Iron(II/III) Halide Complexes Promote the Interconversion of Nitric Oxide and S-Nitrosothiols through Reversible Fe-S Interaction

Article abstract of DOI:10.1021/acs.inorgchem.1c00203

Heme and non-heme iron in biology mediate the storage/release of NO? from S-nitrosothiols as a means to control the biological concentration of NO?. Despite their importance in many physiological processes, the mechanisms of N-S bond formation/cleavage at Fe centers have been controversial. Herein, we report the interconversion of NO? and S-nitrosothiols mediated by FeII/FeIII chloride complexes. The reaction of 2 equiv of S-nitrosothiol (Ph3CSNO) with [Cl6FeII2]2- results in facile release of NO? and formation of iron(III) halothiolate. Detailed spectroscopic studies, including in situ UV-vis, IR, and M?ssbauer spectroscopy, support the interaction of the S atom with the FeII center. This is in contrast to the proposed mechanism of NO? release from the well-studied red product κ1-N bound S-nitrosothiol FeII complex, [(CN)5Fe(κ1-N-RSNO)]3-. Additionally, FeIII chloride can mediate NO? storage through the formation of S-nitrosothiols. Treatment of iron(III) halothiolate with 2 equiv of NO? regenerates Ph3CSNO with the FeII source trapped as the S = 3/2 {FeNO}7 species [Cl3FeNO]-, which is inert toward further coordination and activation of S-nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO?/thiolate and S-nitrosothiol, which has important implications toward how Nature manages the biological concentration of free NO?.

Full text of DOI:10.1021/acs.inorgchem.1c00203

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Iron(II/III) Halide Complexes Promote the Interconversion of Nitric
Oxide and S‑Nitrosothiols through Reversible Fe‑S Interaction
Anna L. Poptic and Shiyu Zhang*
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ABSTRACT: Heme and non-heme iron in biology mediate the
storage/release of NO• from S-nitrosothiols as a means to control
the biological concentration of NO•. Despite their importance in
many physiological processes, the mechanisms of N−S bond
formation/cleavage at Fe centers have been controversial. Herein,
we report the interconversion of NO• and S-nitrosothiols mediated
by FeII/FeIII chloride complexes. The reaction of 2 equiv of S-
nitrosothiol (Ph3CSNO) with [Cl6FeII ]2− results in facile release of
2
NO• and formation of iron(III) halothiolate. Detailed spectroscopic
̈
studies, including in situ UV−vis, IR, and Mossbauer spectroscopy,
support the interaction of the S atom with the FeII center. This is in
contrast to the proposed mechanism of NO• release from the well-
studied “red product” κ1-N bound S-nitrosothiol FeII complex, [(CN)5Fe(κ1-N-RSNO)]3−. Additionally, FeIII chloride can mediate
NO• storage through the formation of S-nitrosothiols. Treatment of iron(III) halothiolate with 2 equiv of NO• regenerates
Ph3CSNO with the FeII source trapped as the S = 3/2 {FeNO}7 species [Cl3FeNO]−, which is inert toward further coordination and
activation of S-nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO•/thiolate and S-
nitrosothiol, which has important implications toward how Nature manages the biological concentration of free NO•.
