Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

Article abstract of DOI:10.1021/acs.inorgchem.0c03427

Recent experimental evidence suggests that the FeMoco of nitrogenase undergoes structural rearrangement during N2 reduction, which may result in the generation of coordinatively unsaturated iron sites with two sulfur donors and a carbon donor. In an effort to synthesize and study small-molecule model complexes with a one-carbon/two-sulfur coordination environment, we have designed two new SCS pincer ligands containing a central NHC donor accompanied by thioether-or thiolate-functionalized aryl groups. Metalation of the thioether ligand with Fe(OTf)2 gives 6-coordinate complexes in which the SCS ligand binds meridionally. In contrast, metalation of the thiolate ligand with Fe(HMDS)2 gives a four-coordinate pseudotetrahedral amide complex in which the ligand binds facially, illustrating the potential structural flexibility of these ligands. Reaction of the amide complex with a bulky monothiol gives a four-coordinate complex with a one-carbon/three-sulfur coordination environment that resembles the resting state of nitrogenase. Reaction of the amide complex with phenylhydrazine gives a product with a rare κ1-bound phenylhydrazido group which undergoes N-N cleavage to give a phenylamido complex.

Full text of DOI:10.1021/acs.inorgchem.0c03427

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Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS
Pincer Ligand
Amy L. Speelman, Kazimer L. Skubi, Brandon Q. Mercado, and Patrick L. Holland*
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ABSTRACT: Recent experimental evidence suggests that the
FeMoco of nitrogenase undergoes structural rearrangement during
N2 reduction, which may result in the generation of coordinatively
unsaturated iron sites with two sulfur donors and a carbon donor.
In an effort to synthesize and study small-molecule model
complexes with a one-carbon/two-sulfur coordination environ-
ment, we have designed two new SCS pincer ligands containing a
central NHC donor accompanied by thioether- or thiolate-
functionalized aryl groups. Metalation of the thioether ligand
with Fe(OTf)2 gives 6-coordinate complexes in which the SCS ligand binds meridionally. In contrast, metalation of the thiolate
ligand with Fe(HMDS)2 gives a four-coordinate pseudotetrahedral amide complex in which the ligand binds facially, illustrating the
potential structural flexibility of these ligands. Reaction of the amide complex with a bulky monothiol gives a four-coordinate
complex with a one-carbon/three-sulfur coordination environment that resembles the resting state of nitrogenase. Reaction of the
amide complex with phenylhydrazine gives a product with a rare κ1-bound phenylhydrazido group which undergoes N−N cleavage
to give a phenylamido complex.
Chart 1
INTRODUCTION
■
Nitrogenases, which convert N2 to bioavailable NH3, play a
vital role in the global nitrogen cycle.1 In Mo-nitrogenase, this
reaction occurs at the unusual iron−molybdenum cofactor
(FeMoco), which in its resting state contains six sulfide-
bridged iron atoms arranged around a central carbide atom.2−5
To bind N2, the FeMoco must undergo further reduction,
which is potentially accompanied by changes in the
coordination environment at the iron site(s).6−8 In particular,
crystallographic studies have revealed that in nitrogenases the
central “belt” sulfides can be replaced by exogenous ligands
during substrate reduction.9−12 The ability to lose a sulfide
implies that prior to N2 binding, the FeMoco may contain one
or more coordinatively unsaturated iron atoms with two S
donors and a C donor (Chart 1, top).
The unusual coordination environment and reactivity of the
Fe atoms in the FeMoco have stimulated the development of
inorganic model complexes that capture features in common
with the potential intermediates.13 While many earlier studies
had abiological donor atoms and/or metals,14,15 there has been
significant recent progress toward iron complexes with S- and
C-containing ligands that more closely mimic the coordination
environment in nitrogenase.16−21 For example, we reported
iron complexes of a m-terphenyl-derived bis(thiolate) ligand
(Chart 1) in which the sulfur donors are accompanied by a
central arene ring that acts as a carbon donor through its π-
system.22,23 Iron complexes with this ligand bind N2 upon
reduction, giving the first example of an Fe−N2 complex
containing only biomimetic S and C donors. However, the π-
Received: November 20, 2020
Published: January 14, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03427
© 2021 American Chemical Society
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Scheme 1. Synthesis of Imidazolium Salts 6a and 6b
acceptor character of the central arene competes with π-
backbonding to N2, leading to weak N2 binding. Furthermore,
the SCS ligand dissociates in the presence of excess acid,
causing decomposition and preventing catalytic N2 reduction.
