Electron-transfer kinetics for generation of organoiron(IV) porphyrins and the iron(IV) porphyrin π radical cations

Article abstract of DOI:10.1021/ja982136r

Homogeneous electron-transfer kinetics for the oxidation of seven different iron(III) porphyrins using three different oxidants were examined in deaerated acetonitrile, and the resulting data were evaluated in light of the Marcus theory of electron transfer to determine reorganization energies of the rate-determining oxidation of iron(III) to iron(IV). The investigated compounds are represented as (P)Fe(R), where P = the dianion of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP) and R = C₆H₅, 3,5-C₆F₂H₃, 2,4,6-C₆F₃H₂, or C₆F₅ or P = the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and R = C₆H₅, 2,4,6-C₆F₃H₂, or 2,3,5,6-C₆F₄H. The first one-electron transfer from (P)Fe(R) to [Ru(bpy)₃]³⁺ (bpy = 2,2'-bipyridine) leads to an Fe(IV) σ-bonded complex, [(P)Fe(IV)(R)]⁺, and occurs at a rate which is much slower than the second one-electron transfer from [(P)Fe(IV)(R)]⁺ tO [Ru(bpy)₃]³⁺ to give [(P)Fe(IV)(R)]·²⁺. The one- or two-electron oxidation of each (OETPP)Fe(R) or (OEP)Fe(R) derivative was also attained by using [Fe(phen)₃]³⁺ (phen = 1,10-phenanthroline) or [Fe(4,7-Me₂phen)₃]³⁺ (Me₂phen = 4,7-dimethyl- 1,10-phenanthroline) as an electron-transfer oxidant. The reorganization energies (kcal mol^-1) for the metal-centered oxidation of (P)Fe(III)(R) to [CP)Fe(IV)(R)]⁺ increase in the order (OEP)Fe(R) (83 ± 4) << (OETPP)Fe(C₆F₅) (99 ± 2) < (OETPP)Fe(2,4,6-C₆F₃H₂) (107 ± 2) < (OETPP)Fe(3,5-C₆F₂H₃) (109 ± 3) < (OETPP)Fe(C₆H₅) (113 ± 3). Each value is significantly larger than the reorganization energies determined for the porphyrin-centered oxidations involving the same two series of compounds, i.e., the second electron transfer of (P)Fe(R). In each case, the first metal-centered oxidation is the rate-determining step for generation of the iron(IV) porphyrin π radical cation. Coordination of pyridine to (OETPP)Fe(C₆F₅) as a sixth axial ligand enhances significantly the rate of electron-transfer oxidation.

Full text of DOI:10.1021/ja982136r

J. Am. Chem. Soc. 1999, 121, 785-790
785
Electron-Transfer Kinetics for Generation of Organoiron(IV)
Porphyrins and the Iron(IV) Porphyrin π Radical Cations
Shunichi Fukuzumi,*,1a Ikuo Nakanishi,1a Keiko Tanaka,1a Tomoyoshi Suenobu,1a
Alain Tabard,1b Roger Guilard,*,1b Eric Van Caemelbecke,1c and Karl M. Kadish*,1c
Contribution from the Department of Material and Life Science, Graduate School of Engineering,
Osaka UniVersity, Suita, Osaka 565-0871, Japan, LIMSAG, UMR 5633, UniVersite´ de Bourgogne,
Faculte´ des Sciences “Gabriel”, 6 BouleVard Gabriel, 21000 Dijon, France, and Department of
Chemistry, UniVersity of Houston, Houston, Texas 77204-5641
ReceiVed June 19, 1998
Abstract: Homogeneous electron-transfer kinetics for the oxidation of seven different iron(III) porphyrins
using three different oxidants were examined in deaerated acetonitrile, and the resulting data were evaluated
in light of the Marcus theory of electron transfer to determine reorganization energies of the rate-determining
oxidation of iron(III) to iron(IV). The investigated compounds are represented as (P)Fe(R), where P ) the
dianion of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP) and R ) C6H5, 3,5-C6F2H3,
2,4,6-C6F3H2, or C6F5 or P ) the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and R ) C6H5,
2,4,6-C6F3H2, or 2,3,5,6-C6F4H. The first one-electron transfer from (P)Fe(R) to [Ru(bpy)3]3+ (bpy ) 2,2′-
bipyridine) leads to an Fe(IV) σ-bonded complex, [(P)FeIV(R)]+, and occurs at a rate which is much slower
than the second one-electron transfer from [(P)FeIV(R)]+ to [Ru(bpy)3]3+ to give [(P)FeIV(R)]•2+. The one- or
two-electron oxidation of each (OETPP)Fe(R) or (OEP)Fe(R) derivative was also attained by using [Fe(phen)3]3+
(phen ) 1,10-phenanthroline) or [Fe(4,7-Me2phen)3]3+ (Me2phen ) 4,7-dimethyl-1,10-phenanthroline) as an
electron-transfer oxidant. The reorganization energies (kcal mol-1) for the metal-centered oxidation of (P)-
FeIII(R) to [(P)FeIV(R)]+ increase in the order (OEP)Fe(R) (83 ( 4) , (OETPP)Fe(C6F5) (99 ( 2) < (OETPP)-
Fe(2,4,6-C6F3H2) (107 ( 2) < (OETPP)Fe(3,5-C6F2H3) (109 ( 3) < (OETPP)Fe(C6H5) (113 ( 3). Each
value is significantly larger than the reorganization energies determined for the porphyrin-centered oxidations
involving the same two series of compounds, i.e., the second electron transfer of (P)Fe(R). In each case, the
first metal-centered oxidation is the rate-determining step for generation of the iron(IV) porphyrin π radical
cation. Coordination of pyridine to (OETPP)Fe(C6F5) as a sixth axial ligand enhances significantly the rate of
electron-transfer oxidation.
