Partial synthetic models of FeMoco with sul
fi
de
and carbyne ligands: Effect of interstitial atom in
nitrogenase active site
Linh N. V. Le
a
, Gwendolyn A. Bailey
a
, Anna G. Scott
a
, and Theodor Agapie
a,1
a
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125
Edited by Marcetta Y. Darensbourg, Texas A&M University, College Station, TX, and approved October 26, 2021 (received for review May 20, 2021)
Nitrogen-
fi
xing organisms perform dinitrogen reduction to ammo-
nia at an Fe-M (M
=
Mo, Fe, or V) cofactor (FeMco) of nitrogenase.
FeMco displays eight metal centers bridged by sul
fi
des and a car-
bide having the MFe
7
S
8
C cluster composition. The role of the car-
bide ligand, a unique motif in protein active sites, remains poorly
understood. Toward addressing how the carbon bridge affects the
physical and chemical properties of the cluster, we isolated syn-
thetic models of subsite MFe
3
S
3
C displaying sul
fi
des and a chelating
carbyne ligand. We developed synthetic protocols for structurally
related clusters, [Tp*M
’
Fe
3
S
3
X]
n
2
,whereM
’
=
Mo or W, the bridg-
ing ligand X
=
CR, N, NR, S, and Tp*
=
Tris(3,5-dimethyl-1-pyrazolyl)-
hydroborate, to study the effects of the identity of the heterometal
and the bridging X group on structure and electrochemistry. While
thenatureofM
’
results in minor changes, the chelating,
μ
3
-bridging
carbyne has a large impact on reduction potentials, being up to 1 V
more reducing compared to nonchelating N and S analogs.
FeMoco model
j
nitrogenase
j
carbide clusters
j
reduction potential
j
iron-sulfur clusters
B
iological dinitrogen conversion to ammonia is performed by
nitrogenases, a class of enzymes displaying several complex
iron-sulfur clusters (1). The site of N
2
reduction in the most effi-
cient nitrogenase is a heterometallic cluster displaying Fe and
Mo, the iron-molybdenum cofactor (FeMoco) (1). Two other
nitrogenases are known where Fe or V are found at the Mo posi-
tion. FeMoco consists of Fe
4
S
3
CandMoFe
3
S
3
C cubanes with
μ
3
-sulfides joined together by a shared interstitial
μ
6
-carbide and
three additional sulfides that bind in
μ
2
-fashion (Fig. 1) (2). The
impact of the carbide ligand on the electronic structure and reac-
tivity of the cofactor, and therefore its role in the catalytic cycle
of N
2
-to-NH
3
conversion, is unclear (3). The carbide ligand is not
lost during catalysis, and it has been suggested that it becomes
protonated before N
2
activation (3). To address the effect of
carbon-based ligands for N
2
activation, such as providing elec-
tronic stabilization and structural flexibility to accommodate mul-
tielectron redox processes, synthetic models have included arene
(4),
N
-heterocyclic carbene (5), aryl (6), and alkyl (7, 8) donors
in mononuclear iron complexes.
Bi- and multimetallic synthetic analogs focused on interro-
gating the role of the interstitial atom and multimetallic effects
have been targeted (7, 9–23), but complexes that display bridg-
ing carbide (11, 24–27) or even carbyne (9, 10, 28) ligands are
rare. Carbide-containing Fe clusters display four to six metal
centers but invariably are rich in CO ligands (24, 25, 27). The
presence of this strong field donor limits the comparison to
FeMoco given the significantly different electronic structure
conferred by the weak field sulfides. Moreover, the formal oxi-
dation state of the Fe centers is significantly more reduced,
between Fe
0
and Fe
II
, than in the protein, between Fe
II
and
Fe
III
(2). Recent promising advances have been made toward
the incorporation of sulfide ligands into carbide-containing iron
carbonyl clusters (10, 11). In order to gain a more accurate
understanding of the impact of the carbide on the properties
of clusters related to FeMoco, metal complexes structurally
related to the biological active site that are multimetallic, have
multiple sulfide ligands and few CO ligands, and display
bridging carbon-based ligands and oxidation states of Fe
II
-Fe
III
are
desirable.
