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Vol. 67
NOTES
which leads to a small increase in A@ for other I a compounds. The changes in chemical shift (A@’) of the @’-aromatic protons in I a compounds are unrelated to the corresponding A@-values. a-Substituents -0Ac
STABILITY CONSTANTS AND STRUCTURES OF SOME METAL COMPLEXES WITH Iil/lIDAZQLE DERIVATIVES’” BYA. CHAKRAYORTY A N D F. A. COTTON Department of Chemistry. Massachusetts Institute of Technology, Cambvidge 59, Massachusetts Received J u l y 88,1883
and H
-N
6
n 0
@U
possess large AB-values but, have only small A@’values. a-Substituents -OXO2 and -Br are iiiterniediate in character and have A@‘-valuesabout one quarter of their A@-values. At the other extreme, a-substituents
Recently the structure of the bis-(histidin0)-zinc(I1) molecule has been revealed by single crystal X-ray and (dZ-CaH&aOz)zstudies on (Z-CsH8N202)2 Zn * 2H202a Zn.5H20.2b I n these molecules, the histidino anions are coordinated primarily through the tertiary imidazole nitrogen and a-amino nitrogen forming stable sixmembered rings-the four nitrogens forming a distorted tetrahedral array about the zinc ion. The two carboxyl oxygens approach the zinc closely enough (2.8-2.9 A,) to be considered as loosely coordinated (I).
-*TJ
I1 I
and
JH2Y-
HN-NH
I11
have very small A@-values but appreciable AB’-.c.alues, these large A@’-values being reduced aImost to zero upon protonation of the nitrogen. The a-hydroxy ketone shows further anomalous behavior in that with change of solvent, from carbon tetrachloride to methanol, the A@’-valueincreases from 0.12 to 0.246. In Ib compounds, the @-substituents do not cause large changes in chemical shift of any of the three types of protons under consideration. @-Substituents of the + type -TU”R2 lead to quite appreciable A@’-values,larger than the Ab’-values for @-substituents of type -XRz. This constitutes a marked reversal of their relative effects when a-substituted. All @-substituted compounds investigated had changes in chemical shift of the y’-aromatic protons (Ay’) comparable to the changes in chemical shift of the @‘-aroniatic protons (A@’), and the Ay’-values were in all instances larger than the AT’-values obtained when the substituent was on the a-carbon. The changes in chemical shift of methyl protons within the substituted alkyl grouping can be predicted with good accuracy despite the group being contained within a fairly complex molecule. On the other hand, within the same molecule, the magnitude of long-range effects upon aromatic protons, several carbon atoms removed from the substituent, is dependent upon the molecular geometry rather than the number of separating carbon atoms. The magnitudes of the long-range effects bear no siniple relationship to the magnitudes of the changes in chemical shift of the methyl protons within the substituted alkyl grouping. Acknowledgment.-This work was supported in part by Grant No. 20,149 from the National Science Foundation.
m
HN-N
IV
In view of this result, it would be interesting to determine the trend of stability constants of the complexes of Zn(I1) with a series of ligands related to histidine, vix., histamine (11), @-(4-imidazolyl)-propionicacid (111, abbreviated henceforth as IPA), and imidazole (IT.’). Thus a decrease in stability in going from histidine to histamine (11) complexes might be correlated, a t least partially, with carboxyl coordination (I). A large decrease is anticipated in passing from complexes of I1 to thoqe of 111, due to the absence of the strongly coordinatiiig -NH2 group in I11 and the increased size of the chelate ring. Lastly, should there be a decrease in stability between complexes of I11 and those of IV, it might be presumed to reflect the measurable coordinating ability of carboxyl oxygen. With these points in mind, the present measurements of the stability constants of complexes of the four ligands mentioned above were undertaken. Apart from Zii(II), Cu(I1) and Ni(I1) systems were also studied to see if the same stability order is manifested, thereby indicating (but not proving) the occurrence of structural variations similar to those in the Zn(I1) case. It is to be recognized that the structure of bis-(histidino)-zinc(I1) molecule in aqueous solution may be different from what it is in the crystal, e.g., the weakly coordinating carboxyl oxygens might be displaced by mater molecules giving a more nearly regular octahedral structure. Experimental &Histidine and histamine were chromatographically pure samples. IPA was prepared from histidine hydrochloridea as white crystals, n1.p. 207” dec. (lit., 206-208” dec.). (1) (a) Financial s q p o r t for this work was provided by t h e Sational Institutes of Health. (2) (a) R. Kretzinger, F. A. Cotton, and E. F. Bryan, Acta Ciyst., 16, G5l (1953); (b) M. M. Harding and S. J. Cole, ibid., 16, 643 (1963).
