Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.7b00741

The syntheses, interconversions, and spectroscopic properties of a set of iron carbonyl clusters containing an interstitial carbide are reported. This includes the low temperature X-ray structures of the six-iron clusters (Y)₂[Fe₆(μ₆-C)(μ₂-CO)₄(CO)₁₂] (1a-c; where Y = NMe₄, NEt₄, PPh₄); the five-iron cluster [Fe₅(μ₅-C)(CO)₁₅] (3); and the novel formulation of the five-iron cluster (NMe₄)₂[Fe₅(μ₅-C)(μ₂-CO)(CO)₁₃] (4). Also included in this set is the novel charge-neutral cluster, [Fe₆(μ₆-C)(CO)₁₈] (2), for which we were unable to obtain a crystallographic structure. As synthetic proof for the identity of 2, we performed a closed loop of interconversions within a family of crystallographically defined species (1, 3, and 4): [Fe₆]^2- → [Fe₆]⁰ → [Fe₅]⁰ → [Fe₅]^2- → [Fe₆]^2-. The structural, spectroscopic, and electronic properties of this "missing link" cluster 2 were investigated by IR, Raman, XPS, and M?ssbauer spectroscopies - as well as by DFT calculations. A single ν_CO feature (1965 cm^-1) in the IR spectrum of 2, as well as a prominent Raman feature (ν_symm = 1550 cm^-1), are consistent with the presence of terminal carbonyls and a {(μ₆-C)Fe₆} arrangement of iron centers around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p_3/2 feature (707.4 eV) as compared to that of 1 (705.5 eV), consistent with the two-electron oxidation induced by treatment of 1 with two equivalents of [Fc](PF₆) under CO atmosphere (for the two added CO ligands). DFT calculations indicate two axial and four equatorial Fe sites in 1, all of which have the same or similar oxidation states, for example, two Fe(0) and four Fe(+0.5). These assignments are supported by M?ssbauer spectra for 1, which exhibit two closely spaced quadrupole doublets with δ = 0.076 and 0.064 mm s^-1. The high-field M?ssbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with δ = -0.18, -0.11, and +0.41 mm s^-1. This indicates three pairs of chemically equivalent Fe sites. The first two pairs arise from irons of a similar oxidation state, while the last pair arises from irons in a different oxidation state, indicating a mixed-valent cluster. Variable field M?ssbauer spectra for 2 were simulated assuming these two groups and a diamagnetic ground state. Taken together, the M?ssbauer results and DFT calculations for 2 indicate two axial Fe(II) sites and four equatorial sites of lower valence, probably Fe(0). In the DFT optimized pentagonal bipyramidal structure for 2, the Fe(II)-C_carbide distances are compressed (～1.84 ?), while the Fe(0)-C_carbide distances are elongated (～2.05 ?). Analysis of the formulations for 1 (closo-square bipyramid) and 2 (nido-pentagonal bipyramid) is considered in the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters, respectively. This redox-dependent intracluster disproportionation of Fe oxidation states is concluded to arise from changes in bonding to the central carbide. A similar phenomenon may be promoted by the central carbide of the FeMoco cluster of nitrogenase, which may in turn stimulate N₂ reduction.

Full text of DOI:10.1021/acs.inorgchem.7b00741

Article
pubs.acs.org/IC
Structures, Interconversions, and Spectroscopy of Iron Carbonyl
Clusters with an Interstitial Carbide: Localized Metal Center
Reduction by Overall Cluster Oxidation
†
‡
†
†
†
Subramaniam Kuppuswamy, Joshua D. Wofford, Chris Joseph, Zhu-Lin Xie, Azim K. Ali,
†
‡
,†
Vincent M. Lynch, Paul A. Lindahl, and Michael J. Rose*
†Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States
‡Department of Chemistry, Texas A&M University, College Station, Texas 77840, United States
*
S Supporting Information
ABSTRACT: The syntheses, interconversions, and spectroscopic
properties of a set of iron carbonyl clusters containing an interstitial
carbide are reported. This includes the low temperature X-ray
structures of the six-iron clusters (Y) [Fe (μ -C)(μ -CO) (CO) ]
2
6
6
2
4
12
(
1a−c; where Y = NMe , NEt , PPh ); the five-iron cluster [Fe (μ -
4
4
4
5
5
C)(CO) ] (3); and the novel formulation of the five-iron cluster
15
(
NMe ) [Fe (μ -C)(μ -CO)(CO) ] (4). Also included in this set is
4 2 5 5 2 13
the novel charge-neutral cluster, [Fe (μ -C)(CO) ] (2), for which we
6
6
18
were unable to obtain a crystallographic structure. As synthetic proof
for the identity of 2, we performed a closed loop of interconversions
within a family of crystallographically defined species (1, 3, and 4):
[
Fe6]2 → [Fe ] → [Fe ] → [Fe ] → [Fe ] . The structural,
−
0
0
2−
2−
6 5 5 6
spectroscopic, and electronic properties of this “missing link” cluster 2
were investigated by IR, Raman, XPS, and Mo
ssbauer spectroscopies
as well as by DFT calculations. A single ν feature (1965 cm ) in the IR spectrum of 2, as well as a prominent Raman feature
̈
−1
CO
−1
(
νsymm = 1550 cm ), are consistent with the presence of terminal carbonyls and a {(μ -C)Fe } arrangement of iron centers
6 6
around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p3/2 feature (707.4 eV) as compared to that of 1 (705.5
eV), consistent with the two-electron oxidation induced by treatment of 1 with two equivalents of [Fc](PF ) under CO
6
atmosphere (for the two added CO ligands). DFT calculations indicate two axial and four equatorial Fe sites in 1, all of which
have the same or similar oxidation states, for example, two Fe(0) and four Fe(+0.5). These assignments are supported by
−1
Mo
Mo
indicates three pairs of chemically equivalent Fe sites. The first two pairs arise from irons of a similar oxidation state, while the
last pair arises from irons in a different oxidation state, indicating a mixed-valent cluster. Variable field Mossbauer spectra for 2
were simulated assuming these two groups and a diamagnetic ground state. Taken together, the Mossbauer results and DFT
calculations for 2 indicate two axial Fe(II) sites and four equatorial sites of lower valence, probably Fe(0). In the DFT optimized
pentagonal bipyramidal structure for 2, the Fe(II)−C distances are compressed (∼1.84 Å), while the Fe(0)−C
̈
ssbauer spectra for 1, which exhibit two closely spaced quadrupole doublets with δ = 0.076 and 0.064 mm s . The high-field
−1
̈
ssbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with δ = −0.18, −0.11, and +0.41 mm s . This
̈
̈
carbide
carbide
distances are elongated (∼2.05 Å). Analysis of the formulations for 1 (closo-square bipyramid) and 2 (nido-pentagonal
bipyramid) is considered in the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters,
respectively. This redox-dependent intracluster disproportionation of Fe oxidation states is concluded to arise from changes in
bonding to the central carbide. A similar phenomenon may be promoted by the central carbide of the FeMoco cluster of
nitrogenase, which may in turn stimulate N reduction.
2
INTRODUCTION
ization, alkene hydrogenation, Fischer−Tropsch synthesis, and
the hydrogenation of amides. Such clusters have also been
investigated as low-temperature catalysts for the water−gas-
■
Polymetallic clusters of the 4d and 5d metals rarely present an
opportunity to study paramagnetic species, especially even-
electron systems, due to the high energetic penalty for spin
unpairing in the greater ligand field of heavy metals. For
example, the closest ruthenium congener of the neutral iron-
carbido cluster described in this work, [Ru (μ -C)(CO) ], is
2
−7
shift reaction. Nitrogenase, the enzyme that catalyzes the six-
8
−11
electron reduction of dinitrogen (N ) to ammonia (NH ),
2
3
contains a carbide in the center of the active site FeMoco
6
6
17
1
diamagnetic. Such metal-carbide clusters have been utilized as
catalysts for chemical transformations such as alkene isomer-
Received: March 29, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
0,11
cluster.
Indeed, a recent review by Holland has delineated a
first-row transition metal clusters. In this Article, we report a
convenient synthetic route for hexa-iron cluster anions with
varying cations (NMe , NEt , and PPh ). These clusters
number of recent attempts at synthetically modeling this
complex bioinorganic target.
12
4
4
4
In the cases of iron carbonyl clusters and iron-carbido
carbonyl clusters, the ground states of nearly all isolated clusters
are diamagnetic due to the strong antiferromagnetic coupling
between equivalent Fe centers, as well as the low-spin
configurations enforced by carbonyl (CO) ligands. There
have been limited reports of iron-carbido carbonyl clusters
since the 1960s, all of which have focused on the isolation and
characterization of multi-Fe clusters. Indeed, the nucleophilic
provide more precise data sets of X-ray structure determi-
nations with respect to the nature and number of the bridging
carbonyls. In addition, controlled oxidation of [Fe6] with an
2
−
outer-sphere redox reagent provides the first access to the
0
oxidized [Fe ] cluster (2). Spectroscopic characterization of 2
6
2−
0
2−
and its interconversions among [Fe ] , [Fe ] , and [Fe ]
6
5
5
confirm its identity.
4
−
carbide (C ) is a potential building unit to assemble
polymetallic clusters via covalent bonds between a central
carbide and metal atoms, thus enforcing strong metal−metal
EXPERIMENTAL SECTION
■
Reagents and Procedures. All manipulations were performed
under an inert atmosphere using Schlenk line or glovebox techniques.
HPLC grade solvents were purchased from EMD, Fisher, Macron, or
J.T. Baker, and dried through an alumina column system (Pure Process
13−16
bonds within the cluster.
