Single-Step Sulfur Insertions into Iron Carbide Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco Analogues

Article abstract of DOI:10.1002/anie.202011517

The one-step syntheses, X-ray structures, and spectroscopic characterization of synthetic iron clusters, bearing either inorganic sulfides or thiolate with interstitial carbide motifs, are reported. Treatment of iron carbide carbonyl clusters [Fe_n

Full text of DOI:10.1002/anie.202011517

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Accepted Article
Title: Single-Step Insertion of Sulfides and Thiolate into Iron Carbide-
Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco
Analogues
Authors: Chris Joseph, Caitlyn R. Cobb, and Michael Rose
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Angewandte Chemie International Edition
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COMMUNICATION
Single-Step Insertion of Sulfides and Thiolate into Iron Carbide-
Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco
Analogues
Chris Joseph, Caitlyn R. Cobb and Michael J. Rose*
[
*]
Prof. Dr. Michael J. Rose
Department of Chemistry
The University of Texas at Austin
Austin, TX 78712
E-mail: mrose@cm.utexas.edu
Supporting information for this article is given via a link at the end of the document.
1
6,17
Abstract: We report the one-step syntheses, X-ray structures, and
interstitial sulfide as found in the nitrogenase P-cluster
and a
spectroscopic characterization of synthetic iron clusters bearing
structure containing an interstitial oxide.18 In a similar vein, the
Holm group reported an iron-sulfur cluster that incorporates a
silyl-nitride into the cluster core. Two reports from our group
demonstrated a series of interconversions between Fe
Fe S clusters in preliminary work for modeling carbon-atom
inorganic sulfides or thiolate with interstitial carbide motifs. Treatment
x
19
of historical iron-carbide-carbonyl clusters [Fe
m = 15,16; x = 0,–2) with electrophilic sulfur sources (S
in the formation of several μ -S ‘dimers of clusters’ (1 and 2), and
moreover the iron-sulfide-(sulfocarbide) clusters 3 and 4. The core
n
(μ
n
-C)(CO)
m
] (n = 5,6;
2
Cl
2
, S
8
) results
2
S
2
and
4
3
insertion as seen in FeMoco biogenesis,2
0,21
although ultimately
4–
‘
carbide’ motif in 3 and 4 is the sulfocarbide unit {C–S} , which serves
carbide insertion proved unsuccessful.
as a structural model for a proposed intermediate in the radical-SAM
biogenesis of the M-cluster. Furthermore, the ‘electrophilic sulfur’
strategy has been extended to provide the first ever thiolato-iron-
carbide complex: An analogous reaction with toluylsulfenyl-chloride
—
(tolS–Cl) affords the cluster [Fe
5
(μ
5
-C)(SC
7
H )(CO)13
7
]
(5). The
‘
electrophilic sulfur donor’ strategy described herein provides a
breakthrough towards developing logical syntheses of biomimetic
iron-sulfur-carbide clusters like FeMoco.
The iron-sulfide-carbide cluster found in the active site of
nitrogenase — particularly the Mo-dependent variant containing
FeMoco — has served as inspiration for many avenues of
research involving the structural, spectroscopic, biosynthetic and
catalytic function of this unique cofactor. However, the chemical
synthesis of the FeMoco cluster still stands as one remaining
‘
Holy Grail’ in synthetic bioinorganic chemistry. The concomitant
Figure 1. Structures of iron-molybdenum nitrogenase cofactor (FeMoco) and
proposed NifB intermediate in FeMoco biogenesis (left); cluster 4 from this work
(right).
assembly of ferrous/ferric sites, sulfides, and an interstitial and
purely inorganic carbide has to date remained elusive.