We reasoned that incorporation of a C-based σ-donor in place
of the arene ring might lead to more stable complexes that are
more electron-rich and therefore better suited for substrate
binding.
sulfide. Both the flexibility and preference for facial
coordination are conducive to mimicking the environment of
iron in the FeMoco. We also report our unsuccessful attempts
to use these complexes as precatalysts for N2 reduction. Finally,
we describe tetrahedral iron complexes containing SCS2− with
nitrogenase-relevant thiolate, phenylhydrazido, and amido
groups.
Although they are not as common as pincers containing N,
C, and P donors, some SCS pincer ligands have been reported
in the literature.24,25 We sought an SCS pincer with both
anionic S donors and a strongly bound C anchor, which led us
to the design of N-heterocyclic carbene (NHC) systems. Iron−
NHC complexes are employed in a variety of catalytic
reactions.26−28 They have been used in iron−sulfur clusters,
where they have been shown to support low oxidation states29
and stabilize alkyl complexes.30−32 Chelating bis(NHC)
ligands can also support iron−thiolate and iron−selenolate
complexes.33,34 Other iron complexes of multidentate NHC
ligands can coordinate N2.35−43 Peters has described a cyclic
alkyl amino carbene complex of iron that can reduce N2 to
ammonia.44
NHC ligands can be modified by changing the substituents
attached to the aryl groups, allowing facile introduction of
additional donor atoms.45−49 Although several NHC ligands
containing accessory S donors have been reported, the
majority have only one sulfur donor, and their syntheses
cannot be easily modified to incorporate a second sulfur
donor.50,51 One notable exception is a set of nickel, platinum,
and palladium complexes with a bis(thiolate) NHC ligand
reported by Sellmann and co-workers (Chart 1).52−54
However, in this system the free ligand was not cleanly
isolated or fully characterized due to its poor solubility in
common organic solvents. We reasoned that the introduction
of bulky organic substituents on the aryl groups of the ligand
would facilitate isolation and purification of both the ligand
and corresponding metal complexes.
RESULTS AND DISCUSSION
■
Synthesis of Bis(thiolate) and Bis(thioether) Imida-
zolium Salts. We targeted an NHC ligand with tert-butyl-
substituted aryl thiolate groups. The initial synthetic steps were
adapted from procedures reported by Sellmann.55 In the
original report, 3,5-di-tert-butylaniline (1) was synthesized by
using a Schmidt reaction, but we wanted to avoid large-scale
reactions with sodium azide. Therefore, we instead prepared 1
from commercially available 3,5-di-tert-butylbromobenzene via
Buchwald−Hartwig amination with lithium bis(trimethylsilyl)-
amide followed by aqueous hydrolysis of the resulting silylated
aniline.56 Following literature procedures,55 1 was converted to
2-aminobenzothiazole 2 and ring-opened to give 2-amino-
thiophenol 3. Crude 3 was then treated with glyoxal to
generate a Schiff base, which is known to rearrange to give 4.55
Upon deprotonation with strong base, 4 rearranges to the
bis(thiolate) Schiff base,55 allowing the net functionalization of
the thiolate groups with electrophiles. Through this strategy,
deprotonation of 4 with KHMDS at −78 °C followed by
addition of methyl iodide afforded S-protected Schiff base 5a
in 90% yield. Our initial attempts to convert 5a to an
imidazolium salt using paraformaldehyde and HCl following
the general procedures used for preparation of common
unsaturated NHC ligands57 afforded only trace amounts of 6a.
We hypothesized that acid-induced decomposition of 5a
occurs more rapidly than reaction with the heterogeneous
paraformaldehyde. Accordingly, sonication of the paraformal-
dehyde pellets with HCl in dioxane to produce a fine
suspension prior to addition to 5a resulted in a significant
increase in yield. Compound 6a was isolated in 35% yield by
triturating the crude product with pentane, followed by
counterion metathesis to the tosylate salt, which precipitated
from dichloromethane solution upon addition of diethyl ether.
