Inorg. Chem. 1994, 33, 3817-3822
3817
Complexation of the Aqua-Iron(II1) Dimer by Tiron: Kinetics of Complex Formation and Dissociation J. Chatlas and R. B. Jordan' Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received February 3, 1994"
The reaction of Tiron (1,2-dihydroxy-3,5-benzenesulfonate) with aqueous iron(II1) has been studied a t moderate acidities (0.1-0.02 M H+) by stopped-flow spectrophotometry at 25 O C in 1.O M NaC104/HC104. Theconcentration of the bis(p-hydroxo) dimer of iron(II1) has been systematically varied by controlling the H+ and iron(II1) concentrations in the iron(II1) solutions before mixing with Tiron solutions at various acidities to give the desired final [H+]. The absorbance change at 660 nm is biphasic, and the absorbance-time profiles for the slower absorbance decrease have been modeled by numerical integration. It is found that the faster absorbance increase is due to reaction of (H20)8Fe~(p-OH)2~+ with Tiron to give a blue complex with k = 5.1 X lo3 M-l s-l, but no protons are released during the complexation, and the formation equilibrium constant is 6.3 X 103 M-1, The dimer complex decomposes to the less strongly colored monomeric complex and F e ( o H ~ ) 6 ~with + k = 0.2 s-I. The possible nature of the dimeric complex is considered, and the relevance of these observations to previous studies on catechols, squaric acid, and ascorbic acid is discussed.
Introduction
This study began as an investigation of the reaction of isopropylideneascorbic acid with aqueous iron(II1). However, it soon became apparent that even the qualitative observations on that system were sufficiently unusual and complex that they would be difficult to unravel without further background information. For that reason, it was decided to investigate the reaction of 1,2-dihydroxy-3,5-benzenesulfonate(Tiron). This system is simpler because the ligand is quite stable under acidic conditions, the reduction potential of Tiron ( E o = 0.955 V)* requires that no oxidation-reduction will occur, and only complexation reactions will be observed. Although this system had been the subject of two previous kinetic s t u d i e ~ ,there ~ - ~ was reason to believe that thecomplexities we wereobserving weredue to the bis(p-hydroxy) dimer of iron(III), (H20)8Fe~(OH)2~+, and that these had been avoided as much as possible in the earlier work. This suspicion was reinforced by a recent study4 of squaric acid and aqueous iron(II1) in which the dimer complexation was found to be an important feature. It may be noted at the outset that some of the observations here have close analogies to those with squaric acid. In the present work, the amount of bis(p-hydroxo) dimer was varied systematically by controlling the total iron(II1) and acidity of the iron( 111)-containingsolution before mixing with the neutral or acidic Tiron solution in the stopped-flow system. The conditions were controlled so that the concentration of the dimer was almost always in reasonable pseudo-first-order excess over Tiron. Although the dimer undergoes some dissociation to monomer on the time scale of some of our observations, the kinetics of this process are known526 and can be taken into account. Results and Analysis
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QualitativeObservations, The reaction is biphasic with an initial absorbance increase which is complete in 100 ms or less. This is followed by an absorbance decrease over a time of -5 s. @Abstractpublished in Advance ACS Abstracts. July 1, 1994. (1) Pelizzetti, E.; Mentasti, E. Z . Phys. Chem. 1977, 205, 21. (2) Xu, J.; Jordan, R. B. Inorg. Chem. 1988, 27, 1502. (3) Mentasti, E.; Pelizzetti, E.; Saini, G. J . Inorg. Nucl. Chem. 1976, 38, 185.
(4) Sisley, M. J.; Jordan, R. B. Inorg. Chem. 1991, 30, 2190. (5) Sommer, B. A.; Margerum, D. W. Inorg. Chem. 1970, 9, 2511. (6) Po, H . N.; Sutin, N. Inorg. Chem. 1971, 10, 428.
Observations on a stopped-flow diode array system show that the increase is associated with formation of a blue species with an absorbance maximum at -660 nm. The decrease is associated with loss in intensity at all wavelengths, but the final absorbance is often finite and even substantial under some conditions. One would expect the absorbance increase due to complexation, but the source of the decrease in absorbance seems problematic, since Tiron is not oxidized by iron(II1). The absorbance-time curves are fitted well by a standard twoexponential first-order model, and the apparent absorbancies of the intermediate (Amax)and rate constants for the faster and slower reactions are summarized in Tables 1 and 2. Since the rate constant for the faster process (ki) is at least 10 times greater than that for the slower process, the values are well defined. Kinetics and Equilibrium for the Faster Reaction. An examination of the magnitude of the absorbance of the intermediate (A,,, in Table 1) reveals that it appears to be independent of the acidity and does not correlate with total iron(II1) concentration, but does correlate with the concentrations of Tiron and iron(II1) dimer. Thevalues of Am,/ [Tiron] have a limitingvalueof 2200 for [dimer] > 18 X 10-4 M and [Tiron] > 1 X 10-4 M. These results are qualitatively consistent with the faster reaction being an equilibrium reaction of the general form of eq 1. The site of
