Kinetic and mechanistic studies on the reaction of nitric oxide with a water-soluble octa-anionic iron(III) porphyrin complex

Article abstract of DOI:10.1021/ic050924t

The polyanionic water-soluble and non-μ-oxo-dimer-forming iron porphyrin iron(III) 5⁴,10⁴,15⁴,20⁴-tetra- tert-butyl-5²,5⁶,15²,15⁶-tetrakis[2, 2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P^8-) Fe^III (1), was synthesized as an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated precursor porphyrins and characterized by standard techniques such as UV-vis and ¹H NMR spectroscopy. A single pK_a1 value of 9.26 was determined for the deprotonation of coordinated water in (P^8-)Fe ^III(H₂O)₂ (1-H2O) present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give the mononitrosyl adduct, (P^8-)Fe^II(NO ⁺)(L), where L = H₂O or OH^-. The kinetics of the binding and release of NO was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters ΔS^? and ΔV ^? for the on and off reactions. The rate constant k_on = 6.2 × 10⁴ M^-1 s ^-1 (24°C), determined for NO binding to the monohydroxo-ligated (P^8-)Fe^III(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for 1-H₂O at lower pH (k_on = 8.2 × 10⁵ M^-1 s ^-1, 24°C, pH 7). The observed decrease in the reactivity is contradictory to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in terms of a mechanistic changeover observed for 1-H₂O and 1-OH in their reactions with NO. The mechanistic interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature, and pressure.

Full text of DOI:10.1021/ic050924t

Inorg. Chem. 2005, 44, 7717−7731
Kinetic and Mechanistic Studies on the Reaction of Nitric Oxide with a
Water-Soluble Octa-anionic Iron(III) Porphyrin Complex
Joo-Eun Jee,† Siegfried Eigler,§ Frank Hampel,§ Norbert Jux,*,§ Maria Wolak,†,‡ Achim Zahl,†
Grazyna Stochel,‡ and Rudi van Eldik*,†
Institute for Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg, Egerlandstrasse 1,
91058 Erlangen, Germany, Institute for Organic Chemistry, UniVersity of Erlangen-Nu¨rnberg,
Henkestrasse 42, 91054 Erlangen, Germany, and Department of Inorganic Chemistry, Jagiellonian
UniVersity, Ingardena 3, 30-060 Krakow, Poland
Received June 8, 2005
The polyanionic water-soluble and non-µ
-oxo-dimer-forming iron porphyrin iron(III) 54,104,154,204-tetra-tert-butyl-
52,56,152,156-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P8-)FeIII (1), was synthesized as
an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated
precursor porphyrins and characterized by standard techniques such as UV−
vis and 1H NMR spectroscopy. A
single pKa1 value of 9.26 was determined for the deprotonation of coordinated water in (P8-)FeIII(H2O)2 (1-H2O)
present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give the
mononitrosyl adduct, (P8-)FeII(NO+)(L), where L
)
H2O or OH-. The kinetics of the binding and release of NO
was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques.
The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange
mechanism based on the positive values of the activation parameters
∆ ∆
Sq and Vq for the “on” and “off” reactions.
1
1
The rate constant kon
) 6.2 × °C), determined for NO binding to the monohydroxo-ligated
104 M- s- (24
(P8-)FeIII(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for
1
1-H2O at lower pH (kon
)
8.2
×
105 M- s-1, 24
°
C, pH 7). The observed decrease in the reactivity is contradictory
to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in
terms of a mechanistic changeover observed for 1-H2O and 1-OH in their reactions with NO. The mechanistic
interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic
porphyrin complex as a function of pH, temperature, and pressure.
Introduction
the tetrapyrrole ring, the polarity of the reaction medium
(solvent or amino acid residues around the active site in heme
proteins), and several other factors. To understand these
structure-function relationships, numerous mechanistic and
structural studies on heme proteins and their biomimetic
models have been performed. In particular, synthetic iron-
(III) porphyrins, the structural analogues of prosthetic groups
in ferriheme proteins, have been extensively studied in
relation to their catalytic and biomimetic properties.
An important aspect of studies on the reactivity of model
iron(III) porphyrins and ferriheme proteins concerns their
interaction with nitric oxide.1-10 These studies were directed,
on the one hand, to explain the nature of interactions
Iron porphyrins, an important class of transition-metal
complexes, continue to attract considerable attention because
of their importance in biology and catalysis. Roles played
by these complexes in electron-transfer processes, metabolic
control, transport and activation of oxygen under biological
conditions, and versatile catalytic processes in vitro are
surprisingly diverse. This rich chemistry stems partially from
the fact that the reactivity of iron porphyrins is finely tuned
by the surroundings of the central iron atom, such as the
identity of axial ligands, the nature of the substituents on
* To whom correspondence should be addressed. E-mail: vaneldik@
chemie.uni-erlangen.de.
† Institute for Inorganic Chemistry, University of Erlangen-Nu¨rnberg.
§ Institute for Organic Chemistry, University of Erlangen-Nu¨rnberg.
‡ Jagiellonian University.
(1) (a) Ford, P. C.; Lorkovic, I. M. Chem. ReV. 2002, 102, 993 and
references cited therein. (b) Ford, P. C.; Laverman, L. E.; Lorkovic,
I. M. AdV. Inorg. Chem. 2003, 54, 2003 and references cited therein.
10.1021/ic050924t CCC: $30.25
Published on Web 10/04/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 22, 2005 7717

Jee et al.
underlying the diverse functions of NO in vivo and, on the
other hand, to probe the reactivity of various iron porphyrins
in ligand substitution reactions. In the latter context, studies
on the mechanism of formation and decay of iron porphyrin
nitrosyls provided information on the influence of the
porphyrin ligand (modified in size and overall charge by the
substituents on the tetrapyrrole ring)1,2,4-7 and protein
environment (in studies concerning heme proteins)1,2,5,8-10
on the reactivity of the central FeIII ion in ligand substitution
reactions. Because the nature and lability of axial ligands
coordinated to the iron center are of crucial importance for
its overall reactivity, information gained from such mecha-
nistic studies contributed to the understanding of various
mechanistic pathways exhibited by iron porphyrins in their
biological and catalytic functions.
It is now firmly established that binding of NO to an iron-
(III) porphyrin leads to the formation of a low-spin diamag-
netic (S ) 0) mononitrosyl complex, in which the Fe-N-O
unit adopts a linear (or close to linear) geometry.1,3 Because
NO binding is accompanied by charge transfer from the NO
ligand to the metal center, the resulting product (representing
a {Fe-NO}6 nitrosyl3) can be formally described as (P)FeII-
NO+. Detailed mechanistic investigations performed in
our5,6,9 and other1,2,4,8,10 laboratories provided evidence that
the dynamics of NO binding in heme proteins and synthetic
iron porphyrins is to a large extent controlled by the ligand
substitution step. Considerable differences in the rates of
binding and release of NO,1,2,4,6,8-10 and varying tendencies
of the resulting nitrosyls to undergo subsequent reactions
(such as reductive nitrosylation)8,11,12 reported in these
studies, apparently reflect the influence of variation in the
immediate surroundings of the iron(III) center (mainly the
types of axial ligands and porphyrin ring substituents) on
the observed reactivity patterns. Despite these studies, a
coherent pattern of a structure-reactivity relationship in the
studied reactions is far from complete. In this context,
systematic studies on the reactivity of water-soluble iron
porphyrins (considered as model compounds for aqueous
porphyrin chemistry) with different types of positively and
negatively charged meso substituents toward NO have been
initiated in order to better assess the correlation between the
porphyrin structure and its reactivity.4,6,7,11-14 The results of
high-pressure NMR, stopped-flow, and laser flash photolysis
investigations performed on these complexes provided
evidence that the negatively charged peripheral substituents
tend to labilize the axial metal-ligand bond by increasing
the electron density on the metal center. A survey of literature
data, however, shows that porphinatoiron(III) complexes may
exist as five- or six-coordinate species in which the d
electrons of the central FeIII ion can be arranged into three
possible spin states, viz., the low spin state (S ) 1/2),
intermediate spin state (S ) 3/2) and the high spin state (S )
5/2).15 In addition, a range of (P)FeIII(L)n complexes (with n
) 1 or 2) with quantum mechanically admixed intermediate
5
3
spin states (S ) /2 and /2) have been reported, in which
5
3
varying ratios of S ) /2 and /2 admixing were observed,
depending on the nature of the axial ligands and porphyrin
ring substituents.15-23 This variety of spin and ligation states
is reflected in distinct structural features observed for
different types of model and naturally occurring iron(III)
porphyrins. While the low-spin (P)FeIII(L)n complexes are
typically six-coordinate and exhibit short axial and equatorial
bonds (due to depopulation of eg* orbitals), purely high-spin
analogues are often five-coordinate, with elongated Fe-Np
bonds and considerable displacement (0.4-0.6 Å) of the iron-
(III) center out of the porphyrin plane.15,22 The structural
manifestations of the intermediate spin state (admixed or
pure) include short Fe-Np equatorial bonds (resulting from
2
2
partial or complete depopulation of the dx -y orbital) and
strongly elongated axial bonds.15,17,21,23-25 Although these
various features are critical in controlling the reactivity of
the iron(III) porphyrins, their influence on the kinetic and
mechanistic features of NO binding and release in different
classes of (P)FeIII(L)n complexes remains obscure. Taking
into account a variety of spin and ligation states in heme
proteins that can interact with NO under biological condi-
tions, an improved characterization of typical reactivity
(2) Hoshino, M.; Laverman, L.; Ford, P. C. Coord. Chem. ReV. 1999,
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(3) (a) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067 and
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R. W. J. Am. Chem. Soc. 2002, 124, 13833 and references cited therein.
(4) Laverman, L. E.; Ford, P. C. J. Am. Chem. Soc. 2001, 123, 11614
and references cited therein.
(13) Laverman, L. E.; Hoshino, M.; Ford, P. C. J. Am. Chem. Soc. 1997,
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(14) Schneppensiepper, T.; Zahl, A.; van Eldik, R. Angew. Chem., Int. Ed.
2001, 40, 1678.
(15) Scheidt, W. R.; Reed, Ch. A. Chem. ReV. 1981, 81, 543.
(16) Toney, G. E.; Gold, A.; Savrin, J.; Haar, L. W.; Sangaiah, R.; Hatfield,
W. E. Inorg. Chem. 1984, 23, 4350.
(17) Nesset, M. J. M.; Cai, S.; Shokhireva, T. K.; Shokhirev, N. V.;
Jacobson, S. E.; Jayaraj, K.; Gold, A.; Walker, F. A. Inorg. Chem.