Both of the electrons stored in
the chemical bonds between (or among) metal ions and open
coordination sites have been employed to achieve chemical
6
8
Technology). Deuterated solvents (d -benzene, d -THF) were
17
transformations such as proton reduction or reactions related
purchased from Cambridge Isotopes or Acros Organics and used as
18
to Fischer−Tropsch chemistry.
received. Diglyme, NEt
nium hexafluorophosphate, iodine, NMe Cl, naphthalene, and PPh Cl
4 2 5 3 2
Cl·H O, [Fe(CO) ], FeCl ·6H O, ferroce-
In recent years, iron-mediated catalytic reactions have
received much attention as promising alternatives for noble
4
4
were used as received. The iron synthons Na [Fe(CO) ] and solutions
2
4
5
−7
of Na [Fe (μ -C)(CO) ] were prepared according to modified
2 6 6 16
22,29
metal catalysts.
In the nitrogenase cofactor, the central
literature reports.
carbide ligand that holds multiple iron atoms in a discrete
cluster may serve to improve multielectron redox processes
with minimal structural changes. Moreover, an interstitial
carbide atom encapsulated within a multimetallic cluster can
provide stability to the cluster core to prevent fragmentation or
large structural changes during catalytic turnover.
Caution: Iron carbonyls are extremely toxic, and disodium
tetracarbonylferrate is pyrophoric. These materials should be handled
carefully in a drybox or well-ventilated fume hood under inert
atmosphere.
Optimized Preparation of [Na Fe(CO) ]. A solution of
2
4
naphthalene (9.5 g, 74 mmol) in THF (100 mL) was chilled at −78
°
C, and sodium pieces (1.7 g, 79 mmol) were slowly added. The
In 1962, Braye and co-workers first characterized an
interstitial carbide atom encapsulated in a discrete five-iron
resulting mixture was gradually warmed to room temperature and
stirred for 2 h to ensure complete in situ formation of the green
sodium naphthalenide. The reaction mixture was again chilled to ice-
1
9
carbonyl cluster. A decade later, Churchill and co-workers
reported the corresponding dianionic six-iron cluster of formula
cold temperature, and to this neat [Fe(CO) ] (5.0 mL, 37 mmol) was
2
−
5
[
Fe (μ -C)(μ -CO) (CO) ] (1 in this report) via degrada-
6 6 2 4 12
added dropwise over a period of 5−10 min. The reaction mixture was
stirred at room temperature for 2 h, and the solution turned red-
brown. All volatiles were removed in vacuo, and the crude residue was
redissolved in THF; subsequent dilution of the mixture with cold
pentane precipitated the analytically pure product. The reddish brown
precipitate of disodium ferrate was isolated by filtration and dried
under vacuum for 30 min. The product was stored at −20 °C under
argon atmosphere until use. Yield: 7.5 g (95%). IR (solid state): 1762
−
tion of the heterometallic anion [MnFe (CO) ] . At that time,
2
12
the room-temperature low-resolution X-ray structure of 1 did
not satisfactorily describe the connectivity or bond metrics of
20,21
2−
the bridging carbonyls.
Interestingly, the [Fe6] cluster has
since been utilized as an inorganic synthon to prepare the
corresponding lower nuclearity cluster complexes, in which the
22,23
central carbide remains bound to the remaining metal ions.
−1
cm .
NMe ) [Fe (μ -C)(μ -CO) (CO) ] (1a). A neat aliquot of [Fe-
Lower nuclearity clusters (five- and four-iron clusters) have
22
(
4
2
6
6
2
4
12
since been investigated to explore the “open-face” Fe −C
n
(
CO) ] (5.0 mL, 37 mmol) was added dropwise to a solution of
2 4 2
5
bonding motif as analogues of heterogeneous catalytic systems.
[
Na Fe(CO) ] (1.56 g, 7.32 mmol) in diglyme (25 mL) under N
Remarkably, the carbide encapsulation stabilizes [Fe6]2 core
−
atmosphere at room temperature over a period of 5 min. The resulting
mixture was refluxed at 160 °C for 6 h, resulting in a violet solution
with concomitant elimination of CO gas. The mixture was then
gradually cooled to room temperature and subsequently washed with
hexanes (3 × 30 mL) to remove any organic-soluble impurities, to
obtain the dark violet residue of disodium salt of Na [Fe (μ -
+
under either an electrophilic substitution with [Au(PPh )] or
3
ligand replacement with nitrous oxide and sulfur dioxide,
2
4−26
However, chemical oxidation of [Fe6]2− with
respectively.
halide-containing oxidants and/or acidic workup yielded lower
nuclearity clusters via the oxidative elimination of
2
6
6
2
2
C)(CO)16]. This material was then extracted with degassed water (400
{
Fe (CO) (X) }units. It is likely that the increase of
l m n
mL); to this was added a solution of NMe Cl (1.77 g, 16.1 mmol) in
4
oxidation state coupled to the presence of halides (and in the
absence of chelating ligands) decreases the metal−metal bond
strength, thus leading to degradation of the six-iron cluster.
However, there are few reports regarding the “pure” redox
activity of [Fe6]2 clusters, although Berben and co-workers
degassed water (50 mL) dropwise at room temperature over a period
of 2 h. During this process, microcrystalline 1a was precipitated, and
the resulting mixture was stirred at room temperature for 12 h. The
purple materials of 1a were filtered out via a medium-pore-size PYREX
glass frit over a thick pad of Celite. The crude material was washed
−
with Et O and then extracted into THF (100 mL). Removal of solvent
have recently investigated both proton reduction and CO
electrocatalysis of nitride-based iron carbonyl clusters.
2
2
2
7,28
in vacuo afforded a violet crystalline material of 1a. Yield: 3.8 g (51%).
X-ray quality crystals of 1a were grown by vapor diffusion of pentane
Herein, we utilize a known series of five- and six-iron clusters
to prepare and study the “missing link” cluster in this series, the
1
8
into a THF solution of 1a at −20 °C. H NMR (400 MHz, d -THF,
13
1
8
ppm) (Figure S1): δ 3.21 (CH , bs). C{ H} NMR (100.5 MHz, d -
3
six-iron neutral cluster [Fe (μ -C)(CO) ] (2).
6
6
18
THF, ppm) (Figure S2): δ 484.7 (μ -C), 229.2 (CO), 56.0 (CH ).
6
3
To further utilize and characterize the six-iron core for
multielectron redox reactions, we have initiated a research
program to address the multielectron redox transformations of
−1
−1
UV/vis in nm (ε = L mol cm ) (Figure S3): 520 (3920). IR (solid
state) (Figure S4): 1910, 1730 cm . Anal. Calcd for C H N O Fe :
−1
29
32
2
17
6
C, 34.29; H, 3.18; N, 2.76. Found: C, 33.93; H, 3.18; N, 2.87.
B
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
NEt ) [Fe (μ -C)(μ -CO) (CO) ] (1b). The same procedure as for
460 (6440), 690 (2050). IR (solid state) (Figure S22): 1910, 1710
4
2
6
6
2
4
12
−1
1
a was followed, except that NEt Cl·H O (2.96 g, 16.1 mmol) was
substituted for NMe Cl. Yield: 44% (3.6 g). X-ray quality single
cm . Anal. Calcd for C O O N Fe : C, 33.22; H, 2.91; N, 3.37.
4
2
23 24 14 2 5
Found: C, 31.99; H, 3.07; N, 3.15.
4
crystals of 1b were grown by vapor diffusion of pentane into a THF
Physical Measurements. NMR spectra were recorded at ambient
1
8
solution of 1b at −20 °C. H NMR (400 MHz, d -THF, ppm) (Figure
temperature on a Varian DirecDrive 400 MHz instrument, and
1
3
1
1
13
1
S5): δ 3.32 (CH , 2H, bs), 1.33 (CH , 3H, bs). C{ H} NMR (100.5
chemical shifts are reported in δ (ppm). For H and C{ H} NMR
2
3
8
MHz, d -THF, ppm) (Figure S6): δ 484.7 (μ -C), 229.2 (CO), 53.2
CH ), 7.7 (CH ). UV/vis in nm (ε = L mol cm ) (Figure 7): 520
3410). IR (solid state) (Figure 8): 1901, 1742 cm . Anal. Calcd for
spectra, the solvent resonance was used as an internal reference; for
6
−
1
−1
31
1
(
P{ H} NMR spectra, 85% H PO was used as an external standard
2
3
3 4
−1
(
(0 ppm). IR spectra were recorded on a Bruker Alpha spectrometer
equipped with a diamond ATR crystal. UV/vis absorption spectra were
obtained using a Varian Cary 6000i spectrometer and 0.1 mM
solutions in 1 cm path length quartz cuvettes. Elemental analyses were
performed at Midwest Micro Lab, IN. The Raman spectra were
C H N O Fe : C, 39.40; H, 4.29; N, 2.48. Found: C, 39.96; H, 4.27;
3
7
48
2
17
6
N, 2.54.
PPh ) [Fe (μ -C)(μ -CO) (CO) ] (1c). The same procedure as for
(
4
2
6
6
2
4
12
1
a was followed, except that PPh Cl (2.2 g, 5.8 mmol) was substituted
4
for NMe Cl. The analytically pure materials of 1c were obtained by
recorded on solid samples using a Renishaw InVia microscope with a
4
+
vapor diffusion of pentane or diethyl ether into a THF solution of 1c
at −20 °C for 2−4 days. Yield: 87% (3.73 g). X-ray quality single
crystals of 1c were chosen from the crystallization described above for
514 nm Ar laser operated below 4 mW. Mo
̈
ssbauer spectra were
collected on a model MS4 WRC low field spectrometer and on a
LHe6T spectrometer (SEE Co.; Edina, MN). Both instruments were
calibrated using α-Fe foil at room temperature.