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protonation of the product is left ambiguous at this stage, but no protons are released by the reaction because A,,, is independent of [H+],and the kinetics described below show no H+dependence for decomposition of the product. From the limited knowledge of the reactivity of the dimer,2*4 it is not surprising that it undergoes more rapid complexation than monomeric aquairon(II1) species. However, it is unexpected that the complexation should proceed without the release of protons and that the equilibrium constant is so large that the equilibrium is essentially completely to the right for the low dimer and Tiron concentrations noted above. These imply an equilibrium constant of 104 M-1 for eq 1 . It is well-known293 that F e ( o H ~ ) 6 ~forms + complexes with a wide range of 1,2-dihydroxybenzene derivatives (H*L, catechols) as shown by eq 2. The equilibrium constants for these reactions (Kfl)depend on the pK, of the catechol, and the value for Tiron2
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0020-1669/94/1333-3817$04.50/0 0 1994 American Chemical Society
Chatlas and Jordan
3818 Inorganic Chemistry, Vol. 33, No. 17, I994 Table 1. Observations on the Increase of Absorbance at 660 nm for the Reaction of Tiron with Aqueous Iron(II1) in 1.0 M NaC104/ HClO4 at 25 OC
104[(H20hoF e ~ ( o H ) 2 ~ + ] [H+], ,
104[Tiron], [iron] M M 2.0 2.0" 2.0 2.0" 0.50 1.o 2.0 3.0 4.0 1 .o 1.o 1 .o 1.o 1.o 1.o 1 .o 1 .o
0.50 0.50 0.50 0.50 0.50 0.50
0.041 0.041 0.041 0.041 0.041 0.041 0.041 0.041 0.041 0.0205 0.041 0.0205 0.041 0.0205 0.0308 0.04 10 0.05 12 0.0410 0.0410 0.0410 0.0410 0.0410 0.0410
M
M
27.4 27.4 27.4 27.4 27.4 27.4 27.4 27.4 27.4 0.413 1.63 18.4 39.1 18.4 35.6 43.7 51.1 27.4 27.4 27.4 27.4 27.4 27.4
0.0241 0.0241 0.1005 0.1005 0.0241 0.0241 0.0241 0.0241 0.0241 0.1057 0.1062 0.1102 0.115 0.110 0.110 0.114 0.117 0.217 0.120 0.0908 0.0618 0.0427 0.0241
Scheme 1
ki, SSI A,,
0.455 0.447 0.434 0.432 0.107 0.211 0.422 0.616 0.832 0.117 0.138 0.213 0.225 0.220 0.236 0.227 0.225 0.102 0.106 0.108 0.111 0.111 0.119
obsd
calcd
35.4 36.8 18.9 19.0 33.8 33.0 34.5 35.5 31.8 (1.24)b (2.91)b 13.0 27.2 13.3 27.5 32.6 36.9 14.5 18.4 20.2 23.3 28.2 34.6
34.7 34.7 19.5 19.5 34.7 34.7 34.7 34.7 34.7 1.32 2.12 13.1 26.4 13.1 24.4 29.5 34.1 16.9 18.7 20.0 22.5 26.0 34.7
not included in the following analysis but are discussed with the slower reaction. The rate constants (ki, Table 1) are consistent with the equilibrium arguments above in that the values increase with increasing dimer concentration and do not change with increasing [H+]. The values of kiare consistent with the reactions in Scheme 1, where Mz(0H)z represents the dimer and Mz(OH)z(OH) is a hydrolyzed dimer species. An alternative k2 path is discussed below. Scheme 1 predicts that the pseudo-first-order rateconstant is given by eq 3, where [Mzlt = [Mz(OH)2] + [M2(OH)z(OH)]
(3) and Kn = kl/k-l. If [H+] >> K,D and [H+] >> KJ, then eq 3 simplifies toeq 4.Least-squares analysis gives kl = (5.05 f 0.25)
0 Under anaerobic conditions in an argon atmosphere. Not used in the least-squares fit to eq 4 because of non-first-order conditions. Analyzed by numerical integration: see Figures 2 and 3B.
Table 2. Observations on the Decrease of Absorbance at 660 nm for the Reaction of Tiron with Aqueous Iron(II1) in 1.0 M NaC104/ HC104 at 25 "C 104[Tiron],
[ironItot,
M
M
104[(H20)IOFez(0Hh4+1, M
2.0 2.0" 2.0 2.0" 0.50 1.Ob 2.0b 3.0b 4.0b 1.OC
0.041 0.041 0.041 0.041 0.041 0.041 0.041 0.041 0.041 0.0205 0.0205 0.0205 0.0308 0.0410 0.0410 0.0410 0.0512 0.0410 0.0410 0.0410 0.0410 0.0410 0.0410
27.4 27.4 27.4 27.4 27.4 27.4 27.4 27.4 27.4 0.413 18.4 18.4 35.6 1.63 39.1 43.7 51.1 27.4 27.4 27.4 27.4 27.4 27.4
1 .o 1 .Od 1 .Od
1.OC 1. o c 1 .Od 1 .Od 0.50 0.50 0.50 0.50 0.50 0.50
[H+l,
M
Ar
ks,
0.0241 0.0241 0.1005 0.1005 0.0241 0.0241 0.0241 0.0241 0.0241 0.1057 0.1102 0.110 0.110 0.1062 0.115 0.114 0.117 0.217 0.120 0.0908 0.0618 0.0427 0.0241
0.197 0.206 0.182 0.185 0.017 0.058 0.179 0.329 0.470 0.050 0.033 0.030 0.060 0.085 0.075 0.080 0.095 0.016 0.015 0.017 0.016 0.019 0.022
s-I
0.335 0.335 0.362 0.367 0.18 0.225 0.324 0.416 0.487 0.176 0.175 0.182 0.234 0.284 0.259 0.281 0.360 0.185 0.184 0.186 0.185 0.185 0.175
Under anaerobic conditions in an argon atmosphere. Calculated and observed results are given in Figures 1 and 3A. Calculated and observed results are given in Figures 2 and 3B. Calculated and observed results are given in Figure 4.
Fe(OH,),3+
-
+ H,L
e (H20),Fe(L)+
+ 2H30+
(2)
is 3 M. Clearly, the reaction with the dimer is different because no protons are released and product formation is much more favorable in acidic solution. Nevertheless, under conditions where the dimer concentration is
