Fe5Mo Cluster with Iron-Carbide and Molybdenum-Carbide Bonding Motifs: Structure and Selective Alkyne Reductions

Article abstract of DOI:10.1021/acs.inorgchem.7b02615

Herein we report the synthesis, X-ray structure, and characterization of the title pentairon (molybdo)carbido cluster. The reaction of the pentairon (μ₅-carbido) dianion [Fe₅(μ₆-C)(μ₂-CO)₂(CO)₁₂]^2- (1) with [Mo(CO)₃(chpt)] (chpt = cycloheptatriene) forms the heterohexanuclear cluster [K(benzo-18-crown-6)]₂[Fe₅Mo(μ₆-C)(μ₂-CO)₃(CO)₁₄] (2). The dianion exhibits a Fe₅Mo(μ₆-C) core structure supported by three bridging (ν_CO = 1788 cm^-1) and terminal (ν_CO = 1943 cm^-1) CO ligands. Cluster 2 provides the selective reduction of diphenylacetylene to cis-diphenylethylene via a spectroscopically observed cluster-hydride intermediate (¹H NMR: δ -26).

Full text of DOI:10.1021/acs.inorgchem.7b02615

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Fe Mo Cluster with Iron-Carbide and Molybdenum-Carbide Bonding
5
Motifs: Structure and Selective Alkyne Reductions
Chris Joseph, Subramaniam Kuppuswamy, Vincent M. Lynch, and Michael J. Rose*
Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States
*
S Supporting Information
obtained. Finally, the cluster is demonstrated to perform selective
reductions with the model substrate diphenylacetylene (DPA).
It was necessary to modify the preparation of several reported
clusters to facilitate crystallization and structure solution along
the synthetic pathway to the final molybdenum-containing
ABSTRACT: Herein we report the synthesis, X-ray
structure, and characterization of the title pentairon
(
(
molybdo)carbido cluster. The reaction of the pentairon
2
−
μ -carbido) dianion [Fe (μ -C)(μ -CO) (CO) ] (1)
5
5
6
2
2
12
with [Mo(CO) (chpt)] (chpt = cycloheptatriene) forms
3
cluster. The neutral species [Fe
according to refs 11 and 12) was reduced with 2.1 equiv of
KC in tetrahydrofuran (THF; Scheme 1), followed by filtration
5 5
(μ -C)(CO)15] (prepared
the heterohexanuclear cluster [K(benzo-18-crown-
6
)] [Fe Mo(μ -C)(μ -CO) (CO) ] (2). The dianion
2 5 6 2 3 14
8
exhibits a Fe Mo(μ -C) core structure supported by
5
6
−1
three bridging (ν = 1788 cm ) and terminal (ν
943 cm ) CO ligands. Cluster 2 provides the selective
=
CO
CO
Scheme 1. Reaction of 1 with [M(CO) (L)] [L = chpt or
3
−1
1
(
THF) ], Which in the Case of M = Mo and L = chpt Affords
3
reduction of diphenylacetylene to cis-diphenylethylene via
the Title Cluster 2
a spectroscopically observed cluster-hydride intermediate
1
(
H NMR: δ −26).
ince the discovery of small-molecule metal clusters with
interstitial light atoms in 1962, the encapsulation of light
S
atoms inside metal clusters has remained a fascinating curiosity in
1
inorganic chemistry. The seminal work by Churchill and others
involving caged carbides, nitrides, and oxides brought forth new
structural possibilities and new routes of studying heterogeneous
of the reaction solution and the addition of 2.1 equiv of benzo-
2
−6
metallic reaction pathways in small-molecule format.
Addi-
18-crown-6. Subsequent crystallization in FPh/Et O afforded
2
tionally, the recent discovery and confirmation of the interstitial
carbide present in the FeMoco active site of molybdenum-
containing nitrogenase have brought renewed attention to this
structural motif. For example, recent structural and spectro-
scopic investigations by DeBeer et al. have led to insight into the
black plates of [K(benzo-18-crown-6)]
Figure S9), which exhibits a nido-[Fe (μ
open site at the basal position of the carbide similar to the
2
[Fe
-C)] core motif with an
5
5 5
(μ -C)(CO)14] (1;
5
10,12
6,13
7
,8
[
NEt ] salt.
