of 19
Tetranuclear Iron Clusters with A Varied Interstitial Ligand:
Effects On Structure, Redox Properties, and Nitric Oxide
Activation
Christopher J. Reed
and
Theodor Agapie
*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
A new series of tetranuclear iron clusters displaying an interstitial
μ
4
-F ligand was prepared for
comparison to
μ
4
-O analogs. With a single NO coordinated as a reporter of small molecule
activation, the
μ
4
-F clusters were characterized in
five
redox states, from Fe
II
3
{FeNO}
8
to
Fe
III
3
{FeNO}
7
, with N–O stretching frequencies ranging from 1680 cm
−1
to 1855 cm
−1
,
respectively. Despite accessing more reduced states with an F
bridge, a two electron reduction of
the distal Fe centers is necessary for the
μ
4
-F clusters to activate NO to the same degree as the
μ
4
-
O system; consequently, NO reactivity is observed at more positive potentials with
μ
4
-O than
μ
4
-F.
Moreover, the
μ
4
-O ligand better translates redox changes of remote metal centers to diatomic
ligand activation. The implication for biological active sites is that the higher charge bridging
ligand is more effective in tuning cluster properties, including the involvement of remote metal
centers, for small molecule activation
SYNOPSIS TOC
A new series of tetranuclear iron clusters displaying a
μ
4
-F ligand allows comparison to
μ
4
-O
analogs to address the effect of the interstitial ligand. With a single NO coordinated as a reporter
of small molecule activation capabilities, the
μ
4
-F clusters were characterized in five redox states.
The higher charge
μ
4
-O bridge results in more effective activation of NO through several effects.
*
Corresponding Author: agapie@caltech.edu.
Notes
The authors declare no competing financial interests.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Crystallographic data files (CIFs)
Supplementary Data (PDF)
HHS Public Access
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Inorg Chem
. Author manuscript; available in PMC 2018 December 03.
Published in final edited form as:
Inorg Chem
. 2017 November 06; 56(21): 13360–13367. doi:10.1021/acs.inorgchem.7b02114.
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Introduction
Transition metal clusters perform diverse functions in proteins, including metal storage,
sensing, electron transfer, and multi-electron small molecule conversions (such as H
2
O
oxidation, CO
2
fixation, and N
2
reduction).
1
A common element of these multinuclear sites
is the presence of highly bridged (≥ μ
3
binding) single atom ligands, such as sulfide,
2
oxide,
3
or carbide.
4
Quantitative measures of the effects these ligands play in small molecule
activation remain rare. This is particularly relevant to understanding the role the interstitial
μ
6
-C ligand in the FeMo cofactor (FeMoco) of nitrogenase (Figure 1A). Synthetic clusters
suitable for structure-function studies of bridging ligands with respect to the activation of a
small molecule are rare, likely because of design constraints hard to overcome by self-
assembly, which is the route typically employed in cluster synthesis. Maintaining the exact
same structure while changing the bridging ligands and redox states, while limiting ligand
binding to a single small molecule, desirable for quantification of the effect and for
mimicking substrate activation by protein active sites, are two major challenges. A host of
iron-carbonyl clusters have been synthesized with a variety of bridging (≥
μ
3
) single atom
ligands, including
μ
6
-C clusters, such as [(μ
6
-C)Fe
6
(CO)
16
]
2−
, with arrangements
reminiscent of the FeMoco structure (Figure 1B, see SI for a comprehensive list).
5
While a
related cluster has been reported displaying a
μ
6
-N ligand, [(
μ
6
-N)Fe
6
(CO)
15
]
3−
, with
potential for structure function studies of the effect of the interstitial ligand, changes in the
structure and number of CO ligands complicates interpretations. In the cases where
completely isostructural clusters can be prepared with bridging elements of the second row
of the periodic table, the large number (≥ 9) of diatomic ligands limits interpretations
regarding the activation of a
single
small molecule substrate, which is most relevant to
biological systems. Recently, in an elegant demonstration of the effect of the
μ
4
-ligand (N vs
C) on reactivity, the hydride ligands in [HFe
4
C(CO)
12
]
2−
and [HFe
4
N(CO)
12
]
have been
shown to have distinct behavior for H
2
and formate generation.
6
Other synthetic clusters
have been studied to address effects of a bridging ligand on redox potentials or to model
FeMoco, but small molecule binding by the clusters with different bridging ligands has not
been reported.
