A Monohydrosulfidodinitrosyldiiron Complex That Generates N2O as a Model for Flavodiiron Nitric Oxide Reductases: Reaction Mechanism and Electronic Structure

Article abstract of DOI:10.1021/acs.inorgchem.1c00429

Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by mediating the reduction of nitric oxide (NO) to nitrous oxide (N2O). The proposed mechanism for the catalytic reduction of NO by FNORs involves a dinitrosyldiiron intermediate with a [hs-{FeNO}7]2 formulation, which produces N2O and a diferric species. Moreover, both NO and hydrogen sulfide (H2S) have been implicated in several similar physiological functions in biology and are also known to cross paths in cell signaling. Here we report the synthesis, spectroscopic and theoretical characterization, and N2O production activity of an unprecedented monohydrosulfidodinitrosyldiiron compound, with a [(HS)hs-{FeNO}7/hs-{FeNO}7] formulation, that models the key dinitrosyl intermediate of FNORs. The generation of N2O from this unique compound follows a semireduced pathway, where one-electron reduction generates a reactive hs-{FeNO}8 center via the occupation of an Fe-NO antibonding orbital. In contrast to the well-known reactivity of H2S and NO, the coordinated hydrosulfide remains unreactive toward NO and acts only as a spectator ligand during the NO reduction process.

Full text of DOI:10.1021/acs.inorgchem.1c00429

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A Monohydrosulfidodinitrosyldiiron Complex That Generates N O
2
as a Model for Flavodiiron Nitric Oxide Reductases: Reaction
Mechanism and Electronic Structure
Nabhendu Pal, Corey J. White, Serhiy Demeshko, Franc Meyer, Nicolai Lehnert,* and Amit Majumdar*
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ABSTRACT: Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by
mediating the reduction of nitric oxide (NO) to nitrous oxide (N O). The proposed mechanism for the catalytic reduction of NO by
2
7
FNORs involves a dinitrosyldiiron intermediate with a [hs-{FeNO} ] formulation, which produces N O and a diferric species.
2
2
Moreover, both NO and hydrogen sulfide (H S) have been implicated in several similar physiological functions in biology and are
2
also known to cross paths in cell signaling. Here we report the synthesis, spectroscopic and theoretical characterization, and N O
2
7
7
production activity of an unprecedented monohydrosulfidodinitrosyldiiron compound, with a [(HS)hs-{FeNO} /hs-{FeNO} ]
formulation, that models the key dinitrosyl intermediate of FNORs. The generation of N O from this unique compound follows a
2
8
semireduced pathway, where one-electron reduction generates a reactive hs-{FeNO} center via the occupation of an Fe−NO
antibonding orbital. In contrast to the well-known reactivity of H S and NO, the coordinated hydrosulfide remains unreactive toward
2
NO and acts only as a spectator ligand during the NO reduction process.
1
5
INTRODUCTION
N O. Quite similar to NO, hydrogen sulfide (H S) has also
2 2
■
16
been recognized as a possible endogenous gasotransmitter
and has been implicated in several physiological processes
such as cardiovascular effects, vasodilation,
Nitric oxide (NO) plays important roles in cardiovascular
1
2−4
17
maintenance, nerve signal transduction,
and the immune
18
19,20
defense mechanism of mammals against invading patho-
and neuro-
3
−5
gens.
While inducible NO synthase can produce up to
21,22
modulation.
H S (pK = 6.9) remains as an equilibrium
2 a
micromolar concentrations of NO as a response to bacterial
−
mixture of hydrosulfide (HS ) and H S with a ratio of 3:1 at a
biological pH of 7.4. Therefore, any or both of these two
species may act as ligands for the metal ions available in
biological systems. NO and H S have also been implicated to
cross paths in cell signaling, via the formation of HNO,
6
2
infection, few microbes such as Thermotoga maritima,
Moorella thermoacetica, and Desulfovibrio gigas express the
enzyme flavodiiron nitric oxide reductase (FNOR), which
reduces the toxic molecule NO to less toxic nitrous oxide
2
7
−10
(
N O).
The FNOR activity thus protects the microbes
2
11
from nitrosative stress under anaerobic conditions. The
active site of FNORs comprises two nonheme Fe centers, each
one usually coordinated by two histidines and a glutamate.
23,24
(
H)SNO, (H)SSNO, and polysulfides.
12,13
Special Issue: Renaissance in NO Chemistry
Received: February 10, 2021
The coordination spheres are completed by two bridging
ligands: a hydroxide and an aspartate. While four different
mechanisms have been proposed for the reduction of NO to
7
−9,12−14
N O by FNORs,
a recent study has revealed that the
2
reaction proceeds through the successive formation of mono-
and dinitrosyldiiron species, of which the latter produces
©
XXXX American Chemical Society
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A

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Functional model complexes of FNORs include cis-
of standing at room temperature, to afford greenish-brown block-
shaped crystals. The crystals were washed several times with
dinitrosyldiiron complexes that can produce N O in
2
25
tetrahydrofuran (THF) and Et O followed by drying under vacuum
quantitative yield either directly (by N.L.) or upon reduction
at room temperature (by N.L.) via a semireduced mechanism
by N.L. and F.M.) or upon photolysis at low temperature.
2
26
to yield 38.0 mg (81%) of 1(BF ) . The identity of the compound
4
2
2
7
28
obtained was confirmed by a single-crystal X-ray structure
determination. Anal. Calcd for C H B F Fe N O S [1(BF ) ]:
(
4
6
57
2
8
2
13
4
1
4 2
However, a trans-dinitrosyldiiron complex was found (by
C, 47.09; H, 4.90; N, 15.52. Found: C, 46.85; H, 5.15; N, 15.39. ESI-
MS in acetonitrile (MeCN; saturated with NO). Found (calcd) for
29
F.M.) to be ineffective toward the production of N O.
2
3
+
Recently, we have reported the only example of a model
[
Fe (N-Et-HPTB)(SH)(NO) ] : m/z 464.1357 (464.1347). Ab-
2
2
3
0
31
−1
−1
mononitrosyldiiron compound and its dinitrosyl analogue,
both of which could produce N O upon reduction by following
sorption spectrum [DMF; λmax, nm (ε , M cm )]: 342 (3450 ±
M
1
−
40), 525(245 ± 35), 600 (210 ± 5). IR: νSH = 2516 cm (KBr
2
31
−1
−1
pellet), νNO = 1785 cm (KBr pellet), 1796 cm (in MeCN).
the superreduced and semireduced pathways, respectively.
−1
−1
Mo
2
̈
ssbauer: δ = 0.65 mm s (ΔE = 1.39 mm s ) at 80 K. XPS: S
p at 162.0 eV. EPR: EPR-silent at 4 K.
However, the effect of coligands on the N O yield and NO
Q
2
reduction activity of model complexes has not been explored
−
Alternative Synthesis of 1(BF Using NO Gas. To a solution of
)
4 2
yet. Furthermore, despite the intriguing reactions of H S/HS
2
2
(BF ) ·DMF (0.04 mmol, 53.28 mg) in 2 mL of DMF was added
4 2
with NO in biological systems, studies on the reactivity of
NO gas (purged) for 2 min. The solution was filtered, and Et O was
−
2
H S/HS toward NO in nonheme diiron complexes are
2
allowed to diffuse into the filtrate overnight at −35 °C with an
additional 1 day of standing at room temperature to afford greenish-
brown block-shaped crystals. The crystals were washed several times
unavailable in the literature. Elucidating the mechanism of NO
reduction by FNORs as well as understanding the interaction
−
of NO with H S/HS in functional nonheme diiron complexes
with THF and Et O followed by drying under vacuum to yield 40.2
2
2
is therefore of significant interest.
mg (85%) of 1(BF ) . The identity of the compound obtained was
4 2
−
confirmed by single-crystal X-ray structure determination and IR
spectroscopy.
The reactivity of H S/HS toward NO in nonheme iron
2
complexes has been explored so far only in the context of
3
2−35
[Fe (N-Et-HPTB)(SH)(NO) (DMF)](ClO ) [1(ClO ) ]. To a mixture
2
2
4 2
4 2
sodium nitroprusside,
complexes,
dinitrosyl- and mononitrosyliron
3
6−40
41−46
of HN-Et-HPTB (0.08 mmol, 57.8 mg), Et
and NaS Bu (0.12 mmol, 15 mg) in 2 mL of DMF was added
3
N (0.12 mmol, 12.1 mg),
and iron−sulfur clusters,
are distinctly different classes of compounds compared with
which, however,
t
Fe(ClO ) ·6H O (0.16 mmol, 58.0 mg) with stirring, and the
4
2
2
nonheme diiron complexes. Considering the importance of the
resultant slurry was stirred for 6 h. The reaction mixture was filtered.