2000, 39, 532 and references cited therein.
(5) Wolak, M.; van Eldik, R. Coord. Chem. ReV. 2003, 230, 263 and
references cited therein.
(6) Theodoridis, A.; van Eldik, R. J. Mol. Catal. A: Chem. 2004, 224,
197.
(7) (a) Imai, H.; Yamashita, Y.; Nakagawa, S.; Munakata, H.; Uemori,
Y. Inorg. Chim. Acta 2004, 357, 2503. (b) Nakagawa, S.; Yashiro,
T.; Munakata, H.; Imai, H.; Uemori, Y. Inorg. Chim. Acta 2003, 349,
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(8) Hoshino, M.; Maeda, M.; Konishi, R.; Seki, H.; Ford, P. C. J. Am.
Chem. Soc. 1996, 118, 5702.
(9) (a) Franke, A.; Stochel, G.; Jung, Ch.; van Eldik, R. J. Am. Chem.
Soc. 2004, 126, 4181. (b) Laverman, L. E.; Wanat, A.; Oszajca, J.;
Stochel, G.; Ford, P. C.; van Eldik, R. J. Am. Chem. Soc. 2001, 123,
285.
(10) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 9568.
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references cited therein.
(19) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 6225 and references
cited therein.
(20) Boersma, A. D.; Goff, H. M. Inorg. Chem. 1982, 21, 581.
(21) Simonato, J.-P.; Pecaut, J.; Pape, L. L.; Oddou, J.-L.; Jeandey, C.;
Shang, M.; Scheidt, W. R.; Wojaczynski, J.; Wolowiec, S.; Latos-
Grazynski, L.; Marchon, J.-C. Inorg. Chem. 2000, 39, 3978.
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Grazynski, L.; Shang, M.; Scheidt, W. R. Inorg. Chem. 1998, 37, 2476.
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Gupta, G. P. Inorg. Chem. 1989, 28, 1591.
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1945.
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43, 5393.
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Chem. 1990, 29, 2741.
7718 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
Figure 1. Synthesis of (P8-)FeIII.
patterns observed in the reactions of NO with model five-
and six-coordinate iron(III) porphyrins with high-spin, low-
spin, and admixed intermediate-spin states of the central iron
atom, respectively, would help to establish the relationship
between the structure of the heme prosthetic group and its
reactivity toward NO in various heme proteins.
Experimental Section
Synthesis and Characterization of 1. Chemicals and solvents
employed for the synthesis of 1 were used as received unless
otherwise noted. Solvents were dried using standard procedures.
Column chromatography was performed on silica gel 60, 32-63
µm, 60 Å (MP Biomedicals). Standard H and 13C NMR spectra
1
were recorded on a Bruker Avance 300 spectrometer (Bruker
Analytische Messtechnik GmbH). Fast atom bombardment (FAB)
mass spectrometry was performed with Micromass Zabspec and
Varian MAT 311A machines. Electrospray ionization mass spectra
(ESIMS) were measured in the negative ion mode on an ESI-
FT-ICR-MS mass spectrometer (Fa. Agilent, ICR: APEX II, Fa.
Bruker Daltonics, 7 T magnet). Standard UV-vis spectra were
recorded on a Shimadzu UV-3102 PC UV-vis-NIR scanning
spectrophotometer. IR spectra (KBr pellets) were recorded with a
FT-IR IFS 88 infrared spectrometer (Bruker Analytische Messtech-
nik GmbH). Elemental analyses were carried out on a CHN-
Mikroautomat (Heraeus). Thin-layer chromatography was carried
out on E. Merck silica gel 60 F254 plates. Zinc(II) 54,104,154,204-
tetra-tert-butyl-52,56,152,156-tetrakis[2,2-bis(ethoxycarbonyl)ethyl]-
5,10,15,20-tetraphenylporphyrin (2) was synthesized as described
previously.28
In a continuation of our earlier mechanistic work on the
reactivity of iron porphyrins toward NO,5,6,9,14,26,27 we now
report on the synthesis and spectroscopic characterization
of a highly negatively charged water-soluble iron(III) por-
phyrin, (P8-)FeIII (1; compare Figure 1) and present the
results of detailed mechanistic studies on its interaction with
nitric oxide. Within this context, in addition to kinetic studies
on the binding and release of NO from the diaqua
(P8-)FeIII(H2O)2 species present in aqueous solution at pH
<9, the reactivity of a high-spin monohydroxo form of the
complex formed at higher pH is reported. Temperature and
pressure effects on the rates of NO binding and release
obtained from stopped-flow and laser flash photolysis
experiments at ambient and elevated pressure at low and high
pH enabled the determination of activation parameters (∆Hq,
∆Sq, and ∆Vq) for the studied reactions. Activation volumes
determined in these studies enabled the construction of
volume profiles for the binding of NO to the diaqua and
monohydroxo forms of the complex, respectively. On the
basis of these data, a feasible explanation for the significant
kinetic and mechanistic differences observed in the reactivity
of these two species is offered. The results are compared
with those reported for other water-soluble porphyrins and
discussed in reference to the relevant literature data on the
structure and reactivity of iron(III) porphyrins toward NO.
54,104,154,204-Tetra-tert-butyl-52,56,152,156-tetrakis[2,2-bis-
(ethoxycarbonyl)ethyl]-5,10,15,20-tetraphenylporphyrin (3). HCl
(6 M, 50 mL) was added to a solution of 2 (174 mg, 0.109 mmol)
in CH2Cl2 (50 mL), and the two layers were shaken vigorously.
The green (organic) layer was shaken once again with 2 M HCl
(50 mL) and twice with water (50 mL each). After neutralization
with a saturated NaHCO3 solution (50 mL) and a final washing
with brine (50 mL), the organic layer was dried with MgSO4 and
the solvent was removed under reduced pressure. The compound
was further cleaned by column chromatography (silica gel, CH2-
Cl2/ethyl acetate 19:1) and obtained as a purple powder. Yield: 160
1
mg (96%, 0.105 mmol). H NMR (300 MHz, CDCl3, rt): δ 8.88
(26) Franke, A.; Stochel, G.; Suzuki, N.; Higuchi, T.; Okuzono, K.; van
Eldik, R. J. Am. Chem. Soc. 2005, 127, 5360.
(27) Wolak, M.; van Eldik, R. J. Am. Chem. Soc. 2005, in press.
(28) Guldi, D. M.; Zilbermann, I.; Anderson, G.; Li, A.; Balbinot, D.; Jux,
N.; Hatzimarinaki, M.; Prato, M. Chem. Commun. 2004, 726.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7719

Jee et al.
(d, 4H, 3J ) 4.78 Hz, â-pyrr-H), 8.69 (d, 4H, 3J ) 4.8 Hz, â-pyrr-
H), 8.15 (d, 4H, 3J ) 8.1 Hz, o-Ar*-H; an asterisk indicates proton
resonances of the charge-carrying aryl ring), 7.76 (d, 4H, 3J ) 8.1
Hz, m-Ar*-H), 7.46 (s, 4H, m-Ar-H), 3.62 (m, 16H, OCH2CH3),
HClO4 or NaOH. The ionic strength (0.1 M) was adjusted with
NaClO4. Argon or nitrogen and gastight glassware were used for
the preparation and handling of deoxygenated solutions.
Measurements. pH measurements were performed on a Meth-
rom 623 pH meter. An NO electrode (World Precision Instruments
isolated nitric oxide meter, model ISO-NO) was used to determine
the concentration of NO gas in aqueous solution. The NO electrode
was calibrated with a freshly prepared KNO2 solution according
to the method suggested by the manufacturer. UV-vis spectra were
recorded in gastight cuvettes on a Shimadzu UV-2100 spectropho-
tometer equipped with a thermostated ((0.1 °C) cell compartment.
Kinetic Studies. (a) Laser Flash Photolysis. Laser flash
photolysis was carried out with the use of the LKS-60 spectrometer
from Applied Photophysics for detection and a Nd:YAG laser
(SURLITE I-10, Continuum) pump source operating in the third
harmonic (λexc ) 355 nm) (100-mJ pulses with ∼7-ns pulse widths).
Spectral changes at 427 and 432 nm (at pH 7 and 11, respectively)
were monitored using a 100-W xenon lamp, monochromator, and
photomultiplier PMT-IP22. The absorbance reading was balanced
to zero before the flash. Data were recorded on a digital storage
oscilloscope DSO HP 54522A and transferred to a computer unit
for subsequent analysis. Gastight quartz flow cuvettes and a pill-
box cell combined with a high-pressure unit were used at ambient
and elevated pressures (up to 170 MPa), respectively. In ambient-
pressure experiments, the deoxygenated solution of the iron
porphyrin was mixed in an appropriate volume ratio with the NO-
saturated solution, transferred to a gastight flow cuvette, and
equilibrated in a thermostated cell holder for 10 min. In the high-
pressure studies, deoxygenated solutions of iron porphyrin and NO
were mixed in an appropriate ratio with the use of gastight syringes,
transferred under an inert atmosphere to the pill-box cell, and
equilibrated for 10 min at the appropriate temperature and pressure
in the high-pressure cell compartment.
3
3
3.04 (t, 4H, J ) 7.8 Hz, CH), 2.80 (d, 8H, J ) 7.8 Hz, Ar-
3
CH2), 1.60 (s, 36H, CH3), 1.51 (s, 36H, CH3), 0.69 (t, 24H, J )
7.2 Hz, OCH2CH3). 13C NMR (75 MHz, CDCl3, rt): δ 168.4, 151.4,
150.5, 139.3, 138.7, 138.4, 134.5, 132.0, 130.1, 124.4, 123.6, 120.2,
115.2, 60.7, 52.2, 34.8, 33.6, 31.6, 31.5, 29.6, 13.4. MS (FAB,
NBA): m/z 1528 (M+.). IR (KBr): ν [cm-1] 3318, 2961, 2933,
2906, 2867, 1755, 1735, 1632, 1468, 1367, 1221, 1146, 1034, 807.
UV-vis (CH2Cl2): λ [nm] (ꢀ [L mol-1 cm-1]) 421 (4.58 × 105),
517 (1.98 × 104), 551 (8.1 × 103), 591 (6.4 × 103), 648 (4.3 ×
103). Anal. Calcd for C92H110N4O16: C, 72.32; H, 7.26; N, 3.67.