Toward developing synthetic methodologies to structures
analogous to FeMoco that include a bridging carbon donor,
we focus our initial efforts on the cubane subsite, MoFe
3
S
3
C
(Fig. 1,
To p R o w
). Because the nature of the
μ
2
-bridging
ligands in FeMoco is variable, with sulfide, selenide (29), CO
(30),orNH(31)(forFeVco)moietiesatthesepositionsas
characterized by crystallography, the primary target was to
matchthecompositionofthecubanecore.Inthiswork,we
present the preparation of a series of heterometallic iron-
sulfur cubane-type clusters containing Mo or W with biologi-
cally relevant
μ
3
bridging ligands X (X
=
N, NR, CR, and S)
incorporated at the Fe
3
face—including examples bearing a
bridging CR ligand. These variations in the bridging ligand
result in a large shift in the biologically relevant M
0
Fe
3
11
+
/
M
0
Fe
3
10
+
redox couple of up to 2 V, with the most reducing
system occurring for the cluster bearing a bridging carbyne.
These results suggest an important role of the interstitial car-
bide ligand in FeMoco in modulating the electronic proper-
ties of the cluster toward rendering it more reducing and
potentially more reactive in N
2
activation and conversion
into NH
3
.
Signi
fi
cance
Nitrogen, a common component of biomolecules, is sourced
from abundant dinitrogen in the atmosphere through conver-
sion to ammonia. Organisms capable of
fi
xing N
2
employ
nitrogenases, metalloproteins that display metal-sul
fi
de clus-
ters that facilitate electron transfers and substrate activation.
The site of N
2
conversion to NH
3
is FeMco (M
=
Mo, Fe, or V),
a cluster of notable complexity in bioinorganic chemistry,
which displays an unusual interstitial carbon ligand. The func-
tion of this bridging ligand remains unclear, and systematic
structure-function studies with bridging C-donors are chal-
lenged by a lack of synthetic methods for analogous clusters.
Herein, we report the
fi
rst synthetic cluster that models a
cubane moiety of FeMco bearing a chelating carbyne ligand
and related structure-property studies.
Author contributions: L.N.V.L., G.A.B., A.G.S., and T.A. designed research; L.N.V.L.,
G.A.B., and A.G.S. performed research; L.N.V.L., G.A.B., A.G.S., and T.A. analyzed data;
L.N.V.L., G.A.B., A.G.S., and T.A. wrote the paper; and T.A. mentored researchers.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Published under the
PNAS license
.
1
To whom correspondence may be addressed. Email: agapie@caltech.edu.
This article contains supporting information online at
http://www.pnas.org/lookup/
suppl/doi:10.1073/pnas.2109241118/-/DCSupplemental
.
Published December 2, 2021.
PNAS
2021 Vol. 118 No. 49 e2109241118
https://doi.org/10.1073/pnas.2109241118
j
1of7
CHEMISTRY
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Results and Discussion
To rationally incorporate different ligands at the
μ
3
-bridging
position corresponding to the carbide, a WFe
3
S
3
cluster sup-
ported by a W-coordinated Tp* ligand,
1-W
, was selected as
precursor bearing a
μ
3
-Cl at the carbide position (Fig. 2) (32).
Although heterometallic iron-sulfur clusters of the MFe
3
types
have been reported with M
=
V, Mo, and W, they typically dis-
play a
μ
3
-S vertex opposite the heterometal that is difficult to
substitute with other donor types relative to chloride (33, 34).
Indeed, starting from a
μ
3
-Cl precursor offers a versatile route
to incorporating biologically relevant light atoms at the bridging
position by ligand metathesis reactions (32). As an example,
the
μ
3
-Cl ligand can be substituted with
μ
3
-S or
μ
3
-NSiMe
3
by
oxidative metathesis with S
8
or Me
3
SiN
3
, respectively (32).
