Electrochemical Reduction of Some Tetrapyridylporphins Bruce P. Neri and George S. Wilson' Department of Chemistry, University of Arizona, Tucson, Ariz. 85721 The electrochemical reductions of meso-tetra(2pyridy1)porphin and (3-pyridy1)porphin in 1M HCI were investigated. The 2-pyridyl derivative is reduced in two reversible one-electron steps. The 3-pyridyl derivative is reduced by a single one-electron step followed by a disproportionation reaction. If unreacted, the one-electron product can be reduced again by another electron to the two-electron product of the disproportionation reaction. The rate constant of the disproportionation reaction was evaluated. Optical spectra for various porphyrin oxidation states of the various derivatives are presented. These results are compared with those of the 4-pyridyl derivative.
n t-
z W
a
[r
0 3
UNTIL RECENTLY, the study of the oxidation and reduction of metal porphyrins has been directed toward the oxidation state of the metal. Recent studies have indicated that it is possible to reduce and oxidize the porphyrin ring instead of the metal to which it is bound. Clack and Hush ( I ) described successive reversible one-electron reductions of the porphyrin ring of several metal porphyrins to form T anion radicals and dianions. In such cases, the lowest unoccupied porphyrin ring molecular orbitals are in a lower energy state than the metal orbitals. If the pertinent orbitals of the central metal atom are comparable in energy to those of the ring, then both the porphyrin ring and the metal can undergo electron transfer (2). Oxidation of metal porphyrins can result in T cation radicals and dications ( 3 , 4 ) , the oxidative analog of T anion radicals and dianions. In these cases the thermodynamics of porphyrin ring electron transfer is influenced by the particular central metal substituent. Also of inteiest is the effect of ring substituents on the course of porphyrin electron transfer and associated reactions. Previous studies (5) had indicated that rather subtle differences in porphyrin reactivity might be detected by electrochemical means. Since water is a major component of many biological systems, it would be of interest to study porphyrin systems in an aqueous medium. Unfortunately, most porphyrins are not sufficiently soluble in water to carry out an uncomplicated investigation of electron transport (6) and nonaqueous solvents are usually employed. Recently (7) a water soluble porphyrin, the tetra n-methyl derivative of meso-tetra(4-pyridy1)porphin (TpPyCH3P), was synthesized. The electrochemistry of this compound has been previously characterized (5). These results will be compared with the meso-tetra(3-pyridy1)porphin and (2-pyridy1)-
I
I
I
I
I
+06 t05 104 t03 t 0 2 t O l 0 0 -01 -02 -03 E vs Ag/AgCL
Figure 1. Cyclic voltammetry of TmPyP diacid. and gold electrode lMHCI, 5 X 10-4MP(0)H42+, A . Scan rate = 0.0176 V/sec B . Scan rate = 0.0176 V/sec C. Scan rate = 0.0016 Visec
porphin (8) which are the subject of the present study. By studying substituent effects on the course of porphyrin reactions, better understanding of enzyme-substrate interactions in biological systems may result. EXPERIMENTAL
(1) D. W. Clack and N. S. Hush, J . Amer. Chem. SOC.,87, 4238 (1965). (2) A. Wolberg and J. J. Manassen, ibid., 92, 2982 (1970).
Apparatus. Polarography and cyclic voltammetric data were obtained using the apparatus described previously (5). All potential measurements were made with respect to a Ag/AgCl electrode in saturated NaCl(5). Reagents. All reagents employed were reagent grade and were made up in distilled water. Meso-tetra(4-pyridyl), (3pyridyl), and (2-pyridy1)porphin were synthesized by a modification of the procedure of Adler et al. (9). All three structures were verified with either NMR or IR. Procedure. Near infrared and visible spectra were obtained in a Cary 14R spectrophotometer used in conjunction with a thin-layer cell (8, IO). EPR spectra were taken as described previously (11). All rapid scan spectrophotometric measurements were made on a rapid scan spectrophotometer
(3) J. Fajer, D. C. Borg, A. Forman, D. Dolphin, and R. H. Felton, J . ibid., p 3451. (4) D. Dolphin, A. Forman, D. C. Borg, J. Fajer, and R. H. Felton, Proc. Nat. Acad. Sci., (US.), 68,614 (1971). (5) B. P. Neri and G. S. Wilson, ANALCHEM.,44, 1002 (1972). (6) A. Ricci, S. Pinamonti, and V. Bellavita, Ric. Sei., 30, 2497 (1960). (7) P. Hambright and E. B. Fleischer. Inorg. Chem., 9, 1757 (1970).