Iron(IV) porphyrin π radical cations play an essential role in
a number of oxidative catalytic processes including biological
systems.2-7 Although high-valent iron porphyrins are usually
extremely reactive and thus difficult to characterize,8 the one-
and two-electron oxidations of σ-bonded iron porphyrins such
as (P)Fe(R), where P is a given porphyrin dianion, will lead
first to an iron(IV) porphyrin and then to an iron(IV) porphyrin
π radical cation, both of which have a stability that depends in
large part upon the nature of the σ-bonded axial ligand (R) and
the porphyrin macrocycle (P).9,10 For example, the one-electron
oxidation of (OETPP)Fe(C6H5) which has a saddle-shaped
nonplanar porphyrin macrocycle (OETPP ) the dianion of
2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphy-
rin) leads to a relatively stable iron(IV) compound, [(OETPP)-
FeIV(C6H5)]+.11,12 In this regard, some octaalkyltetraphenylpor-
phyrins, which are nonplanar as a result of steric crowding of
the peripheral substituents, have been used as model compounds
to investigate the consequences of nonplanar conformational
distortions.13-15 The further one-electron oxidation of [(OETPP)-
(1) (a) Osaka University (Osaka, Japan). (b) Universite´ de Bourgogne
(Dijon, France). (c) University of Houston (Houston, TX).
(2) (a) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997,
30, 251. (b) Traylor, T. G. Pure Appl. Chem. 1991, 63, 265. (c) Cytochrome
P-450: Structure, Mechanism and Biochemistry, 2nd ed.; Ortiz de Mon-
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P. R. Acc. Chem. Res. 1987, 20, 289.
(3) (a) Groves, J. T.; Quinn, R. J. Am. Chem. Soc. 1985, 107, 5790. (b)
Groves, J. T.; Watanabe, Y. J. Am. Chem. Soc. 1988, 110, 8443. (c) Groves,
J. T.; Stern, M. K. J. Am. Chem. Soc. 1988, 110, 8628.
(4) (a) Mansuy, D.; Battioni, P. In Metalloporphyrins in Catalytic
Oxidations; Sheldon, R. A., Ed.; Marcel Dekker: New York, 1994; pp 99-
132. (b) Mansuy, D. Pure Appl. Chem. 1987, 59, 759. (c) Mansuy, D. Coord.
Chem. ReV. 1993, 125, 129.
(5) (a) Meunier, B. Chem. ReV. 1992, 92, 1411. (b) Pitie´, M.; Bernadou,
J.; Meunier, B. J. Am. Chem. Soc. 1995, 117, 2935.
(9) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659.
(10) Kadish, K. M.; Van Caemelbecke, E.; Gueletii, E.; Fukuzumi, S.;
Miyamoto, K.; Suenobu, T.; Tabard, A.; Guilard, R. Inorg. Chem. 1998,
37, 7, 1759.
(6) (a) Bruice, T. C. Acc. Chem. Res. 1991, 24, 243. (b) Ostovic, D.;
Bruice, T. C. Acc. Chem. Res. 1992, 25, 314. (c) Dunford, H. B. AdV. Inorg.
Biochem. 1982, 4, 41.
(7) (a) Guengerich, F. P.; Macdonald, T. L. Acc. Chem. Res. 1984, 17,
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Chemistry; Mariano, P. S., Ed.; JAI Press: Greenwich, CT, 1993; Vol. 3,
pp 191-241. (c) Guengerich, F. P.; Yun, C.-H.; Macdonald, T. L. J. Biol.
Chem. 1996, 271, 27321.
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J.; Smith, K. M.; Tabard, A.; Guilard, R. Inorg. Chem. 1995, 34, 2984.
(12) Kadish, K. M.; Van Caemelbecke, E.; D’Souza, F.; Medforth, C.
J.; Smith, K. M.; Tabard, A.; Guilard, R. Organometallics 1993, 12, 2411.
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10.1021/ja982136r CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/08/1999

786 J. Am. Chem. Soc., Vol. 121, No. 4, 1999
Fukuzumi et al.
