METALLO-TETRAPHENYLPORPHINE-PYRIDINE COMPLEXES
hug. 20, 1952
[CONTRIBUTION FROM
THE
3977
DEPARTMENT O F CHEMISTRY O F WASHINGTON UNIVERSITY]
Pyridinate Complexes of Some Metallo-derivatives of Tetraphenylporphine and Tetraphenylchlorinl BY J. R. MILLERAND G. D. DOROUGH RECEIVEDMARCH3, 1952 By the use of spectrophotonletric techniques, it is shown that the zinc, cadmium, mercury and copper derivatives of tetraphenylporphine and the zinc derivative of tetraphenylchlorin form monopyridinate complexes. The magnesium derivatives of tetraphenylporphine and tetraphenylchlorin are best interpreted as forming dipyridinate complexes. The equilibrium constants, changes in heat content, and standard entropy changes for these reactions are determined. The results illustrate the primary dependence of metalloporphyrin complexing with base substances upon the nature of the central metal substituent.
The ability of iron metalloporphins2 to form complexes with a variety of nitrogen bases is very well known. Numerous other metallo-derivatives of the porphyrin ring systems (see Fig. 1) are also o(
OL
H n
.c
k!
H
pcq
HC
6
to be of this type, and the protein-chlorophyll extracts reported by Anson4 and Smith4 probably contain such bonding. Certainly the interaction of oxygen and ferroheme is a reaction of this type.
C
H
/
\
L
C
H
Fig. 1.-Porphyrin
/
H
13
H
structures: (a) porphine free base; (b) a metalloporphine (divalent metal); ( c ) chlorin free base.
able to complex with basic substances, such as pyridine, but the resulting complexes are observed to vary widely in stability. The free base porphyrins, in which a metal is not present, do not complex with simple nitrogen bases. Complex formation between metalloporphyrins and base substances is thought to be an acid-base reaction in which the metalloporphyrin plays the role of a Lewis acid Plie
HHQ JH3
H
+ : h __ 3 r’
PMe
: h
3
(1)
The orbital employed by the electron pair provided by the base is assumed to be a metal orbital in order to account both for the necessity of having a metal present, and for the variations in the extent of the reaction when the central metal is varied. This type of complexing is of considerable biological importance, for most porphyrin systems play significant biochemical roles only as magnesium and iron metallo-derivatives, both of which readily enter into such reactions. Thus part of the bonding of the protein globin to heme is believed (1) Fifth in a series on “Fundamental Properties of Porphyrin Systems ” Previous articles: THIS J O U R N A L , 1 4 , 3939, 4045 (1950); 1 8 , 4315 (1951); 1 4 , 3974 (1952) (2) For t h e system of nomenclature employed, see ibid., 1 8 , 4315 (1951), footnote 2 (3) For a review of much of t h e work which has been done in this field, see Lemberg a n d Legge, “Hematin Compounds a n d Bile Pigments,” Chapt. V, Interscience Publishers, Inc., New York, N. Y., 1949.
This article deals with a spectrophotometric study of the reaction with pyridine of the magnesium, zinc, cadmium, mercury and copper derivatives of CY, p, y,6-tetraphenylporphine, and the magnesium and zinc derivatives of a,P,y,Gtetraphenylchlorin. The spectrographic data permit in most cases the calculation of the number of pyridine molecules which complex with a given metalloporphyrin, as well as the equilibrium constants, changes in heat content, and standard entropy changes associated with pyridinate formation. The use of corresponding porphines and chlorins makes possible a comparison of these two porphyrin systems with respect .to their relative effect on complex formation. Qualitatively, it is easily demonstrated in numerous cases that complex formation of the type under consideration is a reversible r e a ~ t i o n . ~ For example, if successively small amounts of pyridine are added to a M benzene solution of zinc tetraphenylporphine, there occurs with each addition a change in the’absorption spectrum of the zinc porphine until the pyridine concentration reaches about 0.05 M . Increasing the pyridine concentration above 0.05 results in no appreciable change in the absorption spectrum, but decreasing the pyridine concentration by extraction (4) M . L. Anson, Science, 93, 186 (1941); E . L. Smith, ibid., 88,170 (1938). (5) R. L. Liss, M.S. Thesis, Washington University, 1949.