Et O was added to the filtrate to afford [Fe (N-Et-HPTB)(SH)-
−
reactivity of NO with H S/HS in biology, we planned to
2
2
2
investigate the reactivity of NO toward nonheme diiron(II)
hydrosulfide complexes in the context of FNOR activity.
Recently, one of us (A.M.) reported a series of nonheme
(H O)](ClO ) ·DMF [2(ClO ) ·DMF] as a pale-yellow solid, which
2 4 2 4 2
was used directly in the next step. To a solution of 2(ClO
) ·DMF
4 2
(
0.04 mmol, 54.36 mg) in 1 mL of DMF was added Ph CSNO (0.32
3
47,48
mmol, 96.0 mg) in 1 mL of DMF, and the resulting solution was
stirred for 4 h to obtain a dark-green solution. The solution was
diiron(II) hydrosulfide complexes
that provide an excellent
−
opportunity for investigating the reactivity of HS and NO in
this class of diiron complexes. Here we report the synthesis,
extensive characterization, and NO reduction activity of an
unprecedented monohydrosulfidodinitrosyldiiron compound,
filtered, and Et O was allowed to diffuse into the filtrate overnight at
2
−
35 °C with an additional 1 day of standing at room temperature to
afford greenish-brown block-shaped crystals. The crystals were
washed several times with THF and Et O followed by drying under
2
[
Fe (N-Et-HPTB)(SH)(NO) (DMF)](BF ) [1(BF ) , where
2 2 4 2 4 2
49
vacuum to yield 37.0 mg (77%) of 1(ClO ) . The identity of
4
2
N-Et-HPTB is the anion of N,N,N′,N′-tetrakis[2-(l-ethyl-
1(ClO ) was confirmed by single-crystal X-ray structure determi-
4 2
benzimidazolyI)]-2-hydroxy-1,3-diaminopropane], which fea-
nation. Anal. Calcd for C46
DMF·H O]: C, 45.63; H, 5.16; N, 15.20. Found: C, 45.38; H, 5.59;
N, 14.88. ESI-MS in MeCN. Found (calcd) for [Fe (N-Et-
H57Cl Fe N O S ·DMF·H O [1(ClO ) ·
2 2 13 12 1 2 4 2
7
7
tures a [(HS)hs-{FeNO} /hs-{FeNO} ] formulation. To the
best of our knowledge, there is no analogue with any
dinucleating ligand system in the literature for the unique
structural features of complex 1.
2
2
2
+
HPTB)(SH)(NO) ] : m/z 464.1347 (464.1347). IR (KBr pellet):
2
−1
−1
νSH = 2509 cm , ν = 1782 cm .
NO
[
Fe (N-Et-HPTB)(SH)(NO) (DMF)](OTf) [1(OTf) ]. To a mixture of
2 2 2 2
HN-Et-HPTB (0.08 mmol, 57.8 mg), Et N (0.12 mmol, 12.1 mg),
and NaS Bu (0.12 mmol, 15 mg) in 2 mL of DMF was added
3
EXPERIMENTAL SECTION
t
■
Preparation of Compounds. All reactions and manipulations
were performed under a pure argon atmosphere using either standard
Schlenk techniques or an inert atmosphere box. Solvents were dried
Fe(OTf)
was stirred for 6 h. The reaction mixture was filtered. Et
to the filtrate to afford [Fe (N-Et-HPTB)(SH)(H O)](OTf)
[2(OTf) ·DMF] as a pale-yellow solid, which was directly used in the
next step. To a solution of 2(OTf) ·DMF (0.04 mmol, 58.50 mg) in 1
mL of DMF was added Ph CSNO (0.32 mmol, 96.0 mg) in 1 mL of
DMF, and the resulting solution was stirred for 4 h to obtain a dark-
green solution. The solution was filtered, and Et O was allowed to
2
(0.16 mmol, 56.6 mg) with stirring, and the resultant slurry
O was added
·DMF
2
2
2
2
5
0,51
following standard procedures.
xH O, Fe(OTf) , Cp Fe, Cp Co, Et N, and NaS Bu were obtained
from commercial sources and used without further purification. HN-
Et-HPTB
following the procedures reported in the literature. In the preparations
that follow, all of the filtrations were performed through Celite, and
solvent removal steps were carried out in vacuo inside an inert (argon
gas) atmosphere box. Yields are for recrystallized compounds and are
the average of individual yields obtained from multiple batches of
reactions, calculated using the corresponding molecular weights of the
compound shown in Table S1.
Fe(BF ) ·6H O, Fe(ClO ) ·
2
4
2
2
4 2
t
2
2
2
2
3
2
3
49,52
43
and tritylnitrosothiol (Ph CSNO) were prepared
3
2
diffuse into the filtrate overnight at −35 °C with an additional 1 day
of standing at room temperature to afford greenish-brown block-
shaped crystals. The crystals were washed several times with THF and
Et
1(OTf)
ray structure determination. Anal. Calcd for C H F Fe N O S ·
O followed by drying under vacuum to yield 40.0 mg (74%) of
2
. The identity of 1(OTf) was confirmed by single-crystal X-
2
2
4
8
57
6
2
13 10 3
[
Fe (N-Et-HPTB)(SH)(NO) (DMF)](BF ) [1(BF ) ]. To a solution of
3DMF·Et O [1(OTf) ·3DMF·Et O]: C, 46.04; H, 5.57; N, 14.08.
2
2
4 2
4 2
2 2 2
[
Fe (N-Et-HPTB)(SH)(H O)](BF ) ·DMF [2(BF ) ·DMF; DMF =
Found: C, 45.76; H, 5.44; N, 14.37. ESI-MS in MeCN. Found (calcd)
2
2
4
2
4 2
2
+
N,N-dimethylformamide] (0.04 mmol, 53.28 mg) in 1 mL of DMF
for [Fe (N-Et-HPTB)(SH)(NO) ] : m/z 464.1347 (464.1347). IR
2
2
−
1
−1
was added Ph CSNO (0.32 mmol, 96.0 mg) in 1 mL of DMF, and the
(KBr pellet): νSH = 2532 cm , ν = 1782 cm .
3
NO
resulting solution was stirred for 4 h to obtain a dark-green solution.
Reaction of 1(BF ) with 1 equiv of Cp Co. To a solution of
4
2
2
The solution was filtered, and diethyl ether (Et O) was allowed to
1(BF ) (0.085 mmol, 100 mg) in 5 mL of CH Cl was added a
2
4
2
2
2
diffuse into the filtrate overnight at −35 °C, with an additional 1 day
solution of Cp Co (0.085 mmol, 16.1 mg) in 2 mL of CH Cl . The
2 2 2
B
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color of the solution immediately changed from greenish to dark
brown. The resulting solution was stirred for 1 h and evaporated to
dryness. The residue was washed twice with MeOH, dissolved in 2
Kα X-ray source (1486.8 eV) in the XPS instrument (Omicron Nano
Technology 0571), after the sample surface was etched by argon-ion
sputtering. The peak fitting was done using Peak Fit, version 4.12,
with the Gaussian deconvolution method.
mL of DMF, and filtered. Et O was allowed to diffuse into the filtrate
2
overnight at room temperature to yield a light-brown crystalline solid
N O Yield Calculation. Compound 1(BF ) (about 5 μM) was
2
4 2
(
61 mg).
dissolved in 2 mL of CH Cl in a septum-sealed 25 mL round-
2 2
Reaction of 1(BF ) with 2 equiv of Cp CO. To a solution of
bottomed flask with a 14/20 joint. A total of 0.5, 1, or 2 equiv of
4
2
2
1
(BF ) (0.085 mmol, 100 mg) in 5 mL of CH Cl was added a
CoCp (with respect to the concentration of 2) was dissolved in 0.5
4
2
2
2
2
solution of Cp Co (0.17 mmol, 32.2 mg) in 2 mL of CH Cl . The
mL of CH Cl and syringed into the flask. The solution was stirred for
2
2
2
2
2
color of the solution immediately changed from greenish to dark
brown. The resulting solution was stirred for 1 h and evaporated to
dryness. The residue was washed twice with MeOH, dissolved in 2
exactly 5 min prior to evacuation of the headspace of the flask via
cannula transfer to an evacuated Pike HT gas-IR cell (190 mTorr) for
exactly 20 s. After subtraction of a CH Cl blank taken under the
2
2
mL of DMF, and filtered. Et O was allowed to diffuse into the filtrate
overnight at room temperature to yield a light-brown crystalline solid
exact same conditions, the IR spectrum was converted to absorbance
2
and the N−N stretch of N O was integrated using a straight-line
2
−
1
(
55 mg).
baseline correction from 2150 to 2275 cm . The integration was then
54
General Physical Methods. Elemental analysis was performed
compared to a standard curve.
using a PerkinElmer 2400 series II CHNS analyzer. A cyclic
X-ray Structure Determination. The molecular structures of
1(BF ) , 1(ClO ) , and 1(OTf) were determined by single-crystal X-
−
3
voltammetry study of 1(BF ) (10 M) in DMF was performed
4
2
4
2
4
2
2
using a CHI620E electrochemical analyzer (CH Instruments, USA).
