Slow electron transfer rates for fluorinated cobalt porphyrins: Electronic and conformational factors modulating metalloporphyrin ET

Article abstract of DOI:10.1021/ic034705o

The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl) porphyrinato]cobalt, CoF₂₈TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenyl)porphyrinato]cobalt, CoF₈TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF₂₀TPP, and [5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric digital simulation, in situ UV-vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, ¹⁹F NMR spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as supporting electrolyte, the ET rate constants for the Co^2+/3+ redox couples were found to be strongly substituent dependent; the heterogeneous ET rate constant (k_el) varied by a factor of 10⁴, and the ET self-exchange rate constants (k_ex) varied over 7 orders of magnitude for the compounds studied. The remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical k_el values for all compounds. UV-vis and IR spectroelectrochemistry, bulk electrolysis, and ¹⁹F NMR spectroscopic studies support attribution of different ET rates to widely varying inner sphere reorganization energies (λ_i) for these closely related compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP, CoF₈TPP, CoF ₂₀TPP, and CoF₂₈TPP first oxidations arises mainly from large nuclear reorganization energies primarily associated with core contraction and dilation. Taken together, these studies provide rational design principles for modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar manipulation of ET rates in other porphyrin-based systems: substituents that lower C-C, C-N, and N-M vibrational frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron population will increase k_ET. The heme ruffling apparent in electron transfer proteins such as cytochrome c is interpreted as nature's exploitation of this design strategy.

Full text of DOI:10.1021/ic034705o

Inorg. Chem. 2003, 42, 6032−6040
Slow Electron Transfer Rates for Fluorinated Cobalt Porphyrins:
Electronic and Conformational Factors Modulating Metalloporphyrin ET
Haoran Sun, Valeriy V. Smirnov, and Stephen G. DiMagno*
Department of Chemistry, UniVersity of NebraskasLincoln, Lincoln, Nebraska 68588-0304
Received June 20, 2003
The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro-
5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-
28
tetraphenyl)porphyrinato]cobalt, CoF TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF20TPP, and
8
[5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric
digital simulation, in situ UV−vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, 19F NMR
spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF ) as supporting electrolyte, the ET rate constants for the Co2+/3+ redox couples were
6
found to be strongly substituent dependent; the heterogeneous ET rate constant (kel) varied by a factor of 104, and
the ET self-exchange rate constants (kex) varied over 7 orders of magnitude for the compounds studied. The
remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical kel values for all
compounds. UV−vis and IR spectroelectrochemistry, bulk electrolysis, and 19F NMR spectroscopic studies support
attribution of different ET rates to widely varying inner sphere reorganization energies (λi) for these closely related
compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP,
CoF TPP, CoF TPP, and CoF TPP first oxidations arises mainly from large nuclear reorganization energies primarily
8
20
28
associated with core contraction and dilation. Taken together, these studies provide rational design principles for
modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar
manipulation of ET rates in other porphyrin-based systems: substituents that lower C−C, C−N, and N−M vibrational
frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron
population will increase kET. The heme ruffling apparent in electron transfer proteins such as cytochrome c is
interpreted as nature’s exploitation of this design strategy.
Introduction
the analogous Fe2+/3+ oxidation in iron porphyrins.5 For
example, self-exchange ET rate constants (kex) for 5,10,15,20-
tetraarylmetalloporphyrins fall in the range 10-2 to 104 M-1
s-1 for the Co2+/3+ process,6 and 107 to 108 M-1 s-1 for the
Fe2+/3+ process.7-10 As studies of cobalt cytochrome c have
demonstrated, the reduction in kex for the oxidation of Co2+
porphyrins is primarily due to large inner sphere reorganiza-
tion energies (λi ∼ 2.4 eV).11 Because Co2+ porphyrins are
Owing to their utility as functional models for naturally
occurring hemes,1-4 cobalt porphyrins have been subjected
to a wide range of electrochemical and electron transfer (ET)
studies.5 It has long been noted that the metal-centered
oxidation of cobalt (Co2+/3+) in porphyrins exhibits sluggish
heterogeneous and homogeneous ET rates in comparison to
porphyrin ring oxidation, to ring or metal reduction, or to
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6032 Inorganic Chemistry, Vol. 42, No. 19, 2003
10.1021/ic034705o CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/28/2003

Electron Transfer in Fluorinated Cobalt Porphyrins
1
generally low spin (S ) /2), removal of an electron from
kinetic rate constants for the metal-centered (Co2+/3+) oxida-
tion are unusually small and very sensitive to the degree of
fluorine substitution. These results contrast with electro-
chemical studies of â-brominated Co2+ tetraarylporphyrins,
which show halogen substitution correlated with an increase
in kel for the Co2+/3+ couple.39 The juxtaposition of these
studies generates an interesting puzzle: why should appar-
ently similar â-octahalogenated ligand systems give rise to
such different ET kinetic behavior for the chelated cobalt
ion? The answer to this question provides a general design
strategy for manipulating ET rate constants in metallopor-
phyrins.
2
the predominantly dz -character HOMO results in large
changes in the bonding about the cobalt ion, including
significant alteration of the metal-nitrogen bond distances
and an associated contraction of the macrocycle framework
around the smaller ion.
