1850
ALEXANDER I. POPOV, R A YE. HUMPHREY AND WILLISB. PERSON [CONTRIBUTIOS FROM THE
DEPARTMENT OF
Vol. s2
CHEMISTRY, STATE UNIVERSITY OF IOWA]
Studies on the Chemistry of Halogens and of Polyhalides. XIX. Formation Constants of Halogen Complexes from Infrared Measurements1 BY ALEXANDER 1. POPOV,R A YE. HUMPHREY AND WILLISB. PERSON RECEIVED AUGUST21. 1959 Forination constants of benzene-IC1, dioxane-ICS, metrazole-ICS and pyridine-ICS were determined from the intensities of the infrared absorption bands of these complexes. The values obtained were 0.7 f 0.2, 1.2 f 0.4, 15 f 3 and 51 =t5 , respectively. The formation constant for the first complex compares favorably with t h e value of 0.54 reported in the literature from measurements in the ultraviolet spectral regions. While i t does not seem likely t h a t the use of the infrared spectra of charge-transfer complexes for the deterinination of formation constants will be, in general, preferred over ultraviolet measurements, still, this research illustrates t h a t in certain cases this method can be quite reliable and useful.
Introduction Since the work of Benesi and Hildebrand2 a number of investigations of the formation constants for halogen complexes have been completed using the ultraviolet absorption spectrum of the chargetransfer complex as a measure of the concentration of the ~ o m p l e x . ~Very few such investigations have been made using the infrared absorption ~ p e c t r u m ,although ~ there is nothing in principle which prevents such studies. Since the absorption by the complex in the infrared region is very much weaker than it is in the ultraviolet region, the experimental accuracy with which the formation constant can be determined from the studies in the infrared region leaves much to be desired, thus imposing a practical limitation to the usefulness of such studies. There are occasions when it would be desirable to measure formation constants using the infrared spectrum of the complex. In the course of our studies on the infrared spectra of charge-transfer c o m p l e ~ e s , ~ two - ~ such occasions have arisen. The first case occurs when the absorption by the complex is too far out in the ultraviolet region to be studied easily with conventional spectrometers. This is the case with complexes of iodine cyanide. The other question justifying studies of formation constants in the infrared region of the spectrum has been raised by Orgel and Mulliken.8 They suggest that the weakest charge-transfer complexes are probably not stable complexes in the sense that they are fixed, long-lived chemical entities but t h a t they may be “contact” charge-transfer complexes. They point out that it will be hard to distinguish between these two situations experimentally and that even if measurements in two different spectral regions give the same formation constants, it would not necessarily be a proof of the existence (1) Presented a t t h e 136th Meeting of t h e American Chemical Society, Atlantic C i t y , N. J., Sept. 14, 1969. This is also paper V of t h e series “Infrared Spectra of Charge Transfer Complexes.” (2) H . A. Benesi a n d J. H. Hildebrand, T H I S J O U R N A L , 71, 2703 (1949). (3) See for example t h e studies b y L. J. Andrews a n d R. M. Keefer, ibid., 71, 3044 (1949); 72, 4077, 5039, 5170 (1960); 1 4 , 640, 4500 (1952); 76, 3561 (1953). (4) T h e only one we know of on halogen complexes is t h a t b y D. L. Glusker and H . W. Thompson, J. Chetn. Soc., 471 (1955). ( 5 ) W. B. Peison, R. E . Humphrey, W. A. Deskin a n d A. I. Popov, THISJ O U R N A L , 80, 2049 (1968). ( 1 ; ) W. B. Person, R. E. Humphrey and A. I. Popov, ibid., 81, 273 (1969). (7) W. B. Person, K . E. Erickson and R. E . Buckles, ibid., 82, 29 (19F0). (8) L. E. Orgel and R. S. Mulliken, ibrd., 79, 4839 (1957).