1
8
structural determination. H NMR (400 MHz, d -THF, ppm) (Figure
13
1
S9): δ 7.91 (Ar-H, 1H, bs), 7.77 (Ar-H, 4H, bs). C{ H} NMR
100.52 MHz, d -THF, ppm) (Figure S10): δ 484.6 (μ -C), 229.1
CO), 136.4 (Ar, JP-C = 3.3 Hz), 135.7 (Ar, JP-C = 10.0 Hz), 131.4
Ar, JP-C = 13.3 Hz), 119.5 (Ar, JP-C = 89.5 Hz). P{1H} NMR
161.83 MHz, d -THF, ppm) (Figure S11): δ 23.0. UV/vis in nm (ε =
L mol cm ) (Figure S12): 520 (4430). IR (solid state) (Figure
S13): 1937, 1768 cm . Anal. Calcd for C H O P Fe : C, 52.96; H,
XP spectra were obtained using an X-ray photoelectron
spectrometer (Kratos Axis Ultra) with a monochromated Al Kα X-
ray source (hν = 1486.5 eV). The photoelectron takeoff angle was
normal to the sample surface and 45° with respect to the X-ray beam.
8
(
(
(
(
6
31
8
−9
The pressure of the analysis chamber was sustained around 2 × 10
−1
−1
Torr during measurement. All spectra were obtained with a dwell time
of 1200 ms × 1 sweep (Si atom), 3000 ms × 1 sweep (C atom), 1200
ms × 1 sweep (Br atom), 3000 × 1 sweep (Al atom), and 1800 × 1
−1
65
40 16
2
6
2
.74. Found: C, 53.06; H, 2.81.
Fe (μ -C)(CO) ]·THF (2). A solution of 1a (0.50 g, 0.53 mmol) in
2
[
sweep (Ti atom). A 300 × 700 μm spot and 20 eV of pass energy
6
6
18
THF (15 mL) was combined with solid ferrocenium hexafluorophos-
phate (0.36 g, 1.1 mmol) at −78 °C under CO atmosphere. The
resulting mixture was gradually warmed to room temperature and then
stirred overnight. Insoluble materials were removed by filtration, and
all volatiles were removed in vacuo. The ferrocene byproduct was
thoroughly removed by multiple extractions with pentane (until clear),
followed by extraction of the desired cluster into toluene. The reaction
mixture was then purified through a pipet column packed with
BioBeads (3−4 cm height), and then again through a second pipet
column packed with C18 silica (3−4 cm height). All volatiles were then
removed in vacuo to give the analytically pure material of 2 as a brick-
with resolution of 0.025 eV (Si atom) and 0.1 eV (C, Br, Al, and Ti
atom) were used for the XPS measurement. The obtained spectra were
analyzed by the Casa XPS software (version 2.3.15, Casa Software
Ltd.) using 70% Gaussian and 30% Lorentzian function after
subtraction of Tougaard background.
X-ray Diffraction Data Collection and Crystal Structure
Refinement. The diffraction data for 1a, 1b, and 4 were collected on
either an Agilent Technologies SuperNova Dual Source diffractometer
or a Nonius Kappa CCD diffractometer using a Bruker AXS Apex II
detector. The data for 1c and 3 were collected on a Rigaku AFC12
diffractometer with a Saturn 724+ CCD, all using a graphite
monochromator with Mo Kα radiation. Low temperatures were
maintained using an Oxford Cryostream low temperature device. Data
reductions were performed using either Agilent Technologies
−1
−1
red powder. Yield: 0.30 g (61%). UV/vis in nm (ε = L mol cm )
(
1
2
Figure S14): 472 (1700), 820 (100). IR (solid state) (Figure S15):
−1
965 cm . Anal. Calcd for C H O Fe : C, 29.92; H, 0.87. Found: C,
23 8 19 6
3
0
CrysAlisPro V 1.171.37.3 or Rigaku Crystal Clear version
9.38; H, 1.18.
Fe (μ -C)(CO) ] (3). A slurry of 1a (1.36 g, 1.38 mmol) or 1b
3
1,32
2
2
1
.40.2.
The structure was solved by direct methods using
[
5
5
15
3
3
2
SIR97 and refined by full-matrix least-squares on F with anisotropic
(
3
1.44 g, 1.38 mmol) in toluene (400 mL) was layered with degassed
0 mM aqueous solution (250 mL) of FeCl ·6H O at room
34
displacement parameters for the non-H atoms using SHELXL-2013.
Structure analysis was aided by use of the programs PLATON98 and
3
2
35
temperature under inert atmosphere. The resulting mixture was
vigorously stirred at room temperature for 12 h, and during this period
the toluene layer turned black. Next, the toluene layer was carefully
separated out from the aqueous layer, and subsequently passed
through a medium-pore-size PYREX glass frit packed with a thick pad
of Celite. All volatiles were then removed in vacuo. The crude, black
shiny residue was washed with pentane (removing trace amounts of
green Fe (CO) ) to obtain a black microcrystalline material of 3.
3
6
WinGX. The hydrogen atoms on carbon were calculated in ideal
positions with isotropic displacement parameters set to 1.2 × Ueq of
the attached atom (1.5 × Ueq for methyl hydrogen atoms). Details of
crystal data, data collection, and structure refinement are listed in
Table S1. Experimental details and full thermal ellipsoid plots of 1a−c,
3
, and 4 including counterions are given in the Supporting
Information.
3
12
13
1
Yield from 1a: 63% (0.62 g). Yield from 1b: 25% (0.24 g). C{ H}
6
NMR (100.5 MHz, d -benzene, ppm) (Figure S16): δ 212.2 (CO),
RESULTS AND DISCUSSION
2
09.3 (CO). UV/vis in nm (ε = L mol−1 cm ) (Figure S17): 520
−1
■
2
−
Synthesis. The violet cluster [Fe (μ -C)(CO) ] was
originally isolated as a decomposition product of the
(
5490), 780 (2710). IR (solid state) (Figure S18): 2015, 1975, 1945
6
6
16
−1
cm . Anal. Calcd for C O Fe : C, 27.01. Found: C, 26.90.
1
6
15
5
2
2
−
20
(
NMe ) [Fe (μ -C)(μ -CO)(CO) ] (4). A precooled (−20 °C)
heterometallic anion [MnFe
2
(CO)12
]
by Churchill. We
4
2
5
5
2
13
solution of 3 (0.5 g, 0.7 mmol) in THF (25 mL) was added to a
prepared this cluster according to an alternate report by the
second vessel containing solid KC (0.19 g, 1.4 mmol). The reaction
stoichiometric (in iron) reduction of 5 equiv of [Fe(CO) ]
8
5
2
2
mixture was stirred at room temperature for 2 h, and to this was added
with Na [Fe(CO) ] in diglyme at 160 °C for 6 h (Scheme 1).
2
4
solid NMe Cl (0.16 g, 1.4 mmol). The resulting mixture was stirred
4
This procedure generated a violet solution of Na [Fe (μ -
2
6
6
for 12 h, and then the insoluble materials were removed by filtration;
the volatiles were removed in vacuo. Crystalline material of 4 was
obtained by vapor diffusion of pentane into a THF solution. Yield:
C)(CO) ]. Cation exchange with NMe Cl, NEt Cl, or PPh Cl
16
4
4
4
afforded X-ray quality crystals of (Y) [Fe (μ -C)(μ -
2
6
6
2
1
8
CO) (CO) ] (Y = NMe , NEt , PPh : 1a−c) on multigram
7
3
9% (0.46 g). H NMR (400 MHz, d -THF, ppm) (Figure S19): δ
4
12
4
4
4
13
1
8
scale and in good yields (50−90%). One type of oxidation of 1
.20 (CH , bs). C{ H} NMR (100.5 MHz, d -THF, ppm) (Figure
3
S20): δ 478.3 (μ -C), 229.1 (CO), 227.4 (CO), 223.4 (CO), 221.4
has been reported: reaction with FeCl in aqueous HCl results
3
5
−1
−1
(
CO), 56.0 (CH ). UV/vis in nm (ε = L mol cm ) (Figure S21):
in oxidative elimination of an {Fe(CO)Cl } unit, affording the
3
2
C
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
−
Scheme 1. Synthesis of the Starting [Fe ] Cluster 1 (Top
Biorad) and C -silica gel. The complex is not stable upon
18
6
0
Left), Its Neutral Congener [Fe ] (2, Top Right), and the
Closed Synthetic Loop Including the Crystallographically
Defined Five-Iron Clusters [Fe ] (n = 0 or 2−), Shown at
exposure to underivatized silica or alumina gel, or in
coordinating solvents, or under ambient light conditions.
Such instability is likely the reason for the lack of previous
reports on this complex, despite significant work on the stable
clusters 1a, 3, and 4.
6
n
5
Bottom
To further establish the identity of 2, in relation to its known
[
Fe ] and [Fe ] relatives, we performed synthetic interconver-
6 5
sions according to Scheme 1. Mild oxidation of 2 with 2 more
equiv of [Fc](PF ) under dilute conditions smoothly afforded
6
[
Fe (μ -C)(CO) ] (3, determined by single-crystal X-ray;
5 5 15
Figure 2, left). Subsequent two-electron reduction of 3 with 2
equiv of KC , followed by K → NMe cation-exchange,
8
4
2
−
smoothly afforded the brown [Fe5] species (NMe ) [Fe (μ -
4 2
5
5
C)(μ -CO)(CO) ] (4). Notably, this reaction can be reversed
2
12
by facile two-electron oxidation of 4 under CO atmosphere
with [Fc](PF ) to regenerate 3 in quantitative yield. Finally, to
6
2−
complete the synthetic loop, the [Fe ] cluster 4 can be
5
utilized to reform the parent [Fe6]2 cluster 1 by “capping” it
−
with 0.5 equiv of [Fe (CO) ] in THF at ambient temperature,
2
9
22
which cleanly regenerates 1 in quantitative yield.