Found: C, 71.97; H, 7.31; N, 3.66.
Chloroiron(III) 54,104,154,204-Tetra-tert-butyl-52,56,152,156-
tetrakis[2,2-bis(ethoxycarbonyl)ethyl]-5,10,15,20-tetraphenylpor-
phyrin (4). A solution of FeCl2 (400 mg, 3.15 mmol) in ethanol
(30 mL) was added to a solution of 3 (343 mg, 0.225 mmol) in
CHCl3 and the mixture heated under reflux for 18 h. The solvent
was evaporated and the residue dissolved in CH2Cl2 and washed
with 6 M HCl. The organic layer was separated and washed twice
with water. After drying over MgSO4, the compound was cleaned
by column chromatography (silica gel, 19:1 CH2Cl2/ethyl acetate)
to give a dark green powder. Yield: 318 mg (89%, 0.201 mmol).
1H NMR (300 MHz, CDCl3, rt): δ 82.9 (br s, â-pyrr-H), 80.8 (br
s, â-pyrr-H), 15.8, 14.1, 13.3, 12.2 (br s, aryl-H). MS (FAB,
NBA): m/z 1582 (M+). IR (KBr): ν [cm-1] 2963, 2939, 2907,
2869, 1751, 1734, 1626, 1464, 1445, 1395, 1367, 1332, 1149, 1031,
997, 859, 803, 722. UV-vis (CH2Cl2): λ [nm] (ꢀ [L mol-1 cm-1])
422 (1.18 × 105), 509 (1.4 × 104), 583 (7.3 × 103). Anal. Calcd
for C92H108ClFeN4O16‚CH2Cl2: C, 65.62; H, 6.51; N, 3.29.
Found: C, 65.58; H, 6.68; N, 3.33.
Octasodium Hydroxoiron(III) 54,104,154,204-Tetra-tert-butyl-
52,56,152,156-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tet-
raphenylporphyrin (1-OH). NaOH (1.50 g, 37.5 mmol) was added
to a solution of 4 (200 mg, 0.126 mmol) in ethanol (20 mL), and
the reaction mixture was heated under reflux for 1 h. After cooling
to room temperature, the precipitate was filtered, washed with
ethanol (200 mL), and dried under reduced pressure. Gel permeation
chromatography (Sephadex LH20) in methanol und subsequent
precipitation with diethyl ether gave a dark brown powder, which,
according to the microanalysis, contains sodium hydroxide in the
lattice. Yield: 230 mg (83%, 0.105 mmol; based on the formula
obtained by microanalysis). 1H NMR (300 MHz, unbuffered D2O,
pD ) 13.4, rt): δ 82.7 (br s, â-pyrr-H), 13.2, 12.3 (br s, aryl-H).
ESIMS (MeOH/H2O): 1376.45 {[(P)FeIII(COOH)5(COO)2-
(COONa)]-}, 1354.46 {[(P)FeIII(COOH)6(COO)2]-}. IR (KBr): ν
[cm-1] 3429, 2963, 2924, 2854, 1594, 1445, 884, 805. UV-vis
(H2O, pH 11): λ [nm] (ꢀ [L mol-1 cm-1]) 417 (1.1 × 105), 532
(1.2 × 104). Anal. Calcd for C76H69FeN4Na8O17‚16NaOH: C, 41.68;
H, 3.91; N, 2.56. Found: C, 41.81; H, 4.04; N, 1.56.
Materials. NO gas (Messer Griesheim or Riessner Gase, g99.5
vol %) was cleaned from trace amounts of higher nitrogen oxides
by passing it through a concentrated KOH solution and an Ascarite
II column (NaOH on silica gel, Sigma-Aldrich). CAPS, MES, Tris,
and Bis-Tris buffers were purchased from Sigma-Aldrich. All other
chemicals used in this study were of analytical reagent grade.
Solution Preparation. All solutions were prepared from deion-
ized water. Buffered solutions of the appropriate pH for laser flash
photolysis and stopped-flow measurements were prepared with the
use of Tris (0.05 M), Bis-Tris (0.05 M), CAPS (0.05 M), and TAPS
(0.05 M) buffers. The desired pH was adjusted by the addition of
All kinetic experiments were performed under pseudo-first-order
conditions, i.e., with at least 10-fold excess of NO over the iron
porphyrin. Rate constants reported are the mean values of at least
five kinetics runs, and the quoted uncertainties are based on the
standard deviation.
(b) Stopped-Flow Measurements. Stopped-flow kinetic mea-
surements were performed on an SX 18.MV (Applied Photophysics)
stopped-flow apparatus. In a typical experiment, a deoxygenated
buffer solution was mixed in varying volume ratios with a NO-
saturated solution in a gastight syringe to obtain the appropriate
NO concentration (0.2-1.8 mM). The NO solution was then rapidly
mixed with deoxygenated iron(III) porphyrin in a 1:1 volume ratio
in a stopped-flow apparatus. The kinetics of the reaction was
monitored at 427 and 432 nm at pH 7 and 11, respectively. The
rates of NO binding and release (kon and koff) were determined from
slopes and intercepts of linear plots of kobs versus [NO], respectively,
as described in more detail in the Results and Discussion section.
The NO dissociation rates at different temperatures and pressures
were also measured directly by the NO-trapping method. This
involved rapid mixing of a (P8-)FeII(NO+)(L) solution (2 × 10-5
M; L ) H2O and OH- at pH 7 and 11, respectively) containing a
small excess of NO with aqueous solutions of [RuIII(edta)(H2O)]-
(1-2 mM) to give [RuIII(edta)NO]- and (P8-)FeIII(L), as evidenced
by the observed spectral change. The kinetics of NO release was
followed in a stopped-flow spectrophotometer at 427 nm (pH 7)
or 432 nm (pH 11). The first-order rate constants determined from
the kinetic traces were in acceptable agreement with those
determined from the intercepts of the plots of kobs versus [NO].
High-pressure stopped-flow experiments were performed at
pressures up to 130 MPa on a custom-built instrument described
7720 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
previously.29 The kinetic traces were analyzed with the use of the
OLIS KINFIT (Bogart, GA, 1989) set of programs.
by eq 3. The temperature dependence of ∆ωm was assumed to be
a simple reciprocal function A/T,31a-c where A was determined as
a parameter in the treatment of the line-broadening data. The
exchange rate constant is assumed to have a simple pressure
dependence given by eq 4, where kex0 is the rate constant for solvent
(c) 17O NMR Water-Exchange Measurements. Rate and
activation parameters for water exchange on the paramagnetic
(P8-)FeIII(H2O)2 complex and the corresponding activation param-
eters ∆Hqex, ∆Sqex, and ∆Vq were measured by use of the 17O
ex
kex ) 1/τm ) kex0 exp{(-∆Vqex/RT)P}
(4)
NMR line-broadening technique. Aqueous solutions of 1 (20 mM)
were prepared at pH 7 (0.05 M Bis-Tris buffer) and 11 (0.05 M
CAPS buffer), and 10% of the total sample volume of enriched
17O-labeled water (normalized 19.2% 17O H2O, D-Chem Ltd.) was
added to the solution.
exchange at atmospheric pressure. The pressure-dependent mea-
surements were performed at a temperature close to the optimal
exchange region [i.e., around the maximum of the plot of ln(1/T2r)
versus 1/T]. The reduced relaxation time T2r and the value of ∆ωm
(calculated using the value of A determined from the temperature
dependence and assumed to be pressure-independent31d) were
substituted into eq 2 to determine kex at each pressure. The resulting
plot of ln(kex) versus pressure was linear, and the volume of
activation was calculated directly from the slope (-∆Vexq/RT). The
A sample containing the same components except for 1 was used
as the reference. Variable-temperature and -pressure Fourier
transform 17O NMR spectra were recorded at a frequency of 54.24
MHz on a Bruker Advance DRX 400WB spectrometer. The
temperature dependence of the 17O line broadening was determined
in the range of 278-353 K. A homemade high-pressure probe30
was used for the variable-pressure experiments performed at pH 7
at the selected temperature (283 K) and in the pressure range of
1-150 MPa. The sample was placed in a standard 5-mm NMR
tube cut to a length of 45 mm. The pressure was transmitted to the
sample by a movable macor piston, and the temperature was
controlled as described elsewhere.30 The reduced transverse relax-
ation times (1/T2r) were calculated for each temperature and pressure
from the difference in the line widths observed in the presence and
absence of the metal complex (∆νobs - ∆νsolvent). The reduced
transverse relaxation time is related to the exchange rate constant
kex ) 1/τm (where τm is the mean-coordinated solvent lifetime) and
to the NMR parameters by the Swift and Connick equation (1),31a,b
0
value of kex obtained from the plot of ln(kex) versus P by
extrapolation to atmospheric pressure was in good agreement with
0
the corresponding value of kex from the temperature-dependent
measurements at ambient pressure.
In analogous temperature-dependent 17O NMR measurements
performed for the studied iron(III) porphyrin at pH 11 (0.05 M
CAPS buffer), the line widths observed in the presence and absence
of the metal complex changed little (6-13 Hz within the temper-
ature range of 278-353 K), indicating the absence of a significant
water-exchange process for the 1 form present at higher pH.
Because of the small observed effects (close to the experimental
error limits), the data obtained in the variable-temperature study
did not allow a reliable fit to the Swift and Connick equation.
