The Determination of Axial Ligand Binding Constants for Iron Porphyrins by Cyclic Voltammetry David K. Geiger, Edmund J. Pavlak, and Lawrence T. Kass State University of New York, College at Geneseo, Geneseo, NY 14454 Complex stahility constants are usually determined from spectral or potential measurements ( I ) . In order for spectral techniques to he successful, the various components must have reasonably separated absorption maxima and an isoshestic point must be observed. If these conditions are not met, the system is generally not amenable to spectral determination. In addition, if the metal is in a highly reactive oxidation state, the required spectral measurements become difficult or impossible. These prohlems can he overcome by using electrochemical methods; a number of laboratory experiments employing polarographic or voltammetric techniques for the evaluation of stahility constants and other characteristics of transition metal complexes have appeared in this Journal (2,3). The experiment described herein takes advantage of the shift in reduction potential accompanying the binding of axial ligand(s) to determine the stoichiometry and axial ligand formation constants of an iron porphyrin (4). This method is particularly convenient for Fe(I1) porphyrins, which are extremely air-sensitive (4a). In addition, this experiment lays the groundwork for a discussion of the role of model compounds in the elucidation of the complex structure-function relationships found in metalloproteins. Background Iron(I1) and iron(II1) porphyrins, Fe(P), may take on one or two axial ligands. The complexation equilibria can be described by the following equations: Fe(II)(P)+ L = Fe(II)(P)(L)
(1)
Fe(II)(P)+ 2L = Fe(II)(P)(Lh
(2)
Fe(III)(P)++ L = Fe(III)(P)(L)+
(3)
Fe(III)(P)++ 2L = Fe(III)(P)(Lht
(4)
The counterion for the Fe(II1) species is not shown. In coordinating solvents, solvent molecules are bound in the axial positions prior to ligand binding. The formation constants are then defined as
A superscript I1 or 111 is used to distinguish between the formation constants for the Fe(I1) or Fe(II1) species, respectively. The following equation (4a,5) relates the half-wave reduction potential, Eliz, to the axial ligand formation constants of iron porphyrins: (E,,), = (E,,),
- 0.059 log (B,O"lB,'ed)- 0.059 log
(9)
where (Ellz), and (Eliz), are one-electron half-wave reduction potentials in the presence and absence of ligand, L, respectively, and p and q are the number of ligands hound to the oxidizedand reduced species,respectively. From eq 9 the VS. log [L] is a slope of a linear segment of a plot of (E112)~ multiple, p - q, of -59 mV. T h e y intercept of the same linear segment of the plot is (EI,~),- 0.059 log (B,~"lBqred). In practice, obtaining all four constants depends on the fact that Fe(I)(P) does not bind nitrogenous axial ligands (4),so eq 9 reduces to (E,,),
= (E,/,),
- 0.059 log^," - 0.059 log [LIP
(10)
for theFe(II/I) wave. Upon substitution into eq 9, theformation constants for the Fe(I1) species obtained are used, along with the half-wave potentials for the Fe(III/II) wave, to derive the constants for the Fe(II1) porphyrin (46).
Volume 88 Number 4
April 1991
337
Experimental
Reagents and Apparatus The oorohvrin used for this studv waa (chloro)(meso-tetranhenylporphyrinato)iron(III), Fe(TPP)CL.This complex is commercially available from Aldrieh Chemicals or may he conveniently synthesized by the method of Adler et al. (6).The ligands employed in this study were imidazole, (Im),and 2-methylimidazole, (2-MeIm).Both are available from Aldrich Chemicals. Cyclic voltammetry is used to obtain the half-wave reduction potentials for the Fe(IIIA1) and Fe(II/I) couples in the absence and the presence of various amounts of a " eiven lieand. .. We have found that 0.1 M tetraethvlammonium perchlorate, TEAP, in spectral-grade dimethylformamide, I)MF, performs well as a supporting elertrolyte-solvent SYStQmcomhination. Complenitier msoriated with the displarement of anions from thec8,ordinatiunrphere are minimized in coordinating ~olwnrssuch na DMF 1 4 ~ )An . IRM EC(225 Voltammetric Analyzer was used in roniunction with a three-elertnrde cell supplied hy Bioanalsticel systems and composed of aglassy carbon working electrode, ~t wire auxiliary electrode and a AgIAgC1 reference electrode.