Because methyl protecting groups are often difficult to
remove, we also prepared the analogous benzyl-protected
imine 5b in 72% yield by treating 4 with KHMDS and benzyl
bromide at −78 °C. Cyclization with paraformaldehyde using a
Here, we report the synthesis of two novel SCS pincer
ligands containing a central NHC and two thioether or thiolate
donors (Chart 1), which may also find broader use in
coordination chemistry. We demonstrate that in iron
complexes the ligand binds in a planar fashion with SMeCSMe,
whereas it binds facially with SCS2−, showing the geometric
flexibility of this ligand type. In addition, the facial SCS
coordination resembles the proposed coordination environ-
ment of the iron atoms in FeMoco after dissociation of a
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modified version of the procedure used for preparation of 6a
gave the imidazolium tosylate salt 6b in 36% yield. The benzyl
protecting group was then removed by addition of excess (>3
equiv) potassium naphthalenide at −78 °C in THF followed
by quenching with methanol. The potassium bis(thiolate)
imidazolium salt 7 was isolated in 64% yield after work-up.
We also attempted to isolate the corresponding bis(thiol)
imidazolium chloride salt by quenching the deprotection with
Scheme 3. Synthesis of Bis(thioether) NHC Iron Complexes
1
an ethereal HCl solution instead of with methanol. The H
NMR spectrum of an aliquot of this reaction mixture in
CD2Cl2 indicated complete consumption of the starting
material and formation of a symmetric major species with
tert-butyl resonances at δ 1.36 and 1.59 ppm that are assigned
to the desired bis(thiol) imidazolium salt (Figure S11). Stirring
the solution overnight resulted in degradation of this species to
a lower-symmetry product with tert-butyl resonances at δ 0.80,
1.20, 1.42, and 1.45 ppm. Two doublets (3JH−H = 13 Hz) also
appeared at δ 4.87 and 5.63 ppm, consistent with formation of
an olefin-containing product. These results suggest rearrange-
ment to a benzothiazolium−aminothiol species (Scheme 2)
solution of 8 with pentane and storing at −40 °C resulted in
precipitation of a white solid. Despite multiple attempts, we
were unable to obtain high-quality crystallographic data for 8,
possibly due to its complex speciation in THF (see below). In
a low-resolution crystal structure (R = 12.5%), the ligand is
bound meridionally with a molecule of THF trans to the NHC,
and the two trifluoromethanesulfonate counteranions are
Scheme 2. Proposed Rearrangement of Bis(thiol)
Imidazolium Salt under Acidic Conditions
̈
bound in the remaining sites (Figure S52). The Mossbauer
spectrum of material isolated in this fashion indicates a single
iron-containing species with δ = 1.10 mm/s and |ΔEQ| = 3.74
mm/s (Figure 1, top), which is typical for an octahedral high-
spin iron(II) complex with low symmetry.
with unknown stereochemistry. This reaction occurred slowly
even in the solid state and was also observed when isolated 7
was treated with HCl in THF (Figure S12). Because of this
instability, we were unable to cleanly isolate the fully
protonated bis(thiol) imidazolium form of 7, though the
bis(thiolate) imidazolium form was useful in synthesis, as
described below. Analogous reactivity was observed by
Despagnet-Ayoub et al. during attempts to synthesize an
aniline−NHC−phenol ligand, which suggests that facile
rearrangement may be a general property of N-arylimidazolium
salts containing protic functional groups.58,59
Synthesis of Bis(thioether) NHC Iron Complexes.
Treatment of 6a with KHMDS in THF resulted in formation
of the free N-heterocyclic carbene SMeCSMe, as demonstrated
by the disappearance of the imidazolium proton signal in the
starting material and appearance of a characteristic 13C
resonance for the carbene C at δ 218 ppm in C6D6 (Figure
S10). SMeCSMe was separated from the salt byproduct by
extraction into diethyl ether and isolated in 92% yield. This
material is stable for months when stored in the solid state at
−35 °C under N2.
̈
Figure 1. (top) Mossbauer spectrum of solid 8 isolated by
̈
precipitation from THF with pentane. (middle) Mossbauer spectrum
Slow addition of SMeCSMe to a solution of Fe(OTf)2 in THF
at −78 °C followed by warming to room temperature gave a
colorless iron complex (8). Layering a concentrated THF
̈
of 50 mM 8 in frozen THF. (bottom) Mossbauer spectrum of 9 in the
solid state. The data are shown as circles, the lines correspond to fits
with the parameters in Table 1, and the gray line is the residual.
