Photoredox Chemistry of
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Iron(ll1)Ions
Chemical Physics. (2)J. L. Beauchamp, D. Holtr, S. D. Woodgate. and S. 0. Patt, J. Am. Cbem. Soc.. 94, 2798 (1972). (3)T. B. McMahon, R . J. Blint, D. P. Ridge, and J. L. Beauchamp, J. Am. Chem. Soc., 94, 8934 (1972). (4)R . J. Biint, T. B. McMahon, and J. L. Beauchamp, J. Am. Chem. Soc., 96, 1269 (1974). (5)D. P. Ridge, J. Y. Park, T. B. McMahon, and J. L. Beauchamp, to be submitted for publication. (6)B. S. Freiser and J. L. Beauchamp, J. Am. Chem. SOC., 96, 6260 (1974). (7)R. D. Wieting, R. H. Staley, and J. L. Beauchamp, J. Am. Chem. Soc., 96, 7552 (1974). (8) For related studies, see J. H. J. Dawson, W. G. Henderson, R. M. O'Mailey, and K. R . Jennings, Int. J. Mass Specfrom. /on Phys., 11, 61 (1973). (9)T. B. McMahon and J. L. Beauchamp, Rev. Sci. Insfrum., 43, 509 (1972). (IO)J. A. Kerr, B. V. O'Grady. and A. F. Trotman-Dickenson, J. Chem. SOC., 275 (1969). (11)P. Cadman, M. Day, and A. F. Trotman-Dickenson, J. Chem. SOC. A,
248 (1971). (12)K.C. Kim and D.W. Setser, J. Phys. Cbem., 77, 2021 (1973). (13)For a recent review of ion cyclotron resonance spectroscopy see J. L. Beauchamp, Annu. Rev. Phys. Chem., 22,527(1971). (14)For an exampie where reaction sequences are somewhat modified see J. L. Beauchamp. M. C. Caserio. and T. B. McMahon, J. Am. Chem. SOC.,96, 6243 (1974). (15)J. Y. Park and J. L. Beauchamp, unpublished results. (16)S. G. Lias, A. Viscomi, and F. H. Field, J. Am. Chem. SOC.,96, 359 (1973). (17)For a recent review of fluorinated carbonium ions in solution, see G. A. Oiah and Y . K. Mo. Adv. Fluorine Chem., 7,69-1 12 (1973). (18)G. A. Olah, R. D. Chambers, and M. B. Comisarow, J. Am. Chem. Soc., 89, 1268 (1967);G. A. Oiah and M. B. Comisarow, ibid., 91, 2955 (1969). (19)G.A. Oiah, E. B. Baker, J. C. Evans, W. S. Tolgyesi, J. S. Mclntyre, and I. J. Bastien, J. Am. Chem. Soc., 86,1360 (1964). (20)R. Nakane, A. Natsubori. and 0. Kurihara. J. Am. Chem. SOC.,87, 3597 (1965),and references contained therein. (21)G. A. Dlah and A. M. White, J. Am. Chem. SOC.,91, 5801 (1969).
Photoredox Chemistry of Iron(ll1) Chloride and Iron(ll1) Perchlorate in Aqueous Media. A Comparative Study' Feeya David and P. G. David' Departamento de Quimica, Unlversidade de Brasilia, Brasilia-D.F.,
Brazil (Received September 9, 1975)
Photoredox chemistry of iron(II1) chloride and iron(II1) perchlorate in aqueous media was investigated in the wavelength range 250-425 nm. The effects of incident intensity, iron(II1) and iron(I1) concentrations, wavelength of irradiation, chloride concentration, radical scavengers, pH, time of irradiation, and temperature on product quantum yieIds were investigated. The primary photoreaction is postulated to be Fe3+OHFe2+ -OH in iron(II1) chloride and iron(II1) perchlorate solutions; secondary reactions of .OH affect the product quantum yields. From the variation of quantum yields with the wavelength of irradiation the OHFe(1II) charge-transfer excited state is concluded to be the one responsible for photoreduction. Under identical conditions, the product quantum yield (&JF~z+) for the photoreduction of iron(II1) chloride is less than that of iron(II1) perchlorate.