. ..
~~
~
~
Procedure A 10.0-mL stock solution of 0.100 M lieand in DMF was nre~ared. A 25.0-mL DMF solution 10-'M in F ~ ( P ) Cand I 0.1 M in TEAP ~ s s aLw prepared. A 10.0 rnl. aliquot of the porphyrin solution was placed in the eleccrorhernical cell and deserated for 10 min with a stream of nitrogen. A voltammogram was ohtained using a scan range of t0.3 to -1.4 V versus AgIAgCl and a scan rate of 50 mV1s. Aliquots of the ligand stock solution were added via a microliter syringe until a total of 1.0 mL had been added. A total of eleven additions were made as follows: two 10-aL, four 20-aL, one 100-aL, and four 200-FL diquats. Higher concentrations were ohtained by adding milligram quantities of the solid ligand directly to the solution. The ligand concentration was brought to 0.11 M for (Im) by adding an 8-mg and four 16-mg portions of ligand and 1.6 M for (2MeIm) by adding four S m g , one 80-mg, and seven 180-mg portions. After each addition of ligand, the andyte solution was deaerated for 5 miu and a voltammogram ohtained. , F l w 2. Plots of Ell? for IM Fe(lll1ll)and Fe(ll/l)waves as a function 01 2-
Rawits T h e analyses of data collected for the ligands (2-MeIm) and (Im) are described below. Similar procedures are followed for other lieands. A typical cycli~voltammngramis shown in Figure 1.In the notential ranee scanned. the Fe(IIIA1) and Fe(IIA) . . waves are observed. T h e shifts in the potentials accompanying the addition of (2-Melmr to the oorohvrin . . . solution indicate that theaffinity for this ligand isgreaterfor Fe(llJ(TP1'J than for either Fe(lll)(l'PP)' or Fe(I)(TPPJ-( 4 , 7). Figure 2 shows plots of Eliz versus log [2-MeIm] for both of the reduction waves. An examination of the plot for the Fe(1IA) wave shows that the data can be divided into two
rnethyllmMszole wncentrmlon. regions. At (2-MeIm) concentrations greater than about M, the slope of the curve is -55 mV. This indicates that p - q = +1(i.e., the Fe(I1) species is hound to one more (2MeIm) ligand than the Fe(1) species). As Fe(1) porphyrins do not bind nitrogenous axial ligands (4), the data indicate that the appropriate redox process is: Fe(II)(TPP)(Z-MeIm)t e- = Fe(I)(TPP)-
+ (2-MeIm)
Using eq 10, BI" is evaluated from the y intercept of this linear region of the curve. T h e value ohtained is reported in the table. Examination of the data from the Fe(IIIAI1) wave reveals three regions with significantly different slopes (see Fig. 2). In theconcentrationranae 10-54-10-1s M. theslooe isabout +63 mV, s o p - q = -1. this region, the process involved is Fe(II1)TPP'
+ (2-MeIm) + e- = Fe(II)(TPP)(2-MeIm)
For the concentration range 10-1-L10-'.2 M, the slope is zero, s o p - q = 0, and the redox process observed is Fe(III)(TPP)(2-MeIm)'
+ e-
= Fe(II)(TPP)(2-MeIm)
The constant BI"' is found via eq 9, which simplifies t o
-10
-I
0.4
I
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 V vs. Ag/AgCI Flws 1. Cyclic voltammqlrarnooWllnedfor lO-'M FeCTPPp.71n DMFand 0. I MTEAPataglassy carbonelecwode. Scanrate = 50 mvls. Sollacurve: no(2Melml present. W e d cuve: 0 01 M (2-Mslml.