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1
The H NMR spectrum of 8 in THF-d8 contains multiple
paramagnetically shifted resonances between δ −1 and 106
ppm (Figure S15). On the basis of the relative integrations of
the NMR signals, we estimate that two species containing the
SMeCSMe ligand are present in solution in a ratio of ca. 3:1 (see
the Supporting Information for a detailed discussion of the
1
assignment of the H NMR spectrum of 8). The 19F NMR
spectrum of 8 contains a peak at δ −7.8 ppm (ν1/2 = 220 Hz)
and a much broader peak at δ −33 ppm (ν1/2 = 2100 Hz) also
in a ratio of ca. 3:1 (Figure S16), indicating the presence of
triflate ions in two different coordination environments,
presumably corresponding to the same two species observed
1
̈
in the H NMR spectrum. The Mossbauer spectra of 8 in
frozen THF (Figure 1, middle) and of a THF solution of 8
dried under vacuum (Figure S17) also indicate the presence of
two species. One species has parameters nearly identical to
those of the precipitated 8 described above (δ = 1.07 mm/s
and |ΔEQ| = 3.74 mm/s) and likely corresponds to the
crystallographically determined structure. The second species
has a broader spectrum with nearly the same isomer shift (δ =
1.11 mm/s) but a smaller quadrupole splitting (|ΔEQ| = 2.16
̈
mm/s). The ratio of the two species in the Mossbauer spectra
is nearly 1:1. The difference of this ratio from that observed by
1H NMR spectroscopy may arise from changes in speciation
with concentration and/or freezing. The identity of the other
species in the solution spectra of 8 is unknown but could
correspond to a different ligand binding mode (for example, fac
vs mer binding, or dissociation of one or both thioether
groups).
Figure 2. (top) Thermal ellipsoid plot of the XRD structure of 9 with
thermal ellipsoids shown at the 50% probability level. Hydrogen
atoms, the outer-sphere triflate counterions, and outer-sphere solvent
molecules (CH3CN and diethyl ether) have been omitted. Selected
bond distances (Å): Fe1−S12: 2.2629(8); Fe1−S13: 2.2731(9);
Fe1−C11: 1.884(2). (bottom) View of 9 along the Fe−C bond with
̈
Table 1. Fit Parameters for Mossbauer Spectra of 8−13
CH3CN omitted to show the conformation of SMeCSMe
.
(mm/s)
solid state
frozen THF solution
δ
|ΔEQ|
Γ
δ
|ΔEQ|
Γ
identical with that of 8. Crystals of 9 suitable for XRD were
grown by layering a concentrated CH3CN solution with
diethyl ether at −40 °C. In this structure (Figure 2), the iron is
six-coordinate and SMeCSMe is bound meridionally. The three
remaining open coordination sites are occupied by CH3CN
molecules, and the triflate counterions are outer-sphere.
We attempted catalytic N2 reduction by treating 8
(generated via coevaporation of 9 with THF) with 6 equiv
a
a
a
b
c
b
b
c
8
1.10
3.74
0.28
1.07
1.11
3.63
2.16
0.37
0.56
c
9
0.38
0.67
0.68
0.65
0.67
1.41
1.82
1.76
1.60
1.69
0.30
0.30
0.33
0.32
0.34
10
11
12
13
0.68
0.67
1.78
2.07
0.31
0.42
of KC8 and 12 equiv of acid ([H(OEt2)2][BArF ], BHT,
a
4
Parameters for material precipitated from THF with pentane.
Parameters for major component (53%, see Figure 1). Parameters
for minor component (47%, see Figure 1).
b
c
diphenylammonium triflate, lutidine hydrochloride, H2O, or
HCl) at −78 °C in THF. These reactions generated little or no
NH3 and/or N2H4 (Scheme S1, Supporting Information).
Because of the inability of 8 to catalyze N2 reduction and the
unclear nature of its solution speciation, we turned our
attention to iron complexes of the bis(thiolate) NHC SCS2−.
Synthesis of Bis(thiolate) NHC Iron Complexes.