A three-electrode setup comprised of a glassy carbon working
electrode, a platinum wire auxiliary electrode, and a silver wire
pseudoreference electrode was employed. Tetra-n-butylammonium
hexafluorophosphate (0.1 M) was used as the supporting electrolyte.
Electrochemical potentials were referenced internally to the
ferrocenium/ferrocene couple at 0.0 V. Electronic absorption spectra
were recorded using a Cary 60 UV−vis spectrophotometer. IR spectra
of the solid samples as KBr pellets were recorded using a PerkinElmer
Spectrum BX Fourier transform infrared (FT-IR) spectrometer. The
solution FT-IR data were measured on a PerkinElmer FT-IR
ray structure determinations. Diffraction-quality crystals were
obtained as described in the synthesis of the respective compound.
Single crystals were coated with Parabar oil and mounted under a 100
K nitrogen cold stream. Data collections were performed on a Bruker
D8VENTURE Microfocus diffractometer equipped with a PHOTON
II detector, with Mo Kα radiation (λ = 0.71073 Å), controlled by the
APEX3 (version 2017.3-0) software package. The raw data were
integrated and corrected for Lorentz and polarization effects with the
55
aid of the Bruker APEX II program suite. Absorption corrections
were performed by using SADABS. Space groups were assigned by
systematic absences (determined by XPREP) and analysis of the
56,57
spectrometer (Frontier) instrument. The Mo
recorded using an alternating constant WissEl Mo
̈
ssbauer spectrum was
ssbauer spectrom-
metric symmetry and further checked by PLATON
for additional
̈
symmetry. Structures were solved by direct methods and refined
against all data in the reported 2θ ranges by full-matrix least squares
eter, consisting of an MR 360 Drive Unit, an MV-1000 velocity
transducer, and an LND 45431 proportional counter mounted on an
LINOS precision bench. The system was operated in a horizontal
transmission geometry with the source, absorber, and detector in a
linear arrangement. The temperature was controlled and maintained
using a Janis SHI closed-cycle helium cryostat. Measurements were
performed at 80 K. Data acquisition was performed using a 512-
channel analyzer. Isomer shifts were referenced versus α-iron metal
foil at ambient temperatures. Simulation of the experimental data was
performed using the Mfit program. Magnetic susceptibility measure-
ment of 1(BF ) was conducted on a Quantum-Design MPMS XL-5
2
58
59
on F using the SHELXL program suite in the OLEX2 interface. H
atoms at idealized positions were included in the final refinements.
The OLEX2 interface was used for structure visualization as well as for
60,61
drawing ORTEP
plots. The crystallographic data and final
agreement factors are provided in Table S1. The individual CIFs
include the refinement details and explanations (wherever applicable).
CCDC 2035675−2035677 contain the supplementary crystallo-
graphic data for this paper.
Computational Methods. Optimization and frequency calcu-
62
lations on complex 1 were performed with Gaussian 09 using the
4
2
63,64
65−67
SQUID magnetometer, equipped with a 5 T magnet. The
polycrystalline sample was contained in a gel bucket, covered with a
drop of low-viscosity perfluoropolyether-based inert oil Fomblin Y45
to fix the crystals, and fixed in a nonmagnetic sample holder. The
maximum measuring temperature of 210 K was chosen because of the
pour point of the oil, in order to keep the oil in the frozen state and to
avoid an orientation of the crystals parallel to the magnetic field. Each
raw data file for the measured magnetic moment was corrected for the
diamagnetic contribution of the gel bucket and of the inert oil. The
raw data were corrected for the diamagnetic contribution of the
B3LYP functional
and 6-311G(d) basis set.
Complex 1
contains two antiferromagnetically coupled Fe centers that were
treated by generating four fragments: one fragment containing the N-
−
−
Et-HPTB ligand scaffold, one containing the hydrosulfide (SH )
ligand, and the other two fragments containing one each of the FeNO
units. An initial guess calculation was performed first in Gaussian 09
by making one of the Fe-containing fragments antiferromagnetically
coupled to the other one. This guess was then utilized to begin single-
point and geometry-optimization calculations. Once optimized, a
frequency calculation was performed, and the final optimized
dia
−6
3
−1
68
compound using χM (sample) = −0.5M × 10 cm ·mol . The data
structure was used in a subsequent Orca 4.0.1.2 single-point
53
were fit with the julX program using the appropriate Heisenberg−
Dirac−van Vleck (HDvV) spin Hamiltonian for isotropic exchange
coupling and Zeeman splitting (eq 1).
calculation to analyze the electronic structure and calculate the
̈
Mossbauer parameters. This single-point calculation was performed
on the Gaussian 09 B3LYP-optimized structure (without fragments)
utilizing the B3LYP functional and 6-311G(d) basis set with the def2/
69
̂
̂
̂
⃗
⃗
⃗
J auxiliary basis set. The ferromagnetic single point was calculated
first, and the spins at one of the FeNO units were then flipped using
the Spin Flip operation in Orca.
H = −2JS S + gμ B(S + S )
1
2
B
1
2
(1)
5
A Curie-behaved paramagnetic impurity (PI) with spin S = / was
2
included in the fit, according to χ(calc) = (1 − PI)χ + PIχ(mono).
Electron paramagnetic resonance (EPR) spectra were recorded
using a Bruker X-band EMX spectrometer equipped with an Oxford
liquid-helium cryostat. Spectra were recorded on ∼2 mM frozen
solutions using a 20 mW microwave power and a 100 kHz field
modulation at a 1 G amplitude. IR spectroelectrochemistry experi-
ments were performed using a LabOmak UF-SEC thin-layer cell, with
platinum mesh working and counter electrodes and a silver wire
pseudoreference electrode. X-ray photoelectron spectroscopy (XPS)
measurements were performed using a focused monochromatized Al
RESULTS AND DISCUSSION
Synthesis and Characterization. Treatment of the
diiron(II) hydrosulfide compound, 2(BF ) , with either
■
47
4
2
excess Ph CSNO or excess NO gas yielded 1(BF ) in 81%
3
4 2
and 85% yields, respectively. The molecular structure of
complex 1 revealed an unsymmetrical diiron(II) unit (Figure
1) where one Fe center coordinates the hydrosulfide and one
NO, while the other Fe center is bound to one NO and one
C
https://doi.org/10.1021/acs.inorgchem.1c00429
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These results prompted us to check the possible reactivity of
+
the coordinated hydrosulfide in 2 and 1 with NO (Scheme 1).
Treatment of 2 with 2.5 equiv of (NO)(BF ) (reduction
4
+
potential = 0.87 and 1 V vs Cp Fe /Cp Fe in MeCN and
2
2
CH Cl , respectively) led to formation of the mixed-valent
2
2
diiron(II,III)-SH complex [Fe (N-Et-HPTB)(SH)(H O)-
2
2
3
+
(
DMF)2] (5) in 77% yield. Synthesis [via treatment of 2
with (Cp Fe)(BF )] and characterization of 5 was previously
2
4
4
7
reported by us. Moreover, treatment of 1 (diNOSH
complex) with 1.5 equiv of (NO)(BF ) again yielded 5 in
4
8
1% yield. Reduction of 5 using 1.5 equiv of Cp Co yielded 2
2
in 68% yield. The formation of 5 in the above-mentioned
reactions has been confirmed by single-crystal X-ray structure
determination, IR spectroscopy, mass spectrometry, and cyclic
voltammetry. These results clearly indicate that the coordi-
Figure 1. Molecular structure of complex 1 with 30% probability
thermal ellipsoids and a partial atom-labeling scheme. H atoms are
omitted (except −SH) for clarity.
