Kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to aquated iron(II). An undergraduate text book reaction revisited

Article abstract of DOI:10.1021/ic010628q

A detailed kinetic and mechanistic analysis of the classical brown-ring reaction of [Fe(H₂O)₆]²⁺ with NO was performed using stopped-flow and laser flash photolysis techniques at ambient and high pressure. The kinetic parameters for the on and off reactions at 25 °C were found to be k_on = 1.42 × 10⁶ M^-1 s^-1, ΔH?_on = 37.1 ± 0.5 kJ mol^-1, ΔS?_on = -3 ± 2 J K^-1 mol^-1, ΔV?_on = +6.1 ± 0.4 cm³ mol^-1, and k_off = 3240 ± 750 s^-1, ΔH?_off = 48.4 ± 1.4 kJ mol^-1, ΔS?_off = -15 ± 5 J K^-1 mol^-1, ΔV?_off = +1.3 ± 0.2 cm³ mol^-1. These parameters suggest that both reactions follow an interchange dissociative (l_d) ligand substitution mechanism, which correlates well with the suggested mechanism for the water exchange reaction on [Fe(H₂O)₆]²⁺. In addition, Moessbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H₂O)₅(NO)]²⁺. The Moessbauer and EPR parameters closely resemble those of the {FeNO}⁷ units in any of the other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of high-spin Fe^III antiferromagnetically coupled to NO^- (S = 1) yielding the observed spin quartet ground state (S = 3/2), i.e., [Fe^III(H₂O)₅(NO^-)]²⁺, and not [Fe^I(H₂O)₅(NO⁺)]²⁺ as usually quoted in undergraduate text books.

Full text of DOI:10.1021/ic010628q

Inorg. Chem. 2002, 41, 4−10
Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of
Nitric Oxide to Aquated Iron(II). An Undergraduate Text Book Reaction
Revisited
Alicja Wanat,†,‡ Thorsten Schneppensieper, Gra z3 yna Stochel,* Rudi van Eldik,* Eckhard Bill, and
†
,‡
,†
§
Karl Wieghardt§
Institute for Inorganic Chemistry, UniVersity of Erlangen-N u¨ rnberg, Egerlandstrasse 1,
9
1058 Erlangen, Germany, Department of Inorganic Chemistry, Jagiellonian UniVersity,
Ingardena 3, 30-060 Krakow, Poland, and Max-Planck-Institut f u¨ r Strahlenchemie,
Stiftstrasse 34-36, 45470 M u¨ lheim an der Ruhr, Germany
Received June 13, 2001
2+
A detailed kinetic and mechanistic analysis of the classical “brown-ring” reaction of [Fe(H
2 6
O) ] with NO was
performed using stopped-flow and laser flash photolysis techniques at ambient and high pressure. The kinetic
6
-1 -1
q
parameters for the “on” and “off” reactions at 25 °C were found to be kon ) 1.42 × 10 M s , ∆H on ) 37.1
-
1
q
-1
-1
q
3
-1
-1
q
±
)
0.5 kJ mol , ∆S on ) −3 ± 2 J K mol , ∆V on ) +6.1 ± 0.4 cm mol , and koff ) 3240 ± 750 s , ∆H off
48.4 ± 1.4 kJ mol , ∆S off ) −15 ± 5 J K mol , ∆V off ) +1.3 ± 0.2 cm mol . These parameters suggest
) ligand substitution mechanism, which correlates well with
O) ] . In addition, M o¨ ssbauer spectroscopy
6
5
O) (NO)] . The M o¨ ssbauer and EPR
parameters closely resemble those of the {FeNO} units in any of the other well-characterized nitrosyl complexes.
-
1
q
-1
-1
q
3
-1
that both reactions follow an interchange dissociative (I
the suggested mechanism for the water exchange reaction on [Fe(H
and EPR measurements were performed on the reaction product [Fe(H
d
2
+
2
2+
2
7
III
It is concluded that its electronic structure is best described by the presence of high-spin Fe antiferromagnetically
coupled to NO (S ) 1) yielding the observed spin quartet ground state (S ) / ), i.e., [Fe (H O) (NO )] , and
2 2 5
-
3
III
- 2+
I
+ 2+
2 5
not [Fe (H O) (NO )] as usually quoted in undergraduate text books.
Introduction
During undergraduate training in chemistry, the interaction
of NO with aquated Fe is a reaction used as a spot test
brown ring test) for nitrate in introductory freshman
2
+
The reaction of nitric oxide (NO) with transition metal
ions plays an important role in environmental and biological
processes. This is partly related to the unique redox behavior
of NO as a result of its radical character in the ground state.
This results in the tendency of NO to bind to metal centers
(
laboratory courses. The details of the underlying reaction
mechanism, with respect to the processes involved in the
II
formation of the characteristic green-brown color of the Fe -
+
-
NO species, have not been well understood in the past. We
either as NO or as NO , such as in metmyoglobin where
III
II
+ 1
have in recent work concentrated on the interaction of NO
the Fe center binds NO in the form of Fe -NO , or in
II
II
with a series of polyaminecarboxylate complexes of Fe as
reduced vitamin B12 where Co binds NO in the form of
III
- 2
efficient scavengers for NO in the treatment of gaseous
Co -NO , respectively. Such interactions demonstrate the
4
,5
effluents of power plants. In these reactions, NO is
biological importance of NO, which was announced “mol-
ecule of the year” in 1992.
II
3
reversibly bound by Fe complexes in aqueous solution
according to the overall reaction given in eq 1. The
II
*
Authors to whom correspondence should be addressed. E-mail:
displacement of coordinated water on Fe (L) is characterized
vaneldik@chemie.uni-erlangen.de (R.v.E.); stochel@chemia.uj.edu.pl (G.S.).