To this end, an array of synthetic models intended to emulate
the ligation sphere of the cluster iron and molybdenum sites have
been developed. Peters has reported an elegant series of iron
complexes bearing ligands featuring chelating phosphines either
anchored by an organic carbanion, or bridged by an organic
thiolate.2 Additional complexes featuring various anchor atoms
have provided valuable insight to the flexible mode of Fe–C
Despite this remarkable progress in understanding the synthetic,
structural and functional aspects of iron, sulfur and carbon motifs,
the synthetic challenge of incorporating both the biomimetic
interstitial carbide and inorganic sulfide has proven difficult. A
recent report by Rauchfuss provided the first example of a
synthetic cluster incorporating both these motifs in a multi-iron
construct22 via an iron-carbide-carbonyl cluster precursor.23 While
past literature regarding these clusters has generally reported
1
2
bonding and have even demonstrated catalytic dinitrogen (N )
1–4
reduction. Relatedly, Holland has reported structures derived
from sulfur-containing thiolate5,6 and sulfide ligands7,8 and has
employed multimetallic metal sites that exhibit cooperative N
2
transformations upon the cluster core (e.g. oxidative removal of
binding9 and high-nuclearity iron-sulfur clusters.10 Systematic
work by Agapie has demonstrated electronic and catalytic
modulation of iron cluster sites with interstitial light atoms.11,12 An
incredibly thorough and long-standing research program by
24–26
iron sites, substitution with heterometals);
this family of iron
clusters has proven difficult to control during ligand substitution.
In fact, the sulfide exhibited in the Rauchfuss structure arises from
a three-step removal of oxides from the SO -ligated structure first
2
reported by Shriver,27 and re-utilization of this strategy to
Tatsumi has yielded highly complex iron-sulfur structures of
multiple nuclearities,1
3–15
which include structures bearing an
coordinate a second sulfide proved unstable.28 In this work, we for
1
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COMMUNICATION
2–
OC CO
OC CO
2–
OC
OC
Fe
C
OCOC Fe
OC
OC
CO
OCS
OC CO
O
CO
CO
CO
CO
CO
Fe C
OC
CO
CO OC OC
Fe
Fe
Fe
Fe OC
Fe
Fe
Fe
Fe
Fe
OC
OC
OC
S Cl
2
2
OC
–15 °C OC
CO
Fe
S
OC
OC
+
OC
+
C
CO
CO
C
S
C
Fe
CO OC
OC
S
Fe
S
S
OC
CO
DCE,
Fe
Fe
Fe
Fe
CO
CO
CO
CO
Fe
CO
OC
Fe
1 OC
Fe
CO
OC
CO
S
CO
CO Fe CO
Fe
CO
OC Fe
CO CO
3
OC CO
CO
OC
OC
CO
CO
CO
OC
CO
Fe
50 °C
THF
2–
OC
OC
CO
CO
OC CO
OC
Fe
Fe
OC
OC
OC
OC
Fe
CO
CO
CO
S
OC COCO
CO
CO
OC
OC
CO
C
OC
OC
OC
Fe
OC CO
CO
Fe
Fe
C
Fe
OC Fe
Fe
Fe
OC
Fe
Fe
OC
CO
OC
OC
CO
OC CO
OC
Fe
C
+
OC
OC
OC
Fe
+
Fe
S
1
C
S
/8
DCE/Toluene
15 °C
S8
Fe
OC
OC
Fe
Fe
CO OC
S
S
CO
CO
Fe
CO
CO
CO
OC
Fe
Fe
–
CO
CO
CO
S
OC CO
Fe CO
OC CO
CO
2
4
Scheme 1. Synthetic scheme depicting the electrophilic sulfurization of the Fe
6
C dianion cluster by either S
2 2 5
Cl or elemental sulfur in the presence of a neutral Fe
4
–
cluster affords μ
4
-S cluster (1, 2), a {CS} cluster (3, 4) and the non-carbide cluster [Fe (CO) ].
3
S
2
9
the first time achieve direct CO→sulfide substitution upon the
hexanuclear (NEt [Fe (μ -C)(CO)16 first reported by
We find that treatment of this cluster (and its
relatives) with electropositive or neutral sulfur reagents leads to
cluster oxidation, CO loss, and binding of inorganic sulfide motifs
4 2 5 5 2 4
(NEt ) {[Fe (μ -C)(CO)13] (μ -S)} (2).
)
4 2
6
6
]
Moreover, our efforts to fully characterize the product profiles of
the above reactions resulted in the notable isolation of three multi-
sulfide-containing clusters. First, both reactions generate the well-
Churchill.2
9,30
3 2 9
S (CO)
.31,32 More remarkably,
known, carbide-free cluster Fe
crystallization of the Et O-soluble portion from the synthesis of 1
(Scheme 1).