Due to their more positive redox potentials and higher
stability compared to their nonhalogenated counterparts,
metalloporphyrins possessing halogen (F, Cl, Br) at the
pyrrole (â) positions and/or meso-haloaryl or meso-haloalkyl
substituents have garnered interest as potential catalysts for
the oxidation of organic substrates, including alkane hy-
droxylation and alkene epoxidation.12-34 Recently, we re-
ported the synthesis and unusual properties of â-octafluori-
nated porphyrins and their zinc, cobalt, and rhodium
derivatives.35-38 A key advantage offered by fluorine sub-
stituents is that their relatively small size allows electronic
effects to be studied independently of the profound steric
and/or conformational perturbations that are seen with larger
halogens. Here, we show that the magnitudes of the ET
Experimental Section
Materials. Materials were obtained from Aldrich Chemical Co.,
unless otherwise noted. Benzonitrile and deuterated benzene
(Cambridge Isotope Laboratories) were distilled under reduced
pressure from P2O5 and redistilled from CaH2. Acetonitrile was
distilled from P2O5 and redistilled from AgPF6. Pyridine and CH2-
Cl2 were distilled from CaH2. Tetra-n-butylammonium hexafluo-
rophosphate (TBAPF6) (Fluka) was thrice recrystallized from
acetone/ether and dried under vacuum (18 h, 40 °C) prior to use.
Hexafluorobenzene was used as an internal chemical shift reference
(δ ) -164.9 ppm) for 19F NMR experiments. All other reagents
were of analytical grade. CoF8TPP, CoF28TPP, and CoF20TPP were
synthesized by literature procedures.37 CoTPP was recrystallized
from toluene before use.
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Instrumentation and Methods. Electrochemical experiments
were performed with either a computer-controlled EG&G PARC
M273 potentiostat with M270 electrochemical software or with an
EG&G PARC M173 potentiostat. For cyclic voltammetry, a three-
electrode system consisting of a planar platinum working electrode
(EG&G, 0.0314 cm2), a platinum wire counter electrode, and a
reference electrode was employed. The reference electrode was
either Ag/Ag+ (0.01 M AgNO3 in CH3CN with 0.1 M TBAPF6) or
a silver wire quasi-reference electrode. The observed potential was
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Sci. Catal. 1997, 110, 865-872.
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Sanchez, F. J. Mol. Catal. A: Chem. 1996, 109, 91-98.
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59-73.
corrected by comparison to an internal standard, Fc/Fc+ (E1/2
)
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151 ( 3 mV vs Ag/Ag+). All potentials reported in this paper are
referenced to the Ag/Ag+ couple. The working electrode was
polished and cleaned ultrasonically prior to each use. In situ UV-
vis and FTIR spectroelectrochemical experiments were performed
with an optically transparent, adjustable path length, thin-layer
spectroelectrochemical cell.40 A Pt grid was used as the working
electrode; two Pt sheets were used as the counter electrode. UV-
vis spectra were recorded on an Olis-14 modification of a Cary-14
spectrophotometer. The bulk electrolysis cell possessed two cham-
bers separated by a fine fritted disk. Two large area Pt grid
electrodes were used as working and counter electrodes. A
homemade Ag/Ag+ (0.01 M AgNO3 in CH3CN including 0.1 M
TBAPF6) reference electrode was separated from the bulk of the
solution by a Luggin capillary containing TBAPF6 (0.5 M) in
benzonitrile. UV-vis spectra after bulk electrolysis were determined
in a Schlenk-style cell and compared with the in situ spectroelec-
trochemical results. Digisim 3.05 software (Bioanalytical Systems
Inc.) was used for cyclic voltammetric digital simulation.
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J. Am. Chem. Soc. 1992, 114, 1308-12.
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Further experimental details concerning the X-ray crystal struc-
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T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem. 1993, 32, 4042-8.
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Inorganic Chemistry, Vol. 42, No. 19, 2003 6033

Sun et al.
Table 1. Redox Potential Data and Heterogeneous ET Kinetic Rate Constants for CoF28TPP, CoF20TPP, CoF8TPP, and CoTPP in Benzonitrile
Containing 0.1 M TBAPF6 (E/V vs Ag/Ag+)
oxidation
a∆E2-1
0.722
reduction
O
R
compd
CoTPP
third
second
first
b∆EO-R
first
c∆E2-1
second
-2.285
1.151
0.875
0.153
1.327
-1.174
1.111
1.038
1.000
0.903
0.33
(3.5 × 10-3
)
)
(3.6 × 10-3
)
)
(1.8 × 10-3
)
)
)
)
(4.1 × 10-3
-0.854
)
)
)
)
d
CoF8TPP
CoF20TPP
CoF28TPP
ZnTPPe
1.368
1.210
0.877
0.980
1.119
0.36
0.333
1.187
1.242
1.140
-1.892
(3.0 × 10-3
1.515
(3.0 × 10-3
1.287
(2.7 × 10-5
0.307
(3.8 × 10-3
-0.935
(4.2 × 10-3
)
)
-1.935
(5.9 × 10-6
(5.2 × 10-3
-0.584
1.675
1.16
1.36
0.556
-1.487
(1.3 × 10-7
0.80
(2.7 × 10-3
-1.33
(3.8 × 10-3
-1.66
2.13
(586)f
2.29
ZnF8TPPe
ZnF28TPPe
0.10
1.26
1.70
-1.03
-0.63
(573)f
2.33
0.41
-1.04
(568)f
a ∆E2-1O is the potential separation between the first and the second oxidation. b ∆EO-R is the potential separation between the first reduction and the first
oxidation. c ∆E2-1R is the potential separation between the first and the second reduction. d The data in the parentheses are the heterogeneous ET kinetic rate
constants in units of cm‚s-1 e Data are from ref 38 in methylene chloride containing 0.1 M TBAPF6, vs Ag/AgCl. Fc/Fc+ was incorporated as an internal
.
standard (E1/2 ) 0.48 V vs Ag/AgCl). f Data in parentheses indicate the position of the Q(0,0) transition of neutral compound in methylene chloride (in nm).
quasireversible one electron reduction processes in benzo-
nitrile; the first electron reduces the metal center (Co2+/1+),
while the second reduces the porphyrin ring π-system to form
the corresponding radical dianion. The identities of the
species generated by the sequential reduction processes were
confirmed by in situ spectroelectrochemical studies (vide
infra). Either two (for CoF28TPP) or three (for CoF20TPP,
CoF8TPP, and CoTPP) oxidation processes were observed
within the available benzonitrile potential window.