+
nated −SH does not react even with NO . Furthermore, 1 was
subjected to a base titration study, which was monitored by IR
spectroscopy (Figure S1) in order to check whether
deprotonation of the coordinated −SH may trigger the
reaction with NO. No such reaction, however, could be
observed.
DMF molecule. The reactivity of coordinated thiolates and
31,70,71
NO is quite well-known in the literature,
although there
are few reports for the coexistence of thiolates and NO in the
7
2−75
The Fe2−SH distance of 2.456(2) Å in 1 is longer than
same complex as well.
On the other hand, NO is known
23,24
47
those reported for 2 [2.344(4) Å] and another diiron(II)
to be highly reactive toward H S,
and to the best of our
2
48
bis(hydrosulfide) compound [2.383(1) and 2.377(1) Å].
The Fe−NO distances of 1.761(5) Å (Fe1−N11) and
.751(5) Å (Fe2−N13), N−O distances of 1.106(6) Å
knowledge, there are no reports in the literature for the
coexistence of NO and hydrosulfide in any nonheme dinuclear
complex. It was anticipated that the incoming NO may react
with the coordinated hydrosulfide to generate functionalities
like HSNO on a diiron(II) platform. However, in contrast to
1
(
N11−O2) and 1.150(6) Å (N13−O3), and Fe−N−O angles
of 166.9(5)° (Fe1−N11−O2) and 160.3(5)° (Fe2−N13−O3)
in 1 are comparable with those reported for the FNOR model
the expectation based on the high reactivity of NO and H S,
2
the coordinated hydrosulfide ligand in 2 did not react with the
incoming NO gas, and, moreover, NO and hydrosulfide coexist
in 1 (Figure 1).
compound, [Fe
2
(N-Et-HPTB)(NO)
(DMF) ](BF )
2 2 4 3
3
1
[3(BF ) ], and its mononitrosyl analogue, [Fe (N-Et-
4
3
2
30
HPTB)(NO)(DMF) ](BF ) [4(BF ) ], and other nitro-
3
4 3
4 3
Scheme 1. Reaction of 1 and 2 with NO+
D
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Table 1. Selected Bond Distances, Angles and IR Stretching Frequencies for 1(BF ) , 1(ClO ) , 1(OTf) , and Related
4
2
4 2
2
Compounds from the Literature
Fe---Fe
separation
(Å)
Fe−N
Fe−SH
∠Fe−N−O ∠Fe1−O1−Fe2
νNO (cm−1)
νSH (cm−1
)
NO
(Å)
compound
N−O (Å)
(Å)
Fe−ODMF (Å)
(deg)
(deg)
(as KBr pellet) (as KBr pellet)
1
1
1
(BF4)2
1.761(5),
1.106(6),
1.150(6)
1.159(5),
1.152(5)
1.127(6),
0.973(6)
2.456(2)
2.406(1)
2.435(2)
2.344(4)
2.146(4)
3.749
3.625
3.679
166.9(5),
160.3(5)
167.4(4),
157.0(4)
165.2(5),
157.6(7)
132.4(2)
1785
1782
1782
2516
2509
2532
2515
1
.751(5)
(ClO4)2
(OTf)2
1.759(3),
.761(4)
2.153(3)
2.095(4)
127.9(2)
129.9(2)
1
1.758(5),
.853(6)
1
4
7
2(BF ) )
3.677
3.669
132.1(3)
131.1(2)
4
2
3
1
3(BF4)3
1.729(7),
.748(7)
1.123(8),
1.147(8)
1.066(7)
2.073(6),
2.153(6)
174.9(8),
164.9(7)
160.5(7)
1782
1768
1
3
0
4(BF4)3
1.775(5)
2.159(4),
3.605
129.6(2)
2
2
.339(5),
.082(4)
Figure 2. XPS spectra for 1(BF ) : (a) full range; (b) S 2p.
4
2
2
6,29,76
sylated diiron compounds (Table 1).
The metric
parameters of 1(ClO ) and 1(OTf) (Figure S2) are similar
4
2
2
to those obtained for 1(BF ) (Table 1). The presence of a
4
2
coordinated hydrosulfide in 1 was confirmed by the
appearance of νSH at 2516 cm− (Figure S3), which is nearly
1
−
1
47
identical with that reported for 2 (2515 cm ) and for
48
another diiron(II) hydrosulfide compound. The presence of
sulfur in 1 was further confirmed by XPS of 1(BF ) , which
4
2
4
7,48
showed the characteristic peak for sulfur (S 2p)
at 162.0
eV (Figure 2b). The ν value of 1785 cm− (as KBR pellet,
1
NO
Figure S3; 1796 cm− in MeCN, Figure S4) for 1(BF ) falls
1
4
2
−
1
within the range of 1720−1840 cm typically observed for
7
26,27,29−31,76−78
high-spin (hs) nonheme {FeNO} complexes.
The amide vibration from the metal-coordinated DMF is
observed at 1654 cm− (as KBR pellet), while the solution IR
spectrum shows the vibrations from metal-coordinated DMF
Figure 3. 57Fe Mo
(BF ) at 80 K.
̈
ssbauer spectrum of a polycrystalline sample of
1
1
4
2
−
1
and free DMF at 1654 and 1670 cm , respectively, which is
2
8,30,31
5
7
consistent with literature reports.
The electronic
ligation of the two Fe centers, the Fe Mo
1(BF ) (Figure 3) features only one quadrupole doublet with
̈
ssbauer spectrum of
absorption spectrum of 1(BF ) (Figure S5) shows three
4
2
4 2
−
1
distinct transitions and is in agreement with the data for other
an isomer shift, δ = 0.65 mm s . Very similar isomer shifts
have been reported for 3 (0.64 mm s ) and other complexes
7
9,30,31,76
−1 31
nonheme hs-{FeNO} complexes.
The broad features
−
1
−1
7
27,29−31,76,77
observed at 600 nm (ε = 210 ± 5 M cm ) and 525 nm (ε =
that feature one or more hs-{FeNO} units.
Similar
−
1
−1
31
2
45 ± 35 M cm ) are attributed to mixed d−d and
to 3, complex 1 was also found to be EPR-silent. A
superconducting quantum interference device (SQUID)
magnetometry measurement of solid 1(BF4)2 revealed
−
III
9,79
NO (π*)-to-Fe charge-transfer transition.
Compound
1
±
(BF ) exhibits an additional shoulder at 425 nm (ε = 518
4
2
−
1
−1
−1
75 M cm ), which sits on the tail of the strong UV band
at 342 nm (3450 ± 40 M cm ). Mass spectrometry of 1 (in
MeCN in the presence of NO; Figure S6) showed the
significant antiferromagnetic coupling with J = −30.8 cm
between the two hs-{FeNO} units (Figure 4), which is very
−
1
−1
7
−
1
31
similar to that observed previously (−28.3 cm ) for 3. This
III −
molecular ion peak at m/z 464.1357 for [Fe (N-Et-HPTB)-
result reflects the Fe −NO electronic structure of the two hs-
7
31
2
2
+
(
SH)(NO)2] (calcd m/z 464.1347). Despite the different
{FeNO} units. Together, the molecular structure determi-
E
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Figure 4. SQUID magnetometry measurements of 1(BF ) . The solid
4
2
Figure 5. Cyclic voltammetric trace (multiple scans) for 1(BF ) in
DMF.
3
4 2
red line represents the best fit using S = S = / for 1 and the
1
2
2
indicated parameters.
II
II
II
II
nation, elemental analysis, mass spectrometry, SQUID
magnetometry, electronic absorption, IR, Mossbauer, EPR,
and XPS measurements thus confirm the formulation of 1 as a
Fe −O−Fe /Fe −OH−Fe species, formed from reduction of
II
III
̈
the initial reaction product, a mixed-valent Fe −O−Fe
species, which is generated upon release of N O from reduced
2
7
7
[(HS)hs-{FeNO} /hs-{FeNO} ] species. Complexes 1−3 and
1.
the dinucleating ligand HN-ET-HPTB are shown in Scheme 2.
