Kinetic studies on the reaction of NO with iron(ii) complexes using low temperature stopped-flow techniques

Article abstract of DOI:10.1039/d0dt01764g

Low temperature stopped-flow techniques were used to investigate the reaction of three different iron(ii) complexes with nitrogen monoxide. The kinetic studies allowed calculation of the activation parameters from the corresponding Eyring plots for all three systems. The reaction of iron(ii) chloride with NO leading to the formation of MNIC (mononitrosyl-iron-complex) and DNIC (dinitrosyl-iron-complex) led to activation parameters of ΔH? = 55.4 ± 0.4 kJ mol-1 and ΔS? = 13 ± 2 J K-1 mol-1 for MNIC and ΔH? = 32 ± 6 kJ mol-1 and ΔS? =-193 ± 21 J K-1 mol-1 for DNIC. Formation of MNIC turned out to be much faster in comparison with DNIC. In contrast, activation parameters for the formation of monoculear [Fe(bztpen)(NO)](OTf)2 (bztpen = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-ethylenediamine) ΔH? = 17.8 ± 0.8 kJ mol-1 and ΔS? =-181 ± 3 J K-1 mol-1 supported an associative mechanism. Interestingly, [Fe(bztpen)(CH3CN)](OTf)2 does not react with dioxygen at all. Furthermore, activation parameters of ΔH? = 37.7 ± 0.7 kJ mol-1 and ΔS? =-66 ± 3 J K-1 mol-1 were obtained for the reaction of NO with the dinuclear iron(ii) H-HPTB complex (H-HPTB = N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane), [Fe2(H-HPTB)(Cl)3]. The kinetic data allowed postulation of the mechanisms for all of these reactions.

Full text of DOI:10.1039/d0dt01764g

View Article Online
View Journal
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: P. Specht, M.
Oßberger, P. Klüfers and S. Schindler, Dalton Trans., 2020, DOI: 10.1039/D0DT01764G.
Volume 46
Number 10
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
1
4
March 2017
Pages 3073-3412
Dalton
Transactions
An international journal of inorganic chemistry
rsc.li/dalton
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 0306-0012
COMMUNICATION
Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH , X = S, Se, Te)
+
rsc.li/dalton

Page 1 of 8
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
View Article Online
DOI: 10.1039/D0DT01764G
ARTICLE
Kinetic studies on the reaction of NO with iron(II) complexes using
low temperature stopped-flow techniques
a
b
b
a
Pascal Specht, Martin Oßberger, Peter Klüfers and Siegfried Schindler *
Received 00th January 20xx,
Accepted 00th January 20xx
th
Decicated to Prof. Rudi van Eldik on the occasion of his 75 birthday
DOI: 10.1039/x0xx00000x
Low temperature stopped-flow techniques were used to investigate the reaction of three different iron(II) complexes with
nitrogen monoxide. The kinetic studies allowed to calculate activation parameters from the corresponding Eyring plots for
all three systems. The reaction of iron(II) chloride with NO leading to the formation of MNIC (mononitrosyl-iron-complex)
‡
-1
‡
-1
-1
and DNIC (dinitrosyl-iron-complex) led to activation parameters of ΔH = 55.4 ± 0.4 kJ mol and ΔS = 13 ± 2 J K mol for
‡
-1
‡
-1
-1
MNIC and ΔH = 32 ± 6 kJ mol and ΔS = -193 ± 21 J K mol for DNIC. Formation of MNIC turned out to be much faster in
comparison with DNIC. In contrast, activation parameters for the formation of monoculear [Fe(bztpen)(NO)](OTf) (bztpen
2
=
N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1.2-ethylenediamine) ΔH = 17.8 ± 0.8 kJ mol-1 and ΔS = -181 ± 3 J K mol
‡
‡
-1
-1
supported an associative mechanism. Interestingly, [Fe(bztpen)(CH CN)](OTf) does not react with dioxygen at all.
3
2
‡
-1
‡
-1
-1
Furthermore, activation parameters of ΔH = 37.7 ± 0.7 kJ mol and ΔS = -66 ± 3 J K mol were obtained for the reaction
of NO with the dinuclear iron(II) H-HPTB complex (H-HPTB = N,N,N’,N’-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-
2 3
diaminopropane), [Fe (H-HPTB)(Cl) ]. The kinetic data allowed to postulate mechanisms for all of these reactions.
(
Na
2 5 2
[Fe(CN) (NO)] x 2 H O) that is used as a vasodilator during
5
,6
Introduction
surgery due to the release of NO.
Nitrogen monoxide became well known as a pollutant (one
component of nitrogen oxides NO ) in 2015, the beginning of
Since about 30 years it is known that NO can modify a number
of iron sulphur proteins leading to monomeric dinitrosyl iron
complexes (DNICs) as one of the commonly observed
x
the Volkswagen emissions scandal with regard to the exhaust
system of Diesel engines. While ammonia can be used at higher
temperatures in industry and in cars (formed from an urea
solution, called AdBlue) iron chelate complexes, e. g. iron(II)
edta, can be applied for gas purification under ambient
conditions.1 However, despite the bad image due to its known
toxicity, NO was chosen molecule of the year by Science
7
–9
products.