associated with increased levels of protein S-nitrosation with
simultaneous formation of dinitrosyl iron complexes
(DNICs).10,11 Moreover, the release of NO• from the blood
pressure medication sodium nitroprusside (SNP) [Na]2[Fe-
(CN)5(NO)] is thought to proceed through the decom-
position of an iron(II) S-nitrosothiol adduct, [Fe(CN)5(κ1-N-
RSNO)]3−, known as the “red product”.12,13
Despite the importance of iron centers in facilitating the
formation/decomposition of S-nitrosothiols, the discrete
molecular mechanisms of N−S bond formation/cleavage at
iron sites remain controversial. For example, two mechanisms
have been proposed for S-nitrosothiol formation at heme-iron
sites. The first involves nucleophilic attack of an iron nitrosyl
{FeNO}6 species by a thiolate (Scheme 1, top), whereas the
second invokes an FeIII−thiolate intermediate undergoing
further reaction with free NO• (Scheme 1, bottom).14,15
Despite the structural characterization of S-nitrosated heme-
iron proteins,16 spectroscopic support for both mechanisms
exists.16−18
INTRODUCTION
■
Nitric oxide (NO•) is a crucial secondary signaling molecule
responsible for the regulation of a range of biological processes,
including immune response, smooth muscle relaxation, and
neurotransmission.1 However, NO• has a short lifetime in vivo
and quickly reacts with the bioavailable oxygen and superoxide,
forming reactive oxygen and reactive nitrogen species.2 To
mitigate the formation of these reactive species, Nature has
developed specific regulatory strategies that store NO• as air-
stable S-nitrosothiols (RSNOs).3 RSNOs can also serve as
signaling molecules to participate in the S-nitrosation of
proteins, which is a precisely regulated post-translational
modification. Dysregulation of RSNO homeostasis has been
linked to several diseases, such as Alzheimer’s disease and
Parkinson’s disease, cancer, diabetes, etc.4,5
Formation of S-nitrosothiols from free NO• and thiol
requires one-electron oxidation, which can be facilitated by the
single-electron accepting abilities of FeIII or CuII. Conversely,
FeII and CuI can act as one-electron reducing agents to
promote the release of NO• from S-nitrosothiols via cleavage
of the S−N bond.3,6−9 Several biological iron centers have
been implicated in RSNO decomposition/formation. For
example, heme-iron proteins, such as cytochrome c, are
responsible for the coupling of NO• and low-mass thiols to
RSNOs, i.e., S-nitrosoglutathione (GSNO) or S-nitroso-L-
cysteine (CysSNO).4 In addition to enzymatic heme-iron
centers, exposure of NO• to intracellular labile iron has been
Received: January 21, 2021
Published: March 11, 2021
https://dx.doi.org/10.1021/acs.inorgchem.1c00203
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 5190−5197
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Scheme 1
Scheme 4
Significant research efforts have also been devoted to
understanding the binding mode of S-nitrosothiols to Fe
centers (κ1-N vs κ1-S) during NO• release from SNP (Scheme
2). Spectroscopic studies suggest that the “red product”
NO•. The NO• release from RSNO is reversible, and in the
presence of an additional equivalent of NO•, the iron(III)−
halothiolate captures NO• to afford RSNO. The FeII chloride
byproduct is trapped by a second equivalent of NO• to form
stable {FeNO}7 complexes that are inert toward catalytic
decomposition of RSNO. This unique property of high-spin
iron complexes may be a strategy Nature employs to reversibly
regulate the bio-availability of free nitric oxide.
Scheme 2
RESULTS AND DISCUSSION
■
Reactivity of FeII Chloride with S-Nitrosothiols. We
began our investigation by preparing an iron(II) halide
complex soluble in aprotic solvents. Treatment of PPNCl
(PPN = bis(triphenylphosphine)iminium) with anhydrous
[Fe(CN)5(κ1-N-RSNO)]3− extrudes a thiyl (RS•) radical to
form a [(CN)5FeNO]3− complex ({FeNO}7) which then
releases free NO•.19,20 However, both experimental8,21,22 and
computational23,24,43,44 studies have demonstrated the thiyl
radical (RS•) expulsion process in Scheme 2 should be
unfavorable since κ1-N coordination of S-nitrosothiols
strengthens the S−N bond (Scheme 3). Furthermore, the
FeIICl2 in acetone affords PPN2[Cl6FeII ]. The spectroscopic
2
characterizations of the PPN2[Cl6FeII ] complex match those
2
reported in the literature.38 The analogous TBA salt of iron(II)
trichloride was isolated as colorless crystals by the addition of
TBACl (TBA = tetrabutylammonium) to anhydrous FeIICl2.
Single-crystal X-ray diffraction analysis indicates that the FeII
centers exist in two types of coordination environments, as
Scheme 3
[Cl6FeII ]2− and monomeric [Cl3FeII(H2O)]− (Figure S29).