Treatment of 7 with KHMDS in THF resulted in clean
formation of the bis(thiolate)carbene ligand SCS2−, as
indicated by the loss of the imidazolium signal at δ 9.35
We were unable to fully purify 8 due to its poor solubility in
common noncoordinating organic solvents and its similar
solubility to free Fe(OTf)2 and protonated ligand, which are
consistently present as minor impurities in the isolated
material. We therefore explored the behavior of 8 in a more
strongly coordinating solvent. Upon dissolving 8 in CH3CN,
an immediate color change from white to intense pink
1
1
occurred. The H NMR spectrum of the resulting species in
ppm in the H NMR spectrum and the appearance of a
CD3CN indicates formation of a diamagnetic acetonitrile
resonance at δ 214 ppm in the 13C NMR spectrum (Figures
S13 and S14). Addition of this in situ generated NHC to FeCl2,
FeBr2, or Fe(OTf)2 in THF at −80 °C gave bright red
solutions. However, upon warming to room temperature, these
reaction mixtures turned black and a dark precipitate formed.
Although we have not been able to structurally characterize the
product(s), we suspect that a mixture of oligomeric species is
formed. Dimerization is common for iron complexes with
thiolate-containing ligands and, in particular, has been
adduct (9), and the 19F NMR contains a single sharp
̈
resonance at δ −80 ppm. The Mossbauer spectrum of 9
(Figure 1) contains a single quadrupole doublet with δ = 0.38
mm/s, which is significantly lower than that of 8 (see above)
and is characteristic of a low-spin electronic configuration at
iron(II). The CH3CN binding is reversible; upon dissolving
pink crystals of 9 in THF, a colorless solution is obtained, and
1
the H NMR spectrum of the resulting material in THF-d8 is
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Scheme 4. Synthesis of 10
observed for ligands derived from the o-aminothiophenol
3.60−62 The formation of polymeric species has also been
reported for some iron complexes with a benzimidazole OCO
ligand related to SCS2−.63
In contrast, addition of 7 to Fe(HMDS)2 in THF followed
by addition of 18-crown-6 formed a bright yellow species (10)
that is stable in solution at room temperature in the absence of
air. Crystals of 10 suitable for X-ray diffraction were obtained
from a concentrated diethyl ether solution stored at −40 °C
overnight. XRD analysis reveals a complex in which iron is
coordinated to SCS2− and a bis(trimethylsilyl)amide ligand
with a distorted tetrahedral geometry (τ4 = 0.79) at iron
(Figure 3).64 The ligand is bound facially, which results in the
iron lying 1.18 Å below the plane defined by the donor atoms
of the SCS ligand. This contrasts with 8, 9, and the nickel
complexes reported by Sellmann and co-workers,5254 in which
the metal ion sits in the SCS plane and the flanking aryl groups
twist to accommodate a planar binding mode (see Figure 2).
Structurally analogous bis(phenolate) NHC ligands are also
known to bind in both fac and mer geometries in molybdenum
complexes, illustrating the conformational flexibility of this
type of X(NHC)X ligand scaffold.65
Figure 3. (top) Thermal ellipsoid plot of the XRD structure of 10
with thermal ellipsoids shown at the 50% probability level. One of two
chemically identical but crystallographically inequivalent molecules in
the asymmetric unit is shown. Hydrogen atoms, K(18-crown-6), and
outer-sphere solvent molecules (diethyl ether) are omitted. Selected
bond distances (Å): Fe1−S12: 2.378(2); Fe1−S13: 2.345(2); Fe1−
C11: 2.062(6); Fe1−N14: 1.984(5). (bottom) View of 10 along the
Fe−C bond to illustrate the conformation of SCS2−. Note that the Fe
does not lie in the plane of the imidazolium ring. The bis-
(trimethylsilyl)amide ligand is omitted for clarity.