A cyclic voltammetry study of 1 showed three irreversible
reduction processes at −1.0, −1.18, and −1.26 V (Figure 5). In
comparison, 3 showed two irreversible reduction processes at
Generation of N O by Complex 1. Considering the
similarity in the structural parameters and spectroscopic
2
properties of 1 with the FNOR functional model complex
31
3, the N O production activity of 1 was examined. Complex
2
3
1
−
4
1.18 and −1.30 V, while its mononitrosyl analogue,
1 does not produce N O in solution in the absence of a
reductant (Figure S4), similar to 3 and other previously
reported dinitrosyldiiron complexes
adducts of ribonucleotide reductase and methane monoxyge-
nase. However, 1 generates N O upon chemical (by
cobaltocene) and electrochemical reduction at room temper-
ature. The latter was examined by IR spectroelectrochemistry
2
3
0
(BF ) , showed only one irreversible reduction at −1.02
4
3
30
26,28,76
V under identical conditions. We therefore propose that the
three reduction events for 1 arise because of the partial
dissociation of one NO to generate some amount of a
mononitrosylated species under electrochemical conditions in
solution. This result suggests that the NO ligands in 1 are
and the dinitrosyl
9
2
−
labile. In contrast, complex 3 (without the SH ligand) does
(Figure 6), which confirmed the formation of N O from 1
2
not show this behavior, which indicates that it is likely the NO
ligand coordinated to the Fe−SH unit that is labile. The new
redox couple generated in the return scan at E = −0.570 (E
without any detectable intermediates. Rapid production of
N O upon reduction of 1 by 2 equiv of cobaltocene was also
2
confirmed by IR spectroscopic analysis of the reaction
pa
pc
31
=
−0.630) for 1 may be attributed to the presence of an
headspace. Integration of the N−N stretching band of N O
2
Scheme 2. Complexes 1−3 and the Dinucleating Ligand HN-ET-HPTB
F
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against a calibration curve revealed ∼91% yield of N O [based
2
on the concentration of 1(BF ) ] within 5 min of the
4
2
cobaltocene addition (Figure S10). Furthermore, it was
observed that even 1 equiv of cobaltocene was sufficient for
the production of N O from 1 with 84% yield. Thus, 1 is
2
following a semireduced mechanism for the production of
27
N O. A semireduced mechanism was previously reported by
2
two of us (N.L. and F.M.) for a dinitrosyldiiron compound,
where it was shown that this mechanism constitutes an
efficient pathway for the reduction of NO to N O by model
2
systems and potentially by FNORs as well. Recently, we have
shown that 3 also follows a semireduced pathway for the
1
reduction of NO to N O and that a broad S = / EPR signal
2
2
(
⟨g⟩ = 1.55, 1.79, 2.03) is generated because of the formation
of a mixed-valent diiron(II,III) species upon one-electron
reduction of 3. In line with these results, a mixture of 1 and 1
equiv of cobaltocene indeed generated an S = / EPR signal
31
1
2
(
⟨g⟩ = 2.05, 2.05, 2.03; Figure 7; see also Figure S15 for EPR
simulation of the mixed-valent reduction product) and thus
further confirmed the semireduced mechanism for the
reduction of NO to N O by 1. The semireduced mechanism
2
is also supported by the mass spectrometric study of a reaction
mixture involving 1 and 1 equiv of cobaltocene, which
indicated the formation of a mixed-valent diiron(II,III) species,
2
+
[
Fe (N-Et-HPTB)(μ-O)] , along with its hydroxo-bridged
2
4
+
dimer, [Fe (N-Et-HPTB) (μ-OH) ] (Figures S11 and S12).
4
2
4
On the other hand, a reaction mixture involving 1 and 2 equiv
of cobaltocene indicated the formation of all-ferrous tetrameric
4
+
species, [Fe (N-Et-HPTB) (μ-OH) (H O) ] and [Fe (N-
4
2
2
2
2
4
4
+
Et-HPTB) (μ-OH) ] (Figures S13 and S14). These results
2
2
further indicate that the coordinated hydrosulfide is likely lost
after N O formation.
2
Theoretical Calculation. Because of the asymmetric
nature of 1 (Figure 1), density functional theory (DFT) was
employed to evaluate the electronic structure of its one-
electron-reduced form and the site of reduction. The B3LYP/
Figure 6. Generation of N O upon the electrochemical reduction of
2
6
-311(G)d-optimized structure of 1 shows Fe−NO bond
1
(BF ) . Experimental conditions: [1(BF ) ] = 11.6 mM in CH Cl ;
hold potential = −1.5 V versus silver wire; 0.1 mM (Et N)(BF ).
4 2 4 2 2 2
7
lengths of 1.787 and 1.793 Å at the hs-{FeNO} (SH) and hs-
4
4
7
{
FeNO} (DMF) sites, respectively, and Fe−N−O bond angles
Figure 7. Left: EPR spectra of 1(BF ) and 2(BF ) in MeCN demonstrating that both of the compounds are initially EPR-silent. Upon one-
4
2
4 2
1
electron reduction of 1 at room temperature in CH Cl , a S = / signal arises. EPR conditions: [1(BF ) ] = ∼2 mM, [2(BF ) ] = 2 mM, 9.336
GHz microwave frequency, 20 mW microwave power, 1 G modulation amplitude, and 10.24 ms time constant. EPR data were recorded at 4 K.
2
2
2
4
2
4 2
Right: Spin count simulation of the EPR spectrum resulting from the reduction of 1 with 1 equiv of CoCp (taken in CH Cl ). Simulation
2
2
2
parameters: g = g = 2.05, g = 2.03; s = 0.017.
x
y
z
g(x,y,z)
G
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of 156.2° and 167.6° (Table S2). These values are all typical
7
9
for hs-{FeNO} complexes and are in good agreement with
the structural parameters from the crystal structure of 1
(
Tables 1 and S2). The B3LYP calculations for 1 overestimate
the frequencies of each N−O stretch, predicting values of 1811
and 1861 cm
NO} (DMF) units, respectively, which is commonly observed
when using hybrid functionals like B3LYP.
−
1
7
for the hs-{FeNO} (SH) and hs-{Fe-
7
80
7
The one-electron reduction to the hs-{FeNO} (SH)/hs-
8
{
FeNO} (DMF) state leads to an elongation of the Fe-NO
8
and N−O bond lengths at the hs-{FeNO} (DMF) center from
1
.793 and 1.164 Å to 1.806 and 1.207 Å, respectively (Table
S3). The Fe−N−O bond angle becomes more bent at 149.8°,
and the N−O stretching frequency significantly decreases to
−
1
1
671 cm . It is important to note that in the gas-phase DFT-
7
optimized structure of the hs-{FeNO} (SH)/hs-{Fe-
NO} (DMF) state, the DMF ligand twists such that the
8
DMF does not coordinate to the Fe center but instead
provides a moderately strong hydrogen-bonding interaction to
NO (H−N distance = 2.90 Å). In solution, the reduced Fe
center likely loses the DMF ligand to the bulk solvent. Single-
point calculations on the optimized structure show that the
additional electron occupies the d _π* orbital (Figure 8),
7
8
Figure 9. Overlay of the two hs-{FeNO} (SH)/hs-{FeNO} (DMF)
structures generated from geometry optimizations on the initial hs-
FeNO} (SH)/hs-{FeNO} (DMF) and hs-{FeNO} (SH)/hs-{Fe-
NO} (DMF) states. It is evident from the figure that the main
structural difference lies in the Fe−SH distance.
yz
y
7
8
8
{
7
7
8
different hs-{FeNO} (SH)/hs-{FeNO} (DMF) structures
overlaid in Figure 9 and structural parameters are compared
in Table S3). The structure resulting from the hs-
(
7
8
{
FeNO} (SH)/hs-{FeNO} (DMF) starting point is 5.3 kcal
−
1
mol lower in energy than the structure resulting from the hs-
8
7
{
FeNO} (SH)/hs-{FeNO} (DMF) starting point. In both
cases, Orca single-point calculations on the optimized
structures show that the antibonding d _π* orbital of the
yz
y
8
hs-{FeNO} (DMF) center is occupied upon one-electron
reduction. A PES scan along the hs-{FeNO} (SH)/hs-
FeNO} (DMF) N-N reaction coordinate indicates that N-N
7
8
{
coupling has a 10.9 kcal/mol activation barrier (Figure S16),
similar to previous calculations indicating a 10 kcal/mol
7
activation barrier for N-N coupling within the hs-{FeNO} /hs-
8
−
80
{
FeNO} state of the BPMP ligand scaffold.