1,4-benzoquinone as described in the literature.18 Iron was inserted using
ferrous chloride tetrahydrate in deoxygenated dimethylformamide, and
the formation of (OETPP)FeCl was confirmed by 1H NMR as described
elsewhere.14 The (OETPP)Fe(R) complexes (R ) C6H5, 3,5-C6F2H3,
2,4,6-C6F3H2, C6F5) were prepared by reacting an aryl Grignard reagent
with (OETPP)FeCl according to literature procedures.11,19 The synthesis
of (OEP)Fe(R), where R ) C6H5, 2,4,6-C6F3H2, and 2,3,5,6-C6F4H,
was carried out by reacting the corresponding aryl Grignard reagent
with (OEP)FeCl according to literature procedures.10,16a,20,21 Tris(2,2′-
bipyridine)ruthenium dichloride hexahydrate, [Ru(bpy)3]Cl2‚6H2O, was
obtained commercially from Aldrich. The oxidation of [Ru(bpy)3]Cl2
with lead dioxide in aqueous H2SO4 gives [Ru(bpy)3]3+, which was
isolated as the PF6 salt, [Ru(bpy)3](PF6)3.22 Tris(1,10-phenanthroline)-
iron(II) and tris(4,7-dimethyl-1,10-phenanthroline)iron(II) complexes
were prepared by adding 3 equiv of the corresponding ligand to an
aqueous solution of ferrous sulfate.23 Tris(1,10-phenanthroline)iron-
(III) perchlorate, [Fe(phen)3](ClO4)3, and tris(4,7-dimethyl-1,10-phenan-
throline)iron(III) hexafluorophosphate, [Fe(4,7-Me2phen)3](PF6)3, were
prepared by oxidizing the corresponding iron(II) complexes with ceric
ammonium sulfate or lead dioxide in aqueous H2SO4 followed by the
addition of NaClO4 or KPF6.23,24 Acetonitrile (MeCN) and benzonitrile
(PhCN) were purchased from Wako Pure Chemical Ind., Ltd., and
purified by successive distillation over CaH2 and P2O5, respectively,
according to standard procedures.25 Pyridine (py) was obtained com-
mercially and purified using standard methods.25 Tetra-n-butylammo-
nium perchlorate (TBAP) was purchased from Sigma Chemical Co.,
recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for
at least 1 week prior to use.
Spectral and Kinetic Measurements. Typically, a 10 µL aliquot
of [Ru(bpy)3](PF6)3 (3.0 × 10-3 M) in MeCN was added to a quartz
cuvette (10 mm i.d.) which contained (OETPP)Fe(C6H5) (5.0 × 10-6
M) in deaerated MeCN (3.0 mL). This led to an electron transfer from
(OETPP)Fe(C6H5) to [Ru(bpy)3](PF6)3. UV-vis spectral changes
associated with this electron transfer were monitored using a Shimadzu
UV-2200 spectrophotometer, a Hewlett-Packard 8452A diode array
spectrophotometer, or a Hewlett-Packard 8453 diode array spectro-
photometer. The same procedure was used for spectral measurements
for other σ-bonded iron porphyrins. The coordination of pyridine as a
sixth axial ligand to (OETPP)Fe(C6F5) in MeCN was monitored by
measuring the UV-vis spectral changes as a function of the ligand
concentration. All measurements were carried out in a dark cell
compartment using deaerated solutions. It was confirmed that the
monitoring light did not affect the thermal rates.
Chart 1
FeIV(C6H5)]+ leads to an Fe(IV) porphyrin π radical cation,
formally an Fe(V) compound, and this is followed by migration
of the σ-bonded C6H5 ligand to a nitrogen of the porphyrin ring
to give [(N-C6H5OETPP)FeIII]2+ 11,12
A migration of the σ-bond-
.
ed axial ligand from singly oxidized iron porphyrins with planar
macrocycles such as (OEP)Fe(C6H5) or (TPP)Fe(C6H5) (OEP
) the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin and TPP
) the dianion of 5,10,15,20-tetraphenylporphyrin) has long been
known to occur, and the resulting migration product can be
further oxidized at the metal center to give [(N-C6H5OEP)FeIII]2+
and [(N-C6H5TPP)FeIII]2+ in the presence of excess oxidizing
agent or under the application of an applied oxidizing potential.16
Reversible oxidations have been obtained for both (OETPP)-
Fe(R) and (OEP)Fe(R) by cyclic voltammetry at moderate scan
rates,11,12 but there has so far been no report in the literature on
the kinetics of electron-transfer reactions for generation of iron-
(IV) porphyrins or iron(IV) porphyrin π radical cations prior
to the migration step which occurs on a much longer time scale
than the electron transfer.
This study reports the first kinetic data for the electron-transfer
oxidation of (P)Fe(R) derivatives, where P ) OETPP and R )
C6H5, 3,5-C6F2H3, 2,4,6-C6F3H2, or C6F5 or P ) OEP and R )
C6H5, 2,4,6-C6F3H2, or 2,3,5,6-C6F4H (see Chart 1). In addition,
plots of logarithms of rate constants for electron transfer vs the
free energy change of electron transfer lead to the first evaluation
of reorganization energies (λ) for formation of iron(IV) por-
phyrins and iron(IV) porphyrin π radical cations in light of the
Marcus theory of electron transfer.17 A comparison of the
reorganization energies between (OETPP)Fe(R) and (OEP)Fe-
(R) provides an excellent opportunity to understand the effects
of nonplanar conformational distortion on the intrinsic barrier
for the electron-transfer reactions.
Kinetic measurements of the electron transfer from (P)Fe(R) to the
oxidants were carried out using a Union RA-103 stopped-flow
spectrophotometer under deaerated conditions. Typically, deaerated
MeCN solutions of (OETPP)Fe(C6H5) and [Ru(bpy)3](PF6)3 were
transferred to the spectrophotometric cell by means of a glass syringe
which had earlier been purged with a stream of argon. Rates of electron
transfer from (OETPP)Fe(C6H5) to [Ru(bpy)3]3+ in deaerated MeCN
at 298 K were monitored by following a decrease in absorbance at
431 nm (ꢀ ) 1.04 × 105 M-1 cm-1) due to (OETPP)Fe(C6H5) or an
increase in absorbance at 287 nm (ꢀ ) 7.90 × 104 M-1 cm-1
)
26 due to
Experimental Section
(18) (a) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner,
M. W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851. (b) Medforth, C.
J.; Smith, K. M. Tetrahedron Lett. 1990, 31, 5583. (c) Lindsey, J. S.;
Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M. J. Org.