-
-
+
Introduction Iron(II1) ions in aqueous solutions containing inorganic anionic ligands show a tendency to form ion pairs of the type Fe3+-X- (where X = C1, Br, OH, NCS, Ns, etc.).2-8 In aqueous solutions containing weak anionic ligands, such as Nos- and Clod-, the principal ion pair is Fe3+.0Hformed by h y d r ~ l y s i s while ,~ in the presence of C1- the ion pairs Fe3+-OH- and Fe3+-C1- coexist. The ion pair formation of Fe3+ with OH- and C1- in aqueous media has been in~estigated.~,~-l~ Even though the photochemistry of aqueous iron(II1) chloride solutions dates back to 1949,I6-l8hitherto no general agreement regarding the photochemical mechanism has been reached.lg While it was generally accepted that the primary photoreaction in aqueous solutions of iron(II1) Fe2+ .0H,20-24the primary perchlorate was Fe3+.0Hphotoprocess in aqueous iron(II1) chloride was reported to Fez+ + .Cl.16-18,22However this latter mechbe Fe3+.C1anism has been contradicted by Saha et al.I9 Saha et al. did not detect any appreciable concentration of a C1 end group in the poly(methy1)methacrylate obtained from aqueous solutions containing iron(II1) chloride as photoinitiator of
-
-
+
polymerization. Instead they detected an OH end group in the polymer. Hence, it was believed that a comparative investigation of the photochemistry of iron(II1) chloride and iron(II1) perchlorate in aqueous media would be informative with respect to correlations of both electronic spectra and structural features with photodecomposition modes.
Experimental Section Materials. Iron(II1) chloride (BDH) was purified by the published procedure.25 Iron(II1) perchlorate was prepared following the method of Mulay and Se1w0od.l~Potassium ferrioxalate fm chemical actinometry was prepared and purified by the literature procedure.26,27Purification of methylmethacrylate and sodium benzoate, used for chemical scavenging, has been described.28 Water was distilled over alkaline permanganate, then over EDTA, and stored in polyethylene containers. All other materials used in this study were reagent grade and were used without further purification. Methods. Gross irradiations, from which no quantum yield data were desired, were carried out either by means of a medium-pressure AH-4 mercury source or in a Royonet The Journal of Physical Chemistry, Vol. 80, No. 6. 1976
580
photochemical reactor; RPR-3500 A lamps, which have a maximum intensity a t 350 nm with a band width of approximately 40 nm, were used. Photochemical irradiations, from which quantum yields were to be determined, were performed with a Hanovia 901C-1 150-W xenon lamp dispersed through a Bausch and Lomb 33-86-40 monochromator. Intensity changes were accomplished by Baird-Atomic neutral density filters. Samples were irradiated in a 1-cm rectangular cell (volume 4.5 ml) fitted with ground joint and teflon stopper. The cell was thermostated a t the desired temperature by circulating water through the cell holder from a constant-temperature water bath. In determination of the product quantum yields, conversion of iron(111)to iron(I1) never exceeded 5% of the initial iron(II1) concentration. Since iron(II1) solutions show great tendency to undergo hydrolysis, all solutions were prepared just before use. Source intensities were measured by ferrioxalate actin ~ m e t r y .Iron(I1) ~ ~ , ~ ~was determined spectrophotometrically as the [Fe(phen)3I2+complex (phen = 1,lO-phenant h r ~ l i n e ) .Calculations ~~,~~ of the quantum yield have been d e ~ c r i b e d . ~pH ' measurements were effected using a Corning Model 7 p H meter and a combination glass-calomel electrode (Beckman No. 39142 A8). Electronic absorption spectra and absorbance measurements were obtained using a Zeiss RPQ 20A recording spectrophotometer.
