallow Shallow
Remarks
Violet
Blue
Blue Blue Violet
Color
curve curve
+ F
DETERMINATION
OF
TABLE COMPOUNDS
203
AMINES
3 vs Cull)
NOT TITRATABLE
Compound
loti
I
2.41-2.86 -0.34 3.98 3.9-4.44
Pyridine Hexamethylenetetramine Tris(hydroxymethyl)aminomethane Triethanolamine Diethanolamine
(initial
3.8
pH 8)
(20 ")
This table also gives the logarithm of the first equilibrium constant, K,, for the combination of ligand with cupric ion obtained from the literature (4, 5, 9) according to
ML1 K1=
[M][L]
.
We have attempted to relate log K 1 to the potential of half-complexation, E HC, of the various amines examined in this and the previous studies by using a polynomial regression for 10gKl
= a + h[EH,].
(2)
When all available data points were used the correlation coefficient, Y, was -0.73; when the meuns of all values were used, Y was -0.84. The data was then separated into 2 sets: (1) amines with n s 2 (where n is the number of nitrogen atoms) and (2) amines with n B 2. A summary of the values for r is presented in Table 4. We consider the correlation quite good inasmuch as log K, values reported in the literature (4, 5, 9) were obtained under varying conditions and over many years. Although the difference between r using the means of the values and using all values was quite small, we used the latter. The results are presented graphically TABLE COEFFICIENTS
Data
FOR
4
LocK,vsE,,
Correlation coefficient.
All values Means of all values
-0.729 -0.840
n 4 2, all values n G 2. means of all values
-0.953 -0.935
n 3 2, all values
-0.979 -0.975
n 2 2, means
of values
Y
1.27669 ~2.58351
-0.03357 -0.07746
Amines and diamines 2-Aminoethanol 2-Dimethylaminoethanol 2-Methylaminoethanol 2-Amino-2-methyl1,3-propanediol Diisopropanolamine Triethylenediamine (DABCO) Piperazine 1,7-Diaminoheptane 1 ,PDiaminononane I, 12-Diaminododecane 1 ,I-Diaminooctane 1, IO-Diaminodecane N,N,N’,N’-Tetramethyl-1,2-diaminoethane lJDiamino-2-propanol N.N,N’-Trimethyl-1,2-diaminoethane N, N, N’N’-Tetramethyl-l,2-diaminocyclohexane
Compound
EH,> LOGK,(LITERKTURE
TABLE
5
-230.5
-86.2 -106.2 ~ 110.5 -110.5 - 113.3 -117.6 - 149.2 - 147.3 -170.1 ~ 172.7 - 177.1 - 179.7 - 188.9 -214.6 -220
E,,c
3.32
7.27 9.70
3.32
7.19, 7.20, 9.70 (30”)
7.38
5.22 4.7 5.0
Log K,, average
9.01
4.17 4.84 4.99 4.99 5.08 5.22 6.29 6.22 6.99 7.07 7.22 7.31 7.62 8.48 8.66
Log K,, calculated
POLYAMINES
5.7, 4.73 4.7 5.0
7.30,
log K,
ANDCALCULATEDVALUES)FORVARIOUSAMINESAND
+4.8 -12.6
+89.5
-20.1 +3.0 -0.2
Log K, (lit.) log K 1 (calcd) (SE)
-
F i;
r; E
Polyuminc.7 Diethylenetriamine I ,4,8,1 I-Tetramethyl-1.4.8, I Itetraazacyclotetradecane 1,5,9,13-Tetraazatridecane I ,5,8,12-Tetraazadodecane Triethylenetetramine Tetraethylenepentamine 1,4,8,1 I-Tetraazaundecane I ,4,8,1 I-Tetraazacyclotetradecane 1,4,8,12-Tetraazacyclotetradecane
-279.4 -281.9 -293.3
1 JDiaminopropane 1,2-Diamino-2-methylpropane 1,2-Diaminocyclohexane
17.7 16.63,
-259.8 -260.0 -309.6 -314.3 22.80. 23.9 24.4
-331.7 -334.3 -349.2 -350.5
21.3, 20.1
15.91,
-240.1
22.9,
21.69,
17.3
16.02,
25.25
21.84,
16.17,
21.8
16.7
24.4
23.32 23.9
17.7 16.97 21.66 20.1
16.20
10.78
10.94
10.61.
10.61
10.75 10.4
10.56
10.40,
10.61
10.44,
10.44,
10.66
9.94 10.40
10.50,
10.76,
10.47
10.72,
10.75,
10.09,
9.19 9.69
10.44, 10.64, 10.50 10.71, 10.78 10.4
11.02,
10.76,
10.6,
9.26
-277.8
9.23,
~256.2 ~261.4
9.08, 9.18, 9.77, 9.82, 9.98 9.69, 9.69, 9.72, 9.996, 10.26, 10.40, 10.55
-237.1 -246
N, N-DimethylI ,2-diaminoethane 1,3-Diaminopropane N, N’-Dimethyl-1,2-diaminoethane N-Methyl1,2-diaminoethane 1.2-Diaminoethane
24.57
21.76 23.11 23.31 24.47
17.54 17.56 21.40
16.01
11.12
10.66 10.74
+0.7
+8.3 -0.9 -2.5
~ 1.0
+3.5
1.2 -0.9
~
-0.8 +3.3 +3.2
0
-3.4
10.05 10.61
+0.5 - 1.7 -0.6
9.53 9.88
9.24
cc zm vl
%
0 2.
u z z3 z=i
206
WALTER 12-
I
10 -
SELIG
I
I
I
I
:>,: % .
*
S:
J
;
6\
I
< 4. 2I -250
I -300
I -200
Half-complexation
FIG.