For the installation of a carbon-based ligand at the
μ
3
posi-
tion, we were inspired by the utilization of the strained carbene
bis(diisopropylamino)cyclopropenylidene (BAC) (35) for pro-
moting C-atom transfer to the Fe
≡
N bond of the iron(IV)
nitride [
f
PhB(
i
Pr
2
Im)
3
g
Fe(N)] (
i
Pr
2
Im
=
1,2-diisopropylimida-
zolylidene) (36). This ultimately generated a cyanide ligand,
with the release of alkyne
i
Pr
2
NC
≡
CN
i
Pr
2
as the side product.
Mixing
1-W
with 3 equivalents of BAC in tetrahydrofuran
(THF) in the presence of NaBPh
4
as a chloride abstracting
agent results in the gradual disappearance of the insoluble
1-W
to form a dark red solution, along with the precipitation of a
colorless solid, assigned as NaCl (Fig. 2). Upon filtration, the
vapor diffusion of pentane into the filtrate over one day leads
to the formation of dark purple needles. A single crystal X-ray
diffraction (XRD) study of these crystals confirmed the struc-
ture of the product, where the three terminal chlorides have
been substituted with BAC to give a monocationic cluster,
2-W
,
with a BPh
4
counteranion (Fig. 2). Although MFe
3
S
3
clusters
supported by carbene ligands have not been structurally charac-
terized, the Fe-C distances are in the range of Fe
4
S
4
clusters
supported by NHC ligands (15, 37).
In order to promote the delivery of a C atom or CR group,
at least one C-C bond has to be cleaved, which can be achieved
by methods such as heating (38), photolysis (38), or reduction
(36, 39). While
2-W
remains unchanged when irradiated with a
75-W Xe lamp and decomposes when heated at reflux in THF
under an inert atmosphere, reduction with one equivalent of a
strong reducing agent like potassium naphthalenide leads to
the new cluster
3
. Instead of generating a neutral, one-electron
reduced form of
2-W
and KBPh
4
as byproduct, product
3
loses
C
P
P
i
Pr
2
P
P
P
P
i
Pr
2
H
Fe
Fe
i
Pr
2
i
Pr
2
Fe
Fe
Fe
Fe
C
S
Fe
S
S
CO
CO
CO
CO
CO
CO
CO
CO
OC
CO
OC
OC
CO
C
Fe
Fe
Fe
Fe
Fe
Fe
OC
OC
OC
OC
OC
CO
OC
OC
CO
CO
CO
CO
CO
CO
S
2
C
N
O
S
Fe
Mo
Fig. 1.
(
Top
) Structure of FeMoco in Mo-dependent nitrogenase from the Protein Data Bank structure 3U7Q with a blue circle emphasizing the cubane subsite
and its schematic representation highlighting in color the subsite of focus in this study. (
Bottom
) Carbyne and carbide-containing model complexes (9
–
11).
NN
Tp* =
NN
B
NN
H
Me
Me
Me
Me
Me
Me
Cl
Cl
2
Cl
Fe
S
S
S
Fe
C
Cl
Fe
2 NEt
4
+ 3
Pr
2
N
N
i
Pr
2
i
3 NaBPh
4
THF, RT
16 h
BPh
4
BAC
BAC
BAC
Fe
S
S
S
Fe
C
Cl
Fe
1.1 KC
10
H
8
THF, RT
1 h
BPh
4
3
BAC
BAC
BAC
Fe
S
S
S
Tp* W
Fe
Fe
1.5 KC
8
THF, RT, 48 h
3.5 KC
8
THF, RT,
24 - 48 h
BAC
BAC
Fe
S
S
S
Fe
Tp* W
C
Fe
OTf
4-Mo
3.1 KC
8
THF, RT, 16 h
N
i
Pr
2
N
i
Pr
2
1.2 MeOTf
C
6
H
6
, RT
3 h
N
BAC
BAC
Fe
S
S
S
Fe
Tp* W
C
Fe
N
i
Pr
2
BAC
BAC
Fe
S
S
S
Fe
Tp* Mo
C
Fe
N
i
Pr
2
N
i
Pr
2
BAC
5
4-W
i
Pr
2
3
)
11+
3
)
11+
3
)
11+
3
)
8+
3
)
9+
3
)
9+
Fig. 2.