of the ben7ene solution with water results in n rcversa1 of tlic previous cliaiiges, so t Iiat 11 sullicicirf
nietnllopnrltliyriii concen trxtion in each solution is kept thr saiiic. 'lhe optical densities of the soluextractions are performed, the absorption spectrum tions are determined a t any wave lengths where the of the extracted benzene solution is identical to extinction coefficients of the two species differ apthe original benzene solution. The appearance of preciably (the equations become indeterminate the pure benzene solution is quite different from the when ec = e,,j. For a series of such solutions, a set 0.0q5 II/ pyridine solutinii ; the former 15 pink I I I d values fnr ,Y is readily determined by whtracting color, the latter a bluish-red (see Itig. 2 for spectra). the optical density of the solution coiitaiiiing zero pyridine concentration from the optical densities of the other solutions. If the K of the reaction is fairly large, the value of A will also be known, for the optical density will become a constant in t l i v solutioiis containing the higher pyritline coiiwtitratioiis, and this coiistant value will I)? I ) , whic,tl is rrtluiretl for the evaluation of A , L2'1th h J t h A ;ti1(1 .Y known, K may be calculated from eclunt.o:i i~-kj,xnd the equilibrium constant from equati(Jn (2'1. (The value of (Bj in equation (l'j will hc given by B , - I;',Ct, where Bo is the original pyridine concentration of the mixture, and Ct is defined in footnote 6. Thus Ct must be known if Ba and FcCt are comparable in value.) If the K of the reaction is relatively sinall, the value of D,would not be known since even in pure pyridine the metalloporphyrin would not be coni6i)OO 5.iOi I .?Itoo Illetely complexed. The general expression (5) x i i i +%. may then be employed. A plot of l/X against 1/B Fig. S.--Absorption sprctr:i a t 2!).9' : solid liiie, zinc will give a straight line from which the equilibriuiii tetr:ipheriylporIhiiie ill l)eiizcne; dashed liiir, ziiic trtra- constant iiiay be calculated by evaluating the slope phenylporphinc iiioiiol,yri(liiiate ( 1% -11 pvritliiie i i i bell- and intercept. 111this case the value of Ct need /,talle I. not be known. I t should be emphasized that the above equnThe changes in absorption spectra accompanying tions would fail if the uncomplexed metalloporphyconiplex forniatiori inay be utilized to study the reaction quantitatively. Mathematical relations6 rin were dimerized, if there were any association useful in determining the equilibrium constant between the uncomplexed and complexed metallofrom the optical density of an equilibrium mixture porphyrins, or if the reaction led to a complex other than the monopyridinate. If the equations a t a chosen wave length are fail for the latter reason, the number of base molecules involved in the coniplex may be determined by methods which depend upon the nature of the complexing. For example, if dipyridinate formation should occur in which the constant for the association of the first pyridine is much larger than for the second, the problem may be treated as two (5j nionopyridinate problems using any of the equawhere K is the equilibrium constant for reaction tions ( 2 ) through ( 3 ) . There are many other pos( I ) , ( B ) is the equilibrium pyridine concentration, sible cases, some of which are complicated by the of more than two porphyrin species in the K is the ratio of complexed to uncomplexed metallo- presence solution. There will be no occasion to treat such porphyrin, and Fc is the fraction of metalloporphyrin complexed. X and A are defined as D, - Du cases here. Experimental and Dc - D,,respectively, where Dm is the optical Apparatus.-All optical density readings were taken on a density of an equilibrium mixture a t some given Beckman spectrophotometer model DU equipped with a total metalloporphyrin concentration, and Du and specially constructed cell compartment which was thermoDc are the optical densities which that total metal- stated t o within 1 0 . 1 ' by circulating water from a Precision loporphyrin concentration would exhibit if coni- Scientific Constant Temperature Bath So. 66600. The equilibrium mixtures were prepared in reaction cells made plexing were zero or complete, respectively. from 1 cm. square mandrel Pyrex tubing and equipped with These equations are most conveniently applied ground glass caps. The cells were calibrate1 with reference as follows: X series of benzene solutions are pre- to a standard cell so that corrections for transparency at pared in which the pyridine concentration is varied the wave lengths employed and for cell thickness could be (The corrections were of the order of 1 t o 2% of from zero to any necessary value, but the total applied. the observed optical densities. j (6) These relations may be derived by the reader from Beer's D = eC, a n d the condition t h a t C, f C,, = Ct. where D is t h e optical density (log I/Inl of a solution of 1-cm. thickness, c is t h e molar extinction coefficient, C is concentration in moles/liter, and t h e subscripts c, u a n d t refer t o complexed, uncomplexed, a n d total metalloporphyrin, respectively. Similar equations for a more general case dre derived by W. SI Clark. e1 ai.,.I. B i d . Chcm., 135, 561 (1940). law,