†
University of Erlangen-N u¨ rnberg.
Jagiellonian University.
Max-Planck-Institut f u¨ r Strahlenchemie.
‡
(3) Culotta, E.; Koshland, D. E., Jr. Science 1992, 258, 1862.
(4) Schneppensieper, T.; Finkler, S.; Czap, A.; van Eldik, R.; Heus, M.;
Nieuwenhuizen, P.; Wreesmann, C.; Abma, W. Eur. J. Inorg. Chem.
2001, 491.
§
(1) Laverman, L. E.; Wanat, A.; Oszajca, J.; Stochel, G.; Ford, P. C.; van
Eldik, R. J. Am. Chem. Soc. 2001, 123, 285.
(2) Wolak, M.; Stochel, G.; Zahl, A.; Schneppensieper, T.; van Eldik, R.
J. Am. Chem. Soc. 2001, 123, 9780.
(5) Schneppensieper, T.; Wanat, A.; Stochel, G.; Goldstein, S.; Meyerstein,
D.; van Eldik, R. Eur. J. Inorg. Chem. 2001, 2317.
4
Inorganic Chemistry, Vol. 41, No. 1, 2002
10.1021/ic010628q CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/29/2001

ReWersible Binding of Nitric Oxide to Aquated Iron(II)
II
with sophisticated theoretical calculations provide compelling
[
Fe (L)H O] + NO h [Fe(L)NO] + H O
k , k , K
on off NO
2
2
7
evidence that the {FeNO} unit with spin quartet ground state
(1)
3
(S
t
) /
2
) in the structurally well characterized iron nitrosyl
complexes is best described by a bonding model based on
by the rate constant kon, whereas the reverse displacement
of coordinated NO by water is characterized by the rate
constant koff. The overall binding constant KNO represents
the ratio of the rate constants for the “on” and “off” reactions,
i.e., KNO ) kon/koff. In our studies, a systematic variation of
III
-
high-spin Fe antiferromagnetically coupled to NO (S )
). The conclusion is based on a vast body of consistent data
1
derived from EXAFS and XAS Fe-edge measurements,
absorption and MCD spectroscopy, resonance Raman spec-
troscopy, EPR and magnetic susceptibility measurements,
and applied-field M o¨ ssbauer investigations on complete
2
L resulted in KNO values that varied between 1 × 10 (in the
6
-1
4-
absence of L) and 1 × 10 M (for chelates such as edta
6
/7/8
3-
II
series of isostructural complexes containing the {FeNO}
motif.
and hedtra ). The tendency of Fe (L) to reversibly bind NO
correlated directly with the oxygen sensitivity of the Fe
2
2,23
II
A number of reviews on the reactivity of this
complex, and NO chemistry in general, have been pub-
complexes, suggesting that Fe(L)NO is stabilized in the form
2
4
III
-
lished.
of Fe (L)(NO ) similar to that found for the binding of
III
-
4,5
dioxygen, viz., Fe (L)(O
mination of the kinetic activation parameters have assisted
the clarification of the underlying reaction mechanisms at a
2
). These studies and the deter-
The kinetics of the complex-formation reaction using
6
25
T-Jump techniques was reported by Chang et al. and
2
6
5
-1 -1
Weinstock et al. with kon ) 7.1 × 10 M s , koff ) 1.5
3
-1 11
5
-1 -1
3
molecular level.
× 10 s , and kon ) 6.2 × 10 M s , koff ) 1.4 × 10
-1
II
s
at 25 °C, respectively. The equilibrium constant, deter-
In the absence of L, the reaction product is [Fe (H
2
O)
5
-
2+
7
mined from the kinetic data resulted in K ) k /k ) 470
NO] , i.e., {Fe-NO}, which can formally be stabilized as
NO
on off
-1
I
+
2+
III
-
2+
and 450 M , respectively. The accuracy of these parameters
could recently be confirmed by using flash-photolysis
techniques, resulting in somewhat higher values for the “on”
2 5 2 5
either [Fe (H O) (NO )] or [Fe (H O) (NO )] . The
II
reversible reaction of aquated Fe salts with NO has been
the topic of numerous investigations dating back as far as
the late 19th century. Since 1906, the first quantitative
investigations dealing with the binding of NO to ferrous
complexes, especially from the work by Manchot et al. and
6
-1 -1
and “off” reactions, with kon ) 1.6 × 10 M
s
and koff )
3
-1
3
.2 × 10 s at 25 °C, which results in a KNO value of 500
-1 5
(
140 M . The equilibrium constant measured directly with
7
the aid of an NO sensitive electrode is slightly higher than
others, were published. Much work has also been done on
the values obtained from the kinetic investigations, viz., KNO
the decomposition products of the brown-ring compounds
3
-1
4
8
) 1.2 × 10 M at 25 °C. Although the rate constants for
the “on” and “off” reactions are known, no mechanistic
conclusions could be drawn from these data due to the
in acidic, neutral, and alkaline solutions, which was reviewed
9
by Tarte. Rather conflicting results concerning the binding
mode of NO to the metal center coming from magnetic
susceptibility, ESR, IR, M o¨ ssbauer and other measurements
were published later, from which the reaction product was
q
absence of the corresponding activation parameters (∆H ,
q
q
∆
S , and ∆V ) for these reactions.
I
+ 2+ 10
assigned to be [Fe (H
2
O)
5
(NO )] . However, recent spec-
In this paper we present a complete kinetic and mechanistic
troscopic studies on non-heme iron nitrosyl centers in
analysis of this classical reaction. The employed kinetic
techniques enable detailed insight into the nature of the
1
1-17
18-23
proteins
and inorganic complexes
in conjunction
(
6) Schneppensieper, T.; Wanat, A.; Stochel, G.; Zahl, A.; van Eldik, R.