of one definite complex. Only one such comparison has been made previously; Glusker and Thompson4 obtained a value of 0.7 for the formation constant Kf of the dioxane-iodine complex, from infrared measurements, compared with a value of 0.86 from studies of the ultraviolet and visible s p e ~ t r a . ~ ,It ’ ~thus seemed worthwhile to investigate another complex in the infrared spectral region, When we first considered this problem, we had felt that it was important to measure formation constants in different spectral regions for still another reason. I t had seemed reasonable to us to believe that the apparent formation constant one would obtain from a mixture of complexes would be weighted according t o the molar absorptivities of the different complexes in the spectral region being studied. We had expected that the molar absorptivities of the different geometrical configurations of the complex might have about the same values for the ultraviolet region of the spectrum but that these different configurations would have quite different absorptivities in the infrared region. Thus, we thought that the slight differences reported for the formation constant of the dioxane-iodine complex might be a result of the difference in the weighting of Kr and thus provide some evidence in favor of a mixture of complexes with different geometrical configurations. However, the elegant analysis made by Orgel and Mulliken made i t clear that the measured value of Kf was not weighted according to absorptivities but that it would be exactly the same when measured in any spectral region. Thus, i t seemed clear that the difference in values for the formation constant of dioxane-12 must be due to experimental error alone, and we decided to try one more comparison with ultraviolet data. During our studies of IC15 and ICN6 complexes, we have measured a few formation constants using the absorption of the complex in the infrared region t o measure its concentration. From these measurements formation constants could be estimated. Although these are not very accurate, they are of some interest for the reasons cited above. Experimental Part The chemicals used in these studies, the experimental difficulties due t o reactions and the spcctrometer have all been described earlier.s (9) J. A. A . Ketelaar. C. Dzcubas, Eec. Iran. c h i r n . , 71, (IO) L. S. Lilich and 0 . E. grad. Gosud. Uoiv. im. A . A. No. 12, 3 (1953).
Van d e Stoipe, A. Goudsmit and W . 110-1 (19.52).
Presnikova. “Nauchnye Zapiski. Lenin. Zhdanova,” No. 103, Ser. Khim. Nnuk
FORMATION CONSTANTS OF HALOGEN COMPLEXES
April 20, 1960
Formation constants were determined for the benzeneICl, dioxane-ICN, metrazole-ICN and pyridine-ICN complexes. (Metrazole is the pentamethylenetetrazole of ref. 5 . ) For the benzene-IC1 complex, solutions of 0.1 M IC1 were prepared in carbon tetrachloride and with benzene concentrations of 0.5, 1.0, 1.5 and 2.0 M . For each solution the spectrum of the I-CI stretching fundamental a t 355 cm.-' in the complex was obtained using the fastest rate of scan available for the spectrometer in order t o minimize the reaction between IC1 and benzene (or cell windows). I n order t o avoid investing more time in the experiment than the results would justify, the apparent absorbance [log ( To/ T ) " ] was used to determine concentration instead of the integrated area. This procedure is justified in the Discussion. The results at two different wave lengths are given in Table I together with the calculation of the formation constant by the Scott modification" of the BenesiHildebrand equation A.b.D 1 D =A. Kf.a, -t
1851
tion constant, we have assumed t h a t the stronger band at 469 ern.-' was due to the one-to-one dioxane-ICN complex and the formation constant was calculated by the BenesiHildebrand-Scott procedure outlined above. l'arious concentrations of dioxane were used with a 0.1 M solution of iodine cyanide in chloroform. Values used in calculating K f from data at two different wave numbers are presented in Table 11. The plots show a relatively minor scatter similar t o that in Fig. 1. I 3S5 C Y
60
2.157
I
cu"
a,
TABLE I DATAFOR BEXESI-HILDRBRANWSCOTT EQUATION FOR DETERMINATION OF BENZENE-IC1FORMATION CONSTANT Benzene concn.,
M
0.5 1.0 1.5 2.0 2.5
A.
Xa
355 cm.-*
0.200 .300 ,380 .400 .480
0.250 .333 ,395 ,500 ,520
XQ
A. 367 cm. - 1
0.220 ,330 ,420 .450 .520
0.228 ,300 .359 ,444 .480
Krb 0.72 0.75 amc 7.5 7.8 a X = (Bz).b (ICl)/A#. The path length is expressed in mm. b The concentration of IC1 was 0.1 Ad in all solutions. The units of molar absorptivity are 1. moles-' mm.-'. TABLE I1 EQUATION FOR DEDATAFOR BENESI-HILDEBRAND-SCOTT TERMINATION OF DIOXANE-ICYFORMATION CONSTANT Dioxane concn.,
A.