7
Toward directly interrogating the effect of a cluster’s interstitial ligand on reactivity, we have
developed synthetic methodologies to access site differentiated multinuclear complexes that
allow variation of the bridging ligands. Herein, we present investigations of a series of
tetranuclear iron clusters containing a
μ
4
-F motif, isostructural with our previously reported
μ
4
-O clusters (Figure 1A).
8
These compounds allow for the evaluation of the effects of the
nature of the interstitial atom on cluster properties related to the activation of a single
diatomic ligand (NO).
Results and Discussion
We have recently reported the synthesis of site differentiated tetranuclear clusters, where
three (basal) metal centers are co-ordinated by a hexapyridyl-trisalkoxide framework (
L
3−
,
Figure 1) and bridged to a fourth (apical) metal site through three pyrazolate ligands and a
μ
4
-O ligand.
8
The all ferrous fluoride-bridged cluster,
1
, was synthesized via addition of a
2:1 ratio of phenylpyrazole and potassium phenylpyrazolate along with one equivalent
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anhydrous tetrabutylammonium fluoride to a previously reported trinuclear iron precursor
(
LFe
3
(OAc)(OTf)
2
, Figure 1))
8a
,
9
. The fourth Fe equivalent was delivered as
Fe(N(SiMe
3
)
2
)
2
to complete the tetranuclear cluster (
1
). This redox neutral route of
installing the interstitial F proved to be the most reliable way to avoid the generation of
mixtures, with some
μ
4
-O clusters likely formed due to trace moisture. Subsequent chemical
oxidations afford two additional redox states, Fe
II
3
Fe
III
(
2
) and Fe
II
2
Fe
III
2
(
3
).
Characterization by Mössbauer spectroscopy is consistent with charge localization on each
Fe center and with oxidations occurring exclusively in the basal triiron core, the apical Fe
remaining Fe
II
(Figures S26 – S31), as observed for the
μ
4
-O analogs.
8b
Structural
characterization by single crystal X-ray diffraction (XRD) reveals that the most oxidized
cluster,
3
, displays a five coordinate apical Fe
II
, due to acetonitrile binding (Figure S53).
Removal of this ligand under vacuum results in decomposition. This behavior is in contrast
to the analogous
μ
4
-O clusters, which have been isolated in the Fe
II
2
Fe
III
2
and Fe
II
Fe
III
3
oxidation states, both displaying a four-coordinate apical Fe
II
. This difference suggests that
that the
μ
4
-F clusters are more Lewis acidic than their
μ
4
-O analogues. Consistent with this
interpretation,
μ
4
-O clusters with electron withdrawing substituents show increased
coordination numbers at the apical metal.
8b
Nitric oxide provides a diagnostic vibrational spectroscopic signature for comparing
different complexes to address the effects of the multinuclear supporting platform and the
interstitial ligand on small molecule binding.
10
Studies of the chemistry of iron clusters with
nitric oxide has been principally focused on understanding the biologically relevant
conversion of iron-sulfur clusters to nitrosylated products.
11
However, there are few
examples of multinuclear mononitrosyl complexes containing nearby redox-active metal
centers.
8a
,
12
The clusters targeted here provide insight into the influence of neighboring
metal centers on the chemistry of the metal-nitrosyl moiety. Addition of NO to compound
1
leads to the formation of the corresponding nitrosyl adduct. Cyclic voltammetry of the
monocationic nitrosyl cluster,
1-NO
, displays three electrochemically quasi-reversible
oxidations and one quasi-reversible reduction (Figure 2). Each of the five redox states of the
nitrosyl clusters observed electrochemically was accessed synthetically (Figure 1). Stepwise
treatment of
1-NO
with AgOTf (
2-NO
and
3-NO
) and [(2,4-Br-C
6
H
4
)
3
N][SbCl
6
] (
4-NO
)
provides access to the oxidized NO adducts.
4-NO
decomposes in solution and as a solid on
the time scale of attempted crystallizations, preventing structural characterization. Reduction
of
1-NO
with decamethylcobaltocene in acetonitrile precipitates a purple solid assigned as
5-NO
. Dissolution of
5-NO
in tetrahydrofuran, pyridine, or dichloromethane, leads to rapid
decomposition preventing structural characterization of this complex as well. N
2
O is
detected upon decomposition of
5-NO
, albeit in low yield (~0.1 equivalents, GC-MS).
