(F8TPP)FeII/O2 reactivity studies {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2-)}: Spectroscopic (UV-visible and NMR) and kinetic study of solvent-dependent (Fe/O2 = 1:1 or 2:1) reversible O2-reduction and ferryl formation

Article abstract of DOI:10.1021/ic0105866

In this report, we describe in detail the O₂-binding chemistry of the metalloporphyrin (F₈TPP)Fe^II (1). This complex was synthesized from aqueous dithionite reduction of (F₈TPP)Fe^III-Cl (X-ray structure reported: C₅₅H₃₆ClF₈-FeN₄O; a = 13.6517(2) A, b = 13.6475(2) A, c = 26.3896(4), α = 90°, β = 89.9776(4)°, γ = 90°; monoclinic, P2(1)/c, Z = 4). Complex 1 crystallizes from toluene/heptane solvent system as a bis(toluene) solvate, (F₈TPP)Fe^II·(C₇H₈) ₂, with ferrous ion in the porphyrin plane (C₅₈H₃₆F₈FeN₄; a = 20.9177(2) A, b = 11.7738(2) A, c = 19.3875(2), α = 90°, β = 108.6999(6)°, γ = 90°; monoclinic, C2/c, Z = 4; Fe-N₄(av) = 2.002 A, N-Fe-N (all) = 90.0°). Close metal-arene contacts are also observed at 3.11-3.15 A. Upon oxygenation of 1 at 193 K in coordinating solvents, UV-visible and ²H and ¹⁹F NMR spectroscopies revealed the presence of a reversibly formed dioxygen adduct, formulated as the heme-superoxo complex (S)(F₈TPP)Fe^III-(O₂^-) (2) (S = solvent) [(i) tetrahydrofuran (THF) solvent: UV-visible, 416 (Soret), 536 nm; ²H NMR: δ_pyrrole 8.9 ppm; (ii) EtCN solvent: UV-visible, 414 (Soret), 536 nm; (iii) acetone solvent: UV-visible, 416 (Soret), 537 nm; ²H NMR: δ_pyrrole 8.9 ppm]. Dioxygen-uptake manometry (THF, 193 K) revealed an O₂:1 oxygenation stoichiometry of 1.02:1, consistent with the heme-superoxo formulation of 2. Stopped-flow UV-visible spectrophotometry studies of the (F₈TPP)Fe^II (1)/O₂ reaction in EtCN and THF solvents were able to provide kinetic and thermodynamic insight into the reversible formation of 2 [(i) EtCN: ΔH° = -40 ± 5 kJ/mol; ΔS° = -105 ± 23 J/(K mol); k₁ = (5.57 ± 0.04) × 10³ M^-1 s^-1 (183 K); ΔH? = 38.6 ± 0.2 Jd/mol; ΔS? = 42 ± 1 J/(K mol); (ii) THF: ΔH° = -37.5 ± 0.4 kJ/mol; ΔS° = -109 ± 2 J/(K mol)}. The (F₈TPP)Fe^II (1)/O₂ reaction was also examined at reduced temperatures in noncoordinating solvents (toluene, CH₂Cl₂), where UV-visible and ²H and ¹⁹F NMR spectroscopies also revealed the presence of a reversibly formed adduct, formulated as the peroxo-bridged dinuclear complex [(F₈TPP)Fe^III]₂-(O₂^2-) (3) [CH₂Cl²: UV-visible, 414 (Soret), 535 nm; ²H NMR, δ_pyrrole 17.5 ppm]. Dioxygen-uptake spectrophotometric titrations revealed a stoichiometry of 2 (F₈TPP)Fe^II (1) per O₂ upon full formation of 3. Addition of a nitrogenous base, 4-(dimethylamino)pyridine, to a cold solution of 3 in dichloromethane gave rapid formation of the iron(IV)-oxo ferryl species (DMAP)(F₈TPP)Fe^IV=O (4), based upon UV-visible [417 (Soret), 541 nm] and ²H NMR (δ_pyrrole = 3.5 ppm) spectroscopic characterization. These detailed investigations into the O₂-adducts and ferryl species formed from (F₈TPP)Fe^II (1) may be potentially important for a full understanding of our ongoing heme-copper oxidase model studies, which employ 1 or similar tethered (i.e., covalently attached Cu-chelate) porphyrin analogues in heme/Cu heterobinuclear systems.

Full text of DOI:10.1021/ic0105866

5754
Inorg. Chem. 2001, 40, 5754-5767
(F8TPP)FeII/O2 Reactivity Studies {F8TPP )
Tetrakis(2,6-difluorophenyl)porphyrinate(2-)}: Spectroscopic (UV-Visible and NMR) and
Kinetic Study of Solvent-Dependent (Fe/O2 ) 1:1 or 2:1) Reversible O2-Reduction and
Ferryl Formation
Reza A. Ghiladi,† Ryan M. Kretzer,† Ilia Guzei,‡ Arnold L. Rheingold,‡
Yorck-Michael Neuhold,§ Karen R. Hatwell,§ Andreas D. Zuberbu1hler,§ and
Kenneth D. Karlin*,†
Department of Chemistry, The Johns Hopkins University, Charles and 34th Streets,
Baltimore, Maryland 21218, Department of Chemistry, University of Delaware,
Newark, Delaware 19716, and Institute fu¨r Anorganische Chemie, University of Basel,
CH-4056 Basel, Switzerland
ReceiVed June 4, 2001
In this report, we describe in detail the O2-binding chemistry of the metalloporphyrin (F8TPP)FeII (1). This complex
was synthesized from aqueous dithionite reduction of (F8TPP)FeIII-Cl (X-ray structure reported: C55H36ClF8-
FeN4O; a ) 13.6517(2) Å, b ) 13.6475(2) Å, c ) 26.3896(4), R ) 90°, â ) 89.9776(4)°, γ ) 90°; monoclinic,
P2(1)/c, Z ) 4). Complex 1 crystallizes from toluene/heptane solvent system as a bis(toluene) solvate, (F8TPP)-
FeII‚(C7H8)2, with ferrous ion in the porphyrin plane (C58H36F8FeN4; a ) 20.9177(2) Å, b ) 11.7738(2) Å, c )
19.3875(2), R ) 90°, â ) 108.6999(6)°, γ ) 90°; monoclinic, C2/c, Z ) 4; Fe-N4(av) ) 2.002 Å; N-Fe-N
(all) ) 90.0°). Close metal-arene contacts are also observed at 3.11-3.15 Å. Upon oxygenation of 1 at 193 K
2
in coordinating solvents, UV-visible and H and 19F NMR spectroscopies revealed the presence of a reversibly
formed dioxygen adduct, formulated as the heme-superoxo complex (S)(F8TPP)FeIII-(O2-) (2) (S ) solvent)
2
[(i) tetrahydrofuran (THF) solvent: UV-visible, 416 (Soret), 536 nm; H NMR: δpyrrole 8.9 ppm; (ii) EtCN
2
solvent: UV-visible, 414 (Soret), 536 nm; (iii) acetone solvent: UV-visible, 416 (Soret), 537 nm; H NMR:
δpyrrole 8.9 ppm]. Dioxygen-uptake manometry (THF, 193 K) revealed an O2:1 oxygenation stoichiometry of
1.02:1, consistent with the heme-superoxo formulation of 2. Stopped-flow UV-visible spectrophotometry studies
of the (F8TPP)FeII (1)/O2 reaction in EtCN and THF solvents were able to provide kinetic and thermodynamic
insight into the reversible formation of 2 [(i) EtCN: ∆H° ) -40 ( 5 kJ/mol; ∆S° ) -105 ( 23 J/(K mol);
k1 ) (5.57 ( 0.04) × 103 M-1 s-1 (183 K); ∆Hq ) 38.6 ( 0.2 kJ/mol; ∆Sq ) 42 ( 1 J/(K mol); (ii) THF:
∆H° ) -37.5 ( 0.4 kJ/mol; ∆S° ) -109 ( 2 J/(K mol)}. The (F8TPP)FeII (1)/O2 reaction was also examined
at reduced temperatures in noncoordinating solvents (toluene, CH2Cl2), where UV-visible and 2H and 19F NMR
spectroscopies also revealed the presence of a reversibly formed adduct, formulated as the peroxo-bridged dinuclear
2
complex [(F8TPP)FeIII]2-(O22-) (3) [CH2Cl2: UV-visible, 414 (Soret), 535 nm; H NMR, δpyrrole 17.5 ppm].
Dioxygen-uptake spectrophotometric titrations revealed a stoichiometry of 2 (F8TPP)FeII (1) per O2 upon full
formation of 3. Addition of a nitrogenous base, 4-(dimethylamino)pyridine, to a cold solution of 3 in
dichloromethane gave rapid formation of the iron(IV)-oxo ferryl species (DMAP)(F8TPP)FeIVdO (4), based upon
UV-visible [417 (Soret), 541 nm] and 2H NMR (δpyrrole ) 3.5 ppm) spectroscopic characterization. These detailed
investigations into the O2-adducts and “ferryl” species formed from (F8TPP)FeII (1) may be potentially important
for a full understanding of our ongoing heme-copper oxidase model studies, which employ 1 or similar “tethered”
(i.e., covalently attached Cu-chelate) porphyrin analogues in heme/Cu heterobinuclear systems.
Introduction
heme-protein sensors8,9 of small molecule gases, such as O2
(e.g., FixL),10,11 CO (e.g., CooA),12-14 and NO (e.g., guanylate
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transport and storage by hemoglobin and myoglobin, investiga-
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iron(II) complexes with O2 have unceasingly and deservedly
attracted considerable interest.1-7 A new resurgence into heme-
O2 chemistry has also been ignited by the recent studies on
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* Corresponding author.
† The Johns Hopkins University.
‡ University of Delaware.
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§ University of Basel.
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10.1021/ic0105866 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/10/2001