1
The H NMR spectrum of 10 in C6D6 contains six broad,
paramagnetically shifted resonances between +78 and −13
ppm, suggesting that the crystallographically observed Cs
symmetry is retained in solution. The solution magnetic
moment of μeff = 5.1 0.1 is consistent with a high-spin (S =
Treatment of 10 with 5 equiv of [H(OEt2)2][BArF ] at −80
4
°C resulted in immediate formation of a light orange solution
̈
and precipitation of a white solid. The Mossbauer spectrum of
̈
2) configuration at iron(II). The solid-state Mossbauer
this crude reaction mixture (Figure S49) indicates a
complicated mixture of products, suggesting decomposition
of the complex (perhaps via ligand protonation). Reduction
spectrum of 10 (Figure S26) features a quadrupole doublet
with δ = 0.67 mm/s and |ΔEQ| = 1.82 mm/s, which is also
consistent with high-spin iron(II) in a tetrahedral geometry.
with a mixture of stoichiometric KC8 and [H(OEt2)2][BArF ]
4
̈
The Mossbauer spectra of 10 in the solid state and in a frozen
also led to the formation of multiple species (Figure S50). Our
attempts to use 10 as a precatalyst for N2 reduction in the
presence of excess (55 equiv) KC8 and a variety of acids
THF solution are nearly identical (Figure S27), implying that
the coordination mode observed in the solid-state structure is
retained in coordinating solvents.
([H(OEt2)2][BArF ], diphenylammonium triflate, H2O, and
4
To create a vacant coordination site through loss of
BHT) resulted in the release of only 1 equiv of NH3,
presumably formed via hydrolysis of the bis(trimethylsilyl)-
amide ligand in 10 (Scheme S2).
silylamine, 10 was treated with 1 equiv of [H(OEt2)2][BArF ]
4
in THF at −78 °C, which resulted in immediate color change
̈
from yellow to dark red. The Mossbauer spectrum of a frozen
Generation of SCS-Supported Iron Complexes of
Nitrogenase-Relevant Ligands. By protonating the amide
in 10 with weaker acids, we were able to isolate complexes of
their conjugate bases. Treatment of 10 with a bulky m-
terphenyl monothiol (Scheme 5) gave the tris(thiolate)iron-
(II) complex 11, which has an S3C coordination environment.
The crystallographic structure of this complex (Figure 4)
features a distorted (τ4 = 0.66) tetrahedral geometry in a
coordination environment resembling that of the individual
iron sites in the resting state of nitrogenase. Compared to a
similar complex with an m-terphenyl-derived bis(thiolate)
ligand (Chart 1),22 11 exhibits similar Fe−S bond lengths of
∼2.3 Å. However, in the m-terphenyl analogue, the shortest
Fe−C(arene) distance was 2.484(7) Å, indicating little or no
Fe−C interaction in the iron(II) state. In contrast, the Fe−C
bond length in 11 is 2.035(2) Å, which is similar to the 2.0 Å
aliquot of this solution indicates the presence of at least three
iron-containing species (Figure S47). The same behavior was
observed in an analogous experiment with the weaker acid
̈
[HNEt3][BF4] (Figure S48). Furthermore, the Mossbauer
spectra of reaction mixtures generated under N2 and Ar are
identical (Figure S47), indicating that the complexity of the
reaction mixture is not caused by partial N2 binding. Upon
warming these reaction mixtures to room temperature, a slow
̈
color change to black occurred, and Mossbauer spectra
indicate the presence of at least two iron species different
from those present in aliquots that were frozen before
warming. As discussed earlier, the same behavior was observed
during our attempts to metalate the free carbene SCS2− to
iron(II) salts and may arise from oligomerization of transiently
formed three-coordinate iron complexes.
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the monothiolate ligand are broadened beyond detection with
the exception of a sharp singlet at δ −24 ppm that integrates to
1H. This singlet is assigned to the proton para to the sulfur of
the monothiolate, which is the only proton that is not
exchanged by rotation of the monothiolate about the S−C
bond. Upon cooling to −40 °C, a new set of resonances grows
in (Figure S31). Integration of these resonances indicates that
at low temperature the two mesityl groups are inequivalent,
which is consistent with slow S−C bond rotation. Further-
Scheme 5
̈
more, the Mossbauer spectrum of a frozen THF solution of 11
exhibits significant broadening in comparison to the solid-state
spectrum (Figure S34). Taken together, these studies suggest
that multiple conformations of the monothiolate ligand are
accessible in solution and that they interconvert on the
millisecond−second time scale in solution at room temper-
ature.