Mössbauer spectroscopy was performed on
1-NO
5-NO.
As observed in the
μ
4
-O system,
Mössbauer parameters are consistent with oxidations being localized at the basal triiron core
as characterized previously.
8a–c
,
13
In the Mössbauer spectrum of
1-NO
, the Fe–NO signal is
readily distinguished from the basal iron centers in the cluster, and was fit with an isomer
shift (
δ
) of 0.62 mm/s and a quadrupole splitting value (|ΔE
q
|) of 1.16 mm/s (Figure 3B;
Table 1). The exact Mössbauer parameters for the Fe–NO centers in
2-NO
4-NO
are more
difficult to assign due to spectral overlap with signals from the Fe
III
centers of the triiron
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core. The overlap is consistent, however, with only small changes in the Mössbauer
parameters for the Fe-NO sites in
1-NO
4-NO
(Figures 3C–D and Table 1). These
parameters are also similar to the previously reported
μ
4
-O NO clusters, which have
δ
values
ranging from 0.55 to 0.62 mm/s, and |ΔE
q
| values of 1.94 to 2.38 mm/s.
8a
Overall, these data
are consistent with the {FeNO}
7
formulation, according to Enemark-Feltham notation.
14
The Mössbauer spectrum of
5-NO
was fit with three Fe
II
in the triiron core and an apical
Fe–NO signal distinct from the ones observed for
1-NO
4-NO
, assigned as {FeNO}
8
(
δ
=
0.94 mm/s and |ΔE
q
| = 1.63 mm/s; Figure 3A), consistent with reduction of the Fe–NO
moiety rather than a remote metal site. Compounds
1-NO
,
2-NO
, and
3-NO
were
structurally characterized by XRD. In all cases, binding of NO to the apical Fe occurs in a
linear fashion (≥Fe4–N40–O40 > 175°, Figure 4A). As observed in the
μ
4
-O system and
from Mössbauer spectra (Figure 3B–D), bond metrics are consistent with oxidations being
localized at the basal triiron core of these three clusters (Table 1). The Fe–
μ
4
-F bonds,
which range from 2.07 to 2.24 Å, are longer than the Fe–
μ
4
-O bonds (1.93 to 2.18 Å) despite
the shorter ionic radius of F
which suggests a significantly weaker interaction with the
fluoride resulting in more electron deficient metal centers.
15
The Fe4–F distance increases
0.11 Å upon two oxidation events, similar to the geometry changes observed in the
μ
4
-O
system (0.12 Å).
8a
IR spectroscopy reveals a large range of
ν
N–O
for complexes
1-NO
5-NO
, from 1680 cm
−1
to 1855 cm
−1
(Figure 4B and Figure S22). Comparison of
ν
N–O
for
1-NO
4-NO
(1799
– 1855 cm
−1
) provides insight into the effect of remote redox changes on NO activation.
Oxidation of the Fe centers not bound to NO leads to an average of 19 cm
−1
per redox
change, with redox changes of more reduced clusters having a larger effect. The shift in
ν
N–O
to higher energy upon oxidation is matched by an increase in Fe4-
μ
4
-F distance, and
likely results from a more electron deficient Fe4 center due to this elongation. The nature
and type of interaction with axial ligand has been previously demonstrated to effect the level
of NO activation in mononuclear Fe complexes.
16
Analogous shifts in the distance between
Fe and axial ligands trans to coordinated N
2
have been reported for monoiron models of
nitrogenase.
17
The correlation between the increase in the Fe4-
μ
4
-ligand distance and the increase in the
ν
N–O
frequency observed previously for
μ
4
-O and now with
μ
4
-F interstitial ligands suggests
that this structural parameter generally serves to relay the effect of remote redox changes to
the metal that binds the small molecule. However, the magnitude of change in NO activation
as a result of these distal redox changes varies with the nature of the interstitial atom. For
μ
4
-
O clusters, the
ν
N–O
changes from 1715/1759 to 1823 cm
−1
over two redox events with an
average change of 54/33 cm
−1
per electron transfer, in contrast to only 19 cm
−1
for
μ
4
-F. The
stronger O
2−
ligand roughly doubles the effect of remote redox changes on the activation of
NO compared to F
. This is a unique observation, which relies on the ability to access many
oxidiation states of these clusters, and demonstrates that an interstitial ligand can influence
small molecule activation in two ways: first, by its direct interaction with the small-molecule
binding metal center, and, second, by modulating the degree to which other metals in the
cluster can perturb this metal-interstitial ligand interaction. Structural comparison of the
Fe4–
μ
4
-ligand distances over two oxidiation states shows that redox changes at the remote
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Fe centers shifts the Fe4
–μ
4
-F distance by 0.09
Ǻ
and the Fe4
–μ
4
-O bond by 0.12
Ǻ
(Figure
4B). The more donating interstitial ligand is able to more efficiently translate remote redox
changes in the cluster into NO activation.