However, formation of multinuclear complexes
such as the dinuclear Roussin’s red salt esters (RSEs) can also
occur (Scheme 1).
,2
R
-
S
3
RS
RS
NO
NO
ON
ON
NO
NO
magazine in 1992. This was a consequence of the identification
+ 2 H+
2
Fe
Fe
+ 2 RSH
Fe
of NO as a signalling molecule in the cardiovascular system, a
finding that led to the award of the Nobel prize (in physiology
or medicine) in 1998 to Furchgott, Ignarro and Murad.
Consequently, interest in the solution-phase reactions of NO
S
R
DNIC
RSE
increased since understanding of the reactivity of this molecule Scheme 1: Equilibrium between DNIC and RSE.10
could provide new insights into its physiological role. The
reactions of NO with transition metal ions in the active site of An excellent study on the dynamics of the formation of several
metalloproteins, mainly iron enzymes, are particularly DNICs from the direct reaction of NO with Fe(II) and thiols in
4
aqueous solution has been reported recently.10 Quite complex
interesting. Furthermore, better understanding was gained on
the
reactivity
of
sodium
nitroprusside kinetic behavior was observed due to a series of equilibria
involved. However, the final step of the formation of DNIC could
be kinetically analyzed and activation parameters were
a. P. Specht, Prof. Dr. Siegfried Schindler, Institut für Anorganische und Analytische
Chemie, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 17, 35392 Gießen
‡
-1
‡
-1
-1
obtained (ΔH = +56 ± 3 kJ mol , ΔS = -61 ± 5 J K mol ). The
results supported the mechanism described previously for the
nitrosylation of biomimetic rubredoxin: the DNIC is generated
via a mononitrosyl iron complex (MNIC) that was extremely air-
(Germany).
b.
Dr. Martin Oßberger, Prof. Dr. Peter Klüfers, Department Chemie, Ludwig-
Maximilians-Universität München, Butenandtstraße 5-13, 81377 München
(Germany).
7
,11
and light-sensitive.
†
Electronic Supplementary Information (ESI) available: Details on kinetic
measurements/data. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 2 of 8
ARTICLE
Journal Name
‡
-1
When the influence of an acidic environment was investigated, were
it turned out that depending on the reaction conditions the two ΔS = -3 ± 2 J K mol
calculated
to
ΔH = +37.1 ± 0.5 kJ mol ,
View Article Online
‡
-1
-1
‡
DOI: 10.103
39/D0 D- 1T 01764G
and ΔV = +6.1 ± 0.4 cm mol
thus
quite stable iron nitrogen monoxide complexes MNIC (1), supporting a dissociative interchange (I
d
) mechanism. With
-
-
8,12
2-
[
3 2 2
FeCl (NO)] , and DNIC (2), [FeCl (NO) ] , formed (Scheme 2).
iron(II) chelate complexes, e. g. [Fe(edta)(H
2
O)] the reaction
with NO was even faster, however, followed the same I
d
ligand
2
0
2
-
-
substitution mechanism.
In contrast an associative
S
S
NO
Cl
Cl
Cl
Cl
interchange mechanism was assigned for the related complex
excess NO
+
2 S
Fe
Fe
Fe NO
-
2
[Fe(nta)(H O)
2
] . A much slower reaction rate was observed for
Cl
+
Cl
2
-
2-
the formation of [Fe(CN)
5
(NO)] from [Fe(CN)
5
(H
2
O)] with a
DNIC
-1 -1
reaction rate k
f
of 250 ± 10 L mol s at 25 °C (activation
-
‡
-1
‡
-1
-1
parameters ΔH = +70 ± 1 kJ mol , ΔS = +34 ± 4 J K mol and
NO
i) excess NO
ii) 2 HCl
‡
3
-1 21
ΔV = +17.4 ± 0.3 cm mol ).
Based on these results a
Fe Cl
S + 2 H S
2
dissociative mechanism was proposed with rate-controlling
dissociation of coordinated water and subsequent fast
coordination of NO.
Cl
Cl
MNIC
Scheme 2: Formation of MNIC (1) and DNIC (2) from the reaction of [FeS
from J. Fitzpatrick et al.).8
2
Cl
]2- (redrawn
4
MNIC (1) and DNIC (2) can be synthesized in a different way and
Klüfers and co-workers have reported their physical and
6
chemical properties recently. However, despite the
However, besides the importance of NO in biochemical
reactions it is quite interesting as a non-innocent ligand in
coordination chemistry and several reports have been
published on transition metal complexes with NO ligands and
importance of these complexes no kinetic investigation had
been performed so far on the formation of these two
complexes. According to Scheme 3 iron(II) chloride reacts –
depending on reaction conditions – with nitrogen monoxide to
1
3–15
their reactivity in the past.
Indeed the three different ligand
assignments as NO+ (isoelectronic to CO), NO , and NO
∙
-
3 3
form either NHEt -1 or NHEt -2. Both complexes have been
6
structurally characterized previously.
(
isoelectronic to O ) leads to difficulties in the correct
2
description of the oxidation states of the transition metal ion.