X-ray Structures. Six Iron Clusters. (Y) [Fe (μ -C)(μ -
2
6
6
2
CO) (CO) ] (Y = NMe , NEt , PPh : 1a−c). The original
4
12
4
4
4
(
1971) report of the low-resolution, room-temperature X-ray
structure of 1a by Churchill and co-workers did not
satisfactorily describe the connectivity or bond metrics of the
20,21
bridging carbonyls.
In a recent report, a phase transition for
1a was observed between 200 K and room temperature, in
which one of the bridging carbonyls alternates between two
iron atoms, and as a consequence attains lower symmetry at
38
higher temperatures. To circumvent these limitations and
ambiguities, the precise low temperature structures of 1a−c (Y
5
5
15
2
2
= NMe , NEt , PPh ) were determined (Figure 1, Figures S23−
4 4 4
S25). The structure of 1c (Y = PPh ) provides both a higher
4
we found no reports regarding the preparation of oxidized
resolution and a higher symmetry structure, without the
complications of a phase transition as found for 1a (Tables
S1 and S2). Table 1 lists the pertinent interatomic parameters
for comparison. All of the anions retain the same molecular
formula and are mostly isostructural, but differ in the
connectivity of their (semi)bridging carbonyls.
0
[
Fe ] species. In the present case, an outer-sphere oxidation
6
route was used: reaction of 1 with 2 equiv of [Fc](PF ) under
6
an inert atmosphere leads to the isolation of a mixture of
neutral species, including the carbonyl clusters [Fe (CO) ]
2
9
(
yellow), [Fe (CO) ] (green), the carbide-cluster [Fe (μ -
3 9 5 5
C)(CO) ] (3), and the target [Fe ] cluster (eventually
Cluster 1a crystallizes in space group P2 with two molecules
15
6
1
identified as [Fe (μ -C)(CO) ] (2)). However, the same
in the asymmetric unit. Clusters 1b and 1c crystallize in
6
6
18
reaction under CO atmosphere cleanly affords the red solid of 2
in good yield. To ensure purity, the complex was sequentially
purified through cross-linked polystyrene beads (BioBeads,
orthorhombic and monoclinic space groups P4 /nnm and P2 ,
respectively, and each structure has just one anion per
asymmetric unit. The high crystallographic symmetry of 1b,
2
1
Figure 1. Molecular structures (isotropic view) of the anions in the crystal structures of 1a−c (Y = NMe , NEt , PPh ; from left to right,
4
4
4
respectively); counter-cations and solvents are omitted for clarity. Full thermal ellipsoid plots are shown in Figures S23−25.
D
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. X-ray Diffraction Experimental Details for 1a−c, 3, and 4
1
a
1b
1c
3
4
formula
fw
C H Fe N O
C H Fe N O
C H Fe O P
C Fe O
C H Fe N O
23 24 5 2 14
2
9
32
6
2
17
33 40
6
2
16
65 40
6
16
2
16
5
15
1015.66
133(2)
0.71073
1055.77
1474.01
711.41
100(2)
0.71073
831.69
T (K)
λ (Å)
133(2)
100(2)
100(2)
0.71073
0.71073
0.71073
a (Å)
11.4065(3)
15.6419(5)
21.1122 (7)
90
11.7157(4)
11.7157(4)
16.8663(6)
90
11.0889(16)
14.517(2)
18.902(3)
90
16.599(16)
8.8900(19)
29.696(7)
90
19.5579(13)
10.8967(10)
28.9033(16)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
space group
Z
91.210(4)
90
90
101.209(4)
90
105.337(8)
90
96.463(6)
90
90
3766.0(2)
P21
2315.03(18)
P42/nnm
2
2984.8(8)
P21
4225.9(16)
C2/c
6120.6(8)
C2/c
4
2
8
8
3
Dcalcd (mg/cm )
μ (mm−1)
GOF on F2
1.791
1.515
1.640
2.236
1.805
2.325
1.923
1.545
3.431
19.173
1.054
1.054
1.339
1.067
1.155
R (I > 2σ(I))
0.0352
0.0773
0.0372
41422
17007
0.0345
0.1087
0.0532
47922
0.0363
0.0924
0.033
0.0660
0.1370
0.100
0.1104
0.3320
0.1599
26965
1
a
wR2 (all)
Rint
Nref (all)
Nref (I > 2σ(I))
33022
10481
25556
3716
1430
6220
a
R1 = ∑∥F | − |F ∥/∑|F |; wR2 = {∑[w(F − F ) ]/∑[w(F ) ]}1/2.
2
2
2
2
o
o
o
o
c
2
o
Table 2. Selected Bond Distances for the 1a−c and DFT Optimized Distances (1DFT) Reported in This Work, As Well As the
Published Structure (1
)
pub
21
bonds
1a (NMe4)
1b (NEt4)
1c (PPh4)
1pub
1DFT
Fe1−C1/C4
Fe2−C1/C4
Fe3−C1
Fe4−C1
Fe5−C1
1.896(5), 1.877(5)
1.876(5), 1.890(5)
1.897(5), 1.902(5)
1.908(5), 1.906(5)
1.876(5), 1.869(5)
1.885(5), 1.885(5)
1.948(5), 1.959(5)
1.947(5), 1.960(5)
1.966(5), 1.946(5)
1.965(6), 1.953(6)
1.804(6), 1.799(6)
2.707(0), 2.654(4)
1.809(6), 1.800(6)
2.692(4), 2.709(0)
2.700(1), 2.702(1)
2.684(1), 2.668(1)
2.711(1), 2.679(1)
2.577(1), 2.584(1)
2.671(1), 2.680(1)
2.583(1), 2.574(1)
2.682(1), 2.705(1)
2.659(1), 2.661(1)
2.634(1), 2.662(1)
2.758(1), 2.729(1)
2.745(1), 2.750(1)
2.660(1), 2.647(1)
3.792(5), 3.778(5)
3.784(5), 3.796(0)
3.755(5), 3.754(5)
1.8743(7)
1.8838(5)
1.878(5)
1.874(5)
1.886(5)
1.898(5)
1.897(4)
1.881(4)
1.895(4)
2.064(4)
1.878(4)
2.136(4)
1.882(4)
2.077(4)
1.899(4)
2.036(4)
2.6561(9)
2.6771(9)
2.7559(9)
2.5964(8)
2.6721(9)
2.6050(8)
2.6663(8)
2.6663(9)
2.7139(9)
2.5975(9)
2.5795(8)
2.8186(8)
3.759(3)
3.764(2)
3.777(2)
1.963(36), 1.968(38)
1.888(35), 1.884(38)
1.862(36), 1.821(38)
1.888(35), 1.884(38)
1.834(36), 1.805(38)
1.907(36), 1.949(38)
1.969(60), 1.905(63)
2.326(61), 2.005(63)
1.780(43), 1.791(60)
2.163(43), 2.213(60)
2.222(10), 2.330(2)
2.720(3), 2.583(2)
1.780(43), 1.791(60)
2.163(43), 2.213(60)
2.706(9), 2.688(9)
2.706(9), 2.688(9)
2.662(11), 2.725(10)
2.609(10), 2.553(10)
2.699(9), 2.654(9)
2.632(10), 2.621(9)
2.682(9), 2.695(9)
2.699(9), 2.654(9)
2.682(11), 2.646(10)
2.743(10), 2.729(10)
2.632(10), 2.621(9)
2.682(9), 2.695(9)
3.779(0), 3.787(0)
3.788(4), 3.829(4)
3.752(3), 3.739(6)
1.8697
1.8692
1.8684
1.8679
1.8771
1.8775
1.8219
2.0439
1.8216
2.0459
1.8197
2.0494
1.8201
2.0482
2.6478
2.6448
2.6926
2.601
Fe6−C1
Fe1/Fe2−C4/C2
Fe6/Fe1−C4/C2
Fe2−C7
Fe5−C7
Fe3−C10
Fe6−C10
Fe4−C13
Fe5−C13
Fe1−Fe2
Fe1−Fe4/Fe2#1
Fe1−Fe5
Fe1−Fe6
Fe2−Fe3/Fe2#1
Fe2−Fe5
Fe2−Fe6
Fe3−Fe4
Fe3−Fe5
Fe3−Fe6
Fe4−Fe5
1.869(4)
2.101(4)
2.5955(7)
2.7180(7)
2.6670(8)
2.6428
2.5994
2.6945
2.6429
2.7001
2.6007
2.6012
2.698
Fe4−Fe6
Fe1··Fe3/Fe2··Fe2#1
Fe2··Fe4
Fe5··Fe6/Fe1··Fe1#1
3.764(0)
3.749(0)
3.7356
3.7346
3.7546
in particularly, mandates just two distinct Fe sites. For 1a−c,
bonds. The μ -central carbide atom encapsulated by six
equidistant iron atoms adopts a nearly octahedral geometry.
6
the cluster exhibits an octahedral μ -carbido ligand at its center
6
that supports the tightly knit network of Fe−C and Fe−Fe
The distances between carbide and iron atoms in 1a−c reside
E
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
0
2−
Figure 2. Thermal ellipsoid plots (30% probability) of the molecular structures of the [Fe ] cluster 3 (left) and the [Fe ] cluster 4 (right);
5
5
counter-cations and solvents are omitted for clarity. Full thermal ellipsoid plots are shown in Figures S26 and S27.