A consequence of varying the
μ
4
-ligand in these clusters is that the weaker F
donor
increases the overall cluster charge of a particular redox state by one compared to the O
2−
version. Separating the effect of higher positive charge from the effect of the donating
abilities of the interstitial ligand on NO activation can be addressed by comparing clusters
2-
NO
4-NO
and the
μ
4
-O analogs. For the same cluster redox state, significantly higher
ν
N–O
are observed for the
μ
4
-F ligand compared to
μ
4
-O, as expected. The overall cluster
charge, which is higher by one compared to
μ
4
-O clusters of the same Fe redox states, is not
sufficient to explain the higher NO activation. Comparison of clusters of the same charge for
μ
4
-O and
μ
4
-F, but higher overall Fe redox state for
μ
4
-O (for example (
μ
4
-
F)Fe
II
Fe
III
2
{FeNO}
7
(
3-NO
) with
ν
N–O
= 1842 cm
−1
vs (
μ
4
-O)Fe
III
2
{FeNO}
7
with
ν
N–O
=
1823 cm
−1
), still shows higher degree of NO activation with O
2−
. This difference suggests
that the higher charge interstitial ligand leads to a more electron rich cluster and a lower
ν
N–O
due to its direct interaction with the metal centers rather than solely due to the cluster
charge.
IR spectroscopy of
5-NO
corroborates the Mössbauer data and is consistent with the
formation of a {FeNO}
8
motif; the
ν
N–O
at 1680 cm
−1
, is ~120 cm
−1
lower than
ν
N–O
for
the {FeNO}
7
moiety of
1-NO
. A similarly large shift was observed upon reduction for a
structurally related mononuclear trigonal bipyramidal Fe-NO complex,
18
and more generally
for non-heme {FeNO}
7
/{FeNO}
8
complexes.
19
An analogous species is not observable for
the
μ
4
-O clusters. Comparison of the redox potentials of the
μ
4
-F and the
μ
4
-O system
(Figure 4B)
8a
reveals that the F
ligand shifts the redox potentials positively by
approximately 1 V for the same cluster oxidation states compared to the O
2−
ligand, due to
the lower negative charge and electron donating ability of F
. An analogous effect is
observed for other clusters upon changing the bridging ligand to alter the charge of the
cluster.
6
,
7e
The shift in redox potentials allows access to more reduced states of the
μ
4
-F
clusters within the electrochemical solvent window, which could be beneficial for storing
additional reducing equivalents at more positive potentials. However, this is counterbalanced
by weaker activation of the diatomic ligand, as reflected by IR spectroscopy (vide supra). In
fact, to achieve the same level of NO activation, the
μ
4
-F clusters need to have Fe oxidation
states lower by two levels compared to the
μ
4
-O clusters. This is in contrast to the behavior
observed for certain iron-multicarbonyl clusters, where data is available for isostructural
motifs. For example, [Fe
4
C(CO)
12
]
2−
shows lower average CO activation than the one
electron more reduced, but same-charge cluster, [Fe
4
N(CO)
12
]
2−.
6
,
20
The difference is likely
a result of distribution of charge and small molecule activation over many (12) CO ligands.
In the present system, which displays a more biomimetic, single ligand binding, redox
changes at remote metal centers is relayed through the interstitial atom to a single Fe-NO
moiety, providing a test for the ability of the
μ
4-
ligand to communicate redox changes at
metals not bound to the small molecule. Furthermore, differences in chemical reactivity of
the diatomic ligand are observed. Addition of NO to (
μ
4
-O)Fe
II
2
Fe
III
{FeNO}
7
leads to NO
disproportionation to generate N
2
O and the one electron oxidized nitrosyl cluster.
8a
In
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contrast, addition of NO to
1-NO
, which is one electron more reduced ((
μ
4
-F)Fe
II
3
{FeNO}
7
)
does not result in a reaction. This difference in reactivity as a function of interstitial ligand is
likely due to a more activated NO and a 250 mV lower redox potential for the
μ
4
-O cluster.