NOTES
Dec., 1963 Anal. Calcd. for CJI8N202: C, 51.42; H, 5.75; N, 20.00. Found: C, 51.08; H, 5.70; N, 19.90. Metal ion solutions were prepared from A.R. grade metal nitrates in demineralized COz-free water. Concentrations were accurately determined by analysis. Carbonate-free NaOH was used. Titrations were carried out potentiometrically in a thermostated bath (25 0.1') under an atmosphere of pure nitrogen. Each solution (100 cc.) was initially 0.001 M in KN03. Changes in concentration upon titration with 0.1 M NaOH were neglected in the calculations. Treatment of Data.-The dissociation constants of the ligands4 were determined by titrating them with standard KaOH ,or HClO, as required. 6 (average number of ligand molecules per metal atom) and pA (A = chelating species) were calculated from titration data following standard procedures,6*8assuming formation of l :l and 1 :2 complexes only. The analysis of IPA-metal ion data was similar to that of glycin-metal ion8 data. From the ii us. pA data the stability constants were calculated using a least squares technique7 whenever feasible.8
+
2879
ligands log K1 (and also log Kz) are in close agreement with the l i t e r a t ~ r e . ~ ~ ~ ~ ~ " - " TABLE I1 Loci K I FOR THE METALCOMPLEXES (Medium, 0.2 M KN03) Metal ion
Histidine
IPA
Histamine
Cu( 11) 10.30 8.62 Ni( 11) Zn( 11) 6.57 a Log Kz = 3.89 (compare, imidazole, 3.52).
9.43 6.83 5.62 histidine,
Imidazole
4.56" 4.15 3.32 3.01 3.15 2.13 8.20; histamine, 6.43;
From Table I1 it is clear that the Irving-Williams stability order, Le., Cu(I1) > Ki(I1) > Zn(I1) is satisfied €or each ligand. The trend of particular interest here, however, is the variation of stability constants for the four ligands with any given metal ion, viz., histidine > histamine >>> IPA > imidazole. This supports the Results and Discussion structural speculations presented earlier. It may be The acidity constants of h i ~ t i d i n e ,h~i. ~ t~a m i n e , ~ ~ ~ ~ that log K1 values of IPA and imidazole are very noted and imidazolegcand stability constants of their comclose for Cu(I1) and Ni(I1); the difference is much plexes with metal ions have been determined larger in the case of Zn(I1). The carboxylate oxygen by potentiometric titrations with glass electrodes. apparently makes a proportionately larger contribuWe have redetermined these so as to refer all our results tion in the Zn(1I) complex than in the Cu(I1) and Nito exactly the same experimental conditions. Further, (11) complexes. It also seems likely from the very the present titrations mere done in complete absence of similar trend in pK1 values for the set of ligands with anions other than perchlorate so that any possible formaeach metal ion that the histamine and histadine coordition of halo or nitrato complexes was completely avoided. nate in a similar way to each of the metal ions. The acidity constants of the ligands are given in Table I. INTERACTION OF CHROMOUS BROMIDE AND TABLE I CHROMOUS CHLORIDE WITH HYDROGEN ACLDITY CONSTANTS OF THE LIGANDS CHLORlDE AND HYDROGEN BROMIDE (Medium, 0.2 M KN03)
Ligand
PKCOOH
PKNH+
Histidine Histamine IPA Imidazole
1.77
6.08 6.20 7.56 7.12
I
.