Synthesis of carbyne-containing clusters.
2of7
j
PNAS
Le et al.
https://doi.org/10.1073/pnas.2109241118
Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of
interstitial atom in nitrogenase active site
Downloaded at California Institute of Technology on December 3, 2021
the
μ
3
-Cl ligand as KCl likely driven by precipitation, leaving an
open triangular Fe
3
face, as demonstrated by XRD characteri-
zation (Fig. 2).
Cluster
3
possesses a rare incomplete cubane geometry for
iron-sulfur clusters. The related [Fe
4
S
3
] geometry has only been
reported in the anion [Fe
4
S
3
(NO)
7
]
of Roussin’s black salt (40)
in inorganic compounds, and an oxygen-tolerant [NiFe]-hydroge-
nase in biology (41). Incomplete heterometallic cubanes of the
form M
0
Fe
3
S
3
have only been observed for M
0
=
Mo in a syn-
thetic system, where the Fe atoms are ligated by multiple CO
ligands (42). The open-face Fe
3
triangle resembles the sulfide-
free triiron systems supported by multinucleating trisamide
ligands, which can bind
μ
3
-nitride or
μ
3
-imide moieties (14, 43).
Thus, the open nature of the Fe
3
cluster face in
3
makes it a prom-
ising platform for the rational installation of various bridging
ligands in a
μ
3
mode.
Cluster
3
can further be reduced with an excess of KC
8
to
form the neutral, Et
2
O soluble cluster
4-W
. Gratifyingly, under
these highly reducing conditions, the C-C bond in the BAC
ligand is cleaved and the cyclopropene ring opens, delivering a
carbyne ligand to the bridging position. The cluster loses its C
3
symmetry, resulting in two Fe atoms ligated by BAC and a
unique Fe center, to which the rest of the ring-opened BAC
ligand anchors as a vinyl fragment. This is an example of a syn-
thetic iron-sulfur cluster without CO ligands that displays a car-
byne donor. Aside from the bridging carbyne ligand, the terminal
hydrocarbyl ligand is also notable, given the role of such ligands
in SAM enzymes (44) and their scarcity in synthetic iron-sulfur
cluster chemistry (45). Conveniently,
4-W
can also be synthesized
directly from
2-W
using an excess of KC
8
or potassium naphthale-
nide without isolating
3
. This reaction stops at
3
if conducted at
78
°
C for 1 h, while appreciable conversion to
4-W
can only be
achieved at room temperature over longer reaction times, sug-
gesting that the ring opening and rearrangement of the BAC
ligand is rate-limiting. The vinyl ligand in
4-W
can be alkylated
with MeOTf, leading to a five-membered amine-carbyne chelate
with N
i
Pr
2
bound to the unique Fe (Fig. 2). This cluster is remi-
niscent of a putative NH
3
-bound form of FeMoco, as it displays a
bridging C-based ligand and a nitrogen donor at one of the Fe
centers.
Toward preparing structural analogs of the
μ
3
-carbyne ligand,
3
was investigated as a precursor to a cluster bearing N or S at
the bridging position. Treatment of
3
with NBu
4
N
3
,Me
3
SiN
3
,
and PPh
3
S (or S
8
) leads to the formation of the corresponding
nitride- (
6
), imide- (
7
), and sulfide- (
8
) bridged clusters (Fig. 3).