1
T2r
1
Pm
) π (∆νobs - ∆νsolvent) )
Results and Discussion
2
T2m-2 + (T2mτm)-1 + ∆ωm
Synthesis of 1. Some of us recently reported the synthesis
of a water-soluble anionic zinc porphyrin (2 in Figure 1)
that carries eight carboxylates in four malonate units.28 The
precursor porphyrin 2 carries the malonates in the form of
ethyl esters, which renders the compound soluble in organic
but not in aqueous solvents. We choose 2 as a starting
material because it can be easily made from a bromometh-
ylated zinc porphyrin precursor32 by a typical malonate ester
alkylation protocol. By virtue of its easy demetalation with
concentrated hydrochloric acid, 2 is transformed into the free-
base porphyrin 3, as shown in Figure 1. Subsequent reaction
with ferrous chloride in methanol using lutidine as a proton
scavenger gives the neutral iron(III) porphyrin tetramalonic
1
τm
1
T2os
+
(1)
2
(T2m-1 + τm-1)2 + ∆ωm
{
}
where Pm is the mole fraction of water coordinated to the FeIII ion,
T
2os represents the outer-sphere contribution to T2r arising from long-
range interactions of unpaired electrons of FeIII with the water
outside the coordination sphere, T2m is the transverse relaxation
time of water in the inner coordination sphere in the absence of
chemical exchange, and ∆ωm is the difference in the resonance
frequency of 17O nuclei in the first coordination sphere of the metal
and in the bulk solvent. In the present system, the contributions of
1/T2m and 1/T2os to 1/T2r are negligible, so that eq 1 can be reduced
to eq 2. Taking into account that the temperature dependence of
2
1
ester system, 4. H NMR and UV-vis spectroscopic data
∆ωm
1
T2r
1
Pm
1
τ
) π (∆νobs - ∆νsolvent) )
(2)
2
show that 4 is a paramagnetic iron(III) porphyrin with a
single chloro axial ligand. The two â-pyrrole 1H resonances
at 82.9 and 80.8 ppm (half-width ∼ 300 Hz) indicate the
overall C2V symmetry of the porphyrin and prove the S ) 5/2
spin state.33,34 Saponification of the malonic esters of 4 with
sodium hydroxide in ethanol leads to precipitation of a
brownish material that is soluble in water but totally insoluble
in apolar solvents. Gel permeation chromatography (Sepha-
dex LH20) in methanol and subsequent precipitation with
diethyl ether gave a solid material containing 75% of 1 (with
the impurities being NaOH and H2O). Standard NMR spectra
m τm-2 + ∆ωm
{
}
k
ex is given by eq 3 (taken from transition-state theory), the NMR
kex ) 1/τm ) (kBT/h) exp{(∆Sqex/R) - (∆Hqex/RT)}
(3)
and kinetic parameters were calculated by the use of a nonlinear
least-squares method applied to eq 2, in which 1/τm was replaced
(29) (a) van Eldik, R.; Palmer, D. A.; Schmidt, R.; Kelm, H. Inorg. Chim.
Acta 1981, 50, 131. (b) van Eldik, R.; Gaede, W.; Wieland, S.; Kraft,
J.; Spitzer, M.; Palmer, D. A. ReV. Sci. Instrum. 1993, 64, 1355.
(30) Zahl, A.; Neubrand, A.; Aygen, S.; van Eldik, R. ReV. Sci. Instrum.
1994, 65, 882.
(31) (a) Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307. (b)
Swift, T. J.; Connick, R. E. J. Chem. Phys. 1964, 41, 2553. (c)
Bloembergen, N. J. J. Chem. Phys. 1957, 27, 595. (d) Newman, K.
E.; Meyer, F. K.; Merbach, A. E. J. Am. Chem. Soc. 1979, 101, 1470.
(32) Jux, N. Org. Lett. 2000, 2, 2129.
(33) Woon, T. C.; Shirazi, A.; Bruice, T. Inorg. Chem. 1986, 25, 3845.
(34) Cheng, R.-J.; Latos-Grazynski, L.; Balch, A. Inorg. Chem. 1982, 21,
2412.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7721

Jee et al.
tant gradual shift of the broad band centered at ca. 420 nm
to shorter wavelengths leads to the formation of a Soret band
at 400 nm (ꢀ ) 9.8 × 104 M-1 cm-1) at pH >5. Further
spectral changes, accompanied by clean isosbestic points at
360, 438, 514, and 555 nm, are observed in the pH 8-11
range. The nature of these spectral changes, i.e., shift of the
Soret band to 417 nm (ꢀ ) 1.1 × 105 M-1 cm-1) and
formation of peaks at 334 and 532 nm (ꢀ ) 1.2 × 104 M-1
cm-1), is analogous to that observed for other water-soluble
iron(III) porphyrins on deprotonation of coordinated water
at the Fe(III) center.35-39 The spectral features observed at
high pH are typical for that reported for monomeric mono-
hydroxo-coordinated iron(III) porphyrins in organic33,34 and
aqueous35-39 solvents. No further spectral changes occurred
in solution at high pH on extended standing, and those
observed in the pH 8-11 range were fully reversible and
indicated rapid interconversion of species present at high and
low pH upon changing of the pH. These observations
strongly suggest that the formation and hydrolysis of µ-oxo
dimers do not occur in the studied system, as was further
confirmed by NMR data reported below.
The pKa1 values characterizing the acid-base equilibria
in the studied system were determined by Specfit40 analysis
of the UV-vis spectra recorded in the pH 1-12 range. To
minimize experimental errors resulting from changes in
absorbance due to the observed precipitation of protonated
(H8P)FeIII in the pH 1-5 range, the results of four indepen-
dent titrations were subjected to the Specfit analysis. The
pKa values of 2.9 ( 0.8 and 4.4 ( 0.6 estimated for the low
pH range41a are ascribed to deprotonation of the carboxylic
acid groups on the porphyrin (compare simplified eq 5) on
the basis of their similarity with the pKa1 of carboxylic groups
in benzylmalonic acid (2.56 and 5.22),41b and the observed
changes in the solubility of 1 in this pH range. The pKa1
Figure 2. ORTEP diagram of [(P)Zn(THF)2] (2-THF) visualizing
structural features of the porphyrin ligand in 2. General crystallographic
data for 2-THF are reported in Table S1 of the Supporting Information.
measured in unbuffered D2O ([1] ) 1.5 × 10-2 M, pD )
13.4) have shown that under these conditions 1 exists as a
high-spin monohydroxo complex (P8-)FeIIIOH (â-pyrrole 1H
at 82.7 ppm; see also further text).
To visualize the structure of the porphyrin ligand in 1,
the crystal structure of its zinc precursor 2 crystallized from
THF is shown in Figure 2. As can be seen from the structure,
the zinc atom lies in the N4 porphyrin plane (average Zn-N
distance, 2.053 Å) and interacts weakly with oxygen atoms
of the two axially bound THF molecules (Zn-O distance,
2.419 Å). Two structural features important in terms of the
investigations presented in this report can be extracted from
this structure. First, the malonate groups are located above
and below the porphyrin plane, thus decreasing the possibility
of µ-oxo-dimer formation for steric and electrostatic reasons
(as confirmed by in-depth spectroscopic studies described
below). Second, the malonate groups cannot coordinate to
the metal center in an intramolecular fashion without extreme
distortion of the molecule (the average distance of the
malonate oxygen atoms from the zinc atom, ∼7 Å; the closest
distance, ∼4 Å). Nevertheless, ester or carboxylate groups
of malonate substituents may interact with axial ligands
coordinated to the metal center, as visualized in Figure S1
(in the Supporting Information) for the diaqua-ligated por-
phyrin (P8-)FeIII(H2O)2.
(H8P)FeIII a (P8-)FeIII + 8H+
2.5 < pKa(RCOOH) < 5.2
(5)
value of 9.26 ( 0.01 determined at higher pH is ascribed to
deprotonation of a water molecule in the (P8-)Fe(H2O)2 (1-
H2O) species present in solution at pH <9, according to eq
6. This conclusion is further supported by 1H NMR data and
(P8-)FeIII(H2O)2 a (P8-)FeIII(OH) + H3O+
pKa1 (6)
Speciation of 1 as a Function of pH. To establish the
nature of the iron porphyrin species present in aqueous
solution as a function of pH, a spectrophotometric titration
in the pH 1-12 range and 1H NMR measurements at selected
pD values (7 and 11) were performed for 1. Figure 3 shows
the UV-vis spectral changes observed during the spectro-
photometric titration of 1 at 0.1 M ionic strength (adjusted
with NaClO4). The corresponding plot of absorbance at 418
nm versus pH is shown in the inset of Figure 3b. As can be
seen from these data, the observed spectral changes can be
separated into two regions. In the pH 1-6 range, a continu-
ous increase in absorbance over the whole spectral range is
observed. This results mainly from an increase in the
solubility of the porphyrin complex due to deprotonation of
the carboxylic acid groups upon increasing pH. A concomi-
17O NMR water-exchange measurements performed for 1 at
pD 7 and 11. According to numerous literature reports, H
1
(35) Zipplies, M. F.; Lee, W. A.; Bruice, T. C. J. Am. Chem. Soc. 1986,
108, 4433.
(36) Tondreau, G. A.; Wilkins, R. G. Inorg. Chem. 1986, 25, 2745.
(37) Kobayashi, N. Inorg. Chem. 1985, 24, 3324.
(38) Miskelly, G. M.; Webley, W. S.; Clark, Ch. R.; Buckingham, D. A.
Inorg. Chem. 1988, 27, 3773.
(39) El-Awady, A. A.; Wilkins, P. C.; Wilkins, R. G. Inorg. Chem. 1985,
24, 2053.
(40) Binstead, R. A.; Jung, B.; Zuberbu¨hler, A. D. Specfit. 32, Spectrum
Software Associates, 2000.
(41) (a) Mean pKa values determined in four independent measurements.
(b) Kawassiades, C. Th.; Kouimtzis, Th. A.; Tossidis, J. A. Chem.
Chron. A 1968, 33, 1. The Handbook of Chemistry and Physics (76th
ed.; CRC Press: New York, 1995) reports the values pKa(1) ) 2.83
and pKa(2) ) 5.69.
7722 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
Figure 3. UV-vis spectral changes observed for aqueous solutions of (P8-)FeIII (1) in the pH 1-6 (a) and 6-12 (b) ranges. Inset: plot of absorbance at
418 nm versus pH. Experimental conditions: [1] ) 1 × 10-5 M, 25 °C, I ) 0.1 M (adjusted with NaClO4).