As expected, fluorine substitution induces positive shifts
(∆E ) E°CoFnTPP - E°CoTPP) in the formal potentials relative
to CoTPP. The largest potential shifts are ∆E(-1/-2) ) 798
mV and ∆E(+1/+2) ) 800 mV for the first porphyrin ring
reduction and the first porphyrin ring oxidation waves of
CoF28TPP. In contrast, other halogenated metalloporphyrins,
such as ZnCl8TPP, ZnCl28TPP, ZnBr8TPP, and CoBr8TPP,
typically show larger potential shifts for ring reduction
processes than for ring oxidation, an observation generally
attributed to the nonplanar conformations observed in por-
phyrins substituted with the heavier halogens.41,42 These
effects are not observed with CoF28TPP, which was shown
to have a planar core conformation in both its high-spin and
low-spin states.37
Figure 1. Cyclic voltammograms and digital simulation results of CoF28-
TPP, CoF20TPP, CoF8TPP, and CoTPP in benzonitrile containing 0.1 M
The formal potentials associated with metal-centered
oxidation and reduction reactions of CoF28TPP and CoF8-
TPP are shifted less than those of the corresponding
porphyrin-centered redox events, due to the attenuation of
the substituent effect by the macrocycle framework (Table
1). The differential shifts in the ring and metal electrochemi-
cal potentials are also evident in the energies of the ligand
to metal charge transfer bands on the red edge of the
electronic spectrum; these bands move to higher energy with
increasing fluorine substitution (see Supporting Information).
TBAPF6 as supporting electrolyte. Scan rate ) 100 mV/s. The solid lines
are the experimental results; the broken lines are the digital simulation
results. The scaling factor m ) 5 µA for CoF28TPP, CoF8TPP, and CoTPP,
and m ) 2.5 µA for CoF20TPP.
reactions, digital simulation, near-IR spectroscopy, FTIR spectro-
electrochemistry, and semiempirical calculations are provided in
the Supporting Information.
Results and Discussion
Cyclic Voltammetry. Cyclic voltammograms along with
the corresponding simulation of the current/potential response
for CoF28TPP, CoF20TPP, CoF8TPP, and CoTPP in benzo-
nitrile containing 0.1 M TBAPF6 as supporting electrolyte
are shown in Figure 1; the potentiometric data are sum-
marized in Table 1. The four cobalt porphyrins undergo two
(41) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239-45.
(42) Ochsenbein, P.; Ayougou, K.; Mandon, D.; Fischer, J.; Weiss, R.;
Austin, R. N.; Jayaraj, K.; Gold, A.; Terner, J.; Fajer, J. Angew. Chem.
1994, 106, 355-7. (See also: Angew. Chem., Int. Ed. Engl. 1994,
106 (3), 348-50.)
6034 Inorganic Chemistry, Vol. 42, No. 19, 2003

Electron Transfer in Fluorinated Cobalt Porphyrins
Heterogeneous Electron Transfer Rate Constants. The
shapes of the first oxidation waves (Co2+/3+) in cyclic
voltammograms of CoTPP, CoF8TPP, CoF20TPP, and CoF28-
TPP are strikingly different (Figure 1); the potential separa-
tion of the Co2+/3+ anodic and cathodic peaks is 0.13 V for
CoTPP, 0.50 V for CoF8TPP, and 1.10 V for CoF28TPP at
a sweep rate of 0.1 V/s. These data indicate that the Co2+/3+
heterogeneous ET kinetic rate constant (kel) is reduced in
the fluorinated porphyrins. In contrast, other halogenated
cobalt porphyrins, such as CoBrxTPP, show enhanced ET
rates: peak-to-peak separations for the Co2+/3+ process
decrease with increasing â-Br substitution.43
The results of cyclic voltammetric digital simulations are
indicated as broken lines in Figure 1. (Details are given in
the Supporting Information.) The simulation data show that
kel for the Co2+/3+ process varies by more than 4 orders of
magnitude for the cobalt porphyrins studied here (Table 1).
Only the Co2+/3+ reactions exhibit a large difference in the
heterogeneous ET kinetic rate constants; kel does not vary
by more than a factor of 3 for the other electrode reactions.
To exclude a substituent-induced change in the electron
transfer site (metal- vs ring-localized ET) as an explanation
for the varied heterogeneous ET kinetics of these fluorinated
porphyrins, we performed in situ spectroelectrochemistry,
bulk electrolysis, and 19F NMR experiments to characterize
definitively the products of each electrochemical process
shown in Figure 1.
UV-vis Spectroelectrochemistry. Figure 2 shows the
changes in the UV-vis spectra of CoF28TPP, CoF8TPP, and
CoTPP in benzonitrile containing 0.1 M TBAPF6 for the four
reported redox processes. No low energy transitions indica-
tive of porphyrin ring radical species are evident after the
first oxidation (Figure 2A) or the first reduction (Figure 2C),
indicating that these redox events are primarily metal-
centered.44 Further oxidation (Figure 2B) or reduction (Figure
2D) leads to the telltale signature of ring-centered processes,
broad absorbance bands in the red to near-IR region.45 To
support the assignments that the first oxidation and reduction
processes are metal-centered for CoF28TPP, electrolyte
solutions (0.1 M TBAPF6 in benzonitrile) of [CoF28TPP]-1
and [CoF28TPP]+1 were analyzed by 19F NMR and found to
be diamagnetic. The chemical shifts of the porphyrin ring
fluorine signals are an important diagnostic for metal electron
density; accordingly, the chemical shift values for the ions
([CoF28TPP]-1, δ ) -152.7 ppm; [CoF28TPP]+1, δ )
-142.8 ppm) bracketed the benchmark value for ZnF28TPP
(δ ) -145.4 ppm).35,38
Homogeneous ET Self-Exchange Rates. Marcus’s semi-
classical description of outer sphere ET46-52 and the Marcus
(43) Kadish, K. M.; Li, J.; Van Caemelbecke, E.; Ou, Z.; Guo, N.; Autret,
M.; D’Souza, F.; Tagliatesta, P. Inorg. Chem. 1997, 36, 6292-6298.
