ligands of high basicity and thus additionally affect electron transfer rates. I t has been shown (36) that complexation of imidazole, a biologically significant ligand, with an iron(II1) porphyrin greatly increases the electron transfer rate constant for the iron(II1)-iron(I1) redox system as compared to an iron(II1) porphyrin with solvent in the axial position.
LITERATURE CITED D. G. Davis and R . F. Martin, J. Am. Chem. Soc., 88, 1365 (1966). R. F. Martin and D. G. Davis, Biochemistry, 7, 3906 (1968). D. G. Davis and D. J. Orgeron, Anal. Chem., 38, 179 (1966). K. M. Kadish and J. Jordan, Anal. Lett., 3, 113 (1970). T. M. Bednarski and J. Jordan, J. Am. Chem. SOC., 86, 5690 (196p). D. W. Clark and N. S. Hush, J. Am. Chem. SOC.,87, 4238 (1965). G. Peychai-Heiiingand G. S. Wilson, Anal. Chem., 43, 550 (1971). (8)R . H. Felton and H. Linschitz, J. Am. Chem. SOC.,88, 4238 (1965). (9) N. E. Tokel, C. P. Keszthelyi, and A. J. Bard, J. Am. Chem. SOC.,94, 4872 (1972). (10) R. H. Felton, G. S. Owen, D. Dolphin, and J. Fajer, J. Am. Chem. SOC.. 93, 6332 (1971). (11) K. M. Kadish, G. Larson, D. Lexa, and M. Momenteaux, J. Am. Chem. SOC.,97, 282 (1975). (12) J. Fajer, D. C. Borg, A. Forman, D. Dolphin, and R. H. Felton, J. Am. Chem. SOC.,92, 3451 (1970). (13) A. Stanienda, Naturwissenschaften, 52, 105 (1965). (14) A. Stanienda and G. Biebl, 2.Phys. Chem., 52, 254 (1967). (15) A. Woiberg and J. Manassen, J. Am. Chem. SOC.,92, 2982 (1970). (16) K. M. Kadish and D. G. Davis, Ann. N. Y. Acad. Sci.. 208, 495 (1973). (17) J. H. Fuhrhop. K. M. Kadish, and D. G. Davis, J. Am. Chem. SOC.,95, 5140 (1973). (1) (2) (3) (4) (5) (6) (7)
(18) D. Lexa. M. Momenteau, and J. Mispelter, Biochim. Biophys. Acta, 338, 151 (1974). (19) D. Lexa, M. Momenteau, J. Mispelter. and J. Lhoste, Bioelectrochem. Bioenerg., 1, 108 (1974). (20) H. A. 0. Hill and K. G. Morallee, J. Am. Chem. SOC.,94, 731 (1972). (21) A. D. Adler. F. R. Longo, J. D. Finarelli. J. Goldmacher, J. Assour, and L. Korsakoff, J. Org. Chem., 32, 476 (1967). (22) A. D. Adler, L. Sklar, F. R. Longo, J. D. Finareiii. and M. C. Finarelli, J. Heterocycl. Chem., 5, 669 (1968). (23) A. D. Adier, F. R. Longo, F. Kampas, and J. Kim, J. lnorg. Nucl. Chem., 32, 2443 (1970). (24) R. P.Van Duyne and C. N. Reiiley, Anal. Chem., 44, 145 (1972). (25) J. L. Huntingtonand D. G. Davis, Chem. instrum., 2, 83 (1969). (26) R. H. Felton and H. Linschitz, J. Am. Chem. SOC.,88, 1113 (1966). (27) I. A. Cohen, D. Ostfeid, and B. Lichenstein, J. Am. Chem. SOC., 94, 4522 (1972). (28) G. J. Handschuh, "Spectral Investigations of Iron(1)-Etioporphyrin(1i) and Related Systems", Ph.D. Dissertation, Johns Hopkins University, Baltimore, Md., 1971. (29). J. E. Falk, "Porphyrins and Metalloporphyrins", Elsevier Publishing Company, New York, 1964. (30) S. B. Brown and I. R. Lantzke, Biochem. J., 115, 279 (1969). (31) S. Giasstone, K. J. Laidler, and J. Eyring, "The Theory of Rate Process", McGraw-Hill, New York, 1941. (32) H. A. Degani and D. Fiat, J. Am. Chem. SOC.,93, 4281 (1972). (33) L. A. Wooten and L. P. Hammett, J. Am. Chem. SOC.,57, 2289 (1935). (34) L. M. Epstein. D. K. Straub, and C. Maricondi, lnorg. Chem., 6, 1720 (1967). (35) L. J. Radonovich, A. Bloom, and J. L. Hoard, J. Am. Chem. SOC., 94, 2073 (1972). (36) L. A. Constant and D. G. Davis, in preparation.
RECEIVEDfor review March 28, 1975. Accepted August 6, 1975. The authors thank the National Science Foundation for financial assistance under Grant GP-42479X.
Electrochemistry of Cobalt Tetraphenylporphyrin in Aprotic
L. A. Truxillo and D. G. Davis Department of Chemistry, University of New Orleans, New Orleans, La 70722
The electrochemistry of cobalt tetraphenylporphyrln has been investigated in dimethylsulfoxide, N,Kdimethylacetamide, N,Kdimethylformanlde, n-butyronitrlle, and dichloromethane by cyclic voltammetry and controlled potential coulometry. In addition to the solvent studies, the number and stability constants of axial ligands were determined. Axial ligands included pyrldine, 4-picoline, and plperidine. Generally it was found that one or two axial ligands complex with Co(lll) porphyrin and that ligands were lost on reduction to Co(ll) or Co(l). Some equilibrium constants for complex formation were calculated from the electrochemical data.