According to previous precedents,
we
4 2
reasoned that the open carbide dative site could be capped
with a heterometal to afford the corresponding Fe M carbide
5
electronic structure and spectroscopic signature of an interstitial
9
structure. We thus attempted metalation of the carbide with
chromium, molybdenum, and tungsten carbonyls. Reactions of
carbide. Additionally, recent work by Berben et al. using [Fe ]
4
and [Fe ] nitrides and carbides has shown their utility in selective
5
1
0
[M(CO)
(L)] [where L = chpt (cycloheptatriene) or (THF) ]
3 3
CO reduction.
2
with M = Cr and Mo in THF afforded adducts postulated as
In our research into the synthesis of such metal carbido
clusters and nitrogenase, we have investigated the incorporation
of “heterometals” into otherwise all-iron carbido clusters. In this
work, we report the synthesis, characterization, and X-ray
structure of a stable pentairon (molybdo)carbidocarbonyl
2−
[
Fe M(μ -C)(CO) ] , as judged by their IR spectra (blue-
5 6 17
−1
shifted features ≈ 1940 and 1785 cm ). However, whereas the
Mo adduct proved stable under crystallization conditions, the
{
Cr(CO) } unit spontaneously dissociated over time, even at
3
low temperatures (−30 °C) during crystallization. Treatment of
cluster. While the [NEt4] salt of the title pentairon
2
1 with [W(CO) (THF) ] did not install the {W(CO) } unit to
3
3
3
11
molybdocarbidocarbonyl cluster has been reported before,
the carbide site under any tested condition, possibly because of
the poor orbital overlap between the diffuse W 5d set and the
contracted C 2p set.
the full structure of the cluster is not available in the CCDC. Our
attempts to characterize this compound produced a structure in
2
−
the cubic space group Fm3
rise to ambiguous metal atom identities. We do note, however,
̅
On the basis of the stability of the [Fe Mo] cluster, structural
5
characterization was forthcoming (FPh/Et
O vapor diffusion),
2
affording black plates of [K(benzo-18-crown-6)] [Fe Mo(μ -
that the structure of the [Fe Mo] cluster can be inferred from its
2
5
6
5
6
gold-coordinated derivatives. In this work, we show that, by
inducing an interaction between the cluster and its counterion, a
crystal structure with unambiguous metal ion identities is
Received: October 12, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02615
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
To probe the redox activity of 1 and 2, we performed cyclic
voltammetry (CV) to determine the accessibility of the reduced
congeners of 1 and 2. A cathodic sweep of 1 in THF (Figure S4)
did not exhibit any reduction events. The return anodic sweep,
however, did reveal two oxidation events assigned to the
conversion of the dianionic cluster to the known neutral species.
Alternately, the CV for the Mo-containing cluster 2 (Figure S8)
revealed a set of reversible features with a reduction event at
−
2.00 versus Fc. The accessibility of these redox events to
common chemical reductants, such as KC8 and sodium
perylenide (Na Per), led us to explore reactions of the reduced
2
cluster with model substrates that provide insight into selective
reductions of triply bonded functional groups.
On the basis of these findings and literature precedent
Figure 1. ORTEP diagram (50% thermal ellipsoids) for 2. Bond
14
demonstrating the activation of alkynes by iron carbonyls, we
pursued reactions of the model substrate DPA with each cluster
under reducing conditions in the presence of varying proton
sources. Selected results are delineated in Table 1 (additional
results in Table S3). Previous studies explored the reduction of
DPA using palladium-catalyzed or electrochemical means to
distances (Å): Mo1−C1 = 2.113(6), Fe1 −C1 = 1.937(6), Fe2−C1 =
ax
1
1
2
C17
2
.896(5), Fe3−C1 = 1.902(4), Fe4−C1 = 1.887(5), Fe5−C1 =
.881(4), Mo1−Fe2 = 2.9666(9), Mo1−Fe3 = 3.054(1), Mo1−Fe4 =
.861(1), Mo1−Fe5 = 2.769(1), Mo1−C16
= 1.972(7), Mo1−
term
= 1.961(7), Mo1−C18
= 1.961(8), Mo1−C15bridg
=
term
term
.344(6), K1···O5 = 2.831(5), K2···O11 = 2.916(8).