Manuscript in preparation.
(17) Orville, A. M.; Chen, V. J.; Kriauciunas, A.; Harpel, M. R.; Fox, B.
G.; M u¨ nck, E.; Lipscomb, J. D. Biochemistry 1992, 31, 4602.
(18) (a) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339.
(b) Feltham, R. D.; Enemark, J. H. In Topic in Inorganic and
Organometallic Stereochemistry; Geoffroy, G. L., Ed.; Topics in
Stereochemistry 12; Allinger, N. L., Eliel, E. L., Series Eds.; Wiley:
New York, 1981; p 155. (c) Westcott, B. L.; Enemark, J. H. In
Inorganic Electronic Structure and Spectroscopy; Solomon, E. I.,
Lever, A. B. P., Eds.; Wiley: New York, 1999; Vol. II, p 403.
(19) Wells, F. V.; McCann, S. W.; Wickman, H. H.; Kessel, S. L.;
Hendrickson, D. N.; Feltham, R. D. Inorg. Chem. 1982, 21, 2306.
(20) Feig, A.; Bantista, M. T.; Lippard, S. J. Inorg. Chem. 1996, 35, 6892.
(21) Pohl, K.; Wieghardt, K.; Nuber, B.; Weiss, J. J. Chem. Soc., Dalton
Trans. 1987, 187.
(
7) (a) Manchot, W.; Zechentmayer, K. Annalen 1906, 350, 368. (b)
Manchot, W. Annalen 1910, 372, 179. (c) Manchot, W. Ber. Dtsch.
Chem. Ges. 1914, 47, 1601. (d) Schlesinger, H. I.; van Valkenburgh,
H. B. J. Am. Chem. Soc. 1929, 51, 1323 and literature cited therein.
8) (a) Dunstan; Dymond. J. Chem. Soc. 1887, 51, 646. (b) Usher. Z.
Phys. Chem. 1908, 62, 622. (c) Divers; Hager. J. Chem. Soc. 1885,
(
47, 361. (d) Huffner. Z. Phys. Chem. 1907, 59, 416. (e) Kohlschutter;
Sazanoff. Ber. Dtsch. Chem. Ges. 1911, 44, 1423.
9) Tarte, E. J. Ind. Chim. Belge 1952, 17, 42.
(
(
10) (a) Gray, H. B.; Bernal, I.; Billig, E. J. Am. Chem. Soc. 1962, 84,
3404. (b) Griffith, W. P.; Lewis, J.; Wilkinson, G. J. Chem. Soc. 1958,
3993. (c) Ogura, K.; Watanabe, M. J. Inorg. Nucl. Chem. 1981, 43,
1239. (d) Burlamacchi, L.; Martini, G.; Tiezzi, E. Inorg. Chem. 1969,
8, 2021. (e) Mosbæk, H.; Poulsen, K. G. Acta Chem. Scand. 1971,
25, 2421.
(22) (a) Brown, C. A.; Pavlosky, M. A.; Westre, T. E.; Zhang, Y.; Hedman,
B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1995, 117,
715. (b) Westre, T. E.; Di Cicco, A.; Filipponi, A.; Natoli, C. R.;
Hedman, B.; Solomon, E. I.; Hodgson, K. O. J. Am. Chem. Soc. 1994,
116, 6757.
(
(
(
(
(
(
11) Arciero, D. M.; Lipscomb, J. D.; Huynh B. H.; Kent, T. A.; M u¨ nck,
E. J. Biol. Chem. 1983, 258, 14981.
12) Nocek, J. M.; Kurtz, D. M., Jr.; Sage, J. T.; Xia, Y.-M.; Debrunner,
P.; Shiemke, A. K.; Sanders-Loehr, T. M. Biochemistry 1988, 27, 1014.
13) Rodriguez, J. H.; Xia, Y.-M.; Debrunner, P. J. Am. Chem. Soc. 1999,
(23) Hauser, C.; Glaser, T.; Bill, E.; Weyherm u¨ ller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2000, 122, 4352.
(24) (a) McCleverty, J. A. Chem. ReV. 1979, 79, 53. (b) Connelly, N. G.
Inorg. Chim. Acta, ReV. 1972, 47. (c) Caulton, K. G. Coord. Chem.
ReV. 1975, 14, 317. (d) Bottomley, F. Acc. Chem. Res. 1978, 11, 158.
(e) Johnson, B. F. G.; McCleverty, J. A. Prog. Inorg. Chem. 1966, 7,
277. (f) Feelisch, M.; Stamler, J. S. Methods in Nitric Oxide Research;
John Wiley & Sons: Chichester, 1996.
121, 7846.
14) Bill, E.; Bernhardt, F.-H.; Trautwein, A. X.; Winkler, H. Eur. J.
Biochem. 1985, 147, 177.
15) Haskin, C. J.; Ravi, N.; Lynch, J. B.; M u¨ nck, E.; Que, L., Jr.
Biochemistry 1995, 34, 11090.
16) Chen, V. J.; Orville, A. M.; Harpel, M. R.; Frolik, C. A.; Sureros, K.
M u¨ nck, E.; Lipscomb, J. D. J. Biol. Chem. 1989, 264, 21677.
(25) Littlejohn, D.; Chang, S. G. J. Phys. Chem. 1982, 86, 537.
(26) Kustin, K.; Taub, I. A.; Weinstock, E. Inorg. Chem. 1966, 5, 1079.
