Electrochemistry and Spectral Characterization of Oxidized and Reduced TPPBrxFeCl Where TPPBrx is the Dianion of β-Brominated-Pyrrole Tetraphenylporphyrin and x Varies from 0 to 8

Article abstract of DOI:10.1021/ic960148c

The electrochemistry and spectroelectrochemistry of (TPPBr_x)FeCl (TPPBr_x is the dianion of β-brominated-pyrrole tetraphenylporphyrin and x = 0-8) were examined in PhCN containing tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. Each compound undergoes two reversible to quasireversible one-electron oxidations and either three or four reductions within the potential limits of the solvent. The two oxidations occur at the conjugated porphyrin π ring system, and ΔE_1/2 between these two lectrode reactions increases as the molecule becomes more distorted. The overall reduction of each compound involves the stepwise electrogeneration of an iron(II), iron(I), and iron(I) π anion radical. An equilibrium between chloride-bound and chloride-free iron(II) forms of the porphyrin is observed with association of the anionic ligand being favored for compounds with x > 5. Singly reduced (TPPBr_x)FeCl (x = 0 to x = 6) forms both mono- and bis-CO adducts in CH₂Cl₂. Only the mono-CO adduct is observed for (TPPBr₇)FeCl, and there is no binding at all of CO to (TPPBr₇)FeCl. The v_co of both the mono- and bis-adducts increases with increase in the number of Br groups, but in a nonlinear fashion which is explained in terms of two competing effects. One is the electron-withdrawing affinity of the Br substitutents and the other the nonplanarity of the macrocycle.

Full text of DOI:10.1021/ic960148c

5570
Inorg. Chem. 1996, 35, 5570-5576
Electrochemistry and Spectral Characterization of Oxidized and Reduced (TPPBrx)FeCl
Where TPPBrx Is the Dianion of â-Brominated-Pyrrole Tetraphenylporphyrin and x Varies
from 0 to 8
Pietro Tagliatesta,*,† Jun Li,‡ Marie Autret,‡ Eric Van Caemelbecke,‡ Anne Villard,‡
Francis D’Souza,§ and Karl M. Kadish*,‡
Dipartimento di Scienze e Tecnologie Chimiche, II Universita` degli Studi di Roma, 00133 Roma, Italy,
Department of Chemistry, University of Houston, Houston, Texas 77204-5641, and Department of
Chemistry, Wichita State University, 1845 Fairmont, Wichita, Kansas 67260-0051
ReceiVed February 8, 1996X
The electrochemistry and spectroelectrochemistry of (TPPBrx)FeCl (TPPBrx is the dianion of â-brominated-pyrrole
tetraphenylporphyrin and x ) 0-8) were examined in PhCN containing tetra-n-butylammonium perchlorate (TBAP)
as supporting electrolyte. Each compound undergoes two reversible to quasireversible one-electron oxidations
and either three or four reductions within the potential limits of the solvent. The two oxidations occur at the
conjugated porphyrin π ring system, and ∆E1/2 between these two electrode reactions increases as the molecule
becomes more distorted. The overall reduction of each compound involves the stepwise electrogeneration ïf an
iron(II), iron(I), and iron(I) π anion radical. An equilibrium between chloride-bound and chloride-free iron(II)
forms of the porphyrin is observed with association of the anionic ligand being favored for compounds with x >
5. Singly reduced (TPPBrx)FeCl (x ) 0 to x ) 6) forms both mono- and bis-CO adducts in CH2Cl2. Only the
mono-CO adduct is observed for (TPPBr7)FeCl, and there is no binding at all of CO to (TPPBr8)FeCl. The νCO
of both the mono- and bis-adducts increases with increase in the number of Br groups, but in a nonlinear fashion
which is explained in terms of two competing effects. One is the electron-withdrawing affinity of the Br
substitutents and the other the nonplanarity of the macrocycle.
Introduction
makes the porphyrin π ring system harder to oxidize, thus
stabilizing it against oxidative degradation.
Metallotetraphenylporphyrins bearing halogen substituents on
the pyrrole â-positions of the macrocycle are of great interest
since they show a high catalytic efficiency for the oxidation of
organic substrates.1-18 In the case of synthetic iron derivatives,
this catalytic efficiency has been attributed to the presence of
electron-withdrawing substituents on the macrocycle which
The redox potentials of halogenated metalloporphyrins have,
in several cases, been related to their catalytic efficiency19 or
the planarity of the molecules,20-24 and with this in mind, our
laboratory has examined the electrochemistry of (TPPBrx)M
where TPPBrx ) the dianion of â-brominated-pyrrole tetraphe-
nylporphyrin and M ) Fe (x ) 0-8)24 or Co (x ) 0, 6, 7, 8).25
We now provide further details on the synthesis of the above
series of brominated iron porphyrins, examine the binding of
carbon monoxide by the Fe(II) forms of the porphyrins, and
also report their full electrochemical and spectroelectrochemical
characterization.
(TPP)FeCl (TPP ) the dianion of tetraphenylporphyrin)
undergoes three one-electron reductions and two one-electron
oxidations in most nonaqueous solvents.26 The first two
reductions are metal-centered and involve a stepwise conversion
of Fe(III) to Fe(II) followed by an Fe(II)/Fe(I) reaction at more
negative potentials. The third reduction involves the porphyrin
† II Universita` degli Studi di Roma.
‡ University of Houston.
§ Wichita State University.
X Abstract published in AdVance ACS Abstracts, June 1, 1996.
(1) Lyons, J. E.; Ellis, P. E. Metalloporphyrins in Catalytic Oxidations;
Dekker M.: New York, 1994; pp 297-324.
(2) Mansuy, D. The ActiVation of Dioxygen and Homogeneous Catalytic
Oxidation; Plenum Press: New York and London, 1993; pp 347-
358.
(3) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987, 26, 1338.
(4) Ellis, P. E.; Lyons, J. E. Catal. Lett. 1989, 3, 389.