F. David and P. G. David
0
0.2
0.4
0.8
0.6
1.0
[NaCI], M
Figure 1. Effect of (NaCI] on the product quantum yields for the photoreduction of iron(l1l) chloride ( O ) ,and iron(ll1) Derchlorate (0). initial concentration of iron(ll1) chloride and iron(ll1) perchlorate was
5.0 x 10-3 M.
Results30 Chemical Scavenging E x p e r i m e n t s . Degassed solutions M of iron(II1) chloride and iron(II1) perchlorate (1.0 X and p H 2.0) were photolyzed a t 350 nm in the presence of varying concentrations of methylmethacrylate (0.5-1.0 M). The polymers formed were recovered, purified, and analyzed for C1 and OH end g r o ~ p s . ~The ~ , test ~ ~ for - ~the ~ C1 end group was always negative, while that for the OH end group was strongly positive. Benzoic acid was also employed as a radical scavenger. When aqueous solutions of iron(II1) chloride or iron(II1) perchlorate (0.001 M and pH 2.0) were irradiated in the presence of sodium benzoate (0,002 M), a deep purple color developed showing the formation of salicylic acid and, hence, OH radicals in the solution. In order to verify whether any chlorobenzoic acids were formed in iron(II1) chloride, the irradiated solution was acidified with HC1 and extracted with ether; the ether extract, after washing with water, was evaporated to dryness and the residue subjected to elemental analysis for chlorine. No chlorine was detected, thus showing the absence of any detectable amount of chlorobenzoic acid in the irradiated solution. Effect of A d d e d Concentration of Chloride on Q u a n t u m Y i e l d . Addition of sodium chloride to iron(II1) chloride or iron(II1) perchlorate had a decelerating effect on the photoreduction (Figure 1).The quantum yield for the photoreduction of iron(II1) chloride decreased by a factor of 5 on increasing the added [Cl-] to 1.0 M. For iron(II1) perchlorate this factor was ca. 10. I t should be noted that 0.005 M iron(II1) perchlorate containing 0.015 M chloride gave nearly the same quantum yield as that of 0.005 M iron(II1) chloride which contained 0.015 M chloride in the absence of any added C1-. Variation of Q u a n t u m Yield w i t h Wavelength. As shown in Figures 2 axd 3 the variation of quantum yields with excitation wavelength in the region 250-425 nm for iron(II1) chloride and iron(II1) perchlorate was dependent on the absorbance of the solutions a t the respective waveThe Journal of Physical Chemistry, Vol. 80, No. 6, 1976
Wavelength, nm
Figure 2. Product quantum yields for the photoreduction of iron(ll1) chloride as a function of wavelength of irradiation: (1) wavelength dependence on quantum yield at constant iron(1ll) concentration of 3.5 X M; (2) wavelength dependence on quantum yield at constant absorbance of 0.55 f 0.03; (3) absorbances of the 3.5 X loy3 M solution as a function of wavelength.
24
12 -
E 6-
a
0
/---
2-
-
O
2o;
'
3Ao
I
do
I
400 4h5
Wavelength,nm
Figure 3. Product quantum yields for the photoreduction of iron(l1l) perchlorate as a function of wavelength of irradiation: (1) wavelength dependence on quantum yield at constant iron(ll1) concentraM; ( 2 ) wavelength dependence on quantum yield tion of 7.0 X at constant absorbance of 0.55 It 0 03; (3) absorbances of the 7.0 X M solution as a function of wavelength.
lengths. When a solution of the same iron(II1) concentration was used a t different wavelengths (the absorbances of this solution were different a t different wavelengths), the quantum yields showed maxima a t 300 and 400 nm. When the quantum yields were determined using solutions whose absorbances were nearly constant (ca. 0.5) a t the desired wavelengths (this was achieved by using different iron(II1) concentrations a t different wavelengths), the quantum
581
Photoredox Chemistry of Iron(ll1) Ions
I
24 -
I
I
1
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I
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I
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81
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1
1
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0
1
2
3
4
5
6
7
8
9
10
[Fe3']x
4
chloride ( l ) , and iron(ll1) perchlorate (2), as a function of concentration. (3) and (4) represent the absorbances of iron(ll1) perchlorate and iron(ll1) chloride, respectively, as a function of concentration.