CONCLUSION
In summary, an unprecedented monohydrosulfidodinitrosyl-
diiron compound [1(BF ) ] has been synthesized and
■
4
2
extensively characterized. Complex 1 features a [(HS)hs-
7
7
Figure 8. Contour plot of the Fe−NO antibonding d _π* orbital
{FeNO} /hs-{FeNO} ] formulation and produces N O in
yz
y
2
that is occupied upon one-electron reduction of 1 to the hs-
high yield following a semireduced mechanism, where one-
7
8
{
FeNO} (SH)/hs-{FeNO} (DMF) state.
8
electron reduction generates a reactive hs-{FeNO} center via
the occupation of an Fe−NO antibonding orbital. Isolation of
which is antibonding with respect to the Fe−NO bond. In line
1 confirms that in stark contrast to the known reactivity of H S
2
8
23,24
with this, the spin density at the hs-{FeNO} (DMF) unit
and NO,
the Fe-bound hydrosulfide in 1 does not react
increases (Table S4), suggesting a decreased covalency in the
with NO. Interestingly, the coordinated hydrosulfide in 1 and
2 does not even react with NO either. In spite of featuring a
unique molecular structure, the structural parameters and
+
Fe−NO bond. This finding is also in agreement with the
predicted, dramatic drop in the N−O stretch of the hs-
8
{
FeNO} (DMF) center. The occupation of the antibonding
spectroscopic properties of the FeNO units as well as the N O
2
dyz_π*y orbital leads to a strong activation of the
production activity of 1 are quite similar to that observed for
8
80
corresponding hs-{FeNO} unit, as previously discussed.
Interestingly, the geometry optimizations of the hs-
the previously reported functional model complex of FNORs,
31
3. These results thus strongly indicate that the coordinated
hydrosulfide is acting as a spectator ligand during the
7
8
8
{
FeNO} (SH)/hs-{FeNO} (DMF) and hs-{FeNO} (SH)/
7
hs-{FeNO} (DMF) states ultimately converged to two slightly
generation of N O by 1.
2
H
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(
3) Moncada, S.; Palmer, R. M.; Higgs, E. A. Nitric oxide:
physiology, pathophysiology, and pharmacology. Pharmacol. Rev.
991, 43, 109−142.
4) Ignarro, L. Nitric Oxide: Biology and Pathobiology; Academic
Press: San Diego, CA, 2000.
5) Missall, T. A.; Lodge, J. K.; McEwen, J. E. Mechanisms of
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00429.
■
*
1
(
(
Experimental procedure, spectroscopic data and N O
yield calculation (PDF). (PDF)
2
Resistance to Oxidative and Nitrosative Stress: Implications for
Fungal Survival in Mammalian Hosts. Eukaryotic Cell 2004, 3, 835−
8
46.
Accession Codes
(
6) MacMicking, J.; Xie, Q. W.; Nathan, C. Nitric oxide and
macrophage function. Annu. Rev. Immunol. 1997, 15, 323−350.
7) Kurtz, D. M., Jr. Flavo-diiron enzymes: nitric oxide or dioxygen
reductases? Dalton Trans. 2007, 4115−4121.
8) Khatua, S.; Majumdar, A. Flavodiiron nitric oxide reductases:
CCDC 2035675−2035677 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
(
Recent developments in the mechanistic study and model chemistry
for the catalytic reduction of NO. J. Inorg. Biochem. 2015, 142, 145−
153.
AUTHOR INFORMATION
(9) Berto, T. C.; Speelman, A. L.; Zheng, S.; Lehnert, N. Mono- and
dinuclear non-heme iron−nitrosyl complexes: Models for key
intermediates in bacterial nitric oxide reductases. Coord. Chem. Rev.
■
Corresponding Authors
Nicolai Lehnert − Department of Chemistry, The University of
Michigan, Ann Arbor, Michigan 48109, United States;
orcid.org/0000-0002-5221-5498; Email: lehnertn@
umich.edu
Amit Majumdar − School of Chemical Sciences, Indian
Association for the Cultivation of Science, Kolkata 700032,
West Bengal, India; orcid.org/0000-0003-0522-8533;
Email: icam@iacs.res.in
2
(
013, 257, 244−259.
10) Lehnert, N.; Fujisawa, K.; Camarena, S.; Dong, H. T.; White, C.
J. Activation of Non-Heme Iron-Nitrosyl Complexes: Turning Up the
Heat. ACS Catal. 2019, 9, 10499−10518.
(11) Gardner, A. M.; Helmick, R. A.; Gardner, P. R. Flavorubredox-
in, an Inducible Catalyst for Nitric Oxide Reduction and
Detoxification in Escherichia coli. J. Biol. Chem. 2002, 277, 8172−
8
(
177.
12) Hayashi, T.; Caranto, J. D.; Wampler, D. A.; Kurtz, D. M., Jr.;
Authors
Moënne-Loccoz, P. Insights into the Nitric Oxide Reductase
Mechanism of Flavodiiron Proteins from a Flavin-Free Enzyme.
Biochemistry 2010, 49, 7040−7049.
Nabhendu Pal − School of Chemical Sciences, Indian
Association for the Cultivation of Science, Kolkata 700032,
West Bengal, India
Corey J. White − Department of Chemistry, The University of
Michigan, Ann Arbor, Michigan 48109, United States;
orcid.org/0000-0001-6938-7061
(
13) Silaghi-Dumitrescu, R.; Kurtz, D. M., Jr.; Ljungdahl, L. G.;
Lanzilotta, W. N. X-ray Crystal Structures of Moorella thermoacetica
FprA. Novel Diiron Site Structure and Mechanistic Insights into a
†,‡
Scavenging Nitric Oxide Reductase . Biochemistry 2005, 44, 6492−
501.
14) Blomberg, L. M.; Blomberg, M. A.; Siegbahn, P. M. Theoretical
6
(
Serhiy Demeshko − Institut fu
Universität Göttingen, Göttingen 37077, Germany
Franc Meyer − Institut fur Anorganische Chemie, Universität
Göttingen, Göttingen 37077, Germany; orcid.org/0000-
002-8613-7862
̈
r Anorganische Chemie,
study of the reduction of nitric oxide in an A-type flavoprotein. JBIC,
J. Biol. Inorg. Chem. 2006, 12, 79−89.
̈
(15) Caranto, J. D.; Weitz, A.; Hendrich, M. P.; Kurtz, D. M. The
0
Nitric Oxide Reductase Mechanism of a Flavo-Diiron Protein:
Identification of Active-Site Intermediates and Products. J. Am.
Chem. Soc. 2014, 136, 7981−7992.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00429
(16) Wang, R. Two’s company, three’s a crowd: can H2S be the
third endogenous gaseous transmitter? FASEB J. 2002, 16, 1792−
798.
17) Wang, R. Physiological Implications of Hydrogen Sulfide: A
Author Contributions
1
(
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Whiff Exploration That Blossomed. Physiol. Rev. 2012, 92, 791−896.
(18) Liu, Y.-H.; Lu, M.; Hu, L.-F.; Wong, P. T.-H.; Webb, G. D.;
Bian, J.-S. Hydrogen Sulfide in the Mammalian Cardiovascular
System. Antioxid. Redox Signaling 2012, 17, 141−185.
Notes
The authors declare no competing financial interest.
(19) Hosoki, R.; Matsuki, N.; Kimura, H. The Possible Role of
Hydrogen Sulfide as an Endogenous Smooth Muscle Relaxant in
Synergy with Nitric Oxide. Biochem. Biophys. Res. Commun. 1997, 237,
527−531.
ACKNOWLEDGMENTS
■
0
This work was supported by SERB, India (Grant EMR/2017/
00828 to A.M.) and CSIR India [Grant 01(2972)/19/EMR-
II to A.M.] and by the National Science Foundation (Grant
CHE-2002885 to NL). N.P. acknowledge CSIR, India, for
SRF. F.M. acknowledges support from the Georg-August-
University Göttingen.
(
20) Zhao, W.; Zhang, J.; Lu, Y.; Wang, R. The vasorelaxant effect of
H S as a novel endogenous gaseous K channel opener. EMBO J.
2
ATP
2
(
001, 20, 6008−6016.
21) Abe, K.; Kimura, H. The possible role of hydrogen sulfide as an
endogenous neuromodulator. J. Neurosci. 1996, 16, 1066−1071.
