RONALD H. ERLICH AND ALEXANDER I. POPOV
338 extremely weak and that fluoride is most satisfactory as an alternative “noncomplexing” medium t o the commonly used perchlorate. p1 values in the literature vary over the range 3-5,24 so that it can be seen again that the expected result of p,(I=O) > ,&’(I = 1.0) > &’(I = 4.0) is obtained for chloride complexes of thallium(1).
Summary of Polarographic Results Polarographic results have indicated that the fluoride complex of thallium(1) could be weaker than the per-
chlorate complex. Consequently, fluoride electrolytes should be better than or at least as good as perchlorate as a “noncomplexing” electrolyte in studies on concentration stability constants of thallium(1) complexes. The use of fluoride as a noncomplexing” medium to maintain constant ionic strength at 1.O in polarographic studies has given values of PI’ = 0.32, 0.65, and 2.1 for the complexes TlC104, T1N03, and TlC1, respectively. At an ionic strength of 4.0, values of PI’ = 0.37 and 1.00 and pz’ = 0.36 for the complexes T1N03, TlCl and TlClz-, respectively, were obtained.
Basicity Constants of Cyclopolymethylelaetetrazoles in Formic Acid Solutions by Ronald H. Erlich and Alexander I. Popov Department of Chemistry, Michigan State University, East Lansing, Michigan 48828
(Received July $1, 1969)
Electrical conductance measurements have been carried out on six cyclopolymethylenetetrazoles varying from trimethylenetetraaole to undecamethylenetetrazole as well as on 6,6’-dichloro- and 6,6’-dibromopentamethylenetetrazoles in formic acid solutions at 25’. Basicity constants defined by the reaction Tz HCOOH TzH+ HCOO- as well as limiting equivalent conductances have been calculated by the Fuoss-Shedlovsky method from the conductance data. It is shown that while the above tetrazoles do not have any detectable proton affinity in aqueous solutions the unsubstituted cyclopolymethylenetetrazoles act as fairly strong monoprotic bases in formic acid solutions. The length of the hydrocarbon chain does not influence the basic strength of the tetrazole ring, but the inductive effect of the halogens essentially divests the ring of its proton affinity.
+
Previous studies on cyclopolymethylenetetrazoles have shown that these compounds can form fairly stable
complexes with transition metal and with halogen^.^ The compounds act as unidentate ligands. It is interesting to note, however, that qualitative studies in aqueous solutions indicate essentially complete absence of proton aEnity1-6 although a claim has been made6 for the preparation of a solid complex P M T . HzS04 (PMT = pentamethylenetetrazole). It has also been shown that the tetrazole ring can be protonated in a strongly protogenic solvent such as formic acid and the pKb value for PMT has been determined in this solvent both by potentiometric6 and by conductometric measurements.’ It should be noted that for the study of very weak bases formic acid is a much better solvent than acetic acid since not only does the former have a greater acidic strength, but also, The Journal of Physical Chemiatry
+
+
owing to its high dielectric constant of 56.1, the formation of ion pairs is minimized. Recently a number of new polymethylenetetrazoles have been synthesized8 with n varying from 3 to 11, as well as two halogenated derivatives of PMT, namely, 6,6‘dichloro- and 6,6’-dibromopentamethylenetetraz01es.~ It was of interest to us to see if the variation in the length of the hydrocarbon chain or the halogen substitution had any influence on the proton affinity of the tetrazoles. (1) A. I. Popov and R. D . Holm, J . Amer. Chem. Soc., 81, 3250 (1959). (2) F. M. D’Itri and A. I. Popov, Inorg. Chem., 5, 1670 (1966); 6 , 597, 1591 (1967). (3) D . M. Bowers and A. I. Popov, ibid., 7, 1594 (1968). (4) A. I. Popov, C. C. Bisi, and M. Craft, J . Amer. Chem. Soc., 80, 6513 (1958). (5) A. Dister, J . Pharm. Belg., 3, 190 (1948). (6) A. I. Popov and J. C. Marshall, J . Tnorg. Nuc2. Chem., 19, 340 (1961). (7) T. C. Wehman and A. I. Popov, J . Phys. Chem., 72, 4031 (1968). (8) F. M. D’Itri and A. I. Popov, J . Amer. Chem. SOC.,90, 6476 (1968). (9) F. M. D’Itri, Ph.D. Thesis, Michigan State University, 1968.