(5) Meunier, B. Chem. ReV. 1992, 92, 1411.
(6) Ostovic, D.; Bruice, T. C. Acc. Chem. Res. 1992, 25, 314.
(7) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; Gray, H. B. Science
1994, 264, 1311.
(8) Ellis, P. E.; Lyons, J. E. J. Chem. Soc., Chem. Commun. 1989, 1315.
(9) Ellis, P. E.; Lyons, J. E. Coord. Chem. ReV. 1990, 105, 181.
(10) Lyons, J. E.; Ellis, P. E. Catal. Lett. 1991, 8, 45.
(11) Tsuchiya, S.; Seno, M. Chem. Lett. 1989, 263, 263.
(12) Hoffman, P.; Labat, G.; Robert, A.; Meunier, B. Tetrahedron 1990,
31, 1991.
(13) Wijesekera, T.; Matsumoto, A.; Dolphin, D.; Lexa, D. Angew. Chem.
1990, 102, 1073.
(14) Bartoli, J. F.; Brigaud, O.; Battioni, P.; Mansuy, D. J. Chem. Soc.,
Chem. Commun. 1991, 440.
(15) Gonsalves, A. M.; Johnstone, R. A. W.; Pereira, M. M.; Shaw, J.;
Sobral, J. F. Tetrahedron Lett. 1991, 32, 1355.
(16) Battioni, P.; Brigaud, O.; Desveaux, H.; Mansuy, D.; Traylor, T. G.
Tetrahedron. Lett. 1991, 32, 2893.
(19) Lyons, J. E.; Ellis, P. E., Jr. In Metalloporphyrins in Catalytic
Oxidations; Sheldon, R. A., Ed.; Marcel Dekker Inc.; New York, 1994,
p 314.
(20) Birnbaum, E. R.; Schaefer, W. P.; Labinger, J.-A.; Bercaw, J. E.; Gray,
H. B. Inorg. Chem. 1995, 34, 1751.
(21) Grinstaff, M. W.; Hill, M. G.; Birnbaum, E. R.; Schaefer, W. P.;
Labinger, J.-A.; Gray, H. B. Inorg. Chem. 1995, 34, 4896.
(22) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239.
(23) Takeuchi, T.; Gray, H. B.; Goddard, W. A., III. J. Am. Chem. Soc.
1994, 116, 9730.
(24) Kadish, K. M.; D’Souza, F.; Villard, A.; Autret, M.; Van Caemelbecke,
E.; Bianco, P.; Antonini, A.; Tagliatesta, P. Inorg. Chem. 1994, 33,
5169.
(17) Bruice, T. C. Mechanistic Principles of Enzyme ActiVity; VCH
(25) D’Souza, F.; Villard, A.; Van Caemelbecke, E.; Franzen, M.; Boschi,
T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem. 1993, 32, 4042.
(26) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435.
Publishers: New York, 1988; Chapter 8.
(18) Traylor, T. G. Tetrahedron Lett. 1991, 32, 2893.
S0020-1669(96)00148-6 CCC: $12.00 © 1996 American Chemical Society

Electrochemistry of (TPPBrx)FeCl
Inorganic Chemistry, Vol. 35, No. 19, 1996 5571
Table 1. FAB Mass Spectral and UV-Vis Data for (TPPBrx)FeCl
(x ) 1-8)
π ring system and leads to an Fe(I) π anion radical as a final
reduction product, although a formal Fe(0) species has also been
proposed for electrogenerated [(TPP)Fe]2- 27
The electro-
.
m/z
reduction of each (TPPBrx)FeCl complex also occurs Via the
stepwise electrogeneration of an iron(II), iron(I), and iron(I) π
anion radical, but as will be shown, the number of electrode
processes, as well as their half-wave or peak potentials, depend
directly on the number of Br groups on the macrocycle.
Theoretical calculations23,28,29 predict that the HOMO of
porphyrins is more affected by the nonplanarity of the molecule
than the LUMO, and this has been verified experimen-
tally.13,22,23,30-33 In addition, the E1/2 for the first reversible
oxidation of highly halogenated porphyrins does not vary lin-
early with increase in number of halogen groups. This deviation
from linear free energy relationships has been quantitated in
the case of (TPPBrx)FeCl where x varied from 0 to 8.24
The oxidation of (TFPPX8)Zn (TFPPX8 ) the dianion of
tetrakis(pentafluorophenyl)porphyrin and X ) Cl, Br, or CH3)
has also been reported in the literature.34 The three nonplanar
porphyrins are all oxidized at the macrocycle Via two overlap-
ping one-electron transfers34 rather than Via two well-separated
one-electron steps as is the case for virtually all metallopor-
phyrins,26 including (TPP)Zn and (OEP)Zn26 (OEP ) the
dianion of octaethylporphyrin), both of which are planar. The
two ring-centered oxidations of nonplanar [(TPPBrx)CoIII]+ (x
) 6-8) are almost overlapped in PhCN,25 and this can be
compared to the case of [(TPP)CoIII]+, a planar molecule which
undergoes two well-separated one-electron oxidations at the
macrocycle under the same solution conditions. This result
suggests that there is a relationship between the ∆E1/2 separating
the two ring-centered oxidations of a given cobalt or zinc
metalloporphyrin and the planarity of the macrocycle. However,
the same effect on E1/2 is not observed for all metalloporphyrins,
since the potential separation between the two oxidations (460
mV)30 of nonplanar (OETPP)FeCl (OETPP ) the dianion of
octaethyltetraphenylporphyrin) is larger than ∆E1/2 between the
two oxidations of (TPP)FeCl,35 which is a planar molecule and
has a ∆E1/2 of 260 mV under the same experimental conditions.