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5755
cyclase).15,16 Such inquiries also bear upon dioxygen activation
chemistry,17-20 such as that occurring in the cytochrome P-450
monooxygenase enzymes.17,18,21,22 More recent biomimetic
While an initial O2 interaction with CuB has been postulated,41,42
the latest emerging picture is that the metal-O2 chemistry is
dominated by the heme, with CuB being critical for electron-
transfer and proton-translocation chemistry.37,39,43-46 Thus, an
emphasis of our research program is focused upon elucidating
the fundamental aspects of O2-interactions with heme-copper
centers, by looking at how a copper ion influences (P)FeII/O2
chemistry (P ) porphyrinate) and how hemes impact already
known copper(I)-dioxygen reactivity patterns.
23,24
25-29
efforts into non-heme Fe-O2
or Cu-O2
chemical
systems have focused on other classes of dioxygen-transport
metalloproteins, such as in model compounds for hemerythrin,
the non-heme diiron O2-carrier of marine worms,30-32 and
hemocyanin, a copper-containing arthropodal and molluscan
O2-carrier protein.25,33,34
Our own interests in this area come from a combination of
heme and copper dioxygen chemistry, as is important at the
active site of heme-copper oxidase proton pumps such as
cytochrome c oxidase. Recent biochemical and notable protein
X-ray structural advances35-40 reveal that O2-binding and
reduction occurs at a heterobinuclear active site consisting of a
high-spin heme group (with proximal histidine), located ∼4.5
Å from CuB, which possesses three histidine nitrogen ligands.
To these ends, we have been studying the dioxygen reactivity
of 1:1 mixtures (i.e., “untethered” systems) of the porphyrinate-
iron(II) complex (F8TPP)FeII (1) (see diagram; ArF ) 2,6-
difluorophenyl) with copper(I) complexes, either [(TMPA)CuI-
(MeCN)]+ (TMPA ) tris(2-pyridylmethyl)amine) or [(MePY2)-
CuI(MeCN)]+ (MePY2 ) N,N-bis[2-(2-pyridyl)ethyl]methyl-
amine).47-51 More recent efforts have focused on their analogous
binucleating ligand systems (i.e., “tethered”, see diagram),
wherein a tetradentate (in the case of [(6L)FeIICuI]+ 52) or
tridentate (as in [(4L)FeIICuI]+ 53) chelate for copper is covalently
appended to the periphery of a 2,6-difluorophenyl-substituted
porphyrinate. For all cases, FeII-CuI/O2-reactivity studies reveal
the presence of interesting O2-intermediates detected by low-
temperature spectral studies and/or stopped-flow spectropho-
tometry. Notably, in the case of the tetradendate system
[(6L)FeIICuI]+, a unique high-spin heme-copper peroxo complex,
[(6L)FeIII-(O22-)-CuII]+,52 has been extensively characterized,
with a similar µ-peroxo species formed in the untethered
analogue, [(F8TPP)FeIII-(O22-)-CuII(TMPA)]+.54 Similar O2-
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5756 Inorganic Chemistry, Vol. 40, No. 23, 2001
Ghiladi et al.
Heme-superoxo species such as 2 have been postulated to form
during the initial reaction of dioxygen with the heme-copper
complexes (F8TPP)FeII/[(TMPA)CuI(MeCN)]+ 54 and [(6L)FeII-
CuI]+,52 prior to their forming heme-peroxo-Cu adducts. Ad-
ditionally, 2 is suggested to form as an intermediate in the
(F8TPP)FeII/[(MePY2)CuI(MeCN)]+ 51 oxygenation reaction and
may well be present during O2-binding to [(4L)FeIICuI]+.53 In
contrast to the superoxo dominated chemistry found in coor-
dinating solvents, the reaction of (F8TPP)FeII (1) with dioxygen
in nonpolar solvents leads to a µ-peroxo dinuclear complex,
[(F8TPP)FeIII]2-(O22-) (3); the reVersibility of this reaction
represents an interesting finding not previously observed.
Finally, as also studied by Balch and co-workers,61 we describe
the addition of ligand base to 3, thereby cleaving the O-O bond
and affording the high-valent iron(IV) oxo complex (F8TPP)-
FeIVdO (4). The spectroscopic characterization of this “ferryl”
species with the F8TPP porphyrin system may be potentially
important for a detailed understanding of our heme-copper
oxidase model studies (i.e., [(6L)FeIICuI]+ or [(4L)FeIICuI]+; vide
supra), since heme-copper oxidases afford ferryl (P)FeIVdO
intermediates during the 4e-/4H+ reduction of dioxygen to
water.37,39,43-45
Results and Discussion
Our previous studies on the CuI/O2 chemistry of [(TMPA)-
Synthesis. Procedures for making the free base F8TPPH2,47,51,65
ferric chloride (F8TPP)FeIII-Cl,47 and target (F8TPP)FeII (1)51
porphyrin complexes have been previously described, but
modified synthetic procedures are presented here. The structures
of both the (F8TPP)FeIII-Cl precursor complex and (F8TPP)FeII
(1) were determined by X-ray crystallography; discussion of
the structure of (F8TPP)FeII (1) is found below, while the
unremarkable structure of (F8TPP)FeIII-Cl can be found in the
Supporting Information.
To simplify the interpretation of NMR spectra, a pyrrole-
deuterated analogue of 1, (F8TPP-d8)FeII (1-d8),51 was synthe-
sized, thereby allowing for 2H NMR spectroscopic monitoring
of low-temperature stable dioxygen adducts with unambiguous
assignment of the pyrrole resonances. The synthesis employed
in deuteration of the pyrrole hydrogens is similar to the ones
reported earlier in the literature,66 affording F8TPPH2 with
greater than 90% (by mass spectrometric analysis) deuterium
incorporation into the eight â-pyrrole hydrogen positions. This
route was deemed necessary given the ability to assign oxidation
and spin states of hemes, as well as the types of heme/O2-
adducts formed, based upon the chemical shift of the pyrrole
hydrogens.67-69
26,55
56,57
CuI(MeCN)]+
and [(MePY2)CuI(MeCN)]+
gave us
considerable insight into the O2-complexes involved. However,
to fully understand the nature of both the “tethered” and
“untethered” synthetic heme-copper/O2 reactions described
above, we need to deduce the O2-chemistry of (F8TPP)FeII (1)
itself, with a very similar moiety (i.e., with three instead of four
6
4
2,6-difluorophenyl meso groups) occurring in L or L.
Thus, a detailed study of (F8TPP)FeII (1)/O2 chemistry is the
focus of the present report. First, the X-ray structure of (F8-
TPP)FeII (1), as a toluene solvate, is presented, followed by the
details of our low-temperature UV-visible and 1H, 2H, and 19
F
NMR spectroscopic monitoring of the 1/O2 chemistry. While
following closely the approaches and methods used in the
elegant studies by Balch, co-workers, and collaborators,58-64
we find that the 2,6-difluorophenyl substitution provides
interesting behavior contrasting the heme/O2 chemistry seen for
the well-studied tetraphenyl (TPP) or tetramesityl (TMP)
porphyrinates. In weakly to moderately coordinating polar
solvents, we find that O2-binding to 1 giving (F8TPP)FeIII-(O2
)
-
(2) is nicely reversible, and we describe UV-visible-monitored
stopped-flow studies which provide kinetic and equilibrium data.
X-ray Structure of 1‚(C7H8)2. Experimental data for the
X-ray structure data collection and analysis for 1 are given in
Table 1 and the Supporting Information. The structure of (F8-
TPP)FeII‚(C7H8)2 (1‚(C7H8)2), as the bis(toluene) solvate, is
shown in Figure 1. Compound 1‚(C7H8)2 is a mononuclear,
tetracoordinate iron(II) species, with ligation to the four pyrrole
nitrogens. The geometry around the ferrous ion is square planar
(with the iron lying in the plane of the porphyrin macrocycle)
[ N-Fe-N (all) ) 90.0°, Fe-N4 (av) ) 2.002 Å]. This is
comparable to other four-coordinate metalloporphyrins,70 most
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Karlin, K. D. J. Am. Chem. Soc. 1998, 120, 12960-12961.
(58) Balch, A. L.; Chan, Y.-W.; Cheng, R.-J.; La Mar, G. N.; Latos-
Grazynski, L.; Renner, M. W. J. Am. Chem. Soc. 1984, 106, 7779-
7785.
(59) Balch, A. L. Inorg. Chim. Acta 1992, 198-200, 297-307.
(60) Chin, D.-H.; La Mar, G. N.; Balch, A. L. J. Am. Chem. Soc. 1980,
102, 4344-4350.
(61) Chin, D.-H.; Balch, A. L.; La Mar, G. N. J. Am. Chem. Soc. 1980,
102, 1446-1448.
(65) Nanthakumar, A.; Goff, H. M. Inorg. Chem. 1991, 30, 4460-4464.
(66) Shirazi, A.; Goff, H. M. J. Am. Chem. Soc. 1982, 104, 6318-6322.
(67) Goff, H. M.; La Mar, G. N. J. Am. Chem. Soc. 1977, 99, 6599-6606.
(68) Walker, F. A.; Simonis, U. In Biological Magnetic Resonance; Berliner,
L. J., Reuben, J., Eds.; Plenum Press: New York, 1993; Vol. 12/
NMR of Paramagnetic Molecules, pp 133-274.
(69) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 5/NMR
and EPR, pp 81-184.
(62) Chin, D.-H.; La Mar, G. N.; Balch, A. L. J. Am. Chem. Soc. 1980,
102, 5945-5947.
(63) La Mar, G. N.; de Ropp, J. S.; Latos-Grazynski, L.; Balch, A. L.;
Johnson, R. B.; Smith, K. M.; Parish, D. W.; Cheng, R.-J. J. Am.
Chem. Soc. 1983, 105, 782-787.
(64) Latos-Grazynski, L.; Cheng, R.-J.; La Mar, G. N.; Balch, A. L. J.
Am. Chem. Soc. 1982, 104, 5992-6000.

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5757
The one unique feature of (F8TPP)FeII‚(C7H8)2 (1‚(C7H8)2),
when compared to the other metalloporphyrins in Table 2, is
that the closest M‚‚‚Carene contact is to a meta-carbon of the
toluene solvate. In all other cases, the closest metal-solvate
distance is to an electron-rich ortho- or para-arene carbon atom,
with the metal center acting as an electron acceptor.75 Given
that the fluorine substituents in (F8TPP)FeII (1) should make
the ferrous ion an even better electron acceptor compared to
the unsubstituted TPP, it is somewhat surprising to see that the
closest metal-toluene contact in 1‚(C7H8)2 is to the more
electron-deficient meta carbon. Furthermore, the crystal structure
shows that the 2,6-difluorophenyl groups on the meso carbon
positions are not perpendicular to the porphyrin plane; one pair
of opposing 2,6-difluorophenyl rings is rotated by an angle of
8.1° from perpendicularity, while for the other pair, this angle
is -20.8° (i.e., twisted in the opposite direction).
Table 1. Crystal Data and Structure Refinement for
(F8TPP)FeII‚(C7H8)2 (1‚(C7H8)2)
empirical formula
fw
temperature
wavelength
cryst syst
C58H36F8FeN4
996.76
195(2) K
0.71073 Å
monoclinic
space group
unit cell dimens
C2/c
a ) 20.9177(2) Å, R ) 90°
b ) 11.7738(2) Å, â ) 108.6999(6)°
c ) 19.3875(2) Å, γ ) 90°
4522.71(9) Å3, 4
1.464 g/cm3
volume, Z
density (calcd)
abs coeff
0.413 mm-1
F(000)
cryst size
cryst color
2040
0.50 × 0.50 × 0.10 mm
red plate
θ range for data collection
2.01-28.18°
limiting indices
-24 e h e 26, -13 e k e 15,
-23 e l e 24
Solvent-Dependent Spin State of (F8TPP)FeII (1). Depend-
ing upon the solvent, multiple FeII spin states are observed for
1, as detected by 1,2H NMR spectroscopy. Table 3 shows the
chemical shifts of the pyrrole hydrogens of (F8TPP)FeII (1) in
various solvents at 293 and 193 K, along with their correspond-
ing Evans method magnetic moment measurements (293 K).
In noncoordinating solvents, such as methylene chloride or
toluene, the pyrrole hydrogen resonances are found at ∼δ 3.9-
4.9 ppm. This has been previously characterized for 1 as a four-
coordinate heme in an S ) 1 spin state,49 with the ferrous ion
in the plane of the porphyrin (as also shown by the crystal
structure of 1). Consistent with this intermediate spin-state
formulation was the finding of µB ) 3.0, close to the spin-only
value of 2.8 for two unpaired electrons.
no. of reflns collected
no. of ind reflns
refinement method
14 417
5251 (Rint ) 0.0285)
full-matrix least-squares on F2
no. of data/restraints/params 5251/0/322
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1.006
R1 ) 0.0421, wR2 ) 0.1005
R1 ) 0.0576, wR2 ) 0.1102
0.385 and -0.459 e Å-3
largest diff peak and hole
In weakly to moderately coordinating solvents, such as
acetone and tetrahydrofuran (THF), these pyrrole-H resonances
are paramagnetically shifted downfield, to δ 48 (acetone) and
δ 56 (THF) ppm. This is indicative of a high-spin ferrous (S )
2) complex,68,69,77 with axial solvent ligation, either as a five-
coordinate heme (moderate axial field ligands, i.e., acetone) or
as a six-coordinate heme (i.e., two weak-field axial THF
ligands).78,79 The Evans method magnetic moment measure-
ments now show a µB ) 5.0-5.1, consistent with a high-spin
S ) 2 system having four unpaired electrons (µB ) 4.9; spin-
only value). In MeCN solvent, however, the pyrrole hydrogens
were found at δ 28 ppm [µB ) 4.1; Evans method, 293 K] and
are representative of an equilibrium mixture of high- and low-
spin states. Upon lowering the MeCN solution to 233 K, the
pyrrole hydrogens displayed anti-Curie behavior as they shifted
upfield to δ 10.1 ppm, indicating more complete formation of
a low-spin six-coordinate heme. This temperature-dependent
behavior is consistent with the spin-equilibrium proposed of a
high-spin (five-coordinate, one axially bound MeCN) structure
favored at higher temperature and a low-spin (six-coordinate,
two axially bound MeCN) structure favored at lower temper-
atures.
Figure 1. Perspective view (30% ellipsoids) of the structure of
1‚(C7H8)2. Selected bond lengths (Å) and angles (deg): Fe-N4(av) )
2.002, N-Fe-N (all) ) 90.0, Fe-C(105) ) 3.149, Fe-C(106) )
3.112.
notably (TPP)FeII, which has an average Fe-N distance of 1.972
Å and N-Fe-N (av) ) 90.0°.71
The asymmetric unit consists of half of 1‚(C7H8)2 located on
an inversion center and a molecule of toluene, for an overall
stoichiometry of two toluenes per ferrous heme, one π-stacked
above the plane of the porphyrin macrocycle, the other below
(Figure 1). The stacked toluenes are nearly coplanar with the
porphyrin macrocycle, with a small angle of 10.2° between the
two planes. Weak axial interactions between the ferrous ion and
two carbon atoms of each toluene appear to exist, with distances
of 3.149 and 3.112 Å for Fe-C(105) and Fe-C(106), respec-
tively. Such interactions between hemes and aromatic solvates
are known in other MII or FeIII porphyrinates,72-76 and relevant
data for arene solvates of several metalloporphyrins are given
in Table 2.
Finally, in a strongly coordinating solvent such as pyridine,
the pyrrole resonances are found at δ 8.9 ppm; a diamagnetic
(74) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17,
706-710.
(75) Shelly, K.; Bartczak, T.; Scheidt, W. R.; Reed, C. A. Inorg. Chem.
1985, 24, 4325-4330.
(76) Xie, Z.; Bau, R.; Reed, C. A. Angew. Chem., Int. Ed. Engl. 1994, 33,
2433-2434.
(70) Senge, M. O. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 10.
(71) Collman, J. P.; Hoard, J. L.; Kim, N.; Lang, G.; Reed, C. A. J. Am.
Chem. Soc. 1975, 97, 2676-2681.
(77) La Mar, G. N.; Walker, F. A. In The Porphyrins; Dolphin, D., Ed.;
Academic Press: 1979; Vol. IVB, pp 57-161.
(78) Reed, C. A.; Mashiko, T.; Scheidt, W. R.; Spartalian, K.; Lang, G. J.
Am. Chem. Soc. 1980, 102, 2302-2306.
(79) Thompson, D. W.; Kretzer, R. M.; Lebeau, E. L.; Scaltrio, D.; Ghiladi,
R. A.; Lam, K.-C.; Rheingold, A. L.; Meyer, G. J.; Karlin, K. D.
Manuscript in preparation.
(72) Kirner, J. F.; Reed, C. A.; Scheidt, W. R. J. Am. Chem. Soc. 1977,
99, 1093-1101.
(73) Scheidt, W. R.; Reed, C. A. Inorg. Chem. 1978, 17, 710-714.

