The Role of Adduct Formation in the Extraction of Zinc with Substituted 8-Quinolinols Fa-Chun Chou a n d H e n r y Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 85721 Equilibrium distribution ratios have been determined for the extraction of zinc(l1) with 8-quinolinol and its 2-methy1, 4-methy1, 5-chloro, 5-bromo, 5-iodo, and 5nitro analogs between chloroform and water at 2 5 O C over a range of pH and reagent concentration values. From these data, the overall formation constants of the zinc chelates in water, as well as the adduct formation constants in the organic phase, have been evaluated. All of the reagents, except for the 2-methyl- and 5iodo-8-quinolinols, gave rise to self-adduct complexes of w e formula ZnL,.HL. The influence of chelate stability and of reagent basicity on adduct formation probably counteract one another. Pyridine, 2- and 4-methylpyridines, and 2,4,6-trimethylpyridine have been found to enhance the extraction of zinc(l1) into chloroform with either 8-quinolinol or its 2- or 4-methyl analogs. From a quantitative evaluation of extraction equilibrium data, the adduct formation constants of what proved to be 1:l chelate: nitrogen base adducts were determined. The results are consistent with Lewis acid-base concepts, and the special role of steric factors may be observed. Thermogravimetric analysis of solid crystalline adducts complemented the extraction findings.
MUCHWORK HAS BEEN DONE o n the evaluation of such factors as ligand basicity, steric considerations, and metal ion "acidity" in the study of chelate formation from the aquated metal ion and the chelating agent. Very little is known, however, about the factors affecting the formation of a complex from a metal ion some of whose coordinated water molecules have been replaced by a stronger complexing agent-e.g., a chelating agent. One approach to clarification is through a systematic study of adduct formation. In an earlier study on adduct formation using solvent extraction ( I ) , zinc was found t o extract with 8-quinolinol (HOx) in the form of the complex ZnOx2.HOx, termed a selfadduct complex, in which the HOx was believed t o be acting as a monodentate ligand involving the quinoline nitrogen atom. With 4-methyl-8-quinolino1, whose nitrogen is more basic than that of HOx, a more stable self-adduct was formed. In the case of the 2-methyl analog, instead of a self-adduct, the extractable complex was the simple 2 : l chelate. Such behavior indicated the sensitivity of self-adduct formation t o steric influences. This paper represents an extension of our work t o the investigation of electronic influences on selfadduct formation in the 8-quinolinol family of chelating extractants. In an attempt to unravel the relative importance of steric hindrance and other factors in the simple chelate and in the adducting ligand, it was decided to investigate also the effect of some pyridines including those with 2-methyl substituents on the extraction of zinc(I1) with 8-quinolinol and its 2- and 4-methyl analogs.
apparatus built in this laboratory (2) was used in this work. A Beckman DU spectrophotometer with matching 1-cm silica cells was used for spectrophotometric measurements. Materials. 8-Quinolinol, its 2- and 4-methyl analogs were purified as previously reported (1). 5-Nitro-8-quinolinol which was prepared by nitration of the 5-nitroso- derivative ( 3 ) and purified by recrystallization from ethanol-water had a mp of 180' C [Reported ( 3 ) 173"-8" C]. 5-Bromo-8quinolinol was synthesized by bromination of Cu(I1)-8quinolinolate by the method of Prasad et al. (+modified by the use of H2S to remove Cu(I1) from the reaction product-and was recrystallized from ethanol-water and vacuumsublimed; the mp was 124"-125.5" C [Reported (3) 124" C]. 