22) Kimura, H. Hydrogen sulfide as a neuromodulator. Mol.
(
Neurobiol. 2002, 26, 13−19.
REFERENCES
■
(23) Ivanovic-Burmazovic, I.; Filipovic, M. R. Saying NO to H
2
S: A
−
(
1) Moncada, S.; Higgs, E. A. The discovery of nitric oxide and its
role in vascular biology. Br. J. Pharmacol. 2006, 147, S193−S201.
2) Culotta, E.; Koshland, D. E. NO NEWS IS GOOD-NEWS.
Story of HNO, HSNO, and SSNO . Inorg. Chem. 2019, 58, 4039−
4051.
(
(24) Marcolongo, J. P.; Venancio, M. F.; Rocha, W. R.; Doctorovich,
̂
Science 1992, 258, 1862−1865.
F.; Olabe, J. A. NO/H S “Crosstalk” Reactions. The Role of
2
I
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−
−
Thionitrites (SNO ) and Perthionitrites (SSNO ). Inorg. Chem.
2
(
(40) Tsou, C.-C.; Chiu, W.-C.; Ke, C.-H.; Tsai, J.-C.; Wang, Y.-M.;
Chiang, M.-H.; Liaw, W.-F. Iron(III) Bound by Hydrosulfide Anion
019, 58, 14981−14997.
25) Dong, H. T.; White, C. J.; Zhang, B.; Krebs, C.; Lehnert, N.
III
Ligands: NO-Promoted Stabilization of the [Fe −SH] Motif. J. Am.
Non-Heme Diiron Model Complexes Can Mediate Direct NO
Reduction: Mechanistic Insight into Flavodiiron NO Reductases. J.
Am. Chem. Soc. 2018, 140, 13429−13440.
Chem. Soc. 2014, 136, 9424−9433.
(41) Fitzpatrick, J.; Kim, E. Synthetic Modeling Chemistry of Iron−
Sulfur Clusters in Nitric Oxide Signaling. Acc. Chem. Res. 2015, 48,
2453−2461.
(
26) Zheng, S.; Berto, T. C.; Dahl, E. W.; Hoffman, M. B.;
Speelman, A. L.; Lehnert, N. The Functional Model Complex
(42) Tran, C. T.; Williard, P. G.; Kim, E. Nitric Oxide Reactivity of
[
Fe (BPMP)(OPr)(NO) ](BPh ) Provides Insight into the Mech-
[2Fe-2S] Clusters Leading to H S Generation. J. Am. Chem. Soc.
2
2
4
2
2
anism of Flavodiiron NO Reductases. J. Am. Chem. Soc. 2013, 135,
902−4905.
27) White, C. J.; Speelman, A. L.; Kupper, C.; Demeshko, S.;
2014, 136, 11874−11877.
4
(
(43) Harrop, T. C.; Tonzetich, Z. J.; Reisner, E.; Lippard, S. J.
Reactions of Synthetic [2Fe-2S] and [4Fe-4S] Clusters with Nitric
Oxide and Nitrosothiols. J. Am. Chem. Soc. 2008, 130, 15602−15610.
(44) Tonzetich, Z. J.; Do, L. H.; Lippard, S. J. Dinitrosyl Iron
Complexes Relevant to Rieske Cluster Nitrosylation. J. Am. Chem. Soc.
2009, 131, 7964−7965.
Meyer, F.; Shanahan, J. P.; Alp, E. E.; Hu, M.; Zhao, J.; Lehnert, N.
The Semireduced Mechanism for Nitric Oxide Reduction by Non-
Heme Diiron Complexes: Modeling Flavodiiron Nitric Oxide
Reductases. J. Am. Chem. Soc. 2018, 140, 2562−2574.
(
28) Jiang, Y.; Hayashi, T.; Matsumura, H.; Do, L. H.; Majumdar,
(45) Victor, E.; Lippard, S. J. A Tetranitrosyl [4Fe−4S] Cluster
Forms En Route to Roussin’s Black Anion: Nitric Oxide Reactivity of
A.; Lippard, S. J.; Moënne-Loccoz, P. Light-induced N O production
2
2
−
from a nonheme iron-nitrosyl dimer. J. Am. Chem. Soc. 2014, 136,
[Fe S (LS3)L′] . Inorg. Chem. 2014, 53, 5311−5320.
4
4
1
(
2524−12527.
(46) Harrop, T. C.; Song, D.; Lippard, S. J. Reactivity pathways for
nitric oxide and nitrosonium with iron complexes in biologically
relevant sulfur coordination spheres. J. Inorg. Biochem. 2007, 101,
1730−1738.
29) Kindermann, N.; Schober, A.; Demeshko, S.; Lehnert, N.;
Meyer, F. Reductive Transformations of a Pyrazolate-Based
Bioinspired Diiron−Dinitrosyl Complex. Inorg. Chem. 2016, 55,
1
(
1538−11550.
(47) Pal, N.; Majumdar, A. Transfer of Hydrosulfide from Thiols to
Iron(II): A Convenient Synthetic Route for Nonheme Diiron(II)-
hydrosulfide Complexes. Dalton Trans. 2019, 48, 5903−5908.
(48) Ganguly, T.; Das, A.; Majumdar, A. Iron(II) Mediated
Desulfurization of Organosulfur Substrates Produces Nonheme
Diiron(II)-hydrosulfides. Inorg. Chem. 2019, 58, 9998−10011.
(49) McKee, V.; Zvagulis, M.; Dagdigian, J. V.; Patch, M. G.; Reed,
C. A. Hemocyanin Models-Synthesis, Structure, and Magnetic
Properties of a Binucleating Copper(II) System. J. Am. Chem. Soc.
1984, 106, 4765−4772.
(50) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals, 6th ed.; Butterworth-Heinemann, Elsevier Inc.: Waltham,
MA, 2009.
(51) Arulsamy, N.; Bohle, D. S.; Butt, J. A.; Irvine, G. J.; Jordan, P.
A.; Sagan, E. Interrelationships between Conformational Dynamics
and the Redox Chemistry of S-Nitrosothiols. J. Am. Chem. Soc. 1999,
121, 7115−7123.
(52) Pal, N.; Majumdar, A. Controlling the Reactivity of Bifunctional
Ligands: Carboxylate-Bridged Nonheme Diiron(II) Complexes
Bearing Free Thiol Groups. Inorg. Chem. 2016, 55, 3181−3191.
(53) Bill, E. julX, Program for Simulation of Molecular Magnetic Data;
Max-Planck Institute for Chemical Energy Conversion: Mu
Ruhr, Germany, 2008.
(54) White, C. J. Modelling Flavodiiron Nitric Oxide Reductases:
Geometric and Electronic Structure of Key Intermediates and
Mechanistic Insights. Ph.D. Thesis, University of Michigan, Ann
Arbor, MI, 2020.
(55) APEX II, 2009 ed.; Bruker Analytical X-ray Systems Inc.:
Madison, WI, 2009.
30) Jana, M.; Pal, N.; White, C. J.; Kupper, C.; Meyer, F.; Lehnert,
N.; Majumdar, A. Functional Mononitrosyl Diiron(II) Complex
Mediates the Reduction of NO to N O with Relevance for
2
Flavodiiron NO Reductases. J. Am. Chem. Soc. 2017, 139, 14380−
1
(
4383.
31) Jana, M.; White, C. J.; Pal, N.; Demeshko, S.; Cordes, C.;
Meyer, F.; Lehnert, N.; Majumdar, A. Functional Models for the
Mono- and Dinitrosyl Intermediates of FNORs: Semireduction versus
Superreduction of NO. J. Am. Chem. Soc. 2020, 142, 6600−6616.
(
32) Rock, P. A.; Swinehart, J. H. The Kinetics of the Aqueous
Hydrogen Sulfide-Nitroprusside System. Inorg. Chem. 1966, 5, 1078−
079.
33) Quiroga, S. L.; Almaraz, A. E.; Amorebieta, V. T.; Perissinotti,
1
(
L. L.; Olabe, J. A. Addition and Redox Reactivity of Hydrogen
−
Sulfides (H S/HS ) with Nitroprusside: New Chemistry of Nitro-
2
sosulfide Ligands. Chem. - Eur. J. 2011, 17, 4145−4156.
(
34) Filipovic, M. R.; Ivanovic-Burmazovic, I. The Kinetics and
Character of the Intermediates Formed in the Reaction between
Sodium Nitroprusside and Hydrogen Sulfide Need Further
Clarification. Chem. - Eur. J. 2012, 18, 13538−13540.