(8) A. D. Adler, New England Institute, Ridgefield, Conn. 06877, private communication, 1972. (9) A. D. Adler, F. R . Longo, J. D. Finarelli, J. Goldmacher. J. Assour, and L. Korsakoff, J . Org. Chem., 32,476 (1967). (10) G. Peychal-Heiling and G. S. Wilson, ANAL. CHEM.,43, 550 (1971). (1 1) Ibid., p. 545.
To whom communications concerning this paper should be addressed.
442
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
IO
Figure 2. Spectra of TmPyP diacid and its reduction products in 1M HCl. -1.0 X 10-3MTmPyP, thin-layer cell P(O)H4'+ (-), P( -II)H6+ (-O-O-O-)
(RSS) commercially available from Harrick Scientific Corporation, similar in design to that previously described (12). All measurements were carried out in 1M HC1 so that each derivative existed as the porphyrin diacid P(0)H42+.
Table I. Spectral Data for TmPyP Diacid 1 X 10-3A4, TmPyP (thin-layer cell)
Species
RESULTS AND DISCUSSION
P(O)H4*+
Reduction Mechanism of TmPyP. The cyclic voltammetric measurements of TmPyP diacid are shown in Figure 1. It will be shown subsequently that the first wave at -0.1 V represents a one-electron reduction whose product disproportionates to the diacid P(0)H42+ and the phlorin monocation P( -II)H5+. The following set of reactions demonstrates this sequence of events.
P( --II)H;+
, , ,A
434 585 633 465 745
e x 10-3 (M-1 cm-1) 306 10 17.5 40 12
Conditions lMHC1 1M HC1
The reversibility of Reaction 1 was verified at a very fast scan rate. If the scan rate is slow enough (Figure IC), the second reduction wave decreases in relative height while the first reduction wave increases at the expense of the second wave, because most of the P(-I)H4+. has been removed by the disproportionation reaction and there is a need for two electrons to form P( - II)H5+. If the scan rate is increased (Figure lA), both the first and second waves are observed because there is not sufficient time for P(-I)H4+. to dispro-
portionate. The product formed as the result of the first wave reduction and subsequent disproportionation is oxidized at the same potential as the product formed from the second wave reduction. This indicates that P(-II)H,+ can be produced from either the first or second wave reductions (Figures 1A and 1B). Thin-layer coulometry gave a value of two electrons for both reduction steps. This indicates again that the two electron product P(-II)H5+ can be produced at either the first or second steps by the mechanisms mentioned previously. The thin-layer spectrometry results are given in Table I and Figure 2 . P(0)H42+gives a spectrum indicative of most meso substituted diacids (13). The spectrum attributed to P(-II)Hj+ is similar to that for the phlorin monocation of TpPyP (5). The location of maximum absorbance and the molar absorptivities of the two phlorin monocations are very similar. If the current is turned off during the reduction of P(O)H4'+ at the first wave potential, it is possible to observe an increase
(12) J. W. Strojek, G. A. Gruver, and T. Kuwana, ANAL.CHEM. 41,481 (1969).
(13) A. Stone and E. B. Fleischer, J . Amer. Chem. SOC.,90, 2735 (1968).
+ le- J_ P(-I)H4+. H' +P(0)Hd2+ + P(-II)HS+
P(0)Hb2+ 2P(-I)H4+*
(1) (2)
The first wave one-electron product P(-I)H4+., if unreacted can be reduced irreversibly at -0.23 volt by another one-electron step to produce P(-II)Hj+.
