D. J. KENNEY,G. G. CLINCKEMAILLIE, AND C. L. MICHIELS
410
cently measured values obtained at room temperature.bJ1 It is concluded that reaction 2 can compete favorably with other reactions of OH in systems of various compositions and at various temperatures. This study provides a basis for quantitatively evaluating the con-
tributions of reaction 2 to the over-all scheme. The current body of datal3on the H20zsystem suggests that a clean source of HO2 radicals at high concentration could be obtained in a stream of hot H20zvapor at a point downstream from the region where reaction 2 is going to completion.
Ligand Substitution Reactions of Hexacyanoferrate(II1) and Azide Induced by Flash Photolysis’ by Donald J. Kenney, Guido G. Clinckemaillie,2and Brother Cyril Leo Michiels Department of Chemistry, University of Ddroit, Detroit, Michigan 48881
(Received July 10, 1967)
Aqueous potassium hexacyanoferrate(II1) does not react with sodium azide even upon prolonged standing in a well-lighted room. However, when irradiated strongly with sunlight or a tungsten filament lamp, the yellow solution turns purple because of the formation of azidopentacyanoferrate(II1). Since the purple color disappears rather quickly due to the re-formation of hexacyanoferrate(III), reaction rates were measured after flash photolysis. The results indicate that this bleaching of the purple azidopentacyanoferrate(II1) by cyanide ions proceeds by an S N mechanism. ~ Azide concentration and pH also have a dramatic influence on reaction rates, while ionic strength does not. The thousandfold increase in ligand substitution rate upon flashing suggests a photoexcitedstate mechanism.
Introduction Because of its stability, rates of hydrolysis or photohydrolysis have not previously been measured for solutions of hexacyanoferrate(II1). However, Jaselskis has shown3s4 that if aquopentacyanoferrate(II1) can once be formed in situ or by synthesis, it will give a characteristic color with sodium azide.
+ N3-
Fe(CN)6H202-
Fe(CN)sNP
+ HzO
(1)
Jaselskis generally oxidized Fe(CN)sH203- or Fe(CN)swith hydrogen peroxide in the presence of azide to produce the color. The Fe(CN6)4- and Fe (CNJ8- ions will not yield the azide color under the same conditions. One could conclude from this that normally the aquation reaction 2 is slow while the reverse is quite rapid compared with reaction 1.
Fe(CN)sN33- complex with absorption maximum at 560 mp is moderately fast, the reaction rate can be measured with an oscilloscope. It is also shown that as a competing reaction, Fe(CN)aNa3-is decomposed by cyanide ions at a measurable rate in the dark. Reaction 3 has a first order dependence upon azidopentacyanoferrate(II1) and also cyanide concentration and is reversible to a certain extent when the iron complexes are in a photoexcited state. Fe*(CN)6N3a-
+ CN- E Fe*(CN)s3- + N3-
(3)
Experimental Section Reagents. The following reagents were used without further purification : sodium cyanide, sodium car-
(2)
(1) This work was presented in part a t the 163rd National Meeting of the American Chemical Society, Miami, Fla., April 1967. (2) Based in part on a dissertation submitted by G. G. Clinckemaillie
The current investigation shows that Fe(CN)&- will react directly with azide under the influence of flash photolysis. Although the development of the purple
to the Graduate School of the University of Detroit, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (3) B. Jaselskis and J. C. Edwards, Anal. Chem., 3 2 , 381 (1960). (4) B. Jaselskis, J . Am. Chem. SOC.,83, 1082 (1961).