Chem. 1987, 52, 827.
Materials. Free-base (OETPP)H2 was prepared from benzaldehyde
and 3,4-diethylpyrrole in the presence of BF3‚OEt2, followed by
oxidation of a resulting porphyrinogen with 2,3-dichloro-5,6-dicyano-
(19) (a) Kadish, K. M.; Boisselier-Cocolios, B.; Cocolios, P.; Guilard.
R. Inorg. Chem. 1985, 24, 2139. (b) Cocolios, P.; Guilard, R.; Fournari, P.
J. Organomet. Chem. 1979, 179, 311.
(20) Guilard, R.; Boisselier-Cocolios, B.; Tabard, A.; Cocolios, P.;
Simonet, B.; Kadish, K. M. Inorg. Chem. 1985, 24, 2509.
(21) (a) Cocolios, P.; Lagrange, G.; Guilard, R. J. Organomet. Chem.
1983, 253, 65. (b) Tabard, A.; Cocolios, P.; Lagrange, G.; Gerardin, R.;
Hubsch, J.; Lecomte, C.; Zarembowitch, J.; Guilard, R. Inorg. Chem. 1988,
27, 110.
(22) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
(23) Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5593.
(24) Fukuzumi, S.; Miyamoto, K.; Suenobu, T.; Van Caemelbecke, E.;
Kadish, K. M. J. Am. Chem. Soc. 1998, 120, 2880.
(25) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratry Chemicals; Pergamon Press: Elmsford, NY, 1966.
(15) (a) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.;
Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1994, 116, 8582. (b) Regev, A.;
Galili, T.; Medforth, C. J.; Smith, K. M.; Barkigia, K. M.; Fajer, J.; Levanon,
H. J. Phys. Chem. 1994, 98, 2520. (c) Barkigia, K. M.; Renner, M. W.;
Furenlid, L. R.; Medforth, C. J.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc.
1993, 115, 3627. (d) Renner, M. W.; Cheng, R.-J.; Chang, C. K.; Fajer, J.
J. Phys. Chem. 1990, 94, 8508. (e) Shelnutt, J. A.; Medforth, C. J.; Berber,
M. D.; Barkigia, K. M.; Smith, K. M. J. Am. Chem. Soc. 1991, 113, 4077.
(16) (a) Lanc¸on, D.; Cocolios, P.; Guilard, R.; Kadish, K. M. J. Am.
Chem. Soc. 1984, 106, 4472. (b) Mansuy, D.; Battioni, J.-P.; Dupre, D.;
Sartori, E. J. Am. Chem. Soc. 1982, 104, 6159.
(17) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus,
R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (c) Eberson, L. AdV.
Phys. Org. Chem. 1982, 18, 79.

Organoiron(IV) Porphyrins
J. Am. Chem. Soc., Vol. 121, No. 4, 1999 787
Table 1. Rate Constants (ket, M-1 s-1) and Free Energy Changes (∆G0et, eV) for First One-Electron Oxidation of (P)Fe(R) with Various
Oxidants in Deaerated MeCN at 298 K, Their Redox Potentials (E1ox, E2ox, and E0red, V vs SCE), and Reorganization Energies (λ11, kcal mol-1
for the Self-Exchange Reaction of (P)Fe(R)/[(P)Fe(R)]+
)
(P)Fe(R)
E1ox,a V
E2ox,a V
oxidant
E0red,b V
ket, M-1 s-1 (∆G0et, eV)
λ11, kcal mol-1
(OETPP)Fe(C6H5) (1)
0.27
1.06
[Ru(bpy)3](PF6)3
1.24
1.07
0.90
1.24
1.07
0.90
1.24
1.07
0.90
1.24
1.07
0.90
2.0 × 107 (-0.97)
7.7 × 105 (-0.80)
3.4 × 105 (-0.63)
7.8 × 106 (-0.85)
4.5 × 105 (-0.68)
1.3 × 105 (-0.51)
2.2 × 106 (-0.76)
5.7 × 105 (-0.59)
3.1 × 104 (-0.42)
3.6 × 106 (-0.68)
5.0 × 105 (-0.51)
6.7 × 105 (-0.34)
112
118
110
109
112
106
110
105
106
102
99
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
[Ru(bpy)3](PF6)3
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
[Ru(bpy)3](PF6)3
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
[Ru(bpy)3](PF6)3
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
(OETPP)Fe(3,5-C6F2H3) (2)
(OETPP)Fe(2,4,6-C6F3H2) (3)
(OETPP)Fe(C6F5) (4)
0.39c
0.48c
0.56
0.93c
0.84c
0.80
96
(OEP)Fe(C6H5) (5)
0.48
0.76
0.79
1.30
1.19
1.14
[Ru(bpy)3](PF6)3
1.24
1.07
0.90
1.24
1.07
0.90
1.24
1.07
0.90
too fast (-0.76)
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
[Ru(bpy)3](PF6)3
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
[Ru(bpy)3](PF6)3
too fast (-0.59)
4.7 × 104 (-0.42)
3.3 × 106 (-0.48)
1.1 × 106 (-0.31)
1.8 × 104 (-0.14)
3.0 × 106 (-0.45)
1.6 × 106 (-0.28)
1.3 × 104 (-0.11)
81
88
80
86
86
76
85
(OEP)Fe(2,4,6-C6F3H2) (6)
(OEP)Fe(2,3,5,6-C6F4H) (7)
[Fe(phen)3](ClO4)3
[Fe(4,7-Me2phen)3](PF6)3
a Taken from refs 10 and 11 unless otherwise noted. b E0 values vs SCE in MeCN, 0.1 M TBAP; see Experimental Section. c Determined in
red
this study; see Experimental Section.