2
The aqua ligand could be a result of H2O in either TBACl or
FeIICl2. Next, we monitored the reaction of [Cl6FeII ]2− with S-
2
nitrosotriphenylmethanethiol (Ph3CSNO) with UV−vis spec-
troscopy at low temperature. The structural difference between
the PPN and TBA salts of the FeII chlorides in the solid state
does not impact their reactivity with S-nitrosothiols (Figures
currently proposed mechanism does not explain the rapid
vasodilatory effect of SNP, given the slow rate of NO•
liberation from the {FeNO}7 [(CN)5FeNO]3− (k = 5.0 ×
10−5 s−1, Scheme 2).25
S8, S9). Addition of 2 equiv of Ph3CSNO to [Cl6FeII ]2− in a
2
1:1 mixture of THF and CH3CN at −50 °C affords a dark
brown complex with UV−vis absorption bands at 520 nm (ε =
1000 M−1 cm−1) and 630 nm (ε = 970 M−1 cm−1) (Figure 1,
brown trace), which slowly converts to a green species at room
temperature with absorbances at 365 nm (ε = 1080 M−1
cm−1), 480 nm (ε = 410 M−1 cm−1), and 660 nm (ε = 320
M−1 cm−1) (Figure 1, green trace). The final green complex
was identified as S = 3/2 TBA[Cl3FeNO], which can be
As the debate for the mechanisms of S-nitrosothiol
formation and decomposition at iron sites continues, the
mechanism of RSNO formation mediated by the labile iron
pool (LIP) also remains ambiguous. Iron within the LIP is
present in both FeII/FeIII oxidation states and can be
coordinated by various weak field ligands to form high-spin
FeII/FeIII complexes.10,26−29 While Kim and Lippard have
investigated the reactivity of NO• with synthetic iron-sulfur
clusters30−33 and iron(II) thiolates,34−37 other simple FeII/FeIII
coordination complexes relevant to the LIP and NO•/S-
nitrosothiol interconversion still warrant continued investiga-
tion.
independently synthesized by treatment of [Cl6FeII ]2− with
2
NO• (Figures S1, S4).39 On the basis of the absorptivity of
[Cl3FeNO]−, the yield of TBA[Cl3FeNO] was approximately
quantitative. The [Cl3FeNO]− anion was observed first by
Kohlschu
Beck,42 and Klufers.39 The S = 3/2 spin state of [Cl3FeNO]−
̈
̈
tter40 and structurally characterized by van Eldik,41
Herein, we employ FeII/FeIII chloride complexes as
simplified models for LIP to investigate the potential roles of
FeII/FeIII in RSNO formation/decomposition. We demon-
strate that reaction of RSNO with FeII chloride complexes
leads to the facile release of NO• from RSNO (Scheme 4).
Spectroscopic studies suggest that the FeII center interacts with
the S atom before its conversion to iron(III)−halothiolate and
was established recently based on SQUID measurement39 and
our Evans method study (see Supporting Information (SI)).
The [Cl3FeNO]− complex is remarkably stable to oxygen and
moisture, and no decomposition of PPN[Cl3FeNO] solid was
observed even after storage under ambient conditions for
several days.
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these two mechanisms is the identity of the brown
intermediate prior to the formation of [Cl3FeNO]− (high-
lighted in gray, Scheme 5). In Mechanism A, the brown
intermediate should be [Cl3Fe(κ1-N-Ph3CSNO)]−, whereas,
for Mechanism B, it should be either [Cl3Fe(κ1-S-
Ph3CSNO)]− or [Cl3FeIII-SCPh3]−.