It was therefore of interest to examine whether an increase or
decrease of ∆E1/2 would be observed between the two oxidations
of (TPPBrx)FeCl as the number of Br groups on the macrocycle
was systematically increased from x ) 0 to x ) 8. It was also
of interest to know how the presence of bulky electron-
withdrawing substituents (and the resulting nonplanarity of the
porphyrin macrocycle) would affect the binding of carbon
monoxide by electrogenerated (TPPBrx)FeII and [(TPPBrx)FeIICl]-.
Both of these points are investigated in the present study.
λmaxa (nm)
compound
found
calcd
(TPPBr1)FeCl
(TPPBr2)FeCl
(TPPBr3)FeCl
(TPPBr4)FeCl
(TPPBr5)FeCl
(TPPBr6)FeCl
(TPPBr7)FeCl
(TPPBr8)FeCl
746.2
826.2
906.2
747.5
826.4
905.3
383 (sh)
386 (sh)
375 (sh)
385 (sh)
390 (sh)
406 (sh)
410 (sh)
418 (sh)
418
422
428
432
432
448
452
460
512
514
516
520
520
526
530
536
984.5
984.2
1062.3
1140.5
1221.6
1300.4
1063.1
1141.9
1220.8
1299.7
a In CH2Cl2.
Tetrahydrofuran (THF) from EM Science was used without further
treatment. Tetra-n-butylammonium perchlorate (TBAP), from Sigma
Chemical Co., was recrystallized from ethyl alcohol and dried under
vacuum at 40 °C for at least 1 week prior to use. Silver perchlorate
(99.9%), AgClO4‚H2O, packed under argon, from Johnson Matthey
Electronics was used as received. Sodium borohydride, from Spectrum
Chemical MFG Corp., was used without further purification. All other
reagents were of analytical grade, purchased from Carlo Erba or Aldrich,
and were used without further purification unless otherwise indicated.
Carbon monoxide was purchased from Matheson Co.
Synthesis. (TPP)H2 and (TPP)Zn were synthesized according to
literature procedures.36 N-Bromosuccinimide was purified as reported
in the literature.37
(TPPBrx)H2 Where x ) 1-4 and 6-8. (TPPBr1)H2, (TPPBr2)H2,
(TPPBr3)H2, (TPPBr4)H2, (TPPBr6)H2, (TPPBr7)H2, and (TPPBr8)H2
were synthesized as reported in the literature.25,38
(TPPBr5)H2. (TPP)Zn (677 mg, 1 mmol) was dissolved in 250 mL
of dry (P2O5) CCl4, and N-bromosuccinimide (1246 mg, 7 mmol) was
added to the solution, which was then refluxed for 4 h in air and
protected from moisture with a CaCl2 valve. The solvent was
evaporated and the residue chromatographed on Al2O3. The first green
fraction was collected, the solvent evaporated, and the residue dissolved
in 200 mL of CH2Cl2. Trifluoroacetic acid (3 mL) was added and the
solution stirred under nitrogen for 4 h, after which it was washed first
with water and then with saturated NaHCO3. Evaporation of the solvent
gave a residue that was chromatographed on a silica gel column, using
CHCl3/n-hexane (1:1) as eluent. The second eluted fraction was
collected, evaporated, and recrystallized from CHCl3/n-hexane (1:3)
to give 200.3 mg of the final product. Total yield: 20.1%. lH NMR
(CDCl3, 400 MHz): δ (ppm) ) 8.4-8.8 (multiplet, 3H, pyr), 8.05-
8.2 (multiplet, 8H, o-Hphenyl), 7.65-7.85 (multiplet, 12H, m-Hphenyl and
p-Hphenyl). MS (FAB/NBA): m/z 1007.5 [M - 2H]+. UV-vis
(CHCl3): λmax (nm) ) 440, 536, 687. Anal. Calcd for C44H25N4Br5:
C, 52.37; H, 2.50; N, 5.55. Found: C, 53.01; H, 2.65; N, 5.76.
(TPPBr1)FeCl and (TPPBr2)FeCl. The mono- and dibrominated
iron derivatives were obtained as reported in the literature.36 In a typical
preparation, 100 mg of (TPPBr1)H2 or (TPPBr2)H2 was dissolved in
50 mL of acetic acid, 1 mL of pyridine. FeSO4‚7H2O (saturated
aqueous solution) was added (1 mL), and the resulting solution was
degassed for 10 min under nitrogen and then kept at 60 °C for 1 h
under continued bubbling. Evaporation of the solvent under vacuum
gave a residue that was purified on a silica gel column eluting with
CHCl3. The second eluted fraction was washed with a 5% HCl solution
and dried on NaCl after which it was recrystallized from CH2Cl2/n-
hexane (1:2) to give the desired compounds in yields of 50% for
(TPPBr1)FeCl and 60% for (TPPBr2)FeCl. Mass spectral, UV-vis,
and elemental analysis data are given in Tables 1 and 2.
Experimental Section
Chemicals. Benzonitrile (PhCN) was purchased from Aldrich
Chemical Co. and distilled over P2O5 under vacuum prior to use.
Absolute dichloromethane (CH2Cl2) over molecular sieves was obtained
from Fluka Chemical Co. and used without further purification.
(27) Hammouche, M.; Lexa, D.; Momenteau, M.; Saveant, J.-M. J. Am.
Chem. Soc. 1991, 113, 8455.
(28) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am.
Chem. Soc. 1988, 110, 7566.
(29) (a) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.;
Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627. (b) Ghosh,
A. J. Am. Chem. Soc. 1995, 117, 4631.
(30) Kadish, K. M.; Van Caemelbecke, E.; D’Souza, F.; Medforth, C. J.;
Smith, K. M.; Tabard, A.; Guilard, R. Inorg. Chem. 1995, 34, 2984.
(31) Sparks, L. D.; Medforth, C. J.; Park, M. S.; Chamberlain, J. R.; Ondrias,
M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc.
1993, 115, 581.