2
Reaction of (NEt
4
)
2
[Fe
6
(μ
6
-C)(CO)16] with S
2
Cl
2
resulted in a
afforded red prisms of a CO-supported iron-sulfur cluster
featuring a ‘carbide-like’ site, multiple sulfur atoms, and a ‘dangler’
mixture of products. Crystallization of the charged species and
subsequent X-ray diffraction revealed an asymmetric ‘dimer of
clusters’ structure bridged by a 4-coordinate sulfide (Figure 2,
iron (Figure 3, top) of formula [{Fe
4 2 4 3 3
(κ S–κ C)(CO)10}(μ -S)(μ -
S
2
)Fe(CO) ] (3). The average Fe–C distance of 1.97 ± 0.04 Å is
3
top) of formula (NEt
1). Alternatively, the introduction of elemental sulfur into a
solution of (Et N) [Fe (μ -C)(CO)16] plus the five-iron species
4
)
2
{[(CO)15(μ
6
-C)Fe
6
](μ
4
-S)[Fe
5
(μ
5
-C)(CO)13]}
significantly longer than average for iron-carbonyl-carbide
clusters (~1.90 Å) and is quite close to the average Fe–C distance
found in FeMoco (2.00 ± 0.02 Å).33 While the presence of a C–S
single bond [1.714(5) Å] precludes the central C atom from being
an authentic C4– carbide, this bonding motif is reminiscent of the
proposed biogenesis mode of carbide insertion into the M-cluster
as postulated by Wiig, Hu, Ribbe and Britt (Figure S15),34–36 in
which a S-bound methyl group undergoes H• atom abstraction by
radical S-adenosyl-L-methionine (SAM) before proceeding
through dehydrogenation/deprotonation. Such intermediates
would likely be difficult to isolate for crystallographic
characterization. However, comparative spectroscopic studies of
synthetic cluster compounds have previously demonstrated utility
in determining structural aspects of nitrogenase, such as in the
(
4
2
6
6
5
[Fe (μ
5
-C)(CO)15] (Scheme 1, bottom) led to a symmetric dimer
of Fe
5
clusters bridged by the same 4-coordinate sulfide motif
(Figure 2, bottom), resulting in the simplified formula
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
C
S
C
Fe
Fe
Fe
Fe2
Fe4
Fe1
C0 Fe3
Fe8
Fe9
C29
Fe6
S1
C]2–
assignment of the central carbide (which itself utilized the [Fe
6
Fe5
cluster as a reference compound).3
7,38
The presence of the {C–
Fe7
Fe10
S}4– motif (i.e. tetra-deprotonated methylthiol) in 3 thus serves as
the first rudimentary structural model for intermediates in M-
1
Fe
cluster biosynthesis (see Supplementary Information, S23) or
Fe
Fe
Fe
Fe
Fe
Fe
39
Fe
C
2
possibly alternative N ase-related C–S bond breaking enzymes.
C
S
Fe
Lastly, the Fe–S distances found in 3 are notable as their average
distance of 2.27 ± 0.03 Å is considerably elongated compared to
1 and 2, placing it closer to that of FeMoco (2.25 ± 0.03 Å).
The analogous elucidation of the product profile in the
synthesis of 2 identified yet another cluster: the phylochemically
Fe
Fe1
Fe10
Fe9
Fe6
Fe2
Fe3
Fe5
S1
C0
Fe4
C27
Fe7
Fe8
related,
S) Fe(CO)
C–S}4– motif but dispenses with the persulfide motif in favor of a
bis-sulfide’ motif that again attaches a ‘dangler’ Fe5 site in a
FeS Fe(CO) } coordination motif. The average bond distances in
multi-sulfide
cluster
4 2 4 3
[{Fe (κ S–κ C)(CO)10}(μ -
2
3
] (4) (Figure 3, bottom). This cluster retains the core
2
{
‘
Figure 2. Thermal ellipsoid plots (50% probability) of the anions of the μ
clusters 1 and 2.
4
-S
{
2
3
2
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Angewandte Chemie International Edition
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COMMUNICATION
4
are similar to those in 3 with the marked difference of average
the high Mulliken charge on these Fe sites cannot be entirely
attributed to oxidation state. Similar analysis of 4 (Figure S24)
reveals an Fe5 site that is of median oxidation state relative to the
cluster.