(44) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Inorg. Chem. 1988, 27, 1489-
92.
(45) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am.
Chem. Soc. 1970, 92, 3451-9.
(46) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.
(47) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-78.
(48) Marcus, R. A. J. Chem. Phys. 1957, 26, 872-7.
(49) Marcus, R. A. J. Chem. Phys. 1957, 26, 867-71.
(50) Marcus, R. A. Discuss. Faraday Soc. 1960, 21-31.
Figure 2. In situ UV-vis spectroelectrochemistry of CoF28TPP, CoF8-
TPP, and CoTPP in benzonitrile, 0.1 M TBAPF6: (A) first oxidation, (B)
second oxidation, (C) first reduction, (D) second reduction.
Inorganic Chemistry, Vol. 42, No. 19, 2003 6035

Sun et al.
Table 2. Summary of Cross-Reaction Data
-∆E°′
-∆G
(kcal/mol)
kij
reductant
oxidant
(V)
(M-1 s-1
)
Fca
CoF28TPP+
CoF28TPP+
CoF28TPP+
CoF8TPP+
CoF28TPP+
0.382
0.501
0.249
0.026
0.403
8.82
11.5
5.64
0.60
9.30
6.0 × 104
4.9 × 105
16.3
Me2Fca
CoF20TPPb
CoF20TPPb
CoTPPb
30.7
9.73 × 103
a These data result from stopped flow experiments conducted in CH3CN
(0.05 M NaPF6, 0.05 M TBAPF6). b These data were gathered by
conventional, time-resolved UV-vis spectroscopy (benzonitrile, 0.1 M
TBAPF6).
Table 3. Self-Exchange Rate Constants of Cobalt Porphyrins and
Benchmark Compounds
redox couple
kex (M-1 s-1
)
ref
CoF28TPP+/CoF28TPP
CoF20TPP+/ CoF20TPP
CoF8TPP+/CoF8TPP
CoTPP+/CoTPP
8.0 × 10-4 a
this work
this work
this work
this work
53
6
72
73
20.4
16.8
1.8 × 104
6.5 × 106
9.7
Fc+/Fc
CoTPP(Py)2+/CoTPP(Py)2
Co(Phen)33+/Co(Phen)3
44
2+
CoTTPCl/CoTTPb
2.7 × 104
a An average value (see text). b TTP is 5,10,15,20-tetratolylporphyrin.
2), yielding kex ) 7.0 × 10-4 M-1 s-1 for CoF28TPP. Similar
studies with dimethylferrocene yielded kex ) 8.9 × 10-4 M-1
s-1 for CoF28TPP; thus, we assigned the average value (8 ×
10-4 M-1 s-1) as the standard for further studies.
Figure 3. UV-vis spectral change during the ET reaction of [CoF28TPP]PF6
with excess ferrocene. Time: 0-1 s.
cross-reaction relationship51 (eq 1) were used to estimate the
self-exchange rate constants of CoF28TPP, CoF20TPP, CoF8-
TPP, and CoTPP (Table 3).
Because the Soret band absorption energies differ signifi-
cantly for the â-fluorinated and â-unsubstituted compounds,
cross-reaction rates (Table 2) could be measured easily for
the following pairs of cobalt porphyrins: CoF28TPP(PF6) and
CoF20TPP, CoF8TPP(PF6) and CoF20TPP, and CoF28TPP-
(PF6) and CoTPP. These reactions were sufficiently slow
(Figure 4) to be followed by conventional UV-vis spec-
troscopy. Measured values for kex vary over a 107 range and
mirror the results of the heterogeneous electron transfer
experiments; the more fluorinated porphyrins exhibit reduced
ET rates for the Co2+/3+ process. Thus, specific electrode
interactions are not the origin of the unusual electrochemical
kinetic behavior of the fluorinated complexes, and unusually
large reorganization energies (λ) are implicated as the likely
source of the greater activation barrier for ET.
Structure of Br[CoF28TPP]‚CH3CN. We obtained single
crystal X-ray diffraction data for Br[CoF28TPP] to identify
a possible structural source for the substantial reorganization
energies associated with the oxidation of CoF28TPP. The
structure of the acetonitrile complex, obtained by chemical
oxidation of CoF28TPP with bromine, is rendered in Figure
5. F28TPP is somewhat unusual among the â-perhalogenated
tetraarylporphyrins in that it supports a stable, planar
conformation, as is shown by X-ray crystal structure
determinations of the free base54 and of its rhodium36 and
zinc38 chelates. Two structures also indicate that CoF28TPP
is planar.37 In contrast, the acetonitrile complex of Br[CoF28-
TPP] is significantly ruffled. The largest displacements from
the mean plane of the ring (av ) 0.48 Å) occur at the meso
positions. In terms of its degree of nonplanarity, the structure
k12 ) (k11k22K12f12)1/2
(1a)
(1b)
ln f12 ) (ln K12)2/4 ln(k11k22/Z11Z22)
In eq 1, k12 is the cross-reaction rate constant, k11 and k22
are the self-exchange rate constants for the two redox
partners, f12 is a correction factor (generally close to 1), K12
is the cross-reaction equilibrium constant, which is calculated
from the difference in the redox potentials of the two species,
and Z11 and Z22 are collision frequencies.