5-Iodo-8-quinolinol (Aldrich Chemical) was recrystallized from ethanol-water; the mp was 119"-121" C. Pyridine (A. R. grade, J. T. Baker) was used without further purification but the picolines (Matheson, Colemsn, and Bell) and collidine (Eastman Kodak) were dried over K O H and fractionally distilled. The anhydrous zinc chelates of 8-quinolinol and 2-methyl-8quinolinol were prepared by precipitating the chelates in a n acetic acid buffer solution using a slight (15%) excess of ethanolic reagent; after overnight digestion, they were collected, washed, and dried at 120" C for 2 hours. The zinc content of these chelates, as well as of the adducts, was determined by EDTA titration (5). The adducts were prepared in a dry box by dissolving the anhydrous chelate in the pyridine solvent and partially evaporating the solvent until the adduct, which separated as a crystalline solid, was formed. The still moist crystals, removed by decantation, were redissolved in a fresh portion of solvent and the process was repeated. The final product was in the solvent until just before use, at which time it was filtered and dried under reduced pressure. Carrier free G5Zn radiotracer solutions ( l O P t o 10-6M) were used in all the chelate extraction experiments. The solutions were buffered and the activities were between and lo-* mc/ml. Acetic acid-sodium acetate buffers were used in the p H range 4 t o 6, and ammonia-ammonium chloride buffer solutions in the p H range 7 to 9. Sodium perchlorate was added to the buffers t o maintain a constant ionic strength a t 0.1. All other chemicals were of A.R. grade and used without further purification. PROCEDURES
Distribution Studies of the Self-Adduct Systems. Ten milliliters of buffered 65Zn solution a t a constant ionic strength (0.1) and 10 ml of reagent solution to 1.9M) in chloroform, were equilibrated by vigorous shaking for 0.5 hr, a time period which was adequate for attainment of equilibrium. The mixture was then allowed to stand for about 1 hour for phase separation. The p H value of the aqueous
EXPERIMENTAL
Apparatus. The apparatus used in the extractions has been previously described ( I ) . A thermogravimetric analysis (1) Fa-Chun Chou, Q. Fernando, and H. Freiser, ANAL.CHEM.,37 361 (1965).
34
ANALYTICAL CHEMISTRY
(2) D. Kingston, Ph.D. Dissertation, University of Arizona, 1966. (3) V. Petrow and B. Sturgeon, J. Chem. SOC.(London), 1954, 570. (4) R. Prasad, H. Coffer, Q. Fernando, and H. Freiser, J . Org. Chem., 30, 1251 (1965). ( 5 ) H. A. Flaschka, "EDTA Titrations," Pergamon Press, London (1959).
V
2, 4, 6-TRIMETHYLPYRIDINE
Table I. K D R Values of Pyridine and Its Methyl Analogs between Chloroform and Water Compound Pyridine 2-Methylpyridine
4-METHYLPYRIDINE
A
PK. (6, 7)
pHi/za
5.2 5.9
4.0 4.0 4.0 4.7
6.1 2,4,6-Trimethylpyridine 7.5 Determined experimentally. b Calculated from: Log KDR = pK. CMethylpyridine
2-METHYLPYRIDINE
Log KDR* 1.2 1.9 2.1
PYRIDINE
2.8
0
- pH,/, (8).
phase after extraction was taken as the equilibrium pH value. After phase separation, equal-volume aliquots of both phases were pipetted out and counted separately. Distribution Studies of the Pyridine Adducts. The distribution ratios of the zinc 8-quinolinol pyridine adduct systems were determined by shaking 10 ml of buffered 65Zn solution at a constant ionic strength (O.l), 10 ml of a solution containing a pyridine (0.006 to 3M), and the 8-quinolinol (10-4 to 1.9M) in chloroform solution, for 0.5 hr, a time period which was adequate for attainment of equilibrium. The mixtures were then allowed to stand for 0.5 to 1 hour for phase separation. After phase separation, equal-volume aliquots of both phases were pipetted out and