Flash Photolysis Intermediates of Trioxalatoferrate ( I I I )
2437
Spectrophotometric and Electrochemical Studies of Flash-Photolyzed Trioxalatoferrate(1 I I) J. I. H. Patterson and S. P. Perone* Department of Chemistry, Purdue University, Lafayette, lndiana 47907 February 26, 1973) Publication costs assisted by the
(Received July 78, 7972; Revised Manuscript Received
U.S.Public Health Service
Spectrophotometric and electrochemical monitoring techniques have been used to follow the intermediates generated by the flash photolysis of trioxalatoferrate(II1). The results indicate that there are competing initial photolytic processes followed by a sequence of three secondary reactions. In the first, an oxidizable iron(II1) diradical species, formed by the flash, disappears by a rapid first-order reaction. The rate of disappearance of the second intermediate is dependent on the iron(II1) oxalate concentration. The third step in the mechanism produces the final product, dioxalatoferrate(I1). Of the three sequential steps, only the reaction of the second intermediate can be followed photometrically and has been reported previously. The initial and final reactions can be monitored conveniently electrochemically. The reaction sequence proceeds to completion in less than 1sec.
Recently, from this laboratory, Jamieson and Peronel reported a flash photolytic study of iron(II1) oxalate using electrochemical detection methods. They proposed a mechanism involving initial formation of a diradical intermediate which could be monitored electrolytically. The overall mechanism suggested was not in agreement with proposals of other workers2,3 who have used spectrophotometric monitoring in similar flash photolysis studies. A review of other workers’ results was provided previous1y.l Because the earlier flash photolysis studies utilizing kinetic s p e c t r o ~ c o p y employed ~~~ solution and photolytic conditions different than for our earlier work1 using electrochemical monitoring, the purpose of the work reported here was to apply both electrochemical and spectrophotometric monitoring techniques under identical conditions. Moreover, the instrumentation employed here allowed simultaneous monitoring by both techniques. Thus, it was possible to obtain a more complete outline for the mechanism of the trioxalatoferrate(II1) photoreduction than had been postulated when considering either electrochemical or spectrophotometric data alone. Experimentad Section
Apparatus and Procedures. The apparatus and general procedures for photometric and electrochemical measurements have been described p r e v i ~ u s l y .Photometric ~ determination of the final concentration of iron(I1) oxalate produced was made a t 430 nm. Electrochemical determination of the concentration of electroactive intermediates was done with time-delay potentiostatic current measurement~.~ Reagents. All solutions were prepared as in ref 1. Except as noted in the text, all data are reported for trioxalatoferrate(II1) in 0.4 Moxalate solution a t pH 6.0. Results Spectrophotometric Monitoring. The absorbance us. time behavior following the flash photolysis of iron(II1) oxalate solution is qualitatively the same a t all wavelengths as that reported p r e v i o u ~ l y .In ~ , ~no case was an
increase in absorbance observed after the slow decay, in contrast to the report of Cooper and Degraff3 that a slow increase was noted a t 334 and 313 nm. This, however, may be caused by the slightly different conditions of the experiments reported here. The majority of the experiments in this work were monitored a t the isosbestic point, 410 nm, to eliminate the contribution of the changing absorbances of trioxalatoferrate(II1) and dioxalatoferrate(I1). The absorbance here rises instantaneously with the flash and then decreases until the final absorbance is equal t o the initial absorbance. The decrease in absorbance was plotted according to first-order kinetics from data which spanned a t least oneand-a-half to two half-lives of the reaction. Standard deviations were the order of 2-5%. The observed rate constants agreed within experimental error regardless of the monitoring wavelength used over the range 315-640 nm. At long times (greater than 25 msec), there were no absorbance changes noted which would correspond to the long-lived electrochemically monitored reaction reported below. The dependence of the first-order rate constant on the concentration of iron(II1) oxalate was studied, varying the initial concentration of trioxalatoferrate(II1) over the range of 1 X 10-4-2 X 10-3 M . Figure 1 shows a log-log plot of the observed first-order rate constants us. the concentration of excess trioxalatoferrate(II1). The range covered in this study is somewhat larger than that used by Cooper and DeGraff3 and shows that the dependence is more complex than they have suggested. At low concentrations, the rate constant shows no dependence on the iron(II1) oxalate concentration, while a t higher concentrations the dependence appears half-order as Cooper and DeGraff observed. Electrochemical Monitoring. Potentiostatic analysis of flash-photolyzed trioxalatoferrate(II1) was performed and confirmed the data reported previously by Jamieson and Per0ne.l However, more detailed studies of certain features were conducted as discussed below. Figure 2 shows a typical plot of the total oxidation current, measured a t -0.1 V us. sce, us. time ( 7 ) for 2 x The Journai of Physical Chemistry. Vol. 77, No. 20, 1973
J. I. H. Patterson and S. P. Perone
2438
21-
>, , ,
,
, -
log [Fe'31
Figure 1. Log-log plot of photometrically observed first-order rate constant vs. the concentration of excess iron(ll I ) after the flash.