This study of cobalt tetraphenylporphyrin (CoTPP) was undertaken because of the biological importance of cobalamins which have a porphyrin-like structure. Thus it was hoped that the materials investigated here would act as model compounds for cobalamins and especially vitamin BIZ.The use of nonaqueous solvents was selected to avoid adsorption problems as previously found ( 1 ) and to better mimic the in vivo environment in which cobalt compounds must function. CoTPP lends itself to the study of model biological systems since it can be both oxidized and reduced electrochemically. Vitamin B12 has been shown to undergo similar reactions (2-4). All electrochemical studies have been done in nonaqueous solvent in this work. 2260
Stanienda and Biebl ( 5 ) first reported some anodic Ellz's for CoTPP. They reported an irreversible oxidation of the metal before the oxidation of the ring in two one-electron steps. The two final steps produced the T cation and the dication (6). Kadish and Davis ( 7 ) investigated the oxidations and reductions and reported a heterogeneous (eleccm/sec. for the trochemical) rate constant of 5 X Co(II1) to Co(I1) reduction. Co(1I)TPP was oxidized in benzonitrile (8) and a reversible reaction a t +0.52 volts vs. SCE was found. Felton and Linschitz (9) recorded two reductions of CoTPP in DMSO, one a t -0.82 volt and the other a t -1.87 volt vs. SCE. The large difference between these as compared to other metal porphyrins (10) implies a difference in the nature of the reduction process and suggests that the site of primary reduction may be the metal rather than the ring in the first reduction step. A similar reduction was also reported ( 7 ) and was found to be the reduction of Co(1I)TPP to Co(I)TPP, the existence of cobalt(I) porphyrins having been discussed previously (2, 11). The reduction of Co(1I)TPP to Co(1)TPP has also been studied on mercury and platinum ( 2 ) in dimethylformamide. The purpose of this research was to apply electrochemical techniques to determine E1/2's and heterogeneous rate constants for CoTPP and its complexes with axial ligands.
EXPERIMENTAL Reagents. Cobalt(I1) tetraphenylporphyrin [Co(II)TPP] was initially prepared from tetraphenylporphyrin (obtained from Mad
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
River Chemical Co.) by the method of Adler and coworkers (12). Later this complex was made from tetraphenylporphyrin prepared in this lab by another method of Adler and coworkers (13). T h e metal-free complex was successfully purified by column chromatography on alumina by using reagent grade chloroform as an eluting solvent. The cobalt tetraphenylporphyrin was not purified further since its spectral and electrochemical characteristics agreed with a purified sample of CoTPP obtained from an independent source (See Acknowledgment). With regard to spectra, both samples of CoTPP had identical spectra in DMSO, with significant adsorption bands at 528 and 414 nm. Both samples exhibited two electrochemical reductions in DMSO a t potentials which are identical within experiment,al error. On the basis of these two observations, the CoTPP was not purified further. All of the nonaqueous solvents were checked for reduction or oxidation peaks which would interfere with the observation of the cobalt redox couples. If peaks were seen, a purification process was initiated to remove the impurity causing the peak. Dimethylsulfoxide, DMSO, was treated with Type 3A molecular sieve and used without further purification. Butyronitrile, PrCN, was purified by the method of Van Duyne and Reilley (14). The solvent was twice heated a t 7 5 O C for 3 hr with a mixture of Na2C03 and KMn04. After this treatment, it was distilled twice a t reduced pressure and the middle 60% by volume collected each time. The solvent was then stored under a nitrogen atmosphere over alumina activated a t 350 “ C for 3 hr. Dimethylacetamide, DMA, was treated with Type 3A molecular sieve, distilled a t reduced pressure, and as with PrCN, the middle 60x8 collected. Dimethylformamide, DMF, was purified by stirring for 15 minutes with barium oxide and filtering (this procedure being repeated three times). The solvent was then distilled twice at reduced pressure and the middle 60% kept. This solvent was very unstable even if stored under nitrogen, decomposing usually within one or two days. Dichloromethane, CH2C12, was treated with Type 3A molecular sieve and used without further purification. Unless otherwise stated, all of the experiments were carried out under a nitrogen atmosphere. Commercial tank nitrogen was prepurified by bubbling through a vanadous sulfate solution, then in turn through a chromous sulfate solution to remove traces of oxygen. The nitrogen was then washed by passing through a water bath to remove any acid present. I t then was passed through a chamber containing solvent and any volatile ligand present in the solution to be investigated. Finally, it was passed through a calcium chloride drying tube to remove water vapor. T h e tetrabutylammonium salts, specifically the C1-, Br-, I-, BF4-, and Clod- were used without further purification. Prior to their use, they were dried in a vacuum desiccator over P2Oj. Both tetrabutylammonium fluorborate (TBAF, electrometric grade), and tetrabutylammonium iodide (TBAI, polarographic grade) were obtained from Southwestern Analytical Chemicals, Inc. The tetrabutylammonium perchlorate, (TBAP), tetrabutylammonium chloride, (TBAC), and tetrabutylammonium bromide (TBAB) were obtained from the Eastman Kodak Co. Pyridine, 4-methylpyridine, and piperidine were purified by distillation. Apparatus. A specially constructed all-glass cell was used for the cyclic voltammetric work. The cell head is equipped with five ground glass joints us8ed to house the working electrode whose depth in solution could be regulated, the reference electrode which was connected to the solution uia a salt bridge and Luggin capillary, gas inlets to pass nitrogen either through or over the solution. as well as a gas outlet. The auxiliary electrode consisted of a small piece of platinum wire sealed in the bottom of the cell. The working electrode was a horizontal unshielded platinum disk electrode of area 4.9 mm2, and a commercial saturated calomel electrode was used as the reference electrode. T h e cell used for constant potential coulometry in nonaqueous solutions consisted of ii 25-ml beaker, or alternatively, a small cell with a maximum capacity of 10 ml of solution. In those cases where the 25-ml beaker was employed as a reaction vessel, a working electrode consists of ,a platinum wire-mesh electrode. In those cases where the small cell was used, a circular sheet of platinum foil was employed as an electrode. In both cases, a SCE was used as the reference electrode and the auxiliary electrode was constructed from a small square of platinum foil. The SCE was separated from solution by a salt bridge, and the auxiliary electrode was separated by a medium porosity glass frit. Agitation was achieved by bubbling nitrogen through the solution while the electrolysis was in progress. All cyclic voltammetxic measurements were made with a threeelectrode potentiostatic circuit constructed from operational am-