Complexes
6
,
7
,and
8
are isostructural, with a WFe
3
S
3
X(X
=
N
or S) cubane supported by Tp* at W and one BAC ligand bound
to each iron center (Fig. 4). The presence of three BAC ligands
is a distinct feature relative to
4-W
. Targeting a carbyne analog
with the same number of BAC donors, compound
5
was treated
with BAC; however, no reaction was observed, likely due to a
combination of steric constraints and stability of the chelate.
For closer similarity to FeMoco, a Mo variant of the above
clusters was targeted. The Mo-containing precursor
1-Mo
was conveniently synthesized from
[NEt
4
][Tp*MoS
5
]
via
[NEt
4
]
[Tp*MoS
3
]
(46) generated by sulfur abstraction with PPh
3
(
SI
Appendix
). Adapting the synthetic protocol developed for
2-W
,
chloride substitution with BAC from
1-Mo
allowed for the isola-
tion of
2-Mo
. Ring opening upon reduction with KC
8
resulted in
the formation of
4-Mo
(Fig. 2). Notably, the MoS
3
Fe
3
Ccluster
core of
4-Mo
reproduces one half of the structure of FeMoco,
including the bridging carbon donor. Furthermore, the geometry
of the unique Fe in the C-bridged clusters
4-W
,
4-Mo
,and
5
repro-
duces the four-coordinate, distorted trigonal pyramidal geometry
found in the belt sites of FeMoco (Fig. 2). The S-Fe-S-C and S-
Fe-S-N torsion angles in
4-W
(173.2
°
)and
5
(154.2
°
) approach
180
°
, bringing these four atoms close to coplanar, which corre-
sponds to a distorted trigonal pyramidal geometry at Fe, leaving
the axial site open for potential substrate coordination, as has
been previously invoked for N
2
binding in FeMoco (17). In addi-
tion, the Fe-N distance in
5
is 2.16 A
̊
,closetotheFe-Nbond
length in the previously characterized NH-bound FeVco (2.01
±
0.04 A
̊
) (31). Further studies are being conducted to investigate
reactivity at this site.
A comparison of the structural aspects of the reported clus-
ters and the corresponding subsite of FeMoco is informative
(
SI Appendix
, Table S3
). The W/Mo-S distances vary modestly
(2.36 to 2.39 A
̊
) in the series of cubane complexes, suggesting
that the metal oxidation state remains unchanged. Although
the total redox state of the metal core varies from (M
0
Fe
3
)
8
+
to
(M
0
Fe
3
)
11
+
, it is likely that the formal oxidation state for M
0
lies
within the 3
+
/4
+
range, based on literature assignments for
MoFe
3
S
4
(47) and WFe
3
S
4
(48) in two redox states, (M
0
Fe
3
)
10
+
and (M
0
Fe
3
)
11
+
, as well as the trend in M
0
-S bond length as a
function of oxidation states of M
0
from related species (
SI
Appendix
, Table S4
). Comparison of bond lengths within the
organic fragment supporting the carbyne ligand reveals notable
differences in
4-W
/
4-Mo
versus
5
.In
4-W
/
4-Mo
, the C10-N11
(average 1.36 A
̊
) and C9-N18 (average 1.46 A
̊
) distances are
significantly different, suggesting multiple bonding character in
C10-N11, while in
5
C10-N11 and C9-N12 are more similar [1.
47 (1) and 1.43 (1) A
̊
, respectively]. The orientation of N11 in
4-W
/
4-Mo
is such that the lone pair can engage in delocaliza-
tion within the olefin
π
bond, increasing the N-C bond order
and lowering the C9-C10 bond order (
SI Appendix
, Fig. S33
).