Characterization of the FeIII Halothiolate Intermedi-
ate. To further discern the two proposed mechanisms, we set
out to identify the initial brown intermediate that absorbs at
520 and 630 nm. On the basis of Mechanism B, the brown
intermediate could be [Cl3Fe(κ1-S-Ph3CSNO)]−, or if NO•
release has already occurred, the iron(III)−halothiolate
[Cl3FeIII-SCPh3]−. We began our studies by first attempting
to generate [Cl3FeIII-SCPh3]− from an independent reaction
between TBA[Cl4FeIII] and NaSCPh3. As shown in Figure 2,
Figure 1. In situ UV−vis spectra of the treatment of [Fe2Cl6]2− with 2
equiv of Ph3CSNO at −50 °C in 1:1 THF/CH3CN (0.5 mM). The
brown intermediate [Cl3Fe(SCPh3)]− (λmax1 = 520 nm (ε = 1000
M
−1 cm−1), λmax2 = 630 nm (ε = 970 M−1 cm−1)) was converted after
15 h (scan rate 30 min) to [Cl3FeNO]− (green trace) (λmax1 = 365
nm (ε = 1080 M−1 cm−1), λmax2 = 480 nm (ε = 410 M−1 cm−1), λmax3
= 660 nm (ε = 320 M−1 cm−1) .
Scheme 5
Figure 2. Comparison of initial UV−vis spectra resulting from the
reaction between [Fe2Cl6]2− and Ph3CSNO at −50 °C in 1:1 THF/
CH3CN (0.5 mM, brown trace); the reaction between TBA[Cl4FeIII]
and 1 equiv of NaSCPh3 at −20 °C (0.5 mM, red trace).
the in situ UV−vis spectrum of the reaction of TBA[Cl4FeIII]
with 1 equiv of NaSCPh3 in CH3CN/THF (red trace) shows a
good match with the brown intermediate. Further, the reaction
between FeCl3 and TBA(SCPh3) leads to the formation of the
same spectrum, albeit at a lower spectroscopic yield (Figure
S11). In addition, the predicted UV−vis spectrum of [Cl3FeIII-
SCPh3]− by time-dependent density functional theory (TD-
DFT) at the TPSSh/def2-TZVP level correlates well with the
experimental data (Figure S30). Additional independent
experiments examining the reaction between [Cl6FeII ]2− and
varying equivalents of NaSCPh3 (Figure S12) also excluded
the possibility of an FeII thiolate species being the identity of
the brown intermediate.
2
Two possible mechanisms could explain the formation of
[Cl3FeNO]− (Scheme 5). On the basis of the proposed
mechanism of NO• release from SNP (Scheme 2), one could
envision coordination of Ph3CSNO to the FeII center to form
an FeII κ1-N S-nitrosothiol adduct, followed by thiyl radical
extrusion to afford [Cl3FeNO]− (Mechanism A, Scheme 5,
top). However, Mechanism A contradicts the chemical
reactivity of metal S-nitrosothiols described in Scheme
3.8,21,22 Therefore, an alternative mechanism in which the
FeII center interacts with the S atom warrants consideration
(Mechanism B, Scheme 5, bottom). In this case, NO• is
released from the proposed [Cl3Fe(κ1-S-Ph3CSNO)]− adduct,
̈
Analysis of the brown intermediate with Mossbauer
spectroscopy further supports its assignment as an FeIII thiolate
species. Independent generation of [Cl357FeIII(SCPh3)]− from
the reaction between TBA[Cl457FeIII] and 1 equiv of NaSCPh3
̈
displays a Mossbauer spectrum similar to that produced from
the reaction between TBA2[Cl657FeII ] and 2 equiv of
2
Ph3CSNO (See Supporting Information, Figures S19, S20).
Both spectra display peaks corresponding to two major
speciesthe first one (ca. 40%) exhibiting isomer shift and
quadrupole splitting values identical to that of TBA2[Cl6-
then recaptured by [Cl6FeII ]2− generated from the decom-
2
position of [Cl3FeIII-SCPh3]−. The key difference between
57FeII ] (δ = 1.16 mm s−1, |ΔEQ| = 2.86 mm s−1 (Figure S16)).