4
1
in the narrow range of 1.87−1.91 Å, whereas the original X-ray
structure 1 exhibits a much broader range of distances
(2.60 Å). Interestingly, they are slightly shorter than in
2
−
42−44
[Fe (CO) ] with organic cations (2.78−2.84 Å).
pub‑1
2
8
21
between 1.80 and 1.97 Å. In the room-temperature structure,
the Fe ions with the longest Fe−C bond appear to occupy
Five-Iron Clusters. [Fe
(μ
5
5
-C)(CO)15] (3). The solid-state,
19
room-temperature X-ray structure of 3 was reported. In this
work, we collected the low temperature data to confirm the
connectivity of the atoms within the cluster. The isotropic view
of the molecular structure of 3 is shown in Figure 2 (full
anisotropic thermal ellipsoid plot, Figure S26). Upon first
inspection, the only considerable change in bond parameters
noted in 3 as compared to the reported structure is a dramatic
elongation (∼0.2 Å) along the axis of the axial Fe5 ion (Table
carbide
positions trans from one another, thus revealing a pseudoaxial
configuration about the carbide center. In contrast, all of the
structures determined at low temperature (1a−c) exhibit a
more uniform distribution of Fe−C
distances, indicating a
carbide
more strictly octahedral arrangement of Fe ions about the
central carbide. The primary difference within the set of
matched anions involves the extent of bridging or semibridging
3
). This ion is 1.948(7) Å from the interstitial carbide at low
carbonyls. For example, in cluster 1a (NMe salt), there are two
4
semibridging carbonyls [e.g., Fe−C(O) = 1.799(6), 2.654(4)
Å]. In clusters 1b and 1c, all four bridging carbonyls are more
symmetric [e.g., 1b, Fe−C(O) = 1.869(4), 2.101(4) Å].
Cluster 1b has the highest symmetry, including two
crystallographically distinct Fe sites. The Fe2 ion of 1b
represents four “equatorial” Fe sites, and exhibits an Fe−
Table 3. Selected Bond Distances for 3 and 4 Reported in
This Work, As Well As the Two Previously Published
Structures Determined at Room Temperature
19
45
bonds
3
3‑RT
4
4‑RT‑NBu4
Fe1−C1
Fe2−C1
Fe3−C1
Fe4−C1
Fe5−C1
Fe1−Fe2
Fe1−Fe4
Fe1−Fe5
Fe2−Fe3
Fe2−Fe5
Fe3−Fe4
Fe3−Fe5
Fe4−Fe5
Fe1−C4
Fe2−C4
Fe1−C5
Fe4−C5
1.875(8)
1.90(3)
1.882(13)
1.866(14)
1.853(13)
1.862(14)
1.993(13)
2.657(3)
2.506(3)
2.637(3)
2.651(3)
2.579(3)
2.692(3)
2.594(3)
2.607(3)
2.532(0)
1.813(14)
1.990(16)
1.904(15)
1.87(2)
1.88(2)
1.84(2)
1.87(2)
2.00(1)
C
distance of 1.8838(5) Å. In contrast, the two “axial” Fe1
carbide
1.897(7)
1.96(3)
sites reside slightly closer to the carbide (Fe−C1 = 1.8743(7)
Å). Considering only the carbide and carbonyl groups as
ligands, the axial Fe1 sites in 1b display a distorted trigonal
bipyramidal geometry with two terminal and bridging carbon-
yls, and one bond to the central carbide. In this limited sense−,
the Fe1 ions are five-coordinate, with a total of 7 contributed e
1.865(8)
1.89(3)
1.893(7)
1.88(3)
1.948(7)
1.76(4)
2.6331(15)
2.6496(15)
2.6557(16)
2.6703(15)
2.5869(15)
2.6780(15)
2.6466(16)
2.5997(14)
2.675(6)
2.667(7)
2.650(6)
2.652(7)
2.600(7)
2.636(7)
2.666(7)
2.587(6)
2.597(4)
2.553(3)
2.597(4)
2.680(4)
2.591(2)
2.670(4)
2.590(2)
2.616(4)
2.28(2)
−
−
donors: two terminal CO (2 e each), two bridging CO (1 e
−
each), and one-half of a carbide 2p (1 e ). In contrast, the
z
equatorial Fe2 sites have trigonal pyramidal geometry,
−
consisting of donors that collectively donate 6 e to th−e
−
metal: two terminal CO (2 e each), one bridging CO (1 e
−
each), and one-half of a carbide 2px/y (1 e ).
1.85(2)
On the basis of the assignment of an axially oriented system
about the central carbide, there are two types of Fe−Fe bond
found in the cluster. The adjacent Fe−Fe bonds between the
equatorial sites are in the range of 2.57−2.76 Å [2.6670(8) Å in
2.15(2)
1.82(2)
1
b], whereas the “diagonal” Fe−Fe bonds between Fe and
eq
ax
temperature, as compared to the corresponding Fe−C
carbide
Fe are slightly longer [2.7180(7) Å in 1b]. Closer inspection
distance of 1.76(4) Å at room temperature. Interestingly, closer
inspection of the equatorial Fe−C distances reveals that
of the structure reveals that each of the longer, “diagonal” Fe −
ax
carbide
Fe interactions are supported by a bridging CO unit, whereas
eq
they are anticorrelated with this trend: the longer Fe −C
ax
carbide
the shorter Fe −Fe interactions are not. All of the Fe−Fe
eq
eq
distance of 1.948(7) Å found at low temperature goes along
with comparatively short Fe −C distances (avg = 1.883 Å,
contacts in 1a−c are longer than the Fe−Fe distances in
eq
carbide
39
metallic iron (2.48 Å) or the sum of the covalent radii of two
max = 1.897(7) Å), whereas the shorter Fe −C
distance
ax
carbide
40
iron atoms (2.80 Å). The Fe−Fe distances observed in these
of 1.76(4) Å found at room temperature is accompanied by
−
clusters are comparable to those of the [Fe (CO) ] anion
generally longer Fe −C
distances (avg = 1.908 Å, max =
3
11
eq
carbide
F
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
.96(3)). This reflects the dynamic nature and structural
Article
1
observable features (see DFT Calculations section for differ-
entiation).
The Raman spectra of 1−4 provide insight into the
symmetry of their structures. Dianion 1 exhibits several low
energy features (150−600 cm ) that are assigned to symmetric
breathing modes that radiate from the central carbide. A DFT/
Hessian calculation for 1 (vide infra, DFT section) indicates
that these low-energy Raman modes arise from synchronized
stretches among the Fe−(μ -C), Fe−Fe, and bridging CO
ligands (see DFT calculation results, Figure S29). The Raman
preferences of the iron−carbide bonding motif.
(
NMe ) [Fe (μ -C)(μ -CO)(CO) ] (4). As noted above
4
2
5
5
2
13
regarding modulating the counter cations in clusters 1a−c,
the coordination mode of metal carbonyls swings between
semibridging and bridging, which directly affects the electronics
at the particular metal sites. The cluster type of [Fe5]2 has
been known for several years, and used as a potential precursor
−
1
−
22
for heterometallic cluster synthesis. The reported cluster was
6
2
−
prepared as [Fe (μ -C)(μ -CO) (CO) ] with two disor-
5
5
2
2
12
dered NBu cations in the structure based on data collected at
room temperature. To obtain more precise structural
spectrum of 1a also exhibits a higher energy feature at 1320
cm , which is not readily assignable in the DFT calculation.
4
4
5
−1
information, we reformulated the cluster as the NMe salt
This feature is likely another symmetric breathing mode that
4
and prepared single crystals of 4. The refined molecular
synchronizes the oscillations of the μ -C−Fe skeleton with the
6
6
2
−
structure of the [Fe5] anion is shown in Figure 2; the full
terminal CO ligands.
2−
anisotropic thermal ellipsoid plots containing [Fe5] and
Interestingly, the Raman spectrum of 2 (Figure 4) exhibits
only the higher energy resonance, observed at 1550 cm− (μ6-
1
NMe ions can be found in Figure S27. The most notable
4
difference between 4 and the reported cluster are the distances
at the bridging carbonyl positions. For instance, one of the
bridging iron−carbonyl distances in 4 is considerably longer
(
=
Fe1−C4 = 2.532 Å) than others (Fe2−C4 = 1.813 Å, Fe1−C5
1.990 Å, Fe4−C5 = 1.904 Å); however, more symmetrical
bridging carbonyl distances are observed in the reported
complex (Fe1−C4 = 2.274 Å, Fe2−C4 = 1.851 Å, Fe1−C5 =
2
.149 Å, Fe4−C5 = 1.819 Å). Comparing cluster 4 with 3, the
anion’s metal−metal bond distances are broadly dispersed
across a range of ∼0.2 Å (i.e., 2.506−2.692 Å), whereas the
neutral cluster’s Fe−Fe distances lie in a narrower range of
∼
0.1 Å (i.e., 2.590−2.680 Å). A similar trend is found in the
Fe−C distances (1.853−1.993 Å for 4; 1.865−1.948 Å for 3).
int
In general, the metal−metal and metal−carbide distances of
both 3 and 4 are comparable in 1a−c.
Infrared and Raman Characterization. Figure 3 displays
2
−
the solid-state IR spectra of [Fe6] (1a, left in violet) and
Figure 4. Solid-state Raman spectra of 1a, 2, 3, and 4; λex = 532 nm.
C−Fe plus terminal CO); cluster 2 does not exhibit the lower
6
energy features observed in 1a that are associated with the
symmetric breathing mode of the bridging CO ligands. As
counter-examples, the Raman spectrum of 3 and 4 indicate a
less symmetric (i.e., noncentrosymmetric) coordination envi-
ronment about the interstitial carbide. Neither of these [Fe ]-
5
type clusters exhibits any prominent absorption feature in the
−
1
−1
1
4
50−2000 cm region (1700−2000 cm not shown) (Figure
), consistent with the absence of symmetric breathing modes.