5758 Inorganic Chemistry, Vol. 40, No. 23, 2001
Ghiladi et al.
[(TPP)Fe]+ c
Table 2. Summary of Metalloporphyrin-Arene Contacts.
(F8TPP)FeII a
(TPP)Cra
(TPP)Zna
(TPP)Mna
[(TPP)Fe]+ b
interplanar spacing, Å
3.37
4.5
3.36
ortho
73
3.34
12.0
3.12
para
74
3.30
10.7
3.05
para
72
3.30
7.0
3.34
ortho
75
interplanar angle, deg
closest M‚‚‚Carene contact, Å
carbon atom type
10.2
3.11
meta
this work
13.0
2.94
ortho
76
reference
a Toluene solvate. b Fluorobenzene solvate. c p-Xylene solvate.
Table 3. Chemical Shifts of the Pyrrole Hydrogens at 293 and 193 K and Their Corresponding Magnetic Moments (Evans Method, 293 K) in
Various Solvents.
solvent
THF
acetone
acetonitrile
28
10.1 (233 K)
4.1
CH2Cl2
toluene
3.9
pyridine
8.9
δ
pyrrole (293 K), ppm
pyrrole (193 K), ppm
56
93
5.1
48
84
5.0
4.9
-2.5
3.0
δ
µB (293 K)
0
Scheme 1
Figure 2. UV-visible spectra of the (F8TPP)FeII (1)/(F8TPP)FeIII-(O2
)
-
(2) reversible dioxygenation reaction in THF at 193 K. See text for
further discussion.
Figures S1 and S2 show this cycling as followed by UV-visible
spectroscopy, in THF and EtCN solvents, respectively. Evolution
of O2 by this process was confirmed by using alkaline
pyrogallol, which was employed as a means for detecting
liberated dioxygen (see Experimental Section). Interestingly,
application of a simple vacuum without the Ar bubbling purge
was not able to reverse the dioxygen adduct back to the reduced
complex at low temperature. Additional discussion pertaining
to the reversibility of 2 can be found in the Supporting
Information.
NMR spectrum is observed, suggesting a six-coordinate, low-
spin ferrous center (S ) 0, low-spin d6) with strong axial ligation
by two pyridine ligands. As expected, an Evans method NMR
experiment shows µB ) 0; that is, no paramagnetically shifted
solvent peak was observed downfield from the reference/
capillary solvent resonance.
Reversible Iron-Superoxo Formation: (S)(F8TPP)FeIII-
(O2-) (2). UV-Visible Spectroscopy. Low-temperature UV-
visible spectroscopic monitoring of the (F8TPP)FeII (1)/O2
reaction in coordinating solvents reveals the presence of a
reversibly formed intermediate, 2, which was found to be stable
indefinitely at 193 K. Ultimately, thermal decomposition to the
previously characterized47 ferric hydroxy complex, (F8TPP)-
FeIII-OH (5), occurs upon warming (Scheme 1).
The reduced (F8TPP)FeII (1) complex exhibits UV-visible
features at 422 (Soret) and 542 nm in THF solvent at 193 K
(Figure 2 and Figure S1). Upon bubbling dioxygen directly
through the solution thereby forming 2, an immediate change
in the spectrum is observed [λmax ) 416 (Soret), 536 nm].
Formation of 2 is not limited to THF solvent, however, as similar
chemistry is observed when the oxygenation reaction is per-
formed in EtCN (193 K). Here, addition of O2 to (F8TPP)FeII
(1) [λmax ) 414, 424 (split Soret), 526 nm] results in a new
spectrum [λmax ) 414 nm (Soret), 536 nm] (Figure S2).
Upon bubbling argon through the solution containing the
heme-O2 complex 2 at 193 K, it was possible to reverse the
dioxygen-binding and give back (with only minor decomposition
to 5) the starting complex, reduced species (F8TPP)FeII (1).
Dioxygen-uptake manometry in THF at 193 K revealed a
reaction stoichiometry of 1 Fe:1.02 O2 (see Experimental
Section), confirming the proposed formation of a myoglobin-
like iron(III)-superoxo complex, (S)(F8TPP)FeIII-(O2-) (2),
where S represents a coordinated axial base solvent molecule
(Scheme 1).
The reversibility of O2-binding to (F8TPP)FeII (1) was also
demonstrated employing acetone as solvent; these observations
2
(UV-visible and H NMR spectroscopies) are reported in the
Supporting Information, Figures S3-S5.
NMR Spectroscopic Characterization of (S)(F8TPP-d8)-
FeIII-(O2-) (2-d8). Figure 3 depicts a typical low-temperature
oxygenation reaction of 1-d8 in THF solvent, as monitored by
2H NMR spectroscopy. Complex (F8TPP-d8)FeII (1-d8) gives
rise to a high-spin (vida supra) pyrrole resonance at δ 56 ppm
(Figure 3a, 293 K). The expected temperature-dependent
paramagnetic downfield-shifting of the pyrrole resonance to δ
93 ppm is seen at 193 K (Figure 3b). Upon generation of the
dioxygen intermediate, 2, by bubbling O2 via syringe directly

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5759
Table 4. NMR Data and Pyrrole Chemical Shifts of Heme-Superoxo {PFeIII-(O2-)}, Heme-Peroxo-Heme {(PFeIII)2-(O22-)}, and Ferryl
{PFeIVdO} Complexes
complex
δpyrrole
solvent
THF
acetone
toluene-d8
toluene-d8
CD2Cl2
CD2Cl2
toluene-d8
CH2Cl2
toluene-d8
toluene-d8
toluene-d8
toluene-d8
toluene-d8
toluene-d8
CH2Cl2
temp (K)
ref
(THF)(F8TPP-d8)FeIII-(O2-) (2-d8)
8.9
8.9
8.37
8.2
9.0
9.1
9.8
193
193
196
197
198
198
198
193
203, 243
193
248
193
199
243
193
203
223
193
200
243
this work
this work
(acetone)(F8TPP-d8)FeIII-(O2-) (2-d8)
(N-MeIm)(TMP)FeIII-(O2
)
58
64
64
64
-
(TpivPP)FeIII-(O2
)
-
a
-
a
(T(2,4,6-MeO)3PP)FeIII-(O2
)
(T(2,4,6-EtO)3PP)FeIII-(O2
(T(3,4,5-MeO)3PP)FeIII-(O2
)
-
a
-
a
)
64
[(F8TPP-d8)FeIII]2-(O22-) (3-d8)
17.5
this work
[(TMP)FeIII]2-(O2
)
17.7, 19.0
16.0
16.6
16.2
19.2
16.8
3.5
58
60
64
60
60
64
2-
[(TPP)FeIII]2-(O2
)
2-
[(TpivPP)FeIII]2-(O2
)
2-
[(TmTP)FeIII]2-(O2
)
2-
2-
[(DPDMe)FeIII]2-(O2
)
[(T(3,4,5-MeO)3PP)FeIII]2-(O2
)
2-
(DMAP)(F8TPP-d8)FeIVdO (4-d8)
(TMP)FeIVdO
this work
58
8.4
7.2
toluene-d8
THF-d8
toluene-d8
toluene-d8
toluene-d8
(THF)(TpivPP)FeIVdO
(1-MeIm)(TPP)FeIVdO
(1-MeIm)(TmTP)FeIVdO
(1-MeIm)(TMP)FeIVdO
96
58
61
58
5.05
∼5
4.6
a Denotes five-coordinate, diamagnetic complex due to lack of a suitable axial base; TMP ) meso-tetramesitylporphyrin dianion; TpivPP )
meso-tetrakis(R,R,R,R-o-pivalamidophenyl)porphyrin dianion; T(2,4,6-MeO)3PP ) meso-tetrakis(2,4,6-trimethoxyphenyl)porphyrin dianion; T(2,4,6-
EtO)3PP ) meso-tetrakis(2,4,6-triethoxyphenyl)porphyrin dianion; T(3,4,5-MeO)3PP ) meso-tetrakis(3,4,5-trimethoxyphenyl)porphyrin dianion;
TPP ) meso-tetraphenylporphyrin; TmTP ) meso-tetra-m-tolylporphyrin; DPDMe ) deuterioporphyrin IX dimethyl ester; DMAP ) 4-(dimethyl-
amino)pyridine; N-MeIm ) 1-MeIm ) N-methylimidazole.
Figure 4. 19F NMR spectra of the oxygenation reaction of (F8TPP)-
FeII (1) in THF, 193 K.
peroxide complex [PFeIII-O2]- (∼60 ppm, RT),66 the intermo-
Figure 3. 2H NMR spectra of the (F8TPP-d8)FeII (1-d8) oxgyenation
lecular µ-oxo complex (F8TPP)FeIII-O-FeIII(F8TPP) (13.7 ppm,
reaction in THF at 193 K. See text for explanation.
RT),47 or (F8TPP)FeIII-OH (5) (125 ppm, 193 K).47
into an NMR tube containing 1 in a dry ice/acetone bath, a
Additional characterization of the heme-superoxo complex
(S)(F8TPP)FeIII-(O2-) (2) comes from 19F NMR spectroscopy.
(F8TPP)FeII (1) exhibits a single fluorine resonance at δ -116
ppm (THF, 193 K, Figure 4a), implying a symmetric and
equivalent environment for all fluorine substituents in 1 [i.e., a
six-coordinate (bis-THF) high-spin ferrous heme].79 Upon
oxygenation, two distinctive fluorine resonances of approxi-
mately equal integration appear at δ -111 and -113 ppm
(Figure 4b, 193 K). The origin of these two resonances may be
due to the asymmetry of 2 (S ) THF), with one resonance due
to the fluorines located on the superoxo ligand side of the
porphyrin plane and the second resonance due to those fluorines
on the THF ligand side. This hypothesis is reasonable, since
different chemical shifts for inequivalent porphyrin phenyl
substituents due to the asymmetry imposed by oxygen and/or
axial ligand binding have been shown before by 1H NMR
diamagnetic spectrum is observed (δpyrrole ) 8.9 ppm) (Figure
3c), consistent with the formulation of (THF)(F8TPP)FeIII-(O2
)
-
(2) and possessing a low-spin (S ) 0) six-coordinate iron. This
2H NMR-derived pyrrole-H resonance assignment is in agree-
ment with previously studied five- and six-coordinate diamag-
netic heme-superoxo complexes (Table 4). Warming the solution
of 2 to room temperature (several hours) leads to complete
formation of (F8TPP-d8)FeIII-OH (5-d8) (δpyrrole ) 125 ppm, 193
K) (Figure 3d).
2
Using the assigned pyrrole hydrogen resonances in the H
NMR spectrum of 2-d8 (THF, 193 K), we are able to exclude
the possibility of 2 being the intermolecular µ-peroxo dimer
PFeIII-OO-FeIIIP (∼15-17 ppm, 193 K; vide infra),58,60,64 the
high-valent iron-oxo (ferryl) complex PFeIVdO (∼2-4 ppm,
193-233 K, vide infra),58,61 the anionic mononuclear iron

