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New Insight Into Photochemistry of Ferrioxalate

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New Insight Into Photochemistry of Ferrioxalate 8316 J. Phys. Chem. A 2008, 112, 8316–8322 New Insight into Photochemistry of Ferrioxalate Ivan P. Pozdnyakov,* Oksana V. Kel, Victor F. Plyusnin, Vjacheslav P. Grivin, and Nikolai M. Bazhin Institute of Chemical Kinetics and Combustion, Institutskaya 3, and NoVosibirsk State UniVersity, PirogoVa2, 630090 NoVosibirsk, Russia ReceiVed: May 8, 2008; ReVised Manuscript ReceiVed: June 20, 2008 Optical spectroscopy and nanosecond flash photolysis (Nd:YAG laser, 355 nm, pulse duration 5 ns, mean III 3- energy 5 mJ/pulse) were used to study the photochemistry of Fe (C2O4)3 complex in aqueous solutions. The main photochemical process was found to be intramolecular electron transfer from the ligand to Fe(III) II • 3- ion with formation of a primary radical complex [(C2O4)2Fe (C2O4 )] . The yield of radical species (i.e., • - • - III 3- II • 3- CO2 and C2O4 ) was found to be less than 6% of Fe (C2O4)3 disappeared after flash. [(C2O4)2Fe (C2O4 )] II • - dissociates reversibly into oxalate ion and a secondary radical complex, [(C2O4)Fe (C2O4 )] . The latter reacts II with the initial complex and dissociates to Fe (C2O4) and oxalate radical. In this framework, the absorption spectra and rate constants of the reactions of all intermediates were determined. 1. Introduction Fe(III)-O bond (between one oxalate ligand and Fe(III) ion) and the C-C bond of the ligand, in which biradical complex The photochemical reactions of Fe(III)-(poly)carboxylate [(C O ) FeIII(CO •) ]3- 16 or CO • - radical are assumed as complexes influence many iron-dependent biogeochemical 2 4 2 2 2 2 primary intermediate(s).17,18 processes and are responsible for photochemical production of 1-4 In spite of the serious difference of the proposed mechanisms, CO and CO2 and for oxygen consumption in natural waters. It is assumed that the photolysis of Fe(III)-(poly)carboxylate the successive formation of several (at least two) intermediates complexes is one of the main sources of active oxygen species is supposed in all aforesaid works. One of these species has an • • absorption maximum near 400 nm and disappears in the ( OH, HO2 ,H2O2), the formation of which is catalyzed by Fe(II) and Fe(III) ions in Fenton-like reactions.4-7 Active oxygen millisecond time range. It was also found that the decay of this 16-18 15,17 species influence the chemical composition and the oxitation- intermediate depends on both complex and ligand II 4-17 II • 3- 14,15 reduction capacity of natural water systems, in particular concentrations.TheFe (C2O4)3 or the [(C2O4)2Fe (C2O4 )] III • 2-16 providing oxidation of sulfur- and nitrogen-containing com- and the [(C2O4)2Fe (CO2 )] radical complexes were assumed pounds in the atmosphere.8,9 However, the primary mechanisms as the intermediate with the millisecond lifetime. of photoreaction for most Fe(III)-(poly)carboxylate complexes Unfortunately, information about the spectral and kinetic are still not fully investigated. properties of primary species formed immediately after excita- III 3- Among Fe(III)-(poly)carboxylate complexes, the photo- tion of the Fe (C2O4)3 complex is rather limited. This is due III 3- chemistry of trioxalate Fe(III) complex [Fe (C2O4)3 ]isof to the application of methods with insufficient time resolution intense interest, as this species not only exhibits high photo- in the majority of works devoted to the photochemistry of 8,9 III 3- 14,16,17 chemical activity in natural waters under sunlight but also is Fe (C2O4)3 . So suggestions about the mechanism of 10,11 III 3- widely used as a stable and sensitive chemical actinometer. Fe (C2O4)3 photolysis presented in these works were mainly III 3- The total process of Fe (C2O4)3 photolysis could be presented hypothetical. as12 There are only two works in which the photochemistry of III 3- hν the Fe (C2O4)3 complex was investigated with sufficient time III 3- 98 II 2- 2- + + 2- 15,18 2Fe (C2O2)3 2Fe (C2O4 )2 2CO2 C2O4 resolution, and it is worth noting that the conclusions of these works are in a conflict with each other. In ref 15, two intermediates with lifetimes 30 µs and 1 ms were detected by (1.1) means of a nanosecond flash photolysis technique. The spectrum The quantum yield of Fe(II) formation in reaction 1.1 is of long-lived intermediate corresponds well to transient spectra independent of excitation wavelength in the range 270-365 nm observed in microsecond flash photolysis experiments.14,16,17 The and equals 1.24.10,13 This means that the absorption of UV short-lived intermediate was assigned to the excited state of the III 3- quantum leads to the photoreduction of the excited complex complex (*[Fe (C2O4)3 ]). This state decays due to electron with a 60% probability. transfer from ligand to central ion with formation of the long- II • 3- There are two different points of view