Fe(CN)&
+ H2O
Fe(CN)sH202-
The Journal of Physical Chemistry
+ CN-
SYMPOSIUM ON IKORGANIC PHOTOCHEMISTRY bonate, sodium hydrogen carbonate (Baker and Adamson reagent grade); sodium perchlorate (G. F. Smith rewent grade). Potassium hexacyanoferrate(II1) (Fischer reagent grade) mas recrystallized from water, and sodium azide (Fischer Purified) was recrystallized from water and ethanol. Apparatus. A flash photolysis apparatus was constructed along the lines suggested by Bailey and Herc u l e ~ . ~The flash tube, mounted along one axis of an elliptical reflector, was a modified XFX-47A obtained from Edgerton, Germeshauser, and Grier, Inc., Boston, 0.1 cm of Xe. Spectrcand was filled with 1.5 graphic analysis of the light emitted by this tube, when flashed with the power circuit described here, revealed a continuous background of white radiation with the superimposed line spectrum of xenon as described by Goucz and SeweL6 Mounted along the other axial focus of the elliptical reflector was a cylindrical sample holder, 1.9 cm i.d., 10 em path length, manufactured by Pyrocell 3Ianufacturing Co., Westwood, N. J. When it wns desired to flash the solution with light of wavelength no shorter than 300 mp, as was the case for all of the work reported here, the flash tube was surrounded by two Pyrex cylinders each having a wall thickness of 2 mm. For a typical flash, 31 cc of solution einstein as measured with would then absorb G X the uranyl oxalate actinometric solution of Forbes and Heidt? A Series 130 grating monochromator made by the Farrand Optical Co., Kew York, N. Y., was used withe 14'28 photomultiplier tube to analyze optical density changes in the test solution. A slit width of 0.4 mm was generally employed, and this led to molar extinction coefficients which were about 7% lower than those obtained with a Beckman DU spectrophotometer. The entire optical system was checked for adherence to Beer's law for all of the solutions measured. A trigger pulse for the oscilloscope is obtained from a voltage drop in a short section of the copper tubing connected to the grounded side of the power circuit, and the oscilloscope time sweep is photographed with a Polaroid camera. A Tektronix 533 oscilloscope with a Type CA plug-in unit was employed. The flash lamp power circuit was similar to the one employed by Grossweiner and Matheson,' except that the need for a triggering device was obviated by a Jennings-RH2G vacuum switch. When charged up to 10,OOO V, a bank of five 5-pF capacitors delivers 1250 J upon discharging. The total flash duration, using this circuit, was less than 500 psec, and did not interfere with the reaction kinetic measurements. Kinetic M e m r e m a l s . Since Jaselskis' reported that azidopentacyanoferrate(II1) has an absorption maximum a t 560 mp, whereas hexacyanoferrate(II1)does not absorb a t this wavelength, the oscilloscope deflections were calibrated in terms of per cent transmittancea t 560 mp. A buffered solution of hexacyanoferrate-
*
411
(111) containing azide ions wns placed in the sample cell, and the 100% transmittam line was adjusted to the top grid line of the cathode ray tube. Zero per cent transmittance was simulated by applying the dc filter of the oscilloscope to the photon~ultiplieroutput. The distance between the 100% line and the 0% line was measured, and the scale was then expanded to the desired magnification. After the stability of the magnified 100% line was observed, it wns photographed and the flash was then triggered. A typical photograph of the oscilloscope trace after flnshing is shown in Figure 1. The rapid formation followed by the slower disappearance of the purple azidopentacyanoferrate(II1) can thus be measured a t 560 mp after converting per cent transmittance to absorbancy and concentration of Fe(CX)aN33-. Temperature. Rates of reaction were not found to depend upon temperature in the range from 14 to 30". Hence, no positive control of temperature was employed, and data reported in this paper were generally measured a t ambient conditions of 25 2". I a i c Strength. Rates of reaction were also found not to depend upon ionic strength in the range from 0.5 to 2.0, except insofar as changing the ionic strength changes the pH. Hence, enough of the HC03-CO? buffer was added to all solutions to make the final ionic strength 0.5. Interference b y Ozygen. Flashing solutions in the presence of air did not seem to affect the values of measured reaction rates. After several freezing and vacuum evacuation cycles, solutions responded identically to flashing as did aerated samples of the same concentrations. Therefore, since the solutions were shielded against light of wavelength below 300 m# by Pyrex filters, it was Considered safe to flash aerated solutions of KnFe(CN)6.
*
F - 5 0 0 rnsec.----fl Figure 1. Typical aseillogrnm at 560 rn# after flarhing 10-2 M &Fe(CNh at 24'; pH 9.4; 0.2 M NaN,; 0.013 hf CN-; ionic strength 0.5; maximum Fe(CN)2-lsa- formed: 1.9 x 10-6 M.