plifiers. This apparatus has been previously described (7, 15, 16). A Wenking Model 61-TR Potentiostat (Brinkmann Instruments, Westbury, Calif.) served as the basic unit in combination with a Type 250 Exact Wave Function Generator. Readout was through a Tektronix Type 564B storage oscilloscope equipped with a Type 2A63 and a Type 3A1 plug-in amplifier. Permanent recordings of the traces were accomplished through the use of a Polaroid camera attached to the oscilloscope. The cleaning of the platinum working electrode was a simple procedure in which the electrode was immersed in dichromate cleaning solution for 15-30 minutes and then rinsed first with water and then finally with acetone and dried. Prior to a cyclic voltammetric run, a potential of about -0.1 volt was imposed on the electrode for about 5 minutes during which time nitrogen was bubbled through the solution. From our experience, this procedure appears to create a reproducible surface on the platinum electrode. IR loss was controlled in the cyclic voltammetric cell through the utilization of a system of “positive feed-back’’ in which a potential, proportional current output was returned to the potential input. This method has been utilized in this laboratory previously (17). In this process, the fraction output being fed back into the input is increased until the amplifier breaks into oscillation and is then backed off slightly from this point. For the experiments involving slow scan rates, a variable capacitor was connected between the output of the current follower and its inverse input in order to minimize high frequency noise a t these scans. Capacitance values less than 0.07 pF were used. Controlled potential coulometry experiments were carried out using an Analytical Instruments Potentiostat with Current Integrator, a Beckman Electroscan 30, or a Princeton Applied Research Model 173 potentiostat/galvanostat equipped with a Model 179 plug-in digital coulometer. All absorption spectra were obtained with a Beckman DB recording spectrophotometer using a Sargent Model SR or SRLG recorder and either 1-mm or 1-cly cells. Procedures. The porphyrin concentration was prepared by direct weighing of the porphyrin with subsequent dilution to the desired volume. The concentration of Co(1I)TPP was 0.4 mM. In preparing a series of solutions for study, a concentrated stock solution was prepared and subsequent dilution t o the desired volume was performed. From the Polaroid pictures, the values of the cathodic and anodic peak potentials and peak currents were measured manually. These data were fed into a PDP-11 or PDP-10 computer using a program devised by Constant (17) which calculates the diffusion coefficient and the heterogeneous rate constants using the method of Nicholson (18). This method involves the measurement of the difference in peak potential and calculation of the rate constants from a working curve. The program then performs the statistical Q test to decide if any value should be discarded and calculates the average of the values retained. The average Do value is then used to calculate the k, value from the peak separation a t each scan rate. The statistical procedure is performed and the average k, is reported. The program also calculates i,/V”* and iJiC for diagnostic purposes (19). In addition to the average value, the program also reports the number retained in the average and the percent standard deviation.
RESULTS AN DISCUSSION Metalloporphyrin without Axial Ligands. All of the electrode reactions involving CoTPP can be characterized as quasireversible in nature since the peak separation &?3 varies with the scan rate ( 1 8 ) for each reaction that is reported. This permitted the determination of heterogeneous electron transfer rate constants. In addition, the values for the current function, (ip)c/V1’2, for these redox couples remain constant in the range of scan rates studied. Finally, the value for the ratio (ip)a/(ip)cis independent of scan rate and remains constant. In the tetraphenylporphyrin complex, the cobalt has an oxidation state of 2+ and is air stable, which may be attributed to the decreased basicity of tetraphenylporphyrin. Two metal redox reactions can take place, specifically an oxidation: Co(I1) Co(II1) and a reduction: Co(I1) Co(1). In addition, t w o ring reductions and two ring oxidations should be observed. Since the effect of axial ligand substitution on the reductions of cobalt(II1) to cobalt(I1)
-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
-
2281
*
-:w 1 -2
c06 04
t0.3
0.0 -0.3 - 0 . 6
-0.9
-1.2
-1.3
Flgure 1. Cyclic voltammogram of CoTPP in DMSO 0.4mM CoTPP, 0.1N TBAP. Scan rate, 0.2 volt/sec
and cobalt(I1) to cobalt(1) in CoTPP is the objective of this study, it was necessary to generate Co(1II)TPP prior to the reduction process. Assignments of the type of electrode process (ring or metal reduction or oxidation) occurring are based on observations by Fuhrhop and coworkers (10) who reported that the potential difference between the first and second ring reductions is 0.42 f 0.05 volts vs. SCE, and the potential difference between the first and second ring oxidations is 0.29 f 0.05 V for a variety of metallooctaethylporphyrins. Dimethysulfoxide appears to be the solvent of choice for observation of the redox reactions of CoTPP. Both metal redox processes as well as the first reduction step for the tetraphenylporphyrin ring were observed. A cyclic voltammogram of CoTPP in DMSO is shown in Figure 1. The first two sets of peaks are attributed to the sequence of reactions 1-3. Co(II)P
(1)
7 Co(1)P'-
(2)
CO(III)P+ Co(I1)P
-
The third set of peaks is due to: Co(1)P-
-1.83
V
[co(I)Pl2-
(3)
The potentials found here agree closely with those previously reported (7, 9). Since both cobalt(I1) and cobalt(II1) can add two additional ligands (20), one above and one below the planar porphyrin to give an octahedral complex, it is assumed that molecules of DMSO are occupying the fifth and sixth positions in these complexes. Cobalt(1) porphyrin has not been extensively studied with axial ligands so that the possibility of DMSO being coordinated to Co(1)TPP or the product of the ring reduction remains to be established. The visible spectrum of Co(1I)TPP in DMSO is characterized by an absorption band a t 528 nm ( t = 24,500) and a Soret band a t 414 nm. This spectrum fits the spectrum for Co(1I)TPP reported by Wolberg and Manassen (21) quite closely. Similar looking spectra were obtained in DMA and DMF. In the solvent DMA, CoTPP exhibits a somewhat unusual wave for the Co(II1)-Co(I1) reduction. Although the waves corresponding to the Co(I1)-Co(1) redox process are well-defined, those due to the Co(II1)-Co(I1) redox are not. See Figure 2. The cathodic wave is very ill-defined, in fact, almost nonexistent, while the anodic wave is well-defined, resembling the redox couple in DMSO. This could be due to irreversibility or possibly a CE mechanism. For reasons which are not completely understood, all observations in this solvent are consistent with the presence 2262
00-02-04 - 0 8 -10 - I 2 -14 -16
Flgure 2. Cyclic voltammogram of CoTPP in DMA 0.4mM CoTPP, 0.1 N TBAP. Scan rate, 0.2 volt/sec
-1.8
E,Volts vs SCE
,
02
E,Volts vs SCE
-4
* ANALYTICAL CHEMISTRY, VOL. 47, NO.