[Ru(bpy)3]2+. The rate constants of electron transfer (ket) were
and the two-electron oxidation of all but one investigated (P)-
Fe(R) derivative with [Ru(bpy)3]3+ is therefore energetically
determined either by the second-order plots for the electron-transfer
reactions of (P)Fe(R) with 2 equiv of oxidant or by the pseudo-first-
feasible.
order plots for the electron-transfer reactions in the presence of a large
A stopped-flow technique was used to determine the electron-
excess oxidant. In each case, it was confirmed that the ket values derived
transfer rates. The electron-transfer reaction between (P)Fe(R)
from at least 5 independent measurements agreed within an experi-
(1.0 × 10-5 M) and 2 equiv of [Ru(bpy)3]3+ (2.0 × 10-5 M) in
mental error of (5%. Second- or pseudo-first-order rate constants were
MeCN at 298 K was followed by UV-visible spectrophotom-
etry and indicated an increase in absorbance at 287 nm due to
the generated [Ru(bpy)3]2+ complex (Figure 1a). This is
accompanied by a decrease in the Soret band (e.g., at 431 nm
for (OETPP)Fe(C6H5)) due to loss of the (P)Fe(R) reactant
(Figure 1b). Changes in absorbance due to [Ru(bpy)3]2+
formation and consumption of (P)Fe(R) both obey second-order
kinetics, thus indicating that the two-electron oxidation of (P)-
Fe(R) by [Ru(bpy)3]3+ occurs via an initial rate-determining
electron transfer (Scheme 1). In such a case, the first electron
transfer from (P)Fe(R) to [Ru(bpy)3]3+ is much slower than the
second one-electron transfer between [(P)FeIV(R)]+ and [Ru-
(bpy)3]3+, although this second step is energetically less
favorable. The first electron transfer is known to occur at the
metal center and the second at the porphyrin ring to give [(P)-
FeIV(R)]+ and [(P)FeIV(R)]•2+, respectively.10,11 Thus, the metal-
centered oxidation is kinetically harder than the macrocycle
oxidation.
determined by a least-squares curve fit using a Macintosh microcom-
puter. The second-order plots of (A∞ - A)-1 vs time and the first-order
plots of ln(A∞ - A) vs time (A∞ and A are the final absorbance and the
absorbance at the reaction time, respectively) were linear for 3 or more
half-lives with the correlation coefficient ρ > 0.999.
Cyclic Voltammetry. The E0 values of oxidants in MeCN
red
containing 0.1 M TBAP as supporting electrolyte were determined at
room temperature by cyclic voltammetry under deaerated conditions
using a three-electrode system and a BAS 100B electrochemical
analyzer. The E0 values of (OETPP)Fe(3,5-C6F2H3) and (OETPP)-
ox
Fe(2,4,6-C6F3H2) were determined in PhCN instead of MeCN because
of a solubility problem as previously reported for other (P)Fe(R)
derivatives.11 The working and counter electrodes were platinum while
Ag/AgNO3 (0.01M) was used as the reference electrode. All potentials
are reported as V vs SCE. The E1/2 value of ferrocene used as a standard
is 0.37 V vs SCE in PhCN or MeCN under our solution conditions.27
Results and Discussion
Rates of Electron-Transfer Oxidation of (P)Fe(R). Three
oxidants which are strong enough to oxidize (P)FeIII(R) to [(P)-
FeIV(R)]+ or to [(P)FeIV(R)]•2+ were selected for use in
acetonitrile (MeCN). The one-electron reduction potentials for
the utilized oxidizing agents were determined in this study and
The one- or two-electron oxidation of each (OETPP)Fe(R)
and (OEP)Fe(R) derivative was also attained by using [Fe-
(phen)3]3+ or [Fe(4,7-Me2phen)3]3+ as an electron-transfer
oxidant.28 The observed second-order rate constants for the first
rate-determining electron transfer with all three oxidants are
listed in Table 1, which includes the first and second oxidation
potentials of (OETPP)Fe(R) and (OEP)Fe(R) as well as the one-
electron reduction potentials of the three oxidants (see Experi-
mental Section).
are as follows: E0 ) 1.24 V vs SCE for [Ru(bpy)3](PF6)3
red
(bpy ) 2,2′-bipyridine); E0red ) 1.07 V vs SCE for [Fe(phen)3]-
(ClO4)3 (phen ) 1,10-phenanthroline); E0red ) 0.90 V vs SCE
for [Fe(4,7-Me2phen)3](PF6)3 (4,7-Me2phen ) 4,7-dimethyl-1,-
10-phenanthroline). The first of the three oxidants has an E0
red
Reorganization Energies for Electron-Transfer Oxidation
of (P)Fe(R). Reorganization energies for the self-exchange
which is more positive than the second oxidation potentials of
each (P)Fe(R) complex except for (OEP)Fe(C6H5) (Table 1),10,11
(28) The singly oxidized Fe(IV) porphyrin was generated in all cases
and this was followed by a second electron transfer when the difference in
potential between the porphyrin oxidation and the oxidant reduction is
(26) Braddock, J. N.; Meyer, T. J. J. Am. Chem. Soc. 1973, 95, 3158.