1. Relation
between
I -150 potential,
I -100
-50
mV
Epjc and log K, for amines
with n s 2.
in Fig. 1 for II c 2 and in Fig. 2 for n 3 2. The coefficients a and m for Eq. (2) are also given in Table 4. Finally, Table 5 summarizes the experimental values for EHC,the literature values for log K,, and the calculated values obtained from Eq. (2) using the coefficients given in Table 4, as well as the difference between them in percent. Blanks in Table 5 indicate that no literature values were available. For amines with n G 2, if the high calculated log K, for 2-aminoethanol and piperazine are disregarded, the mean difference between the average log K, reported in the literature and the calculated value was -0.4%; for II 2 2 it was +0.8%. Table 5 shows that only a single value was available for piperazine, while the two values for 2-aminoethanol differed by more than one order of magnitude. Omitting these points and replotting the data yielded a better correlation with Y = -0.98 for the mean, and coefficients
14’
-350
-300
Half-complexation
FIG.
2. Relation
between
E,,
-250 potential,
-200
mV
and log K, for amines
with
n 3 2.
DETERMINATION
OF
AMINES
207
208
WALTER
SELIG
CL= 1.3733 and b = -0.03440. The equations for calculating the known E,, are, for IZ < 2, log K, = 1.3733 - 0.03340 [EHC],
log K, from (3)
and, for n > 2, log K, = -2.5835
- 0.07746 [EHC].
(4)
Inspection of the data in Tables 3 and 5 shows that aliphatic amines with log K, < 4 cannot be determined by titration vs cupric ion. Again piperazine is an exception; our findings indicate that the literature value of 3.32 is probably too low. Another requirement, already stated previously, is that the initial pH of a 10e3 M solution of the amine should be at least 9 for titration with Cu(I1) to succeed. Hall and Amma (2) have discussed the macrocyclic effect in which the stability of a copper complex is additionally enhanced by coordination to a tetraamine macrocyclic ligand, as compared to a similar noncyclic tetraamine ligand. They reported log K, = 28 for the mes~ isomer of .5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane with cupric ion in aqueous solution. We have found this compound not sufficiently soluble in water and titrated it in 20% methanol. The log K, calculated from Eq. (4) was 15.6. Anichini et al. (1) also pointed out the striking changes that cyclization of a linear polyamine ligand produces in the properties of the metal complexes when compared to the equivalent complexes of linear ligands. One of these properties is the high thermodynamic stability which is reflected in the stability constants, which can be several orders of magnitude larger than that for the corresponding linear ligands. These compounds are of particular interest because they can be regarded as simple models for naturally occurring metal-macrocycle centers found in proteins. We present the literature and calculated values of log K 1 for some of these compounds in Table 6. It is evident that Eqs. (3) and (4) are useful for estimating log K, for aliphatic amines. SUMMARY The first equilibrium to the half-complexation
constant for the combination potential, E,,c, by log K,
of ligand
with
cupric
ion, K,, is related
= a + h[E,,,..
The coeffkients differ for n G 2 and n 3 2, where n is the number of nitrogen atoms in the molecule. The correlation coefficients, I, based on data available from the literature were -0.98. The equations can be used for estimating log K, of aliphatic amines. Successful titration of aliphatic amines vs cupric ion requires log K , Z= 4 and an initial pH of a lo-” A4 solution of 29.
DETERMINATION
OF
209
AMINES
ACKNOWLEDGMENT The author evaluation.
wishes
to thank
Hal R. Brand
for help with
computer
programs
for the data
REFERENCES I.
2. 3.
4.
5. 6. 7.
8.
Y. 10.
Anichini, A., Fabbrizzi, L., Paoletti, P.. and Clay. R. M., A microcalorimetric study of the macrocyclic effect. Enthalpies of formation of copper(I1) and zinc(H) complexes with some tetra-aza macrocyclic ligands in aqueous solution. J.C.S. L)ulfon 1978, 577-583. Hall, E. A., and Amma, E. L.. Macrocyclic effect on the stability of copper(I1) tetramine complexes. J. Avw. Chem. SW. 91, 6540-6541 (1969). Hassan, S. S. M., Tadros, F.. and Selig, W., Microdetermination and pK, measurement of some aliphatic amines using the copper-ion-selective electrode. Microc.hr~n. J. 26, 426643.5 (1981). “Stability Constants of Metal-Ion Complexes,” Suppl. 1. Mat-tell. A. E. (compiler), Part II. “Organic. Including Macromolecule, Ligands.” The Chemical Society. London, 1971. Perrin, D. D. (compiler). “‘Stability Constants of Metal-Ion Complexes,” Part B, “Organic Ligands.” Pergamon, Oxford, 1979. Selig, W., Evaluation of toluene as a replacement for benzene in tetrabutylammonium hydroxide titrant. Microchr,rr. .I. 23, 4666468 (1978). Selig, W., “Ion-Selective Electrodes in Organic Elemental and Functional Group Analysis: A Review (1975 to 1978),” pp. 166 18. Lawrence Livermore National Laboratory Report UCRL-52393, Suppl. I, 1978. Selig, W., The Analysis of Composites Containing Ethylenediamine Dinitrate, Ammonium Nitrate, and Hexahydro-1,3,5-trinitro-l,3,5-triazine (RDX).” Lawrence Livermore National Laboratory Report UCID-18934, 1981. Selig, W., Microdetermination of aliphatic diamines and polyamines using cupric ion. Microchrm. J. 27, 102% 111 (1982). Sillen, L. G. (compiler), “Stability Constants of Metal-Ion Complexes,” Section 11, “Organic Ligands.” The Chemical Society, London, 1964.