Only
5-NO
, with an electronically different, {FeNO}
8
moiety, undergoes conversion to N
2
O
with a fluoride interstitial ligand, albeit not cleanly. Over-all, despite more negative
potentials compared to
μ
4
-F analogs of the same redox state, reactivity of NO is observed at
milder potentials with the
μ
4
-O cluster.
Summary
In this report, we have demonstrated the significant effects that the change of interstitial
ligands (
μ
4
-O vs
μ
4
-F) has on the small molecule activation properties of tetranuclear iron
clusters. The more positive redox potentials of
μ
4
-F clusters allow access to more reduced Fe
states. However, this does not result in more efficient activation of small molecule ligands,
as inferred from IR spectroscopy and reactivity of NO complexes. The higher
ν
N–O
values
of the
μ
4
-F species for the same Fe oxidation states compared to the
μ
4
-O analogues are not
due to the difference in cluster charge, but rather the nature of the interactions with the
bridging ligand. To achieve similar NO activation, the cluster needs to be two electrons more
reduced with the
μ
4
-F compared to the
μ
4
-O ligand. Consequently, NO disproportionation is
observed with a
μ
4
-O ligand at higher Fe oxidation states and more positive potentials than
with a
μ
4
-F ligand. Furthermore, the
μ
4
-O ligand is a better relay of remote redox changes.
The structure-function studies described here suggest that a higher charge interstitial ligand,
such as the carbide in FeMoco of nitrogenase, is more efficient at tuning cluster properties in
a variety of ways toward the activation of small molecule. Analogs of the reported
compounds with
μ
4
-C and
μ
4
-N moieties would provide further quantitative measures of NO
activation upon additional increase of the formal negative charge of the interstitial ligand;
their syntheses are being pursued.
Experimental Details
General Considerations
All reactions were performed at room temperature in an N
2
-filled M. Braun glovebox or
using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at
140 °C for at least 2 h prior to use, and allowed to cool under vacuum.
LFe
3
(OAc)(OTf)
2
,
8a
Fe(N(SiMe
3
)
2
)
2,
21
benzyl potassium,
22
1-
H
-3-phenyl pyrazole (HPhPz),
23
anhydrous
[NBu
4
][F]
24
, and [(2,4-Br-C
6
H
3
)
3
N][SbCl
6
]
25
were prepared according to literature
procedures. [(4-Br-C
6
H
4
)
3
N][OTf] was prepared according to a modified literature
procedure.
26
Tetrahydrofuran was dried using sodium/benzophenone ketyl, degassed with
three freeze-pump-thaw cycles, vacuum transferred, and stored over 3Å molecular sieves
prior to use. CH
2
Cl
2
, diethyl ether, benzene, acetonitrile, hexanes, and pentane were dried
by sparging with nitrogen for at least 15 minutes, then passing through a column of activated
A2 alumina under positive N
2
pressure.
1
H and
19
F NMR spectra were recorded on a Varian
300 MHz spectrometer.
13
C NMR spectra were recorded on a Varian 500 MHz spectrometer.
CD
3
CN and CD
2
Cl
2
was purchased from Cambridge Isotope Laboratories, dried over
calcium hydride, degassed by three freeze-pump-thaw cycles, and vacuum transferred prior
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to use. Infrared (ATR-IR) spectra were recorded on a Bruker ALPHA ATR-IR spectrometer
at 4 cm
−1
resolution. Headspace analysis was conducted on a HP 5972 GC-MS.
Physical Methods
Mössbauer measurements—
Zero applied field
57
Fe Mossbauer spectra were recorded
at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped
with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the
centroid of an
α
-Fe foil signal at room temperature. Samples were prepared by mixing
polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing
a concentrated acetonitrile solution in the cup. The data were fit to Lorentzian lineshapes
using WMOSS (
www.wmoss.org
).
Electrochemical measurements—
CVs and SWVs were recorded with a Pine
Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All
measurements were performed in a three electrode cell, which consisted of glassy carbon
(working; ø = 3.0 mm), silver wire (counter) and bare platinum wire (reference), in a N
2
filled M. Braun glovebox at RT. Dry acetonitrile or CH
2
Cl
2
that contained ~85 mM [Bu
4
N]
[PF
6
] was used as the electrolyte solution. The ferrocene/ferrocinium (Fc/Fc
+
) redox wave
was used as an internal standard for all measurements.