Because the carbyne is directly bonded to the olefin, its charac-
ter is linked to the propensity of the amine lone pair to delocal-
ize, therefore rendering
4-W
/
4-Mo
more Fischer-like than
5
(49). The possible changes in the character of the carbyne
makes oxidation states ambiguous, but for consistency, herein
the carbyne is assigned in the same way in all of the com-
pounds. It is worth noting that the nature of the carbide ligand
in FeMoco may also vary as a function of changes in the inter-
actions with the other, remote metal centers. Additional experi-
ments will be necessary to determine the overall redox states
and distribution between metals; nevertheless, these com-
pounds are in the range assigned for FeMoco (2). The struc-
tural parameters for the W and Mo analogs
4-W
and
4-Mo
are
very similar, which suggests analogous redox distribution within
the cluster despite different heterometals M
0
. The Fe-C distan-
ces in
4-W
,
4-Mo
, and
5
are in the range of 1.94 to 1.95 A
̊
, which
are close to the average Fe-C bond length in FeMoco of 2.00 A
̊
BPh
4
NBu
4
N
3
THF, RT
3 h
SPPh
3
or 1/8 S
8
THF, RT, 16 h
3
BPh
4
BPh
4
Me
3
SiN
3
THF, RT, 16 h
6
7
8
BAC
BAC
BAC
Fe
S
S
S
Fe
Tp* W
C
N
Fe
BAC
BAC
BAC
Fe
S
S
S
Fe
Tp* W
C
N SiMe
3
Fe
BAC
BAC
BAC
Fe
S
S
S
Fe
Tp* W
C
S
Fe
BAC
BAC
BAC
Fe
S
S
S
Tp* W
Fe
Fe
3
)
10+
3
)
10+
3
)
10+
3
)
8+
Fig. 3.
Synthesis of nitride, imide, and sul
fi
de-containing clusters.
CHEMISTRY
Le et al.
Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of
interstitial atom in nitrogenase active site
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(50), though shorter, likely due to bridging of the carbide
between more metal centers in the biological system.
In order to probe the impact of structure on the redox poten-
tials of the cubane models of FeMoco, we carried out a compara-
tive cyclic voltammetry (CV) study of compounds
4
to
8
(Fig. 5).
Each cluster displays at least one oxidation and one reduction
event, both reversible. To assign the redox waves to the corre-
sponding redox couple, starting from the structurally character-
ized complexes, the open-circuit potential of the system was
determined prior to scanning reductively. For
6
,
7
,and
8
,thetwo
CV features are assigned to the (M
0
Fe
3
)
11
+
/(M
0
Fe
3
)
10
+
and
(M
0
Fe
3
)
10
+
/(M
0
Fe
3
)
9
+
couples. For
4-W
,
4-Mo
,and
5
,they
correspond to (M
0
Fe
3
)
12
+
/(M
0
Fe
3
)
11
+
and (M
0
Fe
3
)
11
+
/(M
0
Fe
3
)
10
+
.
Compounds
4-W
and
4-Mo
show an additional reversible event
at more positive potentials, assigned to (M
0
Fe
3
)
13
+
/(M
0
Fe
3
)
12
+
,
which might be an indication of the carbyne ligand’s ability to
accommodate expanded redox capabilities. Compounds
4
to
8
canbecomparedusingthe(M
0
Fe
3
)
11
+
/(M
0
Fe
3
)
10
+
couple
(highlighted by boxes in Fig. 5), which they all display.
Although the compared redox event corresponds to the
same formal oxidation state and metal coordination number
across all clusters, there are several structural changes that can
impact the reduction potentials and convolute interpretation:
the identity of the bridging atom (C versus N versus S), the
presence and nature of a chelate attached to the bridge, and
the character of the bridging ligand stemming from its substitu-
ents (i.e., rotation of the amine and delocalization of its lone
pair). Within the carbyne-containing clusters there is little
impact of the identity of the Group 6 metal (M
0
=
Mo versus
W) on the reduction potentials, with a slight increase in redox
potential of 70 mV on changing W to Mo, although the other
biologically relevant Fe or V variants remain to be pursued. In
biomimetic group transfer chemistry with Mo and W, a similarly
modest increase in potentials of about 120 mV is also observed
for a nicotinic acid hydroxylase synthetic analog when Mo is
replaced with W (51). The redox couple shifts positively by
about 0.75 V between
4-W
and
5
, and a combination of struc-
tural changes support this trend: the positive charge of
5
, the
weaker electron donating capability of the N
i
Pr
2
group com-
pared to the vinyl ligand, and the donation of the amine lone
pair into the olefin
π
system. The size of the carbyne chelate
may also impact redox chemistry by changing the electronic
3
4-Mo
5
6
7
8
Fig. 4.
