Axial Ligand Effects on the Spin State and Electrochemistry of Iron Porphyrins An Advanced Laboratory Experiment David K. Geiger State University of New York, College at Geneseo, Geneseo, NY 14454 Transition metals play a n important role in a myriad of biological processes. They and their complexes serve as structural supports, electron transfer relays, oxygen transport and storage depots, and catalytic centers (I). Model compounds have served t o further our understanding of many of these complex systems. T h e complex structure-function relationships of a number of metalloproteins can be rationalized using models of coordination chemistry that are introduced in a basic inorganic chemistry course. Although most modern inorganic chemistry texts have incorporated a chapter on bioinorganic chemistry (2), in a crowded curriculum this topic may be glossed over or neglected entirely. The undergraduate laboratory can provide the medium to explore various aspects of bioinorganic chemistry, and a number of laboratory experiments in bioinorganic chemistry have appeared in this Jour-
nal (3). We have incoroorated a series of experiments in our Inorganic Synthesis and Techniques ~ a b o i a t othat r ~ involve the svnthesis and characterization of iron porphyrins. These &me models are used to illustrate the kffict of the axial h a n d s on soin state (4) and redox chemistry (5). The experimental results are used in conjunction with the interpretation of (1) the stereochemical changes accompanying oxygen binding and the resulting coop&ativity exhibited between heme units in hemoglobin (i.e., the Perutz trigger mechanism) (6) and (2) the effect of heme environment on Fe(III)/Fe(II) redox potential and how this pertains to electron shuttling by cytochrome sequences (5b, 7). In addition, the synthesis of the metalloporphyrin supplements the lecture discussion on macrocycles and the macrocyclic effect. The sequence of experiments described below is carried out over four 4-h laboratorv oeriods. When required by time .. constraints, we have supplied our students withcommerciallv available free-base oorohyrins. The students are broken &to "research teams" wkh-each member of a given team working with a different iron porphyrin analogue. The data obtained are shared within a team so that the results of spectral and electrochemical analyses can be compared. ~~~
Synthesis of the Porphyrln Llgand Our students routinely synthesize tetraphenylporphyrin, TPP; tetra@-tolyl)porphyrin, T T P ; and tetra@-chlorophenyl)porphyrin, TClPP, using a modified version of the one-pot procedure developed by Adler et al. (8).All manipulations including isolation of the product must be performed in a hood. Procedure A stir bar and 0.3 L propionic acid were added to a 500-mL,threeneck, round-bottom flask fitted with a reflux condenser. The acid was brought to a reflux and 0.1 mol of the appropriate substituted benzaldebyde was added. To the refluxing solution, 10 mL (0.1 mol) freshly distilled pyrrole was cautiously added via a dropping funnel. Caution1 The condensation reaction of the pyrrole with the aldehyde is extremely exothermic, and the rate of pyrrole addition
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must be continuously monitored. The solution was refluxed for 30 min, cooled to ambient temperature, and chilled in an ice bath before suction filtering off the purple, crystalline product. The product was washed with boiling water until free from the odor of propionic acid. The absorption spectrum of the product was obtained in chloroform. Typical yields were in the range of 10-20%. of the oroduet was established bv comoarine The -~~~ouritv . ..the wee~, trum with that reported in the literature (91.Thp product was invarialrly oisufficicnt pnrity for use in the merollation renrtim. The abaorptron maxima shift to the red with increasing electrundonating power of the para substituents (9); however, for the porphyrins synthesized in this experiment,shifts are toosmall (about 1 nm) to be detected on a general use UV-vis spectropbotometer.
