May, 1963
]BASICITY IX T.a:-‘!.‘rER
O F Ci,b,y,&TETKA-(4-PYRIDYL)-PORPHIXE
1131
THE BASICITY I N WATER OF ~,P,~,G-TETRA-(~-PYRIDYL) -PORPHINE1 BY EVERLY B. FLEISCHER AND L. E. W E B B ~ George Herbert Jones Laboratory, University of Chicago, Chicago 37, Illinois Received December 1, 1963 The equilibrium constant for the addition of two protons to the free base form of a water-soluble porphyrin, determined by spectrophotometric titration. Spectral data show that the intermediate monocation of this porphyrin is either in low concentration or does not exist in solution. Alkali metal and alkaline earth ions enter into equilibria with the free base form of the porphyrin. Since these ions and hydrogen ions change the porphyrin spectrum in a similar manner, buffer solutions containing alkali metal or alkaline earth ions should not be used for the spectrophotometric determination of porphyrin basicity. a,& y,&tetra-(.l-pyridyl)-porphine, was
The interpretation of data from previous work on the basicities of porphyrin compounds is complicated by the fact that most studies have been carried out in non-aqueous or phase-partioned systems because of the low solubility of porphyrins in water. Studies in aqueous systems have included buffer solutions whose effect on porphyrins was not understood. I n his review of porphyrin ioiiization behavior, Phillips discusses these investigations and some of the difficulties in their interpretation.3 The species involved in the present work are the free base, monocation, and dication as shown in Fig. 1. The terms monocation and dication do not refer to the charge of the entire molecule but only to the positive charge associated with the pyrrole ring nitrogens. The concentration quotients for the dissociation of protons from the pyrrole nitrogen atoms are here defined as k l = c,a/c2 k z = coa/cl
~,~~,d-Tetra-(4-pyridyl)-porphiiie (hereafter denoted as TPyP) was employed in our investigation of porphyrin acid-base equilibria. Aqueous, unbuffered HC1 solutions of TPyP were used in spectrophotometric titrations. The pyridyl groups make this porphyrin soluble in aqueous solutions having a pH lower than about 2.5, and the free base form has the structure shown in Fig. Id, in solutions of this acidity. The hydrogen ions bonded to the pyridine nitrogen atonis would not be expected to dissociate in the pH range of 0 to 2 used in this study. Steric models show that the pyridyl groups cannot be in the plane of the coiijugated ring. Therefore, spectra taken at various concentrations of RCl should not be complicated by changes resulting from interactions of the pyridyl group with the porphyrin ring.
and the thermodynamic equilibrium constants are
a b C Fig. l.---(a) Porphyrin dication; (b) monocation; (e) free base. Side chains are omitted for clarity.
where cg, cl, and c2 refer to the concentrations of the free base, monocation, and dication, respectively, yo, rl, and yz are the activity coefficients of these species, and “a” is the hydrogen ion activity. Molar extinction coefficients will be designated by E , where €0, el, and c2 refer t o the extinctions of the free base, monocation aiid dication, respectively. Almost all of the direct measurements of porphyrin basicities in aqueous solution have been done by Keuberger and Scott, who obtained values for kl and k 2 from spectra which, they said, established the existence of monocations of certain porphyrins in aqueous E O ~ U t i ~ n . ~ However, they used buffer solutions which contained sodium and potassium ions. Since we have found that these ions change the spectrum of the free base form, as shown below, we believe Keuberger aiid Scott’s results to be somewhat questionable. They used porphyrins with carboxylic and sulfonic acid groups attached to the conjugated ring to maintain a desirable water solubility. However, these groups may interact with the ring electronically and cause spectral changes, especially since these groups would be expected to ionize in the pH range used by Neuberger and Scott in their spectrophotometric titrations.
Fig. Id.-Free base form of a,@,y,6-tetra-(4-pyridyl)-porphinein solutions having a pH lower than about 2.5.