on the primary lived [(C2O4)2Fe (C2O4 )] radical complex. It was assumed III 3- II 2- • - photoprocess following by excitation of Fe (C2O4)3 : (1) that radical complex dissociates to Fe (C2O4)2 and C2O4 intramolecular electron transfer from the ligand to Fe(III) ion radical and oxidation of the latter species by the second III 3- in which the long-lived excited state, the radical complex Fe (C2O4)3 leads to formation of final photoreaction products. II • 3- • - III 3- [(C2O4)2Fe (C2O4 )] or the C2O4 radical are proposed as In ref 18, the photochemistry of Fe (C2O4)3 was investi- primary intermediate(s);14,15 or (2) sequential cleavage of the gated by combination of time-resolved extended X-ray absorp- tion fine structure (EXAFS) spectroscopy, flash photolysis with * Corresponding author: e-mail [email protected]. different time resolution (from femto- to milliseconds), a radical 10.1021/jp8040583 CCC: $40.75 2008 American Chemical Society Published on Web 08/15/2008 New Insight into Photochemistry of Ferrioxalate J. Phys. Chem. A, Vol. 112, No. 36, 2008 8317 scavenging technique, and theoretical calculations. Ultrafast K3Fe(C2O4)3 (analytical grade) was recrystallized from aque- formation (<2 ps) of an absorption band with a maximum near ous solution before use. K2C2O4 (analytical grade) and methyl 430 nm was observed, which was assigned to the excited state viologen dichloride hydrate (Aldrich, 98%) were used without III 3- of Fe (C2O4)3 . At 4 and 9 ps after excitation, the length of further purification. Fe(II)-oxalate complexes were obtained the Fe-O bond was found to decrease to 1.93 and 1.87 Å, by mixing freshly prepared oxygen-free solutions of FeSO4 and respectively. When it was taken into account that the experi- K2C2O4. FeSO4 solution was obtained by dissolution of carbonyl mental values of Fe(II)-O bonds are always higher than those iron powder (analytical grade) in 0.1 M sulfuric acid (chemical of Fe(III)-O, the primary photoprocess was proposed to be grade) under constant argon bubbling. Fe(II) concentration in 12 Fe-O bond cleavage in the excited state with formation of five- the FeSO4 solution was determined by complexometry with III 3- coordinated [(C2O3)O-Fe (C2O4)2] intermediate. This pro- 1,10-phenanthroline as a ligand. This method was also used to cess is accompanied by C-C bond cleavage of the ligand, determine the quantity of Fe(II) produced in the flash experiments. III - • - leading to formation of stable Fe (C2O4)2 complex and CO2 The samples were prepared with deionized water. Unless radical: otherwise specified, all experiments were carried out with oxygen-free samples at pH ) 6-7, ionic strength 0.1 (LiClO4, III 3- f III 3- f *[Fe (C2O4)3 ] [(C2O3)OFe (C2O4)2] chemical grade), and 298 K in a 1 cm optical cell. Oxygen was - •- removed by bubbling solutions with gaseous argon. Initial FeIII(C O ) + 2CO (1.2) 2 4 2 2 concentrations of K3Fe(C2O4)3 and K2C2O4 were varied in the -4 -4 III 3- III - •- ranges (1-12) × 10 and (3-300) × 10 M, respectively. *[Fe (C O ) ] f Fe (C O ) + 2CO (1.3) 2 4 3 2 4 2 2 Under the aforesaid conditions, the main form (>90%) of Fe(III) 18 III 3- 20 The authors could not distinguish between reactions 1.2 and in solution was Fe (C2O4)3 complex. Samples for flash 1.3 due to insufficient time resolution of EXAFS method. For photolysis were used until a 10% decrease in UV absorption III 3- III - both [(C2O3)O-Fe (C2O4)2] and Fe (C2O4)2 , theoretical occurred. calculations predict a Fe-O bond length of about 1.9 Å. The existence of reactions 1.2 and 1.3 was also supported by more 3. Results and Discussion than 50% decrease in Fe(II) quantum yield upon the addition 3.1. Optical Spectroscopy of Fe(II) and Fe(III) Oxalate of radical scavenger (thymine) in ferrioxalate solution. Complexes. Depending on the pH and the concentrations of At longer time scales (from tens of nanoseconds to 2 ms), Fe(III) and oxalate ions, complexes with one, two, and three the transient absorption signal exhibits biexponential decay in ligands in the coordination sphere of Fe(III) ion can exist in a similar manner as observed in ref 15. This behavior was aqueous solutions:20 explained by the following set of reactions:18 + - + Fe3 + C O 2 S Fe(C O ) pK ) 9.4 III - + •-f II 4- + aq 2 4 2 4 1 Fe (C2O4)2 CO2 Fe (C2O4)3 CO2 (1.4) + - - Fe3 + 2C O 2 S Fe(C O ) pK ) 16.2 II 4- S II 2- + 2- aq 2 4 2 4 2 2 Fe (C2O4)3 Fe (C2O4)2 C2O4 (1.5) + - - Fe3 + 3C O 2 S Fe(C O ) 3 pK ) 20.2 The last reaction has to explain dependence of the intermediate aq 2 4 2 4 3 3 absorption decay on a ligand concentration observed in the All Fe(III)-oxalate complexes exhibit charge-transfer (CT) previous work.17 So in the mechanism of photoreactions bands with maxima about 260 nm overlapping with a strong proposed,18 the main intermediates on the aforementioned time absorption band of the ligands at 210 nm.6,12,18 Intensity of the scale are the oxalate complexes of Fe(III) and Fe(II).
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