counted separately, The concentrations of the aliquots and the time for counting were adjusted so that the total counts were between l o 4 and lo5 to minimize the statistical counting errors. The pH value of the aqueous phase after extraction was taken as the equilibrium pH value. Determination of Distribution Coefficients, KDR,for Pyridine and Methylpyridines. The KDRvalues of the pyridines were determined by shaking 10 ml of buffer solution of suitable pH and at a constant ionic strength (0.10) with 10 ml of the reagent solution (5 X 10-4M) in chloroform for 0.5 hr. After phase separation, an aliquot of the organic phase was passed through filter paper to remove small water droplets which might have been present, and was placed in a cuvette for spectrophotometric measurement. The concentrations of the aliquots were adjusted to a n absorbance between 0.3 and 0.5 at about 262 mp (256 mp for 4-methylpyridine). The results are shown in Table I. Thermogravimetric Analysis of the Pyridine Adducts. A sample (up to 50 mg) of the adduct was placed on the pan of the calibrated thermobalance and was heated in a stream (24 cc/min) of dry air (Matheson) at a rate of 100" C per hour from room temperature to 600" C. RESULTS AND DISCUSSION
When a zinc chelate extraction system includes an extractable species ZnQz.aB (where H Q represents an 8-quinolinol, B, a pyridine, and a, the number of B molecules in the complex), the distribution ratio, D,of zinc is given by
I
I
I -2
-3
I
I 0
I
I
-I
LOG ( P I 0
Figure 1. Distribution of pyridine adducts of zinc 8-quinolinate between chloroform and water (8-quinolinol)o = 2.29 X 1 0 M ; pH = 5.52 for pyridine, 2-methylpyridine and 4-methylpyridine systems; pH = 6.40 for 2,4,6trimethylpyridine system the only adduct species of importance in the organic phase is ZnQz.aB:
where K,, = K & ' K D C / K D R Z , and K A D is the equilibrium constant of the organic phase reaction of the simple 2 : l chelate and a moles of the adduct-forming base, B. A plot of log D VS. log [Bl0 at constant [HQl0 and constant pH (see Figures 1-3) gives a curve with two linear portions: (a) when [Bl0-+ 0, log D, = log Kez (b) when [Bl0+
a ,log
Do = log D,
+ 2 log [HQIo + 2 PH
+ log KAD+
(3)
log [Blo (4)
a
From the slope of portion (b), the number of B molecules involved in the adduct, a, is obtained. From the intersection of lines (a) and (b), the adduct formation constant, K A D , is given : log
KAD
=
- log [Blo
For the self-adduct systems, B
= HQ.
(5)
Equation 4 be-
comes
D =
+ [ZnQz BIo + . . . + [ZnQz aB10 [ZnBn2+1 [7n2+l + [ZnQ+l + [ZnQzI + : z [ Z n Q z iB1 + -
I
[ZnQzIo
n=N
z=1
n=l
(1) At the lower pH range, [Zn2+]is the predominant zinccontaining species in the aqueous phase. The following expression can be derived upon the further assumption that (6) A. Cero and J. J. Markhaum, J . Org. Chem., 16, 1835 (1951). (7) K. Clarke and Roth Well, J . Chem. Soc. (London),1960, 1885. (8) G. H. Morrison and H. Freiser, "Solvent Extraction in Analytical Chemistry," Wiley, New York and London (1962).
log Db = log
Kez
-I- (2
a) log [HQIo
+ 2 PH
log KAD
(6) When log D is plotted against log [HQI0 at constant pH values, two straight lines, one of slope 2 at lower reagent concentrations and the other of slope 2 a, at higher reagent concentrations, are observed. The intersection of these two lines gives the value for K A D . The Self-adduct Systems. The formation of 1 :1 adducts has been reported previously (I) for 8-quinolinol and its 4methyl derivative. The same was again observed for its 5-nitro-, 5-chloro-, and 5-bromo- analogs. Following the approach outlined here, we have evaluated the adduct forma-
+
VOL 40, NO. l , JANUARY 1968
0
35
v
2, 4, 6-TRIMETHYLPYRIDINE
1, 2-METHYLPYRIDINE .