(1) This work was supported by a Public Health Service Grant and a Block Fund Grant. (2) National Science Foundation Summer Predoctoral Fellow. (3) J. N. Phillips, Rev. Pure. A p p l . Chem. (Australia), 10, 35 (1960). (4)A. Neuberger and J. J. Scott, Proc. Roy. Soc. (London), A213, 307
Data on the TPyP spectra in various solvents have been 1-ep0rted.j The spectrum of an aqueous HCl solution of TPyP a t a pH of 2.3 is similar to the spec-
(1952).
( 5 ) E. B. Fleischer, I n o r y . Chem., 1, 493 (1962).
1132
EVERLY B. FLEISCHER AND L. E. WEBB
Vol. 67
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Wavelength In m p . Fig. 2.-Soret band spectra of aqueous solutions of ~ i , p , y , ~ tetra-(4-pyridyl)-porphine. Solution pH value“ 1 0.00 f 0.05 2 ‘ 57 3 .85 4 1.03 5 1.22 6 1.62 7 2.30 Spectrum number one is shown as a dotted line because it is believed to be shifted from the isobestic point by the high ionic strength. a The experimental values agreed to within 0.03 pH unit with values calculated assuming the pH in the hydrochloric acid t o be given by pH = -log LZHCI.
trum in organic solvents. This indicates that TPyP exists at this pH in the free base form shown in Fig. IC. Experimental Procedures.-The TPyP was synthesized and purified as described previously.6 Aqueous HC1 solutions of the porphyrin were prepared by adding measured volumes of etandard HC1 to TPyP. Measurements of p H were taken on a Coleman Companion p H meter using standardizing buffer solutions. The spectra were taken in 10.0 mm. matched cells in a Gary Model 14 spectrophotometer. All salts added to the TPyP solutions were Reagent Grade.
Results It was found t.hat aqueous TPyP solutions at a pH of 1.0 followed Beer’s lam for porphyrin concentrations ranging from 4.5 X to 9.0 X M in the to 4.0 X M visible region and from 2.0 X in the Soret region. The Soret band spectra for M TPyP are aqueous HC1 solutions of 2.89 X shown in Fig. 2. The pH of the solutions ranged from 2.3 to 0.0. Since the absorbance at 442 mp reaches a maximum a t a p H of zero, the TPyP is assumed to be all in the acid form, either monocation or dication, a t this pH.
tI-
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PH. Fig. 3.-Log ( D A - D ) / ( D - D B )vs. pH a t two wave lengths for aqueous HC1 solutions of a,p,y,d-tetra-(4-pyridyl)-porphine. The slopes are 2.00 a t 442 mp and 1.75 a t 420 mfi.
Several interpretations of the isobestic point are possible. First, the acidity may not be high enough for the dication to be formed. In this case (DA- D ) / (D - D B ) = co/cl where D A is the absorbance a t a pH of 0.0, D B is the absorbance a t a pH of 2.3, and D is the absorbance at an intermediate pH. But, since (H+) (D.4 - D ) / ( D - DB) varies by a t least a factor of five as the acidity is changed, whereas ( H + ) 2 ( D~ D)/ ( D - DB) is relatively constant, cz must not be equal to zero, and the two protons are adding to the free base with the formation of the porphyrin dication. The only other possible conditions which could result in an isobestic point are: (1) c1 = 0 and eo = €2 a t the isobestic point and ( 2 ) c1 # 0 and eo = €1 = €2 a t the isobestic point. If c1 is equal to zero, (DA - D)/ ( D - DB)= co/cz. Since it was found in preliminary calculations that ( H + ) 2 ( D~ D)/(D- DB)was relatively constant, the hypothesis that c1 = 0 and therefore that the quantity [@A - D)/(D- DB)la2 YO/YZ represented the over-all equilibrium constant K = KlK2 was tested with the results shown in Fig. 3 and Table I. TABLEI CALCULATED pK AND I; VALUESFOR TPYP Nave length, ml*
420 442 Av
.