(5) D. N. Bailey and D. M. Henules. J . Chem. Educ., 42. A83 (1885). (6) J. H. Goncz and P. B. Newell. J . Opc. Soc. Am., 56. 87 (19136). (7) 0 . S. F o r b and L. J. Heidt. J . Am. Chm. Soc.. 56,2363 (1934). (8) L. I. Gmssweiner and M. 8. Matheson. J . Phya. (1957).
C h . ,61. 1088
Volume 7% Number X
February 1968
D. J. KENNEY, G. G. CLINCKEMAILLIE, AND C. L. MICHIELS
412
Results The disappearance of the purple Fe(C1\)5S33- after about 100 msec (see Figure 1) was found to follow pseudo-first-order kinetics in the presence of excess CN-, and the pseudo-first-order rate constant, IC, was found to vary with pH and azide concentration. Influence of C N - Concentration. Figure 2 shows a linear dependence of IC upon CN- concentration. Similar results are obtained at pH 9.4, and linearity is to be expected if the rate of disappearance or bleaching of the purple Fe(CN)aN33-is first order in Fe(CN)sN33and also CN-. (See reaction 3.) Influence of N3- Concentration. Figure 3 shows that there is a slight dependence of k on azide concentration, indicating a certain amount of reversibility to the reaction of Fe(CN);Ns3- and CN-. Indeed, the pseudofirst-order plots of log absorbancy vs. time were found to level off after a certain time due to the competing reverse reaction and approaching equilibrium. Influence of p H . Table I shows that the value of k increases with increasing pH. Since the pK, value for HCN is 9.31, it was necessary to mask the obvious influences of pH on CN- concentration. For example, at pH 9.31 half of the sodium cyanide added would be
k (sac-') 2.8
/
0.41
p
4
/ 8
I2
I6
20
24
I
Conc. CN- (mM/I)
Figure 2. Pseudo-first-order rate constant as a function of cyanide concentration a t 25'; pH 10.2; 0.2 M NaNa; i o - 3 M KaFe(CN)e; ionic strength 0.5.
I
k (sec-')
t3
converted to HCN, and a solution buffered at pH 9.31 should be made 0.026 F in XaCN in order to obtain a CN- concentration of 0.013 M. Therefore, all CNconcentrations reported in this paper were first adjusted to account for the various degrees of hydrolysis at different pH values. The rate constants in Table I were all obtained at a calculated CN- concentration of 0.006 M , showing that pH exerts an influence on the bleaching rate of Fe(CN)s?rT&-apart from affecting the CN- concentration. Table I : Pseudo-First-Order Rate Constant as a Function of pH a t 25O, Ionic Strength 0.5, 0.2 M NaN3, 0.006 M CN-, loU3M KaFe(CN)6
PH
880 -1
9.2 9.4 9.8 10.3
1.25 f 0 . 0 7 1.70 f 0.04 2.50 =!= 0.04: 2.60 & 0.12
Discussion It should be remembered that the purple color of azidopentacyanoferrate(II1) is the basis of a colorimetric test3 for Fe(CN);NHP and Fe(CN)sH202-in the presence of Fe(CN)s3- and Fe(CN)a4-. Naturally, the color of Fe(CN)aN33- must be sufficiently stable after developing its full intensity for normal colorimetric measurements. Indeed, standard solutions of Fe(CN)6Ns3- can be stored in the refrigerator overnight, and even in the presence of excess cyanide, these solutions have half-lives in terms of hours9 rather than milliseconds as reported here. Consequently, flashing the solution with light of wavelengths longer than 300 mp must produce an electronically excited state of Fe(CN)$-. Moreover, this photoexcitation must persist for nearly a second while ligands are exchanged to form Fe(CN)jN33- and Fe(CN)a3- and back again. This would also explain the negligible temperature dependence for IC, since an excited complex could have a much lower activation energy relative to ligand exchange. Although curve fitting is not a proof of mechanism, it is convenient to use the following expression to fit the oscillograms of the formation of subsequent decay of Fe(CN) after flashing. slow
B
I
I
0.2
I
I
0.3 0.4 A z i d e Conc. ( M / I )
,
0.5
Figure 3. Pseudo-firsborder rate constant as a function of azide concentration at 25'; pH 10.3; 0.006 M CN-; 10-8 M KgFe(CN)B; ionic strength 0.6 f 0.2. The Journal of Physical Chemistry
aAo(e-kg
fast
- e-*'$)
(4)
Here, B is the concentration of the purple Fe(CN)6N33- at any time t, A O is the initial concentration directly after flashing of some active precursor of B (9) W. H. Geer, "Thermal Decomposition Rate of Azidopentacyanoferrate(II1) in Aqueous Solution," M.S. Thesis, University of Detroit, 1965.