Addition of 1 equiv of phenylhydrazine to 10 in benzene
resulted in a slow color change from yellow to red-orange over
the course of 2 h at room temperature, and 1H NMR
spectroscopy indicated clean formation of a new iron(II)
species (12). The XRD structure of 12 (Figure 5) reveals a
Figure 5. Thermal ellipsoid plot of the XRD structure of 12 with
thermal ellipsoids shown at the 50% probability level. Hydrogen
atoms and K(18-crown-6)(THF)2 are omitted. Selected bond
distances (Å): Fe−S1: 2.3542(7); Fe−S2: 2.3501(8); Fe−C11:
2.057(2); Fe−N1: 1.963(2); N1−N2: 1.470(5).
four-coordinate complex with a distorted tetrahedral geometry
(τ4 = 0.74). Importantly, the observation that the tetrahedral
geometry of 10 and 11 is retained in 12 indicates that the low
coordination number of these complexes is not due solely to
steric congestion at the metal center. Furthermore, the
phenylhydrazido ligand is bound κ1 rather than the typical η2
or bridging binding modes for deprotonated phenylhydrazine.
To our knowledge, 12 is the first structurally characterized
complex featuring a phenylhydrazido ligand with this binding
mode.
Only two other iron(II) phenylhydrazido complexes have
been reported in the literature. In cis-[Fe(dmpe)2(η2-
NH2NPh)]+, the phenylhydrazido moiety is bound side-on
and has an N−N bond length of 1.400(5) Å.66 In the second
example, the phenylhydrazido ligand binds μ-κN1:κN2 between
two sulfide-bridged β-diketiminate-ligated iron centers and has
an N−N bond length of 1.417(8) Å.67 In 12, the N−N bond is
elongated to 1.470(5) Å, which is the same as the bond length
for free phenylhydrazine68 (1.471 Å), suggesting that the N−N
Figure 4. Thermal ellipsoid plot of the XRD structure of 11 with
thermal ellipsoids shown at the 50% probability level. Hydrogen
atoms and K(18-crown-6) are omitted. Selected bond distances (Å):
Fe1−S12: 2.3258(9); Fe−S13: 2.3396(9); Fe1−C11: 2.035(2); Fe1−
S14: 2.2940(8).
Fe−carbide bond lengths in the resting state of the nitrogenase
FeMoco.9
In the crystal structure of 11, the monothiolate ligand is
bound asymmetrically with one mesityl ring lying below the
iron center. However, spectroscopic studies suggest that the
solid-state geometry is not fully preserved in solution. First, in
the 1H NMR spectrum of 11 in THF-d8 at 25 °C, the peaks for
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Figure 7. Thermal ellipsoid plot of the XRD structure of 13 with
thermal ellipsoids shown at the 50% probability level. Hydrogen
atoms except H1 and K(18c6) are omitted. Selected bond distances
(Å): Fe−S1: 2.342(1); Fe−S2: 2.3649(8); Fe−C11: 2.069(3); Fe−
N1: 1.974(3).
1
Figure 6. H NMR spectra (400 MHz, THF-d8, 25 °C) of 12 before
(top) and after (middle) heating at 70 °C for 2.5 h, leading to partial
formation of 13 (35% based on comparison to a nickelocene
standard). The spectrum of independently synthesized 13 is shown in
the bottom panel.
CONCLUSIONS
bond may be weakened relative to those of the other iron
complexes.
■
Here, we have reported new pincer ligands with SCS donor
sets that mimic nitrogenase intermediates. The isolation of
these ligands is complicated by the acid-catalyzed rearrange-
ment to a benzothiazole, which can be overcome with
appropriate modifications. Importantly, the synthetic route
allows incorporation of both thioethers and thiolates as S
donors in a bulky SCS scaffold, which may be useful for a
broad range of applications in addition to nitrogenase
modeling.
Although it is stable in the solid state, 12 slowly decomposes
in THF or benzene solutions over the course of several days at
room temperature or several hours at 70 °C. Although we were
unable to fully purify 12 due to this instability as well as its
high moisture sensitivity, we were able to identify the major
product of its degradation. After heating for 2.5 h at 70 °C, 1H
NMR spectroscopy reveals the presence of a new species
formed in 35% spectroscopic yield (Figure 6). The similarity of
The bis(thiolate) NHC ligand SCS2− is particularly notable
as a ligand that gives four-coordinate high-spin anionic iron(II)
complexes with X ligands in the fourth position. SCS2− can
fold its core resulting in facial binding, which contrasts with the
mer binding of the bis(thioether) NHC analogue SMeCSMe.