M trioxalatoferrate(II1) in 0.4 M oxalate a t pH 6.0. (The data in Figure 2 and all other electrochemical currents referred to in this work were measured by time-delay potentiostatic a n a l y ~ i s . Reduction )~ currents measured a t -1.5 V us. sce 300 ksec after the flash show a decrease from the initial level which is equal, within experimental error, to the minimum oxidation current observed after the flash (see Figure 2 ) . This indicates that there may be a small amount of iron(I1) oxalate formed initially by the flash. (This observation contrasts with the earlier report1 that no significant instantaneous decrease in the reduction current a t -1.5 V occurred with the flash. Because the decrease only slightly exceeds the range of experimental error, it was not recognized earlier.) The observation of the initial decrease in reduction current led t o a closer examination of the initial decay in oxidation current a t -0.1 V us. sce. The measured minimum oxidation current, due to initial production of iron(I1) oxalate, can be subtracted from each of the short-time current measurements. Then, when this corrected current is plotted in the standard first-order manner, good linear plots are obtained with a standard deviation of about 6%. The first-order rate constant observed for this reaction was found to be independent of iron(II1) oxalate concentration and has a value of (2.8 f 0.2) X 103 sec-I. The slow increase of oxidation current measured a t -0.1 V us. sce was plotted using the standard first- and second-order plotting techniques, and both cases were found to display some curvature, with neither order giving an appreciably better fit. The calculated rate constants for both orders are given in Table I. These rate constants display a dependence on iron(II1) oxalate concentration which probably reflects the effect of a preceding reaction. Simultaneous Monitoring. In order to determine that the reactions being monitored photometrically and electrochemically were results of the same photolytic process, the reactions were monitored simultaneously using timedelay potentiostatic analysis a t -0.1 V us. sce and photometric monitoring a t 430 nm. The final observed concentrations of dioxalatoferrate(I1) were calculated with the results shown in Table 11. I t can be seen that the two different systems are monitoring very similar final concentrations, indicating that they are monitoring the same segment of solution. This conclusion allows the pooling of kinetic data obtained from both measurements into a digital simulation of the complete photolytic process as described below. The Journal of Physical Chemistry, Vol. 77, No. 20, 1973
2t I
-3.5
-3.0
-2.5
-2.0
log
-1.5
-1.0
-0.5
00
1
( T ) sec
Figure 2. Current vs. log ( T ) after the flash for 2 FeOXs3- : Eappl = - 0.10V vs. sce.
X
M
TABLE I: Rate Constants for Long Time Increase in Oxidation Current at -0.1 V YS. Sce [Fe(OX)33-], M
k(first order), sec-'
k(second order), M - l sec-l
2 x 10-4 6X 1 x 10-3
2.4 3.3 9.5
7.8 x 104 11.1 x 104 13.5 x 104
TABLE I I: Comparison of [Fe2+]rinalMonitored Electrochemically at -0.1 V vs. Sce and Photometrically
at 430 nm
[Fe3+ ]initial, M
2 x 10-4 4 x 10-4 6X 8X 1 x 10-3
Electrochemical
0.98 x 1.3x 1.6x 1.8x 2.0 x
10-4 10-4 10-4 10-4 10-4
Photometric
0.91 x 1.4x 1.8x 2.0 x 2.1 x
10-4 10-4 10-4 10-4 10-4
Effects of p H and Oxalate. In addition to the experiments outlined above, studies were made of the various reactions observed with varied pH and oxalate concentration. The concentration of oxalate had no measurable effect on any of the reactions observed over the range 0.10.4 M . Likewise, the solution pH had no effect over the range 5.0-6.5. Discussion I t is appropriate here to emphasize the new information available from the combined studies described above. From the photometric kinetic study, the dependence on ferrioxalate concentration was established over a broader range, with a zero-order dependence showing up a t the lowest concentrations. From the electrochemical studies, detection of a small amount of iron(I1) oxalate produced initially with the flash allowed the more accurate kinetic analysis of subsequent data. Finally, the results of simultaneous monitoring allowed the pooling of kinetic data to characterize the complete photolytic process. The work reported here provides a more complete picture of the kinetic pathway for the photolytic reduction of ferric oxalate. It is clear, for example, that a t least three
Flash Photolysis Intermediatesof Trioxalatoterrale(llI )
2439
sequential reactions occur. Thus, the results presented in the preceding section, and the very restrictive fact that the quantum yield of iron(I1) oxalate is not affected by either the concentration of iron(II1) or iron(II), suggests the following basic mechanism. 2Fd)X,,:'
hr
2FeOX;'-
Fd)X,:-
B
+
XO,
b
k
B
OX*-
(1A)
A
(1B)
Y
(2)
A
A
+
Fd)X3>- 2 C
b
b
B C
+
+
A
% !