0.4 .6
0.245 .320 .360 .400 .420 .440
M
.8 1.0 1.2 1.4
Krb
Xa
A.
0.163 ,188 .222 .250 286 .319
0.110 .170 .180 ,225 .230 ,250
469 c m . - 1
I
I
lot
460 cm. - 1
Xa
0.366 ,354 .445 ,444 .522 .560
1.55 0.90 amC 6.5 4.0 a X = (Diox).b.(ICN)/A.. The path length is expressed in mm. The concentration of I C N was 0.1 M in all solutions. The units of molar absorptivity are 1. moles-' mm.-'.
Fig. 1.-Benesi-Hildebrand-Scott plot for the benzene-IC1 complex. For the metrazole-ICN complex, the Benesi-HildebrandScott assumption that the complex be weak is not valid and the procedure was therefore modified. We tried to use the spectrum to determine directly the equilibrium values of the concentrations, which then were used to evaluate K f . The concentration of the complex was determined using a calibration curve and the absorbance of the complex. The calibration curve was constructed by measuring the absorbances of varying concentrations of I C N in a solution of 1 M metrazole in benzene. The ICK concentrations were chosen so that the assumption that all the I C K was complexed would be valid. The resulting Beer's law plots at different wave numbers are shown i n Fig. 2. Using this calibration curve, the concentration of the complex could be determined in an unknown mixture. Knowing the initial concentrations of I C N and metrazole, we could then easily calculate the equilibrium concentrations and thus the formation constant. The results for several solutions are shown in Table 111.
TABLE I11 DATAON DETERMINATION OF THE FORMATIOX CONSTANT O F THE METRAZOLE-ICK COMPLEX Orig. met.
0.050 ,075 ,100 ,125 ,150 .175
Concn. moles/l. Complex Free I C N
0.023 ,030 ,043 ,051 .062 ,066
Free met.
450 cm.-' 0.077 0.027 ,064 ,039 .057 ,057 ,049 .074 ,038 .OS8 .034 ,109
A,
0.090 .140 ,170 ,200 .240 ,260 Av. Kr
Kt
10 14 13 14 18 18 15
where A is the initial molar concentration of the acceptor, D is the initial molar concentration of the donor, b is the cell length, A . the absorbance of the complex; K f the formation constant for the complex (concentration units are moles/l.) and a , is the molar absorptivity at this frequency. If the left-hand side of this equation is plotted against D , 442 a straight line is obtained with a slope of l/a, and a n intercept of l/Kfa,. Some estimate of the error can be obtained 0.075 0.050 0.025 0.025 0.070 13 by carrying out the calculations a t different wave numbers. .030 ,070 .075 .045 ,085 9 I n order to illustrate the reliability of this method, using infrared data, we show the Benesi-Hildebrand-Scott plots .057 .loo .043 ,057 .120 13 for the benzene-IC1 complex in Fig. 1. This figure shows .053 ,047 .125 .072 .150 16 that this method can be applied in the infrared region of the .150 .062 ,038 .OS8 .180 19 spectrum, with proper precautions, to give data which are ,175 .068 .032 ,107 .190 20 not too much different from those obtained using ultraviolet spectra. The resulting formation constant is given in Av. Kr 15 Table Xr. The determination of K f for the pyridine-ICN complex For the dioxane-ICS system, two absorption bands are found in the I-C stretching region.6 In studying the forma- was carried out in the same way as for the metrazole complex. The Beer's law calibration is similar to Fig. 2 and (11) R. L. Scott, Rec. t r a y . chirn.. 75, 787 (19%). the results for several solutions are given in Table I V .