+ 1e- HT, P( -II)H
P( -I)H~+.
5+
(3)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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0.8
Figure 3. Effect of disproportionation rate on porphyrin diacid reduction 1. k0 = 11. mole-' sec-l 2. k z = 10 1. mole-' sec-1 3. kz = 100 1. mole-' sec-1 4. k z = 1000 1. mole-' sec-1 5. k 2 = 10.000 1. mole-' sec-1 Open circles represent data points on theoretical curve 4
IO
20
40
30
TIME
0
ISEC)
m
* 03
c W
02
5
01
0 WAVELENGTH
INMI
Figure 5. Spectra of ToPyP diacid and its reduction products in 1MHCl. -1 X 10-aMToPyP, thin-layer cell to3
to2
t0'1 00 ~vsA . ~ A ~ C P
(P-II)Ha .-. (-O-O-O-) -), P(0)Hd2+(--), P( -I)Hd+ (-.-
-01
Figure 4. Cyclic voltammetry of ToPyP diacid. 1 M HCl, 1 X 10-8M ToPyP, scan rate = 0.09 V/sec, and gold electrode
in the Soret band of P(0)Hb2+. This indicates that it is possible to form P(0)H42+from the one-electron product which is what is predicted by the disproportionation reaction. The absorbance of the diacid at 434 nm (Soret band) was followed during reduction at -0.14 volt. At this potential, the absorbance does not decrease in a simple first-order manner because of the regeneration of the parent compound. If the diacid is reduced at the potential of the second step (-0.25 volt), the Soret band does decrease in a simple first-order manner as expected. Digital simulation of a first-order reaction followed by a disproportionation was carried out. The following set of reactions was used. A L B 2B&A+C The electrochemical step, Reaction 4, was taken as a firstorder reaction because during the time scale of these studies the thin-layer cell reduction is best described by a controlled potential electrolysis model (14). Since the thin-layer cell
has such a small volume, there is not sufficient amount of unreduced porphyrin after one second of reduction for the Cottrell equation to be upheld. Also, the potential is sufficiently negative to drive Reaction 4 quantitatively to the right. The concentration of A was calculated as a function of time. The first-order controlled potential electrolysis rate was determined from the reduction of the diacid at the second wave potentials and from previous reductions of TpPyP diacid. The kz value was then varied so that the theoretical curve could be made to match the experimental curve (Figure 3). A value of lo3 1. mole-' sec-l gave the best fit for the rate of the disproportionation reaction. Several other possible reaction mechanisms, including a reduction followed by a simple chemical reaction and a reduction followed by a reversible disproportionation were examined, but none of these could be made to match the experimental curve. Reduction Mechanism of ToPyP. Cyclic voltammetry of ToPyP indicates a completely reversible system (Figure 4). The 60-mV scan rate independent separation between the cathodic and anodic peaks for both the first and second waves indicates that both reductions are one-electron reversible processes. The following reactions indicate the process that occurs for each reduction.
(14) B. P. Neri, Ph.D. Thesis, The University of Arizona, Tucson, A r k , 1972. 444
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
P(0)H42+
+ le- J_ P(-I)H4+.
(6)
Table 11. Thin-Layer Coulometry of ToPyP 1 x 1 0 - 3 ~T ~ P Y PI ,M HCI Reduction Oxidation From To 7am From To P(0)Hd2+ 1.01 P(-I)Hd+. P(O)Hd*+ P(-I)Hd'. P(O)H&Z+ P(--II)HI? 2.10 P(-II)HI? P(O)Hd*+ P(--II)Ha? 1.05 P(-II)H,? P(-I)Ha+. P(-I)Hd'.