FeCl2 + (NHEt3)Cl + NO
NHEt3[FeCl3(NO)]
2
+
Thus, even the simple complex [Fe(H
since many years from the qualitative analytical test on nitrate
“brown ring” test) had caused a lot of discussion on the
2
O)
5
NO] , well known
MeOH
(1)
NHEt3-1
(
NEt3
I
II
III
NHEt3[FeCl3NO] + 2 NO
NHEt3[FeCl2(NO)2] + MeONO + (NHEt3)Cl
oxidation state of the iron ion (Fe , Fe and Fe has been
reported).1 To keep track of the oxidation states of transition
MeOH
(2)
6,17
NHEt3-1
NHEt -2
3
metal NO complexes the Enemark-Feltham notation
m
(
{Fe(NO)
x
}
with x = number of NO ligands and m = sum of
Scheme 3: Reaction of iron(II) chloride with nitrogen monoxide in methanol leads to the
formation of 1 (eq. 1). In presence of a base and with higher excess of nitrogen monoxide
2 is formed (eq. 2).
metal (d) and NO (π*) electrons) has been introduced leading
for Fe + NO to {FeNO} .
II
7 18
Quite surprisingly little is known about the kinetics and 1 is formed in a fast reaction (eq. 1) without base) and is stable
mechanisms of the reaction of NO with transition metal under these conditions. In the presence of a base 2 is formed in
complexes.4
,19,20
The reasons for this are based on a) the a consecutive reaction (eq. 2), however, at a much slower rate.
difficult handling of NO, b) the extreme fast reaction rates and The two complexes can be easily distinguished by their UV-vis
c) related to b: mechanistic studies were performed mainly in spectra as shown in Figure 1 (FeCl does not absorb under these
2
aqueous solutions that did not allow cooling to slow down conditions in that wavelength range).
reaction rates. In here, we now present kinetic data and
mechanistic interpretation for the formation of three different
iron NO complexes that had been investigated in methanol
using low temperature stopped-flow techniques.
1000
800
MNIC
DNIC
FeCl2
600
400
200
0

Results and discussion
400
500
600
700
MNIC/DNIC
wavelength / nm
Reactions of nitrogen monoxide with iron(II) complexes usually
푐FeII
= 2 ∙10-3 mol L ), DNIC ( = 1 ∙10-3 mol L-1) and
푐FeII
-1
Figure 1: UV-vis spectra of MNIC (
푐FeII
= 1 ∙10-3 mol L ) in methanol.
-1
are extremely fast. For example, the reversible reaction of NO complex solution before the reaction (
with [Fe(H
2 6 4
O) ]SO in an acetate buffered aqueous solution has
6
-1 -1
A big advantage in the investigation of these complexes is based
on the redox-innocent ligand chloride that simplifies the system
a rate constant of kon = (1.42 ± 0.04) ∙10 L mol s at 25 °C and
had to be measured by T-jump or flash photolysis. From the
temperature and pressure dependence activation parameters
1
6
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 8
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
in contrast to non-innocent sulphur ligands in the natural The first step is rate determining and the dissociation of a
View Article Online
1
2
DOI: 10.1039/D0DT01764G
-
systems described in the introduction.
methanol ligand that is followed by the reaction with NO/Cl .
This accounts for the independence of the rate on the NO
concentration. Furthermore, the mechanism is supported by
the fact that the reaction rate is independent on chloride
concentration.
Kinetic investigation of MNIC formation
The formation of 1 can be followed spectroscopically easily
between -87.0 °C and -34.0 °C. The time resolved UV-vis spectra
are presented in Figure 2 with characteristic absorbance
maxima at 464 nm and 587 nm. As described above 1 is stable
under these conditions and did not react further. However, time
of formation and rate constants change fast when the
temperature is varied.
In general, such a dissociative mechanism should be observed
in the biological systems as well (Scheme 2), however - as
pointed out by Ford and co-workers – things are much more
complicated in the natural systems due to complex equilibria
and non-innocent ligands.
1
0
1
1
0
0
0
.5
.2
.9
.6
.3
1.5
1.0
0.5
0.0
Kinetic investigation of DNIC formation
1 reacted with nitrogen monoxide to 2 in a much slower
reaction. Absorbance maxima of 1 and 2 overlap in large parts
(Figure 1), however, it was still possible to separate the two
reactions. 2 has two characteristic absorbance maxima at
0
100
200
time / s
300
5
7
20 nm and 700 nm. With only a small absorbance of 2 at
00 nm this wavelength was used for data fitting. The reaction
follows first order kinetics and a good one exponential fit to the
absorbance vs. time trace is possible. Kinetic studies were
performed in a temperature range between -5.0 °C and 40.0 °C.
The formation of 2 under the applied conditions took 1.5 h with
400
500
wavelength / nm
600
-4 -1
a rate constant of kobs = 6.5 ∙10 s at 20.0 °C (Figure 3). Further
experiments with dependence on nitrogen monoxide
concentration were not carried out because a pseudo first
kinetic could not be guaranteed anymore with lower
concentrations of nitrogen monoxide.