-
-
0
-
-
I
I
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I
I
I
I
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100
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-6
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m
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-0.8
4-
-
-
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g
U 2.0
3.0
Figure 5. Effect of pH on the product quantum yields for the photoreduction of iron(ll1) chloride ( l ) ,and iron(ll1) perchlorate (2). (3) and (4) represent the absorbances of iron(ll1) chloride and iron(ll1) perchlorate, respectively, as a function of pH.
yield values exhibited a single maximum a t 300 nm; the quantum yields decreased on increasing or decreasing the wavelength from 300 nm. E f f e c t of Initial I r o n ( I I I ) Concentration on Q u a n t u m Yield. The dependence of quantum yield on initial concentration of iron(II1) is depicted in Figure 4 along with the absorbances of the solutions. The change in quantum yield was more pronounced at iron(111) concentrations below 0.002 M. When the concentration is decreased from 0.01 to 0.0005 M, the quantum yield increased by a factor of 4 for iron(II1) chloride and by a factor of 7 for iron(II1) perchlorate. Variation of Q u a n t u m Yield w i t h Incident Intensity. The quantum yields for iron(II1) chloride and iron(II1) perchlorate decreased with increase in incident intensity. For iron(II1) chloride the quantum yields a t incident intensity quanta s-l) of 5.0, 2.6, 1.0, 0.50, and 0.23 values ( X were, respectively, 0.011, 0.015, 0.025, 0.035, and 0.068. For iron(II1) perchlorate the quantum yields a t identical incident intensity values were respectively 0.035, 0.045, 0.055, 0.110, and 0.300. The variation of quantum yields was more significant a t incident intensity values less than 1.0 X 1014 quanta s-'. For higher incident intensities the quantum yields showed a tendency to reach a limiting value. p H a n d t h e Q u a n t u m Yield Values. The quantum yield
-4
: .x
-2
s
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9
0
-1
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X
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120
-1.6z
8E -
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-2.0 c
-
;;1 0 -
5
100
Figure 6. Product quantum yields and the concentrations of iron(l1) formed in the photoreduction of iron(ll1) chloride (1 and 3, respectively), and iron(ll1) perchlorate (2 and 4, respectively) as a function of time of irradiation.
I
-2.4
12-*
B
80
60
40
Time of irradiation, h
Figure 4. Product quantum yields for the photoreduction of iron(ll1)
0
0 20
103M
1 1
0
I
I
I
I
2
4
6
8
I
I
1
10 12 14 ([Fe2+lo + $[Fez'],) x 1 05M
1
16
-0
1
Flgure 7. Product quantum yields for the photoreduction of iron(ll1) chloride (B - - B), and iron(ll1) perchlorate (0 - - 0 )as a function of added concentration of iron(l1) perchlorate. A and B are the plots vs. [Fe2+Iav for iron(ll1) perchlorate and iron(ll1) chloride, of 1/anet respectively. Time of irradiation for iron(ll1) perchlorate solutions was 2.5 h.