1
Table 1. â-Pyrrole H NMR Chemical Shiftsa and pKa1 Values of
NMR spectroscopy is a widely recognized method to
characterize (P)Fe(L)n (n ) 1 and 2) complexes in
Synthetic Water-Soluble Iron(III) Porphyrins
solution.16-19,24,42-45 The hyperfine shift patterns observed
for paramagnetic iron porphyrins were shown to strongly
depend on the spin and ligation states of the central iron
atom. In particular, the chemical shift of the â-pyrrole protons
in iron(III) porphyrins has proven to be an excellent probe
to determine the spin state of the iron(III) center.16-19,24,42-45
5
In the case of pure high-spin (S ) /2) iron(III) porphyrins,
the â-pyrrole proton signals are shifted downfield to δ g 80
ppm at 25 °C.18,19,33,34 In contrast, the pure intermediate spin
state (S ) 3/2) complexes exhibit â-pyrrole proton signals at
extremely upfield positions, i.e., at ca. -60 ppm.18 In the
case of admixed intermediate spin state complexes (S ) 5/2,
3/2), â-pyrrole resonances appear between these two extremes,
viz., +80 and -60 ppm. As can be seen from the data
summarized in Table 1, the â-pyrrole shifts observed for
selected water-soluble iron(III) porphyrins fall in the range
of 43-85 ppm. Chemical shifts of δ g 80 ppm, diagnostic
for monomeric monohydroxo-ligated iron porphyrins in
aqueous35,42,43 and nonaqueous18,33,34 solvents, clearly indicate
that these complexes exist in solution as purely high-spin
species. The more upfield signals (43-73 ppm) observed
for diaqua-ligated forms35,43,44 reflect a varying contribution
a Referenced to TMPS. NA ) not assigned. b Values from ref 53 unless
3
3
5
otherwise stated. c This work. d Two separate resonances result from
of S ) /2 in the admixed S ) /2, /2 spin system featuring
(P)Fe(H2O)2, in which the axial sites are occupied by two
weak field H2O ligands (see also the text below). The position
of the â-pyrrole proton resonances at ca. 47 ppm for (P8-)Fe
at pH 7 (compare Table 1 and Figure S2 in the Supporting
Information) is similar to that observed for the diaqua forms
of other negatively charged water-soluble iron(III) porphyrins
chemical inequivalency of â-pyrrole protons in P8- e Reference 7a.
.
f Reference 44. g The small peak at 33.4 ppm assigned to the hydroxo form
of (TPPS)FeIII in ref 44 is probably shifted upfield from the typical value
of ca. 80 ppm because of the exchange of diaqua, monohydroxo, and µ-oxo
dimers present in solution at pD 5-6. h Reference 35. i Reference 27.
j Reference 42. k Reference 43.
and differs remarkably from that observed for the monohy-
droxo species. A peak at ca. 83 ppm (Table 1 and Figure
S2b in the Supporting Information) appears, however, in the
1H NMR spectrum of (P8-)Fe at pD 11, indicating the
presence of the monohydroxo-coordinated form. In addition,
the lack of resonances in the spectral region of 13-14 ppm
(characteristic for â-pyrrole protons in µ-oxo-bridged
(42) (a) La, T.; Miskelly, G. M. J. Am. Chem. Soc. 1995, 117, 3613. (b)
La, T.; Miskelly, G. M.; Bau, R. Inorg. Chem. 1997, 36, 5321.
(43) Reed, R. A.; Rodgers, K. R.; Kushmeider, K.; Spiro, T. Inorg. Chem.
1990, 29, 2883.
(44) Ivanca, M. A.; Lappin, A. G.; Scheidt, W. R. Inorg. Chem. 1991, 30,
711.
(45) Walker, F. A. Proton and NMR spectroscopy of paramagnetic
metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: New York, 1999; Vol. 5.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7723

Jee et al.
Table 2. Rate Constants (at 25 °C) and Activation Parameters for Water-Exchange Reactions on a Series of Water-Soluble Iron(III) Porphyrins
iron(III) porphyrin
δâ-pyrrole (ppm)
Int %a
kex (s-1
)
∆Hqex (kJ mol-1
)
∆Sqex (J mol-1 K-1
)
∆Vqex (cm3 mol-1
)
1-H2Ob
ca. 46
43
24
26
20
6.8
(7.7 ( 0.1) × 106
(2.1 ( 0.1) × 107
(2.0 ( 0.1) × 106
(4.5 ( 0.1) × 105
61 ( 6
61 ( 1
67 ( 2
71 ( 2
+91 ( 23
+100 ( 5
+99 ( 10
+100 ( 6
+7.4 ( 0.4
+11.9 ( 0.3
+7.9 ( 0.2
+7.4 ( 0.4
c
(TMPS4-)Fe(H2O)2
c
(TPPS4-)Fe(H2O)2
52
c
(TMPyP4+)Fe(H2O)2
70.5
a Calculated according to eq 7. b This work. c Reference 14.
dimers18,34,44), shows unequivocally that the studied porphyrin
complex does not undergo dimerization to a µ-oxo dimer at
high pH.
of the data reported in Tables 1 and 2 is that the lability of
water coordinated to the iron(III) center apparently correlates
3
with the contribution of the intermediate S ) /2 spin state
Temperature- and pressure-dependent 17O NMR studies
performed at pH 7 (compare the Experimental Section and
Figure S3a,b in the Supporting Information) indicated that
the (P8-)Fe(H2O)2 form (1-H2O) present at this pH undergoes
a rapid water-exchange reaction. As can be seen from data
presented in Table 2, the rate constant measured for this
process (kex ) 7.7 × 106 s-1 at 25 °C) falls within the range
of kex values reported for diaqua-ligated forms of other water-
soluble iron(III) porphyrins. Significantly positive ∆Sqex and
∆Vqex observed for 1-H2O as well as for other diaqua-ligated
porphyrins indicate the operation of a dissociative (Id or D)
pathway for water exchange on (P)FeIII(H2O)2 complexes
studied to date. In contrast to this observation, variable-
temperature 17O NMR measurements performed for 1 at pH
11 indicated very small variation of the reduced relaxation
time for the bulk water signal (compare Figure S3c in the
Supporting Information). This strongly suggests that the
monohydroxo-ligated form of 1 exists in the five-coordinate
form (P8-)FeIII(OH) [possibly in equilibrium with a minor
fraction of the six-coordinate (P8-)FeIII(OH)(H2O) species].
in the admixed S ) /2, /2 spin system of (P)FeIII(H2O)2
complexes. This contribution can be estimated for iron(III)
complexes of meso-tetraarylporphyrins on the basis of the
â-pyrrole proton chemical shift (δ) according to eq 7.19 The
5
3
Int % ) [(80 - δ)/140] × 100 (%)
(7)
Int % values calculated from eq 7 for water-soluble porphy-
rins reported in Table 2 clearly show that an increasing
contribution of the S ) 3/2 state correlates with an increasing
labilization of the axial water molecules, as indicated by
higher rates of water exchange observed for these complexes.
Taking into account that the strong tetragonal distortion
resulting in long axial bonds is the representative feature of
3
the S ) /2 spin state in iron(III) porphyrins, the increasing
lability of axial H2O apparently reflects the lengthening of
5
3
the Fe-OH2 bond in spin-admixed S ) /2, /2 porphyrins
with increasing contribution of the S ) 3/2 spin state. It has
3
been shown that population of the S ) /2 spin state results
2
2
from destabilization of the dx -y orbital; i.e., as the energy
2
2
1
of the dx -y orbital increases, the ground state in iron(III)
porphyrins with weak field ligands (such as H2O) changes
It is concluded on the basis of the UV-vis, H NMR, and
17O NMR data reported above that the studied porphyrin
complex exists as 1-H2O in the pH 5-8 range and forms
the monomeric monohydroxo species (P8-)FeIII(OH) as a
predominant porphyrin form at pH >9. No evidence for the
formation of a dihydroxo species, (P8-)Fe(OH)2, or µ-oxo-
bridged dimers was obtained in the studied pH range, i.e.,
pH <13.
5
3
from mainly S ) /2 to mainly S ) /2. Hence, the larger
3
contribution of the S ) /2 state in negatively charged
porphyrins, in comparison to that observed for (Pn+)Fe(H2O)2,
2
2
apparently reflects destabilization of the dx -y orbital by
repulsive interactions with the increased electron density on
the pyrrole nitrogens. A similar effect has been reported for
water-insoluble ferric porphyrins with meso substituents of
varying electron-releasing ability.16 It can be expected that
the trend of increasing lability of axial positions observed
for water-soluble porphyrins included in Table 2 will also
be reflected in their reactivity toward NO. In fact, a
As can be seen from data summarized in Table 1, the pKa1
determined for 1-H2O is higher than the reported pKa1 values
characterizing deprotonation of a water molecule in other
water-soluble (P)FeIII(H2O)2 porphyrins. Such a high pKa1
apparently reflects the electronic effects of the negatively
charged meso substituents that increase the electron density
on the metal center (and thus disfavor the release of a proton
from coordinated water), as well as through-space interac-
tions of RCOO- groups with the axially coordinated water
molecules. In the latter case, the formation of hydrogen bonds
with deprotonated carboxylate groups of flexible malonate
substituents (compare Figure S1 in the Supporting Informa-
tion) is expected to stabilize coordinated water and the local
1+ charge of the [FeIII(Np)4]+ unit, thus increasing the pKa1
value. A similar influence of through-space interactions with
3
correlation of the S ) /2 spin admixture and the rates of
NO binding exists for diaqua-ligated (P)FeIII(H2O)2, as
discussed in detail in the following section.
Reactivity of 1-H2O toward NO. The addition of NO
gas to a deoxygenated solution of 1-H2O at pH <9 resulted
in the spectral changes presented in Figure 4. The decrease
in the absorbance at 400 nm accompanied by the appearance
of new bands at 427 (Soret, ꢀ ) 1.5 × 105 M-1 cm-1) and
540 nm indicates the formation of a typical low-spin iron-
(III) porphinatonitrosyl complex4,6 in which the formal charge
distribution can be described as (P8-)FeII(NO+)(H2O) (eq 8).
-
negatively charged SO3 groups increasing the pKa1 value
of coordinated water has been reported for another negatively
charged water-soluble ferric porphyrin, viz., (TanP4-)FeIII-
(H2O)2.7a
(P8-)FeIII(H2O)2 + NO {kon} (P8-)FeII(NO+)(H2O) + H2O
koff
KNO ) kon/koff (8)
An interesting observation that can be added on the basis
7724 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
Figure 4. Spectral changes resulting from NO binding to (P8-)FeIII(H2O)2. Inset: plot of (A - A0)-1 versus [NO]-1 (where A0 ) absorbance at 400 nm
at [NO] ) 0, A ) absorbance at 400 nm at a given NO concentration10). Experimental conditions: pH ) 7 (0.05 M Bis-Tris), I ) 0.1 M (NaClO4), 22 °C.
Bubbling of an inert gas through the resulting solution leads
to reversed spectral changes indicating reversibility of the
reaction, in agreement with eq 8. The combination of UV-
vis spectroscopy and amperometric detection of free NO in
solution allowed the determination of the thermodynamic
equilibrium constant KNO ) (1.5 ( 0.2) × 104 M-1 (compare
the inset in Figure 4).