Figure 2: Time resolved stopped-flow UV-vis spectra of MNIC formation at -81.9 °C in
methanol (ccomplex = 2 ∙10-3 mol L , cNO = 7.25 ∙10-3 mol L , after mixing). Inlay (time
trace): Absorbance vs. time at 464 nm (black: experimental, red: exponential fit).
-1
-1
At -81.9 °C formation of 1 is complete in 300 s and the
absorbance vs. time trace can be perfectly fitted with a one
exponential fit. With NO concentration in excess and with the
1
.5
0
0
0
0
0
.08
.07
.06
.05
.04
2 2
rate law v = kobs x ccomplex (complex = [FeCl (MeOH) ], see below)
-3 -1
a first order rate constant kobs = 15.8 ∙10 s could be
calculated. It turned out that the reaction rate is independent
with regard to NO concentration thus leading to zero order of
NO concentration (see Supporting Information, Figure S1). The
obtained first order rate constants were used for obtaining an
Eyring plot (see Supporting Information, Figure S2, S3) and
1.0
0
400
800
1200
time / s
0.5
activation
parameters
were
calculated
to
‡
-1
‡
-1
-1
ΔH = +55.4 ± 0.4 kJ mol and ΔS = +13 ± 2 J K mol . The
0.0
positive activation entropy indicates
dissociative interchange mechanism.
a dissociative or
400
500
600
wavelength / nm
700
Figure 3: Time resolved stopped-flow UV-vis spectra of DNIC formation at 40.0 °C in
When FeCl
reactant that in consecutive reactions is transformed into
NHEt -1. Based on the results of our kinetic study a reaction
mechanism according to Scheme 4 is proposed.
2 2 2
is dissolved in methanol it forms [FeCl (MeOH) ] as
methanol (ccomplex = 1 ∙10-3 mol L , cNO = 7.25 ∙10-3 mol L , after mixing). Inlay (time
-1
-1
trace): Absorbance vs. time at 700 nm (black: experimental, red: exponential fit).
3
Activation
parameters
could
be
calculated
to
‡
-1
‡
2
-1
-1
ΔH = +32 ± 6 kJ mol and ΔS = -1.9 ± 0.2 ∙10 J K mol (see
Supporting Information, Figure S4). In comparison with the
formation of 1 there is a huge difference in rate. From the Eyring
plot the rate constant for the formation of 1 can be calculated
to kobs = 5.0 ∙10 s at 20.0 °C. So, the reaction rates differ by a
factor of 10 . This huge difference most likely is caused by
[
FeCl (MeOH) ]
[FeCl (MeOH)] + MeOH
(3)
(4)
2
2
2
-
[
FeCl (MeOH)] + NO + Cl-
[FeCl (NO)] + MeOH
2
3
5
-1
1
9
Scheme 4: Proposed mechanism for the formation of 1.
electron transfer during the reaction of 1 to 2. The mononitrosyl
7
9
2
iron complex of {FeNO} -type turns into a {Fe(NO) } -species,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 4 of 8
ARTICLE
Journal Name
(5)
where the latter receives two electrons. Two NO molecules as
ligands and a reducing agent, contribute one electron
respectively. The remaining NO binds to methanolate forming From the slope a second order rate constant k = 19.3 L mol s
methyl nitrite. With regard to the large negative value of ΔS an can be derived at -47.8 °C. With measurements in the
associative mechanism could be suggested. However, this for temperature range between -47.8 °C and -18.4 °C an Eyring plot
sure is not referring to a simple reaction step especially not for allowed calculation of the activation parameters to
푣 = 푘obs ∙ 푐complex ∙ 푐NO
View Article Online
DOI: 10.1039/D0DT01764G
-1 -1
‡
‡
-1
‡
-1
-1
the simple substitution of another chloride ligand in 1. Here we ΔH = +17.8 ± 0.8 kJ mol and ΔS = -181 ± 3 J K mol (see
observe a complex reaction system and not just one reaction.
Supporting Information, Figure S6). The large negative value for
activation entropy indicates an associative mechanism for the
formation of the nitrosyl complex. The postulated mechanism is
presented in Scheme 5.
The Iron bztpen system
In
contrast
to
most
other
iron(II)
complexes
[
Fe(bztpen)(CH
3
CN)](OTf)
2
(bztpen = N-benzyl-N,N’,N’tris(2-
2+
2+
2+
methylpyridyl)-ethylenediamine, Scheme 5) does not react with
dioxygen.2 Only when a large excess of hydrogen peroxide is
used (together with a base) the complex can be oxidised to an
iron(III) peroxido complex. Using iodosobenzene, formation of
an iron(IV) oxido complex can be observed, however, none of
these species could be crystallized and therefore have only been
spectroscopically characterized. In contrast, reacting the
complex with nitrogen monoxide did allow crystallization and
structural characterization of the corresponding nitrosyl
complex.22 Therefore, a kinetic analysis of this reaction was
N
N
N
2,23
N
N
N
N
N
N
NO
- MeCN
Fe
N
Fe
N
NO
Fe
N
N
MeCN
N
MeCN
N
NO
2
4
Scheme 5: Postulated mechanism of the formation of the iron nitrosyl complex of
Fe(bztpen)(MeCN)]2+.