X-ray crystallography—
X-ray diffraction data was collected at 100 K on a Bruker
PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo K
α
(
λ
) =
0.71073 Å or Cu K
α
(
λ
) = 1.54178 Å). All manipulations, including data collection,
integration, and scaling, were carried out using the Bruker APEXII software. Absorption
corrections were applied using SADABS. Structures were solved by direct methods using
XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7
to convergence. All non-hydrogen atoms were refined using anisotropic displacement
parameters. Hydrogen atoms were placed in idealized positions and were refined using a
riding model. Due to the size of the compounds (
1
3
and
1-NO
3-NO
), most crystals
included solvent-accessible voids that contained disordered solvent. In most cases the
solvent could be modeled satisfactorily.
Synthetic Procedures
Synthesis of Potassium 3-phenyl-pyrazolate (KPhPz)—
In the glovebox, a solution
of 1-
H
-3-phenyl-pyrazole (1.54 g, 11.8 mmol) in THF (5 mL) was stirred while a solution of
benzyl potassium (1.70 g, 11.8 mmol) in THF (10 mL) was added drop-wise. Addition
caused the solution to change from colorless to pale yellow. After 30 minutes, the solvent
was removed under reduced pressure to obtain 1.83 g off-white powder (85% yield).
1
H
NMR (300 MHz, CD
3
CN)
δ
7.83 (d, 2H), 7.44 (s, 1H), 7.28 (t, 2H), 7.07 (t, 1H), 6.39 (s,
1H).
13
C NMR (500 MHz, CD
3
CN)
δ
100.01 (Pz NC
C
H), 125.02 (
p
-Ar
C
H), 125.37 (
m
-Ar
C
H), 128.98 (
o
-Ar
C
H), 139.34 (Pz
C
HCHN), 150.27 (Pz N
C
CH). An expected signal ~
138 ppm (
i
-Ar
C
)
8a
could not be observed, likely due to the low solubility of KPhPz.
Synthesis of tris-4-bromo-phenylamininum trifluoromethanesulfonate ([(4-Br-
C
6
H
4
)
3
N][OTf])—
This was prepared through a modification of a literature procedure for
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[(4-Br-C
6
H
4
)
3
N][BF
4
].
26
Tris-4-bromo-phenylamine (1.5 g, 3.11 mmol) was dissolved in 30
mL diethyl ether with silver trifluoromethanesulfonate (AgOTf; 1.2 g, 4.67 mmol). This
light green solution was added to a 100 mL Schlenk tube and cooled to −40 °C under N
2
atmosphere. Iodine powder (0.75 g, 2.96 mmol) was added with a counter-flow of N
2
while
stirring; addition caused the solution to turn dark blue. The Schlenk tube was warmed to
room temperature and filtered over a course porosity frit. The collected precipitate was
filtered with 30 mL CH
2
Cl
2
in the glovebox. To the resulting dark blue solution, 40 mL
diethyl ether was added and the flask was cooled to −40 °C. [(4-Br-C
6
H
4
)
3
N][OTf] was
collected as a dark purple solid upon filtration (1.36 g, 69% yield). Anal. Calc. (%) for
C
19
H
12
Br
3
F
3
NO
3
S: C, 36.16; H, 1.92; N, 2.22. Found: C, 36.70; H, 1.94; N, 2.27.
Synthesis of [LFe
3
F(PhPz)
3
Fe][OTf] (1)—
In the glovebox, a suspension of
LFe
3
(OAc)
(OTf)
2
(1047 mg, 0.76 mmol) in THF (3 mL) was frozen in the cold well. To the thawing
suspension, solutions of potassium 3-phenyl-pyrazolate (190 mg, 1.04 mmol) in THF (3 mL)
and 1-
H
-3-phenyl-pyrzole (220 mg, 1.52 mmol) in THF (3 mL) were added. The suspension
changed color from yellow to orange upon addition of the potassium 3-phenyl-pyrazolate.
[Bu
4
N][F] (208 mg, 0.79 mmol) was added as a suspension in THF (3 mL), causing the
solution to become dark red. A solution of Fe(N(SiMe
3
)
2
)
2
(288 mg, 0.76 mmol) in THF (2
mL) was added. The reaction was stirred for 20 h, after which an orange precipitate was
observed. The suspension was filtered over a bed of celite on a fine porosity glass frit and
washed with 5 mL THF. The orange solid was collected with 60 mL acetonitrile. The solvent
was removed under reduced pressure to obtain [LFe
3
F(PhPz)
3
Fe][OTf] as an orange solid
(950 mg, 75% yield).