V
2, 4, 6-TRIMETHYLPYRIDINE
A
2-METHY LPYRlDl N E
4-h4ETHYLPYRIDINE
v
4-METHYLPYRIDINE
PYRIDINE
0
PYRIDINE
0 0 0
-
s
-
-I
L
-2 -3
-2
-I
0
-4 I 1 -
I
I
I
-2
LOG (PI0
I
I
I
0
-I LOG (P)o
Figure 2. Distribution of pyridine adducts of zinc 4-methyl-8quinolinate between chloroform and water (4-methyl-8-quinolinol)0 = 3.16 X 10-3M; pH = 5.75
tion constant, K A D , characterizing the formation of adduct from the 2 : l chelate and ligand in the organic phase, the overall formation constant of the zinc chelate in the aqueous phase, K , (1, 9), as well as the distribution constant of the chelate, KDc ( I , 9) as shown in Table 11. Also listed are values of the distribution constant of the ligand, K D R , and the sum of its pKa values, used as a measure of basicity. The zinc chelating ability of these ligands showed the expected (10, 11) linear relationship to ligand basicity as can be seen by the plot of 1/2 log K , against the sum of the pK, values shown in Figure 4. The slope of this line is less than unity (0.87) which indicates that for these, and probably for most other members of the 8-quinolinol family, the proton displacement constant (K,K,*), a measure of the ability of the metal ion to compete with the proton for the ligand, increases slightly with decreasing chelate stability. Hence, zinc chelate formation will begin at a lower pH for the 5-nitro- than for the 4-methyl- derivative. Also in Figure 4 is a plot of the adduct formation constant, K A D , GS. the sum of pKaI and pK.,*. The K A D values are seen t o increase only slightly with the reagent basicity. It would seem reasonable to suppose that the Lewis acidity of a metal ion-i.e., its ability t o form complexes-is sensitive to the ligands already in its coordination sphere. On this basis, the metal in more stable chelates should have less tendency t o react further-Le., have less “residual” Lewis acid character. Hence, one would expect a lower K A D for a chelate of higher K , value, provided the adducting ligand is involved. Similarly, for a given chelate, the adduct forming tendency can be expected t o increase with ligand basicity. These two effects must be considered together in self-adduct formation and therefore, as seen in Figure 4, nearly cancel one another. Because 5-nitro-8-quinolinol is the least basic and 4-methyl-8-quinolinol is the most basic, one would expect an adduct of the zinc chelate of the 5-nitro derivative as acceptor with the 4-methyl analog as donor t o have the highest K A D .
Figure 3. Distribution of pyridine adducts of zinc 2methy I-8-quinolinolate between chloroform and water (2-methyld-quinolino1)0 = 3.16 X 10-3M; pH = 5.52 for pyridine, 2-methylpyridine and 4-methylpyridine systems; pH = 6.55 for 2,4,6-trimethylpyridine system Although the failure of the 2-methyl analog t o form a selfadduct zinc complex may be attributed t o adverse steric factors ( I ) , the inability of the 5-iodo-8-quinolinol t o d o so cannot be explained in this manner. Because the 5-nitro compound forms a well-defined self-adduct, the behavior of the 5-iodo-8-quinolinol cannot be explained by its low basicity. To a first approximation, the distribution coefficient K D C is equal to the ratio of the solubilities in one solvent t o that in the other, both solvents being saturated with their counterparts. For an organic reagent as well as its metal chelates, the solubility in the organic solvent generally increases with increasing molecular weight (number of carbon atoms) and decreases with the increasing polarity of the compounds. The reverse is true for its solubility in water. The value of KDccan there-
14
KF
A
1967. (10) M. Calvin and K. W. Wilson, J. Am. Chem. SOC.,67, 2003 (1945). (11) J. Bjerrum, Chem. Reo., 46, 381 (1950).
36
ANALYTICAL CHEMISTRY
K A D , ~ ADDUCT FORMATION CONSTAVT
12 LL y W
IO
4
s a 8
ja 4
4
2
0
6
(9) Fa-Chun Chou, Ph.D. Dissertation, University of Arizona,
OVER-ALL CHELATE FORMATION CONSTANT
8
IO
12
14
16
18
P Ka
Figure 4. Correlation between adduct formation constants (or chelate formation constants) and the basicities of the chelating agents
Table 11. Equilibrium Constants for Zinc Chelates of 8-Quinolinol and Substituted 8-Quinolinols
Parent compound 2-Methyl4Methy'5-Chloro5-Bromo5-IOdO5-Nitro-
2.17
...