The kinetics of the reversible binding of NO to 1-H2O at
pH 7 was studied by laser flash photolysis and stopped-flow
techniques for the “on” and “off” reactions, respectively.
Laser flash photolysis of (P8-)FeII(NO+)(H2O) solutions
resulted in transient spectral changes consistent with the
spectral difference between (P8-)FeII(NO+)(H2O) and (P8-)FeIII-
(H2O)2. In the presence of excess NO, the transient spectrum
decayed exponentially to the initial one. Thus, the processes
occurring in the laser flash experiment can be summarized
as follows.
trochemical measurements. To determine the activation
parameters ∆Sq, ∆Hq, and ∆Vq for the binding and release
of NO, the kinetics was studied at different temperatures (6-
30 °C) and hydrostatic pressures (0.1-170 MPa). The kon
and koff values determined from linear dependences of kobs
versus [NO] at each temperature and pressure allowed the
construction of Eyring plots for the “on” and “off” reactions
(compare Figure 5) and a linear plot of ln(kon) versus pressure
(Figure 6). Activation parameters calculated from the plots
are summarized in Table 3. Because of the small intercepts
in the plots of kobs vs [NO] in the pressure-dependent study,
the activation volume for the “off” reaction could not be
determined accurately in this way. This value, however, could
be measured in a stopped-flow experiment using the NO-
trapping method. Rapid mixing of a (P8-)FeII(NO+)(H2O)
solution with a large excess of [Ru(edta)(H2O)]- (an efficient
scavenger for NO)46 led to re-formation of the (P8-)FeIII(H2O)2
complex, as indicated by the observed spectral change. The
kinetics of the reaction was followed at 427 nm. The kinetic
traces gave good mathematical fits to a single-exponential
function, and the observed reaction rates did not depend on
[Ru(edta)(H2O)-] in the concentration range used in the NO-
trapping experiments (1-2 mM). These observations indicate
that the release of NO from (P8-)FeII(NO+)(H2O) follows
the reaction sequence in Scheme 1, in which koff is the rate-
determining step, such that the kobs values determined from
the kinetic traces equal koff.
The kinetics of the reaction was followed under pseudo-
first-order conditions with at least a 10-fold excess of NO.
As can be expected for the reversible process (eq 9), the
kobs values determined under such conditions by fitting the
kinetic traces to a single-exponential function depend linearly
on [NO] according to eq 10 (see Figure 5a). A linear fit of
As can be seen from the data in Table 3, the rate constants
obtained from variable-temperature NO-trapping measure-
ments are in reasonable agreement with those resulting from
the laser flash photolysis experiments. Small deviations of
the koff values obtained by these two techniques were,
however, observed, particularly at higher temperatures (a
similar tendency was also observed in measurements at pH
11; see further text). This presumably reflects a diminished
kobs ) kon[NO] + koff
(10)
kobs versus [NO] obtained at 24 °C allowed the determination
of kon ) (8.2 ( 0.1) × 105 M-1 s-1 and koff ) 217 ( 16 s-1
from the slope and intercept, respectively. The overall
equilibrium constant calculated from the kinetic data, KNO
) kon/koff ) (3.8 ( 0.2) × 103 M-1 (24 °C), is in reasonable
agreement with the corresponding thermodynamic value of
Keq determined from a combination of UV-vis and elec-
(46) Wanat, A.; Schneppensieper, T.; Karocki, A.; Stochel, G.; van Eldik,
R. J. Chem. Soc., Dalton Trans. 2002, 941.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7725

Jee et al.
Figure 5. (a) Plots of kobs versus [NO] for the reaction of 1-H2O with nitric oxide in the temperature range 6-30 °C measured by laser flash photolysis.
(b) Corresponding Eyring plots for the “on” and “off” reactions. Experimental conditions: [1-H2O] ) 2.0 × 10-5 M, pH 7 (0.05 M Bis-Tris), λirr ) 355
nm, λdet ) 417 nm, I ) 0.1 M (NaClO4).
Figure 6. (a) Plots of kobs versus [NO] for the reaction of 1-H2O with NO in the pressure range of 10-170 MPa. (b) Plot of ln(kon) versus pressure.
Experimental conditions: [1-H2O] ) 2.0 × 10-5 M, pH 7 (Bis-Tris, 0.05 M), λirr ) 355 nm, λdet ) 417 nm, I ) 0.1 M (NaClO4).
Table 3. Rate Constants and Activation Parameters Determined by
efficiency of [RuIII(edta)(H2O)]- as the NO trap at pH >6
Laser Flash Photolysis and Stopped-Flow (NO-Trapping Method)
because of its deprotonation to a less reactive [RuIII(edta)-
(OH)]2- form.46 Nevertheless, the similarity of the activation
parameters ∆Hq and ∆Sq determined from laser flash
Techniques for the Reversible Binding of NO to (P8-)FeIII(H2O)2 at pH
7
a
temp
(°C)
pressure
(MPa)
kon × 10-5
koff
koff
off
off
(M-1 s-1
)
(s-1
)
(s-1
)
photolysis and NO-trapping studies (Table 3) indicates that
the NO-trapping method provides a reliable mechanistic
description of the “off” reaction at pH 7. This technique was
therefore used to determine ∆Vq for the “off” reaction. The
stopped-flow NO-trapping measurements performed in the
pressure range of 0.1-130 MPa allowed the determination
6
10
12
18
24
30
10
2.0 ( 0.1
15 ( 1
10.5 ( 0.5
23.0 ( 0.5
31.0 ( 0.4
81 ( 2
3.4 ( 0.1
5.3 ( 0.2
8.2 ( 0.1
12.7 ( 0.1
2.7 ( 0.1
2.4 ( 0.1
2.2 ( 0.1
1.9 ( 0.1
1.7 ( 0.1
34 ( 2
96 ( 28
217 ( 16
494 ( 24
8.4 ( 0.2b
6.7 ( 0.1b
4.8 ( 0.3b
3.5 ( 0.2b
220 ( 2
432 ( 12
10
50
90
130
170
of ∆Vq ) +16.8 ( 0.4 cm3 mol-1.47
off
A comparison of the rate and activation parameters for
the binding and release of NO obtained for 1-H2O with those
reported in the literature for other water-soluble ferric
porphyrins (compare Table 4) shows that the reactivity of
the diaqa-ligated 1-H2O toward NO is similar to that
observed for other (Pn-)Fe(H2O)2 complexes. Although the
“on” and “off” rate constants reported for (TMPyP4+)FeIII-
(H2O)2 with positively charged meso substituents are mark-
∆Hq (kJ mol-1
∆Sq, (J mol-1 K-1
∆Vq (cm3 mol-1
)
51.1 ( 0.5
+40 ( 2
+6.1 ( 0.1
101 ( 2
+140 ( 7
107 ( 3
+160 ( 10
+16.8 ( 0.4
)
)
a Data obtained by the NO-trapping method using [RuIII(edta)(H2O)]-
(unbuffered solution, pH 7). b Data obtained by the NO-trapping method
at 3 °C.
(47) Plots of ln(koff/T) versus 1/T and ln(koff) versus pressure constructed
from data obtained in the NO-trapping experiments are shown in Figure
S4 in the Supporting Information.
edly smaller in comparison to those observed for (Pn-)Fe-
(H2O)2, the similarity of the activation parameters (in
7726 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
Scheme 1
Table 4. Comparison of Rate Constants and Activation Parameters for Binding and Release of NO for Water-Soluble Diaqua-Ligated Iron(III)
Porphyrins
iron(III) porphyrin
Int %
k at 25 °C
∆Hq (kJ mol-1
)
∆Sq (J mol-1 K-1
)
∆Vq (cm3 mol-1
)
k
on (M-1 s-1
)
1-H2O
24
26
20
7
8.2 × 105
3.8 × 106
5 × 105
51 ( 1
57 ( 3
69 ( 3
67 ( 3
+40 ( 2
+6.1 ( 0.1
+13 ( 1
(TMPS4-)Fe(H2O)2
+69 ( 11
+95 ( 10
+67 ( 11
a
(TPPS4-)Fe(H2O)2
+9 ( 1
a
(TMPyP4+)Fe(H2O)2
2.9 × 104
+3.9 ( 1.0
b
b
k
off (s-1
)
1-H2O
24
26
20
7
217
900
500
66
101 ( 2
84 ( 3
76 ( 6
113 ( 5
+140 ( 7
+94 ( 10
+60 ( 11
+169 ( 18
+16.8 ( 0.4
+17 ( 3
a
(TMPS4-)Fe(H2O)2
a
(TPPS4-)Fe(H2O)2
+18 ( 2
(TMPyP4+)Fe(H2O)2
+16.6 ( 0.2
a Data from ref 4. b Data from ref 6.
the FeII-NO+ bond (resulting in a positive contribution to
∆Vqoff) is in this case accompanied by a further volume
increase due to formal oxidation (FeII f FeIII) and spin-state
change (S ) 0 f S ) 5/2, 3/2) on the iron(III) center, as well
as solvent reorganization due to neutralization of the partial
charge on the FeII-NO+ unit.
It can also be seen from the data in Table 4 that the
increase in the rate of NO binding correlates directly with
the rising contribution of the intermediate S ) 3/2 spin state
5
3
(Int %) in the admixed S ) /2, /2 spin system of the (P)-
FeIII(H2O)2 complexes, as previously observed for the water-
exchange rates. This provides additional evidence that the
lability of the axial water controls to a large extent the
kinetics of NO binding. It can be further concluded from
data in Table 4 that the rate of NO release from a (P)FeII-
(NO+)(H2O) complex tends to increase with increasing
electron-donating influence of meso-porphyrin substituents
Figure 7. Volume profile for the reversible binding of nitric oxide to
(P8-)FeIII(H2O)2.
3
[reflected by the rising contribution of S ) /2 in the spin-
admixed state of (P)FeIII(H2O)2]. Although the correlation
of koff and Int % is not as clear as in the case of the NO-
binding rates, it suggests that the FeII-NO+ bond in (P)-
FeII(NO+)(H2O) is better stabilized by the positively charged
porphyrins than by the negatively charged ones. This is in
line with recent literature reports that, based on resonance
Raman and density functional theory (DFT) results, indicate
that a gradual increase of the electron density within the Fe-
NO unit induced by meso substituents destabilizes the Fe-
NO bond in the porphyrin {FeNO}6 nitrosyls.49
Reactivity of Monohydroxo-Ligated (P8-)Fe(OH) (1-
OH) toward NO. Spectroscopic and kinetic studies on the
reaction of 1 with NO performed at pH >9 indicated
differences in the kinetics of NO binding and release, as well
as in the nature of the nitrosyl species formed at high pH in
comparison to that observed at pH <9. The spectral changes
particular, ∆Sq and ∆Vq) shows that the mechanistic features
of the binding and release of NO are analogous for both
positively and negatively charged (P)FeIII(H2O)2 species.