.~ ~
Metallation of the Free-Base Porphyrln The iron porphyrin is conveniently prepared from the free-base porphyrin by a scaled-down, modified version of the orocedure reoorted bv Adler e t al. (10). The promess of the ieaction is convenientiy monitored by checking the reaction medium for fluorescence. Dilute solutions of the freebase porphyrins fluoresce brightly when irradiated with lonewave UV light in a darkened room, whereas the metallated porphyrinsdo not. Procedure In a 250-mL, three-neck, round-bottom flask equipped with a reflun condenser, a stirred mixture of 1 g of free-base porphyrin in 100-mL dimetbylformamide,DMF, was brought to a reflux. About 0.5 g of FeC12 was added over a 5-min period, and the solution was allowed to reflux far 10 min. Caution! The addition of FeClz to the reaction medium is accompanied with the evolution of heat. The FeClz must be added in small increments. If an examination of the reaction medium revealed fluorescence,additional FeC12 was added and the reaction mixture refluxed for an additional 10 min. The DMF solution was cooled to room temperature and an equal volume ofwater added along with 1mLaf 6M HCl. The mixture was chilled in an ice bath and suction filtered.The dark purple solid was washed with water until the filtrate was colorless and aspirated to dryness. Typically, isolated yields were greater than 90%. An absorption spectrum was obtained in CHClz.Values for absorption maxima and molar absorptivities were compared to literature values as a check of product purity ( 1 1 ) . The presence of any free-base porphyrin was easily discerned. Maanetlc Susceptlblllty Measurements The number and nature of axial ligands are the primary ( 4 ) . Thus. determinants of spin state in metalloporphyrins . . . as the axial ligand field strength decreases, the spin state adopted by an Fe(III)(P), where P is any porphyrin, progresses from low-spin ( S = 1/2), to high-spin ( S = 5/21, and finally to intermediate-spin ( S = 312). This is represented in Figure 1 with simple ligand-field splitting diagrams. An Fe(III)(P) with very weak-field ligands invariably adopts a spin state that is not pure S = 312 but rather quantum mechanically admixed S = 315,512 (4). The effective magnetic moments of the iron porphyrins are determined by the Guoy method using the procedure described by Angelici (12) and in solution via the Evans
decreasing axial ligand field strength L--
''.
7 , '
Figure 1.Ligand fieldspllttlng diagram for five-or six--dinate rins. Adapted hom ref 4.
Fe(ll1)porphy-
technique (13). Samples are corrected for diamagnetism usingvalues of -700 X 10-6cgsfor T P P (14) and, after making adjustments for the presence of methyl or chloro substituents (15), -753 X 10-6cgsfor T T P and -760 X 10-6cgs for TCIPP. -~ -~ For the solution susceptibility, o w students perform the studv in dunlicate: one run involves the measurement of the F~(I;)CIin CHCI, and a second run employs as a solvent 0.1 M imidazole in CHCII. The reference solution for the second solution must contain imidazole in the same concentration. A~ in of ahout 0.01 M works well. The - o -r ~-h v rconcentration presence of the imidazole results in the formation of lowspin complexes, [Fe(P)(Im)2]CI. Magnetic moments for Fe(P)Cl by both the Guoy and Evans mithods are typically in the range 5.3-6.0 FB, corresponding to a high-spin state (S = 512) whereas the values , to obtained for [Fe(P)(Im)2]Cl are 2.42.7 p ~ corresponding a low-spin state (S = 1/2). Electrochernlcal Characterizallon An experiment centered on the effect of complex formation on the redox potential of metal ions has recently appeared in this Journal (16). The electrqchemistry of iron porphyrins has been well characterized (5), and the potentials for the various redox processes mav be correlated with metal spin state, coordinaiion of axial iiginds, the solvent system employed for the study, out-of-por~hvrin-planedisilacementof t h e iron, counterion, and basicity of the porphyrin ring (5c). The electrochemical investigations performed by o w students emphasize the effect of porphyrin basicity and axial ligation on the Fe(IIIA1) and Fe(IIA) redox ~otentials. cYciicvoltammetry is used to obtain the half-wave reduction potentials for the Fe(lIIA1) and Fe(llfl) c o u ~ l e sin the absence and presence of imidazole. gain, the students work in teams so that each poruhvrin in the series is examined and comparisons of the El 1'; can be made. We have found that 0.1 M tetraethvlammonium perchlorate, TEAP, in dimethylformamide, DMF, perfor& well as a supporting electrolyte-solvent system combination. Complexities associated with the displacement of anions from the coordination sphere are minimized in coordinating solvents such as DMF (5~).