of a very slightly reversible wave for the Co(II1)-Co(I1) reduction. In DMA, as well as the remaining solvents to be discussed, no ring reductions were observed because solvent reduction is easier than ring reduction. The visible spectrum in DMA agrees with that previously reported (22). Cobalt tetraphenylporphyrin behaves similarly in the solvents DMF and PrCN. The Co(II1)-Co(I1) redox couple has large peak separations (-150 mV a t slow scan speeds), but both anodic and cathodic peaks are present. The oxidation of CoTPP in PrCN had previously been shown to be irreversible (5). Such large peak separations were found in the studies of FeTPP in DMA and DMF (17, 23). I t has been reported that, in these solvents, the ferric porphyrin and its anion exists as an ion pair while in DMSO they are dissociated. The larger peak separation for the Co(II1)-Co(I1) redox couple in DMA, DMF, and PrCN could be possibly due to the same phenomenon mentioned above for iron (23). With DMSO, the Co(1II)TPP has two DMSO molecules coordinated, but in these other solvents, it may have one coordinated solvent molecule and the electrolyte anion associates to form an ion pair. The removal of the perchlorate ion during reduction should result in a lower rate constant (causing a larger peak separation to be observed), since the activation energy needed to separate this electrolyte anion from the cobalt atom before reduction is increased. The fact that only one set of peaks near 0 volts is observed regardless of the scan speed indicates that the reactions which involve the removal and acquisition of the perchlorate ion prior to reduction and after oxidation of the cobalt porphyrin are very rapid, and thus separate peaks are not observed. The proposed mechanism is depicted in reaction 4: S
clod
I
I 1
Co(III)P+ + S
I
I
+ e-
Co(II)P
I
+
Clod-
(4)
S Neither ring reductions nor oxidations were observed in PrCN. Table I summarizes the electrochemical half-wave potentials measured for CoTPP in the solvents previously discussed, along with the heterogeneous rate constants of electron transfer. Small differences are seen with respect to half-wave potentials and rate constants observed. The one rate constant worthy of note is that for the Co(II1)-Co(I1) reduction in PrCN. I t is quite small compared to the value in DMSO, lending support to the idea of an associated ion pair in this solvent. Unfortunately, large peak separations and other difficulties are factors which interfere with the determination of rate constants in DMF and DMA solutions. The large peak separations observed in these sol-
13, NOVEMBER 1975
S
Table I. Electrochemical D a t a for CoTPPa in Nonaqueous Solvents Co(III)-C0(II) Solvent
SCE)
E 1;2(Vvs.
DMA PrCN DMSO DMF
k,
X
lo3 ( c m / s e c ) b
...
+0.32 +0.26 -0.06 -0.03
2
0.26 2.1 Co(1I) -Co( I )
,
,
14
12
,
,
,
,
,
,
-12 -16
-20
...
-24
16
DMA DMSO DMF PrCN
-0.84 8.7 -0.85 4.7 -0.87 12 -0.89 7.4 a Porphyrin concentration 0.4mM. The errors in the measurement of k , are on the order of 20%.
vents indicate small rate constants for the Co(II1)-COW) reduction andlor complex reactions. Dichloromethane is the solvent which was used most extensively to study the ring oxidations and the effect of inert electrolyte on the metal oxidation and ring oxidations. A careful study of each redox couple (see Figure 3) indicated that the first oxidation process involves oxidation of the metal. This conclusion is based upon a constant potential coulometry experiment in conjunction with the use of visible spectra (20). The data indicated that Co(III)TPP+ is formed rather than [Co(II)TPP+]+,the product of the first ring oxidation, in the first oxidation step. Subsequent oxidations of the solution a t the two other oxidation potentials seen, resulted in the formation of the ring oxidation products. The large shift in the I1 I11 potential may be due to the absence of a complexing environment. Dichloromethane is a relatively poor coordinating solvent, and less likely to complex cobalt than any of the other solvents used. The additional pairs of waves are caused by ring oxidations. Thus, in dichloromethane with the inert electrolyte TBAP the following oxidations occur:
-
Co(1I)TPP === [Co(III)TPP]' (metal)
(5)
[Co(III)TPP]'
[CO(III)TPP]~'(ring)
(6)
[Co(III)TPP12' F=+
[CO(III)TPP]~'(ring)
(7)
On the basis of these ring oxidations in dichloromethane and the ring reduction in DMSO, the potential difference between the first ring reduction and the first ring oxidation was found to be 2.74 volts which is close to the experimental value reported for free tetraphenylporphyrin (17) and for a series of metallooctaethylporphyrins ( 10). The half-wave potentials and heterogeneous rate constants measured in dichloromethane for the metal and ring oxidations are summarized in Table 11. Large anions should possess the ability to donate electron density to the cobalt atom, thus making it easier to oxidize. This effect should be apparent in the rate constants and oxidation potentials observed with the anions. For the metal oxidation Co(I1)-
I O 0 8 0 6 0 4 0 2 00
E , Volts vs SCE Figure 3. Cyclic voltammogram of CoTPP in dichloromethane 0.4mM CoTPP, 0.1N TBAP. Scan rate, 0.2 volt/sec
Co(III), the trend observed in the potentials is Br- > C1- > Clod- > BF4-. Bromide ion, being very large (ionic radius 1.95 A) ( 2 4 ) , should contribute more electron density than fluorborate which is smaller (ionic radius 1.56 A) ( 2 4 ) ; therefore, bromide ion should have permitted CoTPP to be oxidized more easily than fluorborate ion as can be seen in the oxidation potentials. Chloride and perchlorate ion fall between these two values in the order suggested by the oxidation potentials. A similar behavior is observed with vitamin Blz derivatives where the potentials for the primary reduction waves became more negative as the nucleophilicity of the axial ligand increased ( 4 ) . While the oxidation potentials for CoTPP correlate, the rate constants do not. Even though Br- causes the CoTPP to be oxidized easily, the rate constant is relatively small and close in value to that observed for perchlorate. Comparison of the heterogeneous rate constants for the two metal reductions with the rates for the ring oxidations shows what effect the presence of the electrolyte anion may have on the redox process. Generally, rate constants for the ring oxidations are an order of magnitude faster than reductions, which supports the contention that the anion is somewhat effective in supplying electron density to the metalloporphyrin to ease the oxidation process. The increased rates could be attributed to the ease of removal of an electron from the ring as opposed to the metal. The visible spectrum for Co(1I)TPP follows the pattern set previously by Wolberg and Manassen (21), having little change with electrolyte. The difference in the spectra is seen in the oxidation products, oxidations being accomplished chemically with N-bromosuccinimide. The spectra of these complexes fall into two general categories which have been attributed to the presence of different ground states of the radicals (6, 15). The difference in spectra also suggests interaction of the anion with the oxidation products since it has been shown (26) that in a good coordinating solvent, the donor molecule would displace the coordinated anion to give a bis-solvent complex. This would lead to a situation in which each anion gives identical spectra. On the basis of these spectra, the anions C1-, Br-, and Iexhibit Class 1 spectra and BF4- and C104- exhibit Class 2 spectra. Class 1 spectra are associated with a 2A2u ground
Table 11. Electrochemical D a t a for CoTPPa in Dichloromethane with Various Electrolytes l l e t a l oxidation
F1r.t riny oxidation
S e c o n d n n b oxidation
I o n l C _.