2.20 1.95 2.04 ... 1.52
1.41 2.99 (1) 1.97 2.40 3.76 4.00 1.43
fore be expected to increase with the molecular weight of the compound as long as its polarity does not change appreciably. It has been reported by Dyrssen (12) and others (13) that K D R values of a homologous series of organic reagents increase by a factor of 4 per each additional carbon atom. Metal chelates in general have higher distribution coefficients than the reagents, but the increase is much smaller if only from the molecular weight basis. Undoubtedly, the new polar center due t o the presence of metal ion greatly compensates for the large increase in the molecular weight. One would expect a still higher distribution coefficient t o be observed for an adduct because it contains one additional reagent molecule. Unfortunately, the present study does not permit the evaluation of the distribution coefficient of the adduct. Solubility measurements of the adducts in both aqueous and organic solvent media should help solve this problem provided the adduct does not decompose. The distribution coefficients of the chelates of the 8-quinolinol analog, except for 5-nitro- and 2-methyl-8-quinolino1, increase linearly with the molecular weights of the chelates. The KDc value increases by a factor of 2 as the molecular weight of the chelate increases by 20. The K D C value of zinc(I1) 5-nitro-8-quinolinolate is about 20 times smaller than expected on this basis. Probably the effect of the solvation b y water on the large polar nitro group compensates for the large increase in the molecular weight. The same effect appears in the distribution coefficient of the reagent itself. The distribution coefficient of 5-nitro-8quinolinol is essentially the same as the KDR value of the parent compound (IO), even though its molecular weight is much larger. Pyridine Adduct Systems. Applying Equations 3 and 4 to the extraction data, it can be concluded that adduct formation occurs in which l mole of pyridine base is associated with 1 mole of chelate in all of the 12 pyridine adduct systems studied. This can be deduced using Equation 4 from the observation of regions of unit slope in the plots in Figures 1-3. At the higher base concentrations, particularly in the pyridine systems, the slopes of the extraction curves become zero. This change can be related quantitatively t o the formation of significant concentrations of 1: 1 zincpyridine complexes in the aqueous phase and, therefore, further confirms the stoichiometry of the adduct. Using Equation 5, the values of K A D were calculated (Table 111). Whereas in self-adduct systems, the nature of the adduct cannot be varied independently, this variation would be possible in the use of other adducting bases such as pyridines. In relating the values of K A D t o the various factors involved in self-adduct formation, a parallel but more pronounced trend was observed. (12) D. Dyrssen, Summer Symposium, Division of Analytical Chemistry, Tucson (1962). (13) A. J. Fresco and H. Freiser, ANAL.CHEM.,36, 631 (1964).
2.64 (13) 3.22 (13) 3.27 (13) 3.32 (14) 3.51 (14) 3.75 (14) 2.64 (15)
17.06 (1) 15.68 (1) 18.11 (1) 15.58 14.62 14.86 12.14
14.9 (14) 15.9 (14) 15.66 (14) 13.00 (14) 12.80 (14) 11.90 (14) 8.79 (15)
Table 111. Logarithmic Values of Adduct Formation Constants for Various Pyridine Adducts of the Zinc 8-Quinolinolates in Chloroform a t 25" C Adducting base
_Chelating agent 8-Quinolinol 2-MethyI-8-quinolinol 4-Methyl-8-quinolinoI
2,4,6Tri2-Methyl- 4-Methyl- methylPyridine pyridine pyridine pyridine 3.05 1 .60 2.47
2.10 1 .OO
2.00
3.40 1.75 2.87
1.50 0.20 1.50
The adduct formation constant increased with increasing basicity of the donor base. For example, for both the 8-quinolinol and the 4-methyl-8-quinolinol systems, the adduct formation constants of 4-methylpyridine adducts were 0.4 logarithmic unit higher than those of the corresponding pyridine adducts. For the 2-methyl-8-quinolinol system, the corresponding increase was 0.1 5 logarithmic unit. There is a correlation of the adduct formation constants with the chelate stability. This correlation indicates that the general trend is again followed-i.e., the greater the stability of the metal chelate, the smaller the residual Lewis acidity and consequently the less favorable condition for adduct formation. Thus, zinc 4-methyl-8-quinolinol has greater stability than that of 8-quinolinol and as a result, it formed less stable (by 0.5 logarithmic unit) pyridine adducts. A decrease in the adduct stability was observed whenever steric hindrance was encountered. For example, the adduct formation constants for the 2,4,6-trimethylpyridine adducts were lower than those of the corresponding 2-methylpyridine adducts which, in turn were lower than those of pyridine. Likewise, in comparing the chelating ligands, the adduct formation constants for the pyridine adducts of 2-methyl-8quinolinolate were lower than those of the corresponding adducts of 8-quinolinol or 4-methyl-8-quinolinoI chelates. In the 2,4,6-trimethylpyridine and 2-methylquinolinol system, four adjacent methyl groups are present in the 1:l adduct. The resultant crowding would have an adverse effect on adduct formation. Not surprisingly, therefore, the adduct formation constant in this case was found to be very low (of the order of unity). The 1 :1 adduct stoichiometry raises some interesting questions about the nature of these complexes. Zinc(I1) often has a coordination number of six. Because the extrac(14) F. Ashizawa, unpublished data, University of Arizona, Tucson, A r k , 1965. (15) S. Thompson, unpublished data, University of Arizona, Tucson, Ariz., 1965. VOL 40, NO. 1, JANUARY 1968
0
37
Table IV.