Table 111. Spectral Data for ToPyP Diacid 1 x ~O-~MTOP~P ?hPP
0.98 2.03
1.00
Thin-layer coulometry measurements are summarized in Table 11. Reaction 6 was studied by controlled potential electrolysis at +0.11 V while Reaction 7 was measured at +0.01 V. The results are consistent with those of cyclic voltammetry. Figure 5 as well as Table I11 indicates the spectral characteristics of the species observed. The spectrum for P(0)Ha2+ is similar to those of other diacid porphyrins (13). A stable one-electron reduction product of a porphyrin in aqueous acid solution has never been observed to our knowledge. The spectrum attributed to P( -I)H4+. is assigned on the basis of the thin-layer coulometry and EPR results. The spectrum of the two-electron product is similar to previously observed phlorin monocations (5). The spectrum has the distinctive absorbance in the near-infrared region, but in the Soret region there are two bands instead of the usual single band. Therefore, from these results it is not possible to establish whether this spectrum can be attributed to a phlorin monocation, but at least the spectrum must be due to some sort of phlorin derivative. It was possible to obtain the EPR spectrum of the oneelectron product. The spectrum has a line width of 15 gauss but shows no hyperfine structure. Reduction Mechanism of TpPyP. The reduction mechanism of TpPyP in 1 M HC1 was described previously (5). TpPyP is reduced in a single two-electron step (0.07 V) followed by a chemical step to produce a phlorin monocation. The product following the reduction is believed to be an isophlorin where the exterior of the porphyrin ring is completely conjugated. CONCLUSIONS
The electrochemical and spectral properties of the three derivatives are given in Table IV. The reduction potentials for both the one- and two-electron steps are in the order of ToPyP < TpPyP < TmPyP. The ortho derivative is the easiest to reduce, while the meta derivative is the most difficult to reduce. This same order is also observed in the molar absorptivities of the Soret band and the a band of the diacid spectra. The separation between the CY and p bands of the diacid spectra are also in this order. The significance of these results is not yet obvious, however, Gouterman (15) has indicated that both molar absorptivities and positions of bands in the spectra of various porphyrins are largely dependent on the substituents on the ring. Perhaps the observed order of the three derivatives could be a function of electron energies of the porphyrin ring caused by (15) M. Gouterman, J . Chenz. Phys., 30,1129 (1959).
Species P(0)Ha'+ P( --I)Ha+.
P( -1I)Ha ?
E x 10-3 ( A 4 - I cm-1)
Xm,,
439 585 640 435 480(s) 770 1000 400(s) 440 472 755
186
10
Conditions 1MHC1
7.4 100 24 5 4 36 47 45
1M HCl
1M HC1
12 ~
~~
~
Table IV. Electrochemical and Spectral Properties of the Three Tetra-Pyridylporphin Derivatives Ortho Parae Meta EP
Red. Step 1
+0.11
none
-0.10
$0.01
-0.07
-0.23
439 186
443 254
434 306
640 7
640 15
633
585 10
588
585 10
EP
Red. Step 2 Soret band (diacid) (nm) E X 10-3M-1 cm-l CY band (diacid) (nm) E
X 10-3M-1 cm-1
17
/3 band (diacid)
(nm) E
X 10-3 M-1 cm-1
a
See Reference ( 5 ) .
11
the various pyridine substitutions. The pyridine rings could affect the electron energies of the ring either from resonance stabilization or from electrostatic effects that are caused by the solvation of the pyridinium ion. The resonance stabilization of the negatively charged porphyrin ring could occur either through direct conjugation of the pyridine ring with the porphyrin ring through the bridge position or direct interaction of the positively charged pyridinium ion and one of the pyrrole ring peripheral positions. If the porphyrin ring is poorly solvated, the proximity of the positively charged pyridinium ion to the porphyrin ring could lower the electron energies of the porphyrin ring sufficiently to allow for the stabilization of the negative charge. This difference in electron energies caused by the pyridine ring could also account for the entirely different reduction mechanism of the three derivatives. Further investigations are necessary before any definite conclusions in this area can be made. ACKNOWLEDGMENT
We would like to thank A. D. Adler for his generous gift of the pyridyl porphins used in this study. RECEIVED for review September 12, 1972. Accepted November 6,1972. This work was supported in part by the National Science Foundation (GP 28051) and the Office of Naval Research.
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