Inorganic Chemistry, Vol. 41, No. 1, 2002 5

Wanat et al.
underlying reaction mechanism and a correlation with the
lability and exchange mechanism of coordinated water in
II 27
aquated Fe . In addition, M o¨ ssbauer spectroscopy and EPR
measurements were performed on the reaction product {Fe-
7
NO} and allow us to unequivocally resolve the discrepancy
in the literature concerning the bonding character of this
product.
Experimental Section
Materials. All chemicals were of analytical grade and used as
received. Solutions were prepared with distilled and purified water
using a Milli-Q water purification system. The stock solution of
nitric oxide was prepared by degassing 0.2 M acetate buffer solution
(
pH 5.0) and then saturating it with NO. NO gas was purchased
from Linde 93 or Riessner Gase in purity of at least 99.5 vol %
and cleaned from traces of higher nitrogen oxides like N and
NO by passing through an Ascarite II column (NaOH on silica
Figure 1. Absorption spectral changes recorded for the reaction of 3 ×
O
2 3
-3
II
2+
10
M [Fe (H2O)6] with NO. Experimental conditions: 0.2 M acetate
II
2+
2
buffer, pH ) 5.0, 25 °C. Curves: trace a, [Fe (H2O)6] solution saturated
with NO; trace b, a + Ar brief (10 s); trace c, b + Ar brief (10 s); trace d,
c + Ar brief (10 s).
gel, Sigma-Aldrich) via vacuum line techniques. Dilutions of known
concentration were prepared from this saturated solution. All
experiments were performed under strict exclusion of air oxygen.
Buffer solutions were deaerated for extended periods (in general 1
1
8.MV, Applied Photophysics) coupled to an online data acquisition
system. The changes in absorbance were monitored at 451 nm. The
reported rate constants represent the mean values from at least five
kinetic runs.
min per mL solution) with pure N
2
or Ar, before they were brought
II
II
in contact with Fe salt (Fe SO
4
‚7H O) or nitric oxide.
2
Measurements. pH measurements were performed on a Metro-
hm 623 pH meter with a Sigma glass electrode or on a Mettler
Delta 340 pH meter. The concentration of free nitric oxide in
solution was determined with an ISO-NOP electrode connected to
an ISO-NO Mark II nitric oxide sensor from World Precision
Laser flash photolysis kinetic studies were carried out with the
use of the LKS.60 spectrometer from Applied Photophysics for
detection and a Nd:YAG laser (SURLITE Is10 Continuum) pump
source operating in the second (λexc ) 532 nm) harmonic (245 mJ
pulses with ∼7 ns pulse width). Spectral changes at 451 nm were
monitored using a 100 W xenon arc lamp, monochromator, and
photomultiplier tube PMTs1P22. The absorbance reading was
balanced to zero before the flash, and data were recorded on a digital
storage oscilloscope DSO HP 54522A and then transferred to a
computer for subsequent analysis. Gastight quartz cuvettes and a
Instruments. The NO electrode consists of a membrane-covered
-
anode which selectively oxidizes NO to NO
3
ions. The resulting
current is proportional to the concentration of NO in solution. The
NO electrode was calibrated daily with fresh solutions of sodium
nitrite and potassium iodide according to the method suggested by
the manufacturers. The calibration factor (nA/µM) was determined
with a linear fit program. The determination of the stability constant
2
8
pill-box cell combined with high-pressure equipment were used
at ambient and under high pressure (up to 170 MPa), respectively.
At least 30 kinetic runs were recorded under all conditions, and
the reported rate constants represent the mean values of these.
4
K
N
O
w
a
s
p
e
r
f
o
r
m
e
d
a
s
d
e
s
c
r
i
b
e
d
b
e
f
o
r
e
.
UV/vis spectra were recorded on a Shimadzu UV-2100 spec-
trophotometer equipped with a thermostated cell compartment CPS-
60. IR spectra were recorded with the use of a two-mirror ATR
2
Results and Discussion
cell on an ATI Mattson FTIR Infinity spectrophotometer. The
reflection instrument utilizes a Ge crystal as an internal reflection
plate with a size of 50 × 20 × 2 mm and 45° angle of incidence.
Spectroscopic Observations. The UV/vis spectrum of
II
2+
[Fe (H O) ] measured in 0.2 M acetate buffer solution at
pH ) 5.0 exhibits no absorption band between 250 and 800
nm. As can be seen from Figure 1, exposure of a degassed
solution of [Fe (H
increase in absorbance giving band maxima at 336 (ꢀ ) 440),
4
removal of NO from solution with a stream of Ar gas, the
spectral changes were completely reversed, indicating that
the following equilibrium (eq 2) must be valid under the
selected conditions.
2
6
7
For the {Fe-NO} complex, four independent measurements of
100 scans were performed under inert gas atmosphere.
M o¨ ssbauer data were recorded on an alternating constant-
II
2+
2 6
O) ]
to excess NO leads to a significant
acceleration spectrometer. The minimum experimental line width
-1
was 0.24 mm s (full width at half-heigth). The sample temperature
was maintained constant in an Oxford Instruments Variox cryostat;
the 57Co/Rh source (1.8 GBq) was kept at room temperature. Isomer
shifts are quoted relative to iron metal at 300 K. X-band EPR spectra
were recorded on a Bruker ELEXSYS E500 spectrometer equipped
with a helium flow cryostat (Oxford Instruments ESR 910), an
NMR gaussmeter, and a Hewlett-Packard frequency counter. The
solution spectra were simulated with the program GFIT of the ESIM
-
1
-1
51 (ꢀ ) 265), and 585 nm (ꢀ ) 85 M cm ). Upon
KNO
II
2+
II
2+
[
Fe (H O) ] + NO { } [Fe (H O) NO] + H O (2)
2 6 2 5 2
package (by E.B.). The powder simulation procedure uses effective
1
g values for effective spin S ) /
2
and either anisotropic field-
Using UV/vis spectroscopy combined with the electrochemi-
cal detection of NO, the equilibrium constant K was
determined to be (1.15 ( 0.05) × 10 M in 0.1 M acetate
buffer, pH ) 5.0 and 23 °C.
constant line widths or isotropic frequency-constant line widths.