To determine kex for CoF28TPP, the reductions of CoF28-
TPP(PF6) by ferrocene and dimethylferrocene were followed
by stopped flow methods. Ferrocenes were selected as
reducing agents because their kex values are well-established
and have been shown to be relatively solvent insensitive.53
The time-dependent decrease in Soret band intensity of
rapidly mixed acetonitrile solutions (0.05 M NaPF6 and 0.05
M TBAPF6) containing CoF28TPP(PF6) and ferrocene was
followed by UV-vis spectroscopy (Figure 3). With excess
ferrocene present (>20 times porphyrin concentration), good
pseudo-first-order kinetic behavior was observed; the pseudo-
first-order rate constants varied linearly with ferrocene
concentration (see Supporting Information). At 25 °C, the
cross-reaction rate constant was 6.4 × 104 M-1 s-1 (Table
(51) Marcus, R. A. J. Phys. Chem. 1963, 67, 853-7.
(52) Marcus, R. A.; Eyring, H. Annu. ReV. Phys. Chem. 1964, 15, 155-
96.
(53) Nelsen, S. F.; Ismagilov, R. F.; Gentile, K. E.; Nagy, M. A.; Tran, H.
Q.; Qu, Q.; Halfen, D. T.; Odegard, A. L.; Pladziewicz, J. R. J. Am.
Chem. Soc. 1998, 120, 8230-8240.
(54) Leroy, J.; Bondon, A.; Toupet, L. Acta Crystallogr., Sect. C: Cryst.
Struct. Commun. 1999, C55, 464-466.
6036 Inorganic Chemistry, Vol. 42, No. 19, 2003

Electron Transfer in Fluorinated Cobalt Porphyrins
Figure 4. Top: Plot of the single wavelength (396 nm) absorbance change
vs time for the cross-reaction of [CoF8TPP]+ and CoF20TPP. Bottom: Plot
of 1/(Alim - At) vs time for the same data.
of Br[CoF28TPP]‚CH3CN falls within the range seen in the
structures of Co3+ derivatives of TPP.55-61 The porphyrin
core structure (Co-N bonds lengths average 1.971(6) Å, Co
is displaced 0.01 Å from the mean plane of the nitrogen
atoms, Table 4) is also unremarkable. For similarly ruffled
Co3+TPP derivatives, the Co-N bond lengths average
1.955,62 1.964,63 and 1.975 Å,64 respectively, for the aquachlo-
ro, diaquaperchlorate, and chloropyridine complexes. Al-
though no definitive conclusions about the solution structure
(where PF6 is the counterion and benzonitrile or CH3CN is
the coordinating solvent) can be based upon the solid state
structure, the available data indicate that there is nothing
manifestly unusual about the type or degree of nonplanarity
or the metal nitrogen bonding in Br[CoF28TPP]‚CH3CN.
Figure 5. Perspective drawings from the X-ray crystal structure deter-
mination of Br[CoF28TPP]‚CH3CN. The top figure is drawn with 30%
thermal ellipsoids.
Table 4. Summary of Structural Parameters for Br[CoF28TPP]‚CH3CN
Selected Bond Lengths and Angles
Co1-N21 1.965(5) N21-Co1-N11 89.4(2) C12-N11-C15 106.5(6)
Co1-N11 1.968(6) N21-Co1-N31 90.6(2) C12-N11-Co1 125.6(5)
Co1-N31 1.973(6) N11-Co1-N31 179.7(3) C15-N11-Co1 127.9(5)
Co1-N41 1.977(6) N21-Co1-N41 178.5(3) N11-C12-C46 127.0(7)
Co1-N1S 1.990(7) N11-Co1-N41 90.1(2) N11-C12-C13 123.7(7)
Co1-Br1 2.3499(13) N31-Co1-N41 89.9(2) C46-C12-C13 123.7(7)
N11-C12 1.363(9) N21-Co1-N1S 89.0(3) C14-C13-F14 126.6(8)
N11-C15 1.365(9) N11-Co1-N1S 87.4(3) C14-C13-C12 107.0(7)
C12-C46 1.396(10) N31-Co1-N1S 92.9(3) F14-C13-C12 126.2(7)
C12-C13 1.444(11) N41-Co1-N1S 89.5(3) C13-C14-F13 126.7(8)
C13-C14 1.322(11) N21-Co1-Br1 90.77(18) C13-C14-C15 107.7(7)
C13-F14 1.332(8) N11-Co1-Br1 90.01(19) F13-C14-C15 125.5(7)
C14-F13 1.350(9) N31-Co1-Br1 89.7(2) N11-C15-C16 124.5(7)
C14-C15 1.436(10) N41-Co1-Br1 90.68(19) N11-C15-C14 109.2(6)
C15-C16 1.388(10) N1S-Co1-Br1 177.38(19) C16-C15-C14 125.9(7)
C16-C22 1.379(10) C1S-N1S-Co1 164.9(7) C22-C16-C15 122.7(7)
(55) Goodwin, J.; Bailey, R.; Pennington, W.; Rasberry, R.; Green, T.;
Shasho, S.; Yongsavanh, M.; Echevarria, V.; Tiedeken, J.; Brown,
C.; Fromm, G.; Lyerly, S.; Watson, N.; Long, A.; De Nitto, N. Inorg.
Chem. 2001, 40, 4217-4225.