5760 Inorganic Chemistry, Vol. 40, No. 23, 2001
Ghiladi et al.
Table 5. Kinetic (k1, k-1) and Thermodynamic (K1) Parameters for
the Oxygenation of (F8TPP)FeII (1) in EtCN and THF Solvents
parameter
temp, K
propionitrile
tetrahydrofuran
K1 (M-1
)
183
233
293
6 × 105
1 × 105
(6 ( 1) × 103
(3 ( 2) × 101
-40 ( 5
(1.21 ( 0.04) × 103
(7.4 ( 0.2) × 100
-37.5 ( 0.4
∆H° (kJ/mol)
∆S° (J/K mol)
-105 ( 23
-109 ( 2
k1 (M-1 s-1
)
183
233
293
(5.57 ( 0.04) × 103
(6.4 ( 0.1) × 105
(1.6 ( 0.1) × 108
38.6 ( 0.2
∆Hq (kJ/mol)
∆Sq (J/K mol)
42 ( 1
k-1 (s-1
)
183
233
293
9 × 10-3
(1.1 ( 0.2) × 102
(6 ( 5) × 106
78 ( 5
∆Hq (kJ/mol)
∆Sq (J/K mol)
147 ( 23
T (extrapolated) the reaction becomes less favorable (∼101 M-1
at 298 K) due to the strongly unfavorable entropic component,
∆S° ) -105 ( 23 J/(K mol). Enthalpies and entropies of
activation for the O2-binding (k1) and dissociation (k-1) reaction
were obtained (Table 5) and are discussed below.
In THF solvent, the reaction of (F8TPP)FeII (1) with dioxygen
was too fast to be followed by stopped-flow spectrophotometry;
that is, full formation of 2 occurred within the mixing time of
the apparatus (∼1 ms). However, it was possible to extract
equilibrium data (Table 5) for this reversible oxygenation
reaction through temperature-dependent studies by analyzing
the relative amount of 1 (λmax ) 543 nm) and 2 (λmax ) 535
nm) present in equilibrium at a given T (Figure S7); the van’t
Hoff plot is provided in the Supporting Information, Figure S8.
Comparison of the thermodynamic parameters obtained for
both EtCN and THF solvents shows that ∆H° and ∆S° (thus, K
values as well) are very similar. This indicates that as axial
ligands for (F8TPP)FeII (1), THF (an oxygen-atom donor) and
EtCN (a nitrogen-atom donor) are nearly identical in their effect
with respect to formation and stabilization of the heme-superoxo
complex (S)(F8TPP)FeIII-(O2-) (2). This might seem somewhat
surprising, given the differences in electronic structure of
(F8TPP)FeII (1) in these two solvents at low temperature: (i) in
THF, a six-coordinate high-spin heme (δpyrrole ) 93 ppm)
prevails with ferrous ion in the plane of the porphyrin
macrocycle;79 (ii) in EtCN, a six-coordinate, low-spin heme
[δpyrrole ) 10.1 ppm (MeCN, 233 K), UV-visible split Soret
band in EtCN (Figure S2)] is also diagnostic of a low-spin
ferrous heme80) with ferrous ion in the porphyrin plane. An
explanation may reside in a balancing of different effects in
the two solvents: formation of 2 in EtCN with the softer (less
favorable) nitrile ligand (for FeIII, yielding an N-FeIII bond)
and no spin-state change may be roughly equivalent to forming
2 with a relatively more favorable hard THF ligand (giving an
O-FeIII bond), but whose formation is also accompanied by
the less favorable spin-state change.
Figure 5. Time-dependent oxygenation of (F8TPP)FeII (1) (λmax ) 529
nm) in EtCN (183 K, ttotal ) 250 ms) (top). Traces for the formation of
the 538 nm band ascribed to (S)(F8TPP)FeIII-(O2-) (2) and loss of the
529 nm band are shown (bottom).
spectroscopy.58 After warming to room temperature and sub-
sequent rechilling, the decomposition product (F8TPP)FeIII-OH
(5) was observed (Figure 4c, δ -83 ppm), as confirmed by
comparison to a 19F NMR spectrum of authentic material.
Kinetics-Thermodynamics of the Oxygenation of (F8TPP)-
FeII (1) in Coordinating Solvents. Stopped-flow (UV-visible)
spectrophotometry studies were able to provide valuble insight
into the kinetics and thermodynamics of reversible O2-binding
to (F8TPP)FeII (1) forming (S)(F8TPP)FeIII-(O2-) (2) in coor-
dinating solvents such as EtCN and THF.
k1(K1)
-
(F8TPP)FeII (1) + O2 y z (S)(F8TPP)FeIII-(O2 ) (2) (1)
k-1
Spectroscopic monitoring of the oxygenation reaction was
performed between 450 and 725 nm, allowing for complete
resolution of the alpha band region.
For the kinetics of oxygenation of (F8TPP)FeII (1) in EtCN
solvent, data collected over a range 178-205 K were used in
the analysis. Figure 5 depicts a typical stopped-flow UV-visible
data set obtained at 183 K. Upon oxygenation, loss of the 529
nm band, attributed to (F8TPP)FeII (1), and formation of the
538 nm band, (S)(F8TPP)FeIII-(O2-) (2), was observed concur-
rently (t1/2,183K ≈ 25 ms); isosbestic points were seen at 506
and 534 nm. The data were fit to a second-order reversible
reaction; an Eyring plot is given in the Supporting Information
(Figure S6). As can be seen from the results of the analysis
provided in Table 5, the equilibrium constant, K1, for the fully
reversible oxygenation reaction is very favorable at low tem-
perature, 6 × 105 M-1 at 183 K. However, as expected, at higher
Table 6 shows thermodynamic and kinetic comparisons for
O2-binding (yielding heme-superoxo complexes) of (F8TPP)-
FeII (1) with other heme systems. Despite having an extrapolated
(room temperature) forward rate constant (k1) up to 100 times
faster in EtCN (possibly due to the lack of steric hindrance for
O2-binding in 1), the equilibrium constant (K1) for 1 is smaller
(factor of 101-105). This is presumably due to the electron-
withdrawing fluorine substituents, which lead to a weaker Fe-O
bond strength upon O2-binding (hence, the larger k-1 for reaction
(80) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. 3 (Part A), pp 1-165.