PK
IC0
4 . 4 x 10-3 6 5 X 5 . 4 x 10-3
1 18 1 09 1 14
“IC = IcllC2.
The linear plot of Fig. 3 can be explained by the log K - log yo/y2 = equation log (co/c2) = 2pH
+
May, 1963
BASICITY IN
+
2pH log IC. If the activity coefficient ratio is constant and CI == 0, a graph of log [(DA - D)/(DDR)]vs. pH should be linear with a slope of 2. The pK is given by the zero intercept on the plH axis. The linear form of the plot obtained with our data, the fact that the slope is nearly 2.0, and the agreement of the pK values a t two different wave lengths all support the hypothesis that c1 is zero or very small. The second possibility for the explanation of the isobestic point, that of equal extinction coefficients of all three species a t the isobestic point can be eliminated because a plot of log [ ( D A - D)/(D- DB)]us. pH would not be linear. Thus, the assumption in accordance with experiment is that the concentration of the intermediate monocation is zero or negligibly small. The hitherto unrecognized effect of buffer solutions and salts added to maintain constant ionic strength, which are commonly used in spectroscopic study of porphyrins, has contributed to much of the confusion of past work. As shown in Fig. 4, lithium, sodium, and potassium ions change the spectra of the free base form of T P y P at a pH of 2 in water to spectra similar to that of the dicatioii acid form. Apparently the alkali metal ions are associating with the porphyrin in the same way as hydrogen ions. Addition of CdCls, JfgCI,, SrClz, or CaClz also changed the spectrum. The results are summarized in Table 11. TABLEI1 MAXIMUM MOLAREXTINCTION COEFFICIENTS OF TPYP SOLCTIONS 1.8 M IN VARIOUSSALTS il HC1 solution a t a p H of 0.0 with no added salt is included for comparison .Idded Absorption max. (e x 10-4) compound
(mr)
€IC1 LiCl XaC1
442 443 442 442 443 445 445 445
IC01 CdClz MgC1,
SrC1, CaClz
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JvATER OF a,@,?,&TErITtA-(4-PYRIDYL)-PORPHINE
(I. mole-' om.-')
2.9 2.0 1.8 1.7 2.3 2.4 2.4 2.5
Discussion The absence of an intermediate monocation might imply a trimolecular reaction with simultaneous addition of two protons. This has been considered unlikely because of electrostatic repul~ion.~A small or zero value of c1 implies that k z is much greater than kl. I n other words, the addition of the first proton creates
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460
440
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Wavelength in rnp.
Fig. 4.-Effect on addition of alkali metal ions on the spectra of TPyP. Spectrum no. 1 is of the free base form a t a pH of 2.0. The other spectra are of the same solution with salt concentrations as follows: (2) 1.8 iV LiC1; (3) 1.8 144 h'aC1; (4) 1.8M KC1; (5) 1.0 M KG1.
an unstable species which is immediately protonated to the dication form. Lack of evidence for a monocation in spectrophotometric titrations has been reported for other porphyrins.6 The present work leads to the following conclusions. First, the pK for the dissociation of two protons froin the dication of TPyP is about 1.1. The low value of the pK is explained by the high positive charge on the molecule. More importaiit than the absolute pK value is the conclusion that the concentration of the monocation of TPyP is either zero or very small, which implies that Jcz is much greater than kl. Finally, attention is directed to the spectral changes of the TPyP caused by the alkali metal and alkaline earth metal ions. The free base porphyrin and these metal ions enter into an equilibrium which, interestingly, is similar to the hydrogen ion equilibrium with the free base. This phenomenon merits further investigation. Acknowledgments.--We wish to thank Mr. Richard Palmer for his helpful assistance in this investigation. S.Aronoff, J . Phys. Chem., 62, 428 (1958). (7) S.Aronoff and C. A. Weast, J . Org. Chem., 6, 650 (1941).
(6)