2
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The second component (ca. 60%) displays an isomer shift of δ
= 0.38 mm s−1 and quadrupole splitting value of |ΔEQ| = 0.94
mm s−1, which was assigned as [Cl3FeIII(SCPh3)]−. These
parameters are different from those of TBA[Cl457FeIII] (Figure
S18), which has an isomer shift value of δ = 0.23 mm s−1 and a
quadrupole splitting of |ΔEQ| = 0.53 mm s−1 (Table 1).
captures free NO• to form the [Cl3FeNO]− complex (Scheme
5, Mechanism B). Further support of Mechanism B was
provided by an in situ time-resolved solution IR study.
Addition of [Cl6FeII ]2− to a solution with 2 equiv of
2
Ph3CSNO in THF-d8 at −70 °C results in a decrease of the
peak at 1493 cm−1 corresponding to Ph3CSNO. Upon further
warming of the reaction mixture to −50 °C, a new band at
1792 cm−1 appeared, which was attributed to TBA[Cl3FeNO]
(Figure 3, left).39 An 15N-labeling experiment confirmed that
Ph3CSNO and [Cl3FeNO]− were the only species observed
with 15N-sensitive stretches over the course of the reaction (see
SI, Figures S24, S25). Importantly, the initial decay of
Ph3CSNO was not accompanied by the formation of
[Cl3FeNO]− (Figure 3, right). The 15 s time intervals at
which the IR spectra were monitored allowed the time gap (ca.
90 s, see SI, Figures S22, S23) between these two events to be
determined. The delay in [Cl3FeNO]− formation provides
further assent that free NO• was first released into the
headspace, where it was unobservable by solution IR until it
̈
Table 1. Solution Mossbauer Parameters
complex (major component)
TBA2[Cl657FeII
δ (mm/s)
ΔEQ (mm/s)
]
1.16
0.23
0.38
0.38
2.86
0.53
0.94
0.93
2
TBA[Cl457FeIII]
TBA[Cl457FeIII] + NaSCPh3
TBA2[Cl657FeII2] + 2 Ph3CSNO
̈
Despite our UV−vis and Mossbauer experiments providing
spectroscopic evidence for the assignment of the brown
intermediate as an FeIII thiolate species, we were unable to
obtain structural data on the putative [Cl3FeIII-SCPh3]−. Our
inability to structurally characterize [Cl3FeIII-SCPh3]− could be
due to (1) the decomposition of [Cl3FeIII-SCPh3]− to Ph3CS-
was recaptured by [Cl6FeII ]2− as [Cl3FeNO]−, consistent with
2
Mechanism B (Scheme 5, bottom). These results are also in
agreement with our UV−vis studies, where the formation of
[Cl3FeNO]− was observed concurrently with the slow decay of
the brown intermediate.
SCPh3 and [Cl6FeII ]2− (see Supporting Information) or (2)
2
the labile nature of the chloride ligands leading to complicated
ligand scrambling that can generate other [ClxFeIII-
(SCPh3)4−x]− (x = 1, 2, 3, 4). Indeed, titration experiments
of NaSCPh3 into TBA[Cl4FeIII], and TBA(SCPh3) into FeCl3
at −40 °C show formation of distinctly different species as
additional amounts (1−5 equiv) of thiolate are added to FeIII
(Figures S13, S14). Nonetheless, with a 3:1:1 stoichiometry of
Cl−:FeIII:Ph3CS−, we believe the major iron(III) halothiolate
species in solution at the low temperatures at which our studies
were conducted is [Cl3FeIII-SCPh3]−.
̈
In situ UV−vis, Mossbauer, and IR spectroscopic studies
allow us to conclude that NO• release likely proceeds through
coordination of the S atom to FeII prior to S−N bond cleavage.
Otherwise, the concurrent formation of [Cl3FeNO]− and
consumption of Ph3CSNO would be expected. The proposed
mechanism is also consistent with the reactivity profile of metal
S-nitrosothiol complexes−κ1-S coordination weakens the S−N
bond, making the release of NO• more favorable (Scheme
3).13
In Situ IR Study of the Conversion of [Cl6FeII ]2− to
2
[Cl3FeNO]− by Ph3CSNO. The in situ UV−vis and
RSNO Formation from Putative Iron(III) Halothiolate.