Figure 3. Solid-state IR spectra of 1a and 2. The feature at 1730 cm−
in the spectrum of 1a indicates bridging CO ligands.
1
As such, this indicates the retention of some form of a
centrosymmetric (μ -C)-[Fe ] core for 2.
6
6
XPS of 1 and 2. To obtain further information regarding the
formal or spectroscopic oxidation states contained in 1 and 2,
we collected their X-ray photoelectron spectra. Much like X-ray
absorption spectroscopy (e.g., XAS K-edge), the binding
energies observed in XPS correlate quite well to the average
0
[
Fe ] (2, right in red). The terminal and bridging CO ligands
6
in 1a are evident in the absorption features at υ = 1910 and
730 cm , respectively. In contrast, the most prominent
feature in the IR spectrum of 2 corresponds to a terminal υ
CO
−1
1
46
oxidation state(s) of the Fe clusters. The comparative XP
CO
−1
−9
feature at 1965 cm . This blue-shifted feature suggests an
overall higher valence of the iron centers in the oxidized cluster,
thus diminished dπ(Fe) → π*(CO) back-bonding. Addition-
ally, the IR spectra of the crystallographically defined five-iron
clusters 3 and 4 provide further analogy. While the dianion 4
does exhibit bridging carbonyls in its crystal structure and its
solid-state IR spectrum (υCO = 1710 cm ), the neutral cluster
does not. The lack of a bridging CO feature in the IR of 2 is
consistent with either (a) a lack of bridging υCO ligand(s) or
b) a lower symmetry structure with fewer experimentally
spectra (solid samples, room temperature, 1 × 10 Torr) for
1a and 2 are displayed in Figure 5. In our experiment, the XP
spectrum for 1a includes prominent core 2p (3/2, 1/2) features
at 705.5 and 718.3 eV, slightly shifted from the values reported
46
by Sosinksy and co-workers (likely due to a different
standardization procedure). In the case of 1, the component
fit of the spectrum was modeled using primarily one
component, with the exception of a “trailing” feature on both
the 2p3/2 and the 2p1/2 features; this trailing component is
observed even in the case of mononuclear iron complexes that
−1
3
(
G
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Inorganic Chemistry
Article
3
exhibited its 2p edge at 709.6 eV. The +1.5 eV shift observed
2
−
0
for [Fe5] → [Fe ] (4 → 3) is very similar to the +1.9 eV
shift observed for [Fe6] → [Fe ] (1 → 2). Overall, in this
5
2−
0
6
case, the +2 eV change in the 2p edge in going from 1 → 2
likely reflects the analogous two-electron oxidation of the
cluster.
Another indicator that has not been reported previously is
the change in ionization energy of electrons closer to the
valence shell, the 3p feature (Figure 5, right). Cluster 1 exhibits
its 3p feature at 52.1 eV, whereas the corresponding feature in 2
is shifted to higher energy (53.7 eV). Similar to the trend for
the 2p feature, this +1.5 eV shift to higher energy indicates an
overall higher oxidation state in the oxidized cluster 2. Overall,
it is important to note that due to the “high” temperature of the
experiment (298 K), only the average oxidation state across the
cluster can be determined, and the precise distribution of
oxidation states for a mixed-valence cluster cannot be resolved.
Two primary conclusions can be deduced from the XP
spectra and corresponding contextual data (Table 4). First, it is
Figure 5. XP spectra for 1a and 2 in the Fe 2p region (left) and Fe 3p
region (right). Experimental conditions: solid samples, rt, 1 × 10
Torr, Al Kα X-ray source.
−9
47
are unambiguously Fe(II), such as ferrocene. Therefore, it can
be concluded that the 2p spectrum for 1 is indicative of a single,
delocalized oxidation state at room temperature. This is
2
−
0
clear that the oxidation from [Fe6] → [Fe ] (1 → 2) is
6
primarily iron-based. Second, the quantitative shift of about +2
n
eV supports a two-electron oxidation, as observed in the [Fe5]
̈
consistent with the reported room-temperature Mossbauer
4
6
case. Although the XP spectra provided tangible and direct
evidence for the overall oxidation observed in 2, further
spectroscopic studies at low temperatures were required to
investigate any possible mixed valence configuration of the
ground state of 2.
spectrum for 1, which was comprised of a single quadrupole
doublet (vide infra). In contrast, the low temperature
Mossbauer of 2 (vide infra) indicates two sets of irons with
̈
distinct coordination environments and/or oxidation states.
By comparison, the XP spectrum of 2 exhibits its core Fe 2p
features at higher energy: 707.4 and 720.7 eV. This roughly +2
eV shift in ionization energy is consistent with an overall higher
average Fe oxidation state in 2 versus 1. Another interesting
distinction between the 2p regions of 1 and 2 can be seen in the
component fit for each cluster. In contrast to cluster 1, the 2p
region for 2 can only be modeled using two main components
at {707.0/719.8 eV} and {708.8/721.9 eV} (plus the obligate
trailing feature), in a ratio of 1:1.57 (low:high BE). This
suggests fewer iron centers (possibly ∼2/6) in a higher
oxidation state and more (possibly ∼4/6) in a lower oxidation
state. Considering the evidence from the XP spectra of known
clusters and complexes (see next paragraph), the formal charge
on the two sets of irons in 2 could vary by as much as two units.
Thus, the nominal oxidation state configurations for 2 could be
Mo
̈
̈
ssbauer Spectroscopy for 1 and 2. A Mossbauer
spectrum of the reduced cluster (NMe ) [Fe (μ -C)(μ -
4
2
6
6
2
CO) (CO) ] (1a) was collected at 5 K and 0.05 T (γ-rays
4
12
parallel to the applied magnetic field). The spectrum (Figure 6)
consisted of two partially overlapping quadrupole doublets. The
solid red line is a simulation assuming the parameters
delineated in Table 5. Sites 1 and 2 represented 2/6 and 4/6
of the spectral intensity, respectively. Within the context of the
two axial Fe sites plus four equatorial Fe sites described in the
X-ray section, site 1 corresponds to the two axial irons and site
2 to the four equatorial irons. The isomer shifts associated with
both sites are similar, suggesting that the corresponding irons
have the same or similar oxidation states. The Mo
̈
ssbauer
3
8
spectrum of the same cluster at 295 K has been reported;
2
+
0
1+
0.5
−1
−1
(
Fe ) (Fe ) or (Fe ) (Fe ) , when considering the carbide
ΔE
Q
was 0.57 mm s and δ was −0.004 mm s (we calibrated
2
4
2
4
4
−
−1
as C .
The reported XP spectra for the crystallographically defined
five-iron clusters [Fe5] (4) and [Fe ] (3) are insightful in
the reported value of δ = 0.22 mm s vs sodium nitroprusside
−1
48
by subtracting 0.22 mm s ). Minor differences observed
between the δ and ΔEQ values are probably due to
temperature-dependent shifts.
2−
0
5
this regard. In an early report, the reduced cluster 4 exhibited
its Fe 2p edge feature at 707.9 eV, at higher energy as compared
̈
A Mossbauer spectrum of the oxidized cluster 2 was collected
at 5 K (Figure 6B) and at 30 K (Figure S28); the spectra were
essentially indistinguishable. Both consisted of four quadrupole
−
to the dianion 1a (705.5 eV), due to the 2 charge distributed
4
6
0
across fewer irons. By comparison, the oxidized [Fe ] cluster
5
Table 4. Summary of the IR, Raman, and XPS Properties of 1a−c, 2, 3, and 4 from This Work and Previous Reports (As
Indicated in the Table)
infrared (cm−1)
Raman (cm−1
μ -C-[Fe ]-CO(terminal)
)
XPS Fe 2p (eV)
2p1/2
cluster
terminal CO
bridging CO
1730
μ -C-[Fe ]-CO(bridging)
2p3/2
3p
6
6
6
6
−
1
2
3
a [Fe ]2
1910
1965
2015
150−600
1320
1550
705.5
707.4
718.3
720.7
52.1
53.7
6
[Fe ]0
6
[Fe ]0
709.6
a
722.7
a
5
1
1
975
945
[Fe ]2
−
1910
1710
707.9
a
720.4
a
4
5
a
Reference 46.
H
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Article
̈
Figure 6. Mossbauer spectra of 1a (A) and 2 (B) in frozen THF at 5 K
and 0.05 T. Solid red lines are composite simulations using parameters
given in Table 5. The paramagnetic impurity in cluster 2 (blue line in
B) constitutes 16% of the iron in the sample.
doublets, with parameters as listed in Table 5. The parameters
for sites 1 and 2 are similar to those of trigonal bipyramidal
−
1
−1
Fe(CO) (δ = −0.09 mm s and ΔE = 2.57 mm s ) and
5
Q
2−
−1
tetrahedral [Fe(CO) ] (δ = −0.18 mm s and ΔE = 0 mm
4
Q
−1
49
s ). Thus, the isomer shift values for sites 1 and 2 suggest
iron oxidation states between 0 and −2. Site 4 was not
simulated in Figures 6 or 7, as it was ascribed to adventitious
iron.
To further investigate the magnetic properties of 2,
̈
Mossbauer spectra were collected at 4.2 K with applied fields
ranging from 0−6 T (Figure 7). Spectra were fitted assuming
that the spin state of the oxidized cluster was either S = 0 or S =
50,51
1. Site 4 was likely a paramagnetic contaminant,
so spectral
intensity due to this site was ignored in the fitting. Simulations
Figure 7. Variable field 4.2 K Mo
̈
ssbauer spectra for a frozen solution
assumed two cluster sites, including one with δ and ΔE values
Q
of 2 in toluene. Fields were applied perpendicular to the γ radiation.