Procedure An IBM ECl22.5 Analvzer was used in coniunction -~ ~ - Voltammetric ~~~, , with a three-electrodecell supplied by Biosnalytiral Systems. Inc., and composed of a glassy carbon working electrode, Pt aire auxilis~
~
~
~
~
~~~~~
~
~
~
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ry electrode and a AgIAgCl reference electrode.
Volts vs. Aq/AgCI Flgure 2 Cyclic voltammagrams obtain& for 10 ' M FeUPPK: In DMF and 0 1 M TEAP at a glassy csroon elsnrws Scan rate = 50 mvls Soltd curve no mddazole present Don& curve 0 1 M rnndazole
Half-Wave Potentlalo*
TTP
TPP TCIPP 'Potemials
-0.66 -1.53 -0.66 -1.52 -0.63 -1.46 are re~ortedin voln and referenced to
1.85
-060
1.74 1.29
-0.59 -0.53 terrocene
-1.89 -1.82
-1.82 as
tollown: E.., =
A 25.0-mL DVF solution 10.' M in Fe(P)CIand 0.1 M in TEAP was prepared. A 15.0-mLaliquot was placed in t h p electrochrmiral cell and denerared for 10 min with a stream of nitrogen. A voltnmmogram was obtained using a scan range of +0.3 to--1.4 V vs Ag/ AgCL and a scan rate of 50 mV1s. Then 100 mg of imidazole was put into the DMF solution. which was aeain deaerated. A second voltammoaram wasohtained. Finally, fernrrene war added M, thesolution and the wave for the ferrorene-ierricinium couple was recorded for ralrhration of the rell.
Results Typical Ell2 values for the three iron porphyrins are reported in the table. An examination of the half-wave reduction potentials reveals a number of trends that the students are expected t o explain. The differences in the reduction potentials are small but significant. A correlation between the ease of reduction and the substituent on the norohvrin . " is clearly observed: the TClPP derivative is the most easily reduced followed bv the T P P and finallv the T T P derivative. This can be understood by considering the nature of the ~ h e n vsubstituent-as l the basicitv of the ~ v r r o l nitroeens e d e ~ r ~ a s ethe s , iron is more readily reduced..?he pK.'s (9;for the three porphyrins are reported in the table. Relationships between porphyrin basicity and redox potential (5) and spin state (17) are well documented. The relevance to electron shuttling by the cytochromes has been discussed by Walker et al. (5b). The direction of the shift of Ell2 upon ligation of the imidazole is also of interest. Note that the shift is to less negative values for the Fe(IIIAI)(P) couple, indicating that bis(imidazo1e) ligation is more favorable for Fe(II)(P) than Fe(III)(P). However, the shift is to more negative values for the Fe(II/I)(P) couple, which indicates that formation of the six-coordinate species is more favorable for Fe(II)(P) than for Fe(1)P. In fact, Fe(1) porphyrins have little tendency to coordinate nitrogenous bases in the axial sites (5).
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Volume 68
Number 4
April 1991
341
Concluslons
The set of experiments presented above has been performed for the nast few vears in our Inoreanic Technioues Laboratory, which bas a junior-level inorganic chemistry course as a coreauisite. The exneriments are nerformed during the after t i e presentation of ligand fieid theory. The theorv of cvclic voltammetrv is covered in an analvtical chemistry cburse that is a prerequisite for this labor.&ory. Thus, these exneriments not onlv ~rovidefor an introduction t'o bioinorganic chemistry, b i t &so serve to substantiate material covered in lecture. Significantly, students gain some experience working as a team on a "research project." Students are encouraged to discuss findings andinterpretation of the data with one another. Individual laboratory reports are submitted, however. in which students are ex~ectedto discuss iron ~ o r ~ h v rin spin-state/stereochemical~elationshipsand factbrs'cktributine to the modulation of iron norohvriu . . - redox Dotenti& witi reference to pertinent heme proteins.