C16-C51 1.501(10)
C22-C16-C51 118.3(7)
C15-C16-C51 118.8(7)
(56) Munro, O. Q.; Shabalala, S. C.; Brown, N. J. Inorg. Chem. 2001, 40,
3303-3317.
Average Bond Lengths
(57) Ohba, S.; Eishima, M.; Seki, H. Acta Crystallogr., Sect. C: Cryst.
Struct. Commun. 2000, C56, e555-e556.
Co-N
CR-N
CR-Câ
Câ-Câ
CR-Cm
Câ-Fâ
(58) Tse, A. K. S.; Wang, R.-j.; Mak, T. C. W.; Chan, K. S. Chem.
Commun. 1996, 173-4.
1.971
1.370
1.437
1.324
1.386
1.339
(59) Bang, H.; Edwards, J. O.; Kim, J.; Lawler, R. G.; Reynolds, K.; Ryan,
W. J.; Sweigart, D. A. J. Am. Chem. Soc. 1992, 114, 2843-52.
(60) Yamamoto, K.; Iitaka, Y. Chem. Lett. 1989, 697-8.
(61) Doppelt, P.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1984, 106, 5188-
93.
Thus, the solid state structure provides no obvious clue as
to the source of the “extra” reorganization energy associated
with the oxidation of CoF28TPP in comparison to CoTPP.
(62) Iimura, Y.; Sakurai, T.; Yamamoto, K. Bull. Chem. Soc. Jpn. 1988,
61, 821-6.
IR Spectroelectrochemistry. Difference IR spectroelec-
trochemical experiments were performed to interrogate axial
ligation during the Co2+/3+ oxidation. Prominent IR bands
were observed at 2293.5 cm-1 (CoF28TPP), 2292.5 cm-1
(63) Masuda, H.; Taga, T.; Osaki, K.; Sugimoto, H.; Mori, M. Bull. Chem.
Soc. Jpn. 1982, 55, 4-8.
(64) Sakurai, T.; Yamamoto, K.; Seino, N.; Katsuta, M. Acta Crystallogr.,
Sect. B 1975, B31, 2514-17.
Inorganic Chemistry, Vol. 42, No. 19, 2003 6037

Sun et al.
(CoF8TPP), and 2291.1 cm-1 (CoTPP) attributable to axial
coordination of benzonitrile upon oxidation (see Supporting
Information). These data are consistent with similar changes
in benzonitrile coordination to Co3+ for each of the examples,
suggesting that differences in axial ligation are not likely
responsible for the larger reorganization energy observed for
the fluorinated complexes; the changes in the benzonitrile
stretching frequencies are markedly smaller than those
associated with the porphyrin ring.
In addition to increasing the intensity of a number of
absorption bands attributable to ring C-C and C-N stretch-
ing, fluorine substitution on the porphyrin ring gives rise to
strong C-F IR signatures at 1699 cm-1 (CoF28TPP) and 1688
cm-1 (CoF8TPP). Upon either one-electron oxidation or
reduction, these bands downshift dramatically (17 cm-1 for
CoF28TPP and 15 cm-1 for CoF8TPP). In contrast, when the
metal charge is held constant and the ionic radius decreases,
as in the series ZnF28TPP, CuF28TPP, NiF28TPP, the por-
phyrin ring C-F stretching frequency increases. (C-F
stretching frequencies of 1681, 1693, and 1695 cm-1 are
observed for ZnF28TPP, CuF28TPP, and NiF28TPP, respec-
tively.)65 Therefore, the increased charge on cobalt, rather
than the conformational change due to the smaller ionic
radius of Co3+, is the most likely cause of downshifting upon
oxidation. For fluorinated porphyrins, the bond weakening
associated with repolarization is attributable to the substantial
charge borne by the electronegative fluorine atoms in the
Co2+ structure. In contrast, the IR frequency downshifts
associated with CoF28TPP and CoF8TPP reduction can be
ascribed to population of a metal-centered orbital that leads
to significant σ-antibonding interactions within the porphyrin
ring.
reorganization energies (λis) in metalloporphyrins, semi-
empirical calculations (PM3)67,68 were performed on fluori-
nated and brominated model compounds. The model com-
pounds were devoid of axial ligands, so that only those
contributions to λi intrinsic to the porphyrin ligands were
considered. Two simplifications were made in order to
facilitate estimation of λi: (1) we chose Zn2+ as an example
of a “large” +2 ion to obviate any complications arising from
direct comparison of open-shell and closed-shell semi-
empirical calculations, and (2) the metal ions were con-
strained to the plane of the four nitrogen atoms in the
porphyrin core for ruffled geometries to mimic the available
structural data. Finally, we postulated that reorganization
energies for the Co2+/3+ self-exchange event are directly
related to the inner sphere reorganization energies for the
(computationally simpler) instantaneous (Zn2+/Co3+) metal
ion exchange, with the understanding that the core size
changes are somewhat larger for (Zn2+/Co3+) than they are
for (Co2+/Co3+).69 Consequently, λi is expected to be
overestimated using this technique. Aside from the wrinkle
of altering the identity of the ion, calculation of inner sphere
reorganization energies by semiempirical methods has ample
precedent.70
Optimized geometries for the Zn2+ and Co3+ derivatives
of P, F8P, Br8P, F8TPP, F28TPP, and Br8F20TPP were
calculated. Conformational mapping was employed to find
multiple local minima in several cases. Inner sphere reor-
ganization energies derived from metal ion exchange were
then calculated using the following equation:
λi ) (E(CoLZn) - E(CoLCo)) + (E(ZnLCo) - E(ZnLZn))
Recent theoretical work (DFT B3LYP/6-31G(d)) compar-
ing the vibrational spectra of octabrominated, octachlorinated,
octafluorinated, and unsubstituted porphyrins indicates that
ring fluorinated porphyrins show some marked differences
compared to chlorinated or brominated counterparts.66 The
fluorinated porphyrins are unique in that high-frequency
C-C and C-N in-plane bond vibrations are upshifted
significantly relative to porphine.66 Chlorinated and bromi-
nated porphyrins show substantial downshifts for these same
modes. The trends described by DFT underlie a compact
explanation for the slow kinetics associated with the Co2+/3+
redox events; core size change requires significant reorga-
nization in the porphyrin framework. Thus, the larger force
constants for the bond vibrational modes of fluorinated
porphyrins imply that the potential energy wells associated
with expansion or dilation of the ring are steeper in the
fluorinated compounds. This explanation is consistent with
the IR-spectroelectrochemistry results and the X-ray crystal-
lographic data.