(
35) Filipovic, M. R.; Eberhardt, M.; Prokopovic, V.; Mijuskovic, A.;
̈
lheim/
Orescanin-Dusic, Z.; Reeh, P.; Ivanovic-Burmazovic, I. Beyond H2S
and NO Interplay: Hydrogen Sulfide and Nitroprusside React
Directly to Give Nitroxyl (HNO). A New Pharmacological Source
of HNO. J. Med. Chem. 2013, 56, 1499−1508.
(
36) Fitzpatrick, J.; Kalyvas, H.; Filipovic, M. R.; Ivanovic-
́
Burmazovic, I.; MacDonald, J. C.; Shearer, J.; Kim, E. Transformation
́
of a Mononitrosyl Iron Complex to a [2Fe-2S] Cluster by a Cysteine
Analogue. J. Am. Chem. Soc. 2014, 136, 7229−7232.
(56) Spek, A. L. Single-crystal structure validation with the program
PLATON. J. Appl. Crystallogr. 2003, 36, 7−13.
(57) Spek, A. L. Structure validation in chemical crystallography.
Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, 65, 148−155.
(58) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr.,
Sect. A: Found. Crystallogr. 2008, 64, 112−122.
(59) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.
K.; Puschmann, H. OLEX2: a complete structure solution, refinement
and analysis program. J. Appl. Crystallogr. 2009, 42, 339−341.
(60) Farrugia, L. J. ORTEP-3 for Windows - a version of ORTEP-III
with a Graphical User Interface (GUI). J. Appl. Crystallogr. 1997, 30,
565.
(61) Farrugia, L. J. WinGX and ORTEP for Windows: an update. J.
Appl. Crystallogr. 2012, 45, 849−854.
(62) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scalmani, G.;
Barone, V.; Mennucci, B.; Caricato, M.; Li, X.; Hratchian, H. P.;
Hada, M.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Nakai,
(
37) Tsai, M.-L.; Tsou, C.-C.; Liaw, W.-F. Dinitrosyl Iron
Complexes (DNICs): From Biomimetic Synthesis and Spectroscopic
Characterization toward Unveiling the Biological and Catalytic Roles
of DNICs. Acc. Chem. Res. 2015, 48, 1184−1193.
(
38) Tsai, M.-L.; Chen, C.-C.; Hsu, I. J.; Ke, S.-C.; Hsieh, C.-H.;
Chiang, K.-A.; Lee, G.-H.; Wang, Y.; Chen, J.-M.; Lee, J.-F.; Liaw, W.-
F. Photochemistry of the Dinitrosyl Iron Complex [S Fe(NO) ]
−
5
2
2
‑
Leading to Reversible Formation of [S Fe(μ-S) FeS ] : Spectro-
5
2
5
scopic Characterization of Species Relevant to the Nitric Oxide
Modification and Repair of [2Fe−2S] Ferredoxins. Inorg. Chem. 2004,
4
(
3, 5159−5167.
39) Lu, T.-T.; Huang, H.-W.; Liaw, W.-F. Anionic Mixed Thiolate−
Sulfide-Bridged Roussin’s Red Esters [(NO) Fe(μ-SR)(μ-S)Fe-
2
−
(
NO) ] (R = Et, Me, Ph): A Key Intermediate for Transformation
2
of Dinitrosyl Iron Complexes (DNICs) to [2Fe-2S] Clusters. Inorg.
Chem. 2009, 48, 9027−9035.
J
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Forum Article
H.; Vreven, T.; Montgomery, J. A., Jr.; Bearpark, M.; Keith, T.;
Kobayashi, R.; Normand, J.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Zakrzewski, V. G.; Voth, G. A.; Dapprich, S.; Daniels, A. D.;
Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision E.01;
Gaussian Inc.: Wallingford, CT, 2013.
Flavodiiron Nitric Oxide Reductases. Inorg. Chem. 2018, 57, 4252−
4269.
(
63) Becke, A. D. Density-functional thermochemistry. III. The role
of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
64) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti
(
correlation-energy formula into a functional of the electron density.
Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789.
(
65) Perdew, J. P.; Burke, K.; Wang, Y. Generalized gradient
approximation for the exchange-correlation hole of a many-electron
system. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16533−
1
6539.
(
66) Wachters, A. J. H. Gaussian Basis Set for Molecular
Wavefunctions Containing Third-Row Atoms. J. Chem. Phys. 1970,
2, 1033−1036.
67) Hay, P. J. Gaussian basis sets for molecular calculations. The
representation of 3d orbitals in transition-metal atoms. J. Chem. Phys.
977, 66, 4377−4384.
68) Neese, F. Software update: the ORCA program system, version
.0. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2018, 8, e1327.
69) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn.
Phys. Chem. Chem. Phys. 2006, 8, 1057−1065.
70) Hunt, A. P.; Lehnert, N. The Thiolate Trans Effect in Heme
5
(
1
(
4
(
(
6
{
FeNO} Complexes and Beyond: Insight into the Nature of the Push
Effect. Inorg. Chem. 2019, 58, 11317−11332.
71) Hunt, A. P.; Samanta, S.; Dent, M. R.; Milbauer, M. W.;
(
Burstyn, J. N.; Lehnert, N. Model Complexes Elucidate the Role of
the Proximal Hydrogen-Bonding Network in Cytochrome P450s.
Inorg. Chem. 2020, 59, 8034−8043.
(
72) McQuilken, A. C.; Ha, Y.; Sutherlin, K. D.; Siegler, M. A.;
Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Jameson, G. N. L.;
7
Goldberg, D. P. Preparation of Non-heme {FeNO} Models of
Cysteine Dioxygenase: Sulfur versus Nitrogen Ligation and Photo-
release of Nitric Oxide. J. Am. Chem. Soc. 2013, 135, 14024−14027.
(
73) Chiang, C.-Y.; Miller, M. L.; Reibenspies, J. H.; Darensbourg,
M. Y. Bismercaptoethanediazacyclooctane as a N S Chelating Agent
2
2
and Cys−X−Cys Mimic for Fe(NO) and Fe(NO) . J. Am. Chem. Soc.
2
2
(
004, 126, 10867−10874.
74) McQuilken, A. C.; Matsumura, H.; Du
Sheckelton, J. P.; Siegler, M. A.; McQueen, T. M.; Ivanovic-
Burmazovic, I.; Moënne-Loccoz, P.; Goldberg, D. P. Photoinitiated
̈
rr, M.; Confer, A. M.;
́
́
Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}7
Complex with Dioxygen. J. Am. Chem. Soc. 2016, 138, 3107−3117.
(
75) Ekanayake, D. M.; Fischer, A. A.; Elwood, M. E.; Guzek, A. M.;
Lindeman, S. V.; Popescu, C. V.; Fiedler, A. T. Nonheme iron−
thiolate complexes as structural models of sulfoxide synthase active
sites. Dalton Trans. 2020, 49, 17745−17757.
(
76) Feig, A. L.; Bautista, M. T.; Lippard, S. J. A Carboxylate-
Bridged Non-Heme Diiron Dinitrosyl Complex. Inorg. Chem. 1996,
5, 6892−6898.
77) Majumdar, A.; Lippard, S. J. Non-Heme Mononitrosyldiiron
3
(
Complexes: Importance of Iron Oxidation State in Controlling the
Nature of the Nitrosylated Products. Inorg. Chem. 2013, 52, 13292−
1
(
3294.
78) Berto, T. C.; Hoffman, M. B.; Murata, Y.; Landenberger, K. B.;
Alp, E. E.; Zhao, J.; Lehnert, N. Structural and Electronic
Characterization of Non-Heme Fe(II)−Nitrosyls as Biomimetic
Models of the FeB Center of Bacterial Nitric Oxide Reductase. J.
Am. Chem. Soc. 2011, 133, 16714−16717.
(
79) Speelman, A. L.; White, C. J.; Zhang, B.; Alp, E. E.; Zhao, J.;
Hu, M.; Krebs, C.; Penner-Hahn, J.; Lehnert, N. Non-heme High-
Spin {FeNO}6 Complexes: One Ligand Platform Can Do It All. J.
Am. Chem. Soc. 2018, 140, 11341−11359.
−8
(
80) Van Stappen, C.; Lehnert, N. Mechanism of N−N Bond
Formation by Transition Metal−Nitrosyl Complexes: Modeling
K
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