(TPPBrx)FeCl Where x ) 3-8. The Fe(III) porphyrins with three
to eight Br groups were synthesized and purified according to the metal
carbonyl method reported by Groves and Myers.39 In a typical
preparation, 100 mg of (TPPBrx)H2 was dissolved in 25 mL of dry
(36) Fuhrhop, J. H.; Smith, K. M. Porphyrins and Metalloporphyrins;
Elsevier Scientific Publishing Co.: Amsterdam, 1975; Section H, p
757.
(32) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.; Smith,
K. M.; Fajer, J. J. Am. Chem. Soc. 1994, 116, 8582.
(33) Brigaud, O.; Battioni, P.; Mansuy, D. New. J. Chem. 1992, 16, 1031.
(34) Hodge, J. A.; Hill, M. G.; Gray, H. B. Inorg. Chem. 1995, 34, 809.
(35) Kadish, K. M.; Morrison, M. M.; Constant, L. A.; Dickens, L.; Davis,
D. G. J. Am. Chem. Soc. 1976, 98, 8387.
(37) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, 1988; p 105.
(38) Callot, H. J. Bull. Soc. Chim. Fr. 1974, 7-8, 1492.
(39) Groves, J. T.; Myers, R. S. J. Am. Chem. Soc. 1983, 105, 5791.

5572 Inorganic Chemistry, Vol. 35, No. 19, 1996
Tagliatesta et al.
Table 2. Elemental Analyses of (TPPBrx)FeCl Complexes (x )
1-8)
% C
% H
% N
compound
calcd found calcd found calcd found
(TPPBr1)FeCl 67.50 67.58
(TPPBr2)FeCl 61.32 60.95
(TPPBr3)FeCl 56.18 55.98
(TPPBr4)FeCl 51.83 52.02
(TPPBr5)FeCl 48.11 47.60
(TPPBr6)FeCl 44.89 45.02
(TPPBr7)FeCl 42.07 42.77
(TPPBr8)FeCl 39.58 38.92
3.48
3.04
2.68
2.37
2.11
1.88
1.68
1.51
3.53
2.99
2.58
2.47
2.02
1.90
1.86
1.79
7.16
6.50
5.96
5.49
5.10
4.76
4.46
4.20
7.02
6.03
5.99
5.60
5.25
4.99
4.06
3.99
(Na), argon-degassed toluene. Fe(CO)5 (0.1 mL) and I2 (10 mg) were
added, and the solution was kept at 60 °C for 2 h under argon. The
solvent was evaporated under vacuum and the residue chromatographed
on alumina, eluting with chloroform. The second fraction was collected,
washed with 5% HCl, and dried on NaCl. Evaporation of the solvent
and recrystallization from CH2Cl2/n-hexane (1:3) led to the final product
in yields of 70-80%. Mass spectral, UV-vis, and elemental analysis
data are given in Tables 1 and 2.
Instrumentation. Cyclic voltammetry was carried out with an
EG&G Model 173 potentiostat or an IBM Model EC 225 voltammetric
analyzer. Current-voltage curves were recorded on an EG&G Prin-
ceton Applied Research Model RE-0151 X-Y recorder. A three-
electrode system was used, consisting of a glassy carbon or platinum
button working electrode, a platinum wire counter electrode, and a
saturated calomel reference electrode (SCE). This reference electrode
was separated from the bulk of the solution by a fritted-glass bridge
filled with the solvent/supporting electrolyte mixture. All potentials
are referenced to SCE.
Mass spectra were obtained on a VG-4 mass spectrometer using
m-nitrobenzyl alcohol or 2-aminoglycerol as the matrix. 1H NMR
spectra were recorded in CDCl3 on a Bruker AM-400 spectrophotometer
using TMS as internal standard. UV-vis spectra were obtained with
a HP-8452A spectrophotometer. Elemental analyses were performed
by the Analytical Laboratory of the University of Padova, Italy.
UV-visible spectroelectrochemical experiments were performed
with an optically transparent platinum thin-layer electrode of the type
described in the literature.40 Potentials for oxidation or reduction of
each compound were applied with an EG&G Model 173 potentiostat.
Time-resolved UV-visible spectra were recorded with either a Tracor
Northern Model 6500 rapid scan spectrophotometer/multichannel
analyzer or a Princeton Instrument PDA-1024 diode array rapid
scanning spectrophotometer.
Figure 1. Cyclic voltammograms of representative (TPPBrx)FeCl
complexes (x ) 0, 2, 4, 6, 8) in PhCN, 0.1 M TBAP. Scan rate ) 0.1
V/s.
Table 3. Half-Wave Potentials (V vs SCE) for Reduction and
Oxidation of (TPPBrx)FeCl, in PhCN Containing 0.1 M TBAPa
oxidation
IV
1.52 1.20 -0.29 -1.06
reduction
compound
(TPP)FeCl
V
I
II II′
III
-1.73
-1.73
-1.61
-1.60
-1.37
-1.36
(TPPBr1)FeCl 1.52 1.24 -0.26 -1.07
(TPPBr2)FeCl 1.51 1.28 -0.18 -1.01
(TPPBr3)FeCl 1.53 1.27 -0.13 -0.97 -1.26
(TPPBr4)FeCl 1.56 1.26 -0.07 -0.86 -1.18
(TPPBr5)FeCl 1.58 1.26 -0.02 -0.83 -1.14
(TPPBr6)FeCl 1.60 1.24
(TPPBr7)FeCl 1.62 1.21
(TPPBr8)FeCl 1.64 1.19
0.04 -0.73 -1.17b -1.28
0.06 -0.67 -1.13b -1.24
0.10 -0.62 -0.97b -1.20
b
a Identification of processes I-V are given in Figure 1. Epc at 0.1
Infrared spectroelectrochemical measurements were made with an
FTIR Nicolet Magna-IR 550 spectrometer using a specially constructed
light-transparent three-electrode cell.41 The background was obtained
by recording the IR spectrum of (TPPBrx)FeCl in CH2Cl2 under a CO
atmosphere without any applied potential. The IR spectrum of the
electroreduced compound under CO was taken after bubbling CO
through the spectroelectrochemical cell for 5-15 min prior to applying
a potential, and a blanket of CO was maintained above the solution
during the measurement.