These examples illustrate the conformational flexibility of this
scaffold, which contrasts with the more rigid scaffold in a set of
iron complexes with an NHC-containing PCP pincer ligand
recently reported by de Ruiter.73 Though these iron complexes
were not precatalysts for N2 reduction due to their
decomposition in the presence of acid, we were able to
generate complexes of thiolate, hydrazido, and amido groups
that take advantage of the biomimetic scaffold. Most notable is
the unique κ1-phenylhydrazido complex, which models a
potential N2 reduction intermediate and undergoes N−N
cleavage.
1
the H NMR spectrum of this product to those of 11−12,
particularly the sharp singlet at δ −55 ppm that integrates to
1H, suggests that the new product is an iron(II) complex with
an anionic ligand containing a phenyl group. We therefore
reasoned that the new species was likely a phenylamido
complex.
To confirm this hypothesis, the phenylamido complex 13
was independently synthesized by heating 10 with excess
aniline at 65 °C in THF. Crystalline 13 was isolated by layering
a concentrated THF solution with diethyl ether at −40 °C, and
XRD analysis confirms that it is structurally analogous to 10−
12, with a distorted tetrahedral (τ4 = 0.73) geometry (Figure
7). The 1H NMR spectrum of this material is identical to that
of the major NMR-active species formed from thermolysis of
12 (Figure 6). Analysis of the thermolysis reaction mixture by
̈
Mossbauer spectroscopy confirms the presence of 13 and
indicates the presence of multiple additional species which we
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03427.
■
have been unable to identify (Figure S51).
sı
It is notable that this system can break N−N bonds. A
number of previously studied iron systems are also capable of
cleaving the N−N bonds in hydrazines. Some of the literature
systems even contain sulfur-containing ligands, such as β-
diketiminate-supported sulfide-bridged diiron systems67,69 as
well as thiolate-supported,70,71 phosphine-supported,72 and
pentamethylcyclopentadienyl-supported16 diiron complexes. In
our current NHC/thiolate-supported system, the yield of the
N−N cleaved product was too low for mechanistic studies that
would enable this synthetic system to give insight into the
enzymatic N−N cleavage process.
Synthetic procedures and spectroscopic data (PDF)
Accession Codes
CCDC 2035911−2035916 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.
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AUTHOR INFORMATION
■
Corresponding Author
Patrick L. Holland − Department of Chemistry, Yale
University, New Haven, Connecticut 06520, United States;
orcid.org/0000-0002-2883-2031;
Email: patrick.holland@yale.edu
Authors
Amy L. Speelman − Department of Chemistry, Yale
University, New Haven, Connecticut 06520, United States;
orcid.org/0000-0002-5023-4847
Kazimer L. Skubi − Department of Chemistry, Yale University,
New Haven, Connecticut 06520, United States;
orcid.org/0000-0002-9689-1842
Brandon Q. Mercado − Department of Chemistry, Yale
University, New Haven, Connecticut 06520, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03427
(18) Li, Y.; Li, Y.; Wang, B.; Luo, Y.; Yang, D.; Tong, P.; Zhao, J.;
Luo, L.; Zhou, Y.; Chen, S.; Cheng, F.; Qu, J. Ammonia formation by
a thiolate-bridged diiron amide complex as a nitrogenase mimic. Nat.
Chem. 2013, 5, 320−326.
(19) Coric, I.; Holland, P. L. Insight into the Iron-Molybdenum
Cofactor of Nitrogenase from Synthetic Iron Complexes with Sulfur,
Carbon, and Hydride Ligands. J. Am. Chem. Soc. 2016, 138, 7200−
7211.
Notes
The authors declare no competing financial interest.
̌
́
ACKNOWLEDGMENTS
■
We gratefully acknowledge funding from the National
Institutes of Health (R01-GM065313 to P.L.H.; F32-
GM123658 to A.L.S.; and F32-GM126656 to K.L.S.). We
thank Anna Brosnahan for performing preliminary inves-
tigations of sulfur-functionalized NHC ligands.
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Complexes for Understanding N2 Reduction. In Nitrogen Fixation;
Nishibayashi, Y., Ed.; Springer International Publishing: Cham, 2017;
pp 197−213.
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