ridis
A
Electrohte
E1 2
k s x lo2 ( c m sec)
E1 2
k S x lo2
(CU
sec)
E1 2
k S x lo2 ( i m iec)
1.67 TBAP -0.75 0.33 +0.91 2.5 +1.07 3.3 1.56 TBAF +0.94 0.94 +1.09 3.7 +1.26 3.5 1.95 TBAB -0.29 0.18 1.81 TBAC +0.74 1.1 a Porphyrin concentration 0 4mM, electrolyte concentration 0 1N TBAP = tetrabutylammonium perchlorate TBAF = tetrabutylammonium fluoborate. TBAB = tetrabutylammonium bromide, and TBAC = tetrabutylammonium chloride
...
...
... ...
...
...
... ...
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2263
~~
Table 111. Electrochemical D a t a for CoTPPO with Added Pyridineb in Nonaqueous Solvents co(lII)-co(lr)
ks x Solvent
E l,z(V vs. SCE)
DMA
-0.3 1 -0.3 7 -0.26 -0.20
PrCNC
DMSO DMF
k
x IO3 ( c r n i s e c )
P)ridine (M/L)
0.58 0.21 1.2 0.40
-1.01 -0.99 -0.93 PrCNC -1.04 a Porphyrin concentration 0.4mM. 1.24M. Ligand concentration 0.86M.
1.7 1.4 1.7 4.5 Ligand concentration
Table IV. Variation of Heterogeneous R a t e Constant for CoTPP" Reductions with Ligand Pyridine in DMSO k g x lo4 ( c m ) s e c ) Pyridine (JWL)
0.0 0.00124 0.00620 0.0124 0.0620 0.124 0.124
co(rIr)-co(II)
21 ... 2.71
:;
3.0 ligands 2.3 3.2 2.1
0.620 8.8 0.620 ligand 1.24 1.24 14 a Porphyrin concentration 0.4mM.
k s x lo3 ( c m l s e c ) Co(I1)-Co(1)
4.7 Zero
3.3
ligands lost
2.3 4.4
1.2
lost
state and Class 2 spectra with a 2Alu ground state. Interestingly the two anions exhibiting Class 2 spectra have rate constants for the second ring oxidation which are about equal in magnitude. Unfortunately, background current from the halogenated tetrabutylammonium salts prevented the observation of ring oxidations with these electrolytes. Pyridine as an Axial Ligand. Upon the addition of pyridine to the CoTPP solution, the redox potentials of both the Co(II1)-Co(I1) and Co(I1)-Co(1) redox couples shifted to more negative values (See Table 111). The shifts observed are larger for the Co(II1)-Co(I1) redox reaction suggesting that the cobalt(II1) complexes are stabilized to a greater degree than the cobalt(I1) complexes. The general shapes of the voltammograms are similar to those obtained without pyridine. Changes in the visible spectra of the complexes are seen in addition to the observed potential shifts. In DMSO, the bands change from 528 and 414 nm with no pyridine to 534, ( t = 18,500) 432, ( t = 43,500) and 410 nm with 1.24M pyridine. For DMA, changes from 531 ( t = 28,500) and 416 nm to 533, ( e = 22,500) 431 sh, and 410 nm are observed. Similar changes are observed in DMF, but no such changes are seen in PrCN which is extraordinary since a complex is indicated by the potential shifts. CoTPP in pure pyridine has bands a t 544, ( e = 19,250) 435, and 412 nm ( t = 42,000) with a shoulder a t 586 nm. This is evidence that pyridine is indeed coordinated to CoTPP. However, the difference in intensity observed for the bands near 435 and 412 nm between pure pyridine and the solvent with added pyridine may indicate that, in the presence of the nonaqueous solvent and ligand, the same number of pyridine ligands may 2264
.
k, x l o 2 (crnjsec)
I
Co(I1)-Co(1)
0.87 )
.
... ... ... ...
0.56 0.46
*..
...