Materials.-The metalloporphyrins in benzene solution were prepared by methods given previously.z The pyridine used was Mallinckrodt A.R., redistilled from barium oxide. The benzene employed was redistilled Mallinckrodt A.R. Procedure.--Stock solutions of pyridine in benzene covering a wide range of pyridine concentrations were prepared. T w o ml. each of 3 stock nietalloporphyrin solution a n d an
Aug. 20, 1952
METALLO-TETRAPHENYLPORPHINE-PYRIDINE COMPLEXES
appropriate pyridine solution were pipetted into a Pyres reaction cell. Several reaction mixtures were made up simultaneously, each having the same total metalloporphyrin content, but a different pyridine concentration. Three cells, containing, respectively, pure benzene, the metalloporphyrin in pure benzene, and the metalloporphyrin in an appropriate pyridine concentration, were placed in the cell compartment of the Beckman and allowed to come to temperature equilibrium. Optical density readings were taken and recorded a t previously selected wave lengths where the difference in extinction coefficients of the complexed and uncomplexed metalloporphyrins was large. The cell containing the pyridine was replaced by another of different concentration, and after temperature equilibrium was attained, readings were recorded for the new mixture at the chosen wave lengths. This latter procedure was repeated for all the remaining equilibrium mixtures. Since it was found that the wave length dial setting in relation t o the spectral segment selected by the monochromator changed slightly as heat evolved by the tungsten light warmed up the back plate of the Beckman instrument, it was necessary to set the wave length each time to repeat the original optical density reading of the pure benzene solution, rather than merely to reset the wave lengths indicated on the dial.' The pure benzene solution of the metalloporphyrin was thus always left in the Beckman as a reference solution. The temperatures chosen for the measurements were in the neighborhood of 20, 30 and 40'. AH and AS" Determinations.-AH was calculated in the usual way from the variation of the equilibrium constant with temperature. AS" was calculated from this AH and the A F O determined from the equilibrium constant. Absorption Curves of the Complexed and Uncomplexed Metallop0rphyrins.-The absorption curves were determined with the aid of a Beckman spectrophotometer by taking optical density readings every 50 A. by even fifties, and plotting the results in terms of extinction coefficient
3979
50 40 0
I
2 30 X
20 10
6500
6000
5500
5000
x in A. Fig. 3.-Absorption spectra a t 29.9': solid line, zinc tetraphenylchlorin in benzene; dashed line, zinc tetraphcnylchlorin monopyridinate (1.21 U pyridine in benzene).
1.5 n'
I
2 1.0 X w
5
TABLE I Temperature 29.9', total zinc tetraphenylporphin concentration (C,) 2.91 X 10-6 mole/liter Initial
Equilibrium
pyr1-
ovridine ..
dine
concn.
conc n ,
Bo
I
moles/ liter x 108
0 1240 1.24 0.620 .248 .I24
Ohscrved optical density
1 X Fo FOCt) - xFc(D, - Du) ( X / A ) X loa 1 - F c B D, At 5510 A. 0.647 (D,) 0.000 0.0 .369(A) 1 . 0 .278 ( D c ) 0.808 1.22 .298 .349 3450 .251 .681 0.600 .396 3560 .479 .235 .168 .45A 3560 ,301 ,115 .111 ,586 3740
At 5640 A. 0,000 0.0 .:35X(A) 1 0 .29Y 0 . M 4 1.22 ,2,iZ .704 0.599 ,171 .478 .234 .116 .315 .115
Cl t240
1.24 c;%U
u
,248 .124 0 1240 1 .a4
0.620 .248 .124
(w
(Ba -
6500 Equilibrium constant
6000
5500
x in A. Fig. 4.-Absorption spectra a t 20.3": solid line, cadniiuin tetraphenylporphine in benzene; dashed line, cadmium tetraphenylporphine monopyridinate ( 1.24 M pyridine in benzene).
s 3
X w 4120 3970 3910 4000
At 6050 A. 0.036 (D,) 0.000 0.0 .246(A) 1.0 ,282 0.806 1.22 3410 .198 .234 3730 .691 0.600 .206 ,170 ,472 ,234 3820 ,116 .I52 .115 4030 .078 ,317 .I14 Average K 3780d~200
(a)
(7) Thi3 difficulty, which was most serious in the red region of the spectrum, is corrected by the use of one of the water-cooled lamp housings with which the Beckman Spectrophotometers are now equipped.
5
6000 5500 x in A. Fig. 5.-Absorption spectra a t 30": solid line, mercury tetrapheriylporphine iii briizerie; dashed line, Inercury tetraphenylporphine monopyridinate (6.2 M pyridine in benzene). 6500
against wave length. throbgh 8.
The curves are shown in Figs. 2
Discussion Optical density measurenients on a set of equilibrium mixtures of pyridine and zinc tetraphenylporphine a t 29.9' are given in Table 1. The ap-
TIIDLE
~'YRtDlX;\.IE
Ppridines ill corn. _--
Porphyrin
plrx
XPtRI
l'etrapheiiylporphiiie
11
vA\KI