where E(CoLZn) is the calculated enthalpy of formation (∆Hf)
for the Co3+ porphyrin at the optimized Zn2+ porphyrin
geometry, E(CoLCo) is ∆Hf for the optimized Co3+ porphyrin,
E(ZnLCo) is ∆Hf for the Zn2+ porphyrin at the optimized
Co3+ porphyrin geometry, and E(ZnLZn) is ∆Hf for the
optimized Zn2+ porphyrin. Finally, energies of constrained
ruffled geometries (transannular dihedral angles, e.g., C1-
N21-N23-C14, were constrained to 50°, as described
previously)30,71 were calculated for all of the zinc complexes
in order to give an estimate of the energy required for ruffling
of the various porphyrins in the absence of metal ion
exchange. The results of these calculations are summarized
in Table 5. Initially, only the data from calculations involving
a ruffled Co3+ porphyrin geometry are discussed (column 3
in Table 5).
ET in Ruffled Co3+ Porphyrins. Several interesting
trends are seen in the data in Table 5. First, it is apparent
that the calculations follow the general ordering seen in the
(67) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221-64.
(68) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209-20.
(69) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1-70.
(70) Nelsen, S. F.; Blackstock, S. C.; Kim, Y. J. Am. Chem. Soc. 1987,
109, 677-82.
(71) Wertsching, A. K.; Koch, A. S.; DiMagno, S. G. J. Am. Chem. Soc.
2001, 123, 3932-3939.
(72) Haim, A.; Sutin, N. Inorg. Chem. 1976, 15, 476-8.
(73) Chapman, R. D.; Fleischer, E. B. J. Am. Chem. Soc. 1982, 104,
1582-7.
PM3 Semiempirical Modeling of ET. To assess the
impacts of halogenation and ion size upon inner sphere
(65) Walters, V. A.; DiMagno, S. G.; Smirnov, V. V.; de Paula, J. C. Book
of Abstracts, 215th National Meeting of the American Chemical
Society, Dallas, TX, Mar 29-Apr 2, 1998; American Chemical
Society: Washington, DC, 1998, INOR-320.
(66) Nguyen, K. A.; Day, P. N.; Pachter, R. J. Chem. Phys. 1999, 110,
9135-9144.
6038 Inorganic Chemistry, Vol. 42, No. 19, 2003

Electron Transfer in Fluorinated Cobalt Porphyrins
Table 5. Summary of Semiempirical (PM3) Calculations for Assorted
compounds, steric clashes at the porphyrin periphery hinder
contraction about the small Co3+ ion, resulting in reduced
reorganization energies for the instantaneous exchange to the
larger zinc ion.
Porphyrinsa
∆λ
∆λ
∆λ
∆λ
b
c
d
e
compd
λexp
λruf
λsad ∆Hruf (F8)f (Br8)g (Ph4)h (C6F5)i
P
F8P
Br8P
TPP
F8TPP
Br8TPP
F20TPP
F28TPP
Br8F20TPP
88.52
96.24
89.39
21.47 7.72
20.25
20.32
0.88 -9.76
-8.61
The impact of meso phenyl substitution upon λi is indicated
in column 8 of Table 5. Reorganization energies for the pairs
P and TPP, F8P and F8TPP, and Br8P and Br8TPP were
compared. In each case, addition of four meso-phenyl
substituents leads to a decrease in the overall reorganization
energy. By far the largest decrease in λi is seen for phenyl
substitution of Br8P; again, these calculations strongly suggest
that steric effects limiting the extent of macrocycle contrac-
tion about the small Co3+ ion are responsible for the reduced
λi values. The semiempirical calculations also predict that
fluorination of the meso-phenyl substituents leads to high
inner sphere reorganization energies, as is shown by examin-
ing the results from the pairs TPP and F20TPP, F8TPP and
F28TPP, and Br8TPP and Br8F20TPP. These data are sum-
marized in the last column of Table 5.
-16.33
36.17 78.75 71.46 18.87 8.88 -5.69
52.89 87.63 69.49 12.15
73.06 59.79 12.35
52.36 84.95 80.10 19.36 11.97 -11.23
76.11 96.92 85.12 11.19
6.20
9.29
0.65
73.72 63.03 10.58
a All energies are reported in kcal/mol. b Calculated from the self-
exchange ET rate constant and the Marcus equation. c Inner sphere
reorganization energies for instantaneous metal ion exchange, calculated
as described in the text. These data are obtained for ruffled Co(III)
porphyrins. d Inner sphere reorganization energies for instantaneous metal
ion exchange for saddled Co(III) porphyrins. e Energy calculated to distort
Zn(II) porphyrin from its global minimum conformation to one in which
the transannular dihedral angles, e.g., C1-N21-N23-C14, were con-
strained to 50°. f Difference in reorganization energy brought about by
fluorination of the porphyrin ring for each of the porphyrins listed.
g Difference in reorganization energy brought about by bromination of the
porphyrin ring. h Difference in reorganization energy brought about by meso-
phenyl substitution. i Difference in reorganization energy brought about by
fluorination of the phenyl rings.