Thermogravimetric Analysis for the Solid Pyridine Adducts of Zinc 8-Quinolinols
Compound Zinc 8-quinolinoate anhydrous Zinc 2-methyl-8-quinolinoate, anhydrous Zinc 8-quinolinoate dihydrate Pyridine adduct of zinc 8-quinolinoate 2-Methylpyridine adduct of zinc 8-quinolinolate 2,4,6-Trimethylpyridine adduct of zinc 8-quinolinoate Pyridine adduct of zinc 2-methyl-8quinolinolate 2-methylpyridine adduct of zinc 2-methyl8-quinolinolate 2,4,6-Trimethylpyridine adduct of zinc 2methyl-8-quinolinolate
Empirical formula (from TGA analysis)
38
ANALYTICAL CHEMISTRY
Zinc content By EDTA titration
Theoretical
ZnQt ZnQt ZnQt * ( H 2 0 ) n ZnQt W 1 . 5
18.45 f 0.03 17.20 f 0.06
18.49 17.13
13.75 + 0.05
13.78
ZnQ
17.11 + 0.07
17.13
16.30 rt 0.04
16.60
12.63 + 0.08
12.47
15.79 rt 0.04
15.75
15.45 =!= 0.04
15.12
(B)o.~+
ZnQt ( B h +
tion data in this study yielded information about only five coordination sites around zinc ion, it was decided t o isolate and examine these complexes. An attempt was made t o isolate the adducts directly from the extraction system, which was, however, unsuccessful because of the limited solubility of the adduct in chloroform. Although it is recognized that the composition of the complex in the solid need not be the same as that of the complex in solution, information about the former is relevant. The solid adducts were therefore prepared by crystallization from the pure pyridine solvent and their compositions were determined thermogravimetrically. Zinc chelates of both 8-quinolinol and 2-methyl-8-quinolinol form di-adducts with pyridine in the solid state (Table IV). The decomposition of the pyridine adduct is a two-step reaction, For the pyridine adduct of the zinc 8-quinolinol chelate, these two steps are closely spaced in temperature (86" C and 129" C). For the pyridine adduct of the 2-methyl-8-quinolinol chelate, one adducted pyridine molecule is lost some 34" C before that in the corresponding 8-quinolinol case, a fact which indicates a destabilization by the adjacent methyl groups on the chelate rings. The mono-adduct, however, decomposes a t about the same temperature as the corresponding pyridine adduct. After losing one molecule of pyridine, the adduct is more stable probably because there is a change in the configuration from the strained octahedral structure t o a strain-free square-planar pyramidal structure. With the pyridine adduct of the zinc 8-quinolinol chelate in which no steric hindrance is encountered, the mono- and diadducts have essentially the same thermal stability. 2-Methylpyridine forms a mono-adduct with the zinc 8-quinolinol chelate as well as with the zinc 2-methyl-8-quinolinol chelate. These adducts are much less stable (decomposition occurring a t room temperature) than the corresponding pyridine adduct probably because of the presence of adjacent methyl groups o n the chelate rings. According to the x-ray study (16), zinc 8-quinolinolate dihydrate is octahedral, consisting of two trans-coplanar chelate rings with water molecules located at axial positions. The formation of a di-adduct would just involve the replacement of the axial water molecules by pyridine molecules. Because the size of the 8-quinolinol molecule is sufficiently large, the 2-methyl groups on its nucleus are situated far enough away from the central metal atom that they will not (16) L. L. Merritt, Jr., ANAL.CHEM., 25, 718 (1953).