BASICITY CONSTANTS IN CYCLOPOLYMETHYLENETETRAZOLES
Table I : Conductances of Some Cyclopolymethylenetetrazoles in Formic Acid at 25' CsMT
lOaC
CiMT 10sc A
A
4.852 10.26 16.91 22.40 26.91 31.58 6.26 10.63 15.95 21.43 25.28 30.12
31.57" 28.29 25.09 23.15 21.89 20.78 30.72b 28.25 25.41 23.36 22.22 20.99
CaMT lo@
2.790 5.299 8.532 12.37 15.07 17.13 19.65 21.56 24.11 26.33 29.84 3.3019 7.165 10.78 14.30 18.16 21.08 24.32 27.12 31.03 33.25 38.87
6.825 10.59 12.34 18.34 21.39 25.19 29.56 33.69 39.75 47.97 6.209 10.18 14.46 20.00 23.71 26.58 31.04 42.62 51.09
CiMT l0BC A
A
34.06" 33.98 31.92 29.58 28.18 27.26 26.21 25.50 24.66 23.99 23.03 34.6jb 32.89 30.53 28.57 26.79 25.67 24.56 23.76 22.70 22.17 20.99
2.732 6.064 9.457 12.77 18.37 22.23 26.04 31.57 35.02 46.15 2.790 6.274 9.737 13.62 16.94 22.29 25.11 29.32 33.44 43.55
6,6'-Dichloro-CsMT 108C
2.776 7.888 13.18 18.07 24.08 32.00 38.47 47.14
A
0.06446 0.05144 0.05418 0.06252 0.06774 0.07507 0.08066 0.08378
' First run.
42.32" 39.94 38.86 35.76 34.40 32.96 31.50 30.34 28.84 27.16 42.5sb 40.04 37.49 34.78 33.24 32.23 30.81 27.91 26.27
Second run.
32.58" 32.92 30.54 28.72 26.27 24.88 23.70 22.32 21.58 19.62 34.5gb 33.92 31.62 29.35 27.73 25.64 24.69 23.49 22.46 20.47
CsMT
ioac
A
3.628 8.398 14.05 18.98 23.08 27.44 31.14 33.86 38.01 43.23 48.12 4.235 8.295 11.45 15.73 20.50 23.24 27.53 30.32 33.50 36.50 41.64
ioac
39.19" 36.91 33.39 31.00 29.37 27.91 26.84 26.16 25.21 24.15 23.27 37.83b 36.48 33.54 31.32 29.24 28.23 26.87 26.10 25.27 24.58 23.53
CiiMT
5.233 7.794 11.35 13.82 16.54 20.26 22.00 24.48 27.02 29.82 36.42 2.896 4.875 6.872 9.043 11.88 14.07 15.67 17.77 19.96 21.42 28.17
A
29.71" 28.21 26.13 24.88 23.70 22.34 21.77 21.04 20.36 19.69 18.34 29.80b 29.69 28.56 27.30 25.72 24.65 23.96 23.11 22.33 21.84 19.97
6,6'-Dibromo-CsMT 1ov A
2.657 5.308 8.573 11.65 14.34 17.83 20.03
0.09847 0.1254 0.1330 0.1359 0.1438 0.1477 0.1488
339
Experimental Section The solvent (B & A 98--100% formic acid) was allowed to stand 48 hr over anhydrous copper sulfate. It was then slowly vacuum distilled through a l-m Vigreux column at about 20 mm pressure. The retained middle fraction was placed in a 6-1. separatory funnel under nitrogen atmosphere and subjected to a t least four fractional freezings. The pure formic acid obtained in this manner had a freezing point of about 8.5". It was stored frozen until ready for use. The specific conductance of this product was 1 X ohm-' em-*, slightly better than the best literature value.' The cyclopolymethylenetetrazoles were all prepared by the methods of D'Itri.8j9 To remove any possible conducting impurities such as the azide ion, etc., the tetrazoles were dissolved in a minimum amount of 50: 50 water-ethanol solution to which was then added 5 ml of concentrated sulfuric acid and enough 0.3 M KMn04 to completely oxidize the azide as evidenced by the permanent purple color of the excess permanganate. The tetrazoles were extracted into chloroform, evaporated to dryness, and recrystallized several times from the recommended solvent mixture^.^ Since tri- and tetramethylenetetrazole have reasonably high vapor pressure, they were further purified by sublimation under vacuum. Although pentamethylenetetrazole will sublime, it yielded a gray product indicating that some decomposition had taken place; therefore, only recrystallization from ether was used to purify the latter. The remainder of the cyclopolymethylenetetrazoles do not have sufficiently high vapor pressure to allow vacuum sublimation even at temperatures near their melting points. An attempt was also made to measure the basicity constants for 6,6'-dichloro- and 6,6'-dibromopentamethylenetetrazole. These tetrazoles were obtained from Dr. D'Itri of this laboratory and used without further purification. The apparatus and procedures used in this investigation have been described previously.7 I n general, the upper limit of concentration was deter= 3.2 X 10-7D3, mined by the Fuoss equation, C,, where D is the dielectric constant of the solvent, since at high concentrations, the laws of dilute solutions may no longer apply.1° Even for most dilute solutions the specific conductance of the solvent was less than 5% of the specific conductance of the solutions.