0.124 0.620
One 0'56 ligand 0.65 0.58) lost
2.48 1.24 0
lo3 (crntsec)
Co(1II)- Co(I1)
0.0 0.00124 0.00620 0.00620 0.0124 0.0620
co(rr)-co(r)
DMA DMSO DMF
~
Table V. Variation of Heterogeneous R a t e Constant for CoTPPa Reductions with Ligand Pyridine in DMA
0.13
Porphyrin concentration 0.4mM
not be coordinated to the CoTPP. I t is indicated that in pure pyridine, two pyridines are coordinated to Co(II)TPP, but in the presence of solvent, only one pyridine is coordinated in an axial position, the other position being occupied by a solvent molecule. A similar observation has been made for piperidine complexes of cobalt protoporphyrin where the intensities of the bands are different for the fiveand six-coordinate species (27). This hypothesis could be better tested by studying the variation of the half-wave potential with pyridine concentration. Such a plot for DMSO solutions shows the number of ligand molecules exchanged during the reduction as determined from the slopes of the plots. In DMSO, two pyridines are lost a t low pyridine concentrations during the Co(II1)-Co(I1) reduction and no ligands are lost during the Co(I1)-Co(1) reduction. In contrast, a t high concentrations, one pyridine is lost during Co(II1)-Co(I1) reduction, and one pyridine lost during the Co(I1)-Co(1) reduction. Thus, the following reactions are proposed a t low pyridine concentrations: DMSO .py I
Co(1II)P'
t
2DMSO
I
+
e- a C o ( I I ) P
2Py
(8)
DMSO
PY
DMSO Co(1I)P i e-
1
==
+
Co(1)P-
2DMSO
(9)
DMSO while at high pyridine concentrations the following is present: DMSO
PY
I Co(III)P* + DMSO
+
e-
==
Co(1I)P
I
I
PY
PY
+
Py
(10)
, DMSO I
I Co(1I)P
+
e-
Co(1)P-
+
Py
+ DMSO
(11)
PY
The slope values are within 10% of the value expected for the above reactions. This is comparable to what is observed for cobalt hematoporphyrin in aqueous solutions ( I ) , with dissimilarity being a t one point that Co(I1)Hm is coordinated with one pyridine ligand a t low pyridine concentrations and two pyridines a t high pyridine concentrations.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
~
~
~~~
Table VI. Variation of Heterogeneous Rate Constant for CoTPPa Reduction with Ligand Pyridine in D M F k , x 10'
(cmlsec)
Co(l1) -Co(l)
Pyridine (MIL)
0.0 0.00124 0.00620 0.0124 0.0620
0.124 0.372 0.620 0.868 0.124 0.372 0.620 0.868
2 .o
0.124 0.620 1.24 a Porphyrin concentration 0.4mM. DMSO is the only solvent which shows this loss of two pyridines a t low concentrations of pyridine for the Co(II1)Co(I1) reduction. This loss of two pyridines demonstrates the strong coordinating strength of DMSO over the other solvents. When low concentrations of pyridine are present, DMSO should be expected to compete for axial positions but, a t high pyridine concentrations, competition is not as strong between DMSO and pyridine so that only one pyridine is lost. With the other solvents, competition is not as predominant as with DMSO so that such an observation is not made. With the exception of DMF, which shows no loss of pyridine when undergoing a reduction from Co(111)-Co(II), all solvents except DMSO show loss of one pyridine during this reduction. Generally the following takes place in these solvents:
Py
S
I
I
Co(III)P* + Solvent I
+
I
e-
+ CO(II)P + 1I PY
PY
PY (12)
S
I Co(1I)P
+
e-
F=
Co(1)P-
+
Py
+
Solvent
(13)
PY
Yamamoto and Kwan (28) have reported that cobalt(I1) tetraphenylporphyrin has one pyridine coordinated a t low pyridine concentrations which agrees with the data that are presented for DMA and PrCN. Since all spectra are similar, similar structure is suggested; and based on these spectra, Co(1I)TPP is coordinated with one pyridine. The ligand exchange accompanying the Co(II1)-Co(I1) and Co(I1)-Co(1) reductions affects the value of the heterogeneous rate constant. The overall trend is a decrease in rate constant when ligand exchange is indicated. The variation of rate constant with concentration of pyridine in DMSO is shown in Table IV. Here it is observed that, with no ligand present, both redox reactions have larger rate constants than when pyridine is present, suggesting little difference between the oxidized and reduced forms. When ligands are present and exchange occurs, the rate constants decrease in a manner indicative of the number of pyridines being exchanged. During the Co(I1)-Co(1) reduction a t low concentrations of pyridine where no ligands are lost, the rate constant is relatively constant. Both reductions have decreases in the rate constant upon loss of pyridine molecules; however, an increase is observed upon transition from two pyridines lost to one pyridine lost in the Co(II1)Co(I1) reduction. This is expected since only one ligand is being exchanged instead of two. Identical situations are
... ...
0.45 0.72 One 0.73 0.48 >ligand lost 0.93 0.60 0.45 0.45 I'
...
... 0.21
Table VIII. Stability Constants for Formation of CoTPPa Complexes Co(II1)TPP Complex
No
ox
L I ands
2ED
Solvent
E
2
1
DMSO
3.5X10'
2
0
DMSO
7.3 x l o 5
2 2 2
1 1 1
PrCN
1.7
DMA DMA
Ligand
)i. 10" 1.8 X l o i 3 6.5 X l o i 3
Pyridine (High concn) Pyridine (LOW concn) Pyridine Pyridine 4-Picoline
Co(1I)TPP Complex
1 0 DMSO 1.4 X 1 0 PrCN 4.6 X 4.6 X 1 0 DMA 1 0 DMA 1.7 X a Porphyrin concentration 0.4mM.
10' lo5 10' 10'
Pyridine Pyridine Pyridine 4-Picoline
present for DMA, DMF, and PrCN. The results are reported in Tables v, VI, and VII, respectively. Incomplete complex formation a t the low concentrations of pyridine prevented the determination of rate constants a t these concentrations for some of the Co(II1)-Co(1I) reductions. The trend of the rate constants decreasing when ligand exchange accompanies the reduction was also observed in iron porphyrin with pyridine ( 17). Stability constants (6) for the formation of some of these pyridine complexed CoTPP species were determined using equations discussed by Meites (291, and appear in Table VIII. The stability constants reflect the trend observed previously in the reduction potentials. The presence of two ligand molecules on Co(1II)TPP renders this complex more stable to reduction than when one ligand is coordinated. When two pyridines are bonded to Co(III)TPP, the reduction potential shifts more negatively for the Co(II1)-Co(I1) reduction than for the Co(II)-Xo(I) reduction, indicative of a more stable species being formed. The stability constants for the oxidized species, cobalt(III), are large compared to the cobalt(I1) complexes. The cobalt(I1) complexes in most cases are coordinated with one pyridine molecule and are not as stable as indicated by slightly smaller negative shifts in reduction potentials. Bispyridine Co(1II)TPP complexes are found to have larger stability constants for formation of the complexes than for the monopyridine Co(1I)TPP complexes which results in smaller rate constants being measured for the Co(II1)-Co(I1) reduction since ligand exchange must occur. Worthy of note, CoTPP has a measured rate constant for the Co(II1)-Co(I1) reduction that is larger
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2265
Table IX. Electrochemical D a t a for CoTPPa with Added 4-Picolineb
Table XI. Electrochemical D a t a for CoTPPa with Added Piperidine
Co(I1I)-Co( 11) Solvent
E ( V u s . SCE)
DMA DMF DMSO
-0.37 0.47 4.36
Co(II1)-Co(l1)
ks ( c m , sec)
E 1 , ~( V
Solvent
... ... ...