SYMPOSIUM ON INORGANIC PHOTOCHEMISTRY
413 kfe-k’)cm
aA,
s
a
1.83
0.8 200
ke-k)cm
400
300
500
k = kz[CN-]
TIME (msec.)
Figure 4. Theoretical fit of data. Experimental points taken from oscillogram after flashing 10+ M KsFe(CN)a a t 25’; p H 9.4; 0.2 M NaNs; 0.018 M CN’-; ionic strength 0.5. Curve taken from eq 4.
+
0.1 0.2 0.3 0.4 A z i d e Conc. (M/I)
I
0.5
Figure 5. Calculation of pseudo-first-order rate constant k’. Experimental points taken from Figure 3.
such as Fe*(CN)63- or Fe*(CN)P which decays rapidly with a rate constant k‘, and a is the fraction of A. which was predestined in a statistical sense to form B. (Naturally, a reactive species such as A could also react with HzO or CN- and never form B at all, and a will depend on the relative reactivity and concentration of these ligands compared to N3-.) After a sufficient period of time, the last term in eq 4 becomes negligible and pseudo-first-order kinetics results.
B s UA&-~;
(5)
The usual semilog plot not only gives a straight-line slope proportional to k but also an intercept of log (a&). Going back to eq 4, the value for the fast rate constant k’ may be obtained by setting dB/dt equal to zero.
(7)
k4
Here, kz is the second-order rate constant for the reaction of Fe(C1C’)5N38-with CN-, and it is also the slope of the line in Figure 2. The y intercept of this line, k4,has been labeled S N 1 to indicate that Fe(CN)sNP is also disappearing at a rate not controlled by CNconcentration. Naturally, it is difficult to experimentally distinguish between a true S N 1 mechanism ~ of Fe(CN)5NP with solvent and an S N reaction molecules of water. Since the bleaching or disappearance of Fe(CN)5N33- is base catalyzed, it seems likely that hydroxyl ions are also displacing N3- by an S N ~ reaction. The fast pseudo-first-order rate constant k’ may be defined and expressed as -d(ln A)/dt = k’ = kl[Ns-]
1
(6)
Here, tm is the time corresponding to the minimum on the oscillogram or maximum amount of B formed. Using these calculated parameters, a satisfactory fit to the experimental data can be obtained as shown in Figure 4. Again from eq 6, the rate constant k’ for the rapid decay of the active precursor A can be calculated and shown to involve kinetics which are first order in A as well as N3-. (See Figure 5.) Finally, k which is defined by eq 5 may be expressed as
1.0
100
=
+ k3
(8) Here, kl is the second-order rate constant for the reaction of A with N3-, and it is also the slope of the line in Figure 5. The y intercept of this line, kg, has been labeled SN1, again with the reservation that it may ~ with OH- or water molecules. involve an S N reaction There is also the possibility that kg may involve an S Nreaction ~ with cyanide ions if A turns out to be Fe*(CN)b2-.
Conclusions Hexacyanoferrate(II1) can undergo photoexcitation and rapidly exchange ligands with azide upon flash photolysis and form azidopentacyanoferrate(II1) which apparently retains the photoexcitation and exchanges ligands with cyanide and perhaps water molecules. Rate constants for the formation of azidopentacyanoferrate(II1) are of the order of 60 sec-1 and for its decay are of the order of 2 sec-1 for the aqueous solutions tested. The decay or bleaching of the purple Fe(CN)6Na3- is base catalyzed. Acknowledgment. The authors wish to thank Mr. Jack D. Nixon for his design of the apparatus and many helpful discussions.
Volume 72, Number 2 Februarg 1068