Results and Discussion Proton affinity of PMT in aqueous solutions was tested by the following two methods. Distilled water was purged with purified nitrogen until the pH of the solution was 7.00. Enough solid P M T was added to make the solution 0.1 M in this solute. No change in (10) R. M. Fuoss, J . Amer. Chem. SOC.,57, 2604 (1935)
Volume 74, Number 2 January 12,1970
RONALD H. ERLICH AND ALEXANDER I. POPOV
340 Table I1 : Basicity Constants and Limiting Equivalent Conductances a t 25" -Run
7-
---------
Run 2
-1
Tetrszole
PKb
Ao
PKb
CaMT CdMT CsMT CeMT C,MT CiiMT
1.79 f 0.03a 1.74 f 0 . 0 3 1.88 f 0.01 1.82 f 0 . 0 2 1.87 f 0.02 1.91 i 0.03
41.70 f 1.17" 57.33 rt 1.10 55.05 rt 1 . 0 6 45.91 f 0 . 9 8 46.23 i 0 . 9 8 41.86 f 1.04
1.86 f 0 . 0 3 1.76 f 0 . 0 3 1.84 i: 0.01 1.94 rt 0 . 0 3 1.76 f 0.05 1.83 f 0 . 0 3
A0
43.32 rt 1 . 2 3 57.58 5 1 . 5 3 51.87 f 0 . 9 2 48.96 f 1 . 0 3 44.52 f 1.96 40.07 f 0 . 9 3
Standard deviations.
pH was observed. Likewise PMT was added to a dilute solution of acetic acid at pH of 5.00, and again there was no change in the acidity of the solution. Conductance data were obtained for eight cyclopolymethylenetetrazoles in anhydrous formic acid. These data were analyzed by the methods of Fuoss and Shedlovsky'l using a FORTRAN program run on a CDC 3600 computer. The values of equivalent conductance us. concentration are shown in Table I while the values of pKb where Kb is the equilibrium constant for the reaction
Tz
+ HCOOH
TzH+
+ HCOO-; Kb
=
(TzH+)(HCOO-) (T4
and the values of the equivalent conductance at infinite dilution are shown in Table I1 along with the standard deviations for these data. It should be noted that the tetrazoles act as monoprotic bases. The unsubstituted cyclopolymethylenetetraaoles in
The Journal of Physical Chemistry
formic acid solutions behaved as fairly strong electrolytes. The basicity constants shown in Table I1 do not vary appreciably with length of the hydrocarbon chain. On the other hand, halogen substitution drastically decreases the basic strength of the cyclopolymethylenetetraaoles t o such extent that it becomes impossible to measure their basicity constant even in formic acid solutions. In fact, it is seen from Table I that at a given concentration the molar conductance of the dihalo derivatives is nearly three orders of magnitude lower than the conductance of the unsubstituted cyclopolymethylenetetraaoles. Comparison of the conductances of the chloro and the bromo derivatives also indicates the greater electron-withdrawing effect of the former halogens.
Acknowledgment. This work was supported by Research Grant MH-07825 from the Institute of Mental Health. R. H. E. gratefully acknowledges a predoctoral fellowship from the U. S. Public Health Service. (11) R. M. Fuoss and (1949).
T.Shedlovsky, J. Amer. Chem. SOC.,71, 1496