1
H NMR (300 MHz, CD
2
Cl
2
)
δ
104.77, 78.57, 75.13, 48.82, 37.46,
30.48, 27.17, 26.44, 25.63, 19.69, 18.42, 11.60, 10.53, 4.54, 4.22, 3.44, 1.99, 1.27, 1.16,
−1.13, −2.80, −46.96.
19
F NMR (300 MHz, CD
2
Cl
2
)
δ
−78.45. UV-vis (CH
2
Cl
2
) [
ε
(M
−1
cm
−1
)]: 251 nm (9.2 ×10
4
), 463 nm (3.9 ×10
3
). Anal. Calcd. (%) for C
85
H
60
F
4
Fe
4
N
12
O
6
S:
C, 60.88; H, 3.61; N, 10.02. Found: C, 61.16; H, 3.75; N, 9.74.
Synthesis of [LFe
3
F(PhPz)
3
Fe][OTf]
2
(2)—
To a suspension of [LFe
3
F(PhPz)
3
Fe][OTf]
(
1;
94 mg, 0.06 mmol) in THF (2 mL), a solution of AgOTf (14 mg, 0.06 mmol) in THF (2
mL) was added. The color of the suspension changed from orange to brown and, after 2
hours, the solvent was removed under reduced pressure. The brown residue was dissolved in
CH
2
Cl
2
and filtered over a bed of celite on glass filter paper. The solvent was removed under
reduced pressure to obtain [LFe
3
F(PhPz)
3
Fe][OTf]
2
as a brown solid (100 mg, 98% yield).
1
H NMR (300 MHz, CD
2
Cl
2
)
δ
101.33, 87.83, 79.33, 47.73, 46.79, 35.24, 34.14, 28.86,
26.35, 18.15, 16.58, 16.33, 12.10, 8.55, 7.28, 6.79, 6.25, 5.25, 4.63, −42.36.
19
F NMR (300
MHz, CD
2
Cl
2
) −78.19. UV-vis (CH
2
Cl
2
) [
ε
(M
−1
cm
−1
)]: 250 nm (10.9 ×10
4
), 432 nm (4.8
×10
3
). Anal. Calcd. (%) for C
86
H
60
F
7
Fe
4
N
12
O
9
S
2
: C, 56.57; H, 3.31; N, 9.21. Found: C,
56.47; H, 3.13; N, 8.88.
Synthesis of [LFe
3
F(PhPz)
3
Fe(CH
3
CN)][OTf]
3
(3)—
To a stirring solution of
[LFe
3
F(PhPz)
3
Fe][OTf]
2
(
2
; 78.5 mg, 0.04 mmol) in acetonitrile (2 mL), [(p-Br-C
6
H
4
)
3
N]
[OTf] (27.1 mg, 0.04 mmol) was added as an acetonitrile solution (2 mL). The brown
solution became purple upon addition. After 30 minutes, the solution was filtered. 5 mL of
CH
2
Cl
2
was added to the filtrate, then 10 mL pentane, to obtain a purple precipitate. The
Reed and Agapie
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. Author manuscript; available in PMC 2018 December 03.
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supernatant was decanted and the remaining solid was briefly dried under reduced pressure
to obtain [LFe
3
F(PhPz)
3
Fe(CH
3
CN)][OTf]
3
as a purple solid (42.3 mg, 50% yield).
1
H
NMR (300 MHz, CD
3
CN)
δ
125.15, 91.53, 82.45, 80.10, 61.48, 51.98, 43.99, 15.30, 13.93,
12.33, 8.44, 6.48, 5.67, 5.30, 0.46, −5.74, −18.78.
19
F NMR (300 MHz, CD
3
CN) −75.66.
UV-vis (CH
2
Cl
2
) [
ε
(M
−1
cm
−1
)]: 250 nm (10.3 ×10
4
), 465 nm (3.6 ×10
3
). Anal. Calcd. (%)
for C
88
H
62
Cl
2
F
10
Fe
4
N
12
O
12
S
3
(
3
with CH
2
Cl
2
instead of CH
3
CN; compound recrystallized
in CH
2
Cl
2
): C, 51.31; H, 3.03; N, 8.16. Found: C, 51.26; H, 3.04; N, 8.43.