[
possible, without an interference of a side reaction of the NO is approaching and binding to the complex while acetonitrile is
complex with dioxygen.
still coordinated, the rate determining step. Then acetonitrile
dissociates and [Fe(bztpen)(NO)](OTf) is formed.
2
When [Fe(bztpen)(CH
3 2
CN)](OTf) was reacted with NO an
absorbance increase is observed with a maximum at 550 nm The iron HPTB system
and a decrease of the absorbance maximum at 380 nm
Figure 4). The increase as well as the decrease follow first order
Activation of dioxygen at metal ion sites is important with regard to
selective oxygenation of organic substrates in organic synthesis as
well as for the reactivity of metalloenzymes.25–28 Where dioxygen
complex products are not readily accessible, nitrogen monoxide, as
a related diatomic molecule to dioxygen, and its metal complexes
may help lead to a better understanding of dioxygen binding
patterns.29,30
(
kinetics in complex concentration and were fitted with a one
exponential fit (inlay in Figure 4). The formation time of the NO
complex at -47.8 °C is around 40 s with rate constant
-1
k
obs = 0.136 s .
1
0
0
.0
.8
.6
The dinuclear iron(II) complex with the ligand Et-HPTB (N,N,N’,N’-
tetrakis(2-(1-ethylbenzimidazolyl)methyl)-2-hydroxy-1,3-
diaminopropane ([Fe (R-HPTB)(X) Y](anion); see Scheme 6, a
2 2
deprotonated ligand with R = Et in the corresponding iron complex)
has been used as a model compound for the oxygen carrier protein
hemerythrin (as well as related metalloenzymes).
1.0
0.5
0.0
0
10
20
time / s
30
40
50
R
N
R
n+
N
N
N
N
O
N
Fe
X
Fe
X
N
N
R
400
500
wavelength / nm
600
R
Y
N
N
Figure 4: Time resolved stopped-flow UV-vis spectra of [Fe(bztpen)(CH
3
CN)](OTf)
2
with
nitrogen
monoxide
at
-47.8 °C
in
methanol
(ccomplex = 0.5 ∙10-3 mol L-1,
NO = 7.25 ∙10-3 mol L , after mixing). Inlay (time trace): Absorbance vs. time at 384 nm
-1
c
(black: experimental, red: exponential fit).
2 2
(R-HPTB)(X)
Y]) with R = H or Et, X = NO or Cl-,
Scheme 6: The iron(II) R-HPTB complex (Fe
Y = benzoate or Cl-.
A plot of kobs vs. nitrogen monoxide concentration shows a
linear correlation (see Supporting Information, Figure S5) and
thus confirms first order dependence in nitrogen monoxide
concentration leading to the second order rate law:
2 4 2
The reaction of dioxygen with [Fe (Et-HPTB)(benzoate)](BF ) in
nitrile solvents has been investigated in great detail using low
temperature stopped-flow methods.31 More recently some of us
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 8
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
investigated the related complex [Fe
obtained ΔH = +15.0 ± 0.4 kJ mol and ΔS = -146 ± 1 J K mol as exponential fit of absorbance vs. time (inlay in Figure 5) with nitrogen
activation parameters.32 These values are identical (in the range of monoxide concentration in excess leads to the rate constant kobs
2
(H-HPTB)(Cl)
3
] in methanol and absorbance maxima at 452 nm and at 600 nm. AViepweArrfteiccletOonlninee
‡
-1
‡
-1
-1
DOI: 10.1039/D0DT01764G
=
-1
error) with the ones derived from the measurements in propionitrile 0.133 s at -78.2 °C. Activation parameters were calculated from an
‡
-1
‡
-1
‡
-1
-1
for [Fe
2
(Et-HPTB)(benzoate)](BF
4
)
2
(ΔH = +15.4 ± 0.6 kJ mol and Eyring plot to ΔH = +37.7 ± 0.7 kJ mol and ΔS = -66 ± 3 J K mol
‡
-1
-1
ΔS = -121 ± 3 J K mol ). While the reaction most likely must (see Supporting Information, Figure S7). These values are somewhat
proceed in two steps according to the following equations only the different with regard to activation parameters obtained for the
formation of the peroxido complex (c) could be observed in one step. analogous reaction with dioxygen, however, here the product
This easily can be explained by assuming that the formation of the contains a peroxide bridge leading to a highly ordered system instead
iron superoxido complex (b) is rate determining and the consecutive of the two NO ligands coordinated separately to each iron cation.
reaction to the peroxido complex is much faster.
However, similar findings were reported for an iron(II) complex
system with the tripodal ligand 6-Me
3
TPA (6-Me
3
-TPA = tris(6-
of
FeII H-HPTB FeII + O2
FeII H-HPTB FeIII
30
methyl-2-pyridylmethyl)amine.