V/s.
radical and dication;26 this also appears to be the case for each
(TPPBrx)FeCl complex on the basis of comparisons between
the UV-visible spectra of the singly and doubly oxidized
compounds and spectra obtained after the same electrode
reactions of (TPP)FeCl. Examples of the resulting reversible
spectral changes are shown in Figure 2 for the case of (TPPBr2)-
FeCl, while Table 4 summarizes the UV-vis data for neutral
and electrooxidized (TPPBrx)FeCl where x ) 0, 2, 4, and 7.
For all four compounds, the Soret band decreases in intensity
upon oxidation and exhibits a blue shift of 1-14 nm depending
upon the number of Br groups on the macrocycle.
The values of E1/2 for the two oxidations are plotted vs the
number of Br groups in Figure 3a, while Figure 3b shows the
correlation involving ∆E1/2 between processes IV and V and
the number of Br groups on (TPPBrx)FeCl. The first oxidation
of (TTPBrx)FeCl shifts positively in potential upon going from
x ) 0 to x ) 2 but negatively upon going from x ) 3 to x )
8. An opposite trend is seen for the second oxidation; i.e., a
negative shift is seen upon going from x ) 0 to 2 and a positive
one from x ) 3 to 8. Thus, the absolute potential difference
between the two oxidations (∆E1/2) varies with the number of
Br groups on the macrocycle and increases linearly from 230
mV in the case of (TPPBr2)FeCl to 450 mV in the case of
EPR spectra were recorded at 77 K using a Bruker ESP 380 E
spectrometer. The g values were measured with respect to diphenylpi-
crylhydrazyl (g ) 2.0036 ( 0.0003).
Results and Discussion
Cyclic voltammograms of representative (TPPBrx)FeCl com-
plexes in PhCN, 0.1 M TBAP are presented in Figure 1, and
the redox potentials for each electrochemical process are
summarized in Table 3. Each compound undergoes two
oxidations and either three or four reductions within the anodic
or cathodic potential limits of the solvent.
Electrooxidation of (TPPBrx)FeCl. The two oxidations of
(TPPBrx)FeCl (processes IV and V) are reversible to quasire-
versible for all of the studied compounds. (TPP)FeCl undergoes
two one-electron oxidations, leading to an iron(III) π cation
(40) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
(41) Lin, X. Q.; Mu, X. H.; Kadish, K. M. Electroanalysis 1989, 1, 35.

Electrochemistry of (TPPBrx)FeCl
Inorganic Chemistry, Vol. 35, No. 19, 1996 5573
Figure 2. Time-resolved thin-layer electronic absorption spectra of
(TPPBr2)FeCl in PhCN, 0.2 M TBAP upon stepping the potential from
(a) 0.0 to +1.4 V and (b) +1.4 to +1.6 V.
Table 4. Absorption Maxima (λ, nm) and Corresponding Molar
Absorptivities (10-3ꢀ) for the Soret Band of (TPPBrx)FeCl (x ) 0,
2, 4, 7) and Their Electrogenerated Products in PhCN, 0.2 M TBAP
electrode reactions
Figure 3. Plots of (a) E1/2 vs the number of Br groups for the first and
second oxidations of (TPPBrx)FeCl and (b) ∆E1/2 between the first and
second oxidations of (TPPBrx)FeCl vs the number of Br groups.
compound
(TPP)FeCl
process IV
none
process I process II (or II′)a
412 (76) 411 (95) 424 (131)
427 (83)
439 (81)
446 (77)
473 (140)
(TPPBr2)FeCl 417 (76) 421 (92) 438 (129)
(TPPBr4)FeCl 422 (56) 425 (96) 441 (145)
(TPPBr7)FeCl 441 (130) 455 (180) 477 (290)
of electron-withdrawing substituents at the meso positions of
the porphyrin ring and the other a distortion of the porphyrin
macrocycle.
a II′ for (TPPBrx)FeCl with x ) 6-8.
In summary, ∆E1/2 between the two ring-centered oxidations
of porphyrins may increase or decrease with increased non-
planarity of the molecule, but the exact effect seems to depend
upon the type of central metal ion and/or its ligand environment.
For example, ∆E1/2 between the two ring-centered oxidations
of (TPPBr8)FeCl (450 mV) is larger than ∆E1/2 between the
two ring-centered oxidations of (TPP)FeCl (320 mV). This
contrasts with the case of cobalt porphyrins having the same
TPPBrx macrocycles, where ∆E1/2 between the two ring-centered
oxidations of [(TPPBr8)Co]+ (70 mV) is smaller than ∆E1/2
between the two ring-centered oxidations of [(TPP)Co]+ (190
mV).24
Resonance Raman31,46-48 and structural studies49 have re-
vealed that an S4-ruffling of the porphyrin core is characterized
by a shortening of the M-Np bonds (M is a given metal and
Np a ring nitrogen), and this could also lead to an increased
overlap between the π ring and metal d orbitals depending upon
the type of central metal ion and/or the M-Np bond lengths.
The ionic radii50 of Fe(III) (0.64 Å) and Co(III) (0.63 Å) are
(TPPBr8)FeCl. Interestingly, the latter value is virtually identical
to ∆E1/2 between the two ring-centered oxidations of (OETPP)-
FeCl30 (460 mV) and (DPP)FeCl42 (460 mV) (DPP ) the
dianion of dodecaphenylporphyrin), two molecules that possess
a severely distorted macrocycle.31,42-45
The oxidative behavior of (TPPBr8)FeCl, (OETPP)FeCl, and
(DPP)FeCl contrasts significantly with data for other highly
distorted porphyrins such as (TFPPBr8)Zn34 and [(TPPBrx)-
CoIII]+ 25 (x ) 6-8). The latter two series of compounds
undergo two ring-centered one-electron oxidations, both of
which are overlapped in potential to give a global two-electron
transfer. It was suggested34 that the electrogenerated [(TFPPBr8)-
Zn]+ π cation radical is kinetically unstable and that this
instability was related to two factors which influence the relative
energies of the a and b1 orbitals. One factor was the presence
(42) Simpson, M. C.; Showalter, M.; J., M.; Jentzen, W.; Hobbs, J. D.;
Medforth, C. J.; Nurco, D. J.; Forsyth, T. P.; Smith, K. M.; D’Souza,
F.; Van Caemelbecke, E.; Kadish, K. M.; Shelnutt, J. A. Manuscript
in preparation.