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ALEXANDER I. POPOV, RAYE. HUMPHREY AND WILLISB. PERSON
The formation constants thus determined are presented it1 Table 1.. An attempt has been made in each case to deteriiiiiie the range of K i due t o the uncertainty in the esperimental data. These are also indicated in the table. DATAO N
-
TABLE 11. DETERMIXATION OF THE FORMATION COSSTANT O F PYRIDINE-ICNCOMPLEX
Orig. P y
Concn., indes/l.-Free I C N
Complex
7
Free Py
445 c1n. -1 0.046 0,021 .03.5 ,039
0.075 ,100 ,120
0.054 ,065 ,071
O.Oi5
,100 ,120
0.034 .0G4 ,071
430 CII1. --I O.04G 0.021 ,036 .O36 .U29 .010
0.075 ,100 ,120
0.053 ,060 ,067
421 cn1. -1 0.047 0.022 ,010 ,040 ,033 ,053
,089
,019
As
Ki
0.125 ,150 ,165 Av. Ki
56 53 47 52
0.215 56 ,290 6% ,320 47 Av. Kr 55 0.335 49 ,385 48 ,430 40 Av. Ki 46
TADLE X7 FORMATION
Complex
CONSTANTS Ii1
Benzene-IC1 0.; i 0 . 2 Dioxane-ICN 1 . 2 i0 . 4 Metrazole-ICIV 19 f 3 Pyridine-ICK 51 i 5 This work. Concentration in moles/l. urements in the ultraviolet region. Ref. 3. Q
& I#
0.51
From meas-
Discussion From the scatter of the results in the tables, i t would seem t h a t the uncertainty in the formation constants is not unreasonable. Certainly the order of magnitude is well established. Some improvement in accuracy could be attained if more care were taken in measuring initial concentrations, etc., b u t not without a much greater investment in time. \Ye realize t h a t the use of absorbance, rather than integrated absorption, as a nieasure of con-
VOl. 82
centration can be criticized. However, we feel t h a t this procedure is justified for the following reasons. Experimentally, the plots shown in Fig. 2 show t h a t Beer’s law is obeyed quite well for this absorption band. The reason for this can be seen when we realize t h a t the apparent halffor these bands are large intensity widths, h/:, (-15 c n ~ . - I )and ~ ~ the spectral slit width s of the spectrometer is relatively small ( - 3 cm. -1). Thus the ratio AvI/: s is greater than five for all these measurements. If we refer to Ramsay’s paperI2 for the effect of resolution on the band shape, we find that even though the apparent , not equal to the true absorbance, log ( T O I T ) is absorbance log (lo ’1)) the correction factor is very close to 1.00 (about 1.02) and furthermorc it zs constant as log ( T O / T )Lhanges Thus, when AvI/:/A is this large we may expect Beer’s law to hold when absorbance is plotted against concentration. The values for the formation constants of the ICN complexes in Table V are considerably smaller than the corresponding values for IC1 coml 3 as would be expected for this weak acceptor. Perhaps the most important result is the comparison between the value of Ki for the benzeneIC1 complex measured in the infrared region and the value measured in the ultraviolet region of the spectrum. Although our value of 0.7 is somewhat higher than the value of 0.54 reported by Keefer and Sndrews, we believe the agreement between the two numbers is within the experimental error. In fact, there is quite a difference between the concentration of the solutions we have studied and the concentration of the solutions used by Keefer and Sndrews ; there could thus be some difference expected due to the change in the activity coefficients However, we conclude t h a t these constants are the same, as predicted by the equations of Orgel and hlulliken.Y It should be emphasized t h a t these results do not necessarily validate Orgel and Mulliken’s assuniption as to the existence of “contact charge-transfer” complexes IYhile it is true that the Benesi-Hildebrand-Scott treatment of experimental data does not present a distinction between onc complex of definite geometric configuration and a multiplicity of complexes of different configurations, nevertheless, i t seems that other experimental evidence J,6 supports the idea that there is no fundamental difference between the strong and weak halogen complexes. Acknowledgments.-Partial financia! support from the Office of Ordnance Research, U. S. a r m y , and from the Institute of Neurological Diseases and Blindness, P.H.S. (grant R-1095), is gratefully acknowledged. R.E.H. wishes to thank the Graduate College of the State University of Iowa for a research assistantship, and the Eastman Kodak Company for a predoctoral fellowship. I O ~ VCA I r i , Ion A