Fe–S distances of 2.25 ± 0.03 Å, which is exactly on par with
FeMoco (Table S4).
We note that Mössbauer spectroscopy is notoriously difficult to
interpret in iron carbonyl clusters due to the competing effects of
formal oxidation state versus π-backbonding ligands, such that
lower oxidation states in iron-carbonyls typically exhibit higher
isomer shifts than higher oxidation states.40 This counter-intuitive
trend is somewhat realized in the analysis of Mulliken charges on
the Fe sites in cluster 3. The six-coordinate ‘dangler’ site Fe5 in
S
Fe
C
Fe
Fe
S1
S
Fe
S
Fe1
Fe4
S4
S
Fe
C0
S3
Fe2
Fe3 Fe5
S2
{
S
3
Fe(CO)
compared to the adjacent seven-coordinate Fe3 and Fe4 sites
+0.15) in {S (C)(Fe) Fe(CO) } ligation. Meanwhile, the distal
seven-coordinate Fe1 and Fe2 sites in {S(C)(Fe) Fe(CO)
3
} ligation exhibits a higher Mulliken charge (+0.24)
3
(
2
2
2
2
3
}
S
Fe
C
ligation both exhibit the highest extent of π-backbonding based
on carbonyl metrics and DFT as well as the highest Mulliken
charges (+0.25).
Thus, we deemed an electron spectroscopy method to be the
preferred path of investigation. To spectroscopically probe for a
ferrous site in 3, high-resolution XPS data in the Fe 2p region was
collected (Figure 4, left). Component peak fitting reveals three
S1
Fe
Fe
Fe4
Fe
S
S3
Fe1
Fe2
S
C0
S2
Fe
Fe3
Fe5
4
distinct peaks at binding energies (BE) of 711.6, 709.4, and 707.5
3
eV in the 2p( /
2
) region and 724.6, 722.2, and 720.5 eV in the
2
p(1/
2
) region with approximately 1:2:2 peak area ratios. The lower
Figure 3. Thermal ellipsoid plots (50% probability) of the {CS}4– clusters 3 and
3
2
BE 2p( / ) peaks at 709.4, and 707.5 eV are attributed to the four
4.
Fe sites that encircle the carbide. Furthermore, DFT reveals that
the HOMO in 3 (Figure S22) is strongly localized in the Fe1–Fe2
bond, which further indicates that the lowest BE components
The presence of a putatively ferrous ‘dangler’ Fe site in 3
especially, with no Fe–Fe bond), and 4 with a more σ-donating
coordination sphere ({FeS (CO) } in 3; {FeS (CO) } in 4) suggests
that a progressive approach towards biologically relevant Fe /Fe
sites is possible. DFT Mulliken charges analysis (B3PW91/6-31G)
on the carbonyl C sites was evaluated as a proxy for π-
backbonding. In cluster 3, The C(O) sites on Fe5 exhibit the
highest Mulliken charges (Figure 4, right), consistent with a
decreased extent of π-backbonding due to the nominally higher
oxidation state of Fe5. The Fe1 and Fe2 carbonyl C sites
exhibited the greatest extent of CO π-backbonding, indicating that
(
720.5 and 707.5 eV) are attributable to Fe1 and Fe2. These
(
2–
values are slightly higher binding energies relative to 1, [Fe
and [Fe5] (Figure S16), consistent with overall increased
6
] ,
3
3
2
3
0
II
III
‘
average’ Fe oxidation state in 3. Finally, the highest binding
energy feature at 711.6 eV is attributed to the dangler Fe5 site by
both integration and BE, which falls within expected range for a
ferrous and CO-supported iron site.4
1,42
Consistent with all the
above data and interpretation, the overall increase in average and
localized oxidation states in 3 is spectroscopically obvious in the
higher CO stretching frequencies observed in the IR spectrum
Mulliken Charge Analysis
Observed
Fitting Sum
Fe1,Fe2
Fe3,Fe4
Fe5
Fe Sites
0
1
.25
0.15
4
5 0.24
2
3
0.25
0.15
C(O) Sites
0.239
0.192
0.222
0.236
0.246
0.251
0.251
0.222
0
.238
.194
7
35
730
725
720
715
710
705
700
0.246
0
0
.236
Binding Energy (eV)
0.222
Figure 4. Observed high-resolution X-ray photoelectron spectrum (XPS) and component fitting of the iron 2p region of cluster 3 (left) and calculated Mulliken charges
right) on Fe sites (top) and carbonyl C sites (bottom) of clusters 3.