(43) Ochsenbein, P.; Ayougou, K.; Mandon, D.; Fisher, J.; Weiss, R.;
Austin, R. N.; Jayaraj, K.; Gold, A.; Terner, J.; Fajer, J. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 348.
(44) Medforth, C. J.; Smith, K. M. Tetrahedron. Lett. 1990, 31, 5583.
(45) Mandon, D.; Ochsenbein, P.; Fischer, J.; Weiss, R.; Jayaraj, K.; Austin,
R. N.; Gold, A.; White, P. S.; Brigaud, O.; Battioni, P.; Mansuy, D.
Inorg. Chem. 1992, 31, 2044.
(46) Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, L. D.; La
Mar, G. N. J. Am. Chem. Soc. 1982, 104, 4345.
(47) Spiro, T. G. Iron Porphyrins, Part 2; Addison-Wesley: Reading MA,
1983, p 89.
(48) Piffat, C.; Melamed, D.; Spiro, T. G. J. Phys. Chem. 1993, 97, 7441.
(49) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543.

5574 Inorganic Chemistry, Vol. 35, No. 19, 1996
Tagliatesta et al.
almost identical, suggesting that the size of the metal ion may
not be the most important factor to account for the difference
in ∆E1/2 between the two ring-centered oxidations of (TPPBr8)-
FeCl and [(TPPBr8)CoIII]+.
The average of the M-Np bond lengths in (TPPBr8)FeCl
should be higher than that in the singly oxidized product of
(TPPBr8)Co which, in PhCN, 0.1 M TBAP, most likely exists
as the bis-PhCN complex, [(TPPBr8)CoIII(PhCN)2]+. This is
the case for highly distorted51 [(OETPP)CoIII(py)2]+, which has
an average M-Np bond length of 1.931 Å as compared to 1.971
Å for (OETPP)FeCl.31 It should also be noted that (OETPP)-
Cu, which has an average M-Np bond length of 1.977 Å31,
shows a 460 mV potential separation between its two ring-
centered oxidations in butyronitrile,32 and this value is similar
to the 450 mV potential separation between the two ring-
centered oxidations of (TPPBr8)FeCl in PhCN.
Finally it can be pointed out that ∆E1/2 between the two ring-
centered oxidations of (TPPBrx)FeCl in CH2Cl2 is similar to
∆E1/2 in PhCN, thus implying that the solvation of the oxidized
porphyrin, i.e. [(TPPBrx)FeCl)]+, is not the main factor con-
tributing to the correlation between ∆E1/2 and the number of
Br groups on the macrocycle (see Figure 3b). The present data
rather suggest that the M-Np bond lengths, which influence
the overlap between the π ring and metal d orbitals, may play
a key role in determining the potential separation between the
two ring-centered oxidations of nonplanar porphyrins.
Electroreduction of (TPPBrx)FeCl. Each (TPPBrx)FeCl
complex undergoes up to four reductions by cyclic voltammetry
in PhCN. The first, labeled as process I in Figure 1, is reversible
in all cases and involves a conversion of Fe(III) to Fe(II).24 The
second, labeled as process II, is reversible only for compounds
with x ) 0 or 1. As the number of Br groups increases above
2, the cathodic peak current intensity of process II becomes
smaller than that of process I while a new irreversible reduction
peak, labeled as process II′ in Figure 1, appears at a potential
which is 290-460 mV more negative than the E1/2 of process
II (see Table 3 for exact potentials). Process II′ is coupled to
the anodic peak of process II as seen in Figure 1.
Kadish and Rhodes52 have shown that the first reduction of
(TPP)FeCl in PhCN results in a mixture of [(TPP)FeIICl]- and
(TPP)FeII. Similar equilibria between halide-bound and halide-
free Fe(II) forms of the porphyrin also exist after the first
reduction of (TPPBrx)FeCl, and this was confirmed by cyclic
voltammograms of the compounds in PhCN solutions with and
without added AgClO4. For example, the addition of AgClO4
to a PhCN solution containing (TPPBr8)FeCl leads to precipita-
tion of AgCl and the subsequent formation of (TPPBr8)FeClO4,
a compound which is reduced at E1/2 ) +0.49 V. The latter
process, labeled as I′ in Figure 4, is located at a potential that
is 390 mV more positive than E1/2 for the Fe(III)/Fe(II) reaction
of (TPPBr8)FeCl. Thus, by analogy to (TPP)FeClO4 and (TPP)-
FeCl,26 process I′ is assigned to a conversion of (TPPBr8)FeClO4
to (TPPBr8)FeII.
Process II′ almost completely disappears after addition of 3.0
equiv of AgClO4 to the solution, and the Fe(II) reduction occurs
mainly Via process II, as shown in Figure 4. This process is
assigned as a reduction of (TPPBrx)FeII to [TPPBrx)Fe]-, while
process II′ is proposed to involve a reduction of [(TPPBrx)FeIICl]-
to [(TPPBrx)Fe]-. This is also the conclusion which results from
analysis of the UV-vis spectroelectrochemical data (see fol-
Figure 4. Cyclic voltammograms of 0.8 × 10-3 M (TPPBr8)FeCl in
PhCN, 0.1 M TBAP containing 0.0, 1.0, and 3.0 equiv of AgClO4.
lowing section). Finally, E1/2 of process III remains unchanged
after addition of 1.0-3.0 equiv of AgClO4 to solutions of
(TPPBr8)FeCl, and this is consistent with the fact that the Cl-
axial ligand is not coordinated to doubly reduced (TPPBr8)-
FeCl.