Thus, the volume profile constructed from the activation
volumes, ∆Vq and ∆Vqoff, obtained for 1-H2O (Figure 7)
on
reflects a common reaction mechanism for the diaqua forms
of water-soluble porphyrins, in which the “on” reaction is
controlled by substitution of a water molecule according to
a dissociative (Id or D)48 mechanism, as evidenced by the
positive values of ∆Vq 4,6 The rate-determining substitution
.
on
step is followed by a large volume collapse due to a spin-
state change and solvent reorganization accompanying the
formation of the low-spin (P)FeII(NO+)(H2O) product. The
large positive volumes of activation typical for the “off”
reaction clearly indicate that the release of NO also follows
a dissociative mechanism. The rate-determining breakage of
(49) (a) Linder, D. P.; Rodgers, K. R.; Banister, J.; Wyllie, G. R. A.; Ellison,
M. K.; Scheidt, W. R. J. Am. Chem. Soc. 2004, 126, 14136. (b) Linder,
D. P.; Rodgers, K. R. J. Am. Chem. Soc. 2005, 44, 1367.
(48) (a) Helm, L.; Merbach, A. E. Chem. ReV. 2005, 105, 1923. (b) Richens,
D. T. Chem. ReV. 2005, 105, 1961.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7727

Jee et al.
Figure 8. (a) Spectral change accompanying the reaction of 1-OH with NO at pH 11. (b) Spectra of the product obtained in the reaction of 1 with NO in
buffered aqueous solutions in the pH 7-11 range. A plot of ∆Abs434 versus pH is shown in the inset. Experimental conditions: [1] ≈ 7 × 10-6 M, [NO]
) 1 mM, I ) 0.1 M (NaClO4). Buffers used: pH 7, Bis-Tris (0.05 M); pH 7.5-8.8, TAPS (0.05 M); pH 9.1, borate (0.05 M); pH 9.5-11, CAPS (0.05 M).
Figure 9. pH dependence of the rate constants for binding (a) and release (b) of NO from 1 determined from the slopes and intercepts of kobs versus [NO]
plots measured in buffered aqueous solutions in the pH 6.5-11.5 range. Experimental conditions: [1] ) 2.0 × 10-5 M, 10 °C, I ) 0.1 M (NaClO4).
that accompany the reaction of 1-OH with NO at pH 11
(Figure 8a) lead to a final spectrum in which the characteristic
peaks at 317, 432 (Soret, ꢀ ) 1.9 × 105 M-1 cm-1), and
544 nm occur at longer wavelengths as compared to those
observed at lower pH (viz., 310, 427, and 540 nm). The
spectral changes reported in Figure 8b show a gradual change
in the spectrum of the nitrosyl product obtained by the
reaction of the ferric porphyrin with NO in buffered aqueous
solutions as a function of pH in the range 7-11. A fit of the
plot of ∆Abs434 vs pH (inset in Figure 8b) allowed the
determination of pKa(NO) ) 9.1 for the pH-dependent equi-
librium associated with the observed spectral change. This
equilibrium is ascribed to deprotonation of a coordinated
water molecule in (P8-)FeII(NO+)(H2O) according to eq 11.
× 103 M-1 for the reversible binding of NO to 1-OH at pH
11. This value is similar to that derived kinetically from the
rate constants kon ) 5.1 × 104 M-1 s-1 and koff ) 14.9 s-1
(24 °C) determined from the dependence of kobs on [NO] at
pH 11, viz., KNO ) kon/koff ) (3.4 ( 0.1) × 103 M-1. A
comparison of kon and koff measured at pH 11 with those
obtained at pH 7 (kon ) 8.2 × 105 M-1 s-1 and koff ) 217
s-1) shows that both coordination and release of NO
decreases ca. 15 times at high pH. Figure 9 presents the pH
dependence of kon and koff values determined from kobs versus
[NO] plots in the pH 6.5-11.5 range. It can be seen from
these data that the rates of NO binding and dissociation
gradually decrease between pH 7.5 and 10.5 and become
constant at pH >11. The pKa value determined by fitting
the data in Figure 9a to a sigmoidal function, viz., 9.04 (
0.01, is close to the pKa1 ) 9.26 determined from a
spectrophotometric titration of 1, which characterizes depro-
tonation of a water molecule in 1-H2O (eq 6). This indicates
that the change in reactivity observed on increasing pH
(P8-)FeII(NO+)(H2O) a (P8-)FeII(NO+)(OH) + H+
pKa(NO) ) 9.1 (11)
Combined UV-vis and NO electrode measurements allowed
the determination of equilibrium constant KNO ) (4.8 ( 0.5)
7728 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
Scheme 2
activated in 1-OH. This difference is particularly evident on
comparing the volume profile in Figure 7 with that con-
structed for the reversible binding of NO to (P8-)FeIII(OH),
as shown in Figure 11. Notably, the latter profile is very
similar to that reported in the literature for reversible binding
of NO to the high-spin five-coordinate (P)FeIII(Cys) center
9a
in substrate-bound Cytochrome P450cam
.
The activation
volumes ∆Vq ) -7.3 cm3 mol-1 and ∆Vq ) +24 cm3
on
off
mol-1 determined for this ferric protein were interpreted in
terms of the mechanistic scheme outlined in eq 12, in which
rapid formation of an encounter complex,
reflects differences in the kinetics of NO binding to 1-H2O
and 1-OH present in solution at low and high pH, respec-
tively. An analogous sigmoidal fit of the less precise data in
Figure 9b (constructed on the basis of koff values obtained
by extrapolation of kobs versus [NO] plots to [NO] ) 0)
resulted in pKa ) 8.9 ( 0.1, which is also close to that
characteristic for the formation of the hydroxo-ligated
porphyrin (eq 6), as well as that estimated for the process
depicted in eq 11 (pKa(NO) ) 9.1). It is apparent from these
data that the presence of the hydroxo group decreases the
rate of NO binding to (P8-)FeIII, as well as the rate of NO
release from (P8-)FeII(NO+)(OH). The observed reactivity
pattern is summarized in Scheme 2, in which the rate
ka
98
(P)FeIII(Cys) + NO {kkD } {(P)FeIII(Cys)||NO}
-D
(P)FeII(Cys)(NO+) (12)
{(P)FeIII||NO}, is followed by the activation step involving
the formation of the Fe-NO bond.9a
In the above scheme, kD and k-D represent the rate
constants for the diffusion-limited formation and dissociation
of the encounter complex and ka is the rate constant for Fe-
NO bond formation. The relatively small negative value of
∆Vq ) -7.3 cm3 mol-1 observed for Cyt P450cam was
on
ascribed to an activation-controlled mechanism with an
“early” transition state, in which only partial (P)FeIII-NO
bond formation and a change in the spin state of the FeIII
center (S ) 5/2 f S ) 0) occurs on going from the encounter
complex to the transition state {(P)FeIII- - -NO}q. A large
constants for 1-H2O and 1-OH species are denoted by kH O
2
and kOH, respectively.
The observation that the rate of NO binding to (P8-)FeIII
decreases on going from a trans-diaqua to an hydroxo
complex is surprising in view of kinetic data on the
substitution behavior of such metal complexes in solution,
where in the majority of cases an increase in the reactivity
of the hydroxo form is observed as compared to the diaqua
species. Such a trend reflects the significant labilizing
influence of the hydroxo group, which facilitates substitution
of the ligand coordinated in the trans position to OH-.50 The
reversed reactivity pattern observed for 1-OH indicates that
binding of NO to 1-OH is no longer controlled by the lability
of the metal center (where, in such a case, a very fast
diffusion-controlled rate for NO coordination to this five-
coordinate iron(III) complex would be expected). Apparently,
other factors determine the rate of formation of the nitrosy-
lated species at high pH. This conclusion is further substanti-
ated by a detailed study of the reversible binding of NO to
1-OH at pH 11. Figures 10, S5, and S6 (in the Supporting
Information) and Table 5 report the results of stopped-flow
experiments performed at different temperatures and pres-
sures for the reaction of 1-OH with NO at pH 11. As can be
seen from the data summarized in Table 5, the activation
positive ∆Vq value of 24 cm3 mol-1 observed for the
off
backward reaction evidenced a subsequent large volume
collapse on going from the transition state to the low-spin
(P)FeII(NO+) product. The data obtained in the present study
for (P8-)FeIII(OH) strongly suggest an analogous reaction
mechanism, as summarized in Scheme 3. According to this
scheme, initial partial formation of the Fe-NO bond in the
transition state (accounting for the negative ∆Vq ) -6.1
on
cm3 mol-1) is followed by volume collapse (ca. 17 cm3
mol-1) due to complete formation of the FeII-NO+ bond, S
5
)
/ f S ) 0 spin change, and solvent contraction
2
accompanying partial charge transfer from NO to FeIII.
The mechanistic difference observed in the binding of NO
to 1-H2O and 1-OH is further reflected in the kon values
observed for the diaqua- and hydroxo-ligated species,
respectively. The ca. 15-fold decrease in the rate of NO
binding to the five-coordinate (or weakly six-coordinate)
hydroxo form implies that the formation of (P8-)FeII(NO+)-
(OH) is not controlled by the lability of the FeIII center but
rather by the enthalpy and entropy changes associated with
spin reorganization and structural rearrangements upon the
formation of the FeII-NO+ bond. This situation parallels that
previously observed in the reactions of five-coordinate Fe-
(II) hemes with CO, for which an analogous mechanistic
scheme was suggested. In these earlier studies,4 considerably
slower rates of CO coordination to (P)FeII as compared to
NO binding rates determined for the same complexes (which
approached the diffusion limit in water) and an associative-
activated mechanism were interpreted in terms of a signifi-
cant activation barrier associated with spin state/structural
parameters determined for NO binding to 1-OH, viz., ∆Hq
on
) 34.6 kJ mol-1, ∆Sq ) -39 J mol-1 K-1, and ∆Vq
)
on
on
-6.1 cm3 mol-1, are very different from that characterizing
NO binding to the diaqua forms of water-soluble porphyrins
given in Table 4. In particular, negative values of ∆Sqon and
∆Vq contrast the positive ones observed at low pH and
on
indicate a changeover in the mechanism of NO binding from
dissociatively activated (Id) in 1-H2O to associatively
(50) Cusanelli, A.; Frey, U.; Ritchens, D. T.; Merbach, A. E. J. Am. Chem.