values a t various pH (adjusted with HC104) along with the absorbances of the solutions are displayed in Figure 5. For the iron(II1) chloride, the quantum yields showed a maximum a t pH 2.0. Iron(II1) perchlorate exhibited the quantum yield maximum a t pH 1.2. In comparison, the change in quantum yield was more pronounced for iron(II1) perchlorate (a factor of 3.5 between the highest value a t p H 1.2 and the lowest value a t pH 2.8) than for iron(II1) chloride (a factor of 1.7 between the highest value a t p H 2.0 and the lowest value a t p H 2.8). T i m e of Irradiation and Q u a n t u m Yield Values. As shown in Figure 6, the quantum yields for the photoreduction of iron(II1) chloride and iron(II1) perchlorate decreased with the time of irradiation. The plot of [Fe2+] formed vs. time of irradiation showed that the iron(I1) concentration reached a limiting value of ca. 1.4 X M. The extrapolated quantum yield values to zero time of irradiation were 0.077 and 0.033, respectively, for iron(II1) perchlorate and iron(II1) chloride. Variation of Q u a n t u m Yield w i t h Added Concentration o f I r o n ( I I ) . Added concentrations of iron(I1) (as iron(I1) The Journal of Physical Chemisfry, Vol. 80, No. 6, 1976
F. David and P. G. David
582
perchlorate) decelerated the photoreduction (Figure 7), the quantum yields dropping nearly to zero for iron(I1) values M and higher. I t is interesting to note that ca. 1.4 X this concentration was nearly same as the limiting iron(I1) concentration in the plot of [Fez+] vs. time of irradiation (Figure 6). The plots of l/@vs. [Fe2+IaV(where [Fez+],, = [Fe2+]o+ 1/~[Fe2+]f; [Fe2+]ois the concentration of iron(I1) present initially and [Fe2+]fis the concentration of iron(I1) formed photochemically) gave straight lines (Figure 7) with slopes of 6.5 X lo5 and 2.5 X lo5, and intercepts of 26 and 9.5, respectively, for iron(II1) chloride and iron(II1) perchlorate. Dependence of Q u a n t u m Yields on Temperature. Quantum yield for the photoreduction of iron(II1) chloride increased with increase in temperature of the irradiated solution, the quantum yields being 0.015, 0.017, 0.020, 0.025, and 0.032, respectively, a t temperatures 5, 10, 20, 30, and 40 "C. The variation of quantum yields for iron(II1) perchlorate was negligible (the quantum yield decreased ca. 5% as the temperature was raised from 5 to 40 "C).
Discussion The experimental results presented herein are in agreement with the hypothesis that, in aqueous solutions of iron(II1) chloride as well as iron(I11) perchlorate, the primary photoprocess is the homolytic cleavage of the Fe(II1)OH bond.] The primary photoreaction may be represented as hu
Fe3+.0H- --+Fez+
+ OH
Radical scavenging experiments employing benzoate and methylmethacrylate clearly indicated that OH radicals were produced during the irradiation. There was no indication for the production of C1 radicals as was formulated earlier.16-ls Further evidence in support of formulating Fe3+-OH- as the photoactive species is the variation of quantum yield with added concentration of C1- (Figure 1).The concentration of Fe3+.C1- increases with increase in [C1-].15 Hence, a t constant pH, the molar fraction of Fe3+.C1- increases in comparison to Fe3+.OH-. Consequently, an increase in quantum yield for the photoreduction is expected if Fe3+C1-- is the photoactive species, However the results, contrary to this prediction, indicate that Fe3+.0H-, rather than Fe3+.C1-, is the photoactive species. A decrease in molar fraction of Fe3+.OH- with the consequent decrease in the fraction of light absorbed by it might account for the decrease in quantum yield with increase in [Cl-1. The dependence of quantum yield on excitation wavelength might prove to be another strong point in identifying Fe3+.0H- as the photoactive species. Fe3+-C1- has a charge-transfer maximum a t 340 nm15 while Fe3+.0H- has this charge-transfer absorption maximum a t 300 nm.s-lo Both iron(II1) chloride and iron(II1) perchlorate showed quantum yield maxima a t 300 nm (Figures 2 and 3) indicating the photoactive charge-transfer band centered a t 300 nm. The slight decrease in quantum yield values a t wavelengths lower than 300 nm might arise from the partial absorption of incident light by Feaq3+ which is photochemically less active than Fe3+.0H-. The appearance of a second maximum a t 400 nm on employing solutions of constant initial iron(II1) concentration might be a manifestation of extremely low light absorption (