The solution equilibrium which exists between electro-
generated (TPP)FeII and [(TPP)FeIICl]- is displaced toward the
anionic form of the porphyrin by addition of (TBA)Cl to
solution, and the resulting complex shows a UV-vis spectrum
with only two visible bands.52 Three bands are observed in
the visible region of the Fe(II) complexes having zero to four
Br groups, but only two are seen for those complexes with x >
5, thus suggesting that [(TPPBrx)FeIICl]- is the dominant species
in solution for compounds with six to eight Br groups.
Examples of the UV-vis spectra are shown in Figure 5 for the
singly reduced products of (TPPBr2)FeCl and (TPPBr6)FeCl.
The electrogenerated Fe(II) porphyrin with two Br groups has
bands at 529, 563, and 605 nm, while the one with six Br groups
has visible bands at 591 and 637 nm.
Further reduction of [(TPPBrx)FeIICl]- Via process II′ in
Figure 1 leads to a loss of the chloride axial ligand, and
[(TPPBrx)Fe]- is then the only porphyrin present in solution.
The UV-visible spectrum of electrogenerated [(TPPBrx)Fe]-
shows a broad Soret band whose molar absorptivity is smaller
than that for the Soret band of the singly-reduced Fe(II) product
(see Table 4).
Doubly reduced (TPP)FeCl and (TPPBr4)FeCl both contain
iron(I), as indicated by their ESR spectra,53 and a similar metal
oxidation state is proposed for the other seven [(TPPBrx)Fe]-
complexes on the basis of data in the literature and results in
the present study for [(TPPBr7)Fe]-. The doubly reduced form
of (TPPBr7)FeCl was generated by chemical reduction of the
neutral porphyrin using excess sodium borohydride and its ESR
spectrum then recorded in frozen THF. This spectrum is shown
in Figure 6 and is characterized by signals at g ) 2.21 and
1.96. Additional weaker signals are also seen at g ≈ 4.3 and g
(50) Handbook of Chemistry and Physics, 66th ed.; CRC Press, Inc.; Boca
Raton, FL, 1985-1986; p F-164.
(51) X-ray crystallographic data for (OETPP)Ni31 and Ni porphyrins with
eight halogens at the pyrrole â positions46 show that these two types
of metallo complexes are severely distorted and have similar M-Np
bond lengths.
(53) Donohoe, R. J.; Atamian, M.; Bocian, D. F. J. Am. Chem. Soc. 1987,
109, 5593.
(52) Kadish, K. M.; Rhodes, R. K. Inorg. Chem. 1983, 22, 1090.

Electrochemistry of (TPPBrx)FeCl
Inorganic Chemistry, Vol. 35, No. 19, 1996 5575
Scheme 1
eight iron brominated porphyrins undergo a third reduction at
the same site of the molecule.
In summary, the overall electroreduction of (TPPBrx)FeCl
in PhCN, 0.1 M TBAP is proposed to occur as shown in
Scheme 1.
A dissociation of the chloride axial ligand from [(TPPBrx)-
FeIICl]- is favored for compounds with zero to four Br groups
and occurs rapidly for the compounds with x ) 0 or 1. The
electron density at the iron(III) or iron(II) center of the porphyrin
decreases as the number of electron-withdrawing Br groups on
the macrocycle is increased, thus resulting in a strengthening
of the Fe-Cl bond for the higher brominated compounds.
Consequently, the second reduction will occur Via both processes
II and II′ for compounds with x ) 2-5, conditions under which
the relative cyclic voltammetric peak intensities of these
processes will depend upon both their potential separation and
the degree of ligand dissociation.26 The [(TPPBrx)FeIICl]- form
of the Fe(II) porphyrin which is generated Via process I is very
stable when x ) 6-8, and a dissociation of the chloride axial
ligand in these complexes occurs only after the second reduction
and formation of the Fe(I) porphyrin, i.e. after process II′.
CO Binding to Electrogenerated Iron(II) Complexes. It
has long been known that Fe(II) porphyrins form both mono
and bis-CO adducts in nonaqueous media,56-60 with the exact
stoichiometry of the reaction depending upon the partial pressure
of CO and any competitive ligand binding reactions. Little,
however, is known with respect to how systematic changes in
basicity or distortion of the porphyrin macrocycle might affect
the CO binding reactions. These studies were therefore carried
out for each singly reduced product of (TPPBrx)FeCl in the
nonbonding solvent, CH2Cl2. The cyclic voltammograms of
the porphyrins in CH2Cl2, 0.2 M TBAP are very similar in shape
to those obtained in PhCN, and the redox potentials for each
electrode reaction are almost identical in both solvents under
an N2 atmosphere.
Figure 5. Electronic absorption spectra of singly reduced (a) (TPPBr2)-
FeCl and (b) (TPPBr6)FeCl in PhCN, 0.2 M TBAP.
Figure 7 shows the FTIR difference spectra obtained after
the first reduction of 0.7 ( 0.1 mM (TPPBrx)FeCl (x ) 0-8)
in CH2Cl2, 0.2 M TBAP under a CO atmosphere. The spectra
of the compounds with zero to six Br groups show two bands
whose relative intensity depends on the number of Br groups.
Under the same solution conditions, the spectrum of (TPPBr7)-
FeCl shows only a single band and no band at all is seen for
the spectrum of (TPPBr8)FeCl.
The bands located between 1962 and 1980 cm-1 in Figure 7
arise from the νCO of a mono-CO adduct while the higher energy
bands, located between 2028 and 2043 cm-1, are assigned to
the asymmetric stretches of a bis-CO adduct.57 These assign-
ments are consistent with the fact that the band of the bis-CO
adduct first decreases in intensity and then disappears as the
porphyrin concentration is increased from 0.7 to 2.0 mM. The
Figure 6. EPR spectrum of [(TPPBr7)FeI]- in THF at 77 K.