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5761
Table 6. Equilibrium and Kinetic Data for Heme-Dioxygen Complexes (20 °C, unless otherwise stated)
O2
‡
q
P1/2
torr
,
∆H1 ,
∆S1 ,
∆H°,
kJ/mol
∆S°,
J/K mol
compound
k1, M-1 s-1
k-1, s-1
K1, M-1
kJ/mol
J/K mol
ref
(F8TPP)FeII (1)a
(1.6 ( 0.1) × 108 (6 ( 5) × 106 (3 ( 2) × 101
38.6 ( 0.2 42 ( 1
-40 ( 5
-105 ( 23
c
c
(F8TPP)FeII (1)b
(7.4 ( 0.2) × 100
-37.5 ( 0.4 -109 ( 2
myoglobin
[Piv3(5CImP)P]Fe
[Piv3(4CImP)P]Fe
1-2 × 107
4.3 × 108
4.3 × 108
1.06 × 108
1.7 × 107
2.2 × 106
6.2 × 107
10-30
2.9 × 103
2.9 × 103
4.6 × 104
71
4.30 × 108
1.48 × 105
1.48 × 105
2.31 × 103
2.4 × 105
2.4 × 105
1.6 × 104
0.37-1
0.58
0.60
38
0.36
0.36
5.6
2450
139
23
4000
12.6
84.3
57.4
1.02
-64 to -88 -159 to -235 81-83
0.75 ( 0.33 -116 ( 2.5 -68 ( 3.3 -167 ( 13
-70 ( 2.1 -176 ( 8.3
82
82
82
82
82
82,84,85
86
(TpivPP)Fe(1,2-Me2Im)
(MedPoc)Fe(1-MeIm)
(PocPiv)Fe(1-MeIm)
chelated protoheme
(durene-4/4)Fe(1,2-Me2Im)
(durene-4/4)Fe(DcIm)
(C2-cap)Fe(1-MeIm)
(C2-cap)Fe(1,2-Me2Im)
(PocPiv)Fe(1,2-Me2Im)
(TPP)Fe[(NH(CH2)3Im]
(Deut)Fe[O(CH2)3Py]
(TpivPP)Fe(1-MeIm)
10.5 ( 1.6 -95 ( 4
-60 ( 2
-176 ( 8.3
7.1 ( 2.5
35 ( 2.5
-121 ( 8
-45 ( 10
9
4000
-59
-54.8
-64
-44
-41
-146
-190
-207
-117
-150
8.2 × 107
105
820
86
87,88
87,88
82
89
89
1.9 × 106
5 × 107
280
6.8 × 103
1.7 × 103
2.5 × 103
1.4 × 105
2.5 × 104
7 × 104
230
20 × 107
4.30 × 108
90
a EtCN solvent. b THF solvent. c This work; Abbreviations used: Piv3(5CImP)P ) R,â,γ-tris(o-pivalamidophenyl)-δ-[o-(3-N-imidazolylvalera-
mido)phenyl]porphyrin; Piv3(4CImP)P ) R,â,γ-tris(o-pivalamidophenyl)-δ-[o-(3-N-imidazolylbutyramido)]phenylporphyrin; TpivPP ) meso-R,â,γ,δ-
tetrakis(o-pivalamidophenyl)porphyrin; MedPoc ) “medium” pocket porphyrin; PocPiv ) “small” pocket porphyrin; TPP ) meso-tetraphenyl-
porphyrin; durene-4/4 ) durene-capped porphyrin; C2-cap ) 5,10,15,20-[pyromellitoyl(tetrakis(o-oxyethoxyphenyl))]porphyrin; C3-cap ) 5,10,15,20-
[pyromellitoyl(tetrakis(o-oxypropoxyphenyl))]porphyrin; Deut ) deuterioporphyrin; 1-MeIm ) N-methylimidazole; 1,2-Me2Im ) 1,2-
dimethylimidazole; DCIm ) 1,5-dicyclohexylimidazole; Py ) pyridine.
of 2 to give back 1 plus O2). The rather high activation enthalpy
(∆Hq ) 38.6 ( 0.2 kJ/mol) for the formation of 2 may be
attributed to the need for dissociation of the strong EtCN axial
ligand upon O2-binding, with the positive entropy of activation
(∆Sq ) 42 ( 1 J/K mol) being also consistent with the
importance of axial ligand (EtCN) dissociation. Since this ligand
dissociation situation is not present in the other heme systems
listed in Table 5 (which employ tethered axial bases and/or
superstructured O2-binding pockets), their dioxygen-binding
leads to negative activation entropies. Furthermore, the reac-
tion enthalpy (EtCN, ∆H° ) -40 ( 5 kJ/mol; THF, ∆H° )
-37.5 ( 0.4 kJ/mol) and entropy (EtCN, ∆S° ) -105 ( 23
J/K mol; THF, ∆S° ) -109 ( 2 J/K mol) are smaller for 1
than for the majority of the other systems (Table 5), suggesting
again the influence of the fluorine substituents upon dioxygen-
binding: the O2-binding (O2- to FeIII) is weaker in (S)(F8TPP)-
FeIII-(O2-) (2).
Figure 6. UV-visible spectra of the (F8TPP)FeII (1)/[(F8TPP)FeIII]2-
(O22-) (3) reversible dioxygenation reaction in toluene at 193 K.
Reversible Diiron-µ-Peroxo Formation: [(F8TPP)FeIII]2-
(O22-) (3). UV-Visible Spectroscopy. Addition of dioxygen
to a solution of (F8TPP)FeII (1) in CH2Cl2 or toluene at low
temperature revealed the presence (by UV-visible spectroscopy)
of a stable species, 3, which eventually decomposes (thermally)
to the ferric hydroxy complex, (F8TPP)FeIII-OH (5).
As was the case with the mononuclear iron(III)-superoxo
complex, (S)(F8TPP)FeIII-(O2-) (2), the dinuclear peroxo-
bridged complex [(F8TPP)FeIII]2-(O22-) (3) also shows dioxy-
gen-binding reversibility in toluene at 193 K, releasing O2 (as
detected by a qualitative alkaline pyrogallol test; see Experi-
mental Section) and giving back the starting reduced complex
In toluene solvent, (F8TPP)FeII (1) exhibits UV-visible
features at 422 (Soret), 528, and 556 nm (193 K, Figure 6).
Upon exposure to dioxygen, an immediate change in the
spectrum is observed [λmax ) 418 (Soret), 536 nm]; similar
behavior is also observed in CH2Cl2 (see below). Given the lack
of a suitable axial base ligand in these noncoordinating solvents,
we formulate the intermediate species formed as the dinuclear
peroxo-bridged complex [(F8TPP)FeIII]2-(O22-) (3) (Scheme 2),
even though the UV-visible spectrum contains features very
similar to those of (F8TPP)FeIII-(O2-) (2). This formulation is
(82) Collman, J. P.; Brauman, J. I.; Iverson, B. L.; Sessler, J.; Morris, R.
M.; Gibson, Q. H. J. Am. Chem. Soc. 1983, 105, 3052-3064.
(83) Wang, M. R.; Hoffman, B. M.; Shire, S. J.; Gurd, F. R. N. J. Am.
Chem. Soc. 1979, 101, 7394-7398.
(84) Traylor, T. G.; Berzinis, A. P. Proc. Natl. Acad. Sci., U.S.A. 1980,
77, 3171-3175.
(85) Traylor, T. G.; Mitchell, M. J.; Tsuchiya, S.; Campbell, D. H.; Stynes,
D. V.; Koga, N. J. Am. Chem. Soc. 1981, 103, 5234-5236.
(86) David, S.; James, B. R.; Dolphin, D.; Traylor, T. G.; Lopez, M. A. J.
Am. Chem. Soc. 1994, 116, 6-14.
(87) Hashimoto, T.; Dyer, R. L.; Crossley, M. J.; Baldwin, J. E.; Basolo,
F. J. Am. Chem. Soc. 1982, 104, 2101-2109.
2
based upon our dioxygen-uptake measurements and H NMR
(88) Rose, E. J.; Venkatasubramanian, P. N.; Swartz, J. C.; Jones, R. D.;
Basolo, F.; Hoffman, B. M. Proc. Natl. Acad. Sci., U.S.A. 1982, 79,
5742-5745.
spectroscopic supporting evidence, as well as (in part) the
extensive work of Balch and co-workers, which established
µ-peroxo diiron PFeIII-(O22-)-FeIIIP adduct formation with the
TPP and TMP systems.58-60,91
(89) Lavalette, D.; Momenteau, M. J. Chem. Soc., Perkin Trans. 1982,
385-388.
(90) Lavalette, D.; Tetreau, C.; Momenteau, M.; Mispelter, J.; Lhoste, J.-
M.; Wuenschell, G. E.; Reed, C. Laser Chem. 1990, 10, 297-318.
(91) Chin, D.-H.; Del Gaudio, J.; La Mar, G. N.; Balch, A. L. J. Am. Chem.
Soc. 1977, 99, 5486-5488.
(81) Traylor, T. G.; White, D. K.; Campbell, D. H.; Berzinis, A. P. J. Am.
Chem. Soc. 1981, 103, 4932-4936.

5762 Inorganic Chemistry, Vol. 40, No. 23, 2001
Ghiladi et al.
Scheme 2
(F8TPP)FeII (1). Reversibility was accomplished by application
of a vacuum and subsequent bubbling of argon directly into
the solution containing the heme-O2 complex at 193 K; vacuum
alone was unable to accomplish this. Slight decomposition to
(F8TPP)FeIII-OH (5) [UV-visible: 408 (Soret) and 572 nm]
was seen only after several cycles of O2-binding and release
and was most easily followed by UV-visible spectroscopic
monitoring, Figure S9. This behavior contrasts with the ir-
reversible O2-binding observed for other PFeIII-(O22-)-FeIIIP
porphyrinate-iron(III) systems as studied by Balch and co-
workers.59 We speculate that the electron-withdrawing fluorine
substituents weaken the Fe-O bonds in 3 (as compared to
Balch’s TMP or TPP systems), shifting the equilibrium and
thereby allowing for the reversibility to be easily observed under
these mild conditions. Furthermore, toluene as solvent (vs CH2-
Cl2) may be critical for reversibility: the crystal structure of 1,
as the bis(toluene) solvate, shows the close contacts of toluene
to the metal center, and these solvent “ligands” may promote
stabilization of the (F8TPP)FeII (1) complex. Complex 3 was
found to be irreversible in dichloromethane, with all attempts
at reversing the O2-binding leading to formation of the
decomposition complex 5. Additionally, formation of 3 from
reaction of 1 with dioxygen in noncoordinating solvents was
too fast to be observed by stopped-flow spectrophotometry.
Dioxygen-Uptake Stoichiometry of (F8TPP)FeII (1). Con-
sistent with the formation of a µ-peroxo-bridged diiron complex
was the confirmation of the dioxygen uptake of 1, utilizing a
spectrophotometric titration methodology (see Experimental
Section). The UV-visible spectrum of (F8TPP)FeII (1) is shown
in Figure S10 [λmax ) 421 nm (Soret); CH2Cl2, 193 K]. Addition
of 0.3 equiv of dioxygen (via syringe) showed a slow and partial
conversion to the spectrum corresponding to [(F8TPP)FeIII]2-
(O22-) (3) [λmax ) 414 (Soret), 535 nm]. An additional 0.1 equiv
(0.4 equiv total) of dioxygen showed further formation of 3,
which neared completion after 0.5 equiv total dioxygen added.
A final 0.05 equiv of O2 aliquot (0.55 equiv O2 total) showed
the full formation of complex 3, after which the injection of a
large excess (>100 equiv) of dioxygen showed no further
spectral changes. Thus, complete formation of [(F8TPP)FeIII]2-
(O22-) (3) was accomplished with an overall oxygenation
stoichiometry of 0.5-0.55 equiv of O2 per equiv of (F8TPP)-
FeII (1), in agreement with the formulation of 3 as the heme-
peroxo-heme dinuclear complex depicted in Scheme 2.
Figure 7. 2H NMR of the oxygenation reaction of (F8TPP-d8)FeII
(1-d8) in CH2Cl2 at 193 K. Formation of the µ-peroxo dimer [(F8TPP-
d8)FeIII]2-(O22-) (3-d8) and the subsequent O-O bond cleavage reaction
giving (F8TPP-d8)FeIVdO (4-d8) upon addition of DMAP () B) are
also shown. See text for further explanation.
NMR Spectroscopic Characterization of [(F8TPP-d8)-
FeIII]2-(O22-) (3-d8). 2H NMR spectroscopy was used to follow
the oxygenation reaction of 1-d8 in CH2Cl2 (Figure 7). As
mentioned (vide supra), the pyrrole resonance at δ 4.9 ppm
(Figure 7a, 293 K) for 1-d8 is indicative of an intermediate spin
(S ) 1) system,49,92 an observation that shows the lack of an
axial base ligand for the ferrous-heme; this resonance shifts to
δ -3 ppm upon cooling to 193 K (Figure 7b). Addition of
dioxygen (via syringe) to the chilled (dry ice) NMR tube
containing a solution of 1-d8 in CH2Cl2 gives an interesting
result: two pyrrole resonances are observed (Figure 7c), with
the main resonance at δ 17.5 ppm and a second minor one
occurring at δ 8.9 ppm. We assign the δ 17.5 ppm feature to
2-
the dinuclear peroxo-bridged complex [(F8TPP-d8)FeIII]2-(O2
)
(3-d8), which is in very good agreement with the pyrrole-H
resonances of previously characterized heme-peroxo-heme
complexes (Table 4).
We have also assigned the second feature at δ 8.9 ppm as a
mononuclear iron(III)-superoxo complex, (F8TPP)FeIII-(O2-),
similar to 2, but lacking the coordinated axial (solvent) ligand.
The occurrence of the (P)FeIII-(O2-) superoxo species in
noncoordinating solvents was observed earlier by Balch and co-
workers for TMP (toluene solvent, -100 °C, resonance Raman
spectroscopy)58,93 and by others (see Table 4). Complete
transformation to the µ-peroxo complex 3 occurs upon warming
the sample to 223 K for a few minutes, with stabilization of
this species accomplished by rechilling to 193 K; the spectrum
of only the pure dinuclear peroxo-bridged complex [(F8TPP-
d8)FeIII]2-(O22-) (3-d8) is then observed, Figure 7d. Thus,
oxygenation of (F8TPP)FeII (1) in CH2Cl2 not only gives the
expected dinuclear µ-peroxo complex 3 but also shows an initial
(92) Goff, H.; La Mar, G. N.; Reed, C. A. J. Am. Chem. Soc. 1977, 99,
3641-3646.
(93) Mizutani, Y.; Hashimoto, S.; Kitagawa, T. J. Am. Chem. Soc. 1990,
112, 6809-6814.