(27) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28,
2459.
energetically feasible, i.e., ∆G0 < 0.
et

788 J. Am. Chem. Soc., Vol. 121, No. 4, 1999
Fukuzumi et al.
Figure 2. Dependence of log ket on ∆G0 for the rate-determining
et
first one-electron oxidation of (P)Fe(R) with three different oxidants
in MeCN at 298 K (see Scheme 1). The identity of compounds (1-7)
is given in Table 1, and the fit of the curves based on the Marcus theory
of electron transfer (eqs 2 and 3) is shown by the solid lines (a)
(OEP)Fe(R)/[(OEP)Fe(R)]+ (R ) C6H5, 2,4,6-C6F3H2, 2,3,5,6-C6F4H),
(b) (OETPP)Fe(C6F5)/[(OETPP)Fe(C6F5)]+, and (c) (OETPP)Fe(R)/
[(OETPP)Fe(R)]+ (R ) C6H5, 3,5-C6F2H3, 2,4,6-C6F3H2). The broken
line (d) shows the calculated dependence of log ket on ∆G0 for the
et
second electron transfer; see text.
increase in the following order: (OEP)Fe(R) (83 ( 4) ,
(OETPP)Fe(C6F5) (99 ( 2) < (OETPP)Fe(2,4,6-C6F3H2) (107
( 2) < (OETPP)Fe(3,5-C6F2H3) (109 ( 3) < (OETPP)Fe(C6H5)
(113 ( 3). Thus, the electron-transfer rate at the same free
energy change of electron transfer (∆G0et) should also decrease
in this order. This is clearly shown in Figure 2, where the log
Figure 1. Time course of the absorption change (a) at 287 nm due to
formation of [Ru(bpy)3]2+ and (b) at 431 nm due to decay of (OETPP)-
Fe(C6H5) in the electron-transfer reaction from (OETPP)Fe(C6H5) (1.0
× 10-5 M) to [Ru(bpy)3]3+ (2.0 × 10-5 M) in MeCN at 298 K. Inset:
second-order plot.
ket values are plotted against ∆G0 for electron transfer from
et
Scheme 1
(P)Fe(R) to the oxidants in MeCN at 298 K. The fit of the curves
in light of the Marcus theory of adiabatic outer-sphere electron
transfer (eqs 2 and 3)17 using the reorganization energies for
∆Gq ) (λ/4)(1 + ∆G0 /λ)2
(3)
(4)
reaction of (P)Fe(R)/[(P)Fe(R)]+ (λ11) are shown in Table 1 and
were determined by eq 1, which is readily derived from the
Marcus equation,17 where ∆Gq is the activation free energy and
λ22 is the reorganization energy for the self-exchange reaction
of oxidant/(oxidant)-.
et
λ ) (λ11 + λ22)/2
the cross reactions between (P)Fe(R) and the oxidants (λ), which
is given by eq 4, is shown in Figure 2 which indicates that the
rate variations at the same ∆G0 value arise from a difference
et
λ11 ) 2(2∆Gq - ∆G0et + 2[∆Gq(∆Gq - ∆G0 )]1/2) - λ22
et
in the λ value and not from the nonadiabaticity.
The λ values are equal to 54, 49, and 42 kcal mol-1
respectively for the electron-transfer oxidation of (OETPP)FeIII-
(R) to [(OETPP)FeIV(R)]+ (R ) C6H5, 3,5-C6F2H3, and 2,4,6-
C6F3H2), (OETPP)FeIII(C6F5) to [(OETPP)FeIV(C6F5)]+, and
(OEP)FeIII(R) to [(OEP)FeIV(R)]+.30 Each value is significantly
larger than reorganization energies (ca. 24 kcal mol-1 in MeCN)
for the ligand-centered oxidation of free-base porphyrins.31
Rate constants for the second electron transfer from [(P)Fe-
(R)]+ to the oxidants to produce [(P)FeIV(R)]•2+ were also
calculated using the λ value (24 kcal mol-1) for the ligand-
(1)
The λ22 value for the oxidants used in this study can be
neglected.29 The ∆Gq values are obtained from the observed
rate constant of electron transfer (ket) and the diffusion rate
constant (kdiff) using eq 2, where Z is the collision frequency
taken as 1 × 1011 M-1 s-1, the kdiff value in MeCN is 2.0 ×
1010 M-1 s-1, and the other notations are conventional.
∆Gq ) 2.3RT log[Z(ket-1 - kdiff-1)]
(2)
centered oxidation32 and the ∆G0 values based on eqs 2 and
et
(30) Since the λ22 value for the oxidants used in this study can be
neglected (see ref 29), the λ value corresponds approximately to λ11/2.
(31) Marguet, S.; Hapiot, P.; Neta, P. J. Phys. Chem. 1994, 98, 7136.
(32) The reported λ value (24 kcal mol-1) which is mainly the solvent
reorganization energy for the electron-transfer oxidation of free-base
porphyrins (see ref 31) is used for the electron-transfer oxidation of both
[(OETPP)Fe(R)]+ and [(OEP)Fe(R)]+.
Significant differences are observed in the reorganization
energies depending on the type of porphyrin macrocycle and
the σ-bonded axial ligand, and the λ11 values (in kcal mol-1
)
(29) Fukuzumi, S.; Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1980,
102, 2, 2928.