As many NO• release/storage mechanisms are reversible in
Nature,8 we next investigated whether the FeII/FeIII chloride
couple can carry out the formation of S-nitrosothiol. Lancaster
et al. proposed that the LIP can promote RS−NO bond
II 2−
̈
Mossbauer data so far suggest that reaction of [Cl6Fe ]
2
with Ph3CSNO first leads to the formation of an iron(III)
halothiolate and free NO•. The iron(III) halothiolate then
slowly decomposes to disulfide and [Cl6FeII ]2−, which
2
Figure 3. Left: Selected in situ solution IR spectra of the reaction between [Fe2Cl6]2− and Ph3CSNO in THF-d8, 30 mM (from red to purple
traces), showing the decrease in ν(NO) = 1493 cm−1 (Ph3CSNO) and the appearance of ν(NO) = 1792 cm−1 ([Cl3FeNO]−) upon warming.
Right: Normalized peak heights of Ph3CSNO (blue) and TBA[Cl3FeNO] (green) versus time and temperature (yellow) shows a 90 s delay
(shaded gray) in TBA[Cl3FeNO] formation.
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formation based on the observation that the cellular protein S-
nitrosation levels strongly depend on the concentration of
labile/chelatable iron.10,11 To this end, we first generated the
putative [Cl3FeIIISCPh3]− by treating FeIIICl3 with TBA-
(SCPh3) at −78 °C and subjected this reaction mixture to
sequential additions of NO•. In situ solution IR spectroscopy
was utilized to monitor and quantify product formation. After
the addition of 2 equiv of NO• at −78 °C and upon warming
the reaction mixture to −50 °C, we observed concurrent
formation of peaks at 1792 and 1493 cm−1 (Figure 4), assigned
Scheme 6
Scheme 7
3:1 stoichiometry (Figure S6). While the titration of NaSCPh3
to PPN[Cl3FeNO] displays similar UV−vis spectra (Figure
S7), employing tert-butyl thiolate allowed us to calculate the
yield of the product [(tBuS)3FeNO]− based on reported molar
extinction coefficients. The spectroscopic yield of [(tBuS)3-
FeNO]− was determined to be 99%, indicating that the
{FeNO}7 motif is robust, while the Cl− ligand exchange occurs
Figure 4. Selected in situ IR spectra of the reaction between
[Cl3FeIIISCPh3]− and 2 equiv of NO• in 9:1 THF-d8/CD3CN at −80
°C (30 mM). At −50 °C, formation of Ph3CSNO (ν(NO) = 1493
cm−1) and TBA[Cl3FeNO] (ν(NO) = 1792 cm−1) was observed.
as [Cl3FeNO]− and Ph3CSNO, respectively.39,45 The
estimated yields of [Cl3FeNO]− and Ph3CSNO were 67%
and 50%, respectively, based on the IR absorbances (see
Supporting Information, Figures S24, S25). The FeIII center
facilitates the oxidative coupling of thiolate and NO• to form
Ph3CSNO by acting as an electron acceptor and undergoing
reduction to FeII, which may be trapped by additional NO• to
form [Cl3FeNO]−. The presence of an additional equivalent of
NO• is critical to prevent the reverse reaction, that is, the
release of NO• from Ph3CSNO, as FeII complexes are known
to lead to the decomposition of RSNO to disulfide and NO•
(Scheme 6). The robust Fe-NO• interaction impedes the
displacement of the NO• ligand by S-nitrosothiol and prevents
the catalytic decomposition of RSNO (Scheme 6). Such a
“switch effect” of NO• represents a key difference between Fe
and Cu mediated S-nitrosothiol formation; the strong
interaction between high-spin FeIII (S = 5/2) with NO− (S
= 1) is not shared by Cu.6 In fact, some of us, and Hayton et
al., have shown that the interaction of CuII and NO• is highly
reversible.46,47
36
t
with BuS−.