(4.4)
C
(48)
Fd)X2z-
(5)
Although the data do not allow an explicit assignment of all the intermediate species, spectroscopic and electrochemical characteristics of each can be specified. In this mechanism, A is a species which is electrochemically oxidizable a t -0.1 V us. sce and is so short lived that it was not observed photometrically in this work. B is not oxidizable a t -0.1 V US. sce and is the intermediate which is responsible for the first-order decay observed photometrically; C is a species which is not oxidizable a t -0.1 V us. sce and has a spectrum very similar to dioxalatoferrate(11). Therefore, C is not detected photometrically or electrochemically but is inferred from the long-term production of iron(I1) oxalate as observed electrochemically. Y is a reactive intermediate which disappears in a very rapid step and, therefore, is not observed electrochemically or photometrically. The competitive photolytic steps (1A and 1B) are suggested by the fact that there is a small amount of iron(I1) oxalate generated initially by the flash. This reaction must have a quantum yield which is independent of the concentration of iron(I1l) oxalate, as the overall reaction has a quantum yield which is independent of iron(II1) oxalate. Jamieson and Perone' suggested that the initial photolytic intermediate A is an iron(1II) oxalate diradical species. The predicted electrochemical features of this species agree well with the observed photoelectrochemical data.' Also, the suggestion of intermediate A , is supported by other work.6.7
0X.Fe -
I \
0-C
b
A
The second reaction (2) represents the rapid first-order reaction which is observed electrochemically. This reaction shows no dependence on iron(lI1) oxalate concentration; it has a first-order rate constant (kl)of 2.8 x 103 sec-', which is an order of magnitude faster than the reaction monitored photometrically. The most probable unimolecular reaction of A would he an intramolecular oxidation-reduction reaction producing iron(l1) dioxalate. CO?.and a C O Y - radical. However, iron(11) dioxalate is oxidizable a t -0.1 V us. see, while experimental data show that intermediate B is not. Thus, it appears that the
Figure 3. Digital simulation of concentration YS. log ( r ) after the flash. Time scale is 100 psec to 1 sec: (upper photo) 2 X lo-' M FeOX33-, rate constants: k r = 2.8 X lo3 sec-', k 2 = 1.4 X 105 M - I sec-'. k , = 121 sec-l, h , = 2.4 sec-'; initial concentrations: A = 4.1 X 10-5 M, FeOX& = 1.4 X lo-' M. FeOXzz- = 2.9 X M: (lower photo) 1 X lo-? M Fax,'-. rate constants: k , = 2.8 X lo3 sec-'. k p = 1.4 X 105 M - ' sec-', k 3 = 121 sec-', k , = 9.5 sec-': initial concentrations: A = 7.6 X M. FeOX3'- = 8.6 X to-' M. FeOX2*- = 7.5 X M.