-
...
interfere appreciably with any axially coordinated base of sufficiently small size, such as a pyridine or water. The presence of a methyl group o n the 2-position of the pyridine ring, however, would greatly interfere with the chelating plane. Hence, an octahedral complex could not form between 2-methylpyridine and zinc 8-quinolinolate. Between pyridine and zinc 2-methyl-8-quinolinolate, a n octahedral adduct complex formed however. With 2,4,6-trimethylpyridine as the adduct base, greater steric hindrance is encountered. As a result, very unstable adducts, which exist only in the presence of a n excess amount of pyridine, can be formed. The nonintegral number of addkcted base molecules suggests that they may not be coordinated directly t o the central metal atom, but are, rather, occluded in the crystal lattice. The formation of a monoadduct in solution, which is confirmed by solvent extraction data and spectral studies, can be attributed t o the presence of a large excess of pyridine which exerts a mass action effect. In some cases-Le., the pyridine and 4-methylpyridine adducts of the zinc chelate of 8-quinolinol and 4-methyl-8quinolinol-the compositions of the pyridine adducts in the solid state are different from those in the extraction system. Of course, the pyridine concentrations employed in the extractions could not be nearly as high as that used in the preparation of solid adducts. Perhaps the sixth cmrdination site is here occupied by water. On the basis of this discussion, it is proposed that in the extraction system, pyridine and 4-methylpyridine adducts of zinc chelates of 8-quinolinol, 2-methyl-, and 4-methyl-8quinolinol are extracted as hexacoordinate complexes with one molecule of water occupying the sixth coordination site ; whereas, with other pyridines containing the steric hindering methyl g r o u p i . e . , 2-methylpyridine and 2,4,6-trimethylpyridine-the extracted species are anhydrous pentacoordinate adducts. In forming a self-adduct (or pyridine and 4-methylpyridine adducts), one of the water molecules is replaced by one 8-quinolinol molecule (or pyridine molecule) and one remains; whereas, in the extraction of an adduct of 2-methylo r 2,4,6-trimethylpyridine, when one molecule of water is replaced by these ligands, the other water molecule is pushed out of the coordination sphere by the sterically hindering methyl groups. A pentacoordinate adduct with pyramidal structure would be a probable structure for the extractable species in the latter case.
From a practical point of view, adduct formation in general significantly enhances the extraction of a metal ion. I n many cases, it also supplies valuable information about the coordination number. The adduct stability is enhanced by an increase in the basicity of the adducting base and a n increase in the residual Lewis acidity of the metal atom-Le., after chelation. In the self-adduct system, these two factors
tend to compensate for each other so that the overall effect is very small (0.63 log unit)over a wide rangeof reagent basicities. RECEIVEDfor review June 29, 1967. Accepted October 27, 1967. Work supported under the financial assistance of the U. S. Atomic Energy Commission, under Contract No. AT(l1-1)-676.
Heats and Entropies of Formation of Metal Chelates of Certai'n 8-Quino1inols, Quin01ine-8-thiols, and 2,4=Pentanedione George Gutnikov' and Henry Freiser Department of Chemistry, The Unicersity of Arizona, Tucson, Ariz. 85721 A simple twin-differential calorimeter was constructed, which i s capable of determining the heats of reactions in dilute solutions. The heats of reaction of certain oxygen- and sulfur-containing ligands of analytical interest with a number of transition and heavy metal ions were measured in 50 v/v% aqueous dioxane at 2 5 O C and 0.1 ionic strength. The ligands studied were 8-quinolino1, 2-methyl- and 4-methyl-8-quinolinoI, 8-quinolinol-5-sulfonic acid, quinoline-8-thiol, 2-methylquinoline-8-thiol, and 2,4-pentanedione; the metal ions included Mn+2, C O + ~Ni+z, , Cu+*, Zn+2, Cd+2, and Pb+2. The heats of chelation for the quinoline-8-thiols show the metal-sulfur bonds to be stronger than metaloxygen bonds, even for Mn+2. The reversal of the usual stability sequence (Ni > Zn) is due to a more favorable entropy change, which was attributed to the formation of a tetrahedral zinc chelate.