Electronic Structure of [Cl3FeNO]−, [Cl3CuNO]−, and
Cl3FeNO. The inertness of [Cl3FeNO]− toward nitrosation of
thiolate and the irreversible binding of NO• at [Cl3FeII]− are a
crucial part of Mechanism B proposed in Scheme 5. These
properties of [Cl3FeNO]− are in sharp contrast to its Cu
analogue [Cl3CuNO]−, which exhibits both reversible binding
with NO• and nitrosative reactivity toward thiolates. To gain
more insight into the different reactivities of [Cl3FeNO]− and
[Cl3CuNO]−, we utilized complete active space self-consistent
field (CASSCF) computations to further understand their
electronic structure (Figure 5). Some of us46 and
others39,46,48,49 have shown that CASSCF calculations can
provide a detailed understanding of the correlation of
electronic structure and reactivity of metal-nitrosyls. The
state-specific CASSCF calculations were performed in each
complex’s well-established spin state, S = 3/2 for [Cl3FeNO]−
and S = 0 for [Cl3CuNO]−.39,46,50 After screening various
combinations of metal 3d, NO-σ*, and NO-π* orbitals, we
chose an active space of [9e,13o] for [Cl3FeNO]− and
[10e,13o] for [Cl3CuNO]−. These active spaces include (i)
To confirm that [Cl3FeNO]− does not participate in RS−
NO bond formation, we investigated its reaction with thiolate
(Scheme 7). Treatment of PPN[Cl3FeNO] with sodium tert-
butyl thiolate affords a red-colored solution with strong
absorbances at 360 nm (ε = 6670 M−1 cm−1) and 475 nm
(ε = 4360 M−1 cm−1), which were assigned as PPN-
[(tBuS)3FeNO].36 Titration of tert-butyl thiolate to a solution
of PPN[Cl3FeNO] in the UV−vis spectrometer confirmed the
bonding and antibonding orbitals of NO-π* with metal-dxz/yz
,
2
(ii) bonding and antibonding orbitals of NO-σ* with metal-dz ,
2
2
(iii) nonbonding dx −y /xy as well as (iv) the next five
unoccupied orbitals lowest in energy, which are often based
on metal 4d or ancillary ligands Cl− (see Supporting
Information). We found that 4d orbitals and ancillary orbitals
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Figure 5. Partially occupied, covalent metal−NO frontier orbitals resulting from CASSCF calculations of the electronic ground states of (A)
[Cl3FeNO]− [9e,13o], (B) [Cl3CuNO]− [10e,13o], and (C) Cl3FeNO [6e, 13o].
have little effect on the results of the calculations, consistent
[Cl3CuNO]−.46 Post-localization recalculation of the CI
coefficients shows Cl3FeNO exhibits about 93% FeIII-NO•
character as the leading configuration, and FeII-NO+
contributes about 4.6% to the ground state as the second
leading configuration. The increase in NO+ character in the
Cl3FeNO complex as compared to the [Cl3FeNO]− anion is
consistent with our experimental observation that the FeIII
complex, but not FeII, is able to mediate an oxidative coupling
reaction to form S-nitrosothiols.
with previous studies by Klu
̈
fers.39,49 To computationally
estimate the amount of NO−/•/+ character in each complex, a
valence bond-like interpretation of the CAS wave function was
applied by following the method laid out by Radon et al.51 The
́
amount of NO−/•/+ character in each species is summarized in
Table 2. Intuitively, higher percentages of Mn+1-NO+ character
Table 2. Results of Valence-Bond Analysis of Metal
Nitrosyls
SUMMARY AND CONCLUSIONS
[Cl3FeNO]−
[Cl3CuNO]−
Cl3FeNO
■
res. structure
In contrast to the RS• expulsion mechanism from the κ1-N
RSNO species invoked during NO• release from SNP, reaction
of the high-spin [Cl3FeII]− motif with S-nitrosothiol generates
iron(III) thiolate and free NO•. Detailed in situ spectroscopic
M−NO−
M−NO•
M−NO+
others
52%
40%
2%
1%
0.25%
93%
4%
86%
10%
3%
5%
2%
̈
studies, including UV−vis, Mossbauer, and IR, suggest that
coordination of the S atom to the iron(II) center is necessary
to promote S−N bond cleavage. Importantly, iron(III)
chloride complexes can also promote the formation of S-
nitrosothiol when NO• is in excess. Our CASSCF results, when
paired alongside our experimental studies, highlight the
possibility of the use of computation to predict the NO−/•/+
reactivity of a series of metal nitrosyl complexes.