redox reaction may proceed through an intermediate (B) in which electron transfer is incomplete. For example, B might be a species where one C02.- radical acts as a bidentate ligand allowing delocalization of the odd electron. Such a complex would be more difficult to oxidize and could account for the observed loss of oxidation current as reaction 2 proceeds. It would also be unstable, decomposing as indicated by reactions 4A and 4 8 which are obsewed photometrically. Species Y could be a free C02.- radical released by the intramolecular reaction 2. It should react very rapidly with excess iron(II1) oxalate (reaction 3), presumably to form the same product, B, as in reaction 2. Species Y is apparently too short lived to he detected here, although reaction 3 accounts for the fact that the overall quantum yield can be greater than one;s i.e., a second molecule of iron(II1) oxalate must be reduced in the reaction sequence. It is possible that reactions 4A and 4 8 involve the competition of intra- and intermolecular steps which result in the reduction of a central iron(l1l) atom to iron(I1) and oxidation of the radical ligand to COz. Rapid solvolysis could then occur to form a carbonate or bicarbonate complex (C). Experimental data indicate that the product, C . is not oxidizable a t -0.1 V us. sce. Moreover, it must have an absorption spectrum very similar to iron(l1) oxalate, as no absorbance change is observed over the range 315-640 The Jouinal of Physical Chemistry. Vol. 77. NO. 20, 1973
J. I. H. Patterson and S.P. Perone
2440
nm during the final step, reaction 5 These observations are not inconsistent with a carbonate complex. The set of competitive reactions (4A and 4B) involving B, the photometrically observed intermediate, are suggested by the nature of the dependence of the photometrically observed first-order rate constant on the concentration of excess iron(II1) oxalate (Figure 1). I t is possible to reproduce the data presented in Figure 1 by assuming that reaction 4A is a pseudo-first-order reaction in iron(II1) oxalate, while reactions 4A and 4B show first-order dependence on B . Then the photometrically observed first-order rate constant ( h o b s d ) will be kohsd = k ,
+
kz[FeOX:-]
From the data in Figure 1, k3 and kz were calculated to be 121 & 6 sec-I and (14.0 f 0.8) X 10f4 M - I sec-l, respectively. The reaction of B seen here corresponds to the “normally observed” photolytic process reported by others.2~3 The final reaction in this sequence ( 5 ) is probably a ligand exchange process. This could involve the slow displacement of carbonate or bicarbonate ion, which is equivalent to one of the final products, carbon dioxide. This results in the formation of dioxalatoferrate(I1) giving rise to the slow increase in oxidation current which is noted a t long times. The apparent dependence of this final reaction on the concentration of iron(II1) oxalate is probably a result of the preceding reactions (4A and 4B) which show a definite dependence on iron(II1) oxalate. It is interesting to note that the data obtained indicate that the mechanism is divided into three distinctly different tirne segmentsfollowing the initial photolysis. ~h~ first segment is the rapid first-order reaction 2 which is
The Journal of Physical Chemistry, Vol. 77, No. 20, 1973
followed electrochemically. The next segment, which is followed photometrically, involves the “normally observed” reactions (4A and 4B). The final time range is represented by reaction 5 and is followed electrochemically. In order to further substantiate the mechanism prqposed above, digital simulation was performed. In these simulations, all rate constants used are those which were directly observed for each of the three reaction times. Figure 3 shows the results of the simulation of the electrochemical data obtained at 2 x and 1 x 10-3 M iron(111) oxalate. In these simulations, the crosses represent oxidation currents due to intermediates oxidizable a t -0.1 V us. sce. The line represents the simulated results for the two-electron oxidation of A plus the one-electron oxidation of iron(I1) oxalate. The excellent agreement between the simulated and real electroanalytical data is all the more significant because the rate constants used in the simulation were obtained necessarily from the two different monitoring techniques.
Acknowledgment. This work was supported by Public Health Service Grant No. CA-07773 from the National Cancer Institute. References and Notes (1) R. A. Jamieson and S.P. Perone, J. Phys. Chem., 76, 830 (1972). C. A. Parker and C. J. Hatchard, J. Phys. Chem., 63, 22 (1959). G. D. Cooper and B. A. DeGraff, J. Phys. Chem., 75,2897 (1971). J. I. H. Patterson and S. P. Perone, Anal. Chem.. 44, 1978 (1972). S. P. Perone and J. R. Birk. Anal. Chem., 38,1589 (1966). (6) F. R. Dukes, J. Amer. Chem. Soc., 69, 2885 (1947). (7) V . V. Boldyrev, I. S. Nev’yantsev, Yu I . Mikhailov. and E. F. Khalretdinov, Kinet. Katal., 11, 367 (1970). (8) c. A. Parker, Proc. ROY. SOC.,Ser. A, 220, 104 (1953).
(2) (3) (4) (5)