Red lines are composite simulations assuming parameters given in
Table 5 and described in the text. The gold and green lines are
simulations for the equatorial and axial irons, respectively.
that were the average of those parameters for sites 1 and 2
(
Figure 7, yellow line), and the other with δ and ΔE values for
Q
site 3 (Figure 7, green line). The two sites represented 4/6 and
/6 of the remaining spectral intensity, respectively. Simu-
2
lations that assumed S = 0 (Figure 7, red lines) fit the spectra
acceptably well at all applied fields, whereas those that assumed
S = 1 did not. We conclude that the oxidized cluster is
diamagnetic.
DFT Calculations for 1 and 2. To provide further insight
into the electronic structure of 1, as well as structural
possibilities for 2, DFT calculations were pursued. To validate
the calculation parameters, we first compared the X-ray
structural parameters of 1 to the bond metrics derived from
the DFT optimized structure of 1DFT (B3PW91/G6-31). Figure
8 displays the geometry optimized structure of 1DFT. Similar to
̈
Table 5. Mossbauer and Parameters for 1a and 2
Mo
̈
ssbauer parameters
DFT charges
cluster and site
NMe ) [Fe (μ -C)(CO) ] (1a)
δ (mm s−1)
ΔEQ (mm s−1
)
Γ (mm s−1
)
area (%)
Mulliken
(
4
2
6
6
16
site 1 (ax)
site 2 (eq)
carbide
0.076
0.064
0.36
0.57
0.26
0.27
37
63
0.0377, 0.0376
0.449, 0.450, 0.449, 0.453
−1.269
[
Fe (μ -C)(CO) ] (2)
6 6 18
site 1 (eq)
site 2 (eq)
site 3 (axial)
site 4
−0.20
−0.11
0.42
1.37
0.76
2.30
2.8
0.29
0.47
0.32
0.58
22
42
30
16
0.353, 0.374
0.383, 0.428
0.465, 0.586
1.2
carbide
−1.045
I
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Article
Figure 8. Molecular structure of the DFT-optimized structure for 1DFT: (left) complete view of the structure including CO ligands; (right) truncated
2
−
view of the iron-carbide structure to highlight the [Fe (C)] core.
6
Table 6. Summary and Comparisons of the Experimental (X-ray) and Calculated Averages for Fe−C , Fe−C(O), and Fe−Fe
int
Bond Lengths (Å) in 1a−c, 1 , 1 , and 2
pub
DFT
DFT
2−
[
Fe (μ -C)(μ -CO) (CO) ]
[Fe (μ -C)(CO) ]
6 6 18
6
6
2
4
12
2
1
a
1
a
1b
1c
1pub
1DFT
2DFT
1.96 ± 4
Fe−C(carbide) [all]
Fe−C(carbide) [Ax]
Fe−C(carbide) [Eq]
Fe−C(O) [all]
1.889(14)
1.879(6)
1.886(10)
1.888(53)
1.8716 ± 17
1.87730 ± 14
1.8688 ± 3
1.81 ± 3
2.647 ± 11
2.648 ± 17
2.6446 ± 10
2.016 ± 16
1.84 ± 3
1.84 ± 2
2.76 ± 8
2.73 ± 4
2.8 ± 2
2.101(357)
2.671(53)
1.985(164)
2.660(62)
1.983(105)
2.667(70)
2.122(285)
2.671(44)
Fe−Fe [all]
Fe−Fe [Ax−Eq]
Fe−Fe [Eq−Eq]
a
DFT values are presented as avg ± std dev across the multiple instances of that bond in the calculated cluster.
Figure 9. Molecular structure of the DFT-optimized structure for [Fe (μ -C)(CO) ] (2DFT): (left) complete view of the structure including CO
6
6
18
ligands; (right) truncated view of the iron-carbide structure to highlight the [Fe (C)] core.
6
the most symmetric experimental structure 1c, the number of
bridging COs (4) in 1DFT was minimized. As shown in the last
column of Table 2 (X-ray section) and above in the more
concise Table 6, the DFT-optimized average distances for the
Fe−C(carbide) bonds (1.872 Å) and Fe−Fe bonds (2.647 Å)
are within the range of the experimentally determined bond
distances. In contrast, the most clearly variable metric is the
average Fe−C(O) distance, which varies from 1.983−2.122 Å
in the experimental data due to the range of bridging to
semibridging to terminal carbonyls observed in the X-ray
structures. The DFT calculated average Fe−C(O) bond for
1
(1.81 Å) is notably shorter than the shortest experimental
DFT
distances in 1b and 1c (1.985 and 1.983 Å, respectively), which
exhibit more symmetric bridging carbonyl ligands (unlike the
J
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Inorganic Chemistry
Article
semibridging COs found in 1a and 1pub). Overall, one can
conclude that the DFT calculated structure of 1DFT accurately
from site 1 irons of the reduced cluster, forming the site 3 irons
of the oxidized cluster. The 1:2 area ratio of site 1:site 2 in the
reduced cluster suggests that site 1 represents axial irons and
site 2 equatorial irons. The isomer shift of the site 3 doublet in
the oxidized cluster was 0.33 mm s− greater than that for site 1
of the reduced cluster, indicating a substantial oxidation. In
contrast, the δ values of sites 1 and 2 of the oxidized cluster
were ca. 0.2 mm s− lower than those of the reduced cluster.
models the [Fe (C)] core structure of 1a−c, although some
6
question remains regarding the predictive power of the
calculation with respect to Fe−C(O) bond distances. None-
theless, we conclude that the DFT calculation parameters are
appropriate for predicting the core structure and electronic
configuration for a given iron-carbido-carbonyl cluster to a first
approximation. In this case, the given cluster is [Fe (μ -
1
1
̈
The simplest interpretation of the Mossbauer spectra of the
6
6
C)(CO) ] (2), for which we were unable to crystallo-
reduced cluster is that all six irons of the reduced cluster have a
formal oxidation state of +1/3. However, the decreased
symmetry of the cluster and the DFT Mulliken charges suggest
18
graphically determine its structure.
Thus, DFT computations were used to predict the structure
of [Fe (μ -C)(CO) ] (2). In principle, this formulation could
0
+0.5
6
6
18
2 Fe (axial) and 4 Fe (equatorial) sites. In that case, the two
electrons removed during oxidation of the dianion would come
include six {Fe(CO) } units capping the central carbide (only
3
0
II
terminal COs). However, this general structure proved
unstable: using severely restricted gradient perturbation
parameters prevented the DFT system from converging. In
contrast, looser constraints on geometry optimization revealed
a very different core structure for 2 versus that for 1. One face
of the cluster was opened by breaking an equatorial Fe−Fe
bond (Figure 9; new Fe···Fe distance, 3.589 Å), and the
from the two axial Fe ions (which become the “site 3” Fe ’s in
the oxidized cluster). This suggests that sites 1 and 2 become
reduced from Fe+ to Fe upon oxidation of the cluster (Figure
3). We conclude that the irons of the reduced cluster further
disproportionate upon oxidation, with equatorial irons
becoming reduced and axial irons becoming oxidized.
0.5
0
There is some precedence for individual irons of a carbonyl
carbide Fe cluster becoming reduced when the cluster overall
becomes oxidized. Sosinsky et al. reported that δ values decline
remaining equatorial Fe−Fe bonds shortened slightly (Fe −
Eq
Fe = 2.451, 2.596, 2.604 Å), while the axial Fe(CO) units
Eq
3
46
compressed (Fe−C
= 1.817 Å) as compared to the
as the anionic charge on Fe carbonyl clusters increases; for
carbide
0
−
equatorial iron-tricarbonyl units (Fe−C
= 1.981 Å). The
(1.84 Å) remained similar
example, in the series [Fe (CO) ] , [Fe (CO) H] , and
carbide
2 9 2 8
2
−
−1
average Fe−C(O) distance for 2
[Fe (CO) ] , δ = 0.42, 0.33, and 0.18 mm s . However,
2 8
DFT
to that for 1
(Fe−C(O) = 1.81 Å).
trends involving δ were opposite for several iron-carbido
DFT
0
Overall, this “open” structural motif is reminiscent of the
open “butterfly” shape of the four-iron carbido cluster [Fe (μ -
carbonyl clusters: [Fe C(CO) ] had lower δ than did
5
15
2
−
−1
4
4
[Fe C(CO) ] (0.20 vs 0.35 mm s , respectively), which
5
14
3
,23
C) (CO) ], wherein the four-coordinate carbide is exposed
suggests that the iron framework in the neutral (oxidized)
cluster is more electron rich than that in the dianion (reduced)
cluster. These researchers suggested that the additional electron
density was localized on the carbido carbon in the dianion, such
that the irons in the dianion are more oxidized even though the
13
to the solvent on one face of the cluster. In this sense, 2
represents a hybrid of structural motifs borrowed from the six-
2
−
iron, closed structure of the [Fe6] dianion (1), and the open
0
structure of the four-iron cluster [Fe ] . The five-iron clusters
4
46
of intermediate nuclearity (both neutral and dianion) exhibit
their open sites below the basal plane of the square pyramidal
core structure (defined by the carbide and the four equatorial
Fe centers). Perhaps most importantly, the symmetry of the
calculated structure provides three chemically distinct iron sites,
which is consistent with the three Fe sites observed in the
Mossbauer spectrum of 2.
̈
To further substantiate the validity of the DFT calculations
for 1 and 2, we also used those calculations to predict IR
spectra (Hessian calculation) for each cluster. The predicted IR
overall cluster is more reduced. They also suggested that the
2
−
cluster distorts (similar to that seen in [Fe C(CO) ] vs
4
12
0
[
Fe (CO) ] ) to alter the dihedral angle about carbon and
4 13
Fe−C bond lengths.