UNTILRECENTLY, interpretations of the behavior of chelating agents have been based primarily on the free energies of reaction (AG). Because the free energy is comprised of enthalpy (AH) and entropy (AS), terms which frequently tend to compensate each other in dissociation processes (I), more complete information about the factors governing chelate formation can be gained by determining these terms as well. The recent thermochemical studies of EDTA and its analogs by Reilley el d.(2) and Anderegg (3) demonstrate the type of detailed information provided by such additional thermodynamic data. Thermodynamic data have also been reported for various polyamines (2, 4 ) , but for other types of ligands, especially those containing sulfur and oxygen as donor atoms, the data are sparse. Furthermore, of the few AH and AS values available, most were derived from the temperature coefficients of formation constants. That this method yields less Present address, Universal Oil Products Co., Des Plaines, Ill. (1) D. J. G. Ives and P. D. Marsden, J . Cliem. Soc., 1965, 649. (2) D. L. Wright, J. H. Holloway, and C. N. Reilley, ANAL. CHEM.,37, 884 (1965). (3) G. Anderegg, Helc. Cliim. Acta, 46, 1833, 2813 (1963); 47, 1801 (1964); 48, 1712, 1718, 1722 (1965). (4) M. Ciampolini, P. Paoletti, and L. J. Sacconi, in "Advances in the Chemistry of Coordination Compounds," S. Kirschner, Ed., Macmillan, New York, 1961, p. 303.
accurate results than the direct calorimetric method has been well documented (5). I n addition to the above considerations, the questions of the effect of substituting sulfur for an oxygen donor atom and the effect of substituents on the chelating properties of a ligand prompted us to determine calorimetrically the heats of reaction of certain 8-quinolinols (oxines), their thio analogs (thiooxines), and 2,4-pentanedione (acetylacetone) with divalent transition and heavy metal ions (Mn, Co, Ni, Cu, Zn, Cd, and Pb). Previous AH values for the formation of oxinates had been obtained by the temperature coefficient method (6, 7) and by a calorimetric method (8) in which precipitation might have occurred during reaction. N o AH values were available for the formation of thiooxinates. Those for 2,4-pentanedione, determined by the temperature coefficient method, indicated a more exothermic reaction with Ni(I1) than with Cu(I1) (9). These data seemed unusual and confirmation was deemed necessary. EXPERIMENTAL
Reagents. 8-Quinolinol and 2-methyl-8-quinolinol (Eastman Kodak Co., White Label grade) were recrystallized from aqueous ethanol followed by sublimation. The respective mp were 73.0-74.0" C and 71.5-73.0" C. Reported 72-14" C and 74" C. 4-Methyl-8-quinolinol was synthesized according to (10) and purified as above; mp 140.0-141.5" C. Reported 141" C. 8-Quinolinol-5-sulfonic acid (Eastman Kodak Co., White Label grade) was twice recrystallized from boiling 5% HC1 and once from boiling water. Standard solutions of the sodium salt were prepared from the free acid by titration to the isoelectric pH with standard NaOH. Quinoline-8-thiol (thiooxine) and 2-methylquinoline-%thiol were synthesized according to Kealey and Freiser (11 ) . (5) F. J. C. Rossotti, "Modern Coordination Chemistry," J. Lewis and R. G. Wilkins, Eds., Interscience, New York, 1960, p. 68. (6) W. D. Johnston and H. Freiser, Anal. Cliim. Acta, 11, 201 (1954). (7) E. Uusitalo, Ann. Sci. Fenn., A, (87) (1957). (8) D. Fleischer and H. Freiser, J . Phys. Chem., 63, 260 (1959). (9) R. M. Izatt, W. C. Fernelius, and B. P. Block, Ibid.,59, 235 (1955). (10) J. P. Phillips, L. L. Elbinger, and L. C. Merritt, J . Am. Chem. SOC.,71, 3986 (1949). (11) D. Kealey and H. Freiser, Tuluntu, 13, 1381 (1966). VOL 40, NO. 1, JANUARY 1968
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