Organoiron(IV) Porphyrins
J. Am. Chem. Soc., Vol. 121, No. 4, 1999 789
3. The calculated dependence of log ket on ∆G0et for the second
electron transfer is shown by the broken line in Figure 2. A
comparison of the ket values between the first and second
electron transfers clearly indicates that the second electron
transfer from the porphyrin ligand to produce [(P)FeIV(R)]•2+
is much faster than the first electron transfer to produce
[(P)FeIV(R)]+ due to a much smaller reorganization energy
required for the ligand-centered oxidation as compared to the
metal-centered oxidation. Thus, the first electron transfer from
iron(III) to give iron(IV) is the rate-determining step for the
generation of the iron(IV) porphyrin π radical cation,
[(P)FeIV(R)]•2+
.
The crystallographic results obtained for the metal complexes
of OETPP have shown that the size of metal ion alters the degree
of planarity of the porphyrin macrocycle, with a smaller metal
ion favoring a more nonplanar conformation.14 The metal ion
is placed inside the curved surface of the nonplanar porphyrin
macrocycle. Such a distorted conformation results in an ap-
preciably shorter metal-N distance than that of a planar metal
porphyrin.14,33,34 Thus, the stronger binding of metal ion with
the nonplanar porphyrin upon the oxidation of metal ion may
give rise to a larger inner-shell reorganization energy and hence
to the slower electron-transfer rate as compared with that of a
planar metal porphyrin as experimentally observed in this study.
Figure 3. Plot of ket vs [py] for the electron transfer from (OETPP)-
Fe(C6F5) (4.3 × 10-6 M) to [Fe(4,7-Me2phen)3]3+ (4.6 × 10-5 M) in
the presence of pyridine (py) in MeCN at 298 K.
transfer, and the logarithm of the rate constants of electron-
transfer is linearly correlated with the one-electron oxidation
potentials of a series of p-substituted N,N-dimethylanilines.37
The slope of the linear correlation also indicates a large
reorganization energy associated with the electron-transfer
reduction of the Fe(IV) to the Fe(III) porphyrin. Thus, the large
reorganization energy determined for the electron-transfer
oxidation of (P)FeIII(R) to [(P)FeIV(R)]+ (Figure 2) provides the
first experimentally determined energetic basis for the electron
transfer between Fe(III) and Fe(IV) porphyrins which plays an
essential role in biological oxidations and also provides valuable
insights into the mechanistic viability of electron transfer in iron
porphyrin-catalyzed oxidation processes.
The E0 value of (OETPP)Fe(R) (e.g., 0.27 V for R ) C6H5)
ox
is more negative than that of (OEP)Fe(R) (0.48 V for R ) C6H5)
despite the electron-withdrawing effect of extra phenyl groups
in OETPP (Table 1), and this is also ascribed to the stronger
Fe(IV)-N binding which decreases the HOMO level because
of the nonplanar conformation of the OETPP ligand as compared
with the planar OEP ligand.
Effects of a Base on Electron-Transfer Oxidation of
(OETPP)Fe(R). The addition of nitrogenous bases such as
pyridine to five-coordinate iron porphyrins often results in
substantial changes in the redox reactivities.38,39 For this reason
we have also examined the effects of a base on the electron-
transfer oxidation of (OETPP)Fe(R). The addition of pyridine
to the (OETPP)Fe(R)-oxidant system results in a significant
increase in the rate of electron transfer from (OETPP)Fe(R) to
the oxidant. Most electron-transfer rates of (OETPP)Fe(R) in
the presence of pyridine were so rapid as to fall outside the
stopped-flow range. Thus, the least reactive system in Table 1,
i.e., the (OETPP)Fe(C6F5)/[Fe(4,7-Me2phen)3]3+ system, was
chosen to determine the effect of pyridine on the electron-
transfer rate. The electron-transfer rate constant ket increases
linearly with increase in the pyridine concentration as shown
in Figure 3.
As seen in Table 1 the reorganization energy is only slightly
affected by the number of F atoms on the σ-bonded axial ligand
R for all compounds in the (OETPP)Fe(R) and (OEP)Fe(R)
series except for (OETPP)Fe(C6F5), which has a somewhat
smaller λ11 value as compared with the less fluorinated phenyl
σ-bonded Fe complexes of OETPP. Such a difference in the
λ
11 values cannot be ascribed to the difference in the spin state,
since all known (OETPP)Fe(R) complexes are low spin.35 The
spin state of (OEP)Fe(R) is also not a key factor which
determines the reorganization energy, since low-spin (OEP)-
Fe(R) (R ) C6H5) and the high-spin complexes (R ) 2,4,6-
C6F3H2 and 2,3,5,6-C6F4H) have similar λ11 values (Table 1).
Although there have so far been no reports on the electron-
transfer oxidation of Fe(III) porphyrins to Fe(IV) or Fe(IV)
porphyrin π radical cations, the reorganization energy for
reduction of iron oxo complexes of (TPFPP)Fe (TPFPP )
tetrakis(pentafluorophenyl)porphyrin dianion) by N,N-dimeth-
The addition of pyridine to an MeCN solution of (OETPP)-
Fe(C6F5) results in a significant change in the UV-vis spectrum.