Synthesis of [LFe
3
F(PhPz)
3
Fe(NO)][OTf] (1-NO)
Method A:
In the glovebox, a 100 mL Schlenk tube was charged with a solution of
[LFe
3
F(PhPz)
3
Fe][OTf] (
1
; 179 mg, 0.11 mmol) in CH
2
Cl
2
(5 mL). The solution was
degassed by three freeze-pump-thaw cycles. While frozen, gaseous nitric oxide (33 mL, 59
mmHg, 0.11 mmol) was condensed in the tube. The reaction was stirred at room temperature
for 2 h and changed color from orange to brown. The solvent was removed under reduced
pressure to yield [LFe
3
F(PhPz)
3
Fe(NO)][OTf] as a brown solid (181 mg, 99% yield).
1
H
NMR (300 MHz, CD
2
Cl
2
)
δ
98.43, 76.64, 74.24, 42.59, 40.12, 35.92, 32.51, 27.06, 20.05,
15.27, 14.16, 11.24, 10.79, 4.27, 2.46, 1.13, 0.58, 0.46, −10.77, −23.61.
19
F NMR (300
MHz, CD
2
Cl
2
)
δ
−78.71. Anal. Calcd. (%) for C
86
H
62
Cl
2
F
4
Fe
4
N
13
O
7
S (
1-NO
· CH
2
Cl
2
;
compound recrystallized from CH
2
Cl
2
/pentane): C, 57.66; H, 3.49; N, 10.16. Found: C,
57.40; H, 3.46; N, 10.01.
Method B:
In the glovebox, solid LFe
3
F(PhPz)
3
Fe(NO) (
5-NO
; 22 mg, 0.014 mmol) was
cooled to −196 °C in a cold well in a 20 mL vial with a stir bar. AgOTf (3.7 mg, 0.014
mmol) in 0.5 mL thawing tetrahydrofuran was added to the cooled powder. This reaction
was stirred at room temperature for 30 minutes then pumped down. The purple suspension
became a brown solution.
1
H NMR analysis of the crude reaction showed mostly (>90%)
[LFe
3
F(PhPz)
3
Fe(NO)][OTf] (
1-NO
; Figure S22). The brown solid was filtered in CH
2
Cl
2
to obtain 16.8 mg of [LFe
3
F(PhPz)
3
Fe(NO)][OTf] after recrystallization (69% yield).
Synthesis of [LFe
3
F(PhPz)
3
Fe(NO)][OTf]
2
(2-NO)
Method A:
In the glovebox, a 100 mL Schlenk tube was charged with a solution of
[LFe
3
F(PhPz)
3
Fe][OTf]
2
(
2
; 163 mg, 0.09 mmol) in CH
2
Cl
2
(5 mL). The solution was
degassed by three freeze-pump-thaw cycles. While frozen, gaseous nitric oxide (33 mL, 50
mmHg, 0.09 mmol) was condensed in the tube. The reaction was stirred at room temperature
for 2 h, changing color from brown to yellow-green. The solvent was removed under
reduced pressure to yield [LFe
3
F(PhPz)
3
Fe(NO)][OTf]
2
as a dark green solid (162 mg, 98%
yield).
1
H NMR (300 MHz, CD
2
Cl
2
)
δ
100.10, 83.22, 80.63, 66.68, 50.74, 46.79, 41.32,
17.25, 14.62, 14.38, 12.35, 11.71, 3.31, 0.30, −3.31, −17.33.
19
F (300 MHz, CD
2
Cl
2
)
δ
−77.52. Anal. Calcd. (%) for C
86
H
60
F
7
Fe
4
N
13
O
10
S
2
: C, 55.65; H, 3.26; N, 9.81. Found: C,
55.59; H, 3.25; N, 9.53.
Method B:
In the glovebox, a solution of [LFe
3
F(PhPz)
3
Fe(NO)][OTf] (
1-NO
; 160 mg,
0.10 mmol) in acetonitrile (3 mL) was added to a solution of AgOTf (25 mg, 0.10 mmol) in
acetonitrile (2 mL). The solution changed color from brown to yellow-green. After 1 h, the
solvent was removed under reduced pressure. The green residue was dissolved in CH
2
Cl
2
Reed and Agapie
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