NO]2 was studied
+
The kinetics of NO release from [Fe(H
2
O)
5
3
-1
on a thermostated (( 0.1 °C) stopped-flow spectrometer (SX-
(27) (a) Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307. (b)
Ducommun, Y.; Newman, K. E.; Merbach, A. E. Inorg. Chem. 1980,
(28) Spitzer, M.; Gartig, F.; van Eldik, R. ReV. Sci. Instrum. 1988, 59,
19, 3696.
2092.
6
Inorganic Chemistry, Vol. 41, No. 1, 2002

ReWersible Binding of Nitric Oxide to Aquated Iron(II)
2
+
Figure 3. Zero-field M o¨ ssbauer spectrum of [Fe(H2O)5(NO)] (0.2 M
II
Fe solution saturated with NO at 298 K) at 80 K. The bold solid line is
the result of a fit with two Lorentzian doublets to the experimental (×)
7
II
Figure 2. Experimental and simulated X-band EPR derivative spectra of
data (thin line “a”, {FeNO}; thin line “b”, residual Fe ).
2+
[
Fe(H2O)5(NO)] and numerical integrated spectrum (inset) with respective
II
simulations. Experimental conditions: 0.1 mM Fe solution saturated with
NO at 298 K (the sample was an aliquot of the M o¨ ssbauer preparation);
measuring temperature 10 K, microwave frequency 9.4625 GHz, power
(S ) 2) are virtually EPR silent at the selected experimental
conditions, due to the presence of extremely broad spectra
induced by the large zero-field splitting of the integer-spin
1
00 µW, modulation 1.2 mT at 100 kHz. The simulation (trace “S ) 3/2”)
was obtained with effective g values g ) (4.04, 4.04, 2.0) and constant
isotropic line width W ) 5 mT (Gaussian).
species.
The zero-field M o¨ ssbauer spectrum of the 57Fe-enriched
nitrosylated iron solution measured at 80 K shows three lines
which can be deconvoluted into two overlapping quadrupole
doublets with an intensity ratio of about 2:3 (Figure 3). The
major doublet shows a very large isomer shift, d ) 1.39 mm
IR spectroscopy is considered to be a useful tool for the
determination of the binding mode and oxidation state of
24a,29
NO in metal-based complexes.
The ATR-IR spectrum
was measured with a 0.2 M unbuffered solution (pH ) 3.1)
-
1
-1
s , and a large quadrupole splitting, ∆E
Q
) 3.33 mm s ,
II
2+
of [Fe (H
2
O)
6
]
saturated with NO. The formation of a
II
like the precursor Fe SO
4
(aq.). Hence, the contribution (58%)
-
1
single peak at 1810 cm was observed, indicating the
II
2+
2 6
can be assigned to nonreacted [Fe (H O) ] , which perfectly
II
2+
formation of [Fe (H
2
O)
5
NO] . Taking into account that for
agrees with that expected for a saturated NO solution and
the KNO value given above. The formation of the nitrosyl
complex is indicated by the appearance of the resolved peak
II
the Fe (L)NO complexes (L ) polyaminecarboxylate) the
-
1
observed wavenumbers are found to be around 1777 cm
L ) edta ), which are known to bind NO as Fe -NO ,
the slightly shifted peak in [Fe (H
respond to a species formally written as [Fe (H
4
-
III
- 22a
(
-
1
at about 1.8 mm s . The corresponding subspectrum was
successfully simulated by adopting an asymmetric Lorentzian
doublet with identical line intensities but significantly
different line widths. The related M o¨ ssbauer parameters, d
II
2+
2 5
O) NO] should cor-
III
-
2 5
O) (NO )]
with a charge contribution indicating a pronounced electron
donation from the metal to the ligand.
-
1
-1
)
0.76 mm s , ∆E
Q
) 2.1 mm s , are practically identical
(aq), d ) 0.76
mm s , ∆E ) 2.3 mm s . We presume that the line
The X-band EPR spectrum of the nitrosylated solution was
measured in frozen state at 10 K as shown in Figure 2, from
which the corresponding absorption spectrum was obtained
by numerical integration of the experimental signals (inset).
The appearance of two absorption maxima reveals that two
major subspectra are present with similar abundances. The
very asymmetric broad absorption centered at about g ) 1.9
is typical of “free” NO molecules in frozen solution (not
with those previously reported for Fe(NO)SO
4
-
1
-1 10e,30
Q
asymmetry and broadening at 80 K owes its origin to the
onset of spin relaxation with rates intermediate to the nuclear
Larmor precession and the lifetime. The assignment of lines
applied in the Lorentzian fit shown in Figure 3 and validity
of the obtained parameters were confirmed from a measure-
ment at liquid helium temperature (not shown). Since the
simulated). The low g value and the large width of this S )
2 5
M o¨ ssbauer and EPR parameters of the [Fe(H O)
(NO)]2
+
1
2
/ spectrum indicate the presence of substantial spin-orbit
7
complex closely resemble those of the {FeNO} units in any
of the other well-characterized nitrosyl complexes, we infer
that its electronic structure is also best described by the
presence of high-spin Fe antiferromagnetically coupled to
NO (S ) 1) yielding the observed spin quartet ground state
interaction, as expected for the NO radical. The low-field
part of the experimental spectrum, however, could be readily
simulated by adopting an independent subspectrum with
effective g values g ) (4.04, 4.04, 2.0), which are typical of
a spin quartet species (trace “S ) 3/2”) with dominating zero-
III
-
3
(
S ) /
2
).