From these changes, the formation constant (K) for axial ligand
binding of pyridine to (OETPP)Fe(C6F5) was determined as K
) 62 M-1 in MeCN at 298 K. The acceleration of the rate of
electron transfer by the presence of pyridine may be ascribed
to the much faster electron-transfer rate constant for the six
coordinate complex (ket(6)), (OETPP)Fe(C6F5)(py), than that for
the five coordinate complex (ket(5)), (OETPP)Fe(C6F5) as shown
ylanilines has recently been estimated as 47 kcal mol-1 36
, a value
which is as large as the reorganization energy for the oxidation
of (P)FeIII(R) to [(P)FeIV(R)]+ (42-54 kcal mol-1) by our one-
electron oxidants (Figure 2). The reactions of [(TMP)FeIV-
(dO)]•+ (TMP ) 5,10,15,20-tetramesitylporphyrin dianion) with
N,N-dimethylanilines have been shown to proceed via electron
(33) Guilard, R.; Perie´, K. P.; Barbe, J.-M.; Nurco, D. J.; Smith, K. M.;
Van Caemelbecke, E.; Kadish, K. M. Inorg. Chem. 1998, 37, 973.
(34) VanAtta, R. B.; Strouse, C. E.; Hanson, L. K.; Valentine, J. S. J.
Am. Chem. Soc. 1987, 109, 1425.
(35) The inductive effect of five F atoms on the σ-bonded axial ligand
may result in an increase in the contribution of the porphyrin ligand-centered
oxidation which decreases the reorganization energy of the electron-transfer
oxidation.
(37) Goto, Y.; Watanabe, Y.; Fukuzumi, S.; Jones, J. P.; Dinnocenzo, J.
P. J. Am. Chem. Soc. 1998, 120, 10762.
(38) (a) Kadish, K. M. In Iron Porphyrins, Part 2; Lever, A. B. P., Gray,
H. B., Eds.; Addison-Wesley: Reading, MA, 1982; pp 161-249. (b) Kadish,
K. M.; Bottomley, L. A. Inorg. Chem. 1980, 19, 832. (c) Walker, F. A.;
Barry, J. A.; Balke, V. L.; McDermott, G. A.; Wu, M. Z.; Linde, P. F. AdV.
Chem. Ser. 1981, No. 201, 377.
(36) Baciocchi, E.; Lanzalunga, O.; Lapi, A.; Manduchi, L. J. Am. Chem.
Soc. 1998, 120, 5783.
(39) Lanc¸on, D.; Cocolios, P.; Guilard, R.; Kadish, K. M. Organome-
tallics 1984, 3, 1164.

790 J. Am. Chem. Soc., Vol. 121, No. 4, 1999
Fukuzumi et al.
Scheme 2
the reorganization energy associated with the electron-transfer
oxidation of (OETPP)Fe(C6F5)(py). Although the acceleration
effects of pyridine on rates of electron transfer for other
(OETPP)Fe(R) complexes could not be determined quantita-
tively (because of the fast electron-transfer rates for (OETPP)-
Fe(R)(py) which are beyond the detection limit of a stopped-
flow technique), such a rate acceleration effect of pyridine
indicates a significant decrease in the reorganization energy upon
the axial ligand coordination of bases. The self-exchange rate
constant for a six-coordinate iron(III) porphyrin complex,
[(TMpyP)Fe(OH)(H2O)]4+ (TMpyP ) tetrakis(4-N-methylpy-
ridiniumyl)porphyrin dication), has been reported to be at least
3 orders of magnitude greater than that for a five-coordinate
iron(III) complex, [(TMpyP)Fe(H2O)]5+ (1.2 × 106 M-1 s-1).40
The sixth axial coordination of a base may minimize the
structural change associated with the electron transfer, since the
six-coordinate iron atom may remain in the plane of the rather
rigid porphyrin ligand irrespective of the oxidation state.41
in Scheme 2. The observed rate constant of electron transfer in
the presence of pyridine is then given by eq 5. Since K[py] ,
ket ) (ket(5) + ket(6)K[py])/(1 + K[py])
(5)
1 for the pyridine concentrations as indicated in Figure 3, the
slope of the linear plot of ket vs the pyridine concentration in
Figure 3 corresponds to the ket(6)K value. The ket(6) value is then
determined from the ket(6)K value as 3.4 × 109 M-1 s-1, which
is close to the diffusion-limited value. The one-electron oxida-
tion potential is expected to be shifted in a positive direction
by the axial ligand coordination of pyridine, when the electron
transfer oxidation becomes energetically more favorable. How-
ever, the observed positive shift in the oxidation potential in
pyridine as compared to the observed potential in MeCN is only
0.09 V, which cannot account for the remarkable acceleration
of the rate of electron transfer for the pyridine-coordinated
complex as compared with the noncoordinated complex. Thus,
the acceleration in the rate of electron transfer upon axial
coordination of pyridine results from a significant decrease in
Acknowledgment. This work was partially supported by the
International Scientific Research Program (Grant No. 08044083)
and a Grant-in-Aid for Scientific Research Priority Area (Nos.
10149230, 09237239, and 09231226) from the Ministry of
Education, Science, Culture, and Sports of Japan. R.G. ac-
knowledges support from the CNRS. K.M.K. also acknowledges
support from the Robert A. Welch Foundation (Grant E-680).
JA982136R
(40) Pasternack, R. F.; Spiro, E. G. J. Am. Chem. Soc. 1978, 100, 968.
(41) Although the self-exchange rate of a six-coordinate low-spin iron
porphyrin is suggested to be faster than the five-coordinate high-spin iron
porphyrin,40 the spin state is not a key factor to determine the reorganization
energy in the present case.