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5763
Figure 9. UV-visible spectra of (F8TPP)FeII (1), [(F8TPP)FeIII]2-(O22-
(3), and (DMAP)(F8TPP)FeIVdO (4) in CH2Cl2 at 193 K.
)
Figure 8. 19F NMR spectroscopic monitoring of the oxygenation
reaction of (F8TPP)FeII (1) in CH2Cl2 at 193 K.
methylene chloride at 193 K led to the immediate formation of
a new species (Figure 7e, δpyrrole ) 3.5 ppm). This is consistent
with the literature values of the distinctive pyrrole-H resonances
for the previously studied six-coordinate ferryl complexes (Table
4). We thus formulate this species as the six-coordinate high-
valent iron(IV)-oxo (ferryl) complex (DMAP)(F8TPP-d8)-
FeIV)O (4-d8), Scheme 2. Species 4-d8 is stable indefinitely
between 193 and 223 K; however, immediate formation of (F8-
TPP-d8)FeIII-OH (5-d8) (δpyrrole ) 125 ppm, CH2Cl2, 193 K) is
observed upon warming.
We also investigated the ability of tetrahydrofuran, a non-
nitrogen donating base, to possibly effect a similar homolytic
O-O bond cleavage reaction of [(F8TPP)FeIII]2-(O22-) (3) to
form the ferryl species 4. Upon addition of THF (excess) to
3-d8 at 193 K in methylene chloride, the decomposition product
5-d8 was observed. Given that ferryl species are known to be
stable in THF solvent (Tables 4 and 7),96,97 we suggest that the
µ-peroxo dinuclear complex [(F8TPP)FeIII]2-(O22-) (3) instead
undergoes a direct reaction with THF to rapidly form (F8TPP)-
FeIII-OH (5).
UV-visible spectroscopy was also utilized for further
characterization of (B)(F8TPP)FeIVdO (4) (B ) DMAP).
Oxygenation of (F8TPP)FeII (1) [Figure 9: λmax ) 420 (Soret),
528, 558 (sh) nm; CH2Cl2, 193 K] reveals the formation of [(F8-
TPP)FeIII]2-(O22-) (3) [λmax ) 414 (Soret) and 535 nm],
comparable to the spectral features observed for toluene solvent.
Addition of 1 equiv of DMAP (per Fe) gave (B)(F8TPP)FeIVdO
(4) (B ) DMAP) with features at 417 (Soret) and 541 nm,
similar to the other previously characterized FeIVdO ferryl
species, Table 7.
minor formation of a heme-superoxo complex, [(F8TPP)FeIII-
(O2-)] (without axial base), which rearranges at higher tem-
peratures to form 3.
19F NMR spectroscopy was used to further investigate this
chemistry. Complex (F8TPP)FeII (1) exhibits a single fluorine
resonance at δ -107 ppm (CH2Cl2, 193 K, Figure 8a), implying
an equivalent environment for all fluorine substituents. Upon
oxygenation and slight warming to 233 K to form pure
[(F8TPP)FeIII]2-(O22-) (3), two fluorine resonances were ob-
served at δ -106 and -109 ppm, Figure 8b. The asymmetry
of complex 3 can rationalize the differences in the fluorine
environment of the 2,6-difluorophenyl substituents, with one
resonance due to the fluorines on the “inner” peroxo side of
the porphyrin macrocycle and the second resonance being
attributed to those fluorines on the other “outer” side. Upon
decomposition of 3, forming (F8TPP)FeIII-OH (5), a single broad
fluorine resonance is observed (Figure 8c) at δ -79 ppm
(assignment confirmed by 19F NMR spectroscopy on authentic
5).
Iron(IV)-Oxo (Ferryl) Formation: (B)(F8TPP)FeIVdO (4).
2
The H NMR investigations of the dinuclear peroxo-bridged
complex [(F8TPP)FeIII]2-(O22-) (3) led us to study its chemical
reactivity with respect to the addition of exogenous axial base
ligands (B), such as 4-(dimethylamino)pyridine (DMAP) or
tetrahydrofuran, to see if the F8TPPH2 ligand, with its electron-
withdrawing substituents, is able to stabilize a high-valent iron-
(IV) oxo ferryl species. Earlier studies58,61-63,94 established the
formation of ferryl species (by NMR spectroscopy) after addition
of nitrogenous bases (i.e., 1-methylimidazole) to the µ-peroxo
dinuclear complexes, PFeIII-(O22-)-FeIIIP, of tetraarylporphyrin
systems, and we followed the same methodology to establish
this chemistry with the F8TPPH2 porphyrin ligand. Nam and
co-workers95 have also reported the formation of a similar five-
coordinate ferryl species, (F8TPP)FeIVdO, using 3-chloroper-
oxybenzoic acid, but the UV-visible characterization (Table
7) was performed in acetonitrile solvent and without the presence
of an axial base ligand.
Summary
Following closely the comprehensive studies by Balch and
co-workers for other heme systems, we have utilized multi-
nuclear NMR and UV-visible spectroscopies to determine the
dioxygen reactivity of (F8TPP)FeII (1) under various conditions
of solvent and temperature. In coordinating solvents such as
THF, propionitrile, and acetone, (F8TPP)FeII (1) reacts with
dioxygen at low temperature to form a stable heme-superoxo
complex, (S)(F8TPP)FeIII-(O2-) (2) (S ) solvent). As with other
PFeIII-(O2-) complexes, we have shown 2 to be fully reversible
Addition of 1 equiv of DMAP (by syringe) to a chilled NMR
tube containing the diiron peroxo-bridged complex [(F8TPP-
d8)FeIII]2-(O22-) (3-d8) (Figure 7d, δpyrrole ) 17.5 ppm) in
(94) Balch, A. L.; La Mar, G. N.; Latos-Grazynski, L.; Renner, M. W.;
Thanabal, V. J. Am. Chem. Soc. 1985, 107, 3003-3007.
(95) Nam, W.; Lim, M. H.; Moon, S. K.; Kim, C. J. Am. Chem. Soc. 2000,
122, 10805-10809.
(96) Schappacher, M.; Weiss, R.; Montiel-Montoya, R.; Trautwein, A. X.;
Tabard, A. J. Am. Chem. Soc. 1985, 107, 3736.
(97) Yeh, C.-Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2001,
123, 1513-1514.

5764 Inorganic Chemistry, Vol. 40, No. 23, 2001
Ghiladi et al.
ref
Table 7. UV-Visible Data for PFeIVdO (Ferryl) Complexes Under Various Conditionsa
FeIVdO complex
λmax
solvent
temp (K)
(DMAP)(F8TPP)FeIVdO (4)
(F8TPP)FeIVdO
417, 541
n/a, 540b
CH2Cl2
193
233
233
223
223
223
212
233
233
RT
203
203
183
183
183
this work
95
95, 98
96
96
96
97
99
100
101
96
CH3CN/CH2Cl2
CH3CN/CH2Cl2
THF
DMF
THF
(F20TPP)FeIVdO
∼410,b 545
(THF)(Cl8TPP)FeIVdO
(DMF)(Cl8TPP)FeIVdO
(1-MeIm)(Cl8TPP)FeIVdO
(HPX)FeIVdO
417, 551
419, 554
421, 561
418, 555
THF
(TMP)FeIVdO
∼420,b 545
CH2Cl2
CH2Cl2/TBAP
benzene
THF
(TMP)FeIVdO
418, 545
(TMP)FeIVdO
418, 540
419, 550
426, 560
427, 555, 563
416, 543, 555
406, 535, 546
(THF)(TpivPP)FeIVdO
(1-MeIm)(TpivPP)FeIVdO
(1-MeIm)(TPP)FeIVdO
(1-MeIm)(PPDME)FeIVdO
(1-MeIm)(OEP)FeIVdO
THF
96
toluene
toluene
toluene
102
102
102
a F20TPP ) meso-tetrakis(pentaflurorphenyl)porphyrin; Cl8TPP ) meso-tetrakis(2,6-dichlorophenyl)porphyrin; TMP ) meso-tetramesitylporphyrin;
TPP ) meso-tetraphenylporphyrin; TpivPP ) meso-tetrakis(R,R,R,R-o-pivalamidophenyl)porphyrin; PPDME ) protoporphyrin IX dimethyl ester;
OEP ) octaethylporphyrin; DMAP ) 4-(dimethylamino)pyridine; 1-MeIm ) N-methylimidazole. b Exact wavelength not given, but spectrum
available.
benzophenone. Preparation and handling of air-sensitive materials was
carried out under an argon atmosphere using standard Schlenk
techniques. Solvents and solutions were deoxygenated either by repeated
by benchtop UV-visible spectroscopy, and stopped-flow spec-
trophotometry was used to determine the kinetics and thermo-
dynamics of dioxygen-binding to 1. Although the formation of
freeze-pump-thaw cycles or by bubbling of argon (20-30 min)
2 in THF solvent was within the mixing time of the apparatus,
directly through the solution. Solid samples were stored and transferred,
equilibrium data were obtained from temperature-dependent
and samples for UV-visible and NMR spectroscopies were prepared,
investigations. However, O2-binding to 1 in EtCN was suf-
in an MBraun LabMaster 130 inert atmosphere (<1 ppm O2, <1 ppm
ficiently slow as to provide kinetic insights. Confirmation of
H2O) glovebox under nitrogen atmosphere. Ultrahigh-purity O2 (99.994%)
2 as a heme-superoxo complex came from both O2-uptake
manometry and 2H NMR spectroscopy. Furthermore, 19F NMR
spectroscopy was also employed to characterize 2 and is, to
our knowledge, the first example of 19F NMR used to examine
these types of dioxygen adducts.
was purchased from WSC, Baltimore, MD.
NMR spectra were measured on a Varian NMR instrument. All
spectra were recorded in 5 mm o.d. NMR tubes, and chemical shifts
were reported as δ (ppm) values downfield from an internal standard
of Me4Si (1H), as δ (ppm) values calibrated to natural abundance
deuterium solvent peaks with shifts referenced downfield from TMS
(2H), or as δ (ppm) values referenced to an external standard of R,R,R-
trifluorotoluene at δ -63.73 ppm (19F). Low-temperature UV-visible
spectral studies were carried out with a Hewlett-Packard 8453 diode
array spectrometer equipped with HPChemstation software. The
spectrometer was equipped with a variable-temperature dewar and
cuvette assembly as described elsewhere.103,104 Fast atomic bombard-
ment (FAB-MS) mass spectra were obtained with a double-focusing
Vacuum Generator 70-S (VG 70-S) gas chromatography/mass spec-
trometer. Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF-MS) mass spectra were recorded on a Kratos Analytical
Kompact MALDI 4 mass spectrometer equipped with a 337 nm nitrogen
laser (20 kV extraction voltage). No matrix was necessary for the
MALDI-TOF-MS given that the porphyrin ligand was sufficient for
laser desorption.
We have also examined the dioxygen reactivity of 1 in
noncoordinating solvents (CH2Cl2, toluene) to yield the heme-
peroxo-heme dimer [(F8TPP)FeIII]2-(O22-) (3) at low tempera-
ture. Unlike the PFeIII-(O22-)-FeIIIP peroxo-bridged systems
studied by Balch using other porphyrin ligands, the formation
of 3 is fully reversible in toluene solvent, probably due to the
electron-withdrawing nature of the F8TPP porphyrin. Formula-
tion of the dioxygen adduct in noncoordinating solvents as
peroxo complex 3 was confirmed by dioxygen-uptake measure-
ments (spectrophotometric titration) and by 2H NMR spectros-
copy in CH2Cl2. Again, 19F NMR spectroscopy was used to
further characterize 3. The chemical reactivity of 3 to form (B)-
(F8TPP)FeIVdO (4) (B ) DMAP) was also explored by UV-
2
visible and H NMR spectroscopies, by addition of a nitrogen
F8TPPH2. Tetrakis(2,6-difluorophenyl)porphyrin was prepared by
standard benzaldehyde-pyrrole condensation in propionic acid/nitroben-
zene reflux, in a procedure modified from that previously65,105,106
reported: To a 2000 mL two-neck round-bottom flask equipped with
reflux condensor was added 1000 mL of propionic acid, 250 mL of
axial base ligand. Thus, formation of a ferryl species such as
(DMAP)(F8TPP)FeIVdO (4) was confirmed by these studies.
In conclusion, we have studied the dioxygen reactivity of (F8-
TPP)FeII (1) under various conditions and have concluded that
the reaction of 1 with O2 is similar to other heme systems such
as (TPP)FeII and (TMP)FeII, with one notable exception: the
formation of the heme-peroxo-heme dimer, complex 3, is fully
reversible. These studies of (F8TPP)FeII (1) with dioxygen lay
the groundwork for better understanding PFeII/CuI(L)/O2 reac-
tions and will serve to help elucidate the rich chemistry of heme-
Cu systems as models for cytochrome c oxidase.
(98) Nam, W.; Lim, M. H.; Oh, S.-Y.; Lee, J. H.; Lee, H. J.; Woo, S. K.;
Kim, C.; Shin, W. Angew. Chem., Int. Ed. 2000, 39, 3646-3649.
(99) Nam, W.; Lim, M. H.; Oh, S.-Y. Inorg. Chem. 2000, 39, 5572-5575.
(100) Groves, J. T.; Gilbert, J. A. Inorg. Chem. 1986, 25, 123.
(101) Groves, J. T.; Gross, Z.; Stern, M. K. Inorg. Chem. 1994, 33, 5065-
5072.
(102) Oertling, W. A.; Kean, R. T.; Wever, R.; Babcock, G. T. Inorg. Chem.
1991, 29, 2633-2645.
(103) Karlin, K. D.; Cruse, R. W.; Gultneh, Y.; Farooq, A.; Hayes, J. C.;
Zubieta, J. J. Am. Chem. Soc. 1987, 109, 2668-2679.
(104) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Meyer, G. J.; Farooq, A.;
Gultneh, Y.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1988, 110,
1196-1207.
(105) Fuhrhop, J.-H.; Smith, K. M. In Porphyrins and Metalloporphyrins;
Smith, K. M., Ed.; Elsevier: New York, 1975; pp 769-770.
(106) Adler, A. D.; Longo, F. R.; Finarelli, J.; Goldmacher, J.; Assour, J.;
Korsakoff, L. J. Org. Chem. 1967, 32, 476.
Experimental Section
Materials and Methods. Reagents and solvents were purchased from
commercial sources and were of reagent quality unless otherwise stated.
All solvents were distilled under argon prior to use. Dichloromethane
(CH2Cl2), acetonitrile (CH3CN), and heptane were distilled directly from
CaH2. Propionitrile (C2H5CN) was first distilled over P4O10, then
refluxed and distilled over CaH2. Diethyl ether was distilled from Na/