Crystal structures of
3
,
4-Mo
, and
5
to
8
(reference
SI Appendix
for the isostructural cluster
4-W
). Hydrogen atoms, counteranions (for
3
,
5
,
7
, and
8
), and the BAC ligand except for the carbene C are omitted for clarity. Ellipsoids are shown at 50% probability.
4of7
j
PNAS
Le et al.
https://doi.org/10.1073/pnas.2109241118
Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of
interstitial atom in nitrogenase active site
Downloaded at California Institute of Technology on December 3, 2021
character of the ligand. Osmium compounds supported by
dppe versus dppm ligands show a relatively small change of
30 mV (52), but we could not find specific precedent for the
potential range of such effect for carbynes.
The (M
0
Fe
3
)
11
+
/(M
0
Fe
3
)
10
+
couple for the C-containing clus-
ters
4-W
,
4-Mo
, and
5
appear at potentials below
2 V, signifi-
cantly more negative than for
6
to
8
at
0.87 V to
1.16 V.
While
6
to
8
contain three BAC ligands and
4-W
/
5
only have
two, complicating comparison, compound
5
displays a weaker
donor in the tertiary amine compared to BAC in
6
to
8
.
Although a less ambiguous analysis would benefit from an ana-
log of
5
with a BAC ligand instead of the amine, which could
not be accessed (
vide supra
), the greater electron donation abil-
ity of the NHC ligand compared to the tertiary amine is
expected to render that hypothetical version of
5
even more
reducing. Compounds
4-W
,
4-Mo
, and
5
all display chelates,
unlike
6
to
8
. Multidentate phosphines and pyridines show
shifts of less than 0.3 V in redox potentials relative to mono-
dentate variants (52). Although this difference is much smaller
than the differences observed here, because the carbyne inter-
actions may be more strongly impacted by changes in bond
angles, we cannot rule out that changes in reduction potentials
are primarily due the presence of chelates in
4-W
,
4-Mo
, and
5
.
Overall, the combination of carbyne and chelate results in a
remarkable redox potential difference. Compounds
5
and
6
maintain the same formal charge for the bridging ligand (3-) as
carbyne versus nitride. Still, a difference in the (M
0
Fe
3
)
11
+
/
(M
0
Fe
3
)
10
+
redox potentials of 1.12 V is observed, a substantial
impact of the chelating C- versus N-based, though nonchelat-
ing, ligands. Changing the donor from nitride (
6
) to imide (
7
)
or sulfide (
8
) shifts the redox potential less than 200 mV,
highlighting the similar effect of S and N donors on the redox
chemistry, in contrast to the chelating carbyne.
These electrochemical results suggest that the interstitial car-
bon ligand in FeMoco may play an important role in increasing
the reductive power of the clusters. While the specific oxidation
states of the metal centers cannot be verified without additional
spectroscopic studies, the reduced form, (M
0
Fe
3
)
10
+
, corre-
sponds to an average metal oxidation state of 2.5 (or 2 if a
Fischer carbyne resonance is considered), close to the resting
state of FeMoco as Fe
II
3
Fe
III
4
Mo
III
(average metal oxidation
state 2.6) (2). Additional reduction steps lower the average for-
mal oxidation state of FeMoco, but not below 2, and do not
bring it in the range typically observed for mononuclear Fe
complexes studied for N
2
activation (53). Therefore, the ability
of the biological cofactor to perform N
2
activation at high oxi-
dation states is unusual. A possible explanation is charge redis-
tribution within the cluster to increase reducing equivalents at
the site of substrate binding or electronic communication
between different metal sites (21–23, 54). We find here that the
chelating carbyne ligand has a remarkable impact on the cluster
reduction potentials, with very reducing potentials for relatively
high, biologically relevant metal oxidation states. Moreover, the
chelating carbyne clusters are significantly more reducing for
the same redox state compared to N and S analogs. It is impor-
tant to note that the chelation present in all carbynes reported
here and the delocalization of amine lone pair in some of them
may have a substantial effect on the redox chemistry by tuning
the electronic properties of the carbon ligand; conceptually
related, changes in the coordination environment of distal iron
centers in FeMoco may have similar effects on the carbide.