-
1
field splitting, D . hν (0.3 cm at X-band), and vanishing
rhombicity, E/D ) 0. Similar spectra have been observed
for solutions of [Fe(edta)(NO)]2 or [(1,4,7-trimethyl-1,4,7-
Flash-Photolysis Kinetics. The rates of NO binding to
II
2
Fe (L) complexes (where L ) H O, polyaminocarboxylates)
2
are convenient to measure by flash photolysis, which can
be employed to study the kinetics of reactions on a
significantly faster time scale than stopped-flow technique.
3 2
triazacyclononane)Fe(NO)(N )
].23 Hence, the subspectrum
can be assigned to the nitrosylated iron-aqua complex and
3
7
it proves the S ) /
2
ground state of the {FeNO} core. We
II
mention that residual precursor high-spin Fe ions in water
(30) The isomer shifts reported in ref 10e, which are given with respect to
sodium nitroprusside, were converted to values relative to R-iron at
-
1
(29) Mingos, D. M. P.; Sherman, D. J. AdV. Inorg. Chem. 1989, 34, 293.
ambient temperature by subtracting 0.258 mm s .
Inorganic Chemistry, Vol. 41, No. 1, 2002 7

Wanat et al.
Table 1. Rate Constants and Activation Parameters for the Reversible
II
2+ a
Binding of NO to [Fe (H2O)6]
1
koff (s-1
)
temp (°C)
pressure (MPa)
0.1
kon (M- s-1
)
5
5
5
5
(4.4 ( 0.2) × 10
(6.2 ( 0.2) × 10
(8.1 ( 0.1) × 10
760 ( 380
1170 ( 380
1690 ( 290
2380 ( 220
3240 ( 750
4420 ( 590
7050 ( 900
2190 ( 650
2120 ( 100
2060 ( 470
2050 ( 240
2000 ( 290
10
15
20
25
30
35
20
6
6
6
6
6
5
5
5
5
(1.06 ( 0.10) × 10
(1.42 ( 0.04) × 10
(1.86 ( 0.03) × 10
(2.31 ( 0.05) × 10
(1.04 ( 0.04) × 10
(9.27 ( 0.06) × 10
(8.15 ( 0.24) × 10
(7.55 ( 0.12) × 10
(6.97 ( 0.14) × 10
10
5
9
0
0
1
1
30
70
q
∆
H (kJ/mol)
37.1 ( 0.5
-3.3 ( 2.0
+6.1 ( 0.4
48.4 ( 1.4
-15.3 ( 4.7
+1.3 ( 0.2
Figure 4. Transient absorption difference spectra for [Fe(H2O)5NO]2+ after
q
∆S (J/(K mol))
2
+
q 3
laser flash photolysis at 532 nm. Experimental conditions: [Fe(H2O)6
)
]
∆V (cm /mol)
-
4
0.012 M, [NO] ) 3.6 × 10 M, 0.2 M acetate buffer, pH ) 5.0, 22 °C.
a
Experimental conditions: 0.2 M acetate buffer, pH ) 5.0, [NO] ) 3.6
Curves: a, after 25 µs; b, after 50 µs; c, after 100 µs; d, after 150 µs; e,
after 250 µs; f, after 450 µs.
-
4
II
2+
×
10 M, [Fe (H2O)6 ] ) 0.006-0.03 M, λirr ) 532 nm, λdetect ) 451
nm.
Scheme 1
II
2 6
O)
2+] are
In agreement with eq 3, plots of kobs vs [Fe (H
linear with nonzero intercepts as displayed in Figure S2
Supporting Information) for the temperature range 5-35
C. The kon and koff values determined from these plots (Table
) at 25 °C were used to estimate the overall equilibrium
constant K for reaction 2, which resulted in K ) kon/koff
(
°
1
Irradiation at 532 nm induces dissociation of the nitrosyl
complex and rapid release of NO, followed by relaxation of
the system back to the original equilibrium position (Scheme
)
-
1
4
40 ( 110 M . This value is in reasonable agreement with
the thermodynamically determined value reported above; the
deviations are most probably related to errors involved in
the determination of koff via extrapolation of the kinetic data
obtained in the flash-photolysis experiments. The value of
K was also calculated (kon/koff) for the other temperatures,
from which it follows that K decreases with increasing
temperature, i.e., reaction 2 is an exothermic process with
1).
As can be seen from Figure 4, 532 nm flash photolysis of
II
2+
[
Fe (H
2
O)
5
NO] in 0.2 M acetate buffer solution at pH )
5
results in spectral changes which are described by prompt
transient bleaching with the largest change in absorbance
centered at 336, 451, and 585 nm. This is consistent with
II
2+
-
1
-1
the photolabilization of NO from [Fe (H
formation of free NO and [Fe (H O)
2 6
4
2
O)
] . After a delay of
50 µs, the transient bleaching disappears and the spectrum
5
NO] and
∆
mol .