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5765
nitrobenzene, and 25.0 g (176 mmol) of 2,6-difluorobenzaldehyde. The
solution was brought to reflux, after which 12.0 g (179 mmol) of pyrrole
was added dropwise to the reaction flask, causing an immediately
darkening of the yellow solution to black. After 2 h, the reaction mixture
was cooled to room temperature and placed in a freezer (-20 °C) for
several days. The resulting purple solid was collected via filtration over
Celite, washed with water (2000 mL), dried over air, and removed from
Celite with CH2Cl2 (2000 mL). The solution was dried over MgSO4,
filtered, and concentrated to dryness via rotary evaporation. The crude
material was first purified over an alumina column (35 cm × 5.5 cm
o.d.) using methylene chloride (Rf ) 0.99) eluent and then purified
further over a silica column (30 cm × 5.5 cm o.d.) with 50:50 CH2-
Cl2/hexanes (Rf ) 0.57). Solvent was removed via rotary evaporation,
and the solid obtained was dried in vacuo for 2 h. F8TPPH2 was isolated
as a free flowing purple solid in 6.4% yield (8.5 g). UV-vis (CH2-
for ample time (∼5 min) to allow for the separation of the two layers,
after which the organic layer was allowed to slowly pass through a
Schlenk filter tube (coarse porosity) over which was packed a small
plug of anhydrous sodium sulfate (∼4 g). The solvent was concentrated
in vacuo, and addition of deoxygenated heptane precipitated 1 as a
opaque crystalline solid (875 mg, 1.08 mmol) in 91% yield. UV-vis:
THF, 421, 542; propionitrile, 414, 424, 528; toluene, 422, 528, 558
(sh); CH2Cl2, 422, 528, 558 (sh). 1H NMR (THF-d8): δ 56 (s, pyrrole-
H, 8 H), 8.38 (s, p-phenyl-H, 4H), 7.15 (s, m-phenyl-H, 8H). 19F NMR
(THF-d8): δ -111.5 (o-phenyl-F). MALDI-TOF-MS (m/z): 813
(M + H+)+.
(F8TPP-d8)FeII (1-d8). The procedure used is identical to that
described above for (F8TPP)FeII (1), except (F8TPP-d8)FeIIICl was used
2
instead of (F8TPP)FeIIICl. H NMR (THF): δ 56 (s, br, pyrrole-D).
MALDI-TOF-MS (m/z): 820 (M + H+)+.
1
Crystallographic Structural Determination of (F8TPP)FeII‚
(C7H8)2 (1‚(C7H8)2). Recrystallization of 1 from toluene/heptane under
argon atmosphere gave large purple-red crystals suitable for X-ray
diffraction. Crystal data collection and refinement parameters are given
in Table 1. The systematic absences in the diffraction data are consistent
for the C-centered monoclinic space groups, Cc and C2/c. The
E-statistics and the value of Z suggested the centrosymmetric space
group, which yielded chemically reasonable and computationally stable
results of refinement. The structure was solved using direct methods,
completed by subsequent difference Fourier syntheses, and refined by
full-matrix least-squares procedures. The asymmetric unit consists of
half of 1 located on an inversion center and a molecule of toluene. All
non-hydrogen atoms were refined with anisotropic displacement
coefficients, and the hydrogen atoms were treated as idealized contribu-
tions.
Cl2): 412 (Soret), 507, 584 nm. H NMR (CDCl3): δ 8.86 (s, 8 H,
pyrrole-H), 7.79 (m, 4 H, p-phenyl-H), 7.38 (t, J ) 8.39 Hz, 8 H,
m-phenyl-H), -2.75 (s, 2 H, NH); 19F NMR (CDCl3): δ -109.17 (t,
JHF ) 6.0 Hz, o-phenyl-F). MALDI-TOF-MS (m/z): 759.6 (M + H+)+.
F8TPPH2-d8. In a procedure modified from that previously re-
ported,66 the pyrrole-deuterated F8TPPH2-d8 was prepared in a one-pot
reaction in which the pyrrole proton-deuteron exchange was effected
prior to condensation: To a 2000 mL two-neck round-bottom flask
was added under argon 1 L of propionic anhydride (97%) and 175 mL
of D2O (99.9 atom % D), and the solution was brought to reflux for 1
h. To this deuterated propionic acid was added under argon 12.0 g
(179 mmol) of pyrrole, and the reaction mixture was refluxed for an
additional hour. In air, 250 mL of nitrobenzene and 25.0 g (176 mmol)
of 2,6-difluorobenzaldehyde were added. Reflux continued for an hour,
after which the reaction mixture was cooled to room temperature and
placed in a freezer (-20 °C) for several days. The resulting purple
solid was collected and purified as above for F8TPPH2. F8TPPH2-d8
was isolated as a free flowing purple solid in 6.5% yield (8.5 g). UV-
All software and sources of the scattering factors are contained in
the SHELXTL (5.1) program library (G. Sheldrick, Siemens XRD,
Madison, WI). The X-ray table for 1‚(C7H8)2 is found in the text (Table
1), while the full X-ray reports for both 1‚(C7H8)2 and (F8TPP)FeIII-Cl
can be found in the Supporting Information.
1
visible and H and 19F NMR spectroscopic characterization were the
same as for F8TPPH2. 2H NMR (CHCl3): δ 8.94 (s, pyrrole-D). FAB-
MS (m/z): 766 (M + H+)+. Mass spectroscopic analysis reveals that
F8TPPH2-d8 made by this procedure is minimally 90 atom % deuterated
for the pyrrole hydrogens.
Dioxygen-Uptake Manometry of F8TPP)FeII (1) in THF Solvent.
O2 absorption by 1 at 193 K was monitored at constant pressure in a
glass buret as previously described.103,104 A sidearm Schlenk flask
containing 10 mL of deoxygenated THF solution of (F8TPP)FeII (1)
(500 mg, 615 µmol) was attached to the manometer system and cooled
to 193 K. After evacuation and degassing of the solution with repeated
freeze-pump-thaw cycles, the stopcock leading to the reaction flask
was closed and the buret assembly was equilibrated to 1 atm of O2.
The Schlenk flask was then opened to the manometer assembly,
allowing reaction of 1 with O2. The volume of O2 taken up was 15.1
mL at 960 mTorr (after accounting for 4.0 mL of solvent O2 uptake),
corresponding to an Fe:O2 ratio of 1:0.98; a second experiment at 980
mTorr required 15.6 mL, for an Fe/O2 ratio of 1:1.03.
Dioxygen Uptake (Spectrophotometric Titration) of F8TPP)FeII
(1) in CH2Cl2 Solvent. To an air-free cuvette assembly with sidearm
stopcock and female 14/20 joint was added in a glovebox a known
quantity of 1 (typically 1 mg) via stock solution (1 mg 1/g solvent) in
CH2Cl2. A 14/20 septum adapter (Chemglass) was used to seal the
cuvette from the atmosphere, followed by two additional septa (each
successively larger than the previous) wired to the septum adapter (for
a total of three septa). The cuvette assmbly was then removed from
the glovebox, placed into the modified low-temperature UV-visible
spectrophotometer, and cooled to -80 °C. A commercially available
(Potomac Airgas, Baltimore, MD) 1.01 ( 0.02 mol % O2 in Ar gas
mixture was then slowly bubbled through a two-sidearmed Schlenk
flask equipped with rubber septum and oil bubbler, which allowed for
continuous flow of the 1% O2 gas mixture at atmospheric pressure.
Hamilton gastight syringes (250, 500, and 1000 µL) equipped with
three-way valves (for Ar purging of the syringe barrel and needle) were
used to transfer exact molar quanitites of dioxygen to the cooled cuvette
assembly. Typical transfer ratios included 0.3 equiv of (875 µL)
dioxygen (per 1), followed by 2 × 0.1 equiv (2 × 300 µL) and 0.05
equiv (150 µL) additions. Each addition of dioxygen directly into the
solution through the multiple septa was accompanied by gentle shaking
of the cuvette assembly and UV-visible spectroscopic monitoring.
When no further spectral changes were detected after each successive
(F8TPP)FeIIICl. To a refluxing solution of F8TPPH2 (1.0 g, 1.32
mmol) in deoxygenated DMF (250 mL) under Ar atmosphere was added
anhydrous FeCl2 (3.5 g, 27.6 mmol) in approximate 20-fold molar
excess. After continued stirring and reflux for 1 h, the reaction mixture
was cooled and exposed to air, and addition of 1 L of 2 M HCl(aq)
caused a solid purple precipitate to form. The solid material was
collected by filtration using a medium-porosity filter frit and dissolved
in 500 mL of CH2Cl2 to give a dark red solution. This organic solution
was washed with 1 M HCl (2 × 500 mL) and water (1 × 500 mL),
dried over Na2SO4, and filtered, and the crude material (filtrate) was
adsorbed onto silica. Purification over a silica column (35 cm × 4.0
cm o.d.) was accomplished by removing impurities (including unmeta-
lated ligand) with CH2Cl2, followed by elution of the desired product
with 3% MeOH in CH2Cl2 (Rf ) 0.45-0.55). Solvent was removed
via rotary evaporation, and the resulting purple-black solid was
precipitated from CH2Cl2 (50 mL) solution upon addition of heptane
(450 mL). (F8TPP)FeIIICl was isolated (1.03 g, 1.22 mmol) as a black
1
solid in 92% yield. UV-vis (CH2Cl2): 411, 503, 639, 772. H NMR
(CDCl3): δ 81.54 (s, br, 8 H, pyrrole-H), 13.86 (s, 4 H, m-phenyl-H),
12.52 (s, 4 H, m-phenyl-H), 7.6 (s, 4 H, p-phenyl-H). 19F NMR
(CDCl3): δ -76.53 (s, br, o-phenyl-F), -80.28 (s, br). FAB-MS (m/
z): 812 (M - Cl-)+. Anal. Calcd for C44H20ClF8FeN4: C, 62.32; H,
2.38; N, 6.61; Found: C, 62.62; H, 2.44; N, 6.37.
(F8TPP-d8)FeIIICl. The procedure used is identical to that described
above for (F8TPP)FeIIICl, but utilized F8TPPH2-d8 in place of F8TPPH2.
2H NMR (CHCl3): δ 80.6 (s, br, pyrrole-D). FAB-MS (m/z): 819
(M - Cl-)+.
(F8TPP)FeII (1). Under Ar, to a solution of (F8TPP)FeIIICl (1.0 g,
1.18 mmol) in deoxygenated methylene chloride (50 mL) was added
40 mL of 1 M sodium hydrosulfite in deoxygenated water. The two
solutions were agitated with Ar bubbling for 30 min to ensure complete
mixing, during which time the color of the organic later changed from
dark red-brown to bright red. The reaction mixture was allowed to sit