In the clusters that we have investigated, the average isomer
shift of the four equatorial irons of the oxidized neutral cluster
was lower than that of the reduced dianion cluster irons (−0.15
mm s− vs +0.07 mm s ). This follows the same trend
reported by Sosinsky, supporting the idea that the carbido
carbon accepts a significant amount of the additional electron
density associated with the reduced cluster. Upon oxidation,
most electron density comes out of the two axial irons (and
some out of the carbide). It is possible that the bonding of the
equatorial irons to the carbido ion weakens such that these Fe
centers are more easily reduced.
1
−1
spectrum for cluster 1
(Figure S30, Table S4; scaling factor
DFT
6
2
0
.9631) includes two remarkably prominent and exclusive
−
1
features: a single terminal feature at 1891 cm (cf., exp avg =
1
−1
916 cm , Figures S4, S8, and S13), and two less intense
−1
bridging CO features at 1737 and 1721 cm (cf., exp avg =
−
1
1
747 cm ). The results are quite consistent with the
Lee and Peters have investigated a series of three
monocarbonyl Fe complexes in which the Fe oxidation state
experimental IR spectra for 1. Regarding the less symmetric
cluster 2DFT (IR, Figure S31), a broader range of both terminal
52
is formally 0, +1, and +2. Treatment of the complex in the
−1
CO stretches (1958−1972 cm ) and bridging CO stretches
most reduced state with Me SiOTf (trimethylsilylttrifluorome-
3
−1
(
1733−1808 cm ) is observed, consistent with the broader
thansulfonate) afforded an iron carbyne that included a Fe
experimental spectrum. Interestingly, the bridging CO features
cannot be identified in the experimental spectrum for 2 (Figure
S15), likely due to contributions from multiple yet weak
oscillations.
C−O−SiMe unit. Here, a strongly π-accepting Fischer-type
3
carbyne ligand, in which the iron back-donates to the carbon,
0
IV
stabilized the Fe state rather than Fe . These researchers
+
regarded the carbyne as a {COSiMe } closed-shell cation, with
3
one σ donor and two π acceptor orbitals at the carbon. The
DISCUSSION
■
range of isomer shifts is compressed due to strong covalency.
−
Structure, Mo
dized cluster 2 (or [Fe ] ) was obtained from the dianion 1 by
̈
ssbauer, and DFT Interrelations. Oxi-
The isomer shifts of the {Fe−CO} , {Fe−CO}, and {Fe−
0
+
CO} moieties shift positive as the overall charge (and the
6
0
a two-electron oxidation. These two electrons were removed
relative Fe oxidation state) increases from Fe (δ = 0.089 mm
K
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Inorganic Chemistry
Article
−
1
I
−1
II
−1
−1
s ) to Fe (δ = 0.21 mm s ) to Fe (δ = 0.31 mm s ). The
average δ for the cofactor irons is 0.41 mm s , which is
−1
isomer shift of the related carbyne complex (δ = 0.061 mm s )
is essentially the same as that for the equatorial Fe’s of [Fe ] ,
which rules out the Fe designation and supports the Fe
assignment. The isomer shift for d (Fe ) trigonal bipyramidal
carbonyl complexes range from −0.09 to −0.18 mm s . Last,
although the ligand environment is quite different, iron carbide
nanoparticles exhibit Mo
with δ = 0.41 mm s , ΔE = 1.06 mm s at 7 K, which is
rather similar to the parameters of site 3.
Cluster Electron-Counting Perspective. Another lens
through which to view and give context to the present results
are the metallocluster electron counting rules, analogous to
thought to arise from a cofactor with oxidation states
{(Mo )(Fe ) (Fe ) }. Although the isomer shift is similar
0
IV
II
III
6
4 3
IV
0
to that of site 3, the coordination environment of the two
clusters is significantly different; the iron centers of the
nitrogenase cofactor are coordinated by sulfide ions, while
those of site 3 are coordinated by carbonyls. However, these
researchers calculated that the interstitial C has the effect of
increasing average δ for the irons in the FeMoco cluster by 0.15
± 0.03 mm s , and this effect might be transferable to site 3.
That is, the bonding interaction to the central carbide might
explain, to some extent, the unusually high δ of the site 3 irons.
8
0
−1
52
̈
ssbauer spectra that include a doublet
Q
−1
−1
9
−1
54
̈
Munck and co-workers also found that Fe−C interactions in
FeMoco were primarily ionic in character rather than covalent.
53
Wade’s rules for carboranes. Of particular use in this case are
the “14n+2” rule for closo structures, and the “14n+4” rule for
This may favor a Schrock-type Fe−C interaction, which would
15,53
2−
55
nido structures (Scheme 2).
Considering [Fe6] in a
stabilize Fe in a higher valent state, rather than a Fischer-type
interaction, which would stabilize Fe in a lower valent state. We
Scheme 2. General Formulation and Electron-Counting for
Metal−Carbido Clusters of the Closo and Nido Variety
Synthesized Herein and Predicted by This Work
note, however, that the Fe−C(carbide) bond lengths found in
10
the enzyme (all ∼2.0 Å) are approximately ∼0.1 A longer
than those observed in 1, 3, and 4, although the DFT-calculated
Fe−C(carbide) distances in 2 (1.96−2.02 Å) are quite similar
to that of the enzyme.
The function of the interstitial carbide in FeMoco has been
the subject of great interest, but evidence supporting any
56
proposed function is limited. Ribbe and co-workers found
that the carbide does not exchange during turnover and is not
directly involved in substrate binding or turnover. Cramer and
57
co-workers found that the Fe−C stretching force constant
associated with the FeMoco cluster was nearly 10-fold less than
comparable constants in low-spin Fe clusters such as [Fe C-
4
(
CO) ], and that the nitrogenase carbide allows for structural
12
−
changes upon substrate or inhibitor binding. Creutz and
covalent electron-counting rationale, the carbide (4 e ), six iron
58
−
−
Peters have suggested that the carbide controls the reactivity
of the ligated iron center. They reported that the carbanion of
related complexes can accommodate a significant ionic charge
and thus stabilize an ionic Fe−C interaction. Theoretical
calculations by Rao, Xu, and Adamo suggest that the hydrogen-
and nitrogen-based substrates bind directly to the carbide,
leading to a structural rearrangement. Also, Dance has
suggested that the carbide modulates the reactivity of the
cluster through “coordinative allosterism”.
centers (48 e ), 16 CO ligands (32 e ), and two extra electrons
dianionic, 2 e ) afford an 86 e cluster. This predicts a closo-
−
−
(
−
M structure: (14 × 6) + 2 = 86 e .
6
0
Regarding [Fe ] , an attempt to rationalize the same
formulation of this cluster as a closo-[Fe (C)(CO) ] was
unproductive: this cluster is an 88 e formulation, while the
closo-[Fe ] electron-counting predicts only 86 e . An alternate
explanation would be a formulation with one less CO ligand, as
closo-[Fe (C)(CO) ] (86 e ). However, this would require the
6
6
18
−
59
−
6
60
−
6
17
Our results also favor the notion that the carbide ligand is
uniquely poised to control structural changes when accom-
panied by changes in the distribution of redox states. The
concept of “deficit spending” proposed by Seefeldt, Hoffman,
and Dean has been applied mainly to the P-clusters, the idea
being that the P-clusters initially donate electrons (to the
FeMoco and then to substrate) followed by a reduction that
returns the P-clusters to their initial state. Given our results,
one could imagine the initial donation of electrons from
FeMoco to substrate followed by reduction (from the P-
clusters) to return the FeMoco to its initial state. This would
promote a redox-dependent disproportionation of electron
presence of a single bridging CO ligand, and no experimental or
computational evidence for a truly bridging (i.e., not “semi-
bridging”) CO ligand was observed. Continuing with the
[
Fe (C)(CO) ] formulation, we were enthused to find that the
6 18
DFT-optimized structure for 2 was a nido-pentagonal
bipyramid (PBP)i.e., a PBP with one Fe missing; this cluster
−
is, of course, also formulated as an 88 e cluster. However, the
61
nido cluster electron-counting rule for a six-metal cluster with
−
an “open” metal site predicts 14n + 4 = 88 e , which is in
perfect agreement with both the experimental formulation and
the DFT optimized structure. Indeed, the cluster electron-
counting rules should provide a framework for predicting the
isolability of higher nuclearity iron carbide clusters. For
example, the pentanuclear PBP cluster closo-[Fe (C)(CO) ]
density in the cluster, thereby reducing the Fe’s at which N or
2
H bind while other Fe’s in the FeMoco become oxidized.
2
7
21
2
−
Through such intracluster redox disproportionations (as found
in the present cluster 2), particular iron sites in the cofactor
might become unusually powerful nucleophiles/reductants,
or the related dianion closo-[Fe (C)(CO) ] may be isolable
7
20
clusters. Our continuing synthetic work in this area is testing
these hypotheses, to further push the boundaries of hypervalent
carbide bonding motifs.
sufficiently reactive to bind N and reduce the triple bond in
2
the presence of protons.
Relations to Nitrogenase FeMoco. The FeMoco cofactor
of nitrogenase contains a central interstitial carbon atom, and
CONCLUSION
We summarize our findings as follows:
Mu
̈
nck and co-workers have used Mo
̈
ssbauer spectroscopy and
■
54
DFT to better understand the effect of this species. The
L
DOI: 10.1021/acs.inorgchem.7b00741
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
1) Oxidation of the known dianionic [Fe6]2− cluster with an
Article
(
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*
Corresponding Author
E-mail: mrose@cm.utexas.edu.
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ORCID
Transition-Metal Cluster Carbides Which Have an Exposed Carbon
Atom. Organometallics 1984, 3, 962−970.
Chris Joseph: 0000-0002-9763-5162
Michael J. Rose: 0000-0002-6960-6639
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