H° ) -11.4 ( 1.8 kJ mol and ∆S° ) +12 ( 6 J K
II
2+
-
1
The temperature dependence of kon and koff (Table 1) was
returns to the baseline ascribed to the regeneration of the
original nitrosyl complex. The decay of the transient bleach-
ing was followed using single-wavelength detection at 451
nm and could be analyzed as a one-exponential decay as
shown in Figure S1 (Supporting Information). As can be
used to construct linear Eyring plots (Figure S3, Supporting
Information), from which the activation parameters ∆H and
q
q
q
∆
(
S were determined. The values for kon are ∆H on ) 37.1
-1
q
-1
-1
0.5 kJ mol and ∆S on ) -3 ( 2 J K mol ; for koff
q q
-1
these are ∆H off ) 48.4 ( 1.4 kJ mol and ∆S off ) -15 (
II
2+
seen, the rebinding of NO to [Fe (H
2
O)
6
]
is completed
-1
-1
5
J K mol . Since the koff values are obtained by
extrapolation of the kobs vs [Fe (H
determination of ∆S in principle involves an extrapolation
to 1/T ) 0, it is reasonable to expect that the values of ∆S ,
II
2+
within 0.2 ms at [Fe (H
2
O)
6
] ) 0.018 M and 25 °C for
II
2+
2
O)
6
] plots and the
4
-1
which kobs ) 3.0 × 10 s .
q
The rate of the reaction approaching equilibrium with one
q
II
2+
2 6
of the components (in our case [Fe (H O) ] ) in large excess
especially for the “off” reaction, can be subjected to large
is expected to follow pseudo-first-order kinetics to give a
rate constant kobs expressed by eq 3. Accordingly, plots of
error limits. Thus mechanistic conclusions based on the
‡
values reported for ∆S are difficult to reach, especially when
they are close to 0.
A more reliable mechanistic discrimination parameter is
the volume of activation (∆V ) -RT(d ln k
is the rate constant at a particular pressure) derived from
II
2+
kobs ) k [Fe (H O) ] + koff
(3)
on
2
6
q
II
2+] should be linear with slopes equal to
on and intercepts equal to koff. In the studied system the
equilibrium constant KNO is sufficiently small that extrapola-
] ) 0 gives a measurable intercept, i.e.,
off. However, such intercepts are subjected to extrapolation
i
/dP)
T
, where k
i
k
k
obs vs [Fe (H O)
2 6
3
1
the effect of hydrostatic pressure P on the kinetics of the
II
II
2+
reaction. The effect of pressure on the reaction of [Fe -
tion to [Fe (H
2
O)
6
2
+
(H
2
O)
6
]
with NO was studied using a high-pressure flash-
k
errors. All kinetic experiments were performed in 0.2 M
(
31) (a) van Eldik, R.; Asano, T.; le Noble, W. J. Chem. ReV. 1989, 89,
49-688. (b) van Eldik, R.; D u¨ cker-Benfer, C.; Thaler, F. AdV. Inorg.
Chem. 2000, 49, 1.
acetate buffer at pH ) 5.0 with at least a 10-fold excess of
5
II
2+
[
Fe (H
2
O)
6
]
using laser irradiation at λirr ) 532 nm.
8
Inorganic Chemistry, Vol. 41, No. 1, 2002

ReWersible Binding of Nitric Oxide to Aquated Iron(II)
Scheme 2
2
+
NO] . Spectral changes recorded for this reaction (see
Figure S6, Supporting Information) are described by two
clear shifts in the absorbance maxima from 451 to 435 nm
II
and from 585 to 633 nm, indicating the decay of [Fe (H
2 5
O) -
2
+
Figure 5. Volume profile for reaction 2 based on the determined volumes
of activation.
NO] and formation of a new nitrosyl complex, viz.,
Fe (edta)(NO)] . In order to determine the koff value, the
II
2-
[
reaction was studied using stopped-flow techniques. At room
temperature the reaction occurs within the dead time of the
apparatus; hence the rate measurements were performed at
photolysis technique. The flash-photolysis experiments were
carried out in 0.2 M acetate buffer solutions, at 20 °C and
in the pressure range 0.1-170 MPa. The results are reported
5
4
1
°C. At this temperature a rapid increase in absorbance at
in Figure S4 (Supporting Information).
32 nm was observed and the reaction was complete within
II
2+] at different pressures,
) vs
From plots of kobs vs [Fe (H
2
O)
6
-
1
0 ms with kobs ) 788 ( 50 s . A typical stopped-flow
values of kon and koff were determined, and linear ln(k
i
trace for this reaction is shown in Figure S7 (Supporting
Information). The observed rate constants were found to be
independent of the concentration of the trapping complex
and therefore represent the value for koff. The value of koff
determined in this manner is consistent, within experimental
pressure plots (Figure S5, Supporting Information) show that
both rate constants decrease with increasing pressure, result-
q
ing in small positive volumes of activation, viz., ∆V on
)
3
-1
q
3
-1
+
6.1 ( 0.4 cm mol and ∆V off ) +1.3 ( 0.2 cm mol
for the “on” and “off” reactions, respectively. The effect of
pressure on the back reaction is rather small, resulting in
almost the same, within the experimental error limits, koff
values at different pressures (Table 1). The reaction volume
II
error limits, with that obtained from extrapolating kobs vs [Fe -
2
+
(H
2
O)
6
] in the flash-photolysis experiments at 5 °C. It
should be noted that the trapping technique employed enables
a direct measurement of the dissociation rate constant and
in principle must result in a more accurate value for koff than
that obtained using the indirect extrapolation method.
However, the magnitude of koff is such that stopped-flow
techniques could not be used to measure this rate constant
as a function of temperature and pressure.
q
q
(
+
∆V) for the overall equilibrium (∆V ) ∆V on - ∆V off) is
3
-1
4.8 ( 0.6 cm mol , indicating that there is an overall
increase in partial molar volume during the displacement of
coordinated water by NO. A volume profile constructed on
the basis of these data is given in Figure 5, which suggests
that reaction 2 proceeds according to a dissociative inter-
change mechanism (see further discussion).