5766 Inorganic Chemistry, Vol. 40, No. 23, 2001
Ghiladi et al.
addition of dioxygen, the next partial equivalent was added. In this
manner, it was established that full formation of the oxygen intermediate
of 1 occurred between 0.5 equiv (1500 µL 1.01% O2 in argon) and
0.55 equiv (1650 µL 1.01% O2 in argon) of dioxygen. No spectral
changes were observed from direct injection of dioxygen (>100 equiv)
after the total 0.55 equiv was added. Furthermore, a second cuvette
filled with the same stock solution of 1 used for the titration was directly
injected with a large excess of dioxygen, and the spectrum obtained
for this was nearly superimposable (with respect to λmax and absorbance)
to that obtained from the spectrophotometric titration. Finally, a control
experiment was performed in which 1 was injected a total of five times
with 500 µL of dioxygen-free argon in a manner identical to that above,
with no detectable changes in the spectrum of 1 observed.
temperature cuvette assembly equipped with a Schlenk-type sidearm.
After cooling to 193 K, generation of the dioxygen adducts was carried
out by bubbling the chilled solution with dioxygen (ultrahigh purity).
Removal of excess dioxygen was performed by simple application of
vacuum at 193 K. Reversible binding was carried out either by bubbling
the solution with argon using a syringe needle at 193 K for 15-25
min or by warming the solution to room temperature and subsequent
vacuum to remove O2 liberated during the warming process.
Alkaline Pyrogallol Test for Dioxygen Evolution. In the glovebox,
a solution of 1 (40 mg) in the desired solvent (∼15 mL) was prepared
and transferred to a Schlenk flask capped with a rubber septum.
Bubbling of dioxygen via syringe needle at 193 K afforded the desired
dioxygen adduct, either (THF)(F8TPP)FeIII-(O2-) (2) (THF solvent) or
[(F8TPP)FeIII]2-(O22-) (3) (toluene solvent). Removal of excess O2 was
performed with multiple freeze-pump-thaw cycles (thawing temper-
ature was 193 K), yielding a solution whose only source of dioxygen
was the heme-O2 adduct.
To a special Schlenk flask modified with a quartz cuvette and
equipped with a stir bar was added 4.0 g of pyrogallol and 25 mL of
deoxygenated 50% KOH(aq) solution under Ar. The alkaline pyrogallol
solution was a faint beige color, and its UV-visible spectrum was
recorded (λ400 nm ) ∼0.035-0.04). Using a syringe needle, argon was
bubbled directly through the heme-O2 solution at 193 K, liberating O2
into the headspace of the Schlenk flask, which was then passed through
a cannula and bubbled directly into the stirring alkaline pyrogallol
solution, whose color began to darken after 1 min. After approximately
20 min, the pyrogallol solution ceased becoming darker, and its
Variable-Temperature NMR Studies. Both deuterated and undeu-
terated solvents were distilled and degassed prior to use. In the
glovebox, typically a solution containing 10-20 mg of 1 in 1 mL of
solvent (deuterated solvent for 1H and 19F spectra, undeuterated solvent
for 2H) was prepared and transferred to 5 mm o.d. 8 in. Wilmad screw-
1
2
cap sample tubes (528-TR-8). H, H, and/or 19F NMR spectra were
recorded at room temperature before cooling to the desired temperature.
Addition of dioxygen at low temperature was carried out in the
following manner: the NMR tube was removed from the spectrometer
and placed either ina dry ice/acetone bath or directly into dry ice, and
using a Hamilton gastight syringe, excess O2 (99.994%; WSC,
Baltimore) was bubbled directly through the solution using a long
syringe needle. Addition of solutions (i.e., 4-(dimethylamino)pyridine)
was also performed in a similar manner. Typical spectra were recorded
at 100 kHz spectral width with no delay between 90° pulses of 7.2,
17.5, and 5.0 µs for 1H (400 MHz), 2H (61 MHz), and 19F (376 MHz)
spectra, respectively. Referencing/calibration of the spectra was per-
formed as described above for each given temperature, and shift values
are reported at the stated temperature; for 1H and 2H NMR spectroscopy,
those resonances positive to TMS are downfield and those negative
are upfield.
spectrum was re-recorded (λ400
repeated under identical conditions but without 1 present as a control
) ∼0.4-0.45). This process was
nm
and showed no significant increase in absorbance (λ400 ) ∼0.05-
nm
0.06).
Stopped-Flow Spectrophotometry. Rapid kinetics were followed
using an SF-21 variable-temperature stopped-flow unit (Hi-Tech
Scientific) combined with a TIDAS-16 HQ/UV-vis 512-16B diode
array spectrometer (J & M; 507 diodes, 300-720 nm, 1.3 ms minimum
sampling time). Data acquisition (up to 256 complete spectra; up to
four different time bases) was performed by using the Kinspec program
(J & M). For numerical analysis, all data were pretreated by factor
analysis. The solvents were treated prior to use to remove impurities
and water. Propionitrile was dried with phosphorus pentoxide (P2O5),
distilled under normal pressure, and kept dry by storing over calcium
hydride (CaH2, Merck). Tetrahydrofuran was dried over sodium (using
benzophenone as an indicator for the absence of water) and freshly
distilled prior to use.
For (F8TPP)FeII (1) in propionitrile, three series of experiments were
performed with 0.249, 0.109, and 0.052 mM complex concentration
(room temperature), respectively. Data for a total of 251 individual
runs between 178 and 293 K with data collection times between 0.1
and 641 s were obtained. Of these, 119 data sets (over a temperature
range of 178-205 K, where the formation of the Fe-superoxo species
could be observed) were used for the analysis. The concentration profile
based on the kinetic model 1 + O2 ≡ 2 was calculated by numerical
integration using the Glofit110 program. Thermal expansion of propio-
nitrile was taken into account using F(g/mL) ) (+1.0688-9.80) ×
10-4 T (K) for the solvent density as a function of temperature. The
initial O2-concentration in propionitrile was 4.4 mM (room tempera-
ture).111
Evans Method Magnetic Moment Measurements. All Evans
method107,108 magnetic moment measurements were carried out in 5
mm o.d. 8 in. Wilmad screw-cap sample tubes (528-TR-8) equipped
with pierceable Teflon-faced rubber septa. Coaxial insert reference
capillaries were also purchased from Wilmad (WGS-5BL). In a typical
experiment, the sample tube and the coaxial insert were brought into
a glovebox, and a sample of 1 with known concentration and volume
in deoxygenated solvent was placed in the sample tube. The coaxial
insert was filled with deoxygenated solvent as well and then placed
inside the sample tube, which was then sealed with the septum cap.
After removal from the glovebox, 1H and 2H NMR spectra were
obtained and referenced as normal, with the paramagnetically shifted
2
solvent peak occurring downfield of the reference solvent peak. H
NMR was used to confirm the Evans method magnetic moment
measurements since only two resonances, one corresponding to the
paramagnetically shifted deuterated solvent and the second correspond-
ing to the reference capillary deuterated solvent, were observed, thereby
eliminating erroneous peak assignments or unobserveable peaks due
to spectral overlap. The equation µB ) 2.84 ø T/n was used to
x
M
calculate the magnetic moment, where T is the temperature (K) of the
measurement, n is the nuclearity of the complex (n ) 1 for 1), and
øM ) -(3/4π)(∆ν/ν)(1000/c) + øsMol - øD, where øsol is the solvent
M
susceptibility () øg, gram susceptibility of solvent, × MW of the metal
In tetrahydrofuran, (F8-TPP)FeII (1) was studied by using three series
(0.346, 0.230, and 0.090 mM). Data for a total of 189 individual runs
between 182 and 303 K with data collection times between 0.1 and
651 s were obtained. Of these, 80 data sets (over a temperature range
of 228-288 K) were used for the analysis of the fast initial equilibrium.
The concentrations required for the calculation of the equilibrium
constant by the law of mass action were received by Lambert and Beer’s
law (linear regression) using a fixed set of species spectra and
absorbances measured at the first reaction time. Thermal expansion of
complex), øD is the total diamagnetic correction (calculated from
Pascal’s constants),109 ∆ν is the paramagnetic shift of the solvent
solVent
reference
() ν
- νsolVent
sample, a negative value) in Hz, ν is the frequency of the
NMR instrument in Hz, and c is the concentration of the metal complex
(corrected for temperature by use of the temperature dependence of
the solvent density). All reported magnetic moments are the average
of three runs (unless otherwise stated).
Reversible O2-Binding. In the glovebox, a solution of 1 in the
desired solvent was prepared and transferred to a modified low-
(107) Evans, D. F. J. Chem. Soc. 1959, 2003.
(110) Kinfit clone (see ref 111) converted into Matlab’s computer language.
Neuhold, Y.-M. Ph.D. Thesis, University of Basel, Switzerland, 1999.
(111) Karlin, K. D.; Wei, N.; Jung, B.; Kaderli, S.; Niklaus, P.; Zuberbu¨hler,
A. D. J. Am. Chem. Soc. 1993, 115, 9506-9514.
(108) Live, D. H.; Chan, S. I. Anal. Chem. 1970, 42, 791-792.
(109) CRC Handbook of Chemistry and Physics, 74th ed.; Lide, D. R.,
Ed.; CRC Press: Boca Raton, 1993.

(F8TPP)FeII/O2 Reactivity Studies
Inorganic Chemistry, Vol. 40, No. 23, 2001 5767
tetrahydrofuran was taken into account using F(g/mL) ) (+1.1850-
1.028) × 10-3 T (K) for the solvent density as a function of temperature
determined by linear regression of the density data.112 The initial O2-
concentration in tetrahydrofuran was 3.3 mM (room temperature).113
Supporting Information Available: Discussion of the reversible
superoxo formation of (S)(F8TPP)FeIII-(O2-) (2) in THF and EtCN
solvents (Figures S1, S2). Discussion of the dioxygen reactivity of (F8-
TPP)FeII in acetone solvent at 193 K (Figures S3-S5). Eyring plot for
the (F8TPP)FeII (1) oxygenation reaction in EtCN (Figure S6). UV-
visible spectra of the (F8TPP)FeII (1) + O2 reaction as a function of
temperature in THF solvent (Figure S7). Van’t Hoff plot for the (F8-
TPP)FeII (1) oxygenation reaction in THF (Figure S8). UV-visible
spectra of the (F8TPP)FeII (1)/[(F8TPP)FeIII]2-(O22-) (3) reversible
oxygenation (cycling) reaction in toluene at 193 K (Figure S9).
Dioxygen uptake (spectrophotometric titration) of (F8TPP)FeII (1) with
dioxygen in CH2Cl2 (Figure S10). ORTEP representation of (F8TPP)-
FeIII-Cl (Figure S11). Crystal data and structure refinement, atomic
coordinates, bond lengths and bond angles, anisotropic displacement
coefficients, and H-atom coordinates for (F8TPP)FeII‚(C7H8)2 (1‚(C7H8)2)
(Tables S1-S5) and (F8TPP)FeIII-Cl (Tables S6-S10). This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We are grateful to the National Institutes
of Health (K.D.K., GM28962 and GM60353) and the Swiss
National Science Foundation (A.D.Z.) for support of this
research. R.M.K. also acknowledges support from the Howard
Hughes Undergraduate Summer Research Program (Johns
Hopkins University). We would also like to thank Prof. R. J.
Cotter and Dr. A. S. Woods (Johns Hopkins School of Medicine)
for their help in obtaining the MALDI-TOF-MS data.
(112) Carvajal, C.; To¨lle, K. J.; Smid, J.; Szwarc, M. J. Am. Chem. Soc.
1965, 87, 5548-5553.
(113) The solubility of dioxygen in tetrahydrofuran was determined in the
same way as in propionitrile by using an oxygen-sensitive Clark
electrode. For more details see ref 111.
IC0105866