Toward addressing the effect of the interstitial ligands of
FeMoco, we have reported studies of tetranuclear Fe clusters
with
μ
4
-fluoride and oxide ligands (21). In those cases, the
oxide makes the cluster about 1 V more reducing compared to
fluoride for the same redox state while also promoting NO acti-
vation. Additionally, remote metal centers affect reactivity
through interactions with the bridging moiety (O or F). In
those systems, the charge of the interstitial atom could play a
role in changing the potential. Here, this series of clusters
accounts for changes in ligand charge. Chelating carbyne (
5
)
and nitride (
6
) ligands have the same formal charge but result
in
∼
1 V difference in reducing power, in contrast with the
nitride (
6
), imide (
7
), and sulfide (
8
) species that have reduc-
tion potentials within 300 mV. The ability of the chelating car-
byne to increase the reduction power is likely a consequence of
its stronger interaction with the metal centers, an aspect that
will be pursued in a separate study, including nonchelated
targets.
In the context of N
2
reduction, the redox tuning observed
here suggests that the interstitial carbon may allow FeMoco to
access higher reducing power, enabling more facile transfer of
electrons to the N
2
substrate for conversion to NH
3
. This paral-
lels the results from experiments using synthetic iron catalysts
for N
2
reduction, many of which require strong external reduc-
ing agents like KC
8
(8, 55)
.
Considering the potential impact of
replacing the bridging carbyne with a more biologically inex-
pensive sulfide, a much less reducing cluster (
8
) is generated
for the same redox state, (M
0
Fe
3
)
10
+
, and even an additional
reducing equivalent in (M
0
Fe
3
)
9
+
does not match with the
(M
0
Fe
3
)
10
+
carbyne system. Therefore, an interstitial sulfide
may not provide sufficient reducing power to efficiently convert
N
2
into NH
3
, leading instead to the preference for the unusual
bridging carbide motif.
In summary, we have described the synthesis of a series of
heterometallic iron-sulfur clusters of the form M
0
Fe
3
S
3
X(M
0
=
-3
-2.5
-2
-1.5
-1
-0.5
0
Potential vs Fc/Fc
+
/V
50
A
4-Mo
6
7
8
4-W
11+/10+
11+/10+
11+/10+
12+/11+
13+/12+
10+/9+
10+/9+
10+/9+
12+/11+
12+/11+
11+/10+
11+/10+
5
11+/10+
*
13+/12+
Fig. 5.
CV scans for compounds
4
to
8
. Each voltammogram starts from
the open-circuit potential, and the boxed peaks correspond to the
(M
’
Fe
3
)
11
+
/(M
’
Fe
3
)
10
+
couples of interest. The redox assignment is indicated
above each wave in terms of the charge of the (M
’
Fe
3
) metal core. Condi-
tions:
∼
2.5 mM cluster in MeCN with 0.2 M TBAPF
6
, scan rates of 200 mV
s
1
(
4-W
,
5
to
8
) or 250 mV s
1
(
4-Mo
). The asterisk indicates small
amounts of
4-W
impurity in samples of
6
.
CHEMISTRY
Le et al.
Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of
interstitial atom in nitrogenase active site
PNAS
j
5of7
https://doi.org/10.1073/pnas.2109241118
Downloaded at California Institute of Technology on December 3, 2021