Activation parameters
‡
-1
‡
-1
-1
O
(6)
ΔH = +29 kJ mol and ΔS = -77 J K mol for the reaction of this
complex with NO are in good agreement with our data. Furthermore,
activation parameters for the reaction of this complex with dioxygen
O
a
b
‡
-1
‡
-1
-1
FeII H-HPTB FeIII
FeIII H-HPTB FeIII
(ΔH = +17 ± 2 kJ mol and ΔS = -175 ± 20 J K mol ) also fit quite
well
our
data
for
the
[Fe
2
H-HPTB(Cl)
3
-1
]
complex
O
O
O
O
(7)
‡
-1
‡
-1
(ΔH = +15.4 ± 0.6 kJ mol and ΔS = -121 ± 3 J K mol ) taking the
b
c
large errors of the values for the activation entropy into account.
Comparing the activation parameters of these reactions we can now
support the suggestion made earlier, that both the nitrosylation and
oxidation of these complexes share a similar associative mechanism
with a somewhat later transition state in the reaction with NO,
Reacting NO instead of dioxygen with the iron(II) Et-HPTB complex
showed an NO molecule coordinated to each of the two iron ions
Scheme 6, anions = 2 BF
4
-, X = NO, Y = benzoate) in the product. The
molecular structure of this complex has been characterized
previously.33,34 More recently an iron HPTB complex with only one
NO ligand coordinated has been reported, however, not by using NO
itself as a reactant.35 So far no kinetic studies have been performed
with this system that could give more details on the reactivity of the
iron(II) R-HPTB complexes.
(
‡
‡ 30
leading to a partial compensation effect between ΔH and ΔS .
Experimental
Materials and methods
Complexes and Ligands H-HPTB and bztpen were prepared and
characterized as previously described.
3
6,37
Solvents and
2 3
When [Fe (H-HPTB)(Cl) ] in methanol was reacted with NO a fast
reaction was observed. The time resolved UV-vis spectra are
reported in Figure 5.
reagents used were of commercially available reagent quality.
1
3
1
C-NMR and H-NMR spectra were measured on a Bruker
Avance II 400 MHz and Avance III 400 MHz HD spectrometer.
Electrospray-ionization MS (ESI-MS) measurements were
performed on a Bruker micro-TOF mass spectrometer. All
measurements under inert conditions were carried out in argon
or nitrogen atmosphere by standard Schlenk techniques or
working in a glove box (MBraun, Garching, Germany). For these
experiments extra dry solvents were distilled under argon
atmosphere with a drying agents and transferred into the glove
box. For reactions with nitrogen monoxide a gastight syringe
was filled with methanol only and saturated with in-situ
prepared nitrogen monoxide (see below) by bubbling a gas
1
1
0
0
.5
.0
.5
.0
1
0
0
.2
.8
.4
0
10
20
time / s
30
40
-3
-1 38
stream through the solvent for 15 min (14.5 ∙10 mol L ).
Different concentrations of nitrogen monoxide were achieved
by mixing a saturated nitrogen monoxide solution with argon
saturated pure methanol, thus achieving lower concentrations.
Due to the mixing of complex and nitrogen monoxide solution
in the stopped-flow instrument, the maximum nitrogen
4
00
450
500
550
600
650
wavelength / nm
Figure 5: Time resolved stopped-flow UV-vis spectra of [Fe
nitrogen monoxide at -78.2 °C in methanol
NO = 7.25 ∙ 10- mol L , after mixing). Inlay (time trace): Absorbance vs. time at 452 nm
black: experimental, red: exponential fit).
2
(H-HPTB)(Cl)
3
] reacting with
(ccomplex = 1 ∙ 10-3 mol L-1,
monoxide concentration is 7.25 ∙10-3 mol L-1
.
3
-1
c
(
Low-temperature stopped-flow measurements
For low-temperature stopped-flow measurements HI-TECH
Scientific CSF-61DX2 and SF-61SX2 instruments (TgK Scientific,
To slow down the reaction it has been studied in a temperature range
between -81 °C and -40 °C. The nitrosyl complex is observed with
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 6 of 8
ARTICLE
Journal Name
‡
-1
-1
Bratford on Avon, UK) were used. Setup and kinetic ΔS = -65.0 ± 3.0 J K mol for the reaction of the iron H-HPTB
View Article Online
DOI: 10.1039/D0DT01764G
measurements procedure already were described in detail complex are similar to the data obtained for an iron 6-Me
previously. The kinetic data were analysed with the integrated complex
software Kinetic Studio (Version 5.02 Beta, TgK Scientific). For dichloromethane) described previously.30 This is the only report we
3
TPA
in
3
9
‡
-1
‡
-1
-1
(ΔH = +29 kJ mol
and
ΔS = -77 J K mol
-3
-1
the investigation of MNIC formation a 4 ∙10 mol L iron(II)
are aware of, in which such a reaction has been described in the past.
Furthermore, activation parameters for the reaction of this complex
with dioxygen (ΔH = +17 ± 2 kJ mol and ΔS = -175 ± 20 J K mol )
also fit quite well our data for the [Fe (H-HPTB)(Cl) ] complex
chloride tetrahydrate solution with 2.5 times excess of
-3
-1
triethylammonium chloride (10 ∙10 mol L ) was prepared. To
‡
-1
‡
-1
-1
check the NO dependency of MNIC formation, these
2
3
-3
-1
experiments were also carried out with a 0.6 ∙10 mol L
‡
-1
‡
-1
-1
(
ΔH = +15.4 ± 0.6 kJ mol and ΔS = -121 ± 3 J K mol ) taking the
-
3
-1
complex solution (0.6 ∙10 mol L iron(II) chloride tetrahydrate,
1
large errors of the values for the activation entropy into account. In
general, it is quite difficult to compare the activation parameters for
the formation of iron NO complexes because they have been
obtained under quite different reaction conditions.