DMSO *
VS.
SCE)
-0.98 -1.10 -0.98
a P o r p h y r i n concentration 0.4mM.
2.2 x 10-3 3.1 x 10-3 0.4 x 10-3 4-Picoline concentration
0.31 x 10-3 0.23 x 10-3
-0.35 -0.41
DMA"
Co( 11)-c o( I)
C0(II)-Co(I)
DMA DMF DMSO
k, ( c m / s e c )
-0.94 -1.11
DMSO~ D MA" a Porphyrin
concentration
0.4mM.
0.20 x 10-2 0.10 x 10-2 Ligand
concentration
0.05M. L i g a n d concentration 1.01M.
1.02M.
Table X. Variation of Heterogeneous Rate Constant for CoTPP" Reduction with Ligand 4-Picoline in DMA Co(I1)-Co(1) Reduction 4-Picoline ( M I L )
0.0 0.00102 0.00510 0.0102 0.0510 0.102
k
x
lo2 ( c m l s e c )
ligands 1.2 1 .o
0.510 1.02 2.04 a P o r p h y r i n concentration 0.4mM.
in DMSO than in DMA, which is an indication that the bispyridine complex is less stable in DMSO. In DMSO, the stability constant for the formation of the cobalt(II1) complex is smaller than in DMA. O t h e r Nitrogeneous Bases as Axial Ligands. Another nitrogeneous base which complexed CoTPP is 4-picoline. In PrCN, no waves are seen for the Co(II1)-Co(I1) or Co(I1)Co(1) reductions when 4-picoline is added to the test solution. Peak separations (AE)are large for the Co(II1)-Co(I1) reductions in DMSO, DMA, and DMF. The potentials and rate constants measured in these solvents are reported in Table IX. The potentials measured for the two metal reductions indicate the 4-picoline complexes of CoTPP are more stable in DMF than in DMA which is in contrast to DMA having the more stable pyridine complexes. The stability constant p for the formation of the Co(1II)TPP complex in DMA is 6.5 X 1013 ( p for the pyridine complex is 1.8 X and that for the Co(1I)TPP complex in DMA is 1.7 X lo2 ( p for the pyridine complex is 4.6 X lo2). These values have standard deviations of about 10%.The number of 4-picoline molecules that are coordinated to Co(1II)TPP and Co(1I)TPP are two and one, respectively, a situation identical to that with pyridine. Comparing these values of p for the 4-picoline complex with those for the pyridine complexes shows that the more stable complex has a lower rate constant. The effect of ligand concentration on the rate constant is similar to that observed for the ligand pyridine in DMA (See Table X). Piperidine is another nitrogeneous base which complexes CoTPP. In PrCN and DMF, large peak currents are measured for the Co(I1)-Co(1) reduction process and large peak separations seen for the Co(II1)-Co(I1) reduction process. On varying the concentration of piperidine, it was determined that one piperidine molecule is lost during the Co(II1)-Co(I1) reduction. It has been shown (30, 31) that Co(1II)TPP and Co(1I)TPP are coordinated with two pi2266
peridines in the crystalline state. It was difficult to determine the number of ligands exchanged for the Co(I1)-Co(1) reduction due to large peak separations and incomplete complexation at low concentrations of piperidine. The electrochemical data for CoTPP the ligand piperidine are shown in Table XI. As with pyridine, the piperidine complexed species are more stable in DMA than in DMSO, indicating that solvent competition for axial sites is not as predominant in DMA as in DMSO. Again, it is evident that piperidine exchange on reduction causes the rate constants to be lower and comparable to the pyridine-complexed species. CONCLUSIONS Taking into account the absolute rate theory, situations where there is the smallest difference between oxidized and reduced states should result in fastest electron transfer being observed if all other factors are equal. From this study, it is seen that when ligand exchange accompanies the reduction of CoTPP, there is a decrease in the measured heterogeneous rate constants. In these situations, ligand exchange and solvent reorientation would affect the electron transfer rate since an activated complex state would have to be found before electron transfer could occur. Ligand exchange is a prominent occurrence in the conversion of vitamin B12, which is not active as a coenzyme for any known enzymatic reaction, to deoxyadenosyl Bl2 coenzyme which does take part in a number of enzymatic reactions. In addition, the presence of an axially coordinated ligand generally stabilizes the cobalt(II1) state relative to the cobalt(I1) state as seen by the potential shifts observed when a ligand is coordinated. ACKNOWLEDGMENT The authors thank A. Adler for a sample of CoTPP. LITERATURE CITED D. G. Davis and L. A. Truxillo, Anal. Chim. Acta, 64, 55 (1973). D. Lexa and J. M. Lhoste, Experientia Suppl., 8 , 395 (1971). J. W. Collat and J. C. Abbot, J. Am. Chem. Soc., 86, 2306 (1964). P. G. Swetik and D. G. Brown, J. Nectroanal. Chem., 51, 433 (1974). A. Stanienda and G. Biebl, Z.Phys. Chem., 52, 254 (1967). (6)J. Faier. D. C. Bora, A. Forman, D. Dolohin, and R. H. Felton. J. Am. Chem. SOC.,92, 3431 (1970). (7) K. M. Kadish and D. G. Davis, Ann. N. Y. Acad. Sci., 206, 495 (1973). 16) J. - Manassen and A. Bar-llan. J. Catal.. 17. 66 (1970). (9) R. H. Felton and H. Linschitz; J. Am. Chem. Sdc., 88, 1113 (1966). (10) J. H. Fuhrhop, K. M. Kadish, and D. G. Davis, J. Am. Chem. Soc., 95, 5140 (1973). (11) H. W. Whitlock and B. K. Bower, Terrahedron Lett., 4627 (1965). (12) A. D. Adler, F. R. Longo, J. Kampas, and J. Kim, J. lnorg. Nucl. Chem., 32, 2443 (1970). (13) A. D. Adler, L. Skiar, F. R. Longo, J. D. Finarelli, and M. C. Finareili. J. Heterocycl. Chem., 5, 669 (1966). (14) R. P. Van Duyne and C. N. Reilley, Anal. Chem., 44, 145 (1972). (15) D. G. Davis and J. G.Montalvo, Jr.. Anal. Chem., 41, 1195 (1969). (16) D. G. Davis and J. G. Montalvo, Jr., Anal. Chem., 41, 641 (1969). (17) L. A. Constant, Ph.D. Dissertation, LSUNO. 1973. (18) R. S.Nicholson, Anal. Chem., 37, 1351 (1965). (19) R. S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964). (1) (2) (3) (4) (5)