≈ 2.0 and are assigned to an iron peroxo species produced
by reaction of the iron(I) porphyrin with trace oxygen.54
The ESR spectrum of [(TPPBr7)Fe]- in frozen THF is
similar to the ESR spectrum of [(TPP)Fe]- which has g values
at 2.28 and 1.93 in frozen DMF,53 thus suggesting that the
second reduction product of (TPPBr7)FeCl also contains
iron(I).
The third reduction of (TPPBrx)FeCl (labeled as process III
in Figure 1) is quasireversible to reversible by cyclic voltam-
metry. This electrode reaction has been assigned to the
conversion of an iron(I) porphyrin to an iron(I) porphyrin π
anion radical55 in the case of (TPP)FeCl. A linear relationship
exists between E1/2 for process III and the number of Br groups
on (TPPBrx)FeCl (figure not shown), and this suggests that all
(56) Wayland, B. B.; Mehne, L. F.; Swartz, J. J. Am. Chem. Soc. 1978,
100, 2379.
(57) Strauss, H.; Holm, R. H. Inorg. Chem. 1982, 21, 863.
(58) Rougee, M.; Brault, D. Biochem. Biophys. Res. Commun. 1973, 1364.
(59) Rougee, M.; Brault, D. Biochemistry 1975, 14, 4100.
(60) Swistak, C.; Kadish, K. M. Inorg. Chem. 1987, 26, 405.
(54) Valentine, J. S.; McCandlish, E. Frontiers of Biological Energetics,
Electrons to Tissues; Academic Press: New York, 1978; p 936.
(55) Hickman, D. L.; Shirazi, A.; Goff, H. M. Inorg. Chem. 1985, 24, 563.

5576 Inorganic Chemistry, Vol. 35, No. 19, 1996
Tagliatesta et al.
Figure 8. Plots illustrating νCO of mono- and bisadducts of CO versus
the number of Br groups on the porphyrin macrocycle.
the same. These data indicate that a bisadduct is not present in
solutions containing excess Cl-, where only the six-coordinate
[(TPPBrx)Fe(CO)Cl]- species is formed as shown in eq 2.
Figure 8 illustrates plots of νCO versus the number of Br
groups on the porphyrin macrocycle for both the mono- and
bis-adducts. The frequencies of both CO vibrations increase
with increase in the number of Br groups on the macrocycle,
and this is consistent with a decreased electron density at the
Fe(II) center and a weakened Fe-CO bond. As a result, the
octabrominated complex does not bind CO at all while the
heptabrominated species shows only a weak band at 1980 cm-1
(see Figure 7), indicating the presence of a single CO molecule
coordinated to the iron(II) form of the porphyrin, which is
presumed to exist as [(TPPBr7)Fe(CO)Cl]-.
Interestingly, νCO does not show a linear relationship with
the number of Br groups on the macrocycle, and this was also
the case for E1/2 of the first oxidation (see Figure 3a). The fact
that similar trends are observed in Figure 3a and Figure 8
suggests that the ring distortion, which arises from an increase
in the number of Br groups on the porphyrin macrocycle,
stabilizes the Fe-CO bond in both the mono- and bis-CO
adducts. The change in νCO per added Br group is small, but
there is no doubt that two correlations exist, one for compounds
with x ) 0-2 and the other for compounds with x ) 3-7 (or
8). This point is noteworthy, and to our knowledge is the first
report where the axial ligand binding affinity of a nonplanar
porphyrin for a small molecule, such as CO, will be affected
by the nonplanarity of the macrocycle.
Finally, our results have shown that the metal ion and its
environment seem to influence the potential separation between
the ring-centered oxidations of nonplanar porphyrins. Theoreti-
cal and experimental studies with different metal ions should
therefore be undertaken in order to better understand this
relationship.
Figure 7. IR spectra of 0.76 mM (TPPBrx)FeCl (x ) 0-4 and 6-8)
in CH2Cl2, 0.1 M TBAP under a CO atmosphere at a potential slightly
more negative than that of process I (see text).
assignments also agree with the expected trans effect of CO,
which raises νCO for the bis-CO adduct as compared to the
mono.56
The electrochemical and UV-vis spectral data both indicate
that the singly reduced products of (TPPBrx)FeCl are (TPP-
Brx)FeII or [(TPPBrx)FeIICl]-, or a mixture of both, in a
nonbinding solvent, with the degree of Cl- binding to Fe(II)
depending upon the overall basicity of the macrocycle. The
binding of CO to the Fe(II) forms of (TPPBrx)FeCl can therefore
be interpreted by eqs 1-3.
(TPPBrx)FeII + CO a (TPPBrx)FeII(CO)
[(TPPBrx)FeIICl]- + CO a [(TPPBrx)FeII(CO)Cl]- (2)
(TPPBrx)FeII + 2CO a (TPPBrx)FeII(CO)2
(1)
(3)
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K.; Grant E-680), The University of Houston
Energy Laboratory (K.M.K.) and the Italian MURST (P.T.), and
the CNR (P.T.) is gratefully acknowledged. We also thank
Giuseppe D’Arcangelo and Fabio Bertocchi for mass spectral
and NMR measurements and Dr. T. Wasowicz for ESR
measurements.
Kadish et al.60 have shown that [(TPP)FeCl]- can be
converted to [(TPP)Fe(CO)Cl]- in CH2Cl2, 0.1 M TBACl under
a CO atmosphere on the cyclic voltammetry time scale, and
our IR spectral data can be interpreted in a similar manner. The
2028 cm-1 band, assigned to the νCO of (TPP)Fe(CO)2,
disappears upon going from 0.2 M TBAP to 0.2 M (TBA)Cl
while the intensity of the band for the mono-CO adduct remains
IC960148C