15
obtain cis-alkene. In contrast, there is just a single report of a
C)(μ -CO) (CO) ] (2; Figure 1). The structure exhibits a
2−
2
3
14
single-molecule catalyst (an [Fe S ](SR) ] cluster) for DPA
4
4
4
16
“
capped” μ -carbide motif, incorporating the {Mo(CO) } unit at
6
3
reduction. A general, unbalanced formula for the reaction
performed here is given as:
a distance of 2.113(6) Å from the carbide. Regarding the trans
effect of the capping {Mo(CO) } unit, the apical Fe ion is
3
−
+
substantially compressed toward the carbide at Fe−carbide =
[cluster] + 30 e + 30 H + 15 C C →
cis‐CC + trans‐CC + CC
12
1
.937(6) Å versus the five-iron cluster [1.99(1) Å]; the
equatorial Fe−carbide distances remain largely unchanged. The
Mo site exhibits three terminal CO ligands [Mo−C(O) =
First, control reactions with no cluster (KC , LutH·OTf)
8
1
.972(7), 1.961(7), and 1.961(8) Å] and one bridging CO ligand
revealed a poorly selective 0.25 ratio of the intermediately
reduced product (diphenylethylene) to the fully reduced alkane
product (diphenylethane) with 21.8% DPA conversion. None of
the desired cis-alkene was observed in the control reaction. In
[
Mo−C(O) = 2.344(6) Å] with Fe5.
Overall, the Mo center participates in nine bonding
interactions (including one bridging CO), whereas the highest-
coordination-number Fe site (Fe3; also coordination number =
contrast, the presence of catalytic 2 (KC , LutH·OTf) provided
8
9
) exhibits two bridging CO ligands. Thus, while polyhedral
better selectivity for alkene/alkane conversion (1.58 ratio),
indicating over a 6-fold improvement in the selectivity, despite a
diminished 14.1% overall conversion. Within the possible alkene
products (trans and cis), cluster 2 did provide some, albeit
minimal, cis product (cis/trans = 0.22). A secondary control
reaction replaced cluster 2 as a catalyst with its individual metal
skeletal electron pair theory (PSEPT) accurately predicts the
total number of CO ligands, simple inspection of the metal
0
I
oxidation states [Mo versus Fe ] indicates the location of the
extra” CO on the Mo site. Incubation of 2 under a CO
atmosphere results in no change in the IR spectrum or enhanced
“
13
features in the C NMR, indicating no further CO binding and
the validity of the PSEPT rules in this case. The bond metrics of
the terminal and bridging CO ligands on the Fe sites remain
center components. When 5 equiv of Fe(CO) was used in
5
conjugation with 1 equiv of Mo(CO) , moderate selectivity was
6
maintained. However, substrate conversion decreased dramati-
cally (3.6%). A screening of different proton sources revealed
poor activity with phenols (Table S3) and more consistently
beneficial results using nitrogen-based acids. Additionally, it was
largely unchanged [avg Fe−C(O)
= 1.778 ± 0.017; Fe−
term
C(O)
= 1.968 ± 0.141 Å]. Lastly, the crowned K ions exhibit
bridg
close contacts with two terminal CO ligands [K···O = 2.831(5)
and 2.916(8) Å], similar to the K···O contacts found in 1.
found that bulkier, lower pK proton sources yielded higher
a
a
Table 1. Selected Results from Reductions of DPA to cis/trans-Diphenylethylene and Diphenylethane
catalyst
none
reductant
proton source
[LutH]OTf
pKa
temp (°C)
convn (%)
CC/CC ratio
cis/trans ratio
KC8
KC8
KC8
KC8
KC8
6.60
6.60
6.60
6.60
4.37
4.37
4.37
4.37
3.30
3.30
rt
21.8
9.7
0.25
0.00
0.34
0.22
0.82
0.30
0.56
0.03
1.15
0.65
0
Fe (1)
5
[LutH]OTf
[LutH]OTf
[LutH]OTf
rt
0.60
Fe Mo (2)
5
rt
14.1
3.6
1.58
5Fe + Mo
rt
0.62
Fe Mo (2)
5
[MesNH ]OTf
rt
25.4
17.9
28.8
3.7
0.36
3
Fe Mo (2)
5
Na (Per)
2
[MesNH ]OTf
rt
3.01
3
Fe Mo (2)
5
Na (Per)
2
[MesNH ]OTf
−20
60
rt
1.66
3
Fe Mo (2)
5
Na (Per)
2
[MesNH ]OTf
6.55
3
t
Fe Mo (2)
5
Na (Per)
2
[2,4,6- Bu PhNH ]OTf
15.9
0.8
16.0
3
3
t
none
Na (Per)
2
[2,4,6- Bu PhNH ]OTf
rt
CC only
3
3
a
LutH = 2,6-lutidinium; MesNH = 2,4,6-trimethylanilinium; Na (Per) = sodium perylenide; 5Fe + Mo = 5Fe(CO) + Mo(CO) ; Fe = cluster 1;
3
2
5
6
5
Fe Mo = cluster 2. The bold values denote the best result in each column. A complete table of results can be found in the Supporting Information.