\ - ,
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
~
~~~~
~
~
(20) J. E. Falk, "Porphyrins and Metalloporphyrins", Vol. II, Elsevier, New York, 1964. (21) A. Wolberg and J. Manassen, J. Am. Chem. SOC., 92, 2982 (1970). (22) M. Momenteau, M. Fournier, and M. Rougie, J. Chim. Phys., 67, 926 (1970). (23) S. B. Brown and I. R. Lantzke, Biochem. J., 115, 279 (1969). (24) R. T. Sanderson. "Inorganic Chemistry", Reinhold Publishing Corp., New York. 1967, p 136. (25) D. Dolphin, A. Forman, D. C. Borg, J. Fajer, and R. H. Felton, froc. Nat. Acad. Sci. USA, 68, 614 (1971). (26) L. J. Boucher, "Manganese Porphyrin Complexes Il-Electronic Spectroscopy and Structure" in "Coordination Chemistry", Plenum Press, New York, 1969.
(27) D. V . Stynes. H. C.Stynes, B. R. James, and J. A. Ibers.. J. Am. Chem. Soc.. 95. 1796 11973). (28) K. Yamamoto and T. Kwan, J. Cats/., 18, 354 (1970). (29) L. Meites, "Polarographic Techniques", 2nd ed.. Interscience Publlshers, New York, 1966. (30) W. R. Scheldt and J. L. Hoard, J. Am. Chem. Soc., 95, 8281 (1973). (31) W. R. Scheidt, J. Am. Chem. SOC., 96, 84 (1974).
RECEIVEDfor review March 28, 1975. Accepted August 4, 1975. The National Science Foundation (GP-42479X) provided financial assistance.
Application of the Semiintegral Method to the CE Mechanism J. H. Carney Department of Chemistry, York College, Jamaica, N. Y.
1145 1
A successive approximation method of applying the semiintegrai to chemical reactions preceding a reversible electron transfer is described and tested on the cadmlum and lead nitrilotriacetic acid systems. Both the equilibrium and rate constants may be obtained by this method. This particular approach is applicable over a wide range of rate parameters and in the presence of significantly large values of uncompensated resistance.
We wish to report an extension of the semiintegral method to analysis of electrochemical systems represented by Equation 1 which are complicated by a homogeneous reaction that precedes a reversible electron transfer. kf
%+Ox
+ ne-
s Red
(1)
The transform variable is s , and E is the transform of the current. The diffusion coefficients of 2 and Ox are assumed to be equal and are represented by D. For linear sweep voltammetry, the current varies in a complex fashion, but it can be approximated by a series of linear current segments as previously demonstrated (5):
The terms bj are the slopes of the individual segments. The term a0 is the current a t the beginning of the first segment. Experimental conditions can be arranged in linear sweep voltammetry so that the initial current, a0 is zero. The current function of Equation 3 with a0 = 0 is transformed, combined with Equation 2, and inverted to yield Equation 4.
kb
Imbeau and Saveant have already indicated how the semiintegral method could be applied to such systems under conditions of complete kinetic control of the current ( I ) . Lawson and Maloy have treated the EC and ECE complications by calculating numerical working curves of the diffusion controlled semiintegral for double potential step experiments (2). The range of usefulness for the method presented in this communication is the same as Nicholson and Shain's method ( 3 ) of nonlinear curve fitting but possesses the advantage of simplicity of use and allows correction for uncompensated resistance. Furthermore, this method does not require precalculated working curves.
THEORY The necessary equations are most easily derived from consideration of chronopotentiometric theory under conditions of semiinfinite linear diffusion. For the reaction mechanism given by Equation 1, the concentration a t the electrode surface is obtained by taking the inverse Laplace transform indicated by Equation 2 ( 4 ) r
__
K
-
l+K
where C* is the sum of the concentrations of 2 and Ox in the solution bulk, K is the equilibrium constant for the equilibrium between 2 and Ox, and L is the sum (kf kb).
+
where
and
(7) The second term inside the braces of Equation 4 is equivalent to the semiintegral for the diffusion-controlled electron transfer (6). For the diffusion-controlled case as well as for the mechanism considered here, the concentration of Red a t the electrode surface is given by Equation 7. The term x ( t ) in Equation 4 contains the kinetic information. Unfortunately, because of the form of the kinetic term, direct analysis cannot be made. Values for L and the equilibrium constant must both be guessed and Equation 4 evaluated with these parameters, then the results compared with theory. The desired numbers are determined by a two-step approach. First L is adjusted until the ratio C,,(O,t)l C r e d ( O , t ) behaves in Nernst fashion. This behavior is determined by plotting potential vs. the second log term of Equation 8 and adjusting L until the plot is a straight line with slope RTInF.
ANALYTICAL CHEMISTRY, VOL. 47,
NO.
13, NOVEMBER 1975
2267