Overall Reaction Mechanism. From the spectral (UV-
vis, IR, M o¨ ssbauer, EPR) data reported for the nitrosyl
reaction product, it is clear that the complex can be
Stopped-Flow Kinetics. Since the koff values obtained
2+
from the [Fe(H
2
O)
6
]
concentration dependence of kobs are
III
-
2+
I
formulated as [Fe (H
(
2
O)
5
(NO )] and not as [Fe (H
2
O)
4
-
subjected to extrapolation errors, it is important to confirm
their reliability in a more direct way. As it was already
+
2+
10
NO )] as claimed before. This means that displacement
of coordinated water on [Fe (H
formal oxidation of Fe to Fe and the reduction of NO to
NO . It is reasonable to expect that the “on” reaction will
II
2+
2
O)
6
]
by NO results in the
5
shown, the most suitable method to measure koff directly is
II
III
to use an NO-trapping technique. In such a case the nitrosyl
complex is treated with another complex that binds NO
stronger and more rapidly than the system under investiga-
tion. When an excess of the trapping complex is employed,
NO release from the nitrosyl complex becomes the rate-
limiting step in the reaction sequence shown in Scheme 2,
and koff can be determined directly from the observed kinetic
-
to some extent be related to the water exchange reaction on
II
2+
6
-1
q
[
)
+
Fe (H
2
O)
6
] , for which kex ) 4.4 × 10 s at 25 °C, ∆H
-1 q -1 -1 q
41.4 kJ mol , ∆S ) +21.2 J K mol , and ∆V )
3.8 cm mol , in a way similar to that recently reported
for the binding of NO to [Fe (TPPS)(H O) ] . The rate
2 2
3
-1 27
III
3- 32
of water exchange is much faster than the “on” kinetics for
trace, viz., kobs ) koff
.
q
the reaction with NO, and the values of ∆H are indeed very
Two complexes can be employed as efficient NO scav-
similar. In addition, the activation volumes reported for the
water exchange process and the reaction with NO are also
very similar and suggest that both reactions occur according
II
2-
engers in such trapping experiments, viz., [Fe (edta)(H
2
O)]
III
-
or [Ru (edta)(H
2
8
O)] . Both of them react rapidly with NO
-1 -1
5
(
k
on is ca. 1 × 10 M
s
at pH ) 5.0 and 25 °C) to form
31
d
to a dissociative interchange (I ) mechanism. It follows that
the corresponding nitrosyl complexes.
II
O)]2- (in 20-fold
excess) was used to trap nitric oxide released from [Fe (H
In the studied reaction, [Fe (edta)(H
2
(
32) Schneppensieper, T.; Zahl, A.; van Eldik, R. Angew. Chem. 2001, 40,
II
2
O)
5
-
1678.
Inorganic Chemistry, Vol. 41, No. 1, 2002 9

Wanat et al.
II
]2+ controls not only the rate
water exchange on [Fe (H
2
O)
6
We conclude that the detailed spectroscopic and kinetic
studies performed on the reaction of aquated Fe with NO
II
but also the nature of the mechanism for the binding of NO.
This rate-determining displacement of coordinated water is
followed by a rapid intramolecular charge-redistribution
process to lead to the final [Fe (H
During the “off” reaction, the release of NO will involve
electron transfer to form [Fe (H O) NO] , followed by the
2 5
displacement of NO by water. The latter step exhibits a much
higher activation enthalpy than both the water exchange and
on” reactions, which must be related to the stronger Fe-
NO bond and accounts for the significantly slower “off”
reaction. As a result the overall reaction enthalpy for the
binding of NO is -11.4 ( 1.8 kJ mol , i.e., the process is
exothermic, and KNO decreases with increasing temperature.
The activation volume for the “off” reaction is small positive
and also in line with an interchange dissociative (I
substitution mechanism. Alternatively, the release of NO
has resolved the nature of the nitrosyl product and the
mechanism of the “on” and “off” reactions. These are
controlled by ligand substitution processes on the iron metal
center. In addition, it is important to note that although NO
is a radical, it behaves as a normal nucleophile during such
ligand displacement reactions. Its radical nature, however,
does require a formal charge transfer process to occur in
order to stabilize the nitrosyl reaction product. It will now
be appropriate to correct numerous undergraduate chemistry
text books that incorrectly describe this “simple” reaction.
III
-
2+
2 5
O) (NO )] product.
II
2+
“
-
1
Acknowledgment. The authors gratefully acknowledge
financial support from the Deutsche Forschungsgemeinschaft,
the Max-Buchner-Forschungsstiftung, and DAAD (fellow-
ship to A.W.). Studies at the Jagiellonian University were
supported by the State Committee for Scientific Research,
Poland, KBN (3TO9A11515 and 1605/T09/2000/19), and the
Foundation for Polish Science (“Fastkin” No. 8/97).
d
) ligand
III
-
from Fe -NO in the “off” reaction can be visualized as a
homolysis reaction that results in the formation of Fe and
II
NO, in which bond cleavage is accompanied by formal
III
II
reduction of Fe to Fe , and involves the simultaneous entry
of a water molecule, i.e., a water-“assisted” homolysis
reaction. The overall positive reaction volume of +4.8 (
Supporting Information Available: A total of seven figures
in which UV-vis spectra, kinetic traces, and plots of kobs as a
function of concentration, temperature, and pressure are reported
for the reversible binding of NO to aquated iron(II). This material
is available free of charge via the Internet at http://pubs.acs.org.
3
-1
0
.6 cm mol results from the displacement of a water by
an NO molecule in the coordination sphere of Fe and is in
agreement with the small positive value of +12 ( 6 J K
II
-
1
-
1
mol found for ∆S°.
IC010628Q
10 Inorganic Chemistry, Vol. 41, No. 1, 2002