5
B
DOI: 10.1021/acs.inorgchem.7b02615
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
substrate conversion and cis selectivity. In particular, a sterically
encumbered anilinium triflate (2,4,6-trimethylanilinium triflate)
provided the highest overall conversion (25.4%), moderate
alkene/alkane selectivity (0.36), and a higher cis/trans ratio
supported FeMoco cluster in nitrogenase. More synthetic work is
needed to address the substitution of terminal and bridging CO
units to biologically relevant, sulfur-based supports such as
sulfides, thiolates, and thioethers. This work is now underway
and will be reported in due course.
(0.30) than other conditions. By comparison, the Fe-only cluster
1
provided a low overall conversion (9.7%) and only an
intermediate alkene/alkane selectivity (0.60). The less beneficial
results from 1 are most likely due to its inaccessible redox activity
by KC and the decreased stability of the [Fe ] core versus the
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02615.
■
*
S
8
5
[
Fe Mo] core under catalytic conditions.
5
To further optimize the conversion and selectivity of the
reaction, a weaker reducing agent (sodium perylenide, generated
in situ) was used. While substrate conversion dropped modestly
Syntheses, NMR, IR, CV, X-ray experimental and
parameters, and tabulated bond distances (PDF)
(from 25% to 18%), the selectivity of cis-alkene increased
significantly (alkene/alkane = 3.01; cis/trans = 0.56). Interest-
ingly, temperature was found to be inversely correlated with
substrate conversion while being directly correlated with
selectivity toward cis-alkene. Performing the reaction at 60 °C
exhibited the highest selectivity (alkene/alkane = 6.55; cis/trans
Accession Codes
CCDC 1541349 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
=
1.15), although it suffered from poor substrate conversion
(3.7%). Finally, 2,4,6-tri-tert-butylanilinium triflate was used as
an even bulkier proton source, which dramatically increased the
alkene/alkane selectivity (16.0) while the overall conversion and
cis selectivity remained moderate. The corresponding control
reaction (no catalyst) provided only 0.8% overall conversion, and
no cis product was detected.
To establish the integrity of the cluster throughout the
catalytic process, we determined several spectroscopic properties
of the in situ cluster product. After catalysis with 2, the IR
AUTHOR INFORMATION
Corresponding Author
*E-mail: mrose@cm.utexas.edu (M.J.R.).
ORCID
Chris Joseph: 0000-0002-9763-5162
Michael J. Rose: 0000-0002-6960-6639
Author Contributions
The manuscript was written through contributions of all authors,
who all have given approval to the final version of the manuscript.
Notes
■
spectrum of the reaction mixture exhibited a distinct ν feature
CO
at 1963 cm−1 (Figure S11); this is distinct from the starting IR of
, at 1943 cm−1 (Figure S7). To provide context for this
2
observation and to gain insight into the mechanism of substrate
reaction, stoichiometric reactions of 2 with reductants and
substrates were performed (Figure S12). Treatment of cluster 2
The authors declare no competing financial interest.
−1
ACKNOWLEDGMENTS
with 1 equiv of Na (Per) resulted in a blue shift to 1958 cm .
■
2
Subsequent addition of 1 equiv of DPA resulted in no significant
The authors gratefully acknowledge the Robert A. Welch
Foundation (F-1822), the ACS Petroleum Research Fund
(PRF 53542-DN13), and a Cottrell Scholar Award to M.J.R.
(RCSA 23640).
−1
change (νCO at 1959 cm ), indicating no direct interaction of
reduced 2 with DPA. In contrast, treatment of reduced 2 with 1
equiv of [MesNH ]OTf resulted in a slightly shifted ν at 1962
3
CO
−1
−1
cm (closer to the crude mixture value of 1963 cm ), indicating
direct reaction of reduced 2 with the proton source. Also, while
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DOI: 10.1021/acs.inorgchem.7b02615
Inorg. Chem. XXXX, XXX, XXX−XXX