-3
-1
.5 ∙10 mol L triethylammonium chloride) in order to obtain
better conditions for pseudo first order with higher nitrogen
monoxide excess. For the investigation of DNIC formation a
-
3
-1
2
∙10 mol L iron(II) chloride tetrahydrate with five times
-3
-1
excess (10 ∙10 mol L ) triethylammonium chloride and
Understanding the mechanisms of these reactions is not only
important for the biological systems described in the introduction
but furthermore for industrial setups, such as reactions in bubble
flow columns. Knowing kinetic parameters for these reactions e. g.
for the formation of MNIC allow further optimization of future
applications.
-3
-1
triethylamine (2 ∙10 mol L ) was prepared (to avoid formation
of iron precipitation triethylamine was added at the end). The
complex solutions were prepared in a glove box and filled into
gastight syringes. For the reaction of the iron(II) bztpen complex
with nitrogen monoxide a 1 ∙10 mol L solution with
bis(acetonitrile)iron(II) triflate and bztpen in a ratio of 1:1 was
prepared. For the reaction of the diiron(II) H-HPTB complex with
nitrogen monoxide a 2 ∙10 mol L complex solution was
prepared with anhydrous iron(II) chloride in a 2:1 ratio of iron
salt to ligand.
-3
-1
-3
-1
Conflicts of interest
There are no conflicts to declare.
In-situ preparation of nitrogen monoxide
A suspension of 90 g (0.32 mol) iron(II) sulphate heptahydrate Acknowledgements
in 200 ml (2.04 mol) sulphuric acid was filled in a dropping
The authors gratefully acknowledge financial support from the
German Research Foundation (DFG, Deutsche
Forschungsgemeinschaft) received within the scope of SPP 1740
Influence of local transport processes in chemical reactions in
funnel and added in small portions to 140 g (2.03 mol) sodium
nitrite in a Schlenk flask. Before starting to prepare nitrogen
monoxide the whole apparatus was flushed with nitrogen to
remove oxygen. The gas stream was then passed through wash
bottles filled with concentrated sodium hydroxide solution to
remove nitric dioxide and solid sodium hydroxide was used for
drying.
(
bubble flows), SCHI 377/13-1 and KL 624/18-1).
References
1
J. F. Demmink, I. C. F. van Gils and A. A. C. M. Beenackers,
Ind. Eng. Chem. Res., 1997, 36, 4914–4927.
2
FeSO4 + 3 H2SO4 + 2 NaNO2
2 2 4 3
2 NO + 2 H O + Fe (SO ) + 2 NaHSO4
2
M. W. Hlawitschka, M. Oßberger, C. Backes, P. Klüfers and
H. J. Bart, Oil Gas Sci. Technol., 2017, 72, 1–11.
Conclusions
3
4
D. E. Koshland Jr., Science, 1992, 258, 1861–1861.
P. C. Ford and I. M. Lorkovic, Chem. Rev., 2002, 102, 993–
Reactions of iron(II) complexes, for example iron aminocarboxylate
derivatives (e.g. [Fe(edta)]2- complexes and derivatives) react
extremely fast with nitrogen monoxide.20,40 For applications in
industry there is high interest in gaining better understanding of
mass transfer in reactive bubbly flows and detailed kinetic studies
are necessary for high level calculations.2,41 However, reactivity
studies with these complexes were performed in water and it was
necessary to use fast measurement techniques such as laser flash
photolysis to obtain rate constants and activation parameters. To the
best of our knowledge there are nearly no reports of the kinetics of
NO binding to iron(II) complexes in organic solvents. Using methanol
as a solvent and working with a different group of iron(II) compounds
we were able to study the kinetics of iron-NO complex formation in
methanol using low temperature stopped-flow techniques.
1
017.
F. Roncaroli, J. A. Olabe and R. van Eldik, Inorg. Chem.,
002, 41, 5417–5425.
5
6
7
8
9
1
2
A. In-Iam, M. Wolf, C. Wilfer, D. Schaniel, T. Woike and P.
Klüfers, Chem. Eur. J., 2019, 25, 1304–1325.
M. L. Tsai, C. C. Tsou and W. F. Liaw, Acc. Chem. Res., 2015,
48, 1184–1193.
J. Fitzpatrick and E. Kim, Acc. Chem. Res., 2015, 48, 2453–
2461.
A. Buet and M. Simontacchi, Ann. N. Y. Acad. Sci., 2015,
1
340, 39–46.
0
D. R. Truzzi, O. Augusto, A. V. Iretskii and P. C. Ford, Inorg.
Chem., 2019, 58, 13446–13456.
T.-T. Lu, S.-J. Chiou, C.-Y. Chen and W.-F. Liaw, Inorg.