ET in Saddled Co3+ Porphyrins. Although oxidized
cobalt porphyrins generally adopt ruffled conformations, very
sterically demanding substituents allow access to low energy
saddled conformations. Therefore, we explored how a change
in ring conformation for the Co3+ state would alter λi. Local
minima were found for saddled conformations of all of the
Co3+ tetraarylporphyrins, and λi values were calculated as
already described. Interestingly, the reorganization energies
are predicted to be uniformly lower for this distortion mode
(Table 5). Examination of the details used in the λi calculation
indicate that the reduction in reorganization energy is due
to the lower energy of the E(ZnLCo) term for the saddled
conformation compared to the ruffled geometry. In other
words, the saddled core is more receptive to an instantaneous
expansion in ion size than is a ruffled core. This result is
easily interpreted: the saddled core structure reduces the
σ-bonding overlap of the nitrogen orbitals with the metal
experimental results; compounds that exhibit small kex values
are calculated to possess larger λi values. On the whole, the
semiempirical model studies reasonably reproduce the ex-
perimental λi values, although the calculated values for λi
are too large, as might be expected from the nature of the
calculation, and do not show as much spread (λi(F28TPP) -
λi(TPP) ) 18 kcal/mol) as the overall reorganization energies
derived from the kex values (λ(total)(F28TPP) - λ(total)(TPP) )
30 kcal/mol).
The second important trend evident in Table 5 is that
reorganization energies are predicted to be strongly dependent
upon â-substitution in sterically unencumbered porphyrins.
In the series P, F8P, and Br8P, similar degrees of ruffling
are observed for all three of the Co3+ porphyrins, but the
calculated reorganization energies differ by a large margin.
The larger reorganization energy calculated for F8P indicates
that fluorination (unlike bromination) significantly increases
the energetic costs of porphyrin core expansion and contrac-
tion, consistent with DFT studies indicating that â-fluorina-
tion increases C-C and C-N stretching frequencies. More-
over, data from calculations of the constrained, ruffled Zn2+
complexes (Table 5, column 5) show that ruffling of the
halogenated derivatives is somewhat easier than ruffling of
ZnP, indicating that differences arising from nonplanarity
cannot make a large contribution to the overall reorganization
energy. These data and conclusions are consistent with
complementary DFT results showing exceptionally low
bending frequencies for ruffling of porphyrins generally, and
for halogenated porphyrins in particular.66
2
2
2
(dz and dx -y ) orbitals; thus, there is less energetic cost
associated with changes in these orbitals’ populations. Thus,
substituents that induce a saddled conformation in the
porphyrin ring should result in more rapid cobalt-centered
ET reactions than are expected solely on the basis of
electronic substituent effects. As the data in Table 5 show,
the effect of varying the porphyrin distortion mode can be
substantial.
Conclusions
These studies demonstrate that, under identical experi-
mental conditions, ET self-exchange rate constants (kex) for
the metal-centered oxidation in a series of closely related
cobalt porphyrins can be modulated over 7 orders of
magnitude by sterically undemanding substituent modifica-
tion. Widely varying inner sphere reorganization energies
for these closely related compounds are responsible for the
differing ET rates. Spectroscopic, structural, and semi-
empirical (PM3) studies indicate that the divergent kinetic
behavior of CoTPP, CoF8TPP, CoF20TPP, and CoF28TPP first
oxidations arises mainly from reorganization associated with
porphyrin core dilation and contraction. Semiempirical theory
Further substitution of the porphyrin macrocycle leads to
more complex behavior. â-Halogenation of various porphy-
rins with relatively small fluorine atoms leads to uniformly
larger λi values for all compounds studied (Table 5, column
6). In contrast, bromination of P results in a minimal change
in λi, but a substantial lowering of λi is predicted for Br8-
TPP and Br8F20TPP (Table 5, column 7). For the latter
Inorganic Chemistry, Vol. 42, No. 19, 2003 6039

Sun et al.
also shows that enforced saddling of the porphyrin ring
dramatically reduces λi by minimizing M-N σ-bonding
interactions.
Confirmation of this proposed structure/function relationship
could, in principle, be obtained by measuring ET rates in
cytochrome c mutants in which the heme conformation is
constrained to be saddled.
The main conclusion from these studies, that the key
features controlling electron transfer rate in cobalt porphyrins
are (1) the degree of overlap between redox active metal
orbitals and the porphyrin orbitals, and (2) substituents that
alter the bond frequencies associated with porphyrin contrac-
tion and dilation, provide rational design principles for
modulating ET rate constants in metalloporphyrins. These
concepts also provide a plausible functional role for the
ruffling often observed in the hemes of ET proteins such as
cytochrome c. Such ruffling decreases porphyrin π-metal
dπ interactions and minimizes porphyrin core contraction
and expansion associated with the redox event, thereby
providing small inner sphere reorganization energies and
enhancing electron transfer rates in the natural systems.
Acknowledgment. We thank the National Science Foun-
dation (CHE-9817247) and the Office of Naval Research
(N00014-00-1-0283) for support of this research.
Supporting Information Available: Details regarding (1)
determination of formal redox potentials, (2) parameters for cyclic
voltammetric digital simulation, (3) near-IR spectra for cobalt
porphyrins and FTIR spectroelectrochemistry procedures and
spectra, (4) procedures and data used to estimate ET self-exchange
rate constants, (5) semiempirical calculations, and (6) the crystal
structure refinement for Br[CoF28TPP]‚CH3CN. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC034705O
6040 Inorganic Chemistry, Vol. 42, No. 19, 2003