One central question that remains unanswered within the
literature is how Fe centers promote the storage of NO• as S-
nitrosothiols, while also being efficient for the decomposition
of RSNOs to NO• and disulfide. We believe the answer to this
question lies within the ability of NO• to irreversibly bind to
the FeII center ligated by weak field ligands,34−37 shutting
down coordination of S-nitrosothiol, which is a crucial step in
its decomposition (Scheme 3). Since [Cl3FeNO]− can be
easily converted to the DNIC [Cl2Fe(NO)2]− in the presence
of excess NO•,39 our study also provides a potential
explanation for the formation of DNIC and S-nitrosothiol
frequently observed during cellular protein S-nitrosation10,11
and demonstrates a strategy available to Nature to reversibly
modulate biological NO• concentration with labile iron ions.
correlates to NO+ transfer (nitrosative) ability, while Mn-NO•
corresponds to free NO• release. We found the CASSCF
results to be roughly consistent with the observed reactivity of
[Cl3FeNO]− and [Cl3CuNO]−. Specifically, [Cl3FeNO]−
exhibits the lowest NO• character (38%) and NO+ character
(2%), consistent with its inability to release free NO• and
perform S-nitrosation. [Cl3CuNO]− exhibits significant NO•
and NO+ character (86% and 10%, respectively) and is capable
of both NO• release and S-nitrosation.46
We were curious as to whether this qualitative analysis
would allow us to predict the NO+ reactivity of Cl3FeNO, the
product of the one-electron oxidation of [Cl3FeNO]−. This
theoretical interaction of FeIIICl3 with NO• would result in an
{FeNO}6 complex with a spin (S = 2), which would be more
likely to exhibit NO+ transferring ability. The geometry of
putative Cl3FeNO complex was optimized with DFT in a
variety of spin states (S = 0, 1, 2). The two known non-heme
{FeNO}6 complexes both have an experimentally and
computationally determined S = 1 (FeIV-NO−) ground
state;52 however, optimization by B3LYP/def2-TZVP deter-
mined the S = 2 state in Cl3FeNO to be the lowest in energy
by −20.3 kcal/mol. CASSCF calculation on the S = 2 spin
state showed highly covalent bonding character between the Fe
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00203.
■
sı
−
2
dz and NO px orbitals with 1.35 e in the bonding orbital and
0.65 e− in the antibonding orbital (Figure 5C), similar to
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Inorg. Chem. 2021, 60, 5190−5197

Inorganic Chemistry
Experimental details, including characterization data,
pubs.acs.org/IC
Article
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spectra, computational procedures, and results. Crystal-
lographic data for TBA3[Cl6FeII ]·[Cl3FeII(H2O)]
2
(CSD: 2057290) (PDF)
Accession Codes
CCDC 2057290 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Shiyu Zhang − Department of Chemistry & Biochemistry, The
Ohio State University, Columbus, Ohio 43210, United
States; orcid.org/0000-0002-2536-4324;
Email: zhang.8941@osu.edu
Author
Anna L. Poptic − Department of Chemistry & Biochemistry,
The Ohio State University, Columbus, Ohio 43210, United
States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00203
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the Ohio Supercomputer Center for high-
performance computing resources, and Prof. Alexander
Sokolov for help with CASSCF calculations. This publication
is based on work funded by the National Science Foundation
under award no. CHE-1904560. The authors thank The Ohio
State University Department of Chemistry and Biochemistry
for additional financial support.
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