338
0.2
Journal of Chemical Education
under these conditions. Finally, a t higher concentrations of ligand, the slope is about -44 m V s o p - q = +1, which corresponds to the redox process
Equation 9 thus becomes = (Ell& - 0.059 log (B2111/B111) - 0.059 log [L] (12)
They intercept of this linear region of the curve (see Fig. 2) is evaluated to determine &I1'. The formation constants obtained are listed in the table. The data collected for a run employing imidazole is shown in Figure 3. Again, the data from the Fe(II/I) wave was used to evaluate the stoichiometry of the reduction process. The slope of the linear portion of the curve is -133 mV ( p - q = +2) indicative of the process
and eq 10 was used to evaluate Bz". The data from the Fe(III/II) wave was used along with B 2 1 1 to determine the formation constants for the Fe(II1) species. The data can be divided into two regions of interest (see Fig. 3). In the (Im) M, the slope of the curve concentration range 10-3.4 to is +49mV (orp q = -I), so theredox process occurringis
-
+
~e(111)(TPP)(Irn)+(In) + e- = Fe(II)(TPP)(Im), and use of eq 9 yields Eln1.At (Im) concentrations greater thanO.O1 M, thevalue of E l i 2 is constant, indicating t h a t p o = 0 and both members of the redox c o u ~ l are e bound to the same number of (Im) ligands:
~. ~
~
Figwe 3. Plots of El,, for Me Fe(lll/ll)~amlFFeIll) waves as a hmctlon of lmldazole concentration.
~
+
Fe(III)(TPP)(Im),+ e- = Fe(II)(TPP)(Im),
The values of the formation constants are listed in the table. Dlscusslon Besides imidazole and 2-methylimidazole, our students have examined the ligands N-methylimidazole and pyridine. The procedure for data analysis is similar to the examples discussed above. At low ligand concentrations and when the Fe(II1) porphyrin formation constants are very large, much of the ligand will be complexed. Walker (4b) has suggested that the concentration of free lieand be calculated via an iterative procedure using the initial values of the formation constants for the FetIlT). .norvhvrin toobtain refined constants. Wedid . . not take into account the ligand bound by the principal species in solution, Fe(III)(TPP)+. However, as the table shows, our results agree reasonably well with literature values (4,8). The preparation of solutions and collection of the electrochemical data is easily performed in one 4-h laboratory period. Data analysis is facilitated by the use of a spreadsheet program, and our students are encouraged to make use of a computer for the repetitive calculations, least-squares analyses, and graphing. The experiment described above demonstrates the validity of using cyclic voltammetry for the determination of complex stoichiometry and formation constants. The systems examined also point up the role of model compounds in
the study of the heme proteins. Formation constants for both five- and six-coordinate FeW) and Fe(II1) porphyrins depend on a number of factors including the electronic and steric characteri~ricsof the added ligand (8bJ.Interestingly, Hoffman (9)has recently reportedadramatic dependence of electron transfer rates on the heme axial ligand for mixedmetal hemoglobin hybrids. Obviously, an understanding of factors that influence the redox chemistry of iron porphyrins is a prerequisite for the elucidation of the numerous electron transfer mechanisms in biological systems that employ heme groups (7,lO). Acknowledgment This work was supported by an Exxon Education Foundation Grant of Research Corporation.
230.
6. Adle?, A. D.:Longa, F.R.; Kampas, R.: Kim, J. J.lnorg.Nue1. Chem. 1970, 32, 24432445.
7. Kadi8h.K. M.;Bottamley, L. A.lnor8. Chrm. 1980,19,832-836. 8. (a) Lera, D.: Momentesu, M.;Mispelter. J.; Lhorte. J. M. Biodecfrorhem. Riornarg. 1971,1,2~17.(b)Walker,FA.:Lo,1l.W.;Rrc,M.TJ.Am.Chsm.Soc.15749B, sec,-zc