3.95
..
PKNES+
9.17 9.87
BY Pi. W. GREGORY AND J.T. TRACY Department of Chemnistry, Unzverszty of Washzngton, Seattle, Washzngton Recezved August 6 , 1963
..
..
The values for IPA are reported for the first time; t'he other values are in close agreement with those in the l i t e r a t ~ r e . ~ . ~ bIPA ~ ~ - -undoubtedly ~ exists in the zwitterionic form (111). Its high melting point, extreme solubility in water, and insolubility in less polar solvents all support this. Further, its infrared spectrum has a strong band a t 1,605 cm.-l which is characteristic of the -COO- group.lo The stability constants are collected in Table 11. Unfortunately, precipitation preven.ted any accurate determination of the second stability constants for Ni(11) and Zn(I1) complexes with IPA. All comparisons are therefore limited to log IC, and these are the only values recorded in Table 11. For the other three (3) A. Windaus and W. Vogt, Beitr. Chem. Physiol. Pathol., 11,406 (1908). (4) The value was 0.001 M in 0.2 M KNOs; however, t o obtain reliable values of PILOOOHfor hisitidine and IPA, more concentrated solutions (-0.05 M ) were required. Thanks are due t o Dr. B.R. Rabin for suggestions on this point. ( 5 ) R. Leberman and B. R. Rabin, Trans. Faraday Soc., 56, 1660 (1959). (6) L. E. Maley and D. P. Mellor, Australian J. Sci. Res., A2, 57'9 (1949). (7') H. Irving and H. S.Rossotti, J . Chem. Soc., 3397 (1953). (8) The Cu(I1)-histidine system shows abnormal behaviors and K, and were simply determined from the approximate relations log KI = pA at n = 0.5 andlog Kz = pA a t k = 1.5. (9) J. Bjerrum, G. Sohwarzenbach, and L. G. Sill&, "Stability Constants, Part I," Chemical Society, London, 1957: (a) p. 46; (b) p. 31; (e) p. 11. (10) L. J. Bellamy, "The Infrared Spectra of Complex Molecules," Methuen, London, 1962, p. 162.
CrClz and CrBrz crystallize in different systems.1.2 The arrangement of halogens around Cr(I1) is similar in the two compounds; however, chloride ions bridge chromium ions together in a three dimensional array, similar t o CaC12, whereas the bromide octahedra are interlocked in a sandwich layer-type structure. We have made a limited study of solid mixtures of CrClz and CrBr2 at 500' by following the halogen exchange equilibrium between the corresponding hydrogen halides and the solid phases. If CrBr2 and CrC12 remain virtually pure phases, the equilibrium would be represented by the equation CrBra(s)
+ 2HCl(g) = CrCI2(s) + 2HBr(g)
(1) and at a given temperature the equilibrium composition of the gas phase would remain constant as the over-all composition of the solid is changed from the bromide to the chloride. It was found, however, that the equilibrium composition of the gas phase changed as the reaction progressed. One of the pure solids was placed initially in a ca. 850-ml. vessel and the alternate pure hydrogen halide added at a measured pressure. After varying periods of time, the gas phase was removed by condensation (1) J. W. Tracy, N. W. Gregory, E. C. Lingafelter, J. D. Dunitz, H.-C. Mez, R. E. Rundle, C. Sheringer, H. L. Yakel, and M. K. Wilkinson, Acta Cryst., 14, 927 (1961). (2) J. W . Tracy, N. W. Gregory, and E. C. Lingafelter, ibzd., 16, 672 (1962).