Soc. 1996, 118, 5265 and references cited therein.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7729

Jee et al.
Figure 10. (a) Eyring plots of ln(k/T) versus 1/T obtained for the “on” and “off” reactions in stopped-flow variable-temperature studies on reversible
binding of NO to 1-OH. Experimental conditions: [1-OH] ) 2.0 × 10-5 M, pH 11 (0.05 M CAPS buffer), I ) 0.1 M (NaClO4), λdet ) 432 nm. (b) Plots
of ln(k) versus pressure determined for the same reaction by high-pressure stopped-flow measurements in the pressure range of 10-130 MPa. Experimental
conditions: [1-OH] ) 1.2 × 10-5 M, pH 11 (0.05 M CAPS buffer), I ) 0.1 M (NaClO4), λdet ) 430 nm, 1.5 °C.
Table 5. Rate Constants and Activation Parameters for Binding and
Release of NO from (P8-)FeIII(OH) at pH 11
a
b,c
temp
(°C)
pressure
(MPa)
kon × 10-4
koff
(s-1
koff
(s-1
(M-1 s-1
)
)
)
6
12
18
24
30
1.5
1.9 ( 0.1
2.6 ( 0.1
0.70 ( 0.02
2.2 ( 0.3
6.0 ( 0.5
14.9 ( 0.3
33.9 ( 0.7
2.7 ( 0.3
2.4 ( 0.2
1.8 ( 0.1
1.1 ( 0.3
0.86 ( 0.01
1.97 ( 0.02
5.1 ( 0.1
3.7 ( 0.1
5.1 ( 0.2
6.7 ( 0.1
1.67 ( 0.06
1.83 ( 0.03
2.06 ( 0.02
2.29 ( 0.05
11.4 ( 0.3
23.7 ( 0.3
10
50
90
0.16 ( 0.01d
0.12 ( 0.01d
0.09 ( 0.01d
0.06 ( 0.01d
130
∆Hq (kJ mol-1
∆Sq (J mol-1 K-1
∆Vq (cm3 mol-1
)
34.6 ( 0.4
-39 ( 1
-6.1 ( 0.2
107 ( 2
+136 ( 7
+17 ( 3
96 ( 1c
+97 ( 4c
)
)
+21.3 ( 0.4c
a Data obtained from intercepts of plots of kobs versus [NO]. b Data
obtained with the use of [RuIII(edta)(H2O)]- as the NO scavenger.
c Systematic deviations of the koff values from those determined as intercepts
of kobs versus [NO] plots point to a low efficiency of NO trapping by
Ru(edta) at pH 11. Therefore, the activation parameters obtained in
alternative measurements (Figure 10) are preferentially used for mechanistic
characterization of NO release at high pH. d Measured at 3 °C.
Figure 11. Volume profile for the reversible binding of nitric oxide to
(P8-)FeIII(OH).
spin-forbidden transition itself and/or to a considerable
structural reorganization required to enable the spin change.
Consideration of characteristic structural features reported
for five-coordinate high-spin, six-coordinate spin-admixed,
and low-spin (P)FeIII(L)n porphyrins, respectively, suggests
that rather minor structural changes are required for the
formation of low-spin (P)FeII(NO+)(H2O) from spin-admixed
1-H2O species, where in both complexes the iron atom lies
in the porphyrin plane, is six-coordinate, and exhibits short
equatorial Fe-Np bonds. In contrast, the binding of NO to
1-OH presumably involves a change in the coordination
number of the Fe(III) atom, its movement into the porphyrin
plane, and contraction of the Fe-Np equatorial bond; i.e., a
considerable structural change (and thus higher energy
barrier) is expected to occur upon going from the substrate
to product.
changes upon coordination of CO (which, however, was
absent in the reactions with NO). This conclusion was further
substantiated by recent DFT computations performed for the
(P)Fe(II) + CO system.51 The decrease in kon observed in
the present case for NO coordination to the Fe(III) porphyrin
complex 1 at high pH is ascribed to a larger intrinsic
activation barrier for the S ) 5/2 f S ) 0 spin state change,
which occurs upon the formation of (P8-)FeII(NO+)(OH)
from the purely high-spin hydroxo species 1-OH, as com-
pared to that associated with the (S ) 5/2, 3/2 f S ) 0) spin
change occurring for the diaqua-ligated spin-admixed 1-H2O
complex. As pointed out in recent mechanistic studies on a
spin-forbidden proton-transfer reaction in solution,52 such an
increased “intersystem barrier” can be related to a
An additional contribution to the observed decrease in kon
at high pH may arise from a difference in the reducibility of
the FeIII center in (P)FeIII(H2O)2 and (P)FeIII(OH). As
(51) Harvey, J. N. J. Am. Chem. Soc. 2000, 122, 12401.
(52) Shafirovich, V.; Lymar, S. V. J. Am. Chem. Soc. 2003, 125, 6547.
7730 Inorganic Chemistry, Vol. 44, No. 22, 2005

Reaction of Nitric Oxide with a Iron(III) Porphyrin Complex
Scheme 3
indicated by literature data, deprotonation of a water
molecule in (P)FeIII(H2O)2 species shifts the potentials of FeIII
reduction to more negative values in comparison to that
observed for diaqua-ligated forms.53,54 Because the formation
of the FeIII-NO bond is accompanied by charge transfer from
NO and formal reduction of the FeIII center, a decrease in
the rate of this process can be expected for less easily
reducible hydroxo-ligated species in comparison to the (P)-
FeIII(H2O)2 counterparts.
nated water in 1-H2O to the highest value reported to date.
This is ascribed to through-space interactions of negatively
charged substituents with coordinated water, as well as to
electronic effects. The latter effects are reflected in partial
2
2
depopulation of the iron dx -y atomic orbital, leading to ca.
26% contribution of the S ) 3/2 spin state in the spin-admixed
(P8-)FeIII(H2O)2 system. This contribution is relatively high
in comparison with those of other (P)FeIII(H2O)2 species and
correlates with the high lability of coordinated water, as is
indicated by rapid water-exchange and NO coordination rates
observed for the 1-H2O complex. The solution pH determines
the nature of the axial ligands in 1 and thereby controls the
coordination number, spin state, and reactivity of the iron-
(III) center toward NO. The predominantly five-coordinate,
purely high-spin 1-OH formed at pH >9 binds NO according
to an associative mechanism, in contrast to a dissociatively
activated process observed for six-coordinate 1-H2O at lower
pH. In the case of 1-OH, however, the NO binding step is
no longer controlled by the lability of the metal center. Other
(most probably electronic and structural) factors involved
in the rate-limiting Fe-NO bond formation and breakage
(for the “on” and “off” reactions, respectively) apparently
account for the slower rate of NO binding and release
observed at high pH for 1-OH. Correlations between spin/
ligation states and reactivity inferred on the basis of
spectroscopic and kinetic data for other water-soluble iron-
(III) porphyrins studied to date further support this conclu-
sion. As suggested by preliminary studies on other water-
soluble iron(III) porphyrins, the pH-reactivity pattern reported
here for water-exchange and reversible NO binding to 1 is
a common phenomenon for diaqua- and monohydroxo-
ligated (P)FeIII species in aqueous media. Because of the
potential significance of this observation for studies on NO
interactions with heme proteins (in which a variety of spin
and ligation states are observed), the mechanistic conclusions
inferred here will be supplemented and verified by further
studies that will be reported in a subsequent paper.
In view of the fact that the spin changes (and the
accompanying structural changes) are clearly involved in the
rate-determining step for the “off” reaction (i.e., breakage
of the FeII-NO+ bond) in both (P8-)FeII(NO+)(H2O) and
(P8-)FeII(NO+)(OH), it is suggested that the decreased
reaction rate observed for the release of NO at high pH
reflects a larger demand for reorganization of d electrons
upon re-formation of 1-OH from the corresponding nitrosyl
5
complex (i.e., S ) 0 f S ) /2) in comparison to that
5
3
occurring for (P8-)FeII(NO+)(H2O) (S ) 0 f S ) /2, /2).
Taken together, the kinetic and mechanistic data described
above provide strong evidence that the rate of NO binding
and release observed for hydroxo-ligated species is to a large
extent controlled by the degree of spin reorganization
occurring at the FeIII center on going from reactants to
products. Notably, the kon and koff rates determined in the
present study for 1-OH (viz., kon ) 5.1 × 104 M-1 s-1 and
koff ) 14.9 s-1 at 24 °C) are quite similar to those reported
for the (TMPyP4+)Fe(H2O)2, which is almost purely high
spin (Int % ) 7%, kon ) 2.9 × 104 M-1 s-1, koff ) 66 s-1 at
25 °C; compare Table 4), but considerably smaller than those
observed for (Pn-)Fe(H2O)2 species exhibiting a relatively
large contribution of the S ) 3/2 spin state (ca. 26%), further
supporting the above conclusion. The observations on the
influence of the spin and ligation states of the FeIII center
on the dynamics of the reaction with NO are important to
understand kinetic and mechanistic factors governing the
interactions of NO with naturally occurring heme proteins,
in which a variety of spin and ligation states are observed.
This stimulated further in-depth mechanistic studies on the
reversible binding of NO to model (P)FeIII(L)n complexes
with different types of porphyrin and axial ligands, which
will be described in a subsequent report.
Acknowledgment. The authors gratefully acknowledge
financial support from the Deutsche Forschungsgemeinschaft
within SFB 583 “Redox-active metal complexes”, Fonds der
Chemischen Industrie, and the DAAD/KGN exchange pro-
gram.
Conclusions
According to the data reported in this study, introduction
of four flexible malonate substituents on the highly negatively
charged iron(III) porphyrin 1 increases the pKa1 of coordi-
Supporting Information Available: Figures S1-S7, Table S1,
and X-ray crystallographic data for 2 (CIF format). This material
is available free of charge via the Internet at http://pubs.acs.org.
(53) Batinic-Haberle, I.; Spasojevic, I.; Hambright, P.; Benov, L.; Crumbliss,
A. L.; Fridovich, I. Inorg. Chem. 1999, 38, 4011.
(54) Liu, M.; Su, Y. O. J. Electroanal. Chem. 1998, 452, 113.
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