-
~.~~~ ."...,
1. See J. Chem. Educ. 1985.62.917-IWI. This issue contains the praeedings from the
swpoaium on Bioinorganic Cherpistry sponsored by the CHED Dinsian of the ACS.
342
Journal of Chemical Education
2. See, for example, (a1Huheey.J. E. lnorgonie Chemistry, 3 r d d . : Harper & Row: New York, 1983. (b)Cotton, F. A,: Wilkinaon, G.AduancedInorganic Chemistry.5thod.; Wiley: New York, 1988. ic) Douglas, 8. E.; MeDsniel. D. H.; Alexander, J. J. Coneept* ond Models oihorganic Chemistry, 2nd ed.; Wiley: New York, 1983. (dl Shriver. 0. F.: Atkins, P. W.; Langfmd. C. H. Inorganic Chemirfry; Freeman: New "".L
>W"
3. See, forexample, (a1A w e s , D. J.: Paris, M. R. J. Chem Educ. 1989,66,654-856. (b)
Bsekmann.B. A,: Buchmsn,A.;Pssterna.k,R.F.;Rcinpre"hf, J.T.; Voge1.G. C.J. Chem.Edue. 1976.53.387-389, ie) B6rd.D. M. J. Chrm. Edue 1985.62.168-169. 4. Scheidt, W.R.:Reed,C.A. Chem.Reo. 1981,81,543-555. 5. ( 8 ) Bottomley, L. A ; Olson,L : Kadiah, K. M. InEleetrochemicoland Speefrochemia 1 Studies of Biological Redox Components: Kadiah, K. M.. Ed.; Adv. in Chem., 201. American Chemical Society: Washington. DC, 1982:Chapter 13. (b) Wa1ker.F. A ; B m y , J. A: Balk., V. L.: MeDemott, G. A ; Wu. M. 2.; Linde, P. F. Ref 50, Chapter 17. (4Ksdish, K. M. In Iron Porphyriw. Port II: Lever. A. B. P.: Gray, H. B.. Eds.: Addison-Wesley: MA, 1983: Chapter 4. 6. Perutz,M. F.; Fermi, G.: Shaman, L. B.;L i d d i n a n , R. C. Aer. Chem. Res. 1987,%0, 3W. 7. Wilson, D. F.; Dutton, P. L.; Erminaks, M.: Lindsay, J. G.; S&, N. Are. Chem. Rea.
~n ." .-,-,,a,
?"A
8. Ad1er.A. D.: L0ngo.F. R.;Finsrelli, J.D.: Goldmeher, J.:Amur, J.;Korsskoff,L. J. Org. Chem. 1967,32,476. 9. Moet-Ner,M.;Adler,A. D. J A m . Cham.Soe. 1975.97.5107-5111. 10. Adlor, A. D.; Longo, F. R.: Kampas, R.; Kim, J. J . lnorg. Nuel. Chrm. 1970,32,2443"*LC
11. Walker,F.A.;Lo,M. W.:Ree,M. T. J A m . Chsrn.Soc. 1976,98,55S24559. 12. Angelici, R. J. Synlheaia and Technique in lno~ganieChemistry, 2nd ed.; Saunders: Philadelphis, 1977. 13. Evans.0.R. J. Ch~m.Soc.1959.2W3-2WS. 14. Eaton,S. S.:Eaton,G.R.Inorg.Cham. 1980.19. 1095-1096 15. Muisy, L. N. In Physical Method8 of Chemistry; Weisshurger and Rossiter, Eds.; Wiley-Interscience: NeaYork, 1972; Vol I, Part IV. 16. Ibanez, J. G.:Gonlalel. l.;Cardenss,M. A. J. Chom.Educ. 1988,65,173-176. 17. Geiger, D. K.; Scheidt, W. R. Inore. Chem. 1964.23.1970-1972.