(
3
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COMMUNICATION
(
Figure S9) where all νCO values range upwards from 2002 cm–1
(tolS–Cl). The incorporation of multiple sulfur sites and higher
valent iron in 3 — coupled with the now-possible synthetic
versatility of thiolate incorporation — now opens the full toolbox of
bio-inorganic chelation chemistry and ligand design to be applied
to the preparation of elusive synthetic models of FeMoco. We
hereby assert that this method will finally ‘unlock the synthetic
door’ to biomimetic models of the nitrogenase active site cluster.
(additionally consistent with the presence of a ferrous site).
Lastly — and critically — we deemed it essential to demonstrate
the broad applicability of this ‘electrophilic sulfur’ approach
towards sulfur-functionalized iron-carbide clusters relevant to
FeMoco. Therefore, we sought to demonstrate thiolate
incorporation by the analogous approach. As such, p-toluenethiol
was chlorinated with NCS in situ and added into a DCE solution
of the aforementioned [Fe
fluorobenzene were obtained of the thiolate-appended [Fe
cluster (NEt )[Fe (μ -C)(SC
first ever iron-carbide-thiolate structure. The structure of 5 reveals
the elimination of an {Fe(CO) Cl} from the [Fe
]2– cluster,
presumably by ‘nucleophilic’ attack by the dianionic cluster on the
electrophilic tolS–Cl reagent. The extension of the inorganic
43
]2– (Figure 5). Crystals from
6
0
5
]
Acknowledgements
4
5
5
7 7
H )(CO)13] (5), which represents the
This research was funded by the National Science Foundation
3
6
(NSF CHE-1808311) and the Robert A. Welch Foundation (F-
1822). The authors gratefully acknowledge Dr. Vince Lynch for
his technical assistance with the collection and solution of X-ray
crystallographic data and Dr. Hugo Celio for his technical
assistance with X-ray photoelectron spectroscopy.
electrophilic sulfur strategy (S
2 2 8
Cl , S ) to organic sulfenyl-halides
(RS–X) establishes a logical and viable synthetic pathway
towards the assembly of FeMoco-related clusters derived from
historical organometallic iron-carbide-carbonyl clusters.
Keywords: biogenesis • biomimetic • carbide • FeS cluster •
nitrogenase
OC CO
tol–SH
CO
OC
CO
NCS –20 °C
[
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OC
Fe
DCM
1 h
Fe
O
CO
CO
OC
OC
Cl OC
CO
C
H
C
3
Fe
Fe
Fe
Fe CO
[
OC
OC
C
CO
CO
C
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Fe
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Fe
Fe
[
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OC
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OC Fe
CO
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OC
CH3
CO
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of the thiolato-iron-carbide cluster (NEt )[Fe (μ -C)(SC )(CO)13] (5).
4
5
5
7 7
H
[
[
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4–
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synthetic example of an iron cluster containing multiple sulfur sites
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are highly comparable to those in FeMoco (Table S4). The
presence of the inorganic C–S unit is markedly reminiscent of a
proposed biosynthetic pathway towards carbide insertion into
FeMoco, which may provide valuable characterization information
in proposing intermediates that occur throughout biogenesis.
Critically, we have extended the ‘electrophilic sulfur’ approach for
the incorporation of thiolates, where the first ever thiolato-iron-
carbide species 5 was derived from an organic sulfenylchloride
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COMMUNICATION
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Angewandte Chemie International Edition
10.1002/anie.202011517
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Approaching High-Fidelity FeMoco Models: At first glance, CO-supported iron-carbide clusters are an appealing synthetic starting
point for modeling the carbide-containing FeMoco cluster. However, these compounds have historically been uncontrollable towards
traditional CO→S substitution methods. We demonstrate an approach utilizing unconventional, electrophilic sulfur reagents to
achieve carbide-containing multi-sulfide multi-iron clusters.
Institute and/or researcher Twitter usernames: @RoseLabUT
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This article is protected by copyright. All rights reserved.