‡
-1
11
Activation
parameters
of
ΔH = +37.7 ± 0.7 kJ mol
and
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 8
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
Chem., 2006, 45, 8799–8806.
C. T. Tran and E. Kim, Inorg. Chem., 2012, 51, 10086–
0088.
R. Eisenberg and C. D. Meyer, Acc. Chem. Res., 1975, 8, 26–
4.
40
41
M. Wolak and R. van Eldik, Coord. Chem. Rev., 2002, 230,
View Article Online
263–282.
D. Merker, L. Böhm, M. Oßberger, P. Klüfers and M.
Kraume, Chem. Eng. Technol., 2017, 40, 1391–1399.
DOI: 10.1039/D0DT01764G
1
1
2
3
1
3
1
1
1
4
5
6
J. A. McCleverty, Chem. Rev., 1979, 79, 53–76.
J. C. Fanning, Coord. Chem. Rev., 1991, 110, 235–273.
A. Wanat, T. Schneppensieper, G. Stochel, R. van Eldik, E.
Bill and K. Wieghardt, Inorg. Chem., 2002, 41, 4–10.
G. Monsch and P. Klüfers, Angew. Chem. Int. Ed., 2019, 58,
1
1
1
2
2
2
7
8
9
0
1
2
8
566–8571.
J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974,
3, 339–406.
K. A. Pearsall and F. T. Bonner, Inorg. Chem., 1982, 21,
978–1985.
1
1
T. Schneppensieper, A. Wanat, G. Stochel and R. van Eldik,
Inorg. Chem., 2002, 41, 2565–2573.
F. Roncaroli, J. A. Olabe and R. van Eldik, Inorg. Chem.,
2
003, 42, 4179–4189.
T. Nebe, A. Beitat, C. Würtele, C. Dücker-Benfer, R. van
Eldik, C. J. McKenzie and S. Schindler, Dalt. Trans., 2010,
3
9, 7768–7773.
2
2
3
4
A. Hazell, C. J. McKenzie, L. P. Nielsen, S. Schindler and M.
Weitzer, J. Chem. Soc. Dalt. Trans., 2002, 310–317.
D. Wang, K. Ray, M. J. Collins, E. R. Farquhar, J. R. Frisch, L.
Gómez, T. A. Jackson, M. Kerscher, A. Waleska, P. Comba,
M. Costas and L. Que, Chem. Sci., 2013, 4, 282–291.
Y. Liang, J. Wei, X. Qiu and N. Jiao, Chem. Rev., 2018, 118,
2
5
4
912–4945.
2
2
6
7
A. L. Feig and S. J. Lippard, Chem. Rev., 1994, 94, 759–805.
P. P. Chandrachud and D. M. Jenkins, Tetrahedron Lett.,
2
015, 56, 2369–2376.
2
2
3
3
3
3
3
8
9
0
1
2
3
4
M. E. Sosa Torres, J. P. Saucedo-Vázquez and P. M. H.
Kroneck, The magic of dioxygen, 2015, vol. 15.
L. M. Blomberg, M. R. A. Blomberg and P. E. M. Siegbahn, J.
Inorg. Biochem., 2005, 99, 949–958.
S. V. Kryatov, E. V. Rybak-Akimova, V. L. MacMurdo and J.
Que, Inorg. Chem., 2001, 40, 2220–2228.
A. L. Feig, M. Becker, S. Schindler, R. van Eldik and S. J.
Lippard, Inorg. Chem., 1996, 35, 2590–2601.
A. Miska, D. Schurr, G. Rinke, R. Dittmeyer and S. Schindler,
Chem. Eng. Sci., 2018, 190, 459–465.
A. L. Feig, M. T. Bautista and S. J. Lippard, Inorg. Chem.,
1
996, 35, 6892–6898.
Y. Jiang, T. Hayashi, H. Matsumura, L. H. Do, A. Majumdar,
S. J. Lippard and P. Moënne-Loccoz, J. Am. Chem. Soc.,
2
014, 136, 12524–12527.
A. Majumdar and S. J. Lippard, Inorg. Chem., 2013, 52,
3292–13294.
3
3
3
3
3
5
6
7
8
9
1
V. McKee, M. Zvagulis, J. V. Dagdigian, M. G. Patch and C.
A. Reed, J. Am. Chem. Soc., 1984, 106, 4765–4772.
L. Duelund, R. Hazell, C. J. McKenzie, L. P. Nielsen and H.
Toftlund, J. Chem. Soc. Dalt. Trans., 2001, 152–156.
M. D. Lim, I. M. Lorković and P. C. Ford, Methods Enzymol.,
2
005, 396, 3–17.
M. Weitzer, M. Schatz, F. Hampel, F. W. Heinemann and S.
Schindler, J. Chem. Soc. Dalt. Trans., 2002, 686–694.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Dalton Transactions
Page 8 of 8
View Article Online
DOI: 10.1039/D0DT01764G
Table of Contents
Kinetic studies on the reaction of NO with iron(II) complexes using low temperature
stopped-flow techniques
-------------------------------------------------------------
Quite different reaction mechanisms were observed in a kinetic study of NO binding to
iron(II) complexes.